Syntheses of Aluminum Amidotrihydroborate Compounds and Ammonia
Triborane as Potential Hydrogen Storage Materials
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the
Graduate School of The Ohio State University
By
Jason Michael Hoy
Graduate Program in Chemistry
The Ohio State University
2010
Master's Examination Committee:
Dr. Sheldon G. Shore, Advisor
Dr. James A. Cowan
Copyright by
Jason Michael Hoy
2010
Abstract
A number of methods and materials have been synthesized for use as hydrogen storage
materials. However, to date, none of the materials are capable of being used as a sustainable fuel
source as a result of poor recyclability. Therefore, new materials need to be synthesized and
evaluated in order to obtain the goal of creating a hydrogen fuel economy. Furthermore, some
possible hydrogen storage candidates have been ignored as a result of poor and laborious
syntheses. Finding new synthetic routes to these materials opens exploration of their effectiveness
as a hydrogen source.
The reaction of lithium aluminum hydride with ammonia borane has been investigated in
varying ratios. Evaluation of the hydrogen released, proton and boron-11 NMR spectroscopy, and
infrared spectroscopy indicate the formation of a compound of composition LiAl(NHBH3)H2 in
the equimolar reaction of lithium aluminum hydride and ammonia borane followed by subsequent
addition of amidotrihydroborate to this material with the presence of additional ammonia borane
to form LiAl(NHBH3)(NH2BH3)H and LiAl(NHBH3)(NH2BH3)2. It was unclear whether the
reaction in a ratio of 1:4 lithium aluminum hydride to ammonia borane produced LiAl(NH2BH3)4
or if the product was identical to that of the reaction in a 1:3 ratio. All of the compounds made
retain a high gravimetric capacity of hydrogen.
Solvent-free sodium octahydrotriborate was synthesized via a new method en route to
ammonia triborane. Tetrahydrofuran borane complex solution was stirred with an amalgamation
of sodium and mercury to produce the solvent coordinated sodium octahydrotriborate and sodium
ii
borohydride. The product was separated by extraction with ethyl ether and heating to remove
coordinated solvent. Average yield of the final product was approximately 60%.
A literature method for synthesizing ammonia triborane was refined, removing the need
for multiple cooling steps and for the use of column chromatography to purify the product. The
reaction of elemental iodine with tetrabutylammonium octahydrotriborate was allowed to react at
room temperature rather than addition at low temperatures, producing identical results to the
literature procedure. Furthermore, the ammonia triborane was separated from the remaining
residue after removal of the solvent using a mixed solvent of hexanes and ethyl ether. Potassium
octahydrotriborate was also able to be used in the synthesis in place of the tetrabutylammonium
salt.
iii
Dedication
This document is dedicated to my wife, Julia. Without her continued support throughout my
graduate career, I would have long ago forgotten what is important in life.
iv
Acknowledgments
I am thankful to Dr. Sheldon Shore for providing his support and knowledge during my
graduate career at Ohio State University. Dr. Shore provided me with advice and insight into my
research I would not have thought about personally.
I also thank the Shore Group members for providing their daily support and
understanding. In particular, I thank Dr. Matthew R. Sturgeon for all of his technical advice and
Chris Potratz for his help with attempts at X-ray diffraction and crystallography.
I also would like to thank Dr. J.-C. Zhao in the Department of Materials Science and
Engineering for his monetary and educational support. Furthermore, I would like to thank the
Zhao Group members for their perspective of my research from a materials science approach.
v
Vita
June 2002 ............................................................. Twin Valley South High School
2006 ..................................................................... B.S. Chemistry, Ohio University
2006 to 2008 ....................................................... Graduate Teaching Associate, Department of
Chemistry, The Ohio State University
2008 to present …………………………….. Graduate Research Assistant, Department of
Materials Science and Engineering, The Ohio
State University
Fields of Study
Major Field: Chemistry
vi
Table of Contents
Abstract...............................................................................................................................ii
Dedication...........................................................................................................................iv
Acknowledgments...............................................................................................................v
Vita.....................................................................................................................................vi
Table of Contents..............................................................................................................vii
List of Tables......................................................................................................................xi
List of Figures....................................................................................................................xii
List of Abbreviations........................................................................................................xiii
Chapter 1: Introduction ...................................................................................................... 1
1.1 Chemical Hydrogen Storage .............................................................................. 1
1.2 Ammonia Borane and the Amidortrihydroborate Anion ................................... 4
1.2.1 Ammonia Borane ................................................................................ 4
1.2.2 Amidotrihydroborate ........................................................................... 5
1.2.3 Structures of Amidotrihydroborate Compounds ................................. 9
1.2.4 Use of Lithium Amidotrihydroborate in Reduction Reactions ......... 10
vii
1.2.5 Use of Ammonia Borane and Amidotrihydroborate as a Hydrogen
Fuel Source ................................................................................................. 10
1.2.6 Regeneration of Ammonia Borane .................................................... 13
1.3 Ammonia Triborane ......................................................................................... 15
1.3.1 The Octahydrotriborate Anion .......................................................... 15
1.3.2 Syntheses of Ammonia Triborane ..................................................... 17
1.3.3 Reactivity of Ammonia Triborane .................................................... 18
1.4 Statement of the Problem ................................................................................. 18
Chapter 2: Results and Discussion.....................................................................................21
2.1 Lithium Aluminum Hydride Reactions with Ammonia Borane ...................... 21
2.1.1 Rationale for the Aluminum Amidotrihydroborate Compounds ...... 21
2.1.2 Equimolar Reaction of Lithium Aluminum Hydride and Ammonia
Borane ........................................................................................................ 21
2.1.3 The Reaction of Lithium Aluminum Hydride and Ammonia Borane
in a 1:2 Ratio .............................................................................................. 22
2.1.4 The Reaction of Lithium Aluminum Hydride and Ammonia Borane
in a 1:3 Ratio .............................................................................................. 24
2.1.5 The Reaction of Lithium Aluminum Hydride and Ammonia Borane
in a 1:4 Ratio .............................................................................................. 25
2.1.6 Discussion of the Results of the Reactions of LiAlH4 and H3NBH3. 27
viii
2.2 Solvent-Free Sodium Octahydrotriborate ........................................................ 46
2.3 Ammonia Triborane ......................................................................................... 50
2.3.1 Improved Synthesis of Ammonia Triborane ..................................... 50
2.3.2 The Reaction of KB3H8 with I2 ......................................................... 51
2.3.3 The Reaction of Ammonia Triborane with Sodium Hydride ............ 51
Chapter 3: Experimental ................................................................................................... 53
3.1 Equipment and Apparatus ................................................................................ 53
3.1.1 Nuclear Magnetic Resonance Spectroscopy (NMR)......................... 53
3.1.2 X-Ray Powder Diffraction ................................................................ 53
3.1.3 Infrared Spectroscopy........................................................................ 53
3.1.4 Vacuum Line ..................................................................................... 54
3.1.5 Glassware .......................................................................................... 55
3.1.6 Dry Box ............................................................................................. 56
3.2 Solvents and Reagents ...................................................................................... 57
3.2.1 Solvents ............................................................................................. 57
3.2.2 Reagents ............................................................................................ 57
3.3 Syntheses ......................................................................................................... 59
3.3.1 Reaction of 1:1 Lithium Aluminum Hydride and Ammonia Borane 59
3.3.2 Reaction of 1:2 Lithium Aluminum Hydride and Ammonia Borane 60
ix
3.3.3 Reaction of 1:3 Lithium Aluminum Hydride and Ammonia Borane 61
3.3.4 Reaction of 1:4 Lithium Aluminum Hydride and Ammonia Borane 61
3.3.5 Solvent-Free sodium Octahydrotriborate .......................................... 62
3.3.6 Improved Synthesis of Ammonia Triborane ..................................... 63
References ..................................................................................................................... 65
x
List of Tables
Table 1.1 Gravimetric Capacity of Hydrogen for Known NH2BH3- Compounds ........... 13
Table 2.1 Gravimetric Capacity of Aluminum Amidotrihydroborate Compounds ......... 31
Table 2.2 Hydrogen Release Data from LiAlH4 and H3NBH3 Reactions ....................... 31
xi
List of Figures
Figure 2.1 11B NMR of Equimolar LiAlH4 with H3NBH3.................................................32
Figure 2.2 Proton Decoupled 11B NMR of Equimolar LiAlH4 with H3NBH3..................32
Figure 2.3 1H NMR of Equimolar LiAlH4 with H3NBH3 ................................................. 33
Figure 2.4 Boron decoupled 1H NMR of Equimolar LiAlH4 with H3NBH3 .................... 33
Figure 2.5
11
B NMR of 1:2 LiAlH4 with H3NBH3 ........................................................ 34
Figure 2.6 Proton Decoupled 11B NMR of 1:2 LiAlH4 with H3NBH3 ............................ 34
Figure 2.7 1H NMR of 1:2 LiAlH4 with H3NBH3 ........................................................... 35
Figure 2.8 Boron Decoupled 1H NMR of Equimolar LiAlH4 with H3NBH3 .................. 35
Figure 2.9 11B NMR of the Reaction Solution of 1:3 LiAlH4 with H3NBH3 ................... 36
Figure 2.10 1H Dec. 11B NMR of the Reaction Solution of 1:3 LiAlH4 with H3NBH3 .... 36
Figure 2.11 11B NMR of Isolated 1:3 LiAlH4 with H3NBH3............................................ 37
Figure 2.12 1H Dec. 11B NMR of Isolated 1:3 LiAlH4 with H3NBH3 .............................. 37
Figure 2.13 1H NMR of Isolated 1:3 LiAlH4 with H3NBH3 ............................................. 38
Figure 2.14 11B Dec. 1H NMR of Isolated 1:3 LiAlH4 with H3NBH3 .............................. 38
Figure 2.15 11B NMR of Isolated1:3 LiAlH4 with H3NBH3 After Extra Pumping .......... 39
Figure 2.16 1H Dec. 11B NMR of Isolated 1:3 LiAlH4 with H3NBH3 Extra Pumping ..... 39
Figure 2.17 1H NMR of Isolated 1:3 LiAlH4 with H3NBH3 After Extra Pumping .......... 40
Figure 2.18 11B Dec. 1H NMR of Isolated 1:3 LiAlH4 with H3NBH3 Extra Pumping ..... 40
xii
Figure 2.19 11B NMR of the Reaction Solution of 1:4 LiAlH4 with H3NBH3 ................. 41
Figure 2.20 1H Dec. 11B NMR of the Reaction Solution of 1:4 LiAlH4 with H3NBH3 .... 41
Figure 2.21 11B NMR of Isolated 1:4 LiAlH4 with H3NBH3............................................ 42
Figure 2.22 1H Dec. 11B NMR of Isolated 1:3 LiAlH4 with H3NBH3 .............................. 42
Figure 2.23 1H NMR of Isolated 1:4 LiAlH4 with H3NBH3 ............................................. 43
Figure 2.24 11B Decoupled 1H NMR of Isolated 1:4 LiAlH4 with H3NBH3 .................... 43
Figure 2.25 Infrared Spectra of 1:1 and 1:2 LiAlH4 and H3NBH3 ................................... 44
Figure 2.26 Infrared Spectra of 1:3 and 1:4 LiAlH4 and H3NBH3 ................................. 44
Figure 2.27 Infrared Spectra of All Reaction Products of LiAlH4 and H3NBH3 ............ 45
Figure 2.28 Infrared Spectra of LiAlH4 and H3NBH3 ...................................................... 45
xiii
Abbreviations
kg
Kilogram
L
Liter
MOF
Metal organic framework
B-N
Boron to nitrogen bond
THF
Tetrahydrofuran
IR
Infrared
NMR
Nuclear magnetic resonance
δ
NMR chemical shift in ppm
ppm
parts per million
s
singlet
q
quartet
br
broad
xiv
Chapter 1
Introduction
1.1 Chemical Hydrogen Storage
Hydrogen is an attractive fuel for automobiles as a result of the clean emissions
formed in its use. In terms of practicality, a hydrogen powered automobile must contain a
light weight hydrogen storage system with the highest percentage of hydrogen by mass as
possible. The current Department of Energy targets for hydrogen systems is a gravimetric
capacity of 9% or volumetric capacity of 0.081 kg-H2/L by the year 2015.26 Two issues
have arisen that have thus far made the use of hydrogen fuel out of reach, one being a
safe on-board hydrogen storage system capable of meeting the standards set and the other
being a means of recycling the spent fuel for a sustainable source.
Use of compressed gaseous or cryogenic liquid hydrogen is not feasible as a fuel
source. Compressed hydrogen tanks lack adequate storage to allow for a vehicle to travel
500 kilometers without refueling, must be able to withstand high pressures (10,000 psi in
the most efficient to date), and can be potentially seriously dangerous in the circumstance
of a break or leak, making their use impractical.16,21 Liquid hydrogen also requires a tank
able to withstand very high pressures and requires large quantities of energy to condense,
eliminating it as a reasonable material for use in mobile applications.21 Moreover, the use
of such fuels would require a complete overhaul of the current methods of distribution.18
1
Metal-organic frameworks and complex metal hydride systems have been
synthesized to store hydrogen through adsorption both on the frameworks surface and in
the voids within the material. MOF materials do not, however, have the storage capacity
to meet the standards set by the Department of Energy, mostly as a result of weak
interaction between the framework and hydrogen.17 Complex metal hydride systems have
also been restricted to low capacities of stored hydrogen, less than 8% even if
disregarding the weight of the rest of the system (tanks, lines, etc.).18
Zeolite materials and carbon based structures, such as nanotubes, nanofibers,
activated carbon, or solid foams, have been the center of some research on hydrogen
adsorption and release. The carbon based materials suffer from the same set back as the
MOF materials as hydrogen adsorption requires very low temperature and yields a
maximum of about 5% hydrogen by mass. Zeolites also have low hydrogen adsorption,
as the materials are made up of heavy atoms that reduce the gravimetric capacity.24
Chemical hydrogen storage has been a topic of increased interest in recent years18
and is the subject of this thesis. An early example of this form of storage was exhibited in
the NatriumTM car, which was fueled by the hydrolysis of sodium borohydride with a
heterogeneous ruthenium catalyst. The demonstration brought to light the need to be able
to regenerate chemical hydrogen storage fuels in order to create a sustainable fuel source.
The commercialization of the borohydride hydrolysis system was unable to proceed as a
result of the lack of an energy efficient method to recycle the boric acid formed.18-20
Some small molecule, cyclic carbon compounds have been shown to work as a
source of chemically stored hydrogen. The molecules consist of saturated rings that form
2
aromatic rings upon dehydrogenation.25 These materials require the use of a catalyst and
relatively high temperatures (typically over 200 ⁰C) to release hydrogen. Furthermore,
the materials have a low gravimetric capacity.25
A number of simple metal hydride systems have been evaluated as potential
chemical hydrogen storage materials, including sodium borohydride mentioned in the
above example. These compounds include alkali and alkaline earth metal hydrides, low
molecular weight metal borohydrides, alkali metal aluminum hydrides, aluminum
hydride, and lithium amide mixed with lithium hydride.21-24 Many of these systems are
energy inefficient to recycle after use as fuel and may have gravimetric material
capacities that are too low to meet the on-board hydrogen storage requirements.24
Chemical hydrides are hydrides involving lighter elements than found in most
metal hydrides and therefore contain a higher gravimetric capacity. The use of such
materials requires the cleavage of covalent bonds between the elements of the compound
and hydrogen. Necessarily, a successful chemical hydride fuel would require the breaking
of the bonds to be a slightly endothermic reaction, preventing spontaneous hydrogen
release, but allowing regeneration of the fuel under reasonable conditions. Most known
materials of this nature with a high mass percentage of hydrogen have enthalpies for
hydrogen release that are too exothermic or endothermic to provide a useful source.
Reaction kinetics may also prevent a material from being useful if the release or
adsorption rates of hydrogen are unfavorable.24
Small boron molecules and clusters, examples of chemical hydrides, have been
evaluated as candidates for chemical hydrogen storage. Boron has a low molecular
3
weight and many boron materials have a high gravimetric capacity for hydrogen.
Molecules containing boron and hydrogen atoms exclusively can be used to produce
hydrogen gas by thermal degradation or by hydrolysis, but a distinct problem has arisen
in using this type of material. Thermal decomposition of small boron compounds tends to
cause the formation of stable and/or dangerous boron molecules or clusters and
hydrolysis forms borates in an irreversible reaction, preventing easy regeneration for a
sustainable fuel source.23
Boron-nitrogen compounds, also examples of chemical hydrides, have many
advantages over the materials described previously. Boron and nitrogen are lightweight
elements and can bond to multiple hydrogen atoms, giving many of the compounds a
high gravimetric capacity. While hydrolysis of these molecules yields the same issues as
discussed with the boron hydrides, thermal decomposition can yield materials that are
much easier to break apart in recycling due to the electronegativity difference between
the two elements. Furthermore, B-N compounds tend to have hydridic and protic
hydrogen atoms, leading to more efficient hydrogen release.18,24 The investigation of B-N
materials as potential hydrogen fuel sources is the topic of this work and further
discussion of relative materials will be elaborated subsequently.
1.2 Ammonia Borane and the Amidotrihydroborate Anion
1.2.1 Ammonia Borane
The first reported synthesis for ammonia borane was reported by Shore and Parry
in 19551. The compound was isolated as a result of the reaction of an ammonium salt and
lithium borohydride in diethyl ether (Equation 1.1 and 1.2). Furthermore, it was reported
4
the molecule H3NBH3 could also be produced from the diammoniate of diborane,
[H2B(NH3)2][BH4], by reaction with ammonium chloride (Equation 1.3). The product
could then be extracted with diethyl ether with a trace of ammonia. To verify the
molecular structure of the product was actually that of ammonia borane as a monomeric
species, the molecular weight was determined via both freezing point depression of
dioxane and vapor pressure depression of diethyl ether. Ammonia borane was also found
to be monomeric both in liquid ammonia2 and as a solid material,3,4 as determined by xray powder diffraction.
NH4Cl + LiBH4
H3NBH3 + LiCl + H2
(NH4)2SO4 + 2LiBH4
2H3NBH3 + Li2SO4 + 2H2
[H2B(NH3)2][BH4] + NH4Cl
[H2B(NH3)2]Cl +H3NBH3 + H2
(1.1)
(1.2)
(1.3)
Other synthetic procedures have since been developed to isolate ammonia borane,
including the use of other borohydrides in similar reactions, the reaction of diborane with
ammonia under conditions to produce symmetric cleavage of B2H6, and the reaction of
L∙BH3, where L is a coordinated solvent, with ammonia.83
1.2.2 Amidotrihydroborate
The electronegativity difference between the boron atom and the nitrogen atom in
the ammonia borane molecule creates a dipole, making the hydrogen atoms bonded to
nitrogen have a slightly positive charge, while the hydrogen atoms bonded to boron have
a slightly negative charge. As a result, H3NBH3 can undergo reactions involving these
hydrogen atoms. The hydrides are sufficiently negative that a reaction with hydrochloric
5
acid occurs, yielding the product H3NBH2Cl with hydrogen gas evolution.5-6 Of more
interest, ammonia borane can react with a hydride or a sufficiently reducing metal to
produce the amidotrihydroborate anion.5-9
The first evidence of the existence of the amidotrihydroborate anion was
illustrated in the work of Schlesinger and Berg in 1938.8,9 The observation that the
product isolated from the reaction of trimethylamine with borane carbonyl did not react
with sodium, while the product of ammonia with the etherate of borane did to yield 0.5
equivalents of H2 gas, led to the proposal that the compound Na[NH2BH3] had been
formed. This observation was further supported by Shore and Parry2 after the isolation
and identification of the ammonia borane molecule. The reaction of confirmed,
monomeric ammonia borane with sodium yielded 0.5 equivalents of hydrogen gas
(Equation 1.4). The reaction was also found to occur with potassium metal with the same
quantity of hydrogen evolution5.
H3NBH3 + M
M[NH2BH3] + 0.5H2
(M = Na, K)
(1.4)
The rare earth metals erbium and ytterbium were able to react with two
equivalents of ammonia borane in liquid ammonia to produce the amidotrihydroborate
anion (Equation 1.6).7 The hydrogen evolved was measured, indicating 0.89 equivalents
of the gas were formed in the reaction. Replacing the coordinated ammonia in the product
with pyridine indicated the formation of (py)nLn[NH2BH3]2 in the context of elemental
analysis. No structural information was able to be determined.7 Extracting the crude
product of the ytterbium reaction with 1,2-dimethoxyethane led to the isolation of
6
crystals of (DME)2Yb[NH2BH3]2, confirmed by NMR and single crystal X-ray
crystallography.10
2 H3NBH3 + Ln + xNH3
(NH3)xLn[NH2BH3]2
(1.5)
Later work produced the same anion using the hydrides of lithium, sodium, and
potassium (Equation 1.5). All alkali hydrides were insoluble in tetrahydrofuran, while the
amidotrihydroborate salts of sodium and lithium were soluble in THF, allowing isolation
of the products. The potassium salt was insoluble in tetrahydrofuran, but dissolved in
liquid ammonia. Li[NH2BH3], Na[NH2BH3], and K[NH2BH3]5,6 were characterized by 1H
and 11B NMR(d8-THF or ND3), X-ray powder diffraction, and IR spectroscopy.
H3NBH3 + MH
M[NH2BH3] + H2 (M= Li, Na, K)
(1.6)
The lithium salt of amidotrihydroborate was also able to be produced by the use of
alkyl lithium reagents, as seen first in the work of DeGraffenreid5 and published by
Myers, Yang, and Kopecky (Equation 1.7).11
H3NBH3 + LiR
Li[NH2BH3] + HR
(1.7)
Lithium aluminum hydride was able to react in a four to one ratio with ammonia
borane to produce nearly four equivalents of hydrogen, indicating the compound
LiAl[NH2BH3]4 had formed (Equation 1.8).6 The 11B NMR spectrum of the material
consisted of a single quartet, showing the existence of a –BH3 group in the product.
Attempts at crystallization of the material failed. The reaction was also attempted in a 1:1
ratio of reactants, but two equivalents of hydrogen were obtained as opposed to the
expected single equivalent. All spectra of the compound were broad and uncharacteristic.
7
Elemental analysis of the 1:1 reaction product indicated a 1:1:1:1 ratio of lithium,
aluminum, nitrogen, and boron. Structural information was unable to be obtained.6
LiAlH4 + 4H3NBH3
LiAl[NH2BH3]4 + 4H2
(1.8)
Calcium hydride exhibited the same reactivity with ammonia borane as the hydrides
reported previously. The reaction was performed in THF to produce the compound
(THF)2Ca[NH2BH3]2 (Equation 1.9)12. The coordinated tetrahydrofuran could be
removed via vacuum to yield the solvent-free Ca[NH2BH3]2. The THF structure was
crystalline in nature, making it the first reported single crystal structure of an
amidotrihydroborate compound.12 The structure will be discussed in the following
section.
CaH2 + H3NBH3 + 2THF
(THF)2Ca[NH2BH3]2 + 2H2
(1.9)
The amidotrihydroborate compounds Cp2Ti[NH2BH3], Cp2Hf[NH2BH3]Cl, and
Cp2Hf[NH2BH3]H were made from the metathesis reaction of Na[NH2BH3] and
titanocene dichloride or hafnocene dichloride (Equations 1.10-1.12).10 The titanium was
reduced in the reaction from titanium (IV) to titanium (III), owing to the reduction
capability of the amidotrihydroborate anion. Attempts at replacing both chlorides on
hafnocene dichloride resulted in a hydride abstraction, presumably from Na[NH2BH3].
Single crystal structures of the compounds will be discussed in the following section.
Cp2TiCl2 + 2Na[NH2BH3]
Cp2Ti[NH2BH3] + NaCl(s)
+1/x(NH2BH2)x(s) + 0.5H2
Cp2HfCl2 + Na[NH2BH3]
Cp2Hf[NH2BH3]Cl + NaCl(s)
8
(1.10)
(1.11)
Cp2HfCl2 + 2Na[NH2BH3]
Cp2Hf[NH2BH3]H
+ 2NaCl(s) + 1/x(NH2BH2)x(s)
(1.12)
1.2.3 Structures of Amidotrihydroborate Compounds
The first crystal structure of an amidotrihydroborate species was the THF
coordinated calcium amidotrihydroborate discussed previously, (THF)2Ca[NH2BH3]2.
The structure was polymeric with η3,µ2-NH2BH3 with bonding via hydride bridges from
the boron atom to one calcium atom and the nitrogen atom directly interacting with the
other calcium atom. The hydrogen atoms were arranged in a staggered configuration on
the anion. The structure collected exhibited disorder for each atom over two positions.
Using a large, bulky ligand disrupted the polymer formation in the calcium
amidotrihydroborate compound, resulting in a monomeric species, [(DIPPnacnac)Ca(NH2BH3)(THF)2].11 Single crystal data for the compound exhibited a direct
electron donation from the nitrogen atom to the metal, while the boron atom was also
linked to the calcium atom by a hydrogen bridge.
High resolution X-ray powder crystallography was used to identify the crystalline
structure of Li[NH2BH3], Na[NH2BH3]13, and solvent-free Ca[NH2BH3]2.14 The
structures of the lithium and sodium salts are identical, described as a staggered
configuration of the hydrogen atoms, with the alkali metal occupying the location of the
abstracted hydrogen atom from ammonia borane. Later work identified the atomic
coordinates of the lithium salt from powder crystallography.14 The metal is 6 coordinate
in the structure of Ca[NH2BH3]2. Four ligands are bonded to the calcium atom through
9
hydride bridges between the boron atoms and the metal, resulting in a total of eight
bridges to each calcium atom. The two other coordination sites are filled by direct
interaction between the nitrogen of two other amidotrihydroborate anions and the metal
cation.14
Single crystal structures were determined for the amidotrihydroborate compounds
synthesized by Wilson.10 The amidotrihydroborate compounds formed with the
cyclopentadienyl coordinated transition metal dichlorides as well as the ytterbium
compound all exhibited the same coordination of the [NH2BH3]- ligand. The nitrogen
atom directly donates a pair of electrons to the transition metal and the boron atom
connects to the metal via a 3-center 2-electron bond,10 as exhibited in the previously
described calcium structure with a bulky ligand group.
1.2.4 Use of Lithium Amidotrihydroborate in Reduction Reactions
Lithium amidotrihydroborate was used to reduce tertiary amides to primary
alcohols, serving as a nucleophilic hydride source.11 The yields of the reduction reactions
performed were high for most tertiary amides, falling mostly in the 70 to 95% range. The
alcohols formed also presented high enantioselectivity, with greater than 90%
enantiomeric excess. The reduction process tended to form tertiary amines instead of the
primary alcohols with increasing steric effects on the starting material amide, as had been
observed previously in experiments with other reducing agents.11
1.2.5 Use of Ammonia Borane and Amidotrihydroborate as a Hydrogen Fuel Source
Ammonia borane has been of much interest in the search for a chemical hydrogen
storage material. The molecule contains 19.6% hydrogen by mass and favors
10
decomposition by elimination of hydrogen gas over breaking the coordination between
the nitrogen and boron atoms. The compound is also easily handled and stable at room
temperature.
Hydrolysis of ammonia borane can occur using a metal29-34 or acid catalyst27,28
(Equations 1.13 and 1.14). Hydrolysis of ammonia borane allows for the generation of
three equivalents of hydrogen gas per equivalent of ammonia borane and is faster than
thermal degradation. The disadvantage of the hydrolysis mechanism for hydrogen release
is the inability to recycle the fuel, as stated previously. Furthermore, the reaction
produces the ammonium cation as a product, leading to equilibrium with the water
medium to form some ammonia gas, contaminating the hydrogen released. Ammonia is
corrosive and could potentially damage the fuel system.
H3NBH3 + H+ + 3H2O
H3NBH3 + 2H20
NH4+ + B(OH)3 + 3H2
NH4+ + BO2- + 3H2
(1.13)
(1.14)
Several metal catalysts in organic solvents can decompose ammonia borane to yield
hydrogen gas. Metal-based catalysts have shown the formation of single products in the
decomposition of ammonia borane, the fastest release of the first equivalent of hydrogen,
and can generate up to 2.5 equivalents of hydrogen gas.50,51 However, the use of solvents
and the heavy metal catalysts employed decrease the gravimetric capacity of the system.
Thermal decomposition of ammonia borane yields hydrogen gas and a multitude of
B-N compounds. The first equivalent of hydrogen gas is released over a wide range of
temperatures, 77-112 ⁰C,35-39 followed by a second equivalent over an even larger range,
110-200 ⁰C, and finally releases the third equivalent of hydrogen in excess of 500 ⁰C.36
11
The extreme temperature required to release the final equivalent of hydrogen is too high
for practical use. As a result, the gravimetric capacity of ammonia borane from thermal
dehydrogenation is in reality 13.06% hydrogen. Borazine is produced in small amounts
during the removal of the second equivalent of hydrogen gas.36 Borazine is a vapor under
the reaction conditions and contaminates the fuel, which subsequently forms a solid
material upon cooling. The contamination is significant, as the material can be distributed
throughout the hydrogen storage system as a vapor, condense, and form a solid material
that cannot be easily removed. The reaction scheme for the thermal release of hydrogen
from ammonia borane is shown below (Equations 1.15-1.18).
n H3NBH3
(H2NBH2)n + nH2
(H2NBH2)n
(HNBH)n + nH2
(H2NBH2)n
n/3 N3B3H6 + nH2
(HNBH)n
(-21.3 kJ/mol)40
(1.15)
(131.4 kJ/mol)40
(1.16)
(1.17)
(562.3 kJ/mol)40
nBN + nH2
(1.18)
The amidotrihydroborate anion has less hydrogen by mass than ammonia borane, yet
still has a high gravimetric capacity. A selection of amidotrihydroborate species with
mass percentage of hydrogen are shown in Table 1.1. The thermal release of hydrogen
gas from LiNH2BH3 and NaNH2BH3 has been reported in the literature. Approximately 8
wt% and 6 wt% of hydrogen was released from LiNH2BH3 and NaNH2BH3, respectively,
heating the samples at 91 ⁰C for about one hour. After 19 hours of heating at the same
temperature, 10.9 wt% and 7.63 wt% of hydrogen was released, corresponding to two
equivalents.41 In comparison, ammonia borane yields slightly more hydrogen (~12 wt%)
12
released at approximately 150 ⁰C over the same time frame.41 A separate report indicates
the hydrogen is released from the lithium salt of amidotrihydroborane at ~92 and 120 ⁰C
over three hours.14 The thermal decomposition of LiNH2BH3 and NaNH2BH3 did not
form borazine during the process. Ca[NH2BH3]2 also releases two equivalents of
hydrogen for each anion at 100 and 140 ⁰C with no indication of borazine synthesis.14
The amidotrihydroborate cannot be regenerated by direct re-hydrogenation.
MNH2BH3
MNHBH2 + H2
(1.19)
MNHBH2
MNBH + H2
(1.20)
Compound
Weight % Hydrogen
LiNH2BH3
13.70%
NaNH2BH3
9.54%
KNH2BH3
7.31%
Ca[NH2BH3]2
10.10%
Table 1.1: Amidotrihydroborate species known and gravimetric capacity of hydrogen.
Actual hydrogen available is only two equivalents and the full capacity cannot be
recovered.
1.2.6 Regeneration of Ammonia Borane
The resulting boric acid from the hydrolysis of ammonia borane cannot be
efficiently regenerated, as stated previously. The enthalpy of hydrolysis to release three
equivalents of hydrogen gas and form B(OH)3 is approximately -227 kJ/mol. Even partial
13
hydrolysis to form 1/n[B3N3H4]n and 2.3 equivalents of hydrogen has an enthalpy of
hydrolysis of -60.8 kJ/mol. The energy barrier to direct hydrogenation of the spent fuel in
the hydrolysis system prevents the process from occurring without significant energy
input.14 Other means have been attempted to form B-H bonds and regenerate ammonia
borane, such as the reaction of sodium hydride with boric acid and by an electrochemical
process (Equations 1.21 and 1.22), but these methods are also inefficient and require
significant energy input.14
BO2- + 3H2 +2e2OH- + H2
BH4- + 2OH- (cathode reaction)
2H2O + 2e-
(anode reaction)
(1.21)
(1.22)
After thermal decomposition of ammonia borane, the resulting material, of
composition NBHx, cannot be directly re-hydrogenated. As a result, the fuel must be
regenerated by digestion and reformation. One method published is the reaction of the
NBHx material with hydrochloric acid, forming boron trichloride and ammonium
chloride. Hydrobromic acid can also be used in the process to produce ammonium
bromide and boron tribromide.42,43 The ammonium salt can be thermally decomposed or
can undergo a base-exchange reaction to reform ammonia gas.43 BCl3 can be converted to
B-H species through a variety of methods.43-46 Reacting the spent fuel with trifluoroacetic
acid produces B(OOCF3)3 and B(OOCF3)4-, which can react with trimethylamine-alane to
form borane.47
Polyborazyline, a partial decomposition product of ammonia borane after
dehydrogenation by a metal catalyst in an organic solvent, has been digested using thiols
and subsequently reacted with Bu3SnH or Bu2SnH2 to reform ammonia borane with 67%
14
yield. The tin hydride compounds can be regenerated. The same reaction scheme can be
used to regenerate ammonia borane from borazine, another product formed in some
dehydrogenation reactions.52
1.3 Ammonia Triborane
1.3.1 The Octahydrotriborate Anion
The octahydrotriborate anion was first synthesized by the direct action of
diborane on sodium metal in the work of Stock.53 Further study of this reaction led to the
realization that ether solvents and the amalgamation of sodium metal with mercury
promoted the reaction to yield a 1:1 mixture of sodium borohydride and sodium
octahydrotriborate (Equation 1.23).54,55,60 In ethyl ether, solvent free sodium
octahydrotriborate was obtained through this method. The reaction also occurs using
other alkali metal amalgams with mercury and other solvents.59,74 Furthermore,
dispersing potassium, rubidium, or cesium metal into tetrahydrofuran with naphthalene as
a carrier allows a reaction to commence with diborane, forming initially M2BH3,
followed by M2B2H6, and finally forming MBH4 and MB3H8 with continued addition of
diborane.57,58 In a similar reaction, sodium-mercury amalgam was shown to react with
tetraborane in ethyl ether to produce NaB3H8.56
2Na/Hg + 2B2H6
NaB3H8 + NaBH4
(1.23)
Reactions that normally produce diborane at room temperature have been used to
form NaB3H8 in high boiling ether solvents at 100 ⁰C. NaBH4 reacts with iodine or boron
trifluoride etherate in diglyme at the temperature described to produce sodium
octahydrotriborate. 61-63 The reaction of sodium borohydride with benzyldimethylamine
15
complexes of boron trihalides in diglyme yields identical results.64 After the reaction, the
diglyme cannot be removed completely as the solvent binds strongly to the sodium
cation.65 To form the solvent-free alkali metal salt from the reaction product in diglyme, a
two step procedure is required. The solvated sodium is replaced by the
tetrabutylammonium cation, followed by a second replacement reaction to displace the
cation with sodium tetraphenylborate to yield solvent-free NaB3H8.67,68
Tetrahydrofuran-borane complex reacts with alkali metal amalgams to form the
octahydrotriborate anion and borohydride at room temperature. The literature reports the
potassium, rubidium, and cesium salts of octahydrotriborate, purified by filtering away
the insoluble borohydride salts and removal of the solvent by vacuum. The procedure
works using an ytterbium-mercury amalgam as well.66
A number of reports claim various synthetic routes to ammonium
octahydrotriborate. The compound can be made via the reaction of pentaborane with
basic ammonium salts,75,76 ion replacement with an octahydrotriborate salt using an NH4+
type ionic-exchange resin,77 and by the reaction of tetramethylammonium
octahydrotriborate and an ammonium halide in liquid ammonia.78 The compound is
stable up to 60 ⁰C when isolated, at which point it releases hydrogen. The compound also
releases hydrogen in ethyl ether or in benzene. Upon release of one equivalent of
hydrogen, the compound NH3B3H7, ammonia triborane, is formed, which contains a
strong coordination between ammonia and a single boron atom of the boron ring.76 As
stated previously, B-N compounds are preferred for thermal dehydrogenation in order to
prevent the problems of stable cluster formation or dangerous, volatile boron compounds.
16
In addition, ammonia triborane contains protic hydrogen atoms in addition to hydridic
hydrogen atoms, allowing for more efficient hydrogen release.18,24
The thermal decomposition of octahydrotriborate compounds generates a mix of
species, including B5H9, B2H6, BH4- and hydrogen. In some cases, further decomposition
leads to the formation of B12H12-, a very stable boron cluster. B5H9 and B2H6 are
explosive in contact with oxygen, and B12H12- is too stable to provide a sustainable fuel.67,
69-73
Thus, octohydrotriborate is a poor choice to use for hydrogen storage.
1.3.2 Syntheses of Ammonia Triborane
Most syntheses in the literature for ammonia triborane involve primary
abstraction of a hydride from the octahydrotriborate anion followed by coordination to
ammonia. In the case of ammonium octahydrotriborate the hydride extraction and
ammonia coordination occur simultaneously. A proton on the ammonium cation reacts
with a hydride on the anion to release hydrogen gas and form ammonia triborane.76 The
first reported synthesis of NH3B3H7, in the work of Kodama, Parry, and Carter, is, in
effect, identical to later reports of hydrogen release from ammonium octahydrotriborate.
In the synthesis, ammonium chloride is reacted with sodium octahydrotriborate in ethyl
ether at room temperature yielding ammonia triborane (Reaction 1.26). In the same work,
a separate synthesis is detailed in which tetrahydrofuran and tetrahydropyran cleaves
tetraborane to form ether coordinated B3H7. Addition of ammonia displaces the ether to
form NH3B3H7.79
A more recent synthesis of ammonia triborane has been published by Yoon and
Sneddon. In the procedure, elemental iodine is reacted with tetrabutylammonium
17
octahydrotriborate in dimethoxyethane to produce solvent coordinated triborane. As in
the work of Kodama and Parry, liquid ammonia is added to the solution to displace the
solvent and form ammonia triborane. The product was then purified by column
chromatography.80,81
1.3.3 Reactivity of Ammonia Triborane
Ammonia triborane reacts with trimethylamine at room temperature to produce
trimethylamine borane and a solid that decomposes through hydrogen release.79 NH3B3H7
also reacts with excess sodium, releasing hydrogen gas and forming sodium borohydride
as the only distinguishable product. In the reaction, the first half equivalent of H2 releases
within 20 minutes and continued evolution of hydrogen eventually reaches a total
hydrogen gas release of one mole of H2 per mole of ammonia triborane.79 Ammonia
triborane is also reported to react with metal hydrides and metal borohydrides to reform
the octahydrotriborate anion.82
Ammonia triborane undergoes hydrolysis both by acid catalysis and more rapidly
by metal catalysis. Eight moles of hydrogen gas are produced in the reaction per mole of
ammonia triborane in each method.81 Borates are formed in the hydrolysis processes, just
as in the hydrolysis of ammonia borane, preventing efficient recycling of the material,
and, therefore, use as a sustainable fuel source with current technology.
1.4 Statement of the Problem
While recent work has provided a multitude of compounds for use as a hydrogen
source, no compound currently available can be regenerated efficiently. Hydrolysis of
any boron containing compound results in the formation of borates or boric acid which
18
cannot currently be regenerated efficiently and addition of water and catalysts decreases
the gravimetric capacity of the system, making the method highly improbable to be used
as a hydrogen fuel source for mobile applications. Boron compounds also seem to be
impractical for such uses, forming stable clusters with high energetic barriers to
regeneration or even volatile, highly reactive small boranes. B-N compounds form
products after thermal decomposition that can be digested and regenerated to some
extent, but the yields are low and the processes tend to require many steps. Furthermore,
ammonia borane decomposes into multiple materials, including borazine, turning interest
in thermal decomposition to other B-N type compounds.
In the interest of easier regeneration, addition of aluminum to B-N materials may
serve to catalyze regeneration or aid in digestion. Though the equimolar and 4:1 reactions
of ammonia borane with lithium aluminum hydride have been previously studied, the
reaction series was not completed. To this extent, a series of reactions involving lithium
aluminum hydride and ammonia borane have been investigated in the attempt to form
amidotrihydroborate-aluminum materials, measuring the released quantities of hydrogen
and characterizing the products via NMR and IR spectroscopy.
Ammonia triborane is another B-N type compound that serves as a potential
candidate for hydrogen storage. Though the hydrolysis of NH3B3H7 has been
investigated, other reactions involving the compound are rather unexplored. The methods
to synthesize ammonia triborane rely on octahydrotriborate compounds and are low yield
reactions or laborious in nature. To refine a method for easy synthesis, a new method for
obtaining solvent-free sodium octahydrotriborate has been pursued. Furthermore,
19
improvements to a literature procedure for the synthesis of ammonia triborane from
tetrabutylammonium octahydrotriborate has been sought to make the process less time
consuming and less expensive. NMR spectroscopy was used in confirmation of the
achievement of these goals.
20
Chapter 2
Results and Discussion
2.1 Lithium Aluminum Hydride Reactions with Ammonia Borane
2.1.1 Rationale for the Aluminum Amidotrihydroborate Compounds
Previous alkali metal and alkaline earth amidotrihydroborate compounds
synthesized are of interest as borazine is not formed in the thermal dehydrogenation
process. The problem in using such compounds for an on-board hydrogen storage system
is the lack of an efficient means to recycle the spent fuel. As a result, the use of other
metal atoms in such compounds is of interest to possibly facilitate easier rehydrogenation after thermal degradation. The work presented here focuses on research
towards an aluminum-amidotrihydroborate compound.
Previous work has shown the reaction in a 1:1 ratio of lithium aluminum hydride
and ammonia borane releases two equivalents of hydrogen, while a 1:4 ratio releases
slightly less than the expected four equivalents. However, no products were isolated from
the reactions. NMR and IR data of the 1:1 product exhibited only very broad peaks, while
the 1:4 reaction exhibited a quartet in the 11B NMR.6
2.1.2 Equimolar Reaction of Lithium Aluminum Hydride with Ammonia Borane
The 1:1 reaction of the material was repeated in an attempt to better characterize
the product. Hydrogen released from the material was identical to that reported
21
previously. A single peak in the 11B NMR spectrum is centered at -22.8 ppm, which
sharpens upon decoupling from hydrogen. The proton NMR spectrum consists of two
broad peaks, one spanning the range from 2.5 to 3.6 ppm and the other from 0.6 to 2.2
ppm. Decoupling the spectrum from boron affects only the peak farther upfield, which
collapses to a complicated pattern. In light of the broadness of the peaks and complicated
patterns, identification of the material was not possible from NMR spectroscopy. The
NMR spectra are shown in Figures 2.1 to 2.4 in section 2.1.6.
The infrared spectrum of the compound exhibits N-H stretching frequencies at
3265 and 3115 cm-1, C-H stretching frequencies at 2983 and 2879 cm-1, and broad bands
between 1900 and 2500 cm-1 (B-H stretch) and 1500 and 1900 cm-1 (Al-H band). The IR
spectrum is shown in Figure 2.25 in section 2.1.6.
Attempts to crystallize the material were not lucrative, even when attempting to
use large, bulky cations, such as tetrabutylammonium and triphenylphosphine. X-ray
powder diffraction indicated an amorphous material. The product may be polymeric in
nature.
2.1.3 The Reaction of Lithium Aluminum Hydride with Ammonia Borane in a 1:2 Ratio
The reaction of lithium aluminum hydride with ammonia borane in a 1:2 ratio,
respectively, released slightly more than three equivalents of hydrogen gas over several
hours. Upon completion of the reaction, the solution was slightly cloudy and was filtered.
The resulting clear solution was examined by 11B NMR spectroscopy. The NMR
spectrum consisted of a single broad peak centered at the same shift as in the 1:1 reaction.
22
Decoupling from hydrogen in the boron-11 NMR also produced similar results to the 1:1
reaction, with a slight narrowing of the peak.
The compound was isolated after NMR analysis by removal of the solvent,
leaving a sticky residue. The material was washed multiple times with ethyl ether to
remove remaining starting materials and a white solid material remained. 1H and 11B
NMR and infrared spectroscopy was then performed on the isolated product.
The boron spectrum is unchanged as compared with the reaction solution before
purification. The proton NMR spectrum indicates solvent remained in the material, both
THF and ethyl ether. The spectrum also includes a broad peak between 4.0 and 4.5 ppm,
in the same range as the shift for the ammonia group of H3NBH3, indicating the peak
probably corresponds to protons on the nitrogen atom in the reaction product. A
complicated pattern is shown between 0.2 and 2.4 ppm that appears to be a broad,
featureless peak overlaying a second peak. Decoupling the proton NMR from boron
collapses the broad peak into a large, less broad yet still featureless peak centered at 1.36
ppm, with a shoulder on the peak. The second peak in the region, as seen in the nondecoupled boron spectrum, seems to be unchanged and still resides at 1.25 ppm. The
NMR spectra are shown in Figures 2.5 to 2.8 in section 2.1.6.
The infrared spectrum of the compound consists of N-H stretching frequencies at
3316 and 3265 cm-1, C-H stretching frequencies at 2983 and 2879 cm-1, a B-H stretching
frequency between 2000 and 2500 cm-1, a peak at 1836 cm-1 (Al-H), and an N-H
bending signal at 1539 cm-1. The IR spectrum is shown in Figure 2.25 in section 2.1.6.
23
Attempts to crystallize the material were fruitless, even with the use of large,
bulky catons. X-ray powder diffraction was attempted on the material, but the reaction
product is amorphous and no diffraction peaks were present.
2.1.4 The Reaction of Lithium Aluminum Hydride with Ammonia Borane in a 1:3 Ratio
Lithium aluminum hydride and ammonia borane were mixed in a 1:3 ratio and
allowed to react in tetrahydrofuran. Over the course of several hours, hydrogen gas was
released, collected, and measured. The average release of hydrogen during the reaction
was slightly more than four equivalents. After the evolution of hydrogen had ceased, the
solution was filtered to remove a fine white precipitate present in minute quantities. The
solution was examined by boron-11 NMR spectroscopy, which exhibited a broad quartet
centered at -22.7 ppm, shifted slightly from the quartet signal for ammonia borane (-22.0
ppm). Upon decoupling from hydrogen, the quartet condensed into a broad singlet, also at
-22.7 ppm.
The solvent was removed under vacuum from the reaction solution. The resulting
residue was washed with ethyl ether to remove any ammonia borane or lithium aluminum
hydride still present. The material was again pumped dry, leaving a solid, white powder.
The solid was evaluated with infrared and 1H and 11B NMR spectroscopy.
The 11B NMR spectrum of the material consists of a much broader peak which
appears to be a quartet centered at the same shift as the peak found prior to isolation of
the product. Decoupling the spectrum from hydrogen causes the broad quartet to collapse
into a singlet, much broader than the peak found in the proton decoupled spectrum of the
reaction solution, at -22.7 ppm. The proton NMR of the compound exhibits a very broad
24
peak ranging from 3.7 to 4.3 ppm which most likely corresponds to N-H hydrogen atoms.
Another broad peak ranges from 0.8 to 2.2 ppm in the proton spectrum, which changes
upon decoupling from boron to a large, broad singlet centered at 1.4 ppm. Large peaks
for tetrahydrofuran and ethyl ether are also visible in the proton NMR spectrum. With
further exposure of the reaction product to vacuum, the signals for the solvent are
smaller, but still present. Furthermore, the boron-11 NMR spectrum shows a narrower
quartet with a much less broad singlet at -22.7 ppm. A small amount of borohydride can
be seen in the spectrum as well at -41.9 ppm, which, notably, is shifted from the normal
signal of lithium borohydride in THF at -41.3 ppm. There is also a small amount of a
third boron compound, which appears to correspond to lithium amidotrihydroborate. All
NMR spectra for the 1:3 reaction of lithium aluminum hydride and ammonia borane are
shown in Figures 2.9 to 2.18 in section 2.1.6.
The infrared spectrum of the isolated material has N-H stretching frequencies at
3316 and 3261 cm-1, C-H stretching frequencies at 2983 and 2879 cm-1, a B-H stretching
signal between 2000 and 2500 cm-1, and an N-H bending signal at 1545 cm-1. The IR
spectrum is shown in Figure 2.26 in section 2.1.6.
The product of the reaction could not be crystallized and was found to be
amorphous upon examination with x-ray powder diffraction. The compound may be
polymeric as the peaks are broad without distinguishing features in the NMR spectra with
the exception of the quartet in the boron-11 NMR spectrum, which identifies a -BH3
group exists in the compound.
2.1.5 The Reaction of Lithium Aluminum Hydride and Ammonia Borane in a 1:4 Ratio
25
Lithium aluminum hydride and ammonia borane were combined in a 1:4 ratio and
tetrahydrofuran was added. Over the course of the reaction, slightly less than four
equivalents of hydrogen were released, measured via Toepler pump. When hydrogen
evolution had ended, the resulting solution was filtered to remove the small quantity of
white precipitate that had formed. The filtrate was then examined by 11B NMR
spectroscopy, exhibiting a single quartet at -22.7 ppm, which condensed into a singlet
with a shoulder upon decoupling.
The solvent was removed via vacuum and the resulting residue was washed with
ethyl ether to remove starting materials. The product remained as a sticky residue even
upon further pumping. The material was then examined by infrared and 1H and 11B NMR
spectroscopy.
The boron-11 spectrum consists of a broad quartet at -22.7 ppm, a small amout of
lithium borohydride, and a small peak that may be lithium amidotrihydroborate. The
spectrum appears to be identical to the boron-11 NMR of the 1:3 product. Decoupling
from hydrogen, the boron-11 NMR now has a singlet with a more pronounced shoulder
than observed in the reaction solution. The proton NMR indicates the presence of N-H
type hydrogen atoms with a broad peak spanning 3.6 to 4.2 ppm. Also shown in the
proton NMR spectrum is a broad peak between 0.4 and 2.2 ppm, which collapses into a
broad peak with slight doublet splitting upon decoupling from boron. The NMR spectra
are shown in Figures 2.19 to 2.24 in section 2.1.6.
Occasionally, after the reaction had completed, the product of the reaction would
seemingly decompose or change structure, resulting in the formation of much higher
26
concentrations lithium amidotrihydroborate and lithium borohydride. The cause for the
formation of lithium species remained unidentified and was a rare occurrence.
The infrared spectrum of the residue has signals corresponding to N-H stretching
at 3316 and 3249 cm-1, C-H stretching from the remaining solvents at 2983 and 2879
cm-1, B-H stretching frequencies of 2383, 2348, and 2278 cm-1, and N-H bending at
1546 cm-1. The IR spectrum is shown in Figure 2.26 in section in 2.1.6.
As the material was not solid, x-ray powder diffraction data was unable to be
obtained. Furthermore, the product was unable to be crystallized upon repeated attempts
with various methods.
2.1.6 Discussion of the Results of the Reactions of LiAlH4 and H3NBH3
Structural information regarding the compounds made in the reactions of lithium
aluminum hydride and ammonia borane in varying ratios is difficult to discern. The
amorphous nature of the materials made any diffraction studies impossible. Therefore,
analysis of the compounds fell to spectroscopy alone.
The peaks in the NMR spectra for the compounds tend to be broad and
featureless. The proton NMR of the compounds indicates the presence of N-H and B-H
type hydrogen atoms, as there are broad peaks in a similar range to where the ammonia
shift is observed in ammonia borane and there are broad peaks which change upon
decoupling from boron. The boron-11 NMR of the 1:3 and 1:4 reaction products indicates
the presence of the –BH3 group in both, while that of the 1:1 and the 1:2 reaction
products yields no structural information, though the chemical shift of all of the
compounds is identical at -22.7 ppm. The broadness of the peaks in the boron-11 NMR
27
spectra of the 1:1 and 1:2 reaction products may be correlated to the existence of Al-H
bonds in the compound, as the 1:3 and 1:4 reaction products exhibit no Al-H bands in the
infrared spectrum and have much more defined peaks in the NMR spectra. The
equivalent shift of the peaks indicates the boron atoms that are in the compounds are in
similar environments. Of particular note, the lithium borohydride formed in some of the
reactions is shifted from its usual position in the boron-11 NMR spectrum by 0.6 ppm
even though the machine was properly calibrated. This value is approximately the shift of
the peaks in the products of the reactions from the signal for the –BH3 of ammonia
borane.
The infrared spectra are the most useful data in attempting to identify the results
of the reactions. In ammonia borane, there are absorption frequencies at 3319, 3251, and
3197 cm-1 that correspond to the N-H stretch, as well as at 2381, 2335, 2281, and 2218
cm-1 from the B-H stretching and at 1603 cm-1 from the N-H bending. In comparison, the
N-H stretch in the materials isolated from the reaction discussed only have two bands in
the region 3200 to 3350 cm-1 and, for the 1:2, 1:3, and 1:4 reaction ratios, an N-H
bending signal around 1540 cm-1. In regards to this information, it can be concluded that
the nitrogen is bonded to two hydrogen atoms and is a secondary amine, which is
consistent with the presence of an amidotrihydroborate group. The 1:1 reaction product
appears in comparison to have only one hydrogen atom on the nitrogen as the bending
signal is absent from the IR spectrum of the material and the bands for the N-H region are
weak and not well defined. The hydrogen release data also supports this claim. A B-H
stretching frequency is also apparent in all of the compounds as a broad band between
28
2000 and 2500 cm-1. The wide band between 1500 and 2000 cm-1 in the 1:1 reaction
product appears to be a signal for the Al-H bond when compared to the infrared spectrum
of the lithium aluminum hydride starting material, which has two signals in this region. A
narrower peak in the 1:2 reaction product in the same region probably corresponds to an
Al-H bond as well. In correspondence with the hydrogen data, two equivalents of
hydrogen released in the 1:1 reaction would leave two hydrides present on the aluminum,
which would be responsible for band in the IR spectrum. Addition of another equivalent
of ammonia borane in the 1:2 reaction releases only one extra equivalent of hydrogen gas
as compared to the 1:1 reaction. Therefore, one hydride remains on the aluminum, and
may explain the band. Further addition of ammonia borane would remove all of the
hydrides from the aluminum in this circumstance, which explains the absence of peaks
corresponding to the Al-H bond in the 1:3 and 1:4 reaction products. The IR spectra of
ammonia borane, lithium aluminum hydride, all reaction products, and sodium
amidotrihydroborate (for comparison to another amidotrihydroborate species) are
included in Figures 2.25 To 2.28 A summary of the hydrogen released in the reactions of
lithium aluminum hydride and ammonia borane is also presented in Table 2.2.
Examining all of the data collected, structural determination is not definitively
accessible. From the NMR, -BH3 groups are definitely present in the 1:3 and 1:4 reaction
products, and are probably present in all of the products made. From the infrared spectra,
it can be determined that N-H stretching and bending are both present in the 1:2, 1:3, and
1:4 reactions and in the proper number of bands to conclude the presence of a primary
amine group, -NH2. Therefore, the conclusion that can be reached is that a reaction
29
occurs in a 1:1 ratio, releasing two equivalents of hydrogen. The 1:1 reaction product
then reacts with extra ammonia borane to add amidotrihydroborate to the compound. The
hydrogen release data matches this hypothesis as approximately three equivalents of
hydrogen gas are evolved in the 1:2 reaction. Upon a third exposure to a third equivalent
of ammonia borane, the material adds another amidotrihydroborate anion to the structure,
using the last of the available hydrides as evident in the absence of the peak between
1500 and 2000 cm-1 in the IR spectrum of the 1:3 reaction product. As the compounds are
amorphous and the NMR peaks are broad, the compounds may also be polymeric, in
which case the fractions of hydrogen measured may be associated with a competing
reaction to the amidotrihydroborate addition in which the polymer chain would be
extending or branching, releasing extra hydrogen gas. However, the fractions of hydrogen
gas could also be the result of the error in the Toepler pump calibration. With the current
data, it is difficult to conclude whether the 1:4 reaction is adding four equivalents of
amidotrihydroborate to the aluminum atom, or if the reaction only proceeds to the 1:3
reaction point and stops, leaving extra ammonia borane in solution. The hydrogen
released, the NMR spectra, and the IR spectra of both compounds are similar to the point
where an inability to distinguish one from the other exists.
Though the compounds cannot be identified from the data collected, the
remaining hydrogen on the materials is still high and may result in a good hydrogen
source. A summary of the proposed composition of the compounds and the mass
percentage of hydrogen remaining on each is presented in Table 2.1.
30
Ratio of Reaction
(LAH/AB)
1:1
1:2
1:3
1:4
Apparent Composition
LiAl(NHBH3)H2
LiAl(NHBH3)(NH2BH3)H
LiAl(NHBH3)(NH2BH3)2
LiAl(NH2BH3)4 ?
mass % hydrogen
9.34%
10.76%
11.43%
13.04%
Table 2.1: Percent hydrogen by mass of proposed products from the reaction of lithium
aluminum hydride and ammonia borane in varying ratios
2LiAlH4 +
2NH3BH3
2LiAlH4 +
4NH3BH3
1
4.06 mmolesH2
6.25 mmoles H2
8.3 mmoles H2
7.6 mmoles H2
2
3.99 mmoles H2
6.4 mmoles H2
8.5 mmoles H2
7.4 mmoles H2
Trial
2LiAlH4 +
6NH3BH3
2LiAlH4 +
8NH3BH3
Table 2.2: Hydrogen release data in the various reaction ratios of lithium aluminum
hydride with ammonia borane
31
Figure 2.1: 11B NMR spectrum of the product of the 1:1 reaction of lithium aluminum
hydride and ammonia borane
Figure 2.2: Proton decoupled 11B NMR of the 1:1 reaction product of lithium aluminum
hydride and ammonia borane
32
Figure 2.3: 1H NMR spectrum of the reaction product of 1:1 lithium aluminum hydride
and ammonia borane
Figure 2.4: Boron decoupled 1H NMR of the reaction product of 1:1 lithium aluminum
hydride and ammonia borane
33
Figure 2.5: 11B NMR of the reaction product of 1:2 lithium aluminum hydride and
ammonia borane
Figure 2.6: Proton decoupled 11B NMR of the reaction product of 1:2 lithium aluminum
hydride and ammonia borane
34
Figure 2.7: 1H NMR of the reaction product of 1:2 lithium aluminum hydride and
ammonia borane
Figure 2.8: Boron decoupled 1H NMR of the reaction product of 1:2 lithium aluminum
hydride and ammonia borane
35
Figure 2.9: 11B NMR of the reaction solution before isolation of 1:3 lithium aluminum
hydride and ammonia borane
Figure 2.10: Proton decoupled 11B NMR of the reaction solution of 1:3 lithium aluminum
hydride and ammonia borane
36
Figure 2.11: 11B NMR of the isolated reaction product of 1:3 lithium aluminum hydride
and ammonia borane
Figure 2.12: Proton decoupled 11B NMR of the isolated reaction product of 1:3 lithium
aluminum hydride and ammonia borane
37
Figure 2.13: 1H NMR of the isolated reaction product of 1:3 lithium aluminum hydride
and ammonia borane
Figure 2.14: Boron decoupled 1H NMR of the isolated reaction product of 1:3 lithium
aluminum hydride and ammonia borane
38
Figure 2.15: 11B NMR of the isolated reaction product of 1:3 lithium aluminum hydride
and ammonia borane after washing and pumping overnight
Figure 2.16: Proton decoupled 11B NMR of the isolated reaction product of 1:3 lithium
aluminum hydride and ammonia borane after washing and pumping overnight
39
Figure 2.17: 1H NMR of the isolated reaction product of 1:3 lithium aluminum hydride
and ammonia borane after washing and pumping overnight
Figure 2.18: Boron decoupled 1H NMR of the isolated reaction product of 1:3 lithium
aluminum hydride and ammonia borane after washing and pumping overnight
40
Figure 2.19: 11B NMR of the reaction solution of 1:4 lithium aluminum hydride and
ammonia borane
Figure 2.20: Proton decoupled 11B NMR of the reaction solution of 1:4 lithium aluminum
hydride and ammonia borane (inset shows the same spectrum in a more focused range)
41
Figure 2.21: 11B NMR of the isolated reaction product of 1:4 lithium aluminum hydride
and ammonia borane
Figure 2.22: Proton decoupled 11B NMR of the isolated reaction product of 1:4 lithium
aluminum hydride and ammonia borane
42
Figure 2.23: 1H NMR of the isolated reaction product of 1:4 lithium aluminum hydride
and ammonia borane
Figure 2.24: Boron decoupled 1H NMR of the isolated reaction product of 1:4 lithium
aluminum hydride and ammonia borane (inset shows the B-H region of the spectrum to
clarify the doublet splitting)
43
44
45
2.2 Solvent-Free Sodium Octahydrotriborate
A method to make sodium octahydrotriborate was needed both for use in the
synthetic process to ammonia triborane, included in this work, and to prepare solvent-free
alkaline earth metal octahydrotriborate compounds through ball milling. The methods
previously employed to synthesize sodum octahydrotriborate have had solvent
coordinated to the sodium, involved expensive reagents, and/or involved dangerous
reagents such as diborane, which can react explosively with air. Therefore, a safe, easy,
and relatively efficient method to produce the compound was refined (Equations 2.1-2.2).
2Na/Hg + 4THF∙BH3
(THF)xNaB3H8
(THF)xNaB3H8 + NaBH4
NaB3H8 + xTHF↑
(2.1)
(2.2)
Tetrahydrofuran-borane complex reacted with sodium mercury amalgam at room
temperature to form sodium octahydrotriborate. The purity of the mercury was of
particular importance. Attempts to simply wash the mercury with water, THF, and
acetone led to boron impurities detectable in the NMR spectrum, albeit very small
quantities. Remaining THF∙BH3 and most of the THF solvent was removed by vacuum.
The resulting material was an oil with high viscosity. Removal of the remaining THF,
including that coordinated to the sodium cation, was achieved by heating the material
under vacuum to 75 ⁰C overnight. The crude product, containing sodium borohydride and
sodium octahydrotriborate, was extracted with ethyl ether, filtered, the solvent was
removed under vacuum, and additional heating for two hours removed all traces of THF.
An additional extraction with ethyl ether, filtering, removal of solvent under vacuum, and
46
washing of the material on a frit with dry hexanes ensured the removal of all sodium
borohydride and any of the boron impurities. The pure product was a white solid with
approximately 60% yield on average.
Ethyl ether can be completely removed under vacuum at room temperature from
sodium octahydrotriborate to yield the solvent-free product, as described in the
introduction. Removal of the coordinated tetrahydrofuran, therefore, is the main objective
in the synthesis described. To that extent, deuterated ethyl ether was chosen for NMR
experiments. The 11B NMR exhibits a single nonet corresponding to the
octahydrotriborate anion, centered at -30.4 ppm. The 1H NMR spectrum includes a peak
with a splitting of ten for the protons on the octahydrotriborate ring centered at 0.2 ppm,
peaks at 1.12 and 3.42 ppm corresponding to the ethyl ether used as the medium for the
NMR spectrum, and impurities already present in the solvent before use. The
tetrahydrofuran peaks, at 1.73 and 3.58, are absent in the spectrum, indicating success of
the method. Acetonitrile can also be used as a solvent for NMR to identify the absence of
both THF and ethyl ether, but is a somewhat poor choice as it is much more difficult to
dry completely. Any water present in the solvent leads to the formation of hydrolysis
products in the spectrum.
The sodium octahydrotriborate was extremely hygroscopic. Exposure to air for
very short periods of time, on the order of several minutes, resulted in a water solution.
The material was also solvoscopic, coordinating to vapors of tetrahydrofuran when
exposed.
47
48
49
2.3 Ammonia Triborane
2..3.1 Improved Synthesis of Ammonia Triborane
The procedure outlined in the work of Yoon, Carrol, and Sneddon works well to
produce small quantities of ammonia triborane.80,81 In order to obtain larger amounts of
material at lower cost, the procedure was refined to generate pure ammonia triborane
without the need of one as many cooling steps or the chromatography column described
in the literature article.
A solution of iodine in dimethoxyethane was added dropwise to a stirring solution
of tetrabutylammonium octahydrotriborate over a period of approximately 15 minutes at
room temperature in an inert atmosphere. As each drop of iodine came into contact with
the octahydrotriborate solution, the purple color quickly faded to colorless. A white
precipitate formed during the reaction (Bu4NI). The solution was transferred to a vacuum
line and cooled to -78 ⁰C. Ammonia, dried over sodium metal, was condensed into the
solution. The cold bath was removed and the ammonia was allowed to slowly evaporate
from the stirring solution with slight vacuum through a liquid nitrogen cold trap to keep
the gas just above atmospheric pressure to prevent evacuation into the laboratory. After
evaporation of the ammonia was complete, the solution was filtered in an inert
atmosphere and the solvent was removed via vacuum. The remaining oil was then
extracted by ethyl ether, filtered, concentrated, and again extracted with a mixture of
ether and hexanes. Upon pumping on the solution, the ether evaporated first at which
point NH3B3H7 precipitated and was collected by vacuum filtration.
50
The boron-11 NMR of the crude ammonia triborane exhibits in majority two
peaks corresponding to the B3H7 ring, with impurities of remaining octahydrotriborate,
ammonia borane, and a compound containing B-O bonds (Figure 2.31). After extracting
with mixed solvents, the boron-11 NMR clearly shows pure ammonia triborane (Figure
2.32).
2.3.2 The Reaction of KB3H8 with I2
KB3H8 was use in place of Bu4NB3H8 in the reaction to synthesize ammonia
triborane. The reaction occurred in the same manner as in the original procedure,
producing impure NH3B3H7 which subsequently could be extracted with a mixed solvent
of hexanes and ethyl ether for purification. The NMR spectra matched exactly with that
reported in the literature and in the improved synthesis.
Potassium or sodium octahydrotriborate was used in the synthesis of
tetrabutylammonium octahydrotriborate. The ability to use the potassium salt in place of
the tetrabutylammonium salt in the synthesis of ammonia triborane further simplified the
procedure presented in the literature.
2.3.3 The Reaction of Ammonia Triborane with Sodium Hydride
Sodium hydride was reacted with ammonia triborane in a 1:1 reaction ratio in
THF. A gas formed during the reaction, but only sparingly. The reaction solution was
examined by 11B NMR spectroscopy. Contrary to the previous report that the reaction
produces sodium octahydrotriborate,82 the reaction predominantly formed sodium
borohydride. Addition of an excess of sodium hydride converted all of the ammonia
triborane in the reaction to sodium borohydride and unknown boron compounds.
51
Figure 2.31: 11B NMR of ammonia triborane as extracted from the reaction of Bu4NB3H8
and I2
Figure 2.32: 11B NMR of clean ammonia triborane after extraction with mixed solvent
52
Chapter 3
Experimental
3.1 Equipment and Apparatus
3.1.1 Nuclear Magnetic Resonance Spectroscopy (NMR)
NMR spectra were collected with a Bruker AM-250 spectrometer operating at
250.13 MHz for 1H NMR and 80.25 MHz for 11B NMR. 11B NMR was calibrated using a
boron trifluoride etherate standard (δ = 0.00). Chemical shifts are reported in ppm.
3.1.2 X-Ray Powder Diffraction
X-ray powder diffraction data for the reactions of lithium aluminum hydride and
ammonia borane was collected on a Bruker D8 Advance X-ray powder diffractometer,
which employs a Vario monochromator at the X-ray tube with Cu-Kα1 radiation (λ =
1.5406 Å). The samples were ground with a mortar and pestle and packed into a 0.5 mm
Lindeman glass capillary under an inert atmosphere in the dry box.
3.1.3 Infrared Spectroscopy
The infrared spectra of the lithium aluminum hydride and ammonia borane
reaction products were collected on a Bruker Tensor 27 FT-IR spectrophotometer.
Resolution was set at 2 cm-1 over 32 scans. The program OPUS was used for refinement
of the spectra, including base line correction and peak picking. The samples were pressed
53
into KBr pellets under inert atmosphere in the dry box. The materials were loaded into an
air tight cell consisting of two NaCl salt plates on either side of the IR nut. The apparatus
was sealed from air using rubber O-rings between the nut and the plates, held together by
clamping the cell.
3.1.4 Vacuum Line
Dynamic vacuum was created by a Pyrex glass vacuum line consisting of a
mechanical vacuum pump capable of reducing pressure to 10-6 mm Hg and four working
manifolds. The oil in the pump was drained and replaced every ten to fifteen weeks. Two
cold traps, one typically held at -78 ⁰C via dry ice in isopropanol and the other held at 196 ⁰C with liquid nitrogen, were placed between the manifolds and the vacuum pump to
protect the pump from solvent vapors. The cold traps were emptied daily.
A main manifold separated the working manifold from the vacuum pump and
allowed for isolation of the individual manifolds via grease stopcocks. The stopcocks
were greased with Dow Corning High Vacuum Grease. Two of the working manifolds
were attached to four U-tubes, calibrated for use in gas titrations. The other two
manifolds were separated by a glass disk and a Teflon Kontes valve that could be opened
or closed to connect the two. Each manifold had four ports consisting of Kontes valves
and either Fischer-Porter joints or 14/35 ground glass male joints. The ports were
connected to mercury blowouts to ensure gas pressure in flasks connected to the manifold
would not build pressure beyond atmospheric level to prevent pressure explosions. A
cold trap could be connected between the two manifolds using custom made glassware
and the innermost port on each manifold.
54
A Toepler pump was connected to the working manifold to measure noncondensable gases. Gas in the manifold was allowed to flow into a chamber over a large
volume of mercury. Room pressure was allowed under the mercury, causing the mercury
to rise, forcing the gas above the mercury through a cold trap and into three calibrated
bulbs connected to a manometer. A shifting glass valve moves up to allow gas to flow out
of the chamber and into the calibrated bulbs as the mercury forces the gas upwards, but
seals when the mercury level falls upon removal of the atmospheric pressure below to
prevent backflow of gas into the chamber.
3.1.5 Glassware
Reactions were performed exclusively in Pyrex glass flasks. The flasks typically
all had rounded bottoms and were equipped with Fischer-Porter joints on the neck. The
flasks were attached to adapters with Kontes valves using Teflon joints to connect the
Fischer-Porter joints and plastic clips and screw joints to apply appropriate pressure to
ensure an air tight seal. The adapters had appropriate joints to connect the flasks to the
vacuum line manifolds.
Filtration of air sensitive materials was performed using a traditional glass
vacuum line extractor, separating the solution bulb to be filtered from the collection flask
by a porous glass frit of porosity of 70-100 μ. The extractor had two Kontes valves, one
to open the collection flask to or isolate it from the vacuum line manifold, and one to
open to equalize the pressure between the two sides of the glass frit. Dry celite was used
on the glass frit to ensure adequate filtration.
55
Figure 3.1: a) Example of the flask and Kontes valve adaptors used in reactions; b)
example of the glass frit extractor described
3.1.6 Dry Box
All chemicals used and produced in this work were handled and stored under an
inert atmosphere in a Vacuum Atmosphere (model: HE-43-2) glove box equipped with a
drying train (model: MO-40-1H). The internal nitrogen (99.998% N2 supplied by OSU
stores) was maintained at 1 in. H2O above ambient external pressure to allow adequate
purging and to prevent backfilling of air. The oxygen level was evaluated by an
Innovative Technology Inc. oxygen sensor. An analytical balance and a refrigerator were
maintained inside the box. An external vacuum pump was connected to the box to allow
vacuum filtration or pumping on materials inside the dry box.
The air in the dry box was pumped by a standard circulator pump through the
drying train and into a layer of round molecular sieves, a catalyst (13% CuO2 on silica),
and type 13X sodium aluminosilicate molecular sieves, present in equal quantities. The
layer removed solvent, O2, and any moisture from the atmosphere of the dry box. The
56
materials in the layer were acquired from Vacuum Atmospheres. The catalyst mixture
was regenerated upon standing O2 sensor readings greater than 2 ppm oxygen using
Praxair regeneration gas (5% H2 in N2).
3.2 Solvents and Reagents
3.2.1 Solvents
Ammonia, NH3 (PraxAir) was dried over sodium and distilled as needed.
Dichloromethane, CH2Cl2 (Fisher, 99.5%) was purified by washing with concentrated
sulfuric acid followed by washings with a 5% aqueous solution of sodium bicarbonate
and then water. The solvent was then preliminarily dried over magnesium sulfate,
followed by storage over phosphorous pentoxide. The solvent was distilled as needed.
Dimethoxyethane, DME (Fisher, 99.8%) was dried and stored over sodium with
benzophenone as a carrier for the metal and distilled when needed.
Ethyl Ether, Et2O, (Fisher, 99.9%) was dried and stored over sodium with benzophenone
as a carrier for the metal and distilled when needed.
Hexanes (Mellinckrodt, 98.5%) was purified by mixing with sulfuric acid, followed by
washing with a 5% solution of sodium bicarbonate, washing with water, and preliminary
drying over magnesium sulfate. The solvent was then distilled into a flask containing
benzophenone as a carrier and sodium metal, in which it was stored. The solvent was
distilled from the flask as needed.
Tetrahydrofuran, THF (Fisher, ~100.0%) was dried and stored over a mixture of sodium
metal and benzophenone and was distilled as needed.
3.2.2 Reagents
57
All chemicals were used as received unless otherwise noted.
Ammonia Borane, H3NBH3, was made by the reaction of ammonium carbonate and
sodium borohydride in anhydrous tetrahydrofuran. The crude product was recrystallized
from water, dissolved in anhydrous tetrahydrofuran, filtered to remove boric acid
contamination, and isolated as a pure, white flaky material. The material was stored in the
dry box.
Ammonium Carbonate, (NH4)2CO3 (Aldrich, ~100.0%) was stored on the shelf.
Barium Oxide, BaO (Aldrich, 97%) was stored in a dessicator.
Benzophenone, C13H10O (Fisher) was stored on the shelf.
Celite filter aid, diatomiaceous earth (Aldrich) was heated to 180 ⁰C under vacuum
overnight to remove moisture and was stored in the dry box.
Iodine, I2 (Aldrich, ≥ 99.8%) was ground with barium oxide with a mortar and pestle and
sublimed before use.
Lithium Aluminum Hydride, LiAlH4 (Aldrich, 95%) was stored in the dry box.
Magnesium sulfate, MgSO4 (Aldrich, 99%) was stored in a closed container on the shelf.
Mercury, Hg (Bethlehem, 99.999999% ) was stored on the shelf.
Phosphorous pentoxide, P2O5 (Fisher, 99.6%) was stored under N2 on the shelf.
Potassium, K (Aldrich, 99.5%) was stored under mineral oil in the dry box.
Potassium bromide, KBr (Aldrich, 99.99%) was stored in an oven in a glass vial at over
100 ⁰C.
Potassium Octahydrotriborate, KB3H8 was made from the reaction of tetrahydrofuran
borane complex and potassium-mercury amalgam. The product was collected by filtering,
58
pumping away the solvent on the filtrate, and washing the resulting solid with ethyl ether.
The material was stored in the dry box.
Sodium, Na (Aldrich, 99.95%) was stored under mineral oil on the shelf.
Sodium borohydride, NaBH4 (Aldrich, ~98%) was stored in the dry box.
Sodium hydride, NaH (Aldrich, 95%) was stored in the dry box.
Tetrabutylammonium bromide, Bu4NBr (Aldrich, ≥98%) was stored in the dry box.
Tetrabutylammonium octahydrotriborate, Bu4NB3H8 was made via cation replacement.
Sodium octahydrotriborate was dissolved tetrahydrofuran, to which a saturated aqueous
solution of tetrabutylammonium bromide was added in slight excess of the molar quantity
needed to replace the cation. Ethyl ether was added to the material, precipitating
Bu4NB3H8. The material was recrystallized several times from dry dichloromethane and
ethyl ether, followed by several recrystallizations from dry tetrahydrofuran and ethyl
ether.
Tetrahydrofuran borane complex solution, THF∙BH3 (Aldrich, 1.0 M in THF) was stored
in the refrigerator in a sure-seal bottle.
3.3 Syntheses
3.3.1 Reaction of 1:1 Lithium Aluminum Hydride and Ammonia Borane
Lithium aluminum hydride (79.8 mg, 2 mmoles after adjusting for 95% purity)
was mixed with ammonia borane (61.6 mg, 2 mmoles) as a dry powder in a round-bottom
flask equipped with a Fisher-Porter joint. The flask was equipped with a Kontes valve
adapter and transferred to the vacuum line. The flask was evacuated and 25 mL of dry
tetrahydrofuran was distilled into the flask at -196 ⁰C. The flask was allowed to warm
59
slowly to first -78 ⁰C, at which point the reaction began, and then to room temperature.
After approximately two hours, hydrogen evolution had ceased. 3.99 mmoles of
hydrogen gas were released, measured via Toepler pump. The resulting solution was
filtered in the dry box to remove the white precipitate that had formed. The filtrate was
collected and the solvent was evacuated. The resulting residue was washed multiple times
with anhydrous ethyl ether. The product was collected as a white powder. 11B NMR (d8THF): -22.7 ppm (br), 1H NMR (d8-THF): 0.6-2.2 ppm (br), 2.5-3.6 ppm (br), IR: νN-H
= 3265 and 3115 cm-1(stretch), νC-H = 2983 and 2879 cm-1 (stretch), νB-H = 1900-2500 cm1
(stretch), νAl-H = 1500-1900 cm-1.
3.3.2 Reaction 1:2 Lithium Aluminum Hydride and Ammonia Borane
Lithium aluminum hydride (79.8 mg, 2 mmoles after adjusting for 95% purity)
was mixed with ammonia borane (123.2 mg, 4 mmoles) as a dry powder in a roundbottom flask equipped with a Fisher-Porter joint. The flask was equipped with a Kontes
valve adapter and transferred to the vacuum line. The flask was evacuated and 25 mL of
dry tetrahydrofuran was distilled into the flask at -196 ⁰C. The flask was allowed to warm
slowly to first -78 ⁰C, at which point the reaction began, and then to room temperature.
After approximately two hours, hydrogen evolution had ceased. 6.25 mmoles of
hydrogen gas were released as measured using a Toepler pump. The resulting solution
was filtered in the dry box to remove the white precipitate that had formed. The filtrate
was collected and the solvent was evacuated. The resulting residue was washed multiple
times with anhydrous ethyl ether. The product was collected as a white powder. 11B NMR
(d8-THF): -22.7 ppm (br), 1H NMR (d8-THF): 0.6-2.4 ppm (br), 4.0-4.5 ppm (br), IR:
60
νN-H = 3316 and 33265 cm-1(stretch) 1539 cm-1 (bend), νC-H = 2983 and 2879 cm-1
(stretch), νB-H = 2000-2500 cm-1 (stretch), νAl-H = 1836 cm-1.
3.3.3 Reaction of 1:3 Lithium Aluminum Hydride and Ammonia Borane
Lithium aluminum hydride (79.8 mg, 2 mmoles after adjusting for 95% purity)
was mixed with ammonia borane (184.8 mg, 6 mmoles) as a dry powder in a roundbottom flask equipped with a Fisher-Porter joint. The flask was equipped with a Kontes
valve adapter and transferred to the vacuum line. The flask was evacuated and 25 mL of
dry tetrahydrofuran was distilled into the flask at -196 ⁰C. The flask was allowed to warm
slowly to first -78 ⁰C, at which point the reaction began, and then to room temperature.
After approximately two hours, hydrogen evolution had ceased. 8.3 mmoles of hydrogen
gas were released, measured via Toepler pump. The resulting solution was filtered in the
dry box to remove the white precipitate that had formed. The filtrate was collected and
the solvent was evacuated. The resulting residue was washed multiple times with
anhydrous ethyl ether. The product was collected as a white powder. 11B NMR (d8-THF):
-22.7 ppm (q), 1H NMR (d8-THF): 0.8-2.2 ppm (br), 3.7-4.3 ppm (br), IR: νN-H = 3316
and 3261 cm-1(stretch), 1545 cm-1 (bend), νC-H = 2983 and 2879 cm-1 (stretch), νB-H =
2000-2500 cm-1 (stretch).
3.3.4 Reaction of 1:4 Lithium Aluminum Hydride and Ammonia Borane
Lithium aluminum hydride (79.8 mg, 2 mmoles after adjusting for 95% purity)
was mixed with ammonia borane (184.8 mg, 6 mmoles) as a dry powder in a roundbottom flask equipped with a Fisher-Porter joint. The flask was equipped with a Kontes
valve adapter and transferred to the vacuum line. The flask was evacuated and 25 mL of
61
dry tetrahydrofuran was distilled into the flask at -196 ⁰C. The flask was allowed to warm
slowly to first -78 ⁰C, at which point the reaction began, and then to room temperature.
After approximately two hours, hydrogen evolution had ceased. 8.3 mmoles of hydrogen
gas were released, measured via Toepler pump. The resulting solution was filtered in the
dry box to remove the white precipitate that had formed. The filtrate was collected and
the solvent was evacuated. The resulting residue was washed multiple times with
anhydrous ethyl ether. The product was collected as a sticky, colorless material. 11B
NMR (d8-THF): -22.7 ppm (q), 1H NMR (d8-THF): 0.4-2.2 ppm (br), 3.6-4.2 ppm (br),
IR: νN-H = 3316 and 3249 cm-1(stretch), 1546 cm-1 (bend), νC-H = 2983 and 2879 cm-1
(stretch), νB-H = 2383, 2348, and 2278 cm-1 (stretch).
3.3.5 Solvent-Free Sodium Octahydrotriborate
Pure sodium metal (0.414 g, 0.018 mmoles) was added in small pieces to a flask
equipped with a glass coated magnetic stirring bar and 12 mL of quadruple distilled
mercury. The amalgam was formed over 1 hour of stirring, at which point the flask had
cooled from the extremely exothermic reaction. 54 mL of a 1.0 M tetrahydrofuran borane
complex was added to the amalgam and the solution and amalgam were stirred together
for 24 hours. After the reaction was complete, the solution was filtered to remove some
of the sodium borohydride that formed. The filtrate was collected and the solvent was
removed via dynamic vacuum, leaving an oily residue. The residue was heated to
approximately 75 ⁰C under dynamic vacuum overnight. The solid material remaining in
the flask was extracted multiple times with ethyl ether to separate the NaB3H8 from any
remaining sodium borohydride. The solvent was removed and the solid was again heated
62
to 75 ⁰C for several hours to ensure complete removal of solvent. The NaB3H8 was
washed with dry hexanes to remove minute quantities of impurities, leaving 36.2 mg
(63.3% yield) of the pure material as a white solid. NMR: 11B NMR (d10-Et2O): -30.5
ppm (nonet), 1H NMR (d10-Et2O): 0.23 ppm (decet).
3.3.6 Improved Synthesis of Ammonia Triborane
Bu4NB3H8 (1.0 g, 3.53 mmoles) was dissolved in anhydrous dimethoxyethane
(10.0 mL) in a 100-mL round-bottomed flask equipped with a Teflon coated magnetic
stirring bar in the dry box. To the flask, a solution of elemental iodine (0.44 g, 1.73
mmoles) in dimethoxyethane (10.0 mL) was added drop-wise with stirring over the
course of 15 minutes. During the addition, bubbling was observed as hydrogen was
released in the reaction. After the addition was complete, the flask was equipped with a
Kontes valve adapter and transferred to the vacuum line. Under liquid nitrogen, the flask
was evacuated of non-condensable gases. The flask was warmed to -78 ⁰C, at which point
anhydrous ammonia (25 mL) was condensed into the reaction with vigorous stirring. The
reaction was allowed to stir for 30 minutes under these conditions. The solution was then
allowed to warm to just below the normal boiling point of ammonia, at which point
ammonia was slowly removed via dynamic vacuum through a liquid nitrogen cold trap
until completely removed. The dimethoxyethane was then removed by dynamic vacuum.
The resulting residue was extracted with a mix of 70% hexane and 30% ethyl ether
multiple times to remove all of the ammonia triborane, leaving an oil behind. The extract
was returned to the vacuum line and the ethyl ether solvent was removed, at which point
the pure ammonia triborane precipitates as fine, white crystals suspended in hexanes. The
63
solution was filtered and the ammonia triborane was collected from the filter frit. NMR:
11
B NMR (in hexane/ether mix): -9.97 ppm (br, two boron atoms on B3H7), -33.9 ppm
(br, B-N boron atom of B3H7).
64
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