What is Boron?
Boron is comparatively unabundant in our universe. It constitutes the
waterways and crustal epidermal layer of the Earth to an average extent of about
9 ppm. This makes its presence on our planet rather less abundant than lithium
(18 ppm) or lead (13 ppm), overwhelmingly less abundant than carbon (180 ppm),
but similar to the presence of thorium (8.1 ppm) for example. It occurs in nature
almost invariably as compound with oxygen mainly in the form of borate minerals
Na2[BxOy(OH)z] or as borosilicates. Commercially valuable deposits are rare, but
where they do occur, as in California or Turkey, they can be vast. Isolated
deposits are also worked in the USSR, Tibet, and Argentina. All major deposits
occur in areas of former volcanic activity and appear to be associated with the
waters from former hot springs. Exposure and subsequent weathering resulted
in leaching by surface waters, leaving residues of the minerals. Total annual
production of these minerals in the USA amounted to just over a million tonnes
(2003 figures). Production in Turkey has expanded dramatically in the past
decades and has now surpassed that of the USA to become the world’s leading
producer at around 1.5 million tonnes per annum (2003 figures). Production in
the USSR is around the same as that in the US (2003 figures). At current levels
of consumption, world resources are adequate for the foreseeable future.
Borate minerals were used by the ancient
Egyptians as a desiccant and disinfectant in the
process of mummification, an application that has
since developed into their general use as
detergents, cleaners and soaps.
The same
minerals are also a very important
component of glass, used since
ancient times, added especially to
increase the heat resistance of the
glass. As a trace mineral, borates are crucial to the healthy
growth of vegetation, and are concordantly a constituent of
fertilizer. The reported distribution pattern for boron compounds
consumed in the US in 2001 was as follows: glass and ceramics,
78%; soaps and detergents, 6%; agriculture, 4%; fire retardants,
3%; and other, 9%.
Boron is the only light element with two abundant naturally occurring isotopes,
B and 11B, providing for interesting nuclear
properties. The exceptionally high nuclear crosssection of 10B for the capture of thermal neutrons
(over 1000 times that of any of its neighbours)
leads to interest in nuclear applications such as
boron neutron capture therapy (BNCT) in the
treatment of cancer and has found application in
the nuclear power industry as a neutron absorber.
10
Boron has been known to be one of the
elementary substances since the electrolytic
experiments of Sir Humphrey Davy. Its
position next to carbon in the
Periodic Table has suggested
to numerous chemists of the
past and present that it should
have a many-faceted hydride
chemistry. However, its great affinity for oxygen prevented exploitation of this
promise until the development of anaerobic techniques in 1912, particularly by
Alfred Stock.[1] This same affinity for oxygen gives it a potential as a high-energy
fuel and for other applications where chemical energy must
be packaged in as concentrated a form as possible. This
possibility led to an intensive exploration of the chemistry of
the boron hydrides (or boranes) during the 1950’s as a
possible new high-energy fuel. The results of the research
found that the use of boron hydrides for this purpose is
inappropriate and ill-conceived as combustion of these
materials in the presence of air leads to the formation of
oxygenated boron compounds that are, at the temperature
of a running engine, a glassy liquid, which rapidly engenders
the malfunctioning and eventual failure of any combustion
engine. The considerable monies invested into research into the boranes did
however reveal a hydride chemistry more extensive than any other element from
the periodic table other than carbon. Intensive research laid the foundations to
current application or development of application of borane compounds in
medical areas such as in the fight against HIV/Aids [2] and cancer [3], catalytic
synthesis [4], and micro-electronics and the doping of silicon with boron to form ptype semi-conductors.[5]
These application trends reveal a wind-change in the direction of research and
use of boron hydride materials from high-energy fuels and propellants (1950’s
and 1970’s) toward medical applications (1990’s and
2000’s) and high-tech materials (2000’s). This trend is
evident from a review of all current research literature
published on the boranes, and from the types of borane
materials commercially available.
Structure, bonding and classification of the boranes
In all of chemistry there is a continuum of structural behaviour between very
condensed metal clusters, which can be described as being based on metallic
lattice fragments, and the open chains, rings and mononuclear species more
typical of organic and P-block inorganic chemistry. The chemistry of the boranes
(boron hydrides) is characterized by structures towards the centre of this
continuum.
Figure 1. A structural continuum: From condensed metal clusters (left) to open chains and rings
(right). Bridging these extremes are the cages of the boranes (middle).
From the beginning, the boranes posed interesting problems of constitution,
structure and bonding. Stock's earliest publications on the first boron hydrides [6]
predated the Lewis-Langmuir octet theory of electron-pair bonds [7] by some five
years and the boranes proved to be the most troublesome exception to that
theory. Boron is in Group III of the Periodic Table, which might in the first
instance imply that the simplest hydride would be BH3. Yet, despite extensive
and diligent experimentation, no sign of BH3 was ever encountered by Stock, the
simplest member of the series being diborane, B2H6. The stoichiometry is clearly
similar to that of ethane, C2H6, but as the boron atom has one electron fewer
than carbon there are apparently insufficient electrons to form two-centre twoelectron bonds between every adjacent pair of atoms. The boranes were
therefore said to be 'electron-deficient' and presented scientists with a genuine
mystery.
H
H
H
H
H
C
C
B
B
H
H
H
H
H
H
H
Figure 2. Ball and stick representations of the structures of ethane (C2H6) and diborane (B2H6).
Note the divalent ‘bridging’ hydrogen atoms in the diborane structure.
The solution to this mystery was found by H.C. Longuet-Higgins[8] by means of
his concept of the three-centre two-electron bond: if electrons are in short supply
then a pair of electrons can bond three atoms in a triangular array. He
successfully established and interpreted the bridged dimeric structure of B2H6
(see Figure 2) and then, generalising from three-centre to multi-centre bonding,
made the brilliant prediction of stable polyhedral dianions of then unprecedented
structures, namely the octahedral cluster [B6H6]2- and the icosahedral cluster
[B12H12]2-. (Synonymous with many borane compounds is the word cluster. This
description comes about because, in order to achieve efficient interatomic
bonding, boron atoms often aggregate together, i.e. form clusters of atoms) The
structural and bonding systematics were developed into a topological discourse
by W.N. Lipscomb.[9] In this description of the bonding in boron hydride
compounds, bonding schemes are described in terms of both two-centre and
three-centre orbital arrangements.
Using Libscomb's system for a given borane of general formula, BpHp+q, several
geometric and bonding relationships may be determined. For example, R.E.
Williams [10] observed that borane clusters fall into three geometrically distinct
stoichiometric series which he
called closo-[BnHn]2-, nido-BnHn+4
and arachno-BnHn+6. The structural
relations between Williams' three
series are shown in Figure 3 with
nido-borane geometries formed by
the removal of one highly
connected
vertex
from
the
corresponding closo polyhedron,
and arachno borane geometries by
removal of two adjacent vertices.
The predominant structural motifs of
the boranes are fragments of
triangulated polyhedral clusters, or
'deltahedra'. In viewing these and
similar structures straight lines
joining pairs of atoms do not
necessarily represent pairs of
electrons, but indicate geometrical
connectivities:
three-centre and
multi-centre bonding is the rule.
Figure 3. The relationships between the
closo, nido and arachno polyhedral boron
hydrides.
Polyhedral borane cluster structures may be regarded as being based on BH
units held together by multi-centre bonding which is often delocalized over the
cluster surface.
sp-hybridised boron orbital in overlap with the 1s
hydrogen atom electronic orbital, creating a B-H
bond pointing away from the borane cluster
H
full p-character orbitals aligned
tangentedly to the cluster surface
B
Figure 4 (above). Ball and stick
representation of the molecular
structure of the icosahedral closo[B12H12]2- anion.
Figure 5 (right). The associated
atomic orbitals of a cluster {BH}
unit as encircled in Figure 1.
sp-hybridised orbital extending
externally and perpendicualarly from
the cluster surface
full p-character orbitals aligned
tangentedly to the cluster surface
sp-hybridised orbital pointing
directly into the cluster
The structural, bonding, and electron-counting “rules” that govern polyhedral
cluster chemistry, which together constitute the “Polyhedral Skeletal Electron Pair
Theory”,[9, 11-14] are now generally well-known and are comprehensively dealt with
in modern undergraduate textbooks. However, as these rules will be used
extensively in this article, it is appropriate to summarize here some specific
pertinent points.
In this article, the descriptors closo, nido and arachno have a dual role. First,
they describe systems with, respectively, n+1, n+2, and n+3 skeletal electron
pairs, where n is the number of cluster vertices. Second, they describe
corresponding n-vertex clusters with closed polyhedral (closo) geometries, and
clusters derived from the closed polyhedral by the successive removal of one
(nido) and two (arachno) vertices.
General properties of the boranes
Generally speaking, the boranes are colourless, white or pale yellow,
diamagnetic, molecular compounds. The lower members are gases at room
temperature but with increasing molecular weight they become volatile liquids or
solids. Boranes are generally chemically reactive species and several are
spontaneously flammable in air. Arachno-boranes tend to be more reactive (and
less stable to thermal decomposition) than nido-boranes and reactivity
diminishes with increasing molecular weight. Closo-borane anions (such as
closo-[B12H12]2- in figure 1) are exceptionally stable and their general chemical
behaviour has suggested the term “three-dimensional aromaticity”.
References
1. A. Stock, “Hydrides of Boron and Silicon”, Cornell University Press, Ithaca, New
York, 1933.
2. P. Cigler et al., Proc. Nat. Acad. Sci. 102(43), 15394-15399, 2005.
3. see, for example, http://web.mit.edu/nrl/www/bnct/info/description/description.html
4. see, for example, C. A. Reed, Acc. Chem. Res., 31, 133, 1998; J. Am. Chem. Soc., 121,
6314, 1999.
5. J. Plešek, Chem. Rev., 92, 269-278, 1992; X. Lu, L. Shao, X. Wang, Q. Chen, J Liu, J.
Bennet, L. Larson, P. Ling, Proc. Electrochem. Soc., 2001-2009, 337-344, 2001; Y.
Kawasaki, T. Kuroi, T. Yamashita, K Horita, T. Hayashi, M. Ishibashi, M. Togawa,
Y. Ohno, M. Yoneda, T. Horsky, D. Jacobson, W. Krull, Nucl. Instrum. Methods
Phys. Res., Sect. B, 237, 25-29, 2005.
6. A. Stock and C. Massenez, Ber., 45, 3529, 1912; A. Stock and K. Friederici, Ber., 46,
1959, 1913; A. Stock, K. Friederici and O. Priess, Ber., 46, 3353, 1913.
7. I. Langmuir, J. Am. Chem. Soc., 41, 868, 1919.
8. H. C. Longuet-Higgins, J. Chem. Phys., 46, 275, 1949.
9. W. N. Lipscomb, Boron Hydrides, Benjamin, New York, 1963.
10. R. E. Williams, Adv. Inorg. Chem. Radiochem., 18, 67, 1976.
11. R. Mason and D. M. P. Mingos, Int. Rev. Sci., Phys. Chem. Ser. 2, 11, 1975, p.121.
12. D. M. P. Mingos, Nat. Phys. Sci., 236, 99, 1972.
13. R. W. Rudolph, Acc. Chem. Res., 9, 446, 1976.
14. R. W. Rudolph and W. R. Pretzer, Inorg. Chem., 11, 1974, 1972.