DOI: 10.1021/cg900985w
Crystallization Behavior of Coordination Polymers. 1. Kinetic and
Thermodynamic Features of 1,3-Bis(4-pyridyl)propane/MCl2 Systems
2009, Vol. 9
5024–5034
Lucia Carlucci,† Gianfranco Ciani,† Juan Manuel Garcı́a-Ruiz,§ Massimo Moret,*,‡
Davide M. Proserpio,† and Silvia Rizzato*,†,#
†
Dipartimento di Chimica Strutturale e Stereochimica Inorganica and #Facolt
a di Farmacia,
Universit
a degli Studi di Milano, Via G. Venezian 21, I-20133 Milano, Italy, §Laboratorio de Estudios
Cristalogr
aficos, IACT (CSIC-Universidad de Granada), Avda. del Conocimiento s/n, P.T. Ciencias de
a degli Studi di
la Salud, 18100 Armilla, Spain, and ‡Dipartimento di Scienza dei Materiali, Universit
Milano Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy
Received August 18, 2009; Revised Manuscript Received October 1, 2009
ABSTRACT: A series of one-, two-, and three-dimensional (1D, 2D, and 3D) coordination polymers has been crystallized from
solution as well as in gelled media, by controlled reaction of the flexible ligand 1,3-bis(4-pyridyl)propane with MCl2 salts (M =
Mn, Fe, Co, Ni, Cu, Cd, Zn). Attention was paid to the role of kinetic and thermodynamic control upon selection of the final
products that can be obtained with different optimized crystallization techniques. It is shown that the correct use of
crystallization techniques allows tuning of the metal to ligand ratio and absolute concentrations and, hence, control of the
crystallization of metal-organic frameworks, including their solvent-mediated phase transitions.
Introduction
Recent years have witnessed a surge of interest in supramolecular chemistry,1 and, in particular, there is hope for
future technological applications2 of coordination polymers,
a specific class of supramolecular objects also known as metalorganic frameworks (hereafter MOFs).3,4 The intriguing variety of architectures and topologies5 exhibited by MOFs are
explored in crystal engineering,6 as they can reveal interesting
properties for opto-electronic7 and magnetic8 functional materials, molecular sieving,9 ion exchange,10 catalysis11-13 as
well as stable and highly porous scaffolds14 for sorption of
volatile organic compounds and gas storage.15-18 Attempts to
build novel and useful coordination networks rely upon
almost all metal cations available in the periodic table of
elements, that is, transition, post-transition, lanthanide,19-21
and actinide21 metals. The counterpart of ligands shows an
even wider variety of choices being based on multifunctional
organic or inorganic moieties, displaying diversified local
stereochemistries supposedly able to force the self-assembling
of MOFs to follow a pre-established structural motif as
wished by crystal engineers.22-24 Many structurally relevant
MOFs which do have promising properties for key technological applications and high commercial value25 have been
synthesized starting from nitrogen-based multifunctional
ligands.3,16c,d,26 Among them, pyridine-based ligands differing in length, flexibility, and conformational freedom produced a variety of supramolecular networks with different
metal cations.12
While the structural characterization of MOFs by X-ray
diffraction is well established, there is no comparable understanding of their solution chemistry, under ambient or solvothermal conditions.27,28 Particularly unclear are the molecular
recognition and assembling steps which eventually lead to a
coordination polymer. This failure partly stems from the
complexity of chemical equilibria involving these polymers
*Corresponding authors. (M.M.) Phone: þ39 0264485218; fax: þ39
0264485400; e-mail: [email protected]; (S.R.) phone: þ39
0250314442; fax: þ39 0250314454; e-mail: [email protected].
pubs.acs.org/crystal
Published on Web 10/23/2009
which fragment upon solubilization, and whose high molecular weight frameworks arise from mostly unknown solution
species,29 apart from a few noteworthy oligomeric entities.30,31
Thus, no accurate model for the network formation from
solution is presently available; chemical synthesis usually is
performed following a buy-and-mix strategy. However, it is
also largely acknowledged that the final outcome of reactants
mixing can be driven by subtle kinetical and thermodynamical
details of the chemical processes. As several authors noticed,
supramolecular inorganic coordination frameworks represent
a dynamic challenge for structural chemists.32,33 Starting from
the mother solutions, the overall supramolecular synthesis of
MOFs includes molecular recognition and self-assembly of
solution species through nucleation of crystalline phases,
growth of mature crystals, and possibly phase transitions
according to the relative kinetic prevalence and thermodynamic stability. Therefore, solution coordination chemistry is
deeply entangled with nucleation and growth mechanisms of
MOFs crystals to determine the final products and their
properties, through a complicated interplay between kinetics
and thermodynamics.
We have previously synthesized coordination polymers
with reversible uptake/release of guest solvent molecules, for
example, compounds [Cu5(bpp)8(SO4)4(EtOH)(H2O)5](SO4) 3
EtOH 3 25.5H2O34 (bpp=1,3-bis(4-pyridyl)propane; EtOH=
ethanol) and [Cu(bipy)2(CF3SO3)2] 3 2CH2Cl2 3 H2O,35 (bipy =
4,40 -bipyridyl); both exhibit interesting microporous behavior
arising from novel 3D architectures associated with cavities or
tunnels, respectively, from which guest molecules can be easily
and reversibly removed due to some degree of flexibility36 of
the metal-organic framework. Moreover, the self-assembly
of polymeric networks from different Ag(I) salts and the
flexible ligand 1,3-bis(4-pyridyl)propane (bpp) has been systematically investigated in order to obtain basic information
useful for the crystal engineering of coordination frames upon
variation of the counterions.37
We have also studied the crystallization of inorganic polymers based on the flexible nitrogen ligand bpp and metal(II)
chlorides, trying to improve understanding of the self-assembly
r 2009 American Chemical Society
Article
Crystal Growth & Design, Vol. 9, No. 12, 2009
processes of coordination networks. Reaction of aqueous
CuCl2 with bpp ligand under different conditions afforded a
family of coordination polymers with one-, two-, and threedimensional (1D, 2D, and 3D) polymeric network dimensionalities. A preliminary account on these systems has already
been published38 mainly dealing with the structural features of
the Cu(II)/bpp systems but also gathering preliminary data on
how different metal cations assemble with bpp to build
coordination networks.
The present study aims at a better characterization of the
formation of coordination polymers, that is, crystal nucleation, crystal growth, and kinetical or thermodynamical selection of the products, factors which for MOFs are intimately
connected with self-assembly and building of the polymeric
species. We choose a simple but flexible ligand such as 1,3bis(4-pyridyl)propane reacted with the chlorides of a series of
divalent cations (Mn2þ, Fe2þ, Co2þ, Ni2þ, Cu2þ, Cd2þ, and
Zn2þ) aiming at forcing the chloride anion in the inner
coordination sphere to obtain neutral polymers, thus removing the role of the counterion as a possible templating agent
during crystallization of MOFs.37 The present paper expands
the case study of the CuCl2/bpp system38 and improves
description of the subtle interplay between experimental conditions under which MOFs based on the MCl2/bpp couple are
synthesized, including effects of concentration, metal/ligand
ratio, nature of solvent, and kinetic or thermodynamic competition between different solid phases.
Experimental Procedures
Materials and Methods. All reagents (metal salts: CdCl2 3 2H2O,
CuCl2 3 2H2O, CoCl2 3 6H2O, FeCl2 3 4H2O, MnCl2 3 4H2O, NiCl2 3 6H2O,
ZnCl2; ligand 1,3-bis(4-pyridyl)propane) and solvents were commercially available high-purity materials (from Aldrich Chemicals
and Pharmacia LKB Biotechnology), and used as supplied without
further purification apart from the bpp ligand which was purified by
sublimation. Deionized Milli-Q water was used for electroconductance measurements using a Hanna HI255 conductimeter with a
vessel thermostatted at 25 °C ( 0.05 °C. The Job plot absorbance
data points were collected with a Hewlett-Packard 8453 UV/vis
spectrophotometer by using CuCl2 and bpp solutions both 10.7 or
20.0 mM. Optical micrographs were taken with a polarizing SZX12
Olympus stereomicroscope equipped with a digital camera for
single shot or time sequence images acquisition. X-ray powder
diffraction spectra were collected on a Philips PW 3050 verticalscan diffractometer. For the chemical synthesis, no crystal yields
are reported due to unavoidable difficulties during crystal recovery
from gels and the very long times required to reach chemical
equilibrium. A selection of yields is reported in Table 2S in
Supporting Information for precipitation of microcrystalline powders at high concentrations.
Synthesis of [M(bpp)3Cl2] 3 2H2O (M = Cd (1Cd), Co (1Co), Fe
(1Fe), Ni (1Ni)). Compounds 1Cd and 1Co were crystallized by the
slow evaporation method from a mixture of reagents using water/
acetone for 1Cd (CdCl2 3 2H2O: 28.3 mg, 0.129 mmol in 10 mL of
water added to bpp: 51.5 mg, 0.260 mmol in 10 mL of acetone) and
water for 1Co (CoCl2 3 6H2O: 32.4 mg, 0.136 mmol dissolved in
10 mL of water added to bpp: 53.4 mg, 0.269 mmol in 10 mL of water)
as solvent. In the case of 1Cd a white precipitate formed immediately. Both reaction mixtures were filtered and left to evaporate at
ambient temperature until crystals were obtained, colorless for 1Cd
and pink for 1Co.
1Fe has been prepared by the liquid-liquid diffusion technique
using aqueous solutions of reagents and a 1:2 molar ratio
(FeCl2 3 4H2O: 15.6 mg, 0.078 mmol dissolved in 3.0 mL of water
layered on bpp: 31.2 mg, 0.157 mmol dissolved in 3.0 mL of water).
Because of the oxidation of Fe(II) a brown powder precipitates from
the solution; however, in a few days formation of aggregates of
yellow crystals of 1Fe has been observed.
5025
1Ni: 1.00 g of solid PEO (poly (ethylene oxide), average MW
∼ 2 000 000) was added to 20 mL of a 0.060 M solution of bpp in water.
After a few minutes of stirring, a stable suspension was obtained.
Part of this solution was poured into a test tube (10 mm i.d. and
115 mm long) approximately to half height. In the same way 20 mL
of a 0.060 M solution of NiCl2 3 6H2O was gelled with 0.50 g of PEO
and then poured above the bpp suspension until the top of the tube
was reached. The tube was closed and the mixture was left to stand
at room temperature to crystallize. Crystals appeared within a few
days as clusters of green plates.
Synthesis of [M(bpp)2Cl2] 3 solv (M=Cd (3Cd), Co (3Co), Fe (3Fe),
Mn (3Mn), Ni (3Ni) solv = MeOH, EtOH, H2O). 3Cd: crystallization was accomplished using single gel diffusion technique. The
crystals were grown in a single glass tube of length 115 mm and
diameter 10 mm. A 1.0% (w/v) agarose gel was obtained by adding
agarose powder (Tg = 42 °C) to 4.5 mL of an aqueous solution of
CdCl2 3 2H2O (32.0 mg, 0.146 mmol) under continuous stirring and
then heating the mixture in a water bath at a temperature above the
agarose melting point until a clear solution was obtained. The hot
sol was then poured in the test tube and cooled. After setting of the
gel, an ethanolic solution of bpp (19.1 mg, 0.406 mmol) was carefully layered over it. In a few weeks, white well-formed bymiramidal
crystals of 3Cd were formed at the gel-solution interface and in the
supernatant ethanolic solution.
3Co: 0.75 g of a bead-formed dextran gel, Sephadex LH-20
(Pharmacia LKB Biotechnology), was added to 5 mL of a
CoCl2 3 6H2O methanolic solution (34.7 mg, 0.146 mmol). The gel
was allowed to swell for several hours into a test tube (10 mm i.d.
and 115 mm long). The excess of solution was removed before
layering on top of the packed beds an equal volume of a bpp solution
at required molarity (0.030 M in methanol). The tube was closed and
allowed to stand at room temperature. Pink well-formed crystals,
with characteristic bipyramidal shape, grew in a few days together
with blue crystals of species 6Co both within the body of the gel and
in the supernatant solution.
3Mn and 3Fe: Both compounds were crystallized by carefully
layering 3 mL of a methanolic solution of the metal salt (MnCl2 3
4H2O: 42.3 mg, 0.214 mmol; FeCl2 3 4H2O: 9.1 mg, 0.0458 mmol)
over a solution of bpp in methanol (82.1 mg, 0.414 mmol, 3 mL) for
3Mn and dichloromethane (17.6 g, 0.0887 mmol, 3 mL) for 3Fe at
room temperature. In both cases, the mixture was left to stand at
ambient temperature for one day to yield typical bipyramidal
crystals (colorless for 3Mn and yellow for 3Fe).
3Ni: solid PEO (0.40 g, average MW ∼ 2 000 000) was dissolved
by stirring in 20 mL of a dichloromethane solution containing
bpp (80.6 mg, 0.406 mmol) to give a gel-like high viscosity fluid.
Approximately 3 mL of the gel material, characterized by the presence of gas bubbles, were transferred into a test tube (10 mm i.d.
and 115 mm long). The bubbles were removed by letting the sample
stand for several hours. Then, an equal volume of NiCl2 3 6H2O
(11.7 mg, 0.0492 mmol) methanolic solution was layered above the
gel. The tube was closed and the mixture was left to stand at room
temperature to crystallize. Many small green crystals, with a tetragonal bipyramidal shape, were formed at the gel-solution interface
in a few days.
Synthesis of 5Cu. This compound was obtained as a pure crystalline
powder by a counter-diffusion method using isopropanol as solvent,
suitable concentrations of reactants, and a 1:1 metal-ligand molar
ratio (bpp solution: 24.6 mg, 0.124 mmol in 3.0 mL, layered over a
CuCl2 3 2H2O solution: 20.9 mg, 0.123 mmol in 3.0 mL)
Synthesis of [Co(bpp)Cl2] (6Co). Single crystals of 6Co were
obtained by using a modification of the gel-acupuncture method.39
A 1.0% (w/v) agarose gel was prepared and poured into a test tube
(13 mm i.d. and 160 mm long) until a layer of 10 mm in height was
obtained. A 100 mm long glass tube, open at both ends, with a
2.0 mm inner diameter, was immediately inserted into the bigger
tube and fixed with one side of the tube into the sol-gel. After
completion of gelling of the agarose sol, 1.5 mL of a methanolic
solution of bpp (11.9 mg, 0.060 mmol) and 6.0 mL of a solution of
CoCl2 3 6H2O (47.6 mg, 0.200 mmol) in the same solvent were
poured respectively in the inner and in the outer tube, and stored
at room temperature. Well-formed prismatic blue crystals were
formed in the bpp solution in a few weeks.
5026
Crystal Growth & Design, Vol. 9, No. 12, 2009
Synthesis of [Zn(bpp)Cl2] (6Zn). Crystals of polymeric Zn-bpp
were grown by liquid-liquid diffusion method using both water and
methanolic solutions of reagents and 1:2 or 1:1 metal-ligand molar
ratio (for example: ZnCl2 3.4 mg, 0.025 mmol, 2.5 mL of H2O/bpp
5.0 mg, 0.025 mmol, 2.5 mL of H2O; ZnCl2 3.6 mg, 0.026 mmol,
3.0 mL of H2O/bpp 5.1 mg, 0.026 mmol, 3 mL of MeOH).
Synthesis of [Cu(bpp)1.5Cl2] 3 (THF) 3 0.75(H2O) (7Cu). Crystallization of compound 7Cu was accomplished by layering on a solution of bpp in tetrahydrofuran (THF) (4.6 mg, 0.030 mmol, 4 mL)
an isopropanolic solution of CuCl2 3 2H2O (11.9 mg, 0.060 mmol,
4 mL). The mixture was left to stand at ambient temperature for one
day to yield small prismatic light-blue crystals. The nature of the
bulk material as pure 7Cu was confirmed by X-ray powder diffraction data compared with the pattern calculated from the singlecrystal X-ray structure.
Determination of Phase Diagrams. The batch technique40 has
been used to perform a screening of crystallization conditions and to
build the phase diagram as a function of metal salts and ligand
concentrations. Crystallization trials were rapidly set up in a
standard 24-well plate made of polystyrene (aqueous systems) or
polypropylene (methanolic systems). Each well was air-tightly
sealed with a 22 mm diameter glass coverslip and vacuum grease.
A set of solutions with different concentrations of metal salts and
bpp ligand were prepared in deionized water or methanol. Crystallization trials were performed by mixing the appropriate amount of
reagent solutions and pure solvent so that a final volume of 2.0 mL
for each fixed concentration of the components was obtained. The
concentrations of metal salt and bpp ligand were systematically
varied along the rows and the columns of the plate. The samples
were monitored, by using an optical microscope coupled to a CCD
camera, until equilibrium conditions were reached.
An extensive study was carried out only for copper and cobalt
chlorides in water and methanol solvent systems. For all other metal
cations (Mn2þ, Fe2þ, Ni2þ, Cd2þ, Zn2þ), the nature and purity of
the products formed by precipitation from water or methanol were
checked by XRPD analysis by comparing the experimental patterns
with the simulated ones from the single crystal structures. Microcrystalline powder samples were prepared following a standard
procedure: typically, a water (or methanolic) solution of bpp of
appropriate concentration was added to an aqueous (or methanolic)
solution of the metal salt while stirring. In water a 1:3 metal-ligand
molar ratio was used while in methanol the reagents were mixed in
1:1 and 1:2 ratio according to the stoichiometry of the two cobalt
species isolated from this solvent. Precipitate was formed and the
reaction mixture was left to react for ∼2 h. The precipitate was
filtered through a Buchner funnel, washed with small portions of
solvent and dried in air. A summary of the results and further details
about crystallization conditions for each plate and powder samples
are given as Supporting Information.
Gel Double Diffusion Experiments. Two similar crystallization
experiments, based on the double diffusion technique, were carried
out by using U-shaped tubes41 (5 mm i.d. and 130 mm long with two
identical arms of 30 mm length). The tubes were filled up with
agarose gel of high strength prepared at 1% (w/v) and then solutions
of the reagents (∼0.5 mL) of the appropriate concentrations were
layered above the gel on opposite arms of the tubes. The two
crystallization trials differ both for the solvent system and the
concentration of reactants. In the first experiment (Figure 6) equimolar (0.128 M) aqueous solutions of bpp and copper salt (CuCl2 3
2H2O) were employed while in the second one (Figure 7) methanol
and lower concentrations (0.099 M) were used.
Phase Transformations. The crystal transformations among different MOFs were monitored by means of an optical microscope
coupled to a CCD camera connected to a computer equipped with a
frame-grabber. Time-lapse acquisition for automatic archiving of
image series was performed at a suitable frequency.
1D f 2D Transformation. A crystal of the 1D copper species 1Cu
was placed in a small cell to which 0.2 mL of a 0.1 M CuCl2 aqueous
solution was added. The transformation took place at 25 °C in
about 4 h (Figure 3).
1D f 3D Transformation. A crystal of 1Cu was placed in a small
cell to which pure ethanol was added. The transformation took
place at 25 °C in about 5 min.
Carlucci et al.
2D f 1D Transformation. Crystals of the 2D copper species 2Cu
were spontaneously nucleated in a small cell by preparing an
aqueous solution 4 mM and 12 mM with respect to CuCl2 and
bpp, respectively. Subsequent appearance of 1Cu crystals starts
within 15-30 min; equilibrium is reached at 25 °C in about 3-5 days
with complete comsumption of 2Cu crystals (see Figure 1S, Supporting Information).
2D f 3D Transformation. A crystal of 2Cu was placed in a small
cell to which pure ethanol was added. The transformation took
place at 25 °C in about 50 min.
Electroconductance Measurements. Solubilities of compounds
1Co, 1Ni, 1Cu, and 1Cd have been determined at 25.0 ( 0.1 °C
using a thermostatic bath. Metal(II) chloride and bpp ligand solutions were mixed in a 1:3 M/L ratio and molar concentration
sufficient to give a microcrystalline precipitate (1Cd) or crystals
(1Co, 1Ni, 1Cu) of the corresponding 1D MOF species. Chemical
equilibrium was reached after 3 weeks. Solubilities were determined
by comparison of electroconductance value of the saturated solution after diluting 1:2 with deionized water against a set of standard
solutions of the pertinent 1D species.
Single Crystal X-ray Structure Characterization. Crystal data for
all the compounds are listed in Table 1S, Supporting Information.
The data collections were performed with Mo KR (λ = 0.71073 Å)
on a Bruker APEX II CCD area-detector diffractometer for 1Co,
1Ni, 3Cd, 3Co, 3Fe, 3Mn, 3Ni, 6Co, and 7Cu and on an EnrafNonius CAD4 for 1Cd, by the ω-scan method. An empirical
absorption correction was applied42 for the structures collected
with the CCD detector, while the ψ-scan43 method was used
for 1Cd. The structures were solved by direct methods (SIR9744
or SHELX-S97)45 and refined by full-matrix least-squares on
F2 (SHELXL-97)46 with WINGX interface.47 Anisotropic thermal
parameters were commonly assigned to all the nondisordered nonhydrogen atoms except for the partially occupied atoms of the
clathrate solvent molecules in some of the structures. In compounds
1Cd, 1Co, and 1Ni one of two independent ligands was found
disordered and refined isotropically using two models with occupancies of 52 and 48% (1Cd), 50 and 50% (1Co), 59 and 41% (1Ni).
In the same structures the water molecules were refined anisotropically but with half occupancy. The hydroxyl H atom associated with
the ethanol molecule in 3Mn was located on a difference Fourier
map and refined freely, with Uiso(H)=1.2Ueq(O), while all the other
ones were placed in geometrically calculated positions and thereafter refined using a riding model with the correct occupancy for
disordered models. All the structure diagrams were performed using
the TOPOS48 SCHAKAL9949 programs.
Results and Discussion
As the nature of coordination polymers can be modified by
the conformation of the chosen ligands, coordination geometry preferred by the metal, counterions, solvent system, and
metal-to-ligand ratio, in the following sections we discuss the
phenomenology and the interplay among some of the chemical parameters relevant to the selection of the crystalline
species obtained after mixing of metal(II) chlorides and ligand
bpp.50
The CuCl2/bpp/Water System. We already reported38 on
the possibility to obtain a whole family of coordination
polymers based on the couple CuCl2/bpp in water, with
new interesting 1D, 2D, and 3D topologies (Figure 1) depending on the choice of several chemical parameters. The
CuCl2/bpp system subsequently afforded another new species when the reaction was conducted in THF solutions (see
next section). In ref 38, we also briefly explored the networking properties of the chlorides of other M2þ cations with
suitable ionic radius and coordination stereochemistry that
in principle can afford species isostructural to those obtained
from CuCl2 (see Table 1).
By working with aqueous CuCl2, a 1D species ([Cu(bpp)3Cl2] 3 2H2O, 1Cu), consisting of interdigitated zigzag chains
Article
Crystal Growth & Design, Vol. 9, No. 12, 2009
containing dangling monocoordinated bpp ligands, and a
2D coordination polymer ([Cu(bpp)2Cl]Cl 3 1.5H2O, 2Cu),
that now can be better defined as a 3D array formed from
inclined polycatenation of 2D square layers in the diagonal-diagonal mode (density of catenation DOC = (2/2))5a
have been obtained using different Cu/bpp ratios and
Figure 1. Coordination polymers obtained with the CuCl2/bpp/
water system showing new interesting 1D, 2D, and 3D topologies.
5027
concentrations. Independently from us another group prepared a 1D MnCl2/bpp coordination polymer isostructural
with our Cu(II) 1D system.51 On the contrary, the presence
of alcohols or other organic solvents (acetone, dichloromethane) leads to the formation of a 3D 4-fold interpenetrated diamondoid network ([Cu(bpp)2Cl2] 3 2.75H2O, 3Cu)
(see next section). Curiously, while in these systems phenomena of polymorphism and supramolecular isomerism are
often encountered, this seems to be not the case here; indeed,
compounds 2Cu and 3Cu have very similar composition
differing only in the amount of solvated water molecules,
but the chloride ions play a different role in these two species
so that it is hard to consider them supramolecular isomers.
During initial exploration of the behavior of aqueous
solutions of CuCl2 and bpp in a 1:2 ratio we obtained crystals
of the 1D phase 1Cu by slow solvent evaporation at RT.
While trying to improve size and quality of 1Cu crystals by
varying crystallization conditions, that is, M/L ratio, absolute concentrations, and crystal growth technique (see later),
we observed the formation of a new crystalline MOF (species
2Cu), easily recognized due to its deep blue color and
different crystal morphology compared to 1Cu. However,
upon standing for a few days in the presence of the mother
solution the deep blue crystals of 2Cu disappeared leaving
only pale blue crystals of 1Cu. Therefore, under the selected
conditions, 1Cu was the thermodynamically stable product
(showing indeed a lower solubility) whose formation is
preceded by the nucleation of crystals of 2Cu, the kinetically
favored species. This sequence of events follows Ostwald’s
rule of stages52 which allows the possibility that different
crystalline species (e.g., polymorphs, or different hydrates or
solvates) can nucleate sequentially starting from the least
stable one that in turn transforms into a thermodynamically
more stable species and so on until only the most stable
species survive owing to its lowest solubility. Supersaturation conditions that provoke at first the nucleation of 2Cu
followed by the more stable 1Cu (but see later for the
opposite 1Cu to 2Cu transformation) are mirrored in the
large number of small single crystals of 2Cu while crystals of
1Cu are generally heavily twinned rosettes. Nucleation of
2Cu occurs within minutes after mixing of reactants (e.g.,
CuCl2 4.0 mM and bpp 12.0 mM), while the subsequent
solvent-mediated transformation into 1Cu requires several
days to go to completion as monitored by means of time
lapse optical microscopy (Figure 1S, Supporting Information).
Table 1. Summary of Crystallographic Data for Coordination Polymers Obtained from Different MCl2/bpp/Solvent Systems
compound
MOF
1Cda
1Coa
1Cub
1Fea
1Mna
1Nia
2Cub
3Cda
3Coa
3Cub
3Fea
3Mnc
3Nia
6Coa
6Zna,d
7Cua
1D
1D
1D
1D
1D
1D
2D
3D
3D
3D
3D
3D
3D
1D
1D
2D
a
a (Å)
b (Å)
c (Å)
16.516(4)
16.580(3)
17.191(1)
16.517(2)
16.571(3)
16.484(3)
16.314(1)
17.536(1)
17.187(1)
17.221(1)
17.081(2)
17.201(2)
16.991(4)
5.189(1)
5.2254(4)
22.062(9)
26.779(6)
27.108(3)
16.242(1)
26.922(4)
26.888(5)
26.901(3)
18.211(1)
17.536(1)
17.187(1)
17.221(1)
17.081(2)
17.201(2)
16.991(4)
12.980(1)
12.9371(9)
16.984(8)
17.705(3)
17.122(6)
26.859(2)
17.221(2)
17.525(3)
16.973(6)
36.247(3)
42.966(1)
42.299(3)
40.965(2)
42.554(5)
42.697(4)
41.947(9)
10.493(1)
10.5425(6)
15.647(7)
This work. b Ref 38. c Ref 51. d Ref 54.
β (deg)
91.59(1)
93.589(2)
94.247(3)
118.183(6)
crystal system
sp group
orthorhombic
orthorhombic
monoclinic
orthorhombic
orthorhombic
orthorhombic
orthorhombic
tetragonal
tetragonal
tetragonal
tetragonal
tetragonal
tetragonal
monoclinic
monoclinic
monoclinic
Ibca
Ibca
I2/a
Ibca
Ibca
Ibca
P212121
I41/a
I41/a
I41/a
I41/a
I41/a
I41/a
P21/m
P21/m
C2/c
5028
Crystal Growth & Design, Vol. 9, No. 12, 2009
This behavior is highly reproducible and has been checked
several times. In the context of MOFs chemistry several
other systems displayed crystal-to-crystal transformations.53
After this accidental discovery, we pursued a more rational approach by studying the influence of the Cu2þ/bpp
Figure 2. Equilibrium phase diagram for aqueous CuCl2/bpp system with indication of the stability regions for 1D polymer 1Cu, 2D
polymer 2Cu, and Cu4Cl2(OH)6 (4) species as a function of [L] þ [M]
total concentration and [L]/[M] ratio (M = Cu2þ, L = bpp).
Carlucci et al.
ratio and absolute concentration values on kinetics and final
equilibrium composition. Considering that solid 1Cu and
2Cu have Cu2þ/bpp ratios respectively of 1:3 and 1:2, a
systematic exploration of Cu2þ/bpp ratio and total concentration of reactants has been undertaken with the batch
approach.40 Results for aqueous CuCl2/bpp system are
summarized in Figure 2 which extends up to ca. 15 mM for
both CuCl2 and bpp (solubility of bpp being of ca. 60 mM in
water at RT). The equilibrium phase diagram clearly shows
dominance of the 1D polymer over the 2D species starting
from a bpp/CuCl2 ratio of ∼2, while the absolute concentration value has a negligible influence over stability regions of
the two species. The phase diagram also shows the presence
of the microcrystalline species Cu4Cl2(OH)6 (4) identified by
means of XRPD. This copper(II) hydroxochloride is produced when the Lewis basicity of bpp is overcome by its
Broensted-Lowry basicity at low ligand concentrations.
From the point of view of kinetics, increasing the bpp/CuCl2
ratio reduces the nucleation time for species 1Cu to an extent
for which the preliminary nucleation and appearance of
crystals of 2Cu becomes negligible or not discernible under
the microscope at the highest magnification. Low ligand-tometal ratios favor formation of 2Cu at the expense of 1Cu.
Hence, when proper conditions are used, the 1 T 2 reversible
transformation can be driven to completion at will so that we
are able to selectively nucleate and obtain pure crystal
Figure 3. Selected frames from a video microscopy record of the solvent mediated 1Cu f 2Cu transformation induced by a proper increase of
the CuCl2 concentration (time lapse between subsequent images is 15 min; crystal size is ca. 2 mm).
Article
samples of both species through equilibration. Indeed, based
on our knowledge of chemical equilibria of aqueous CuCl2/
bpp system, it has been possible to drive to completion in
about 4 h also the solvent-mediated 1Cu f 2Cu transformation as shown in Figure 3.
Since compounds 1Cu and 2Cu are characterized by
different Cu/bpp stoichiometries the 1Cu f 2Cu transformation produces the release of the excess of bpp as a ghostly
halo surrounding the transforming crystals (last frames in
Figure 3). Clearly, the unidentified solution species leading
to 1Cu or 2Cu depending on chemical conditions can be
manipulated to move equilibrium by mass action law toward
the desired product, independently from the kinetic features
of the system previously described. A hint about the nature
of the solution species that self-assemble to give 1Cu or 2Cu
(the nucleation step in term of crystal growth mechanisms)
has been collected by constructing a Job plot55 with CuCl2
and bpp solutions 10.7 and 20.0 mM. Unfortunately, stability constants for complexes between the bpp ligand and
copper(II) are not very high and the Job plot of Figure 4
shows a significant rounding of absorbance vs molar fraction
ascending and descending arms which hampers a clear-cut
analysis of data. Indication is that at total [Cu2þ] þ [bpp] =
10.7 mM the average formula of solution species lies between
Figure 4. Job plot for the CuCl2/bpp aqueous system with indication of the average composition of solution species at χCu ∼ 0.28,
that is, intermediate between the stoichiometry of 1Cu and 2Cu.
Crystal Growth & Design, Vol. 9, No. 12, 2009
5029
a 1:2 and 1:3 M:L ratio, while at 20.0 mM total concentration
the M:L ratio slightly shifts toward 1:3, but basically without
statistical significance. This trend is however in accordance
with the decreasing nucleation time required for 1Cu when
increasing the bpp/Cu ratio.
Demonstration that with proper knowledge of thermodynamics and kinetics of a metal-ligand system it is possible
to control the outcome of MOFs synthesis is a relevant
message to all those involved with this kind of solid-state
preparation; in fact, the selected pair of ligand and metal salt
can afford several different species in a perfectly controllable
manner when the system is carefully studied (Figure 5).
The previous results could be further refined in terms of
capability to selectively obtain the desired product by exploring the parameters relevant to nucleation and growth of
crystals of the different species. By crystallizing in gels56
within U-shaped tubes and choosing experimental conditions on the basis of the phase diagram (Figure 2), it was
possible to afford in the same crystallization tube the three
species observed in water, that is, Cu4Cl2(OH)6 (4), 2Cu, and
1Cu in order of increasing bpp to Cu ratio. On the right arm
of the tube in Figure 6 was put an aqueous solution of CuCl2
while on the left reservoir an aqueous solution of bpp was
charged; the horizontal branch of the tube contained a 1% w/
w agarose gel. Counter diffusion leads to concentration
gradients of the two reactants which enabled us to obtain
the aforementioned species in a single experiment according
to the phase diagram, that is, 4 at very low, 2Cu at intermediate, and 1Cu at high bpp/Cu2þ ratios, respectively
(Figure 6). This experimental setup is inter alia very useful
to tailor reactants concentration in order to obtain good
single crystals avoiding excessive nucleation, spherulitic,
dendritic or twinned crystals, all representative of a very
high supersaturation. By properly selecting concentrations,
it has also been possible to drive the system toward production of pure samples of 2Cu, without 1Cu or 4 as byproducts.
Figure 6. Crystallization in a U-shaped tube by counterdiffusion of
bpp (left arm) and CuCl2 (right arm) showing the concomitant
presence of species Cu4Cl2(OH)6 (4, microcrystalline), 2Cu and 1Cu
in order of increasing bpp to Cu ratio from right to left. The
horizontal part of the U-shaped tube is filled with a gel of high
strength agarose to avoid convective mixing of the reactants.
Figure 5. Scheme illustrating the reversible solvent mediated chemical transformation 1Cu T 2Cu.
5030
Crystal Growth & Design, Vol. 9, No. 12, 2009
The same experimental setup can be used to produce morphodromes, that is, a detailed study of dependence of crystal
morphology on supersaturation or ratio between reacting
species, a feature that can be of relevance for technological
applications of MOFs. It must be mentioned that morphology of crystals can become a delicate issue when planning
practical applications of crystalline materials, for example,
for catalysis or gas sorption. These results, in view of the fact
that no general and reliable models for the formation of
coordination networks from solution are presently available,
can be a useful guide for careful exploration and optimization of conditions for synthesis of crystalline coordination
polymers. An illustrative example of the severe difficulties
when trying to reproducibly synthesize and study the archetypal structure MOF-5 are discussed in a recent paper.27 Also
temperature plays a fundamental role on determining kinetics and thermodynamics of coordination polymer synthesis
and we briefly discussed it in ref 38. Recently, temperature
has been exploited to select the final product and polymer
topology through variation of conformation of a flexible
ligand.57
The CuCl2/bpp/Organic Solvent Systems. Other crystallization trials involved use of organic solvents (methanol,
ethanol, isopropanol, acetone, dichloromethane) for dissolving the ligand bpp and copper chloride. Presence of an
organic solvent directs the self-assembling process toward
the 3D species 3Cu, [Cu(bpp)2Cl2] 3 2.75H2O, that does not
contain any solvated organic molecule within the cavities of
the 3D metal framework.38 Therefore, it must be assumed
that the organic solvent, without entering the coordination
network, modifies the nature of the unknown solution
species and/or their self-organization which eventually assemble into the interpenetrated structure of 3Cu. The role of
methanol is well illustrated in a refined crystallization experiment with aqueous agarose gel which, thanks to composition gradients, in particular those involving methanol,
produced both species 3Cu and 2Cu on sides of the growth
tube where bpp concentration was higher or lower, respectively (Figure 7). Hence, beyond the presence of the organic
solvent also the bpp/Cu2þ ratio comes again into play on
determining which species can nucleate and develop into
mature crystals.
Aiming at a better control of chemical equilibria involving
our polymeric coordination species, we tried to transform
1Cu and 2Cu, obtained in the absence of organic solvents,
into 3Cu by selecting the proper conditions. Indeed, crystals
of 1Cu and 2Cu were unstable in the presence of pure organic
solvents such as methanol, ethanol, or methylene chloride.
As an example, upon immersing a crystal of 1Cu in ethanol
we could observe its quick conversion into microcrystalline
aggregates pseudomorphic of the starting crystal and whose
composition was showed by XRPD to be that of 3Cu
(Figure 8). Analogously, 2Cu can be converted into 3Cu
with the same procedure but with a process that takes longer
times to reach equilibrium. Therefore, as highlighted in
Scheme 1, we could devise procedures to reversibly convert
1Cu into 2Cu and vice versa, together with conversion of 1Cu
and 2Cu into 3Cu.
Parallel to formation of 3Cu, crystallization from organic
solvents produced also a yellow sparingly soluble, and almost amorphous, powder (5Cu), represented by the yellow
zones in the phase diagram and appearing at low total concentrations and bpp/Cu ratios. The formation of compound
5Cu has been observed in several preparations involving
Carlucci et al.
Figure 7. Crystallization by counterdiffusion in a U-shaped tube of
methanolic solutions of bpp (diffusing from the left arm) and CuCl2
(diffusing from the right arm). Note the concomitant presence of
microcrystalline Cu4Cl2(OH)6 (4), 2Cu and 3Cu in order of increasing bpp/Cu ratio from right to left. The horizontal part of the
U-shaped tube is filled with a gel of high strength agarose to avoid
convective mixing of the reactants.
organic solvents such as methanol, isopropanol, or amyl
acetate, but it has been possible isolate it in a crystalline pure
form only by the method described above. In any case, the
analysis of crystallization trials performed using a wide
selection of conditions and configurations has confirmed
that a low ligand-to-metal molar ratio favors unknown
species 5Cu as evidenced by the phase diagram. The elemental analysis of the yellow phase58 indicated that the
stoichiometric composition for 5Cu corresponds to a 1:1
combination of ligand and metal ion. These results and the
similarities between the copper diagram in methanol and the
cobalt one led us to hypothesize that the structures of yellow
5Cu and the blue 1D-polymer 6Co, containing cobalt ions
tetrahedrically coordinated, could be closely related. Indeed,
there are many examples of tetrahedral Cu(II) complexes
containing N-donor ligands, some of which, but not all,
show a yellow color. Unfortunately, other derivatives, as
pyridinium salts of copper halide,59 could precipitate under
the experimental conditions used, so we cannot be sure about
the polymeric nature of the yellow phase.
We also built the equilibrium phase diagram for the CuCl2/
bpp/MeOH system with a batch procedure (Figure 9). Deviations from the correct 3:1 bpp/Cu stoichiometry of 3Cu
produce significant variations of the crystal morphology,
but the combined effects upon supersaturation hamper
interpretation of morphodromes (Figure 9 right). The feature common to all growth conditions is that, on average,
species 3Cu exhibits a fast growing [001] direction, along
which the 4-fold interpenetrated networks stack on top of
each other. This feature has also been observed in other
diamantoid networks based on nitriles.
When using THF to dissolve ligand bpp the new species
[Cu(bpp)1.5Cl2] 3 (THF) 3 0.75H2O (7Cu) has been obtained,
exemplifying the different role for this solvent, that can be
supposed to be an active templating agent. Compound 7Cu
consists of a two-dimensionally extended hexagonal array
(Cu 3 3 3 Cu 13.349 and 13.489 Å) with open pores that accommodate the noncoordinating THF and water molecules stacked
along the [201] direction with an ABAB sequence. The copper
atoms exhibit a distorted square pyramidal co-ordination
geometry with the basal plane formed by two nitrogens of
pyridyl groups and two chloride atoms arranged in trans
configuration, while another pyridine nitrogen occupies the
apical position at a rather long distance 2.27(1) Å. The two
independent bpp ligands display both a trans-trans (TT)
conformation (N-to-N of 9.644 and 9.859 Å). A single layer
of 7Cu is shown in Figure 2S in the Supporting Information.
Article
Crystal Growth & Design, Vol. 9, No. 12, 2009
5031
Figure 8. Video recording of the 1Cu f 3Cu transformation in ethanol by optical microscopy with polarized light. Time interval between
frames is 20 s. Development of opacity tracks conversion of a single crystal of 1Cu into microdomains of crystalline 3Cu.
Scheme 1. Chemical Reactions Relative to the Feasible
Interconversions among Species 1Cu-3Cu
The MCl2/bpp/Water (M = Mn, Fe, Co, Ni, Cd) Systems.
As preliminarly reported in ref 38 substitution of copper(II)
chloride with other bivalent cations allowed us to isolate
coordination polymers isostructural to 1Cu (Table 1), while
no analogues of 2Cu have been recovered. This different
behavior has to be related to the octahedral coordination of
copper in 1Cu, a local stereochemistry easily available also
for Mn, Fe, Co, Ni and Cd 2þ ions, at variance with the
square pyramidal pentacoordination of copper in 2Cu which
is found less frequently for the other 2þ cations.60 Differences
in ionic radii induced only minor structural effects on the 1D
polymers of Co(II), Ni(II), and Cd(II) even though their
crystal structures are described in different crystal systems
(Table 1). Electroconductance measurements of saturated
solutions and comparison between crystallization trials indicate that for 1D polymers of formula [M(bpp)3Cl2] 3 2H2O
1Cu, 1Cd, 1Co, 1Ni, 1Mn, and 1Fe the solubility order is
Mn > Fe > Co > Ni > Cu > Cd. Values for the solubility
at 25 °C for 1Co, 1Ni, 1Cu, and 1Cd are 2.81, 1.66, 0.73, and
0.51 mM, respectively.
Further insights about similarities and differences between
copper and the other divalent cations were gathered from the
experimental determination with the batch technique of the
phase diagrams for MCl2/bbp systems in water. As an
example, concentrations had to be increased up to 50 mM
for both bpp and Co2þ, about three times higher than that
used for the less soluble copper polymer. A thorough analysis of batch experiments for the 1D polymer 1Co shows,
compared to copper, a shrinking of the nucleation zone
moved at higher concentrations. From a practical point of
view, the increased solubility of all 1D networks with the
exception of Cd, is reflected in more difficulties to obtain
crystals on moving from copper to cobalt and nickel, the
latter one being the most soluble system. Crystals of the
derivative 1Ni could be obtained only from isothermal
evaporation of solutions containing Ni2þ cation and bpp
ligand because there is no possibility to reach the critical
concentration necessary for nucleation by simply mixing
cation and ligand. On the contrary, trials in high density
hydrogels evidenced for the cadmium derivative 1Cd a prompt
5032
Crystal Growth & Design, Vol. 9, No. 12, 2009
Carlucci et al.
Figure 9. (left) Equilibrium phase diagram for CuCl2/bpp/MeOH with indication of the stability regions for 3D polymer 3Cu (blue area) and
5Cu (yellow area) species as a function of [L] þ [M] total concentration and [L]/[M] ratio (M=Cu2þ, L=bpp). (right) Morphodrome showing
dependence of crystal morphology and habit upon metal and ligand concentration.
crystallization due its very low solubility. In fact, crystallization
of 1Cd must be carefully controlled in order to avoid dendritic
or spherulitic growth owing to the very high supersaturation
reachable in the mother solution. A high growth rate can be
easily obtained producing several centimeters long dendritic
crystals in a few minutes after contact of the bpp and CdCl2
solutions.
The MCl2/bpp/Organic Solvent (M = Mn, Fe, Co, Ni, Cd)
Systems. Similarities among Cu and Mn, Fe, Co, Ni, and Cd
2þ cations were further investigated by using an alyphatic
alcohol (methanol, ethanol, isopropanol) as solvent for both
MCl2 and bpp or alternatively an alcohol for dissolving the
metal salt and dichloromethane to dissolve bpp. In this way,
it has been possible to obtain crystalline coordination polymers akin to 3Cu for all metal cations studied. Solubility
ranking for these 3D metal-organic networks is analogous
to that discussed for the 1D polymers crystallizing from
water as confirmed by the reaction yields obtained from
methanol at high concentrations of the reagents and at
metal/bpp molar ratio of 1:2. The formation of pure products in all cases (with exception of Cd) was checked by
XRPD analysis, by comparing the experimental patterns
with the simulated ones from the single crystal structures.
These results evidence once more the necessity to explore for
any solvent system chosen as reaction media a wide range of
concentrations and reagents molar ratio. In fact, data collected for the phase diagram of the CoCl2/bpp/methanol
system with the batch technique evidenced a shift toward
higher concentrations with respect to the case of copper,
together with a higher bpp/M2þ molar ratio. Analysis of the
crystallization batch (Figure 10) revealed also the presence of
the new species 6Co characterized by blue crystals appearing
just beneath the metastable zone of the 3D polymer 3Co.
Also the prismatic morphology of species 6Co is clearly
different from that typically bipyramidal (with or without
the (100) pinacoid) of the 3D polymer 3Cu but also for Ni,
Cd, Mn, and Fe. The crystal structure of 6Co has the formula
[Co(bpp)Cl2] and is isomorphous to the published 1D polymer [Zn(bpp)Cl2]54 obtained via hydrothermal synthesis
using a mixture of ZnCl2 in H2O and bpp ligand in ethanol.
This species, which is probably related to compound 7Cu
discussed in the case of copper, consists of festoon chains of
Co atoms interconnected by bpp ligands in TT conformation. The distorted tetrahedral coordination geometry of
cobalt cations are completed by two chloride anions. To
Figure 10. Equilibrium phase diagram for methanolic CoCl2/bpp
system with indication of the stability regions for 1D polymer 6Co
(blue area), species 3Co (pink area), and an intermediate region of
coexistence, as a function of [L] þ [M] total concentration and
[L]/[M] ratio (M = Co2þ, L = bpp).
confirm the primary role of the tetrahedral coordination
around the metal ion in the assembly of Zn and Co 1D
polymers, we have reprepared the Zn-bpp compound 6Zn, in
both powder and single crystal forms, following a more
conventional way of crystallization (i.e., using the liquidliquid diffusion method) than the hydrothermal reaction
used by Yao and co-workers.54 We have found that the
better solvent system to prepare this compound in a suitable
crystalline form for X-ray diffraction measurements is water
and a combination of water and methanol.
Conclusion
We discussed kinetical and thermodynamical features of
the crystallization behavior of MOFs based on MCl2 salts and
the ligand 1,3-bis(4-pyridyl)propane. Our systematic approach, using different crystallization techniques, to explore
conditions enabling the synthesis of new MOF systems is of
general application to a variety of coordination polymers. In
particular, the self-assembly of coordination networks and the
phase transformations here described between species with
different metal to ligand ratios and/or clathrated solvents are
Article
Crystal Growth & Design, Vol. 9, No. 12, 2009
intimately related to kinetic and thermodynamic factors active
during the crystallization processes. We showed that MOF
species which are kinetically favored can be substituted by the
thermodynamical product and conversion among different
species can be performed after proper chemical knowledge has
been gained.
The present work, while not resolving the very nature of the
self-assembling processes leading to polymeric coordination
networks, shows, however, that it is possible to finely tune and
control the synthetic route toward a desired MOF. The
capability to explore extensively the physicochemical variables of the metal-ligand solution equilibria is a major
improvement over a simple buy-and-mix approach.
Acknowledgment. M.M. and S.R. thank Fondazione
CARIPLO for financial support. J.M.G.-R. acknowledges
MICINN project “Factorı́a de Crystalizaci
on, ConsoliderIngenio-2010”.
Supporting Information Available: X-ray crystallographic information files (CIF) for compounds 1Cd, 1Co, 1Fe, 1Ni, 3Cd, 3Co,
3Fe, 3Mn, 3Ni, 6Co, 7Cu, Figure 1S showing crystallization of 2Cu
followed by its transformation into 1Cu, Figure 2S with details of
the structure of 7Cu, Figures 3S and 4S showing batch crystallization results for 1,3-bis(4-pyridyl)propane þ CuCl2 3 2H2O in
methanol and CoCl2 3 6H2O in water, respectively, Table 1S with
crystal data and Table 2S with a summary of microcrystalline
powder samples prepared with bpp and Mn, Fe, Ni, Cd, Zn
chlorides in water and methanol. This information is available free
of charge via the Internet at http://pubs.acs.org.
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