Comparison of chiral and racemic forms of zinc cyclohexane trans-1,2-dicarboxylate frameworks: A structural, computational, and calorimetric study

Article abstract of DOI:10.1002/anie.200802564

(Figure Presented) An integrated study of the organic-inorganic framework material zinc cyclohexane trans-1,2-dicarboxylate involving synthesis,structure elucidation, computer simulation,and calorimetry shows that the chiral R,R form (right in picture) is less stable than its racemic R,R/S,S analogue (left) and adopts a layered structure with a fundamentally different topology. The results point to the possibility that the structural diversity of racemic frameworks and their homochiral analogues may be much greater than has hitherto been suspected.

Full text of DOI:10.1002/anie.200802564

Communications
DOI: 10.1002/anie.200802564
Hybrid Materials
Comparison of Chiral and Racemic Forms of Zinc Cyclohexane trans-
1,2-Dicarboxylate Frameworks: A Structural, Computational, and
Calorimetric Study**
Andrew J. Bailey, Clare Lee, Russell K. Feller, James B. Orton, Caroline Mellot-Draznieks,
Ben Slater, William T. A. Harrison, P. Simoncic, A. Navrotsky, Martin C. Grossel, and
Anthony K. Cheetham*
The field of hybrid organic–inorganic framework materials is
one of the major growth areas of materials chemistry. Hybrid
frameworks show an enormous diversity of chemical and
structural types, including coordination polymers, porous
metal-organic frameworks (MOFs), and extended inorganic
hybrids.[1–5] Chiral hybrid frameworks are of particular
interest since they can readily be made from commercially
available homochiral ligands[6] and show promise in applica-
tions such as enantiomerically selective catalysis and separa-
tions.[7–13] The comparative structures of racemic hybrid
frameworks and their homochiral analogues have not been
discussed in detail, but a few examples have been reported. In
the nickel aspartates, for example, it has been found that the
homochiral materials contain single-handed helical chains
while the racemic phase contains both hands of the same
chain.[14,15] However, there may be other ways of accommo-
dating the constraints imposed by having only a single
enantiomer with which to build the framework. For example,
recent work on magnesium tartrates revealed entirely differ-
ent architectures for the racemic materials and their d-
tartrate analogues.[16] In the present work we explore the
racemic and homochiral forms of zinc cyclohexane trans-1,2-
dicarboxylate, [Zn(C8H10O4)]. Recent work on the cis and
trans zinc 4-cyclohexene-1,2-dicarboxylates has shown that
these materials form under thermodynamic control and give
rise to layered coordination polymers,[17] but the possibility of
using the R,R or the S,S enantiomers of the trans-ligand was
not examined at the time. In the present work we compare the
structure and energetics of the chiral zinc R,R-cyclohexane
trans-1,2-dicarboxylate with those of its racemic R,R/S,S
analogue. This particular system was chosen for a detailed
study of this type because of the availability of appropriate
forcefields for the calculations and the availability of high-
purity samples of each phase for calorimetric studies.
[*] Dr. J. B. Orton, Prof. Dr. A. K. Cheetham
Department of Materials Science and Metallurgy, University of
Cambridge, Pembroke Street, Cambridge, CB2 3QZ (UK)
Fax: (+44)1223-334567
Reactions of the racemic mixture of trans-R,R and trans-
S,S-cyclohexane dicarboxylic acids (1 equiv) with zinc acetate
(1 equiv) heated hydrothermally in water at 1508C for 48 h
produced single crystals of a colorless phase, 1, which was
studied by single-crystal X-ray diffraction. Enantiomerically
pure trans-R,R-cyclohexane dicarboxylic acid was prepared
by resolution of commercially available racemic material
following the procedure of Berkessel et al.[18] using (R)-(+)-1-
phenylethylamine. A resolved sample having > 99% ee was
obtained after three recrystallizations from hot ethanol/
toluene (1:1). Heating of the resolved acid with zinc acetate,
as described for 1 above, led to the formation of a colorless
solid product, 2, which gave an X-ray powder pattern that was
quite different from that obtained using the racemic mixture
of dicarboxylic acids. X-ray intensity data were obtained from
small single crystals of 2 on a laboratory diffractometer. The
structures of 1 and 2 were solved and refined by conventional
single-crystal methods.[19] Figure 1 shows the individual car-
boxylic acid ligands used in the hydrothermal synthesis, and
their distinctive stacking in the layered hybrid structures, 1
and 2.
E-mail: akc30@cam.ac.uk
Homepage: http://www.msm.cam.ac.uk/fihm/index.html
A. J. Bailey, Dr. M. C. Grossel
School of Chemistry, University of Southampton (UK)
R. K. Feller, Dr. P. Simoncic
Materials ResearchLaboratory, University of California, Santa
Barbara (USA)
C. Lee, Dr. W. T. A. Harrison
Department of Chemistry, University of Aberdeen (UK)
Dr. C. Mellot-Draznieks, Dr. B. Slater
Department of Chemistry, University College London (UK)
Prof. A. Navrotsky
Peter A. Rock Thermochemistry Laboratory, University of California,
Davis (USA)
[**] This work was supported by the National Science Foundation under
Award No. DMR05-20415 to the MRSEC center at UCSB and Award
No. DMR04-09848 to the International Center of Materials Research
at UCSB. R.K.F. is grateful for financial support from Unilever plc.
C.M.D. thanks the EPSRC for an EPSRC Advanced Research
Fellowship. We thank the EPSRC for use of their facilities at the
National Crystallography Service at Southampton.
¯
The structure of 1 is centrosymmetric (space group P1),
while that of 2 is chiral in space group P212121. Both 1 and 2
are layered 2-D coordination polymers, comprising double
rows of ZnO4 tetrahedra with the layers propagating in the ab
plane (Figures 2 and 3). Pendant cyclohexane rings decorate
the tops and bottoms of the layers, giving rise to non-covalent
interlayer interactions. Assuming that the reactions proceed
Supporting information for this article (crystallographic tables and
powder X-ray diffraction patterns for 1 and 2, the procedure for the
resolution of the trans-1,2-dicarboxylate acids, details of the force-
field and simulation procedure, comparison of observed and
calculated structures, hypothetical racemic structure) is available on
the WWW under http://dx.doi.org/10.1002/anie.200802564.
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Angew. Chem. Int. Ed. 2008, 47, 8634 –8637

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Chemie
À À
are bridged by pairs of carboxylate groups whose O C O
bonds run diagonal to the double rows and are parallel with
À À
À À
C
all other O C Ogroups. In the chiral compound, the O
Obonds zigzag along the double rows, lying in two roughly
perpendicular directions, both being diagonal to the chain.
The layer stacking is also very different between 1 and 2.
In 1, the cyclohexane rings are arranged in a parallel fashion
(Figure 2c), while those in the chiral compound adopt a
herringbone geometry (Figure 3c). This results in the distance
between neighboring sheets in the homochiral phase
(13.12 Š) being significantly less than that in the racemic
compound (13.63 Š), suggesting that there might be stronger
interactions between the layers in 2. In addition, the density is
1.862 gcmÀ3 for the homochiral phase 2 compared with
1.721 gcmÀ3 for the racemic phase 1, a difference of 8.2%,
raising doubts as to whether 1 is indeed more stable than 2.
In order to explore this question more thoroughly, we
have estimated the lattice energies of the two phases by using
a recently developed forcefield for the Zn–Ointeraction [20] in
combination with a standard cvff forcefield[21] for the organic
parts of the structure. The total energies were decomposed
into their respective contributions from the isolated layers
and the interactions between the layers (Table 1). The results
Figure 1. a,b) The trans-R,R-cyclohexane dicarboxylic acid (a) and trans-
S,S-cyclohexane dicarboxylic acid (b). c) The racemic hybrid Zn-dicar-
boxylate structure 1. d) The homochiral hybrid Zn-dicarboxylate struc-
ture 2.
Table 1: Comparison between energies E (in kJmolÀ1) of the racemic (1)
and homochiral structures (2), normalized to the unit-cell composition
[Zn2(C8H10O4)2] of the racemic structure.
E
ECoulomb
Ebonded+non-bonded
Total E
racemic phase 1
single layer of 1
inter-layer energy in 1
À2038.1
À2037.7
À0.4
À202.4
À146.4
À56.0
À2240.5
À2184.1
À56.4
Figure 2. The crystal structure of 1, the racemic R,R/S,S-zinc cyclohex-
ane trans-1,2-dicarboxylate: a) Plan view of the layer; b) view of a chain
showing zinc connectivity; c) side view showing the layer.
homochiral phase 2
single layer of 2
inter-layer energy in 2
À2041.3
À2040.7
À0.6
À177.3
À106.0
À71.3
À2218.6
À2146.7
À71.9
for the energy minimizations with geometry optimization
(varying the cell parameters at constant pressure) for 1 and 2
are normalized to the cell content of the racemic structure,
per [Zn2(C8H10O4)2] unit. Observed and calculated cell
parameters, fractional coordinates, and bond lengths/angles,
which agree rather well, are given in the Supporting
Information. The results indicate that the less dense racemic
structure 1 is indeed more stable than the chiral phase 2 by
21.9 kJmolÀ1, in agreement with our experimental observa-
tion that the racemic R,R/S,S phase forms in preference to a
50:50 mixture of the R,R and S,S chiral phases.
In order to corroborate the computational results and to
assess their accuracy, the relative stabilities of racemic and
chiral zinc cyclohexane trans-1,2-dicarboxylate were deter-
mined by solution calorimetry. A commercial CSC 4400
isothermal microcalorimeter operating at 298 K was used to
measure the enthalpies of solution of phases 1 and 2. Samples
were pressed into pellets of about 15 mg and dropped into
25 g of 2m NaOH (standardized solution, Alfa Aesar).
Mechanical stirring was used for better sample dissolution.
Calibration used the heat of solution of 15 mg pellets of KCl
(NIST Standard Reference Material 1655)[22] in 25 g of
Figure 3. The crystal structure of 2, the chiral R,R-zinc cyclohexane
trans-1,2-dicarboxylate: a) Plan view of the layer; b) view of a chain
showing zinc connectivity; c) side view showing the layer.
under thermodynamic control,[17] the racemic R,R/S,S product
must be more stable than a 50:50 mixture of the R,R and S,S
chiral phases. If the reverse were true, we would expect to see
the racemic acid spontaneously resolve into its two chiral
hybrids (R,R and S,S) on crystallization.
One of the most striking findings of our work is that the
connectivities within the layers of 1 and 2 are fundamentally
different. The layer of 1 is constructed from 4T-rings in which
four tetrahedrally coordinated zinc ions are bridged by four
carboxylate groups (Figure 2b), whereas the layers in 2
contain 3T-rings (Figure 3b). The bridging topology of
ZnO4 tetrahedra within each double row also differs between
the two compounds. In the racemic compound, the tetrahedra
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Communications
deionized water under identical experimental conditions.
than has hitherto been suspected. In addition, we have shown
that forcefield calculations on our hybrid structures made
reliable predictions of their relative energies based upon the
excellent agreement with our calorimetric results. Further-
more, we have shown that calculations of this kind can shed
considerable light on the factors that control the energies of
different crystalline forms.
Although the chiral zinc S,S-cyclohexane trans-1,2-dicarbox-
ylate phase was used for the calorimetric experiments, the
energetics of R,R and S,S chiral phases are identical. The
calorimetric results are summarized in Table 2. Enthalpies of
solution of structures 1 and 2 are exothermic, structure 1
(racemic) having a less exothermic DHsol than structure 2
(chiral). This indicates that structure 1 is energetically more
Received: June 2, 2008
Revised: August 18, 2008
Published online: October 2, 2008
stable than structure
2
by an amount, DHtrs = 19.6 Æ
1.3 kJmolÀ1, which is in excellent agreement with the
calculations (21.9 kJmolÀ1).
Keywords: calorimetry · chirality · crystal engineering ·
molecular modeling · organic-inorganic hybrid composites
.
Table 2: Calorimetric data (in kJmolÀ1) for the racemic (1) and chiral (2)
zinc cyclohexane trans-1,2-dicarboxylates, normalized to unit-cell com-
position [Zn2(C8H10O4)2] of the racemic structure.
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DHsol (2)
DHtrs
À137.5Æ0.5
À157.1Æ1.0
19.6Æ1.3
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The relative stabilities of the racemic 1 and chiral 2 phases
can be rationalized in terms of the differences in the internal
intra-layer energies of the component single layers and the
non-bonded, inter-layer interactions (see Table 1). The inter-
layer energy is stronger in phase 2 by 15.5 kJmolÀ1 and is
composed almost entirely (> 99%) of dispersive interactions.
However, the reason for the more favorable lattice energy of
1 stems from the intra-layer energy, which exceeds that in
phase 2 by 37.4 kJmolÀ1, yielding a greater stability for phase
1 of 21.9 kJmolÀ1 measured relative to 2.
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It is interesting to speculate as to whether there might be
other possible structures of similar or even lower energies. We
examined a number of candidates, of which one was quite
stable and could be energy minimized. Specifically, a racemic
structure in which the stable layers of 1 are preserved, but
alternate sheets are inverted so that R,R faces R,R and S,S
faces S,S across the non-bonding regions (as in the closer
packed single enantiomer structures) gives a total energy of
À2235 kJmolÀ1, only 5 kJmolÀ1 less stable than the observed
racemic structure. The inter-layer interaction is indeed
stronger (À68 kJmolÀ1), as in 2, but the energy-minimized
single layer energy is significantly less favorable than that in 1
(À2167 kJmolÀ1). Details are given in the Supporting Infor-
mation. The calculations on alternative models underline the
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[19] Crystal data for 1: [Zn(C8H8O4)], Mr = 233.52, crystal size 0.2 ”
¯
0.15 ” 0.08 mm; triclinic, P1 (No. 2), a = 4.9223(7), b =
In summary, we report the first integrated study of hybrid
organic–inorganic framework materials involving synthesis,
structure, computer simulation, and calorimetry. We have
shown that the chiral zinc R,R-cyclohexane trans-1,2-dicar-
boxylate is less stable than its racemic R,R/S,S analogue and
adopts a layered structure that has a fundamentally different
topology. It is not yet clear whether such behavior is common
in hybrid framework materials, but our observations point to
the possibility that the structural diversity of racemic frame-
works and their homochiral analogues may be much greater
6.8499(11), c = 13.864(2), a = 93.048(3)8, b = 99.600(2)8, g =
98.501(2)8, V= 454.38(12) Š3, Z = 2, m = 2.680 mmÀ1, 1calcd
=
1.721 MgmÀ3, F(000) = 240, R1 = 0.069, wR2 = 0.147, GoF =
1.06. Bruker SMART CCD diffractometer, MoKa radiation,
l = 0.71073 Š, T= 293 K, 3423 reflections scanned (2qmax
=
52.78), 1746 unique (RInt = 0.034). Crystal data for 2: [Zn-
(C8H8O4)], Mr = 233.52, crystal size 0.1 ” 0.04 ” 0.01 mm; ortho-
rhombic, P212121 (No. 19), a = 4.8436(2), b = 6.6125(3), c =
26.2382(13) Š, a = 908, b = 908, g = 908, 840.37(7) Š3, Z = 4,
m = 2.898 mmÀ1, 1calcd = 1.862 MgmÀ3, F(000) = 480, R1 = 0.069,
wR2 = 0.130, GoF = 1.04. Nonius KappaCCD diffractometer,
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Angewandte
Chemie
MoKa radiation, l = 0.71073 Š, T= 293 K, 5908 reflections
scanned (2qmax = 54.98), 1919 unique (RInt = 0.062). All
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atoms were geometrically placed and refined as riding with
Uiso(H) = 1.2Ueq(C). CCDC 688064 (1) and 688065 (2) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[22] National Bureau of Standards Certificate—Standard Reference
Material 1655.
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8637