Coordination polymers formed in solution and in solvent-free environment. Structural transformation due to interstitial solvent removal revealed by X-ray powder diffraction

Article abstract of DOI:10.1039/b503748d

The crystal-to-powder transformation induced by solvent removal has been examined through a direct comparison of the structures of the solvated and the unsolvated coordination products determined by single crystal and powder X-ray diffraction, respectively. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b503748d

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COMMUNICATION
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Coordination polymers formed in solution and in solvent-free
environment. Structural transformation due to interstitial solvent
removal revealed by X-ray powder diffraction{
Evgeny V. Dikarev,*a Bo Li,a Vladimir V. Chernyshev,b Roman V. Shpanchenkob and Marina A. Petrukhina*a
Received (in Columbia, MO, USA) 12th March 2005, Accepted 10th May 2005
First published as an Advance Article on the web 27th May 2005
DOI: 10.1039/b503748d
synthesis of such isomers, we tested two synthetic approaches:
solution and solvent-free coordination reactions. The former
technique may favor the formation of closed architectures (dilute
systems, slow crystal growth, low temperatures), while the high
temperature solid state reactions should inevitably yield the infinite
chain motif. This is due to the fact that assembling processes for
discrete and extended products from the starting building units are
essentially different. For a polymer, each assembling step exactly
repeats the previous one, and therefore, high temperatures should
favor this indiscriminatory process. For a discrete structure, there
is a unique ‘‘intelligent’’ step that requires closing of a cycle that
may happen at low temperatures in solution. While dicyanoben-
zenes were widely used as bidentate angular spacers in solution
assembling reactions, mainly extended coordination networks have
been isolated as products.6 The only metallotriangle was reported
for 1,2-phenylene diisocyanide.7
The crystal-to-powder transformation induced by solvent
removal has been examined through a direct comparison of
the structures of the solvated and the unsolvated coordination
products determined by single crystal and powder X-ray
diffraction, respectively.
There has been extensive interest in controlled formation of metal–
organic frameworks having different topologies due to possible
applications in gas storage, separation, guest recognition, catalysis,
and magnetism.1 Various synthetic procedures utilizing the
preconstructed building blocks for self-assembling are usually
carried out in solutions. In this case the formation of multi-
dimensional structures is often accompanied by incorporation of
solvent molecules into open cavities or channels. Moreover, a
template effect of various solvents in self-assembling reactions is
well recognized.2 Solvents are known to affect formation,
structures and properties of supramolecular architectures.3 The
evacuation of guest solvent molecules often causes structural
transformations including a total collapse of porous networks.
While X-ray powder diffraction (XRPD) is widely used as a
general method to detect these changes, an application of XRPD
for structural characterization of powders formed as a result of
such transformations is very rare.4 Importantly, the recent
developments in XRPD method and programming can now be
utilized for direct elucidation of the structures of microcrystalline
materials formed after the solvent- or temperature-induced
structural transitions which are often accompanied by the loss of
single crystalline properties. Herein, to study such solvent-
determined transformation we applied a combination of single
crystal and powder X-ray diffraction techniques. This allowed us
to examine an interesting solvent-directing effect in the syntheses of
coordination polymers by directly comparing the structures of the
solvated and unsolvated products.
From solution coordination reaction between the dirhodium
complex and 1,3-dicyanobenzene, single crystals of 1 have been
isolated in good yield. When removed from solution crystals were
losing solvent quickly, and that prevented obtaining the accurate
chemical analysis data for 1. IR spectra of 1 confirmed the presence
of both building units: the stretches at 1664 and 1478/1460 cm21
are due to nasym(CO) and nsym(CO) of carboxylates, while weak
bands at 2275 and 2239 cm21 correspond to n(CN) vibrations. The
single crystal X-ray structural characterization revealed a 1D chain
For this study we have selected a model system composed of a
strongly-coordinating angular bidentate dicyanobenzene ligand,
1,3-C6H4(CN)2, and very electrophilic bidentate linear metal
complex, [Rh2(O2CCF3)4]. This system is of interest since the
interaction of the above building blocks may afford coordination
isomers.5 Thus, for a 1 : 1 composition, a hexagon comprised of six
dimetal units and six ligands as well as various conformations of
the 1D chain can be envisioned (Scheme 1). To attempt targeted
{ Electronic Supplementary Information (ESI) available: Synthesis,
characterization, X-ray details, XRPD patterns, and additional plots of
the structures. See http://www.rsc.org/suppdata/cc/b5/b503748d/
*dikarev@albany.edu (Evgeny V. Dikarev)
Scheme 1 Discrete hexagon (a) vs. 1D polymer (b) in 1 : 1 complex of
dimetal tetracarboxylate with m-dicyanobenzene.
marina@albany.edu (Marina A. Petrukhina)
3274 | Chem. Commun., 2005, 3274–3276
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structure of 1, [Rh2(O2CCF3)4?C6H4(CN)2]?0.5CH2Cl2?1.75C6H6,
with solvent molecules filling the interchain voids (Fig. 1a,c,e).
The 1D chains in 1 run in two different directions (69.3u) to
leave interstitial cavities that are occupied by the disordered
benzene and dichloromethane molecules. Each hybrid chain
composed of alternating dimetal units and organic spacers is
…
˚
build on strong Rh N axial interactions of 2.197(4) A. The Rh–
Rh–N and Rh–N–C angles are both close to linear, being 179.5(1)
and 175.6(4)u, respectively. The Rh–Rh distance within the
Scheme 2 Syntheses and transformations of 1 and 2.
˚
dimetal unit of 2.4137(6) A is comparable with that in other
(Scheme 2). The XRPD patterns of the desolvated and the
unsolvated powders obtained by different synthetic routes were
identical (Fig. 2).
dirhodium(II) trifluoroacetate adducts having axially bound
N-donors.8
The defining feature of the solid state structure of 1 is
intermolecular interactions between the solvent and the ligand
p-systems. Although benzene molecules are not coplanar with the
To understand the above crystal-to-powder transformation
upon solvent evacuation from 1, the structural characteristics of
the microcrystalline product 2 were needed. It is noteworthy that 2,
having the ratio of building units of 1 : 1, as determined by
elemental analysis, is insoluble in common organic solvents and
cannot be melted without decomposition thus precluding the use
of single crystal X-ray diffraction. Recently, we have successfully
applied XRPD for structural characterization of coordination
polymers that resisted single crystal growth techniques.10 Herein,
the structure of 2 was solved based on XRPD data using
procedures described in the ESI. This application provided a
unique opportunity to follow the desolvation process and to
understand the role of solvent in the structure.
…
aromatic planes of the ligands, close C C contacts of 3.36(2)–
˚
3.45(2) A exist between the edges of the rings. The characteristic
parameters for these interactions, following the criteria of Janiak,9
are presented in the ESI.{
The TGA data for 1 indicate that partial loss of solvent
(probably CH2Cl2) starts at room temperature. A complete
removal of solvents from the single crystalline material is achieved
around 85 uC corresponding to the weight loss of 15.8% (assuming
that all benzene is removed only upon heating, this value also
includes the loss of residual dichloromethane, ca. 25.4%, based on
the crystal structure). The polymeric product 1 starts to decompose
at temperatures above 225 uC. Using these data, we obtained the
desolvated product 2 in the form of fine microcrystalline powder
by heating the crystals of 1 under reduced pressure at 85 uC. The
process of solvent removal from 1 was accompanied by a
noticeable structural transformation, as recorded by X-ray powder
diffraction (ESI). Importantly, the same powder product 2 was
obtained in quantitative yield by direct solid state reaction of the
1 : 1 mixture of the dirhodium complex and 1,3-dicyanobenzene in
the absence of solvents in an evacuated ampoule at 70 uC
The hybrid powder 2 is comprised of rigid building blocks,
[Rh2(O2CCF3)4] and C6H4(CN)2, and the knowledge of geometries
of both units11,12 facilitated the structure solution based on the
rigid body approach. The Rietveld plot for 2 showed good
correspondence between the model and the observed data (Fig. 3).
The structure features zig-zag chains [Rh2(O2CCF3)4?C6H4(CN)2]‘
that have the same conformation as those in 1 but run parallel to
each other (Fig. 1b). The major geometric characteristics of these
1D chains are very close in both structures, although the
parameters obtained from powder diffraction data for 2 are not
as accurate as those extracted from the single crystal X-ray analysis
of 1. A noticeable feature of the solid state packing in 2 is weak off-
centered p–p interactions between the 1,3-dicyanobenzene mole-
cules of the neighboring chains. The arene planes of ligands exhibit
parallel displacement so that the shortest contacts of 3.33 and
˚
3.37 A occur between the ring edges and carbon atoms of the
cyano-groups. The estimated interplanar distance between the
˚
arene rings of 3.33 A in 2 is even shorter than that in the solid
11
stacks of 1,3-dicyanobenzene itself (3.47 A). Recent analysis of
˚
p–p interactions in metal complexes with N-containing aromatic
ligands9 shows that such non-covalent interactions play a very
Fig. 1 Representation of the structural transformation from 1 to 2 upon
solvent evacuation. Perspective views are arbitrary chosen for three almost
perpendicular directions to show: rotation of the [Rh2L]‘ chains (a,b);
chain shift (c,d); and contraction of the interchain space (e,f) in the
structures of 1 and 2, respectively. Only Rh atoms (dark blue) of metal
complex are shown. N light blue, C grey, H-atoms are removed for clarity.
Fig. 2 Observed X-ray powder patterns for the desolvated (yellow) and
unsolvated (red) powders 2 at room temperature.
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 3274–3276 | 3275

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This work was supported by the American Chemical Society
Petroleum Research Fund (39039-G3) and the National Science
Foundation (NSF-01300985).
Evgeny V. Dikarev,*a Bo Li,a Vladimir V. Chernyshev,b
Roman V. Shpanchenkob and Marina A. Petrukhina*a
aDepartment of Chemistry, University at Albany, SUNY, Albany,
New York, 12222, USA. E-mail: dikarev@albany.edu;
marina@albany.edu; Fax: +1-518-442-3462;
Tel: +1-518-442-4401 (E. V. D.) Tel: +1-518-442-4406 (M. A. P.)
bDepartment of Chemistry, Moscow State University, Moscow, 119992,
Russia
Notes and references
{ Crystal data for 1: C27H15.5ClF12N2O8Rh2, M 5 965.18, orthorhombic,
˚
Pnma (no. 62), a 5 16.3302(10), b 5 24.7253(15), c 5 8.7079(5) A,
V 5 3516.0(4) A , Z 5 4, T 5 173(2) K, m(Mo–Ka) 5 1.126 mm21, 4260
3
˚
reflections measured, 3849 unique (Rint 5 0.0550). The final R1 5 0.0595,
wR2 5 0.1437 for all data. For 2: the data were collected on an automated
˚
STADI-P (STOE) diffractometer, Cu Ka1-radiation, l 5 1.5406 A, 2h 5 6–
65.0u, step 0.01u, C16H4F12N2O8Rh2, M 5 786.03, monoclinic, C 2/c (no.
Fig. 3 The Rietveld plot of the powder diffraction pattern for
[Rh2(O2CCF3)4?C6H4(CN)2] (2) at 293 K. The observed pattern (black
crosses), the best calculated fit (red), and the difference profile (blue) are
given. The black lines at the bottom show the allowed peak positions.
˚
15), a 5 19.411(20), b 5 16.879(19), c 5 8.348(4) A, b 5 114.91(2)u,
3
˚
V 5 2481(4) A , Z 5 4, T 5 293(2) K, 438 reflections used in calculations.
The final Rp 5 0.0324, Rwp 5 0.0426, and x2 5 1.246. CCDC 266399–
266400. See http://www.rsc.org/suppdata/cc/b5/b503748d/ for crystallo-
graphic data in CIF or other electronic format.
important role in crystal packing due to their low p-electron
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3276 | Chem. Commun., 2005, 3274–3276
This journal is ß The Royal Society of Chemistry 2005