Crystal Packing in Equilibrating Systems: A Single Crystal Containing Three Isomers of CuCl(1-pyridin-2-yl-3-p-tolyl-thiourea)2 Joseph T.
Lenthall,†
Kirsty M.
Anderson,†
Stephen J.
Smith,‡
and Jonathan W.
Steed*,†
Department of Chemistry, Durham UniVersity, South Road, Durham, DH1 3LE, UK, and Hull Research and Technology Centre, BP Chemicals, Saltend, Hull HU12 8DS, UK
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1858-1862
ReceiVed June 12, 2007
ABSTRACT: The X-ray crystal structure of the labile copper(I) complex CuCl(1-pyridin-2-yl-3-p-tolyl-thiourea)2 comprises three different isomers and has profound implications for work on the understanding and prediction of low-symmetry crystal packing. A chloroform co-crystal of one of these isomers is also reported along with a related compound of (6-methyl-benzothiazol-2-yl)pyridin-2-yl-amine. There is a great deal of recent interest in the small proportion of crystals that pack with more than with one crystallographically independent molecule (Z′ > 1).1-9 It is speculated that molecular crystals generally pack with only one crystallographically independent molecule (Z′ ) 1) because the “molecule” is long-lived on the time scale of the crystallization process.10 In some cases, it has been suggested that long-lived intermolecular associations (e.g., in an aggregate joined by strong hydrogen bonds) may result in the crystallization of (possibly metastable11,12) crystal morphs with Z′ > 1.1,6,10 In other words, a stable dimer or trimer, for example, may behave like a single molecule during crystal nucleation and growth. While this hypothesis does not explain all occurrences of structures with Z′ > 1,10 it is certainly feasible in cases where the resulting molecular aggregate either lacks internal symmetry or when that internal symmetry is incompatible with the symmetry of a space group that results in optimal packing of such an aggregate. Conversely, a very labile chemical system should adopt the opposite kind of behavior. If exchange between a number of isomeric or oligomeric complexes is fast on the crystallization time scale then one of three outcomes are possible. (1) The crystallization process results in the formation of pure crystals of the form that is most stable in the solid state. This may not be the same as the predominant form in solution, as observed for [(η6-C6H6)(OH)Ru(µ2-OH)2Ru(H2O)(η6-C6H6)], for example.13 (2) Crystallization may result in the formation of a mixture of different crystals each containing a single form of the compound. We have recently reported a case in which such crystals co-deposit epitaxially.14 (3) It is feasible that the crystallization process can “pick and choose” between the various solution species to form a single crystal composed of a number of conformers or isomers. A series of cocrystals containing 19 different conformers of 4,4-diphenyl2,5-cyclohexadienone has been reported,15 for example, and a number of other cases are known.16-18 Because these conformers differ only by rotation about a single bond they are in fast equilibrium in solution but are crystallographically distinct. Moreover, this distinction usually results in Z′ > 1 since the various conformers are not likely to be related by crystallographic symmetry. * To whom correspondence should be addressed. Tel: +44 (0)191 334 2085. Fax: +44 (0)191 384 4737. E-mail:
[email protected]. † Durham University. ‡ BP Chemicals.
In addition to the presence of various conformers in a single crystal, the presence of labile bonds, particularly in coordination complexes, can result in the simultaneous presence of different isomers or oligomers that can interconvert by bond breaking and re-formation. Good examples are the co-crystal formed by the rac-diphenyl[2.2]paracyclophanylphosphine, complexes [{Pd{PPh2(C16H15)}Cl(µ-Cl)}2] and trans-[Pd{PPh2(C16H15)}2Cl2],19 and the dimeric isomeric pair of Cu(I) complexes [{Cu(PTU)(µ-PTU)Cl}2] and [{Cu(PTU)2(µ-Cl)}2] (PTU ) N-phenyl-N′2-propenoylthiourea).20 In both cases, there are two crystallographically independent coordination complexes (as well as lattice solvent), but because they are chemically distinct it is better to assign Z′ ) 1 (0.5 in the case of the Cu(I) complex because each dimer sits on an inversion center20) with a formula unit reflecting the different identities of both compounds. However, since these systems are so labile there is an argument in the Cu(I) case for example for defining the formula unit as “Cu(PTU)2Cl” and assigning Z ) 4 and Z′ ) 2. The Cambridge Structural Database (CSD21) assigns Z′ ) 0.5 (refcode SOGZAP), while the original authors assign Z ) 4 and hence Z′ ) 2.20 The coordination sphere of the d10 ion Cu(I) in particular is highly labile and easily distorted, with regular and irregular linear, trigonal, and tetrahedral coordination geometries being well-documented.22 We have previously shown that Cu(I) thiourea complexes are very susceptible to crystal packing forces, exhibiting remarkable plasticity in the Cu‚‚‚Cu distance and adopting either dimeric or polymeric forms depending on interactions to crown ethers, for example.23 As part of a program involving the chemistry of pyridyl ureas,24-26 we now report a remarkable result involving copper(I) complexes of 1-pyridin2-yl-3-p-tolyl-thiourea (L).
Pyridyl thiourea L is readily prepared from the reaction of 2-aminopyridine with p-tolylisothiocyanate and has been structurally characterized by West27 as part of a wide-ranging study of related compounds.28 Reaction of L with copper(II) chloride dihydrate in acetonitrile solution results in a redox reaction to give the copper(I) species Cu(L)2Cl (1) as a pale yellow crystalline solid that was characterized by X-ray crystallography. The crystal structure of 1q reveals a remarkable arrangement
10.1021/cg070533k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/17/2007
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Figure 1. The formula unit (two asymmetric units) in 1, 2[Cu(L)2Cl]‚ 2[{Cu(L)(µ-L)Cl}2]‚[{Cu(L)2(µ-Cl)}2]. Monomer 2 is shown in red, the centrosymmetric chloride-bridged dimer is in blue, and the sulfurbridged dimer is in green.
comprising a total of four crystallographically independent Cu(L)2Cl units in three chemically distinct, isomeric forms. The formula unit of the crystal (Figure 1) is best defined as comprising two monomers [Cu(L)2Cl] (2), two S-bridged dimers [{Cu(L)(µ-L)Cl}2] (3) and one Cl-bridged dimer [{Cu(L)2(µCl)}2] (4), i.e., 2[Cu(L)2Cl]‚2[{Cu(L)(µ-L)Cl}2]‚[{Cu(L)2(µCl)}2]. The material crystallizes in space group P2/c with the chloride-bridged dimer situated on a crystallographic inversion center, thus with the above formula unit, Z ) 2 and Z′ ) 0.5. This Z′ ) 0.5 value does not, of course, do justice to the complexity of the formula unit that is conceptually analogous to a compound with Z′ ) 4. The molecular structures of complexes 2-4 are shown in Figure 2. Monomer 2 adopts a trigonal planar geometry about the Cu(I) center, while the remaining copper(I) ions are distorted tetrahedral. The Cu‚‚‚Cu distance in the S-bridged complex 3 of 2.810 Å is dramatically shorter than in the Cl-bridged complex 4 (3.641 Å), consistent with related compounds.20 The chloride bridges in 4 are asymmetrical with Cu-Cl distances of 2.357 and 2.651 Å, suggesting a weak association of two monomer units of type 2. Indeed, the geometry about the copper centers in 4 is very similar to that in 2. Similarly, compound 3 is asymmetrically bridged with two sets of Cu-S distances averaging 2.30 and 2.55 Å. Both copper ions approach the same side of the (RNH)2CdS group, and hence it seems likely that only one of the two sulfur lone pairs is engaged in bonding to the two copper ions, resulting in electron-deficient three-center, two-electron interactions and hence a short Cu‚‚‚Cu distance and some metal-metal bonding. In our previous work, we showed that constraints on the thiourea position induced by crystal packing can reduce this distance still further.23 The pyridyl thiourea ligand L in every case adopts a syn, anti conformation observed in a variety of related examples27,28 that allows the formation of an intramolecular six-membered hydrogen-bonded ring between one NH group and the pyridyl nitrogen atom. This conformation leaves one NH group available for NH‚‚‚Cl-Cu hydrogen bonding interactions. In every case, the N atom of the two unique NH groups per metal is approximately equidistant from the coordinated chloride and copper(I) atoms. Each Cu-Cl unit thus forms a pair of interactions that might be termed an intramolecular multicenter
Figure 2. Molecular structures (50% ellipsoids) of (a) [Cu(L)2Cl] (2), (b) [{Cu(L)(µ-L)Cl}2] (3), and (c) [{Cu(L)2(µ-Cl)}2] (4). Selected bond lengths (Å): (2) Cu1-S2 2.2211(13), Cu1-S1 2.2307(12), Cu1-Cl1 2.2793(12), (3) Cu3-S201 2.2711(12), Cu3-S202 2.2993(12), Cu3Cl20 2.3502(12), Cu3-S203 2.5230(13), Cu3‚‚‚Cu4 2.8102(8), Cu4S204 2.2767(12), Cu4-S203 2.2989(12), Cu4-Cl21 2.3260(12), Cu4S202 2.5711(13), (4) Cu2-S101 2.2542(12), Cu2-S102 2.2690(12), Cu2-Cl10′ 2.3568(12), Cu2-Cl10 2.6510(13), Cu2‚‚‚Cu2′ 3.641(10).
heteroacceptor (IMH) after the nomenclature proposed by Braga et al.29 with the electron-rich copper(I) center acting as one of the acceptors. In each case, the crystallographically located position of the H atom is closer to the Cl atom than the Cu atom by ca. 0.3 Å. A bulk sample of compound 1 was checked for possible polymorphism by differential scanning calorimetry (DSC) in the temperature range -170 to +50 °C. This scan revealed no evidence for any phase change, however. The bulk sample was
1860 Crystal Growth & Design, Vol. 7, No. 9, 2007
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based on Cu(1) exhibits a Cu‚‚‚Cu separation of 3.19 Å, and there are no intermolecular interactions to the coordinated chloride ligand. In contrast, the complex based on Cu(2) exhibits a much shorter Cu‚‚‚Cu separation of 2.83 Å with additional hydrogen bonding from the cocrystallized chloroform CH moieties, Figure 4. The cause and effect relationship (if there is one) between the Cu‚‚‚Cu distance and the presence of the chloroform molecules is not obvious; however, it is clear even from the poor refinement we were able to obtain that the two independent dimers are very different from one another in terms of both their environment within the crystal and their conformational characteristics. Figure 3. Crystal structure (50% ellipsoids) of [Cu(L)2Cl]‚CHCl3 (6). Selected bond lengths (Å): Cu1-Cl1 2.2962(12), Cu1-S1 2.2266(11), Cu1-S2 2.2239(11).
shown to be reproducibly the same as the single-crystal structure by comparison of the powder X-ray diffraction (PXRD) pattern with that calculated from the single-crystal data (see Supporting Information). A different crystal form of the monomer [Cu(L)2Cl] (2) was obtained by crystallization from chloroform solution, to give the compound as a chloroform cocrystal, [Cu(L)2Cl]‚CHCl3 (6). Crystal 6 exhibits a single unique copper(I) monomer similar to molecule 2 in crystal 1 and one cocrystallized molecule of chloroform, thus Z′ ) 1 and Z′′ ) 2 (where Z′′ is the total number of molecules in the asymmetric unit30). It seems likely that in chloroform solution the monomer-dimer equilibrium involving complexes 2-4 is either significantly shifted in favor of 2 or does not occur. The structure of 6 is shown in Figure 3. The only significant difference between the copper(I) monomer complex in crystal 6 and crystal 1 is the angle of the p-tolyl rings relative to the metal coordination plane. In 6 the aryl rings are rotated out of plane to allow room for the cocrystallized CHCl3 to engage in a CH‚‚‚π interaction with one of the aromatic rings (H‚‚‚centroid 2.86 Å). Such a specific solventsolute interaction may be sufficient to disfavor the approach of a second copper(I) monomer in solution to form a dimer. The colorless crystals of formula 6 form in chloroform over the course of a few hours. However, after a several days, a further crop of yellow crystals similar in color to 1 was observed in the chloroform sample. These crystals proved to be twinned, and their analysis by X-ray crystallography was extremely challenging. No satisfactory twinned refinement was achieved (see experimental section); however, the crystallographic data are of sufficient quality to clearly reveal the overall structure of the material. This product proved to be a copper(I) complex of L and of a ligand that is a result of an intramolecular cyclization rearrangement of ligand L to give (6-methylbenzothiazol-2-yl)-pyridin-2-yl-amine, L2. Copper(I) has been shown to catalyze this process.31 The new crystals are thus of formula [{CuCl(κ-N-L2)(µ-κ-S-L)}2]‚CHCl3 (7) in which the L2 ligands are terminally bound through the imine nitrogen atoms, while ligand L binds to each Cu(I) center through a bridging thiourea sulfur atom, as in compound 3, Figure 2b. In this sample, there are two independent copper(I) centers giving rise to an asymmetric unit comprising two half-dimers (related by inversion symmetry in P1h) and a single chloroform molecule. This formally Z′ ) 1 and Z′′ ) 2 situation is conceptually indistinguishable from a structure with Z′ ) 2, particularly since the two crystallographically independent half-dimers are qualitatively different from one another. Both dimers exhibit intramolecular hydrogen bonding from NH to Npyridyl in ligand L, and NH‚‚‚Cl-Cu in the same way as all three complexes in crystal 1 and the monomer in crystal 6. However, the dimer
In conclusion, we have shown that the geometry of the Cu(L)2Cl unit and a related compound is stable in an approximately trigonal planar arrangement because of supporting IMH hydrogen bonding interactions. These units can associate by the formation of long Cu-Cl or Cu-S interactions, the latter possibly involving some Cu‚‚‚Cu bonding. The lability of the Cu(I) core suggests that all three complexes 2-4 are present in acetonitrile solution, while this association may be disfavored in chloroform. In this case, a stable cocrystal of all three complexes is also formed. Formally, this cocrystal should be assigned as Z′ ) 0.5; however, if the long Cu-S and Cu-Cl interactions are regarded as intermolecular interactions then the crystal is conceptually indistinguishable from a Z′ ) 4 packing arrangement of Cu(L)2Cl molecules (cf. the highly unsymmetrical Z′ ) 0.5 arrangement in oligomers of Ag(I) terpyridine complexes).32 We can thus explain the formation of a “high Z′ structure” as resulting from frustration between directional CuS/Cl intermolecular interactions and the rigid, flat molecular shape of Cu(L)2Cl. Similar high Z′ values have been observed for related flat, trigonal Ag(I) complexes.26 The situation thus parallels the packing arguments put forward for high Z′ structures in the case of alcohols. The present study highlights several general points. First the definition of the parameter Z′ can mask cases of interesting, low-symmetry packing in the CSD. Second, the argument of frustration between shape and directional interactions as a root cause of high Z′ behavior is applicable beyond hydrogen bonding. Third, very labile coordination complexes may prove a rich ground for future studies on crystals with complex packing arrangements (cf. the Z′ ) 5 structure of NaOtBu33), and finally there may be a link between solution speciation or conformational distribution and high Z′ packing. Experimental Procedures Synthesis of Compound 1. CuCl2·2H2O (10 mg, 0.06 mmol) was dissolved in 8 mL of MeCN to form a green solution, while L (28.5 mg, 0.12 mmol) was dissolved in 10 mL of MeCN to form a colorless solution. The L solution was slowly added to the copper solution, where upon some fine dark precipitate formed and redissolved immediately. Upon full addition, the green solution was allowed to slowly evaporate, and after a few days, yellow crystals suitable for X-ray crystallography had grown. After a period of days, the crystals had decomposed to form a dark brown solid. The reaction was repeated on a larger scale with CuCl2·2H2O (0.1 g, 0.58 mmol) and L (0.29 g, 0.13 mmol) in 100 mL of acetonitrile. A short period after addition some yellow precipitate formed, and when the volume of solvent was reduced by 50%, more precipitate formed. This precipitate was recovered by filtration. Yield ) 0.23 g, 0.39 mmol,
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Figure 4. (a) Structures of the two independent copper(I) complexes [{CuCl(κ-N-L2)(µ-κ-S-L)}2] found in crystal 7: (a) dimer based on Cu(1), (b) dimer based on Cu(2). 67%. 1H NMR (400 MHz, CDCl3, J/Hz) δ (ppm): 2.35 (3H, s, CH3), 7.04 (1H, br, CHAr), 7.18 (2H, d, 3J ) 7.2, CHAr), 7.35 (2H, d, 3J ) 7.2, CHAr), 7.51 (1H, br, CHAr), 7.72 (1H, br, CHAr), 8.20 (1H, br, CHAr), 11.08 (1H, s, NH), 13.59 (1H, s, NH). {1H}-13C NMR (100.6 MHz, CDCl3) δ (ppm): 19.21, 112.58, 117.12, 118.75, 123.82, 127.61, 127.82, 133.06, 137.16, 143.48, 151.71, 174.94. Analysis: calc’d for CuC26H26N6S2Cl: C 53.32, H 4.47, N 14.35%; found: C 53.29, H 4.46, N 14.29% ESI-MS: 549.2, 100% rel. abundance with Cu isotope for [63Cu(T-TUP)2]+. IR: 3213 (w, N-H), 3105 (w, N-H), 1627 (m, N-H), 1606 (s, N-H), 1183 (m, CdS), 1149 (m, CdS), 1123 (w), 1097 (w). Crystal data for 1: C208H208Cl8Cu8N48S16, M ) 4685.10, yellow block, 0.30 × 0.20 × 0.20 mm3, monoclinic, space group P2/c (No. 13), a ) 25.271(2) Å, b ) 19.6049(14) Å, c ) 21.8179(16) Å, β ) 101.369(3)°, V ) 10597.4(14) Å3, Z ) 2, Dc ) 1.468 g cm-3, F000 ) 4832, Bruker Smart 1000, Mo KR radiation, λ ) 0.71073 Å, T ) 120(2) K, 2θmax ) 58.2°, 151 259 reflections collected, 28 416 unique (Rint ) 0.1798). Final GOF ) 1.019, R1 ) 0.0776, wR2 ) 0.1045, R indices based on 14 717 reflections with I > 2σ(I) (refinement on F2), 1305 parameters, 0 restraints. Lp and absorption corrections applied, µ ) 1.110 mm-1. Diffraction-quality single crystals of [Cu(L)2Cl]‚CHCl3 (6) were obtained by slow evaporation of a chloroform solution of solid 1. After several days. this same solution also serendipitously precipitated crystal 7. Crystal data for 6: C27H27Cl4CuN6S2, M ) 705.01, colorless block, 0.20 × 0.20 × 0.10 mm3, triclinic, space group P1h (No. 2), a ) 10.6215(12) Å, b ) 11.8452(13) Å, c ) 14.1250(16) Å, R ) 109.281(2), β ) 90.098(2), γ ) 113.112(2)°, V ) 1525.1(3) Å3, Z ) 2, Dc ) 1.535 g/cm3, F000 ) 720, SMART 1K, Mo KR radiation, λ ) 0.71073 Å, T ) 120(2) K, 2θmax ) 58.4°, 22 258 reflections collected, 8129 unique (Rint ) 0.0846). Final GOF ) 0.991, R1 ) 0.0590, wR2 ) 0.0936, R indices based on 4742 reflections with I > 2σ(I) (refinement on F2), 378 parameters, 0 restraints. Lp and absorption corrections applied, µ ) 1.233 mm-1. Crystal data for 7: C53H49Cl5Cu2N12S4, M ) 1286.61, yellow block, 0.30 × 0.20 × 0.10 mm3, triclinic, space group P1h (No. 2), a ) 14.237(3) Å, b ) 14.275(2) Å, c ) 14.525(3) Å, R ) 89.983(14), β ) 78.980(16), γ ) 78.002(12)°, V ) 2831.9(9) Å3, Z ) 2, Dc ) 1.509 g/cm3, F000 ) 1316, SPIDER, Mo KR radiation, λ ) 0.71073 Å, T ) 0(2) K, 2θmax ) 45.1°, 11 638 reflections collected, 4513 unique (Rint ) 0.1327). Final GOF ) 1.602, R1 ) 0.1777, wR2 ) 0.4275, R indices based on 2747 reflections with I > 2σ(I) (refinement on F2), 684 parameters, 0 restraints. Lp and absorption corrections applied, µ ) 1.183 mm-1. Despite samples that appeared high quality on visual inspection, attempts to collect data at 120 K failed as the crystal cracked even if cooled slowly. The data are almost certainly twinned. A twin law was derived corresponding to a rotation of 179.97° around the normal to (-1.00, -4.78, -0.01), equivalent to a rotation around [0.00, -1.00, 0.00]; however, no twin refinement was successful. The data are therefore of poor quality with large residuals. The data/parameter ratio is poor as only the reflections from one twin component have been
used. Despite these problems, the connectivity of the compound is not in doubt; however, the precision of the refinement is very poor.
Acknowledgment. We thank the EPSRC for funding a postdoctoral grant (KMA) and the web resource www.dur.ac.uk/ zprime, and EPSRC and BP Chemicals Ltd. for a CASE studentship (J.T.L.). Supporting Information Available: Crystallographic information files for all structures presented herein (.cif) along with figures of DSC and PXRD data. This material is available free of charge via the Internet at http://pubs.acs.org.
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1862 Crystal Growth & Design, Vol. 7, No. 9, 2007 (24) Turner, D. R.; Smith, B.; Goeta, A. E.; Evans, I. R.; Tocher, D. A.; Howard, J. A. K.; Steed, J. W. CrystEngComm 2004, 6, 633-641. (25) Turner, D. R.; Henry, M.; Wilkinson, C.; McIntyre, G. J.; Mason, S. A.; Goeta, A. E.; Steed, J. W. J. Am. Chem. Soc. 2005, 127, 1106311074. (26) Turner, D. R.; Smith, B.; Spencer, E. C.; Goeta, A. E.; Evans, I. R.; Tocher, D. A.; Howard, J. A. K.; Steed, J. W. New J. Chem. 2005, 29, 90-98. (27) Valdes-Martinez, J.; Hernandez-Ortega, S.; West, D. X.; Ackerman, L. J.; Swearingen, J. K.; Hermetet, A. K. J. Mol. Struct. 1999, 478, 219-226.
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CG070533K
