Crystal supramolecularity: extended aryl embraces in dimorphs of [Cu(1,10-phen)2I]I3 O4PE Caitlin Horn, Basem Ali, Ian Dance, Marcia Scudder and Don Craig School of Chemistry, University of New South Wales, Sydney 2052, Australia. E-mail: I.Dance@unsw.edu.au Received 22nd December 1999, Accepted 27th January 2000, Published 7th February 2000 The crystal structures of monoclinic and triclinic dimorphs of [Cu(phen)2I]+ I3– are reported, and the crystal packing of each is analysed. Local interactions between [Cu(phen)2I]+ cations are parallel fourfold aryl embraces (P4AE) of the phen ligands, supplemented by offset face-to-face interactions, to form layers of embracing cations. The tri-iodide ions are associated with the faces and the peripheries of the phen ligands, and recurring motifs are identified.Calculations of the intermolecular energies for the [Cu(phen)2I]+···[Cu(phen)2I]+ motifs and the [Cu(phen)2I]+···I3– motifs and the negligible I3–···I3– interactions are presented. While the energy contributions for the two structures have significant differences, the totals are very similar, consistent with the dimorphism. There are significant differences in molecular coordination stereochemistry for [Cu(phen)2I]+ in the two structures, and we conclude that this difference is a consequence of the alternative crystal packings. P4PE 3]z+ of it.11 This is consistent with the presence of an offset face-to-face (OFF) local interaction in the P4AE but not in the 6AE, which is comprised only of edge-to-face (EF) local interactions: the larger surface area of phen relative to bipy would favour the OFF interaction.These multiple aryl embraces all involve cationic complexes, which are mutually attractive because the van der Waals energy exceeds electrostatic energies. The crystal supramolecularity of these cations is naturally dependent on the associated anions, specifically their size, shape, number, and intermolecular forces. Polyiodides I z– provide a prolific assortment of anions, varying in charge, size and shape.12–14 We have exploited this variation in an investigation of the numerous and diverse polyiodide crystals containing the cation [Fe(phen)3]2+.15 There is a complementarity between the orthogonality of the ligand planes in [M(phen)3]z+ and the angularity of the larger polyiodides.12–14 One of the characteristics of the polyiodides of [Fe(phen)3]2+ is polymorphism, which is a general reflection of the similar energies of (a) embraces between [Fe(phen)3]2+ cations, (b) [Fe(phen)3]2+···Ix interactions, and (c) Ix···Ix interactions. In this paper we describe dimorphs of [Cu(phen)2I] I3.A monoclinic form of this compound is mentioned in an abstract16 and listed [ref. code BASHIM] in the Cambridge Structural Database (CSD),17,18 but without coordinates. We have crystallised and determined the structures of a new triclinic form, 1 and the monoclinic form, 2. These structures and their crystal packing are described and compared here, together with calculations of intermolecular energies.Introduction In earlier papers we have described multiple phenyl embraces, the supramolecular motifs which strongly influence the crystal packing of the numerous compounds containing the Ph4P+ cation or the Ph3P ligand.1–8 The most common of these are the sixfold phenyl embrace (6PE), the orthogonal fourfold phenyl embrace (O4PE), and the parallel fourfold phenyl embrace (P4PE), pictured in Fig. 1. 6PE Fig. 1 The three main types of multiple phenyl embrace, illustrated for Ph4X groups. Recently, similar supramolecular motifs were recognised in the crystal structures of metal complexes containing heteroaryl groups such as 2,2’-bipyridine (bipy)9 and 2,2’:6’,6’’-terpyridine (terpy).10 A general crystal packing motif for the group of compounds [M(bipy)3]z+ is the linear chain of sixfold aryl embraces (6AE)9 with [M(bipy) forming a double-ended version of the 6PE.Complexes [M(terpy)2]2+ and similar complexes with related ligands form two-dimensional networks comprised of parallel fourfold aryl embraces (P4AE)10 analogous to the P4PE. The ligand 1,10-phenanthroline (phen) is similar to bipy, but with a larger aryl surface. Multiple aryl embraces (MAE) could therefore be expected for compounds [M(phen)3]z+. We have found that [M(phen)3]z+ in crystals rarely forms the 6AE, but that complexes [M(phen)3]z+ and [M(phen)2L2]z+ more commonly form P4AE, and variants CrystEngComm, 2000, 2 x Results Crystallisation As is common with polymorphs, reproducible crystallisation methods were obtained only after considerable investigation.Most of the preparative methods and crystallisation conditions investigated yielded the triclinic form 1, but the monoclinic crystal 2 was obtained on two occasions. The preparative reactions included [Cu(phen)2I]+ + I2, [Cu(phen)2]2+ + I2 + I–, and [Cu(phen)2]2+ + I2, in various molar ratios, and in solvent(a) (b) (c) Fig. 2 (a) ORTEP drawing of [Cu(phen)2I] from 1. The trigonal bipyramidal stereochemistry about the Cu atom is evident. Selected dimensions for the two structures are Cu–I1 2.620(1), 2.602(1) (for 1 and 2, respectively); Cu–N1A 2.102(6), 2.079(5); Cu–N2A 2.002(5), 2.005(5); Cu–N1B 2.142(6), 2.147(5); Cu–N2B 1.992(6), 1.978(5); I1–Cu–N1A 133.6(2), 143.7(2); I1–Cu–N2A 95.7(2), 96.1(2); I1–Cu–N1B 114.2(2), 118.0(1); I1–Cu–N2B 93.8(2), 91.4(2); N1A–Cu–N2A 81.5(2), 81.6(2); N1A–Cu–N1B 112.1(2), 98.2(2); N1A–Cu–N2B 91.9(2), 93.1(2); N2A–Cu–N1B 95.9(2), 95.0(2); N2A–Cu–N2B 170.5(3), 172.4(2); N1B–Cu–N2B 80.2(2), 80.3(2).Click image or here to see a larger version. (b) and (c) Click images or here to access 3D models of structures 1 and 2, respectively. systems acetone, DMF, and acetone + acetonitrile + ethanol. Since the crystallisation conditions are critical in understanding the subtle factors which control polymorphism, we provide full details in the experimental section. Molecular structure The coordination geometry about the Cu in both complexes is close to trigonal bipyramidal, with the axial positions being occupied by N of the two ligands (Fig.2). Details of the geometry about the Cu atoms in the two structures are given in the caption to Fig. 2. Despite the similarity in the overall dimensions of the two cations, there is a very clear difference between them, which is illustrated in Fig. 3. A measure of this difference is given by the angle N1A–Cu–N1B, which is 112° in 1 but 98° in 2 (the N–Cu–I equatorial angles are 134, 114° in 1 and 118, 144° in 2). The position of the equatorial iodo ligand is much closer to that of regular trigonal bipyramidal coordination stereochemistry in 1 than in 2. 2 1 Fig. 3 Edge on views of [Cu(phen)2I]+ in 1 and 2, showing the difference in the coordination stereochemistry and in the angle between the phen wings of the cations.A survey of 36 five-coordinate Cu(phen)2X structures in the CSD indicates that this flexibility at the metal atom is common. When the fifth ligand is H2O or NCS, the N–Cu– N angle in the equatorial plane of the bipyramidal structure is large (116–140°). For structures where the fifth coordination position is occupied by halogen or CN, this angle varies markedly ranging from 93 to 124°. The molecular geometries for [Cu(phen)2X]+ (X = Cl, Br and I) have been examined in detail.19,20 Crystal packing: overall features Full cell packing diagrams for 1 (space group 1 P ) and 2 (space group P21/c) are shown in Fig. 4. The general arrangement of cations and anions in the cell is similar for the two structures, even though the size of the unit cell for 1 is half that for the higher symmetry structure 2.Crystal packing: [Cu(phen)2I]+ ··· [Cu(phen)2I]+ interactions We now describe the packing of the [Cu(phen)2I]+ and I3– ions in the lattices of 1 and 2. The cations come together in centrosymmetric pairs, shown in Fig. 5. The embrace motif is P4AE, involving a central OFF interaction, as expected for the large aryl surface of phen, but the P4AEs are different with greater overlap of the phen rings in the OFF of 1.(a) (c) (d) (b) Fig. 4 (a) and (c): full packing diagrams for 1 and 2, respectively. The general arrangement of components is similar for each when viewed down the a axis. (b) and (d): click images or here to access 3D models of 1 and 2, respectively.(a) (b) Fig.5 The aryl···aryl interactions present in 1 (left) and 2 (right). (a) The P4AE involves both edge-to-face and offset face-to-face interactions. (b) Views with 90° rotation about the horizontal axis relative to (a) showing that the extent of the overlap in the OFF interaction is greater in 1 than in 2. Click images to see larger versions. When a pair of [Cu(phen)2I]+ molecules form a P4AE there is a differentiation of the phen ligands. As labelled in Fig. 5(a), one phen ligand (A) is the acceptor for an EF interaction, while the other (B) participates in the OFF interaction and is also the donor for an EF interaction. The pairs of molecules involved in the P4AE have exposed aryl surfaces on the outside of both the A and B ligands. In the terpy embrace10 of complexes [M(terpy)2]2+, shown in Fig.6, these surfaces participate in further EF interactions and OFF interactions in an infinite two dimensional array where each ligand wing is equivalent. In both 1 and 2, the extended interactions are OFF between the outer surfaces of A ligands, giving rise to the chains shown in Fig. 7. Contiguous chains in both structures are stepped. This is shown in Fig. 8, which is an end-on view of two chains in each structure. In both, there are OFF interactions between B ligands of adjacent chains in the formation of the layer. These interactions occur at the central regions of Fig. 8. Both structures can therefore be described as being made up of stepped layers of cations.The layer structures of 1 and 2 are shown in Fig. 9. In both 1 and 2, there are additional interactions between adjacent chains within the layer which take the form of Cu– I···H–aryl contacts. The shortest Cu–I···H–C distances are 3.0 and 3.1 Å, respectively for the two structures, and in each there are three different contacts of this type, all less than 3.3 Å. In 1, the coordinated iodine is also adjacent to an aromatic ring, with the shortest Cu–I···C–aryl distance being 3.9 Å. Fig. 6 The terpy embrace, exhibited by many [M(terpy)2]2+ complexes, where there is two dimensional propagation generated by P4AE. Click image or here to see a larger version. Crystal packing: [Cu(phen)2I]+ ··· I3– interactions We now describe the locations of the tri-iodide anions in these two lattices.Fig. 10 shows space filling representations of the associations of tri-iodide anions with the aggregates of embracing cation for the two structures, and the difference between them is evident.12Fig. 7 Chains of molecules formed by offset face-to-face association of the pairs in Fig. 5, 1 (top) and 2 (bottom). Note the difference in the overall geometry of the chain resulting from the difference in the angular arrangement of the phen ligands of the molecules. In both instances, the chains are formed by OFF interactions between the outer surfaces of pairs of A ligands. Fig. 8 An end-on view of two contiguous chains of cations in 1 and 2.12 12Fig. 9 The cation layers in 1 and 2. 1 2 Fig. 10 Space filling representations of the association of I3– ions with the layers of cations in 1 and 2. In these views the coordinated I has been coloured orange to distinguish it from the I3– ions. The differences in the positions and orientations of the anions in the two structures are clear. Also evident in these views is the stepping of the cation chains down the layers, from bottom to top in each picture. Click images to see larger versions.(a) 3 3 Analysis of the details of the local interactions between [Cu(phen)2I]+ and I3– reveals several geometrical types, which are presented in Fig. 11. The anion can lie across and approximately parallel to the surface of a phen ligand, as in Fig.11(a), at a distance of ca. 4 Å from the ligand plane. Alternatively, I – can be approximately orthogonal to the phen plane, with C···I ca. 3.8 Å [Fig. 11(b)]. There are three arrangements of I – around the periphery of phen ligands: in the ligand plane [Fig. 11(c)], normal or oblique to the ligand plane [Fig. 11(d) and (e)], or oblique to a pair of phen ligands in the OFF interaction [Fig. 11(f)]. C–H···I distances of ca. 3–3.5 Å occur, as marked on Fig. 11, and are suggestive of a weakly directional favourable interaction. These local interactions are common in other crystalline polyiodides of metal phenanthroline complexes.15 (b) Fig. 12 shows the cations which surround each anion. Six – in 1, and seven in cations make close contact with each I3 2.3 3 Interactions between I – are minimal in both structures. In 1 there are end···end interactions, with I···I = 4.3 Å. There are no other inter-anion I···I distances < 5 Å in either structure, but in 1, there are contacts of 4.6 Å of the type I –···I–Cu and similar contacts in 2 are 4.7 and 4.8 Å in length. (c) Calculated intermolecular energies (d) (e) We have estimated the intermolecular energies (using the summed atom–atom energy approximation,21,22 Etotal = SEij) for the interactions between cations and for the cation– anion associations which occur in these two crystals. Our intermolecular potential for atoms with charges qi, qj separated by dij is given by eqn.(1) and (2). The coulombic component of the energy is significant, providing directionality for the embraces between [Cu(phen)2I]+, but its estimation is dependent on the atomic partial charges and the permittivity. We have evaluated atom charges by the QEq procedure of Rappe and Goddard.23 The atom parameters ea, da and q are obtained by analyses to be described and justified in detail in a separate paper:24 here we comment that the van der Waals attractive energy ea for carbon in phen has been increased slightly in accordance with the increased polarisability of these p-delocalised complexes, and the permittivity is set at e = 2 dij for similar reasons. The parameters used for the calculations reported in this paper are contained in Table 1.Eij = eija [ (dij/dija)–12 – 2 (dij/dija)–6 ] + qi qj/ e dij (1) dij j i ij j a = r + r ; e = (e e )0.5 (2) a a a a a i (f) The two principal motifs between the cations in these crystal structures are the P4AE and the two different "backto-back offset" linkages, A···A which creates the chains (Fig. 7) and B···B which then creates the layers (Fig. 9). Table 2 contains the calculated energies of these motifs in each structure. Fig. 11 Two views of each of the local interactions between [Cu(phen)2I]+ cations and I3– anions. (a) I3– side to phen face in 2, (b) I3– end to phen face in 1 and 2, (c) I3– parallel and peripheral to phen in 2, (d) I3– orthogonal and peripheral to phen in 2, (e) I3– oblique and peripheral to phen in 1, (f) I3– oblique and peripheral to a pair of phen faces in 1.Table 1 Atom parameters used in the calculation of supramolecular energies ea/kcal mol–1 ra/Å 0.11 0.02 0.11 0.15 0.6 Van der Waals parameters Atom CHNCu ICalculated atom charges for [Cu(phen)2I]+ 7 4 3 5 Cu NC2 C3 C4 H4 C5 H5 C6 H6 C7 H7 IOther atom charges [I–I–I]– 12 Nq+0.35 –0.29 +0.17 –0.01 –0.08 +0.16 –0.10 +0.16 +0.03 +0.14 –0.11 +0.17 –0.31 –0.39, –0.22, –0.39 There is a significant difference between the energy of the P4AE in 1 and 2.The reason for this is to be found in Fig. 5. The extent of the overlap in the offset-face-to-face interaction in 2 is much less than that found in 1, so the attraction would be expected to be diminished.The A–A motif in 1 is more favourable than that for B–B and this is also a direct result of the extent of the OFF overlap. In 2, the A···A interaction is only slightly more attractive than that for B···B. As described above, the environments of the anions in the two structures are different. Table 3 lists the I – ···[Cu(phen)2I]+ interaction energies for each of the six nearest neighbour cations. Also listed in the table are references to Fig. 11 which indicate the type of interaction observed. If the relative orientations of the cation and anion are appropriate, it is possible for one cation to make two interactions, one with each of its phen ligands. For 1, the most favourable interaction is a combination of two oblique, peripheral interactions [Fig.11(e)], while for 2, it is a combination of one surface interaction [Fig. 11(a)] and one orthogonal peripheral interaction [Fig. 11(d)]. There are no attractive interactions between anions [the closest anion···anion contact is end···end in 1 for which the energy is calculated to be +2.4 (–0.6, 3.0) kcal mol–1]. Other anion···anion interaction energies are weakly repulsive. 2 6 N Cu Fig. 12 Space filling diagrams showing the environment of the I3– anions in 1 and 2. The angular interactions are evident in the top figure and the orthogonal interactions in the bottom figure. These geometric differences match those found for the cations themselves (see Fig.3). The slots between phen ligands which are apparent in the top figure are filled by another such ligand which is part of the P4AE. Click images to see larger versions. 1.95 1.68 1.95 2.17 2.40 3Table 2 Calculated energies for the different types of [Cu(phen)2I]+···[Cu(phen)2I]+ interactions Etotal (vdW, coul.)/kcal mol–1 per {[Cu(phen)2I]+}2 pair 1 2 –17.1 (–21.1, 4.0) –15.4 (–16.4, 1.0) –10.9 (–11.2, 0.3) P4AE A–A B–B The total interaction energy for each structure could be considered to be the sum of two major components: cation···cation and cation···anion. Each cation is involved in one P4AE, one A–A and one B–B interaction. So, the total cation···cation energy per cation is half the sum of the three values listed in Table 2.The energy per cation is therefore –21.7 and –19.0 kcal mol–1 for 1 and 2, respectively. These are not very different, and as pointed out above, the additional attractive energy in 1 can be directly attributed to the increased OFF overlap in the P4AE. When the cation···anion interaction energies (Table 3) are summed for the six nearest neighbour cations, the totals per anion are –38.2 and –43.0 kcal mol–1. Summing the energy per cation and the energy per anion gives total calculated energies of –59.9 and –61.9 kcal mol–1, for 1 and 2, respectively. While these calculations are not the lattice energies of the two structures, they include the major components, and they indicate that the total energies of the two arrangements are similar, consistent with the dimorphism.2I]+ Discussion – ions around the cation assemblies: there are local 3 Polymorphism is an enigmatic phenomenon.25–29 The dimorphs described here illustrate the two typical attributes, which are the subtleties of crystallisation conditions, and the subtleties of the supramolecular isomerism.28 In 1 and 2 the supramolecular isomerism occurs within the same collection of P4AE and OFF motifs between [Cu(phen) molecules and resulting layers of cations (Fig. 9). The overall packing of cations is similar in the dimorphs, but the isomerism is clearly evident in the calculated energies (Table 2). The isomerism appears also in the juxtapositions of the I motifs occurring in these [Cu(phen)2I]+···I3– relationships, which reappear in many other crystals containing metal– phen cations with polyiodide anions.15 A noticeable difference between 1 and 2 occurs in the molecular coordination stereochemistry (Fig.3). One question is whether this difference drives the dimorphism, or the reverse. As already mentioned, Cu(phen)2X structures in the CSD show considerable intramolecular variability. There is one other structure for [Cu(phen) the CSD, with I– and H 2I]+ in 2O, [VITTOH],† and its Table 3 Calculated energies for nearest neighbour I3–···[Cu(phen)2I]+ interactions, Etotal (vdW, coul.)/kcal mol–1 per {[Cu(phen)2I]+/I3–} combination [ref. to Fig. 11] 1 2Experimental details Crystallisations of the triclinic dimorph, 1 A. A solution of I2 (0.16 g, 0.63 mmol) in 2 mL acetone was added dropwise to a stirred blue–green suspension of [Cu(phen)2][BF4]2 (0.10 g, 0.17 mmol) in 20 mL of a 1 : 1 CH3CN : ethanol mixture.On addition of the I2 solution, the mixture turned deep brown and became opaque. Subsequently, a solution of NaI (0.03 g, 0.2 mmol) in 2 mL acetone was added. A deep brown–black crystalline solid was immediately apparent. The mixture was left undisturbed for 30 min, then the solid was collected by suction filtration and rinsed with a little ethanol. The product was air-dried for 5 min, and it was characterised by X-ray crystallography without further purification or recrystallisation. B. A solution of I2 (0.43 g, 1.7 mmol) in 10 mL acetone was added to a stirred blue–green suspension of [Cu(phen)2][BF4]2 (0.10 g, 0.17 mmol) in 10 mL acetone.The mixture turned deep brown on addition of the I solution. No solid was immediately apparent. The mixture was left undisturbed and unsealed overnight, after which time a small amount of black crystals were present. These were collected by suction filtration, then rinsed with ether and used for single crystal X-ray diffraction experiments. The mother liquor was retained, and solid was deposited for up to 3 d. C. A solution of I2 (1.27 g, 5.0 mmol) in 20 mL acetone was added to a deep brown suspension of [–12.6 (–7.0, –5.5)] [(a) + (d)] [–8.4 (–4.8, –3.5)] [(a)] [–7.8 (–3.6, –4.2)] [(b) + (c)] [–5.7 (–1.4, –4.3)] [(c)] [–4.3 (–2.9, –1.5)] [(d)] [–4.3 (–1.5, –2.8)] [Undefined] –9.2 (–4.1, –5.1) [2 (e)] –8.5 (–3.7, –4.7) [(e)] –6.1 (–4.1, –2.1) [(b)] –5.6 (–1.8, –3.8) [(e)] –4.7 (–1.3, –3.4) [(e)] –4.1 (–1.2, –2.9) [Undefined] –7.1 (–11.2, 4.0) –16.1 (–17.3, 1.3) –14.7 (–16.4, 1.7) intramolecular geometry is such that the N–Cu–N angle between the phen ligands is 115.4°, so its ligand planes are splayed even further than those for 1.Therefore we conclude that the intramolecular differences between 1 and 2 are a consequence of the different crystal packing, and that this is another illustration that packing forces can have quite large (in angular terms) influences on molecular coordination stereochemistry,22 which might otherwise be incorrectly interpreted.2X-Ray crystallography Table 4 Crystal data for 1 and 2a Properties Formula MCrystal system Space group 2 qmax Min transmission factor Max transmission factor Unique reflections Observed reflections Rmerge Ra/Å 9.212(7) 12.662(9) b/Å 11.086(7) 20.519(5) c/Å 13.886(11) 10.326(6) a/° 101.28(4) 90 b/° 97.93(4) 92.17(3) g/° 103.61(4) 90 V/Å3 1326(2) 2681(3) Dc /g cm–3 2.33 2.31 Z 2 4 mMo/cm–1 54.0 50 0.39 0.65 4711 2893 0.015 0.031 0.039 aRw Click here for full crystallographic data (CCDC no. 1350/8). [Cu(phen)2I][BF4] (0.318 g, 0.5 mmol) in 30 mL acetone. A small amount of paler brown precipitate was immediately apparent. The mixture was stirred for 30 min, then left undisturbed and unsealed for three d, after which time black crystals were present in small amount.These were collected by suction filtration, then rinsed with ether and characterised by single crystal X-ray diffraction. D. Recrystallisation of the above samples from acetone, acetonitrile, or ethanol also returned 1. Crystallisations of the monoclinic dimorph, 2 A standard solution was prepared in 50 mL of DMF, by dissolving [Cu(phen)2I][BF4] (0.319 g, 0.50 mmol), then adding I2 (0.635 g, 2.50 mmol) with stirring. The resulting solution was 10 mM in [Cu(phen)2I]+ and 50 mM in I2. 20 mL of this solution was kept in a sealed sample tube at ambient temperature, and after 12 h a dark brown fine crystalline solid had formed. The crystals were collected by decantation, and washed with a little diethyl ether, and characterised by single-crystal X-ray diffraction.Acknowledgements Support from the Australian Research Council is gratefully acknowledged. We thank Professor Lars Kloo for discussion and provision of ref. 14. References 1 I. G. Dance and M. L. Scudder, J. Chem. Soc., Chem. Commun., 1995, 1039. 2 I. Dance and M. Scudder, Chem. Eur. J., 1996, 2, 481. 3 I. Dance and M. Scudder, J. Chem. Soc., Dalton Trans., 1996, 3755. 4 C. Hasselgren, P. A. W. Dean, M. L. Scudder, D. C. Craig and I. G. Dance, J. Chem. Soc., Dalton Trans., 1997, 2019. 1 2 C24H16CuIN4I3 931.6 Triclinic 1 P 54.5 50 0.39 0.53 4641 3331 0.044 0.040 0.053 C24H16CuIN4I3 931.6 Monoclinic P21/c 5 M.Scudder and I. Dance, J. Chem. Soc., Dalton Trans., 6 M. Scudder and I. Dance, J. Chem. Soc., Dalton Trans., 7 M. Scudder and I. Dance, J. Chem. Soc., Dalton Trans., 1998, 329. 1998, 3155. 1998, 3167. 8 I. Dance and M. Scudder, New J. Chem., 1998, 481. 9 I. Dance and M. Scudder, J. Chem. Soc., Dalton Trans., 1998, 1341. 10 M. L. Scudder, H. A. Goodwin and I. G. Dance, New J. Chem., 1999, 23, 695. 11 V. M. Russell, C. Horn, D. Craig, M. L. Scudder and I. G. Dance, in preparation. 12 N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1985. 13 A. J. Blake, F. A. Devillanova, R. O. Gould, W.-S. Li, V. Lippolis, S. Parsons, C. Radek and M. Schroder, Chem. Soc. Rev., 1998, 27, 195.14 P. H. Svensson, PhD Thesis, Lund University, Sweden, 1998. 15 C. Horn and I. G. Dance, in preparation. 16 B. Freckmann and K. F. Tebbe, Acta Crystallogr., C: Cryst. Struct. Commun., 1981, 37, 228. 17 F. H. Allen, J. E. Davies, J. J. Galloy, O. Johnson, O. Kennard, C. F. Macrae and D. G. Watson, Chem. Inf. Comput. Sci., 1991, 31, 204. 18 F. H. Allen and O. Kennard, Chem. Des. Autom. News, 1993, 8, 131. 19 G. Murphy, P. Nagle, B. Murphy and B. J. Hathaway, J. Chem. Soc., Dalton Trans., 1997, 2645. 20 G. Murphy, C. O’Sullivan, B. Murphy and B. Hathaway, Inorg. Chem., 1998, 37, 240. 21 A. J. Pertsin and A. I. Kitaigorodsky, The atom–atom potential method. Applications to organic molecular solids, Springer Series in Chemical Physics, Springer- Verlag, Berlin, 1987.22 I. G. Dance, in The Crystal as a Supramolecular Entity, ed. G. R. Desiraju, John Wiley, New York, 1996, p. 137. 23 A. K. Rappe and W. A. Goddard, J. Phys. Chem., 1991, 95, 3358. 24 B. F. Ali, I. G. Dance and M. L. Scudder, in preparation. 25 G. R. Desiraju, Crystal Engineering: The Design of Organic Solids, Elsevier, Amsterdam, 1989. 26 J. Bernstein, J. Phys. D: Appl. Phys., 1993, 26, B66. 27 J. D. Dunitz and J. Bernstein, Acc. Chem. Res., 1995, 28, 193. 28 J. D. Dunitz, in Perspectives in Supramolecular Chemistry: The Crystal as a Supramolecular Entity, ed. G. R. Desiraju, Wiley, Chichester, 1996, p. 1. 29 G. R. Desiraju, Chem. Commun., 1997, 1475. Footnote †As part of our investigation, we have crystallised this compound and determined its structure. The cell dimensions quoted in the CSD are apparently in error. Our cell is a = 10.163(3), b = 11.658(6), c = 12.054(4) Å, a = 66.06(3), b = 65.83(3), g = 71.96(3)°. The non-reduced cell quoted in the CSD for VITTOH is 12.860, 12.049, 10.169, 114.07, 120,44, 87.57, which on reduction yields 10.169, 11.674, 12.049, 72.03, 65.93, 71.76. However, replacement of g in the quoted cell by (180 – g ) and reduction yields 10.169, 11.674, 12.049, 66.30, 65.93, 71.76, which compares well with our cell. R for our structure determination is 0.028. Paper a910231k CrystEngComm © The Royal Society of Chemistry 2000