The crystal packing and engineering prospects of Hellwinkel's salt: [P(2,2'-biphenylyl)2]z[P(2,2'-biphenylyl)3]2 aSchool of Chemistry, University of New South Wales, Sydney NSW 2052, Australia. E-mail: I.Dance@unsw.edu.au bResearch School of Chemistry, Australian National University, Canberra ACT 0200, Australia cInsitut fu»r Anorganische Chemie, Ludwig-Maximilian University, Munich D-81377, Germany Received 3rd October 2000, Accepted 14th November 2000 Published on the Web 18th December 2000 The crystal structure of Hellwinkel's salt [P(2,2'-biphenylyl)2]z[P(2,2'-biphenylyl)3]2 (1) is reported and the crystal packing analysed. The cation [P(2,2'-biphenylyl)2]z (Pz) is analogous to Ph4Pz, and the anion [P(2,2'- biphenylyl)3]2 (P2) is analogous to [M(bipy)3]z complexes, both of which form multiple aryl embraces in crystals, but not previously with each other.The chirality of P2 is potentially valuable for the resolution of chiral cations. The crystal supramolecularity of 1 is comprised of (1) a P2ºP2 sixfold aryl embrace with a calculated energy of 8.9 kcal mol21 attraction, (2) a tight homochiral embrace PzºP2 embrace using offsetface-to-face interactions between biphenylyl ligands (calculated attraction 21.0 kcal mol21), (3) a parallel fourfold aryl embrace PzºPz (calculated attraction 1.2 kcal mol21), and (4) several non-standard PzºP2 embraces (calculated attractions 13.9, 18.3 kcal mol21). Comparisons with related crystals are made. The prognosis for application of P2 to the resolution of phenylated molecules and cationic metal complexes with phenylphosphonium or phenylarsonium ligands is good.Introduction The salt [P(2,2'-biphenylyl)2]z[P(2,2'-biphenylyl)3]2 1 was prepared by Hellwinkel in 1965.1,2 In the following we will use the symbols Pz for [P(2,2'-biphenylyl)2]z and P2 for [P(2,2'-biphenylyl)3]2. We are interested in 1 for four reasons. One relates to the general similarity of Pz with Ph4Pz, which is commonly used for the crystallisation of anions, and which engages in multiple phenyl embraces of the type that we have investigated in detail.3,4Multiple phenyl embraces between molecules are concerted combinations of local edge-to-face (EF) and the offset-face-to-face (OFF) geometries for a pair of aryl or heteroaryl rings: the sixfold phenyl embrace (6PE) is a concerted cycle (EF)6, the parallel fourfold phenyl embrace (P4PE) is (OFF)(EF)2, and the orthogonal fourfold phenyl embrace (O4PE) is (EF)4.Crystal supramolecular extensions in one-, two- and three-dimensions have been described.5±7 The crucial difference in molecular geometry and conformation between Pz and Ph4Pz is that the phenyl rings are linked in pairs in the former but are separate and largely unconstrained in the latter. Second, the 2,2'-biphenylyl ligands in 1 are directly analogous to the 2,2'-bipyridyl (bipy) ligands in metal complexes, and chiral P2 is very similar in structure to [M(bipy)3]z complexes. We have previously described the high symmetry crystal lattices with inÆnite chains of sixfold aryl DOI: 10.1039/b007964m This journal is # The Royal Society of Chemistry 2000 Bettina Jee,a Johann Zank,b Ian Dance,*a S.Bruce Wild,b P. Klu»fersc and Anthony Willisb Paper embraces (6AE, analogous to 6PE) that occur for complexes [M(bipy)3]zz, z ~ 3, 2 and 0.8 The attractive energy in these 6AE motifs is most favourable when z~ 0. The ion P2 could engage in the same threefold symmetric 6AE interaction, either as pairs, (P2)2, or as inÆnite chains, (P2)`. Third, the salt 1 provides a combination of embracing entities that has not been achieved previously: it is analogous to a molecular crystal containing both Ph4Pz and [M(bipy)3]zz, and the embrace motifs between such species are not yet known. Finally, P2 is chiral, and has the potential for interaction with chiral cations in the solid state and in solution.(°)-P2 has been resolved by fractional crystallisation of its N-methylbrucinium salt.9 The ability of the enantiomers of P2 to achieve resolution will be related to the crystal supramolecular motifs it forms. We note that while the term crystal engineering is credited to Schmidt,10 and it is now widely used, there is a large opus of earlier crystal engineering implicit in the design of diastereomeric lattices for resolution of chiral molecules and ions by fractional crystallisation.11 These considerations raised the question of the intermolecular interactions between Pz and P2 in crystalline 1. Here we report the crystal structure of 1 (Table 1) and an analysis of its crystal packing and crystal supramolecular motifs in the context of these related embrace motifs and possible applications of P2 to resolutions and chiral discriminations.Synthesis Compound 1 was prepared and isolated as previously described.1 Calculation of inter-ion energies Inter-ion energies were calculated using the summed atom± atom method, with van der Waals and electrostatic compo- CrystEngComm, 2000, 35, 1±5Table 1 Crystal data for 1a Properties r calcd/g cm23 Empirical formula MCrystal system, space group a/A b/A c/A b/©ø V/A 3 Zrm/cm21 F(000) R, Rw T/K No. of unique re�ªections No. of re�ªections observed aYellow plates 0.3060.2860.15 mm. Click here for full crystallographic data (CCDC no.1350/39). nents expressed by eqns. (1) to (4). Einter¢§ion~ t {12 ij Eij~eaij X iqj ¢§ ¢§ ij=da {2 dij=da z q = edij d (2) eaij~ eai eaj The parameters ea and ra for the Lennard-Jones potential describing the van der Waals energy are: P, ea ~0.20 kcal mol21, ra ~ 2.1 A ; C, ea ~ 0.093 kcal mol21, ra ~1.945 A ; H, ea ~ 0.02 kcal mol21, ra ~ 1.6 A . The unique atom charges are presented in the following diagrams, and permittivity e in eqn. (2) is distance-dependent, e ~d Crystal structure and crystal packing The space group is P21/a, with one cation and one anion in the asymmetric unit, and no solvent in the lattice. The bond lengths and angles in Pz and P2 are normal, each of the �¡ve biphenylene ligands is coplanar to better than 0.1 A , and the coordination stereochemistry of P2 is close to octahedral.The two biphenylene ligands in Pz are twisted slightly from orthogonality, with a dihedral angle between the ligand planes of 85.34©ø. The principal interest in the structure is the crystal packing. We �¡rst describe the array of ions in the lattice and then focus on the details of each of the local inter-ion motifs. The ion centres are shown (as P atoms only) in Fig. 1: the cations Pz are coloured orange and the anions P2 magenta. The array of ions can be described as stacked layers, each layer being approximately hexagonal with oppositely charged ions alternating within each layer. The P¡¾P separations for Pz��P2 pairs in the layer are 6.56, 8.23 and 8.41 A .There are two 2 CrystEngComm, 2000, 35, 1¡¾5 Values 1/a (no. 14) C60H40P2 822.92 Monoclinic, P2 16.3355(2) 16.3158(2) 16.9505(2) 111.0154(8) 4217.26(9) 41.296 1.4 1720 0.038, 0.042 200 9677 6257 (Iw2s(I)) Fig. 1 The array of P atoms for the cations Pz (orange) and anions P2 (magenta) in the crystal lattice of 1. The space group is P21/a. All P¡¾P distances less than 9 A are marked. These distance symbols are used to identify the inter-ion interaction motifs. (1) ij Eij {6 ij (3) daij~raizraj0:5 (4) stacking modes relating adjacent layers: one set of adjacent layers is effectively eclipsed in projection normal to the layers, with oppositely charged ions all separated by P¡¾P~ 7.96 A . The other juxtaposition of contiguous layers involves adjacent ions of the same sign, with P¡¾P for Pz��Pz at 8.53 A and for P2��P2 at 7.80 A . In this way the stack of layers is comprised of alternating homo-ion and hetero-ion interactions.Each ion, Pz or P2, is surrounded by one ion of the same charge and four of the opposite charge. An important inter-ion motif is the association of two anions, P2��P2, at P¡¾P~ 7.80 A . This motif is an excetrosymmetric sixfold aryl embrace (6AE) between a pair of P2: (a) space-�¡lled side view; (b) skeletal end view showing the eclipsed ions. One end of each biphenylyl ligand forms an edge-to-face (EF) interaction across the interaction domain with another ligand, and the cycle of six EF primary motifs is concerted.This is the 7.80 A motif of Fig. 1. The H��C distances in the EF interactions range from 2.83 to 3.3 A .Fig. 3 Two orthogonal views of the parallel fourfold aryl embrace (P4AE) between two Pz ions. This is the 8.53 A motif of Fig. 1. The interplanar separation in the OFF is 3.55 A , and the HºC distances in the EF motifs are y3.3 A . Fig. 4 The embrace between Pz (left, orange) and P2 (right, magenta) at P±P~6.56 A . The (non-crystallographic) twofold axes of both Pz and P2 are close to collinear with the motif axis, which is horizontal in this view. There are two OFF primary motifs, at the upper and lower regions of the interaction domain in this view.Note that the two ions are homochiral: they have the same helicity. Click image or here to access a 3D representation. sixfold aryl embrace (6AE), shown in detail in Fig. 2. One end of each biphenylyl ligand forms an EF interaction across the interaction domain, generating the characteristic cycle of concerted (EF)6. The 6AE is crystallographically centrosymmetric and closely approaches S6 symmetry (Fig. 2(b)). Fig. 2 shows that the EF motifs are very well-formed, with H atoms on the edge directed over the C atoms of the face. Both ends of P2 are equivalent along the molecular threefold axis, and therefore P2 could form 6AE at both ends (as do the [M(bipy)3] complexes), but does not in 1. The homo-cation PzºPz embrace (PºP 8.53 A , Fig.1) is a parallel fourfold aryl embrace, P4AE, shown in Fig. 3. This Fig. 5 The 7.96 A motif in 1. The threefold axis of P2 is close to the motif axis, which is horizontal in this view. There are two welldeveloped primary EF motifs, at the front of this picture. The ions are heterochiral. Fig. 6 The PzºP2 embrace, OFFz EF, at 8.23 A . centrosymmetric embrace is comprised of one OFF motif in which the H atoms of one ring are over C atoms of the other (see Fig. 3(a)), and two EF motifs. In addition to these embraces of each ion with another ion of the same charge, each ion is surrounded by four ions of the opposite charge, at PºP distances of 6.56, 7.96, 8.23 and 8.41 A . Fig. 4 shows the close motif at 6.56 A .The P2 ion is oriented with a twofold axis close to the motif PzºP2 axis, and a twofold axis of Pz is also close to collinear with these two axes. This allows the two ligand planes of Pz to become almost parallel with two ligand planes of P2, and generates two pseudo-OFF primary motifs. This parallelism of the ligand planes and absence of EF primary motifs permits the two ions to approach each other more closely than other motifs where EFs occur, and accounts for the relatively short P±P distance. The secondary motif here is effectively (OFF)2, which is analogous to the threefold (OFF)3 embrace between MePh3Pz and [Cr(oxalate)3]32.12 This embrace type with parallel ligand Øanges is very rare,13 and is geometrically awkward to the extent of being impossible for two [M(bipy)3] or [M(phen)3].It is the Øatter array of the two biphenylene ligands in Pz that allows this (OFF)2 motif in 1. An important consequence of the (OFF)2 motif is that the two species involved in the embrace are homochiral, in contrast to the heterochiral 6AE. The crystal lattice is centrosymmetric, and so is a racemate containing homochiral embraces of both helicities. The 7.96 A embrace is shown in Fig. 5. Apart from two good EF motifs there is no good development of concerted primary motifs in this secondary motif. The two ions are heterochiral. The two other interion motifs, at 8.23 and 8.41 A , are portrayed in Figs. 6 and 7. Both have one biphenyl ligand of Pz inserted between two biphenyls of P2, allowing one OFF and one EF primary motif in each case.Having described the main embrace motifs, it is necessary to examine higher levels of association and check for extended embraces in the crystal packing. Fig. 8 (with the same orientation as Fig. 1) shows that the crystal structure is essentially layered parallel to the ab plane. There are two Fig. 7 The PzºP2 embrace, OFFz EF, at 8.41 A . CrystEngComm, 2000, 35, 1±5Fig. 8 The layers parallel to ab in crystalline 1: cations Pz have C red; anions P2 have P magenta and C green; H atoms are omitted. The 6AE P2ºP2 motif and the P4AE PzºPz motif occur between the upper two layers, across z~0.5, and the 7.96 PzºP2 motif occurs between the lower two layers, across z ~0.different interfaces between the layers: one involves the 7.96 A motifs, and the other the 6AE and P4AE embraces. All PzºP2 motifs except the 7.96 A are within the layers. The energies of the secondary motifs have been calculated using our standard procedures: the results are contained in Table 2. The hetero-ion PzºP2 motifs where the coulombic contributions augment the van der Waals attractions are more attractive than the homo-ion motifs, but both homo-ion motifs are attractive, at 28.9 kcal mol21 for the 6AE (P2ºP2) and 21.2 kcal mol21 for the P4AE (PzºPz). We note that the tight hetero-ion (OFF)2 embrace is particularly stable, at 221.0 kcal mol21. The layering in the crystal is consistent with these energies, in that the three most attractive secondary motifs occur within the layers and the interlayer motifs are appreciably less attractive. Discussion The sizes of the Pz and P2 ions in 1 are not markedly different, and, in the absence of local supramolecular motifs, a cubic array (fcc or bcc) of ions with hetero-charge primary contacts would be expected. However, multiple aryl embraces, as secondary motifs comprised of concerts of the primary OFF and EF motifs, play a signiÆcant role in the crystal packing of 1.The 6AE between two P2 ions is very well-developed, and calculated to contribute an attractive energy that is of the order of half the attractive energy for hetero-charged PzºP2 motifs, and it is evident that the optimum crystal packing in 1{ has incorporated both homo-charged and hetero-charged motifs.We note that inÆnite chains of P2 linked by 6AE, analogous to Table 2 Calculated energies [kcal mol21] of the secondary motifs in 1 Total energy Coulombic energy van der Waals energy Motif label Secondary motif 2 P2ºP2, 6AE PzºPz, P4AE PzºP2, (OFF) PzºP2 PzºP2 PzºP2 z4.2 z5.1 26.6 25.3 27.1 26.8 28.9 21.2 221.0 213.9 218.3 218.3 7.80 8.53 6.56 7.96 8.23 8.41 213.1 26.3 214.4 28.6 211.2 211.5 {We are exploring for polymorphs of 1. 4 CrystEngComm, 2000, 35, 1±5 Fig. 9 The stack of molecules Si(bipy)2 3 (in crystalline BIPYSI), in skeletal and space-Ælling representations. In each bipy ligand, each pyridyl ring functions as `donor' and `acceptor' in two edge-to-face EF local motifs.The edge is inclined at about 70� to the face. There are four such EF motifs between each pair of molecules. 4Pz ° the inÆnite chains of 6AE for complexes [M(bipy)3]zz,8 do not occur. This alternative packing would require corresponding chains of cations Pz, calculated to be less attractive, and these chains may not be commensurate with the anion chain, or allow good inter-ion embraces between chains. The crystal packing of 1 can be compared with that of Ph Ph4B2 2, in which the oppositely charged ions are much closer in shape and size.14 The tetragonal lattice of 2 is dominated by translational columns of orthogonal fourfold phenyl embraces (O4PE) with alternation of oppositely charged ions along the columns.Less signiÆcant attractive motifs occur between the columns and inØuence the registry between columns.15 The net supramolecular energies between homo-charged XPh4 ions are not repulsive, as is often assumed.16 The enantio-isomerism of the crystal supramolecular motifs in 1 is of particular interest to us, since P2 could be used for the resolution of cations by fractionallisation of diastereomers. The 6.56 hetero-charged motif is homo-chiral, while the 7.80 homo-charged 6AE motif is heterochiral. Our analysis shows that both the 6AE between two P2 and a hetero-charged 6AE or similar embrace involving P2 are predicted to be Fig. 10 The pseudo-hexagonal array of stacks of Si(bipy)2 in crystalline BIPYSI.Fig.11 Comparison of the stacks of Si(bipy)2 with (EF)4 secondary motifs between molecules (a) and a typical stack of O4PE formed by Ph4Pz (b). The two Ægures are drawn on the same scale. The repeat distances are 5.1 A in (a) and 7.1 A in (b). particularly stable. The prognosis is good for application of P2 to the resolution of phenylated molecules and of metal complexes with phenylphosphonium or phenylarsonium ligands. Metal complexes containing Ph3P ligands commonly form 6PE secondary motifs.17 The crystal supramolecularity of the Pz ion can be understood further by inspection of the crystal structure of the closely related compound Si(bipy)2 3 [CSD refcode BIPYSI].18 The molecular stereochemistry of this compound is partially Øattened tetrahedral: the dihedral angle between the ligand planes is about 75� in 3, and 85� in 1.Molecules of 3 are stacked parallel to the c axis in the space group Pbca, as shown in Fig. 9, such that the Si±Si distance is 5.07 A and the Si±Si±Si angle is 175.5�. This stack is shown in Fig. 9, and the pseudohexagonal array of stacks is shown in Fig. 10. These stacks of Si(bipy)2 allow EF primary motifs between molecules: there are four EF motifs between each pair of molecules. Each pyridyl ring functions as both donor (at the edge) and acceptor (at the face) in EF motifs. This stack is thereby analogous to the linear inÆnite chain of orthogonal fourfold phenyl embraces (O4PE) previously described in detail for molecules of the type XAr4.5 The two stack motifs are compared and contrasted in Fig. 11.In each case there are four aryl groups and four EF motifs in the embrace domain: the difference is due to the constraining linkage of pairs of rings in Si(bipy)2, in contrast to the conformational freedom of the rings in XAr4. The consequence of this is that the EF interactions in the Si(bipy)2 stack have a lesser inclination of the two rings in each EF, while in XAr4 the EF interactions can be more strongly canted and approach orthogonality. This in turn affects the repeat distance along the stack, as is evident in Fig. 11: the repeat distance in the O4PE stack shown in Fig. 11(b) is 7.1 A , which contrasts the 5.1 A repeat for Si(bipy)2 (Fig. 11(a)). The columnar motif demonstrated by Si(bipy)2 in BIPYSI is feasible for Pz in its crystals with other anions.A conceivable alternative crystal structure for 1 is a column of Pz as in Fig. 11(a) with a parallel inÆnite column of P2 engaged in 6AE at either end, as are known for [M(bipy)3]z.8 However, these columns would be incommensurate. Further experiments with Pz and P2 are in progress. Acknowledgements This work was supported by the Australian Research Council and the Australian National University, and was carried out by Bettina Jee for her Diplomarbeit at the Ludwig-Maximilian University, Munich. References 1 D. Hellwinkel, Chem. Ber., 1965, 98, 576. 2 D. Hellwinkel, Angew. Chem., Int. Ed. Engl., 1965, 4, 356. 3 I. G. Dance and M. L. Scudder, J. Chem. Soc., Chem. Commun., 1995, 1039. 4 I. Dance and M. Scudder, Chem. Eur. J., 1996, 2, 481. 5 I. Dance and M. Scudder, J. Chem. Soc., Dalton Trans., 1996, 3755. 6 M. Scudder and I. Dance, J. Chem. Soc., Dalton Trans., 1998, 3167. 7 M. Scudder and I. Dance, J. Chem. Soc., Dalton Trans., 1998, 3155. 8 I. Dance and M. Scudder, J. Chem. Soc., Dalton Trans., 1998, 1341. 9 D. Hellwinkel, Chem. Ber., 1966, 99, 3628. 10 G. M. Schmidt, J. Pure Appl. Chem., 1971, 27, 647. 11 J. Jaques, A. Collet and S. H. Wilen, Enantionmer, Racemates, and Resolutions, Wiley-Interscience, New York, 1981. 12 V. M. Russell, D. C. Craig, M. L. Scudder and I. G. Dance, CrystEngComm, 2000, 3 (http://www.rsc.org/ej/ce/2000/a909749j/ index.htm) 13 V. M. Russell and I. G. Dance, unpublished results. 14 M. A. Lloyd and C. P. Brock, Acta Crystallogr., Sect. B, 1997, 53, 773. 15 B. F. Ali, Crystal Supramolecularity for Inorganic Molecules and Clusters, PhD, University of New South Wales, Sydney, 1998. 16 M. A. Lloyd and C. P. Brock, Acta Crystallogr., Sect. B, 1997, 53, 780. 17 I. G. Dance and M. L. Scudder, J. Chem. Soc., Dalton Trans., 2000, 1587. 18 R. Morancho, P. Pouvreau, G. Constant, J. Jaud and J. Galy, J. Organomet. Chem., 1979, 166, 329. CrystEngComm, 2000, 35, 1&pl