Dimorphic intra- and intermolecular aryl motifs in symmetrical hexafaceted molecules (ArnX)3Y-Z-Y(XArn)3

A variety of crystals containing molecules formulated generally as (ArnX)3Y-Z-Y(XArn)3 with exact or pseudo S6 symmetry have been analysed for the presence of intra- and intermolecular multiple aryl embraces composed of offset-face-to-face (OFF) and edge-to-face (EF) local motifs. Ar is phenyl or substituted phenyl; n = 1, 2 or 3; the linkage Z is linear diatomic, linear monatomic, absent (Y-Y bond), or void (Y atoms unconnected). Representative molecular systems analysed are (PhCH2)3Si-O-Si(CH2Ph)3, (PhCH2)3Sn-Sn(CH2Ph)3, (4-Me-C6H4CH2)3Ge-O-Ge(CH2C6H4-4-Me)3, [(PhS)3Fe-O-Fe(SPh)3]-, [(PhCH2)3P-Cu-P(CH2Ph)3]+ and [(Ph3P)3Ag-CN-Ag(PPh3)3]+. One characteristic intramolecular motif is a sixfold phenyl embrace (6PE) in which aryl groups fold back to the central domain of the molecule forming a rhombohedral box maintained by (EF)6. When the aryl groups are in exo positions there is an (EF)3 motif, like a tractor wheel, at each end of the molecule: [(Ph2N)3Ti-O-Ti(NPh2)3] possesses both intramolecular motifs as (EF)3-(EF)6-(EF)3. The molecules in this set are hexafaceted, and the crystal packing is generally based on intermolecular EF or OFF motifs with these faces, either from the central (EF)6 set or the end (EF)3-(EF)3 sets. Three types of subtle crystal packing isomerism occur: 1) the faces of the rhombohedral boxlike molecules slide over each other with variation of the intermolecular motifs, forming dimorphic crystals in space groups R3 or P1; 2) the faces of the tractor wheels (EF)3 slide over each other or 3) very similar molecules crystallise with the rhombohedral box or tractor wheel structures. In general the molecules considered are shape auspicious rather than shape awkward; solvent is included in a small proportion of the crystals and the crystal packing in these compounds is evidently efficient. Some principles for elaboration of these systems and the design of molecular crystal lattices are considered.