A STUDY OF CRYSTAL PACKING IN A SERIES OF CLOSELY-RELATED SQUARE-PLANAR PALLADIUM(II) AND PLATINUM(II) COMPLEXES

The crystal structures of the series of four-coordinate complexes [PdL(1)Cl].CH2Cl2 (1.CH2Cl2), [PtL(1)Cl].CH2Cl2 (2.CH2Cl2), [PdL(3)Cl][PF6].CH2Cl2 (3.CH2Cl2) and [PdL(4)Cl][PF6] (4) [HL(1)= 6-(2-hydroxyphenyl)-2,2'-bipyridine; L(3) = 6-(2-dimethylaminophenyl)-2,2'-bipyridin; L(4) = 2-(2-dimethylaminophenyl)-1,10-phenanthroline] are compared and contrasted to examine the extent to which aromatic pi-stacking interactions contribute to crystal packing. Complexes 1 and 2 have very similar planar molecular structures but stack in different ways; molecules of 1 form a linear stack in which overlap of the aromatic ligands is maximized between adjacent molecules and there is no Pd-Pd interaction, whereas in 2 there are axial Pt-Pt interactions within the linear stack but less overlap between adjacent aromatic ligands. In 3 the ligand L(3) is not planar but substantially twisted about the bond between the phenyl and bipyridyl fragments. The phenyl rings of the complex cations form an interleaved stack in the crystal, but the bipyridyl fragments are not involved in any close stacking interactions. Replacing the bipyridyl group by a phenanthrolinyl group in 4 changes the crystal packing; the structure of the individual complex cations is very similar to that of 3, but both fragments of the twisted ligand (the phenyl and phenanthrolinyl parts) are involved in separate stacking interactions with adjacent molecules.