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 c ontrasted to examine the extent to which aromatic pi-stacking interact ions contribute to crystal packing. Complexes 1 and 2 have very simila r planar molecular structures but stack in different ways; molecules o f 1 form a linear stack in which overlap of the aromatic ligands is ma ximized between adjacent molecules and there is no Pd-Pd interaction, whereas in 2 there are axial Pt-Pt interactions within the linear stac k but less overlap between adjacent aromatic ligands. In 3 the ligand L(3) is not planar but substantially twisted about the bond between th e phenyl and bipyridyl fragments. The phenyl rings of the complex cati ons form an interleaved stack in the crystal, but the bipyridyl fragme nts are not involved in any close stacking interactions. Replacing the bipyridyl group by a phenanthrolinyl group in 4 changes the crystal p acking; the structure of the individual complex cations is very simila r to that of 3, but both fragments of the twisted ligand (the phenyl a nd phenanthrolinyl parts) are involved in separate stacking interactio ns with adjacent molecules.