Molecular structures and packing in crystals of ZnX2L2 complexes, having L as a monodentate oxo-ligand

Abstract

Abstract The changes in the molecular and crystal structures of the four-coordinate ZnX2L2 complexes through the chloride, bromide, iodide series and with L = acetophenone (acp), pyridine-N-oxide (pyo), dimethylformamide (dmf), dimethylsulfoxide (dmso) and trimethylamine-oxide (tmno) are reviewed. Tmno, as the strongest of these monodentate oxo-ligands, gives the shortest Zn–O and longest Zn–X bonds. As to molecular packing, the chlorides and bromides of the pyo, dmso and tmno series are not isotypic while the bromides and iodides – apart from the pyo complexes – are similar. Evidence from inter-molecular approach distances in the pyo and dmso series indicates a field strength sequence CH––Cl > CH––O > CH––Br and this dominates the molecular packing. It enhances Cl/Br differences, but would not reduce Br/I similarities. The structures of the acp complexes, including the chain-polymeric [{ZnCl2(acp)} n ] are dominated by an approach to parallel packing of the large planar ligands. In the pyo series parallel orientation of the planar rings provides efficient packing, but each set of rings is oriented to meet CH––X attraction requirements. The ring orientation is particularly sensitive to halogen size. With the complexes from the non-planar ligands dmso and tmno, structural variations arise from the influence of the methyl substituents as well as from differences in the halogen sizes; but CH––O approaches again become relatively more important in the bromides and iodides. In the dmf series because there is a strong preference for a particular molecular orientation, there is no change in space group, though with the iodide packing instability does lead to a phase change at low temperature.