cis-Protected palladium(ii) based binuclear complexes as tectons in crystal engineering and the imperative role of the cis-protecting agent

Abstract

A survey of the Cambridge Structural Database (CSD) for crystal structures of binuclear coordination complexes, formed by the combination of suitable ligand(s) and Pd(II) centres which are protected in a cis fashion by 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen), tetramethylethylenediamine (tmeda) or ethylenediamine (en), was carried out. The structures of these binuclear complexes could be classified into five different categories – “opened jaws”, “helicate”, “plateau”, “step” and “bow-shaped” based on the relative spatial orientation of the Pd(II) square planes of the cations and the conformation of the ligand(s). Investigation of the molecular packing in these complexes revealed involvement of the cis-protected Pd(II) units in the self-assembly of the cations. Cations in the complexes that contained bpy and phen moieties often assembled via π⋯π stacking and C–H⋯π interactions between the aromatic rings of the cis-protecting Pd(II) units. In contrast, in the en and tmeda complexes, molecular self-organisation in most cases occurred through noncovalent interactions between the cis-protected Pd(II) units and the ligand, counteranion or included solvent molecules. Hence, the cations can be defined as ‘tectons’, each of which contains two cis-protecting units, which play the role of ‘supramolecular synthons’ in the self-assembly of these binuclear complexes. The study of these crystal structures provides information about preferred patterns of molecular association in these complexes, which is invaluable for the crystal engineering of pre-designed self-assembled coordination complexes for desired functions or properties.