The effect of phenyl substituents on supramolecular assemblies containing directly linked bis-pyridylimine ligands: synthesis and structural characterisation of mononuclear nickel(ii) and dinuclear silver(i) and cobalt(iii) complexes of (2-pyridyl)phenylketazine

Abstract

The effects of phenyl substituents introduced at the imine carbon of directly-linked bis-pyridylimine ligands is investigated. With octahedral metals the (2-pyridyl)phenylketazine ligand acts as an N₃ donor ligand with a non-coordinated imine unit. The triple-helical [M₂L₃]⁴⁺ complexes (observed with the unsubstituted ligand and in which the ligand uses all four nitrogen atoms) are not observed. A mononuclear [NiL₂]²⁺ complex containing mer-coordinated N₃ ligands and a dinuclear [Co₂L₂(OAc)(OH)₂]³⁺ containing fac-coordinated ligands are structurally characterised. With silver(I) the library of supramolecular architectures observed for the unsubstituted ligand is driven to a single double-helical architecture by the presence of the phenyl substituents. A dimeric [Ag₂L₂]²⁺ double-helical cation is structurally characterised. In addition to the N₄ coordination afforded by the ligands, one of the silver centres in each helicate forms a short contact to a methanol solvent or a tetrafluoroborate anion. Although the double-helical structure can support the addition of four phenyl substituents, it appears that the triple-helical structure is unable to accommodate the steric requirements of six phenyl substituents. Thus in this system, the phenyl substituents destabilise the triple-helical architectures but stabilise double-helical architectures (relative to the alternative competing structures).