Co-ordination chemistry of mixed pyridine–phenol ligands: polynuclear complexes of 6-(2-hydroxyphenyl)-2,2′-bipyridine with NiII, CdII, MnII and MnIIMnIII

Abstract

A series of complexes with the N,N,O-terdentate ligand 6-(2-hydroxyphenyl)-2,2′-bipyridine (HL¹) has been prepared and structurally characterised. The complexes are [Ni₂L¹₂(dmf)₂(H₂O)₂][BPh₄]₂1(dmf = dimethylformamide), [Cd₂L¹₂(HL¹)(dmf)₂][BPh₄]₂2, [Mn₄L¹₆][BPh₄]₂3 and [Mn₂L¹₃(MeCN)][PF₆]₂4. In the centrosymmetric complex 1 one ligand L¹ is associated with each metal but the phenolates are bridging, giving a Ni^II₂(µ-O)₂ core; the remaining two co-ordination sites are occupied by H₂O and dmf (O-co-ordinated) ligands. In complex 2 one Cd^II ion is octahedrally co-ordinated by two ligands L¹; the two phenolates bridge to the second Cd^II, whose co-ordination sphere is completed by two dmf ligands and the two pyridyl donors of another equivalent of HL¹, whose pendant phenol remains protonated and forms hydrogen-bonding interactions to lattice solvent molecules. Complex 3 in contrast is an unusual example of a linear chain, with a Mn^II(µ-O)₂Mn^II(µ-O)₂Mn^II(µ-O)₂Mn^II core (where O denotes a bridging phenolate) and highly distorted six-co-ordinate Mn^II centres: the chain formation is apparently facilitated by the absence of any stereoelectronic preference for the Mn^II ions which can readily tolerate the distorted geometries. Complex 4 contains a Mn^II centre, co-ordinated by two ligands L¹, and a Mn^III centre which shares two phenolates with the Mn^II and also has a terdentate L¹ and a MeCN ligand to complete the co-ordination sphere. The Mn^III centre shows Jahn–Teller elongation along one of the axes. The contributions of factors such as the metal oxidation state, the co-ordination modes available to the ligand, and non-covalent interactions (hydrogen bonding, aromatic π-stacking) to the structures of the complexes are discussed.