The co-ordination chemistry of mixed pyridine–phenol ligands; synthesis of 6-(2-hydroxyphenyl)-2,2′-bipyridine (HL) and the crystal structures of [Cu2L2(µ-MeCO2)][PF6]·1.5CH2Cl2 and [CoL2][PF6]·MeCN

Abstract

The new N,N,O-tridentate ligand 6-(2-hydroxyphenyl)-2,2′-bipyridine (HL) has been prepared in 41% yield overall by reaction of 2-lithioanisole with 2,2′-bipyridine, followed by rearomatisation with KMnO₄ and subsequent conversion of the anisole group to a phenol with pyridinium hydrochloride. Complexes with Co^III, Ni^II and Cu^II have been prepared, which possess different structures and metal:ligand ratios. The structure of [CoL₂][PF₆]·MeCN, established by X-ray diffraction, is mononuclear with a near octahedral cis-N₄O₂ co-ordination sphere; the presence of the phenolates stabilises the Co^III oxidation state by about one volt relative to [Co(terpy)₂]³⁺(terpy = 2,2′ : 6′,2″-terpyridine). The crystal structure of [Cu₂L₂(µ-MeCO₂)][PF₆]·1.5CH₂Cl₂ shows two near-planar CuL fragments stacked adjacent to each other such that the coplanar aromatic rings of the ligands are separated by an average of 3.54 Å, comparable to the stacking distances in graphite and nucleic acids. Both phenolate residues and the acetate bridge the two copper(II) centres, giving a Cu₂(µ-O)₂(µ-1,3-MeCO₂) core. By contrast the nickel(II) complex has the formulation [NiL₂]·HPF₆.