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]·1.5CH2Cl2and [CoL2][PF6]·MeCN

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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 CoIII, NiII and CuII 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 CoIII 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.

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