Iron(II) and cobalt(II) complexes of tris-azinyl analogues of 2,2':6',2'-terpyridine

The syntheses of 2,6-di(pyrid-2-yl)pyrazine (L(1)), 2,6-di(pyrazinyl)pyridine (L(2)), 2,2':6',2''-terpyrazine (L(3)), 2,6-di(pyrimidin-4-yl)pyridine (L(4)), 2,6-di(1,2,4-triazin-3-yl)pyridine (L(5)), 4-hydroxy-2,6-di(pyrazinyl)pyridine (L(6)) and 4-hydroxy-2,6-di(pyrimidin-2-yl)pyridine (L(7)) are described. Homoleptic iron(II) and cobalt(II) complexes of these ligands have been prepared and, in four cases, structurally characterised. The iron complexes are all low-spin. However, while the cobalt complexes of the pyrazine-rich ligands L(2), L(3) and L(6) are all predominantly low-spin in the solid state, the other cobalt complexes are essentially high-spin between 5-300 K. The voltammetric M(III)/(II) (M = Fe or Co) oxidations and metal- or ligand-based reductions all become more anodic as the nitrogen content of the ligands increases, which correlates well with Lever's additive electrochemical parameters for the heterocyclic donor groups in each complex.