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.