Redox and spin state control of Co(II) and Fe(II) N-heterocyclic complexes

Two series of complexes with the general formulas [Co(L)2]2+ and [Fe(L)2]2+, where L represents the tridentating ligands 2,2′:6′;2′′-terpyridine (L1), 6-(N-pyrazolyl)-2,2′-bipyridine (L2); and 2,6-bis(N-pyrazolyl)pyridine (L3), were prepared and characterized. The electronic properties of both series of compounds were investigated as a function of stepwise variation in coordinated pyridines relative to pyrazoles. In the cobalt series, the Co(III/II) redox potential measured an average anodic shift of 0.16 V for each pyridine coordination site replaced by pyrazole, while only a slight shift was observed for the Fe(III/II) couple. Solution magnetic susceptibility measurements showed a general shift toward a high-spin ground state as pyridine was replaced by coordinated pyrazoles. The cobalt series exhibited a μeff of 3.2 B.M. for bis-terpy (L1) which is close to the spin crossover, while the μeff for [Co(L2)2]2+ and [Co(L3)2]2+ were measured at 4.5 and 4.6 B.M., respectively. For the iron series, [Fe(L1)2]2+ and Fe(L2)2]2+ exhibited a stable diamagnetic low-spin state; however, [Fe(L3)2]2+ measured a μeff of 4.6 B.M. which implies a spin equilibrium that is predominantly high-spin with possible low-spin contribution. The observed redox and spin state regulation correlate with the weaker σ-donor and π-acceptor properties of coordinated pyrazole relative to pyridine.