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 "-terpyridi ne (L-1), 6-(N-pyrazolyl)-2,2'-bipyridine (L-2); and 2,6-bis(N-pyrazolyl)py ridine (L-3), were prepared and characterized. The electronic properties of both series of compounds were investigated as a function of stepwise varia tion 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 f or each pyridine coordination site replaced by pyrazole, while only a sligh t shift was observed for the Fe(III/II) couple. Solution magnetic susceptib ility measurements showed a general shift toward a high-spin ground state a s pyridine was replaced by coordinated pyrazoles. The cobalt series exhibit ed a mu(eff) of 3.2 B.M. for bis-terpy (L-1) which is close to the spin cro ssover, while the mu(eff) for [Co(L-2)(2)](2+) and [Co(L-3)(2)](2+) were me asured at 4.5 and 4.6 B.M., respectively. For the iron series, [Fe(L-1)(2)] (2+) and Fe(L-2)(2)](2+) exhibited a stable diamagnetic low-spin state; how ever, [Fe(L-3)(2)](2+) measured a mu(eff) of 4.6 B.M. which implies a spin equilibrium that is predominantly high-spin with possible low-spin contribu tion. The observed redox and spin state regulation correlate with the weake r sigma-donor and pi-acceptor properties of coordinated pyrazole relative t o pyridine. (C) 2000 Elsevier Science S.A. All rights reserved.