SPIN-STATE REGULATION OF IRON(III) CENTERS BY AXIAL LIGANDS WITH TETRADENTATE BIS(PICOLINAMIDE) INPLANE LIGANDS

Mononuclear iron(III) complexes trans-[Fe(bpb)X2]+/- [H2bpb=1,2-bis(py ridine-2-carboxamido)benzene; X = pyridine (py) 1, N3- 2. MeCO2- 3 or CN- 4] and trans- [Fe(bpc)X2]+/- [H2bpc = 5-dichloro-1,2-bis(pyridine- 2-carboxamido)benzene; X = py 5, Cl- 6 or MeCO2- 7] have been synthesi sed and characterized. Complexes 1 and 4 are low-spin (mu(eff) = 2.50 and 2.1 0 at 298 K, solution phase) whereas 2, 3, 6 and 7 (mu(eff) = 5 .86-5.98 at 298 K, solution phase) are high spin. Solid-state magnetic susceptibility (40-300 K) and Mossbauer spectral measurements at 300 and 77 K of 1 and 7 confirmed the spin state. The complex [NEt4][Fe(bp c)(MeCO2)2].CHCl3 7 has been characterized by X-ray crystallography: s pace group P2(1)/c, a = 12.283(3), b = 18.819(3), c = 16.437(3) angstr om, beta = 101.02(2)-degrees, Z = 4, R = R' = 0.041 for 5069 observed reflections. The solid-state EPR spectra (300 and 77 K) of 1, 4 and 6 are typical of low-spin iron(III) and have been analysed to determine the ligand-field parameters. All the complexes display ligand-to-metal charge-transfer transitions in the visible region (550-800 nm). Cycli c voltammetric measurements of 1 in pyridine and 4 in dimethylformamid e revealed a quasi-reversible Fe(III)-Fe(II) reduction (E1/2 = -0.06 V , DELTAE(p) = 150 mV for 1 and E1/2 = -0.83. V, DELTAE(p) = 1 00 mV fo r 4 vs. saturated calomel electrode (SCE)]. The high-spin complexes ex hibit an electrochemically irreversible but chemically reversible Fe(I II)-Fe(II) reduction (E(pc) = -0.44 to -0.83 V) in acetonitrile soluti on. Complexes 2,4 and 6 show a reversible one-electron oxidation (E1/2 = +0.66 to +1.05 V vs. SCE).