Syntheses and Characterization of Dinuclear Iron(II, II) and Iron(II, III) Complexes with a Dinucleating Ligand, 2,6-Bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenolate (1—)

Abstract

Abstract Dinuclear iron(II, II) complexes, [Fe2(L-py)(RCOO)2]BF4·nH2O (RCOO : CH3COO (1a) and C6H5COO (2a)), and two types of dinuclear mixed valence iron(II, III) complexes [Fe2(L-py)(RCOO)2](BF4)2·nH2O (RCOO : CH3COO (1b) and C6H5COO (2b)) and [Fe2(L-py)(CH3COO)(OH)](BF4)2·2H2O (3) were prepared, where L-py represents 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methyl-phenolate(1–). Only one of the two iron(II) ions in the iron(II, II) complexes is easily oxidized with molecular oxygen in methanol to form the mixed valence complexes, [Fe2(L-py)(RCOO)2](BF4)2·nH2O, which were characterized by chemical analyses and Mössbauer measurements. Magnetic susceptibility measurements over the temperature range 80–300 K revealed that high-spin iron(II) and iron(III) ions are antiferromagnetically coupled with the spin-exchange coupling constants J=−3–−8cm−1. The complexes exhibited an ESR signal of gav≈1.7 near liquid helium temperature. Such an ESR signal arises from an antiferromagnetically spin-coupled high-spin iron(II, III) dimer. An inter-valence charge-transfer transition was observed in the region 7000–8500 cm−1. The electron delocalization parameter α is estimated to be ca. 0.08, which corresponds to the class II mixed valence type in the classification of Robin and Day. Cyclic voltammogram of the complex 2b in acetonitrile showed two reversible couples at +0.05 and +0.73 V (vs. SCE), which were assigned to the redox reactions of Fe(II, III)/Fe(II, II) and Fe(III, III)/Fe(II, III), respectively. The comproportionation constants of 1b and 2b at 20 °C are 1.6×1012 and 5.0×1011, respectively, implying that the present mixed valence complexes are significantly stabilized.