Two types of dinuclear high-spin mixed valence iron(II,III) complexes, [Fe2(L-Bzim)(RCOO)2](BF4)2·nH2O (RCOO=CH3COO (1) or C6H5COO (2)), and [Fe2(L-Bzim)(C6H5COO)2(OH)]BF4·H2O (3), and dinuclear high-spin iron(III,III) complexes, [Fe2(L-Bzim)(CH3COO)2](ClO4)3·3.5H2O (4) and [Fe2(L-py)(C6H5COO)2](ClO4)3·CH3CN·H2O (5) were prepared, where L-Bzim is 2,6-bis[bis(2-benzimidazolylmethyl)aminomethyl]-4-methylphenolate(1—) and L-py is 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenolate(1—). Mössbauer spectra of the mixed valence complexes revealed that they involve high-spin iron(II) and (III) ions in equimolar amount. Their ESR spectra exhibited a signal at gav=1.7 near liquid helium temperature characteristic of an antiferromagnetically spin coupled high-spin iron(II,III) dimer. Magnetic susceptibility measurements over the temperature range 80–300 K revealed that weak antiferromagnetic spin-exchange interactions are present between iron(II) and iron(III) ions (J≈−5 cm−1). The complexes showed an intervalence absorption band in the near infrared region, indicating that they belong to class II mixed valence type on the classification of Robin and Day. Cyclic voltammogram of 3 in acetonitrile showed two sets of reversible redox couples at 0.00 and 0.66 V vs. SCE, which are assigned to the redox reactions of Fe(II,III)/Fe(II,II) and Fe(III,III)/Fe(II,III), respectively. The complexes 4 and 5 exhibited weak antiferromagnetic interaction (J=−9–−12 cm−1). ESR spectra of 4 and 5 showed a significant temperature dependence.

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.

No abstract

No abstract

No abstract