No 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.

The Mössbauer spectra, magnetic susceptibilities, cyclic voltammograms, and electronic spectra of the dinuclear iron(II,III) complex, [Fe2(bpmp)(ena)2](BF4)2, were measured and the crystal structure was determined at 293 K, where Hbpmp represents 2,6-bis[bis(2-pyridylmethyl)aminoethyl]-4-methylphenol and Hena is heptanoic acid. The valence states of the irons of the complex are localized below about 200 K in the Mössbauer time scale (10−7 s), and are delocalized above 260 K. The activation energy of the electron interexchange between Fe2+ and Fe3+ was evaluated to be 15 kJ mol−1 at between 200 and 230 K based on an analysis of the Mössbauer spectra. The complex crystallizes in the monoclinic system, space group P2/c, with a = 10.129(4), b = 23.295(30), c = 11.324(7) Å, β = 104.55(4)°, dcalc = 1.34 g cm−3, Z = 2, and molecular formula Fe2F8O5N6C47B2H59. The structure was solved by the heavy-atom method and was refined anisotropically by the least-squares method, employing 1981 unique reflections with |F| > 4σ(Fo). The hydrogen atoms of bpmp were located and refined by an isotropic approximation. The final R and Rw were 7.0 and 8.2%, respectively. The Fe–Fe distance was 3.365 Å, the average Fe–O value 2.033 Å, the average Fe–N value 2.188 Å, and the Fe–O–Fe angle 112.3(4)°. These values support the idea that both iron atoms are in an averaged valence state at 293 K.