SOLUTION STUDIES ON THE CUBOIDAL MIXED-METAL COMPLEX [MO3FES4(H2O)10]4+

Solution properties of the title complex, prepared by reaction of Fe metal with [Mo3S4(H2O)9]4+ (preferred route) and characterized previously by X-ray crystallography, are reported. Alternative preparative routes involve NaBH4 or electrochemical reduction of [Mo3S4(H2O)9]4+ in the presence of Fe2+. Solutions of [Mo3FeS4(H2O),0]4+ purified by Dowex chromatography (normally in 2 M HC104), undergo air oxidation, 2Mo3FeS44+ + O2 + 4H+ → 2Mo3S44+ + 2Fe2+ + 2H20 (t,/2 ~ 30 min), and have to be stored air-free. A solid sample of [Mo3FeS4(H2O)10](p-CH3C6H4SO3)4-7H2O gave a magnetic susceptibility of 2.83 μ at 22 °C corresponding to two unpaired electrons. No EPR spectrum was observed at temperatures down to 4.2 K for solutions in 2.0 M HC104 (2 mM) or 2.0 M HC1 (10 mM) or for the edta complex (0.3 mM) [Mo3FeS4(edta)2]4~ at pH ~ 6.5. Zero-field Mössbauer spectra at 4.2 K give a chemical isomer shift (= 0.52 mm/s), quadrupole splitting (AEq = 0.22 mm/s), and linewidth (= 0.29 mm/s) that are consistent with Fe in oxidation state III, spin-coupled, to give an effective overall spin of zero. With Cl in the range 0-0.10 M, strong 1:1 complexing to yield [Mo3FeS4(H20)9Cl]3+, K = 560 M’1, is observed at 25 °C, 7 = 2.00 M. The reaction is fast (2 X 104 M1 s1) and is assigned as substitution at the tetrahedral Fe. Oxidation with [Co(dipic)2], Mo3FeS44+ + 2Com → Mo3S44+ + Fe2+ + 2Co2+, rate law ytCo[Mo3FeS44+][Com], ArCo(25 °C) = 87 M1 s1, is independent of [H+] in the range 0.5-1.8 M. With [Fe(H20)6]3+ as oxidant, the same stoichiometry and form of rate law are obtained, but in this case there is an [H+] dependence, k?t = a + [H+], with a = 4.8 M1 s1 and b = 4.0 s1. Path b is assigned as an inner-sphere reaction of [Fe(H20)50H]2+ at the more labile Fe site on the cluster. © 1990, American Chemical Society. All rights reserved.