Heterometal Cubane-Type MFe3S4Clusters (M = Mo, V) Trigonally Symmetrized with Hydrotris(pyrazolyl)borate(1−) and Tris(pyrazolyl)methanesulfonate(1−) Capping Ligands

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Abstract
A series of heterometal cubane-type clusters containing [VFe3S4]2+ and [MoFe3S4]3+,2+ cores has been prepared. Ligand substitution of [(DMF)3VFe3S4Cl3]- affords [(Tpms)VFe3S4L3]2- (L = Cl- (8), EtS- (9), p-MeC6H4S-, p-MeC6H4O-). A new procedure for the preparation of molybdenum single cubanes is introduced by the reaction of recently reported [(Tp)MoS(S4)]- with FeCl2/NaSEt to afford [(Tp)MoFe3S4Cl3]- (1, 75% yield). This procedure is more efficient that the existing multistep synthesis of single cubanes, which generally affords clusters of mirror symmetry. Also prepared were [(Tp)MoFe3S4L3]- (L = EtS- (2), p-MeC6H4S-). Reduction of 1 with borohydride gives [(Tp)MoFe3S4Cl3]2- (5, 67%). Owing to the nature of the heterometal ligand, all clusters have idealized trigonal symmetry, reflected in their 1H NMR spectra. Trigonal structures are demonstrated by crystallography of (Bu4N)[1,2], (Bu4N)2[5]·MeCN, and (Me4N)2[8,9]. The availability of 1 and 5 allows the first comparison of structures and 57Fe isomer shifts of [MoFe3S4]3+,2+ in a constant ligand environment. Small increases in most bond distances indicate that an antibonding electron is added in the reduction of 1. Collective synthetic and electrochemical results from this and other studies demonstrate the existence of the series of oxidation states [VFe3S4]3+,2+,1+ and [MoFe3S4]4+,3+,2+ whose relative stabilities within a given series are strongly ligand dependent. Isomer shifts indicate that the reduction of 1 largely affects the Fe3 subcluster and are consistent with the formal descriptions [MoFe3+2Fe2+S4]3+ (1) and [MoFe3+Fe2+2S4]2+ (5). Reaction of 1 with excess Li2S in acetonitrile affords the double cubane {[(Tp)MoFe3S4Cl2]2(μ2-S)}2-, whose sulfide-bridged structure is supported by two sequential reductions separated by 290 mV, in analogy with previously reported double cubanes of higher charge. Trigonally symmetric single cubanes eliminate isomers in the formation of double cubanes and other cluster structures, and may be of considerable value in the preparation of new types of M−Fe−S clusters. (Tpms = tris(pyrazolyl)methanesulfonate(1−); Tp = hydrotris(pyrazolyl)borate(1−).)

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