Stabilization of Reduced Molybdenum−Iron−Sulfur Single- and Double-Cubane Clusters by Cyanide Ligation

Recent work has shown that cyanide ligation increases the redox potentials of Fe4S4 clusters, enabling the isolation
of [Fe4S4(CN)4]4-, the first synthetic Fe4S4 cluster obtained in the all-ferrous oxidation state (Scott, T. A.; Berlinguette,
C. P.; Holm, R. H.; Zhou, H.-C. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The generality of reduced cluster
stabilization has been examined with MoFe3S4 clusters. Reaction of single-cubane [(Tp)MoFe3S4(PEt3)3]1+ and edge-bridged double-cubane [(Tp)2Mo2Fe6S8(PEt3)4] with cyanide in acetonitrile affords [(Tp)MoFe3S4(CN)3]2- (2) and
[(Tp)2Mo2Fe6S8(CN)4]4- (5), respectively. Reduction of 2 with KC14H10 yields [(Tp)MoFe3S4(CN)3]3- (3). Clusters
were isolated in ∼70−90% yields as Et4N+ or Bu4N+ salts; clusters 3 and 5 contain all-ferrous cores, and 3 is the
first [MoFe3S4]1+ cluster isolated in substance. The structures of 2 and 3 are very similar; the volume of the reduced
cluster core is slightly larger (2.5%), a usual effect upon reduction of cubane-type Fe4S4 and MFe3S4 clusters.
Redox potentials and 57Fe isomer shifts of [(Tp)MoFe3S4L3]2-,3- and [(Tp)2Mo2Fe6S8L4]4-,3- clusters with L =
CN-, PhS-, halide, and PEt3 are compared. Clusters with π-donor ligands (L = halide, PhS) exhibit larger isomer
shifts and lower (more negative) redox potentials, while π-acceptor ligands (L = CN, PEt3) induce smaller isomer
shifts and higher (less-negative) redox potentials. When the potentials of 3/2 and [(Tp)MoFe3S4(SPh)3]3-/2- are
compared, cyanide stabilizes 3 by 270 mV versus the reduced thiolate cluster, commensurate with the 310 mV
stabilization of [Fe4S4(CN)4]4- versus [Fe4S4(SPh)4]4- where four ligands differ. These results demonstrate the
efficacy of cyanide stabilization of lower cluster oxidation states. (Tp = hydrotris(pyrazolyl)borate(1−)).

近期研究表明，氰基配位可提升四铁四硫（Fe4S4）簇的氧化还原电位，成功分离出[Fe4S4(CN)4]4-——这是首例以全亚铁氧化态得到的合成四铁四硫簇（Scott, T. A.; Berlinguette, C. P.; Holm, R. H.; Zhou, H.-C. 《美国国家科学院院刊》, 2005, 102, 9741）。
科研人员以钼铁三硫（MoFe3S4）簇为模型，考察了还原态簇合物稳定化效应的普适性。将单立方烷型[(三吡唑基硼酸盐(1−)（hydrotris(pyrazolyl)borate(1−), 缩写Tp）)MoFe3S4(三乙基膦(PEt3))3]1+与桥边双立方烷型[(Tp)2Mo2Fe6S8(PEt3)4]在乙腈中与氰化物反应，分别得到[(Tp)MoFe3S4(CN)3]2-（化合物2）与[(Tp)2Mo2Fe6S8(CN)4]4-（化合物5）。用KC14H10还原化合物2，得到[(Tp)MoFe3S4(CN)3]3-（化合物3）。
所有簇合物均以四乙基铵（Et4N+）或四丁基铵（Bu4N+）盐的形式分离得到，产率约为70%~90%；其中化合物3与5的簇核均为全亚铁结构，而化合物3是首例分离得到的[MoFe3S4]1+型簇合物。
化合物2与3的结构极为相似，还原态簇核的体积略增2.5%，这是立方烷型Fe4S4与MFe3S4簇合物还原后的典型现象。
本文对比了配体L为CN-、苯硫基（PhS-）、卤离子与三乙基膦（PEt3）的[(Tp)MoFe3S4L3]2-,3-与[(Tp)2Mo2Fe6S8L4]4-,3-簇合物的氧化还原电位与57Fe同质异能位移。带有π给体配体（如卤离子、苯硫基）的簇合物展现出更大的同质异能位移与更低（更负）的氧化还原电位；而π受体配体（如CN-、三乙基膦）则会导致更小的同质异能位移与更高（负性更弱）的氧化还原电位。
对比化合物3/2与[(Tp)MoFe3S4(SPh)3]3-/2-的氧化还原电位可知，相较于还原态硫醇盐簇，氰基使化合物3的电位升高了270 mV；这与[Fe4S4(CN)4]4-相较于[Fe4S4(SPh)4]4-的310 mV电位提升相符——两者的四个配体均不相同。
上述结果证实了氰基可有效稳定低氧化态簇合物。（注：Tp = 三吡唑基硼酸盐(1−)，英文全称为hydrotris(pyrazolyl)borate(1−)）