High-Nuclearity Sulfide-Rich Molybdenum−Iron−Sulfur Clusters: Reevaluation and Extension

High-nuclearity Mo−Fe−S clusters are of interest as potential synthetic precursors to the MoFe7S9 cofactor cluster
of nitrogenase. In this context, the synthesis and properties of previously reported but sparsely described trinuclear
[(edt)2M2FeS6]3- (M = Mo (2), W (3)) and hexanuclear [(edt)2Mo2Fe4S9]4- (4, edt = ethane-1,2-dithiolate; Zhang,
Z.; et al. Kexue Tongbao 1987, 32, 1405) have been reexamined and extended. More accurate structures of 2−4
that confirm earlier findings have been determined. Detailed preparations (not previously available) are given for
2 and 3, whose structures exhibit the C2 arrangement {[(edt)M(S)(μ2-S)2]2FeIII}3- with square pyramidal Mo(V)
and tetrahedral Fe(III). Oxidation states follow from 57Fe Mössbauer parameters and an S = 3/2 ground state from
the EPR spectrum. The assembly system 2/3FeCl3/3Li2S/nNaSEt in methanol/acetonitrile (n = 4) affords (R4N)4[4]
(R = Et, Bu; 70−80%). The structure of 4 contains the [Mo2Fe4(μ2-S)6(μ3-S)2(μ4-S)]0 core, with the same bridging
pattern as the [Fe6S9]2- core of [Fe6S9(SR)2]4- (1), in overall C2v symmetry. Cluster 4 supports a reversible three-member electron transfer series 4−/3−/2− with E1/2 = −0.76 and −0.30 V in Me2SO. Oxidation of (Et4N)4[4] in
DMF with 1 equiv of tropylium ion gives [(edt)2Mo2Fe4S9]3- (5) isolated as (Et4N)3[5]·2DMF (75%). Alternatively,
the assembly system (n = 3) gives the oxidized cluster directly as (Bu4N)3[5] (53%). Treatment of 5 with 1 equiv
of [Cp2Fe]1+ in DMF did not result in one-electron oxidation but instead produced heptanuclear [(edt)2Mo2Fe5S11]3-
(6), isolated as the Bu4N+salt (38%). Cluster 6 features the previously unknown core Mo2Fe5(μ2-S)7(μ3-S)4 in molecular
C2 symmetry. In 4−6, the (edt)MoS3 sites are distorted trigonal bipramidal and the FeS4 sites are distorted tetrahedral
with all sulfide ligands bridging. Mössbauer spectroscopic data for 2 and 4−6 are reported; (mean) iron oxidation
states increase in the order 4 < 5 ≈ 1 < 6 ≈ 2. Redox and spectroscopic data attributed earlier to clusters 2 and
4 are largely in disagreement with those determined in this work. The only iron and molybdenum−iron clusters with
the same sulfide content as the iron−molybdenum cofactor of nitrogenase are [Fe6S9(SR)2]4- and [(edt)2Mo2Fe4S9]3-,4-.