Molybdenum−Iron−Sulfur Clusters of Nuclearities Eight and Sixteen, Including a Topological Analogue of the P-Cluster of Nitrogenase

Transformations of the edge-bridged double cubane cluster [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4] (1) under reducing
conditions have been investigated as synthetic approaches to the clusters of nitrogenase. Cluster 1 is a versatile
precursor to different Mo−Fe−S cluster types. The reaction system 1/K(C14H10) in THF yields the reduced cluster
[(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4]1- (2), which as its crystalline Et4N+ salt retains the edge-bridged structure of
1. X-ray structural and Mössbauer spectroscopic results indicate an unsymmetrical electron distribution with
localized [MoFe3S4]2+,1+ cubane-type units. The system 1/2K(C14H10)/2HS- in THF/acetonitrile affords
[(Cl4cat)4(Et3P)4Mo4Fe12S20K3(DMF)]5- (3), whose structure was determined as the Ph3PMe+ salt. The cluster
consists of two isostructural Mo2Fe6S9 fragments connected by two μ2-S bridges. Three potassium ions are bound
between the two fragments. In each fragment, the iron atoms are present in tetrahedral FeS4 and the molybdenum
atoms in octahedral MoO2PS3 coordination units, and two MoFe3(μ3-S)3 cuboidal units are bridged by a common
μ6-S atom. The fragments have idealized mirror symmetry and are isostructural with two of the fragments present
in the previously reported high-nuclearity cluster [(Cl4cat)6(Et3P)6Mo6Fe20S30]8- (4) (Osterloh, F.; Sanakis, Y.;
Staples, R. J.; Münck, E.; Holm, R. H. Angew. Chem., Int. Ed. Engl. 1999, 38, 2066). On the basis of overall
shape, atom connectivities, and metric features, the Mo2Fe6S9 fragment is a topological analogue of the P-cluster
of nitrogenase in the PN (reduced) state. A third cluster type, formed as a minor byproduct in the reaction system
leading to 2, was crystallographically identified as [(Cl4cat)2(Et3P)2Mo2Fe6S8(PEt3)4]4-, whose core is made up
of two MoFe3(μ3-S)3 cuboidal units bridged by two μ2-S atoms and connected by a direct Fe−Fe bond. Full
structural details and the redox properties of 2 and 3 are reported.