Edge-Bridged Mo2Fe6S8 to PN-Type Mo2Fe6S9 Cluster Conversion: Structural Fate of the Attacking Sulfide/Selenide Nucleophile

Reaction of the edge-bridged double cubane cluster [(Tp)(2)M(2)Fe(6)S(8)(PEt(3))(4)] (1; Tp = hydrotris(pyrazolyl)borate(1-)) with hydrosulfide affords the clusters [(Tp)(2)M(2)Fe(6)S(9)(SH)(2)](3)(-)(,4)(-) (M = Mo (2), V), which have been established as the first structural (topological) analogues of the P(N) cluster of nitrogenase. The synthetic reaction is an example of core conversion, resulting in the transformation M(2)Fe(6)(mu(3)-S)(6)(mu(4)-S)(2) (C(i)) --> M(2)Fe(6)(mu(2)-S)(2)(mu(3)-S)(6)(mu(6)-S) (C(2)(v)), the reaction pathway of which is unknown. The most prominent structural feature of P(N)-type clusters is the mu(6)-S atom, which bridges six iron atoms in two MFe(3)S(3) cuboidal halves of the cluster. The initial issue in core conversion is the origin of the mu(6)-S atom. Utilizing SeH(-) as a surrogate reactant for SH(-) in the system 1/SeH(-)/L(-) in acetonitrile, a series of selenide clusters [(Tp)(2)Mo(2)Fe(6)S(8)SeL(2)](3)(-) (L(-) = SH(-) (4), SeH(-) (5), EtS(-) (6), CN(-) (7)) was prepared. The electrospray mass spectra of 4 and 6 revealed inclusion of one Se atom in each cluster, and (1)H NMR spectra and crystallographic refinements of 4-7 indicated that this atom was disordered over the two mu(2)-S/Se positions. The clusters {[(Tp)(2)Mo(2)Fe(6)S(9)](mu(2)-S)}(2)(5)(-) (8) and {[(Tp)(2)Mo(2)Fe(6)S(8)Se](mu(2)-Se)}(2)(5)(-) (9) were prepared from 2 and 5, respectively, and shown to be isostructural. They consist of two P(N)-type cluster units bridged by two mu(2)-S or mu(2)-Se atoms. It is concluded that, in the preparation of 2, the probable structural fate of the attacking nucleophile is as a mu(2)-S atom, and that the mu(3)-S and mu(6)-S atoms of the product cluster derive from precursor cluster 1. Cluster fragmentation during P(N)-type cluster synthesis is unlikely.