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

Reaction of the edge-bridged double cubane cluster [(Tp)2M2Fe6S8(PEt3)4] (1; Tp = hydrotris(pyrazolyl)borate(1−)) with hydrosulfide affords the clusters [(Tp)2M2Fe6S9(SH)2]3-,4- (M = Mo (2), V), which have been established as the first structural (topological) analogues of the PN cluster of nitrogenase. The synthetic reaction is an example of core conversion, resulting in the transformation M2Fe6(μ3-S)6(μ4-S)2 (Ci) → M2Fe6(μ2-S)2(μ3-S)6(μ6-S) (C2v), the reaction pathway of which is unknown. The most prominent structural feature of PN-type clusters is the μ6-S atom, which bridges six iron atoms in two MFe3S3 cuboidal halves of the cluster. The initial issue in core conversion is the origin of the μ6-S atom. Utilizing SeH- as a surrogate reactant for SH- in the system 1/SeH-/L- in acetonitrile, a series of selenide clusters [(Tp)2Mo2Fe6S8SeL2]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 1H NMR spectra and crystallographic refinements of 4−7 indicated that this atom was disordered over the two μ2-S/Se positions. The clusters {[(Tp)2Mo2Fe6S9](μ2-S)}25- (8) and {[(Tp)2Mo2Fe6S8Se](μ2-Se)}25- (9) were prepared from 2 and 5, respectively, and shown to be isostructural. They consist of two PN-type cluster units bridged by two μ2-S or μ2-Se atoms. It is concluded that, in the preparation of 2, the probable structural fate of the attacking nucleophile is as a μ2-S atom, and that the μ3-S and μ6-S atoms of the product cluster derive from precursor cluster 1. Cluster fragmentation during PN-type cluster synthesis is unlikely.