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

Abstract

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.

Developments in the Biomimetic Chemistry of Cubane-Type and Higher Nuclearity Iron–Sulfur Clusters

“…The foregoing clusters may be distinguished from others such as the diclusters 75 104,105,111,112,128 and the tricluster 76 . 138 whose subclusters are bridged at formerly terminal positions by μ 2 -Q ligands.…”

“…The molybdenum cluster is presumably also generated in the 4− state but is isolated in the 3− state formulated as [Mo 3− 2 Fe 2+ 5 Fe 3+ S 9 ] 1+ , again a consequence of a very low redox potential ( E 4−/3− = −1.80 V). The core supports terminal ligand replacement with ligands that maintain low cluster redox potentials 111 such as RS − , CN − , and OH − . 112 The signal structural feature of these clusters is a μ 6 -S atom, highly unusual but not unprecedented in molecular compounds, acting as a vertex in a structural element with a large external Fe-S-Fe angle of ca.…”

“…It utilizes the pronounced chemical similarity between sulfur and selenium in the same oxidation state and their ready distinction by crystallography. 111 When the reaction is conducted with three equiv of hydroselenide, selenium is incorporated in the core of product 62 . The mass spectrum of [(Tp) 2 Mo 2 Fe 6 S 8 Se(SEt) 2 ] 3− (derived from 62 ) discloses one selenide per cluster that, from X-ray structure analyis, is disordered over the two doubly bridging positions.…”

“…It was further shown that selenium incorporation does not occur by Se/S exchange of all-sulfide 41b and hydroselenide. Consequently, we conclude that “the probable structural fate of the attacking sulfide nucleophile is as a μ 2 -S bridging atom in the P N -type topology.” 111 …”

Hydroxide-Promoted Core Conversions of Molybdenum−Iron−Sulfur Edge-Bridged Double Cubanes: Oxygen-Ligated Topological PN Clusters

“…10 The values of θand bridge angle β = Fe-Q-Fe follow the usual periodic trend for atoms Q, while also the larger values for Q = O and the linear bridges in the planar ring of 6 presumably arise from Fe-O π-bonding contributions. In solution, the clusters are readily distingushed by the 1 H NMR spectra (Figure 3).…”