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

Abstract

The occurence of a heteroatom X (C, N, or O) in the MoFe 7 S 9 X core of the iron-molybdenum cofactor of nitrogenase has encouraged synthetic attempts to prepare high nuclearity M-Fe-S-X clusters containing such atoms. We have previously shown that reaction of the edge-bridged double cubane [(Tp) 6 (μ 6 -S)] core topology very similar to the P N cluster of nitrogenase. The reaction system 1/Et 4 NOH in acetonitrile/methanol yields the P N -type cluster [(Tp) 2 Mo 2 Fe 6 S 9 (OMe) 2 (H 2 O)] 3− (5). The system 1/Me 3 SnOH/F − affords the oxo-bridged double P N -type cluster {[(Tp) 2 Mo 2 Fe 6 S 9 (μ 2 -O)] 2 } 5− (7), convertible to the oxidized cluster {[(Tp) 2 Mo 2 Fe 6 S 9 (μ 2 -O)] 2 } 4− (6), which is prepared independently from [(Tp) 2 Mo 2 Fe 6 S 9 F 2 (H 2 O)] 3− /(Bu 3 Sn) 2 O. In the preparations of 3-5 and 7, hydroxide liberates sulfide from 1 leading to the formation of P N -type clusters. Unlike reactions with HQ − , no oxygen atoms are integrated into the core structures of the products. However, the half-dimer composition [Mo 2 Fe 6 S 9 O] relates to the MoFe 7 S 9 consitution of the putative native cluster with X = O. (Tp = hydrotris(pyrazolyl) borate(1-)).

Stepwise formation of P-cluster in nitrogenase MoFe protein

“…Error is reported as F/no. of points, where F ϭ [⌺k 6 (exptl Ϫ calcd) 2 / ⌺k 6 exptl 2 ] (27). of the precursor (Fig.…”

“…Such a geometry suggests that the P-cluster is likely assembled by the fusion of two [Fe 4 S 4 ]-like subclusters (3). This reaction mechanism is well established in synthetic inorganic chemistry and successfully realized by the recent synthesis of P-cluster topologs (4)(5)(6). Biological evidence in this regard was supplied by a FeMoco-deficient form of MoFe protein (designated ⌬nifH MoFe protein), which was isolated from a nifHdeletion strain of A. vinelandii (7).…”

“…The structure of P-cluster can be viewed as a symmetric double cubane in which two [Fe 4 S 4 ] cubanes share a central 6 -sulfur (S) atom. Such a geometry suggests that the P-cluster is likely assembled by the fusion of two [Fe 4 S 4 ]-like subclusters (3).…”

Biosynthesis of the Metalloclusters of Molybdenum Nitrogenase

“…Such a reaction mechanism has been well established in synthetic chemistry (38) and successfully realized by the recent synthesis of P-cluster topologs (19,40,41,57,59). The biological evidence in this regard was supplied by the characterization of a MoFe protein isolated from the nifH deletion background of A. vinelandii (designated the ⌬nifH MoFe protein).…”

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

“…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. 150°.…”