Synthesis of New [8Fe-7S] Clusters: A Topological Link between the Core Structures of P-Cluster, FeMo-co, and FeFe-co of Nitrogenases

A coordinatively unsaturated dinuclear iron(II) complex of bulky thiolates, [(TipS)Fe(μ-SDmp)]2
(1; Tip = 2,4,6-iPr3C6H2, Dmp = 2,6-(mesityl)2C6H3), was synthesized from stepwise reactions of
Fe{N(SiMe3)2}2 with 1 equiv of HSDmp and then with 1 equiv of HSTip. Complex 1 was found to react with
elemental sulfur (S8) in toluene to generate a new class of [8Fe-7S] cluster, [(DmpS)Fe4S3]2(μ-SDmp)2(μ-STip)(μ6-S) (2). The cluster 2 was also produced from one-pot reactions of Fe{N(SiMe3)2}2 + HSDmp +
HSTip + S8 (8:6:10:7/8) and Fe3{N(SiMe3)2}2(μ-STip)4 + HSDmp + S8 (8/3:16/3:7/8), where another [8Fe-7S] cluster, [(TipS)Fe4S3]2(μ-SDmp)2{μ-N(SiMe3)2}(μ6-S) (3), was also found as a minor byproduct. In either
of the clusters, two Fe4S3 incomplete cubane units are connected by three anionic ligands, namely three
thiolate S atoms for 2 or two thiolate S atoms and one amide N atom for 3, and one hexa-coordinate S
atom resides at the center of the [8Fe-7S] core. They have a common Fe(II)5Fe(III)3 oxidation states, and
an S = 1/2 ground spin state was indicated by rhombic EPR signals at 10 K with g = 2.19, 2.07, and 1.96
for 2 and g = 2.13, 2.06, and 1.93 for 3. The structural relevance of clusters 2 and 3 to P-cluster, FeMo-co,
and FeFe-co of nitrogenases is discussed.