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(micro-SDmp)]2 (1; Tip = 2,4,6-(i)Pr(3)C(6)H(2), Dmp = 2,6-(mesityl)(2)C(6)H(3)), was synthesized from stepwise reactions of Fe{N(SiMe(3))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)Fe(4)S(3)]2(micro-SDmp)2(micro-STip)(micro(6)-S) (2). The cluster 2 was also produced from one-pot reactions of Fe{N(SiMe(3))2}2 + HSDmp + HSTip + S8 (8:6:10:7/8) and Fe3{N(SiMe(3))2}2(micro-STip)4 + HSDmp + S8 (8/3:16/3:7/8), where another [8Fe-7S] cluster, [(TipS)Fe(4)S(3)]2(micro-SDmp)2{micro-N(SiMe(3))2}(micro(6)-S) (3), was also found as a minor byproduct. In either of the clusters, two Fe(4)S(3) 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)(5)Fe(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.