Precursors to Clusters with the Topology of the PN Cluster of Nitrogenase: Edge-Bridged Double Cubane Clusters [(Tp)2Mo2Fe6S8L4]z: Synthesis, Structures, and Electron Transfer Series

Abstract

Members of the cluster set [(Tp)₂Mo₂Fe₆S₈L₄]^z contain the core unit M₂Fe₆(μ₃-S)₆(μ₄-S)₂ in which two MoFe₃S₄ cubanes are coupled by two Fe−(μ₄-S) interactions to form a centrosymmetric edge-bridged double cubane cluster. Some of these clusters are synthetic precursors to [(Tp)₂Mo₂Fe₆S₉L₂]^3-, which possess the same core topology as the P^N cluster of nitrogenase. In this work, the existence of a three-member electron-transfer series of single cubanes [(Tp)MoFe₃S₄L₃]^z (z = 3−, 2−, 1−) and a four-member series of double cubanes [(Tp)₂Mo₂Fe₆S₈L₄]^z (z = 4−, 3−, 2−, 1−) with L = F^-, Cl^-, N₃, PhS^- is demonstrated by electrochemical methods, cluster synthesis, and X-ray structure determinations. The potential of the [4−/3−] couple is extremely low (<−1.5 V vs SCE in acetonitrile) such that the 4− state cannot be maintained in solution under normal anaerobic conditions. The chloride double cubane redox series was examined in detail. The members [(Tp)₂Mo₂Fe₆S₈Cl₄]^4-,3-,2- were isolated and structurally characterized. The redox series includes the reversible steps [4−/3−] and [3−/2−]. Under oxidizing conditions, [(Tp)₂Mo₂Fe₆S₈Cl₄]^2- cleaves with the formation of single cubane [(Tp)MoFe₃S₄Cl₃]^1-. The quasireversible [2−/1−] couple is observed at more positive potentials than those of the single cubane redox step. Structure comparison of nine double cubanes suggests that significant dimensional changes pursuant to redox reactions are mainly confined to the Fe₂(μ₄-S)₂ bridge rhomb. The synthesis and structure of [(Tp)₂Mo₂Fe₆S₉F₂·H₂O]^3-, a new topological analogue of the P^N cluster of nitrogenase, is described. (Tp = hydrotris(pyrazolyl)borate(1-)).