On [Fe₄S₄]²⁺−(μ₂-SR)−M^II Bridge Formation in the Synthesis of an A-Cluster Analogue of Carbon Monoxide Dehydrogenase/Acetylcoenzyme A Synthase

The construction of a synthetic analogue of the A-cluster of carbon monoxide dehydrogenase/acetylcoenzyme synthase, the site of acetylcoenzyme A formation, requires as a final step the formation of an unsupported [Fe₄S₄]−(μ₂-SR)−Ni^II bridge to a preformed cluster. Our previous results (Rao, P. V.; Bhaduri, S.; Jiang, J.; Holm, R. H. Inorg. Chem. 2004, 43, 5833) and the work of others have addressed synthesis of dinuclear complexes relevant to the A-cluster. This investigation concentrates on reactions pertinent to bridge formation by examining systems containing dinuclear and mononuclear Ni^II complexes and the 3:1 site-differentiated clusters [Fe₄S₄(LS₃)L‘]^2- (L‘ = TfO^- (14), SEt (15)). The system 14/[{Ni(L_O-S₂N₂)}M(SCH₂CH₂PPh₂)]⁺ results in cleavage of the dinuclear complex and formation of [{Ni(L_O-S₂N₂)}Fe₄S₄(LS₃)]^- (18), in which the Ni^II complex binds at the unique cluster site with formation of a Ni(μ₂-SR)₂Fe bridge rhomb. Cluster 18 and the related species [{Ni(phma)}Fe₄S₄(LS₃)]^3- (19) are obtainable by direct reaction of the corresponding cis-planar Ni^II−S₂N₂ complexes with 14. The mononuclear complexes [M(pdmt)(SEt)]^- (M = Ni^II, Pd^II) with 14 in acetonitrile or Me₂SO solution react by thiolate transfer to give 15 and [M₂(pdmt)₂]. However, in dichloromethane the Ni^II reaction product is interpreted as [{Ni(pdmt)(μ₂-SEt)}Fe₄S₄(LS₃)]^2- (20). Reaction of Et₃NH⁺ and 15 affords the double cubane [{Fe₄S₄(LS₃)}₂(μ₂-SEt)]^3- (21). Cluster 18 contains two mutually supportive Fe−(μ₂-SR)−Ni^II bridges, 19 exhibits one strong and one weaker bridge, 20 has one unsupported bridge (inferred from the ¹H NMR spectrum), and 21 has one unsupported Fe−(μ₂-SR)−Fe bridge. Bridges in 18, 19, and 21 were established by X-ray structures. This work demonstrates that a bridge of the type found in the enzyme A-clusters is achievable by synthesis and implies that more stable, unsupported single thiolate bridges may require reinforcement by an additional covalent linkage between the Fe₄S₄ and nickel-containing components. (LS₃ = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3−); L_O-S₂N₂ = N,N‘-diethyl-3,7-diazanonane-1,9-dithiolate(2−); pdmt = pyridine-2,6-methanedithiolate(2−); phma = N,N‘-1,2-phenylenebis(2-acetylthio)acetamidate(4−); TfO = triflate.)