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

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journal contribution
posted on 16.02.2005, 00:00 by P. Venkateswara Rao, Sumit Bhaduri, Jianfeng Jiang, Daewon Hong, R. H. Holm
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)−NiII 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 NiII complexes and the 3:1 site-differentiated clusters [Fe4S4(LS3)L‘]2- (L‘ = TfO- (14), SEt (15)). The system 14/[{Ni(LO-S2N2)}M(SCH2CH2PPh2)]+ results in cleavage of the dinuclear complex and formation of [{Ni(LO-S2N2)}Fe4S4(LS3)]- (18), in which the NiII 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 NiII−S2N2 complexes with 14. The mononuclear complexes [M(pdmt)(SEt)]- (M = NiII, PdII) with 14 in acetonitrile or Me2SO solution react by thiolate transfer to give 15 and [M2(pdmt)2]. However, in dichloromethane the NiII reaction product is interpreted as [{Ni(pdmt)(μ2-SEt)}Fe4S4(LS3)]2- (20). Reaction of Et3NH+ and 15 affords the double cubane [{Fe4S4(LS3)}22-SEt)]3- (21). Cluster 18 contains two mutually supportive Fe−(μ2-SR)−NiII 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−); LO-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.)