RESUMO
Recent work has shown that cyanide ligation increases the redox potentials of Fe(4)S(4) clusters, enabling the isolation of [Fe(4)S(4)(CN)4]4-, the first synthetic Fe(4)S(4) cluster obtained in the all-ferrous oxidation state (Scott, T. A.; Berlinguette, C. P.; Holm, R. H.; Zhou, H.-C. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 9741). The generality of reduced cluster stabilization has been examined with MoFe(3)S(4) clusters. Reaction of single-cubane [(Tp)MoFe(3)S(4)(PEt(3))3]1+ and edge-bridged double-cubane [(Tp)2Mo(2)Fe(6)S(8)(PEt(3))4] with cyanide in acetonitrile affords [(Tp)MoFe(3)S(4)(CN)3]2- (2) and [(Tp)2Mo(2)Fe(6)S(8)(CN)4]4- (5), respectively. Reduction of 2 with KC(14)H(10) yields [(Tp)MoFe(3)S(4)(CN)3]3- (3). Clusters were isolated in approximately 70-90% yields as Et(4)N+ or Bu(4)N+ salts; clusters 3 and 5 contain all-ferrous cores, and 3 is the first [MoFe(3)S(4)]1+ cluster isolated in substance. The structures of 2 and 3 are very similar; the volume of the reduced cluster core is slightly larger (2.5%), a usual effect upon reduction of cubane-type Fe(4)S(4) and MFe(3)S(4) clusters. Redox potentials and 57Fe isomer shifts of [(Tp)MoFe(3)S(4)L3]2-,3- and [(Tp)2Mo(2)Fe(6)S(8)L(4)]4-,3- clusters with L = CN-, PhS-, halide, and PEt3 are compared. Clusters with pi-donor ligands (L = halide, PhS) exhibit larger isomer shifts and lower (more negative) redox potentials, while pi-acceptor ligands (L = CN, PEt3) induce smaller isomer shifts and higher (less-negative) redox potentials. When the potentials of 3/2 and [(Tp)MoFe(3)S(4)(SPh)3]3-/2- are compared, cyanide stabilizes 3 by 270 mV versus the reduced thiolate cluster, commensurate with the 310 mV stabilization of [Fe(4)S(4)(CN)4]4- versus [Fe(4)S(4)(SPh)4]4- where four ligands differ. These results demonstrate the efficacy of cyanide stabilization of lower cluster oxidation states. (Tp = hydrotris(pyrazolyl)borate(1-)).
Assuntos
Cianetos/química , Ferro/química , Molibdênio/química , Enxofre/química , Modelos Moleculares , Oxirredução , Difração de Raios XRESUMO
Members of the cluster set [(Tp)2Mo2Fe6S8L4]z contain the core unit M2Fe6(mu3-S)6(mu4-S)2 in which two MoFe3S4 cubanes are coupled by two Fe-(mu4-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(mu4-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-)).