Syntheses, structures, bonding, and redox behavior of 1,4-bis(ferrocenyl)butadiyne coordinated osmium clusters

The reaction of Os-3(CO)(n)(NCMe) (1) with 1,4-bis(ferrocenyl)butadiyne (3) yielded two products: Os-3(CO)(10)(mu(3)-eta(2)-FcC(4)Fc) (4) and Os-3(CO)(11)(mu(3)-eta(4)-FcC(4)Fc) (5). Compound 4 was obtained in better yield (94%) from the reaction of Os-3(CO)(10)(NCMe)(2) (2) with 3 at 67 degreesC. Compound 4 contains a bis(ferrocenyl)butadiyne ligand coordinated to a triangular triosmium cluster via one of its alkyne groups in a triply bridging fashion. Compound 5 contains an open triosmium cluster with the two C-C triple bonds coordinated in a parallel fashion to the three metal atoms. Compound 2 reacts with 3 at 97 degreesC to yield the compound Os-2(CO)(6)-(mu-eta(4)-FcC(2)Cequivalent toCFc)(2) (6) as a result of cluster fragmentation. Compound 6 exhibits a ferrole-type structure formed by the coupling two molecules of diyne 3. At 97 degreesC 4 is transformed into the new compound Os-3(CO)(9)(mu(3)-eta(2)-C=CFc)(mu-eta(2)-Cequivalent toCFc) (7), which contains two bridging ferrocenylacetylide ligands formed by cleaving the C-C bond between the two alkyne groups. All four products were characterized by IR, H-1 NMR, and single-crystal X-ray diffraction analyses. The two ferrocenyl units in 4, 6, and 7 are inequivalent. The redox potentials of the ferrocenyl groups in 4 are very closely spaced, DeltaE(p) = 0.057 V, suggesting that there is little electrocommunication between them. The redox potentials of the ferrocenyl groups in 6 are widely spaced, probably due to their intrinsic inequivalence. The two ferrocenyl units in 5 are equivalent but show two resolved one-electron redox processes, DeltaE(p) = 0.184 V, which indicates that there is significant electrocommunication between them. This is attributed to greater interactions in the pi-orbital network in the ligand 3 induced by coordination to the metal atoms. A molecular orbital model based on a butatrienediyl group is proposed to explain the coordination and pi-bonding in the 1,4-bis(ferrocenyl)butadiyne ligand in 5.