Syntheses, Structures, and Electrochemical Properties of Os₃(CO)₉_-<i>_n</i>(CNCH₂Ph)<i>_n</i>(μ₃-η²:η²:η²-C₆₀) (<i>n</i> = 2−4)

Benzyl isocyanide-substituted C60-triosmium complexes, Os3(CO)9-n(CNCH2Ph)n(μ3-η2:η2:η2-C60) (n = 2 (3), 3 (4), and 4 (5)), have been prepared by reactions of either Os3(CO)9(μ3-η2:η2:η2-C60) (1) or its lesser isocyanide-substituted complexes with appropriate amounts of (triphenylphosphino)benzylimine (PhCH2NPPh3). Compounds 3−5 have been characterized by spectroscopic (IR, MS, 1H and 13C NMR) methods, cyclic voltammetry, and X-ray diffraction studies. Single-crystal X-ray diffraction studies reveal that isomer 3a has two inequivalent equatorial isocyanide ligands as a cis,trans-1,2-isomer and isomer 4a has three equivalent equatorial isocyanide ligands as a 1,2,3-isomer with C3 symmetry. In compound 5, one benzyl isocyanide is axially coordinated to an osmium atom, whereas the other three benzyl isocyanide ligands are equatorially coordinated to each osmium atom. 1H and 13C NMR data, however, indicate that compound 3 exists as a mixture of 1,2- (3a) and 1,1-isomers (3b) in a ratio of 7:1, compound 4 as a mixture of 1,2,3- (4a) and 1,1,2-isomers (4b) in a ratio of 1:1, and compound 5, interestingly, as a single species of a 1,1,2,3-isomer in solution. The cyclic voltammetric studies reveal that all the CVs of 3−5 and related Os3(CO)8(CNCH2Ph)(μ3-η2:η2:η2-C60) (2) show four reversible redox waves that correspond to a one-electron process each with the third and fourth waves overlapped within the chlorobenzene solvent potential window. As more isocyanide ligands are coordinated in 2−5, all the corresponding half-wave potentials are gradually shifted to more negative potentials, reflecting the electron-donor property of the isocyanide ligand. Furthermore, C60-mediated electron delocalization from C60 to the triosmium center takes place in the trianionic species of 2−5. The two isomers of 3 and 4 apparently undergo an equivalent electrochemical process, respectively.