Syntheses, structures, and electrochemical properties of Os ₃(CO)_9-n(CNCH₂Ph)_n(μ₃- η²: η²:η²-C₆₀) (n = 2-4)

Benzyl isocyanide-substituted C₆₀-triosmium complexes, Os ₃(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 Os ₃(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 Os ₃(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 electrondonor 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.