Synthesis and characterization of η2-C60 and μ3-η2,η2,η2-C60 triosmium cluster complexes

Various eta(2)-C-60 and mu(3)-eta(2),eta(2),eta(2)-C-60 triosmium carbonyl cluster complexes Os-3(CO)(11)(eta(2)-C-60) (1), Os-3(CO)(10)(NCMe)(eta(2)-C-60) (2), Os-3(CO)(10)(PPh3)(eta(2)-C-60) (3), Os-3(CO)(9)(PR3)(2)(eta(2)-C-60) (4, R = Ph; 5, R = Me), Os-3(CO)(9)(mu(3)-eta(2),eta(2),eta(2)-C-60) (6), and Os-3(CO)(8)(PMe3)(mu(3)-eta(2),eta(2),eta(2)-C-60) (7) have been isolated as crystalline solids and characterized by spectroscopic (IR, MS, and H-1, P-31, and C-13 NMR) and analytical data. The molecular structure of complex 1 has been determined by a single-crystal X-ray diffraction study. The structure of 1 is derived from that of Os-3(CO)(12) by replacing an equatorial carbonyl ligand with an eta(2)-C-60 ligand coordinated through a 6-6 ring fusion. The structural assignment of 2-7 is made on the basis of spectroscopic results. Compound 2 exists as two isomers in solution in a ratio of 2:1 (2a: 2b). VT C-13 NMR spectra of 2a and 5 indicate that both complexes undergo similar fluxional processes of restricted equilibration of in-plane equatorial C-60 and carbonyl ligands via a triply bridged intermediate with identical values of Delta G(c)(not equal) = 12.7 +/- 0.1 kcal/mol. Thermolysis of 2 in refluxing chlorobenzene affords Os-3(CO)(9)(mu(3)-eta(2),eta(2),eta(2)-C-60) (6) in 23% yield, which can be alternatively prepared in 32% yield from the reaction of Os-3(CO)(10)(NCMe)(2) (2 equiv) and C-60 (1 equiv). Decarbonylation of 6 with Me3NO/MeCN reagent in the presence of excess PMe3 gives Os-3(CO)(8)(PMe3)(mu(3)-eta(2),eta(2),eta(2)-C-60) (7) in a quantitative yield. VT C-13 NMR spectra of both 6 and 7 reveal a localized-3-fold rotation of carbonyl and phosphine ligands on each osmium center.