Ligand-induced conversion of π to σ C60-metal cluster complexes:: Full characterization of the μ3-η1:η2:η1-C60 bonding mode

The reaction Of OS3(CO)(9)(mu(3)-eta(2):eta(2):eta(2)-C-60) (1) with PhCH2N=PPh3 in chlorobenzene affords the benzyl isocyanide substituted product Os-3(CO)(8)(CNR) (mu(3)-eta(2):eta(2):eta(2)-C-60) (2a, R = CH2Ph) in 76% yield. Photolysis of 1 in the presence of an excess of benzyl isocyanide gives the isocyanide-inserted product Os-3(CO)(9)(mu(3)-CNR)mu(3)-eta(1):eta(2):eta(1)-C-60) (2b) in 11% vield. Thermal reaction of 2a with an excess of RNC in chlorobenzene at 80 degreesC produces two isocyanide-inserted, isomeric products, Os-3(CO)(8)(ONR)(mu(3)-CNR)(mu(3)-eta(1): eta(2):eta(1)-CGO) (3a, 22%; 3b, 27%). The two isomers 3a and 3b can be selectively prepared by photolysis of 2a in the presence of excess isocyanide in 49% yield and by decarbonylation of 2b with Me3NO/MeCN and subsequent reaction with isocyanide in 56% yield, respectively. Compounds 2a,b and 3a,b have been characterized by microanalytical and spectroscopic methods. In particular, the sp(3) carbons of M-C(C-60) sigma-interactions in mu(3)-eta(1): eta(2): eta(1)-C-60 complexes exhibit unique upfield resonances around 39 ppm in the C-13 NMR spectra, compared to the pi-bonded sp(3) carbon resonances observed in the range of 95-65 ppm. The X-ray crystallographic studies for 2a,b and 3a,b reveal that mu(3)-eta(2): eta(2): eta(2)-C-60 ligands in 1 and 2a transform into new sigma-type mu(3)-eta(1):eta(2:) eta(1)-C-60 ligands in 2b and 3a,b by insertion of the isocyanide ligand concomitant with Os-Os bond cleavage, forming a bent triosmium framework. In compounds 2b and 3b, a boat-shaped 1,4-cyclohexadiene-like C-6 ring in the C-60 molecule is clearly observed and the M-C-60 sigma-bond distances (average 2.24 Angstrom) are shorter than the pi-bond lengths (average 2.41 Angstrom). This study represents a novel ligand-induced conversion of pi to sigma C-60-metal complexes.