Ligand-Induced Conversion of <i>π</i> to <i>σ </i>C₆₀−Metal Cluster Complexes: Full Characterization of the <i>μ</i>₃-<i>η</i>¹:<i>η</i>²:<i>η</i>¹-C₆₀ Bonding Mode

The reaction of Os3(CO)9(μ3-η2:η2:η2-C60) (1) with PhCH2NPPh3 in chlorobenzene affords the benzyl isocyanide substituted product Os3(CO)8(CNR)(μ3-η2:η2:η2-C60) (2a, R = CH2Ph) in 76% yield. Photolysis of 1 in the presence of an excess of benzyl isocyanide gives the isocyanide-inserted product Os3(CO)9(μ3-CNR)(μ3-η1:η2:η1-C60) (2b) in 11% yield. Thermal reaction of 2a with an excess of RNC in chlorobenzene at 80 °C produces two isocyanide-inserted, isomeric products, Os3(CO)8(CNR)(μ3-CNR)(μ3-η1:η2:η1-C60) (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 sp3 carbons of M−C(C60) σ-interactions in μ3-η1:η2:η1-C60 complexes exhibit unique upfield resonances around 39 ppm in the 13C NMR spectra, compared to the π-bonded sp3 carbon resonances observed in the range of 95−65 ppm. The X-ray crystallographic studies for 2a,b and 3a,b reveal that μ3-η2:η2:η2-C60 ligands in 1 and 2a transform into new σ-type μ3-η1:η2:η1-C60 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 C6 ring in the C60 molecule is clearly observed and the M−C60 σ-bond distances (average 2.24 Å) are shorter than the π-bond lengths (average 2.41 Å). This study represents a novel ligand-induced conversion of π to σ C60−metal complexes.