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(μ₃-η²:η² :η²-C₆₀) (1) with PhCH₂N=PPh₃ in chlorobenzene affords the benzyl isocyanide substituted product Os₃(CO)₈(CNR)((μ₃-η²:η ²: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-η ¹:η²:η¹-C₆₀) (2b) in 11% yield. Thermal reaction of 2a with an excess of RNC in chlorobenzene at 80 °C produces two isocyanideinserted, isomeric products, Os₃(CO)₈(CNR)(μ₃CNR)((μ₃-η ¹: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.