Ligand-Induced Conversion of <i>π</i> to <i>σ </i>C<sub>60</sub>−Metal Cluster Complexes: Full Characterization of the <i>μ</i><sub>3</sub>-<i>η</i><sup>1</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>1</sup>-C<sub>60</sub> Bonding Mode

The reaction of Os<sub>3</sub>(CO)<sub>9</sub>(<i>μ</i><sub>3</sub>-<i>η</i><sup>2</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>2</sup>-C<sub>60</sub>) (<b>1</b>) with PhCH<sub>2</sub>NPPh<sub>3</sub> in chlorobenzene affords the benzyl isocyanide substituted product Os<sub>3</sub>(CO)<sub>8</sub>(CNR)(<i>μ</i><sub>3</sub>-<i>η</i><sup>2</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>2</sup>-C<sub>60</sub>) (<b>2a</b>, R = CH<sub>2</sub>Ph) in 76% yield. Photolysis of <b>1</b> in the presence of an excess of benzyl isocyanide gives the isocyanide-inserted product Os<sub>3</sub>(CO)<sub>9</sub>(<i>μ</i><sub>3</sub>-CNR)(<i>μ</i><sub>3</sub>-<i>η</i><sup>1</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>1</sup>-C<sub>60</sub>) (<b>2b</b>) in 11% yield. Thermal reaction of <b>2a</b> with an excess of RNC in chlorobenzene at 80 °C produces two isocyanide-inserted, isomeric products, Os<sub>3</sub>(CO)<sub>8</sub>(CNR)(<i>μ</i><sub>3</sub>-CNR)(<i>μ</i><sub>3</sub>-<i>η</i><sup>1</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>1</sup>-C<sub>60</sub>) (<b>3a</b>, 22%; <b>3b</b>, 27%). The two isomers <b>3a</b> and <b>3b</b> can be selectively prepared by photolysis of <b>2a</b> in the presence of excess isocyanide in 49% yield and by decarbonylation of <b>2b</b> with Me<sub>3</sub>NO/MeCN and subsequent reaction with isocyanide in 56% yield, respectively. Compounds <b>2a</b>,<b>b</b> and <b>3a</b>,<b>b</b> have been characterized by microanalytical and spectroscopic methods. In particular, the sp<sup>3</sup> carbons of M−C(C<sub>60</sub>) <i>σ</i>-interactions in <i>μ</i><sub>3</sub>-<i>η</i><sup>1</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>1</sup>-C<sub>60</sub> complexes exhibit unique upfield resonances around 39 ppm in the <sup>13</sup>C NMR spectra, compared to the <i>π</i>-bonded sp<sup>3</sup> carbon resonances observed in the range of 95−65 ppm. The X-ray crystallographic studies for <b>2a</b>,<b>b</b> and <b>3a</b>,<b>b</b> reveal that <i>μ</i><sub>3</sub>-<i>η</i><sup>2</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>2</sup>-C<sub>60</sub> ligands in <b>1</b> and <b>2a</b> transform into new <i>σ</i>-type <i>μ</i><sub>3</sub>-<i>η</i><sup>1</sup>:<i>η</i><sup>2</sup>:<i>η</i><sup>1</sup>-C<sub>60</sub> ligands in <b>2b</b> and <b>3a</b>,<b>b</b> by insertion of the isocyanide ligand concomitant with Os−Os bond cleavage, forming a bent triosmium framework. In compounds <b>2b</b> and <b>3b</b>, a boat-shaped 1,4-cyclohexadiene-like C<sub>6</sub> ring in the C<sub>60</sub> molecule is clearly observed and the M−C<sub>60</sub> <i>σ</i>-bond distances (average 2.24 Å) are shorter than the <i>π</i>-bond lengths (average 2.41 Å). This study represents a novel ligand-induced conversion of <i>π</i> to <i>σ</i> C<sub>60</sub>−metal complexes.