Synthesis and Characterization of (μ-H)2Os3(CO)9(PPh3)(μ-CH2) and Related Compounds. Phosphine Ligand Effects on the Methylene/Methyl Tautomerization

Treatment of the phosphine ligand derivatives (μ-H)2Os3(CO)9L (L = PEt3, PEt2Ph, PPh3, PiPrPh2, PCy3) with excess ethereal diazomethane at −78 °C provides a set of compounds with the formula Os3(CO)9L(“CH4”). As isolated solids these compounds exist as methylene ligand complexes, and the structure of (μ-H)2Os3(CO)9(PPh3)(μ-CH2) has been determined by single-crystal X-ray crystallography. A triangular array of osmium atoms supports a methylene moiety bridging one Os−Os vector, and the triphenylphosphine ligand is bound to the remaining osmium center in an "in-plane” position. The hydride ligands have been located and refined; one also bridges the same edge as the methylene ligand, and the other forms a single bridge in a position cis to the phosphine ligand. Analysis of the solution 1H and 13C NMR spectra of (μ-H)2Os3(CO)9(PPh3)(μ-CH2) indicates that the solid-state structure is maintained in solution. However, also present in solution is a methyl tautomer, (μ-H)Os3(CO)9(PPh3)(μ-CH3), the structure of which has been inferred by a combination of spectroscopy, labeling, and reactivity studies as having both the methyl group and the hydride ligand bridging the edge adjacent to the phosphine ligand. For the various derivatives, the equilibrium amount of the methyl tautomer depends strongly on the size of the phosphine ligand, ranging from 1:12 for L = PEt3, through 1:56 for L = PPh3, to essentially zero for L = PCy3. Protonation of (μ-H)2Os3(CO)9L(μ-CH2) (L = PPh3, PCy3) occurs at an Os−C bond of the methylene ligand to produce a cationic methyl compound. In this case the methyl group bridges the same edge as in the methylene precursor: that is, a position remote from the phosphine ligand. Pyrolysis of (μ-H)2Os3(CO)9(PPh3)(μ-CH2) in refluxing toluene forms the methylidyne derivative (μ-H)3Os3(CO)8(PPh3)(μ-CH) in high yield. The reaction of the methyl/methylene tautomer mixture with ethylene to eliminate methane and form a hydrido vinyl compound shows a strong dependence on the size of the phosphine ligand, with inhibition by bulkier ligands. However, the action of the tautomer mixture to catalyze the decomposition of excess diazomethane into nitrogen and a hydrocarbon polymer depends on the electronic properties of the phosphine ligand, with stronger donors showing slower rates.