Synthesis and characterization of (mu-H)(2)Os-3(CO)(9)(PPh3) (mu-CH2) and related compounds. Phosphine ligand effects on the methylene/methyl tautomerization

Treatment of the phosphine ligand derivatives (mu-H)(2)Os-3(CO)(9)L (L = PE t3, PEt2Ph, PPh3, PiPrPh(2), PCy3) with excess ethereal diazomethane at -78 degrees C provides a set of compounds with the formula Os-3(CO)(9)L("CH4") AS isolated solids these compounds exist as methylene ligand complexes, an d the structure of (mu-H)(2)Os-3(CO)(9)(PPh3)(mu-CH2) has been determined b y single-crystal X-ray crystallography. A triangular array of osmium atoms supports a methylene moiety bridging one Os-Os vector, and the triphenylpho sphine ligand is bound to the remaining osmium center in an "in-plane" posi tion. The hydride ligands have been located and refined; one also bridges t he same edge as the methylene ligand, and the other forms a single bridge i n a position cis to the phosphine ligand. Analysis of the solution H-1 and C-13 NMR spectra of (mu-H)(2)Os-3(CO)(9)(PPh3)(mu-CH2) indicates that the s olid-state structure is maintained in solution. However, also present in so lution is a methyl tautomer, (mu-H)-Os-3(CO)(9)(PPh3)(mu-CH3), the structur e of which has been inferred by a combination of spectroscopy, labeling, an d reactivity studies as having both the methyl group and the hydride ligand bridging the edge adjacent to the phosphine ligand. For the various deriva tives, the equilibrium amount of the methyl tautomer depends strongly on th e size of the phosphine ligand, ranging from 1:12 for L = PEt3, through 1:5 6 for L = PPh3, to essentially zero for L = PCy3. Protonation of (mu-H)(2)O s-3(CO)(9)L(mu-CH2) (L = PPh3, PCy3) occurs at an Os-C bond of the methylen e ligand to produce a cationic methyl compound. In this case the methyl gro up bridges the same edge as in the methylene precursor: that is, a position remote from the phosphine ligand. Pyrolysis of (mu-H)(2)Os-3(CO)(9)(PPh3)( mu-CH2) in refluxing toluene forms the methylidyne derivative (mu-H)(3)Os-3 (CO)(8)(PPh3)(mu-CH) in high yield. The reaction of the methyl/ methylene t automer mixture with ethylene to eliminate methane and form a hydride vinyl compound shows a strong dependence on the size of the phosphine ligand, wi th inhibition by bulkier ligands. However, the action of the tautomer mixtu re 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.