Synthesis of Ruthenium(II) complexes containing hydroxymethylphosphines and their catalytic activities for hydrogenation of supercritical carbon dioxide

Ligand substitution of RuCl2[P(C6H5)(3)](3) and Cp*RuCl(isoprene) (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) complexes with hydroxymethylphosphines was investigated to develop new catalyst systems for CO2 hydrogenation. A reaction of P(C6H5)(2)CH2OH with RuCl2[P(C6H5)(3)](3) in CH2Cl2 gave Ru(H)Cl(CO)[P(C6H5)(2)CH2OH](3) (1), which was characterized by NMR spectroscopy and X-ray crystallographic analysis. An isotope labeling experiment using P(C6H5)(2)(CH2OH)-C-13 indicated that the carbonyl moiety in complex 1 originated from formaldehyde formed by degradation of the hydroxymethylphosphine. Elimination of formaldehyde from PCy2CH2OH (Cy = cyclohexyl) was also promoted by treatment of RuCl2[P(C6H5)(3)](3) in ethanol to give RuCl2(PHCy2)(4) under mild conditions. On the other hand, the substitution reaction using Cp*RuCl(isoprene) with the hydroxymethylphosphine ligands proceeded smoothly with formation of Cp*RuCl(L)(2) [2a-2c; L = P(C6H5)(2)CH2OH, PCy(CH2OH)(2), and P(CH2OH)(3)] in good yields. The isolable hydroxymethylphosphine complexes 1 and 2 efficiently catalyzed the hydrogenative amidation of supercritical carbon dioxide (scCO(2)) to N,N-dimethylformamide (DMF).