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)2CH2OH with RuCl2[P(C6H5)3]3 in CH2Cl2 gave Ru(H)Cl(CO)[P(C6H5)2CH2OH]3 (1), which was
characterized by NMR spectroscopy and X-ray crystallographic analysis. An isotope labeling experiment using
P(C6H5)213CH2OH 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)2CH2OH, 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 (scCO2) to N,N-dimethylformamide (DMF).