Novel homogeneous catalyst comprising ruthenium and trimethylphosphite for the hydrolysis of sodium borohydride

Abstract

Homogeneous catalytic hydrolysis of sodium borohydride starting with Ru(acac) 3 (acac = acetylacetonate) and P(OMe) 3 was followed by monitoring the hydrogen evolution and the UV–vis electronic absorption spectra which shows the conversion of all ruthenium(III) to a ruthenium(II) species, most likely acting as catalyst. This active catalyst is alive only under reducing conditions and converted mainly to Ru(acac) 3 along with other minor complexes when the catalytic reaction is over. A ruthenium(II) complex was isolated from the reaction solution after the complete catalytic hydrolysis of sodium borohydride and characterized to be [Ru{P(OMe) 3} 4H 2] by single crystal XRD, MS, UV–vis, FTIR, 1H, 13C and 31P NMR spectroscopy. [Ru{P(OMe) 3} 4H 2] complex crystallizes in the triclinic space group P-1 with a = 10.066(2) Å, b = 19.108(3) Å, c = 20.938(4) Å, α = 78.839(14)°, β = 87.308(16)° and γ = 79.506(14)°. This ruthenium(II) complex was found not to be the active catalyst in the hydrolysis of sodium borohydride, rather one of its conversion products after catalysis. Although the active catalyst could not be isolated from the reaction solution, it could be stabilized by adding chelating 2,2′-bipyridine into the solution during the catalysis. Thus, a stabilized form of the active catalyst, [Ru(acac)(bipy){P(OMe) 3}H], could be isolated and characterized by MS, UV–vis, FTIR, 1H, 13C and 31P NMR spectroscopy. [Ru(acac)(bipy){P(OMe) 3}H] is expectedly not as active as the ruthenium(II) species formed in situ during the catalysis. Taking all the results together reveals that the active catalyst is a ruthenium(II) complex, either [Ru(acac){P(OMe) 3} 3H] or most likely its dissociation product, [Ru(acac){P(OMe) 3} 2H]. Control experiments showed that 2,2′-bipyridine can replace only the trimethylphosphite ligands but not the acetylacetonato ligand in the ruthenium(II) complex.