Homogeneous catalysts for the hydrolysis of Sodium Borohydride : synthesis, characterization and catalytic use

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2010
Masjedi, Mehdi
Recent study has shown that ruthenium(III) acetylacetonate acts as a homogeneous catalyst in the hydrolysis of sodium borohydride. When two equivalents of trimethylphosphite per ruthenium is added to the reaction solution containing sodium borohydride and ruthenium(III) acetylacetonate in the mixture of water and tetrahydrofuran, the rate of hydrogen generation is practically stopped (or reduced to the level of self hydrolysis). However, the catalytic hydrogen evolution of sodium borohydride restarts at an unexpectedly high rate in a certain period of time (induction time) after addition of trimethylphosphite. Consequently, trimethylphosphite known to be a poison in the hydrolysis, is involved in the formation of a new active catalyst (ruthenium species containing trimethylphosphite ligands) which has much higher catalytic activity in comparison with sole ruthenium(III) acetylacetonate. The same rate enhancement is observed by addition of two equivalents of triphenylphosphite per ruthenium into the medium. Varying the phosphorus compound affects not only the life time of catalyst but also the kinetic and activation parameters of the hydrolysis of sodium borohydride. However, varying the mole ratio of phosphorus compound to ruthenium does not affect the rate of hydrolysis or in other words, the rate of hydrogen generation is independent of phosphite concentration. Trans- and cis-[Ru(acac)2{P(OMe)3}2] complexes do not show significant catalytic activity in hydrogen generation of sodium borohydride. However, catalytic activity of cis-isomer is highly increased in the presence of two equivalents of trimethylphosphite, showing that the active catalyst formed during hydrolysis of sodium borohydride starting with Ru(acac)3 or cis-[Ru(acac)2{P(OMe)3}2], has more than two phosphine ligands. For the first time, a ruthenium(I) complex was isolated from aqueous solution after finishing the catalytic hydrolysis of sodium borohydride starting with ruthenium(III) acetylacetonate and trimethylphosphite. Hydridotetrakis(trimethylphosphite)ruthenium(I), [Ru{P(OMe)3}4H] was isolated and characterized by single crystal X-ray diffraction, Mass, UV-visible, FTIR, 1H, 13C and 31PNMR spectroscopy. Following the catalytic reaction by UV-Visible spectroscopy shows in-situ formation of a Ru(II) species which is mostly converted back to ruthenium(III) acetylacetonate after hydrolysis reaction along with formation of [Ru{P(OMe)3}4H] complex as a minor product. Although Ru(II) species could not be isolated, adding 1 equivalent of 2,2'-bipyridine yielded [Ru(acac)(bipy){P(OMe)3}H] complex which could be isolated and characterized by Mass, UV-Visible, FTIR, 1H, 13C and 31PNMR spectroscopy. In-situ generated Ru(II) species has much higher catalytic activity in comparison with its stabilized form [Ru(acac)(bipy){P(OMe)3}H] or [Ru{P(OMe)3}4H] complex. Conclusively, the fac-[Ru(acac){P(OMe)3}3H] complex is believed to be the in-situ generated Ru(II) species and the active catalyst in the hydrolysis of sodium borohydride.
Citation Formats
M. Masjedi, “Homogeneous catalysts for the hydrolysis of Sodium Borohydride : synthesis, characterization and catalytic use,” Ph.D. - Doctoral Program, Middle East Technical University, 2010.