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Testing the ruthenium(iii) acetylacetonate and 1,2-bis(diphenylphosphino)ethane system as homogeneous catalyst in the hydrolysis of sodium borohydride

Demiralp, Tülin
Recent studies have shown that ruthenium(III) acetylacetonate is acting as homogeneous catalyst in the hydrolysis of sodium borohydride. Although trimethlyphosphite is found to be a poison for the catalytic hydrolysis of sodium borohydride, a longer observation of the reaction in the presence of ruthenium(III) acetylacetonate and 2 equivalent trimethylphosphite shows an unexpected enhancement in the catalytic activity after an induction period. The same rate enhancement is observed when 2 equivalent triphenylphosphine is added into the reaction solution. Addition of 1 equivalent 1,2-bis(diphenylphosphino)ethane, dppe, into the solution shows similarly a rate enhancement in the hydrolysis of sodium borohydride catalyzed by ruthenium(III) acetylacetonate. The effect of 1,2-bis(diphenylphosphino)ethane on the catalytic activity of ruthenium(III) acetylacetonate in the hydrolysis of sodium borohydride was studied by varying mole ratio of dppe / Ru(acac)3, ruthenium concentration, substrate concentration and temperature. The highest enhancement in the rate of hydrolysis was obtained when 1 equivalent dppe was used and therefore, this mole ratio of dppe / Ru(acac)3 was used in the further studies. The rate of the reaction was found to be first order in catalyst concentration and zero order in substrate concentration. From the evaluation of rate constant versus temperature data, the activation parameters for the hydrolysis of sodium borohydride catalyzed by ruthenium(III) acetylacetonate plus 1 equivalent dppe were found to be Ea= 59 ± 2 kJ/mol, H≠ = 60 ± 1 kJ.mol-1 and S≠ = -50 ± 3 J.(mol.K)-1. A series of control experiments were performed to characterize the active catalyst. However, the only useful information could be obtained by comparison of the UV-vis electronic absorption spectra taken from the solution during the catalytic reaction, is that, ruthenium(III) is reduced to ruthenium(II) in the course of reaction. It was concluded that a ruthenium(II) species is formed as a transient and may be the active catalyst in the reaction. After the reaction, the only ruthenium species isolated from the solution was the ruthenium(III) acetylacetonate.