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An investigation into the synthesis and catalytic hydrogenation activity of rhodium - stannous - chloride complexes

University of British Columbia

RhCl₃:3H₂O in 3M HCl or ethanol was treated with various amounts of SnCl₂:2H₂O and R₄NCl (R = Me, Et) or [Et₄N][SnCl₃]. Kinetic products obtained include [RhCl₂(SnCl₃)₄]³⁻ and the new complex [Rh(SnCl₃)₄(SnCl₄)]³⁻. At longer reaction times [RhCl(SnCl₃)₅]³⁻, [RhCl₃(SnCl₃)₃]³⁻ and [Rh(SnCl₃)₄(SnCl₄)]⁵⁻, amongst other uncharacterised complexes, were found. The relative proportions of the complexes formed were dependent on the use of aerobic or anaerobic conditions, temperature, reaction time, precipitant and light. The products were characterised using UV-VIS and infrared spectroscopy and where possible by elemental analysis, conductance and tin Mossbauer spectroscopy. Based on the variability of product mixture under different synthetic conditions, a Rh(I) catalysed substitution at Rh(III) centres is believed to afford the initial Rh(III) anionic complexes. Other Rh(I) products form via reduction of Rh(III) to Rh(I) by Sn(II). Thermodynamic products result from slow equilibration of the initial product mixture. The anionic products obtained were found to be light-sensitive in solution. A preliminary study of similar complexes, formed in situ, as catalyst precursors for the hydrogenation of fumaric and maleic acids was undertaken. In 3M HCl (or 3M DCl/D₂O) at 80°C and under 450 mmHg of H₂ (or D₂) the most catalytically active systems were those containing Sn(II) and Rh(III) with a ten-fold excess of olefin over Rh(III). An increase to thirty-fold excess of olefin markedly decreased activity although activity was independent of the presence of Sn(II). A stoichiometric reduction by Sn(II), or rhodium-tin chloride complexes plus two protons, was found to compete with the catalytic processes. Deuterium scrambling was observed for both catalytic and stoichiometric systems. In the absence of Rh, this suggested the intermediacy of tin hydrides formed via β-elimination of tin-alkyl intermediates. As a working hypothesis, a conventional catalytic mechanism is proposed to operate via a 'hydride route' involving rhodium mono-hydrides formed by heterolytic splitting of H₂ (or D₂). Evidence also suggests that stoichiometric reduction of the olefin by a rhodium(I)-tin chloride complex resulting in oxidation of Rh(I) to Rh(III) may be coupled to the hydrogen reduction of Rh(III) to Rh(I) in the most active hydrogenation system.

Text

2010-04-22

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http://hdl.handle.net/2429/24052

Chemistry

University of British Columbia

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