Protonation of pi-alkene-rhodium(I) complexes leads to sigma-alkyl-rhodium(III) - an NMR study

Journal article, 1998

The reaction of Rh(beta-diketonato)(alkene)(2) complexes with CF3SO3H in THF gives Rh(beta-diketonato)(alkyl) species when the alkene is ethylene, cis-butene or 5-methylene-cycloheptene whereas 1,5-cyclooctadiene complexes are unreactive. The reaction was followed by means of NMR-spectroscopy at low temperature and the products were characterized in solution by 2D-NMR techniques. Three possible reaction mechanisms are discussed: ligand assisted proton transfer, hydride transfer and direct protonation of the alkene. The Rh(III)ethyl species are stable against degradation by beta-hydride elimination. However, Rh(III)(beta-diketonato)(n-butyl) complexes lose butene rapidly when ethylene is added and Rh(beta-diketonato)(ethyl) complexes are formed. A mechanism preventing the formation of higher oligomers is proposed, where beta-hydride elimination is followed by rapid alkene exchange and hydride re-insertion. (C) 1998 Elsevier Science S.A. All rights reserved.