Halide bridged trinuclear rhodium complexes and their inhibiting influence on catalysis

Abstract

The addition of halides to the cationic solvate complexes of the type [Rh(PP)(solvent)₂][anion] leads to the formation of trinuclear μ₃-halide-bridged complexes. The corresponding complexes [Rh₃(PP)₃(μ₃-X)₂][BF₄] with X = Cl or Br and diphosphines PP Me-DuPHOS, Et-DuPHOS, DIPAMP, t-Bu-BisP* and Tangphos were characterized – in most cases – also by X-ray analysis. By reducing the concentration of the active catalyst, the in situ formation of these μ-halide-bridged multinuclear complexes in catalytic reactions leads to a decrease in activity or even to a total inactivity. The required halides – in most cases chloride – are usually present as impurities in the substrates (also when produced industrially). The extent of deactivation, known from enzyme catalysis as competitive inhibition, depends on several factors: the type of halide, the ratio of stability constants of multinuclear halide complexes and of substrate complexes, and the concentration of halide and substrate in solution.