Kinetics of reversible chelate ring-opening and ring-closure substitution reactions of rhodium(III)–o-dimethylaminophenyldimethylarsine complexes in acidic methanolic solutions

Abstract

The effect of acidity on the rate of reversible interconversions (i) has been investigated in methanol (X = Cl, Br, or I; L and L′=o-dimethylaminophenyldimethylarsine-NAs and -As respectively). The forward reactions are acid trans-[RhL₂Cl₂]⁺+ X^–⇌mer-[RhL(L′)XCl₂](i) catalysed. The catalytic efficiency of acidity is found to increase in the order I < Br < Cl. The reverse reactions are inhibited by the presence of acid, with an inhibition efficiency of acidity increasing in the order I < Br < Cl. The kinetic results are interpreted in terms of a slow forward step involving a chelate-ring opening through a dissociative mechanism and formation of a highly reactive intermediate whose basic end NMe₂, made available by dechelation, undergoes fast reversible protonation. This step is followed by nucleophilic attack of the entering X^– on both the protonated and unprotonated intermediates.