Kinetic and mechanistic investigation of the substitution reactions of four and five co-ordinated rhodium stibine complexes with a bulky phosphite

Abstract

The substitution reaction of trans-[Rh(Cl)(CO)(SbPh₃)₂] (1) with tris(2,4-di-tert-butylphenyl)phosphite (2,4-TBPP) to form trans-[Rh(Cl)(CO)(2,4-TBPP)₂] (4) in two consecutive steps has been investigated by UV-vis stopped-flow spectrophotometry. The experiments were performed in dichloromethane and in ethyl acetate, at 298 K and 268 K respectively for the first reaction step, and for the second reaction step over a temperature range from 278 to 313 K in both solvents. The first step is very fast (up to 1630 s⁻¹) and on the limit of what is observable with the stopped-flow technique. Introduction of the five-coordinate complex trans-[Rh(Cl)(CO)(SbPh₃)₃] (2) in equilibrium with (1), by adding an excess SbPh₃, led to a significant decrease in overall reaction rate for the formation of the intermediate trans-[Rh(Cl)(CO)(SbPh₃)₂(2,4-TBPP)] (3). Activation parameters for the second substitution reaction, in which 3 is converted to 4, has been determined as ΔH^‡ = 22.85 ± 0.17 and 28.38 ± 0.10 kJ mol⁻¹ and ΔS^‡ = −144.7 ± 0.6 and −100.9 ± 0.4 J mol⁻¹ K⁻¹ for CH₂Cl₂ and EtOAc respectively, supporting an associative pathway. A strongly coordinating solvent promotes both reactions. In all reaction steps a strong tendency for stibines to promote 5-coordinated, fairly stable intermediates is manifested.