Thermochemical Investigation of Phosphine Ligand Substitution Reactions Involving trans-(PR_3)_2Cl_2RuCH−CHCPh_2 Complexes

Abstract

The enthalpies of reaction of (PPh_3)_2Cl_2RuCH−CHCPh_2 (1) with a series of tertiary phosphine ligands, leading to the formation of (PR_3)_2Cl_2RuCH−CHCPh_2 complexes (PR_3 = P^iPr_3, PBz_3, PPh_2Cy, PPhCy_2, PCy_3, and PiBu_3) have been measured by solution calorimetry in CH_2Cl_2 at 30 °C. The range of reaction enthalpies spans some 6.5 kcal/mol and helps to establish a relative order of complex stability for these six ruthenium carbene complexes. The enthalpies of reaction for tertiary phosphine complexes, trans-(PR_3)_2Cl2RuCH−CHCPh_2, are as follows (PR_3, kcal/mol):  PPh_3, 0; PBz_3, −1.3 (0.2); PCyPh_2, −1.8 (0.2); PCy_2Ph, −3.8 (0.2); PiBu_3, −3.8 (0.2); P^iPr_3, −5.2 (0.1); PCy_3, −6.5 (0.3). The thermodynamics of the exchange of these sterically demanding tertiary phosphine ligands for PPh_3 in 1 provides a measurement of the relative importance of phosphine steric and electronic properties/character to the enthalpy of reaction in this system. Correlations of various factors gauging the electron-donating properties of the phosphine ligands clearly show the electronic factor to be the overwhelming contributor to the enthalpy of reaction in this system.