Thermochemical Investigation of Phosphine Ligand Substitution Reactions Involving trans-(PR₃)₂Cl₂RuCH−CHCPh₂ Complexes

The enthalpies of reaction of (PPh 3 ) 2 Cl 2 Ru CH−CH CPh 2 ( 1 ) with a series of tertiary phosphine ligands, leading to the formation of (PR 3 ) 2 Cl 2 Ru CH−CH CPh 2 complexes (PR 3 = P i Pr 3, PBz 3, PPh 2 Cy, PPhCy 2, PCy 3, and P i Bu 3 ) have been measured by solution calorimetry in CH 2 Cl 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 ) 2 Cl 2 Ru CH−CH CPh 2, are as follows (PR 3, kcal/mol): PPh 3, 0; PBz 3, −1.3 (0.2); PCyPh 2, −1.8 (0.2); PCy 2 Ph, −3.8 (0.2); P i Bu 3, −3.8 (0.2); P i Pr 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.