COORDINATION CHEMISTRY AND MECHANISMS OF METAL-CATALYZED CC-COUPLING REACTIONS .10. LIGAND DISSOCIATION IN RHODIUM-CATALYZED HYDROFORMYLATION - A THEORETICAL-STUDY

The equilibrium structures of rhodium complexes HRh(CO)(m)(PR(3))(4-n) and HRh(CO)(n)(PR(3))(3-n) with n = 1-3 and R = H have been calculate d using density functional theory (DFT) with both local and gradient-c orrected functionals. These molecules are model systems for the analog ous complexes with R = alkyl or aryl involved in homogeneously catalyz ed hydroformylation, the largest scale organometallic catalysis known to date. For the compounds with n = 1 and n = 2, ab initio calculation s at the HF and MP2 level of theory have been performed as well. Basis sets of valence double-zeta plus polarization quality were used, and in the case of the ab initio calculations effective core potentials fo r rhodium and phosphorus were applied. The ligand dissociation energie s derived from single-point CCSD(T) calculations revealed that MP2 str ongly overestimates bond strengths for these systems. The DFT calculat ions using gradient-corrected functionals yielded values very close to the CCSD(T) energies. Comparison with experimental results for PPh(3) complexes shows that the phosphine dissociation energies are calculat ed too low when PH3 serves as the model phosphine. The situation is im proved significantly when PMe(3) is used instead: the dissociation ene rgy is 7.2 kcal/mol larger than for PH3 (n = 1).