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) n (PR 3 ) 4 - n and HRh(CO) n (PR 3 ) 3 - n with n = 1−3 and R = H have been calculated using density functional theory (DFT) with both local and gradient-corrected functionals. These molecules are model systems for the analogous complexes with R = alkyl or aryl involved in homogeneously catalyzed hydroformylation, the largest scale organometallic catalysis known to date. For the compounds with n = 1 and n = 2, ab initio calculations at the HF and MP2 level of theory have been performed as well. Basis sets of valence double-ζ plus polarization quality were used, and in the case of the ab initio calculations effective core potentials for rhodium and phosphorus were applied. The ligand dissociation energies derived from single-point CCSD(T) calculations revealed that MP2 strongly overestimates bond strengths for these systems. The DFT calculations 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 calculated too low when PH 3 serves as the model phosphine. The situation is improved significantly when PMe 3 is used instead: the dissociation energy is 7.2 kcal/mol larger than for PH 3 ( n = 1).