Role of Electronegative Substituents on the Bond Energies in the Grubbs Metathesis Catalysts for M = Fe, Ru, Os

Coupled cluster theory [CCSD­(T)] with the aug-cc-pVDZ/aug-cc-pVDZ-PP basis sets is used to predict the thermodynamic properties of models of the Grubbs catalyst, H2ImM­(PH3)­(CRR′) and H2ImM­(C2H4)­(CRR′) for M = Fe, Ru, and Os and CRR′ = CH2, CHF, and CF2. The PH3 and C2H4, imidazolinium carbene (H2Im), and CRR′ bond dissociation energies (BDEs) are reported. Because of low metal-carbene BDEs, the M = Fe complexes are unlikely to form, so they will not be good catalysts for olefin metathesis. The metal–carbene BDE is an important component in metathesis catalyst design and correlates with the singlet–triplet splitting in the carbene. The two metallacyclobutane intermediates (cis and trans to the imidazolinium carbene) formed by reaction of the CRR′CRR′ with the 14-electron active species (H2ImM­(CRR′)) (R and R′ either H or F) are investigated at the same level of theory. The metallacycles cis to the nitrogen heterocyclic carbene are lower in energy than the trans conformer with the exception of four M = Fe metallacycles in the gas phase and in CH2Cl2 solution at 298 K. The olefin π complex for the simplest CH2 ligand plus C2H4 reactant combination is more stable in the gas phase, but in CH2Cl2 solution at 298 K, the cis metallacycles are more stable than the π complexes or the trans metallacycles on the free energy scale as a result of the large dipole moments in the cis metallacycles. The results show that the best energy balance is achieved with M = Ru, a CH2 carbene substituent, and a C2H4 reactant. The energetics for the Grubbs catalysts are shown to differ from those of the Schrock catalysts.