Bond Energies in Models of the Schrock Metathesis Catalyst

Abstract

Heats of formation, adiabatic and diabatic bond dissociation energies (BDEs) of the model Schrock-type metal complexes M(NH)(CRR′)(OH)2 (M = Cr, Mo, W; CRR′ = CH2, CHF, CF2) and MO2(OH)2 compounds, and Brønsted acidities and fluoride affinities for the M(NH)(CH2)(OH)2 transition metal complexes are predicted using high level CCSD(T) calculations. The metallacycle intermediates formed by reaction of C2H4 with M(NH)(CH2)(OH)2 and MO2(OH)2 are investigated at the same level of theory. Additional corrections were added to the complete basis set limit to obtain near chemical accuracy (±1 kcal/mol). A comparison between adiabatic and diabatic BDEs is made and provides an explanation of trends in the BDEs. Electronegative groups bonded on the carbenic carbon lead to less stable Schrock-type complexes as the adiabatic BDEs of M═CF2 and M═CHF bonds are much lower than the M═CH2 bonds. The Cr compounds have smaller BDEs than the W or Mo complexes and should be less stable. Different M(NH)(OH)2(C3H6) and MO(OH)2(OC2H4) metallacycle intermediates are investigated, and the lowest-energy metallacycles have a square pyramidal geometry. The results show that consideration of the singlet–triplet splitting in the carbene in the initial catalyst as well as in the metal product formed by the retro [2 + 2] cycloaddition is a critical component in the design of an effective olefin metathesis catalyst in terms of the parent catalyst and the groups being transferred.