Conformational Analysis of Olefin-Carbene Ruthenium Metathesis Catalysts

We settle a long-standing disagreement of DFT with experiment (both solution and gas phase) for the phosphine dissociation process in Grubbs metathesis catalysis. Our findings with the M06 functional provide further support to gas-phase experimental work, concluding that for the ring-closing metathesis of norbornene, the resting state is the alkylidene-olefin complex and the rate-determining step is the loss of norbornene as a ligand and generation of the 14-electron activated species. Comparing to recent solution NMR data on olefin-carbene Ru complexes relevant to olefin metathesis, we find that the M06 density functional leads to accurate predictions for the stability of conformers, similar to 0.5 kcal/mol better than is found by B3LYP. Using this methodology, we suggest that Piers and co-workers observed the cis -dichloro "down" isomer exclusively following the ring opening of acenaphthalene.