Cycloreversion of Rhenium(V) Diolates Containing the Hydridotris(3,5-dimethylpyrazolyl)borate Ancillary Ligand

Hydridotris(3,5-dimethylpyrazolyl)boratorhenium(V) diolates cyclorevert to Tp‘ReO 3 and alkene at elevated temperatures in nonpolar solvents. Comparison of activation parameters to those of the analogous pentamethylcyclopentadienyl complexes show the former have similar Δ S ⧧ 's, but higher Δ H ⧧ 's by 2−4 kcal/mol. Computation of the frontier orbitals for this ligand system show that the appropriate acceptor orbital on the metal is pushed to higher energy compared to the case where the ligand is pentamethylcyclopentadienyl, consistent with predictions for the proposed stepwise mechanism of this reaction.