Kinetics and Thermodynamics of Alkene Complexation in d0Metal−Alkyl−Alkene Complexes

Alkene dissociation from the yttrium chelate Cp* 2 Y[η 1,η 2 -CH 2 CH 2 CH 2 CH CH 2 ] ( 7-on ) became slow enough below −100 °C to be measured by dynamic 1 H NMR spectroscopy [Δ G ⧧ (−110 °C) = 7.5 kcal mol - 1, Δ H ⧧ = 9.3 kcal mol - 1 ]. Coalescence of the Cp* resonances of Cp* 2 Y[η 1,η 2 -CH 2 CH 2 CH(CH 3 )CH CH 2 ] ( 8-on ) requires alkene dissociation plus inversion at yttrium and occurred substantially slower than simple alkene dissociation [Δ G ⧧ (−72 °C) = 9.6 kcal mol - 1, Δ H ⧧ = 10.8 kcal mol - 1 ]. The binding energy of a disubstituted alkene to a d 0 yttrium center was determined to be Δ H ° = 2.6 kcal mol - 1 by direct observation of the equilibrium between Cp* 2 Y(η 1,η 2 -CH 2 CH 2 CH 2 C(CH 3 ) CH 2 ) ( 6-on ) and Cp* 2 Y(η 1 -CH 2 CH 2 CH 2 C(CH 3 ) CH 2 ) ( 6-off ). The significantly greater Δ H ⧧ of alkene dissociation compared with Δ H ° of alkene binding can be attributed either to stabilization of the dissociated yttrium alkyl by a β-agostic interaction or to destabilization of the transition state leading to alkene dissociation by increased strain in the chelate tether. Binding energies of monosubstituted alkenes were determined indirectly by comparing the relative binding energies of 2,5-dimethyl-THF to both 6 and monosubstituted alkene chelates. Binding energies of monosubstituted alkenes were found to be greater than for disubstituted alkenes; for Cp* 2 Y(η 1,η 2 -CH 2 CH 2 CH 2 CH CH 2 ) ( 7-on ), Δ H ° = 4.0 kcal mol - 1 . The rate of alkene dissociation from these yttrium chelates is much faster than reversible intramolecular insertion of the coordinated alkene into the metal alkyl bond to produce cyclobutylmethylmetal compounds.