Kinetics and thermodynamics of alkene complexation in d0 metal-alkyl-alkene complexes

Alkene dissociation from the yttrium chelate Cp*Y-2[eta(1),eta(2)-CH2CH2CH2CH=CH2] (7-on) became slow enough below -100 degrees C to be measured by dynamic H-1 NMR spectroscopy [Delta G double dagger(-110 degrees C) = 7.5 kcal mol(-1), Delta H double dagger = 9.3 kcal mol(-1)]. Coalescence of the Cp* resonances of Cp*Y-2[eta(1),eta(2)-CH2CH2CH(CH3)CH = CH2] (8-on) requires alkene dissociation plus inversion at yttrium and occurred substantially slower than simple alkene dissociation [Delta G double dagger(-72 degrees C) = 9.6 kcal mol(-1), Delta H double dagger = 10.8 kcal mol(-1)]. The binding energy of a disubstituted alkene to a d(0) yttrium center was determined to be Delta H degrees = 2.6 kcal mol(-1) by direct observation of the equilibrium between Cp*2Y(eta(1),eta(2)-CH2CH2CH2C(CH3)=CH2) (6-on) and Cp*2Y(eta(1)-CH2CH2CH2C(CH3)=CH2) (6-off). The significantly greater Delta H double dagger of alkene dissociation compared with Delta H degrees of alkene binding can be attributed either to stabilization of the dissociated yttrium alkyl by a beta-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*Y(eta(1),eta(2)-CH2CH2CH2CH= CH2) (7-on), Delta H degrees = 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.