REVERSIBLE MIGRATORY INSERTION BETA-ALKYL ELIMINATION IN ALPHA-AGOSTIC ALKYLNIOBIUM ALKYNE COMPLEXES

Ethyl-, n-propyl- and [(trimethylsilyl)methyl]niobium (4e)-alkyne comp lexes TpNb(Cl)(CH(2)R)(PhC=CR') (Tp* = hydrotris(3,5-dimethylpyrazoly l)borate; R = Me, R' = Me (2a), Et (2b), n-Pr (2c); R = Et, R' = Me (3 a), Et (3b); R = SiMe(3), R' = Me (4a)) exhibit alpha-agostic structur es in solution (H-1 and C-13 NMR) and in the solid state (X-ray struct ure at 173 K for 4a). The phenylpropyne complexes 2a, 3a, and 4a under go a thermally induced rearrangement which yields niobium methyl compl exes TpNb(Cl)(Me)(PhC=CEt), Tp*Nb(Cl)(Me)(PhC=C-n-Pr), and Tp*Nb(Cl)( Me)(PhC=CCH(2)SiMe(3)), respectively. This reaction follows a first-or der rate law. The rate decreases with increased steric bulk of the alk yl group in the starting complex. For 2a, a full kinetic analysis yiel ds a high enthalpic barrier and an entropy of activation close to zero . Heating a toluene solution of TpNb(Cl)(n-Pr)-(PhC=CEt) (3b) establi shes a 1:1 equilibrium between 3b itself and TpNb(Cl)(Et)(PhC=C-n-Pr) (2c). Heating a solution of TpNb(Cl)(Et)(PhC=CEt) (2b) and PhC=CMe y ields the eta(1)-alkenyl (4e)-phenylpropyne complex TpNb(Cl)(eta(1)-C Ph=CEt(2))(PhC=CMe) (6), demonstrating that alkyl migration is a key s tep of the rearrangement. Under pseudo-first-order conditions, this re action does not depend on the phenylpropyne concentration. Potential 2 e-donor ligands fail to act as efficient traps. Accordingly, a success ful trapping experiment involves dinitrogen extrusion from N3P(N-i-Pr- 2)(2) in the presence of 2b which yields an unprecedented (phosphinoim ido)niobium complex: TpNb(Cl)(eta(1)-CPh=CEt(2))[=N-P(N-i-Pr-2)(2)] ( 7) The crystal structure of 7 confirms the eta(1)-alkenyl formulation and reveals a formal Nb-N triple bond, a linear Nb-N-P link, and a pyr amidal P. Deoxygenation of propene oxide and desulfurization of ethyle ne sulfide gives, respectively, the oxo and sulfide eta(1)-alkenyl com plexes TpNb(Cl)(eta(1)-CPh=CEt(2))(=X) (X = 0 (8), S (9)). The observ ed thermal rearrangements are unique examples of reversible migratory insertion/beta-alkyl elimination in transition metal alkyl alkyne comp lexes. In the absence of efficient trapping agent, the unsaturated eta (2)-alkenyl intermediate that is first generated reversibly ring opens and ring closes. This realizes the intramolecular C-C bond activation which leads to the observed alkyl switch. Kinetic and trapping data i ndicate that the migratory insertion of the alkyl group on to the alky ne is the rate-determining step of the whole process.