Abstract
Ethyl-, n-propyl- and [(trimethylsilyl)methyl]niobium (4e)-alkyne complexes Tp*Nb(Cl)(CH2R)(PhC⋮CR‘) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; R = Me, R‘ = Me (2a), Et (2b), n-Pr (2c); R = Et, R‘ = Me (3a), Et (3b); R = SiMe3, R‘ = Me (4a)) exhibit α-agostic structures in solution (1H and 13C NMR) and in the solid state (X-ray structure at 173 K for 4a). The phenylpropyne complexes 2a, 3a, and 4a undergo a thermally induced rearrangement which yields niobium methyl complexes Tp*Nb(Cl)(Me)(PhC⋮CEt), Tp*Nb(Cl)(Me)(PhC⋮C−n-Pr), and Tp*Nb(Cl)(Me)(PhC⋮CCH2SiMe3), respectively. This reaction follows a first-order rate law. The rate decreases with increased steric bulk of the alkyl group in the starting complex. For 2a, a full kinetic analysis yields a high enthalpic barrier and an entropy of activation close to zero. Heating a toluene solution of Tp*Nb(Cl)(n-Pr)(PhC⋮CEt) (3b) establishes a 1:1 equilibrium between 3b itself and Tp*Nb(Cl)(Et)(PhC⋮C−n-Pr) (2c). Heating a solution of Tp*Nb(Cl)(Et)(PhC⋮CEt) (2b...
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