Reversible Migratory Insertion/β-Alkyl Elimination in α-Agostic Alkylniobium Alkyne Complexes

Ethyl-, n -propyl- and [(trimethylsilyl)methyl]niobium (4e)-alkyne complexes Tp*Nb(Cl)(CH 2 R)(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 = SiMe 3, R‘ = Me ( 4a )) exhibit α-agostic structures in solution ( 1 H and 13 C 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⋮CCH 2 SiMe 3 ), 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 ) and PhC⋮CMe yields the η 1 -alkenyl (4e)-phenylpropyne complex Tp*Nb(Cl)(η 1 -CPh CEt 2 )(PhC⋮CMe) ( 6 ), demonstrating that alkyl migration is a key step of the rearrangement. Under pseudo-first-order conditions, this reaction does not depend on the phenylpropyne concentration. Potential 2e-donor ligands fail to act as efficient traps. Accordingly, a successful trapping experiment involves dinitrogen extrusion from N 3 P(N− i -Pr 2 ) 2 in the presence of 2b which yields an unprecedented (phosphinoimido)niobium complex: Tp*Nb(Cl)(η 1 -CPh CEt 2 )[ N-P(N− i -Pr 2 ) 2 ] (7) . The crystal structure of 7 confirms the η 1 -alkenyl formulation and reveals a formal Nb−N triple bond, a linear Nb−N−P link, and a pyramidal P. Deoxygenation of propene oxide and desulfurization of ethylene sulfide gives, respectively, the oxo and sulfido η 1 -alkenyl complexes Tp*Nb(Cl)(η 1 -CPh CEt 2 )( X) (X = O ( 8 ), S ( 9 )). The observed thermal rearrangements are unique examples of reversible migratory insertion/β-alkyl elimination in transition metal alkyl alkyne complexes. In the absence of efficient trapping agent, the unsaturated η 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 indicate that the migratory insertion of the alkyl group on to the alkyne is the rate-determining step of the whole process.