Structural and Electronic Noninnocence of α-Diimine Ligands on Niobium for Reductive C-CI Bond Activation and Catalytic Radical Addition Reactions

摘要

A d~0 niobium(V) complex, NbCl_3(α-diimine) (la), supported by a dianionic redox-active N,N'-bis(2,6-diisopropyl-phenyl)-l,4-diaza-2,3-dimethyl-l,3-butadiene (α-diimine) ligand (ene-diamido ligand) served as a catalyst for radical addition reactions of CCl_4 to α-olefins and cyclic alkenes, selectively affording 1:1 radical addition products in a regioselective manner. During the catalytic reaction, the α-diimine ligand smoothly released and stored an electron to control the oxidation state of the niobium center by changing between an η~4-(σ~2,π) coordination mode with a folded MN_2C_2 metallacycle and a k~2-(N,N') coordination mode with a planar MN_2C_2 metallacycle. Kinetic studies of the catalytic reaction elucidated the reaction order in the catalytic cycle: the radical addition reaction rate obeyed first-order kinetics that were dependent on the concentrations of the catalyst, styrene, and CCl_4, while a saturation effect was observed at a high CCl_4 concentration. In the presence of excess amounts of styrene, styrene coordinated in an η~2-olefinic manner to the niobium center to decrease the reaction rate. No observation of oligomers or polymers of styrene and high stereoselectivity for the radical addition reaction of CCl_4 to cyclopentene suggested that the C-C bond formation proceeded inside the coordination sphere of niobium, which was in good accordance with the negative entropy value of the radical addition reaction. Furthermore, reaction of 1a with (bromomethyl)cyclopropane confirmed that both the C- Br bond activation and formation proceeded on the α-diimine-coordinated niobium center during transformation of the cyclopropylmethyl radical to a homoallyl radical. With regard to the reaction mechanism, we detected and isolated NbCl_4(α-diimine) (6a) as a transient one-electron oxidized species of 1a during reductive cleavage of the C-X bonds; in addition, the monoanionic α-diimine ligand of 6a adopted a monoanionic canonical form with selective one-electron oxidation of the dianionic ene-diamido form of the ligand in 1a.