Structural and Electronic Noninnocence of α‑Diimine Ligands on Niobium for Reductive C–Cl Bond Activation and Catalytic Radical Addition Reactions

A d0 niobium­(V) complex, NbCl3(α-diimine)
(1a), supported by a dianionic redox-active N,N′-bis­(2,6-diisopropyl­phenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene
(α-diimine) ligand (ene-diamido ligand) served as a catalyst
for radical addition reactions of CCl4 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 η4-(σ2,π) coordination mode with a folded MN2C2 metallacycle and a κ2-(N,N′) coordination mode with a planar
MN2C2 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 CCl4, while a saturation effect was observed at a high CCl4 concentration. In the presence of excess amounts of styrene,
styrene coordinated in an η2-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 CCl4 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 NbCl4(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.