Unusual Benzyl Migration Reactivity in NHC-Bearing Group 4 Metal Chelates: Synthesis, Characterization, and Mechanistic Investigations

The
reaction of 1 equiv of [M­(CH2Ph)4] (M
= Zr, Hf) and 1,3-bis­(3,5-di-tert-butyl-2-hydroxyphenyl)­imidazolinium
chloride [tBu(OCO)­H3, 1] cleanly yielded the corresponding M-NHC chloro benzyl derivatives
[[tBu(OCO)]­M­(Cl)­(CH2Ph)] (2Zr and 2Hf) along with 3 equiv of toluene. For both metal complexes,
the effective formation of a (κ3-OCO) metal chelate
and the coordination of a benzyl ligand onto the M­(IV) metal center
were established by NMR and elemental analysis. In contrast, under
identical conditions, the reaction of Ti­(CH2Ph)4 with the imidazolinium proligand 1 yielded the unexpected
rearranged dimer product 3Ti,
arising from the migration of the Ti-Bn group from
the metal center to the Ccarbene atom. The molecular structure
of 3Ti was established by analogy
with the X-ray-determined Zr analogue 3Zr. Compound 3Zr quantitatively
formed upon heating a benzene solution of 2Zr at 60 °C. In the solid state, compound 3Zr consists of two seven-coordinate
mononuclear Zr fragments that are associated by two bridging μ2-chloride atoms, confirming the migration of the Zr-Bn moiety from the metal center to the Ccarbene atom. Carrying out the reaction of [M­(CH2Ph)4] (M = Ti, Zr, Hf) with imidazolinium proligand 1 in
THF led to the quantitative formation of the corresponding rearranged
monomeric THF adduct [[tBu(OC­(Bn)­O)]­M­(Cl)­(THF)]
(4Ti-THF, 4Zr-THF, and 4Hf-THF), as established by X-ray
crystallographic studies in the case of 4Ti-THF. Such a THF-promoted benzyl migration was
also observed with the dibenzyl Zr and Hf complexes [[tBu(OCO)]­M­(CH2Ph)2] (5Zr and 5Hf), leading to the formation of the corresponding THF-rearranged
products [[tBu(OC­(Bn)­O)]­M­(CH2Ph)­(THF)] (6Zr-THF and 6Hf-THF).
The addition of 1 equiv of methylmagnesium bromide (CH3MgBr) or phenylmagnesium bromide (PhMgBr) to 1 equiv of the zirconium
dichloro NHC complex [[tBu(OCO)]­Zr­(Cl)2(THF)] (8) in THF yielded the rearranged products
[[tBu(OC­(Me)­O)]­M­(Cl)­(THF)] (9Me) and [[tBu(OC­(Ph)­O)]­M­(Cl)­(THF)] (9Ph),
respectively, as deduced from NMR data. Kinetic studies were carried
out on the THF-promoted rearrangement reaction of the benzyl chloro
Hf derivative 2Hf in the presence
of THF to produce 4Hf-THF. These data are consistent with the reaction rate law being first
order both in THF and in the THF adduct 2Hf-THF. DFT calculations on the Ti, Zr, and Hf systems
support a benzyl migration reaction occurring at a transient heptacoordinated
bis-THF adduct species of the type [[tBu(OCO)]­M­(Cl)­(Bn)­(THF)2], which may readily form upon THF
coordination to 2Hf-THF.