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)H-3, 1] cleanly yielded the corresponding M-NHC chloro benzyl derivatives [[(tBu)(OCO)]-M(Cl)(CH2Ph)] (2(Zr) and 2(Hf)) along with 3 equiv of toluene. For both metal complexes, the effective formation of a (k(3)-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 3(Ti), arising from the migration of the Ti-Bn group from the metal center to the C-carbene atom. The molecular structure of 3(Ti) was established by analogy with the X-ray-determined Zr analogue 3(Zr) Compound 3(Zr) quantitatively formed upon heating a benzene solution of 2(Zr), at 60 degrees C. In the solid state, compound 3(Zr), consists of two seven-coordinate mononuclear Zr fragments that are associated by two bridging mu(2)-chloride atoms, confirming the migration of the Zr-Bn moiety from the metal center to the C-carbene 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)] (4(Ti)-THF, 4(Zr)-THF, and 4(Hf)-THF), as established by X-ray crystallographic studies in the case of 4(Ti)-THF. Such a THF-promoted benzyl migration was also observed with the dibenzyl Zr and Hf complexes [[(tBu)(OCO)]M(CH2Ph)(2)] (5(Zr), and 5(Hf)), leading to the formation of the corresponding THF-rearranged products [[(tBu)(OC(Bn)O)]M(CH2Ph)(THF)] (6(Zr)-THF and 6(Hf)-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)] (9(Me)) and [[(tBu)(OC(Ph)O)]M-(Cl)(THF)] (9(Ph)), 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 4(Hf)-THF. These data are consistent with the reaction rate law being first order both in THF and in the THF adduct 2(Hf)-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 2(Hf)-THF.