American Chemical Society, Organometallics, 20(34), p. 4854-4863, 2015
DOI: 10.1021/om501143t
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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.