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

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• 作者：Charles Romain ; David Specklin ; Karinne Miqueu ; Jean-Marc Sotiropoulos ; Christophe Fliedel ; St茅phane Bellemin-Laponnaz ; Samuel Dagorne
• 刊名：Organometallics
• 出版年：2015
• 出版时间：October 26, 2015
• 年：2015
• 卷：34
• 期：20
• 页码：4854-4863
• 全文大小：481K
• ISSN：1520-6041

文摘

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)] (2_Zr and 2_Hf) along with 3 equiv of toluene. For both metal complexes, the effective formation of a (魏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 掳C. In the solid state, compound 3_Zr consists of two seven-coordinate mononuclear Zr fragments that are associated by two bridging 渭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 2_Hf 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.