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.

将1当量的[M(CH₂Ph)₄]（M=锆(Zr)、铪(Hf)）与1,3-双(3,5-二叔丁基-2-羟基苯基)咪唑啉鎓氯化物[tBu(OCO)H₃, 1]反应，可干净地得到相应的金属-氮杂环卡宾(M-NHC)氯代苄基衍生物[[tBu(OCO)]M(Cl)(CH₂Ph)]（2Zr和2Hf），同时生成3当量的甲苯。通过核磁共振波谱与元素分析，证实了两种金属配合物均成功形成了κ^3-OCO型金属螯合物，且苄基配体配位至M(IV)金属中心。与之相反，在相同条件下，四(苄基)钛(Ti(CH₂Ph)₄)与咪唑啉鎓前配体1的反应却得到了意料之外的重排二聚产物3Ti，该产物源于Ti-Bn基团从金属中心迁移至卡宾碳原子。3Ti的分子结构通过与X射线衍射测定的锆类似物3Zr类比得以确定。将2Zr的苯溶液于60℃加热时，可定量生成化合物3Zr。固态下，3Zr由两个七配位单核锆结构单元组成，二者通过两个μ2-桥联氯原子相连，这证实了Zr-Bn基团从金属中心迁移至卡宾碳原子的过程。在四氢呋喃(THF)中，[M(CH₂Ph)₄]（M=钛、锆、铪）与咪唑啉鎓前配体1的反应可定量生成相应的重排单核THF加合物[[tBu(OC(Bn)O)]M(Cl)(THF)]（4Ti-THF、4Zr-THF和4Hf-THF），该产物的结构通过4Ti-THF的X射线晶体学研究得以确认。该THF促进的苄基迁移过程也在二苄基锆和铪配合物[[tBu(OCO)]M(CH₂Ph)₂]（5Zr和5Hf）中被观察到，生成了相应的THF重排产物[[tBu(OC(Bn)O)]M(CH₂Ph)(THF)]（6Zr-THF和6Hf-THF）。在四氢呋喃中，向1当量的二氯代氮杂环卡宾锆配合物[[tBu(OCO)]Zr(Cl)₂(THF)]（8）中加入1当量的溴化甲基镁(CH₃MgBr)或溴化苯基镁(PhMgBr)，分别得到了重排产物[[tBu(OC(Me)O)]M(Cl)(THF)]（9Me）和[[tBu(OC(Ph)O)]M(Cl)(THF)]（9Ph），该结果由核磁共振数据推导得出。对THF存在下氯代苄基铪衍生物2Hf的THF促进重排反应（生成4Hf-THF）进行了动力学研究，结果符合该反应的速率定律：对THF和加合物2Hf-THF均为一级反应。对钛、锆、铪体系的密度泛函理论(DFT)计算结果支持，苄基迁移反应发生在瞬态七配位双THF加合物[[tBu(OCO)]M(Cl)(Bn)(THF)₂]中，该加合物可通过THF与2Hf-THF配位轻易形成。