Kinetic and Thermodynamic Selectivity of Intermolecular C–H Activation at [Tp′Rh(PMe3)]. How Does the Ancillary Ligand Affect the Metal–Carbon Bond Strength?
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Tp′Rh(PMe3)(CH3)H was synthesized as a precursor to produce the coordinatively unsaturated fragment [Tp′Rh(PMe3)], which reacts with benzene, mesitylene, 3,3-dimethyl-1-butene, 2-methoxy-2-methylpropane, 2-butyne, acetone, pentane, cyclopentane, trifluoroethane, fluoromethane, dimethyl ether, and difluoromethane at ambient temperature to give only one product in almost quantitative yield in each case. All of the complexes Tp′Rh(PMe3)(R)H were characterized by NMR spectroscopy, and their halogenated derivatives were fully characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The active species [Tp′Rh(PMe3)] was also able to activate the alkynyl C–H bond of terminal alkynes to give activation products of the type Tp′Rh(PMe3)(CCR)H (R = t-Bu, SiMe3, hexyl, CF3, Ph, p-MeOC6H4, and p-CF3C6H4). The measured relative rhodium–carbon bond strengths display two linear correlations with the corresponding carbon–hydrogen bond strengths, giving a slope of 1.54 for α-unsubstituted hydrocarbons and a slope of 1.71 for substrates with α-substitution. Similar trends of energy correlations were established by DFT calculated metal–carbon bond strengths for the same groups of substrates.
Tp′Rh(PMe₃)(CH₃)H 被合成为配位不饱和片段 [Tp′Rh(PMe₃)] 的前驱体。该活性片段在室温下可与苯、均三甲苯、3,3-二甲基-1-丁烯、2-甲氧基-2-甲基丙烷、2-丁炔、丙酮、戊烷、环戊烷、三氟乙烷、氟甲烷、二甲醚及二氟甲烷发生反应，每一例反应均以近乎定量的产率仅生成单一产物。所有 Tp′Rh(PMe₃)(R)H 型配合物均通过核磁共振波谱法完成表征，其卤代衍生物则通过核磁共振波谱法、元素分析与X射线晶体学实现全面表征。活性物种 [Tp′Rh(PMe₃)] 还可活化端炔烃的炔基C–H键，生成 Tp′Rh(PMe₃)(C≡CR)H 型活化产物，其中R为叔丁基（t-Bu）、三甲基硅基（SiMe₃）、己基、三氟甲基（CF₃）、苯基（Ph）、对甲氧基苯基（p-MeOC₆H₄）以及对三氟甲基苯基（p-CF₃C₆H₄）。测得的铑-碳键相对强度与对应碳-氢键强度呈现两条线性相关关系：无α取代的烃类底物对应的相关斜率为1.54，带有α取代的底物对应的相关斜率为1.71。通过密度泛函理论（Density Functional Theory, DFT）对同一组底物计算得到的金属-碳键强度，同样呈现出相似的能量相关趋势。
