Kinetic and Thermodynamic Selectivity of Intermolecular C–H Activation at [Tp′Rh(PMe₃)]. How Does the Ancillary Ligand Affect the Metal–Carbon Bond Strength?

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)­(CCR)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.