Relations between H–C and M–C bond strengths: influence of the organic ligand and the metal

This review describes the thermodynamics of C–H bond activation in various hydrocarbons, especially (poly)fluorinated benzenes, by transition metal complexes through experiments and computational studies with a focus on the latter. Experimental metal‑carbon bond dissociation enthalpies (BDEs), determined relative to an internal reference, were used to establish the relationship between H–C and M–C BDEs. In parallel, bond dissociation enthalpies or energies were computed with DFT methods. For a given metal complex and a set of sufficiently similar organic substrates a linear correlation with a characteristic slope R M-C/H-C was obtained. Since far more H–C BDEs are known than M–C BDEs, these correlations provide a powerful predictive tool. Both experimental and computational studies indicate that the slope is always greater than 1 and quantitatively agree on its value when comparisons could be made, suggesting a systematic thermodynamic preference for cleaving stronger hydrogen‑carbon bonds. The R M-C/H-C value is particularly high for (poly)fluorobenzenes, with the greatest increase in M–C BDE occurring when fluorine atoms are ortho to the M–C bond, as found by experiments. Values of R M-C/H-C almost up to 3 have been reported. The polarity of the M–C bond, greater than that of the H–C bond, is suggested to be the cause of the high value of R M-C/H-C and the associated observed high selectivity in C–H bond activation. Recent experimental studies suggested that Co is more selective than Rh and Ir for C–H bond activation in (poly)fluorobenzenes. A computational study of the thermodynamics of C–H activation in C 6 F n H 6-n ( n = 0, 5) by CpM(CO) (M = Co, Rh, Ir) confirms that R M-C/H-C is highest for Co and shows again a relation between R M-C/H-C and the polarity of the M–C bond. Additionally, the R M-C/H-C value is shown to be inversely related to the strength of the M–C bond since the Co–C BDE is the weakest for group 9 transition metals. Further studies are needed to confirm the generality of the finding that 3d transition metal complexes could be more selective in C–H activation than their heavier congeners. Correlations are also found between M–X and H–X (X = O, N) BDEs with slopes varying from 0.9 to 1.4. The ranking of M–X BDEs (M–C < M–N < M–O) is discussed as well as the magnitudes of the correlation slopes that are generally higher for aromatic substituents. Although this review is centred on reactions with thermodynamically controlled product distributions, some reactions are recognized in which kinetically controlled products follow thermodynamics. For all systems, computations appear to provide accurate information and can replace experiments if needed. Prediction of relative bond strengths in many organometallic species from knowledge of bond strengths for cognate organic molecules can be achieved.