Catalyst Inhibition by Ground State Stabilization

As shown above. Fig. 7.11, many systems that activate CH bonds are now known and it is possible that if these systems can be made stable that some could be used as the basis for development of catalysts for alkane oxidation. In many cases, the CH activation rates of these reported systems are quite rapid when rates are extrapolated to temperatures of -200 °C. On the basis of these observations and assuming that stable motifs can be identified that would facilitate CH activation and functionalization, it might be assumed that acceptable catalysis rates can be readily obtained by simply basing catalyst designs on these CH activation systems. However, this is not likely to be the case and other considerations need to be taken into account 

As discussed for the Hg(II) and (bpym)Pt(II) systems, an issue with many of the reported CH activation systems is severe inhibition of the catalyst by water and/or methanol. Indeed, the CH activation is typically a rapid reaction only when the reaction system is carefully chosen such that the alkane is the most (or only) coordinating species present. However, under conditions where useful products such as alcohols or other coordinating species can be produced, many of the reported CH activation systems, even if they can be made stable, would exhibit very low rates. Significantly, this type of inhibition is likely to be a general issue with oxidation systems based on the CH activation reaction (as defined above). 

One fundamental reason for this is that the alkane CH bond, unlike CC double bonds of olefins or other functional groups, are among the poorest known ligands and unlikely to compete well with other more coordinating species for the metal center. This may well be the Achilles heel of hydroxylation catalysts based on the CH activation reaction. Consistent with the poor ligating capability of alkanes, other than by spectroscopic methods and mechanistic studies, only 

These considerations point to a fundamental issue that must be overcome in developing hydroxylation catalyst systems based on the CH activation reactions coordination reactions with alkanes (whether associative or dissociative) leading to weakly bound, intermediate alkane complexes, or direcdy to a transition state leading to CH cleavage, can be expected to be subject to severe ground state inhibition by the desired products such as methanol or by most media or reac- 

As can be seen from Fig. 7.21, the more stable this state is, the higher the expected activation barrier for CH Activation. 

---

Catalyst Modifications that Minimize Catalyst Inhibition by Ground State Stabilization... 

---