A comparison of C-F and C-H bond activation by zerovalent Ni and Pt: a density functional study

Density functional theory indicates that oxidative addition of the C−F and C−H bonds in C6F6 and C6H6 at zerovalent nickel and platinum fragments, M(H2PCH2CH2PH2), proceeds via initial exothermic formation of an η2-coordinated arene complex. Two distinct transition states have been located on the potential energy surface between the η2-coordinated arene and the oxidative addition product. The first, at relatively low energy, features an η3-coordinated arene and connects two identical η2-arene minima, while the second leads to cleavage of the C−X bond. The absence of intermediate C−F or C−H σ complexes observed in other systems is traced to the ability of the 14-electron metal fragment to accommodate the η3-coordination mode in the first transition state. Oxidative addition of the C−F bond is exothermic at both nickel and platinum, but the barrier is significantly higher for the heavier element as a result of strong 5dπ−pπ repulsions in the transition state. Similar repulsive interactions lead to a relatively long Pt−F bond with a lower stretching frequency in the oxidative addition product. Activation of the C−H bond is, in contrast, exothermic only for the platinum complex. We conclude that the nickel system is better suited to selective C−F bond activation than its platinum analogue for two reasons: the strong thermodynamic preference for C−F over C−H bond activation and the relatively low kinetic barrier.