Controlling the Selectivity of C–H Activation in Pyridinium Triazolylidene Iridium Complexes: Mechanistic Details and Influence of Remote Substituents

Iridium complexes containing a triazolylidene ligand with an appended methylpyridinium site undergo either aromatic C­(sp²)–H bond activation or exocyclic C­(sp³)–H bond activation of the N-bound methyl group. The selectivity of these bond activations is controlled by the remote substituent R of the triazolylidene ligand. Iterative computational and synthetic experiments provide evidence for more facile C­(sp²)–H bond activation for a variety of remote substituents with R = Me, CH₂C₆F₅, CH₂CH₂C₆H₅. For triazolylidene ligands with a benzylic substituent, C­(sp²)–H bond activation of this benzylic group is the lowest energy pathway and is competitive with aromatic pyridinium C–H bond activation. The generated cyclometalated species is metastable and undergoes, via an oxidative addition/reductive elimination sequence, a transcyclometalation with exclusive activation of the methyl C–H bond and thus leads to the C­(sp³)–H bond activated product. An experimental determination of activation energies as well as isomer ratios of the intermediates validates the computed pathways. The application of a transcyclometalation procedure to activate more challenging C­(sp³)–H bonds is unprecedented and constitutes an attractive concept for devising catalytic processes.