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.