Alkane Dehydrogenation and H/D Exchange by a Cationic Pincer-Ir(III) Hydride: Cooperative C–H Addition and β‑H Elimination Modes Induce Anomalous Selectivity

We report that the cationic iridium complex (iPrPCP)­IrH+ catalyzes the transfer-dehydrogenation of alkanes
to give
alkenes and hydrogen isotope exchange (HIE) of alkanes and arenes.
Contrary to established selectivity trends found for C–H activation
by transition metal complexes, strained cycloalkanes, including cyclopentane,
cycloheptane, and cyclooctane, undergo C–H addition much more
readily than n-alkanes, which in turn are much more
reactive than cyclohexane. Aromatic C–H bonds also undergo
H/D exchange much less rapidly than those of the strained cycloalkanes,
but much more favorably than cyclohexane. The order of reactivity
toward dehydrogenation correlates qualitatively with the reaction
thermodynamics, but the magnitude is much greater than can be explained
by thermodynamics. Accordingly, the cycloalkenes corresponding to
the strained cycloalkanes undergo hydrogenation much more readily
than cyclohexene, despite the less favorable thermodynamics of such
hydrogenations. Computational (DFT) studies allow rationalization
of the origin of reactivity and the unusual selectivity. Specifically,
the initial C–H addition is strongly assisted by β-agostic
interactions, which are particularly favorable for the strained cycloalkanes.
Subsequent to α-C–H addition, the H atom of the β-agostic
C–H bond is transferred directly to the hydride ligand of (iPrPCP)­IrH+ to give a dihydrogen ligand. The overall
processes, C–H addition and β-H-transfer to hydride,
are calculated to generally have minima on the IRC surface although
not necessarily on the enthalpy or free energy surfaces; these minima
are extremely shallow such that the 1,2-dehydrogenations are effectively
concerted although asynchronous.