Origin of Regioselectivity in the Dehydrogenation of Alkanes by Pincer-Iridium Complexes: A Combined Experimental and Computational Study

PCP-pincer (kappa(3)-2,6-C6H3(CH2PR2)(2)) iridium complexes have been reported to catalyze the transfer dehydrogenation of n-alkanes with high regioselectivity for the terminal position. We find that the very closely related PCOP (kappa(3)-2,6-C6H3(CH2PR2)(OPR2)) and POCOP (kappa(3)-2,6-C6H3(OPR2)(2)) complexes, in contrast, afford no such regioselectivity. The difference is a true kinetic phenomenon, i.e., it is not a result of isomerization subsequent to the formation of free alpha-olefin. In addition to direct observation of the distribution of n-alkane dehydrogenation products over time, the pronounced difference in regioselectivity is confirmed through intermolecular competition studies of the reverse reaction (olefin transfer hydrogenation) and of the dehydrogenation of cycloalkane vs n-alkane. Electronic structure (DFT) calculations indicate that the rate-and selectivity-determining step for dehydrogenation by the (PCP)Ir complexes is beta-H transfer. C-H activation at the primary position is much more favorable than at secondary positions, but this is not responsible for the terminal regioselectivity; indeed, the formation of alpha-olefin via C2-H addition and transfer of the C1-H bond is calculated to be slightly more favorable than dehydrogenation proceeding via C1-H addition. For both PCP and POCOP complexes, the formation of the alpha-olefin iridium dihydride complex is more facile than the formation of internal-olefin complexes. The next step in the catalytic pathway, loss of olefin, is calculated to have an activation energy that is significantly greater than the metal-ligand (thermodynamic) bond energy. In the case of POCOP complexes, the loss of olefin, rather than beta-H transfer, is the rate-and selectivity-determining step. The hydrocarbon moiety in the transition state for olefin loss has the character of a fully formed olefin; this favors the formation of internal olefin. The different regioselectivity of (POCOP)Ir vs (PCP)Ir catalysts is thus attributable to the different rate-determining steps of their respective catalytic cycles; this in turn can be explained in terms of different electronic effects of O versus CH2 linker exerted through the pincer aromatic ring.

Automated summary

The PCP-pincer iridium complexes exhibit high regioselectivity for the transfer dehydrogenation of n-alkanes, while the closely related PCOP and POCOP complexes do not. The difference in regioselectivity is a true kinetic phenomenon, not a result of isomerization subsequent to the formation of free alpha-olefin. The different regioselectivity of the POCOPIr vs PCPIr catalysts is attributable to the different rate-determining steps of their respective catalytic cycles, which can be explained in terms of different electronic effects of O versus CH2 linker exerted through the pincer aromatic ring.