posted on 2021-09-14, 15:09authored bySoumik Biswas, Michael J. Blessent, Benjamin M. Gordon, Tian Zhou, Santanu Malakar, David Y. Wang, Karsten Krogh-Jespersen, Alan S. Goldman
PCP-pincer
(κ3-2,6-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 (κ3-2,6-C6H3(CH2PR2)(OPR2)) and
POCOP (κ3-2,6-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 α-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 β-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 α-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 α-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 β-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.