Alkane Dehydrogenation Catalyzed by a Fluorinated Phebox Iridium Complex

We
report an iridium acetate complex with a fluorinated Phebox
ligand (2,6-bis­(4,4-dimethyl-4,5-dihydrooxazol-2-yl)-3,5-bis­(trifluoromethyl)­phenyl)
that is a highly effective catalyst for acceptorless dehydrogenation
of alkanes. Under typical acceptorless dehydrogenation conditions,
a high turnover frequency is obtained, which is limited by the rate
of expulsion of H2 from the reaction solution. Rates and
turnover numbers for acceptorless dehydrogenation are significantly
greater than found for the nonfluorinated analogue. As in the case
of the nonfluorinated analogue, Na+ acts as a cocatalyst
with the fluorinated catalyst again yielding greater rates and total
turnovers. Computational studies shed light on the possible mechanistic
pathways. The initial alkane activation is a net Ir–H/C–H
bond metathesis leading to the formation of an Ir–alkyl bond
and loss of H2; this is the slowest chemical step in the
cycle. The lowest-energy pathway is calculated to proceed via concerted
metalated deprotonation (CMD) of the alkane. Pathways proceeding via
transition states with oxidative addition (Ir­(V)) character, however,
are calculated to be only slightly higher in energy. These transition
states can lead either to Ir­(V) intermediates, which then lose H2, or connect directly to a dihydrogen complex. The role of
Na+ is largely to promote dechelation by coordinating to
an acetate oxygen, opening a vacant coordination site that allows
reaction with the alkane. This coordination by Na+ prevents
the CMD mechanism from operating, but it significantly lowers the
energy of the Ir­(V) TSs. NBO analysis shows a net transfer of charge
from the alkane atoms to the metal complex in the Ir­(V) TSs, with
and without coordinated Na+. Thus, the oxidative addition
is actually reductive in nature, driven in part by electrophilicity
of the metal center. The Na+ cation further increases electrophilicity
in addition to promoting dechelation. The greater activity of the
fluorinated catalyst compared with the parent complex can also be
explained in terms of the electrophilic nature of the reaction. The
fluorinated catalyst is also more resistant to decomposition than
the nonfluorinated analogue.