Electrophilic Organoiridium(III) Pincer Complexes on Sulfated Zirconia for Hydrocarbon Activation and Functionalization

Single-site
supported organometallic catalysts bring together the favorable aspects
of homogeneous and heterogeneous catalysis while offering opportunities
to investigate the impact of metal–support interactions on
reactivity. We report a (dmPhebox)­Ir­(III)
(dmPhebox = 2,6-bis­(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl)
complex chemisorbed on sulfated zirconia, the molecular precursor
for which was previously applied to hydrocarbon functionalization.
Spectroscopic methods such as diffuse reflectance infrared Fourier
transformation spectroscopy (DRIFTS), dynamic nuclear polarization
(DNP)-enhanced solid-state nuclear magnetic resonance (SSNMR) spectroscopy,
and X-ray absorption spectroscopy (XAS) were used to characterize
the supported species. Tetrabutylammonium acetate was found to remove
the organometallic species from the surface, enabling solution-phase
analytical techniques in conjunction with traditional surface methods.
Cationic character was imparted to the iridium center by its grafting
onto sulfated zirconia, imbuing high levels of activity in electrophilic
C–H bond functionalization reactions such as the stoichiometric
dehydrogenation of alkanes, with density functional theory (DFT) calculations
showing a lower barrier for β-H elimination. Catalytic hydrogenation
of olefins was also facilitated by the sulfated zirconia-supported
(dmPhebox)­Ir­(III) complex, while the homologous
complex on silica was inactive under comparable conditions.