Supported Organoiridium-Pincer Catalysts for the Nonoxidative Dehydrogenation of High-Density Polyethylene

The iridium-pincer complex {p-OP(tBu)2-C6H2-2,6-OP(tBu)2]2}Ir(C2H4) (P[Ir]) has been reported
as a stable and
active catalyst toward alkane dehydrogenation in homogeneous and supported
heterogeneous systems. Dehydrogenation has been shown as a practical
method toward functional polyolefins, with dehydrogenated high-density
polyethylene (deHDPE) demonstrated as a valuable synthon for upcycling,
as orthogonal C–H strategies are key to end-of-life upcycling.
The heterogenization of P[Ir] on oxides (SiO2, Al2O3, and TiO2; P[Ir]/EyOx) yields a
mixture of organometallic Ir-fragments whose catalytic nonoxidative
dehydrogenation activity is modulated by the binding modes of the
active metal on the surface. The binding mode was elucidated by a
combination of solid-state NMR and XAFS analyses and supported by
DFT calculations. Surface binding through the ligand enables active
organoiridium that catalyzes internal olefination of deHDPE up to
1.23 mol % at 200 °C under dynamic vacuum. Alternatively, when
the organoiridium is bonded though the metal center (Ir–OSiO), catalyst activity is negligible. Furthermore, the catalytic
activity of P[Ir]/SiO2 showed comparable reactivity
with the homogeneous analogue under the same catalytic conditions,
and the heterogenized catalyst can be reused up to three cycles. This
work highlights the importance of understanding how organometallic
precursors react with hydroxylated metal oxide surfaces to establish
structure–property relationships.