<p>Pincer-ligated iridium
complexes of the <a>form [Ir(<sup>R4</sup>PCP)L] (<sup>R4</sup>PCP
= κ<sup>3</sup>-C<sub>6</sub>H<sub>3</sub>-2,6-(XPR<sub>2</sub>)<sub>2</sub>; X
= CH<sub>2</sub>, O; R = <i>t</i>Bu, <i>i</i>Pr) </a>have previously been shown
competent for acceptorless alkane dehydrogenation when supported on silica. It
was observed by post-catalysis solid-state NMR that silica-tethered <a>[Ir(C<sub>2</sub>H<sub>4</sub>)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] </a>(<b>3-C<sub>2</sub>H<sub>4</sub></b>) was
converted fully to [Ir(CO)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)]
(<b>3-CO</b>) at 300 °C. In this work, the
characterization of species under dehydrogenation reaction conditions far from
equilibrium between butane and butenes (approach to equilibrium <i>Q</i>/<i>K</i><sub>eq</sub>
= 0.3 at 300 °C) is performed with <i>operando
</i>Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) to
show the kinetics of species conversion from <b>3-C<sub>2</sub>H<sub>4</sub></b> to <b>3-CO</b>. It is further found that [IrClH(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)]
(<b>3-HCl</b>), a species considered to be
a precatalyst for alkane dehydrogenation, is also fully converted to <b>3-CO</b>. A mechanism of decomposition is
proposed that implicates surface silanol groups, while carbon monoxide acts as
a “stabilizer” for the catalyst by promoting their reductive elimination and
maintaining the complex in the I oxidation state. </p>
Pincer-ligated iridium complexes of the form [Ir(R4 PCP)L] (R4 PCP = κ 3-C6H3-2,6-(XPR2)2; X = CH2, O; R = tBu, iPr) have previously been shown competent for acceptorless alkane dehydrogenation when supported on silica. It was observed by post-catalysis solid-state NMR that silica-tethered [Ir(C2H4)(≡SiO-tBu4 POCOP)] (3-C2H4) was converted fully to [Ir(CO)(≡SiO-tBu4 POCOP)] (3-CO) at 300 °C. In this work, the characterization of species under dehydrogenation reaction conditions far from equilibrium between butane and butenes (approach to equilibrium Q/Keq = 0.3 at 300 °C) is performed with operando Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) to show the kinetics of species conversion from 3-C2H4 to 3-CO. It is further found that [IrClH(≡SiO-tBu4 POCOP)] (3-HCl), a species considered to be a precatalyst for alkane dehydrogenation, is also fully converted to 3-CO. A mechanism of decomposition is proposed that implicates surface silanol groups, while carbon monoxide acts as a "stabilizer" for the catalyst by promoting their reductive elimination and maintaining the complex in the I oxidation state.
The production of olefins via on-purpose dehydrogenation of alkanes allows for a more efficient, selective and lower cost alternative to processes such as steam cracking. Silica-supported pincer-iridium complexes of the form [(≡SiO-R4 POCOP)Ir(CO)] (R4 POCOP = κ 3-C 6 H 3-2,6-(OPR 2) 2) are effective for acceptorless alkane dehydrogenation, and have been shown stable up to 300 °C. However, while solution-phase analogues of such species have demonstrated high regioselectivity for terminal olefin production under transfer dehydrogenation conditions at or below 240 °C, in open systems at 300 °C, regioselectivity under acceptorless dehydrogenation conditions is consistently low. In this work, complexes [(≡SiO-tBu4 POCOP)Ir(CO)] (1) and [(≡SiO-iPr4 PCP)Ir(CO)] (2) were synthesized via immobilization of molecular precursors. These complexes were used for gas-phase butane transfer dehydrogenation using increasingly sterically demanding olefins, resulting in observed selectivities of up to 77%. The results indicate that the active site is conserved upon immobilization. File list (2) download file view on ChemRxiv MS3 v22 preprint.pdf (751.71 KiB) download file view on ChemRxiv SI v11.pdf (1.59 MiB)
<p>While several metal phosphides have
attracted significant attention in the last several years due to their
potential use as photocatalytic and hydrotreating catalysts, iridium phosphide
has remained largely unexplored. In this work, silica-supported pincer-iridium
species are thermolyzed, resulting in deconstruction of the tridentate ligand
precursor and formation of a sub-nanometer iridium phosphide phase
characterized by <sup>31</sup>P magic angle spinning nuclear magnetic resonance
(<sup>31</sup>P-MAS-NMR), X-ray absorption spectroscopy (XAS), and high angle
annular dark field scanning transmission electron microscopy (HAADF-STEM). The
support material was found to play an active role in determining the product of
the surface thermolysis, with the silica supported material generating
phosphorus rich iridium phosphide nanoparticles. The resulting silica-supported
iridium phosphide phase is explored as a thermocatalyst for non-oxidative butane
dehydrogenation, achieving high initial reaction rates up to 900 mol<sub>butenes</sub> mol<sub>catalyst</sub><sup>-1</sup>
hr<sup>-1 </sup>and a terminal olefin selectivity of up to 70 %.</p>
<p>While several metal phosphides have
attracted significant attention in the last several years due to their
potential use as photocatalytic and hydrotreating catalysts, iridium phosphide
has remained largely unexplored. In this work, silica-supported pincer-iridium
species are thermolyzed, resulting in deconstruction of the tridentate ligand
precursor and formation of a sub-nanometer iridium phosphide phase
characterized by <sup>31</sup>P magic angle spinning nuclear magnetic resonance
(<sup>31</sup>P-MAS-NMR), X-ray absorption spectroscopy (XAS), and high angle
annular dark field scanning transmission electron microscopy (HAADF-STEM). The
support material was found to play an active role in determining the product of
the surface thermolysis, with the silica supported material generating
phosphorus rich iridium phosphide nanoparticles. The resulting silica-supported
iridium phosphide phase is explored as a thermocatalyst for non-oxidative butane
dehydrogenation, achieving high initial reaction rates up to 900 mol<sub>butenes</sub> mol<sub>catalyst</sub><sup>-1</sup>
hr<sup>-1 </sup>and a terminal olefin selectivity of up to 70 %.</p>
<p>Pincer-ligated iridium
complexes of the <a>form [Ir(<sup>R4</sup>PCP)L] (<sup>R4</sup>PCP
= κ<sup>3</sup>-C<sub>6</sub>H<sub>3</sub>-2,6-(XPR<sub>2</sub>)<sub>2</sub>; X
= CH<sub>2</sub>, O; R = <i>t</i>Bu, <i>i</i>Pr) </a>have previously been shown
competent for acceptorless alkane dehydrogenation when supported on silica. It
was observed by post-catalysis solid-state NMR that silica-tethered <a>[Ir(C<sub>2</sub>H<sub>4</sub>)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] </a>(<b>3-C<sub>2</sub>H<sub>4</sub></b>) was
converted fully to [Ir(CO)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)]
(<b>3-CO</b>) at 300 °C. In this work, the
characterization of species under dehydrogenation reaction conditions far from
equilibrium between butane and butenes (approach to equilibrium <i>Q</i>/<i>K</i><sub>eq</sub>
= 0.3 at 300 °C) is performed with <i>operando
</i>Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) to
show the kinetics of species conversion from <b>3-C<sub>2</sub>H<sub>4</sub></b> to <b>3-CO</b>. It is further found that [IrClH(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)]
(<b>3-HCl</b>), a species considered to be
a precatalyst for alkane dehydrogenation, is also fully converted to <b>3-CO</b>. A mechanism of decomposition is
proposed that implicates surface silanol groups, while carbon monoxide acts as
a “stabilizer” for the catalyst by promoting their reductive elimination and
maintaining the complex in the I oxidation state. </p>
<p>Pincer-ligated iridium
complexes of the <a>form [Ir(<sup>R4</sup>PCP)L] (<sup>R4</sup>PCP
= κ<sup>3</sup>-C<sub>6</sub>H<sub>3</sub>-2,6-(XPR<sub>2</sub>)<sub>2</sub>; X
= CH<sub>2</sub>, O; R = <i>t</i>Bu, <i>i</i>Pr) </a>have previously been shown
competent for acceptorless alkane dehydrogenation when supported on silica. It
was observed by post-catalysis solid-state NMR that silica-tethered <a>[Ir(C<sub>2</sub>H<sub>4</sub>)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)] </a>(<b>3-C<sub>2</sub>H<sub>4</sub></b>) was
converted fully to [Ir(CO)(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)]
(<b>3-CO</b>) at 300 °C. In this work, the
characterization of species under dehydrogenation reaction conditions far from
equilibrium between butane and butenes (approach to equilibrium <i>Q</i>/<i>K</i><sub>eq</sub>
= 0.3 at 300 °C) is performed with <i>operando
</i>Diffuse Reflectance Infrared Fourier-Transform Spectroscopy (DRIFTS) to
show the kinetics of species conversion from <b>3-C<sub>2</sub>H<sub>4</sub></b> to <b>3-CO</b>. It is further found that [IrClH(≡SiO-<i><sup>t</sup></i><sup>Bu4</sup>POCOP)]
(<b>3-HCl</b>), a species considered to be
a precatalyst for alkane dehydrogenation, is also fully converted to <b>3-CO</b>. A mechanism of decomposition is
proposed that implicates surface silanol groups, while carbon monoxide acts as
a “stabilizer” for the catalyst by promoting their reductive elimination and
maintaining the complex in the I oxidation state. </p>
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