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Our laboratory has reported that (CX3Phebox)Ir(H)(OAc) (X = H, F) catalysts are highly active for the acceptorless dehydrogenation of n-alkanes1, particularly in the presence of Lewis acids. In this work we report the synthesis of isoelectronic (Pybox)Os(H)(OAc) and (Pybox)Ru(H)(OAc), and investigation of these complexes for alkane dehydrogenation. DFT calculations predict (Pybox)Ru(H)(OAc) to catalyze acceptorless alkane dehydrogenation with a barrier lower than that for (CH3Phebox)Ir(H)(OAc), while the barrier calculated for (Pybox)Os(H)(OAc) is even lower. The rate-limiting step chem. for the catalytic cycle is calculated to be a net M-H/C-H σ-bond metathesis reaction, although expulsion of H2 from the reaction mixture was found to be rate-determining under typical conditions for acceptorless n-alkane dehydrogenation catalyzed by (CF3Phebox)Ir(H)(OAc). H/D exchange experiments were used to probe the kinetics of C-H activation yielding the order of activity: (Pybox)Os(H)(OAc) > (Pybox)Ru(H)(OAc) > (CF3Phebox)Ir(H)(OAc). Exptl. investigation of catalysis by (Pybox)Ru(H)(OAc) and (Pybox)Os(H)(OAc) is still in progress but the Ru complex, unfortunately, does not appear to be stable at the high temperatures required for acceptorless alkane dehydrogenation. We have also reported that (CH3Phebox)Ir(C2H4)2 catalyzes selective dehydrogenative coupling of ethylene to butadiene via an iridacyclopentane complex.2 In this work we used the precursor (Pybox)OsH4 to investigate the same catalytic reaction and appears to result in and analogous dehydrogenative coupling of ethylene to form butadiene via an osmacyclopentane.
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Regioselective Gas‐Phase n ‐Butane Transfer Dehydrogenation via Silica‐Supported Pincer‐Iridium Complexes
Abstract 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−R4POCOP)Ir(CO)] (R4POCOP=κ3‐C6H3‐2,6‐(OPR2)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−tBu4POCOP)Ir(CO)] (1) and [(≡SiO−iPr4PCP)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.
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- Award ID(s):
- 1705746
- PAR ID:
- 10236534
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- ChemCatChem
- Volume:
- 13
- Issue:
- 1
- ISSN:
- 1867-3880
- Format(s):
- Medium: X Size: p. 407-415
- Size(s):
- p. 407-415
- Sponsoring Org:
- National Science Foundation
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