N‑Bridged Pincer Iridium Complexes for Highly Efficient Alkane Dehydrogenation and the Relevant Linker Effects

• Published in: ACS catalysis 2020-06, Vol.10 (11), p.6475-6487
• Main Authors: Zhang, Xin, Wu, Song-Bai, Leng, Xuebing, Chung, Lung Wa, Liu, Guixia, Huang, Zheng
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.0c00539

A series of iridium complexes (RPOCR′NP)–HCl (4–HCl) and (RPSCR′NP)­Ir–HCl (5–HCl) (R = substituent on P; R′ = substituent on N) supported by N-linked pincer ligands, with the other linker being O- or S-atom, have been synthesized. Among them, complexes with phosphino-iPr substituents ( iPrPOCR′NP)Ir (4b, 4e, and 4g) exhibit very high catalytic activity for transfer dehydrogenation (TD) of both cyclic and linear alkanes. In the prototypical TD reaction of cyclooctane (COA) with tert-butylethylene (TBE), for example, 4g affords 14 720 turnovers at 200 °C, which are more than twofold of that obtained by the most efficient catalyst reported so far. Moreover, these complexes are highly effective for acceptorless dehydrogenation of 1,2,3,4-tetrahydronaphthalene, giving a turnover frequency of 8.8 min–1 for H2 production within the first 4 h. The effects of the linkers at the positions ortho to the Ir center have been elucidated by a systematic comparison of electronic and steric properties of these O/N- and S/N-linked pincer systems to those of C/C-linked iPrPCP, O/O-linked iPrPOCOP, and S/O-linked iPrPSCOP systems. Examination of the structure–activity relationships reveals that alkane C–H bond addition to the 14e (pincer)Ir fragment (the rate-determining step in the TD reaction) is more favored by iPrPOCR′NP relative to other pincers, largely due to the strong N-linker → C­(aryl) π-donation. Compared to the parent iPrPOCOP complex with similarly π-donating O-linkers, the energy of the alkene-bound out-of-cycle resting state is raised by iPrPOCR′NP primarily due to steric factors. Consequently, the incorporation of the N-linker exerts an overall favorable effect on the catalytic rates. In addition, we found that the thermal stability of these catalysts with different linker combinations differs significantly.