Highly Active Catalysts for the Transfer Dehydrogenation of Alkanes: Synthesis and Application of Novel 7–6–7 Ring‐Based Pincer Iridium Complexes

Abstract

A series of Ir-PCP pincer precatalysts [(7-6-7-(R) PCP)Ir(H)(Cl)] and [(7-6-7-(Ar) PCP)Ir(H)(Cl)(MeCN)] bearing a novel "7-6-7" fused-ring skeleton have been synthesized based upon the postulate that the catalytic species would have durability due to their rather rigid structure and high activity owing to the low but sufficient flexibility of their backbones, which are not completely fixed. Treatment of these precatalysts with NaOtBu gave rise to the active 14 electron (14e) species [(7-6-7-(iPr) PCP)Ir] and [(7-6-7-(Ph) PCP)Ir], which can trap hydrogen and were spectroscopically characterized as the tetrahydride complexes. Both [(7-6-7-(iPr) PCP)Ir] and [(7-6-7-(Ph) PCP)Ir] were found to be highly effective in the transfer dehydrogenation of cyclooctane with tert-butylethylene as the hydrogen acceptor, the initial reaction rate at high temperature (230 °C) being higher for [(7-6-7-(iPr) PCP)Ir] than [(7-6-7-(Ph) PCP)Ir], and the turnover number (TON) of the overall hydrogen transfer being higher for the latter. Nonetheless, the estimated TONs were as high as 4600 and 4820 for the two complexes at this temperature, respectively, which are unprecedented absolute values. In terms of durability, the [(7-6-7-(Ph) PCP)Ir] complex is the catalyst of choice for this reaction. Structural analysis and computational studies support the importance of the low flexibility of the ligand core.

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

“…1 Examples of pincer-iridium complexes that have been investigated for alkane dehydrogenation Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes groups have been modified (Fig. 1) [58][59][60][61][62][63][64][65][66][67][68][69][70]. The Brookhart and Jensen groups reported a new class of catalysts wherein the benzylic CH 2 linkers of the PCP backbone were replaced by O linkers to obtain the ( R POCOP)Ir (21, R¼t-Bu;22, R¼i-Pr) systems [58][59][60]66].…”

“…1) for alkane transfer dehydrogenation [63]. These catalysts were tested with the hope that the rigid structure would impart high thermal stability, while the flexibility of the backbone would enhance catalytic activity.…”

“…Yamamoto and coworkers have also noted that 35 was more selective for primary dehydrogenation giving a higher ratio of COE:1,3-COD compared to 34. Catalyst 34 also gave high activity for the dehydrogenation of n-octane (6.0 M) with NBE (1.1 M) at 150 C; 35, however, showed poor solubility in n-octane and was not active for catalytic n-octane transfer dehydrogenation [63]. A schematic representation of our current best understanding of the catalytic cycles, illustrated for n-alkane/TBE transfer dehydrogenation and 1,2-α-olefin olefin isomerization by (pincer)Ir complexes, is shown in Scheme 1.…”

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product

“…3 Catalysts 1 and 2 were also found to be effective for the acceptorless dehydrogenation of alkanes. 2,4 Work with these complexes has been followed by reports of numerous catalytically active variants with the (PCP)Ir motif, [5][6][7][8][9] including other bis-phosphines, [10][11][12][13][14] bis-phosphinites (POCOP), [15][16][17][18] hybrid phosphine-phosphinites (PCOP), 19,20 arsines (AsOCOAs), 21 hybrid phosphine-thiophosphinites (PSCOP) 22 and hybrid amine-phosphinites (NCOP) 23 . In addition to simple alkane dehydrogenation, these complexes have been employed for numerous other catalytic transformations of hydrocarbons, including alkane metathesis, 6,8,9,20,[24][25][26] alkyl group metathesis, 27 dehydroaromatization, 19,28,29 alkanealkene coupling reactions, [30][31][32] borylation of alkanes 23 and the dehydrogenation of several non-alkane substrates.…”

Catalytic alkane dehydrogenations

“…In 2013, Yamamoto and co-workers [68] reported the synthesis of PCP pincer iridium precatalysts bearing fused ring backbones {(7-6-7-R 4 PCP)Ir(H)Cl} (R = iPr, 22a; R = Cy, 22b) and {(7-6-7-Ph 4 PCP)Ir(H)(Cl)(MeCN)} (22c) (Eq. (18)).…”