Highly Active and Recyclable Heterogeneous Iridium Pincer Catalysts for Transfer Dehydrogenation of Alkanes

Abstract

Pincer-ligated iridium complexes have proven to be highly effective catalysts for the dehydrogenation and transfer-dehydrogenation of al-A C H T U N G T R E N N U N G kanes. Immobilization onto a solid support offers significant potential advantages in the application of such catalysts particularly with respect to catalyst separation and recycling. We describe three approaches toward such immobilization: (i) covalent attachment to a Merrifield resin, (ii) covalent bonding to silica via a pendant alkoxysilane group, and (iii) adsorption on g-alumina (g-Al 2 O 3 ), through basic functional groups on the para-position of the pincer ligand. The simplest of these approaches, adsorption on g-Al 2 O 3 , is also found to be the most effective, yielding catalysts that are robust, recyclable, and comparable to or even more active than the corresponding species in solution. Spectroscopic evidence (NMR, IR) and studies of catalytic activity support the hypothesis that binding occurs at the para-substituent and that this has only a relatively subtle and indirect influence on catalytic behavior.

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

“…Over the years many research groups have reported variants of 11 where either the aryl backbone has been altered [54][55][56][57] Fig. 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.…”

“…Varying the electronic component by introduction of methoxy (16/17), dimethylamino (18), and ester (19) functionalities to the para-position of the arene backbone in PCP pincer-iridium systems was found to have a modest effect on catalytic activity [54,55,59]. Roddick and coworkers have reported the synthesis of a series of PCP iridium complexes with bis(trifluoromethyl)phosphino groups [85][86][87].…”

“…Based on results by Brookhart and Goldman in which p-substituted tBu4 PCP complexes, including ( p-Me 2 N-tBu4 PCP)Ir [55,103] were anchored to solid supports such as γ-alumina, the Brookhart group synthesized 65d with a dimethylamino group on the triptycene backbone. Catalysts 65a and 65d (1 mM) gave comparable activities (2,800 and 2,040 TON, respectively, after 20 h) for solution-phase COA (3.9 M)/TBE (3.9 M) transfer dehydrogenation at 200 C. Attempting to anchor 65d on basic γ-alumina and acidic γ-alumina, however, resulted in catalyst decomposition yielding only 185 and 70 TON, respectively, for the same transfer dehydrogenation (also at 200 C).…”

Catalytic alkane dehydrogenations

“…Recently, Brookhart and co-workers [93] prepared Merrifield resin-, silica-, and c-Al 2 O 3 -supported pincer iridium complexes and investigated their activity for alkane dehydrogenation. The POCOP pincer catalysts supported by Merrifield resin and silica showed low to moderate activity for the COA/TBE transfer dehydrogenation reaction.…”

A catalytic synthesis of dialkylamines from alkylamines using neopentyl-substituted PNP pincer–iridium complex

“…Among them, bulky and electron-rich pincer/Ir system has been utilized to dehydrogenation of alkanes [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. The key to succeed this unique transformation may be due to remarkable thermodynamic stability of pincer-ligated metal complex, resulted from tight coordination to the metal center.…”