Acceptorless Alkane Dehydrogenation Catalyzed by Iridium CCC-Pincer Complexes

Abstract

Iridium complexes of CCC-pincer bis-N-heterocyclic carbenes, including a newly synthesized trifluoromethyl-substituted complex, were examined as catalysts for the acceptorless dehydrogenation of cyclooctane and n-undecane. Up to 103 turnovers were observed for the dehydrogenation of cyclooctane, and up to 97 turnovers were observed for the dehydrogenation of n-undecane. The catalysts showed high initial turnover frequencies, followed by a gradual loss of activity over 24 h. Experiments indicate that this loss of activity is due to catalyst decomposition rather than product inhibition. Stoichiometric reactivity was investigated for the precatalysts, focusing on the synthesis of dihydride and trihydride complexes as well as the dissociation and addition of neutral ligands.

Recent Advances in Alkane Dehydrogenation Catalyzed by Pincer Complexes

“…There are, however, reports of alkane dehydrogenation catalyzed by non-phosphine-based pincer-iridium complexes, mainly based on NCN [92,93] and CCC [84,[94][95][96] pincer motifs. Goldberg and coworkers have recently reported the use of a NCN-type pincer-type complex, ( dm Phebox)Ir(OAc) 2 (OH 2 ) (36) for the stoichiometric dehydrogenation of n-octane at 200 C resulting in the formation of ( dm Phebox)Ir(OAc)(H) (61) and octenes (7) [93].…”

“…Chianese and coworkers have investigated the acceptorless dehydrogenation of alkanes catalyzed by CCC pincer-iridium complexes (37)(38)(39)(40)(41)(42) [94]. Complexes 37, 38, and 42 were inactive for acceptorless dehydrogenation under the conditions applied.…”

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

“…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. 22,33,34 Several pincer motifs more recently explored, such as (CCC)Ir, [35][36][37][38] (PCP)Ru 39-41 , (PCP)Os 42 , and (NCN)Ir 43,44 have been found to show promise for alkane dehydrogenation but as of yet none have proven to be competitive with the well investigated PCP-type iridium-based systems. 26 In early alkane dehydrogenation studies 45 3,3-dimethyl-1-butene (TBE) was found by Crabtree to be a singularly effective hydrogen acceptor.…”

Synthesis and complexes of imidazolinylidene-based CCC pincer ligands

“…6 Based on the reported procedures for B, iridium complexes of the N-aryl substituted pro-ligands, 5a and 5b, were obtained as analytically pure materials from reaction between [Ir(COE) 2 Cl] 2 , 1a/1b, and Et 3 N in acetonitrile at elevated temperature, but in low isolated yields of 13 and 11%, respectively (Scheme 3). 18 Under these conditions analysis, by 1 H NMR spectroscopy, of similar reactions carried out with 1c and 1d, however, showed no meaningful evidence for formation of the desired hydride containing products.…”