Iridium Complexes of Bulky CCC-Pincer N-Heterocyclic Carbene Ligands: Steric Control of Coordination Number and Catalytic Alkene Isomerization

Abstract

Three new iridium complexes of meta-phenylene-bridged bis-N-heterocyclic carbene CCC-pincer ligands were synthesized and characterized. For a pincer ligand with 2,6-diisopropylphenyl N-substituents, a six-coordinate iridium(III) complex of the formula Ir(CCC)HCl(MeCN) was formed. In contrast, ligands with t-butyl or adamantyl N-substituents gave five-coordinate iridium(III) complexes of the formula Ir(CCC)HCl. These iridium complexes, along with two previously described iridium complexes, were tested for activity in the catalytic transfer-dehydrogenation of n-octane at 150 °C. The new complexes were inactive for this reaction, while two previously reported catalysts were modestly active: a mesityl-substituted derivative gave 12 turnovers, and a 3,5-di-t-butylphenyl-substituted variant gave 10 turnovers. In contrast, these complexes were shown to be highly active catalysts for the isomerization of terminal alkenes, under conditions much milder than those required for transfer-dehydrogenation.

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].…”

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.…”

Alkene Isomerisation Catalysed by a Ruthenium PNN Pincer Complex

“…A reductive elimination can take place in two directions and will provide either the olefin isomer or the starting olefin. [42] Among the catalysts developed for this reaction, only a few belong to the family of the pincer complexes [43][44][45][46][47] and, to our surprise, the reactivity of PNN (PNN: 2-di-tert-butylphosphinomethyl-6-diethylaminomethylpyridine) ruthenium complexes with alkenes has not been explored. In particular, the pyridinebased PNN ruthenium pincer complexes are of great interest, due to the unique participation of the ligand in the reaction (non-innocent ligand).…”