Volume 151, 2011

Homogeneous dehydrogenation of liquid organic hydrogen carriers catalyzed by an iridium PCP complex

Abstract

The activity of the PCP pincer complex IrH2{C6H3-2,6-(OPBut2)2}, (3) as a catalyst for the dehydrogenation of the potential liquid organic hydrogen carriers: perhydro-dibenzofuran (4), perhydro-indole (5), N-methyl perhydro-indole (6), 4,4′-bipiperridine (7), 4-amino-methylpiperridine (8), and aminomethylcyclohexane (9) was investigated. Only low levels of dehydrogenation were achieved with 4 at 150 and 200 °C except in the presence a hydrogen acceptor, 3,3-dimethyl-1-butene (tbe). At temperatures above 150 °C, 3 showed high activity with 5 but only for the release of H2 across the C–N bond. Dehydrogenation of 6 at 150 °C was found to release up to 2.9 wt% H2, giving N-methyl tetrahydroindole in 92% yield, but only 7% N-methyl indole. However, efficient dehydrogenation of the aliphatic hydrogens occurred at 200 °C giving mixtures that approached a ∼1 : 3 equilibrium between N-methyl indole and N-methyl tetrahydroindole at longer reaction times. The pincer catalyst was observed to be effective for the dehydrogenation of 7 and 8 at 200 °C but the products polymerize. The catalytic dehydrogenation of 9 was very efficient at 200 °C but gave primarily the undesired products dibenzylamine (68%), benzylidenebenzylamine (22%), ammonia and only minor amount of benzonitrile (10%). At 160 °C, 85% of the substrate was consumed after 24 h of heating but only a minor amount of cyclohexanecarbonitrile (2%) was detected with the major products being biscyclohexylmethylamine (45%) and cyclohexylmethyl-cyclohexylimine (38%). The nitrile yield is remarkably improved to 97% when the dehydrogenation was carried out at 160 °C in the presence of 10 mol% NaOBut but heating the reaction mixtures containing the base to 200 °C resulted in the decomposition of the pincer catalyst.

Article information

Article type
Paper
Submitted
04 Jan 2011
Accepted
24 Jan 2011
First published
18 May 2011

Faraday Discuss., 2011,151, 297-305

Homogeneous dehydrogenation of liquid organic hydrogen carriers catalyzed by an iridium PCP complex

Z. Wang, J. Belli and C. M. Jensen, Faraday Discuss., 2011, 151, 297 DOI: 10.1039/C1FD00002K

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