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Volume 151, 2011
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Homogeneous dehydrogenation of liquid organic hydrogen carriers catalyzed by an iridium PCP complex

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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.

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Article information


Submitted
04 Jan 2011
Accepted
24 Jan 2011
First published
18 May 2011

Faraday Discuss., 2011,151, 297-305
Article type
Paper

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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