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-(OPBu(t)2)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 degrees C except in the presence a hydrogen acceptor, 3,3-dimethyl-1-butene (tbe). At temperatures above 150 degrees C, 3 showed high activity with 5 but only for the release of H2 across the C-N bond. Dehydrogenation of 6 at 150 degrees 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 degrees C giving mixtures that approached a approximately 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 degrees C but the products polymerize. The catalytic dehydrogenation of 9 was very efficient at 200 degrees C but gave primarily the undesired products dibenzylamine (68%), benzylidenebenzylamine (22%), ammonia and only minor amount of benzonitrile (10%). At 160 degrees 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 degrees C in the presence of 10 mol% NaOBu(t) but heating the reaction mixtures containing the base to 200 degrees C resulted in the decomposition of the pincer catalyst.