Abstract
New iridium(III)-bipyridonate complexes having cyclopentadienyl ligands with a series of alkyl substituents were synthesized for the purpose of tuning the catalytic activity for acceptorless dehydrogenation reactions. A comparison of the catalytic activity was performed for the reaction of alcoholic substrates such as 1-phenylethanol, 2-octanol, and benzyl alcohol. The 1-t-butyl-2,3,4,5-tetramethylcyclopentadienyl iridium complex exhibited the best performance, which surpassed that of the 1,2,3,4,5-pentamethylcyclopentadienyl (Cp*) iridium catalyst in the dehydrogenation reaction of alcohols. The catalytic activity in the dehydrogenation of 2-methyl-1,2,3,4-tetrahydroquinoline was also examined. The highest efficiency was obtained in the reaction catalyzed by the same t-butyl-substituted cyclopentadienyl iridium complex.
Highlights
Dehydrogenation of small organic molecules without using external oxidants is an attractive transformation reaction from the viewpoint of excellent atomic efficiency [1,2,3,4]
A catalytic system has been applied to the dehydrogenation reaction of N-heterocyclic compounds for use in hydrogen storage (Scheme 1b) [23,24,25,26,27,28,29,30]
Our research group has consistently studied the catalytic activity of pentamethylcyclopentadienyl (Cp*) iridium complexes for the hydrogen transfer process of alcoholic substrates [31,32,33,34,35]
Summary
Dehydrogenation of small organic molecules without using external oxidants (i.e., acceptorless dehydrogenation) is an attractive transformation reaction from the viewpoint of excellent atomic efficiency [1,2,3,4]. The resulting hydrogen gas can be used as a promising energy carrier owing to its high weight energy density and carbon neutrality These characteristics make the significance of acceptorless dehydrogenation much greater in the field of organic synthesis as well as energy science [4,5,6,7,8,9]. By combining hydroxypyridine or dihydroxybipyridine derivatives as non-innocent ligands, the Cp* iridium complex shows an extremely high catalytic activity in acceptorless dehydrogenation reactions of alcoholic substrates and. A cyclopentadienyl ligand complexes donates six that electrons to a metal center with tridentate coordination mode, which results(Cp) in stable are widely used as catalysts.
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