Homogeneous Catalyst for Alkane Dehydrogenation

The removal of hydrogen from alkanes to give alkenes is an important commercial objective, as alkenes are widely used as organic feedstocks in industrial processes involving chemical synthesis and polymerisation. This reaction is, however, significantly endothermic, needing up to 30 kcal per mole, and normally occurring at temperatures exceeding 400°C in the presence of heterogeneous supported metal catalyst systems in the gas phase. The conversion of alkanes to aromatic molecules (as part of 'catalytic reforming') is even more important and is also effected using supported metal catalysts, such as platinum on high surface area alumina. For these kinds of reactions to be achieved with a homogeneous catalyst in solution, under milder conditions, the use of hydrogen acceptors has been thought essential; that is, the conversions have been achieved via 'transfer hydrogenation'. A possible alternative means for achieving the dehydrogenation reaction would be irradiation with UV light, but this is not usually industrially attractive. In early work, R. H. Crabtree and co-workers (1) used IrH, { O,CCF,} (PPr'& to effect the dehydrogenation of alkanes to arenes via oxidative addition at 150°C in the presence of the hydrogen acceptor, ten-butylethylene. However, it was thought that this system was not catalytic because hydrogenolysis of the phosphine P-C bonds occurs at temperatures of 135°C or above: the temperatures needed to release arenes from the intermediate complexes. The five-co-ordinate iridium P-C-P pincer complex (PCP)IrH, 0, where PCP = q3-C0H3[CH,P(m-CJ19)2]2-1,3, was reported by Jensen, Kaska and their colleagues (2 ) to be a highly active homogeneous catalyst for the transfer dehydrogenation of cyclooctane, with unusually long-term stability for temperatures as high as 200°C. This catalyst was subsequently demonstrated to be useful for the catalytic transfer dehydrogenation of cycloalkanes to arenes (3). For example, cyclohexane was dehydrogenated to cyclohexene and benzene in the presence of 0 and ten-butylethylene, the latter being converted almost quantitatively into reen-butylethane. This special reactivity is ascribed to the presence of the P-C-P ligand, which renders the metal centre reactive with saturated hydrocarbons, but restricts its access to the ligand P-C bonds. It was claimed that the P-C-P pincer complex could promote accessibility to previously unattainable catalytic pathways.