A pincer iridium(III) complex, (Phebox)Ir(OAc)2OH2 (Phebox = 3,5-dimethylphenyl-2,6-bis(oxazolinyl)), has been reported to dehydrogenate n-octane stoichiometrically, via C-H activation of n-octane followed by β-H elimination. Previous mechanistic study shows that β-H elimination is the rate-determining step. We report that the addition of simple Lewis acids such as Na+ or B(C6F5)3 dramatically catalyzes the β-H elimination step. For example, the rate of β-H elimination of (Phebox)Ir(OAc)(n-octyl) was determined to be 5.7 x 10-4 s-1 at 55 °C (half-life of ca. 20 min) in the presence of 0.5 mM Na[BArF] (BArF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), as compared with a much slower reaction at 200 °C in the absence of Na[BArF]. DFT calculations reveal the mechanism underlying this effect. In addition, the C-H activation step is also catalyzed by Lewis acid, although its effect on this step is relatively modest. With the assistance of Lewis acid, both acceptorless and transfer alkane-dehydrogenation systems catalyzed by phebox-ligated iridium complexes have been developed.

Alkane dehydrogenation co-catalyzed by an iridium(III) complex and Lewis acids. Catalyst design and mechanistic study

251st National Meeting (2016)

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