Catalysts which are selective for terminal dehydrogenation of alkanes are desirable for production of long-chain hydrocarbons from short-chain ones via dehydrogenation, subsequent alkene metathesis and hydrogenation. The mechanism of alkane dehydrogenation by pincer iridium complexes is thought to involve C-H bond activation, beta-hydride elimination and dissociation of the newly-formed olefin. The final step is hypothesized to be rate-determining for (C6H3)(OP(tBu)4)2Ir ((POCOP)Ir) but not (C6H3)(CH2P(tBu)4)2Ir ((PCP)Ir) based on density-functional theory calculations of the energies of the transition states, and the energetic barrier for olefin dissociation is hypothesized to depend significantly on the stability of the olefin in question. Therefore, internal dehydrogenation of linear alkanes is hypothesized to be kinetically favored for (POCOP)Ir because the olefin dissociation transition state, involving a relatively stable internal alkene, will be lower in energy than for the transition state in the case of terminal dehydrogenation. We test this hypothesis by determining the relative energies of various cyclic mono-enes by creating a system where transfer-dehydrogenation occurs between the desired cyclic alkenes and alkanes. Thermodynamic parameters for these reactions are reported, and these experimental results are compared to corresponding computationally-derived values from density functional theory. Kinetic isotope effect dehydrogenation competition studies are reported to establish if the rate-determining step of the dehydrogenation process is different for (PCP)Ir and (POCOP)Ir. Also, dehydrogenation competition experiments which determine the relative rate of dehydrogenation of different cyclic mono-enes for (PCP)Ir and (POCOP)Ir are also reported in order to establish if the lower-energy alkenes are preferentially dehydrogenated.

Regioselectivity of catalytic alkane dehydrogenation by pincer iridium complexes

248th National Meeting (2014)

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