Rational design and synthesis of highly active pincer-iridium catalysts for alkane dehydrogenation

"PCP" -pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ( ^R4PCP)Ir ( ^R4PCP = κ ³-C ₆H ₃-2,6-(CH ₂PR ₂) ₂; R = ¹Bu or Me). DFT calculations for ( ^R4PCP)Ir species (R ₄ = ^tBu ₄ or ^tBu ₃Me) indicate that the ratedetermining step in the n-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by ( ^R4PCP)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ( ^tBu3MePCP)Ir than for ( ^tBu4PCP)Ir (relative to the corresponding free ( ^R4PCP)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ( ^tBu3MePCP)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for- ^tBu substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ( ^tBu3MePCP)IrH ₄ has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ( ^tBu4PCP)IrH ⁴ for alkane transfer dehydrogenation. ( ^tBu2Me2PCP) IrH ⁴ was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ( ^tBu3MePCP)IrH ₄. However, synthetic precursors of ( ^tBu2Me2PCP)IrH ⁴ tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of ( ^tBu2Me2PCP)Ir vs ( ^tBu3MePCP)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.