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 <i>tert</i>-butyl groups of (^R4PCP)Ir (^R4PCP = κ³-C₆H₃-2,6-(CH₂PR₂)₂; R = ^tBu or Me). DFT calculations for (^R4PCP)Ir species (R₄ = ^tBu₄ or ^tBu₃Me) indicate that the rate-determining step in the <i>n</i>-alkane/1-alkene transfer dehydrogenation cycle is β-H elimination by (^R4PCP)Ir(<i>n</i>-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)IrH₄ 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)IrH₄ for alkane transfer dehydrogenation. (^tBu2Me2PCP)IrH₄ was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than (^tBu3MePCP)IrH₄. However, synthetic precursors of (^tBu2Me2PCP)IrH₄ 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.