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 = 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 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)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.