Investigation of Iridium ^CF₃PCP Pincer Catalytic Dehydrogenation and Decarbonylation Chemistry

Abstract

The iridium fluorinated pincer complex (CF3PCP)Ir(cod) (CF3PCP = 2,6-C6H3(CH2P(CF3)2)2) catalyzes hydrogen transfer from cyclooctane (coa) to tert-butylethylene (tbe) in 1/1 coa/tbe at 200 °C to give cyclooctene (coe) and neohexane (tba) at an initial rate of 40 TO h–1. In 5/1 coa/tbe, higher initial activity (155 TO h–1) and higher turnovers (2580 TON’s after 1450 min) are found. Samples of 95% tbe contain significant amounts of isoprene (2-methyl-1,3-butadiene), which reacts with (CF3PCP)Ir(cod) to initially form (CF3PCP)Ir(isoprene). Alkene inhibition studies show that (CF3PCP)Ir is only modestly inhibited (67% reduced initial activity) in the presence of 800 equiv of added coe. Unlike donor pincer systems, no decrease in activity is noted under 1 atm of N2 or in the presence of excess water. Hydrogenation of (CF3PCP)Ir(L) (L = cod, isoprene) did not produce (CF3PCP)Ir(H)x but instead afforded the first example of the unusual aryl-bridged bimetallic complex [(μ-1κ2(P,C),2κ2(P′,C)-CF3PCP)Ir(H)2]2(μ-CF3PCPH)(μ-H), which has been isolated and crystallographically characterized. Ir(I) pincer complexes (CF3PCP)Ir(L) (L = MeP(C2F5)2, CO, dfepe (dfepe = (C2F5)2PCH2CH2P(C2F5)2)) also serve as moderately active aldehyde decarbonylation catalyst precursors for 2-naphthaldehyde with similar activities in diglyme (1.7 TO h–1, 152 °C) and in 1,4-dioxane (0.052 TO h–1, 94 °C). The catalyst resting states are the corresponding five-coordinate carbonyl complexes (CF3PCP)Ir(MeP(C2F5)2)(CO), (CF3PCP)Ir(CO)2, and [(CF3PCP)Ir(CO)]2(μ-dfepe). DFT studies indicate that the preferred catalyst resting state for alkane dehydrogenation, (CF3PCP)Ir(cod), can be ascribed to the lower steric requirements of the CF3-substituted pincer ligand.