Investigation of Iridium CF3PCP Pincer Catalytic Dehydrogenation and Decarbonylation Chemistry

The iridium fluorinated pincer complex ((PCP)-P-CF3)Ir-(cod) ((PCP)-P-CF3 = 2,6-C6H3(CH2P(CF3)(2))(2)) catalyzes hydrogen transfer from cyclooctane (coa) to tert-butylethylene (tbe) in 1/1 coa/tbe at 200 degrees 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% the contain significant amounts of isoprene (2-methyl-1,3-butadiene), which reacts with ((PCP)-P-CF3)Ir(cod) to initially form ((PCP)-P-CF3)Ir(isoprene). Alkene inhibition studies show that ((PCP)-P-CF3)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 N-2 or in the presence of excess water. Hydrogenation of ((PCP)-P-CF3)Ir(L) (L = cod, isoprene) did not produce ((PCP)-P-CF3)Ir(H)(x) but instead afforded the first example of the unusual aryl-bridged bimetallic complex [(mu-1 kappa(2)(P,C),2 kappa(2)(P',C)-(PCP)-P-CF3)Ir(H)(2)](2)(mu-(PCPH)-P-CF3)(mu-H), which has been isolated and crystallographically characterized. Ir(I) pincer complexes ((PCP)-P-CF3)Ir(L) (L = MeP(C2F5)(2), CO, dfepe (dfepe = (C2F5)(2)PCH2CH2P(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 degrees C) and in 1,4-dioxane (0.052 TO h(-1), 94 degrees C). The catalyst resting states are the corresponding five-coordinate carbonyl complexes ((PCP)-P-CF3)Ir(MeP(C2F5)(2))(CO), ((PCP)-P-CF3)Ir(CO)(2), and [((PCP)-P-CF3)Ir-(CO)](2)(mu-dfepe) (CO)](2)(mu-dfepe). DFT studies indicate that the preferred catalyst resting state for alkane dehydrogenation, ((PCP)-P-CF3)Ir-(cod), can be ascribed to the lower steric requirements of the CF3-substituted pincer ligand.