Synthesis, structure, and reactivity of O-donor Ir(III) complexes: C-H activation studies with benzene

Various new thermally air- and water-stable alkyl and aryl analogues of (acac-O,O)₂lr(R)(L), R-Ir-L (acac-O,O = κ²-O,O- acetylacetonate, -Ir- is the trans-(acac-O,O)₂Ir(III) motif, R = CH₃, C₂H₅, Ph, PhCH₂CH₂, L = Py) have been synthesized using the dinuclear complex [Ir(μ-acac-O,O, C³)-(acac-O,O)(acac-C³)]₂, [acac-C-Ir] ₂, or acac-C-Ir-H₂O. The dinuclear Ir (III) complexes, [Ir(μ-acac-O,O,C³)-(acac-O,O)(R)]₂ (R = alkyl), show fluxional behavior with a five-coordinate, 16 electron complex by a dissociative pathway. The pyridine adducts, R-Ir-Py, undergo degenerate Py exchange via a dissociative mechanism with activation parameters for Ph-Ir-Py (ΔH‡ = 22.8 ± 0.5 kcal/mol; ΔS‡ = 8.4 ± 1.6 eu; ΔG‡_298K = 20.3 ± 1.0 kcal/mol) and CH ₃-Ir-Py (ΔH‡ = 19.9 ± 1.4 kcal/mol; ΔS‡ = 4.4 ± 5.5 eu; ΔG‡_298K = 18.6 ± 0.5 kcal/mol). The trans complex, Ph-Ir-Py, undergoes quantitatively trans-cis isomerization to generate cis-Ph-Ir-Py on heating. All the R-Ir-Py complexes undergo quantitative, intermolecular CH activation reactions with benzene to generate Ph-Ir-Py and RH. The activation parameters (ΔS‡ = 11.5 ± 3.0 eu; ΔH‡ = 41.1 ± 1.1 kcal/mol; ΔG‡_298k = 37.7 ± 1.0 kcal/mol) for CH activation were obtained using CH₃-Ir-Py as starting material at a constant ratio of [Py]/[C₆D₆] = 0.045. Overall the CH activation reaction with R-Ir-Py has been shown to proceed via four key steps: (A) pre-equilibrium loss of pyridine that generates a trans-five-coordinate, square pyramidal intermediate; (B) unimolecular, isomerization of the trans-five-coordinate to generate a cis-five-coordinate intermediate, cis-R-Ir-□; (C) rate-determining coordination of this species to benzene to generate a discrete benzene complex, cis-R-Ir-PhH; and (D) rapid C-H cleavage. Kinetic isotope effects on the CH activation with mixtures of C ₆H₆/C₆D₆ (KIE = 1) and with 1,3,5-C₆H₃D₃ (KIE ∼3.2 at 110 °C) are consistent with this reaction mechanism.