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)2Ir(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 CH3−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 C6H6/C6D6 (KIE = 1) and with 1,3,5-C6H3D3 (KIE ∼3.2 at 110 °C) are consistent with this reaction
mechanism.