Synthesis, Structure, and Reactivity of O-Donor Ir(III) Complexes: C−H Activation Studies with Benzene
Gaurav Bhalla, Xiang Yang Liu, Jonas Oxgaard, William A. Goddard III, Roy A. Periana
Abstract
Various new thermally air- and water-stable alkyl and aryl analogues of (acac-O,O)_2Ir(R)(L), R−Ir−L (acac-O,O = κ^2-O,O-acetylacetonate, −Ir− is the trans-(acac-O,O)_2Ir(III) motif, R = CH_3, C_2H_5, Ph, PhCH_2CH_2, L = Py) have been synthesized using the dinuclear complex [Ir(μ-acac-O,O,C^3)−(acac-O,O)(acac-C^3)]_2, [acac-C−Ir]_2, or acac-C−Ir−H_2O. The dinuclear Ir (III) complexes, [Ir(μ-acac-O,O,C^3)−(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 CH_3−Ir−Py as starting material at a constant ratio of [Py]/[C_6D_6] = 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_6H_6/C_6D_6 (KIE = 1) and with 1,3,5-C_6H_3D_3 (KIE ∼3.2 at 110 °C) are consistent with this reaction mechanism.
Group Members
Bhalla, G., Liu, X. Y., Oxgaard, J., III, W. A. G., & Periana, R. A. (2005). Synthesis, Structure, and Reactivity of O-Donor Ir(III) Complexes: C−H Activation Studies with Benzene. *J. Am. Chem. Soc.*, *127*(32), 11372-11389.