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Published August 17, 2005 | Supplemental Material
Journal Article Open

Synthesis, Structure, and Reactivity of O-Donor Ir(III) Complexes: C−H Activation Studies with Benzene

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.

Additional Information

© 2005 American Chemical Society. Received 9 March 2005. Published online 23 July 2005. Published in print 1 August 2005. We gratefully acknowledge financial support of this research by the Chevron Texaco Energy Research and Technology Co. and thank Dr. William Schinski of Chevron Texaco for helpful discussions. The authors would also like to thank Ms. Irina Tysba, Mr. NamHat Ho, Mr. Muhammed Yousufuddin, and Prof. Bau for X-ray crystallography. We thank David Laviska of Rutgers, The State University of New Jersey for helping with the editing of this manuscript.

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Supplemental Material - ja051532osi20050520_010731.cif

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Created:
August 19, 2023
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