Photochemical and Time Resolved Spectroscopic Studies of Intermediates Relevant to Iridium-Catalyzed Methanol Carbonylation: Photoinduced CO Migratory Insertion

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• 作者：Maurizio Volpe ; GuangWu ; Alexei Iretskii ; and Peter C. Ford
• 刊名：Inorganic Chemistry
• 出版年：2006
• 出版时间：February 20, 2006
• 年：2006
• 卷：45
• 期：4
• 页码：1861 - 1870
• 全文大小：209K
• 年卷期：v.45,no.4(February 20, 2006)
• ISSN：1520-510X

文摘

Photoreaction, time-resolved infrared (TRIR), and DFT studies were utilized to probe transformations betweeniridium complexes with possible relevance to the mechanisms of the iridium/iodide-catalyzed methanol carbonylationto acetic acid. Solution-phase continuous and laser flash photolysis of the tetraphenylarsonium salt of the fac-[CH₃Ir(CO)₂I₃]^- anion (1a) under excess carbon monoxide resulted in migratory insertion to give the acyl complexion mer,trans-[Ir(C(O)CH₃)(CO)₂I₃]^- (2a). The latter was isolated as its AsPh₄⁺ salt, and its X-ray crystal structurewas determined. TRIR spectra indicate that several transients are generated upon flash photolysis of 1a. Theprincipal photoreaction is CO dissociation, and this is proposed to generate the isomeric complexes fac-[CH₃Ir(CO)(Sol)I₃]^- (I_CO(fac), Sol = solvent) and mer,trans-[CH₃Ir(CO)(Sol)I₃]^- (I_CO(mer)). I_CO(fac) reacts with CO toregenerate 1a with a second-order rate constant (k_CO) ~2.5 × 10⁷ M^-1 s^-1 in ambient dichloroethane, while I_CO(mer) is the apparent precursor to 2a. Kinetics studies indicate the photoinduced formation of a third intermediate(I_M), hypothesized to be the anionic acyl complex fac-[Ir(C(O)CH₃)(CO)(Sol)I₃]^-. In the absence of added CO,these intermediates undergo dimerization to form a mixture of isomers with the apparent formula [Ir(C(O)CH₃)(CO)I₃]₂^2-.One of these dimers was isolated as the AsPh₄⁺ salt, and the crystal structure was determined. Addition of excesspyridine to a solution of the dimers gave the neutral complex mer,trans-[Ir(C(O)CH₃)(CO)(py)₂I₂], which wascharacterized by FTIR, NMR, and X-ray crystallography. These transformations, especially the unprecedentedphotoinduced CO insertion reaction, are discussed and interpreted in terms of the factors favoring migratory insertiondynamics.