Identification of the Reactive cis,mer Isomer of [Ir(CO)2I3Me]−: Relation to the Mechanism of Iridium-Catalyzed Methanol Carbonylation

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• 作者：Anthony Haynes ; Anthony J. H. M. Meijer ; James R. Lyons ; Harry Adams
• 刊名：Inorganic Chemistry
• 出版年：2009
• 出版时间：January 5, 2009
• 年：2009
• 卷：48
• 期：1
• 页码：28-35
• 全文大小：241K
• 年卷期：v.48,no.1(January 5, 2009)
• ISSN：1520-510X

文摘

Thermal dissociation of CO from cis,fac-[Ir(CO)₂I₃Me]⁻ (1a) gives the iodide-bridged dimer [{Ir(CO)I₂(μ-I)Me}₂]²⁻, which was characterized crystallographically as its Ph₄As⁺ salt. This dimer reacts with CO at ambient temperature to give the acetyl complex trans,mer-[Ir(CO)₂I₃(COMe)]⁻. An intermediate in this reaction is the previously unobserved cis,mer-[Ir(CO)₂I₃Me]⁻ (1b), which was characterized by IR and NMR spectroscopy. Carbonylation of 1b is much faster (t_1/2 1 min at 25 °C) than for the cis,fac isomer 1a and also faster than the neutral tricarbonyl [Ir(CO)₃I₂Me]. The observations show that the relative positioning of carbonyl ligands influences reactivity more than than their number, and that CO insertion is particularly accelerated when a CO ligand is placed trans to the migrating methyl group. DFT calculations indicate that the Ir−CO bond trans to methyl contracts significantly as 1b approaches the transition state for methyl migration, facilitating stronger π-backbonding from Ir to the spectator CO ligand, which stabilizes the transition state. The results confirm the recently proposed CO-loss mechanism for the photochemical carbonylation of 1a and suggest that the cis,mer isomer may play a minor role in the catalytic cycle for methanol carbonylation.