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Comparison of Isomerization Rates of the Metal Carbonyl Cluster Cp*IrCp2Co2(CO)3 in Three Oxidation States (47e, 48e, 49e):  Dramatic Rate Enhancements in the Odd-Electron Species

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journal contribution
posted on 26.03.1997, 00:00 by William E. Geiger, Michael J. Shaw, Martin Wünsch, Craig E. Barnes, Frank Holger Foersterling
The 48-electron cluster Cp*IrCp2Co2(CO)3 has two known isomers, one with a terminal carbonyl ligand and two edge-bridging carbonyls (1) and the other with three edge-bridging carbonyls (2). The rate of their interconversion is dramatically dependent on the number of electrons in the cluster. NMR studies establish that 2 is the thermodynamically favored isomer and that the isomerization is slow at ambient temperatures in the 48 e- complex (kisom ≈ 10-6 s-1 at 298 K). In contrast, isomerization proceeds very rapidly (kisom = 400 s-1) through the 47-electron monocation as part of an efficient electron-transfer-catalyzed process. Cyclic voltammetry and square-wave voltammetry were used to measure the isomerization rate of the monocation. The catalytic nature of the anodically-induced isomerization was diagnosed by theoretical modeling of the electrode responses and by infrared spectroelectrochemistry using a fiber-optic probe of an electrolysis solution. Reductions of the cluster isomers give 49-electron monoanions. The anion 1- isomerizes over the period of a bulk electrolysis to 2-, setting the limits of kisom between 10-1 and 10-3 s-1 for 1-. The relative rate of the cluster isomerization increases, therefore, in the order 48 e- ≪ 49 e- ≪ 47 e-, with relative rates of 1:≈104:108. Rate enhancements are rationalized in terms of changes in occupancies involving redox orbitals either bonding or antibonding with respect to the trimetallic framework. The results constitute a rare example of the determination of a reaction rate through three oxidation states of a complex.