Control of Ligand Substitution and Addition Reactions of (Arene)Cr(CO)₃ Complexes by Attachment of a Self-Closing Redox Switch

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Abstract

The ferrocenyl group attached to the benzene ring in (C6H5Fc)Cr(CO)3 (3) functions as a reversible redox switch by providing a mechanism for the chromium center to be oxidatively activated to ligand substitution and addition reactions. Because initial oxidation of 3 occurs at the ferrocenyl group, the potentials required for activation are far lower than would otherwise be the case, and this allows for controlled reactions without the unproductive decomposition that often accompanies oxidation of (arene)Cr(CO)3 complexes. Although the chromium in 3+ is predominantly an 18-electron center, it is sufficiently activated to undergo very rapid CO substitution by P(OEt)3 to afford (C6H5Fc)Cr(CO)2P(OEt)3+ (4+). Internal electron transfer from Cr to Fe accompanies this CO substitution, with the result that the ferrocenium group in 3+ reverts to ferrocenyl in 4+, thus closing the redox switch. Upon oxidation of 4+ to 42+, the switch reopens and the chromium center is again activated, this time being attacked by P(OEt)3 to give the purple addition complex, (C6H5Fc)Cr(CO)2[P(OEt)3]22+ (52+), which is readily isolated. The formation of 52+ from 42+ is also accompanied by internal electron transfer from Cr to Fe and automatic switch closing to afford a product with 18-electron centers. The degree of activation of 3 upon oxidation of the ferrocenyl switch is shown to be directly related to the difference in oxidation potentials of the chromium and iron centers. These results suggest that the attachment of a remote redox switch may be a useful general methodology for the quantitative control of the reactivity of a metal center.

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