Are Reactions Between Metal Cyanides and Aryl Diazonium Ions Really Outer-Sphere Electron Transfer Processes?

Substitution-inert complexes such as Fe(CN)64− are usually considered to react by outer-sphere electron transfer (ET) with most electron acceptors, including aryl diazonium ions (ZC6H4N2+, where Z denotes a substituent on the benzene ring). However, in contrast to the conclusion drawn in a previous report (J. Am. Chem. Soc. 1987, 109, 1536−1540), kinetic studies and identification of products from the reactions of 4-nitro- and of 4-methoxybenzenediazonium with an excess of Fe(CN)64− show that this is not the case and that the reactions actually go via the formation of an adduct, a diazoisocyanide complex [ZC6H4N2+ + Fe(CN)64− → ZC6H4N2(NC)Fe(CN)53−]. The adduct decomposes heterolytically by expulsion of nitrogen either to form an isocyanide complex [ZC6H4N2(NC)Fe(CN)53− → ZC6H4(NC)Fe(CN)53− + N2] or the 4-substituted benzonitrile via a ligand exchange [ZC6H4N2(NC)Fe(CN)53− → ZC6H4CN + Fe(CN)53− + N2]. A competing homolytic decomposition resulting in an overall ET reaction occurs only to a minor extent, giving small amounts of Fe(CN)63−, ZC6H5, and various organic compounds. In oxygenated solutions ZC6H4N2(NC)Fe(CN)53− decomposes to Fe(CN)63− and ZC6H4OH. The measurements with Fe(CN)64− were supplemented by the study of the analogous reactions of Os(CN)64−, Mo(CN)84−, and W(CN)84−. The observation that isocyanide and even short-lived diazoisocyanide complexes are formed is in accordance with an inner-sphere mechanism. Further support of this conclusion comes from the observation that the slope of the activation-free energy plots for the reactions of NO2C6H4N2+ and MeOC6H4N2+ with the four metal cyanides is higher than that expected for an outer-sphere ET mechanism. The implication of these results are discussed in the context of the previous report (vide supra) on the extraction of the self-exchange reorganization energies for substituted benzenediazonium salts from their reactions with Fe(CN)64− and decamethylferrocene. Our conclusion is that Marcus theory is not applicable in the interpretation of the measured rate constants, thereby also precluding a determination of such energies.