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.