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

Substitution-inert complexes such as Fe(CN)<sub>6</sub><sup>4−</sup> are usually considered to react by outer-sphere electron transfer (ET) with most electron acceptors, including aryl diazonium ions (ZC<sub>6</sub>H<sub>4</sub>N<sub>2</sub><sup>+</sup>, 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)<sub>6</sub><sup>4−</sup> show that this is not the case and that the reactions actually go via the formation of an adduct, a diazoisocyanide complex [ZC<sub>6</sub>H<sub>4</sub>N<sub>2</sub><sup>+</sup> + Fe(CN)<sub>6</sub><sup>4−</sup> → ZC<sub>6</sub>H<sub>4</sub>N<sub>2</sub>(NC)Fe(CN)<sub>5</sub><sup>3−</sup>]. The adduct decomposes heterolytically by expulsion of nitrogen either to form an isocyanide complex [ZC<sub>6</sub>H<sub>4</sub>N<sub>2</sub>(NC)Fe(CN)<sub>5</sub><sup>3−</sup> → ZC<sub>6</sub>H<sub>4</sub>(NC)Fe(CN)<sub>5</sub><sup>3−</sup> + N<sub>2</sub>] or the 4-substituted benzonitrile via a ligand exchange [ZC<sub>6</sub>H<sub>4</sub>N<sub>2</sub>(NC)Fe(CN)<sub>5</sub><sup>3−</sup> → ZC<sub>6</sub>H<sub>4</sub>CN + Fe(CN)<sub>5</sub><sup>3−</sup> + N<sub>2</sub>]. A competing homolytic decomposition resulting in an overall ET reaction occurs only to a minor extent, giving small amounts of Fe(CN)<sub>6</sub><sup>3−</sup>, ZC<sub>6</sub>H<sub>5</sub>, and various organic compounds. In oxygenated solutions ZC<sub>6</sub>H<sub>4</sub>N<sub>2</sub>(NC)Fe(CN)<sub>5</sub><sup>3−</sup> decomposes to Fe(CN)<sub>6</sub><sup>3−</sup> and ZC<sub>6</sub>H<sub>4</sub>OH. The measurements with Fe(CN)<sub>6</sub><sup>4−</sup> were supplemented by the study of the analogous reactions of Os(CN)<sub>6</sub><sup>4−</sup>, Mo(CN)<sub>8</sub><sup>4−</sup>, and W(CN)<sub>8</sub><sup>4−</sup>. 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 NO<sub>2</sub>C<sub>6</sub>H<sub>4</sub>N<sub>2</sub><sup>+</sup> and MeOC<sub>6</sub>H<sub>4</sub>N<sub>2</sub><sup>+</sup> 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)<sub>6</sub><sup>4−</sup> 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.