A new Fe(IV) superoxide: The perferrate isomerization re-examined

The perferrate anion [FeVIIO4]-, the elusive iron analog of permanganate, has been extensively investigated, yet key questions remain unresolved concerning its stability with respect to isomerization. A previous computational work had been unable to reproduce the stability trend between the peroxide and perferrate: [FeVO2(η2-O2)]- > [FeVIIO4]-. Herein, the manifold of possible structures is revisited, for the first time, using an MRCISD+Q/x2c-TZVP//CASPT2(25,17)/ANO-RCC-VDZP level of theory. Computationally, we not only reproduce the correct stability trend but also find that the most stable species is a cyclometalated superoxide [FeIVO2(η2-O2)]- (6B1 ground state), perferrate being less stable by +29.2 kcal mol-1. In parallel, gas-phase ions of [FeO4]- stoichiometry were generated via an alternative electrospray ionization-based approach and probed by collision-induced dissociation mass spectrometry, providing complementary evidence for the presence of O-O motifs in the experimentally generated ions, in agreement with the computational results.