Use of weakly coordinating anions to develop an integrated approach to the tuning of ΔE1/2 values by medium effects

An integrated approach to the control of Delta E-1/2 values, and therefore comproportionation equilibria, through medium effects was delineated for multi-step redox reactions involving cationic products. Delta E-1/2 values (defined as E-1/2(2+/1+) - E-1/2(1+/0)) of the two one-electron oxidations of bis(fulvalene)dinickel, 1, were measured under 45 different conditions of solvent and supporting electrolyte. The smallest value, 212 mV, was found in anisole/0.1 M [NBU4]Cl and the largest, 850 mV, in CH2Cl2/0.02 M Na[B(C6H3(CF3)(2))(4)]. By systematically changing the solvent properties, the degree of ion-pairing strengths of the supporting electrolyte ions, and the concentration of the electrolytes, a set of ideal properties was found for maximizing Delta E-1/2 values involving positively charged electrode products. Most importantly (i) the solvent must be of lower polarity and low donor strength and (ii) the supporting electrolyte must have a weakly coordinating anion (WCA). The contrast in ion-pairing tendencies of 1(2+) with WCAs (on the weak side) and halides (on the strong side) mimics that of dianions in THF, where longer chain tetraalkylammonium ions (weak ion pairing) contrast with alkali metal ions (strong ion pairing). The broad picture of medium effects is that of a "mirror image" of solvent and electrolyte properties that influence the tuning of Delta E-1/2 values for multi-electron systems. Application was made both to the four-step oxidation process and to the two-step reduction process of the tetraferrocentyl complex Ni(S2C2FC2)(2), Fc = Fe(C5H5)(C5H4), 2. The two-electron process 2(0/2-) is observed either as a single two-electron voltammetric wave or as two well-separated one-electron waves, depending on the medium. The consequences of the present model for the interpretation of Delta E-1/2 values in mixed-valence chemistry are discussed.