Use of medium effects to tune the Delta E(1/2) values of bimetallic and oligometallic compounds

The redox potentials of bis(fulvalene)dinickel, 1, and the tetrakis(ferrocenyl)nickel dithiolene complex 2 have been measured in a variety of nonaqueous electrolytes. The difference in E1/2 values of the two successive one-electron oxidations of 1 (i.e., DeltaE1/2 values) increased from a low of 212 mV in anisole/[NBu4]Cl to a high of 850 mV in CH2Cl2-Na[B(C6H3(CF3)2)4], reflecting an increase of over 1010 in the comproportionation constant (Kcomp = [1+]2/[1][12+]). Six reversible one-electron processes are possible for compound 2, the four oxidations arising from the ferrocenyl substituents, and the two reductions arising from the Ni dithiolene moiety. The E1/2 spreads of the four oxidation waves and the two reduction waves are both highly sensitive to medium effects. For both 1 and 2, the largest DeltaE1/2 values for cationic products are found in solvents of low polarity and donor strength containing electrolyte salts having large anions and small cations. Conversely, the smallest DeltaE1/2 values for anionic products are found under these conditions, culminating in the observation of a single two-electron reduction wave for 2/22- in CH2Cl2-Na[B(C6H3(CF3)2)4]. A combination of solvation and ion-pairing effects must be considered, and may be used to advantage, when using DeltaE1/2 values as a measure of electronic interactions between redox centers in compounds containing two or more electron-transfer sites.