Use of Weakly Coordinating Anions to Develop an Integrated Approach to the Tuning of ΔE_1/2Values by Medium Effects

Abstract

An integrated approach to the control of deltaE(1/2) values, and therefore comproportionation equilibria, through medium effects was delineated for multi-step redox reactions involving cationic products. deltaE(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 deltaE(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 deltaE(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 tetraferrocenyl 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 deltaE(1/2) values in mixed-valence chemistry are discussed.

Anion Effects to Electrochromic Properties of Ru-based Metallo-supramolecular Polymers

“…These results indicate that the "hard" anions such as Cl -and CO 3 2-prevent the redox of Ru ions due to the strong ion pair formation with "hard" Li ions, such as Li + -H 2 O-Cl -and Li + -CO 3 2-, in the electrolyte solution. [18][19][20] On the contrary, the "soft" anions such as TFSI -, CF 3 SO 3 -, BF 4 -, ClO 4 -and PF 6 -have less ion-pairing effects because of the low charge-to-size ratio. [18] Therefore, it is considered that the smooth anion transfer in the film enables the reversible redox of Ru ions in the polymer film, because the anion transfer is essential to compensate the positive charge change of Ru ions during the redox.…”

“…[18][19][20] On the contrary, the "soft" anions such as TFSI -, CF 3 SO 3 -, BF 4 -, ClO 4 -and PF 6 -have less ion-pairing effects because of the low charge-to-size ratio. [18] Therefore, it is considered that the smooth anion transfer in the film enables the reversible redox of Ru ions in the polymer film, because the anion transfer is essential to compensate the positive charge change of Ru ions during the redox. BF 4 -is a "soft" anion, but the redox wave of polyRu-BF 4 has a large peak separation.…”

Mixed-Valency and Metal-Metal Coupling in Polynuclear Metal Complexes: Recent Progress and Prospects

“…10). 64,65) The differential-pulse voltammograms (DPVs) showed how the four Fc-based oxidations of this compound become spread out in CH 2 Cl 2 as the supporting electrolyte is changed (Fig. 11) The coordination-driven self-assembly is a hot subject in supramolecular chemistry.…”

Ferrocene‐ and Biferrocene‐Containing Macrocycles towards Single‐Molecule Electronics

“…Additionally, CVs were run in [ n Bu 4 N][BAr F 4 ] solution (BAr F 4 =tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate). Owing to the weak ion pairing capabilities of BAr F 4 and increased electrostatic interactions between the redox sites, half‐wave potential splittings were significantly enhanced 31, 32. This pushes the third oxidation wave close to the anodic stability limit of our electrolyte with concomitant changes in the voltammetric response (Supporting Information, Figure S16).…”