Chemistry of tetravalent nickel and related species. 2. Cyclic voltammetry of oxidation-reduction equilibriums involving protons

Abstract

The redox behavior of nickel(II) and nickel(IV) species derived from a few hexadentate ligands of type 1 (H 2 RR'L) have been thoroughly studied in buffered media over a wide range of pH using cyclic voltammetry and coulometry. The results for the system having R=R'= Me are representative. At pH 2 L) 2+ =Ni(H 2 Me 2 L) 2+ (E° 298 =0.71 V vS. SCE) is observed. At pH >6. two distinct reversible one-electron processes arc observed. One of these has no proton involvement: Ni(Me 2 L) 2+ -Ni(Me 2 L) + has E° 298 =0.42 V vs. SCE The second couple may or may not involve protons: pH∼7, Ni(Me 2 L) + -Ni(HMe 2 L)+, E° 298 = 0.64 V vs. SCE; pH >8.45, Ni(Me 2 L) + -Ni(Me 2 L), E° 298 =0.15 V vs. SCE. The nickel(II) species involved in each couple is what is expected from acid dissociation data: pK 2 = 5.90, pK 2 = 7.80. The observed transition of the single twaclecanyi couple into two one-electron couples as the pH is raised can be rationalized on the basis of the higher proton affinity of the nickel(II) species. It is estimated that the couple Ni(Me 2 L) + -Ni(H 2 Me 2 L) 2+ has E° 298 =1.01 V. The nickel(III) intermediate Ni(Me 2 L) + is relatively unstable. However a paramagnetic solid containing nickel(III) has been obtained by oxidation of Ni(H 2 Me 2 L) 2+ . In the polycrystalline phase it shows room-temperature EPR signals characteristic of axial symmetry (g=2.04:g⊥= 216). The one-electron redox behavior of Fe(H 2 Me 2 L) 2- is briefly reported and an empirical relationship between E° 298 values of nickel(III)-nickel(II) and iron(III)-iron(II) couples is noted.