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 (H₂RR'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 <5, a single reversible two-electron, two-proton couple, Ni(Me₂L)²⁺=Ni(H₂Me₂L)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(Me₂L)²⁺-Ni(Me₂L)₊ has E°₂₉₈=0.42 V vs. SCE The second couple may or may not involve protons: pH∼7, Ni(Me₂L)⁺-Ni(HMe₂L)+, E°₂₉₈= 0.64 V vs. SCE; pH >8.45, Ni(Me₂L)⁺-Ni(Me₂L), E°₂₉₈=0.15 V vs. SCE. The nickel(II) species involved in each couple is what is expected from acid dissociation data: pK₂= 5.90, pK₂= 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(Me₂L)⁺-Ni(H₂Me₂L)²⁺ has E°₂₉₈=1.01 V. The nickel(III) intermediate Ni(Me₂L)⁺ is relatively unstable. However a paramagnetic solid containing nickel(III) has been obtained by oxidation of Ni(H₂Me₂L)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(H₂Me₂L)^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.