Nickel(II) Complex of a Hexadentate Ligand with Two o‑Iminosemiquinonato(1−) π‑Radical Units and Its Monocation and Dication

Aerobic reaction of a hexadentate redox-active o-aminophenol-based ligand, H₄L³ = N,N′-bis­(2-hydroxy-3,5-di-tert-butylphenyl)-2,2′-diamino­(diphenyldithio)-ethane, in CH₃OH with Ni^II(O₂CCH₃)₂·4H₂O and Et₃N afforded isolation of a reddish-brown crystalline solid [Ni­(L³)] 1. Cyclic voltammetry (CV) experiment exhibits two oxidative responses at E_1/2 = 0.09 and 0.53 V vs SCE (saturated calomel electrode). Chemical oxidation of 1 in air by [Fe^III(η⁵-C₅H₅)₂]­[PF₆] and AgBF₄ in CH₂Cl₂ led to the isolation of one-electron oxidized species [1]¹⁺ as purple [1]­[PF₆]·CH₂Cl₂ and two-electron oxidized species [1]²⁺ as dark purple [1]­[BF₄]₂·CH₂Cl₂, respectively. X-ray crystallographic analysis at 100(2) K unambiguously established that the ligand is present in [Ni^II{(L^ISQ_O,N)^•–}­{(L^ISQ_O,N)^•–}­{(L_S,S)⁰}] 1, [Ni^II{(L^IBQ_O,N)⁰}­{(L^ISQ_O,N)^•–}­{(L_S,S)⁰}]­[PF₆]·CH₂Cl₂, and [Ni^II{(L^IBQ_O,N)⁰}­{(L^IBQ_O,N)⁰}­{(L_S,S)⁰}]­[BF₄]₂·CH₂Cl₂, as monoanionic o-iminosemiquinonate­(1−) π-radical (S_rad = 1/2) (L^ISQ)^•– and neutral o-iminoquinone (L^IBQ)⁰ redox-levels. Complexes 1, [1]­[PF₆]·CH₂Cl₂, and [1]­[BF₄]₂·CH₂Cl₂ possess an S = 2, 3/2, and 1 ground-state, respectively, established by temperature-dependent (2–300 K) magnetic behavior of 1 and [1]­[PF₆]·CH₂Cl₂, and a μ_eff value of [1]­[BF₄]₂·CH₂Cl₂ at 300 K. Both 1 and [1]­[PF₆]·CH₂Cl₂ exhibit ferromagnetic exchange-coupling between the two electrons of Ni^II and two/one ligand π-radicals, respectively. The redox processes are shown to be ligand-based. Spectroscopic and redox properties, and density functional theory (DFT) calculations at the CAM-B3LYP-level of theory adequately describe the electronic structure of 1, [1]¹⁺, and [1]²⁺. The observed UV–vis–NIR absorptions for 1, [1]­[PF₆]·CH₂Cl₂, and [1]­[BF₄]₂·CH₂Cl₂ have been assigned, based on time-dependent (TD)-DFT calculations.