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

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journal contribution
posted on 27.05.2016, 19:04 by Akram Ali, Debanjan Dhar, Suman K. Barman, Francesc Lloret, Rabindranath Mukherjee
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 NiII(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 E1/2 = 0.09 and 0.53 V vs SCE (saturated calomel electrode). Chemical oxidation of 1 in air by [FeIII5-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 [NiII{(LISQO,N)•–}­{(LISQO,N)•–}­{(LS,S)0}] 1, [NiII{(LIBQO,N)0}­{(LISQO,N)•–}­{(LS,S)0}]­[PF6]·CH2Cl2, and [NiII{(LIBQO,N)0}­{(LIBQO,N)0}­{(LS,S)0}]­[BF4]2·CH2Cl2, as monoanionic o-iminosemiquinonate­(1−) π-radical (Srad = 1/2) (LISQ)•– and neutral o-iminoquinone (LIBQ)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 μeff value of [1]­[BF4]2·CH2Cl2 at 300 K. Both 1 and [1]­[PF6]·CH2Cl2 exhibit ferromagnetic exchange-coupling between the two electrons of NiII 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.