Single-Oxidation of Ni(II) Complexes of Bidentate Fused N‑Heterocyclic Carbene–Phenol Conjugates

The pro-ligands (H2LtBu)+ and (H2LOMe)+ were synthesized as chloride salts. They associate an imidazo [1,5-a] pyridinium ring and an electron-rich 2,4-(di-tert-butyl)phenol or 2-(tert-butyl)-4-methoxyphenol, respectively. The homoleptic Ni(II) complexes of H2LtBu (1) and H2LOMe (2) were prepared and structurally characterized, showing a four-coordinate nickel center with two ligands bound by one phenolate oxygen and one carbene each. Complexes 1 and 2 show two reversible oxidation waves in their cyclic voltammetry curve, at E1/21 = 0.19 V and E1/22 = 0.59 V, and E1/21 = 0.03 V and E1/22 = 0.32 V, respectively. The one-electron oxidized forms were generated and characterized by spectroscopy, while 2+ could be isolated as single crystals. They both demonstrate two remarkable NIR bands indicating radical species in CH2Cl2. The EPR parameters are 2.097, 2.023, and 1.997 (gaverage = 2.039) for 1+ and 2.030, 2.016, and 1.994 (gaverage = 2.013) for 2+. Both gaverage values are slightly higher than those reported for genuine phenoxyl radicals but far below those of Ni(III) complexes, supporting a metalloradical character of 1+ and 2+, whereby one metal d-orbital contributes to the singly occupied molecular orbital (SOMO). A very good agreement is found between DFT calculated and experimental vis–NIR and EPR data. Both metalloradical cations could be converted into Ni(III) species by addition of pyridine. The redox isomerism could be monitored by EPR spectroscopy: the [1•Py]+ species exhibits for instance the g-values 2.157, 2.136, and 2.014 (gaverage = 2.103). The Ni(III) adducts are thermally unstable and undergo reduction back to the neutral precursor at room temperature.