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

Thepro-ligands (H2LtBu)(+) and(H2LOMe)(+) were synthesized as chloridesalts. They associate an imidazo [1,5-a] pyridiniumring and an electron-rich 2,4-(di-tert-butyl)phenolor 2-(tert-butyl)-4-methoxyphenol, respectively.The homoleptic Ni(II) complexes of H2LtBu (1) and H2LOMe (2) wereprepared and structurally characterized, showing a four-coordinatenickel center with two ligands bound by one phenolate oxygen and onecarbene each. Complexes 1 and 2 show tworeversible oxidation waves in their cyclic voltammetry curve, at E (1/2) (1) = 0.19 V and E (1/2) (2) = 0.59 V, and E (1/2) (1) = 0.03 V and E (1/2) (2) = 0.32 V, respectively. The one-electron oxidized forms were generatedand characterized by spectroscopy, while 2 ( + ) could be isolated as single crystals. They both demonstratetwo remarkable NIR bands indicating radical species in CH2Cl2. The EPR parameters are 2.097, 2.023, and 1.997 (g(average) = 2.039) for 1 ( + ) and 2.030, 2.016, and 1.994 (g(average) = 2.013) for 2 ( + ). Both g(average) valuesare slightly higher than those reported for genuine phenoxyl radicalsbut far below those of Ni(III) complexes, supporting a metalloradicalcharacter of 1 ( + ) and 2 ( + ), whereby one metal d-orbital contributesto the singly occupied molecular orbital (SOMO). A very good agreementis found between DFT calculated and experimental vis-NIR andEPR data. Both metalloradical cations could be converted into Ni(III)species by addition of pyridine. The redox isomerism could be monitoredby EPR spectroscopy: the [1 center dot Py] ( + ) species exhibits for instance the g-values 2.157, 2.136, and2.014 (g(average) = 2.103). The Ni(III) adducts are thermallyunstable and undergo reduction back to the neutral precursor at roomtemperature.