Département de chimie moléculaire

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, resp. The homoleptic Ni(II) complexes of H2LtBu (1) and H2LOMe (2) were prepared and structurally characterized, showing a four-coordinate Ni center with two ligands bound by one phenolate O 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, resp. The 1-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 MO (SOMO). A very good agreement is found between DFT calculated and exptl. visible-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