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Anilino Radical Complexes of Cobalt(III) and Manganese(IV) and Comparison with Their Phenoxyl Analogues

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journal contribution
posted on 22.09.2000, 00:00 by Frank N. Penkert, Thomas Weyhermüller, Eckhard Bill, Peter Hildebrandt, Sophie Lecomte, Karl Wieghardt
The pendent arm macrocyclic aniline ligands 1-(2-amino-3,5-di-tert-butylbenzyl)-4,7-dimethyl-1,4,7-triazacyclononane, H[L1], and 1,4,7-tris(2-amino-3,5-di-tert-butylbenzyl)-1,4,7-triazacyclononane, H3[L2], have been synthesized. The reaction of these ligands in methanol (1:1) and/or ethanol with CoII(ClO4)2·6H2O and potassium di-tert-butylacetylacetonate (1:1) or MnII(acetate)2·4H2O in the presence of air produced the anilido complexes [CoIII(L1)(Bu2acac)](ClO4) (1) and [MnIV(L2)]X (X = ClO4 (2), BPh4 (2)), respectively. The reaction of H3[L2] and CuCl2·2H2O in ethanol produced upon addition of NaClO4 blue crystals of [CuII(H3L2)Cl](ClO4) (3) containing two coordinated and one uncoordinated aniline pendent arm. Complexes 1, 2, and 3 have been structurally characterized by X-ray crystallography. Electrochemically, 1 is reversibly oxidized with formation of the paramagnetic anilino radical species [CoIII(L1•)(Bu2acac)]2+ (S = 1/2) whereas the cyclic voltammogram of 2 displays three ligand-centered one-electron-transfer oxidation waves where complexes [MnIV(L2•)]2+, [MnIV(L2••)]3+, and [MnIV(L2•••)]4+ containing one, two, and three coordinated anilino radicals are formed successively. These anilino radical species have been characterized by UV−vis, X-band EPR, and resonance Raman spectroscopy. The characteristic spectroscopic features of coordinated anilino radicals have been elucidated. In the MnIV radical species strong intramolecular exchange coupling between the metal ion (t2g3 configuration, S = 3/2) and one, two, or three anilino radicals (Srad = 1/2) produces the ground states St = 1, St = 1/2, and St = 0 in the di-, tri-, and tetracation of the parent complex 2, respectively.