Co^II Complexes with a Tripyridine Ligand, Containing a 2,6-Di-tertbutylphenolic Fragment: Synthesis, Structure, and Formation of Stable Radicals

Interaction of a tripyridine ligand bearing a 2,6-di-tert-butylphenolic fragment (L, 2,6-di-tert-butyl-4-(3,5-bis(4-pyridyl)pyridyl)phenol) with Co^II pivalate or chloride led to the formation of one-dimensional coordination polymers [Co(L)Cl₂]_n·nEtOH (1) and [Co₃(L)₂(OH)(Piv)₅]_n (2) or a trinuclear complex Co₃(H₂O)₄(L)₂Cl₆ (3) (Piv^– = pivalate). Chemical oxidation of L and 1–3 by PbO₂ or K₃[Fe(CN)₆], as well as exposure of L (in solution or solid state) and 2 (in solid state) to UV irradiation, led to the formation of free radicals with g = 2.0024, which probably originated because of oxidation of 2,6-di-tert-butylphenolic groups. These radicals were stable for several days in solutions and more than 1 month in solid samples. Irradiation and oxidation of the solid samples probably caused formation of the phenoxyl radical only on their surface. It was shown by density functional theory calculations that exchange coupling between the unpaired electron of the phenoxyl radical and Co^II ions was negligibly weak and could not affect the electron paramagnetic resonance signal of the radical, as well as exchange coupling of Co^II ions could not be transmitted by L. The latter conclusion was confirmed by the analysis of magnetic properties of 1: temperature dependency of magnetic susceptibility (χM) of 1 could be simulated by a simple model for isolated Co^II ions.