Synthesis, structure, catalytic and magnetic properties of a pyrazole based five coordinated di-nuclear cobalt(II) complex

The reaction of 4-methyl-2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)phenol and Co(NO₃)₂·6H₂O in MeCN gives rise to [Co₂(L)₂(N₃)₂] (1), a dinuclear five coordinated complex of Co(II), in satisfactory yield. The complex has been characterized by C, H and N microanalyses, FT-IR and UV-Vis spectral measurements. A single crystal X-ray diffraction study of 1 reveals that each Co(II) atom is in a distorted square-pyramidal geometry. Electrochemical studies in CH₃CN showed that the Co(II)-Co(III)/Co(III)-Co(III) redox couple appears to be quasi-reversible (E_1/2 = 0.409 V), while the corresponding Co(II)-Co(III)/Co(II)-Co(II) couple is mostly irreversible (E_1/2 = -0.447 V). This complex displays modest catalytic activity toward the oxidation of various allylic compounds using TBHP as an oxidant in MeCN under mild conditions. A radical trapped experiment carried out in the presence of 4-tert-butylphenol clearly reveals that the epoxidation reactions occur purely through a radical pathway rather than a concerted one. Low temperature magnetic studies showed that the present Co(II) dimeric complex possesses a high positive magnetic anisotropy (D = +29.8 cm^-1) and a weak ferromagnetic exchange interaction (2J = +10.3 cm^-1).