TETRAOXOLENE RADICAL STABILIZATION BY THE INTERACTION WITH TRANSITION-METAL IONS

Metal complexes of formula M2(CTH)2(DHBQ)Y2 (M = Mn, Fe, Ni; CTH = dl-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; Y = ClO4, PF6) have been obtained by reaction of the appropriate M(CTH)2+ cation and 2,5-dihydroxy-1,4-benzoquinone (H2DHBQ) in alkaline medium. On the basis of their spectral and magnetic properties, it is suggested that these complexes are dinuclear with the DHBQ2- ligand bridging the two dipositive metal ions. The coupling between the metal ions is weakly antiferromagnetic. When Cr(CTH)2+ is used as reacting species, complexes of formula Cr2(CTH)2(DHBQ)Y3 are obtained. A compound of the same stoichiometry is obtained by oxidizing the above diiron(II) derivative. On the basis of IR, electronic, EPR, and Mossbauer spectra, evidence is found that these compounds contain the radical species DHBQ3- bridging the tripositive metal ions. Electrochemical data support this formulation. Magnetic susceptibility measurements in the range 4-298 K are consistent with a S = 5/2 and a S = 9/2 ground state for the chromium(III) and iron(III) derivatives, respectively, as a result of a strong antiferromagnetic coupling between the metal ions and the bridging radical. The stabilization of the radical ligand in the latter derivatives is discussed on the basis of the electronic properties of the tetraoxolene moiety. The unusual EPR spectra are discussed and interpreted in terms of a general approach valid for high-spin values.