Structural, spectroscopic, and chemical properties of the first low-spin iron(III) semiquinonate complexes in the solid state and in solution

The reaction of iron(III) perchlorate with the tetraazamacrocyclic ligand N,N'-dimethyl-2,11-diaza[3.3]-(2,6)pyridinophane (L-N₄Me₂) and 3,5-di-tert-butyl-1,2-benzoquinone in 96% ethanol yields the blue compound [Fe(L-N₄Me₂)(dbsq)](ClO₄)₂·2.5 H₂O (3a, dbsq^- = 3,5-di-tert-butyl-1,2-benzosemiquinonate). On the basis of structural, Mossbauer spectroscopic, and magnetic evidence, this compound was identified as a low-spin iron(III) semiquinonate complex, the first of its kind, in which the unpaired electron of the coordinated semiquinonate radical is strongly antiferromagnetically coupled with the unpaired electron of the lowspin iron(III) ion. In acetonitrile solution, [Fe(L-N₄Me₂)(dbsq)]²⁺ (3) is in equilibrium with uncoordinated dbq and with the low-spin iron(III) complex [Fe(L-N₄Me₂)(MeCN)₂]²⁺ (5), as demonstrated by NMR, Mossbauer, and UV/Vis spectroscopic data, as well as by the electrochemical results. The equilibrium constant for the reaction 3+2MeCN⇆ dbq+5 was determined to be 7.97 x 10^-6M^-1 at 25°C, and the pseudo-first-order rate constant for the forward reaction k=k(f)[MeCN]² to be 2.85 s^-1 by NMR spectroscopy and electrochemical methods, respectively. This equilibrium constant and the redox potentials, determined for the involved species, were used to calculate the formation constants for the complexation of dbsq^and 3,5-di-tert-butylcatecholate (dbc^2-) by [Fe(L-N₄Me₂)(MeCN)₂]^2+,3+ ions. Solutions of complex 3 in acetonitrile are found to be stable towards molecular oxygen. In addition, the reaction of the iron(III) semiquinonate complex 3 with superoxide quantitatively yields the corresponding iron(III) catecholate complex [Fe(L-N₄Me₂)(dbc)]⁺ (2). Therefore, the reactivity of 3 with molecular oxygen and with superoxide demonstrates that the correct oxidation states of both the metal ion and the coordinated dioxolene unit are required for the occurrence of the well-established cleavage of the intradiol C-C bond of 3,5-di-tert-butyl-catecholate ligand coordinated to the iron(III) ion in 2 by molecular oxygen and that the cleavage reaction does not occur through an initial electron-transfer step, resulting in the formation of an iron(III) semiquinonate as intermediate, but instead by the direct attack of the oxygen molecule on the iron(III) catecholate moiety.