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(II) per chlorate with the tetraazamacrocyclic lig and 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 compoun d [Fe(L-N4Me2)(dbsq)](ClO4)(2). 2.5 H2O (3 a, dbsq(-) = 3,5-di-tert-bu tyl-1,2-benzo-semiquinonate). On the basis of structural, Mossbauer sp ectroscopic, and magnetic evidence, this compound was identified as a low-spin iron(III) semiquinonate complex, the first of its kind, in wh ich the unpaired electron of the coordinated semiquinonate radical is strongly antiferromagnetically coupled with the unpaired electron of t he low-spin iron(III) ion. In acetonitrile solution, [Fe(L-N4Me2)(dbsq )](2+) (3) is in equilibrium with uncoordinated dbq and with the low-s pin iron(Ii) complex [Fe(L-N4Me2)(MeCN)(2)](2+) (5), as demonstrated b y NMR, Mossbauer, and UV/Vis spectroscopic data, as well as by the ele ctrochemical results. The equilibrium constant for the reaction 3+2MeC N reversible arrow dbq+5 was determined to be 7.97 x 10(-6) M-1 at 25 degrees C, and the pseudo-first-order rate constant for the forward re action k = k(f)[MeCN](2) to be 2.85 s(-1) by NMR spectroscopy and elec trochemical methods, respectively. This equilibrium constant and the r edox potentials, determined for the involved species, were used to cal culate the formation constants for the complexation of dbsq- and 3,5-d i-tert-butylcatecholate (dbc(2-)) by [Fe(L-N4Me2)(MeCN)(2)](2+,3+) ion s. Solutions of complex 3 in acetonitrile are found to be stable towar ds molecular oxygen. In addition, the reaction of the iron(III) semiqu inonate complex 3 with superoxide quantitatively yields the correspond ing iron(III) catecholate complex [Fe(L-N4Me2)(dbc)](+) (2). Therefore , the reactivity of 3 with molecular oxygen and with superoxide demons trates that the correct oxidation states of both the metal ion and the coordinated dioxolene unit are required for the occurrence of the wel l-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 initia l electron-transfer step, resulting in the formation of an iron(III) s emiquinonate as intermediate, but instead by the direct attack of the oxygen molecule on the iron(III) catecholate moiety.