SYNTHESIS, CHARACTERIZATION, AND REACTIVITY OF CATECHOLATO ADDUCTS OF IRON(III) TRIAZAAZAMACROCYCLIC AND TETRAAZAMACROCYCLIC COMPLEXES - CHEMICAL EVIDENCE OF THE ROLE OF THE METAL-ION IN THE OXIDATIVE CLEAVAGE

Iron(III)-catecholato complexes of formulas [Fe(SS-CTH)(Cat)]Y, [Fe(Cyclam)(Cat)]Y,and [Fe(TACN)(Cat)Cl] (SS-CTH = (SS)-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; Cyclam = 1,4,8,11-tetraazacyclotetradecane; TACN = 1,4,7-triazacyclononane; Cat = DTBCat = 3,5-di-tert-butylcatecholate; TCCat = tetrachlorocatecholato; Y = PF6, ClO4) have been synthesized. Magnetic measurements and EPR spectra indicate that all these complexes have to be formulated as high-spin iron(III) derivatives. Cyclic voltammetry experiments show that all of these complexes undergo electron transfer reactions involving either the metal ion (iron(III)-iron(II) couple) or the coordinated dioxolene ligand (catecholato to semiquinonato for all the derivatives and semiquinonato to quinone for DTBCat complexes). The electronic spectra of these complexes are dominated by LMCT transitions which were assigned with the help of the CD spectra obtained with the (-)SS-CTH derivatives. A striking correlation between the energy of the first LMCT electronic transitions and the free energy changes of the process Fe(III)(Cat)-Fe(II)(SQ) (SQ=semiquinonato), calculated from electrochemical data, is observed. The tetraazamacrocycle derivatives of DTBCat are inert toward dioxygen, whereas the triazamacrocycle one shows an enhanced reactivity in polar solvents yielding either quinone and extradiol cleavage reaction products. The different chemical behavior of these complexes strongly supports the suggestion that the catechol cleavage reaction first requires the coordination of the dioxygen molecule to a coordinatively unsaturated iron(II) center. This result is believed to be relevant in order to understand the enzymatic reaction mechanism of dioxygenases.