Octahedral Non-Heme Non-Oxo Fe(IV) Species Stabilized by a Redox-Innocent N-Methylated Cyclam−Acetate Ligand

Abstract

The ligand 1,4,8-tri-N-methyl-1,4,8,11-tetraazacyclotetradecane-11-acetic acid (Me3cyclam-acetic acid) has been synthesized by Eschweiler-Clarke methylation of cyclam-acetic acid, and the iron(III) complex [(Me3cyclam-acetate)FeN3]PF6, 1, has been synthesized, which has been found to have significantly different properties than its unmethylated analogue, [(cyclam-acetate)FeN3]PF6, 2. Whereas the iron ion in 2 is low spin with S = 1/2, 1 is found to be high spin at temperatures above 100 K, though low-spin species are observed at lower temperatures, indicating a spin crossover phenomenon. The iron(II) species 1red is electrochemically more accessible than 2red since the Fe2+/3+ redox wave in 1 appears approximately 350 mV more positive than the corresponding wave in 2. Also, 1 displays a reversible Fe3+/4+ redox wave, which is irreversible in 2, denoting that the Fe(IV) species 1ox is kinetically stable. 1red and 1ox have been generated electrochemically in solution and studied spectroscopically. Mössbauer spectroscopy has confirmed that, in both reduction and oxidation, iron is the redox center, that 1red is high spin (S = 2), and that 1ox is low spin (S = 1), in contrast to 2red which is low spin and 2ox which could not be isolated.