J.AM. CHEM. SOC. 2010, 132, 9774–9781.
W. Chadwick Ellis, Camly T. Tran, Riddhi Roy, Marte Rusten, Andreas Fischer, Alexander D. Ryabov, Bruce Blumberg, and Terrence J. Collins.
Abstract: We describe the synthesis, characterization, aqueous behavior, and catalytic activity of a new generation of FeIII−TAML (tetraamido macrocycle ligand) activators of peroxides (2), variants of [Fe{(OC)2(o,o′-NC6H4NCO)2CMe2}(OH2)−] (2d), which have been designed to be especially suitable for purifying water of recalcitrant oxidizable pollutants. Activation of H2O2 by 2 (kI) as a function of pH was analyzed via kinetic studies of Orange II bleaching. This was compared with the known behavior of the first generation of FeIII−TAMLs (1). Novel reactivity features impact the potential for oxidant activation for water purification by 2d and its aromatic ring-substituted dinitro (2e) and tetrachloro (2f) derivatives. Thus, the maximum activity for 2e occurs at pH 9, the closest yet to the EPA guidelines for drinking water (6.5−8.5), allowing 2e to rapidly activate H2O2 at pH 7.7. In water, 2e has two axial water ligands with pKa’s of 8.4 and 10.0 (25 °C). The former is the lowest for all FeIII−TAMLs developed to date and is key to 2e’s exceptional catalytic activity in neutral and slightly basic solutions. Below pH 7, 2d was found to be quite sensitive to demetalation in phosphate buffers. This was overcome by iterative design to give 2e (hydrolysis rate 2d > 100 × 2e). Mechanistic studies highlight 2e’s increased stability by establishing that to demetalate 2e at a comparable rate to which H2PO4− demetalates 2d, H3PO4 is required. A critical criterion for green catalysts for water purification is the avoidance of endocrine disruptors, which can impair aquatic life. FeIII−TAMLs do not alter transcription mediated by mammalian thyroid, androgen, or estrogen hormone receptors, suggesting that 2 do not bind to the receptors and reducing concerns that the catalysts might have endocrine disrupting activity.