Macrocyclic Pyridylideneamide Metal Complexes for Sustainable Catalytic Oxidation

Abstract

Clean water constitutes the most fundamental resource for human well-being and healthy ecosystems. Thus, the fact that the world’s limited water supplies are increasingly contaminated with persistent organic micropollutants, many of which cause serious harm to humans and wildlife, poses a tremendous threat to sustainable and socio-economic advancement. This research thesis contributes to the development of more sophisticated water purification systems by introducing the novel biomimetic oxidation catalyst Fe(III)L1Cl for the catalytic degradation of organic compounds in water. The macrocyclic ligand framework L1 of this catalyst was broadly derived from reported tetra-amido macrocyclic ligands (TAMLs) but incorporates hybrid pyridylideneamide (PYA) functionalities instead of classical carboxamidates or sulfonamidates. Metalation of the proligand H2L1 forming complex Fe(III)L1Cl and the transition metal analogues Co(II)L1, Ni(II)L1, Cu(II)L1, and Pd(II)L1 proceeded under surprisingly mild conditions (91-96% yields), which was attributed to the capability of the PYAs of H2L1 to facilitate the metalation. The synthesised metal complexes, proligand H2L1, and its precursor H4L1(OTf)2 have been thoroughly characterised through various techniques including X-ray crystallography and cyclic voltammetry. The iron complex Fe(III)L1Cl is a highly active catalyst for the oxidative degradation (bleaching) of the model substrate orange II in water with hydrogen peroxide and follows the same kinetic mechanism that is established for iron-TAML activators. The kinetic rate constants associated with the catalyst activation (k1), substrate oxidation (k2), and catalyst inactivation (ki), which reflect the catalytic performance of Fe(III)L1Cl, were determined: k1 = 290 52 M-1 s-1, k2 = 6700 1200 M-1 s-1, and ki = 0.0064 0.0011 s-1 at pH 7.0. Importantly, Fe(III)L1Cl is highly stable towards acid-promoted demetalation in aqueous buffer solution at pH 5.0 or above, in striking contrast to the analogous iron-TAML that does not contain the two pyridinium groups. The hybrid PYA donors of L1, therefore, introduce greatly enhanced acid stability to Fe(III)L1Cl without substantially compromising catalytic activity.