Catalytic Cooperativity, Nuclearity, and O2/H2O2 Specificity of Multi‐Copper(II) Complexes of Cyclen‐Tethered Cyclotriphosphazene Ligands in Aqueous Media

Abstract

Three ligands L1, L2, and L3 with 2, 4, and 6 1,4,7,10‐tetraazacyclododecane (cyclen) moieties attached to a cyclotriphosphazene core, respectively, were synthesized, and oxidation activities of their CuII complexes were investigated. Aerobic oxidation of catechol by these complexes follows an intramolecular dinuclear pathway with significant cooperativity (i.e., θ ≈ 1.5 out of a maximum of 2 for two potential substrate binding sites) and kinetic constants (i.e., kcat = 17.5 × 10–3 s–1, Km = 2.8 mm, and quite remarkable catalytic specificity kcat/Km 12.5 m–1 s–1 per di‐Cu center), while that by untethered CuII–cyclen follows a bimolecular dinuclear pathway without noticeable cooperativity (θ = 0.96) and fourfold lower kcat, despite their similar dinuclear mechanisms. The proximity of CuII centers is suggested by EPR spectra and relaxations, showing a broad spectral component particularly in Cu6L3. Thermodynamic parameters also indicate the significance of multi‐CuII sites in the oxidative catalysis. Air is a more specific oxidation agent for the representative complex Cu2L1, showing 3.2‐fold higher catalytic specificity kcat/Km than H2O2 toward a catechol substrate. The research provides further molecular basis for future design of O2/H2O2‐specific oxidation of multi‐domain Cu complexes.