Tris(imidazolyl) dicopper(i) complex and its reactivity to exert the catalytic oxidation of sterically hindered phenol substrates via a [Cu2O]2+ core

Abstract

The Cu ion ligated with histidine residues is a common active site motif of various Cu-containing metalloenzymes exerting versatile catalytic oxidation reactions. Due to the scarcity of structurally characterized biomimetic binuclear Cu(I)–imidazolyl complexes, the bonding interactions between the Cu(I) center and imidazolyl donor ligands as well as their stoichiometric/catalytic oxidation reactivities remain relatively unexplored. In this study, we successfully synthesized a tris(imidazolyl) dicopper(I) complex [Cu^I(μ-bimeta)₃Cu^I][PF₆]₂ (1) characterized by single-crystal X-ray diffraction, cyclic voltammetry, and UV-vis absorption spectroscopy. The coordination environment around each Cu(I) center of complex 1 is best described as a regular trigonal geometry and the distance between two Cu(I) centers is ∼3.0521(18) Å. From the O₂-/PhIO-titration reactions of complex 1, only 0.5 equiv. of O_2(g) and one equiv. of PhIO are required to produce the corresponding oxygenated product complex 1^ox, respectively, exhibiting a distinct UV-vis absorption band at ∼640 nm. From the characterization of ESI mass spectrometry, IR, UV-vis spectroscopy, and O₂-/PhIO-titration reactions, the molecular identity of complex 1^ox is tentatively assigned as [Cu(μ-O)(μ-bimeta)₃Cu]²⁺, which is further corroborated by the spectroscopically calibrated DFT calculations. The oxidation reactivities of complex 1^ox were investigated using the well-understood sterically hindered phenol substrates DTBP and TBBP in both stoichiometric and catalytic fashions. In stoichiometric reactions, one equiv. of 1^ox is capable of fully converting TBBP into TBOBF in the CH₃CN solution at room temperature within 1 h. The optimized selectivity of catalytically oxidizing DTBP into TBOBF (>95% selectivity, ∼100% conversion rate) could be achieved with a 10 mol% loading of complex 1^ox using air as the oxidant. From the time-course product distributions of this catalyst system, ∼40% of DTBP was converted into the corresponding TBBP product in the first 5 min. From 10 to 120 min, the oxidation of DTBP to TBBP and the oxidation of TBBP to TBOBF simultaneously proceeded, which was evidenced by the decrease of TBBP and the increase TBOBF.