Cu(i) complexes obtained via spontaneous reduction of Cu(ii) complexes supported by designed bidentate ligands: bioinspired Cu(i) based catalysts for aromatic hydroxylation

Abstract

Copper(I) complexes [Cu(L^1–7)₂](ClO₄) (1–7) of bidentate ligands (L¹–L⁷) have been synthesized via spontaneous reduction and characterized as catalysts for aromatic C–H activation using H₂O₂ as the oxidant. The single crystal X-ray structure of 1 exhibited a distorted tetrahedral geometry. All the copper(I) complexes catalyzed direct hydroxylation of benzene to form phenol with good selectivity up to 98%. The determined kinetic isotope effect (KIE) values, 1.69–1.71, support the involvement of a radical type mechanism. The isotope-labeling experiments using H₂¹⁸O₂ showed 92% incorporation of ¹⁸O into phenol and confirm that H₂O₂ is the key oxygen supplier. Overall, the catalytic efficiencies of the complexes are strongly influenced by the electronic and steric factor of the ligand, which is fine-tuned by the ligand architecture. The benzene hydroxylation reaction possibly proceeded via a radical mechanism, which was confirmed by the addition of radical scavengers (TEMPO) to the catalytic reaction that showed a reduction in phenol formation.