Synthesis, structure, and H₂O₂-dependent catalytic functions of disulfide-bridged dicopper(I) and related thioether-copper(I) and thioether-copper(II) complexes

A disulfide-bridged dicopper(I) complex, [Cu₂(Py₂SSPy₂)](ClO₄)₂ (1) (Py₂SSPy₂ = bis{2-[N,N-bis(2-pyridylethyl)-amino]-1,1-dimethylethyl}disulfide), a thioether-copper(I) complex, [Cu((i)PrSPy₂)](ClO₄) (2) ((i)PrSPy₂ = N-(2-isopropylthio-2-methyl)propyl-N,N-bis-2-(2-pyridyl)ethylamine), and a thioether-copper(II) complex, [Cu-(PheSPy₂)(H₂O)](ClO₄)₂ (3) (PheSPy₂ = N-(2-methyl-2-phenethylthio)propyl-N,N-bis-2-(2-pyridyl)ethylamine), were newly synthesized by the reactions of Cu(ClO₄)₂·6H₂O with a thiol ligand of Py₂SH (N,N-bis[2-(2-pyridyl)-ethyl]-1,1-dimethyl-2-mercaptoethylamine) and thioether ligands of (i)PrSPy₂ and PheSPy₂, respectively. For complexes 1 and 2, X-ray analyses were performed. Complex 1 crystallizes in the triclinic space group P1, and complex 2 crystallizes in the orthorhombic space group Pbca with the following unit cell parameters: for 1, a = 15.165 (3) Å, b = 22.185 (4) Å, c = 14.989 (3) Å, α = 105.76 (1)°, β = 90.82 (2)°, γ = 75.23 (1)°, and Z = 2; for 2, a = 17.78 (2) Å, b = 17.70 (1) Å, c = 15.75 (1) Å, and Z = 8. Complex 1 is the first structurally characterized example obtained by the redox reaction Cu(II) + RSH → Cu(I) + RSSR and has two independent structures (1a, 1b) which mainly differ in S-S bond distances, Cu(I)···Cu(I) separations, and C-S-S-C dihedral angles of the disulfide units. The S-S bond distances of 2.088(7) Å, in 1a and 2.070(7) Å, in 1b are indicative of significant activation of the S-S bonds by the dicopper centers. Fragment molecular orbital (FMO) analyses and molecular orbital overlap population (MOOP) analyses based on the extended Huckel method clarify the preferable formation of the disulfide S-S bond in 1 rather than the formation of a thiolate-copper(II) complex within the Py₂S^- ligand framework. Catalytic functions of complexes 1-3 were investigated with peroxides (H₂O₂ and (t)BuOOH) as oxidants. Complex 1 catalyzed the selective oxidation of cyclohexane to cyclohexanol and mediated the cyclohexene epoxidation in the presence of H₂O₂. A transient dark green intermediate observed in the reaction of 1 with H₂O₂ is characterized by UV-vis, EPR, and resonance Raman spectroscopies, identifying it as a Cu(II)-OOH species, 1(OOH). The resonance Raman features of the v(O-O) bands at 822 and 836 cm^-1, which are red-shifted to 781 and 791 cm^-1, respectively, upon introduction of H₂¹⁸O₂, are indicative of formation of two kinds of Cu-OOH species rather than the Fermi doublet and the significant weakening of the O-O bonds. These mechanistic studies demonstrate that by virtue of the electron-donating ability of the disulfide unit the Cu-OOH species can be actually activated for one-electron oxidation, which has been reported so far unfavorable for other vibrationally characterized Cu-OOH species.