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, [Cu-2(Py2SSPy2)](ClO4)(2) (I) (Py2SSPy2 = bis{2-[N,N-bis(2-pyridylethyl)amino]-1,1-dimethylethyl}disulfide), a thioether-copper(I) complex, [Cu((PrSPy2)-Pr-i)](ClO4) (2) ((PrSPy2)-Pr-i = N-(2-isopropylthio-2-methyl)propyl-N,N-bis-2-(2-pyridyl)ethylamine), and a thioether-copper(II) complex, [Cu(PheSPy(2))(H2O)](ClO4)(2) (3) (PheSPy(2) = N-(2-methyl-2-phenethylthio)propyl-N,N-bis-2-(2-pyridyl)ethylamine), were newly synthesized by the reactions of Cu(ClO4)(2). 6H(2)O with a thiol ligand of Py2SH (N,N-bis[2-(2-pyridyl)ethyl]-1,1-dimethyl-2-mercaptoethylamine) and thioether ligands of (PrSPy2)-Pr-i and PheSPy(2), respectively, For complexes 1 and 2, X-ray analyses were performed. Complex 1 crystallizes in the triclinic space group P (1) over bar, and complex 2 crystallizes in the orthorhombic space group Pbca with the following unit cell parameters: for 1, a = 15.165 (3) Angstrom. b = 22.185 (4) Angstrom, c = 14.989 (3) Angstrom, alpha = 105.76 (1)degrees, beta = 90.82 (2)degrees, gamma = 75.23 (1)degrees, and Z = 2; for 2, a = 17.78 (2) Angstrom, b = 17.70.(1) Angstrom, c = 15.75 (1) Angstrom, 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 (la, Ib) 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) Angstrom in 1a and 7.070(7) Angstrom 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 (BuOOH)-Bu-t) 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 nu(O-O) bands at 822 and 836 cm(-1), which are red-shifted to 781 and 791 cm(-1), respectively, upon introduction of (H2O2)-O-18, 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.