| Abstract
| - A disulfide-bridged dicopper(I) complex, [Cu2(Py2SSPy2)](CIO4)2 (1), synthesized by the redox reaction of a Cu(II) salt with a thiol-containing N3S-donor ligand, N,N-bis[2-(2-pyridyl)ethyl]-2-mercapto-2-methylpropylamine (Py2SH), is demonstrated to catalyze epoxidation and hydroxylation with H2O2 as an oxidant. In the early stage of the oxidation a Cu−OOH intermediate was characterized by resonance Raman spectroscopy. Related thioether−copper(I) and -(II) complexes 2 and 3 were also newly prepared and characterized spectroscopically, and H2O2-dependent catalytic C−H bond activation by 2 and 3 was investigated.
- 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(iPrSPy2)](ClO4) (2) (iPrSPy2 = 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 iPrSPy2 and PheSPy2, respectively. Forcomplexes 1 and 2, X-ray analyses were performed. Complex 1 crystallizes in the triclinic space group P1̄, andcomplex 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 structurallycharacterized example obtained by the redox reaction Cu(II) + RSH → Cu(I) + RSSR and has two independentstructures (1a, 1b) which mainly differ in S−S bond distances, Cu(I)···Cu(I) separations, and C−S−S−C dihedralangles 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 ofsignificant activation of the S−S bonds by the dicopper centers. Fragment molecular orbital (FMO) analyses andmolecular orbital overlap population (MOOP) analyses based on the extended Hückel method clarify the preferableformation of the disulfide S−S bond in 1 rather than the formation of a thiolate−copper(II) complex within thePy2S- ligand framework. Catalytic functions of complexes 1−3 were investigated with peroxides (H2O2 andtBuOOH) as oxidants. Complex 1 catalyzed the selective oxidation of cyclohexane to cyclohexanol and mediatedthe cyclohexene epoxidation in the presence of H2O2. A transient dark green intermediate observed in the reactionof 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 ν(O−O) bands at 822 and 836 cm-1, which arered-shifted to 781 and 791 cm-1, respectively, upon introduction of H218O2, are indicative of formation of twokinds of Cu−OOH species rather than the Fermi doublet and the significant weakening of the O−O bonds. Thesemechanistic studies demonstrate that by virtue of the electron-donating ability of the disulfide unit the Cu−OOHspecies can be actually activated for one-electron oxidation, which has been reported so far unfavorable for othervibrationally characterized Cu−OOH species.
|
