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Syntheses, Characterization, and Dioxygen Reactivities of Cu(I) Complexes with cis,cis-1,3,5-Triaminocyclohexane Derivatives:  A Cu(III)2O2 Intermediate Exhibiting Higher C−H Activation

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posted on 16.04.2007 by Yuji Kajita, Hidekazu Arii, Takahiro Saito, Yamato Saito, Shigenori Nagatomo, Teizo Kitagawa, Yasuhiro Funahashi, Tomohiro Ozawa, Hideki Masuda
Six Cu(I) complexes with cis,cis-1,3,5-triaminocyclohexane derivatives (R3CY, R = Et, iBu, and Bn), [Cu(MeCN)(Et3CY)]SbF6 (1), [Cu(MeCN)(iBu3CY)]SbF6 (2), [Cu(MeCN)(Bn3CY)]SbF6 (3), [Cu(CO)(Et3CY)]SbF6 (4), [Cu(CO)(iBu3CY)]SbF6 (5), and [Cu(CO)(Bn3CY)]SbF6 (6), were prepared to probe the ability of copper complexes to effectively catalyze oxygenation reactions. The complexes were characterized by elemental analysis, electrochemical and X-ray structure analyses, electronic absorption spectroscopy, IR spectroscopy, 1H NMR spectroscopy, and ESI mass spectrometry. The crystal structures of 13 and 6 and the CO stretching vibrations (νCO) of 46 demonstrate that the ability of R3CY to donate electron density to the Cu(I) atom is stronger than that of the previously reported ligands, 1,4,7-triazacyclononane (R3TACN) and 1,4,7-triazacyclodecane (R3TACD). Reactions of complexes 13 with dioxygen in THF or CH2Cl2 at −105 to −80 °C yield bis(μ-oxo)dicopper(III) complexes 79 as intermediates as confirmed by electronic absorption spectroscopy and resonance Raman spectroscopy. The Cu−O stretching vibrations, ν(Cu−O) for 7 (16O2:  553, 581 cm-1and 18O2:  547 cm-1) and 8 (16O2:  571 cm-1 and 18O2:  544 cm-1), are observed in a lower energy region than previously reported for bis(μ-oxo) complexes. The decomposition rates of complexes 79 in THF at −90 °C are 2.78 × 10-4 for 7, 8.04 × 10-4 for 8, and 3.80 × 10-4 s-1 for 9. The decomposition rates of 7 and 8 in CH2Cl2 were 5.62 × 10-4 and 1.62 × 10-3 s-1, respectively, and the thermal stabilities of 79 in CH2Cl2 are lower than the values measured for the complexes in THF. The decomposition reactions obeyed first-order kinetics, and the H/D isotope experiments for 8 and 9 indicate that the N-dealkylation reaction is the rate-determining step in the decomposition processes. On the other hand, the decomposition reaction of 7 in THF results in the oxidation of THF (acting as an exogenous substrate) to give 2-hydroxy tetrahydrofuran and γ-butyrolactone as oxidation products. Detailed investigation of the N-dealkylation reaction for 8 by kinetic experiments using N−H/D at −90 °C showed a kinetic isotope effect of 1.25, indicating that a weak electrostatic interaction between the N−H hydrogen and μ-oxo oxygen contributes to the major effect on the rate-determining step of N-dealkylation. X-ray crystal structures of the bis(μ-hydroxo)dicopper(II) complexes, [Cu2(OH)2(Et3CY)2](CF3SO3)2 (10), [Cu2(OH)2(iBu3CY)2](CF3SO3)2 (11), and [Cu2(OH)2(Bn3CY)2](ClO4)2 (12), which have independently been prepared as the final products of bis(μ-oxo)dicopper(III) intermediates, suggest that an effective interaction between N−H and μ-oxo in the Cu(III)2(μ-O)2 core may enhance the oxidation ability of the metal−oxo species.