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
2007-04-16T00:00:00Z (GMT) by
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 1−3 and 6 and the CO stretching vibrations (νCO) of 4−6 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 1−3 with dioxygen in THF or CH2Cl2 at −105 to −80 °C yield bis(μ-oxo)dicopper(III) complexes 7−9 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 7−9 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 7−9 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.