Syntheses, characterization, and dioxygen reactivities of Cu(I) complexes with cis,cis-1,3,5-triaminocyclohexane derivatives: A Cu(III)(2)O-2 intermediate exhibiting higher C-H activation

摘要

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)(iBu(3)CY)]SbF6 (2), [Cu(MeCN)(Bn3CY)]SbF6 (3), [Cu(CO)(Et3CY)]SbF6 (4), [Cu(CO)(iBu(3)CY)]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, H-1 NMR spectroscopy, and ESI mass spectrometry. The crystal structures of 1-3 and 6 and the CO stretching vibrations (nu(CO)) 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 (R(3)TACN) and 1,4,7-triazacyclodecane (R(3)TACD). Reactions of complexes 1-3 with dioxygen in THF or CH2Cl2 at -105 to -80 degrees C yield bis(mu-oxo)dicopper(III) complexes 7-9 as intermediates as confirmed by electronic absorption spectroscopy and resonance Raman spectroscopy. The Cu-O stretching vibrations, nu(Cu-O) for 7 (O-16(2): 553, 581 cm(-1)and O-18(2): 547 cm(-1)) and 8 (O-16(2): 571 cm(-1) and O-18(2): 544 cm(-1)), are observed in a lower energy region than previously reported for bis(mu-oxo) complexes. The decomposition rates of complexes 7-9 in THF at -90 degrees C are 2.78 x 10(-4) for 7, 8.04 x 10(-4) for 8, and 3.80 x 10(-4) s(-1) for 9. The decomposition rates of 7 and 8 in CH2Cl2 were 5.62 x 10(-4) and 1.62 x 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 gamma-butyrolactone as oxidation products. Detailed investigation of the N-dealkylation reaction for 8 by kinetic experiments using N-H/D at -90 degrees C showed a kinetic isotope effect of 1.25, indicating that a weak electrostatic interaction between the N-H hydrogen and mu-oxo oxygen contributes to the major effect on the rate-determining step of N-dealkylation. X-ray crystal structures of the bis(mu-hydroxo)dicopper(II) complexes, [Cu-2(OH)(2)(Et3CY)(2)](CF3SO3)(2) (10), [Cu-2(OH)(2)(iBu(3)CY)(2)](CF3SO3)(2) (11), and [Cu-2(OH)(2)(Bn3CY)(2)](ClO4)(2) (12), which have independently been prepared as the final products of bis(mu-oxo)dicopper(III) intermediates, suggest that an effective interaction between N-H and mu-oxo in the Cu(III)(2)(mu-O)(2) core may enhance the oxidation ability of the metal-oxo species.