Carboxyester Hydrolysis Catalyzed by a Novel Dicopper(II) Complex with an Alcohol-Pendant Macrocycle

A novel hexaaza macrocycle bearing two hydroxyethyl pendants (L), 3,6,9,16,19,22-hexaaza-6,19-bis(2-hydroxyethyl)tricyclo[22,2,2,211,14]triaconta-1,11,13,24,27,29-hexaene, was synthesized as a potential binucleating ligand. The corresponding Cu(II) complex [Cu2LCl2]Cl2·5.5H2O was isolated as a blue crystal, triclinic, space group P1̄, with a = 9.4920(19) Å, b = 4.783(3) Å, c = 16.553(3) Å, α = 63.87(3)°, β = 86.10(3)°, γ = 83.8(3)°, V = 2072.8(7) Å-3, Z = 2, R1 = 0.0658, and wR2 = 0.1839. Both Cu ions adopt the geometry of a distorted trigonal bipyramid in a pentacoordinated environment. A complexation study on the novel title complex has revealed that the alcoholic OH groups of the complex Cu2L exhibit an obvious acidity with rather low pKa values at 25 °C. The Cu(II)-bound alkoxides, which act as reactive nucleophiles toward the hydrolysis of 4-nitrophenyl acetate in 10% (v/v) CH3CN at 25 °C, with I = 0.10 (NaNO3) and pH 9.3, have shown a second-order rate constant, 0.41 ± 0.02 M-1 s-1, a value that is approximately 10 times greater than the corresponding value for the mononuclear Cu(II) complex formed by a relatively simple tripodal ligand (L1). The pH−rate profile gave a sigmoidal curve. The possible catalytic mechanism has been proposed, and the reason for the high catalytic activity of the title complex has been discussed.