Reversible methanol addition to copper Schiff base complexes: a kinetic, structural and spectroscopic study of reactions at azomethine CN bonds

Abstract

The reversible methanolysis of an azomethine CN in a series of copper(II) Schiff base complexes has been investigated through combined spectroscopic, structural, and kinetic studies. Pentadentate copper(II) complexes [L₁-Cu(X)]Y (L₁ = 1,2-bis[(1-methyl-2-imidazolyl)methyleneamino]ethane; X = Y = ClO₄⁻ (1); X = Y = TfO⁻ (2); X = Y = BF₄⁻ (3); X = H₂O, Y = (ClO₄⁻)₂ (4) spontaneously add methanol in a ligand centered reaction to yield stable, isolable hemiaminal ether product complexes 5–8. In methanol free solution, 5–8 spontaneously release alcohol to regenerate 1–4. The methanol addition reaction is first-order in methanol and first-order in complex with second-order rate constants varying from 1.1 × 10⁻⁴ to 187 × 10⁻⁴ M⁻¹ s⁻¹ dependent on the donor ability of the axial ligand. Rate constants for methanol elimination vary from 0.67 to 3.7 × 10⁻⁴ s⁻¹ with dependence on the counterion and water content of the solvent. Equilibrium constants for methanolysis range from 1.5 to 51 M⁻¹. Structural comparisons of the Schiff base complexes 1–4 and the hemiaminal ether complexes 5–8 suggest methanol addition is favored by the release of ligand strain associated with three planar five-membered chelates in 1–4.