Reversible methanol addition to copper Schiff base complexes: a kinetic, structural and spectroscopic study of reactions at azomethine C[double bond, length as m-dash]N bonds.

The reversible methanolysis of an azomethine C[double bond, length as m-dash]N in a series of copper(ii) Schiff base complexes has been investigated through combined spectroscopic, structural, and kinetic studies. Pentadentate copper(ii) complexes [L-Cu(X)]Y (L = 1,2-bis[(1-methyl-2-imidazolyl)methyleneamino]ethane; X = Y = ClO (1); X = Y = TfO (2); X = Y = BF (3); X = HO, Y = (ClO) (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 to 187 × 10 M s dependent on the donor ability of the axial ligand. Rate constants for methanol elimination vary from 0.67 to 3.7 × 10 s with dependence on the counterion and water content of the solvent. Equilibrium constants for methanolysis range from 1.5 to 51 M. 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.