Synthesis and molecular recognition of chiral salen Zn with imidazoles and pyridines

Chiral salen 1 and chiral salen Zn complex 2 were synthesized and characterized. The electronic spectra and circular dichroism spectra properties were discussed. By method of UV-vis spectrophotometer technique, the equilibrium constants of axial coordination reaction were measured between complex 2 and series of imidazoles or pyridines. The molecular recognition of the host 2 was studied. The results show that the equilibrium constants decrease in the orders of K (Im) > K (2-MeIm) > K (SMIm) > K (EMIm) (imidazoles) and K (Py) > K (3-Py) > K(3,5-Py) > K(2,4-Py) > K(2,4,6-Py) (pyridines). The thermodynamic parameters Delta(r)G(m)(circle minus), Delta(r)H(m)(circle minus), and Delta(r)S(m)(circle minus) of the recognition process were determined. It is found that the reaction is an exothermic process with entropy decreasing. In addition, the conformations with minimal energy of each host-guest molecular system were sought by molecular dynamics method. Quantum chemical calculations were performed on these conformations to explain the experimental data. A model of the recognition mechanism was constructed.