Synthesis of a bis(oxazoline)-N-carboxylate ligand and its Zn(II) complex that shows C-H center dot center dot center dot Cl hydrogen bonding

Abstract

An efficient synthesis of a novel N3O-type chelate and its Zn(II) complex is described. The Zn(II) complex shows C-H center dot center dot center dot Cl-Zn hydrogen bonding both in the solid and solution states. The chelate, (S,S)-[bis{(4-isopropyl-4,5-dihydrooxazol-2-yl)methyl}amino]acetic acid sodium salt, is a new and branched type of N3O ligand, featuring C-2-symmetric bis(oxazoline)-N-carboxylate. A key step of the ligand synthesis involved a stepwise introduction of the oxazoline rings to suppress side products otherwise resulted from one-step approach. Treatment of the sodium bis(oxazoline)-N-carboxylate with ZnCl2 and subsequent removal of NaCl resulted in the corresponding Zn(II) complex. The Zn(II) complex showed an unusual downfield shift by the isopropyl methine protons on its H-1 NMR spectrum, which suggests the C-H center dot center dot center dot Cl-Zn hydrogen bonding. An X-ray crystallographic analysis of the single crystals of the Zn(II) complex supports the C-H center dot center dot center dot Cl-Zn hydrogen bonding, showing a microchannel structure that encapsulates a linear array of chloroform molecules through other intermolecular hydrogen bonding interactions. (C) 2006 Elsevier Ltd. All rights reserved.