Structural and Spectral Studies on the Ni(II) Complexes of 1,5‐Diazacyclooctane (DACO) Bearing Heterocyclic Pendants: Formation of a Two‐dimensional Network via Hydrogen Bonds and π‐π Stacking Interactions

Abstract

Medium-sized cyclic ligands containing seven-to ten-membered rings occupy a very important position between acyclic and macrocyclic ligands for they can usually provide a steric effect along with a strong ligand field effect. ' 1,5-Diazacyclooctane (DACO) , the most typical example of the diazamesocyclic ligands, offers several attractive features as a framework for ligand development with exceptionally strong ligand fields , unique conformational requirements and the potential for further functionalization. 24 Since DACO has only two nitrogen donors, two or more ligands are required for the formation of one metal center. 2*4*s However, appropriately substituted rnesccycles can enhance the pre-organization of the ligand, thereby facilitating complexation. The first example of the DACO derivative, 1 ,5-diaza~yclooctane N , N'-diacetate (DACODA) and its Co( II) , Ni( II) , Cu( 11) and Zn( II) complexes were reported by Leg and co-workers.' The crystal structures show that DACO exhibits the interesting " W c h a i r " configuration in its metal complexes. The hydrogen atoms of the trimethylene p u p on the backbone effectively block the sixth-coordination to the metal center to form complexes with penta-coordination mode' and the flap-over ring could provide a sterically protected metal coordination site capable of sup-