Coordination chemistry of a mono-dibenzofuran derivative of 1,4,7,10-tetraazacyclododecane

Abstract

Copper(II) and cadmium(II) complex of a mono-dibenzofuran derivative of 1,4,7,10-tetraazacyclododecane (cyclen), L1, have been prepared by reaction of the mono-dibenzofuran-substituted cyclen with either copper(II) or cadmium(II) perchlorate in acetonitrile. This yielded the corresponding divalent metal complexes, [Cu(L1)](ClO₄)₂·H₂O (C1) and [Cd(L1)](ClO₄)₂·H₂O (C2), which were isolated as single crystals suitable for X-ray crystallography by diffusing diethyl ether into an acetonitrile solution of each complex. For the copper(II) complex, C1, the X-ray crystal structure revealed a distorted square pyramidal Cu(II) coordination sphere occupied by four amine nitrogens from the cyclen ring and one oxygen from the amide linkage present in L1. On the other hand, the metal center in the cadmium complex is seven coordinate, with two weakly bound acetonitrile molecules occupying two additional coordination sites about the Cd(II) center; these bind cis to the amide oxygen. The coordination geometry is best described as monocapped trigonal prismatic. In both complexes, the secondary amine nitrogens on the cyclen macrocycle form hydrogen bonds with perchlorate counterions present in the crystal lattice. Titration of L1 with various metal ions (Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Cd²⁺) in acetonitrile revealed a gradual quenching of the benzofuran emission with increasing [M²⁺] consistent with formation of metal complexes with a 1 : 1 M²⁺: L1 stoichiometry. From the emission data, the conditional stability constant was determined to be 10–20 times higher for the Cu(II) complex than the other divalent metal complexes investigated.

Keywords:

• Coordination chemistry
• Macrocyclic ligands
• Crystal structures
• Fluorescence
• Metal ion-binding affinity

Acknowledgements

This research was funded in part by the National Health and Medical Research Council through Program Grant 400121 (D.C.R.) and the Australian Research Council through a Discovery Grant and Discovery Outstanding Research Award (L.S., DP130100816), as well as a Future Fellowship (B.G., FT130100838). J.B. and T.J. are grateful for the award of Monash Graduate Scholarships, Monash International Postgraduate Research Scholarships, and a Postgraduate Publication Award (T.J.).