Synthesis and complexation chemistry of new n-functionalised tetra-arzamacrocycles

Abstract

Tetraazamacrocycles occupy a unique position in coordination chemistry, owing to their ability to form very stable complexes with a large range of metal ions. The inclusion of additional pendent coordinating groups is of particular interest, due to the effect on the properties of these compounds as ligands. The regioselective synthesis of N,N’-dialkylated-1,4,8,11-tetraazacyclotetradecane in positions 1,5-, 1,8- and 1,11- has been accomplished using different substituents, including the potentially coordinating 2-pyridylmethyl group. A number of synthetic strategies have been investigated. The functionalisation of the related, cyclic dioxotetramines, 1,4,8,11-tetraazacyclotetradecane-a,b-diones (a,b = 2,3, 5,7 and 5,12) at the amino nitrogens has also been carried out. The nickel, copper and chromium complexes of some of these ligands have been prepared and characterised. In several cases, solid state structures have been elucidated by X-ray crystallography, and have revealed a number of distinct and intriguing structural types and macro cyclecon formations. UV-Visible absorption spectroscopy, cyclic voltammetry and (forchromium) luminescence spectroscopy have been used to investigate the properties insolution. New octadentate ligands based on the 1,4,7,10-tetraazacyclododecane ring, tris-N-functionalised with acetate groups, have been synthesised, for preparation of water soluble lanthanide complexes of high thermodynamic and kinetic stability. In one class of ligand, the eighth donor atom is the carbonyl oxygen of a covalently-linked para-substituted acetophenone . Efficient sensitisation of europium(III) luminescence is observed following excitation of the acetophenone, with the para-substituent exerting a significant effect on the photophysical properties. In the second class, the macrocycle is connected through a three- or four-atom linker, containing a coordinating amide group, to a benzophenone moiety, found to act asan efficient sensitiser of both europium(ffl) and terbium(M) bound to the macrocycle. In both classes, lifetime measurements in H 20 and D20 reveal the presence of a water molecule occupying the ninth coordination site of the metal ion. In the third class, 8-benzyloxyquinoline has been linked to the macrocycle, through a single 2-methylene unit. In this system, the nitrogen of the aromatic group is appropriately positioned to bind to the coordinated metal ion. No additional water molecule is observed in this case, and it is proposed that this is due to the benzyloxy group successfully occupying the site normally taken by the water molecule.