Binding and structural aspects of nitrile-and amino-functionalised pendant arm derivatives of 1,4,7-triazacyclononane ([9]aneN3)

Abstract

The coordination chemistry of the tridentate macrocycle 1,4,7-triazacyclononane ([9]aneN3) and its N-alkylated analogues (R3-[9]aneN3) with main group and transition metal ions has been comprehensively studied over the last thirty years [1-7]. In addition to kinetically inert and thermodynamically stable sandwich complexes, a series of polynuclear complexes have also been prepared in the presence of suitable bridging ligands. In these species, the [9]aneN3 macrocyclic framework serves as a face-capping group, allowing variation of the donor array at the remaining coordination sites of a coordinated metal ion. The binding properties of [9]aneN3 to metal cations can be finely adapted through sequential functionalisation of the secondary amines with pendant arms bearing additional coordinating groups to generate ligands with increasing number of donor atoms. Symmetric N-functionalisation of [9]aneN3 via incorporation of three identical pendant arms terminated with neutral or anionic N-, O-, S-, or P-donor groups has afforded a great variety of effective hexadentate ligands. These often confer remarkable stability upon metal centres in low nuclearity complexes, and can adapt to their binding mode to the preferred coordination geometries and oxidation states of metal centre(s) [8-15]. However, much less work has been reported on selectively N-functionalised derivatives of [9]aneN3 bearing only one or two pendant arms having identical coordinating donor groups or "innocent" alkyl substituents [8-10, 16-21]. Even less work has been reported on the synthesis of asymmetric derivatives of [9]aneN3 having different N-attached pendant-donors groups, reflecting the increased and significant synthetic difficulty encountered in the preparation of these types of ligands using a multistep approach [22, 23]. Nevertheless, pendant arm derivatives of [9]aneN3 and other polyaza-macrocycles have garnered great interest in recent years for their use in many different chemical applications such as catalysis [24-30], selective cation binding [30-32], surfactants [33, 34], mimicry of enzymes and siderophores [35-39], tumor-directed radioisotope carriers, and use in magnetic resonance imaging reagents [40]. The synthesis, structure, and coordination properties of [9]aneN3 and other polyazamacrocycles bearing N-attached coordinating pendant groups have been reviewed [1, 8-10, 40]. The present review covers the last four years of our work in this field and is only focussed on synthetic aspects and coordination properties of nitrile-and aminofunctionalised pendant arms derivatives of [9]aneN3. Their use in developing new highyield synthetic routes for the preparation of asymmetric derivatives of [9]aneN3 with more than one type of pendant donors and featuring amino-functionalised pendant arms are also discussed. © 2005 Springer. Printed in the Netherlands.