Di- and trinuclear copper(II) complexes of polyaza macrocycles and cryptands as anion receptors Pedro Mateus, Luís M.P. Lima, Rita Delgado ⇑ Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Av. da República – EAN, 2780-157 Oeiras, Portugal article info Article history: Available online 1 August 2012 Dedicated to Alfred Werner on the 100th Anniversary of his Nobel prize in Chemistry in 1913. Keywords: Copper N-donor macrocycles Dinuclear complexes Cascade complexes Anion recognition abstract Di- and trinuclear copper(II) complexes of ligands containing several polyamine coordination sites have been widely used as receptors for the recognition of anions. Here, a selection of relevant examples of di- and trinuclear copper(II) complexes of polyamine macrocycles and cryptands based on varied binding units, outlining their behaviour as receptors for binding of anionic substrates, is reviewed. The selected examples are discussed taking into account the anion binding mode, the association strength, the selec- tivity, and the structural features of the recognition phenomena. A comparison of the advantages and lim- itations of each receptor design is also undertaken in the perspective of achieving selective recognition of the anionic substrates. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Selective recognition of anions by artificial receptors is a very challenging goal to achieve. However, primarily due to the potential biomedical and environmental applications that can be devised, this research field continues to interest many supramolec- ular chemists [1–3]. The difficulty of selective binding of anions in aqueous solution stems from the intrinsic characteristics of anions: larger sizes when compared to isoelectronic metal ions, variety of shapes, and pH dependency. Furthermore, the solvent itself plays a very important role in the binding process, as water is the most active competitor in the recognition process by strongly solvating recep- tors and anions and by competing for the hydrogen bonding sites. The binding process requires desolvation of the partners, which gives rise to large energetic penalties. One of the most common approaches to bind anions in aqueous solution is to combine hydrogen bonding with electrostatic inter- actions. In this regard, protonated macrocyclic and macrobicyclic polyamines are among the most successful groups of compounds used in the recognition of anions in aqueous solution, due to the binding properties of the ammonium group and the encapsulating abilities of the cyclic architectures [4–9]. However, even electro- static interactions combined with hydrogen bonding frequently give rise to low affinities and no selectivity unless both binding partners are multiply charged. Another possibility is the formation of cascade species by the selective coordination of an anionic guest between the metal ions of a dinuclear complex of a ditopic macrocycle or cryptand, see Scheme 1 [10–14]. This strategy has the advantage of having less interference from water, which being a relatively poor Lewis base is weakly coordinated. The energy of a coordination bond is rather high when compared to hydrogen bonds, consequently higher affinities for the anions are expected. Copper is frequently the metal of choice in the design of metal- based anion receptors. The reason for this is primarily because, among divalent first-row transition metal ions, copper(II) estab- lishes the strongest coordination bonds with most donor atoms. Moreover, the intrinsic kinetic lability of copper complexes allows fast exchange of anions, a required property for sensing purposes. In addition, coordination numbers of 4–6, along with a variety of different geometries (square planar, square pyramidal, trigonal bipyramidal or octahedral), are usual with copper(II) depending on the type of ligand. This leaves room for a range of conceivable ligand architectures and donor atom sets for the design of copper complexes as anion receptors. Macrocyclic and macrobicyclic ligands, notably those of the polyamine family, may coordinate two or more metal cations in- side its molecular cavity while leaving at least one vacant binding site on each metal. These vacant binding sites are originally occu- pied by weakly coordinated water molecules or counterions (fre- quently omitted throughout this review for simplicity purposes). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.07.073 ⇑ Corresponding author. E-mail address: delgado@itqb.unl.pt (R. Delgado). Polyhedron 52 (2013) 25–42 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly