The Supramolecular Chemistry of Anion Binding and Sensing
We are interested in the development of new paradigms for anion binding and sensing within the realm of supramolecular chemistry. Anions are ubiquitous throughout Nature. They are involved in myriad biological processes, are associated with numerous medical diseases, and are implicated in environmental pollution. Our work in this area is leading to paradigm shifts within anion coordination chemistry and sensing.
Supramolecular Anion Coordination Chemistry
The affects anions exert, are primarily due to the way they interact with their environment. Nevertheless, anion coordination chemistry per se has only recently become systematically investigated and our under-standing of the processes involved is still relatively under-developed. Our work is addressing some of these critical and fundamental questions and we have made significant contributions to the area including several ‘firsts' including the first report of a helicate complex assembled around an anion template, Figure 3. Metal ion-driven assemblies had dominated this field until our report.
Figure 3: Molecular structure of [(H4LCl)2]·6Cl·H2O: a dinuclear double helicate complex self-assembled around two chloride ions (see Chem. Commun., 2001, 2192)
Indeed, we have contributed another recent first with the deliberate synthesis of dinuclear triple metallo-helicate species specifically designed to bind anionic species within their intra-helical cavity, Figure 4. These complexes are currently being developed in an attempt to take advantage of the chiral space with the intra-helical cavity to see whether the formation of host complexes capable of entio-discrimination of guest species is possible. We are also investigating these species as potential nano-space reaction vessels and as potential anion sensors.
Figure 4: Molecular model of a dinuclear triple helicate complex [Fe2(L)3(Cl)2]2+ showing the proposed mode of binding of two chloride ions within the intra-helical cavity (see Dalton Trans., 2006 , 1277; see also Chem. Eng. News, 2006, 84(3), 26.)
Supramolecular Anion Sensing
Our recent work in this area has seen the development of naked-eye anion sensors capable of detecting anionic species in extremely competitive aqueous media where the recognition event between host (sensor) and guest iss mediated through H-bonding interactions at a thio-urea or urea-based receptor. Upon the interaction of the anionic species with the host molecule a striking colour change occurs that is visible to the naked-eye, Figure 5. This was the first report of its kind.
Figure 5: Chromogenic response of solutions of sensor 1 (1 × 10-3 M, DMSO) upon interaction with various anions: (a) 1; (b) 1 + 1 eq. AcO–; (c) 1 + 1 eq. H2PO4– ; (d) 1 + 1 eq. F–; (e) 1 + F–(excess) (see J. Org, Chem., 2005 , 70 , 10875; Chem. Eng. News 2006 , 84(7) , 83-85).
This work resulted from the culmination of much research effort in this area by our group and in collaboration with the group of Prof Thorri Gunnlaugsson at Trinity College Dublin. We are currently investigating the applicability of incorporating these molecules within an inert support to hopefully lead to inexpensive test-kits for anionic species in aqueous solution. We have recently reviewed this area: ‘ Anion Recognition in Organic and Aqueous Media Using Luminescent and Colorimetric Sensors', Coord. Chem. Rev., 2006, 250, 3094-3117.