A theoretical study of aromaticity in squaramide complexes with anions David Qui ~ nonero, Rafel Prohens, Carolina Garau, Antonio Frontera * , Pau Ballester, Antonio Costa, Pere M. Dey a DepartamentdeQu õmica,UniversitatdelesIllesBalears,Crta.deValldemossakm7,5,07071PalmadeMallorca,Spain Received 27 June 2001; in ®nal form 27 September 2001 Abstract A systematic estimation of aromaticity in squaramide complexes with anions has been studied using structural and magnetic criteria. Results based on Nucleus Independent Chemical Shift NICS) predict that squaramide is less aro- matic than its complexes with several anions. We had previously observed the same behavior in squaramide complexes with cations. This bifunctionality can be used to design ditopic receptors. In fact a 1:1:1 complex between squaramide and both ammonium cation and formiate anion is more aromatic than any of the other two 1:1 squaramide-cation/ anion complexes. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction In spite of the fact that anion complexation is the less developed area of supramolecular chem- istry, anions play essential roles in chemical, bio- logical and environmental recognition processes [1,2]. In nature phosphate and sulfate binding proteins are important receptors for transport systems in cells. Molecular recognition of anions by synthetic receptors is currently an expanding ®eld of research [3]; including catalysis [4], sepa- rations [5] and anion transport and sensing [3,6]. Synthetic receptors for anions are usually based on macrocyclic polyammonium/guanidinium [7,8], amides [9], urea/thiourea [10,11] and calixarenes [12]. The binding of anions to neutral receptors is ofspecialsigni®cance;®rst,toavoidthecompeting counterion complexes present if cationic hosts are used. Second, the selectivity is modest in cationic receptors due to the dominance of nondirectional electrostaticinteraction[13].Thehighspeci®cityof neutral anion binding proteins is due to a recog- nition site where the anion guest is bonded simply via multiple hydrogen bonds [14]. Obviously, each nonbonded interaction is weaker than Coulombic interactions present in positively charged recep- tors, however an enough number of nonbonded interactions can lead to selective neutral receptors for anions. Recently, ditopic receptors, i.e., receptors that complex both anions and cations, have been used to accelerate the transport rate in separation 4 Janauary 2002 Chemical Physics Letters 351 2002) 115±120 www.elsevier.com/locate/cplett * Corresponding author. Fax: +34-971-17-34-26. E-mailaddress: tonif@soller.uib.es A. Frontera). 0009-2614/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0009-261401)01295-7