FULL PAPER A DFT Study of Tin- and Crown-Ether-Based Host Molecules Capable of Binding Anions and Cations Simultaneously Ricardo Vivas-Reyes, [a,b] Frank De Proft, [b] Monique Biesemans, [c] Rudolph Willem, [c] and Paul Geerlings* [b] Keywords: Acidity / Basicity / Crown compounds / Density functional calculations / Host-guest systems / Tin DFT calculations are reported for a series of tin- and crown- ether-based host compounds {i.e., the trimethyl derivatives [18]-crown-6-C 6 H 3 COOSn(CH 3 ) 3 and [15]-crown-5- C 6 H 3 COOSn(CH 3 ) 3 } capable of binding cations and anions simultaneously. The B3LYP functional together with the 6- 31G* basis set was used for the atoms C, H, N, O, S, Na and K and the 3-21G* basis set for Sn in order to obtain insights into the factors determining the nature of the interactions of these compounds with the neutral molecules acetone and H 2 O, the SCN - anion, and the Na + and K + cations. The inter- action strength pattern with these molecules was explained by the use of a series of reactivity descriptors such as the Fukui function, hardness, local softness, and the MEP (mo- lecular electrostatic potential). In all cases studied in this pa- per, the complexes with Na + were more stable than those with K + , correlating with the size of the cation and the vol- ume available in the crown ether. Moreover, this finding is also in accordance with the greater hardness of Na + relative to K + , in combination with the hard environment of the crown ether moiety. This region was also analysed by computation 1. Introduction The complexation of host molecules capable of binding an anion and a cation simultaneously is still a relatively unexplored topic in chemistry. [1] Numerous studies, how- ever, have been devoted either to the interactions between cations and preorganised host molecules, [2] or to fixation of anions by neutral molecules containing Lewis acidic metal centres. [3] Studies on interactions between crown ethers and various inorganic and organic cations have spawned wide applications in chemistry, biology, medicine and techno- logy, [4] while the Lewis acidity of the tin atom in, for in- [a] Grupo de Quimica Cuantica y Teorica, Universidad de Cartagena, Chemistry Department, A.A 1661 Cartagena, Colombia [b] Eenheid Algemene Chemie, Faculteit Wetenschappen, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium Fax: (internat.) + 32-2/6293317 E-mail pgeerlin@vub.ac.be [c] High Resolution NMR Centre, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium Eur. J. Inorg. Chem. 2003, 1315-1324  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434-1948/03/0407-1315 $ 20.00+.50/0 1315 of electrostatic potentials, which showed that highly negative values are associated with the inside region of the cavity of the crown, this region thus being amenable to electrophilic attack. The HSAB principle, characterising the reactive sites on the basis of local softness and the Fukui function, pro- vided a firm explanation of the reactivity of the tin atom of the crown ether benzocarboxylate towards SCN - , acetone and water. The HSAB concept was also successfully used to explain the preference of the tin atoms in both crown ethers to bind with the nitrogen atom rather than the sulfur atom of SCN - . This result is a confirmation that the tin atoms in the compounds under consideration behave as hard atoms. Overall, these results fit remarkably well with previous ex- perimentally measured NMR spectroscopy data and demon- strate that the interactions of this kind of molecules can be predicted and interpreted by the use of DFT calculations and DFT-based reactivity descriptors, as well as MEP calcula- tions. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) stance, organotin halides is well documented. [5] Fewer stud- ies have been reported on interactions of Lewis bases with organotin carboxylates, [6] despite widespread interest in their in vitro antitumour activity [7] and their rich structural diversity in the solid state. [8] We recently reported a novel type of salt complexation by a new class of host molecules, containing both a crown ether susceptible to interaction with the cation and a Lewis acidic tin centre potentially acting as an anion carrier. [9] It was demonstrated that tri-n-butyl and triphenyltin derivat- ives of 4-carboxybenzo-[18]crown-6 or [15]crown-5 are re- ceptors capable of binding M + SCN - ion pairs heterotop- ically. The Lewis acidic complexation of Sn by the thiocyan- ate anion cooperates with the crown ether complexation by the alkali metal cation and gives rise to a large charge sep- aration, as evidenced by X-ray data in the crystalline state for the triphenyltin derivatives and by NMR spectroscopic data in solution for the tri-n-butyltin analogues. This paper presents a theoretical investigation of the tri- methyltin analogues of these compounds {i.e., [18]crown-6- C 6 H 3 COOSn(CH 3 ) 3 and [15]crown-5-C 6 H 3 COOSn(CH 3 ) 3 ,