shows the complexes resulting from
shows the complexes resulting from interaction of the proposed anion receptors with the anions F, Cl, NO, and SO. No imaginary frequency was observed for the optimized complexes indicating the complexes are in potential minima and they are not transition state structures. summarizes the calculated ∆H and ∆G values for interactions of the anion receptors with F, Cl, NO, and SO anions. The negative values of the calculated enthalpies (−∆H) are considered as anion affinity. The F affinity of the anion receptors is more than their Cl affinity. The difference may be due to the larger size of Cl and its smaller charge density compared to F. The affinities of the anion receptors toward Cl and NO are comparable. The simplest anion receptors and without any electron withdrawing group show F affinities of 265.6 and 390.0 kJ/mol, respectively, which are among the highest anion affinities reported so far. The proposed anion receptors exhibit approximately the same affinity toward F and SO anions. Substitution of strong electron withdrawing group CN enhances the anion affinities so that the SO affinities of the BRD 7552 mg is 487.1 kJ/mol. These values show that the anion affinities of these simple anion receptors are comparable with those reported for the strong anion receptors [,]. To estimate effect of aromaticity/antiaromaticity switching on the anion affinities, F affinity of NHBH was computed. The calculated F affinity of NHBH is 100.0 kJ/mol which is significantly smaller than the corresponding values for the anion receptors – (266–470 kJ/mol). This comparison confirms that the high anion affinities of the anion receptors are not only due to electron deficiency of boron and beryllium atoms and our proposed strategy has led in so high anion affinities. The monodentate anion receptors shown in were extended to bidentate ones by fusing two pyrrole rings. The optimized structures of the bidentate anion receptors – are shown in Fig. S1 (Supplementary materials). shows the optimized structures for the interactions of the bidentate anion receptors with F, Cl, NO, and SO anions. Only the most stable structures are shown in . Other possible structures have been shown in Fig. S2 (Supplementary materials). The calculated vibrational frequencies reveal that some of the complexes in Fig. S2 are transition state (TS) structures due to their imaginary frequency. In the bidentate receptors including boron atoms only one boron atom interacts with the F, Cl, and NO anions. However, in the case of SO, both the B atoms of the anion receptors interact with two oxygen atoms of this anion. The bidentate anion receptors with beryllium atoms tend to interact with the anions via their both Be atoms. The NB and NBe bond lengths in the bidentate anion receptors are about 1.42 and 1.53 Å, respectively (Fig. S1). Interaction of the anions with anion receptors increases the NB and NBe bond lengths so that the NB and NBe bond lengths in the complexes of are about 1.50–1.58 and 1.57–1.63 Å, respectively. summarizes the calculated enthalpies (∆H) and Gibbs free energies (∆G) for interactions of the bidentate anion receptors with F, Cl, NO, and SO anions. The anion affinity of the bidentate anion receptors – is larger than that of the corresponding monodentate anion receptors –. However, the difference in the anion affinities of the boron-based anion receptors toward F, Cl, and NO is not significant; because both the monodentate and bidentate anion receptors interact with these anions via only one of their boron atoms. However, the bidentate anion receptors exhibit large anion affinity toward SO because both the boron atoms participate in the interactions. In the case of the beryllium-based anion receptors the difference in the anion affinities of the monodentate and bidentate receptors are considerable. The anion affinities of the beryllium-based anion receptors are generally more than those of boron-based anion receptors. The anion receptor without any electron withdrawing group shows F affinity of 538.3 kJ/mol, which is among the highest anion affinities reported so far. The anion affinities of show that the proposed anion receptors are stronger than or comparable with many strong anion receptors [,,]. Generally, the anion receptors show a high affinity toward SO anion, therefore, they may be considered as selective SO receptors. The highest SO affinity was obtained for the compound with anion affinity of 808 kJ/mol. To the best of our knowledge, it is the highest anion affinity that have been reported for a so small anion receptor. The anion affinities of the bidentate beryllium-based anion receptors are comparable with those reported by Yanez et al.  for 1,8-diBeX-naphtalene (200–770 kJ/mol) however, our anion affinities are slightly larger due to antiaromaticity/aromaticity switching.