The compounds containing beryllium act as strong

The compounds containing beryllium act as strong Lewis acids because of electron deficiency of the Be GW-1100 []. Interactions of X-Be-Y compounds with different Lewis bases have been studied by Yanez and co-workers [], extensively. Recently, they proposed a class of extremely strong bidentate Be-based anion receptors with anion (F, Cl, Br, NO, CN, SO) affinities between 230 and 770 kJ/mol []. In this work, we use -boryl and -beryllium pyrrole derivatives to design a new type of anion receptors. The mechanism of these anion receptors is based on formation of an aromatic compound after interaction of the anions with the boron and beryllium atoms. Structures of the molecules, anions, and complexes were fully optimized employing B3LYP functional and 6-311++G(d,p) basis set in gas phase. The frequency calculations were performed at 298.15 K at the same level of theory to obtain the enthalpies and Gibbs free energies of the interactions. Nuclear-independent chemical shift (NICS) parameter [], as an index of aromaticity, was computed using the gauge-independent atomic orbital (GIAO) [] method at the same level of theory. Isotropic, NICS, and out-of-plane components, NICS, of the chemical shift tensor were used as aromaticity indices. The NICS values at the ring surface, NICS(0), and at 1 Å above the ring, NICS(1), were also calculated using the Bq dummy atom as a probe []. All calculations were carried out using Gaussian 09 software []. According to the Hückel theory of aromaticity, the monocyclic compounds with 4n + 2 π-electrons are aromatic []. Pyrrole, CHN, is an aromatic molecule because the lone pair on the nitrogen atom and four 2p electrons of carbons make an aromatic ring with 6 π-electrons. shows an -boryl pyrrole compound which has two resonance structures. The structure A1 has an aromatic ring in which the lone pair on the N atom participates in the electron delocalization of the 5-membered ring. Boron is an atom with electron deficiency, therefore, it may withdraw the lone pair electrons of the nitrogen to form an anti-aromatic zwitterion isomer, A2. Relative abundance of the isomers A1 and A2 can be controlled by inclusion electron withdrawing (EWG) and electron donating groups (EDG) instead of X. It is expected that the EWGs promote formation of the zwitterion A2 and increase its relative abundance. When the -boryl pyrrole derivatives interact with an anion via their boron atom, the entering anion compensates the electron deficiency of the boron atom. Therefore, in the produced anion complex, the boron atom no longer need the lone pair electrons of the N atom and allows them to go and participate in the aromaticity of the 5-membered ring (). The same discussion is applicable about -beryllium pyrroles. shows the optimized structures of the -boryl and -beryllium derivatives of pyrrole and their adducts with anion F. The aromaticities of these compounds are compared in using NICS(1) index. Other NICS values including NICS(0), NICS(0), and NICS(1) are summarized in Table S1 (Supplementary materials). The molecules with more negative NICS values are more aromatic. The -beryllium pyrroles are generally more aromatic than the corresponding -boryl pyrroles; therefore, it can be concluded that the ability of boron to withdraw the electron pair of the nitrogen atom is more than that of beryllium. The compounds with the strong electron withdrawing group CN are less aromatic than those with X = H, F. In other words, CN substituent intensifies the electron deficiency of B and Be and facilitates formation of the antiaromatic zwitterionic isomer. Comparison of the NICS(1) values of the anion receptors and their corresponding adducts with F shows that the latters are more aromatic. In addition, the NB and NBe bond lengths in the neutral anion receptors are significantly shorter than those in the anionic adducts. Comparison of the aromaticities and the bond lengths confirms that boron and beryllium act as Lewis acids accepting the electron pair on the nitrogen atom. This leads in a dative bond and consequently shorter NB and NBe bonds in the anion receptors. Therefore, the neutral anion receptors are mainly in the zwitterionic form and antiaromatic. After interaction with F, the electron pair of N is delocalized on the ring, the aromaticity increases, and the NB and NBe bond lengths become longer.