Boron has been becoming more and more intriguing element for the last years. Having less valence electrons than neighboring carbon boron usually forms non-trivial bonds going far beyond classic paradigms of bonding. Now pure boron clusters are known up to borospherene B40- which was obtained recently and became a sensation. Another field of great interest relates to boron compounds including other mostly electron-donor atoms which allows to perform so called electronic transmutation resulting in carbon-like structures formation. Here we would like to report two electronically enriched novel boron clusters B2Al3- and NaBH3-. B2Al3- turned out to be a planar triangular cluster in which two B atoms are linked directly. We performed bonding study using the AdNDP method as a powerful extension of classic NBO approach to decipher bond character in this structure (Fig1, A). We revealed that this is a case of B≡B triple bond and taking into account four electrons transferred to B atoms we went even beyond the transmutation to carbon and obtained N≡N isoelectronic analogue (Fig1, B). In case of B2Al3- occupation numbers deviating from ideal 2.00 values indicate some delocalization which is common for ionic clusters. We expected the same transmutation behavior in NaBH3- cluster under acceptance of two electrons by BH3 fragment but the real bonding picture even exceeded our guess. Combining experiment and theory we found that the real bonding is Na-(BH3) meaning that Na- forms a dative bond with neutral BH3 moiety. This bonding remains the one in classical Lewis pair NH3-BH3 (Fig1, C, D). BDE values also show that Na-B bond is covalent like in Na2 (BDEs are 17.1 and 16.3 kcal/mol, respectively). To the best of our knowledge, this is the first example of inverse Lewis pair formed by negatively charged alkali metal.

Dualism of electronically enriched boron clusters: From transmutation to nitrogen to formation of inverse Lewis pair

2019 Fall National Meeting

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