Deciphering chemical bonding in B_nH_n²⁻(n = 2–17): flexible multicenter bonding

Abstract

It is well known that closo-borane dianions B n H n 2À are stable aromatic cages and possess n + 1 valence electron pairs, in accordance with Wade's rules. However, the electronic structures of closo-B n H n 2À cannot be explained by a Lewis structure because of their electron-deficient character, while recent theoretical and experimental studies reveal that multicenter bonding is a key part in their electronic structures. In this work, flexible multicenter bonding of B n H n 2À (n ¼ 2-17) is studied using the adaptive natural density partitioning (AdNDP) method in order to get insight into their stability and aromaticity. The large HOMO-LUMO gaps and negative NICS values indicate their close-shell electronic structure. Further chemical bonding analysis shows that all 2n + 2 delocalized valence electrons in closo-B n H n 2À are involved in various delocalized s and p multicenter bonding systems on their cage surface, well matched with different symmetric configurations. There are five types of multicenter bonds, including an open 3c-2e BBB bond, closed 3c-2e BBB bond, 4c-2e bond, 8c-2e bond and a fully delocalized bond, which are delocalized on a B 3 broken-line, B 3 triangle, B 4 rhombus, B 8 double-ring and the whole surface of the boron cage, respectively. Our work reveals the flexibility of multicenter bonding in diversified closo-B n H n 2À clusters, giving new insights into the bonding nature of B n H n 2À .

Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes

“…To gain insight in thermodynamic hydricity of polyhedral closo-boranes, we calculated at first stage HDA MeCN of small dianionic boranes [BnHn] 2− and their lithium salts Li2[BnHn] (n = 2-4) (Figure 2), which can be formally viewed as building blocks or prototypes of polyhedral closo-boranes. Whereas [B2H2] 2− is linear, and [B3H3] 2− is planar, [B4H4] 2− has been described as having an "intermediate configuration between planar and tetragonal geometry" [133], which is actually disphenoid. In each structure, all B-Hterm bonds are equivalent (rBHterm = 1.191 Å for [B2H2] 2− ; rBHterm = 1.209 Å for [B3H3] 2− and rBHterm = 1.206 Å for [B4H4] 2− ).…”

“…The decrease of HDA MeCN values in this series (Scheme 7) is apparently associated with a better delocalization of electron density and with an increase in the number of possible resonance structures as the cluster size grows. In [B2H2] 2− featuring a triple B≡B bond, there are two 2c-2ē π-bonds, whereas in [B3H3] 2− and [B4H4] 2− featuring π-aromatic systems, there is one 3c-2ē and one 4c-2ē delocalized π-bond, respectively [133]. Further increase of borane cluster size results in the formation of polyhedral closo-boranes structures Li 2 [B n H n ] (n = 5-17) (Figure 3), which are formed following the Wade's rules [134,135].…”

“…Due to the 3D aromaticity, closo-boranes are characterized by higher HDA MeCN values (Figure 4, Figure S2) than small [B n H n ] 2− clusters (n = 2-4). Formal addition of BH to [B 4 H 4 ] 2− yields [B 5 H 5 ] 2− , which is the least stable member of the polyhedral closo-boranes [B n H n ] 2− according to formation enthalpy [50], and has never been synthesized [128,133]. The structure of [B 5 H 5 ] 2− is typical for the polyhedral closo-boranes-there are two types of boron atoms-two B atoms form caps, and a group of three other B atoms has the geometry of a B 3 H 3 triangle, forming a belt of the polyhedron (for further description of structural features of polyhedral closo-boranes, see in SI).…”

“…In the case of polyhedral closo-boranes due to the rigid frame of the boron cluster [133,148], several quasi-borinium cations were observed, localized on vertices from which hydride was torn off.…”

[(Cp2M)2B9H11] (M = Zr or Hf): early transition metal ‘guarded’ heptaborane with strong covalent and electrostatic bonding

“…As a result, an improved synthesis and complete structural characterization of [B 7 H 7 ] 2– were of general interest. Although a number of theoretical studies on the structure and bonding of [B 7 H 7 ] 2– have been reported, 10 its convenient synthesis and full structural characterization were established only recently by Bernhardt in 2011. 11 In an effort to synthesize the neutral hypercloso -heptaborane analogue, Siebert reported [B 7 Cl 5 (PMe 3 ) 2 ], which was mainly characterized using 11 B NMR.…”