Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes

Abstract

Thermodynamic hydricity (HDA^MeCN) determined as Gibbs free energy (ΔG°[H]⁻) of the H⁻ detachment reaction in acetonitrile (MeCN) was assessed for 144 small borane clusters (up to 5 boron atoms), polyhedral closo-boranes dianions [B_nH_n]²⁻, and their lithium salts Li₂[B_nH_n] (n = 5–17) by DFT method [M06/6-311++G(d,p)] taking into account non-specific solvent effect (SMD model). Thermodynamic hydricity values of diborane B₂H₆ (HDA^MeCN = 82.1 kcal/mol) and its dianion [B₂H₆]²⁻ (HDA^MeCN = 40.9 kcal/mol for Li₂[B₂H₆]) can be selected as border points for the range of borane clusters’ reactivity. Borane clusters with HDA^MeCN below 41 kcal/mol are strong hydride donors capable of reducing CO₂ (HDA^MeCN = 44 kcal/mol for HCO₂⁻), whereas those with HDA^MeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B₂H₆, B₄H₁₀, B₅H₁₁, etc.). The HDA^MeCN values of closo-boranes are found to directly depend on the coordination number of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDA^MeCN.

1. Introduction

Boron-based chemistry is vast, diverse, and fascinating due to the ability of boron to form electron-deficient structures of various shapes (such as cages, clusters, etc.) with delocalized electrons and multicenter bonding. Boron hydrides, i.e., boranes (e.g., BH₄⁻, [B₃H₈]⁻, [B_nH_n]²⁻ n = 6–12), are of great interest because of their use as ligands in inorganic chemistry [1,2,3,4,5], as building blocks in material chemistry [3,6,7,8], and as materials for the energetic purposes—components of batteries [9,10], rocket fuels [11,12,13] and systems of hydrogen storage [14,15,16,17]. Polyhedral boranes are widely used as sources of boron (¹⁰B) in boron neutron capture cancer therapy [18,19,20,21,22,23], in the creation of luminescent materials [24,25,26], thermally stable polymers [5], liquid crystals and nonlinear optical materials [27,28], as well as precursors of nanostructured materials [29,30].

Thermodynamic hydricity, i.e., hydride donating ability (HDA), determined as Gibbs free energy (ΔG°[H]⁻) for the reaction of hydride ion, H⁻, detachment, is a very important characteristic of transition metal hydrides [31] and main group hydrides [32,33,34] that describes their reactivity and is used for the rational design of catalytic reactions. In our recent work, we have demonstrated the existence of an inversely proportional dependence of the hydride-transfer ability of Li[L₃B–H] on Lewis acidity of L₃B [34]. High values of HDA indicate high Lewis acidity of parent borane L₃B, whereas low HDA values indicate high hydride transfer ability of Li[L₃B–H]. For that reason, the hydride donating ability (HDA) of boranes can be used as a measure of the Lewis acidity of parent neutral borane or boron cations.

However, there are significant problems in the experimental determination of the thermodynamic hydricity for hydrides of main group elements (E–H, where E = Si, C, B, Al) because of their instability in polar solvents (MeCN, H₂O), which are typically used for that [32,35]. Besides, the E–H bond of main group hydrides is characterized by low polarizability compared to transition metal hydrides and, therefore, in many cases, the detachment of the hydride ion (H⁻) is challenging even in the presence of a large excess of strong Lewis acid. Due to these problems, there is a huge gap in our knowledge about the reactivity of boranes towards hydride transfer [32,35,36].

In our previous paper [34], we focused on the DFT investigation of thermodynamic hydricity of tetracoordinate borohydrides Li[L₃B–H]. However, despite the attempts to evaluate the electron-donating properties in polyhedral boranes by ¹H NMR [37], there is a lack of knowledge of the reactivity of B–H bond in small borane clusters and polyhedral boron hydrides [36]. Thus, in this paper, we report on the results of the DFT analysis of thermodynamic hydricity in MeCN (HDA^MeCN) for both well-known, structurally characterized boranes and prospective reaction intermediates, some of which were previously known from theoretical works.

2. Results and Discussion

Borane clusters (such as B₂H₆, [B₂H₇]⁻, [B₃H₈]⁻, [B₁₂H₁₂]²⁻, [B₁₀H₁₀]²⁻, etc.) were discovered as intermediate products of borohydrides thermal decomposition [38,39,40,41,42,43,44,45,46,47,48,49,50]. These compounds are widely used in direct synthesis of other boranes [51,52], organoboron compounds [53], and new materials [5]. For example, hydride ion abstraction from borane anions (such as BH₄⁻, [B₃H₈]⁻, [B₄H₉]⁻, etc.) by Lewis acids BX₃ (X = F, Cl, Br) is the most convenient route for the preparation of higher boranes [54,55]. Many small borane clusters are highly reactive and unstable species, so their reactivity is hard to assess experimentally. Others, like polyhedral boron hydrides, require the presence of an excess of Lewis or Brønsted acids for generation of boron-centered quasi-borinium cations [36]. Thus, the reactivity of such compounds can be characterized in terms of their ability to hydride transfer as thermodynamic hydricity (HDA) through DFT calculations [32,34].

In this paper, we performed the DFT calculations of the thermodynamic hydricity in MeCN of small borane clusters (containing up to 5 boron atoms) and polyhedral boron hydrides [B_nH_n]²⁻ (n = 5–17). For anionic species we use their lithium salts to offset the effect of ionic species and to make a correct comparison with the HDA^MeCN values for neutral boranes. 2.1. Thermodynamic Hydricity of Small Borane Clusters

Recently, the decomposition pathways of [LiBH₄]_n, n = 2–12 clusters were investigated by DFT calculations [48]. In the case of dimeric [LiBH₄]₂ clusters, the release of up to 4 equivalents of H₂ was found, as well as [LiBH_m]₂ (m = 6, 4, 2) reaction intermediates, during the decomposition. Our calculations show that during the decomposition of [LiBH₄]₂ the first H₂ release leads to the decrease of HDA^MeCN by 15.9 kcal/mol (Scheme 1). Further H₂ release leads to an increase of HDA^MeCN due to formation unsaturated diboranes with double and triple bonds.

2.1.1. General Pattern in Thermodynamic Hydricity of Borane Clusters

To gain insight into an effect of borane (BH₃) aggregation in small clusters, we used the monomers LiBH₄, Li₂BH_3, LiBH_2, and Li₂BH along with neutral BH₃ and BH species to construct a homologous series of boron clusters (containing up to 5 boron atoms) by consequent addition of BH₃ (Table 1, Schemes S1 and S2). Another two homologous series were constructed based on Li[B₂H₃] and B₂H₂. In each series there are the most stable boranes (namely BH₄⁻, B₂H₆, [B₂H₆]²⁻, [B₃H₈]⁻, B₄H₁₀, [B₃H₇]²⁻, [B₄H₉]⁻, and B₅H₁₁), which are generally observed during thermal decomposition of metal borohydrides [38,39,40,41,42,43,44,45,46,47,48,49,50].

According to computed heats of formation [50], neural boranes (such as B₂H₆, B₄H₁₀, and B₅H₁₁) are less stable than anionic ones (BH₄⁻, [B₃H₈]⁻ and [B₄H₉]⁻); however, the former may be derived from the latter as a result of hydride abstraction by Lewis acids BX₃ (X = F, Cl, Br) [55]. This lower stability of neutral boranes compared to anionic boranes can be explained by higher Lewis acidity. In our previous research, we demonstrated that Lewis acidity of parent borane (R₃B) can be estimated by the analysis of thermodynamic hydricity of their product of hydride addition [R₃BH]⁻. Higher HDA^MeCN values of [R₃BH]⁻ correspond to higher Lewis acidity of R₃B.

Thermodynamic hydricity values of diborane B₂H₆ (HDA^MeCN = 82.1 kcal/mol) and its dianion [B₂H₆]²⁻ (HDA^MeCN = 40.9 kcal/mol for Li₂[B₂H₆]) can be selected as border points for the range of borane clusters’ reactivity. Previously [57], the formal division of the thermodynamic hydricity scale into weak (above 80 kcal/mol), medium (between 80 and 50 kcal/mol) and strong hydride donors (less 50 kcal/mol) was suggested.

Our study and literature analysis show that borane clusters with HDA^MeCN below 41 kcal/mol are strong hydride donors capable of reducing CO₂ (HDA^MeCN = 44 kcal/mol for HCO₂⁻ [31]); however, those with HDA^MeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and less prone to hydride transfer than to proton transfer (e.g., B₂H₆ [58], B₄H₁₀, B₅H₁₁, etc. [59]). Moreover, in higher boranes, the deprotonation of B–H–B bridges leads to B–B bond formation, in which boron is nucleophilic and susceptible to the attack of electrophiles [59]. Thus, higher borane anions could be generated by the insertion of such electrophile molecules as BH₃, B₂H₂ or BH.

Based on the data obtained for the homologous series (Table 1, Schemes S1 and S2), an evolution of thermodynamic hydricity can be traced on the example of B₃ clusters transformations (Scheme 2). Thus, neutral boranes B₃H₉, B₃H₇, and B₃H₅ feature the highest Lewis acidity (highest HDA^MeCN values). Monoanionic boranes Li[B₃H₁₀] etc. have lower Lewis acidity than parent neutral boranes. The two-electron reduction of the neutral boranes leads to a significant decrease of HDA^MeCN values (by 41.1 kcal/mol for Li₂[B₃H₉] and by 20.0 kcal/mol for Li₂[B₃H₇]).

Although it is generally acknowledged that, in clusters, borane anions’ hydridic hydrogen becomes less reactive due to increasing charge distribution across the cluster [8], our calculations show that HDA^MeCN values are not directly connected with the size of borane cluster (Figure S1). However, if we plot computed HDA^MeCN values for all series of boranes (n = 1–5; Figure 1), it appears that minimum HDA^MeCN values (nucleophilic boron) are typical for the dianionic species and maximum HDA^MeCN (electrophilic boron) for the neutral boranes, whereas borane monoanions are in the intermediate position. Since neutral transition metal hydrides are better hydride donors than cationic ones [31], it is obvious that anionic boranes should be much more better hydride donors than neutral boranes.

An increasing size of borane cluster in neutral B_nH_3n, B_nH_3n−2, and B_nH_3n−4 and dianionic Li₂[B_nH_3n] and Li₂[B_nH_3n−2] series results in a drop of HDA^MeCN values. This leads to a smoothing of the extrema in the graph of HDA^MeCN for larger borane clusters as B₄ and B₅. Moreover, in the B₃ clusters due to triangle form an effective charge distribution occurs, which leads to the systematically lower values of HDA^MeCN. It is worth noting that a decrease of saturation of the borane cluster results in an increase of HDA^MeCN that is especially pronounced for neutral diboranes.

2.1.2. Features of Thermodynamic Hydricity in Homologous Series

The Li[B_nH_3n+1] series is formed by consecutive BH₃ addition to tetrahydroborate-anion BH₄⁻ (Scheme 3). The first addition of BH₃ yields [B₂H₇]⁻ and leads to the HDA^MeCN decrease by 16.4 kcal/mol, while the subsequent BH₃ addition leads to a gradual HDA^MeCN increase due to the delocalization of electrons between a greater number of atoms.

Tetrahydroborate anions BH₄⁻ [4,60,61,62,63,64,65,66,67,68] and [B₂H₇]⁻ [69,70] are used as ligands in the synthesis of transition metal complexes. Structures of other borohydride anions were accessed by DFT calculations [58,71], but only branched isomers of Li[B₄H₁₃] and Li[B₅H₁₆] have been reported [71]. However, we found that their linear isomers were energetically more favorable (see ∆G°_MeCN, Scheme 3, Table S1). It is important to note that regardless of the structure of the isomer, the hydride detachment only occurs from the terminal BH₃ groups, due to their increased hydricity.

The B_nH_3n series represents neutral boranes formed by consecutive BH₃ aggregation (Scheme 4), which causes a gradual decrease of HDA^MeCN at each step (from 108.4 to 55.1 kcal/mol). Borane BH₃ has high HDA^MeCN value (HDA^MeCN = 108.4 kcal/mol) and cannot be isolated due to its high Lewis acidity; however, it can be stabilized by dimerization into diborane B₂H₆ molecule (HDA^MeCN = 82.1 kcal/mol) or by interaction with Lewis bases (Scheme 5). The Lewis acid–base complexes of BH₃ are characterized by reduced HDA^MeCN and some of them, especially amine and phosphine boranes, are able to form σ-complexes with transition metals [72,73,74,75,76,77].

The diborane-based structures B₂H₅(µ-H)[(BH₂)(µ-H)]_mBH₃ (m = 1–3) were found to be the most stable isomers for higher borane clusters (Scheme 4, Table S2). In previous theoretical works, cyclic structures of B₃H₉ and B₄H₁₂ have been reported to be the most stable isomers in the gas phase [78,79,80]. However, according to our optimization, [M06/ and MP2/6-311++G(d,p) theory levels in MeCN using SMD] is the most stable configuration for B₃H₉ is B₂H₅(µ-H)BH₃, whereas butterfly-like and cyclic structures are slightly less favorable in terms of Gibbs free energy scale (∆G°_MeCN = 0.5 kcal/mol and 2.3 kcal/mol for M06 and 0.7 kcal/mol and 1.3 kcal/mol for MP2, respectively, Table S3).

The Li₂[B_nH_3n] series is formed by the consecutive addition of BH₃ to Li₂[BH₃] (Scheme S3). Both [BH₃]²⁻ [81] and [B₂H₆]²⁻ [82,83,84,85,86,87,88], which could be obtained by the reduction of B₂H₆ [89], are used as ligands in transition metal complexes. Boranes of this series formally could be obtained by two-electron reduction from their neutral analogues B_nH_3n, leading to a significant decrease in their thermodynamic hydricity by 24–65 kcal/mol. High reactivity of [B₂H₆]²⁻ towards hydride transfer (HDA^MeCN = 40.9 kcal/mol for Li₂[B₂H₆]) results in full substitution of terminal BH hydrides in (Cp*M)₂(κ²-B₂H₆) (M = V, Nb, Ta) in chlorinated solvents (CH₂Cl₂ and CHCl₃) [86,88]. Higher boranes were not isolated, which is apparently related to their low HDA^MeCN (Table 1); however, according to our DFT calculations, the structures of [B₃H₉]²⁻, [B₄H₁₂]²⁻ and [B₅H₁₅]²⁻ are based on [B₂H₆]²⁻ geometry (Scheme S3, Table S4).

The Li[B_nH_3n−1] series is formed via the consecutive addition of BH₃ to LiBH₂ (Scheme S4). Along this series, HDA^MeCN decreases from 64.4 kcal/mol for LiBH₂ to 44.5–45.1 kcal/mol for Li[B₄H₁₁] and Li[B₅H₁₄]. The most stable structure in this series is Li[B₃H₈], and this anion is used as a ligand in transition metal complexes [70,90,91,92,93,94]. In the case of Li[B₄H₁₁] and Li[B₅H₁₄], the isomers, the structures of which are based on [B₃H₈]⁻, are found by 19.2 and 24.0 kcal/mol lower in ∆G°_MeCN scale than the isomers having a cyclic structure (Scheme S4, Table S5).

According to the literature, there are several possibilities for synthesizing [B₃H₈]⁻ (Scheme 6) [52,58,95]. Formation of the octahydrotriborate anion [B₃H₈]⁻ in the reaction of BH₄⁻ with B₂H₆ or with B₂H₄ can be explained by the electrophilic nature of neutral boranes (evidenced by higher HDA^MeCN, Table 1 and Scheme S1) [58,95].

The B_nH_3n−2 series is formed via the consecutive addition of BH₃ to BH (Scheme S5). In this series, except for the B₅H₁₃, all compounds were obtained experimentally. The first BH₃ addition leads to an increase in HDA^MeCN by 14.5 kcal/mol, the second addition causes its reduction by 30.9 kcal/mol, whereas for the next members in the series, B₃H₇, B₄H₁₀, and B₅H₁₃, HDA^MeCN varies in a narrow range of 74.5–78.2 kcal/mol.

Boron monohydride radical is known to be generated in the gas phase by photodissociation of BH₃CO [96,97]. B₂H₄ can be obtained by the reaction of B₂H₆ with F radicals in the gas phase [98]. Due to its high electrophilicity (HDA^MeCN = 105.4 for B₂H₄ is comparable to HDA^MeCN = 108.4 for BH₃), there is a lack of transition metal complexes with B₂H₄ [99]; however, it can be stabilized in the form of a Lewis acid–base complex (Me₃P)₂B₂H₄ [100,101,102,103,104]. In turn, (Me₃P)₂B₂H₄ can act as a bidentate ligand in transition metal complexes [102,103,104,105]. B₃H₇ can be easily obtained from B₃H₈⁻ by reaction with a non-oxidizing acid (Scheme 6) in ether solvents (e.g., THF) or in the presence of other Lewis bases (L = R₃N, R₃P) [52,106,107] with the formation of L∙B₃H₇ adduct [108]. The most stable borane in this series, B₄H₁₀, was characterized rather a long time ago, and its structure was determined in the gas phase by gas-phase electron diffraction [109,110]. B₄H₁₀ geometry is preserved in the structure of B₅H₁₃ (Scheme S5, Table S6), which is characterized by a slightly lower HDA^MeCN value.

In the Li₂[B_nH_3n−2] series formed by the consecutive addition of BH₃ to Li₂BH (Scheme S6), the HDA^MeCN values decrease by 20–52 kcal/mol relative to the corresponding neutral boranes B₃H_3n−2 (Table 1 and Scheme 2). Both [B₂H₄]²⁻ [99,102,104,111] and [B₃H₇]²⁻ [82,112,113,114] are used as ligands in transition metal complexes. The most stable structures, [B₄H₁₂]²⁻ and [B₅H₁₅]²⁻, are based on the geometry of [B₃H₇]²⁻, whereas cyclic structures are energetically less favorable by 7.4 and 16.8 kcal/mol (∆G°_MeCN), respectively (Table S7).

The Li[B_nH_3n−3] series is formed by the addition of BH₃ to B⁻ (Scheme S7, Table S8). The change in the thermodynamic hydricity in this series is uneven. The first BH₃ addition leads to a significant drop in HDA^MeCN by 41.9 kcal/mol, the second addition causes its reduction by 13.3 kcal/mol, and the third addition leads to an increase of HDA^MeCN by 11.3 kcal/mol. The first two members of this series, [B₂H₃]⁻ [115,116] and [B₃H₆]⁻ [80], are observed in the gas phase during collision-induced dissociation [117], and their structures were predicted by DFT calculations. [B₄H₉]⁻ can be synthesized in quantitative yield by deprotonation of B₄H₁₀, whereas the addition of B₂H₆ to [B₄H₉]⁻ yields [B₅H₁₂]⁻ [51,118]. [B₄H₉]⁻ is used as a ligand in transition metal complexes [70,119,120,121,122], and it is the most stable structure in this series (HDA^MeCN = 60.2 kcal/mol).

Finally, the B_nH_3n−4 series is formed via the consecutive addition of BH₃ to B₂H₂ (Scheme S8, Table S9). Upon the BH₃ addition, HDA^MeCN gradually decreases from an extremely high value of 168.6 kcal/mol for B₂H₂ to 85.5 kcal/mol for B₅H₁₁. B₂H₂ [100,123], B₃H₅ [100] and B₄H₈ [124,125] are obtained as a result of higher borane cleavage (e.g., B₅H₉) and need to be stabilized by the interaction with Lewis bases (Me₃P, Me₃N, Me₂S) due to their high Lewis acidity. Their Lewis acid–base adducts (such as (Me₃P)₂∙B₂H₂) have used as ligands in transition metal complexes [104,126,127].

2.2. Thermodynamic Hydricity of Polyhedral Closo-Boranes

During thermal decomposition of metal tetrahydroborates, the formation of stable metal dodecaborane [B₁₂H₁₂]²⁻ with an admixture of [B₁₀H₁₀]²⁻ via intermediate boron clusters such as [B₂H₇]⁻ and [B₃H₈]⁻ was observed [42,45,46,47,50]. In octahedral [B₆H₆]²⁻ and icosahedral [B₁₂H₁₂]²⁻ closo-borane dianions, all terminal BH groups are equivalent. However, in their lithium salts Li₂[B_nH_n], cation–anion interaction causes asymmetry in the bond lengths of terminal BH groups interacting with Li atoms. Therefore, thermodynamic hydricity of polyhedral closo-boranes was assessed for both [B_nH_n]²⁻ dianions and their lithium salts Li₂[B_nH_n] (n = 5–17).

Theoretical calculations show that the thermodynamic stability of polyhedral closo-boranes increases with an increase in the number of boron atoms participating in the cluster formation from [B₅H₅]²⁻ to [B₁₂H₁₂]²⁻ [50]. According to thermodynamic stabilities, closo-borane dianions such as [B₁₇H₁₇]²⁻ and [B₁₆H₁₆]²⁻ should be even more stable than [B₁₂H₁₂]²⁻ [128]. Indeed, DFT calculations predict that the formation of large clusters (such as [B₄₂H₄₂]²⁻, [B₆₀H₆₀]²⁻, [B₉₂H₉₂]²⁻) is possible [129,130,131,132]. However, one should keep in mind that the more B–H bonds present in a polyhedral closo-borane, the higher its stability, since more energy is stored in these chemical bonds [50].

To gain insight in thermodynamic hydricity of polyhedral closo-boranes, we calculated at first stage HDA^MeCN of small dianionic boranes [B_nH_n]²⁻ and their lithium salts Li₂[B_nH_n] (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-H_term bonds are equivalent (r_BHterm = 1.191 Å for [B₂H₂]²⁻; r_BHterm = 1.209 Å for [B₃H₃]²⁻ and r_BHterm = 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 Li₂[B_nH_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_nH_n]²⁻ clusters (n = 2–4). Formal addition of BH to [B₄H₄]²⁻ yields [B₅H₅]²⁻, which is the least stable member of the polyhedral closo-boranes [B_nH_n]²⁻ according to formation enthalpy [50], and has never been synthesized [128,133]. The structure of [B₅H₅]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 B₃H₃ triangle, forming a belt of the polyhedron (for further description of structural features of polyhedral closo-boranes, see in SI).

Boron atoms in the belt and caps of the polyhedron have different coordination numbers (CN)—4 and 5, respectively. That in turn leads to a difference in B–H bonds’ lengths (r_BH^ap = 1.194 Å and r_BH^eq = 1.202 Å, Figure 3) and anisotropy in the charge distribution across the polyhedron (Figure S2). According to natural bond population analysis (NPA) of [B₅H₅]²⁻ the capping boron atoms are more negatively charged (q^MeCN = −0.437) than the equatorial ones (q^MeCN = −0.331÷ −0.332). Thus, in the case of [B₅H₅]²⁻, the apical BH groups (82.0 kcal/mol) have higher HDA^MeCN values than equatorial ones (60.0 kcal/mol) (Figure S2). This HDA^MeCN(BH^ap)/HDA^MeCN(BH^eq) ratio is conserved when going to larger dianions [B₁₀H₁₀]²⁻, [B₁₅H₁₅]²⁻ and [B₁₇H₁₇]²⁻.

To gain insight into the thermodynamic hydricity of polyhedral closo-boranes, it is highly important to consider the difference in geometry along with the charge distribution in their dianions (Figure S2, Table S10). These parameters suggest different reactivity of boron atoms forming the polyhedron’s caps and belt (Figure 3). Although different bond lengths of terminal B-H already indicate different properties of these centers, it is not possible to estimate their thermodynamic hydricity, since there is no general relationship between r_BHterm and HDA^MeCN (Figure S4).

Thermodynamic hydricity was assessed for both [B_nH_n]²⁻ dianions (Figure 4, Table S10) and their lithium salts Li₂[B_nH_n] (n = 5–17) (Figure S3, Table S11). In lithium salts, the cation–anion interaction causes asymmetry in BH bond length and leads to different HDA^MeCN values for the same vertex types, which pushes us to use HDA^MeCN for dianions to make a correct comparison inside the [B_nH_n]²⁻ series. HDA^MeCN values for lithium salts (typically higher by 10.2–22.3 kcal/mol than those for dianions) are provided for the comparison with the neutral and monoanionic hydrides (Table S11). HDA^MeCN of closo-boranes directly depends on the coordination number (Table S10) of the boron atom, for which the hydride abstraction and stabilization of quasi-borinium cation take place. In general, the larger the coordination number (CN) of a boron atom, the lower the value of HDA^MeCN. Deviation from this rule is observed only for [B₁₁H₁₁]²⁻, [B₁₃H₁₃]²⁻ and [B₁₄H₁₄]²⁻, where the boron atoms with the highest CN have the largest HDA^MeCN values. The probable explanation is that despite that the boron atoms forming the polyhedron belt have a lower CN than the boron atoms of the cape, the interaction between them and the surrounding atoms is stronger. This is suggested by shorter r_(B-B) for borons with lower CN, as in, e.g., [B₁₁H₁₁]²⁻ (1.773–1.802 Å), [B₁₃H₁₃]²⁻ (1.739–1.826 Å) and [B₁₄H₁₄]²⁻ (1.732–1.904 Å), in comparison to longer bonds at boron atoms with higher CN (r_(B-B) is 1.738–1.971 Å for [B₁₁H₁₁]²⁻, 1.842–1.904 Å for [B₁₃H₁₃]²⁻ and 1.904 Å for [B₁₄H₁₄]²⁻).

In most cases, except for [B₇H₇]²⁻ and [B₁₆H₁₆]²⁻, the equatorial BH groups have the lowest HDA^MeCN values. The trend in the ability to hydride transfer (Scheme 8, for Li₂[B_nH_n] see Scheme S10) correlates with the calculated Gibbs free energy per unit (G°/n) for both Li₂[B_nH_n] and [B_nH_n]²⁻ (n = 5–17) (Figures S5 and S6) and with previously reported energy per unit by PRDDO calculations [136]. The data obtained indicate that the most stable closo-boranes [B₁₂H₁₂]²⁻, [B₁₀H₁₀]²⁻ and [B₆H₆]²⁻ are least prone to the reaction of hydride transfer. At the same time, closo-boranes which differ from [B₆H₆]²⁻ and [B₁₂H₁₂]²⁻ by one BH group (B₅/B₇ and B₁₁/B₁₃, respectively) and [B₈H₈]²⁻ are the most reactive in hydride-transfer reactions. The high reactivity of [B₁₃H₁₃]²⁻, which HDA^MeCN (39.7 kcal/mol) is comparable to that of “superhydride” Li[Et₃BH] (HDA^MeCN = 37.3 kcal/mol) [34], which apparently explains the challenge of jumping over the so-called “icosahedral barrier” in the synthesis of higher closo-borane clusters. The higher propensity of some closo-boranes for hydride transfer could be used for the directed generation of quasi-borinium cations, which are postulated as intermediates of the electrophile-induced nucleophilic substitution [36,137].

Previously, electron-donating properties of polyhedral boranes were assessed by ¹H NMR, which revealed the electron-donating ability decrease in the row [2-B₁₀H₉]²⁻ > [B₁₂H₁₂]²⁻ > [1-B₁₀H₉]²⁻ [37]. According to our calculations, the HDA^MeCN value for [B₁₂H₁₂]²⁻ (79.6 kcal/mol) is higher than HDA^MeCN for equatorial BH groups in [B₁₀H₁₀]²⁻ (75.6 kcal/mol), but lower than for apical BH terminal groups in [B₁₀H₁₀]²⁻ (89.0 kcal/mol).

3. Materials and Methods Computational Details

In the present manuscript, DFT/M06 was used to allow a comparison to hydride donating ability of tetracoordinated boron hydrides previously calculated by the same method [34]. Additionally, the values obtained can be used as a reference for assessing the effectiveness of the activation of B-H bonds by transition metals. In this regard, M06 is a more versatile method than M06-2X (generally recommended for calculation thermochemistry of main group elements) and can be used in the cases where multi-reference systems are or might be involved since it has been parametrized for both main group elements and transition metals [138].

All calculations were performed without symmetry constraints using the M06 hybrid functional [138] and MP2 implemented in the Gaussian09 (Revision D.01) (Wallingford, CT, USA) [139] software package, using the 6-311++G(d,p) basis set [140].

Vibrational frequencies were calculated for all optimized complexes at the same level of theory to confirm a character of local minima on the potential energy surface. Visualization of the optimized geometries was realized using the Chemcraft 1.8 graphical visualization program [141].

The inclusion of nonspecific solvent effects in the calculations was performed by using the SMD method [142]. Acetonitrile (MeCN, ε = 35.7) was chosen as a solvent for the geometry optimization because a large amount of data on reduction potentials, pKa values, and experimental hydride donating ability (HDA) of transition metal hydride complexes were determined in MeCN [31,143].

The calculations were carried out with an ultrafine integration grid and a very tight SCF option to improve the accuracy of the optimization procedure and thermochemical calculations. Hydride donating ability in MeCN (HDA^MeCN) was calculated as Gibbs free energy of hydride transfer [HDA^MeCN ≡ ΔG°[H]⁻_Solv = G°_Solv (E⁺) + G°_Solv (H⁻) − G°_Solv (E–H)].

From the data obtained during the geometry optimization, for each molecule, the most stable configuration of each of its conformers was chosen. To find the most stable configuration of cationic boranes, the terminal hydrogen atoms (B–H_term) were torn off from each vertex in optimized molecules. As is widely known, bridge hydrogen atoms (B–H_br–B) have an increased acidity [5,58,144,145,146,147]; therefore, their participation in the hydride transfer was not considered herein. For most of the small borane clusters, due to the structural rearrangements during the geometry optimization only one stable configuration of cationic boranes was found.

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.

4. Conclusions Our DFT study of thermodynamic hydricity (HDA^MeCN) revealed that for small borane clusters (up to 5 boron atoms), the hydride detachment occurs only from the ending terminal BH_n (n = 1–3) group having the largest number of hydrogen atoms. The experimental data and the HDA^MeCN pattern obtained suggest that stable borane clusters have HDA^MeCN between 48 and 82 kcal/mol. Hydrides with lower HDA^MeCN would be hydrolytically unstable, and those with higher HDA^MeCN tend to aggregation in larger clusters. Neutral boranes with high Lewis acidity such as B₂H₂ (168.6 kcal/mol), B₃H₅ (109.9 kcal/mol), BH₃ (108.4 kcal/mol), B₂H₄ (105.4 kcal/mol) and B₄H₈ (89.2 kcal/mol) could be stabilized in the form a Lewis acid–base complex (L∙B_xH_y, where L = R₃N, R₃P, THF, etc.), the formation of which increases the BH group’s hydricity (lowers HDA^MeCN). These HDA^MeCN border lines are exemplified by diborane B₂H₆ (HDA^MeCN = 82.1 kcal/mol) and its dianion [B₂H₆]²⁻ (HDA^MeCN = 40.9 kcal/mol for Li₂[B₂H₆]). Borane clusters with HDA^MeCN less than 41 kcal/mol are strong hydride donors capable of reducing CO₂ (HDA^MeCN = 44 kcal/mol for HCO₂⁻), whereas those with HDA^MeCN over 82 kcal/mol, predominately neutral boranes, are weak hydride donors and could even serve as proton donors (e.g., B₂H₆, B₄H₁₀, B₅H₁₁ etc.).

In closo-boranes, HDA^MeCN depends on the coordination number (CN) of the boron atom from which hydride detachment and stabilization of quasi-borinium cation takes place. Thus, HDA^MeCN for equatorial boron vertices (75.6 kcal/mol, CN = 6) in [B₁₀H₁₀]²⁻ is lower than those for apical boron vertices (89.0 kcal/mol, CN = 5). That could explain the observed experimental reactivity row [2-B₁₀H₉]²⁻ > [B₁₂H₁₂]²⁻ > [1-B₁₀H₉]²⁻ [37].

Supplementary Materials

The following are available online. Scheme S1. Scheme of homologous series of neutral and monoanionic boranes clusters. Scheme S2. Homologous series of neutral and dianionic boranes clusters, Figure S1. Plots of HDAMeCN against the number of boron atoms in borane clusters derived from different starting species. Table S1. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for Li[BnH3n+1] series. Table S2. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for BnH3n series. Table S3. Difference in Gibbs energy (∆G°MeCN in kcal/mol) relative to the most stable isomer of B3H9 in MeCN (∆G°MeCN in kcal/mol), B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol). Scheme S3. Li2[BnH3n] series. Table S4. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for Li2[BnH3n] series; Scheme S4. Li[BnH3n-1] series. Table S5. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for Li[BnH3n-1] series. Scheme S5. BnH3n-2 series. Table S6. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for BnH3n-2 series. Scheme S6. Li2[BnH3n-] series. Table S7. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for Li2[BnH3n-2] series. Scheme S7. Li[BnH3n-3] series. Table S8. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for Li[BnH3n-3] series. Scheme S8. BnH3n-4 series. Table S9. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol) for BnH3n-4 series. Structural features of polyhedral closo-boranes. Figure S2. NPA charge distribution (showed in blue-green-red scale from -0.40 to 0.05), calculated for M06-optimized geometries of dianions [BnHn]2- (n = 5-17) of polyhedral closo-boranes in MeCN. Table S10. Coordination numbers (CN) of boron atom in polyhedral closo-boranes. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔH°[H]-MeCN in kcal/mol). Table S11. Computed [M06/6-311++G(d,p)] B-H terminal bond length (in Å), hydride donating ability (HDAMeCN alias ΔG°[H]-MeCN in kcal/mol) and enthalpy of hydride detaching reaction (ΔG°[H]-MeCN in kcal/mol). Figure S3. HDAMeCN of polyhedral closo-boranes Li2[BnHn] (n = 5-17). Scheme S9. General trend of HDAMeCN for Li2[BnHn] (n = 2-4). Scheme S10. General trend of HDAMeCN for polyhedral closo-boranes Li2[BnHn] (n = 5-17). Figure S4. Plot HDAMeCN vs bond length of terminal B-H bond for Li salts polyhedral closo-boranes. Figure S5. Free Gibbs energy per BH unit calculated for dianions [BnHn]2- (n = 5-17) of polyhedral closo-boranes and their Li-salts Li2[BnHn] in (n = 5-17) MeCN vs number boron atoms. Figure S6. Graph of normalized lowest HDAMeCN of and free Gibbs energy per BH unit for dianions [BnHn]2- (n = 5-17) of polyhedral closo-boranes vs number boron atoms. Table S12. DFT-optimized geometries (Cartesian coordinates) and electronic energies.

Author Contributions

Funding acquisition, I.E.G.; Investigation, I.E.G.; Resources, O.A.F.; Supervision, L.M.E. and E.S.S.; Visualization, V.A.K.; Writing-original draft, I.E.G.; Writing-review & editing, O.A.F. and N.V.B. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Russian Science Foundation (grant number 19-73-00309).

Conflicts of Interest

The authors declare no conflict of interest.

References

1. Greenwood, N.N. Electron-Deficient Boranes as Novel Electron-Donor Ligands. In ACS Symposium Series; American Chemical Society (ACS): Washington, WA, USA, 1994; Volume 565, pp. 333-345.

2. Greenwood, N.N. The concept of boranes as ligands. Co-ord. Chem. Rev. 2002, 226, 61-69.

3. Alexandrova, A.N.; Boldyrev, A.I.; Zhai, H.-J.; Wang, L.-S. All-boron aromatic clusters as potential new inorganic ligands and building blocks in chemistry. Co-ord. Chem. Rev. 2006, 250, 2811-2866.

4. Besora, M.; Lledós, A. Coordination Modes and Hydride Exchange Dynamics in Transition Metal Tetrahydroborate Complexe. In Contemporary Metal Boron Chemistry I: Borylenes, Boryls, Borane Sigma-complexes, and Borohydrides; Springer: Berlin/Heidelberg, Germany, 2008; Volume 130, pp. 149-202.

5. Hansen, B.R.S.; Paskevicius, M.; Li, H.-W.; Akiba, E.; Jensen, T.R. Metal boranes: Progress and applications. Coord. Chem. Rev. 2016, 323, 60-70.

6. Mohtadi, R.; Remhof, A.; Jena, P. Complex metal borohydrides: Multifunctional materials for energy storage and conversion. J. Phys. Condens. Matter 2016, 28, 353001.

7. Roll, M.F. Ionic borohydride clusters for the next generation of boron thin-films: Nano-building blocks for electrochemical and refractory materials. J. Mater. Res. 2016, 31, 2736-2748.

8. Huang, Z.; Wang, S.; Dewhurst, R.D.; Ignat'Ev, N.V.; Finze, M.; Braunschweig, H. Boron: Its Role in Energy-Related Processes and Applications. Angew. Chem. Int. Ed. 2020, 59, 8800-8816.

9. Lu, Z.; Ciucci, F. Metal Borohydrides as Electrolytes for Solid-State Li, Na, Mg, and Ca Batteries: A First-Principles Study. Chem. Mater. 2017, 29, 9308-9319.

10. Guzik, M.N.; Mohtadi, R.; Sartori, S. Lightweight complex metal hydrides for Li-, Na-, and Mg-based batteries. J. Mater. Res. 2019, 34, 877-904.

11. Huang, S.; Qi, X.; Zhang, W.; Liu, T.; Zhang, Q. Exploring Sustainable Rocket Fuels: [Imidazolyl-Amine-BH2]+-Cation-Based Ionic Liquids as Replacements for Toxic Hydrazine Derivatives. Chem. Asian J. 2015, 10, 2725-2732.

12. Huang, S.; Qi, X.; Liu, T.; Wang, K.; Zhang, W.; Li, J.; Zhang, Q. Towards Safer Rocket Fuels: Hypergolic Imidazolylidene-Borane Compounds as Replacements for Hydrazine Derivatives. Chem. Eur. J. 2016, 22, 10187-10193.

13. Liu, T.; Qi, X.; Huang, S.; Jiang, L.; Li, J.; Tang, C.; Zhang, Q. Exploiting hydrophobic borohydride-rich ionic liquids as faster-igniting rocket fuels. Chem. Commun. 2016, 52, 2031-2034.

14. Ley, M.B.; Jepsen, L.H.; Lee, Y.-S.; Cho, Y.W.; Von Colbe, J.B.; Dornheim, M.; Rokni, M.M.; Jensen, J.O.; Sloth, M.; Filinchuk, Y.; et al. Complex hydrides for hydrogen storage-new perspectives. Mater. Today 2014, 17, 122-128.

15. Mohtadi, R.; Orimo, S.-I. The renaissance of hydrides as energy materials. Nat. Rev. Mater. 2016, 2, 16091.

16. Callini, E.; Atakli, Z.; Özlem, K.; Hauback, B.C.; Orimo, S.-I.; Jensen, C.; Dornheim, M.; Grant, D.M.; Cho, Y.W.; Chen, P.; et al. Complex and liquid hydrides for energy storage. Appl. Phys. A 2016, 122, 353.

17. Fisher, S.P.; Tomich, A.W.; Lovera, S.O.; Kleinsasser, J.F.; Guo, J.; Asay, M.J.; Nelson, H.M.; Lavallo, V. Nonclassical Applications of closo-Carborane Anions: From Main Group Chemistry and Catalysis to Energy Storage. Chem. Rev. 2019, 119, 8262-8290.

18. Hawthorne, M.F. New horizons for therapy based on the boron neutron capture reaction. Mol. Med. Today 1998, 4, 174-181.

19. Barth, R.F.; Coderre, J.A.; Vicente, M.G.H.; Blue, T.E. Boron Neutron Capture Therapy of Cancer: Current Status and Future Prospects. Clin. Cancer Res. 2005, 11, 3987-4002.

20. Issa, F.; Kassiou, M.; Rendina, L. Boron in Drug Discovery: Carboranes as Unique Pharmacophores in Biologically Active Compounds. Chem. Rev. 2011, 111, 5701-5722.

21. Scholz, M.S.; Hey-Hawkins, E. Carbaboranes as Pharmacophores: Properties, Synthesis, and Application Strategies. Chem. Rev. 2011, 111, 7035-7062.

22. Detlef, G. Boron clusters in medicinal chemistry: Perspectives and problems. Pure Appl. Chem. 2015, 87, 173-179.

23. Leśnikowski, Z. Challenges and Opportunities for the Application of Boron Clusters in Drug Design. J. Med. Chem. 2016, 59, 7738-7758.

24. Li, X.; Yan, H.; Zhao, Q. Carboranes as a Tool to Tune Phosphorescence. Chem. Eur. J. 2016, 47, 1888-1898.

25. Mukherjee, S.; Thilagar, P. Boron clusters in luminescent materials. Chem. Commun. 2016, 52, 1070-1093.

26. Nunez, R.; Tarrés, M.; Ferrer-Ugalde, A.; De Biani, F.F.; Teixidor, F. Electrochemistry and Photoluminescence of Icosahedral Carboranes, Boranes, Metallacarboranes, and Their Derivatives. Chem. Rev. 2016, 116, 14307-14378.

27. Kaszyński, P.; Douglass, A.G. Organic derivatives of closo-boranes: A new class of liquid crystal materials. J. Organomet. Chem. 1999, 581, 28-38.

28. Ringstrand, B.; Kaszynski, P. Functionalization of the [closo-1-CB9H10]- Anion for the Construction of New Classes of Liquid Crystals. Acc. Chem. Res. 2013, 46, 214-225.

29. Han, Y.-F.; Jin, G.-X. Half-Sandwich Iridium- and Rhodium-based Organometallic Architectures: Rational Design, Synthesis, Characterization, and Applications. Acc. Chem. Res. 2014, 47, 3571-3579.

30. Housecroft, C.E. Carboranes as guests, counterions and linkers in coordination polymers and networks. J. Organomet. Chem. 2015, 798, 218-228.

31. Wiedner, E.S.; Chambers, M.B.; Pitman, C.L.; Bullock, R.M.; Miller, A.J.M.; Appel, A.M. Thermodynamic Hydricity of Transition Metal Hydrides. Chem. Rev. 2016, 116, 8655-8692.

32. Heiden, Z.M.; Lathem, A.P. Establishing the Hydride Donor Abilities of Main Group Hydrides. Organometallics 2015, 34, 1818-1827.

33. Alherz, A.; Lim, C.-H.; Hynes, J.T.; Musgrave, C.B. Predicting Hydride Donor Strength via Quantum Chemical Calculations of Hydride Transfer Activation Free Energy. J. Phys. Chem. B 2018, 122, 1278-1288.

34. Golub, I.E.; Filippov, O.A.; Belkova, N.V.; Epstein, L.M.; Shubina, E.S. Hydride donating abilities of the tetracoordinated boron hydrides. J. Organomet. Chem. 2018, 865, 247-256.

35. Lathem, A.P.; Treich, N.R.; Heiden, Z.M. Establishing the Steric Bulk of Main Group Hydrides in Reduction Reactions. Isr. J. Chem. 2015, 55, 226-234.

36. Sivaev, I.B.; Bregadze, V.I. Lewis acidity of boron compounds. Coord. Chem. Rev. 2014, 270, 75-88.

37. Sivaev, I.B.; Prikaznov, A.V.; Anufriev, S.A. On relative electronic effects of polyhedral boron hydrides. J. Organomet. Chem. 2013, 747, 254-256.

38. Greenwood, N.N.; Greatrex, R. Kinetics and mechanism of the thermolysis and photolysis of binary boranes. Pure Appl. Chem. 1987, 59, 857-868.

39. Hwang, S.-J.; Bowman, R.; Reiter, J.W.; Soloveichik, G.L.; Zhao, J.-C.; Kabbour, H.; Ahn, C.C. Rijssenbeek NMR Confirmation for Formation of [B12H12]2- Complexes during Hydrogen Desorption from Metal Borohydrides. J. Phys. Chem. C 2008, 112, 3164-3169.

40. Ozolins, V.; Majzoub, E.H.; Wolverton, C. First-Principles Prediction of Thermodynamically Reversible Hydrogen Storage Reactions in the Li-Mg-Ca-B-H System. J. Am. Chem. Soc. 2009, 131, 230-237.

41. Zhang, Y.; Majzoub, E.; Ozolins, V.; Wolverton, C. Theoretical prediction of different decomposition paths for Ca(BH4)2 and Mg(BH4)2. Phys. Rev. B 2010, 82, 174107.

42. Friedrichs, O.; Remhof, A.; Hwang, S.-J.; Züttel, A. Role of Li2B12H for the Formation and Decomposition of LiBH4. Chem. Mater. 2010, 22, 3265-3268.

43. Zhang, Y.; Majzoub, E.; Ozolins, V.; Wolverton, C. Theoretical Prediction of Metastable Intermediates in the Decomposition of Mg(BH4)2. J. Phys. Chem. C 2012, 116, 10522-10528.

44. Pitt, M.; Paskevicius, M.; Brown, D.H.; Sheppard, D.A.; Buckley, C.E. Thermal Stability of Li2B12H12 and its Role in the Decomposition of LiBH4. J. Am. Chem. Soc. 2013, 135, 6930-6941.

45. Yan, Y.; Remhof, A.; Rentsch, D.; Lee, Y.-S.; Whan Cho, Y.; Zuttel, A. Is Y2(B12H12)3 the main intermediate in the decomposition process of Y(BH4)3? Chem. Commun. 2013, 49, 5234-5236.

46. Yan, Y.; Remhof, A.; Rentsch, D.; Züttel, A. The role of MgB12H12 in the hydrogen desorption process of Mg(BH4)2. Chem. Commun. 2015, 51, 700-702.

47. Schouwink, P.; Sadikin, Y.; van Beek, W.; Černý, R. Experimental observation of polymerization from BH4- to B12H122- in mixed-anion A3BH4B12H12 (A = Rb+, Cs+). Int. J. Hydrogen Energy 2015, 40, 10902-10907.

48. Huang, Z.-Q.; Chen, W.-C.; Chuang, F.-C.; Majzoub, E.H.; Ozolins, V. First-principles calculated decomposition pathways for LiBH4 nanoclusters. Sci. Rep. 2016, 6, 26056.

49. McKee, M.L. Deconvoluting the Reaction Path from B10H14 Plus BH4- to B12H122-. Can Theory Make a Contribution? In Boron: The Fifth Element; Hnyk, D., McKee, M., Eds.; Springer International Publishing: Cham, Switzerland, 2015; pp. 121-138.

50. Sethio, D.; Daku, L.M.L.; Hagemann, H.; Kraka, E. Quantitative Assessment of B-B-B, B-Hb-B, and B-Ht Bonds: From BH3 to B12H122-. ChemPhysChem 2019, 20, 1967-1977.

51. Remmel, R.J.; Johnson, H.D.; Jaworiwsky, I.S.; Shore, S.G. Preparation and nuclear magnetic resonance studies of the stereochemically nonrigid anions nonahydrotetraborate(1-)dodecahydropentaborate(1-), undecahydrohexaborate(1-), and dodecahydroheptaborate(1-). Improved syntheses of pentaborane(11) and hexaborane(12). J. Am. Chem. Soc. 1975, 97, 5395-5403.

52. Liu, X.-R.; Chen, X.-M.; Zhang, J.; Jensen, T.R.; Chen, X. The interconversion between THF·B3H7 and B3H8-: An efficient synthetic method for MB3H8 (M = Li and Na). Dalton Trans. 2019, 48, 5140-5143.

53. Fernández, E.; Whiting, A. Synthesis and Application of Organoboron Compounds; Springer International Publishing: Cham, Switzerland, 2015; Volume 49.

54. Leach, J.B.; Toft, M.A.; Himpsl, F.L.; Shore, S.G. New, systematic, good yield syntheses of boron hydrides: Preparation of tetraborane(10) and pentaborane(11). A practical conversion of pentaborane(9) to decaborane(14). J. Am. Chem. Soc. 1981, 103, 988-989.

55. Toft, M.A.; Leach, J.B.; Himpsl, F.L.; Shore, S.G. New systematic syntheses of boron hydrides via hydride ion abstraction reactions: Preparation of B2H6, B4H10, B5H11, and B10H14. Inorg. Chem. 1982, 21, 1952-1957.

56. Nguyen, M.T.; Matus, M.H.; Dixon, D.A. Heats of Formation of Boron Hydride Anions and Dianions and Their Ammonium Salts [BnHmy-][NH4+]y with y = 1-2. Inorg. Chem. 2007, 46, 7561-7570.

57. Ilic, S.; Alherz, A.; Musgrave, C.B.; Glusac, K.D. Thermodynamic and kinetic hydricities of metal-free hydrides. Chem. Soc. Rev. 2018, 47, 2809-2836.

58. Chen, X.-M.; Ma, N.; Zhang, Q.-F.; Wang, J.; Feng, X.; Wei, C.; Wang, L.-S.; Zhang, J.; Chen, X. Elucidation of the Formation Mechanisms of the Octahydrotriborate Anion (B3H8-) through the Nucleophilicity of the B-H Bond. J. Am. Chem. Soc. 2018, 140, 6718-6726.

59. Shore, S.G. Studies of the smaller boron hydrides and their derivatives. Pure Appl. Chem. 1977, 49, 717-732.

60. Marks, T.J.; Kolb, J.R. Covalent transition metal, lanthanide, and actinide tetrahydroborate complexes. Chem. Rev. 1977, 77, 263-293.

61. Makhaev, V.D. Structural and dynamic properties of tetrahydroborate complexes. Russ. Chem. Rev. 2000, 69, 727-746.

62. Visseaux, M.; Bonnet, F. Borohydride complexes of rare earths, and their applications in various organic transformations. Co-ord. Chem. Rev. 2011, 255, 374-420.

63. Golub, I.E.; Filippov, O.A.; Gutsul, E.I.; Belkova, N.V.; Epstein, L.M.; Rossin, A.; Peruzzini, M.; Shubina, E.S. Dimerization Mechanism of Bis(triphenylphosphine)copper(I) Tetrahydroborate: Proton Transfer via a Dihydrogen Bond. Inorg. Chem. 2012, 51, 6486-6497.

64. Belkova, N.V.; Bakhmutova-Albert, E.V.; Gutsul, E.I.; Bakhmutov, V.I.; Golub, I.E.; Filippov, O.A.; Epstein, L.M.; Peruzzini, M.; Rossin, A.; Zanobini, F.; et al. Dihydrogen Bonding in Complex (PP3)RuH(η1-BH4) Featuring Two Proton-Accepting Hydride Sites: Experimental and Theoretical Studies. Inorg. Chem. 2013, 53, 1080-1090.

65. Golub, I.E.; Filippov, O.A.; Belkova, N.V.; Epstein, L.M.; Rossin, A.; Peruzzini, M.; Shubina, E.S. Two pathways of proton transfer reaction to (triphos)Cu(η1-BH4) via a dihydrogen bond [triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane]. Dalton Trans. 2016, 45, 9127-9135.

66. Belkova, N.V.; Golub, I.E.; Gutsul, E.I.; Lyssenko, K.A.; Peregudov, A.S.; Makhaev, V.D.; Filippov, O.A.; Epstein, L.M.; Rossin, A.; Peruzzini, M.; et al. Binuclear Copper(I) Borohydride Complex Containing Bridging Bis(diphenylphosphino) Methane Ligands: Polymorphic Structures of [(µ2-dppm)2Cu2(η2-BH4)2] Dichloromethane Solvate. Crystals 2017, 7, 318.

67. Golub, I.E.; Filippov, O.A.; Belkova, N.V.; Gutsul, E.I.; Epstein, L.M.; Rossin, A.; Peruzzini, M.; Shubina, E.S. Competition between the Hydride Ligands of Two Types in Proton Transfer to [{κ3-P-CH3C(CH2CH2PPh2)3}RuH(η2-BH4)]. Eur. J. Inorg. Chem. 2017, 2017, 4673-4682.

68. Safronov, S.V.; Gutsul, E.I.; Golub, I.E.; Dolgushin, F.M.; Nelubina, Y.V.; Filippov, O.A.; Epstein, L.M.; Peregudov, A.S.; Belkova, N.V.; Shubina, E.S.; et al. Synthesis, structural properties and reactivity of ruthenocene-based pincer Pd(II) tetrahydroborate. Dalton Trans. 2019, 48, 12720-12729.

69. Grebenik, P.D.; Green, M.L.H.; Kelland, M.A.; Leach, J.B.; Mountford, P.; Stringer, G.; Walker, N.M.; Wong, L.-L. Formation of three-vertex metallaboranes from monoborane precursors: X-ray crystal structures of the molybdenum and ruthenium complexes [Mo(η-C5H5)2H(η2-B2H5)] and [Ru(η-C5Me5)(PMe3)(η2-B2H7)]. J. Chem. Soc. Chem. Commun. 1988, 12, 799.

70. Green, M.L.H.; Leach, J.B.; Kelland, M.A. Synthesis and Interconversion of Some Small Ruthenaboranes: Reaction of a Ruthenium Borohydride with Pentaborane(9) to Form Larger Ruthenaboranes. Organometallics 2007, 26, 4031-4037.

71. Srivastava, A.K.; Misra, N. Designing new electrolytic salts for Lithium Ion Batteries using superhalogen anions. Polyhedron 2016, 117, 422-426.

72. Alcaraz, G.; Sabo-Etienne, S. Coordination and Dehydrogenation of Amine-Boranes at Metal Centers. Angew. Chem. Int. Ed. 2010, 49, 7170-7179.

73. Titov, A.A.; Guseva, E.A.; Smol'Yakov, A.F.; Dolgushin, F.M.; Filippov, O.A.; Golub, I.E.; Krylova, A.I.; Babakhina, G.M.; Epstein, L.M.; Shubina, E.S. Complexation of trimeric copper(i) and silver(i) 3,5-bis(trifluoromethyl)pyrazolates with amine-borane. Russ. Chem. Bull. 2013, 62, 1829-1834.

74. Johnson, H.C.; Hooper, T.N.; Weller, A.S. The Catalytic Dehydrocoupling of Amine-Boranes and Phosphine-Boranes. In Topics in Organometallic Chemistry; Springer International Publishing: Cham, Switzerland, 2015; Volume 49, pp. 153-220.

75. Rossin, A.; Peruzzini, M. Ammonia-Borane and Amine-Borane Dehydrogenation Mediated by Complex Metal Hydrides. Chem. Rev. 2016, 116, 8848-8872.

76. Todisco, S.; Luconi, L.; Giambastiani, G.; Rossin, A.; Peruzzini, M.; Golub, I.E.; Filippov, O.A.; Belkova, N.V.; Shubina, E.S. Ammonia Borane Dehydrogenation Catalyzed by (κ4-EP3)Co(H) [EP3 = E(CH2CH2PPh2)3; E = N, P] and H2 Evolution from Their Interaction with NH Acids. Inorg. Chem. 2017, 56, 4296-4307.

77. Colebatch, A.L.; Weller, A.S. Amine-Borane Dehydropolymerization: Challenges and Opportunities. Chem. Eur. J. 2018, 25, 1379-1390.

78. McKee, M.L.; Lipscomb, W.N. Symmetry lowering in boranes B3H9 and B4H12. Inorg. Chem. 1985, 24, 2317-2319.

79. Duke, B.; Gauld, J.W.; Schaefer, H.F. Ab Initio Characterization of a Triborane(9) Isomer with a Pentacoordinated Central Boron Atom. J. Am. Chem. Soc. 1995, 117, 7753-7755.

80. Korkin, A.A.; Schleyer, P.V.R.; McKee, M.L. Theoretical ab Initio Study of Neutral and Charged B3Hn (n = 3-9) Species. Importance of Aromaticity in Determining the Structural Preferences. Inorg. Chem. 1995, 34, 961-977.

81. Parshall, G.W. Borane Complexes of Transition Metals. J. Am. Chem. Soc. 1964, 86, 361-364.

82. Aldridge, S.; Shang, M.; Fehlner, T.P. Synthesis of Novel Molybdaboranes from (η5-C5R5)MoCln Precursors (R = H, Me; n = 1, 2, 4). J. Am. Chem. Soc. 1998, 120, 2586-2598.

83. Weller, A.S.; Shang, M.; Fehlner, T.P. Synthesis of Mono- and Ditungstaboranes from Reaction of Cp*WCl4and [Cp*WCl2]2 with BH3·thf or LiBH4(Cp* = η5-C5Me5). Control of Reaction Pathway by Choice of Monoboron Reagent and Oxidation State of Metal Center. Organometallics 1999, 18, 53-64.

84. Geetharani, K.; Tussupbayev, S.; Borowka, J.; Holthausen, M.C.; Ghosh, S. A Mechanistic Study of the Utilization of arachno-Diruthenaborane [(Cp*RuCO)2B2H6] as an Active Alkyne-Cyclotrimerization Catalyst. Chem. Eur. J. 2012, 18, 8482-8489.

85. Sharmila, D.; Mondal, B.; Ramalakshmi, R.; Kundu, S.; Varghese, B.; Ghosh, S. First-Row Transition-Metal-Diborane and -Borylene Complexes. Chem. Eur. J. 2015, 21, 5074-5083.

86. Prakash, R.; Bakthavachalam, K.; Varghese, B.; Ghosh, S. Chlorination of the terminal hydrogen atoms in the hydrogen-rich group 5 dimetallaboranes (Cp*M)2(B2H6)2 (M = Nb, Ta). J. Organomet. Chem. 2017, 846, 372-378.

87. Prakash, R.; Pradhan, A.N.; Jash, M.; Kahlal, S.; Cordier, M.; Roisnel, T.; Halet, J.-F.; Ghosh, S. Diborane(6) and Its Analogues Stabilized by Mono-, Bi-, and Trinuclear Group 7 Templates: Combined Experimental and Theoretical Studies. Inorg. Chem. 2020, 59, 1917-1927.

88. Haridas, A.; Kar, S.; Raghavendra, B.; Roisnel, T.; Dorcet, V.; Ghosh, S. B-H Functionalization of Hydrogen-Rich [(Cp*V)2(B2H6)2]: Synthesis and Structures of [(Cp*V)2(B2X2)2H8] (X = Cl, SePh; Cp* = η5-C5Me5). Organometallics 2019, 39, 58-65.

89. Godfroid, R.A.; Hill, T.G.; Onak, T.P.; Shore, S.G. Formation of [BH3]2- and [B2H6]2- From the Homogeneous Reduction of B2H6. J. Am. Chem. Soc. 1994, 116, 12107-12108.

90. Lippard, S.J.; Melmed, K.M. Transition metal borohydride complexes. III. Structure of octahydrotriboratobis(triphenylphosphine)copper(I). Inorg. Chem. 1969, 8, 2755-2762.

91. Ghosh, S.; Beatty, A.M.; Fehlner, T.P. The Reaction of Cp*ReH6, Cp* = C5Me5, with Monoborane to Yield a Novel Rhenaborane. Synthesis and Characterization of arachno-Cp*ReH3B3H8. Collect. Czechoslov. Chem. Commun. 2002, 67, 808-812.

92. Kim, D.Y.; You, Y.; Girolami, G.S. Synthesis and crystal structures of two (cyclopentadienyl)titanium(III) hydroborate complexes, [Cp∗TiCl(BH4)]2 and Cp2Ti(B3H8). J. Organomet. Chem. 2008, 693, 981-986.

93. Ramalakshmi, R.; Bhattacharyya, M.; Rao, C.E.; Ghosh, S. Synthesis, structure and chemistry of low-boron containing molybdaborane: Arachno -[Cp*Mo(CO)2B3H8 ]. J. Organomet. Chem. 2015, 792, 31-36.

94. Joseph, B.; Saha, K.; Prakash, R.; Nandi, C.; Roisnel, T.; Ghosh, S. Chalcogenolato-bridged dinuclear half sandwich complexes of ruthenium and iridium. Inorganica Chim. Acta 2018, 483, 106-110.

95. Titov, L.V.; Levicheva, M.D.; Psikha, S.B. Synthesis and Thermal Decomposition of Magnesium, Calcium, and Strontium Octahydrotriborates Solvated with Diglyme. Russ. J. Inorg. Chem. 1984, 29, 386-389.

96. Rice, J.K.; Caldwell, N.J.; Nelson, H.H. Gas-phase reaction kinetics of boron monohydride. J. Phys. Chem. 1989, 93, 3600-3605.

97. Garland, N.L.; Stanton, C.T.; Fleming, J.W.; Baronavski, A.P.; Nelson, H.H. Boron monohydride reaction kinetics studied with a high-temperature reactor. J. Phys. Chem. 1990, 94, 4952-4956.

98. Ruscic, B.; Schwarz, M.; Berkowitz, J. Molecular structure and thermal stability of B2H4 and B2H+4 species. J. Chem. Phys. 1989, 91, 4576-4581.

99. Reddy, K.H.K.; Jemmis, E.D. Stabilization of diborane(4) by transition metal fragments and a novel metal to π Dewar-Chatt-Duncanson model of back donation. Dalton Trans. 2013, 42, 10633.

100. Kameda, M.; Kodama, G. Cleavage reaction of pentaborane(9). Formation of a new hypho triborane adduct. Inorg. Chem. 1980, 19, 2288-2292.

101. Snow, S.A.; Shimoi, M.; Ostler, C.D.; Thompson, B.K.; Kodama, G.; Parry, R.W. Metal complexes of the neutral borane adduct (B2H4∙2P(CH3)3). Inorg. Chem. 1984, 23, 511-512.

102. Katoh, K.; Shimoi, M.; Ogino, H. Syntheses and structures of [M(CO)5{B2H4∙2P(CH3)3}] and [M(CO)4{B2H4∙2P(CH3)3}] (M = chromium, molybdenum, tungsten). Inorg. Chem. 1992, 31, 670-675.

103. Parry, R.; Kodama, G. Coordination compounds formed using three-center hydrogen bridge bonds: An extension of the Lewis donor-acceptor coordinate bond. Co-ord. Chem. Rev. 1993, 128, 245-260.

104. Hata, M.; Kawano, Y.; Shimoi, M. Synthesis and Structure of a Dichromatetraborane Derivative [{(OC)4Cr}2(η4-H,H',H'',H'''-BH2BH2·PMe2CH2PMe2)]. Inorg. Chem. 1998, 37, 4482-4483.

105. Shimoi, M.; Katoh, K.; Tobita, H.; Ogino, H. Syntheses and properties of bis{bis(trimethylphosphine)tetrahydrodiboron}copper(1+) halide (halide = chloride, iodide) and x-ray crystal structure of the iodide. Inorg. Chem. 1990, 29, 814-817.

106. Ring, M.A.; Witucki, E.F.; Greenough, R.C. Tautomerism exchange in B3H7.N(CH3)3 and B3H7.-THF. Inorg. Chem. 1967, 6, 395-396.

107. Dewkett, W.; Beall, H.; Bushweller, C. Quadrupolar relaxation and intramolecular exchange in (C6H5CH2)2NCH3(B3H7). Inorg. Nucl. Chem. Lett. 1971, 7, 633-636.

108. Driess, M.; Nöth, H. Molecular Clusters of the Main Group Elements; Wiley-VCH: Weinheim, Germany, 2008.

109. Nordman, C.E.; Lipscomb, W.N. The molecular structure of B4H10. J. Am. Chem. Soc. 1953, 75, 4116-4117.

110. Brain, P.T.; Morrison, C.A.; Parsons, S.; Rankin, D.W.H. Tetraborane(10), B4H10: Structures in gaseous and crystalline phases. J. Chem. Soc. Dalton Trans. 1996, 24, 4589.

111. Kaufmann, E.; Schleyer, P.v.R. Dilithiodiborane(6) (Li2B2H4). An experimentally viable species with a B=B double bond. Planar no-bond-double-bond isomers with pentacoordinate boron? Inorg. Chem. 1988, 27, 3987-3992.

112. Muetterties, E.L.; Kane, A.R. Metalloboranes. VI. A B3H72- derivative of platinum. A possible π-allyl analog based on boron. J. Am. Chem. Soc. 1971, 93, 1041-1042.

113. Guggenberger, L.J.; Kane, A.R.; Muetterties, E.L. Metalloboranes. VII. Synthesis and chemistry of π-borallyl complexes and the crystal structure of [(CH3)2PC6H5]2PtB3H7. J. Am. Chem. Soc. 1972, 94, 5665-5673.

114. DiPasquale, A.; Lei, X.; Fehlner, T.P. Metallaborane Reactivity. Complexities of Cobalt Carbonyl Fragment Addition to 1,2-{Cp*RuH}2B3H7, Cp* = η5-C5Me5, and Characterization of a Diruthenium Analogue of Pentaborane(11) 1,2-{Cp*Ru}2(CO)2B3H7. Organometallics 2001, 20, 5044-5049.

115. Qu, Z.-W.; Li, Z.-S.; Ding, Y.-H.; Sun, C.-C. Theoretical Study of the Gas-Phase Reaction of Diborane(3) Anion B2H3-with CO2. J. Phys. Chem. A 2000, 104, 11952-11960.

116. Guermoune, A.; Jarid, A.; Ouassas, A.; Chafiq, S.; Es-Sofi, A. DFT and MP2 investigation of the B2H3- anion potential energy surface. Chem. Phys. Lett. 2004, 399, 190-195.

117. Krempp, M.; Damrauer, R.; DePuy, C.H.; Keheyan, Y. Gas-Phase Ion Chemistry of Boron Hydride Anions. J. Am. Chem. Soc. 1994, 116, 3629-3630.

118. Shore, S.G.; Johnson, H.D. Deprotonation of tetraborane(10) by ammonia. The temperature-dependent boron-11 nuclear magnetic resonance spectrum of B4H9-. J. Am. Chem. Soc. 1970, 92, 7586-7587.

119. Shore, S.G.; Parry, R.W. The crystalline compound ammonia-borane,1 H3NBH3. J. Am. Chem. Soc. 1955, 77, 6084-6085.

120. Boocock, S.K.; Toft, M.A.; Inkrott, K.E.; Hsu, L.Y.; Huffman, J.C.; Folting, K.; Shore, S.G. Irida-, rhoda-, nickela-, and cuprapentaboranes derived from the arachno-[B4H9]- ion. Crystal structure of [Ir(η4-B4H9)(CO){P(CH3)2C6H5}2], an analog of arachno-B5H11 and [Ir(η4-C4H6)(CO){P(CH3)2C6H5}2]+. Inorg. Chem. 1984, 23, 3084-3091.

121. Kawano, Y.; Matsumoto, H.; Shimoi, M. Syntheses of Diruthenaborane Clusters, ({Cp*Ru(µ-H)}2B3H7), ({(Cp*Ru)2(µ-H)}B4H9), and ((Cp*Ru)2(µ-H)(PMe3)(µ-η4-B2H5)). Chem. Lett. 1999, 28, 489-490.

122. Lenczyk, C.; Roy, D.K.; Oberdorf, K.; Nitsch, J.; Dewhurst, R.D.; Radacki, K.; Halet, J.-F.; Marder, T.B.; Bickelhaupt, F.M.; Braunschweig, H. Toward Transition-Metal-Templated Construction of Arylated B4 Chains by Dihydroborane Dehydrocoupling. Chem. Eur. J. 2019, 25, 16544-16549.

123. Hertz, R.K.; Denniston, M.L.; Shore, S.G. Preparation and characterization of B2H4∙2P(CH3)3. Inorg. Chem. 1978, 17, 2673-2674.

124. Dodds, A.R.; Kodama, G. Isolation and characterization of trimethylamine-tetraborane(8). Inorg. Chem. 1979, 18, 1465-1470.

125. Kameda, M.; Shimoi, M.; Kodama, G. Tetraborane(8) adducts of strongly basic phosphines. Inorg. Chem. 1984, 23, 3705-3709.

126. Snow, S.A.; Kodama, G. Novel coordination of a neutral borane adduct to nickel(0). Formation of Ni(CO)2[B2H4∙2P(CH3)3]. Inorg. Chem. 1985, 24, 795-796.

127. Shimoi, M.; Katoh, K.; Kawano, Y.; Kodama, G.; Ogino, H. Fluxional behavior of chromium and tungsten complexes of monodentate bis(trimethylphosphine)diborane(4), [M(CO)5(η1-B2H4·2PMe3)] (M = Cr, W):. J. Organomet. Chem. 2002, 659, 102-106.

128. Schleyer, P.V.R.; Najafian, K.; Mebel, A.M. The Large closo-Borane Dianions, BnHn2- (n = 13-17) Are Aromatic, Why Are They Unknown? Inorg. Chem. 1998, 37, 6765-6772.

129. Derecskei-Kovacs, A.; Dunlap, B.I.; Lipscomb, W.N.; Lowrey, A.; Marynick, D.S.; Massa, L. Quantum Chemical Studies of Boron Fullerene Analogs. Inorg. Chem. 1994, 33, 5617-5619.

130. Wang, Z.-X.; Schleyer, P.V.R. A "Sea Urchin" Family of Boranes and Carboranes: The 6m + 2n Electron Rule. J. Am. Chem. Soc. 2003, 125, 10484-10485.

131. Balakrishnarajan, M.M.; Hoffmann, R.; Pancharatna, P.D.; Jemmis, E.D. Magic Electron Counts and Bonding in Tubular Boranes. Inorg. Chem. 2003, 42, 4650-4659.

132. Pancharatna, P.; Marutheeswaran, S.; Austeria, M.P.; Balakrishnarajan, M.M. Deltahedra with holes: Structural preferences of supraicosahedral boranes. Polyhedron 2013, 63, 55-59.

133. Shen, Y.-F.; Xu, C.; Cheng, L. Deciphering chemical bonding in BnHn2-(n = 2-17): Flexible multicenter bonding. RSC Adv. 2017, 7, 36755-36764.

134. Wade, K. The structural significance of the number of skeletal bonding electron-pairs in carboranes, the higher boranes and borane anions, and various transition-metal carbonyl cluster compounds. J. Chem. Soc. D 1971, 792-793.

135. Mark, A.F.; Ken, W. Evolving patterns in boron cluster chemistry. Pure Appl. Chem. 2003, 75, 1315-1323.

136. Bicerano, J.; Marynick, D.S.; Lipscomb, W.N. Large closo boron hydrides. Inorg. Chem. 1978, 17, 2041-2042.

137. Bregadze, V.I.; Timofeev, S.V.; Sivaev, I.B.; Lobanova, I.A. Substitution reactions at boron atoms in metallacarboranes. Russ. Chem. Rev. 2004, 73, 433-453.

138. Zhao, Y.; Truhlar, D.G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: Two new functionals and systematic testing of four M06 functionals and 12 other functionals. Theor. Chem. Accounts 2008, 119, 525.

139. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, USA, 2009.

140. Krishnan, R.; Binkley, J.S.; Seeger, R.; Pople, J.A. Self-consistent molecular orbital methods. XX. A basis set for correlated wave functions. J. Chem. Phys. 1980, 72, 650-654.

141. Adrienko, G.A. Chemcraft, Version 1.8 (build 574b). ChemCraft-Graphical Program for Visualisation of Quantum Chemistry Computations. Available online: http://www.chemcraftprog.com (accessed on 24 June 2020).

142. Marenich, A.V.; Cramer, C.J.; Truhlar, D.G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113, 6378-6396.

143. Waldie, K.M.; Ostericher, A.L.; Reineke, M.H.; Sasayama, A.F.; Kubiak, C.P. Hydricity of Transition-Metal Hydrides: Thermodynamic Considerations for CO2 Reduction. ACS Catal. 2018, 8, 1313-1324.

144. Pitzer, K.S. Electron Deficient Molecules. I. The Principles of Hydroboron Structures. J. Am. Chem. Soc. 1945, 67, 1126-1132.

145. Goebbert, D.J.; Hernandez, H.; Francisco, J.S.; Wenthold, P.G. The Binding Energy and Bonding in Dialane. J. Am. Chem. Soc. 2005, 127, 11684-11689.

146. Kühl, O. The coordination chemistry of the proton. Chem. Soc. Rev. 2011, 40, 1235-1246.

147. Kaese, M.S.T.; Budy, B.S.H.; Bolte, M.; Lerner, H.-W.; Wagner, M. Deprotonation of a Seemingly Hydridic Diborane(6) To Build a B-B Bond. Angew. Chem. Int. Ed. 2017, 56, 7546-7550.

148. King, R.B. Three-dimensional aromaticity in polyhedral boranes and related molecules. Chem. Rev. 2001, 101, 1119-1152.