Boronium cation

Amine—Boronium Cations. The most extensively studied boronium cation is the diammoniate of diborane [23777-63-1] (16—18). 

Hydrolytic and oxidative stability of the cations increase with the base strength of the ligand. Bistrimethylamine-boronium cations are unusually stable. They have been recovered, apparently unchanged after prolonged periods of time at 100°in concentrated acids (HC1, H2SO4, andHNOs) and 10% NaOH. On the other hand bisdialkyl sulfide boronium ions are rapidly attacked by cold water. The ion [BH2(OH2)2]+ is perhaps the most unstable thermally and hydrolytically of the boronium ions reported. Evidence has been claimed for its existence in 8 MHC1 solution at 70° 95,96). 

Among the more novel examples of boronium cations are homo-morphs of norbomane 103>... 

The bidentate ligands (RR C(pz)2) react with R"2BX compounds, where X is a leaving group, to form boronium cations [(RR C(pz)2)BR"2]+ isosteric with pyrazaboles and isolable as PF6 salts.202... 

Hypercoordinate Onium-Carbonium Dications and Isoelectronic Onium-Boronium Cations... 

In general, a broad correlation was found between the calculated NMR chemical shifts of onium-carbonium dications and the calculated B NMR chemical shifts of the corresponding isoelectronic onium-boronium cation analogs (Fig. 5.16). However, the correlation line for Figure 5.16 [Eq. (5.37)] has a slope of 0.45 contrast 0.33 for Equation (5.36) ... 

Figure 5.16. Correlation between the calculated NMR chemical shifts of onium-boronium cations and the calculated NMR chemical shifts of onium-carbonium dications (259-268, Scheme 5.17).

An isomeric species (106), which formally contains a borate anion and a boronium cation center was also reported. Similar isomeric structures were discovered in the case of imidazaboles (107) and (108).195.196 Diborylated bipyridine derivatives (109 R = NMe2, 2-thienyl) were also studied. While amino substituents stabilize a monomeric structure through n bonding to boron, the thienyl derivative (109 R=Th) is believed to adopt an oligomeric... 

Chemistry of NHCs and Charged Boron Compounds While neutral boranes are Lewis acidic in and of themselves, boron cations are even more so. Boron cations come in three forms varying in the coordination number of the boron center borinium cations are two-coordinate, borenium cations three-coordinate, and boronium cations four-coordinate [7]. Borinium cations are the most reactive of the three and to date there are no examples in the literature. Borenium and boronium cations on the other hand have been produced in a number of different ways, usually via stabilization with a Lewis base, a role that NHCs can fill quite nicely. 

An amino-pendant NHC-stabilized boronium cation was prepared from the reaction of a LiBr adduct of an NHC with borane in THF, giving a bromide salt. Subsequent reaction with silver triflate and lithium borohydride gave the amino-pendant NHC adduct with two equivalents of borane, one at the carbeneic carbon and one at the pendant nitrogen (Scheme 15.9). A four-membered metallacycle intermediate with a B-H-B motif has been postulated [108]. 

Scheme 15.9 Preparation of an amino-pendant NHC-stabiiized boronium cation and its subsequent reaction with LiBH4 and AgOTf [108].

Figure 15.40 Structure of the NHC stabilized diphosphorous-boronium cation [225],...

Wang, K., Zhang, Y, Chand, D., Parrish, D.A. and Shreeve, J.M., Boronium-cation-based ionic liquids as hypergolic fluids, Chem. Eur. J. 18 (52), 16931-16937 (2012). 

A different tetracoordinate boron species, where the boron bears a positive charge is called boro-nium cation (Figure 29.4, left side). In 2010, Riither et al. described boronium-cation-based RTILs as novel electrolytes for rechargeable Lithium batteries (Figure 29.4, right side). 

FIGURE 29.4 Boronium cation (L = Lewis base, e.g., NR3, A-Alkyl-imidazole) and boronium-cation-based RTILs. (Adapted from T. Riither, et al., Chem. Mater. 2010, 22,1038.)... 

Boron anions in the form of tetravalent boron compounds can serve as anions in ILs, combined with the conventionally used cations such as iV, Af -dialkylimidazolium, A(-alkylpyridinium, tetraalkylam-monium, and tetraalkylphosphonium. The properties of the salts are very similar to those of salts with other anions. Boronium-cation-based RTILs are also promising candidates as electrolytes in... 

T. Riither, T. D. Huynh, J. Huang, A. F. HoUenkamp, E. A. Salter, A. Wierzbicki, K. Mattson, A. Lewis, J. H. Davis, Stable Cycling of Lithium Batteries Using Novel Boronium-Cation-Based Ionic Liquid Electrolytes. Chem. Mater. 2010, 22, 1038. 

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