Reaction of the Tricyanoborate Dianion [B(CN)₃]^2– with HgCl₂

The very reactive [B­(CN)₃]^2– dianion has a strongly nucleophilic boron atom and can be used for the synthesis of tricyanoborates, which otherwise are difficult to access. Herein the reaction of this anion with HgCl₂ is reported. The main product is the anionic mercury complex [Hg­(B­(CN)₃)₂]^2–. Heteronuclear NMR spectroscopic experiments shows that the reaction proceeds via the intermediate [ClHgB­(CN)₃]^2–. Even though [Hg­(B­(CN)₃)₂]^2– is the main product, it is difficult to obtain it in pure form, because it slowly decomposes in the presence of water and air to [(NC)­HgB­(CN)₃]⁻. All three anions were fully characterized by hetereonuclear NMR spectroscopy (¹¹B, ¹³C, and ¹⁹⁹Hg). Single-crystal X-ray diffraction studies of the salts K­[ClHg­B­(CN)₃], [Ph₄P]₂[Hg­(B­(CN)₃)₂], K­[(NC)­Hg­B­(CN)₃], and [Ph₄P]­[(NC)­Hg­B­(CN)₃] revealed linear coordination environments around mercury for all anions. The Hg–B bonds range from 2.219(5) Å in [Hg­(B­(CN)₃)₂]^2– to 2.148(11) Å in [ClHgB­(CN)₃]⁻, are in accord with the sum of the covalent radii of mercury and boron, and can be described as covalent single bonds. A comparison with related complexes indicates that the [B­(CN)₃]^2– dianion is a stronger ligand than chloride, cyanide, or carbenes. [Hg­(B­(CN)₃)₂]^2– hydrolyses in solution only in the presence of oxygen. It is suggested that <i>cis</i>-[Hg­(OH)₂­(B­(CN)₃)₂]^2– is formed as a very unstable intermediate, which decomposes very fast to [(NC)­Hg­B­(CN)₃]⁻ and other products. The anion <i>cis</i>-[Hg­(OH)₂­(B­(CN)₃)₂]^2– would contain mercury in the unusual oxidation state +IV. Quantum-chemical calculations were performed to support this assumption

Improving the Solubility of Halogenated 1‑Ammonio-<i>closo</i>-dodecaborate Anions

The partly halogenated and <i>N</i>-alkylated <i>closo</i>-dodecaborates [B₁₂Cl₆H₅N­(propyl)₃]⁻ and [B₁₂Br₆H₅NR₃]⁻ (R = ethyl–pentyl) were prepared by alkylation of [B₁₂H₁₁NH₃]⁻ and subsequent halogenation with elemental chlorine or <i>N</i>-bromosuccinimide. Simple metathesis reactions yielded the [HNMe₃]⁺, [C₆mim]⁺, [NBu₄]⁺, and Na⁺ salts, which were characterized by heteronuclear NMR and IR spectroscopy as well as electrospray ionization mass spectrometry. The crystal structures of the salts [HNMe₃]­[B₁₂Br₆H₅N­(ethyl)₃]·CH₃CN, [HNMe₃]­[B₁₂Br₆H₅N­(propyl)₃], Na­[B₁₂Br₆H₅N­(butyl)₃], and [HNMe₃]­[B₁₂Cl₇H₄N­(propyl)₃]·CH₃CN were determined by single-crystal X-ray diffraction. The [C₆mim]⁺ salts are thermally stable to temperatures higher than 300 °C. The melting points are between 57 and 80 °C, which classify the [C₆mim]⁺ salts of [B₁₂Cl₆H₅N­(propyl)₃]⁻ and [B₁₂Br₆H₅NR₃]⁻ (R = propyl–pentyl) as ionic liquids. The anions are oxidized only at potentials higher than 2 V versus Fc^0/+ as determined by cyclic voltammetry. The solubility of the sodium salts in CH₂Cl₂ solution was determined by NMR spectroscopy. With the increasing length of the alkyl chain attached to the ammonio group the solubility is significantly enhanced. A solubility up to 125 mmol/L for Na­[B₁₂Br₆H₅N­(pentyl)₃] in dichloromethane was determined. In addition, the trialkylation of the perchlorinated anion [B₁₂Cl₁₁NH₃]⁻ was investigated in detail. A Hofmann elimination was observed to occur at higher temperatures, when alkyl groups with β-hydrogen atoms were introduced. Organic substituents without β-hydrogen atoms gave more stable compounds; however, trialkylation proved to be difficult presumably due to steric hindrance. The crystal structure of the byproduct [PPh₄]₂[B₁₂Cl₁₁N­(propargyl)₂] was determined

Theoretical and Synthetic Study on the Existence, Structures, and Bonding of the Halide-Bridged [B₂X₇]⁻ (X = F, Cl, Br, I) Anions

While hydrogen bridging is very common in boron chemistry, halogen bridging is rather rare. The simplest halogen-bridged boron compounds are the [B₂X₇]⁻ anions (X = F, Cl, Br, I), of which only [B₂F₇]⁻ has been reported to exist experimentally. In this paper a detailed theoretical and synthetic study on the [B₂X₇]⁻ anions is presented. The structures of [B₂X₇]⁻ anions have been calculated at the MP2/def2-TZVPP level of theory, and their local minima have been shown to be of <i>C</i>₂ symmetry in all cases. The bonding situation varies significantly between the different anions. While in [B₂F₇]⁻ the bonding is mainly governed by electrostatics, the charge is almost equally distributed over all atoms in [B₂I₇]⁻ and additional weak iodine···iodine interactions are observed. This was shown by an atoms in molecules (AIM) analysis. The thermodynamic stability of the [B₂X₇]⁻ anions was estimated in all phases (gas, solution, and solid state) based on quantum-chemical calculations and estimations of the lattice enthalpies using a volume-based approach. In the gas phase the formation of [B₂X₇]⁻ anions from [BX₄]⁻ and BX₃ is favored in accord with the high Lewis acidity of the BX₃ molecules. In solution and in the solid state only [B₂F₇]⁻ is stable against dissociation. The other three anions are borderline cases, which might be detectable under favorable conditions. However, experimental attempts to identify [B₂X₇]⁻ (X = Cl, Br, I) anions in solution by ¹¹B NMR spectroscopy and to prepare stable [PNP]­[B₂X₇] salts failed

Electronic Structure and Stability of [B₁₂X₁₂]^2– (X = F–At): A Combined Photoelectron Spectroscopic and Theoretical Study

The stability and electron loss process of numerous multiply charged anions (MCAs) have been traditionally explained in terms of the classical Coulomb interaction between spatially separated charged groups. An understanding of these processes in MCAs with not well-separated excess charges is still lacking. We report the surprising properties and physical behavior of [B₁₂X₁₂]^2–, X = F, Cl, Br, I, At, which are MCAs with not well-separated excess charges and cannot be described by the prevailing classical picture. In this series of MCAs, comprising a “boron core” surrounded by a “halogen shell”, the sign of the total charge in these two regions changes along the halogen series from X = F–At. With the aid of experimental photoelectron spectroscopy and highly correlated <i>ab initio</i> electronic structure calculations, we demonstrate that the trend in the electronic stability of these MCAs is determined by the interplay between the Coulomb (de)­stabilization originating from the “boron core” and “halogen shell” and the extension of the overlap between the orbitals from both regions. The second excess electron is <i>always</i> taken from the most <i>positively</i> charged region, viz., the “boron core” for X = F, Cl, and the surrounding “halogen shell” for X = I, At. This change in the physical behavior is attributed to the position of the highest occupied molecular orbital, which dwells in a region that is spatially separated from the one containing the excess negative charge. The unusual intrinsic electronic structure of the [B₁₂X₁₂]^2– MCAs provides the basis for a molecular level understanding of their observed unique physical and chemical properties and a new paradigm for understanding the properties of these MCAs with not well-separated charges that departs from the prevailing model used for spatially separated charges that is based on their classical Coulomb interaction