Insights into the Topology and the Formation of a Genuine ppσ Bond: Experimental and Computed Electron Densities in Monoanionic Trichlorine [Cl3 ]. Angew Chem Int Ed Engl 2021;60:2569-2573. [PMID: 33151006 PMCID: PMC7898528 DOI: 10.1002/anie.202013727]

Reversible Cl/Cl- redox in a spinel Mn3O4 electrode

A unique prospect of using halides as charge carriers is the possibility of the halides undergoing anodic redox behaviors when serving as charge carriers for the charge-neutrality compensation of electrodes. However, the anodic conversion of halides to neutral halogen species has often been irreversible at room temperature due to the emergence of diatomic halogen gaseous products. Here, we report that chloride ions can be reversibly converted to near-neutral atomic chlorine species in the Mn3O4 electrode at room temperature in a highly concentrated chloride-based aqueous electrolyte. Notably, the Zn2+ cations inserted in the first discharge and trapped in the Mn3O4 structure create an environment to stabilize the converted chlorine atoms within the structure. Characterization results suggest that the Cl/Cl- redox is responsible for the observed large capacity, as the oxidation state of Mn barely changes upon charging. Computation results corroborate that the converted chlorine species exist as polychloride monoanions, e.g., [Cl3]- and [Cl5]-, inside the Zn2+-trapped Mn3O4, and the presence of polychloride species is confirmed experimentally. Our results point to the halogen plating inside electrode lattices as a new charge-storage mechanism.

Unraveling the Bonding Complexity of Polyhalogen Anions: High-Pressure Synthesis of Unpredicted Sodium Chlorides Na2Cl3 and Na4Cl5 and Bromide Na4Br5

The field of polyhalogen chemistry, specifically polyhalogen anions (polyhalides), is rapidly evolving. Here, we present the synthesis of three sodium halides with unpredicted chemical compositions and structures (tP10-Na₂Cl₃, hP18-Na₄Cl₅, and hP18-Na₄Br₅), a series of isostructural cubic cP8-AX₃ halides (NaCl₃, KCl₃, NaBr₃, and KBr₃), and a trigonal potassium chloride (hP24-KCl₃). The high-pressure syntheses were realized at 41-80 GPa in diamond anvil cells laser-heated at about 2000 K. Single-crystal synchrotron X-ray diffraction (XRD) provided the first accurate structural data for the symmetric trichloride Cl₃^- anion in hP24-KCl₃ and revealed the existence of two different types of infinite linear polyhalogen chains, [Cl]_∞^n- and [Br]_∞^n-, in the structures of cP8-AX₃ compounds and in hP18-Na₄Cl₅ and hP18-Na₄Br₅. In Na₄Cl₅ and Na₄Br₅, we found unusually short, likely pressure-stabilized, contacts between sodium cations. Ab initio calculations support the analysis of structures, bonding, and properties of the studied halogenides.

Structure and Bonding of Halonium Compounds

The geometrical parameters and the bonding in [D···X···D]⁺ halonium compounds, where D is a Lewis base with N as the donor atom and X is Cl, Br, or I, have been investigated through a combined structural and computational study. Cambridge Structural Database (CSD) searches have revealed linear and symmetrical [D···X···D]⁺ frameworks with neutral donors. By means of density functional theory (DFT), molecular electrostatic potential (MEP), and energy decomposition analyses (EDA) calculations, we have studied the effect of various halogen atoms (X) on the [D···X···D]⁺ framework, the effect of different nitrogen-donor groups (D) attached to an iodonium cation (X = I), and the influence of the electron density alteration on the [D···I···D]⁺ halonium bond by variation of the R substituents at the N-donor upon the symmetry, strength, and nature of the interaction. The physical origin of the interaction arises from a subtle interplay between electrostatic and orbital contributions (σ-hole bond). Interaction energies as high as 45 kcal/mol suggest that halonium bonds can be exploited for the development of novel halonium transfer agents, in asymmetric halofunctionalization or as building blocks in supramolecular chemistry.

On pyridine chloronium cations

We present the first solid-state structural evidence of mono- and bis(pyridine)chloronium cations. The latter was synthesized from a mixture of pyridine, elemental chlorine and sodium tetrafluoroborate in propionitrile at low temperatures. The mono(pyridine) chloronium cation was realized with the less reactive pentafluoropyridine, using ClF, AsF₅, and C₅F₅N in anhydrous HF. During the course of this study, we also investigated pyridine dichlorine adducts and found a surprising disproportionation reaction of chlorine that depended on the substitutional pattern of the pyridine. Electron richer dimethylpyridine (lutidine) derivatives favor full disproportionation into a positively and a negatively charged chlorine atom which forms a trichloride monoanion, while unsubstituted pyridine forms a 1 : 1 py·Cl₂ adduct.

The Rise of Trichlorides Enabling an Improved Chlorine Technology

Chlorine plays a central role for the industrial production of numerous materials with global relevance. More recently, polychlorides have been evolved from an area of academic interest to a research topic with enormous industrial potential. In this minireview, the value of trichlorides for chlorine storage and chlorination reactions are outlined. Particularly, the inexpensive ionic liquid [NEt₃ Me][Cl₃ ] shows a similar and sometimes even advantageous reactivity compared to chlorine gas, while offering a superior safety profile. Used as a chlorine storage, [NEt₃ Me][Cl₃ ] could help to overcome the current limitations of storing and transporting chlorine in larger quantities. Thus, trichlorides could become a key technique for the flexibilization of the chlorine production enabling an exploitation of renewable, yet fluctuating, electrical energy. As the loaded storage, [NEt₃ Me][Cl₃ ], is a proven chlorination reagent, it could directly be employed for downstream processes, paving the path to a more practical and safer chlorine industry.

Benzene and Borazine, so Different, yet so Similar: Insight from Experimental Charge Density Analysis

Although benzene and borazine are isoelectronic and isostructural, they have very different electronic structures, mainly due to the polar nature of the B-N bond. Herein, we present an experimental study of the charge density distribution obtained from the multipole model formalism and Hirshfeld atom refinement (HAR) based on high-resolution X-ray diffraction data of borazine B₃N₃H₆ (1) and B,B',B″-trichloroborazine (2) crystals. These data are compared to those obtained from HAR for benzene (4) and 1,3,5-trichlorobenzene (5) and further compared with values obtained from density functional theory calculations in the gas phase, where N,N',N″-trichloroborazine (3) was also included. The results confirm that, unlike benzene, borazines are only weakly aromatic with an island-like electronic delocalization within the B₃N₃ ring involving only the nitrogen atoms. Furthermore, delocalization indices and interacting quantum atom energy for bonded and non-bonded atoms were found to be highly suitable indicators capable of describing the origin of the discrepancies observed when the degree of aromaticity in 2 and 3 is evaluated using common aromaticity indices. Additionally, analysis of intermolecular interactions in the crystals brings further evidence of a weakly aromatic character of the borazines as it reveals surprising similarities between the crystal packing of borazine and benzene and also between B,B',B″-trichloroborazine and 1,3,5-trichlorobenzene.

Novel synthetic pathway for the production of phosgene

Chloride ions are efficient catalysts for the synthesis of phosgene from carbon monoxide and elemental chlorine at room temperature and atmospheric pressure. Control experiments rule out a radical mechanism and highlight the role of triethylmethylammonium trichloride, [NEt₃Me][Cl₃], as active species. In the catalytic reaction, commercially available [NEt₃Me]Cl reacts with Cl₂ to form [NEt₃Me][Cl₃], enabling the insertion of CO into an activated Cl─Cl bond with a calculated energy barrier of 56.9 to 77.6 kJ mol⁻¹. As [NEt₃Me]Cl is also a useful chlorine storage medium, it could serve as a catalyst for phosgene production and as chlorine storage in a combined industrial process.

When Electrons Step in: Polarizing Effects Explored with Triisobutylaluminum

High-resolution X-ray diffraction data of triisobutylaluminum were collected, and unexpected structural features were observed, hinting toward yet unnoticed polarization effects. To approach these, a multipole refinement using the Hansen and Coppens method, followed by a topological analysis using Bader's quantum theory of atoms in molecules, was employed. The electron localization function based on density functional theory calculations supported the experimental findings. Thereby, unobserved electron shifts within the isobutyl group become detectable. It is shown that the impact of this electron shift is dependent mainly on whether the ⁱBu substituent of the homoleptic triisobutylaluminum dimer [AlⁱBu₃]₂ (1) is connected by a directional (σ) or a multicenter (μ) bond to the metal. The effect found is assumed not only to be of paramount importance for organoaluminum compounds, widely used in synthesis and in the industrial value chain, but also to be present in organometallic chemistry in general.

Insights into the Topology and the Formation of a Genuine ppσ Bond: Experimental and Computed Electron Densities in Monoanionic Trichlorine [Cl3 ]. Angew Chem Int Ed Engl 2021;60:2569-2573. [PMID: 33151006 PMCID: PMC7898528 DOI: 10.1002/anie.202013727]

So far, several publications have discussed the bonding concepts in polyhalides on a theoretical basis. In particular, the trichlorine monoanion is of great interest because its structure should be symmetrical and show two equidistant Cl-Cl bonds. However, apart from matrix-isolation studies, only asymmetric trichlorine anions have been reported so far. Herein, the trichlorine monoanions in 2-chloroethyltrimethylammonium trichloride [NMe₃ EtCl][Cl₃ ], 1, tetramethylammonium trichloride [NMe₄ ][Cl₃ ], 2, and tetrapropylammonium trichloride [NnPr₄ ][Cl₃ ], 3, are analysed. High-resolution X-ray structures and experimental charge density analyses supported by periodic quantum-chemical calculations provide insight into the influence of the crystalline environment on the structure of these [Cl₃ ]^- anions as well as into the progress of the bond formation between a dichlorine molecule and a Cl^- anion.

Conductivity and Redox Potentials of Ionic Liquid Trihalogen Monoanions [X3 ]- , [XY2 ]- , and [BrF4 ]- (X=Cl, Br, I and Y=Cl, Br)

The ionic liquid (IL) trihalogen monoanions [N2221 ][X3 ]- and [N2221 ][XY2 ]- ([N2221 ]+ =triethylmethylammonium, X=Cl, Br, I, Y=Cl, Br) were investigated electrochemically via temperature dependent conductance and cyclic voltammetry (CV) measurements. The polyhalogen monoanions were measured both as neat salts and as double salts in 1-butyl-1-methyl-pyrrolidinium trifluoromethane-sulfonate ([BMP][OTf], [X3 ]- /[XY2 ]- 0.5 M). Lighter IL trihalogen monoanions displayed higher conductivities than their heavier homologues, with [Cl3 ]- being 1.1 and 3.7 times greater than [Br3 ]- and [I3 ]- , respectively. The addition of [BMP][OTf] reduced the conductivity significantly. Within the group of polyhalogen monoanions, the oxidation potential develops in the series [Cl3 ]- >[BrCl2 ]- >[Br3 ]- >[IBr2 ]- >[ICl2 ]- >[I3 ]- . The redox potential of the interhalogen monoanions was found to be primarily determined by the central halogen, I in [ICl2 ]- and [IBr2 ]- , and Br in [BrCl2 ]- . Additionally, tetrafluorobromate(III) ([N2221 ]+ [BrF4 ]- ) was analyzed via CV in MeCN at 0 °C, yielding a single reversible redox process ([BrF2 ]- /[BrF4 ]- ).