Revisiting the covalent nature of halogen bonding: a polarized three-center four-electron bond

Charge-transfer Adducts vs Iodine(I) Complexes: Dual Role of Halogen Bonding in Reactions of Diiodine with N-donor Bases

The interaction of diiodine with quinuclidine (QN) and 4-dimethylaminopyridine (DMAP) in solutions with 1 : 1 molar ratio of reactants at room temperature produced (in essentially quantitative yields) pure charge-transfer QN⋅I2 adducts and iodine(I) salt [DMAP-I-DMAP]I3, respectively. In comparison, the quantitative formation of pure iodine (I) salt [QN-I-QN]I5 was observed for the room-temperature reactions of QN with a 50 % excess of I2, and the charge-transfer adducts of I2 with DMAP (and other pyridines) were formed when reactions were carried out at low temperatures. Computational analysis related the switch from the formation of charge-transfer adducts to iodine(I) complexes in these systems to the strength of the halogen bonding of diiodine to the N-donor bases. It shows that while the halogen-bonded adducts represent critical intermediates in the formation of iodine(I) complexes, exceedingly strong halogen bonding between diiodine and the base prevents any subsequent transformations. In other words, while halogen bonding usually facilitates electron and halogen transfer, the halogen-bonded complexes may serve as "black holes" hindering any follow-up processes if this intermolecular interaction is too strong.

Rational electrolyte design and electrode regulation for boosting high-capacity Zn-iodine fiber-shaped batteries with four-electron redox reactions

Aqueous Zn ion-based fiber-shaped batteries (AZFBs) with the merits of high flexibility and safety have received much attention for powering wearable electronic devices. However, the relatively low specific capacity provided by cathode materials limits their practical application. Herein, we first propose a simple strategy for fabricating high-capacity Zn-iodine fiber-shaped batteries with a high concentration electrolyte and a reduced graphene oxide fiber (GF) cathode. It was found that oxygen functional groups in the graphene sheet demonstrate strong interaction with polyiodides but hinder electron conductivity; thus, the optimal balance between the specific capacity and coulombic efficiency of the GF electrode can be a function of the surface properties at different hydrothermal temperatures. Besides, the regulated high concentration electrolyte effectively suppresses the diffusion of polyiodides, which is attributed to the constrained freedom of water. More importantly, a four-electron redox mechanism was experimentally revealed through in situ Raman spectra. As a result, this fiber-shaped battery delivers a superior high reversible capacity of 390 mA h cm-3 at 1 A cm-3, an excellent rate performance of 125.7 mA h cm-3 at a high current density of 8 A cm-3 and outstanding cycling life with 82% capacitance retention after 2500 cycles.

Synergistic Ion Sieve and Solvation Regulation by Recyclable Clay-Based Electrolyte Membrane for Stable Zn-Iodine Battery

The high dissolution of polyiodides and unstable interface at the anode/electrolyte severely restrict the practical applications of rechargeable aqueous Zn-iodine batteries. Herein, we develop a zinc ion-based montmorillonite (ZMT) electrolyte membrane for synergizing ion sieve and solvation regulation to achieve highly stable Zn-iodine batteries. The rich M-O band and special cation-selective transport channel in ZMT locally tailor the solvation sheath around Zn2+ and therefore achieve high transference number (t+ = 0.72), benefiting for uniform and reversible deposition/stripping of Zn. Meanwhile, the mechanisms for three-step polyiodide generation and shuttle-induced Zn corrosion are highlighted by in situ characterization techniques. It is confirmed that the strong chemical adsorption between O atoms in ZMT and polyiodides species is the key to effectively inhibit the shuffle effect and side reactions. Consequently, the ZMT-based Zn-iodine battery delivers a high capacity of 0.45 mAh cm-2 at 1 mA cm-2 with a much improved Coulombic efficiency of 99.5% and outstanding capacity retention of 95% after 13 500 cycles at 10 mA cm-2. Moreover, owing to its high durability and chemical inertness and structural stability, ZMT-based electrolyte membranes can be recycled and applied in double-sided pouch cells, delivering a high areal capacity of 2.4 mAh cm-2 at 1 mA cm-2.

Quasi-atomic orbital analysis of halogen bonding interactions

A quasi-atomic orbital analysis of the halogen bonded NH3⋯XF complexes (X = F, Cl, Br, and I) is performed to gain insight into the electronic properties associated with these σ-hole interactions. It is shown that significant sharing of electrons between the nitrogen lone pair of the ammonia molecule and the XF molecule occurs, resulting in a weakening of the X-F bond. In addition, the N-X bond shows increasing covalent character as the size of the halogen atom X increases. While the Mulliken outer complex NH3⋯XF appears to be overall the main species, the strength of the covalent interaction of the N-X bond becomes increasingly similar to that of the N-X bond in the [NH3X]+ cation as the size of X increases.

Halogen Bonding and/or Covalent Bond: Analogy of 3c-4e N···I···X (X = Cl, Br, I, and N) Interactions in Neutral, Cationic, and Anionic Complexes

X-ray structural measurements and computational analysis demonstrated the similarity of the geometries and electronic structures of the X-I···N (X = Cl, Br, I, and N) bonding in strong halogen-bonded (HaB) complexes and in the anionic or cationic halonium ions. In particular, I···N bond lengths in the solid-state associations formed by strong HaB donors (e.g., I2, IBr, ICl, and N-iodosuccinimide) and acceptors (e.g., quinuclidine or pyridines) were in the same range of 2.3 ± 0.1 Å as those in the halonium ions [e.g., the bis(quinuclidine)iodonium cation or the 1,1'-iodanylbis(pyrrolidine-2,5-dione) anion]. In all cases, bond lengths were much closer to those of the N-I covalent bond than to the van der Waals separations of these atoms. The strong N···I bonding in the HaB complexes led to a substantial charge transfer, lengthening and weakening of the I···X bonds, and polarization of the HaB donors. As a result, the central iodine atoms in the strong HaB complexes bear partial positive charges akin to those in the halonium ions. The energies and Mayer bond orders for both N···I and I···X bonds in such associations are also comparable to those in the halonium ions. The similarity of the bonding in such complexes and in halonium ions was further supported by the analysis of electron densities and energies at bond critical (3, -1) points in the framework of the quantum theory of atoms in molecules and by the density overlap region indicator. Overall, all these data point out the analogy of the symmetric N···I···N bonding in the halonium ions and the asymmetric X···I···N bonding in the strong HaB complexes, as well as the weakly covalent character of these 3c-4e interactions.

Supramolecular axial chirality in [N-I-N]+-type halogen bonded dimers

Axial chiral molecules are extensively used as skeletons in ligands for asymmetric catalysis and as building blocks of chiroptical materials. Designing axial chirality at the supramolecular level potentially endows a material with dynamic tunability and adaptivity. In this work, for the first time, we have reported a series of halogen-bonded dimeric complexes with axial chirality that were formed by noncovalent bonds. The [N-I-N]+-type halogen bond is highly directional and freely rotatable with good linearity and ultra-high bond energy; this bond was introduced to couple quinoline moieties with chiral substitutes. The resultant dimers were stable in solutions with thermo-resistance. Prominent steric effects from the 2' chiral pendant allowed the chirality to be transferred to aryl skeletons with induced preferred axial chirality and optical activities. Halogen-bonded complexation presented visible emissions to afford luminescent axial chiral materials, whereby circularly polarized fluorescence and phosphorescence were achieved. The [N-I-N]+-type halogen bond performed as a powerful tool to construct functional axial chiral compounds, enriching the toolbox for asymmetric synthesis and optics.

A Two-Dimensional Porphyrin Coordination Supramolecular Network Cathode for High-Performance Aqueous Dual-Ion Battery

Iodine has great potential in the energy storage, but high solubility of I₃ ^- has seriously delayed its promotion. Benefited from abundant active sites and the open channel, two-dimensional coordination supramolecular networks (2D CSNs) is considered to be a candidate for the energy storage. Herein, a 2D porphyrin-CSN cathode named Zn-TCPP for aqueous iodine dual-ion battery (DIB) shows an excellent specific capacity of 278 mAh g^-1 , and a high energy density of 340 Wh kg^-1 at 5 A g^-1 , as well as a durable cycle performance of 5000 cycles and a high Coulombic efficiency of 98 %. Molecular orbital theory, UV/VIS, Raman spectroscopy and density functional theory (DFT) calculations reveal charge-transfer interaction between the donor of porphyrin nitrogen and the acceptor of I₃ ^- , and computational fluid dynamics (CFD) simulations demonstrate the contribution of 2D layered network structure of Zn-TCPP to the penetration of I₃ ^- .

Anchoring I3- via Charge-Transfer Interaction by a Coordination Supramolecular Network Cathode for a High-Performance Aqueous Dual-Ion Battery

Iodine is considered to have broad application prospects in the field of electrochemical energy storage. However, the high solubility of I₃^- severely hampers its practical application, and the lack of research on the anchoring mechanism of I₃^- has seriously hindered the development of advanced cathode materials for iodine batteries. Herein, based on the molecular orbital theory, we studied the charge-transfer interaction between the acceptor of I₃^- with a σ* empty antibonding orbital and the donor of pyrimidine nitrogen with lone-pair electrons, which is proved by the results of UV-vis absorption spectroscopy, Raman spectroscopy, and density functional theory (DFT) calculations. The prepared dual-ion battery (DIB) exhibits a high voltage platform of 1.2 V, a remarkable discharge-specific capacity of up to 207 mAh g^-1, and an energy density of 233 Wh kg^-1 at a current density of 5 A g^-1, as well as outstanding cycle stability (operating stably for 5000 cycles) with a high Coulombic efficiency of 97%, demonstrating excellent electrochemical performance and a promising prospect in stationary energy storage.

Theoretical study on L-H+-L with identical donors: short strong hydrogen bond or not?

Short strong hydrogen bonds (SSHBs) play crucial role in many chemical processes. Recently, as the representative of SSHBs, [F-H-F]- was experimentally observed. [F-H-F]- has a symmetric structure, which can be described as a H+ acid shared by two terminal F- donors (F--H+-F-). To explore whether two identical donors are bound to result in SSHBs, we performed theoretical studies on a series of compounds (L-H+-L) with two identical electron donors (L corresponds to donors containing group 14, 15, 16 and 17 elements). The results show that identical donors do not definitely lead to SSHBs. Instead, typical hydrogen bonds also exist. We found that both electronegativity and basicity contribute to the patterns of hydrogen bonds, where more electronegative and weaker donors benefit to SSHBs. Besides, it was found that zero-point energies also respond to the hydrogen bonding systems. This systemic work is expected to provide more insights into SSHBs.

Halogenated Baicalein as a Promising Antiviral Agent toward SARS-CoV-2 Main Protease

The coronavirus disease pandemic is a constant reminder that global citizens are in imminent danger of exposure to emerging infectious diseases. Therefore, developing a technique for inhibitor discovery is essential for effective drug design. Herein, we proposed fragment molecular orbital (FMO)-based virtual screening to predict the molecular binding energy of potential severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) main protease inhibitors. The integration of quantum mechanical approaches and trajectory analysis from a microsecond molecular dynamics simulation was used to identify potential inhibitors. We identified brominated baicalein as a potent inhibitor of the SARS-CoV-2 main protease and confirmed its inhibitory activity in an in vitro assay. Brominated baicalein did not demonstrate significant toxicity in either in vitro or in vivo studies. The pair interaction energy from FMO-RIMP2/PCM and inhibitory constants based on the protease enzyme assay suggested that the brominated baicalein could be further developed into novel SARS-CoV-2 protease inhibitors.