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1. Anion–anion and anion–neutral triel bonds

   https://doi.org/10.1039/D0CP06547A  

   Wysokiński, Rafał; Michalczyk, Mariusz; Zierkiewicz, Wiktor; Scheiner, Steve (March 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   The ability of a TrCl 4 − anion (Tr = Al, Ga, In, Tl) to engage in a triel bond with both a neutral NH 3 and CN − anion is assessed by ab initio quantum calculations in both the gas phase and in aqueous medium. Despite the absence of a positive σ or π-hole on the Lewis acid, strong triel bonds can be formed with either base. The complexation involves an internal restructuring of the tetrahedral TrCl 4 − monomer into a trigonal bipyramid shape, where the base can occupy either an axial or equatorial position. Although this rearrangement requires a substantial investment of energy, it aids the complexation by imparting a much more positive MEP to the site that is to be occupied by the base. Complexation with the neutral base is exothermic in the gas phase and even more so in water where interaction energies can exceed 30 kcal mol −1 . Despite the long-range coulombic repulsion between any pair of anions, CN − can also engage in a strong triel bond with TrCl 4 − . In the gas phase, complexation is endothermic, but dissociation of the metastable dimer is obstructed by an energy barrier. The situation is entirely different in solution, with large negative interaction energies of as much as −50 kcal mol −1 . The complexation remains an exothermic process even after the large monomer deformation energy is factored in. 

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2. Crystallographic and Theoretical Evidences of Anion⋅⋅⋅Anion Interaction

   https://doi.org/10.1002/cphc.202100132  

   Wysokiński, Rafał; Zierkiewicz, Wiktor; Michalczyk, Mariusz; Scheiner, Steve (April 2021, ChemPhysChem)

   Abstract Planar (HgCl₃)⁻anions are stacked fairly closely together in a slipped parallel arrangement within several crystal structures. Quantum chemical analysis shows evidence of strong noncovalent spodium bonds between the Hgπ‐hole of one unit and the Cl atom of an adjacent unit. Anion⋅⋅⋅anion spodium bonds work in tandem with crystal packing forces. 

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3. Triel bonds within anion ··· anion complexes

   https://doi.org/10.1039/D1CP04296C  

   Michalczyk, Mariusz; Zierkiewicz, Wiktor; Wysokiński, Rafał; Scheiner, Steve (November 2021, Physical Chemistry Chemical Physics)

   The ability of two anions to interact with one another is tested in the context of pairs of TrX 4 − homodimers, where Tr represents any of the triel atoms B, Al, Ga, In, or Tl, and X refers to a halogen substituent F, Cl, or Br. None of these pairs engage in a stable complex in the gas phase, but the situation reverses in water where the two monomers are held together by Tr⋯X triel bonds, complemented by stabilizing interactions between X atoms. Some of these bonds are quite strong, notably those involving TrF 4 − , with interaction energies surpassing 30 kcal mol −1 . Others are very much weaker, with scarcely exothermic binding energies. The highly repulsive electrostatic interactions are counteracted by large polarization energies. 

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4. The 9‐Borataphenanthrene Anion

   https://doi.org/10.1002/anie.202002125  

   Bartholome, Tyler A.; Kaur, Aishvaryadeep; Wilson, David J. D.; Dutton, Jason L.; Martin, Caleb D. (May 2020, Angewandte Chemie International Edition)

   Abstract The 9‐borataphenanthrene anion is easily accessed by deprotonation of a 9,10‐dihydro‐9‐boraphenanthrene and its diverse reactivity is investigated. Alkylation occurs at the carbon atom adjacent to boron, and room temperature hydroboration occurs across the B=C bond. The π‐manifold of the central BC₅ring coordinates to chromium in an η⁶fashion while only the B=C unit binds η²to gold, indicating versatility of the 9‐borataphenanthrene anion as a ligand. Supporting calculations rationalize the reactivity and aromaticity is corroborated by nucleus‐independent chemical shift (NICS) indices. 

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5. Orthogonal, modular anion–cation and cation–anion self-assembly using pre-programmed anion binding sites

   https://doi.org/10.1039/D2SC05121D  

   Dhara, Ayan; Fadler, Rachel E.; Chen, Yusheng; Köttner, Laura A.; Van Craen, David; Carta, Veronica; Flood, Amar H. (March 2023, Chemical Science)

   Subcomponent self-assembly relies on cation coordination whereas the roles of anions often only emerge during the assembly process. When sites for anions are instead pre-programmed, they have the potential to be used as orthogonal elements to build up structure in a predictable and modular way. We explore this idea by combining cation (M + ) and anion (X − ) binding sites together and show the orthogonal and modular build up of structure in a multi-ion assembly. Cation binding is based on a ligand (L) made by subcomponent metal-imine chemistry (M + = Cu + , Au + ) while the site for anion binding (X − = BF 4 − , ClO 4 − ) derives from the inner cavity of cyanostar (CS) macrocycles. The two sites are connected by imine condensation between a pyridyl-aldehyde and an aniline-modified cyanostar. The target assembly [LM-CS-X-CS-ML], + generates two terminal metal complexation sites (LM and ML) with one central anion-bridging site (X) defined by cyanostar dimerization. We showcase modular assembly by isolating intermediates when the primary structure-directing ions are paired with weakly coordinating counter ions. Cation-directed (Cu + ) or anion-bridged (BF 4 − ) intermediates can be isolated along either cation–anion or anion–cation pathways. Different products can also be prepared in a modular way using Au + and ClO 4 − . This is also the first use of gold( i ) in subcomponent self-assembly. Pre-programmed cation and anion binding sites combine with judicious selection of spectator ions to provide modular noncovalent syntheses of multi-component architectures. 

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