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1. Hydrogen‐Bonding as a Factor to Determine the Regioselectivity for Pd‐mediated C−H Activation of Pyridine

   https://doi.org/10.1002/cctc.202001658  

   Haines, Brandon_E; Musaev, Djamaladdin_G (December 2020, ChemCatChem)

   Abstract Direct and regioselective functionalization of pyridine is a topic of high scientific and technological importance. In spite of extensive efforts, the regioselective functionalization of pyridine still remains a significant challenge due to their low reactivity and presence of Lewis‐basic sp²nitrogen. Here, we studied the effect of hydrogen bonding interactions on the regiochemical outcome of Pd‐mediated C−H activation of pyridine by utilizing DFT calculations. We demonstrated that hydrogen bonding can act as a second independent factor to override the inherent regioselectivity of pyridine. This novel approach complements previously reported strategies, such as: (a) coordination of pyridine to transition metal center via its N‐center, (b) installation of directing group (DG) and then coordination of pyridine to the transition metal center via this DG (i. e. chelation assistant strategy), (c) protection of its nitrogen lone pair with N‐oxide or N‐imino groups or with Lewis acids, (d) the inherent positional reactivity of C−H bonds based on the electronic or steric properties of the substituents, and (e) by the identity of the oxidant used. We have also demonstrated that the oxidation state of the Pd catalyst has impact on the regiochemical outcome of the C−H activation step in pyridine. The implications of our study for regioselective C−H functionalization catalyst design of heteroarenes are twofold: It demonstrates (1) hydrogen bonding as a viable design principle, and (2) Pd(IV) as a catalyst for C−H functionalization. 

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2. Inverse Electron Demand Diels–Alder Reaction on M ₃ N@C ₈₀ (M=Lu, Sc): Reactivity and Reversibility Enable Chemical Separation of Metallofullerenes

   https://doi.org/10.1002/anie.202424776  

   Sun, Yue; Abella, Laura; Emge, Thomas_J; Zhu, Sheng; Li, Yanbang; Ferraro, Ian; Li, Anyin; Stevenson, Steven; Poblet, Josep_M; Rodríguez‐Fortea, Antonio; et al (February 2025, Angewandte Chemie International Edition)

   Abstract Endohedral metallofullerenes are chemically more inert compared to empty fullerenes, primarily due to their intramolecular electron transfer. In this work, we report an inverse electron demand Diels–Alder (IEDDA) reaction on M₃N@C₈₀(M=Lu, Sc), where they show significantly higher reactivity than empty fullerenes. The molecular structures of the [4+2] cycloadducts were unambiguously characterized. Moreover, the cycloadducts can fully revert to pristine M₃N@C₈₀via retro‐cycloaddition upon thermal treatment. With the unusual reactivity and reversibility, the IEDDA reaction enables an effective separation approach for metallofullerenes from their soot extracts, opening path to efficient and economical scale‐up synthesis of metallofullerenes in laboratory and industrial settings. 

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3. Synthesis and supramolecular organization of the iodide and triiodides of a polycyclic adamantane-based diammonium cation: the effects of hydrogen bonds and weak I⋯I interactions

   https://doi.org/10.1039/D0CE01730B  

   Mezentsev-Cherkes, Ivan A.; Shestimerova, Tatiana A.; Medved'ko, Aleksei V.; Kalinin, Mikhail A.; Kuznetsov, Alexey N.; Wei, Zheng; Dikarev, Evgeny V.; Vatsadze, Sergey Z.; Shevelkov, Andrei V. (March 2021, CrystEngComm)

   A careful selection of organic and inorganic components enables the production of unusual structure types with promising practical properties by facile syntheses. In this paper, we describe novel supramolecular architectures comprising organic adamantane-like divalent building blocks and iodide or polyiodide anions. Highly acidic conditions facilitated the formation of a doubly protonated organic ligand out of 5,7-dimethyl-1,3-diazaadamantane that generates three different crystal structures with inorganic counterions. In these structures, cationic substructures are constructed by transforming neutral organic ligands into [(C 10 N 2 H 20 )I] + or [(C 10 N 2 H 20 )(H 2 O)] 2+ cations, which crystallize with charge-compensating iodine-based anions of different complexities. All three crystal structures are characterized by various noncovalent forces, ranging from strong (N)H⋯I, (O)H⋯I, and (N)H⋯O hydrogen bonds to secondary and weak I⋯I interactions. Raman and diffuse reflectance spectroscopy as well as DFT calculations were employed to describe the electronic structures and optical properties of new supramolecular architectures, with particular attention to the role of non-covalent interactions. 

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4. Pattern of covalent and non‐covalent interactions within the pentaiodide anion in the structure of (3‐HOC ₅ H ₉ NH ₂ )I ₅

   https://doi.org/10.1002/zaac.202200039  

   Shestimerova, Tatiana A.; Bykov, Andrey V.; Kuznetsov, Alexey N.; Grishko, Alexey Y.; Wei, Zheng; Dikarev, Evgeny V.; Shevelkov, Andrei V. (March 2022, Zeitschrift für anorganische und allgemeine Chemie)

   Abstract 3‐Hydroxypiperidinium pentaiodide was synthesized by a facile reaction in concentrated aqueous HI. Its crystal structure comprises 3‐hydroxypiperidinium cations and pentaiodide anions, the latter having geometry ofcis‐shaped chains composed of I₂and I₃⁻building units. The analysis of interatomic distances, Raman spectroscopy data, and results of DFT calculations, including non‐covalent interaction analysis, showed that the title compound exhibits a complex pattern of covalent and non‐covalent interactions. Those include I−I covalent bonds and I⋅⋅⋅I halogen bonds within the I₅⁻anion as well as (N)H⋅⋅⋅I and (O)H⋅⋅⋅I hydrogen bonds and even weaker (C)H⋅⋅⋅I van‐der‐Waals interactions between the cations and anions. 

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5. Structural insights into supramolecular interactions in isostructural salts of 2,4,6-triaminopyrimidinium with various heterocyclic carboxylates

   https://doi.org/10.1107/S2053229624008787  

   Mohana, Marimuthu; Gomathi, Sundaramoorthy; Thomas_Muthiah, Packianathan; Butcher, Ray J (November 2024, Acta Crystallographica Section C Structural Chemistry)

   2,4,6-Triaminopyrimidine is an interesting and challenging molecule due to the presence of multiple hydrogen-bond donors and acceptors. Its noncovalent interactions with a variety of carboxylic acids provide several supramolecular aggregates with frequently occurring molecular synthons. The present work focuses on the supramolecular interactions of 2,4,6-triaminopyrimidinium 3-(indol-3-yl)propionate–3-(indol-3-yl)propionic acid (1/1), C₄H₈N₅⁺·C₁₁H₁₀NO₂⁻·C₁₁H₁₁NO₂, (I), 2,4,6-triaminopyrimidinium 2-(indol-3-yl)acetate, C₄H₈N₅⁺·C₁₀H₈NO₂⁻, (II), 2,4,6-triaminopyrimidinium 5-bromothiophene-2-carboxylate, C₄H₈N₅⁺·C₅H₂BrO₂S⁻, (III), and 2,4,6-triaminopyrimidinium 5-chlorothiophene-2-carboxylate, C₄H₈N₅⁺·C₅H₂ClO₂S⁻, (IV). All four salts exhibit robust homomeric and heteromericR₂²(8) ring motifs. Salts (I) and (II) develop sextuple [in (I)] and quadruple [in (I) and (II)] hydrogen-bonded arrays through fused-ring motifs. Salt (II) exhibits a rosette-like architecture. Salt (IV) is isostructural and isomorphous with salt (III), exhibiting an identical crystal structure with a different composition and an identical supramolecular architecture. In salts (III) and (IV), a linear hetero-tetrameric motif is formed and, in addition, both salts exhibit halogen–π interactions which enhance the crystal stability. All four salts develop a supramolecular hydrogen-bonded pattern facilitated by several N—H...O and N—H...N hydrogen bonds with multiple furcated donors and acceptors. 

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