Chalcogen Bonding in Co-Crystals: Activation through 1,4-Perfluorophenylene vs. 4,4'-Perfluorobiphenylene Cores

Selective Activation of Chalcogen Bonding: An Efficient Structuring Tool toward Crystal Engineering Strategies

ConspectusAmong the noncovalent interactions available in the toolbox of crystal engineering, chalcogen bonding (ChB) has recently entered the growing family of σ-hole interactions, following the strong developments based on the halogen bonding (XB) interaction over the last 30 years. The monovalent character of halogens provides halogen bonding directionality and strength. Combined with the extensive organic chemistry of Br and I derivatives, it has led to many applications of XB, in solution (organo-catalysis, anion recognition and transport), in the solid state (cocrystals, conducting materials, fluorescent materials, topochemical reactions, ...), in soft matter (liquid crystals, gels,···), and in biochemistry. The recognition of the presence of two σ-holes on divalent chalcogens and the ability to activate them, as in XB, with electron-withdrawing groups (EWG) has fueled more recent interest in chalcogen bonding. However, despite being identified for many years, ChB still struggles to make a mark due to (i) the underdeveloped synthetic chemistry of heavier Se and Te; (ii) the limited stability of organic chalcogenides, especially tellurides; and (iii) the poor predictability of ChB associated with the presence of two σ-holes. It therefore invites a great deal of attention of molecular chemists to design and develop selected ChB donors, for the scrutiny of fundamentals of ChB and their successful use in different applications. This Account aims to summarize our own contributions in this direction that extend from fundamental studies focused on addressing the lack of directionality/predictability in ChB to a systematic demonstration of its potential, specifically in crystal engineering, and particularly toward anionic networks on the one hand, topochemical reactions on the other hand.In this Account, we share our recent results aimed at recovering with ChB the same degree of strength and predictability found with XB, by focusing on divalent Se and Te systems with two different substituents, one of them with an EWG, to strongly unbalance both σ-holes. For that purpose, we explored this dissymmetrization concept within three chemical families, selenocyanates R-SeCN, alkynyl derivatives R-C≡C-(Se/Te)Me, and o-carborane derivatives. Such compounds were systematically engaged in cocrystals with either halides or neutral bipyridines as ChB acceptors, revealing their strong potential to chelate halides as well as their ability to organize reactive molecules such as alkenes and butadiynes toward [2+2] cycloadditions and polydiacetylene formation, respectively. This selective activation concept is not limited to ChB but can be effectively used on all other σ-hole interactions (pnictogen bond, tetrel bond, etc.) where one needs to control the directionality of the interaction.

77Se and 125Te solid-state NMR and X-ray diffraction structural study of chalcogen-bonded 3,4-dicyano-1,2,5-chalcogenodiazole cocrystals

Three novel chalcogen-bonded cocrystals featuring 3,4-dicyano-1,2,5-selenodiazole (C₄N₄Se) or 3,4-dicyano-1,2,5-tellurodiazole (C₄N₄Te) as chalcogen-bond donors and hydroquinone (C₆H₆O₂), tetraphenylphosphonium chloride (C₂₄H₂₀P⁺·Cl^-) or tetraethylphosphonium chloride (C₈H₂₀P⁺·Cl^-) as chalcogen-bond acceptors have been prepared and characterized by single-crystal X-ray diffraction (XRD), powder X-ray diffraction and ⁷⁷Se/¹²⁵Te magic-angle spinning solid-state NMR spectroscopy. The single-crystal XRD results show that the chalcogenodiazole molecules interact with the electron donors through two σ-holes on each of the chalcogen atoms, which results in highly directional and moderately strong chalcogen bonds. Powder XRD confirms that the crystalline phases are preserved upon moderate grinding of the samples for solid-state NMR experiments. Measurement of ⁷⁷Se and ¹²⁵Te chemical shift tensors via magic-angle spinning solid-state NMR spectroscopy confirms the number of magnetically unique chalcogen sites in each asymmetric unit and reveals the impact of chalcogen-bond formation on the local electronic structure. These NMR data are further assessed in the context of analogous data for a wider range of crystalline chalcogen-bonded systems.

Topochemical, Single‐Crystal‐to‐Single‐Crystal [2+2] Photocycloadditions Driven by Chalcogen‐Bonding Interactions

The face‐to‐face association of (E)‐1,2‐di(4‐pyridyl)ethylene (bpen) molecules into rectangular motifs stabilized for the first time by chalcogen bonding (ChB) interactions is shown to provide photoreactive systems leading to cyclobutane formation through single‐crystal‐to‐single‐crystal [2+2] photodimerizations. The chelating chalcogen bond donors are based on original aromatic, ortho‐substituted bis(selenocyanato)benzene derivatives 1–3, prepared from ortho‐diboronic acid bis(pinacol) ester precursors and SeO₂ and malononitrile in 75–90 % yield. The very short intramolecular Se⋅⋅⋅Se distance in 1–3 (3.22–3.24 Å), a consequence of a strong intramolecular ChB interaction, expands to 3.52–3.54 Å in the chalcogen‐bonded adducts with bpen, a distance (<4 Å) well adapted to the face‐to‐face association of the bpen molecules into the reactive position toward photochemical dimerization.

Various Sorts of Chalcogen Bonds Formed by an Aromatic System

The chalcogen Y atom in the aromatic ring of thiophene and its derivatives YC₄H₄ (Y = S, Se, Te) can engage in a number of different interactions with another such unit within the homodimer. Quantum calculations show that the two rings can be oriented perpendicular to one another in a T-shaped dimer in which the Y atom accepts electron density from the π-system of the other unit in a Y···π chalcogen bond (ChB). This geometry best takes advantage of attractions between the electrostatic potentials surrounding the two monomers. There are two other geometries in which the two Y atoms engage in a ChB with one another. However, instead of a simple interaction between a σ-hole on one Y and the lone pair of its neighbor, the interaction is better described as a pair of symmetrically equivalent Y···Y interactions, in which charge is transferred in both directions simultaneously, thereby effectively doubling the strength of the bond. These geometries differ from what might be expected based simply on the juxtaposition of the electrostatic potentials of the two monomers.

Topochemical Polymerization of a Diacetylene in a Chalcogen-Bonded (ChB) Assembly

The successful topochemical polymerization of bis(selenocyanatomethyl)butadyine 1 is achieved upon association in a 1 : 1 co-crystal with 1,2-bis(2-pyridyl)ethylene (2-bpen) through strong and linear (NC)-Se⋅⋅⋅N_Py chalcogen bonding (ChB) interactions, allowing for an appropriate parallel alignment of the diacetylene moieties toward the solid-state reaction. Co-crystal 1⋅(2-bpen) undergoes polymerization upon heating at 100 °C. The reaction progress was monitored by IR, DSC and PXRD. An enhancement of the polymer conductivity by 8 orders of magnitude is observed upon iodine doping. Strikingly, the course of polymerization is accompanied with sublimation of the ChB acceptor molecules 2-bpen, providing the polymer in a pure form with full recovery of the co-former, at variance with the usual hydrogen-bonded co-crystal strategies toward polydiacetylenes.

Participation of S and Se in hydrogen and chalcogen bonds

The heavier chalcogen atoms S, Se, and Te can each participate in a range of different noncovalent interactions. They can serve as both proton donor and acceptor in H-bonds. Each atom can also act as electron acceptor in a chalcogen bond.