Abstract
We report experimental 125Te magic-angle spinning solid-state nuclear magnetic resonance (MAS NMR) measurements of the tellurium chemical shift (CS) tensors in three [K(18-crown-6)]+ 3,4-dicyano-1,2,5-telluradiazole-XCN− (X = O, S, Se) salt cocrystals featuring chalcogen bonds. These data are compared to those for pure 3,4-dicyano-1,2,5-telluradiazole (1). A reduction in the span of the 125Te CS tensor is consistently noted in the salt cocrystals compared to pure 1. Isotopically 15N-labelled [K(18-crown-6)]+[1-OC15N]−, which features a chalcogen bond between Te and the cyanate nitrogen atom, is synthesized using KOC15N, and the nitrogen CS tensors are measured for both samples via 15N slow MAS NMR spectroscopy. Possible dynamic disorder of the cyanate ions in KOCN is ruled out. Two crystallographically distinct nitrogen sites are resolved for the salt cocrystal. Upon formation of [K(18-crown-6)]+[1-OC15N]−, the 15N isotropic CS and CS tensor span both decrease relative to the values for pure KOC15N, and the axial symmetry of this tensor is lost. These findings are supplemented with a series of density functional theory calculations of magnetic shielding tensors using cluster models or periodic boundary conditions. Inclusion of spin–orbit relativistic effects in the calculation of tellurium shielding tensors is particularly important in achieving agreement with experiment.
Introduction
Non-covalent bonds play important and unique roles in determining molecular structure, dynamics, and function. Such bonds may include hydrogen bonds, halogen bonds (Desiraju et al. 2013), chalcogen bonds (Aakeroy et al. 2019), tetrel bonds (Bauzá et al. 2019), pnictogen bonds (Resnati et al. 2023), and other more recently explored element-based electrostatic bonds such as regium bonds (Frontera and Bauzá 2018) or coinage metal bonds (Legon and Walker 2018), triel bonds (Grabowski 2020), spodium bonds (Bauzá et al. 2020), and matere bonds (Daolio et al. 2021), for example. In this terminology, the bond donor is named after the element participating in the bond and which accepts electrons (Cavallo et al. 2014). The bond acceptor donates electrons. The prevailing paradigm for understanding these types of non-covalent bonds often invokes the concept of a σ-hole which forms on a donor atom opposite a covalently bonded electron-withdrawing substituent (Politzer et al. 2017). The σ-hole is an area of elevated electrostatic potential, typically coinciding with decreased electron density. Because of the way in which the σ-hole is created, the resulting non-covalent bonds tend to be highly predictable, directional, and tunable. Evidence for π character in halogen bonds has also been presented (Kellett et al. 2020). σ-hole interactions are now well established as important tools in catalysis, supramolecular chemistry, materials chemistry, and in biological systems. In the solid state, σ-hole interactions also offer a wide range of unique opportunities in crystal engineering.
Chalcogen bonds and related chalcogen–chalcogen interactions are of particular interest from a fundamental perspective and in a range of applications (Werz et al. 2002; Gleiter et al. 2003; Gleiter et al. 2018; Aakeroy et al. 2019; Scilabra et al. 2019; Haberhauer and Gleiter 2020). Selenium and tellurium are the most common chalcogen bond donor elements, and many examples of chalcogen-bonded systems featuring these elements have been discussed, including their applications in catalysis, materials chemistry, and synthesis (Mahmudov et al. 2017; Vogel et al. 2018; Yan et al. 2021). To understand the structural, electronic, and crystallographic properties of chalcogen bonds, as well as the role they may play in crystal engineering applications, it is important to develop novel analytical methods for their analysis. In addition to single-crystal X-ray diffraction, a wealth of information is available from 77Se and 125Te solid-state nuclear magnetic resonance (NMR) studies of chalcogen-bonded solids and cocrystals (Collins et al. 1988; Stanford et al. 2014; Sanz Camacho et al. 2015; Sanz Camacho et al. 2016; Sanz Camacho et al. 2018; Kumar et al. 2018; Xu et al. 2020; Kumar et al. 2020a; 2020b; 2022; Nag et al. 2022).
