Experimental Evidence for Non-Fermi-Contact J Coupling Across Chalcogen Bonds in Ionic Salt Cocrystal Polymorphs

A pair of novel polymorphic ionic cocrystals of 3,4-dicyanotelluradiazole and tetraphenylphosphonium bromide are synthesized and are characterized by single-crystal XRD. Strong and directional non-covalent chalcogen bonds (ChB) between Te and Br are analyzed via solid-state NMR to reveal large and anisotropic J(125Te,79/81Br) coupling tensors, providing unequivocal evidence for non-Fermi contact contributions across ChBs. Along with large 79/81Br quadrupolar couplings for the Br- anions, these data provide new tools to characterize chalcogen bonds and to differentiate between ChB polymorphs.

Solid-state NMR spectra of amino acid enantiomers and their relative intensities

Under normal experimental conditions in an achiral environment, NMR spectra of enantiomers have chemical shifts and J couplings which are not differentiable. In this work, the reproducibility of spectral intensities for pairs of amino acid enantiomers, as well as factors influencing these intensities, is assessed using 13C and 15N cross-polarization magic-angle spinning (CP/MAS) NMR spectroscopy. Prompted by a recent literature debate over a possible influence of the chirality-induced spin selectivity (CISS) effect on spectral intensities obtained in CP/MAS NMR experiments carried out on enantiomers, a number of control experiments were performed with recycle delays of at least 5T1. These included the analysis of proton-decoupled Bloch decay solid-state NMR spectra as well as solution NMR spectra where the cross polarization process is absent. Bloch decay and CP/MAS NMR spectra yield the same relative intensities for pairs of enantiomers while solution NMR spectra provide relative intensities closest to unity. Differences of plus-or-minus a few percent in the D/L spectral intensity ratios observed in all solid-state NMR experiments are due to sample preparation (i.e., grinding, particle size, partial amorphization) and limitations on sample purity. As previously described in the literature, more drastic intensity differences on the order of 50% are easily created by ball milling the samples. Finally, apodization is shown to invert the apparent D/L ratio in low signal-to-noise 15N CP/MAS NMR spectra of aspartic acid enantiomers. In summary, no spectral intensity differences attributable to enantiomerism are identified.

Double-rotation (DOR) NMR spectroscopy: Progress and perspectives

Double-rotation (DOR) solid-state NMR spectroscopy is a high-resolution technique developed in the late 1980s. Although multiple-quantum magic-angle spinning (MQMAS) became the most widely used high-resolution method for half-integer spin quadrupoles after 1995, development and application of DOR NMR to a variety of chemical and materials science problems has endured. This Trend article recapitulates the development of DOR NMR, discusses various applications, and describes possible future directions. The main technical limitations specific to DOR NMR are simply related to the size of the double rotor system. The relatively large outer rotor (and thus coil) used for most applications over the past 35 years translates into relatively low rotor spinning frequencies, a low filling factor, and weak radiofrequency powers available for excitation and for proton decoupling. Ongoing developments in NMR instrumentation, including ever-shrinking MAS rotors and spherical NMR rotors, could solve many of these problems and may augur a renaissance for DOR NMR.

Non-covalent matere bonds in perrhenates probed via ultrahigh field rhenium-185/187 NMR and zero-field NQR spectroscopy

Matere bonds (MaB) to rhenium in a set of organic perrhenates are probed via185/187Re solid-state NMR in applied magnetic fields of up to 35.2 T, and via185/187Re NQR. 185/187Re quadrupolar couplings distinguish between MaB samples and control samples, and their precise values are governed by shear strain of the ReO4- anions.

Rapid Access to Encapsulated Molecular Rotors via Coordination-Driven Macrocycle Formation

Macrocycle formation that relies upon trans metal coordination of appropriately placed pyridine ligands within an arylene ethynylene construct provides rapid and reliable access to molecular rotators encapsulated within macrocyclic stators. Showing no significant close contacts to the central rotators, X-ray crystallography of AgI -coordinated macrocycles provides plausibility for unobstructed rotation or wobbling of rotators within the central cavity. Solid-state 13 C NMR of PdII -coordinated macrocycles supports the notion of unobstructed movement of simple arenes in the crystal lattice. Solution 1 H NMR studies indicate complete and immediate macrocycle formation upon the introduction of PdII to the pyridyl-based ligand at room temperature. Moreover, the formed macrocycle is stable in solution; a lack of significant changes in the 1 H NMR spectrum upon cooling to -50 °C is consistent with the absence of dynamic behavior. The synthetic route to these macrocycles is expedient and modular, providing access to rather complex constructs in four simple steps involving Sonogashira coupling and deprotection reactions.

Modulation of Rotational Dynamics in Halogen-Bonded Cocrystalline Solids

Dynamic processes are responsible for the functionality of a range of materials, biomolecules, and catalysts. We report a detailed systematic study of the modulation of methyl rotational dynamics via the direct and the indirect influence of noncovalent halogen bonds. For this purpose, a novel series of cocrystalline architectures featuring halogen bonds (XB) to tetramethylpyrazine (TMP) is designed and prepared using gas-phase, solution, and solid-state mechanochemical methods. Single-crystal X-ray diffraction reveals the capacity of molecular bromine as well as weak chloro-XB donors to act as robust directional structure-directing elements. Methyl rotational barriers (Ea) measured using variable-temperature deuterium solid-state NMR range from 3.75 ± 0.04 kJ mol-1 in 1,3,5-trichloro-2,4,6-trifluorobenzene·TMP to 7.08 ± 0.15 kJ mol-1 in 1,4-dichlorotetrafluorobenzene·TMP. Ea data for a larger series of TMP cocrystals featuring chloro-, bromo-, and iodo-XB donors are shown to be governed by a combination of steric and electronic factors. The average number of carbon-carbon close contacts to the methyl group is found to be a key steric metric capable of rationalizing the observed trends within each of the Cl, Br, and I series. Differences between each series are accounted for by considering the strength of the σ-hole on the XB donor. One possible route to modulating dynamics is therefore via designer cocrystals of variable stoichiometry, maintaining the core chemical features of interest between a given donor and acceptor while simultaneously modifying the number of carbon close contacts affecting dynamics. These principles may provide design opportunities to modulate more complex geared or cascade dynamics involving larger functional groups.

Tetrel bond in the triphenyltin(IV) chloride-cyclo-hexyl-diphenyl-phosphane oxide (1/1) cocrystal

The single-crystal X-ray diffraction structure of the title compound, [SnCl(C6H5)3]·C18H21OP, is reported. The 1:1 cocrystal features a short and directional tetrel bond between tin and oxygen. The tin-oxygen distance is 2.346 (4) Å, representing 62% of the sum of the van der Waals radii of Sn and O. The Cl-Sn⋯O angle is 174.0 (1)° and this nearly linear arrangement is consistent with a tetrel bond formed via a σ-hole opposite the tin-chlorine covalent bond. Some weak C-H⋯Cl inter-actions are noted between adjacent mol-ecules.

Tuneable tetrel bonds between tin and heavy pnictogens

The first example of a binary cocrystal, comprised of SnPh₃Cl and PPh₃, whose components are organized via short and directional tetrel bonds (TtB) between tin and phosphorus, is described. DFT elucidates, for the first time, the factors influencing the strength of TtBs involving heavy pnictogens. A CSD survey reveals that such TtBs are also present and determinative in single component molecular systems, highlighting their significant potential as tuneable structure-directing elements.

Anticooperativity and Competition in Some Cocrystals Featuring Iodine-Nitrogen Halogen Bonds

Phenomena such as anticooperativity and competition among non-covalent bond donors and acceptors are key considerations when exploring the polymorphic and stoichiomorphic landscapes of binary and higher-order cocrystalline architectures. We describe the preparation of four cocrystals of 1,3,5-trifluoro-2,4,6-triiodobenzene with N-heterocyclic compounds, namely acridine, 3-aminopyridine, 4-methylaminopyridine, and 1,2-di(4-pyridyl)ethane. The cocrystals, which are characterized by single-crystal and powder X-ray diffraction experiments, all show moderately strong and directional iodine⋅⋅⋅nitrogen halogen bonds with reduced distance parameters ranging from 0.79 to 0.92 and carbon-iodine⋅⋅⋅nitrogen bond angles ranging from 165.4(3) to 175.31(7)°. The cocrystal comprising 1,3,5-trifluoro-2,4,6-triiodobenzene and acridine provides a relatively rare example where all three halogen bond donor sites form halogen bonds with three acceptor molecules, overcoming an anticooperative effect. This effect manifests itself through the lengthening of non-halogen-bonded C-I bonds, weakening their potential to form halogen bonds. The effect is only observed once two halogen bonds have been formed to 1,3,5-trifluoro-2,4,6-triiodobenzene; one such bond does not appear to be adequate. Among the four cocrystals studied, competition between the pyridyl nitrogen atoms and the amine nitrogen atoms suggests that the former are the preferred halogen bond acceptors. Analysis by Hirshfeld fingerprint plots and ¹³ C and ¹⁹ F magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy provides additional insights into the prevalence of various short contacts in the crystal structures and into the spectral response to halogen-bond-induced cocrystallization.

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.

Solid-state multinuclear magnetic resonance and X-ray crystallographic investigation of the phosphorus...iodine halogen bond in a bis(dicyclohexylphenylphosphine)(1,6-diiodoperfluorohexane) cocrystal

Halogen bonding to phosphorus atoms remains uncommon, with relatively few examples reported in the literature. Here, the preparation and investigation of the cocrystal bis(dicyclohexylphenylphosphine)(1,6-diiodoperfluorohexane) by X-ray crystallography and solid-state multinuclear magnetic resonance spectroscopy is described. The crystal structure features two crystallographically unique C-I...P halogen bonds [d_I...P = 3.090 (5) Å, 3.264 (5) Å] and crystallographic disorder of one of the 1,6-diiodoperfluorohexane molecules. The first of these is the shortest and most linear I...P halogen bond reported to date. ¹³C, ¹⁹F, and ³¹P magic angle spinning solid-state NMR spectra are reported. A ³¹P chemical shift change of -7.0 p.p.m. in the cocrystal relative to pure dicyclohexylphenylphosphine, consistent with halogen bond formation, is noted. This work establishes iodoperfluoroalkanes as viable halogen bond donors when paired with phosphorus acceptors, and also shows that dicyclohexylphenylphosphine can act as a practical halogen bond acceptor.

To what extent do bond length and angle govern the 13C and 1H NMR response to weak CH⋯O hydrogen bonds? A case study of caffeine and theophylline cocrystals

Weak hydrogen bonds are important structure-directing elements in supramolecular chemistry and biochemistry. We consider here weak CH⋯O hydrogen bonds in a series of cocrystals of theophylline and caffeine and assess to what extent the CH⋯O distance and angle govern the observed ¹³C and ¹H isotropic chemical shifts. Gauge-including projector-augmented wave density functional theory (GIPAW DFT) calculations consistently predict a decrease in the ¹³C and ¹H magnetic shielding constants upon hydrogen bond formation on the order of 2-5 ppm (¹³C) and 1-2 ppm (¹H). These trends are reproduced using the machine-learning approach implemented in ShiftML. Experimental ¹³C and ¹H chemical shifts obtained for powdered samples using one-dimensional NMR spectroscopy as well as heteronuclear correlation (HETCOR) spectroscopy correlate well with the GIPAW DFT results. However, the experimental ¹³C NMR response only correlates moderately well with the hydrogen bond length and angle, while the experimental ¹H chemical shifts only show very weak correlations to these local structural elements. DFT computations on isolated imidazole-formaldehyde models show that the ¹³C and ¹H chemical shifts generally decrease with the C⋯O distance but show no clear dependence on the CH⋯O angle. These results demonstrate that the ¹³C and ¹H response to weak CH⋯O hydrogen bonding is influenced significantly by additional weak contacts within cocrystal heterodimeric units.

Predictability of Chalcogen-Bond-Driven Crystal Engineering: An X-ray Diffraction and Selenium-77 Solid-State NMR Investigation of Benzylic Selenocyanate Cocrystals

We describe a series of new chalcogen-bonded cocrystals featuring 1,2-bis(selenocyanatomethyl)benzene (DSN) and 1,2,4,5-tetrakis(selenocyanatomethyl)-benzene (TSN) as the donor moieties and a variety of Lewis bases such as onium halides, N-oxides, and pyridine-containing heterocycles as the acceptors. Single-crystal X-ray diffraction demonstrates that, in every case, the selenocyanates consistently interact with the acceptor molecules through strong and directional Se···X chalcogen-bonds (ChBs) (X = halides, oxygen, and nitrogen). ⁷⁷Se solid-state nuclear magnetic resonance spectroscopy was applied to measure selenium chemical shift tensor magnitudes and to explore potential correlations between these tensor elements and the local ChB geometry. In every case, the isotropic ⁷⁷Se chemical shift decreases, and the chemical shift tensor span increases upon cocrystallization of DSN with the various ChB acceptors. This work contributes to a growing body of knowledge concerning the predictability and robustness of chalcogen bonds in crystal engineering as well as the NMR response to the establishment of chalcogen bonds. In particular, among the systems studied here, highly linear chalcogen bonds are formed exclusively at the stronger σ-hole of each and every selenium atom regardless of the size, charge, or denticity of the electron donor moiety.

1,3,5-Trifluoro-2,4,6-triiodobenzene–piperazine (2/1)

The single-crystal structure of the title compound, C4H10N2·2C6F3I3, features a moderately strong halogen bond between one of the three crystallographically distinct iodine atoms and the nitrogen atom. The iodine–nitrogen distance is 2.820 (3) Å, corresponding to 80% of the sum of their van der Waals radii. The C—I...N halogen bond angle is 178.0 (1)°, consistent with the linear interaction of nitrogen via a σ-hole opposite the carbon–iodine covalent bond. The other two iodine atoms do not engage in halogen bonding. Some weak C—H...F and —H...I interactions are also observed. The complete piperazine molecule is generated by symmetry.

Stoichiomorphic halogen-bonded cocrystals: a case study of 1,4-diiodotetrafluorobenzene and 3-nitropyridine

The concept of variable stoichiometry cocrystallization is explored in halogen-bonded systems. Three novel cocrystals of 1,4-diiodotetrafluorobenzene and 3-nitropyridine with molar ratios of 1:1, 2:1, and 1:2, respectively, are prepared by slow evaporation methods. Single-crystal X-ray diffraction analysis reveals key differences between each of the nominally similar cocrystals. For instance, the 1:1 cocrystal crystallizes in the P21/n space group and features a single chemically and crystallographically unique halogen bond between iodine and the pyridyl nitrogen. The 2:1 cocrystal crystallizes in the [Formula: see text] space group and features a halogen bond between iodine and one of the nitro oxygens in addition to an iodine–nitrogen halogen bond. The 1:2 cocrystal crystallizes with a large unit cell (V = 9896 Å3) in the Cc space group and features 10 crystallographically distinct iodine-nitrogen halogen bonds. Powder X-ray diffraction experiments carried out on the 1:1 and 2:1 cocrystals confirm that gentle grinding does not alter the crystal forms. 1H → 13C and 19F → 13C cross-polarization magic angle spinning (CP/MAS) NMR experiments performed on powdered samples of the 1:1 and 2:1 cocrystals are used as spectral editing tools to select for either the halogen bond acceptor or donor, respectively. Carbon-13 chemical shifts in the cocrystals are shown to change only very subtly relative to pure solid 1,4-diiodotetrafluorobenzene, but the shift of the carbon directly bonded to iodine nevertheless increases, consistent with halogen bond formation (e.g., a shift of +1.6 ppm for the 2:1 cocrystal). This work contributes new examples to the field of variable stoichiometry cocrystal engineering with halogen bonds.

Field-stepped ultra-wideline NMR at up to 36 T: On the inequivalence between field and frequency stepping

Field-stepped NMR spectroscopy at up to 36 T using the series-connected hybrid (SCH) magnet at the U.S. National High Magnetic Field Laboratory is demonstrated for acquiring ultra-wideline powder spectra of nuclei with very large quadrupolar interactions. Historically, NMR evolved from the continuous-wave (cw) field-swept method in the early days to the pulsed Fourier-transform method in the modern era. Spectra acquired using field sweeping are generally considered to be equivalent to those acquired using the pulsed method. Here, it is shown that field-stepped wideline spectra of half-integer spin quadrupolar nuclei acquired using WURST/CPMG methods can be significantly different from those acquired with the frequency-stepped method commonly used with superconducting magnets. The inequivalence arises from magnetic field-dependent NMR interactions such as the anisotropic chemical shift and second-order quadrupolar interactions; the latter is often the main interaction leading to ultra-wideline powder patterns of half-integer spin quadrupolar nuclei. This inequivalence needs be taken into account to accurately and correctly determine the quadrupolar coupling and chemical shift parameters. A simulation protocol is developed for spectral fitting to facilitate analysis of field-stepped ultra-wideline NMR spectra acquired using powered magnets. A MATLAB program which implements this protocol is available on request.

π-Complexes of Diborynes with Main Group Atoms

We present herein an in‐depth study of complexes in which a molecule containing a boron‐boron triple bond is bound to tellurate cations. The analysis allows the description of these salts as true π complexes between the B−B triple bond and the tellurium center. These complexes thus extend the well‐known Dewar‐Chatt‐Duncanson model of bonding to compounds made up solely of p block elements. Structural, spectroscopic and computational evidence is offered to argue that a set of recently reported heterocycles consisting of phenyltellurium cations complexed to diborynes bear all the hallmarks of π‐complexes in the π‐complex/metallacycle continuum envisioned by Joseph Chatt. Described as such, these compounds are unique in representing the extreme of a metal‐free continuum with conventional unsaturated three‐membered rings (cyclopropenes, azirenes, borirenes) occupying the opposite end.

Double Chalcogen Bonds: Crystal Engineering Stratagems via Diffraction and Multinuclear Solid-State Magnetic Resonance Spectroscopy

Group 16 chalcogens potentially provide Lewis-acidic σ-holes, which are able to form attractive supramolecular interactions with electron rich partners through chalcogen bonds. Here, a multifaceted experimental and computational study of a large series of novel chalcogen-bonded cocrystals, prepared using the principles of crystal engineering, is presented. Single-crystal X-ray diffraction studies reveal that dicyanoselenadiazole and dicyanotelluradiazole derivatives work as promising supramolecular synthons with the ability to form double chalcogen bonds with a wide range of electron donors including halides and oxygen- and nitrogen-containing heterocycles. Extensive ⁷⁷ Se and ¹²⁵ Te solid-state nuclear magnetic resonance spectroscopic investigations of cocrystals establish correlations between the NMR parameters of selenium and tellurium and the local chalcogen bonding geometry. The relationships between the electronic environment of the chalcogen bond and the ⁷⁷ Se and ¹²⁵ Te chemical shift tensors were elucidated through a natural localized molecular orbital density functional theory analysis. This systematic study of chalcogen-bond-based crystal engineering lays the foundations for the preparation of the various multicomponent systems and establishes solid-state NMR protocols to detect these interactions in powdered materials.

Direct investigation of chalcogen bonds by multinuclear solid-state magnetic resonance and vibrational spectroscopy

We report a multifaceted experimental and computational study of three self-complementary chalcogen-bond donors as well as a series of seven chalcogen bonded cocrystals.

Mechanochemical Preparations of Anion Coordinated Architectures Based on 3-Iodoethynylpyridine and 3-Iodoethynylbenzoic Acid

The halogen bond has previously been explored as a versatile tool in crystal engineering and anion coordination chemistry, with mechanochemical synthetic techniques having been shown to provide convenient routes towards cocrystals. In an effort to expand our knowledge on the role of halogen bonding in anion coordination, here we explore a series of cocrystals formed between 3-iodoethynylpyridine and 3-iodoethynylbenzoic acid with halide salts. In total, we report the single-crystal X-ray structures of six new cocrystals prepared by mechanochemical ball milling, with all structures exhibiting C≡C-I⋅⋅⋅X- (X=Cl, Br) halogen bonds. Whereas cocrystals featuring a pyridine group favoured the formation of discrete entities, cocrystals featuring a benzoic acid group yielded an alternation of halogen and hydrogen bonds. The compounds studied herein were further characterized by 13C and 31P solid-state nuclear magnetic resonance, with the chemical shifts offering a clear and convenient method of identifying the occurrence of halogen bonding, using the crude product obtained directly from the mechanochemical ball milling. Whereas the 31P chemical shifts were quickly able to identify the occurrence of cocrystallization, 13C solid-state NMR was diagnostic of both the occurrence of halogen bonding and of hydrogen bonding.

Mechanochemical Preparations of Anion Coordinated Architectures Based on 3-Iodoethynylpyridine and 3-Iodoethynylbenzoic Acid

Invited for this month's cover picture is the group of Professor David Bryce at the University of Ottawa. The cover picture shows a stylized depiction of the mechanochemical halogen-bond induced cocrystallization of 3-iodoethynylpyridine with tetraphenylphosphonium bromide. Read the full text of their Full Paper at 10.1002/open.201900194.

SCFit: Software for single-crystal NMR analysis. Free vs constrained fitting

The design and implementation of a software package for the analysis of single-crystal NMR data is presented. The SCFit software can treat spectra arising from various interactions: (i) chemical shift tensor only; (ii) chemical shift tensor and quadrupolar coupling tensor; (iii) dipolar and indirect nuclear spin-spin coupling tensors; (iv) all four interactions. The software is demonstrated on recently reported ¹⁷O and ³¹P single-crystal NMR data for triphenylphosphine oxide and for two of its halogen-bonded cocrystals. The ¹⁷O single-crystal NMR data represent a case where all four above-mentioned interactions simultaneously affect the spectra. SCFit can fit the chemical shift and quadrupolar coupling in two ways: (i) through an unconstrained fitting process where all tensor parameters are freely optimized or (ii) through a constrained fitting process where the principal components of the tensors may be fixed to values known previously with high precision via the analysis of powder samples. The second strategy is explored in an effort to reduce the number of unknowns in the fitting process; an improvement in the precision of the resulting tensor orientations is noted in some cases.

Definition of the chalcogen bond (IUPAC Recommendations 2019)

This recommendation proposes a definition for the term “chalcogen bond”; it is recommended the term is used to designate the specific subset of inter- and intramolecular interactions formed by chalcogen atoms wherein the Group 16 element is the electrophilic site.

Rapid Identification of Halogen Bonds in Co-Crystalline Powders via 127 I Nuclear Quadrupole Resonance Spectroscopy

¹²⁷ I nuclear quadrupole resonance (NQR) spectroscopy is established as a rapid and robust method to indicate the formation of iodine-nitrogen halogen bonds in co-crystalline powders. Once the relevant spectral frequency range has been established, diagnostic ¹²⁷ I NQR spectra can be acquired in seconds. The method is demonstrated for a series of co-crystals of 1,4-diiodobenzene. Changes in the ¹²⁷ I quadrupolar coupling constant (C_Q ) by up to 74.4 MHz correlate with the length of the C-I donor covalent bond and inversely with the I⋅⋅⋅N halogen-bond length. The predictive power of this technique is validated on two previously unknown co-crystalline powders prepared mechanochemically. Single-crystal growth via co-sublimation and structure determination by single-crystal X-ray diffraction cross-validates the findings. Natural localized molecular-orbital analyses provide insight into the origins of the quadrupolar coupling constants.

Single-Crystal NMR Characterization of Halogen Bonds

Oxygen-17-enriched triphenylphosphine oxide and three of its halogen-bonded cocrystals featuring 1,4-diiodotetrafluorobenzene and 1,3,5-trifluoro-2,4,6-triiodobenzene as halogen bond donors have been characterized by ³¹P and ¹⁷O single-crystal NMR spectroscopy. Single-crystal NMR allows for the measurement of not only the magnitudes of various NMR interaction tensors, but also their orientations relative to the crystal lattice and therefore relative to the halogen bonds themselves. ³¹P chemical shift tensors, ¹⁷O chemical shift tensors, ¹⁷O quadrupolar coupling tensors, and ³¹P-¹⁷O indirect nuclear spin-spin (J) coupling tensors are reported here for P═O···I halogen bonds. The angular deviations in the directions of the pseudo-unique components of the ³¹P chemical shift tensors, the ¹⁷O chemical shift tensors, and the ¹⁷O quadrupolar coupling tensors from the direction of the oxygen-iodine halogen bond correlate with the deviations in linearity of the P═O···I halogen bond. There is also a clear decrease in anisotropy and an increase in asymmetry of the J(³¹P,¹⁷O) coupling tensors attributable to the formation of iodine-oxygen halogen bonds. The small but quantifiable changes in the tensors are consistent with the weak nature of these halogen bonds relative to the P═O motif. Overall, this work establishes single-crystal NMR as a novel probe of halogen bonds in solids. Analysis of the results has provided insights into the correlations between the magnitude and orientation of various NMR interaction tensors and the local geometry of the halogen bond. Gauge-including projector-augmented wave computations corroborate the experimental findings.

Halogen bonding as a supramolecular dynamics catalyst

Dynamic processes have many implications in functional molecules, including catalysts, enzymes, host-guest complexes, and molecular machines. Here, we demonstrate via deuterium NMR relaxation experiments how halogen bonding directly impacts the dynamics in solid 2,3,5,6-tetramethylpyrazine cocrystals, catalyzing the methyl group rotation. On average, we observe a reduction of 56% in the rotational activation energy of the methyl groups in the halogen bonded cocrystals, contrasting the reduction of 36% in the hydrogen bonded cocrystals, with respect to pure 2,3,5,6-tetramethylpyrazine. Density functional theory calculations attribute this superior catalytic ability of the halogen bond to the simultaneous destabilization of the staggered conformation and stabilization of the gauche conformation, overall reducing the rotational energy barrier. Furthermore, the calculations suggest that the catalytic ability of the halogen bond may be tuneable, with stronger halogen bond donors acting as superior dynamics catalysts. Thus, halogen bonding may play a role in both assembly and promoting dynamical processes.

The halogen bond is well known for its ability to assemble supramolecules. Here, using NMR experiments, the authors reveal the role of these bonds in dynamic processes, finding that the halogen bond directly catalyzes dynamical rotation in solid cocrystals by reducing the associated energy barrier.

121/123Sb Nuclear Quadrupole Resonance Spectroscopy: Characterization of Non-Covalent Pnictogen Bonds and NQR Crystallography

Pnictogen (or pnicogen) bonding is an attractive interaction between the electrophilic region of group 15 elements (N, P, As, Sb, Bi) and a nucleophile. This interaction for which unique applications in catalysis have recently been uncovered continues to gain popularity. Here, we investigate a series of pnictogen-bonded cocrystals based on SbF₃ and SbCl₃, prepared via mechanochemical ball milling, with ^121/123Sb ( I = ⁵/₂ and ⁷/₂, respectively) nuclear quadrupole resonance (NQR) spectroscopy. Observed NQR frequency shifts upon cocrystallization are on the order of 0.1 to 10 MHz and are clearly diagnostic of the formation of pnictogen bonds to antimony. Further evidence for pnictogen bonding is obtained by complementary ¹³C cross-polarization magic-angle spinning solid-state NMR experiments. DFT calculations of NMR parameters as well as natural localized molecular orbital analyses support the experimental findings and elucidate the electronic origins of the experimental NQR frequency shifts. This work provides insights into the changes in the antimony quadrupolar coupling constant upon pnictogen bonding: strikingly, the decreases noted here parallel those known for hydrogen bonds, but contrast with the increases reported for halogen bonds. The utility of the observed antimony nuclear quadrupolar coupling constants in constraining structural models of cocrystals for which diffraction-based structures are unavailable, i.e., a rudimentary implementation of NQR crystallography, is established. Overall, this work offers a new approach to understand emerging classes of electrophilic interactions and to contextualize them in the broader landscape of established chemical bonding paradigms.

New frontiers for solid-state NMR across the periodic table: a snapshot of modern techniques and instrumentation

Selected highlights of the recent literature on solid-state NMR of some of the lesser studied nuclei are provided. The roles of ultrahigh magnetic fields, radiofrequency pulse sequences, dynamic nuclear polarization, isotopic enrichment, and nuclear quadrupole resonance in opening up the periodic table to in-depth study are discussed.

Recent advances in solid-state nuclear magnetic resonance spectroscopy of exotic nuclei

We present a review of recent advances in solid-state nuclear magnetic resonance (SSNMR) studies of exotic nuclei. Exotic nuclei may be spin-1/2 or quadrupolar, and typically have low gyromagnetic ratios, low natural abundances, large quadrupole moments (when I > 1/2), or some combination of these properties, generally resulting in low receptivities and/or prohibitively broad line widths. Some nuclides are little studied for other reasons, also rendering them somewhat exotic. We first discuss some of the recent progress in pulse sequences and hardware development which continues to enable researchers to study new kinds of materials as well as previously unfeasible nuclei. This is followed by a survey of applications to a wide range of exotic nuclei (including e.g., ⁹Be, ²⁵Mg, ³³S, ³⁹K, ⁴³Ca, ^47/49Ti, ⁵³Cr, ⁵⁹Co, ⁶¹Ni, ⁶⁷Zn, ⁷³Ge, ⁷⁵As, ⁸⁷Sr, ¹¹⁵In, ¹¹⁹Sn, ^121/123Sb, ^135/137Ba, ^185/187Re, ²⁰⁹Bi), most of them quadrupolar. The scope of the review is the past ten years, i.e., 2007-2017.

79/81Br nuclear quadrupole resonance spectroscopic characterization of halogen bonds in supramolecular assemblies

One- and two-dimensional bromine-79/81 NQR spectroscopy of halogen bond donors in a series of cocrystals shows changes in resonance frequency of up to 20 MHz and differentiates between crystallographically non-equivalent bromine sites.

Despite the applicability of solid-state NMR to study the halogen bond, the direct NMR detection of ^79/81Br covalently bonded to carbon remains impractical due to extremely large spectral widths, even at ultra-high magnetic fields. In contrast, nuclear quadrupole resonance (NQR) offers comparatively sharp resonances. Here, we demonstrate the abilities of ^79/81Br NQR to characterize the electronic changes in the C–Br···N halogen bonding motifs found in supramolecular assemblies constructed from 1,4-dibromotetrafluorobenzene and nitrogen-containing heterocycles. An increase in the bromine quadrupolar coupling constant is observed, which correlates linearly with the halogen bond distance (d_Br···N). Notably, ^79/81Br NQR is able to distinguish between two symmetry-independent halogen bonds in the same crystal structure. This approach offers a rapid and reliable indication for the occurrence of a halogen bond, with experimental times limited only by the observation of ^79/81Br NQR resonances.

3-(1,2,2-Triiodoethenyl)benzoic acid

The title compound, C9H5I3O2, has a layered structure exhibiting O—H...O hydrogen bonds, as well as C—I...C and C—I...O halogen bonding. The C atoms of the ethenyl group are disordered over two sets of sites with refined occupancies of 0.545 (18) and 0.455 (18).

Structural Insights from 59 Co Solid-State NMR Experiments on Organocobalt(I) Catalysts

A series of fumarate-based organocobalt(I) [CoCp(CO)(fumarate)] catalysts is synthesized and characterized by X-ray crystallography, multinuclear (¹³ C and ⁵⁹ Co) solid-state NMR spectroscopy, and ⁵⁹ Co NQR spectroscopy. Given the dearth of ⁵⁹ Co solid-state NMR studies on Co^I compounds, the present work constitutes the first systematic characterization of the ⁵⁹ Co electric field gradient and chemical shift tensors for a series of cobalt complexes in this oxidation state. Using X-ray crystallography, the molecular geometry about the Co^I centre is found to be nearly identical in all compounds studied herein. Owing to the ⁵⁹ Co nucleus' large chemical shift range, solid-state NMR experiments are found to be able to detect small structural differences between the individual organocobalt(I) compounds. With the aid of density functional theory calculations on these complexes, it is shown that the ⁵⁹ Co chemical shift anisotropy and the ⁵⁹ Co quadrupolar coupling constant are both extremely sensitive gauges of the Fu-Co-Cp bond angle, providing a link between these ⁵⁹ Co NMR observables and the catalysts' structures.

Halogen-bond driven self-assembly of triangular macrocycles

Halogen bonds drive the self-assembly of 2-iodoethynylpyridine and 2- iodoethynyl-1-methyl-imidazole into discrete supramolecular triangles.

A rare example of a phosphine as a halogen bond acceptor

The cocrystal of triphenylphosphine with 1,3,5-trifluoro-2,4,6-triiodobenzene features a rare, moderately strong, and linear phosphorus–iodine halogen bond.

Halide ion recognition via chalcogen bonding in the solid state and in solution. Directionality and linearity

Benzylic selenocyanates are versatile anion receptors which operate in solution and in the solid state via chalcogen bonding interactions.

Mechanochemistry and cocrystallization of 3-iodoethynylbenzoic acid with nitrogen-containing heterocycles: concurrent halogen and hydrogen bonding

Halogen-bonded and hydrogen-bonded cocrystals of 3-iodoethynylbenzoic acid and several nitrogen-containing heterocycles are formed using mechanochemical and solvent-based slow evaporation methods.