As a crystallographer, nothing is sadder than failed crystal growth. As an “experienced” crystallographer (a basketball ref couldn't hand signal my number of years), I’ve thrown away more than my share of oily smudges (always according to established hazardous waste procedures, of course). Fortunately, my students are more curious than I. Recently we reported the first halogen-bonding-based deep eutectic solvent, which consisted of a mixture of 1,3-dithiane and o- diiodotetrafluorobenzene – a system that simply refused to cooperate during crystal growth (Peloquin et al. Angew Chem, Int. Ed.2021, 60, 22983–22989). Based on this result and memories of many other liquid samples, we have been exploring a number of different systems, most consisting of tetraalkylammonium triiodides with a variety of organoiodines. As it turns out, many of these systems that resist crystal growth are doing exactly what they are “supposed” to do. Most of these are pseudo binary systems with two or more eutectic points and at least one cocrystalline composition (see phase diagram below). The preparation and characterization of these systems by thermal analysis and, in the case of cocrystals, structural characterization will be discussed. Efforts to correlate halogen bonding in triiodide-based cocrystals with halogen bonding in triiodide-based deep eutectic solvents are now in progress.

We present the application of an extreme gradient boosting model (eutXG) to predict the melting point (MP) of deep eutectic solvents (DES). The model is based on XGBoost, a decision tree ensemble based on gradient boosting designed to be highly scalable that enables superior training speed and prediction accuracy. The selected model-trained with molecular fingerprints, molar ratios, and selected chemical descriptors-enabled the prediction of the MPs of DES with an average accuracy of 97.6%, which represents a difference of just +/- 2.4% with respect to the values reported in the literature. Using SHapley Additive exPlanations (SHAP), further insights into the relative importance of different inputs used to train the machine learning model were identified. Moreover, the generalization ability of the eutXG model was critically assessed by comparing the predicted vs the experimentally determined MP of a series of novel DES based on halogen bonding, developed by mixing tetraalkylammonium triiodide salts (NPe(4)I(3) or NHex(4)I(3)) with organoiodines, such as 1,2-diiodotetrafluorobenzene (o-F4DIB), 1,3-diiodotetrafluorobenzene (m-F4DIB), or 2,5-diiodothiophene (2,5-DIT), demonstrating its ability to predict the actual melting with a difference of only 2 K. Our results not only reinforce the importance of having (at least some) representative data for the training step to increase the accuracy of the model's predictions but also demonstrate the ability of eutXG to accelerate the development of novel applications for this entirely new class of hydrophobic DES, potentially impacting a wide range of fields from pharmaceuticals to agrochemicals.

A series of cocrystals of halogen bond donors 1,4-diiodotetrafluorobenzene (p-F4DIB) and tetraiodoethylene (TIE) with five aromatic heterocyclic diazine mono-N-oxides based on pyrazine, tetramethylpyrazine, quinoxaline, phenazine, and pyrimidine as halogen bonding acceptors were studied. Structural analysis of the mono-N-oxides allows comparison of the competitive occurrence of N···I vs O···I interactions and the relative strength and directionality of these two types of interactions. Of the aromatic heterocyclic diazine mono-N-oxide organoiodine cocrystals examined, six exhibited 1:1 stoichiometry, forming chains that utilized both N···I and O···I interactions. Two cocrystals presented 1:1 stoichiometry with exclusive O···I interactions. Two cocrystals displayed a 2:1 stoichiometry-one characterized solely by O···I interactions and the other solely by N···I interactions. We have also compared these interactions to those present in the corresponding diazines, some of which we report here and some which have been previously reported. In addition, a computational analysis using density functional theory (M062X/def2-SVPD) was performed on these two systems and has been compared to the experimental results. The calculated complex formation energies were, on average, 4.7 kJ/mol lower for the I···O halogen bonding interaction as compared to the corresponding N···I interaction. The average I···O interaction distances were calculated to be 0.15 Å shorter than the corresponding I···N interactions.

Triiodide salts are of interest for a variety of applications, including but not limited to electrochemical and photochemical devices, as antimicrobials and disinfectants, in supramolecular chemistry and crystal engineering, and in ionic liquids and deep eutectic solvents. Our work has focused on the design of salt–solvate cocrystals and deep eutectic solvents in which the triiodide anion interacts as a halogen bond acceptor with organoiodine molecules. To understand structure–property relationships in these hybrid materials, it is essential to have benchmark structural and physical data for the parent triiodide salt component. Herein, we report the structure and thermal properties of eight new triiodide salts, three of which exhibit polymorphism: tetrapentylammonium triiodide (1a and 1b), tetrahexylammonium triiodide (2), trimethylphenylammonium triiodide (3), trimethylbenzylammonium triiodide (4), triethylbenzylammonium triiodide (5), tri-n-butylbenzylammonium triiodide (6), 3-methylbenzothizolium triiodide (7a and 7b), and 2-chloro-1-methylpyridinium triiodide (8a and 8b). The structural features of the triiodide anion, Raman spectroscopic analysis, and melting and thermal decomposition behavior of the salts, as well as a computational analysis of the polymorphs, are discussed. The polymorphic pairs here are distinguished by symmetric versus asymmetric triiodide anions, as well as different packing patterns. Computational analyses revealed more subtle differences in their isosurface plots. Importantly, this study provides reference data for these new triiodide salts for comparison to hybrid cocrystals and deep eutectic solvents formed from their combination with various organoiodines.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Utilizing the N-heterocyclic chalcogenones hexahydro-1,3-bis(2,4,6- trimethylphenyl)-2H-1,3-diazepine-2-thione (SDiazMesS) and hexahydro-1,3-bis(2,4,6-trimethylphenyl)-2H-1,3-diazepine-2-selone (SDiazMesSe) as halogen-bond acceptors, a total of 24 new cocrystals were prepared. The solid-state structures of the parent molecules were also determined, along with those of their acetonitrile solvates. Through the reaction of the chalcogen atom with molecular diiodine, a variety of S-I-I and Se-I-I fragments were formed, spanning a wide range of I-I bond orders. With acetone as a reaction solvent, molecular diiodine causes the oxidative addition of acetone to the chalcogen atom, resulting in new C-S, C-Se and C-C covalent bonds under mild conditions. The common halogen-bond donors, iodopentafluorobenzene, 1,2-, 1,3- and 1,4-diiodotetrafluorobenzene, 1,3,5-trifluorotriiodobenzene and tetraiodoethylene resulted in halogen-bond-driven cocrystal formation. In most cases, the analogous SDiazMesS and SDiazMesSe cocrystals are isomorphic.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Through the combination of heterocyclic thiones with variation in the identity of the heterocyclic elements, namely, imidazolidine-2-thione, 2-mercaptobenzimidazole, 2-mercapto-5-methylbenzimidazole, 2-mercaptobenzoxazole, and 2-mercaptobenzothiazole with the common halogen-bond donors 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene, 1,3,5-trifluorotriiodobenzene, and tetraiodoethylene, a series of 18 new crystalline structures were characterized. In most cases, N-H...S hydrogen bonding was observed, with these interactions in imidazole-containing structures typically resulting in two-dimensional motifs (i.e. ribbons). Lacking the second N-H group, the thiazole and oxazole hydrogen bonding resulted in only dimeric pairs. C-I...S and C-I...I halogen bonding, as well as C=S...I chalcogen bonding, served to consolidate the packing by linking the hydrogen-bonding ribbons or dimeric pairs.

Utilizing the facile addition–elimination reaction of thiosemicarbazide with acetone or aldehydes, nine thiosemicarbazones were synthesized. Aldehydes were chosen which contain additional heteroatoms to increase the diversity of possible intermolecular interactions. Further, the thiosemicarbazone synthesis was conducted in situ with one of the common halogen bond donors 1,2-, 1,3-, or 1,4-diiodotetrafluorobenzene, 1,3,5-trifluoro-2,4,6-triiodobenzene, or tetraiodoethylene. These reactions resulted in the characterization of 12 new cocrystals showcasing halogen bonding. The dimerization of two thiosemicarbazone units through a pair of N‒H···S hydrogen bonds was a universal feature of the solid-state structures in this series, with the hydrogen bond network often extending these motifs into chains. The organoiodines serve to link chains through either I···S or I···N halogen bonding, or less commonly, S···I chalcogen bonding. This variety of intermolecular interactions leads to the formation of double-stranded chains, ribbons, and sheets. Utilizing the facile addition–elimination reaction of thiosemicarbazide with acetone or aldehydes, nine thiosemicarbazones were synthesized, seven of which were isolated as cocrystals with common halogen bond donors. Significant N–H···S hydrogen bonding was observed in all, with S···I halogen and chalcogen bonding contributing to the long-range packing in the cocrystals.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.