Planting the Seeds of a Decision Tree for Ionic Liquids: Steric and Electronic Impacts on Melting Points of Triarylphosponium Ionic Liquids.

While machine learning and artificial intelligence offer promising avenues in the computer-aided design of materials, the complexity of these computational techniques remains a barrier for scientists outside of the specific fields of study. Leveraging decision tree models, inspired by empirical methodologies, offers a pragmatic solution to the knowledge barrier presented by artificial intelligence (AI). Herein, we present a model allowing for the qualitative prediction of melting points of ionic liquids derived from the crystallographic analysis of a series of phosphonium-based ionic liquids. By carefully tailoring the steric and electronic properties of the cations within these salts, trends in the melting points are observed, pointing toward the critical importance of π interactions to forming the solid state. Quantification of the percentage of these π interactions using modern quantum crystallographic approaches reveals a linear trend in the relationship of C-Hπ and π-π stacking interactions with melting points. These structure-property relationships are further examined by using computational studies, helping to demonstrate the inverse relationship of dipole moments and melting points for ionic liquids. The results provide valuable insights into the features and relationships that are consistent with achieving low Tm values in phosphonium salts, which were not apparent in earlier studies. The data gathered are presented in a simple decision tree format, allowing for visualization of the data and providing guidance toward developing yet unreported compounds.

New type of tin(IV) complex based turn-on fluorescent chemosensor for fluoride ion recognition: elucidating the effect of molecular structure on sensing property.

A novel type of chemosensor based on tin(IV) complexes incorporating hydroxyquinoline derivatives has been designed and investigated for selectively detecting fluoride ions. Sn(meq)2Cl2 (meq = 2-methyl-8-quinolinol) (complex 1) exhibits a significant enhancement in luminescence upon the introduction of fluoride ions. This enhancement greatly surpasses that observed with Snq2Cl2 and Sn(dmqo)2Cl2 (q = 8-hydroxyquinnoline; dmqo = 5,7-dimethyl-8-quinolinol). Furthermore, complex 1 displays excellent sensitivity and selectivity for fluoride detection in comparison to halides and other anions. As a result, complex 1 serves as an outstanding turn-on fluorescent chemosensor, effectively sensing fluoride ions. The Benesi-Hilderbrand method and Job's plot confirmed that complex 1 associates with F- in a 1 : 2 binding stoichiometry. Also, complex 1 exhibited a large binding constant (pKb = 10.4 M-2) and a low detection limit (100 nM). To gain a deeper insight into the photophysical properties and the underlying mechanism governing the formation of the tin(IV) fluoride complex via halide exchange, we successfully synthesized partially fluorinated Sn(meq)2F0.67Cl1.33 (2) and fully fluorinated Sn(meq)2F2 (3), all of which were characterized through computational studies, thereby elucidating their photophysical properties. DFT studies reveal that converting Sn(meq)2Cl2 to Sn(meq)2F2, an endergonic process, leads to greater stability due to reducing steric hindrance about the metal center. Furthermore, the fluorinated complex significantly increases dipole moment, resulting in high affinity toward the F- ion.

Are nature's strategies the solutions to the rational design of low-melting, lipophilic ionic liquids?

Ionic liquids (ILs) have emerged as a new class of materials, displaying a unique capability to self-assemble into micelles, liposomes, liquid crystals, and microemulsions. Despite evident interest, advancements in the controlled formation of amphiphilic ILs remain in the early stages. Taking inspiration from nature, we introduced the concept of lipid-like (or lipid-inspired) ILs more than a decade ago, aiming to create very low-melting, highly lipophilic ILs that are potentially bio-innocuous - a combination of attributes that is frequently antithetical but highly desirable from several application-specific standpoints. Lipid-like ILs are a subclass of functional organic liquid salts that include a range of lipidic side chains such as saturated, unsaturated, linear, branched, and thioether while retaining melting points below room temperature. It was observed in several homologous series of [Cnmim] ILs that elongation of N-appended alkyl chains to greater than seven carbons leads to a substantial increase in melting point (Tm) - which is the most characteristic feature of ILs. Accordingly, it is challenging to develop ILs with low Tm values while preserving their hydrophobicity and self-organizing properties. We found that two alternative Tm depressive approaches are useful. One of these is the replacement of the double bonds with thioether moieties in the alkyl chains, as detailed in several published papers detailing the chemistry of these ILs. Employing thiol-ene and thiol-yne click reactions is a facile, robust, and orthogonal method to overcome the challenges associated with the synthesis of alkyl thioether-functionalized ILs. The second approach involves replacing the double bonds with the cisoid cyclopropyl motif, mimicking the strategy used by certain organisms to modulate cell membrane fluidity. This discovery has the potential to greatly impact the utilization of lipid-like ILs in various applications, including gene delivery, lubricants, heat transfer fluids, and haloalkane separations, among others. This feature article presents a concise, historical overview, highlighting key findings from our work while offering speculation about the future trajectory of this de novo class of soft organic-ion materials.

Alkyl-templated cocrystallization of long-chain 1-bromoalkanes by lipid-like ionic liquids.

The serendipitous discovery of an unorthodox ionic cocrystallization system using 2-mercaptothiazolium-based ionic liquids as a crystallization milieu paves the way for the first report of crystal structures of long-chain 1-bromoalkanes. We used single crystal X-ray diffraction to determine the structures of 1-bromo-hexadecane and 1-octadecane with the aid of ionic liquids with alkyl side chains of equivalent length to the bromoalkane at room temperature. Long alkyl chains in combination with σ-hole interactions from strategically placed sulfur motifs synergistically function to crystallize the 1-bromoalkanes.

Regiodivergent sulfonylation of terminal olefins via dearomative rearrangement.

Sulfones are fascinating and highly used functional groups, but current syntheses still have limitations. Here, a regiodivergent transition metal-free approach towards sulfones [(E)-allylic sulfones and α-sulfonyl-methyl styrenes] is reported. The method employs commercially available olefins, bases, additives, solvents, and sodium sulfinates (RSO2Na) and produces adducts in good yields. Considering that up to 4 reactions (bromination, dearomative rearrangement, E2, and SN2) are happening, this approach is very efficient. The structures of key adducts were confirmed by X-ray crystallography.

Cyclopropane as an Unsaturation "Effect Isostere": Lowering the Melting Points in Lipid-like Ionic Liquids.

The replacement of unsaturation with a cyclopropane motif as a (bio)isostere is a widespread strategy in bacteria to tune the fluidity of lipid bilayers and protect membranes when exposed to adverse environmental conditions, e.g., high temperature, low pH, etc. Inspired by this phenomenon, we herein address the relative effect of the cyclopropanation, both cis and trans configurations, on melting points, packing efficiency, and order of a series of lipid-like ionic liquids via a combination of thermophysical analysis, X-ray crystallography, and computational modeling. The data indicate there is considerable structural latitude possible when designing highly lipophilic ionic liquids that exhibit low melting points. While cyclopropanation of the lipid-like ionic liquids provides more resistance to aerobic degradation than their olefin analogs, the impact on the melting point decrease is not as pronounced. Our results demonstrate that incorporating one or more cyclopropyl moieties in long aliphatic chains of imidazolium-based ionic liquids is highly effective in lowering the melting points of such materials relative to their counterparts bearing linear, saturated, or thioether side chains. It is shown that the cyclopropane moiety effectively disrupts packing, favoring formation of gauche conformer in the side chains, resulting in enhancement of fluidity. This was irrespective of the configuration of the methylene bridge, although marked differences in the effect of cis- and trans-monocyclopropanated ILs on the melting points were observed.

Bridging the crystal and solution structure of a series of lipid-inspired ionic liquids.

A series of 1,2-dimethylimidazolium ionic liquids bearing a hexadecyl alkyl chain are thoroughly examined via X-ray crystallography. The crystal structures reveal several key variations in the non-covalent interactions in the lipid-like salts. Specifically, distinct cation-cation π interactions are observed when comparing the bromide and iodide structures. Changing the anion to bis(trifluoromethane)sulfonimide (Tf2N-) changes these cation-cation π interactions with anion⋯π interactions. Additionally, several well-defined geometries of the cations are noted based on torsion and core-plane angles of the alkyl chains. Hirshfeld surface analysis is used to distinguish the interactions and geometries in the solid state, helping to reveal characteristic structural fingerprints for the compounds. The solid-state structures of the ionic liquids are correlated with the solution-state structures through UV-vis spectroscopic studies, further emphasizing the importance of the π interactions in the formation of aggregates. Finally, we investigated the thermal properties of the ionic liquids, revealing complex phase transitions for the iodide-containing species. These phase transitions are further rationalized via the analysis of the data gathered from the structures of the other crystallized salts.

Anticancer Agents as Design Archetypes: Insights into the Structure-Property Relationships of Ionic Liquids with a Triarylmethyl Moiety.

A fundamental challenge underlying the design principles of ionic liquids (ILs) entails a lack of understanding into how tailored properties arise from the molecular framework of the constituent ions. Herein, we present detailed analyses of novel functional ILs containing a triarylmethyl (trityl) motif. Combining an empirically driven molecular design, thermophysical analysis, X-ray crystallography, and computational modeling, we achieved an in-depth understanding of structure-property relationships, establishing a coherent correlation with distinct trends between the thermophysical properties and functional diversity of the compound library. We observe a coherent relationship between melting (T m) and glass transition (T g) temperatures and the location and type of chemical modification of the cation. Furthermore, there is an inverse correlation between the simulated dipole moment and the T m/T g of the salts. Specifically, chlorination of the ILs both reduces and reorients the dipole moment, a key property controlling intermolecular interactions, thus allowing for control over T m/T g values. The observed trends are particularly apparent when comparing the phase transitions and dipole moments, allowing for the development of predictive models. Ultimately, trends in structural features and characterized properties align with established studies in physicochemical relationships for ILs, underpinning the formation and stability of these new lipophilic, low-melting salts.

Ionic Liquids Containing the Sulfonyl Fluoride Moiety: Integrating Chemical Biology with Materials Design.

The persistent achievements of ionic liquids in various fields, including medicine and energy necessitate the efficient development of novel functional ionic liquids that exhibit favorable characteristics, alongside the development of practical and scalable synthetic methodologies. Ionic liquids are fundamentally understood as materials in which structure begets function, and the function and applicability of ILs is of utmost concern. It was recently reported that "full fluorosulfonyl" electrolyte is compatible with both the Li metal anode and the metal-oxide cathode that is crucial for the development of high-voltage rechargeable lithium-metal batteries. Inspired by these results, for the first time, we reported the synthesis of a series of ionic liquids with a sulfonyl fluoride motif using an highly effective and modular fluorosulfonylethylation procedure. Herein, we present a detailed analysis of novel sulfonyl fluoride-based ionic liquids paired with the hexafluorophosphate anion. We employed a combination of computational modeling and X-ray crystallographic studies to gain an in-depth understanding of their structure-property correlations.

1-Methyl-5-nitro-imidazolium chloride.

The title salt, C4H6N3O2 +·Cl-, exhibits multiple hydrogen-bonding inter-actions involving the nitro-imidazolium cation and the chloride anion. Strong hydrogen bonds between the amine hydrogen atom and the chloride anion link the ionic moieties. Of note, with respect to H⋯Cl inter-actions, the central aromatic hydrogen atom displays a shorter inter-action than the other aromatic hydrogen atom. Finally, inter-actions are observed between the nitro moiety and methyl H atoms. While no π-π stacking is observed, anion-π inter-actions are present. The crystal was refined as a two-component twin.

Contrasting the Noncovalent Interactions of Aromatic Sulfonyl Fluoride and Sulfonyl Chloride Motifs via Crystallography and Hirshfeld Surfaces.

A heteroaryl sulfonyl(VI) fluoride, 4-chloro-7-fluorosulfonyl-2,1,3-benzoxadiazole, was synthesized from its chloride counterpart (4-chloro-7-chlorosulfonyl-2,1,3-benzoxadiazole) and the X-ray structure analysis of these compounds and the interactions in the solid-state were thoroughly examined. Hirshfeld surface analysis is used to provide a thorough and complete picture of the changes arising from the different halides in the functional groups. Surface analysis reveals that the fluoride does not participate in any hydrogen interactions as opposed to the chloride. However, the fluorine atom is observed to form close interactions with several π bonds. For both moieties, however, the sulfonyl oxygens show comparable interactions with respect to both magnitude and interatomic distances. The Hirshfeld surface analysis is coupled with computational studies to help elucidate the observed interactions that are found from the distinct nitrogen, chlorine, and oxygen atoms present in the molecules, providing new physical insights to the correlation between their structures and properties.

Developing Structural First Principles for Alkylated Triphenylphosphonium-Based Ionic Liquids.

While ionic liquids have proved to be versatile materials for a wide spectrum of applications, e.g., energy, materials, and medicine, several challenges remain concerning the rational design of novel materials. In light of this, a series of four triphenylphosphonium-based ionic liquids have been synthesized for the first time. These compounds exhibit high thermal stability with decomposition temperatures up to 450 °C. Their solid-state structures are characterized by single-crystal X-ray diffraction and the intermolecular interactions rigorously analyzed via Hirshfeld surface analysis. It was found that the unique geometries of the anions used in the study form distinct interactions with the cations. The interactions in the crystalline state are correlated with the thermal properties of the four ionic liquids to rationalize the melting points and phase transitions for each compound. The observed arrangements of the alkyl chains on the cations are investigated computationally to gain an understanding of how rotational freedom may impact the thermal properties of the compounds. By intention, each IL reported in this work offers a unique property profile and contributes to the ever-growing ionic liquid catalog.

Crystal structure and spectroscopic properties of (E)-1,3-dimethyl-2-[3-(4-nitro-phen-yl)triaz-2-enyl-idene]-2,3-di-hydro-1H-imidazole.

The title compound (E)-1,3-dimethyl-2-[3-(4-nitro-phen-yl)triaz-2-enyl-idene]-2,3-di-hydro-1H-imidazole, C11H12N6O2, has monoclinic (C2/c) symmetry at 100 K. This triazene derivative was synthesized by the coupling reaction of 1,3-di-methyl-imidazolium iodide with 1-azido-4-nitro benzene in the presence of sodium hydride (60% in mineral oil) and characterized by 1H NMR, 13C NMR, IR, mass spectrometry, and single-crystal X-ray diffraction. The mol-ecule consists of six-membered and five-membered rings, which are connected by a triazene moiety (-N=N-N-). In the solid-state, the mol-ecule is found to be planar due to conjugation throughout the mol-ecule. The extended structure shows two layers of mol-ecules, which present weak inter-molecular inter-actions that facilitate the stacked arrangement of the mol-ecules forming the extended structure. Furthermore, there are several weak pseudo-cyclical inter-actions between the nitro oxygen atoms and symmetry-adjacent H atoms, which help to arrange the mol-ecules.

Design Principles of Lipid-like Ionic Liquids for Gene Delivery.

We developed lipid-like ionic liquids, containing 2-mercaptoimidazolium and 2-mercaptothiazolinium headgroups tethered to two long saturated alkyl chains, as carriers for in vitro delivery of plasmid HEK DNA into 293T cells. We employed a combination of modular design, synthesis, X-ray analysis, and computational modeling to rationalize the self-assembly and desired physicochemical and biological properties. The results suggest that thioamide-derived ionic liquids may serve as a modular platform for lipid-mediated gene delivery. This work represents a step toward understanding the structure-function relationships of these amphiphiles with long-range ordering and offering insight into design principles for synthetic vectors based on self-assembly behavior.

Pyridinium 3-nitro-benzoate-3-nitro-benzoic acid (1/1).

The crystal structure of the product of the neutralization reaction between 3-nitro-benzoic acid and pyridine is reported. The entities that crystallized are a pyridinium cation, a 3-nitrobenzoate anion and a 3-nitrobenzoic acid molecule in a 1:1:1 molar ratio, C5H6N+·C7H4NO4 -·C7H5NO4. Distinct sets of hydrogen bonds link the pyridinium and benzoate ions (N-H⋯O) and the acid and benzoate moieties (O-H⋯O). The hydrogen bonding along with π-π stacking between the acid and benzoate moieties accounts for the long-range ordering of the crystal.

Quinine di-hydro-chloride hemihydrate.

Numerous non-covalent inter-actions link together discrete mol-ecules in the crystal structure of the title compound, 2C20H26N2O2 2+·4Cl-·H2O {systematic name: 4-[(5-ethenyl-1-azonia-bicyclo-[2.2.2]octan-2-yl)(hy-droxy)meth-yl]-6-meth-oxy-quinolin-1-ium dichloride hemihydrate}. A combination of hydrogen bonding between acidic H atoms and the anions in the asymmetric unit forms a portion of the observed hydrogen-bonded network. π-π inter-actions between the aromatic portions of the cation appear to play a role in the formation of the long-range ordering. One ethyl-ene double bond was found to be disordered. The disorder extends to the neighboring carbon and hydrogen atoms.

Synthesis and Characterization of a Linear Triiron(II) Extended Metal Atom Chain Complex with Fe-Fe Bonds.

Extended metal atom chain (EMAC) complexes of first-row transition metals with metal-metal bonds have the potential to elicit unique magnetic properties and reactivities. Until now, the library of EMAC complexes with late-first-row transition metals was incomplete because of the omission of a triiron species with Fe-Fe bonding. Herein we report the synthesis and preliminary investigation of the first linear, triiron(II) complex containing close Fe-Fe interactions. The complex is supported by three dianionic 2,6-bis[(trimethylsilyl)amido]pyridine ligands (L), with an overall composition of Fe3L3, and pseudohelical ligand coordination stabilizing the local trigonal-planar geometry at each iron. Fe3L3 was characterized by X-ray diffraction, 1H NMR, cyclic voltammetry, electronic absorption, and Mössbauer spectroscopies. Evans method analysis indicated a large uncompensated spin and an S = 6 ground state, suggesting ferromagnetic coupling in the triiron chain, likely due to direct exchange.

2,3-Dimethyl-1H-imidazol-3-ium benzene-sulfonate-1,2-dimethyl-1H-imidazole co-crystal.

In the title co-crystal, C5H9N2 +·C6H5O3S-·C5H8N2, the two 1,2-di-methyl-imidazole rings exist as partially protonated moieties in the asymmetric unit as a two-part disordered unit wherein the acidic hydrogen atom is bound to each ring. The two imidazolium cations share a strong hydrogen bond via the acidic hydrogen atom, which is disordered between two positions, being bonded to the first versus second imidazole ring in a 0.33 (2) to 0.67 (2) ratio. A benzene sulfonate anion is present for charge balance and inter-acts with the aromatic H atoms on both imidazole rings as well as with the methyl groups on the rings.

2,3-Dimethyl-1H-imidazol-3-ium chloride.

The title salt, C5H9N2 +·Cl-, exhibits multiple hydrogen-bonding inter-actions between the cationic imidazole moiety and the chloride anion. The protonated aromatic nitro-gen moiety displays the shortest hydrogen-bonding inter-actions while weaker hydrogen bonding is observed between the aromatic H atoms and the chloride anion. The crystal studied was refined as a two-component inversion twin with a twin ratio of 0.71 (5) to 0.29 (5).

2,2'-[Methyl-enebis(sulfanedi-yl)]bis-(pyridine 1-oxide).

The title compound, C11H10N2O2S2, crystallizes with one complete mol-ecule in the asymmetric unit. In the crystal, weak hydrogen bonding is observed between the N-oxide moieties and several C-H units.