A Strategy to Form Eutectic Molecular Liquids Based on Noncovalent

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A Strategy to Form Eutectic Molecular Liquids Based on Noncovalent Interactions Dongkun Yu, and Tiancheng Mu J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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The Journal of Physical Chemistry

A Strategy to Form Eutectic Molecular Liquids Based on Noncovalent Interactions Dongkun Yu and Tiancheng Mu* Department of Chemistry, Renmin University of China, Beijing 100872, P.R. China

Abstract: The concept of eutectic molecular liquids (EMLs) was defined, and a strategy to form EMLs based on noncovalent interactions was proposed. We verified the formation and investigated the properties, interaction sites and interaction energies of the obtained 16 EMLs. Moreover, two new forms of noncovalent interactions, κ-hole and μ-hole bonding interactions were proposed, which broaden the understanding of intermolecular interactions. Numerous EMLs can be strategically designed and prepared by simply mixing two parent molecule components based on noncovalent interactions, including hydrogen bonding interactions; π-π stacking; and σ-hole (halogen, chalcogen, pnicogen, tetrel bonds), π-hole, κ-hole, and μ-hole bonding interactions. The properties of EMLs can be finely tailored by selecting or even designing appropriate parent compounds for task-specific applications. Our work presents a substantial step toward the innovative development of liquid systems.

INTRODUCTION Room-temperature liquid compounds can be used as solvents, electrolytes, heat transfer media, etc. These compounds are used in more than 70% of all chemical and chemical engineering processes. Numerous room-temperature liquids such as traditional volatile organic solvents and ionic liquids (ILs) have been developed and used in various fields.1 The term “eutectic” originated in metallurgy, and eutectic phenomena have been known for thousands of years. One of the most famous examples of the eutectic concept is the Hall-Héroult process for aluminum electrolysis.2 With the aid of cryolite Na3AlF6 (m. p. 1010 °C), aluminum electrolysis from Al2O3 (m. p. 2072 °C) can be achieved at 940980 °C. In general, steric hindrance and interactions inhibit the ability of the parent compounds to crystallize, therefore decreasing the melting point of mixtures of ionic compounds. For example, deep eutectic solvents (DESs) can be formed by mixing an ionic salt as a hydrogen bond acceptor and a molecular compound as a hydrogen bond donor.3-4 Inspired by the eutectic concept, molecular compounds can also be used to form eutectics based on noncovalent interactions. If the eutectics formed by molecular compounds are liquid at or near room temperature, they can be defined as eutectic molecular liquids (EMLs). The noncovalent interactions occurring in the formation of EMLs include hydrogen bonding interactions, σ-hole bonding interactions (including aerogen, halogen, chalcogen, pnicogen, and tetrel bonds) and π-hole bonding interactions, as well as two new bonding interactions, κ-hole and μ-hole bonding

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interactions, which are defined for the first time in this study. The EMLs formed via these interactions have potential applications not only in the fields of material preparation and environmental, biological and chemical engineering and technology but also in molecular recognition, self-assembly, and so on. 5-11 EMLs differ from ILs and DESs in nature (Table S1). EMLs are composed of two kinds of molecular compounds based on noncovalent interactions. ILs are composed of cations and anions based primarily on Coulomb forces, while DESs are composed of cations, anions and molecules based on Coulomb forces and hydrogen bonding interactions, categorized according to the four types of DESs defined by Abbott.12 EMLs can be formed because noncovalent interactions reduce the charge density in polar compounds, thus depressing the melting point (or glass transition temperature) of EMLs. Noncovalent interactions have played an important role in supramolecular chemistry and molecular recognition.13 In the 21st century, halogen bonds have received renewed attention, although these bonds were proposed two hundred years ago.14 Very recently, other noncovalent interactions including chalcogen (group VIA elements), pnicogen (group VA elements), tetrel (group IA elements) and even aerogen (group VIIIA elements) bonding have been discovered and defined.15-21 These “bonds”—which are actually bond paths—can be explained by the presence of a region of positive electrostatic potential (ESP), known as the σ-hole, on the outermost portion of the atomic surface, centered on the A-B axis.22-23 Thus, they are denoted as σ-holes, meaning “on the back of” the σ-bond (Scheme 1a). In addition, the π-hole bond (Scheme 1b) has also been studied.24 As stated by Feynman, the only nature of any interatomic interaction is electrostatic, which encompasses many other common contributions, including covalency, London dispersion forces, and polarization.25 Herein, we observed two other types of noncovalent interactions based on a positive hole between two neutral molecules. We term one interaction the κ-hole bond (following the Greek word “κάθετη” for “perpendicular”) because this type of hole is perpendicular to all σ-bonds. The other interaction is denoted the μ-hole bond (following the Greek term “μόνος” for “lone”), which means that the positive hole is centered on the A-lone pair axis (Scheme 1c, d).

Scheme 1. Location of various positive holes. (a) σ-hole, (b) π-hole, (c) κ-hole and (d) μ-hole. The blue ellipses mark the locations of positive holes. Substituent groups are omitted.

RESULTS AND DISCUSSION


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Formation of positive holes. Positive holes (Scheme 1), including the σ-hole, πhole, κ-hole and μ-hole, are the basis for the formation of hole bonding interactions. When an atom (A) bonds with a halogen atom (X), positive charge is more likely to be concentrated on the surface of atom A due to electron attraction from atom X, the highly electronegative atom. In this way, a variety of holes can form. For the first time, we define two new noncovalent interactions, the κ-hole and μ-hole bonding interactions. Detailed information regarding the two new noncovalent interactions is given below. To identify these holes intuitively, ESP maps of electron donors and electron acceptors are presented. The ESP V(r) created at a point by a molecule’s nuclei and electrons can be given rigorously by V(𝑟𝑟) = � 𝐴𝐴

𝑍𝑍𝐴𝐴 𝜌𝜌(𝑟𝑟 ′ )𝑑𝑑𝑟𝑟 ′ −� ′ |𝑅𝑅𝐴𝐴 − 𝑟𝑟| |𝑟𝑟 − 𝑟𝑟|

(1)

where ZA is the charge of nucleus A located at position rA and ρ is the electron density at position r. The former term accounts for the ESP generated by the atomic nuclei, which is positive, and the latter term denotes the ESP of the electron, which is negative. ESP maps of parent components. In this work, 16 EMLs were prepared based on hydrogen bonding interactions, π-π stacking, and σ-hole, π-hole, κ-hole and μ-hole bonding interactions at ambient temperature. ESP maps of each individual component are shown in Figure 1 (names and associated information are given in Table S2). Although the EMLs are formed by mixing two solid components, most of the compounds are liquid under ambient conditions.



Figure 1. M06-2X/def2-pVTZ-calculated ESP maps and structures of parent component. (a) 1,3,5-trimethoxybenzene, (b) iodine monobromide, (c) cyanogen bromide, (d) selenium tetrabromide, (e) phosphorus oxybromide, (f) antimony trichloride, (g) trimethyl tin chloride, (h) tin tetrabromide, (i) gallium trichloride, (j) N-nitrosodiphenylamine, (k) trans-methyl-butenoic acid, (l) diphenylmethanol, (m) diphenylamine, (n) N-phenyl-1-naphthylamine, (o) octafluoronaphthalene, (p) 3,4,5,6-tetrafluorophthalonitrile, (q) 1,4-diiodotetrafluorobenzene, (r) antimony trichloride. It should be noted that r is the bottom view of f. The less obvious hole, the μhole, is indicated by a red circle. The substance represented by a is an electron donor, and the substances represented by b to q are electron acceptors. The magnitude is 0.02 a.u., and the blue shading represents the strength of the positive hole.

1,3,5-Trimethoxybenzene (TB) was chosen as an electron donor because it has both lone-pair and π electrons. Negative charge is more likely to accumulate on three oxygen atoms and a benzene ring (Figure 1a). Compared to compounds that have only lonepair electrons, such as 18-crown-6, TB can form π-π stacking and a variety of σ-hole···π


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interactions. All of these interactions are conducive to forming eutectics. EMLs can be created by simply mixing equal molar amounts of TB and electron acceptors in sample vials, stirring at 20-70 °С for a few minutes until the mixture changes to a liquid and then cooling the liquid to room temperature (Figure 2). In this work, heating was applied to accelerate the formation of liquids. However, many of the EMLs can be obtained via stirring at room temperature. σ-Holes in molecules. Halogen bonds and hydrogen bonds are relatively common noncovalent interactions. Over the past 50 years, reports on halogen bonds have increased significantly (Figure S2). Since their definition was recommended by the International Union of Pure and Applied Chemistry (IUPAC) in 2013, halogen bonds have become more common.26 Iodine was the first element to inspire researchers to explore halogen bonds because it is simple to form a σ-hole on the surface of an iodine atom. It is generally accepted that the sequence of ability to form a halogen bond is At, I, Br, Cl, and F.27-28 Iodine monobromide (1) (Figure 1b) possesses two types of σ-holes, which are both categorized as σVII-holes (a σVII-hole is a σ-hole on the surface of a Group VIIA atom; similar definitions apply for σVI-holes, σV-holes and σIV-holes). These two σ-holes are more “natural” than that in cyanogen bromide (2) (Figure 1c). In 2, there is a strong electron-withdrawing group, cyano, which attaches to a bromine atom. The electronegativity of carbon is less than that of nitrogen and bromine, so positive charges are more likely to concentrate on carbon atoms. However, there is only a slight positive charge on C atoms; for this reason, a σ-hole can form more easily on bromine, as a halogen atom, compared to tetrel atoms. Similar to halogen atoms, electropositive regions may appear around chalcogen atoms (O, S, Se, Te, Po) when they bond to strong electron-withdrawing groups. In most reports, the chalcogen atoms in chalcogen bonds are sp2-hybridized, with one σVIhole.17 In other reports, the sp3-hybridized chalcogen atoms engaged in chalcogen bonds have two covalent bonds and two lone pairs, which exhibit two σVI-holes.29 However, in selenium tetrabromide (3), the hybridization is neither sp2 nor sp3. According to valance shell electron pair repulsion (VSEPR) theory, selenium tetrabromide should be sp3d-hybridized. The geometry of 3 is a seesaw, and interestingly, the surface also shows two σVI-holes, similar to the sp3-hybridized case. In the molecular ESP map (Figure 1d), there are obvious differences between two longitudinal and two parallel bromines, which can be explained by the molecular orbit of 3 (Figure S3d). In general, pnicogen atoms (N, P, As, Sb, Bi) and tetrel atoms (C, Si, Ge, Sn, Pb) are more likely to form σ-holes when the compounds are sp3 hybridized.30 In phosphorus oxybromide (4), there is a region of positive molecular ESP collinear with and opposite to the P=O bond (Figure 1e). This region may be caused by the electrostatic attraction of an oxygen atom and three bromine atoms to the phosphorus atom. In antimony trichloride (5), the positive regions are larger and more obvious than those in 4 (Figure 1f). The electronegativity of antimony is 1.96 (in Pauling electronegativity), while the electronegativity of chlorine is 3.16, with a difference of 1.20. Similarly, the difference in ESP between phosphorus and bromine is 0.77, which explains why the σ-hole is more



obvious in 5 than in 4. In the ESP maps of trimethyl tin chloride (6) and tin tetrabromide (7), there are one and four σIV-holes, respectively (Figure 1g, 1h). These σIV-holes arise from halogen atoms. κ-Holes in molecules. Boron belongs to the IIIA group of the periodic table. The discovery of the structure of the borane compound was a complex process, which may be due to the electrical complexity of the boron compound.31 However, no reports on σ-holes in IIIA group (B, Al, Ga, In, Tl) elemental compounds have been published. Boron group atoms have an ns2(n-1)p1 valence-shell configuration, where n is the principal quantum number and usually engage in chemical bonding as sp2 hybridization. As a result, the hybridized molecules are electron-deficient compounds (6 electrons), which are also the exception to the 8-electron rule. Thus, positive charge is more likely to concentrate in the center of sp2-hybridized molecules, such as gallium trichloride. Unlike σ-holes, the positively charged region on the ESP surface of this molecule is not on the outermost portion of the gallium surface but is perpendicular to the A−B axis (Scheme 1c, Figure 1i). We denote this type of hole as a “κ-hole” because it is perpendicular to all σ bonds in molecules. Currently, κ-hole bonding interactions are sometimes called “charge transfer” or “coordination interactions”.32-33 The electron donor and acceptor system are called donor-acceptor complexes. However, the nature of the attraction in a donor-acceptor complex is not a true chemical bond. Thus, the charge transfer is weaker than that in covalent interactions. For this reason, we use the term “κ-hole” because the so-called charge transfer is also a type of noncovalent interaction and is essentially similar to the σ-hole bonding interaction. Hydrogen in molecules. Hydrogen bonds are very common and widely known. The aromatic-aromatic interaction or π–π stacking is an important noncovalent intermolecular force similar to the hydrogen bonding interaction.34-35 In this work, Nnitrosodiphenylamine (9) was chosen as an electron acceptor to interact with TB via the π-π stacking interaction. We take (E)-2-methyl-2-butenoic acid (10) as an example (Figure 1j). The ESP map shows that the negative charge is primarily concentrated on the oxygen while the positive charge is primarily concentrated on the hydrogen (Figure 1k). Moreover, we can study the π-π stacking interaction along with the hydrogen bond by controlling the molecular structure. Thus, diphenylmethanol (11), diphenylamine (12) and N-phenyl-1-naphthylamine (13) were chosen to interact with TB. In benzene, carbon with a π electron is more likely to accumulate a negative charge. Hydrogen, on the periphery of the aromatic circle, is positive in the ESP maps (Figure 1l-n). Nitrogen and oxygen can withdraw electrons from the aromatic system; thus, they show a high electron density in ESP maps. π-Holes in molecules. When hydrogen is the only substituent in an aromatic ring, the electron cloud of a conjugated large π-bond is evenly distributed above and below the ring plane, and the quadrupole moment is negative. However, the quadrupole moment can be adjusted by a substituent of cyclic carbon.24, 36 The opposite characteristics of benzene and hexafluorobenzene in ESP maps show that the latter is a classic example of a π-hole (Figure S4). Thus, octafluoronaphthalene (14), 1,2-dicyano3,4,5,6-tetrafluorobenzne (15) and 1,2,4,5-tetraflouro-3,6-diiodobenzene (16) were


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chosen as electron acceptors (Figure 1o-q). μ-Holes in molecules. After repeatedly observing 5, we noticed that the σ-hole (including the σV-hole and σVII-hole) was not the only type of positive hole in the ESP map (Figure 1r). The new type of hole is termed the μ-hole, which is centered on the Alone pair axis, rather than a covalent bond axis. Interestingly, some pnicogen compounds (such as PCl3) have been studied, but the μ-hole has been ignored.37 Previous researchers have observed only the σ-hole in target molecules. Notably, an apparent μ-hole appears under the following two conditions: 1, the central atom is a VA group atom that is sp3 hybridized; 2, all substituent groups are atoms that are more electronegative than the central atom. Otherwise, it may be difficult to verify whether the positive hole originates from the central atom. For example, when a phosphine molecule interacts with an ammonia molecule, it is difficult to determine whether hydrogen or phosphorus interacts with nitrogen. Although the μ-hole is inferior and insignificant compared to the σV-hole in 5 (Figure 1f, 1r), it is indeed a kind of positive hole that can form noncovalent interactions with electron-rich groups. This finding can be applied to enrich noncovalent interactions. Table 1. Tg and Tm of EMLs TB+1 to TB+16.[a] Compound

Tm (°С)[b]

System

Tg (°С)

Tm (°С)

TB 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

54.5 ̶ 52 123 55-56 73.4 37.5 31 77.9 66.5 64.5 69 53-54 61 87.5 83 109-111

̶ TB+1 TB+2 TB+3 TB+4 TB+5 TB+6 TB+7 TB+8 TB+9 TB+10 TB+11 TB+12 TB+13 TB+14 TB+15 TB+16

̶ -44.3 -46.7 -37.5 -80.3 -37.6 ̶ ̶ -51.1 -70.9 ̶ ̶ -68.6 -59.3 ̶ ̶ ̶

̶

[a] All

̶ ̶ ̶ ̶ ̶ 18.9 20.9 ̶ 28.3 35.4 30.7 20.7 29.2 30.2 40.1 38.1

values are obtained from the rising temperature curve. [b] Obtained from Web of Science.

Experimental characterization of noncovalent interactions in 16 EMLs. Thermal gravimetric analysis (TGA) indicates that the EML TB+5 (molar ratio 1:1) is less stable than its parent components (Figure 2a), which is powerful evidence for the formation of an EML. The intrinsic mechanism governing this phenomenon is the strong intermolecular interactions between TB and 5. Scheme 1c shows that the crystal



structures of TB and 5 are destroyed when the EML is formed. Compared to a liquid, a crystalline structure usually undergoes a melting process before weight is lost. Thus, for a temperature increase of 10 °C/min, weight loss of the crystalline structure requires a higher temperature (more energy). The Tonset values of other EMLs can be seen in Table S3 and Figure S5. Differential scanning calorimetry (DSC) curves show the EML composed of 5 and TB (molar ratio 1:1) does not have a melting point (Tm) in the exact sense, but only a glass transition temperature (Tg) (Figure 2b). This behavior is similar to that of DESs, and multicomponent eutectic systems may possess this feature.38 EMLs possess either a melting point, a glass transition temperature, or both (Figure S6). The Tm/Tg values of EMLs are lower than the melting point of each individual component (Table 1). In order to further prove these eutectic systems, the phase diagrams of some systems were obtained (Figure S7). As discussed above, halogen contributes to the formation of positive holes. Thus, inorganic compounds with multi-halogen substituents were chosen for investigation. However, some of these compounds are commonly used for substituent reactions.39 It is important to identify whether TB reacts with these electron acceptors after they form EMLs. Fortunately, UV-vis spectroscopy and nuclear magnetic resonance (NMR) experiments demonstrated that in most EMLs, no obvious chemical reactions can be observed.


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Figure 2. Thermal and dynamical characterization of EML. (a) TGA curves of TB, 5 and TB+5. (b) DSC curves of TB+5. The glass transition temperature is marked by an arrow. (c) UV-vis spectra of TB and TB+5. The solvent is CH2Cl2. (d) 13C NMR spectra of TB and TB+5. The solvent is CD2Cl2. (e) IR spectra of 5, TB and TB+5 (molar ratio 1:1 and 1:3). (f) Dynamic IR spectra of TB+5 (molar ratio 1:3). EML TB+5 evaporates at 1 atm, 25 °С. Curves from top to bottom correspond to the time from 0 min to 210 min, with an interval of 6 min. (g) Contour map and sectional view of variations in dynamic spectra. Red represents the degree of reduction, while purple represents the degree of increase. (h) 3D color mapping of the surface map of dynamic IR spectra. Figure S11a reveals that in the case of natural evaporation, the speed reaches a maximum at 7.5 min, and the rate of decrease was significantly lower after 45 min.

To minimize the effects of solvents, CH2Cl2 was chosen to dissolve the EML systems, because it does not possess active hydrogen or any polar functional groups. Therefore, changes could be considered to arise from noncovalent interactions between electron donors and acceptors. UV-vis spectroscopy shows that in pure TB, two absorbance peaks at 226.4 nm and 266.6 nm are observed, which correspond to π-π* transitions (Figure 2c). In some EML systems, the two peaks overlap; thus, the peak near 266 nm cannot be easily identified. However, in all EML systems, the slight shift approximately 226.4 nm (224.6~226.6 nm, details are shown in Table S5 and Figure S8) indicates the



presence of only noncovalent interactions between electron donors and acceptors. 13 C NMR spectroscopy was employed to further confirm the absence of chemical reactions between the two components in EMLs. Similar to the UV-vis spectra, the characteristic peaks of TB showed little change (Figure 2d, Table S6). This finding further indicates that in most of the EMLs, no obvious chemical reaction occurred between the two parent compounds; therefore, the EMLs are primarily formed by noncovalent interactions. The 13C NMR spectra of 16 EMLs are given in the supporting information (Figure S9). Bonding sites demonstrated by IR spectra. The infrared (IR) spectrum of 5 (antinomy trichloride) is simple. The peak at 3600 cm-1 corresponds to the stretching of Sb−Cl, and the peak at 1610 cm-1 corresponds to the bending of Sb−Cl (Figure S10b). In TB, the band at 1600 cm-1 corresponds to the typical mode of C=C (aromatic carbon) stretching, while in TB+5 (1:1), there are two peaks near 1600 cm-1 (Figure 2e). To correctly distinguish and assign the two peaks near 1600 cm-1 in TB+5 to Sb−Cl bending and C=C (aromatic carbon) stretching, the IR spectra of TB+5 with molar ratios 1:1 and 1:3 were recorded. The result showed that as the ratio of 5 increases, the intensity of the peak at 1635 cm-1 increases dramatically. This result confirms that the typical mode of C=C at 1605 cm-1 shifts to 1600 cm-1 in TB+5 (results for ratio 1:1 and ratio 1:3 are almost the same). This result may arise because when 5 is added, the nucleophilic C=C interacts with the positive σ-hole. The positive hole lowers the electron density of the benzene ring, so the C=C stretching is weakened. This result is consistent with the change in electron donor in DESs, that is, the hydrogen bond acceptor.40 Other EML systems were also tested by IR spectroscopy, and different degrees of redshifting were found (details are given in Figure S10 and Table S7). The intensity of the IR absorption band depends on the variation of the dipole moment during vibration. The vibration of a polar bond can induce a change in the dipole moment. Since TB is a highly symmetrical molecule, if its three methoxy groups have the same mode of stretching vibration, there is no change in dipole moment and no IR absorption occurs. In pure TB, the band at 1206 cm-1 corresponds to the typical stretching mode of C−O. The band splits into two bands because the vibration mode of TB changes after the EML is formed. The asymmetry of the vibration is increased, and the instantaneous dipole moment is changed. This striking change indicates that C−O is another site at which the σ-hole interacts. TGA curves (Figure 2a) indicate that the EMLs may possess a relatively poor thermal stability. Considering the weight loss mode of DESs, in situ experiments were carried out to further explore the interactions in EMLs. EML TB+5 with a high molar ratio presented a more interesting phenomenon compared to that with a low molar ratio (Figure S11b). EML TB+5 (molar ratio 1:3) was placed at ambient temperature and pressure to carry out an in situ IR experiment. As shown in Figure 2f, with the loss of 5, the intensity of the characteristic absorption peak (ν=3599 cm-1) decreases. Surprisingly, the intensity of some peaks of TB (ν1=2834 cm-1, ν2=2966 cm-1) increases. The change in these absorption peaks indicates that the C−H bonds in TB are also affected when TB and 5 form an EML. In the contour map and side view of variations in dynamic IR spectra, red represents the degree of decrease (Figure 2g). Combined


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with Figure 2h (3D view of Figure 2g), the results show that the absorption peak with the strongest increase in intensity corresponds to the absorption of Sb−Cl; the absorption peak with the greatest decrease in intensity corresponds to the absorption of C−H. This result indicates that with the loss of 5, the polarization of C−H in TB becomes stronger, perhaps because the negative charge on the oxygen atom has increased by the removal of positive hole sites. In conclusion, IR spectroscopy reveals that the positive σ-hole primarily affects electron-rich regions in TB, including C=C and C−O. The C−H bonds in the benzene ring also exhibit an effect. In this case, oxygen is the most likely atom to act as a site for hole bonding interactions. Discussion of noncovalent interactions based on DFT calculations. Based on the results obtained by IR spectroscopy, the oxygen atom in TB was chosen to bond with a σ-hole, κ-hole, μ-hole, and hydrogen atom. We proposed that the π-hole interacts with the benzene ring in TB rather than the oxygen atom because of its large steric hindrance. As shown in Figure 3a to 3q, the interaction energies (Eint) range from -3.75 to -23.77 kcal/mol. For the σ-hole bonding interaction, the energy ranges from 11.75 to -4.35 kcal/mol. In the EMLs studied, the bond angle of the σ-hole bonding interaction is greater than 170°, while the bond angle of the hydrogen bond is less than 170° (Figure 3r). The bond lengths of the κ-hole bond and hydrogen bonds are similar, ranging from 1.85 to 2.11 Å. All σ-hole bonds possess a bond length between 2.80 and 3.15 Å, except for 4 (Figure 3s). Interestingly, the lengths and angles of one σ-hole bond (4 and TB) and μ-hole bond are similar. This result may arise because of the relatively large steric hindrance around their center atoms. Compared to that concluded from some published data, the σ-hole bonding interactions studied in this work possess considerable strength. Hydrogen bonding and π-π stacking have similar energies, corresponding to moderate noncovalent interactions.41 The κ-hole bond in this work is the strongest bond, and although the μhole bond is the weakest, it possesses a nonnegligible Eint that is comparable to that of some σ-hole bonds.42-43 The results of density functional theory (DFT) calculations indicate that there are nonnegligible noncovalent interactions between C−O and various positive holes, which is consistent with the conclusion drawn from the IR spectra. Energy analysis was performed to explore the possible forms of interactions between two types of molecules in EMLs, as well as the essence of eutectic phenomena in EMLs. Further discussion on the form of interactions between two single components is provided in the Supporting Information (Figure S12).



Figure 3. Theoretical calculation of various hole bonding interactions. M06-2X/def2-pVTZoptimized geometries of complexes pairing TB with 1 to 16. (a) to (g) present σ-hole bonding interactions; the edges are enclosed by red dotted lines. (h) presents a κ-hole; the edges are enclosed by black dotted lines. (i) presents π-π stacking; the edges are enclosed by green dotted lines. (j) to (m) present hydrogen bonding interactions; the edges are enclosed by blue dotted lines. (n) to (p) present π-hole bonding interactions; the edges are enclosed by yellow dotted lines. (q) presents μhole bonding interactions; the edges are enclosed by purple dotted lines. The energy of the interaction (kcal/mol) is listed to indicate the decrease in energy after a dimer complex is formed. The interacting atoms are marked by black dotted lines. The π-hole bond and π-π stacking are marked by gray dotted lines because the bonding sites cannot be precisely determined. (r) Bond


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angle of σ-hole bonds and hydrogen bonds in this work. (s) Scattering plots of stabilization energy and bond length. The black square with a green dotted circle represents the σ-hole bonding interaction between 4 and TB. (t) Molecular structure of 5 and its LUMO, LUMO+1, LUMO+2, LUMO+3. In the ball-stick molecular models, the purple and green balls represent Sb and Cl atoms. The positions of the antimony atoms are marked by gray dotted lines.

Discussion on the formation of σ-holes and μ-holes. Here, we attempt to describe how σ-holes and μ-holes arise in a molecule. By observing the lowest unoccupied molecular orbit (LUMO) of 1-16 (Figure S3a-q), one can find that the location of a σ-hole coincides with a distribution of LUMO, which means that the σhole corresponds to the LUMO. The highest occupied molecular orbit (HOMO) has the highest electron energy and is the least restrained; thus, it is very active. The LUMO has the lowest energy among the unoccupied orbits and can easily accept electrons.44 Therefore, the HOMO and LUMO determine the electron gain and loss (transfer ability) of molecules and important chemical properties such as the spatial orientation of the intermolecular reaction. Just as there is no essential difference between covalent bonds and ionic bonds, there is no clear boundary between covalent bonds and noncovalent interactions. In some cases, very strong hydrogen bonds can match the bond energy of covalent bonds.45 It may be easy to imagine that a covalent bond is formed by electron transfer between the HOMO and LUMO, while a noncovalent interaction is formed by a partial electron transfer between one HOMO and another LUMO. Moreover, this approach can also be used to explain the difference between the two kinds of bromine atoms in 3 (Figure S3d). Here, we discuss the formation of holes based on only the properties of the HOMO and LUMO in the frontier orbital theory (FOT) and do not include reaction conditions such as symmetry matching. The distribution of the LUMO also corresponds well to κ-holes (Figure S3i). However, for μ-holes, the situation is slightly different. It is difficult to observe the LUMO distribution at the center of an Sb atom (Figure 3t). As the energy level rises, there is a greater distribution at the center of the Sb atom, particularly for LUMO+2 and LUMO+3. However, the contributions of LUMO+2 and LUMO+3 are inferior to that of LUMO; thus, the μ-hole is not obvious, and the μ-hole bonding interaction is relatively weak (Figure 1r, Figure 3q).

CONCLUSION In conclusion, 16 EMLs were prepared based on various noncovalent interactions, which include well-known hydrogen bonding interactions, π-π stacking, and σ-hole and π-hole bonding interactions, as well as the newly proposed κ-hole and μ-hole bonding interactions. Importantly, κ-hole and μ-hole bonding interactions were proposed as new forms of noncovalent interactions between two molecules, which broaden our understanding of intermolecular interactions. Most of the studied EMLs exhibit fluidity near room temperature, and DSC analysis demonstrated the formation of EMLs. UVvis and 13C NMR spectroscopy indicated the absence of any obvious chemical reactions in most of the EMLs, further proving that the interactions in these binary systems are noncovalent. Fourier transform IR (FT-IR) and dynamic IR spectra demonstrated that



in EMLs, oxygen atoms are the primary bonding sites of electron acceptors. The results of DFT calculations demonstrated that most of the bond interactions are moderatestrength noncovalent interactions, which lays the foundation for the acceptance of κhole and μ-hole bonds as a bond path. The analysis of LUMO and other orbits provided theoretical support for μ-holes. Numerous other new EMLs can be strategically designed and prepared based on the characteristics of ESP maps of the two parent components. Finally, as a new type of polar liquid complex, EMLs can be applied to material preparation and environmental, biological, and chemical engineering and technology. More importantly, the properties of EMLs can be finely tailored by selecting or even designing appropriate parent compounds to form EMLs for task-specific applications, such as molecular recognition and self-assembly.

MATERIALS AND METHODS Materials. 1,3,5-Trimethoxybenzene (98%, Shanghai Aladdin Bio-Chem Technology Co., Ltd.), iodine monobromide (98%, J&K Scientific Ltd.), cyanogen bromide (97%, Shanghai Aladdin Bio-Chem Technology Co., Ltd.), selenium tetrabromide (99%, J&K Scientific Ltd.), phosphorus oxybromide (99%, J&K Scientific Ltd.), antimony trichloride (99%, Shanghai Aladdin Bio-Chem Technology Co., Ltd.), trimethyl tin chloride (98%, Tokyo Chemical Industry Co., Ltd.), tin tetrabromide (99%, Strem Chemicals, Inc.), gallium trichloride (99.99%, Acros Scientific Inc.), (E)-methyl-butenoic acid (98%, Shanghai Aladdin Bio-Chem Technology Co., Ltd.), N-nitrosodiphenylamine (95%, Shanghai Macklin Biochemical Co., Ltd.), diphenylmethanol (99%, Shanghai Macklin Biochemical Co., Ltd.), diphenylamine (99%, Shanghai Macklin Biochemical Co., Ltd.), N-phenyl-1naphthylamine (98%, Shanghai Aladdin Bio-Chem Technology Co., Ltd.), octafluoronaphthalene (96%, Shanghai Macklin Biochemical Co., Ltd.), 3,4,5,6tetrafluorophthalonitrile (97%, Shanghai Macklin Biochemical Co., Ltd.), and 1,4diiodotetrafluorobenzene (98%, Tokyo Chemical Industry Co., Ltd.) were used without purification. Synthesis. The 16 EMLs were prepared by simple mixing, assisted by heating and stirring in a glove box. Temperature for systems TB+1, TB+4, TB+5, TB+6 and TB+12 is 20 °C. Temperature for systems TB+2, TB+7, TB+9, TB+11, TB+13 and TB+14 is 50 °C. Temperature for systems TB+3, TB+8, TB+10, TB+15 and TB+16 is 70 °C. Instruments and characterization. TGA curves were acquired by using a TGA Q4000 (PerkinElmer Instruments Inc.). The temperature was increased by 10 °C/min to the terminal temperature. The balance gas was nitrogen, with a purge flow of 40.0 mL/min, and the sample gas was nitrogen, with a purge flow of 60 mL/min. The crucibles used in TGA were made of Al2O3. The weight of each sample was approximately 5 mg. DSC measurements were performed in a DSC Q200 (TA Instruments Inc.), with a sample weight of approximately 5 mg. Samples were analyzed in aluminum hermetic crucibles. The unit was calibrated with indium and zinc standards.


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The temperature was increased by 20 °C/min to the terminal temperature, with helium at 25.0 mL/min and nitrogen at 25.0 mL/min. UV-vis absorption spectra were recorded on a Shimadzu UV3600 spectrophotometer, and 13C NMR spectra were recorded on a Bruker 400 spectrometer. FT-IR and dynamic IR spectra were acquired by a Bruker Tensor 27. Solid samples were mixed with KBr and tested, while liquid samples were tested after being dropped on a blank KBr substrate. Dynamic IR spectroscopy was performed in a closed, water-free environment. TGA and DSC curves and IR and dynamic IR spectra were drawn using the software Origin8.0 based on original data. DFT calculations. We studied the structures and stabilities of 16 EMLs using DFT calculations. We chose the M06-2X function, which has been shown to give good results for noncovalent interactions, combined with the def2-TZVP basis set to fully optimize the structures, which were confirmed as true minima by frequency calculations.46-50 The interaction energies (Eint) of complexes A···B were calculated by the equation Eint=E(AB) - E(A) - E(B). All DFT calculations were performed with the Gaussian 09 package, revision C01.51 ESP maps were built on isosurfaces with an electron density of 0.02 au using GaussView.52

ASSOCIATED CONTENT Supporting Information. Experimental and computational details, discussion of σholes, detailed analysis of TGA and DSC curves, UV and IR spectra, 13C NMR spectra, additional strategically designed EMLs and LUMO of compound TB, HOMO of compound 1-16 (PDF)

AUTHOR INFORMATION Corresponding Author *Tel: 8610-62514925. E-mail: [email protected]. ORCID Tiancheng Mu: 0000-0001-8931-6113. NOTES The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (21773307).

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A Strategy to Form Eutectic Molecular Liquids Based on Noncovalent

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