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Article

Selective Benzene Recognition in Competitive Solvent System (Cyclohexene, Cyclohexane, Tri- and Hexafluorobenzenes) Using Perfluorinated Dinuclear Cu(II) Complex

by
Kazuki Shiomoto
,
Nanako Oimatsu
,
Satoshi Hirano
and
Akiko Hori
*
Department of Applied Chemistry, Graduate School of Engineering and Science, Shibaura Institute of Technology, Fukasaku 307, Minuma-ku, Saitama 337-8570, Japan
*
Author to whom correspondence should be addressed.
Crystals 2025, 15(4), 322; https://doi.org/10.3390/cryst15040322
Submission received: 10 March 2025 / Revised: 25 March 2025 / Accepted: 25 March 2025 / Published: 28 March 2025
(This article belongs to the Special Issue Crystallisation Advances)
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Abstract

:
The selective adsorption and separation of benzene from structurally similar six-membered hydrocarbons and fluorocarbons remain a significant challenge due to their comparable physical properties. In this study, we investigated the molecular recognition and separation properties of a perfluorinated triketonate Cu(II) complex (1) as a Nonporous Adaptive Crystal (NAC). In addition to the previously reported benzene (2)-encapsulated crystal of 1•(2)3, we report here the crystal structures of guest-free 1 and cyclohexene (3)-encapsulated 1•(O)2•3, where (O)2 represents two water molecules. Single-crystal analysis demonstrated that 1 selectively encapsulates 2 while excluding other hydrocarbons, including 3, cyclohexane (4), trifluorobenzene (5), and hexafluorobenzene (6). Gas adsorption experiments confirmed this high affinity for 2, as reflected in its preferential adsorption behavior in mixed solvent and vapor environments. The molecular selectivity of 1 was attributed to strong π-hole···π and metal···π interactions, which favor electron-rich aromatic guests. Additionally, crystallization experiments in competitive solvent systems consistently led to the formation of 1•(2)3, reinforcing the high selectivity of 1 for 2. These findings highlight the unique molecular recognition capabilities of NACs, providing valuable insights into the rational design of advanced molecular separation materials for industrial applications involving aromatic hydrocarbons. Hirshfeld surface analysis revealed that the contribution of F···F interactions to crystal packing decreased upon guest recognition (48.8% in 1, 34.2% in 1•(O)2•3, and 22.2% in 1•(2)3), while the contribution of F···H/H···F interactions increased (8.6% in 1, 22.2% in 1•(O)2•3, and 35.4% in 1•(2)3). Regarding Cu interactions, the self-assembled columnar structure of 1 results in close contacts at the coordination sites, including Cu···Cu (0.1%), Cu···O (0.7%), and Cu···C (1.3%). However, in the guest-incorporated structures 1•(O)2•3 and 1•(2)3, the Cu···Cu contribution disappears; instead, 1•(O)2•3 exhibits a significant increase in Cu···O interactions (1.2%), corresponding to water coordination, while 1•(2)3 shows an increase in Cu···C interactions (1.5%), indicative of the metal···π interactions of benzene.

1. Introduction

Benzene (C6H6) is a fundamental unit of aromatic compounds [1,2] and a crucial raw material in the chemical industry, widely used in the synthesis of various organic compounds [3,4,5,6,7]. However, its separation from other six-membered hydrocarbons, such as cyclohexene and cyclohexane, remains challenging due to their similar physical properties, including vapor pressure and polarity [8,9,10,11,12,13]. Conventional separation techniques, such as distillation and solvent extraction, are often ineffective. Therefore, the development of molecular crystals capable of selectively adsorbing and separating benzene is expected to contribute to improved energy efficiency and simplified separation processes. Various porous materials, such as Metal–Organic Frameworks (MOFs), have been reported for benzene [14,15,16,17,18], cyclohexene, and cyclohexane encapsulations [19,20,21,22]. However, in recent years, the development of more advanced molecularly selective adsorption and separation materials has focused on Nonporous Adaptive Crystals (NACs) [23,24,25]. NACs, despite lacking permanent voids, undergo dynamic structural changes upon exposure to specific guest molecules, enabling selective molecular recognition and encapsulation within the crystal lattice. For example, Huan et al. reported the NACs capable of separating six-membered hydrocarbons with similar physical properties, including benzene, cyclohexene, and cyclohexane, which highlights the potential of NACs as a promising approach for molecular separation [8,25].
Meanwhile, the separation of benzene from fluorinated aromatic compounds, such as trifluorobenzene and hexafluorobenzene, also presents an intriguing challenge due to their similar physical properties and shared aromaticity. These fluorinated compounds differ in their electronic nature; trifluorobenzene is electronically neutral, whereas hexafluorobenzene possesses an electron-deficient aromatic ring as a result of the electron-withdrawing effect of fluorine atoms. This electron deficiency leads to the formation of π-holes [26,27,28,29,30,31], which are positively charged regions on the molecular surface. These π-holes can interact electrostatically with electron-rich aromatic molecules such as benzene, offering a promising strategy for selective molecular recognition and separation in mixed systems. Consequently, the development of adsorption materials capable of efficiently separating benzene from fluorinated aromatic compounds could significantly enhance organic synthesis processes and separation technologies involving fluorinated compounds. In this study, we utilize the nonporous molecular crystal of a perfluorinated triketonate Cu(II) dinuclear complex 1 [31,32] as a NAC to elucidate the co-crystallization behavior and separation properties of benzene (2) in competitive solvent environments, including cyclohexene (C6H10, 3), cyclohexane (C6H12, 4), trifluorobenzene (C6H3F3, 5), and hexafluorobenzene (C6F6, 6), as shown in Figure 1. The comparison of guest molecules 2–4 serves as a good example for elucidating whether aromaticity plays a crucial role in molecular recognition. Additionally, this comparison is significant in the industrial process of hydrogenating benzene to synthesize cyclohexene (3, b.p. 82 °C), where the byproduct cyclohexane (4, b.p. 81 °C) and the raw material benzene (2, b.p. 80 °C) need to be separated from the cyclohexene product [2,8,9,10,11,12,13]. Furthermore, the comparison of the aromatic guests, 2, trifluorobenzene (5, b.p. 76 °C), and hexafluorobenzene (6, b.p. 81 °C), can clarify the electrophilic nature of the π-hole characteristics of the host molecule. It also helps to determine whether the π-hole···π interaction preferentially stabilizes electron-rich aromatic compounds with the negative quadrupole moment Qzz, such as compound 2 [26,33,34,35]. By investigating the role of crystal structure and electrostatic interactions in the selective adsorption and separation of six-membered hydrocarbons, this study aims to provide insights into the rational design of highly selective molecular separation materials.

2. Materials and Methods

2.1. General

Complex 1 was prepared using previously reported protocols [31] with the corresponding ligand [36]. Solvents were of reagent grade and were used without further purification. Single crystal 1•(2)3 (CCDC1900742) was reported from the crystallization of a CH2Cl2 solution of 1 with 2 [31]. The thermogravimetric (TG) analysis was performed using a TA TGA-Q500 (TA Instruments, Tokyo, Japan). Gas adsorption isotherms were obtained using BELSORP MAX II (MicrotracBEL Corp., Osaka, Japan). The ESP was calculated by DFT using the Spartan’20 package (V1.1.4) with ωB97X-D/6-31G* (Wavefunction Inc., Tokyo, Japan).

2.2. Quadrupole Moments and Electrostatic Potential of 2–6

The quadrupole moments along the principal axis (Qzz) [26,33,34,35] and electrostatic potentials (ESPs) provide complementary insights into the electronic environments of the studied guest molecules 2–6. The Qzz values quantify the electron density distribution along the principal axis, while ESPs visualize the electrostatic potential surrounding the molecule, highlighting regions of electron richness or deficiency (Figure 2). The ESP at the ring center of 2, calculated using density functional theory (DFT) at the ωB97X-D/6-31G* level, is −94 kJ mol−1 (red region), with a quadrupole moment of −29.0 × 10−40 C m2 [26]. For 3, an electron-rich site located above the double bond (red region) exhibits an ESP of −97 kJ mol−1, indicating the delocalization of electron density. The ring center of 3 also exhibits an electron-rich bias, reaching a maximum of −38 kJ mol−1, due to π-bond delocalization. In contrast, 4 shows minimal charge polarization, with an ESP of approximately −7 kJ mol−1 at the ring center. The hydrogen atoms, which carry partial positive charges (ESP of +34 kJ mol−1), protrude outward, leading to an overall electron-deficient molecular surface for the principal axis (C3 rotate axis). The degree of fluorination significantly influences both Qzz and ESP values. The ESP at the ring center of 5 is relatively small (ESP = +5.0 kJ mol−1, Qzz = 1.9 × 10−40 C m2), whereas 6 exhibits a strongly electron-deficient π-hole character (ESP = +97 kJ mol−1, Qzz = +31.9 × 10−40 C m2) [26,33].

2.3. Crystal Structure Determination

The single crystal X-ray structures were determined by a Bruker D8 QUEST diffractometer with a graphite monochromator and MoKα radiation (λ = 0.71073 Å) generated at 50 kV and 1 mA. Crystals were coated with paratone-N oil and measured at 173 K. The SHELXT program was used to solve the structures 2018/2 [37]. Refinement and further calculations were carried out using SHELXL 2019/1 [38]. The crystal data and structure refinements of new crystals, 1 and 1•(O)2•3, are summarized in Table 1 with the previously reported data of 1•(2)3 [31] and Appendix A. In the crystal structure of 1•(O)2•3, two water molecules were modeled based on electron density for the oxygen atoms, while the hydrogen atoms could not be located and were not included in the refinement. All H atoms were placed in geometrically idealized positions and refined as riding atoms, with aromatic C-H = 0.95 Å and Uiso(H) = 1.2Ueq(C).

3. Results and Discussion

3.1. Preparation and Vapor Adsoption of 1

It has been reported that complex 1 forms crystalline inclusion compounds with benzene 2 and methylated derivatives, such as toluene, xylene, mesitylene, durene, and anisole. Typically, when a CH2Cl2 solution of 1 was crystallized under benzene vapor diffusion conditions, green prismatic crystals of 1•(2)3 were exclusively obtained, incorporating three benzene molecules per unit of 1. The high affinity of 1 for benzene is attributed to the π-hole of the pentafluorophenyl groups and the enhanced electrophilicity of the Cu(II) centers, which preferentially interact with aromatic guest molecules like 2. The complex was purified by extraction with CHCl3/H2O following the literature procedure to remove residual metal ion impurities, and was isolated as a pure substance after at least one recrystallization. The resulting crystals were dried at 100 °C for 2 h and used in all experiments.
Immediately prior to the N2 adsorption/desorption measurements, the powder samples were further activated by heating at 100 °C for 3 h under vacuum. The N2 adsorption isotherm for 1 at 77 K follows a type-III profile, yielding 10.5 cm3(STP) g−1 (equivalent to 0.5 molecules per 1) at 0.90 P/P0, and increasing to 109.5 cm3(STP) g−1 (5 molecules per 1) at 0.99 P/P0 (Figure 3a). The desorption curve traced the adsorption, indicating that 1 behaves as a nonporous material, lacking typical features of physisorption or chemisorption. This result is consistent with the nature of molecular crystals classified as NACs, and a similar trend has been observed in previously synthesized fluorinated metal complexes [30].
Meanwhile, the vapor adsorption experiments using benzene (2) and hexafluorobenzene (6) clearly demonstrated the unique characteristics of the fluorinated complex 1 (Figure 3b). The benzene vapor adsorption for 1 at r.t. began to increase around 0.10 P/P0, reaching 66.4 cm3(STP) g−1 (3 molecules per 1) at 0.36 P/P0. The adsorption isotherm gradually increased, reaching 87.8 cm3(STP) g−1 (4 molecules) at 0.98 P/P0. The desorption curve exhibited a clear hysteresis, retaining 70.8 cm3(STP) g−1 (3 molecules), even at 5 × 10−3 P/P0. In contrast, when using hexafluorobenzene, 1 showed minimal adsorption up to approximately 0.65 P/P0, followed by a gradual increase, ultimately reaching 35.4 cm3(STP) g−1 (1.6 molecules) at 0.98 P/P0. The desorption process also exhibited hysteresis, retaining 20.4 cm3(STP) g−1 (1 molecule) down to approximately 0.2 P/P0. A comparison of the adsorption/desorption isotherms of 2 and 6 on 1 revealed that benzene exhibited adsorption at lower relative pressures and showed a greater overall adsorption capacity. This indicates that 1 has a higher affinity for 2 than for 6. These results are in good agreement with the previously observed crystal structure of 1•(2)3, further confirming the NAC properties of 1, where the encapsulation of 2 is primarily driven by metal···π and π-hole···π interactions [31].

3.2. Single-Crystal Analysis with Guest-Molecules 3–6

This study investigates whether 1 can accommodate cyclohexene (3), which possesses a partially conjugated six-membered ring, and cyclohexane (4), a fully aliphatic hydrocarbon, within its crystal structure. To evaluate this, CH2Cl2 solutions of 1 were mixed with either 3 or 4, followed by crystallization through natural evaporation. As a result, green block-like crystals were obtained from 3, while green needle-like crystals were obtained from 4, both of which were clearly distinct from the prismatic crystal of 1•(2)3.
Single-crystal X-ray diffraction analysis revealed that the green block-like crystals obtained from 3 corresponded to 1•(O)2•3, indicating the successful incorporation of 3 into the crystal lattice. In contrast, the needle-like crystals obtained from 4 corresponded to the pure form of 1, with neither CH2Cl2 nor 4 incorporated. The calculated densities (Table 1) show a decreasing trend upon guest inclusion: 2.106 g cm−3 for 1, 1.895 g cm−3 for 1•(O)2•3, and 1.705 g cm−3 for 1•(2)3. This density reduction reflects the expansion of the lattice to accommodate guest molecules and is consistent with the adaptive behavior of NACs. The relationship between crystal morphology and lattice parameters is summarized in Table 2. Similarly, crystallization was attempted using solutions containing 1,3,5-trifluorobenzene (5) and hexafluorobenzene (6), which are structurally and physically similar to benzene. In this case, only needle-like crystals were obtained, and structural analysis confirmed that these corresponded to pure 1, indicating that neither 5 nor 6 was incorporated. In NACs, molecular recognition occurs when host–guest interactions dominate over host–host interactions. The comparison of guest molecules 2–4 demonstrated that aromaticity plays a crucial role in molecular recognition. Furthermore, the fact that only 2 was selectively incorporated among aromatic guests 2, 5, and 6 suggests that the host’s π-hole is highly electrophilic, favoring interactions with electron-rich aromatics such as benzene via π-hole···π interactions.

3.3. Crystal Structure of 1

The crystal structure of non-solvated 1, with atom-labeling schemes, is shown in Figure 4a. This crystal, appearing as green needles, was obtained by natural evaporation from a CH2Cl2 solution of 1 with 5. In the crystal structure of 1, the asymmetric unit contains half of the coordination complex 1, which is centrosymmetric and consists of two Cu(II) ions and two ligands, forming a dinuclear Cu(II) complex with square-planar coordination geometries. The intramolecular Cu1···Cu1i distance (symmetric code i: −x + 1, −y, −z + 1) is 3.0282(12) Å. The dinuclear coordination core (Cu2O6) in 1 is planar, and the dihedral angles between the pentafluorophenyl groups (Ring-A and Ring-B) and the coordination plane are 38.49° and 52.28°, respectively, due to the steric hindrance of the ortho-substituted fluorine atoms. The final difference Fourier map exhibited no significant residual electron density, with a highest peak of 0.37 e Å−3 and a deepest hole of −0.49 e Å−3, supporting the absence of guest molecules in the crystal lattice. The structures of two adjacent molecules, viewed from the front and side, are shown in Figure 4b. The molecules exhibit significant overlap, and the corresponding columnar stacking is observed along the b-axis. The offset distance between the two molecules is 3.816 Å. The overlap between Ring-A of the pentafluorophenyl groups involves a slip of 1.540 Å, with a perpendicular distance of 3.4918(18) Å, while the overlap between Ring-B involves a slip of 1.549 Å, with a perpendicular distance of 3.4876(18) Å, approaching the van der Waals radius of C···C (3.4 Å). Additionally, along the a- and c-axes, fluorine atoms are oriented in a manner that fills the intermolecular voids (Figure 4c), resulting in pronounced anisotropy along the b-axis. This structural feature suggests that the crystal preferentially grows along the b-axis, leading to its characteristic needle-like morphology.

3.4. Crystal Structure of 1•(O)2•3

The molecular structure of 1•(O)2•3 is shown in Figure 5a. In the asymmetric unit, the entire complex 1, consisting of two Cu(II) ions, two ligands, and two coordinated water molecules (O7 and O8), was observed along with the solvated guest molecule 3. Unfortunately, the hydrogen atoms of the water molecules could not be assigned, and 3 was modeled as a thermally disordered species with large and distorted thermal ellipsoids. However, the results clearly indicate the 1:1 co-crystallization of 3 in 1, and the presence of two unexpected water molecules, likely incorporated as moisture, was essential for sufficient crystal growth. The Cu(II) ions exhibit a square-pyramidal coordination geometry; the average equatorial bond distance is 1.92 Å for both Cu1-O (O1, O2, O5, O6) and Cu2-O (O2, O3, O4, O5). The axial bond distances for the coordinated water molecules are 2.312(5) Å (Cu1-O7) and 2.463(5) Å (Cu2-O8), which are characteristic of the Jahn–Teller effect in the Cu(II) complex. The intramolecular Cu1···Cu2 distance is 3.0372(7) Å. For the molecular structure of 3, the C36-C37 bond is the shortest at 1.283(13) Å, indicating a localized double bond; the other bond lengths of C35-C36, C37-C38, C38-C39, C39-C40, and C40-C35 are 1.368(10), 1.376(18), 1.50(2), 1.475(18), and 1.512(13) Å, respectively. The supramolecular arrangement of the crystal is shown in Figure 5b, indicating a dimeric form of two complexes. The intermolecular distance Cg(Cu2O2)···Cg(Cu2O2) is 4.1234(15) Å, with a corresponding slippage of 2.558 Å, forming a zig-zag arrangement along the c-axis. The cyclohexene was incorporated diagonally above the coordination site without metal···π or π-hole···π interactions. However, two hydrogen atoms, H35A and H35B, on C35, closely interact with O8 (the coordinated water molecule) through pseudo-hydrogen bonds. A detailed discussion of these interactions is hindered by the disorder of the solvated molecule 3.
In the TG analysis, dried solid samples of 1 were exposed to the vapor of 3 for 24 h. To eliminate the influence of 3 adhering to the crystal surface, the samples were naturally dried at room temperature for 3 h before measurement. The results showed a mass loss of −10.8% up to 95 °C, which closely matched the theoretical total mass loss of −10.5% corresponding to the desorption of two water molecules and one molecule of 3 (Figure 6). This agreement strongly indicates that 1•(H2O)2•3 is the primary product. The simultaneous desorption of water along with 3 aligns well with the absence of the aqua complex of 1•(H2O)2 as an experimentally observed intermediate. This finding suggests that, in this system, the four-coordinate structure is fundamentally more stable than the five-coordinate alternative. Furthermore, the presence of water appears to assist in the formation of single crystals by filling voids in the crystal packing during the encapsulation of 3. In the crystal structure of 1•(H2O)2•3, both water and 3 are accommodated in spatially separated environments and do not form strong interactions with the host framework or with each other, which likely facilitates their cooperative desorption as the crystal lattice relaxes upon heating.

3.5. Hirshfeld Surface Analysis of 1, 1•(O)2•3, and 1•(2)3

To better understand intermolecular interactions, Hirshfeld surface (HS) analysis [39,40] was performed for each complex using Crystal Explorer 17.5 [41]. The results of dnorm-mapped surfaces (dnorm is a normalized contact distance used to highlight significant intermolecular interactions, where red regions indicate shorter contacts than the van der Waals sum) and the front and back views of shape index-mapped surfaces (used to visualize complementary concave and convex features at contact regions) for 1, 1•(O)2•3, and 1•(2)3, along with their corresponding fingerprint plots, are shown in Figure 7. In the dnorm-maps, red spots indicate regions of short intermolecular contacts, indicating notable intermolecular interactions. The prominent red regions are observed in F···F interactions in 1, coordinated water molecules in 1•(O)2•3, and interactions between fluorine atoms of the pentafluorophenyl groups and hydrogen atoms of benzene in 1•(2)3. The corresponding fingerprint plots illustrate the overall molecular interaction landscape and the specific contributions from the Cu(II) center and F···F interactions. These calculations were conducted by selecting a single unit of 1 and analyzing its interaction contributions with the surrounding molecules. Regarding Cu interactions, the self-assembled columnar structure of 1 results in close contact at coordination sites, including Cu···Cu (0.1%), Cu···O (0.7%), and Cu···C (1.3%). However, in the guest-incorporated structures 1•(O)2•3 and 1•(2)3, the Cu···Cu contribution disappears. Instead, 1•(O)2•3 exhibits a significant increase in Cu···O interactions (1.2%), corresponding to coordination to water molecules, while 1•(2)3 shows an increase in Cu···C interactions (1.5%), indicative of metal···π interactions with benzene. The Cu···H interactions observed in 1•(O)2•3 and 1•(2)3 do not arise from electrostatic properties of the Cu center; instead, they result from the broad dinuclear coordination plane interacting with hydrogen atoms of the guest molecules positioned above it. Guest recognition significantly reduces the contribution of F···F interactions to crystal packing (48.8% in 1, 34.2% in 1•(O)2•3, and 22.2% in 1•(2)3), while F···H/H···F interactions increase (8.6% in 1, 22.2% in 1•(O)2•3, and 35.4% in 1•(2)3). This trend suggests that electron-rich fluorine atoms repel each other, leading to a preferential interaction with hydrogen atoms of electron-deficient hydrocarbons, facilitating guest recognition. Moreover, not only does the relative contribution of F···F interactions decrease, but the reduced sharpness in the fingerprint plots also indicates an overall weakening of interactions between the complex units.
In the shape index maps, the red and blue regions correspond to concave (hollow) and convex (bump) areas, respectively. Pairs of complementary red and blue triangles, often forming hourglass-like patterns, are indicative of close intermolecular contacts. In crystal 1, the shape index is equivalent on the front and back, with complementary red and blue triangles distributed on the surface. In crystal 1•(O)2•3, a red concave is observed on the front side where water and 3 molecules are present, indicating the proximity of guest molecules, while complementary red and blue triangles are observed on the back side due to dimerization. In crystal 1•(2)3, clear red concaves are observed on both sides where benzene molecules are located. In addition, the pentafluorophenyl groups show red and blue hourglass-shaped patterns in all structures, indicating complementary intermolecular contacts likely involving π-interactions. The C···C contacts, which are indicative of aromatic overlap, further support this interpretation; their contributions are 9.3% in 1, 4.7% in 1•(O)2•3, and 6.0% in 1•(2)3, suggesting the presence of infinite columnar stacking in 1, dimer formation in 1•(O)2•3, and host–guest interactions between 1 and 2 in 1•(2)3.

3.6. High Selectivity for Benzene from Mixed Solvents

To compare the affinity of the fluorinated complex in a mixed solvent system, crystallization experiments were conducted using the following three combinations: (1) 2 and 3, both of which have been confirmed to form inclusion compounds; (2) 2, 3, and 4 to compare the differences between aromatic and aliphatic hydrocarbons; and (3) 2, 5, and 6 to examine the effects of quadrupole moments. Each component was mixed in equimolar ratios and subjected to crystallization. As a result, prismatic crystals suggestive of the formation of 1•(2)3 were precipitated in all combinations. This finding indicates that complex 1 exhibits a high affinity for 2 among six-membered hydrocarbons (Table 3). This selectivity is largely attributed to π-hole···π interactions and metal···π interactions between 1 and 2.
Subsequently, interactions between the solid 1 and various vapors were evaluated using a method distinct from crystallization. First, crystalline 1 was heat-dried and sealed in a small vial. Each guest molecule (2–6) was sealed separately in a larger vial, into which the smaller vial was placed. The vial system was sealed and left undisturbed overnight to expose 1 to the guest vapors. The uptake of guest vapors by 1 was then examined using powder X-ray diffraction (pXRD).
The pXRD measurements were conducted using the capillary method with a MoKα X-ray source using the same equipment used for single-crystal X-ray diffraction (scXRD). Unlike conventional open-system measurements, this method is suitable for analyzing inclusion compounds but is characterized by broader peaks due to the influence of the capillary glass. In this study, the peak patterns obtained from scXRD structure analysis were simulated to evaluate structural changes associated with guest uptake. The results showed significant changes in the pXRD peak patterns for the samples exposed to 2 and 3, whereas the peak patterns for the samples exposed to 4, 5, and 6 were almost identical to those of crystalline 1 (Figure 8a). This suggests that 5 and 6 were not incorporated into the crystal structure of 1. In contrast, the mass loss observed in TGA and the peak shifts in pXRD confirmed the uptake of 2 and 3 into the crystal interior. These results are consistent with the findings from the crystallization experiments. Regarding 4, although TGA indicated mass loss, the pXRD peak patterns remained unchanged compared to those of 1, suggesting that 4 was not incorporated into the crystal, but adsorbed on its surface instead. Additionally, it is possible that 4 was adsorbed into microscopic voids created during the desorption of 2. Furthermore, vapor exposure experiments were conducted under mixed guest conditions, including benzene, similar to the crystallization experiments, followed by pXRD analysis. The results showed that in all cases, the observed pXRD patterns were in good agreement with the simulated data based on the co-crystal structure of 1•(2)3. This finding suggests that 1 preferentially incorporates 2 even in mixed vapor environments, demonstrating that among the five six-membered hydrocarbons, 1 exhibits the highest affinity for 2, which is aromatic and has a negative Qzz.
Subsequently, to compare the differences in adsorption under mixed guest conditions, powder samples of 1 were exposed to equimolar mixed solutions of {2 and 3}, {2, 3, and 4}, and {2, 5, and 6}, followed by pXRD measurements (Figure 8b). The selected guest molecules (2 and 3) were previously confirmed to be incorporated into the crystal structure. As a result, in all cases, the pXRD patterns obtained were identical to that of the co-crystal 1•(2)3, where three equivalents of 2 were incorporated into 1. This pattern was also completely consistent with that obtained when 1 was exposed to pure 2 vapor. The pXRD pattern of guest-free crystal 1, marked by the green dashed line, completely disappeared, while the observed pattern precisely matched the simulated pattern based on the scXRD data of 1•(2)3, marked in orange. These results clearly demonstrate that 1 preferentially incorporates 2 even in mixed vapor environments, despite the five guest molecules having nearly identical boiling points and similar polarity. This selectivity highlights the fundamental property of NACs, which selectively open their internal space only for the preferred guest molecules. Furthermore, it provides a rational demonstration that the π-hole generated by fluorinated aromatic substitution selectively recognizes aromatic, electron-rich π-systems.

4. Conclusions

In this study, we explored the selective molecular recognition and adsorption properties of a perfluorinated triketonate Cu(II) complex (1) toward various six-membered hydrocarbons. Single-crystal analysis revealed that 1 selectively incorporates benzene (2) while excluding cyclohexene (3), cyclohexane (4), trifluorobenzene (5), and hexafluorobenzene (6). This selectivity was found to be primarily driven by π-hole···π and metal···π interactions, which enhance the affinity of 1 for electron-rich aromatic guests. Vapor adsorption experiments further demonstrated that 1 preferentially absorbs 2 at lower pressures and exhibits a higher adsorption capacity compared to 6, indicating a strong affinity for benzene in competitive environments. The crystallization experiments reinforced these observations, as prismatic crystals of 1•(2)3 were consistently obtained, even in the presence of multiple guest molecules. In contrast, neither fluorinated aromatics nor aliphatic hydrocarbons were incorporated into the crystal structure of 1, confirming the crucial role of aromaticity in molecular recognition.
Furthermore, TG and pXRD analyses revealed that while 3 was partially adsorbed, 4 remained on the crystal surface without significant incorporation. The results strongly support the idea that 1 functions as a selective molecular separation material, particularly in distinguishing benzene from other six-membered hydrocarbons. Overall, this study provides fundamental insights into the design of NAC-based molecular recognition materials. The exceptional selectivity of 1 for benzene over structurally similar compounds highlights its potential in industrial hydrocarbon separation and recovery. Future work will focus on extending this approach to other aromatic separation systems and optimizing the structural features of NACs to further enhance selectivity and adsorption efficiency.

Author Contributions

Formal analysis, data curation, investigation, validation, writing—original draft preparation, K.S.; data curation, investigation, N.O. and S.H.; conceptualization, writing—review and editing, funding acquisition, methodology, supervision, A.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Grant-in-Aid for Scientific Research B (no. 23K21122) of JSPS KAKENHI.

Data Availability Statement

Crystallographic data have been deposited at CCDC.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre and the deposition CCDC numbers are 2430009 and 2430010 for 1 and 1•(O)2•3, respectively. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/structures/ (accessed on 10 March 2025).

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Crystals 15 00322 g001
Figure 1. Molecular structures of complex 1 and guest molecules 2–6.
Figure 1. Molecular structures of complex 1 and guest molecules 2–6.
Crystals 15 00322 g001
Crystals 15 00322 g002
Figure 2. The electrostatic potential maps of 2–6 from the optimized structures: the color of the potential is shown between −100 kJ mol−1 (red) and +100 kJ mol−1 (blue).
Figure 2. The electrostatic potential maps of 2–6 from the optimized structures: the color of the potential is shown between −100 kJ mol−1 (red) and +100 kJ mol−1 (blue).
Crystals 15 00322 g002
Crystals 15 00322 g003
Figure 3. (a) N2 and (b) benzene and hexafluorobenzene adsorption isotherms at r.t. for 1: N2, black circles; benzene 2, blue square; hexafluorobenzene 6, red triangles; filled, adsorption; open, desorption.
Figure 3. (a) N2 and (b) benzene and hexafluorobenzene adsorption isotherms at r.t. for 1: N2, black circles; benzene 2, blue square; hexafluorobenzene 6, red triangles; filled, adsorption; open, desorption.
Crystals 15 00322 g003
Crystals 15 00322 g004
Figure 4. (a) The molecular structures of 1 at 173 K, showing the atom-labeling schemes and (b) supramolecular association and (c) a part of the packing structure of 1. Displacement ellipsoids are drawn at the 50% probability level. Symmetry code: (i) −x + 1, −y, −z + 1. Color scheme: C, gray; H, white; Cu, orange; F, green; O, red.
Figure 4. (a) The molecular structures of 1 at 173 K, showing the atom-labeling schemes and (b) supramolecular association and (c) a part of the packing structure of 1. Displacement ellipsoids are drawn at the 50% probability level. Symmetry code: (i) −x + 1, −y, −z + 1. Color scheme: C, gray; H, white; Cu, orange; F, green; O, red.
Crystals 15 00322 g004
Crystals 15 00322 g005
Figure 5. Crystal structures of 1•(O)2•3 at 173 K: (a) asymmetric unit and (b) supramolecular association with views from the top and side. Color scheme: C, gray; H, white; Cu, orange; F, green; O, red.
Figure 5. Crystal structures of 1•(O)2•3 at 173 K: (a) asymmetric unit and (b) supramolecular association with views from the top and side. Color scheme: C, gray; H, white; Cu, orange; F, green; O, red.
Crystals 15 00322 g005
Crystals 15 00322 g006
Figure 6. TG curves of 1•(H2O)2•3. The scan rate was 5.0 °C min−1.
Figure 6. TG curves of 1•(H2O)2•3. The scan rate was 5.0 °C min−1.
Crystals 15 00322 g006
Crystals 15 00322 g007
Figure 7. HS analysis mapped with dnorm and shape index, along with corresponding fingerprint plots, showing the contributions of intermolecular interactions involving molecule 1 in the crystals of (a) 1, (b) 1•(O)2•3, and (c) 1•(2)3.
Figure 7. HS analysis mapped with dnorm and shape index, along with corresponding fingerprint plots, showing the contributions of intermolecular interactions involving molecule 1 in the crystals of (a) 1, (b) 1•(O)2•3, and (c) 1•(2)3.
Crystals 15 00322 g007
Crystals 15 00322 g008
Figure 8. pXRD patterns of (a) 1 exposed to guest molecules 2–6, and (b) 1 exposed to guest mixtures {2 and 3}, {2, 3, and 4}, and {2, 5, and 6}. Simulated patterns of 1 and 1•(2)3 are shown in green and orange, respectively. Green dashed lines and orange arrows indicate the main peak positions of 1 and 1•(2)3, respectively.
Figure 8. pXRD patterns of (a) 1 exposed to guest molecules 2–6, and (b) 1 exposed to guest mixtures {2 and 3}, {2, 3, and 4}, and {2, 5, and 6}. Simulated patterns of 1 and 1•(2)3 are shown in green and orange, respectively. Green dashed lines and orange arrows indicate the main peak positions of 1 and 1•(2)3, respectively.
Crystals 15 00322 g008
Table 1. Crystal data and structure refinements for 1 and 1•(O)2•3 with 1•(2)3.
Table 1. Crystal data and structure refinements for 1 and 1•(O)2•3 with 1•(2)3.
1•(2)3 [31]11•(O)2•3
Descriptionprismaticprismaticblock
Chemical formulaC52H22Cu2F20O6C34H4Cu2F20O6C40H14Cu2F20O8
Formula weight1249.771015.451129.59
Crystal systemorthorhombicmonoclinicmonoclinic
Space groupFdd2C2/cP21/n
a [Å]28.776(4)26.567(8)14.3647(9)
b [Å]52.939(7)3.8163(12)10.1169(8)
c [Å]6.3912(9)32.269(9)27.646(2)
β [°]90101.815(9)99.765(2)
V [Å3]9736(2)3202.3(17)3959.5(5)
Z844
Dc [Mg m−3]1.7052.1061.895
μ [mm−1]1.0021.4961.224
F(000)496019762224
Rint0.02990.07150.0556
GOF1.0671.0501.031
R [(I) > 2σ (I)]0.04280.04100.0493
wR (Fo2)0.12020.09490.1484
CCDC No.190074224300092430010
Table 2. Crystal appearances and cell parameters for 1 with guest molecules 3–6. Unit cell parameters were obtained from the integrated data.
Table 2. Crystal appearances and cell parameters for 1 with guest molecules 3–6. Unit cell parameters were obtained from the integrated data.
Guest3456
AppearanceCrystals 15 00322 i001Crystals 15 00322 i002Crystals 15 00322 i003Crystals 15 00322 i004
Descriptionblockneedleneedleneedle
Colordeep greengreengreengreen
Crystal systemmonoclinicmonoclinicmonoclinicmonoclinic
a [Å]14.243(6)26.56(2)26.60(1)26.60(4)
b [Å]10.191(4)3.819(3)3.816(1)3.823(6)
c [Å]27.736(11)32.22(2)32.27(1)32.30(4)
β [°]100.103(12)101.83(2)101.75(2)101.87(3)
V [Å3]3963(5)3198(6)3207(4)3214(13)
Table 3. Crystal appearances and cell parameters for 1 with guest mixtures 2–6. Unit cell parameters were obtained from the integrated data; the volume differences from 2 [=1•(2)3] are attributed to differences in instrumentation [31].
Table 3. Crystal appearances and cell parameters for 1 with guest mixtures 2–6. Unit cell parameters were obtained from the integrated data; the volume differences from 2 [=1•(2)3] are attributed to differences in instrumentation [31].
2 [=1•(2)3]2 + 32 + 3 + 42 + 5 + 6
AppearanceCrystals 15 00322 i005Crystals 15 00322 i006Crystals 15 00322 i007Crystals 15 00322 i008
Descriptionprismaticprismaticprismaticprismatic
Colorgreengreengreengreen
a [Å]28.776(4)28.724(4)28.787(12)28.728(3)
b [Å]52.939(7)53.068(8)53.20(2)53.149(6)
c [Å]6.3912(9)6.4344(10)6.451(3)6.4356(7)
β [°]90909090
V [Å3]9736(2)9808(3)9879(7)9826(2)
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Shiomoto, K.; Oimatsu, N.; Hirano, S.; Hori, A. Selective Benzene Recognition in Competitive Solvent System (Cyclohexene, Cyclohexane, Tri- and Hexafluorobenzenes) Using Perfluorinated Dinuclear Cu(II) Complex. Crystals 2025, 15, 322. https://doi.org/10.3390/cryst15040322

AMA Style

Shiomoto K, Oimatsu N, Hirano S, Hori A. Selective Benzene Recognition in Competitive Solvent System (Cyclohexene, Cyclohexane, Tri- and Hexafluorobenzenes) Using Perfluorinated Dinuclear Cu(II) Complex. Crystals. 2025; 15(4):322. https://doi.org/10.3390/cryst15040322

Chicago/Turabian Style

Shiomoto, Kazuki, Nanako Oimatsu, Satoshi Hirano, and Akiko Hori. 2025. "Selective Benzene Recognition in Competitive Solvent System (Cyclohexene, Cyclohexane, Tri- and Hexafluorobenzenes) Using Perfluorinated Dinuclear Cu(II) Complex" Crystals 15, no. 4: 322. https://doi.org/10.3390/cryst15040322

APA Style

Shiomoto, K., Oimatsu, N., Hirano, S., & Hori, A. (2025). Selective Benzene Recognition in Competitive Solvent System (Cyclohexene, Cyclohexane, Tri- and Hexafluorobenzenes) Using Perfluorinated Dinuclear Cu(II) Complex. Crystals, 15(4), 322. https://doi.org/10.3390/cryst15040322

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