Crystalline Supramolecular Gyroscope with a Water Molecule as an

Jun 5, 2017 - For convenience, we label the phase below and above TC(heating)/TC(cooling) as the α and β phase, respectively. For such phase transit...
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Crystalline Supramolecular Gyroscope with a Water Molecule as an Ultrasmall Polar Rotator Modulated by Charge-Assisted Hydrogen Bonds Wang Li,†,∥ Chun-Ting He,‡,∥ Ying Zeng,† Cheng-Min Ji,§ Zi-Yi Du,*,† Wei-Xiong Zhang,*,‡ and Xiao-Ming Chen‡ †

Key Laboratory of Jiangxi University for Functional Materials Chemistry, College of Chemistry and Chemical Engineering, Gannan Normal University, Ganzhou 341000, China ‡ MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China § Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China S Supporting Information *

Scheme 1. Topological Diagram of the “Stator−Rotator− Stator” (a), “Stator−Rotator···Stator” (b) and “Stator··· Rotator···Stator” (c) Models for Molecular and Supramolecular Gyroscopes, Respectively

ABSTRACT: A new strategy for the construction of crystalline molecular rotors is presented. The combination of a conformation-modifiable macrocyclic host and two cooperative guests affords one supramolecular gyroscopelike compound, (t-BuNH3)(18-crown-6)[ZnCl3(H2O)], in which the coordinated water molecule functions as an ultrasmall polar rotator, revealed by its significant dielectric relaxation and the molecular dynamics simulations. In addition, such a compound can reversibly undergo a polarto-polar phase transition triggered by the changed conformation of the 18-crown-6 host, leading to a switchable on/off rotation of water molecule, well controlled by strength and direction of charge-assisted hydrogen bonds.

aggregate may be generated through a one-pot supramolecular approach, with a self-assembly of the suitable rotator and stator fragments. (ii) Although rotators in the molecular gyroscopes are restricted to some common organic groups such as phenyl, those in supramolecular gyroscopes are not limited to organic but can be extended to some inorganic species, expanding the source of rotators (especially polar rotators). For example, even a coordinated water molecule can function as a dipole rotator. (iii) Within the stator−rotator···stator system, the directional rotating shaft is maintained due to retention of a covalent axle at one side of the rotor. More importantly, the rotational energy barrier may be modulated by noncovalent interaction at the other side of the rotor, such as the hydrogen (H)-bond strength and direction, to control rotational rate and even its on/off states. Until now, although few supramolecular gyroscope-like compounds designed as “stator···rotator···stator” (Scheme 1c) have been reported,29−32 those corresponding to a “stator−rotator··· stator” model are hardly observed. Generally, the two stators in the “stator···rotator···stator” system are structurally identical, whereas those in the “stator−rotator···stator” system can be designed to be different species, facilitating introduction of more variables to control the rotation state of diversified rotators owing to their asymmetrical rotating space. To address the above-mentioned issue, we conceive a tetrahedron-like [MX3(H2O)]− anion can be viewed as a “stator−rotator” fragment, and to search for a well-matched “stator” fragment to construct the expected “stator−rotator···

M

olecular rotors, as a component of artificial molecular machinery, have attracted attention over the past few decades.1−15 The basic structure of a molecular rotor contains a rotatable part (rotator) and an axis of rotation. Sometimes the large piece attached to the axis is also part of the rotor (stator), which remains stationary while the rotator is rotating. Most understanding of molecular rotors has been acquired by synthesizing complex organic compounds with designed structures and studying their properties in solution. Recognizing macroscopic rotors are usually in solid state or anchored on a solid substrate, recent efforts shifted toward such targets. For solid-state molecular rotors, molecular compounds structurally or functionally mimicking macroscopic gyroscope have aroused interest,16−28 which possess a central rotator linked by two covalent rotary axles to two stators at both ends (Scheme 1a). In such “stator−rotator−stator” structure, crystalline rotation space of the rotator could be maintained and protected by the stators. However, it often requires a high-cost and laborious organic synthesis to obtain such compounds. As an alternative, the “stator−rotator−stator” model can be modified to “stator−rotator···stator” (Scheme 1b). Three advantages of the latter model are as follows: (i) Instead of experiencing a multistep organic synthesis route to obtain the complex organic compound, a gyroscope-like supramolecular © 2017 American Chemical Society

Received: March 25, 2017 Published: June 5, 2017 8086

DOI: 10.1021/jacs.7b02981 J. Am. Chem. Soc. 2017, 139, 8086−8089

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Journal of the American Chemical Society stator” structure, we noticed a charge-balanced [R−NH3(18crown-6)]+ macrocyclic group may be a good candidate. Although the R−NH3+ guest anchors on one side of the 18crown-6 host, there is possibility for the conformationally highly diverse 18-crown-6 molecule to bind simultaneously another guest on the other side of its macrocycle, taking into account its number of O atoms as H-bond acceptors. With this strategy, our research yielded a supramolecular gyroscope-like compound (tBuNH3)(18-crown-6)[ZnCl3(H2O)] (1), which exhibits a reversible polar-to-polar phase transition below room temperature. The coordinated water in this compound can function as an ultrasmall polar rotator, with its on/off rotation well controlled by the H-bonding strength/direction that regulated by a conformation-modifiable 18-crown-6 host. The inherent molecular dynamics (MD) of water rotator was uncovered by the variabletemperature dielectric measurements and MD simulations. Compound 1 was synthesized as colorless block-shaped crystals from slow evaporation of the aqueous hydrochloric solution of a mixture of ZnCl2, 18-crown-6 and (t-BuNH3)Cl in the molar ratio of 1:1:1 (see Supporting Information). The thermogravimetric analysis (TGA) curve shows it can be stable up to 93 °C, whereupon a dehydration process occurs (Figure 1).

Figure 2. Hydrogen bonds in 1α at 200 K (a) and 1β at 296 K (b), respectively, and overlay map of molecular units of them, with [ZnCl3(H2O)]− anions overlap (c). Crystallographically disordered water H atoms and related bonds in (b) are shaded in transparent. All H atoms in panel c have been omitted for clarity.

bond donors. The three-in-one “guest−host−guest” complex of (t-BuNH3)(18-crown-6)[ZnCl3(H2O)] is mainly associated by charge-assisted H-bonding interactions, which simultaneously contain two cooperative H-bonds and a weak Coulomb interaction. These complexes are further assembled into a three-dimensional supramolecular aggregate via van der Waals forces (Figure S2). The structural differences between α and β phases can be attributed to a modifiable conformation of the 18-crown-6 host and the synergistic displacements of both its guests, which lead to a symmetry breaking and the resultant structural phase transition. In β phase, a 3-fold axis runs across all three components, i.e., the macrocycle center of 18-crown-6, the C−N bond of (t-BuNH3)+ cation, and the Zn−O bond of [ZnCl3(H2O)]− anion (Figure 2b). The pattern of the O−C−C−O torsion angles in the crown ether ring (labeled as g+, g−, or t if the angle is 60° ± 60°, −60° ± 60°, and 180° ± 60°, respectively33), beginning at O1 and increasing clockwise, is g+g−g+g−g+g−, showing a nonfolding conformation of the macrocycle (Figure S3b). Striking in 1β phase is the disorderly distributed two water H atoms over three symmetry-related sites owing to the crystallographically imposed C3-axis, with a dipole moment nonparallel to the rotation axis, strongly hinting the water molecule is in a dynamic rotation state. From β to α, the C3-symmetrical crown ether ring distorts to some extent, indicated by obvious changes of torsions (Figure S3a), to form an asymmetrical conformation. The pattern of the O−C−C−O torsion angles for the new conformation is changed as g+g+g−g−g+g−. Also, its two H-bond associated guests shift synergistically to cater for the conformational change of the 18crown-6 host, and the rotating water molecule turns stationary, switching to an ordered state. Before and after phase transition, the relative displacement between N1···O1W of the two ion-pair guests is about 0.03 Å. If the [ZnCl3(H2O)]− anions of 1α and 1β phases are fixed at a same location, the overlay map of molecular units indicates the angular deviation of the N1···O1W line after the phase transition is about 8.7° (Figure 2c). Overall, the conformational change of the 18-crown-6 host not only makes itself lose the molecular C3-symmetry but also breaks the crystallographic C3-symmetry of its two H-bond associated guests, leading to a structural phase transition. The rotation of the water molecule in 1β phase and its on/off switch between α and β phases were further confirmed by variable-temperature dielectric spectroscopy. The complex dielectric permittivity (ε = ε′ − iε″, where ε′ and ε″ are the real and imaginary parts, respectively) of 1 was measured over a wide frequency range (from 0.5 to 1000 kHz) as a function of temperature (from 150 to 330 K). As shown in Figure 3, an abrupt step-like anomaly can be perceived at around the phase

Figure 1. TGA curve for 1 (left) and the DSC measurement on a heating−cooling cycle (right).

Differential scanning calorimetry (DSC) curves of 1 show a pair of endothermic/exothermic peaks at heating/cooling runs, revealing it can undergo a reversible structural phase transition below room temperature. For convenience, we label the phase below and above TC(heating)/TC(cooling) as the α and β phase, respectively. For such phase transition, the sharp peak shape, prominent enthalpy change, and a thermal hysteresis of ca. 4 K between the heating and cooling runs reveal the discontinuous character of the transition, indicative of a first-order phase transition. To understand the crystal structures of 1 in both phases, singlecrystal X-ray diffraction was performed for 1 at two different temperatures, revealing it crystallizes in the polar space groups Cc at α phase and R3 at β phase (Table S1). The noncentrosymmetric structures of both phases were further confirmed by its variable-temperature second-harmonic-generation (SHG) signals (Figure S1), which exhibit a SHG efficiency ∼0.8 times that of KDP (KH2PO4). The crystal structures in both phases can be viewed as a three-component host−guest system, containing a 18-crown-6 host, one (t-BuNH3)+ cation guest and one [ZnCl3(H2O)]− anion guest. As excepted, the two ion-pair guests locate just above and below the 18-crown-6 macrocycle, respectively. The 18-crown-6 host provides six O atoms as potential H-bond acceptors, whereas the (t-BuNH3)+ and [ZnCl3(H2O)]− guests contain three ammonium H atoms and two water H atoms, respectively, as H-bond donors (Figure 2a,b and Table S2). For [ZnCl3(H2O)]− anion, the coordination of water molecule makes its H atoms more labile to function as H8087

DOI: 10.1021/jacs.7b02981 J. Am. Chem. Soc. 2017, 139, 8086−8089

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shown in the MD snapshots of 1β at 296 K (Figure 4), the orientational changes of the water H atoms demonstrate the

transition temperature. Notably, significant dielectric relaxation is revealed in β phase.

Figure 4. Snapshots of NVT dynamic simulation for 1β at 296 K (a−h), showing conformational changes over the simulation time, and derived from the MD simulations, several representative conformations of the [ZnCl3(H2O)]− anions are overlapped (i), to display their dynamic rotations (water H atoms are highlighted in ball to guide the eye).

Figure 3. Temperature-dependent ε′ (a) and ε″ (b) at various ac frequencies for powder-pressed pellet sample of 1. Insets in panel b are shown in Arrhenius representation, providing linear fitting of ln(ω) versus 1/Tpeak.

water molecule rotates dynamically around the Zn−O bond, whereas [ZnCl3]− and [(t-BuNH3)(18-crown-6)]+ fragments themselves behave as stators. Analysis reveals water rotation is synergistically accompanied by vibration of the 18-crown-6 molecule, indicated by the somewhat large anisotropic displacement ellipsoids of the 18-crown-6 molecule in the crystal structure of 1β and its overlap map derived from the MD simulations (Figure S7). Especially, the ether O1 atom H-bonded with the water molecule has a more intense vibration than O2 atom H-bonded with the (t-BuNH3)+ group, in favor of the water rotating. When the simulated temperature falls to 200 K, all the atoms only vibrate slightly in the vicinity of their equilibrium positions, without translation or rotation, in accordance with results of structural analysis and variable-temperature dielectric spectroscopy in α phase. To identify quantitatively and statistically locations of the water H atom within 1, the radial distribution function (RDF)40,41 derived from MD simulations were calculated from equation gij(r) = V/(4πr2ΔrNiNj), where r is the distance between species i and j, is the ensemble-averaged number of species j around i within a shell from r to r + Δr, V is the system volume, and Ni and Nj are the numbers of species i and j. Figure S8 shows the g(r) of the water H and Cl atoms around the specified O atom of 18-crown-6 at 200 and 296 K, respectively. The one sharp and two overlap peaks of the H···O curve at 296 K (β phase) reveal the highlighted water H atom has three relatively stable sites (r ≈ 2.0, 3.1, and 3.3 Å, two of which are very close), matching well with optimized H-bond geometries based on crystal structure (three different H···O distances of 2.03, 3.37, and 3.41 Å, see inset map of Figure S8). Thus, the water H atom can move from one to another within its three stable sites. In contrast, the only one peak with a slightly narrower peak width for the H···O curve at 200 K demonstrates water H is relatively static in α phase. In addition, the Cl···O curves at 200/296 K indicate the nonrotating Cl atom in both phases. Overall, RDF analysis coincides well with structural analyses, as well as dielectric relaxation. In summary, a supramolecular gyroscope-like compound, i.e, 1, was synthesized and studied by the combined techniques of

The frequency-dependent dielectric responses can be explained in terms of dielectric relaxation of the rotatable molecular dipoles.34−37 The relaxation process can be demonstrated by frequency dependence of ε″, in which the peak maxima away from TC were found at Tpeak values of 269.3 274.2, 279.1, 284.7, 288.1, 292.2, and 296.5 K for f = 180, 250, 350, 500, 630, 800, and 1000 kHz, respectively. Based on the Debye-type relaxation process, the temperature-dependent ε″ can be expressed as ε″(T) = ωτ(T)/[1 + ω2τ(T)2], where ω is the angular frequency of the test field and τ(T) is the relaxation time, a function of the temperature obeying the Arrhenius law: 1/τ = ω0exp[−Ea/(kBT)], where ω0 is a pre-exponential factor, Ea is an activation energy, and kB is the Boltzmann constant. The ε″ value reaches a maximum when ωτ(T) = 1. Thus, test frequency can be used to estimate the value of τ at Tpeak. Accordingly, an ω0 value of 1.85 × 1014 s−1 and an Ea of 10.1 kcal·mol−1 (0.44 eV) were obtained from the plot of 1/Tpeak versus ln(ω) (Figure 3b, inset). To confirm the observed dielectric relaxation is attributed to rotation of water molecule, we investigated its deuteration effect. The dehydrated sample of 1 can absorb moisture in the air, returning to the original phase (Figure S4). Thus, the fumigation of deuterated water for the dehydrated sample of 1 can afford the deuterated compound 2 (t-BuNH 3 )(18-crown-6)[ZnCl3(D2O)], further evidenced by IR spectra (Figure S5). Similar phase transition and dielectric relaxation were observed from the temperature-dependent dielectric spectrum of 2 (Figure S6), but with an ω0 value of 8.74 × 1013 s−1 and an Ea of 9.68 kcal· mol−1. Thus, the τ ratio for 2 and 1 at 296 K was calculated to be 2.1, verifying the kinetic isotope effect of a rotating water molecule. As well-known, MD simulation reveals intuitively the microscopic molecular motion process (e.g., vibration, translation, or rotation) over time and/or temperature.38,39 To gain direct insight into the rotational dynamics of water molecule in 1, we performed MD simulations with Dreiding force field at 200 and 296 K, respectively, based on the corresponding unit-cell parameters of α or β phase (keeping volumes unchanged). As 8088

DOI: 10.1021/jacs.7b02981 J. Am. Chem. Soc. 2017, 139, 8086−8089

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variable-temperature X-ray structure analysis and dielectric measurement as well as MD simulations. Delicately, 1 can be viewed as a unique “stator−rotator···stator” system, where the [(t-BuNH3)(18-crown-6)]+ and [ZnCl3]− species are two different stators whereas the coordinated H2O functions as an ultrasmall polar rotator, revealed by its significant dielectric relaxation and the results of MD simulations. In addition, such a compound can reversibly undergo a polar-to-polar phase transition triggered by the changed conformation of the 18crown-6 host, leading to a switchable on−off rotation of the water well controlled by the strength/direction of charge-assisted hydrogen bonds. This compound represents a new class of supramolecular rotor corresponding to a “stator−rotator··· stator” model, affording a strategy in seeking of a new type of molecular rotors. We are currently extending such a technique by replacing Cl− with Br− or changing the R−NH3+ species, to regulate the speed of the water rotator. A detailed study is now underway.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02981. Synthesis and characterizations of 1; CCDC 1538784 and 1538785 (PDF) X-ray crystallographic data for 1 at 200 K (CIF) X-ray crystallographic data for 1 at 296 K (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Zi-Yi Du: 0000-0002-2794-7670 Wei-Xiong Zhang: 0000-0003-0797-3465 Xiao-Ming Chen: 0000-0002-3353-7918 Author Contributions ∥

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21661004, 21361002, 21290173, and 21671202). C.-T.H. is thankful to the National Postdoctoral Program for Innovative Talents (BX201600195).

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REFERENCES

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DOI: 10.1021/jacs.7b02981 J. Am. Chem. Soc. 2017, 139, 8086−8089