Article pubs.acs.org/crystal
Proton Conducting Compound of Benzimidazole with Sebacic Acid: Structure, Molecular Dynamics, and Proton Conductivity Adam Rachocki,†,* Katarzyna Pogorzelec-Glaser,† Paweł Ławniczak,† Maria Pugaczowa-Michalska,† Andrzej Łapiński,† Bozė na Hilczer,† Michał Matczak,† and Adam Pietraszko‡ †
Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland Institute of Low Temperature and Structure Research, Polish Academy of Sciences, 50-422 Wroclaw, Poland
‡
S Supporting Information *
ABSTRACT: Benzimidazole salt of sebacic acid, a new proton conductor from the family of benzimidazole compounds of dicarboxylic acids, was crystallized to search for the factors which determine the hydrogen bond motif and the structure of the crystals. The molecular structure of benzimidazole-sebacic acid salt was solved by using X-ray diffractions and confirmed by 1H and 13C MAS NMR experiments combined with DFT calculations. The salt of sebacic acid, with 10 carbon atoms in the chain, was found to exhibit an undulated layer-type structure with banana-shaped acid molecules linked by O−H···O bonds into rectangular-type wavy chains and flat base molecules attached to the carboxylic groups by N−H···O bonds. The undulated layers are not linked with hydrogen bonds. Comparison of the architecture of benzimidazole salts with weak dicarboxylic acids of shorter carbon chains, studied by us earlier, points at the role of the acid chain length in the formation of structural and hydrogen bond motifs. The dynamics of protons in the ordered crystalline phase and disordered surface layers was characterized by NMR spin−lattice relaxation measurements, whereas complex impedance studies yielded information on the activation energy of proton diffusion in the both phases.
1. INTRODUCTION Though hydrogen bonding appears to be one of the most effective tools for crystal engineering, it is still difficult to predict and control the intermolecular interactions which determine the motifs of molecular packing in organic crystals. The idea that the directional character of the hydrogen bonds can be the driving force to organize individual molecules into well-defined crystal structures appears to be broken because many organic molecules have more than one functional group and thus can form more than one hydrogen-bond pattern. Recently special attention was paid to metal organic frameworks (MOFs) based on transition-metal ions and organic ligands the architecture of which can be designed to create channels with controlled guest arrangements. One of the most interesting functionalities provided by MOFs appears the proton-conducting pathway within the channels. Many papers have been devoted to control the proton conductivity by designing the ligand functional groups and host−guest interactions with carboxyl-containing cationic ions and/or nitrogen containing heterocycles.1−6 A variety of channel dimensions and hydrogen-bond networks have been designed, and within various combinations proposed, one can distinguish a system exhibiting superprotonic conductivity of 10−2 S/cm at room temperature. The material has been obtained by using a framework of 2-D hydrogen bond networks among adipic acid molecules, ammonium ions, water molecules, and oxalic ions and providing additional protons by putting carboxyl end groups of adipic acid in the honeycomb-shaped channels.2 © 2014 American Chemical Society
To gain insight into the factors determining the structure preferences and hydrogen bond network in benzimidazole (BIm) salts with weak dicarboxylic acids we crystallized and studied the structure and proton conductivity of the salts with acids of different chain lengths. Dicarboxylic acids are known to form easily hydrogen bonds, whereas the benzimidazole molecule belongs to nitrogen containing heterocycles which form a hydrogen bond network similar to that of water.7−9 Previously we reported the structure and proton conductivity of BIm salts with glutaric acid (GLU), containing five carbon atoms in the acid chain, pimelic acid (PIM) with seven carbon atoms in the chain, and azelaic acid (AZE) which has a chain of nine carbon atoms.10,11 Though the acidities of the dicarboxylic acids were very close (Table 1), the base:acid stoichiometry in BIm−GLU salt was found to be 1:1, whereas that in BIm−PIM and BIm−AZE salts was established as 2:1. Moreover, the structural motif of BIm−PIM and BIm−AZE salts was found to be similar and consisted of layers formed by acid molecules linked into rectangular chains by O−H···O hydrogen bonds and alternated by layers of benzimidazole molecules connected by N−H···N bonds into dimers. The two types of layers were linked together by two hydrogen bonds formed of the nitrogen atoms of the BIm dimer and oxygen atoms from the hydroxyl group of acid molecules in Received: November 20, 2013 Revised: January 28, 2014 Published: February 6, 2014 1211
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with CCD area detector. Mo Kα radiation (0.7107 Å) generated at 50 kV and 25 mA was used in the study and the measurements were done at room temperature. The unit cell was indexed from all the frames and the positional data were refined with diffractometer constants to yield the final unit cell parameters. Integration, scaling and absorption corrections were performed with CrysAlis program - version 172.32.18 OXFORD DIFFRACTION and unique data sets, corrected for Lorentz polarization effects, were obtained. The structure was solved using SHELXS97 program16 and refined using full-matrix least-squares methods in the program. The CIF file has been deposited at Cambridge Crystallographic Data Centre (CCDC 941831). 2.4. NMR Study. Solid-state 1H−13C CP/MAS NMR measurements were carried out at room temperature with a Bruker Avance DMX-300 spectrometer operating at the 1H resonance frequency of 300.17 MHz and equipped with a double-resonance MAS probe. Well powdered material was placed in a 4-mm zirconia rotor and spun at 10 kHz. Powder glycine sample was used both to set the Hartmann− Hahn condition and to calibrate the 13C chemical shift scale (43.5 and 176.2 ppm with reference to TMS). The 13C NMR spectra were recorded with the following parameters: proton π/2 pulse length of 3.9 μs, contact time of 5 ms, recycle delay of 60 s and spectral width of 25 kHz. The ramped amplitude shape pulse was applied during the crosspolarization.17 The two-pulse phase-modulated sequence with tp pulse duration of 7 μs and a phase difference of 15° was used for proton decoupling.18 2.5. GIPAW Calculations. Density functional theory (DFT) projected augmented-wave (PAW) approach19 as implemented in the gauge-including projector-augmented wave (GIPAW) module20 of the Quantum Espresso code21 was used to calculate 1H and 13C NMR chemical shifts in BIm−SEB. Electron exchange and correlation effect are modeled using a generalized gradient approximation in the form of Perdew−Burke−Ernzerhof.22 Well converged plane wave basis sets were applied for calculations using norm-conserving pseudopotentials with a kinetic energy cutoff chosen at 80 Ry. Integrals over the Brillouin zone were performed using a 3 × 3 × 3 Monkhorst-Pack k point grid.23 The value of 10−8 Ry was used as convergence criterion for the total energy. 2.6. Vibration Spectroscopy Study. Room temperature Raman spectrum was obtained with a Jobin-Yvon HORIBA LabRAM HR 800 spectrometer (λext = 633 nm) in backscattering geometry with a resolution of 2 cm−1. The power of the laser beam focused on the sample was kept below 4 mW to avoid a damage. FT-IR absorption spectra were measured with spectral resolution of 2 cm−1 using a FT-IR Bruker Equinox 55 spectrometer equipped with KBr beam splitter and DTGS detector. Benzimidazole, sebacic acid, and BIm−SEB salt samples dispersed KBr (c = 1:2000) were measured at room temperature. Theoretical calculations of IR and Raman spectra were performed with the Gaussian 03 sets of codes.24 B3LYP hybrid density functional which combines Becke’s three-parameter nonlocal exchange potential with nonlocal correlation functional of Lee, Yang, and Parr25 was applied and 6-311++G(d,p) Pople style basis set was used.26 The optimization process was performed with initial geometry of the molecules based on our single crystal X-ray diffraction data refined for BIm−SEB. 2.7. Electric Conductivity Study. The electric conductivity was determined by a standard impedance spectroscopy method for samples in the form of pellets made of powdered BIm−SEB crystals. Circular surfaces of the pellet were electroded with Hans Wolbring GmbH silver paste. Complex impedance of the samples studied was measured using an Alpha-A high performance frequency analyzer (Novocontrol GmbH) in the frequency range from 1 Hz to 10 MHz (with the voltage oscillations of ±1 V) and in the temperature range of 270−353 K. The temperature was controlled using a Quatro Cryosystem with accuracy better then ±0.1 K.
Table 1. pKa Values of the First Ionisable Carboxylic Group of Weak Dicarboxylic Acids12,13 acid
chemical formula
pKa value
succinic glutaric adipic pimelic suberic azelaic sebacic
HOOC−(CH2)2−COOH HOOC−(CH2)3−COOH HOOC−(CH2)4−COOH HOOC−(CH2)5−COOH HOOC−(CH2)6−COOH HOOC−(CH2)7−COOH HOOC−(CH2)8−COOH
4.19 4.34 4.42 4.48 4.52 4.55 4.72
neighboring layers.10,11 The architecture of the BIm−GLU structure was found to be of simple layer-type with flat layers parallel to the (−102) plane containing rectangular acid chains linked with O−H···O bonds. The benzimidazole molecules were attached to the acid chains by N−H···O bonds.11 Such a difference in the structure of the crystallization product may appear for small differences in ΔpKa in pKa values of the conjugated base and that of the acid.14 As the value pKa for benzimidazole is equal to 5.4915 the differences ΔpKa are really very small amounting to 1.15, 1.01, and 0.94 for BIm−GLU, BIm−PIM, and BIm−AZE salts, respectively. To look for any regularity we have grown crystals of succinic acid (four carbon atoms in the chain) and sebacic acid (10 carbon atoms in the chain) with benzimidazole and solved their crystal structure. It appears that the structural motif of BIm salt with succinic acid (BIm−SUC) was found to be similar to that characteristic of BIm−GLU salt, whereas the structure of BIm salt with sebacic acid (BIm−SEB) is intriguing and the structural motif is considerably different from those of BIm−SUC and BIm−GLU as well as from those of BIm−PIM and BIm−AZE. Therefore here we report the molecular structure and the results of molecular dynamics studies by means of NMR and vibrational spectroscopy and impedance spectroscopy measurements of the BIm−SEB salt.
2. EXPERIMENTAL SECTION 2.1. Synthesis and Crystallization. Benzimidazole (Fluka) and sebacic acid (Aldrich 98%) were dissolved separately in acetone (Aldrich pro analysis). The substrate solutions were mixed together and stirred at 320 K until a white precipitate disappeared. Reaction product the benzimidazole salt of sebacic acid (BIm−SEB) was separated and washed with anhydrous ethyl acetate. The crystallization of the salt was carried out by slow evaporation of the solvent at room temperature. Transparent needle-like crystallites approximately 6 mm in length and 0.5 mm thick were obtained. 2.2. Sample Characterization. Room temperature crystallization of BIm−SEB salt yields small needle-like crystallites which were used to X-ray diffraction studies only. NMR and vibration spectroscopy studies were performed for powdered samples obtained by milling the crystals in an agate mortar for 3 min, whereas for conductivity studies the powder was pressed at room temperature under 30 MPa for 1 min into pellets 1.5 mm thick and 6.7 mm in diameter. Room temperature scanning electron microscopy (SEM) images of BIm−SEB crystallites and powder samples were obtained using a Fei NovaSEM 650 microscope in vacuum better than 6 × 10−4 Pa. The samples were coated with 25 nm gold film. Thermal stability of BIm−SEB salt samples was characterized by differential scanning calorimetry (DSC) with a NETZSCH DSC 200F3 calorimeter. Powdered samples with a mass of few mg were measured in an Al pan on heating at a rate of 10 K/min in the temperature range from 300 to 390 K. 2.3. Structural Study. Single crystal X-ray diffraction measurements of BIm−SEB salt were performed using an X-ray four-circle XCALIBUR Diffractometer (Oxford Diffraction Company) equipped
3. RESULTS AND DISCUSSION 3.1. Crystal Structure. The crystal structure of benzoimidazole-sebacic acid compound belongs to a triclinic system with 1212
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centro-symmetric space group P-1. Table 2 contains the results of XRD studies of benzimidazole salts with sebacic acid at room temperature. Table 2. Crystal Data and Structure Refinement of Benzimidazole Sebacic Acid Salt (BIm−SEB) at 293 K empirical formula formula weight temperature (K) wavelength (Å) crystal system, space group unit cell dimensions volume (Å3) Z calculated density (g cm−3) absorption coefficient (mm−1) F(000) crystal size (mm)
C17H24N2O4 320.38 293(2) 0.71073 triclinic, P1̅ a = 4.9004(10) Å, b = 14.202(3) Å, c = 14.325(3) Å, α = 118.43(3)°, β = 93.44(3)°, γ = 97.06(3)° 862.0(3) 2 1.234 0.088 344 0.40 × 0.26 × 0.14
The H atoms of the heterocyclic molecules and carboxylic acid molecules were located from the difference-Fourier map and their positional parameters were refined freely, however, with an isotropic displacement parameter 1.2 times of that of the neighboring N or O atoms. The atomic coordinates and equivalent isotropic displacement parameters of BIm−SEB compound (Table SI1), the bond lengths and angles (Table SI2) and the torsion angles (Table SI3) are given in the Supporting Information. The unit cell contains two benzimidazole (BIm) and two sebacic acid (SEB) molecules coupled by symmetry center, respectively. Figure 1 shows a BIm−SEB single molecule with the atom numbering.
Figure 2. Projection of the BIm−SEB crystal structure along the b axis (a) and projection of a single layer onto the (102) plane (b).
The sebacic acid molecules are linked by strong O−H···O hydrogen bonds into rectangular-type wavy chains with a period containing an acid dimer. The BIm molecules are attached to the carboxylic groups of the acid molecules by N− H···O hydrogen bonds. There are no hydrogen bonds between the layers and adjacent undulated layers are held together by weak interactions of van der Waals type. The details of the O− H···O and N−H···O hydrogen bonds are collected in Table 3. 3.2. SEM Imaging and Thermal Stability. Figure 3a presents an example of an SEM image of an as-grown BIm− SEB crystallite surface. The main surface of the crystallite is normal to the [010] triclinic axis and one can observe welldefined grown edges parallel to the [201̅] direction and the (102) plane. The molecular arrangement on the projection along the b axis, as well as that onto the (102) plane is visible in Figure 2. For NMR and IR studies BIm−SEB crystallites were crushed in an agate mortar and the surface of the grains obtained was found to be seriously developed and moreover, partial degradation of the surface is apparent as seen in Figure 3b. The BIm−SEB was used in NMR and Raman spectroscopy experiments, whereas for IR studies the powdered sample was dispersed in KBr pellets and impedance spectroscopy measurements were done with pellets obtained by pressing the powder under 30 MPa for 1 min. Figure 4 shows the DSC heating scan of the BIm−SEB powdered sample. One can observe that the melting process of the salt is stretched over about 15 K, starts at ∼350 K, is
Figure 1. Single molecule of benzimidazole salt of sebacic acid with atom numbering.
The BIm rings are almost flat but the molecules of sebacic acid are of banana shape with the following dihedral angles: the angle between planes [C1B, C2B, C3B, C4B, ..., C9B, C10B] and [O1B C1B O2B] amounts to 56.69°, whereas the angle between [C1B, C2B, C3B, C4B, ..., C9B, C10B] and [O3B, C1OB, O4B] planes is equal to 35.07°. The packing of the BIm−SEB molecules form a layer-type crystal structure (Figure 2a) with undulating layers parallel to the (102) plane. Figure 2b shows the projection of a single wave-like layer of the BIm−SEB crystal onto the (102) plane. 1213
dx.doi.org/10.1021/cg401742b | Cryst. Growth Des. 2014, 14, 1211−1220
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Table 3. Hydrogen Bonds in Benzimidazole−Sebatic Acid (BIm−SEB) at Room Temperature (Å and deg) D−H···Aa N(1A)−HN1···O N(2A)−HN2···O O(3B)−HO3···O O(1B)−HO1···O a
(2B) (4B) #1 (3B) #2 (1B) #3
d(D−H)
d(H···A)
d(D···A)
