Channel-Assisted Proton Conduction Behavior in Hydroxyl-Rich

Apr 20, 2017 - Two lanthanide-based magnetic metal−organic frameworks show channel-assisted proton conduction behavior, a magnetocaloric effect, and...
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Channel-Assisted Proton Conduction Behavior in Hydroxyl-Rich Lanthanide-Based Magnetic Metal−Organic Frameworks Soumava Biswas,† Jayita Chakraborty,‡ Vijay Singh Parmar,† Siba Prasad Bera,† Nirmal Ganguli,§ and Sanjit Konar*,† †

Department of Chemistry, Indian Institute of Science Education and Research (IISER) Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462 066, India ‡ Institute for Functional Materials and Quantum Technologies, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany § Department of Physics, Indian Institute of Science Education and Research (IISER) Bhopal, Bhauri, Bhopal 462066, India S Supporting Information *

ABSTRACT: Two new lanthanide-based 3D metal−organic frameworks (MOFs), {[Ln(L)(Ox)(H2O)]n·xH2O} [Ln = Gd3+ and x = 3 (1) and Dy3+ and x = 1.5 (2); H2L = mucic acid; OxH2 = oxalic acid] showing interesting magnetic properties and channel-mediated proton conduction behavior, are presented here. Single-crystal X-ray structure analysis shows that, in complex 1, the overall structure originates from the mucate-bridged gadolinium-based rectangular metallocycles. The packing view reveals the presence the two types of hydrophilic 1D channels filled with lattice water molecules, which are strongly hydrogen-bonded with coordinated water along the a and b axes, whereas for complex 2, the 3D framework originates from a carboxylatebridged dysprosium-based criss-cross-type secondary building block. Magnetic studies reveal that 1 exhibits a significant magnetic entropy change (−ΔSM) of 30.6 J kg−1 K−1 for ΔH= 7 T at 3 K. Our electronic structure calculations under the framework of density functional theory reveal that exchange interactions between Gd3+ ions are weak and of the antiferromagnetic type. Complex 2 shows fieldinduced single-molecule-magnetic behavior. Impedance analysis shows that the proton conductivity of both complexes reaches up to the maximum value of 4.7 × 10−4 S cm−1 for 1 and 9.06 × 10−5 S cm−1 for 2 at high temperature (>75 °C) and relative humidity (RH; 95%). The Monte Carlo simulations confirm the exact location of the adsorbed water molecules in the framework after humidification (RH = 95%) for 1. Further, the results from computational simulation also reveal that the presence of a more dense arrangement of adsorbed water molecules through hydrogen bonding in a particular type of channel (along the a axis) contributes more to the proton migration compared to the other channel (along the b axis) in the framework.

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INTRODUCTION There has been enormous interest for exploration of the proton conduction behavior in metal−organic frameworks (MOFs) in recent times.1 It has been claimed that the MOFs could be superior materials for proton-exchange membranes for fuel-cell technologies.1 There are several advantages of MOFs over commercially used Nafion (e.g., tunable structural features, high chemical and thermal stability, and highly ordered structure).1 However, the design strategy for a good proton-conducting MOF depends on several factors, such as the presence of a good proton carrier, water-filled nanochannels, acidic pores, and a hydrogen-bonded arrangement. 2 The use of a polyhydroxyl-group-containing ligand could be a beneficial strategy to constructing such MOFs having well-directed hydrophilic channels and pores because such groups can offer better hydrogen-bonding interactions throughout the framework.3 Normally, proton conduction is observed in the MOF materials having water-filled nanochannels. However, the observation of channel-directed proton conductivity is quite uncommon in the case of a MOF having different types of © 2017 American Chemical Society

hydrophilic nanochannels along different directions. It could be interesting to study the proton conduction behavior of such MOFs because a specific alignment (or higher density) of proton carriers in a particular direction improves their solid electrolytic nature. Moreover, the exploration of anisotropic proton conduction in such MOF materials will be ambitious because of their potential applications in the fabrication of nanodevices.1a,3p It was surprisingly observed that much less attention has been gained in examining the anisotropic proton conductivity inside MOFs.4 Because of the highly ordered regular architecture of MOFs, the anisotropic motion of hydrogen carriers is quite possible in well-directed channels.4 On the other hand, lanthanide ions could be useful metal nodes as an oxophilic center which can offer acidic proton from the metal-bound water molecules.5 Lanthanide-based protonconducting MOFs could be very useful to understanding the phenomena of spin protonics because of their diverse magnetic Received: December 23, 2016 Published: April 20, 2017 4956

DOI: 10.1021/acs.inorgchem.6b03147 Inorg. Chem. 2017, 56, 4956−4965

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Inorganic Chemistry behavior and oxophilic nature.6 Considering this, we have investigated the proton conduction behavior of two lanthanidebased 3D MOFs, {[Ln(L)(Ox)(H2O)]n·xH2O} [Ln = Gd3+ and x = 3 (1) and Dy3+ and x = 1.5 (2); [H2L = mucic acid; OxH2 = oxalic acid], using a polyhydroxyl ligand (mucic acid). The magnetic properties of complexes 1 and 2 were also studied. The magnetic exchange interactions within complex 1 are also studied by first-principles electronic structure calculations in detail. Complex 1 shows a cryogenic magnetocaloric effect, whereas 2 shows magnetic-field-dependent singlemolecule-magnetic (SMM) behavior below 10 K. The presence of hydrophilic channels, pore surface, and continuous arrangement of hydrogen-bonded water molecules makes the complexes ideal candidates for a good proton conductor. Impedance analyses of both complexes show that the highest conductivities obtained at high temperatures and humidity are σ = 4.7 × 10−4 S cm−1 for 1 and σ = 9.06 × 10−5 S cm−1 for 2. From the computational simulation studies (for 1), the exact location of the adsorbed water molecules in the framework was confirmed. The presence of the more densely packed arrangement of adsorbed water molecules in a particular type of channel that contributes more to proton migration in the framework has been revealed.

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(found) for C7H13DyO12.5: C, 18.3 (18.52); H, 2.85 (2.59). Selected IR data (4000−400 cm−1, KBr pellets); 3329(b), 2661(b), 1664(s), 1591(m), 1436(m), 1317(m), 1097(s), 1047(m), 837(m), 800(m), 667(m), 514(s). Synthesis of the Magnetically Diluted Sample 2′. A magnetically diluted sample was prepared by following the same procedure as that for complex 1 by using Dy(NO3)3·xH2O (27 mg, 0.062 mmol) and Y(NO3)3·6H2O (72 mg, 0.187 mmol) together (molar ratio of 1:3) instead of Gd(NO 3 ) 3 ·6H 2 O. Elem anal. Calcd (found) for C7H13Y0.75Dy0.25O12.5: C, 20.78 (20.62); H, 3.24 (3.17). The possible formula and molar ratio were calculated from energy-dispersive X-ray analysis. Physical Measurements. Elemental analyses of the complexes were carried out on an Elementar Micro vario Cube elemental analyzer. Fourier transform infrared spectra (4000−400 cm−1) were recorded on KBr pellets with a PerkinElmer Spectrum BX spectrometer. Powder X-ray diffraction (PXRD) data were collected on a PANalytical EMPYREAN instrument using Cu Kα radiation. Water vapor adsorption studies were performed using a BELSORP MAX (BEL JAPAN) volumetric adsorption analyzer. All of the magnetic data are collected using a Quantum Design SQUID MPMS3 magnetometer. The measured data were corrected for the experimentally measured contribution of the sample holder, while the derived susceptibilities were corrected for the diamagnetism of the samples, calculated from Pascal’s tables.11 Alternating-current (AC) impedance analysis measurements were carried out with a Solartron SI 1260 impedance analyzer using the conventional quasi-four-probe method on a pellet sample (diameter of 13 mm and thickness of ≈0.85−0.9 mm). The temperature and humidity were controlled by a programmable humidification chamber (JEIOTECH, TH-PE series). Simulation of a water-adsorbed structure was obtained through grand canonical Monte Carlo (GCMC) calculation with Material Studio 6.1 software (sorption module).

EXPERIMENTAL SECTION

Materials. All of the reagents and solvents were commercially available and were used as obtained. Mucic acid, Gd(NO3)3·6H2O, and Dy(NO3)3·xH2O were obtained from Sigma-Aldrich Chemical Co. and used as received. Single-Crystal X-ray Diffraction. Suitable diffraction-quality single crystals of both complexes were mounted on a Bruker D8 diffractometer equipped with a graphite monochromator and Mo Kα (λ = 0.71073 Å, 120 K) radiation. Data collection was done using φ and ω scans. The structures were solved using direct methods followed by full matrix least-squares refinements against F2 (all data in HKLF 4 format) using SHELXTL.7 Subsequent difference Fourier synthesis and least-squares refinement revealed the positions of the remaining non-hydrogen atoms. Determinations of the crystal system, orientation matrix, and cell dimensions were performed according to established procedures. Lorentz polarization and multiscan absorption correction were applied. Non-hydrogen atoms were refined with independent anisotropic displacement parameters, and hydrogen atoms were placed geometrically and refined using the riding model. For complex 1, the presence of “Short Inter D-H..H-D” in alert B (CheckCIF) may be due to some spatial disorder for the respective hydrogen atoms. These factors contribute for inaccurate automatic placement of the hydrogen atoms. However, for complex 2, the presence of “Short Inter D···A Contact” in alerts A and B (CheckCIF) can be due to the crystal packing and high thermal parameters of the oxygen atoms in the noncoordinated water molecules. This makes donor (D) and acceptor (A) atoms come closer to each other. All calculations were carried out using SHELXL 2014/7,8 PLATON 99,9 and WinGXsystem, version 1.64.10 Data collection and structure refinement parameters and crystallographic data for the two complexes are given in Table S1. Synthesis of 1. Gd(NO3)3·6H2O (113 mg, 0.25 mmol) and mucic acid (79 mg. 0.38 mmol) were added in 10 mL of water. After 30 min of stirring, the solution was transferred to a Teflon vessel sealed in a stainless steel container and heated at 120 °C for 72 h, followed by cooling to room temperature (∼30 °C). Colorless block-shaped crystals were separated and washed with cold water. Yield: 32% (based on gadolinium). Elem anal. Calcd (found) for C7H16GdO14: C, 17.46 (17.58); H, 3.35 (3.19). Selected IR data (4000−400 cm−1, KBr pellets): 3290(b), 3174(s), 1595(s), 1382(s), 1309(m), 1238(m), 1093(m), 1035(m), 977(m), 916(m), 665(m), 6540(s), 470(s). Synthesis of 2. Complex 2 was prepared following the same procedure as that for complex 1 by using Dy(NO3)3·xH2O (87 mg, 0.25 mmol) instead of Gd(NO3)3·6H2O. Yield: 35%. Elem anal. Calcd

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RESULTS AND DISCUSSION Complexes 1 and 2 were synthesized by hydrothermal reaction using the corresponding lanthanide salts and mucic acid. Mucic acid was chosen as a linker because of the presence of four hydroxyl groups, which can offer hydrophilic and acidic pore surfaces as well as the continuous arrangement of hydrogen bonding with lattice and adsorbed water molecules upon humidification. Structural Description of 1 and 2. Both complexes are crystallized in the triclinic P1̅ space group. The structural refinement parameters are listed in Table S1, and the overall structures are very similar. The relevant bond parameters around the lanthanide centers are listed in Tables S2 and S3. The single-crystal structural discussion of 1 has been demonstrated in detail. The asymmetric unit of 1, {[Ln(L)(Ox)(H2O)]n·3(H2O)n} consists one Gd3+ ion, two halfmucate ligands having different coordination modes, half of the oxalate, and one coordinated water (Figure S1). Systematic analysis of the coordination geometry around the metal using SHAPE 212 suggests that the nine-coordinate Gd3+ ion adopts a geometry that is best described as a spherical tricapped trigonal prism (minimum CShM value of 0.674; Figure S2). Complete results of the geometry analyses are summerized in Table S4. The molecular entity of the complexes is similar to that of the reported analogues of a Tb3+-based 3D MOF.13 For both complexes, the presence of oxalate comes from the thermal decomposition of mucic acid in hydrothermal conditions. The overall framework consists of gadolinium mucate zigzag layers (Figure 1), which are further connected through oxalate anions along the c axis. It has been observed that there are two types of bridging modes of mucate present in the structure (Figure S3). The 4957

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Systematic analysis of the coordination geometry for Dy3+ using SHAPE 212 confirms that the eight-coordinate Dy3+ ion adopts a biaugmented trigonal-prismitic geometry (minimum CShM value of 1.254; Figure S8). The complete results of geometry analyses are summarized in Table S5. The overall structure of 2 is also a 3D pillar-layered framework. The layer of the framework consists of 1D oxodysprosium chains through the alternative linking of oxalate and two types of μ2-connected mucate (Figure S9). These chains are further joined by a μ4connected mucate linker, which propagate along the b axis to form the overall 3D framework (Figure 4). The secondary building unit of the framework structure is illustrated in Figure S10. For 2 also, two types of 1D channels are visible along the a and c axes (Figure S11).

Figure 1. View of the zigzag 2D lanthanide mucate layer along the b axis (for 1).

zigzag layers are linked together by a μ2-connected oxalate anion along the b axis to form the 3D framework (Figure S4). It is found that the overall extended 3D framework is formed by repetition of a rectangular metallomacrocycle-type secondary building unit (Figure S5). Further structural investigation reveals the existence of two types (A and B) of hydrophilic channels in the framework along the two different directions. The presence of lattice water molecules in the rectangular channel (type A) along the a axis is illustrated in Figure 2. In another channel (type B), lattice water molecules are strongly hydrogen-bonded with the metal-bound water moleculaes that are pointed toward the channels (Figures 3 and S6).

Figure 4. View of the 3D framework along the c axis (for 2).

PXRD and Thermogravimetric Analysis (TGA) Characterizations. To confirm the phase purity, PXRD was performed for the complexes. The powder patterns of bulk samples match well with the simulated pattern from singlecrystal data indicating phase purity (Figure S12). TGA was performed in the temperature range of 30−600 °C under N2 flow with a heating rate of 10 min−1. The TGA plot for complex 1 reveals a weight loss of ∼10% (calcd ∼11%) in the temperature range of 80−190 °C, which might be due to the loss of three noncoordinated lattice water molecules from the framework (Figure S13), whereas for complex 2, the complete removal of one and half lattice water molecules (per formula unit) occurred around 230 °C because of a weight loss of around ∼6% (calcd ∼7%). After that, the framework collapsed with an increase in the temperature. Magnetic Property Studies. Direct-current (dc) magnetic susceptibility measurements of both complexes were performed using a polycrystalline powder in the experimental temperature range of 2−300 K at an applied dc field of 1000 Oe. In 1, the room temperature χMT (χM = molar magnetic susceptibility) value is found to be 7.82 cm3 K mol−1, which is consistent with the spin-only χMT value of 7.87 cm3 K mol−1 for one Gd3+ ions (8S7/2 and g = 1.99). With a decrease in the temperature, the χMT value remains almost constant down to 25 K and then decreases rapidly to reach a minimum of 7.27 cm3 mol−1 K at 2 K (Figure S14). The overall shape of the susceptibility plot surely indicates the occurrence of antiferromagnetic coupling between Gd3+ ions throughout the framework. The Curie− Weiss fitting for susceptibility data provides the parameters as C = 8.83 cm3 mol−1 K and θ = −0.12 K, which further confirms that the antiferromagnetic interactions are weak in nature (Figure S15). Isothermal magnetization data for 1 were measured in the temperature range of 2−10 K. Figure S14 (inset) reveals that the M/NμB value increases with increasing magnetic field (H)

Figure 2. Representation of lattice water molecules in the channel (along a-axis) of complex 1.

Figure 3. Representation of lattice water molecules in the channel (along b-axis) of complex 1.

For complex 2, the asymmetric unit is similar to that of 1, although the binding mode of mucate is different (Figure S7). 4958

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Oe, the appearance of peak maxima in out-of-phase (χ″M) signals (Figure S21) observed and became quite prominent in the temperature range of 5.6−7 K under an applied field of 3000 Oe (Figures 6 and S22), although the presence of a long

and a saturation value of 6.97 NμB is obtained at 7 T and 2 K. A plot of M/NμB versus H/T (Figure S16) illustrates that all magnetization isotherms merge on the same master curve, indicating the isotropic nature of a Gd3+ ion. The magnetic entropy change (−ΔSm) for complex 1 is extracted from the magnetization data using the Maxwell relationship14 −ΔSm(T ) =

∫ [∂M(T , H)/∂T ]H dH

(1)

The experimentally obtained value of −ΔSm at 2 K for ΔH = 7 T is 30.6 J kg−1 K−1 (Figure 5), which is lower than the

Figure 6. Temperature dependence of the out-of-phase ac susceptibility (χM″) for 2 for an applied field of 3000 Oe at the indicated frequencies.

tail in the temperature range of 2−3.5 K suggests that the QTM is not fully suppressed even at higher dc field.18e The anisotropic energy barrier was extracted using the Arrhenius equation

Figure 5. ΔSm calculated by using the magnetization data of 1 at different fields and temperatures.

1/τ = (1/τ0) exp( −Ueff /kBT )

theoretically calculated full entropy change value of 35.9 J kg−1 K−1 (calculated from R ln(2S + 1), where S = 7/2). The experimental value is quite comparable with those of other gadolinium-based MOFs and coordination polymers.15 The volumetric entropy change is calculated to be 73.1 mJ cm−3 K−1.16 For 2, the observed room temperature χMT (χM = molar magnetic susceptibility) value is 13.97 cm3 mol−1 K. The theoretical χMT value for one Dy3+ cation (6H15/2 and g = 1.33) is 14.17 cm3 mol−1 K. Experimentally, the χMT value slowly decreases down to around 75 K and then decreases rapidly to a value of 9.42 cm3 mol−1 K at 2 K (Figure S17). The overall shape of the χMT curve does not surely suggest the presence of antiferromagnetic interactions between Dy3+ centers because of the large orbital angular momentum, strong spin−orbit coupling, and thermal depopulation of excited Stark sublevels of Dy3+.17 Consequently, magnetization data of 2 do not show complete saturation even at the highest field measured, 7 T and at 2 K (Figure S17, inset). Moreover, the M/NμB versus H/T plot (Figure S18) shows that all isotherm magnetization curves do not collapse on the same master curve, signifying the high magnetic anisotropy and crystal-field effects. To investigate the dynamic magnetic behavior of 2, ac susceptibility measurements were performed under a 0 Oe dc field, and a 3.5 Oe ac magnetic field was applied (Figure S19). Observation of frequency dependencies of the out-of-phase (χ″M) signals confirms the occurrence of slow relaxation of magnetization (Figure S19). No peak maxima are observed for out-of-phase (χ″M) signals in the temperature range of 2−10 K. The probable reason is the fast quantum tunnelling of magnetization (QTM) at low temperature.18 Because it is quite impossible to reliably obtain the anisotropic energy barrier from the collected data, ac susceptibility measurements were again performed in the presence of some external dc magnetic field (1000, 2000, and 3000 Oe) to suppress the QTM (Figures S20 and S21).18 For an applied field of 2000

(2)

where Ueff is the effective anisotropic energy barrier, kB is the Boltzmann constant, and 1/τ0 is the preexponential factor. The least-squares fit using eq 2 (Figure S23) afforded the energy barrier of Ueff/kB = 36.5 K and the relaxation time τ0 = 7.73 × 10−6 s, which is quite consistent with the characteristic relaxation times 10−6−10−11 s for SMMs.19 The fitting of the Cole−Cole plot is shown in Figure S24, evidence of the broad distribution of magnetic relaxation process occurring in complex 2. Further, to understand the dipolar interactions between neighboring dysprosium ions and the origin of the SMM behavior, detailed magnetic characterization has been carried out on an analogous diluted yttrium-based (1:3 molar ratio of dysprosium and yttrium) framework 2′ (details are given in Figure S25). The χMT value of 2′ slowly decreases to around 100 K, and after that, it decreases rapidly to 2.53 cm3 mol−1 K at 2 K (Figure S26), whereas complete saturation is not observed in the isothermal magnetization isotherm (Figure S26). The ac susceptibility measurement for the diluted complex 2′ shows the absence of peak maxima at 0 and 1000 Oe dc field (Figures S27 and S28). At an applied dc field of 2000 Oe, clear peak maxima are observed in the out-of-phase ac susceptibility data (Figure S29). The absence of a long tail in the lower temperature region for the out-of-phase ac susceptibility data confirms the suppression of fast QTM.18g,h The effective energy barrier and relaxation time are extracted using the Arrhenius equation. The higher energy barrier (Ueff/ kB = 43.6 K) and smaller relaxation time (τ0 = 2 × 10−6 s) compared to complex 2 further confirm the suppression of interaction-driven QTM in the diluted complex (Figure S30).18h,19g,i The small deviation in the Arrhenius plot and broadening of peaks in the out-of-phase ac susceptibility plot is probably due to the presence of the Orbach process along with additional relaxation pathways.19g 4959

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Inorganic Chemistry Electronic Structure Calculations. In order to critically analyze the magnetic properties of the gadolidium-based MOF, we performed ab initio electronic structure calculation with density functional theory (DFT). Our calculations were carried out using the projector-augmented-wave20 method along with plane-wave basis set, as implemented in the VASP21 code. The exchange-correlation part is approximated through a generalized gradient approximation (GGA) due to Perdew, Burke, and Ernzerhof.22 We used a plane-wave energy cutoff of 500 eV and a Γ-centered k mesh of 11 × 8 × 5. The electronic structure obtained from our calculation assuming that the magnetic moments of gadolinium aligned parallel to each other is displayed in the form of spin-polarized density of states (DoS) in Figure 7. The total DoS as well as its

Figure 8. (a) Magnetic exchange interactions path and (b) AF1, (c) AF2, (d) AF3, and (e) AF3 antiferromagnetic spin configurations.

E(AFM 2) = − (4J1 − 2J2 − 2J3 − 2J3)N 2/4 E(AFM 3) = − (4J1 − 2J2 − 2J3 + 2J3)N 2/4

E(AFM 4) = (4J1 + 2J2 − 2J3 + 2J4 )N 2/4

The results of our calculations are displayed in Table 1. We gather from Table 1 that the dominant exchange interactions J1, Table 1. Exchange Interaction Strengths between Gadolinium Atoms along Different Exchange Paths exchange parameters

distance (Å)

exchange interaction (cm−1)

J1 J2 J3 J4

6.11 6.40 7.63 7.78

−0.011 −0.026 −0.003 0.002

Figure 7. Spin-polarized total DoS is shown in part a, while parts b and c represent the projected DoS corresponding to the Gd 4f and O 2p states.

J2, and J3 are antiferromagnetic and J4 is weakly ferromagnetic. Our calculations reveal that the spin interactions are very weak because of the localized nature of the Gd 4f states. J2 is the strongest interaction. Although the Gd−Gd distance is large, the relatively strong value of the antiferromagnetic exchange interaction is due to short O−O distances (2.27 Å; comparable to the van der Waals distance) in the exchange pathway. We estimated the Curie−Weiss temperature θ within the meanfield approximation using the following formula:

projection onto Gd 4f and O 2p have been shown separately for majority and minority spins. The contributions from carbon and hydrogen were found to be in the same energy range as that of oxygen but much smaller in magnitude. The figure reveals that the majority-spin Gd 4f states are completely filled and the minority-spin ones are completely empty, as one would expect for a nominal Gd3+ oxidation state. The O 2p states do not show any significant sign of magnetization, suggesting that the magnetization primarily comes from the Gd 4f states. We further observe that the Gd 4f states are located far away from the Fermi energy, leaving hardly any possibility of strong magnetic interaction among the gadolinium atoms. In order to extract the various exchange interactions, the relative energies of these ordered spin states determined from the GGA calculations are mapped onto the corresponding energies obtained from the total spin-exchange energies of the Heisenberg spin Hamiltonian.23

θ=

s(s + 1) ∑i ziJi 3KB

where s is the spin quantum number of each spin site (s = 7/2), the summation runs over all nearest neighbors of a given spin site, zi is the number of nearest neighbors connected by the spin-exchange parameter Ji. The calculated value of θ is −0.38 K. This is in good agreement with the experiment in view of the fact that DFT calculations within GGA often slightly overestimate the exchange interaction strengths. Proton Conduction Studies. The presence of water-filled channels and the free hydroxyl groups of mucate make both complexes potential candidates as good proton conductors. ac impedance measurements were performed on a pelletized sample at anhydrous as well as controlled humidity conditions and different temperatures.1 The conductivities of the complexes were calculated from the fitting of the Nyquist plots.1 Because for both complexes the proton conductivities were found to be very low in anhydrous condition and room temperature, the samples were subjected to controlled humidification for 48 h. After humidification with 95% relative

H = −∑ Jij Si·Sj i,j

We have simulated five magnetic configurations, as shown in Figure 8. The total spin energies with N unpaired spins per spin site (N = 7) of these configurations are written as E(FM) = − (4J1 + 2J2 + 2J3 + 2J3)N 2/4 E(AFM 1) = − ( − 4J1 + 2J2 + 2J3 + 2J3)N 2/4 4960

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Inorganic Chemistry humidity (RH), both complexes started to show significant conductivity at different temperatures (Figures S31 and S32). It is worth mentioning that the conductivities of both complexes increase with increasing temperature (Figure S33). At a RH of 95%, the room temperature conductivity values of the complexes are 2.37 × 10−6 S cm−1 for 1 and 1.84 × 10−6 S cm−1 for 2. The values are very much comparable with the conductivity value of MIL-53-based MOFs (σ = 10−6−10−9 S cm−1 at 25 °C and 95% RH)24 or R-MaMb MOFs6c and lower than that of PCMOF-3.3e Surprisingly, at 80 °C the conductivity reaches up to maximum values of 4.7 × 10−4 S cm−1 for 1 (Figure 9) and 9.06 × 10−5 S cm−1 for 2 (Figure 10)

For complex 1, the presence of two different types of hydrophilic channels in the framework prompts us to explore the exact proton-conducting pathway and proton migration mechanism from a structural point of view.4e,27 As in the cavity of channel B, coordinated water molecules are pointed inward, which could be a suitable source of H+ compared to channel A. To gain insight into the molecular level, we performed a GCMC simulation study on complex 1. To locate the position of adsorbed water molecules in the framework after humidification (95% RH) at high temperature (80 °C), hydrated frameworks using the Material Studio 6.1 sorption module were generated.28 The number of water molecules (eight) present in the framework was confirmed from TGA after completion of temperature dependency measurement at highest humidity (95%; Figure S13). Then the adsorbed water molecules were exactly located in the framework through computational simulation (Figure 11). Compared to the as-

Figure 9. Nyquist plot for proton conduction for 1 at 95% RH and 80 °C. Figure 11. Simulated unit cell of complex 1 from humiliation at 95% RH and 353 K (hydrogen-bonding interaction is indicated by a dotted line).

synthesized framework, the packing arrangements of the lattice water molecules are different. After humidification (95% RH), the formation of a dense hydrogen-bonding network occurred at high temperature (80 °C). Adsorbed water molecules are strongly hydrogen-bonded with the coordinated water as well as with the carboxylic oxygen. The humidified framework forms a more regular hydrogen-bonding network along both channels (A and B) compared to the as-synthesized one. From the results of the computational simulation, it is observed that density of adsorbed water molecules is more and continuous in channel A compared to those of channel B (Figures S34 and S35). The activation energy for proton conduction was extracted from the temperature dependence data by using eq 3. For both complexes, the activation energy indicates that the vehicle mechanism operates for proton conduction (Ea = 0.88 eV for 1 and Ea = 0.70 eV for 2; Figure S36).29

Figure 10. Nyquist plot for proton conduction for 2 at 95% RH and 80 °C.

⎛E ⎞ σT = σ0 exp⎜ a ⎟ ⎝ kT ⎠

These highest conductivity values are well comparable to most of the proton conductive magnetic MOFs (95% RH; Table S6).6 However, the value is lower than that of PCMOF21/2.5a Here, in the framework metal-bound water molecules act as potential proton sources because of the Lewis acidic nature of lanthanide ions.25 The higher conductivity in 1 compared to 2 may be due to the structural difference as well as the presence of more lattice water molecules in 1. The increase in the conductivity with the temperature is certainly caused by thermal activation of the water molecules, which help with the transport of protons in the channel2b and the retention of strong hydrogen-bonding interaction at high temperatures.26

(3)

The hydronium ions generated from the metal-bound as well as absorbed water molecules may act as a vehicle for proton conduction. The water vapor adsorption and humidity dependency of proton conduction for both complexes reveal the role of adsorbed water molecules to form hydrogenbonding pathways in the framework (Figures S37 and S38). Complex 1 is a very rare example that shows channel-directed proton conduction behavior, and the results could offer a better understanding of the hydrogen-ion-transport dynamics as well as anisotropic proton conduction in highly ordered molecular 4961

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Inorganic Chemistry systems.4 Furthermore, for both complexes, the highest conductivity value was found to be at high temperature. Only a few lanthanide-based magnetic MOF materials exist to show high-temperature conductivity at high-humidity conditions.6

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CONCLUSIONS We thoroughly investigated the magnetic and proton conduction behavior of two lanthanide-based 3D MOFs. The present synthetic strategy adopts the use of a polyhydroxyl carboxylate ligand, which offers hydrophilic nanochannels in the resultant structures. Complex 1 acts as a cryogenic magnetic refrigerant, whereas field-dependent SMM behavior was observed for complex 2. The proton conductivities of both complexes reach the maximum values at higher temperature and high humidity. A Monte Carlo simulation study confirms the exact location of the adsorbed water molecules featuring the major proton migration pathway in the framework for complex 1, which demonstrates a rare example of a channel-directed anisotropic proton conductor. The overall results could deliver an emerging outlook of material chemistry in designing smart molecular materials as well as understanding the ion-transport dynamics through different molecular channels.

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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/acs.inorgchem.6b03147. PXRD patterns of compounds, TGA, and some additional figures and tables (PDF) X-ray crystallographic data in CIF format for 1 (CIF) X-ray crystallographic data in CIF format for 2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sanjit Konar: 0000-0002-1584-6258 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.B. thanks IISER Bhopal for the fellowship. V.S.P. thanks DST for an Inspire fellowship. S.P.B. thanks the University Grants Commission, India, for a fellowship. N.G. gratefully acknowledges the use of the HPC facility of IISER Bhopal. S.K. thanks the Council of Scientific and Industrial Research [Grant 01(2822)/15/EMR-II], Government of India, and IISER Bhopal for generous financial support. The authors thank Amit Kumar Mondal for helpful scientific discussion.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.6b03147 Inorg. Chem. 2017, 56, 4956−4965