High heat of hydrogen adsorption and guest-responsive magnetic modulation in a 3D porous pillared-layer coordination framework

Abstract

A bimetallic pillared-layer coordination framework {[Mn₃(bipy)₃(H₂O)₄][Cr(CN)₆]₂·2(bipy)·4(H₂O)}_n has been constructed using a cyanometallate anion ([Cr(CN)₆]³⁻) and an organic linker (4,4′-bipyridyl) that provides high heat of hydrogen adsorption (∼11.5 kJ mol⁻¹) and shows guest dependent magnetic modulation.

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Metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) are promising materials for applications in gas storage, separation, sensing, catalysis, magnetism, drug delivery, etc.¹ Multifunctional materials, i.e. materials which combine a set of well defined properties (e.g. porosity and magnetism, porosity and optical) for specific applications, are gaining importance recently.^1a,e,2 Such a synergism, where two different functionalities are combined, would open up the possibility and prospect of finding novel physical phenomena for designing smart materials. Magnetism has generic dependence on distance while the porosity is enhanced with long linkers and hence combination of these two is not always easy or straightforward. A rational concept and design approach would be to choose a polycyanometallate anion, [(M_A(CN)_n]^m− (n = 2–8), as a hub which can link to another metal M_B to create a magnetic pathway M_B–NC–M_A–CN–M_B.³ Once the magnetic platform has been constructed, it can be further linked by a long organic pillar (e.g.4,4′-bipyridyl) to invoke porosity in the material. The presence of porosity provides the opportunity to study gas storage property, selectivity as well as guest-dependent magnetism⁴ and magnetic sensing which are of particular interest for applications such as magnetic devices and sensors (Scheme 1).^2b,5 Several fascinating properties like guest-dependent spin crossover,^4d,6 magnetic ordering sensitive to guest removal/exchange,⁷ and different magnetic behaviour corresponding to reversible solvent-induced structural changes⁸ have been reported in porous magnets. On the other hand, the porous property of MOFs is gaining increased attention with time and the framework materials have emerged as an excellent storage alternative to high pressure and liquefied hydrogen tanks.^1a,9 In this respect, recent reports indicate that the presence of unsaturated metal site (UMS) is crucial for strong interaction of H₂ molecule and also largely enhances the value of enthalpy of adsorption.¹⁰ This report tries to combine various aspects: (i) use of cyanometallate anion [Cr(CN)₆]³⁻ as magnetic hub; (ii) use of long linker 4,4′-bipyridyl (bipy) to invoke porosity; (iii) generation of UMSs to enhance enthalpy of hydrogen adsorption, together in a single framework in cyanometallate systems for the first time, to the best of our knowledge (Scheme 1). Our continuous efforts towards bimetallic systems¹¹ have resulted in a new MnIICrIII framework {[Mn₃(bipy)₃(H₂O)₄][Cr(CN)₆]₂·2(bipy)·4(H₂O)}_n (1) (bipy = 4,4′-bipyridyl) that exhibits guest-dependent magnetic modulation and selective sorption of CO₂/H₂ over N₂. It is noteworthy that the presence of UMSs in the framework leads to a very high value of enthalpy of adsorption for H₂ and is close to the highest reported value of 13.5 kJ mol⁻¹.^10b

Scheme 1 Bimodal functionality in a framework material.

Light yellow block type single crystals of 1 were grown by slow diffusion of K₃[Cr(CN)₆] and bipy with MnCl₂·4H₂O in an EtOH/H₂O medium (see ESI).‡ The IR spectrum of 1 shows a band at 2125 cm⁻¹ which corresponds to ν(C≡N) stretch of terminal cyanide groups. The high frequency bands at 2155 and 2170 cm⁻¹ correspond to the bridging cyanide groups (Fig. S1, ESI‡). X-ray single crystal structure determination reveals that 1 has a neutral 3D coordination framework of MnII built by [Cr(CN)₆]³⁻ and bipy with the formulation {[Mn₃(bipy)₃(H₂O)₄][Cr(CN)₆]₂·2(bipy)·4(H₂O)}_n (Fig. S2, ESI‡). The framework of 1 is isomorphous to our previously reported MnIIFeIII system¹¹ where the 2D layers formed by [Cr(CN)₆]³⁻ and MnII are pillared by the bipy linkers to generate a 3D pillared-layer framework with two kinds of channel along the c-axis (Fig. 1, S3 and S4, ESI‡). The channels are occupied by water (both coordinated and guest) and bipy molecules. Upon removal of the coordinated water and guest (water and bipy) molecules framework provides 37.7% void space to the total crystal volume with coordinatively unsaturated MnII centers on the pore surface. Despite of rigid connectivity in three directions, 1 has also significant inter- (H-bonding: Cr1–CN⋯O1w⋯CN–Cr1 and Mn2–O2⋯bipy⋯O2–Mn2) and intralayer (H-bonding: Mn1–O1⋯O2–Mn2 and Mn1–O1⋯CN–Cr1) supramolecular interactions (Fig. S5 and Table S4, ESI‡) that would have pronounced influence on its properties.

Fig. 1 3D view of compound 1 along c direction shows two different type of channels; one square occupied by water molecules and the other distorted triangular occupied by water and guest bipy molecules.

Thermal studies were carried out to analyse the stability and integrity of the framework (Fig. S6, ESI‡). TG analysis under nitrogen flow suggests two-step release of guest and coordinated water molecules (obs. 9.1%; calc. 9.55%) in the temperature range 40–180 °C. However, when 1 is heated at 165 °C under high vacuum (to prepare 1a) we observed the loss of guest bipy together with guest and coordinated water molecules. This was confirmed by IR spectroscopy and elemental analysis (see ESI‡). Powder X-ray diffraction (PXRD) pattern of {[Mn₃(bipy)₃][Cr(CN)₆]₂}(1a) recorded after heating the sample at 165 °C under vacuum suggests that the 3D framework structure remains intact even if it sustains loss of water (guest and coordinated) and guest bipy molecules (Fig. S7, ESI‡). However, decrease in intensity of peaks in 1a indicates that crystallinity of the deguest sample decreases.

The desolvated framework 1a was used for gas adsorption studies to establish the permanent porosity of the compound. N₂ adsorption at 77 K reveals a type II profile indicating only surface adsorption (Fig. S8, ESI‡). On the contrary, CO₂ adsorption measurement at 195 K shows a typical type I profile with a steep uptake at low pressure regions (Fig. 2). The Langmuir surface area calculated from CO₂ profile turns out to be 353 m² g⁻¹. The hysteretic sorption and large isosteric heat of adsorption (q_st, ϕ) value of ∼34.3 kJ mole⁻¹ (calculated using Dubinin–Radushkevich equation)¹² suggests a strong interaction of CO₂ molecules with 1a. This could be correlated with the well established fact that the electric field generated in the framework by UMSs and aromatic π-cloud interacts firmly with the quadrupole moment of CO₂ (−1.4 × 10⁻³⁹ C m²) causing a rapid uptake at low pressure. 1a was also tested for its H₂-storage capacity at cryogenic temperature (inset of Fig. 2). High pressure adsorption measurement at 77 K reveals a type I profile with a steep uptake at low pressure and the final uptake volume is ∼81 mL g⁻¹ at ∼45 bar. Interestingly, a careful measurement at 77 K and low pressure (P ≈ 1 atm) (Fig. S8, ESI‡) shows a prominent hysteresis in the adsorption profile suggesting a strong interaction of H₂ molecules with the pore surface. The high pressure profiles measured at 77 and 87 K were used for the calculation of enthalpy of adsorption (ΔH_ads) applying Clausius–Clapeyron equation which provides a value of ∼11.5 kJ mol⁻¹ (Fig. S9–S11, see ESI‡).^10a The high value of ΔH_ads can be linked to the highly polar nature of 1a decorated with cyanide groups (free as well as coordinated) and unsaturated MnII sites (Mn1 and Mn2) which bind H₂ molecules strongly in the pore. This is also in line with the hysteresis obtained from the low pressure measurement. It is noteworthy that the value of ΔH_ads obtained for 1a is one of the highest among the reported values.¹⁰ The polar nature of 1a is also reflected in H₂O sorption that displays an unusual type I profile (Fig. S12, ESI‡) with a steep uptake at the low pressure region where the framework shows uptake of eight water molecules resulting in {[Mn₃(bipy)₃][Cr(CN)₆]₂·8H₂O} (1b).

Fig. 2 CO₂ adsorption isotherm of 1a measured at 195 K. Inset shows the high pressure H₂ isotherm (only adsorption) of 1a measured at 77 K (a) and 87 K (b).

We have successively measured magnetic data of as-synthesized 1, 1a and 1b (rehydrated). The dc magnetic susceptibilities are shown in the form of χ_Mvs. T (Fig. S13, ESI‡) and χ_MT vs. T (1 and 1a) in Fig. 3. The χ_M value for 1 at 300 K is 0.0531 cm³ mol⁻¹(χ_MT = 16.17 cm³ mol⁻¹ K) which agrees well with the spin only value (0.0558 cm³ mol⁻¹; 16.74 cm³ mol⁻¹ K) expected for magnetically isolated three MnII (S = 5/2) and two CrIII (S = 3/2) ions. The χ_MT value gradually decreases with decreasing temperature to reach a minimum value of 14.96 cm³ mol⁻¹ K at 85 K, and then rapidly increases to a maximum value of 2114.96 cm³ mol⁻¹ K at 53 K and then again decreases to 161.65 cm³ mol⁻¹ K at 2.5 K. The 1/χ_Mvs. T plot in the temperature range of 300–150 K obeys the Curie–Weiss law (Fig. S14, ESI‡) with a Weiss constant θ = −39.3 K, which suggests the antiferromagnetic interaction between the adjacent MnII and CrIII ions through cyanide bridges as has already been observed.¹³ The rapid increase in χ_MT value indicates a ferrimagnetic ordering, which was determined accurately as 80 K by a dM/dT differential plot (Fig. S15, ESI‡). The decrease in χ_MT value after 53 K suggests further interaction between the layers through the bipy linker. The magnetic data of 1a also show a typical ferrimagnetic ordering below 50 K and the dM/dT plot suggests that the T_c is about 45.3 K (Fig. S16, ESI‡). The Weiss constant θ was calculated as −59.8 K (300–150 K, Fig. S17, ESI‡) suggesting stronger antiferromagnetic interaction in 1a compared to 1. The change in Weiss constant, T_c and maxima in the χ_MT value suggest that there are changes in the magnetic pathways in 1a. The value of T_c depends on the overall magnetic interaction via covalent (cyanide or as well as bipy bridges) and noncovalent interactions (like H-bond, π–π interactions). In 1a upon removal of the guest water, bipy and coordinated water molecules all the H-bonding interactions (Fig. S5, ESI‡) (vide supra) between the magnetic centres are destroyed. These noncovalent interactions provide ferromagnetic interactions in 1 and their absence in 1a might be responsible for lowering the Curie temperature. Moreover, the structural strain in the deguest phase (like the bent Cr–CN–Mn pathway) also provides the negative contribution of T_c.^3c The increase in T_c (59 K) in 1b (Fig. S18–S20, ESI‡) also unequivocally suggests the role of water molecules in ferromagnetic interactions as they are involved in several H-bonding interactions. The field dependence magnetization curve of 1 shows a rapid increase to saturate at 8 kOe which is also the signature of magnetic ordering within the compound (Fig. 4). The saturation magnetization value in 1 is 10 Nβ which is in good agreement with the theoretical value (9 Nβ) expected for a ferrimagnet where antiferromagnetic coupling between CrIII and MnII exists. The decrease in magnetization value (6 Nβ) in 1a at high field suggests strong antiferromagnetic interaction operating between CrIII and MnII which is also in line with observed high Weiss constant and low T_c.

Fig. 3 Temperature dependence of the magnetic susceptibility of 1 and 1a in an applied field of 500 Oe under zero field cooled condition. Inset shows the same for 1a to give a clear understanding of ordering at low T.

Fig. 4 Magnetization plot for 1 and 1a measured at 2.5 K.

In conclusion, we have conceived a rational strategy to assemble 2D magnetic layers into a 3D rigid porous framework that exhibits permanent porosity and ferrimagnetic ordering at low temperature. The highly polar nature of 1a was found to be exceedingly promising for high heat of H₂ adsorption and establishes itself to be the first member of cyanometallate systems having heat of H₂ adsorption value of ∼11.5 kJ mol⁻¹. The framework demonstrates an interesting guest responsive magnetic modulation with a change in T_c as observed from dc susceptibility measurements. Current efforts are underway to synthesize frameworks with different pillar modules and study their porosity and magnetic properties.

TKM gratefully acknowledges the financial support from DST, Govt. of India (fast track proposal). PK acknowledges CSIR, India for SRF. The authors are thankful to Mr N. Kumar and Prof. A. Sundaresan for helping in magnetic measurements at JNCASR.

Notes and references

1. (a) L. J. Murray, M. Dincă and J. R. Long, Chem. Soc. Rev., 2009, 38, 1294; (b) S. Kitagawa, R. Kitaura and S. Noro, Angew. Chem., Int. Ed., 2004, 43, 2334; (c) J. R. Li, R. J. Kuppler and H.-C. Zhou, Chem. Soc. Rev., 2009, 38, 1477; (d) J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450; (e) D. Maspoch, D. Ruiz-Molina and J. Veciana, Chem. Soc. Rev., 2007, 36, 770; (f) S. Achmann, G. Hagen, J. Kita, I. M. Malkowsky, C. Kiener and R. Moos, Sensors, 2009, 9, 1574; (g) P. Horcajada, C. Serre, G. Maurin, N. A. Ramsahye, F. Balas, M. Vallet-Regi, M. Sebban, F. Taulelle and G. Férey, J. Am. Chem. Soc., 2008, 130, 6774; (h) P. Kanoo, R. Matsuda, M. Higuchi, S. Ktagawa and T. K. Maji, Chem. Mater., 2009, 21, 5860.
2. (a) N. R. Champness, Dalton Trans., 2006, 877; (b) C. J. Kepert, Chem. Commun., 2006, 695; (c) Z. M. Wang, B. Zhang, Y. J. Zhang, M. Kurmoo, T. Liu, S. Gao and H. Kobayashi, Polyhedron, 2007, 26, 2207; (d) P. Kanoo, K. L. Gurunatha and T. K. Maji, J. Mater. Chem., 2010, 20, 1322.
3. (a) M. Ohba, K. Yoneda and S. Kitagawa, CrystEngComm, 2010, 12, 159; (b) T. Mallah, S. Thiebaut, M. Verdaguer and P. Veillet, Science, 1993, 262, 1554; (c) W. Kaneko, M. Ohba and S. Kitagawa, J. Am. Chem. Soc., 2007, 129, 13706.
4. (a) X. N. Cheng, W. X. Zhang, Y. Y. Lin, Y. Z. Zheng and X. M. Chen, Adv. Mater., 2007, 19, 1494; (b) M. Kurmoo, H. Kumagai, K. W. Chapman and C. J. Kepert, Chem. Commun., 2005, 3012; (c) D. Maspoch, D. Ruiz-Molina, K. Wurst, N. Domingo, M. Cavallini, F. Biscarini, J. Tejada, C. Rovira and J. Veciana, Nat. Mater., 2003, 2, 190; (d) G. J. Halder, C. J. Kepert, B. Moubaraki, K. S. Murray and J. D. Cashion, Science, 2002, 298, 1762.
5. (a) D. Maspoch, D. Ruiz-Molina and J. Veciana, J. Mater. Chem., 2004, 14, 2713; (b) E. Coronado, F. Palacio and J. Veciana, Angew. Chem., Int. Ed., 2003, 42, 2570.
6. (a) V. Niel, A. L. Thompson, M. C. Munoz, A. Galet, A. S. E. Goeta and J. A. Real, Angew. Chem., Int. Ed., 2003, 42, 3760; (b) Z. M. Wang, B. Zhang, H. Fujiwara, H. Kobayashi and M. Kurmoo, Chem. Commun., 2004, 416; (c) M. Kurmoo, H. Kumagai, M. Akita-Tanaka, K. Inoue and S. Takagi, Inorg. Chem., 2006, 45, 1627.
7. (a) M. H. Zeng, X. L. Feng, W. X. Zhang and X. M. Chen, Dalton Trans., 2006, 5294; (b) L. G. Beauvais and J. R. Long, J. Am. Chem. Soc., 2002, 124, 12096.
8. (a) K. Barthelet, J. Marrot, D. Riou and G. Férey, Angew. Chem., Int. Ed., 2002, 41, 281; (b) C. Livage, N. Guillou, J. Chaigneau, P. Rabu, M. Drillon and G. Férey, Angew. Chem., Int. Ed., 2005, 44, 6488; (c) C. Serre, F. Millange, C. Thouvenot, M. Nogues, G. Marsolier, D. Louer and G. Férey, J. Am. Chem. Soc., 2002, 124, 13519.
9. K. M. Thomas, Dalton Trans., 2009, 1487.
10. (a) S. S. Kaye and J. R. Long, J. Am. Chem. Soc., 2005, 127, 6506; (b) J. G. Vitillo, L. Regli, S. Chavan, G. Ricchiardi, G. Spoto, P. D. C. Dietzel, S. Bordiga and A. Zecchina, J. Am. Chem. Soc., 2008, 130, 8386; (c) B. Chen, X. Zhao, A. Putkham, K. Hong, E. B. Lobkovsky, E. J. Hurtado, A. J. Fletcher and K. M. Thomas, J. Am. Chem. Soc., 2008, 130, 6411; (d) S. Mohapatra, K. Hembram, U. Waghmare and T. K. Maji, Chem. Mater., 2009, 21, 5406.
11. T. K. Maji, S. Pal, K. L. Gurunatha, A. Govindaraj and C. N. R. Rao, Dalton Trans., 2009, 4426.
12. M. M. Dubinin, Chem. Rev., 1960, 60, 235.
13. (a) Y. Yoshida, K. Inoue and M. Kurmoo, Chem. Lett., 2008, 586; (b) T. Glaser, M. Heidemeier, T. Weyhermüller, R. D. Hoffmann, H. Rupp and P. Müller, Angew. Chem., Int. Ed., 2006, 45, 6033; (c) A. Marvilliers, S. Parsons, E. Rivière, J. P. Audière and T. Mallah, Chem. Commun., 1999, 2217.

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Footnotes

† This article is part of the ‘Emerging Investigators’ themed issue for ChemComm.

‡ Electronic supplementary information (ESI) available: Extensive figures and full experimental details of crystallography, physical measurements, magnetic measurements. CCDC 784351. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc02490b

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