Encapsulation, storage and controlled release of sulfur hexafluoride from a metal–organic capsule

Abstract

A metal–organic cage is shown to bind SF₆—the most potent greenhouse gas known—and to release it under well-defined conditions.

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Sulfur hexafluoride is long-lived in the atmosphere¹ and absorbs infrared light with great efficiency,² making it the most potent greenhouse gas known.³ Small reductions in the amount of this gas released from industrial processes are thus highly effective in abating global warming, with each gram of SF₆ climatically equivalent to 24 kg of CO₂.²SF₆ represents a challenging target for sequestration because of a property that renders it industrially useful: it is chemically inert. Here we report the reversible, selective trapping of inert, hydrophobic SF₆ within the central cavity of a self-assembled metal–organic cage, representing the first example of gas binding within such a discrete metal–organic capsule in solution.

Nature employs a range of metal–organic complexes to bind and transport gases as well as catalyse their conversion to useful substrates.⁴ Organic hosts and metal–organic frameworks (MOFs) have been demonstrated to bind a variety of gas molecules with potential applications in gas storage, transportation and purification.^5,6 In contrast to MOFs, which are constructed of an infinite lattice of atoms, self-assembled metal–organic capsules are discrete complexes which have been shown to selectively bind guests within their central cavities,⁷ thereby inducing novel reactivity⁸ or rendering pyrophoric materials stable.⁹ MOFs are structurally defined only in the solid state, whereas the solubility of cages allows inclusion complexes to be observed in solution. Three recent examples of discrete metallo-supramolecular systems have been shown to exhibit MOF-like gas binding in the solid state¹⁰ but not in solution. The present system thus represents the first example of the encapsulation, storage and controlled release of a gas molecule within a dissolved metal–organic capsule.

We have previously reported the synthesis of the tetramethylammonium salt of 1 through subcomponent self-assembly (Scheme 1).^11,12 The twelve sulfonate groups decorating the exterior of the cage provide water solubility while the aromatic rings that define the edges of the cage provide a hydrophobic interior. This design allows the encapsulation of guests within the central cavity in water. SF₆ is a non-toxic, colourless, odourless gas with limited water solubility (2.2 × 10⁻⁴ M at 298 K and 1 atm);^3,13 its potent climatic impact made it a relevant target for sequestration within 1.

Scheme 1 Synthesis of 1 (left) and its sequestration of SF₆ to form [SF₆⊂1] (right).

Bubbling SF₆ through a solution of 1 in D₂O resulted in the formation of [SF₆⊂1], as observed by NMR. The aromatic protons of [SF₆⊂1] displayed a significant downfield shift relative to those observed in the spectrum of guest-free 1 (Fig. 1). The spectrum, however, did not indicate complete encapsulation; at most 75% of 1 was observed as [SF₆⊂1], regardless of reaction time and volume of gas added.

Fig. 1 ¹H NMR (top) and ¹⁹F NMR (bottom) spectra of [SF₆⊂1] at 50% guest uptake. Blue peaks in the ¹H NMR correspond to [SF₆⊂1], black peaks to uncomplexed 1. Peaks marked with dots indicate the diamine subcomponent, present as a minor impurity.

Signals corresponding to both encapsulated and uncomplexed SF₆ were observed in the ¹⁹F NMR spectrum (Fig. 1), confirming the encapsulation and indicating slow exchange on the NMR timescale.

Encapsulation of SF₆ within the cavity of 1 was also observed by X-ray diffraction.§ Crystals of (NMe₄)₄[(SF₆)_0.5⊂1]·15H₂O suitable for diffraction studies were grown by vapour diffusion of 1,4-dioxane into a H₂O solution of [SF₆⊂1]. Although the quality and resolution of the data obtained were limited, they are of sufficient quality to unambiguously assign the cage's structure and the presence of the SF₆ guest within the cage (Fig. 2). As observed previously,^9,12 the metal centres are located at the vertices of a tetrahedron with six ligands bridging them;¹² there is an average metal–metal separation of 12.9 Å and the cage encapsulates a volume of 141 ų.‡ The bound SF₆ is located at the centre of the cage and is stabilised by van der Waals interactions, suggesting that the hydrophobic effect is a significant driver of encapsulation.¹⁴

Fig. 2 Representation of the ordered portion of the X-ray crystal structure of [SF₆⊂1].

SF₆ occupies 53% of the void volume enclosed by the cage, which is larger than the reported optimal volume ratio of 40% for a gas encapsulated within an organic host.⁶‡ The host–guest binding constant of the system (K_a) is 1.3 × 10⁴ M⁻¹, similar to values obtained with cucurbit[6]uril hosts.¹⁵ The observation of such strong binding suggests the possibility of using 1 to competitively bind SF₆ and hence employing 1 to selectively remove SF₆ from a mixture of gases: in preliminary experiments 1 was observed to have no affinity for Xe, Ar, N₂, O₂, C₂H₄, CO₂ or N₂O.

This binding constant indicated that the maximum loading of SF₆ into a saturated solution of 1 in water is 6.6 mM at ambient temperature and pressure, which is 30 times higher than the solubility of SF₆ in the absence of the host under similar conditions. SF₆ can readily be stored under ambient conditions within 1. A solution of [SF₆⊂1] left open to the atmosphere at room temperature showed negligible loss of gas after one week.

The amount of gas present in the complex in the solid state (as indicated by X-ray diffraction)§ suggests the concentration of SF₆ is significantly greater than in solution. The crystal density of 1.341 g cm⁻³ equates to a gas concentration of 0.17 M, or 4.1 cm³ of gas for every cm³ of solid. Assuming ideal gas behaviour, 1.88 kJ mol⁻¹ would be required to compress a gas to this extent. The ability of 1 to compress and store the gas both as a solid and in aqueous solution under ambient conditions may be rationalised in terms of the ‘pressure’ experienced by the guest upon encapsulation.¹⁶ Assuming a hard-sphere model of [SF₆⊂1] and ideal gas behaviour of SF₆, the guest molecule experiences 288 atm of pressure. Under this pressure at 298 K, the bulk phase of SF₆ would not be a gas.¹⁷

The application of three different chemical and physical stimuli allowed for the controlled release of SF₆ from 1. First, increasing the temperature induced guest desorption: after 96 hours less than 10% of the SF₆ remained within 1 in a solution heated to 323 K and open to the atmosphere. Second, upon addition of acid, 1 opened releasing the guest molecule. Third, the addition of tris(2-ethylamino)amine to a solution of [SF₆⊂1] removed the iron(II) and the 2-formylpyridine residues from 1,¹² releasing the guest. Thermal release could form the practical basis of a process by which SF₆ could be scrubbed from a waste stream.

In conclusion, we have demonstrated the ability of 1 to strongly bind, and hence markedly increase the water solubility of, the potent greenhouse gas SF₆ under ambient conditions. Encapsulated SF₆ is significantly compressed and can be readily stored in solution. Furthermore, the application of three mild physical and chemical stimuli allows for the gas to be controllably released from the host molecule. These results point the way towards the application of discrete supramolecular metal–organic cages for gas separation and manipulation, particularly in the context of SF₆ recycling.

We thank Dr J. E. Davies for assistance with collecting the X-ray data. This work was supported by the Netherlands Organisation for Scientific Research-NWO (M.M.J.S.), the Marie Curie IIF scheme of the 7th EU Framework Program (J.K.C.), the Walters-Kundert Charitable Trust (J.R.N.) and the EPSRC.

Notes and references

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Footnotes

† This manuscript is part of the Emerging Investigators theme issue for ChemComm.

‡ Electronic supplementary information (ESI) available: Experimental details and spectroscopic data; details of calculations; CIF for (N(Me)₄)₄[(SF₆)_0.5⊂1]·15H₂O. CCDC 784594. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0cc02573a

§ Crystal data for (N(Me)₄)₄[(SF₆)_0.5⊂1]·15H₂O, formula C₁₆₀H₁₇₄F₃Fe₄N₂₈O₅₁S_12.50, M 3986.42, trigonal, space groupP3̄c1(#165), a 33.788(12), b 33.788(7), c 39.938(8) Å, V 39 486(18) ų, D_c 1.341 g cm⁻³, Z 8, crystal size 0.32 × 0.10 × 0.05 mm, colour purple, habit block, temperature 180(2) K, λ(MoKα) 0.71073 Å, μ(MoKα) 0.505 mm⁻¹, T(SORTAV)_min,max 0.792, 0.966, 2θ_max 30.06, hkl range −15 24, −24 18, −28 28, N 23 796, N_ind 5327 (R_merge 0.1287), N_obs 3684 (I > 2σ(I)), N_var 308, residuals R₁(F) 0.1335, wR₂(F²) 0.3690, GoF(all) 1.343, Δρ_min,max −0.596, 1.063 e⁻ Å⁻³.

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