Abstract
Porous coordination polymers are molecule-based materials presenting a high degree of tunability, which offer many advantages for targeted applications over conventional inorganic materials. We demonstrate here that the hydrophilic-hydrophobic character of Prussian blue analogues having a lipophilic feature may be tuned to optimize the gas adsorption properties. The role of the coordinatively unsaturated metal sites is emphasized through a combination of theoretical and experimental study of water, ethanol and n-hexane adsorption. Porous coordination polymers are exciting molecule-based materials, made of metal ion nodes and molecular building blocks, which have attracted a great deal of attention for several decades not only from the fundamental point of view, but also due to potential technological applications in several fields including catalysis, gas storage, separation and purification. The most explored families of porous coordination polymers belong to Metal Organic Frameworks (MOFs) [1,2] and Prussian Blue Analogues (PBA) [3] have been investigated as promising alternatives for activated carbon or zeolite materials usually used in industry for gas storage [4] or separation/purification processes. [5-7] They present many advantages in comparison with conventional inorganic porous adsorbers consisting in: (i) "soft" chemistry routes for their synthesis, (ii) thermal and hydrothermal stability, (iii) comparable or higher adsorption capacities, (iv) structural flexibility providing the possibility to easily adjust the pore size and topology, (v) adjustable chemical composition permitting to optimize the physico-chemical properties of adsorption sites, such as the Coordination Unsaturated Sites (CUS), (vi) functionalization ability offering the possibility to tune the affinity between the host network and the guest molecules. [3,8] In particular, the adjustability of the hydrophobic/hydrophilic character of these materials, independently to their lipophilic/lipophobic balance, to finely adapt the interaction between guest molecules and host frameworks by using different approaches appears as a fundamental point for optimizing the adsorption properties. [9,10] In the targeted applications where adsorption is the main process, the surface chemistry of materials is indeed of prime importance. [11] In the case of MOFs, several strategies have been undertaken in order to adapt the affinity of the host network either for favouring or disfavouring interactions with sorbates: (i) the post-synthetic [12-14] or in-situ [15-17] functionalization of the framework, (ii) the modification of the CUS nature or their saturation to prevent strong interactions between metal center and sorbate molecules, [18] or (iii) the introduction of extra-framework charges by using for instance hydrophilic alkali or hydrophobic organic cations. [19], [20] In all these cases, the hydrophobic/hydrophilic character of the network is governed by the nature of the organic functionalities, metal ions, CUS, which modify the hydrophilic-hydrophobic balance by varying the nature of the interactions (strong covalent interactions, electrostatic or van der Waals interactions, H-bond interactions and hydrophobic interactions) of the solid network with guest species. Thus, in general cases, such modification of the hydrophobic/hydrophilic character is irreversible, except for MIL-53, which offers a hydrophobic-hydrophilic transition originating from a structural phase transition. Indeed the network is hydrophobic in the large pores form and becomes hydrophilic when water enters in the structure for the narrow pores form. [21][22] In contrast, PBA have been much less investigated for gas adsorption [23] despite an excellent hydrothermal stability and high adsorption capacities observed for the adsorption of water [24,25] , CO2 [26-28] or ammonia. [29] In this frame, Kawamoto et al highlighted the potential of PBAs for gas storage using different strategies, but to the best of our knowledge, their surface properties, in terms of hydrophobicity and hydrophilic character have never really been clarified up to now. [24] Recently, we reported on a series of lipophilic PBA materials, highly efficient for the separation of different vapors, such as water and hydrocarbons. [25] It was demonstrated that among this series of compounds, the Co[Co III (CN)6]0.66 PBA exhibits an important hydrothermal stability and a high adsorption capacity allowing it to be greatly efficient for the hydrocarbons separation in both, dry or humid atmospheres. In the present communication, we demonstrate for the first time the ability of this lipophilic Co[Co III (CN)6]0.66 PBA to reversibly switch its hydrophobic-hydrophilic character by coordination of water molecules on the CUS and optimize then its affinity for adsorption of molecules with different character, such as ethanol and n-hexane in humid and dry atmospheres. The PBA Co(H2O)x[Co III (CN)6]0.66•yH2O (x + y = 5.2 as determined by thermogravimetric analysis (Figure S1, Electronic Supporting Information (ESI)) 1 was obtained using the usual self-assembly reactions (see ESI). Figure 1 shows the fcc crystallographic structure of 1, where the Co 2+ and Co 3+ ions are connected through the cyano-bridge forming a 3D cubic structure with pores of ca. 0.5 nm. The electroneutrality in the structure is ensured by [Co III (CN)6] 3 vacancies, which generate randomly