The structure and chemistry of porous metal-organic frameworks (MOFs) affects their suitability for laboratory and industrial applications. Many MOFs exhibit structural flexibility, accommodating large structural changes upon adsorption/desorption of guest or under a mechanical stimulus without losing crystallinity.1,2 Framework properties can be modified by changing the metal or ligand species, or less commonly, through disorder (e.g. defects or substitutional disorder). Defect engineering is studied in UiO-type frameworks but the role of disorder on mechanical properties is rarely studied in other MOF structure-types.3
Here, the effect of ligand ratio and distribution on the structural behaviour of zeolitic imidazolate framework, ZIF-60 (Zn(Im)1.5(mIm)0.5, where Im is imidazolate and mIm is 2-methylimidazolate) is examined by single crystal X-ray diffraction under ambient and high-pressure conditions.4 Three new ZIF-60 derivatives are synthesised through solvothermal methods. Two of the frameworks feature mIm partially occupying one coordination site in different quantities, with formulae of Zn(Im)1.8(mIm)0.2 (A) and Zn(Im)1.9(mIm)0.1 (B). The third features mIm in two coordination sites with a formula of Zn(Im)1.5(mIm)0.5 (C). Framework C is sterically hindered due the presence of an intraframework steric clash between methyl groups on the mIm ligands. All the frameworks are isostructural and crystallise in the tetragonal space group, I4/mmm, as-synthesised from either N,N-diethylformamide (DEF) or N,N-dimethylformamide (DMF). The frameworks are perforated by interconnected channels.
To assess the effect of guest-exchange on the framework structures, each material was characterised before and after being soaked in immersion oil, ethanol or water. Frameworks A and B underwent phase transitions from large-pore to intermediate-pore or narrow-pores structure, with symmetry-lowering from I4/mmm to P42/nmc. Framework A retains the large-pore structure in ethanol but forms the intermediate-pore phase (P42/nmc) in immersion oil or water. By contrast, B forms the intermediate-pore phase in ethanol and immersion oil and forms the narrow-pore phase (P42/nmc) in water, highlighting the importance of the ligand ratio on the frameworks' structural properties. Sterically hindered C is unaffected by guest-exchange, highlighting the importance of ligand distribution.
Structural flexibility of frameworks A and C is probed by hydrostatic compression of their single crystals to 7 GPa in pressure-transmitting media of DEF/DMF and methanol/ethanol. Compression of A in both media forces guest into the pores, resulting in gate-opening rotation of the mIm ligand. In DEF, A undergoes an isostructural phase transition at 2.03 GPa, with sudden shortening along the channel direction corresponding to desorption of guest from the pores. In methanol/ethanol, A undergoes a re-entrant phase transition from I4/mmm and Pmnn and back to I4/mmm at 1.13 GPa and 2.47 GPa, respectively, corresponding to adsorption and desorption of guest, which is facilitated by the smaller size of MeOH/EtOH media. Additional steric hindrance in framework C prevents or delays gate-opening and sterically blocks phase transitions in both media.