Crystal size versus paddle wheel deformability: selective gated adsorption transitions of the switchable metal-organic frameworks and † Switchable pillared layer metal-organic frameworks M 2 (2,6-ndc) 2 (dabco) (DUT-8(M), M ¼ Ni, Co, 2,6-ndc ¼ 2,6naphthalenedicarboxylate, dabco ¼ 1,4-diazabicyclo-[2.2.2]octane, DUT -Dresden University of Technology) were synthesised in two different crystallite size regimes to produce particles up to 300 mm and smaller particles around 0.1 mm, respectively. The textural properties and adsorption-induced switchability of the materials, obtained from both syntheses, were studied by physisorption of N 2 at 77 K, CO 2 at 195 K and nbutane at 273 K, revealing pronounced differences in adsorption behavior for Ni and Co analogues. While the smaller nano-sized particles (50-200 nm) are rigid and show no gating transitions confirming the importance of crystallite size, the large particles show pronounced switchability with characteristic differences for the two metals resulting in distinct recognition effects for various gases and vapours. Adsorption of various vapours demonstrates consistently a higher energetic barrier for the "gate opening" of DUT-8(Co) in contrast to , as the "gate opening" pressure for Co based material is shifted to a higher value for adsorption of dichloromethane at 298 K. Evaluation of crystallographic data, obtained from single crystal and powder X-ray diffraction analysis, showed distinct geometric differences in the paddle wheel units of the respective MOFs.These differences are further disclosed by solid-state UV-vis, FT-IR and Raman spectroscopy. Magnetic properties of DUT-8(Co) and DUT-8(Ni) were investigated, indicating a high-spin state for both materials at room temperature. Density functional theory (DFT) simulations confirmed distinct energetic differences for Ni and Co analogues with a higher energetic penalty for the structural "gate opening" transformation for DUT-8(Co) compared to DUT-8(Ni) explaining the different flexibility behaviour of these isomorphous MOFs.underlying structural phase transitions are triggered by adsorption or desorption of guest molecules and generally characterised by an activation energy barrier, which causes hysteresis in physisorption experiments. Due to their switchable nature, so porous crystals are oen discussed as materials with huge application potential in gas storage, 11 separation processes, 12,13 sensor technology 14,15 and catalysis. 16 Despite rapidly growing research in the eld of exible MOFs, 1,17-21 the role of critical factors inuencing and controlling framework switchability are barely understood. 22 As MOFs are modular networks, the exibility of the linker but also the hinges of the metal node are key features that affect switchability. The importance of metal-node hinges and their energetics for framework switchability has been widely investigated for compounds such as M(bdp) (M ¼ Co, Fe, bdp ¼ 1,4-benzenedipyrazolate), 11,23,24 M(m-OH)(bdc) (MIL-53, M ¼ Cr, Al, Fe, bdc ¼ 1,4-benzenedicarboxylate)...

Switchable metal-organic frameworks (MOFs) have been proposed for various energy-related storage and separation applications, but the mechanistic understanding of adsorption-induced switching transitions is still at an early stage. Here we report critical design criteria for negative gas adsorption (NGA), a counterintuitive feature of pressure amplifying materials, hitherto uniquely observed in a highly porous framework compound (DUT-49). These criteria are derived by analysing the physical effects of micromechanics, pore size, interpenetration, adsorption enthalpies, and the pore filling mechanism using advanced in situ X-ray and neutron diffraction, NMR spectroscopy, and calorimetric techniques parallelised to adsorption for a series of six isoreticular networks. Aided by computational modelling, we identify DUT-50 as a new pressure amplifying material featuring distinct NGA transitions upon methane and argon adsorption. In situ neutron diffraction analysis of the methane (CD 4 ) adsorption sites at 111 K supported by grand canonical Monte Carlo simulations reveals a sudden population of the largest mesopore to be the critical filling step initiating structural contraction and NGA. In contrast, interpenetration leads to framework stiffening and specific pore volume reduction, both factors effectively suppressing NGA transitions.

Controlled nucleation in a micromixer and further crystal growth were used to synthesize Ni(2,6-ndc)dabco (2,6-ndc - 2,6-naphthalenedicarboxylate, dabco - 1,4-diazabicyclo[2.2.2]octane), also termed DUT-8(Ni) (DUT = Dresden University of Technology), with narrow particle size distribution in a range of a few nm to several μm. The crystal size was found to significantly affect the switching characteristics, in particular the gate opening pressure in nitrogen adsorption isotherms at 77 K for this highly porous and flexible network. Below a critical size of about 500 nm, a type Ia isotherm typical of rigid MOFs is observed, while above approximately 1000 nm a pronounced gating behaviour is detected, starting at p/p = 0.2. With increasing crystal size this transition gate becomes steeper indicating a more uniform distribution of activation energies within the crystal ensemble. At an intermediate size (500-1000 nm), the DUT-8(Ni) crystals close during activation but cannot be reopened by nitrogen at 77 K possibly indicating monodomain switching.