Exceptional adsorption-induced cluster and network deformation in the flexible metal–organic framework DUT-8(Ni) observed by in situ X-ray diffraction and EXAFS

Abstract

The "gate opening" mechanism in the highly flexible MOF Ni2(2,6-ndc)2dabco (DUT-8(Ni), DUT = Dresden University of Technology) with unprecedented unit cell volume change was elucidated in detail using combined single crystal X-ray diffraction, in situ XRD and EXAFS techniques. The analysis of the crystal structures of closed pore (cp) and large pore (lp) phases reveals a drastic and unique unit cell volume expansion of up to 254%, caused by adsorption of gases, surpassing other gas-pressure switchable MOFs significantly. To a certain extent, the structural deformation is specific for the guest molecule triggering the transformation due to subtle differences in adsorption enthalpy, shape, and kinetic diameter of the guest. Combined adsorption and powder diffraction experiments using nitrogen (77 K), carbon dioxide (195 K), and n-butane (272.5 K) as a probe molecules reveal a one-step structural transformation from cp to lp. In contrast, adsorption of ethane (185 K) or ethylene (169 K) results in a two-step transformation with the formation of intermediate phases. In situ EXAFS during nitrogen adsorption was used for the first time to monitor the local coordination geometry of the metal atoms during the structural transformation in flexible MOFs revealing a unique local deformation of the nickel-based paddle-wheel node.

Pillared-layered metal–organic frameworks for mechanical energy storage applications

“…Our simulated low angle XRD patterns illustrate that especially the rst peaks should change in relative intensity when the structure contracts. This trend is also observed in experimental in situ studies of guest-or temperature-induced breathing of pillared-layered MOFs 44,49,53,96 Going from the op state to the lp state, we predict a general shi of the peaks towards higher angles in agreement with the volume contraction. The rst peaks shi further to the right in the np and cp congurations, but a second large peak arises in the latter, which may be used in experiment to identify the state of the sample.…”

Raman spectroscopy studies of the terahertz vibrational modes of a DUT-8 (Ni) metal–organic framework

“…The crystal structure of the model for the BOMD simulations of the flexible DUT-8(Ni) MOF was taken from the ESI in ref. 23. The geometry of the crystal structure of the open pore form of DUT-8(Ni) was first optimized using the method mentioned above.…”

“…22,23 The peculiarity of this MOF is that it can be synthesized in ''rigid'' and ''flexible'' forms, depending on the synthetic conditions. 24 The rigid version (DUT-8(Ni)_rigid), consisting of nano sized crystallites, can be desolvated without any phase transition and shows a typical ''Type Ia'' nitrogen physisorption isotherm at 77 K. In contrast, the flexible form (DUT-8(Ni)_flex), crystalized as macro crystals, undergoes a transformation into a closed phase (cp) upon desolvation and can be reversibly transformed into the open phase (op) by adsorption of gases or liquids.…”

Nanoscale Metamaterials: Meta-MOFs and Framework Materials with Anomalous Behavior

“…A family of isostructural materials named DUT-8 (comprised of transition metal paddlewheel units with 1,4-benzenedicarboxylate and 1,4-diazabicyclo [2,2,2]octane ligands) shows significant deformation of the paddlewheel unit which permits a large degree of flexibility [98,99]. DUT-8(Ni) exhibits gate-opening adsorption processes where a complete collapse after solvent removal is reversible following the adsorption of gas at a specific pressure and temperature.…”