Different Breathing Mechanisms in Flexible Pillared-Layered Metal–Organic Frameworks: Impact of the Metal Center

Abstract

The pillared-layered metal−organic framework compounds M 2 (BME-bdc) 2 (dabco) (M 2+ = Zn 2+ , Co 2+ , Ni 2+ , Cu 2+ ; BME-bdc 2− = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate; dabco = diazabicyclo[2.2.2]octane) exhibit structural flexibility and undergo guest and temperature-induced reversible phase transitions between a narrow pore (np) and a large pore (lp) form. These transitions were analyzed in detail by powder X-ray diffraction ex and in situ, isothermal gas adsorption measurements and differential scanning calorimetry. The threshold parameters (gas pressure or temperature), the magnitude of the phase transitions (volume change) as well as their transition enthalpies are strikingly dependent on the chosen metal cation M 2+ . This observation is assigned to the different electronic structures and ligand field effects on the coordination bonds. Accordingly, in situ powder X-ray diffraction measurements as a function of CO 2 pressure reveal different mechanisms for the np to lp phase transition during CO 2 adsorption.

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

“…In particular, it is experimentally conrmed that the bulk modulus for DMOF-1(Cu) is higher than for its Zn-analogue. The higher rigidity of the copper paddle wheel as compared to a zinc paddle wheel was previously ascribed to electronic effects 38,53 such as orbital directing effects only present in the copper variant. For the latter, we considered various possible magnetic congurations (ESI Section I.C †), as each metal ion has one unpaired delectron.…”

“…37 While theoretical investigations of DMOF-1 are relatively limited, 5,17,21,38,39 the unusual and reversible breathing behavior has been highlighted by several experimental groups. 29,[40][41][42][43][44][45][46][47][48][49][50][51][52][53][54] On the one hand, with guests present in the pores, DMOF-1(Zn) was found to contract upon adsorption. Specic guest molecules such as benzene, methanol, ethanol and isopropanol (IPA) trigger breathing, 29,40,42,50 while N,N-dimethylformamide (DMF) distorts the structure in a different way.…”

“…More interestingly, enormous thermal expansion behavior and breathing transitions could be obtained when functionalizing the BDC linker. 39,48,51,53 Particularly, the attachment of pendant alkoxy chains greatly modied the structural exibility. Functionalization also allowed to inuence the guest-dependent behavior.…”

“…[41][42][43][44]47,49,51,52 Very recently, Fischer and co-workers discussed into detail the signicant impact of the metal ion on the breathing mechanisms in a functionalized DMOF-1. 53 Previous studies highlighted how the threshold gas pressure and transition temperature for the breathing transition can be tuned by the choice of the functional groups and metal ion. So far the response of this MOF upon mechanical pressure has barely been investigated.…”

Effect of the Metal within Regioisomeric Paddle‐Wheel‐Type Metal–Organic Frameworks

“…There are several reports about the relationshipsb etween flexibility (as well as breathing behavior and gate opening) and the metal cation on MOFs. [14][15][16][17][18][19] In particular, Kaskel and co-workers prepared as eries of DUT-8M OFs (DUT = Dresden University of Te chnology) with Ni, Cu, Co, and Zn and studied the magnitude of the changes in flexibility with different metal nodes. DUT-8 species are also DABCO-based pillared MOFs with as lightly tilted 2,6-naphthalenedicarboxylic acid).…”