An Unprecedented Stimuli‐Controlled Single‐Crystal Reversible Phase Transition of a Metal–Organic Framework and Its Application to a Novel Method of Guest Encapsulation

Abstract

The flexibility and unexpected dynamic behavior of a third-generation metal-organic framework are described for the first time. The synthetic strategy is based on the flexibility and spherical shape of dipyridyl-based carborane linkers that act as pillars between rigid Co/BTB (BTB: 1,3,5-benzenetricarboxylate) layers, providing a 3D porous structure (1). A phase transition of the solid can be induced to generate a new, nonporous 2D structure (2) without any loss of the carborane linkers. The structural transformation is visualized by snapshots of the multistep single-crystal-to-single-crystal transformation by single-crystal and powder X-ray diffraction. Poor hydrogen bond acceptors such as MeOH, CHCl or supercritical CO induce such a 3D to 2D transformation. Remarkably, the transformation is reversible and the 2D phase 2 is further converted back into 1 by heating in dimethylformamide. The energy requirements involved in such processes are investigated using periodic density functional theory calculations. As a proof of concept for potential applications, encapsulation of C is achieved by trapping this molecule during the reversible 2D to 3D phase transition, whereas no adsorption is observed by straight solvent diffusion into the pores of the 3D phase.

Reversible Phase Transition of Porous Coordination Polymers

“…This phenomenon indicated that the RPT behavior of YNU‐1 opened up a new strategy to encapsulate guests that exceeded the pore size of MOFs. A similar phenomenon was also reported by Planas et al …”

“…B) Optimized conditions for the bulk reversible 1 to 2 transformation and C 60 encapsulation (Reproduced with permission from ref. ).…”

“…Planas et al reported aM OF,n amely, [Co 3 (BTB) 2 (mCB) 2 ]·x DMF (1;B TB:1 ,3,5-benzenetricarboxylate, mCB = 1,7-bis{(piridin-4-yl)methanol}-1,7-dicarba-closo-dodecaborane), that consisted of trinuclear[ Co 3 (COO) 6 N 4 ]u nits;e ach unit coordinated to six different BTB ligands, which createda six-pointed star motif of Co/BTB rigid layers extending in the bc plane. [44] Then these layers were pillared by mCB ligands to form a3 Df ramework. Poor hydrogen-bond acceptors, such as MeOH,C HCl 3 ,o rs upercritical CO 2 ,c an induce such a3 D( 1)t o 2D (2)t ransformation.O nt he contrary,t he 2D form can convert back to the 3D form upon heating in DMF and possess potentialapplications( Figure 2B).…”

“…A) Reversible adsorption of CH 2 Cl 2 ,CHCl 3 ,a nd CCl 4 based on ap ristine MOF and single-crystals tructures (Reproduced with permission from ref [43]. 2011, The Royal SocietyofC hemistry).B )Optimizedc onditionsf or the bulk reversible 1 to 2 transformationa nd C 60 encapsulation (Reproduced with permission from ref [44]…”

Metal–Organic Frameworks in Modern Physics: Highlights and Perspectives

“…This MOF behaves like a spin-crossover magnet, where strong mutual switching between photo-emitted Fe II and neighboring Nb IV atoms works through C-N bridges. [97] Light can also stimulate the transition between two structural states in MOFs with special photochromic molecules as ligands or guests. [96] The 3D niccolite MOF mentioned earlier, [(CH 3 CH 2 ) 2 NH 2 ] [Fe III Fe II (HCOO) 6 ], also appeared to demonstrate magnetic hysteresis regulated by a thermal and magnetic state at low temperatures, [80] a phenomenon which is the response of the magnetization of Fe II and Fe III sublattices to external stimuli.…”