A Bistable Porous Coordination Polymer with a Bond‐Switching Mechanism Showing Reversible Structural and Functional Transformations

Abstract

In recent years, porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) with flexible structures have attracted considerable attention, owing to their potential as functional materials.[1] Unlike rigid frameworks, where, in most cases, structure and properties remain unchanged after removal of guest molecules, flexible frameworks are very sensitive to the presence of guests and undergo structural variations depending upon the amount and nature of the guest molecules inside the framework.[2] Such compounds are capable of forming bistable phases and are expected to exhibit not only structural variations but also modulation of their physical properties, such as chirality as well as optical and magnetic behavior, to achieve multiple functions that are not observed in robust frameworks or other conventional solids. Structural flexibility has also been observed in inorganic frameworks, although the detected changes are not as drastic as those of PCPs. To create such systems, we have focused on modulation of the crystal structure by reversible removal of guest molecules, to produce multifunctional bistable coordination polymers. If the host framework undergoes singlecrystal-to-single-crystal (SCSC) transformations, [3] crystallographic analysis is a very useful tool for deepening our understanding of the dynamic behavior, and correlating the bistable structures with physical properties, such as magnetism.[1d, 4] PCPs containing flexible ligands and metal ions with variable coordination numbers have great potential for this purpose, as such flexibility allows for stability in various structural forms.Porous crystals based on 2D frameworks have several useful characteristics with regard to dynamic guest-responsive phenomena: 1) interlayer separations, which play a major role in guest-inclusion; 2) framework flexibility, as a result of sliding of the 2D layers and closing/opening of the channel spaces by the guest molecules; 3) rearrangement of the frameworks by cleavage and formation of coordination bonds between the layers, leading to expansion/shrinkage of the 2D layered structures by the guest molecules. Most importantly, the transformation between the 2D and 3D frameworks, with cleavage and generation of new bonds, is expected to lead to large differences in their structural and functional behaviors. These unique materials have a high potential for realizing new functions, such as switching and sensing. To make bistable compounds with drastic structural differences, 2D frameworks containing a flexible ligand and a metal ion with variable coordination numbers may easily undergo transformation to a 3D structure upon guest removal by contracting/expanding and concurrent sliding of the 2D layers, using the bond-switching mechanism of the coordination bonds around the metal centers (Figure 1). [5][6][7][8] In this regard, the Cu II metal ion is very promising, as it has versatile coordination chemistry and can readily adopt octahedral, square-pyramidal, trigonal bipyramidal, and square-planar geometries. H...

Alcohol responsive 2D coordination network of 3-(4-pyridyl)benzoate and Zinc(II)

“…Thus in recent years, solid state chemists have developed a series of bottom-up fabrication schemes to achieve two-dimensional open coordination networks, frequently with limited domain size, using non covalent interactions such as hydrogen bonding [3], metal directed assembly [4], and the organisation of flexible species [5]. Porous materials based on 2D frameworks have received attention owing to useful properties endowed by dynamic guest-responsive phenomena [6]. The interlayer separation plays a major role in guest inclusion while framework flexibility can be achieved as a result of sliding of the 2D layers.…”

Structure and magnetism of a porous three-dimensional metal-organic framework based on planar tetranuclear copper(II) cluster units

“…Tripodal carboxylic acid ligands are known to be good for designing MOF materials with varied topologies due to their flexible coordination modes [20][21][22][23], and because the carboxylic acid groups can connect metals to form polynuclear metal cluster units which show excellent magnetic properties [24,25]. The infinite metal cluster units can be further linked through tripodal skeletons to form a porous polymer [26,27].…”

Recent advance in porous coordination polymers from the viewpoint of crystalline-state transformation

“…However, in some case, guest molecules resided in the void space between adjacent layers [49] and unfamiliar phenomena may be observed during the removal or recovery of guest molecules. A 2D PCPs {[Cu 2 (tci)(OH) (H 2 O) 3 ]·1.5H 2 O} n (7·1.5H 2 O) (tci = tris(2-carboxyethyl) isocyanurate) was reported by Kitagawa S and co-workers [66], in which water guest molecules resided in the void space between adjacent layers ( Figure 5(a)). After removing water molecules completely, the color of the compound changed from greenish-blue to deep blue.…”

“…A 2D PCPs {[Cd(azim) 2 (DMF) 2 ] (ClO 4 ) 2 ·2DMF} n (8) was obtained from bis(4-imidazol-1-ylphenyl)diazene (azim) incorporating an azo moiety at the Figure 5 (a) Perspective views of the as-synthesized phase, water molecules resided in the void space between adjacent layers; (b) perspective views of the 3D structure of the dehydrated structure after guest removal (ref. [66]). center and two imidazole groups at the terminals [67].…”