Solid Solutions of Soft Porous Coordination Polymers: Fine‐Tuning of Gas Adsorption Properties

Abstract

Control of the physical and chemical properties of porous materials has been an ongoing challenge for the optimization of functions, such as gas storage, separation, and catalysis. For example, a high surface area is important for large-volume gas uptake, and the control of pore shape is also significant for molecular separation.[1] These requirements are also valid for porous coordination polymers (PCPs) or metal-organic frameworks (MOFs), which consist of metal ions and organic linkers.[2] This class of adsorbent has received attention because of its structural versatility and physical properties, such as magnetism and redox activity.[3] Among the PCP compounds, flexible frameworks have been identified as a unique type of porous material because of their guestresponsive transformations.[4] This structural transformation is often directly related to the functionality of these frameworks; gas recognition separation or slow drug release are good examples in this respect. [5] The flexibility of the network must be modulated to precisely control these functions and to tailor the network performance, and much effort has been expended in creating flexible compounds.[6] However, there have been few reports on the rational incorporation of flexibility in the known PCP compounds as their synthesis is difficult.[7] Herein, we describe the preparation of ligand-based solid solutions of flexible PCPs and our attempts to overcome difficulties in the precise flexibility control and resulting gas sorption properties. A few approaches toward ligand-based solid solutions of robust metal-organic framework have been reported, [8] although corresponding structural information and control of their adsorptive functions has not been observed. We have synthesized two distinct interdigitated frameworks that contain different organic ligands, and have created a series of solid solutions based on these frameworks. These compounds exhibited a range of flexible adsorption properties, and their bimodal properties enabled them to show an improved performance compared with the two pure compounds CID-5 and CID-6 in the separation of a CO 2 /CH 4 mixture.The two flexible compounds with an interdigitation motif of 2D layers, [{Zn(5-NO 2 -ip)(bpy)}(0.5DMF·0.5MeOH)] n (CID-5'G; 5-NO 2 -ip = 5-nitroisophthalate, bpy = 4,4'-bipyridyl, and CID = coordination polymer with an interdigitated structure), and [{Zn(5-MeO-ip)(bpy)}(0.5 DMF·0.5 MeOH)] n (CID-6'G; 5-MeO-ip = 5-methoxyisophthalate), were prepared from Zn(NO 3 ) 2 ·6 H 2 O and either of the ligands in a 1:1 v/v DMF/MeOH mixture. The crystal structures are shown in Figure 1. For both compounds, two carboxylate groups coordinate to Zn 2+ ions to form eight-membered rings, and the bpy groups coordinate to the axial position of the Zn 2+ ions to create the 2D layered structure. The layers are assembled in an interdigitated fashion with micropores formed within the structure. Both frameworks can be

Substituent effects in 4-connected zinc isophthalate coordination polymers with a pyridylnicotinamide ligand: From dimer-based layers to ribbons with embedded infinite water molecule chains

“…Extension of the tether by one methylene unit resulted in a four-fold interpenetrated 6 6 dia net in [Zn(mip)(dpp)] n (dpp = 1,3-bis(4-pyridyl)propane) [12]. A sterically bulkier alkylether substituent in 5-methoxyisophthalate (meoip, Scheme 1) resulted in stacked 2-D (4,4) grids in both {[Zn(meoip)(bpmp)] Á8H 2 O} n [13] and {[Zn(meoip)(bpy)] (0.5DMFÁ0.5MeOH)} n [14]. By forming solid solutions with an isostructural 5-nitroisophthalate compound, the latter phase can absorb CO 2 selectively from a CH 4 /CO 2 gaseous mixture [14].…”

“…A sterically bulkier alkylether substituent in 5-methoxyisophthalate (meoip, Scheme 1) resulted in stacked 2-D (4,4) grids in both {[Zn(meoip)(bpmp)] Á8H 2 O} n [13] and {[Zn(meoip)(bpy)] (0.5DMFÁ0.5MeOH)} n [14]. By forming solid solutions with an isostructural 5-nitroisophthalate compound, the latter phase can absorb CO 2 selectively from a CH 4 /CO 2 gaseous mixture [14]. In comparison to the nearly ubiquitious rigid-rod bpy ligand [15], the kinked dipyridylamide 3-pyridylnicotinamide (3-pna, Scheme 1) has been employed far less frequently as a coligand for the construction of zinc coordination polymers [16].…”

“…The aromatic meta-dicarboxylate isophthalate (ip, Scheme 1) ligand has proven beneficial for the construction of divalent zinc coordination polymers in tandem with dipyridyl-type coligands that can play a key structure-directing role [9][10][11][12][13][14]. Using the rigid-rod tethering ligand 4,4 0 -bipyridine (bpy), the 2-D (4,4) grid compound {[Zn(ip)(bpy)(H 2 O)]Á1.5H 2 O} n was obtained [9].…”

Dependence of adsorption-induced structural transition on framework structure of porous coordination polymers

“…24 Moreover, the gating behavior can be more finely controlled by making a solid solution of two parental PCPs with similar crystal structures. 25,28,29 The solid-solution PCP shows an intermediate adsorption isotherm between those of the parental PCPs, and the gate-opening and -closing pressures can be tuned continuously by varying the ratio of the parental PCPs. More interestingly, a phase-separated PCP 28 consisting of the inner crystal of one parental PCP and the outer crystal of the other parental PCP exhibits a different adsorption isotherm from that of the solid-solution PCP with the same chemical formula.…”

An interdigitated 3D framework with 5-connected BN topology

“…As known, the interdigitated porous frameworks constructed from two-dimensional (2D) layers display tunable flexibility associated with layer-layer weak interactions, including H-bonds, π-electron stacking and van der Waals interactions [14][15][16][17][18][19][20][21][22]. Recently, Kitagawa's group reported a series of interdigitated structures with selective guest sorption and gas separation, and their pore size and shape can be tuned by varying the V-shaped dicarboxylate ligands [18][19][20][21][22]. Herein we report a new flexible coordination polymer with interdigitated motif of 2D layer by exploring the V-shaped ligand, 4,4′-(hexafluoroisopropylidene)-bis-(benzoic acid) (H 2 hfipbb) [23][24][25].…”