Enhancing the stability and porosity of penetrated metal–organic frameworks through the insertion of coordination sites

Guided by the insertion of coordination sites within ligands, an interpenetrated metal–organic framework (MOFs) NKU-112 and a self-penetrated framework NKU-113 were obtained. The enhanced stability and porosity of NKU-113 prove the efficiency of the method for the structure and properties modulation of penetrated MOFs.


Introduction
Metal-organic frameworks (MOFs) composed of metal ions/ clusters connected by organic linkers have emerged as a class of attractive porous materials. 1 Owing to their tailorable porous structures, MOFs have shown great potential in various applications such as gas storage and/or separation, catalysis, sensing, etc. 2 Though signicant progress has been made in the structure and property modulation of MOFs, the practical applications of MOFs have been limited by their relatively low stability. 3 Although several strategies have been proposed to enhance the stability of MOFs, including ligand or metal ion changes, surface modication, interpenetration and the construction of multi-walls, 4 the development of a facile and straightforward design and construction strategy for stable MOFs is still a desired research goal. 5 Framework interpenetration frequently occurs in MOF structures, particularly when extended organic ligands are used for MOF construction. In spite of the reduction of the pore volume of the framework, framework interpenetration in MOFs has been observed to not only enhance MOF stability but also regulate the pore size, thus augmenting their gas sorption properties originating from the promoted interactions between the individual networks. For example, Zhou et al. have compared the H 2 sorption performances of non-interpenetrated and two-fold interpenetrated MOFs to nd that framework interpenetration could benet the stability of the framework and gas sorption at low pressure. 6 However, the enhancement of framework stability by interpenetration is somewhat unmanageable, since the van der Waals interactions between the organic ligands of the individual networks may not be strong enough to prevent structure deformation and framework slippage in response to the removal of guest species in the framework. On the other hand, the coordination geometry of secondary building units (SBUs) composed of metal centers with coordinated solvent molecules may vary in response to the removal of solvents, which could also affect the stability of the MOFs.
Focusing on the stability enhancement of MOFs, the combination of framework penetration and the stabilization of SBUs could be a rational strategy. In principle, if the coordination sphere of the metal center is broadened and additional metal-ligand bonds are introduced to increase the ligancy of the metal cluster and to enhance the strength of the interactions between the individual networks, the resulting selfpenetrated framework may become more stable than the original interpenetrated framework. However, realizing the abovementioned scenario remains to be a challenging task, mainly because of the mismatched distance between frameworks as well as unfavorable coordination environments for additional metal ions.

Structures of NKU-112 and NKU-113
A solvothermal reaction of Ni(NO 3 ) 2 $6H 2 O and H 4 L1 in DMF and acetonitrile affords crystals of NKU-112, whereas crystals of NKU-113 are obtained from the solvothermal reaction of Co(NO 3 ) 2 $6H 2 O and H 4 L2 in DMF, acetonitrile and H 2 O. The structures of NKU-112 and NKU-113 were determined using single-crystal X-ray diffractometry. The bulk samples of NKU-112 and NKU-113 were characterized using IR (Fig. S2 †), and their phase purities were veried by the well matched powder Xray diffraction (PXRD) patterns of the as-synthesized samples and the simulated ones (Fig. 2). Thermogravimetric (TG) analyses showed that NKU-112 can retain its original structure up to a temperature of 370 C, whereas NKU-113 can retain its structure up to 400 C (Fig. S3 †).
Single crystal X-ray diffraction reveals that NKU-112 crystallises in the cubic space group Ia 3. Apart from the guest molecules, the asymmetric unit contains two Ni 2+ cations, one L1 4À anion, three water and two DMF molecules. The Ni1 is six coordinated in a distorted octahedron geometry with four carboxylate oxygen atoms (O1, O15, O22 and O24) from four different organic ligands, one oxygen atom (O3) from a terminal water molecule and one oxygen atom (O13) from a m 2 -water molecule. In contrast, the coordination sphere of Ni2 includes two carboxylate oxygen atoms (O23 and O25), two oxygen atoms from terminal DMF molecules (O9 and O29), one oxygen atom from a terminal water molecule (O8) and one oxygen atom from a m 2 -water molecule (O13), which can also be described as a distorted octahedral geometry (Fig. S4 †). Ni1 and Ni2 are bridged by two carboxylate groups and one m 2 -H 2 O molecule to form a discrete [Ni 2 (COO) 4 (m 2 -H 2 O)(H 2 O) 2 (DMF) 2 ] cluster in which a water molecule is inserted between the Ni 2+ ions, and the Ni1-O13-Ni2 angle is close to 120 (117.463 to be precise), a completely different value to that of a classical [M 2 (COO) 4 O 2 ] SBU ( Fig. S5 †). The SBUs were further expanded by the organic linker to form a three-dimensional (3D) network. Careful examination of the network structure reveals that the framework is composed of three different types of cage. As shown in Fig. 1, the octahedral cage A, with a diameter of ca. 6.8Å, is dened by six Ni clusters, each being at a vertex of the octahedron, and twelve isophthalate moieties along the edges. The other octahedral cage B, with a diameter of ca. 21.6Å, is composed of six Ni clusters and twelve organic ligands, whilst the third type of cage, the distorted cuboctahedral cage C with a diameter of ca. 15.8Å, consists of twelve Ni clusters and six organic ligands (Fig. 1b). As for the packing of NKU-112, each distorted cuboctahedral C cage is surrounded by four A cages and four B cages. Cage C shares three Ni clusters and one isophthalic moiety on the truncated surface with cage A, and three Ni clusters and L1 4À with cage B (Fig. 3a). Topologically, since cage A occupies vertices of cage B and C, cage A can be regarded as nodes for clarity and the framework features a uninodal 12connected net, which is classied as fcu as determined using TOPOS soware (Fig. S6a †). 7 Due to the large void in the network, two-fold interpenetration occurs in NKU-112, and the solvent-accessible volume of the interpenetrated NKU-112 is estimated to be 45.7% per unit cell by PLATON. 8 The transformation between interpenetrated and selfpenetrated MOFs has been studied in the past. 9 Aer specic connecting components are added, two or more folds of interpenetrated frameworks are covalently linked to each other to form a single framework. If these components are removed, the framework can be restored to its original topology. The selfpenetrated structure can also be constructed using elaborate ligand design, and the key is nding a suitable ligand. Typically, as two major coordination groups, carboxylate groups and Ncontaining heterocyclic groups are employed in the majority of MOF constructions. 10 Among the various groups, a chelating bipyridine moiety could form a relatively stable coordination structure with a metal center to stabilize the SBU. Accordingly, a bipyridine group was selected to be inserted in the backbone of the organic ligand H 4 L1, and the resulting H 4 L2 ligand was used to construct NKU-113.
Single-crystal X-ray diffraction reveals that NKU-113 crystallises in the cubic space group Fd 3m. The asymmetric unit comprises two Co 2+ cations, one L2 4À anion, and three water Scheme 1 Framework and modification strategy diagram. The bluecolored balls and the silver-colored sticks represent metal secondary building units and ligands, respectively. The yellow-colored sticks represent inserted coordinate bonds, which covalently connect two sets of frameworks to each other. The red-colored cylinders represent pores, which are enlarged after ligand modification. molecules. Co1 and Co2 are connected to each other through a bridge comprising two carboxylate groups from L2 4À and a m 2water molecule. Co1 is coordinated by four L2 4À anions and two water molecules in a distorted trigonal bipyramid dened by O4, O5 and four O1 atoms from four L2 4À anions. Co2 is coordinated by two carboxyls from L2 4À , two nitrogen atoms from  L2 4À and two water molecules that together form another distorted trigonal bipyramid dened by two O2 atoms, two N1 atoms, O5 and O6 (Fig. S2 †). These two metal ions are coordinated by the nitrogen atoms of L2 4À , featuring a different scenario to that of the Ni cluster in NKU-112 (Fig. S5 †). The ligand in NKU-113 has two-fold disorder with an occupancy of 0.5 each, and Co2 has four-fold disorder with an occupancy of 0.25 each. There are four kinds of cages in NKU-113: the octahedron cage D, with a diameter of ca. 10.6Å, is dened by six Co clusters on the vertices and twelve isophthalic moieties in L2 4À ; octahedral cage E, with a diameter of ca. 28.6Å, is dened by six Co clusters and twelve L2 4À ligands; distorted cuboctahedral cage F features four triangles combined with three Co clusters on the cross-section and six L2 4À on the edge, and distorted octahedron cage G with a diameter of ca. 22.0Å, which has no corresponding structure in NKU-112, dened by six Co clusters and six L2 4À ligands ( Fig. 1a and 3b). As for the packing mode of NKU-113, each distorted cuboctahedral cage F is surrounded by four octahedral D cages by sharing three Co clusters and three isophthalic moieties on the surface; it is additionally surrounded by twelve distorted octahedral G cages by sharing two Co clusters and half of an L2 4À ligand on the edge of the crosssection. Half of the F cages wrap one octahedral E cage, with the other half of the F cages wrapping four G cages and a D cage ( Fig. 1d and S7 †). Cages D and E & F possess two kinds of pores: a smaller pore in the range of 0.5-0.9 nm in cage D, and a larger pore in the range of 1.9-4.5 nm in cage E & F. Beneting from the presence of the coordinating bipyridine moiety in the backbone of the L2 4À ligand, the pyridine group bonds to a different cluster in the framework of NKU-113, resulting in the formation of a self-penetrated structure. Accordingly, the tiling structure of the framework of NKU-113 also changed with respect to that of NKU-112 because of its self-support property (Fig. S6b †). Topologically, by regarding cage D as nodes, the framework of NKU-113 could be simplied as a 3D binodal (3,18)-connected net with a point symbol of (4 2 $6) 6 (4 60 $6 93 ), as determined by the TOPOS soware. The solvent-accessible volume of NKU-113 is estimated by PLATON to be 67.2% per unit cell.
Compared with NKU-112, the structure deployment in NKU-113 causes several changes. The SBU in NKU-112, with a formula of [Ni 2 (COO) 4 (m 2 -H 2 O)(H 2 O) 2 DMF 2 ], is similar to that of [Co 2 (COO) 4 (m 2 -H 2 O)(H 2 O) 2 ] in NKU-113 except for the fact that in the latter cluster the chelating bipyridine moiety in the L2 4À ligand replaces the coordinating DMF molecules (Fig. S5 †). Furthermore, because of the additional coordination of the bipyridine moiety to the metal cluster, NKU-113 features an additional octahedral cage composed of six L2 4À ligands and six metal clusters (Fig. 1a) with respect to NKU-112. It is known that the coordination of solvent molecules will diminish the rigidity and stability of SBUs, and therefore replacing these coordinated solvent molecules with coordinate bonds between the SBU and the framework can strengthen the overall structure.
In detail, if Co-N bonds in NKU-113 are ignored, the packing mode of the two frameworks is different from that in NKU-112. By regarding cage A and cage D as nodes and other ligands as linkers, one structure can be simplied into an fcc-like framework (Fig. S8 †). Cages A & D construct tetrahedral and octahedral voids. In NKU-112, the second framework occupies the centres of larger octahedral voids. In contrast, the second framework occupies the centers of smaller tetrahedral voids in NKU-113 ( Fig. S9 and S10 †). This structure change is induced by the presence of new coordination bonds since the smaller tetrahedral voids are suitable for the linking of the penetrated framework through Co-N bonds. According to the changes in packing mode, the nestication mode in NKU-113 has also varied ( Fig. S11 and S12 †). The presence of additional coordinate bonds causes a reduction of the distance between the two structures, which contributes to the increase in porosity and stability observed in NKU-113 with respect to NKU-112. In summary, although NKU-112 and NKU-113 have similar building blocks, the insertion of linkers in the latter is associated with obvious differences that lead to an enhancement in the structure stability and porosity.

Stabilities and adsorption properties of NKU-112 and NKU-113
To evaluate the porosity and gas storage/separation potential of NKU-112 and NKU-113, gas adsorption experiments have been performed. Before the measurements, the samples of NKU-112 and NKU-113 were soaked in ethanol for solvent exchange and supercritically dried using carbon dioxide. However, the framework of NKU-112 partially collapsed aer the activation procedures (Fig. 2a), and NKU-113 showed a well retained framework structure (Fig. 2b). It should be noted that the collapse of NKU-112 can be ascribed to the loss of coordinated solvents during activation in the SBU as discussed above.
N 2 sorption tests were rst conducted at 77 K to characterize the porosity of the materials (Fig. 4a). The N 2 adsorption isotherm of NKU-113 has the typical characteristics of a type I prole, and the Brunauer-Emmett-Teller (BET) surface area and Langmuir surface area are 1486 m 2 g À1 and 1966 m 2 g À1 , respectively. The pore distribution analysis performed using the Horvath-Kawazoe (H-K) method shows a main distribution of 0.7-1.2 nm and 1.9-4.2 nm (Fig. S13 †), indicating that two kinds of pore existed in the framework, which is consistent with the crystal structures.
Considering the highly porous framework of NKU-113 and the presence of amide groups which may benet its gas sorption performance, a series of sorption tests were performed with C 2 H 6 , C 3 H 8 , CO 2 , and CH 4 (Fig. 4b-d). The gas uptake of NKU-113 at 273 K is 16 cm 3 g À1 (STP) for CH 4 , 63 cm 3 g À1 (STP) for C 2 H 6 , 135 cm 3 g À1 (STP) for C 3 H 8 and 77 cm 3 g À1 (STP) for CO 2 . At 298 K, the framework shows gas uptakes of 10 cm 3 g À1 (STP) for CH 4 , 36 cm 3 g À1 (STP) for C 2 H 6 , 105 cm 3 g À1 (STP) for C 3 H 8 , and 36 cm 3 g À1 (STP) for CO 2 . On the basis of the investigation of capacities, the heat of sorption of different gases was also investigated (Fig. S14 †). The initial heat of sorption values are 15.4 kJ mol À1 for CH 4 , 28.2 kJ mol À1 for C 2 H 6 , 27.2 kJ mol À1 for C 3 H 8 and 30.4 kJ mol À1 for CO 2 . The considerable uptakes and heat of adsorption values of NKU-113 towards alkanes and CO 2 suggest its potential in gas storage and separation applications.

Conclusions
In summary, through the tuning of coordination sites in organic ligands, interpenetrated MOF NKU-112 and selfpenetrated MOF NKU-113 have been constructed. Owing to the additional chelating bipyridine coordination sites introduced through the ligand, the NKU-113, featuring a selfpenetrated framework, reveals enhanced stability and porosity compared with those of the interpenetrated framework of NKU-112. The approach reported herein may provide a valuable method for the stability and gas sorption performance enhancement of penetrated MOFs.

Conflicts of interest
There are no conicts to declare.