Enriching the Reticular Chemistry Repertoire: Merged Nets Approach

Publication Date (Web): June 20, 2018 ... We report our latest addition to the design toolbox in reticular chemistry, namely the concept of merged-net...
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Enriching the Reticular Chemistry Repertoire: Merged Nets Approach for the Rational Design of Intricate Mixed-linker MOF Platforms. Hao Jiang, Jiangtao Jia, Aleksander Shkurenko, Zhijie Chen, Karim Adil, Youssef Belmabkhout, Lukasz J. Weselinski, Ayalew H. Assen, Dong-Xu Xue, Michael O'Keeffe, and Mohamed Eddaoudi J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b04745 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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The merged-net approach for the rational construction of mixed-linkers MOFs; The merging of sph net and hxg net into a new sph net for the construction of sph-MOFs 82x44mm (300 x 300 DPI)

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Enriching the Reticular Chemistry Repertoire: Merged Nets Approach for the Rational Design of Intricate Mixed-linker MOF Platforms Hao Jiang,† Jiangtao Jia,† Aleksander Shkurenko,† Zhijie Chen, † Karim Adil,† Youssef Belmabkhout,† Łukasz J. Weseliński, † Ayalew H. Assen, † Dong-Xu Xue,† Michael O’Keeffe, ‡ Mohamed Eddaoudi*,† †Functional Materials Design, Discovery and Development Research Group (FMD 3), Advanced Membranes and Porous Materials Center (AMPMC), Division of Physical Sciences and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), 23955-6900, Kingdom of Saudi Arabia ‡School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States

ABSTRACT: Rational design and construction of MOFs with intricate structural complexity are of prime importance in reticular chemistry. We report our latest addition to the design toolbox in reticular chemistry, namely the concept of merged-nets based on merging two edge-transitive nets into a minimal edge-transitive net for the rational construction of intricate mixed-linker MOFs. In essence, a valuable net for design enclosing two edges (not related by symmetry) is rationally generated by merging two edgetransitive nets, namely (3,6)-coordinated spn and 6-coordinated hxg. The resultant merged-net, (3,6,12)-coordinated sph net with net transitivity [32] enclosing three nodes and two distinct edges, offers potential for deliberate design of intricate mixed-linker MOFs. We report the implementation of the merged-net approach for the construction of isoreticular RE mixed-linker MOFs, sph-MOF-1 to 4, based on the assembly of 12-c hexanuclear carboxylate-based MBBs, displaying cuboctahedral building units, 3-c tritopic ligands, and 6-c hexatopic ligands. The resultant sph-MOFs represent the first examples of MOFs where the underlying net is merged from two 3-periodic edge-transitive nets, spn and hxg. Distinctively, the sph-MOF-3 represents the first example of a mixed-linker MOF to enclose both trigonal and hexagonal linkers. The merged-nets approach allowed the logical practice of isoreticular chemistry by taking into account the mathematically correlated dimensions of the two ligands to afford the deliberate construction of a mixedlinker mesoporous MOF, sph-MOF-4. Merged-net equation and two key parameters, ratio constant and MBB constant, are disclosed. Merged-net strategy for design of mixed-linker MOFs by strictly controlling the size ratio between edges is introduced.

■ INTRODUCTION Over the past two decades, metal-organic frameworks (MOFs), a distinctive class of hybrid crystalline materials constructed by linking metal-based units (metal ions or metal clusters) with polytopic organic linkers, have attracted wide interest in academia and industry alike due to their high degree of porosity, unique functionalized structures and readily modular construction.1 The MOFs’ readily adjustable pore system metrics and functionality positions MOFs as ideal candidate porous materials to address the enduring challenges pertinent to energy and environmental sustainability such as gas storage,2 gas separation,3 catalysis,4 and chemical sensing.5 The institution of reticular chemistry paved the way for the design, discovery and development of novel functional crystalline solid-state materials including MOFs.6 Principally, the molecular building block (MBB) approach has emerged as a remarkable pathway toward the design and synthesis of novel functional MOFs.7 Purposely, prior the assembly process, the desired geometric and connectivity features, functionalities and properties can be encompassed in preselected MBBs at the design stage. Certainly, the prospective for the effective design is reliant on the ability to access and deploy building blocks with geometrical information and encoded connectivity affording the points of extension to match the vertex figures of the targeted net. Convincingly, edge-transitive nets (all edges are equivalent by symmetry) are regarded as suitable design targets in reticular chem-

istry and crystal chemistry.8 The past two decades have witnessed the burgeoning of MOF chemistry with the design and construction of a large myriad of MOF materials based on the reticulation of edge-transitive nets9 or their derived nets.10 Indeed, MOFs based on edge-transitive nets are the dominant class of materials in MOF chemistry due to the relative ease of their isoreticulation and functionalization, prompting their exploration in myriad applications.2a, 9f, 11 Logically, expanding the rational design of MOFs to include multiple distinct linkers based on the reticulation of multi-edge nets is of prime importance as it offers the prospective to deliberately access intricate materials with assorted functionalities needed for prospect applications.12 Nevertheless, the majority of mixed-linker intricate MOFs encompassing multiple ligands with distinct shapes and dimensions were realized primarily by the tedious trial-and-error approach.13 Markedly, the practice of isoreticulation for intricate MOF platforms based on multiedges nets remains an ongoing challenge as the connecting polytopic ligands are mathematical correlated and their relative expansion is interrelated and needs to be coordinated/synchronized; i.e. it is critical to elect the appropriate combination of linkers with suitable dimensions to afford the requisite net expansion and construct the looked-for isoreticular MOF.14 It is to be noted that various relatively simple examples of mixedlinker structures were reported by linking 0-periodic polyhedra or by pillaring 2-periodic layers in an axial-to-axial fasion,15 or

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by inserting/placing a second linker into specific MOFs containing “accepting” sites such as open metal sites16 or by exchanging terminal coordinating groups (e.g. hydroxide, acetate or benzoate groups).17 Evidently, despite the notable success on designing MOFs based on edge-transitive nets, the rational design of mixed-linker MOFs with higher complexity, in a purposeful one-pot synthesis, using reticular chemistry has yet to be demonstrated and rationalized. Our continuous quest to deploy practical strategies for the logical design of MOFs,10b, 15, 18 has prompted us to closely analyze the edge-transitive nets with the aim to uncover a plausible correlation/inter-relation between pairs of nets.19 Recently, we remarked the prominence of deploying highly-connected building units in combination with minimal edge-transitive derived net (transitivity [32]), based on splitting one type of vertex in the edgetransitive net into groups of linked vertices of lower coordination (e.g., one 12-c to one 6-c and one type of 3-c), as a powerful strategy for the design and construction of MOFs where branched ligands are employed for the deliberate access of the required and intricate highly coordinated net-cBUs.10d, 20 Reasonably, we realized that merging two edge-transitive nets (transitivity [21] or [11]) with the premise to afford shared nodes might lead to a new minimal edge-transitive net8, 20 (transitivity [32]) with higher complexity, encompassing the structural properties from

both parent nets and offering great prospective for the materials’ designer. Practically, we envisioned that two frameworks can merge together through a shared inorganic MBB, thus allowing the successful practice of reticular chemistry for the design of intricate mixed-linker MOFs. Accordingly, we conceived and pursued some key perquisites for the plausible design of mixedlinker MOFs: (i) the designate merged two MOFs are based on minimal edge-transitive nets; (ii) the embedded partial frameworks linking inorganic building units and each organic linkers are based on edge-transitive nets; (iii) the two partial frameworks have to merge and connect to each other via shared inorganic MBBs. Here, we introduce a systematic design principle, named merged net strategy, targeting the design of highly complex mixed-linker MOFs. Notably, the relevance of this concept has been asserted using a mathematical relationship, named merged net equation, that take into consideration the inherent geometrical features of the net that compose the resultant merged net. Markedly, it is becoming evident the employed pair of linkers need to be extended linearly, but not proportionally, in order to construct isoreticular mixed-linker MOFs, addressing the longstanding problem of reliable design of isoreticular mixed-linker MOFs.

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Figure 1. Schematic representation showing the assembly of sph-a net and the sph-MOF with sph-MOF-3 as an example. (a) The 12-c cuboctahedral building unit can be split into two groups, namely, 6-c trigonal antiprism and 6-c hexagonal building unit. (b) The corresponding RE hexanuclear cluster shown in atom mode (left) and polyhedral mode (middle and right). (c) Linking the split building units with triangular and hexagonal building unit results in the formation of spn-a net and hxg-a net. (d) The corresponding spn and hxg partial framework assembled from BTTC and BHPB linker. (e) The spn-a net and hxg-a net merge to form sph-a net. (f) The corresponding spn and hxg partial frameworks merge to sph-MOF. RE, C, O and S are represented by purple, grey, red and dark yellow, respectively, and H atoms are omitted for clarity. Cages are illustrated with yellow and green balls.

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The election of building units with the requisite connectivity and geometrical information, coding the targeted net, is central to the practice of reticular chemistry and the design of MOFs.21 For instance, the augmentation of the (3,6)-coordinated spn net permits the identification of the spn net vertex figures as the 3-c trigonal and the 6-c trigonal antiprismatic, and subsequently provides the coded information embedded in the building units for the reticulation of the spn-a net. Similarly, the uninodal 6-coordinated hxg net requires hexagonal building unit for its prospective reticulation. However, a plausible relationship/connection between the aforementioned two edge-transitive nets was not evident and a possible conjunction of their associated building units was unfortunately dismissed and not considered over the past two decades. Actually, the juxtaposition of the trigonal antiprism (a building unit of spn net) and the hexagon (a building unit of hxg net) results in a cuboctahedral building unit; i.e. the cuboctahedral building unit offers two prospective building units by deliberately separating the points of extension into two sets to give the trigonal antiprism (a building unit of spn net) and the hexagon (a building unit of hxg net). Proposedly, each set of points of extension can be linked to additional building units (triangle for spn net and hexagon for hxg net) to form a 3-periodic net, asserting the potential to employ the cuboctahedral building unit for the introduction of the spn and hxg nets at the same time in the same resultant 3-periodic structure. Practically, the precise control of the ratio between the associated edges of the spn and hxg nets, all trigonal antiprism building units in the spn net and half of the hexagonal building units in the hxg net can merge to form cuboctahedral building units. As a result, the two edge-transitive nets, (3,6)-c spn net (transitivity [21]) and 6-c hxg net (transitivity [11]), can perfectly merge to form a new minimal edge-transitive (3,6,12)-c net (transitivity [32]) with higher complexity, we named sph net (“sp” from “spn” and “h” from “hxg”). The concept of merging two 3-periodic edge-transitive nets to afford a new minimal edge transitive sph net offers great potential for the deliberate design of sph-MOF platforms. Rationally, our recently introduced 12-c cuboctahedron building units based on the 12-connected rare-earth (RE) hexanuclear [RE6(μ3-OH)8(μ3-O)2(O2C−)12] carboxylate-based cluster inspired us to pursue the design of mixed-linker MOF platforms based on the reticulation of sph net.22 To put into practice the merged nets approach for the design of mixed-linkers sph-MOF platform, we derived some key prerequisites in addition to the deployment of the rare earth hexanuclear clusters as the requisite12-c cuboctahedral MBB: (i) 3-c trigonal MBB provided by tricarboxylate ligands, (ii) 6-c hexagonal MBB, afforded by predesigned hexacarboxylate ligands or two π-π interacting 3-c tricarboxylate ligands, (iii) derive the mathematical equation that defines the relationship between the dimensions of the requisite 3-c and 6-c organic MBBs. Certainly, a series of highly symmetric isoreticular mixedlinker MOFs, sph-MOF-1 to 4, were rationally designed and synthesized based on the assembly of 12-c hexanuclear carboxylate-based MBBs, displaying cuboctahedral building units, 3-c tritopic ligands, and 6-c hexatopic ligands or π-π interacting paired 3-c tritopic ligands. Markedly, it is to be noted that the sph-MOF platform represents the first example where the underlying net is a minimal edge-transitive net merged from two

3-periodic edge-transitive nets. In addition, the sph-MOF-3 represents the first example of a mixed-linker MOF encompassing both a 3-c trigonal linker and a 6-c hexagonal linker. Our introduced merged nets approach allowed the logical practice of isoreticular chemistry, the expansion of the 3-c and 6-c polytopic ligands was done in a concerted fashion, taking into account the mathematically correlated dimensions of the two ligands, to afford the deliberate construction of a mixed-linker mesoporous MOF, sph-MOF-4, with cage dimension of 22 Å as confirmed by argon sorption studies. ■ RESULTS AND DISCUSSION Synthesis of single-linker Tb-spn-MOF-1. Various polycarboxylate ligands were explored with the aim to construct a prototype MOF that is amenable to the rational design of mixedlinker MOFs. During the screening of different ligands, an spnMOF was obtained based on Terbium and 5-(4H-1,2,4-triazol4-yl)-isophthalate (TIA), which provided the opportunity for the design of mixed-linker MOFs based on sph net. Solvothermal reactions of Tb(NO3)3·5H2O and H2TIA in a N,Ndimethylformamide (DMF)/water solution in the presence of 2fluorobenzoic acid (2-FBA) for 48 hours at 115 °C yielded colorless octahedral single crystals of Tb-spn-MOF-1. Singlecrystal X-ray diffraction studies (SCXRD) revealed the compound crystallized in the cubic space group Fd-3m with a unit cell parameter a = 38.309(4) Å. The compound formulated as [Tb6(μ3-OH)8(TIA)2(2-FBA)6(H2O)6]·(solv)x, which is also confirmed by thermogravimetric (TGA) (Figure S9, right) and elemental analysis. The phase purity of the crystalline material was confirmed by the similarities between the calculated powder X-ray diffraction (PXRD) pattern derived from the associated SCXRD data and the experimental PXRD pattern of the assynthesized material (Figure S9, left). It is worth mentioning that Tb-spn-MOF-1 is the first spn-MOF constructed by lower symmetric TIA linkers with two carboxylate group and one triazole group. In the structure of Tb-spnMOF-1, each inorganic MBB is connected to six TIA linkers, and each of the linkers is coordinated to three inorganic MBBs. In addition, each inorganic MBB is also coordinated to six 2fluorobenzoate, terminal ligands, which account for the charge balance. Structural and topological analysis revealed that the hexanuclear terbium cluster, a 6-c MBB, linked to the ligand TIA, a 3-c MBB, to form a 3-periodic MOF with the underlying (3,6)-c spn net. The 1,3,5-position carbon atoms of the benzene ring of TIA are acting as points of extension of the 3-c node. The carbon atoms of the coordinated carboxylate moieties and the 1position nitrogen atoms of the coordinated triazole moieties are acting as points of extension of the 6-c node. The overall framework of Tb-spn-MOF-1 contains two types of cages. The larger cages are about 19.5 Å in diameter, while the smaller cages have an internal pore diameter of about 5.4 Å and are delimited by four TIA linkers.

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Figure 2. Schematic showing the design of sph-MOFs by utilizing the merged net equation. (a) When the edge ratio between spn and hxg nets exactly meet the constant, the two nets can merge to sph net. (b) The size of half Tb hexanuclear cluster is 4.7 Å. (c) The merged net equation shows the relationship between spn and hxg linkers. (d) The size of both linkers in sph-MOFs could be designed from the merged net equation. Design of mixed-linker sph-MOF platform based on merged net. The analysis of the crystal structure of Tb-spn-MOF-1 revealed that it possesses the appropriate structural features for accepting a second linker using the traditional installation methodology. More importantly, the careful analysis of the resulted MOF drew our attention that the partial framework based on the sole second linkers can also be described as a 3-periodic framework with an underlying edge-transitive hxg net. In other words, the underlying net of resultant mixed-linker MOF can be deconstructed into two different nets, namely spn net and hxg net by taking into consideration that the cuboctahedron building unit can be deconstructed into a trigonal antiprism and a hexagon building unit. Concretely, the underlying net of the targeted framework is merged from both parent nets (Figure 1a-c). Accordingly, the mixed-linker MOFs with variable sizes can be directly designed based on the underlying merged net by utilizing the knowledge of reticular chemistry. The topological analysis revealed that the perceived mixedlinkers MOF adopts a new underlying net that we called sph. The rationalization of this merging process was demonstrated mathematically using a geometrical based relationship that takes into consideration the inherent geometrical features of the sph net. Indeed, the analysis/deconstruction of the resultant sph net shows a clear correlated relationship between the dimension/length of the associated edges of spn and hxg nets, respectively set up as x and y (Figure 2a). Precisely, the relationship between x and y could be calculated according to the Law of sines as depicted in Figure S2:

(𝟏)

𝑥

𝑦 𝜋 = sin ( 6) sin (2𝜋) 3

From the equation (1), the dimension ratio of edges between the spn net and the hxg net is a constant, which is an intrinsic property of sph net. Namely, spn and hxg nets can merge to sph net only when their edge ratio meets a constant, noted ratio constant CR. 𝑥 √3 (𝟐) C = = R 𝑦 3

Alternatively, the approximate value of ratio constant CR could also be calculated from the coordinate of nodes of the sph net, which we deposited in the Reticular Chemistry Structural Resource (RCSR) database.24 Set the coordinates of 3-c node as (a1, b1, c1), the coordinates of 6-c node as (a2, b2, c2), and the coordinates of 12-c node as (as, bs, cs), then, √(a1-as)2+(b1-bs)2+(c1-cs)2 (𝟑) C = R √(a -a )2+(b -b )2+(c -c )2 2

s

2

s

2 s

Obviously, the ratio constant CR is the key factor for determining the dimensions of linkers in mixed-linker sph-MOFs. However, it is worth taking into consideration that the size ratio between the edges of two parent nets is not equal to the size ratio of just ligands but the sum of ligands and half MBBs (Figure 2b). The merged net equation of sph net is as exemplified by the following equation (4).

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𝐿spn + CM = CR , 𝐿hxg + CM

where Lspn is the length from the center of the 3-c linker to the corresponding carbon atom of carboxylate group, Lhxg is the length from the center of the 6-c linker to the corresponding carboxylate carbon atom, and CM is the constant size defined as the half size of the MBB, length calculated from carboxylate carbon to the center of the cluster. In the case of terbium hexanuclear MBB, the MBB constant CM is equal to ca. 4.7 Å by assessing the reported structures.9e, 22, 25 Practically, by introducing this value of CM into the merged net equation of sph net (4), the relationship between Lspn and Lhxg can be clarified: (𝟓) 𝐿spn = 0.58 𝐿hxg – 2.0 Å

or 𝐿hxg= 1.73 𝐿spn + 3.5 Å.

By employing equation (5), the materials’ designer can determine the appropriate size of the complementary linker needed for the design of the looked-for mixed-linker sph-MOF. Moreover, this equation (5) also determines the condition of applying the isoreticular chemistry within the mixed-linker system as exemplified here by the design of four new compounds, namely sph-MOF-1, 2, 3 and 4 (Figure 2d). By checking the size of available 3-c or 6-c linkers, four pairs of ligands based on three pairs of different sizes were predicted to be precisely matching the sph net and consequently allowed us to deliberately synthesize their corresponding mixed-linker sph-MOFs (Figure 3). Design and synthesis of sph-MOF-1. Based on previous results, TIA was chosen as the first linker of spn part for the synthesis of sph-MOF-1. The size of TIA linker can be calculated as 3.5 Å based on the weighted average value since the linker is asymmetrical. The corresponding size of the hxg linker can be calculated by the merged net equation as 9.5 Å, which is matching with the acylamide-functionalized group based linker (Figure 3b). However, our attempts to obtain the corresponding hexacarboxylate linker was unsuccessful due to the encountered synthetic difficulties in inserting six acylamide functional moieties into one benzene ring. Instead, a tricarboxylate linker 4,4',4''-((benzene-1,3,5-tricarbonyl) tris(azanediyl)) tribenzote (BTCB) was synthesized and used since the packing of two BTCB linkers could possibly serve as a 6-c building block (Figure S3). As expected, solvothermal reactions of Tb(NO3)3·5H2O, H2TIA and H3BTCB in a DMF/water solution in the presence of 2-FBA for 48 hours at 115 °C yielded colorless octahedral single crystals of sph-MOF-1. The SCXRD studies revealed that the compound crystallized in the cubic space group Fd-3m with a unit cell parameter a = 38.985(5) Å. The compound formulated as [Tb6(μ3-OH)8(TIA)2(BTCB)2(H2O)6]·(solv)x, which is also confirmed by TGA (Figure S10, right) and elemental analysis. The phase purity of the bulk crystalline materials for sph-MOF-1 was confirmed on the basis of similarities between the calculated and as-synthesized PXRD patterns (Figure S10, left). The presence of linkers TIA and BTCB were also confirmed by 1H NMR of the HCl digested samples (Figure S31). In the structure of sph-MOF-1, each Tb6 cluster links to twelve linkers in two groups. Six TIA linkers occupied the trigonal antiprism position of the cuboctahedron, and six BTCB linkers occupied the planar hexagonal positions (Figure S3). Structure

and topological analysis revealed that two tricarboxylate linkers BTCB are packed together and serve as a 6-c MBB. The hexanuclear terbium cluster, a 12-c MBB, linked to the ligand TIA, a 3-c MBB, and the paired BTCB moiety, a 6-c MBB, to form a 3-periodic (3,6,12)-c MOF with the unprecedented sph underlying topology. The 1,3,5-position carbon atoms of the benzene ring of TIA are acting as points of extension of the 3-c nodes. The 1,3,5-position carbon atoms of the center benzene ring of double BTCB moieties are acting as points of extension of the 6-c node. The carbon atoms of the coordinated carboxylate moieties and the 1-position nitrogen atoms of the coordinated triazole moieties are acting as points of extension of the 12-c node. As anticipated, by further topologically analysis of sph-MOF1, the underlying (3,6,12)-c sph net was found to be an assembly of two edge-transitive nets, (3,6)-c spn net and 6-c hxg net. The overall framework of sph-MOF-1 contains two types of open cages. The larger truncated tetrahedral cages, having diameters of about 6.5 Å, are delimited by four pairs of BTCB and four TIA linkers, while the smaller tetrahedral cages, having diameters of about 2.5 Å, which are inaccessible by nitrogen or argon, are delimited by four TIA linkers (Figure S6). The corresponding solvent accessible volume for sph-MOF-1 was estimated to be 54%, by summing voxels more than 1.2 Å away from the framework using PLATON software.23 The permanent porosity of sph-MOF-1 has been examined by argon adsorption experiment carried out at 87 K, exhibiting a fully reversible type-I isotherm characteristic of microporous materials (Figures S14-16). The apparent Brunauer-Emmett−Teller (BET) surface area and Langmuir surface area were estimated to be 1020 m2·g− 1 and 1120 m2·g− 1, respectively. The experimental total pore volume was calculated to be 0.37 cm3·g− 1, which is consistent with the theoretical pore volume of 0.44 cm3·g− 1, based on the associated crystal structure. Design and synthesis of sph-MOF-2. The successful design of sph-MOF-1 proofed the effectiveness of the merged net equation, defining the mathematical correlation between the two linkers, and encouraged us to explore isoreticular sph-MOFs with expanding linkers to afford relatively larger cages. According to the merged net equation of the sph net, if sizes of both linkers are linearly increasing by a coefficient of 1.73, the resulted linker pair will still be suitable for the synthesis of sphMOFs. Precisely, the linker responsible for the spn net can be expanded by 1.0 Å if the corresponding hxg linker is increased by 1.73 Å. Based on this principle, a second linker pair was elected. Indeed, the size (5.1 Å) of linker benzo-tris-thiophene carboxylate (BTTC) was found to be matching with the size (12.3 Å) of linker 4,4',4''-(benzene-1,3,5-triyl-tris(benzene-4,1diyl))tribenzoate (BTPB) (Figure 3). As expected, solvothermal reactions of Tb(NO3)3·5H2O, H3BTTC, and H3BTPB in a DMF/water solution in the presence of 2-FBA for 48 hours at 115 °C yielded colorless octahedral single crystals of sph-MOF-2. The SCXRD studies revealed that sph-MOF-2 crystallized in the cubic space group Fd-3 with a unit cell parameter a = 45.6614(5) Å. The compound formulated as [(CH3)2NH2]2[Tb6(μ3-OH)8(BTTC)2(BTPB)2(H2O)6] (solv)x, which is also confirmed by TGA (Figure S11, right) and elemental analysis. The presence of linkers BTTC and BTPB were also confirmed by 1H NMR of the HCl digested samples (Figure S32). The experimental PXRD is consistent with the PXRD pattern calculated based on the crystal structure, which confirmed the phase purity of sph-MOF-2 (Figure S11, left).

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Figure 3. Schematic showing the ligands used in the synthesis of sph-MOFs, (a) ligands of the spn partial frameworks, the sizes showing distances from the center of linkers to the carboxylic carbon or the center of triazole; (b) ligands of the hxg partial frameworks; (c) structures of sph-MOF-1 to 4.

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In the structure of sph-MOF-2, each 12-c Tb6 cluster links to twelve linkers in two groups. Six 3-c BTTC linkers occupied the trigonal antiprism position of the cuboctahedron, and six 3-c BTPB linkers occupied the planar hexagonal position (Figure 1a, 4). The paired BTPB moiety works as a 6-c MBB to form a 3-periodic MOF with the underlying (3,6,12)-c sph net, which is isoreticular to sph-MOF-1. The carbon atoms, which are adjacent to the sulfur atoms in BTTC, are acting as points of extension of the 3-c nodes. The 1,3,5-position carbon atoms of the center benzene ring of double BTPB moieties are acting as points of extension of the 6-c nodes. For the 12-c nodes, unlike the nodes in sph-MOF-1, which has points of extension from both carboxylate moieties and triazole moieties, only the carbon atoms of the coordinated carboxylates are acting as points of extension in sph-MOF-2. As a result of the linker extension, both of the open cages in sph-MOF-2 are enlarged. The larger truncated tetrahedral cages, having diameters of about 12.5 Å, are delimited by four pairs of BTPB and four BTTC linkers. The smaller tetrahedral cages, having diameters of about 6.5 Å, are enclosed by four BTTC linkers (Figure S6). The corresponding solvent accessible volume for sph-MOF-2 was estimated to be 64%, by summing voxels more than 1.2 Å away from the framework using PLATON software.23 The permanent porosity of sph-MOF-2 has been examined by argon adsorption experiment at 87K, showing a fully reversible type-I isotherm characteristic (Figures S17-19) of a microporous material. The apparent BET surface area and Langmuir surface area were estimated to be 1820 m2·g− 1 and 2000 m2·g− 1, respectively. The experimental total pore volume was estimated to be 0.67 cm3·g− 1, which is consistent with the theoretical pore volume of 0.70 cm3·g− 1, based on the associated crystal structure. Design and synthesis of sph-MOF-3. The calculated size (12.3 Å) of hxg linker provides the opportunity to synthesize sph-MOFs with the 6-c ligand. Instead of H3BTPB in sphMOF-2, a hexacarboxylate ligand hexakis(4-(4-carboxyphenyl) phenyl) benzoic acid (H6BHPB) was designed and synthesized. The BHPB linker remains the same size as BTCB and matches with BTTC (5.1 Å) for the merged net equation. Solvothermal reactions of Tb(NO3)3·5H2O and H3BTTC, H6BHPB in a DMF/water solution in the presence of 2-FBA for 48 hours at 115 °C yielded colorless octahedral single crystals of sph-MOF-3. The SCXRD studies revealed that sph-MOF-3 crystallized in the cubic space group Fd-3m with a unit cell parameter a = 46.0411(4) Å, which is about 0.4 Å larger than sph-MOF-2. The compound formulated as [(CH3)2NH2]2[Tb6(μ3-OH)8(BTTC)2(BHPB)(H2O)6]·(solv)x, which is also confirmed by TGA (Figure S12, right) and elemental analysis. The presence of linkers BTTC and BHPB were also confirmed by 1H NMR of the HCl digested samples (Figure S33). The phase purity of the bulk crystalline materials for sph-MOF-3 was also confirmed on the basis of similarities between the calculated and as-synthesized PXRD patterns (Figure S12, left). The topological analysis of sph-MOF-3 showed that the hexanuclear terbium cluster, a 12-c MBB, linked to the ligand BTTC, a 3-c MBB, and the ligand BHPB, a 6-c MBB, to form a 3-periodic MOF with the underlying (3,6,12)-c sph net, which is isoreticular to sph-MOF-1and sph-MOF-2. The carbon atoms, which are adjacent to the sulfur atoms in BTTC,

can be viewed as points of extension of the 3-c triangular node. The 1,2,3,4,5,6-position carbon atoms of the center benzene ring of BHPB moieties can be considered as points of extension of the 6-c hexagonal node. The carbon atoms from the coordinated carboxylates can be regarded as points of extension of the 12-c cuboctahedral node in sph-MOF-3. Comparing to sph-MOF-2, the 6-c BHPB linker makes the structure of sph-MOF-3 more regimented with less rotation of the cluster, which can be clearly seen from the horizontal view of the linker (Figure 4). The overall framework of sph-MOF-3 also contains two types of open cages. The larger truncated tetrahedral cages are delimited by four BHPB and four BTTC linkers. The diameters of the large cages are about 13 Å, which are slightly larger than sph-MOF-2 since the BHPB linkers occupy less space than two BTCB linkers (Figure 4b, 4d). The smaller tetrahedral cages, having diameters of about 6.5 Å, are enclosed by four BTTC linkers (Figure S6). The corresponding solvent accessible volume for sph-MOF-3 was estimated to be 65%, by summing voxels more than 1.2 Å away from the framework using PLATON software.23

Figure 4. Schematic showing the comparison between sphMOF-2 and sph-MOF-3. The difference of chemical environment between BTPB and BHPB linker in sph-MOF-2 and sph-MOF-3 are shown in (a) top view and (b) horizontal view. Correspondingly, the shape and size of cages, (c) small tetrahedral cages and (d) large truncated tetrahedral cages, are also slightly tuned from sph-MOF-2 to sph-MOF-3.

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The permanent porosity of sph-MOF-3 has been examined by argon adsorption experiment at 87K, showing a fully reversible type-I isotherm (Figures S20-22) characteristic of a microporous material. The apparent BET surface area and Langmuir surface area was estimated to be 1930 m2·g− 1 and 2250 m2·g− 1, respectively. The experimental total pore volume was estimated to be 0.72 cm3·g−1, which is consistent with the theoretical pore volume of 0.73 cm3·g− 1, based on the associated crystal structure. Design and synthesis of mesoporous sph-MOF-4. In order to test the effectiveness of merged net equation for the design of mesoporous MOFs, sizes of both linkers are further extended. The linker 4,4',4''-s-Triazine-2,4,6-triyl-tribenzoate (TATB) with size about 7.4 Å and the linker 4,4',4''-(benzene1,3,5-triyl-tris(biphenyl-4,4'-diyl))tribenzoate (BTBPB) with size about 16.3 Å was found to match the equation. It is worth mentioning that no structure has been reported with BTBPB linker by searching the Cambridge Structural Database (CSD),26 which reveals the difficulty of obtaining MOFs with such an elongated tricarboxylate linker. As anticipated, solvothermal reactions of Tb(NO3)3·5H2O and H3TATB, H3BTBPB in a DMF solution in the presence of 2FBA for 48 hours at 115 °C yielded colorless octahedral single crystals of sph-MOF-4. The SCXRD studies revealed that sph-MOF-4 crystallized in tetragonal space group I41 with a unit cell a = b = 39.740(1) and c = 56.261(2). The compound formulated as [(CH3)2NH2]2[Tb6(μ3-OH)8(TATB)2(BTBPB)2 (H2O)6]·(solv)x, which is also confirmed by TGA (Figure S13, right) and elemental analysis. The presence of linkers TATB and BTBPB were confirmed by 1H NMR of the HCl digested samples (Figure S34). The phase purity of the bulk crystalline sph-MOF-4 was confirmed by similarities between the calculated and as-synthesized PXRD patterns (Figure S13, left). The topological analysis of sph-MOF-4 revealed that the 12c hexanuclear terbium clusters are linked to the 3-c TATB ligands and the 6-c paired BTBPB moieties to form a 3-periodic MOF with the underlying (3,6,12)-c sph net (Figure 1, Figure S5). The carbon atoms of the center triazine in TATB can be regarded as points of extension of the 3-c nodes. The 1,3,5position carbon atoms of the center benzene ring of double BTBPB moieties can be viewed as points of extension of the 6-c nodes. The carbon atoms of the coordinated carboxylates can be presented as points of extension of the 12-c nodes. The successful synthesis of the isoreticular structure sph-MOF-4 demonstrated the effectiveness of our mathematic calculation, even when the size of linkers are highly expanded. The extension of the linkers provides both microporous cages and mesoporous cages in the structure of sph-MOF-4. The larger truncated tetrahedral cages, having diameters of about 22 Å, are delimited by four pairs of BTBPB and four TATB linkers, while the smaller tetrahedral cages, having diameters of about 9.5 Å, are enclosed by four TATB linkers (Figure S6). The corresponding solvent accessible volume for sphMOF-4 was estimated to be 75%, by summing voxels more than 1.2 Å away from the framework using PLATON software.23 The permanent porosity of sph-MOF-4 has been examined by argon adsorption experiment at 87K (Figures S23-25). An increase at p/p0 = 0.25 on the argon adsorption isotherm corresponds to a mesoporous cage of ca. 2.2 nm in sph-MOF-4.

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The apparent BET surface area was estimated to be 2170 m2·g− 1. The experimental total pore volume was estimated to be 1.14 cm3·g− 1, which is consistent with the theoretical pore volume of 1.27 cm3·g− 1, based on the associated crystal structure.

Figure 5. Ar adsorption isotherm at 87K of Tb-sph-MOFs. Gas Sorption Studies of sph-MOF-1. In light of the known/ existing large library of adsorption data on MOFs and their relationships to important applications like CO2 separation3 and natural gas upgrading,27 we aimed to access single gas adsorptive performance of Tb-sph-MOF-1 in this mixed-linker system and evaluate the adsorption properties relationships as compared to the MOF benchmarks. The isosteric heat of adsorption (Qst), calculated using CO2 adsorption isotherms at 258, 273, 288 and 298 K (Figures S26-27), show a decreasing tendency. The Qst value at low loadings is 29.4 kJ mol-1, further leveling off at the typical value for pore filling, shows that the acylamide functionality in Tb-sph-MOF-1 generate the same energetic effect in the pores reported in other acylamide based MOFs.28 Further, light hydrocarbon adsorption (CnH2n+2; i.e. C2H6, C3H8, n-C4H10 and iso-C4H10) as compared to CO2, CH4 and N2 in wide range of pressure (Figures S28-S30) show that mixed ligand based sph MOF platform, having the most confined pore system show a typical enhancement behavior of affinity as function or elevating the polarizabilities of the probe molecules. This exemplified in CO2 having less adsorption affinity to the Tb-sph-MOF-1 framework than CnH2n+2, similar to Rare-earth fcu-based MOFs25, 29 and divergent for typical 13X zeolites. The above reported adsorption results, demonstrate that tuning and exploring further the mixed ligand sph platform, could lead to powerful separation, storage and sensing agents. ■ CONCLUSION We introduced and described a new and systematic design principle in reticular chemistry, named merged nets approach, asserting the suitability of minimal edge-transitive nets merged from edge-transitive nets as ideal blueprints for the design of intricate mixed-linker MOFs. The fundamental

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equation, merged net equation, revealed the law of net merging and solved the long-held problem of reliably designing isoreticular mixed-linker MOFs. We reported the design and construction of a series of highly symmetric isoreticular RE mixed-linker MOFs, sph-MOF-1 to 4, based on the assembly of 12-c hexanuclear carboxylatebased MBBs, displaying cuboctahedral building units, 3-c tritopic ligands, and 6-c hexatopic ligands or π-π interacted paired 3-c tritopic ligands. The sph-MOFs represent the first examples of MOFs where the underlying net is merged from two 3-periodic edge-transitive nets, (3,6)-c spn net and 6-c hxg net. The sph-MOF-3 represents the first example of a mixedlinker MOF based on the assembly of the trigonal linker and hexagonal linker. By concerted extension of both linkers, a mesoporous mixed-linker MOF, sph-MOF-4, was constructed to enclose cages with a diameter of about 22 Å. The noted unique control using merged nets to access intricate mixed-linkers MOFs paves the way for the development of advanced made-to-order MOFs with encoded distinct functionalities. Work is in progress to enumerate other merged nets form pairs of edge-transitive nets, prospectively enriching the reticular chemistry repertoire. ■ EXPERIMENTAL SECTION Synthesis of single-linker Tb-spn-MOF-1 (1). Tb(NO3)3·5H2O (50 mg, 0.115 mmol), H2TIA (13 mg, 0.058 mmol), 2-FBA (300 mg, 2.14 mmol), DMF (1.5 ml) and H2O (0.25 ml) were combined in a 20 ml scintillation vial, sealed and heated to 115 oC for 48 h and cooled to room temperature. The colorless polyhedral crystals were collected and washed with DMF. The as-synthesized material was found to be insoluble in H2O and common organic solvents. FT-IR (4000−650 cm−1): 3383 (br), 1650 (s), 1610 (s), 1484 (m), 1451 (s) 1384 (vs), 1299 (w), 1252 (m), 1217 (m), 1161 (w), 1095 (s), 1058 (w), 1010 (w), 858 (w), 796 (w), 760 (s), 734 (m), 710 (m). Elemental analysis found (calculated) for Tb6C65N7H55O29F6: C%: 31.1 (31.6), H%: 1.99 (2.25), N%: 4.06 (3.98). Synthesis of Tb-sph-MOF-1 (2). Tb(NO3)3·5H2O (50 mg, 0.115 mmol), H2TIA (13 mg, 0.058 mmol), H3BTCB (33 mg, 0.058 mmol), 2-FBA (300 mg, 2.14 mmol), DMF (1.5 ml) and H2O (0.25 ml) were combined in a 20 ml scintillation vial, sealed and heated to 115 oC for 48 h and cooled to room temperature. The colorless polyhedral crystals were collected and washed with DMF. The as-synthesized material was found to be insoluble in H2O and common organic solvents. FT-IR (4000−650 cm−1): 3485 (br), 2930 (w), 1651 (vs), 1495 (m), 1437 (m), 1386 (s), 1315 (w), 1253 (m), 1177 (w), 1093 (s), 1060 (w), 864 (w), 786 (m), 777 (m), 732 (m), 709 (m). Elemental analysis found (calculated) for Tb6C80N12H84O43: C%: 31.8 (33.6), H%: 2.82 (2.97), N%: 5.96 (5.89). Synthesis of Tb-sph-MOF-2 (3). Tb(NO3)3·5H2O (8.3 mg, 0.0191 mmol), H3BTTC (2.5 mg, 0.0066 mmol), H3BTPB (5.0 mg, 0.0075 mmol), 2-FBA (75.0 mg, 0.535 mmol), DMF (1.5 ml) and H2O (1.0 ml) were combined in a 20 ml scintillation vial, sealed and heated to 115 oC for 48 h and cooled to

room temperature. The colorless polyhedral crystals were collected and washed with DMF. The as-synthesized material was found to be insoluble in H2O and common organic solvents. FT-IR (4000−650 cm−1): 3399 (br), 2929 (w) 1652 (s), 1604 (s), 1517 (m) 1385 (vs), 1253 (m), 1094 (m), 1004 (w), 833 (w), 786 (s), 735 (m), 702 (m). Elemental analysis found (calculated) for Tb6C124N2H106O43S6: C%: 41.1 (43.1), H%: 3.24 (3.09), N%: 0.98 (0.81). Synthesis of Tb-sph-MOF-3 (4). Tb(NO3)3·5H2O (8.3 mg, 0.0191 mmol), H3BTTC (2.5 mg, 0.0066 mmol), H6BHPB (5.0 mg, 0.0039 mmol), 2-FBA (75.0 mg, 0.535 mmol), DMF (1.5 ml), H2O (1.5 ml) and 3.5M HNO3 (0.2ml) were combined in a 20 ml scintillation vial, sealed and heated to 115oC for 48 h and cooled to room temperature. The colorless polyhedral crystals were collected and washed with DMF. The assynthesized material was found to be insoluble in H2O and common organic solvents. FT-IR (4000−650 cm−1): 3209 (br), 1651 (m), 1603 (m), 1557 (m), 1392 (vs), 1178 (w), 1099 (m), 1005 (w), 838 (w), 776 (s), 736 (m), 703 (m). Elemental analysis found (calculated) for Tb 6C118N2H98O42S6: C%: 39.8 (42.1), H%: 2.91 (2.94), N%: 0.74 (0.83). Synthesis of Tb-sph-MOF-4 (5). Tb(NO3)3·5H2O (7.5 mg, 0.0173 mmol), H3TATB (2.5 mg, 0.0057 mmol), H3BTBPB (5.0 mg, 0.0055 mmol), 2-FBA (75.0 mg, 0.535 mmol) and DMF (1.5 ml) were combined in a 20 ml scintillation vial, sealed and heated to 115oC for 48 h and cooled to room temperature. The colorless polyhedral crystals were collected and washed with DMF. The as-synthesized material was found to be insoluble in H2O and common organic solvents. FT-IR (4000−650 cm−1): 3027 (br), 1651 (w), 1600 (m), 1507 (s), 1398 (vs), 1355 (vs), 1101 (w), 1017 (w), 1003 (w), 816 (s), 775 (vs), 700 (s). Elemental analysis found (calculated) for Tb6C178N8H218O54: C%: 46.7 (49.8), H%: 4.97 (5.13), N%: 2.69 (2.61).

ASSOCIATED CONTENT Supporting Information Detailed procedures for PXRD, TGA, NMR, the synthesis of the organic ligands, additional structural figures, detailed topological analysis, low- and high-pressure gas adsorption isotherms and Xray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from King Abdullah University of Science and Technology (KAUST), Kingdom of Saudi Arabia.

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