Topological control of 3,4-connected frameworks based on the Cu2-paddle-wheel node: tbo or pto, and why?

Two trigonal tritopic ligands with different conformational degree of freedom: conformationally labile H3tcbpa (tris((4-carboxyl)phenylduryl)amine) and conformationally obstructed H3hmbqa (4,4′,4′′-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1]quino-lizino[3,4,5,6,7-defg]acridine-2,6,10-triyl)tribenzoic acid) are assembled with square-planar paddle-wheel nodes with the aim of selective engineering of the frameworks with tbo and pto underlying net topologies. In the case of H3tcbpa, both topological types were obtained forming non-interpenetrated MOFs namely DUT-63 (tbo) and DUT-64 (pto). Whereas synthesis of DUT-63 proceeds under typical conditions, formation of DUT-64 requires an additional topology directing reagent (topological modifier). Solvothermal treatment of the conformationally hindered H3hmbqa ligand with the Cu-salt results exclusively in DUT-77 material, based on the single pto net. The possibility to insert the salen based metallated pillar ligand into networks with pto topology post-synthetically results in DUT-78 and DUT-79 materials (both ith-d) and opens new horizons for post-synthetic insertion of catalytically active metals within the above-mentioned topological type of frameworks.


Introduction
Metal-organic frameworks (MOFs) are a class of crystalline porous solids constructed from inorganic and organic building units by modular approach 1 possessing a very bright application potential. 2,3 By combination of the topological and isoreticular synthesis approaches, an infinite number of frameworks with target structure and properties could be theoretically predicted. 4,5 However, an unambiguous prediction is not always possible, due to the plurality of factors influencing the MOF formation, namely framework interpenetration, secondary building unit (SBU) assembly formed under defined synthetic conditions, solvent or ligand degradation processes, preferred ligand conformation in solution etc.
Nevertheless, crystal engineering plays a key role in the synthesis of novel MOF materials for desired applications. Nowadays, the reticular chemistry offers several approaches for the synthesis of MOF materials with specific textural properties that are based on the control of the underlying topology of the framework. 6,7 In this regards, tritopic ligands are very attractive systems due to the restricted topological diversity of resulting structures, especially if the latter is combined with a square-planar 4-connected inorganic nodes. [8][9][10] Thus, application the isoreticular approach to the HKUST-1 (ref. 11) structure (tbo topology) by expanding the btc 3− (1,3,5-benzenetricarboxylate) ligand to btb 3− (btb -1,3,5benzenetribenzoate) has first led to the interpenetrated structure of MOF-14 (ref. 12) with two interwoven pto type frameworks in the structure. Just a slight modification of the synthesis conditions by adding small amounts of pyridine leads to the formation of DUT-33 and DUT-34 materials (DUT -Dresden University of Technology) consisting of doubly interpenetrated tbo and single pto frameworks, respectively. 8,13 The non-interpenetrated tbo net could be achieved only by using the H 3 btb ligand with bulky substituents at peripheral phenyl ring 14 or triazine based H 3 tatb (H 3 tatb -4,4′,4″-striazine-2,4,6-triyltribenzoate). 15,16 In both cases the peripheral phenyl rings are nearly coplanar with the central one. Further isoreticular expansion of the H 3 btb to the H 3 bbc (H 3 bbc -4,4′,4″-(benzene-1,3,5-triyl-trisĲbenzene-4,1-diyl))tribenzoic acid) results in the MOF-399 structure with tbo underlying topology. 17 Very recently, using even longer bteb 3− (H 3 bteb -1,3,5-benzene-trisethynylbenzoic acid) as a ligand and slightly different synthetic conditions, Schmitt and coworkers succeed in the synthesis of two MOFs isoreticular to MOF-399, namely TCM-4 and TCM-8 containing interpenetrated tbo and pto frameworks, respectively. 18 Obviously, an alkyne functional group, introduced between the central and peripheral phenyl ring, leads to the minimization of the energy difference between two ligand conformations, favouring the particular topology. Calculations demonstrated that the potential pore volume, surface area and N 2 uptake capacity of the tbo structure, TCM-8, are substantially higher than those of the pto counterpart.
Schmid and co-workers have theoretically screened possible network topologies of copper paddle-wheel based systems with the tritopic linkers btc 3− and btb 3− with respect to their relative stability. 19 Their results demonstrate, that the intrinsic conformational preferences of the building blocks often dictate the network topology formed.
The H 3 tcbpa ligand has been already widely used for the design and synthesis of various MOFs, [20][21][22][23][24][25][26][27][28] including those based on the paddle-wheel SBU, 23,29 however neither tbo nor pto type structures are reported up to now. The H 3 hmbqa ligand could be viewed as a chemically fixed conformation of the first one, since their internal phenyl rings are connected to the quasi-planar core by three methylene bridges. By variation of the synthesis conditions, both tbo (DUT-63) and pto (DUT-64) structures could be obtained with H 3 tcbpa, which confirms the conformational lability of the latter. In contrast, the rigid core of the H 3 hmbqa ligand forces the formation of the pto structure (DUT-77) exclusively. Moreover, using nitrogen functionalized metal-salen-complex, (further named as Cu(salen), Fig. 1, formed by reaction of 5-methyl-4-oxo-1,4dihydropyridine-3-carbaldehyde with ethylenediamine and copper nitrate) as a crosslinking ligand, the frameworks of pto topology could be post-synthetically transformed into framework with ith-d underlying topology.

Experimental part
All commercially available chemicals and solvents were used as purchased without further purification. Silica gel for column chromatography with particle sizes of 0.063-0.200 mm and SIL G/UV254 ALUGRAM® aluminium sheets for thin layer chromatography were purchased from Macherey-Nagel. Reactions under inert gas (Argon 4.6, Linde AG) were done with standard Schlenk techniques.

Synthesis of H 3 tcbpa
H 3 tcbpa ligand was synthesized using slightly modified procedure described in ref. 20. Bromine (7.51 mL, 147 mmol, 3 eq.) dissolved in chloroform (20 mL) was injected to a solution of triphenylamine (12. TrisĲ4-bromophenyl)amine (5.00 g, 10.37 mmol), 4-methoxycarbonylphenylboronic acid (11.20 g, 62.24 mmol, 6 eq.), palladiumĲII) acetate (0.140 g, 0.62 mmol, 6 mol%), triphenylphosphine (0.544 g, 2.08 mmol, 20 mol%) and Cs 2 CO 3 (20.28 g, 62.24 mmol, 6 eq.) were stirred at reflux in dry tetrahydrofuran for 72 h under argon atmosphere. The yellow suspension was cooled down to room temperature and the inorganic salts were removed by filtration through a pad of Celite®. After removal of the solvent in vacuum, the resulting yellow-brown solid was dissolved in DCM and washed with water. After drying of the organic phase the solvent was removed by evaporation. The resulting yellow needles were filtered and dried under vacuum. In a round bottom flask trisĲ4′-methoxycarbonylbiphenyl)amine (6.10 g, 9.41 mmol) was dissolved in 200 mL tetrahydrofuran and a solution of KOH (10.56 g, 188 mmol, 20 eq.) in 50 mL H 2 O was added. This mixture was refluxed for 18 h. After cooling down to room temperature, THF was evaporated and 300 mL H 2 O was added to dissolve all obtained solid. The water phase was acidified with conc. HCl until no further precipitate was detected. The yellow solid was collected by filtration, washed with water, small amounts of tetrahydrofuran

Physical measurements
The elemental analyses for C, H, N were performed with CHNS 932 analyzer (LECO) or EA 3000 Euro Vector (HEKAtech). Theoretical calculations of surface area, pore volume and pore limiting diameter were performed using Zeo++ software. 32 Single crystal X-ray diffraction All datasets were collected at Helmholtz-Zentrum Berlin für Materialien und Energie on BL-14.2 or BL-14.3 beamlines at BESSY-MX, equipped with a Mar MX-225 CCD detector (Rayonics, Illinois). 33 Images were recorded at room temperature using φ-scan technique with a scan width of 1°and an exposure time of 1.2 s per frame. Datasets were integrated and scaled with CCP4i software package. 34 Structures were solved by direct methods and refined in anisotropic approximation for all non-hydrogen atoms by full-matrix least squares on F 2 using SHELXTL program package. 35 The hydrogen atoms were placed in geometrically calculated positions and refined using a "riding" model. Cu(salen) ligand in the crystal structure of DUT-78 is disordered over two positions with occupancies 0.64 and 0.36, correspondingly. In the part with lower occupancy, only Cu atom could be localized from the difference Fourier map, whereas nitrogen and carbon atoms could not be localized unambiguously. The lattice solvent molecules as well as terminal ligands could not be located from difference Fourier map due to disorder in the highly symmetric space group. The influence of disordered solvent molecules on the reliability factors was eliminated by applying the SQUEEZE procedure, implemented in PLATON program. 36 Detailed experimental data for the single crystal X-ray diffraction experiments are given in Table S1

Results and discussion
MOFs containing tcbpa ligand cubic F-centered lattice. The crystal structure of the framework is constructed from Cu paddle-wheels, interconnected by trigonal tcbpa 3− linkers forming 3D framework with tbo underlying topology. Thus, DUT-63 is isoreticular to HKUST-1. The phase purity of DUT-63 "as made" phase was confirmed by X-ray powder diffraction (Fig. S1, ESI ‡). Similar to HKUST-1, DUT-63 has 3 types of pores with 15.5, 28.0 and 32.2 Å in diameter (Fig. 2f). According to Zeo++ program, 32 the molecules with maximal diameter of 18.9 Å could penetrate into the pores ( Table 1). The geometric surface area and pore volume are 4768 m 2 g −1 and 4.83 cm 3 g −1 , respectively. Thus, the material has crystallographic porosity ranking among the most porous MOFs. Unfortunately, all attempts to remove the solvent from the pores without framework collapse failed. The reason might be in the ultra-low density of 0.17 g cm −3 , resulting in a high fragility of the desolvated framework. This is corroborated by a high value of solvent accessible void of 91.4% calculated from the crystal structure.
Inspired by our previous work, 37 and in order to force the formation of ith-d framework, a neutral linear diamine linker, namely 3,6-diĲpyridin-4-yl)-1,2,4,5-tetrazine (bpta), potentially suitable for interconnection of the paddlewheels, [38][39][40] was added during the synthesis. This resulted in the formation of cuboctahedral green crystals, crystallizing in the Pm3n space group, according to the single crystal X-ray diffraction analysis. The crystal structure of this novel material (DUT-64) adopts a single 3,4-binodal framework with pto topology (Fig. 2g). Interestingly, the bpta ligand was not found neither during the structure solution from the X-ray data, nor during the 1 H-NMR analysis after digestion of DUT-64 (Fig. S7, ESI ‡). It seems, bpta serves in this case only as a topological modifier for the structure formation, but does not integrate into the structure. Similar observations were made by Schmitt and co-workers during the synthesis of TCM-4. 18 In this case, 4,4-bipyridine serves as topology directing agent to enable the formation of the pto net.
The pore system of DUT-64 involves two types of pores (Fig. 2g). The large mesopore with 26.3 Å in diameter is built from twelve paddle-wheel units, arranged icosahedral and interconnected by eight tcbpa 3− linkers. The mesopores are connected by twelve micropores with five pentagonal pore windows each. The micropores with diameter of 16.5 Å are formed by four SBUs and four tcbpa 3− linkers. The solvent accessible volume, calculated using PLATON is lower, than that of DUT-63 and amounts to 89.5%. Despite of lower pore volume, DUT-64 exhibits a higher geometrical surface area of 5817 m 2 g −1 (Table 1). Due to the topological reasons, the pore limiting diameter for the DUT-64 structure amounts to 11.9 Å and therefore is significantly lower in comparison to DUT-63.
As the bpta molecule seems to be too long for paddlewheel crosslinking (N⋯N distance 11.1 Å), a shorter neutral diamine ligand (CuĲsalen), N⋯N distance 10.82 Å, 30 Fig. 1) was used for post-synthetic incorporation into the DUT-64 yielding a MOF with ith-d topology (DUT-78). The crystallinity and phase purity of as synthesised sample was proved by PXRD (Fig. S4, ESI ‡). The successful incorporation of Cu(salen) was proven by 1 H NMR in solution after dissolving the MOF (Fig. S9, ESI ‡). To confirm the coordination mode of the Cu(salen), the single crystal of DUT-78 was subjected to the single crystal X-ray diffraction at synchrotron. Refinement of the structure shows the incorporation of the  ρ crystcrystallographic density, SAVsolvent accessible void, SSAspecific surface area, V ppore volume, d maxmaximum pore diameter, d limitlimiting pore diameter.
Cu(salen) ligand unambiguously. The interconnection of paddle-wheels in the crystal structure caused only minimal changes in the conformation of the tcbpa 3− linker ( Fig. 2b and d, Table 2). It has also nearly no influence on the pore accessibility. Thus, the theoretical pore size distribution, shows nearly no influence of the cross-linking on the size of the large pore. The small pore size ranged from 8.8 (in DUT-79) to 16.5 Å (in DUT-64) depending on the linkers (Fig.  S6, ESI ‡). The only parameters that are slightly decreased due to the functionalization are geometrical surface area and pore volume (Table 1). Unfortunately all attempts to preserve the framework integrity during the removing of the solvent molecules failed (Fig. S13, ESI ‡). Presumably, the weak spot in this structure is a central nitrogen atom that could distort from sp 2 planar configuration under the harsh conditions that lead to the structural collapse. In prospective, functionalization of the MOF by neutral metallo-ligand is advantageous for catalysis, since not only Cu, but also other catalytically active metals such as Ni, Pd, Pt etc. can be introduced into this highly accessible porous system.

Conformational considerations
The geometry of both ligands was analysed using models, obtained by geometrical optimization of the H 3 tcbpa and H 3hmbqa molecules by Forcite tools (UFF force field), implemented in Materials Studio 5.0 software. 41 Since dihedral angles between the outer phenyl rings in the trigonal ligand are mostly responsible for the formation of the framework with tbo or pto underlying topology, 17 we analysed these angles in the optimized linker models (Fig. 1) in comparison to those found in the DUT-63 and DUT-64 (Fig. 2, Table 2). Thus, the dihedral angle between the outer and inner phenyl rings in H 3 tcbpa is equal to the interplanar angle between the outer rings and core plane in H 3 hmbqa. Similarly, the interplanar angle between the inner rings in H 3tcbpa is identical to the interplanar angle between the outer rings in H 3 hmbqa. At the same time, the H 3 tcbpa has one additional degree of rotation and therefore a larger potential for topological diversity in combination with paddle-wheel units.
According to Furukawa et al. the formation of tbo net is possible if all three carboxylates in the linker do not show significant twist angles between them. 17 The detailed analysis of the geometry in the coordinated tcbpa linker shows that the dihedral angle between carboxylates in DUT-63 (tbo) is only 17°that completely match with the previous statement.
The dihedral angle between two peripheral phenyl rings 1 and 2 is 34° (Fig. 2a, Table 2). The ring 2 and carboxylate group are nearly coplanar, with dihedral angle of 12°between them. It should be mentioned that some attempts to combine the copper paddle-wheels with tcbpa 3− were reported by Shi et al. 23 however resulting in a 2D catenated structure.
The detailed analysis of the linker conformation in DUT-64 shows a notable difference in comparison with the latter in DUT-63 structure. Thus, the peripheral phenyl rings 1 and 2 as well as phenyl ring 2 and carboxylate group are nearly co-planar with dihedral angles of 5°and 2°, correspondingly ( Table 2, Fig. 2b). Consequently, the dihedral angle between the carboxylates in the ligand is defined mostly by the angle between phenyl rings 1 and 1′, which is critical for the formation of the certain framework topology. As a result, the dihedral angle between carboxylates of 60°leads to the formation of the pto net ( Table 2, Fig. 2b).
Thus, due to the conformational flexibility of the H 3 tcbpa ligand, the assembly with the Cu 2 paddle-wheels leads to the formation of either tbo or pto nets (DUT-63 and DUT-64, respectively), depending on the synthesis conditions.

Topological control
For a better control of the topology by linker conformation, a novel tritopic ligand namely H 3 hmbqa with similar distance between carboxylates but a quasi-planar inner core and restricted conformational flexibility in comparison with H 3tcbpa ligand was synthesized. In many cases, the pto topology is more beneficial in comparison to tbo, because of the possibility to crosslink the paddle-wheel with additional neutral ligand to achieve ith-d topology (as shown for DUT-64/ DUT-78). Such crosslinking opens the possibility to stabilize the framework (as in the case of DUT-23 and DUT-34), 8 as well as to incorporate additional functionalities into the open porous framework, such as catalytic centers, chirality (DUT-24), 8 semiconductivity, or luminescence. 8,37,42 The solvothermal treatment of the H 3 hmbqa with CuĲNO 3 ) 2 in DMF (independently of the presence or absence of bpta as topological modulator) leds to the formation of the green cuboctahedral crystals of DUT-77. Single crystal X-ray diffraction analysis confirms the expected structure, isoreticular to DUT-64 with similar unit cell parameters and the pto underlying topology. The phase purity of the bulk sample was confirmed by PXRD measurements (Fig. S3, ESI ‡). The digestion 1 H NMR shows the presence of the H 3 hmbqa ligand exclusively (Fig. S8, ESI ‡). The detailed analysis of the  2  50  48  34  5  28  8  31  2-2′  83  62  9  56  52  55  58  2-3  0  0  1 2  2  5  3  1  3-3′  83  62  17  60  60  60  59 This journal is © The Royal Society of Chemistry 2016 ligand conformation shows a dihedral angle between the carboxylic groups of 60°, which is close to that in DUT-64. This angle is of paramount importance for the formation of the pto net. The planarity of the inner core forces the outer phenyl ring to twist on 28°. The similarity in size and geometry of the ligand, combined with the same topology of the framework, results in very similar textural properties of DUT-77 and DUT-64 structures calculated from the crystal structural data. However, because of the bulky and rigid core of the hmbqa 3− linker in DUT-77, all parameters involving solvent accessible void, theoretical surface area, total pore volume and pore dimensions have slightly lower values.
An approach analogous to that applied to DUT-64 was used for post-synthetic cross-linking of the paddle-wheels by Cu(salen) complex in DUT-77. The resulting material, DUT-79, with ith-d topology shows nearly no conformational changes in hmbqa 3− linker in comparison to the parent DUT-77 ( Table 2, Fig. 2c and e). The incorporation of the ligand was proven by single crystal X-ray diffraction analysis (Table S1 ‡) and 1 H NMR techniques (Fig. S10, ESI ‡).

Nitrogen adsorption experiments
To study the gas accessible porosity of the investigated frameworks, all samples were desolvated using supercritical CO 2 drying. Unfortunately it was not possible to preserve framework integrity completely during the solvent removal. According to the XRD analysis, all compounds are amorphous after activation ( Fig. S4 and S5, ESI ‡). Nevertheless, the highest nitrogen uptake at 77 K at 1 bar could be reached with DUT-79 ( Fig. S11-S14, ESI ‡). The BET surface area and pore volume derived from the isotherm is 980 m 2 g −1 and 0.59 cm 3 g −1 , respectively.

Conclusions
In summary, using trigonal and square-planar building blocks, the topology of the resulting frameworks can be controlled either by addition of topology directing agents or by varying the conformational degree of freedom in the trigonal ligand. This concept exemplified producing of five novel materials namely DUT-63, DUT-64, DUT-77, DUT-78, and DUT-79. In the case of conformationally labile H 3 tcbpa, the formation of framework polymorphs makes the prediction of resulting compound difficult. For conformationally restricted ligands such as H 3 hmbqa, only pto frameworks could be obtained under conditions investigated. Moreover, post synthetic functionalization of DUT-64 and DUT-78 by Cu(salen) ligand leads to the interconnection of the paddlewheels with the formation of ith-d net within DUT-78 and DUT-79 materials. We believe that this could improve the application potential for these MOFs in the heterogeneous catalysis, light emitting devices and other functional systems.