Versatile rare earth hexanuclear clusters for the design and synthesis of highly-connected ftw-MOFs

A targeted rare earth ftw-MOF platform offers the potential to assess the effect of pore functionality and size on gas adsorption via ligand functionalization and/or expansion.


Introduction
Metal Organic Frameworks (MOFs), a unique and emerging class of highly tunable porous materials, have attracted wide interest in recent years due to their unprecedented tunability and high degree of porosity. 1The ability to introduce desired functionality during MOF assembly 2 or post-synthetically 3 contributes to their great potential in addressing technological challenges in areas of gas storage, 4,5 gas separation, 6,7 catalysis, 8 and chemical sensing. 9Various design strategies offer pathways to synthesize materials for specic applications through exploitation of the modular nature of MOF materials.The molecular building block (MBB) approach offers the potential to design MOFs where structural information is included into the building blocks (i.e. the organic ligands and inorganic clusters).
Successful implementation of reticular chemistry and the MBB approach requires isolation of the synthetic conditions that promote the formation of a desired inorganic MBB to minimize the number of possible resulting framework topologies.Our group, among others, seeks to leverage the knowledge that MOFscomposed of highly-connected building blocks which limits the number of possible topological outcomes and thus leads to a greater degree of predictability in structure 10 are suitable platforms to purposely tune the resultant materials properties.Recently researchers have identied reaction conditions conducive to the formation of a hexanuclear Zr/Hf based cluster.3][14] Our recent work has elucidated the reaction conditions (i.e.incorporation of uorine containing ligands or using 2-uorobenzoic acid as a reaction modulator) necessary to promote the formation of a similar highly connected (12-connected) rare earth based hexanuclear MBB (Fig. 1). 15,16Uniquely, this RE cluster presents advantages over the Zr/Hf cluster as it offers potential to tune the resulting material properties 15 through use of different rare earth metals.It was shown that linking these 12-connected RE-MBBs through linear ligands results in the formation of MOFs with fcu topology. 15Specically, in the hexanuclear [RE 6 (OH) 8 (O 2 C-) 12 ] carboxylate-based cluster the carboxylate carbon atoms of the coordinated ligands act as points of extension and correspond to the vertices of the fcu-a net (vertex gure of the fcu net with a cuboctahedron geometry (Fig. 1)).
The aforementioned RE-based hexanuclear cluster is a suitable MBB for the rational design of highly-connected MOFs as its inherent geometry and high connectivity permit only a limited number of possible topological outcomes for subsequent assembly, making it ideal for the effective practice of reticular chemistry.In this work, the focus is on utilizing 4connected quadrangular carboxylate-based ligands (Fig. 2 and  3) to synthesize rare earth MOFs having both 12-c and 4-c nodes.There are a total of three known (4,12)-c edge transitive binodal nets, all of which are constructed from one kind of edge and therefore offer the prospect of constructing MOFs with such topologies through use of a single type of symmetrical ligand.Of the three (4,12)-c nets (shp, w, ith), 10 the w topology is the only one composed of 12-connected nodes matching the cuboctahedron vertex gure in the 12-c RE hexanuclear MBB, [RE 6 (OH) 8 (O 2 C-) 12 ], (Fig. 1).
Accordingly RE-MOFs with an underlying topology of w (w-MOFs) have been purposely targeted 13,14,[17][18][19] by employing 4-connected quadrangular tetracarboxylate ligands in   combination with the 12-c cuboctahedron MBB.This targeted RE w-MOF platform offers the potential to systematically assess the effect of pore functionality and size via ligand functionalization and/or expansion (Fig. 3) by investigating the adsorption properties of relevant gases.
Herein we report a successful reticular chemistry method, where the newly isolated 12-c RE MBB allows the synthesis of a series of highly porous RE-MOFs including Y-w-MOF-2, which to the best of our knowledge has the highest specic surface area ever reported for a RE-MOF.Specically, a tetratopic carboxylate-based ligand with a square-like geometry was reacted with a variety of rare earth metal salts under appropriate reaction conditionsincluding the use of a uorinated modulator 15 to predictably form w-MOFs (w-MOF-1).Furthermore, ligand expansion was achieved via elongating the central core and/or extending the four arms, resulting in two relatively more open w-derived MOFs (w-MOF-2 and w-MOF-3, see Fig. 2 and 3).The versatility of this platform was evidenced by the construction of w-MOFs where the central core of the ligand was functionalized/decorated (i.e. three related ligands having a central phenyl (w-MOF-2), naphthalene (w-MOF-2 (Naphth)), or anthracene (w-MOF-2 (Anth)) based core, see Fig. 2 and 3).

Results and discussion
w-MOF based on a geometrically square ligand In our efforts to target the rst example of a RE w-MOF, we designed a exible tetracarboxylate ligand, 1,2,4,5-tetrakis[(4carboxy)phenoxymethyl]benzene (TetPOMB) (Fig. 2), which can act as a square/quadrangular MBB.1][22] The exibility of this type of ligand permits adoption of the necessary geometry (a square) to allow the formation of a MOF with the ideal w topology.
As anticipated, based on previous work from our group, the in situ formation of the 12-c rare earth MBB was facilitated by the addition of excess 2-uorobenzoic acid (2-FBA), a modulator and a directing agent for the in situ formation of highlyconnected polynuclear carboxylate-based clusters. 15This allowed for the synthesis of yttrium, terbium, and ytterbium w-MOF-1 analogs.Y-w-MOF-1 was structurally characterized by single-crystal X-ray diffraction (SCXRD) and phase purity of the Y, Tb and Yb analogs was conrmed by Whole Prole Pattern Matching using the Le Bail method (Fig. S4a, ESI †, Table 1). 23CXRD studies revealed that w-MOF-1 crystallized in the cubic Pm 3m space group and was formulated as |(DMA) 2 | [(Y 6 (m 3 -OH) 8 (H 2 O) 6 )(TetPOMB) 3 ]$(solv) x ; DMA ¼ dimethylammonium cations, solv ¼ solvent.Topological analysis conrmed that the material had the expected w topology (Fig. S1, ESI †).The w-MOF-1 structure can be viewed as a primitive cubic packing of the 12-c rare earth MBBs, which are positioned in the center of the unit cell.The 12-c rare earth MBBs are connected through 12 bis-monodentate carboxylates from the 12 separate TetPOMB ligands (Fig. 4), thus forming one central cubic cage $19 Å in diameter that is delimited by six TetPOMB ligands (Fig. 4).Due to the exibility of the ether linkages of the ligand, its central phenyl core is disordered over four positions in the crystal structure.Preliminary CO 2 and N 2 adsorption studies revealed that w-MOF-1 was not permanently porous using conventional activation procedures.In fact, numerous attempts to activate the w-MOF-1 samples for gas sorption analysis resulted in loss of crystallinity.We attributed this lack of robustness to the exibility in the ligand, which plausibly resulted in the collapse of the material upon guest molecule removal.

w-MOF based on rigid and expanded rectangular ligand
Further structural examination revealed that a MOF with w topology could potentially be accessed using rectangular ligands.A rotation of the inorganic 12-c MBB, as well as alternation in the orientation of the 4-c ligand (the organic MBB), appeared to be required in order to accommodate the lower symmetry rectangular ligands for the construction of w-MOFs (Fig. 2 and S1-S3, ESI †).A similar phenomenon was reported recently in structurally related Zr w-MOFs. 13Convincingly, in order to synthesize a more rigid and permanently porous w-MOF, a rigid expanded analog of the TetPOMB linker was synthesized: 3,3 00 ,5,5 00 -tetrakis(4-carboxyphenyl)-p-terphenyl (TCPT, Fig. 2 and 3).Under similar reaction conditions, including the use of 2-FBA, crystalline yttrium, ytterbium, and terbium w-MOF-2 analogs were synthesized.Y-w-MOF-2 was characterized by SCXRD, and the phase purity of the Y, Tb, and Yb analogs was conrmed by Whole Prole Pattern Matching using the Le Bail method (Fig. S4a and S4b, ESI †). 23  view, the resultant structure can be described as a trinodal (3,3,12)-c net with transitivity [3443], given that the 4-c ligand comprises two 3-c triangular nodes. 24Consequently, the employment of a quadrangular 4-c ligand, based on linked 3-c triangular building units, permits us to disclose a novel (3,3,12)c net derived from the parent w-net with a new topology yet to be recognized (Fig. S3, ESI †). 10,13Accordingly, we assigned the three letter code kle (KAUST Luebke Eddaoudi) to the wderived net. 10 The w-MOF-2 structure is composed of ligands with a paraterphenyl core covalently linked in the 3,3 00 ,5,5 00 positions to the para position of benzoate moieties, thus forming a rectangular 4-c MBB (Fig. 5

w-MOF functionalization via ligand decoration
To illustrate the tunability of the w-MOF platform, and subsequently investigate the impact of additional aromatic functionality on the associated sorption properties, naphthalene and anthracene functionalized analogs of w-MOF-2 were synthesized (Y-w-MOF-2 (Naphth) and Y-w-MOF-2 (Anth)).In these analogs, the central 1,4-substituted phenyl ring of TCPT was replaced with a 1,4-substituted naphthalene core and a 9,10-substituted anthracene core (Fig. 2 and 3).
SCXRD studies revealed that Y-w-MOF-2 (Naphth) crystallized in the cubic Im Experimental PXRD patterns matched the calculated PXRD from the modeled structure.This conrmed the attainment of the anticipated isoreticular structure, Y-w-MOF-2 (Naphth).The structure of the Y-w-MOF-2 (Naphth) analogue differs from the parent w-MOF-2 with regard to the orientation of the bicyclic central core of the ligands: the bulkier naphthalene core is rotated 90 out of plane (Fig. S13 and S14, ESI †).Similarly, the resultant functionalized w-MOFs enclose two distinct cages.Namely, a highly symmetric cubic cage, with T h symmetry, delimited by six ligands which are puckered outward with the bulk of the naphthalene core pointed toward the center of the cage, and with a minimum cage diameter of $9.5 Å.The 12-c MBBs are rotated $13 out of perfect alignment along all three axes (i.e. in the x, y and z directions).Adjacent to, and sharing one face of, the cubic cage, are six symmetry related cuboidal cages with D 2h symmetry.These cages, with minimum diameters of approximately 14 Å, 13 Å and 10 Å, are delimited by six ligands, two of which are puckered outward having the bulk of the naphthalene core pointing inward, and the other four ligands are puckered inward with the bulk of the naphthalene core pointing outward into the adjacent cage.Conrmation of the successful synthesis of Y-w-MOF-2 (Anth) was only possible by unit cell indexing from a poor quality highly twinned crystal and further comparing the experimental powder pattern to a modeled and geometry optimized Y-w-MOF-2 (Anth) structure (Fig. S7, ESI †).The determined structure was consistent with the Y-w-MOF-2 (Naphth) structure with regard to connectivity and ligand geometry.Accordingly, the Y-w-MOF-2 (Anth) structure was modeled based on Y-w-MOF-2 (Naphth), and further functionalized and geometry optimized in Materials Studio.The Y-w-MOF-2 (Anth) encloses two unique cages.The rst is a highly symmetric cubic cage, with T h symmetry and a minimum diameter of $7 Å (Fig. S15, ESI †).This cage is delimited by six ligands which are puckered inward; the anthracene core is rotated perpendicular to the plane of ligand.The 12-c MBBs are oriented similarly to Y-w-MOF-2 (Naphth), i.e. rotated $13 out of perfect alignment along all three axes (i.e. in the x, y and z directions).The second cage is a cuboidal cage with D 2h symmetry (Fig. S16, ESI †), and minimum diameters of approximately 14 Å, 12 Å and 5 Å.It is delimited by six ligands, two of which are puckered inward while the other four ligands are puckered outward: the anthracene core is rotated and adopts an orientation perpendicular to the plane of the ligand.

Gas sorption analysis
As envisioned, the inherent rigidity of the TCPT-based ligands led to the attainment of w-MOFs with permanent porosity.Ar adsorption studies at 87 K showed that Y-w-MOF-2, Y-w-MOF-2 (Naphth) and Y-w-MOF-2 (Anth) exhibited a fully reversible Type-I isotherm (Fig. S20a, S21a and S22a, ESI †), characteristic of microporous materials.Table 2 summarizes the apparent BET and Langmuir surface areas and pore volumes estimated from the Ar adsorption isotherms.The pore size distribution (PSD) for each of the Y-w-MOF-2 analogs (Fig. S20b, S21b and S22b, ESI †) was determined and found to be in good agreement with pore size derived from the corresponding structures.
Similar to the recently reported rare earth based (3,18)-connected gea-MOF-1, 25 Y-w-MOF-2 maintained its crystallinity up to 400 C (Fig. S5a, ESI †) and retained its optimal porosity upon heating beyond 200 C in vacuo, indicative of a high degree of thermal stability.It is worth mentioning that most other examples of highly thermally stable MOFs are not oen supported by porosity measurements aer such high temperature treatment.
In order to assess the structure-properties relationship for Y-w-MOF-2, gas adsorption measurements were undertaken for CO 2 , N 2 , CH 4 and H 2 at 298 K at low and high pressures.H 2 and CO 2 showed weak interactions with Y-w-MOF-2, as evidenced by their relatively low isosteric heats of adsorption, Q st : 5.7 and 27 kJ mol À1 for H 2 and CO 2 , respectively, at low loading.The Q st values were determined from variable temperature adsorption isotherms at 258, 268 and 278 K for CO 2 (Fig. S23 and S24, ESI †) and at 77 K and 87 K for H 2 (Fig. S27, ESI †).
The CO 2 adsorption isotherm at high pressure (Fig. S30a and S31a, ESI †) indicated a nearly full saturation of the pore system at 25 bar and 273 K (24.62 mmol g À1 ).This corresponds to a pore volume of 1.17 cm 3 g À1 that is in close agreement with the theoretical pore volume calculated from the Ar isotherm at 87 K (1.26 cm 3 g À1 ).The volumetric CO 2 uptake at 25 bar and 298 K (278 cm 3 (STP) cm À3 ) was slightly lower ($7%) than UTSA-20 (ref.7) (300 cm 3 (STP) cm À3 ) under similar conditions.
Encouraged by the higher CH 4 uptake of Y-w-MOF-2, adsorption of larger and relatively highly polarizable probe molecules such as C 2 H 6 , C 3 H 8 and n-C 4 H 10 and iso-C 4 H 10 (C 2+ ) were also investigated (Fig. 7).The results were compared with the corresponding CO 2 and CH 4 adsorption data and subsequently the C 2+ /CH 4 and n-C 4 H 10 /iso-C 4 H 10 separation factors were derived (Fig. S32, ESI †).Interestingly, C 2+ adsorption isotherms were much steeper at low pressures than for CH 4 and CO 2 , indicative of a plausibly higher affinity of Y-w-MOF-2 for C 2+ .Examination of single adsorption isotherm data using IAST  In order to assess the effect of the bulkier naphthalene core on the CO 2 and C 2+ uptakes, gas sorption studies were performed on Y-w-MOF-2 (Naphth).Based on single component adsorption isotherm data, the added functionality (bulkier core, reduced pore size) had a minimal impact on the adsorption properties and selectivity toward CO 2 and C 2+ (Fig. S33-S37, ESI †).It is foreseeable that the inclusion of aliphatic or polarized groups may offer more promising results. 15Accordingly, further experiments are ongoing to construct w-MOFs decorated with suitable functionalities for gas separation.Regarding w-MOF-3, work is in progress to determine the optimum activation conditions prior to performing detailed sorption studies.

Conclusion
A newly isolated 12-c RE MBB, RE 6 (m 3 -OH) 8 (O 2 C-) 12 , was successfully employed to construct a (4,12)-c MOF platform based on w topology, illustrating the importance of highly-connected building blocks for the practice of reticular chemistry where the assembly of desired MOFs is guided by the limited number of related highly-connected nets.This unique RE-based w-MOF platform is amenable to functionalization and/or expansion, as illustrated by the synthesis of ve isoreticular w-MOFs.w-MOF-1, based on a square-like ligand, was synthesized and its adaptability to various rare earth metals was evidenced by the formation of its isostructures.Extending the central core, through use of a rigid expanded tetracarboxylate, permitted the synthesis of permanently porous isoreticular w-MOFs.Specically, the Y-w-MOF-2 based on the TCPT ligand exhibited a record-high specic surface area (e.g.3690 m 2 g À1 ) for RE-based MOFs; it also offers the potential for sorption based separation of hydrocarbons.Further expansion via lengthening the arms of TCPT lead to a theoretically more open w-MOF structure (w-MOF-3) based on a relatively more symmetrical ligand.Finally the w-MOF structural features permitted additional functionalization in the central core, as evidenced by the successful intra-framework introduction of naphthalene and anthracene groups.Synthesis of Y-w-MOF-2 (Naphth).To a 23 ml glass scintillation vial containing TCDPN (8.5 mg, 0.011 mmol) was added 0.5 ml 0.068 M Y(NO 3 ) 3 $6H 2 O in DMF (0.034 mmol), 1.0 ml 2 M 2-FBA (2.0 mmol) in DMF and 0.5 ml DMF.The vial was sealed and placed into a preheated oven at 115 C for 24 h.Colorless cubic block shaped crystals were obtained.Yield: 8.5 mg.
Synthesis of Y-w-MOF-3.To a 23 ml glass scintillation vial containing TCEPT (9 mg, 0.011 mmol) was added 0.5 ml 0.068 M Y(NO 3 ) 3 $6H 2 O in DMF (0.034 mmol), 0.5 ml 2 M 2-FBA (2.0 mmol) in DMF and 2.0 ml DMF.The vial was sealed and placed into a preheated oven at 115 C for 24 h.The as synthesized pale yellow cubic block shaped crystals were obtained.The product was washed with copious amounts of DMF to remove impurities.Yield: 4.3 mg.

Fig. 1
Fig. 1 Left, the 12-connected rare earth hexanuclear cluster, [RE 6 (m 3 -OH) 8 (O 2 C-) 12 ]. 11Right, illustration of the MBB coordination geometry: note the positions of the carbon atom of the carboxylates match the vertices of the cuboctahedron.

Fig. 2
Fig. 2 Schematic showing the assembly of ftw-MOFs by combining 4-connected ligands (TetPOMB, TCPT, TCEPT, TCDPN, and TCDPA) with 12-connected rare earth hexanuclear MBBs (with a square or rectangle combined with a cuboctahedron vertex figure respectively), resulting in the ftw topology shown here as the augmented ftw net.

Fig. 3
Fig. 3 Pathways to modify quadrangular ligands for the synthesis of ftw-MOFs, including examples of ligands and their vertex figures.(Top) Modification of the organic quadrangular MBB through ligand expansion.(Bottom) Modification of the quadrangular organic MBB through ligand functionalization.
SCXRD studies revealed that Y-w-MOF-2 crystallized in the cubic Im 3 space group, and was formulated as |(DMA) 2 |[(Y 6 (m 3 -OH) 8 (H 2 -O) 6 )(TCPT) 3 ]$(solv) x .Topological analysis conrmed that the material was based on an augmented w net (w-a), the expected (4,12)-c w topology when considering the ligand as a 4-c node.Alternatively, and from a purely topological point of ).A cubic cage $12 Å in diameter, with T h symmetry, is delimited by six ligands: the carboxylates coordinate to the four 12-c RE MBBs positioned at the vertices of the cube face on which the ligand is situated.Compared to the parent w-MOF-1, the 12-c MBBs (the hexanuclear [RE 6 (OH) 8 -(O 2 C-) 12 ] carboxylate-based clusters) are rotated $15 out of perfect alignment along all three axes (i.e. in the x, y and z directions), adapting the orientation to allow for the accommodation of the rectangular ligand (Fig. 5).Consequently, the ligands on the faces of the central cage are all puckered inward.The ligands in the yz-plane are oriented lengthwise in the z-direction; the ligands in the xy-plane are oriented lengthwise in the y-direction; and the ligands in the xz-plane are oriented lengthwise in the x-direction (Fig. 6 and S9, ESI †).Adjacent to, and sharing one face of the cubic cage, are six larger cages with D 2h symmetry.This larger cage (21 Å Â 21 Å Â 12 Å) is a distorted cube delimited by six ligands.The ligands on any two parallel faces are oriented in the same direction with respect to one another and puckered inward; the remaining four faces are puckered outward and the ligands are oriented end to end lengthwise (Fig. 6 and S10, ESI †).

Fig. 6 (
Fig. 6 (Top) Representation of the ftw-MOF-2 framework: green ellipsoids represent the larger asymmetrical cage; yellow spheres represent the smaller symmetrical cages.Purple polyhedra represent the rare earth hexanuclear MBB; blue polygons represent the 4-connected quadrangular organic MBB.(Bottom left and center) Diagram of the asymmetrical cage along two axes; (bottom right) diagram of the symmetrical cage.

Table 1
Crystallographic data and structural refinement

Table 2
Porosity information derived from Ar low pressure adsorption data for ftw-MOF-2 compounds at 87 K a Based on modeled structure from unit cell parameters.