Studies on the structural diversity of MOFs containing octahedral siloxane-backboned connectors

https://doi.org/10.1016/j.poly.2018.09.050 0277-5387/ 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). ⇑ Corresponding author. E-mail addresses: p.lickiss@imperial.ac.uk (P.D. Lickiss), r.davies@imperial.ac.uk (R.P. Davies). Luke C. Delmas , Peter N. Horton , Andrew J.P. White , Simon J. Coles , Paul D. Lickiss , Robert P. Davies a,⇑

We recently reported the synthesis of hexakis(4-carboxyphenyl)disiloxane (L-H 6 ; Fig. 1), its hydrogen-bonded superstructure, and the preparation of a porous Zn-based MOF (IMP-18) derived from it [33]. Here, we further evaluate the role of this connector in MOF chemistry by reporting upon the synthesis, characterisation and topological evaluation of four new MOFs built from L with a focus on the diversity of structural features with different metals. The inorganic Si-O-Si linkage in L adopts a roughly linear arrangement minimizing steric interactions of the surrounding phenyl rings. This results in the coordinating carboxylate groups being disposed in a pseudo-octahedral conformation which is a rare but highly desirable [35] linker geometry for MOF construction. This has prompted us to explore thoroughly the incorporation of L in novel 3D MOFs by allowing L-H 6 to react with a variety of metal salts under solvothermal conditions. Four new MOFs have been isolated and their structures elucidated by single crystal X-ray diffraction. The underlying nets of these materials have been determined through topological analysis of their deconstructed frameworks.

Materials and methods
All commercially-available chemical reagents were used as received without further purification. Hexakis(4-carboxyphenyl)disiloxane (L-H 6 ) was prepared according to the literature protocol [33]. IR spectra were recorded on a Perkin Elmer instrument in the range 600-4000 cm À1 . Powder X-ray diffraction (PXRD) studies were performed using a Panalytical MPD X-ray diffractometer with Cu Ka (1.54 Å) radiation. Thermogravimetric analysis was carried out under a nitrogen atmosphere using a Mettler Toledo instrument under a constant stream of dry nitrogen gas (flow rate 50 mL min À1 ) over the temperature range 30-800°C and at a heating rate of 5°C min À1 . X-ray data was collected using a Rigaku FRE+ (IMP-29 and IMP-31), Rigaku 007HF (IMP-30) or Agilent Xcalibur PX Ultra (IMP-28) diffractometer. Solutions were solved and refined using SHELX and SHELXTL [36,37], as well as Olex-2 [38], and WinGX [39]. The SQUEEZE routine within PLATON or the solvent masking routine in Olex-2 [38] was used to remove heavily disordered solvents from the MOF structures as specified in the text [40]. Graphics were generated using the Crystalmaker software suite [41]. A summary of the crystallographic data is presented in Table 1.

Synthesis of [Cd 3 (L)(DMA) 2 (H 2 O) 2 ] (IMP-28)
L-H 6 (50 mg, 63 mmol) and Cd(NO 3 ) 2 Á4H 2 O (58 mg, 188 lmol) were introduced into a screw-cap vial to which DMA (3.0 mL) and water (3.0 mL) were added. The mixture was agitated in an ultrasound bath for approximately 2 min with intermittent shaking to ensure complete dissolution of the reagents. The reaction mixture was gradually warmed to 85°C using a programmable oven over a 3 h period, held at 85°C for 42 h and allowed to cool slowly to room temperature over 3 h. Colourless crystals were isolated by suction filtration. The crystals were washed with fresh DMA (3 Â 4 mL) and air dried. Yield = 40 mg (40% based on L-H 6 ).

Synthesis of [Ce 2 (L)(DMF) 2 (H 2 O) 2 ] (IMP-29)
L-H 6 (10 mg, 13 mmol) and Ce(NO 3 ) 3 Á7H 2 O (42 mg, 113 lmol) were introduced into a screw-cap vial to which DMF (2.0 mL) and water (0.5 mL) were added. The mixture was agitated in an ultrasound bath for approximately 2 min with intermittent shaking to ensure complete dissolution of the reagents. The reaction mixture was gradually warmed to 80°C using a programmable oven over a 3 h period, held at 80°C for 42 h and allowed to slowly cool to room temperature over 3 h. Colourless crystals were isolated by suction filtration. The crystals were washed with fresh DMF (3 Â 4 mL) and air dried. Yield = 22 mg (86% based on L-H 6 ).  were introduced into a screw-cap vial to which DMF (2.0 mL) and water (0.5 mL) were added. The mixture was agitated in an ultrasound bath for approximately 2 min with intermittent shaking to ensure complete dissolution of the reagents. The reaction mixture was gradually warmed to 80°C using a programmable oven over a 3 h period, held at 80°C for 42 h and allowed to slowly cool to room temperature over 3 h. Colourless crystals were isolated by suction filtration. The crystals were washed with fresh DMF (3 Â 4 mL) and air dried. Yield = 13 mg (52% based on L-H 6 ).  were introduced into a screw-cap vial to which DMF (6.0 mL) was added. The mixture was agitated in an ultrasound bath for

Characterisation of [Cd 3 (L)(DMA) 2 (H 2 O) 2 ] (IMP-28)
Reaction of L-H 6 with Cd(NO 3 ) 2 Á4H 2 O in a 1:1 mixture of DMA/ H 2 O at 85°C in a sealed vial for 2 days afforded colourless needle crystals of IMP-28, where IMP is short for Imperial College London. These crystals were determined by single crystal X-ray analysis to be [Cd 3 (L)(DMA) 2 (H 2 O) 2 ] comprising a 3D-connected MOF built from discrete trimetallic Cd-based nodes linked together by the fully deprotonated ligand L (Fig. 2).
IMP-28 crystallizes in the orthorhombic space group Pnna (no. 52) and the asymmetric unit contains half a fully-deprotonated ligand L, 1.5 Cd atoms, one coordinating DMA molecule and one coordinating H 2 O molecule. There is some disorder in the structure with one of the phenyl rings expressing an alternative rotational position, an alternative orientation of the coordinating DMA mole-cules and small variation in the position of Cd1 (see ESI for further information). For clarity, only the major occupancy orientations are discussed in the following analysis. Using the SQUEEZE routine of PLATON [40], it has been estimated that the pores in the framework contain two molecules of water and one molecule of DMA per asymmetric unit to give an overall formula of Cd 3 (L)(DMA) 2 The siloxane linkage in the ligand is near linear [<Si-O-Si 179.3 (6)°] with staggered aryl groups, resulting in the six carboxylate branches being arranged in a pseudo-octahedral disposition as seen previously in IMP-18 [33] and in the other new MOFs described in this paper. Each of the six carboxylate groups in L bind to a different metal node and thus the linker can be considered to be 6-connected. The trimetallic secondary building unit (SBU) present in the structure consists of three Cd(II) ions bridged by two m 2 -g 2 : g 1 and two dimonodentate (m 2 -g 1 :g 1 ) carboxylate groups (as shown in Fig. 2 In order to better understand the framework topology of IMP-28, the Cd 3 SBU can be simplified to a distorted trigonal prismatic 6-c node and L can be reduced to an octahedral 6-c point node to give overall the (6,6)-c nia basic net as shown in Fig. 2. MOFs with the nia topology remain scarce in the literature though some examples are known with other hexacarboxylate linkers [42][43][44][45]. An alternative topological description for IMP-28 in which the 6c ligand based vertex is replaced by two 4-c silicon centred vertices, however, gives the nia-derived xby topology, rarely encountered in MOFs; see also Fig. S3.
The IMP-28 framework has solvent-filled channels along the crystallographic b direction with a cross section of approximately 5 Â 5 Å 2 . After theoretical removal of both the coordinated and non-coordinated solvent, PLATON [40] estimates the solventaccessible void volume for IMP-28 to be 3567 Å 3 or 53.4% of the unit cell volume. PXRD analysis of the bulk sample revealed peaks assignable to IMP-28 as the main product, however the additional presence of a number of unassigned peaks indicated the presence of an impurity which we have been unable to identify (see Fig. S4). Nevertheless, evacuation of the bulk material was attempted but this led to its decomposition and a complete loss of porosity.
IMP-29 crystallizes in the monoclinic space group C2/c (no. 15) and the asymmetric unit contains half a fully-deprotonated ligand L, one Ce ion, one coordinating DMF molecule and one coordinating H 2 O molecule. There is disorder of the central siloxane oxygen atoms about a 2-fold axis and in the orientation of the coordinating DMF molecules (see ESI for further information). For clarity, only the major occupancy orientations are discussed in the following analysis. Using the solvent masking routine of Olex-2 [38], it has been estimated that the pores in the framework contain two molecules of water and four molecules of DMF per asymmetric unit to give an overall formula of [Ce 2 (L)(DMF) 2 The siloxane linkage in the connector is near linear [<Si-O-Si 169(2)°] with the six carboxylate branches disposed in an octahedral manner. Each of the six carboxylate groups in L bind to a different metal node and thus the linker can be considered to be 6connected. The bimetallic SBU present in the structure consists of two Ce(III) ions held together by two chelating/bridging carboxylates (m 2 -g 2 :g 1 ) and two bridging dimonodentate (m 2 -g 1 :g 1 ) carboxylate groups (as shown in Fig. 3  cule (Ce-O = 2.537(4) Å). Each of these coordinating carboxylate groups belong to distinct molecules of L and thus the overall geometry of this SBU is a 6-connected distorted octahedron.
Looking closely at the framework topology of IMP-29, both the Ce 2 SBU and L ligand can be reduced to pseudo-octahedral 6-c point nodes to give the well-known 6-c pcu basic net as shown in Fig. 3. An alternative topological description in which the 6-c ligand based vertex is replaced by two 4-c silicon centred vertices, however, gives the less commonly encountered 4,6-c fsh topology [47][48][49]; see also Fig. S7.
The IMP-29 framework has solvent-filled channels with the largest window size being ca. 6 Â 8 Å 2 in the 110 crystallographic direction. After theoretical removal of both the coordinated and non-coordinated solvent, PLATON [40] estimates the solventaccessible void volume for IMP-29 to be 4446 Å 3 or 58.0% of the unit cell volume. PXRD measurements on a bulk sample of IMP-29, however, suggested its decomposition during isolation and drying procedures.

Characterisation of [Y 2 (L)(DMF) 2 (H 2 O) 2 ] (IMP-30)
Reaction of L-H 6 with Y(NO 3 ) 3 Á6H 2 O in a 4:1 mixture of DMF/ H 2 O at 80°C in a sealed vial for 2 days afforded colourless prism crystals of IMP-30. These crystals were characterised by single crystal X-ray analysis to be [Y 2 (L)(DMF) 2 (H 2 O) 2 ]. The crystals were found to comprise a 3D-connected MOF built from discrete bimetallic Y-based nodes linked together by the fully deprotonated ligand L (Fig. 4).
IMP-30 crystallizes in the triclinic space group P 1 À (no. 2) and the asymmetric unit contains half a fully-deprotonated ligand L, one Y ion, one coordinating DMF molecule and one coordinating H 2 O molecule. The orientation of the ligand L in the structure is disordered over two orientations with 63:37 occupancy. The relative orientations of the ligand share a fixed central oxygen and are related by a roughly 69°rotation. Despite this, both arrangements result in six coordinating carboxylate groups being in approximately the same positions, allowing the Y(III) ions to remain in a fixed position within the lattice. There is also disorder in the orientation of the coordinating DMF molecules (see ESI for further information). For clarity only the major occupancy orientations are discussed here. Using the solvent masking routine of Olex-2 [38], it has been estimated that the pores in the framework contain 5.5 molecules of DMF per asymmetric unit to give an overall formula of [Y 2 (L)(DMF) 2 (H 2 O) 2 ]Á5.5DMF.
The siloxane linkage in the ligand is linear [<Si-O-Si 180.0°] (the central siloxane oxygen sits on a centre of symmetry) with the six carboxylate branches disposed in an octahedral manner. Each of the six carboxylate groups in L bind to a different metal node and thus the linker can be considered to be 6-connected. The bimetallic SBU present in the structure consists of two Y(III) ions held together by two bridging dimonodentate (m 2 -g 1 :g 1 ) carboxylate groups (as shown in Fig. 4)   late groups belong to distinct molecules of L and thus the overall geometry of this SBU is a 6-connected distorted octahedron.
Looking closely at the framework topology of IMP-30, both the bimetallic Y 2 SBU and L ligand can be reduced to pseudo-octahedral 6-c point nodes to give a 6-c pcu basic net as shown in Fig. 4. Thus, while IMP-29 and IMP-30 are not isostructural crystallographically, from a topological point of view they both have the same connectivity of their underlying nets. The major difference between the two systems is in the geometry of the deconstructed metallic nodes; IMP-29 displays a higher degree of distortion away from the ideal octahedral geometry of the metal-based 6-c vertex. An alternative topological description of IMP-30 in which the 6-c ligand node is replaced by two 4-c vertices (centred on the terminal silicon atoms of L) gives the 4,6-c fsh topology as seen in IMP-29; see also Fig. S11.
The IMP-30 framework has solvent-filled channels with the largest window size being ca. 6 Â 8 Å 2 in the crystallographic c direction. After theoretical removal of both the coordinated and noncoordinated solvent, PLATON [40] estimates the solvent-accessible void volume for IMP-30 to be 1186.4 Å 3 or 59.8% of the unit cell volume. As seen with IMP-29, attempted PXRD measurements on bulk samples of IMP-30 after isolation and air drying suggested its decomposition during isolation and drying procedures.
IMP-31 crystallizes in the triclinic space group P 1 À (no. 2) and the asymmetric unit contains half a tetra-anionic ligand LH 2 , one Zn(II) ion, and two distinct halves of 4,4 0 -bipyridine linkers. The orientation of the ligand LH 2 in the structure is disordered over two orientations with 79:21 occupancy. The relative orientations of the ligand share a fixed central oxygen and are related by a 81°rotation. Despite this, both arrangements result in the coordinating carboxylate groups being in approximately the same positions, allowing the Zn(II) ions to remain in a fixed position within the lattice. There is further positional disorder of the noncoordinating carboxylate groups (see ESI for further information). For clarity in the discussion only the major occupancy orientation of each case is discussed. Using the SQUEEZE routine of PLATON  [40], it has been estimated that the pores in the framework contain 4 molecules of DMF per asymmetric unit to give an overall formula of [Zn 2 (LH 2 )(4,4 0 -bipy) 2 ]Á4DMF.
The siloxane linkage in the ligand is linear [<Si-O-Si 180.0°] (the central siloxane oxygen sits on a centre of symmetry) with the six carboxylate branches disposed in an octahedral manner. Only four of the six carboxylate groups in LH 2 bind to zinc nodes and thus the linker can be considered to be 4-connected. Charge balance requires that the remaining two carboxylic acid groups (which are non-binding) are protonated and, though these protons could not be located in the crystallographic DF map, this is supported by the asymmetric nature of the of the C47-O41/42 bonds which show C-O distances of 1.215(10) and 1.240(11) Å, respectively. The mononuclear Zn(II) nodes present in the structure are coordinated by two monodentate carboxylate groups from two different LH 2 ligands and two aromatic nitrogens from two separate 4,4 0 -bipyridine molecules (as shown in Fig. 5). The Zn-O distances are 1.961(3) and 1.997(8) Å while the Zn-N distances are 2.028(2) and 2.102(3) Å. The overall geometry of this SBU is consequently a 4-connected distorted tetrahedron.
From a topological point of view, the LH 2 ligand can be simplified to a square planar node owing to the fact that two of its carboxylic acid groups are non-binding. The Zn-based nodes are tetrahedral with the 2-connected 4,4 0 -bipyridine units act as linear links between Zn vertices. Thus, the two types of 4-c nodes connect together to give the less commonly encountered 4,4-c mog topology [50][51][52][53] as depicted in Fig. 5. An alternative topological description in which the 4-c ligand based vertex is replaced by two 3-c silicon centred vertices, however, gives a new 3,4-c topology whose point symbol is {6 3 }{6 5 Á8} with stoichiometry (3-c)(4c); see also Fig. S15.
The IMP-31 framework has solvent-filled channels with the largest window size being ca. 5 Â 10 Å 2 in the crystallographic b direction. After theoretical removal of both the coordinated and non-coordinated solvent, PLATON [40] estimates the solventaccessible void volume for IMP-31 to be 987.6 Å 3 or 45.3% of the unit cell volume. PXRD of the bulk material however suggested decomposition of the IMP-31 framework during isolation procedures thus precluding further studies on this material.

Conclusions
The results presented here extend our studies on the octahedral siloxane-based hexacarboxylate linker L for MOF construction to other d-block and f-block metals. Thus, treatment of L-H 6 with Cd II , Ce III , and Y III salts has been demonstrated to result in the assembly of the MOF materials IMP-28, IMP-29, and IMP-30 respectively. These novel MOFs all comprise metal-based clusters which are cross-linked by fully deprotonated molecules of L to afford 3D networks with solvent filled channels. Hexatopic and in particular octahedral connectors remain scarce in the literature, and thus the metal complexes of L presented herein display uncommon MOF topologies including nia and fsh [35]. Combining L-H 6 with Zn II and 4,4 0 -bipyridine gave the mixed linker MOF IMP-31 which features less commonly-encountered monoatomic Zn nodes and possesses the rare mog topology. Although polytopic linkers have been noted to produce MOFs of enhanced stability [54], the MOFs reported here were found to be unstable to evacuation. This is likely due to the structures of the metallic clusters in these new materials. The SBUs are of low nuclearity ( 3 metal ions per SBU) and linking to the organosilicon node is in many cases via coordination of a chelating carboxylate group to a terminal metal ion i.e. there are relatively few bridging carboxylates holding the clusters together. Work is ongoing in our laboratories towards preparing MOFs with L and more highly connected multimetallic clusters (e.g. Zr 6 O 8 ) which are expected to impart significantly improved stability on these systems.