An Expanded MIL-53-Type Coordination Polymer With a Reactive Pendant Ligand

4-Styrylpyridine A mixture of 4-methylpyridine (18.6 g, 19.5 cm, 0.2 mol), benzaldehyde (21.2 g, 20.3cm, 0.2 mol) and acetic anhydride (20.4 g, 18.9 cm, 0.2 mol) was stirred and heated to reflux overnight. The mixture was cooled and poured slowly with stirring into water (200 cm) and allowed to stand with occasional stirring for 15 min by which time a brown solid had formed. This was collected by filtration, washed with a little water, dried with suction and then recrystallised from the minimum volume of hot ethanol. The crystals were filtered and dried under vacuum. The H NMR spectrum of the material matched that previously reported. Yield 60%, mp 126127 °C (Lit. 2 128 °C). H NMR (300 MHz, CDCl3) 8.59 (2H, br d), 7.36(5H, m), 7.31 (1H, d, J = 16 Hz) and 7.03 (1H, d, J = 16 Hz).

Materials with the MIL-53 structure represent one of the most widely studied of all families of metal-organic frameworks.The simple structure, containing infinitely linked trans-corner shared octahedral metal centres cross-linked by bridging 1,4benzene-dicarboxylate linkers, has an open net with diamondshaped channels running parallel to the metal oxide chains. 1 This 'wine-rack' structure is now renowned for its structural flexibility, where, in response to pressure, temperature or molecular guests, variation in the hinge angles of the link of the ligand to the metals allows a reversible opening and closing of the structure. 2This so-called 'breathing' effect has been widely investigated for the capture and release of small molecules in the gas and liquid phase, offering responsive, porous adsorbents, 3 a property now realised and rationalised using computer simulation. 4The structure was first reported in a vanadium form, also known as MIL-47, where oxidation leads to tetravalent metal cations bridged by oxide anions in the inorganic chain, 5 but most work has been done on MIL-53 materials containing trivalent cations (such as Cr 3+ , 6 Al 3+ , 7 Fe 3+ , 8 and Ga 3+ (ref.9)), with hydroxide as the bridging anion.A related set of materials can be formed for divalent cations (Co 2+ , Mn 2+ , Mg 2+ ) where the atom bridging the metals in the inorganic chain is the oxygen of a pyridine-N-oxide: 10 this pendant ligand may occupy the channel space of the framework, rendering the structure non-porous, but its steric bulk can be used to distort the framework, further illustrating the inherent flexibility of the MIL-53 structure. 11n isoreticular form of MIL-53, in which the 1,4-benzenedicarboxylate linker is replaced by the extended 4,4′-biphenyl dicarboxylate linker was reported by Senkovska et al. and named DUT-5. 12Although the parent aluminium form of the material was reported to have permanent porosity with a rigid, non-breathing structure, a vanadium form COMOC-2, showed pore opening and closing with gas pressure, 13 suitable for separation of hydrocarbons. 14In this communication we describe how an expanded form of MIL-53 can be generated from divalent metals and 4,4′-biphenyl dicarboxylate in combination with an extremely bulky pendant pyridine-N-oxide ligand, which in turn provides the possibility of introducing reactive functionality to the framework for post-synthesis chemistry.
The ligand 4-styrylpyridine-N-oxide (4S-PNO) was synthesised via 4-styrylpyridine using a method adapted from the literature (see ESI †).Initial attempts to crystallise a coordination polymer with the 4S-PNO ligand and 1,4-benzenedicarboxylate, as in previous work on divalent metals and pyridine-N-oxides, 10 yielded only a co-crystal of the ligand and 1,4-benzene-dicarboxylic acid (ESI †), with the metal ions presumably remaining in solution, so we instead selected the extended 1,4-biphenyl dicarboxylate (BPDC) linker.Thus for either Co 2+ or Mg 2+ isostructural coordination polymers MĲ4S-PNO)ĲBPDC)•nH 2 O were isolated as single crystals suitable for structure determination (ESI †).The amount of water in the magnesium form was found to be negligible, but extraframework electron density in the cobalt form could be accounted for by 0.25 water molecules per formula unit.Fig. 1a shows a view of the crystal structure of CoĲ4S-PNO)ĲBPDC)•0.25H 2 O illustrating the 1-dimensional channels, with Fig. 1b and c   The phase purity of MĲ4S-PNO)ĲBPDC) (M = Mg, Co) was confirmed by powder X-ray diffraction (XRD), while thermogravimetric analysis corroborated the identity of the bulk sample (for M = Mg, expected total mass loss on combustion to MgO is 91.3%, observed 89.7%, while for M = Co, expected total mass loss on combustion to CoO is 85.1%, observed 88.6%).Thermodiffractometry in flowing air was then used to investigate the phase stability and the possibility of any thermally induced structural deformation, as shown in    Fig. 3.This shows how the material collapses just above 200 °C, and despite continued evolution of the mass loss curve seen by thermogravimetric analysis (TGA) until above 500 °C there is complete loss of crystallinity by 250 °C.Prior to collapse there are no shifts in the Bragg peak positions indicating that the framework is rigid and non-breathing, i.e. , expansion to give a fully open framework, as in COMOC-2, does not occur with temperature.
Previous modifications of the DUT-5 structure have involved adding functionality to the BPDC linker: for example, sulfone modified versions were prepared for tuning the adsorption of hydrocarbons, 15 while a set of functionalised linkers variants were prepared and post synthesis modifications performed. 16For our new material, the pendant ligand has the potential as a site for a distinctly different type of post-synthesis modification.We thus investigated the bromination of this group to establish this possibility.The magnesium form of the material was chosen since it is white, allowing for any effect of bromination to be visually verified, and diamagnetic, allowing spectroscopic observation using solid-state NMR.Furthermore, the presence of a redox inactive cation would minimise additional side reactions occurring.A sample of 150 mg MgĲ4S-PNO)ĲBPDC) was stirred in a solution of bromine (100 mg) in chloroform (5 mL) for 3 days in darkness.The recovered solid, yellow in colour, was allowed to dry in air at room temperature before characterisation.Powder XRD clearly shows that the framework of the material is intact after bromine treatment, with little shift in Bragg peak positions, Fig. 4a, but that the relative intensity of the Bragg peaks is dramatically altered.In particular, the low-angle peaks noticeably reduced in intensity compared to those at higher angle, and this is entirely consistent with the presence of higher electron density species within the structure, as seen for zeolites, for example, when extra-framework space is occupied. 17The powder XRD does show some loss of crystallinity, however, and the presence of an amorphous background, which limits quantifying structural information.Further evidence for the inclusion of bromine is provided by element mapping using energy dispersive X-ray analysis on the scanning electron microscope, Fig. 4b and ESI.† This shows that the bromine is dispersed throughout the crystallites analysed as homogeneously as the other elements present.Bulk chemical analysis shows the presence of 15.8% Br by mass, which would correspond to, on average, just over one half of the carbon-carbon double bonds being dibrominated to give chemical formula MgĲC 13 H 10 BrNO)-ĲBPDC) with an expected Br content of 14.8%.Solution-state NMR spectroscopy of an acid-digested sample, a method that has been commonly used to analyse the post-synthetic modification of MOFs, including in bromination studies, 18 was inconclusive for our material.This may in part be due to the reactivity of the 4S-PNO ligand and its complex fragmentation in acid solution, and indeed this is a shortcoming of the acid-digestion method for analysis of reactive organic components.As an alternative, we investigated the use of 13 C and 81 Br solid-state NMR spectroscopy to provide evidence of the chemical nature of the bromine in the modified MOF. 13 C MAS NMR spectra indicated retention of the organic components of the framework and a structural change, although no direct evidence of C-Br bonds (see ESI †). 81Br is not widely studied due to its large quadrupole moment and Sternheimer antishielding factor, yielding extremely broad spectral resonances, 19 and using high field (20.0 T) wideline WURST-QCPMG experiments 20 our measured spectrum reveals a very broad signal, Fig. 4c, that is not inconsistent with the presence of C-Br bonds.Importantly there is no evidence for the presence of MgBr 2 , 21 which may have conceivably formed upon breakdown of the MOF in contact with bromine.
Despite the bulk of the pendant ligand calculation of the potential void space in the structure (see ESI †) shows that ∼25.0% of the unit cell volume is free space, and, furthermore, when considering the one-dimensional nature of the channels in which the ligand resides, this could be accessible (see ESI † for a space-filling representation of the structure).This would explain how bromination of the ligand is possible.
The literature now contains various reports of postsynthetic bromination of MOFs including by reaction between bromine and carbon-carbon double or triple bonds. 18ll of these, however, are associated with the linker making up the MOF structure and make use of highly porous frameworks.For example, Wang and Cohen used a 2-amino-1,4-benzenedicarboxylate as linker to prepare IRMOF-5 and then introduced bromine via an alkenyl linker modification in a tandem process, 18a while Marshall et al. prepared an isoreticular version of zirconium UiO-66 with a 4,4ethynylenedibenzoate linker that could be dibrominated to modify the mechanical properties of the framework.18e,f For the material described herein, the use of a pendant co-ligand provides a distinct approach for the introduction of a reactive site in metal carboxylate frameworks for post-synthesis modification.This will be of relevance to the design of new materials that may have applications in sensing, catalysis or molecular capture.It would also be interesting to investigate the preparation of analogous materials using other cations, or mixtures of cations, since in the case of MIL-53 this has been proven to have an influence on the structural flexibility of the framework and so may allow the properties to be tuned. 22 perpendicular views, showing the connectivity of the structure and the orientation of the pendant 4S-PNO ligand.

Fig. 2
Fig.2compares the structures of the Mg and Co forms of the new coordination polymer with the related materials DUT-5 and COMOC-2, with structural features identified to provide a quantitative comparison (Table1).This shows that while COMOC-2 presents a fully open structure (D1/D2 ≈ 1), and

Fig. 1
Fig. 1 Structure of MĲ4S-PNO)ĲBPDC) (M = Mg, Co) drawn using the coordinates of the Co form of the material: (a) parallel to the inorganic chains, and (b) and (c) in perpendicular directions, showing the orientation of the BPDC linker and 4S-PNO ligand, respectively.The metal-centred octahedra are shaded blue with carbon, nitrogen and oxygen as grey, olive and red spheres, respectively.Only one orientation of the disordered 4S-PNO and BPDC are shown in each case and the partially filled water sites are shown in (a).

Table 1
Quantitative comparison of the crystal structures of MĲ4S-PNO)ĲBPDC) (M = Mg, Co) with those published for DUT-5 and COMOC-2; see Fig.2for definition of structural parameters