Received 4 September 2013
Distortions of a flexible metal-organic framework from substituted pendant ligands
Four new variants of the 1,4-benzenedicarboxylate MIL-53 structure have been prepared for CoII under solvothermal conditions and their structures solved and refined from single-crystal X-ray data. All materials contain pendant pyridine-N-oxide ligands that bridge pairs of CoII atoms in the inorganic backbone of the structure via O. By the use of the ligands 3-bromopyridine-N-oxide, 4-methoxypyridine-N-oxide, isoquinoline-N-oxide and 4-phenylpyridine-N-oxide, materials are prepared with the same topology but distinct structures. These illustrate how the MIL-53 structure is able to distort to accommodate the bulk of the various substituents on the pyridine ring. The bulkiest pendant ligand, 4-phenylpyridine-N-oxide, results in a distortion of the diamond-shaped channels in an opposite sense to that seen previously in expanded forms of the parent MIL-53 structure. By comparison with published crystal structures for MIL-53 with various occluded guests, the structural distortions that take place to accommodate the pendant ligands are quantified and it is shown how a twisting of the 1,4-benzenedicarboxylate ligand, instead of a hinging about the 2-carboxylate-metal connection, allows the new structures that are observed.
The structural flexibility of certain metal-organic framework (MOF) materials is one of their most striking properties. In MOFs, atomic displacements of several Å may be brought about by an external stimulus, such as temperature, pressure or the addition and/or removal of occluded molecules, whilst crystallinity and, in some cases, also atomic connectivity, is maintained (Férey & Serre, 2009). In contrast, purely inorganic open-framework solids, such as the silicate zeolites, undergo only relatively small atomic displacements in response to similar stimuli. As well as the potentially practical implications of solids with open framework structure whose porosity changes depending on the conditions to which they are exposed, the flexibility of MOFs is of fundamental interest due to the unusual physical properties that may result, such as negative thermal expansion (Han & Goddard, 2007) anisotropic elasticity (Ortiz et al., 2013) and other anomalous mechanical properties (Ogborn et al., 2012). Examples of flexible MOFs have been provided by the work of Kitagawa and co-workers, who have classified various types of these so-called `soft porous crystals' (Kitagawa & Uemura, 2005; Bureekaew et al., 2008), while Rosseinsky and co-workers have demonstrated that the use of peptide linkers can give extended structures with conformational flexibility resembling proteins (Rabone et al., 2010). Among the most widely studied flexible metal organic frameworks are those of the MIL-n family of materials reported by Férey and co-workers, some of which show a large `breathing' effect with no breaking of chemical bonds, which may be of practical application in molecular separation, storage and release (Férey & Serre, 2009).
The material known as MIL-53 is now one of the archetypal flexible MOFs. The framework has the empirical chemical formula M(OH)(BDC), where M is trivalent and may be Cr (Millange et al., 2002), Fe (Millange, Guillou et al., 2008), Sc (Mowat et al., 2011), V (Leclerc et al., 2011), Al (Loiseau et al., 2004), Ga (Volkringer et al., 2009) or In (Anokhina et al., 2005) and BDC = 1,4-benzenedicarboxylate. As shown in Figs. 1(a) and (b) the MIL-53 structure contains octahedrally coordinated metal centres that share trans corners to give extended, linear inorganic chains. The 2 bridging atoms that form the trans corners are the hydroxide ions (sometimes partially replaced by fluoride ions) and the remaining four positions of the octahedron are made up by pairs of 2 carboxylates from the two linkers, one positioned on each opposite side of the octahedron. These linkers each form interchain bridges with their second carboxylate group to produce an extended structure with diamond-shaped channels running parallel to the inorganic chains. The MIL-53(Cr) and MIL-53(Al) materials have been particularly well studied since the as-prepared hydrated materials show large, and reversible, unit-cell expansion upon removal of water at close to 373 K (Serre et al., 2002), and also, in the case of the Cr material, an expansion on exposure to excess water (Guillou et al., 2011). The topology of the structure is maintained during the changes in dimensions. In contrast, MIL-53(Fe) shows no expansion either upon loss of water or addition of extra water (superhydration), but when exposed to a wide variety of small molecules, such as alcohols, aromatic hydrocarbons and heterocycles, the water is displaced in preference to the new guest and structural expansion readily occurs (Millange, Serre et al., 2008). This flexible nature of porous MIL-53 materials has been widely investigated for use in novel adsorbents, both in the liquid phase (Millange, Serre et al., 2008; El Osta et al., 2012; Van de Voorde et al., 2013) and the gas phase (Férey et al., 2003; Bourrelly et al., 2005; Serre et al., 2007; Trung et al., 2008; Llewellyn et al., 2009; Hamon et al., 2011), and for storage and release of species, such as drug molecules (Horcajada et al., 2008).
| || Figure 1 |
(a) and (b) Two views of the structure of MIL-53(Fe)[pyridine] showing the hydrogen-bond interaction between the pyridine and framework hydroxyl as a dotted line in (b) (crystal data taken from Millange et al., 2010), and (c) and (d) Co(BDC)(PNO) (crystal data taken from Munn et al., 2013). (a) and (c) show views parallel to single diamond-shaped channels that form part of the extended structure. Grey spheres are C, red O and cyan N, while the blue polyhedra represent both Fe- and Co-centred octahedral. H atoms are omitted for clarity.
Both experimental and computational methods have been used to understand the structural flexibility of MIL-53 materials. For example, Liu et al. (2008) showed using high-resolution powder neutron diffraction that the fully expanded structure of dehydrated MIL-53(Al) spontaneously closes upon cooling to 77 K, and then used inelastic neutron scattering to suggest that low-energy librational motions of the BDC linkers were the driving force for the closing/opening of the structure. A combination of force-field modelling and neutron diffraction as a function of pressure and temperature allowed the CO2 adsorption isotherms of MIL-53(Cr) over a wide range of pressures and temperatures to be rationalized (Ghoufi et al., 2012). Vapour-phase adsorption experiments over MIL-53(Cr) have shown that for alcohols, host-guest hydrogen bonds are important in determining the expansion of the framework (Bourrelly et al., 2010). Modelling of elastic constants by Ortiz et al. (2013) suggested that the flexibility of MIL-53(Al) is characterized by highly anisotropic elastic properties (Young's and shear moduli, and Poisson's ratios) and directions of negative compressibility.
Recently, Xu et al. (2010) reported an MnII form of MIL-53 in which the 2 bridging atoms are the O atoms of a pendant pyridine-N-oxide (Figs. 1c and d), and we recently extended the work to produce CoII, NiII and mixed-metal analogues (Munn et al., 2013). In this present paper we have investigated the extent to which substituted pyridine-N-oxides may be incorporated into the structure. The aim of this work was to prepare MIL-53 analogues that may present new structural distortions, which in turn may help to understand further the structural flexibility of this widely studied family of materials.
Synthesis was performed under solvothermal conditions from cobalt(II) nitrate hexahydrate (150 mg, Sigma-Aldrich 98%), 1,4-benzenedicarboxylic acid, and the chosen pyridine-N-oxide (see below; all of which were used as supplied by Sigma Aldrich) which were added in an appropriate mass to give a molar ratio as shown in the supporting information.1 The reagents were stirred in 10 ml of N,N-dimethylformamide (DMF) in an 18 ml Teflon cup before being sealed in a steel autoclave and heated at 373-393 K for periods of between 2 and 7 d (see the supporting information ). After cooling, the solids were recovered by suction filtration and dried in air at 343 K. Powder X-ray diffraction was used to verify the purity of the materials produced; this was performed using a Siemens D5000 diffractometer and patterns were compared against those generated from the single-crystal structures reported below (see the supporting information ). Thermogravimetric analysis was performed using a Mettler Toledo TGA/DSC1 instrument in which approximately 10 mg of powder was loaded into an alumina crucible and the sample was heated in air to 1273 K at a rate of 10 K min-1. Combustion to Co3O4 thus allowed the empirical formula of the bulk product to be verified (see the supporting information ).
In a control experiment, the pyridine-N-oxide was omitted from the solvothermal reaction. Under these conditions, the sole crystalline product was Co(BDC)(DMF), a material previously reported (Fu et al., 2004). This material has a MIL-53 type structure, but the 2 atom in the inorganic chain is provided by the O of coordinated N,N-dimethylformamide. In addition, we found that not all substituted pyridine-N-oxides are robust enough to withstand the solvothermal reaction conditions. For example, if 4-pyridinecarboxaldehyde N-oxide is used under similar synthetic conditions (a molar ratio of 1:3:3 Co:1,4-benzenedicarboxylic acid:4-pyridinecarboxaldehyde-N-oxide), then the material [Co3(BDC)4]·DMA+·solvent is formed, where DMA is dimethlyammonium, which may be formed from hydrolysis of the DMF. This material has previously been reported by a number of groups (Damgaard Poulsen et al., 2006; Clausen et al., 2008; Luo et al., 2009; Zhang et al., 2010), and has a structure unrelated to MIL-53 in which trimers of corner-shared Co-centred octahedra are bridged in three dimensions by 1,4-benzenedicarboxylate ligands. This shows that the 4-pyridinecarboxaldehyde-N-oxide has a structure-directing effect, and possibly decomposes under the reaction conditions, but is not incorporated in the product formed.
Single-crystal data were measured using an Oxford Diffraction Gemini four-circle system with Ruby CCD area detector, with the crystals held at constant temperature using an Oxford Cryosystems Cryostream Cobra, or by the EPSRC Crystallographic Service (Coles & Gale, 2012), where a Rigaku AFC12 diffractometer with Saturn724+ CCD was used, see Table 1. Structures were solved by direct methods using SHELXS (Sheldrick, 2008) with additional light atoms located by Fourier methods. H atoms were added at calculated positions and refined using a riding model with freely rotating methyl groups. Anisotropic displacement parameters were refined for all non-H atoms and H atoms were given isotropic displacement parameters equal to 1.2 (or 1.5 for methyl H) times the equivalent isotropic displacement parameter of the atom to which they are attached.
The inclusion of 3-bromo-pyridine-N-oxide (3-Br-PNO) in the solvothermal synthesis yields a phase-pure sample of Co(BDC)(3-Br-PNO). The crystals were small, highly twinned and weakly diffracting. The structure was solved from the major twin component since it proved impossible to perform a reasonable twin refinement. The material crystallizes in the space group P21/c and the asymmetric unit contains two half-occupied Co sites, lying on inversion centres at ½,½,½ and 0,½,½, a 1,4-benzenedicarboxylate anion and a 3-bromopyridine-N-oxide. Chains of -Co-O-Co- run parallel to a, where the 2 oxygen is the O atom of the pendant 3-bromopyridine-N-oxide, Fig. 2. The 3-bromopyridine groups are parallel but the Br substituents force the pyridine rings away from each other so that they are offset, preventing any - interactions. Interestingly, in the crystal structure of pyridine-N-oxide itself there are apparently no - interactions but instead one intermolecular interaction is characterized by a CH distance of 2.72 Å from the one pyridine-N-oxide to the centroid of the aromatic ring of the next (in addition to C-HO interactions not possible in the metal-organic frameworks; Shishkin et al., 2013). In the metal-organic framework, the neighbouring 3-bromo-pyridine-N-oxides are too far apart for such an interaction; instead there is evidence for a CH interaction between H16 and a centroid of the aromatic ring of the closest 1,4-benzenedicarboxylate linker (3.19 Å).
| || Figure 2 |
Views of the structure of Co(BDC)(3-Br-PNO): (a) along the a axis showing the filling of the diamond-shaped channels by the pendant ligand; (b) and (c) two views along the c axis showing the relative orientation of the pendant ligands and the bridging 1,4-benzenedicarboxylates, respectively. The inorganic chains in (b) and (c) are numbered according to the system in (a). H atoms are omitted for clarity.
With 4-methoxypyridine-N-oxide (4-MeO-PNO), the material formed, Co(BDC)(4-MeO-PNO), crystallizes in the space group Pnma. The asymmetric unit contains a Co atom with half a 1,4-benzenedicarboxylate and a disordered 4-methoxypyridine-N-oxide. The 4-methoxypyridine-N-oxide is disordered about the mirror plane and its pyridine ring was constrained to be a hexagon and refined as two components with 50:50 site occupancy about the mirror plane. Atoms Co1 and O6 lie on the mirror plane and the 1,4-benzenedicarboxylate lies on an inversion centre. In this material (Fig. 3) the O-Co-O chains run along a, with the 2 oxygen from the 4-methoxypyridine-N-oxide, and as with the 3-Br-PNO material the pyridine rings are offset thus preventing any - interactions between them. The pendant linkers instead interact with each other via methyl C-H interactions, with the distance between methyl C-H and the centroid of the aromatic pyridine-N-oxide ring being on average 3.08 Å.
| || Figure 3 |
Views of the structure of Co(BDC)(4-MeO-PNO): (a) along the a axis showing the filling of the diamond-shaped channels by the pendant ligand (only one orientation of the 4-MeO-PNO ligand is shown); (b) the two orientations of the 4-MeO-PNO ligand; (c) and (d) two views along the c axis showing the relative orientation of the pendant ligands and the bridging 1,4-benzenedicarboxylates, respectively. The inorganic chains in (c) and (d) are numbered according to the system in (a). H atoms are omitted for clarity.
With the bulky isoquinoline-N-oxide co-ligand, the structure of the phase-pure material produced is distorted significantly, Fig. 4. This phase crystallizes in the space group Pnma. The asymmetric unit contains half a 1,4-benzenedicarboxylate, half an isoquinoline-N-oxide, half a Co atom and a water molecule (with refined occupancy of 1/8). Atoms C2, C3, C6 and C7 of the 1,4-benzenedicarboxylate sit on the mirror plane (Wyckoff position c). A small amount of residual electron density was refined as a water molecule (O10) at occupancy of 1/8, reflecting both its multiplicity on a special position (mirror plane Wyckoff position c) and low fractional occupancy. No H atoms were located for this water molecule, but they are included in the formula to calculate the correct density. The 1,4-benzenedicarboxylate was refined as disordered over two positions by a small rotation of the aromatic ring, with site occupancies constrained to be 50:50. Although they are offset from each other, there is possible stacking between adjacent, symmetry-related isoquinoline-N-oxides: the aromatic surfaces are parallel (related by an inversion centre) with the closest atomic contact being C15 to C17 of a symmetry-related ring of 3.641 Å.
| || Figure 4 |
Views of the structure of Co(BDC)(isoquinoline-N-oxide): (a) along the a axis showing the filling of the diamond-shaped channels by the pendant ligand (only one orientation of the pendant ligand shown), and (b) and (c) two views along the c axis showing the relative orientation of the pendant ligands and the bridging 1,4-benzenedicarboxylates, respectively. The inorganic chains in (b) and (c) are numbered according to the system in (a). Only one orientation of the 1,4-benzenedicarboxylate linker is shown for clarity and H atoms are omitted.
4-Phenylpyridine-N-oxide (4-phenyl-PNO) as a co-ligand introduces the largest structural distortions in the materials that we have studied, Fig. 5. The material crystallizes in space group C2/c. The asymmetric unit contains half a Co atom, half a 1,4-benzenedicarboxylate and half a 4-phenylpyridine-N-oxide. The 1,4-benzenedicarboxylate lies on an inversion centre at the centre of the benzene ring. The 4-phenylpyridine-N-oxide lies on a twofold axis through the phenyl, pyridine and N-O bond. The Co atom lies on a centre of inversion. The phenyl rings of adjacent 4-phenyl-PNO ligands are parallel, the distance between the C atom of one ring and the centroid of the adjacent ring is 3.43 Å, and the closest atom-atom contact between the parallel rings is 3.45 Å (C13-C14), indicating a - interaction.
| || Figure 5 |
Views of the structure of Co(BDC)(4-Ph-PNO): (a) along the a axis showing the filling of the diamond-shaped channels by the pendant ligand, and (b) and (c) two views along the c axis showing the relative orientation of the pendant ligands and the bridging 1,4-benzenedicarboxylates, respectively. H atoms are omitted for clarity.
In comparing the structures of the four new CoII materials, it is informative to consider their relationship to the related MIL-53 materials that have been reported in the literature. The flexibility of the MIL-53 structure is best illustrated by its response to the presence of guest molecules, since published crystal structures of MIL-53[guest] materials are available. For the FeIII analogue of MIL-53, Fe(OH,F)(BDC), where the 2 atom between neighbouring Fe centres along the inorganic chain is a statistical mixture of hydroxide and fluoride, its structure has been refined from high-resolution powder X-ray diffraction data, both in hydrated and dehydrated forms and in the presence of a variety of organic guest molecules (Millange, Guillou et al., 2008; Millange et al., 2010; El Osta et al., 2012). These structural data allow us to parameterize the structural distortions when the MIL-53(Fe) structure is expanded by varying degrees. Férey and Serre have previously suggested that the flexibility of the 1,4-benzenedicarboxylate structures MIL-88b and MIL-53 is largely accounted for by two main structural distortions (Serre et al., 2007): a hinge angle (coined the `knee-cap') that relates to the connection of one of the carboxylate groups of the 1,4-benzenedicarboxylate linker to a pair of metal centres forming the inorganic chain, Fig. 6(a) and a dihedral angle that describes the rotation of the benzene ring of the 1,4-benzenedicarboxylate in relation to its carboxylate groups, Fig. 6(b). We have labelled these and , respectively. A further angle is useful to consider: the M-X-M angle that forms the inorganic backbone of the structure, where X is the 2 O/F atom in MIL-53(Fe) or the O atom of pyridine-N-oxide or DMF in the CoII materials. This angle, which we have denoted , describes the response of the inorganic chain parallel to the channels to the presence of guest molecules in the channels, Fig. 6(c). Finally, the dimensions of the diamond-shaped channels are a useful way to describe the degree of `openness' of the MIL-53 structure (Millange, Serre et al., 2008): these are conveniently expressed in terms of the lengths of the two diagonals, d and D, as labelled in Fig. 5(d). Here, d is the distance between metal atoms across the diamond in the direction that OH groups project into the channels (or the direction of the O-N bond of the pyridine-N-oxide), and D is the perpendicular interchain metal-metal distance. The ratio d/D is then a measure of the shape of the diamond (and can be related to the area of the channels).
| || Figure 6 |
Definition of parameters defining the structural distortions of the MIL-53 structure: (a) hinge angle of carboxylate link to inorganic chain (), (b) carboxylate O-C-C-C dihedral angle (), (c) intrachain M-O-M angle () and (d) dimensions of the projection of the diamond-shaped channels. Crystal data were taken from the structure of Co(BDC)(3-Br-PNO): cyan atoms are Co, red O, large grey C and small grey H.
Table 2 contains these various structural parameters for a set of MIL-53(Fe)[guest] materials, two hydrated MIL-53(Cr) materials (Guillou et al., 2011) and the Co(BDC)(ligand) materials (where ligand = pyridine-N-oxide or DMF). (Note that some of the hinge and dihedral angles are mean values in some cases, depending on the crystal symmetry of the material and the number of crystallographically unique metal sites and BDC linkers.) It is immediately apparent that the M-O-M angle shows no correlation with the ratio d/D, showing that the inorganic chain is essentially invariant with the degree of expansion of the structure. On the other hand, the angles and , which describe the orientation and twist of the 1,4-benzenedicarboxylate, respectively, each show a clear relationship with the d/D ratio, and this is shown graphically in Fig. 7. For the MIL-53 materials, the opening of the structure (an increase in d/D from 0.3 to 1) is reflected in a decreasing angle (Fig. 7a); the structural expansion is thus accommodated by the hinging of the 1,4-benzenedicarboxylate linker about the O-O pair binding to a pair of metal atoms, while all bond distances remain largely unchanged. For the Co(BDC)(PNO) materials there is a similar trend, except notably for Co(BDC)(4-phenyl-PNO) for which the hinge angle is as would be expected for the closed MIL-53 structures. For the MIL-53 materials, the angle , the dihedral angle describing the twist of the BDC linker, remains close to 180° whatever the degree of expansion (Fig. 7b). For Co(BDC)(PNO) we see some interesting differences: while the angles and for Co(BDC)(PNO) (i.e. containing unsubstituted pyridine-N-oxide) are rather similar to those seen in MIL-53(Fe)[pyridine] (in which the pyridine is held by hydrogen bonding to the framework 2 hydroxide ions), the angle also varies with increasing d/D as substituents are added to the pyridine ring of the pendant PNO. While for Co(BDC)(4-MeO-PNO) and (Co(BDC)(isoquinoline-N-oxide) and both decrease with increasing d/D, for 4-phenyl pyridine-N-oxide only the angle is involved in the distortion of the structure and this drops to 142.55°, reflecting a considerable twisting of the BDC from planarity even though the hinge angle is at the value for the closed structures of MIL-53(M)[H2O]. Thus, the structure is able to accommodate bulky, substituted ligands by a distortion that is distinct from that seen in the parent MIL-53 materials, but introducing the additional variable of the dihedral angle . For comparison, the angle in solid 1,4-benzenedicarboxylic acid is 174.67 and 178.97° in its two common crystalline forms (Bailey & Brown, 1967); this highlights the distortion brought about by the bulky pendant ligands. It is interesting to note that the angle has also been implicated in the expansion/contraction of the MIL-88b MOF structure (Serre et al., 2007).
| || Figure 7 |
Plots of d/D versus (a) and (b) . Points are taken from data in Table 2 and the blue lines are linear fits to the points for the MIL-53 materials.
The introduction of co-ligands into the widely investigated MIL-53 structure provides a convenient route to introduce unusual structural distortions in this archetypal flexible metal-organic framework material. The structure is able to accommodate the bulk of substituted pyridine-N-oxide ligands by distortions not usually seen when guest molecules are introduced from solution or vapour into pre-made porous MIL-53 materials. Particularly striking is the case of 4-phenylpyridine-N-oxide, where the diamond-shaped channels are stretched in the opposite sense to that observed when bulky guest molecules are added to MIL-53, and this results in a twist of the BDC linkers around the benzene ring, rather than their hinging about the carboxylate connection to the metals. Although the choice of the metal in the MIL-53 structure is known to influence the flexibility of the structure, when comparing the CoII(BDC)(PNO) materials with FeIII(OH)(BDC) MIL-53 materials it is clear that the nature of the substituted PNO ligand induces unusual structural distortions. This suggests that new forms of MIL-53 itself may be possible in the presence of bulky guest molecules that are sterically more demanding than those so far investigated.
Some of the equipment used in materials characterization at the University of Warwick was obtained through the Science City Advanced Materials project `Creating and Characterising Next Generation Advanced Materials' with support from Advantage West Midlands (AWM) and part funded by the European Regional Development Fund (ERDF). We thank the EPSRC National Crystallographic Service for recording one of the single-crystal diffraction data sets, Luke Daniels (Warwick) for measuring the thermogravimetric data and Franck Millange (Institut Laviosier Versailles) for useful discussions.
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