The impact of N , N ′-ditopic ligand length and geometry on the structures of zinc-based mixed-linker metal – organic frameworks †

Combining Zn(NO3)2·6H 2 O with a series of dicarboxylic acids in the presence of the N,N′-ditopic ligand di(4-pyridyl)-1H-pyrazole (Hdpp) results in a series of mixed-linker metal-organic frameworks (MOFs) that have been crystallographically characterised. The reaction with 1,4-benzenedicarboxylic acid (H2bdc) gives [Zn2bdc)2(Hdpp)2]·2DMF 1, which shows Zn2(μ-carboxylate)2(carboxylate)2 secondary building units (SBUs) linked by bdc ligands into sheets, and these are pillared by the Hdpp linkers into a doublyinterpenetrated three-dimensional network. The reaction with 1,4-naphthalene dicarboxylic acid (H2ndc-1,4) gives two products: [Zn2(1,4-ndc)2(Hdpp)]·4DMF 2a forms a three-dimensional network in which sheets, formed from Zn2(carboxylate)4 'paddle-wheel' SBUs being linked by 1,4-ndc, are connected together by Hdpp pillars, whereas [Zn(1,4-ndc)(Hdpp)]·DMF 2b forms a fourfold interpenetrated structure based on diamondoid networks with single zinc centres as nodes. The reaction with 1,3-benzenedicarboxylic acid (H2mbdc) produces [Zn(mbdc)(Hdpp)]·DMF 3, which forms a two-dimensional network with (4,4) topology in which ZnO2N2 nodes are interlinked by mbdc and Hdpp linkers. The reaction with 5-methyl-1,3-benzenedicarboxylic acid (H2mbdc-Me) also forms a two-dimensional network structure, [Zn2(mbdc-Me)2(Hdpp)2]·DMF 4, albeit wherein dicarboxylates bridge between zinc-dicarboxylate tapes, themselves formed by interlinking of Zn2(μ-carboxylate)2(carboxylate)2 SBUs similar to those in 1. Finally, the reaction with 2,6-naphthalene dicarboxylic acid (H2ndc-2,6) yields two crystalline species, both having the formula [Zn2(2,6-ndc)2(Hdpp)]·DMF 5a/5b and possessing infinite zinc-carboxylate chain motifs interlinked by both naphthalene rings and Hdpp linkers into a three-dimensional framework. In compounds 1, 2b, 3 and 4, the pyrazole NH groups are involved in hydrogen bonding that serves to link either interpenetrated networks or neighbouring sheets together. However, in 2a and 5a/5b the NH groups project into the pores of the framework enabling interactions with guest molecules. Disciplines Medicine and Health Sciences | Social and Behavioral Sciences Publication Details Burrows, A. D., Chan, S., Gee, W. J., Mahon, M. F., Richardson, C., Sebestyen, V. M., Turski, D. & Warren, M. R. (2017). The impact of N, N ′-ditopic ligand length and geometry on the structures of zinc-based mixedlinker metal-organic frameworks. CrystEngComm, 19 (37), 5549-5557. Authors Andrew D. Burrows, Siobhan Chan, William Gee, Mary F. Mahon, Christopher Richardson, Viorica Sebestyen, Domenyk Turski, and Mark Warren This journal article is available at Research Online: http://ro.uow.edu.au/smhpapers/4985


Introduction
Over recent years there has been a rapid growth of interest in metal-organic framework (MOF) materials. 1This has largely been driven by their potential for porosity, which affords ac-cess to a wide range of applications, including gas storage, 2 catalysis 3 and separations. 4urrently, there is intense interest in the introduction of chemical functionalities onto the linkers, as a means of tuning the properties of the resultant MOFs. 5 This can be achieved either by using functionalised linkers in the synthesis, 6 or by post-synthetic modification protocols. 7Both approaches take advantage of the fact that many MOFs form isoreticular series 8 -MOFs that have the same structural topology, but differ in the length and/or functionalities on the linkers.
We reasoned that Hdpp had the potential to act in a similar manner to other neutral N,N′-ditopic ligands in polycarboxylate-containing MOFs.We therefore sought to investigate the effect of the longer linker length with respect to bpy and dabco on the network structures formed with zinc dicarboxylates.In addition, we reasoned that the central fivemembered pyrazole ring in Hdpp would reduce the angle between the pyridyl nitrogen donors from 180°as observed in bpy and dabco to approximately 156°, and that this difference might also have structural consequences.Finally, it was anticipated that the presence of the hydrogen bond donor and acceptor on the pyrazole ring would provide the potential for the network to act as a selective host for guests containing complementary hydrogen bonding faces.

Experimental
General experimental details and the synthesis of Hdpp 20 are provided in the ESI.†

Synthesis of 1-5
[Zn 2 Ĳbdc) 2 ĲHdpp) 2 ]•2DMF 1. Hdpp (0.044 g, 0.20 mmol), ZnĲNO 3 ) 2 •6H 2 O (0.119 g, 0.40 mmol) and H 2 bdc (0.066 g, 0.40 mmol) were dissolved in 20 cm 3 of anhydrous DMF with gentle heating and stirring.The colourless solution was placed in a 40 cm 3 Ace pressure tube, sealed and heated at 120 °C for 2 days.The resultant colourless hexagonal crystals were separated by filtration and washed with fresh DMF.Yield 0.050 g (48%).The PXRD pattern of the sample matched that simulated from the X-ray single crystal structure (Fig. S1 † [Zn 2 Ĳ1,4-ndc) 2 ĲHdpp)]•4DMF 2a and [ZnĲ1,4-ndc)ĲHdpp)] •DMF 2b.Hdpp (0.022 g, 0.10 mmol), 1,4-naphthalene dicarboxylic acid (0.043 g, 0.20 mmol) and ZnĲNO 3 ) 2 •6H 2 O (0.027 g, 0.10 mmol) were dissolved in 6 cm 3 of anhydrous DMF with gentle stirring.The colourless solution was placed in a Biotage pressure vial, capped and heated at 130 °C for 2 days.The resultant colourless crystals were separated by filtration and washed with fresh DMF.Two different crystal structures (2a and 2b) were obtained from these crystals, confirming that the sample was not phase-pure.This was further evidenced by the PXRD pattern, which showed the dominant species to be 2a (Fig. S2 †).Crystals of compound 2b were observed to undergo a morphological change when removed from the reaction medium and exposed to air, as can be seen in Fig. S6.† [ZnĲmbdc)ĲHdpp)]•DMF 3. Hdpp (0.111 g, 0.50 mmol), ZnĲNO 3 ) 2 •6H 2 O (0.297 g, 1.00 mmol) and H 2 mbdc (0.166 g, 1.00 mmol) were dissolved in 10 cm 3 anhydrous DMF with gentle heating and stirring.The colourless solution was placed in a 20 cm 3 Ace pressure tube, sealed and heated at 120 °C for 2 days.The colourless hexagonal crystals were separated by filtration and washed with fresh DMF.Yield 0.20 g (76%).The PXRD pattern of the sample matched that simulated from the X-ray single crystal structure (Fig. S4 † [Zn 2 Ĳ2,6-ndc) 2 ĲHdpp)]•DMF 5a, b.Hdpp (0.022 g, 0.10 mmol), 2,6-naphthalene dicarboxylic acid (0.043 g, 0.20 mmol) and ZnĲNO 3 ) 2 •6H 2 O (0.027 g, 0.1 mmol) were dissolved in 6 cm 3 anhydrous DMF with gentle stirring.The colourless solution was placed in a vial, capped and heated at 130 °C for 2 days.The resultant block-like pale yellow crystals showed signs of degradation at low temperatures during the course of collecting single crystal X-ray data, whereas collection at room temperature only gave poor resolution X-ray data.However, when the solvent was removed from the reaction vial and the crystals were left at room temperature for several weeks, a change in morphology was observed, resulting in crystals with better defined edges.Single crystal X-ray diffraction analysis revealed that two types of crystal were present with different unit cell parameters and space groups (5a and 5b), though both were shown to have the same gross structure.PXRD studies showed a reasonable match between the bulk sample and that simulated from the crystal structure of 5b (Fig. S5 †).Anal.calcd.for C 44.5 H 41.5 N 6.5 O 11.5 Zn 2 (982.12),5•H 2 O•1.5DMF: C, 54.42; H, 4.26; N, 9.27%.Found: C, 54.61; H, 3.96; and N, 9.31%.

Crystallography
The zinc compounds prepared in this study were all structurally characterised using single-crystal X-ray diffraction techniques.While full data have been provided in the ESI, † key data are summarised below: Crystal data for C 96 H 84 N 20 O 20 Zn 4 (1): The structures were solved using SHELXS 21a and refined using full-matrix least squares in SHELXL 21b using the OLEX-2 interface.21c Details of the final refinements are provided in the ESI.† Unless noted therein, all non-hydrogen atoms were refined anisotropically in the final least squares cycles, and hydrogen atoms were included at calculated positions.The search for solvent accessible voids in the structures and their analysis was performed using the SQUEEZE subroutine of PLATON. 22

Results and discussion
In this study, the reaction of zincĲII) nitrate hexahydrate with Hdpp was undertaken in the presence of a variety of dicarboxylic acids in order to observe the influence of the coligand on the network structure adopted.The carboxylic acids used are shown in Fig. 1 alongside the structure of Hdpp.In the cases of 1,4-benzenedicarboxylic acid (H 2 bdc), 1,3benzenedicarboxylic acid (H 2 mbdc) and 5-methyl-1,3benzenedicarboxylic acid (H 2 mbdc-Me), a single product was obtained regardless of the stoichiometry employed.For the two isomers of naphthalene dicarboxylic acid, the reaction mixtures contained more than one phase, regardless of the reaction conditions or stoichiometry employed.For 1,4naphthalene dicarboxylic acid (H 2 ndc-1,4), two separate compounds were identified (2a and 2b), though both refinements suffered from poor data quality.For 2,6-naphthalene dicarboxylic acid (H 2 ndc-2,6), data sets were collected on two sets of crystals which had different unit cells.However, in this case, the two structures (5a and 5b) were shown to have similar gross structures, in contrast to observations with 2a and 2b.
Fig. 1 The structures of Hdpp and the diacids used in this study.
This journal is © The Royal Society of Chemistry 2017 The asymmetric unit of 1 consists of four zinc centres, four bdc ligands, four Hdpp ligands and four included DMF molecules.The compound adopts a structure based on Zn 2 Ĳcarboxylate) 4 building blocks, though not the common paddlewheel motifs in which all four carboxylates bridge between the zincĲII) centres.Instead, two carboxylate groups bridge the zinc centres and the remaining two each bind single zinc centres in an asymmetric chelating mode (d (Zn-O) = 1.968Ĳ3)-2.063Ĳ3)Å, and 2.486Ĳ3)-2.979Ĳ3)Å for four unique contacts each).These secondary building units (SBUs) are linked into sheets by the benzene rings of the dicarboxylates (Fig. 2a).The Hdpp ligands coordinate in both axial positions on each zinc centre, and link each dimer into chains (Fig. 2b).Overall, these interactions serve to connect the zincbdc sheets into three-dimensional networks, which are doubly-interpenetrated (Fig. 2c).All of the pyrazole NH groups are involved in N-H⋯O hydrogen bondingtwo form hydrogen bonds to those carboxylate oxygen atoms in the neighbouring framework that form the long coordination bonds [NĲ6)⋯OĲ16) 2.687(5), HĲ6)⋯OĲ16) 1.83 Å, NĲ6)-HĲ6)⋯OĲ16) 164°; NĲ14)⋯OĲ4) 2.752(5), HĲ14)⋯OĲ4) 1.94 Å, NĲ14)-HĲ14)⋯OĲ4) 153°] and these contacts link the interpenetrated frameworks together.The other two NH groups form hydrogen bonds with included DMF molecules [NĲ3)⋯OĲ19A) 2.66(4), HĲ3)⋯OĲ19A) 1.79 Å, NĲ3)-HĲ3)⋯OĲ19A) 171°; NĲ11)⋯OĲ17) 2.811(5), HĲ11A)⋯OĲ17) 1.96 Å, NĲ11)-HĲ11A)⋯OĲ17) 153°].It is notable that both the SBU in 1 and the chains formed by linking the Zn 2 Ĳcarboxylate) 4 dimers with the Hdpp ligands mirror similar structural features observed in [Zn 2 ĲOAc) 4 ĲHdpp) 2 ]•MeOH. 19he pair of Hdpp ligands bridging between pairs of SBUs are parallel to each other with the pyrazole carbon atoms 3.70 Å apart.While the Hdpp ligands are held together by coordination to the zinc centres, the relatively larger Zn⋯Zn separation of 3.95-3.97Å suggests there may be π⋯π interactions between these ligands.Similar interactions are present in the crystal structure of Hdpp•3H 2 O. 23 A study of the Cambridge Structural Database (CSD) reveals that 38 structures containing zinc, bdc (or a substituted version) and bpy (or a substituted version) have been previously reported.Of these, only two -[Zn 2 Ĳbdc) 2 Ĳbpy) 2 ] (LOTXOH) 24 and [Zn 2 {bdc-(OH) 2 } 2 Ĳbpy) 2 ] (SUPLOF) 25 form similar networks to 1 though, due to the shorter length of bpy with respect to Hdpp, these are non-interpenetrated.The most common structural type observed for zinc-bdc-bpy compounds has the general formula [Zn 2 Ĳdicarboxylate) 2 Ĳbpy)] and consists of Zn 2 Ĳcarboxylate) 4 paddlewheels that are linked into sheets by the benzene rings of the dicarboxylates, which are connected into three-dimensional networks by the bpy ligands.This is observed in 19 of the 38 structures, with the majority of these structures being interpenetrated.However, no evidence for a compound of the formula [Zn 2 Ĳbdc) 2 ĲHdpp)] was observed in this study.
The asymmetric unit of 2a consists of half of a zinc centre, half an 1,4-ndc ligand and one half of a Hdpp linker at 50% occupancy, as well as some diffuse solvent.Interpretation of the X-ray data was hampered by symmetry issues which negatively impacted on the residuals that accompany the refinement (see ESI †), but unambiguous assignment of the framework was achieved.
Each pair of zincĲII) centres is bridged by four 1,4-ndc carboxylate groups, giving rise to the ubiquitous paddlewheel motif.In addition, each zincĲII) centre is coordinated to a pyridyl group from a Hdpp ligand.The bridging 1,4-ndc ligands correspondingly yield a (4,4) square net with paddlewheel nodes.The naphthalene group of each 1,4-ndc ligand is disordered over two positions.On average, each (4,4) 'square' adopts one of three conformations with regards to the naphthalene groups; in 50% of instances the opposing pairs of naphthalene groups are oriented both above and below the plane defined by the SBUs within each sheet, in 25% of instances all of the naphthalene groups are oriented below this plane, and in the final 25% all are oriented above this plane (Fig. 3).
The bridging Hdpp ligands link the (4,4) nets formed from zincĲII) and 1,4-ndc, and the entire Hdpp ligand is disordered over two positions.Sterics play a dominant role in the structure of 2a, with the bent nature of Hdpp influencing the orientation of the naphthalene groups within the 1,4-ndc sheets (Fig. 4).This provides justification for the pattern of naphthalene group ordering shown in Fig. 3. Interpenetration in 2a is not observed, and it is likely that the steric bulk of the naphthalene moiety disfavours interpenetration within this type of framework.As a consequence of this, channels surround the Hdpp linkers.The poor quality of the diffraction data precluded assignment of the solvent, though this is modelled as one molecule of DMF per asymmetric unit on the basis of NMR analyses of the digested product.

[ZnĲ1,4-ndc)ĲHdpp)]•DMF 2b
The asymmetric unit of 2b consists of four zinc centres, four 1,4-ndc ligands and four Hdpp linkers, and some diffuse solvent modelled as one molecule of DMF per zincĲII) centre.The same issue regarding X-ray data quality applies in this case as for 2a, though for 2b the data were primarily compromised by a rapid decrease in intensity with increasing 2θ.In addition, evidence of a phase-transition from 2b to 2a was inferred for this material (see ESI †).As for 2a, these deficiencies have affected the refinement residuals, which are higher than desirable.However, as for 2a, the data do permit unambiguous assignment of the framework, and this dictates the limit of discussion herein.
Each zincĲII) centre is coordinated to two 1,4-ndc carboxylate groups and two Hdpp pyridyl groups.The metal centres therefore act as tetrahedral nodes, and the bridging ligands interlink them into three-dimensional diamondoid networks (Fig. 5a).The structure is fourfold-interpenetrated (Fig. 5b).
The relative proximity of pyrazole nitrogen atoms and carboxylate oxygen atoms suggests the presence of hydrogen bonding, which would interlink the networks in a pairwise manner.
1,4-Naphthalene dicarboxylate is a bulkier linker than 1,4benzenedicarboxylate and it is likely that steric interactions   between the aromatic rings prevent the formation of a structure analogous to that of 1.It is notable that none of the known zinc-bpy-dicarboxylate structures have a similar topology to 2b, regardless of the observed ratio of the components.

[ZnĲmbdc)ĲHdpp)]•DMF 3
The asymmetric unit of 3 consists of one zinc centre, one mbdc ligand, one Hdpp ligand and an included DMF molecule.The zincĲII) centre is coordinated to two mbdc carboxylate groups and two Hdpp pyridyl groups.The carboxylates are coordinated in an asymmetrical manner, with the longer contacts 2.497(2) Å and 2.813(2) Å, ensuring that the metal centres have distorted octahedral geometry but act as 4-connected nodes, bonding to two mbdc and two Hdpp ligands.These linking ligands connect the metal centres into two-dimensional networks with the (4,4) topology (Fig. 6).
The asymmetric unit of 4 consists of two zinc centres, two mbdc-Me ligands, two Hdpp ligands and two half-occupancy DMF molecules, only one of which lent itself to being modelled in the refinement of the structural model.Compound 4 contains similar Zn 2 Ĳcarboxylate) 4 SBUs to 1, with two carboxylate groups bridging between the zinc centres and a carboxylate group asymmetrically coordinated to each zinc centre.
The 120°angle present between the carboxylate groups in the mbdc-Me ligand leads to the SBUs being connected into chains (Fig. 7a) rather than the sheets observed for 1. Hdpp ligands coordinate in the axial positions of the zinc centres, and link the chains into sheets (Fig. 7b).
The topology observed in 4 is similar to that observed in the CID class of MOFs reported by Kitagawa and coworkers, 30 and this structural type accounts for 13 of the 49 crystal structures present for zinc-bpy compounds with 1,3-benzene dicarboxylates in the CSD.These MOFs are of particular note for their flexibility, which is facilitated by the The asymmetric unit of 5a consists of two zinc centres, one and two halves of 2,6-ndc ligands, one Hdpp ligand and a disordered DMF molecule.Each zinc centre is coordinated to three carboxylates and one Hdpp ligand.The full and partial occupancy 2,6-ndc ligands play very different structural roles.The two carboxylate groups in the linker containing OĲ1)-OĲ4) bridge between metal centres connecting them into zinc-carboxylate chains, which are linked into pairs by the naphthalene groups (Fig. 8a).π⋯π interactions are also present between the naphthalene rings, with a closest distance between atoms in neighbouring rings being 3.3 Å.The 2,6-ndc linkers containing OĲ5)-OĲ8) connect the pairs of chains into a three-dimensional network, which is further supported by the Hdpp ligands, which bridge zinc centres (Fig. 8b).The gross structure of 5b is very similar to that of 5a despite occupying a higher symmetry space group (P2 1 /c for 5b, P1 ¯for 5a).
The NH groups of the Hdpp ligands are not involved in hydrogen bond interactions to the carboxylate oxygen atoms, and project into the void space occupied by disordered DMF molecules.There is, however, stacking of the Hdpp ligands, with the closest distance between atoms being 3.4 Å.
There are eight previously reported structures containing zinc, 2,6-ndc and bpy (or a substituted analogue) instead of Hdpp.All adopt very different structures to 5a, with zinc-2,6ndc sheets linked into doubly-or triply-interpenetrated threedimensional networks by the bpy linkers. 32

Discussion
This research programme set out to evaluate how changing from bpy to Hdpp influences the nature of the zinc MOF products that contain both dicarboxylate and N,N′-ditopic ligands.Hdpp differs from bpy in three waysthe distance between the nitrogen donors, the angle between the nitrogen donors and the presence of hydrogen bonding groups in the pyrazole ring of the Hdpp ligand.
Insight into the importance of these factors can be acquired by comparing the structures of the new compounds 1-5 with those of their bpy analogues, as noted above, and compounds containing other N,N′-ditopic linkers.Structures involving 1,4-diĲ4-pyridyl)benzene (dpb) are particularly informative, given the similar length of the Hdpp and dpb ligands.Seven crystal structures containing zinc, bdc and either dpb or a substituted analogue have been reported.Five  of these form doubly-interpenetrated DMOF-1 type structures, whereas the other two form structures with a similar topology, but possess a different arrangement of the carboxylate groups around the zinc paddlewheel unit.These seven Znbdc-dpb structures all differ from those adopted by 1 and 2b, but are similar to that of 2a, demonstrating that the distortion from linearity of Hdpp does not prevent it from forming analogous structures to those with linear linkers.The zincbdc sheets present in 1 are less accessible to 1,4-ndc due to unfavourable steric interactions that would be present with the bulkier dicarboxylate.
Eight crystal structures containing zinc, mbdc and dpb or substituted analogues of these ligands have been reported.Two of these compounds are isoreticular with 4, while four of the others adopt three-dimensional networks based on zincdicarboxylate sheets that are topologically similar to those present in 1.
In four of the seven new structures reported in this paper, hydrogen bonding between the NH group in the pyrazole ring in the centre of the Hdpp ligand and a carboxylate oxygen serves to connect either interpenetrated three-dimensional networks or interdigitated two-dimensional networks together.The exceptions to this are 2a and 5a, b, for which the NH groups project into pores occupied by disordered solvent molecules.

Conclusions
We have shown that diĲ4-pyridyl)-1H-pyrazole (Hdpp) can act as a N,N′-ditopic co-linker in zinc dicarboxylate MOFs.X-ray structural analyses reveal that reaction of ZnĲNO In some cases, the products are isoreticular with those adopted by linear N,N′-ditopic ligands such as bpy and dpb, but the presence of the pyrazole group provides a means to interlink interpenetrated or interdigitated networks by hydrogen bonding.However, the structures of 2a and 5a, b reveals that this is not inevitable, as in these cases the hydrogen bonding groups project into the pores and are available to interact with guest molecules.Current research is focussed on the interactions of these frameworks with guest molecules and exploiting these interactions for selective separations.