Synthesis, crystal structure, and topology of a polycatenated bismuth coordination polymer.

Keywords: bismuth; coordination polymer (CP); crystal structure; solvothermal synthesis; topology; tricarboxylic acid

Dedicated to Professor Christian Näther on the occasion of his 60th birthday.

Coordination polymers (CPs) and metal-organic frameworks (MOFs) made from Bi3+ ions are rather underdeveloped compared to those made of other metal ions, despite their often unique structures and properties [[1]]. The use of Bi3+ ions in CPs has a number of appeals as bismuth is well known for its low toxicity and antimicrobial properties [[3]]. In recent years these materials have been receiving increasing focus for use in catalysis [[4]], [[5]], [[6]], [[7]], particularly in CO2 reduction, as well as for the capture of various environmental pollutants [[8]], [[9]], [[10]].

While many CPs and MOFs can be designed by reticular chemistry, the structures of those containing Bi3+ ions remain less predictable, which has been attributed to the inconsistent and in some cases flexible coordination geometry of Bi3+. Bi-CPs have been reported to form a wide range of dense and porous structures including rods and layers that either stack or interpenetrate, and 3D frameworks [[11]].

A number of CPs and MOFs are also known for their flexibility, with structures that respond to their environment [[13]]. In most of the iconic flexible MOFs, such as MIL-53 and MIL-88B [[15]], the coordination geometry of the metal cation is unaffected and flexibility arises instead from tilting of the linkers through changes in the torsion angles. However, in the case of the Bi-MOF SU-100 ([Bi(BPT)]·2CH3OH, H3BPT = biphenyl-3,4′,5-tricarboxylic acid), the inorganic building unit (IBU), composed of Bi2O12 clusters, is flexible and undergoes significant changes of the Bi coordination geometry when the material is placed in various solvents [[17]].

Much attention has been placed on Bi-CPs and –MOFs synthesized employing tricarboxylic acids such as benzene-1,3,5-tricaboxylic acid, trimellitic acid, 1,3,5-tris(4-carboxyphenyl)benzene, triazine-2,4,6-triyl-tribenzoic acid, and biphenyl-3,4′,5-tricarboxylic acid among others [[10], [12], [17]], [[18]], [[19]], [[20]], [[21]], all of which are fairly rigid with little possibility of altering their molecular geometries except for the rotation of the carboxylate groups. The structural investigations of the bismuth compounds have often proved challenging, owing to their tendency of forming microcrystalline materials – an aspect that has largely hindered structure determination using conventional methods, such as single-crystal X-ray diffraction. However, through the development of three-dimensional electron diffraction (3DED) [[22]] it has been possible to elucidate the atomic arrangement of microcrystalline phases, including bismuth coordination compounds such as the long-used active pharmaceutical ingredient bismuth subgallate [[24]].

In this investigation we have combined Bi3+ ions with a highly flexible tricarboxylic acid linker: 1,3,5-tris[4-(carboxyphenyl)oxamethyl]-2,4,6-trimethylbenzene (H3TBTC, Figure 1), which has previously been used to synthesize a number of CPs and MOFs of varying dimensionality, and resolved the crystal structure using 3DED [[25]], [[26]], [[27]].

Graph: Figure 1: Molecular structure of 1,3,5-tris[4-(carboxyphenyl)oxamethyl]-2,4,6-trimethylbenzene (H3TBTC).

All chemicals were purchased from Sigma–Aldrich, Chemsoon, or Walter CMP and were used without any further purification. All syntheses were carried out in sealed 5 mL borosilicate 3.3 glass tubes (Duran 12 × 100 mm, DWK Life Sciences) certified for temperatures up to 180 °C, equipped with a polybutylene terephthalate (PBT) cap containing a polytetrafluoroethylene (PTFE) seal. As the reactions were carried out under solvothermal conditions, care should be taken when reproducing any of the syntheses mentioned in this section, as high pressures are formed inside the reaction vessels, and any scaling of the reactions should come with a proportionate scaling of the employed vessels.

PXRD measurements were carried out using a Panalytical X'pert Pro diffractometer (Cu Kα1,2, λ1 = 1.540598 Å, λ2 = 1.544426 Å) using a Bragg–Brentano geometry. Three-dimensional electron diffraction data was collected using a JEOL JEM2100 TEM, equipped with a Timepix detector from Amsterdam Scientific Instruments, while continuously rotating the crystal at 0.45° s−1. Data collection was carried out using Instamatic [[28]]. Pawley refinements of the PXRD data were carried out using Topas -Academic V6 [[29]]. Scanning electron microscopy images were collected on a Hitachi TM3000 microscope. Topological analysis of the framework structure was carried out using the software packages ToposPro [[30]] and Systre [[31]]. Three-letter net codes throughout the manuscript are listed as seen in the Reticular Chemistry Structure Resource (RCSR) database [[32]]. Thermal analysis (thermogravimetric analysis and differential thermal analysis) was carried out using a Linseis STA PT 1000 instrument (heating rate = 4 K min−1, airflow = 6 L h−1). IR spectra were measured with a Bruker Alpha-P ATR FTIR spectrometer. Elemental analysis for the elements C, H, N and S was carried out using an Elementar vario MICRO cube. Gas/vapor sorption isotherms (CO2, H2O at 20 °C, N2 at −196 °C) were recorded on a Micromeretics ASAP2020 surface area and porosity analyzer. The as-synthesized Bi(TBTC) was activated at 100 °C under dynamic vacuum (10−4 Pa) for 6 h before the isotherms were recorded.

Bi(TBTC) was prepared from a 1:1 molar ratio of Bi3+:linker with 50 mg (0.09 mmol) H3TBTC, 43 mg (0.09 mmol) of Bi(NO3)3·5H2O in 3 mL of CH3OH. The starting materials and the solvent were transferred to a 5 mL glass vial, sealed and heated at 120 °C for 1 h using an Al heating block. The white microcrystalline reaction product was filtered off and rinsed with a small amount of CH3OH and dried in air. Yield: 49 mg (0.06 mmol, 67%), Elemental analysis, measured (calculated, assuming the composition Bi(TBTC)·CH3OH): C/at.% = 49.15 (50.50), H/at.% = 3.59 (3.86).

The crystal structure of Bi(TBTC) was determined from 3DED data, collected from the micrometer-sized plate-shaped crystals (Figures S1 and S2). The collected 3DED data was integrated and merged (from five individual crystals) using XDS [[33]]. Structure solution was performed using Shelxt [[34]], locating all non-hydrogen atoms of the structure, with a subsequent refinement using Shelxl [[35]]. From the subsequent difference Fourier map, some of the hydrogen positions could be elucidated, yet in the final structure all hydrogen positions were refined through the aid of a riding model. All refinement details as well as a graphic of the asymmetric unit with displacement ellipsoids can be found in the Supplementary Information (Table S1, Figure S3).

Solvothermal reaction of Bi(NO3)3·5H2O with H3TBTC in methanol at 120 °C results in the formation of the new Bi-CP with the composition Bi(TBTC), which is obtained as a microcrystalline powder. Hence conventional structure determination from single crystal X-ray diffraction data could not be carried out and structure solution from powder X-ray diffraction data did not result in a sensible structural model.

The crystal structure of Bi(TBTC) (Figure 2a) could be resolved from 3DED data (Figure S2) in space group $P1‾$ where the asymmetric unit contains one Bi3+ cation and one TBTC3− linker trianion (Figure S3).

Graph: Figure 2: Crystal structure of Bi(TBTC). a) Part of a single layer found in the Bi(TBTC) structure, showing the Bi2O12 clusters as blue polyhedra, which are joined together by the TBTC3− ions and b) the same section deconstructed into a kgd net, where the TBTC3− ions are represented as 3-c nodes and the Bi2O12 IBUs are simplified as 6-c nodes.

Phase purity could be confirmed through Pawley fits (Figures S4 and S5, Tables S2 and S3) as well as thermal and elemental analysis (Figure S6 and experimental section). The full deprotonation of the TBTC3− ion indicated by the 3DED refinement was validated by IR measurements (Figure S7), showing the absence of a strong band at about 1700 cm−1, which would be expected for the presence of a carboxylic acid group. The Bi–O distances in the structure range from 2.3 to 2.9 Å, which is in good agreement with distances of known bismuth carboxylates. Each bismuth cation is coordinated by seven oxygen atoms, and is a part of a Bi2O12 cluster, which forms the IBU. Each dinuclear unit is thus bonded to six TBTC3− ions, forming layers which have an alternating arrangement of six- and three-coordinated (6-c and 3-c) building blocks (Figure 2). As such, the structure can be deconstructed into a kgd net, i.e., the dual of the regular Kagome net, kgm. The dual of any net can be acquired by placing a vertex in the middle of each face and joining the resulting vertices to those of adjacent faces [[36]].

However, in the crystal structure of Bi(TBTC), these kgd layers are distorted and catenated, forming a material of higher periodicity. Formally, a three-periodic structure is formed through the catenation of the two-periodic kgd nets, as has been described for other nets in a number of CPs and MOFs [[37]]. Following a similar terminology, the polycatenation of Bi(TBTC) can be described as an alternating AB-stacking of the distorted kgd nets normal to the layers of the Bi(TBTC) structure (Figure 3a and b, green-blue-red layers), where the layers are shifted approximately along [110] (i.e., along the viewing direction of Figure 3c and d). A schematic view of the polycatenation is depicted in Figure 4, and an extended illustration of the AB-stacking in Figure S8. Using a similar cluster-based approach of simplifying the structure, a handful of examples of CPs with a kgd net have been reported, yet none, to the best of our knowledge, are polycatenated. One particularly relevant example, due to the similarity of the linker (in that case an analogous thioether variant bearing hydrogens on the central ring), is a layered Cu-based CP which also adheres to the kgd net [[39]].

Graph: Figure 3: Stacking and intercalation of the layers in Bi(TBTC). a) The structure of Bi(TBTC) in a polyhedral representation, b) the same view as in a) but using a space-filling representation, c) the structure of Bi(TBTC) as viewed along the ab-diagonal, with each layer colored differently (green, blue, and red), and d) the same view of Bi(TBTC) as in c) but using a space-filling representation.

Graph: Figure 4: A schematic presentation of three catenated kgd nets, shown a) slightly offset from the stacking direction and b) from the side.

When exposing as-synthesized Bi(TBTC) to higher temperatures (120 °C in air), an apparent shrinking of the unit cell occurs (1671 → 1522 Å3) as evidenced by changes to the PXRD patterns (Figure 5), corresponding to a decrease of the unit cell volume of 9% (Figures S4 and S5). This likely occurs as a relative shifting of adjacent layers upon the removal of methanol in the intra-layer void space, but could also arise from the flexibility of the IBU, which has been observed in another bismuth tricarboxylate material containing the same Bi2O12 units [[17]].

Graph: Figure 5: A comparison of PXRD patterns of as-synthesized Bi(TBTC), as well as Bi(TBTC) which has been heated at 120 °C in air for 1 h (and subsequently allowed to cool down under ambient conditions), showing large changes in the pattern observed (Cu Kα, λ1 = 1.540598 Å, λ2 = 1.544426 Å).

The potential porosity of the material was probed by gas and vapor sorption measurements (Figures S9–S11). Bi(TBTC) showed little porosity towards N2 at −196 °C (Figure S9) with an estimated Brunauer–Emmett–Teller (BET) specific surface area of ∼12 m2 g−1 and an estimated pore volume of 0.025 cm3 g−1 (at p/p0 = 0.99), suggesting that any possible pores on Bi(TBTC) are likely to be too narrow for N2 to diffuse through. Two other molecules, CO2 and H2O, were used in an attempt to probe the porosity of Bi(TBTC) at higher temperatures. CO2 and H2O were chosen based on their smaller kinetic diameters (0.33 nm and 0.28 nm, respectively) as compared to N2 (0.36 nm). The CO2 adsorption isotherm shown in Figure S10 has a linear shape with a relatively low uptake of ∼0.3 mmol g−1 (1 bar, 20 °C), although the presence of a hysteresis suggested that some CO2 may possibly be trapped in very narrow pores during the absorption/desorption process. The water sorption/desorption isotherms of Bi(TBTC) showed that the material has some porosity to water molecules. The S-shaped isotherm (Figure S11) suggests that Bi(TBTC) is not hydrophilic and does not have very strong interaction with water. The absence of any noticeable hysteresis during desorption also suggested that water molecules could diffuse into and out of the pores freely. The total pore volume estimated using the water adsorption isotherm was ∼0.073 cm3 g−1 (at p/p0 = 0.94). Thus the gas/vapor sorption experiments suggest that very narrow pores do exist in Bi(TBTC) to a limited extent. Comparing this to the crystallographic model, the porosity likely originates from small voids between the Bi2O12 units of neighboring layers, seen as the space between blue and green polyhedra in Figure 3a. This was also indicated from the thermal analysis measurements, showing a mass loss corresponding to one methanol molecule per formula unit, Bi(TBTC), when heating the material up to temperatures between 100 and 300 °C. Further heating beyond ∼300 °C results in a rapid mass loss, likely due to the decomposition of TBTC linkers and a resulting disintegration of the framework. As such, the composition of the as-synthesized material, Bi(TBTC)·CH3OH, is likely attributed to methanol molecules occupying void spaces in the material, having roughly 50 Å3 of accessible pore space per formula unit in the 3DED model (as determined using the Calc Solv function of P laton) [[40]], which are removed from the inter-layer void space upon heating, permitting a shift in the layer arrangement that creates a denser form of the framework.

In summary, a polycatenated bismuth CP, Bi(TBTC), has been synthesized and fully characterized. The layers found in the material consists of dinuclear Bi2O12 IBUs joined through TBTC ligands, forming a kgd type net. Polycatenation of the layers results in a three-periodic material through an AB-type stacking, which upon heating shows some degree of structural flexibility, evidenced by a 9% decrease in unit cell volume. The origin of the flexibility could be two-fold, arising from an inter-layer shift as well as a change in the coordination geometry of the Bi2O12 units found within the structure. While the material is not porous to N2, it shows some uptake of CO2 and water vapor.

Difference plots for the Pawley fits, scanning electron microscopy images, thermal analysis results, 3DED tables, and sorption isotherms are given as supplementary information available online (link).

CCDC 2127767 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data%5frequest/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; fax: + 44 1223 336033.