Mechanochemistry unlocking stoichiometric control in alkali metal carboxylate coordination polymers

Abstract

Alkali metal-based coordination polymers represent a promising alternative to transition metal systems, yet their development remains limited due to the inherent challenges in controlling and predicting the assembly of structures based on alkali cations. In this report we demonstrate a comprehensive study on the preparation of Na, K and Rb 1,3,5-benzenetricarboxylates using both self-assembly in an aqueous solution and the solid-state mechanochemical methods, revealing a remarkable degree of structural diversity in these seemingly simple systems. Notably, the developed mechanochemical procedures enabled excellent control over the metal-to-linker stoichiometry, as demonstrated by the selective formation of two M₂HBTC-type phases (M = K, Rb; BTC = 1,3,5-benzenetricarboxylate), which were not readily accessible by solution-based synthesis. These results demonstrate the unique advantages of mechanochemistry in enabling stoichiometric control and accessing coordination polymers that remain out of reach for conventional solution-based methods.

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Introduction

The growing awareness of undesired environmental impacts of chemical processes calls for the development of efficient transformations that would offer reduction of cumbersome steps and the overall use of solvents. This challenge has spurred the renaissance of mechanochemistry in which chemical transformations are effected by mechanical force in the absence of solvents.^1–4 In this regard, mechanochemistry has emerged as a robust and increasingly adopted synthetic approach to organic and inorganic molecular products.^5,6 It is also widely used for synthesis of diverse functional materials, such as metal oxides,⁷ metal halide perovskites^8,9 and others, owing to its operational simplicity, reduced reliance on bulk solvents, and ability to access reaction pathways unavailable under conventional conditions. Mechanochemical methods have proven particularly effective in the synthesis of coordination polymers,^10–12 enabling the formation of several archetypal MOFs,^13–19 including drug-loaded MOFs,^20,21 as well as previously unreported MOF polymorphs and metastable structures that are not accessible through solution-based methods.^22–24 Another advantage of mechanochemistry is the potential for precise control over reaction stoichiometry, which can be especially valuable in multicomponent systems (e.g. cocrystals, coordination polymers) where solution-based approaches often lead to non-stoichiometric mixtures, kinetic by-products, or phase segregation.²⁵ For example, mixed-metal MOF-74 materials incorporating bimetallic nodes (selected from: Zn, Mg, Co, Ni, Cu, Ca) in controlled 1 : 1 ratio have been synthesised by mechanochemical route, which were not accessible using solution-based methods.²⁶ Furthermore, in some cases mechanochemical synthesis has enabled control over the metal-to-linker ratio in coordination polymers, as shown for reaction systems involving Zn/Cd with cyanoguanidine²⁷ or Ag with ethylenethiourea,²⁸ yielding coordination polymers that are challenging to prepare via solution-based methods. However, such examples remain limited, highlighting the unique yet still underexplored potential of mechanochemistry for precise stoichiometric control in coordination polymer synthesis.

Coordination polymer chemistry, recognized by the 2025 Nobel Prize, has traditionally focused on frameworks built around transition metals, whose directional bonding preferences, variable oxidation states, and ability to form well-defined coordination geometries and discrete clusters facilitate the design of robust extended frameworks. Compared to the transition metal centres, the interactions of alkali metal cations with carboxylate ligands are more ionic, making the structure less rigid and harder to predesign. However, their high natural abundance and low toxicity make alkali metals appealing building units for functional coordination networks, particularly for large-scale and biomedical applications.^29–32 The most prominent example of alkali metal-based MOFs is the family of CD-MOFs, originally developed by Stoddart et al.³³ using cyclodextrins as organic linkers, with promising application potential in various fields.³⁴ Furthermore, carboxylate-based coordination polymers using alkali metals as nodes have been applied in drug delivery systems, gas mixture separation, chemical sensors, and electrochemical devices.^29–32

Moreover, alkali metal-based MOFs serve as excellent templates for porous carbons, as the homogeneous distribution of metal ions ensures uniform dispersion within the carbon matrix upon pyrolysis, promoting the formation of highly porous materials with promising applications in energy storage and catalysis.^35,36 Overall, the inherent properties of the alkali metal-based coordination networks make them a very interesting alternative to classical transition metal-based structures, but their design and efficient application remains a considerable challenge.^29–32 Similarly to the transition metal-based counterparts, the alkali metal-based coordination polymers are usually prepared using solvothermal, hydrothermal or diffusion-based strategies.^29–31 To the best of our knowledge, only two recent reports demonstrated that the mechanochemical approach utilizing simple alkali hydroxides as metal sources can be a very efficient strategy for preparation of alkali metal-based coordination networks.^37,38

Herein, in the course of our ongoing efforts towards development of wet and mechanochemically driven solid-state synthetic methods of 1D,³⁹ 2D,^40,41 3D^19,20 coordination polymers and alkali–metal-based systems in particular,³⁵ we present a systematic study on the synthesis of alkali metal 1,3,5-benzenetricarboxylates (BTC) involving Na⁺, K⁺, and Rb⁺, employing both low-temperature self-assembly in aqueous solutions and mechanochemical methods. While alkali metal 1,3,5-benzenetricarboxylates have been the focus of earlier, investigations,^42–48 our results demonstrate that their chemistry still holds considerable untapped potential, with many aspects of their formation and structural variability remaining to be elucidated. As a result of our study, a series of four new 2D and 3D coordination polymers were prepared and characterized revealing a striking degree of structural diversity among these deceptively simple systems. Moreover, two additional coordination polymers, previously reported only from solution-based synthesis,were successfully obtained under mechanochemical conditions.³⁵ Notably, the ability to control the metal-to-linker stoichiometry played a critical role in directing product formation, particularly under mechanochemical conditions (Fig. 1). In two cases, the desired coordination networks could be obtained in high yield only via mechanochemical synthesis, as repeated attempts to isolate these phases from solution were unsuccessful or led to impure or low-yield products. These findings underscore the unique capacity of mechanochemistry not only to streamline synthesis under mild and scalable conditions, but also to access stoichiometrically defined coordination polymers that are otherwise inaccessible through conventional solution-based methods.

Fig. 1 Conceptual representation of the stoichiometry control in formation of alkali metal-based coordination polymers presented in this report.

Experimental section

Materials and methods

All chemicals were purchased from commercial sources and used without further purification. Spectroscopic grade organic solvents and deionized water were used for the syntheses. Mechanochemical reactions were conducted in 10 ml stainless steel reactors equipped with stainless steel balls (10 mm diameter, one ball per reactor). The Retsch MM400 shaker mill was used to conduct the mechanochemical reactions (30 Hz, 20 min). Powder X-Ray Diffraction (PXRD) data were collected on the Empyrean diffractometer (PANalytical) employed with Ni-filtered Cu Kα radiation (40 kV, 40 mA) using Bragg–Brentano geometry and Si zero-background holder. Elemental analyses were carried out with an UNICUBE Elementar Analyser (Elementar GmbH). Scanning Electron Microscopy (SEM) imaging was performed using FEI Nova NanoSEM 450 system equipped with a field emission electron gun operating at 2 kV. Thermogravimetric analysis (TGA) was carried out with the TA Instruments Q600 analyser (25–1000 °C temperature range) in argon flow (100 mL min⁻¹) in open alumina crucibles.

General procedure for preparation of aqueous M₃BTC solutions (M = Na, K, Rb). Respective MOH (7.14 mmol) was dissolved in 10 ml of water. Next H₃BTC was added (500 mg, 2.38 mmol). The mixture was heated to 50 °C and shaken until the observed pH (tested using standard indicator paper) was close to neutral. Afterwards, the mixture was cooled to the room temperature and filtered.

Synthesis of Na₃BTC·3H₂O. Solution method has been reported previously.³⁵Mechanochemical method: NaOH (57 mg, 1.43 mmol) and H₃BTC (100 mg, 0.48 mmol) and DMF (20 µl) were ball milled in a stainless steel jar (20 min, 30 Hz). The resulting Na₃BTC·3H₂O powder was washed with acetone and dried. Yield: 149 mg (95%). Elemental analysis (%) found: C 32.80, H 2.79; cacld for the formula [C₉H₃O₆Na₃·3H₂O]: C 32.74, H 2.75.

Synthesis of Na₂HBTC·3H₂O. Solution method: Solutions of NaOH (37 mg, 0.96 mmol) 1 ml of water and H₃BTC (100 mg, 0.48 mmol) in 2 ml of EtOH were mixed, which resulted in an immediate precipitation of Na₂HBTC·3H₂O as a white solid. The product was separated by centrifugation, washed with acetone and dried. Yield: 132 mg (89%). Elemental analysis (%) found: C 35.01, H 3.31; cacld for the formula [C₉H₄O₆Na₂·3H₂O]: C 35.08, H 3.27. Mechanochemical method: NaOH (38 mg, 0.96 mmol), H₃BTC (100 mg, 0.48 mmol) and H₂O (8 µl, 0.5 mmol) were ball milled in a stainless steel jar (20 min, 30 Hz) yielding Na₂HBTC·3H₂O as white powder. Yield: 137 mg (93%). Elemental analysis (%) found: C 35.12, H 3.23; cacld for the formula [C₉H₄O₆Na₂·3H₂O]: C 35.08, H 3.27.

Synthesis of K₃BTC·3H₂O. Solution method has been reported previously.³⁵Mechanochemical method: KOH (80 mg, 1.43 mmol) and H₃BTC (100 mg, 0.48 mmol) were ball milled in a stainless steel jar (20 min, 30 Hz). The resulting mixture was put in a closed glass contained and aged for 30 days resulting in formation of K₃BTC·3H₂O as a white powder. Yield: 173 mg (96%). Elemental analysis (%) found: C 28.48, H 2.45; cacld for the formula [C₉H₃O₆K₃·3H₂O]: C 28.56, H 2.40.

Synthesis of K₂HBTC·H₂O. KOH (54 mg, 0.96 mmol) and H₃BTC (100 mg, 0.48 mmol) and DMF (20 µl) were ball milled in a stainless steel jar (20 min, 30 Hz). The resulting K₂HBTC·H₂O powder was washed with acetone and dried. Yield: 134 mg (92%). Elemental analysis (%) found: C 35.48, H 2.01; cacld for the formula [C₉H₄O₆K₂·H₂O]: C 35.51, H 1.99.

Synthesis of Rb₃BTC·5H₂O. Solution method: To the aqueous Rb₃BTC solution (2 ml, 0.24 M) 10 ml of iPrOH was added, which resulted in immediate precipitation of Rb₃BTC·5H₂O as white solid. The product was separated by centrifugation, washed with acetone and dried. Yield: 236 mg (89%). Elemental analysis (%) found: C 19.49, H 2.41; cacld for the formula [C₉H₃O₆Rb₃·5H₂O]: C 19.52, H 2.37. Mechanochemical method: RbOH (147 mg, 1.44 mmol), H₃BTC (100 mg, 0.48 mmol) and EtOH (20 µl) were ball milled in a stainless steel jar (20 min, 30 Hz). The resulting Rb₃BTC·5H₂O powder was washed with acetone and dried. Yield: 255 mg (96%). Elemental analysis (%) found: C 19.50, H 2.34; cacld for the formula [C₉H₃O₆Rb₃·5H₂O]: C 19.52, H 2.37.

Synthesis of Rb₂HBTC·2H₂O. RbOH (98 mg, 0.96 mmol) and H₃BTC (100 mg, 0.48 mmol) and DMF (20 µl) were ball milled in a stainless steel jar (20 min, 30 Hz). The resulting Rb₂HBTC·2H₂O powder was washed with acetone and dried. Yield: 187 mg (94%). Elemental analysis (%) found: C 26.09 H 1.93; cacld for the formula [C₉H₄O₆Rb₂·2H₂O]: C 26.04, H 1.95.

X-ray structure determination of alkali metal-based coordination compounds. The single crystals of, Na₂HBTC·3H₂O, K₂HBTC·H₂O, Rb₃BTC·5H₂O and Rb₂HBTC·2H₂O were selected under Paratone-N oil, mounted on the nylon loops and positioned in the cold stream on the diffractometer. The SCXRD data were processed with CrysAlisPro, Data Collection and Processing Software for Agilent X-ray Diffractometers.⁴⁹ The structures Na₂HBTC·3H₂O, K₂HBTC·H₂O, Rb₃BTC·5H₂O and Rb₂HBTC·2H₂O were solved by direct methods using the SHELXT program and were refined by full matrix least-squares on F² using the program SHELXL.⁵⁰ implemented in the OLEX2⁵¹ or WinGX⁵² suite. All non-hydrogen atoms were refined with anisotropic displacement parameters. H atoms on C atoms were added to the structure model at geometrically idealized coordinates and refined as riding atoms. H atoms of H₂O molecules were located from a Fourier map (Tables S1–S7). Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as a supplementary publication. CCDC 2359250 (Na₂HBTC·3H₂O), 2359252 (K₂HBTC·H₂O), 2450580 (Rb₃BTC·5H₂O) and 2359487 (Rb₂HBTC·2H₂O).

Results and discussion

Following our previous report on the use of alkali 1,3,5-benzenetricarboxylates as precursors to porous materials with promising applications in energy storage,³⁵ we wondered if similar alkali metal-based coordination networks could be prepared using different metal : linker ratio via solution or mechanochemical approach.

Sodium-BTC networks

Initially, we investigated the mechanochemical preparation of the Na-BTC reaction system in order to verify the accessibility of the previously reported Na₃BTC·3H₂O³⁵ phase. In this regard, a series of mechanochemical reactions involving NaOH and H₃BTC (3 : 1 molar ratio) were conducted under various conditions (neat grinding or liquid-assisted grinding (LAG) using EtOH or DMF, see Table S5 and Fig. S2). While all of those attempts allowed for preparation of the desired Na₃BTC·3H₂O phase, the DMF LAG procedure was most favourable due to increased phase purity and crystallinity of the product as evidenced by the PXRD data. Encouraged by the successful preparation of Na₃BTC·3H₂O, we explored further mechanochemical transformations aimed at preparation of product with different stoichiometry by grinding NaOH with H₃BTC in 2 : 1 ratio, which resulted in the preparation of a new 2 : 1 coordination network Na₂HBTC·3H₂O (Fig. 2) as a white microcrystalline powder. The high-quality single crystals of Na₂HBTC·3H₂O could be grown from concentrated aqueous solution of Na₃BTC at room temperature, which highlights the role of acid–base equilibria in directing product composition within the studied reaction system. Furthermore, we found that Na₂HBTC·3H₂O phase could also be directly precipitated by mixing aqueous solution of NaOH and EtOH solution of H3BTC (2 : 1 molar ratio, Fig. S1d, see the Experimental part and SI for more details). Notably, this compound can also be readily converted into the Na₃BTC · 3H₂O phase by simple grinding with one equivalent of NaOH, as evidenced by PXRD (Fig. S2e).

Fig. 2 Crystal structure of Na₂HBTC·3H₂O: 2D polymeric SBU (a and b), 3D coordination network (c). Na = yellow, O = red, C = grey, H atoms have been omitted for clarity. Single crystals of Na₂HBTC·3H₂O were prepared from concentrated aqueous Na₃BTC solution at RT.

The SCXRD investigations of Na₂HBTC·3H₂O showed that it forms a 3D supramolecular structure and crystallizes in P2₁2₁2₁ space group (Table S1). All of the metal cations in Na₂HBTC·3H₂O are coordinated by 5 or 6 oxygen atoms from organic linker or solvent molecules. The SBUs structures in Na₂HBTC·3H₂O, form extended 2D layered structures perpendicular to the crystallographic a axis (Fig. 2a and b). Finally, the layered SBUs are linked by the BTC molecules, forming extended 3D structure (Fig. 2c).

Potassium-BTC networks

Having demonstrated the formation of Na₃BTC·3H₂O and Na₂HBTC·3H₂O in both solution and solid state we followed the course of investigation towards products involving heavier cations: K and Rb. As demonstrated in our previous report, the K₃BTC solution prepared by reaction of KOH with H₃BTC (3 : 1 eq.) could be used for precipitation of K₃BTC·3H₂O phase by introduction of organic solvent.³⁵ However, in the course of our investigations we found that small amount of another product: K₂HBTC·H₂O can also be crystallised upon exposition of the aqueous K₃BTC solution to MeOH vapours. Despite considerable efforts including room temperature and hydrothermal reactions conducted in various stoichiometries, no high-yielding procedure of K₂HBTC·H₂O preparation from solution was established. Instead, the K₃BTC·3H₂O phase is preferably formed in most synthetic attempts. Nevertheless, efficient preparation of K₂HBTC·H₂O was possible by application of the mechanochemical approach (see the Mechanochemistry study below).

The structural investigation of the K₂HBTC·H₂O using SCXRD showed that it forms a 3D coordination network crystallised in P2₁/c space group (Table S2). All of the potassium cations in K₂HBTC·H₂O are coordinated by 6 oxygen atoms. The metal centres in K₂HBTC·H₂O are linked by the carboxylate groups forming 1D SBU chains extending in the direction of the a crystallographic axis (Fig. 3a and b). The BTC linkers interconnect the 1D SBUs in K₂HBTC·H₂O into the 3D coordination network (Fig. 3c).

Fig. 3 Crystal structure of K₂HBTC·H₂O: 1D polymeric SBU (a and b), 3D coordination network (c). K = green, O = red, C = grey, H atoms have been omitted for clarity. Single crystals of K₂HBTC·H₂O (mixed with single crystals of K₃BTC·3H₂O) were prepared by exposition of aqueous K₃BTC solution to MeOH vapours at RT.

In order to investigate the possibility of K₃BTC·3H₂O and K₂HBTC·H₂O preparation in the solid state, we have conducted a mechanochemical screening involving reactions in various stoichiometries and conditions (for details see Table S5 and Fig. S3, S4). Consequently, we have established that the K₂HBTC·H₂O phase can be prepared via a DMF LAG process involving KOH and H₃BTC in 2 : 1 molar ratio. In the case of K₃BTC·3H₂O, we found that while it can be prepared directly by mechanochemical processes (neat grinding or LAG), the products were not phase-pure (Fig S4a–c). To overcome this limitation, we studied the post-synthetic treatment of the mechanochemical products of KOH and H₃BTC reaction (the 3 : 1 molar ratio), leading to the conclusion that pure K₃BTC·3H₂O phase can be prepared either by simple aging (30 days, Fig. S4d) or DMF soaking (2 days, Fig. S4e). Interestingly, the aging-related recrystallisation can be directly observed by Scanning Electron Microscopy (SEM) displaying clear changes in crystal size an morphology (Fig. S17 and S18). Furthermore, we found that the K₂HBTC·H₂O phase can be easily transformed to K₃BTC·3H₂O in the solid state by grinding with addition of KOH and H₂O (the 1 : 1 : 1 molar ratio, Fig. S4g).

Rubidium-BTC networks

Extending the scope of our investigations towards rubidium we observed significant similarities with regard to the potassium-based systems discussed above as well as in the previous report.³⁵ Initially the reaction involving RbOH and H₃BTC (3 : 1 molar ratio) in water was performed, yielding Rb₃BTC solution. After addition of iPrOH to the Rb₃BTC solution, Rb₃BTC·5H₂O immediately formed as white precipitate, which could also be prepared as high quality single crystals by exposing the aqueous Rb₃BTC solution to the iPrOH vapours, as confirmed using PXRD (Fig. S6) and SCXRD (Table S3) analyses. Moreover, we found that upon exposure of the aqueous Rb₃BTC solution to EtOH vapours small amount of single crystals of Rb₂HBTC·3H₂O can be obtained. Despite several attempts, we did not succeed in establishing a procedure for selective preparation of Rb₂HBTC·3H₂O from solution in satisfactory yield, but it could be easily prepared via mechanochemical strategy, similarly to the potassium-based network K₂HBTC·H₂O (see the discussion of the Mechanochemical experiments below).

The SCXRD of the Rb₃BTC·5H₂O single crystals revealed that it forms a closely packed 3D coordination network in P2₁ space group (Table S3). The coordination geometry around the Rb⁺ cations involves 7 and 8 oxygen atoms, but due to the ionic character of the Rb–O interactions the bond lengths are diverse, making the geometry highly irregular and often distorted from ideal polyhedral shapes. The Rb centres in Rb₃BTC·5H₂O are linked by carboxylate groups into formally defined 2D SBUs, connected by BTC linkers into 3D network (Fig. 4). Finally, the SCXRD analysis of the Rb₂HBTC·3H₂O (space group P1̄, Table S4) showed that all of the Rb centres are coordinated to 7 or 8 oxygen atoms. The SBU structure of Rb₂HBTC·3H₂O was similar to the material K₂HBTC·H₂O discussed above (Fig. 3), with 1D chain-type assembly parallel to the a crystallographic axis (Fig. 5a and b). However, the 1D SBUs in Rb₂HBTC·3H₂O are connected by the BTC ligands into 2D layers perpendicular to the c axis (Fig. 5c) forming a stacked supramolecular structure.

Fig. 4 Crystal structure of Rb₃BTC·5H₂O. Rb = purple, O = red, C = grey, H atoms have been omitted for clarity. Single crystals of Rb₃BTC·5H₂O were prepared by exposition of aqueous Rb₃BTC solution to iPrOH vapours at RT.

Fig. 5 Crystal structure of Rb₂HBTC·3H₂O: 1D polymeric SBU (a and b), 2D coordination network (c). Rb = purple, O = red, C = grey, H atoms have been omitted for clarity. Single crystals of Rb₂HBTC·3H₂O were prepared by exposition of aqueous Rb₃BTC solution to EtOH vapours at RT.

The accessibility of the developed Rb-based coordination networks using mechanochemical strategy was also investigated as an integral part of the study. To this end a series of mechanochemical reactions have been tested involving 2 : 1 and 3 : 1 stoichiometries and various reaction conditions (Table S5). As a result we found that both the Rb₂HBTC·3H₂O and Rb₃BTC·5H₂O phases can be prepared via DMF LAG (optimal for Rb₂HBTC·3H₂O), EtOH LAG (optimal for Rb₃BTC·5H₂O) or neat grinding (Fig. S5 and S6). Furthermore, similarly to the Na- and K-bases systems we have observed that the mechanochemical transformation from Rb₂HBTC·3H₂O to Rb₃BTC·5H₂O was easily achieved by milling the former with addition of RbOH and H₂O (1 : 1 : 1 molar ratio).

We would like to emphasise that the successful solid state preparation of K₂HBTC·H₂O and Rb₂HBTC·3H₂O demonstrates a valuable advantage of the mechanochemical approach over the solution-base synthesis. The distinct reactivity observed among the alkali metal systems likely reflects the interplay between ionic size, solvation, and reaction environment. The selective formation of the M₂HBTC phases (M = K⁺, Rb⁺) under mechanochemical conditions may arise from the larger, less strongly solvated cations having a reduced tendency to stabilize the partially deprotonated HBTC²⁻ species in solution. In contrast, Na⁺, with its smaller ionic radius and higher hydration enthalpy, appears able to promote comparable deprotonation equilibria in aqueous media, thus allowing Na₂HBTC to form both in solution and in the solid state. Under solvent-free mechanochemical conditions, the absence of solvation equilibria and the direct acid–base interactions between H₃BTC and MOH constitute a fundamental difference in reaction environment; therefore, the formation of distinct phases compared to solution-based synthesis can be anticipated. While the direct reasons for the selective formation of the M₂HBTC phases under mechanochemical conditions are difficult to pinpoint, it may be speculated that mechanical treatment alters nucleation and crystal growth pathways by continuously renewing reactive surfaces, creating defects, and generating transient local temperature and pressure gradients.⁵³ These effects, combined with the absence of solvation equilibria and the non-equilibrium nature of the mechanochemical environment, can modify both the kinetic and thermodynamic profiles of the reaction.⁵⁴ As a result, mechanochemical synthesis may favour metastable intermediates or alternative phases that remain inaccessible under solution-based conditions.⁵⁵

While the mechanochemical approach has been demonstrated to provide unmatched stoichiometric precision, it can also yield products of lower crystallinity or inferior properties. In order to examine this possibility, we have decided to investigate the thermal properties (using thermogravimetric analysis (TGA)), and crystal size and morphology (using SEM) of all of the materials discussed here. The TGA data (Fig. S7–S12) clearly show that the M₃BTC·xH₂O materials prepared by the solid state and solution methods exhibit very close thermal decomposition profiles with the only essential difference being the small weight loss (∼2%) observed at around 380 °C for samples prepared in solid state. This observation might suggest minor contamination of the mechanochemically-prepared M₃BTC·xH₂O samples with the M₂HBTC·xH₂O phases, as the latter all displayed significant weight loss steps at around 380–400 °C (Fig. S7–S12). Notably, the decomposition steps observed around 100 °C for all of the samples matched the expected weight loss related to the transformation from the hydrated to dehydrated phase, which additionally confirms the composition of the investigated samples. Finally, we have employed SEM imaging technique in order to directly compare the crystal size and morphology of samples prepared using different methods. In general, the samples prepared via solution crystallisation consisted of well-defined crystals with average size from ca. 3–4 µm (for Na₃BTC·3H₂O and K₃BTC·3H₂O, Fig. S13 and S16) to ca. 30–40 µm (for Rb₃BTC·5H₂O, Fig. S20). Unsurprisingly, the mechanochemically-prepared samples displayed much smaller crystal size, mostly below 0.5 µm, but precise measurements were hindered by the irregular size of the crystallites (Fig. S14, S15, S17, S19, S21, S22). Nevertheless, we found that the limitations of the mechanochemical procedures can be mitigated by the post synthetic treatment, as demonstrated by the K₃BTC·3H₂O sample, which was subjected to aging (30 days) or DMF soaking (2 days) resulting in the enhanced phase purity and crystal size (Fig. S4c–e, S17 and S18).

Conclusions

In conclusion, we demonstrated that the seemingly simple alkali 1,3,5-benzenetricarboxylates can form a wide range of diverse structures in solid state involving products of various stoichiometry and hydration state. Importantly, all of the demonstrated materials were prepared using simple aqueous route and structurally characterized in detail using SCXRD technique. Moreover, we investigated the use of solvent-free mechanochemical strategy to prepare the developed alkali metal-based coordination networks, which revealed that most of them can be easily prepared by ball milling of 1,3,5-benzenetricarboxylic acid and respective alkali hydroxide for just ca. 20 minutes. Significantly, two of the reported alkali metal-based coordination networks (K₂HBTC·H₂O and Rb₂HBTC·3H₂O) could only be prepared in high-yield using the mechanochemical approach. This selectivity reflects the superior stoichiometric control afforded by mechanochemical synthesis, which eliminates solvation equilibria and enables direct solid-state acid–base reactions between the components. In contrast to the solution synthesis, where cation size, solvation strength, and deprotonation equilibria govern the accessible phases, the mechanochemical route provides a pathway to phases that are thermodynamically or kinetically inaccessible in solution. In general, the demonstrated results indicate that mechanochemistry is not only a green alternative to traditional solution-based chemistry, but can also be a powerful tool for achieving precise stoichiometric and phase control in solid-state coordination chemistry. Overall, this comprehensive study offers a well-rounded insight into the synthesis and structural characteristics of alkali 1,3,5-benzenetricarboxylates. Further studies extending the scope to other organic linkers and transformations of alkali metal-based coordination polymers into functional porous carbons³⁵ are currently underway.

Conflicts of interest

There are no conflicts to declare.

Data availability

Supplementary information (SI): additional data concerning SCXRD, PXRD, TGA and SEM experiments. See DOI: https://doi.org/10.1039/d5dt01954k.

CCDC 2359250 (Na₂HBTC·3H₂O), 2359252 (K₂HBTC·H₂O), 2359487 (Rb₂HBTC·2H₂O) and 2450580 (Rb₃BTC·5H₂O) contain the supplementary crystallographic data for this paper.^56a–d

Acknowledgements

The authors would like to acknowledge the financial support by the ENERGYTECH-1 project granted by the Warsaw University of Technology under the program Excellence Initiative: Research University (IDUB).

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