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Article

s-Triazine-Based Ligands Possessing Identical Heteroatom-Bridged Substituents—Unexpected Triazine-O Bond Cleavage

by
Vanya B. Kurteva
1,*,
Rusi I. Rusew
2,
Zhanina S. Petkova
1,3,
Magdalena Angelova
2 and
Boris L. Shivachev
2,4,*
1
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 9, 1113 Sofia, Bulgaria
2
Institute of Mineralogy and Crystallography “Acad. Ivan Kostov”, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 107, 1113 Sofia, Bulgaria
3
Centre of Competence “Sustainable Utilization of Bio-Resources and Waste of Medicinal and Aromatic Plants for Innovative Bioactive Products” (BIORESOURCES BG), 1000 Sofia, Bulgaria
4
National Centre of Excellence Mechatronics and Clean Technologies, 8 bul. Kliment Ohridski, 1756 Sofia, Bulgaria
*
Authors to whom correspondence should be addressed.
Molecules 2025, 30(18), 3811; https://doi.org/10.3390/molecules30183811
Submission received: 26 August 2025 / Revised: 16 September 2025 / Accepted: 17 September 2025 / Published: 19 September 2025
(This article belongs to the Special Issue 30th Anniversary of Molecules—Recent Advances in Organic Chemistry)
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Abstract

Metal–organic frameworks (MOFs) are materials with extremely valuable properties. The latter depend largely on the ligand used; therefore, the design of new organic linkers is a priority task today. A series of s-triazines possessing variable heteroatom-bridged identical substituents, useful ligands for the synthesis of MOFs, is obtained in good to excellent yields. The problem of obtaining free carboxyl groups without forming salts with nitrogen atoms is solved. The products are characterized by NMR spectra and single crystal XRD of selected samples. Unexpected O-triazine bond cleavage under basic hydrolysis conditions is observed.

1. Introduction

Metal–organic frameworks (MOFs) are a relatively new class of functional materials presenting hybrid crystalline two- or three-dimensional structures built from coordinately bonded metal nodes and polydentate organic linkers. Effective MOFs possess unprecedentedly high degree of porosity, significant internal surface area, crystallinity, chemical and structural flexibility, and ability for functional design and tuning [1,2,3,4,5]. These characteristics determine their extraordinary applications [6,7,8,9] in a range of key areas, such as medicine [10,11,12,13,14,15], technology [16,17,18], water and air purification [19,20,21,22,23,24,25], gas storage [26,27,28], heterogeneous catalysis [29,30,31,32,33], energy [34,35,36,37,38], etc.
The structure and properties of MOFs are strongly dependent on the ability of the ligand and the complexing agent to form stable bonds. Variable organic compounds are used in the synthesis of these remarkable materials. Generally, these are symmetrical molecules containing coordination centers, either donor or acceptor, that complex with metal salts. Among the broad variety of ligands, polycarboxylates are the most widely exploited. Compared to symmetrically substituted benzene derivatives, triazine-based ligands are less well examined [39,40,41]. The most part represent directly connected to triazine ring donor or acceptor aromatics, while the number of MOFs built from triazines with bridged substituents are quite limited. Among the latter, the materials generated from 2,4,6-tris-(4-carboxyphenoxy)-1,3,5-triazine (H3TCPT) [42,43,44,45,46,47,48], 4,4′,4″-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))tribenzoic acid (H3TATAB) [49,50,51,52,53,54], and 3,3′,3″-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))tribenzoic acid (H3TATMB) [55,56,57] have exhibited exceptional surface area for gas adsorption, catalysis, and sensing.
Compounds containing donor pyridine substituents constitute another prevalent class of ligands, which have been widely used for MOF synthesis, most often as linkers in combination with carboxylates [58,59,60,61,62]. To the best of our knowledge, there are no records in the literature on molecules possessing pyridine substituents attached to a s-triazine scaffold via heteroatom bridge.
Recently, we reported on the synthesis of a novel Zn-based MOF from H3TCPT [63]. Herein, we describe in detail the synthetic protocol and characterization of a series of s-triazine ligands bearing a variety of heteroatom-bridged substituents, either carboxylates or heterocycles. An unexpectedly observed Ar-O bond cleavage during basic hydrolysis is also mentioned.

2. Results and Discussion

The subject of the present work is the synthesis of a series of symmetrically substituted s-triazine-based ligands designed to tune several parameters that would be important in the synthesis of MOFs, such as the type and flexibility of the bridging group, length, position and strength of the coordinating substituent, and type of the coordinating center. These ligands can be divided into three types depending on the substituents, as shown in Scheme 1. The first group includes derivatives having carboxyl groups in the substituents, which are directly linked to an aromatic ring. The position of the carboxyl group, the type of bridging heteroatom, and the length of the bridging unit are varied. In the second series, the carboxyl group is connected to an aromatic ring via a bridging chain of diverse length and type. The goal is to obtain ligands with increased mobility in the region carrying the coordinating carboxyl function, allowing for variations in orientation. The third group contains pyridine or other nitrogen containing heterocyclic substituents, differing in the location and number of the nitrogen atoms, the type of bridging heteroatom, and the length of the linker.
A modified well-known and widely applied protocol was used—a reaction between cyanuric chloride (1) and various aromatic compounds (HX-R, 2) in basic conditions (Scheme 1), followed by acidification in case of carboxylic groups containing products. The conditions were optimized on the example of the known ligand H3TCPT (3a) as already published by us [63]. The main attempts were focused on solving two key points: (1) to achieve complete conversion, i.e., to avoid contamination from either starting agents or bis-substituted triazines; and (2) to obtain the compounds containing COOH groups in neutral form, not as sodium salts or hydrochlorides. The first problem was overridden by using slight excess of the reagents 2, followed by washing with appropriate solvent, while the second one was solved by adjusted the pH to 2–3 by nitric acid instead of hydrochloric acid. The best results were obtained when the reaction was performed in aqueous solution in the presence of sodium hydroxide at room temperature and the isolated yield of 87% pure neutral ligand 3a was achieved. This optimized protocol was applied to all examples. The results are summarized in Table 1.
All compounds from the first series were isolated in good to excellent yields except for 3g and 3h, which were obtained as very fine precipitates leading to serious losses during their isolation. On the contrary, the ligands of the second group 3j–3n formed coarsely crystalline precipitates and were isolated in excellent yields. The third series was the most problematic. Initial attempts were focused on various pyridine derivatives. The reaction was performed by using 4-hydroxy, amino or mercapto pyridine, 3-hydroxy or amino pyridine, 2-mercaptopyridine, 4-hydroxymethyl or aminomethylpyridine, and 3-hydroxymethylpyridine, but the solid masses formed show complete insolubility in any solvent or solvent system in all cases (Table S1). Even those with methylene-bridged heteroatom and pyridine, designed in order to tune both the ligand’s flexibility and solubility, followed the same pattern. Based on these disadvantageous results the series was extended towards 2-hydroxy, amino or mercaptopyrimidinyl, and 2-hydroxy or aminopyrazinyl derivatives. The only soluble product obtained was 3o possessing S-bridged pyrimidinyl substituents. For that reason, variable mercapto heterocycles available on the market, namely 5-methyl-1,3,4-thiadiazole-2-thiol, 4-methyl-4H-1,2,4-triazole-3-thiol, and 4-methyl-4H-1,2,4-triazole-3-thiol, were examined and ligands 3p–3r were isolated in moderate to good yields.
Although the ligands were designed as potential MOF linkers, numerous experiments have been conducted to obtain frameworks from a part of the compounds, resulting in mostly amorphous materials. The only crystalline product obtained to date was synthesized from the well-known ligand 3a [63]. The lack of MOF formation with the synthesized ligands may be attributed to several factors but is mainly related to the poor solubility in common solvents. The limited solubility hindered controlled crystallization and prevented the establishment of extended structural networks. In addition, bulky substituents or highly rigid geometries likely introduced steric and conformational constraints that reduced the accessibility of the donor groups for metal coordination. For some heteroatom-bridged systems, mismatched coordination geometry and excessive flexibility may have favored dense or amorphous phases over porous frameworks. Finally, electronic effects related to the nature of the heteroatom bridge may have reduced the binding strength of some donor sites. Taken together, these factors illustrate the challenges of translating ligand design into a successful MOF assembly. These results provoked us to increase the distance between the carboxyl group and the triazine moiety, but to preserve the geometry of the ligand by extending the aromatic system. Thus, compounds 3s and 3t possessing O- or NH-bridges biphenyl carboxylic acids, respectively, were also obtained. While ligand 3s can be integrated with series 2, 3t does not fit into any group and thus, we designated these two similar products as additional.
The structures of the ligands were assigned by 1D and 2D NMR spectra and were confirmed by a single crystal XRD of selected samples. The NMR spectra of some of the ligands, mainly N-bridged, show broad signals due to a slow exchange between two states at room temperature, which is why they are recorded at higher temperatures (Supporting Information). Compounds from group 3, possessing a N-methyl adjacent to the sulfur bridge, 3q and 3r, show an interesting pattern of behavior. The spectra of 3r correspond to two forms in 7:3 ratio, which do not change after staying at room temperature. On the contrary, a change is observed in the spectra of ligand 3q. In the proton spectrum of a freshly dissolved sample in DMSO at room temperature, two signals for methyl groups at 3.868 and 3.886 ppm in a ratio of 3:1 are observed, which after 1 day in the tube change their ratio to 1:12, and a third signal at 4.082 ppm appears with an intensity commensurable with that of the smaller signal. At 353 K the latter is the main one along with two new low-intensity signals at 4.096 and 4.152 ppm (Figure 1).
Apparently, the signal for the N-methyl group in the second position of the substituent is significantly affected by rotation around the six C–S bonds (three triazine-S and three tetrazole-S), falling into different shielding zones of the neighboring substituents. It can be assumed that the rotational barriers in 3q are lower than those in 3r, since changes are observed even at room temperature. The shifting of the methyl group signal with time or at elevated temperature to a lower field suggests that upon rotation it falls into the deshielding zone of another substituent.
Single-crystal X-ray diffraction analyses were carried out for ligands 3j, 3o, and 3p (Figure 2 and Table S2). Compound 3j crystallizes from DMSO in the trigonal space group R-3 with one-third molecule in the asymmetric unit. The remaining parts of the molecule are achieved through symmetry operations. The unit cell is quite large (V = 5797(5) Å3) and contains significant solvent-accessible voids, partially filled with solvent-disordered molecules of water and DMSO. The presence of such solvent along with the possibility to dissolve 3j only in DMF or DMSO (hindering crystal growth of higher quality) are the main reasons for the relatively high refinement indicators (R1 = 0.240, wR2 = 0.547). On the other hand, such “weak” solubility is suitable and may be exploited in the construction of stable MOF frameworks. In the molecule of 3j the triazine core and aromatic cresol adopts a nearly planar conformation. The peripheral substituents (C-COOH) display a certain degree of rotational freedom. The crystal packing is stabilized by weak hydrogen-bonding interactions, mainly O–H···O and C–H···S contacts. (Table S3). These non-covalent contacts interconnect the molecules into a loose supramolecular network, consistent with the limited solubility of 3j in strongly coordinating solvents such as DMSO or DMF.
Compound 3o crystallizes from isopropanol in the monoclinic space group P21/n with four molecules in the unit cell. The parameters are identical with those of the crystals obtained from chloroform-methanol [64]. The asymmetric unit contains one ligand molecule with a nearly planar triazine fragment. The S-bridged pyrimidine substituents are oriented to maximize intermolecular interactions. The crystal packing is sustained by strong C–H···S contacts (C10–H10···S213, D···A = 3.654(2) Å) supplemented by additional weak C–H···N interactions (Table S4). These interactions organize the molecules into a three-dimensional framework.
Compound 3p crystallizes from benzene in the triclinic space group P-1 with two molecules in the unit cell. The triazine core is essentially planar and the substituents adopt orientations that allow efficient intermolecular hydrogen bonding. The supramolecular architecture is dominated by N–H···N and C–H···S contacts, including C13–H13C···N11 (D···A = 3.570(3) Å) and C27–H27B···S92 (D···A = 3.929(4) Å), complemented by shorter C–H···S interactions (Table S5). These hydrogen bonds generate extended chains and layers, producing a compact three-dimensional network. In summary, all three structures demonstrate that the triazine core with heteroatom-bridged substituents engages in multiple non-covalent interactions (C–H···S, C–H···N, N–H···S), which play a key role in stabilizing the supramolecular packing. While 3j is complicated by solvent disorder, 3o and 3p provide well-resolved models with clear supramolecular features. Importantly, the abundance of donor and acceptor sites and their demonstrated ability to generate extended hydrogen-bonded architectures underscore the suitability of these ligands as versatile linkers for the construction of stable and functional MOFs.
Finally, it is useful to mention an unexpected result obtained during the optimization of the synthetic protocol on the example of ligand 3a. Several parameters were varied to obtain efficiently the product in pure form, such as reagents’ proportions, dilution, the organic solvent, and the reaction duration. A two-step procedure passing through the corresponding ester was also tested with the idea of easy purification due to the solubility of the intermediate in organic solvents. The initial reaction between cyanuric chloride and ethyl 4-hydroxybenzoate led to pure ester 4 in almost quantitate yield, which was further submitted to alkaline hydrolysis (Scheme 2). Surprisingly, instead of the desired product, 4-hydroxybenzoic acid was isolated in high yield.
A literature survey shows that such a cleavage of the C–O bond is quite unexpected under these specific conditions. The cleavage of the C–O bond in diaryl ethers, which are widely distributed fragments in nature, has been extensively studied and it has been shown that the break of this strong C–O bond usually requires a metal catalyst, most often in combination with hydrogen, as summarized in a number of review articles [65,66,67,68,69]. Metal catalysis cannot be assumed in this case, as deionized water was used, indicating that triazine-based ligands should be treated with caution.

3. Materials and Methods

3.1. General

All reagents are purchased from Aldrich, Merck, and Fluka and are used without any further purification. The deuterated solvents are purchased from Deutero GmbH. The melting points are determined in capillary tubes on SRS MPA100 OptiMelt (Sunnyvale, CA, USA) automated melting point system with a heating rate of 1 °C per min. The NMR spectra are recorded on Bruker Avance NEO 400 spectrometer (Rheinstetten, Germany) in DMSO-d6 or CDCl3; the chemical shifts are quoted in ppm in δ-values against solvent peak (DMSO) or TMS as internal standard (chloroform) and the coupling constants are calculated in Hz. The assignment of the signals is confirmed by applying two-dimensional HSQC and HMBC techniques. The spectra are processed with Topspin 3.6.3 program. The mass spectra are recorded using a Q Exactive Plus Hybrid Quadrupole-Orbitrap Mass Spectrometer Thermo Scientific (HESI HRMS) in positive and/or negative mode. The spectra are processed by Thermo Scientific FreeStyle program version 1.8 SP1 (Thermo Fisher Scientific Inc., Waltham, MA, USA).

3.2. Synthesis of Ligands 3

The yields are given in Table 1. The melting points of the majority of the ligands possessing carboxyl groups are above 380 °C and therefore cannot be determined by the equipment used. The proton NMR spectra of some of the ligands show broad signals due to slow exchange between two states at room temperature. At the same time, some signals in the carbon spectra are not detected and are therefore extracted from 2D experiments. For some compounds, carbon and 2D spectra are recorded by using impure samples in order to achieve higher concentrations. For simplicity, the nuclei of the aromatic substituents are indicated as Ar and the triazine unit as Tr.
Method 1. Concerning ligands possessing COOH groups (Series 1 and 2). A mixture of the acid 2a–2n, 2s, or 2t (31 mmol), and NaOH (60 mmol) in deionized water (100 mL) was stirred until full dissolution and then a solution of cyanuric chloride (1, 10 mmol) in THF (30 mL) was added. The mixture was stirred at room temperature for 20 h. The solution was acidified with nitric acid to pH 2–3. The solid phase formed was filtered off and washed successively with water, methanol, and THF to give the pure ligand.
Ligand 3a: colorless solid [70]; 1H NMR (DMSO-d6) 7.32 (AA’BB’, 6H, JAB = 8.8, CH-2+6 Ar), 7.96 (AA’BB’, 6H, JAB = 8.8, CH-3+5 Ar), 13.07 (bs, 3H, COOH); 13C NMR (DMSO-d6) 122.1 (CH-2+6 Ar), 129.1 (Cq-4 Ar), 131.4 (CH-3+5 Ar), 155.0 (Cq-1 Ar), 164.7 (Cq Tr), 167.3 (C=O); HRMS (HESI+) m/z calcd. for C24H16N3O9+ [M + H]+ 490.0881, found 490.0882, Δ = 0.1 mDa; HRMS (HESI−) m/z calcd. for C24H14N3O9− [M − H]− 488.0736, found 488.0736, Δ = 0 mDa.
Ligand 3b: colorless solid; 1H NMR (DMSO-d6) 7.51–7.53 (m, 6H, CH-5+6 Ar), 7.73–7.74 (m, 3H, CH-2 Ar), 7.82–7.84 (m, 3H, CH-4 Ar); 13C NMR (DMSO-d6) 122.6 (CH-2 Ar), 126.5 (CH-6 Ar), 127.4 (CH-4 Ar), 130.4 (CH-5 Ar), 129.1 (Cq-3 Ar), 151.7 (Cq-1 Ar), 166.8 (C=O), 173.4 (Cq Tr); HRMS (HESI+) m/z calcd. for C24H16N3O9+ [M + H]+ 490.0881, found 490.0874, Δ = −0.7 mDa; HRMS (HESI−) m/z calcd. for C24H14N3O9− [M − H]− 488.0736, found 488.0736, Δ = 0 mDa.
Ligand 3c: colorless solid [49]; 1H NMR (DMSO-d6; 373 K) 7.77–7.81 (m, 6H, CH-2+6 Ar), 7.89–7.93 (m, 6H, CH-3+5 Ar); 13C NMR (DMSO-d6; 373 K) 120.9 (CH-2+6 Ar), 126.3 (Cq-4 Ar), 130.4 (CH-3+5 Ar), 142.9 (Cq-1 Ar), 166.9 (C=O), 173.2 (Cq Tr); HRMS (HESI+) m/z calcd. for C24H19N6O6+ [M + H]+ 487.1361, found 487.1356, Δ = −0.5 mDa; HRMS (HESI−) m/z calcd. for C24H17N6O6− [M − H]− 485.1215, found 485.1216, Δ = 0.1 mDa.
Ligand 3d: colorless solid [55]; 1H NMR (DMSO-d6; 373 K) 7.42 (t, 3H, J = 7.9, CH-5 Ar), 7.67 (dd, 3H, J = 7.7, 1.1, CH-6 Ar), 7.97 (dd, 3H, J = 8.0, 1.2, CH-4 Ar), 8.12 (s, 3H, CH-2 Ar), 10.00 (s, 3H, NH); 13C NMR (DMSO-d6; 373 K) 122.6 (CH-2 Ar), 124.9 (CH-6 Ar), 125.9 (CH-4 Ar), 129.2 (CH-5 Ar), 132.1 (Cq-3 Ar), 139.0 (Cq-1 Ar), 164.8 (Cq Tr), 167.4 (C=O); HRMS (HESI+) m/z calcd. for C24H19N6O6+ [M + H]+ 487.1361, found 487.1353, Δ = −0.8 mDa; HRMS (HESI−) m/z calcd. for C24H17N6O6− [M − H]− 485.1215, found 485.1215, Δ = 0 mDa.
Ligand 3e: colorless solid; 1H NMR (DMSO-d6; 373 K) 3.38 (s, 9H, CH3-N), 7.45 (AA’BB’, 6H, JAB = 8.7, CH-2+6 Ar), 7.93 (AA’BB’, 6H, JAB = 8.7, CH-3+5 Ar); 13C NMR (DMSO-d6; 373 K) 38.6 (CH3-N), 126.9 (CH-2+6 Ar), 129.7 (Cq-4 Ar), 130.5 (CH-3+5 Ar), 146.9 (Cq-1 Ar), 160.6 (Cq Tr), 167.0 (C=O); HRMS (HESI+) m/z calcd. for C27H25N6O6+ [M + H]+ 529.1830, found 529.1820, Δ = −1.0 mDa; HRMS (HESI−) m/z calcd. for C27H23N6O6− [M − H]− 527.1685, found 527.1685, Δ = 0 mDa.
Ligand 3f: colorless solid; 1H NMR (DMSO-d6; 373 K) 3.41 (s, 9H, CH3-N), 7.46 (bt, 3H, J = 7.8, CH-5 Ar), 7.55 (bd, 3H, J = 7.9, CH-6 Ar), 7.81 (bd, 3H, J = 7.2, CH-4 Ar), 7.85 (bs, 3H, CH-2 Ar); 13C NMR (DMSO-d6; 373 K) 38.0 (CH3-N), 127.46 (CH-4 Ar), 127.53 (CH-2 Ar), 129.3 (CH-5 Ar), 131.2 (CH-6 Ar), 132.3 (Cq-3 Ar), 144.0 (Cq-1 Ar), 165.3 (Cq Tr), 167.1 (C=O); HRMS (HESI+) m/z calcd. for C27H25N6O6+ [M + H]+ 529.1830, found 529.1819, Δ = −1.1 mDa; HRMS (HESI−) m/z calcd. for C27H23N6O6− [M − H]− 527.1685, found 527.1680, Δ = −0.5 mDa.
Ligand 3g: colorless solid; 1H NMR (DMSO-d6; 373 K) 1.12 (t, 9H, J = 7.1, CH3- CH2-N), 3.89 (q, 6H, J = 7.1, CH2-N), 7.44 (AA’BB’, 6H, JAB = 8.6, CH-2+6 Ar), 8.01 (AA’BB’, 6H, JAB = 8.6, CH-3+5 Ar); 13C NMR (DMSO-d6; 373 K) 13.0 (CH3-CH2-N), 46.6 (CH2-N), 128.2 (CH-2+6 Ar), 130.3 (Cq-4 Ar), 131.0 (CH-3+5 Ar), 144.2 (Cq-1 Ar), 157.9 (Cq Tr), 167.0 (C=O); HRMS (HESI+) m/z calcd. for C30H31N6O6+ [M + H]+ 571.2300, found 571.2347, Δ = 4.7 mDa; HRMS (HESI−) m/z calcd. for C30H29N6O6− [M − H]− 569.2154, found 569.2197, Δ = 4.3 mDa.
Ligand 3h: colorless solid; m. p. 339.1 °C (decomp. with intense gas evolution); 1H NMR (DMSO-d6; 373 K) 4.51 (bs, 6H, CH2-N), 7.36 (bd, 6H, J = 8.4, CH-2+6 Ar), 7.86 (bd, 6H, J = 8.4, CH-3+5 Ar); 13C NMR (DMSO-d6; 373 K) 44.2 (CH2-N), 127.68 (CH-2+6 Ar), 129.72 (CH-3+5 Ar), 130.2 (Cq-4 Ar), 144.6 (Cq-1 Ar), 165.9 (Cq Tr), 167.7 (C=O); HRMS (HESI+) m/z calcd. for C27H25N6O6+ [M + H]+ 529.1830, found 529.1819, Δ = −1.1 mDa; HRMS (HESI−) m/z calcd. for C27H23N6O6− [M − H]− 527.1685, found 527.1685, Δ = 0 mDa.
Ligand 3i: colorless solid; m. p. 332.1 °C (decomp. with intense gas evolution); 1H NMR (DMSO-d6) 7.57 (AA’BB’, 6H, JAB = 8.6, CH-2+6 Ar), 7.85 (AA’BB’, 6H, JAB = 8.6, CH-3+5 Ar), 13.08 (bs, 3H, COOH); 13C NMR (DMSO-d6) 129.8 (CH-3+5 Ar), 131.4 (Cq-1 Ar), 131.8 (Cq-4 Ar), 134.4 (CH-2+6 Ar), 166.4 (C=O), 179.0 (Cq Tr); HRMS (HESI+) m/z calcd. for C24H16N3O3S3+ [M + H]+ 538.0196, found 538.0195, Δ = −0.1 mDa; HRMS (HESI−) m/z calcd. for C24H14N3O3S3− [M − H]− 536.0050, found 536.0049, Δ = −0.1 mDa.
Ligand 3j: colorless solid; m. p. 249.6–249.8 °C (decomp.); 1H NMR (DMSO-d6) 3.60 (s, 6H, CH2-N), 7.18 (AA’BB’, 6H, JAB = 8.7, CH-2+6 Ar), 7.30 (AA’BB’, 6H, JAB = 8.7, CH-3+5 Ar), 12.39 (bs, 3H, COOH); 13C NMR 40.4 (CH2-N), 121.7 (CH-2+6 Ar), 131.0 (CH-3+5 Ar), 133.3 (Cq-4 Ar), 150.5 (Cq-1 Ar), 173.0 (C=O), 173.6 (Cq Tr); HRMS (HESI+) m/z calcd. for C27H22N3O9+ [M + H]+ 532.1351, found 532.1346, Δ = −0.5 mDa; HRMS (HESI−) m/z calcd. for C27H20N3O9− [M − H]− 530.1205, found 530.1205, Δ = 0 mDa.
Ligand 3k: colorless solid; 1H NMR (DMSO-d6) 6.52 (d, 3H, J = 16.0, =CH-COOH), 7.29 (AA’BB’, 6H, JAB = 8.7, CH-2+6 Ar), 7.59 (d, 3H, J = 16.0, =CH-Cq-4), 7.74 (AA’BB’, 6H, JAB = 8.7, CH-3+5 Ar); 13C NMR (DMSO-d6) 119.9 (=CH-COOH), 122.4 (CH-2+6 Ar), 129.9 (CH-3+5 Ar), 132.7 (Cq-4 Ar), 143.3 (=CH-Cq-4), 153.0 (Cq-1 Ar), 168.0 (C=O), 173.4 (Cq Tr); HRMS (HESI+) m/z calcd. for C30H22N3O9+ [M + H]+ 568.1351, found 568.1343, Δ = −0.8 mDa; HRMS (HESI−) m/z calcd. for C30H20N3O9− [M-H]− 566.1205, found 566.1205, Δ = 0 mDa.
Ligand 3l: colorless solid; m. p. 248.1–248.2 °C (decomp.); 1H NMR (DMSO-d6) 3.80 (s, 9H, OCH3), 6.58 (d, 3H, J = 16.0, =CH-COOH), 7.24 (d, 3H, J = 8.2, CH-6 Ar), 7.28 (dd, 3H, J = 8.2, 1.8, CH-5 Ar), 7.48 (d, 3H, J = 1.7, CH-3 Ar), 7.57 (d, 3H, J = 16.0, =CH-Cq-4); 13C NMR (DMSO-d6) 56.5 (OCH3), 112.6 (CH-3 Ar), 120.2 (=CH-COOH), 121.8 (CH-5 Ar), 123.2 (CH-6 Ar), 134.1 (Cq-4 Ar), 141.7 (Cq-1 Ar), 143.6 (=CH-Cq-4), 151.3 (Cq-2 Ar), 168.0 (C=O), 173.4 (Cq Tr); HRMS (HESI+) m/z calcd. for C33H28N3O12+ [M + H]+ 658.1668, found 658.1671, Δ = 0.3 mDa; HRMS (HESI−) m/z calcd. for C33H26N3O12− [M − H]− 656.1522, found 656.1523, Δ = 0.1 mDa.
Ligand 3m: colorless solid; 1H NMR (DMSO-d6) 6.56 (d, 3H, J = 16.1, =CH-COOH), 7.31 (ddd, 3H, J = 8.1, 2.3, 0.8, CH-6 Ar), 7.42 (t, 3H, J = 8.0, CH-5 Ar), 7.56 (d, 3H, J = 16.0, =CH-Cq-4), 7.58 (dd, 3H, J = 8.0, 0.8, CH-4 Ar), 7.61 (t, 3H, J = 1.9, CH-2 Ar); 13C NMR (DMSO-d6) 120.9 (=CH-COOH), 121.1 (CH-2 Ar), 123.6 (CH-6 Ar), 126.5 (CH-4 Ar), 130.4 (CH-5 Ar), 136.4 (Cq-3 Ar), 143.2 (=CH-Cq-4), 152.2 (Cq-1 Ar), 167.8 (C=O), 173.5 (Cq Tr); HRMS (HESI+) m/z calcd. for C30H22N3O9+ [M + H]+ 568.1351, found 568.1345, Δ = −0.6 mDa; HRMS (HESI−) m/z calcd. for C30H20N3O9− [M − H]− 566.1205, found 566.1205, Δ = 0 mDa.
Ligand 3n: colorless solid; 1H NMR (DMSO-d6) 6.56 (d, 3H, J = 16.1, =CH-COOH), 7.31 (ddd, 3H, J = 8.4, 7.4, 0.8, CH-4 Ar), 7.36 (dd, 3H, J = 8.3, 1.2, CH-6 Ar), 7.46 (ddd, 3H, J = 8.3, 7.3, 1.6, CH-5 Ar), 7.61 (d, 3H, J = 16.1, =CH-Cq-4), 7.86 (dd, 3H, J = 7.9, 1.5, CH-3 Ar); 13C NMR (DMSO-d6) 120.1 (=CH-COOH), 123.5 (CH-6 Ar), 126.6 (Cq-2 Ar), 127.0 (CH-4 Ar), 128.2 (CH-3 Ar), 131.6 (CH-5 Ar), 136.9 (=CH-Cq-4), 150.2 (Cq-1 Ar), 167.7 (C=O), 173.6 (Cq Tr); HRMS (HESI+) m/z calcd. for C30H22N3O9+ [M + H]+ 568.1351, found 568.1339, Δ = −1.2 mDa; HRMS (HESI−) m/z calcd. for C30H20N3O9− [M − H]− 566.1205, found 566.1205, Δ = 0 mDa.
Ligand 3s: colorless solid; 1H NMR (DMSO-d6; 353 K) 7.36 (bd, 6H, J = 8.6, CH-2+6 Ar), 7.72 (bd, 6H, J = 8.2, CH-2′+6′ Ar), 7.74 (bd, 6H, J = 8.6, CH-3+5 Ar), 7.98 (bd, 6H, J = 8.2, CH-3′+5′ Ar); 13C NMR (DMSO-d6; 353 K) 122.5 (CH-2+6 Ar), 127.1 (CH-2′+6′ Ar), 128.5 (CH-3+5 Ar), 130.3 (CH-3′+5′ Ar), 130.5 (Cq-4′ Ar), 137.5 (Cq-4 Ar), 143.8 (Cq-1′ Ar), 152.1 (Cq-1 Ar), 167.4 (C=O), 173.7 (Cq Tr); HRMS (HESI+) m/z calcd. for C42H28N3O9+ [M + H]+ 718.1820, found 718.1821, Δ = 0.1 mDa; HRMS (HESI−) m/z calcd. for C42H26N3O9− [M − H]− 716.1674, found 716.1681, Δ = 0.7 mDa.
Ligand 3t: colorless solid; 1H NMR (DMSO-d6; 353 K) 7.71–7.75 (m, 8H, CH-3+5 + ½ CH-2+6), 7.77–7.80 (m, 8H, CH-2′+6′ + ½ CH-2+6), 8.02 (bd, 6H, J = 8.2, CH-3′+5′); 13C NMR (DMSO-d6; 353 K) 122.3 (CH-2′+6′), 126.8 (CH-2+6), 127.6 (CH-3+3), 130.1 (Cq-4′), 130.4 (CH-3′+3′), 135.3 (Cq-4), 138.3 (Cq-1), 144.3 (Cq-1′), 167.5 (C=O), 171.8 (Cq Tr); HRMS (HESI+) m/z calcd. for C42H31N6O6+ [M + H]+ 715.2300, found 715.2297, Δ = 0.3 mDa; HRMS (HESI−) m/z calcd. for C42H29N6O69− [M − H]− 713.2154, found 713.2171, Δ = 1.7 mDa.
Method 2. Concerning ligands possessing heterocyclic substituents (Series 3). A mixture of the heterocyclic thiol 2o–2r (31 mmol) and NaOH (31 mmol) in deionized water (100 mL) was stirred until full dissolution and then a solution of cyanuric chloride (1, 10 mmol) in THF (30 mL) was added. The mixture was stirred at room temperature for 20 h. The solid phase formed was filtered off and washed with water (for 3o and 3p) or successively with water, methanol, and THF (for 3q and 3r) to give the pure ligand.
Ligand 3o: colorless solid; m. p. 161.8–161.9 °C (decomp.; lit. 180–181 °C [64]); 1H NMR (CDCl3) 7.20 (t, 3H, J = 4.9, CH-4 Ar), 8.66 (d, 6H, J = 4.9, CH-3+5 Ar); 13C NMR (CDCl3) 119.9 (CH-4 Ar), 158.2 (CH-3+5 Ar), 165.0 (Cq-1 Ar), 179.2 (Cq Tr); HRMS (HESI+) m/z calcd. for C15H10N9S3+ [M + H]+ 412.0216, found 412.0207, Δ = −0.9 mDa.
Ligand 3p: pale yellow solid; m. p. 179.9–180.1 °C (decomp.); 1H NMR (CDCl3) 2.88 (s, 9H, CH3); 13C NMR (CDCl3) 16.0 (CH3), 154.6 (Cq-1 Ar), 171.0 (Cq-3 Ar), 178.1 (Cq Tr); HRMS (HESI+) m/z calcd. for C12H10N9S6− [M + H]+ 471.9378, found 471.9372, Δ = −0.6 mDa.
Ligand 3q: colorless solid; 1H NMR (DMSO-d6) m. p. 226.1 °C (decomp. with intense gas evolution); 1H NMR (DMSO-d6; predominant form at 300 K) 3.89 (s, 9H, CH3-N); 1H NMR (DMSO-d6; predominant form at 353 K) 4.0790 (s, 9H, CH3-N);13C NMR (DMSO-d6; predominant form at 353 K) 34.4 (CH3-N), 149.8 (Cq-1 Ar), 173.8 (Cq Tr); HRMS (HESI+) m/z calcd. for C9H10N15S3+ [M + H]+ 424.0400, found 424.0396, Δ = −0.4 mDa.
Ligand 3r: pale yellow solid; m. p. 269.7 °C (decomp. with intense gas evolution; 175.3 °C—sharp shrinkage with color change to orange); 1H NMR (DMSO-d6; predominant form) 3.48 (s, 9H, CH3-N), 8.76 (s, 3H, CH-4 Ar); 13C NMR (DMSO-d6) 32.9 (CH3-N), 144.4 (CH-4 Ar), 162.9 (Cq Tr), 168.3 (Cq-1 Ar); HRMS (HESI+) m/z calcd. for C12H13N12S3+ [M + H]+ 421.0543, found 421.0534, Δ = −0.9 mDa; HRMS (HESI−) m/z calcd. for C12H11N12S3− [M − H]− 419.0397, found 419.0396, Δ = −0.1 mDa.
Method 3. Two-step protocol going via ester 4. Step 1. A mixture of ethyl 4-hydroxybenzoate (31 mmol) and NaOH (30 mmol) in water (100 mL) was stirred until full dissolution and then a solution of cyanuric chloride (1, 10 mmol) in THF (30 mL) was added. The mixture was stirred at room temperature for 6 h. The solid phase formed was filtered off, washed with water, and dried on air. The product was purified by flash chromatography on silica gel by using DCM as a mobile phase to give the pure ester 4 in 96% yield: colorless solid; m. p. 113.3–113.4 °C; 1H NMR (CDCl3) 1.39 (t, 9H, J = 7.1, CH3), 4.38 (q, 6H, J = 7.1, CH2), 7.20 (AA’BB’, 6H, JAB = 8.9, CH-2+6 Ar), 8.07 (AA’BB’, 6H, JAB = 8.9, CH-3+5 Ar); 13C NMR (CDCl3) 14.3 (CH3), 61.2 (CH2), 121.4 (CH-2+6 Ar), 128.6 (Cq-4 Ar), 131.3 (CH-3+5 Ar), 154.7 (Cq-1 Ar), 165.6 (C=O), 173.3 (Cq Tr); HRMS (HESI−) m/z calcd. for C30H26N3O9− [M − H]− 572.1674, found 572.1685, Δ = 1.1 mDa.
Step 2. A solution of ester 4 (5 mmol) and NaOH (50 mmol) in water (50 mL) was refluxed with stirring for 20 h. The solution was acidified with nitric acid to pH 2–3. The solid phase formed was filtered off and washed with water to give 4-hydroxybenzoic acid in 86% yield.

3.3. Crystallography

A single crystal of each compound was fixed on the top of a magnetic mount using paraton N, transferred and centered on the Kappa goniometer of a Bruker D8 Venture diffractometer. Data collection was performed at ambient conditions (room temperature, 290 K) using micro-focus Mo-Kα radiation (λ = 0.71013 Å). The data reduction, e.g., unit cell parameter determination, data integration, scaling, and absorption corrections were conducted using Apex6 [71]. The structures were solved via intrinsic methods with ShelxT-2018 [72]. The structural refinement was carried out through multiple cycles of full-matrix least-squares refinement on F2 using the ShelxL-2018 package [73]. Nitrogen-bound hydrogen atoms were located from different Fourier maps, while all other hydrogen atoms were placed in idealized positions. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model [73]. The most important data collection and refinement parameters for 3j, 3o, and 3p are presented in Table S1.
The crystallographic data (including atomic coordinates and structure factors) in the Crystallographic Information File (CIF) were validated using the IUCr checkCIF/PLATON tool [74]. Crystallographic data from the structural analysis were deposited at the Cambridge Crystallographic Data Centre (CCDC) as follows: 2482023, 2482024 and 2482025. A copy of this data can be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: +44 1223 336 033, e-mail: deposit@ccdc.cam.ac.uk, or www.ccdc.cam.ac.uk, accessed on 22 August 2025.
Molecular visualizations of the asymmetric units of 3j, 3o, and 3p were generated using Oak Ridge Thermal-Ellipsoid Plot Program-ORTEP [75], while Mercury [76] was employed to depict the three-dimensional packing and hydrogen bonding interactions.

4. Conclusions

Three series of identically trisubstituted ligands based on s-triazine are obtained in good to excellent yields. The compounds possess variable heteroatom-bridged substituents, namely aromatic acids, which are directly connected to the heteroatom or distant via a chain of diverse length and type, and heterocyclic units linked through a sulfur atom. The synthetic protocol is an eco-friendly and cost-effective procedure, stirring at room temperature in water, that finishes with an easy work-up, just filtration and washing. Two important key points are solved. To avoid contamination from bis-substituted triazines, the reagents are used in slight excess and are eliminated from the final products by washing with appropriate solvents. The problem associated with releasing the sodium carboxylates without forming salts at the nitrogen atoms is solved by acidification with nitric acid. Unexpected cleavage of O-triazine bond is observed during alkaline hydrolysis of the ethyl ester of H3TCPT leading to almost quantitative yield of 4-hydroxybenzoic acid. Current attempts are focused on the synthesis of novel crystalline MOF materials from the ligands reported herein, both alone and in combination with a second linker.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/molecules30183811/s1, Table S1: Reagents leading to insoluble solid masses; Table S2: Crystal data and structure refinement for 3j, 3o, and 3p.; Table S3: Hydrogen bonds for 3j; Table S4: Hydrogen bonds for 3o; Table S5: Hydrogen bonds for 3p; Figures S1–S67: NMR spectra; Figures S68–S105: HRMS spectra.

Author Contributions

Conceptualization, V.B.K. and B.L.S.; methodology, V.B.K. and B.L.S.; investigation, V.B.K., R.I.R., Z.S.P., M.A. and B.L.S.; writing—original draft preparation, V.B.K. and B.L.S.; writing—review and editing, V.B.K., R.I.R., Z.S.P., M.A. and B.L.S.; visualization, V.B.K., R.I.R., M.A. and B.L.S.; supervision, V.B.K. and B.L.S.; project administration, V.B.K. and B.L.S.; funding acquisition, V.B.K. and B.L.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Bulgarian National Science Fund (BNSF), grant numbers KP-06-COST/2 and KP-06-COST/3.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

Financial support by the European Union, COST Action CA22147 European metal–organic framework network: combining research and development to promote technological solutions (EU4MOFs), by Bulgarian National Science Fund, grant numbers KP-06-COST/2 and KP-06-COST/3, and by the Bulgarian Ministry of Education and Science, the Operational Program “Research, Innovation and Digitization for Smart Transformation” 2021–2027, co-funded by the EU, under the Projects Centre of Competence “Sustainable Utilization of Bio-resources and Waste of Medicinal and Aromatic Plants for Innovative Bioactive Products” (BIORESOURCES BG), project BG16RFPR002-1.014-0001 (for Bruker Avance NEO 400 NMR spectrometer and Q Exactive Plus Hybrid Quadrupole-Orbitrap MS equipment), and Centre of Excellence “National centre of mechatronics and clean technologies”, project BG16RFPR002-1.014-0006 (for Bruker D8venture XRD equipment), is gratefully acknowledged.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
MOF(s)Metal–organic framework(s)
H3TCPT2,4,6-tris-(4-carboxyphenoxy)-1,3,5-triazine
H3TATAB4,4′,4″-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))tribenzoic acid
H3TATMB3,3′,3″-((1,3,5-triazine-2,4,6-triyl)tris(azanediyl))tribenzoic acid
NMRNuclear magnetic resonance
XRDX-Ray diffraction
HESI HRMSHeated electrospray ionization high resolution mass spectrometry
DMSO-d6Deuterated dimethyl sulfoxide
CDCl3Deuterochloroform
TMSTetramethyl silane
HSQCHeteronuclear single quantum coherence
HMBCHeteronuclear multiple bond correlation
ROESYRotating-frame nuclear overhauser effect spectroscopy
THFTetrahydrofuran
DCMDichloromethane
COSTCooperation in Science and Technology

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Molecules 30 03811 sch001
Scheme 1. General synthetic procedure and structures of ligands 3.
Scheme 1. General synthetic procedure and structures of ligands 3.
Molecules 30 03811 sch001
Molecules 30 03811 g001
Figure 1. 1H NMR spectra of 3q in DMSO-d6: freshly dissolved sample at 300 K (bottom); the same sample at 300 K after 24 h (middle); the same sample at 353 K (top).
Figure 1. 1H NMR spectra of 3q in DMSO-d6: freshly dissolved sample at 300 K (bottom); the same sample at 300 K after 24 h (middle); the same sample at 353 K (top).
Molecules 30 03811 g001
Molecules 30 03811 g002
Figure 2. Depiction of the molecules of compounds (a) 3j, (b) 3o, and (c) 3p; thermal ellipsoids are displayed at 50% probability, hydrogen atoms are represented as spheres with arbitrary radii while minor disorder component (in 3j and 3p) is shown as dashed lines; color scheme: C = blue, H = white, O = red, N = purple, and S = yellow.
Figure 2. Depiction of the molecules of compounds (a) 3j, (b) 3o, and (c) 3p; thermal ellipsoids are displayed at 50% probability, hydrogen atoms are represented as spheres with arbitrary radii while minor disorder component (in 3j and 3p) is shown as dashed lines; color scheme: C = blue, H = white, O = red, N = purple, and S = yellow.
Molecules 30 03811 g002
Molecules 30 03811 sch002
Scheme 2. Alkaline hydrolysis of compound 4.
Scheme 2. Alkaline hydrolysis of compound 4.
Molecules 30 03811 sch002
Table 1. Synthesis of ligands 3a–3t.
Table 1. Synthesis of ligands 3a–3t.
Series 1Series 2Series 3
Substituent XRYield, %Substituent OArYield, %Substituent XArYield, %
Molecules 30 03811 i0013a87 1Molecules 30 03811 i0023j99Molecules 30 03811 i0033o84
Molecules 30 03811 i0043b89Molecules 30 03811 i0053k96Molecules 30 03811 i0063p87
Molecules 30 03811 i0073c91Molecules 30 03811 i0083l91Molecules 30 03811 i0093q88
Molecules 30 03811 i0103d93
Molecules 30 03811 i0113e65Molecules 30 03811 i0123m96Molecules 30 03811 i0133r67
Molecules 30 03811 i0143f66
Molecules 30 03811 i0153g48Molecules 30 03811 i0163n88Additional
Molecules 30 03811 i0173s89
Molecules 30 03811 i0183h55 Molecules 30 03811 i0193t69
Molecules 30 03811 i0203i90
1 Published in ref. [63].
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MDPI and ACS Style

Kurteva, V.B.; Rusew, R.I.; Petkova, Z.S.; Angelova, M.; Shivachev, B.L. s-Triazine-Based Ligands Possessing Identical Heteroatom-Bridged Substituents—Unexpected Triazine-O Bond Cleavage. Molecules 2025, 30, 3811. https://doi.org/10.3390/molecules30183811

AMA Style

Kurteva VB, Rusew RI, Petkova ZS, Angelova M, Shivachev BL. s-Triazine-Based Ligands Possessing Identical Heteroatom-Bridged Substituents—Unexpected Triazine-O Bond Cleavage. Molecules. 2025; 30(18):3811. https://doi.org/10.3390/molecules30183811

Chicago/Turabian Style

Kurteva, Vanya B., Rusi I. Rusew, Zhanina S. Petkova, Magdalena Angelova, and Boris L. Shivachev. 2025. "s-Triazine-Based Ligands Possessing Identical Heteroatom-Bridged Substituents—Unexpected Triazine-O Bond Cleavage" Molecules 30, no. 18: 3811. https://doi.org/10.3390/molecules30183811

APA Style

Kurteva, V. B., Rusew, R. I., Petkova, Z. S., Angelova, M., & Shivachev, B. L. (2025). s-Triazine-Based Ligands Possessing Identical Heteroatom-Bridged Substituents—Unexpected Triazine-O Bond Cleavage. Molecules, 30(18), 3811. https://doi.org/10.3390/molecules30183811

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