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Article

Coordination Flexibility of Zn(II) in Trimesate-Based Metal–Organic Frameworks: Formation of Distinct Secondary Building Units

by
Giovanni Bella
,
Francesco Nicolò
,
Giuseppe Bruno
,
Amine Assel
and
Antonio Santoro
*
Department of Chemical, Biological, Pharmaceutical and Environmental Sciences, University of Messina, Viale F. Stagno d’Alcontres 31, 98166 Messina, Italy
*
Author to whom correspondence should be addressed.
Crystals 2026, 16(5), 290; https://doi.org/10.3390/cryst16050290
Submission received: 30 March 2026 / Revised: 20 April 2026 / Accepted: 23 April 2026 / Published: 28 April 2026
(This article belongs to the Special Issue Metal Coordination Complexes for Optical and Chiro-Optical Applications)
Browse Figures
Versions Notes

Abstract

Two zinc(II)-trimesate metal–organic frameworks were synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. Although both compounds originate from the same Zn(II)-benzene-1,3,5-tricarboxylate (BTC) chemical system, they crystallize in different space groups and exhibit distinct coordination environments and secondary building units (SBUs). One framework adopts a cubic structure and is built from a binuclear Zn paddlewheel type SBU, characterized by a short Zn–Zn internuclear distance and four μ2-bridging carboxylate groups. In contrast, the second framework crystallizes in a tetragonal lattice and features mixed Zn(II) coordination environments, with the coexistence of tetrahedral and octahedral metal centers assembled into a fundamentally different SBU. The comparison between these two structures highlights the coordination flexibility of Zn(II) and the sensitivity of Zn–BTC frameworks to crystallization conditions, such as solvent composition. These results underline the importance of detailed crystallographic analysis in revealing SBU diversity and provide insight into how variations in local coordination chemistry can lead to distinct framework architectures from identical chemical building blocks.

1. Introduction

Nanomaterials and their derivatives exhibit unique photonic, electronic, magnetic and chiroptical properties that arise from their structural characteristics in the nanoscale domain [1,2,3,4,5,6,7,8,9,10]. In this broad landscape, metal–organic frameworks (MOFs) represent a prominent class of crystalline coordination polymers composed of metal ions or metal clusters interconnected by multitopic organic ligands into extended networks. Owing to their modular nature, MOFs exhibit exceptional structural diversity, allowing structural tunability in terms of topology, pore size, and functionality through suitable selection of metal nodes, organic linkers and, importantly, synthetic conditions [11,12,13,14,15,16,17]. Within the framework of reticular chemistry, which describes the design of extended crystalline networks through the assembly of pre-defined molecular building units, MOFs can be viewed as assemblies of well-defined units whose geometry and connectivity are reflected in pore dimensions, architectural features and tunable physicochemical properties [11,12,13,14,15,16,17,18]. Zinc(II) is one of the most extensively employed metal atoms in MOF synthesis due to its closed-shell electronic configuration, flexible coordination behavior, chemical stability, and an ionic radius compatible with a wide range of carboxylate-based ligands [19,20,21,22,23,24,25]. Zn(II) centers can adopt different coordination geometries such as tetrahedral, trigonal bipyramidal, or octahedral, that can be chosen varying the synthetic conditions, the nature of the partner ligands and the stoichiometry. The flexible coordination behavior of Zn(II) often leads to the formation of distinct SBUs, defined as recurring inorganic or metal–organic clusters that act as nodes within the framework [26,27,28]. SBUs encode the local coordination chemistry of the metal centers and play a fundamental role in determining framework dimensionality, topology, and structural stability [29,30,31]. Importantly, SBUs are not uniquely dictated by the chemical identity of the building blocks, but may vary depending on crystallization conditions, including the presence of auxiliary ligands, solvent molecules of different nature, or guests that stabilize specific coordination environments [32,33,34,35,36]. Among polycarboxylate ligands, benzene-1,3,5-tricarboxylic acid (trimesic acid, H3BTC) has attracted particular interest due to its trigonal geometry and ability to bridge multiple metal centers simultaneously. Upon full deprotonation, BTC3- can coordinate through a variety of binding modes, including monodentate, chelating, and bridging carboxylate groups, often leading to the formation of multinuclear SBUs in Zn–based systems [37,38,39]. Numerous Zn–BTC frameworks have been reported, displaying significant structural diversity in terms of metal coordination environments, cluster nuclearity, and overall network topology [21,38,39,40,41,42,43,44,45,46,47]. Solvent molecules may act as templating agents, participate directly in metal coordination, or occupy framework voids, thereby stabilizing specific SBUs and influencing the final network architecture [46,47,48,49,50]. As a result, minor changes in crystallization conditions can yield distinct frameworks or solvated forms that are not preliminarily identifiable from stoichiometry alone. In this context, single-crystal X-ray diffraction (SCXRD) is essential for resolving the coordination environments, ligand binding modes, and SBU organization of Zn–BTC frameworks [51,52,53]. In this work, we report the synthesis and SCXRD structural characterization of two Zn(II)-trimesate frameworks obtained under different crystallization conditions. Although both structures originate from the same Zn–BTC system and the same stoichiometry, they exhibit distinct SBUs. Despite the extensive literature on Zn–BTC frameworks, the relationship between modest changes in crystallization conditions and SBU selection remains not fully predictable, particularly when identical metal-ligand systems are employed. In many cases, small changes in solvent composition or reaction environment can lead to markedly different coordination modes and framework topologies, highlighting the need for detailed comparative crystallographic studies. One framework (1) features a binuclear Zn unit characterized by a short Zn–Zn separation, four μ2-bridging carboxylate groups in the equatorial plane, and an axially coordinated solvent molecule, consistent with a paddlewheel type Zn2(O2CR)4 motif. In contrast, the second framework (2) displays non-equivalent Zn(II) coordination environments, involving tetra- and hexa-coordinated metal centers interconnected by BTC ligands and stabilized by coordinated solvent molecules, resulting in a radically different SBU and network architecture. By comparing these two structures, this study highlights how the coordination flexibility of Zn(II) combined with crystallization conditions governs SBU selection in Zn–BTC frameworks, leading to alternative framework architectures derived from identical chemical building blocks.

2. Materials and Methods

The single crystal of 1 reactor and heated at 120 °C for 48 h. After completion of the reaction, the system was obtained by hydrothermal synthesis using zinc nitrate and benzene-1,3,5-tricarboxylic acid (H3BTC). Equimolar aqueous solutions (5 × 10−3 M) of Zn(NO3)2 and H3BTC were mixed in a 1:1 (v/v) water/ethanol solvent system. The resulting mixture was sealed in a batch was allowed to cool slowly to room temperature over 24 h. Colorless single crystals were obtained, among which a crystal of suitable size and quality was selected for SCXRD analysis. The single crystal of 2 was prepared under hydrothermal conditions starting from zinc nitrate and H3BTC. Equimolar solutions (5 × 10−3 M) of Zn(NO3)2 and H3BTC were combined in a 1:1 (v/v) water/Dimethylformamide (DMF) solvent mixture. The reaction was conducted in batch mode at 120 °C for 48 h, followed by slow cooling to room temperature over 24 h. The two synthesis are summarized in the Scheme 1.
The choice of solvent systems was guided by the different coordination behavior of protic and aprotic solvents. While ethanol is expected to act primarily as a weakly coordinating medium, favoring regular carboxylate bridging modes, DMF can directly coordinate to metal centers, potentially stabilizing higher coordination numbers and more complex local environments. These differences were expected to influence the formation of distinct secondary building units during the crystallization process. Colorless single crystals suitable for single-crystal X-ray diffraction were isolated from the resulting solution. Suitable crystals were selected, and their diffraction data were collected at room temperature using a single-crystal Bruker ASX Inch. APEX-II CCD diffractometer, Madison, WI, USA and the APEX2 package (v.2024) [54,55], which was used for data reduction and structure solution (SHELXT (v.2024) [51]), while refinement was carried out using the full-matrix least-squares refinement on F2 of SHELXL (v.2024) [52] in OLEX2 (v.2024) [56], which also prepared the publication material. The non-hydrogen atoms were refined anisotropically, while the aromatic H atoms were introduced in calculated positions and their bond geometry and isotropic displacement parameters were constrained to ride on their parent atoms. The crystal packings of both compounds are governed by the high symmetry of the 3D framework, with the coordinated solvent molecules (water and DMF, respectively) being highly disordered at the axial positions of the Zn center located on a special crystallographic site. In compound 1, the hydrogens of the disordered coordinated water were omitted owing to the lack of a reasonable interpretation of their significant electron density residuals surrounding the O atom. In both crystal structure models, the presence of disordered solvent in the cavities of the corresponding porous 3D-network has been treated by the solvent mask method in Olex2 Package [56]. Selected crystallographic parameters and structural determination features are provided in Table 1. All chemicals were purchased from Merck, Darmstadt, Germany.

3. Results

3.1. Synthesis and Crystallization

Although both frameworks were obtained from the same Zn(II)-BTC chemical system, their crystallization under different solvent compositions resulted in distinct SBUs and coordination environments. In particular, hydrothermal synthesis in a water/ethanol mixture leads to the formation of a binuclear Zn paddlewheel type SBU, characterized by uniform metal coordination environments. In contrast, crystallization of 2 from a water/DMF mixture yields a framework featuring mixed Zn(II) coordination environments, with the coexistence of tetrahedral and octahedral Zn centers. These observations suggest that solvent composition may play a role in modulating Zn(II) coordination preferences during framework assembly. Protic solvents such as water and ethanol may favor more regular carboxylate bridging modes, promoting the formation of binuclear SBUs, whereas the presence of a coordinating polar aprotic solvent such as DMF may stabilize higher coordination numbers and heterogeneous local environments. These findings are consistent with previous observations in MOF chemistry, where solvent molecules can act not only as reaction media but also as structure-directing agents, influencing both the nuclearity and geometry of the resulting SBUs. While no definitive mechanistic conclusions can be drawn from the present data alone, the comparison between these two structures highlights the sensitivity of Zn–BTC frameworks to marginal variations in crystallization conditions and underscores the importance of solvent environment and thermal parameters in SBU selection. A systematic investigation of the synthetic plan would be required to fully elucidate the mechanisms governing the formation of these distinct architectures.

3.2. Crystal Structure of the Zn–BTC Framework of Compound 1

Compound 1 crystallizes in the cubic space group Fm 3 ¯ m and is characterized by a highly symmetric three-dimensional framework. The asymmetric unit contains one Zn(II) center, which participates in the formation of a binuclear Zn2 SBU located at the intersection of three orthogonal crystallographic mirror planes. In the crystal packing it can be observed that each Zn(II) ion is coordinated by four oxygen atoms belonging to four distinct carboxylate groups from BTC ligands, arranged in an approximately square-planar equatorial environment, as described in Figure 1. In addition, one oxygen atom from a coordinated solvent molecule occupies an axial position, completing the coordination preference of the metal center. Two such Zn(II) centers are linked through four μ2-bridging carboxylate groups, giving rise to a Zn2(O2CR)4 core. The presence of a short Zn–Zn separation (2.968(1) Å) confirms the binuclear nature of this SBU. This coordination motif is consistent with a paddlewheel-type SBU [19,21]. The BTC ligands act as multitopic linkers, connecting adjacent Zn2 SBUs and propagating the structure into a three-dimensional network. The high crystallographic symmetry of the framework results in a regular arrangement of these SBUs throughout the lattice, as depicted in Figure 2.

3.3. Crystal Structure of the Zn–BTC Framework of Compound 2

Conversely, the MOF 2 crystallizes in the tetragonal space group I4cm and exhibits a markedly different coordination scheme. The asymmetric unit contains two crystallographically independent Zn(II) centers, displaying distinct coordination geometries.
One Zn(II) site adopts a tetrahedral coordination environment defined by three oxygen atoms from BTC carboxylate groups and one μ3-bridging oxygen atom belonging to the trinuclear Zn–based unit. The second Zn(II) site is hexa-coordinated, with an octahedral geometry defined by oxygen atoms from carboxylate groups and coordinated solvent molecules, as illustrated in Figure 3. No short Zn–Zn separations indicative of direct metal-metal interactions are observed in this structure, ruling out the presence of a binuclear paddlewheel motif. The coexistence of tetra- and hexa-coordinated Zn(II) centers leads to the formation of a mixed-coordination SBU, fundamentally different from the binuclear unit observed in the cubic framework. BTC ligands bridge these non-equivalent Zn sites through different carboxylate coordination modes, generating an extended three-dimensional architecture stabilized by coordinated solvent molecules. The lower symmetry of the tetragonal lattice reflects the increased complexity of the local coordination environments and the reduced regularity of the SBUs (see Figure 4).

3.4. Comparison of SBUs and Framework Architectures

Although both compounds originate from the same Zn–BTC chemical system, their crystal structures reveal two distinct SBUs, highlighting the strong influence of crystallization conditions on framework assembly. In the cubic structure of 1, the formation of a binuclear Zn(II) paddlewheel SBU is associated with uniform metal coordination environments and high lattice symmetry. The presence of axial solvent coordination appears to stabilize this binuclear motif, promoting the propagation of identical SBUs throughout the framework. In contrast, the tetragonal structure of 2 features heterogeneous Zn coordination environments, resulting in a mixed-coordination SBU without direct Zn–Zn interactions. In this case, the coordination flexibility of Zn(II), combined with the participation of solvent molecules, favors the stabilization of both tetra- and hexa-coordinated metal centers within the same framework. These observations demonstrate that SBU selection in Zn–BTC frameworks is not solely dictated by ligand geometry but is highly sensitive to subtle variations in the crystallization environment. Solvent molecules play a dual role, acting both as ligands that complete metal coordination spheres and as structure-directing agents that influence SBU nuclearity and geometry.

3.5. Pore Structure Analysis

A geometric analysis of the pore structure was performed in order to evaluate the accessible voids in the two frameworks. The cubic framework of compound 1 exhibits a high degree of porosity, with a calculated network-accessible surface area of 1762 m2/g and a geometric accessible volume of 0.734 cm3/g. The pore system is characterized by a three-dimensional connectivity, as indicated by the three percolated dimensions, and by pore diameters ranging from approximately 5.36 Å (pore limiting diameter) to 11.32 Å (maximum pore diameter). These features are consistent with the high symmetry of the structure and the regular arrangement of the Zn2 paddlewheel SBUs. In contrast, compound 2 displays a significantly lower surface area (623 m2/g) and accessible volume (0.451 cm3/g). The pore system is characterized by one-dimensional percolation, indicating that continuous channels are present only along a single crystallographic direction. The pore diameters range from 7.05 Å to 8.22 Å, suggesting a more constrained and less interconnected void structure. The comparison highlights how the different SBUs and coordination environments directly influence the pore architecture. The binuclear paddlewheel SBU in compound 1 promotes the formation of a highly connected three-dimensional pore network, whereas the mixed coordination SBU in compound 2 leads to reduced connectivity and more limited pore accessibility.

3.6. Implications for SBU Selection in Zn–BTC Systems

The comparison between these two Zn–BTC frameworks underscores the central role of Zn(II) coordination flexibility in governing SBU formation. The ability of Zn(II) to adopt multiple coordination geometries enables the emergence of alternative SBUs from identical chemical components, leading to distinct framework architectures. These results highlight the importance of detailed single-crystal structural analysis in identifying and rationalizing SBU diversity in MOF systems. Understanding how crystallization conditions influence SBU selection is essential for controlling framework topology and, ultimately, for the rational design of Zn–based MOFs with targeted structural features. From a broader perspective, the ability of Zn(II) to generate different SBUs under closely related crystallization conditions has important implications for the rational design of MOFs. Although the present study is primarily focused on structural characterization, the observed SBU variability is directly reflected in the resulting pore architecture, as demonstrated by the differences in accessible surface area, pore dimensions, and connectivity between the two frameworks. In particular, the transition from a three-dimensional pore network in compound 1 to a one-dimensional percolated system in compound 2 highlights how coordination environment and SBU topology can govern pore accessibility and dimensionality. These observations suggest that careful control of the solvent environment may represent an effective strategy to direct SBU formation and, consequently, to tune the structural and geometric properties of Zn–based frameworks.

4. Conclusions

In this work, two Zn(II)-trimesate frameworks obtained under different hydrothermal conditions were structurally characterized by single-crystal X-ray diffraction. Although both compounds originate from the same Zn–BTC chemical system, they exhibit markedly different coordination environments and secondary building units (SBUs), highlighting the structural versatility of Zn(II) in carboxylate-based frameworks. The framework of compound 1 crystallizes in a cubic lattice and is built from a well-defined binuclear Zn2 paddlewheel type SBU, characterized by a short Zn–Zn separation and four μ2-bridging carboxylate groups. In contrast, the framework of compound 2 adopts a tetragonal structure and features mixed Zn(II) coordination environments, with the coexistence of tetrahedral and octahedral metal centers assembled into a fundamentally different SBU. The comparison between these two structures underscores the sensitivity of Zn–BTC frameworks to crystallization conditions, particularly solvent composition, which appears to influence Zn(II) coordination preferences and SBU selection. While no mechanistic conclusions can be drawn from the present data alone, these results emphasize the importance of detailed crystallographic analysis in revealing subtle structure–assembly relationships in metal–organic frameworks. Overall, this study contributes to the understanding of SBU diversity in Zn–trimesate systems and illustrates how identical chemical building blocks can give rise to distinct framework architectures through variations in local coordination chemistry. Such insights are relevant for the rational design of Zn–based MOFs and for future investigations aimed at controlling framework structure through targeted synthetic strategies. The pore analysis further confirms that SBU selection not only affects the structural topology but also significantly impacts the resulting pore architecture and connectivity, leading to marked differences in accessible surface area and dimensionality of the pore system. Future work will focus on systematically varying crystallization parameters in order to establish clearer correlations between solvent environment and SBU formation. In addition, further studies will be aimed at investigating the physicochemical properties of these frameworks, including their potential porosity and guest inclusion behavior.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst16050290/s1. Figure S1. ORTEP view of the binuclear Zn2(O2CR)4 moiety with the four μ2-bridging ligands in the cubic Zn-BTC framework of 1, showing the atomic numbering scheme of the crystallographic asymmetric unit. Ellipsoids are drawn at 50% probability level while H-atom size is arbitrary, and hydrogens of the coordinated water molecule are omitted; Figure S2. Packing representations of the cubic cell Zn-BTC framework of 1 illustrating the regular three-dimensional arrangement of Zn2 paddlewheel secondary building units; Figure S3. View illustrating the position of the molecular unit of compound 1 relative to the selected crystallographic mirror planes; Figure S4. ORTEP view of the trinuclear moiety in the tetragonal Zn-BTC framework of 2, showing the atomic numbering scheme of the crystallographic asymmetric unit. Ellipsoids are drawn at 50% probability level while H-atom size is arbitrary; Figure S5. Packing representations of the tetragonal Zn-BTC framework of 2 showing the three-dimensional network formed by mixed-coordination Zn-based secondary building units; Figure S6. View illustrating the position of the molecular unit of compound 2 relative to the selected crystallographic mirror planes; Table S1. Crystal data and structure refinement for 1; Table S2. Fractional Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for 1. Ueq is defined as 1/3 of the trace of the orthogonalised UIJ tensor; Table S3. Anisotropic Displacement Parameters (Å2 × 103) for 1. The Anisotropic displacement factor exponent takes the form: −2π2[h2a*2U11 + 2hka*b*U12 + …]; Table S4. Bond Lengths for 1; Table S5. Bond Angles for 1; Table S6. Torsion Angles for 1; Table S7. Hydrogen Atom Coordinates (Å × 104) and Isotropic Displacement Parameters (Å2 × 103) for 1; Table S8. Solvent masks information for 1; Table S9. Crystal data and structure refinement for 2; Table S10. Fractional Atomic Coordinates (×104) and Equivalent Isotropic Displacement Parameters (Å2 × 103) for 2. Ueq is defined as 1/3 of the trace of the orthogonalised UIJ tensor; Table S11. Anisotropic Displacement Parameters (Å2 × 103) for 2. The Anisotropic displacement factor exponent takes the form: −2π2[h2a*2U11 + 2hka*b*U12 + …]; Table S12. Bond Lengths for 2; Table S13. Bond Angles for 2; Table S14. Torsion Angles for 2; Table S15. Hydrogen Atom Coordinates (Å × 104) and Isotropic Displacement Parameters (Å2 × 103) for 2; Table S16. Atomic Occupancy for 2; Table S17. Solvent masks information for 2.

Author Contributions

Conceptualization, A.S. and G.B. (Giuseppe Bruno); methodology, F.N.; software, F.N.; validation, G.B. (Giovanni Bella), A.A. and G.B. (Giuseppe Bruno); formal analysis, F.N.; investigation, A.S.; resources, G.B. (Giuseppe Bruno); data curation, G.B. (Giovanni Bella); writing—original draft preparation, A.S.; writing—review and editing, G.B. (Giovanni Bella), G.B. (Giuseppe Bruno) and F.N.; visualization, A.A.; supervision, F.N.; project administration, A.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Crystallographic data are exhaustively reported in Supporting Information, and the .cif files are available on the CCDC by using the code 2539925 for 1 and 2539926 for 2.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 16 00290 sch001
Scheme 1. Reaction scheme for compound 1 and 2.
Scheme 1. Reaction scheme for compound 1 and 2.
Crystals 16 00290 sch001
Crystals 16 00290 g001
Figure 1. ORTEP representation (50% probability ellipsoids) of the binuclear Zn2(O2CR)4 secondary building unit in the cubic Zn–BTC framework of 1, showing the four μ2-bridging carboxylate groups in the equatorial plane and the axially coordinated water molecule. Atom labels are omitted for clarity; full labeling is reported in the representation of the asymmetric unit in Figure S1 of Supporting Information.
Figure 1. ORTEP representation (50% probability ellipsoids) of the binuclear Zn2(O2CR)4 secondary building unit in the cubic Zn–BTC framework of 1, showing the four μ2-bridging carboxylate groups in the equatorial plane and the axially coordinated water molecule. Atom labels are omitted for clarity; full labeling is reported in the representation of the asymmetric unit in Figure S1 of Supporting Information.
Crystals 16 00290 g001
Crystals 16 00290 g002
Figure 2. Packing view of the cubic Zn–BTC framework of 1 along the c crystallographic axis, illustrating the regular three-dimensional arrangement of Zn2 paddlewheel secondary building units and highlighting the hollow columns along the 4-fold axes. Additional views of the 3D packing are provided in the Supporting Information.
Figure 2. Packing view of the cubic Zn–BTC framework of 1 along the c crystallographic axis, illustrating the regular three-dimensional arrangement of Zn2 paddlewheel secondary building units and highlighting the hollow columns along the 4-fold axes. Additional views of the 3D packing are provided in the Supporting Information.
Crystals 16 00290 g002
Crystals 16 00290 g003
Figure 3. ORTEP representation (50% probability ellipsoids) of the Zn–based secondary building unit in the tetragonal Zn–BTC framework of 2, showing the trinuclear unit through one crystallographic mirror plane and highlighting the coexistence of one octahedral and two equivalent tetrahedral Zn(II) coordination environments (1:2 ratio). Atom labels are omitted for clarity; full labeling is reported in the representation of the asymmetric unit in Figure S3 of Supporting Information.
Figure 3. ORTEP representation (50% probability ellipsoids) of the Zn–based secondary building unit in the tetragonal Zn–BTC framework of 2, showing the trinuclear unit through one crystallographic mirror plane and highlighting the coexistence of one octahedral and two equivalent tetrahedral Zn(II) coordination environments (1:2 ratio). Atom labels are omitted for clarity; full labeling is reported in the representation of the asymmetric unit in Figure S3 of Supporting Information.
Crystals 16 00290 g003
Crystals 16 00290 g004
Figure 4. Packing view of the tetragonal Zn–BTC framework of 2 along the crystallographic c axis, showing the three-dimensional network formed by mixed-coordination Zn–based secondary building units and highlighting the hollow columns along the 4-fold axes. Additional views of the 3D packing are provided in the Supporting Information.
Figure 4. Packing view of the tetragonal Zn–BTC framework of 2 along the crystallographic c axis, showing the three-dimensional network formed by mixed-coordination Zn–based secondary building units and highlighting the hollow columns along the 4-fold axes. Additional views of the 3D packing are provided in the Supporting Information.
Crystals 16 00290 g004
Table 1. Comparison of crystallographic and structural features of the two Zn–BTC frameworks.
Table 1. Comparison of crystallographic and structural features of the two Zn–BTC frameworks.
Feature12
CCDC code25399252539926
Crystal systemCubicTetragonal
Space groupFm 3 ¯ mI4cm
Unit cell parameters (Å)a = b = c = 26.582(2)a = b = 20.586(4); c = 17.797(5)
Unit cell volume (Å3)18782(5)7542(3)
Molecular unit (ASU + solvent)C6H4O5ZnC21H13NO14Zn3
Z488
Crystal model quality
Final R indexes [I ≥ 2σ (I)]		R1 = 0.0382, wR2 = 0.0986
GooF (F2) = 0.956		R1 = 0.0533, wR2 = 0.1526
GooF (F2) = 1.063
Crystallographically independent Zn sites12
Zn coordination environmentsEquivalent Zn(II) centersNon-equivalent Zn(II) centers
Zn coordination number(s)5 (4 equatorial + 1 axial)4 (tetrahedral) and 6 (octahedral)
Zn–Zn interaction (Å)2.968(1)Not observed
Secondary building unit (SBU)Binuclear Zn2(O2CR)4Mixed-coordination Zn–based SBU
SBU nuclearityBinuclearPolynuclear/extended
Carboxylate coordination modeμ2-bridging (equatorial)Bridging and chelating modes
Axial/terminal ligandsCoordinated solvent (H2O)Coordinated solvent (DMF)
Role of solventAxial ligand stabilizing Zn2 SBUCoordination and stabilization of mixed Zn environments
Framework dimensionality3D3D
Overall framework symmetryHighLower
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Bella, G.; Nicolò, F.; Bruno, G.; Assel, A.; Santoro, A. Coordination Flexibility of Zn(II) in Trimesate-Based Metal–Organic Frameworks: Formation of Distinct Secondary Building Units. Crystals 2026, 16, 290. https://doi.org/10.3390/cryst16050290

AMA Style

Bella G, Nicolò F, Bruno G, Assel A, Santoro A. Coordination Flexibility of Zn(II) in Trimesate-Based Metal–Organic Frameworks: Formation of Distinct Secondary Building Units. Crystals. 2026; 16(5):290. https://doi.org/10.3390/cryst16050290

Chicago/Turabian Style

Bella, Giovanni, Francesco Nicolò, Giuseppe Bruno, Amine Assel, and Antonio Santoro. 2026. "Coordination Flexibility of Zn(II) in Trimesate-Based Metal–Organic Frameworks: Formation of Distinct Secondary Building Units" Crystals 16, no. 5: 290. https://doi.org/10.3390/cryst16050290

APA Style

Bella, G., Nicolò, F., Bruno, G., Assel, A., & Santoro, A. (2026). Coordination Flexibility of Zn(II) in Trimesate-Based Metal–Organic Frameworks: Formation of Distinct Secondary Building Units. Crystals, 16(5), 290. https://doi.org/10.3390/cryst16050290

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