Variable Dimensionality of Europium(III) and Terbium(III) Coordination Compounds with a Flexible Hexacarboxylate Ligand

A reaction between 4,4′,4″-(benzene-1,3,5-triyltris(oxy))triphthalic acid (H6L) and lanthanide(III) nitrates (Ln = Eu3+, Tb3+) in water under the same conditions gave a molecular coordination compound [Tb(H4.5L)2(H2O)5]∙6H2O in the case of terbium(III) and a one-dimensional linear coordination polymer {[Eu2(H3L)2(H2O)6]∙8H2O}n in the case of europium(III). The crystal structures of both compounds were established by single-crystal X-ray diffraction, and they were further characterized by powder X-ray diffraction, thermogravimetric analysis and infrared spectroscopy. The compounds demonstrated characteristic lanthanide-centered photoluminescence. The lanthanide-dependent dimensionality of the synthesized compounds, which are the first examples of the coordination compounds of hexacarboxylic acid H6L demonstrates its potential as a linker for new coordination polymers.


Introduction
Aromatic polycarboxylic acids are important building blocks for the construction of metal-organic frameworks (MOFs), since the carboxylate groups are able to form strong coordination bonds with most metal cations [1][2][3][4]. Rigid di-, tri-and tetracarboxylic acids were successfully used for the preparation of robust MOFs [5][6][7]. In recent years, an increasing amount of attention has been paid to flexible MOFs, which are able to expand or contract their networks upon external stimuli, such as temperature, pressure or electromagnetic irradiation [8][9][10][11][12][13]. The flexibility of MOFs can be induced by the conformational flexibility of their organic linkers; the latter may be achieved by introducing ether groups into their structures [14][15][16][17]. Increasing the number of the carboxylic groups in MOF linkers may leave some of them uncoordinated by the metal nodes and improve the functional properties of the resulting MOF, such as proton conductivity, adsorption selectivity for gases from their mixtures or metal ions from solutions [18].
ligands are especially suitable for the formation of lanthanide coordination polymers because of their ability to form strong metal-oxygen bonds and saturate the high lanthanide coordination numbers [31,32]. Scheme 1. Synthesis of Tb(III) and Eu(III) coordination compounds 1 and 2.

Synthesis of the Coordination Compounds
Terbium(III) and europium(III) coordination compounds were prepared as single crystals by the reaction between H6L and metal nitrate hexahydrates (1:1 molar ratio) in an aqueous solution under hydrothermal conditions (Scheme 1). It was found that specific pH conditions are necessary for the formation of the products. Thus, 3 equivalents (relative to the amount of the metal salt) of the base (KOH) were necessary to carry out the reaction at 90 °C, followed by neutralization with HNO3 (2.5 equivalents) and prolonged crystallization at room temperature. Numerous attempts to carry out the reaction under other conditions (no acid of base added, only acid or base added, variable temperature) did not result in the formation of any solid product. When the amounts of acid or base used were different from the ones indicated above, only amorphous solids were obtained that were not further characterized. It is interesting to note that under identical synthetic conditions, Tb 3+ formed a molecular complex with a 1:2 M:L ratio, while Eu 3+ gave a linear coordination polymer of a 1:1 M:L composition. Other M:L ratios from 6:1 to 1:6 were tested as well, but they gave no solid products or only amorphous precipitates. Scheme 1. Synthesis of Tb(III) and Eu(III) coordination compounds 1 and 2.

Synthesis of the Coordination Compounds
Terbium(III) and europium(III) coordination compounds were prepared as single crystals by the reaction between H 6 L and metal nitrate hexahydrates (1:1 molar ratio) in an aqueous solution under hydrothermal conditions (Scheme 1). It was found that specific pH conditions are necessary for the formation of the products. Thus, 3 equivalents (relative to the amount of the metal salt) of the base (KOH) were necessary to carry out the reaction at 90 • C, followed by neutralization with HNO 3 (2.5 equivalents) and prolonged crystallization at room temperature. Numerous attempts to carry out the reaction under other conditions (no acid of base added, only acid or base added, variable temperature) did not result in the formation of any solid product. When the amounts of acid or base used were different from the ones indicated above, only amorphous solids were obtained that were not further characterized. It is interesting to note that under identical synthetic conditions, Tb 3+ formed a molecular complex with a 1:2 M:L ratio, while Eu 3+ gave a linear coordination polymer of a 1:1 M:L composition. Other M:L ratios from 6:1 to 1:6 were tested as well, but they gave no solid products or only amorphous precipitates.

Crystal Structure of the Coordination Compounds
Single-crystal X-ray diffraction analysis revealed that compound 1 crystallizes in a monoclinic crystal system, space group C2/c. The compound is a molecular complex, consisting of one Tb 3+ ion, five coordinated water molecules, two crystallographically equivalent ligand molecules and six lattice water molecules (Figure 1a). Each Tb 3+ ion coordinates nine oxygen atoms, four of which are from the carboxylic groups of two organic ligands and the remaining five are from the coordinated water molecules. According to the Shape 2.1 software package [33], the coordination environment is best described by a muffin configuration (MFF) [34] (Figure 1b, Table S1). The Tb-O distances are 2.397(2) and 2.547(2) Å for the carboxylate ligand and vary from 2.330(2) Å to 2.418(2) Å for the coordinated water molecules, which are typical values for such types of coordination. The proton of the carboxylic group at the ortho-position to the coordinated carboxylate group participates in an intramolecular hydrogen bond (d(D···A) = 2.394(3) Å, d(H···A) = 1.25(4) Å, d(D-H) = 1.14(4) Å, ∠(D-H···A) = 174(3) • ) and is disordered over two equivalent positions in two anionic ligands ( Figure S1). To achieve electroneutrality, the terbium(III) coordination sphere should therefore be composed as [Tb(H 4.5 L) 2 (H 2 O) 5 ]. The benzene rings in compound 1 participate in intermolecular π-π stacking interactions with the centroid-tocentroid separation of 3.791 Å (the angle between the benzene ring planes is 10.9 • , Figure  S2), and the water molecules are involved in multipoint hydrogen bonds (Table S2), which connect the molecules of compound 1 into a three-dimensional supramolecular framework ( Figure 1c).

Crystal Structure of the Coordination Compounds
Single-crystal X-ray diffraction analysis revealed that compound 1 crystallizes in a monoclinic crystal system, space group C2/c. The compound is a molecular complex, consisting of one Tb 3+ ion, five coordinated water molecules, two crystallographically equivalent ligand molecules and six lattice water molecules (Figure 1a). Each Tb 3+ ion coordinates nine oxygen atoms, four of which are from the carboxylic groups of two organic ligands and the remaining five are from the coordinated water molecules. According to the Shape 2.1 software package [33], the coordination environment is best described by a muffin configuration (MFF) [34] (Figure 1b, Table S1). The Tb-O distances are 2.397(2) and 2.547(2) Å for the carboxylate ligand and vary from 2.330(2) Å to 2.418(2) Å for the coordinated water molecules, which are typical values for such types of coordination. The proton of the carboxylic group at the ortho-position to the coordinated carboxylate group participates in an intramolecular hydrogen bond (d(D  Figure S1). To achieve electroneutrality, the terbium(III) coordination sphere should therefore be composed as [Tb(H4.5L)2(H2O)5]. The benzene rings in compound 1 participate in intermolecular π-π stacking interactions with the centroidto-centroid separation of 3.791 Å (the angle between the benzene ring planes is 10.9°, Figure S2), and the water molecules are involved in multipoint hydrogen bonds (Table S2), which connect the molecules of compound 1 into a three-dimensional supramolecular framework (Figure 1c). Although the Eu-compound 2 was synthesized by the same method as the Tb-compound 1, the single crystal X-ray diffraction analysis of compound 2 reveals a completely different crystal structure. Compound 2 crystallizes in a triclinic crystal system with space Although the Eu-compound 2 was synthesized by the same method as the Tb-compound 1, the single crystal X-ray diffraction analysis of compound 2 reveals a completely different crystal structure. Compound 2 crystallizes in a triclinic crystal system with space group P-1; the asymmetric unit contains one Eu 3+ ion, one H 3 L 3ligand, three coordinated water molecules and four lattice water molecules (Figure 2a). Similarly to compound 1, the coordination polyhedron of Eu 3+ is close to the muffin shape (Table S1), but in contrast to the Tb-compound 1, two Eu 3+ cations are joined by the bridging carboxylate groups into binuclear 8-connected [Eu 2 ] secondary building units (Figure 2b), which are connected by two H 3 L 3linkers into 1D infinite chains parallel to the ac-plane (Figure 2c). The Eu-O distances for the coordinated carboxylate groups are 2.385(2) Å and 2.526(2) Å, typical for europium(III) carboxylate compounds. Further, a 3D supramolecular structure is formed between one-dimensional chains through the hydrogen bonding interactions, involving the coordinated carboxylate and the uncoordinated carboxylic groups (Table S3,  Figures 2d and S3). group P-1; the asymmetric unit contains one Eu ion, one H3L ligand, three coordinated water molecules and four lattice water molecules (Figure 2a). Similarly to compound 1, the coordination polyhedron of Eu 3+ is close to the muffin shape (Table S1), but in contrast to the Tb-compound 1, two Eu 3+ cations are joined by the bridging carboxylate groups into binuclear 8-connected [Eu2] secondary building units (Figure 2b), which are connected by two H3L 3-linkers into 1D infinite chains parallel to the ac-plane (Figure 2c). The Eu-O distances for the coordinated carboxylate groups are 2.385(2) Å and 2.526(2) Å, typical for europium(III) carboxylate compounds. Further, a 3D supramolecular structure is formed between one-dimensional chains through the hydrogen bonding interactions, involving the coordinated carboxylate and the uncoordinated carboxylic groups (Table S3, Figures  2d and S3). Different coordination behaviors of Tb 3+ and Eu 3+ towards H 6 L ligand under the same conditions may be attributed to the known lanthanide contraction effect. Despite a weak monotonic change of the ionic radii in the lanthanide series, for a certain Ln 3+ ion, a structure may cease to be stable due to the increased steric hindrance in the ever-shrinking coordination sphere of the metal ion. This can lead to a change in the ligand connectivity, including a change in dimensionality and topology. Gadolinium, which stands between the europium and terbium in the lanthanide series, often appears to be a breaking point in such alternations and one product is formed for lanthanides lighter than Gd and a different product is obtained for heavier lanthanides [35][36][37].
The conformational flexibility of the potentially hexacarboxylate ligand H 6 L is evident from the variability of the dihedral angles between the benzene rings connected by ether bonds in Tb-compound 1 and Eu-compound 2 from 61.5 • to 88.8 • (Figure 3). Different coordination behaviors of Tb 3+ and Eu 3+ towards H6L ligand under the same conditions may be attributed to the known lanthanide contraction effect. Despite a weak monotonic change of the ionic radii in the lanthanide series, for a certain Ln 3+ ion, a structure may cease to be stable due to the increased steric hindrance in the ever-shrinking coordination sphere of the metal ion. This can lead to a change in the ligand connectivity, including a change in dimensionality and topology. Gadolinium, which stands between the europium and terbium in the lanthanide series, often appears to be a breaking point in such alternations and one product is formed for lanthanides lighter than Gd and a different product is obtained for heavier lanthanides [35][36][37].
The conformational flexibility of the potentially hexacarboxylate ligand H6L is evident from the variability of the dihedral angles between the benzene rings connected by ether bonds in Tb-compound 1 and Eu-compound 2 from 61.5° to 88.8° (Figure 3).

X-ray Powder Diffraction, IR Spectroscopy and Thermogravimetric Analysys
The phase purity of compounds 1 and 2 was confirmed by powder X-ray diffraction. As shown in Figure S4, the experimental diffractograms recorded at room temperature and the simulated patterns obtained from single crystal data are in good agreement.
The IR spectra of the H6L ligand, Tb-compound 1 and Eu-compound 2 are shown in Figure S5. The IR spectrum of Tb-compound 1 features an absorption peak near 3037 cm −1 , which is attributed to the C-H stretching vibrations of the aromatic ring. The benzene ring vibration bands were observed at 1582 cm −1 and 1549 cm −1 . In addition, the spectrum of

X-ray Powder Diffraction, IR Spectroscopy and Thermogravimetric Analysys
The phase purity of compounds 1 and 2 was confirmed by powder X-ray diffraction. As shown in Figure S4, the experimental diffractograms recorded at room temperature and the simulated patterns obtained from single crystal data are in good agreement.
The IR spectra of the H 6 L ligand, Tb-compound 1 and Eu-compound 2 are shown in Figure S5. The IR spectrum of Tb-compound 1 features an absorption peak near 3037 cm −1 , which is attributed to the C-H stretching vibrations of the aromatic ring. The benzene ring vibration bands were observed at 1582 cm −1 and 1549 cm −1 . In addition, the spectrum of Tb-compound 1 demonstrates a wide O-H stretching vibration peak near 3390 cm −1 from water molecules and protonated carboxylate groups involved in hydrogen bonding. The TG curve of Tb-compound 1 exhibited a weight loss of 14.3%, which occurred in two steps within 30-200 • C ( Figure S6). The first step in the range of 30-150 • C is due to the removal of free water molecules (found: 7.4%, calc.: 6.8%), and the second step at 150-200 • C is due to the removal of coordinated water molecules (found: 6.9%, calc.: 5.7%). After dehydration, Tb-compound 1 remains stable up to 360 • C, indicative of good thermal stability.

Luminescent Properties of the Ligand and the Coordinaion Compounds
The solid-state luminescent spectra of H 6 L ligand, Tb-compound 1 and Eu-compound 2 were measured for the powdered samples at room temperature. As shown in Figure S7, the H 6 L ligand showed a broad emission peak with a maximum of 456 nm (λ ex = 370 nm). The excitation spectra of both Tb-compound 1 and Eu-compound 2 showed broad bands with maxima near 300 nm ( Figure S8), suggesting ligand-centered absorption. When excited at 300 nm, the Tb-compound 1 exhibited the following four characteristic emission peaks from the Tb 3+ ion: 490, 545, 584 and 622 nm, attributed to 5 D 4 → 7 F 6 , 5 D 4 → 7 F 5, 5 D 4 → 7 F 4 , 5 D 4 → 7 F 3 transitions [38], respectively ( Figure 4a). Likewise, upon excitation at 310 nm, the Eu-compound 2 exhibited the following four characteristic emission peaks from the Eu 3+ ion: 593, 615, 649 and 694 nm, attributed to 5 D 0 → 7 F 1 , 5 D 0 → 7 F 2 , 5 D 0 → 7 F 3 , 5 D 0 → 7 F 4 transitions [39], respectively (Figure 4b). The luminescence lifetime of the Tb-compound 1 obeys a single exponential equation (characteristic lifetime of 0.68 ms), which indicates that a single coordination environment exists for Tb 3+ ion (Figure 4c). The luminescence lifetime of the Eu-compound 2 was 0.28 ms (Figure 4c). The quantum yields of the Tb-compound 1 and the Eu-compound 2 were 8% and 2%, respectively. Relatively low quantum yields may be due to the conformational flexibility of the ligand, leading to vibrational non-radiative energy dissipation, often observed for lanthanide coordination polymers with flexible ligands [40,41]. In addition, the presence of five or three coordinated water molecules in the lanthanide coordination sphere in compounds 1 and 2 also leads to deactivation through O-H vibrations [42].
As shown in Figure 4d, the emission of compounds 1 and 2 is characterized by the chromaticity coordinates (0.3318, 0.5769) and (0.6425, 0.3464). The color temperature of the green emission of Tb-compound 1 was 5600 K, and the red emission of the Eu-compound 2 had a color temperature of 8870 K, both of which correspond to the cool colors (>5000 K are called cool colors).
In order to gain insight into the photoluminescence mechanism, TD-DFT calculations for the H 6 L ligand and its triply deprotonated form (as lithium salt, Li 3 H 3 L) were carried out. In the optimized structure of the Li 3 H 3 L model, the dihedral angles corresponding to the rotation of the phthalate rings are in good agreement with the values obtained from the X-ray crystal structure of compound 2, suggesting that the predicted conformation of the anionic ligand is approximately the same as in the structure of compound 2 ( Figure S9, Table S4). Therefore, the obtained geometry of Li 3 H 3 L may be used for further calculations of the photophysical properties. According to TD-DFT calculations, the UV-Vis absorption of H 6 L is associated with the S 0 →S 1 excitation; the calculated maximum is 302 nm. The S 0 →S 1 excitation is accompanied by the following three major electron transitions: HOMO→LUMO (contribution 49%), HOMO-1→LUMO (contribution 41%), HOMO-2→LUMO (contribution 10%). As one can see from the localization of the molecular orbitals, the S 0 →S 1 excitation is a π→π* transition with the charge transfer between the aromatic rings of H 6 L ( Figure S10).  As shown in Figure 4d, the emission of compounds 1 and 2 is characterized by the chromaticity coordinates (0.3318, 0.5769) and (0.6425, 0.3464). The color temperature of the green emission of Tb-compound 1 was 5600 K, and the red emission of the Eu-compound 2 had a color temperature of 8870 K, both of which correspond to the cool colors (>5000 K are called cool colors).
In order to gain insight into the photoluminescence mechanism, TD-DFT calculations for the H6L ligand and its triply deprotonated form (as lithium salt, Li3H3L) were carried out. In the optimized structure of the Li3H3L model, the dihedral angles corresponding to the rotation of the phthalate rings are in good agreement with the values obtained from the X-ray crystal structure of compound 2, suggesting that the predicted conformation of the anionic ligand is approximately the same as in the structure of compound 2 ( Figure  S9, Table S4). Therefore, the obtained geometry of Li3H3L may be used for further calculations of the photophysical properties. According to TD-DFT calculations, the UV-Vis absorption of H6L is associated with the S0→S1 excitation; the calculated maximum is 302 nm. The S0→S1 excitation is accompanied by the following three major electron transitions: HOMO→LUMO (contribution 49%), HOMO-1→LUMO (contribution 41%), HOMO-2→LUMO (contribution 10%). As one can see from the localization of the molecular The calculated position of the absorption maximum of Li 3 H 3 L is 316 nm, which is in reasonable agreement with the experimentally observed value of 308 nm for compound 2. The S 0 →S 1 excitation is characterized by the following two major electronic transitions: HOMO→LUMO (68%) and HOMO-1→LUMO (32%). The charge transfer accompanying the π→π* excitation in Li 3 H 3 L is even more pronounced compared to the protonated ligand H 6 L ( Figure 5). aromatic rings of H6L ( Figure S10).
The calculated position of the absorption maximum of Li3H3L is 316 nm, which is in reasonable agreement with the experimentally observed value of 308 nm for compound 2. The S0→S1 excitation is characterized by the following two major electronic transitions: HOMO→LUMO (68%) and HOMO-1→LUMO (32%). The charge transfer accompanying the π→π* excitation in Li3H3L is even more pronounced compared to the protonated ligand H6L ( Figure 5). Relatively long luminescence lifetimes of the coordination compounds 1 and 2 suggest an emission due to f-f lanthanide transitions. At the same time, the broad absorption bands near 310 nm suggest a ligand-centered excitation; therefore, an intersystem crossing S1-T1 process must be assumed, followed by an energy transfer from the T1 state to 5 D0 or 5 D4 states of Eu 3+ or Tb 3+ ions. A comparison of the energies of these states indicates that in both cases, such transitions are energetically favorable ( Figure 6). Relatively long luminescence lifetimes of the coordination compounds 1 and 2 suggest an emission due to f-f lanthanide transitions. At the same time, the broad absorption bands near 310 nm suggest a ligand-centered excitation; therefore, an intersystem crossing S 1 -T 1 process must be assumed, followed by an energy transfer from the T 1 state to 5 D 0 or 5 D 4 states of Eu 3+ or Tb 3+ ions. A comparison of the energies of these states indicates that in both cases, such transitions are energetically favorable ( Figure 6).

Starting Materials and Synthetic Procedures
All reagents were commercially available and used without further purification. The

Single-Crystal X-ray Diffraction
Diffraction data for single crystals compounds 1 and 2 were collected with a Bruker D8 Venture diffractometer with a CMOS PHOTON III detector and IµS 3.0 source (mirror optics, λ(CuKα) = 1.54178 Å). The ϕ-and ω-scanning techniques were employed to measure the intensities. The crystal structures were solved and refined by means of the SHELXT [43] and SHELXL [44] programs using OLEX2 GUI [45]. Atomic displacement parameters for nonhydrogen atoms were refined anisotropically. Hydrogen atoms were placed geometrically and refined in the riding model. The crystallographic parameters and the details of the diffraction experiment are given in Table 1. The bond lengths and bond angles for the Tb-compound 1 and Eu-compound 2 are provided in Tables S5 and S6.

Computational Chemistry Details
The calculations were performed using Gaussian 09 package [46]. The isolated H 6 L molecule was used to model the free ligand, while a triply deprotonated form (balanced by three lithium cations, Li 3 H 3 L) was used to represent the anionic ligand in compound 2. Singlet ground state geometry optimizations of H 6 L were carried out in the gas phase at the DFT level of theory employing the three-parameter hybrid B3LYP functional [47][48][49][50] and 6-31 + G(d) basis set [51][52][53][54]. An empirical dispersion correction was applied using the D3 version of Grimme's empirical dispersion with Becke-Johnson damping [55]. The frequency calculations in a harmonic approximation were performed for the optimized geometries in order to establish the nature of the stationary points, lack of imaginary vibration modes for the optimized structures indicates that the stationary points found corresponded to minima on the potential energy surface. The first singlet and triplet exited states of H 6 L and Li 3 H 3 L were computed at time-dependent DFT (TD-DFT) level, using the optimized ground state geometry and the same functional and basis set used for the ground state calculations.

Conclusions
In summary, the coordination chemistry of a flexible aromatic triether-bridged hexacarboxylate ligand (4,4 ,4 -(benzene-1,3,5-triyltris(oxy))triphthalic acid) was studied for the first time, and two new lanthanide coordination compounds were prepared and characterized. It was found that the nature of the lanthanide affects the dimensionality of the compounds formed. Thus, in identical reaction conditions, Tb 3+ forms a discrete coordination compound, while Eu 3+ yields a 1D coordination polymer. Both compounds demonstrated characteristic lanthanide-centered emission and ligand-centered excitation, in accordance with the experimental absorption spectra and TD-DFT calculations. Therefore, the aromatic ligand acts as an antenna for lanthanide excitation and further studies of its coordination chemistry may lead to the preparation of efficient light emitters. It should also be noted that the observed difference in the reactivity of Eu 3+ and Tb 3+ may contribute to solving the problem of lanthanide separation. carboxylic groups; Figure S2. Fragment of the crystal structure of compound 1 showing π-π interaction between the benzene rings of two molecules (green dashed line); Figure S3. Fragment of the crystal structure of compound 2 showing hydrogen bonds between the coordinated carboxylate and uncoordinated carboxylic groups in two different coordination polymer chains; Figure S4. Calculated and experimental PXRD patterns of the compounds 1 (a) and 2 (b); Figure S5. FT-IR spectra of H 6 L ligand, compound 1 and compound 2; Figure S6. Thermogravimetric analysis curve for the compound 1; Figure S7. The solid-state excitation (λ em = 450 nm) and emission (λ ex = 370 nm) spectra of H 6 L at room temperature; Figure S8. The solid-state excitation spectra of the compounds 1 (λ em = 545 nm) and 2 (λ em = 615 nm) at room temperature; Figure S9. Optimized geometry of H6L obtained at B3LYP[GD3BJ] 6-31 + G(d) level of theory; Figure S10. Isosurfaces (at 0.02 e/Bohr 3 ) of the molecular orbitals of H 6 L ground state calculated at B3LYP[GD3BJ] 6-31 + G(d) level of theory; Table S1. Continuous shape measures criteria (S) for nine-coordinated metal centers in the compounds 1 and 2; Table S2. Geometrical parameters of the hydrogen bonds in compound 1; Table S3. Geometrical parameters of the hydrogen bonds in compound 2; Table S4. Calculated and experimental geometrical parameters of H 3 L 3-; Table S5. Selected bond distances (d) and angles (ϕ) for compound 1; Table S6. Selected bond distances (d) and angles (ϕ) for the compound 1; CIF files and CheckCIF reports for the compounds 1 and 2.