Lanthanide-Aromatic Iminodiacetate Frameworks with Helical Tubes: Structure, Properties, and Low-Temperature Heat Capacity.

A series of lanthanide coordination polymers [LnL(H2O)2] n [Ln = Pr (1), Nd (2), Sm (3), Eu (4), and Gd (5), H3L = N-(4-carboxy-benzyl)iminodiacetic acid] was hydrothermally prepared and structurally characterized. All the five compounds have been confirmed as 3D Ln-CPs with one-dimensional helical tunnels composed of four helical chains, although there are different coordination geometries around Ln3+. Enantiomeric helixes in 1-3, and absolute left-handed and right-handed helical chains in 4 and 5, respectively, lead to different tunnel spaces. Their conformations can also be featured by different space groups and unit cell dimensions. Photoluminescence measurement on 3 and 4 show characteristic emission peaks of Sm3+ and Eu3+ ions, respectively. The low-temperature heat capacity of 1-4 has been investigated in the temperature range of 1.9-300 K. Their heat capacity values are nearly equal below 10 K and display a crossover with the value order C p,m(2) > C p,m(1) ≈ C p,m(4) > C p,m(3) above 10 K. The measured heat capacities have been fitted, and the corresponding thermodynamic functions were consequently calculated based on the fitting parameters. The standard molar entropies at 298.15 K have been determined to be (415.71 ± 4.16), (451.32 ± 4.51), (308.53 ± 3.09), and (407.62 ± 4.08) J·mol-1·K-1 for 1, 2, 3, and 4, respectively.

Introduction

For decades, lanthanide-based coordination polymers (Ln-CPs) as a new type of organic–inorganic hybrid porous materials have attracted a great deal of research interest. Due to their functional properties, versatile applications in different fields have been explored such as gas adsorption and separation,[1−4] magnetic materials,[5−8] molecular recognition[9−14] and detection, dye enrichment and separation,[15,16] and so on. Among them, lanthanide complexes with luminescent properties have been of substantial importance in many applications such as biological imaging, sensing, analysis optical data transfer, and lighting devices.[17−34] Although there is a possibility of the luminescence of Ln3+ from intraconfigurational f–f transitions, in the case of irradiation with ultraviolet (UV) radiation, some luminescence in the visible or near-infrared spectral regions can be observed. Such emissions are weak owing to the low absorption of the forbidden f–f transitions. Therefore, some strategies need to be carried out in the process of preparing luminescent Ln3+ materials in order to enhance emissive efficiency and decrease the internal nonradiative decay processes. As an important method, appropriate ligands with π-conjugated organic aromatic systems tend to be selected to play a role of “antenna effect”. These ligands, such as 1,10-phenanthroline (phen), β-diketones, aromatic carboxylic acid, and so forth, have been proved to be effective in pumping up Ln3+ emission and optimize luminescence of complexes.[35−41]

The design and synthesis of new types of Ln-CP-functional materials with intriguing structures are as important as the measurement of their physical properties. Helical structures are among the most explored and best investigated supramolecular architectures. Many single-, double-, meso-, and multihelical complexes generated by self-assembly processes of coordination or supramolecular interactions have been reported.[42−44] What is fascinating about them is not only enchanting motifs but also broad applications in the fields of chiral synthesis, optical devices, sensory functions, and drug delivery.[45−53] However, 3D helical frameworks with helical tunnels (not straight channel formed by helical chains) are not common to our knowledge.

As a useful tool of thermodynamic methods, heat capacity measurement has been utilized to investigate thermal properties of materials at low temperature. From heat capacity, researchers can derive thermodynamic functions of materials, such as entropy, enthalpy, and Gibbs free energy, and further investigate and understand lattice vibrations, metals, superconductivity, electronic and nuclear magnetism, dilute magnetic systems, and structural transitions.[54−58]

As an effective approach, organic compounds with amine appending tend to be used for improving the adsorption properties of metal–organic frameworks,[59−64] and some flexible ligand containing amino derivatives have been synthesized and used to prepare and stabilize some complexes due to their special coordination characters as tripodal ligands.[65−67] In the present work, as a flexible ligand with aromatic carboxylic acid, N-(4-carboxy-benzyl)iminodiacetic acid (Scheme ), named H3L, has been used to synthesize some Ln-Cps. A series of coordination polymers with the formula [LnL(H2O)2] have been synthesized (Scheme ) and structurally characterized. Their fluorescent properties, magnetic properties, and thermal stability have also been studied. Furthermore, the heat capacities of 1–4 have been measured over the temperature range (from 1.9 to 300 K) using a PPMS calorimeter. The experimental data have been fitted to a series of theoretical functions, and the corresponding thermodynamic functions have been calculated based on these fits.

Scheme 1

Synthetic Route and Coordination Mode of 1–5

Results and Discussion

Crystal Structure Descriptions

Structure of 1–3

1, 2, and 3 are isomorphous and crystallized in the monoclinic space P2(1)/n. Therefore, the molecular structure of 3 is described in detail. As shown in Figure a, the asymmetric unit [SmL(H2O)2] is composed of one Sm3+ ion, one L3– ligand, and two H2O molecules. The Sm3+ ion is surrounded by an environment of NO8. Six oxygen atoms are supported by three carboxylate groups of different L3– ligands. The rest two O atoms come from coordinated H2O molecules. All the Sm–O bond lengths fall in the range of 2.42–2.67 Å. The N atom is provided by the L3– ligand with a distance of 2.727(6) Å to Sm3+. All Sm–N/O distances are in agreement with literature data.[68,69] As illustrated in Figure , the coordinated NO8 around Sm3+ forms a distorted single-capped antitetraprism. In the process of binding with Sm3+, three carboxylic groups of the deprotonated ligand L3– show two kinds of coordination modes: syn-anti 211 (acetic group) and common 111 (carboxylic group).[70] Neighboring Sm3+ ions are linked by two acetic groups via syn-anti 211 in the ac plane, resulting in a perfect 2D planar with a 4.4 topology and Sm...Sm distances of 6.18 and 6.05 Å. Furthermore, the 4.4 2D networks (Figure b) extend to become a twisted 3D framework by η-L3– (Figure c,d). The Sm...Sm distance bridged by ligands is 12.03 Å. It is worth noting that the 3D framework constructed by L3– and Ln3+ is different from the known one-dimensional (1D) structures,[71] in which partially deprotonated H3L coordinated with Ln3+ and formed 1D linear arrangement. In our research, three complexes exhibit a Ln-CP framework with some tunnel vacancies, whose sizes (regardless of the van der Waals radius) have been listed in Table S1.

Figure 1

Coordination environment of Sm1 (a), 2D layer structure (b), 3D framework along the a direction (c), and 3D topology (d) (−COO bridges and the backbone of L3– are replaced by red and blue lines, respectively) in 3.

Figure 2

Coordination geometries around Ln3+ in 1–5.

Coordination geometries around n class="Chemical">Ln3+ in 1–5.

To our surprise, an interesting 1D tunnel can be observed along the b direction, as illustrated in Figure a,b,f. They are not linear or Z-shaped tunnels but helical tunnels. Each tunnel was constructed by four helical chains, as illustrated in Figure f, in which two are left-handed helical chains (Figure c) and the other two are right-handed helical chains. Each couple of helical chains was bridged by −COO groups and formed a spiral belt (Figure d). Further carboxylic linking between two spiral belts eventually led to a helical tunnel with enantiomeric conformation (Figure e) in 3. For this, the tunnels along the b direction become inhomogeneous. As for 1, the structure and framework are similar to those of 3, except for the three-cap triangle prism geometry (Figure ) around Pr3+ ions. All Pr–O/N lengths in the range of 2.449–2.530 Å together with longer Pr1-O1#1 [2.693(4) Å] and Pr1-N1 [2.745(5) Å] are normal compared with some reported Pr3+ compounds.[72] Other relevant data about 1–3 are listed in Tables S1 and S2.

Figure 3

(a) 3D topological framework of 3. (b) and (f) 1D helical tunnel along the b direction. (c) 1D left-handed helical chain formed by Sm3+ and backbones of L3–. (d) 1D helical belt formed by the double left-handed helical chain. (e) 1D helical tunnel formed by two enantiomers.

Structure of 4

4 also crystallized in a monoclinic system but has a P2(1) space group. The structure of the asymmetric unit [EuL(H2O)2] is similar to that of 1 except for a different three-cap triangle prism geometry around Eu3+. The Eu–O distances are in the range of 2.39–2.486 Å along with two longer bonds Eu1-O2#1 [2.630(15) Å] and Eu1-N1 [2.73(2) Å], which constructed a distorted triangle prism with edge lengths of 2.213, 3.477, and 3.501 Å. Due to flexible single bond of −CH2–N, the phenyl ring can rotate to some extent, which may lead to different molecule conformations. Such an interesting phenomenon just exists in 4. The framework (Figure a) is inconsistent with 3 (Figure c), although its topology structure (Figure b), also with a1D helical tunnel, is similar to that of 3. Further study revealed that all the four chains in the 1D tunnel in the b direction are left-handed helices (Figure c). Two helical chains were bridged by −COO groups, leading to a left-handed helical belt like that in 3. Then, still linked by carboxylic bridges, two left helical belts formed a helical tunnel.

Figure 4

3D framework (a) and (d), topology structure (b) and (e), and 1D helical tunnel (c) and (f) for 4 and 5, respectively.

Structure of 5

5 crystallized in monoclinic system with a P2(1) space group. The asymmetric fragment [GdL(H2O)2] is similar to that of 3 with a single-capped antitetraprism environment around Gd3+. Gd–O/N distances are 2.335–2.467 Å besides long Gd1-O2 [2.74(3) Å] and Gd1-N1 [2.740(14) Å], all of which lie in the normal Gd–O/N bond length range.[72] Obviously, it is a distorted antitetraprism. The spatial structure of 5 is similar to that of 4, as seen in Figure d,e. While, the difference can be found out whether its 1D chain, belt, or tunnel form is a right-handed helix (Figure f).

The relevant intermetallic distance and angles can be found in Table S1.

Powder X-ray Diffraction Patterns and Thermogravimetric Analysis Data

To confirm the phase purity, powder X-ray diffraction (XRD) on crystal samples 1–5 was carried out in the 2θ range of 5–60° at room temperature (Figure S1). The experimental XRD patterns were compared to their corresponding simulated patterns calculated based on single crystal diffraction data. The result indicates that all of the samples are in the pure phase.

The thermogravimetric analysis (TGA) data in the temperature range of 30–700 °C for 3 and 5 and 20–800 °C for 1, 2, and 4 are shown in Figures S2 and S3. Upon heating, samples 1 and 4 lost their weight at approximately 100 °C and then showed three and two steps of weight loss above this temperature, respectively. For 1, two-step weight loss in the ranges of 100–213 and 213–353 °C occurred with values of 4.73 and 8.4%, respectively, consistent with the weight of one coordinated H2O (calcd 4.27%) and one −COO (10.45%). While the loss of H2O and −COO in 4 are consecutive with a sum value of 13.6% (calcd 14.34%) in the range of 100–320 °C. 1 and 4 exhibited quick decomposition above 378 and 335 °C, with the remainder weight 54 and 50.84% at 700 °C, respectively. 2, 3, and 5 also lost weight from 100 °C and showed obvious stages in the range of 285–375, 262–362, and 272–371 °C, respectively. The losses were 9, 9.4, and 8.9% in the lower-temperature region, nearly equal to the weight of two H2O molecules (calcd 8.17, 8.06, and 7.94% for 2, 3, and 5, respectively). 2, 3, and 5 decomposed at higher temperatures.

Photoluminescence Properties

The solid-state photoluminescence properties of 1–5 were measured at room temperature. Under the excitation at 275 nm, 1 and 2 show no lanthanide-based emission peaks, indicating the mismatch of the energy levels of L3– so that the energy could not be effectively transmitted. When excited at 265 nm, 3 and 4 gave rise to lanthanide-based emission, as shown in Figures and 6, showing efficient ligand-sensitized f–f emission. The characteristic Sm3+ emission peaks are observed at 560, 595, and 642 nm, which are attributed to 4G5/2 → 6H5/2, 4G5/2 → 6H7/2, and 4G5/2 → 6H9/2, respectively. The expected Eu3+ characteristic emission peaks for 4 appear at 579, 591, 617, 651, and 697 nm, which belong to the transitions 5D0 → 7F (J = 0–4) (Figure ). According to the transition rules of Eu3+ ions, when Eu3+ ions occupy the inversion centers, the magnetic dipole transitions (5D0 → 7F1) are dominant, emitting orange light; when Eu3+ions dwell in the noninversion sites, the electric dipole transitions (5D0 → 7F2) are regnant, emitting red light.[73−75] The high ratio of intensity (5D0 → 7F2/5D0 → 7F1) suggests that the Eu3+ ions occupy the low-symmetry coordination sites with no inversion centers, which is consistent with the result of the XRD structural analysis. When exposed under an UV lamp, 3 and 4 emit dark red and bright red light [Figures and 6 (insets)], respectively. Time-resolved luminescence measurement on 3 and 4 has been carried out. The result showed lifetimes of 2 and 8 μs (calculated by the formula [τ] = (A1τ1 + A2τ2)) obtained via double-exponential fitting [Figures and 6 (inserted)].

Figure 5

Solid-state emission spectra, emission decay patterns, and emission photo under UV light (insets) of 3.

Figure 6

Solid-state emission spectra, emission decay patterns, and emission photo under UV light (insets) of 4.

Magnetic Properties

The d.c. molar magnetic susceptibilities of 1–5 were measured on polycrystalline samples in the temperature range 2–300 K under an applied magnetic field of 1 kOe. The experimental results are shown in Figure in the form of χMT products. At room temperature, the χMT products of 1–5 have values of 0.845, 1.589, 0.918, 2.254, and 7.561 cm3 K mol–1, respectively. They are in agreement with the expected theoretical values for a single Pr3+ (1.6 cm3 K mol–1, 3H4, g = 4/5), Nd3+ (1.64 cm3 K mol–1, 4I9/2, g = 8/11), and Gd3+ (7.88 cm3 K mol–1, 8S7/2, g = 2). Free Sm3+ ions have the 6H5/2 ground term, which is split into six states by spin–orbit coupling at room temperature; however, the ground state 6H5/2 is not well separated from the excited ones (around 700 cm–1 from the ground state to the first excited state), so both the crystal field effect and possible thermal population of the high states should be evaluated for the Sm3+ complex. The χMT value at 300 K is larger than the expected spin-only value for isolated Sm3+ (0.088 cm3 K mol–1, 6H5/2, g = 2/7). The value of χMT decreases rapidly to 0.0554 cm3 K mol–1 at 2 K and is nearly linear over the whole temperature range, which is similar to that of other reported Sm3+ complexes.[76] As for the case of 4, because the low-lying 7F1 first excited state is partly populated at room temperature, a finite magnetic moment appears at room temperature, although the 7F0 ground state of EuIII possesses no magnetic moment, and thus, Eu3+ should be diamagnetic at low temperature. Finally, the χMT product of 4 decreases continuously upon cooling and reaches 0.203 cm3 K mol–1 at 2 K, reflecting the thermal depopulation of the 7F1 first excited state upon cooling, likely indicating mixing of the 7F0 ground state with excited states possessing a magnetic moment.[77] The χMT products of 1 and 2 remain nearly constant down to approximately 100 K, wherefrom it begins to decrease upon further cooling to reach 0.037 and 0.505 cm3 K mol–1 at 2 K, respectively. This behavior could be an indication of weak antiferromagnetic exchange interactions. Such results could also be predicted from the classical single syn-anti-OCO bridge within the 2D layer, which is the most likely pathway for the antiferromagnetic exchange.[78,79] The χMT product of 5 is nearly unchanged up to 10 K. Then, the plot shows an increasing trend in χMT up to 7.6 cm3 K mol–1 at 2 K (Figure ), which is possibly due to weak ferromagnetic interactions between adjacent Gd3+ centers in 5. Compared with some Gd3+ compounds,[80] the deviation of χMT is not evident. The χM–1versus T over different temperature ranges for 1, 2, and 5 were fitted to the Curie–Weiss law χM = C/(T – θ) (Figure b). 5 obeys the Curie–Weiss law over the entire temperature range with the Curie constant C = 7.56 cm3 K mol–1 and the Weiss constant θ = −0.002, indicating the faint interactions between neighboring Gd3+. Since χM–1vs T deviated from the Curie–Weiss law in low-temperature zone, the fitting of magnetic data was carried out over 10–298 K for 1 and 80–298 K for 2, respectively, with the Curie constant C and the Weiss parameter θ of 0.99 cm3 K mol–1 and −56.89 K for 1 and 1.98 cm3 K mol–1 and −77.2 K for 2, respectively.

Figure 7

(a) Plot of χMT versus T for 1–5. (b) Plot of χM–1versus T for 1, 2, and 5.

Low-Temperature Heat Capacity

The heat capacity of crystalline samples 1–4 measured by the PPMS are given in Tables S3–S6 and plotted vs T in Figure . With the temperature increasing, it can be observed that the heat capacity increases smoothly and no thermal anomalies are found in the total experimental region. In fact, there is an interesting phenomenon seen from the graphs in Figure S4. The experimental heat capacity tendency below 10 K is hardly distinguished for them. With heating, larger and larger Cp,m differences were observed among them. In general, the heat capacity data order is Cp,m(2) > Cp,m(1) ≈ Cp,m(4) > Cp,m(3) in the range of 10–300K. Outwardly, the analogous coordination geometry around Ln3+ in 2 and 3 should lead to approximate heat capacity values. While, the case is not that. The large difference in Cp,m values between them may be effected by lanthanide contraction. Although effected by lanthanide contraction, the Cp,m difference between 1 and 4 is small, which may be due to the distortion of geometric configuration around Ln3+ ions.

Figure 8

Plot of the experimental and fitted heat capacities of 1–4 as a function of temperature ranging from 2 to 300 K. The inset shows the data below 10 K.

Modeling heat capacity data with theoretical functions at low temperature can provide valuable information about magnetic, electronic, and vibrational properties of a sample.[81,82] All the total heat capacities can be viewed as a sum of energetic contributions from the various physical properties of a material. The lattice contribution is relatively small below around 10 K, where the other properties can be extracted by fitting the experimental heat capacity to theoretical functions. The heat capacity data below around 10 K were fitted to the following equationwhere the linear fashion with γT represents the electronic contribution of conductors and defect or oxygen vacancies for insulators; the odd powers in temperature represent the lattice.[83] For compounds 1–3, the values of n are 3, 5, and 7, respectively. However, n = 7 does not necessarily fit to the data of 4.

The data in the middle temperature range for 1–4 are fitted to an arbitrary polynomial of eq . For the high temperature, the data are fitted to eq ,[83,84] a combination of Debye and Einstein functions, where D(θD) and E(θE) are Debye and Einstein functions, respectively, and T and T2 terms constitute an approximation of (Cp,m – CV,m). All the fitting parameters are listed in Table , and the fitted heat capacity is shown in Figure .

Table 1

Summary of Heat Capacity Fitting Parameters of 1–4

parameters	1 (Pr)	2 (Nd)	3 (Sm)	4 (Eu)
low T fits
γ/(J·mol–1·K–2)	0.019504	0.003471	0.001827	0.001229
B3/(J·mol–1·K–4)	0.001618	0.001928	0.001981	0.002551
B5/(J·mol–1·K–6)	2.65 × 10–5	1.57 × 10–5	3.41 × 10–6	–2.1 × 10–6
B7/(J·mol–1·K–8)	–1.7 × 10–7	–8.7 × 10–8	–2.5 × 10–8
%RMS	2.434516	1.687415	2.879716	2.736678
range/K	6.94	9.26	7.96	10
mid. T fits
A0	0.163477	–0.82247	–1.08956	–0.96548
A1	–0.01039	0.602612	0.66634	0.536592
A2	–0.01855	–0.16616	–0.16089	–0.12081
A3	0.007377	0.023535	0.021321	0.016657
A4	–0.00035	–0.00124	–0.00116	–0.00088
A5	8.49 × 10–6	3.52 × 10–5	3.41 × 10–5	2.52 × 10–5
A6	–1.2 × 10–7	–5.6 × 10–7	–5.6 × 10–7	–4.1 × 10–7
A7	9.13 × 10–10	4.65 × 10–9	4.78 × 10–9	3.45 × 10–9
A8	-3 × 10–12	–1.6 × 10–11	–1.7 × 10–11	–1.2 × 10–11
%RMS	1.1062	0.729392	1.525526	1.343216
range/K	from 6.94 to 67	from 9.26 to 67.9	from 7.96 to 57.1	from 10 to 55.05
high T fits
m	6.89743	9.949089	3.013084	5.856802
θD/K	230.0283	208.8382	206.135	216.1289
n	8.761887	14.2764	3.889093	7.563872
θE/K	626.667	566.2246	496.7288	492.2791
A1/(J·K–2·mol–1)	0.049089	–0.93601	0.368435	–0.03802
A2/(J·K–3·mol–1)	0.000548	0.002298	0.000381	0.001261
%RMS	0.67775	0.767591	0.661001	0.553843
range/K	above 67	above 67.9	above 57.1	above 55.05

As seen in Figure , the fits match the experimental heat capacity well. Low-temperature fitting suggests that although the γT contributions for 1–4 are different, the Cp values are comparable since the difference can be weakened by the contribution from their lattice vibration modes. With the temperature increasing, the electronic contribution becomes insignificant, while the contribution of lattice vibration to Cp is dominant.

Based on these fitting results, the corresponding thermodynamic functions have been calculated, and the calculated values of ΔSm and ΔHm have been listed in Tables S3–S6. The standard molar entropy and standard molar enthalpy at T = 298.15 K for 1–4 are (415.71 ± 4.16) J·mol–1·K–1 and (64.29 ± 0.64) kJ·mol–1, (451.32 ± 4.51) J·mol–1·K–1 and (71.03 ± 0.71) kJ·mol–1, (308.53 ± 3.09) J·mol–1·K–1 and (48.05 ± 0.48) kJ·mol–1, and (407.62 ± 4.08) J·mol–1·K–1 and (64.45 ± 0.64) kJ·mol–1, respectively. The four complexes display an Δ0298.15Sm order: (2) > (1) ≈ (4) > (3), which is consistent with their Cp,m order.

Conclusions

A series of lanthanide coordination polymers were isolated as single crystals under hydrothermal conditions. All of them formed 2D planar 4.4 layers via −COO bridges and extended to be 3D frameworks through L3– bridges. Although there are helical tunnels in all of the five compounds, the framework conformations are different due to the flexible L3– and various coordination geometries around metal centers. The helical tunnels in 1–3 are constructed by two pairs of enantiomeric helical chains, which gave rise to large intervals and small space. While, four pure left-handed and right-handed helical chains in 4 and 5, respectively, construct a uniform tunnel space. The changes in the tunnel space may be possible in transport of some ions or small molecules. Further studies need to be carried out to reveal this property. Characteristic Sm3+ and Eu3+ emitting can be seen from emission spectra. 3 emits dark-red luminescence and 4 emits bright-red luminescence under UV light. Unfortunately, the luminescent lifetime is short owing to the coordination of H2O to metal ions and absence of a long conjugate system. Single syn-anti-OCO bridges between Ln3+ ions lead to weak magnetic interactions. The heat capacities of 1–4 have been studied using a Quantum Design PPMS calorimeter over the temperature range from (1.9 to 300) K, and the standard thermodynamic functions have been derived based on the fitted experimental data. The standard molar entropies at T = 298.15 K have been calculated to be (415.71 ± 4.16)——, (451.32 ± 4.51)——, (308.53 ± 3.09)——, and (407.62 ± 4.08) J·mol–1·K–1, respectively.

Experimental Section

Materials and Methods

All chemicals and solvents were of the analytical grade and were obtained from commercial sources and used without further purification. The starting ligand N-(4-carboxy- benzyl)imino diacetic acid (H3L) was synthesized according to the methods already reported.[85,86]

Elemental analyses were carried out on an Elementer Vario Micro Cube analyzer. The FT-IR spectra were measured on a Thermo Niclet IR00 FT-IR spectrometer using KBr pellets (range of 4000–500 cm–1). A SIMADZU DTG-60 thermo analyzer was used to perform the TGA under nitrogen at a heating rate of 5 °C min–1 from room temperature to 700 °C. Powder XRD (PXRD) data under the 2θ range 5–60° were recorded using Cu Kα radiation (κ = 0.1542 nm) on a Bruker D8A A25 X-ray diffractometer, with the X-ray tube operated at 40 kV and 40 mA at room temperature. Luminescence properties were recorded on an FLS980 spectrophotometer with a Xe lamp as the light source at room temperature.

Preparation

[LnL(H2O)2]

A mixture of Ln(NO3)3·6H2O (0.20 mmol) (Ln = Pr 1, Nd 2, Sm 3, Eu 4 and Gd 5) and H3L (0.20 mmol) in 8 mL of distilled water was sealed into a 25 mL Teflon-lined stainless steel reactor and heated at 120 °C for 3 days (Scheme ). Then, the mixture was cooled to room temperature at an approximate rate of 5 °C h–1. Suitable single crystals for XRD were obtained by filtration.

[PrL(H2O)2] (1)

Yield: 32% (based on n class="Chemical">Pr). Anal. Calcd for C12H14NO8Pr (%): C, 32.64; H, 3.17; N, 3.17. Found (%): C, 32.88; H, 2.97; N, 3.13. IR (KBr, cm–1): 3423 (vs), 2937 (w), 1582 (vs), 1524 (s), 1397 (vs), 1324 (s), 1252 (s), 1092 (m), 1009 (w), 975, 931 (m), 848 (m), 785, 703 (m), 538 (w).

[NdL(H2O)2] (2)

Yield: 36% (based on Nd). Anal. Calcd for n class="Chemical">C12H10NO8Nd (%): C, 32.40; H, 3.15; N, 3.15. Found (%): C, 32.72; H, 2.84; N, 3.40. IR (KBr, cm–1): 3384 (vs), 2939 (w), 1594, 1575 (vs), 1522 (s), 1400 (vs), 1332 (s), 1251 (m), 1085 (s), 1008, 972, 932 (m), 855, 815, 785 (m), 716 (m), 540 (w).

[SmL(H2O)2] (3)

Yield: 34% (based on Sm). Anal. Calcd for C12H14NO8Sm (%): C, 31.96; H, 3.11; N 3.11. Found: C, 31.46; H, 3.44; N, 3.08. IR (KBr, cm–1): 3426 (vs), 2934 (w), 1586 (vs), 1530 (s), 1404 (vs), 1327 (s), 1249 (m), 1095 (s), 1018, 983, 933 (m), 863, 814, 779 (m), 547 (w).

[EuL(H2O)2] (4)

Yield: 38% (based on Eu). Anal. Calcd for C12H14NO8Eu (%): C, 31.84; H, 3.10; N, 3.10. Found (%): C, 32.07; H, 3.23; N, 3.17. IR (KBr, cm–1): 3433 (vs), 2933 (w),1577 (vs), 1525 (s), 1405 (vs), 1321 (s), 1243 (m), 1092 (s), 1009, 972, 931 (m), 852, 816, 779 (m), 534 (w).

[GdL(H2O)2] (5)

Yield: 29% (based on Gd). Anal. Calcd for n class="Chemical">C12H14NO8Gd (%): C, 31.48; H, 3.06; N, 3.06. Found (%): C, 31.41; H, 2.75; N, 3.16. IR (KBr, cm–1): 3422 (vs), 2940 (w), 1590 (vs), 1519 (vs), 1416 (vs), 1320 (s), 1243 (m), 1103 (m), 1023, 979, 936 (w), 863, 819, 776 (m), 543 (w).

X-Ray Crystallography

Single-crystal XRD data of the complexes 1–5 were collected on a Bruker SMART APEXII CCD diffractometer equipped with graphite monochromated Mo Kα radiation (λ = 0.71073 Å). The SADABS programs were used to apply the empirical absorption correction. Direct methods and full-matrix least-squares methods on F2 were performed to solve and refine the structure using the program SHEXL-2014.[87−89] All nonhydrogen atoms were refined anisotropically. The hydrogen atoms whose positions and thermal parameters were fixed during the structure refinement were located by geometrical calculations. The crystallographic data and details (Table S7) and selected bond lengths and angles (Table S2) are summarized in the Supporting Information.