Novel Lanthanide (III) Complexes Derived from an Imidazole–Biphenyl–Carboxylate Ligand: Synthesis, Structure and Luminescence Properties

A series of neutral mononuclear lanthanide complexes [Ln(HL)2(NO3)3] (Ln = La, Ce, Nd, Eu, Gd, Dy, Ho) with rigid bidentate ligand, HL (4′-(1H-imidazol-1-yl)biphenyl-4-carboxylic acid) were synthesized under solvothermal conditions. The coordination compounds have been characterized by infrared spectroscopy, thermogravimetry, powder X-ray diffraction and elemental analysis. According to X-ray diffraction, all the complexes are a series of isostructural compounds crystallized in the P2/n monoclinic space group. Additionally, solid-state luminescence measurements of all complexes show that [Eu(HL)2(NO3)3] complex displays the characteristic emission peaks of Eu(III) ion at 593, 597, 615, and 651 nm.

Lanthanides complexes, especially Eu(III) and Tb(III) complexes, are studied due to their luminescence properties [41][42][43], such as the strong emission bands of f-f transitions, broad range lifetimes, and high quantum yields that make them fluorescence materials with applications in medical-diagnostics [44,45]. Wu's group demonstrated that the Eu-MOF, based on Eu and Gd, are promising biomarkers for a carcinoid tumor where as π-conjugated organic ligands are used carboxylates ligands (i.e., H 4 BTEC = 1,2,4,5-benzenetetracarboxylic acid and H 2 TDA = thiazolidine 2,4-dicaboxylic acid) [46]. Furthermore, due to their structural tunability of CPs and MOFs, their use as biomarkers in the diagnosis and/or detection of cancer disease has expanded significantly during the last decade [47].
During past years, our group has been involved in the research of synthesis and characterization of metal complexes with different ligands, and further exploring of their

Synthesis and Characterization of [Ln(HL) 2 (NO 3 ) 3 ] Complexes
The present work deals with the multi-step synthesis of the rigid imidazole-biphenylcarboxylate ligand HL [48]. The synthesis of HL involved the Ullmann coupling of 1,4diiodobenzene with imidazole, followed by the Suzuki coupling of the reaction product with 4-carboxyphenylboronic acid as presented in Scheme 1. The mononuclear Ln 3+ complexes were obtained by mixing hydrated lanthanide nitrate and the bidentate ligand, at the ratio 3:1 (metal salt/ligand) in different solvents. Several approaches for the complexation of ligand HL were explored (Scheme 1). First, the complexation reactions were attempted in an ethanol-water mixture at 80 • C for two to five days. Under these experimental conditions (see Strategy A), a mixture of product, starting material, and also unidentified complexes was obtained, as shown in the PXRD patterns in Figure S1 (Supplementary Materials). When the synthesized complexes were performed in solvents as acetonitrile and water (see Strategy B), at 120 • C (on the programmable oven), for two days, the products were realized in good yield and high purity. The conversion and the purity of the products were monitored by PXRD, as presented in Figure S2

Spectroscopic Details
The ATR spectra of the synthesized lanthanide complexes [Ln(HL) 2 (NO 3 ) 3 ] are very similar and are presented in the Supplementary Materials ( Figures S3-S9). The infrared spectra of the linker HL and [Ln(HL) 2 (NO 3 ) 3 ] (Ln = La, Nd, Eu, Gd, Dy, Ho) complexes show weak and medium absorption bands attributed to the asymmetric and symmetric C-H at 3123 cm −1 and 3084 cm −1 (for the HL) and 3136 cm −1 , 3070 cm −1 (for the complexes), respectively. Additionally, the infrared spectra presented in Figures S3-S9 show week absorption bands between 2850 cm −1 and 2925 cm −1 , which are residual artifacts from acetonitrile used as solvent. Also, as can be seen in Figure 1, the characteristic peak at 1679 cm −1 present in the IR spectrum of the HL is ascribed to ν(C-O), ν(O-H) stretching and δ(O-H) bending, corresponding to the COOH groups. The strong and medium absorption bands were observed in the range 1602-1460 cm −1 and are assigned to the aromatic stretch ν(C = C), ν(C = N), and asymmetric/symmetric vibrations of the complete deprotonation of the carboxylic acid (COO − ). The bands from 1325 to 1300 cm −1 correspond to the ν as (NO 2 ) vibrations of the coordinated nitrate [49], and bands at 1292 cm −1 are due to the ν(C = C) and ν(C-O) stretch. Furthermore, the strong band at 1031 cm −1 and medium bands at 684 and 625 cm −1 , are assigned to Ln-O stretching vibrations for the mentioned complexes.

Powder X-ray Diffraction Studies (PXRD)
The PXRD analysis of synthesized compounds provides important information about purity. All PXDR patterns were recorded after the compounds were washed, filtrated, airdried and exposed to air at least one week. The powder X-ray diffraction patterns (PXRD) of the synthesized HL and [Nd(HL) 2 (NO 3 ) 3 ] are shown in Figure 2. The PXDR pattern of the [Nd(HL) 2 (NO 3 ) 3 ] presents diffraction patterns different from the PXDR pattern of ligand (HL), which indicates the formation of new complexes. Diffraction peaks appearing in lower angles for 3 and 2θ at 8.56, 10.96, 11.8, 13.2, 15.8 17.5 and 24.6 value is rather typical for coordination networks containing imidazole-biphenyl-carboxylate ligands as could be seen for other complex, for example [ZnLAc] n complex [50]. Furthermore, the diffraction peaks of the synthesized [Ln(HL) 2 (NO 3 ) 3 ] compounds are in good agreement with the simulated data, confirming that the [Ln(HL) 2 (NO 3 ) 3 ] were successfully synthesized as shown in Figure S2 (see Scheme 1, Strategy B). The powder materials obtained by the solvothermal method in ethanol and water as solvents (see Scheme 1, Strategy A) were impure, as shown in Figure S1

Thermogravimetric Analysis (TGA)
The TGA analysis of synthesized materials was carried out from 30 to 700 • C, at a heating rate of 5 • C/min, under the nitrogen atmosphere. The TGA profiles of isostructural compounds [Ln(HL) 2 (NO 3 ) 3 ] (1-9) are quite similar, except small differences in the weight mass loss due to the differences in mass of the metal (see Supplementary Materials, Figures S10-S13). The TGA curves of [Ln(HL) 2 (NO 3 ) 3 ] show multi decomposition steps. Figure 3 presents the thermal decomposition of [Nd(HL) 2 (NO 3 ) 3 ] complex, compared to HL, as a representative example for explaining the dehydration and decomposition of all complexes: (i) the first stage was in the temperature range of 260-360 • C, and the percentage of mass loss is attributed to the release of three nitrate groups (inner ions) NO 3 − (Found~20 percent and calc.~21 percent); (ii) the second stage corresponds to the loss of part of the ligand in the range between 360-500 • C (found~9% mass loss); and (iii) the third stage until 700 • C reflected the loss of the other part of organic ligand (found~13% mass loss). The residual weight of~57% at 700 • C is attributed to the metal oxide and organic residue from the linker and above this temperature the decomposition is still continued (the framework continued to collapse with the loss of other molecules).

Luminiscence
An important characteristic property of the complexes-class with lanthanides is the spectral narrow emission obtained from the Ln 3+ 4f → 4f intra-atomic transitions and long radioactive lifetime [31,42]. Luminescence spectra of synthesized ligand and complexes with Ln 3+ were recorded at room temperature as solid state. As can be seen in Figure 4, upon excitation at a wavelength of 315 nm, the ligand HL displays a broad fluorescence emission band between 356 nm and 556 nm with a maximum at 413 nm and a longer tail after the bigger wavelength value. The materials doped with Eu 3+ exhibit 5 D 0 → 7 F J transition. Furthermore, we proved that the complex containing Eu 3+ also exhibits as well 5 D 0 → 7 F 4 transition. Upon excitation at 315 nm, the [Eu(HL) 2 (NO 3 ) 3 ] exhibits characteristic sharp emissions at λ = 593 nm (weak, 5 D 0 → 7 F 0 ), λ = 597 nm (weak, 5 D 0 → 7 F 1 ), λ = 615 nm (very strong, 5 D 0 → 7 F 2 ), λ = 651 nm (very weak, 5 D 0 → 7 F 3 ), and λ = 686 nm (very weak, 5 D 0 → 7 F 4 ). A high hypersensitivity of 5 D 0 → 7 F 2 transition at 615 nm (red light), followed by 5 D 0 → 7 F 0 transition at 593 nm, could be remarked in the spectra of [Eu(HL) 2 (NO 3 ) 3 ] complex. In addition, the luminescence characteristics of three additional complexes ([Ln(HL) 2 (NO 3 ) 3 ], Ln = Dy, Gd, Ho) upon excitation at 315 nm ( Figure S14) as well as excitation spectra at 440 nm ( Figure S15) are shown in the Supplementary Materials. Besides, the emission band (350-560 nm) of the intermolecular transition of the ligand was not observed for the coordinated complexes. Experimentally, in this study, it was proved that the ligand and complexes display intense and narrow emissions at room temperature that make them good candidates for red and green emitter luminescence materials.

Crystallographic Studies
In order to elucidate the crystal structure of the synthesized compounds, a complete single-crystal X-ray diffraction study for 1-7 compounds have been carried out. According to X-ray crystallography the investigated compound form a series of isostructural crystals: they crystallize in P2/n space group of monoclinic system with close unit cell parameters (see Table 1). That is why, below, as an example, only the crystal structure for compound [Eu(HL) 2 (NO 3 ) 3 ] will be presented. The asymmetric part of the unit cell ( Figure 5) comprises one Eu 3+ cation, one imidazole-biphenyl-carboxylate zwitterion (HL) and one and a half NO 3 − anions, so that the charge balance is in agreement with the formation of species [Eu(HL) 2 (NO 3 ) 3 ]. Three positive charges provided by Eu 3+ ions are balanced by three negative charges of the nitrate anions acting as bidentate ligands. There no co-crystalized solvate molecules in 1-7 crystals. The HL zwitterion behaves as a non-bridging ligand being coordinated to Eu atom through carboxylate group in к 2 O,O bidentate-chelating mode. The Eu atom, located in special position on two-fold axis, is ten-coordinated by two carboxylate groups and three nitrate anions. It's coordination polyhedral can be characterized as a distorted bicapped square antiprism in C 4v local geometry. A view of the coordination environment of Eu atom is shown in Figure 6. The further analysis of the crystal structure has shown, that non-coordinated nitrogen atom N2 is involved in the intermolecular hydrogen bonding towards carboxylate oxygen from adjacent unit ( Figure 7a). As a result, the asymmetric units are assembled into one-dimensional supramolecular zig-zag double chains, as depicted in Figure 7b. The crystal structure is built up with the quite dense packing of 2D layers consisted from 1D double chains. The partial view of the crystal structure along b axis is depicted in Figure 7c,d.    (7) 11.6491 (5) 11.6264 (8) 11.6229 (7) 11.6154 (7) 11.5987 (4) (9) 14.0436 (7) 14.0056 (8) 14.0020 (8) 13.9905 (7) 13.9615 (5) (7) 109.849 (5) 109.845 (7) 109.798 (7) 109.817 (7) 109.871 (4)

Material and Methods
All metal salts and solvents were purchased from Sigma Aldrich, Alfa Aesar, or other commercial sources, and used as received without additional purification. Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vertex 70 FTIR spectrometer (Bruker Optics GmbH, Ettlingen, Germany). The measurements were performed in ATR Golden Gate ® (Attenuated Total Reflectance) mode in the 600-4000 cm −1 range at room temperature with a resolution of 4 cm −1 over a total of 64 scans. Thermogravimetric analyses (TGA) were performed under nitrogen stream with a flow rate of 50 mL min −1 using a STA 449 F1 Jupiter (Netzsch, GmbH, Selb, Germany) device. The temperature range used for measurements was between 30 to 700 • C with a heating rate of 5 • C min −1 and the data was processed with the NETZSCH Proteus 4.2 software. The Powder X-ray diffraction patterns (PXRD) were collected employing a Bruker AXS D8 Phaser powder diffractometer with a monochromated Cu-Kα (k = 1.5418 Å) radiation in Bragg-Brentano arrangement, with scan speed of 4 s/step and a step size of 0.02 • (2θ) at RT in the range of 2θ = 3.5-50 • . Simulated PXRD patterns were calculated with CCDC Mercury 4.1 program using the single-crystal data of the compounds. CHN elemental analysis was performed on a VARIO-EL-III elemental analyzer. The photoluminescence spectra were measured for solid state of complexes at room temperature on a Horiba Fluoromax-4 fluorescence spectroscopy instrument using a 370 nm long pass filter from Edmund Optics.

General Procedure for [Ln(HL) 2 (NO 3 ) 3 ]
The organic linker HL (4 -(1H-imidazol-1-yl)biphenyl-4-carboxylic acid) was prepared using a slightly modified procedure described by our group and NMR; IR spectroscopy was used to establish its structure and purity [48]. Starting with the functionalized aromatic building blocks HL, the desired products [Ln(HL) 2 (NO 3 ) 3 ] ( Ln = La, Ce, Nd, Eu, Gd, Dy, Ho) were prepared using two different strategies: Strategy A: A mixture of Ln(NO 3 ) 3 ·yH 2 O (0.3 mmol; Ln = La, Nd, Gd, Dy, Ho), HL (0.1 mmol), ethanol (1 mL) and water (0.5 mL) was transferred to a 10 mL culture tub with a PTFE-lined PBT screw and kept under static conditions for two to five days at 80 • C. After cooling, the crystalline product formed was filtered, washed with ethanol (2 mL), and then dried under air. The materials obtained were investigated by PXRD and from them, only three crystals (La, Nd, Dy) were suitable for single-crystal X-ray diffraction analysis. Strategy B: A mixture of Ln(NO 3 ) 3 ·yH 2 O (0.3 mmol; Ln = La, Ce, Nd, Eu, Gd, Dy, Ho), HL (0.1 mmol), acetonitrile (10 mL) and water (1 mL) was transferred to a 10 mL culture tub with a PTFE-lined PBT screw. The tube with suspension was kept under static conditions in a controlled Memmert oven, for two days at 120 • C. The heating and cooling programs were run with a heating/cooling rate of 1 • C/min. At the end of the reaction time, the crystals were collected by filtration or centrifugation, and washed with acetonitrile three times. Finally, the colored compound (yellow to brawn) was allowed to dry at room temperature. The amount, stoichiometry and yields (based on the linker) are given in Table 2.

Single Crystal X-ray Diffraction
X-ray diffraction measurements were carried out with a Rigaku Oxford-Diffraction XCALIBUR E CCD diffractometer equipped with graphite-monochromated MoKα radiation. The unit cell determination and data integration were carried out using the CrysAlis package of Oxford Diffraction [51]. The structures were solved by Intrinsic Phasing using Olex2 [52] software with the SHELXT [53] structure solution program and refined by full-matrix least-squares on F 2 with SHELXL-2015 [54] using an anisotropic model for non-hydrogen atoms. All H atoms were introduced in idealized positions (d CH = 0.96 Å) using the riding model. The H-atoms attached to nitrogen were localized from difference Fourier maps and their positional parameters verified according to H-bonds geometry. The molecular plots were obtained using the Olex2 program. The crystallographic data and refinement details are quoted in Table 1

Conclusions
This work was focused on the structure, thermal stability, and luminescence properties of lanthanides complexes with an imidazole-biphenyl-carboxylate ligand. Here, we report the optimized synthesis conditions of a series of neutral seven coordinate lanthanide complexes. These complexes were obtained by solvothermal reactions of simple salts with acid ligands, in good yields and high purity. According to X-ray crystallography, the studied complexes form a series of isostructural [Ln(HL) 2 (NO 3 ) 3 ] molecular species. Furthermore, the complex with Eu 3+ shows good luminescence properties, and this could be further developed, investigated and applied in medical imaging.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are not available from the authors.