Crystal Structures, Thermal and Luminescent Properties of Gadolinium(III) Trans-1,4-cyclohexanedicarboxylate Metal-Organic Frameworks

Four new gadolinium(III) metal-organic frameworks containing 2,2′-bipyridyl (bpy) or 1,10-phenanthroline (phen) chelate ligands and trans-1,4-cyclohexanedicarboxylate (chdc2−) were synthesized. Their crystal structures were determined by single-crystal X-ray diffraction analysis. All four coordination frameworks are based on the binuclear carboxylate building units. In the compounds [Gd2(bpy)2(chdc)3]·H2O (1) and [Gd2(phen)2(chdc)3]·0.5DMF (2), the six-connected {Ln2(L)2(OOCR)6} blocks form a 3D network with the primitive cubic (pcu) topology. In the compounds [Gd2(NO3)2(phen)2(chdc)2]·2DMF (3) and [Gd2Cl2(phen)2(chdc)2]·0.3DMF·2.2dioxane (4), the four-connected {Ln2(L)2(X)2OOCR)4} units (where X = NO3 for 3 or Cl− for 4) form a 2D squaregrid (sql) network. The solid-state luminescent properties were investigated for the synthesized frameworks. Bpy-containing compound 1 shows no luminescence, possibly due to the paramagnetic quenching by Gd3+ cation. In contrast, the phenathroline-containing MOFs 2–4 possess yellow emission under visible excitation (λex = 460 nm) with the tuning of the characteristic wavelength by the coordination environment of the metal center.


Introduction
Metal-organic frameworks (MOFs) are an important class of coordination compounds extensively studied in recent years. Their porosity as well as a wide variability of metal centers and organic ligands unveil a route to design materials with highly tunable adsorption, catalytic, optical and other physico-chemical properties. In particular, lanthanide(III) MOFs deserve a great interest due to the unique f n electron configuration of the metal center and subsequent applications in magnetic and luminescent materials [1][2][3][4][5][6][7][8][9].
Gadolinium(III), having a half-filled f 7 sublevel, takes a special place in the lanthanide row. This is a most relevant paramagnetic center in the development of contrast agents for magnetic-resonance tomography and visualization [10][11][12][13][14]. The most intensive electron transitions in Gd 3+ occur in the ultraviolet region of 290-318 nm with the narrow-banded emission and have been applied in common lasers. Under a soft UV and visible excitation, Gd(III) is a non-emissive center and the luminescence of its coordination compounds is ligand-centered [15][16][17][18][19][20][21][22][23].
A new molecular complex [Gd(DMF) 2 (phen)]Cl 3 (5) not containing trans-1,4cyclohexanedicarboxylate bridge (see Figure A1 in Appendix A) was crystallized during this work. The successful synthesis of such a series with a non-photoactive bridging ligand and a non-emissive paramagnetic metal center makes it possible to investigate the impact of the metal ion coordination environment on the N-donor ligand-centered luminescence in the corresponding coordination networks.

Instruments
IR spectra in KBr pellets were recorded in the range 4000−400 cm −1 on a Bruker Scimitar FTS 2000 spectrometer. Elemental analysis was conducted with a VarioMICROcube analyzer. Powder X-ray diffraction (PXRD) analysis was performed at room temperature on a Shimadzu XRD-7000 diffractometer (Cu-Kα radiation, λ = 1.54178 Å, or Co-Kα radiation, λ = 1.78897 Å). Thermogravimetric analysis was carried out using a Netzsch TG 209 F1 Iris instrument under Ar flow (30 cm 3 ·min −1 ) at a 10 K·min −1 heating rate. Photoluminescence spectra were recorded with a spectrofluorometer Horiba Jobin Yvon Fluorolog 3 equipped with ozone-free Xe-lamp 450W power, cooled photon detector R928/1860 PFR technologies with refrigerated chamber PC177CE-010 and double grating monochromators. The spectra were corrected for source intensity and detector spectral response by standard correction curves. The absolute quantum yield was measured using a G8 (GMP SA, Switzerland) spectralon-coated integrating sphere, which was connected to a Fluorolog 3 spectrofluorometer. Diffraction data for single crystals of 1 were obtained on the 'Belok' beamline [28,29] (λ = 0.74539 Å) of the National Research Center 'Kurchatov Institute' (Moscow, Russian Federation) using a Rayonix SX165 CCD detector. The data were indexed, integrated and scaled, and absorption correction was applied using the XDS program package [30]. Diffraction data for single crystals of 2-5 were collected on an automated Agilent Xcalibur diffractometer equipped with an area AtlasS2 detector (graphite monochromator, λ(MoKα) = 0.71073 Å). Integration, absorption correction and determination of unit cell parameters were performed using the CrysAlisPro program package [31]. The structures were solved by the dual-space algorithm (SHELXT [32]) and refined by the full-matrix least squares technique (SHELXL [33]) in the anisotropic approximation (except hydrogen atoms). Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the riding model. The crystallographic data and details of the structure refinements are summarized in Appendix Table A1.
In all the structures 1-3, Gd(III) adopts a similar capped square-antiprismatic environment consisting of two N atoms of diimine chelate ligand and seven O atoms, which belong to the carboxylic groups in 1 and 2 or to carboxylic groups and terminal nitrate in 3 (Figure 1a-c). The selected coordination bond lengths are listed in Table 1. Two symmetry-equivalent Gd(III) ions form binuclear carboxylate blocks {Gd 2 (bpy) 2 (RCOO- . The coordination frameworks in 1 and 2, consisting of six-connected building units and trans-1,4-cyclohexanedicarboxylate bridges, adopt a very distorted three-dimensional primitive cubic topology (pcu) and contain small voids (Figure 2a,b) interconnected by very narrow (2 × 2 Å 2 ) windows with 5% calculated total void volume. In the coordination framework in 3, four-connected binuclear blocks are interconnected by cyclohexane moieties into a two-dimensional square-grid network (sql) (Figure 2c). The layers in 3 are packed in a one-layer (AA) manner to form channels with 26% general void volume. These channels are occupied by the localized DMF solvent molecules.  Compound [Gd2Cl2(phen)2(chdc)2]•0.3DMF•2.2dioxane (4) crystallizes in the monoclinic crystal system with the P21/n space group. The coordination environment of Gd(III) consists of two N atoms of diimine chelate ligand, five O atoms of the carboxylic groups and one Cl atom. The Gd-N and Gd-O bond lengths are close to those in 1-3 (see Table  1) and the Gd-Cl distance is 2.649(2) Å (Figures 1 and 2). The structure of a binuclear Compound [Gd 2 Cl 2 (phen) 2 (chdc) 2 ]·0.3DMF·2.2dioxane (4) crystallizes in the monoclinic crystal system with the P2 1 /n space group. The coordination environment of Gd(III) consists of two N atoms of diimine chelate ligand, five O atoms of the carboxylic groups and one Cl atom. The Gd−N and Gd−O bond lengths are close to those in 1-3 (see Table 1) and the Gd−Cl distance is 2.649(2) Å (Figures 1 and 2). The structure of a binuclear carboxylate block {Gd 2 (phen) 2 (Cl) 2 (µ-RCOO-κ 1 ,κ 1 ) 2 (µ-RCOO-κ 1 ,κ 2 ) 2 } in 4 is analogous to the nitrate-containing unit in 3, except for the reduction of the coordination number to 8 due to the substitution of the bidentate nitrate anion with the larger chloride (Figure 1d), which acts as a monodentate ligand. Four-connected binuclear blocks in 4 are interconnected by cyclohexane moieties (Figure 2d) in a similar AA manner to 3, with the channels of 32% general void volume. These channels are occupied by solvent molecules. Only one dioxane molecule per formula unit was localized directly, while the non-ordered residual electron density was analyzed by the PLATON/SQUEEZE [37] procedure (69 e − in 265 Å per formula unit) and assigned to 0.3DMF + 1.2dioxane (69.6 e − and ca. 209 Å volume estimated from the liquid densities). Compound [Gd2Cl2(phen)2(chdc)2]•0.3DMF•2.2dioxane (4) crystallizes in the monoclinic crystal system with the P21/n space group. The coordination environment of Gd(III) consists of two N atoms of diimine chelate ligand, five O atoms of the carboxylic groups and one Cl atom. The Gd-N and Gd-O bond lengths are close to those in 1-3 (see Table  1) and the Gd-Cl distance is 2.649(2) Å (Figures 1 and 2). The structure of a binuclear carboxylate block {Gd2(phen)2(Cl)2(μ-RCOO-κ 1 ,κ 1 )2(μ-RCOO-κ 1 ,κ 2 )2} in 4 is analogous to the nitrate-containing unit in 3, except for the reduction of the coordination number to 8 due to the substitution of the bidentate nitrate anion with the larger chloride (Figure 1d), which acts as a monodentate ligand. Four-connected binuclear blocks in 4 are interconnected by cyclohexane moieties (Figure 2d) in a similar AA manner to 3, with the channels of 32% general void volume. These channels are occupied by solvent molecules. Only one dioxane molecule per formula unit was localized directly, while the non-ordered residual electron density was analyzed by the PLATON/SQUEEZE [37] procedure (69 ein 265 Å per formula unit) and assigned to 0.3DMF + 1.2dioxane (69.6 eand ca. 209 Å volume estimated from the liquid densities).

Thermal Properties
Thermogravimetric analyses for the compounds 1-4 were performed (Figure 3). For 1, the stepwise decomposition starts at ca. 350 °C. Only 1% weight loss before 300 °C corresponds well to the low content of guest solvent molecules (calculated as 1.5%) determined by X-ray crystallography, and 33% residual weight at 600 °C matches well to the gadolinium(III) oxide (calculated as 32%).
2 slowly losses solvent molecules at the temperature up to 300 °C, much higher than the boiling points of both DMF and water. Such feature is apparently attributed to the low

Thermal Properties
Thermogravimetric analyses for the compounds 1-4 were performed (Figure 3). For 1, the stepwise decomposition starts at ca. 350 • C. Only 1% weight loss before 300 • C corresponds well to the low content of guest solvent molecules (calculated as 1.5%) deter-Crystals 2021, 11, 1375 6 of 11 mined by X-ray crystallography, and 33% residual weight at 600 • C matches well to the gadolinium(III) oxide (calculated as 32%).
2 slowly losses solvent molecules at the temperature up to 300 • C, much higher than the boiling points of both DMF and water. Such feature is apparently attributed to the low size of the windows (~2 Å) in the coordination framework of 2 and the resulting kinetic hindrance of the guest diffusion. The first step of lattice decomposition occurs in the range 340-440 • C and corresponds well to the loss of phen molecules (66% residue at 440 • C; calculated for Gd 2 (chdc) 3 : 66.5%). Further weight loss starting at ca. 460 • C corresponds to the decomposition of the bridging ligand and leads to the Gd 2 O 3 (34% residue at 600 • C, calculated: 29%) being apparently contaminated by carbon admixture due to the incomplete evaporation of the organic moieties.  Compound 3 loses solvents at ca. 120 °C. The first step of coordination lattice decomposition occurs in the range 350-400 °C and corresponds well to the loss of phen molecules (58% residue at 440 °C; calculated for Gd2(NO3)2(chdc)2: 59%). Further weight loss, corresponding to the decomposition of the nitrate and chdc 2-ligands, starts at ca. 430 °C. The TG profile of 4 is close to 3 and includes solvent loss at ca. 130 °C and two steps of the lattice decomposition occurring in the range 320-410 °C and above 490 °C. In summary, thermal stability characteristics of the coordination framework in 1-4 are high [38][39][40][41] and quite similar to each other. However, their stability is limited by the evaporation of the neutral N-donor chelate, occurring below 400 °C.

Luminescence Spectrocopy
Solid-state luminescence measurements were performed for the synthesized compounds. The bpy-containing 1 appeared to possess no luminescent activity, possibly due to the paramagnetic quenching of the emission by the Gd(III) cation. In contrast, the phencontaining compounds 2-4 demonstrate yellow wide-banded emission under a visible light excitation at λex = 460 nm. The corresponding emission spectra are shown in Figure  4a. The maxima of the spectra appear at λ = 537 nm for three-dimensional 2 based on the six-connected carboxylate units, λ = 522 nm for the nitrate-containing 3 and λ = 556 nm for the chloride-containing 4. The observed red-shift of both the maxima and the characteristic wavelengths in the row nitrate < carboxylate < chloride apparently correlates to the electron donor properties of the corresponding ligands. The calculated (x,y) coordinates on the CIE 1931 chromaticity diagram and characteristic wavelength values are shown in Figure 4b and visualize the integral yellow color of the wide-banded emission of 2-4. Compound 3 loses solvents at ca. 120 • C. The first step of coordination lattice decomposition occurs in the range 350-400 • C and corresponds well to the loss of phen molecules (58% residue at 440 • C; calculated for Gd 2 (NO 3 ) 2 (chdc) 2 : 59%). Further weight loss, corresponding to the decomposition of the nitrate and chdc 2− ligands, starts at ca. 430 • C. The TG profile of 4 is close to 3 and includes solvent loss at ca. 130 • C and two steps of the lattice decomposition occurring in the range 320-410 • C and above 490 • C. In summary, thermal stability characteristics of the coordination framework in 1-4 are high [38][39][40][41] and quite similar to each other. However, their stability is limited by the evaporation of the neutral N-donor chelate, occurring below 400 • C.

Luminescence Spectrocopy
Solid-state luminescence measurements were performed for the synthesized compounds. The bpy-containing 1 appeared to possess no luminescent activity, possibly due to the paramagnetic quenching of the emission by the Gd(III) cation. In contrast, the phencontaining compounds 2-4 demonstrate yellow wide-banded emission under a visible light excitation at λ ex = 460 nm. The corresponding emission spectra are shown in Figure 4a. The maxima of the spectra appear at λ = 537 nm for three-dimensional 2 based on the six-connected carboxylate units, λ = 522 nm for the nitrate-containing 3 and λ = 556 nm for the chloride-containing 4. The observed red-shift of both the maxima and the characteristic wavelengths in the row nitrate < carboxylate < chloride apparently correlates to the electron donor properties of the corresponding ligands. The calculated (x,y) coordinates on the CIE 1931 chromaticity diagram and characteristic wavelength values are shown in Figure 4b and visualize the integral yellow color of the wide-banded emission of 2-4.

Conclusions
To summarize, four new gadolinium(III) metal-organic frameworks were synthesized and characterized. Compounds 1 and 2 containing six-connected binuclear metal-carboxylate blocks adopt a distorted primitive cubic topology with narrow pores. A partial substitution of carboxylate by nitrate or chloride in the Gd(III) coordination sphere leads to two-dimensional square-layered networks 3 and 4. Thermal and luminescent properties of the synthesized compounds were investigated. Phenanthrolinebased structures 2-4 emit in the yellow region under a visible blue excitation at 460 nm. The observed red-shift in the row of coordinated ligands, nitrate < carboxylate < chloride was attributed to the donor ability of the corresponding ligands.
Supplementary Materials: The following are available online at www.mdpi.com/xxx/s1: Figure S1: Experimental PXRD pattern for 1 compared to the theoretical one; Figure S2: Experimental PXRD pattern for 2 compared to the theoretical one; Figure S3: Experimental PXRD pattern for 3 compared to the theoretical one; Figure S4: Experimental PXRD pattern for 4 compared to the theoretical one; Figure S5: IR spectra for 1-5.
Author Contributions: P.A.D., original draft preparation, single-crystal XRD, graphing; A.A.V., synthesis, characterization, graphing; V.A.L., synchrotron single-crystal XRD; A.A.R., solid-state luminescence measurements; V.P.F., manuscript review and editing, project administration and funding acquisition. All authors have read and agreed to the published version of the manuscript.

Conclusions
To summarize, four new gadolinium(III) metal-organic frameworks were synthesized and characterized. Compounds 1 and 2 containing six-connected binuclear metalcarboxylate blocks adopt a distorted primitive cubic topology with narrow pores. A partial substitution of carboxylate by nitrate or chloride in the Gd(III) coordination sphere leads to two-dimensional square-layered networks 3 and 4. Thermal and luminescent properties of the synthesized compounds were investigated. Phenanthroline-based structures 2-4 emit in the yellow region under a visible blue excitation at 460 nm. The observed red-shift in the row of coordinated ligands, nitrate < carboxylate < chloride was attributed to the donor ability of the corresponding ligands.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/cryst11111375/s1: Figure S1: Experimental PXRD pattern for 1 compared to the theoretical one; Figure S2: Experimental PXRD pattern for 2 compared to the theoretical one; Figure S3: Experimental PXRD pattern for 3 compared to the theoretical one; Figure S4: Experimental PXRD pattern for 4 compared to the theoretical one; Figure S5: IR spectra for 1-5.
Author Contributions: P.A.D., original draft preparation, single-crystal XRD, graphing; A.A.V., synthesis, characterization, graphing; V.A.L., synchrotron single-crystal XRD; A.A.R., solid-state luminescence measurements; V.P.F., manuscript review and editing, project administration and funding acquisition. All authors have read and agreed to the published version of the manuscript.