Elaboration of Luminescent and Magnetic Hybrid Networks Based on Lanthanide Ions and Imidazolium Dicarboxylate Salts: Influence of the Synthesis Conditions

The syntheses and characterization of four new hybrid coordination networks based on lanthanide ions (Ln = Nd, Sm) and 1,3-carboxymethylimidazolium (L) salt in the presence of oxalic acid (H 2 ox) are reported. The influence of the synthesis parameters, such as the nature of the lanthanide ion (Nd 3+ or Sm 3+), the nature of the imidazolium source (chloride [H 2 L][Cl] or zwitterionic [HL] form) and the presence or not of oxalic acid (H 2 ox), is discussed. In the presence of oxalic acid, the samarium salt gives only one compound [Sm(L)(ox)(H 2 O)]·H 2 O, whatever the nature of the imidazolium ligand, while the neodymium salt leads to three different compounds, [Nd(L)(ox)(H 2 O)]·H 2 O, [Nd(L)(ox) 0.5 (H 2 O) 2 ][Cl] or [Nd 2 (L) 2 (ox)(NO 3)(H 2 O) 3 ][NO 3 ], depending on the imidazolium ligand. In the absence of oxalic acid, gels are obtained, except for the reaction between the neodymium salt and [H 2 L][Cl], which leads to [Nd(L)(ox)(H 2 O)]·H 2 O. All compounds crystallized and their structures were determined by single crystal diffraction. The description of these new phases was consistently supported by ancillary techniques, such as powder X-ray diffraction, thermal analyses and UV-visible-near infrared spectroscopy. The luminescent and magnetic properties of the three pure compounds [Sm(L)(ox)(H 2 O)]·H 2 O, [Nd(L)(ox)(H 2 O)]·H 2 O and [Nd 2 (L) 2 (ox)(NO 3)(H 2 O) 3 ][NO 3 ] were also studied.


Introduction
Hybrid coordination polymers have been the subject of intense research for a few decades.Primarily investigated for their porosity and related properties, coordination polymers are promising for many applications, like gas separation and storage, catalysis or drug delivery, for example [1][2][3][4][5].The versatility of the synthesis of such metal coordination polymers is now exploited to generate new functional hybrid networks with specific electronic properties (luminescence, magnetism, conductivity, etc.) [6][7][8][9][10].
Compared to the first row transition metals, the coordination number of 4f elements is more diverse.Even if a wide range of coordination numbers for lanthanides can make the prediction and control of the final structure of the networks difficult, it can be an advantage for the generation of (multi)functional systems.This (multi)functionality stems essentially from the intrinsic physical properties of the lanthanide ions.Especially due to luminescent properties, lanthanide-based compounds can be used in many potential applications, such as light-emitting devices, sensing, imaging agents in the biomedical area, as well as solar energy conversion [11][12][13][14][15][16].Among the different networks based on lanthanides, those containing Tb 3+ and Eu 3+ ions are certainly the most studied since they exhibit a characteristic green and red emission, respectively [17,18].Moreover, lanthanide ions present a strong magnetic anisotropy, which confers them interesting magnetic properties, such as single molecule magnet or single chain magnet behavior [19][20][21].
The synthesis of hybrid coordination networks is very versatile since different parameters, such as the solvent, the temperature, the pH, the nature and the concentration of the reactants, can have a great influence on the final product [1,22].Recently, the synthesis of hybrid coordination networks has been realized in ionic liquid media, which is called ionothermal synthesis.The use of ionic liquids (belonging to the family of the imidazolium or ammonium salts, for example) allows obtaining new compounds that are only available in these kinds of conditions [23][24][25].However, real control of ionothermal synthesis is still limited, especially because the role of ionic liquids, acting as a solvent, structuring agent or charge compensator, or even a combination of these three possibilities, is rather unpredictable.To circumvent this problem, we have chosen to design imidazolium salts functionalized by carboxylate functions to force the role of the imidazolium salts to that of the ligand.Such a method has already proven its efficiency to get hybrid networks [26][27][28][29][30].
We have recently reported the synthesis of two isostructural hybrid coordination networks based on transition metal ions (M = Co 2+ , Zn 2+ ) [31] and a series of uranyl hybrid coordination networks [32] in the presence of the imidazolium dicarboxylate salt named 1,3-bis(carboxymethyl)imidazolium chloride or [H 2 L][Cl].To go further into the exploration and the understanding of such a system, we analyzed the behavior of the lanthanide ions, and in particular, we investigated the behavior of the Nd 3+ and Sm 3+ ions.
In this paper, we report the effect of the nature of the imidazolium salt (either [H 2 L][Cl] or 2-(1-(carboxymethyl)-1H-imidazol-3-ium-3-yl)acetate denoted [HL]) on the structure of lanthanide-based compounds obtained by the reaction with Nd(NO 3 ) 3 •6H 2 O or Sm(NO 3 ) 3 •6H 2 O in the presence of oxalic acid (H 2 ox) in a water/ethanol mixture.The effect of the presence of oxalic acid is also investigated.3+ or Sm 3+   The diffraction analysis reveals that the compounds [Nd(L)(ox)(H 2 O)]•H 2 O and [Sm(L)(ox)(H 2 O)]•H 2 O are isostructural (see Table 1 and Figure   In the refined model, the asymmetric unit (Figure 1) contains one Nd 3+ cation, one fully-deprotonated L − ligand, two half oxalate ligands, one coordinated water molecule and one non-coordinated water molecule distributed on two positions (O10A and O10B) with the occupancy rates of 0.54 and 0.46, respectively.

Crystal Structure of [Ln(L)(ox)(H 2 O)]•H 2 O with Ln = Nd
In the refined model, the asymmetric unit (Figure 1) contains one Nd 3+ cation, one fully-deprotonated L − ligand, two half oxalate ligands, one coordinated water molecule and one non-coordinated water molecule distributed on two positions (O10A and O10B) with the occupancy rates of 0.54 and 0.46, respectively.In the network, Nd 3+ ions are surrounded by nine oxygens in a distorted monocapped square antiprism.Oxygen atoms belong to three different imidazolium ligands (four oxygens), two different oxalate ligands (four oxygens) and one to the coordinated water molecule.One imidazolium ligand coordinates three Nd 3+ ions and displays a μ3-μ2O3; κ 2 O3O4; κ'O1 coordination mode (Figure 2).One carboxylate function of the imidazolium ligand is coordinated in monodentate mode (O1).The second carboxylate function of the imidazolium ligand bridges two Nd 3+ ions by O3 while it is coordinated to one Nd 3+ ion with O4.The oxalate ligand coordinates two Nd 3+ ions in bis-bidentate bridging mode as already reported [33].These different modes of coordination give rise to dimers of lanthanide ions.These dimeric units are extended in a two-dimensional (2D) network with layers parallel to the a0c plan through oxalates with perpendicular orientation and di-oxo bridges (Figures 2 and 3a).The cavities of the network are filled by free water molecules almost equally distributed on the two different positions O10A and 10B (Figures 2 and 3b).In the network, Nd 3+ ions are surrounded by nine oxygens in a distorted monocapped square antiprism.Oxygen atoms belong to three different imidazolium ligands (four oxygens), two different oxalate ligands (four oxygens) and one to the coordinated water molecule.One imidazolium ligand coordinates three Nd 3+ ions and displays a µ 3 -µ 2 O 3 ; κ 2 O 3 O 4 ; κ'O 1 coordination mode (Figure 2).One carboxylate function of the imidazolium ligand is coordinated in monodentate mode (O1).The second carboxylate function of the imidazolium ligand bridges two Nd 3+ ions by O3 while it is coordinated to one Nd 3+ ion with O4.The oxalate ligand coordinates two Nd 3+ ions in bis-bidentate bridging mode as already reported [33].These different modes of coordination give rise to dimers of lanthanide ions.These dimeric units are extended in a two-dimensional (2D) network with layers parallel to the a0c plan through oxalates with perpendicular orientation and di-oxo bridges (Figures 2  and 3a).The cavities of the network are filled by free water molecules almost equally distributed on the two different positions O10A and 10B (Figures 2 and 3b).In the refined model, the asymmetric unit (Figure 1) contains one Nd 3+ cation, one fully-deprotonated L − ligand, two half oxalate ligands, one coordinated water molecule and one non-coordinated water molecule distributed on two positions (O10A and O10B) with the occupancy rates of 0.54 and 0.46, respectively.In the network, Nd 3+ ions are surrounded by nine oxygens in a distorted monocapped square antiprism.Oxygen atoms belong to three different imidazolium ligands (four oxygens), two different oxalate ligands (four oxygens) and one to the coordinated water molecule.One imidazolium ligand coordinates three Nd 3+ ions and displays a μ3-μ2O3; κ 2 O3O4; κ'O1 coordination mode (Figure 2).One carboxylate function of the imidazolium ligand is coordinated in monodentate mode (O1).The second carboxylate function of the imidazolium ligand bridges two Nd 3+ ions by O3 while it is coordinated to one Nd 3+ ion with O4.The oxalate ligand coordinates two Nd 3+ ions in bis-bidentate bridging mode as already reported [33].These different modes of coordination give rise to dimers of lanthanide ions.These dimeric units are extended in a two-dimensional (2D) network with layers parallel to the a0c plan through oxalates with perpendicular orientation and di-oxo bridges (Figures 2 and 3a).The cavities of the network are filled by free water molecules almost equally distributed on the two different positions O10A and 10B (Figures 2 and 3b    The Nd-O distances range from 2.413(3) Å to 2.731(3) Å (and from 2.385(4) to 2.716(3) Å for the compound [Sm(L)(ox)(H2O)]•H2O).These bond lengths are comparable to those observed in similar compounds [26,34].Nd-Nd distances are equal to 4.221(1) Å ( 4.189(1) Å for Sm-Sm) through the O3 di-oxo bridge and 6.285(1) Å ( 6.234(1) Å for Sm-Sm) through the oxalate ligand.
The Nd1 and Nd2 ions in the asymmetric unit are connected by the oxalate ligand (O10, O8, O9 and O13) in a bis-bidentate bridging mode.The carboxylate functions (O1 and O2, O14 and O5) of two different imidazolium ligands link together asymmetric units along the a axis.
The Nd1 ions are linked together through the carboxylate functions (O11 and O6) of two different imidazolium ligands in a bridging bidentate mode (the Nd1-Nd1 distance is 5.20 Å), whereas the Nd2 ions are interconnected by the carboxylate functions of two other ligands (the Nd2-Nd2 distance is 10.88 Å) (Figure 5).These carboxylate functions are involved in a bidentate chelate coordination mode through O3 and O4 and in a bridging bidentate coordination mode through O4 (Figure 5).The Nd1 ions are linked together through the carboxylate functions (O11 and O6) of two different imidazolium ligands in a bridging bidentate mode (the Nd1-Nd1 distance is 5.20 Å), whereas the Nd2 ions are interconnected by the carboxylate functions of two other ligands (the Nd2-Nd2 distance is 10.88 Å) (Figure 5).These carboxylate functions are involved in a bidentate chelate coordination mode through O3 and O4 and in a bridging bidentate coordination mode through O4 (Figure 5).As for the nitrate anions, one is coordinated to Nd1 in a bidentate chelate mode, while the second is free as previously reported for the compound [Nd(L)2(H2O)2][NO3].3H2O [26].The bidentate chelate coordination mode of the nitrate anion is often reported in the literature [35,36].Moreover, the imidazolium ligand is coordinated in a trans mode contrary to the previous structure [Ln(L)(ox)(H2O)]•H2O with Ln = Nd 3+ or Sm 3+ where a cis mode is observed.The trans mode gives rise to a 3D structure showing staircase linking of the Nd 3+ ions/oxalate chains (see Figure 6).The free nitrate anions are located in the cavities of the structure.The Nd-O distances vary from 2.360(8) Å to

Crystal Structure of [Nd(L)(ox)0.5(H2O)2][Cl]
The compound [Nd(L)(ox)0.18)° (see Table 1).The asymmetric unit contains one Nd 3+ ion, one ligand L − , one half oxalate ligand, one free chloride anion and two coordinated water molecules (Figure 7).In this structure, the Nd 3+ ions are surrounded by nine oxygens belonging to two water molecules (O6 and O8), one oxalate ligand (O3 and O4) and three different imidazolium ligands (O1, O2, O5, O5x,y,−1+z and O7).The two carboxylate functions of each imidazolium ligand show different coordination modes.One carboxylate function coordinates two Nd ions through O1 and O7 in a bridging mode (O1, O7), while the second coordinates two Nd ions through O2 and O5 in a chelating bridging mode (μ2O5; κ 2 O5O2).These different coordination modes are alternated, leading to the formation of a chain of dimeric units linked together by the oxalate ligand forming sheets parallel to the a,b plane.In addition, each chain is linked to another by the imidazolium ligand giving rise to a tridimensional network (Figure 8a).The chloride anion is present in the cavities of the network and is involved in hydrogen bonds with the coordinated water molecules (Figure 8b).
These different coordination modes are alternated, leading to the formation of a chain of dimeric units linked together by the oxalate ligand forming sheets parallel to the a,b plane.In addition, each chain is linked to another by the imidazolium ligand giving rise to a tridimensional network (Figure 8a).The chloride anion is present in the cavities of the network and is involved in hydrogen bonds with the coordinated water molecules (Figure 8b)., the physical properties are not presented in the following since this compound was not obtained as a pure phase (Figure S2).
All features in the FTIR powder spectra are consistent with the single crystal structures (Figure S3) and the SEM analyses in composition mode confirm the composition of the different structures (Figure S4).
The thermal analysis of [Sm(L)(ox)(H2O)]•H2O reveals a first endothermic weight loss at 240 °C corresponding to the departure of the uncoordinated and the coordinated water molecules (calc.7.87%; exp.7.70%).The second weight loss observed between 300 °C and 800 °C is associated with exothermic peaks and corresponds to the decomposition of the organic species (i.e., oxalate and imidazolium ligand) and the formation of the oxide Sm2O3 (calc.58.62%; exp.57.25%).
[Nd(L)(ox)(H2O)]•H2O shows a similar behavior.It shows a first exothermic weight loss at 190 °C corresponding to the loss of the water molecules (calc.7.98%; exp.8.98%) and a second one between 300 °C and 700 °C (calc.59.48%; exp.[Cl] is reminiscent of that reported in other structures involving lanthanide ions (La 3+ or Dy 3+ ) and linear imino diacetic acid [37].In the latter, a "pillared" structure was observed, the ligand linking lanthanide layers; while in the present case, due to the geometry of the 1,3-carboxymethylimidazolium ligand, a staircase linking is observed between adjacent chains.
All features in the FTIR powder spectra are consistent with the single crystal structures (Figure S3) and the SEM analyses in composition mode confirm the composition of the different structures (Figure S4).

Thermal Analyses
The thermal analyses of the three compounds [Nd 2 (L)(ox 9.
The thermal analysis of [Sm(L)(ox)(H 2 O)]•H 2 O reveals a first endothermic weight loss at 240 • C corresponding to the departure of the uncoordinated and the coordinated water molecules (calc.7.87%; exp.7.70%).The second weight loss observed between 300 • C and 800 • C is associated with exothermic peaks and corresponds to the decomposition of the organic species (i.e., oxalate and imidazolium ligand) and the formation of the oxide Sm 2 O 3 (calc.58  C and 140 • C corresponds well to the departure of nitric acid (calc.5.87%; exp.5.43%).This loss is immediately followed by an endothermic event between 130 • C and 240 • C, which corresponds to the departure of the two water molecules and one hydroxide (calc.6.73%; exp.5.94%).The successive exothermic weight losses between 200 • C and 650 • C correspond to the decomposition of the organic species (i.e., imidazolium and oxalate ligands) and the nitrate concomitant with the formation of Nd 2 O 3 (calc.50.84%; exp.52
The successive exothermic weight losses between 200 °C and 650 °C correspond to the decomposition of the organic species (i.e., imidazolium and oxalate ligands) and the nitrate concomitant with the formation of Nd2O3 (calc.50.84%; exp.52.79%).
The successive exothermic weight losses between 200 °C and 650 °C correspond to the decomposition of the organic species (i.e., imidazolium and oxalate ligands) and the nitrate concomitant with the formation of Nd2O3 (calc.50.84%; exp.52.79%).

Luminescent Properties
The luminescent properties of ] have been investigated in the solid state at room temperature.
The compound [Nd(L)(ox)(H2O)]•H2O does not display luminescence.This quenching of luminescence may be due to the presence of the uncoordinated water molecules in the interstitial sites, as previously reported [42,43].
The compound [Nd(L)(ox)(H2O)]•H2O does not display luminescence.This quenching of luminescence may be due to the presence of the uncoordinated water molecules in the interstitial sites, as previously reported [42,43].
The χT product for [Sm(L)(ox)(H2O)]•H2O decreases linearly from 0.37 emu•K•mol −1 at 300 K to 0.02 emu•K•mol −1 at 1.8 K (see Figure 13).The value of the χT product at 300 K is the expected value for the isolated Sm 3+ ion (S = 5/2, gJ = 2/7) [44].At 52 K, the value of the χT product is equal to  Fitting the χ vs. T curve with the above expression was not successful.We thus took into account a mean magnetic interaction between z neighboring Sm 3+ ions zJ'.The expression of the susceptibility then becomes: where gJ is the Zeeman factor for Sm 3+ ions.A good fit of the magnetic data was obtained between 300 K and 25 K with the best refined values λ = 256.5(2)cm −1 and zJ' = −4.11(3)cm −1 .These values are consistent with other values reported in the literature [33,44].
The spin-orbit coupling parameter allows then to determine the gap between the 6 H5/2 ground state and the first excited state 6 H7/2 of the Sm 3+ ion.The gap is given by E = 7λ/2 = 898(1) cm −1 [46].This value is consistent with the value determined by the emission spectra (760 cm −1 ) despite the free ion approximation.
The magnetic behavior of [Nd2(L)(ox)(NO3)(H2O)3][NO3] is presented in Figure 14a.The χT product decreases from 1.51 14b is similar.The χT product decreases from 1.37 emu•K•mol −1 at 300 K to 0.58 emu•K•mol −1 at 1.8 K. Fitting the χ vs. T curve with the above expression was not successful.We thus took into account a mean magnetic interaction between z neighboring Sm 3+ ions zJ'.The expression of the susceptibility then becomes: where g J is the Zeeman factor for Sm 3+ ions.A good fit of the magnetic data was obtained between 300 K and 25 K with the best refined values λ = 256.5(2)cm −1 and zJ' = −4.11(3)cm −1 .These values are consistent with other values reported in the literature [33,44].
The spin-orbit coupling parameter allows then to determine the gap between the 6 H 5/2 ground state and the first excited state 6 H 7/2 of the Sm 3+ ion.The gap is given by E = 7λ/2 = 898(1) cm −1 [46].This value is consistent with the value determined by the emission spectra (760 cm −1 ) despite the free ion approximation.Such behavior is commonly encountered for the isolated Nd 3+ ion [33,[47][48][49].The values of the χT products at 300 K are close to that expected for the free Nd 3+ ion (1.64 emu•K•mol −1 for gJ = 8/11).The decreasing of the χT product stems from the thermal depopulation of the low lying crystal-field states.For the Nd 3+ ion, the first excited state is located at 2000 cm −1 above the ground state, and then, only the ground state is thermally populated even at 300 K. To go further, we have considered that Nd 3+ ions may exhibit a splitting of mj levels in an axial crystal field.The magnetic susceptibility can be described with the following expression [50] where Δ is the zero field splitting parameter, N the Avogadro number, β the Bohr magneton, k the Boltzmann constant and g is the Zeeman factor for Nd 3+ ions.The fit was performed between 75 K and 300 K (red line in Figure 14a,b), and the best refined values Δ are equal to 2.79(1) cm −1 and 3.06(1) cm −1 for [Nd2(L)(ox)(NO3)(H2O)3][NO3] and [Nd(L)(ox)(H2O)]•H2O, respectively.These values are slightly higher than those encountered in the literature [33,44].

Discussion
In the case of the samarium nitrate (see Figure 15), one structure [Sm(L)(ox)(H2O)]•H2O is obtained when oxalic acid is added to the reaction medium whatever the nature of the imidazolium ligand (i.e., zwitterionic [HL] or chloride salt [H2L][Cl]).When the same reaction is realized without oxalic acid, the formation of a gel is observed whatever the nature of the imidazolium ligand ([HL] or [H2L][Cl]).Such behavior is commonly encountered for the isolated Nd 3+ ion [33,[47][48][49].The values of the χT products at 300 K are close to that expected for the free Nd 3+ ion (1.64 emu•K•mol −1 for g J = 8/11).The decreasing of the χT product stems from the thermal depopulation of the low lying crystal-field states.For the Nd 3+ ion, the first excited state is located at 2000 cm −1 above the ground state, and then, only the ground state is thermally populated even at 300 K. To go further, we have considered that Nd 3+ ions may exhibit a splitting of m j levels in an axial crystal field.The magnetic susceptibility can be described with the following expression [50]:

Discussion
In the case of the samarium nitrate (see Figure 15), one structure [Sm(L)(ox)(H 2 O)]•H 2 O is obtained when oxalic acid is added to the reaction medium whatever the nature of the imidazolium ligand (i.e., zwitterionic [HL] or chloride salt [H 2 L][Cl]).When the same reaction is realized without oxalic acid, the formation of a gel is observed whatever the nature of the imidazolium ligand ([HL] or [H 2 L][Cl]).
In the case of the neodymium nitrate (see Figure 15), the situation is slightly different and more complicated since three different structures are obtained depending on the synthesis conditions.In the presence of oxalic acid, the use of imidazolium ligand in its zwitterionic form The in situ formation of the oxalate ligand is worth noticing here.Such an occurrence was previously reported and attributed to four possible main mechanisms: (i) the decomposition of the organic species [44,51,52]; (ii) the decarboxylation of the organic species followed by a reductive coupling of the carbon dioxide [53]; (iii) the oxidation of ethanol in the presence of nitrate anions [54]; and (iv) the hydrolysis followed by an oxidation and a decomposition of the organic species [55].However, in the conditions used here in the presence of nitrates and alcohol, it is difficult to discuss these mechanisms further.In the case of the neodymium nitrate (see Figure 15), the situation is slightly different and more complicated since three different structures are obtained depending on the synthesis conditions.In the presence of oxalic acid, the use of imidazolium ligand in its zwitterionic form The in situ formation of the oxalate ligand is worth noticing here.Such an occurrence was previously reported and attributed to four possible main mechanisms: (i) the decomposition of the organic species [44,51,52]; (ii) the decarboxylation of the organic species followed by a reductive coupling of the carbon dioxide [53]; (iii) the oxidation of ethanol in the presence of nitrate anions [54]; and (iv) the hydrolysis followed by an oxidation and a decomposition of the organic species [55].However, in the conditions used here in the presence of nitrates and alcohol, it is difficult to discuss these mechanisms further.
Nevertheless, in light of our results, it appears that the nature of the coordination networks obtained depends on three factors, which are more or less entangled: (i) the nature of the imidazolium ligand; (ii) the presence or not of oxalic acid in the parent mixture; and (iii) the nature of the lanthanide.Concerning the compounds obtained with Sm 3+ ions, the situation is relatively simple since obtaining crystalline networks depends mainly on the presence or absence of oxalic acid.However, the situation is more complicated for the compounds obtained with Nd 3+ ions.It is difficult to draw definitive conclusions, but it seems that the chloride anion of the imidazolium salt plays a role in the formation of the oxalate ligand and is in competition with the oxalate ligand when they are both present as starting reactants.
The modification of the obtained structures through the conditions of reaction leads for the compounds based on Nd 3+ ions to the quenching of the luminescence properties.Indeed, in the case Nevertheless, in light of our results, it appears that the nature of the coordination networks obtained depends on three factors, which are more or less entangled: (i) the nature of the imidazolium ligand; (ii) the presence or not of oxalic acid in the parent mixture; and (iii) the nature of the lanthanide.Concerning the compounds obtained with Sm 3+ ions, the situation is relatively simple since obtaining crystalline networks depends mainly on the presence or absence of oxalic acid.However, the situation is more complicated for the compounds obtained with Nd 3+ ions.It is difficult to draw definitive conclusions, but it seems that the chloride anion of the imidazolium salt plays a role in the formation of the oxalate ligand and is in competition with the oxalate ligand when they are both present as starting reactants.
The modification of the obtained structures through the conditions of reaction leads for the compounds based on Nd 3+ ions to the quenching of the luminescence properties.Indeed, in the case of [Nd(L)(ox)(H 2 O)]•H 2 O, the luminescence is quenched due to the presence of free water molecules, which is not the case with [Nd 2 (L)(ox)(NO 3 )(H 2 O) 3 ][NO 3 ] possessing free nitrate anions.On the other hand, the magnetic behavior stays almost identical whatever the structure and is typical of isolated 4f ions with low antiferromagnetic interactions (through the oxalate ligand essentially) for the [Sm(L)(ox)(H 2 O)]•H 2 O.The value of these antiferromagnetic interactions is in the same range than those reported in the literature [33,44].Moreover, the detailed study of the luminescence allows corroborating the approximation of free ion used in the analysis of the magnetic data.

Conclusions
The synthesis and the characterization of four new hybrid networks based on imidazolium dicarboxylate salts and Nd 3+ or Sm 3+ ions have been reported.The effect of the nature of the imidazolium salts, of the lanthanide ions, as well as of the presence of oxalic acid has been highlighted for the obtained networks.For the Sm 3+ ions, the compound [Sm(L)(ox)(H 2 O)]•H 2 O is obtained only in the presence of oxalic acid whatever the nature of the imidazolium salt.For the compounds based on Nd 3+ ions, three different compounds can be obtained according to the reaction conditions.In the presence of oxalic acid, the chloride form of the imidazolium salt leads to a biphasic product constituted of [Nd(L)(ox) 0.

Figure 2 .
Figure 2. Selected view showing the different coordination modes in the structure of [Nd(L)(ox)(H 2 O)]•H 2 O (red: oxygen; grey: carbon; blue: nitrogen; and green: neodymium).H atoms are omitted for clarity.

Figure 3 .
Figure 3. Selected packing view of the crystal structure of [Nd(L)(ox)(H2O)]•H2O showing: (a) the 2D character along the c axis; and (b) the cavities of the network along the b axis (red: oxygen; grey: carbon; blue: nitrogen; and green: neodymium).H atoms are omitted for clarity.

Figure 3 .
Figure 3. Selected packing view of the crystal structure of [Nd(L)(ox)(H 2 O)]•H 2 O showing: (a) the 2D character along the c axis; and (b) the cavities of the network along the b axis (red: oxygen; grey: carbon; blue: nitrogen; and green: neodymium).H atoms are omitted for clarity.

Figure 3 .
Figure 3. Selected packing view of the crystal structure of [Nd(L)(ox)(H2O)]•H2O showing: (a) the 2D character along the c axis; and (b) the cavities of the network along the b axis (red: oxygen; grey: carbon; blue: nitrogen; and green: neodymium).H atoms are omitted for clarity.

Figure 8 .
Figure 8. Selected view: (a) of the packing along the a axis; and (b) of the hydrogen bondings between the chloride anion and the networks (blue line) in [Nd(L)(ox)0.5(H2O)2][Cl](red: oxygen; grey: carbon; blue: nitrogen; and green: neodymium).H atoms are omitted for clarity.

Figure 8 .
Figure 8. Selected view: (a) of the packing along the a axis; and (b) of the hydrogen bondings between the chloride anion and the networks (blue line) in [Nd(L)(ox) 0.5 (H 2 O) 2 ][Cl] (red: oxygen; grey: carbon; blue: nitrogen; and green: neodymium).H atoms are omitted for clarity.

Figure 13 .
Figure 13.Plots of χ (closed circles) and χT (open circles) versus T for [Sm(L)(ox)(H2O)]•H2O.The red curves correspond to the fit of the data following the expressions mentioned in the text.

1 )Figure 13 .
Figure 13.Plots of χ (closed circles) and χT (open circles) versus T for [Sm(L)(ox)(H 2 O)]•H 2 O.The red curves correspond to the fit of the data following the expressions mentioned in the text.
[HL] leads to the formation of the pure phase [Nd 2 (L)(ox)(NO 3 )(H 2 O) 3 ][NO 3 ], while the use of the imidazolium ligand in its chloride form [H 2 L][Cl] leads to a biphasic product, where the two phases have been identified as being [Nd(L)(ox)(H 2 O)]•H 2 O and [Nd(L)(ox) 0.5 (H 2 O) 2 ][Cl].In the absence of oxalic acid, the same reaction realized with [HL] leads to a gel, while the use of [H 2 L][Cl] leads to crystallized [Nd

Figure 15 .
Figure 15.Recapitulative scheme indicating the compounds obtained in the water/ethanol mixture as a function of the synthesis conditions.

Figure 15 .
Figure 15.Recapitulative scheme indicating the compounds obtained in the water/ethanol mixture as a function of the synthesis conditions.