Closing the Circle of the Lanthanide-Murexide Series : Single-Molecule Magnet Behavior and Near-Infrared Emission of the Nd III Derivative

Up to now, even if murexide-based complexometric studies are performed with all 3d or 4f ions, the crystal structures of the light-lanthanide derivatives of the lanthanide-murexide series are unknown. In this work, we report the crystal structure of the NdIII derivative named NdMurex. Contrary to all known complexes of the 3d or 4f series, a dimeric compound was obtained. As for its already reported DyIII and YbIII parents, the NdIII complex responsible for the color-change behaves as a single-molecule magnet (SMM). This behavior was observed on both the crystalline (NdMurex: Ueff = 6.20(0.80) K, 4.31 cm−1; τ0 = 2.20(0.92)× 10−5 s, Hdc = 1200 Oe) and anhydrous form (NdMurexAnhy: Ueff = 6.25(0.90) K, 4.34 cm−1; τ0 = 4.85(0.40) × 10−5 s, Hdc = 1200 Oe). The SMM behavior is reported also for the anhydrous CeIII derivative (CeMurexAnhy: Ueff = 5.40(0.75) K, 3.75 cm−1; τ0 = 3.02(1.10) × 10−5 s, Hdc = 400 Oe). The Near-Infrared Emission NIR emission was observed for NdMurexAnhy and highlights its bifunctionality.


Introduction
The interest for multifunctional magnetic material has been boosted by the (re)introduction of lanthanide ions [1] in the field of molecular magnetism [2][3][4].Indeed, in addition to their appealing magnetic properties they have also intrinsic assets such as catalytic activity or photo-luminescent properties [5][6][7][8][9][10].This last point opens the way to magnetic and luminescent materials that can be of particular interest [11][12][13].
Murexide ligand (Scheme 1) is also known as purpuric acid or ammonium salt of 2,6-dioxo-5-[(2,4,6-trioxo-5-hexahydropyrimidinylidene)amino]-3H-pyrimidin-4-olate, abbreviated (NH 4 L) [14,15].This ligand has been widely used in complexometric studies with 3d or 4f ions [16][17][18][19].The complexation triggered a very strong color-change from purple to red-orange either in water or in organic solvent [14,[16][17][18][19].The crystal structure of the complex responsible of this color-change is known for 3d ions [20-27], but this is only very recently that some of us managed to determine it for lanthanides from Sm III to Lu III [28,29].The obtained neutral Ln(L) 3 •xH 2 O complexes were found to behave as single-molecule magnets (SMM) for Ln = Dy [28] or Yb [29] (respectively DyMurex and YbMurex).For DyMurex, the original N 3 O 6 environment of the Dy III ion permitted to demonstrate the crucial role played by the local symmetry of the oxygen surrounding over the nitrogen one.It shows also how the dipolar electrostatic field around the Dy III (and not the local charge) is the governing factor of its magnetic behavior [28].For YbMurex, significant near-infrared (NIR) emissions have been observed that permitted to perform one of the first NIR magneto-luminescent correlations.It evidenced also that the contribution of the Yb III excited-states to the emissions have to be carefully considered in such procedures [29].
(NIR) emissions have been observed that permitted to perform one of the first NIR magneto-luminescent correlations.It evidenced also that the contribution of the Yb III excited-states to the emissions have to be carefully considered in such procedures [29].
In this study, we have been able to obtain information on the crystal structure of two of the missing light lanthanide ions from the series (Ln = Ce III and Nd III ).Light lanthanide ions have so far drawn less attention than heavier ones in the field of molecular magnetism [30].This is somewhat intriguing as most permanent magnets are made of Nd III or Sm III ions [31].We report here the synthesis and magnetic properties of the Ce III (CeMurex) and Nd III derivatives (NdMurex).For the later a bifunctional compound was obtained as NIR emission was observed.

Crystal Structure
NdMurex crystallizes in the triclinic system, P-1 space group (n°2, see Table S1).The asymmetric unit is made of one Nd III ion coordinated by two murexide ligands and three water molecules.The murexide ligand acts a tri-dentate ligand and a capped N2O7 coordination environment around the Nd III ion formed.Nd-O and Nd-N distances are in the 2.42 Å-2.67 Å and 2.74 Å-2.86 Å range, respectively (Table S2).SHAPE analysis revealed that the coordination polyhedron was either a distorted capped square antiprism (C4v) or a spherical capped square antiprism (C4d) (Figure 1, Table S3) [32,33].One free murexide ligand ensures the electroneutrality of the asymmetric unit and seven uncoordinated water molecules were found.An inversion center located close to one of the two murexide ligands creates a dimeric structure [34] with Nd-Nd distance of 7.55 Å (see Figures 1 and 2, Figures S1-S3) and fourteen water molecules in the intermolecular space.The overall formula of NdMurex is then [Nd2(L)4(H2O)6](L)2(H2O)14 (Figure 1).The shortest intermolecular Nd-Nd distance is 9.58 Å.
It is interesting to compare this compound with the ones obtained with heavier lanthanide ions (Ln = Sm to Lu), such as DyMurex [28] and YbMurex [29] where neutral monomeric complex of formula Ln(L)3•xH2O where formed.The striking originality in NdMurex is the ability of the murexide ligand to bind two different Nd III ions either via its tris-dentate clamp but also via one of the two external carbonyl group that were left uncoordinated in the case of Ln(L)3•xH2O.This extra coordination is made via the longest Nd-O distance (Nd-O6 = 2.67 Å) and is stabilized via strong stacking between two symmetry-related murexide ligand (Figure 2).It is likely that the ionic size of the lanthanide ion is the governing factor in stabilizing the dimeric crystalline structure over the monomeric one.In this study, we have been able to obtain information on the crystal structure of two of the missing light lanthanide ions from the series (Ln = Ce III and Nd III ).Light lanthanide ions have so far drawn less attention than heavier ones in the field of molecular magnetism [30].This is somewhat intriguing as most permanent magnets are made of Nd III or Sm III ions [31].We report here the synthesis and magnetic properties of the Ce III (CeMurex) and Nd III derivatives (NdMurex).For the later a bifunctional compound was obtained as NIR emission was observed.

Crystal Structure
NdMurex crystallizes in the triclinic system, P-1 space group (n • 2, see Table S1).The asymmetric unit is made of one Nd III ion coordinated by two murexide ligands and three water molecules.The murexide ligand acts a tri-dentate ligand and a capped N 2 O 7 coordination environment around the Nd III ion formed.Nd-O and Nd-N distances are in the 2.42 Å-2.67 Å and 2.74 Å-2.86 Å range, respectively (Table S2).SHAPE analysis revealed that the coordination polyhedron was either a distorted capped square antiprism (C 4v ) or a spherical capped square antiprism (C 4d ) (Figure 1, Table S3) [32,33].One free murexide ligand ensures the electroneutrality of the asymmetric unit and seven uncoordinated water molecules were found.An inversion center located close to one of the two murexide ligands creates a dimeric structure [34] with Nd-Nd distance of 7.55 Å (see Figures 1  and 2, Figures S1-S3) and fourteen water molecules in the intermolecular space.The overall formula of NdMurex is then [Nd 2 (L) 4 (H 2 O) 6 ](L) 2 (H 2 O) 14 (Figure 1).The shortest intermolecular Nd-Nd distance is 9.58 Å.
It is interesting to compare this compound with the ones obtained with heavier lanthanide ions (Ln = Sm to Lu), such as DyMurex [28] and YbMurex [29] where neutral monomeric complex of formula Ln(L) 3 •xH 2 O where formed.The striking originality in NdMurex is the ability of the murexide ligand to bind two different Nd III ions either via its tris-dentate clamp but also via one of the two external carbonyl group that were left uncoordinated in the case of Ln(L) 3 •xH 2 O.This extra coordination is made via the longest Nd-O distance (Nd-O6 = 2.67 Å) and is stabilized via strong stacking between two symmetry-related murexide ligand (Figure 2).It is likely that the ionic size of the lanthanide ion is the governing factor in stabilizing the dimeric crystalline structure over the monomeric one.

Thermal Stability
The purple crystals of NdMurex are stable for some hours out of their mother solution.Then, cracks appear on the crystal's surfaces associated with the loss of crystallization water molecules.This has been confirmed by thermogravimetric analysis (TGA/DTA) where 18% weight loss, compatible with the loss of 14 water molecules, was observed between 20 and 200 • C (Figure 3).The anhydrous derivative formed, named NdMurexAnhy, was stable up to 300 • C. Similar dehydration was observed for an overnight room-temperature drying of NdMurex crystals.A crystalline powder was obtained and powder X-ray diffraction evidenced a clear phase transition upon dehydration (Figure S4).The Ce III derivative was obtained only as this anhydrous form named CeMurexAnhy and was isostructural to NdMurexAnhy (Figure S4).

Thermal Stability
The purple crystals of NdMurex are stable for some hours out of their mother solution.Then, cracks appear on the crystal's surfaces associated with the loss of crystallization water molecules.This has been confirmed by thermogravimetric analysis (TGA/DTA) where 18% weight loss, compatible with the loss of 14 water molecules, was observed between 20 and 200 °C (Figure 3).The anhydrous derivative formed, named NdMurexAnhy, was stable up to 300 °C.Similar dehydration was observed for an overnight room-temperature drying of NdMurex crystals.A crystalline powder was obtained and powder X-ray diffraction evidenced a clear phase transition upon dehydration (Figure S4).The Ce III derivative was obtained only as this anhydrous form named CeMurexAnhy and was isostructural to NdMurexAnhy (Figure S4).

Magnetic Characterization
Static magnetic measurements of NdMurex have been performed at 1000 Oe on fresh crystals crushed into teflon pellets.The room temperature χMT value was 3.16 emu•K•mol −1 in good agreement with the theoretical value expected for two isolated Nd III ion ( 4 I9/2, gJ = 8/11, J = 9/2, χMT300K = 2 × 1.64 emu•K•mol −1 ).This confirms that most of crystallization water molecules were retained at this point (all crystallization water molecules considered in NdMurex molecular weight).As the temperature was lowered, χMT decreased to reach 1.11 emu•K•mol −1 at 2 K (Figure 4).This decrease was attributed to a standard depopulation of Nd III Stark's sub-levels as long intra-and intermolecular distances preclude the occurrence of Nd-Nd magnetic interactions.

Magnetic Characterization
Static magnetic measurements of NdMurex have been performed at 1000 Oe on fresh crystals crushed into teflon pellets.The room temperature χ M T value was 3.16 emu•K•mol −1 in good agreement with the theoretical value expected for two isolated Nd III ion ( 4 I 9/2 , g J = 8/11, J = 9/2, χ M T 300K = 2 × 1.64 emu•K•mol −1 ).This confirms that most of crystallization water molecules were retained at this point (all crystallization water molecules considered in NdMurex molecular weight).As the temperature was lowered, χ M T decreased to reach 1.11 emu•K•mol −1 at 2 K (Figure 4).This decrease was attributed to a standard depopulation of Nd III Stark's sub-levels as long intra-and intermolecular distances preclude the occurrence of Nd-Nd magnetic interactions.

Thermal Stability
The purple crystals of NdMurex are stable for some hours out of their mother solution.Then, cracks appear on the crystal's surfaces associated with the loss of crystallization water molecules.This has been confirmed by thermogravimetric analysis (TGA/DTA) where 18% weight loss, compatible with the loss of 14 water molecules, was observed between 20 and 200 °C (Figure 3).The anhydrous derivative formed, named NdMurexAnhy, was stable up to 300 °C.Similar dehydration was observed for an overnight room-temperature drying of NdMurex crystals.A crystalline powder was obtained and powder X-ray diffraction evidenced a clear phase transition upon dehydration (Figure S4).The Ce III derivative was obtained only as this anhydrous form named CeMurexAnhy and was isostructural to NdMurexAnhy (Figure S4).

Magnetic Characterization
Static magnetic measurements of NdMurex have been performed at 1000 Oe on fresh crystals crushed into teflon pellets.The room temperature χMT value was 3.16 emu•K•mol −1 in good agreement with the theoretical value expected for two isolated Nd III ion ( 4 I9/2, gJ = 8/11, J = 9/2, χMT300K = 2 × 1.64 emu•K•mol −1 ).This confirms that most of crystallization water molecules were retained at this point (all crystallization water molecules considered in NdMurex molecular weight).As the temperature was lowered, χMT decreased to reach 1.11 emu•K•mol −1 at 2 K (Figure 4).This decrease was attributed to a standard depopulation of Nd III Stark's sub-levels as long intra-and intermolecular distances preclude the occurrence of Nd-Nd magnetic interactions.A pellet of overnight dried NdMurexAnhy was measured and the room temperature χ M T value was 3.22 emu•K•mol −1 .Temperature dependence of its magnetic behavior was then highly similar to the one of NdMurex (Figure 4).Same procedures were used with CeMurexAnhy with a room temperature χ M T value of 1.82 emu•K•mol −1 in agreement with the expected value ( 2 F 5/2 , g J = 6/7, J = 5/2, χ M T 300K = 2 × 0.80 emu•K•mol −1 ) (Figure 4).
Dynamic magnetic measurements were performed on NdMurex and NdMurexAnhy and the optimum external H dc field to observe magnetic slow relaxation was found to be 1200 Oe (Figures S5  and S6).For both samples, frequency dependence of the out-of-phase susceptibility was observed in the 1.8-4.4K region (Figure 5 and Figure S7).The temperature dependence of the relaxation was found to be close to a linear regime and was estimated considering an Orbach relaxation where τ = τ 0 exp(−U eff /k B T) with τ 0 the characteristic relaxation time and U eff the energy barrier for spin reversal.On both samples, very small variations were observed, and U eff almost identical (U eff = 6.20(0.80)K, 4.31 cm −1 (τ 0 = 2.20(0.92)× 10 −5 s) and U eff = 6.25(0.90)K, 4.34 cm −1 (τ 0 = 4.85(0.40)× 10 −5 s) for NdMurex and NdMurexAnhy respectively).Indeed, the dehydration of NdMurex impacts only, and in a moderate way, the characteristic relaxation time (Figures 6 and 7).On both samples the distribution of the relaxation times α was investigated using normalized Cole-Cole plots and an extended Debye equation (Figures 6 and 7).On both samples, stable α values around 0.2 were observed that indicate a moderate distribution of the relaxation times.Large relaxing fractions were observed with (1 − χ S /χ T ) = 91% and 80% at 2 K for NdMurex and NdMurexAnhy, respectively (with χ S = adiabatic susceptibility, χ T isothermal susceptibility, see Tables S4-S7).Overall, these behaviors highlight that the magnetic behavior of NdMurex was not sensitive to dehydration, and consequently that intermolecular interactions play a minor role in its magnetic slow relaxation.
Dynamic magnetic measurements were performed on NdMurex and NdMurexAnhy and the optimum external Hdc field to observe magnetic slow relaxation was found to be 1200 Oe (Figures S5  and S6).For both samples, frequency dependence of the out-of-phase susceptibility was observed in the 1.8-4.4K region (Figure 5 and S7).The temperature dependence of the relaxation was found to be close to a linear regime and was estimated considering an Orbach relaxation where τ = τ0exp(−Ueff/kBT) with τ0 the characteristic relaxation time and Ueff the energy barrier for spin reversal.On both samples, very small variations were observed, and Ueff almost identical (Ueff = 6.20(0.80)K, 4.31 cm −1 (τ0 = 2.20(0.92)•10−5 s) and Ueff = 6.25(0.90)K, 4.34 cm −1 (τ0 = 4.85(0.40)•10−5 s) for NdMurex and NdMurexAnhy respectively).Indeed, the dehydration of NdMurex impacts only, and in a moderate way, the characteristic relaxation time (Figures 6 and 7).On both samples the distribution of the relaxation times α was investigated using normalized Cole-Cole plots and an extended Debye equation (Figures 6 and 7).On both samples, stable α values around 0.2 were observed that indicate a moderate distribution of the relaxation times.Large relaxing fractions were observed with (1 − χS/χT) = 91% and 80% at 2 K for NdMurex and NdMurexAnhy, respectively (with χS = adiabatic susceptibility, χT isothermal susceptibility, see Tables S4-S7).Overall, these behaviors highlight that the magnetic behavior of NdMurex was not sensitive to dehydration, and consequently that intermolecular interactions play a minor role in its magnetic slow relaxation.The cerium derivative, CeMurexAnhy, has been was measured with an optimum external Hdc field of 400 Oe (Figure S8).Frequency dependence of the out-of-phase susceptibility was observed between 1.8 K and 4 K (Figure S9).The magnetic relaxation was also governed by Orbach process  The cerium derivative, CeMurexAnhy, has been was measured with an optimum external Hdc field of 400 Oe (Figure S8).Frequency dependence of the out-of-phase susceptibility was observed between 1.8 K and 4 K (Figure S9).The magnetic relaxation was also governed by Orbach process The cerium derivative, CeMurexAnhy, has been was measured with an optimum external H dc field of 400 Oe (Figure S8).Frequency dependence of the out-of-phase susceptibility was observed between 1.8 K and 4 K (Figure S9).The magnetic relaxation was also governed by Orbach process with U eff = 5.40(0.75)K, 3.75 cm −1 and τ 0 = 3.02(1.10)× 10 −5 s (Figure 7).As for the Nd III derivatives, stable α values around 0.15 were observed, with 92% relaxing fraction at 2 K (Figure 6, Tables S8 and  S9).
The first report of an Nd-SMM was a Nd III with a full nitrogen environment N 9 with an almost perfect D 3h symmetry, but its effective energy barrier was low (U eff = 2.8 cm −1 ) (Table 1) [41].Other examples on sandwich complexes from the COT family (COT = bis(trimethylsilyl)cyclooctatetraenyl dianion) where the Nd III was in C 16 environment shows SMM behavior (U eff = 14.6 cm −1 ) [39].Another organometallic Nd III SMM was reported with Cp* as main ligands (U eff = 29 cm −1 ) [42].Macrocyclic complexes of Nd III ions show mixed magnetic relaxation processes in which U eff ≈ 21 cm −1 or U eff ≈ 51 cm −1 depending on the crown-ether that is considered (O 12 or O 10 N 2 environment) [37].Cadmium-based metalacrowns stabilize hexagonal bipyramidal environment (O 8 ) for Nd III ions with U eff ≈ 15 cm −1 but strong influence of Raman-like relaxation process was observed [40].Nd III derivatives from the polyoxometallates family have also been studied and energy barriers of U eff = 51.4cm −1 [43] and U eff = 34 cm −1 [44] were reported.A detailed study [35] of the magnetic anisotropy of complexes with light lanthanide ions highlights that Ising-type anisotropy can be found in the well-known N 2 O 6 environment that is very common for Dy-based SMMs and U eff ≈ 20 cm −1 for the Nd III derivative is reported.To the best of our knowledge only two polynuclear Nd-SMMs (dimers in these cases) have been reported.The first one is made of 2-fluorobenzoate ligands that bridges two phenantroline-chelated Nd III ions [45].The N 2 O 8 environment that is created, induces a mixed Orbach and Raman-like relaxation with U eff = 9.5 cm −1 .Nd III luminescence is observed but no magneto-luminescent correlation has been possible.On the contrary, such correlation has been possible on the second example [46] that is a carboxylate-bridged dimer with terminal bipyridine ligands (N 2 O 7 environment, mixed Orbach and Raman-like relaxation with U eff = 8 cm −1 ).The splitting of the lowest five Stark levels of the J = 9/2 state of the 4 I 9/2 level is clearly visible.The separation between the ground and first excited m J state is significantly bigger than U eff and confirms the presence of a mixed Orbach and Raman relaxation.
Several one-dimensional compounds made of Nd III display SMM behavior.These are onedimensional assembly of Nd III in O 9 environment (U eff = 18.8 cm −1 ) [47] or benzene carboxylate-based chains with Nd III in O 8 environment that shows mixed Orbach and Raman relaxation processes and U eff values in the 18-21 cm −1 range [36,48].
It is interesting to note that all these examples have been measured using an external static field H dc that ranges from 100 Oe to 3500 Oe.The only examples of zero-field Nd-SMMs reported so far are based on complexes made of phosphonic diamide-or phosphine oxide-based ligands [49,50].The extremely strong geometric constrain [51] imposed by the ligands around the Nd III ions imposes a highly symmetric D 5h pentagonal bipyramid environment (O 7 ) with the apical oxygen atoms from the phosphine groups closer of the Nd III ions than the five equatorial water molecules.This induces a suitable electrostatic surrounding of the Nd III ion that is an oblate ion [52].However the nature of the magnetic relaxation in these two compounds is debated as the initial Orbach mechanisms identified in the first report (U eff = 17.2 cm −1 ) [50] has been revised to pure Raman or mixed Raman and QTM processes with no evidence of Orbach magnetic relaxation [49].It can be noted that some examples may also be considered as relaxing via field-induced paramagnetism and not via standard SMM relaxation pathways.We cannot totally discard this possibility for the three samples we report here.
Last, it is worth noting that U eff values have always been found significantly lower than ab-initio energy gaps when available.Such discrepancy is also visible when magneto-luminescence correlations have been targeted [41].The most striking example is found on the previously cited trispyrazolylborate-based Nd-SMM where U eff = 2.8 cm −1 is observed while a 115 cm −1 energy gap is extrapolated from low-temperature absorption measurements.This indicates that under-barrier fast magnetic relaxation mechanisms can bevery efficient on Nd-SMMs.
In this study, we have measured the luminescence properties of NdMurexAnhy as NdMurex is not suitable for NIR emission because of its numerous crystallization water molecules (Figure 8).At room temperature no emission has been detected, once again the remaining three coordination water molecules are very likely to quench Nd III emission.At 77 K and under λ exc = 629 nm (f -f sensitization) the broad and weak 4 F 3/2 → 4 I 9/2 transition centered on 870 nm is observed together with the characteristic 4 F 3/2 → 4 I 11/2 transition at 1050 nm.For this emission peak, it has not been possible to deconvoluate the different crystal-field contributions of the Stark sub-levels.NdMurexAnhy is thus a NIR-emissive SMM where no magneto-luminescent correlations can be performed [54].

General Procedures and Methods
All chemicals and reagents were of reagent grade (TCI chemicals) and used without further purification.Powder X-ray diffraction patterns were measured with a Panalytical X-Pert Pro diffractometer equipped with an X'celerator detector.Recording condition were 45 kV, 45 mA with Cu Kα (λ = 1.542Å) in θ-θ mode.Simulated patterns from X-ray crystal structure have been obtained with Mercury 3.0 from CCDC.Thermogravimetric and thermodifferential analyses were performed under nitrogen atmosphere (heating rate 5 °C/min) with a PerkinElmer Pyris-Diamond thermal analyzer.Magnetic measurements were performed with a quantum design Magnetic properties Measurement System (MPMS) magnetometer on Teflon pellets.Diamagnetic corrections were applied using Pascal's constants [74].Luminescence measurements performed using a Horiba-Jobin-Yvon Florolog-3 spectrometer equipped with a 450 W Xe lamp and an infrared photodiode cooled by nitrogen liquid (InGaAs, sensitivity 800-1600 nm).Recording conditions were T = 77 K, integ.time 300 ms (10 accumulation ) for excitation spectrum and T = 77 K, integration.time 300 ms (50 acc.).

Synthetic Procedure
0.1 mmol of murexide (ammonium purpurate, "NH4L", 28.42 mg) was solubilized in 20 mL of water under strong stirring and a deep purple solution was obtained.0.1 mmol of Nd(NO3)3•6H2O (45.78 mg) was dissolved in 2 mL of water and quickly added to the NH4L solution.A color change was visible from dark to light purple after stirring for 5 min.At that point, the solution was filtered on sintered glass N°3 for some minutes and left undisturbed for one month at room temperature.Small prismatic purple crystals were then obtained.The Ce III derivative was obtained using a similar procedure but it was not possible to obtain X-ray quality single crystals.Therefore, only its anhydrous derivative was studied here.Elem anal.Calcd for NdMurex C48H64N30Nd2O56: C, 25

Crystal Structure
Crystals were mounted on a Bruker AXS diffractometer (APEXII) and crystal structures were collected at 150 K with Mo Kα (λ = 0.71073 Å) radiation.Data reduction and cell refinement were performed with Denzo and Scalepack [70,71].Crystal structures were solved by SIR97 [72] and

General Procedures and Methods
All chemicals and reagents were of reagent grade (TCI chemicals) and used without further purification.Powder X-ray diffraction patterns were measured with a Panalytical X-Pert Pro diffractometer equipped with an X'celerator detector.Recording condition were 45 kV, 45 mA with Cu K α (λ = 1.542Å) in θ-θ mode.Simulated patterns from X-ray crystal structure have been obtained with Mercury 3.0 from CCDC.Thermogravimetric and thermodifferential analyses were performed under nitrogen atmosphere (heating rate 5 • C/min) with a PerkinElmer Pyris-Diamond thermal analyzer.Magnetic measurements were performed with a quantum design Magnetic properties Measurement System (MPMS) magnetometer on Teflon pellets.Diamagnetic corrections were applied using Pascal's constants [74].Luminescence measurements performed using a Horiba-Jobin-Yvon Florolog-3 spectrometer equipped with a 450 W Xe lamp and an infrared photodiode cooled by nitrogen liquid (InGaAs, sensitivity 800-1600 nm).Recording conditions were T = 77 K, integration time 300 ms (10 accumulation ) for excitation spectrum and T = 77 K, integration time 300 ms (50 accumulation).

Synthetic Procedure
0.1 mmol of murexide (ammonium purpurate, "NH 4 L", 28.42 mg) was solubilized in 20 mL of water under strong stirring and a deep purple solution was obtained.0.1 mmol of Nd(NO 3 ) 3 •6H 2 O (45.78 mg) was dissolved in 2 mL of water and quickly added to the NH 4 L solution.A color change was visible from dark to light purple after stirring for 5 min.At that point, the solution was filtered on sintered glass N • 3 for some minutes and left undisturbed for one month at room temperature.Small prismatic purple crystals were then obtained.The Ce III derivative was obtained using a similar procedure but it was not possible to obtain X-ray quality single crystals.Therefore, only its anhydrous derivative was studied here.Elem anal.

Figure 1 .
Figure 1.The best view of NdMurex with one of the two free murexide molecules and labeling scheme.Inset view of the Nd1 coordination polyhedron.Hydrogen atoms and water molecules are omitted for clarity.

Figure 2 .
Figure 2. Best views of the NdMurex dimeric structure with (a) the asymmetric unit highlighted, and (b) a view parallel to the stacking of murexide ligands.Hydrogen atoms and water molecules are omitted for clarity.

Figure 1 . 14 Figure 1 .
Figure 1.The best view of NdMurex with one of the two free murexide molecules and labeling scheme.Inset view of the Nd1 coordination polyhedron.Hydrogen atoms and water molecules are omitted for clarity.

Figure 2 .
Figure 2. Best views of the NdMurex dimeric structure with (a) the asymmetric unit highlighted, and (b) a view parallel to the stacking of murexide ligands.Hydrogen atoms and water molecules are omitted for clarity.

Figure 2 .
Figure 2. Best views of the NdMurex dimeric structure with (a) the asymmetric unit highlighted, and (b) a view parallel to the stacking of murexide ligands.Hydrogen atoms and water molecules are omitted for clarity.

Figure 5 .
Figure 5. Frequency dependence of the out-of-phase susceptibility of NdMurex (a) and NdMurexAnhy (b) with Hdc = 1200 Oe for temperatures from 1.8 (blue) to 4.4 K (red).Lines are guide to the eye.

Figure 5 .
Figure 5. Frequency dependence of the out-of-phase susceptibility of NdMurex (a) and NdMurexAnhy (b) with H dc = 1200 Oe for temperatures from 1.8 (blue) to 4.4 K (red).Lines are guide to the eye.

Figure 6 .
Figure 6.Cole-Cole plots measured for temperatures from 1.8 (blue) to 5 K (red) and for NdMurex (Hdc = 1200 Oe, a), NdMurexAnhy (Hdc = 1200 Oe, b), and CeMurexAnhy (Hdc = 400 Oe, c) with selected fits and simulation of the ideal distribution of the magnetic relaxation as red curve.

Figure 6 . 14 Figure 6 .
Figure 6.Cole-Cole plots measured for temperatures from 1.8 (blue) to 5 K (red) and for NdMurex (H dc = 1200 Oe, a), NdMurexAnhy (H dc = 1200 Oe, b), and CeMurexAnhy (H dc = 400 Oe, c) with selected fits and simulation of the ideal distribution of the magnetic relaxation as red curve.

Table 1 .
Table of Nd III -based Single-Molecule Magnets with U eff values and references associated.