A New { Dy 5 } Single-Molecule Magnet Bearing the Schiff Base Ligand N-Naphthalidene-2-amino-5-chlorophenol

A new {Dy5} cluster compound has been synthesized and structurally characterized from the initial use of the Schiff base ligand N-naphthalidene-2-amino-5-chlorophenol (nacpH2) in coordination chemistry. The 1:1 reaction between Dy(hpd)3·2H2O and nacpH2, in a solvent mixture comprising CH2Cl2 and MeOH, afforded orange crystals of [Dy5(OH)2(hpd)3(nacp)5(MeOH)5] (1) in 70% yield, where hpd− is the anion of 3,5-heptadione. The {Dy5} complex can be described as two vertical {Dy3(μ3-OH)} triangles sharing a common vertex; such a metal topology is unprecedented in 4f -metal cluster chemistry. Direct current (dc) magnetic susceptibility studies revealed the presence of some weak ferromagnetic exchange interactions between the five DyIII ions at low temperatures. Alternating current (ac) magnetic susceptibility measurements at zero applied dc field showed that complex 1·3MeOH·CH2Cl2 exhibits temperatureand frequency-dependent out-of-phase signals below ~20 K, characteristics of a single-molecule magnet (SMM). The resulting relaxation times were used to construct an Arrhenius-type plot and determine an effective energy barrier, Ueff, of 100 K for the magnetization reversal. The application of a small dc field of 200 Oe resulted in the surpassing of the quantum tunneling process and subsequently the increase of the Ueff to a value of 170 K. The reported results are part of a long-term program aiming at the preparation of structurally and magnetically interesting lanthanide complexes bearing various Schiff base chelating/bridging ligands.


Introduction
The coordination chemistry of 4f -metal ions has recently re-attracted the interest of the scientific community for three primary reasons: (i) their ability to form oligo-and polynuclear metal complexes (clusters) with unprecedented and nanosized structures [1], (ii) the unusual photoluminescence properties they exhibit [2], arising from forbidden 4f -4f electronic transitions that emerge when coordinated organic ligands are involved, and (iii) their pivotal role in the development of efficient single-molecule magnets (SMMs) [3].SMMs are individual molecular compounds that exhibit slow relaxation of their magnetization in the absence of an external magnetic field [4].Such a magnetization reversal is accomplished by the presence of an anisotropic energy barrier, U, which operates through various mechanisms that are either spin-lattice processes (Raman, Orbach and direct mechanism) or quantum tunneling of magnetization (QTM) [5].Due to their molecular properties (i.e., solubility, crystallinity, etc.), SMMs have been proposed as promising candidates for various modern technological advancements, such as in information and data storage, molecular spintronics and quantum computing [6].
In the case of 4f -metal ions, the energy barrier U (or, more explicitly, the effective energy barrier, U eff ) arises from the spin-orbit coupled electronic ground states ( 2S+1 L J ) which can be further split into ±m J microstates in the presence of an appropriate crystal field [7].Therefore, the crystal field has a pronounced effect on the resulting magnetization dynamics.Subsequently, the choice of the coordinated ligands becomes an obvious synthetic challenge.In addition to the bistable ground state, another requirement for a 4f -SMM is the large total angular momentum (J).This is also assisted by the ligand-field around the metal ion(s), which can stabilize the largest projection of the total angular momentum.Based on these prerequisites, it becomes apparent why Dy III ( 6 H 15/2 ) is a key element to the synthesis of SMMs with interesting properties.The non-integer value of J in Dy III ions results in a Kramers doublet ground state, i.e., a degenerate pair of ±m J microstates, which produces a bistable magnetic moment that can be used to store information at a molecular level.Thus, the optimization of the lanthanide's coordination environment towards highly-symmetric polyhedra and ideal geometries has become a sine qua non in the field of SMMs [8].It was theoretically predicted and practically proven that high-symmetry coordination geometries render possible the control of the fast QTM through the disappearance of certain terms in the crystal field Hamiltonian [9].
To gain access into some of these exciting properties, the synthesis of new Dy III complexes is apparently necessary for both the development of structure-magnetism relationships and the elaboration of key factors that affect the chemical and magnetic dynamics of a molecular compound.To this end, we have recently started a program aimed at the employment of various Schiff base ligands (Scheme 1) in Dy III chemistry as a means of obtaining structurally new and magnetically interesting complexes.Indeed, the use of N-salicylidene-o-aminophenol (saphH 2 ) in Dy III chemistry has resulted in the formation of a {Dy 7 } SMM exhibiting a very small energy barrier of ~4 K for the magnetization reversal [10].Our next synthetic attempts were oriented towards the synthesis of Dy III /sacbH 2 complexes, where sacbH 2 is the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid; a new {Dy 2 } SMM was prepared and it exhibited a large energy barrier of ~109 K [11].The reasonable next step to follow was the exploration of the coordination capabilities of N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH 2 ), which is similar to sacbH 2 but with an additional aromatic ring on the N-salicylidene moiety.Although we have been successful with the synthesis of many 3d-metal complexes [12], we have been unable to date to isolate any 4f -metal compound bearing the ligand nacbH 2 .
The last member of this tetralogy of Schiff base ligands was N-naphthalidene-2-amino-5chlorophenol (nacpH 2 ), in structural, electronic and chemical analogy to saphH 2 (and in part to nacbH 2 ).We here report the detailed synthesis and characterization of the ligand nacpH 2 , and its first use in coordination chemistry that allowed us to isolate and study a new {Dy 5 } cluster compound with a bent bowtie topology, and SMM behavior with a large energy barrier of 100 K.
various modern technological advancements, such as in information and data storage, molecular spintronics and quantum computing [6].
In the case of 4f-metal ions, the energy barrier U (or, more explicitly, the effective energy barrier, Ueff) arises from the spin-orbit coupled electronic ground states ( 2S+1 LJ) which can be further split into ±mJ microstates in the presence of an appropriate crystal field [7].Therefore, the crystal field has a pronounced effect on the resulting magnetization dynamics.Subsequently, the choice of the coordinated ligands becomes an obvious synthetic challenge.In addition to the bistable ground state, another requirement for a 4f-SMM is the large total angular momentum (J).This is also assisted by the ligand-field around the metal ion(s), which can stabilize the largest projection of the total angular momentum.Based on these prerequisites, it becomes apparent why Dy III ( 6 H15/2) is a key element to the synthesis of SMMs with interesting properties.The non-integer value of J in Dy III ions results in a Kramers doublet ground state, i.e., a degenerate pair of ±mJ microstates, which produces a bistable magnetic moment that can be used to store information at a molecular level.Thus, the optimization of the lanthanide's coordination environment towards highly-symmetric polyhedra and ideal geometries has become a sine qua non in the field of SMMs [8].It was theoretically predicted and practically proven that high-symmetry coordination geometries render possible the control of the fast QTM through the disappearance of certain terms in the crystal field Hamiltonian [9].
To gain access into some of these exciting properties, the synthesis of new Dy III complexes is apparently necessary for both the development of structure-magnetism relationships and the elaboration of key factors that affect the chemical and magnetic dynamics of a molecular compound.To this end, we have recently started a program aimed at the employment of various Schiff base ligands (Scheme 1) in Dy III chemistry as a means of obtaining structurally new and magnetically interesting complexes.Indeed, the use of N-salicylidene-o-aminophenol (saphH2) in Dy III chemistry has resulted in the formation of a {Dy7} SMM exhibiting a very small energy barrier of ~4 K for the magnetization reversal [10].Our next synthetic attempts were oriented towards the synthesis of Dy III /sacbH2 complexes, where sacbH2 is the Schiff base ligand N-salicylidene-2-amino-5chlorobenzoic acid; a new {Dy2} SMM was prepared and it exhibited a large energy barrier of ~109 K [11].The reasonable next step to follow was the exploration of the coordination capabilities of N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH2), which is similar to sacbH2 but with an additional aromatic ring on the N-salicylidene moiety.Although we have been successful with the synthesis of many 3d-metal complexes [12], we have been unable to date to isolate any 4f-metal compound bearing the ligand nacbH2.
The last member of this tetralogy of Schiff base ligands was N-naphthalidene-2-amino-5chlorophenol (nacpH2), in structural, electronic and chemical analogy to saphH2 (and in part to nacbH2).We here report the detailed synthesis and characterization of the ligand nacpH2, and its first use in coordination chemistry that allowed us to isolate and study a new {Dy5} cluster compound with a bent bowtie topology, and SMM behavior with a large energy barrier of 100 K.

Synthetic Comments
Many synthetic routes to oligo-and polynuclear 4f -metal complexes rely on the employment of β-diketones either alone or in conjunction with various chelating/bridging ligands [13,14].This is clearly because β-diketonate groups are excellent capping, and occasionally bridging ligands, thus fulfilling the coordination needs of the lanthanide ions.Furthermore, they are O-donor ligands which can satisfy the oxophilicity of the Ln III ions and form stable five-membered chelate rings that enhance the stability of the resulting compounds.Lastly, β-diketonate ions are also known for their ability to act as Lewis bases and consequently abstract the H-atoms from the OH-groups of the corresponding organic chelating/bridging ligands, such as nacpH 2 in the present case.Among all the available β-diketones, only 3,5-heptadione was able to facilitate the formation of a crystalline product in this work.Therefore, the reaction between Dy(hpd)

Synthetic Comments
Many synthetic routes to oligo-and polynuclear 4f-metal complexes rely on the employment of β-diketones either alone or in conjunction with various chelating/bridging ligands [13,14].This is clearly because β-diketonate groups are excellent capping, and occasionally bridging ligands, thus fulfilling the coordination needs of the lanthanide ions.Furthermore, they are O-donor ligands which can satisfy the oxophilicity of the Ln III ions and form stable five-membered chelate rings that enhance the stability of the resulting compounds.Lastly, β-diketonate ions are also known for their ability to act as Lewis bases and consequently abstract the H-atoms from the OH-groups of the corresponding organic chelating/bridging ligands, such as nacpH2 in the present case.Among all the available β-diketones, only 3,5-heptadione was able to facilitate the formation of a crystalline product in this work.Therefore, the reaction between Dy(hpd)3•2H2O and nacpH2 in a 1:1 molar ratio, in a solvent mixture comprising CH2Cl2 and MeOH, afforded orange crystals of a new [Dy5(OH)2(hpd)3(nacp)5(MeOH)5] (1) cluster in yields as high as 70%.The general formation of 1 is summarized by Scheme 2.
Scheme 2. General formation of complex 1.
Complex 1 is very stable under the reported conditions, and its identity is not dependent on either the presence of an external base (i.e., NEt3, NMe3, NPr3, and Me4NOH) or the molar ratio of the reagents.However, its crystallinity and yields are largely affected by the appropriate solvent mixture (CH2Cl2/MeOH).More specifically, the same reaction in only CH2Cl2 or MeOH gave amorphous solids or microcrystalline materials, respectively, which we were either unable to further characterize (in the case of CH2Cl2 only) or able to identify as the {Dy5} compound based on IR spectroscopic studies and elemental analyses (in the case of MeOH only).Furthermore, analogous reactions but in 2:1 and 1:2 Dy(hpd)3•2H2O:nacpH2 molar ratios gave crystals of 1 in very low yield (i.e., 5−15%).

Description of Structure
A partially labelled structure of complex 1 is shown in Figure 1.The molecular compound crystallizes with three MeOH and one CH2Cl2 solvate molecules, which appear in the crystal lattice of 1.A view of the core of 1 and its metallic skeleton are illustrated in Figure 2 (top and bottom, respectively).Selected interatomic distances and angles are listed in Table S1.The crystallographically established coordination modes of the ligands hpd − and nacp 2− present in complex 1 are shown in Figure S1.
Complex 1 is very stable under the reported conditions, and its identity is not dependent on either the presence of an external base (i.e., NEt 3 , NMe 3 , NPr 3 , and Me 4 NOH) or the molar ratio of the reagents.However, its crystallinity and yields are largely affected by the appropriate solvent mixture (CH 2 Cl 2 /MeOH).More specifically, the same reaction in only CH 2 Cl 2 or MeOH gave amorphous solids or microcrystalline materials, respectively, which we were either unable to further characterize (in the case of CH 2 Cl 2 only) or able to identify as the {Dy 5 } compound based on IR spectroscopic studies and elemental analyses (in the case of MeOH only).Furthermore, analogous reactions but in 2:1 and 1:2 Dy(hpd) 3 •2H 2 O:nacpH 2 molar ratios gave crystals of 1 in very low yield (i.e., 5−15%).

Description of Structure
A partially labelled structure of complex 1 is shown in Figure 1.The molecular compound crystallizes with three MeOH and one CH 2 Cl 2 solvate molecules, which appear in the crystal lattice of 1.A view of the core of 1 and its metallic skeleton are illustrated in Figure 2 (top and bottom, respectively).Selected interatomic distances and angles are listed in Table S1.The crystallographically established coordination modes of the ligands hpd − and nacp 2− present in complex 1 are shown in Figure S1.
Magnetochemistry 2018, 4, x FOR PEER REVIEW 4 of 13 structure of 1 is stabilized by several strong intramolecular H-bonds that involve the hydroxido, phenoxido (from nacb 2− ) and MeOH groups (both coordinated and lattice).In addition, the lattice CH2Cl2 molecules appear to occupy the voids created by the supramolecular arrangement of the {Dy5} clusters.structure of 1 is stabilized by several strong intramolecular H-bonds that involve the hydroxido, phenoxido (from nacb 2− ) and MeOH groups (both coordinated and lattice).In addition, the lattice CH2Cl2 molecules appear to occupy the voids created by the supramolecular arrangement of the {Dy5} clusters.All Dy III ions are eight-coordinate albeit with different geometries.The coordination geometries were determined by the continuous shape measures (CShM) approach of the SHAPE program [19], which allows one to numerically evaluate by how much a particular polyhedron deviates from the ideal shape.The best fits were obtained for the square antiprismatic (CShM values = 1.53 and 1.10 for Dy3 and Dy5, respectively), triangular dodecahedral (CShM values = 1.40 and 1.03 for Dy1 and Dy4, respectively), and biaugmented trigonal prismatic (CShM value = 1.52 for Dy2) geometries (Figure 3, Table S2).Values of CShM between 0.1 and 3 usually correspond to a not negligible, but still small distortion from ideal geometry [20].All Dy III ions are eight-coordinate albeit with different geometries.The coordination geometries were determined by the continuous shape measures (CShM) approach of the SHAPE program [19], which allows one to numerically evaluate by how much a particular polyhedron deviates from the ideal shape.The best fits were obtained for the square antiprismatic (CShM values = 1.53 and 1.10 for Dy3 and Dy5, respectively), triangular dodecahedral (CShM values = 1.40 and 1.03 for Dy1 and Dy4, respectively), and biaugmented trigonal prismatic (CShM value = 1.52 for Dy2) geometries (Figure 3, Table S2).Values of CShM between 0.1 and 3 usually correspond to a not negligible, but still small distortion from ideal geometry [20].All Dy III ions are eight-coordinate albeit with different geometries.The coordination geometries were determined by the continuous shape measures (CShM) approach of the SHAPE program [19], which allows one to numerically evaluate by how much a particular polyhedron deviates from the ideal shape.The best fits were obtained for the square antiprismatic (CShM values = 1.53 and 1.10 for Dy3 and Dy5, respectively), triangular dodecahedral (CShM values = 1.40 and 1.03 for Dy1 and Dy4, respectively), and biaugmented trigonal prismatic (CShM value = 1.52 for Dy2) geometries (Figure 3, Table S2).Values of CShM between 0.1 and 3 usually correspond to a not negligible, but still small distortion from ideal geometry [20].

Solid-State Magnetic Susceptibility Studies
Direct current (dc) magnetic susceptibility measurements on a crystalline sample of complex 1•3MeOH•CH 2 Cl 2 were performed in the 2-300 K range under an applied magnetic field of 0.1 T (Figure 4).The room-temperature χ M T value of 70.48 cm 3 Kmol −1 is very close to the theoretical value of 70.85 cm 3 Kmol −1 for five non-interacting Dy III ions ( 6 H 15/2 , S = 5/2, L = 5, g = 4/3).The χ M T product declines gradually on cooling until ~20 K, it then plateaus at a value of ~63 cm 3 Kmol −1 from 20 to 10 K, and it finally increases to reach a value of 64.78 cm 3 Kmol −1 at 2.5 K.The decrease of the χ M T product in the 300-20 K region is mainly due to the depopulation of the crystal field (CF) m J states, whereas the low-temperature incline may be attributed to some weak intramolecular ferromagnetic exchange interactions between the Dy III ions.The drop of the χ M T product below 2.5 K could be assigned to effects from the magnetic anisotropy, the presence of intermolecular interactions and the applied dc field (Zeeman effect).The field dependence of the magnetization at 1.9, 3 and 5 K shows a relatively fast increase at low fields without reaching saturation at 7 T, which presages significant magnetic anisotropy (Figure 4, inset).Furthermore, the magnetization value at 7 T is ~30 Nµ B , much lower than the expected value for five Dy III ions (M S /Nµ B = ng J J = 50 Nµ B ), which is due to the CF effects that induce strong magnetic anisotropy.

Solid-State Magnetic Susceptibility Studies
Direct current (dc) magnetic susceptibility measurements on a crystalline sample of complex 1•3MeOH•CH2Cl2 were performed in the 2-300 K range under an applied magnetic field of 0.1 T (Figure 4).The room-temperature χΜT value of 70.48 cm 3 Kmol −1 is very close to the theoretical value of 70.85 cm 3 Kmol −1 for five non-interacting Dy III ions ( 6 H15/2, S = 5/2, L = 5, g = 4/3).The χΜT product declines gradually on cooling until ~20 K, it then plateaus at a value of ~63 cm 3 Kmol −1 from 20 to 10 K, and it finally increases to reach a value of 64.78 cm 3 Kmol −1 at 2.5 K.The decrease of the χΜT product in the 300-20 K region is mainly due to the depopulation of the crystal field (CF) mJ states, whereas the low-temperature incline may be attributed to some weak intramolecular ferromagnetic exchange interactions between the Dy III ions.The drop of the χΜT product below 2.5 K could be assigned to effects from the magnetic anisotropy, the presence of intermolecular interactions and the applied dc field (Zeeman effect).The field dependence of the magnetization at 1.9, 3 and 5 K shows a relatively fast increase at low fields without reaching saturation at 7 T, which presages significant magnetic anisotropy (Figure 4, inset).Furthermore, the magnetization value at 7 T is ~30 NμB, much lower than the expected value for five Dy III ions (MS/NμB = ngJJ = 50 NμB), which is due to the CF effects that induce strong magnetic anisotropy.To probe the magnetic dynamics of 1•3MeOH•CH2Cl2, alternating current (ac) magnetic susceptibility measurements, as a function of both temperature (Figure 5) and frequency (Figure 6), were initially performed in zero applied dc field.The compound displays frequency-dependent tails of signals in the χΜ′ (in-phase susceptibility) and χΜ′′ (out-of-phase susceptibility) versus T plots at temperatures below ~15 K, suggesting the presence of slow relaxation of the magnetization consistent with an SMM behavior.The tails of signals are suggestive of the presence of fast quantum tunneling of magnetization (QTM), without precluding though the concurrent presence of thermally-assisted processes.To this end, we studied the dependence of the χΜ′ and χΜ′′ susceptibilities on the ac frequency at each temperature (1.9-13 K, Figure 6).To probe the magnetic dynamics of 1•3MeOH•CH 2 Cl 2 , alternating current (ac) magnetic susceptibility measurements, as a function of both temperature (Figure 5) and frequency (Figure 6), were initially performed in zero applied dc field.The compound displays frequency-dependent tails of signals in the χ M (in-phase susceptibility) and χ M (out-of-phase susceptibility) versus T plots at temperatures below ~15 K, suggesting the presence of slow relaxation of the magnetization consistent with an SMM behavior.The tails of signals are suggestive of the presence of fast quantum tunneling of magnetization (QTM), without precluding though the concurrent presence of thermally-assisted processes.To this end, we studied the dependence of the χ M and χ M susceptibilities on the ac frequency at each temperature (1.9-13 K, Figure 6).

Figure 6.
Frequency dependence of the in-phase (χΜ′, top) and out-of-phase (χΜ′′, bottom) ac magnetic susceptibilities at zero dc field for 1•3MeOH•CH2Cl2, measured in a 3.0 G ac field at the temperature range 1.9-13 K. Solid lines represent fits to the data, as described in the main text.
The peak maxima allowed us to fit the data to a generalized Debye function [21] and extract the temperature-dependent relaxation times (τ), which subsequently allowed us to assess the operative magnetic relaxation for an SMM.In an ideal case, when an activation barrier exists with respect to the magnetization reversal, the system must exchange energy with the lattice (in the form of phonons) to ascend to the top of the barrier before relaxation can take place [22].Such a relaxation process, known as the Orbach mechanism, results in an exponential dependence of τ upon temperature.Consequently, an Arrhenius-type plot can be constructed and from this the effective energy barrier, Ueff, and pre-exponential factor, τ0, can be derived.As shown in Figure 7 (left), the plot of lnτ versus 1/T exhibits a linear shape at high temperatures, corresponding to a thermally-activated Orbach process.The curvature of the plot at lower temperatures suggests that additional relaxation processes are still operative.Therefore, we fitted the lnτ versus 1/T data of 1•3MeOH•CH2Cl2 using Equation (1), which accounts for the presence of QTM (first term), Raman (second term) and Orbach (last term) relaxation processes over the entire temperature range.A very good fit of the data was obtained with the following best-fit parameters: Ueff = 100(1) K, τ0 = 1.0(1) × 10 −7 s, n = 6.90, C = 5.0 × 10 −4 s −1 K −n and τQTM = 0.10 s.

𝜏
The deviation of the lower-temperature relaxation data from linearity is due to the interplay between QTM and thermally assisted relaxation processes.This is a usual phenomenon in cluster compounds of Kramers ions at different coordination geometries [23].It is very likely that the presence of easy-axis anisotropy, dipole-dipole and hyperfine interactions in 1 allow the mixing of the ground states of the five Kramers ions, thus promoting the QTM over a thermally-assisted pathway [24,25].In fact, the Cole-Cole plots (Figure S6, top) for 1•3MeOH•CH2Cl2 in the temperature range 1.9-13 K exhibit semicircular shapes and the data were fit by using a generalized Debye model.The α values were in the range 0.44-0.17,suggesting the presence of multiple relaxation processes [26].
Frequency (Hz)  The peak maxima allowed us to fit the data to a generalized Debye function [21] and extract the temperature-dependent relaxation times (τ), which subsequently allowed us to assess the operative magnetic relaxation for an SMM.In an ideal case, when an activation barrier exists with respect to the magnetization reversal, the system must exchange energy with the lattice (in the form of phonons) to ascend to the top of the barrier before relaxation can take place [22].Such a relaxation process, known as the Orbach mechanism, results in an exponential dependence of τ upon temperature.Consequently, an Arrhenius-type plot can be constructed and from this the effective energy barrier, U eff , and pre-exponential factor, τ 0 , can be derived.As shown in Figure 7 (left), the plot of lnτ versus 1/T exhibits a linear shape at high temperatures, corresponding to a thermally-activated Orbach process.The curvature of the plot at lower temperatures suggests that additional relaxation processes are still operative.Therefore, we fitted the lnτ versus 1/T data of 1•3MeOH•CH 2 Cl 2 using Equation (1), which accounts for the presence of QTM (first term), Raman (second term) and Orbach (last term) relaxation processes over the entire temperature range.A very good fit of the data was obtained with the following best-fit parameters: U eff = 100(1) K, τ 0 = 1.0(1) × 10 −7 s, n = 6.90, C = 5.0 × 10 −4 s −1 K −n and τ QTM = 0.10 s.
The deviation of the lower-temperature relaxation data from linearity is due to the interplay between QTM and thermally assisted relaxation processes.This is a usual phenomenon in cluster compounds of Kramers ions at different coordination geometries [23].It is very likely that the presence of easy-axis anisotropy, dipole-dipole and hyperfine interactions in 1 allow the mixing of the ground states of the five Kramers ions, thus promoting the QTM over a thermally-assisted pathway [24,25].In fact, the Cole-Cole plots (Figure S6, top) for 1•3MeOH•CH 2 Cl 2 in the temperature range 1.9-13 K exhibit semicircular shapes and the data were fit by using a generalized Debye model.The α values were in the range 0.44-0.17,suggesting the presence of multiple relaxation processes [26].To further study the magnetic dynamics of 1•3MeOH•CH2Cl2 and attempt to suppress the effective QTM, ac measurements were also carried out under an applied, optimum dc field of 200 Oe (Figure S2).The peak maxima in the χΜ″ versus frequency plot (Figure 8) allowed us to fit the data to a generalized Debye model and the extracted relaxation times were then used to construct the Arrhenius-type plot (Figure 7, right), from which we derived the following fitting parameters (using Equation ( 1)): Ueff = 170(1) K, τ0 = 1.6(1) × 10 −9 s, n = 4.74, C = 0.10 s −1 K −n and τQTM = 0.01 s.The obtained parameters agree well with an elimination of the QTM and consequently an increase of the Ueff barrier.The Cole-Cole plots (Figure S6, bottom) for 1•3MeOH•CH2Cl2 in the temperature range 1.9-15 K (under 200 Oe field) exhibit again semicircular shapes; the data were fit and the corresponding α values were in the range 0.46-0.05.Finally, it is worthy to note that among the {Dy5} clusters with a bowtie topology [15][16][17][18], complex 1 is the only one exhibiting ferromagnetic exchange interactions between the metal centers and it has the second largest energy barrier for the magnetization reversal.Tang, Li and coworkers reported an asymmetric {Dy5} bowtie cluster exhibiting SMM behavior with a barrier of 197 K; the Dy III atoms were 8-and 9-coordinate and antiferromagnetic exchange interactions were observed [15].All the remaining {Dy5} complexes with a bowtie metal topology possess various Dy III coordination geometries and weak SMM properties.It is difficult to conclude at this stage of research which factor contributes mostly to the enhancement of the SMM properties within the reported {Dy5} complexes with a bowtie metal arrangement.It is very likely that the combination of the appropriate Dy III coordination geometries, the distortion of the {Dy5} core, and the nature of the predominant magnetic exchange interactions play a vital role on the onset of the magnetization dynamics.
Frequency (Hz)  To further study the magnetic dynamics of 1•3MeOH•CH 2 Cl 2 and attempt to suppress the effective QTM, ac measurements were also carried out under an applied, optimum dc field of 200 Oe (Figure S2).The peak maxima in the χ M " versus frequency plot (Figure 8) allowed us to fit the data to a generalized Debye model and the extracted relaxation times were then used to construct the Arrhenius-type plot (Figure 7, right), from which we derived the following fitting parameters (using Equation ( 1)): U eff = 170(1) K, τ 0 = 1.6(1) × 10 −9 s, n = 4.74, C = 0.10 s −1 K −n and τ QTM = 0.01 s.The obtained parameters agree well with an elimination of the QTM and consequently an increase of the U eff barrier.The Cole-Cole plots (Figure S6, bottom) for 1•3MeOH•CH 2 Cl 2 in the temperature range 1.9-15 K (under 200 Oe field) exhibit again semicircular shapes; the data were fit and the corresponding α values were in the range 0.46-0.05.Finally, it is worthy to note that among the {Dy 5 } clusters with a bowtie topology [15][16][17][18], complex 1 is the only one exhibiting ferromagnetic exchange interactions between the metal centers and it has the second largest energy barrier for the magnetization reversal.Tang, Li and coworkers reported an asymmetric {Dy 5 } bowtie cluster exhibiting SMM behavior with a barrier of 197 K; the Dy III atoms were 8-and 9-coordinate and antiferromagnetic exchange interactions were observed [15].All the remaining {Dy 5 } complexes with a bowtie metal topology possess various Dy III coordination geometries and weak SMM properties.It is difficult to conclude at this stage of research which factor contributes mostly to the enhancement of the SMM properties within the reported {Dy 5 } complexes with a bowtie metal arrangement.It is very likely that the combination of the appropriate Dy III coordination geometries, the distortion of the {Dy 5 } core, and the nature of the predominant magnetic exchange interactions play a vital role on the onset of the magnetization dynamics.To further study the magnetic dynamics of 1•3MeOH•CH2Cl2 and attempt to suppress the effective QTM, ac measurements were also carried out under an applied, optimum dc field of 200 Oe (Figure S2).The peak maxima in the χΜ″ versus frequency plot (Figure 8) allowed us to fit the data to a generalized Debye model and the extracted relaxation times were then used to construct the Arrhenius-type plot (Figure 7, right), from which we derived the following fitting parameters (using Equation ( 1)): Ueff = 170(1) K, τ0 = 1.6(1) × 10 −9 s, n = 4.74, C = 0.10 s −1 K −n and τQTM = 0.01 s.The obtained parameters agree well with an elimination of the QTM and consequently an increase of the Ueff barrier.The Cole-Cole plots (Figure S6, bottom) for 1•3MeOH•CH2Cl2 in the temperature range 1.9-15 K (under 200 Oe field) exhibit again semicircular shapes; the data were fit and the corresponding α values were in the range 0.46-0.05.Finally, it is worthy to note that among the {Dy5} clusters with a bowtie topology [15][16][17][18], complex 1 is the only one exhibiting ferromagnetic exchange interactions between the metal centers and it has the second largest energy barrier for the magnetization reversal.Tang, Li and coworkers reported an asymmetric {Dy5} bowtie cluster exhibiting SMM behavior with a barrier of 197 K; the Dy III atoms were 8-and 9-coordinate and antiferromagnetic exchange interactions were observed [15].All the remaining {Dy5} complexes with a bowtie metal topology possess various Dy III coordination geometries and weak SMM properties.It is difficult to conclude at this stage of research which factor contributes mostly to the enhancement of the SMM properties within the reported {Dy5} complexes with a bowtie metal arrangement.It is very likely that the combination of the appropriate Dy III coordination geometries, the distortion of the {Dy5} core, and the nature of the predominant magnetic exchange interactions play a vital role on the onset of the magnetization dynamics.

Materials, Physical and Spectroscopic Measurements
All manipulations were performed under aerobic conditions using materials (reagent grade) and solvents as received unless otherwise noted.The precursor Dy(hpd) 3 •2H 2 O (hpd − = 3,5-heptadione anion) was synthesized following previously reported methods for various lanthanide(III) β-diketonate starting materials [27].The Schiff base ligand N-naphthalidene-2-amino-5-chlorophenol (nacpH 2 ) was prepared in a manner similar to that of N-naphthalidene-2-amino-5-chlorobenzoic acid (nacbH 2 ) [12].Infrared spectra were recorded in the solid state on a Bruker's FT-IR spectrometer (ALPHA's Platinum ATR single reflection) in the 4000-400 cm −1 range.Elemental analyses (C, H, and N) were performed on a Perkin-Elmer 2400 Series II Analyzer.Melting points were measured on an Electrothermal Apparatus-9100.Electrospray ionization (ESI) mass spectra (MS) of nacpH 2 in MeCN were taken on a Bruker HCT Ultra mass spectrometer.NMR spectra were obtained on a Bruker Avance DPX-400 MHz instrument and are referenced to the residual proton signal of the deuterated solvent for 1 H and 13 C spectra according to published values.Variable-temperature direct and alternating current (dc and ac, respectively) magnetic susceptibility studies were conducted at the temperature range 1.9-300 K using a Quantum Design MPMS XL-7 SQUID magnetometer equipped with a 7 T magnet.Pascal's constants were used to estimate the diamagnetic correction [28], which was subtracted from the experimental susceptibility to give the molar paramagnetic susceptibility (χ M ).

Synthesis of nacpH 2
The organic ligand N-naphthalidene-2-amino-5-chlorophenol (nacpH 2 ) was prepared in quantitative yields (~95%) by the condensation reaction of 2-amino-5-chlorophenol (7.18 g, 50 mmol) with 2-hydroxy-1-naphthaldehyde (8.61 g, 50 mmol) in a molar ratio of 1:1 in 100 mL of refluxing MeOH.The reaction was stirred under reflux for 3 h and then cooled down to room temperature, during which time a brown microcrystalline solid had appeared.The brown solid was collected by filtration, dried under vacuum, and analyzed as solvent-free.Anal.calc.for C 17 H 12 NO   To a stirred, brown suspension of nacpH 2 (0.06 g, 0.20 mmol) in CH 2 Cl 2 (10 mL) was added a yellowish solution of Dy(hpd) 3 •2H 2 O (0.14 g, 0.20 mmol) in MeOH (20 mL).The resulting dark orange solution was stirred for 20 min, filtered and the filtrate was left to evaporate slowly at room temperature.After three days, X-ray quality orange plate-like crystals of 1•3MeOH•CH 2 Cl 2 were formed and these were collected by filtration, washed with cold MeOH (2 × 3 mL), and dried in air.The yield was 70% (based on the ligand available).The air-dried solid was satisfactorily analyzed as lattice-solvent free, i.e., as 1.Anal.

Single-Crystal X-ray Crystallography
Single-crystal X-ray diffraction data were collected on an orange plate-like crystal (0.05 × 0.04 × 0.02 mm) of complex 1•3MeOH•CH 2 Cl 2 using a Rigaku Oxford Diffraction XtaLAB Synergy diffractometer equipped with a HyPix-6000HE area detector at 100 K and utilizing Mo Kα (λ = 0.71073 Å) from PhotonJet micro-focus X-ray source.The structure was solved using the charge-flipping algorithm, as implemented in the program SUPERFLIP [29], and refined by full-matrix least-squares techniques against F o 2 using the SHELXL program [30] through the OLEX2 interface [31].
The non-hydrogen atoms were successfully refined using anisotropic displacement parameters, and hydrogen atoms bonded to the carbon of the ligands and those of the hydroxido groups were placed at their idealized positions using appropriate HFIX instructions in SHELXL.All these atoms were included in subsequent refinement cycles in riding-motion approximation with isotropic thermal displacement parameters (U iso ) fixed at 1.2 or 1.5 × U eq of the relative atom.Various figures of the structure were created using the Diamond 3 program package [32].Unit cell parameters, structure solution and refinement details for 1•3MeOH•CH 2 Cl 2 are summarized in Table S3.Further crystallographic details can be found in the corresponding CIF file provided in the ESI.Crystallographic data for the structure of 1 have been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication number: CCDC-1868805.Copies of these data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 2EZ, UK; FAX: (+44) 1223 336033, or online via www.ccdc.cam.ac.uk/data_request/cif or by emailing data_request@ccdc.cam.ac.uk.

Conclusions and Perspectives
In conclusion, we have shown in this preliminary work that the Schiff base ligand N-naphthalidene-2-amino-5-chlorophenol (nacpH 2 ), in conjunction with the capping anion of 3,5-heptadione, is indeed capable of yielding new Dy III complexes with rare structural motifs and interesting SMM properties.The reported [Dy 5 (OH) 2 (hpd) 3 (nacp) 5 (MeOH) 5 ] (1) cluster has no similarities with any of the previously reported {Dy III x } clusters bearing the structurally similar Schiff base ligands saphH 2 and sacbH 2 (Scheme 1).This emphasizes the bridging versatility of the dianion of nacpH 2 towards coordination with 4f -metal ions, and presages the creation of a rich, new library of cluster compounds with potentially exciting structural and magnetic properties.
Our future efforts are directed towards the chemical reactivity studies of nacpH 2 with 3d-and 4f -metal ions as a means of exploring the coordination capacity of this ligand in the presence of both transition metal ions and lanthanides.Another immediate target is the detailed study of the optical response of 1 to seek for any possible synergy between the SMM and photoluminescence properties.All these endeavors are in progress and the results will be reported in due course.
Author Contributions: D.I.A. and A.A.A. conducted the syntheses, crystallization, conventional characterization and interpretation of the structural and magnetic data for the organic ligand and the {Dy 5 } cluster.D.S. collected single-crystal X-ray crystallographic data, solved the structure and performed the complete refinement.T.C.S. coordinated the research, contributed to the interpretation of the results and wrote the paper based on the reports of his collaborators.All the authors exchanged ideas and comments regarding the interpretation of the results and discussed upon the manuscript at all stages.

Scheme 1 .Scheme 1 .
Scheme 1. Structural formulas and abbreviations of the Schiff base ligands discussed in the text; the blue box encloses the tridentate ligand nacpH2 used in the present work.Scheme 1. Structural formulas and abbreviations of the Schiff base ligands discussed in the text; the blue box encloses the tridentate ligand nacpH 2 used in the present work.

Figure 1 .
Figure 1.Partially labeled representation of the molecular structure of 1. Color scheme: Dy III , yellow; Cl, purple; O, red; N, green; C, gray.H atoms are omitted for clarity.

Figure 1 .
Figure 1.Partially labeled representation of the molecular structure of 1. Color scheme: Dy III , yellow; Cl, purple; O, red; N, green; C, gray.H atoms are omitted for clarity.

Figure 1 .
Figure 1.Partially labeled representation of the molecular structure of 1. Color scheme: Dy III , yellow; Cl, purple; O, red; N, green; C, gray.H atoms are omitted for clarity.

Figure 3 .
Figure 3. Square antiprismatic, triangular dodecahedral and biaugmented trigonal prismatic coordination geometries of the Dy atoms in 1. Points connected by the gray thin lines define the vertices of the ideal polyhedra.

Figure 3 .
Figure 3. Square antiprismatic, triangular dodecahedral and biaugmented trigonal prismatic coordination geometries of the Dy atoms in 1. Points connected by the gray thin lines define the vertices of the ideal polyhedra.

Figure 3 .
Figure 3. Square antiprismatic, triangular dodecahedral and biaugmented trigonal prismatic coordination geometries of the Dy atoms in 1. Points connected by the gray thin lines define the vertices of the ideal polyhedra.

Figure 5 .Figure 5 . 13 Figure 5 .Figure 6 .
Figure 5. Temperature dependence of the in-phase (χΜ′, top) and out-of-phase (χΜ′′, bottom) ac magnetic susceptibilities at zero dc field for 1•3MeOH•CH2Cl2, measured in a 3.0 G ac field oscillating at the indicated frequencies.The solid lines are guides only.

Figure 6 .
Figure 6.Frequency dependence of the in-phase (χ M , top) and out-of-phase (χ M , bottom) ac magnetic susceptibilities at zero dc field for 1•3MeOH•CH 2 Cl 2 , measured in a 3.0 G ac field at the temperature range 1.9-13 K. Solid lines represent fits to the data, as described in the main text.

Magnetochemistry 2018, 4 , 13 Figure 7 .
Figure 7. Arrhenius plots of 1•3MeOH•CH2Cl2 at zero (left) and 200 Oe (right) applied dc fields.The green lines correspond to the fit of the data; see the text for the fit parameters.

Figure 7 .
Figure 7. Arrhenius plots of 1•3MeOH•CH 2 Cl 2 at zero (left) and 200 Oe (right) applied dc fields.The green lines correspond to the fit of the data; see the text for the fit parameters.

Magnetochemistry 2018, 4 , 13 Figure 7 .
Figure 7. Arrhenius plots of 1•3MeOH•CH2Cl2 at zero (left) and 200 Oe (right) applied dc fields.The green lines correspond to the fit of the data; see the text for the fit parameters.

Figure 8 .
Figure 8.Frequency dependence of the out-of-phase (χ M ) ac magnetic susceptibility at 200 Oe dc field for 1•3MeOH•CH 2 Cl 2 , measured in a 3.0 G ac field at the temperature range 1.9-15 K. Solid lines represent fits to the data, as described in the main text.
Figure S5: Positive ESI-MS spectra of the ligand nacpH 2 in MeCN.The inset spectra show a zoomed picture of the experimental data (top) and the theoretical simulation of the isotope model of the protonated ion of nacpH 2 (bottom).
Figure S6: Cole-Cole plots for 1 obtained using the ac susceptibility data at zero (top) and 200 Oe (bottom) applied dc fields.The solid lines are the best fit obtained from a generalized Debye model.Table 3 •2H 2 O and nacpH 2 in a 1:1 molar ratio, in a solvent mixture comprising CH 2 Cl 2 and MeOH, afforded orange crystals of a new [Dy 5 (OH) 2 (hpd) 3 (nacp) 5 (MeOH) 5 ] (1) cluster in yields as high as 70%.The general formation of 1 is summarized by Scheme 2.