Field-Induced Slow Magnetic Relaxation of Mono-and Dinuclear Dysprosium ( III ) Complexes Coordinated by a Chloranilate with Different Resonance Forms

We synthesized the dinuclear and mononuclear dysprosium(III) complexes [{Dy(Tp)2}2(Cl2An)]·2CH2Cl2 (1) and [Co(Cp)2][Dy(Tp)2(Cl2An)] (3), where Cl2An and Tp− are the chloranilate and hydrotris(pyrazolyl)borate ligand, respectively. In addition, the magnitude of the magnetic coupling between the lanthanide centers through the Cl2An bridge has been probed through the synthesis of [{Gd(Tp)2}2(Cl2An)]·2CH2Cl2 (2), which is a gadolinium(III) analogue of 1. Complexes 1–3 were characterized by infrared (IR) spectroscopy, elemental analysis, single-crystal X-ray diffraction, and SQUID measurements. IR and single-crystal X-ray structural analyses confirm that the coordination environments of the lanthanide(III) centers in 1 and 3 are similar to each other; i.e., eight-coordinated metal centers, each occupied by an N6O2 donor set from two Tp− ligands and one Cl2An ligand. The coordination geometries of the lanthanide(III) centers in 1 and 2 are distorted triangular dodecahedral, while that in the mononuclear complex 3 is square antiprismatic, where the Cl2An ligand takes the bi-separated delocalized form in 1 and 2, and the o-quinone form in 3. Alternating-current (AC) magnetic studies clearly reveal that both 1 and 3 exhibit field-induced slow relaxations of magnetization that occur via Raman and direct processes. Complexes 1 and 3 exhibit different spin relaxation behavior, which reflects the coordination geometry around each DyIII center and its nuclearity, as well as the molecular packing in the crystal lattice. Although the magnetic analysis of 2 revealed negligible magnetic coupling, Cl2An bridges with small biases may form in the dinuclear complexes, which play roles in the spin relaxation dynamics through dipolar interactions.

Early surveys of SMMs were focused on polynuclear cluster complexes with high nuclearities due to the realization of large ground spin states [2].However, it is rather difficult to combine a large ground spin state with a large magnetic anisotropy, since the entire molecular symmetry in a polynuclear cluster complex often increases with an increasing nuclearity, leading to a small magnetic anisotropy as a consequence.Therefore, the realization of polynuclear cluster complexes with large spin ground states and large magnetic anisotropies is still a significant challenge.
On the other hand, more recent frontline SMMs studies have tended to focus on enhancing magnetic anisotropy, rather than augmenting ground spin states, which is a much more informative method for increasing the blocking temperature and the activation energy barrier.From these perspectives, studies into SMMs, and more recently, SIMs, based on trivalent lanthanide (Ln III ), ions have proceeded at a rapid pace because of their significant magnetic anisotropies that arise from both spin-orbit coupling and crystal-field splitting, where crystal-field splitting is normally much smaller than spin-orbit coupling in Ln III ions [22,23].Hence, it is very important to investigate small changes in ligand-field strength associated with the coordination geometries of Ln III ions.However, the relationship between the coordination geometry of the Ln III ion and the slow magnetic relaxation of SMMs or SIMs is still largely unclear, because it is very difficult to systematically control the coordination geometries and the ligand-field strengths of Ln III ions.We noticed that chloranilate (Cl 2 An 2− ) is a promising organic ligand for investigating the above-mentioned Ln III -based SMMs or SIMs.The dianionic form of Cl 2 An 2− has a delocalized π system with resonance structures that are represented by p-quinone, o-quinone, tetraquinone, and bi-separated delocalized forms (Scheme 1) [24,25].The coordination geometry and the ligand-field strength of the central Ln III ion can be fine-tuned by combining Cl 2 An 2− with a suitable ancillary ligand, while maintaining a similar coordination environment.Hydrotris(pyrazolyl)borate (Tp − ) ligands are scorpionate ligands [26] that exhibit hard σ-donating characteristics and have been used to stabilize trivalent Ln III metal centers.Thus, we chose Tp − as the ancillary ligand.
Early surveys of SMMs were focused on polynuclear cluster complexes with high nuclearities due to the realization of large ground spin states [2].However, it is rather difficult to combine a large ground spin state with a large magnetic anisotropy, since the entire molecular symmetry in a polynuclear cluster complex often increases with an increasing nuclearity, leading to a small magnetic anisotropy as a consequence.Therefore, the realization of polynuclear cluster complexes with large spin ground states and large magnetic anisotropies is still a significant challenge.
On the other hand, more recent frontline SMMs studies have tended to focus on enhancing magnetic anisotropy, rather than augmenting ground spin states, which is a much more informative method for increasing the blocking temperature and the activation energy barrier.From these perspectives, studies into SMMs, and more recently, SIMs, based on trivalent lanthanide (Ln III ), ions have proceeded at a rapid pace because of their significant magnetic anisotropies that arise from both spin-orbit coupling and crystal-field splitting, where crystal-field splitting is normally much smaller than spin-orbit coupling in Ln III ions [22,23].Hence, it is very important to investigate small changes in ligand-field strength associated with the coordination geometries of Ln III ions.However, the relationship between the coordination geometry of the Ln III ion and the slow magnetic relaxation of SMMs or SIMs is still largely unclear, because it is very difficult to systematically control the coordination geometries and the ligand-field strengths of Ln III ions.We noticed that chloranilate (Cl2An 2− ) is a promising organic ligand for investigating the above-mentioned Ln III -based SMMs or SIMs.The dianionic form of Cl2An 2− has a delocalized π system with resonance structures that are represented by p-quinone, o-quinone, tetraquinone, and bi-separated delocalized forms (Scheme 1) [24,25].The coordination geometry and the ligand-field strength of the central Ln III ion can be fine-tuned by combining Cl2An 2− with a suitable ancillary ligand, while maintaining a similar coordination environment.Hydrotris(pyrazolyl)borate (Tp − ) ligands are scorpionate ligands [26] that exhibit hard σ-donating characteristics and have been used to stabilize trivalent Ln III metal centers.Thus, we chose Tp − as the ancillary ligand.Based on the above idea, we successfully synthesized mononuclear and dinuclear dysprosium(III) complexes bearing the Cl2An 2− ligand and Tp − as auxiliary ligands, in which the Cl2An 2− ligand adopts the -quinone and bi-separated delocalized forms, respectively.Comparisons of the spin-relaxation dynamics for the mononuclear and dinuclear complexes will aid in our understanding of dipolar interactions mediated by bridging Cl2An 2− ligands in dinuclear complexes.Herein, we report the syntheses, single-crystal X-ray structures, and detailed magnetic properties of two types of complexes, namely [{Dy(Tp)2}2(μ-Cl2An 2− )]•2CH2Cl2 and [Co(Cp)2][Dy(Tp)2(Cl2An 2− )], which display slow magnetic relaxations under small static direct-current (DC) magnetic fields.
Based on the above idea, we successfully synthesized mononuclear and dinuclear dysprosium(III) complexes bearing the Cl 2 An 2− ligand and Tp − as auxiliary ligands, in which the Cl 2 An 2− ligand adopts the o-quinone and bi-separated delocalized forms, respectively.Comparisons of the spin-relaxation dynamics for the mononuclear and dinuclear complexes will aid in our understanding of dipolar interactions mediated by bridging Cl 2 An 2− ligands in dinuclear complexes.Herein, we report the syntheses, single-crystal X-ray structures, and detailed magnetic properties of two types of complexes, namely [{Dy(Tp) 2 } 2 (µ-Cl 2 An 2− )]•2CH 2 Cl 2 and [Co(Cp) 2 ][Dy(Tp) 2 (Cl 2 An 2− )], which display slow magnetic relaxations under small static direct-current (DC) magnetic fields.

Syntheses and Characterizations
The [{Ln(Tp) 2 } 2 (µ-Cl 2 An 2− )] dinuclear complexes (1: Ln = Dy; 2: Ln = Gd) are readily formed by the reaction of 4 equiv. of KTp and 1 equiv. of Na 2 Cl 2 An with 2 equiv. of the corresponding lanthanide chloride salt in water.The subsequent slow diffusion of hexane into a concentrated CH 2 Cl 2 solution of 1 gave purple rod-shaped crystals, with two CH 2 Cl 2 solvates per dinuclear unit, suitable for single-crystal X-ray diffraction studies [27].On the other hand, reaction of 1 with 1 equiv. of cobaltocene (Co(Cp) 2 , Cp − = cyclopentadienide) in a N 2 -filled glovebox led to the formation of the mononuclear [Co(Cp) 2 ][Dy(Tp) 2 (Cl 2 An 2− )] complex (3); 3 crystallized without any solvent molecules.Therefore, to the best of our knowledge, complexes 1-3 are the first heteroleptic systems containing the Cl 2 An 2− and Tp − ligands.Infrared (IR) spectroscopy provides complementary information to that obtained by single-crystal X-ray studies with respect to structural features (Figure 1).Complexes 1 and 2 have similar masses (162.5 amu for 1 and 157.25 amu for 2), so large differences in their IR spectra were not observed, although some peaks are shifted.The IR absorptions associated with the bridging Cl 2 An 2− ligand in both 1 and 2 are typified by predominant C-Cl and C-O vibrations at ~850 and 1528 cm −1 , respectively.In contrast, 3 has three characteristic absorption bands centered at 833, 1540, and 1643 cm −1 .The first two absorption bands are assigned to C-Cl and C-O vibrations, while the third absorption band is considered to correspond to C=O vibrations.This result is a promising indication that the Cl 2 CA 2− ligand in 3 is in the o-quinone form [28]. Furthermore, the presence of the ancillary Tp -ligands in 1-3 is confirmed by characteristic absorptions in their IR spectra at approximately 2470 cm −1 (c.f., KTp − , νB-H = ~2440 cm −1 ) [29].
Inorganics 2017, 5, 7 3 of 13 CH2Cl2 solution of 1 gave purple rod-shaped crystals, with two CH2Cl2 solvates per dinuclear unit, suitable for single-crystal X-ray diffraction studies [27].On the other hand, reaction of 1 with 1 equiv. of cobaltocene (Co(Cp)2, Cp -= cyclopentadienide) in a N2-filled glovebox led to the formation of the mononuclear [Co(Cp)2][Dy(Tp)2(Cl2An 2− )] complex (3); 3 crystallized without any solvent molecules.Therefore, to the best of our knowledge, complexes 1-3 are the first heteroleptic systems containing the Cl2An 2− and Tp − ligands.Infrared (IR) spectroscopy provides complementary information to that obtained by single-crystal X-ray studies with respect to structural features (Figure 1).Complexes 1 and 2 have similar masses (162.5 amu for 1 and 157.25 amu for 2), so large differences in their IR spectra were not observed, although some peaks are shifted.The IR absorptions associated with the bridging Cl2An 2− ligand in both 1 and 2 are typified by predominant C-Cl and C-O vibrations at ~850 and 1528 cm −1 , respectively.In contrast, 3 has three characteristic absorption bands centered at 833, 1540, and 1643 cm −1 .The first two absorption bands are assigned to C-Cl and C-O vibrations, while the third absorption band is considered to correspond to C=O vibrations.This result is a promising indication that the Cl2CA 2− ligand in 3 is in the o-quinone form [28]. Furthermore, the presence of the ancillary Tp -ligands in 1-3 is confirmed by characteristic absorptions in their IR spectra at approximately 2470 cm −1 (c.f., KTp − , νB-H = ~2440 cm −1 ) [29].

Molecular Structure Descriptions
The molecular structures of neutral dinuclear complexes 1 and 2 are isostructural and belong to the monoclinic P21/n space group (No. 14) (Figure 2).Both 1 and 2 are centrosymmetric complexes with two Ln III centers bridged by a bis-bidentate Cl2An 2− ligand; consequently, only half of the dimer unit is unique.The coordination environments around the Ln III centers in 1 and 2 are eight-coordinated with six N atoms from the two Tp − capping ligands, and two O atoms from the Cl2An 2− bridge.The eight-coordinate Ln III centers adopt slightly distorted triangular dodecahedral geometries based on continuous shape measures (CShMs) [30,31] of 0.785 for 1 and 0.831 for 2.
The bond distances within the rings of quinoidal ligands, such as Cl2An 2− , bound to metal ions provide strong information about the electronic structures of the ligands (Scheme 1).The o-quinone form is usually found in mononuclear complexes through terminal bidentate coordination modes, whereas the bi-separated delocalized form is often found in polynuclear complexes and coordination polymers through bis-bidentate coordination modes.

Molecular Structure Descriptions
The molecular structures of neutral dinuclear complexes 1 and 2 are isostructural and belong to the monoclinic P2 1 /n space group (No. 14) (Figure 2).Both 1 and 2 are centrosymmetric complexes with two Ln III centers bridged by a bis-bidentate Cl 2 An 2− ligand; consequently, only half of the dimer unit is unique.The coordination environments around the Ln III centers in 1 and 2 are eight-coordinated with six N atoms from the two Tp − capping ligands, and two O atoms from the Cl 2 An 2− bridge.The eight-coordinate Ln III centers adopt slightly distorted triangular dodecahedral geometries based on continuous shape measures (CShMs) [30,31] of 0.785 for 1 and 0.831 for 2.
The bond distances within the rings of quinoidal ligands, such as Cl 2 An 2− , bound to metal ions provide strong information about the electronic structures of the ligands (Scheme 1).The o-quinone form is usually found in mononuclear complexes through terminal bidentate coordination modes, whereas the bi-separated delocalized form is often found in polynuclear complexes and coordination polymers through bis-bidentate coordination modes.The anionic mononuclear complex 3 cocrystallized with [Co(Cp)2] + in its crystal lattice, and was found to belong to the monoclinic P21/c space group (No. 14) (Figure 3).The coordination environment around the Dy III center of the anionic monomer 3 is structurally similar to those in dimers 1 and 2, differing only slightly in bond distances and angles.The eight-coordinate Dy III center features a distorted square antiprismatic geometry, according to the CShM value of 1.116, in which the two square faces are defined by N1, N5, N9, and O1, and N3, N7, N11, and O2.The Cl2An 2− moiety of 3 acts as a terminal capping ligand, unlike in dimers 1 and 2. Hence, the resonance structure of Cl2An 2− in the anionic monomer 3 differs significantly from those in 1 and 2 (vide supra).Indeed, the Cl2An    4).The value of χMT at 295 K for 1 was determined to be 28.29 cm 3 •K•mol −1 , which is in good agreement with the expected value of 28.34The anionic mononuclear complex 3 cocrystallized with [Co(Cp) 2 ] + in its crystal lattice, and was found to belong to the monoclinic P2 1 /c space group (No. 14) (Figure 3).The coordination environment around the Dy III center of the anionic monomer 3 is structurally similar to those in dimers  The anionic mononuclear complex 3 cocrystallized with [Co(Cp)2] + in its crystal lattice, and was found to belong to the monoclinic P21/c space group (No. 14) (Figure 3).The coordination environment around the Dy III center of the anionic monomer 3 is structurally similar to those in dimers 1 and 2, differing only slightly in bond distances and angles.The eight-coordinate Dy III center features a distorted square antiprismatic geometry, according to the CShM value of 1.116, in which the two square faces are defined by N1, N5, N9, and O1, and N3, N7, N11, and O2.The Cl2An 2− moiety of 3 acts as a terminal capping ligand, unlike in dimers 1 and 2. Hence, the resonance structure of Cl2An 2− in the anionic monomer 3 differs significantly from those in 1 and 2 (vide supra).Indeed, the Cl2An    4).The value of χMT at 295 K for 1 was determined to be 28.29 cm 3 •K•mol −1 , which is in good agreement with the expected value of 28.34

Static Magnetic Properties
DC magnetic susceptibility (χ M ) measurements on 1 were carried out in the 1.8-295 K temperature (T) range in an applied 0.1 T DC field (Figure 4).The value of χ M T at 295 K for 1 was determined to be 28.29 cm 3 •K•mol −1 , which is in good agreement with the expected value of 28.34 cm 3 •K•mol −1 for two uncoupled Dy III ions ( 6 H 15/2 with g = 4/3).The χ M T value remained constant over the 100-300 K T range, below which it decreased smoothly to a value of 23.83 cm 3 •K•mol −1 at 1.8 K, due to the depopulation of excited Stark sublevels in the crystal-field state.The magnetization (M) of 1 increased rapidly with an increasing magnetic field (H) to 7 T (M = 10.55 µ B ) without saturation at 1.8 K, indicating very strong magnetic anisotropy (Figure S1, Supplementary Materials).Moreover, no hysteresis was observed for 1 above 1.8 K using a conventional SQUID.The detailed determination of the magnetic-exchange coupling constant (J), i.e., magnitude and sign (ferromagnetic or antiferromagnetic), of a polynuclear Ln complex is difficult because of its complicated 4f electronic structure.In contrast, the electronic structure of the Gd III ion contains a half-filled 4f orbital system, leading to no orbital angular momentum, and can be analyzed as a simple spin-only case ( 8 S 7/2 with g = 2).Hence, the qualitative magnitude of J for 1 can be indirectly determined from that of the Gd III analogue (2).T-dependent χ M T measurements were performed in the 1.8-300 K T range in an applied DC magnetic field of 0.05 T (Figure 4).The χ M T value of 2 was determined to be 15.75 cm 3 •K•mol −1 at 300 K, which corresponds to the spin-only value of two uncoupled Gd III ions with g = 2.00; the χ M T value remained constant to 10 K. Further lowering of the temperature to 1.80 K lowered the χ M T value to 15.02 cm 3 •K•mol −1 .Furthermore, the H-dependent isothermal M data at 1.8 K for 2 show a steady increase with increasing magnetic field, reaching saturation at a value of 14.0 µ B at 7 T (Figure S1, Supplementary Materials).The magnetic behavior of 2 is the result of the competition between antiferromagnetic exchange between the Gd III centers via the Cl 2 An 2− bridge, and single-ion anisotropy arising from the Gd III ion.To determine whether intermolecular isotropic magnetic exchange or single-ion magnetic anisotropy dominates, plots of χ M T vs. T, and M vs. H for 2 were constructed using the spin Hamiltonian given by Equation (1): where S Gd1 and S Gd2 are the spin operators for Gd III ions (S = 7/2), J Gd1-Gd2 is the isotropic magnetic exchange coupling constant between two Gd III centers, D Gd and E Gd are the axial and rhombic zero-field splitting (ZFS) parameters that are associated with an S = 7/2 Gd III ion, respectively, g is the isotropic g factor, and µ B is the Bohr magneton.This model fitted the χ M T vs. T plot well over the entire 1.8 to 300 K temperature range, and the M vs. H plot at 1.80 K yielded J Gd1-Gd2 /k B = −0.0066K, D Gd /k B = 0.17 K, and E Gd /k B = 1.1 × 10 −5 (≈ 0) K, with g = 2.00 (Figures 4 and S1, Supplementary Materials).The calculated g factor is close to the spin-only value, and a negligible antiferromagnetic interaction between the two Gd III centers through the Cl 2 An 2− bridge, and a large ZFS contribution (|D| >> |J|) were found for 2. Except for organic radical bridging ligands, magnetic interactions between paramagnetic Ln III centers via common organic bridging ligands are well-known to be very weak in general, because 4f orbitals are shielded by outer 6s and 5d orbitals.The two Gd III centers in 2 are also well-separated by a diamagnetic spacer, namely the Cl 2 An 2− bridge (vide supra).Therefore, the small calculated J value might be meaningless.As an alternative, we subsequently attempted to fit the magnetic data for 2 by considering only single-ion anisotropy with the mean-field approximation (zJ ), as given by Equation 2: This model also fitted the χ M T vs. T plot well over the entire 1.8 to 300 K temperature range, and the M vs. H plot at 1.80 K provided D Gd /k B = 0.18 K, E Gd /k B = 1.8 × 10 −4 (≈ 0) K, and g = 2.00 with a negligible zJ /k B value (~−10 −5 K).Hence, the χ M T value for 3 at 300 K is 14.16 cm 3 •K•mol −1 , which is in good agreement with that of the free Dy III ion ( 6 H 15/2 with g = 4/3).The χ M T product gradually decreased with decreasing T, and finally reached a minimum value of 11.87 cm 3 •K•mol −1 at 1.8 K.This magnetic behavior is ascribable to the thermal depopulation of the crystal-field state.The M vs.
H plot of 3 at 1.8 K displays significant magnetic anisotropy and does not reach saturation up to 7 T (M = 5.10 µ B ).

Dynamic Magnetic Properties
To study the slow relaxation of the magnetization of both 1 and 3, alternate-current (AC) susceptibility measurements were performed at a fixed T of 1.8 K with a DC field (HDC) that varied between 0 and 8000 Oe (Figures 5 and 6).The χM signals for 1 and 3 in the absence of an applied field increased with increasing frequency (ν), but exhibited only tailing, and no apparent peaks were observed due to blocking in the available ν range.Subsequently, ν-dependent non-zero χM signals clearly appeared at small static HDC.This result clearly indicates that zero-field QTM [13] and/or dipolar interactions [32] are suppressed, and/or the non-negligible transverse magnetic anisotropy [33] is broken by the application of HDC in the ground state.Each χM peak maximum for 1 shifted to lower ν with an increasing applied HDC up to ~950 Oe.Further increases in HDC resulted in the maximum χM shifting to higher ν.Similarly, the maximum χM value for 3 shifted to lower ν, reaching a minimum at ~1500 Oe, above which it shifted to higher ν.Therefore, the optimum HDC values for 1 and 3 were determined to be ~950 Oe for 1 and ~1500 Oe for 3, which was used in the Tand ν-dependent AC susceptibility experiments (Figures 7, 8 and S2, Supplementary Materials).

Dynamic Magnetic Properties
To study the slow relaxation of the magnetization of both 1 and 3, alternate-current (AC) susceptibility measurements were performed at a fixed T of 1.8 K with a DC field (H DC ) that varied between 0 and 8000 Oe (Figures 5 and 6).The χ M " signals for 1 and 3 in the absence of an applied field increased with increasing frequency (ν), but exhibited only tailing, and no apparent peaks were observed due to blocking in the available ν range.Subsequently, ν-dependent non-zero χ M " signals clearly appeared at small static H DC .This result clearly indicates that zero-field QTM [13] and/or dipolar interactions [32] are suppressed, and/or the non-negligible transverse magnetic anisotropy [33] is broken by the application of H DC in the ground state.Each χ M " peak maximum for 1 shifted to lower ν with an increasing applied H DC up to ~950 Oe.Further increases in H DC resulted in the maximum χ M " shifting to higher ν.Similarly, the maximum χ M " value for 3 shifted to lower ν, reaching a minimum at ~1500 Oe, above which it shifted to higher ν.Therefore, the optimum H DC values for 1 and 3 were determined to be ~950 Oe for 1 and ~1500 Oe for 3, which was used in the Tand ν-dependent AC susceptibility experiments (Figures 7, 8 and S2, Supplementary Materials).
In addition, the other striking feature for both 1 and 3 is the clear presence of two distinct relaxation processes at higher H DC fields: the H DC -dependent component and the H DC -independent component, which are thermal and QTM relaxation modes, respectively (Figure 6).
Both 1 and 3 show ν-dependent χ M " at their respective optimum H DC .The relaxation time (τ) can be extracted at each blocking temperature using a generalized Debye model (Figure 7).Here, Argand plots [34] (χ M vs. χ M " plot) for 1 and 3 were semicircular with small α parameters in the 0.0063-0.31range for 1 and the 0.035-0.44range for 3 (Figure 8).In addition, the other striking feature for both 1 and 3 is the clear presence of two distinct relaxation processes at higher HDC fields: the HDC-dependent component and the HDC-independent component, which are thermal and QTM relaxation modes, respectively (Figure 6).In addition, the other striking feature for both 1 and 3 is the clear presence of two distinct relaxation processes at higher HDC fields: the HDC-dependent component and the HDC-independent component, which are thermal and QTM relaxation modes, respectively (Figure 6).Both 1 and 3 show ν-dependent χM at their respective optimum HDC.The relaxation time (τ) can be extracted at each blocking temperature using a generalized Debye model (Figure 7).Here, Argand plots [34] (χM vs. χM plot) for 1 and 3 were semicircular with small α parameters in the 0.0063-0.31range for 1 and the 0.035-0.44range for 3 (Figure 8).The linear regions at higher T in the semi-logarithmic plots of τ vs. 1/T (Arrhenius plots) for 1 and 3 were used to calculate the thermal energy barrier (Δeff) for magnetization reversal, known as Orbach relaxation [35], which provided Δeff/kB = 44.7 K and τ0 = 3.43 × 10 −7 s for 1, and Δeff/kB = 49.4K and τ0 = 2.12 × 10 −6 s for 2 (Figure 9).However, the Arrhenius plots for 1 and 3 deviate from linearity at lower T.Such magnetic relaxation behavior for 1 and 3 suggests the coexistence of thermal relaxation processes and other operative relaxation processes.Hence, τ for 1 and 3 in the given temperature range were analyzed by Equation 3: Argand plots [34] (χM vs. χM plot) for 1 and 3 were semicircular with small α parameters in the 0.0063-0.31range for 1 and the 0.035-0.44range for 3 (Figure 8).The linear regions at higher T in the semi-logarithmic plots of τ vs. 1/T (Arrhenius plots) for 1 and 3 were used to calculate the thermal energy barrier (Δeff) for magnetization reversal, known as Orbach relaxation [35], which provided Δeff/kB = 44.7 K and τ0 = 3.43 × 10 −7 s for 1, and Δeff/kB = 49.4K and τ0 = 2.12 × 10 −6 s for 2 (Figure 9).However, the Arrhenius plots for 1 and 3 deviate from linearity at lower T.Such magnetic relaxation behavior for 1 and 3 suggests the coexistence of thermal relaxation processes and other operative relaxation processes.Hence, τ for 1 and 3 in the given temperature range were analyzed by Equation 3: The linear regions at higher T in the semi-logarithmic plots of τ vs. 1/T (Arrhenius plots) for 1 and 3 were used to calculate the thermal energy barrier (∆ eff ) for magnetization reversal, known as Orbach relaxation [35], which provided ∆ eff /k B = 44.7 K and τ 0 = 3.43 × 10 −7 s for 1, and ∆ eff /k B = 49.4K and τ 0 = 2.12 × 10 −6 s for 2 (Figure 9).However, the Arrhenius plots for 1 and 3 deviate from linearity at lower T.Such magnetic relaxation behavior for 1 and 3 suggests the coexistence of thermal relaxation processes and other operative relaxation processes.Hence, τ for 1 and 3 in the given temperature range were analyzed by Equation 3: where the first, second, third, and fourth terms represent Orbach, Raman [35][36][37][38][39], direct [35][36][37][38][39], and QTM processes, respectively.Data for 1 and 3 were analytically reproduced using Equation 3 with appropriate terms for the Raman and direct processes.The best fit was obtained with C = 6.12 × 10 −3 s −1 •K −7.02 and A = 0.323 s −1 •K −1 for 1, while C = 7.22 × 10 −3 s −1 •K −5.71 and A = 0.106 s −1 •K −1 were best for 3 (Figure 9).Generally, the power index (n) of T for a Raman process is 9 because of Kramers ions.The n values of 1 and 3 are lower than the expected value, which might be attributable to the involvement of both optical and acoustic Raman processes during magnetic relaxation [38].Therefore, both Raman and direct processes dominate the spin dynamics of 1 and 3. Complexes 1 and 3 exhibit different spin-relaxation behavior, which reflect the coordination geometries around their Dy III centers and their nuclearities, as well as crystal-lattice molecular packing.In addition, although the magnetic analysis of 2 did not reveal any magnetic exchange coupling via the Cl 2 An 2− ligand, the dinuclear complex 1 might create a small bias that allows for tunneling at zero field.
Inorganics 2017, 5, 7 9 of 13 where the first, second, third, and fourth terms represent Orbach, Raman [35][36][37][38][39], direct [35][36][37][38][39], and QTM processes, respectively.Data for 1 and 3 were analytically reproduced using Equation 3 with appropriate terms for the Raman and direct processes.The best fit was obtained with C = 6.12 × 10 −3 s −1 •K −7.02 and A = 0.323 s −1 •K −1 for 1, while C = 7.22 × 10 −3 s −1 •K −5.71 and A = 0.106 s −1 •K −1 were best for 3 (Figure 9).Generally, the power index (n) of T for a Raman process is 9 because of Kramers ions.The n values of 1 and 3 are lower than the expected value, which might be attributable to the involvement of both optical and acoustic Raman processes during magnetic relaxation [38].Therefore, both Raman and direct processes dominate the spin dynamics of 1 and 3. Complexes 1 and 3 exhibit different spin-relaxation behavior, which reflect the coordination geometries around their Dy III centers and their nuclearities, as well as crystal-lattice molecular packing.In addition, although the magnetic analysis of 2 did not reveal any magnetic exchange coupling via the Cl2An 2− ligand, the dinuclear complex 1 might create a small bias that allows for tunneling at zero field.

Materials and Methods
Co(Cp)2, DyCl3•6H2O, GdCl3•6H2O, Na2Cl2An•3H2O, and all of the solvents were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) KTp was purchased from Tokyo Chemical Industry (TCI) Co., Ltd (Tokyo, Japan).All of the chemicals were reagent grade and were used as received.Both CH2Cl2 and hexane were super dehydrated grade and deoxidized grade.Unless otherwise noted, reactions and subsequent manipulations were performed under aerobic conditions at room temperature, while compound 3 was prepared under an inert N2 atmosphere using standard glovebox techniques.

Co
and all of the solvents were purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan) KTp was purchased from Tokyo Chemical Industry (TCI) Co., Ltd.(Tokyo, Japan).All of the chemicals were reagent grade and were used as received.Both CH 2 Cl 2 and hexane were super dehydrated grade and deoxidized grade.Unless otherwise noted, reactions and subsequent manipulations were performed under aerobic conditions at room temperature, while compound 3 was prepared under an inert N 2 atmosphere using standard glovebox techniques.

Synthesis of [{Dy(Tp
This complex was prepared from modification of the original method of Kaizaki et al. [26].An aqueous solution (5 mL) of DyCl 3 •6H 2 O (75.4 mg, 0.2 mmol) was added dropwise to a stirred aqueous solution (5 mL) of Na 2 Cl 2 An•3H 2 O (30.7 mg, 0.1 mmol) and KTp (100.8 mg, 0.4 mmol), resulting in a fine purple precipitate immediately.After stirring for 10 min, the purple solid was isolated via filtration, washed with a small amount of H 2 O (2 × 5 mL) and Et 2 O (3 × 5 mL), and dried in vacuo.The purple solid was then dissolved in CH 2 Cl 2 , and the resultant purple solution was filtered.Diffusion of hexane into the filtrate yielded purple rod-shaped crystals suitable for single-crystal X-ray diffraction analysis within 1 week.Yield: 144.5 mg (52% In a N 2 -filled glovebox, Co(Cp) 2 dissolved in PhMe (5 mL) was added to a solution of 1 in the same solvent (5 mL).The color of the reaction mixture changed to yellowish green.Upon stirring the mixture, bright yellow microcrystalline solid appeared immediately, which was collected by filtration, washed with a small amount of Et 2 O (5 mL), and allowed to dry on the filter for 15 min.The yellowish green solid was then dissolved in MeCN, and the resulting pale yellow solution was filtered.Slow diffusion of Et 2 O into the filtrate gave red block-shaped crystals suitable for single-crystal X-ray diffraction analysis for two weeks.Yield: 83%.Anal

Physical Measurements
Elemental analyses were performed by the Technical Support Division, Research and Analytical Centre for Giant Molecules, Graduate School of Science, Tohoku University.Fourier transform infrared (FTIR) spectra were acquired as KBr disks at room temperature on a JASCO FT/IR-410 spectrophotometer (JASCO Corporation, Tokyo, Japan).Magnetic data were collected on a Quantum Design MPMS3 or MPMS-5S superconducting quantum interference device (SQUID) magnetometer (Quantum Design Japan, Inc., Tokyo, Japan).All of the magnetic measurements were performed on finely ground microcrystalline powders in a calibrated gelatin capsule.DC susceptibility measurements were performed in the T range of 1.8-300 K in a DC field of 0.05 T or 0.1 T. Field-dependent dc magnetization measurements were performed from −7 to +7 T at 1.8 K. AC susceptibility measurements were performed in the T range of 1.8-20 K in a 3 Oe AC field, oscillating at frequencies of 1 and 997 or 1488 Hz in different applied DC fields.Magnetic susceptibility data were corrected for diamagnetic contributions from the sample holder as well as for the core diamagnetism of each sample estimated using Pascal's constants [44].

Conclusions and Outlook
The dinuclear and mononuclear [{Dy(Tp) 2 } 2 (µ-Cl 2 An)] and [Co(Cp) 2 ][Dy(Tp) 2 (Cl 2 An)] eight-coordinate complexes were synthesized.The two complexes have similar N 6 O 2 coordination environments that are based on the dianionic Cl 2 An 2− ligand and Tp − as auxiliary ligands.However, each Cl 2 An 2− moiety adopts the bi-separated delocalized form in dinuclear [{Dy(Tp) 2 } 2 (µ-Cl 2 An)], and the o-quinone form in mononuclear [Co(Cp) 2 ][Dy(Tp) 2 (Cl 2 An)].Therefore, in this work, both complexes were magnetically characterized to examine how the donor strength affects the slow relaxation of the magnetization.Dynamic magnetic measurements clearly reveal that both of the complexes exhibit temperature-dependent slow relaxations of magnetization facilitated by multiple relaxation pathways.Their spin dynamics are different, reflecting the coordination geometries around the Dy III centers and their nuclearities, as well as crystal-lattice molecular packing.The redox-active Cl 2 An 2− ligand is a good candidate for the future construction of redox-switchable SMMs, as anilate derivatives readily undergo redox reactions to form both diamagnetic and paramagnetic species; such redox-switchable SMMs and/or SIMs that may be applied in future technologies, such as molecular spintronics.Furthermore, we are currently constructing dinuclear complexes of all other lanthanides bridged with the Cl 2 An 2− moiety.This effort will provide valuable insight into the electronic structures, magnetic properties, and trends observed in the series of dinuclear [{Ln(Tp) 2 } 2 (µ-Cl 2 An)] complexes.

Supplementary Materials:
The following are available online at www.mdpi.com/2304-6740/6/1/7/s1. Cif and cif-checked files.Figure S1: Field dependence of magnetization plots of 1-3 at 1.8 K.The solid red lines were fitted to the data as described in the text.; Figure S2: Temperature dependence of χ M and χ M " products at given frequencies in an applied DC field of 950 Oe for 1 and 1500 Oe for 3, respectively.Solid black lines serve as guides.

Figure 2 .
Figure 2. Molecular structure of 1. H atoms and lattice solvent molecules are omitted for clarity.

Figure 3 .
Figure 3. Molecular structure of 3. H atoms are omitted for clarity.

Figure 2 .
Figure 2. Molecular structure of 1. H atoms and lattice solvent molecules are omitted for clarity.

Figure 2 .
Figure 2. Molecular structure of 1. H atoms and lattice solvent molecules are omitted for clarity.

Figure 3 .
Figure 3. Molecular structure of 3. H atoms are omitted for clarity.

2. 3 .
Magnetic Properties 2.3.1.Static Magnetic Properties DC magnetic susceptibility (χM) measurements on 1 were carried out in the 1.8-295 K temperature (T) range in an applied 0.1 T DC field (Figure

Figure 3 .
Figure 3. Molecular structure of 3. H atoms are omitted for clarity.

Figure 4 . 3 .
Figure 4. T dependence of χMT products for 1-3.The solid red line represents fit to the data, which is described in the text.

Figure 4 .
Figure 4. T dependence of χ M T products for 1-3.The solid red line represents fit to the data, which is described in the text.

Figure 5 .
Figure 5. Variable-field χM for 1 (a and b) and 3 (c) at 1.8 K for direct-current (DC) fields of 0-8000 Oe, over the frequency range of 0.1-1000 Hz.Solid black lines serve as guides.

Figure 6 .
Figure 6.HDC-dependece of τ for 1 and 3 at 1.8 K indicating two distinct relaxation processes: HDC-dependent component (blue open circles for 1 and blue open circles for 3) and HDC-independent ones (blue filled circles for 1 and black filled circles for 3).Solid lines serve as guides.

Figure 5 .of 13 Figure 5 .
Figure 5. Variable-field χ M " for 1 (a and b) and 3 (c) at 1.8 K for direct-current (DC) fields of 0-8000 Oe, over the frequency range of 0.1-1000 Hz.Solid black lines serve as guides.

Figure 6 .
Figure 6.HDC-dependece of τ for 1 and 3 at 1.8 K indicating two distinct relaxation processes: HDC-dependent component (blue open circles for 1 and blue open circles for 3) and HDC-independent ones (blue filled circles for 1 and black filled circles for 3).Solid lines serve as guides.

Figure 6 .
Figure 6.H DC -dependece of τ for 1 and 3 at 1.8 K indicating two distinct relaxation processes: H DC -dependent component (blue open circles for 1 and blue open circles for 3) and H DC -independent ones (blue filled circles for 1 and black filled circles for 3).Solid lines serve as guides.

Figure 7 .
Figure 7. ν-dependent χM and χM in the T range 1.8-8.0K in an applied HDC of 950 Oe for 1 (a) and in the T range 1.8-11.0K in an applied HDC of 1500 Oe for 3 (b), respectively.Solid black lines represent best fits to a generalized Debye model.

Figure 8 .
Figure 8. Argand plots at given temperatures in an applied DC field of 950 Oe for 1 (a) and 1500 Oe for 3 (b), respectively.Solid black lines are best-fit lines.

Figure 7 .
Figure 7. ν-dependent χ M and χ M " in the T range 1.8-8.0K in an applied H DC of 950 Oe for 1 (a) and in the T range 1.8-11.0K in an applied H DC of 1500 Oe for 3 (b), respectively.Solid black lines represent best fits to a generalized Debye model.

Figure 7 .
Figure 7. ν-dependent χM and χM in the T range 1.8-8.0K in an applied HDC of 950 Oe for 1 (a) and in the T range 1.8-11.0K in an applied HDC of 1500 Oe for 3 (b), respectively.Solid black lines represent best fits to a generalized Debye model.

Figure 8 .
Figure 8. Argand plots at given temperatures in an applied DC field of 950 Oe for 1 (a) and 1500 Oe for 3 (b), respectively.Solid black lines are best-fit lines.

Figure 8 .
Figure 8. Argand plots at given temperatures in an applied DC field of 950 Oe for 1 (a) and 1500 Oe for 3 (b), respectively.Solid black lines are best-fit lines.

Figure 9 .
Figure 9. T-dependece of τ for 1 and 3 at 950 and 1500 Oe, respectively.Solid lines were fitted to the data as described in the text.

Figure 9 .
Figure 9. T-dependece of τ for 1 and 3 at 950 and 1500 Oe, respectively.Solid lines were fitted to the data as described in the text.