Assembly of Homochiral Magneto-Optical Dy6 Triangular Clusters by Fixing Carbon Dioxide in the Air

A new hydrazone Schiff base bridging ligand (H2LSchiff (E)-N′-((1-hydroxynaphthalen-2-yl)methylene)pyrazine-2-carbohydrazide) and L/D-proline were used to construct a pair of homochiral Dy6 cluster complexes, [Dy6(CO3)(L-Pro)6(LSchiff)4(HLSchiff)2]·5DMA·2H2O (L-1, L-HPro = L-proline; DMA = N,N-dimethylacetamide) and [Dy6(CO3)(D-Pro)6(LSchiff)4(HLSchiff)2]·5DMA·2H2O (D-1, D-HPro = D-proline), which show a novel triangular Dy6 topology. Notably, the fixation of CO2 in the air formed a carbonato central bridge, playing a key role in assembling L-1/D-1. Magnetic measurements revealed that L-1/D-1 displays intramolecular ferromagnetic coupling and magnetic relaxation behaviours. Furthermore, L-1/D-1 shows a distinct magneto-optical Faraday effect and has a second harmonic generation (SHG) response (1.0 × KDP) at room temperature. The results show that the immobilization of CO2 provides a novel pathway for homochiral multifunctional 4f cluster complexes.

Molecules 2024, 29, x FOR PEER REVIEW 2 of 12 magnetochiral dichroism (MChD) is expected to be used in the optical readout of magnetic storage data [47].Generally, homochiral Schiff base ligands [39] and homochiral βdiketone ligands [40][41][42] are selected to construct such homochiral SMMs, as these two types of ligands have characteristic Cotton peaks in the circular dichroism (CD) spectra, which facilitates the study of magneto-optical Faraday effects using magnetic circular dichroism (MCD) spectra.We hope that this research will be extended to the construction of magneto-optical SMMs with simpler and cheaper chiral carboxylic acid ligands.In this paper, we adopted L/D-proline as the homochiral ligand and a new hydrazone Schiff base ligand condensed with 1-hydroxy-2-naphthaldehyde and pyrazinoic acid hydrazide as the bridging ligand (H2LSchiff, Scheme 1) to assemble a pair of

Synthesis and Crystal Structures
When the Dy(III) complexes are assembled with hydrazone Schiff base ligands in an alkaline solution, the automatic fixation of CO2 in the air can occasionally occur, and the carbonato-bridged Dy(III) cluster complexes are finally obtained [2,4,6,8,12], which include Dy6 complexes with topologies of the triangular prism [2,8,12] and the fusion of three capped triangular Dy3 motifs [4].L-1/D-1 was yielded by the reaction of 0.25 mmol of H2LSchiff, 0.25 mmol of Dy(CF3SO3)3, 0.25 mmol of L/D-proline, and 0.75 mmol of LiOH•H2O in 5 mL of DMA for 24 h at room temperature.In general, the reaction at room temperature can effectively prevent the rapid racemization of chiral complexes at high temperatures [48].Good-quality single crystals of L-1/D-1 could be obtained by recrystallization with CH2Cl2-MeOH (1:1 v/v), which crystallizes in the monoclinic C2 space group.If DMA is changed to DMF, the product with the same structure can also be obtained, but the lattice solvent DMA becomes DMF, and the single crystal diffraction data will be worse.Notably, the Dy(III) cluster complexes constructed with L/D-proline are very rare [49], and L-1/D-1 represents the first complex constructed from the hydrazone Schiff base ligand H2LSchiff.
As shown in Figure 1a, the carbonato ligand formed by the automatic fixation of CO2 in the air acts as the structural core of L-1, which directly bridges three Dy 3+ ions in a triangular shape in a hexadentate µ3-mode, with Dy…Dy separation distances of 4.793, 4.867, and 4.745 Å for Dy2-Dy3, Dy3-Dy5, and Dy2-Dy5, respectively.Such a CO3 2−bridged Dy3 central triangle is further linked to three additional Dy 3+ ions through the HLSchiff − /LSchiff 2− and L-Pro − mixed bridges, eventually forming a larger Dy6 triangular cluster, which contains three small outer Dy3 triangles.Such a triangular Dy6 topology is different from the triangular prism [2,8,12] and the fusion of three capped triangular Dy3 Scheme 1. Synthesis pathway of the new hydrazone Schiff base ligand H 2 L Schiff .

Synthesis and Crystal Structures
When the Dy(III) complexes are assembled with hydrazone Schiff base ligands in an alkaline solution, the automatic fixation of CO 2 in the air can occasionally occur, and the carbonato-bridged Dy(III) cluster complexes are finally obtained [2,4,6,8,12], which include Dy 6 complexes with topologies of the triangular prism [2,8,12] and the fusion of three capped triangular Dy 3 motifs [4].L-1/D-1 was yielded by the reaction of 0.25 mmol of H 2 L Schiff , 0.25 mmol of Dy(CF 3 SO 3 ) 3 , 0.25 mmol of L/D-proline, and 0.75 mmol of LiOH•H 2 O in 5 mL of DMA for 24 h at room temperature.In general, the reaction at room temperature can effectively prevent the rapid racemization of chiral complexes at high temperatures [48].Good-quality single crystals of L-1/D-1 could be obtained by recrystallization with CH 2 Cl 2 -MeOH (1:1 v/v), which crystallizes in the monoclinic C 2 space group.If DMA is changed to DMF, the product with the same structure can also be obtained, but the lattice solvent DMA becomes DMF, and the single crystal diffraction data will be worse.Notably, the Dy(III) cluster complexes constructed with L/D-proline are very rare [49], and L-1/D-1 represents the first complex constructed from the hydrazone Schiff base ligand H 2 L Schiff .
As shown in Figure 1a, the carbonato ligand formed by the automatic fixation of CO 2 in the air acts as the structural core of L-1, which directly bridges three Dy 3+ ions in a triangular shape in a hexadentate µ 3 -mode, with Dy. ..Dy separation distances of 4.793, 4.867, and 4.745 Å for Dy2-Dy3, Dy3-Dy5, and Dy2-Dy5, respectively.Such a CO 3 2−bridged Dy 3 central triangle is further linked to three additional Dy 3+ ions through the HL Schiff − /L Schiff 2− and L-Pro − mixed bridges, eventually forming a larger Dy 6 triangular cluster, which contains three small outer Dy 3 triangles.Such a triangular Dy 6 topology is different from the triangular prism [2,8,12] and the fusion of three capped triangular Dy 3 motifs [4] in carbonato-bridged Dy 6 cluster complexes.The outer Dy. ..Dy separation distances are 3.987 Å for Dy2-Dy4, 3.930 Å for Dy5-Dy4, 3.959 Å for Dy1-Dy2, 3.906 Å for Dy1-Dy3, 3.926 Å for Dy3-Dy6, and 3.931 Å for Dy5-Dy6, which are smaller than those in the Dy 3 central triangle (4.745-4.867Å).Notably, the µ 3 -η 2 :η 2 :η 2 coordination mode of the carbonato ligand in L-1 is also observed in the triangular prism-like Dy 6 cluster complexes assembled by other hydrazone Schiff base ligands [2,8,12] but is different from the µ 3 -η 2 :η 2 coordination mode in the Dy 6 cluster complex with a topology of the fusion of three capped triangular Dy 3 motifs [4] and the µ 4 -η 1 :η 2 :η 1 :η 1 coordination mode in the tub-like Dy 8 cluster complex [2,6].The Schiff base ligands in L-1 have not only a deprotonated form (L Schiff 2− ) but also a protonated form (HL Schiff − ); both bridge the two Dy 3+ ions on the outer Dy-Dy edge of the outer small Dy 3 triangle, and this Dy-Dy edge is simultaneously bridged by the carboxyl group of one L-Pro − anion; the latter adopts a unique µ 2-η 1 :η 2 coordination mode in which the pyrrolidine N atom does not participate in the coordination, while it usually uses both the carboxyl O atom and the pyrrolidine N atom to participate in coordination [49].Notably, the µ3-η 2 :η 2 :η 2 coordination mode of the carbonato ligand in L-1 is also observed in the triangular prism-like Dy6 cluster complexes assembled by other hydrazone Schiff base ligands [2,8,12] but is different from the µ3-η 2 :η 2 coordination mode in the Dy6 cluster complex with a topology of the fusion of three capped triangular Dy3 motifs [4] and the µ4-η 1 :η 2 :η 1 :η 1 coordination mode in the tub-like Dy8 cluster complex [2,6].The Schiff base ligands in L-1 have not only a deprotonated form (LSchiff 2− ) but also a protonated form (HLSchiff − ); both bridge the two Dy 3+ ions on the outer Dy-Dy edge of the outer small Dy3 triangle, and this Dy-Dy edge is simultaneously bridged by the carboxyl group of one L-Pro − anion; the latter adopts a unique µ2-η 1 :η 2 coordination mode in which the pyrrolidine N atom does not participate in the coordination, while it usually uses both the carboxyl O atom and the pyrrolidine N atom to participate in coordination [49].The Dy 3+ ions located on the central Dy3 triangle (Dy2, Dy3, and Dy5) are 10-coordinated (Figure 1), which are coordinated by two O atoms from the carbonato ligand, four Ocarboxyl atoms from two L-Pro − anions, two Ocarboxyl atoms and two Npyrazine atoms from two LSchiff 2− /HLSchiff − ligands.An accurate analysis by the Shape 2.1 software [50] revealed that the coordination configurations of these Dy 3+ ions are the bicapped square antiprism J17 (D4d), and the CShM values are 2.886 for Dy2, 3.156 for Dy3, and 3.052 for Dy5 (Table S1).However, the Dy 3+ ions located at the vertices of the large Dy6 triangle (Dy1, Dy4, and Dy6) are 8-coordinated (Figure 1), which are bonded by two Ocarboxyl atoms from two L-Pro − anions, two Ocarboxyl atoms, two Npyrazine atoms and two Ophenol atoms from two LSchiff 2− /HLSchiff − ligands.An accurate analysis by the Shape 2.1 software [50] showed that the coordination configurations of these three Dy 3+ ions are the following: the biaugmented trigonal prism J50 (C2v) for Dy1 and Dy4, with CShM values of 2.249 and 2.679, respectively (Table S2); the biaugmented trigonal prism (C2v) for Dy6, with a CShM value of 2.118 (Table S2).D-1 is an enantiomer of L-1 and thus has a structure that is similar and mirror symmetrical to L-1 (Figure 1b).

Magnetic Properties
The temperature-dependent DC magnetic susceptibility of L-1 measured at 1000 Oe revealed that its χMT value (85.10 cm 3 K mol −1 ) at 300 K is in agreement with the theoretical  S1).However, the Dy 3+ ions located at the vertices of the large Dy 6 triangle (Dy1, Dy4, and Dy6) are 8-coordinated (Figure 1), which are bonded by two O carboxyl atoms from two L-Pro − anions, two O carboxyl atoms, two N pyrazine atoms and two O phenol atoms from two L Schiff 2− /HL Schiff − ligands.An accurate analysis by the Shape 2.1 software [50] showed that the coordination configurations of these three Dy 3+ ions are the following: the biaugmented trigonal prism J50 (C 2v ) for Dy1 and Dy4, with CShM values of 2.249 and 2.679, respectively (Table S2); the biaugmented trigonal prism (C 2v ) for Dy6, with a CShM value of 2.118 (Table S2).D-1 is an enantiomer of L-1 and thus has a structure that is similar and mirror symmetrical to L-1 (Figure 1b).

Magnetic Properties
The temperature-dependent DC magnetic susceptibility of L-1 measured at 1000 Oe revealed that its χ M T value (85.10 cm 3 K mol −1 ) at 300 K is in agreement with the theoretical value of six isolated Dy 3+ ions (85.02 cm 3 K mol −1 ) (Figure 2).The χ M T value of L-1 decreases as it cools down, reaching a minimum of 65.44 cm 3 K mol −1 at 10 K, and then suddenly increases to 69.55 cm 3 K mol −1 at 2 K.Such a hook-like χ M T-T curve is the result of a combination of two main factors: the depopulation of the m J levels of the Dy 3+ ions and the intramolecular ferromagnetic coupling among the Dy 3+ ions [12,40,42,51].Similar intramolecular ferromagnetic interactions between Dy 3+ ions are also present in another carbonato-bridged dysprosium (III) complex [6].Moreover, the magnetic field-dependent magnetization measurements of L-1 showed no overlap in the M-H/T curves at 2-6 K (Figure S1), indicating magnetic anisotropy.
as it cools down, reaching a minimum of 65.44 cm K mol at 10 K, and then suddenly increases to 69.55 cm 3 K mol −1 at 2 K.Such a hook-like χMT-T curve is the result of a combination of two main factors: the depopulation of the mJ levels of the Dy 3+ ions and the intramolecular ferromagnetic coupling among the Dy 3+ ions [12,40,42,51].Similar intramolecular ferromagnetic interactions between Dy 3+ ions are also present in another carbonato-bridged dysprosium (III) complex [6].Moreover, the magnetic field-dependent magnetization measurements of L-1 showed no overlap in the M-H/T curves at 2-6 K (Figure S1), indicating magnetic anisotropy.The temperature-dependent AC magnetic susceptibility of L-1 was measured in the range of 50-1399 Hz at 0 Oe to investigate its SMM properties.The χ″ vs. T curves show frequency dependence, but no peaks, and the χ″ signal appears only in the lower temperature region (<5 K) (Figure 3), suggesting that the effective energy barrier value of L-1 is very small.Furthermore, the χ″-T curve at 1399 Hz also shows no signs of peaking when a common DC field of 2000 Oe is applied (Figure S2), suggesting that the failure to show peaks is not caused by the quantum tunnelling effect.Such a phenomenon also occurs in the Dy5 cluster complexes constructed from L/D-proline [49] and other Dy(III) cluster complexes formed by the immobilized CO2 [5,7].
The Debye model based on Equation (1), in which ω (=2πν) represents the angular frequency [52], can generally be used to roughly estimate the Ueff/k value and the τ0 value [53].ln(χ″/χ′) = ln(ωτ0) + Ueff/kT (1) It is worth mentioning that such a fitting method is only suitable for when the χ″-T curves do not have peaks, and the Ueff/k and τ0 values may be different when fitting with χ″-T curves of different frequencies [54].The best fitting of ln(χ″/χ')-1/T curves for L-1 leads to Ueff/k values of 6.5-8.3K and τ0 values of 2.0 × 10 −7 -6.5 × 10 −7 s (Figure S3).The Ueff/k value of L-1 is comparable with that of the hourglass-like {Dy7} cluster complex (~4.6 K) [53].In Dy 3+ multinuclear cluster complexes, the magnetic axes of different ions are difficult to regularize, and factors such as J-type exchange coupling and intramolecular and intermolecular dipole interactions are often not conducive to good SMM performance [32]; these factors are also likely to be the reason for the smaller Ueff/k value of L-1.The temperature-dependent AC magnetic susceptibility of L-1 was measured in the range of 50-1399 Hz at 0 Oe to investigate its SMM properties.The χ ′′ vs. T curves show frequency dependence, but no peaks, and the χ ′′ signal appears only in the lower temperature region (<5 K) (Figure 3), suggesting that the effective energy barrier value of L-1 is very small.Furthermore, the χ ′′ -T curve at 1399 Hz also shows no signs of peaking when a common DC field of 2000 Oe is applied (Figure S2), suggesting that the failure to show peaks is not caused by the quantum tunnelling effect.Such a phenomenon also occurs in the Dy 5 cluster complexes constructed from L/D-proline [49] and other Dy(III) cluster complexes formed by the immobilized CO 2 [5,7].

Circular Dichroism Spectra (CD) and Magnetic Circular Dichroism Spectra (MCD)
In order to verify the properties of the enantiomers, the CD spectra of L-1 and D-1 in DMF solution were measured along with their UV-vis spectra (Figure 4).The UV-vis spectrum of L-1 is similar to that of D-1 (Figure 4, below), the strong peak at 322 nm may be ascribed to the π-π* conjugation of the naphthalene ring and the C=N bond of the hydrazone Schiff base ligand, while the strong peak at 454 nm and its shoulder peak at a higher wavelength are attributed to the n-π* transition of the C=N bond of the hydrazone Schiff base ligand [39,41,42,45,46].In contrast, the UV-vis spectrum of the H2LSchiff ligand in DMF solution can clearly distinguish the absorption peaks of the pyrazine group (323 nm), 1-naphthol group (338 nm) [55], and the π-π* conjugation of the naphthalene ring and the C=N bond (373 nm), while the absorption peak of the n-π* transition of the C=N bond at 475 nm is very weak (Figure S4).The CD spectra of L-1 and D-1 exhibit excellent mirror symmetry, confirming their enantiomeric properties (Figure 4, above).The Cotton peak at 298 nm is assigned to the π-π* transitions of the conjugated group in the hydrazone Schiff base ligand, while the Cotton peaks at 374 nm and 428 nm are caused by the π-π* and n-π* transitions of the C=N chromophore, respectively [42,45,46].The Debye model based on Equation (1), in which ω (=2πν) represents the angular frequency [52], can generally be used to roughly estimate the U eff /k value and the τ 0 value [53].
ln(χ ′′ /χ ′ ) = ln(ωτ 0 ) + U eff /kT (1) It is worth mentioning that such a fitting method is only suitable for when the χ ′′ -T curves do not have peaks, and the U eff /k and τ 0 values may be different when fitting with χ ′′ -T curves of different frequencies [54].The best fitting of ln(χ ′′ /χ')-1/T curves for L-1 leads to U eff /k values of 6.5-8.3K and τ 0 values of 2.0 × 10 −7 -6.5 × 10 −7 s (Figure S3).The U eff /k value of L-1 is comparable with that of the hourglass-like {Dy 7 } cluster complex (~4.6 K) [53].In Dy 3+ multinuclear cluster complexes, the magnetic axes of different ions are difficult to regularize, and factors such as J-type exchange coupling and intramolecular and intermolecular dipole interactions are often not conducive to good SMM performance [32]; these factors are also likely to be the reason for the smaller U eff /k value of L-1.

Circular Dichroism Spectra (CD) and Magnetic Circular Dichroism Spectra (MCD)
In order to verify the properties of the enantiomers, the CD spectra of L-1 and D-1 in DMF solution were measured along with their UV-vis spectra (Figure 4).The UV-vis spectrum of L-1 is similar to that of D-1 (Figure 4, below), the strong peak at 322 nm may be ascribed to the π-π* conjugation of the naphthalene ring and the C=N bond of the hydrazone Schiff base ligand, while the strong peak at 454 nm and its shoulder peak at a higher wavelength are attributed to the n-π* transition of the C=N bond of the hydrazone Schiff base ligand [39,41,42,45,46].In contrast, the UV-vis spectrum of the H 2 L Schiff ligand in DMF solution can clearly distinguish the absorption peaks of the pyrazine group (323 nm), 1-naphthol group (338 nm) [55], and the π-π* conjugation of the naphthalene ring and the C=N bond (373 nm), while the absorption peak of the n-π* transition of the C=N bond at 475 nm is very weak (Figure S4).The CD spectra of L-1 and D-1 exhibit excellent mirror symmetry, confirming their enantiomeric properties (Figure 4, above).The Cotton peak at 298 nm is assigned to the π-π* transitions of the conjugated group in the hydrazone Schiff base ligand, while the Cotton peaks at 374 nm and 428 nm are caused by the π-π* and n-π* transitions of the C=N chromophore, respectively [42,45,46].

Circular Dichroism Spectra (CD) and Magnetic Circular Dichroism Spectra (MCD)
In order to verify the properties of the enantiomers, the CD spectra of L-1 and D-1 in DMF solution were measured along with their UV-vis spectra (Figure 4).The UV-vis spectrum of L-1 is similar to that of D-1 (Figure 4, below), the strong peak at 322 nm may be ascribed to the π-π* conjugation of the naphthalene ring and the C=N bond of the hydrazone Schiff base ligand, while the strong peak at 454 nm and its shoulder peak at a higher wavelength are attributed to the n-π* transition of the C=N bond of the hydrazone Schiff base ligand [39,41,42,45,46].In contrast, the UV-vis spectrum of the H2LSchiff ligand in DMF solution can clearly distinguish the absorption peaks of the pyrazine group (323 nm), 1-naphthol group (338 nm) [55], and the π-π* conjugation of the naphthalene ring and the C=N bond (373 nm), while the absorption peak of the n-π* transition of the C=N bond at 475 nm is very weak (Figure S4).The CD spectra of L-1 and D-1 exhibit excellent mirror symmetry, confirming their enantiomeric properties (Figure 4, above).The Cotton peak at 298 nm is assigned to the π-π* transitions of the conjugated group in the hydrazone Schiff base ligand, while the Cotton peaks at 374 nm and 428 nm are caused by the π-π* and n-π* transitions of the C=N chromophore, respectively [42,45,46].To study the magneto-optical properties of L-1 and D-1, we further measured their CD spectra when a magnetic field (±1.6 T) was applied (Figure 5).It should be noted that the positive (+1.6 T, NS) and negative magnetic fields (−1.6 T, SN) refer to the parallel and antiparallel direction of the magnetic field to the polarized light, respectively.As shown in Figure 5, the CD spectra of L-1 and D-1 under the ±1.6 T magnetic field also have mirror symmetry: the D-1 (NS, +1.6 T) curve is mirror symmetrical with L-1 (SN, −1.6 T), while the D-1 (SN, −1.6 T) curve is mirror symmetrical with L-1 (NS, +1.6 T).Notably, the positions of the Cotton peaks of the CD spectra of L-1 and D-1 at ±1.6 T are similar to those of their CD spectra at 0 T. The pure MCD signals L-1 and D-1 could be obtained based on MCD = [CD(NS) − CD(SN)]/2/M (here M = 1.6 T) [38,40,41,44,45].The pure MCD spectra show strong troughs at 350 nm and 437 nm for L-1 and at 350 nm and 437 nm for D-1 (Figure S5); these two troughs for each isomer are ascribed to the π-π* and n-π* transitions of the C=N chromophore, respectively [42,45,46].hydroxybutan-2-yl)imino)methyl)-5-methoxyphenol; HL2 = 2-hydroxy-4-methoxybenzaldehyde} (0.012 T -1 ) [44], but are obviously smaller than those of 4f SMMs based on homochiral β-diketone ligands (≥0.34 T -1 ) [40][41][42] and Cu-Dy zero-field SMMs (≥0.44 T -1 ) [39,45].This may be related to the fact that the SMM properties of L-1 and D-1 are not as good as the latter.

Nonlinear Optical Properties
Since L-1/D-1 is a chiral crystalline complex, we investigated its second-order nonlinear optical properties.The second harmonic generation (SHG) efficiency of L-1 is as  [39,45].This may be related to the fact that the SMM properties of L-1 and D-1 are not as good as the latter.

Nonlinear Optical Properties
Since L-1/D-1 is a chiral crystalline complex, we investigated its second-order nonlinear optical properties.The second harmonic generation (SHG) efficiency of L-1 is as strong as that of the reference sample KDP (KH 2 PO 4 ) (Figure 6), and the SHG intensity of L-1 [DyZn 2 (S,S-L Schiff )Br 2 (MeOH)](CF 3 SO 3 ) 2 (1.0 × KDP) [57], but is obviously larger than those of chiral Dy(III) SMMs (0.7 × KDP) [58] and other chiral Zn-Ln SMMs (≤0.2 × KDP) [26,59,60].Therefore, L-1/D-1 is a potential second-order nonlinear optical molecular material.By the way, simultaneous measurements showed that L-1/D-1 does not show any third-order nonlinear optical properties, which may be due to the lack of a strong electronic push-pull group in the ligand of L-1.
larger than those of chiral Dy(III) SMMs (0.7 × KDP) [58] and other chiral Zn-Ln SMMs (≤0.2 × KDP) [26,59,60].Therefore, L-1/D-1 is a potential second-order nonlinear optical molecular material.By the way, simultaneous measurements showed that L-1/D-1 does not show any third-order nonlinear optical properties, which may be due to the lack of a strong electronic push-pull group in the ligand of L-1.

Experimental Section
All chemicals were analytically pure, purchased from commercial suppliers, and ready for use.

Materials and Instruments
The elemental analyses (C, H, N) were determined using a Thermo Flash Smart (Thermo Fisher Scientific, Waltham, MA, USA) elemental analyzer.The infrared spectra were recorded on a Bruker VERTEX 70v spectrophotometer (BRUKER OPTICS, Karlsruhe, Germany) using pressed KBr pellets.The UV-vis spectrum of H2LSchiff was measured on a UH5700 Spectrophotometer (Hitachi hi-tech Co., Ltd., Tokyo, Japan).The circular dichroism (CD) spectra were recorded on a JASCO J-1700 spectropolarimeter (JASCO, Tsukuba, Japan) at room temperature with a 5 mm optical path, and a permanent magnet (+1.6 or −1.6 T) was used to measure MCD spectra.Magnetic properties were recorded on a Quantum Design MPMS-XL5 (SQUID) magnetometer (Quantum Design, San Diego, CA, USA).The diamagnetism of the molecule was corrected with Pascal's constant.

Nonlinear Optical (NLO) Response Measurements
An ultrafast fiber laser (NPI Lasers, Rainbow 1550 OEM) (PL OPTICS, Nanjing, China) was utilized as the excitation source to output 100 fs pulses at 1550 nm with a repetition rate of 80 MHz.This laser beam was then focused by an aspheric lens with a numerical aperture of 0.8, forming a laser spot with a beam waist radius of 2 µm.The

Experimental Section
All chemicals were analytically pure, purchased from commercial suppliers, and ready for use.

Materials and Instruments
The elemental analyses (C, H, N) were determined using a Thermo Flash Smart (Thermo Fisher Scientific, Waltham, MA, USA) elemental analyzer.The infrared spectra were recorded on a Bruker VERTEX 70v spectrophotometer (BRUKER OPTICS, Karlsruhe, Germany) using pressed KBr pellets.The UV-vis spectrum of H 2 L Schiff was measured on a UH5700 Spectrophotometer (Hitachi hi-tech Co., Ltd., Tokyo, Japan).The circular dichroism (CD) spectra were recorded on a JASCO J-1700 spectropolarimeter (JASCO, Tsukuba, Japan) at room temperature with a 5 mm optical path, and a permanent magnet (+1.6 or −1.6 T) was used to measure MCD spectra.Magnetic properties were recorded on a Quantum Design MPMS-XL5 (SQUID) magnetometer (Quantum Design, San Diego, CA, USA).The diamagnetism of the molecule was corrected with Pascal's constant.

Nonlinear Optical (NLO) Response Measurements
An ultrafast fiber laser (NPI Lasers, Rainbow 1550 OEM) (PL OPTICS, Nanjing, China) was utilized as the excitation source to output 100 fs pulses at 1550 nm with a repetition rate of 80 MHz.This laser beam was then focused by an aspheric lens with a numerical aperture of 0.8, forming a laser spot with a beam waist radius of 2 µm.The spectra of the second-harmonic generation (SHG) and the third-harmonic generation (THG) were obtained using a cooled fiber optic spectrometer (Ideaoptics, NOVA, Shanghai, China).For comparison, the same integration time (T int = 0.5 s) was used to obtain SHG and THG signals for the sample and reference material.The SHG and THG mappings were monitored by integrating SHG and THG spectra at different locations over an integration range of 500-550 nm and 750-800 nm for THG and SHG, respectively.

Synthesis of H 2 L Schiff
1-Hydroxy-2-naphthaldehyde (10 mmol) and pyrazinoic acid hydrazide (10 mmol) were added to 30 mL of methanol; the reaction mixture was refluxed for 3 h and then cooled to room temperature.The resulting light-yellow precipitate was collected by filtration and washed with 3 × 10 mL of ethanol before being dried in air.

X-ray Single-Crystal Structure Measurement
Single-crystal diffraction data for L-1 and D-1 were collected on a Rigaku MM007HF (RIGAKU, Tokyo, Japan) diffractometer () with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) at 173 K.The two structures were solved using the olex2.solvestructure solution program and refined using the ShelXL-2015 refinement package.The squeeze technology was adopted to refine these two structures since the lattice solvent molecules DMA and H 2 O in L-1 and D-1 were disordered.Notably, the pyrrolidine ring in L/D-proline ligands was disordered in two positions and idealized by bond distances.All non-hydrogen atoms were refined anisotropically, and all hydrogen atoms were allowed for as riding atoms.The crystallographic data and refinement parameters are listed in Table 1.

Conclusions
In summary, a pair of enantiomers with the novel triangular Dy 6 core was prepared using the new hydrazone Schiff base bridging ligand (E)-N ′ -((1-hydroxynaphthalen-2yl)methylene)pyrazine-2-carbohydrazide and the chiral ligand L/D-proline.During the reaction, CO 2 in the air was fixed to form the carbonato ligand, which plays a critical role in the construction of the homochiral Dy 6 complexes as a central bridging ligand.This pair of homochiral Dy 6 clusters exhibited intramolecular ferromagnetic coupling and a frequency-dependent AC magnetic susceptibility and displayed a distinct magneto-optical Faraday effect and an SHG response as strong as KDP.This study demonstrates that the fixation of CO 2 in the air can be used to construct chiral cluster complexes, which provides a unique synthetic pathway for multifunctional molecular materials.

Figure 1 .
Figure 1.Crystal structures of L-1 (a) and D-1 (b).Color code: C, grey; N, blue; O, red; Dy, green.All disordered pyrrolidine rings show only one of two sets, and all H atoms and solvent molecules are omitted for clarity.

Figure 1 .
Figure 1.Crystal structures of L-1 (a) and D-1 (b).Color code: C, grey; N, blue; O, red; Dy, green.All disordered pyrrolidine rings show only one of two sets, and all H atoms and solvent molecules are omitted for clarity.The Dy 3+ ions located on the central Dy 3 triangle (Dy2, Dy3, and Dy5) are 10coordinated (Figure 1), which are coordinated by two O atoms from the carbonato ligand, four O carboxyl atoms from two L-Pro − anions, two O carboxyl atoms and two N pyrazine atoms from two L Schiff 2− /HL Schiff − ligands.An accurate analysis by the Shape 2.1 software [50] revealed that the coordination configurations of these Dy 3+ ions are the bicapped square antiprism J17 (D 4d ), and the CShM values are 2.886 for Dy2, 3.156 for Dy3, and 3.052 for Dy5 (TableS1).However, the Dy 3+ ions located at the vertices of the large Dy 6 triangle (Dy1, Dy4, and Dy6) are 8-coordinated (Figure1), which are bonded by two O carboxyl atoms from two L-Pro − anions, two O carboxyl atoms, two N pyrazine atoms and two O phenol atoms from two L Schiff 2− /HL Schiff − ligands.An accurate analysis by the Shape 2.1 software[50] showed that the coordination configurations of these three Dy 3+ ions are the following: the biaugmented trigonal prism J50 (C 2v ) for Dy1 and Dy4, with CShM values of 2.249 and 2.679, respectively (TableS2); the biaugmented trigonal prism (C 2v ) for Dy6, with a CShM value of 2.118 (TableS2).D-1 is an enantiomer of L-1 and thus has a structure that is similar and mirror symmetrical to L-1 (Figure1b).

Table 1 .
Crystal data and structural refinement parameters for L-1 and D-1.