Three Polyhydroxyl-Bridged Defective Dicubane Tetranuclear Mn III Complexes : Synthesis , Crystal Structures , and Spectroscopic Properties

Three polyhydroxyl-bridged tetranuclear MnIII complexes [Mn4(L)2(μ3OMe)2(μ2-OMe)2(MeOH)2] (1), [Mn4(L)2(μ3-OMe)2(μ2-OMe)2(H2O)2] (2), and [Mn4(L)2(μ3OMe)2(μ2-OMe)2(H2O)2] (3) derived from Mnn+-promoted reactivity of Schiff base ligands (HL1 = 1-(4-{[(E)-3,5-dichlorine-2-hydroxybenzylidene]amino}phenyl)ethanone O-benzyloxime, HL2 = 1-(4-{[(E)-3-bromine-5-chloro-2-hydroxybenzylidene]amino}phenyl) ethanone O-benzyloxime, and HL3 = 1-(4-{[(E)-3,5-dibromine-2-hydroxybenzylidene]amino}phenyl)ethanone O-benzyloxime) have been synthesized and characterized. In the MnIII complexes 1, 2, and 3, the newly formed ligands (L1a)4−, (L2a)4−, and (L3a)4− are derived from the chemoselective cleavage of the C=N bond in the original Schiff base ligands HL1, HL2, and HL3 to form corresponding halogenated salicylaldehyde, 3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, and 3,5-dibrominesalicylaldehyde, respectively. Then, the further addition of acetone to two halogenated salicylaldehyde molecules in situ α,α double aldol reaction promoted by Mnn+ ions in the presence of base to give the new ligands ((Lna)4−. X-ray crystallographic analyses of the MnIII complexes 1, 2, and 3 show that the three complexes are all tetranuclear structure and crystallizes in the triclinic system, space group P-1. The four MnIII ions and bridging alkoxido groups are arranged in a face-shared dicubane-like core with two missing vertices. In the three MnIII complexes, the asymmetric unit contains two kinds of different MnIII ions (Mn1 and Mn2), where the MnIII ions are all hexacoordinated with slightly distorted octahedral geometries. Simultaneously in the synthesis of multinuclear Mnn+ complexes above, we explored the crystal structure, spatial configuration, and spectroscopic properties of the multinuclear MnIII complexes with different halogen substituents.


Introduction
There is a wide range of research space in aldol condensation reactions, in particular, in its asymmetric and catalytic field, via an asymmetric organic catalyst or chelating agent of the metal complex unit.Normally, the B, Ti, or Sn ions because of their specific Lewis acid properties are used for promoting enolization followed by aldol addition [1][2][3].To a lesser extent, the other first row of transition metal Zr, Co, Ni, Cu, or Zn ions also have been used [4][5][6].In addition, we have recently reported a tetranuclear Zn II complex using a double aldol ligand formed in situ by α,α-double aldol addition of acetone to 3,5-dichlorosalicylaldehyde promoted by Zn II ion in the presence of base [7].In efforts to design new multidentate ligands which can form novel polynuclear complexes with attractive structural features, we are probing into this reaction using Lewis acid metal assistance.This favorable reaction will allow to obtaining ligands that are extremely difficult to separate by classical anionic organic chemistry under conventional conditions.This, in turn, will allow the synthesis of novel-innovative metal complexes that are pre-restricted by ligand design [8][9][10][11][12][13][14][15][16][17].Based on this, the transition metal Mn ion has become our target of choice due to its various oxidation states and Lewis acidity [18], which acts as a potential promoter to study the extent of the in situ aldol reaction of acetone and salicylaldehyde derivatives.Up to now, the use of Mn ions for the synthesis of aldol products has been relatively little explored, as reported, a unique one-pot α,α-double aldol addition of acetone to two o-vanillin molecules promoted by Mn n+ ions in situ, leading to a novel multidentate ligand, further obtained a rare defect-dicubane {Mn 4 } complex [19].Therefore, we hope that we can synthesize a variety of multinuclear Mn complexes, explore the spatial structure and crystal parameters, and perhaps determine the laws.Moreover Schiff base ligands and their complexes have been application in many fields [20][21][22][23], such as biological activity reagents [24][25][26][27][28][29][30][31][32][33][34], magnetic materials [35][36][37][38][39][40][41][42][43], luminescent materials [44][45][46][47][48][49][50][51][52].
Based on this, we designed and synthesized three Schiff base ligands with different halogen substituents and their Mn III complexes 1, 2, and 3, respectively.In the synthesis of this Mn III complexes, due to the hydrolysis chemoselective cleavage of the C=N bond of the original Schiff base ligands (HL 1 , HL 2 , and HL 3 ), forms the corresponding halogenated salicylaldehyde molecules and further the α,α double aldol addition of acetone to two halogenated salicylaldehyde molecules promoted by Mn n+ ions in situ, leading to a unique corresponding multidentate polyhydroxyl ligand (L na ) 4− , acetone-disalicylaldehyde aldol (Scheme 1).On the basis of the synthesis of multinuclear Mn III complexes, we have investigated the crystal structure and spatial configuration of the multinuclear Mn III complexes with different halogen substituents.To the best of our knowledge, a few aldol additions were previously reported as a one-pot reaction or a specific Mn n+ -promoted reaction.
Based on this, we designed and synthesized three Schiff base ligands with different halogen substituents and their Mn III complexes 1, 2, and 3, respectively.In the synthesis of this Mn III complexes, due to the hydrolysis chemoselective cleavage of the C=N bond of the original Schiff base ligands (HL 1 , HL 2 , and HL 3 ), forms the corresponding halogenated salicylaldehyde molecules and further the α,α double aldol addition of acetone to two halogenated salicylaldehyde molecules promoted by Mn n+ ions in situ, leading to a unique corresponding multidentate polyhydroxyl ligand (L na ) 4− , acetone-disalicylaldehyde aldol (Scheme 1).On the basis of the synthesis of multinuclear Mn III complexes, we have investigated the crystal structure and spatial configuration of the multinuclear Mn III complexes with different halogen substituents.To the best of our knowledge, a few aldol additions were previously reported as a one-pot reaction or a specific Mn n+ -promoted reaction.

Synthesis of HL
1-(4-Aminophenyl)ethanone O-benzyl oxime was synthesized according to an analogous method reported early [7].To an ethanol solution (5 mL) of 1-(4-aminophenyl)ethanone (272.0 mg, 2.2 mmol) was added an ethanol solution (5 mL) of O-benzylhydroxylamine (270.0 mg, 2.2 mmol).The mixture solution was stirred at 328 K for 18 h.Cooled to room temperature, and the precipitate was filtered and washed successively with ethanol and n-hexane, respectively.The product was dried under vacuum and purified with recrystallization from ethanol to obtain 411.80  Add ({4-amino}phenyl)ethanone O-benzyloxime (240.0 mg, 1.0 mmol) into ethanol solution (7 mL) of 3,5-dichlorosalicylaldehyde (191.5 mg, 1.0 mmol).The mixture solution was stirred at 333 K for 18 h.After cooling to room temperature, the precipitate was filtered and washed successively with ethanol and ethanol/n-hexane (1/4), respectively.The product was dried under reduced pressure to obtain 301.84  The ligands HL 2 and HL 3 were prepared by a method similar to that of HL 1 except substituting 3,5-dichlorosalicylaldehyde with 3-bromine-5-chlorosalicylaldehyde or 3,5-dibrominesalicylaldehyde, respectively.HL 2 : 351.33  1-(4-Aminophenyl)ethanone O-benzyl oxime was synthesized according to an analogous method reported early [7].To an ethanol solution (5 mL) of 1-(4-aminophenyl)ethanone (272.0 mg, 2.2 mmol) was added an ethanol solution (5 mL) of O-benzylhydroxylamine (270.0 mg, 2.2 mmol).The mixture solution was stirred at 328 K for 18 h.Cooled to room temperature, and the precipitate was filtered and washed successively with ethanol and n-hexane, respectively.The product was dried under vacuum and purified with recrystallization from ethanol to obtain 411.80  Add ({4-amino}phenyl)ethanone O-benzyloxime (240.0 mg, 1.0 mmol) into ethanol solution (7 mL) of 3,5-dichlorosalicylaldehyde (191.5 mg, 1.0 mmol).The mixture solution was stirred at 333 K for 18 h.After cooling to room temperature, the precipitate was filtered and washed successively with ethanol and ethanol/n-hexane (1/4), respectively.The product was dried under reduced pressure to obtain 301.84  The ligands HL 2 and HL 3 were prepared by a method similar to that of HL 1 except substituting 3,5-dichlorosalicylaldehyde with 3-bromine-5-chlorosalicylaldehyde or 3,5-dibrominesalicylaldehyde, respectively.HL 2 : 351.33 III Complexes 1, 2, and 3 Complex 1: A solution of Mn(OAc) 2 •4H 2 O (2.5 mg, 0.01 mmol) in methanol (5 mL) was added dropwise to a solution of HL 1 (10.2 mg, 0.02 mmol) in acetone (3 mL) containing three drops of trimethylamine at room temperature.The color of the mixed solution turned red-brown immediately, then stirred for 0.5 h at room temperature.The mixture solution was filtered and the filtrate was allowed to stand at room temperature for about two weeks.The solvent was partially evaporated and obtained several red-brown prismatic single crystals suitable for X-ray crystallographic analysis.The yield was 52% (based on the total available Mn). Anal.

X-Ray Crystallography
The selected single crystals of complexes 1, 2, and 3 were put in a sealed tube, and the measurement was performed on a Bruker Smart Apex-II CCD diffractometer (Karlsruhe, Germany).The reflections were collected by a graphite monochromated Cu Ka radiation (λ = 1.54184Å) at 293(2) K and 296.15(2)K for Mn III complexes 1 and 3, respectively, and that of 2 was collected by a graphite monochromated Mo Ka radiation (λ = 0.71073 Å) at 296.15(2) K.The SMART and SAINT software packages [61] were used for data collection and reduction respectively.Absorption corrections based on multiscans using the SADABS software [62] were applied.The structures were solved by direct methods and refined by full-matrix least-squares against F 2 using the SHELXL program [63].All non-hydrogen atoms were refined anisotropically.The positions of hydrogen atoms were calculated and isotropically fixed in the final refinement.Details of the crystal parameters, data collection, and refinements for complexes 1, 2, and 3 are summarized in Table 1.Supplementary crystallographic data for this paper have been deposited at the Cambridge Crystallographic Data Centre (1538187, 1538179, and 1538146 for complexes 1, 2, and 3) and can be gained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Synthesis
Synthesis of Mn III complexes 1, 2, and 3 consisting of the addition of Schiff base ligands (HL 1 , HL 2 , and HL 3 ) to the acetone solution containing a small amount of triethylamine, respectively, followed by addition of a solution of Mn(OAc) 2 •4H 2 O in methanol (5 mL).It is worth noticing that the C=N bond of the original Schiff base ligands HL 1 , HL 2 , and HL 3 have chemoselectively cleaved due to the hydrolytic action and gave the corresponding halogenated salicylaldehyde derivatives, 3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, and 3,5-dibrominesalicylaldehyde, respectively.Then a unique one-pot α,α double aldol addition in situ of acetone to two corresponding halogenated salicylaldehyde molecules promoted by Mn n+ ions, resulting to the corresponding new multidentate ligands H 4 L 1a , H 4 L 2a , and H 4 L 3a , acetone-disalicylaldehy aldol (Scheme 1), respectively.In the process of synthesis of Mn III complexes 1, 2, and 3, triethylamine acts as a base in this reaction forming bridging oxides, methoxides, and ligands after deprotonation.
It is worth noting that, in order to explore the oxidation of manganese ions, the same reactions to synthesis the Mn III complexes 1, 2, and 3 were carried out under air-free conditions to prevent the oxidation of Mn II to Mn III .As well as the reaction were carried out under acetone-free and Mn salt-free conditions (see Supporting Information).However, these reactions did not give any crystals of the complex product nor any aldol product in the solution (analyzed by NMR), which leads us to believe that Mn III is more likely to promote the α,α double aldol addition, and the acetone is also a necessary factor.Based on the above findings, our proposed mechanism to rationalize the formation of the new generated polyhydroxyl multidentate ligands, H 4 L 1a , H 4 L 2a , and H 4 L 3a , by Mn n+ -promoted α,α-double aldol additions [7,19] is described in Scheme 3.
Firstly, the initial Schiff base ligands (HL 1 , HL 2 , and HL 3 ) underwent hydrolysis to get 1-(4-aminophenyl)ethanone O-benzyl oxime and corresponding salicylaldehyde derivatives (3,5-dichlorosalicylaldehyde, 3-bromine-5-chlorosalicylaldehyde, 3,5-dibrominesalicylaldehyde), respectively (Scheme 3A).Then, a molecule of acetone is deprotonated by the hydroxide moiety of the base, triethylamine, yielding enolate i, which is stabilized by the keto-enolate mesomeric effect as well as coordination to Mn n+ (Scheme 3B).Moreover, the phenol position of salicylaldehyde derivatives is also deprotonated by the hydroxide base leading to a Mn n+ -phenolate complex ii.The concomitant coordination of the Mn n+ ion to the neighboring carbonyl oxygen atom of salicylaldehyde derivatives allows the activation of the aldehyde function which undergoes the first aldol addition of the acetone enolate i , yielding the first aldol product iii (Scheme 3C).The latter can then be further deprotonated either in the α or α position from the carbonyl of the acetone residue.Deprotonation of iii occurs predominantly in the α position due to the thermodynamic conditions used which favor deprotonation on the most substituted α position (Scheme 3D).The formation of enolate iv, stabilized by the keto-enolate mesomeric effect, is also promoted by Mn n+ coordination assistance.Enolate iv can undergo another aldol addition on a second molecule of salicylaldehyde derivatives, activated by its coordination to Mn n+ , leading to the ligand acetone-di-salicylaldehyde aldol ν (H 4 L 1a , H 4 L 2a and H 4 L 3a ) which is quadruply deprotonated and coordinated to two Mn n+ ions

IR Spectra
The FT-IR spectra of Mn III complexes 1, 2, and 3 exhibit various bands in the 500-4000 cm −1 region.The most important FT-IR bands are listed in Table 2.In the Mn III complexes 1, 2, and 3, the bands appear at about 3437, 3416, and 3445 cm −1 , respectively, are attribute to O-H stretching frequency coordination methanol or water molecules, which confirmed by the crystal structure [64][65][66][67][68][69].No characteristic C=N stretching band is found in complexes 1, 2, and 3 indicating that the ligands HL 1 , HL 2 , and HL 3 are converted into the polyhydroxy deprotonation ligands and coordinated to the Mn III ions.The frequency of Ar-O and C-O stretching vibration shows a strong band at 1244 and 1156 cm −1 , 1296 and 1167 cm −1 , and 1275 and 1173 cm −1 in the Mn III complexes 1, 2, and 3, respectively [70][71][72][73].The characteristic stretching of the carbonyl (C=O) group appears at 1587, 1602, and 1620 cm −1 as the strong bands in the Mn III complexes 1, 2, and 3, respectively.The moderate and weak vibrations appearing around 698 and 503 cm −1 in 1 (698 and 503 cm −1 in 2, 698 and 503 cm −1 in 3) correspond to the asymmetric and symmetric stretching vibrations of the Mn-O-Mn, respectively, indicating that the Mn-O bond forms at the Mn III ions and the oxygen atoms.

UV-vis Absorption Spectra
The UV-vis spectra of the Mn III complexes 1, 2, and 3 were recorded in 1.0 × 10 −5 mol•L −1 DMF solution at room temperature shown in Figure 1.

UV-vis Absorption Spectra
The UV-vis spectra of the Mn III complexes 1, 2, and 3 were recorded in 1.0 × 10 −5 mol•L −1 DMF solution at room temperature shown in Figure 1.A broad absorption band at 443, 417, and 410 nm in complexes 1, 2, and 3 are observed, respectively, which can be attributed to the L→M charge-transfer transitions.In the complex 2, the peak at 297 can be assigned to the n-π* charge transition of the C=O bond of the new ligand (L 2a ) 4− unit, probably arising from the charge transfer of an oxygen atom in the methoxy group to the Mn III center.The shoulder band at 472 nm can attribute to the d-d transitions for elongated octahedral Mn III ions [74][75][76][77], however, which are not observed in complexes 1 and 3 maybe because of the A broad absorption band at 443, 417, and 410 nm in complexes 1, 2, and 3 are observed, respectively, which can be attributed to the L→M charge-transfer transitions.In the complex 2, the peak at 297 can be assigned to the n-π* charge transition of the C=O bond of the new ligand (L 2a ) 4− unit, probably arising from the charge transfer of an oxygen atom in the methoxy group to the Mn III center.The shoulder band at 472 nm can attribute to the d-d transitions for elongated octahedral Mn III ions [74][75][76][77], however, which are not observed in complexes 1 and 3 maybe because of the weaker d-d transitions absorption peak of Mn III ions are obscured by the stronger L→M charge-transfer absorption peak of the complexes.

Fluorescence Properties
The emission spectra of the Mn(III) complexes 1, 2, and 3 were investigated in dilute DMF solution (5.0 × 10 −5 mol/L) at room temperature (Figure 13).The complexes exhibit the bluish violet photoluminescence with maximum emissions at 486, 494, and 501 nm (π-π*) upon excitation at 320 nm.The changes of the maximum emission wavelength and fluorescence intensity may be related to the different substituents on complexes 1, 2, and 3 [94][95][96][97][98][99][100].As discussed above, because of the coordination of Mn III ions to the ligand, which resulting in increasing of the delocalization of electrons and reducing the energy gaps between the π-π* molecular orbitals of the ligand in the complexes (from 3, 1 to 2).
photoluminescence with maximum emissions at 486, 494, and 501 nm (π-π*) upon excitation at 320 nm.The changes of the maximum emission wavelength and fluorescence intensity may be related to the different substituents on complexes 1, 2, and 3 [94][95][96][97][98][99][100].As discussed above, because of the coordination of Mn III ions to the ligand, which resulting in increasing of the delocalization of electrons and reducing the energy gaps between the π-π* molecular orbitals of the ligand in the complexes (from 3, 1 to 2).

Conclusions
Based on the above data, description and discussion, three tetranuclear Mn III complexes with defective double-cubane cores, namely [Mn4(L 1a )2(μ3-OMe)2(μ2-OMe)2(MeOH)2] (1), [Mn4(L 2a )2(μ3-OMe)2(μ2-OMe)2(H2O)2] (2), and [Mn4(L 3a )2(μ3-OMe)2(μ2-OMe)2(H2O)2] (3) have been synthesized and characterized.X-ray crystal structure determinations revealed that the structural features of complexes 1, 2, and 3 are similar except for the differences in the coordinated solvent molecules and the substituent of the ligands.There are worth noting that when the Schiff base ligand reacted with Mn III acetate tetrahydrate, they undergo an one-pot chemoselective cleavage of the C=N bond and further the α,α double aldol addition of acetone to two salicylaldehyde derivatives molecules promoted by Mn n+ ions in situ, leading to the novel multidentate polyhydroxyl ligand (L 1a ) 4− , (L 2a ) 4− , and (L 3a ) 4− .This has proved an effective route to obtain the multidentate ligands and their tetranuclear Mn III compounds.In these Mn III complexes, all hexa-coordinated Mn III atoms adopt elongated slightly distorted octahedral geometries.In addition, the Mn III complex 1 possess a self-assembling infinite 2D supramolecular structure, whereas complexes 2 and 3 show the 1D chain.Interestingly, the existence of substituent effect in complexes 1, 2, and 3 may be responsible for the slight differences in their coordination geometries, supramolecular structure, and fluorescence properties.

Conclusions
Based on the above data, description and discussion, three tetranuclear Mn III complexes with defective double-cubane cores, namely [Mn 4 (L 1a ) 2 (µ 3 -OMe) 2 (µ 2 -OMe) 2 (MeOH) 2 ] (1), [Mn 4 (L 2a ) 2 (µ 3 -OMe) 2 (µ 2 -OMe) 2 (H 2 O) 2 ] (2), and [Mn 4 (L 3a ) 2 (µ 3 -OMe) 2 (µ 2 -OMe) 2 (H 2 O) 2 ] (3) have been synthesized and characterized.X-ray crystal structure determinations revealed that the structural features of complexes 1, 2, and 3 are similar except for the differences in the coordinated solvent molecules and the substituent of the ligands.There are worth noting that when the Schiff base ligand reacted with Mn III acetate tetrahydrate, they undergo an one-pot chemoselective cleavage of the C=N bond and further the α,α double aldol addition of acetone to two salicylaldehyde derivatives molecules promoted by Mn n+ ions in situ, leading to the novel multidentate polyhydroxyl ligand (L 1a ) 4− , (L 2a ) 4− , and (L 3a ) 4− .This has proved an effective route to obtain the multidentate ligands and their tetranuclear Mn III compounds.In these Mn III complexes, all hexa-coordinated Mn III atoms adopt elongated slightly distorted octahedral geometries.In addition, the Mn III complex 1 possess a self-assembling infinite 2D supramolecular structure, whereas complexes 2 and 3 show the 1D chain.Interestingly, the existence of substituent effect in complexes 1, 2, and 3 may be responsible for the slight differences in their coordination geometries, supramolecular structure, and fluorescence properties.

Scheme 3 .Scheme 3 .
Scheme 3. Proposed mechanism for the Mn n+ -promoted α,α double aldol additions.(A) Hydrolysis of Ligands HL 1 , HL 2 , and HL 3 .(B) Deprotonation of the acetone.(C) Activation of the salicylaldehyde derivatives and aldol addition.(D) Deprotonation of the aldol product and second aldol addition.Scheme 3. Proposed mechanism for the Mn n+ -promoted α,α double aldol additions.(A) Hydrolysis of Ligands HL 1 , HL 2 , and HL 3 .(B) Deprotonation of the acetone.(C) Activation of the salicylaldehyde derivatives and aldol addition.(D) Deprotonation of the aldol product and second aldol addition.

Figure 2 .
Figure 2. (a) Molecule structure and atom numbering of complex 1 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 1 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 2 .
Figure 2. (a) Molecule structure and atom numbering of complex 1 (hydrogen atoms are omitted for clarity); (b) the planar Mn 4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 1 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 3 .
Figure 3. (a) Molecule structure and atom numbering of complex 2 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 2 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 4 .
Figure 4. (a) Molecule structure and atom numbering of complex 3 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 3 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 3 .
Figure 3. (a) Molecule structure and atom numbering of complex 2 (hydrogen atoms are omitted for clarity); (b) the planar Mn 4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 2 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 3 .
Figure 3. (a) Molecule structure and atom numbering of complex 2 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 2 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 4 .
Figure 4. (a) Molecule structure and atom numbering of complex 3 (hydrogen atoms are omitted for clarity); (b) the planar Mn4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 3 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 4 .
Figure 4. (a) Molecule structure and atom numbering of complex 3 (hydrogen atoms are omitted for clarity); (b) the planar Mn 4 rhombus and (c) the coordination polyhedra for Mn III atoms of complex 3 (symmetry code: a = 1 − x, 1 − y, 1 − z).

Figure 8 .
Figure 8.View of the 2D layered motif within Mn III complex 1 (hydrogen atoms, except those forming hydrogen bonds, are omitted for clarity).

complexes 1, 2 and 3 .
Author Contributions: Y.-X.S. conceived and designed the experiments; H.-R.J. and J.C. performed the experiments; H.-J.Z. and J.L. analyzed the data; H.-R.J., and Y.-X.S. wrote the paper.Funding: This research received no external funding.