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Enantiopure Magnetic Heterometallic Coordination Cubic Cages [MII8CuII6] (M = Ni, Co) Yang Yang, Yi Wu, Jian-Hua Jia, Xiu-Ying Zheng, Qian Zhang, Ke-Cai Xiong, Ze-Min Zhang, and Quan-Ming Wang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00554 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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Crystal Growth & Design

Enantiopure Magnetic Heterometallic Coordination Cubic Cages [MII8CuII6] (M = Ni, Co) Yang Yang, a* Yi Wu,a Jian-Hua Jia,b Xiu-Ying Zheng,c Qian Zhang,a Ke-Cai Xiong,a Ze-Min Zhang,d* Quan-Ming Wangc,e*

a. School of Chemistry and Material Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou 221116, China. b. MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China c. Chemistry Department, Xiamen University, Xiamen 361005, China d. School of Eco-environment Technology, Guangdong Industry Polytechnic, Guangzhou, 510300, China. e. Chemistry Department, Tsinghua University, Beijing 100084, China. E-mail:

ACS Paragon Plus Environment

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Abstract

A

series

of

enantiopure

magnetic

heterometallic

coordination

cubic

cages

[MII8CuII6L24(MeCN)12](ClO4)28 (1:M = Ni, 2:M = Co, L=chiral ditopic ligand) have been synthesized. These cages have cubic structures enclosing 2250 Å3 interior cavity with controllable chirality. Both the vertexes and the faces of the cubic cages are in chiral arrangements, which are dictated by the chiral amines used in forming the ligands.

R-1-

phenylethylamine leads to 8Λ-6P configuration, while S-1-phenylethylamine results in 8∆-6M ones. The magnetic behaviors of the cages are varied by the metals. They all show weak antiferromagnetic interaction. These cages represent the first series of enantiopure magnetic cubic cages, which offer a platform for the study of host-guest chemistry of potential chiral magnetic guests.

Introduction Self-assembled cage compounds are attractive due to their appealing structures and fascinating properties.1-5 The confined spaces enclosed by cages facilitate numerous applications in field of catalysis, gas adsorptions, mixture separation, and stabilization of labile substances.6-8 Besides of the confined space, additional properties are introduced to the cage frameworks for special functionalities, such as luminescence,9 photoresponsed ability,10-12 redox activity,13, synthetic reactivity,15,

16

chirality17 and magnetism.18,

19

14

post-

Coordination cages containing

paramagnetic metal ions exhibit interesting magnetism. For example, Batten et al. showed that [Fe6Cu8] cages had magnetic response to external stimulus20 and Brechin et al. reported a class of exchange-coupled paramagnetic metals containing cages, which are good hosts for the study of


2

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host-guest magnetic interaction.18, 19, 21 The synergistic effects between multiple properties could endow novel functionalities.22, 23 Enantiomerically pure chiral coordination cages with specific aim at chiral guests are applied in chiral recognition/separation and asymmetric catalysis.17, 23, 24 The incorporation of confined space with chirality and magnetism may result in magneto-optical properties25 as well as host-guest chemistry for chiral-magnetic guest. So far, all homochiral magnetic cage compounds are cynaometalate compounds reported by Oshio et al.26-29 With short cyanide-bridge severing as backbone, their sizes of the cavities are restricted, thus the guests are all small ones such as water molecule26, 29 or K+/Cs+ ions.30 Chiral magnetic cages with large cavity are imperative and remaining rare. Previously, we successfully constructed a novel cubic [Al8Pd6] cage31 by employing rigid metalloligands and we diastereoselectively synthesized chiral cubic [Zn8Pd6] cages by rational ligand designed.32 As a continuous work, we extend our study to enantiopure magnetic cubes. Herein, we report the synthesis, structures and properties of a series enantiomerically pure magnetic heterometallic coordination cubes with formula as [MII8CuII6L24(MeCN)12](ClO4)28, (1:M = Ni, 2:M = Co), where L is a chiral ditopic ligand containing both bidentate pyridylimine and monodentate pyridine donors by reacting a precursor ligand with chiral amine. Several paramagnetic metals containing cube-like cages have been reported18, 20, 33-39. To the best of our knowledge, enantiopure magnetic cubic cages are unprecedented.

Experimental section General Method. The C, H, N microanalyses were carried out with a CE instruments EA 1110 elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range 4000-400 cm-1 with a Nicolet AVATAR FT-IR360 spectrometer. Circular dichroism (CD) was measured


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using a Jasco J-810 spectrodichrometer. Mass spectra were recorded on a Waters SYNAPTTM G2-Si HDMS. Magnetic susceptibility measurements were performed with a Quantum Design MPMS-XL7 SQUID. EDS was measured by Hitachi SU-8100. TGA spectra were recorded on TGA Q50. All reagents employed were commercially available and used without further puriﬁcation. The solvents used were of analytical grade. Precursor ligand [3,4']bipyridinyl-6carbaldehyde was synthesized according to previous report. 32 Syntheses. Cage R/S-1:

[3,4']Bipyridinyl-6-carbaldehyde (50 mg, 271.7 µmol) was dissolved

with 10 mL acetonitrile, R-1-phenylethylamine (34.7 µL, 271.7 µmol) was injected into the solution. The mixture was refluxed at 70°C for 3 h to give ligand LR. After cooling to room temperature, nickel(II) perchlorate hexahydrate (33.1 mg, 90.6 µmol) was added. Then the solution was stirred for 6 hours before copper(II) perchlorate hexahydrate (25.2 mg, 67.9 µmol) was added. The resultant solution was further stirred overnight then filtrated. The filtrate was transferred to thin tubes, and diethyl ether was layered on them. After several days, brown block crystals

of

R-1

were

collected.

Yield:ca.68%.

Anal.Calcd.

for

C480H444O112N84Cl28Ni8Cu6·(CH3CN)20(%): C 52.72, H 4.29, N 12.30. Found: C 52.45, H 4.52, N 12.15. IR (KBr, cm-1): υ1618.69 (br, C=N). Cage S-1 (the enantiomer of R-1) was prepared in the similar manner by used S-1-phenylethylamine instead. Cage R/S-2: The synthesis process of S-2 is similar to that of cage S-1 by substituting nickel(II) perchlorate hexahydrate to equivalent cobalt(II) perchlorate hexahydrate. Pale-brown crystals of S-2 were obtained after two weeks. Yield:ca.35%. Anal.Calcd. for C480H444O112N84Cl28Co8Cu6·(CH3CN)25(%): C 52.81, H 4.34, N 12.67. Found: C 52.45, H 4.45, N 12.38. IR (KBr, cm-1): υ1618.88 (br, C=N). Cage R-2 (the enantiomer of S-2) was prepared in the similar manner by used R-1-phenylethylamine instead.


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X-ray Crystallography. The crystals were fragile and easily lost solvent molecules. Thus low temperature and rapid handing of the samples was needed. Diffraction data of R-1 was collected on an Agilent SuperNova X-Ray diffractometer at 173 K using micro-focus X-ray sources (Cu Kα, λ=1.54184 Å). Absorption corrections were applied by using the program CrysAlisPro (multi-scan). Diffraction data of S-2 were collected on Bruker SMART APEX II X-Ray diffractometer at 150 K using Mo-Kα (λ=0.71073 Å) X-ray sources. Data were processed with the INTEGRATE program of the APEX2 software for reduction and cell refinement. Multi-scan absorption corrections were applied by using the SCALE program for area detector. Using Olex2,40 the structure was solved with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package using Least Squares minimization.41 Non-hydrogen atoms except some solvent molecules and counteranions were refined anisotropically. The hydrogen atoms of organic ligands were generated geometrically. Due to the large unit cell and small size of crystals, the diffractions were relatively weak. Not all the counter-anions were found because of the weak data and high symmetry. Only fifteen perchlorate anions per cage for R-1 and ten perchlorate anions per cage for S-2 were located and refined. The correct chemical formula reported in CIF took the undefined counter-ions into consideration for charge balance. This resulted in an A level alerts in checkcif warning that the calculated and reported molecular weights have large difference. Both crystals have large solvent accessible voids since a large number of disordered solvent molecules and counter-anions were not defined. Thus SQUEEZE routines in PLATON were employed in the structural refinements. The crystallographic data of the two compounds are given in Table 1.The detail structure data are available at Cambridge Crystallographic Data Centre with CCDC number 1587228 (for R-1) and 1587229 (for S-2).


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Table 1. Crystallographic Data and Structure Refinement for R-1 and S-2 R-1

S-2

Formula

C494H465N91O112Cl28Ni8Cu6

C480H444N84O112Cl28Co8Cu6

Formula weight

11312.06

11026.45

Crystal system

Trigonal

Trigonal

Space group

R32

R32

a [Å]

37.3687(6)

37.477(2)

b [Å]

37.3687(6)

37.477(2)

c [Å]

84.2974(10)

41.995(5)

α [°]

90

90

β [°]

90

90

γ [°]

120

120

V [Å3]

101944(3)

51081(9)

Z

6

3

T [K]

173.0(1)

150.0(2)

F (000)

35016.0

17022

Crystal size (mm3)

0.06*0.06*0.06

0.08*0.05*0.04

ρcalcd (g/cm3)

1.106

1.075

µ (mm-1 )

2.002a

0.548b

Rint

0.0556

0.0429

R1, wR2 (I ≥ 2σ (I))

0.0840 0.2393

0.0583 0.1670

R1, wR2 (all data)

0.0996 0.2582

0.0717 0.1781

Flack factor

0.049(7)

0.04(2)

a

Cu Kα radiation. bMo Kα radiation.


6

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Results and discussion

Synthetic procedures The synthesis processes of [MII8CuII6L24](ClO4)28 are similar, as Scheme 1 shown. Precursor ligand bearing a formylpyridyl and a pyridyl group was first reacted with enantiomerically pure R-1-phenylethylamine or S-1-phenylethylamine to form chiral ligand LR or LS, which was further coordinated to MII perchlorate salts following with CuII perchlorate salts stepwise in a ratio of 24 : 8 : 6 to form the cage products R-1, R-2, or S-1, S-2, respectively. Crystalline products suitable for X-ray structural determinations were obtained by diffusion of diethyl ether into the acetonitrile solutions in a moderate yields. The simultaneous addition of the two kinds metals also resulted in the same product as the stepwise ones but with relative lower yields. The identities of the products were verified by multiple techniques like single crystal diffraction, elemental analysis, IR and so on. The heterometallic compositions of both cages are supported by energy dispersive X-ray spectroscopy (EDS), with M:Cu metal ratio close to 4:3 (Figure S1S2). The Jahn–Teller effect results in an elongated axis (dz2) when six coordinated CuII in a octahedral mode, thus CuII ions prefer the face other than the vertices leading to self-sorting of the metals. The cubes could still be formed by adding CuII ions before MII ions. However, no face-ligand of CuL4 moiety could be isolated. Addition of sole metal salts, respectively, no crystalline product was formed under the same condition. The counter-ions were not specifically to be perchlorate ions. Nitrate salts and tetrafluoroborate salts also led to the formation of the cages. Replacing the homochiral 1-phenylethylamine with racemic ones led to amorphous precipitate, ruling out the spontaneous resolution by conglomerate crystallization.42 The CD spectrum of the amorphous precipitate shows no peaks (Figure S3). Racemic ligands chiral self-


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sorting processes are reported accompanying with crystallization.43-45 Base on previous studies of similar ligands32, 46 and no crystals were obtained, we infer the absence of chiral self-sorting of the ligands in this case.

Scheme 1. Synthetic routes of the ligands and the cubic cages.

Crystallography

Crystals of R-1 and S-2 were applied for single crystal structural determinations, which revealed that they all crystallized in chiral trigonal space group R32 (Table 1.). Cation parts of the molecules adopt cubic structure, as Figure 1 shown. The vertices of the cube are occupied by MII chelated by tri-pyridylimine units, while the six square faces are capped by CuII ions coordinated to four terminal pyridine and two acetonitrile at the axial direction. The MII are all in


8

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a regular {MIIN6} octahedral environment, with Ni-N, Co-N bond length in the ranges of 2.082.09 Å, 2.14-2.15 Å, respectively. CuII ions are in Jahn-Teller distorted {CuN6} geometry. Four equatorial Cu-N of R-1 are among 2.02-2.03 Å, and two axial Cu-N are among 2.41-2.61 Å. The equatorial Cu-N of S-2 are slightly longer, among 2.03-2.09 Å. Cu···Cu distances between two paralleled faces of both crystal are about 16 Å. And the MII···MII of the adjacent vertices are among 13.59-14.36 Å , which imply that these cages are distorted from a perfect cube. The diameters of the whole molecules are about 3.3 nm. The frameworks enclose a volume of about 2250 Å3, calculated by 3V Volume Assessor programme employing an outer probe of 8.0 Å and an internal probe of 1.5 Å (Figure 1b).47 In fact, the thermogravimetric analyses (TGA, Figure S4-S5) show the crystals contain large amount of solvent molecules. Eight perchlorate counterions are found to be located inside the cage. There are twelve rhombus windows with diameter about 4.3 Å on the edges, which will be available for the penetration of suitable guests.48 The pyridylimine units wrap around MII in a chiral spiral manner in Λ or ∆ configurations, and the four terminal pyridine coordinated to CuII also exhibit either clockwise or anticlockwise propeller-like rotational patterns in P or M configurations.49 The chiralities communicate through π···π interactions of neighbouring ligands at one vertex and through CuII propellers between vertices. For one molecule, eight vertices are homochiral, so as the six faces. The chirality is dictated by the chiral amines. R-1-phenylethylamines lead to 8Λ-6P configurations (R-1), while S-1-phenylethylamines result in 8∆-6M configurations (S-2), as Figure 3c shown, analogy to the previous reported diamagnetic [ZnII8PdII6L24](BF4)28.32 The chiralities of the vertices correlate to the face ones. For example, ∆ configurations of the vertices are associated with M configurations of the faces. Structure simulation of ∆ configurations of the vertices with P configurations of the


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faces finds out large dihedral angle of the two pyridyl rings of the ligand, which means higher energy with less preference. (Figure S6) All molecules of the whole crystal are in identical chirality. The absolute configurations and enantiopurities are confirmed by the Flack factor (Table 1.) and circular dichroism (CD) studies (vide infra).


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Figure 1. X-ray crystal structures of the cubic cages a) R-1 with eight perchlorate anions encapsulated; b) Representation of the calculated available internal cavity space in cage R-1; c) X-ray crystal structures of R-1 with 8Λ-6P configuration (left), S-2 with 8∆-6M configuration (right). Hydrogen atoms and counter ions are omitted for clarity in b) and c). Color legend: purple Ni; pink Co; green Cu; blue N; gray C; yellow Cl; red O; white H.

Mass spectrometry studies

Cages 1-2 are insoluble in dichloromethane or tetrahydrofuran and slightly

soluble in

methanol. But they are highly soluble in acetonitrile, from which they could be recrystallized. Due to the paramagnetic metals of these cages, we are unable to study their solution behaviour by NMR techniques. But the ESI-MS spectra of the solutions are informative. The molecular formula [Ni8Cu6(C19H17N3)24(ClO4)28] is straightforward verified by ESI-MS study of acetonitrile solution of R-1. A +5 and a +4 multicharged molecular ions peaks derived from the loss of corresponding numbers of ClO4- counterions and all the coordinated

acetonitrile

molecules of R-1 are clearly identified from other fragments, as Figure 2a shown. The fragmentation occurred under ESI conditions is commonplace. The isotopic patterns match well with the simulated ones. According the formula confirmed by ESI-MS, the heterometallic composition is undoubted. No metals ratio disordered molecular peak is observed, confirming the self-sorting of the metals. The axial coordinated acetonitriles observed in crystal structure are easy to dissociate. The coordination unsaturated metal sites are always valuable for potential applications such as catalysis or selectively adsorption.50 ESI-MS spectra of 2 under the same condition give only futile fragments which are hard to identify (Figure S7). It is reasonable that Co-N and Cu-N bond lengths of S-2 in the crystal structures are all slightly longer than those of


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Ni-N and Cu-N in R-1, which represent weaker bond strength. The stability of this series cage is in line with the expectation of Irving–Williams series.51

Figure 2. a) ESI-MS spectrum of R-1 in acetonitrile, molecular ion peaks with +5 and +4 multicharges peak are highlighted with red and green respectively, inset: the experimental (red trace) and

simulated

(blue

trace)

isotopic

patterns

of

the


molecular

ion

peaks

of

12

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[Ni8Cu6(LR)24(ClO4)23]5+; b) CD spectra of 1.43×10-5 mol/L acetonitrile solution of R-1 (black), S-1 (red), R-2 (green), S-2 (blue).

Circular dichroism (CD) studies In solid state CD spectra, samples prepared from single crystals of R-1 and R-2 show unambiguous cotton effect in the range of 240-350 nm, while S-1 and S-2 have mirror images respectively (Figure S8-S9). Bulk samples show identical profile with single crystal ones ruling out the formation of conglomerates. These evidences suggest that these cages are diastereoselectively synthesized by using the enantiopure chiral amine as chirality director. The solution CD studies were applied to further explore the chiroptical properties of these cubes. As Figure 2b shown, acetonitrile solution of R-1 also shows strong positive exciton couplet around 240-350 nm, and a negative mirror image for S-1. The corresponding absorption band is assigned to pyridylimine ligand centered π–π* transitions (Figure S10-S11).46 Exciton theory predicts a positive exciton couplet to be associated with Λ configuration, which is agreed with the crystal structure.52 The R/S-2 are isomorphic compounds of R/S-1, thus sharing similar profile but with a relatively weaker intensity. Previous studies indicated that the structural rigidity may correlate to the CD intensity.32, 53 The weaker bond strength of Co-N than Ni-N leading to the poorer rigidity may account for weaker CD intensity.

SQUID magnetometry studies

The temperature dependence of the magnetic susceptibilities of crystalline powder samples of complexes R-1 and R-2 were measured in the temperature range of 2–300 K with an applied


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direct-current (dc) magnetic field of 1000 Oe. As shown in Figure 3a, the observed χMT values of R-1(Ni8Cu6) and R-2(Co8Cu6) at room-temperature are 12.10 and 28.10

cm3 mol-1 K,

respectively, which are larger than the calculated spin-only values of 10.25 and 17.25 cm3 mol-1 K (g = 2.0, NiII: S = 1, CoII: S = 3/2, CuII: S = 1/2). This may due to the loss of lattice solvent during measurement leading to uncertainty in the molar mass. Upon cooling, the χMT value of R1 decreases slowly down to 10 K before sharply drops to 10.69 cm3 mol-1 K at 2 K. The data obey Curie-Weiss law well in the whole measurement range, with C = 12.45 cm3 mol-1 K and θ = -1.37 K, suggesting weak antiferromagnetic interaction. For cage R-2, the χMT value decreases slowly between 300 and 80 K, then decreases more abruptly down to minimum value of 16.92 cm3 mol-1 K. The data deviate from Curie-Weiss law (C = 28.47 cm3 mol-1 K and θ = -9.72 K) at around 45 K (Figure 3b). For R-1 and R-2, the intra-cage magnetic interactions are expected very weak due to their long intra-cage MII···CuII distances (ca. 10 Å) and twisting of bridging 2,4’-bipyridine moiety. The symmetrical six-coordinated CoII ions show no axial distortion, thus, the ground state should be 4T1g and consequently it shows spin-orbit coupling. The shape of the graph is characteristic of this interaction. In our opinion, the most important contribution to the magnetic behaviour is the spin-orbit coupling of the CoII ions, with a minor contribution of the antiferromagnetic interaction between CoII···CuII. The field (H) dependence of the magnetizations (M) of cage R-1 and R-2 were also measured between 0 and 7 T at 2 K. As shown in Figure 3c, the magnetization values for cage R-1 and R-2 are 22.5 and 23.1 µB at 7 T, respectively. The magnetization values of R-1 are close to saturation values of 22.0 µB, in agree with the very weak antiferromagnitc coupling. For R-2, the experimental values are far below the saturation values of 30.0 µB, due to the magnetic anisotropy of the molecule and crystal-field effect on the CoII ions


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Similar cubic cage systems [MIII8CuII6] (MIII = Cr or Fe) have been reported with CuII taking the same face central positions of the cube. Magnetic properties of the same style cage [MIII8CoII6] are also reported.18, 21 CoII ions are in the place of the CuII ions, with an axial distortion (CoN4O2 core). The magnetic behaviour is explained by small magnetic exchange of M···Co and significant zero-field splitting of the CoII ion, which is similar to our case.18 The JM-Cu , which is the isotropic exchange parameter between Cu and MIII centers, have been quantitative determined to be -0.18 cm-1 (MIII = Cr ) and 0.10 cm-1( MIII = Fe). The exchange J values in our case are estimated to be smaller than these values with longer MII···CuII distances .

Figure 3. Plot of χmT vs. T (left), χm-1 vs. T (right) and corresponding liner fit of a) R-1 (Ni8Cu6) and b) R-2 (Co8Cu6) ; c) field dependence of the magnetizations at 2.0 K of R-1 and R-2.

Conclusions


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In summary, we have successfully synthesized a series of enantiomerically pure paramagnetic heterometallic coordination cubes [MII8CuII6L24(MeCN)12](ClO4)28 (MII = Ni, Co). The structures, chiralities and magnetisms of these cubic cages have been studied. They have large interior cavity with controllable chirality by switching chiral amines. The magnetic properties of these cages have been studied. They represent the first series of enantiopure magnetic cubic cages, which offer a platform for the study of host-guest magnetic interactions. Even though the longer ligand backbone for larger enclosed space lead to weak intermolecular magnetic interactions, but it would facilitate the studies of host-guest magnetic interaction of potential magnetic or chiralmagnetic guests, such as paramagnetic metal containing chiral-at-metal complex.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.XXXXXX. The following files are available free of charge. Additional figures.(PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. [email protected]


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[email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21701064), Natural Science Foundation of Jiangsu Province (BK20170230), Foundation of Jiangsu Normal University (16XLR014) and the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. We thank Dr. Shang-Fu Yuan for the help of ESI-MS. REFERENCES 1.

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For Table of Contents Use Only Title: Enantiopure Magnetic Heterometallic Coordination Cubic Cages [MII8CuII6] (M = Ni, Co) Author: Yang Yang, Yi Wu, Jian-Hua Jia, Xiu-Ying Zheng, Qian Zhang, Ke-Cai Xiong, ZeMin Zhang, Quan-Ming Wang TOC graphic:

Synopsis: A series of enantiopure magnetic heterometallic coordination cubic cages enclosing large interior cavity have been present. Both the vertexes and the faces of the cubic cages are in chiral arrangements, which are dictated by the chiral amines used in forming the ligands. The magnetic behaviors of the cages are varied by the metals.


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Enantiopure Magnetic Heterometallic Coordination ... - ACS Publications

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