Communication pubs.acs.org/IC
Enantioenriched Cobalt Phosphonate Containing Δ‑Type Chains and Showing Slow Magnetization Relaxation Bei Liu, Yan Xu, Song-Song Bao, Xin-Da Huang, Min Liu, and Li-Min Zheng* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, P. R. China S Supporting Information *
cnapp)(OH)(H2O)2 (1). It shows a three-dimensional framework structure in which the Δ-type chains of corner-sharing Co3(μ3-OH) triangles are cross-linked by the organic groups of the phosphonate ligands. Moreover, slow magnetization relaxation with spin-canted structure is observed in compound 1 at low temperature. As far as we are aware, this is the first example of enantioenriched metal phosphonate resulting from symmetry breaking that shows slow magnetization relaxation. Compound 1 was synthesized through the hydrothermal reaction of a mixture of CoCl2 and 4-cnappH3, with the pH adjusted to 7.80 by NaOH, at 140 °C for 3 days. The deep-blue needlelike crystals were collected as a single phase, confirmed by powder X-ray diffraction (PXRD) measurement. Single-crystal structural analysis reveals that compound 1 crystallizes in orthorhombic space group P212121.11 The Flack parameter is 0.04(4), suggesting that the crystal is enantiomerically pure. The asymmetric unit contains two Co2+, one 4-cnapp3−, one OH−, and two H2O. Co1 has a distorted octahedral geometry, surrounded by two phosphonate oxygen atoms (O3B and O3C), one carboxylate oxygen atom (O4), two hydroxyl oxygen atoms (O6 and O6A) atoms, and one coordination water molecule (O1W) [Co−O = 2.051(5)−2.133(6) Å], while Co2 has a distorted trigonal-bipyramidal environment. The equatorial plane is filled with two phosphonate oxygen atoms (O1D and O2B) and one carboxylate oxygen atom (O5) [Co−O = 1.983(5)−2.028(5) Å] with a sum of neighboring O−Co−O angles of 358.6°, whereas the axial positions are occupied by the hydroxyl group (O6) and a water molecule (O2W) [Co−O = 2.065(5)−2.120(5) Å] (Figure 1a). The Co−O distances are comparable to those in the other cobalt phosphonate− carboxylate compounds.12 4-cnapp3− is hexadentate. It binds four cobalt atoms through its three phosphonate oxygen atoms (O1, O2, and O3) and two cobalt atoms via its two carboxylate oxygen atoms (O4 and O5; Scheme 1). The hydroxyl group (O6) serves as a μ3 bridge, connecting two equivalent Co1 and one Co2 atoms into a triangular unit of Co3(μ3-OH). The Co···Co distances within the triangular unit are 3.113(2) Å for Co1···Co1A, 3.607(3) Å for Co1···Co2A, and 3.431(3) Å for Co1A···Co2A. The Co−O6− Co angles are 98.2(2)−121.5(2)°. The Co3(μ3-OH) triangles are corner-shared with each other, forming an infinite Δ-type chain running along the a axis. The adjacent chains are cross-linked by the organic groups of 4-cnapp3−, forming a three-dimensional framework structure (Figure 1b). Notably, weak hydrogen-
ABSTRACT: By using an achiral (4-carboxynaphthalen1-yl)phosphonic acid (4-cnappH3), compound Co2(4cnapp)(OH)(H2O)2 (1) is isolated crystallizing in orthorhomibic space group P212121. It shows a threedimensional framework structure in which the Δ-type chains of corner-sharing Co3(μ3-OH) triangles are crosslinked by the organic groups of the phosphonate ligands. Interestingly, the bulk sample of compound 1 is enantioenriched, thus providing a rare example of symmetry breaking upon crystallization from achiral starting materials. Slow magnetization relaxation is observed at low temperature.
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norganic−organic hybrid materials combining chirality with magnetism are of great interest because they can bring new multifunctional materials and/or new effects such as multiferroicity1 and magnetochiral dichroism.2 Metal phosphonates are an important class of inorganic−organic hybrid materials. Although a large amount of metal phosphonates with versatile structures have been reported during the past 2 decades,3 chiral or enantioenriched ones are extremely rare; most of them were obtained using chiral phosphonic acids or other chiral building units as starting materials.4,5 Because the preparation of chiral ligands is tedious and expensive, symmetry breaking upon crystallization using achiral precursors is more desirable but rather uncontrollable, often resulting in conglomerates or racemates. 6,7 Only in three cases, where achiral (2pyridylmethylamino)methylphosphonic,8 (2-carboxyphenyl)phosphonic,9 or thiophene-2-phosphonic acids10 were used as starting materials, does symmetry breaking operate, giving products of enantiomeric excess. However, none exhibits lowor high-dimensional magnet behavior at low temperature. In this paper, we report a rare example of enantioenriched metal phosphonate using an achiral (4-carboxynaphthalen-1yl)phosphonic acid (4-cnappH3; Scheme 1), e.g., Co2(4Scheme 1. Molecular Structure (Left) and Coordination Mode (Right) of 4-cnappH3
Received: July 31, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.inorgchem.6b01854 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
whereas the bulk sample is enantioenriched in favor of one chiral form of the same crystal structure. The origin of such an unusual symmetry-breaking phenomenon is still not clear for us and could be related to the cooperative effect of the asymmetric ligand of 4-cnapp3− and the presence of different chiral centers (cobalt and phosphorus). The temperature-dependent magnetic susceptibilities of 1 were measured in the temperature range 1.8−300 K under an applied field of 1 kOe. Figure 3a shows the χMT versus T plot.
Figure 1. (a) Inorganic chain structure and (b) packing diagram of compound 1.
bonding interactions are observed between the adjacent chains, forming supramolecular layers in the ab plane. The shortest Co··· Co distances between the chains are 5.718(3) and 9.543(7) Å along the b and c axes, respectively. It is interesting that single crystals of compound 1 are chiral, although all of the starting materials are achiral. In order to determine whether the spontaneous resolution upon crystallization results in conglomerates or enantioenriched products, solid-state circular dichroism (CD) spectra of bulk samples from five parallel syntheses were measured. In each measurement, 2 mg of bulk sample was mixed with 48 mg of dried KCl powder. The mixture was then ground and pressed into a disk. As shown in Figure 2, all show positive Cotton effects at ca. 212, 263, and
Figure 3. (a) χMT versus T plot of 1. Inset: Magnetization hysteresis measured at 1.9 K. (b) Temperature-dependent ac magnetic susceptibilities of 1, measured under Hac = 5 Oe.
The χMT value per Co2 unit at 300 K is 6.09 cm3 K mol−1, which is higher than the spin-only value of 3.75 cm3 K mol−1 for two spin-only Co2+ ions (S = 3/2 and g = 2) due to the orbital contribution of the high-spin octahedral Co2+. Upon cooling, the χMT value decreases continuously to 1.8 K. The susceptibility data above 50 K obey the Curie−Weiss law with a Curie constant of C = 7.09 cm3 K mol−1 and a Weiss constant of θ = −46.3 K, indicating that antiferromagnetic interactions are dominant between the magnetic centers.13 The field-dependent magnetization measured at 1.9 K reveals a hysteresis loop with a coercive field of ca. 1.38 kOe, indicating that 1 is a soft magnet at 1.9 K. The remnant magnetization (Mr) is 0.04 Nβ, far from the saturation value (Ms) for two Co2+ ions (∼4.2 Nβ). Hence, the observed spontaneous magnetization is due to spin canting. According to the equation sin α = Mr/Ms,14 the canting angle is estimated to be 0.55°. Usually the spin-canting effect arises from antisymmetric interactions related to the symmetry of the exchange pathways (Dzyaloshinsky−Moriya interaction) and/or single-ion anisotropy. The weak ferromagnetism observed in compound 1 may be attributed to its structure, where an inversion center is absent between the neighboring cobalt ions and also the single-ion anisotropy of Co2+. A similar phenomenon has been found in other cobalt phosphonates.15 Figure 3b shows the temperature dependence of the alternating-current (ac) magnetic susceptibilities, measured
Figure 2. Solid-state CD spectra for five parallel bulk samples of compound 1.
323 nm, suggesting that the bulk samples are enantioenriched. It is noted that the bulk sample contains different clusters of needlelike crystals, and 20 clusters were randomly selected from the same batch for CD measurements. A total of 17 of them show positive Cotton effects, whereas the remaining three show negative Cotton effects (Figure S4). To check whether the single cluster is enantiopure, seven single crystals were randomly cut from the same cluster and were used for single-crystal structural determination. Interestingly, they all crystallize in space group P212121, and the Flack parameters are less than 0.02 (Table S2). The results indicate that the single cluster could be enantiopure, B
DOI: 10.1021/acs.inorgchem.6b01854 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry under a zero static field and 2 Oe oscillating field at frequencies of 72−488 Hz. Both in-phase (χ′) and out-of-phase (χ″) signals exhibit peaks below 7 K. Interestingly, the signals are frequencydependent. The shift of the peak temperature (Tp) of χ″ can be quantified by ϕ = (ΔTp/Tp)/Δ(log f). The obtained ϕ value is 0.19, which is larger than that for normal spin glasses (ϕ = 0.004−0.08)16 but closer to the value for a superparamagnet. A least-squares fit based on the Arrhenius relationship τ(T) = τ0 exp(ΔE/kBT), where τ0 is the preexponential factor and ΔE is the energy gap, leads to parameters τ0 = 3.3 × 10−9 s and ΔE/kB = 59.6 K. The τ 0 value is comparable to a few other antiferromagnetic chains showing spin canting and single-chain magnet (SCM) behavior,17 suggesting that magnetization relaxation of compound 1 could originate from the SCM-like behavior. However, considering the interchain magnetic interactions through the hydrogen bonds and organic ligands, the contribution of the domain wall movement to the magnetic dynamics cannot be fully excluded.18 In summary, an enantioenriched metal phosphonate 1 is reported based on an achiral asymmetrical 4-cnappH3. This compound has a three-dimensional framework structure containing Δ-type chains of corner-sharing Co3(μ3-OH) triangles. Slow magnetic relaxation is observed, attributed to the SCM-like behavior with spin-canted structure and/or domain wall movement. Further work is in progress to achieve homochiral or enantioenriched metal phosphonates with multifunctions.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01854. Synthetic details, table of crystal data, figures of PXRD, IR, CD, and thermogravimetric analysis, additional figures of the structure, and magnetic properties (PDF) Crystallographic information file for 1 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the National Basic Research Program of China (Grant 2013CB922102) and the National Natural Science Foundation of China (Grant 21371094).
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b01854 Inorg. Chem. XXXX, XXX, XXX−XXX
