Synthesis, Crystal Structures, HF-EPR, and Magnetic - ACS Publications

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Series of Single-Ion and 1D Chain Complexes Based on Quinolinic Derivative: Synthesis, Crystal Structures, HF-EPR, and Magnetic Properties Huidan Lou,† Lei Yin,† Biquan Zhang,‡ Zhong-Wen Ouyang,† Bao Li,*,‡ and Zhenxing Wang*,† Wuhan National High Magnetic Field Center & School of Physics and ‡Key Laboratory of Material Chemistry for Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

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S Supporting Information *

ABSTRACT: By utilizing the quinolinic derivative, 8-carboxymethoxy-2-carboxylicquinoline (L), five transition metal coordination complexes, [M(L)(H2O)3]·H2O] (M = Mn (1), Co (2)), [Ni(L)(H2O)2] (3), and {[M(L)](H2O)}n (M = Ni (4), Cu (5)), were synthesized by hydrothermal methods employing similar synthetic strategies. The crystal structures, magnetism and high-field EPR were characterized for the obtained compounds. 1−3 are mononuclear compounds. 1 and 2 have pentagonal bipyramidal geometry, while 4 and 5 exhibit one-dimensional zig-zag chain. Direct current magnetic and EPR studies demonstrate that compound 2 has large and positive D value (∼70.4 cm−1), indicating the easy plane magnetic anisotropies of 2. This D value is the largest one in the reported Co(II) complexes with pentagonal bipyramidal geometry. Field-induced slow magnetic relaxation behavior was observed for 2 by the dynamic ac magnetic susceptibility measurements. The dc magnetic susceptibility studies of 4 and 5 give similar weak MII−MII antiferromagnetic interactions (J = −1.50 and −3.55 K for 4 and 5, respectively). High-field EPR results show that 4 can be considered as a quantum antiferromagnet. could be seen as the key factors that greatly affect the final structures.10 The coordination habits and conformation of selected ligands would determine the coordination environment of center atoms, including the coordination polyhedron or coordination number.11 To explore the possibility of high coordination number of metal centers in complexes, 8carboxymethoxy-2-carboxylicquinoline (H2L), a derivative derived from quinoline had been designed and synthesized with the following considerations: (1) The derivatives possess multiple coordination sites and semirigid conformations. (2) The decorated carboxylate groups could supply versatile coordination modes and effectively transfer the magnetic coupling between adjacent metal centers. (3) The angles of the three coordination sites on quinolinic ring are close to 60°, which is very vital to construct the pentagonal bipyramidal geometry. (4) The additional aliphatic carboxyl group is

1. INTRODUCTION The field of coordination compounds on magnets have aroused growing interest over past few years. These fascinating nanomagnets, including SMMs,1 SIMs,2 and SCMs,3 have greatly potential applications in molecular spintronics and ultrahigh density information storage.4 Since the Long group found the field-induced SIM behavior in a mononuclear FeII complex,5 the new transition-metal ions complex with SIM behavior has aroused great attention.2,6 For example, the highspin cobalt(II) based complexes had been attracted more attention, because its relaxation behavior can be easily seen for S = 3/2, which is because this spin state should minimize quantum-tunneling effects.7 The different coordination geometries and environments of cobalt(II) based complex with SIM behavior had been reported, which normally exhibits 4−6 coordination numbers;8 Co-based SIMs with pentagonal bipyramidal geometry (D5h) have rarely been explored.9 The construction of coordination complexes depends on several factors, in which the choice of versatile bridging ligands © XXXX American Chemical Society

Received: March 26, 2018

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DOI: 10.1021/acs.inorgchem.8b00812 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry flexible, which would be effective for releasing the strain caused by the chelating mode and ensure the formation of environment with high coordination number. In accordance with our assumptions, a serial of transition metal complexes exhibiting pentagonal bipyramidal geometry of metal centers have been constructed with H2L via hydrothermal methods, where the metal ions conclude Co, Ni, Mn, and Cu. The synthesis steps and corresponding characterization of these complexes have been detailed as follows.

respectively, the O7 is from coordinated water, while two O atoms (O6, O8) of coordination water molecules coordinates in the two axial positions. The axial bond angles (O6−Co1− O8) is 174.447(7)°, and Co−O or −N bond lengths are in the range of 2.085(2)−2.214(2) Å. Furthermore, the singlenuclear basic units form the 3D supramolecular via intermolecular hydrogen bonding and π−π interactions. Different to the structure of 1 or 2, the nickle center in 3 adopts the six-coordinated octahedral environment, and coordinates with two water molecules. Ni−O or −N bond lengths are about 1.991(2)−2.370(5) Å. 4 and 5 are isostructural and have similar crystallography parameters, therefore only the magneto-structural correlations of 4 will be detailed described. As shown in Figure 2, 4 exhibits 1D chain structure, the chain along the b axis and its space group is monoclinic (P21/n). The central Ni atom is sixcoordinated. The central nickel atoms are alternately distributed, separated by the bulky ligands, and connected together by the carboxylic oxygen atoms on the ligand. The shortest distance of intrachain isolated Ni(II) is about 5.2928(4) Å, and the distance of the interchain Ni(II)···Ni(II) is about 8.1003(5) Å for 4 (5.3212(8) and 8.1903(1) Å for 5). In 4, one N atom (N1) and three O atoms (O2, O3, O5) occupy the equatorial planes, while two O atoms (O1, O4) coordinates in the two axial positions. The O2, O4, and O5 come from the carboxylic oxygen atoms and ether oxygen atoms on the ligand, respectively, and the O3 is from coordinated water. As shown in Figure 2b, the syn-syn carboxylate bridges connect the isolated nickel ions to form 1D chain structure and further to form the 3D supramolecular architecture by the hydrogen bonding and π−π interactions between the adjacent chains. In the five structures, seven-coordination (bipyramidal geometry) can be found in 1 or 2, while 3−5 have sixcoordination (octahedron) geometry. Two of the possible reason for these differences are as follows: (1) The covalent radius of Mn2+(1), Co2+(2), Ni2+(3, 4), and Cu2+(5) are decreasing, and the manganese and cobalt ions are large enough to have a flexible mode of coordination, but nickel and copper ions are relatively fixed. (2) It is seen that the coupling loops of ligands and metal ions are all quintuple rings, and the angles of the O−M−O and N−M−O all smaller than 90°, which promotes the formation of seven-coordination structures. The difference of coordination numbers couple can strongly affect the residence of unpaired electrons in d-orbitals, which further results in the difference of g- and D-factors.14 Detailed magnetic studies of 1−5 are as following. 2.3. Magnetic Properties of 1−5. The direct-current (dc) magnetic susceptibility data for 1−3 were carried out on crystalline samples at 1000 Oe in the temperature range of 2− 300 K (Figures 3 and S6). For 1, under the temperature of 300 K, the χMT value is about 4.48 cm3 K mol−1; this experimental data is closed to the spin-only value with one magnetically isolated divalent manganese ion (4.375 cm3 K mol−1). With the decrease of temperature, the χMT values gradually decrease, quickly decline below 30 K, and decrease to 3.26 cm3 K mol−1 at 2 K, which could be mainly ascribed to the zero field splitting of the Mn(II) ion. The magnetizations at high fields are close to saturation, indicating the magnetic isotropy of 1. As for 2 and 3, at 300 K the χMT values are about 3.12 and 1.23 cm3 K mol−1, respectively, larger than the theoretical spinonly value of g = 2.00 (1.875 and 1.000 cm3 K mol−1). This can be ascribed to the significant magnetic anisotropy in 2 and

2. RESULTS AND DISCUSSION 2.1. Synthesis. The ligand was prepared by modifying 8hydroxy-2-methylquinoline (Scheme S1). This five compounds were synthesized by hydrothermal method with good yields. Although with the similar starting materials, the as-synthesized compounds show the different coordination structures, which is probably because the metal ion centers exhibits different radii, coordination habits, and electronic structures.12 The 1−5 were characterized by PXRD to further confirm the purity of the samples (Figure S2), which show the agreement of the simulated and experimental patterns. As for the IR spectra, the carboxylate groups can be confirmed from the strong bands around 1600 cm−1. Band can be found in the region of 1690− 1730 cm−1 if there have H atoms in the carboxylate groups. No band was found in this range in the IR spectra, which confirmed that there was no H atom in the functional groups. 2.2. Structural Description. The X-ray diffraction measurements revealed that 1 and 2 were isostructural with similar cell parameters, and they had been previously reported.13 Here, we only briefly describe 2 for discussions of the following magneto-structural correlations. As shown in Figures 1 and S3, 2 is a single-nuclear compound, and its space group is triclinic (P1̅). As for the crystallographic structure of 2, the central Co ion is seven-coordinated with one ligand anion, and three coordinated water molecules. One N atoms (N1) and four O atoms (O1, O3, O5, O7) occupies the equatorial planes, the O1, O5, and O3 come from the carboxylic oxygen atoms and ether oxygen atom on the ligand,

Figure 1. (a) Basic structure in 2. (b) 3D structure of 2. The green dotted line represents intermolecular H bonds. Other H atoms are omitted for clarity. B


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Inorganic Chemistry

Figure 2. (a) Basic unit in 4. (b) 1D chain structure along the b axis. (c) The adjacent chain interacted with each other through π−π stacking in 4. (d) 3D network of 4. Pink dotted lines represent intermolecular H bonds.

Table 1. Fitting Results of the Magnetic Data for 1−3 comp.

gx = gy

gz

D (cm−1)

E (cm−1)

zj (cm−1)

1 2 3

2.01(2) 2.65(2) 2.31(2)

2.01(2) 2.12(2) 2.31(2)

−0.61(8) 70.41(0) 16.76(9)

0 8.89(7) −3.40(2)

−0.02(1) −0.01(1) −0.13(2)

The temperature-dependent magnetic susceptibilities for 4 and 5 are shown in Figure 4. Under the 300 K, the χMT values

Figure 3. ΧMT versus T plot at 1000 Oe for 2. Insert: M versus H plot at 2−5 K. Solid lines are the fittings according to Hamiltonian (1).

3. As temperature is lowered, in the range of 300−100 K, the χMT value for 2 decreases slightly and then quickly to about 1.86 cm3 K mol−1 at 2 K. As for 3, the χMT curves has a similar trend, a slow decrease in the temperature range of 300−50 K, and a minimum value 0.247 cm3 K mol−1 at 2 K, mainly due to the zero field splitting of the central metal ion. The field dependence of the magnetization for each compound was measured with the magnetic field up to 7 T in the temperature range of 2−5 K. For 2, under the low magnetic fields the magnetization curves show the quickly increase and then slowly increase to the values of 2.4 NμB at 7 T and 2 K. The maximum value deviates from the theoretical saturation value (3 NμB for S = 3/2, g = 2), revealing the significant magnetic anisotropy in 2. For 3, the nonsuperimposed magnetization curves at 2−5 K also indicates that 3 exhibits big magnetic anisotropy. The χMT versus T and M versus H curves of 1−3 were further fitted via PHI program15 to quantify the anisotropy parameters. Good fits were obtained using the following spin Hamiltonian:

Figure 4. ΧMT versus T plots for 4 (a) and 5 (b) at 1000 Oe; the solid line presents the fitting with Bonner−Fisher model. Inset: 1/χM versus T with fit to Curie−Weiss law.

2 2 2 Ĥ = gμB B ·S ̂ + D[Sẑ − S(S + 1)/3] + E(Sx̂ − Sŷ )

are 1.63 and 0.51 cm3 K mol−1, higher than one magnetically isolated M(II) (M = Ni (4), Cu (5)) ion with g = 2.0 (1 and 0.375 cm3 K mol−1, respectively). In the temperature range of 300−50 K, the χMT values decreases slightly and then quickly to the minimum value about 0.247 cm3 K mol−1 at 2 K. As shown from the curves of the χMT versus T, 4 and 5 exhibit antiferromagnetic behaviors. Using the Curie−Weiss law to fit the 1/χM versus T plot for 4 and 5 (inset of Figure 4),

(1)

where μB, B, S, D, and E stand for the Bohr magneton, magnetic field vector, the spin operator, and the axial and rhombic ZFS parameters, respectively. The best fitting values for 1−3 are shown in Table 1. It is noted that the zero-field splitting parameter D of 2 is the largest value in the reported Co(II) complexes with pentagonal bipyramidal geometry and showing SIM properties (Table S4). C


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Inorganic Chemistry Curie constants of 1.687(9) and 0.526(1) cm3 K mol−1, and the Weiss temperature of −8.5 (1) and −13.5(1) K were obtained, respectively. In order to quantitatively estimate the antiferromagnetic interaction and obtain corresponding parameters, eqs 2−4 were applied to fit the magnetic data.16 n−1

Ĥ = −2J ∑ S Â i ·S Â i+1 i=1

(2)

For S = 1, J < 0, χM =

Ng 2β 2 2.0 + 0.0194x + 0.777x 2 kT 3.0 + 4.346x + 3.232x 2 + 5.834x 3

(3)

For S = 1/2, J < 0, χM =

Figure 6. Resonance field versus microwave frequency (quantum energy) for 2. Solid lines are the linear fits to each resonance branches. The vertical dashed line indicates the frequency (140 GHz) used in Figure 5.

Ng 2β 2 0.25 + 0.074975x + 0.075235x 2 kT 1 + 0.9931x + 0.172135x 2 + 0.757825x 3 (4)

where x = |−2J|/kT, and J describes the intrachain exchange coupling amplitude. For 4, the best fitting parameters were g = 2.32(1), and J = −1.50(2) K. For 5, g = 2.22(1), and J = −3.55(2) K. 2.4. HF-EPR Studies of 2 and 4. It is known that the accurate value of the D and E usually cannot be obtained from the dc magnetic data, especially their signs, so high-frequency electron paramagnetic resonance (HF-EPR) measurements were performed to determine the accurate anisotropy parameters of 2. The polycrystalline powder of 2 was studied over the frequency range of 60−259 GHz. As shown in Figure 5, we can easily get the EPR spectra containing three main

By comparing the experimental EPR spectrum with the simulated spectra (Figure 5), we found that the simulation with D > 0 fit better to the experiment data than did those with D < 0, confirming that the D is positive. The positive D value might derive from the spin−orbital coupling of the ground state and excited state electrons, which further demonstrates the easy plane magnetic anisotropies of 2.9b,17 2.5. Dynamic (ac) Magnetic Properties of 2. The ac susceptibilities were performed at 1.8−3.0 K for 2 to explore its magnetic relaxation behaviors. When applying the zero dc field, out-of-phase ac susceptibility (χ″) signal cannot be seen (Figure S7). Once dc field was applied, the signal could be observed, which indicates the field-induced slow magnetic relaxation behavior of 2. To find the optimum dc field where the QTM effect is suppressed, various dc fields were used in the ac susceptibility measurements at 2 K (Figure S8). It turned out that at 1500 Oe we found the slowest relaxation, and then under this dc field, the following ac measurements were carried out (Figures 7, S9, and S10).

Figure 5. Experimental HF-EPR spectrum of 2 at 2 K (black) with its theoretical simulations (red, D > 0; blue, D < 0).

modes, which is the typical feature of a high spin cobalt(II) ion compound with S = 3/2. No inter-Kramer transitions between MS = ±1/2 and MS = ±3/2 were seen, probably because the complex exhibits very large D value which exceeds the limitation of the microwave frequency. All the EPR signals can be interpreted as from the intra-Kramers transitions within the lowest doublet MS = ±1/2 multiplet with ΔMS = ±1. The relationship of the resonance fields and its corresponding various microwave frequencies curve were shown in Figure 6. The resonance fields were simulated using the |D| value of 70.4 cm−1 from SQUID measurements while adjusting E (transverse zero-field splitting parameter) and g values to get the well-estimated data. The corresponding parameters obtained are gx = gy = 2.53(2), gz = 2.29(2), and |E| = 23.2(5) cm−1 (|E/ D| ∼ 0.33).

Figure 7. Frequency dependence of the out-of-phase (χ″) for 2 under a 1500 Oe dc field. Insert: Arrhenius plot with ln(τ) versus T−1 for 2. The blue line represents the fit to the Arrhenius law at 1.8−3 K.

The Cole−Cole plots were generated from the ac magnetic susceptibilities of 2 (Figure S11) and fitted the semicircles data according to the Debye model.18 The corresponding fitting values of are listed in Table S3, including the isothermal susceptibility χT, the adiabatic susceptibility χS, relaxation time τ, and α (α represent the distribution of relaxation times). By fitting the experiment data the α parameters are found in the range of 0.20−0.22, indicating the distribution of the relaxation D


Inorganic Chemistry

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time are narrow. The Arrhenius plots with ln(τ) versus T−1 is shown in Figure 7 (inset). The blue line shows the fit to the Arrhenius expression τ−1 = τ0−1 exp(−Ueff/kBT). The corresponding parameter values were obtained: Ueff = 5.27 K and τ0 = 1.35 × 10−5 s. 2.6. Spin Gap Properties of 4. The representative EPR spectra of 4 measured at 2 K are shown in Figure 8. A single

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00812. Synthesis, crystal data, TG spectra, XRD, and magnetic data (PDF) Accession Codes

CCDC 1830240−1830244 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (B.L.). *E-mail: [email protected] (Z.W.). ORCID

Figure 8. HF-EPR spectra of 4 at various frequencies under 2 K.

Bao Li: 0000-0003-1154-6423 Zhenxing Wang: 0000-0003-2199-4684

broad resonance peak is observed. As the frequency increases, the position of the resonance peak moves toward the lower field. The frequency-field (f−H) relationship is linear. An extrapolation of the f−H curve gives rise to a zero-field spin gap of Δ = 420 GHz (20.2 K). This spin gap suggests the presence of quantum fluctuations in 4, which is in accordance with fact that 4 is a 1D spin-chain system and does not exhibit long-range magnetic ordering down to 2 K. Thus, the observed resonance peak originates from the transition from the single state (S = 0) to the lower branch (SZ = −1) of the triplet states. Figure 8 also shows the spin gap will be closed at 15.3 T (20.4 K), corresponding to the value of spin gap with g = 1.98. Our EPR results show that 4 can be considered a quantum antiferromagnet. We note that the spin gap observed by EPR (20.2 K) is much larger than that by dc susceptibility measurements (1.466 K). This is probably because the model of fitting susceptibility is the best way to describe the magnetic behavior of 4, which deserves further studies.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the National Natural Science Foundation of China (21701046, 21471062) for financial support. This work is also supported by the Fundamental Research Funds for the Central Universities (2018KFYXKJC010). We also thank Prof. Ming-Hua Zeng in Hubei University for the precise help in sample synthesis.

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REFERENCES

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3. CONCLUSION In summary, by utilizing the quinolinic derivative ligand, a family of 3d transition metal complexes have been successfully constructed, which were further structurally characterized. In accordance with our assumption, 1−3 are mononuclear compounds. 1 and 2 have a pentagonal bipyramidal geometry, while 4 and 5 exhibit 1D zigzag chain. The differences between the coordination environment and final packing structures were ascribed to the effect of metal centers, whose coordination habits and radii would favor the proper structures. In addition, the designed and synthesized chelating ligands would be favored for assembling the rare pentagonal bipyramidal geometry of Co or Mn centers. The analysis of magnetic properties reveal that 2 has large and positive D value (∼70.4 cm−1), indicating the easy plane magnetic anisotropies. In addition, 2 exhibits the typical field-induced slow magnetic relaxation behavior, which could be an important supplement for Co SIM family. 4 and 5 give similar weak MII−MII antiferromagnetic interactions (J = −1.50 and −3.55 K, respectively). Through studies by HF-EPR, 4 can be considered a quantum antiferromagnet. E


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Inorganic Chemistry

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