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. 2020 Apr 1;5(14):8347–8354. doi: 10.1021/acsomega.0c00853

Study on the Magneto-Structural Correlation of a New Dinuclear Cobalt(II) Complex with Double μ-Phenoxo Bridges

Xiao-jiao Song 1,*, Xiao-ming Xue 1
PMCID: PMC7161063  PMID: 32309745

Abstract

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A new μ-phenoxo-bridged dinuclear cobalt(II) complex, [Co2(L)2(acac)2(H2O)] (1), has been synthesized by employing a new ligand, (4-methyl-2-formyl-6-(((2-trifluoromethyl)phenyl)methyliminomethyl) phenol) (HL). Structural analysis of complex 1 reveals that the geometry around cobalt centers is best described as a distorted octahedron and the distance of cobalt neighbors is 3.128(0) Å. The magnetic property studies indicate that complex 1 exhibits strong spin–orbit coupling effects and weak ferromagnetic coupling between two high-spin Co(II) centers linked by double μ-Ophenoxo bridges, with J = 1.87(2) cm–1. The studies show that not only the Co–O–Co angle affects the alignment of the cobalt spins but also the dihedral angle between the CoOCo plane and the phenyl plane plays an important role in the magnetic coupling in this [Co2O2] system. Thus, the small bridging angles (96.96(11) and 96.91(11)°) and the large dihedral angles between the CoOCo plane and the phenyl plane (63.0(1) and 30.6(1)°) induce intramolecular ferromagnetic exchange interaction in complex 1.

Introduction

The phenoxo bridge is an excellent linking group to construct complexes with short metal–metal distances.1−3 The phenoxo-bridged complexes have attracted extensive attention for their optical, catalytic, biomimetic, redox, and magnetic properties.4−13 Particular interest has been focused on the relationship of the structure and the magnetic property because the complexes with various phenoxides exhibit different magnetic behavior.14−19 Previous research has shown that the exchange coupling constant between metal centers can be affected by several structural parameters, such as the distance between the adjacent metal ions (M), the magnitude of the M–Ophenoxo–M angle and the out-of-plane shift of the phenyl ring, the substituents on the phenyl ring, and so on.20−24

The high-spin Co(II) complexes exhibit excellent magnetic properties, which arise from the strong orbital angular momentum,25 such as several phenoxo-bridged cobalt(II) complexes behaving as single-molecule magnets.26,27 Dinuclear complexes are suitable for investigating the magnetic interactions between paramagnetic centers transmitted through the phenoxo bridge because of their simple structures.28,29 Over the past decades, a lot of reports have been made on dinuclear bis(μ-phenoxo)-bridged cobalt(II) complexes; in addition, most of them exhibit antiferromagnetic coupling (AF) between cobalt(II) centers,20,23,30−45 and only four cases show ferromagnetic coupling (F) between cobalt(II) centers.23,45−47 To further understand the magnetic behavior of this system, more new μ-phenoxo-bridged Co(II) complexes need to be obtained and studied. Herein, we designed and synthesized a new Schiff-base ligand, (4-methyl-2-formyl-6-(((2-trifluoromethyl)phenyl)methyliminomethyl) phenol) (HL). We constructed a new dinuclear cobalt(II) complex, [Co2(L)2(acac)2(H2O)] (1), based on HL. In this article, we report the syntheses and crystal structures of HL and complex 1. Magnetic studies indicate the presence of intramolecular ferromagnetic coupling between the high-spin Co(II) centers in complex 1, which arises most prominently from the Co–O–Co super-exchange pathway. In addition, the magneto-structural correlation of complex 1 has also been investigated and discussed in detail. To predict the critical angle for switching the sign of the coupling constant (J) in the Co2O2 system, the magnetic properties and the structural parameters of the reported bis(phenoxo)-bridged dinuclear cobalt(II) complexes have also been collected, analyzed, and summarized.

Experimental Section

Materials and Methods

All chemicals for syntheses are commercially available and used without further purification.

Elemental analysis (C, H, and N) was performed with a Heraeus CHN-O-Rapid elemental analyzer. Fourier transform infrared (FT-IR) spectroscopy was recorded with KBr Pellets using a Thermo Scientific Nicolet iS10 spectrometer. Crystallographic data were collected using a Bruker CCD area-detector diffractometer. Diffraction data were measured with the SAINT program48 and absorption correction was processed by the SADABS program.49 Structures were solved by direct method and refined by full-matrix least-squares on F2 using the SHELXTL-2014 package.50 All nonhydrogen atoms were located by different Fourier maps and refined anisotropically. The hydrogen atoms were added geometrically and refined isotropically. The details for the structural analysis of ligand HL and complex 1 are shown in Table 1. Selected bond distances and angles for HL and complex 1 are listed in Tables 2 and S1, respectively. The CCDC numbers of HL and complex 1 are 1 976 333 and 1 976 334, respectively.

Table 1. Crystal Structural Data and Refinement Parameters for HL and Complex 1.

complexes HL 1
formula C17H14F3NO2 C44H42F6N2O9Co2
FW 321.29 974.65
crystal system monoclinic monoclinic
space group Cc C2/c
a (Å) 4.420(2) 28.201(9)
b (Å) 43.948(9) 14.449(4)
c (Å) 7.635(2) 20.928(8)
α (deg) 90 90.00
β (deg) 97.64(3) 91.058(16)
γ (deg) 90 90.00
V (Å3) 1469.9(8) 8526(5)
Z 4 8
ρcalcd/g cm–3 1.452 1.519
F(000) 664 4000
θ (deg) 2.781–24.992 2.72–27.063
reflections collected 5173 18 007
independent reflections 1966 7563
observed data [I > 2σ(I)] 1845 5119
data/restraints/parameters 1966/2/211 7563/0/545
GOF on F2 1.028 1.060
Rint 0.0418 0.0476
R1, wR2 [I > 2σ(I)] 0.0367, 0.0843 0.0524, 0.1330
R1, wR2 (all data) 0.0337, 0.0871 0.0836, 0.1458
residual electron density (e Å–3) 0.125, −0.128 0.355, −0.437
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Table 2. Selected Bond Lengths [Å] and Angles [Deg] of Ligand HL and Complex 1.

bond lengths [Å] and bond angles [deg]
Co1–O5 2.025(3) Co2–O3 2.016(3)
Co1–O6 2.046(3) Co2–O7 2.035(3)
Co1–N1 2.077(3) Co2–O8 2.061(3)
Co1–O1 2.078(3) Co2–O2 2.090(3)
Co1–O9 2.117(3) Co2–N2 2.090(3)
Co1–O3 2.162(3) Co2–O1 2.100(3)
O5–Co1–O6 88.46(11) O7–Co2–O2 89.47(12)
O5–Co1–O1 96.25(11) O8–Co2–N2 177.53(13)
O6–Co1–O9 88.16(11) O3–Co2–O1 84.48(10)
O1–Co1–O9 86.72(11) O2–Co2–O1 85.28(11)
N1–Co1–O3 168.97(11) N2–Co2–O1 94.68(11)
Co1–O1–Co2 96.96(11) Co2–O3–Co1 96.91(11)
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The magnetic data were collected using a Quantum Design MPMS-XL 7 SQUID magnetometer. Correction for the diamagnetic contribution was estimated from Pascal’s constant.51

Synthesis of 4-Methyl-2-formyl-6-(((2-trifluoromethyl)phenyl)methyliminomethyl) Phenol (HL)

The ligand HL was synthesized following the literature methods with some modification,52 as shown in Scheme 1. 2-Trifluoromethyl-1-phenylmethanamine (0.429 g, 4 mmol) in 10 mL of acetonitrile was added to a solution of 4-methyl-2,6-diformylphenol (0.656 g, 4 mmol) in 20 mL of acetonitrile. The reaction mixture was refluxed under stirring for 4 h, then cooled to room temperature. The reaction solution was concentrated on a rotary evaporator to about 10 mL. The yellow single crystals suitable for X-ray diffraction were obtained after about 3 days by slow evaporation at room temperature in open atmosphere (yield = 1.052 g, 82%). Anal. Calcd. For C17H14F3NO2: C, 63.55%; H, 4.39%; N, 4.36%. Found: C, 63.86%; H, 4.57%; N, 4.32%. Main IR peaks (KBr)/cm–1: 2858(m), 1676(s), 1637(m), 1598(s), 1476(m), 1449(m), 1335(s), 1318(s), 1261(m), 1229(m), 1198(s), 1166(s), 1114(s), 1071(s), 1037(s), 968(s), 951(m), 913(m), 875(s), 800(s), 746(m), 703(s), 671(s), 633(s), 586(m), 568(m).

Scheme 1. Synthesis Procedure of Ligand HL.

Scheme 1

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Synthesis of [Co2(L)2(acac)2(H2O)] (1)

Acetylacetone cobalt(II) (0.6 mmol, 0.1543 g) and 4-methyl-2-formyl-6-(((2-trifluoromethyl)phenyl) methyliminomethyl) phenol (0.6 mmol, 0.1928 g) were added to 10 mL of acetonitrile solution. The metallic salt and ligand were dissolved through stirring, and the solution became red. The solution was continuously stirred for 1 h at room temperature and then for 1 h at 85 °C. The mixture was cooled to room temperature, then filtered to remove all precipitate or undissolved material. Red single crystals suitable for X-ray diffraction were obtained from the filtrate by slow evaporation at room temperature after 7 days. Anal. Calcd. For C44H42F6N2O9Co2: C, 54.22%; H, 4.34%; N, 2.87%. Found: C, 54.48%; H, 4.55%; N, 3.38%. Main IR peaks (KBr)/cm–1: 3307(m), 2870(m), 1663(s), 1636(m), 1597(s), 1542(s), 1519(s), 1454(s), 1392(m), 1327(s), 1257(m), 1235(s), 1201(m), 1165(s), 1116(s), 1074(s), 1054(w), 1015(m), 920(m), 877(w), 821(w), 800(w), 773(m), 701(m), 659(w), 555(w), 500(w), 410(w).

Results and Discussion

FT-IR Spectrum

The FT-IR data were recorded in the range of 4000–400 cm–1 for ligand HL and complex 1, as shown in Figures S1 and S2, respectively. The bands in the region 2800–3000 cm–1 for HL and 1 are attributed to the stretching vibration of the C–H bond.52 The νC=O band is located at 1676 cm–1 for the ligand, and it shifts to 1663 cm–1 for complex 1, which indicates the coordination of the C=O group to the cobalt ions.53 Both HL and 1 exhibit bands at around 1636 cm–1, which is attributed to the stretching vibration of the C=N bond. For complex 1, the strong band at 1542 cm–1 is related to the coordinated acetylacetone anions, and the broad band at around 3307 cm–1 is assigned to the coordinated water molecule.54

Structural Description

The molecular structure of the ligand HL was determined by single-crystal X-ray diffraction analysis, which crystallized in the monoclinic system with space group Cc. The asymmetric unit only contains one Schiff-base ligand HL, as shown in Figure 1. The bond length of C9=N1 is 1.282(4) Å, within the accepted range for double bond. The bond length of C10–N1 is 1.472(4) Å, which is in agreement with the value proposed for single bond. Similarly, the C1=O1 bond length is 1.217(4) Å, which is consistent with double bond character. The C8–O2 bond length is 1.340(3) Å, which means the existence of a single bond. Hydrogen bond interaction was found between O2 and N1, as shown in Table S2 and Figure S3.

Figure 1.

Figure 1

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Structure of ligand HL.

The single-crystal X-ray diffraction analysis reveals complex 1 crystallized in the monoclinic system with space group C2/c. The structure of the asymmetric unit contains two Co(II) centers, two deprotonated Schiff-base ligands (L–), two acetylacetone anions (acac–), and one coordinated water molecule (H2O), as shown in Figure 2. There is no solvent molecule in the lattice. Two cobalt ions are bridged through two phenoxy-O atoms from two L– ligands and form a nearly planar Co2(μ-O)2 core with the dihedral angle of 175.77(2)° between Co1O1Co2 plane and Co1O3Co2 plane. In Co2(μ-O)2 core, the Co1–O1–Co2 and Co1–O3–Co2 angles (γ) are 96.96(11) and 96.91(11)°, respectively, as shown in Figure S4. The dihedral angles (δ) between the CoOCo plane and the phenyl plane are 63.0(1) and 30.5(1)°, respectively, as shown in Figure S5. Both cobalt metal centers show a six-coordinate environment [CoO5N]. We use the SHAPE 2.1 software55 to analyze the configuration of Co(II) ions and the results show that both cobalt centers adopt a distorted octahedral geometry with a little higher distortion for Co1, as shown in Table S3. Around the Co1 center, four atoms (O1, O5, O6, and O9) compose an equatorial plane, and two atoms (O3 and N1) occupy the axial position. In the equatorial plane, the values of the angles O5–Co1–O1, O5–Co1–O6, O6–Co1–O9, and O1–Co1–O9 are close to a right angle. Around the Co2 center, four atoms (O1, O2, O3, and O7) form an equatorial plane, and two atoms (N2 and O8) occupy the axial position. The cobalt centers are linked by double μ-phenoxo bridges, leading to a Co1···Co2 distance of 3.128(0) Å. There are hydrogen bonding interactions in complex 1, as shown in Table S2. In this dinuclear molecule, the oxygen (O9) from coordinated H2O is involved in intramolecular hydrogen bonding with oxygen (O8) from the acetylacetone anion. Furthermore, the dinuclear moieties are linked by intermolecular hydrogen bonding between O9 from coordinated H2O and O6(i) (i = −x + 1/2, −y + 3/2, −z + 1) from the acetylacetone anion of the neighboring molecule. The intermolecular hydrogen bonding interactions lead to the formation of a tetranuclear [Co1Co2]2 units, as shown in Figure S6.

Figure 2.

Figure 2

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Asymmetric unit of complex 1 with selected atom labeling. Hydrogen atoms have been omitted for clarity.

Magnetic Properties

Direct current (dc) magnetic measurement has been performed on complex 1 in the temperature range of 1.8–300 K under a 2.0 kOe field. The plot of χMT versus T for the [Co2(L)2(acac)2(H2O)] unit is shown in Figure 3. At room temperature, the χMT value is 6.03 cm3 mol–1 K, which is larger than the expected value for two isolated octahedral Co(II) ions (3.75 cm3 mol–1 K with g = 2.0 and S = 3/2). This phenomenon is owing to the strong spin–orbit coupling. Upon cooling, the χMT value first decreases to a value of 4.68 cm3 mol–1 K at 18 K, then increases to a value of 4.74 cm3 mol–1 K at 8 K, and finally rapidly decreases to a value of 3.35 cm3 mol–1 K at 1.8 K. This behavior is mainly because of spin–orbit coupling effects and ferromagnetic interaction between two cobalt centers. The final decrease at low temperature could be ascribed to the partial depopulation of excited magnetic states, zero-field splitting effects, and/or intermolecular interactions.

Figure 3.

Figure 3

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Thermal variation of the χMT product per [CoII2O2] cluster in complex 1; the solid line is the best fit result by PHI. Inset: experimental M versus H plots at the indicated temperatures for complex 1; the solid line is the best fit result by PHI.

The field-dependence magnetizations of complex 1 were measured in the filed range of 0–7 T at temperatures 1.8, 2.5, 5.0, and 10.0 K, as shown in the inset of Figure 3. The magnetization at 1.8 K under 7 T field is 4.35 NμB, which is considerably lower than the expected saturated value (>6.0 NμB). This behavior may be due to a strong magnetic anisotropy. Besides, the non-superposition of the M versus H/T plots indicates the presence of strong magnetic anisotropy, as shown in Figure S7. The susceptibilities and the magnetizations of complex 1 have been simulated using the program PHI.56 The spin Hamiltonian is given in eq 1, where λ, α, and B20 represent the spin–orbit coupling constant, orbital reduction parameter, and crystal field parameter, respectively57,58

graphic file with name ao0c00853_m001.jpg 1

Generally, for Co(II) complexes of Oh symmetry with a weak ligand field, λ is nearly 150 cm–1, and α lies below 1.50. The parameters were obtained as λ = −144.64(7) cm–1, α = 1.35(1), B20 = 151.55(27) cm–1, J = 1.87(2) cm–1, and zj = −0.016(1) cm–1 for 1, in good agreement with the experimental results. Deviation of the fitted λ can be caused by the distortion from an octahedral geometry.

The alternating current (ac) magnetic susceptibility measurement was performed on complex 1 in the frequency range of 1–999 Hz under 0 Oe dc field. No signal of out-of-phase ac susceptibility (χM″) was observed in complex 1, which could be a result of quantum tunneling of magnetization,47,59 as shown in Figure S8.

Magneto-Structural Correlation Analysis

To understand super-exchange pathways of complex 1, the magnetic orbital coupling between Co1(II) and Co2(II) was discussed. The coordination environment of Co(II) ions and the defined axes are shown in Scheme 2. For two cobalt centers, the energy level split of 3d orbitals was analyzed based on simple ligand field considerations.60,61 The distribution of the 3d electrons on their splitting orbitals is shown in Scheme 3. The dxy orbital of Co1 and all magnetic orbitals (dx′y′, dz′2, dx′2–y′2) of Co2 result in zero overlap integral, leading to the ferromagnetic interaction between two cobalt centers (Jxy_x′y′, Jxy_z′2, Jxy_x′2–y′2 > 0, as shown in Scheme 3). However, the overlap integral between the dz2, dx2–y2 orbital of Co1 and all magnetic orbitals of Co2 is nonzero, leading to the antiferromagnetic interaction between two cobalt centers (Jz2_x′y′, Jz2_z′2, Jz2_x′2–y′2, Jx2–y2_x′y′, Jx2–y2_z′2, Jx2–y2_x′2–y′2 < 0, as shown in Scheme 3).62,63 Some of these interactions will be more important than others by comparing the overlaps between the different localized orbitals.64 Most μ-phenoxo-bridged dinuclear Co(II) complexes reveal antiferromagnetic interaction because of more and stronger AF contributions (Jz2_x′y′, Jz2_z′2, Jz2_x′2–y′2, Jx2–y2_x′y′, Jx2–y2_z′2, Jx2–y2_x′2–y′2), as well as fewer and weaker F contributions (Jxy_x′y′, Jxy_z′2, Jxy_x′2–y′2) in the Co2O2 system. The most important AF contributions are Jx2–y2_x′2–y′2AF and Jz2_x′2–y′2AF, followed by Jx2–y2_z′2AF and Jz2_z′2AF, and these four AF contributions strongly depend on the angle of Co–O–Co. The leading AF contributions (Jx2–y2_x′2–y′2AF, Jz2_x′2–y′2AF, Jx2–y2_z′2AF and Jz2_z′2AF) decrease with reducing of Co–O–Co angle, whereas the main F contributions (Jxy_x′2–y′2F and Jxy_z′2F) are not affected by reducing the angle of Co–O–Co.64 Therefore, the small Co–O–Co angle (96.96(11) and 96.91(11)°) is the primary factor that causes complex 1 to exhibit weak ferromagnetic interaction.

Scheme 2. Coordination Environment of Co(II) Ions in Complex 1 with an Indication of the Relative Orientation of Jahn–Teller Elongation.

Scheme 2

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Scheme 3. Local Magnetic Orbitals Around Co(II) Ions and Electron Exchange Pathways in Complex 1.

Scheme 3

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Bold lines symbolize antiferromagnetic couplings (AF), while dotted lines stand for ferromagnetic couplings (F).

As well known, the magnetic interaction can also be affected by other structural parameters besides the M–O–M angles (γ), such as the dihedral angle (δ) between the MOM plane and the phenyl plane, which also plays a key role in the bis(phenoxo)-bridged M2O2 systems (see Scheme 4).16,21,65,66 The effect of dihedral angle on magnetic interaction for the Co2O2 system can be explained with the help of magnetic orbitals (Scheme 3). The main F contributions are almost not affected by opening the dihedral angle. However, a large dihedral angle could result in a decrease of the leading AF contributions.14,21,67 To understand the magneto-structural corrections of the Co2O2 system, we have studied the reported bis(phenoxo)-bridged Co(II) complexes and listed their magnetic couplings and important structural parameters in Table 3. According to the data of Table 3, the scatter plot of the dihedral angle (δ) versus the Co–O–Co angle (γ) was drawn for the dinuclear bis(phenoxo)-bridged Co(II) complexes (see Figure 4). There is no clear correlation between γ and δ for the Co2O2 system. It seems that larger dihedral angles appear preferentially for smaller Co–O–Co angles in many cases. For the near planar Co2O2 system (δ = 1.0°), the critical value of Co–O–Co angle (γ) is 97.0° for switching the sign of the coupling constant (J), while the critical angle (γCo–O–Co) increases to 99.7° when the dihedral angle (δ) increases to 35.3°. However, if the Co–O–Co angle is large enough (γ > 99.7°), bis(phenoxo)-bridged cobalt(II) complexes always exhibit antiferromagnetic coupling regardless of the magnitude of the dihedral angle (δ). In summary, the exchange coupling in the dinuclear bis(phenoxo)-bridged Co(II) complexes can be predicted with the help of Table 4. The smaller the Co–O–Co angle (γ < 99.7°) and the larger the dihedral angle between the CoOCo plane and the phenyl plane (δ > 35.3°), the more favorable is the formation of ferromagnetic coupling for bis(phenoxo)-bridged Co2O2 system.

Scheme 4. Schematic Structure of M2O2 Core with the M–O–M Angle (γ) and the Dihedral Angle (δ).

Scheme 4

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Table 3. Structures and Magnetic Properties of Relevant Phenoxo-Bridged Dinuclear Co(II) Complexesa,b.

complex coupling J (cm–1) d (Å) γ (deg) δ (deg) refs
[Co2(L)2(acac)2(H2O)] F 1.87 3.127 97.0 63.0 this work
96.9 30.6
[CoL(phen)]2 F 7.8 3.106 97.1 62.7 (23)
67.9
[Tl2Co(OC8F6H3)4·toluene]2 F   3.251 99.2 65.9 (45)
[Co2(L)2(N3)2] F 5.15 3.198 99.5 35.6 (46)
99.9 34.9
[Co(L)(acac)]2 F 2.06 2.893 90.1 89.0 (47)
[CoL]2·2CH3CN AF –7.5 3.155 102.0 30.1 (32)
[Co2Cl4(4-CH3C6H4O)2] AF –1.2 3.032 99.9 15.6 (20)
[CoL(MeOH)]2 AF –2 3.134 100.2 30.1 (23)
[Co2(H2L1)2(H2O)2(MeOH)2]Cl2·2MeOH AF –6.9 3.092 96.9 1.0 (30)
[Co2(py2ald)2](ClO4)2·0.7CH3OH AF –3.56 3.203 101.5 23.8 (31)
101.6 31.0
[Co2(py2ald)2](BF4)2·CH3OH AF –3.30 3.197 101.3 24.3 (31)
101.5 30.7
[Co2(sym-hmp)2](BPh4)2 AF –27.3 3.237 104.9 34.4 (33)
104.5 37.2
[Co2II(L)2]·0.5(1,4-dioxane) AF –6.1 3.200 102.5 21.5 (34)
102.8 23.9
[Co2(L2)2(CH3OH)]·2CH3OH·0.5C4H10O AF   3.163 100.0 24.1 (35)
99.7 5.34
[Co2L2]·H2O AF –4.2 3.119 102.0 35.5 (36)
99.8 29.6
[CoII(NCS)(bip)]2·dmf AF –7.37 3.244 104.0 27.1 (37)
103.9 25.2
[CoII(N3)(bip)]2·CH3OH AF –12.6 3.254 104.9 26.6 (37)
[CoII(NCO)(bip)]2·CH2Cl2 AF –8.06 3.242 104.0 28.4 (37)
[Co2IIL21] AF   3.134 100.2 31.1 (38)
Co2(L)(H2O)Cl2 AF –1.25 2.929 98.0 17.0 (39)
97.8 19.3
[Co2II(L1)2]·2CHCl3 AF –1.84 3.139 100.2 19.2 (40)
[Co2II(L2)2] AF –1.32 3.193 101.2 21.6 (40)
[Co2II(L3)2] AF –5.70 3.172 102.0 30.8 (40)
23.0
[Co2(tidf)(ClO4)2(H2O)2] AF –10.3 3.104 100.1 16.0 (41)
3.099 99.7 8.30
[LCo2(NCS)2]n AF   3.092 101.2 9.1 (42)
[Co2(hmp)2](BPh4)2·2H2O·2C3H6O AF   3.231 104.3 37.6 (43)
[Co2L2Cl2(CH3OH)2] AF –4.22 3.131 99.5 1.5 (44)
[Co(OArF)2(DME)]2 AF   3.139 99.4 22.4 (45)
104.0 72.0
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a

Coupling: F = ferromagnetic coupling, AF = antiferromagnetic coupling.

b

d: the distance of Co···Co, γ: the Co–O–Co bond angle, δ: the dihedral angle between the CoOCo plane and the phenyl plane.

Figure 4.

Figure 4

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Scatter plot of the dihedral angle between the CoOCo plane and the phenyl plane (δ) versus the Co–O–Co angle (γ) for the dinuclear bis(phenoxo)-bridged Co(II) complexes whose structures and magnetic properties were characterized by pioneers. For the Co2O2 complexes with low symmetry, the means of γ and the means of δ were calculated and used to draw this figure. Red squares correspond to ferromagnetic coupling complexes and blue circles correspond to antiferromagnetic coupling complexes.

Table 4. Relationship between the Exchange Coupling and Structural Parameters (γ, δ) for the Co2O2 Systema.

Co–O–Co angle γ (deg) dihedral angle δ (deg) coupling
large (>97.0) very small (≈1.0) AF
small (<99.7) large (>35.3) F
larger (>99.7) any value (0–90) AF
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a

F: ferromagnetic coupling, AF: antiferromagnetic coupling.

Conclusions

A new complex 1 based on HL ligand was synthesized by the slow solvent evaporation method under room temperature. The cobalt ions adopt a distorted octahedral geometry. Complex 1 exhibits ferromagnetic coupling between the cobalt centers linked by phenoxo bridges, with J = 1.87(2) cm–1. The results show that not only the Co–O–Co angle plays a key role in the magnetic coupling but also the dihedral angle between the CoOCo plane and the phenyl plane has an important effect on magnetic properties for the Co2O2 system. A prediction can be made that the critical Co–O–Co angle (γ) for switching the sign of the coupling constant is 99.7° by studying all of the dinuclear bis(phenoxo)-bridged Co(II) complexes with reliable magnetic and structural data. Furthermore, in the Co2O2 system, ferromagnetic coupling interaction is observed only when the Co–O–Co angle is small (<99.7°) and the dihedral angle is large (>35.3°). This result can guide us to control the magnetic coupling between metal ions linked by a phenoxo bridge and then to design and synthesize high-spin clusters.

Acknowledgments

This work was supported by Fundamental Research Funds for the Central Universities (LGZD201807), Pre-research Fund of Nanjing Forest Police College (LGY201701), and the Natural Science Foundation of Jiangsu Province (BK20181339).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c00853.

  • IR spectra; structural and magnetic characterization (PDF)

  • X-ray crystallographic data HL (CIF)

  • Co2L2 (CIF)

The authors declare no competing financial interest.

Supplementary Material

ao0c00853_si_001.pdf (775.2KB, pdf)
ao0c00853_si_002.cif (226.2KB, cif)
ao0c00853_si_004.cif (587.3KB, cif)

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