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. 2019 Nov 25;4(24):20634–20643. doi: 10.1021/acsomega.9b02764

Magnetic Properties of End-to-End Azide-Bridged Tetranuclear Mixed-Valence Cobalt(III)/Cobalt(II) Complexes with Reduced Schiff Base Blocking Ligands and DFT Study

Abhisek Banerjee †, Snehasis Banerjee ‡, Carlos J Gómez García §, Samia Benmansour §, Shouvik Chattopadhyay †,*
PMCID: PMC6906786  PMID: 31858049

Abstract

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Two tetranuclear mixed-valence cobalt(III/II) complexes having the general formula [(μ1,3-N3){CoII(Ln)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (where H2L1 and H2L2 are two reduced Schiff base ligands) have been synthesized and characterized. The structures of both complexes show cobalt(II) and cobalt(III) centers with a distorted octahedral geometry with cobalt(III) and cobalt(II) centers located at the inner N2O2 and outer O4 cavities of the reduced Schiff base ligands, respectively. The oxidation states of both cobalt centers have been confirmed by bond valence sum (BVS) calculations. The magnetic properties show that both compounds behave as cobalt(II) dimers connected through an end-to-end azido bridging ligand and show moderate antiferromagnetic Co(II)–Co(II) couplings of −11.0 and −14.4 cm–1 for 1 and 2, respectively, as also corroborated by DFT calculations, Jtheo = −13.07 cm–1 for 1 and −12.49 cm–1 for 2. The calculated spin densities of both complexes at the cobalt(II) centers are −2.75 and +2.75, respectively, clearly supporting that they are the magnetic centers.

Introduction

High-nuclearity complexes containing paramagnetic metal centers have attracted huge interest due to their interesting physical properties, architectural beauty, and magnetostructural correlations in the field of coordination chemistry.1,2 During the process of their synthesis, partial oxidation of manganese(II), iron(II), and cobalt(II) usually produces mixed-valence polynuclear complexes.3−8 Focusing on mixed-valence cobalt complexes, they are less abundant compared to those of manganese and iron but much more interesting because of their wide range of applications in magnetism and electrochromism.9,10 The archetypal normal spinel Co3O4 is probably the best-known mixed-valence complex of cobalt. It presents low-spin cobalt(III) centers occupying octahedral holes and high-spin cobalt(II) centers occupying tetrahedral ones.11

Schiff bases have been widely used by various groups to prepare many mixed-valence cobalt(III)/cobalt(II) complexes,12−29 including a few trinuclear examples.12−20 Macrocyclic Schiff bases prepared by condensation of different diamines with 4-alkyl-2,6-diformylphenols have also been used to prepare dinuclear cobalt mixed-valence complexes21−25 although their magnetic properties have not been reported. On the other hand, many hydroxide-rich Schiff bases have also been used to form dinuclear mixed-valence complexes, including a few examples showing single-molecule magnet (SMM) behavior.26−29

In the present work, we have used reduced Schiff bases to prepare phenoxido-bridged dinuclear cobalt(III)–(O)2–cobalt(II) moieties, which are further connected by end-to-end azide bridges to form tetranuclear mixed-valence Co(II/III) complexes with CoIII–(O)2–CoII–(N3)–CoIII–(O)2–CoII cores. Since Co(III) ions are low-spin diamagnetic centers, from a magnetic point of view, the tetrameric moieties can be considered as Co(II) dimers connected via end-to-end azide bridges in both compounds.

Here, we report a new strategy, using two different compartmental reduced Schiff base ligands and azide as a coligand, for the synthesis of two tetranuclear mixed-valence cobalt(III/II) complexes: [(μ1,3-N3){CoII(L1)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (1) where H2L1 = (1,3-propanediyl)-bis(iminomethylene)bis(6-methoxyphenol), and [(μ1,3-N3){CoII(L2)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (2), where H2L2 = (1,3-propanediyl)bis(iminomethylene)bis(6-ethoxyphenol) (2). Both complexes have been structurally characterized by single-crystal X-ray diffraction studies and also by spectral and elemental analyses. The magnetic properties indicate the presence of two high-spin (S = 3/2) cobalt(II) centers in both complexes with moderate antiferromagnetic couplings. To obtain a better understanding of the magnetic exchange mechanism, quantum mechanical (DFT) calculations have been performed.

Experimental Section

Materials and Methods

All chemicals used were purchased from Sigma-Aldrich and were of reagent grade. They were used as received, without further purification.

Although no problems were encountered in this work, care should be taken while handling as perchlorate salts and organic ligands in the presence of azide are potentially explosive. Only a small amount of the material should be prepared, and it should be handled with care.

Synthesis of Reduced Schiff Base Ligands

Synthesis of H2L1 [(1,3-Propanediyl)bis(iminomethylene)bis(6-methoxyphenol)]

A solution of 3-methoxysalicylaldehyde (608 mg, 4 mmol) and 1,3-propanediamine (0.22 mL, 2 mmol) in 20 mL of methanol was refluxed for 2 h to prepare the Schiff base ligand N,N′-bis(3-methoxysalicylidene)-1,3-propanediamine, H2La. The resulting solution was cooled to 0 °C, and solid sodium borohydride (150 mg, 4 mmol) was gently added to this methanolic solution with continuous stirring. The solution was further acidified with glacial acetic acid (10 mL) and placed under reduced pressure in a rotary evaporator (60 °C). The residue was then dissolved in water (15 mL) and extracted with dichloromethane (15 mL). The dichloromethane fraction was dried using anhydrous sodium acetate, and the solution was filtered to remove the solid sodium acetate to obtain a solution of the desired reduced Schiff base ligand, H2L1. It was then extracted in methanol and was directly used for the synthesis of the cobalt complex 1 (see below).

Synthesis of H2L2 [(1,3-Propanediyl)bis(iminomethylene)bis(6-ethoxyphenol)]

This ligand was prepared as H2L1 but using 3-ethoxysalicylaldehyde (664 mg, 4 mmol) instead of 3-methoxysalicylaldehyde. The obtained ligand N,N′-bis(3-ethoxysalicylidene)-1,3-propanediamine, H2Lb, was also reduced with sodium borohydride, and the final resulting solution was directly used for the synthesis of the cobalt complex 2 (see below).

Synthesis of Compounds 1 and 2

Synthesis of [(μ1,3-N3){CoII(L1)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (1)

A solution of cobalt(II) perchlorate hexahydrate (732 mg, 2 mmol) in methanol (10 mL) was added to the methanol solution of the reduced Schiff base ligand, H2L1, with constant stirring to obtain a dark brown solution. A solution of 3-nitrobenzoic acid (334 mg, 2 mmol) in methanol (20 mL) was then added to the dark brown solution followed by the addition, with constant stirring, of a solution of sodium azide (130 mg, 2 mmol) in 10 mL of methanol/water (in a 4:1 ratio). The resulting solution was stirred for 1 h, and a solution of NH4PF6 (163 mg, 1 mmol) in methanol (5 mL) was then added followed by 30 min of stirring. Single crystals, suitable for X-ray diffraction, were obtained after 5–6 days on slow evaporation of the solution in an open atmosphere. X-ray powder diffraction confirmed the phase purity of the sample (see the Supporting Information).

Yield: 248.26 mg (∼65%, based on Co). Anal. Calcd for C52H56Co4N15O16F6P (FW = 1527.81 g/mol): C, 40.84; H, 3.66; N, 13.74%. Found: C, 40.8; H, 3.7; N, 13.8%. FT-IR (KBr, cm–1): 3238 (υN–H), 2940 (υC–H), 2023 (υN3), 2121 (υN3), 1477 (υCOO–), 1532 (υCOO–), 848 (PF6–). UV–vis, λmax (nm), [εmax (dm3 mol–1 cm–1)] (CH3CN), 615 (5.43 × 102), 330 (3.36 × 103), 226 (1.06 × 104).

Synthesis of [(μ1,3-N3){CoII(L2)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (2)

Compound 2 was synthesized following a similar synthetic procedure used for the synthesis of complex 1 except that H2L2 was used as a reduced Schiff base ligand instead of H2L1. Single crystals, suitable for X-ray diffraction, were obtained after 3–4 days on slow evaporation of the solution in an open atmosphere. X-ray powder diffraction confirmed the phase purity of the sample (see the Supporting Information).

Yield: 237.15 mg, (∼60%, based on Co). Anal. Calcd for C56H64Co4N15O16F6P (FW = 1583.91 g/mol): C, 42.42; H, 4.04; N, 13.25%. Found: C, 42.4; H, 3.9; N, 13.3%. FT-IR (KBr, cm–1): 3228 (υN–H), 2985 (υC–H), 2028 (υN3), 2117 (υN3), 1470 (υCOO–), 1534 (υCOO–), 830 (PF6–). UV–vis, λmax (nm), [εmax (dm3 mol–1 cm–1)] (CH3CN), 620 (5.29 × 102), 330 (4.19 × 103), 240 (0.92 × 104).

Physical Measurements

Elemental analyses (C, H, and N) were performed using a PerkinElmer 240C elemental analyzer. IR spectra in KBr pellets (4500–500 cm–1) were recorded with a PerkinElmer Spectrum Two spectrophotometer. Electronic spectra in acetonitrile (900–200 nm) were recorded on a PerkinElmer Lambda 35 UV–visible spectrophotometer. Magnetic measurements were performed with a Quantum Design MPMS-XL-7 SQUID magnetometer with an applied magnetic field of 0.1 T in the 2–300 K temperature range on polycrystalline samples with masses of 8.092 and 25.454 mg for compounds 1 and 2, respectively. The isothermal magnetization measurements were done with fields up to 7 T at 2 K. The susceptibility data were corrected for the sample holder and for the diamagnetic contribution of the salts using Pascal’s constants.30 The X-ray powder diffractograms were collected for polycrystalline samples of both compounds using a 0.7 mm glass capillary that was mounted and aligned on an Empyrean PANalytical powder diffractometer, using Cu Kα radiation (λ = 1.54177 Å). A total of three scans were collected at room temperature in the 2θ range 5–40°.

X-ray Crystallography

Suitable single crystals of both complexes were used for data collection using a “Bruker D8 QUEST area detector” diffractometer equipped with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by the direct method and refined by full-matrix least squares on F2 using the SHELXL-18 package.31 Non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms attached to nitrogen atoms were located by difference Fourier maps and were kept at fixed positions. All other hydrogen atoms were placed in their geometrically idealized positions and constrained to ride on their parent atoms. Multiscan empirical absorption corrections were applied to the data using the program SADABS.32 A summary of the crystallographic data is listed in Table 1. Selected bond lengths and angles are listed in Table S1 in the Supporting Information.

Table 1. Crystal Data and Refinement Details of Complexes 1 and 2a.

compound 1 2
formula C52H56Co4N15O16F6P C56H64Co4N15O16F6P
formula weight 1527.81 1583.91
crystal system monoclinic monoclinic
space group C2/c C2/c
a (Å) 19.000(5) 19.559(3)
b (Å) 13.808(4) 13.632(4)
c (Å) 24.205(8) 24.520(4)
β (°) 104.680(9) 106.255(4)
V (Å3) 6143(3) 6276.3(2)
Z 4 4
d(calc) [g cm–3] 1.652 1.676
μ [mm–1] 1.185 1.164
F(000) 3112 3240
total reflections 88 166 33 731
unique reflections 5838 5652
observed data [I > 2 σ(I)] 5469 3474
R(int) 0.023 0.145
R1, wR2 (all data) 0.0383, 0.0962 0.1233, 0.2331
R1, wR2 [I > 2 σ(I)] 0.0368, 0.0948 0.0720, 0.1855
residual electron density (e Å–3) 1.108, −0.518 0.665, −1.236
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a

R1 = ∑||Fo| – |Fc||/∑|Fo| and wR2 = ∑w(|Fo|2 – |Fc|2)2/∑w(|Fo|2)1/2.

Computational Details

A DFT study is carried out to understand the electronic structure of the investigated complex. All geometry optimizations of the complex are carried out using the density functional theory method at the B3LYP level with the Gaussian 09 program package. Los Alamos effective core potentials lanL2DZ basis set was employed for the Co atom. On the other hand, the split-valence 6-31G(d) basis set was applied for the other atoms. The starting structure of the investigated complex was used from its X-ray crystallographic data. The geometry optimization is performed without any constraint, and the nature of stationary points was confirmed by normal-mode analysis. The traditional broken-symmetry (BS) scheme for the same functional, as implemented in ORCA,33 was applied to calculate magnetic exchange coupling constants (J). The energy difference between the high-spin and broken-symmetry solutions along with their spin expectation values ⟨S2⟩ was used in the Yamaguchi34,35 formula as shown in the following equation

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Results and Discussion

Synthesis

The two hexadentate N2O2O′2 donor compartmental Schiff base ligands, H2La and H2Lb, were synthesized by refluxing in methanol 1,3-propanediamine with 3-methoxysalicylaldehyde or 3-ethoxysalicylaldehyde, respectively. The resulting Schiff base solutions were reduced with sodium borohydride in methanol to obtain the corresponding reduced Schiff base ligands, H2L1 and H2L2, respectively. The reaction of Co(ClO4)·6H2O with the reduced Schiff bases gave two mixed-valence Co(II/III) complexes, [(μ1,3-N3){CoII(L1)(μ-(NO2)PhCOO)CoIII(N3)}2]PF6 (1) and [(μ1,3-N3){CoII(L2)(μ-(NO2)PhCOO)CoIII(N3)}2]PF6 (2), which are stabilized by the presence of bridging 3-nitrobenzoate and azide ligands. The synthetic procedure of both complexes is shown in Scheme 1.

Scheme 1. Synthetic Route to Ligands H2L1 (R = Me) and H2L2 (R = Et) and Complexes 1 (R = Me) and 2 (R = Et); the Non-Coordinated PF6– Ion Has Been Omitted for Clarity.

Scheme 1

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Description of the Structures of [(μ1,3-N3){CoII(L1)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (1) and [(μ1,3-N3){CoII(L2)(μ-O2CC6H4NO2)CoIII(N3)}2]PF6 (2)

X-ray crystal structure determination reveals that both complexes crystallize in the monoclinic space group C2/c and that they contain two Co(II) and two Co(III) ions, in agreement with the charge balance, the Co–N and Co–O bond distances, and bond valence sum (BVS) calculations (vide infra).36−41

The asymmetric unit of each complex consists of one Co(III) and one Co(II) center, one complete reduced deprotonated Schiff base ligand: (L1)2– in 1 and (L2)2– in 2, one 3-nitrobenzoate anion, one terminal N3– anion, half bridging N3– anion, and half PF6– anion. The presence of a C2 axis passing through the central N atom (N6) of the bridging N3– anion generates Co2IICo2III tetramers, as depicted in Figures 1–3. The total cationic charge of the Co2IICo2III tetramer (+10) is compensated by two (NO2)PhCOO– ligands, two terminal N3– ligands, two deprotonated reduced Schiff base ligands: (L1)2– in 1 and (L2)2– in 2, the bridging N3– ligand, and the isolated PF6– anion.

Figure 1.

Figure 1

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View of complex 1 with selected atom labeling in one of the dimers. Hydrogen atoms have been omitted for clarity.

Figure 3.

Figure 3

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View of the tetranuclear Co2IICo2III complex in compound 1 (similar for 2) with the labeling scheme. Only the atoms around the metal centers have been shown for clarity.

Figure 2.

Figure 2

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View of complex 2 with selected atom labeling in one of the dimers. Hydrogen atoms have been omitted for clarity.

In both complexes, the Co(III) centers (Co1) occupy the inner N2O2 cavities of the ligands and show a fac-CoO3N3 coordination environment. The Co(II) centers (Co2) occupy the outer O2O′2 cavities and present a CoO5N coordination environment. Both cobalt atoms (Co1 and Co2) are hexacoordinated and adopt a distorted octahedral geometry with a much higher distortion for Co2, as shown by the SHAPE analysis of both cobalt ions (see the Supporting Information).42 In fact, this SHAPE analysis shows that the coordination geometry of both Co2 ions is intermediate between distorted octahedral and trigonal prism.43

The central Co2IICo2III clusters in compounds 1 and 2 are very similar. Thus, in both compounds, the equatorial plane of Co1 is formed by two amine nitrogen atoms (N1 and N2) and two phenoxido oxygen atoms (O1 and O2) from the deprotonated reduced Schiff base ligand. The axial positions of Co1 are occupied by a N atom (N3) from a terminal azide ligand and by an oxygen atom (O5) from the carboxylate group of the 3-nitrobenzoate ligand (Figure 3). The equatorial plane of Co2 in both compounds is formed by four oxygen atoms (O1, O2, O3, and O4) from the reduced Schiff base ligand, and the axial positions are occupied by the other oxygen atom (O6) of the carboxylate group of the 3-nitrobenzoate ligand and by a N atom (N4) from a bridging N3– ligand. Therefore, Co1 and Co2 are connected through a double oxido bridge (O1 and O2 from the reduced Schiff base ligand) and by a syn–syn carboxylate bridge (O5 and O6 form the 3-nitrobenzoate ligand). This triple bridge gives rise to very short Co1···Co2 distances of 2.994(3) Å in 1 and 2.999(2) Å in 2. The two Co2 atoms are only connected through a single μ1,3-N3– bridge (Figure 3) with a Co2···Co2 separation of 5.59(3) Å in 1 and 5.61(1) Å in 2.

The saturated 6-membered chelate rings, Co1–N1–C9–C10–C11–N2 in complex 1 and Co1–N1–C10–C11–C12–N2 in complex 2, present chair conformations with puckering parameters of θ(2) = 0.569(4) Å and φ = 181(2)° in 1 and θ(2) = 0.573(7) Å and φ = 357(5)° in 2.44

Both amine nitrogen centers {N(1) and N(2)} in each complex reside in an identical chiral environment, and the configurations of both centers are R and S for N1 and N2, respectively, as shown in Figure 4.

Figure 4.

Figure 4

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Configurations of two chiral centers {N(1) and N(2)} in 1.

BVS Calculations

Based on crystallographically determined metal–ligand bond distances, the oxidation state of the metal ions in the crystalline solids can be evaluated using BVS calculations. Mathematically, the bond valences (sij) can be evaluated using eq 1.36−41

graphic file with name ao9b02764_m002.jpg 1

where i is the donor center, j is the metal center; r0 is the reported bond length between i and j, rij is the observed bond length, sij is the valence of a bond between two atoms i and j, and b is usually considered to be 0.37. The bond valence sum is calculated according to eq 2

graphic file with name ao9b02764_m003.jpg 2

where zj is the valence of atom j connecting i–j bonds with all neighboring i atoms.

The calculated bond valence sum (BVS) values for Co1 (2.96 in 1 and 2.94 in 2) and Co2 (2.02 in 1 and 2.01 in 2) clearly confirm that the oxidation state of Co1 is +3, whereas that of Co2 is +2.

Supramolecular Interactions

Table 2 lists the hydrogen bonding interactions in complexes 1 and 2. In both complexes, two hydrogen atoms of the reduced Schiff base ligand (H1 and H2) are available for hydrogen bonding. Thus, H1 (from the amine nitrogen atom N1) is involved in intramolecular H-bonding with a fluorine atom (F1 in 1 and F3(b) in 2; (b) = −x, 1 – y, 1 – z) of the non-coordinated PF6– anion. The other H atom (H2, from the amine nitrogen atom N2) is H-bonded to the nitrobenzoate oxygen atom (O8(a) in 1 and O7(c) in 2; (a) = 3/2 – x, 3/2 – y, 1 – z and (c) = 1/2 – x, 3/2 – y, 1 – z). All of these interactions led to the formation of chainlike structures as shown in Figures 5 and 6 for compounds 1 and 2, respectively. None of the compounds show significant C–H···π and π···π interactions.

Table 2. H-Bonding Distances (Å) and Angles (°) in Compounds 1 and 2a.

compound D–H···A D–H (Å) H···A (Å) D···A (Å) ∠D–H···A (°)
1 N1–H1···F1 0.89(3) 2.26(3) 3.09(7) 156(2)
  N2–H2···O8(a) 0.88(3) 2.56(3) 3.32(6) 148(2)
2 N1–H1···F3(b) 0.97(9) 2.19(10) 3.11(3) 159(7)
  N2–H2···O7(c) 1.06(7) 2.23(6) 3.09(12) 138(5)
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a

Symmetry transformations: (a) = 3/2 – x, 3/2 – y, 1 – z; (b) = −x, 1 – y, 1 – z; (c) = 1/2 – x, 3/2 – y, 1 – z.

Figure 5.

Figure 5

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View of the zigzag chain formed by the H-bonding interactions in compound 1. Only the coordinating atoms around the metal centers, the bridging 3-nitrobenzoate group, and relevant H atoms have been shown for clarity. Symmetry transformation: (a) = 3/2 – x, 3/2 – y, 1 – z.

Figure 6.

Figure 6

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View of the zigzag chain formed by the H-bonding interactions in compound 2. Only the coordinating atoms around the metal centers, the bridging 3-nitrobenzoate group, and relevant H atoms have been shown for clarity. Symmetry transformations: (b) = −x, 1 – y, 1 – z; (c) = 1/2 – x, 3/2 – y, 1 – z.

IR and Electronic Spectra

The IR and electronic spectra of both tetranuclear cobalt complexes are in agreement with their crystal structures (see the Supporting Information). Sharp bands observed in the 3240–3190 cm–1 range in the IR spectra of both complexes, attributed to the N–H stretching vibration, confirm the presence of the reduced Schiff base.14 The bands corresponding to the alkyl C–H stretching can be observed in the range of 2985–2950 cm–1 for both complexes. The symmetric and antisymmetric carboxylate stretching vibrations can be observed as sharp absorption bands respectively at 1477 and 1532 cm–1 in 1 and 1470 and 1534 cm–1 in 2.45−46 A very strong double band observed at 2023 and 2121 cm–1 in 1 and 2028 and 2117 cm–1 in 2 confirms the presence of two types of azide groups (terminal and bridging) in both complexes.

The UV–vis spectra of compounds 1 and 2 are very similar (see the Supporting Information). They show three absorption bands at ca. 618 nm (ε ≈ 102 M–1 cm–1), ca. 330 nm (ε ≈ 103 M–1 cm–1), and ca. 235 nm (ε ≈ 104 M–1 cm–1). The higher-energy absorption band at around 235 nm is assigned to intraligand π–π* transitions,47 whereas the band at ca. 330 nm may be attributed to a ligand-to-metal charge transfer transition. Finally, the band at ca. 618 nm can be attributed to any of the two spin-allowed d–d transitions (1A1g → 1T1g and 1A1g → 1T2g) that are expected in low-spin cobalt(III) complexes.16 The other spin-allowed transition may be hidden by the strong LMCT transition at ca. 330 nm or might be located at higher energies, out of the scan range.48 The bands corresponding to the electronic transitions from cobalt(II) are not observed since they are hidden by the d–d transitions from the Co(III) centers.13

Magnetic Properties

As expected, the magnetic properties of compounds 1 and 2 are very similar. Thus, the product of the molar magnetic susceptibility per Co2IICo2III tetramer times the temperature (χmT) shows a value of ca. 5.3 cm3 K mol–1 at room temperature for both compounds and a continuous decrease when the temperature is lowered to reach a value of 0.30 and 0.15 cm3 K mol–1 at 2 K for compounds 1 and 2, respectively (Figure 7a). This room temperature value is slightly below than the expected value for two isolated octahedral Co(II) ions (2.7–3.4 cm3 K mol–1 per Co(II) ion). The low temperature value is well below the expected value for two isolated Co(II) ions (1.5–1.8 cm3 K mol–1 per Co(II) ion),49 indicating the presence of a moderate antiferromagnetic Co–Co coupling. The isothermal magnetization at 2 K (Figure 7b) shows a linear increase of magnetization in both compounds at high fields without reaching saturation at 7 T, confirming the presence of a moderate antiferromagnetic coupling. Furthermore, the lower values of magnetization for compound 2 suggest that the magnetic coupling is slightly stronger in this compound. Since the two Co(III) ions are low-spin d6 diamagnetic ions, we can assume that the magnetic properties are due to the two Co(II) ions in the cluster and that they are antiferromagnetically coupled through the μ1,3-N3– bridge. Accordingly, we have performed a simultaneous fit of the χmT product and of the isothermal magnetization at 2 K of both compounds to a Co(II) dimer model using the program PHI.50 The model used includes the spin–orbit coupling constant (λ), the orbital reduction factor (α = κA), and the crystal field distortion of the octahedral geometry (Δ) to account for the relatively high distortions of the coordination geometry of the Co2 centers.49 This model reproduces very satisfactorily the thermal variation of the χmT product and the magnetization at 2 K of both complexes in the 2–300 K range with the following set of parameters: α = 1.49, λ = −130 cm–1, J = −11.0 cm–1, and Δ = 221 cm–1 for 1 and α = 1.49, λ = −17.5 cm–1, J = −14.4 cm–1, and Δ = −100 cm–1 for 2 (solid lines in Figure 7, the Hamiltonian is written as H = −JS1S2). To reproduce the divergence in the χm vs T plot at very low temperatures (see the Supporting Information), we have included a monomeric Co(II) impurity of 5.5% for 1 and 3.5% for 2. Finally, to reduce the number of parameters in the fit, we have corrected a fixed temperature-independent paramagnetism of 2 × 10–3 cm3 mol–1 to account for the two Co(III) ions, a value close to those observed in other Co(III)-containing compounds.51,52

Figure 7.

Figure 7

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(a) Thermal variation of the χmT product per Co2IIICo2II cluster in compounds 1 and 2. (b) Isothermal magnetization at 2 K for compounds 1 and 2. Solid lines are the best fit to the model (see the text).

The J constant coupling is antiferromagnetic in both complexes, in agreement with the observed behavior in most of the reported Co(II) complexes with single μ1,3-N3 bridges.53−63 Note that, as expected, both coupling constants are very similar, given the similar structural parameters of the azido bridges in both compounds (all of the bond distances and angles are almost identical, see the Supporting Information).

Theoretical Studies

Orbital Analysis

The isodensity plots of α-HOMOs of complexes 1 and 2 are shown in Figure 8. The isodensity plots of the vacant LUMO and six singly occupied molecular orbitals (SOMOs) of both complexes are gathered in Figures S8 and S9 (Supporting Information). The calculated results (Tables 3 and 4) show that, for all SOMOs, the bridging azide and the d orbitals of cobalt(II) have practically no contributions and therefore the SOMO components do not favor magnetic exchange between the two cobalt(II) centers via the bridging azide. However, in the α-HOMO, a significant contribution of the bridging azide (68% for 1 and 63% of 2) and the dz2 orbital of two Co2 centers (ca. 11% for both 1 and 2) has been observed. This indicates that the magnetic exchange coupling has high contribution from bridging azide and Co centers to the α-HOMO. This is also obvious from the isodensity plot of α-HOMO (Figure 8).

Figure 8.

Figure 8

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Isodensity plots of α-HOMOs of (a) complex 1 and (b) complex 2.

Table 3. Frontier Molecular Orbital Energies (eV) and Compositions (%) of the Complex 1.

    contribution
α-MO energy Co1 Co2 Co3 Co4 SB1a SB2a L1b L2b N3_brg N3_1 N3_2
L + 9 –2.1 7 0 7 0 40 40 3 3 0 0 0
L + 8 –2.12 11 4 11 4 33 33 2 2 0 0 0
L + 7 –3.2 0 0 0 0 0 0 49 49 0 0 0
L + 6 –3.21 0 0 0 0 0 0 49 49 0 0 0
L + 5 –3.99 1 0 1 0 0 0 49 49 0 0 0
L + 4 –3.99 0 0 0 0 0 0 49 49 0 0 0
L + 3 –4.12 29 0 29 0 6 6 5 5 0 10 10
L + 2 –4.13 29 0 29 0 6 6 4 4 0 10 10
L + 1 –4.17 32 0 32 0 17 17 0 0 0 0 0
LUMO –4.18 32 0 32 0 17 17 0 0 0 0 0
SOMO1 –7.53 1 1 1 1 48 48 0 0 0 0 0
SOMO2 –7.56 1 1 1 1 48 48 0 0 0 0 0
SOMO3 –7.64 1 1 1 1 48 48 1 1 0 0 0
SOMO4 –7.71 3 0 3 0 1 1 0 0 0 46 46
SOMO5 –7.72 3 0 3 0 2 2 0 0 0 45 45
SOMO6 –7.85 1 1 1 1 46 46 1 1 0 1 1
HOMO –8.25 0 11 0 11 8 8 1 1 68 0 0
H – 1 –8.37 4 1 4 1 2 2 2 2 27 28 28
H – 2 –8.38 5 0 5 0 2 2 3 3 0 40 40
H – 3 –8.39 2 3 2 3 6 6 2 2 55 11 11
Open in a new tab
a

SB1 and SB2: Reduced Schiff base ligands.

b

L1 and L2: Nitrobenzoate ligands.

Table 4. Frontier Molecular Orbital Energies (eV) and Compositions (%) of Complex 2.

    contribution
α-MO energy Co1 Co2 Co3 Co4 SB1a SB2a L1b L2b N3_brg N3_1 N3_2
L + 9 –2.1 12 12 0 0 36 36 2 2 0 0 0
L + 8 –2.12 14 14 5 5 29 29 1 1 0 0 0
L + 7 –3.2 0 0 0 0 0 0 49 49 0 0 0
L + 6 –3.21 0 0 0 0 0 0 49 49 0 0 0
L + 5 –3.99 0 0 0 0 0 0 49 50 0 0 0
L + 4 –3.99 0 0 0 0 0 0 50 49 0 0 0
L + 3 –4.12 30 29 0 0 8 8 4 4 0 9 9
L + 2 –4.13 30 30 0 0 9 9 3 3 0 8 8
L + 1 –4.17 32 32 0 0 15 15 1 1 0 2 2
LUMO –4.18 32 32 0 0 14 14 1 1 0 3 3
SOMO1 –7.53 1 1 1 1 48 48 0 0 0 0 0
SOMO2 –7.56 1 1 1 1 48 48 0 0 0 0 0
SOMO3 –7.64 1 1 1 1 48 48 1 1 0 0 0
SOMO4 –7.71 3 3 0 0 2 2 0 0 0 46 45
SOMO5 –7.72 3 3 0 0 1 1 0 0 0 46 46
SOMO6 –7.85 1 1 1 1 47 47 1 1 0 1 1
HOMO –8.25 0 11 0 10 9 9 2 2 63 1 1
H – 1 –8.37 5 6 0 0 2 2 3 3 1 39 39
H – 2 –8.38 5 5 0 0 3 3 3 3 2 39 39
Open in a new tab
a

SB1 and SB2: Reduced Schiff base ligands.

b

L1 and L2: Nitrobenzoate ligands.

Magnetism and Spin Population Analyses

The magnetic coupling mechanism in the tetranuclear mixed-valence cobalt(III/II) complexes has been studied using DFT. The value of J has been calculated using X-ray characterized geometries, and excellent agreement has been found for both complexes (J = −13.07 cm–1 in 1 and −12.49 cm–1 in 2).

Graphical representation of spin population of both complexes is shown in Figure 9. The spin density at the cobalt(II) centers is ∼2.7 e– in both complexes (high spin value), thus confirming their oxidation states as +2, with some spin density being delocalized on the ligand (to the atoms directly bonded to cobalt). The spin densities calculated using the broken-symmetry approach of +2.75 e– on one cobalt(II) and −2.75 e– on the other cobalt(II) center confirm that they are the magnetic centers.

Figure 9.

Figure 9

Open in a new tab

Graphical representation of the spin densities (contour 0.002 e Å–3) on complex 1 (a) and complex 2 (b).

Conclusions

The synthesis of mixed-valence cobalt compounds is an interesting research topic because of the potential application of these complexes as magnetic materials. In the current work, two tetranuclear mixed-valence cobalt(III/II) compounds containing CoIII–(O)2–CoII–(N3)–CoIII–(O)2–CoII cores have been synthesized using two N2O2O′2 donor compartmental “reduced Schiff base” blocking ligands. They have been characterized by elemental and spectral analyses. The structures have been confirmed by the single-crystal X-ray diffraction study. Smaller cobalt(III) was placed in the inner N2O2 compartment, and larger cobalt(II) was placed in the outer O2O′2 compartment in both complexes. In each compound, cobalt centers exhibit pseudo-octahedral geometry as confirmed by SHAPE analysis. Oxidation states of cobalt centers were ascertained from charge balance the Co–N and Co–O bond distances and bond valence sum (BVS) calculations and also from calculated (using DFT) spin density at different cobalt centers. Magnetic studies demonstrated that in each complex both cobalt(II) centers are antiferromagnetically coupled through the μ1,3-azide bridge with J = −11.0 cm–1 (for complex 1) and −14.4 cm–1 (for complex 2). This was well supported by the DFT calculations. This observation will obviously open up new interesting possibilities in the synthesis of similar mixed-valence cobalt(III/II) compounds of different nuclearity and diverse magnetic properties.

Acknowledgments

A.B. thanks the UGC, India, for awarding a Junior Research Fellowship. The authors also thank the Spanish MINECO (project CTQ2017-87201-P AEI/FEDER, UE) and the Generalidad Valenciana (Prometeo/2019/076) for financial support.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02764.

  • X-ray powder diffraction analysis; experimental and simulated X-ray powder diffractograms for compounds 1 and 2; bond length and angles, IR and UV-Vis spectra; SHAPE analysis of the coordination geometry of Co1 and Co2; isodensity plot of FMOs of complex 1; isodensity plot of FMOs of complex 2 (PDF)

  • Crystallographic data (CIF) (CIF)

The authors declare no competing financial interest.

Supplementary Material

ao9b02764_si_001.pdf (675.1KB, pdf)
ao9b02764_si_002.cif (2.6MB, cif)
ao9b02764_si_003.cif (1MB, cif)

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Supplementary Materials

ao9b02764_si_001.pdf (675.1KB, pdf)
ao9b02764_si_002.cif (2.6MB, cif)
ao9b02764_si_003.cif (1MB, cif)

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