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Abstract

Coordination complexes constructed from the nitronyl nitroxide radical NIT-Pyra-3-Isobu and M^II(hfac)₂(H₂O)₂ as building blocks (NIT-Pyra-3-Isobu = 2-(3-isobutyl-pyrazole)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, hfac = hexafluoroacetylacetonato, and M = Mn or Co) are successfully synthesized. The crystal structures of the coordination compounds reveal the usefulness of the functionalized radical in providing discrete or extended architectures. Single-crystal X-ray diffraction analyses indicated that the ligand is coordinated through the oxygen atom of NO• and the N atom of the pyrazole linkage to the metal, and the metals adopt an octahedral geometry leading to one-dimensional zig-zag chain systems. The magnetic behavior of the manganese and cobalt shows overall antiferromagnetic interactions between the metal center and the organic radical.

Keywords

magnetic properties nitronyl nitroxide radical X-ray crystal structure

Introduction

Studies on metal−organic radical complexes have attracted more and more attention in recent decades due to the intriguing magnetic interactions between metal ions and organic spin carriers.^1–12 Among the commonly used organic radicals, nitronyl nitroxide radicals (NITRs) are particularly attracting because they are stable, easy to synthesize, and participate in numerous coupling pathways that favor the onset of bulk magnetic properties in the solid state.^13–18 However, the popularity of nitronyl nitroxides mainly stems from their coordination properties. Their metal complexes are characterized by strong direct exchange couplings between organic and metal unpaired electrons, and their properties are highly dependent on the structure of the free radical and on the nature of the metal ion. In addition, modification of the substituted R groups of NITR with functional coordination groups can generate various metal–radical complexes with diverse magnetic properties. For example, three-dimensional (3D) metal–NITR complexes with different R groups can form, such as mono- and poly-nuclear complexes, as well as one-dimensional (1D) and higher dimensional architectures.^19–27

Herein, we have synthesized the nitronyl nitroxide ligand 2-(3-isobutyl-pyrazole)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (NIT-Pyra-3-Isobu) (Scheme 1). And then, we report the syntheses, crystal structures, and magnetic properties of two complexes which were constructed from NIT-Pyra-3-Isobu and M^II(hfac)₂(H₂O)₂ as building blocks, where M = Mn or Co.

Scheme 1.

Structures of the organic radical NIT-Pyra-3-Isobu and the inorganic complexes M^II(hfac)₂: M = Mn or Co.

Results and discussion

Crystal structures of complexes 1 and 2

The molecular structures of 1 and 2 were successfully determined by means of X-ray crystallographic analysis at 296 K (Table 1 shows selected crystallographic data). Complexes 1 and 2 crystallize in the monoclinic space group C2/c with Z = 8. For both compounds, the ligand NIT-Pyra-3-Isobu coordinated through the oxygen atom of the NO• and the N atom of pyrazole linking to the metal, leading to a zig-zag chain, as depicted in Figure 1 (bottom). The geometry of the pyrazole functional groups is analogous to other related transition metal complexes. As shown in Figure 1 (bottom), the asymmetric unit of complex 1 consists of two half of manganese atoms, one with a nearly octahedral coordination (Mn1) and hfac in trans-positions which are occupied by four oxygen atoms (O3 and O4) from two hfac units, two oxygen atoms (O1) from two nitroxide ligands, and the other is of elongated octahedron geometry (Mn2) with hfac oxygen atoms (O5 and O6) at the base and one pyrazole nitrogen atom (N3) at the apex. The Mn–O(radical) bond length is 2.158(2) Å, while the Mn–O(hfac) are closer than the bond and the Mn–N_pyra bond length is 2.339(3) Å. The angle of N–O_Rad–Mn is 129.9(6)°. The packing arrangement of the chains of 1 is shown in Figure 2. The nearest intrachain metal–metal (Mn•••Mn) distance is 8.055(3) Å, while the interchain distance is 9.893(9) Å. In addition, there are two slightly longer intermolecular F•••F interactions (F2•••F4 = 3.19(6) Å and F9•••F9 = 3.177(7) Å), which bridge the chains into layers as shown in Figure 3.

Table 1.

Crystallographic data and refinement parameters for complexes 1 and 2 at 296 K.

	1	2
Formula	C₂₄H₂₅MnF₁₂N₄O₆	C₂₄H₂₅CoF₁₂N₄O₆
Formula weight	748.42	752.41
Crystal system	Monoclinic	Monoclinic
Space group	C2/c	C2/c
a (Å)	30.6644(16)	30.529(6)
b (Å)	9.8939(5)	9.7875(18)
c (Å)	26.1398(14)	26.164(5)
β (°)	125.7454(13)	125.110(5)
V (Å³)	6436.6(6)	6395(2)
D_c (g cm⁻³)	1.545	1.563
Z	8	8
Reflections total	60,973	49,519
Unique parameter	7902	7946
	450	433
R_int	0.0346	0.0314
R₁/wR₂ (I>2σ(I))	0.0737	0.0702
	0.2189	0.2067
R₁/wR₂ (all data)	0.1184	0.0992
	0.2553	0.2355
Goodness of fit	1.039	1.032
Completeness	0.985	0.995
Theta (°)	28.297	28.341

Figure 1.

The X-ray crystal structure complex 1. Structural ellipsoids are drowned at 50% probability (top). Fragment of the chain (bottom) of complex 1. Fluorine, hydrogen, and some of the carbon atoms are omitted for clarity.

Figure 2.

The crystal packing details of compound 1. Fluorine, hydrogen, and some of the carbon atoms are omitted for clarity.

Figure 3.

Details of the crystal packing of complex 1 of the F•••F interchain short contacts are shown in green. Hydrogen and some fluorine and carbon atoms are omitted for clarity.

For complex 2, the Co–O(radical) bond length is 2.151(3) Å, while the Co–O(hfac) are close to the bond and the Co–N_pyra bond length is 2.280(3) Å. The angle of N–O_Rad–Co is 127.8(8)°. The nearest intrachain metal–metal (Co•••Co) distance is 8.015(1) Å, while the interchain distance is 10.978(1) Å.

Magnetic properties

The temperature-dependent magnetic susceptibilities of the complexes were measured in the temperature range 2–300 K in an external magnetic field of 5000 Oe, and the data are corrected for the diamagnetism of the components.

The thermal dependence of the χ_m (square) and χ_mT (circle) product for complex 1 are depicted in Figure 4. For complex 1, χT is equal to 3.68 cm³ K mol⁻¹ at 300 K, a value much lower than that expected for non-interaction S_Mn = 5/2 and S_rad = 1/2 spins. As the temperature is lowered to 6 K, χT gradually decreases to reach a plateau value of 3.00 cm³ K mol⁻¹. This value is close to the S = 2 state expected for isolated Mn^II ions antiferromagnetically coupled to a nitroxide radical. The gradual increase of the χT values at high temperatures indicates thermal population of a higher spin excited state of S = 3, reaching a room temperature value intermediate between those corresponding to S = 2 and S = 3.^28,29 Below 5 K, the decrease of the χT value can be attributed to weak intermolecular antiferromagnetic interactions. The reciprocal susceptibility versus temperature was fitted to the Curie–Weiss law leading to Curie and Weiss constants of C = 3.13 cm³ K mol⁻¹ and θ = −2.6 K. Although complex 1 crystallizes as a 1D framework, the magnetic data were reproduced by a simplified model, considering an isotropic interaction between one manganese and one nitroxide radical (Ĥ = −J (S_Mn • S_rad)) and a mean field approximation in order to account for the weak magnetic interactions between Mn–Rad dimers (equations (1) and (2)).^29,30 The solid line in Figure 4 shows the best fit achieved with J = −42.66 cm⁻¹ and zJ’ = −0.03 cm⁻¹. The J value is in good agreement when compared to similar compounds in which the oxygen atom of the nitroxide group is coordinated to an Mn^II ion.^29–31 A very weak intermolecular antiferromagnetic interaction exists at low temperature, which was indicated by the negative zJ’ value. Complex 1 reaches an almost saturated value of 3.9 Bohr magnetons at a magnetic field of 5 T (see Figure 6, circle) at 2 K. The magnetic data for the complex 1 show that it behaves as expected for an S = 2 Mn^II–Rad antiferromagnetic dimer, being close to theory expected (4 Nµ_B).

Figure 4.

Temperature dependence of χ_m (squares) and χ_mT (circles) for complex 1. The solid lines show the best fits.

χ_{m} = \frac{N β^{2} g^{2}}{κ T} [\frac{28 + 10 \exp (- 6 J / κ T)}{7 + 5 \exp (- 6 J / κ T)}]

(1)

{χ^{'}}_{m} = \frac{χ_{m}}{1 - z J^{'} χ_{m}}

(2)

The thermal dependence of the χ_m (square) and χ_mT (circle) product for complex 2 is depicted in Figure 5. The χ_mT value at 300 K is 3.63 cm³ K mol⁻¹, which is close to the expected value for one Co^II center with one radical ligand.³² Upon decreasing the temperature from 300 K, the χ_mT value decreases slightly to 90 K. Below 90 K, χ_mT(T) for 2 drops precipitously to 0.76 cm³ K mol⁻¹ at 2 K. The Curie constant and Weiss constant were estimated to be 3.58 cm³ K mol⁻¹ and −6.34 K from the Curie–Weiss fit of the reciprocal susceptibility versus temperature curve for 2 (blue line). The Curie constant is again consistent with the sum of the individual carriers considering the orbital contribution in the case of Co(II). However, the Weiss constant reflects the effects of both exchange and spin–orbit for Co(II).^2,33 It is generally difficult to separate the two from susceptibility measurements. The antiferromagnetic interactions among the spin carriers are weak. The solid line in Figure 6 shows the best fit achieved with g_Co = 2.63, g_rad = 2.00 (fixed), and J = −14.98 cm⁻¹.^34–36 The fitting results indicate that the magnetic coupling between the Co(II) atom and the coordinated nitroxide radical is weakly antiferromagnetic. Furthermore, the field dependence of magnetization has been determined at 2 K in the 0–50 kOe field range (squares in Figure 6). Upon increasing the applied field, M increases up to 1.69 Nµ_B at 50 kOe, but does not reach saturation.

Figure 5.

Isothermal magnetization at 2 K of complexes 1 and 2.

Figure 6.

Temperature dependence of χ_m (squares) and χ_mT (circles) for complex 2. The solid lines show the best fits.

Conclusion

1D chain complexes {(NIT-Pyra-3-Isobu)[Mn(hfac)₂]}_n and {(NIT-Pyra-3-Isobu)[Mn(hfac)₂]}_n based on metal(II) and NITRs have been successfully prepared. The single-crystal structures show that both compounds exhibit a 1D chain structure in which Co or Mn ions are linked by the oxygen atoms of NO• groups and the N atoms of the pyrazole rings. The magnetic studies revealed antiferromagnetic interactions for both compounds and the magnetic data were fitted using simplified models through metal–radical pairs. Their magneto-structural correlations have been explained based on the crystal structures.

Experimental

Materials and methods

All reagents were obtained from commercial sources and used without further purification. The nitroxide radical was prepared as reported previously with only minor modifications (Scheme 2).^24,37–39

Scheme 2.

Synthesis of the NIT-Pyra-3-Isobu radical.

General characterizations

Infrared spectra were recorded on a JASCO FT/IR-660 PLUS spectrometer by transmission through KBr pellets in the range 400–4000 cm⁻¹. Elemental analyses for C, H, and N were carried out using a PerkinElmer series II CHNS/O Analyzer 2400. The magnetization measurements on the crystalline solids were carried out using Quantum Design MPMS-5S and MPMS-2 SQUID magnetometers. The magnetic field was varied from −50 to 50 kOe and the temperature from 2 to 300 K. The data are corrected for sample diamagnetism using Pascal’s constants.^40,41

1-Isobutyl-1H-pyrazole-4-carboxaldehyde

1-Isobutylpyrazole (12.52 g, 0.1 mol) was dissolved in DMF (36 g, 0.5 mol), and POCl₃ was added dropwise (30.66 g, 0.2 mol). The reaction mixture was stirred for 5 h at 70–75 °C and then poured in to cold water (200 mL). The pH of the solution was brought to 7 or 8 with saturated NaHCO₃ solution. The precipitate was filtered of and dried in air. The yield is 8.87 g (58%).

Preparation of NIT-Pyra-3-Isobu

A solution of 2,3-bis-(hydroxyamino)-2,3-dimethylbutane sulfate hydrate (3.11 g, 11.7 mmol) in water (50 mL) and a mixture of 1-isobutyl-1H-pyrazole-4-carboxaldehyde (1.39 g, 10.0 mmol) in Et₂O (15 mL) was stirred for 4 h at room temperature and then treated with NaHCO₃ until CO₂ ceased to evolve. The resulting precipitate was filtered off, washed with water and acetone, and dried in a vacuum desiccator. The obtained adduct (1.32 g, 4.9 mmol) was stirred in CH₃OH (7 mL) in the presence of MnO₂ (6.6 g, 75.9 mmol) for 1.5 h. The reaction mixture was filtered and the precipitate washed with CH₃OH. The filtrate was evaporated and the residue dissolved in a minimum amount of ethyl acetate. The resulting solution was filtered through a silica gel layer (1.5 cm × 15 cm) and evaporated. Yield: 1.17 g (84%). FTIR (KBr, cm⁻¹): ν = 3418 (m), 3124 (w), 2962 (s), 2875 (s), 1678 (s), 1585 (s), 1541 (m), 1510 (m), 1455 (s), 1396 (s), 1358 (s), 1321 (m), 1221 (m), 1173 (s), 1141 (s), 1058 (w), 1012 (w), 962 (w), 871 (m), 830 (m), 791 (w), 761 (w), 656 (w), 541 (m).

Preparation of Mn(hfac)₂·2H₂O

hfac (1 g, 7.1 mmol) was added to a suspension of Mn(CH₃COO)₂·H₂O (0.68 g, 3.55 mmol) in H₂O (17 mL) at room temperature and then NaHCO₃ (0.071 g, 0.8 mmol) was added with stirring. After 10 min, a light blue precipitate was obtained which was washed three times with n-hexane. The light blue solid was dried at 50 °C for 3 days to give Mn(hfac)₂·2H₂O (0.74 g) was obtained. FTIR⁴² (KBr, cm⁻¹): 3398 (s), 1618 (s), 1561 (s), 1534 (s), 1497 (s), 1250 (s), 1212 (s), 800 (s), 767 (w), 743 (w), 667 (s), 590 (s).

Preparation of Co(hfac)₂·2H₂O

The procedure for the synthesis of Co(hfac)₂·2H₂O was the same as that used for except that Co(CH₃COO)₂·H₂O. FTIR⁴² (KBr, cm⁻¹): 3367 (s), 1645 (s), 1563 (s), 1536 (s), 1488 (s), 1236 (s), 1144 (s), 806 (s), 746 (w), 676 (s), 590 (s).

Preparation of {(NIT-Pyra-3-Isobu)[Mn(hfac)₂]}_n (1)

Mn^II(hfac)₂·2(H₂O) (1 mmol, 51.5 mg) was dissolved in hot n-heptane 10 mL and then the solution was cooled to about 50 °C. NIT-Pyra-3-Isobu (1 mmol, 28.1 mg), dissolved in CHCl₃ (2 mL), was added with constant stirring. The solution was stirred for ca. 10 min and then cooled to room temperature. The filtrate was kept under an N₂ stream at room temperature until a solid appeared. A few drops of CHCl₃ were added until the solid disappeared and then the flask was sealed and placed in a refrigerator. After 2–3 days, dark blue crystals were obtained. Yield 81%. Anal. calcd for C₂₄H₂₅MnN₄O₆ are: C, 38.48; H, 3.34; N, 7.48; found: C, 38.04; H, 3.21; N, 7.51. FTIR (KBr, cm⁻¹): ν = 2969 (m), 1648 (s), 1595 (m), 1555 (m), 1532 (m), 1482 (s), 1399 (w), 1358 (m), 1263 (s), 1212 (s), 1148 (s), 872 (w), 850 (w), 800 (s), 769 (w), 747 (w), 676 (s), 532 (m).

Preparation of {(NIT-Pyra-3-Isobu)[Co(hfac)₂]}_n (2)

The method for the synthesis was similar to used for preparation of {(NIT-Pyra-3-Bu)[Mn(hfac)₂]}_n. Dark blue crystals in ca. 82% yield were obtained under similar conditions. Anal. calcd for C₂₄H₂₅CoN₄O₆ are: C, 38.28; H, 3.32; N, 7.44; found: C, 38.24; H, 3.26; N, 7.57. FTIR (KBr, cm⁻¹): ν = 2969 (m), 1645 (s), 1596 (m), 1559 (m), 1531 (m), 1482 (s), 1398 (w), 1358 (m), 1266 (s), 1211 (s), 1148 (s), 872 (w), 851 (w), 800 (s), 766 (w), 746 (w), 676 (s), 535 (m).

Single-crystal X-ray diffraction studies

X-ray diffraction intensities were collected for the selected single crystals individually mounted on glass fibers at room temperature using Bruker diffractometers: SMART-APEX II with a CCD and D8-QUEST with a CMOS area detector. Both employed graphite-monochromated MoKα (λ = 0.71073 Å) radiation. Data reduction was made using SAINT, and the intensities were corrected for absorption by SADABS.^43,44 The structures were solved by direct methods and refined by full-matrix least-squares against F² using ShelXL.⁴⁵ Part of the hydrogen atoms was located in the difference Fourier maps, and those not found were added at theoretical positions using the riding model. The details can be obtained from the cif files deposited at the Cambridge Crystallographic Data Centre. The crystal and refinement data are summarized in Table 1.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Science and Technology Program of Shaanxi Province (nos 2017GY-131 and 2018GY-086), the Doctoral Scientific Research Foundation of Yulin University (no. 18GK24), the Shaanxi Provincial Science and Technology Department Innovation Talents Promotion Plan Project of China (no. 2018KJXX-078), the Shaanxi Provincial Science and Technology Resources Open Sharing Platform Project (no. 2019PT-18), and the Project of Production and Study of Yulin City (no. 2016CXY-01).

ORCID iD

Yan-Li Gao

Supplemental material

Supplementary crystallographic data for this paper (CCDC 2063820 for complex 1 and CCDC 2063821 for complex 2). The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via .

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Synthesis,crystal structures,and magnetic properties of nitroxide radical-coordinated manganese(II) and cobalt(II) complexes

Abstract

Keywords

Introduction

Results and discussion

Crystal structures of complexes 1 and 2

Magnetic properties

Conclusion

Experimental

Materials and methods

General characterizations

1-Isobutyl-1H-pyrazole-4-carboxaldehyde

Preparation of NIT-Pyra-3-Isobu

Preparation of Mn(hfac)2·2H2O

Preparation of Co(hfac)2·2H2O

Preparation of {(NIT-Pyra-3-Isobu)[Mn(hfac)2]}n (1)

Preparation of {(NIT-Pyra-3-Isobu)[Co(hfac)2]}n (2)

Single-crystal X-ray diffraction studies

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

Supplemental material

References

Preparation of Mn(hfac)₂·2H₂O

Preparation of Co(hfac)₂·2H₂O

Preparation of {(NIT-Pyra-3-Isobu)[Mn(hfac)₂]}_n (1)

Preparation of {(NIT-Pyra-3-Isobu)[Co(hfac)₂]}_n (2)