Hongdao
Li
*a,
Lu
Xi
b,
Pei
Jing
b,
Jianke
Tang
a,
Qi
Wang
a,
Hong
Yang
a,
Lijun
Zhai
a,
Yulan
Niu
a,
Lifeng
Ding
a and
Zhenjun
Song
*bc
aDepartment of Chemistry and Chemical Engineering, Taiyuan Institute of Technology, Taiyuan 030008, China. E-mail: lihong.dao@163.com
bDepartment of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry and Tianjin Key Laboratory of Metal and Molecule-based Material Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China. E-mail: songzj@mail.nankai.edu.cn
cSchool of Pharmaceutical and Materials Engineering, Taizhou University, Taizhou 318000, China
First published on 18th November 2020
Taking advantage of a nitronyl nitroxide radical ligand 8-QNNIT (1) (8-quinolyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and Ln(hfac)3·2H2O (LnIII = Tb 2 and Dy 3; hfac = hexafluoroacetylacetonate), two Ln–radical one-dimensional chains [Ln(hfac)3(8-QNNIT)]n were constructed. The magnetic properties, thermodynamics and optical behavior of both compounds were observed, exhibiting polyfunctionality of the 2p–4f system. Complexes 2 and 3 show typical single-chain magnet (SCM) properties with temperature-dependent relaxation peaks giving an effective energy barrier (Ueff/kB) of 47.4 K (2) and 64.6 K (3). Luminescence spectra and ultravioletvisible (UV-vis) absorption spectra of the Tb compound, and UV-vis absorption spectra and thermodynamics properties of the Dy complex were investigated. As far as we know, the Dy complex is the first SCM with thermodynamics behavior and optical properties comparable to those of multifunctional magnetic materials. This study aims to provide a new way and field of vision for the construction of 2p–4f heterospin multi-functional materials.
On the other hand, compared to an intensive study of the magnetic properties, thermodynamics behavior and luminescence properties of lanthanide–radical complexes have rarely been explored. Moreover, in recent years, molecules with multifunctional functionalities are currently of great interest in coordination chemistry. For example, Deun et al. prepared a series of 4f-based nanosheets for optical luminescence thermometry.8 And the dual functionality of an Yb compound for slow magnetic relaxation and luminescence thermometry was explored.9 Quite lately, tetranuclear lanthanide complexes based on a functionalized nitronyl nitroxide biradical, displaying slow magnetic relaxation behavior and luminescence performance, were obtained successfully.10 In addition, Vaz et al. constructed lanthanide compounds with a dppnTEMPO radical displaying two relaxation processes and the presence of two distinct luminescent centers.11 However, to date, lanthanide–nitronyl nitroxide complexes with SCM propertyies, thermodynamics or/and optical behavior coexisting in single molecules have not been reported.
In this regard, to investigate multifunctional properties of 2p–4f heterospin SCMs, herein, we synthesize a new nitronyl nitroxide radical 8-QNNIT (1) (8-quinolyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and utilize the radical ligand to achieve two heterospin 1D compounds, namely, [Ln(hfac)3(8-QNNIT)]n (LnIII = Tb 2, Dy 3; hfac = hexafluoroacetylacetonate). Additionally, the magnetic, optical and thermodynamics properties of the 2p–4f system have been systematically investigated. Tb and Dy compounds display frequency-dependent out-of-phase signals indicating SCM behavior. Moreover, the Tb–radical chain shows the characteristic emission peaks of metal TbIII ions. The heat capacity of the Dy compound is also studied in the paper. To our knowledge, this is the first report on nitronyl nitroxide-based LnIII compounds with SCM properties and optical or/and thermodynamics behavior coexisting in a molecular entity.
Complex | 1 | 2 | 3 |
Empirical formula | C16H18N3O2 | C31H21F18N3O8Tb | C31H21F18N3O8Dy |
M r | 284.33 | 1064.42 | 1067.98 |
T (K) | 293(2) | 150(2) | 150(2) |
Crystal system | Orthorhombic | Monoclinic | Monoclinic |
Space group | Pca21 | P21/c | P21/c |
a/Å | 15.2203(3) | 22.868(5) | 22.7736(11) |
b/Å | 8.09480(16) | 16.664(3) | 16.6481(8) |
c/Å | 11.9026(3) | 22.826(5) | 22.7247(11) |
α/° | 90 | 90 | 90 |
β/° | 90 | 117.20(3) | 117.2050(10) |
γ/° | 90 | 90 | 90 |
V/Å3 | 1466.46(5) | 7736(3) | 7662.7(6) |
Z | 4 | 4 | 4 |
D calcd/g cm−3 | 1.288 | 1.828 | 1.852 |
μ/mm−1 | 0.703 | 1.966 | 2.090 |
θ/° | 5.46–67.23 | 2.890–25.906 | 2.015–26.420 |
F(000) | 604 | 4152 | 4160 |
Reflections collected | 8594 | 78![]() |
83![]() |
Unique reflns/Rint | 2494/0.0235 | 14![]() |
15![]() |
GOF (F2) | 1.055 | 1.058 | 1.086 |
R 1, wR2 (I > 2σ(I)) | 0.0308, 0.0829 | 0.0729, 0.1822 | 0.0613, 0.1656 |
R 1, wR2 (all data) | 0.0323, 0.0837 | 0.1075, 0.2062 | 0.0837/0.1829 |
Complex | 2 | 3 |
---|---|---|
Ln–O(hfac) | 2.345(6)–2.393(7) | 2.322(5)–2.384(5) |
Ln–O(rad) | 2.333(6)–2.389(7) | 2.316(5)–2.384(5) |
Ln–O–N | 137.2(6)–149.2(6) | 137.5(4)–148.7(4) |
Crystallographic studies manifest that isostructural 2 and 3 are crystallized in monoclinic space group P21/c (Fig. 2 and Fig. S2, ESI†). Taking compound 3 for example, the compound displays a one-dimensional 4f-system, which is supported by 8-QNNIT radicals. Each DyIII ion ligates two oxygen atoms of NO groups, and the rest of the coordination sphere is completed with six oxygen atoms by using three hfac− coligands. Both crystallographically independent Dy(hfac)3(8-QNNIT) moieties belong to different 1D chains. The SHAPE measure analysis16 gives these values (0.372 (Dy1) and 0.246 (Dy2)) for a triangular dodecahedron geometry (D2d symmetry) (Fig. S5 and Table S3, ESI†). The DyIII–Ohfac bond lengths are in the range of 2.322(5)–2.384(5) Å, while DyIII–Orad distances vary in the range of 2.316(5)–2.384(5) Å.
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Fig. 2 Single-crystal X-ray diffraction structure of complex 3, in which H and F atoms are omitted for clarity. |
As can be found, Dy–radical chains are well separated (Fig. 3), with the nearest Dy⋯Dy distance being 11.65 Å for 3, which can avert significant intermolecular interactions.
χM = (C/T)[(1 + 5.7979916K + 16.902653K2 + 29.376885K3 + 14.036981K5)/(1 + 2.7979916K + 7.0086780K2 + 8.6538644K3 + 4.5743114K4)]2/3 |
The best fit values are J = 0.11 cm−1, zJ′ = −0.009 cm−1, C = 0.3755 emu K mol−l, and g = 2.0. In addition, the reciprocal susceptibility vs. the temperature abides by the Curie–Weiss law (2–300 K) with Weiss constant θ = 0.152 K and Curie constant C = 0.40 cm3 K mol−1, implying the existence of ferromagnetic interactions.
The observed positive values of J and θ may result from dominant ferromagnetic coupling, derived from the pseudo 1D chain (Fig. 5), in which the bridging sp2 carbon atoms (C10) and the oxygen atoms (O1) from the NO groups of the neighboring 8-QNNIT radicals carry the opposite sign spin densities, following McConnell's law.17 The negative value of zJ′ arises from short NO–ON contacts and π⋯π interactions.
The static direct-current (dc) magnetic susceptibility properties for compounds 2 and 3 were measured under a dc field of 1 kOe from 300 to 2 K (Fig. 6). At 300 K, the χMT values are 12.27 cm3 K mol−1 for 2 and 14.94 cm3 K mol−1 for 3, which are slightly more than the theoretical values (12.19 cm3 K mol−1 for 2 and 14.54 cm3 K mol−1 for 3) for one non-interacting LnIII ion (TbIII: 7F6, S = 3, L = 3, g = 3/2, C = 11.82 cm3 K mol−1; DyIII: 6H15/2, S = 5/2, L = 5, g = 4/3, C = 14.17 cm3 K mol−1) and one 8-QNNIT radical (S = 1/2, C = 0.375 cm3 K mol−1).
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Fig. 6 Plots of χMT–T for 2 (left) and 3 (right). Insets: ln(χMT)–1/T plots. The red solid lines represent the best fits. |
Upon cooling, the χMT value of compound 2 decreases steadily to 7 K and the value of 3 remains almost constant above 100 K. Then χMT values go down, followed by an abrupt increase for both complexes, and finally drop to 6.07 cm3 K mol−1 for 2 and 4.52 cm3 K mol−1 for 3 at 2.0 K. At low temperature, the χMT values of both compounds display a degree of rise, suggesting the presence of nearest-neighbor heavy lanthanide metal–radical ferromagnetic coupling (Fig. 6).
For the Ising-like or anisotropic Heisenberg 1D chain, the value of χMT could follow an exponential behavior (χMT ≈ Ceffexp(Δξ/T)), in which the energy needed to create a domain wall along the chain is represented by Δξ.1a For 2 and 3, the ln(χMT) vs. 1/T plot displays a linear region between 3.5 and 4.5 K with Δξ = 1.35 K for 2 and between 4.5 and 6 K with Δξ = 2.54 K for 3, verifying the 1D Ising-like character (Fig. 6, inset).
The isothermal field-dependent magnetization data for Tb and Dy compounds display continuous increases up to 4.86 and 6.66 Nβ at 2 K and 70 kOe (Fig. 7 and Fig. S6, ESI†) with the lack of high-field saturation, illustrating the crystal-field effects and low-lying excited states. Besides, the non-superposition of the M vs. H/T plots at different temperatures (2 K, 3 K and 5 K) is indicative of the presence of significant magnetic anisotropy (Fig. 7 and Fig. S7, ESI†).
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Fig. 7 Field dependences of magnetization for compound 3 at temperatures of 2, 3, and 5 K. Insets: Plots of the reduced magnetization M versus HT−1. |
It is worth mentioning that the M vs. H curve of the Dy complex displays a sigmoid, suggesting a metamagnetic system. The dM/dH curve shows that a three-step field induced transition at critical fields of H1 = 93 Oe, H2 = 10 kOe, and H3 = 15 kOe (Fig. 8).
At around 93 Oe, a transition is to surmount relatively weak interchain antiferromagnetic couplings. The second metamagnetic step is related to the transformation of the system from the spin-canted phase to the spin-flop state.18 The third step shows that when the magnetic field is larger than the critical field, the intrachain antiferromagnetic interactions can be induced to transform to ferromagnetic couplings.19 Moreover, there is a narrow hysteresis at 2 K for the Dy complex (Fig. 9), indicating that a dramatic slowing down of the relaxation time does not occur as the temperature is lowered below critical temperature.
Toward further understanding the spin dynamic behaviors of compounds 2 and 3, alternating current magnetic susceptibilities in the variable-temperature and variable-frequency patterns were implemented. In the absence of a dc field, out-of-phase (χ′′) signals display temperature- and frequency-dependence obviously, revealing the slow magnetization relaxation (Fig. 10 and Fig. S8–S13, ESI†). To rule out the possibility of glassiness, the parameter φ was evaluated (φ = (ΔTp/Tp)/Δ(logf)).20 The results show that the φ value is 0.191 for 2 and 0.090 for 3 to exclude the possibility of a spin-glass (0.01 < φ < 0.08).21
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Fig. 10 Frequency dependence of the out-of-phase (χ′′) for 2 (left) and 3 (right) under zero dc field. The solid lines represent the best fitting. |
The equation τ = τ0exp
(Δτ/kBT) was employed to analyze the relaxation time under 0 Oe DC fields (Fig. 11), giving τ0 = 1.63(5) × 10−8 s, Δτ/kB = 47.4(8) for compound 2 and τ0 = 5.26(8) × 10−12 s, Δτ/kB = 64.6(1) K for compound 3, which correspond to the values reported for rad-4f SCMs.6e,22 This energy barrier (Δτ) is made up of two parts, namely the correlation energy (Δξ) and the blocking energy (ΔA).23 And the latter is about 46.05 K for 2 and 62.06 K for 3, which are related to the magnetic anisotropy of the 4f ion unit.
The Cole–Cole plots (Fig. 12) can be fitted by the Debye model to afford α factors in the range of 0.14–0.19 for compound 2 with a middle distribution and 0.050–0.601 for compound 3 with a wide distribution.
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Fig. 13 UV-vis absorption (purple line) and luminescence (green line, emission) spectra of compound 2 in dichloromethane. |
As depicted in Fig. 14, the 8-QNNIT radical ligand shows three absorption bands around 238, 318 and 390 nm, on account of the π–π* and n–π* transitions of the aromatic nucleus of the radical. For Dy(hfac)3·2H2O, an intense absorption around 299 nm is observed. Complex 3 exhibits weak absorption bands at 237, assigned to the radical ligand, which is blue shifted slightly compared with the 8-QNNIT radical due to the influence of DyIII ion coordination. And, the strong absorption of compound 3 around 299 nm is related to the hfac− coligand. Besides, the wavelengths of the UV-vis absorption peaks of the Tb complex are 225 and 299 nm, respectively (Fig. 13).
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Fig. 15 Temperature dependence of molar heat capacities for compound 3. The solid line represents the best fitting. |
in which X denotes the reduced temperature (X = [T − (Tmax + Tmin)/2]/[(Tmax − Tmin)/2]), T represents the experimental temperature, and Tmax and Tmin are the maximum and minimum temperatures (346.71 and 263.01 K).
4f-Based 1D complex | The minimum distance between interchain 4f ions (Å) | Energy barrier (K) | Relaxation time (s) | Ref. |
---|---|---|---|---|
{[DyCu(hfac)5(NIT-Ph-p-OCH2trz)]·0.5C6H14} | 9.75 | 29.0 | 6.1 × 10−10 | 6d |
[Dy(hfac)3NIT3BrPhOMe]n | 10.13 | 39.8 | 2.43 × 10−10 | 27 |
[Dy(hfac)3NITPh(OMe)2]n | 11.53 | 70.3 | 8.68 × 10−15 | 6c |
[Dy(hfac)3(8-QNNIT)]n | 11.98 | 64.6 | 5.26 × 10−12 | This work |
On the other hand, in the field of 4f-based hetero-spin complexes involving nitronyl nitroxide, compared with well-studied SMM and SCM behaviors, the thermodynamics behavior and optical properties of lanthanide complexes have rarely been documented and multifunctional molecular materials are still in their infancy (Table 4). As far as we know, [Ln(hfac)3(8-QNNIT)]n (LnIII = Tb 2, Dy 3) is the first SCMs with thermodynamics behavior as multifunctional magnetic materials.
Multifunctional molecular materials | Diverse properties | Ref. | ||
---|---|---|---|---|
[Tb(acac)3NIT2Py·0.5H2O] | Luminescence | SMM behavior | 28 | |
[Ln2(hfac)6(H2O)2(dppnTEMPO)] (LnIII = Tb, Dy) | Luminescence | Field induced SMM behavior | 9 | |
Ln(hfac)3(NITPhOCF3)2 (LnIII = Tb, Dy) | Thermodynamics | Slow magnetic relaxation | 25 | |
[{Ln(hfac)3}3{Cu(hfac)2}{NIT-Ph(OMe)2}4]n (LnIII = Gd, Tb) | Magnetocaloric effect | Slow magnetic relaxation | 29 | |
[Ln(hfac)3(8-QNNIT)]n (LnIII = Tb, Dy) | Optical property | SCM behavior | Thermodynamics | This work |
Lanthanide SCMs/SMMs as multifunctional molecular magnetic materials, such as luminescent magnets, magnetoelectric materials, conducting magnets, chiral magnets, magnetic refrigeration materials, etc., displaying specific functionalities, have gradually caused extensive concern in the molecular material field on account of their potential applications in magneto-luminescence sensing, luminescence thermometry,8,9 photocatalysis,30 single molecule detection, magnetic cooling technology, etc. Among them, photomagnets act as a kind of multifunctional magnetic material, in which the magnetic properties can be regulated or/and controlled by photons, arousing great interest of scientific research. From the point of view of practical application, photomagnets can be extended to the Ln–radical system for designing multifunctional materials involving nitronyl nitroxide, which may become the research focal point in the future.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2024373–2024375. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0tc04266h |
This journal is © The Royal Society of Chemistry 2021 |