Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Two Dy(III) Single-Molecule Magnets with Their Performance Tuned by Schiff Base Ligands Shui Yu,† Zhaobo Hu,† Zilu Chen,*,† Bo Li,*,‡ Yi-Quan Zhang,*,§ Yuning Liang,† Dongcheng Liu,† Di Yao,† and Fupei Liang*,†,#
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State Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry and Pharmaceutical Sciences, Guangxi Normal University, Guilin 541004, P. R. China ‡ College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, P. R. China § Jiangsu Key Laboratory for NSLSCS, School of Physical Science and Technology, Nanjing Normal University, Nanjing 210023, P. R. China # Guangxi Key Laboratory of Electrochemical and Magnetochemical Functional Materials, College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, P. R. China S Supporting Information *
ABSTRACT: To develop new lanthanide single-molecule magnets (SMMs), two new complexes of [Dy2(MeOH)2(HL1)2(NO3)2]·2MeOH (1) and [Dy6(μ3OH)2(L2)2(HL2)2(H2L2)2Cl2(EtOH)2]Cl2·3EtOH·CH3CN (2) were obtained by reacting Dy(NO)3·6H2O with 3-amino1,2-propanediol in the presence of 2-hydroxynaphthaldehyde for 1 and by reacting DyCl 3 ·6H 2 O with 1,1-di(hydroxymethyl)ethylamine in the presence of 2-hydroxynaphthaldehyde for 2, respectively, in which the Schiff base ligands of 3-(((2-hydroxynaphthaen-1-yl)methylene)amino)propane-1,2-diol (H3L1) and 2-(β-naphthalideneamino)-2(hydroxylmethyl)-1-propanol (H3L2) were in situ formed. The two Dy(III) ions in 1 are linked by two Oalkoxy atoms of two (HL1)2− ligands to build a dinuclear skeleton. Complex 2 presents a nearly planar hexanuclear skeleton constructed from four edge-shared triangular Dy3 units with the two peripheral Dy3 units consolidated by two μ3-O bridges and the two central Dy3 units consolidated by one μ3-O bridge. Obviously, they exhibit a different topological arrangement resulting from the linkage of the Schiff base ligands. Both of them are typical SMMs under zero dc fields, with a Ueff/kB value of 34 K for 1 and 40 K for 2, respectively. Multiple processes are involved in the relaxation processes of 1 and 2. The different SMM performances of the two titled complexes reveal a tuning effect of Schiff base ligands through tuning the coordination environments and topological arrangements of dysprosium(III) ions, which is supported by the theoretical calculations.
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INTRODUCTION Single-molecule magnets (SMMs) bearing high performance show wide application prospects in fields such as data storage, high-speed computing, and molecular spintronics,1−5 which has promoted explosive growth for SMM-based magnetic materials in the latest decades. Since the discovery of Mn12 cluster as the first SMM showing slow magnetic relaxation (SMR),6,7 many SMMs have sprang out, which include homonuclear and heteronuclear clusters.8−14 Recently, the development of high-performance lanthanide SMMs has drawn special attention, probably due to the coexistence of several aspects in lanthanide ions, such as large magnetic anisotropy, big magnetic moment, weak crystal-field splitting, and strong spin−orbit coupling, although the weak intermetallic magnetic interactions are unfavorable for improving the performance of lanthanide SMMs to some degree.12,15−21 Thus it is of great importance to optimize the nature of the lanthanide ion itself © XXXX American Chemical Society
by tuning its coordination environment and the weak intermetallic magnetic interactions by tuning the bridging groups. Much effort has already been made on lanthanide SMMs, especially on Dy(III) SMMs, reaching a relatively high energy barrier as well as a high blocking temperature.20−27 Although the extensive research in recent years has brought some guidelines and strategies for the rational design of new SMMs, it is still a challenging mission to undertake thorough studies for disclosing the intrinsic essence of lanthanide SMMs. As is well known, the rational design of coordination geometries and ligand fields for lanthanide ions is the key factor for constructing SMMs. The selection of appropriate ligands can help to achieve this goal not only by tuning the nature for the targeted lanthanide SMMs also including their Received: September 16, 2018
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DOI: 10.1021/acs.inorgchem.8b02637 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry easy axes but also by promoting the magnetic interactions between lanthanide ions.28−34 Up to now, a deep insight into the role of ligand in optimizing the performance of SMMs has been on the way.35−38 The Layfield group obtained an SMM presenting the highest reported energy barrier (1837 K) and blocking temperature (60 K) at that time by the use of 1,2,4tri(tertbutyl)cyclopentadienyl ligand for the enhancing magnetic anisotropy of Dy3+ in combination with the removal of equatorial ligands.35 The influences of different terminal solvent ligands36 and substitutions on ligands37 as well as the steric hindrance and charge-driving effects of ligands38 on SMM performances were also investigated. It is believed that the ligands play their roles through tuning the type and strength of ligand fields for metal ions and also through tuning the topologies. Many novel topological structures, such as “hourglass”, “double triangle”, “ship”, and “bottlebrush” were synthesized for optimizing the magnetic properties of SMMs.39−42 These systematic studies help us to explore the origin of SMR and to develop high-performing SMMs. The ligands for SMMs are to be designed or selected based on the considerations of adjusting ligand spheres around metal atoms for maximizing their anisotropies and bridging metal ions for propagating magnetic interactions. Oxygen atom has special coordination affinities to lanthanide ions and can effectively propagate magnetic interactions through versatile bridging modes with different M−O−M bond angles. On the basis of the above considerations, we designed two polyhydroxylamine-derived Schiff bases, which were in situ generated from the reactions of 2-hydroxynaphthaldehyde with 3-amino-1,2-propanediol and 1,1-di(hydroxymethyl)ethylamine in the presence of dysprosium(III) salts (Scheme 1), respectively. The presence of naphthyl group in these
yield of 22% calculated from Dy(NO)3·6H2O. Anal. calcd for C32H42Dy2N4O16: C, 36.13; H, 3.98; N, 5.27%. Found: C, 35.89; H, 4.13; N, 5.54%. IR (KBr pellet, cm−1): 3416(m), 2934(m), 2837(m), 2754(m), 2681(m), 1625(s), 1546(m), 1507(m), 1462(s), 1384(m), 1343(m), 1295(m), 1250(m), 1184(m), 1120(m), 1033(m), 987(m), 951(w), 899(w), 831(w), 748(m), 631(m), 556(w), 507(w), 460(w). Synthesis of [Dy6(μ3-OH)2(L2)2(HL2)2(H2L2)2Cl2(EtOH)2]Cl2· 3EtOH·CH3 CN (2). 2-Hydroxy-1-naphthaldehyde (0.1 mmol, 0.0172 g), triethylamine (20 μL), DyCl3·6H2O (0.10 mmol, 0.0386g), and 1,1-di(hydroxymethyl)ethylamine (0.1 mmol, 0.0105 g) in a mixed solvent of ethanol (1.5 mL) with acetonitrile (0.5 mL) were all carefully sealed within a Pyrex tube under vacuum, which was then heated to 80 °C over 2 days. After being cooled overnight, the thus-formed light-yellow crystals were separated. Yield: 18% (calculated from DyCl3·6H2O). Anal. calcd for C102H125Cl4Dy6N7O25: C, 41.31; H, 4.25; N, 3.31%. Found: C, 41.96; H, 4.60; N, 3.66%. IR (KBr pellet, cm−1): 3432(s), 1623(s), 1533(w),1442(w), 1366(m), 1159(w), 1063(m), 825(w), 743(w), 635(w), 568(w), 482(w). Collection of Crystal Data and Structural Refinement. The diffraction intensities of single crystals were recorded with a Bruker Smart APEX-II CCD for 1 and a SuperNova diffractometer for 2, respectively. Their structures were solved and subsequently refined using SHELXS43 and SHELXL,44 respectively. All non-hydrogen atoms (C, N, O, Cl, Dy) in both complexes were located by difference Fourier maps, subsequently refined anisotropically. All of the C-bound H atoms were refined at the geometrical sites, with the O-bound H atoms found from difference Fourier map. The ligand of nitrate on Dy(III) ion and the free methanol molecule of 1, as well as one free ethanol molecule in 2, were modeled into disorder. The detailed parameters for the structures are shown in Table 1. The related structural data such as bond lengths and angles for both titled complexes are presented in Tables S1 and S2, respectively. CCDC 1860651 and 1860652 for 1 and 2, respectively, can be obtained from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif, which is free of charge.
Scheme 1. In Situ Formed Schiff Base in the Two Titled Complexes
Table 1. Crystallographic Data of Both Titled Complexes
ligands can adjust the steric effects, which help to form discrete structures and to tune the coordination environments around dysprosium atoms. The coordination of in situ formed Schiff bases to Dy(III) ions gave two SMMs [Dy2(MeOH)2(HL1)2(NO3)2]·2MeOH (1) and [Dy6(μ3OH)2(L2)2(HL2)2(H2L2)2Cl2(EtOH)2]Cl2·3EtOH·CH3CN (2).
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EXPERIMENTAL SECTION
Synthesis of [Dy2(MeOH)2(HL1)2(NO3)2]·2MeOH (1). Methanol (2 mL) and triethylamine (5 μL) were mixed with 2-hydroxynaphthaldehyde (0.1 mmol, 0.0172 g), 3-amino-1,2-propanediol (0.1 mmol, 0.0091 g), and Dy(NO)3·6H2O (0.1 mmol, 0.0568 g) in a Pyrex tube. It was then mixed and sealed under vacuum. The resulting yellow solution was left at 80 °C over 3 days, then cooled overnight before filtration. The clear solution obtained from filtration was then evaporated over 10 days, giving yellow crystals of complex 1 with a B
complex
1
2
formula fw λ/Å T/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z μ/mm−1 Dc/g cm−3 F(000) θ/deg reflns collected reflns unique Rint GOF on F2 Rn [I > 2σ(I)] wR2 [I > 2σ(I))] R1 (all data) wR2 (all data)
C32H42Dy2N4O16 1063.69 0.71073 296(2) monoclinic C2/c 16.222(3) 27.327(5) 9.3497(17) 90 105.839(3) 90 3987.4(13) 4 3.791 1.772 2088 2.403 to 25.018 20741 3386 0.0381 1.021 0.0207 0.0479 0.0279 0.0513
C102H125Cl4Dy6N7O25 2965.88 0.71073 296(2) triclinic P 13.1233(3) 14.4709(5) 17.1334(5) 65.581(3) 78.613(2) 85.956(2) 2904.12(15) 1 3.971 1.696 1450 3.148 to 25.010 38677 10226 0.0426 1.08 0.0364 0.087 0.0483 0.098 DOI: 10.1021/acs.inorgchem.8b02637 Inorg. Chem. XXXX, XXX, XXX−XXX
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literatures.45−47 (HL1)2− acts as a μ2-bridge (Figure S7) using its Ophenolic, Nimino, and one Oalkoxy atoms to chelate one of the Dy(III) ions and using its two Oalkoxy atoms to chelate the other Dy(III) ion. Dy1 and Dy1A in 1 are thus linked by two (HL1)2− ligands through their two Oalkoxy atoms to form a dinuclear skeleton (Figure 1) with the Dy−O−Dy angle and the Dy···Dy distance of 110.72(9)° and 3.7073(4) Å, respectively, which are similar to the reported values.36,48−51 Crystal Structure of [Dy6(μ3OH)2(L2)2(HL2)2(H2L2)2Cl2(EtOH)2]Cl2·3EtOH·CH3CN (2). The single-crystal structural analysis result for 2 demonstrates that its crystals have P1̅ space group in the triclinic system. As depicted in Figure 2 and Figure S8, it possesses a
RESULTS AND DISCUSSION Preparation and Characterization. Bulky crystalline products for the titled complex of 1 were obtained by evaporating the filtered reaction solution of 2-hydroxynaphthaldehyde with 3-amino-1,2-propanediol in the presence of Dy(NO)3·6H2O in methanol at 80 °C in a sealed Pyrex tube. However, the crystals of 2 were formed by directly cooling the reaction solution of 2-hydroxynaphthaldehyde with 1,1di(hydroxymethyl)ethylamine in the presence of DyCl3· 6H2O in ethanol and acetonitrile at 80 °C in sealed Pyrex tube. Increasing the reaction temperature to 100 °C resulted in much poorer crystal quality and lower yield. Upon lowering the temperature to 70 °C or using routine solution reaction methods, no crystalline products of 1 and 2 were found. The use of a sole solvent of ethanol or acetonitrile for the preparation of 2 did not work as expected. Their thermogravimetric (TG) analyses are presented in Figures S1 and S2. Upon increasing the temperature from ambient temperature, complex 1 underwent a slow weight loss of 2.9% before 118 °C with a subsequent much faster weight loss of 3.7%, which agrees well with two free methanol molecules (calcd 6.02%) per formula unit of 1. The weight losses that followed were due to losing methanol ligands and the Schiff base ligands in 1, which did not come to an end even when the temperature was increased at 800 °C. No weight loss was found for the sample of complex 2 before 54 °C. Upon heating, a continuous weight loss was found. It underwent a fast weight loss before 105 °C with a subsequent slower weight loss. The weight loss of 6.68% before 105 °C is due to the loss of three free ethanol molecules and one free acetonitrile molecule (calcd 6.04%) per formular unit. The weight loss that followed was incomplete even upon being heated to 800 °C. The experimental PXRD curves of complexes 1 and 2 correspond well to their fitted ones obtained from their corresponding crystal structures, as shown in Figures S3 and S4, respectively, which indicates the purities for their bulky samples. Crystal Structure of [Dy 2 (MeOH) 2 (HL 1 ) 2 (NO 3 ) 2 ]· 2MeOH (1). The crystals of 1 have a C2/c space group in the monoclinic system, as revealed from single-crystal structural analyses. The components per formular unit of 1 (Figure 1 and Figure S5) include two Dy(III) ions and two
Figure 2. (a) Hexanuclear coordination unit of 2 omitting H atoms for clarity with labels for selected atoms. (b) Hexanuclear skeleton of 2. Symmetry code: (A) −x + 1, −y, −z + 1.
centrosymmetric hexanuclear coordination cation that consists of six Dy(III) ions, six Schiff base ligands with three different deprotonated forms ((L2)3−, (HL2)2−, and (H2L2)−), two μ3OH− ions, two Cl− ions, and two ethanol molecules. All Dy(III) ions in 2 show eight-coordinated bicapped trigonal prism geometries that are distorted to different degrees (Figure S9). The eight coordination atoms (one Cl atom, one N atom, and six O atoms) of Dy1 are from one Cl− ion, one μ3-OH−, one (L2)3−, one (HL2)2−, and one (H2L2)− ligand. The seven O atoms and one N atom in the coordination sphere of Dy2 are from one μ3-OH− ion, one ethanol molecule, two (L2)3− ligands, and one (H2L2)− ligand. The eight coordination atoms of Dy3 are seven O atoms and one N atom from one μ3-OH−, one (HL2)2−, and two (L2)3− ligands. The bond lengths of Dy(III)-O/N and the bond angles of O-Dy(III)-O/N are 2.198(5)−2.562(4) Å and 60.58(13)−155.47(15)°, respectively. The Dy−Cl bond in 2 has a bond length of 2.666(2) Å. The three deprotonated forms ((L2)3−, (HL2)2−, and (H2L2)−)
Figure 1. Dinuclear skeleton of 1 with labels for selected atoms. For clarity, hydrogen atoms and the disordered nitrate part are omitted. Symmetry code: (A) −x + 1/2, −y + 1/2, −z + 1.
(HL1)2−, two NO3−, and two methanol ligands in the coordination unit, together with two free methanol molecules. The coordination sites of Dy(III) for 1 are occupied by one N and seven O atoms from one methanol, one NO3−, and two (HL1)2− ligands, forming a bicapped trigonal prism (Figure S6). The bond lengths of Dy(III)-O/N are 2.231(2)− 2.535(17) Å, agreeing well with the reported values in C
DOI: 10.1021/acs.inorgchem.8b02637 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
values of complex 1 decrease when the temperature of its sample is lowered, reaching a minimum value (27.12 cm3 K mol−1) at a temperature of 25 K. The further cooling of the sample to 2 K led to an abrupt upturn of the χMT of 1. When the temperature of the sample was decreased from 300 to 242 K, the χMT values of 2 remain nearly constant. Upon further cooling the sample to 2 K, the χMT of 2 undergoes a slow decrease, with a subsequent quick decrease to an ultimate minimum of 39.68 cm3 K mol−1 at last. The obviously different χMT versus T profiles for complexes 1 and 2 are probably attributed to thermal depopulation in Stark sublevels together with their different magnetic interactions. Obviously, the Dy(III) ions from 1 are ferromagnetically coupled. This is rarely seen in the reported SMMs with similar skeletons,48 which usually present antiferromagnetic interactions.36,48−51,58,59 The completely different χMT versus T profile of 2 from that of 1 seems to suggest a dominant antiferromagnetic interaction in 2, although it is also possible for a ferromagnetic interaction to exist in 2. These hypotheses are supported by the related simulation. The isothermal magnetizations measured at different temperature for 1 and 2 were plotted as M versus H/T curves (Figure 4) and M versus H curves (Figures S11 and S12).
of the six Schiff base ligands in 2 present different bridging modes, as depicted in Figure S10. Dy1, Dy2, and Dy3A as well as Dy1A, Dy2A, and Dy3, are double-bridged through one μ3-Oalkoxy atom and one μ3-OH− ligand to build triangular Dy3 units in 2. The adjacent edges from the two triangular units are parallel to each other with the four Dy(III) ions on the two edges connected by another two μ3-Oalkoxy atoms, which leads to the construction of a nearly planar hexanuclear skeleton of 2 (Figure 2b) featuring four edge-shared triangular Dy3 units. This kind of connection is much different from those with the two triangular units edgeto-edge bridged through one η4-O2− ion52 or vertex-to-vertex linked by double μ2-O atoms.53,54 The six Dy(III) ions in 2 are further consolidated by six μ2-O atoms with Dy−O−Dy angles of 91.69(13)−110.06(15)°. The intratrigonal Dy···Dy distances have a range of 3.4903(4) to 3.8883(6) Å. The Dy6(μ3O)6(μ2-O)6 skeleton was also seen in refs 55−57, but our case is much different from the reported cases in at least two features. First, the six Dy(III) ions of the skeleton of 2 (our case) are nearly planar; however, the six Dy(III) ions for those reported cases present a chair-shaped conformation. Second, there are only two μ3-OH− ligands affording μ3-O atoms in our case, which is different from the reported cases with four μ3OH− ligands affording μ3-O atoms. Magnetic Properties. Molar magnetic susceptibilities for both titled complexes collected with a direct current (dc) magnetic field (1 kOe) are plotted into temperature-dependent χM and χMT plots (Figure 3). The χMT values of complexes 1 and 2 at 300 K are 28.58 and 82.74 cm3 K mol−1, being near to those theoretically calculated based on two and six isolated dysprosium(III) ions with 6H15/2 multiplet ground state (28.34 and 85.02 cm3 K mol−1), respectively. The experimental χMT
Figure 4. M versus HT−1 plots under different temperatures for both 1 (a) and 2 (b).
Their magnetizations undergo a fast increase first upon increasing the fields, with a followed slow increase to the maximum of 11.57 and 33.83NμB at 70 kOe and 1.8 K for 1 and 2, respectively, which are not saturated. The M versus H/T curves of the two complexes at 1.8, 2.5, 5, and 10 K do not superpose with each other apparently. These features, as revealed by the M versus H and M versus H/T curves, clearly indicate the occurrence of low-lying energy states or anisotropies for both titled complexes.52,60,61 As indicated by
Figure 3. Temperature-dependent χM and χMT plots of 1 (a) and 2 (b). D
DOI: 10.1021/acs.inorgchem.8b02637 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
χ′′ curves under 1 kOe external field for 1 (Figure S16) and under zero external field for 2 (Figure S17), with their peaks moving to higher temperature upon increasing the measured frequencies. Both χ′ versus v (Figure S18) and χ′′ versus v (Figure 6a−c) of complexes 1 (zero and 1 kOe external field) and 2 (an external field of zero) measured at different temperatures show large differences from each other. The χ′′ versus v curves of both 1 and 2 show maximums shifting toward higher frequencies upon increasing the measured temperature. In comparison, those maximums of χ′′ versus v curves of 2 appear at higher frequencies than those for 1. The maximums of χ′′ versus v curves of 1 under 1000 Oe dc field appear at lower frequencies than those under zero dc field. All of these features confirm that both 1 and 2 are SMMs.47,52,64 To explore the relaxation processes of 1 and 2, their Cole− Cole diagrams (χ′′ vs χ′ isothermal curves) were plotted from their frequency-dependent ac susceptibilities (Figure 6d−f). The profiles of these Cole−Cole diagrams for both titled complexes are near-semicircle. The simulation to these curves using the Debye model gave the α parameters of 0.14 to 0.28, 0.14 to 0.36, and 0.17 to 0.29 and τ values of 1.50 × 10−4 to 1.24 × 10−3, 1.46 × 10−4 to 1.07 × 10−2, and 1.99 × 10−4 to 7.22 × 10−4 for 1 under an external field of zero, 1 under an external field of 1000 Oe, and 2 under an external field of zero, respectively. Apparently, an obvious lengthening of relaxation time was observed under the 1000 Oe external field for complex 1. The plots for the change of ln(τ/s) with T−1 are drawn in Figure 7 based on these derived data. The ln(τ/s) versus T−1 curves of 1 derived at the zero and 1000 Oe field show clearly linear regimes within 10−6.5 K; however, clearly nonlinear curves are shown in the whole measured temperature range. As is well known, the relaxation mechanism for the lanthanide-based SMMs may involve four possible terms of Orbach (τ0−1 exp(−Ueff/kBT)), Raman (CTn), QTM (τQTM−1), and direct relaxation processes (AHmT), as shown in eq 1.65−67 The last term is usually neglected when the relaxation is performed without external field. The fitting ln(τ/s) versus T−1 curves of the titled complex of 1 under zero external field (Figure 7a) according to eq 1 by omitting the last term agree well with the experimental data, giving parameters τ0 = 9.5 × 10−6 s, Ueff/kB = 34 K, τQTM = 0.0025 s, C = 219 s−1 K−1.2, and n = 1.2. The similar simulation for 1 under 1000 Oe dc field (Figure 7b) was successful only when the QTM and direct processes were ignored, giving parameters τ0 = 4.5 × 10−6 s, Ueff/kB = 36.4 K, and C = 40.4 s−1 K−1.38, and n = 1.38. The obvious lengthening of relaxation time and higher Ueff/kB values for 1 under 1000 Oe than those under 0 Oe, as well as their τQTM values, clearly indicate the presence of QTM for 1, which might be suppressed under external field.68 The plot of ln(τ/s) versus T−1 (Figure 7c) for 2 derived from the related magnetic data under zero field was simulated according to eq 1 by omitting the last two terms, giving Ueff/kB = 40 K, τ0 = 1.68 × 10−6 s, and C = 372 s−1 K−2.12, and n = 2.12. The Ueff/kB for 2 is higher than those (
