Mononuclear Lanthanide Complexes: Energy-Barrier Enhancement

Article pubs.acs.org/IC

Mononuclear Lanthanide Complexes: Energy-Barrier Enhancement by Ligand Substitution in Field-Induced DyIII SIMs Sourav Biswas,† Kochan S. Bejoymohandas,‡,§ Sourav Das,∥ Pankaj Kalita,⊥ Mundalapudi L. P. Reddy,*,‡,§ Itziar Oyarzabal,□ Enrique Colacio,*,# and Vadapalli Chandrasekhar*,†,⊥ †

Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology, Council of Scientific and Industrial Research, Thiruvananthapuram 695 019, India § Academy of Scientific and Innovative Research, New Delhi 110001, India ∥ Department of Chemistry, Institute of Infrastructure Technology Research and Management, Ahmedabad 380026, India ⊥ National Institute of Science Education and Research Bhubaneswar, HBNI, Jatni 752050, Odisha, India # Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Avenida de Fuentenueva s/n, 18071 Granada, Spain □ Departamento de Química Aplicada, Facultad de Química, Universidad del País Vasco UPV/EHU, Paseo Manuel de Lardizabal, no. 3, 20018 Donostia-San Sebastián, Spain ‡

S Supporting Information *

ABSTRACT: The sequential reaction of 2-((6(hydroxymethyl)pyridin-2-yl)-methyleneamino)phenol (LH2), LnCl3·6H2O, and 1,1,1-trifluoroacetylacetone (Htfa) in the presence of Et3N afforded [Ln(LH) (tfa)2] [Ln = Dy3+ (1), Ln = Tb3+ (2), and Ln = Gd3+ (3)], while under the same reaction conditions, but in the absence of the coligand, another series of mononuclear complexes, namely, [Ln(LH)2]·Cl· 2MeOH] [Ln = Dy3+ (4) and Tb3+ (5)] are obtained. Singlecrystal X-ray diffraction analysis revealed that the former set contains a mono-deprotonated [LH]− and two tfa ligands, while the latter set comprises of two mono-deprotonated [LH]− ligands that are nearly perpendicular to each other at an angle of 86.9°. Among these complexes, 2 exhibited a ligand-sensitized lanthanide-characteristic emission. Analyses of the alternating current susceptibility measurements reveal the presence of single-molecule magnet behavior for 1 and 4, in the presence of direct-current field, with effective energy barriers of 4.6 and 44.4 K, respectively. The enhancement of the effective energy barrier of the latter can be attributed to the presence of a large energy gap between the ground and first excited Kramers doublets, triggered by the change in coordination environments around the lanthanide centers.

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in single-ion magnets (SIMs) in particular.3 On the one hand, in comparison to SMMs that involve two or more metal ions, SIM behavior is due to a single metal ion, and hence understanding magnetic behavior is relatively easier.5 On the other hand, it has been shown that SIMs are very sensitive toward the ligand field; the latter lift the degeneracy of the ground state of the lanthanide ions into various ±MJ sublevels, which influence the energy barrier for magnetization relaxation.6 A careful modulation of ligand environment around the lanthanide metal ion can lead to a ground state with a larger |Jz|, separating other sublevels with a substantial energy gap.6 In spite of the fact that the number of SIMs has increased since the original discovery of Ishikawa, many of these are still based

INTRODUCTION

The coordination complexes of 4f metal ions are gaining increasing interest in recent years in view of their potential applications, particularly in the field of molecular magnetism.1 Many lanthanide ions have a high-spin ground state and possess an Ising-type magneto-anisotropy making them attractive candidates for the preparation of molecular compounds, which can be potential single-molecule magnets (SMMs).2 This field received a fillip by the discovery of Ishikawa and coworkers that the double-decker complexes, [LnPc2]− (Ln = DyIII and TbIII; Pc = pthalocyanin)3 behaved as molecular magnets possessing significant energy barriers for magnetic relaxation. Until this point, polynuclear complexes, mainly those involving transition-metal ions, were the subject of investigations for SMM behavior.4 Ishikawa’s complexes spurred interest in lanthanide-based complexes in general and © 2017 American Chemical Society

Received: March 23, 2017 Published: June 29, 2017 7985

DOI: 10.1021/acs.inorgchem.7b00689 Inorg. Chem. 2017, 56, 7985−7997

Article

Inorganic Chemistry on macrocyclic ligands,3,7 although a few are based on other ligand types.8 Interestingly, some SIMs containing Dy3+ have been shown to exhibit the highest effective energy barriers for the reversal of the magnetization (Ueff = 1815 K) and the highest temperatures at which magnetic hysteresis has been ever observed for SMMs (∼20 K).9 Another reason for interest in lanthanide complexes is because of their photophysical properties. Many lanthanide complexes, particularly those involving Eu3+ and Tb3+, show interesting photoluminescence;10 however, this property can be accentuated by energy transfer involving antenna ligands.11 A perusal of the literature reveals that, while lanthanide complexes have been investigated separately for their magnetic and photophysical properties, complexes that exhibit simultaneously these properties are sparse.12 One of the challenges in this area is to design suitable ligands that can satisfy the coordination requirements of the metal ions and at the same time transfer energy efficiently to the metal center. Recently, we reported a series of trinuclear heterometallic [M2Ln; M = Zn 2+ and Mg2+] complexes by employing the flexible Schiff base ligand, 2-(2hydroxy-3-(hydroxymethyl)-5-methylbenzylideneamino)-2methylpropane 1,3-diol (Figure 1).13 The DyIII analogue of this

Figure 2. Cubane-shaped homometallic [Ln4(L)4(μ2-η1η1Piv)4] complexes.18

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EXPERIMENTAL SECTION

Solvents and other general reagents used in this work were purified according to standard procedures.14 2,6-Bis(hydroxymethyl)pyridine, activated manganese(IV) dioxide (MnO2), DyCl3·6H2O, TbCl3·6H2O, GdCl3·6H2O, and 1,1,1-trifluoro-2,4-pentanedione were obtained from Sigma-Aldrich Chemical Co. and were used as received. 2-Aminophenol and sodium sulfate (anhydrous) were obtained from SD Fine Chemicals, Mumbai, India, and were used as such. 6-Hydroxymethyl2-pyridinecarboxaldehyde and 2-((6-(hydroxymethyl)pyridin-2-yl)methyleneamino)phenol (LH2) were prepared according to a literature procedure.15 Instrumentation. Melting points were measured using a JSGW melting point apparatus and are uncorrected. IR spectra were recorded as KBr pellets on a Bruker Vector 22 FT IR spectrophotometer operating at 400−4000 cm−1. Elemental analyses of the compounds were obtained from Thermoquest CE instruments CHNS-O, EA/110 model. Electrospray ionization mass spectrometry (ESI-MS) was performed on a Micromass Quattro II triple quadrupole mass spectrometer. 1H NMR spectra were recorded in CD3OD solutions on a JEOL JNM LAMBDA 400 model spectrometer operating at 500.0 MHz. Chemical shifts are reported in parts per million (ppm) and are referenced with respect to internal tetramethylsilane (1H). Magnetic Measurements. Direct (dc) and alternating (ac) current susceptibility measurements were performed with a Quantum Design SQUID MPMS XL-5 device. The ac experiments were performed using an oscillating ac field of 3.5 Oe and frequencies ranging from 1 to 1500 Hz. The experimental susceptibilities were corrected for the sample holder and diamagnetic contributions (molecular mass/2 ≈ −350 × 10−6 cm3 mol−1 per compound). Pellets of the different samples were cut into small pieces and placed in the sample holder to avoid any orientation of the microcrystals by the magnetic field. Photophysical Characterization. Absorption data were measured on a Shimadzu, UV-2450 UV/vis/near-IR spectrophotometer. Photoluminescence (PL) spectra were recorded on a Spex-Fluorolog FL22 spectrofluorimeter. The latter was equipped with a double grating 0.22 m Spex 1680 monochromator and a 450 W Xe lamp as the excitation source and a Hamamatsu R928P photomultiplier tube detector. Corrections were applied to the emission and excitation spectra with regard to source intensity (lamp and grating) by using standard correction curves. Lifetime measurements were performed at room temperature using a Spex 1040D phosphorimeter. Solution-state luminescence quantum efficiencies were calculated by comparing the emission intensities of the standard sample and the unknown sample according to eqn 1.

Figure 1. Trinuclear hetero-bimetallic complexes13 [M2Ln(LH3)4]· 3NO3.

family exhibited SMM behavior, while the TbIII analogue exhibited luminescence property.13 These results, as well as the fact that ligand environments around the lanthanide centers have profound effects on the magnetism, prompted us to investigate other ligand systems. Previously, we utilized a multidentate ligand, 2-((6-(hydroxymethyl)pyridin-2-yl)methyleneamino)phenol (LH2), for the preparation of a Ln4 family possessing a cubane-type geometry (Figure 2). Interestingly, we now find that under altered reaction conditions, mononuclear complexes can be accessed using the same ligand. Thus, LH2 upon reaction with LnCl3·6H2O (Ln = Dy, Tb, and Gd) in the presence of 1,1,1-trifluoro-2,4pentanedione (Htfa), afforded a series of mononuclear lanthanide complexes, namely, [Ln(LH) (tfa)2] [Ln = Dy (1), Ln = Tb (2), and Ln = Gd (3)]. However, in the absence of the tfa coligand, under the same reaction conditions, another series of mononuclear lanthanide complexes, [Ln(LH)2]·Cl· 2MeOH], [Ln = Dy3+ (4) and Tb3+ (5)] were formed (Scheme 1). The synthesis, structure, magnetism, and photophysical properties of these complexes are discussed herein.

Φunk = Φstd(Iunk /Istd)(A std /A unk )(ηunk /ηstd)2 7986

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Inorganic Chemistry Scheme 1. Synthesis of Mononuclear Complexes [Ln(LH) (tfa)2] (1−3) and [Ln(LH)2]·Cl·2MeOH (4−5)

Table 1. Crystal Data and Structure Refinement Parameters of 1−5 1 formula M, g crystal system space group a, Å b, Å c, Å α (deg) β (deg) γ (deg) V, Å3 Z ρc, g cm−3 μ, mm−1 F(000) cryst size (mm3) θ range (deg) limiting indices

reflns collected ind reflns completeness to θ refinement method data/restraints/ parameters goodness-of-fit on F2 final R indices [I > 2θ(I)] R indices (all data)

2

3

4

5

C23H19N2O6F6Dy 695.90 orthorhombic Pbca 19.630(5) 11.909(5) 21.507(5) 90 90 90 5028(3) 8 1.839 3.058 2712 0.27x 0.15 × 0.09 4.11 to 25.02 −23 ≤ h ≤ 22 −12 ≤ k ≤ 14 −25 ≤ l ≤ 25 32 207 4419 [R(int) = 0.0422] 99.5% full-matrix least-squares on F2 4419/1/324

C23H19N2O6F6Tb 692.32 orthorhombic Pbca 19.622(5) 11.918(3) 21.521(6) 90 90 90 5033(2) 8 1.827 2.896 2704 0.23 × 0.21 × 0.15 4.11 to 25.03 −23 ≤ h ≤ 22 −12 ≤ k ≤ 14 −25 ≤ l ≤ 21 31 409 4424 [R(int) = 0.0992] 99.4% full-matrix least-squares on F2 4424/6/323

C23H19N2O6F6Gd 691.66 monoclinic P21/n 12.964(5) 12.124(5) 16.512(5) 90 102.251(5) 90 2536.2(16) 4 1.811 2.700 1352 0.19 × 0.14 × 0.08 4.08 to 25.03 −15 ≤ h ≤ 15 −10 ≤ k ≤ 14 −19 ≤ l ≤ 19 13 359 4257 [R(int) = 0.0924] 99.4% full-matrix least-squares on F2 4257/2/348

C28H30N4O6ClDy 716.51 monoclinic P21 7.9475(9) 21.638(2) 8.0528(9) 90 91.634(2) 90 1384.2(3) 2 1.719 2.845 714 0.24 × 0.17 × 0.11 4.11 to 25.02 −9 ≤ h ≤ 9 −20 ≤ k ≤ 25 −9 ≤ l ≤ 8 7225 4080 [R(int) = 0.0378] 99.5% full-matrix least-squares on F2 4080/5/317

C28H30N4O6ClTb 712.93 monoclinic P21 7.9356(4) 21.6488(10) 8.0487(4) 90 91.6210(10) 90 1382.18(12) 2 1.713 2.705 712.0 0.02 × 0.02 × 0.02 4.11 to 25.03 −9 ≤ h ≤ 9 −25 ≤ k ≤ 25 −9 ≤ l ≤ 9 16491 4864 [R(int) = 0.0395] 100% full-matrix least-squares on F2 4864/7/365

1.064 R1 = 0.0363 wR2 = 0.0866 R1 = 0.0458 wR2 = 0.0915

1.048 R1 = 0.0446 wR2 = 0.0782 R1 = 0.1092 wR2 = 0.1256

1.032 R1 = 0.0692 wR2 = 0.1391 R1 = 0.1854 wR2 = 0.2375

1.041 R1 = 0.0353 wR2 = 0.0403 R1 = 0.0731 wR2 = 0.0788 0.004 (15)

1.067 R1 = 0.0244 wR2 = 0.0548 R1 = 0.0272 wR2 = 0.0556 0.003 (12)

Flack parameter

where Φunk and Φstd are the luminescence quantum yields of the unknown sample and the standard sample, respectively, and Iunk and Istd are the integrated emission intensities of the unknown sample and standard sample solution, respectively. Aunk and Astd are the absorbances of the unknown sample and standard sample solution at their excitation wavelengths, respectively. The ηunk and ηstd terms represent the refractive indices of the corresponding solvents (pure

solvents were assumed). Quinine sulfate monohydrate in 0.05 M H2SO4 was used as the standard.16 X-ray Crystallography. Crystal data were collected on a Bruker SMART CCD diffractometer (Mo Kα radiation, λ = 0.710 73 Å; Table 1). The following programs were used: SMART,17a for collecting frames of data, indexing reflections, and determining lattice parameters; SAINT17a for integration of the intensity of reflections 7987


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Inorganic Chemistry

Figure 3. (a) Full-range ESI-MS spectrum of complex 1. (b) Experimental and (c) simulated mass spectral pattern of the species [C23H19DyF6N2O6 − H+ + MeCN]−. 0.051 g, 56.6% (based on Dy3+). mp > 260 °C. FT-IR (KBr) cm−1: 3432 (b), 2927(b), 2583 (m), 1629 (s), 1588 (s), 1576 (w), 1553 (w), 1527 (m), 1485 (s), 1447 (s), 1361 (m), 1297 (s), 1254 (m), 1220 (m), 1184 (s), 1126 (s), 1044 (m), 1016 (m), 936 (w), 851 (w), 826 (w).,771 (m), 752 (w). ESI-MS m/z, ion: 735.965, [C23H19DyF6N2O6 − H+ + MeCN] − Anal. Calcd for C23H19DyF6N2O6 (695.90): C, 39.70; H, 2.75; N, 4.03. Found: C, 39.54; H, 2.88; N, 4.23%. [Tb(LH) (tfa)2] (2). Quantities: LH2 (0.03 g, 0.13 mmol), TbCl3· 6H2O (0.048 g, 0.13 mmol), Et3N (0.052 mL, 0.39 mmol), Yield: 0.046 g, 51.7% (based on Tb3+). mp > 260 °C. FT-IR (KBr) cm−1: 3430 (b), 2925(b), 2587 (m), 1622 (s), 1592 (s), 1571 (w), 1543 (w), 1515 (m), 1482 (s), 1444 (s), 1361 (m), 1293 (s), 1259 (m), 1221 (m), 1188 (s), 1128 (s), 1042 (m), 1018 (m), 933 (w), 850 (w), 824 (w).,771 (m), 753 (w). ESI-MS m/z, ion: 693.049, [C23 H19 Dy F6 N2 O6 + H+]+. Anal. Calcd for C23H19F6N2O6Tb (692.33): C, 39.90; H, 2.77; N, 4.05. Found: C, 39.73; H, 2.92; N, 4.17%. [Gd(LH) (tfa)2] (3). Quantities: LH2 (0.03 g, 0.13 mmol), GdCl3· 6H2O (0.048 g, 0.13 mmol), Et3N (0.052 mL, 0.39 mmol), Yield: 0.041 g, 45.9% (based on Gd3+). mp > 260 °C. FT-IR (KBr) cm−1: 3437 (b), 2923(b), 2584 (m), 1629 (s), 1587 (s), 1570 (w), 1548 (w), 1510 (m), 1476 (s), 1445 (s), 1367 (m), 1292 (s), 1253 (m), 1228 (m), 1185 (s), 1134 (s), 1040 (m), 1015 (m), 938 (w), 851 (w), 822 (w),771 (m), 759 (w). ESI-MS m/z, ion: 729.959, [C23H20F6GdN2O6 − H+ + MeCN] − Anal. Calcd for C23H20F6GdN2O6 (691.66): C, 39.94; H, 2.91; N, 4.05. Found: C, 40.11; H, 2.78; N, 4.22%.

and scaling; SADABS17b for absorption correction; SHELXTL17c,d for space group and structure determination and least-squares refinements on F2. SHELXL-201417e using Olex-217f were used to solve the crystal structures and for refinement by full-matrix least-squares methods against F2. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen positions were fixed at calculated positions and refined isotropically. Additional crystallographic information is available in the Supporting Information. General Synthetic Procedure for the Preparation of Complexes 1−3. All the metal complexes (1−3) were synthesized by following a similar procedure as follows. LH2 (0.03g, 0.13 mmol) was dissolved in methanol (40 mL). To this, LnCl3·6H2O (0.13 mmol) was added, and the resulting deep red colored solution was stirred for 10 min at room temperature. At this stage, Et3N (0.052 mL, 0.39 mmol) was added dropwise to this solution, and stirring was continued for 30 min. Then, 1,1,1-trifluoro-2,4-pentanedione (0.016 mL, 0.26 mmol) was added dropwise, and the resulting mixture was stirred for 12 h at room temperature. A light red colored solution that was obtained at this stage was evaporated in vacuo to afford a redcolored residue, which was washed twice with diethyl ether and dried before being dissolved in MeOH/CHCl3 (1:1 v/v). Red crystals, suitable for X-ray diffraction, were obtained through slow evaporation of solvent for 12 d. Specific details of each reaction and the characterization data of the compounds are outlined below. [Dy(LH) (tfa)2] (1). Quantities: LH2 (0.03 g, 0.13 mmol), DyCl3· 6H2O (0.049 g, 0.13 mmol), Et3N (0.052 mL, 0.39 mmol), Yield: 7988


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Inorganic Chemistry General Synthetic Procedure for the Preparation of Complexes 4 and 5. The metal complexes (4−5) were synthesized by the following procedure. To a vigorously stirred solution of methanol (50 mL) containing LH2 (0.03g, 0.13 mmol), LnCl3·6H2O (0.065 mmol) was added, and the resulting solution was stirred for 10 min. After that, Et3N (0.035 mL, 0.26 mmol) was added dropwise to afford red colored solution, which was stirred for 12 h at room temperature. Then, the solution was stripped off the solvent in vacuo to get a red residue, which was washed twice with diethyl ether and dried. This material was finally dissolved in methanol and kept for slow evaporation. After ∼8 d, block-shaped red colored crystals suitable for X-ray study were obtained. The details of the characterization data for these complexes are given below. [Dy(LH)2]·Cl·2MeOH (4). Quantities: LH2 (0.03 g, 0.13 mmol), DyCl3·6H2O (0.024 g, 0.065 mmol), Et3N (0.035 mL, 0.26 mmol), Yield: 0.029 g, 63.5% (based on Dy3+). mp > 260 °C. 1 FT-IR (KBr) cm−1: 3392 (b), 2976(b), 2938 (s), 2739 (m), 2677 (s), 2625 (m), 2492 (s), 1612 (w), 1569 (s), 1544 (s), 1475(s), 1397 (m), 1378 (m), 1366 (m), 1308 (m), 1289 (s), 1277 (m), 1254 (m), 1156 (w), 1112 (s), 1072 (w), 879 (m), 829 (w). 748 (w), 731 (w). ESI-MS m/z, ion: 618.091, [C26H22DyN4O4]+. Anal. Calcd for C28H30ClDyN4O6(716.51): C, 46.94; H, 4.22; N, 7.82. Found: C, 46.56; H, 4.37; N, 8.03%. [Tb(LH)2]·Cl·2MeOH (5). Quantities: LH2 (0.03 g, 0.13 mmol), TbCl3·6H2O (0.024 g, 0.065 mmol), Et3N (0.035 mL, 0.26 mmol), Yield: 0.026 g, 56.5% (based on Tb3+). mp > 260 °C. 1 FT-IR (KBr) cm−1: 3394 (b), 2973(b), 2942 (s), 2735 (m), 2671 (s), 2623 (m), 2496 (s), 1619 (w), 1569 (s), 1547 (s), 1479(s), 1392 (m), 1370 (m), 1364 (m), 1303 (m), 1287 (s), 1278 (m), 1251 (m), 1155(w), 1109 (s), 1075 (w), 874 (m), 823 (w). 749 (w), 737 (w). ESI-MS m/z, ion: 613.092, [C26H22TbN4O4]+. Anal. Calcd for C28H30ClN4O6Tb (712.94): C, 47.17; H, 4.24; N, 7.86. Found: C, 47.33; H, 4.39; N, 7.91%.

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) of 1 bond length around Dy3+ Dy(1)−O(1) = 2.420(4) Dy(1)−O(2) = 2.298(4) Dy(1)−N(2) = 2.500(4) Dy(1)−O(5) = 2.338(4) Dy(1)−O(4) = 2.318(4) Dy(1)−N(1) = 2.511(4) Dy(1)−O(3) = 2.347(4) Dy(1)−O(6) = 2.332(4)

bond angle around Dy3+ O(1)−Dy(1)−N(2) = 127.14 (3) O(1)−Dy(1)−N(1) = 63.92(14)

O(2)−Dy(1)−O(5) = 93.74(14) O(2)−Dy(1)−O(4) = 94.53(14) O(2)−Dy(1)−N(1) = 129.38(14) O(2)−Dy(1)−O(3) = 85.94(15) O(2)−Dy(1)−O(6) = 78.89(14) N(2)−Dy(1)−N(1) = 64.19(14) O(5)−Dy(1)−O(1) = 85.85(14) O(5)−Dy(1)−N(2) = 81.99(14) O(5)−Dy(1)−N(1) = 69.15(13) O(5)−Dy(1)−O(3) = 145.43(14) O(4)−Dy(1)−O(1) = 94.68(15) O(4)−Dy(1)−N(2) = 67.87(15) O(2)−Dy(1)−O1 = 165.44(14)

O(4)−Dy(1)−O(3) 72.04(14) O(4)−Dy(1)−O(6) 145.52(14) O(3)−Dy(1)−O(1) 86.19(14) O(3)−Dy(1)−N(2) 128.26(14) O(3)−Dy(1)−N(1) 134.73(15) O(6)−Dy(1)−O(1) 87.15(14) O(6)−Dy(1)−N(2) 135.06(14) O(6)−Dy(1)−O(9) 72.31(13) O(6)−Dy(1)−N(1) 132.66(13) O(6)−Dy(1)−O(3) 73.74(14) O(4)−Dy(1)−O(5) 142.17(14) O(2)−Dy(1)−N(2) 66.46(14) O(4)−Dy(1)−N(1) 77.18(14)

= = = = = = = = = = = = =

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RESULTS AND DISCUSSION Synthetic Aspects. Multidentate, flexible, Schiff base ligands13 possessing well-defined coordination environments,

Figure 5. Coordination mode of [LH]− and tfa in 1.

Figure 4. Molecular structure of 1 (hydrogen atoms are omitted for the sake of clarity).

Figure 6. Molecular structure of 4 (selected hydrogen atoms, chloride, and solvent molecules are omitted for the sake of clarity).

have been employed successfully for assembling lanthanide complexes. Recently, we utilized a multidentate flexible Schiff base ligand, namely, (E)-2-((6-(hydroxymethyl)pyridin-2-yl)methyleneamino)phenol (LH2) for assembling cubane-shaped homometallic Ln4 complexes [Ln4(L)4(μ2-η1η1Piv)4] (Figure 2) (Ln = Dy3+, Tb3+, and Gd3+). In this family, the DyIII analogue showed a two-step relaxation of magnetization.18 Note that the formation of these complexes is assisted by

bridging pivalate ligands, which connect alternate lanthanide centers. From a synthetic point of view it was of interest to see what happens if the pivalate ligand is replaced by a chelating ligand. We anticipated that the absence of the bridging coordination action of the pivalate would lead to the formation of mononuclear compounds. A similar result should also be possible if a coligand was completely absent. Both of these 7989


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Inorganic Chemistry Table 3. Selected Bond Lengths (Å) and Bond Angles (deg) of 4 bond length around Dy3+ Dy1−O1 = 2.383(9) Dy1−O2 = 2.233(7) Dy1−O3 = 2.438(7) Dy1−O4 = 2.296(6)

O1−Dy1−N4 = 82.0(3)

O4−Dy1−N2 = 86.3(3)

O2−Dy1−O1 = 162.7(2) O2−Dy1−O3 = 90.8(2) O2−Dy1−O4 = 93.9(2)

O4−Dy1−N3 = 131.3(3) O4−Dy1−N4 = 68.0(3) N1−Dy1−N3 = 126.3(3) N2−Dy1−N1 = 65.6(3)

Dy1−N1 = 2.504(11)

O2−Dy1−N1 = 133.6(3) O2−Dy1−N2 = 68.0(2)

Dy1−N2 = 2.482(8)

O2−Dy1−N3 = 90.3(2)

Dy1−N3 = 2.506(8)

O2−Dy1−N4 = 83.8(2)

Dy1−N4 = 2.473(8)

O3−Dy1−N1 = 82.8(3) O3−Dy1− N2 = 80.6(2) O3−Dy1−N3 = 64.5(2)

bond angles around Dy3+ O1−Dy1−O3 = 90.3(3) O1−Dy1−N1 = 63.6(3)

O3−Dy1−N4 = 128.5(2) O1−Dy1−N3 = 74.6(3)

N2−Dy1−N3 = 138.8(3) N4−Dy1−N1 = 134.4(3) N4−Dy1−N2 = 140.5(3) N4−Dy1−N3 = 64.3(3) O4−Dy1−N1 = 82.4(3)

Figure 9. Temperature dependence of the χMT product for compounds 1−5. The solid line represents the best fit of the experimental data.

Table 5. Direct-Current Magnetic Data for the Complexes Studied in This Work

compound

free-ion χMT values (cm3 K mol−1)a

experimental χMT300K/ χMT2K (cm3 K mol−1)

experimental M value (T = 2 K, H = 50 kOe) (NμB)

theoretical Msat value (NμB)b

1 2 3 4 5

14.17 11.48 7.875 14.17 11.48

13.93/11.16 11.58/7.01 7.86/7.51 14.34/12.18 11.98/6.99

5.32 5.85 7.07 5.02 5.92

10 9 7 10 9

O4−Dy1−O1 = 89.9(3) O4−Dy1−O3 = 163.3(2) O1−Dy1−N2 = 129.2(3)

a b

Table 4. Comparison of Bond Lengths of 1 and 4 (Å) Dy−Oalcohol

Dy−NPy

Dy−Nimine

Dy−Ophenolate

1 4

2.419 2.409

2.509 2.505

2.500 2.476

2.297 2.264

Nβ 2 2 {g j J(J 3k

+ 1)}.

M = NJμB ; J = L + S ; gJ =

3 2

+

ST (ST + 1) − L(L + 1) . 2J(J + 1)

mononuclear lanthanide complexes [Ln(LH) (tfa)2] [Ln = Dy3+ (1), Ln = Tb3+ (2), and Ln = Gd3+ (3)]. On the other hand, a similar reaction in the absence of the coligand afforded monocationic mononuclear lanthanide complexes, [Ln(LH)2]· Cl·2MeOH] [Ln = Dy3+ (4) and Tb3+ (5)] (Scheme 1). In the latter, two [LH]− ligands are involved in contrast to one [LH]− ligand in the formation of 1−3. ESI-MS studies on 1−5 revealed stable parent ion peaks (Supporting Information). A representative ESI-MS spectrum of 1 is given in Figure 3; the rest are given in Supporting Information (Figures S1−S4). X-ray Crystallography. Single-crystal X-ray diffraction analysis revealed that the homometallic mononuclear complexes 1−3 are neutral and isostructural. 1 and 2 crystallized in the orthorhombic system in the space group Pbca (Z = 8),

Figure 7. (a) Distorted square antiprism geometry around DyIII in 1. (b) Distorted triangular dodecahedron geometry around DyIII in 4.

complex

χM T =

hypotheses were proved correct in the formation of compounds 1−5 (Scheme 1). Thus, on the one hand, the reaction of LH2 and LnCl3·6H2O in the presence of 1,1,1-trifluoroacetylacetone afforded neutral

Figure 8. Formation of 1D zigzag chain through O−H···O interactions in 1 (selected hydrogen atoms and fluorine are omitted for clarity). Hydrogen-bonding parameters. Distance: O1−H1H···O, 1.782 Å. Angle: O1−H1H−O, 166.66°. 7990


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Table 6. Absorption Properties of the Ligand and the Complexes (1−5) in Methanol at 298 K compound

absorbance λ [nm] (εmax [1 × 103 M−1 cm−1]) in solution

1 2 3 4 5 ligand (LH2)

452(6.94), 337 (7.05), 293 (24.65) 454 (8.32), 338 (49.53), 292 (29.56) 442 (9.55), 337 (63.39), 291 (32.52) 451 (15.84), 336 (15.43), 282 (10.11) 451 (15.39), 336 (15.55), 282 (10.30) 360 (9.72), 291 (11.55)

Figure 10. Temperature dependence of the out-of-phase χ″M component of the ac susceptibility at different frequencies under an applied magnetic field of 2000 G for complex 1. (inset) Arrhenius plots for the relaxation times.

Figure 13. Phosphorescence spectrum of the gadolinium derivative 3 at 77 K.

Figure 11. Temperature dependence of the out-of-phase χM″ component of the ac susceptibility at different frequencies under an applied magnetic field of 1000 G for complex 1. (inset) Arrhenius plots for the relaxation times.

Figure 14. Room-temperature excitation and emission spectra for complex 2 in 5 μM methanol solution (λex = 302 nm).

Table 7. 5D0/5D4 Lifetimes (τobs), Radiative Lifetimes (τRAD), Intrinsic Quantum Yields (ΦLn), Energy-Transfer Efficiencies (Φsen), and Overall Quantum Yields (Φoverall) for Complex 2 Figure 12. UV/vis absorption spectra for the ligand (LH2) and the complexes 1−5 in CH3OH solution (10−5 M).

compound

τobs [μs]

τRADa [μs]

ΦLn [%]

Φsen [%]

Φoverall [%]

2

276 ± 2

680 ± 11

40

4.5

1.8 ± 0.1

a

whereas complex 3 crystallized in the monoclinic system in the space group P21/n (Z = 4). In view of the similarity of the molecular structures of 1−3, [Dy(LH) (tfa)2] (1) was chosen

τRAD = observed lifetime at 77 K in CD3OD.

as a representative example to elucidate the salient structural features of these complexes. A perspective view of the 7991


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Figure 15. Phosphorescence decay of complex 2 (a) at 298 K in CH3OH and (b) at 77 K in CD3OD. The emission was monitored at 545 nm (5D4→7F5).

dimensional (1D) zigzag chain (Figure 8). On the other hand, 4 does not have any significant intermolecular interactions. Magnetic Properties. The temperature dependence of the χMT product for complexes 1−5 (χM being the molar magnetic susceptibility per mononuclear LnIII unit) in the 2−300 K temperature range was measured with an applied magnetic field of 1 kOe for complexes 1, 4, and 5 and with a field of 3 kOe for complexes 2 and 3 (Figure 9). At room temperature, the observed χMT values for 1−5 are close to those calculated for independent LnIII ions in the freeion approximation (Table 5). Values larger than the free-ion value, in particular, the relatively large deviation from the freeion value for 5, could be due to a loss of solvent molecules during the sample manipulation for SQUID measurements or after doing vacuum in the SQUID sample chamber. Lowering temperature, the χMT product of complexes 1, 2, 4, and 5 decreases steadily until ∼20 K and then more sharply to 2 K. The observed behavior is mainly due to the depopulation of the MJ sublevels of the LnIII ions, which arise from the splitting of the ground term by the ligand field as well as the possible existence of intermolecular dipolar interactions. The magnetization versus field plots for these complexes at T = 2 K (Figure S8) exhibit a fast increase of the magnetization up to ∼10 kOe and then a slow increase with the field without reaching saturation at 50 kOe, which is mainly due to the presence of significant magnetic anisotropy.20 The magnetization values at the highest applied dc magnetic field of 50 kOe are however almost the half of those calculated for noninteracting LnIII ions (Table 5), which can be mainly attributed to crystal-field effects giving rise to significant magnetic anisotropy.19,21 With regard to GdIII complex 3, the room-temperature χMT value of 7.86 cm3·K·mol−1 is close to the calculated value of 7.88 cm3·K·mol−1 for the ground state of the GdIII ion (4f,7 S = 7/2, gJ = 2). When cooled, the χMT product remains constant until ∼25 K and then decreases to reach a value of 7.51 cm3·K· mol−1 at 2 K. This behavior is probably due to the combined action of very weak intermolecular exchange interactions between the GdIII ions in the 1D zigzag chain, very small zero-field splitting (ZFS) of the ground state, which sometimes is observed for this essentially isotropic ion, and Zeeman saturation effects. The field dependence of the magnetization at

molecular structure of 1 is given in Figure 4, while those of 2 and 3 are given in the Supporting Information (Figures S5 and S6). The bond parameters of 1 are summarized in Table 2. The mononuclear complex 1 is formed by the coordination action of a mono-deprotonated ligand [LH]− and two deprotonated coligands, 1,1,1-trifluoroacetylacetonate (tfa). In the formation of 1, the tetradentate [LH]− utilized all the coordinating centers: the coordination mode being μ4−η1:η1:η1:η1. The two tfa ligands function in a chelating manner (Figure 5). All the Dy−Otfa bond distances are nearly similar, 2.318− 2.347 Å, indicating the delocalized form of tfa. These bond lengths are longer than the Dy−Ophenolate, 2.298 Å, but shorter than the Dy−Oalcohol, 2.420 Å. The average Dy−N bond length is found to be 2.506 Å, which is consistent with literature precedents.12,13a,13c,13e, Complexes 4 and 5 are monocationic and isostructural. These crystallize in the monoclinic system in the chiral space group P21 (Z = 2), however, as a racemic mixture. Compound 4 is described below as a representative example (Figure 6). Selected bond parameters of 4 are given in Table 3, while those of 5 are given in the Supporting Information. Complex 4 is assembled by the cumulative coordination actions of two mono-deprotonated ligands [LH]−, which are nearly perpendicular to each other at an angle of 86.9°. A few comments about the bond distances found in 4. The average bond distance of Dy−Ophenolate is 2.264 Å, which is longer than the average bond length found for Dy−Oalcohol, 2.410 Å. Dy−N bond lengths fall in a narrow range, 2.473−2.504 Å, which is consistent with those found in literature.13 In both 1 and 4 DyIII is 8-coordinated: in 1, DyIII is surrounded by six O and two N atoms in a distorted square antiprism geometry, whereas in 4 DyIII possesses a distorted triangular dodecahedron geometry with coordination environment of 4N, 4O (Figure 7). On the one hand, while several mononuclear lanthanide complexes are now known3,7−9 none of these are similar to the present case. On the other hand, the similarity of bond distances in 1 and 4 can be noticed (Table 4). On the one hand, detailed analysis of the crystal packing of 1 reveals the presence of strong intermolecular hydrogen bonding, which leads to formation of a supramolecular one7992


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Dynamic ac measurements of complex 4 under an applied field of 0.1 T show temperature- and field-dependent in-phase and out-of-phase signals with two peaks of different intensity in the ranges of 3.5 K (100 Hz)−4 K (1400 Hz) and 4.0 K (50 Hz)−7.0 K (1400 Hz), the latter relaxation process being much stronger than the former one (Figure 11). The presence of two thermal relaxation processes has been previously observed in other Dy mononuclear SIMs.21 In this case, where all the Dy3+ ions are crystallographically equivalent, this behavior could be due to the existence of two competing relaxation pathways via excited states.22 The χM″ versus frequency data at different temperatures were fitted to the generalized Debye model, but only the relaxation times for the most intense relaxation process could be obtained. From the extracted data the Arrhenius plot was built, which led to following parameters: τo = 3.2 × 10−7 s and Ueff = 44.4(1) K (Figure 11). The Cole−Cole plots show semicircular shapes with α values in the 0.15 (5 K)−0.05 (7 K) region, thus indicating the existence of a dominant relaxation process in this temperature range (Figure S14). The above results clearly show that the substitution of two bidentate β-diketone ligands in 1 by a tetradentate N2O2 ligand, LH−, to afford 4 takes place with a large increase of the effective energy barrier Ueff. To explain this fact, we must analyze the distribution of donor atoms and Dy−donor distances in the Dy3+ ion coordination sphere, which play a central role in dictating the SMM behavior. This is so because low-symmetry Dy3+ complexes like 1 and 4 generally exhibit an axial ground state (with a large contribution of mJ = ±15/2 to its wave function), which has a disc shape electron density distribution. This oblate electron density can be stabilized by an axial crystal field, where the donor atoms with the largest electron densities are located above and below the equatorial plane, thus reducing the electrostatic repulsive interactions.23 The donor atoms exercising the largest electrostatic repulsion with the Dy3+ electron density are those possessing the shortest Dy−donor distances, so that to reduce these repulsive interactions, the Dy3+ disc surface electron density is positioned almost perpendicular to the shortest Dy−donor bonds.24 Consequently, the magnetic moment that is perpendicular to the electron density disc is found in the direction of the shortest Dy−donor bonds. In the case of the heteroleptic complex 1, the shortest Dy−donor distances involve the phenoxide and βdiketone ligand oxygen donor atoms (Dy−O ≈ 2.3 Å). The distribution of these donor oxygen atoms around the Dy3+ is not appropriate to create an axial crystal field, and therefore a small axial anisotropy is expected. As a result, the energy gap between the ground and first excited Kramers doublets (KD) states, which determine the thermal energy barrier in an Orbach mechanism, is expected to be also small, which agrees well with the experimental results. Conversely for complex 4, the Dy−O2phenoxido distance (2.233 Å) is rather shorter than the other Dy−donor distances, which favors a larger axial anisotropy with the anisotropy axis lying parallel to the Dy− O2phenoxido direction. In fact, the calculated direction of the anisotropy axis of the Dy3+ ion by using a simple electrostatic model6c is almost coincident with Dy−O2 bond (Figure S17). Note that other eight-coordinated Dy3+ complexes with a Dy− Ophenoxide distance of ∼2.2 Å, which is at least 0.1 Å shorter than the rest of Dy−donor distances, exhibit strong axial anisotropy in the ground KD with large calculated energy separation between the ground and first excited KDs.24b In view of this, a strong axial anisotropy is also expected in compound 4, which

2 K (Figure S8) shows a relatively rapid increase of the magnetization up to 20 kOe and then a linear increase to reach a value of 7.07 μB at 50 kOe, which is very close to the theoretical saturation value for a GdIII ion with g = 2.0 (7 μB). The magnetic susceptibility and magnetization data of 3 were analyzed simultaneously with the following Hamiltonian: H = −zJ ′⟨S⟩S + gμB SGdH

where −zJ′⟨S⟩S accounts for the intermolecular interactions by means of the molecular field approximation, g is the g factor, μB is the Bohr magneton, and H is the applied magnetic field. The simultaneous best fit of susceptibility and magnetization data afforded the following set of parameters: zJ′ = −0.0063(5) cm−1, g = 2.01(2), and R = 1.1 × 10−5 (R = ∑(χMTexp − χMTcalc)2/(χMTexp)2). These results show that, as expected, the intermolecular interactions are anti-ferromagnetic and very weak. Temperature and frequency dynamic ac magnetic susceptibility measurements performed on these complexes did not show a significant frequency dependence of the in-phase (χM′) and out-of-phase (χM″) signals at zero field above 2 K. This behavior could be due to either a very low energy barrier for the flipping of the magnetization, which is not high enough to trap the magnetization above 2 K, or the existence of a very fast resonant zero-field quantum tunneling of the magnetization (QTM), which leads to a flipping rate that is too fast to observe the maximum in the χM ″ above 2 K. When the ac measurements were performed in the presence of a small external field of 2000 Oe to partly or fully suppress QTM (this field was selected because the relaxation process was shown to be the slowest), complexes 1 and 4 showed slow relaxation of the magnetization (Figures 10, 11, and S9−S14), whereas the rest of complexes exhibit a very weak frequency dependence of the ac susceptibility signals without any clear maximum of the out-of-phase susceptibility signal above 2 K (Figures S15 and S16). It is worth noting that the application of a small dc field to eliminate, at least partly, QTM fast relaxation, generally gives rises to a slowdown of the relaxation with a concomitant increase of Ueff and a decrease in τ0. Complex 1 presents slow relaxation of the magnetization below 10 K, with maximum of the out-of-phase signal (χM″) at 3.5 K at the highest measured frequency (1488 Hz; Figure 10). The relaxation times (τ) were obtained from the fitting of the frequency dependence of χM″ at each temperature to the generalized Debye model (Figure S10). Nevertheless, the extracted values for the relaxation times should be taken with caution, as no well-defined maxima were observed above 2.5 K in the frequency dependence of the of χM″ susceptibility. The fit of the relaxation times to the Arrhenius equation in the 2− 3.5 K temperature range afforded an effective energy barrier for the reversal of the magnetization (Ueff) and τo values of 4.6 K and 3.66 × 10−5 s, respectively. The τo value is larger than those usually found for Dy-based SMMs, which suggests the presence of competing relaxation modes (probably the QTM has not been fully eliminated). In fact, the Cole−Cole plots show semicircular shapes with α values in the 0.45 (2 K)−0.25 (3.5 K) region, thus confirming the presence of a distribution of relaxation processes (Figure S11). The α parameter determines the width of the distribution of relaxation times, so that α = 1 corresponds to an infinitely wide distribution of relaxation times, whereas α = 0 represents a process with only a single relaxation time. 7993


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Inorganic Chemistry could lead to a large Ueff thermal energy barrier, in good agreement with the experimental results. Photophysical Properties. Electronic Spectra of the Complexes (1−5). The UV/vis absorption spectra, recorded in CH3OH solution (c = 1 × 10−5 M) at 298 K for LH2 and complexes 1−5, are depicted in Figure 12. The ligand-centered absorption properties of complexes 1−5 are summarized in Table 6. The ligand (LH2) displays absorption maxima at 291 and 360 nm. These are the result of spin-allowed singlet π−π* transitions. The absorption spectra of 1−5 reveal evidence of perturbation by metal coordination. 1−3 show absorption maxima at 293, 292, and 291 nm, respectively, which may arise due to the n−π* transitions of the trifluoroacetylacetonate (tfa) ligand motif present in the complexes. In addition, absorption peaks are observed at 341 and 445 nm, which may be due to the ligand LH2. Finally, a broad absorption peak is observed at 445 nm, which is due to the metal-to-ligand charge transfer (1MLCT) transition. The molar absorption coefficients are summarized in Table 6. Complexes 4 and 5 reveal the presence of absorptions at 451 and 336 nm. It may be noted that the molar absorption coefficients in the complexes are quite high suggesting that the ligand could be involved for sensitization of the lanthanide luminescence. Photoluminescent Properties. To understand the photophysical properties of 1−5, in particular, the energy migration pathways, it is important to determine the singlet and triplet energy levels of LH2. The UV/vis upper absorption edge of the Gd3+ complex (3) was used to calculate the singlet (1ππ*) energy level of LH2. On the one hand, the value obtained from the absorption edge of the 1MLCT band (530 nm; 18 867 cm−1) is below the 5D4 level of the terbium ion (20 500 cm−1) suggesting that LH2 cannot be used for sensitizing the lanthanide ion. On the other hand, the upper absorption edge (1ππ* peak) due to the tfa ligand (Figure 12; 340 nm (29 411 cm−1) is appropriate for sensitization. The triplet energy (3ππ*) level was determined by an experiment involving a low-temperature (77 K) phosphorescence measurement of the gadolinium derivative (Figure 13), and the value obtained is 422 nm (23 696 cm−1). From the literature it is well-established that Gd3+ complexes are ideal for determining triplet energy levels (3ππ*) of the ligand because of the following reasons: (1) the lowest excited energy level (6P7/2) for Gd3+ is at 32 150 cm−1, which does not allow energy transfer to the gadolinium ion from the ligand and (2) the heavy paramagnetic ion effect of Gd3+ increases intersystem crossing (ISC) from the singlet to the triplet state.25a Thus, the luminescence observed for the gadolinium complexes can be considered as arising due to the ligand. It has been established that ISC is effective if the singlet and triplet energy gap {ΔE(1ππ*−3ππ*)} of the ligand is close to 5000 cm−1.25b,c In the present study, as shown above for 3, it is 5715 cm−1. Thus, this ligand manifold has the capability for inducing efficient ISC. Among all the complexes studied only the TbIII derivative (2) displays a metal-centered luminescence in 5 μM solution in methanol. The steady-state excitation and emission spectra of 2 at room temperature is shown in Figure 14. The excitation spectra of 2 reveals a broad band in the 260330 nm region (centered at ca. 302 nm because of the π−π* transitions of the ligand). The absence of a ligand-centered emission indicates an efficient energy-transfer process from the ligand excited states to the metal ion. Upon excitation at the ligand energy level (λex

= 302 nm), 2 exhibits sharp emission bands at 490, 545, 585, and 620 nm. These are characteristic of Tb3+ emission resulting from the deactivation of 5D4 excited state to 7FJ ground state (J = 6, 5, 4, 3).25b Among the emission peaks the most intense emission at 545 nm corresponds to the transition of 5D4→7F5. The room-temperature excited-state 5D4 (Tb3+) luminescence lifetime values (monitored at 545 nm) were found to be τ0 = 0.276 ms for compound 2 with a single exponential decay curve. This observation suggests a single terbium-emitting center. The 5D4 lifetime values of the Tb3+ complex 2 is found to be temperature-dependent. This is indicated by the variation in τRAD (Table 7) from 298 (Figure 15) to 77 K. Thus, it seems likely that in this system, such as what is found in other luminescent lanthanide complexes, temperature-dependent vibrational quenching is dominant.24d To quantify the ability of the ligands to sensitize the luminescence of lanthanides, we analyzed the emission in terms of Equation 2 (below), where Φoverall and ΦLn represent the ligand-sensitized and intrinsic luminescence quantum yields of Ln3+; Φsen is the efficiency of the ligand-to-metal energy transfer, and τobs and τRAD are the observed and the radiative lifetimes of Ln3+.25e Φoverall = Φsen × ΦLn = Φsen × (τobs/τRAD)

(2)

The intrinsic quantum yield for Tb3+ (ΦTb) was estimated using Equation 3 with the assumption that the decay process at 77 K in a deuterated solvent is purely radiative.25f ΦTb = τobs(298 K)/τobs(77 K)

(3)

Table 7 summarizes the Φoverall, ΦLn, and Φsen. In the case of terbium luminescence, solution-state quantum yields were calculated relative to quinine sulfate as the standard (0.05 M H2SO4 solution with Φstd = 54%). The large difference in the ligand singlet−triplet energy difference (5715 cm−1) in complex 2 suggests a less-efficient energy transfer as evidenced from the poor sensitization efficiency of 4.5%, which in turn leads to a weak quantum yield of 1.8%.

■

CONCLUSION In summary, structural, photophysical, and magnetic properties of two series of mononuclear lanthanide complexes assembled by a Schiff base ligand have been described. The structural differences between these two series of complexes can be attributed to the presence of the two tfa ligands along with a Schiff base ligand in one series, while the other contains only Schiff base ligands. The dynamic ac susceptibility measurements in the presence of dc field reveal the SMM behavior of compounds 1 and 4 with anisotropic energy barriers of 4.6 and 44.4 K, respectively. The enhancement of the Ueff in 4 seems to be the result of a strong axial anisotropy, which is imposed by the existence in its coordination sphere of a Dy−O bond distance that is much shorter than the remaining Dy−donor distances. Detailed photophysical measurements revealed that complex 2 exhibited a series of sharp emission bands at 490, 545, 585, and 620 nm, affirming the ability of the tfa ligand to sensitize the luminescence of Tb3+.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00689. 7994


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V.; van Leusen, J.; Herrera, J. M.; Koegerler, P.; Colacio, E.; Chandrasekhar, V. Homometallic DyIII Complexes of Varying Nuclearity from 2 to 21: Synthesis, Structure and Magnetism. Chem. - Eur. J. 2017, 23, 5154. (i) Biswas, S.; Das, S.; van Leusen, J.; Kogerler, P.; Chandrasekhar, V. Pentanuclear [2.2] spirocyclic lanthanide (III) complexes: slow magnetic relaxation of the DyIII analogue. Dalton Trans. 2015, 44, 19282−19293. (j) Biswas, S.; Das, S.; Gupta, T.; Singh, S. K.; Pissas, M.; Rajaraman, G.; Chandrasekhar, V. Observation of Slow Relaxation and Single-Molecule Toroidal Behavior in a Family of Butterfly-Shaped Ln4 Complexes. Chem. - Eur. J. 2016, 22, 18532− 18550. (k) Biswas, S.; Das, S.; van Leusen, J.; Kogerler, P.; Chandrasekhar, V. Tetranuclear [2× 2] Square-Grid Lanthanide(III) Complexes: Syntheses, Structures, and Magnetic Properties. Eur. J. Inorg. Chem. 2014, 2014, 4159−4167. (l) Mazarakioti, E. C.; CunhaSilva, L.; Bekiari, V.; Escuer, A.; Stamatatos, T. C. New structural topologies in 4f-metal cluster chemistry from vertex-sharing butterfly units: {LnIII7} complexes exhibiting slow magnetization relaxation and ligand-centred emissions. RSC Adv. 2015, 5, 92534−92538. (m) Biswas, S.; Jena, H. S.; Adhikary, A.; Konar, S. Two Isostructural 3D Lanthanide Coordination Networks (Ln = Gd3+, Dy3+) with Squashed Cuboid-Type Nanoscopic Cages Showing Significant Cryogenic Magnetic Refrigeration and Slow Magnetic Relaxation. Inorg. Chem. 2014, 53, 3926−3928. (n) Biswas, S.; Mondal, A.; Konar, S. Densely Packed Lanthanide Cubane Based 3D Metal−Organic Frameworks for Efficient Magnetic Refrigeration and Slow Magnetic Relaxation. Inorg. Chem. 2016, 55, 2085−2090. (o) Mondal, A.; Parmar, V. S.; Biswas, S.; Konar, S. Tetrahedral MII based binuclear double-stranded helicates: single-ion-magnet and fluorescence behavior. Dalton Trans. 2016, 45, 4548−4557. (2) (a) Mannini, M.; Pineider, F.; Danieli, C.; Totti, F.; Sorace, L.; Sainctavit, P.; Arrio, M. A.; Otero, E.; Joly, L.; Cezar, J. C.; Cornia, A.; Sessoli, R. Quantum tunnelling of the magnetization in a monolayer of oriented single-molecule magnets. Nature 2010, 468, 417−421. (b) Gonidec, M.; Biagi, R.; Corradini, V.; Moro, F.; De Renzi, V.; del Pennino, U.; Summa, D.; Muccioli, L.; Zannoni, C.; Amabilino, D. B.; Veciana, J. Surface supramolecular organization of a terbium(III) double-decker complex on graphite and its single molecule magnet behavior. J. Am. Chem. Soc. 2011, 133, 6603−6612. (c) Urdampilleta, M.; Klyatskaya, S.; Cleuziou, J. P.; Ruben, M.; Wernsdorfer, W. Supramolecular spin valves. Nat. Mater. 2011, 10, 502−506. (d) Candini, A.; Klyatskaya, S.; Ruben, M.; Wernsdorfer, W.; Affronte, M. Graphene spintronic devices with molecular nanomagnets. Nano Lett. 2011, 11, 2634−2639. (e) Alexandru, M. G.; Visinescu, D.; Shova, S.; Andruh, M.; Lloret, F.; Julve, M. Synthesis, Crystal Structures, and Magnetic Properties of Two Novel CyanidoBridged Heterotrimetallic {CuIIMnIICrIII} Complexes. Inorg. Chem. 2017, 56, 2258−2269. (3) Ishikawa, N.; Sugita, M.; Ishikawa, T.; Koshihara, S. -y.; Kaizu, Y. Lanthanide double-decker complexes functioning as magnets at the single-molecular level. J. Am. Chem. Soc. 2003, 125, 8694−8695. (4) (a) Zadrozny, J. M.; Xiao, D. J.; Long, J. R.; et al. Mössbauer Spectroscopy as a Probe of Magnetization Dynamics in the Linear Iron(I) and Iron(II) Complexes [Fe(C(SiMe3)3)2]1−/0. Inorg. Chem. 2013, 52, 13123−13131. (b) Vaidya, S.; Upadhyay, A.; Singh, S. K.; Gupta, T.; Tewary, S.; Langley, S. K.; Walsh, J. P. S.; Murray, K. S.; Rajaraman, G.; Shanmugam, M. A synthetic strategy for switching the single ion anisotropy in tetrahedral Co(II) complexes. Chem. Commun. 2015, 51, 3739−3742. (c) Zadrozny, J. M.; Atanasov, M.; Bryan, A. M.; Lin, C.-Y.; Rekken, B. D.; Power, P. P.; Neese, F.; Long, J. R. Slow magnetization dynamics in a series of two-coordinate iron(II) complexes. Chem. Sci. 2013, 4, 125−138. (d) Zadrozny, J. M.; Xiao, D. J.; Long, J. R.; Atanasov, M.; Neese, F.; Grandjean, F.; Long, G. J. Magnetic blocking in a linear iron(I) complex. Nat. Chem. 2013, 5, 577−581. (e) Baldoví, J. J.; Cardona-Serra, S.; Clemente-Juan, J. M.; Coronado, E.; Gaita-Ariño, A.; Palii, A. Rational design of single-ion magnets and spin qubits based on mononuclear lanthanoid complexes. Inorg. Chem. 2012, 51, 12565−12574. (f) Botezat, O.; van Leusen, J.; Kravtsov, V. C.; Kogerler, P.; et al. Ultralarge 3d/4f Coordination Wheels: From Carboxylate/Amino Alcohol-Supported {Fe4Ln2} to

ESI-MS spectra, molecular structure, list of bond lengths and angles of the complexes 2, 3, and 5. Field dependence of the magnetization at 2 K for complexes 1−5, temperature dependence of the in-phase χM′ component of the ac susceptibility at different frequencies under an applied magnetic field of 2000 G, variable-temperature frequency dependence of the χM″ signal and Cole−Cole plots for complexes 1 and 4. Temperature dependence of the in-phase χM′ (inset) and out-of-phase χM″ components of the ac susceptibility at different frequencies under an applied magnetic field of 2000 G for complexes 2 and 3. Orientation of the magnetic moment for complex 4 (PDF) Accession Codes

CCDC 1535356−1535359 and 1535361 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (V.C.) *E-mail: [email protected]. (E.C.) *E-mail: [email protected]. (M.L.P.R.) ORCID

Vadapalli Chandrasekhar: 0000-0003-1968-2980 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Dept. of Science and Technology (DST), India, for financial support, including support for a single-crystal CCD X-ray diffractometer facility at IIT-Kanpur. V.C. is grateful to the DST for a J. C. Bose fellowship. S.B. would like to thank Science and Engineering Research Board (SERB), New Delhi, India for a National Post-Doctoral fellowship (PDF/2016/ 001362). This work was supported by the Junta de Andaluciá (FQM-195 and the Project of Excellence P11-FQM-7756), MINECO of Spain (Project CTQ2014-56312-P), the Univ. of Granada, and the Univ. of The Basque Country UPV/EHU (Project GIU14/01).

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