Influence of Radicals on Magnetization Relaxation Dynamics of

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Influence of Radicals on Magnetization Relaxation Dynamics of Pseudo-Octahedral Lanthanide Iminopyridyl Complexes Chinmoy Das, Apoorva Upadhyay, and Maheswaran Shanmugam* Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, Maharashtra, India

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S Supporting Information *

ABSTRACT: Controlling quantum tunneling of magnetization (QTM) is a persistent challenge in lanthanide-based single-molecule magnets. As the exchange interaction is one of the key factors in controlling the QTM, we targeted lanthanide complexes with an increased number of radicals around the lanthanide ion. On the basis of our targeted approach, a family of pseudo-octahedral lanthanide/transition-metal complexes were isolated with the general molecular formula of [M(L•−)3] (M = Gd (1), Dy (2), Er (3), Y (4)) using the redox-active iminopyridyl (L•−) ligand exclusively, which possess the highest ratio of radicals to lanthanide reported for discrete metal complexes. Direct current magnetic susceptibility studies suggest that dominant antiferromagnetic interactions exist between the radical and lanthanide ions in all of the complexes, which is strongly corroborated by magnetic data fitting using a Heisenberg−Dirac−Van Vleck (HDVV) Hamiltonian (−2J Hamiltonian). A good agreement between the fit and the experimental magnetic data obtained using g = 2, Jrad‑rad = −111.9 cm−1 for 4 and g = 1.99, Jrad‑rad = −111.9 cm−1, JGd‑rad = −1.85 cm−1 for 1. Complex 2 shows frequency-dependent slow magnetization relaxation dynamics in the absence of an external magnetic field, while 3 shows field-induced frequency-dependent χM′′ signals. An ideal octahedral geometry around the lanthanide ion is predicted to be unsuitable for the design of a single-molecule magnet (SMM); nevertheless, complex 2 exhibits slow relaxation of magnetization with a record high anisotropy barrier for a six-coordinate Dy(III) complex. A rationale for this unusual behavior is detailed and reveals the strength of the synthetic methodology developed.

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INTRODUCTION When an anisotropic lanthanide ion placed in a suitable ligand field, the ligand field lifts the degeneracy of mj levels, which leads to a slow magnetization relaxation called single-molecule magnets (SMMs). Such SMM behavior was found in [Tb(Pc)2]− complex by Ishikawa and co-workers in 2003 which shows the largest anisotropic barrier1 in comparison to any transition-metal-based SMMs reported to date.2 Later on several mononuclear3 and polynuclear lanthanide-based SMMs flooded into the literature4 with a record high blocking temperature (60 K) reported for a two-coordinate mononuclear Dy(III) complex by Chilton, Mills, and co-workers.3e,f Equally fascinating slow relaxation was brought about by a toroidal arrangement of the magnetization vector in {CrDy6},5 a {Dy3} triangle (spin chirality),6 and other polynuclear lanthanide clusters. 4b,7 However, controlling the rapid quantum tunneling of magnetization (QTM) in the ground state mj levels is still a challenging task in the realization of molecular-based storage devices.4b,7b,4c Recently, Long and coworkers elegantly established in a series of dimeric [Ln2(N2)3−]8 and [Ln2(Lredox)] (where Lredox = pyrimidine and tetrapyridylpyrazine) complexes9 that, by enhancing the exchange interaction,10 QTM can be quenched significantly, which results in isolation of a [Tb2(N2)3−] complex with a TB value of 14 K.11 A similar conclusion equally holds true in © XXXX American Chemical Society

certain 3d−4f and 4f metal complexes such as [Cr2Dy2], [Dy2], [Ni2Dy2], etc. reported by Murray and co-workers, Powell and co-workers, and us, respectively.12a−c,7a,12d−f On a similar note, Gao and co-workers have pointed out an alternate strategy (i.e., enhancing the metal ligand covalency) which facilitates the quenching of QTM in a two-coordinate Co(II) complex. This resulted in a record high magnetization blockade barrier (413 cm−1 (636 K)) for this complex in comparison to any transition-metal-based SIM or SMM known in the literature.13 As pointed out earlier, in order to quench QTM in lanthanide ions, intramolecular exchange interactions need to be increased significantly. In comparison to transition-metal ions or certain other radical-containing ligands, such as nitronyl nitroxide and semiquinones,14 the exchange interaction exerted by a redox-active ligand is expected to be strong; therefore, we have focused and employed a redox-active iminopyridyl ligand (L•−), not only to isolate discrete lanthanide complexes but also to increase the number of radicals per lanthanide ions. In this article, we report four structurally analogous pseudo-octahedral lanthanide/transition-metal complexes [M(L•−)3] (where M = Gd (1), Dy (2), Er (3), Y (4)) using L•− exclusively. Due to the presence Received: April 11, 2018

A

DOI: 10.1021/acs.inorgchem.8b00979 Inorg. Chem. XXXX, XXX, XXX−XXX

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crystal X-ray diffraction. Data for 1: yield 0.059 g, 24.68% (based on Gd). Elemental analysis (Calcd %; argon-dried sample): C, 67.82; H, 6.96; N, 8.79, Found (%): C, 67.66; H, 6.82; N, 8.69. Synthesis of [M(L•−)3] (M = Dy (2), Er (3), Y (4)). A synthetic procedure similar to that for 1 was followed, but Gd[N(SiMe3)2]3 was replaced by the corresponding lanthanide/transition-metal silylamide precursor [M[N(SiMe3)2]3 (M = Dy, 0.161 g; M = Er, 0.162 g; M = Y, 0.143 g). Data for 2: yield 0.054 g, 22.45% (based on Dy). Elemental analysis (Calcd %; argon-dried sample): C, 67.45; H, 6.92; N, 8.74. Found (%): C, 67.27; H, 6.86; N, 8.61. Data for 3: yield 0.062 g, 25.68% (based on Er). Elemental analysis (Calcd %; argondried sample): C, 67.11; H, 6.88; N, 8.7. Found (%): C, 67.12; H, 6.73; N, 8.61. Data for 4: yield 0.052 g, 23.42% (based on Y). Elemental analysis (Calcd %; argon-dried sample): C, 73.03; H, 7.49; N, 9.46. Found (%): C, 72.82; H, 7.39; N, 9.38. Synthesis of 50% Diluted Sample of 2. The same synthetic procedure as for 2 was followed, but half the amounts of Dy[N(SiMe3)2]3 (0.0804 g, 0.125 mmol) and Y[N(SiMe3)2]3 (0.07125 g, 0.125 mmol) were used in place of the pure dysprosium silylamide precursor. Green single crystals were grown after 5 days, which were isolated and dried under inert atmosphere before use for ac relaxation measurements.

of exchange interactions between lanthanide ions and radicals, zero-field SMM behavior was observed in a pseudo-octahedral Dy(III) (complex 2), a geometry otherwise predicted to be nonideal to reveal SMM behavior in zero applied magnetic field, which is supported by ab initio calculations (vide infra).

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EXPERIMENTAL SECTION

Materials and General Methods. Unless otherwise stated, all reactions were carried out under an inert (Ar/N2) atmosphere. The αiminopyridine ligand (L; 2,6-diisopropyl-N-(2-pyridinylmethylene)phenylamine)15 and M[N(SiMe3)2]3 (M = Gd(III), Dy(III), Er(III), Y(III)) precursors were synthesized as per the literature reports.16 THF, hexane, and DME solvents were dried with sodium and benzophenone and distilled prior to use. All other chemicals were purchased from commercially available sources (Alfa Aesar and Sigma-Aldrich). Single-crystal data were collected on a Rigaku Saturn CCD diffractometer using a graphite monochromator (Mo Kα, λ = 0.71073 Å). The selected crystals were mounted on the tip of a glass pin using mineral oil and placed in the cold flow produced with an Oxford Cryo-cooling device. Complete hemispheres of data were collected using ω and φ scans (0.3°, 16 s per frame). Integrated intensities were obtained with Rigaku Crystal Clear-SM Expert 2.1 software, and they were corrected for absorption correction. Structure solution and refinement were performed with the SHELX package. The structures were solved by direct methods and completed by iterative cycles of ΔF syntheses and full-matrix least-squares refinement against F2 (CCDC numbers: 1444293−1444296). The magnetic susceptibility measurements were obtained with the use of a MPMS-XL SQUID magnetometer equipped with 70 kOe superconducting magnet. Measurements were performed on polycrystalline samples and the magnetic data were corrected for the sample holder and diamagnetic contribution. Samples were prepared using glovebox techniques under an argon atmosphere. To shed light on the mechanism of relaxation, g-tensors and blockade barrier ab initio calculations were performed on the lanthanide(III) ions for complexes 2 and 3 using the MOLCAS 8.0 suite of software.17 Relativistic effects were taken into account on the basis of the Douglas−Kroll Hamiltonian.18 The spin-free Eigen states were achieved by the complete active space self-consistent field (CASSCF) method.19 To avoid the various issues regarding the radical calculations, inclusion of the radical associated with the ligand was ignored and hence all of the ligands were considered as neutral with a total charge of +3 on each metal complex. We have employed the [ANO-RCC···8s7p5d3f2g1h] basis set for Dy and Er atoms,20 the [ANO-RCC···3s2p] basis set for C atoms, the [ANO-RCC···2s] basis set for H atoms, and the [ANORCC···3s2p1d] basis set for N atoms. The CASSCF calculations that were performed included 9 electrons across 7 4f orbitals of the Dy(III) ion (9, 7) and 11 electrons across 7 4f orbitals of the Er(III) ion (11, 7) in 2 and 3, respectively. With this active space, 21 roots were computed for complex 2. For complex 3, 35 quartets and 112 doublets using the configuration interaction (CI) procedure were considered to compute the anisotropy. After computing all these excited states, we mixed all 35 quartets and all 112 doublets using the RASSI-SO module to compute the spin−orbit coupled states.21 Moreover, these computed SO states were considered in the SINGLE_ANISO program to compute the g tensors of complexes 2 and 3.22 The High Cholesky decomposition for two-electron integrals was employed throughout in the calculations to reduce the disk space. Crystal-field parameters were extracted using the SINGLE_ANISO code, as implemented in MOLCAS 8.0.17 Synthesis of [Gd(L•−)3] (1). The neutral ligand (L; 0.200 g, 0.7508 mmol) was reduced with sodium metal (0.0184 g, 0.8008 mmol) in dry THF, with stirring for 4 h. Gd[N(SiMe3)2]3 (0.1597 g, 0.2503 mmol) was added and the mixture stirred for another 16 h at room temperature. The solvents were removed under reduced pressure, and the product of interest was extracted with dry hexane. Again, hexane was removed under reduced pressure, and the resultant crude product was crystallized from hexane/DME mixture. Green single crystals were grown after 4 days which were suitable for single-

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RESULTS AND DISCUSSION The reaction of 1 equiv of M[N(SiMe3)2]3 (M = Gd or Dy or Er or Y) with 3 equiv of one-electron-reduced iminopyridyl ligand (L•−) in THF, followed by crystallization in a DME/ hexane mixture, yielded block-shaped green crystals that were suitable for single-crystal X-ray diffraction (Scheme 1). Scheme 1. General Synthetic Procedure Followed To Isolate Complexes 1−4

All four complexes are structurally analogous to each other, which is evidently reflected from their crystallographic parameters (see Table 1). All the complexes crystallized in a trigonal system with R3̅ space group. A representative crystal structure of 2 is shown in Figure 1 (see also Figure S1 in the Supporting Information). The asymmetric unit of complexes 1−4 contains only one-third of the molecule with one M(III) and L•−, and the remaining fragment is generated by rotational and inversion symmetry. In all the complexes, M(III) ion lies in a special position and the pseudo-C3 symmetry axis passes through it. The trivalent cationic charge of the lanthanide ions in all the complexes is neutralized by three singly reduced iminopyridyl radical anionic ligands. The one-electron reduction of this redox-active ligand was confirmed by the C16−N12 bond lengths, which were found to be 1.346(1), 1.351(1), 1.354(2), and 1.360(1) Å in complexes 1−4, respectively, consistent with the literature reports.23 The M1−N12(imine) bond lengths were found to be 2.413(1), 2.386(1), 2.357(1), and 2.372(1) Å for complexes 1−4, respectively. Similarly, the M1− B

DOI: 10.1021/acs.inorgchem.8b00979 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Parameters for Complexes 1−4 formula Size (mm) system space group a (Å) b (Å) c (Å) γ (deg) V (Å3) Z ρcalcd (g cm−3) 2θmax (deg) radiation λ (Å) T (K) no. of rflns no. of indep rflns no. of rflns with >2σ(I) R1 wR2

1

2

3

4

C54H66N6Gd 0.095 × 0.057 × 0.052 trigonal R3̅ 18.7069(8) 18.7069(8) 25.453(2) 120.000 7713.9(9) 6 1.235 58.4 Mo Kα 0.71073 100 15953 4618 4062 0.0611 0.1344

C54H66N6Dy 0.10 × 0.07 × 0.06 trigonal R3̅ 18.6701(9) 18.6701(9) 25.4871(17) 120.000 7693.9(9) 6 1.245 58.2 Mo Kα 0.71073 100 35546 4605 4307 0.0328 0.0859

C54H66N6Er 0.13 × 0.087 × 0.079 trigonal R3̅ 18.618(4) 18.618(4) 25.516(5) 120.000 7660(4) 6 1.257 58.3 Mo Kα 0.71073 100 32071 4599 4403 0.0420 0.1005

C54H66N6Y 0.168 × 0.154 × 0.080 trigonal R3̅ 18.7115(9) 18.7115(9) 25.575(2) 120.000 7754.7(10) 6 1.141 58.25 Mo Kα 0.71073 100 18986 4633 3457 0.0593 0.1301

Selected bond lengths and bond angles for all of the complexes are given in Table S1 in the Supporting Information. The significantly shorter bond length of M1− N12(imine) in comparison to that of M1−N11(pyridine) is likely due to the additional radical electron density distributed over π* orbitals of the imino (−CN) group in L•−. The closest M(III)···M(III) distance in the packing diagrams (Figure S2 for complex 2) of all complexes are observed to be Gd···Gd = 8.382(6) Å, Dy···Dy = 8.353(6) Å, Er···Er = 8.338(6) Å, and Y···Y = 8.363(6) Å. The coordination potential and electronic structures of the employed iminopyridyl-containing transition-metal or maingroup-metal complexes have been very well established and heavily investigated in the literature.23a,b,24,23c−e On the other hand, lanthanide complexes with an exclusively redox active ligand such as L•− are extremely scarce in the literature.25 To be more specific, using L•− exclusively, there has been only one Yb(III) complex reported by Trifonov et al. elsewhere without any detailed magnetic studies.26 Lanthanide complexes isolated with redox-active ligands are often supplemented by other auxiliary ligands.9 Another interesting aspect of this reported series is that each lanthanide complex contains three radicals, which is again unprecedented in the literature. The majority of the discrete lanthanide complexes reported in the literature contain a maximum of two radicals,9b while a rare 2D-extended structure reported by Dunbar and co-workers possesses even an higher metal to radical ratio.27 In each complex (1−4) the metal ion is surrounded by three chelating iminopyridyl ligands and exists in a distorted-octahedral geometry. The Continuous Shape Measurement (CShM) software28 indeed supports that the geometry around M(III) is in close agreement with an octahedral rather than trigonal-prismatic (Figure S3 in the Supporting Information) geometry. A coordination number of 6 for a lanthanide ion is rather unusual, considering its large ionic radii.29a,4a,29b,c,16,14 dc Magnetic Susceptibility Measurements. Variabletemperature direct current magnetic susceptibility measurements were performed on polycrystalline samples of 1−4 in

Figure 1. (A) Representative crystal structure of 2. Side chain carbon atoms and hydrogen atoms are removed for clarity. (B) Polyhedral view showing the distorted-octahedral geometry around the Dy(III) ion in 2.

N11(pyridine) bond lengths were observed to be 2.465(1), 2.439(1), 2.413(3), and 2.431(1) Å in 1−4, respectively. C

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value of 5.27 cm3 K mol−1 at 2.0 K. Several factors are likely to play roles in this scenario such as antiferromagnetic exchange coupling between the Gd(III) ion and the radical anionic ligands, intermolecular antiferromagnetic interaction, and/or magnetic anisotropy and/or ASE interaction. To infer the strength of exchange coupling in complex 1, we have fitted the experimental χMT(T) data31 using the HDVV Hamiltonian given in eq 2.

the presence of an external magnetic field (1 kOe) between 2.0 and 300 K (Figure 2). The room-temperature χMT values of

Ĥ = −2J1[(S1̂ ·S2̂ ) + (S2̂ ·S3̂ ) + (S1̂ ·S3̂ )] − 2J2 [(Gd1·S1̂ ) + (Gd1·S2̂ ) + (Gd1·S3̂ )] + gμB S ·H (2)

In the above equation J1 represents the intramolecular radical−radical coupling and J2 represents the intramolecular exchange coupling between the Gd(III) ion and the radical anionic ligands. Attempt to fit the magnetic data of 1 only with J1 yielded a poor fit (Figure S4 in the Supporting Information). However, inclusion of J2 gives an excellent fit to the magnetic data (Figure 2) using the SH parameters J1 = −111.9 cm−1, J2 = −1.85 cm−1, g = 1.995, and zj = −0.011 cm−1. The extracted parameters unambiguously confirm that there is a significant exchange interaction between the Gd(III) ion and the paramagnetic radical anionic ligands. The observed J2 value in complex 1 is lower than the J values reported for [Gd2(N2)3−]− and [Gd2(Lredox)] (where Lredox = pyrimidine and tetrapyridylpyrazine) complexes,8,9 and is an order of magnitude higher than the Gd···Gd exchange interaction mediated by phenoxylate, carboxylate, or hydroxo- or azidobridged complexes reported in the literature.32,7a,c A similar or even stronger radical−radical exchange coupling is witnessed in many transition-metal and/or main-group-metal complexes with iminopyridyl ligands.33a,23a,24,33b,23c−e On the other hand, χMT(T) profiles of both complexes 2 and 3 show a trend similar to that for 1 over the entire temperature range measured. This qualitatively ascertains that the magnitude of the antiferromagnetic exchange interaction of Dy(III) or Er(III) with the radical anionic ligand is similar to that of Gd(III) ions in 1. To calculate empirically the exchange interaction between the radicals and the anisotropic ion in 2 and 3, the exchange interaction extracted for 1 (JGd‑rad) was rescaled by multiplying by values of 5/2 (spin of Dy(III), for 2) and 3/2 (spin of Er(III), for 3) and dividing by 7/2 (spin value of Gd(III)), resulting in JDy‑rad = −1.32 cm−1 and JEr‑rad = −0.79 cm−1. Such an empirical approach was successfully employed to estimate the exchange interaction between Cr(III) and Dy(III) ions in a heteronuclear {Cr2Dy2} complex by Murray and co-workers and similarly by us.7a,12c Overall dc magnetic susceptibility studies of these complexes unmistakably demonstrate that the paramagnetic radical ligand is exchange-coupled to the Gd(III), Dy(III), and Er(III) ions and that is likely to have a huge effect on the magnetization relaxation of these complexes (vide infra). Field-dependent magnetization for complexes 1−4 was recorded between 2 and 6 K (Figure S5 in the Supporting Information), and the nonsuperimposable nature of the reduced magnetization curves evidently show magnetic anisotropy associated with complexes 2 and 3 (Figure S6 in the Supporting Information). The fielddependent magnetization data collected on a polycrystalline sample of 1 at 2.0 K tend to saturate around 5.6 NμB, consistent with the overall ground state (S) value of 3 for 1 (Figures S5 and S7 in the Supporting Information).

Figure 2. Variable-temperature dc magnetic susceptibility measurements performed on polycrystalline samples of 1−3 (see inset for 4) in the presence of 1 kOe magnetic field. The solid red line represents the best fit obtained for 1 and 4 using the parameters described in the main text.

8.37, 15.29, 12.60, and 1.11 cm3 K mol−1 for 1−4, respectively, are in close agreement with the expected values for 2 and 3 (15.30 and 12.60 cm3 K mol−1 for 2 and 3, respectively), while they are slightly lower than the Curie calculated values of 9.0 and 1.125 cm3 K mol−1 for 1 and 4, respectively, for three magnetically uncoupled radical anions (in 4 (g = 2)) and lanthanide ions (Gd(III), g = 2 and 8S; Dy(III), g = 4/3 and 6 H15/2; Er(III), g = 6/5; 4I15/2). To understand the intramolecular radical−radical coupling in 4, its magnetic data were analyzed initially. The χMT value abruptly decreases from room temperature up to 21 K for 4. This unmistakably suggests that the radicals in the iminopyridyl ligands are coupled antiferromagnetically. When the temperature is lowered further, a plateau around 0.39 cm3 mol−1 K suggests that a ground state of S = 1/2 tends to stabilize at this temperature. Below 15.0 K, the χMT value decreases again and reaches a final value of 0.09 cm3 K mol−1 at 2.0 K. Using an HDVV (eq 1) Hamiltonian, we could fit the magnetic data up to 17 K from room temperature using the parameters J1 = −111.9 cm−1, g = 2, and TIP = 1.5 × 10−3 cm3 K mol−1 (see Figure S4 in the Supporting Information). Ĥ = −2J1[(S1̂ ·S2̂ ) + (S2̂ ·S3̂ ) + (S1̂ ·S3̂ )] + gμB S ·H

(1)

The pseudo-equilateral-triangular arrangement and strong antiferromagnetic exchange interaction (J1) between the radicals in 4 result in spin frustration. Such a scenario in transition-metal complexes has led to antisymmetric exchange (ASE)30 due to the strong spin−orbit coupling (SOC). Since SOC is extremely weak in organic radicals, the existence of ASE in 4 is very unlikely. Hence, by consideration of intermolecular antiferromagnetic coupling (zj = −1.55 cm−1), a reasonably good fit was obtained for χMT below 17 K (inset of Figure 2). In contrast, the χMT value of 1 gradually decreases upon lowering the temperature to 45 K from room temperature. Below this temperature the χMT value precipitously drops to a D

DOI: 10.1021/acs.inorgchem.8b00979 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (A, B) Frequency-dependent in-phase and out-of-phase magnetic susceptibility of 2 in the absence of dc field, respectively. (C) Cole− Cole plot of the complex 2 measured in the absence of a dc field (Hdc= 0 Oe). The solid red lines are the best fit obtained for complex 2 using a generalized Debye model with broad distribution of relaxations (0 < α1 < 0.41; 0.1 < α2 < 0.34). (D) Arrhenius plot constructed from the relaxation time extracted from the Cole−Cole fit.

Figure 4. (A, B) Frequency-dependent in-phase and out-of-phase magnetic susceptibility of 2 in the presence of a bias field (Hdc = 600 Oe). (C) Cole−Cole plot of complex 2 measured in the absence of a dc field. The solid red lines are the best fit obtained for complex 2 using a generalized Debye model with broad distribution of relaxations (0.02 < α1 < 0.62; 0.03 < α2 < 0.43). (D) Arrhenius plot constructed from the relaxation time extracted from the Cole−Cole fit. The experimental data have been fitted with Orbach, Raman, and QTM processes.

E

DOI: 10.1021/acs.inorgchem.8b00979 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Investigation of Magnetization Relaxation Dynamics for 2 and 3. Ac relaxation dynamics were measured for anisotropic complexes 2 and 3 with an oscillating field of 3.5 Oe in the temperature range 1.8−20 K with and without an optimum external magnetic field. Complex 2 shows an out-ofphase susceptibility (χM′′) signal in the absence of an external magnetic field, indicative of SMM behavior. In the χM′′ = f(ν) plot, however, the maxima observed only at higher frequency suggest that the magnetization vector relaxes much more quickly than anticipated. An attempt to fit the Cole−Cole plot of 2 with a single relaxation process yielded a poor fit; therefore, χM′′ = f(χM′) curves were fitted by considering two relaxation processes (fast relaxation (FR) and slow relaxation (SR)) using the modified Debye equation, resulting in reasonably good agreement between the fit and the experimental data. The two different relaxation times (τ1 and τ2) extracted from the fit were used to construct an Arrhenius plot (Figure 3 and Table S2 in the Supporting Information), and the barriers to the magnetization reversal (Ueff) were estimated to be 23.5 cm−1 (τ0 = 8.1 × 10−6 s) and 10.3 cm−1 (τ0 = 6.9 × 10−5 s; see also Table S3 in the Supporting Information). In order to determine the origin of χM′′ in 2 due to a single molecule or bulk phenomenon, we performed ac relaxation dynamics on the magnetically diluted sample (50%). We wish to point out that, in the ac data collected on 5%, 10%, and 25% diluted samples, the signals were too noisy (data not shown) in Hdc = 0. Again two different relaxation processes are witnessed from a Cole−Cole data fit with the increased effective energy barriers for both processes (SR, Ueff = 37.7 cm−1, τ0 = 6.3 × 10−6 s; FR, Ueff = 12.3 cm−1, τ0 = 4.1 × 10−5 s; see Figure S8 and Tables S3 and S4 in the Supporting Information), suggesting a non-zero contribution of dipolar interaction to the magnetization relaxation dynamics. On the other hand, when the spin−lattice relaxation time was measured in the presence of an optimum magnetic field (600 Oe) (Figure S9 in the Supporting Information) on a pure crystalline complex of 2, χM′′ signal maxima are shifted to lower frequency upon decreasing the temperature. This indicates that an under-barrier mechanism is effectively active still in 2 despite non-zero exchange interaction (between the Dy(III) and radicals) in the absence of an external magnetic field. A nearly 2-fold increase in the extracted energy barrier (in comparison to the pure crystalline state of 2) for both slow and fast relaxation processes (SR, Ueff = 48 cm−1, τ0 = 2.3 × 10−6 s; FR, Ueff = 19 cm−1, τ0 = 1.3 × 10−5 s; Hdc ≠ 0) is observed (see Figure 4 and Tables S3 and S5 in the Supporting Information). The opening of the hysteresis loop observed for polycrystalline sample of 2 at 1.8 K further confirms the SMM behavior (Figure 5 and inset). Observation of slow relaxation of magnetization for a distorted-octahedral complex in the absence of an external magnetic field is extremely rare in the literature due to its inherent electronic structure.29b,c,34 Recently Tong, Chibotaru, and co-workers have given an elegant explanation for the apparent unsuitability of an ideal octahedral (Oh) crystal field symmetry for Ln-SMM design.29b Unlike 2, complex 3 does not show frequency- and temperature-dependent χM′′ signals in the absence of an external magnetic field. The triggering of fast relaxation in 3 in the absence of zero field is likely due to the unsuitable geometry around the prolate Er(III) (in comparison to the oblate Dy(III) ion), which might facilitate a close energy gap

Figure 5. Hysteresis loop measurement performed on a polycrystalline sample of 2 at the indicated temperature and sweep rate.

between the ground state Kramers doublet and first excited Kramers doublet (vide infra). Indeed, 3 shows an χM′′ signal in the presence of an optimum external magnetic field of 2 kOe indicative of SMM behavior. A reasonably good fit was obtained upon fitting the Arrhenius plot of 3 using multiple relaxation processes (Orbach (Ueff = 41 cm−1, τ0 = 1.9 × 10−11 s), Raman (C = 0.03 s−1 K−9 and n = 9), and QTM (τQTM = 0.009 s)) (Figure 6).35 This suggests that, even in the presence of an external bias field, QTM is still operative. Multiple relaxations have already been observed in the literature for several monomeric lanthanide complexes.36a,4e,36b To the best of our knowledge 3 is the first six-coordinate (pseudo-octahedral) Er(III) complex that exhibits fieldinduced slow relaxation of magnetization behavior. Computational Studies. In order to understand the role of radicals in the relaxation dynamics of 2 and 3, we performed ab initio CASSCF SINGLE_ANISO calculations on these complexes without taking into account the exchange interaction between the radicals and Ln(III) ions (i.e., the ligand is considered as neutral and the overall charge on the complex taken as +3 for calculations) while maintaining the same geometry around Ln(III) as observed in the crystal structures of 2 and 3. This is now labeled as 2+3 or 3+3 to avoid confusion. The lowest Kramers doublet (KD) in 2+3 is found to be predominately mJ = ±1/2 and its associated g tensors are transverse in nature (gx = 10.42, gy = 9.75, gz = 1.25; see Table S6 and Figure S10 in the Supporting Information). The wave function decomposition analysis clearly emphasizes that the ground KD is heavily mixed with other excited states (KDs) (0.27|±1/2|+0.15|±5/2|+0.16|±7/2|; see Table S6 and Figure S10 in the Supporting Information). It is very well exemplified in the literature that a ground state KD with an easy plane anisotropy does show only field-induced SIM behavior.29a,b,37,35a However, experimentally this is not what is observed: i.e., 2 shows evidently slow magnetization of relaxation in the absence of zero field. Further, it is observed that all the eight low-lying KDs of 2+3 are span in the range of 607.7 cm−1 and the gzz orientation of all the excited KDs of 2+3 are nearly coaxial with the gzz orientation of ground state KD (Tables S6 and S8 in the Supporting Information). On the basis of the experimentally extracted Ueff values for 2 (23.5 F

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Figure 6. (A, B) In-phase (χM′) and out-of-phase ac magnetic susceptibility (χM′′) of complex 3 in the presence of a 2 kOe applied magnetic field. (C) Arrhenius plot constructed for complex 3. The experimental data have been fitted with Orbach, Raman, and QTM processes.

cm−1 (Hdc = 0); 48 cm−1 (Hdc = 600 Oe)), we surmise that the magnetization vector is likely to relax via the third excited state. To confirm this scenario unambiguously, calculations on the exchange-coupled systems are necessary. However, the usual Lines model within the POLY_ANISO module employed widely in the literature to understand the exchange interaction between the paramagnetic centers cannot be used for systems/ molecules with strong exchange interaction (as in the case of 2 or 3).38 Nevertheless, the preliminary CASSCF calculations performed above demonstrate that, without the radicals, a pseudo-octahedral geometry of Dy(III) in 2 is unlikely to show zero-field SMM behavior (see Figure S3 in the Supporting Information).29b,4c,34a This is the scenario observed for other discrete octahedral Dy(III) complexes isolated with diamagnetic ligands reported to date: i.e., none of the discrete octahedral complexes exhibit SIM behavior in the absence of an external dc magnetic field (Figure S3).4a,29b,c,34a,14 Overall it can be concluded that the presence of an exchange interaction between the Dy(III) and radical anion quenches the QTM to some extent via the ground KD despite its easy plane nature to reveal zero field SMM behavior. On the other hand, SINGLE_ANISO calculations performed on 3+3 reveal the lowest KD found to be axial (gx = 0.30, gy = 0.35, gz = 15.32; mj = ±15/2) in nature with a non-zero transverse component (see Tables S7 and S9 and Figure S11 in the Supporting Information). The axial nature of the ground state KD combined with radical exchange coupling is expected to give better SMM behavior for 3 in comparison to 2. However, this is absolutely in contrast with the experimental observation. The faster magnetization relaxation triggered in 3+3 (Hdc = 0) is likely due to the reduced energy gap between the ground and

first excited KD (1.51 cm−1) in comparison to 2+3 in conjunction with the larger tilt angle observed in the gzz orientation of first excited KD in 3+3 in comparison to 2+3 (Table S7 in the Supporting Information). This suggests that not only the exchange interaction but also the right choice of lanthanide ion with suitable geometry is mandatory in the design of an SIM. This scenario was particularly emphasized by various authors; for example Tong and co-workers and Murugavel and co-workers pointed out the importance of a pentagonal-bipyramidal geometry in stabilizing a large blocking temperature, which is further validated by computational calculations.3a,c,d,29b

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CONCLUSION

To conclude, four structurally analogous lanthanide/transitionmetal complexes (1−4) were isolated using exclusively a redox-active iminopyridyl ligand. Detailed dc magnetic susceptibility measurements suggest that non-negligible superexchange interactions exist between the lanthanide ion and the radical anions. An increase in the strength of exchange coupling of Dy(III) and radicals quenches the QTM to some extent. This finding was indirectly supported by ab initio calculations. The novel synthetic approach reported here reveals SMM behavior in a Dy(III) ion even if the geometry around the lanthanide ion is non-ideal to reveal SMM behavior. By modulation of the number of radicals and steric hindrance of the ligand, the crystal field around the lanthanide ion could be altered, with the expectation of showing better SMM behavior. This work is currently progress in our laboratory. G

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Single-Molecule Magnet. Angew. Chem., Int. Ed. 2016, 55 (52), 16071−16074. (c) Gupta, S. K.; Rajeshkumar, T.; Rajaraman, G.; Murugavel, R. An air-stable Dy(III) single-ion magnet with high anisotropy barrier and blocking temperature. Chem. Sci. 2016, 7 (8), 5181−5191. (d) Liu, J.; Chen, Y.-C.; Liu, J.-L.; Vieru, V.; Ungur, L.; Jia, J.-H.; Chibotaru, L. F.; Lan, Y.; Wernsdorfer, W.; Gao, S.; Chen, X.-M.; Tong, M.-L. A Stable Pentagonal Bipyramidal Dy(III) SingleIon Magnet with a Record Magnetization Reversal Barrier over 1000 K. J. Am. Chem. Soc. 2016, 138 (16), 5441−5450. (e) Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P. Molecular magnetic hysteresis at 60 K in dysprosocenium. Nature 2017, 548 (7668), 439−442. (f) Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamaeki, A.; Layfield, R. A. A Dysprosium Metallocene SingleMolecule Magnet Functioning at the Axial Limit. Angew. Chem., Int. Ed. 2017, 56 (38), 11445−11449. (4) (a) Blagg, R. J.; Ungur, L.; Tuna, F.; Speak, J.; Comar, P.; Collison, D.; Wernsdorfer, W.; McInnes, E. J. L.; Chibotaru, L. F.; Winpenny, R. E. P. Magnetic relaxation pathways in lanthanide singlemolecule magnets. Nat. Chem. 2013, 5 (8), 673−678. (b) Woodruff, D. N.; Winpenny, R. E. P.; Layfield, R. A. Lanthanide single-molecule magnets. Chem. Rev. 2013, 113 (7), 5110−5148. (c) Layfield, R. A. Organometallic Single-Molecule Magnets. Organometallics 2014, 33 (5), 1084−1099. (d) Upadhyay, A.; Singh, S. K.; Das, C.; Mondol, R.; Langley, S. K.; Murray, K. S.; Rajaraman, G.; Shanmugam, M. Enhancing the effective energy barrier of a Dy(III) SMM using a bridged diamagnetic Zn(II) ion. Chem. Commun. 2014, 50 (64), 8838−8841. (e) Das, C.; Upadhyay, A.; Vaidya, S.; Singh, S. K.; Rajaraman, G.; Shanmugam, M. Origin of SMM behaviour in an asymmetric Er(III) Schiff base complex: a combined experimental and theoretical study. Chem. Commun. 2015, 51 (28), 6137−6140. (f) Liu, K.; Shi, W.; Cheng, P. Toward heterometallic single-molecule magnets: Synthetic strategy, structures and properties of 3d-4f discrete complexes. Coord. Chem. Rev. 2015, 289−290, 74−122. (g) Liu, K.; Zhang, X.; Meng, X.; Shi, W.; Cheng, P.; Powell, A. K. Constraining the coordination geometries of lanthanide centers and magnetic building blocks in frameworks: a new strategy for molecular nanomagnets. Chem. Soc. Rev. 2016, 45 (9), 2423−2439. (5) (a) Vignesh, K. R.; Soncini, A.; Langley Stuart, K.; Wernsdorfer, W.; Murray Keith, S.; Rajaraman, G. Ferrotoroidic ground state in a heterometallic {Cr(III)Dy(III)6} complex displaying slow magnetic relaxation. Nat. Commun. 2017, 8 (1), 1023. (b) Vignesh, K. R.; Langley, S. K.; Swain, A.; Moubaraki, B.; Damjanovic, M.; Wernsdorfer, W.; Rajaraman, G.; Murray, K. S. Slow Magnetic Relaxation and Single-Molecule Toroidal Behaviour in a Family of Heptanuclear {CrIIILnIII6} (Ln = Tb, Ho, Er). Angew. Chem., Int. Ed. 2018, 57 (3), 779−784. (6) (a) Tang, J.; Hewitt, I.; Madhu, N. T.; Chastanet, G.; Wernsdorfer, W.; Anson, C. E.; Benelli, C.; Sessoli, R.; Powell, A. K. Dysprosium triangles showing single-molecule magnet behavior of thermally excited spin states. Angew. Chem., Int. Ed. 2006, 45 (11), 1729−1733. (b) Lin, S.-Y.; Wernsdorfer, W.; Ungur, L.; Powell, A. K.; Guo, Y.-N.; Tang, J.; Zhao, L.; Chibotaru, L. F.; Zhang, H.-J. Coupling Dy3 Triangles to Maximize the Toroidal Moment. Angew. Chem., Int. Ed. 2012, 51 (51), 12767−12771. (c) Ungur, L.; Lin, S.Y.; Tang, J.; Chibotaru, L. F. Single-molecule toroics in Ising-type lanthanide molecular clusters. Chem. Soc. Rev. 2014, 43 (20), 6894− 6905. (7) (a) Ahmed, N.; Das, C.; Vaidya, S.; Langley, S. K.; Murray, K. S.; Shanmugam, M. Nickel(II)-Lanthanide(III) Magnetic Exchange Coupling Influencing Single-Molecule Magnetic Features in {Ni2Ln2} Complexes. Chem. - Eur. J. 2014, 20 (44), 14235−14239. (b) Feltham, H. L. C.; Brooker, S. Review of purely 4f and mixedmetal nd-4f single-molecule magnets containing only one lanthanide ion. Coord. Chem. Rev. 2014, 276, 1−33. (c) Das, C.; Vaidya, S.; Gupta, T.; Frost, J. M.; Righi, M.; Brechin, E. K.; Affronte, M.; Rajaraman, G.; Shanmugam, M. Single-Molecule Magnetism, Enhanced Magnetocaloric Effect, and Toroidal Magnetic Moments in a Family of Ln4 Squares. Chem. - Eur. J. 2015, 21 (44), 15639− 15650.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00979. Selected bond distances, packing diagrams, supporting dc and ac magnetic data along with the energies of Kramers doublets of complexes 2 and 3 with g tensors, and crystal field parameters (PDF) Accession Codes

CCDC 1444293−1444296 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 Author

*E-mail for M.S.: [email protected]. ORCID

Maheswaran Shanmugam: 0000-0002-9012-743X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.S. thanks the funding agencies SERB (EMR/2015/000592), INSA (SP/YSP/119/2015/1264), and IRCC-IIT Bombay for financial support.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.8b00979 Inorg. Chem. XXXX, XXX, XXX−XXX