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Slow Magnetic Relaxation in Intermediate Spin S = 3/2 Mononuclear Fe(III) Complexes Xiaowen Feng, Seung Jun Hwang, Jun-Liang Liu, Yan-Cong Chen, Ming-Liang Tong, and Daniel G. Nocera J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09699 • Publication Date (Web): 25 Oct 2017 Downloaded from http://pubs.acs.org on October 26, 2017
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Journal of the American Chemical Society
Slow Magnetic Relaxation in Intermediate Spin S = 3/2 Mononuclear Fe(III) Complexes Xiaowen Feng,†,§ Seung Jun Hwang,†,§ Jun-Liang Liu,‡ Yan-Cong Chen,‡ Ming-Liang Tong,‡ and Daniel G. Nocera*,† †
Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, United States
‡
Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, P. R. China Supporting Information Placeholder ABSTRACT: Magneto-structural studies of mononuclear intermediate S = 3/2 Fe(III) complexes, (PMe3)2FeCl3 (1) and
(PMe2Ph)2FeCl3 (2), demonstrate the influence of local symmetry on magnetic anisotropy. Symmetric compound 1 is characterized by zerofield splitting (ZFS) parameter of D = –50(2) cm–1, leading to the observation of slow magnetic relaxation with an energy barrier of 81 cm–1 along with magnetic hysteresis up to 4 K, whereas symmetrically perturbed compound 2 displays a much reduced ZFS parameter of D = –17(1) cm−1 and energy barrier of Ueff = 46 cm–1.
Paramagnetic molecules displaying single-molecule magnet (SMM) behavior exhibit slow magnetic relaxation at low temperatures.1–3 This phenomenon arises from an energy barrier (Ueff) to spin inversion, generated by magnetic anisotropy (D) acting on a high spin ground state (S). The capacity of SMMs to maintain their magnetization at higher temperatures leads to potential applications in high-density information storage and quantum computing.4–10 To this end, SMMs made from noncritical elements is a beneficial goal for their implementation as materials for large-scale information storage. Efforts to increase Ueff have centered on synthesizing polynuclear transition metal complexes possessing high S. 11 However, theoretical and experimental studies suggest that an increase of S is compensated by a corresponding decrease in overall D. 12–15 To overcome this problem, molecules containing highly anisotropic lanthanide ions have been introduced as spin centers owing to their strong spin-orbit coupled ground state;16–25 energy barriers exceeding 1837 K (1277 cm−1) have been reported, an order of magnitude larger than that of the original Mn12 SMMs.26,27 As an alternative to the critical f-block elements, interest has turned to mononuclear complexes of non-critical first-row transition metals with large magnetic anisotropies where orbital angular momentum is left unquenched.28–30 Additionally, in the first-row, fast quantum tunneling due to the mixing of ground states can be circumvented by utilizing non-integer spin systems as predicted by Kramers theorem.31 Mononuclear transition metals of S = 3/2 are promising systems for such an approach.32–36 For example, a linear two-coordinate Fe(I) complex displays an energy barrier of 354 K (246 cm−1) and magnetic hysteresis at
temperatures of 6.5 K, comparable to highest SMM performing lanthanide complexes.37 To date, systems with S = 3/2 mononuclear transition metal ions have been largely confined to Co(II) systems, which exhibit moderate magnetic anisotropy, and in this body of work, a Co(II) complex with three-fold symmetry establishes control of the local symmetry as a good strategy to design SMMs with large anisotropy.38 Accordingly, (PNP)FeCl2 (PNP = N[2P(CHMe2)2-4-methylphenyl]2 is a notable exception as a ironbased S = 3/2, system that exhibits slow magnetic relaxation with D = –11 cm−1.33 This finding suggests the opportunity for realizing an iron-based intermediate spin S = 3/2 SMM with large anisotropy for ligand fields that enforce the appropriate local symmetry. Herein we show this to be the case with the observation of slow magnetic relaxation for the S = 3/2 mononuclear Fe(III) complex, (PMe3)2FeCl3 (1), which possesses almost perfect three-fold symmetry. The compound displays an energy barrier of 116 K (81 cm–1) arising from a zero-field splitting parameter of D = –50(2) cm–1, which represents the largest magnetic anisotropy for any Fe(III) molecule yet reported. More fundamentally, the system reported herein elegantly demonstrates the criticality of local symmetry on the magnetic anisotropy. By breaking three-fold symmetry of the complex by substitution the axial PMe3 ligands with PMe2Ph, the ((PMe2Ph)2FeCl3 (2) complex shows a significantly reduced magnetic anisotropy, highlighting the influence of local symmetry on the overall anisotropy of SMMs. The Fe(III) trichloride complexes 1 and 2 were prepared by
Figure 1. Thermal ellipsoid plots of (a) 1 and (b) 2 in which H atoms have been removed for clarity. Ellipsoids are drawn at 50% probability. (c) Qualitative d-orbital splitting diagram fora trigonal bipyramidal FeCl3L2 complex of intermediate spin S = 3/2.
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The magnetic anisotropy of the ground states of 1 and 2 was probed with variable field magnetization measurements collected from 2 to 20 K. The resulting magnetization plot (Figures 2b and S15) exhibits separated iso-field curves, and saturation behavior at 1.94 Nβ for 1 and 2.40 Nβ for 2 under an applied field of 7 T. The magnetization would be expected to saturate at 3 Nβ (M = gS) for a spin-only S = 3/2 ion; deviation from this value arises from the presence of magnetic anisotropy. That saturation in 1 occurs at a significantly lower value than 3 Nβ, indicates a highly anisotropic spin ground state. To quantify the zero-field splitting parameter further, variable temperature, variable field magnetization was fit to the spin Hamiltonian,43 Ĥ = DŜz2 + β(gxŜx + gyŜy + gzŜz)H
Figure 2. Variable temperature magnetic susceptibility data under 0.5 T and magnetization data collected at 7 fields (1−7 T) over the temperature range 2.0 to 20 K for complex 1. Solid lines are the best fits to the experimental data. The fits of the individual curves are better seen for each individual trace presented in Figure S7.
modifying the method reported by Poli39 with the treatment of FeCl3 in PhCF3 solution with 2.5 equiv of corresponding phosphines, PMe3 and PMe2Ph, respectively. The molecular structures of 1 and 2 have been determined by X-ray diffraction and they are shown in Figure 1 (and Figures S3 and S4). The mononuclear Fe(III) center resides in a 5-coordinate trigonal bipyramid (TBP) as defined by the five-coordinate geometric parameter τ (τ5 = 0.90 for 1, τ5 = 0.83 for 2) ligand field.40 Complex 1 has almost perfect three-fold local symmetry as demonstrated by Cl–Fe–Cl angles of 122.475(6)°, 115.023(12)°, and 122.476(6)°. This local three-fold symmetry is broken in 2 by the presence of the unique phenyl ring on the phosphine ligand. Whereas the PMe3 groups in 1 are perfectly eclipsed with a P–Fe–P angle of 176.604(12)°, the structure of 2 shows the PMe2Ph groups to be slightly staggered (20 °) with a P–Fe–P angle of 175.11(2)°. In 1 and 2, one of the Fe–Cl bonds is longer than the others; in 1 this lengthening is 0.0187 Å as compared to 0.0577 Å) in 2. Selected bond distances and angles for 1 and 2 are summarized in Table S3. Zero-field 57Fe Mössbauer analysis of 1 and 2 in Figures S5 and S6 revealed the presence of a single iron environment with parameters (isomer shift, δ = 0.46 and 0.48 mm s−1 and a quadrupole splitting, |ΔEQ| = 1.93 and 2.22 mm s−1 for 1 and 2, respectively), in line with other intermediate spin S = 3/2 Fe(III) complexes previously synthesized. 41 , 42 The larger quadrupole splitting of 2 is consistent with a significant symmetry distortion about Fe, as observed in solid-state structure. The influence of local symmetry on magnetic anisotropy was examined by performing dc susceptibility measurements under an applied field of 5000 Oe. For 1, at 300 K, the value of χMT (Figure 2a) yields 2.29 cm3 K mol–1, corresponding to a S = 3/2 spin ground state. As temperature decreases, the value of χMT remains constant until 40 K, before dropping off at lower temperatures. In contrast, the value of χMT for 2 in Figure S11 shows a significant temperature dependence over the range of 50 to 300 K, indicative of gradual S = 3 /2 to S = 5/2 thermal spin crossover behavior.33
(1)
For 2, an isotropic g tensor sufficiently fit the data, yielding D = –17(1) cm–1 and gx = gy = gz = 2.49(1) where an isotropic g value was used so as not to overfit the observed data; spin crossover in 2 prevents the use of dc susceptibility to provide constraints on g values. This is not the case for 1 where spin crossover is not observed thus allowing both χMT (Figure 2a) and magnetization data (Figure 2b) be used to fit eq (1), yielding an anisotropic g tensor (gx = gy = 1.91(1) and gz = 2.49(1)) and D = −50(2) cm−1. These magnetic data were fit to high temperatures (1.8–20 K for M vs HT−1 and 2-300 K for χMT vs T) to account for the magnetic contribution arising from thermal population of the doublet excited state. The much larger magnitude of D in 1 results from three electrons in the doubly degenerate (e′′) orbital set, resulting in unquenched orbital contribution and thus magnetic anisotropy.30–29 In comparison, due to the local distortion from three-fold symmetry in 2, the degeneracy of the e′′ orbitals is removed, leading to a significantly reduced zero-field splitting parameter. These results emphasize the sensitivity of the magnetic anisotropy to subtle changes in local symmetry, which may be further accentuated by second-order spin-orbit coupling effects35, though the small differences in bond lengths between the primary coordination spheres (
