A Ferric Semiquinoid Single-Chain Magnet via Thermally-Switchable

May 10, 2018 - We report the synthesis of a semiquinoid-bridged single-chain magnet, as generated through a thermally induced metal–ligand electron ...
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A Ferric Semiquinoid Single-Chain Magnet via Thermally-Switchable Metal-Ligand Electron Transfer Jordan A. DeGayner, Kunyu Wang, and T. David Harris J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03949 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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A Ferric Semiquinoid Single-Chain Magnet via ThermallySwitchable Metal-Ligand Electron Transfer Jordan A. DeGayner, Kunyu Wang, and T. David Harris* Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3313, United States Supporting Information Placeholder ABSTRACT: We report the synthesis of a semiquinoid-bridged single-chain magnet, as generated through a thermally-induced metal-ligand electron transfer. Reaction of FeCl3 with 2,5dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) in the presence of (NMe4)Cl gave the compound (NMe4)2[LFeCl2]. Together, variable-temperature X-ray diffraction, Mössbauer spectra, Raman spectra, and dc magnetic susceptibility reveal a transition from a chain containing (L2−)FeII units to one with (L3−•)FeIII upon decreasing temperature, with a transition temperature of T1/2 = 213 K. Dc magnetic susceptibility measurements show strong metalradical coupling within the chain, with a coupling constant of J = −81 cm−1, and ac susceptibility data reveal slow magnetic relaxation, with a relaxation barrier of Dt = 55(1) cm−1. To our knowledge, this compound provides the first example of a semiquinoid-bridged single-chain magnet. Over the past two decades, certain one-dimensional compounds have been shown to exhibit slow magnetic relaxation upon removal of an applied magnetic field.1 These compounds, termed single-chain magnets, may find use in spintronics applications such as high-density information storage and processing. However, small spin relaxation barriers, and thus low magnetic blocking temperatures, constitute a key limitation toward this end, with 14 K representing the highest reported blocking temperature.2 In contrast to single-molecule magnets, where the relaxation barrier is governed only by the spin (S) and the magnitude of magnetic anisotropy (D), the barrier of a single-chain magnet also increases with the strength of magnetic coupling (J) between spins.3 As S and D are inversely related and thus difficult to simultaneously maximize,4 increasing J represents an important synthetic challenge to realize high-barrier single-chain magnets. One strategy toward this end is to install direct magnetic coupling in a chain featuring paramagnetic metal ions linked by radical bridging ligands. Indeed, a number of single-chain magnets based on nitronyl nitroxide1a,2,5 and organonitrile6 radical bridging ligands have been reported. Nevertheless, the bis-monodentate binding nature and low charges of these ligands hinder their synthetic predictability and versatility, and limit the strength of metal-radical coupling in the resulting chain compounds. In contrast, bis-bidentate semiquinoid radical bridging ligands have been shown to exhibit extremely strong coupling with metal ions in molecular complexes.7 Although several benzoquinoidbridged chains have been reported, the literature features only one semiquinoid radical-bridged chain.8 While this chain was formed through post-synthetic reduction, the crystal packing adopted by most compounds precludes the employment of post-synthetic redox chemistry. Alternatively, judicious selection of building units can give spontaneous redox chemistry upon solid formation, thereby generating ligand radicals in situ. Indeed, this approach has recently been employed to synthesize tetraoxolene radical-

Figure 1. Upper: Synthesis and structure of 1. Orange, green, red, and gray spheres represent Fe, Cl, O, and C, respectively; NMe ions are omitted for clarity. Lower: Structure of 1 at 250 K (left) and 100 K (right), highlighting relevant bond distances (Å). 4

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based networks.9,10 Herein, we demonstrate thermally-induced electron transfer in a chain compound to afford (NMe4)2[(L3−•)FeIIICl2]. This compound features strong metalradical magnetic coupling and slow magnetic relaxation, thereby providing the first example of a semiquinoid-bridged single-chain magnet. Reaction of FeCl3·6H2O with LH2 and excess (NMe4)Cl in DMF at 130 °C afforded brown, plate-shaped crystals of (NMe4)2[LFeCl2]. (1). At 250 K, the structure of 1 features dianionic chains comprised of FeCl2 units linked by Ln−. Each Fe center resides in a distorted octahedral coordination environment and is ligated by two cis-disposed Cl− ions and two cis-disposed O donor atoms from each of two Ln− ligands (see Figures 1, S1, and S2). This cis geometry, along with alternating Δ and Λ chirality at Fe, gives a one-dimensional, zig-zag chain. The negative charge of the chain is compensated by NMe4+ cations (see Figure S1). The chains are arranged in a staggered parallel configuration along the crystallographic c axis, with a closest interchain Fe…Fe distance of 8.906(2) Å. Overall, each repeating unit of the chain is reduced by one electron relative to the starting materials. Such a spontaneous reduction has been observed in similar chain syntheses.11,12

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Figure 3. Solid-state Raman spectra for 1, collected at 220 (red), 210 (green), and 200 K (blue) following excitation at 532 nm.

Figure 2. Mössbauer spectra for 1, collected at 220 (upper), 210 (middle), and 200 K (lower). Black crosses and solid lines represent experimental data and fits, respectively.

A comparison of bond distances in 1 at 100 K and 250 K reveals key differences (see Table S1). Within Ln−, the mean C–C distance at 100 K is 1.421(2) Å, 1.6% shorter than that of 1.444(5) Å at 250 K. The C1-C3 distance is particularly temperaturedependent, with a 3.7% difference between 1.523(9) Å at 250 K and 1.466(4) Å at 100 K. Conversely, the mean C–O distance of 1.291(2) Å at 100 K is 2.8% longer than that of 1.256(5) Å at 250 K. The distances at 250 K are consistent with other chains featuring diamagnetic L2−,11 while those at 100 K are characteristic of L3−•, as observed in molecular complexes7a-e and networks.9 Moreover, the mean Fe–O distance at 250 K of 2.140(4) Å is 5.6% longer than that of 2.027(2) Å at 100 K, consistent with high-spin FeII in similar coordination.11 Together, these structural observations firmly support an electronic structure of (L2−)FeII at 250 K and of (L3−•)FeIII at 100 K. To our knowledge, 1 represents only the second example of a semiquinoid-bridged chain compound.8 The oxidation state of Fe in 1 was further probed by variabletemperature Mössbauer spectroscopy. At 220 K, the spectrum exhibits a single quadrupole doublet, with a fit giving an isomer shift of d = 1.086(3) mm/s and a quadrupole splitting of DEQ = 2.112(6) mm/s (see Figures 2 and S3). These parameters can be unambiguously assigned to high-spin FeII, consistent with structural analysis.11a,12 In contrast, the spectrum at 210 K features two superimposed doublets, one with parameters of d = 1.146(5) mm/s and DEQ = 2.08(1) mm/s and the other with d = 0.517(4) mm/s and DEQ = 1.129(9) mm/s. The latter doublet is consistent with high-spin FeIII.9,12 At 200 K, the spectrum resolves into a single doublet, with parameters d = 0.555(3) mm/s and quadrupole splitting of DEQ = 1.061(4) mm/s, indicative of high-spin FeIII. The ligand oxidation state in 1 was further probed by variabletemperature Raman spectroscopy. From 300 K to 220 K, prominent vibrations at 1346 and 1570 cm−1 are assigned to the nCC and nCO modes of L2−, respectively, based on other chloranilatebridged compounds (see Figures 3 and S4).9a Upon lowering the

temperature to 210 K, nCC broadens and increases in energy to 1355 cm−1, while nCO decreases in intensity as a new feature at 1481 cm−1 concomitantly appears. Upon moving to 200 K, nCC shifts slightly in energy to 1356 cm−1, while nCO resolves into a single peak at 1481 cm−1. The respective increase and decrease in energy of nCC and nCO upon cooling is consistent with formation of L3−•, consistent with structural analysis. The presence of two energetically distinct νCO vibrations at 210 K suggests that electron transfer between L2− and L3−• is slow relative to the Raman timescale. To investigate the magnetic behavior of 1, variable-temperature dc magnetic susceptibility data were collected, and the resulting plot of cMT vs T is shown in Figure 4. At 300 K, cMT = 3.72 cm3·K/mol, corresponding to magnetically isolated FeII ions with g = 2.23. Upon lowering the temperature, cMT remains nearly constant until ca. 220 K, where a sharp, sigmoidal upturn occurs (see Figure 4, inset). This observation is consistent with a transition from a chain comprised of high-spin, S = 2 FeII and diamagnetic L2− to one of coupled S = 5/2 FeIII and S = ½ L3−•. A plot of the derivative of cMT with respect to T vs T provides a transition temperature of T1/2 = 213 K (see Figure S5). No hysteresis was observed between cooling and warming cycles (see Figure S6). The above characterization demonstrates a reversible, temperature-induced metal-to-ligand electron transfer, often termed valence tautomerism. This phenomenon has been observed in numerous metal-quinoid complexes, often accompanying a spincrossover or other large coordination sphere rearrangement.13–16 In contrast, the observation of electron transfer from high-spin FeII to form high-spin FeIII upon cooling is unusual, as it is accompanied by only a small distortion of the Fe coordination sphere.17 Indeed, such a transition has been verified in only one example and proposed to accompany an Fe-based spin-crossover in another.19 Tetraoxolene-bridged zig-zag chains with FeII and the diamagnetic form of the ligand have been reported.11 However, these compounds exclusively feature neutral s donating terminal ligands. In 1, we propose that the combination of the anionic charge and the π-donating ability20 of Cl− serves to cathodically shift the FeII/FeIII redox couple, thereby leading to a ground state of (L3−•)FeIII. Regarding the temperature dependence of electronic configuration, previous studies have found valence tautomerism 18

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Figure 5. Left: Variable-frequency ac susceptibility data for 1. Solid lines are guides to the eye. Right: Arrhenius plot of relaxation time. Figure 4. Variable-temperature dc magnetic susceptibility for 1, collected under an applied dc field of 1000 Oe. The black line corresponds to a fit to the data. Inset: Expanded view of the high-temperature region.

to be an entropy-driven process stemming primarily from changes in metal-ligand bond lengths and access to lower-energy vibrational modes upon increasing temperature, with only slight contribution from any associated increase in spin degrees of freedom.22 A similar rationale may apply to 1, as (L2−)FeII features longer Feligand bond distances than does (L3−•)FeIII. Upon decreasing temperature from 213 K, cMT increases exponentially to a maximum of 83 cm3·K/mol at 12 K, consistent with strong metal-radical coupling and magnetic correlation along the chain. A fit to the data gives a coupling constant of J = −81 cm−1,23,24 corresponding to uncompensated antiferromagnetic coupling between FeIII and L3−•. While relatively large, J is smaller than that of −186 cm−1 reported for a tetraoxolene radicalbridged FeIII2 complex, possibly owing to antiferromagnetic radical-radical interactions within the chain.7a Note that, relative to other electronic configurations, high-spin d5 ions give weak metal-radical coupling due to competing interactions between the ligand radical and d electrons occupying orbitals of both s and p symmetry.25 Accordingly, we anticipate that metal ions with magnetic orbitals of homogenous symmetry will engender much stronger coupling. Variable-field magnetization data for 1 reveal the presence of magnetic hysteresis. At 1.8 K, a coercive field of Hci = 2180 Oe was observed at a sweep rate of 0.6 Oe/s (see Figure S7). The data at 1.8 K reach a value of M = 3.94 µB at 70 kOe, close to the expected saturation value of 4 µB for a repeating S = 2 spin unit. While no magnetic hysteresis was observed above 1.8 K, the plot of M vs H features an inflection point up to 12 K (see Figures S8 and S9), suggesting an ordered ground state arising from weak, antiferromagnetic inter-chain coupling.26 Variable-frequency ac susceptibility data for 1 feature frequency dependence in both in-phase (cMʹ) and out-of-phase (cMʺ) components (see Figures 5 and S10). A fit to the corresponding Arrhenius plot of relaxation time gives a relaxation barrier Dt = 55(1) cm−1, with t0 = 3.9(8) × 10−12 s, demonstrating that 1 is indeed a single-chain magnet. The barrier in 1 is remarkable given the small magnetic anisotropy expected for high-spin FeIII. To our knowledge, 1 represents the only example of a single-chain magnet comprised of high-spin d5 metal ions, and this behavior likely reflects the contribution of strong magnetic coupling to the barrier. To probe this contribution, ac susceptibility data were collected at 1 Hz from 1.8 to 100 K (see Figure S11). The resulting plot

of ln(cMʹT) vs 1/T features a linear region from 19 to 44 K, which can be fit to give a correlation energy, the energy required to create a domain wall, of Dx = 22 cm−1. This relatively small value is consistent with a chain in the Heisenberg limit, where Dx is dependent on magnetic anisotropy.3, The increase in intensity of the cMʺ data with increasing temperature further supports the ordered antiferromagnetic ground state suggested by the M vs H data.26a While application of a 400 Oe dc field caused the peaks in cMʺ vs T to shift to lower frequencies, as predicted from the magnetic phase diagram, the extracted relaxation barrier is nearly identical to that at zero-field (see Figures S12-15). The foregoing results provide the first example of a semiquinoid-bridged single-chain magnet. Efforts are underway to enhance the relaxation barrier through introduction of anisotropic metal ions and ligands, and to exploit the redox activity of quinoid ligands to construct electrically conductive single-chain magnets. Toward this end, a preliminary current-voltage measurement revealed a conductivity of s = 5.7 × 10−8 S/cm for 1 at ambient temperature (see Figure S16). 27

ASSOCIATED CONTENT Supporting Information Experimental details, characterization data, and crystallographic information files (CIF) for 1 at 250 and 100 K. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was funded by the National Science Foundation (DMR-1351959) and Northwestern University. We thank Dr. R. Clérac for helpful discussions.

REFERENCES

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