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Nov 22, 2017 - ABSTRACT: We report the serendipitous discovery and magnetic characterization of a dysprosium bis(ammonia) metallocene complex ...
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Slow Magnetic Relaxation in a Dysprosium Ammonia Metallocene Complex Selvan Demir,*,†,‡ Monica D. Boshart,§ Jordan F. Corbey,§ David H. Woen,§ Miguel I. Gonzalez,† Joseph W. Ziller,§ Katie R. Meihaus,† Jeffrey R. Long,*,†,∥,⊥ and William J. Evans*,§ †

Department of Chemistry, University of California, Berkeley, California 94720-1460, United States Department of Chemical and Biomolecular Engineering, University of California, Berkeley, California 94720, United States ⊥ Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡ University of Goettingen, Institute of Inorganic Chemistry, Tammannstrasse 4, 37077 Goettingen, Germany § Department of Chemistry, University of California, Irvine, California 92697-2025, United States

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S Supporting Information *

ABSTRACT: We report the serendipitous discovery and magnetic characterization of a dysprosium bis(ammonia) metallocene complex, [(C5Me5)2Dy(NH3)2](BPh4) (1), isolated in the course of performing a well-established synthesis of the unsolvated cationic complex [(C5Me5)2Dy][(μ-Ph)2BPh2]. While side reactivity studies suggest that this bis(ammonia) species owes its initial incidence to impurities in the DyCl3(H2O)x starting material, we were able to independently prepare 1 and its tetrahydrofuran (THF) derivative, [(C5Me5)2Dy(NH3)(THF)](BPh4) (2), from the reaction of [(C5Me5)2Dy][(μ-Ph)2BPh2] with ammonia in THF. The lowsymmetry complex 1 exhibits slow magnetic relaxation under zero applied direct-current (dc) field to temperatures as high as 46 K and notably exhibits an effective barrier to magnetic relaxation that is more than 150% greater than that previously reported for the [(C5Me5)2Ln][(μ-Ph)2BPh2] precursor. On the basis of fitting of the temperature-dependent relaxation data, magnetic relaxation is found to occur via Orbach, Raman, and quantum-tunneling relaxation processes, and the latter process can be suppressed by the application of a 1400 Oe dc field. Fieldcooled and zero-field-cooled dc magnetic susceptibility measurements reveal a divergence at 4 K indicative of magnetic blocking, and magnetic hysteresis was observed up to 5.2 K. These results illustrate the surprises and advantages that the lanthanides continue to offer for synthetic chemists and magnetochemists alike.

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electronics.7 The highest spin-relaxation barriers are observed in mononuclear dysprosium complexes,8 predominantly because of the fact that Dy3+ exhibits a large magnetic anisotropy and is a Kramers ion (possessing an odd number of unpaired electrons). The latter property ensures that the degeneracy of ±MJ ligand-field states (which arise from the effect of the ligand-field symmetry on the ground J state) is guaranteed in the absence of a direct-current (dc) magnetic field.9 Although slow magnetic relaxation can often occur irrespective of the ligand-field symmetry for a Kramers ion, tailoring the ligand field for a particular ion can lead to enhanced single-ion anisotropy and relaxation behavior.10 For example, a strongly axial coordination environment for free Dy3+ (J = 15/2 ground state)11 can effect the stabilization of its maximal MJ = 15/2 state and impart maximal single-ionmagnetic anisotropy.10

INTRODUCTION The field of single-molecule magnetism has made great progress since the discovery of slow magnetic relaxation in the lanthanide complexes [LnPc2]− (Ln = Tb, Dy; Pc2− = phthalocyanine dianion) over a decade ago.1 Since then, scientists have contributed computationally, theoretically, and experimentally to understanding more fully the underlying causes that give rise to slow magnetic relaxation.2 Although great advancements have been achieved with transition metals3 and the actinides,4 the most significant progress toward singlemolecule-magnet-based technology has been accomplished through the utilization of some of the later lanthanides, owing to their inherently high magnetic moments and large magnetic anisotropies arising from unquenched orbital angular momentum and strong spin−orbit coupling. The ability of some single-molecule magnets to retain spin information for long periods of time at low temperatures has rendered these systems intriguing for application in high-density information storage,5 although systems with shorter relaxation times also have proposed utility in quantum computing6 and spin-based © 2017 American Chemical Society

Received: September 18, 2017 Published: November 22, 2017 15049

DOI: 10.1021/acs.inorgchem.7b02390 Inorg. Chem. 2017, 56, 15049−15056

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Inorganic Chemistry

a Jasco FT/IR-4700 spectrometer. [(C5Me5)2Dy(NH3)2](BPh4) (1) and (C5Me5)Dy(C3H5)2(THF) were initially prepared by the general procedures used to produce the known complexes in Scheme S1.12b,16 The specific preparations are given in the Supporting Information (SI). Synthesis of [(C5Me5)2Dy(NH3)2](BPh4) (1). In an argon-filled glovebox, [(C5Me5)2Dy][(μ-Ph)2BPh2] (0.100 g, 0.133 mmol) and toluene (20 mL) were combined in a Schlenk flask equipped with a magnetic stir bar to form a pale-yellow suspension. The flask was sealed, brought outside of the glovebox, and connected to a vacuum line. The solution was cooled to −78 °C and excess ammonia gas (∼20 Torr) was condensed into the flask with stirring. The pale-yellow suspension immediately turned colorless. The solution was warmed to room temperature and briefly exposed to a vacuum to remove excess ammonia. The flask was then brought into a glovebox, and toluene was removed under reduced pressure to yield 1 (97 mg, 92%) as a colorless solid. Colorless single crystals of 1 were grown by dissolving the product in boiling toluene and allowing it to slowly cool to room temperature. Anal. Calcd for C44H56BN2Dy: C, 67.22; H, 7.18; N, 3.56. Found: C, 67.41; H, 7.25; N, 3.28. Synthesis of [(C5Me5)2Dy(NH3)(THF)](BPh4) (2). In an argonfilled glovebox, a Schlenk flask containing a magnetic stir bar was charged with [(C5Me5)2Dy][(μ-Ph)2BPh2] (0.100 g, 0.133 mmol), toluene (20 mL), and THF (0.1 mL) to generate a colorless slurry. The flask was sealed, brought outside of the glovebox, and connected to a vacuum line. The solution was frozen using a liquid-nitrogen bath, and the flask was placed under reduced pressure to remove argon. Excess ammonia gas was condensed into the flask, the flask was subsequently sealed, and the solution was warmed to room temperature with immediate stirring. After 1 h, the flask was placed under reduced pressure to remove excess ammonia and then brought into a glovebox. The resulting colorless slurry was heated to boiling, and the hot slurry was filtered to produce a clear and colorless solution. After 2 h, needle-like single crystals were obtained, one of which was identified as the product by X-ray diffraction. X-ray Structure Determinations. Crystallographic details for compounds 1, (C5Me5)Dy(C3H5)2(THF), and 2 are summarized in the SI. Magnetic Measurements. A sample of 1 was prepared by briefly drying the material under vacuum, grinding it into a fine crystalline powder, and loading this powder into a 5 mm i.d./7 mm o.d. quartz tube with a raised platform in an inert glovebox atmosphere. Eicosane was added to saturate and cover the sample to prevent crystallite torqueing and provide good thermal contact between the sample and cryogenic bath. The tube was then fitted with a Teflon-sealed adapter, evacuated on a Schlenk line, and flame-sealed under vacuum using a H2/O2 flame. Magnetic susceptibility measurements were collected using a Quantum Design MPMS2 SQUID magnetometer. Dc magnetic susceptibility data measurements were performed at temperatures ranging from 2 to 300 K using applied fields of 1000, 5000, and 10000 Oe. Alternating-current (ac) magnetic susceptibility measurements were performed using a 4 Oe switching field. All data were corrected for diamagnetic contributions from the eicosane and core diamagnetism estimated using Pascal’s constants.17 Cole−Cole plots were fitted using formulas describing χ′ and χ″ in terms of the frequency, constant-temperature susceptibility (χT), adiabatic susceptibility (χS), relaxation time (τ), and a variable representing the distribution of relaxation times (α).18 Fits were performed utilizing all data points to give 0.08 ≤ α ≤ 0.21 (for Hdc = 0 Oe) and α ≤ 0.09 (for Hdc = 1400 Oe).

Although somewhat rare to date, possibly because of their challenging synthesis, dysprosium metallocene complexes featuring slow magnetic relaxation have begun to represent an important class of compounds in single-molecule magnetism. Indeed, a number of mononuclear dysprosium metallocenes have been shown to exhibit strikingly high relaxation barriers (Ueff) and blocking temperatures (Tb),2i,8c,d,12 and the current record blocking temperature of 60 K is held by the complex [Cp t tt 2 Dy] + [Cp t tt = 1,2,4-tris(tert-butyl)cyclopentadienide].8c,d Previously, some of us detailed magnetic characterization of the structurally comparable complex [(C5Me5)2Dy][(μ-Ph)2BPh2], wherein a bis(pentamethylcyclopentadienyl)dysprosium cation coordinates agostically to a tetraphenylborate anion. In magnetically dilute form (doped with an isostructural yttrium complex), [(C5Me5)2Dy][(μ-Ph)2BPh2] exhibited slow magnetic relaxation with Ueff = 349 cm−1 under zero applied field, as well as magnetic hysteresis up to 5.3 K.12a For this complex, we proposed that the magnetic anisotropy is dictated primarily by the distorted axial ligand field provided by the two (C5Me5)− rings. Nonetheless, the weak agostic interactions with the tetraphenylborate anion in the equatorial plane likely served as the predominant contributor to observed tunneling of the magnetization at low temperatures. Given the ease of displacing the tetraphenylborate anion, we were interested in exploring complexes of the [(C5Me5)2Dy]+ cation with alternate ancillary ligands. Serendipitously, in the course of preparing a new batch of [(C5Me5)2Dy][(μPh)2BPh2], we isolated the compound [(C5Me5)2Dy(NH3)2][BPh4] (1), containing two ammonia (NH3) ligands in place of the tetraphenylborate anion. Herein, we briefly divulge synthetic endeavors to identify the initial source of NH3, which further led to isolation of a rare bis(allyl) complex, (C5Me5)Dy(C3H5)2(THF). We also report the successful direct synthesis of 1 from the reaction of [(C5Me5)2Dy][(μPh)2BPh2] with ammonia in toluene and synthesis of the THF adduct [(C5Me5)2Dy(NH3)(THF)][BPh4] (2) from reaction of the tetraphenylborate species with NH 3 in THF. Comprehensive magnetic characterization of the bis(ammonia) complex 1 reveals it to be a single-molecule magnet under zero dc field, with an effective relaxation barrier of 546 cm−1, nearly double that of [(C5Me5)2Dy][(μ-Ph)2BPh2] in spite of similar relaxation dynamics and magnetic hysteresis behavior between the two compounds.

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EXPERIMENTAL SECTION

The syntheses and manipulations described below were conducted under a nitrogen or an argon atmosphere with rigorous exclusion of air and water by Schlenk, vacuum-line, and glovebox techniques. Solvents were sparged with ultra high purity argon and dried over columns containing Q-5 and molecular sieves. NMR solvents (Cambridge Isotope Laboratories) were dried over a NaK alloy, degassed by three freeze−pump−thaw cycles, and vacuum-transferred before use. The compound (HNEt3)(BPh4) was prepared according to a previously published procedure.13 DyCl3 was dried according to a literature procedure by heating a mixture of the hydrated trichloride with an excess of NH4Cl.14 K[N(SiMe3)2] (Aldrich, 95%) was purified via toluene extraction before use. C5Me5H was dried over molecular sieves and degassed using three freeze−pump−thaw cycles. KC5Me5 was synthesized by deprotonation of C5Me5H with K[N(SiMe3)2].15 Anhydrous ammonia gas (Aldrich, 99.99+%) was used as received. 1H NMR spectra were recorded on Bruker GN500 and CRYO500 MHz spectrometers (13C{1H} at 125 MHz) at 298 K, unless otherwise stated, and referenced internally to residual protiosolvent resonances. IR samples were prepared as KBr pellets, and spectra were obtained on

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RESULTS AND DISCUSSION Syntheses and Structures. We previously established a synthetic route for the preparation of [(C5Me5)2Dy][(μPh)2BPh2] via the reaction of (C5Me5)2Dy(C3H5) with (HNEt3)(BPh4) in the absence of a coordinating solvent (Scheme S1).12a,b,16b−d,19 Serendipitously, during a typical 2-gscale preparation, recrystallization of the isolated product 15050

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To our knowledge, 1 and 2 represent the first ammonia adducts of a rare-earth metallocene cation,21 although related adducts such as [(C5Me5)2LnL2]+ solvated with L = THF,22 tetrahydrothiophene (C4H8S),23 and pyridine, acetone, and OPPh316b have been crystallographically characterized. Previously, the only crystallographically characterized ammonia complexes of rare-earth metallocenes were (C5Me5)2Yb(SPh)(NH3),21a (C5Me5)2Yb(TePh)(NH3),21b and an adduct of divalent ytterbium (C5Me5)2Yb(THF)(NH3),21c although the tris(cyclopentadienyl)ammonia complexes (C5H5)3Ln(NH3) have been known since the 1960s24 and Sm, Gd, Dy, Ho, Er, and Yb examples were crystallographically characterized in 2006.25 The structure of 1 (Figures 1 and S1) is similar to that of other bis(pentamethylcyclopentadienyl) rare-earth metallocenes (see the SI for structural details),12a,b,16 with average Dy−N bond distances of 2.47(1) Å that agree with or are slightly shorter than other Dy−N bond distances involving a neutral N-donor ligand.26 The Cnt−Dy−Cnt (Cnt = C5Me5 ring centroid) angle of 140.2° is larger than the analogous angle of 134.0° observed in the precursor tetraphenylborate complex, a direct result of the less sterically demanding NH3 ligand relative to the larger tetraphenylborate anion. The Cnt−Dy− Cnt angle in 2 is 139.2°, slightly smaller than that observed for 1 perhaps because of the presence of a more sterically demanding THF ligand. The Dy−Cnt distances in 1 and 2 are comparable, at 2.350 and 2.370 Å, respectively, while both Dy− N distances in 1 are similar to the single Dy−N distance in 2 [2.466(3)/2.476(3) and 2.486(2) Å, respectively; see the SI for complete crystallographic details]. As previously demonstrated, [(C5Me5)2Dy][(μ-Ph)2BPh2] is a single-molecule magnet exhibiting characteristic slow magnetic relaxation under zero dc field and, in magnetically dilute form (doped with isostructural [(C 5Me5)2Y][(μPh)2BPh2]), was found to have an effective relaxation barrier of Ueff = 339 cm−1.12a While 1 retains the [(C5Me5)2Dy]+ backbone, the remaining coordination sites are now occupied by two neutral ammonia ligands. This electronic modification, in conjunction with the increase in the Cnt−Dy−Cnt angle, may be considered to enforce a more strongly axial coordination environment for the Dy3+ ion in 1 relative to [(C5Me5)2Dy][(μ-Ph)2BPh2], a potential boon for maximizing single-ion-magnetic anisotropy.10 We therefore sought to fully characterize 1 via both dc and ac magnetic susceptibility measurements, and these data are discussed in detail below with a comparison to that previously collected for [(C5Me5)2Dy][(μ-Ph)2BPh2]. All magnetic susceptibility data were collected on samples of 1 synthesized via the direct route in Figure 1. Magnetic Characterization of 1. Variable-temperature dc magnetic susceptibility data collected for a sample of 1 under a field of 1000 Oe (for 5000 Oe, see Figure S4) show non-Curie law behavior, indicating the presence of substantial magnetic anisotropy (Figure 2). At 300 K, the χMT value is 14.11 cm3·K/ mol, in agreement with the calculated value of 14.17 cm3·K/ mol for the corresponding free Dy3+ ion. As the temperature is lowered, χMT progressively declines to 12.21 cm3·K/mol before dropping abruptly to 4.53 cm3·K/mol at 2 K. The initial gradual decline of χMT data with decreasing temperature is typical behavior for a mononuclear Dy3+ complex, resulting from depopulation of the ligand-field excited states within the ground J state. The drastic drop in χMT below 5 K, however, is indicative of magnetic blocking. In the case of mononuclear single-molecule magnets, this phenomenon arises when the barrier to moment inversion is much greater than the system

yielded colorless crystals of 1 instead of the tetraphenylborate adduct. The crystals could be grown under multiple conditions from toluene and benzene, in gloveboxes containing coordinating and noncoordinating solvents, at room temperature and −30 °C. We sought to identify the source of this ammonia by investigating the individual precursors encountered in the synthesis (Scheme S1), starting with the dehydrated form of DyCl3. We found no evidence of nitrogen (for example, from unreacted NH4Cl) in the elemental analysis of the latter, although IR spectroscopy did show a weak, broad feature near 3300 cm−1. The precursor (C5Me5)2DyCl2K(THF)2 could be cleanly made from the same batch of DyCl3 and was used to prepare (C5Me5)2Dy(C3H5)(THF) (Scheme S1). While the expected mono(allyl) compound was the predominant product, we also isolated and identified by X-ray crystallography single crystals of the complex (C5Me5)Dy(C3H5)2(THF) (Figure S3), notably a rare example of a lanthanide complex with a single (C5Me5)− ring and only one of three structurally characterized bis(allyl) rare-earth complexes.20 Attempts to isolate (C5Me5)Dy(C3H5)2(THF) directly were unsuccessful (see the SI for additional synthetic and crystallographic details). Although the source of ammonia in the initial preparation of 1 could not be identified, it was possible to isolate 1 cleanly and in high yield via the direct addition of excess ammonia gas to a toluene solution of crystalline [(C5Me5)2Dy][(μ-Ph)2BPh2] at low temperature (Figure 1, left-hand side). When the same

Figure 1. Synthesis of compounds 1 and 2 starting from [(C5Me5)2Dy][(μ-Ph)2BPh2]. The structures of the [(C5Me5)2Dy(NH3)2]+ cation in 1 and the [(C5Me5)2Dy(NH3)(THF)]+ cation in 2 (see also Figures S1 and S2, respectively) are presented in the lower left and lower right; green, red, blue, gray, and white spheres correspond to Dy, O, N, C, and H atoms, respectively. All H atoms except those belonging to the NH3 ligands have been omitted for clarity.

reaction is carried out with THF as the solvent, the THF adduct 2 can instead be isolated (Figure 1, right-hand side). Importantly, although the mechanism of formation of this ammonia complex is not yet understood, the successful direct synthesis is promising as a convenient and general route to rare-earth metallocene ammonia complexes. 15051

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Figure 2. Variable-temperature dc magnetic susceptibility data for a restrained polycrystalline sample of 1 collected under a 1000 Oe dc field. Inset: Plot of the magnetization versus temperature during FC (purple triangles) and ZFC (red circles) measurements revealing a bifurcation at the approximate blocking temperature.

thermal energy, and instead of being free to fluctuate, the magnetic moments are frozen along the magnetic easy axes of the individual molecules. In a randomly ordered crystalline powder sample, the overall magnetization will therefore approach zero at very low temperatures due to cancellation of the individual magnetic moments. Upon heating of the sample, the thermal energy grows steadily larger and thermal magnetic fluctuations again become possible. The temperature associated with the drop in χMT provides an estimate of the blocking temperature, below which pinning of the magnetization occurs (∼5 K for 1), at least in the presence of an applied dc field. Another method of estimating the blocking temperature is to compare the results of field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements, wherein, as the names imply, the sample magnetization is measured after first cooling the sample under a zero dc field. In a system exhibiting magnetic blocking, a comparison of the FC and ZFC data should reveal a bifurcation at Tb. Indeed, a comparison of the FC and ZFC measurements conducted on 1 reveals a sharp divergence at 4 K (Figure 2, inset). To examine the magnetization relaxation dynamics for 1, variable-temperature ac magnetic susceptibility measurements were carried out under zero dc field with Hac = 4 Oe, oscillating at frequencies ranging from 1 to 1500 Hz. As shown in Figure 3, 1 exhibits an out-of-phase component in the ac magnetic susceptibility (χM″), a hallmark of slow magnetic relaxation. At the lowest probed temperature of 2 K, χM″ reaches a maximum at a frequency of 5.8 Hz, and this frequency position holds steady, while the peak intensity decreases with increasing temperature up to 8 K (Figure S5). Upon further heating of the sample above 8 K, the χM″ maximum begins to shift steadily toward larger frequencies until it moves beyond the limit of the SQUID magnetometer (1500 Hz) at 46 K (Figure 3). The observed frequency independence of the χM″ signal at lower temperatures can be ascribed to the presence of quantum tunneling of magnetization, which begins to give way in favor of thermally activated relaxation with increasing temperature. Tunneling can also be short-circuited via the application of a dc magnetic field, and we sought to probe this effect for 1 via variable dc field ac susceptibility measurements carried out at 10 K. Notably, upon application of a 200 Oe dc field, the intensity of the zero dc field χM″ peak diminishes in

Figure 3. Top: In-phase (χM′, top) and out-of-phase (χM″, bottom) components of the ac magnetic susceptibility for 1 under a zero dc field from 8 K (blue triangles) to 46 K (red circles). Solid lines represent fits to the data as described in the main text. The temperature regime for relaxation is shown from 8 to 46 K for clarity, although the χM″ signal was observed to temperatures as low as 2 K (Figure S5).

conjunction with the emergence of a second, lower-frequency χM″ signal. As the field is increased in 200 Oe increments, the high-frequency peak diminishes in intensity and the lowerfrequency χM″ peak saturates at 0.3 Hz under an applied field of 1400 Oe (Figure 4, aqua trace).

Figure 4. Out-of-phase ac magnetic susceptibility (χM″) collected on pure 1 at 10 K under dc fields ranging from 0 to 1400 Oe in 200 Oe increments. Solid lines are guides for the eye.

This low-frequency shift in the χM″ signal under an applied field indicates an overall lengthening of the sample molecular relaxation times because thermal relaxation pathways begin to dominate over quantum tunneling and other rapid relaxation processes. Indeed, variable-temperature ac magnetic susceptibility data collected under Hdc = 1400 Oe clearly show a greater 15052

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3(1) × 10−13 s. This slight increase in Ueff under an applied field highlights the difficulty in extracting an accurate value for the relaxation barrier from ac susceptibility data when other relaxation processes are active. Indeed, given the limited temperature range that could be probed and the presence of other spin−lattice processes under the applied field, the actual magnitude of U, representing exclusive Orbach relaxation through the highest possible excited state, may be even larger than that determined here. As anticipated from the long low-temperature relaxation times extrapolated for 1 under a dc field (tens of seconds), the molecule also exhibits magnetic hysteresis in response to variable-field magnetization measurements. Sweeping out to absolute fields as high as 1 T with an average rate of 0.002 T/s revealed butterfly-shaped hysteresis at 1.8 K and as high as 5.3 K, with full loop closure at 5.8 K (Figure 6). We ascribe the lack

temperature dependence than the zero dc field data (Figure S7). In particular, the temperature-independent regime observed from 2 to 8 K is effectively obsolete under 1400 Oe. In the absence of strong quantum tunneling, thermal relaxation of magnetization in single-molecule magnets occurs via spin−lattice relaxation, wherein energy is exchanged between the “spin” system and surroundings via crystal lattice vibrations (phonons). When the energy exchanged corresponds to the quantized excited states (here, ±MJ) within a molecule, relaxation occurs via the Orbach mechanism.27 This thermally activated mode exhibits temperature dependence described by the equation τ = τ0 exp(Ueff/kBT) (where τ is the relaxation time, τ0 is the preexponential factor, Ueff is the effective barrier to magnetic relaxation, and kB is the Boltzmann constant). For relaxation occurring predominantly via this process, a plot of the natural logarithm of τ versus inverse temperature should therefore yield a straight line, the slope of which is Ueff (in units of Kelvin = (0.695−1)·cm−1). Temperature-dependent values of τ were extracted for 1 by fitting plots of χM′′ versus χM′ (Cole− Cole plots; Figures S8 and S10) using a generalized Debye model,18 and Figure 5 shows the resulting plots of ln(τ) versus

Figure 6. Variable-field magnetization data for compound 1 collected from 1.8 to 5.8 K at an average sweep rate of 0.002 T/s.

Figure 5. Arrhenius plots showing variable-temperature relaxation time data for 1. Solid black lines correspond to fits to multiple relaxation processes according to eq 1, as described in the text and the SI. The best fits for 1 yield values of Ueff of 546(6) cm−1 (blue circles; Hdc = 0 Oe) and 609(44) cm−1 (orange triangles; Hdc = 1400 Oe), respectively, and values of τ0 = 2(1) × 10−12 and 3(1) × 10−13 s, respectively.

of remnant magnetization at zero field to the tunneling of magnetization that was also evident in the ac susceptibility data under Hdc = 0 Oe. This hysteresis behavior exhibited by 1 is notably similar to that observed for [(C5Me5)2Dy][(μPh)2BPh2],12a although the measured Ueff value for the former is nearly double that of the tetraphenylborate complex under zero and applied dc fields [e.g., Ueff = 609(44) vs 331 cm−1 under fields of 1400 and 1600 Oe, respectively]. The analogous hysteresis behavior suggests that, at low temperatures, relaxation in the two Dy3+ systems is dominated by similar processes, i.e., quantum tunneling or direct relaxation. With increasing temperature, however, the population of molecules favoring Orbach relaxation begins to increase and distinctions become clear in the energies accessible for thermally activated relaxation. In particular, we ascribe the greater relaxation barrier of 1 to a reduction in the transverse molecular anisotropy that is effected by replacing the tetraphenylborate anion of [(C5Me5)2Dy][(μ-Ph)2BPh2] with the ammonia ligands. Structurally, this ancillary ligand substitution is accompanied by an increase of the Cnt−Dy− Cnt angle from 134.0° in [(C5Me5)2Dy][(μ-Ph)2BPh2] to 140.2° in 1, leading to a symmetry for the latter that more closely approaches axial. On the surface, the result of this change is the larger measured Ueff, which may arise from changes in the extent of MJ state mixing as well as allowed phonon transitions,28 and does not necessarily indicate a substantially different separation of magnetic energy levels in 1 and the tetraphenylborate compound. A comparison of the

1/T for ac magnetic susceptibility data collected under Hdc = 0 Oe and Hdc = 1400 Oe. Both sets of data exhibit linearity at high temperatures and curvature as the temperature is lowered to about 15 K, beyond which the zero-field data exhibit a plateau because temperature-independent quantum tunneling dominates the relaxation. This evidence of non-Orbach behavior under zero and applied fields is indicative of a mixture of temperature-dependent magnetization relaxation pathways, and accordingly each data set was fit using multiple relaxation processes, with the goal of achieving a more accurate estimation of Ueff. Successful modeling of the Arrhenius plots warranted the utilization of eq 1, which includes terms for quantum tunneling and Raman relaxation, in addition to the Orbach term from above (see the SI for more details). ⎛ −U ⎞ 1 1 = + CT n + τ0−1 exp⎜ eff ⎟ τobs τQTM ⎝ kBT ⎠

(1)

For relaxation data collected under Hdc = 0 Oe, eq 1 yielded values of Ueff = 546(6) cm−1 and τ0 = 2(1) × 10−12 s, while under Hdc = 1400 Oe, it afforded Ueff = 609(44) cm−1 and τ0 to 15053

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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.

calculated electronic structure for both compounds would provide additional insight in this regard. To the best of our knowledge, 1 is the first ammoniacoordinated complex to display slow magnetic relaxation, and it exhibits the second highest spin-reversal barrier for an organometallic compound, using the definition that at least one metal−carbon bond must be present within the molecule. The barrier for 1 is also among the highest yet reported for a mononuclear dysprosium single-molecule magnet,29 comparable to effective barriers for [L2Dy(H2O)5][I]3·L2·H2O [L = t BuPO(NHiPr)2; Ueff = 511 cm−1] and (NNTBS)DyI(THF)2 [NNTBS = fc(NHSitBuMe2)2 with fc = 1,1′-ferrocenediyl; Ueff = 536 cm−1],30 but is surpassed by the barriers exhibited by the pentagonal-bipyramidal complexes [Dy(bbpen)Br] (Ueff = 712 cm−1)8a and [Dy(OtBu)2(py)5][BPh4] (Ueff = 1261 cm−1)8b and the organometallic compound [(Cpttt)2Dy][B(C6F5)4], featuring a base-free metallocenium cation [(Cpttt)2Dy]+ (Ueff = 1277 cm−1).8c,d The results described herein drive home the concept of targeting highly axial ligand environments to enhance the single-ion magnetic anisotropy of the Dy3+ ion. Conversely, another way of viewing this approach is to focus on minimizing the transverse anisotropy. Indeed, the recent Ueff breakthroughs were all achieved in systems exhibiting highly axial ligand environments and weak transverse anisotropies. In the pentagonal-bipyramidal complexes, high axiality was achieved with two opposing anionic oxo ligands and weakly bound, neutral equatorial ligands, while the metallocenium cation [Cpttt2Dy]+, with no equatorial ligands, exhibits a Cnt− Dy−Cnt angle of 152°, even larger than that of 1.8c,d

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*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Selvan Demir: 0000-0001-7983-9850 Jordan F. Corbey: 0000-0002-3273-3044 David H. Woen: 0000-0002-5764-1453 Jeffrey R. Long: 0000-0002-5324-1321 William J. Evans: 0000-0002-0651-418X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation through Grants CHE-1565776 (to W.J.E.) and CHE-1464841 (to J.R.L.). We thank Jason R. Jones for assistance with X-ray crystallography, Lucy E. Darago for experimental assistance, and Dr. Joseph M. Zadrozny for helpful discussions.

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REFERENCES

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CONCLUSIONS AND OUTLOOK We have reported the serendipitous discovery of 1, which can be reproducibly made in good yield via the addition of NH3 to [(C5Me5)2Dy][(μ-Ph)2BPh2] in toluene at low temperature. The THF adduct 2 can also be synthesized directly via the addition of NH3 to [(C5Me5)2Dy][(μ-Ph)2BPh2] in THF. Notably, the exploration of side reactions intended to understand the initial source of 1 led to isolation of the rare bis(allyl) complex (C5Me5)Dy(C3H5)2(THF). Magnetic characterization of the bis(ammonia) adduct 1 revealed that it is a zero-field single-molecule magnet like its forerunner, [(C5Me5)2Dy][(μ-Ph)2BPh2], and is the first ammonia adduct of a lanthanide to exhibit slow magnetic relaxation. Notably, the replacement of the tetraphenylborate anion with two ammonia ligands results in more than 150% enhancement in the effective relaxation barrier, a result that we ascribe to a reduction of the transverse anisotropy in 1 relative to [(C5Me5)2Dy][(μPh)2BPh2]. Efforts are now underway to investigate the impact of other coordinating ligands on the magnetization dynamics of the metallocenium cation [(C5Me5)2Dy]+.

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02390. Additional experimental details, crystallographic details, X-ray data collection, structure solution, and refinement, and additional magnetic characterization data (PDF) Accession Codes

CCDC 1417998−1417999 and 1551034 contain the supplementary crystallographic data for this paper. These data can be 15054

DOI: 10.1021/acs.inorgchem.7b02390 Inorg. Chem. 2017, 56, 15049−15056

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Inorganic Chemistry

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