Electronic Structure of a Mixed-Metal Fluoride-Centered Triangle

Dec 4, 2015 - In compound 1, the nickel and chromium atoms are fully disordered over the three metallic sites of the triangle. The occupancies were se...
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Electronic Structure of a Mixed-Metal Fluoride-Centered Triangle Complex: A Potential Qubit Component James P. S. Walsh,†,‡ Sarah B. Meadows,† Alberto Ghirri,§ Fabrizio Moro,†,∥ Martin Jennings,† William F. Smith,⊥ Darren M. Graham,⊥ Takumi Kihara,# Hiroyuki Nojiri,# Iñigo J. Vitorica-Yrezabal,† Grigore A. Timco,† David Collison,*,† Eric J. L. McInnes,† and Richard E. P. Winpenny† †

School of Chemistry and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom ⊥ School of Physics and Astronomy and Photon Science Institute, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom § S3 Centre, Institute Nanoscience (CNR), via G. Campi 213/A, 41125 Modena, Italy # Institute for Materials Research, Tohoku University, Katahira, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: A novel fluoride-centered triangular-bridged carboxylate complex, [Ni2Cr(μ3-F)(O2CtBu)6(HO2CtBu)3] (1), is reported. Simple postsynthetic substitution of the terminal pivalic acids in 1 with pyridine and 4-methylpyridine led to the isolation of [Ni2Cr(μ3-F)(O2CtBu)6(C5H5N)3] (2) and [Ni2Cr(μ3-F)(O2CtBu)6((4-CH3)C5H4N)3] (3). Structural and magnetic characterizations carried out on the series reveal a dominating antiferromagnetic interaction between the nickel and chromium centers leading to an S = 1/2 ground state with a very unusual value of geff = 2.48.

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INTRODUCTION Trimetallic complexes with a central μ3-atom bridge have a history that can be traced as far back as 1908, when Weinland and Werner independently gave the first reports of trimetallic chromium carboxylate compounds.1,2 The triangular arrangement was not immediately apparent and was conclusively determined by X-ray diffraction studies only almost 60 years later.3 Since then, a huge number of compounds containing the same core structure has been reported.4 The vast majority have the general molecular formula [M3XB6L3], where M is the transition-metal ion, X is the central μ3 atom (usually oxygen), B is the edge-bridging unit (we restrict our discussion here to the overwhelmingly common carboxylates), and L is the terminal ligand. The {M3X} core is typically planar, with the six edge-bridging μ2-carboxylates orienting themselves above and below the plane defined by the metals and with the three terminal M−L bonds lying within this plane. The large number of known compounds with this structure stems from the variability of each component. For example, the core can, in principle, be composed of any combination of transition metals (iron,5−7 manganese,8−11 vanadium,12,13 chromium,14 cobalt,15 and heavier 4f and 5f metals16,17 are known), and the edge bridges can be any carboxylate (formate,16 acetate,5,6 pivalate,15 benzoate,12 etc.). Additionally, if the terminal ligands are sufficiently labile, then simple © XXXX American Chemical Society

postsynthetic substitution reactions can allow for the generation of families of closely related compounds sharing the same core. In fact, the only aspect of these compounds that is not readily changed is the identity of the central μ3 atom: Only two carboxylate triangular-bridged complexes are known where the μ3 atom is not oxygen, namely, M[NiII3(μ3F)(O2CF3)6(L)3] and M[CoII3(μ3-F)(O2CF3)6(L)3] (M = alkali metal), both reported by Tereshchenko et al.18,19 Triangular-bridged complexes are of particular interest to magnetochemists for a number of reasons, including (i) the ability to study the effects of systematic chemical changes (e.g., in M or the carboxylate) on the exchange interactions within a rigid core structure; (ii) the fact that a triangle is the smallest discrete geometry in which important physical effects such as spin frustration can be studied, leading to studies of, for example, antisymmetric exchange effects;17 and (iii) the utility of triangles as excellent precursors for the supramolecular synthesis of larger polymetallic clusters and frameworks.15,20 Although the use of oxides/hydroxides as bridging ligands in such clusters is common, compounds utilizing the isoelectronic fluoride remain rare. This is unfortunate, as fluorides could be particularly useful in real-world technological applications Received: October 20, 2015

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DOI: 10.1021/acs.inorgchem.5b01898 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystallographic Data for Compounds 1−3 formula fw cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) T (K) Z ρcalcd (g cm−3) λ (Å)/μ (mm−1) no. of reflns collected/2θmax (deg) no. of reflns: unique/I > 2θ(I) no. of params/restraints R1/goodness of fit wR2 [I > 2θ(I)] residual density (e Å−3)

1

2

3

Ni2CrFC45H84O18 1101.5 orthorhombic Cmcm 18.4480(10) 20.356(2) 17.1920(10) 90 6456.1(8) 230(2) 4 1.133 0.71073/0.801 37659/50.7 3168/2680 221/419 0.0634/1.061 0.1907 0.48/−0.58

Ni2CrFC45H69N3O12 1032.5 monoclinic P21/n 22.3995(4) 22.9275(3) 22.7329(5) 113.317(2) 10721.3(4) 150(2) 8 1.279 0.71073/0.954 45067/50.70 21726/15144 1191/36 0.0493/1.045 0.1038 0.65/−0.81

Ni2CrFC48H75N3O12 1074.5 monoclinic P21 11.6150(11) 20.4827(11) 12.3905(14) 110.780(7) 2805.4(5) 100(2) 2 1.272 0.71073/0.914 10573/50.70 8853/6819 605/49 0.0738/1.045 0.1713 1.59/−0.79

days along with small crystals, including crystals suitable for X-ray diffraction studies. All crystals were washed briefly with cold hexane. Large crystals: Anal. Found: C, 49.24; H, 7.77; Ni, 10.90; Cr, 4.64. Calcd for Ni2CrFC45H84O18: C, 49.07; H, 7.69; Ni, 10.66; Cr, 4.72. Small crystals: Anal. Found: C, 49.41; H, 7.98; Ni, 10.80; Cr, 4.62. Calcd for Ni2CrFC45H84O18: C, 49.07; H, 7.69; Ni, 10.66; Cr, 4.72. [Ni2Cr(μ3-F)(O2CtBu)6(C5H5N)3] (2). A dark green solution of 1 (0.5 g, 0.5 mmol) in hexane (50 mL) was treated with an excess of pyridine (0.2 mL, 2.48 mmol), giving a turquoise solution almost instantly. The solution was left stirring for 24 h in a closed flask at ambient temperature. The resulting solution was then left under slow evaporation, giving large single crystals suitable for X-ray diffraction studies after 2 days. These were collected by filtration, washed with cold MeCN, and left to dry under dinitrogen. Yield: 0.42 g (81%). Anal. Found: C, 52.21; H, 6.74; N, 4.01; Ni, 11.27; Cr, 5.14. Calcd for Ni2CrFC45H69O12N3: C, 52.35; H, 6.74; N, 4.07; Ni, 11.37; Cr, 5.04. ES+ MS, m/z 771.9 ([Ni2CrF(O2CtBu)5(py)]+), 692.0 ([Ni2CrF(O2CtBu)5]+), 511.0 ([NiCr(O2CtBu)4]+). [Ni2Cr(μ3-F)(O2CtBu)6((4-CH3)C5H4N)3]} (3). The same procedure as described for 2 was employed, but using excess 4-methylpyridine (0.2 mL, 2.05 mmol). Yield: 0.39 g (78%). Anal. Found: C, 53.71; H, 7.10; N, 3.95; Ni, 10.65; Cr, 5.05. Calcd for Ni2CrFC48H75O12N3: C, 53.65; H, 7.04; N, 3.91; Ni, 10.93; Cr, 4.84. ES+ MS, m/z 878.0 ([Ni2CrF(O2CtBu)5(4-pic)2]+), 786.9 ([Ni2CrF(O2CtBu)5(4-pic)]+), 525.1 ([Ni2F(O2CtBu)2(4-pic)2]+). X-ray Crystallography. Data Collection. For compounds 1−3, single crystals were mounted in the nitrogen cold stream of an Agilent SuperNova diffractometer (1 and 2) and an Oxford Diffraction XCalibur2 diffractometer (3). Graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) was used in all studies. Crystal Structure Determination. X-ray data were processed and reduced using the CrysAlisPro suite of programs. Absorption correction was performed using empirical methods based on symmetry-equivalent reflections combined with measurements at different azimuthal angles.23 Atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and refined using idealized geometries (riding model) and fixed isotropic displacement parameters. Many of the t-butyl groups exhibited significant disorder, especially on the terminal pivalic acid ligands in compound 1. This was modeled by allowing for two conformations of the t-butyl group and refining their occupancy factors. Atomic displacement parameter (adp) values on some of the problematic carbon and oxygen atoms were also restrained using the RIGU, EADP, and SIMU commands. In compound 1, the nickel and chromium atoms are fully disordered over the three metallic sites of the triangle.

because of their lack of redox activity (i.e., an increased chemical stability compared to oxides).21 Moreover, fluorides can promote interesting superexchange interactions, a recent example being the generation of a strong ferromagnetic interaction in a near-linear M−F−M dimer.22 This study reports the synthesis and magnetic characterization of the first mixed-metal fluoride-centered triangular bridged complex, [Ni2Cr(μ3-F)(O2CtBu)6(HO2CtBu)3] (1), along with two derivatives formed by postsynthetic substitution of the terminal pivalic acid groups for N-donor pyridines [pyridine (2) and 4-methylpyridine (3)].

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EXPERIMENTAL SECTION

Synthesis. Unless stated otherwise, all reagents were purchased from Sigma-Aldrich and used without further purification. Acetonitrile and hexane were distilled over CaH2. Extra-dry acetone was purchased from Acros Organics. The Erlenmeyer Teflon FEP flasks were supplied by Fisher. With the exception of the procedure carried out in Teflon FEP Erlenmeyer flasks, all procedures were performed under a dinitrogen atmosphere. [Ni2Cr(μ3-F)(O2CtBu)6(HO2CtBu)3] (1). Pivalic acid (30 g, 294 mmol), 2-amino-2-(hydroxymethyl)-1,3-propanediol (1.0 g, 8.25 mmol), CrF3·4H2O (3.00 g, 16.57 mmol), and 2NiCO3·3Ni(OH)2· 4H2O (3.90 g, 6.64 mmol) were stirred in an open Teflon FEP Erlenmeyer flask in a preheated oil bath at 140 °C for 5 h. During this time, the chromium fluoride dissolved, and a green crystalline product began to precipitate. The flask was then allowed to cool to room temperature, acetonitrile (50 mL) was added, and the mixture was stirred for 0.5 h to complete the precipitation. The precipitate was collected by filtration and washed with acetonitrile (3 × 50 mL) and then with cold acetone (3 × 5 mL). The solid was left to dry under a flow of dinitrogen and then extracted into hexane (100 mL). This solution was filtered, and the solvent was removed from the filtrate, giving a green polycrystalline product. Yield: 5.6 g (31%). Anal. Found: C, 49.07; H, 7.97; Ni, 10.92; Cr, 4.59; F, 1.74. Calcd for Ni2CrFC45H84O18: C, 49.07; H, 7.69; Ni, 10.66; Cr, 4.72; F, 1.72. 19F NMR (chloroform-d, ppm): − 175 (bs). Preparation of Single Crystals. Compound 1 (2.0 g) was dissolved in refluxing hexane (12 mL) under stirring for 5 min. The clear solution obtained was allowed to cool slowly to room temperature and was then left undisturbed at ambient temperature under dinitrogen. Large, well-shaped crystals were collected after 5 B

DOI: 10.1021/acs.inorgchem.5b01898 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry The occupancies were set to 1/3 and 2/3 for the chromium and nickel atoms, respectively, and the adp values were constrained to be equal using the EADP command. Final cell constants were obtained from least-squares fits to all measured reflections. All structures were solved by direct methods using SHELXS-2015.24 In 2 and 3, significant differences in the electron densities and local bond lengths enabled determination of the positions of each metal. Crystallographic data are presented in Table 1 (CCDC numbers 1055991−1055992, 1415891). Physical Measurements. Variable-temperature (2−300 K) magnetic-susceptibility measurements were recorded in a 0.1 T magnetic field on a SQUID magnetometer (Quantum Design MPMS-XL). The same instrument was used to measure the field-dependent magnetization of all compounds between 0 and 7 T and at temperatures of 2− 5 K. The simulations of both magnetization and susceptibility were performed using the computer program PHI.25 Additional magnetization measurements were performed at 0.5 K using a conventional inductive probe in a pulsed magnetic field system that incorporated a 3 He cryostat. High-frequency electron paramagnetic resonance (EPR) measurements were performed on the same high-field pulsed magnet system26 using a range of Gunn oscillators with frequencies ranging from 135 to 405 GHz. The data were collected in absorption mode and were then computationally converted into derivative-mode spectra to facilitate analysis. X- and Q-band continuous-wave EPR data were collected on a Bruker EMX spectrometer. Pulsed measurements were performed on a Bruker E580 spectrometer equipped with a dielectric resonator and used a two-pulse echo decay sequence (π/2−τ−π−τ− echo). EPR spectra were simulated using Weihe’s SimEPR program,27 using routines described elsewhere,28,29 with further calculations performed with EasySpin.30 Analytical data were obtained by the microanalytical service of the University of Manchester. 19F NMR was carried out at the University of Manchester. Heat-capacity measurements were carried out at the Institute Nanoscience in Modena using a Quantum Design PPMS and were modeled using PHI.

bases will often template the formation of {Cr7Ni} ring structures under these conditions.31 (Indeed, such compounds were being targeted when compound 1 was discovered.) The nickel and chromium ions retain their original oxidation states, leading to an overall charge-neutral molecule. Upon cooling, compound 1 undergoes a phase change at 168 K, wherein its crystal system transforms from orthorhombic to monoclinic. Multiple data sets were collected above and below this temperature, and no significant geometrical changes were observed between the phases (averages of the salient bond lengths differed by at most 0.04 Å). For this reason, we report here only the highest-quality data collected for this compound, which were measured at 230 K. The terminal pivalic acid ligands in 1 are relatively labile, facilitating postsynthetic substitution reactions in the presence of an excess of the N-donors pyridine and 4-methylpyridine to give compounds 2 and 3, respectively (Figure 2). The substitutions are fast and are accompanied by a distinct color change from dark green to turquoise/blue that typically occurs within minutes of stirring at room temperature. All three compounds exhibit an essentially planar {Ni2Cr(μ3F)} unit, bound by six μ2-pivalates. The maximum displacement of the central μ3-fluorine from the {Ni2Cr} plane is 0.018(10) Å (in 2). Two pivalates bridge each pair of metals, oriented above and below the {Ni2Cr} plane, and terminal ligands trans to the central μ3-F ion complete the octahedral environment of each metal. In compound 2 the locations of the metals can be determined from significant differences in the bond lengths (Table 2) and electron densities at the metal sites. The difference in bond lengths is not as clear-cut for 3, but a model with the metal positions fixed gives a significantly better agreement to the data during refinement, and so we retained it. There are virtually no differences in the metrics of the metal sites in 1, which is also the only member of the series not to have a complete molecule in the asymmetric unit. We therefore assumed full disorder of the metals in this compound and refined with each site carrying a fixed occupancy of 2/3 nickel and 1/3 chromium. The orientation of the pyridine ring planes with respect to the {Ni2Cr} plane is different for 2 and 3. In 2, two of the pyridines lie perpendicular to the plane, with the remaining pyridine lying parallel to it, whereas in 3, all of the pyridines lie parallel to the plane. Magnetic Measurements. The temperature-dependent molar magnetic-susceptibility (χM) behavior is qualitatively identical across the series (Figure 3). At 300 K, the χMT values fall in the narrow range of 3.94−4.04 cm3 K mol−1 and exhibit a steady downward trend upon cooling, before leveling off at the lowest temperatures to values between 0.45 and 0.49 cm3 K mol−1. These data indicate a dominant antiferromagnetic interaction resulting in a nonzero-spin ground state (which has to be the case for two SNi(II) = 1 ions and one SCr(III) = 3/2 ion). Field-dependent molar magnetization (M) plots measured over 2−5 K (Figure S1, Supporting Information) are also similar across the series, failing to saturate at 7 T. In each case, the M(H,T) traces cross each other between 6 and 7 T. These data indicate that the ground state is not well isolated in the higher field range. To investigate this issue further, we performed very-high-field magnetization studies at 0.5 K. In all three compounds, an inflection point in M(H) is apparent at around 11.5−14.0 T (Figures S2−S4, Supporting Information)

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of 1 (Figure 1) is a straightforward one-pot reaction that involves heating pivalic acid in the presence of nickel(II) carbonate hydroxide tetrahydrate and chromium(III) trifluoride. The presence of a sterically bulky base is required [we used 2amino-2-(hydroxymethyl)-1,3-propanediol] because less bulky

Figure 1. Crystal structure of 1. The light green atoms are nickel, and the dark green atom is chromium; however, both are disordered around the triangle. Hydrogen atoms are omitted for clarity. C

DOI: 10.1021/acs.inorgchem.5b01898 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Crystal structures of 2 (left) and 3 (right); colors as defined in Figure 1. The identities of the metal sites are known, as explained in the text. Hydrogen atoms are omitted for clarity.

Table 2. Average Bond Distances (Å) and Angles (deg) for 1−3a 1 M1−O M2−O

1.994(2) 2.009(2)

M1−L M2−L

2.039(6) 2.045(5)

M1−F1 M2−F1

2.008(4) 1.996(2)

Ni1−O Ni2−O Cr1−O Ni1−L Ni2−L Cr1−L Ni1−F1 Ni2−F1 Cr1−F1

2

3

2.027(2)/2.019(2) 2.029(2)/2.018(2) 1.966(2)/1.951(2) 2.079(3)/2.063(3) 2.068(3)/2.068(3) 2.082(3)/2.089(3) 2.049(2)/2.039(2) 2.039(2)/2.023(2) 1.922(2)/1.925(2)

2.002(9) 1.983(9) 1.990(9) 2.061(9) 2.084(9) 2.072(10) 2.010(5) 2.003(5) 1.999(6)

a Values calculated using ∑xi/n, where x is the bond metric and n is the number of values averaged. L represents the metal-bound atom at the terminal sites. The M−O lengths are the averages of the four equatorial M−O bonds, where the axis is defined by F1−M−L.

Figure 4. Single-crystal heat-capacity measurements for compound 1, normalized to R = 8.314 J mol−1 K−1. Solid lines are simulations using the parameters: J1 = −3.2 cm−1, J2 = −4.2 cm−1, gNi = 2.20, gCr = 1.98, TD = 51 K, and α = 2.6.

lattice contribution, which can be modeled using the phenomenological formula ⎛ T ⎞α C latt = 234⎜ ⎟ R ⎝ TD ⎠

(1)

with TD = 51 K and α = 2.5. Below 5 K, the magnetic contribution becomes visible. At zero field and T < 1 K, C is vanishingly small, ruling out any significant zero-field-splitting (ZFS) effects in a total spin multiplet and hence implying a ground state with S = 1/2. A Schottky anomaly is observed centered at around 2 K, implying an excited state at roughly 6 K (or 4 cm−1). A further Schottky anomaly is induced by an applied magnetic field, and its field dependence confirms that the ground state has a total spin of S = 1/2. EPR Spectroscopy. All three compounds were investigated using variable-frequency EPR spectroscopy. Figure 5 shows low-temperature (4.2−5 K) powder EPR spectra for compound 1 measured over a range of frequencies between 9.4 and 405 GHz. The complete set of spectra of 1 over the temperature range of 4.2−50 K is given in Figure S5 (Supporting Information). At all frequencies, the spectra are dominated by a single feature centered at g = 2.48, with significantly weaker features appearing at around g ≈ 2.1 (seen more clearly at higher frequencies). Upon warming, the g ≈ 2.1 feature

Figure 3. Temperature-dependent molar magnetic susceptibilities of compounds 1−3 measured under a static field of 0.1 T. Solid traces are simulations calculated using the following parameters: J1 = −3.2 cm−1; J2 = −3.6 cm−1 (red), − 3.9 cm−1 (green), and −4.2 cm−1 (blue); gNi = 2.20; and gCr = 1.98.

because of the crossover of the spin ground state with an excited state. These field positions are defined more clearly in dM/dH, giving peaks at 13.6, 12.4, and 11.6 T for 1, 2, and 3, respectively. Heat-Capacity Measurements. Figure 4 shows measurements of specific heat (C) as a function of temperature for a sample of 1. Above 5 K, the C(T) curves are dominated by the D

DOI: 10.1021/acs.inorgchem.5b01898 Inorg. Chem. XXXX, XXX, XXX−XXX

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typical values of gNi = 2.20 and gCr = 1.98, we obtain geff = 2.42 for the |2, 1/2⟩ state and g = 1.83 for the |1, 1/2⟩ state. The former is in excellent agreement with the EPR-detected ground state. Hence, we have a |2, 1/2⟩ ground state with a low-lying |1, 1 /2⟩ excited state. This requires the interaction between the nickel and chromium ions to be antiferromagnetic. There are two unique exchange interactions: J1 between the nickel centers and J2 between the nickel and chromium ions. The simplest possible (isotropic) spin Hamiltonian is given by 3

Ĥ =

∑ giβsîH⃗ − 2J1s1̂ ·s2̂ − 2J2 (s1̂ ·s3̂ + s2̂ ·s3̂ ) i=1

This can be handled by a straightforward Kambe treatment, giving the eigenvalues listed in the Supporting Information. The two S = 1/2 states have energies of E(|2, 1/2⟩) = −2J1 + 9J2 and E(|1, 1/2⟩) = 2J1 + 5J2. The energy gap between these two states is 4(J1 − J2). Hence, for |2, 1/2⟩ to be lower in energy than |1, 1/2⟩ (as is observed) requires J1/J2 < 1.0, as shown in the Kambe plot Figure S8 (Supporting Information). In other words, the Ni···Ni interaction has to be less antiferromagnetic (or ferromagnetic) than the Ni···Cr interaction. For J1/J2 < 1.0, the lowest-lying excited states are |1, 1/2⟩ and |2, 3/2⟩ at relative energies of 4(J1−J2) and 3J2, respectively (Figure S8, Supporting Information). We can roughly estimate J1 and J2 from the M(H) and C(T) data. The peaks in the dM/dH curves observed at ca. 13 T must be due to the level crossing between the lowest Zeeman states of the |2, 1/2⟩ ground state and the lowest-energy S > 1/2 excited state, namely, |2, 3/2⟩. (The relative energies of the |2, 1 /2⟩ and |1, 1/2⟩ states do not change other than because of the difference in g, and given that the g value of the |1, 1/2⟩ state is smaller than that of the |2, 3/2⟩ state, they are not expected to cross at any field.) The difference in the Zeeman energy of the M = −1/2 and M = −3/2 states is simply gβH (ca. 1 cm−1 T−1); hence, the energy separation of |2, 1/2⟩ and |2, 3/2⟩ at zero field (3J2) must be around 13 cm−1. This energy gap is much too big to explain the zero-field Schottky anomaly in C(T); hence, this latter gap (4 cm−1) must correspond to the |1, 1/2⟩ state, giving 4(J1−J2) = 4 cm−1. Combining these data gives J1 ≈ − 3 cm−1 and J2 ≈ − 4 cm−1. A conventional magnetochemical treatment to determine J1 and J2 would be to perform a simple free fit to χMT(T) with the Hamiltonian given in eq 2. Doing this for 1, with gNi and gCr fixed to the values above, gives best-fit parameters of J1 = −4.3 and J2 = −4.7 cm−1, in reasonable agreement with those above. However, the fit is in fact rather insensitive to J1 within the range of a few cm−1. Moreover, using these J values to calculate high-field, low-temperature dM/dH values gives a peak at ca. 15 T, which is significantly higher than observed experimentally. Hence, we refined these parameters using the Hamiltonian in eq 2 to attempt to simultaneously model (i) χMT(T), (ii) the high-field peak in dM/dH, (iii) g ≈ 2.48 in the low-temperature EPR spectra, and (iv) C(T,H). The calculated positions of the M(H) steps are entirely dependent on the magnitude of J2 (see above), and the experimental values were found to be J2 = −4.2, − 3.9, and −3.6 cm−1 for 1, 2, and 3, respectively (Figures S2− S4, Supporting Information). The accuracy of these J2 values is dependent on the dM/dH peak width, which is narrowest for 2 and broadest for 1. These parameters are all consistent with the experimental χMT(T) values because they fall within a narrow range of −3.9 ± 0.3 cm−1.

Figure 5. Solid-state powder EPR spectra for compound 1 measured at low temperature (4.2−5 K) with microwave frequencies ranging from 9.4 to 405 GHz. Red traces are simulations using the parameters given in Table 3. The features at g ≈ 2.1 in the calculated highfrequency spectra arise from transitions within the |2, 3/2⟩ multiplet; these transitions are not well-resolved in the experimental data for the reasons discussed in the main text.

increases in intensity, whereas the g = 2.48 feature diminishes, suggesting that they arise from an excited state and a ground state, respectively. An S = 1/2 ground state with g = 2.48 is consistent with the base-temperature value of χMT (0.49 cm3 K mol−1). Table 3. Global Parameter Set Used for the Simulations of Compounds 1−3 gNi gCr J1 (cm−1) J2 (cm−1)

1

2

3

2.20 1.98 −3.2 −4.2

2.20 1.98 −3.2 −3.9

2.20 1.98 −3.2 −3.6

(2)

The spectra of 2 and 3 (Figures S6 and S7, Supporting Information) are similar in form to those of 1 but with a slight splitting of the g = 2.48 feature and greater relative intensity of the g ≈ 2.1 feature. A general feature in the spectra of all three compounds is an increase in the intensity of the transitions around g ≈ 2.1 at higher fields and higher temperatures. The phase-memory times, TM, of the two main features (at g = 2.48 and g ≈ 2.1) were measured for compound 1 using a stimulated echo (π/2−τ−π−τ−echo) pulse sequence. These experiments are detailed in the Supporting Information. For the g = 2.48 feature, TM in a frozen toluene solution is found to be 643 ± 7 ns, whereas for the g ≈ 2.1 feature, TM is found to be 44 ± 8 ns. Modeling the Data. Combining the data allows us to formulate a sensible starting point for our model, which, to a first approximation, can be used to explain the data for all three compounds (because the data are almost identical across the series). First, each of the techniques supports a ground state with S = 1/2 character along with low-lying excited states. We have a triangle of spins comprising S1 = S2 = 1 and S3 = 3 /2. There are two total-spin S = 1/2 states for the triangle, namely, |S12, S⟩ = |1, 1/2⟩ and |S12, S⟩ = |2, 1/2⟩. These are predicted to have values of g = −2gNi/3 + 5gCr/3 and g = 2gNi − gCr, respectively, in the strong-exchange limit.32 When using E

DOI: 10.1021/acs.inorgchem.5b01898 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Calculated C(T,H) curves are quite sensitive to J1 and require this parameter to be nonzero to reproduce the experimental data for 1. Calculating C(T,H) for 1 with a fixed value of J2 = −4.2 cm−1 gives good simulations with J1 = −3.2 cm−1 up to 3 T (Figure 4). These parameter ranges of J2 = −3.9 ± 0.3 cm−1 with J1 ≈ − 3 cm−1 correctly give the |2, 1/2⟩ ground state with |1, 1/2⟩ and | 2, 3/2⟩ as the first and second excited states, respectively. The experimental EPR spectra are dominated by the g = 2.48 resonance of the ground state at all frequencies measured (9− 405 GHz), that is, with resonance fields up to ca. 12 T. This feature is reproduced in calculated EPR spectra at all frequencies (Figure 5) using the parameters above. At higher frequencies, the calculated spectra also give a significant resonance at around g = 2.1. This arises from transitions within the |2, 3/2⟩ state, which is predicted to have g = 4gNi/5 + gCr/5 = 2.16. This feature becomes more prominent at higher frequencies because the higher magnetic fields bring the |2, 3/2⟩ state closer in energy to the |2, 1/2⟩ ground state. This feature is also clearly present in the experimental high-frequency spectra, becoming more prominent with increasing temperature (Figures S5−S7, Supporting Information), but nevertheless, it remains less prominent than in the calculated spectra. It is tempting to “dampen” this transition by increasing J2 and, hence, increasing the energy gap to |2, 3/2⟩; however, this is not compatible with the M(H) data. Instead, the reason for the diminished amplitude of this transition in the experimental spectra is its much larger line width compared to the groundstate (g = 2.48) resonances. EPR transitions within S > 1/2 states tend to be broader than those within S = 1/2 states because the former can be broadened by ZFS effects and also by strain in the ZFS parameters. Moreover, additional relaxation pathways become possible for multilevel states (i.e., S > 1/2), leading to higher relaxation rates.32 This is experimentally observed in phase-memory measurements of this transition (Supporting Information). Our simple isotropic model does not incorporate ZFS and, hence, artificially enhances the amplitude of the excited state (g = 2.1) peak. It would be straightforward to introduce a broadening of this peak through the introduction of local ZFS parameters (dM) at Ni(II) and Cr(III) and by adding a strain in these parameters. However, even though dNi would be expected to be largest in magnitude, we found that, for moderate values of dM (less than ca. 1 cm−1, assuming colinear dM tensors for simplicity), the spectra are actually more sensitive to dCr. This is because the projection factors for dCr and dNi on the |2, 3/2⟩ state are 1/3 and nil, respectively, in the strong-exchange limit. However, in the absence of any well-resolved fine structure, there is little point in introducing these extra parameters purely to broaden a peak in a calculated spectrum. The same overall modeling approach was used for compounds 2 and 3, which show data similar to those obtained for 1. The slight changes observed in the curvature of the magnetic-susceptibility traces of 1−3 are caused by the subtle changes in J2 (the calculated curves being much more sensitive to J2 than to J1). The susceptibility trace is also quite insensitive to the introduction of ZFS. The splitting of the g ≈ 2.4 feature in the EPR spectra of 2 and 3 can be simply explained by a slightly axial gNi with gz < gx,y. The higher relative amplitude of the g ≈ 2.1 features in 2 and 3 is partly a result of this splitting of the g = 2.48 signal and also of the lower energy of the excited state as shown by the M(H) results (see above). The only chemical difference in the three complexes is a change in the

coordination spheres of the metal ions from M{O5F} (1) to M{O4FN} (2 and 3). Such a change could easily introduce a greater g anisotropy and might also be expected to enhance the ZFS at Ni(II). However, for the reasons given above, the model is relatively insensitive to dNi.

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CONCLUSIONS A novel fluoride-bridged triangle complex has been isolated and characterized by X-ray crystallography, mass spectrometry, and microelemental analysis, clearly confirming the inclusion of fluorine in the center of the triangular complex. The presence of a central fluoride rather than a conventional oxideand the resulting effect on the charge-balance requirementshas allowed the isolation of a new heterometallic combination. In other words, we have used the monoanionic fluoride to isolate [CrNi2F(O2CR)6(L)3] rather than [Cr2NiO(O2CR)6(L)3], which is the product obtained with dianionic oxides.33 In both cases, the formation of the charge-neutral product facilitates its isolation from ionic byproducts (e.g., {Cr3O}+, {Cr2NiF}+, or {Ni3F}−). To the best of our knowledge, this is the first example of a {MII2M′III} triangle complex. Magnetic-susceptibility measurements combined with highfield magnetization, high-field/-frequency EPR spectroscopy, and heat-capacity measurements can be explained by a surprisingly simple isotropic model. Although there must be anisotropy in the magnetic properties (due to local ZFS parameters or anisotropic/antisymmetric exchange effects), their effect on the low-temperature physics (i.e., on the lowest lying spin multiplets) is minimal in these compounds. The studies reveal a dominant antiferromagnetic exchange interaction between nickel and chromium of J2 ≈ −4 cm−1, with a weaker interaction between the nickel ions of J1 ≈ −3 cm−1. The J2 value can be compared with the JCrNi = −47 cm−1 value determined for the closely related, but oxo-centered, {Cr2Ni} triangle [Cr2NiO(OAc)6(py)3] (with JCrCr = −26 cm−1).33 Accordingly, exchange interactions appear to be much weaker through fluoride than through oxide in these triangular structures. This has also been observed for linear F-/O-bridged dimers.21 We can make a further comparison with the {Cr7Ni} ring-like structures of the general formula (cation)[Cr7NiF8(O2CR)16],31 where each neighboring pair of metal ions is bridged by a fluoride and two carboxylates, as in 1−3. In these compounds, JCrNi has a similarly small value of ca. −7 cm−1 as determined by magnetic, EPR, and inelastic neutron scattering studies.28,29,34 Replacement of fluoride bridges in these {Cr7Ni} structures with alkoxides increases |J| by ca. 50%,35 consistent with the change of JCrNi in the triangles, albeit less dramatic in magnitude. As in 1−3, the antiferromagnetic nearest-neighbor exchange interactions in the {Cr7Ni} rings lead to an S = 1/2 ground state. The ground state of {Cr7Ni} has nearly axially symmetric g values, with gz = 1.74, gx,y = 1.78. These unusually low g values are in stark contrast with the unusually high value of g = 2.48 found for 1−3, despite the fact that both S = 1/2 ground states arise from antiferromagnetic coupling between Cr(III) and Ni(II) ions. Both can be justified on the basis of simple vectorcoupling schemes, and this illustrates how coordination chemistry can be used to engineer very different g values even when starting from the same building blocks. An emerging trend in molecular magnetism is the assembly of supramolecular systems as a bottom-up approach toward quantum devices. In these systems, control of quantum states is F

DOI: 10.1021/acs.inorgchem.5b01898 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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ACKNOWLEDGMENTS This work was supported by the United Kingdom Engineering and Physical Sciences Research Council (Grant EP/J002518/ 1), the ICC-IMR of Tohoku University, and the Italian MIUR (FIRB Project RBFR12RPD1). R.E.P.W. holds a Royal Society Wolfson Research Merit Award. J.P.S.W. and W.F.S. thank the North West Nanoscience DTC for their Ph.D. studentships. We thank the EPSRC UK National EPR Facility at the University of Manchester. We also thank Dr. Kate Tucker at the University of Manchester for measuring the 19F NMR spectrum. Note that the data associated with this article are openly available from The University of Manchester eScholar Data Repository at http://dx.doi.org/10.15127/1.275455.

a major target, as this will eventually enable the implementation of quantum algorithms capable of outperforming their classical counterparts. One critical problem with these assemblies is the issue of addressability. It must be possible to control individual components of the assembly without inadvertently affecting the quantum states of the other components. One way to achieve this would be to target the separate parts of the molecule spatiallybut such an effort is far from trivial. A more elegant approach is to tune the magnetic states of the components such that they respond to only a single welldefined input signal, that is, to design a system containing two or more components that can be excited by significantly different photon energies under the same conditions. A way to achieve this goal is to assemble a system in which the states being targeted have significantly different energetic spacings between their resonant levels. The dominant influence on such splitting in a magnetic field is the effective g value of the states in question. Control of this parameter, or so-called g engineering,36,37 offers an exciting route towards the assembly of fully addressable systems. The example here, in which the same metal ionsCr(III) and Ni(II)can be used to give very different g values through the control of chemistry, is rather appealing. The phase-memory time of the ground state (TM = 643 ± 7 ns) is comparable to those of other trimetallic complexes that have been studied in the context of potential qubits (van Slageren and co-workers reported TM = 1.3 μs for a hydroxo-centered copper triangle,38 and Papavassiliou and coworkers reported TM = 2.6 μs for an oxo-centered iron triangle39), and it should be possible to extend this lifetime by several multiples through simple modifications of the ligand set, as we have shown with other Cr−Ni clusters.40 The fact that the terminal ligands of 1 can be readily substituted for pyridyls with minimal changes in the magnetic properties suggests that such supramolecular structures should be readily achievable with {CrNi2} through diimines such as 4,4′-bipyrdine, as we previously demonstrated with {Cr7Ni} rings.41 We are working towards this goal.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01898.

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Article

EPR spectra, field-dependent magnetization (PDF) Crystallographic data for Ni 2 CrFC 4 5 H 8 4 O 1 8 , Ni2CrFC45H69N3O12, and Ni2CrFC48H75N3O12 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ‡

Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States. ∥ School of Physics and Astronomy, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom. Notes

The authors declare no competing financial interest. G

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DOI: 10.1021/acs.inorgchem.5b01898 Inorg. Chem. XXXX, XXX, XXX−XXX