Heteroleptic Fe(II) Complexes with N4S2 Coordination as a Platform

Article pubs.acs.org/IC

Heteroleptic Fe(II) Complexes with N4S2 Coordination as a Platform for Designing Spin-Crossover Materials Sandugash Yergeshbayeva,†,‡ Jeremy J. Hrudka,† Jeff Lengyel,† Rakhmetulla Erkasov,‡ Sebastian A. Stoian,†,§,⊥ Alina Dragulescu-Andrasi,*,† and Michael Shatruk*,† †

Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States Department of Chemistry, L.N. Gumilyov Eurasian National University, 5 Munaitpasov Street, 010008 Astana, Kazakhstan § National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States ‡

S Supporting Information *

ABSTRACT: Heteroleptic complexes [Fe(bpte)(bim)]X2 and [Fe(bpte)(xbim)]X2 (bpte = S,S′-bis(2-pyridylmethyl)-1,2-thioethane, bim = 2,2′-biimidazole, xbim = 1,1′-(α,α′-o-xylyl)-2,2′biimidazole, X = ClO4−, BF4−, OTf−) were prepared by reacting the corresponding Fe(II) salts with a 1:1 mixture of the ligands. All mononuclear complexes exhibit temperature-induced spin crossover (SCO) with the onset above room temperature. The SCO is rather gradual, due to low cooperativity of interactions between the cationic complexes, as revealed by crystal structure analyses. These complexes expand the range of the recently discovered Fe(II) SCO materials with {N4S2} coordination environment.

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INTRODUCTION Spin-crossover (SCO) complexes of first-row transition metals attract significant interest due to their ability to exhibit magnetic state switching coupled to dramatic changes in structural and optical properties.1 Such multichannel switching, which can occur abruptly and hysteretically with changes in temperature, pressure, or photoexcitation, makes SCO complexes promising for applications in memory, display, and sensing devices. Fe(II) complexes with a coordination sphere consisting of six nitrogen-donor atoms, {N6}, dominate the field, accounting for the majority of known SCO complexes.2 We and others have recently demonstrated the occurrence of SCO in Fe(II) complexes with {N4S2} coordination.3−5 More specifically, reactions of the Fe(II) ion with S,S′-bis(2pyridylmethyl)-1,2-thioethane (bpte), in combination with NCE coligands (E = S, Se, BH3), result in mononuclear complexes [Fe(bpte)(NCE)2], which exhibit gradual SCO. Together with several reports on the occurrence of SCO in Fe(II) complexes with {N4O2} coordination,6−19 these results provide the opportunity to design new Fe(II) SCO materials that do not rely solely on the {N6} coordination environment. Prompted by the observation of SCO in the aforementioned complexes with the tetradentate bpte ligand, we sought for alternative coligands that could replace the two monodentate NCE− moieties, which essentially represent a dead end in terms of synthetic variability. We have previously shown that bidentate 2,2′-biimidazole (bim) and its alkylated derivative, 1,1′-(α,α′-o-xylyl)-2,2′-biimidazole (xbim), offer ligand field strengths comparable to that of a pair of NCE− ligands.20 Herein we demonstrate that the combination of bpte with © 2017 American Chemical Society

either bim or xbim (Scheme 1) successfully leads to heteroleptic mononuclear complexes [Fe(bpte)(bim)]X2 and Scheme 1. Ligands Used in This work

[Fe(bpte)(xbim)]X2 (X = ClO4−, BF4−, OTf−), which exhibit SCO above room temperature. We present the syntheses, crystal structures, and magnetic properties of these new SCO complexes with {N4S2} coordination environment.

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MATERIALS AND METHODS

Synthesis. All reactions were performed under an inert N2 atmosphere using standard Schlenk techniques. Reagents were purchased from Aldrich, except for glyoxal (Alfa Aesar) and 1,2ethanedithiol (Fluka). All reagents were used as received. [Fe(OTf)2(MeCN)2]21 and ligands bim,22 xbim,23 and bpte24 were synthesized according to published procedures. Anhydrous commerReceived: June 6, 2017 Published: August 30, 2017 11096

DOI: 10.1021/acs.inorgchem.7b01415 Inorg. Chem. 2017, 56, 11096−11103

Article

Inorganic Chemistry Table 1. Data Collection and Structure Refinement Parameters for Complexes 1a−1c and 2a−2c formula T, K CCDC no. fw space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z cryst color cryst size, mm3 dcalc, g cm−3 μ, mm−1 λ, Å 2θmax,deg total reflns Rint unique reflns params refined restraints used R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) goodness of fitb diff peak/hole, e Å−3 a

FeS2Cl2O9N6C21H26 (1a·MeOH)

FeS2F8ON6C22B2H28 (1b·EtOH)

FeS4F6O6N6C22H22 (1c)

FeS2Cl2O9N6C29H32 (2a·MeOH)

FeS2F8ON6C28B2H30 (2b·H2O)

FeS4F6O7N6C31H32 (2c·MeOH)

240 1549788 697.35 C2/c 14.03(1) 20.61(1) 11.34(1) 90 121.00(5) 90 2812(4) 4 purple 0.14 × 0.04 × 0.04 1.647 0.936 0.71073 28.56 16279 0.043 3407 194 9 0.039, 0.094

230 1549789 686.09 P21/c 8.354(1) 32.688(5) 10.662(2) 90 103.500(2) 90 2831.2(7) 4 purple 0.33 × 0.12 × 0.05 1.610 0.763 0.71073 28.47 17037 0.046 6218 409 0 0.053, 0.127

230 1549791 764.54 P21/c 15.035(2) 23.001(3) 8.750(1) 90 91.978(2) 90 3024.2(6) 4 red 0.22 × 0.09 × 0.02 1.679 0.861 0.71073 28.57 35043 0.035 7282 406 0 0.039, 0.109

230 1549790 799.47 Pbca 16.423(3) 17.564(4) 23.291(5) 90 90 90 6718(2) 8 red 0.27 × 0.11 × 0.05 1.581 0.795 0.71073 28.59 66288 0.059 8278 451 13 0.065, 0.205

230 1549792 760.17 Pbca 16.338(5) 17.388(5) 23.192(6) 90 90 90 6589(3) 8 red 0.30 × 0.20 × 0.03 1.533 0.664 0.71073 28.57 72232 0.080 8128 438 6 0.074, 0.207

230 1549793 898.71 P1̅ 11.698(3) 13.713(3) 13.969(3) 96.731(3) 112.523(3) 110.656(3) 1852.5(7) 2 red 0.26 × 0.15 × 0.07 1.611 0.718 0.71073 28.57 16050 0.026 8429 505 16 0.067, 0.187

0.058, 0.103

0.093, 0.154

0.057, 0.131

0.104, 0.230

0.134, 0.251

0.098, 0.214

1.024 0.67, −0.33

1.039 0.82, −0.49

1.028 0.59, −0.34

1.041 1.44, −0.68

1.042 0.96, −0.83

1.066 0.84, −0.64

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. bGoodness-of-fit = [∑[w(Fo2 − Fc2)2]/(Nobs − Nparams)]1/2, based on all data. [Fe(bpte)(bim)](OTf)2 (1c). Complex 1c was prepared in a manner similar to that described for 1a, starting from [Fe(OTf)2(MeCN)2] (87.2 mg, 0.2 mmol), bim (26.8 mg, 0.200 mmol), and bpte (55.3 mg, 0.200 mmol) in a MeOH/EtOH solvent mixture (5 mL, 1:4 v/v). Plate-shaped red crystals were recovered by filtration, washed successively with EtOH and Et2O, and dried by suction. Yield = 55.3 mg (36%). Anal. Calcd (found) for FeC22H24S4N6O7F6 (1c· H2O): C, 33.77 (33.84); H, 3.09 (3.03); N, 10.74 (10.80). [Fe(bpte)(xbim)](ClO4)2 (2a). Fe(ClO4)2·6H2O (36.3 mg, 0.100 mmol) was dissolved in 2 mL of MeOH and added to a solution of xbim (23.6 mg, 0.100 mmol) in 2 mL of MeOH, followed by addition of a solution of bpte (27.6 mg, 0.100 mmol) in 2 mL of MeOH. The reaction mixture was stirred for 1 h and filtered under an inert atmosphere. The filtrate was layered with Et2O and left undisturbed overnight. Plate-shaped red crystals that formed were recovered by filtration, washed with Et2O, and dried by suction. Yield = 28.7 mg (37%). Anal. Calcd (found) for FeC28H28Cl2N6O8S2 (2a): C, 43.82 (43.59); H, 3.68 (3.73); N, 10.95 (11.08). [Fe(bpte)(xbim)](BF4)2 (2b). Complex 2b was prepared in a manner similar to that described for 2a, starting from Fe(BF4)2·6H2O (67.6 mg, 0.200 mmol), xbim (47.2 mg, 0200 mmol), and bpte (55.3 mg, 0.200 mmol) in 10 mL of MeOH. Plate-shaped red crystals were recovered by filtration, washed with Et2O, and dried by suction. Yield = 44.8 mg (30%). Anal. Calcd (found) for FeC28H28B2F8N6S2 (2b): C, 45.31 (45.55); H, 3.80 (3.98); N, 11.32 (11.51). [Fe(bpte)(xbim)](OTf)2 (2c). Complex 2c was prepared in a manner similar to that described for 2a, starting from [Fe(OTf)2(MeCN)2] (43.6 mg, 0.100 mmol), xbim (23.6 mg, 0.100 mmol), and bpte (27.6 mg, 0.100 mmol) in 3.5 mL of MeOH. Plateshaped red crystals were recovered by filtration, washed with Et2O, and dried by suction. Yield= 47.4 mg (53%). Anal. Calcd (found) for

cial solvents were additionally purified by passing through a doublestage drying/purification system (Glass Contour Inc.). Elemental analyses were carried out by Atlantic Microlab, Inc. (Norcross, GA, USA). Caution! The complexes between metal ions and organic ligands with perchlorate counterion are potentially explosive. The compounds should be prepared in small amounts and handled with great care! [Fe(bpte)(bim)](ClO4)2 (1a). A solution of Fe(ClO4)2·6H2O (72.6 mg, 0.200 mmol) in 1 mL of MeOH was added to a suspension of bim (26.8 mg, 0.200 mmol) in 4 mL of EtOH. The mixture was stirred for 5 min, followed by addition of a solution of bpte (55.3 mg, 0.200 mmol) in 1 mL of MeOH. The reaction mixture was stirred for additional 30 min and filtered under an inert atmosphere. The filtrate was layered with Et2O and left undisturbed overnight. Plate-shaped light-brown crystals were recovered by filtration, washed with Et2O, and dried by suction. Yield = 73.3 mg (54%). Anal.: Calcd (found) for FeC20.5H24Cl2O8.5S2N6 (1a·0.5MeOH): C, 36.14 (35.98); H, 3.48 (3.61); N, 12.33 (12.19). [Fe(bpte)(bim](BF4)2 (1b). A mixture of Fe(BF4)2·6H2O (67.6 mg, 0.200 mmol) and bim (26.8 mg, 0.200 mmol) was dissolved in 1 mL of MeOH with vigorous stirring, followed by addition of 4 mL of EtOH. The mixture was stirred for another 5 min, after which a solution of bpte (55.3 mg, 0.200 mmol) in 2 mL of MeOH was added. The reaction mixture was then stirred for 30 min and filtered under an inert atmosphere. The filtrate was layered with Et2O and left undisturbed overnight. Plate-shaped red crystals that formed were recovered by filtration, washed successively with EtOH and Et2O, and dried by suction. Yield = 89.1 mg (67%). Anal. Calcd (found) for FeC21H26B2F8O1S2N6 (1b·MeOH): C, 37.53 (37.38); H, 3.90 (3.67); N, 12.50 (12.64). 11097


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Figure 1. Molecular structure of the cations [Fe(bpte)(bim)]2+ (a) and [Fe(bpte)(xbim)]2+ (b) in the crystal structures of 1a and 2a, respectively. The thermal ellipsoids are shown at the 50% probability level. FeC30H28F6N6O6S4 (2c): C, 41.58 (41.24); H, 3.26 (3.38); N, 9.70 (9.93). Magnetic Measurements. For all complexes, magnetic susceptibility measurements were carried out on polycrystalline samples. The measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL). Direct current (DC) susceptibility was measured in an applied field of 0.1 T in the 1.8−400 K temperature range, at a cooling/warming rate of 2 K/min. The data were corrected for temperature-independent paramagnetism (TIP), due to the contribution from the excited states of the d6 Fe(II) ion,25 for diamagnetic contribution from sample holder, and for the intrinsic diamagnetism using tabulated constants.26 Mö s sbauer Spectroscopy. 57Fe Mö ssbauer spectra were collected on constant acceleration instruments at room temperature and 4.2 K. Samples were prepared from neat polycrystalline solids, which were either placed directly in custom-made polyethylene cups closed with lids or dispersed in eicosane at a temperature slightly above eicosane’s melting point of 35−37 °C and allowed to solidify inside cups. For 4.2 K measurements, the samples were loaded into a liquid He filled cryostat of the Mössbauer spectrometer. Samples were mounted outside the cryostat for room temperature measurements. Typically, the loading process took ∼2 min, thus flash-cooling the samples from room temperature to 4.2 K. Isomer shifts are quoted relative to Fe metal foil at room temperature. Thermal Analysis. The thermal stability of complexes 1b, 1c, 2b, and 2c was studied on microcrystalline samples using a simultaneous differential thermal analyzer Q600 (TA Instruments). The measurements were performed in the 300−850 K temperature range at the heating rate of 5 K/min. X-ray Crystallography. Single-crystal X-ray diffraction experiments were performed on a Bruker APEX-II CCD X-ray diffractometer equipped with a graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å). In a typical experiment, a selected single crystal was suspended in Paratone-N oil (Hampton Research) and mounted on a cryoloop, which was placed in an N2 cold stream and cooled down to the desired data collection temperature. The data sets were recorded as ω-scans at 0.3° step width and integrated with the Bruker SAINT software package.27 In all the experiments, a multiscan adsorption correction was applied based on multiple equivalent measurements (SADABS).28 The space group determination was performed with XPREP,29 and the crystal structure solution and refinement were carried out using the SHELX program suite.30 The final refinement was performed with anisotropic atomic displacement parameters for all non-hydrogen atoms. All H atoms were placed in calculated positions and refined in the riding model. A

summary of pertinent information relating to data collection and refinements is provided in Table 1.

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RESULTS AND DISCUSSION Synthesis. Heteroleptic complexes 1a−1c and 2a−2c were obtained in moderate yields by simple stoichiometric reactions of precursor Fe(II) salts with a 1:1 mixture of bpte and bim or xbim, respectively. Whereas the syntheses of xbim-containing complexes were performed in MeOH, the poorer solubility of bim required the use of MeOH/EtOH solvent mixture to facilitate the complexation of the bidentate bim ligand to the Fe(II) ion. All resulting complexes were well soluble in the reaction medium, and the crystallization was achieved by slow diffusion of Et2O into solutions containing the complexes. Alternatively, it was also possible to crystallize the complexes by slow cooling of the reaction mixtures. Although syntheses and crystallizations were carried out under an inert N2 atmosphere to prevent oxidation of complexes in solution, all complexes, once crystallized, are sufficiently stable toward air and can be stored as microcrystalline materials in a desiccator without any additional precautions. Thermal gravimetric and differential thermal analyses performed on the microcrystalline samples showed that the complexes remain thermally stable up to 500 K (Figure S1). Crystal Structures. Single-crystal X-ray diffraction was used to determine the crystal structures of the six heteroleptic complexes (Figure 1). Crystal structure refinement showed that all complexes, with the exception of 1c, contain one molecule of interstitial solvent per formula unit of the complex (Table 1). Complexes 2a and 2b are isomorphous, differing only in the nature of the counterion (ClO4− vs BF4−) and the interstitial solvent molecule (MeOH vs H2O), whereas the other complexes have distinctly different combinations of space group symmetry and unit cell size. The elemental analyses indicated that bulk samples of 2a−2c did not contain any interstitial solvent, suggesting that the solvent is lost upon filtration and drying of these complexes. Analysis of metric parameters of the crystal structures determined at 230 K revealed that in all complexes the Fe− N and Fe−S bond lengths fall within narrow ranges of 1.98− 2.02 Å and 2.22−2.25 Å, respectively, which are characteristic of Fe(II) ions in low-spin state. For comparison, Table 2 also 11098


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Table 2. Metal−Ligand Bond Lengths in Complexes 1a−1c and 2a−2c in Comparison to the Distances in the HS and LS States of [Fe(bpte)](NCBH3)2]4 complex temp, K bond lengths, Å Fe−N

Fe−S

1a

1b

1c

2a

2b

2c

[Fe(bpte)](NCBH3)2]

230

230

230

230

230

230

150 (LS)

295 (HS)

2.003(2) 2.003(2) 2.004(2) 2.004(2) 2.236(2) 2.236(2)

1.993(3) 2.001(3) 2.003(3) 2.007(3) 2.236(9) 2.244(1)

1.992(2) 2.006(2) 2.010(2) 2.016(2) 2.2315(7) 2.2321(7)

1.981(3) 1.991(3) 2.001(3) 2.006(3) 2.228(1) 2.238(1)

1.982(4) 1.984(4) 1.994(4) 2.006(4) 2.234(1) 2.234(1)

1.990(3) 1.995(3) 1.998(3) 2.003(3) 2.229(1) 2.242(1)

1.960(2) 1.960(2) 2.008(2) 2.008(2) 2.250(5) 2.250(5)

2.093(2) 2.093(2) 2.161(1) 2.161(1) 2.523(9) 2.523(9)

Figure 2. Crystal packing of 1a: the top view of double columns of [Fe(bpte)(bim)]2+ cations down the [11̅0] direction (a) and the side view of two adjacent double columns (b). The π−π contact between the imidazole rings in one of the columns is highlighted with a red double arrow in panel a and with a dashed red oval in panel b. The H atoms have been omitted for clarity. Color scheme: Fe = magenta, S = yellow, Cl = yellow, O = red, N = blue, C = gray.

lists the Fe−N and Fe−S bond lengths established for the lowspin (LS) and high-spin (HS) states of the previously reported complex [Fe(bpte)(NCBH3)2].4 Note that for this complex the change in the Fe−S bond lengths between the LS and HS structures is nearly twice as large as the change in the Fe−N bond lengths, and therefore the former serves as an excellent indicator of the spin-state change. In the crystal structure of 1a, the [Fe(bpte)(bim)]2+ cations are packed in double columns that run along the [11̅0] crystallographic direction (Figure 2a). Within the columns, the cations appear as dimers weakly coupled by π−π interactions between the imidazole rings of bim (Figure 2b), with the interplanar distance of 3.45 Å. The interactions between the columns are even weaker. The space between and within the columns is filled by the ClO4− anions and MeOH molecules. The structure of 1b reveals even weaker interactions between the [Fe(bpte)(bim)]2+ cations, which can be viewed as arranged in layers parallel to the bc plane (Figure 3), but within each layer the cations are effectively separated by the BF4− anions and EtOH molecules and do not exhibit any substantial interactions with each other. Similar to the structure of 1b, the packing of the [Fe(bpte)(bim)]2+ cations in the crystal structure of 1c is quite loose, as the cationic species are interspersed with the OTf− counterions and lack effective cation−cation interactions. The isomorphous structures of 2a and 2b feature a layerlike arrangement of the [Fe(bpte)(xbim)]2+ cations (Figure 4a). Within the layers, which are parallel to the bc plane, the cations are interacting only via weak van der Waals contacts (Figure 4b). This is in striking contrast to the structure of the previously reported similar complex, [Fe(tpma)(xbim)](ClO4)2 (tpma = tris(2-pyridylmethyl)amine),20 in which cationic complexes form efficient crystal packing with double columns that feature extensive π−π interactions between the

Figure 3. A view of the crystal packing of 1b down the c axis, showing the layers of [Fe(bpte)(bim)]2+ cations. The H atoms have been omitted for clarity. Color scheme: Fe = magenta, S = yellow, F = sky blue, O = red, N = blue, C = gray, B = coral.

pyridyl rings coordinated to the Fe(II) centers. As will be shown below, these structural differences have a substantial impact on the magnetic behavior of the complexes. Complex 2c represents the only structure in the present series with substantial intermolecular interactions between the [Fe(bpte)(xbim)]2+ cationic units. The cations form chains that run along the c axis and exhibit extensive intrachain π−π interactions between the pyridyl rings of bpte and xylene rings of xbim (Figure 5a). The interplanar distances are 3.43 Å for the pyridyl−pyridyl π−π contact and 3.50 Å for the xylene− xylene π−π contact. The interactions between the chains are less pronounced (Figure 5b). The crystal structures discussed above suggest that the combination of bpte with either bim or xbim, in general, does not provide for efficient intermolecular interactions between the cationic complexes in the solid state. This conclusion, 11099


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Figure 4. Crystal packing of 2a: a view of the layers of [Fe(bpte)(xbim)]2+ cations down the c axis (a) and a top view of a single layer (b). The H atoms have been omitted for clarity. Color scheme: Fe = magenta, S = yellow, Cl = yellow, O = red, N = blue, C = gray.

cooperative interactions, such as those seen in the structure of 2c. Magnetic Properties. Variable-temperature magnetic susceptibility measurements were carried out on polycrystalline samples of 1a−1c and 2a−2c in an applied magnetic field of 0.1 T. The temperature was varied at a rate of 2 K/min. At low temperatures, the Fe(II) center in all complexes adopts the LS state, as shown by nearly zero values of the χT product (Figure 6). Upon warming up, an onset of SCO is observed for 1a around 230 K (Figure 6a). The LS → HS conversion for this complex is rather gradual and occurs in two steps, as manifested by an inflection point in the χT vs T curve at ∼300 K. The value of χT at this point is 1.60 emu·K·mol−1. A compositional estimate that corresponds to this χT product can be obtained by assuming (χT)LS = 0 emu·K·mol−1 for the S = 0 LS state and (χT)HS = 3.50 emu·K·mol−1 for the S = 2 HS state. The latter value was reported for the HS state of the Fe(II) ion in other SCO compounds, including the related [Fe(bpte)(NCE)2] complexes (E = S, Se, BH3).4 This estimation indicates a ∼ 1:1 mixture of LS and HS [Fe(bpte)(bim)]2+ cations in 1a at 300 K. The χT value reaches ∼2.45 emu·K·mol−1 at 385 K, suggesting that ∼70% of the Fe(II) ions have converted to the HS state by this temperature. The T1/2 value, defined as the SCO midpoint at which the fractions of the HS and LS species are equal, is estimated to be T1/2 = 330 K for 1a. In the case of 1b, the onset of SCO is observed slightly below room temperature. The maximum χT observed at 390 K is 1.25

Figure 5. Crystal packing of 2c: a fragment of a chain of the [Fe(bpte)(bim)]2+ cations interacting via π−π contacts highlighted with red double arrows (a) and the packing of chains viewed down the c axis (b). The H atoms have been omitted for clarity. Color scheme: Fe = magenta, S = yellow, F = sky blue, O = red, N = blue, C = gray.

however, is only valid for the set of anions used in the present work. The use of other counterions might modify the crystal packing and result in structures with more extensive

Figure 6. Temperature dependence of χT for complexes (a) 1a (red ●), 1b (blue ■), and 1c (Δ), and (b) 2a (red ●), 2b (blue ■), and 2c (Δ). The inset in panel a shows an enlarged high-temperature part of the plot, with the results of measurements in the second heating cycle (●) overlaid on those from the first heating cycle (Δ). 11100


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Inorganic Chemistry Table 3. Mössbauer Parameters of Fe(II) Ions in Samples of 1a and 1b f LS, f HS complex

T, K

FeII spin state

δ, mm s−1

ΔEQ, mm s−1

Γ, mm s−1

Mössbauera

magnetismb

1a

4.2 298

4.2 298

0.480(4) 0.37(2) 1.00(1) 1.00(1) 0.46(1) 0.38(1) 1.25(1)

0.180(3) 0.25(3) 3.30(1) 2.60(1) 0.20(1) 0.21(1) 3.00(2)

0.270(5) 0.34(3) 0.40(1) 0.40(2) 0.30(1) 0.32(1) 0.31(1)

0.98(2) 0.52(1) 0.24(1) 0.24(1) 0.99(1) 0.93(3) 0.07(2)

0.98(1) 0.54(1) 0.46(1)

1b

LS LS HS (A) HS (B) LS LS HS

0.99(1) 0.95(1) 0.05(1)

Values refer to spectral areas and are connected to relative fractions only through the intermediate of the intrinsic “intensity” factors, the recoilless fraction f. bValues estimated from the magnetic susceptibility measurements by dividing the χT product at specified temperatures by (χT)HS = 3.50 emu·K·mol−1, a value reported for the S = 2 state of the Fe(II) ion in other SCO compounds with {N4S2} coordination environment. a

emu·K·mol−1, a value that suggests that the LS → HS conversion for 1b reaches ∼35%. Complex 1c exhibits an onset of SCO only at 325 K. The χT value of 1.4 emu·K·mol−1 attained at 400 K indicates ∼40% LS → HS conversion. Similar to complex 1a, the SCO appears to proceed stepwise, but the first step observed between 325 and 400 K is rather abrupt (Figure 6a). Although the structure of 1c contains no interstitial solvent (as confirmed by the crystal structure refinement, Table 1), the elemental analysis suggested the presence of some interstitial water. Given that the rise of χT is rather abrupt for a material lacking efficient intermolecular interactions between its complex-cation units in the solid state, we questioned whether it might be driven by solvent loss at higher temperatures. Thus, we have conducted successive magnetic measurements where the temperature was cycled between room and high temperatures (Figure 6a, inset). These measurements indicated reproducible reversible SCO behavior. In contrast to complexes 1a−1c, complexes 2a−2c are characterized by increased stability of the LS state, as the onset of SCO is observed only above 350 K (Figure 6b). The increase in the χT value is only slightly visible for 2a and 2b, whereas for 2c the χT value measured at 400 K was 0.8 emu·K·mol−1, indicating ∼20% LS → HS conversion. Thus, it appears that the replacement of bim with xbim increases the ligand field strength and shifts the SCO to higher temperatures. This finding agrees with our previous comparison of the strength of these ligands that was performed by evaluating the ligand field splitting in their Ni(II) complexes by means of optical absorption spectroscopy.20 Although complexes with gradual temperature-induced SCO usually exhibit weak or no photoinduced LS → HS conversion, we examined complex 1a for the presence of such effect. This complex exhibits the lowest T1/2 value in the present series, hence it should offer the smallest difference in the zero-point energies between the LS and HS states. The sample of 1a was irradiated with a 650 nm diode laser at 10 K for 2 h, but no notable changes in the χT product were observed (Figure S2), indicating the lack of photomagnetic effects in this material. 57 Fe Mö ssbauer Spectroscopy. As shown by magnetic data, complex 1a undergoes SCO to a greater extent than the other complexes in the series, with the estimated 50% LS → HS conversion at room temperature, and complex 1b has a SCO onset occurring slightly below room temperature. Their LS → HS conversions are only partially completed even at temperatures as high as 390 K. Given these partial conversions, together with the fact that magnetic measurements reflect the bulk magnetic moment of a sample, the fractions of LS and HS species in the sample cannot be accurately determined solely

from magnetic data. Thus, Mössbauer spectroscopy was employed to establish the oxidation and spin states of the Fe ions in 1a and 1b and to directly provide accurate compositional information regarding the ratio of LS and HS fractions in these samples. Spectral analyses afforded a set of Mössbauer parameters, based on which we have assigned the spin and oxidation states of the Fe sites in samples 1a and 1b (Table 3). The Mössbauer spectra of complexes 1a and 1b were obtained at 4.2 and 298 K (Figure 7), in the absence of applied

Figure 7. Zero-field 57Fe Mössbauer spectra recorded at 4.2 and 298 K for 1a (a) and 1b (b). Red lines in the 4.2 K plots are spectral simulations representing the LS Fe(II) contribution. The solid gray lines overlaid over the 298 K spectra are simulations obtained from the sum of contributions from the LS (red traces) and HS (blue traces) Fe(II) sites.

magnetic field. The spectra recorded at cryogenic temperatures for 1a and 1b were simulated each with one quadrupole doublet having nearly identical isomer shift (δ) and quadrupole splitting (ΔEQ) parameters, δ/ΔEQ (1a) = 0.48/0.18 mm s−1 and δ/ΔEQ (1b) = 0.46/0.20 mm s−1, which are characteristic of the LS Fe(II) ion. These results indicate that, at low temperatures, 1a and 1b contain only LS [Fe(bpte)(bim)]2+ cations, in agreement with the results of magnetic measurements. The spectrum of 1a collected at 298 K exhibits one LS Fe(II) doublet and two additional doublets with parameters characteristic of HS Fe(II). The ratio between the LS fraction and the total HS fraction determined from the Mössbauer data is close to the 1:1 ratio evaluated from the magnetic susceptibility data at this temperature. The ratio between the two HS components observed in this Mössbauer spectrum is also 1:1. The distinct 11101


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difference between the HS components suggests a possibility of slightly different geometric environment associated with specific structural distortions around the HS Fe(II) centers. Such situation can lead to the formation of a superstructure with the increased unit cell or lowered symmetry. 31 Unfortunately, we could not perform crystal structure determination of 1a at room temperature because of the quick loss of crystallinity, which might have occurred due to the loss of interstitial solvent. The room temperature Mössbauer spectrum of 1b contains two components. The major contribution originates from LS Fe(II), which accounts for ∼95% of the total Fe in the sample, whereas the minor contribution is attributed to HS Fe(II). This composition is also in agreement with magnetic data, which showed an onset of SCO slightly below room temperature and the estimated value of f(HS) ∼ 7% at 300 K.

CCDC 1549788−1549793 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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

Sebastian A. Stoian: 0000-0003-3362-7697 Michael Shatruk: 0000-0002-2883-4694

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Present Address ⊥

Department of Chemistry, University of Idaho, 875 Perimeter Dr, Moscow, ID 83844

CONCLUDING REMARKS We have recently shown that the strength of a bidentate diimine ligand correlates well with the separation between the metal-binding N-donor sites in the free (uncoordinated) ligand, with shorter N···N distances leading to stronger ligand fields.32 As a result, the decrease in the N···N distance from 2.991 Å in bim to 2.768 Å in xbim increases the stability of the LS state relative to the HS state in the Fe(II) complexes containing the xbim ligand. The contrast between the magnetic behaviors displayed by complexes 2a−2c and 1a−1c is in good agreement with this empirical rule. Complexes 2a−2c exhibit an onset of SCO above 350 K (Figure 6b), notably higher than the SCO onset temperatures observed for complexes 1a−1c (Figure 6a). Instrumental limitation prevented us from measuring the temperature dependence of χT above 400 K. Nevertheless, it is interesting to note that, in contrast to the bim-containing complexes 1a−1c, for which the tetrafluoroborate (1b) and triflate (1c) complexes exhibit the most stable LS state (with the highest T1/2 values), the xbim-containing triflate complex 2c shows a larger extent of the LS → HS conversion (∼25% at 390 K) as compared to complexes 2a and 2b (Figure 2b). This might be related to the presence of extensive one-dimensional π−π interactions between the SCO cations in the crystal structure of 2c (Figure 5a), which are absent in all other complexes of this series. Such elastic intermolecular interactions are known to enhance vibronic coupling between the metal centers, which is crucial to the occurrence of SCO.33−36 The magnetic properties of the complexes examined in this work also indicate that the bpte ligand provides a slightly stronger ligand field as compared to the similar tetradentate ligand, tpma, which was used by us previously to obtain heteroleptic SCO complexes [Fe(tpma)(bim)](ClO4)2·0.5H2O and [Fe(tpma)(xbim)](ClO4)2, with T1/2 of 190 and 200 K, respectively.20 Thus, while the typical coordination environment of Fe(II) SCO complexes contains six N-donor sites, our results demonstrate that the {N4S2} coordination appears to be a promising platform for the design of new SCO materials.

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AUTHOR INFORMATION

Corresponding Authors

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (Award CHE-1464955 to M.S.). The Mössbauer spectroscopy experiments were performed at the NHMFL, which is supported by the U.S. National Science Foundation (Award DMR-1157490) and by the State of Florida. The Mössbauer instrument was purchased using the NHMFL User Collaboration Grant Program (UCGP-5064) awarded to Dr. Andrzej Ozarowski.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01415. Thermal gravimetric and differential thermal analyses and results of photomagnetic studies (PDF) 11102


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