Spin Crossover in Fe(II) Complexes with N4S2 Coordination

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Spin Crossover in Fe(II) Complexes with N4S2 Coordination Alejandra Arroyave,† Anders Lennartson,‡ Alina Dragulescu-Andrasi,† Kasper S. Pedersen,§ Stergios Piligkos,§ Sebastian A. Stoian,∥ Samuel M. Greer,†,∥ Chongin Pak,† Oleksandr Hietsoi,† Hoa Phan,† Stephen Hill,∥,⊥ Christine J. McKenzie,*,‡ and Michael Shatruk*,† †

Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States Department of Physics, Chemistry and Pharmacy, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark § Department of Chemistry, University of Copenhagen, Universitetsparken 5, Copenhagen, Denmark ∥ National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310, United States ⊥ Department of Physics, Florida State University, 315 Keen Building, Tallahassee, Florida 32306, United States ‡

S Supporting Information *

ABSTRACT: Reactions of Fe(II) precursors with the tetradentate ligand S,S′-bis(2-pyridylmethyl)-1,2-thioethane (bpte) and monodentate NCE− coligands afforded mononuclear complexes [Fe(bpte)(NCE)2] (1, E = S; 2, E = Se; 3, E = BH3) that exhibit temperature-induced spin crossover (SCO). As the ligand field strength increases from NCS− to NCSe− to NCBH3−, the SCO shifts to higher temperatures. Complex 1 exhibits only a partial (15%) conversion from the high-spin (HS) to the low-spin (LS) state, with an onset around 100 K. Complex 3 exhibits a complete SCO with T1/2 = 243 K. While the γ-2 polymorph also shows the complete SCO with T1/2 = 192 K, the α-2 polymorph exhibits a two-step SCO with the first step leading to a 50% HS → LS conversion with T1/2 = 120 K and the second step proceeding incompletely in the 80−50 K range. The amount of residual HS fraction of α-2 that remains below 60 K depends on the cooling rate. Fast flash-cooling allows trapping of as much as 45% of the HS fraction, while slow cooling leads to a 14% residual HS fraction. The slowly cooled sample of α-2 was subjected to irradiation in the magnetometer cavity resulting in a light-induced excited spin state trapping (LIESST) effect. As demonstrated by Mössbauer spectroscopy, an HS fraction of up to 85% could be achieved by irradiation at 4.2 K.

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complexes, in which the FeII ion resides in the {N4S2} coordination provided by one {N2S2}-tetradentate ligand, S,S′bis(2-pyridylmethyl)-1,2-thioethane (bpte), and two N-bound coligands.7 The complex [Fe(bpte)(NCSe)2] crystallized as four distinct polymorphs, of which three showed temperatureinduced SCO.7a Very recently, another SCO complex with the {N4S2} coordination was reported by Brooker and co-workers.8 The {N4S2} coordination environment, while unusual for FeII SCO complexes, offers new possibilities in the design and synthesis of such materials. Inspired by the initial discovery, we have set out to perform a detailed study of SCO in [Fe(bpte)(NCSe)2], as well as to expand this rare class of SCO compounds by substituting either NCS− or NCBH3− for the NCSe− ligands. Since the ligand field strength is known to vary as NCS− < NCSe− < NCBH3−,9 such substitutions should produce a shift in the SCO toward both lower and higher temperatures. Herein we report the synthesis, crystal structures, and magnetic and spectroscopic studies on the family of neutral

INTRODUCTION The ability of certain complexes of 3d transition metal ions to exhibit both high-spin (HS) and low-spin (LS) electronic configurations has been demonstrated for the CrII and MnIII (d4),1 MnII and FeIII (d5),1,2 FeII and CoIII (d6),1,3 and CoII and NiIII (d7) ions.4 The spin crossover (SCO) between the HS and LS states can be actuated by changes in temperature and pressure, or by irradiation. A vast majority of known SCO materials are based on octahedral FeII ions bearing N-donor ligands, which furnish a {N6} coordination sphere. Reports on the observation of SCO in FeII complexes with coordination environments other than {N6} are scarce, most of them dealing with the {N4O2} coordination.5 The occurrence of SCO in FeII complexes with ligands featuring S atom donors, such as thioethers, is rare; to the best of our knowledge, only four such examples have been reported, all involving {N4S2} donor sets. Grillo et al. described the temperature-dependent magnetic properties of [FeII([9]aneN2S)2](ClO4)2 ([9]aneN2S = 1-thia4,7-diazacyclononane) in both solid state and solution, revealing an incomplete LS ↔ HS conversion in the solid state.6 More recently, one of our groups has reported SCO © XXXX American Chemical Society

Received: February 2, 2016

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DOI: 10.1021/acs.inorgchem.6b00246 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

(0.03 M, 1 mL), and on top of this solution, a solution of [Fe(OTf)2(MeCN)2] (109 mg, 0.25 mmol) and bpte (69 mg, 0.25 mmol) in degassed acetonitrile (2 mL) was layered. Crystals started to grow as the reactants diffused together. After 3 days, the mixture was filtered and the crystals washed with acetonitrile (2 mL) and dried by suction. Yield = 74 mg (55%). IR (KBr), ν, cm−1: 3433 (br), 2949 (w), 2057 (s), 1600 (m), 1567 (m), 1481 (m), 1438 (m), 1264 (m), 1156 (m), 1100 (w), 1057 (m), 1016 (m), 868 (w), 770 (m), 755 (m), 713 (m), 640 (m). Anal. Calcd (Found) for Se2FeS2N4C16H16 (2), %: C, 35.44 (35.78); H, 2.97 (2.73); N, 10.33 (10.22). Method B. To the solution of [Fe(H2O)6](BF4)2 (84 mg, 0.25 mmol) in 2 mL of MeCN was added bpte (69 mg, 0.25 mmol). The mixture was stirred for 5 min in a Schlenk tube to afford a clear purple solution. On top of this solution was layered a degassed aqueous solution of KNCSe (72 mg, 0.50 mmol) and ascorbic acid (0.03 M, 1 mL). The yellow solution obtained was left undisturbed, and the crystal growth of 2 became apparent after ∼45 min. After 1 day, the crystals were collected by filtration, rinsed with MeCN (2 mL), and dried by suction. Yield = 72.7 mg (54%). Anal. Calcd (Found) for Se2FeS2N4C16H16 (2), %: C, 35.44 (35.59); H, 2.97 (3.11); N, 10.33 (10.51). [Fe(bpte) (NCBH3)2] (3). Method A. NaNCBH3 (90 mg, 1.4 mmol) was dissolved in MeCN (1 mL). The solution was filtered through a Celite plug and added to a solution of [Fe(OTf)2(MeCN)2] (218 mg, 0.50 mmol) and bpte (138 mg, 0.50 mmol) in MeCN (1 mL). Within a few minutes bluishgray crystals of 3 started to grow. The mixture was allowed to stand for 1 h, after which time the crystals were collected by filtration, washed with MeCN (2 mL), and dried by suction. Yield = 190 mg (92%). IR (KBr), ν, cm−1: 3034 (w), 2963 (w), 2335 (s), 2182 (s), 1603 (s), 1568 (m), 1484 (m), 1439 (s), 1406 (m), 1318 (w), 1264 (m), 1201 (w), 1158 (m), 1120 (s), 1060 (m), 1019 (m), 902 (w), 867 (m), 770 (s), 713 (m), 642 (m), 600 (w). Anal. Calcd (Found) for FeC16H22B2S2N4 (3): C, 46.64 (46.15); H, 5.38 (5.28); N, 13.60 (13.71). Method B. Complex 3 was prepared in a manner similar to that described in the section on Method B for 2, using [Fe(H2O)6](BF4)2 (84 mg, 0.25 mmol), bpte (69 mg, 0.25 mmol), and NaBH3CN (31.42 mg, 0.50 mmol) as starting materials. The solution obtained was left undisturbed for 2 days to allow for the growth of crystals. X-ray quality single crystals of 3 were obtained after 2 days, rinsed with MeCN (2 mL), and dried by suction. Yield = 25.1 mg (24%). Anal. Calcd (Found) for FeS2N4C16B2H22 (3), %: C, 46.64 (46.50); H, 5.38 (5.29); N, 13.60 (13.48). Physical Measurements. Infrared (IR) spectra of the complexes as KBr discs were measured using a Hitachi 270-30 IR spectrometer in the 400−4000 cm−1 range. Mössbauer Spectroscopy. 57Fe Mössbauer spectra were collected on constant acceleration instruments at temperatures between 4.2 and 293 K. Samples were prepared from neat polycrystalline solid materials, 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, then transferred into cups, and allowed to solidify. The samples were loaded into a liquid He-filled cryostat of the Mössbauer spectrometer. Typically, the loading process took ∼2 min, thus flash-cooling the samples from room temperature to 4.2 K. After obtaining the 4.2 K spectra, the samples were gradually warmed up for the variable-temperature measurements. The variable-temperature spectra were collected

mononuclear complexes, [Fe(bpte)(NCE)2], where E = S (1), Se (2), or BH3 (3) (Scheme 1). Complexes 1−3 exhibit SCO, Scheme 1. General Structure of Complexes [Fe(bpte)(NCE)2], Where E = S (1), Se (2), or BH3 (3)

and similarly to other FeII SCO systems with N-bound NCE− coligands,9 the SCO temperature is modulated in accord with the increase in the ligand field strength from NCS− to NCSe− to NCBH3−. Moreover, the α-polymorph of 2 exhibits temperature-induced excited spin state trapping (TIESST) and light-induced excited spin state trapping (LIESST) effects.

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MATERIALS AND METHODS Synthesis. All reactions were performed under an inert N2 atmosphere using standard Schlenk techniques. Most of the reagents were purchased from Aldrich, except for NaNCS (Ferak) and (Et4N)Cl (Fluka). All reagents were used as received. [Fe(OTf)2(MeCN)2]10 and S,S′-bis(2-pyridylmethyl)-1,2-thioethane (bpte)11 were prepared by published procedures. Anhydrous commercial solvents were additionally purified by passing through a double-stage drying/purification system (Glass Contour Inc.). Elemental analyses were performed by Atlantic Microlab, Inc. (Norcross, GA). [Fe(bpte) (NCS)2] (1). Method A. Solid NaNCS (40 mg, 0.5 mmol) was layered with 1 mL of water followed by a solution of [Fe(OTf)2(MeCN)2] (109 mg, 0.25 mmol) and bpte (69 mg, 0.25 mmol) in 2 mL of MeCN. Yellow crystals grew slowly in the interface. After a few days, the mixture was filtered, and the crystals were washed with MeCN (2 mL) and dried by suction. Yield = 91 mg (81%). IR (KBr), ν, cm−1: 3435 (br), 2916 (w), 2060 (s), 1599 (m), 1567 (m), 1481 (m), 1438 (m), 1265 (m), 1157 (m), 1101 (w), 1056 (m), 1015 (m), 967 (w), 867 (w), 757 (m), 713 (m), 636 (m), 597 (w), 480 (w). Anal. Calcd (Found) for FeS4N4C16H16 (1), %: C, 42.85 (43.15); H, 3.60 (3.50); N, 12.50 (12.54). Method B. KSCN (48.6 mg, 0.50 mmol) and [Fe(H2O)6](BF4)2 (84 mg, 0.25 mmol) were added to different sides of a medium H-tube. A degassed aqueous solution of ascorbic acid (0.03 M, 1 mL) was cannulated into the side containing KNCS. To the side containing Fe(BF4)2·6H2O was added bpte (69 mg, 0.25 mmol) resulting in purple coloration. About 7.5 mL of MeCN was added to each side of the H-tube, until the solutions came into contact in the horizontal connecting bar. X-ray quality single crystals that appeared after several days were collected, rinsed with MeCN (2 mL), and dried by suction. Yield = 42.7 mg (38%). Anal. Calcd (Found) for FeS4N4C16H16 (1), %: C, 42.85 (42.77); H, 3.60 (3.65); N, 12.49 (12.55). [Fe(bpte)(NCSe)2] (2). Method A. The synthesis of this compound was reported by us earlier7a and is repeated here for the sake of completeness. Solid KNCSe (72 mg, 0.50 mmol) was layered with a degassed aqueous solution of ascorbic acid B


Article

Inorganic Chemistry Table 1. Data Collection and Structure Refinement Parameters for Complexes 1 and 3 formula T, K CCDC number fw space group a, Å b, Å c, Å β, deg V, Å3 Z crystal color crystal 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) GOF (all data)b diff peak/hole, e Å−3 a

FeC16H16N4S4 (1)

FeC16H22B2N4S2 (3)

90(2) 1450642 448.42 Pbcn 13.803(1) 9.932(8) 13.968(1)

230(2) 1450643 448.42 Pbcn 13.857(6) 9.851(4) 14.252(6)

1914.9(3) 4 yellow 0.22 × 0.13 × 0.09 1.555 1.230 0.710 73 28.28 19824 0.026 2318 114 0 0.035, 0.095 0.038, 0.096 1.119 0.85, −0.26

1945(1) 4 yellow 0.22 × 0.13 × 0.09 1.531 1.211 0.710 73 28.33 15503 0.027 2319 118 0 0.034, 0.079 0.045, 0.085 1.059 0.48, −0.49

150(2) 1450644 411.96 C2/c 20.324(1) 7.9693(5) 14.0085(9) 123.205(1) 1898.5(2) 4 dark gray 0.18 × 0.06 × 0.02 1.441 1.020 0.710 73 26.37 7294 0.026 1934 115 0 0.026, 0.064 0.033, 0.067 1.064 0.30, −0.23

295(2) 1450646 411.96 C2/c 20.82(1) 8.093(4) 14.260(7) 122.860(5) 2019(2) 4 dark gray 0.42 × 0.18 × 0.10 1.355 0.960 0.710 73 28.26 8234 0.017 2355 115 0 0.025, 0.069 0.028, 0.071 1.085 0.25, −0.18

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. bGOF = [∑[w(Fo2 − Fc2)2]/(Nobs − Nparams)]1/2, based on all data.

at different temperatures in warming mode (heating rate of ∼10 K min−1). For slow cooling experiments, samples were cooled down at rates of ∼0.25−2 K min−1. Spectral simulations were generated using WMOSS (WEB Research, Edina, MN). Isomer shifts are quoted relative to Fe metal foil at room temperature. To prepare the sample of α-2 used for the investigation of the LIESST effect, crystalline materials were dispersed in light mineral oil, placed into a transparent Mössbauer cup, and then subjected to flash freezing inside an optical liquid He-filled cryostat. The frozen sample (4.2 K) was subsequently irradiated for 10 min with a white-light source (halogen bulb, 100 W cm−2) kept at room temperature. Before reaching the sample, the incident light was passed through a water-bath filter to minimize local heating by eliminating radiation in the IR region of the spectrum. Magnetic and Photomagnetic Measurements. Magnetic susceptibility measurements were carried out on polycrystalline samples of 1−3. The measurements were performed with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-XL). Direct current susceptibility was measured in an applied field of 0.1 T in the 1.8−400 K temperature range. The data were corrected for the diamagnetic contribution from the sample holder and for the intrinsic diamagnetism using tabulated constants.12 Photomagnetic measurements were performed with the SQUID, using a fiberoptic sample holder (Quantum Design) and a continuous-wave 650 nm diode laser operating at the power density of 15 mW cm−2. The sample was cooled from 300 to 70 at 10 K min−1 and allowed to equilibrate at 70 K for 8 h. Then, the sample was cooled down to 10 K at 0.2 K min−1 and irradiated at that temperature for 3 h. After the laser was turned off, the magnetic susceptibility was measured in the warming mode at a heating rate of 0.3 K min−1.

Powder X-ray Diffraction (PXRD). Room-temperature PXRD patterns were recorded on a PANalytical X’Pert Pro diffractometer with an X’Celerator detector using Cu Kα radiation (λ = 1.541 87 Å). The samples were ground and mounted on a zero-background holder. The data were processed with the HighScore software package.13 X-ray Crystallography. Single-crystal X-ray diffraction experiments were performed on a Bruker APEX-II CCD Xray diffractometer equipped with a graphite-monochromated Mo Kα radiation source (λ = 0.710 73 Å). In a typical experiment, a single crystal was suspended in Paratone-N oil (Hampton Research) and mounted on a cryoloop, which was placed in a 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.14 In all the experiments, a multiscan adsorption correction was applied by fitting a function to the empirical transmission surface sampled by multiple equivalent measurements (SADABS).15 The space group determination was performed with XPREP,16 and the crystal structure solution and refinement were carried out using the SHELX program suite.17 The final refinement was performed with anisotropic atomic displacement parameters for all non-hydrogen atoms, with the exception of some strongly disordered counteranions that were refined isotropically. 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. The preparations of the tetradentate ligand bpte and its FeII complexes 1−3 were carried out under inert N2 atmosphere. Compounds 1−3 were obtained as air-stable, C


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Inorganic Chemistry

observed for the SCO polymorphs of 2. A detailed discussion of the structural aspects of all polymorphs of 2 was provided in our earlier publication.7a The packing of the molecules in the crystal structures of 1 and 3 are similar. Ribbons of stacked molecules are found parallel with the b axis (Figures S1 and S2). The two Fe-NCS arms of 1 and the two Fe-NCBH3 arms of 3 “embrace” the SCH2CH2S backbones of the neighboring complex along the ribbon. Within the ribbon, the closest distance between the Fe atoms in 3 are nearly 2 Å shorter compared to the analogous distance in 1 (∼8.0 Å for 3 and ∼9.9 Å for 1). This seems consistent with more possibilities for intermolecular H-bonding in 3. Two ethylene backbone H atoms point toward the center of the C−B bonds of the next molecule. In addition, the terminal BH3 groups interact with pyridine C−H groups on adjacent molecules from two parallel ribbons. As the SCO translates into changes in the local distortion of the FeII coordination environment, we also examined the distortion parameter ∑ obtained by summing deviations of 12 cis angles at the octahedral FeII center from the ideal value of 90°. The distortion is usually observed to decrease upon change from the HS to the LS structure.18 The ∑ values calculated for 1 at 230 and 90 K are nearly the same (67.4(3)° and 68.2(2)°, respectively), and both are similar to the ∑ value calculated for 3 at 295 K (62.8(2)°), but the ∑ value for 3 at 150 K is substantially smaller (41.7(2)°). Thus, the changes in the distortion parameter also agree with the persistence of the HS state in complex 1 down to 90 K and with the occurrence of temperature-driven SCO in complex 3 between 295 and 150 K. The bulk materials obtained for each of the complexes 1, α-2, γ-2, and 3 that were subsequently used for magnetic measurements were also characterized by PXRD. The experimental PXRD patterns were found to be in good agreement with the ones calculated from the crystal structures (Figure 2), thus confirming the identity between single crystals and bulk polycrystalline materials. Magnetic Properties. Variable-temperature magnetic susceptibility measurements were carried out on polycrystalline samples of 1−3. Magnetic data were collected for each sample in both cooling and warming modes, in order to detect possible thermal hysteresis effects. We have reported previously the magnetic behavior of three different polymorphs of 2. Here we compare the magnetic behavior of γ-2 to that of 1 and 3. (Note that a further discussion and comparison of the magnetic behavior of α-2 and γ-2 will be provided in the Mössbauer Spectroscopy and TIESST and LIESST Effects sections below.) For all complexes, the room-temperature χT value is close to 3.5 emu K mol−1 (Figure S3), consistent with the presence of one HS FeII ion (S = 2) with a slight orbital contribution to the magnetic moment.19 The temperature dependence of the HSFeII fraction (f HS) reveals that all three complexes exhibit gradual temperature-induced SCO although the extent of the spin state conversion differs (Figure 3). Complex 3 undergoes essentially complete SCO between 200 and 300 K, with T1/2 = 243 K. [T1/2 is defined as the SCO midpoint, at which the fractions of the HS and LS species are equal, f HS = f LS = 0.5.] For γ-2, the SCO is also complete but shifted to lower temperature, T1/2 = 192 K. Finally, the χT value of 1 remains nearly constant down to ∼100 K, below which the χT exhibits a gradual drop to 3.0 emu K mol−1 at 70 K, remains constant to ∼25 K, and then decreases abruptly below this temperature due to zero-field splitting effects of the HS FeII ion. The magnetic behavior of 1 thus indicates an incomplete thermally induced

solvent-free neutral complexes, starting from either presynthesized [Fe(OTf)2(MeCN)2]10 or from commercially available [Fe(H2O)6](BF4)2. The substantially higher yields achieved with the former reagent are probably due to its higher purity. Crystals of 1 are yellow at room temperature and show low solubility in most organic solvents and water. They turn green on cooling. This color change is comparable to that seen for the three polymorphs of 2, α, γ, and δ, which exhibit SCO (the βpolymorph is a HS complex).7a The bluish-gray crystals of 3 darken on cooling and become paler on heating. The coordination of the NCE− coligands was verified by FT-IR, which showed νC≡N stretches at 2060 cm−1 for 1, 2057 cm−1 for 2, and 2182 cm−1 for 3, values that fall in the range observed for other FeII complexes with N-bound NCE− ligands.9c In contrast to complex 2, which exhibits four crystalline polymorphs, only one crystalline phase was observed for either 1 or 3. Crystal Structures. Single-crystal X-ray diffraction was used to determine the crystal structures of 1 and 3. Similar to complex 2 reported earlier,7a the FeII ion is coordinated by one tetradentate bpte and two monodentate N-bound NCE− ligands in both 1 and 3. Two NCE− ligands and the thioether S-donor atoms of the bpte ligand form the equatorial plane of the octahedral coordination, while the pyridyl N atoms of bpte occupy the axial sites (Figure 1). In each structure, the

Figure 1. Crystal structures of 1 (a) and 3 (b), with the thermal ellipsoids at 50% probability level. In both structures, the atoms labeled with prime have been generated from the asymmetric unit atoms by the (x, y, 1/2 − z) operation.

[Fe(bpte)(NCE)2] molecule lies on a 2-fold rotational axis that passes through the Fe atom and bisects the ECN−Fe−NCE angle. The bond lengths and angles around the Fe center in 1 and 3 are listed in Table 2, which for comparison also includes metric parameters for the α-2 and γ-2 polymorphs. In the crystal structure of 1 determined at 230 K, the Fe− NNCS, Fe−Nbpte, and Fe−S bond lengths are 2.075(2), 2.206(2), and 2.556(1) Å, respectively. Very close structural parameters are also observed in the structure of 1 determined at 90 K, suggesting that the FeII ion remains in the HS state down to this temperature. In contrast, the crystal structure determination for 3 revealed substantial changes in the Fe− NNCS and Fe−Nbpte bond lengths, respectively, from 2.093(2) and 2.161(1) Å at 295 K to 1.960(2) and 2.008(2) Å at 150 K. The Fe−S bond length also decreased from 2.5233(9) Å at 295 K to 2.2503(5) Å at 150 K. These changes indicate the crossover from the HS to LS electronic configuration at the FeII center. Similar changes in the metal−ligand bond lengths were D


Article

Inorganic Chemistry Table 2. Selected Bond Lengths and Angles in Crystal Structures of 1, α-2, γ-2, and 3

The data were taken from ref 7a. bThe data are given only for the site that exhibits a complete HS → LS conversion in the first, higher-temperature SCO step (see the Magnetic Properties section).

a

Figure 2. Experimental (black traces) and calculated (red traces) X-ray powder diffraction patterns of 1, α-2, γ-2, and 3.

centers become ineffective and the HS to LS state conversion “freezes” at ∼60−70 K. Further support for this explanation will be provided below. While the magnetic data are coherent with the projected trends associated with the increase in ligand field strength in the order NCS− < NCSe− < NCBH3−, it is clear from the previous study that crystal structure can override this expectation.7a While the SCO curves for the three of the polymorphs of 2 lie within the extremes represented by 1 and 3, one of the polymorphs, namely, the β-2 phase, is HS over the entire measured temperature range. Mössbauer Spectroscopy. We have used variable-temperature Mössbauer spectroscopy at 4.2−293 K to confirm the oxidation and spin states of Fe sites in 1−3. Whereas the

SCO between 100 and 70 K, with only a small fraction of the FeII ions changing their electronic configuration from HS to LS in response to temperature changes. The temperature-dependent magnetic properties of 1 and 3 are also in agreement with the crystallographic data. The magnetic behavior of 1−3 correlates well with the known change in the strength of the NCE− ligands. As one would expect, the SCO shifts to lower temperatures upon going from the stronger NCBH3− ligand in 3 to the weaker NCSe− ligand in 2 and to the even weaker NCS− ligand in 1. Moreover, the SCO occurs only to a limited extent in complex 1. The incomplete SCO in the latter case can be explained by a kinetic effect: at low temperatures, due to the depopulation of higher energy phonons, vibronic interactions between the SCO E


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Inorganic Chemistry

apparent singlet assigned to the LS-FeII species could not be satisfactorily fit by a single Lorentzian or Gaussian function, indicating the presence of an unresolved doublet.) The major contribution is from HS FeII, which accounts for 85% of the total Fe in the sample, whereas the remaining 15% is LS FeII (Figure 4a). This composition is in agreement with f HS = 0.86 at 4.2 K, as determined from magnetic susceptibility measurements. At 293 K, only the contribution from HS-FeII ions is present in the spectrum, which is also in accord with magnetic data. At cryogenic temperatures, a polycrystalline sample of γ-2 contains only one Fe component as seen from the single resonance in the 4.2-K Mössbauer spectrum (Figure 4b). Parameters δ/ΔEQ = 0.46/0.16 mm s−1 are virtually identical to those of the LS-FeII species observed for 1 at 4.2 K. At 293 K, the spectrum contains a doublet with parameters characteristic of HS-FeII (Table 3). These observations are in agreement with the complete, temperature-induced HS → LS SCO inferred for γ-2 from the f HS versus T plot (Figure 3). The Mössbauer spectra of 3 collected at 4.2 K (Figure S4) and 160 K (Figure 4c) contain only one component, with parameters identical to those observed for the LS-FeII site in 1 at 4.2 K (Table 3). The spectra of 3 collected at higher temperatures exhibit both LS-FeII and HS-FeII doublets, with the HS-FeII component visibly growing as the temperature increases. The fraction of HS-FeII ions determined from the Mössbauer data at 240 and 260 K equals 0.44 and 0.74, respectively, which agrees well with the f HS values of 0.44 and 0.75, respectively, calculated from the magnetic susceptibility data at these temperatures. Overall, the Mössbauer spectroscopy results confirm that a complete HS → LS conversion takes place in complexes γ-2

Figure 3. Temperature dependence of HS-FeII fraction ( f HS) for 1 (□), α-2 (o), γ-2 (◊), and 3 (Δ) measured in the cooling mode at the rate of 1 K min−1.

magnetic measurements reflect the bulk averaged magnetic moment of a sample, Mössbauer spectroscopy provides a direct probe to estimate the relative ratio of distinct Fe species at various temperatures along the LS ↔ HS conversion. Spectral analyses afforded a set of Mössbauer parameters, based on which we assigned the type of Fe ions present in the samples (Table 3). The Mössbauer spectra of 1 were obtained at 4.2 and 293 K, in zero magnetic field. The low-temperature spectrum was simulated with two quadrupole doublets with parameters δ/ ΔEQ = 0.48/0.15 mm s−1 and δ/ΔEQ = 1.10/2.70 mm s−1 that are characteristic of LS-FeII and HS-FeII ions, respectively. (The

Table 3. Mössbauer Parameters of FeII Ions in Samples of 1, α-2, γ-2, and 3 complex

T [K]

1

4.2

α-2

293 4.2 (fast) 4.2 (slow) 100 150 170

α-2 after irradiation

4.2 (fast)

γ-2

4.2 293 4.2 160 240

3

260

FeII spin state

δ [mm s−1]

ΔEQ [mm s−1]

Γ [mm s−1]a

f LS, f HS Möss.b

f LS, f HS magnc

LS HS HS LS HS LS HS LS HS LS HS LS HS LS HS LS HS LS LS LS HS LS HS

0.48(2) 1.10(1) 1.03(2) 0.48(1) 1.08(4) 0.49(1) 1.10(1) 0.48(3) 1.08(4) 0.48(2) 1.06(7) 0.53(3) 1.08(3) 0.48(1) 1.07(1) 0.46(1) 0.95(1) 0.49(1) 0.50(3) 0.50(1) 1.06(1) 0.50(1) 1.05(6)

0.15(6) 2.79(2) 2.37(1) 0.15(2) 2.68(1) 0.21(1) 2.60(1) 0.14(2) 2.65(4) 0.17(1) 2.40(1) 0.18(1) 2.32(2) 0.15(2) 2.70(1) 0.16(1) 1.85(1) 0.15(1) 0.17(1) 0.17(1) 2.36(2) 0.20(2) 2.18(2)

0.30(1) −0.26(1) 0.33(2) 0.27(1) 0.28(1) 0.28(1) 0.30(1) 0.27(1) 0.26(1) 0.24(1) 0.31(1) 0.33(1) 0.27(1) 0.30(1) 0.27(1) 0.27(1) 0.34(1) 0.38(1) 0.28(1) 0.33(1) 0.32(2) 0.45(5) 0.32(2)

0.15(2) 0.85(2) 1.00 0.55(1) 0.45(1) 0.86(1) 0.14(1) 0.50(2) 0.49(2) 0.25(3) 0.75(3) 0.13(3) 0.87(3) 0.15(5) 0.85(5) 1.00 1.00 1.00 1.00 0.56(2) 0.44(2) 0.26(2) 0.74(2)

0.14(1) 0.86(1) 1.00 n/d n/d 0.82(1) 0.18(1) 0.53(1) 0.47(1) 0.35(1) 0.65(1) 0.10(1) 0.90(1) n/d n/d 0.98(1) 1.00 1.00 1.00 0.54(1) 0.44(1) 0.25(1) 0.75(1)

a

Positive values indicate Lorentzian line shapes; negative values indicate Gaussian line shapes. bValues refer to spectral areas and are connected to relative fractions only through the intermediate of the intrinsic “intensity” factors, the recoilless fraction f. cValues established from the magnetic susceptibility measurements by dividing the χT product at specific temperature by the maximum χT product observed when f HS = 1. F


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Inorganic Chemistry

Figure 4. 57Fe Mössbauer spectra of 1 (a), γ-2 (b), and 3 (c) recorded at various temperatures in zero applied magnetic field. The solid gray lines overlaid on the experimental spectra are simulations obtained from the sum of the two components: HS-FeII sites shown in red and LS-FeII sites shown in blue.

Figure 5. Temperature dependence of χT for α-2 measured in cooling (blue curves) and warming (red curves) modes. All data were acquired at a temperature sweep rate of |0.4| K min−1. (a) The sample was cooled down from 300 to 2 K and then warmed up to 300 K. (b) The sample was flashed-cooled at 40 K and then cooled down to 2 K, warmed up to 300 K, and recooled to 2 K.

The kinetic trapping observed in the lower-temperature SCO step was probed in two ways. First, the measurement of magnetic susceptibility in the warming mode allowed the observation of a thermal hysteresis (Figure 5a). As the temperature approached 50 at 0.4 K min−1, the χT value began to decrease slightly from 0.6 emu K mol−1, thus deviating from the curve recorded in the cooling mode and reaching the minimum of 0.25 emu K mol−1 at 70 K. Then, it increased steeply and coincided with the cooling curve observed above 80 K. Such an effect was described in detail by Hauser et al. and classified as an “apparent hysteresis”, i.e., a kinetic hysteresis observed for a thermally nonequilibrated system.20 This nonequilibrium was demonstrated by cooling the sample of α-2 to 70 at 10 K min−1 and maintaining it at that temperature while monitoring the χT product. The value of χT gradually decreased from 1.4 to 0.5 emu K mol−1 as more FeII centers converted from the HS to the LS state (Figure S5). Second, the kinetic trapping was probed by rapidly lowering the sample into the SQUID cavity maintained at 40 K. After flash-cooling, the magnetic susceptibility was measured down to 2 K at 0.4 K min−1. Then the measurement was performed in the warming mode at 0.4 K min−1, and finally in the cooling

and 3 as the temperature is lowered, while only a partial (15%) HS → LS conversion is observed for complex 1. TIESST and LIESST Effects in Complex α-2. As mentioned in the original report on various polymorphs of complex 2,7a only γ- and δ-phases exhibit complete SCO. The β-phase exists only in the HS form, while the α-phase shows a two-step SCO. Further studies of the α-polymorph revealed interesting SCO kinetics and quenching effects that we describe herein. At room temperature, α-2 exhibits the pure HS state, with the χT value of 3.6 emu K mol−1. As the temperature is lowered, the first SCO step takes place, with an onset at ∼200 K and T1/2 = 160 K (Figure 5a). After a plateau region at ∼1.8 emu K mol−1 between 120 and 80 K, which corresponds to the 1:1 ratio of the HS and LS species ( f HS = 0.5), the second SCO step takes place down to ∼50 K, but this step is incomplete, leading to a low-temperature plateau of 0.6 emu K mol−1 below 50 K. Such behavior can be explained by kinetic trapping of the HS state, with f HS = 0.22(1) achieved upon cooling at 0.4 K min−1. (Note that incomplete SCO at low temperature was observed for complex 1 as well.) Further decrease in the χT product below 10 K is due to zero-field-splitting effects. G


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Inorganic Chemistry mode down to 2 K at the same sweep rate (Figure 5b). A plateau for the χT product below 50 K was observed at 1.2 emu K mol−1 upon flash-cooling, corresponding to f HS = 0.33, even higher than observed in the cooling experiments at 10 K min−1. Above 50 K, the χT value began to decrease, in agreement with the relaxation of the trapped metastable HS state to the LS ground state. These data conclusively show that α-2 exhibits a partial temperature-induced excited spin state trapping (TIESST) effect. Above 80 K, the χT curves obtained in the cooling and warming modes coincided, confirming the reversibility of the high-temperature SCO step. It is important to note that the TIESST effect itself does not guarantee the presence of apparent hysteresis. The latter phenomenon arises in systems that exhibit incomplete HS to LS crossover as the temperature is lowered and the phonon energy becomes insufficient to maintain effective vibronic coupling required for the spin state conversion. Raising the temperature allows the kinetically trapped HS species to “unfreeze” and continue converting to the LS species, as the system moves closer to the thermodynamic equilibrium. Under such a scenario, the more effective kinetic trapping, manifesting as a higher residual f HS value, results in a smaller hysteresis observed at the same warming rate. This effect can be clearly seen from Figure 5, in which the warming curves for both slowly cooled and flash-cooled samples were measured at 0.4 K min−1. The higher fraction of the HS state trapped by flashcooling results in the TIESST relaxation curve returning to the thermodynamic equilibrium curve also at the higher f HS value, and the hysteresis loop is smaller. As an example, a similar effect of TIESST-dependent apparent hysteresis was observed by Weber and co-workers for a 1-D coordination polymorph that upon slow cooling (0.5 K min−1) showed an incomplete SCO below 100 K.5k Interestingly, when α-2 was dispersed in hexadecane at room temperature and subsequently flash-cooled to 40 K, a plateau for the χT product was observed at about 1.1 emu K mol−1 while the sample was cooled from the initial temperature of 40 to 2 K at 0.4 K min−1 (Figure S6). Upon heating the sample at the same rate (0.4 K min−1), the same plateau was reproduced up to ∼50 K, whereupon continuing heating the χT product began to decrease to reach a minimum at ∼70 K and then increased again upon heating to 90 K, in an analogous way as the one described above (Figure 5b). Further cooling and heating cycles at the same rate (±0.4 K min−1) evidenced the gradual relaxation of this flash-cooled trapped metastable HS state (Figure S6). This behavior can be explained by better thermal contact of the polycrystalline α-2 with the thermal bath, when dispersed in hexadecane. The TIESST effect in α-2 was also observed by Mössbauer spectroscopy measurements, the results of which are in excellent agreement with the magnetic susceptibility data (Table 3). The Mö ssbauer spectra of α-2 contain two components (Figure 6) which, by comparison to the spectra of 1 and 3, can be assigned to the LS-FeII (small ΔEQ) and HSFeII (large ΔEQ) ions. When the sample was flashed-cooled to 4.2 K, the HS-FeII fraction measured was f HS = 0.45. Thus, the faster lower-temperature quenching in the Mössbauer cryostat allowed trapping of a larger fraction of the HS-FeII ions than the flash-cooling to 40 K performed in the SQUID magnetometer (f HS = 0.33). The flash-cooled sample was warmed up, and the Mössbauer spectra were acquired at 100, 150, and 170 K (Figure 6a), resulting in f HS values of 0.49, 0.75, and 0.87, respectively. These values compare well with the ones derived

Figure 6. Variable-temperature, zero-field 57Fe Mössbauer spectra recorded on a sample of α-2 that was flashed-cooled at 4.2 K and (a) gradually warmed to the measurement temperatures; (b) irradiated with white light at 4.2 K. The solid gray lines are spectral simulations obtained from the sum of the HS-FeII (red) and LS-FeII (blue) components.

from the magnetic susceptibility measurements (0.47, 0.65, and 0.90, respectively). We also note that when the sample of α-2 was cooled down more gradually inside the He-filled cryostat, the Mössbauer spectrum obtained at 4.2 K revealed f HS = 0.15, in reasonable agreement with the magnetic data ( f HS = 0.22). Typically, the observation of TIESST behavior in SCO systems suggests that it also should be possible to achieve lightinduced excited spin state trapping (LIESST) by irradiating the sample at lower temperatures. Consequently, the sample of α-2 was cooled down to 4.2 K and irradiated with white light inside the Mössbauer spectrometer. While the spectrum obtained prior to irradiation indicated nearly equal amounts of the HS and LS species ( f HS = 0.45), the spectrum of the irradiated sample revealed a strong suppression of the absorption line associated with the LS-FeII ions, leading to f HS = 0.85 (Figure 6b). The Mössbauer parameters of the photoinduced HS state are similar to those of the residual HS state present in the sample that was trapped by flash-cooling prior to light irradiation. The LIESST effect was also observed by magnetic measurements. The sample of α-2 was cooled down to 70 K and maintained at that temperature for 8 h to achieve thermal equilibrium. Then, the sample was cooled down to 10 K at 0.2 K min−1 and irradiated with a 650 nm continuous-wave diode laser. Irradiation was stopped after 3 h, and the measurement was performed in the warming mode at 0.3 K min−1. The HS fraction increased from f HS = 0.10 before to f HS = 0.68 after irradiation (Figure 7). Warming up the sample led to the thermally activated relaxation of the photoinduced HS state, with a relaxation temperature TL = 40 K determined by the minimum derivative of the χT versus T plot. We also attempted to achieve a photoinduced LS → HS conversion for complex 3, but did not observe any measurable concentration of the HS species under irradiation at 10 K (Figure S7). H


Inorganic Chemistry

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Article

AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation (awards CHE-1464955 to M.S. and DMR1309463 to S.H. and Graduate Research Fellowship DGE1449440 to S.M.G.) and in part by DoD (DOTC 13-01INIT518), Chemring, Inc., and The Danish Council of Independent Research |Natural Sciences (DFF-4181-00329). Work performed at the NHMFL 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 (UCGP5064) awarded to Dr. Andrzej Ozarowski. S.A.S. acknowledges support from the NHMFL Jack E. Crow Postdoctoral Fellowship.

Figure 7. Temperature dependence of HS-FeII fraction (f HS) for α-2 measured before irradiation (○) with the following cooling profile: cooling from 300 to 70 at 10 K min−1, relaxation for 8 h at 70 K, cooling down to 10 at 0.2 K min−1; after irradiation with 650 nm light at 10 K (red △) while warming at 0.3 K min−1.

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CONCLUSIONS The present study reports the first comprehensive investigation of spin crossover behavior observed in Fe(II) complexes with the N4S2 coordination environment, which is very rarely encountered among SCO compounds. The occurrence of SCO has been conclusively shown by a combination of crystallographic, magnetic, and Mössbauer measurements. The LS ↔ HS interconversion temperature was gradually increased by increasing the strength of the ancillary ligands from NCS− to NCSe− to NCBH3−, with complete SCO observed for the complexes containing the latter two ligands. The α-polymorph of [Fe(bpte)(NCSe)2] was the only complex in this series that showed temperature- and light-induced trapping of the metastable HS state at low temperature. The TIESST and LIESST effects were demonstrated both in the SQUID magnetometer and in the Mössbauer spectrometer. Typical Fe(II) SCO complexes exhibit the {N6} coordination, which in principle should offer a stronger ligand field than the {N4S2} ligand set used in this work. Nevertheless, our study conclusively confirms the occurrence of SCO in the complexes that contain the Fe(II) ion in the octahedral {N4S2} coordination. Further studies to improve understanding of this unusual SCO behavior are currently underway in our laboratories, and their results will be reported in due course.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00246. Crystal packing diagrams for complexes 1 and 3, additional magnetic plots, and Mö ssbauer spectra (PDF) Crystallographic data for 1 at 90 K (CIF) Crystallographic data for 1 at 230 K (CIF) Crystallographic data for 3 at 150 K (CIF) Crystallographic data for 3 at 295 K (CIF) I


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Spin Crossover in Fe(II) Complexes with N4S2 Coordination

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