Investigation of the High-Temperature Spin-Transition of a

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Investigation of the High-Temperature Spin-Transition of a Mononuclear Iron(II) Complex Using X‑ray Photoelectron Spectroscopy Alexander R. Craze,† Kyle J. Howard-Smith,† Mohan M. Bhadbhade,‡ Outi Mustonen,‡ Cameron J. Kepert,§ Christopher E. Marjo,*,‡ and Feng Li*,† †

School of Science and Health, Western Sydney University, Locked Bag 1797, Penrith, New South Wales 2751, Australia Mark Wainwright Analytical Centre, University of New South Wales, Sydney, New South Wales 2052, Australia § School of Chemistry, University of Sydney, Sydney, New South Wales 2006, Australia ‡

S Supporting Information *

ABSTRACT: This study presents a new mononuclear complex (1) of the form [FeL](BF4)2, incorporating the thiazolylimine donor moiety, which was found to exhibit a high-temperature spin-transition. The effect of scan rate was investigated, with magnetic susceptibility being measured at 4, 2, and 1 K min−1. The magnetic susceptibility results were confirmed by variable temperature X-ray photoelectron spectroscopy (XPS) (100, 270, 400, and 500 K) and single crystal X-ray diffraction (150 and 400 K) experiments. A rare example of a high-temperature (400 K) single crystal structure of 1 has been reported. The high-spin fraction was calculated indirectly from XPS data, presenting a method for analyzing the spin-state in the surface layers of spincrossover materials.

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INTRODUCTION The discipline of spin-crossover (SCO) materials continues to receive research attention from many fronts. Be it theoretical, experimental, or from an application standpoint, understanding and controlling the magnetic properties of molecular materials are of high priority in the chemistry, physics, and materials sciences.1−4 The most thoroughly studied compounds to this point are those based on Fe(II) (d6 electronic configuration), for which the transition from the paramagnetic high-spin (HS) state (S = 2, 5T2) to the diamagnetic low-spin (LS) state (S = 0, 1 A1) has been observed to occur with temperature under an appropriate ligand field.2,5−11 An important aspect of SCO research lies in the ability to control the temperature of spin-transition and expand the range of transition temperatures achievable, which still represents a significant challenge.12 Recently we,13 along with Halcrow14 and Lü t zen,15 have investigated the scarcely reported thiazolylimine chemical moiety in a mononuclear thiazolylimine donor SCO complex and our very recent dinuclear triple helicate compound.16 A great deal of previous work has focused © XXXX American Chemical Society

on various derivatives of the imidazolylimine donor group, which tends to demonstrate SCO behavior at lower temperatures.17−20 Nitrogen substitutions at various positions in the imidazole ring have the advantage of hydrogen-bond donor and acceptor addition.18 On the other hand, the thiazolylimine group has presented the ability to stabilize the LS electronic state at higher temperatures, resulting in higher SCO temperature.13−16 Such SCO materials may potentially be useful for above room-temperature applications, such as molecular sensing devices. Commonly, the design of supramolecular coordination assemblies utilizes relatively rigid ligands, as these molecules afford greater control in supramolecular design.21,22 Herein, we describe the preparation and characterization of a new mononuclear, high-temperature iron(II) SCO complex 1 ([FeL](BF4)2) incorporating the flexible, N6 hexaazadentate l i g a n d L ( b i s - 4 - t h i o i m i d az o l e - 1 , 2 - d i p r o p y l a m i n o Received: March 5, 2018

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DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Compound 1 was prepared in a one-pot reaction. The Schiff base condensation of 1,2-bis(3-aminopropylamino)ethane and thiazole-4-carboxaldehyde in a 1:2 ratio in ethanol catalyzed by trace amounts of acetic acid was proceeded by the addition of 1 equiv of Fe(II) tetrafluoroborate hexahydrate to form a dark red precipitate. The X-ray quality crystals were obtained by diethyl ether diffusion into acetonitrile. This compound has been fully characterized by UV−vis, CHN, TGA, FT-IR, HRMS, PXRD, and SEM-EDS. An SEM micrograph reveals that compound 1 crystallizes in large rectangular blocks (Figure S1). In the HR-mass spectrum (Figure S3), a major peak for [FeL]2+ was observed at m/z 210.0468, and the isotopic distribution was in agreement with the simulated pattern (inset in Figure S3). The measured PXRD pattern of 1 (Figure S5) at RT shows a distinct similarity to the corresponding PXRD pattern calculated from the single crystal data at 150 K, suggesting that a single-phase material has been prepared. In addition, CHN and TGA also confirmed that there is no solvent in the sample. Magnetic Susceptibility Measurements. A polycrystalline sample of 1 was cycled between 250 and 400 K at scan rates of 4, 2, and 1 K min−1, with a field of 0.5 T (Figure 2).

(aminoethane)) in Figure 1. Ligands with this degree of flexibility can produce many interesting coordination arrange-

Figure 1. A chemical representation of L.

ments under different conditions, and the ability to manipulate metallo-supramolecular structure in these compounds allows their physical properties to be altered. Conformational flexibility can also be an important factor in the design of ligands that facilitate intermolecular interactions in their consequent coordination compounds.10,23−26 The temperature-dependent spin-transition of 1 was explored using the Quantum Design Versalab Measurement System with a vibrating sample magnetometer (VSM) attachment, and further evidence was provided by variable temperature (VT) single crystal X-ray diffraction and variable temperature X-ray photoelectron spectroscopy (VT-XPS) experiments. These experiments all demonstrated excellent correlation of the SCO behavior. The compound was fully characterized by powder X-ray diffraction (PXRD), carbon hydrogen nitrogen analysis (CHN) , thermogravimetric analysis (TGA), highresolution electrospray ionization mass spectrometry (HRESIMS), scanning electron microscopy−energy Dispersive X-Ray Spectroscopy (SEM-EDS), Fourier transform infrared (FT-IR) spectroscopy and UV−vis spectroscopy. Previously, we investigated the spin-transition in a dinuclear Fe(II) triple helicate in detail using VT-XPS experiments, which clearly showed the progression from the 1A1 LS to the 5 T2 HS state, passing through a very interesting, clearly observable splitting of the Fe(II) 2p1/2 peak at temperatures where proportions of both the HS and LS state Fe(II) centers were present in the material.16 The Fe(II) HS and LS isomers can be noted in XPS by an increased intensity of 2p1/2 and 2p3/2 satellite (“shake-up”) bands in the HS state and an increase in the binding energy of the 2p peaks.13,27−31 Additionally, mixed levels of HS and LS centers can be seen in a splitting of the 2p1/2 peak. As an extension of our previous studies mentioned above, this investigation sought to identify whether the same Fe 2p splitting could be seen at intermediate temperatures in a mononuclear complex (also utilizing the same thiazolylimine moiety), allowing the spin-transition to be followed using XPS measurements and to identify if the trend in VT-XPS again matches that of the magnetic susceptibility experiments. Our interests also lie in how these XPS spectra differed when the number of Fe(II) metal centers in the discrete compound changed from dinuclear to mononuclear. Interestingly, the spin state of an Fe(II) center in 1 can be clearly traced from the majority LS state, to a mix of LS/HS states and finally to a majority HS state by three distinct “phases” of the XPS spectrum, which are discussed below by following the 2p peak shape. The ability of XPS to analyze the surface layers of SCO materials serves as a strong advantage, as this may allow for thin-film analysis of future electronic or sensing SCO devices without interference by the underlying substrate.

Figure 2. A χMT vs T plot of 1, showing the heating and cooling modes at 4 K min−1 (black squares), 2 K min−1 (blue triangles), and 1 K min−1 (red circles).

The molar magnetic susceptibility (χMT) at 250 K was 0.06358 cm3·K·mol−1, corresponding to an almost completely diamagnetic LS (1A1, S = 0) state. The χMT increased gradually in a single step manner to a value of 1.76 cm3·K·mol−1 at 400 K, at which point ca. 50% of the Fe(II) metal centers had undergone a thermally induced spin-transition to the HS (5T2, S = 2) electronic configuration. At a rate of 4 K min−1, the heating and cooling modes proceeded with a scan-rate induced thermal loop of 5 K (Figure 2), with a T1/2 of 370 ↓ and 375 ↑ in the cooling and heating modes, respectively. At scan rates of 2 and 1 K min−1, the spin-transition profile retained its shape, although the loop was found to shrink with slower scan rates, until 1 K min−1 where no loop was observed. The loop observed at 4 K min−1 is therefore scan rate induced, the susceptibility of the material lagging behind the change in temperature. The heating and warming modes demonstrated a similar dependence with scan rate. Magneto-Structural Correlation. At 150 K, compound 1 crystallized in the monoclinic space group P21/n. The coordination sphere is composed of six nitrogen donors: two adjacent thiazolylimine nitrogens, two imine nitrogens, and two B

DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry adjacent amine nitrogens (Figure 3). The hexadentate ligand wraps around the Fe(II) metal center in a distorted octahedral

Figure 4. Schematic representation of the unit cell of 1, in which two Δ (red) and two Λ (green) compounds are present.

Figure 3. Schematic representation of the crystal structure of 1 at 150 K, with the two enantiomers Δ (red) and Λ (green) shown. Anions have been excluded for clarity. White represents hydrogen atoms, black - carbon, blue - nitrogen, yellow - sulfur, and purple - iron.

manner. Along the b-axis on the other hand, these rows of similar enantiomers consist of undulating complexes inverted relative to one another (Figure 5b). Along the b-axis, hydrogen bonding between amine N−H and tetrafluoroborate counterions (N···F 2.95 and 2.92 Å) links adjacent complexes in a chain-like manner (Figure 6). The compounds connected by hydrogen bonding interactions along the b-axis are of the same enantiomeric identity, forming chains of Λ---Λ and Δ---Δ. Intermolecular interactions, particularly hydrogen bonding, have been demonstrated to be an important factor in the cooperativity of spin-transitions.10,32,35,36 The gradual nature of the spin-transition profile could be influenced by the hydrogen bonding along only one axis, which provides a relatively small degree of cooperativity between Fe(II) centers of the crystal lattice, this being a direct result of the

geometry (Σ 56.1°), and this, along with average Fe−N bond lengths of 2.00 Å (Table 1), is indicative of a LS Fe(II) center, which is in accord with the magnetic susceptibility results.10,32−34 Furthermore, two enantiomers are observed within the crystal lattice, characterized by the direction with which the distinct “out−in” pairs of N donors proceed around the metal center−in a left- (Λ - green) or right- (Δ - red) handed direction (Figure 3). The unit cell consists of two Λ and two Δ molecules (Figure 4). As can be seen in Figure 5a, these enantiomers are distributed in straight rows of a single species along the c-axis, with adjacent rows possessing the opposite enantiomer. The adjacent rows pack together in an undulating

Table 1. Crystallographic Data of Compound 1 at 150 and 400 K

empirical formula formula weight temperature/K crystal system space group a/Å b/Å c/Å α/° β/° γ/° volume/Å3 Z ρcalc, g/cm3 μ/mm−1 F(000) radiation 2Θ range for data collection/° index ranges reflections collected independent reflections data/restraints/parameters goodness-of-fit on F2 final R indexes [I ≥ 2σ(I)] final R indexes [all data] largest diff. peak/hole/e Å−3 average Fe···N bond lengths (Å) Σ (deg) unit cell volume (Å) spin-state of Fe(II)

1 at 150 K

1 at 400 K

[C16H24FeN6S2](BF4)2 594.00 149.89 monoclinic P21/n 9.77(5) 17.50(8) 13.67(7) 90.0 100.5(2) 90.0 2297.90(2) 4 1.717 0.922 1208.0 MoKα (λ = 0.71073) 3.822−55.062 −12 ≤ h ≤ 12, −22 ≤ k ≤ 22, −17 ≤ l ≤ 17 62967 5289 [Rint = 0.0806, Rsigma = 0.0405] 5289/0/316 1.052 R1 = 0.0350, wR2 = 0.0673 R1 = 0.0596, wR2 = 0.0766 0.56/−0.51 2.00 56.1 2298.10 LS

[C16H24FeN6S2](BF4)2 594.00 400.02 monoclinic P21/n 10.06(13) 17.42(2) 14.22(17) 90 98.7(4) 90 2462.70(5) 4 1.602 0.860 1208.0 MoKα (λ = 0.71073) 3.724−54.366 −11 ≤ h ≤ 12, −22 ≤ k ≤ 22, −18 ≤ l ≤ 18 45318 5471 [Rint = 0.0832, Rsigma = 0.0501] 5471/1/338 1.046 R1 = 0.0699, wR2 = 0.1817 R1 = 0.1187, wR2 = 0.2176 0.89/−0.40 2.08 58.6 2462.70 mixed spin state population (∼50% HS)

C

DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX

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atoms.10,38 This can be attributed to the relative flexibility of L, allowing the complex to adapt to the change in bond lengths with a smaller change in ∑ as a result of compensative changes in the confirmation of L. The small change in the distortion of the octahedron may also contribute to the high stability of the single crystal at such temperatures, producing a smaller change in the packing orientation and volume of the cell, resulting in a reduced strain on the crystal lattice. The unit cell volume increases by 164.6 Å3 from 2298.1 Å3 at 150 K to 2462.70 Å3 at 400 K. The other cell parameters (other than the two 90.0° monoclinic angles) also increase in the 400 K structure (Table 1), although interestingly, while the a- and c-axes both exhibit elongation with increased temperature, at 400 K the b-axis decreases by 0.085 Å (Table 1). This contraction of the cell along the b-axis with increased temperature may be explained by the combination of increasing coordinate bond lengths between N donor atoms and the Fe(II) center as a result of the spin-transition, and the shifting degree of hydrogen bonding between the hydrogen of the secondary amine and fluorine atom of a BF4− counterion along the crystallographic b-axis. At 150 K the F···NH distance is 2.94 Å, well within the distance for hydrogen bonding interactions to be present. On the other hand, at 400 K this distance increases to 2.99 and 3.07 Å (two disordered, partially occupied fluorine atoms present), resulting in a weakening of the hydrogen bonding interactions between these two groups. As a result, the Nsecondary−Fe−Nthioimidazole angle increases from 90.65 in the 150 K structure, to 93.09° at 400 K, as the ligand relaxes back from any conformational stresses caused by hydrogen bonding interactions. Furthermore, the Nthioimidazole−Fe−Nthioimidazole angle decreases from 93.63° to 90.91 as the relatively flexible ligand rearranges. Both of these changes cause a contraction of the ligand away from the BF4− along the b-axis (Figure 7).

Figure 5. Schematic representation of the crystal packing of 1, showing the packing of isomers (a) in straight rows along the c-axis, and (b) in undulating rows along the b-axis.

Figure 6. Schematic representation of the hydrogen bonding interactions (represented by light blue dotted lines) present within the crystal lattice of 1. Hydrogen bonding links complexes in rows along the b-axis.

thiazolylimine donor moiety affording no external hydrogen bonding interactions. Being enantiomers, these compounds will exhibit identical ligand fields, and accordingly only one step is observed in the spin-transition profile. Although as only half of the material has undergone a LS to HS transition at 400 K, we cannot comment on the nature of the curve beyond 400 K, and this will be subject to further investigation. At 400 K, the crystal structure of 1 retains a monoclinic space group P21/n. The blood red crystals showed excellent stability at higher temperatures, the 400 K data finishing with a Rint of 8.32% and a R1 of 6.99% (Table 1). The 400 K structure shows identical packing arrangements, and similarly, the two enantiomers are also present, with two of each in the unit cell. The average Fe···N bond lengths and octahedral distortion parameter (Σ) were found to increase slightly to values of 2.08 Å and 58.6° respectively. These bond lengths are indicative of a mixed HS and LS spin-state population of Fe(II) centers within the crystal lattice, which confirms magnetic susceptibility data that suggests a population of around 50% of the Fe(II) centers are in the HS state at 400 K.32−34,37 Although the change in the octahedral distortion parameter is only slight, and commonly a larger change in Σ is observed in Fe(II) SCO compounds with an N6 arrangement of donor

Figure 7. An overlay of the X-ray crystal structures of 1 at 150 (blue) and 400 K (yellow) demonstrating the mechanism of the contraction along the crystallographic b-axis with increased temperature.

VT-XPS Investigation of 5T2 HS Fe(II) and 1A1 LS Fe(II). Previous studies have identified the primary indication of the spin state of iron complexes as being from the Fe 2p bands in the region between 708 and 722 eV.30,31 In the 1A1 LS state at 100 K, the 2p1/2 and 2p3/2 peaks appear narrow and show minimal satellite peak intensity (Figure 8a). At 270 K, highenergy peaks from the 5T2 HS electronic configuration appear for the 2p1/2 and 2p3/2 electrons that slightly develop in intensity, along with two broad satellite peaks. The origin of the HS peaks is due to four unpaired 3d-electrons in HS Fe(II) that enable a greater degree of spin−orbit coupling with the ejected photoelectron.30 Reduced ligand-to-metal-charge-transfer (LMCT) in the paramagnetic HS state results in increased coordinate bond lengths and ionicity, reducing the “screening” of the effective nuclear charge felt by the 2p photoelectron. As a D

DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 2. Binding Energies (eV) of the Fe 2p Peaks of the XPS of 1 at 100, 270, 400, and 500 K temperature (K)

1/2

3/2

100 270 400 500

719.73 719.68 719.72 720.02

706.93 706.88 706.92 706.92

substantial satellite features toward higher binding energies. This is indicative of a significant proportion of Fe(II) compounds existing in the 5T2 configuration at this temperature, which agrees with crystallographic and magnetic susceptibility results, suggesting around half of the Fe(II) metal centers have transitioned to the HS state at 400 K. As previously shown by our group,16 the shoulder of the 2p1/2 peak toward a higher BE that was very minor in the LS spectra at 100 and 270 K is now roughly the same size as the original LS 2p1/2 peak, demonstrating an increase in the abundance of the HS state in the sample. This is observed as a splitting of the 2p1/2 peak. This trend continues in the 500 K data, where the new 2p1/2 peak at higher binding energy is now dominant (the peak obtained is a superposition of the two, with the original LS 2p peak now a right-side shoulder to the HS peak of higher BE), and increasingly evident satellite structures again result in broadened Fe 2p peaks. This suggests a higher proportion of Fe(II) centers occupying the HS state at this temperature, although magnetic measurements were unable to be carried out at such temperatures, and as a result, a precise HS fraction could not be used to compare with this XPS data. The appearance of a second 2p1/2 peak at higher BE with heating is a result of decreased LMCT in the HS 5T2 state, reducing the degree of shielding of the Fe 2p core electrons and therefore the BE.30,31 The measurement of spin state of Fe(II) ions in 1 using variable temperature XPS can be semiquantified by fitting the spectra at each temperature extreme, then calibrating against the results obtained using the magnetic susceptibility measurements (Figure 8b). Semiquantification of the XPS data in this way has the added potential of investigating spin states of device surfaces essential for device fabrication using thin films of 1 on semiconductor wafers.

Figure 8. (a) XPS spectrum of 1 showing Fe(II) 2p region at 100, 270, 400, and 500 K showing the low-spin (LS) 1A1 features at 100 K, and the high-spin (HS) and satellite 5T2 features that develop from 270 to 500 K. (b) Calibration of spectral fraction to HS% with magnetic data, assuming 50% HS at 375 K.

consequence, the binding energy(BE) of the 2p photoelectron increases.27−31,39 As discussed in our previous work on a dinuclear Fe(II) triple helicate,16 the transition from HS → LS can be monitored in three main stages: (1) The predominantly LS state characterized by sharp narrow peaks and relatively void of satellite structure, (2) the coexistence of similar LS and HS fractions characterized the emergence of 2p satellite peaks and splitting of the 2p1/2 peak, with a second peak emerging at a slightly higher BE, of roughly the same intensity, and (3) the predominantly HS state, indicated by the increase of the higher BE 2p1/2 peak to dominance over the original LS peak, and a further increase of satellite structure, leading to much broader 2p1/2 and 2p3/2 peaks. When cooled to 100 K, the 2p1/2 and 2p3/2 peaks are narrow (Table 2 and Figure 8a) and shake-up satellites appear extremely weak relative to the main 2p lines, suggesting a dominant LS configuration as expected from magnetic susceptibility measurements. The spectrum at 270 K closely resembles that at 100 K, with the 1A1 configuration still dominant at this temperature, although some satellite features are noticeable. Upon warming to 400 K, 30 K above the T1/2 ↑ value, both members of the 2p spin−orbit doublet demonstrate

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CONCLUSIONS In summary, a new Fe(II) SCO compound 1 has been synthesized and fully characterized. The magnetic susceptibility studies demonstrated a high-temperature thermally induced spin-transition with a small thermal loop of 5 at 4 K min−1 (T1/2 of 370 ↓ and 375 ↑). The structural aspects of the spintransition at variable temperature were further investigated with single crystal X-ray diffraction at 150 and 400 K and XPS experiments at 100, 270, 400, and 500 K, all of which show excellent agreement with the magnetic susceptibility data. Furthermore, VT-XPS measurements of the surface layers of SCO materials should be further investigated to allow enhanced characterization of thin-film devices, as such studies have shown how XPS can be used as a complementary technique to magnetic susceptibility measurements in order to determine the HS fraction of mononuclear and dinuclear complexes. Future work will also focus on multinuclear Fe(II) complexes. E

DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX

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with a pass energy of 100 eV with high-resolution scans performed at 20 eV. The complex was measured at 100 K, 170 K, 400 K, and then 500 K. Data were processed using the Avantage software package (Thermo Scientific, UK). Peaks were calibrated using the Fe 2p3/2 peak at 706.9 eV and background corrected using the Shirley method.44

EXPERIMENTAL SECTION

Materials and Methods. All chemicals and reagents were sourced and purchased from commercial sources and used without further purification. A Waters Xevo QToF mass spectrometer was used to collect all HRESI-MS data in positive-ion mode. A Waters lock spray system was used to calibrate the high-resolution masses. A Bruker Vertex 70 with a diamond ATR crystal was used to obtain all FT-IR measurements. Using an Agilent Cary 100 UV−vis with WinUV software, all solid-state UV−vis spectra were measured in Nujol at ambient room temperature, and the spectra were collected at a scan rate of 600 nm per minute from 900 to 200 nm. SEM-EDS analysis was conducted on a Phenom XL tabletop instrument. Samples were run at 15 kV in high vacuum without surface coating. TGA measurement was performed using a Netzsch STA449 C Jupiter instrument. The measurement was performed using an aluminum crucible under argon atmosphere across the temperature range of 300−420 K at a heating rate of 10 K min−1. Elemental analyses for C, H, N, and S were performed by Microanalytical Service at the University of Queensland. Synthesis of Complex [FeL](BF 4 ) 2 (1). 1,2-Bis(3aminopropylamino)ethane (230 mg, 1.33 mmol) in ethanol (10 mL) was added dropwise to thiazole-4-carboxaldehyde (300 mg, 2.66 mmol, 10 mL) with trace amounts of acetic acid. The reaction mixture was then refluxed for 6 h under nitrogen. To the mixture, iron(II) tetrafluoroborate hexahydrate (447 mg, 1.33 mmol, 10 mL) was added dropwise. The resulting blood red solution was stirred for 4 h. The precipitate was filtered, washed with ethanol, and dried in air. This crude product was then dissolved in acetonitrile, and the vapor diffusion of diethyl ether resulted in the formation of dark red crystals which were air-dried, with a yield of 69%. FT-IR (ATR, νmax/cm−1): 3296, 1589, and 1033; UV/vis (solid state in Nujol): λmax 406, 509, 560 nm; elemental analysis (%) (calcd., found for C16H24B2F8FeN6S2): C (32.35, 32.37), H (4.07, 4.01), N (14.15, 13.88), S (10.79, 10.48); ESI-HRMS (positive-ion detection, CH3CN, m/z): cald. for [FeL]2+, 210.0391; found, 210.0468; single crystals were taken from the same sample and used directly in the X-ray study. Magnetic Susceptibility Measurements. Data for magnetic susceptibility measurements were collected on a Quantum Design Versalab Measurement System with a vibrating sample magnetometer (VSM) attachment. Measurements were taken continuously under an applied field of 0.5 T. A polycrystalline sample of 1 was cycled over the temperature range 250−400 K at heating rates of 4, 2, and 1 K min−1. Powder X-ray Diffraction. Powder X-ray diffraction measurements were conducted on a Bruker D8 ADVANCE diffractometer with a LynxEye position sensitive detector (PSD). The X-ray source was a copper K-α1 at 1.54 Å at 40 kV and a current of 40 mA. The sample scan range was 5−55 degrees 2θ with a step size of 0.019°. Data processing was conducted using Bruker’s EVA software. Single Crystal X-ray Diffraction. Crystallographic data were collected using a Bruker kappa-II CCD diffractometer at 150 K, employing an IμS Incoatec Microfocus Source with Mo−Kα radiation (λ = 0.710723 Å). For 400 K measurements, the crystal was mounted on a glass fiber, and secured with superglue. Data integration and reduction were undertaken with CrysAlisPro.40 The structures were solved by direct methods and the full-matrix least-squares refinements were carried out using a suite of SHELX programs41,42 via the Olex2 interface.43 Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in idealized positions and refined using a riding model. The crystallographic data in CIF format has been deposited at the Cambridge Crystallographic Data Centre with CCDC nos. 1585168 and 1585169. It is available free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1 EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc. cam.ac.uk. X-ray Photoelectron Spectroscopy. XPS was performed on an ESCALAB250Xi (Thermo Scientific, UK) using a monochromated Al K alpha line (energy 1486.68 eV) at 150W (13 kV × 12 mA) with a spot size of 500 μm on the sample. Electron optics were arranged at 90 deg with respect to the surface plane. Survey scans were performed

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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.8b00576. Additional characterization details of 1 including SEMEDS, FT-IR, ESI-HRMS, solid-state UV−vis, PXRD, TGA, and crystallographic data details and specific refinement for each structure (PDF) Accession Codes

CCDC 1585168−1585169 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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AUTHOR INFORMATION

Corresponding Authors

*(F.L.) E-mail: [email protected]. *(C.E.M.) E-mail: [email protected]. ORCID

Feng Li: 0000-0001-8465-9678 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by Western Sydney University (WSU). The authors acknowledge the AMCF and Mass Spectrometry facilities at Western Sydney University. XPS and single crystal experiments were conducted at the Mark Wainwright Analytical Centre at the University of New South Wales. Magnetic measurements were performed at The University of Sydney. A.R.C and K.J.H.-S. acknowledge the Western Sydney University Master of Research scholarship program. A.R.C. also acknowledges the AINSE honours scholarship program.

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DEDICATION This paper is dedicated to Professor Karsten Gloe on the occasion of his 70th birthday. REFERENCES

(1) Halcrow, M. A. Spin-Crossover Materials: Properties and Applications; John Wiley & Sons Ltd: Oxford, 2013. (2) Goodwin, H. A.; Gütlich, P. Spin Crossover in Transition Metal Compounds; Topics in Current Chemistry; Springer, 2004. (3) Bousseksou, A.; Molnár, G.; Salmon, L.; Nicolazzi, W. Molecular Spin Crossover Phenomenon: Recent Achievements and Prospects. Chem. Soc. Rev. 2011, 40, 3313−3335. (4) Senthil Kumar, K.; Ruben, M. Emerging Trends in Spin Crossover (SCO) Based Functional Materials and Devices. Coord. Chem. Rev. 2017, 346, 176−205. (5) Dupouy, G.; Marchivie, M.; Triki, S.; Sala-Pala, J.; Gómez-García, C. J.; Pillet, S.; Lecomte, C.; Létard, J.-F. Photoinduced HS State in the F

DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00576 Inorg. Chem. XXXX, XXX, XXX−XXX