Ambient-Temperature Spin-State Switching Achieved by Protonation

Jul 29, 2016 - Ambient-Temperature Spin-State Switching Achieved by Protonation of the Amino Group in [Fe(H2Bpz2)2(bipy-NH2)]. Yang-Hui. Luo† ...
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Ambient-Temperature Spin-State Switching Achieved by Protonation of the Amino Group in [Fe(H2Bpz2)2(bipy-NH2)] Yang-Hui. Luo,† Masayuki Nihei,‡ Gao-Ju Wen,† Bai-Wang Sun,*,† and Hiroki Oshio*,‡ †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, People’s Republic of China Faculty of Pure and Applied Science, University of Tsukuba, Tennodai 1-1-1, Tsukuba 305-8571, Japan

‡

S Supporting Information *

ABSTRACT: Magnetism of a complex [Fe(H2Bpz2)2(bipy-NH2)] (H2Bpz2 = dihydrobis(1-pyrazolyl)borate, bipy-NH2 = 4,4′-diamino-2,2′-bipyridine) has been altered from paramagnetic to spin-crossover (SCO) behavior, through protonation of one amino group of bipy-NH2 with CF3SO3H. Complete SCO transition, both in solid state and in solution, occurs at ambient temperature.

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INTRODUCTION Spin-crossover (SCO), which was first observed in the 1930s,1 continues to attract scientific attention, not only from a fundamental point of view, but also because of their potential to play a significant role in the fields of ultra-high-density memory, devices, sensors, electronics, and spintronics in nanoscience.2−5 Fe(II) SCO complexes show switching between diamagnetic low-spin (LS) and paramagnetic high-spin (HS) electronic states under the application of external stimuli such as temperature, pressure, light, or magnetic field, including changes of metal-ligand bond distances, optical properties, and magnetic properties.6,7 Until now, many SCO compounds have been reported, but only a handful of them have roomtemperature switching properties required for practical applications.8 Hence, the search for such SCO materials is of significant importance. The transition temperature (T1/2) and cooperativity character of SCO compounds are dependent on ligand field strength of the coordinating ligands, as well as the intermolecular interactions in crystal lattice and in the molecules themselves.9 The key step is to modify the ligands for the purpose of tuning either their σ-donor and/or πacceptor character to approach appropriate ligand field strength for SCO point.10 Bipy (2,2′-bipyridine), for instance, represents one of the classic imine ligands for SCO systems, and its electronic and structural modifications have been made to bring the ligand field’s strength into the crossover range. Modifications including substitutions of the pyridine rings,11 replacement of the pyridine rings with six- or five-membered heterocycles,12 and incorporation of substituents into the heterocycles.13 While much effort have been made, room© XXXX American Chemical Society

temperature SCO materials with such imine systems have yet to be achieved. The bipy-related SCO complexes, [Fe(H2Bpz2)2(bipy)] (pz = pyrazolyl), which was first prepared by Real and co-workers in 1997,14 exhibits a SCO transition at ∼160 K, and the pressure15 and light-induced excited spin state trapping (LIESST) effects16 have been investigated. In addition, Tuczek et al.17 fabricated these compounds into the thin film, where the LIESST and SCO behavior were reserved. Dougherty et al.18 deposited these compounds on Au(111) from the bilayer film to the multilayer film regime. Halcrow et al.19 further expanded this system with a series of annelated bipyridyl co-ligands, and gradual SCO transition were obtained. Very recently, the switching between HS and LS with light at room temperature in solid-state form for these types of compounds have been achieved by Khusniyarov et al.,20 which may open a new area in the construction of photo-controlled molecular devices. Secondary noncovalent interactions (host-anion, host-guest, hydrogen bonding, etc.) have emerged as powerful tools to affect the spin state of SCO complexes.21,22 Complexes with an anion receptor can form efficient anion−ligand interactions with the uncoordinated anions; these interactions can increase the σ-donating ability of ligands bound to Fe(II) ions in the solid state remarkably,23 which thus contribute to stabilization of the LS state.24 For [Fe(H2Bpz2)2(bipy)]-related complexes, one possible approach to generate anion−ligand interactions is changing the solution pH by adding acid/base to protonate/ deprotonate the functional groups (amino, carboxyl, etc.) on the bipy ligand. Ruben and co-workers25 and Hasserodt et al.26 Received: May 24, 2016

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DOI: 10.1021/acs.inorgchem.6b01193 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Crystal structure of compound 1·CH3OH. (b) Formation of dimers through N−H···π interactions (illustrated by magenta dashed lines) involving the amino group and pyrazole moiety. (c) Connection of adjacent dimers into a one-dimensional (1D) chain through N−H···O and O−H···N hydrogen bonding interactions (illustrated by orange dashed lines) involving the amino-MeOH and pyrazole nitrogen (N3). (d and e) 1D chain motif of 1·CH3OH viewed from different directions (the H atoms and interactions were omitted for the sake of clarity). (f) Classification of the interactions involving the amino group.

interactions, as illustrated in Figure 1f, the amino group that does not bind to MeOH forms two different types of N−H···π interactions: one is for interchain interactions, the other is for intrachain interactions; the latter interactions connect the 1D chain structure to form a three-dimensional (3D) motif (Figure S3 in the Supporting Information). Protonation-Induced Ultraviolet−Visible Light (UVvis) Absorption Spectra. Protonation experiments were performed to investigate possible modification of electronic states for 1·CH3OH. Ultraviolet−visible light (UV-vis) spectra in n-butyronitrile at 285 K are shown in the range of 700−300 nm (see Figure 2). The n-butyronitrile solution of complex 1· CH3OH exhibited color changes from pale pink to purple by adding of CF3SO3H (Figure 2, inset). In the UV-vis absorption spectra, the broad-band center at ∼550 nm can be assigned to a metal-to-ligand charge transfer (MLCT) band from the metal

have proposed that pH-tuned spin state switching can be used as pH-dependent contrast agents. Hence, in this work, we have employed the foregoing approach to switch the spin state of a new compound, [Fe(H2Bpz2)2(bipy-NH2)] (1, where bipyNH2 = 4,4′-diamino-2,2′-bipyridine and H2Bpz2 = dihydrobis(1-pyrazolyl)borate)), where one amino group of ligand bipyNH2 has been protonated by CF3SO3H. Magnetic and UV-Vis absorption measurements revealed that compound 1 exhibits paramagnetic behavior over the entire temperature range, while the protonated species 1·CF3SO3H displays SCO in an nbutyronitrile solution. To the best of our knowledge, this is the first example of protonation-achieved spin-state switching.

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RESULTS AND DISCUSSION Crystal Structure. Compound 1 was prepared by similar procedures that have been described previously in the literature.14 It was crystallized as a light orange bulk solvate, 1·CH3OH. Single-crystal X-ray diffraction (XRD) at 100 K revealed that 1·CH3OH crystallized in the triclinic P1̅ space group (see the Experimental Section). The Fe(II) ion has a distorted octahedral coordination geometry with six coordinating nitrogen atoms from two H2Bpz2 and one bipy-NH2 ligand (see Figure 1a). The Fe−N coordination bond distances at 100 K are in the range of 2.167(17)−2.218(16) Å, and the distortion parameter Σ15 is found to be 58.2(2)°. These values are characteristic of a HS Fe(II) ion. In the crystal, the adjacent two molecules of 1 form dimers through N−H···π interactions involving the amino group and pyrazole moiety (Figure 1b), and the adjacent dimers are then connected by N−H···O (N··· O distances of 2.954 (8) Å) and O−H···N (O···N distances of 3.098 (4) Å) hydrogen bonding interactions involving the amino−MeOH and pyrazole nitrogen (N3) (Figure 1c) into a one-dimensional (1D) chain structure (Figures 1d and 1e). Note that these hydrogen bonding interactions are responsible for the longest coordination bond length in 1·CH3OH (Fe−N3 = 2.219 (16) Å) and the stabilization of MeOH until 200 °C (verified by TGA measurements; see Figure S2 in the Supporting Information). The amino groups in this complex have occupied a dominant position for the formation of

Figure 2. Ultraviolet−visible light (UV-vis) absorption spectral change of 1·CH3OH in n-butyronitrile upon addition of CF3SO3H at 285 K. Insets: the color change of 1·CH3OH in n-butyronitrile before and after the addition of 1 equiv of CF3SO3H, respectively. B

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Figure 3. (a) UV-vis absorption spectra of compound 1·CF3SO3H in n-butyronitrile solution between 303 K and 273 K and (b) the related γHS vs T plot.

dπ-orbitals into π*-orbitals of the ligands.17,27 The addition of CF3SO3H, with the stoichiometric ratio of 1·CH3OH/ CF3SO3H, from 1:0.1, 1:0.2, 1:0.3, ..., 1:1, resulted in the growth of the broad-band center at ∼550 nm, as well as generation of a new band center at 400 nm; the latter can be assigned to the d−d transition of the LS state Fe(II) ions.28,29 The foregoing spectral change suggested that the protonation of amino groups resulted in partial spin state conversion to give a mixture of HS and LS state species. It then can be presumed that the protonation of amino groups, on the one hand, leads to an increase in the π-accepting ability of ligand bipy-NH2 (as have been confirmed by the growth of the MLCT band); on the other hand, it generates strong anion-ligand electrostatic interactions between CF3SO3−NH2 and bipy−NH2. These two effects both contribute to the stabilizing of the LS states Fe(II) ions.23,24 Note that further additions of acid (beyond the 1:1 stoichiometric ratio) leads to the disappearance of the d−d transition bands for the LS state Fe(II) ions (see Figure S4), as well as the purple color of the solution, suggesting that the protonation of both amino groups on 1 leads to decomposition of the complex. Slow evaporation of the above n-butyronitrile solution containing 1:1 stoichiometric ratio of 1·CH 3 OH and CF3SO3H under refrigerator conditions (4−10 °C) give birth to red-brown powders of complex 1·CF3SO3H (single crystals could not be obtained). The composition of 1·CF3SO3H was confirmed by elemental analysis, infrared (IR) spectra (Figure S1), and 1H NMR spectroscopy (Figure S5 in the Supporting Information). For complex 1·CH3OH, the vibrational peaks located at 3488 and 3385 cm−1 can be assigned to the stretch vibration of OH and NH2 groups, respectively. Whereas, for 1· CF3SO3H, the vibrational peak of OH was vanished and the peak of NH2 was shifted to 3380 cm−1; the new peak centered at 3340 cm−1 can be assigned to the stretch vibration of the NH3+ group. In the 1H NMR spectroscopy, the δ-shift of NH2 group in 1·CH3OH is observed at ∼6.3 ppm, whereas, for 1· CF3SO3H, this δ-shift of NH2 is reserved, and the newly emerged δ-shift at 8.0 ppm can be assigned to the protonated NH3+ group, which thus provided direct evidence of the formation of one amino protonated species. In addition, powder XRD patterns (Figure S6 in the Supporting Information) revealed that the crystal packing between 1· CH3OH and 1·CF3SO3H are completely different.

Temperature-Dependent UV-vis Absorption. The SCO behavior of the 1:1 protonated species in n-butyronitrile solution was investigated by means of UV-vis absorption spectroscopy in the temperature range of 303−273 K (Figure 3a). At 303 K, only HS MLCT bands was observed at the ∼550 nm, characteristic HS state. As the solution temperature decreases from 303 K to 273 K, the d−d transition band of the LS state Fe(II) ions appeared and then increased. Note that further cooling (from 273 K to 253 K) of the solution does not cause any change of the spectra, indicating that all HS species was converted to LS species at 273 K. Moreover, the spectral changes were reversible in the temperature variation between 273 K and 303 K with no hysteresis loop was observed. The relative HS fractions (γHS) of 1:1 protonated species in nbutyronitrile solution were estimated from the normalized absorption at 520 nm vs temperature, and the γHS vs T plot is shown in Figure 3b, giving a slow SCO transition at ambient temperature with T1/2 = 290 K. For comparison, temperaturedependent UV-vis absorption spectra of unprotonated species 1·CH3OH in n-butyronitrile solution have been performed in the temperature range of 303−223 K (Figure S7 in the Supporting Information), which indicate that the high-spin nature (see below) of 1·CH3OH in the solid state is a genuine reflection of its molecular ligand field, rather than imposed by crystal packing effects. Magnetic Properties. The magnetic property of solid samples of complex 1·CF 3 SO 3 H was investigated for comparison. Figure 4 shows χmT vs T plots for 1·CF3SO3H, together with 1·CH3OH. 1·CH3OH exhibited paramagnetic behavior in the entire temperature range measured, with a constant χmT value of 3.33 cm3 mol−1 K at 300 K, the value was in good agreement with the theoretical value (3.00 cm3 mol−1 K) expected for a HS Fe(II) ion (S = 2 and g = 2.0) and consistent with the related complexes.14−20 Note that the gradual decrease of the χmT value below 50 K is due to intermolecular antiferromagnetic interactions and/or the contribution of spin orbit coupling of the Fe(II) ion. On the other hand, 1·CF3SO3H exhibited a complete SCO transition. The χmT value is 3.23 cm3 mol−1 K at 350 K, which is a typical value for a HS Fe(II) ion but slightly smaller than the value for 1·CH3OH. Upon cooling, the χmT values showed a gradual decrease, starting from 325 K, and the value reached the minimum value of 0.05 cm3 mol−1 K at 250 K, indicating the C

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Figure 5. Energy-level diagram depicting frontier molecular orbitals of compound 1 and 1·H+. The protonation to the amino groups lead to the weakened ligand field strength of Δ, which is defined as the energy difference of the dπ-orbitals and dz2-orbitals.

Figure 4. Magnetic susceptibility vs temperature curves of compounds 1·CH3OH and 1·CF3SO3H under an applied magnetic field of 2000 Oe with a cooling rate of 2 K/min.

occurrence of a complete SCO transition from the HS to the diamagnetic LS state with T1/2 = 291 K, which is a transition temperature that is almost identical to that in solution. Discussions. [Fe(H2Bpz2)2(bipy)]-related complexes have been reported to show complete spin transition.14−20 However, on the one hand, the introduction of the electron-donating amino groups into the bipy ligand can reduce the π-acceptor character, leading to the weaker ligand field strength and stabilizing the HS state. On the other hand, the presence of MeOH in the crystal of 1·CH3OH can form strong hydrogen bonding interactions with the coordination N3 atom of pyrazole moiety, leading to the longer Fe−N bond length, which favors the HS state. [The investigation of the influence of MeOH on the magnetic property of 1 cannot be performed, because the removal of MeOH resulted in damage of the complex (see the TGA profiles in Figure S2 and the powder XRD patterns in Figure S6).] We speculate that the addition of CF3SO3H resulted in protonation of the amino groups, and the formation of electron-withdrawing ammonium groups (bipyNH3+) increased the π-accepting character, accompanied by the efficient anion−ligand interactions (involving the bipy-NH3+ and CF3SO3−), giving the appropriate ligand field strength (Δ) for the ambient temperature SCO behavior. Theoretical Calculations. To probe the above hypothesis, we carried out density functional theory (DFT) calculations for 1 and the protonated species 1·H+, with the B3LYP* functional30 at the 3-311+G** basis set,31 using Gaussian 09.32 The structural data were generated from the X-ray data of 1·CH3OH. Geometry optimization of 1 resulted in a structure with the HS state of S = 2, which was in close agreement with the X-ray structure data (see Figure 5). The HS state for 1 can be attributed to the relatively small energy difference (Δ) between the highest dπ-orbital and the dz2-orbital, while the protonated 1·H+, which involved the formation of bipy-NH2· H+, turned out to reveal a relatively large energy splitting of Δ (Figure 5) and stronger π-accepting character, which stabilized the LS state (Figure S8 in the Supporting Information).

complete ambient temperature SCO transition both in solid and solution state, because of the increase in ligand field strength of the bipy−NH2·H+ ligand and anion−ligand interactions. We believe that the strategy in this work may open new pathways to tuning the electronic state of magnetic materials.

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EXPERIMENTAL SECTION

Materials and Methods. All syntheses were performed under ambient conditions. Fe(ClO4)2·6H2O, KH2(Bpz2), and 4,4′-diamino(2,2′-bipyridine) were all obtained commercially and used as received. Elemental analyses for 1·CF3SO3H were performed using a Vario-EL III elemental analyzer for carbon, hydrogen, and nitrogen. IR spectra were recorded on a Shimadzu Model Prestige-21 FTIR-8400S IR spectrometer in the spectral range of 4000−500 cm−1, with the samples in the form of potassium bromide pellets. Thermogravietric analysis (TGA) profiles of 1·CH3OH were performed using a Mettler−Toledo TGA/DSC STARe System at a heating rate of 10 K min−1, under an atmosphere of dry N2 flowing at a rate of 20 cm3 min−1 over a temperature range from 50 °C to 500 °C. Variabletemperature UV/vis absorption spectra were recorded on a Shimadzu Model UV-3150 spectrometer that was equipped with a UNISOKU USP-203-A cryostat. Magnetic susceptibility data for 1·CH3OH were collected using a Quantum Design Model MPMS-5S SQUID magnetometer with an applied magnetic field of 2000 Oe in the temperature range of 5−300 K with a cooling rate of 2 K/min. Magnetic susceptibility data for 1·CF3SO3H were collected using a Quantum Design vibrating sample magnetometer in a physical property measurement system (PPMS), in the temperature range of 350−200 K with a cooling rate of 2 K/min. Single-crystal XRD data for compound 1·CH3OH were collected using a Bruker SMART APEXII diffractometer that was equipped with a CCD-type area detector. Synthesis. 1·CH3OH was prepared as follows. A mixture of Fe(ClO4)2·6H2O (144 mg, 0.4 mmol) and KH2Bpz2 (149 mg, 0.8 mmol) in methanol (20 mL) was stirred for 20 min at room temperature (RT), under a N2 atmosphere; the white precipitates were removed by filtration. The filtrate was added to 10 mL of methanol and bipy-NH2 (72.5 mg, 0.39 mmol) to obtain a yellow solution. After stirring for 20 min, the yellow solution was filtered and kept undisturbed under ambient conditions. Single crystals of 1·CH3OH suitable for XRD were obtained within 2 days, and the light-orange crystals were collected by filtration (187 mg, 82%). Elemental analysis calcd (%) for C22H26B2FeN12·CH3 OH: C, 48.57; H, 5.32; N, 29.57. Found: C, 48.44; H, 5.46; N, 29.51. IR (KBr pellet, cm−1): 3488, 3385,

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CONCLUSIONS In summary, we present a new approach to induce SCO transition through protonation of a HS state compound 1· CH3OH, the protonated compound, 1·CF3SO3H, exhibits D

DOI: 10.1021/acs.inorgchem.6b01193 Inorg. Chem. XXXX, XXX, XXX−XXX

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2403, 2289, 1633, 1562, 1497, 1400, 1291, 1199, 1161, 1052, 1009, 879, 754, 641. Red-brown powder samples of 1·CF3SO3H were obtained by slow evaporation of the n-butyronitrile solution containing 1:1 stoichiometric ratio of 1·CH3OH and CF3SO3H under refrigerated conditions (4−10 °C). Elemental analysis calcd (%) for C22H26B2FeN12· CF3SO3H: C, 40.22; H, 3.96; N, 24.49. Found: C, 40.12; H, 3.87; N, 24.32. IR (KBr pellet, cm−1): 3380, 3340, 2424, 2348, 1617, 1552, 1513, 1259, 1172, 1030, 836, 765, 641, 570. Crystal data for 1·CH3OH at 100 K: C23H30B2FeN12O, triclinic, P1̅, a = 10.4015(17) Å, b = 11.5188(19) Å, c = 12.280(2) Å, α = 89.149(2), β = 80.014(2), γ = 67.328(2)°, V = 1334.9(4) Å3, Z = 2, final R1 = 0.0401, wR2 = 0.0921. Supplementary crystallographic data for this compound can be obtained from the Cambridge Crystallographic Data Centre (No. CCDC-1444829).

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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.6b01193. IR spectra, 1H NMR spectroscopy, and HOMO distribution of compounds 1·CH3OH and 1·CF3SO3H; PXRD patterns of complexes 1·CH3OH, 1·CF3SO3H, and the heated species of 1·CH3OH; TGA profile; CF3SO3H tuned (1:1 to 1:3) and VT (303−223 K) UV/ vis absorption spectra of complex 1·CH3OH; 3D stacking motif of 1·CH3OH (PDF) Crystal data of 1·CH3OH (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-025-52090614. Fax: +86-025-52090614. E-mail: [email protected] (B.-W. Sun). *E-mail: [email protected] (H. Oshio). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based on work supported by the Natural Science Foundation of China (Grant No. 21371031), JSPS KAKENHI (Grant No. 26288021), and PAPD of Jiangsu Higher Education Institutions. Y.-H.L. thanks the Scientific Research Foundation of Graduate School of Southeast University (No. YBPY1409) for financial support.

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DOI: 10.1021/acs.inorgchem.6b01193 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01193 Inorg. Chem. XXXX, XXX, XXX−XXX