Neutral Molecular Iron(II) Complexes Showing Tunable Bistability at

Article Cite This: Cryst. Growth Des. 2017, 17, 6006-6019

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Neutral Molecular Iron(II) Complexes Showing Tunable Bistability at Above, Below, and Just Room Temperature by a Crystal Engineering Approach: Ligand Mobility into a Three-Dimensional Flexible Supramolecular Network Hiroaki Hagiwara,*,† Takuya Masuda,† Takuya Ohno,† Mika Suzuki,† Taro Udagawa,‡ and Kei-ichiro Murai§ †

Department of Chemistry, Faculty of Education and ‡Department of Chemistry and Biomolecular Science, Faculty of Engineering, Gifu University, Yanagido 1-1, Gifu 501-1193, Japan § Department of Chemical Science and Technology, Graduate School of Advanced Technology and Science, Tokushima University, Minami-Josanjima 2-1, Tokushima 770-8506, Japan S Supporting Information *

ABSTRACT: Room temperature (RT) bistable switching materials continue to fascinate the scientists since they can be utilized for a new class of molecular-based switches or memories. While the spin crossover (SCO) compound is categorized into these attractive materials, designing of a SCO system showing desirable bistability (i.e., wide hysteresis loop spanning RT) in a rational way is still a very challenging issue. We report herein a new family of neutral molecular iron(II) complexes showing hysteretic SCO in a wide range of switching temperatures (239−409 K) and hysteresis widths (1−31 K) spanning RT. These materials were obtained as single crystals of two solvent-free compounds [FeII(ptm2-dmpn)(NCS)2] (1; ptm2-dmpn = N,N′-bis[(1-phenyl-1H-1,2,3-triazol-4yl)methylene]-2,2-dimethylpropane-1,3-diamine) and [FeII(p-ttm2-dmpn)(NCS)2] (2; p-ttm2-dmpn = N,N′-bis[(1-p-tolyl-1H1,2,3-triazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine), and two solvatomorphs [FeII(p-ttm2-etpn)(NCS)2]·solvent (pttm2-etpn = N,N′-bis[(1-p-tolyl-1H-1,2,3-triazol-4-yl)methylene]-1-ethylpropane-1,3-diamine, and solvent = 0.5H2O and 0.5MeCN·0.5MeOH·H2O for 3a and 3b, respectively). All compounds are constructed of a three-dimensional (3D) flexible supramolecular network by multiple weak CH···S hydrogen bonds incorporating an additional orderly dimensional structure by aromatic interactions (i.e., π−π and CH···π interactions) between adjacent aromatic rings of tetradentate ligands. These 3D networks can accommodate a number of molecular motions such as (1) NCS bending, (2) tetradentate ligand biting, (3) aromatic ring rotation, and (4) propylene fragment oscillation to various degrees depending on a slight modification of small alkyl substituents and lattice solvents. The present crystal engineering approach of introducing concerted multimolecular motions into a 3D flexible network can be a new designing concept for controlling switching temperature and cooperativity in a systematic way toward the discovery of unprecedented RT bistable SCO materials.

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INTRODUCTION

The stimuli-responsive spin state switching between a highspin (HS) and a low-spin (LS) state, the so-called “spin crossover” (SCO), is one of the pronounced discoveries of an unprecedented switching phenomenon during the 20th century.9,10 After the appearance of the first SCO complex by Cambi et al. in 1931,9 SCO compounds have attracted much attention since their switching ability by external stimuli (such as temperature, pressure, or light) can be utilized for molecular switches or memories.11−13 For practical application, in particular, as memory devices, SCO systems must be required to show a remarkable bistability, i.e., a wide hysteresis loop

The discovery of a new stimuli-responsive switching property captures the hearts of scientists since it leads them into a study of unexplored scientific theories and development of new functional materials. During the years from the late 19th century to the 20th century, there have been some findings such as piezoelectricity of quartz by Curie in 1880,1 photochromism of spiropyran by Fisher in 1952,2 and thermosalient effect of (phenylazophenyl)palladium hexafluoroacetylacetonate (PHA) by Etter and Siedle in 1983.3 If two different switchable states have a bistable feature, i.e., both states can be populated at one definite condition [ideally, at around room temperature (RT)], materials having this feature can be applicable to molecular-based high-density information storages, switching devices, sensors, and displays.4−8 © 2017 American Chemical Society

Received: August 14, 2017 Revised: September 21, 2017 Published: September 22, 2017 6006

DOI: 10.1021/acs.cgd.7b01141 Cryst. Growth Des. 2017, 17, 6006−6019

Crystal Growth & Design

Article

spanning RT.14−16 While long-time efforts have been devoted to control critical temperature (T1/2) and hysteresis width (ΔT) of SCO compounds, designing a SCO system showing on-demand switching properties in a rational way is still a very challenging issue. One of the designing approaches is linking SCO metal sites by bridging ligands for constructing cooperative clusters17−21 or multidimensional networks. Starting from the discovery of near RT bistable 1,2,4-triazole-based one-dimensional (1D) coordination polymers by Lavrenova et al. in 1986,22 various types of 1D,23−28 two-dimensional (2D),29−33 and three-dimensional (3D)34−38 frameworks have been prepared, and some notable results for modulating T1/2, ΔT (and also stepwise nature) by counteranions28 or guest molecules27,28,33,36,37 have been reported. Another designing approach is based on intermolecular interactions, such as hydrogen-bonding,39−46 π−π,47−51 XH···π (X = C or N),52,53 and N···π54 interactions, for constructing cooperative supramolecular networks.55 In this approach, there are also some interesting results for modulating T1/2 and ΔT by substituents,41,42,56−59 coligands,41,60,61 counteranions,40,62−64 or guest molecules.65−67 In the early years of the 21th century, a neutral molecular iron(II) complex having Jäger-type N2O22− tetradentate ligand, [FeIIL(Him)2] (L = {diethyl (E,E)-2,2′-[1,2-phenylbis(iminomethylidyne)]bis[3oxobutanoate](2−)-N,N′,O3,O3′} and HIm = imidazole as axial ligand) have been reported by Weber et al., and it exhibits one of the most desirable properties, namely, reproducible, symmetric, and hysteretic LS ↔ HS SCO spanning RT with ΔT = 70 K.68 This remarkable bistability arises from the 2D NH···O hydrogen-bonding network. Recently, Halcrow has pointed out that “A large structural difference between the high- and low-spin states will lead to strong cooperativity in the solid state, as long as the lattice is sufficiently flexible to accommodate these changes”.69 The supramolecular network of the Webers’ compound is a successful case exemplifying the key point of a crystal engineering design noted by Halcrow. Last year, we have also succeeded to construct the 3D flexible supramolecular network in a neutral molecular iron(II) complex [FeIILMe(NCS)2] (LMe = N,N′-bis[(1-methyl-1,2,3triazol-4-yl)methylene]-propane-1,3-diamine).70 It forms a 3D supramolecular structure solely through intermolecular multiple weak CH···X hydrogen bonds (X = S and additional N), and surprisingly, the compound crystallizes in the HS and LS polymorphs at RT, both showing narrow hysteretic SCO with polymorphism-dependent remarkable T1/2 shift of 100 K that spans RT due to differences of the CH···X hydrogen-bonding assembly. The results indicate that supramolecular assembly through multiple weak hydrogen bonds, which are distinct from conventional hydrogen bonds,71 can bring sufficient flexibility to accommodate structural change between HS and LS molecules and to propagate such change toward the lattice for producing the hysteretic feature. This assembly is also flexible and effective since the modification of T1/2 in a wide temperature range including RT can be achievable by a subtle change of the weak CH···X hydrogen-bonding environment as evidenced by the polymorphism dependence of this system. In the present work, we focus on the capability of the abovementioned 3D multiple CH···S hydrogen-bonding network for accommodating a great structural change of SCO molecules into it. Moreover, we conceive an idea that a great structural change can be brought by concerted multimolecular motions related to the ligand mobility. Hence, in this work we have designed new ligand frameworks having (1) aromatic rings

instead of 1-methyl groups and (2) dimethyl or ethyl substituents into the propylene fragment by the modification of the previously reported compound [FeIILMe(NCS)2] (Figure 1). In this system, an individual molecule possibly has four

Figure 1. Molecular design for the construction of multiple intermolecular CH···S hydrogen-bonding interactions with partial aromatic interactions (a). Possible molecular motions: NCS bending (b), tetradentate ligand biting (c), aromatic ring rotation (d), and propylene fragment oscillation (e). Formulas of the neutral molecular iron(II) complexes reported here (f).

molecular motions such as (1) NCS bending, (2) tetradentate ligand biting, (3) aromatic ring rotation, and (4) propylene fragment oscillation during SCO. Aromatic rings can also construct partial intermolecular aromatic interactions such as π−π and CH···π interactions, into the 3D flexible hydrogenbonding network for propagating ring rotations toward the lattice effectively. Concretely speaking, we have prepared two solvent-free compounds [FeII(ptm2-dmpn)(NCS)2] (1; ptm2dmpn = N,N′-bis[(1-phenyl-1H-1,2,3-triazol-4-yl)methylene]2,2-dimethylpropane-1,3-diamine) and [FeII(p-ttm2-dmpn)(NCS)2] (2; p-ttm2-dmpn = N,N′-bis[(1-p-tolyl-1H-1,2,3triazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine), and two solvatomorphs [FeII(p-ttm2-etpn)(NCS)2]·solvent (p-ttm2etpn = N,N′-bis[(1-p-tolyl-1H-1,2,3-triazol-4-yl)methylene]-1ethylpropane-1,3-diamine, and solvent = 0.5H 2 O and 0.5MeCN·0.5MeOH·H2O for 3a and 3b, respectively). A new family of the present neutral molecular iron(II) complexes surely construct 3D flexible CH···S hydrogenbonding supramolecular network incorporating additional aromatic interactions, accommodating aforementioned molecular motions to various degrees, and showing hysteretic SCO in a wide range of T1/2 (239−409 K) and ΔT (1−31 K) spanning RT depending on a slight modification of small alkyl substituents and lattice solvents. In this contribution, we report an availability of a new crystal engineering approach of introducing concerted multimolecular motions into a 3D flexible network for systematic control of T1/2 and ΔT toward 6007



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up to RT, suggesting the occurrence of thermal hysteresis spanning RT. The infrared (IR) spectra of all compounds were measured at RT (Figure S3). The spectrum of 1 showed strong bands at 2106 and 1594 cm−1, assignable to the NCS stretching mode (νNCS) and the C = N stretching vibration of the Schiff-base ligand (νC=N) in the LS FeII state, respectively.41,70,76,77 The spectra of 3a and 3b also showed similar stronger bands at 2107 and 1605 cm−1, and 2115 and 1606 cm−1, respectively, assignable to the LS FeII state. In addition, when the spectrum of annealed sample of 3b (5 h at 125 °C (398 K) in a Sibata GTO-350D glass tube oven) was measured, absorption bands of 2060 and 1640 cm−1 were increased compared to those of as-synthesized 3b, suggesting the occurrence of spin transition to the HS state. On the other hand, only 2 is essentially HS state at RT, since the IR spectrum showed an intense νNCS band at 2067 cm−1 and the characteristic νC=N absorption at 1635 cm−1. These results are consistent with the sample color at RT. Magnetic Properties. The temperature dependence of the χMT product was measured for all compounds both in the cooling and heating modes in the settle mode (where χM is the molar magnetic susceptibility). The results are shown in Figure 2. The remarkable feature at first viewing is that all compounds

the discovery of unprecedented RT bistable SCO materials by the comprehensive study of the present SCO molecules through SQUID magnetometry, differential scanning calorimetry (DSC), Mössbauer spectroscopy, density functional theory (DFT) calculations, variable-temperature (VT) single crystal Xray studies, and powder X-ray diffraction (PXRD) studies.

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RESULTS AND DISCUSSION Synthesis and Characterization. The 1-R-1H-1,2,3triazole-4-carbaldehyde (R = phenyl or p-tolyl) was prepared through general three steps from commercially available starting reagents, i.e. (1) azidation of aromatic amine,72,73 (2) azide−alkyne click reaction of aromatic azide and 2-propyn-1ol,73,74 and (3) oxidation of alcohol to aldehyde by MnO2.73,75 Subsequently, the tetradentate ligand was prepared by the 2:1 condensation reaction of 1-R-1H-1,2,3-triazole-4-carbaldehyde (R = pheny or p-tolyl), and 2,2-dimethyl-1,3-propanediamine or 1,3-diaminopentane in appropriate solvents, and the resulting ligand solution was used for the syntheses of FeII complex without isolation. Surprisingly, only 1 was obtained in air as dark red-purple needle crystals by slow diffusion of a MeCN solution of NaNCS into a MeOH solution containing the tetradentate ligand ptm2-dmpn and FeIICl2·4H2O. On the other hand, 2 was obtained as red-orange rod-like crystals by mixing a MeOH solution containing the ligand p-ttm2-dmpn, FeIICl2·4H2O and a small amount of L-ascorbic acid with a MeOH solution of NaNCS under nitrogen atmosphere. Fortunately, both 1 and 2 have no crystal solvent confirmed by elemental analyses and thermogravimetric analyses (TGA; Figure S1). Solvatomorphs 3a and 3b were selectively prepared under nitrogen atmosphere by mixing a MeOH solution of Fe(NCS)2 containing a small amount of L-ascorbic acid with a solution of the ligand p-ttm2-etpn in MeOH/DMF and MeOH/MeCN mixed solvent for 3a and 3b, respectively. 3a and 3b were isolated as dark-red platelet crystals and dark-red rod-like crystals, respectively. The formula of both solvatomorphs were also confirmed by elemental analyses and TGA (Figure S1). When the powdered sample of 3a was heated from 30 °C (303 K) at a rate of 5 K min−1, a gradual one-step weight loss was observed, and the total weight loss was 1.3% at 106 °C (379 K) in agreement with the calculated weight percent of 0.5H2O per [FeII(p-ttm2-etpn)(NCS)2]·0.5H2O (1.4%). On the other hand, when the powdered sample of 3b was heated from 30 °C (303 K) at a same heating rate, an abrupt one-step weight loss was observed, and the total weight loss was 8.7% at 71 °C (344 K) in agreement with the calculated weight percent of 0.5MeCN·0.5MeOH·H2O per [FeII(p-ttm2-etpn)(NCS)2]· 0.5MeCN·0.5MeOH·H2O (8.2%). The phase purity of the present compounds was confirmed by PXRD patterns compared to the simulated powder diffractogram from the single-crystal X-ray structural data (Figure S2). 1 showed thermochromism in the solid state from dark purple-red at RT to pinkish-orange at ca. 440 K on a hot plate, while 2 showed color change from pinkish-orange at RT to dark purple-red at liquid nitrogen temperature, suggesting the occurrence of SCO at a different temperature region between two compounds straddled above and below RT. 3a and 3b also showed thermochromism from dark purple-red at RT to brownish-yellow at ca. 390 K for 3a and ca. 370 K for 3b on a hot plate. Interestingly, thermally induced brownish-yellow color of 3b was appreciably retained even after cooling down to RT, and it turned to the initial dark purple-red by cooling in a freezer. Then, the recovered initial color was kept after warming

Figure 2. Temperature dependence of the χMT product of 1 (black), 2 (orange), 3a (blue-violet), and 3b (green) collected in the settle mode in the heating (▲) and cooling (▼) modes. The solid lines are a guide for the eye.

show a complete HS ↔ LS spin transition with a thermal hysteresis at above, below, or just at RT. The SCO behavior of each compound is described in turn as follows. At first, for 1, the χMT value is 0.0 cm3 K mol−1 at 320 K, which indicates an agreement with the theoretical value for LS FeII complex. Upon heating from 320 K, the compound remains in the LS state and undergoes an abrupt LS → HS transition at above RT with T1/2↑ = 409 K. At 418 K, the χMT product is ca. 3.3 cm3 K mol−1, which is in the typical region for the HS FeII complex. Upon further heating, the compound remains in the HS state at least up to 440 K. Upon cooling, a relatively gradual SCO to the LS state is observed at T1/2↓ = 389 K, and a χMT value reaches 0.0 cm3 K mol−1 at 372 K, indicating the occurrence of ΔT = 20 K. Surprisingly, for 2, at RT the χMT product is ca. 3.3 cm3 K mol−1, indicative of a sample being in the HS state, and it shows an abrupt SCO with a narrow hysteresis below RT with T1/2↓ = 239 K, T1/2↑ = 240 K and the resulting ΔT = 1 K. More interestingly, for 3a and 3b, both are in the LS state at RT and show complete SCO with a wide thermal hysteresis at slightly above (for 3a) and just (for 3b) RT with T1/2↓ = 325 K, T1/2↑ = 6008



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Figure 3. Temperature dependence of the χMT product over consecutive thermal cycles for 1 (a) and 2 (b) at a sweep rate of 1 K min−1, and 3a (c) and 3b (d) at a sweep rate of 2 K min−1, respectively. The solid lines are a guide for the eye.

351 K, and ΔT = 26 K for 3a, and T1/2↓ = 293 K, T1/2↑ = 324 K, and ΔT = 31 K for 3b, respectively. The increase of ΔT for all compounds from that of magnetic data in settle mode to magnetic data in sweep mode at 1 K min−1, 2 K min−1 and DSC data at 5 K min−1 (vide infra) indicates the scan rate dependency of thermal spin transition in this system (Table S1). The reproducibility of the hysteresis loop of all compounds was confirmed by additional magnetic measurements of several consecutive thermal cycles (Figure 3). T1/2 and ΔT values in each cycle are listed in Table S1. For 1, only the T1/2↓ is shifted to higher temperature, leading to a slight narrowing of ΔT from first to fifth cycles. After that, T1/2↓ shift is converged with T1/2↓ = 396 K, T1/2↑ = 412 K, and ΔT = 16 K in the sixth cycle. For 2, the magnetic profiles are reproducible in three consecutive thermal cycles. For 3a, both T1/2↑ and T1/2↓ are shifted to lower temperature with ΔT narrowing from first to at least fourth cycles (T1/2↓ = 310 K, T1/2↑= 335 K, and ΔT = 25 K in the fourth cycle). It is noted that the T1/2↑ shift from first to second run is wider than that of further cycles due to a loss of crystal solvents during the first heating process supported by the TGA results. Finally, although 3b shows the T1/2↓ lowering which is similar to that of 3a, the T1/2↑ lowering is only observed from the first to second run related to the desolvation process. As a result, the ΔT of 3b narrows from first to second cycle, then widens and recovers to the initial hysteresis width in the fourth cycle (T1/2↓ = 285 K, T1/2↑ = 322 K, and ΔT = 37 K in the fourth cycle). These reproducible natures of hysteretic SCO are also investigated by DSC studies (vide infra). Mössbauer Spectra. Mössbauer spectra for all compounds were recorded in transmission mode (Figure 4). The spectrum of 1 at 300 K consists of a doublet attributable to the LS FeII

Figure 4. 57Fe Mössbauer spectra of 1, 2, 3a, and 3b at 300 K, and 2 at 220 K. The spectra are deconvoluted into HS and LS sites (red and blue lines, respectively).

species (isomer shift δ = 0.34 mm s−1, quadrupole splitting ΔEQ = 0.74 mm s−1). The spectra of 3a and 3b at 300 K are also similar to that of 1, indicating the LS FeII state (δ = 0.37 mm s−1 and ΔEQ = 0.74 mm s−1 for 3a and δ = 0.36 mm s−1 and ΔEQ = 0.71 mm s−1 for 3b). On the other hand, the spectrum of 2 at 300 K is dominated by an HS Fe(II) doublet with δ = 1.01 mm s−1 and ΔEQ = 1.89 mm s−1 (area ratio = 88%), while possessing a low percentages of LS Fe(II) with δ = 6009



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0.43 mm s−1 and ΔEQ = 0.61 mm s−1 (12%). On lowering the temperature to 220 K, the area ratio of LS species reaches the value of 100% with δ = 0.44 mm s−1 and ΔEQ = 0.38 mm s−1. The ΔEQ value of LS 2 is lower than that of LS 1, 3a, and 3b, indicating a more distorted character of LS octahedron of 2 than that of other complexes,28 which is also evidenced by single crystal X-ray studies (vide infra). As a consequence, different spin states between the LS state for 1, 3a, and 3b and the HS state for 2 at RT and a complete spin conversion to the LS state of 2 at low-temperature is confirmed by these Mössbauer data. DSC Studies. DSC data for all compounds were collected at a sweep rate of 5 K min−1 (Figure 5). The peak top temperatures (Tmax) and overall enthalpy (ΔH) and entropy (ΔS) variations both in the cooling and heating modes are listed in Table S1. For 1, one endothermic and two partially overlapped exothermic peaks with Tmax↑ = 414 K, Tmax↓ = 388, and 397 K of concomitant smaller peak are observed in the first heating to cooling cycle. As the thermal cycling is repeated, exothermic peak at 388 K decreases with an increasing exothermic peak at 397 K until the fifth thermal cycle, and then only a pair of endo- and exothermic peaks with Tmax↑ = 414 K and Tmax↓ = 397 K is detected. Further thermal cycling induces the peak broadening in both heating and cooling directions with a slight shift of Tmax to the opposite directions. In conjunction with these peak shape changes, the ΔH and ΔS values decreases gradually at least until 13 cycles from 30.6 to 28.0 kJ mol−1 and 74.0 to 67.2 J K−1 mol−1, respectively, for the heating mode, and from 30.4 to 28.6 kJ mol−1 and 78.5 to 72.3 J K−1 mol−1, respectively, for the cooling mode. For 2, a pair of sharp endo- and exothermic peaks with Tmax↑ = 240 K and Tmax↓ = 232 K is detected and is reproducible over three thermal cycles. The ΔH and ΔS values are 16.4−16.6 kJ mol−1 and 68.2−69.5 J K−1 mol−1, respectively, for the heating mode, and 16.8−17.0 kJ mol−1 and 72.4−73.3 J K−1 mol−1, respectively, for the cooling mode. For 3a, in the first cooling to heating cycle, there is only an endothermic peak with Tmax↑ = 353 K, ΔH = 19.8 kJ mol−1, and ΔS = 56.1 J K−1 mol−1 possibly due to the spin transition coincided with desolvation. In the second cycle, a pair of two overlapped endo- and exothermic peaks is detected (Tmax↑ = 336 and 341 K of relatively small peak, Tmax↓ = 313 and 324 K of concomitant smaller peak). As the thermal cycling is repeated, the shape of DSC peaks becomes broader in both heating and cooling directions at least until the sixth cycle with lowering of Tmax and the ΔH and ΔS values are 21.7−22.5 kJ mol−1 and 66.2−67.0 J K−1 mol−1, respectively, for the heating mode, and 20.8−22.0 kJ mol−1 and 69.3−70.5 J K−1 mol−1, respectively, for the cooling mode. The occurrence of a pair of two overlapped endo- and exothermic peaks presumably reflects close but somewhat different SCO profiles of two distinct (and also disordered) metal sites confirmed by single crystal X-ray study (vide infra). These ΔH and ΔS values are somewhere in the range of typical values for mononuclear SCO systems (ΔH = 10−20 kJ mol−1 and ΔS = 50−80 J K−1 mol−1),12,78 except for the ΔH values of highertemperature SCO compounds 1 and 3a. That said, the DSC peaks of 1, 2, and 3a are comparable to the T1/2 values of magnetic measurements of them, respectively. Finally, for 3b, in the first cooling to heating cycle, there is only an endothermic peak with Tmax↑ = 369 K, ΔH = 121.4 kJ mol−1, and ΔS = 329.0 J K−1 mol−1. The Tmax↑, ΔH, and ΔS values are not consistent with the T1/2↑ of magnetic result in the first heating mode since this endothermic peak mainly consists

Figure 5. DSC curves over consecutive thermal cycles of 1 (a), 2 (b), 3a (c), and 3b (d) in the heating (red) and cooling (blue) modes at a sweep rate of 5 K min−1.

of a desolvation process of a number of solvent molecules, that is, 0.5MeCN, 0.5MeOH, and one H2O molecules, concealing the SCO peak behind the lower temperature broaden area. In the second cycle, two distinct endothermic peaks (Tmax↑(1) = 269 K and Tmax↑(2) = 318 K) and one exothermic peak (Tmax↓ = 242 K) with overlapped another broad peak ranging from 298 6010



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Table 1. Crystallographic Data for 1, 2, 3a, and 3b

a

complex

1

temperature (K)

150

448

220

250

133

100

spin state

LS

HS

LS

HS

LS

LS

formula formula weight crystal system space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z Dc, g cm−3 μ, mm−1 R1a (I > 2σ(I)) wR2b (all data)

C25H24N10S2Fe 584.51 monoclinic P21/n 7.1683(7) 28.727(3) 13.4456(14) 90 102.5753(13) 90 2702.3(5) 4 1.437 0.748 0.0374 0.0930

P21/c 8.161(9) 27.77(2) 13.115(13) 90 105.419(18) 90 2865(5) 4 1.355 0.705 0.0965 0.2174

C27H28N10S2Fe 612.56 triclinic P1̅ 7.706(16) 12.12(3) 16.21(4) 106.804(14) 94.26(3) 105.02(3) 1381(5) 2 1.473 0.735 0.1685 0.3499

monoclinic P21/c 8.2645(15) 29.772(5) 12.744(2) 90 108.188(2) 90 2979.0(9) 4 1.366 0.682 0.0592 0.1572

C54H58N20OS4Fe2 1243.14 triclinic P1̅ 14.009(5) 14.746(4) 15.181(6) 76.605(12) 84.906(13) 74.296(13) 2935.8(18) 2 1.406 0.694 0.0921 0.2575

C28.5H33.5N10.5O1.5S2Fe 667.13 monoclinic P21/c 14.839(13) 13.809(10) 16.940(14) 90 111.305(15) 90 3234(5) 4 1.370 0.638 0.1518 0.3079

2

3a

3b

R1 = Σ∥Fo| − |Fc∥/Σ|Fo|. bwR2 = [Σw(|Fo2| − |Fc2|)2/Σw|Fo2|2]1/2.

Table 2. Selected Bond Lengths, Angles and Structural Parameters for 1, 2, 3a, and 3ba

a

complex

1

temperature (K)

150

448

220

250

133

100

spin state

LS

HS

LS

HS

LS

LS

average Fe−N, Å Fe−N−C (NCS) bent angle, deg Ntriazole−Fe−Ntriazole bite angle, deg Σ,79 deg Θ,80 deg CShM’sb octahedral volume, Å3

1.949 170.0(1) 177.7(1) 101.25(6)

2.165 173.0(8) 160.0(9) 119.0(2)

1.934 177(1) 170(1) 105.2(6)

2.159 165.5(2) 160.7(3) 117.98(8)

48.6 81.7 0.443 9.739

79.0 203.2 2.142 12.843

50.2 111.5 0.743 9.464

77.4 204.6 2.130 12.772

Fe1: Fe1: Fe2: Fe1: Fe2: Fe1: Fe1: Fe1: Fe1:

2

3a

3b

1.944; Fe2: 1.953 170.5(6), 168.6(4) 157.9(5), 176.0(5) 100.11(18) 100.26(17) 46.3; Fe2: 44.7 80.1; Fe2: 81.4 0.421; Fe2: 0.420 9.678; Fe2: 9.811

1.931 176.5(9) 174.6(9) 101.2(4) 42.5 83.7 0.468 9.470

CShM’s = continuous shape measures. bThe reference shape is the regular octahedron with center.

Figure 6. ORTEP drawing of 1 at 150 K (a), 2 at 220 K (b), 3a at 133 K (c), and 3b at 100 K (d). Covalent bonds of disordered site B of 3a are drawn with broken lines. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are shown with a 30% probability.

cibilities (with slight thermal variation) of the hysteresis loop for all compounds were confirmed by DSC, which are comparable to the magnetic results. Structural Analyses. Crystal structures of solvent-free 1 and 2 in both the LS and HS phases and solvatomorphs 3a and 3b in the LS phase were determined. The crystallographic data are summarized in Table 1. Selected structural parameters for the compounds are given in Table 2 (detailed coordination bond lengths and angles are also given in Table S2). 1 crystallizes in the monoclinic space group P21/n at 150 K and P21/c at 448 K. In this case, the structural phase transition

to 252 K are observed. As the thermal cycling is repeated, overlapped two exothermic peaks are combined into one very broad peak, and then, alteration of endo- and exothermic peaks are not discernible anymore. Since these peaks are broad, ΔH and ΔS values are roughly estimated, that is, ΔH(1) = 1.2−3.9 kJ mol−1 and ΔS(1) = 4.5−14.3 J K−1 mol−1, and ΔH(2) = 5.9− 10.7 kJ mol−1 and ΔS(2) = 18.6−33.8 J K−1 mol−1, for the heating mode and ΔH = 15.2−17.2 kJ mol−1 and ΔS = 62.0− 71.0 J K−1 mol−1, for the cooling mode. These peak profiles suggest that the SCO of 3b is correlated to some kind of interlocking structural phase transition. As a whole, reprodu6011



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Figure 7. CH···S interactions (red dotted line) of 1 at 150 K (a) and 448 K (b). Face-to-face π−π interactions (black dotted line; A and B) of 1 at 150 K (c) and face-to-face π−π and edge-to-face CH···π interactions (black dotted line; A−D) at 448 K (d). Four nearest neighbors connected via aromatic interactions are distinguished by red, blue, green, and magenta colors. Crystal packing of 1 viewed along the bc plane at 150 K (e) and 448 K (f). Color code: green = phenyl group, blue = 2,2-dimethylpropylene fragment, yellow = remaining molecular framework. Hydrogen atoms have been omitted for clarity.

crystal packing (vide infra). The octahedral distortion parameters Σ and Θ,79−81 continuous shape measures (CShM’s) relative to the regular octahedron with the center as the reference shape and octahedral volume for 1 decrease significantly from at 448 K (in the HS state) to at 150 K (in the LS state), also indicating a rearrangement of the N6 coordination sphere to a more regular octahedral geometry upon HS → LS SCO. While HS 2 also shows the decreasing of Σ, Θ, CShM’s and octahedral volume upon HS → LS SCO similar to that of HS 1, ΔΘ = 93.1° and ΔCShM’s = 1.387 are considerably smaller than that of 1 (ΔΘ = 121.5° and ΔCShM’s = 1.699) due to the strong distortion of LS 2. At the same time, ΔΣ and ΔΘ for 1 and 2 are quite larger than that of previously reported analogous complex having similar tetradentate ligand backbone (ΔΣ = ca. 18.0° and ΔΘ = ca. 83.0°).70 One final point about the molecular structures of 1 and 2 is bending of NCS coligands. Fe−N−C bent angles of HS states are generally smaller than those of LS states for both compounds except for the Fe1−N9−C24 angle of 1. In the case of 3a, the Fe−Nave of Fe1 and Fe2 sites at 133 K are 1.944 and 1.953 Å, respectively, indicating that both sites are in the LS state. In addition, these two metal sites have a disordered structure with different manner; namely, the ethyl group of propylene fragment of Fe1 site is disordered over two positions vertically from an equatorial tetradentate ligand plane, while that of Fe2 site is disordered over two positions horizontally from the ligand plane (Figure 6c). In 3b, the Fe−Nave at 100 K is 1.931 Å, which is typical for the LS state. It should be noted that the thermal ellipsoid of the ethyl group in 3b is larger than that of other atoms, indicating the occurrence of a strong thermal motion (vertically from a ligand plane) of the ethyl group even at 100 K (Figure 6d). Finally, structural parameters such as Fe−N−C bent angle, bite angle, Σ, Θ, CShM’s and octahedral volume for 3a and 3b are similar to those of LS 1, reflecting their LS configuration. As an exception,

occurs since lattice constants of both temperature are conformed to the same setting. Interestingly, 2 also shows the structural phase transition from the monoclinic space group P21/c at 250 K to the triclinic space group P1̅ at 220 K. Solvatomorphs 3a and 3b crystallize in a different manner, namely, 3a has the triclinic space group P1̅ at 133 K, while 3b has the monoclinic space group P21/c at 100 K. Unfortunately, the single-crystal X-ray data of 3a and 3b could not be obtained at above RT due to the loss of crystallinity presumably associated with desolvation. The molecular structures are shown in Figure 6. The asymmetric units contain a neutral complex molecule of [FeIIL(NCS)2] except for 3a, which contains the two distinct complex molecules with different disordered structures (vide infra). In addition, in 3a, a H2O molecule per two [FeII(p-ttm2etpn)(NCS)2] molecules is present in the lattice. On the other hand, in 3b, half a MeCN, half a MeOH, and a H2O molecules per [FeII(p-ttm2-etpn)(NCS)2] coexist as disordered solvent molecules in the lattice. In all complexes, the FeII ions assume an octahedral N6 coordination environment including four equatorial N donor atoms of the tetradentate ligand and two apically coordinated N atoms of NCS− coligands. The average Fe−N bond length (here after abbreviated as Fe−Nave) of 1 at 150 K and 2 at 220 K is 1.949 and 1.934 Å, respectively, indicating that both complexes are in the LS state.41,70,77 At higher temperature, the Fe−Nave (2.165 Å for 1 at 448 K and 2.159 Å for 2 at 250 K) are typical for the FeII HS state.41,70,77 In addition to coordination bond lengths, the N3−Fe−N6 bond angle, the so-called “bite angle”, also shows a remarkable difference between the HS and LS states, i.e., 119.0(2)° at 448 K and 101.25(6)° at 150 K, resulting in Δθ = 17.8° for 1, and 117.98(8)° at 250 K and 105.2(6)° at 220 K, resulting Δθ = 12.8° for 2. It is noted that the bite angle difference between HS and LS states of 2 is smaller than that of 1, attributed to the 6012



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Figure 8. CH···S interactions (red dotted line) of 2 at 220 K (a) and 250 K (b). π−π and CH···π interactions (black dotted line; A and B) of 2 at 220 K (c) and 250 K (d). Two nearest neighbors connected via aromatic interactions are distinguished by red and blue colors. Crystal packing of 2 viewed along the bc plane at 220 K (e) and 250 K (f). Color code: green = p-tolyl group, blue = 2,2-dimethylpropylene fragment, yellow = remaining molecular framework. Hydrogen atoms have been omitted for clarity.

combination of face-to-face π−π and edge-to-face CH···π interactions (Figure 7f). It is noted that aforementioned slipping event causes a large variation of bite angle upon SCO (Figures 7a−d and Table 2). The other is the vertical flip of the 2,2-dimethylpropylene fragment of the tetradentate ligand. As a result, an alternate arrangement of neighboring propylene fragments along the c-axis in the LS state (↑···↓···↑···↓···; indicated as red arrows in Figure 7e) changes into the combination of alternate arrangements in the HS state (↑···↑ ···↑··· and ↓···↓···↓···; Figure 7f). As a whole, these variations trigger the structural phase transition, while it is still accommodated in the 3D CH···S network. Upon this HS → LS transition, the cell volume reduces 5.7%. In 2 at 220 K (LS state), in addition to the 3D CH···S network of [FeII(p-ttm2-dmpn)(NCS)2] molecules (Figure 8a and Table S3), additional 1D chains are constructed by intermolecular π−π and C−H···π interactions between neighboring aromatic rings (p-tolyl and a part of triazole rings) along the a-axis (Figure 8c). In the case of 2, at 250 K (HS state), there is only one structural transformation, i.e., the vertical flip of the 2,2-dimethylpropylene fragment of the tetradentate ligand, resulting the rearrangement of propylene fragments in the lattice from ↑···↓···↑···↓··· in the LS state (Figure 8e) to the combination of ↑···↑···↑··· and ↓···↓···↓··· (Figure 8f), which is same as that of 1. The rotation of p-tolyl rings is prohibited since these are locked into the space of the lattice (Figure 8c,d). This packing arrangement also increases the structural distortion of LS 2. As a whole, large molecular distortion and fewer structural transformation of 2 responsible for the quite lowering of T1/2 (ca. 170 K spanning RT) and narrowing of ΔT (ca. 19 K) compared to that of 1, while 2 still shows the hysteretic SCO associated with the structural phase transition. In 3a at 133 K (LS state), at first glance, its assembly structure is similar to that of LS 1, while 3a has two distinct

only the Fe2A−N19A−C53A bent angle of 3a is much smaller than that of other bent angles of 3a and 3b, and is rather close to the value for the HS state. It suggests that the NCS coligands of [FeII(p-ttm2-etpn)(NCS)2] can flexibly bend depending on the nature of the 3D multiple CH···S hydrogen bonding network (vide infra), and these bent angles do not directly affect the spin state of the metal center. Intermolecular Interactions and Crystal Packing with Ligand Mobility. Intermolecular interactions and packing arrangements for the present compounds are described in Figures 7−9, and relevant distances of intermolecular contacts are listed in Table S3−S8. At the beginning, it is noteworthy that all complexes have a 3D supramolecular network via multiple intermolecular CH···S hydrogen-bonding interactions between H atoms of the tetradentate ligand and S atoms of NCS ligands of adjacent molecules both in the HS and LS state. C···S separations of these ranging from 3.52 to 4.09 Å show that individual CH···S interaction is significantly weak.41,70,71 Further, the number of CH···S contacts are different depending on crystal packing, suggesting that the 3D CH···S network of the present system is flexible (Figures 7a,b, 8a,b, and 9a,b,f). In 1 at 150 K (LS state), in addition to the above-mentioned 3D CH···S network (Figure 7a and Table S3), [FeII(ptm2dmpn)(NCS)2] molecules form 2D networks by intermolecular π−π interactions between neighboring aromatic rings (phenyl and triazole rings; Figure 7c and Table S5). In this π−stacked network, phenyl rings are accumulated along the (a + c)/2 direction. As a result, LS 1 has a well-organized 3D packing arrangement (Figure 7e). On the other hand, at 448 K (HS state), two remarkable structural transformations are observed. One is the rotation of one phenyl ring of all molecules (rotational directions are indicated as black arrows in Figure 7c) and subsequent slipping of the rotated ring into the neighboring space of tetradentate ligand embrace (Figure 7d). As a result, new 2D network are constructed by the 6013



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Figure 9. CH···S interactions (red dotted line) of 3a at 133 K for Fe1 site (a) and Fe2 site (b). π−π and CH···π interactions (black dotted line; A− D) of 3a at 133 K for Fe1 site (c) and Fe2 site (d). Four nearest neighbors connected via aromatic interactions are distinguished as in Figure 7. Crystal packing of 3a at 133 K (e). Color code for Fe1 site: green = p-tolyl group, blue = 1-ethylpropylene fragment, yellow = remaining molecular framework. Color code for Fe2 site: light green = p-tolyl group, light blue = 1-ethylpropylene fragment, pink = remaining molecular framework. CH···S interactions of 3b at 100 K (f). Strongly offset π−π (A and B) and CH···π interactions (C) of 3b at 100 K (g). Crystal packing of 3b at 100 K (h). C, N, and O atoms of solvent molecules are indicated by gray, light purple, and red colors, respectively. Hydrogen atoms have been omitted for clarity.

intermolecular π−π and C−H···π interactions are highly slipped (offset), which can make p-tolyl groups rotationable rings in the lattice, (2) there are two large solvent-including spaces which can accommodate large structural change of complex molecule after desolvation, (3) ethyl group of propylene fragment is located near the p-tolyl ring of neighboring molecule and solvent-including void, and a large thermal motion of this ethyl group possibly affects a motion of neighboring p-tolyl ring after desolvation. These features in 3a and 3b are responsible for extending ΔT ca. 30 K and the slightly higher T1/2 of 3a than 3b arises due to the similarity of the assembly structure between LS 3a and LS 1. VT-PXRD Studies for 1, 3a, and 3b. To reveal the thermal cycle dependent variation of HS → LS SCO for 1, VT-PXRDs were measured over three thermal cycles between 370 K (LS state) and 430 K (HS state). First, PXRD patterns at 370 and 430 K in the first heating agree well with the simulated patterns by single crystal X-ray diffraction data in the LS and HS state,

metal sites in an alternate arrangement. On close inspection, there are three additional features: (1) in addition to intermolecular π−π interactions, there are C−H···π interactions, and the resulting overlapped area of the tetradentate ligands are broader than that of LS 1 (Figure 9c,d), (2) there are one vacant and one possibly vacant (including one water molecule as-synthesized) spaces of tetradentate ligand embrace, which can accommodate the possibly rotating and subsequently slipping p-tolyl ring into them (Figure 9e). (3) Disordered 1ethylpropylene fragments gather into a layer along the ab plane, and these disordered fragments possibly affect each other in a strongly flexible manner upon SCO instead of vertical flip motion as observed for 1 and 2 (Figure 9e). On the other hand, in 3b at 100 K (LS state), in addition to the 3D CH···S network (Figure 9f), there is a 3D network constructed of π−π and C− H···π interactions between neighboring molecules, which is a quite different assembly than that of 3a (Figure 9g,h). In the case of 3b, there are three characteristic features: (1) 6014



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Figure 10. VT-PXRD patterns in a thermal cycle from 300 to 400 K for 3a (a) and from 300 to 375 K for 3b (b) as well as the simulated patterns at 133 K for 3a (LS state) and 100 K for 3b (LS state). RT patterns after SQUID measurements (experienced four thermal cycles) for 3a and 3b, which indicate the in situ desolvated LS state, are also represented.

intrinsic ligand field strength to determine the spin states of the complexes. First, we performed single point DFT energy calculations of both HS and LS states on experimental crystal structures at RT (Table S9). The HS−LS energy difference (ΔEHS−LS) for 2 (−111.3 kJ mol−1) reveals that the HS state is more stable. On the other hand, the ΔEHS−LS values are 100.4, 107.9, 90.0, and 132.6 kJ mol−1 for 1, Fe1 site of 3a, the other site of 3a and 3b, respectively, suggesting that the LS states are more stable in these complexes. These tendencies are very consistent with the single crystal X-ray structure analyses, magnetic properties, and Mössbauer spectra. In addition, we have performed DFT geometry optimizations of all LS complexes for a better understanding of the crystal packing effect. The optimized structures are shown in Figure S5, and the geometrical parameters in DFT optimized structures are summarized in Table S10. The Fe−Nave bond lengths are 1.962, 1.963, 1.961, 1.961, and 1.961 Å for 1, 2, Fe1 site of 3a, the other site of 3a, and 3b, respectively. All Fe−Nave bond lengths are similar to each other irrespective of the alkyl substituents R1−R3. In addition, the ΔEHS−LS values for these DFT optimized structures (Table S9) are also similar to each other, which are 76.6, 76.6, 79.1, 77.8, and 79.9 kJ mol−1 for 1, 2, Fe1 site of 3a, the other site of 3a, and 3b, respectively. These results indicate that all ligands studied here have almost the same ligand field strength. We also performed DFT partial geometry optimizations on model complexes x−y (x and y = 1, 2, 3a, and 3b), which were built by replacing substituents R1−R3 in the experimental structure of x with those of y. Only atomic coordinates of the replaced substituents were optimized, and other atoms were kept fixed during the optimization. The ΔEHS−LS values for model complexes (Table S11) depend only on x and are hardly affected by the replacement of the substituents. The above analyses revealed that the electronic states of the complexes were dominantly determined via a crystal packing effect.

respectively (Figure S4). In a further thermal cycling, only a peak broadening is observed at 370 K, while some additional peaks similar to that of at 370 K are observed at 430 K (e.g., 2θ = 9.24, 13.1, 13.38, 16.04 and 26.14° in the third heating). These correspond to the thermal cycle dependent variation of T1/2↓ and Tmax↓ of magnetic and DSC results, respectively, and may be due to the increase of the fraction of LS-like molecular assembly in the high-temperature HS structure upon consecutive LS → HS transitions. In the absence of the single-crystal X-ray diffraction data for 3a and 3b in the HS state, VT-PXRDs of 3a and 3b were measured in a thermal cycle from RT to high temperature, namely, 300, 319, 340, 351, and 400 K for 3a (Figure 10a) and 300, 313, 330, and 375 K for 3b (Figure 10b). In 3a, the PXRD pattern at 400 K is obviously different from that of at 300 K indicates the occurrence of the structural phase transition upon LS → HS SCO. After cooling from 400 to 300 K, the peak pattern perfectly turns to the initial pattern, showing the reversibility of the structural phase transition. The patterns at 300 K before and after thermal treatment in the PXRD device are also similar to the data at 300 K after SQUID measurement (experienced four thermal cycles), also indicating that in situ desolvation does not alter the crystal packing. Finally, the hysteretic nature of the SCO of 3a is structurally confirmed by the difference of the patterns between in the heating and in the cooling direction at 340 and 351 K. In 3b, the PXRD pattern at 375 K is obviously different from that of at 300 K, which indicates the occurrence of the structural phase transition upon LS → HS SCO. During the cooling direction from 375 to 300 K, the patterns remain constant, indicating the retention of the HS structure at 300 K. The data at 300 K after SQUID measurement (LS state; experienced four thermal cycles) is different from that at 300 K of the as-synthesized sample, suggesting that the occurrence of the desolvation-induced phase transition. DFT Calculations. DFT calculations for all complexes were performed for understanding the effect of crystal packing versus 6015



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Schlenk techniques. Other synthetic procedures were carried out in air. Elemental C, H, and N analyses were performed on a J-Science Lab MICRO CORDER JM-10. IR spectra were recorded at RT using either a JASCO FT/IR 460Plus spectrophotometer or a PerkinElmer Spectrum100 FT-IR spectrometer with the samples prepared as KBr disks. 1H NMR spectra were recorded on a JEOL ECA-600 spectrometer. The melting point was measured through a Yanaco MP-S3 micro melting point meter and was uncorrected. Preparation of {1-(4-Methylphenyl)-1H-1,2,3-triazole-4-yl}methanol (b). Hydrochloric acid (10%) (16 mL) was added to a solid of p-toluidine (2.143 g, 20 mmol), and the mixture was cooled to 3 °C in an ice bath. To the mixture was added a solution of NaNO2 (1.656 g, 24 mmol) in H2O (10 mL) dropwise, and then a solution of NaN3 (1.365 g, 21 mmol) in H2O (10 mL) dropwise at 3 °C in an ice bath, and the mixture was stirred at RT for 2 h. The reaction mixture was extracted with CH2Cl2 (3 × 10 mL); the extracts were washed with H2O (30 mL), dried (MgSO4), filtered, and concentrated under reduced pressure to give an oily product of 4-methylphenyl azide (a). To a solution of a in MeOH (10 mL), 2-propyn-1-ol (1.121 g, 20 mmol), powdered CuIISO4·5H2O (0.250 g, 1 mmol), and a solution of sodium ascorbate (0.396 g, 2 mmol) in H2O (10 mL) were added, and the mixture was stirred at RT for 24 h. The reaction mixture was evaporated under reduced pressure to obtain the crude products. The crude products were recrystallized from MeOH (60 mL), during which time the pale orange needle crystals were precipitated. They were collected by suction filtration and dried in vacuo. Yield: 2.484 g (66%). Mp 127−128 °C (lit.,82 124−125 °C), 1H NMR (600 MHz; CDCl3; Me4Si): δ 7.96 (s, 1H), 7.59 (d, 2H, J = 8.2 Hz), 7.31 (d, 2H, J = 8.2 Hz), 4.89 (d, 2H, J = 2.8 Hz), 2.58 (s, 1H), 2.43 (s, 3H). Anal. Calcd for C10H11N3O: C, 63.48; H, 5.86; N, 22.21. Found: C, 63.21; H, 5.77; N, 22.06%. Preparation of 1-(4-Methylphenyl)-1H-1,2,3-triazole-4-carbaldehyde (c). MnO2 (90%) (9.660 g, 100 mmol) was added to a solution of b (1.892 g, 10 mmol) in acetone (50 mL), and the mixture was stirred at RT for 46 h. The reaction mixture was passed through a Celite pad, and the filtrate was evaporated under reduced pressure to obtain the white solids. They were collected by suction filtration and dried in vacuo. Yield: 1.453 g (78%). mp 109−110 °C (lit.,83 105−106 °C), 1H NMR (600 MHz; CDCl3; Me4Si): δ 10.23 (s, 1H), 8.48 (s, 1H), 7.64 (d, 2H, J = 8.2 Hz), 7.37 (d, 2H, J = 8.2 Hz), 2.45 (s, 3H). Anal. Calcd for C10H9N3O: C, 64.16; H, 4.85; N, 22.45. Found: C, 63.89; H, 4.75; N, 22.50%. Preparation of [FeII(ptm2-dmpn)(NCS)2] (1). 2,2-Dimethyl-1,3propanediamine (0.204 g, 2 mmol) in MeOH (10 mL) was added to a solution of 1-phenyl-1H-1,2,3-triazole-4-carbaldehyde (0.693 g, 4 mmol) in MeOH (20 mL). The resulting mixture was stirred at RT for 1.5 h. To a solution of the ligand (2 mmol) thus prepared, a solution of FeIICl2·4H2O (0.398 g, 2 mmol) in MeOH (50 mL) was added, and the resulting solution was further stirred at RT for 10 min and then filtered. Dark red-purple needle crystals were obtained by slow diffusion of the MeCN solution (80 mL) of NaNCS (0.324 g, 4 mmol) into the filtrate (liquid−liquid diffusion) for 6 days at RT. Yield: 0.309 g (26%). Anal. Calcd for [FeII(ptm2-dmpn)(NCS)2] (1) = C25H24N10S2Fe: C, 51.37; H, 4.14; N, 23.96. Found: C, 51.25; H, 4.08; N, 23.98%. IR (KBr): νC=N 1594, 1644, νNCS 2061, 2106 cm−1. Preparation of [FeII(p-ttm2-dmpn)(NCS)2] (2). 2,2-Dimethyl1,3-propanediamine (0.102 g, 1 mmol) in MeOH (5 mL) was added to a suspension of c (0.374 g, 2 mmol) in MeOH (10 mL). The resulting mixture was stirred at RT for 3 h. To a solution of the ligand (1 mmol) thus prepared, a mixture of FeIICl2·4H2O (0.199 g, 1 mmol) in MeOH (5 mL) and L-ascorbic acid (0.018 g, 0.1 mmol) in MeOH (2 mL) was added, and the resulting solution was further stirred at RT for 1 h and then filtered. The filtrate and the MeOH solution (15 mL) of NaNCS (0.162 g, 2 mmol) were mixed under nitrogen atmosphere, and the resulting mixture was left to stand for 8 days at RT, during which the precipitated red-orange rod-like crystals were collected by suction filtration. Yield: 0.240 g (39%). Anal. Calcd for [FeII(p-ttm2dmpn)(NCS)2] (2) = C27H28N10S2Fe: C, 52.94; H, 4.61; N, 22.87. Found: C, 52.78; H, 4.66; N, 22.83%. IR (KBr): νC=N 1635, νNCS 2067 cm−1.

CONCLUSIONS In conclusion, we have reported a new family of neutral molecular iron(II) complexes [FeIIL(NCS)2] (1, 2, 3a, and 3b) having a flexible 3D CH···S hydrogen-bonding supramolecular network incorporating an additional orderly dimensional structure by aromatic interactions (i.e., π−π and CH···π interactions). While these molecules essentially favor the LS state in the gas-phase (individual molecule) confirmed by DFT calculations, they show hysteretic SCO in a wide range of T1/2 (239−409 K) and ΔT (1−31 K) spanning RT depending on a slight modification of small alkyl substituents and lattice solvents in the solid state. In the case of 1 and 2, we could access the deep insight of our new concept, that is, a crystal engineering approach of introducing concerted multimolecular motions into a 3D flexible network for systematic control of T1/2 and ΔT. First, both 1 and 2 crystallize as a 3D supramolecular assembly constructed of multiple weak CH···S hydrogen bonds with additional aromatic interactions. In this network, both have the same flipping motion of the 2,2-dimetylpropane moiety in the crystal lattice upon SCO, inducing the structural phase transition. In addition, NCS bending is also observed which may adjust the 3D CH···S network for accommodating the large molecular shape change by large molecular motions into this flexible network. These features have an important role for exhibiting abrupt SCO with hysteresis. Second, 1 has an aromatic ring rotation upon SCO, inducing the drastic rearrangement of π-stacking assembly, which is not observed in 2 since the space of tetradentate ligand plane of 2 is tightly occupied by p-tolyl group of neighboring molecule. This difference also affects the bite angle variation, that is, in 1, phenyl group rotation and subsequent slipping of the phenyl group into the space of tetradentate ligand plane cause the remarkable bite angle expansion, while the expansion of the bite angle of 2 is moderate. These features have a significant role for enhancing the cooperativity to widen a hysteresis width in 1. The p-tolyl groups of 2 trapped in the lattice with a highly tilted manner possibly enhance the molecular distortion which destabilizes the LS structure. As a result, the T1/2 of 2 drastically shifts to the lower temperature region below RT. We have also attempted to confirm the effectiveness of the crystal design by changing the 2,2-dimethylpropylene fragment of less cooperative 2 into the asymmetrical 1-ethylpropyrene fragment. As a result, the obtained two solvatomorphs 3a and 3b show a wide ΔT (ca. 30 K) at just or slightly above RT, and these wide ΔT values are retained after desolvation and several thermal cycles. Although we could not determine the HS structure of 3a and 3b, the LS crystal structure of both indicates the mobility of the 1-ethylpropyrene fragment, and hightemperature PXRD patterns of both suggest that the concerted molecular motions associated with the structural phase transition exist in the lattice. As a whole, the present system succeeds both the wide range control and fine-tuning of T1/2 and ΔT and leads us to the finding of the new RT bistable compound 3b.

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Syntheses. All reagents and solvents were purchased from commercial sources and used for the syntheses without further purification. 1-Phenyl-1H-1,2,3-triazole-4-carbaldehyde was prepared according to the literature method70 which was constructed by the reported procedures.72,74,75 Complexation and crystallization of 2, 3a, and 3b were performed under nitrogen atmosphere using standard 6016



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Preparation of [FeII(p-ttm2-etpn)(NCS)2]·0.5H2O (3a). 1,3Diaminopentane (0.102 g, 1 mmol) in MeOH (15 mL) was added to a solution of c (0.374 g, 2 mmol) in DMF (5 mL). The resulting mixture was stirred at RT for 1 h. L-Ascorbic acid (0.018 g, 0.1 mmol) in MeOH (5 mL) was added to a solution of FeIISO4·7H2O (0.278 g, 1 mmol) in MeOH (5 mL) followed by the addition of KNCS (0.194 g, 2 mmol) in MeOH (10 mL), and the resulting mixture was stirred at RT for 3 min. The reaction mixture was filtered, and the filtrate and the ligand solution in a MeOH/DMF mixture (3:1, 20 mL) were mixed under nitrogen atmosphere. The resulting mixture was left to stand for 9 days at RT, during which the precipitated dark-red platelet crystals were collected by suction filtration. Yield: 0.188 g (30%). Anal. C a l c d f o r [ Fe I I ( p - t t m 2 - e t p n ) ( N C S ) 2 ] · 0 . 5 H 2 O ( 3 a ) = C27H29N10O0.5S2Fe: C, 52.17; H, 4.70; N, 22.53. Found: C, 51.91; H, 4.53; N, 22.48%. IR (KBr): νC=N 1605, 1638, νNCS 2061, 2107 cm−1. Preparation of [FeII(p-ttm2-etpn)(NCS)2]·0.5MeCN·0.5MeOH· H2O (3b). 1,3-Diaminopentane (0.102 g, 1 mmol) in MeOH (10 mL) was added to a suspension of c (0.374 g, 2 mmol) in MeCN (4.5 mL). The resulting mixture was stirred at RT for 1 h. L-Ascorbic acid (0.018 g, 0.1 mmol) in MeOH (5 mL) was added to a solution of FeIISO4· 7H2O (0.278 g, 1 mmol) in MeOH (5 mL) followed by the addition of KNCS (0.194 g, 2 mmol) in MeOH (5.5 mL), and the resulting mixture was stirred at RT for 3 min. The reaction mixture was filtered, and the filtrate and the ligand solution in a MeOH/MeCN mixture (2:1, 14.5 mL) were mixed under nitrogen atmosphere. The resulting mixture was left to stand for 15.5 h at RT, during which the precipitated dark-red rod-like crystals were collected by suction filtration. Yield: 0.394 g (59%). Anal. Calcd for [FeII(p-ttm2etpn)(NCS)2]·0.5MeCN·0.5MeOH·H2O (3b) = C28.5H33.5N10.5O1.5S2Fe: C, 51.31; H, 5.06; N, 22.05. Found: C, 50.88; H, 4.79; N, 21.99%. IR (KBr): νC=N 1606, 1644, νNCS 2062, 2115 cm−1. TGA. TGA data were collected on a TG/DTA6300 (SII Nano Technology Inc.) instrument at a rate of 10 K min−1 for 1 and 2 and 5 K min−1 for 3a and 3b, respectively, under a nitrogen atmosphere (200 mL min−1). PXRD Studies. PXRD patterns were recorded at RT on polycrystalline powders deposited on a glass plate, using a Mac Science MXP3 V diffractometer at Cu Kα (λ = 1.5418 Å) radiation operated at 1.8 kW power (40 kV, 45 mA). A small amount of samples after SQUID measurements was placed on a nonreflecting silicon plate, and PXRD patterns were recorded at RT using a Rigaku MiniFlex600 diffractometer with Cu Kα radiation operated at 0.4 kW power (40 kV, 10 mA). VT-PXRD patterns were recorded on polycrystalline powders placed on a platinum plate, using a Rigaku Smart lab diffractometer with a high-temperature oven at Cu Kα radiation operated at 9 kW power (45 kV, 200 mA) at Tokushima University, Tokushima, Japan. The simulated patterns were generated by the Mercury program.84 Magnetic Measurements. Magnetic susceptibility data were collected using either a MPMS-7 or a MPMS XL7 SQUID magnetometer (Quantum Design) applied field of 0.5 or 1 T at Institute for Molecular Science (IMS), Okazaki, Japan. A special heating setup of a sample space oven option was used for the measurements of 1 in a high-temperature region. Quartz glass tubes with a small amount of glass wool filler were used as sample containers. The sample was wrapped in an aluminum foil except for 2 and was then inserted into the quartz glass tube. The calibration was done with palladium metal. Corrections for diamagnetism of the sample were made using Pascal’s constants,85 and a background correction for the sample holder was applied. Mö ssbauer Spectroscopy. Mössbauer spectra were recorded using a conventional spectrometer (WissEl GmbH or Topologic Systems Inc.) and a proportional counter at Nagoya Institute of Technology (NIT), Nagoya, Japan. 57Co(Rh) moving in a constant acceleration mode was used as the radioactive source. Hyperfine parameters were obtained by a least-squares fitting of the Lorentzian peaks. Isomer shifts were reported relative to iron foil at 300 K. The

sample temperature was controlled by means of a liquid nitrogen transfer refrigerator (Oxford Instruments) within an accuracy of ±1 K. DSC. DSC measurements were performed with a DSC6200 (SII Nano Technology Inc.) at a rate of 5 K min−1 under a nitrogen atmosphere (30 mL min−1) using aluminum hermetic pans with an empty pan as a reference. Single Crystal X-ray Diffraction Studies. X-ray diffraction data were collected by a Rigaku AFC7R Mercury CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71075 Å) operated at 5 kW power (50 kV, 100 mA) for 1, 2, and 3a, or Mo Kα radiation operated at 0.8 kW power (50 kV, 16 mA) through a confocal mirror for 3b at IMS, Okazaki, Japan. The temperature of the crystal was maintained at the selected value by means of a Rigaku cooling device (for 1, 2, and 3a) or a Japan Thermal Engineering XR-HR10K cryostat (for 3b) with liquid nitrogen flow to within an accuracy of ±2 K. A crystal was mounted on a glass fiber for 1, 2, and 3a, and on a cryoloop for 3b with liquid paraffin. In 1, first, the diffraction data were collected at 150 K. Following the measurement at 150 K, the crystal was then warmed to 448 K, and the subsequent measurements were performed. In 2, the diffraction data were collected at 250 K. And then, the crystal was rapidly cooled to 220 K, and the subsequent measurements were performed. The diffraction data of 3a and 3b were collected at 133 and 100 K, respectively, to avoid a loss of crystal solvents. The data were corrected for Lorentz, polarization, and absorption effects. The structures were solved by the direct method86,87 and refined on F2 data using the full-matrix least-squares algorithm using SHELXL201488 with anisotropic displacement parameters for non-hydrogen atoms except for atoms of solvent molecules of 3b. 3b has two solvent sites and one site was refined as a mixture of H2O and MeCN molecules with the occupancies of 0.5 (0.25 × 2): 0.5, and the other site was refined as a mixture of H2O and MeOH molecules with the occupancies of 0.5 (0.25 × 2): 0.5. Non-hydrogen atoms of these crystal solvents were refined isotropically. Hydrogen atoms were fixed in calculated positions except for solvent molecules and refined by using a riding model. 3a has two distinct metal sites and ethylsubstituted propylene fragments of both were disordered over two possible conformations. The occupancy factors for the possible two positions of C atoms (C12A−C14A, C39A, C40A: C12B−C14B, C39B, C40B = 0.506:0.494) were refined using the tools available from the SHELXL-2014 program package. All calculations were performed by using the Yadokari-XG software package.89 The CShM’s of the FeII centers were calculated by SHAPE 2.1.90 The octahedral volumes of the FeII centers were calculated by OLEX2.91 Computational Details. DFT calculations were performed in gasphase by using (U)PBE1PBE functional and SVP basis set for all atoms except for iron. SDD pseudo potential was adopted for iron. All DFT calculations were performed with the aid of GAUSSIAN 09 program. 92 We used the structures of single crystal X-ray crystallography as the initial geometries for geometry optimizations. Normal mode analyses were performed on the fully optimized geometries to characterize them, and we confirmed that all DFT optimized geometries have no imaginary frequencies. The stabilities of wave functions were confirmed with the Stable=Opt keyword.

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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.cgd.7b01141. TG/DTA curves, PXRD patterns, IR spectra, SCO parameters, coordination bond lengths and angles, distances of intermolecular contacts, and parameters and optimized structures of DFT calculation (PDF) Accession Codes

CCDC 1550391−1550396 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 6017



Article

(23) Kröber, J.; Codjovi, E.; Kahn, O.; Grolière, F.; Jay, C. J. Am. Chem. Soc. 1993, 115, 9810−9811. (24) Kahn, O.; Martinez, C. J. Science 1998, 279, 44−48. (25) Roubeau, O.; Castro, M.; Burriel, R.; Haasnoot, J. G.; Reedijk, J. J. Phys. Chem. B 2011, 115, 3003−3012. (26) Bauer, W.; Schlamp, S.; Weber, B. Chem. Commun. 2012, 48, 10222−10224. (27) Lochenie, C.; Bauer, W.; Railliet, A. P.; Schlamp, S.; Garcia, Y.; Weber, B. Inorg. Chem. 2014, 53, 11563−11572. (28) Dîrtu, M. M.; Naik, A. D.; Rotaru, A.; Spinu, L.; Poelman, D.; Garcia, Y. Inorg. Chem. 2016, 55, 4278−4295. (29) Garcia, Y.; Bravic, G.; Gieck, C.; Chasseau, D.; Tremel, W.; Gütlich, P. Inorg. Chem. 2005, 44, 9723−9730. (30) Agusti, G.; Muñoz, M. C.; Gaspar, A. B.; Real, J. A. Inorg. Chem. 2008, 47, 2552−2561. (31) Martínez, V.; Gaspar, A. B.; Muñoz, M. C.; Bukin, G. V.; Levchenko, G.; Real, J. A. Chem. - Eur. J. 2009, 15, 10960−10971. (32) Kang, S.; Shiota, Y.; Kariyazaki, A.; Kanegawa, S.; Yoshizawa, K.; Sato, O. Chem. - Eur. J. 2016, 22, 532−538. (33) Murphy, M. J.; Zenere, K. A.; Ragon, F.; Southon, P. D.; Kepert, C. J.; Neville, S. M. J. Am. Chem. Soc. 2017, 139, 1330−1335. (34) Halder, G. J.; Kepert, C. J.; Moubaraki, B.; Murray, K. S.; Cashion, J. D. Science 2002, 298, 1762−1765. (35) Bonhommeau, S.; Molnár, G.; Galet, A.; Zwick, A.; Real, J. A.; McGarvey, J. J.; Bousseksou, A. Angew. Chem., Int. Ed. 2005, 44, 4069− 4073. (36) Ohba, M.; Yoneda, K.; Agustí, G.; Muñoz, M. C.; Gaspar, A. B.; Real, J. A.; Yamasaki, M.; Ando, H.; Nakao, Y.; Sakaki, S.; Kitagawa, S. Angew. Chem., Int. Ed. 2009, 48, 4767−4771. (37) Southon, P. D.; Liu, L.; Fellows, E. A.; Price, D. J.; Halder, G. J.; Chapman, K. W.; Moubaraki, B.; Murray, K. S.; Létard, J.-F.; Kepert, C. J. J. Am. Chem. Soc. 2009, 131, 10998−11009. (38) Muñoz-Lara, F. J.; Gaspar, A. B.; Aravena, D.; Ruiz, E.; Muñoz, M. C.; Ohba, M.; Ohtani, R.; Kitagawa, S.; Real, J. A. Chem. Commun. 2012, 48, 4686−4688. (39) Leita, B. A.; Moubaraki, B.; Murray, K. S.; Smith, J. P. Polyhedron 2005, 24, 2165−2172. (40) Yamada, M.; Hagiwara, H.; Torigoe, H.; Matsumoto, N.; Kojima, M.; Dahan, F.; Tuchagues, J.-P.; Re, N.; Iijima, S. Chem. - Eur. J. 2006, 12, 4536−4549. (41) Bréfuel, N.; Shova, S.; Lipkowski, J.; Tuchagues, J.-P. Chem. Mater. 2006, 18, 5467−5479. (42) Nishi, K.; Matsumoto, N.; Iijima, S.; Halcrow, M. A.; Sunatsuki, Y.; Kojima, M. Inorg. Chem. 2011, 50, 11303−11305. (43) Abhervé, A.; Clemente-León, M.; Coronado, E.; Gómez-García, C. J.; López-Jordà, M. Dalton Trans. 2014, 43, 9406−9409. (44) Fujinami, T.; Koike, M.; Matsumoto, N.; Sunatsuki, Y.; Okazawa, A.; Kojima, N. Inorg. Chem. 2014, 53, 2254−2259. (45) Zheng, S.; Reintjens, N. R. M.; Siegler, M. A.; Roubeau, O.; Bouwman, E.; Rudavskyi, A.; Havenith, R. W. A.; Bonnet, S. Chem. Eur. J. 2016, 22, 331−339. (46) Li, Z.-Y.; Ohtsu, H.; Kojima, T.; Dai, J.-W.; Yoshida, T.; Breedlove, B. K.; Zhang, W.-X.; Iguchi, H.; Sato, O.; Kawano, M.; Yamashita, M. Angew. Chem., Int. Ed. 2016, 55, 5184−5189. (47) Hayami, S.; Gu, Z. -z.; Yoshiki, H.; Fujishima, A.; Sato, O. J. Am. Chem. Soc. 2001, 123, 11644−11650. (48) Real, J. A.; Gaspar, A. B.; Muñoz, M. C. Dalton Trans. 2005, 2062−2079. (49) Schäfer, B.; Rajnák, C.; Šalitroš, I.; Fuhr, O.; Klar, D.; SchmitzAntoniak, C.; Weschke, E.; Wende, H.; Ruben, M. Chem. Commun. 2013, 49, 10986−10988. (50) Iasco, O.; Rivière, E.; Guillot, R.; Buron-Le Cointe, M.; Meunier, J.-F.; Bousseksou, A.; Boillot, M.-L. Inorg. Chem. 2015, 54, 1791−1799. (51) Zhang, X.; Xie, H.; Ballesteros-Rivas, M.; Wang, Z.-X.; Dunbar, K. R. J. Mater. Chem. C 2015, 3, 9292−9298. (52) Arata, S.; Torigoe, H.; Iihoshi, T.; Matsumoto, N.; Dahan, F.; Tuchagues, J.-P. Inorg. Chem. 2005, 44, 9288−9292.

Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hiroaki Hagiwara: 0000-0003-0396-7965 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the Koshiyama Research Grant and by Gifu University (GU) for the promotion of the general research (to H.H.). A part of this work was conducted in IMS and NIT, supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors would like to thank Prof. O. Sakurada and Assoc. Prof. N. Katsuta (GU, Japan) for their assistance collecting PXRD data. This work was supported by JSPS KAKENHI Grant Numbers 16K17851 (to T.U.). Portion of the computations were performed at Research center for Computational Science (RCCS), Okazaki.

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Neutral Molecular Iron(II) Complexes Showing Tunable Bistability at

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