In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV

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

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In Situ Ligand Transformation for Two-Step Spin Crossover in FeII[MIV(CN)8]4− (M = Mo, Nb) Cyanido-Bridged Frameworks Shintaro Kawabata,† Szymon Chorazy,‡ Jakub J. Zakrzewski,‡ Kenta Imoto,† Takashi Fujimoto,† Koji Nakabayashi,† Jan Stanek,§ Barbara Sieklucka,‡ and Shin-ichi Ohkoshi*,† †

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Krakow, Poland § Institute of Physics, Jagiellonian University, Łojasiewicza 11, 30-348 Krakow, Poland

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 04/19/19. For personal use only.

‡

S Supporting Information *

ABSTRACT: We report a unique synthetic route toward the multistep spin crossover (SCO) effect induced by utilizing the partial ligand transformation during the crystallization process, which leads to the incorporation of three different FeII complexes into a single coordination framework. The 3acetoxypyridine (3-OAcpy) molecules were introduced to the self-assembled FeII−[MIV(CN)8]4− (M = Mo, Nb) system in the aqueous solution which results in the partial hydrolysis of the ligand into 3-hydroxypyridine (3-OHpy). It gives two novel isostructural three-dimensional {FeII 2(3-OAcpy) 5 (3-OHpy)3[MIV(CN)8]}·nH2O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) coordination frameworks. They exhibit an unprecedented cyanido-bridged skeleton composed of {Fe3M2}n coordination nanotubes bonded by additional Fe complexes. These frameworks contain three types of Fe sites differing in the attached organic ligands, [Fe1(3-OAcpy)4(μ-NC)2], [Fe2(3-OHpy)4(μ-NC)2], and [Fe3(3-OAcpy)3(3OHpy)(μ-NC)2], which lead to the thermal two-step FeII SCO, as proven by X-ray diffraction, magnetic susceptibility, UV− vis−NIR optical absorption, and 57Fe Mössbauer spectroscopy studies. The first step of SCO, going from room temperature to the 150−170 K range with transition temperatures of 245(5) and 283(5) K for FeMo and FeNb, respectively, is related to Fe1 sites, while the second step, occurring at the 50−140 K range with transition temperatures of 70(5) and 80(5) K for FeMo and FeNb, respectively, is related to Fe2 sites. The Fe3 site with both 3-OAcpy and 3-OHpy ligands does not undergo the SCO at all. The observed two-step SCO phenomenon is explained by the differences in the ligand field strength of the Fe complexes and the role of their alignment in the coordination framework. The simultaneous application of two related pyridine derivatives is the efficient synthetic route for the multistep FeII SCO in the cyanido-bridged framework which is a promising step toward rational design of advanced spin transition molecular switches.

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INTRODUCTION

exhibiting strong elastic interaction between spin centers which favors cooperative spin transition with a thermal hysteresis loop, enabling the memory effect.15,16 Recently, particular attention is gained by the stepwise SCO, opening the possibility to improve the information storage capacity.17,18 There are two main routes toward multistep spin transition: the presence of structurally inequivalent SCO sites with different transition temperatures,19,20 and the spontaneous generation of distinct SCO sites by exploring elastic interactions in the crystal lattice, which is usually accompanied by symmetry breaking in the intermediate phase.21−26 Spin crossover phenomenon has been widely investigated in mononuclear complexes of transition metal ions with the d4− d7 valence configuration surrounded by the appropriate ligand

Investigation of molecule-based switches, which magnetic, electronic, optical, or mechanical properties are controlled by external stimuli, such as light, temperature, or pressure, is a hot topic in modern materials science.1−6 Among them, a plethora of switching phenomena were presented for spin crossover (SCO) complexes, in which external stimuli drive a phase transition between two accessible spin states.7−9 The SCO effect originates from the energy gap between the high-spin (HS) and low-spin (LS) states which is of the order of magnitude of the thermal energy, and, thus, it can be overcome by thermal treatment, pressure gradient, light irradiation, and chemical stimuli.8−11 Moreover, the change in the spin-state results in the dramatic shift in magnetic, optical, and electrical properties, making the SCO systems attractive for applications in chemical sensors, data storage materials, and display devices.12−14 Of special interest are the SCO materials © XXXX American Chemical Society

Received: February 6, 2019

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DOI: 10.1021/acs.inorgchem.9b00361 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry field. 27,28 Besides a series of Co II , Mn III , and Fe III complexes,28−30 the extensive studies have been devoted to octahedral FeII complexes, showing the transition between a paramagnetic and weakly colored HS state, [(t2g)4(eg)2], and a diamagnetic and strongly colored LS state, [(t2g)6(eg)0].27,31−33 Such attractive FeII centers were applied for the construction of coordination frameworks and clusters,34−37 which resulted in a diversity of SCO effects manipulated by external stimuli.8,10,36 In this regard, a great potential has been found in combination of FeII with anionic polycyanidometallates, which produces heterometallic cyanido-bridged frameworks exhibiting efficient externally tunable SCO.38−40 It originates from the cyanide ligands coordinating FeII sites by N atoms which, together with supporting organic ligands, ensures the appropriate ligand field for SCO to occur.31 The great achievements in this area were found for two- and three-dimensional Hofmann-type frameworks built of tetracyanido- [MII(CN)4]2− (M = Ni, Pd, Pt) or dicyanido- [MI(CN)2]− (M = Ag, Au) anions and FeII units showing the remarkable spin-state switching programmable by the reversible inclusion of guest molecules,38,39,41,42 sometimes along with the multistep SCO,20,22 and photoinduced SCO realized by light-induced spin-state trapping (LIESST) phenomenon.43,44 Octacyanidometallates [MIV/V(CN)8]4−/3− (M = Mo, W, Re, Nb) are promising prerequisites for SCO materials because they offer up to eight cyanide bridges, resulting in a diversity of coordination topologies.45−47 These large cyanide metal complexes are useful in construction of functional moleculebased materials,47−50 especially molecular magnets combining magnetic phenomena (spin ordering, slow magnetic relaxation, etc.) with extra functionalities, including luminescence,51,52 optical activity,53,54 second harmonic generation,55 and proton conductivity.56 [M(CN)8]-based heterometallic frameworks have been fruitfully used for synthesis of molecule-based switches, exploring magnetic sponge-like behavior, thermo- or photoswitchable charge-transfer phase transitions, and photoinduced spin switching of [Mo/W(CN)8]4− ions.57−61 The [M(CN)8]n− building blocks have been also employed in the preparation of SCO materials based on FeII.37,62−71 The best results were found for the relatively rare, paramagnetic [NbIV(CN)8]4− ion, which together with FeII produced a series of tetragonal 3-D coordination networks, {[FeII(L)4]2[NbIV(CN)8]}·n(solvent) (L = pyridine derivatives or pyrazole).66−70 They enabled the unique discoveries of a light-induced SCO magnet,67 pressure-induced SCO-based photomagnetism,68 and the 90° photoswitching of second harmonic light in a chiral SCO photomagnet.69 In addition, the extremely rare [ReV(CN)8]3− ion combined with FeII enabled the observation of thermal SCO effect in the nanosized {Fe9[Re(CN)8]6} coordination clusters.37,71 Following these results, we aimed at the application of FeII(L)−[M(CN)8]n− system for the generation of multistep SCO phenomenon. To induce such an effect, we could manipulate the supporting organic ligand L, taking into account various pyridine derivatives providing the proper ligand field for the observation of FeII SCO.17,19,31 To achieve the stepwise spin transition, we expected that the selection of two similar, yet distinguishable, pyridine derivatives should be able to induce the FeII(L1/L2)− [M(CN)8]n− coordination network with mixed Fe(L1)n and Fe(L2)n sites. By checking a number of pyridine derivatives, we found that two closely related ligands, 3-acetoxypyridine (3OAcpy) and 3-hydroxypyridine (3-OHpy), could appear simultaneously in the aqueous solution of FeII(3-OAcpy)−

[M(CN)8]n− system when starting from only 3-OAcpy, which undergoes the partial hydrolysis into 3-OHpy (Scheme 1).72 Scheme 1. Hydrolysis of 3-OAcpy into 3-OHpy Which Occurs Partially in the Aqueous Solution during the Crystallization of FeMo and FeNb

Exploration of such in situ ligand transformation has been already introduced in the area of functional molecular materials, especially in the synthesis of coordination clusters.73−75 Using this unusual approach, which could be not simply reconstructed using the mixture of organic ligands as starting precursors, we achieved the coordination system with both 3-OAcpy and 3-OHpy. Thus, we report the nontrivial synthetic route, crystal structure, and magnetic and optical properties of two isostructural 3-D {FeII2(3-OAcpy)5(3OHpy)3[MIV(CN)8]}·nH2O (M = Mo, n = 0, FeMo; M = Nb, n = 1, FeNb) cyanido-bridged frameworks showing the thermal two-step FeII SCO effect presented by the temperature-variable structural, magnetic, and spectroscopic studies.

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EXPERIMENTAL SECTION

Materials. Ammonium iron(II) sulfate hexahydrate, (NH4)2FeII(SO4)2·6H2O, 3-acetoxypyridine (3-OAcpy), L-ascorbic acid, and sodium L-ascorbate were purchased from Wako Pure Chemical Industries, Ltd. and used without further purification. The K4[MoIV(CN)8]·2H2O and the K4[NbIV(CN)8]·2H2O cyanide precursors were prepared following the published procedures.76,77 Synthetic Procedures for FeMo and FeNb. The powder samples of FeMo and FeNb were prepared as follows. The 0.4 mmol portion of (NH4)2FeII(SO4)2·6H2O, the 4.0 mmol portion of 3OAcpy, and the 0.4 mmol portion of L-ascorbic acid (FeMo) or sodium L-ascorbate (FeNb) were dissolved together in 40 mL of distilled water, which was followed by the addition of the 0.2 mmol portion of the respective K4[MIV(CN)8]·2H2O salt dissolved in 40 mL of distilled water. It resulted in the precipitation of yellow or violet polycrystalline products of FeMo and FeNb, respectively. After additional stirring at 30 °C for 8 h, the powder samples were filtrated and dried in vacuum for 12 h to obtain the air-stable products. The compositions of {FeII2(3-OAcpy)5(3-OHpy)3[MIV(CN)8]} of FeMo and {FeII2(3-OAcpy)5(3-OHpy)3[MIV(CN)8]}·H2O of FeNb were determined by CHN elemental analyses and ICP/MS method for metals analyses. The yields of the syntheses varied in the 60−90% range for both FeMo and FeNb. The yellow block single crystals of FeMo were obtained by a slow diffusion method using the concentrated solutions, which were divided by the buffer solution of distilled water to slow down the crystallization process. All attempts to prepare the single crystals of FeNb by the analogous method were unsuccessful, most likely due to the decomposition of [NbIV(CN)8]4− ions during the long crystallization in the aqueous solution. IR spectra (paraffin oil, cm−1, Figure S1, Supporting Information). CN − stretching vibrations, FeMo: 2119s, 2126sh, 2132s, 2139s, 2158w; FeNb: 2117m, 2128w, 2138m, 2147m, 2161sh. Anal. Calcd for Fe2Mo1C58H50N16O13 (FeMo, MW = 1386.7 g·mol−1): Fe, 8.1%; Mo, 6.9%; C, 50.2%; H, 3.6%; N, 16.2%. Found: Fe, 7.9%; Mo, 6.8%; C, 50.1%; H, 3.9%; N, 16.0%. Anal. Calcd for Fe2Nb1C58H52N16O14 (FeNb, MW = 1401.7 g·mol−1): Fe, 8.0%; Nb, 6.6%; C, 49.7%; H, 3.7%; N, 16.0%. Found: Fe, 7.7%; Nb, 6.6%; C, 49.3%; H, 3.8%; N, 15.5%. X-ray Diffraction Methods. A single-crystal X-ray diffraction experiment for the selected crystal of FeMo was performed using a Rigaku R-AXIS Rapid diffractometer equipped with an imaging plate B


Article

Inorganic Chemistry

Calculations. Continuous shape measure analysis for the determination of the geometry of eight-coordinated [MoIV(CN)8]4− ions in FeMo and [NbIV(CN)8]4− ions in FeNb at various temperatures was performed using the SHAPE software, version 2.1.80

area detector and graphite monochromated MoKα radiation. The measurements were performed at various temperatures on the same single crystal taken directly from the mother solution, dispersed in a Paratone N oil, and mounted on Micro Mounts holder. The details of the crystal data and structure refinement for the experiments performed at 7 different temperatures are gathered in Table S1. The crystal structure of FeMo was solved using SHELXS-97 and was refined by a full matrix least-squares technique using SHELXL-2014/ 7.78 The preliminary refinement procedures were performed using Crystal Structure software package, and the further steps within the WinGX (version 2014.1) integrated system. All non-hydrogen atoms were found independently and refined anisotropically. The hydrogen atoms attached to pyridine rings and methyl groups were calculated assuming the idealized positions, and a riding model was used for their refinement. The hydrogen atoms of hydroxyl groups of 3-OHpy ligands were found from the electron density map and refined isotropically. The position of the related H5 atom coordinated to O5 atom of 3-OHpy was restrained by the DFIX commands to ensure the proper geometry and convergence of the refinement procedure. The DELU restraints on the thermal ellipsoids and the DFIX commands on the C−C distances were applied for part of the acetoxy groups of 3-OAcpy ligands exhibiting relatively large structural disorder. CCDC reference numbers for the crystal structures of FeMo at 295(2), 235(2), 190(2), 175(2), 150(2), 115(2), and 20(2) K are 1876170, 1876172, 1876169, 1876167, 1876171, 1876166, 1876168, respectively. The powder X-ray diffraction patterns for FeMo at 295(2) K and for FeNb at 11 different temperatures from the 20−300 K range were collected on a RIGAKU Ultima IV diffractometer equipped with the Cu Kα radiation. The crystal structures of FeNb at the indicated temperatures were found and refined by the Rietveld analysis using a RIGAKU PDXL software, taking the crystal structure of FeMo as a initial model. The details of the crystal data and structure refinement for powder samples of FeNb are shown in Table S4. CCDC reference numbers for the crystal structures of FeNb at 300(2), 270(2), 240(2), 200(2), 140(2), 120(2), 100(2), 80(2), 60(2), 40(2), and 20(2) K are 1884485, 1884487, 1884484, 1884483, 1884482, 1884481, 1884480, 1884489, 1884488, 1884479, 1884486, respectively. Structural figures were prepared with Mercury 3.10.3. software. Physical Techniques. Infrared absorption spectra were measured on the powder samples mixed with paraffin oil (nujol) using a JASCO FT/IR-4100 spectrometer. Temperature variable UV−vis−NIR absorption spectra were collected on the powder samples dispersed in the Apiezon N grease and mounted between two CaF2 plates using a Shimadzu UV-3100 spectrometer. The temperature conditions were controlled with the rate of 2 K/min using an Oxford Instruments MicrostatHe. Magnetic measurements were performed using a Quantum Design MPMS-XL SQUID device. Magnetic data were corrected for the diamagnetic contributions from the sample and the holder. 57Fe Mössbauer spectra of FeMo were performed on the airdried powder sample at three temperatures of 10(2), 155(2), and 300(2) K using a Wissenschaftliche Elektronik Mössbauer spectrometer with 57Co in a Rh cooled source. In this case, no correction for the finite absorber thickness was performed. More precise measurements of 57Fe Mössbauer spectra were performed for FeNb at five different temperatures of 297(2), 270(2), 220(2), 155(2), and 80(2) K. The spectra were recorded in the transmission geometry using a Wissel spectrometer with a bath cryostat and 57Co in Rh source kept at room temperature. The stabilization of the temperature was ca. 0.1 K. The spectra were fitted assuming two quadrupole doublets, assigned to different local spin states of iron. Because in lowtemperature region the resonant absorption effect was quite strong, ∼ 5%, the finite absorption correction of the spectra was applied.79 It turned out that such a correction was statistically significant for the estimation of the ratio of the areas of the assumed doublets only for the spectra at 80 and 155 K. For the spectra measured at 270 and 300 K, the effect was so small that the contribution of the absorption background, caused by same residual iron impurity in the cryostat and detector windows, had to be counted. The background (∼0.01%) measured in a blank run was subtracted from these spectra.

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RESULTS AND DISCUSSION

Structural Studies. The FeMo compound could be obtained as yellow crystals suitable for the single-crystal Xray diffraction analysis at room temperature (T = 295 K), and the resulting structural data are presented in Figures 1−2 and S2−S3 and Tables S1−S3 (Supporting Information). FeMo is a 3-D cyanido-bridged network composed of the coordination nanotubes {Fe3Mo2}n positioned along the c crystallographic axis which are further bonded by additional Fe complexes within the perpendicular directions (Figure 1). The {Fe3Mo2}n coordination nanotubes are constructed of hexagonal corrugated metal-cyanide rings involving six [MoIV(CN)8]4− ions alternately arranged with six Fe complexes depicted as Fe3 sites. The resulting 12-metal-membered rings are combined along c axis by the triplets of Fe1 complexes bridged by cyanides into MoIV centers which produce the nanosized chains (Figure 1b). These chains are linked by Fe1 complexes creating the Mo−Fe1−Mo linkages positioned along both a and c crystallographic axes. Such combined intermetallic bridging modes ensure the trigonal coordination framework crystallizing in the R3̅c space group, which is very different from the typically observed tetragonal coordination network based on divalent transition-metal ions and [MIV(CN)8]4− (M = Mo, W, Nb) metalloligands.66−70 This can be explained by the presence of three distinguishable six-coordinated octahedral FeII complexes detected in FeMo (Figures 1b and 2 and S2). The Fe1 centers, which combine the 12-metal rings of coordination nanotubes along the c axis, coordinate two bridging cyanides in a trans configuration and four 3-OAcpy ligands, resulting in the [Fe1(μ-NC)2(3-OAcpy)4] composition. The Fe2 sites, lying between the nanosized chains, are very different as they contain two bridging cyanides and four 3OHpy ligands, resulting in the [Fe2(μ-NC)2(3-OHpy)4] composition. The Fe3 complexes, which create the hexagonal rings within the coordination nanotubes, are composed of two cyanides and mixed organic coordination with single 3-OHpy and three 3-OAcpy ligands, resulting in the [Fe3(μ-NC)2(3OAcpy)3(3-OHpy)] composition. The Fe1 and Fe2 centers are placed at the special crystallographic positions with only half occupancies (Figure 2), thus, the overall detailed formula of FeMo is given as {[Fe(3-OAcpy)4]0.5[Fe(3-OHpy)4]0.5[Fe(3-OAcpy)3(3-OHpy)][Mo(CN)8]}. All Fe complexes in FeMo exhibit the geometry close to an ideal octahedron, while the [MoIV(CN)8]4− ions bearing four bridging and four terminal cyanides show the geometry of a distorted square antiprism (Table S3). At 295 K, the Fe−N bond lengths related to cyanide bridges are similar for all Fe sites, and they are within the range of 2.08−2.09 Å, typical for the HS FeII state (Table S2).37 The Fe−N bond lengths toward pyridine rings are longer, within the range of 2.21−2.28 Å, also typical for the HS FeII state. However, noticeably shorter are the bond lengths within the Fe1 site with the average value of 2.170 Å versus 2.189 and 2.202 Å for Fe2 and Fe3, respectively, which can suggest the small admixture of the LS state at the Fe1 position.67,69 The coordination framework of FeMo is densely packed, and the interstitial space is only occupied by the external parts of organic ligands, especially more expanded 3OAcpy molecules. There are channels visible within ab and C


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Inorganic Chemistry

Figure 2. Comparison of the asymmetric units of FeMo (a) and FeNb (b) at T = 295 K with the labeling of the metal centers and atoms directly coordinated to the Fe sites. The anisotropic refinement obtained within the single-crystal X-ray diffraction analysis of FeMo was represented by showing the atoms as thermal ellipsoids at the 25% probability level. The Rietveld analysis using the PXRD method was performed for FeNb, which results in the isotropic refinement giving atoms represented by spheres with the fixed size.

further physical characterization was suggested also by the CHN elemental analysis showing the perfect agreement of the experimental results with the expected composition. The FeNb compound could not be obtained in the form of single crystals with a sufficient quality. Therefore, only the polycrystalline sample of FeNb was characterized by using the powder X-ray diffraction (PXRD) experiment followed by the Rietveld analysis performed based on the structure of FeMo as a starting model (Tables S4 and Figures S5−S6). The PXRD structural analysis proves that FeNb is isostructural with FeMo, crystallizing in the identical R3̅c space group with only a slightly larger unit cell. As a result, FeNb can be considered as the analogous 3-D cyanido-bridged network bearing three distinguishable octahedral Fe sites, Fe1 with the formula of [Fe1(μ-NC)2(3-OAcpy)4], Fe2 with the formula of [Fe2(μNC)2(3-OHpy)4], and Fe with the mixed [Fe3(μ-NC)2(3OAcpy)3(3-OHpy)] composition (Figure S7). At 300 K, the respective Fe−N bond lengths of Fe complexes in FeNb are similar to those found in FeMo, indicating the dominant HS FeII state. However, the average Fe−N bond length for Fe1 is

Figure 1. Crystal structure of FeMo at T = 295 K: (a) The 3-D cyanido-bridged network shown within the ab plane, and (b) the representative coordination nanotube lying along the c axis and the insight into the Fe sites. The FeNb compound is isostructural showing the analogous cyanido-bridged network.

(101) planes, but possible solvent molecules within them were not found by the diffraction method, as only very small residual electron density was preserved in the supramolecular network. This is in good agreement with the lack of solvent molecules suggested by the CHN elemental analysis (see Experimental Section). The identity of the bulk powder sample with the structural model coming from the single-crystal X-ray diffraction experiment and the lack of any crystalline impurities were proved by the powder X-ray diffraction method (Figure S4). The overall good purity of the powder samples used in the D


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Inorganic Chemistry

from the HS FeII‑HS to the LS FeII‑LS state.27 It is going selectively on the Fe1 site as the other sites, Fe2 and Fe3, and shows only the minimal contraction due to the standard positive thermal expansion. On further cooling, the average Fe−N bond lengths for Fe1 site remain unchanged, keeping the LS state. The Fe3 site, containing the mixed 3-OAcpy/3OHpy surroundings, exhibits also the lack of any significant changes within the experimental error, thus, it preserves the HS state. On the contrary, below 100 K, the Fe2 center, which coordinates two cyanides and four 3-OHpy ligands, exhibits the decrease of the average Fe−N bond length. In FeMo, the related shift is from 2.186 Å at 150 K to 2.118 Å at 20 K (decrease of ca. 3.1%), while in FeNb, the change is from 2.208 to 2.131 Å at 20 K (decrease of ca. 3.6%). It indicates the occurrence of the thermal SCO effect on the Fe2 sites in FeMo and FeNb below 100 K. The incomplete SCO effect can be reasonably explained by the very low transition temperature, well below 100 K where the spin transitions become hampered due to the extinction of the necessary vibrational modes.7,8 The FeII‑LS fraction is higher in FeNb, indicating the higher completeness of the spin transition. The above results show that the SCO effect occurs in two distinguishable steps, the first one down from room temperature to ca. 150 K and the second one below 100 to ca. 40 K. Interestingly, the first step occurs for Fe1 coordinating only the 3-OAcpy, and the second step occurs for Fe2 coordinating only the 3-OHpy, while the SCO effect is not induced thermally for the Fe3 site exhibiting the mixed coordination by 3-OAcpy and 3-OHpy. Taking into account the ligand field strength as the main parameter to observe the SCO effect, this is a surprising result. The Fe3 site is expected to exhibit the ligand field of the intermediate strength between the strongest ligand field for Fe1 and the weakest for Fe2 site, thus, the Fe3 site should reveal the SCO effect in the intermediate temperature. This indicates the unexpected blocking of the SCO within the Fe3 sites which can be due to their arrangement in the coordination framework. The changes in the cyanido-bridged skeleton of FeMo during the thermal spin transition are visualized in Figure 4. The first SCO step occurs for the Fe1 site lying within the coordination nanotube where there is a relatively large free space for the easy rearrangement of the Fe complexes upon phase transition. Upon the SCO effect from 295 to 150 K, the distances between the 12-metal-rings of the coordination nanotubes decreases from 10.68 to 10.40 Å, which is related to the shortening of Fe1-NC-Mo1 cyanide bridges (Figure 4a). This leads to the elongation of the 12-metal-rings along the c axis, which is visible in the decreasing of their hexagonal crosssection (Figure 4b). Such deformation of the coordination nanotubes hampers any possible SCO effect on the Fe3 sites, which are situated within these 12-metal-rings. Thus, they are stretched between two Mo centers which moved toward opposite directions upon the SCO effect on Fe1 site. As a result, the Fe3 site remains in the HS state. The SCO effect is observed at very low temperatures for the Fe2 sites positioned between the coordination nanotubes. They have enough space for the rearrangement during the SCO effect, but their spin transition involves the shortening of the distance between the large nanosized chains from 10.67 to 10.56 Å. Such movement is difficult to achieve, and, therefore, this spin transition appears as incomplete. Thus, at the very low temperatures, the coordination framework contains the Fe3 sites which form the {Fe6Mo6} metal rings of the HS Fe sites well isolated by the LS

2.178 Å, which is much smaller than 2.208 Å for Fe2 and 2.222 Å for Fe3, suggesting a more significant contribution of the LS state in the Fe1 site than in FeMo (Tables S2 and S5). The [NbIV(CN)8]4− moiety of FeNb exhibits the geometry of a slightly deformed square antiprism, identical to that detected for FeMo (Table S6). The presence of FeII centers surrounded by N atoms of cyanides and N atoms of pyridine derivatives suggests the possible occurrence of a thermal SCO effect.62−71 Therefore, the temperature variable X-ray diffraction studies for the selected single crystal of FeMo, and the polycrystalline sample of FeNb, were performed (Tables S1 and S4, Figures S5−S6). The space group and the whole view of the 3-D coordination frameworks of FeMo and FeNb remain unchanged with decreasing temperature. However, on cooling, the significant contraction of the unit cell and the changes in the bond lengths within Fe and Mo/Nb complexes are observed. The thermal variation of the crystal structures of FeMo and FeNb is the best visualized by the temperature dependence of the average Fe−N bond lengths for different FeII centers (Figure 3). On

Figure 3. Temperature dependence of the Fe−N average bond lengths for different FeII complexes in FeMo (a) and FeNb (b), taken from the single-crystal and powder X-ray diffraction analyses, respectively.

cooling from 300 K down to ca. 150 K, the dramatic decrease of the Fe−N bond lengths are detected only for the Fe1 site bearing cyanides and 3-OAcpy ligands only. From 300 K to 140−150 K, the average Fe−N bond lengths for Fe1 decrease from 2.170 to 1.984 Å and from 2.178 to 1.998 Å, in FeMo and FeNb, respectively, indicating the thermal SCO effect E


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Inorganic Chemistry

Figure 4. Structural changes upon the thermally induced SCO transition in FeMo presented within the cyanido-bridged nanotubes at three indicated temperatures: (a) views along the c axis and (b) views of two neighboring decorated nanotubes within the ab plane. The HS and LS states of Fe(II) are presented as red and blue balls, respectively. The analogous structural changes upon SCO transition were found in FeNb.

state centers of Fe1 and (partially) Fe2. A similar scenario is valid for FeNb. However, they exhibit the spin transition steps at higher temperatures than FeMo, and their completeness seems to be better, which can be related to the slightly stronger ligand field offered by [NbIV(CN)8]4− ions. Magnetic Properties. Thermal SCO effect in FeMo and FeNb was investigated by the temperature variable measurement of magnetic susceptibility (Figure 5). The χMT versus T dependence related to the {Fe2Mo} and {Fe2Nb} units was gathered in the 320−1.8 K range. At 320 K, FeMo exhibits the χMT value of 7.8 K·cm3·mol−1 for the {Fe2Mo} unit which corresponds well to the range of 7.3−7.9 expected for the combined contributions from two HS FeII centers with S = 2, and the range of the g factor of 2.2−2.3, typical for FeII‑HS within heterometallic cyanido-bridged systems.37 On cooling, the χMT decreases in a broad range of 300−160 K, which can be easily explained by the first step of the SCO transition from the paramagnetic FeII‑HS to the diamagnetic FeII‑LS. The structural studies suggested that only the Fe1 sites undergo the spin transition in this range. These Fe sites describe the 25% of all Fe centers in FeMo, following the formula of {[Fe1(3OAcpy)4]0.5[Fe2(3-OHpy)4]0.5[Fe3(3-OAcpy)3(3-OHpy)][Mo(CN)8]}. Thus, the χMT value after the first SCO step should be within the range of 5.4−6.0 K·cm3·mol−1 for the {Fe2Mo} unit, and the experimental value at 150 K of 6.3 K· cm3·mol−1 is close to this limit. On further cooling, the χMT

Figure 5. Direct current (dc) magnetic properties of FeMo (orange) and FeNb (violet) presented by means of the temperature dependence of the magnetic susceptibility−temperature product (Hdc = 2000 Oe) together with the indicated steps of thermal SCO effect, estimated on the basis of the first derivative of the magnetic signal (Figure S9). The χMT values are shown for the {Fe2Mo} and {Fe2Nb} units for FeMo and FeNb, respectively.

F


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Inorganic Chemistry product is almost constant up to 100 K and again decreases significantly in the 100−40 K range. This can be ascribed to the second step of the SCO effect, assignable to Fe2 sites suggested by the structural data. If all Fe2 sites undergo the SCO transition, the χMT value should be in the 3.6−4.0 K·cm3· mol−1 range. At 50 K, the χMT value of FeMo is 5.3 K·cm3· mol−1, which is significantly higher, indicating the incomplete spin transition, as supported by the structural studies. Below 30 K, the χMT value decreases fast, reaching 3.1 K·cm3·mol−1 at 2 K. This effect should be assigned to the antiferromagnetic interactions between the remaining HS FeII sites separated by diamagnetic MoIV complexes and the zero-field splitting effect on these FeII‑HS units.34 Therefore, the magnetic data are in line with interpretation of the two-step thermal SCO effect in FeMo. From the temperature dependence of the first derivative of the χMT product, we estimated the critical transition temperatures which are 235(5) and 65(5) K for the first and the second steps, respectively (Figure S8). The qualitatively similar magnetic data were found for FeNb (Figures 5 and S8). However, the magnetic signal is lower in the whole temperature range. At 320 K, the χMT value is 6.9 K· cm3·mol−1 for the {Fe2Nb} unit, which is lower than the expected range of 7.6−8.3 K·cm3·mol−1 calculated for two FeII‑HS centers (S = 2.0, g in the range of 2.2−2.3) and one NbIV (S = 1/2, g = 2.0). This can be ascribed to the nonnegligible contribution of the LS state, most likely on the Fe1 centers, as suggested by the structural data and strongly supported by the 57Fe Mössbauer spectroscopic studies (see below). On cooling, the χMT value quickly decreases to ca. 5.6 K·cm3·mol−1, which is almost constant from 200 to 140 K. This value is in a good agreement with the range of 5.8−6.3 K· cm3·mol−1 calculated for the HS to LS spin transition occurring for the Fe1 sites in FeNb. Further cooling below 140 K results in the second decrease of the signal occurring in the broad range up to 50 K, until the value of 4.4 K·cm3·mol−1 is reached. This value is in the range of 4.0−4.4 K·cm3·mol−1, which is expected for the complete spin transition for both Fe1 and Fe2 sites. This proves that in FeNb, the spin transition on these two types of Fe centers is almost complete. The further low temperature decrease of the χMT product to the very low value of 1.8 K·cm3·mol−1 at 2 K is mainly due to the antiferromagnetic interactions, which are much stronger than in FeMo, as they involved not only the remaining HS FeII sites but also the paramagnetic NbIV spin centers. The first derivative of the χMT product enabled the magnetic estimation of the spin transition temperatures which are 280(5) K and 75(5) K, higher than found for FeMo (Figure S8). UV−vis Absorption Spectroscopy. Temperature variable solid-state UV−vis absorption spectra were performed for the powder samples of FeMo and FeNb, to confirm the thermal FeII SCO effect which is intrinsically connected with changes in the absorption properties due to the different electronic structure of HS and LS spin states of FeII.12 At 300 K, FeMo exhibits a yellow color which is related to the strong absorption ranging from the UV range to ca. 450 nm (Figure 6a). This UV−vis absorption is connected mainly with the chargetransfer transitions within [MoIV(CN)8]4−.81 However, the band at ca. 400 nm and the absorption tail positioned up to 650 nm can be ascribed partially to the weak d−d transitions within the HS FeII centers present in FeMo at room temperature. On cooling, there is a significant increase of the absorbance within the band centered around 400 nm. In addition, the weak broad absorption is gradually appearing in

Figure 6. Temperature variable solid-state absorption properties of FeMo (a, b) and FeNb (c, d) presented in the form of absorption spectra at various temperatures with the indicated assignment of the most representative bands (a, c), and the temperature dependence of the absorbance at three indicated wavelengths with the indicated steps of thermal SCO effect (b, d), estimated on the basis of the first derivative of the absorption signal (Figure S10). G


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Inorganic Chemistry the 500−600 nm range (Figure 6a). These changes can be interpreted in terms of the FeII SCO effect producing the LS centers with the enhanced absorbance in the visible range related to the spin-allowed 1A1g to 1T2g and 1A1g to 1T1g d−d transitions.27 The absorbance within the broad band at the edge of visible and UV regions, at 380 and 410 nm, as well as the absorbance within the broad weak peak at 540 nm exhibit strong temperature dependence (Figure 6b). All of these peaks increase their intensity upon cooling, confirming the SCO effect. Moreover, the stepwise increase of the indicated absorbance values is in a perfect agreement with the twostep spin transition proved by structural and magnetic studies. Using the first derivative of the absorbance, we could estimate the transition temperatures of 255(5) and 75(5) K (Figure S9) in a reasonable agreement with those found in the magnetic susceptibility studies. FeNb exhibits again the analogous behavior, showing the strong temperature dependence of the optical absorption (Figure 6a). On the contrary to FeMo, the FeNb compound exhibits a violet color at room temperature, which is not only due to the nonzero contribution from the LS FeII centers existing even at 320 K but also to the characteristic FeII−NbIV metal-to-metal charge-transfer (MMCT) transition giving the strong broad band at ca. 580 nm.66,67 Upon cooling, the absorbance in the visible region of 520−650 nm, and at the edge of the UV−vis range, significantly increases, which can be explained by the appearing absorption related to the d−d transitions of the LS FeII states and the related modification of the Fe−Nb MMCT transition. The absorbance at three selected wavelengths of 380, 410, and 594 nm in the function of temperature confirms the two-step FeII SCO phenomenon (Figure 6d). The analysis of the thermal variation of the first derivative of the absorbance at these wavelengths allowed to estimate the transition temperatures, 285(5) and 85(5) K, higher than found in FeMo, which is in line with the previous findings in the magnetic studies. The optical spectra confirm the two-step SCO transition in FeMo and FeNb, with the higher transition temperatures assigned to FeNb (Table 1). Table 1. Transition Temperatures for the Two-Step Fe(II) SCO Effect in FeMo and FeNb Obtained from Magnetic (Magnetic Susceptibility Measurements) and Optical (Absorption Spectra) Studies compound FeMo FeNb

SCO step magnetic transition T (K) step step step step

1 2 1 2

a

235(5) 65(5) 280(5) 75(5)

optical transition T (K) 255(5)b 75(5) 285(5) 85(5)

Estimated from the first derivative of χMT product (Figure S8). Estimated from the first derivative of absorbance at three selected wavelengths from the UV−vis range of the spectrum (Figure S9). a

b

Optical estimation gives slightly higher transition temperatures to those found from magnetic studies which is likely due to the much slower stabilization of the optical signal. The heating rate of 2 K/min, used in optical studies, may be slightly too fast for sufficient thermal stabilization, and the SCO-induced changes in the optical spectra are consequently shifted toward higher temperatures, as visible in Table 1. 57 Fe Mössbauer Spectroscopic Studies. The temperature variable 57Fe Mössbauer spectroscopic measurements have been performed to additionally confirm that FeMo and FeNb exhibit the SCO effect on FeII centers (Figures 7 and

Figure 7. 57Fe Mössbauer spectra of FeNb at the indicated temperatures. Experimental results are shown as black points and the fitted results as solid lines. Colors: HSFeII (red), intermediate HS/LS II Fe (violet), and total calculated fit (green).

S10−S11 and Tables 2 and S7). The more precise characterization at five different temperatures was executed for FeNb H


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Inorganic Chemistry Table 2. T (K) 297 270 220 155 80

57

Fe Mössbauer Spectral Parameters for FeNb

assignment to Fe site

δ (mm· s−1)

ΔEQ (mm· s−1)

area contribution (%)

Γ (mm· s−1)

estimated percentagea of HS state in intermediate HS/LSFeII (%)

HS FeII LS/HS FeII HS FeII LS/HS FeII HS FeII LS/HS FeII HS FeII LS/HS FeII HS FeII LS/HS FeII

1.03(2) 0.53(2) 1.03(1) 0.45(1) 1.07(1) 0.40(1) 1.12(1) 0.41(1) 1.17(1) 0.42(1)

1.38(3) 0.44(2) 1.47(3) 0.46(2) 1.62(2) 0.49(2) 1.81(2) 0.54(2) 1.94(2) 0.62(2)

64(3) 36(3) 68(3) 32(3) 69(3) 31(3) 68(3) 32(3) 60(2) 40(2)

0.38(3) 0.99(10) 0.38(2) 0.52(5) 0.36(2) 0.32(2) 0.37(1) 0.30(1) 0.39(1) 0.32(1)

− 33(3) − 23(2) − 12(2) − 5(1) − 0

total percentage of HS FeIIb (%)

average spin value ⟨s⟩ for Fe sitec

76(3)

1.52(3)

75(3)

1.51(3)

72(3)

1.45(3)

69(3)

1.39(3)

60(3)

1.20(3)

a

Percentage of HS Fe(II) in the intermediate HS/LS Fe(II) contribution to the spectrum was calculated from the deviation of the respective isomer shift from the typical temperature dependence simulated for the LS Fe(II) state (see text and Figure S10 for details). bTotal percentage of HS Fe(II) state in FeNb was calculated as the sum of area contribution from pure HS Fe(II) state and estimated percentage of HS state in intermediate HS/LS Fe(II) contribution to the spectrum. cAverage spin value, ⟨s⟩ for Fe site is given as S = 2 multiplied by the total percentage of HS Fe(II).

HS and LS doublets, while we expect the higher frequency doublet with narrow lines. In this studied case, we still may estimate the amount of the HS FeII state by using the change in the isomer shift, Δδexp compared with the expected difference between the isomeric shifts for the HS and LS states, Δδ = 0.72 mm·s−1 (Figure S10). The contribution of the HS configuration in the intermediate HS/LS component is given by the simple Δδexp/Δδ ratio. The resulting values at various temperatures, given in Table 2, are changing from 0% at 80 K to 33(3)% at 297 K, indicating the SCO effect in FeNb. To clarify the conclusions in this part, we have also calculated the average spin values for the Fe site at various temperatures, collected in Table 2. The average spin is starting from 1.20(3) at 80 K, which shows that at this temperature, ca. 60% of the Fe sites are still in the HS state. This is in a reasonable agreement with the magnetic and structural data showing that at the 80 K point, only the Fe1 sites (the 25% of all Fe sites) and partially the Fe2 sites (the next 25% of all Fe sites) are in the LS state. On heating, the average spin value increases, as expected for the SCO effect, reaching 1.52(3) at 297 K (ca. 76% of the Fe sites in the HS state), which indicates the noncomplete recovery of the HS state, also indicated by magnetic studies. It is important to note that three different Fe1−Fe3 sites are detectable in FeNb, as indicated by the structural data, which should theoretically result in the complex splitting of the Mössbauer spectra with even six doublets for three Fe sites partially in the HS and LS spin states. However, the overall signal was too broad to reliably perform such detailed fitting which can be assigned to the spin-state disorder. It gives the significant line broadening, visible also in the line widths of our fitted doublets, especially at high temperature (Table 2). The analogous 57Fe Mössbauer spectra as found for FeNb were gathered for FeMo at three representative temperatures, and they can be interpreted in the same way as FeNb, which supports the overall interpretation of the thermal SCO FeII phenomenon in both materials (Figure S11 and Table S7).

(Figure 7), while the qualitatively identical but limited data were gathered for FeMo (Figure S11). The 57Fe Mössbauer spectra for FeNb at all temperatures could be reliably deconvoluted into two quadrupole doublets whose hyperfine parameters are collected in Table 2. At the lowest temperature of 80 K, the dominant doublet with the isomer shift (δ) of 1.17(1) mm·s−1 and the quadrupole splitting (ΔEQ) of 1.94 mm·s−1 can be obviously assigned to the FeII‑HS sites.27 The isomer shift of the second doublet, of 0.42(1) mm·s−1 at 80 K, with the smaller ΔEQ of 0.62(2) mm·s−1 could suggest the FeII‑LS sites. This simple consideration gives the false conclusion as the area contributions related to these two doublets seem to be not dependent on temperature, meaning that the first doublet takes ca. 60−70% of the whole signal, while the remaining part is always ca. 30−40% (Table 2). This seems to be not consistent with the available structural, magnetic, and optical data (see above), indicating the occurrence of the thermal spin transition in FeNb. Therefore, we precisely examined the details of the temperature dependence of the isomeric shifts for both components, and we compared them with the temperature dependence of the isomeric shift (or more accurately of the central shift) fitted according to the Debye model of a phonon spectrum (Figure S10).82 While the isomeric shift for the FeII‑HS doublet strictly follows the theoretical one, the second doublet deviates at high temperatures from the dependence characteristic of the FeII‑LS state. The increase of the isomeric shifts with increasing temperature clearly points out the change of the electron density at the 57Fe nuclei toward the value expected for the HS FeII state. Such unusual temperature dependence of the isomeric shift is explained by the dynamic HS−LS transition on the FeII site.83 Therefore, this doublet could be assigned to the intermediate HS/LS FeII contribution (Table 2 and Figure S10). Then, the observed thermally induced increase of isomeric shift of the originally LS FeII state is related to an increasing admixture of the HS configuration to the LS state observed within the characteristic for 57Fe observation time τ = 141 ns, the lifetime of the excited 14.4 keV nuclear level of 57 Fe. The strong increase of the line width of this intermediate HS/LS doublet points out that we do not observe “average” spin state of FeII, but the fluctuation of its spin with the frequency is comparable with 1/(141 × 10−9) s ∼ 107 s−1. For much lower frequencies, we would observe two well-resolved

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CONCLUSIONS We report two novel three-dimensional cyanido-bridged FeII− [MIV(CN)8]4− (M = Mo, Nb) frameworks exhibiting the twostep thermal spin crossover effect. Due to the application of the 3-acetoxypyridine (3-OAcpy) ligand, which undergoes partial I


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Inorganic Chemistry

Research on Innovative Area Soft Crystals (area no. 2903, 17H06367), Quantum Leap Flagship Program (Q-LEAP) from MEXT, and the National Science Centre, Poland within the OPUS-14 project, grant no. 2017/27/B/ST5/00947. The Cryogenic Research Center in The University of Tokyo and the Center for Nano Lithography & Analysis in The University of Tokyo, supported by MEXT, are acknowledged. S.K. acknowledges the support of the Advanced Leading Graduate Course for Photon Science (ALPS).

in situ transformation into the 3-hydroxypyridine (3-OHpy) ligand in an aqueous crystallization solution, we achieved the very unique coordination framework with three inequivalent Fe sites differing in the amount and the type of attached organic ligands. It modulates the cyanido-bridged skeleton toward the unprecedented trigonal coordination network composed of coordination nanotubes bonded by the additional Fe complexes. More importantly, the presence of three different Fe sites induced the thermal two-step and incomplete SCO phenomenon. The highest, near room-temperature transition is observed for the [Fe(3-OAcpy)4(NC)2] moiety which is followed by the SCO effect for the [Fe(3OHpy)4(NC)2] units at very low temperatures below 100 K, and the lack of thermal spin transition for the mixed [Fe(3OHpy)(3-OAcpy)3(NC)2] complex. Such an unusual sequence of the stepwise spin transition is related to the equilibrium between the modified ligand field strength and the alignment of the Fe sites within the complex coordination framework. We show the novel synthetic route toward a highly desired multistep SCO by using the spontaneously formed mixture of two closely related organic ligands which produce the inequivalent Fe sites and the nontrivial coordination topology. It opens the pathway to the impressive multi-step SCO effect with the perspective selective switching of selected spin centers by various external stimuli such as pressure or light irradiation, which will be the subject of future work.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00361. Infrared absorption spectra. Crystal data and structure refinement. Detailed structural views and parameters. PXRD data. Rietveld analysis plots for FeNb. Results of continuous shape measure analysis. Temperature dependence of the first derivatives of χMT product and absorbance. Additional 57Fe Mössbauer spectra (PDF) Accession Codes

CCDC 1876166−1876172 and 1884479−1884489 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]. uk, 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 Author

*E-mail: [email protected]. ORCID

Szymon Chorazy: 0000-0002-1669-9835 Barbara Sieklucka: 0000-0003-3211-5008 Shin-ichi Ohkoshi: 0000-0001-9359-5928 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work financed by the Japan Society for the Promotion of Science, within the Grant-in-Aid for Specially Promoted Research, grant no. 15H05697, Grant-in-Aid for Scientific J


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