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- Oxford University Press
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- Copyright © 2022 China Science Publishing & Media Ltd. (Science Press)
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- 2095-5138
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- 2053-714X
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- 10.1093/nsr/nwy033
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Abstract
Downloaded from https://academic.oup.com/nsr/article/5/4/507/4931058 by DeepDyve user on 16 July 2022 National Science Review 5: 507–515, 2018 RESEARCH ARTICLE doi: 10.1093/nsr/nwy033 Advance access publication 12 March 2018 CHEMISTRY Manipulation of successive crystalline transformations to control electron transfer and switchable functions 1 1 1 1 1 Cheng-Qi Jiao , Wen-Jing Jiang , Yin-Shan Meng , Wen Wen , Liang Zhao , 1 1 2 1 1,∗ Jun-Li Wang , Ji-Xiang Hu , Gagik G. Gurzadyan , Chun-Ying Duan and Tao Liu ABSTRACT Electron transfer in solid is crucial to switchable magnetic, electrical, optical and mechanical properties. However, it is a formidable challenge to control electron-transfer behaviors via manipulation of crystalline phases, especially through dynamic crystalline transformation. Herein, three crystalline phases of an {Fe Co } compound were obtained via enhancement of intermolecular π ···π interactions inducing 2 2 successive single-crystal-to-single-crystal transformations, from solvated 1·2CH OH·4H O, to desolvated 3 2 1 and its polymorph 1a accompanying electron transfer. 1·2CH OH·4H O showed thermally induced 3 2 reversible intermetallic electron transfer in mother liquor. No electron-transfer behavior was observed in 1. 1a showed reversible intermetallic electron transfer upon thermal treatment or alternative irradiation with 808- and 532-nm lasers at cryogenic temperatures. The electron-transfer behaviors significantly change the magnetic and optical properties, providing a strategy to realize different electron-transfer behaviors and switchable functions via π ···π interactions manipulated dynamic crystalline transformation. Keywords: electron-transfer, dynamic structural transformation, successive crystalline transformations, reversible, polymorphs modulation of electron-transfer behavior requires INTRODUCTION chemical modification, such as ligand substitution, State Key Laboratory Electron transfer is a common phenomenon in of Fine Chemicals, anion and solvent exchange in solution reaction nature and plays important roles in biology, en- Dalian University of [11,17,20]. It is a formidable challenge to realize ergy, materials, catalysis and other fields [ 1–4]. In- Technology, Dalian different electron-transfer behaviors by manipulat- termetallic electron transfer not only changes the 116024, China and ing dynamic structural transformations in the solid 2 valence states and electron configurations of the Institute of Artificial state, especially through physical stimuli-induced participant metal ions, but also switches the cou- Photosynthesis, State single-crystal-to-single-crystal (SCSC) transfor- pling interactions between them [2,5–7]. Therefore, Key Laboratory of Fine mations. Such transformation processes usually Chemicals, Dalian the control of electron transfer is an efficient way involve the movement of atoms in the crystal and University of to tune the magnetic, electric and optical proper- rearrangement of chemical bonds, which result in Technology, Dalian ties of materials [8–22]. Thermally and/or photo- drastic changes in not only the molecular structure, 116024, China induced electron transfers have been utilized to but also the physical/chemical properties [25–28]. induce paramagnetic and diamagnetic transforma- On the other hand, SCSC transformations can Corresponding tions, presenting photo-switchable magnet behavior provide access to compounds that are difficult author. E-mail: [11–13]. Moreover, the polarity and dielectric prop- or impossible to be directly obtained by solution liutao@dlut.edu.cn erties can be switched by utilizing electron-transfer- reactions [28]. More importantly, the procedure induced changes in charge distribution [23,24]. of SCSC transformations can directly and accu- Received 5 Typical examples showing external stimuli- rately provide a molecular-level understanding December 2017; tuned intermetallic electron-transfer behaviors are of the mechanism of the transformation, and Revised 1 February cyanide-bridged complexes, wherein the electron- 2018; Accepted 8 help to gain more insights into the correlation transfer behaviors depend on the metallocyanate March 2018 between the structures and properties [29,30]. building blocks and ancillary ligands [11]. The The Author(s) 2018. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. All rights reserved. For permissions, plea se e-mail: journals.permissions@oup.com Downloaded from https://academic.oup.com/nsr/article/5/4/507/4931058 by DeepDyve user on 16 July 2022 508 Natl Sci Rev, 2018, Vol. 5, No. 4 RESEARCH ARTICLE It has been reported that two crystalline phases tetrakis (pyrazolyl)borate). 1 ·2CH OH ·4H O 3 2 with different electron-transfer behaviors could be undergoes successive SCSC transformations as obtained upon solvation and desolvation, because a result of enhancement of intermolecular π ···π hydrogen-bonding interactions between coordi- interactions in the process of desolvation and struc- nated solvents and the framework can tune the tural rearrangement, forming a pair of polymorphs III Pz II redox potentials of metal ions [31–36]. However, [Fe ( Tp)(CN) ] Co (dpq) ·2ClO (1) and 3 2 2 4 4 II Pz III the formation and breakage of hydrogen bonds [Fe ( Tp)(CN) ] Co (dpq) ·2ClO (1a). The 3 2 2 4 4 could only induce one-step SCSC transformation three crystalline phases present different electron- and generate two crystalline phases with different transfer behaviors upon thermal treatment and electron-transfer behaviors [32–36]. Moreover, light irradiation. Especially, it is the first time that the crystallinity is often disrupted owing to break- crystalline transformation between a pair of poly- age of the hydrogen-bonding interactions that morphs in intermetallic electron-transfer-related are important to maintain the integrity of the compounds has been observed. crystalline framework [25,31], restricting further investigation of the electron-transfer mechanism. How to induce successive crystalline transformation RESULTS to obtain more than two crystalline phases with different electron-transfer behaviors is interesting Crystal structure of 1·2CH OH·4H O 3 2 but still remains a challenge, especially for poly- and its successive crystalline morphs with different electron-transfer behaviors. transformations To induce successive SCSC transformations, in- troducing flexible π ···π interactions may provide Single-crystal X-ray diffraction analysis revealed a rational strategy [9,37], as the distances of π ···π that 1·2CH OH·4H O crystallizes in the triclinic 3 2 interactions can be modulated in a continuous range space group P1 (see Supplementary Table 1). The (3.3–3.8 A) [38]. Moreover, the variations in π ···π crystal structure consists of cationic tetranuclear 2+ – interactions between ligands around the metal ions {Fe Co } square units, ClO anions, uncoor- 2 2 4 can induce different distortions of coordination dinated methanol and water molecules (Fig. 1). spheres and the strength of the ligand field, which At 298 K, the Fe–C and Co–N bond lengths can tune the redox potential of metal centers and are 1.877(7)–1.889(7) and 1.879(5)–1.946(5) A, result in different electron-transfer behaviors. respectively (see Supplementary Table 2), which II Herein, we were intrigued by the possibil- are consistent with those observed for {Fe (μ- LS III ity of introducing intermolecular cooperative CN)Co } (LS = low spin) linkages [17,33]. LS π ···π interactions to manipulate successive When the green crystals are slowly heated to 360 K crystalline transformations to obtain different in the mother liquor, a dramatic color change from crystalline phases featuring different electron- green to red is observed, indicating a possible trans- III II transfer behaviors. Inspired by this, we adopt an formation to {Fe (μ-CN)Co } (HS = high LS HS ancillary ligand prazino[2, 3-f][1, 10]phenan- spin) linkages. On cooling to 298 K, the crystals throline (dpq), with an extended π-conjugation return to the initial green color. This phenomenon system, to prepare a tetranuclear {Fe Co } com- indicates that 1·2CH OH·4H O undergoes a ther- 2 2 3 2 II Pz III pound [Fe ( Tp)(CN) ] Co (dpq) · 2ClO mally induced reversible electron transfer in the 3 2 2 4 4 Pz ·2CH OH ·4H O(1·2CH OH ·4H O, Tp = mother liquor (see Supplementary Movie 1). 3 2 3 2 II III III Co Co Co II II Fe Fe III Fe III Co III II Fe II Fe Fe II III Co Co 1a 1 2CH OH 4H O 3 2 Figure 1. Crystal structures of 1·2CH OH·4H O, 1 and 1a. The hydrogen atoms and ClO anions are omitted for clarity. Fe, 3 2 4 dark yellow; Co, turquoise; C, gray; N, blue; B, orange; O, red. Downloaded from https://academic.oup.com/nsr/article/5/4/507/4931058 by DeepDyve user on 16 July 2022 RESEARCH ARTICLE Jiao et al. 509 (calcd: 6.2%). After this weight loss, a long plateau is observed until 540 K, suggesting the formation of a new stable phase. When the green crystals Water vapor are slowly heated to ∼350 K in air, the color of the crystals changes from green to red, indicating the formation of a new redox state. Moreover, the crystallinity is well retained (Fig. 2 and see Supplementary Movie 2). The crystallographic LT 1 1a data demonstrate that the red crystals retain the {Fe Co } tetranuclear structure and have the 2 2 Pz formula of [Fe( Tp)(CN) ] Co (dpq) ·2ClO 3 2 2 4 4 TT Δ (1,Fig. 1). At 298 K, the Fe–C and Co–N bond lengths are 1.925(8)–1.944(9) and 2.097(7)– 2.169(7) A, respectively (see Supplementary Table 2), corresponding with those observed III II for {Fe (μ-CN)Co } linkages [17,39]. LS HS These results indicate that electron transfer occurs in the desolvation process with a transforma- II III tion from {Fe (μ-CN)Co } linkages in LS LS III II 1·2CH OH·4H O to {Fe (μ-CN)Co } 3 2 LS HS linkages in 1. Furthermore, an endothermic peak is seen in the DSC curve of 1·2CH OH·4H O, further 3 2 HT 1a 1 2CH OH 4H O 3 2 confirming the first-order phase transition from 1·2CH OH·4H O to 1 (see Supplementary Fig. 4). 3 2 Figure 2. Photographic images showing the SCSC transformation. Images show the With the transformation from 1·2CH OH·4H O 3 2 conversions among these three species through subsequent desolvation and vapor in- to 1, the average π ···π interaction distance be- duction. tween the dpq ligands decreases from 3.753(2) to 3.664(1) A, and the average C–H···π interaction There are two important structural charac- Pz distance between the C–H moieties of the dpq/ Tp teristics in 1·2CH OH·4H O.Thefirstisthat 3 2 Pz ligands and the pyrazol rings of the Tp ligands uncoordinated water and methanol molecules are 2+ located between the {Fe Co } square units, decreases from 3.148(1) to 3.102(1) A. Such results 2 2 with hydrogen-bonding interactions between un- suggest that the intermolecular interactions are en- coordinated water molecules and terminal cyanide hanced in the process of crystalline transformation nitrogen atoms (see Supplementary Fig. 1 and Table (see Supplementary Fig. 5 and Table 6). 2+ 4). The second is that the {Fe Co } square units Interestingly, when the red crystals of 1 were 2 2 are linked via π ···π interactions (average distance placed in water vapor and heated at 100 Cfor = 3.753(2) A) between the dpq ligands and C– 24 h, the color of the crystals changed from red to H···π interactions (average distance = 3.148(1) A) green (Fig. 2, see Supplementary Fig. 6), suggesting Pz between the C–H moieties of the dpq/ Tp ligands the formation of a new phase, as confirmed by Pz and the pyrazol/pyrazine rings of the Tp/dpq lig- single-crystal X-ray diffraction (Fig. 1) and powder ands (see Supplementary Fig. 2 and Table 5). These XRD analyses (see Supplementary Fig. 7). The intermolecular interactions are very important to new phase exhibits a composition consistent with Pz stabilize the crystalline framework and make it pos- the formula [Fe( Tp)(CN) ] Co (dpq) ·2ClO 3 2 2 4 4 sible to undergo SCSC transformations upon desol- (1a). Although 1 and 1a are a pair of polymorphs, vation. Furthermore, the formation and destruction they exhibit different electron-transfer behaviors. of hydrogen bonding can significantly affect the For 1a at 298 K, the Fe–C and Co–N bond lengths are 1.852(4)–1.914(4) and 1.877(3)–1.949(3) redox potential of the iron centers [31,33], provid- A, respectively (see Supplementary Table 2), ing the possibility of desolvation-induced electron II III indicating the existence of {Fe (μ-CN)Co } transfer. Thus, two crystalline phases with different LS LS linkages. When the crystals of 1a are slowly heated electron-transfer behaviors are reasonably expected. to 350 K, a dramatic color change from green to red The TGA (see Supplementary Fig. 3) of is observed. The corresponding Co–N bond lengths 1·2CH OH·4H O was measured to explore the 3 2 possibility of desolvation-induced SCSC trans- are 2.077(4)–2.145(3) A (see Supplementary III formation. The plot shows a weight loss of 6.0% Table 2), indicating the formation of {Fe (μ- LS II from 300 to 360 K, corresponding well to the CN)Co } linkages [17,39]. Moreover, the HS loss of two methanol and four water molecules crystals return to the initial green color on cooling to Desolvation Downloaded from https://academic.oup.com/nsr/article/5/4/507/4931058 by DeepDyve user on 16 July 2022 510 Natl Sci Rev, 2018, Vol. 5, No. 4 RESEARCH ARTICLE However, 1 exhibits a different electron-transfer 1 2CH OH 4H O in heating process 3 2 3 −1 behavior. Its χT valueis6.51cm mol K at 400 K 1 2CH OH 4H O in desolvated process 3 2 III (Fig. 3), corresponding to paramagnetic {Fe (μ- LS 1a II CN)Co } linkages. As the temperature is low- 6 HS ered, χT values gradually decrease, reaching a min- 3 −1 imum value of 5.36 cm mol Kat24K.Be- low this temperature, the χT value increases up 3 −1 to 5.44 cm mol K at 14 K and then decreases 3 −1 rapidly to 3.50 cm mol K at 2 K. This represents a typical paramagnetic behavior without electron- transfer-induced spin transition. The transformation 0 100 200 300 400 Temperature (K) of the magnetic behavior in the desolvation pro- cess was monitored for as-synthesized crystals of Figure 3. Magnetic characteristics for 1·2CH OH·4H O, 1 3 2 1·2CH OH·4H O. χT values remain essentially 3 2 3 −1 and 1a.Plots of χT vs temperature for 1·2CH OH·4H O, 1 3 2 constant at 0.36 cm mol K below 310 K and 3 −1 and 1a. 1·2CH OH·4H O was soaked in the mother liquor 3 2 reach a maximum value of 6.43 cm mol Kat upon cooling, and cycling the temperature back to 400 K; 1 360 K, which is in agreement with paramagnetic upon cooling; 1·2CH OH·4H O upon heating and concomi- 3 2 III II {Fe (μ-CN)Co } linkages as observed for LS HS tant desorption of methanol and water; 1a upon cooling and 1. Such magnetic behavior confirms that the elec- reheating to 400 K (temperature sweeping rate: 1 K/min for tron transfer occurs in the desolvation process from 10–400 K and 0.5 K/min for 2–10 K). 1·2CH OH·4H O to 1. This electron-transfer pro- 3 2 cess seems analogous to that observed during the 298 K, indicating the thermally induced reversible heating of 1·2CH OH·4H O in mother liquor, but 3 2 intermetallic electron transfer. With the transfor- with a lower transition temperature. This is due to mation from 1 to 1a, the average π ···π interaction the difference in the intermetallic electron-transfer distance between the dpq ligands decreases from mechanism, as the former stems from the loss of sol- 3.664(1) to 3.457(1) A at room temperature, and vent molecules, while the latter arises from thermal the average C–H···π interaction distance between stimuli. Pz the C–H moieties of the dpq/ Tp ligands and For 1a, χT values remain nearly constant be- Pz the pyrazol/pyridine rings of the Tp/dpq ligands 3 −1 tween 2 and 315 K at 0.43 cm mol K (Fig. 3). decreases from 3.102(1) to 3.002(1) A, leading Heating from 315 to 360 K causes an increase in the to the enhanced intermolecular interactions (see 3 −1 χTvalueto6.47cm mol K. The χT value returns Supplementary Fig. 8 and Table 7). When heated to its initial value with a small thermal hysteresis to 350 K, both the average π ···π interaction and ∼5 K wide, showing reversible electron-transfer be- C–H···π interaction distances of 1a increase slightly havior that involves transformation between the dia- (see Supplementary Fig. 9 and Table 8). II III magnetic {Fe (μ-CN)Co } (LT phase = low LS LS temperature phase) linkages and the paramagnetic III II {Fe (μ-CN)Co } (HT phase = high temper- LS HS Magnetic characterization ature phase) linkages. Consistently with magnetic The magnetic properties of the three crystalline data, endothermic/exothermic peaks for 1a are ob- phases were subsequently investigated (Fig. 3). The served with T = 331.2 and 325.3 K, indicating max χT versus T curve for 1·2CH OH·4H O in mother 3 2 the occurrence of the first-order phase transition liquor shows that χT values remain essentially con- (see Supplementary Fig. 10). 3 −1 stant at 0.46 cm mol K below 355 K, confirming II III the existence of {Fe (μ-CN)Co } linkages. LS LS Optical studies Upon heating, χT values abruptly increase to 3 −1 6.44 cm mol K at 390 K, which are in agreement The solid-state UV–vis–NIR absorption spectra of 3 −1 with the theoretical value of 6.67 cm mol K 1·2CH OH·4H O, 1 and 1a were measured at 3 2 III II expected for two LS Fe and two HS Co ions room temperature to study their color changes [16]. When the temperature is lowered from 400 K, and to further support the electronic state as- χT values decrease rapidly to its initial value at signments deduced from the structural and mag- 355 K with a small thermal hysteresis loop of ∼8K, netic analyses (Fig. 4a). The absorption spectra of II indicating that the system regains the {Fe (μ- 1·2CH OH·4H O and 1a are similar, presenting LS 3 2 III CN)Co } linkages. Thus, 1·2CH OH·4H O bands at 427 and 763 nm, respectively. The absorp- LS 3 2 shows thermally induced reversible intermetallic tion band at 427 nm may be assigned to ligand- II electron transfer upon thermal treatment in the to-metal charge transfer (LMCT) of the Fe chro- mother liquor. mophore [17]. The broad band at 763 nm can be 3 -1 χT (cm mol K) Downloaded from https://academic.oup.com/nsr/article/5/4/507/4931058 by DeepDyve user on 16 July 2022 RESEARCH ARTICLE Jiao et al. 511 (a) (b) 1.4 1.4 1 2CH OH 4H O 298 K 3 2 1.2 1 1.2 1a 1.0 1.0 0.8 0.8 0.6 0.6 0.4 0.4 360 K 0.2 0.2 0.0 0.0 300 600 900 1200 1500 300 600 900 1200 1500 Wavelength (nm) Wavelength (nm) Figure 4. Optical spectra for 1·2CH OH·4H O, 1 and 1a. (a) Solid-state UV–vis spectra for 1·2CH OH·4H O, 1 and 1a at room temperature. 3 2 3 2 (b) Variable-temperature solid-state UV–vis spectra for 1a in heating (298→360 K) mode. II III 3 −1 assigned as the Fe → Co intervalence charge- a maximum value of 5.61 cm mol K (Fig. 5a). transfer (IVCT) band [17,35]. In contrast, a broad When the sample is heated from 2 K after irradi- absorption band at 459 nm with a small shoulder ation, χT values first increase steeply to a sharp 3 −1 band at 534 nm is observed for 1. The higher-energy maximum of 6.38 cm mol K at 8.2 K, indi- band at 459 nm is assigned to a spin- and Laporte- cating an almost complete conversion from dia- II III allowed LMCT transition, and the small shoulder magnetic {Fe (μ-CN)Co } to paramagnetic LS LS II III II band at 534 nm can be assigned to the Co → {Fe (μ-CN)Co } linkages (Fig. 5b). The in- LS HS III Fe IVCT band [39]. These results confirm that crease in χT values from 2 to 8.2 K is attributed to the presence of intermolecular antiferromag- 1·2CH OH·4H O and 1a possess the diamagnetic 3 2 II III netic interactions and/or zero field splitting [ 16]. {Fe (μ-CN)Co } linkages, whereas 1 pos- LS LS III II sesses the paramagnetic {Fe (μ-CN)Co } Upon further heating to 73 K, the photo-induced LS HS III II linkages at room temperature. metastable paramagnetic {Fe (μ-CN)Co } LS HS II Variable-temperature solid-state UV–vis–NIR linkages relax to the initial diamagnetic {Fe (μ- LS III absorption spectra of 1a were measured in the CN)Co } linkages, indicating that magnetiza- LS temperature range of 298–360 K (Fig. 4b). As tion can be increased by light irradiation and recov- the temperature increases, there is a gradual ered with thermal treatment. decrease in the broad absorption centered at 763 nm On the basis of the optical studies, 1a in HT II III II III for the characteristic band of the Fe → Co IVCT, phase displays a Co → Fe IVCT band at 527 nm. and the LMCT/MLCT band at 427 nm is shifted The photo-induced metastable phase after irradia- to the lower-energy region associated with the ap- tion with an 808 nm laser was further irradiated II III pearance of the Co → Fe IVCT band at 527 nm. with a 532 nm laser in order to investigate the The observed spectral change confirms the occur- photo-induced reversibility. As a result, χT val- 3 −1 rence of thermally induced electron transfer with the ues decrease from 5.61 to 2.27 cm mol K after II III transformation from the {Fe (μ-CN)Co } to 120 min irradiation at 20 K (Fig. 5b), indicat- LS LS III II {Fe (μ-CN)Co } linkages. ing 59.5% recovery of the diamagnetic state. This LS HS successive photoreversibility of the magnetization can be well repeated (Fig. 5a and see Supple- Photomagnetic characterization mentary Fig. 11). This magnetic change verifies The photomagnetic effects of 1a were examined to the occurrence of photo-induced reversible elec- determine the possibility of photo-induced electron tron transfer in 1a. The relaxation of the photo- II III induced metastable state was monitored at dif- transfer. Because an Fe → Co IVCT band was ferent temperatures in order to probe the sta- observed at 763 nm for the LT phase of 1a,an II bility of the photo-induced phases (see Supple- 808 nm laser was selected to stimulate the Fe → III Co IVCT band and used for photomagnetic ex- mentary Fig. 12). In the low-temperature (10– periments. When the sample is irradiated at 20 K 40 K) region, relaxation time τ was less de- for 120 min, χT values rapidly increase and reach pendent on temperature. This result additionally Absorbance Absorbance Downloaded from https://academic.oup.com/nsr/article/5/4/507/4931058 by DeepDyve user on 16 July 2022 512 Natl Sci Rev, 2018, Vol. 5, No. 4 RESEARCH ARTICLE (a) (b) 808 nm Before irradition 532 nm 808 nm 6 6 532 nm 0 150 300 450 600 750 0 20 40 60 80 100 Time (min) Temperature (K) Figure 5. Photoreversibility of the magnetization for 1a. (a) Plots of χT vs time under cycles of successive irradiation at 808 and 532 nm at 20 K for 1a. (b) Plots of χT vs temperature for 1a before irradiation, after irradiation at 808 nm, the metastable-induced state irradiated at 532 nm, and after thermal annealing treatment at 100 K. confirms that the photoreversibility changes were tion at 808 nm, the intensity of the cyanide stretch- II III induced by light rather than the thermal relaxation ing bands attributed to the {Fe (μ-CN)Co } LS LS effect. linkages decreases and a new peak appears at −1 III 2162 cm , which is attributed to the {Fe (μ- LS II CN)Co } linkages. The intensity of the new HS IR spectra peak gradually increases with irradiation time. Con- versely, the photo-induced metastable phase was ir- The IR spectra analysis of 1·2CH OH·4H O, 3 2 radiated at 532 nm, inducing a decrease in the in- 1 and 1a also supports the redox state assign- tensity of the cyanide stretching band attributed to ments deduced from the structural and magnetic III II the {Fe (μ-CN)Co } linkages, and the in- LS HS analyses. For 1·2CH OH·4H O, the typical ab- 3 2 II tensity of the peaks attributed to the {Fe (μ- LS sorption bands of cyanide groups (2107, 2089 and III −1 CN)Co } linkages increases. By normalization of LS 2071 cm , see Supplementary Fig. 13) indicate III II the peak intensities for the {Fe (μ-CN)Co } LS HS that the compound possesses the diamagnetic II III linkages vs irradiation time upon irradiation at {Fe (μ-CN)Co } linkages [17,33]. Three LS LS II 808 and 532 nm, the recovery of the {Fe (μ- LS absorption bands for cyanide groups are observed III CN)Co } linkages is estimated to be 60.0%, LS in the IR spectrum of 1 (see Supplementary which is comparable to the value obtained from Fig. 13), corresponding to the stretching vi- photomagnetic measurements (see Supplementary brations for the bridging cyanide ions in the III II Fig. 15). Therefore, the observed spectral changes {Fe (μ-CN)Co } linkages (2148 and LS HS −1 further confirm the occurrence of photo-induced 2143 cm ) and the terminal cyanide ions in the Pz III – −1 reversible electron transfer with interconversion [ TpFe (CN) ] anions (2122 cm )[20]. III II II between {Fe (μ-CN)Co } and {Fe (μ- LS HS LS At 300 K, 1a shows absorption bands at 2069, III −1 CN)Co } linkages through successive and alter- LS 2085 and 2106 cm , confirming that 1a possesses II III native irradiation at 808 and 532 nm. the {Fe (μ-CN)Co } linkages in the LT LS LS phase (see Supplementary Fig. 14). When the temperature increases to 360 K, the intensity of II Structure and property discussion the cyanide stretching bands for the {Fe (μ- LS III CN)Co } linkages decreases, and new bands Air-stable 1a can be obtained by two-step-wise LS III II for the {Fe (μ-CN)Co } linkages appear at SCSC transformations from the air-unstable LS HS −1 2150 and 2159 cm as seen in 1. Furthermore, 1·2CH OH·4H O. This unprecedented succes- 3 2 upon cooling to room temperature, the IR spectra sive single-crystalline transformations-induced of 1a return to their initial state, suggesting that the electron transfer inspired us to investigate their thermally induced intermetallic electron transfer is structural correlations. First, the structure of reversible. 1·2CH OH·4H O indicates that hydrogen- 3 2 The irradiation-time dependence of the IR spec- bonding interactions are formed between the tra was measured to further verify the occurrence nitrogen atom of the terminal cyanide and an of photo-induced reversible electron transfer in 1a uncoordinated water molecule with the N···O upon successive and alternative irradiation with 808 distance of 2.713(12) A (see Supplementary Fig. 1 and 532 nm lasers at 20 K (Fig. 6). Upon irradia- and Table 4). Hydrogen-bonding strength can 3 -1 χT (cm mol K) 3 -1 χT (cm mol K) Downloaded from https://academic.oup.com/nsr/article/5/4/507/4931058 by DeepDyve user on 16 July 2022 RESEARCH ARTICLE Jiao et al. 513 (a) (b) 0 min 1 min 808 nm irradiating 0 min 532 nm irradiating 2 min 0.2 min 3 min 0.5 min 6 min 2 min 10 min 4 min 12 min 6 min 18 min 8 min 24 min 12 min 30 min 16 min 36 min 24 min 42 min 32 min 48 min 54 min 2200 2150 2100 2050 2000 2200 2150 2100 2050 2000 -1 -1 Wavenumber (cm ) Wavenumber (cm ) Figure 6. Photo-induced IR spectra for 1a. Irradiation-time dependence of the IR spectra for 1a irradiated at 808 nm (a) and the photoreversibility upon irradiation at 532 nm (b) at 20 K. have a significant effect on redox potential and (C–H···π and π ···π) between dpq ligands. This may have a significant effect on the intermetallic can be observed directly in the deviation of the electron-transfer behavior. Hydrogen bonding as Co–N≡C bond angles (see Supplementary Ta- an electron-withdrawing effect can result in the ble 3). Compared with the bond angles [171.8(5) II ◦ positive shifting of Fe redox potential [31,33]. and 168.6(6) ]in 1·2CH OH·4H O, the differ- 3 2 II ◦ Fe ions are more stable in this situation. Thus, ence value (8.9 ) of the bond angles [171.1(6) and II ◦ 1·2CH OH·4H O exhibits stable {Fe (μ- 162.2(7) ]in 1 is larger, indicating larger devia- 3 2 LS III CN)Co } linkages in the solvated phase. When tion from the ideal octahedron for the latter. For LS 1·2CH OH·4H O loses the solvent molecules 1a, the bond angles [166.1(3) and 166.7(3) ]are 3 2 and transforms into the desolvated phase 1,the nearly the same, which suggests that the distortion hydrogen bonding is destroyed, accompanied of the CoN octahedron becomes smaller. Contin- II by a negative shift in the redox potential of Fe . uous shape measurements (CShM) analysis and the II III The {Fe (μ-CN)Co } linkages become parameter (the sum of |90 − α| for the 12 cis-N– LS LS III unstable and tend to transform into the {Fe (μ- Co–N angles around the Co atoms) were also cal- LS II CN)Co } linkages. Therefore, the electron culated (see Supplementary Table 2). The CShM HS transfer from 1·2CH OH·4H O to 1 occurs as a of cobalt is 0.178 (1·2CH OH·4H O), 1.014 (1), 3 2 3 2 result of thermally induced desolvation. 0.276 (LT phase of 1a) and 1.216 (HT phase of 1a), Second, the average distances of π ···π and respectively. A smaller value is generally associated C−H···π interactions between the adjacent with a stronger ligand field, leading to an LS state of Pz dpq/ Tp ligands are 3.753 and 3.148 Afor the metal ion, whereas the larger value corresponds 1·2CH OH·4H O, 3.665 and 3.102 Afor 1 to a weaker ligand field and support an HS state. 3 2 and 3.457 and 3.002 Afor 1a, indicating that From 1·2CH OH·4H O to 1 and 1a, the values first 3 2 the interactions gradually become stronger from increase and then decrease, which suggests that the 1·2CH OH·4H O to 1 and 1a. The enhancement of strength of the ligand field changes from strong to 3 2 intermolecular interactions increases the synergistic weak and back to strong. Thus, the Co ion exhibits effects between molecules and causes an energy HS state in 1 and the LS state in 1·2CH OH·4H O 3 2 decrease to a more stable structure, providing an and 1a. This unusual behavior serves to illus- important driving force for the successive two-step trate how subtle change in C–H···π and π ···π irreversible SCSC transformations. In comparison, interactions can actually have a profound effect only polycrystalline powder 1a can be obtained on SCSC transformations and electron-transfer directly using solvothermal conditions at 100 C properties. starting from the precursors, as confirmed by pow- der XRD analyses (see Supplementary Fig. 16). This results show the importance of successive crystalline CONCLUSION transformations. In addition, the coordination spheres of cobalt The enhancement of intermolecular π ···π in- centers bring out different distortion accompa- teractions drives successive single-crystalline nying the changes in intermolecular interactions transformations and achieves three crystalline Transmission (a.u.) Transmission (a.u.) 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Journal
National Science Review – Oxford University Press
Published: Jul 1, 2018