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Pressure Sensor with a Color Change at Room Temperature Based on Spin-Crossover Behavior Dameng Gao,† Yan Liu,† Bing Miao,† Chao Wei,† Jian-Gong Ma,*,† Peng Cheng,†,‡ and Guang-Ming Yang† †

College of Chemistry, Key Laboratory of Advanced Energy Material Chemistry (MOE), Nankai University, Tianjin 300071, P. R. China ‡ Co-Innovation Center of Chemistry and Chemical Engineering of Tianjin, Nankai University, Tianjin 300072, P. R. China

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S Supporting Information *

(bttmb) have been reported previously by our group for cobalt complexes.12 Compound 1 exhibits a 1D chain structure without a SCO phenomenon, whereas compound 2 displays a 2D 4,4 network with remarkable PSCO behavior with a color change from colorless to purple at room temperature, which could be an ideal candidate as a pressure sensor and be valuable for the synthesis of new PSCO compounds toward the production of “pressure-testing kits”. Complex 1 crystallizes in the triclinic space group P1̅ (Table S1) and has a 1D chain structure (Figure 1b), which exhibits a

ABSTRACT: Two new iron(II) complexes with 1D chain and 2D network structures have been successfully synthesized and characterized. One of the complexes exhibits a pressure-induced spin-crossover property with a reversible color change from white to purple at room temperature. The special property makes it a suitable candidate as a pressure sensor.

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ressure-induced switches and sensors have attracted significant attention and have been applied in a vast range because they are important for soil sciences,1 submarine equipment,2 space detection,3 and so on to monitor the pressure change in the corresponding equipment/spaces and to warn the operators about the pressure abnormalities. Spin-crossover (SCO) molecular materials, which have a switchable center and could convert between two states [high spin (HS) and low spin (LS)] under an external stimulus like the temperature, irradiation, magnetic field, or pressure,4,5 have been considered to be potential effective pressure sensors for geochemistry,6 geophysics,7 and biology8 in recent decades because of their low cost, high sensitivity, easily turned properties, and especially obvious color changes along with pressure variation.9 If we can hold a series of pressure-driven SCO (PSCO) compounds with sensitivities toward different pressures accompanied by obvious color changes at the environmental temperature, a system of “pressure-testing kits” may be constructed. However, although numerous SCO compounds have been synthesized and characterized,5,10 the study of SCO compounds with piezochromic properties is still quite rare in comparison with temperature-induced SCO compounds.11 Another barrier for applying the PSCO compounds as pressure sensors is that most of the known PSCO compounds demand low temperature to process and maintain their SCO behavior, and PSCO compounds with SCO behavior and a color-changing phenomenon at room temperature are still rare. As a result, continuous research about the PSCO compounds is still ongoing for their fascinating phenomena and potential application in sensors. Herein, we report that the design and synthesis of two new iron(II) compounds were obtained with the formulas [Fe(btmtmb) 2 (SCN) 2 ] n (1) and [Fe(bttmb)2(SCN)2]n (2). The ligands 1,3-bis(1,2,4-triazol-1ylmethyl)-5-methoxy-2,4,6-trimethylbenzene (btmtmb) and 1,3-bis(1,2,4-triazol-1-ylmethyl)-2,4,6-trimethylbenzene © XXXX American Chemical Society

Figure 1. Structures of complex 1: (a) coordination environment of iron(II) ions in 1; (b) 1D chain structure of 1.

configuration similar to that of the cobalt complex [Co(btmtmb)2(SCN)2]n.12 Each iron(II) ion is six-coordinated in a distorted octahedral environment. Two NCS− ions occupy the axial positions, and the equatorial plane is defined by four nitrogen atoms from four btmtmb ligands (Figure 1a). The Fe− N distances in 1 are in the order of Fe−Ntrz > Fe−NNCS−, and the Fe···Fe distances are 10.48 Å. The average Fe−N bond length is 2.18 Å (Table S2), which falls in the range of a value typical for Received: August 28, 2018

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DOI: 10.1021/acs.inorgchem.8b02408 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry HS Fe−N bonds.13 Iron(II) ions are linked via btmtmb ligands to form the Fe2(btmtmb)2 basic structural units, which are further self-assembled into a 1D chain. Consecutive chains are further packed into a 3D supramolecular structure through interlayer π···π interactions (Figure S1). Complex 2 crystallizes in the monoclinic space group P21/c (Table S1) and shows a structure similar to that of the previously reported cobalt complex [Co(bttmb)2(SCN)2]n.12 Each iron(II) ion exhibits an six-coordinate octahedral geometry defined by four nitrogen atoms from four bttmb ligands and two nitrogen atoms from two NCS− groups (Figure 2a). The axial

bar. The colorless transparent crystal of 2 (10 mg) and white KBr solid (100 mg) were mixed, ground, and then handled under select pressure. A gradual color change and darkening were observed with increasing pressure, as shown in Figure 3a.

Figure 3. (a) Color change of complex 2 (10 mg) mixed with KBr (100 mg) under different pressures and then color recovery by heating under vacuum. (b) Color change of 2 under pressure from colorless transparent crystals to purple and then color recovery by heating under vacuum.

The color of the obtained purple tablet could not be turned back to white just by heating. However, when the purple powder of the ground tablet was heated under vacuum, a white powder was obtained. The color change and recovery of 2/KBr indicated the PSCO behavior of the iron(II) centers in 2. Similarly, when the pure crystal of 2 was placed under a certain pressure (600 bar), the colorless crystal changed to a purple sheet, as shown in Figure 3b, which could be turned back to colorless/white by heating under vacuum as well as the 2/KBr mixture (Figure 3b), further confirming the occurrence of the PSCO phenomenon. Powder X-ray diffraction (PXRD) was used to characterize the structure of 2 (Figure S5), which showed the same patterns before and after color change under pressure, indicating maintenance of the structure. Considering that iron(II) might go through oxidation, we carried out X-ray photoelectron spectroscopy (XPS) for 2 (Figure S6). Two peaks at 723.75 and 709.85 eV as well as two satellite peaks were observed in the XPS spectra for the synthesized crystals of 2, which were attributed to the 2p1/2 and 2p3/2 electrons of iron(II),16 respectively, which confirmed that the iron ions were at 2+ state in 2. After being pressed, the purple powders of 2 exhibited the same XPS spectra as those of synthesized 2, indicating that the iron ions kept in their 2+ state during the whole color-change process, which confirmed that the color change of 2 originated from its PSCO behavior. Variable-temperature magnetic susceptibility measurements were applied to study the magnetic behaviors of 1 and 2 in detail at 2−300 K with a magnetic field of 1 kOe. Under ambient air pressure, both 1 and 2 showed no SCO phenomenon. As shown in Figure S8a, the χMT value of 1 at room temperature was 4.03 cm3 K mol−1. Upon cooling, the χMT value slowly decreased from 300 to 50 K. In the range from 50 to 1.8 K, the χMT value decreased abruptly, reaching a remnant value of 0.70 cm3 K mol−1 at 2 K, which was possibly due to the results of an orbital contribution to the paramagnetic susceptibility. The χMT value of 2 at room temperature was 3.99 cm3 K mol−1 (Figure S8b). Upon cooling, the variation trend of χMT value was similar to that of 1. The absence of pressure-driven thermal SCO behavior of 1 was mostly ascribed to the strong steric hindrance and weak ligand field originating from the additional methoxyl on the benzene ring, which forbid the occurrence of pressure-driven or thermal SCO behavior.17 It is important to compare the

Figure 2. Structures of complex 2: (a) coordination environment of iron(II) ions in 2; (b) 2D structure of 2.

positions of the coordination octahedron are occupied by two NCS− groups associated with the N7 and N7A atoms, while the equatorial plane is formed by four ligand bttmb nitrogen atoms (N1, N1A, N4, and N4A). All iron(II) ions in each layer are in the plane, and the Fe···Fe distances are 13.22 and 11.39 Å. The average Fe−N bond length is 2.17 Å (Table S2), which falls in the range of a value typical of HS Fe−N bonds.13 The Fe−N distances in 2 decrease in the order of Fe−Ntrz > Fe−NNCS−, as in other related iron(II) complexes.14,15 Iron(II) ions are linked via bttmb ligands to form Fe4(bttmb)4 structural units, which are further assembled into a 2D 4,4 grid layer (Figure 2b). Consecutive layers are further packed into a 3D supramolecular structure through interlayer π···π interactions (Figure S2). Interestingly, when we were preparing the KBr tablet of 2 for IR measurement, a purple tablet was obtained under the tablet machine, indicating the occurrence of the PSCO phenomenon. Contrarily, no color change of 1 was observed upon external pressure stimulation. In order to gain further insight, we studied the color change of 2 with the pressure varying from 100 to 600 B

DOI: 10.1021/acs.inorgchem.8b02408 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the activation energy ΔW°; thus, the iron(II) ions in 2 favored the LS state.20 After removal of the external pressure, the structure and Fe−N distances as well as the relatively high ΔΕ° remained and thus the color of 2 remained purple with the LSstate iron(II) ions, while heating under reduced pressure/ vacuum recovered ΔΕ° and ΔW° and the spin state of the iron(II) ions as well as the color of 2 turned back. The special performance of 2 makes it a suitable candidate as a visible pressure sensor. In conclusion, two new iron(II) complexes have been synthesized and characterized. As revealed by single-crystal Xray analysis, one has a 1D chain structure and the other has a 2D structure. Complex 2 exhibits an abrupt and complete spin transition under different pressures accompanied by a color change and recovery. A directly observed phenomenon is that additional pressure makes 2 favor the LS state and increase the SCO temperature. The results indicate that it is necessary to pay more attention to the pressure effects on SCO because PSCO is expected to make it into the solid-state sensors for temperature and pressure, which have potential applications in the aerospace,21 nuclear power domain,22 and automotive sectors.23 What is more, the sensors in a single device detect two thermodynamic parameters (temperature and pressure) to save measurements in time and space. The PSCO molecular material would be a suitable candidate for this purpose.8 Furthermore, 2 may be used as a visible pressure sensor because of its special PSCO property.

magnetic behaviors of 1 and 2 with their corresponding isostructural cobalt complexes.12 Although similar ligand fields were supplied by structures similar to those of the iron(II) and cobalt(II) ions, they exhibited totally different χMT versus T character. Both [Co(btmtmb) 2 (SCN) 2 ] n and [Co(bttmb)2(SCN)2]n showed large χMT values at room temperature and strong antiferromagnetic interactions, and no SCO phenomenon could be observed under either ambient or enhanced pressure.12 These different magnetic properties should clearly be attributed to the different electronic configurations of FeII d6 and CoII d7, which leads to their different responses toward similar coordination conformations and ligand fields. To study the pressure effects on the spin transition of 2, the variable-temperature magnetic susceptibilities were measured under pressures of 0.40 and 0.61 kbar, as shown in Figure 4.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02408. Experimental details, NMR and IR spectrospic data, crystallographic data, additional crystal structure diagrams, and PXRD and TGA patterns and XPS spectra of the complexes (PDF)

Figure 4. χMT versus T plots of 2 in the temperature range of 2−300 K under pressures of 0.40 and 0.61 kbar in the cooling and heating modes, respectively.

Under a pressure of 0.40 kbar, the χMT value of 2 was 2.51 cm3 K mol−1 at 300 K, which was smaller than that under 1 atm, probably because external pressures shortened the Fe−N bond lengths toward the distance favoring the LS state.11e,19a−c Upon cooling, the χMT value decreased slowly from 300 to 125 K and then decreased abruptly to 100 K. Between 100 and 2 K, the χMT value decreased slowly to 0.25 cm3 K mol−1, which was ascribed to the contribution from the paramagnetism of the HS iron(II) ions. The χMT values in the whole temperature range indicated that the spin transition was complete and centered at 113 K in the cooling and warming modes. When the pressure was increased to 0.61 kbar, the transition temperature dramatically increased. The χMT value dropped to 0.5 cm3 K mol−1 from 300 to 200 K, remained almost constant in the range from 200 to 25 K, and then continued to decrease to 0.25 cm3 K mol−1 at 2 K. Under pressures of both 0.40 and 0.61 kbar, the χMT value decreased to 0.25 cm3 K mol−1 at 2 K instead of the zero calculated for LS iron(II) ions, which could be mainly attributed to the residual population of HS iron(II) ions because some iron(II) ions on the crystal surface were not fully coordinated by the bttmb ligands.18 As the external pressure was increased, the spin transition moved to a higher temperature more gradually because additional pressure enhanced the ligand field around the iron(II) ions and reduced the difference of the chemical environments adjacent to the iron(II) sites, which was consistent with the literature report.19 The external pressure increased the zero-point energy difference ΔΕ° and decreased

Accession Codes

CCDC 1822297−1822298 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jian-Gong Ma: 0000-0001-7407-5521 Peng Cheng: 0000-0003-0396-1846 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the NFSC (Grant 21671111) and the NFS of Tianjin (Grant 17JCYBJC17700). REFERENCES

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DOI: 10.1021/acs.inorgchem.8b02408 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02408 Inorg. Chem. XXXX, XXX, XXX−XXX