Reversible Thermochromism of Nickel(II) Complexes and Single-Crystal-to-Single-Crystal Transformation.

A molecular Ni(II)-NNN pincer complex (1) exhibited unprecedented reversible single-crystal-to-single-crystal transformation and color change upon heating and cooling due to a subtle change in the N-Ni(II) bond length and ligand conformation. UV-vis, thermogravimetric, differential scanning calorimetry, single-crystal structural data, temperature-dependent powder X-ray diffraction, and Raman and computational studies supported the structural change of the Ni(II) complex with temperature.

Introduction

Smart materials that exhibit modulation in physical properties such as color, fluorescence, and magnetic and electronic properties with respect to external stimuli have received significant attention in recent years due to their potential applications ranging from biology to materials science.[1] In particular, temperature-induced color-changing (thermochromic) materials have found applications in thermochromic coatings, smart dyes, switches, security markers, thermal warning, medical strips, and sensors.[2] Among the thermochromic materials, inorganic materials are having additional advantages such as good thermal stability and durability in extensive range of temperatures over organic materials, though latter provide synthetic versatility. Inorganic semiconducting oxides (V2O5, PbO, and TiO2) and mixed-metal halides exhibited reversible thermochromism.[3] Inorganic salts such as copper(I) iodide, ammonium metavanadate, and manganese violet have shown irreversible phase transition and color change with temperature.[4] Thermochromic behavior of inorganic materials is attributed to the change of coordination geometry, guest absorption/desorption, bond breakage/formation dynamic structural change, strength, charge transfer, and mode of coordinating ligand.[5,6] For instance, yttrium-iron garnet, Y3Fe5O12, displayed reversible thermochromic properties due to a temperature-induced red shift of the O2––Fe3+ ligand-to-metal charge-transfer band.[7] Thermal expansion of axial Cu–O bonds in the 3d–4f hetero-bimetallic complex, Ho2Cu(TeO3)2(SO4)2, with temperature exhibited reversible thermochromism.[8] Single-crystal-to-single-crystal (SCSC) transformation via dehydration and rehydration of the mixed-metal complex, [Co2(ppca)2(H2O)(V4O12)0.5], exhibited reversible thermochromism between red and brown.[9] Fu and co-workers have reported low-temperature thermochromism in various iodometallate-based inorganic–organic hybrids.[10] Inorganic–organic hybrids, {(MV)2[Pb7Br18]}, exhibited thermoswitchable conductance intervened via thermochromism-controlled electron transfer.[11] Metal–organic hybrids based on polyhalobismuthate, [Dim]2[Bi2X10] (X = Cl, Br, and I), revealed photoluminescence with long emission lifetime as well as reversible thermochromism in a solid state.[12] The alteration of coordination equilibrium with temperature in ionic-liquid-based cationic nickel(II) complexes leads to color change from blue-green to orange.[13] Temperature-induced reversible phase transition and coordination geometry have been observed in the nickel(II)–isopropylamine complex.[14] Thus, thermochromism in Ni(II) complexes was mostly caused by the change in coordination number and coordination geometry.[15,16] However, the mechanism of color change of nickel(II) complexes (metal–organic materials) at different temperatures has not been well understood and also scarcely reported. Hence, nickel(II) complexes that retain their single-crystalline character after color change could provide structural evidence for thermochromism and its relevant mechanism. Herein, we report a series of nickel(II)–NNN pincer complexes (1–4) that exhibited reversible thermochromism. Complex 1 showed octahedral coordination geometry of Ni(II). Temperature-dependent powder X-ray diffraction (PXRD) and Raman spectra of 1 suggested a clear structural change at 140 °C that contributed for thermochromism. Significantly, the single-crystal X-ray diffraction study of 1 shows that it maintains its same single-crystal structure before and after heating. The structurally similar Ni(II)–NNN pincer complexes of 2–4 also exhibited reversible thermochromism.

Results and Discussion

The reaction of the tridentate pincer ligand Py–NNN (L1–L4) with NiCl2·6H2O in methanol at room temperature (RT) produced nickel–NNN pincer complexes (1–4) in good yield (Scheme ).

Scheme 1

Synthesis of Nickel(II)–NNN Pincer Complexes (1–4)

Slow diffusion of diethyl ether vapor into methanol solution of 1 produced light green crystals (Figure ). Interestingly, green crystals converted into purple upon heating at 150 °C (1B). Although it was slow, the purple crystals were reversed to green completely upon cooling to room temperature in a duration of 5 h. The complex 1 was melted at 235 °C and produced a dull sand-colored powder material upon cooling to room temperature. However, recrystallization of the sand-colored powder from methanol produced pale green crystals (1D). Heating of 1D at 150 °C showed a color change from pale green to purple, whereas cooling produced a color change from purple to green slowly, which perfectly matched with 1. Thus, complex 1 exhibited clear reversible thermochromism between green and purple upon heating and cooling (Figure ). Further clear color switching of 1 without significant decomposition indicated its good thermal stability.

Figure 1

Reversible thermochromic behavior and SCSC transformation of complex 1.

Single-crystal analysis of 1 showed monoclinic space group P21/n (Table S1). Nickel(II) center adopted a distorted octahedral coordination geometry with three-nitrogen coordination from the Py–NNN ligand and oxygen coordination from water and two chloride ions (Figure a). In complex 1, the bond lengths of Ni(1)–N(1), Ni(1)–N(2), and Ni(1)–N(1) are 1.9945(14), 2.3520(14), and 2.3709(15)Å, respectively, and Ni(1)–Cl(1), Ni(1)–Cl(2), and Ni(1)–O(3) bond lengths are 2.4266(5), 2.3979(5), and 2.0260(14) Å, respectively. The bond angles in N(1)–Ni(1)–O(3), N(2)–Ni(1)–N(3), and Cl(2)–Ni(1)–Cl(1) are 178.67(6), 154.37(5), and 172.230(17)°, respectively (Table S2). The coordinated water molecule formed intermolecular H-bonding interaction with Cl of another complex and produced a dimeric structure that was further stabilized by other weak H-bonding interactions between morpholine oxygen and hydrogen. The relevant O–H···Cl and C–H···O hydrogen bonding distances are 2.275 and 2.919(1) Å, respectively, and hydrogen bond angles in O(3)–H(2)···Cl(1) and C(2)–H(2A)···O(2) are 161.80(1) and 123.52(1)°, respectively (Figure ). Interestingly, the crystal of 1 when heated at 150 °C showed complete color change from green to purple (1B) as well as retained its single-crystalline character [SCSC transformation]. The digital image taken at room temperature undoubtedly confirmed the color change from green to purple (Figure b). Hence, we expected that single-crystal analysis of 1B would provide clear reason for color change by heating. But structural analysis of 1B also revealed the same monoclinic space group P21/n and almost indistinguishable structure from 1 (Table S3). Ni(II) center displayed distorted octahedral geometry with three-nitrogen coordination from the pincer ligand, one water molecule, and two chloride ions (Figure b). A closer look of the 1B structure revealed only a very slight conformational change in the morpholine ring and orientation of chloride ions. It is noted that 1 showed thermochromism at 150 °C but single-crystal analysis was performed at room temperature. Thus, contrary to our expectation, the structure reversed to the initial state while cooling though the crystal retain purple color.

Figure 2

(a) ORTEP diagram of complex 1 with 50% probability ellipsoids. (b) Structure and single-crystal digital images of 1, 1B, 1C, and 1D. C (gray), N (blue), O (red), Cl (green), and Ni (pink). Hydrogen atoms were omitted for clarity.

Figure 3

Supramolecular dimeric structures in the crystal lattice of 1, 1B, 1C, and 1D. C (gray), N (blue), O (red), Cl (green), and Ni (pink). H-atoms are omitted for clarity. H-bonding (broken line) distances are marked in Å.

The purple crystal (1B) slowly converted back to a green crystal (1C) upon cooling for a longer time (Figure ). The structure of 1C also perfectly matched with 1 and 1B. Similarly, structural analysis of pale green single crystals (1D) also revealed the same structure of 1 (Table S8). A comparison of the Ni–N and Ni–O bond lengths of 1, 1B, 1C, and 1D showed insignificant variation in bond lengths (Figure S1, Tables S2, S4, S6, and S8); all four structures showed a similar dimeric structure via H-bonding between coordinated water and chloride ions (Figure and Table ).

Table 1

Hydrogen Bonds for 1–1D

compound	dO–H···Cl	dC–H···O	∠OHCl	∠CHO
1	2.275(0)	2.919(1)	161.80(1)	123.52(1)
1B	2.294(1)	2.906(10)	163.34(2)	123.52(1)
1C	2.300(1)	2.904(10)	163.24(2)	123.81(1)
1D	2.298(1)	2.899(10)	162.52(2)	124.26(1)

Overall single-crystal analysis of 1, 1B, 1C, and 1D divulged only structural integrity of 1 upon heating/cooling/melting, which confirmed that there is no water/chlorine breakage from the coordination sphere. However, it also did not provide any clear structural evidence to explain thermochromism. To gain insight into structural-change-induced thermochromism, we have performed temperature-dependent PXRD (Figure a). Interestingly, the PXRD pattern of 1 did not show any pattern change up to 140 °C, but exhibited a clear change of PXRD pattern when heating at 160 °C. This clearly indicates that 1 undergoes a structural change upon heating at above 140 °C that causes the color change. The PXRD pattern of the cooled crystals perfectly matched with 1, which confirmed structural reversibility (Figure b). However, the exact structural change of 1 at higher temperature could not be obtained at this time. To verify that the bond stretching frequency variation occurs with temperature, we have performed Raman spectral studies (Figure S2). The Raman spectrum of 1 recorded at room temperature showed intense peaks, whereas decrease of peak intensity and disappearance of some peaks were observed at 150 °C. It is noted that Raman spectra did not show a significant change up to 140 °C. Thus, Raman studies also supported the phase change of 1 upon heating at above 140 °C temperature. The differential scanning calorimetry (DSC) studies of 1 showed a clear phase transition at 146 °C before melting at 235 °C (Figure S3). Thermogravimetric analysis (TGA) of 1 indicated no loss of coordinated water or chloride ions upon heating up to 150°C (Figure S4).

Figure 4

(a) Temperature-dependent PXRD patterns of 1. (b) PXRD patterns of 1 at room temperature (black) and of the sample cooled from 160 °C to room temperature (blue).

Electron spin resonance (EPR) studies of 1 and 1B did not show a significant change in the structures except a slightly strong peak between 900 and 1200 gauss (Figure S6). Fourier transformed infrared (FTIR) studies did not show a significant change in the peak position and suggest that 1 did not undergo any structural change chemically after heating (Figure S7). The electronic spectrum of complex 1 was recorded in the solid state to understand the color change (Figure ). Absorption spectra of 1 clearly showed differences before and after heating in the longer wavelength range (Figure ). Before heating, 1 showed ligand-based π–π* transitions at 261 and 312 nm, whereas the ligand-to-metal (L → M) charge-transfer transition was observed at 413 nm. The absorption at 706 nm could be assigned to the d–d transition.[15]

Figure 5

Solid-state UV–vis absorption spectra of 1 and 1B.

The ligand-centered transitions did not show significant change after heating. However, 1B exhibited a red-shifted L → M charge-transfer transition and blue-shifted d–d transition and appeared as a broad band between 460 and 580 nm.

Thus, absorption spectra of 1 clearly evidenced the color change before and after heating and supported thermochromism. The heating and cooling thermochromic cycle of 1 has also been performed four times to confirm the stability and reversibility (Figure S8). Further, to investigate the influence of side arms of nickel complexes on thermochromic behavior, Ni(II) complexes with a different substituent (piperidine, piperazine, and methylpiperazine) instead of morpholine have been synthesized to evaluate the structural robustness on thermochromism (2–4).

Interestingly, the complexes 2 and 3 exhibited clear reversible thermochromism upon heating and cooling of solids (Figure ). In contrast, heating of complex 4 at 90 °C resulted in melting, affording a brown liquid, which upon cooling to room temperature turned green without solidification. Significantly, the green compound showed reversible thermochromic behavior. Upon heating the green liquid at 90 °C, it turned brown, and upon cooling, the green color was retained (Figure ). Thus, by keeping the coordination element constant, different substituents on the side arms of the pincer ligand could alter the thermochromism of nickel complexes. Unfortunately, our efforts to grow single crystals of 2–4 were futile and hence solid-state structural studies for 2–4 could not be performed. However, the structures of 2–4 were confirmed using mass and FTIR spectroscopy studies.

Figure 6

Photographs of reversible thermochromic behavior for complexes 1–4 demonstrating the color change of the complexes by means of temperature.

Conclusions

The simple Ni(II)–NNN pincer complex showed reversible thermochromism in the solid state upon heating and cooling due to reversible structural change. Temperature-dependent PXRD and Raman studies supported the structural change of 1 at higher temperature. TGA and DSC studies indicated that the compound 1 did not undergo any decomposition or lose of coordinated ligands. However, only the structure determination at higher temperature can provide a clear mechanism of thermochromism.

Experimental Section

Materials and Methods

All solvents and chemicals were purchased from commercial suppliers (Aldrich, Alfa Aesar, and TCI Chemicals) and used as received. Solvents were purified with appropriate drying agents when required. All moisture-sensitive reactions were carried out under a nitrogen atmosphere. The UV–vis absorption spectra were recorded with UV–vis spectrophotometer (SHIMADZU 01174) with BaSO4 as a reference, equipped with a diffuse reflectance accessory. Fourier transformed infrared (FTIR) measurement was carried out with a Shimadzu IRAffinity-1S spectrophotometer with KBr pellets. HRMS was recorded using a fast-atom bombardment double-focusing magnetic sector mass spectrometer and the electron impact ionization technique (magnetic sector–electric sector double-focusing mass analyzer). 1H NMR (500 MHz) and 13C NMR (200 MHz) spectra were analyzed on the NMR spectrometer. EPR experiments were carried out using the Bruker EMX plus X-Band EPR spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded on a Philips PW3040/60 high-resolution diffractometer.

Synthesis of L1

To CH3CN (30 mL), solution of 2,6-bis(bromomethyl)pyridine (0.3 g, 1.13 mmol), K2CO3 (0.468 g, 3.39 mmol), and morpholine (0.197 g, 2.26 mmol) was added under the nitrogen atmosphere and stirred at 85 °C for 14 h. After completion of the reaction, water was added into the mixture and the product was extracted using chloroform. The organic fraction was dried and evaporated using a rotavapor under reduced pressure. Yield (0.280 g, 89%). IR (KBr): ν in cm–1 = 2800 (m), 1575 (m), 1454 (m), 1298 (m), 1111 (s), 906 (m), 864 (m). 1H NMR (500 MHz, CDCl3) δ 7.65–7.52 (m, 1H), 7.31 (d, J = 7.6 Hz, 2H), 3.84–3.69 (m, 8H), 3.66 (s, 4H), 2.51 (s, 8H). 13C NMR (126 MHz, CDCl3) δ 157.7, 136.7, 121.4, 77.3, 76.7, 66.9, 64.8, 53.7. ESI-MS: calcd for C15H23N3O2: 277.36; found [M + H]+: 278.

Synthesis of L2

Solution of 2,6-bis(bromomethyl)pyridine (1 g, 3.77 mmol) in CH3CN (40 mL) was added dropwise into the solution of 1-boc-peprazine (1.4037 g, 7.54 mmol) and K2CO3 (1.56 g, 1.13 mmol) in CH3CN (20 mL) under stirring. The solution was allowed to stir at 85 °C for 14 h. After the completion of the reaction, the reaction mixture was cooled and filtered. The filtrate was evaporated, extracted in chloroform, and dried over anhydrous Na2SO4, which afforded a pale yellow oil as a product (PMP1). Yield (0.8 g, 80%). 1H NMR (400 MHz, CDCl3) δ 7.78–7.48 (m, 1H), 7.31 (dd, J = 13.3, 7.0 Hz, 2H), 4.12–3.52 (m, 4H), 3.58–3.30 (m, 8H), 2.72–2.28 (m, 7H), 1.94 (s, 1H), 1.73–1.18 (m, 18H). 13C NMR (400 MHz, CDCl3): δ 157.77, 154.81, 136.74, 121.43, 79.60, 64.45, 53.08, 28.43. HRMS (ESI): calcd for C25H41N5O4 [M + H]+ 475.62; found 476.3244. To a solution of PMP1 (0.8 g) in methanol (15 mL), 1 N HCl (8 mL) was added and allowed to stir for 6 h at 60 °C. After cooling to room temperature, the reaction mixture was neutralized with 5% NaHCO3. Then, the solvent was evaporated. The resulting residue was washed with ethanol and concentrated in vacuum to get a yellow oily product. Yield (0.750 g, 75%). IR (KBr): ν in cm–1 = 3429 (w), 2923 (m), 1639 (s), 1461 (m), 1267 (m), 742 (s). 1H NMR (300 MHz, CDCl3) δ 7.68–759 (t, 1H), 7.45–7.29 (d, 2H), 3.92–3.64 (s, 4H), 3.15–2.72 (m, 8H), 2.71–2.42 (s, 8H), 2.43–2.21 (s, 2H). HRMS (ESI): calcd for C15H25N5 [M + H]+ 275.39; found 276.21.

Synthesis of L3

2,6-Bis(bromomethyl)pyridine (0.3 g, 1.13 mmol) in CH3CN (30 mL) was added dropwise to the mixture of piperidine (0.192 g, 2.26 mmol) and K2CO3 (0.468 g, 3.39 mmol) in CH3CN (15 mL), and the resulting reaction mixture was stirred for 14 h at 85 °C. The compound was extracted with chloroform and evaporated in vacuum under reduced pressure. Yield (0.282 g, 91%). IR (KBr): ν in cm–1 = 2939 (m), 2372 (m), 1627 (m), 1581 (m), 1456 (m), 1276 (m). 1H NMR (500 MHz, CDCl3) δ 8.19–7.31 (m, 2H), 7.19 (d, J = 7.7 Hz, 1H), 3.78–3.46 (m, 4H), 2.47–1.95 (m, 8H), 1.75–1.41 (m, 8H), 1.37 (d, J = 3.8 Hz, 4H). 13C NMR (400 MHz, CDCl3): δ 159.83, 146.45, 137.89, 136.53, 127.67, 124.37, 121.37, 65.23, 58.31, 54.80, 25.97, 24.22, 20.20. ESI-MS: calcd for C17H27N3: 273.41; found [M + H]+: 274.

Synthesis of L4

2,6-Bis(bromomethyl)pyridine (0.8 g, 3.0 mmol) in CH3CN (45 mL) was added dropwise to a stirred solution of 1-methylpiperazine (0.6 g, 6.0 mmol). After addition, K2CO3 (1.249 g, 9.0 mmol, in CH3CN (20 mL)) was added. The resulting reaction mixture was stirred for 14 h at 85 °C. After completion of the reaction, the compound was extracted using chloroform and water. The organic layer was collected and evaporated in vacuum under reduced pressure. Yield (0.815 g, 90%). IR (KBr): ν in cm–1 = 3396 (m), 2934 (m), 1610 (m), 1465 (s), 1296 (m), 1120 (m), 979 (m). 1H NMR (500 MHz, CDCl3) δ 7.59 (s, 1H), 7.28 (s, 2H), 3.66 (s, 4H), 2.54 (s, 8H), 2.46 (s, 8H), 2.28 (s, 6H). 13C NMR (126 MHz, CDCl3) δ 158.0, 136.5, 121.2, 77.2, 76.7, 64.4, 55.1, 53.2, 46.1. ESI-MS: calcd for C17H29N5: 303.44; found [M + H]+: 304

Synthesis of Complex 1

To a solution of ligand L1 (0.1 g, 0.36 mmol) in MeOH (10 mL) was added NiCl2·6H2O (0.085 g, 0.36 mmol). Then, the reaction mixture was stirred at RT for 3 h. Subsequently, the reaction mixture was filtered. Diethyl ether was allowed to diffuse onto the resulting mixture for 2 days. Then, green crystals were formed, which were washed with diethyl ether and collected. Yield (0.156 g, 91%). Anal. calcd (%) for C15H25Cl2N3NiO3: C, 42.39; H, 5.93; N, 9.89. Found: C, 41.98; H, 6.11; N, 10.23. IR (KBr): ν = 3341 (s), 3215 (m), 2957 (m), 1627 (s), 1460 (s), 1295 (m), 1108 (s), 863 (s), 634 (m).

Synthesis of Complex 2

To a stirred solution of ligand L2 (0.05 g, 0.18 mmol) in MeOH (8 mL) was added NiCl2·6H2O (0.043 g, 0.18 mmol). Then, the reaction mixture was stirred at RT for 3 h. The resulting mixture was evaporated in a vacuum under reduced pressure. Yield (0.069 g, 91%). Anal. calcd (%) for C15H27Cl2N5NiO: C, 42.59; H, 6.43; N, 16.56. Found: C, 42.18; H, 6.25; N, 16.10. IR (KBr): ν = 3343 (w), 2831 (m), 2492 (m), 1611 (s), 1460 (m), 1367 (m), 1289 (m), 1166 (m), 1036 (s), 989 (m), 860 (m), 810 (m).

Synthesis of Complex 3

To a stirred solution of L3 (0.05 g, 0.18 mmol) in MeOH (8 mL) was added NiCl2·6H2O (0.043 g, 0.18 mmol). Then, this reaction mixture was stirred at RT for 3 h, and the resulting solution was evaporated in a vacuum under reduced pressure. Yield (0.071 g, 92%). Anal. calcd (%) for C17H29Cl2N3NiO: C, 48.50; H, 6.94; N, 9.98. Found: C, 48.82; H, 7.12; N, 9.09. IR (KBr): ν = 3382 (w), 2929 (s), 1606 (s), 1461 (s), 1037 (m), 862 (m).

Synthesis of Complex 4

To a stirred solution of L4 (0.05 g, 0.16 mmol) in MeOH (8 mL) was added NiCl2·6H2O (0.039 g, 0.16 mmol). Then, the reaction mixture was stirred at RT for 3 h, and the resulting solution was evaporated in a vacuum under reduced pressure. Yield (0.071 g, 96%). Anal. calcd (%) for C17H31Cl2N5NiO: C, 45.27; H, 6.93; N, 15.53. Found: C, 45.02; H, 6.59; N, 14.84. IR (KBr): ν = 3379 (w), 2947 (m), 161 (s), 1465 (m), 1299 (m), 978 (m), 829 (m).