Synthesis and Characterization of Two Organic-Inorganic Hybrid Copper（Ⅱ）Compounds with the Reversible Thermochromism

LI Ming-Li ZHANG Xiu-Xiu RAO Wen-Jun WEI Zhen-Hong WANG Ling-Yu CAI Hu

(College of Chemistry,Nanchang University,Nanchang 330031,China)

Abstract:Reactions of N，N-dimethyl-1，5-diazabicyclo[3.2.1]octane iodide([3.2.1-Me2dabco]I2)and 1-amino-1，4-diazabicyclo[2.2.2]octane iodide([2.2.2-NH2dabco]I)with anhydrous copper chloride in concentrated HCl aqueous solution afforded two organicinorganic hybrid compounds[3.2.1-Me2dabco][CuCl4](1)and[2.2.2-NH2dabco][CuCl4](2)，respectively.The single-crystal X-ray diffraction revealed that the inorganic component in compounds 1 and 2 was a distorted tetrahedral geometry[CuCl4]2-.Compounds 1 and 2 showed an intriguing reversible thermochromism with color change from yellow to red which may be derived from the deformation of[CuCl4]2-tetrahedron.CCDC:1983284，1;1983282，2.

Keywords:1，4-Diazabicyclo[2.2.2]octane;copper chloride;thermochromism;organic-inorganic hybrid;crystal structure

Thermochromism，the phenomenon of reversible change of color with change of temperature has been studied for decades due to their potential applications in the smart windows，temperature sensors，visual thermometers and solar control glazing[1-6].In past years，immense efforts have been devoted to preparation an extensive range of novel thermochromic material classes，including the conventional inorganic solids like vanadium dioxide[7]，hydrogels[8]，polymers[9-10]，synthetic dyes[11]，organic crystals[12]，coordination complexes[13-14]，and organic-inorganic hybrid compounds[15-16].

Among these different kinds of thermochromic materials，organic-inorganic hybrid compounds have received extraordinary research attention because of their structural advantages[17-18].In these hybrids，the presences of inorganic components supply excellent electronic properties and mechanical and thermal stability，and the organic components ensure structural flexibility and synthetic versatility[19-20].Detailed studies on the reversible thermochromic properties of organicinorganic hybrid compounds revealed the origins were most attributed to the change of coordination geometry，bond breakage/formation，dynamic structural change，charge transfer，and mode of coordinating ligand[21].

On the other hand，temperature-induced reversible phase transitions have been common observed in the copper（Ⅱ） organic-inorganic complexes[22-25]，which were mostly caused by the change in coordination number and coordination geometry[26-28].This is because that the color is limited by the energy of d-d electronic transitions of central Cu2+ion in organic-inorganic Cu（Ⅱ）halide system，where the color change generally in the range from green to red，including green to yellow，yellow to brown and yellow to red[29-31].For examples，the known organic-inorganic hybrid compounds[(C2H5)2NH2]CuCl4[32]，[(2-aminobenzothiazolium)2]CuCl4[33]and[DMe-DABCO]CuCl4(N，N′-dimethyl-1，4-diazoniabicyclo[2.2.2]-octonium)[34]have been reported to exhibit reversible thermochromism.

In this paper，two spherical organic molecules 1-amino-1，4-diazabicyclo[2.2.2]octane-1，4-diium iodide([2.2.2-NH2dabco]I)and N，N-dimethyl-1，5-diazabicyclo[3.2.1]octane iodide([3.2.1-Me2dabco]I2)were synthesized and chosen to react with CuCl2，and finally led to the formation of tetrahedrally coordinated organicinorganic hybrid compounds [3.2.1-Me2dabco][CuCl4](1)and[2.2.2-NH2dabco][CuCl4](2).Herein，we report the crystal structure and reversible thermochromism of 1 and 2.

1 Experimental

1.1 Instruments and materials

The starting materials anhydrous CuCl2，1，4-diazabicyclo [2.2.2]octane， 1，5-diazabicyclo[3.2.1]octane and the concentrated hydrochloric acid are commercially available and used without further purification.1-Amino-1，4-diazabicyclo[2.2.2]octane-1，4-diium iodide ([2.2.2-NH2dabco]I)[35]and N，N-dimethyl-1，5-diazabicyclo[3.2.1]octane iodide([3.2.1-Me2dabco]I2)[36]were synthesized according the reported literatures.Powder X-ray diffraction data of 1 and 2 were recorded on an X-ray powder diffractometer(Beijing Persee Instrument Co.，Ltd.XD-3)with Cu Kα radiation(λ=0.154 06 nm)operating at 40 kV/15 mA with a Kβ foil filter at a speed of 2°·min-1in a range of 5.00°≤ 2θ≤ 55.00°.Elemental analysis of C，H and N were conducted on a Vario EL III elemental analyzer.

1.2 Synthesis

1.2.1 N，N-dimethyl-1，5-diazabicyclo[3.2.1]octane iodide([3.2.1-Me2dabco]I2)

To a 40 mL methanol solution of 1，5-diazabicyclo[3.2.1]octane(4 g，36 mmol)was dropwise added CH3I(76 mmol，10.5 mL).After addition，the reaction was refluxing for 3.5 hours，and led to the formation of gray precipitate，which was isolated by filtration and washed with hot ethyl acetate，dried in vacuo.Yield:1.6 g，95%.1H NMR(400 MHz，D2O): δ 5.35(s，2H)，4.45(d，J=6.7 Hz，2H)，4.24(dd，J=16.1，11.0 Hz，2H)，4.10～3.85(m，4H)，3.55(s，6H)，2.79(m，J=25.6，19.3，12.8，6.5 Hz，1H)，2.55(d，J=16.4 Hz，1H).

1.2.2 1-Amino-1，4-diazabicyclo[2.2.2]octane iodide([2.2.2-NH2dabco]I)

To an aqueous solution (20 mL)of 2.2.2-dabco(16.80 g，150.0 mmol)was dropwise added an aqueous solution(10 mL)of H2NOSO3H(5.65 g，50.0 mmol)at the temperature of 90℃under the N2atmosphere.After the mixture was cooled to room temperature，K2CO3(6.90 g，50.0 mmol)was added.The resulting mixture was stirring for 10 minutes，then the solvent was removed by vacuum-rotary evaporation，and the crude residue was washed by tetrahydrofuran(THF)for 3 times.The residue was collected and dissolved in 30 mL EtOH，and K2CO3was removed by filtration.The filtrate was acidized by the addition of 7.0 mL hydriodic acid (11.0 g，57%(w/w)).The reaction mixture was allowed to be stirring at-30℃for 1 hour to give pale-yellow precipitates，which were filtered and washed with EtOH to afford the desired product as a white solid.Yield:6.5 g，34%.1H NMR(400 MHz，D2O):δ 3.87～3.78 (m，6H)，3.67～3.58(m，6H);13C NMR(100 MHz，D2O):δ 56.4，44.7.

1.2.3 [3.2.1-Me2dabco][CuCl4](1)

To an aqueous solution of[3.2.1-Me2dabco]I2(1.98 g，5 mmol)was added Ag2CO3(0.7 g，2.54 mmol)solid.The mixture was kept stirring for 2 hours and filtered.The filtrate was acidized with 8 mL hydrochloric acid(36%(w/w))，and anhydrous CuCl2(0.67 g，5 mmol)was added.The yellow block crystal was obtained by volatilizing the solution at 40℃for three days.Yield:2.69 g，80%.Anal.Calcd.for C8H16N2CuCl4(%):C，27.81;H，4.67;N，8.11.Found(%):C，28.12;H，4.45;N，8.28.

1.2.4 [2.2.2-NH2dabco][CuCl4](2)

To a concentrated HCl aqueous solution(10 mL，36%)was added[2.2.2-NH2dabco]I(2.55 g，10.0 mmol)and anhydrous CuCl2(1.35 g，10.0 mmol).Evaporation of the solution gave the yellow rod-like crystals at room temperature in three days.The crystals were filtered and washed with ethanol and dried in vacuo.Yield:2.51 g，75%.Anal.Calcd.for C6H15N3CuCl4(%):C，21.54;H，4.52;N，12.56.Found(%):C，21.25;H，4.32;N，13.02.

1.3 X-ray crystallography

Diffraction data of the single crystals with dimensions of 0.14 mm×0.18 mm×0.15 mm for 1 and 0.26 mm×0.10 mm×0.12 mm for 2 were collected on a Bruker SMART CCD area detector diffractometer with graphite monochromated Mo Kα radiation(λ=0.071 073 nm).The crystal structures were solved by direct methods using SHELXL[37].Non-hydrogen atoms were first refined isotropically followed by anisotropic refinement by full matrix least-squares calculations based on F2.Hydrogen atoms on carbon and nitrogen atoms were placed in idealized positions and treated as riding atoms.Crystal data and structure refinement results of compounds 1 and 2 were summarized in Table 1.

CCDC:1983284，1;1983282，2.

Table 1 Crystallographic data for compounds 1 and 2

2 Results and discussion

2.1 Synthesis and characterization

The two caged organic amines[3.2.1-Me2dabco]I2and [2.2.2-NH2dabco]I were synthesized according to the literatures with little modifications[35-36]，and their corresponding organic-inorganic hybrid copper（Ⅱ）compounds[3.2.1-Me2dabco][CuCl4](1)and [2.2.2-NH2dabco][CuCl4](2)were obtained by reactions with CuCl2on the basis of molar ratio of1∶1 in concentrated hydrochloric acid.Their phase purities were verified using the elemental analysis (EA)and powder X-ray diffraction (PXRD)patterns.As shown in Fig.1，the experimental PXRD curves of 1 and 2 were matched very well with the simulated ones in terms of the single-crystal X-ray data.

Fig.1 PXRD patterns of compounds 1(above)and 2(below)with the simulated one in red and the experimental one in black

2.2 Crystal structure analysis

Compound 1 crystallizes in the monoclinic system with P21space group，and the asymmetric unit contains one independent[CuCl4]2-anion and one[3.2.1-Me2dabco]2+cation，as shown in Fig.2.In the[3.2.1-Me2dabco]2+cation，the two positive charges are supplied by two quaternary nitrogen atoms.In the[CuCl4]2-anion，the central Cu2+ion adopts a tetrahedral geometry coordinated by four Cl-ions.As shown in Table 2，compound 1 has four independent Cu-Cl bond distances ranging from 0.221 62(17)to 0.223 05(11)nm and six independent bond angles∠Cl-Cu-Cl varying from 100.28(4)°to 128.28(4)°，indicating a slightly distortion of the [CuCl4]2-tetrahedron in 1.

Fig.2 Crystal structures of 1 in the asymmetric unit with the displacement ellipsoids drawn at the 50%probability level

Table 2 Selected bonds(nm)and angles(°)for 1 and 2

Fig.3 Crystal structures of 2 in the asymmetric unit with the displacement ellipsoids drawn at the 50%probability level

Compound 2 crystallizes in the monoclinic system with a space group of P21/c and the asymmetric unit contains one protonated[2.2.2-NH2dabco]2+cation and one [CuCl4]2-anion，as shown in Fig.3.In the[2.2.2-NH2dabco]2+cation，only the tertiary amine nitrogen atom N(3)is protonated，and the primary amine nitrogen N(1)can′t be protonated.The two positive charges in[2.2.2-NH2dabco]2+cation are expressed by one quaternary nitrogen atom N(2)and one tertiary protonated N(3)atom.Similar to compound 1，the metal center Cu2+ion in the[CuCl4]2-anion is also tetrahedrally coordinated by four Cl-ions.As shown in Table 2，the Cu-Cl bond lengths in compound 2 varied from 0.223 28(6)to 0.226 19(6)nm，and the bond angles∠Cl-Cu-Cl range from 97.79(2)°to 130.63(3)°，which are larger than the corresponding ones in compound 1，revealing the[CuCl4]2-tetrahedron in compound 2 is slightly distorted than that in compound 1.

In compound 1，there is no hydrogen bond between the cation and the [CuCl4]2-anion，and the∠Cl-Cu-Cl angles are less than 130°.While，in compound 2 the cations and anions are connected by three kinds of N-H…Cl hydrogen bonds extending indefinitely in the two-dimensional(2D)ac plane as shown in the Fig.4.Due to the existing of N-H…Cl hydrogen bonding(Table 3)，the maximum ∠Cl-Cu-Cl angle of 130.63(3)°in compound 2 is slightly larger than 130°.

Fig.4 Hydrogen bonds connected the cations and anions in compound 2

Table 3 Hydrogen bonds in compound 2

2.3 Thermochromism

Interestingly，the crystals 1 and 2 both exhibited yellow color at room temperature.When the temperature exceeds 60℃，the yellow crystals 1 and 2 gradually changed from yellow into red.Conversely，the red crystals were reversed to yellow completely upon cooling to room temperature in just few minutes，exhibiting reversible thermochromism，in Fig.4.To our best knowledge，thermochromism is commonly seen in other tetrahedrally coordinated Cu（Ⅱ） halides，where the Cu2+ion has the 3d9electronic configuration，and the d-level is not completely filled[38-39].Consequently，there are d-d electronic transitions within the 3d9electronic configuration.Since the position of the absorption maximum for the d-d transition is correlated with the coordination geometry of central Cu2+ion，the deformation of Cu（Ⅱ） halide polyhedron in the structure will cause the shift of electronic absorption bands，which induces the color change of organic-inorganic Cu（Ⅱ） halides.Therefore，in 1 and 2，the remarkable deformation of[CuCl4]2-tetrahedron is responsible for the yellow-to-red color change.

Fig.5 Reversible thermochromism in 1(top)and 2(bottom)

3 Conclusions

In summary， reactions ofN，N-dimethyl-1，5-diazabicyclo[3.2.1]octane iodide([3.2.1-Me2dabco]I2)and 1-amino-1，4-diazabicyclo [2.2.2]octane iodide([2.2.2-NH2dabco]I)with anhydrous CuCl2led to two organic-inorganic hybrid compounds[3.2.1-Me2dabco][CuCl4](1)and [2.2.2-NH2dabco][CuCl4](2)which were characterized by element analysis，single X-ray diffraction and powder X-ray diffraction.Crystals 1 and 2 showed reversible thermochromism，and the deformation of [CuCl4]2-tetrahedron was responsible for the yellow to red color change.

Acknowledgement:Wethank the National Natural Science Foundation of China (Grants No.21571094，21661021，21865015)for financial support.