Chinese Chemical Letters  2017, Vol. 28 Issue (5): 1027-1030   PDF    
A highly stable and luminescent mononuclear Cu(Ⅰ) bis-{5-tert-butyl-3-(6-methyl-2-pyridyl)-1H-1,2,4-triazole} complex
Yan-Sheng Luo, Jing-Lin Chen, Xue-Hua Zeng, Lu Qiu, Li Hua, Sui-Jun Liu, He-Rui Wen     
School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China
Abstract: A new emissive mononuclear homoleptic Cu(Ⅰ) complex of 5-tert-butyl-3-(6-methyl-2-pyridyl)-1H-1,2,4-triazole (bmptzH), [Cu(bmptzH)2](ClO4) (1), has been synthesized by treatment of [Cu(PPh3)2(CH3CN)2](ClO4) or[Cu(CH3CN)4](ClO4) with the bmptzH ligand. It is revealed that complex 1 displays a distorted N4 tetrahedral arrangement formed by two bmptzH chelates, in which bmptzH adopts a neutral bidentate chelating coordination mode using the N atom of the pyridyl ring and the 4-N not 2-N atom of the 1,2,4-triazolyl ring. It is shown that complex 1 is highly stable and exhibits good luminescence properties in solution and solid states at room temperature due to the introduction of a methyl group at the ortho-position of the pyridyl ring.
Key words: Cu(Ⅰ) complex     1,2,4-Triazole     6-Methyl-2-pyridyl     Luminescence     Crystal structure    
1. Introduction

There has been a rapidly increasing interest in the fundamental properties of emissive transition metal complexes, because of their potential applications in organic light-emitting devices, lightemitting electrochemical cells, chemical sensors/probes, and biological labeling [1]. In moving toward realization of various luminescent devices, much attention has been focused on the design and synthesis of the metal-based phosphors with the emission wavelengths in the entire visible region, particularly the late transition-metal emissive complexes [24]. However, due to the high cost and limited availability of these noble metals, more and more attention has been devoted to the exploitation of inexpensive alternatives over the past two decades. Recently, copper(Ⅰ) systems have become an important class of luminescent transition metal complexes, owing to their high relative abundance, low cost, and promising application perspectives in optoelectronics [510].

Cu(Ⅰ) complexes with two diimine chelates and one diimine plus one bis-phosphine mixed-ligands are one of the most extensively studied types [5, 11, 12]. However, this type of tetrahedral Cu(Ⅰ) complexes generally emit weak and short-lived emissions in solution, because their excited states readily suffer the Jahn–Teller based geometric distortion, which results in a more flattened conformation and thus facilitates relaxation back to the ground state via non-radiative pathways [5]. It is known that structural modification of the diimine ligands such as 1,10-phenanthroline is an effective approach for tuning luminescence properties of Cu(Ⅰ) complexes. On introducing various substituents displaying different steric and conformational effects on the Cu(Ⅰ) core, the steric hindrance is increased and the Jahn–Teller based distortion is more difficult to occur and thus luminescence properties are improved. Herein, we report the synthesis, structure and photophysical properties of a new Cu(Ⅰ) bis-{5-tert-butyl-3-(6-methyl-2-pyridyl)-1H-1,2,4-triazole} complex.

2. Results and discussion 2.1. Synthesis and characterization

Mononuclear Cu(Ⅰ) bis-diimine complex [Cu(bmptzH)2](ClO4) (1), instead of the anticipated mononuclear Cu(Ⅰ)-phosphinediimine complex [Cu(PPh3)2(bmptzH)](ClO4), was prepared by reaction of [Cu(PPh3)2(CH3CN)4](ClO4) with an equivalent amount of 5-tert-butyl-3-(6-methyl-2-pyridyl)-1H-1,2,4-triazole (bmptzH) in CH2Cl2 at room temperature (Scheme 1). This suggests that the possible reaction intermediate [Cu(PPh3)2(bmptzH)](ClO4) further rearranges into a thermodynamically stable product [Cu (bmptzH)2](ClO4) [11, 12], as supported by formation of another colorless crystalline species [Cu(PPh3)4](ClO4). Moreover, complex 1 could be obtained in good yield by treatment of [Cu(CH3CN)4] (ClO4) with two equivalent amounts of bmptzH in CH2Cl2 (Scheme 1). In the 1H NMR spectra of 1 in CD2Cl2 (Fig. S1 in Supporting information), besides the pyridyl proton signals at 7.38–8.20 ppm as two doublet peaks and one triplet peak and the C–H resonances from the tert-butyl at 1.26 ppm and the methyl at 2.31 ppm as two singlet peaks, the characteristic N–H proton signal of the 1,2,4-triazolyl ring occurs at 12.35 ppm as a broad singlet peak, suggesting that bmptzH directly coordinates to the Cu(Ⅰ) atom as a neutral ligand without deprotonation of the 1,2,4-triazolyl-NH, as revealed by X-ray crystallography. Thermogravimetric analysis of 1 was carried out in a nitrogen atmosphere with a heating rate of 10℃/min. As depicted in Fig. S3 (Supporting information), the structure of 1 remains stable up to 225℃ and only the little weight loss (obs. 2.51%) occurs, which is assigned to the release of CH2Cl2 and CH3OH solvate molecules. The observed loss is much less than the calculated value (16.42%) based on [Cu (bmptzH)2](ClO4)·CH2Cl2·CH3OH revealed by X-ray crystallography. This is perhaps due to the volatilization loss of CH2Cl2 and CH3OH solvate molecules from the surface of the crystalline sample. The weight loss (obs. 46.27%) between 225℃ and 1000℃ is attributed to the decomposition of two bmptzH ligands and is far less than the calculated value (60.72%), which is due to the retention carbon in the solid residue (black in color). The similar thermal decomposition behavior was also observed in an indium compound with organic ligand [13].

Download:
Scheme 1. Synthetic route of complex 1.

The exact structure of 1 was established by single-crystal X-ray crystallography. As indicated in Fig. 1, the Cu(Ⅰ) atom of 1 is fourcoordinated by four N atoms from two pyridyl and two 1,2,4-triazolyl rings, and these four N atoms further constitute a N4 distorted tetrahedral geometry with the N–Cu–N angles deviating obviously from the ideal angle (109°28'') of the regular tetrahedron due to the restricted bite angle of the bmptzH chelate, in which bmptzH adopts a charge-neutral chelating coordination mode using the N atom of the pyridyl ring and the 4-N atom of the 1,2,4-triazolyl ring not the 2-N atom of the 1,2,4-triazolyl ring [11, 12, 14]. The Cu– Npyridyl lengths (2.111(4) and 2.096(4) Å) are slightly longer than the Cu–Ntriazolyl lengths (2.036(3) and 2.021(3) Å), suggesting a stronger bonding of the Cu(Ⅰ) ion to the 1,2,4-triazolyl-N atom relative to the pyridyl-N atom [11, 12].

Download:
Figure 1. Perspective drawing of the cation of 1 showing 30% probability thermal ellipsoids and atom-labeling scheme.

2.2. Photophysical properties

Absorption spectra of bmptzH and its derivative 1 were measured in CH2Cl2 solution at ambient temperature (Fig. 2). The free ligand bmptzH displays two strong absorption bands in the UV region ( < 310 nm), attributable to the ligand-centered 1ππ* transitions inside bmptzH. Analogously, compound 1 also exhibits two strong absorption peaks in the same wavelength range, originating from the similar 1ππ* transitions within bmptzH. Moreover, a relatively weak low-energy absorption band is clearly observed for 1 in the range of 340–450 nm. On the basis of the previous work about the related Cu(Ⅰ) complexes [5, 11, 12, 15], the weak low-energy absorption band of 1 can be tentatively identified as the metal-to-ligand charge-transfer (1MLCT, Cu (3d)→bmptzH) state, perhaps mixed with some intra-ligand (1IL) charge-transfer transitions inside bmptzH.

Download:
Figure 2. Absorption spectra of bmptzH and complex 1 in CH2Cl2 solution.

Complex 1 gives a broad single emission band at ambient temperature in degassed CH2Cl2 solution (Fig. 3), peaking at 575 nm with the quantum yield of 0.020 and the emission lifetime of 8.7 µs. The emission intensity of 1 is drastically quenched by the presence of oxygen in aerated CH2Cl2 solution. This, together with the emission lifetime of several microseconds, suggests that the emission may be phosphorescent in origin. Thus, it is believed that the solution emission of 1 may originate from the metal-to-ligand charge-transfer (3MLCT, Cu(3d)→bmptzH) transition, probably mixed with some intra-ligand (3IL) charge-transfer character inside bmptzH. The solid-state emission of 1 was also investigated at room temperature using powder sample (Fig. 3). Complex 1 is highly emissive in the solid state at room temperature, and emits a strong emission with the maximum at 560 nm, the quantum yield of 0.784 and the emission lifetime of 17.1 µs. The solid-state emission of 1 has a hypsochromic shift of 15 nm compared to its solution emission, as a result of the rigid medium. Such an emission behavior is often observed in luminescent Cu(Ⅰ) complexes [7, 9, 11, 12].

Download:
Figure 3. Emission spectra of 1 in CH2Cl2 solution and in the solid state.

3. Conclusion

We have synthesized a new homoleptic mononuclear Cu(Ⅰ) bis-{5-tert-butyl-3-(6-methyl-2-pyridyl)-1H-1,2,4-triazole} complex. Its structural feature and photophysical properties have been well investigated. It is revealed that this Cu(Ⅰ) complex gives a distorted N4 tetrahedral geometry and 5-tert-butyl-3-(6-methyl-2-pyridyl)-1H-1,2,4-triazole acts as a charge-neutral chelating coordination mode using the N atom of the pyridyl ring and the 4-N not 2-N atom of the 1,2,4-triazolyl ring. It is shown that this Cu(Ⅰ) complex is highly stable and exhibits good luminescence properties in solution and solid states at room temperature, as a result of the introduction of a methyl group at the ortho-position of the pyridyl ring. The results presented herein might provide a new insight into the design and synthesis of phosphorescent materials based on highly stable Cu(Ⅰ) bis-diimine complexes with potentially high emission efficiency.

4. Experimental 4.1. Materials and instrumentation

All reactions were carried out under a N2 atmosphere, using anhydrous solvents or solvents treated with an appropriate drying reagent. Commercially available reagents were used without further purification unless otherwise stated. 5-tert-Butyl-3-(6-methyl-2-pyridyl)-1H-1,2,4-triazole (bmptzH) was synthesized according to the literature method [11]. The 1H NMR spectrum was recorded on a Bruker Avance Ⅲ (400 MHz) NMR spectrometer with SiMe4 as the internal reference (Fig. S1). Infrared (IR) spectrum was recorded on a Bruker Optics ALPHA FT-IR spectrometer using KBr pellets (Fig. S2). Thermogravimetric analysis was performed on a Perkin-Elmer Pyris Diamond TG/ DTA 6300 instrument under a nitrogen gas atmosphere with a heating rate of 10℃/min from room temperature to 1000℃ (Fig. S3). Elemental analyses (C, H and N) were conducted on a Perkin-Elmer model 240C elemental analyzer, where all the crystal samples are used after grinding and drying under vacuum. UV–vis absorption spectra were measured on a Shimadzu UV-2550 spectrometer. The photoluminescence properties in solution and solid states were determined on an Edinburgh analytical instrument (F900 fluorescence spectrometer) with a thermoelectrically cooled Hamamatsu R3809 photomultiplier tube. The emission quantum yields (Φem) in CH2Cl2 solution at room temperature were calculated by Φsr(Br/Bs)(nr/ns)2(Ds/Dr) using fluorescein in H2O as the standard (Φem=0.79), where the subscripts "r" and "s" denote the reference standard and the sample solution, respectively, and n, D, and Φ are the refractive index of the solvents, the integrated intensity, and the emission quantum yield, respectively [16, 17]. The quantity B is calculated by B=1-10-AL, where A is the absorbance at the excitation wavelength and L is the optical path length. An integrating sphere (Lab sphere) was applied to measure the emission quantum yield in the solid state.

Caution! The perchlorate salts are potentially explosive and should be handled carefully in small amount.

4.2. Synthesis of [Cu(bmptzH)2](ClO4) (1) 4.2.1. Method 1

A CH2Cl2 solution of [Cu(PPh3)2(CH3CN)2](ClO4) (63.9 mg, 0.083 mmol) and bmptzH (18.0 mg, 0.083 mmol) was stirred at room temperature for 3 h to give a yellow solution. The solvent was then removed at reduced pressure. The resulting residue was again dissolved in CH2Cl2, and slow diffusion of a mixture of methanol– petroleum ether (1:10, v/v) into the above solution yielded yellow crystals of 1 (20.5 mg, 0.034 mmol, 41% based on [Cu(PPh3)2(CH3CN)2](ClO4)). 1H NMR (CD2Cl2, 400 MHz): δ 12.35 (s, 2H), 8.19 (d, 2H, J=8 Hz), 7.90 (t, 2H, J=8 Hz), 7.39 (d, 2H, J=8 Hz), 2.31 (s, 6H), 1.26 (s, 18H). Selected IR (KBr, cm-1): 1108 s (ClO4-). Anal. calcd. for C24H32ClCuN8O4: C, 48.40; H, 5.42; N, 18.81. Found: C, 48.44; H, 5.40; N, 18.83.

4.2.2. Method 2

A CH2Cl2 solution of [Cu(CH3CN)4](ClO4) (17.4 mg, 0.053 mmol) and bmptzH (23.2 mg, 0.107 mmol) was stirred at ambient temperature for 3 h to give a yellow solution. The solvent was then removed at reduced pressure. The resulting residue was again dissolved in CH2Cl2, and slow diffusion of a mixture of methanol– petroleum ether (1:10, v/v) into the above solution gave yellow crystals of 1 (27.4 mg, 0.046 mmol, 87% based on [Cu(CH3CN)4] (ClO4)).

4.3. Crystallographic data and structure refinements

The measurement of 1 was performed on a Bruker D8 QUEST diffractometer at room temperature using a graphite-monochromated Mo Kα radiation (λ=0.71073 Å). The program CrystalClear was used for integration of the diffraction profiles. The structure was solved by direct method and refined by full-matrix leastsquares technique using the SHELXTL software package. The heavy atoms were located from E-maps and other non-hydrogen atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F2. The hydrogen atoms were generated theoretically onto the specific atoms and refined isotropically with fixed thermal factors. The crystallographic data and structure refinement details of 1 are summarized in Table 1, and the selected bond lengths and angles are given in Table 2.

Table 1
Crystal data and structure refinement parameters for 1.

Table 2
Selected bond lengths (Å) and angles (°) for 1.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Nos. 21561013, 21501077), the Major Program of Jiangxi Provincial Natural Science Foundation of China for Young Scholar (Nos. 20143ACB21017, 20161ACB21013), the Jiangxi Provincial Natural Science Foundation of China (Nos. 20142BAB203001, 20151BAB213003), and the Program for Qingjiang Excellent Young Talents of Jiangxi University of Science and Technology.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cclet.2016.12.015.

References
[1] V. Balzani, S. Campagna, Photochemistry and Photophysics of Coordination Compounds, Springer, Berlin Heidelberg, 2007.
[2] D. Kumaresan, K. Shankar, S. Vaidya, R. H. Schmehl, Photochemistry and photophysics of coordination compounds: osmium, in: V. Balzani, S. Campagna (Eds. ), Photochemistry and Photophysics of Coordination Compounds Ⅱ, Springer, Berlin Heidelberg, 2007, pp. 101-142.
[3] L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura, F. Barigelletti, Photochemistry and photophysics of coordination compounds: iridium, in: V. Balzani, S. Campagna (Eds. ), Photochemistry and Photophysics of Coordination Compounds Ⅱ, Springer, Berlin Heidelberg, 2007, pp. 143-203.
[4] J. A. G. Williams, Photochemistry and photophysics of coordination compounds: platinum, in: V. Balzani, S. Campagna (Eds. ), Photochemistry and Photophysics of Coordination Compounds Ⅱ, Springer, Berlin Heidelberg, 2007, pp. 205-268.
[5] A. Barbieri, G. Accorsi, N. Armaroli. Luminescent complexes beyond the platinum group:the d10 avenue. Chem. Commun. 44 (2008) 2185–2193.
[6] Q. Zhang, Q. Zhou, Y. Cheng, et al., Highly efficient green phosphorescent organic light-emitting diodes based on Cu complexes. Adv. Mater. 16 (2004) 432–436. DOI:10.1002/(ISSN)1521-4095
[7] C.W. Hsu, C.C. Lin, M.W. Chung, et al., Systematic investigation of the metalstructure-photophysics relationship of emissive d10-complexes of group 11 elements:the prospect of application in organic light emitting devices. J. Am. Chem. Soc. 133 (2011) 12085–12099. DOI:10.1021/ja2026568
[8] L.H. He, J.L. Chen, J.Y. Wang, et al., Structures and luminescence properties of two copper(Ⅰ) halide complexes featuring 2-(2-benzimidazolyl)-6-methylpyridine. Chin. Chem. Lett. 23 (2012) 1169–1172. DOI:10.1016/j.cclet.2012.07.014
[9] J.L. Chen, Z.H. Guo, H.G. Yu, et al., Luminescent dinuclear copper(Ⅰ) complexes bearing 1,4-bis(diphenylphosphino)butane and functionalized 3-(2'-pyridyl) pyrazole mixed ligands. Dalton Trans. 45 (2016) 696–705. DOI:10.1039/C5DT03451E
[10] Y.Q. Chen, G.R. Li, Z. Chang, et al., A Cu(Ⅰ) metal-organic framework with 4-fold helical channels for sensing anions. Chem. Sci. 4 (2013) 3678–3682. DOI:10.1039/c3sc00057e
[11] J.L. Chen, X.F. Cao, J.Y. Wang, et al., Synthesis characterization, and photophysical properties of heteroleptic copper(Ⅰ) complexes with functionalized 3-(2'-pyridyl)-1,2,4-triazole chelating ligands. Inorg. Chem. 52 (2013) 9727–9740. DOI:10.1021/ic4002829
[12] J.L. Chen, Z.H. Guo, Y.S. Luo, et al., Luminescent monometallic Cu(Ⅰ) triphenylphosphine complexes based on methylated 5-trifluoromethyl-3-(2'-pyridyl)-1,2,4-triazole ligands. New J. Chem. 40 (2016) 5325–5332. DOI:10.1039/C5NJ03529E
[13] C. Chen, Y.L. Liu, S.H. Wang, et al., Hydrothermal syntheses, structural characterizations, and photoluminescence properties of three fluorinated indium phosphates with bipyridyl ligands:In3F2(2. 2'-bipy)2(HPO4)2(H1.5PO4)2, In2F2(H2O)(2,2'-bipy)2(HPO4)2, and In2F2(H2O)(2,2'-bipy-5-amine)2(HPO4)2, Chem. Mater. 18 (2006) 2950–2958.
[14] J.L. Chen, X.Z. Tan, X.F. Fu, et al., Synthesis and characterization of emissive mononuclear Cu(Ⅰ) complexes with 5-tert-butyl-3-(pyrimidine-2-yl)-1H-1,2,4-triazole. J. Coord. Chem. 67 (2014) 1186–1197. DOI:10.1080/00958972.2014.908465
[15] N. Armaroli, G. Accorsi, F. Cardinali, A. Listorti, Photochemistry and photophysics of coordination compounds: copper, in: V. Balzani, S. Campagna (Eds. ), Photochemistry and Photophysics of Coordination Compounds Ⅰ, Springer, Berlin Heidelberg, 2007, pp. 69-115.
[16] H.B. Xu, X.M. Chen, Q.S. Zhang, L.Y. Zhang, Z.N. Chen. Fluoride-enhanced lanthanide luminescence and white-light emitting in multifunctional Al3Ln2(Ln=Nd, Eu, Yb) heteropentanuclear complexes. Chem. Commun. 45 (2009) 7318–7320.
[17] G.A. Crosby, J.N. Demas. The measurement of photoluminescence quantum yields. J. Phys. Chem. 75 (1971) 991–1024. DOI:10.1021/j100678a001