Synthesis and characterization of copper(II) complexes with heterocyclic aromatic selenium compounds

Sui Chaoxia1, Yang Fang1,2, Zheng Wenjie1,2, Bai Yan1
(1 Department of Chemistry, Jinan University; 2 Research Center of Hydrobiology, Jinan University, 510632, China)


Abstract A series of complexes CuL2Cl2 (L= NS, MB, BS) were synthesized, based on the liquid-phase reaction of piazselenole(NS), 5-methyl- piazselenole(MB), 4,5-benzopiazselenole(BS), which acted as ligands and Cu2+ as cation. The physicochemical properties of the complexes were determined by ICP-AES, EA, FT-IR analysis, UV-Vis, XRD, FS etc. The coordination mechanism between ligands and Cu2+ was also discussed.

Selenium is the trace element controlled by gene in human body[1]. Heterocyclic aromatic selenium compounds (Piaselenole, Pis) are a sort of important compounds derived by the reaction between aromatic ortho-aromatic diamine and selenium dioxide. Pis have attracted considerable interests in many fields, such as new functional materials, analytical reagents and new drugs etc[2-6]. Copper is also an essential element in vivo, and it is also sulphophile element. A series of complicated reactions take place between copper and sulphur (selenium) in the metabolism. However, the biochemistry mechanism and the reciprocity of the reaction between selenium and copper is not clear at the present time[7]. Consequently, the coordination mechanism between Cu2+ and the small molecule of selenium compounds, especially heterocyclic aromatic selenium compounds will be helpful to explain metabolic mechanisms of copper and selenium, which will be significant to develop new drugs and new functional materials with selenium.

1.1 Reagents and chemicals
o-phenylenediamine, 3,4-Diaminotoluene, 2,3-Diaminonaphthale, all ortho-aromatic diamines were purchased from Sigma and used without any further purification, CuCl2(anhydrous), SeO2, anhydrous ethanol, ethanol (95%).
1.2 Synthesis of ligands
The ligands were synthesized according to literature [4]. The ligands synthesized in the article included: 2,1,3-napheno[3,4-c]selenadiazole (BS), 5-methy-2,1,3-benzo[3,4-c]selenadiazole( MB), 2,1,3-benzo[c]selenadiazole (NS), the structural formula were as follows:

NS                                 MB                          BS

1.3 Synthesis of CuL2Cl2 complexes
The complexes CuL2Cl2 (L= BS/ MB/ NS) were synthesized by modified methods in literature [8]. The mole ratio of ligands to CuCl2 was 7:1. The ligands and CuCl2 were dissolved in anhydrous ethanol separately, then mixed and stirred for 1.5h at 55¡æ, and a great deal of precipitate came into being while stirring. The precipitate was filtered quickly and washed several times with ethanol (95%).
1.4 Characterization of CuL2Cl2 complexes
The contents of Se and Cu were analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) on Optime 2000 DV spectrometer (Perkin-Elmer company, America). The contents of C, H, N were determined by Vario EL spectrometer (ELEMENTAR company, Germany). FT-IR spectra of complexes were taken with potassium bromide pellets on a Equinox55 spectrometer. Scan range: 400-4000 cm-1. UV-Vis spectra were recorded on a UV-260 spectrophotometer in N£¬N-dimethylformamide solution. Fluorescence spectra were recorded on a 970CRT fluorescent spectrophotometer (Shanghai SANCO Instrument Co. Ltd) in N£¬N-dimethylformamide solution. X-ray diffraction patterns were recorded on a XD-98 diffractometer equipped with Cu-anode, Radiation of Cu Ka1 (0.15405981nm), Power setting of 40KV, 20mA. Scan speed of 5s/step, Divergence slits of 1 deg., Receiving slits of 0.15mm, Soller slits of 1. All experiments were carried out at room temperature.

2.1 Composition analysis
Elemental analysis (EA) results of the complexes were given in Table 1. The productivity and color of the complexes were given in Table 2, FT-IR spectra. UV-Vis spectra and FS spectra for ligands and complexes were shown in Table. 3-4. X-ray diffraction patterns and crystal degree were shown in Fig. 1 and Table 5 respectively.

Table 1
Elemental analysis results of complexes (%)

complexes C H N Cu2+ Se
Cu( NS)2Cl2 28.92(28.75)* 1.66(1.60) 11.36(11.18) 12.56(12.67) 31.74(31.54)
Cu(MB)2Cl2 31.19(31.76) 2.24(2.27) 10.54(10.59) 12.03(12.01) 29.50(29.87)
Cu(BS)2Cl2 40.80(39.97) 1.88(1.99) 9.21(9.33) 10.51(10.58) 26.15(26.30)

©~The theoretical value is in bracket.

The content of element calculated from the results of elemental analysis was consistent with the theoretical value of CuL2Cl2 according to Table 1, which indicated that reaction between ligands and Cu2+ was complete and the CuL2Cl2 complexes were synthesized.

Table2 The productivity and color of the complexes

¡¡ Cu( NS)2Cl2 Cu(MB)2Cl2 Cu(BS)2Cl2
Color Brilliant green Yellow green Purple black
Productivity 55.45 32.48 50.36

Table 3 FT-IR data of ligands and complexes (cm-1)

Compounds n (Ar-N-) n (-C-H) Vibration of framework of aromatic ring n (N-Se-N)[9,10]
NS 3448s 2923w, 2855w 1633s 442w, 488w, 744s
Cu (NS)2Cl2 3442s 2925w, 2854w 1632s 498w, 579w,729w
MB 3448s 2913s, 2858w 1626vs 498w, 574s, 708w
Cu (MB)2 Cl2 3437s 2926w, 2857w 1632s 499w, 572w, 798s
BS 3445s 2925w, 2854w 1633s 493w, 530w
Cu (BS)2 Cl2 3447s 2923s, 2853w 1681w, 1636s 490w, 594s, 709w

As seen in Table 3, a strong stretching peak appeared at 3450 cm-1 which attributed to Ar-N- group in the ligands. The peaks had red shift or blue shift to a certain degree after the coordination. There were hardly change at 2850~2950cm-1, which indicated that framework of aromatic rings had no effect on the coordinate reaction of Se and Cu. There were obvious differences about the peaks at 490~750cm-1 attributed to N-Se-N group between the complexes and the ligands, which indicated that N-Se-N group was the coordinate site of ligand and Cu2+.

Table 4 Data of UV-Vis spectra for ligands and complexes

Compounds lmax/n(e/104)
NS 222(0.31) 238(0.21) 254(2.64) 262(0.94) 332(1.17)
Cu( NS)2Cl2 224(0.30) 236(0.35) 240(0.36) 256(0.48) 265(1.41) 332(1.12)
MB 233(2.02) 239(0.91) 243(0.68) 264(0.82) 336(0.76)
Cu(MB)2Cl2 237(1.02) 264(1.29) 335(0.50)
BS 227(0.226) 236(0.23) 240(0.22) 248(0.23) 265(0.81) 379(0.31)
Cu (BS)2Cl2 222(0.75) 230(0.80) 266(3.82) 368(1.34) 379(2.13)

The UV-Vis spectra of ligands and complexes were shown in Table 4, and there were obvious differences between the complexes and the ligands. Each of the characteristic peak had red shift or blue shift to a certain degree, and some of them disappeared and new peaks appeared after the coordination. Absorption peaks of BS at 240, 248nm disappeared and absorption peaks shifted from 227, 236nm to 222, 230nm respectively, while new absorption peaks appeared at 368nm after the coordination. Similarly new absorption peaks appeared at 240nm and the peak at 222nm had blue shift in NS. Absorption peaks at 233, 243nm disappeared in MB after the coordination. The coordinate effect was the reason of changes in the UV-Vis spectra.

Table 5 Data of Fluorescence spectra for the ligands and complexes

Compounds concentration
Ex (nm) Em (nm) F
NS 2.2 ¡ª ¡ª ¡ª
Cu( NS)2Cl2 2.2 ¡ª ¡ª ¡ª
MB 2.0 380 434 486.24
Cu(MB)2Cl2 2.0 380 434 622.30
BS 1.9 378 580 905.57
Cu(BS)2Cl2 1.9 378 573 448.09

As seen in Table 5, NS had no characteristic fluorescence and BS had intensive fluorescence peak at room temperature, as was described in the literature [11]. After the coordination between ligands and CuCl2, NS still had no fluorescence spectra, while fluorescence intensity of MB and BS had great changes and the peak position had no change, which indicated that the luminescence of complex was the ligand. It seemed as if fluorescence intensity of MB increased after coordinating with CuCl2, but actually it decreased because the ligand concentration in the complex was two times of the ligand. Fluorescence intensity obviously dropped after coordination with CuCl2. The fluorescence data indicated that CuCl2 could induce fluorescence quenching of complex after coordinating with ligands.
It can be seen from the above analysis that molecular orbital energy level of the ligands had great changes after the coordination between heterocyclic aromatic selenium compounds and CuCl2, which caused the change of intramolecular electron transition energy of the ligands and resulted in the absorption changes of the complexes.
2.2 X-ray diffraction
X-ray diffraction of ligands, CuCl2 and complexes were shown in Fig. 1. It could be seen from Fig. 1 that X-ray diffraction of the complexes was different from the ligands and CuCl2 obviously. The chief diffraction peaks of the reactants disappeared and new diffraction peaks appeared in the patterns of complexes, which indicated that new compound was formed.
The crystal diffraction and the non-crystal diffraction were identified by using the multiple separation method in computer, then the crystal degree of ligands and complexes were calculated and the results were given in Table 6. As seen in Table 6, crystal degree of the ligands has descended obviously after coordinating with CuCl2.

Table 6 Crystal degree of ligands and complexes (crystal degree¦Öc% )

BS Cu (BS)2Cl2 MB Cu(MB)2Cl2 NS Cu( NS)2Cl2 CuCl2
42.3 11.3 32.0 19.3 46.3 23.7 25.6

Fig. 1 XRD patterns of complexes CuL2Cl2 ligands and CuCl2

    Molecular formula of the complex was CuL2Cl2 (L= NS / MB/ BS), which was the result of reaction between heterocyclic aromatic selenium compounds and CuCl2. Valence shell electron configuration of the central ion Cu2+ is 3d94s04p04d0, which can form quadridentate (sp3) or sexadentate (sp3d2) complexes.

Fig. 2 The possible coordinate formula of the complex

Three ligands all had structure of attribute. Two nitrogen atoms of the structure adopted sp3 hybridization. One hybrid orbital accommodated the lone pair on nitrogen. The bonding of the selenium atom was various and sophisticated because it had d orbital. Selenium atom could use sp2 hybridization if not considering d orbital, so a pair of p electrons had a share in conjugate and lone electron pair hold one sp2 hybrid orbital. Consequently, two nitrogen atoms and one selenium atom all had lone electron pair which could coordinate with CuCl2. In addition, the electron-rich five member cycloalkane could coordinate with CuCl2. That is to say, the site of coordination where three ligands coordinated with CuCl2 was the five member cycloalkane. The above deduction had been proved by IR and UV-Vis. Further considering configuration and geometric of the ligands as well as lone electron pair space orientation of hetero-atom, it can be seen that the coordinated lone pair of nitrogen atom had great influences on the steric hindrance originating from the C-H bond of 4-location and 5-loaction. However, there is little probability of coordination of the five member cycloalkane. So it was most likely that the coordination site was Cu2+ and selenium atom. As to the complexes CuL2Cl2 (L= NS / MB/ BS), copper was quadridentate sp3 or sexadentate sp3d2 depending on whether chloride ion formed bridge bond or not. When chloride ion coordinated through bridge bond, the complex could form chain structure. In this structure, copper ion formed sp3d2 hybridization utilizing outer-shell d orbital; the four chloride ions and copper ion was in the same plane, and the two ligands were vertical to the plane and located just above and under the plane. The structural formula of the complex was shown in Fig. 2.