CHEM. RES. CHINESE UNIVERSITIES 2010, 26(6), 1011 1015 Spectroscopic Studies on Co(II), Ni(II), Zn(II) Complexes with 4,4 -Bipyridine SHI Xiu-min 1,2, WANG Hai-yan 3, LI Yan-bing 2, YANG Jing-xiu 1, CHEN Lei 1, HUI Ge 1, XU Wei-qing 1 and ZHAO Bing 1* 1. State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, P. R. China; 2. College of Chemical Engineering, Changchun University of Technology, Changchun 130012, P. R. China; 3. The Second Hospital, Jilin University, Changchun 130041, P. R. China Abstract Fourier-transform infrared(ftir), Raman and ultraviolet-visible spectra of 4,4 -bipyridine and its metal-organic coordination compounds synthesized from 4,4 -bipyridine and nitrate of Co(II), Ni(II) and Zn(II) were measured and analyzed, respectively. The main FTIR and Raman bands were assigned in detail. The relationship between these characteristic bands and the structure of ligands and coordination compounds were discussed. Keywords 4,4 -Bipyridine; Infrared spectrum; Raman spectrum; Ultraviolet-Visible spectrum Article ID 1005-9040(2010)-06-1011-05 1 Introduction Constructing metal-organic coordination compounds with highly-ordered structures through the reaction of multi-dentate ligand with transition metal ions via coordinative bonds, hydrogen bonds, π-π interactions and hydrophobic interactions has been a hot research field in coordination chemistry, supramolecular chemistry and material chemistry. 4,4 -Bipyridine(4,4 -BPy) is a linear non-branched-chainoriented bidentate ligand, which can form a variety of spatial structures including linear, zigzag, rail type, square lattice, and octahedron structures in self-assembly processes with metal. In addition, the circumvolution of the two pyridine ring around the intervening carbon-carbon odd bond adds the diversity of spatial structures of bridged complexes. In recent years, various kinds of functional materials with 4,4 -BPy as ligands have been developed and applied to material separation and purification, catalysis and ion exchange [1,2]. At present, the studies of complexes with 4,4 -BPy as ligand have mainly been focused on their synthesis and the characterization of their crystal structure [3 7], however, the spectroscopic properties of the ligand and its complexes have not been reported. In this paper, three kinds of metal-organic coordination compounds(co-bpy, Ni-BPy and Zn-BPy) were (FTIR), Raman and ultraviolet-visible(uv-vis) spectra of 4,4 -BPy and its metal-organic coordination compounds were investigated, respectively. The main bands of FTIR and Raman were assigned in detail. The results of the characterization were discussed. 2 Experimental 2.1 Reagents and Preparation of Complexes 4,4 -BPy was purchased from Shanghai Yuanfan Auxiliary Factory(China) and used without further purification. Acetone, anhydrous ethanol, Co(NO 3 ) 2 6H 2 O, Ni(NO 3 ) 2 6H 2 O and Zn(NO 3 ) 2 6H 2 O were analytical pure reagents, which were purchased from Beijing Chemical Plant(China). Co(NO 3 ) 2 6H 2 O, Ni(NO 3 ) 2 6H 2 O and Zn(NO 3 ) 2 6H 2 O were used without further purification. Complexes were synthesized according to the literature [8], their chemical general formula is {[M 2 (4-4 -BPy) 3 (NO 3 ) 4 ] xh 2 O} n (M-BPy), M=Co, x=4; M=Ni, x=2; M=Zn, x=2. 2.2 Equipments and Characterization FTIR spectra of samples were recorded at room temperature by a Bruker Vertex 80 V equipped with a DTGS detector at a resolution of 4 cm 1 via KBr disk technique, scan range 400 4000 cm 1, and scan 32 times. Raman spectra were recorded on JY-T64000 synthesized with 4,4 -BPy. Fourier-transform infrared *Corresponding author. E-mail: zhaobing@jlu.edu.cn Receive November 30, 2009; accepted January 22, 2010. Supported by the National Natural Science Foundation of China(Nos.20773044, 20873050, 20921003 and 20973074) and the 111 Project of Minstry of Education of China(No.B06009).
1012 CHEM. RES. CHINESE UNIVERSITIES Vol.26 Raman spectrometer. Excitation source was provided by an Ar-ion laser of 514.5 nm with 2 mw on the samples. Tested samples were solid powders. UV-Vis absorption spectra of samples were obtained on a UV-3600 spectrophotometer(shimadzu). Scan range was 300 800 nm, optical slit 5 nm. 3 Results and Discussion 3.1 FTIR Spectral Analysis Fig.1 shows FTIR spectra of 4,4 -BPy and three as-synthesized complexes. The FTIR data of ligand and coordination compounds are listed in Table 1, the main IR bands are assigned according to the literature [9 11]. Fig.1 IR spectra of 4,4 -BPy(a) and complexes Co-BPy(b), Ni-BPy(c) and Zn-BPy(d) Table 1 FTIR data and assignments of complexes M-BPy and 4,4 -BPy /cm 1 Assignment 4,4 -BPy Co-BPy Ni-BPy Zn-BPy 497m 512w 506w 506w ν(c C), ν(c N), δ(c H) 571w 576w 576w 576w Ring breathing 608s 634m 638m 640m Ring breathing 735w 735w 735w 735w Ring breathing 807vs 814s 814s 814s γ(c H) 850w 874vs 859w 866w γ(c H) 966w 967vw 967vw 967vw Ring breathing 987m 1012w 1012w 1012w Ring breathing 1038w 1038w 1050w 1044w Ring breathing 1076w 1070w 1076w 1070w δ(c H) 1219m 1223m 1222s 1217w δ(c H) 1300s 1301s 1294m 1323w 1322m 1326s 1323m δ(c H) 1384vs 1384s 1384vs ν(no 3 ) 1405s 1416m 1416s 1416s Ring breathing 1486w 1467s 1473s 1461m ν(c C), δ(c H),δ(C N) 1531m 1537w 1537m 1537w Ring breathing 1589s 1608s 1608vs 1612s ν(c C), ν(c N) 3025m 3083w 3062w 3073w ν(c H) 3442w 3363m 3459w 3.1.1 Metal-sensitive Bands As 4,4 -BPy molecule is coordinated to metal ion, a new M N coordination bond is formed. Consequently, the changes in the vibration frequencies of the C N, C=C, C C and C H bonds occur in the complexes. For example, the absorption band of the composite vibration band, ν(c C) and ν(c N), for 4,4 -BPy at 1589 cm 1 blue-shifted to 1608, 1608 and 1612 cm 1 for the complexes, respectively. Another example, the absorption band of γ(c H), for 4,4 -BPy at 807 cm 1 blue-shifted to 814 cm 1 for the complexes, respectively. The spatial effect is considered to be the main factor responsible for the frequency changes of the bands observed in infrared absorption spectra. Three factors, including field effect, steric effect and ring strain, could contribute to the spatial effect. First, owing to the field effect [12], the bond force constant of pyridine ring changed and further caused the shift of the vibration frequency. Second, owing to steric effect [13], the conjugated systems of the complexes were not well coplanar so that the conjugated systems were incomplete in the molecules. Two spatial effects just presented above made the corresponding fundamental characteristic frequency shifted to higher wavenumber. In Table 1 and Fig.1, the composite vibration band including ν(c C), δ(c H) and δ(c N) for 4,4 -BPy at 1486 cm 1 red-shifted to 1467, 1473 and 1461 cm 1 for the complexes, respectively. Moreover, the band has changed from a weak band into two strong bands and a moderate-intensity absorption band. The numbered shift is due to the bending vibration of the ring, which is in favor of ligand ring vibration and thus makes the frequency and relative intensity changed. On account of the difference of ionic radius and electronic structure of metal, the changes of metal-sensitive bands are different. In the infrared spectra, there are certain differences in frequency and intensity of the band for three metal organic coordination compounds. The change of wavenumbers about absorption bands at 497, 608, 850 and 1038 cm -1 are sensitive to the different metals. 3.1.2 Absorption Bands of Pyridine Ring The absorption bands of 4,4 -BPy at 1076, 1219 and 1323 cm 1 and the absorption bands of compounds(co-bpy, Ni-BPy, Zn-BPy) at 1070, 1076 and 1070 cm 1 ; 1223, 1222 and 1217 cm 1 ; 1322, 1326 and 1323 cm 1 were all attributed to in-plane CH
No.6 SHI Xiu-min et al. 1013 bending vibration in the ring of 4,4 -BPy [9]. The absorption band of ligand at 571, 735, 966, 987, 1405 and 1531 cm 1 and the absorption band of compounds (Co-BPy, Ni-BPy, Zn-BPy) at 576, 735, 967, 1012, 1416 and 1537 cm 1 were attributed to the breathing vibration of pyridine ring [9,11]. From Table 1, it can be seen that these bands almost did not increase with the difference of the radii of metal ions. 3.1.3 C H Bands of Pyridine Ring and Characteristic Bands of Complexes ν(c H) of pyridine ring bands in ligand appeared at 3025 cm -1, but appeared at 3083, 3062 and 3073 cm 1 in the complexes(co-bpy, Ni-BPy, Zn-BPy) respectively. The intensity of the band changed from strong to weak, the difference between the ligand and one of the complexes was due to the formation of intramolecular hydrogen bonds in the complexes. The important symbol of the complexes different from the ligand was a wide band which occurred at 3442, 3363 and 3459 cm 1, respectively for the three complexes. In addition, as is seen in Fig.1, the main difference of the infrared absorption bands of the complexes (Co-BPy, Ni-BPy, Zn-BPy) from that of ligand is a strong wide band appearing between 1222 and 1537 cm 1, and the center of the band and the most powerful spot appeared at 1384 cm 1, assigned to NO 3 stretching vibration. And the bands of the three complexes are very similar, which is due to that they have the same spatial configuration. It could be validated that the three complexes have the same general formula of the molecule from the view of spectra. 3.2 Raman Spectral Analysis Fig.2 shows Raman spectra of 4,4 -BPy and the three complexes. the Raman band data of the ligand and coordination compounds are listed in Table 2. The main Raman bands were assigned according to the literature [9 11]. 3.2.1 Metal-sensitive Bands Raman and infrared spectroscopies can complement and proof each other, and they can provide more information on the molecular structure. In the Raman spectra, the same change trend of the corresponding C N bond, C=C, C C bond and C H bond were observed. For example, the composite vibration band including ν(c C) and ν(c N) for 4,4 -BPy at 1513 cm 1 blue-shifted to 1521 cm 1 for the three complexes. Another example, the Raman vibration band, Fig.2 Raman spectra of 4,4 -BPy(a) and complexes Co-BPy(b), Ni-BPy(c) and Zn-BPy(d) Table 2 Raman shift and assignments of complexes M-BPy and 4,4 -BPy Raman shift/cm 1 4,4 -BPy Co-BPy Ni-BPy Zn-BPy Assignment 263w 287w 293w 295w 325w 311w 309w 309w Ring breathing 383m 389m 404m 392m Ring breathing 575w 575w 577w 575w Ring breathing 660w 660w 659w 656w Ring breathing 762w 778w 779w 782w Ring breathing 882w 859w 863w 861w γ(c H) 1001vs 1025s 1028s 1030s Ring breathing 1076w 1083w 1083w 1077w ν(c C), ν(c N), δ(c H) 1231w 1237w 1239w 1234w δ(c H) 1302vs 1295s 1295s 1299s Ring breathing, δ(c H) 1513m 1521m 1521m 1521m ν(c C),ν(C N) 1617s 1621vs 1621vs 1621vs ν(c C) 3057w 3089w 3090w 3091w ν(c H) ν(c C), for 4,4 -BPy at 1617 cm 1 also blue-shifted to 1621 cm 1 for the three complexes. In comparison to that of 4,4 -BPy, the bands of 4,4 -BPy in the three complexes change into strong ones. The biggest change of band type and band intensity took place at 1001 cm 1, which can be assigned to ring breathing vibration of the ligand. The band appeared about 20 cm 1 shifted for the coordination complexes, and the bands changed into a medium band compared with that of 4,4 -BPy. These observations can be understood on the basis of the bonding involved in the interaction between the metal and the ligand. The π-electrons of the ligand involved in the bond imply strengthening of the C N bond, and consequently an increase of the bond order and a decrease in the bond length. This result is in good agreement with the coordination of the nitrogen atom of the 4,4 -BPy to the metal ions. The band of the ligand at 1302 cm 1 with high intensity feature, which can be assigned to the
1014 CHEM. RES. CHINESE UNIVERSITIES Vol.26 ring breathing and δ(c H) vibration, showed downward shift to 1295, 1295 and 1299 cm 1 for the coordination complexes, respectively. The reason of the change about the above-mentioned Raman spectra is mainly due to the spatial effect and the change of spatial symmetry of 4,4 -BPy [14]. 3.2.2 Bands of Pyridine Ring The two bands of 4,4 -BPy at 575 and 659 cm 1 and the two bands of M-BPy at 575, 660 cm 1 ; 577, 659 cm 1 ; and 575, 656 cm 1 can be assigned to ring breathing vibration. As is seen in Fig.2 and Table 2, those bands basically do not increase with the change of the metal ion radii. 3.2.3 C H Bands of Pyridine Ring and Characteristic Bands of Complexes The band of 4,4 -BPy at 3057 cm 1 assigned to the (C H) vibration of pyridine ring band appeared at 3089, 3090 and 3091 cm 1 and became weak for the metal-ligand complexes(co-bpy, Ni-BPy, Zn-BPy), respectively. For the three complexes, the band at 1621 cm 1 is in high intensity and consistent type. And the range from 1025 cm 1 to 1083 cm 1 are all composed of three gradually weak bands and all consistent in the three complexes. Besides, the weak bands of the three complexes from 575 cm 1 to 782 cm 1 are very similar, among which the band at 659 cm 1 is relatively the highest band. In the low wavenumber area, the band-types of the three complexes are similar, which are of great difference from that of the ligand. Thus, from the Raman spectra, the three complexes have the same spatial configuration. 3.3 UV-Vis Spectral Analysis Fig.3 shows the UV-Vis absorption spectra of 4,4 -BPy and the three complexes. As seen from Fig.3, there is an absorption band of 4,4 -BPy(363 nm) in spectrum d, a band of Zn-BPy(342 nm) in spectrum c. There are three absorption bands for Co-BPy(471, 514, 623 nm), and two absorption bands for Ni-BPy(363, 606 nm). There are three electronic transitions involved in the coordination complexes in solid state [15 17] : (1) a d-d transition or an f-f transition on the metal ion. Such transition is called ligand-field transition. The absorption band is called ligand-field absorption band. It often occurs in the visible and near infrared regions, and the intensity of the band is weak; (2) a charge transfer based on the ligand, this charge transfer(lc) is similar to that of a general organic compound, often occurs in the Ultraviolet region; (3) charge-transfer absorption spectra are produced by charge transfer between the ligand and metal ion, concretely including two types: 1) metal-to-ligand charge transfer(mlct); 2) ligand-to-metal charge transfer(lmct), which often occur in the ultraviolet and visible regions. Because 4,4 -BPy and Zn-BPy complex are colorless, there is a spatial configuration of d 10 track in the metal ion Zn(II) and the d orbital is fully filled, d-d electronic transition can not happen. The band of 4,4 -BPy is at 363 nm, whereas the band of Zn-BPy complex is at 342 nm. They are assigned to LC bands. Co-BPy and Ni-BPy complexes are colorful, and there are spatial configurations of d 7 and d 8 tracks in the metal ions Co(II) and Ni(II), and the d orbit is not filled, d-d electronic transition can occur. Thus, the three bands at 471, 514 and 623 nm, whose assignment are d-d transition bands. The band at 363 nm is assigned to LC band and the band at 606 nm is assigned to d-d transition in Ni-BPy complex. Comparing the UV-Vis spectrum of 4,4 -BPy with those of its three complexes, one can see that when the same ligand was coordinated to different metals ions, the electronic absorption spectra of the complexes varied greatly, due to the different extranuclear electron distribution of the metals. 4 Conclusions Fig.3 UV-Vis spectra of complexes Co-BPy(a), Ni-BPy(b) and Zn-BPy(c) and 4,4 -BPy(d) The three coordination complexes of Co 2+, Ni 2+, Zn 2+ with 4,4 -BPy were synthesized and characterized by FTIR, Raman and UV-Vis spectroscopic techniques. The vibrational spectra of the compounds are very similar and in agreement with the crystal data. The highest intensity band is at 1384 cm 1 in the FTIR spectra of the three complexes. In the Raman spectra, the three bands at 1295, 1521 and 1621 cm 1 assigned
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