Zhong Yun, Xu Zhiguang, Zhang Weiguang, Yin Xia, Tan Minyu
School of Basic Sciences, East China Jiaotong University, Nanchang 330013; School of Chemistry and Environment, South China Normal University, Guangzhou 510631; School of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China)
Abstract:Theoretical studies on zinc complexes with dithiocarbamate Zn(SCNBz，Zn(SCNBzPy (Bz = benzyl, Py = pyridine) have been performed by density function theory at B3LYP/631G* level, and their IR spectra have been calculated and analyzed exactly. According to the change of energies of frontier molecular orbitals, charge distributing, components of HOMO orbital , IR spectra and other related data, it was found that by adding nitric heterocycle it would reduce the stability and symmetry of these complexes, especially weakened SZn bond, which had been interpreted by charge transfer, change of orbital energies and bond valence sum analysis.
Dithiocarbamate is a kind of common ligand containing sulfur, which could form complexes with all kinds of transition metals, B,B metals and rare earth. For the wide application in extracting metal ions^{[1]}, lubricant industry^{[2]} and rubber vulcanization accelerant^{[3]}, these complexes have been more and more studied. What’s more, these compounds could be the precursors for / materials^{[4]}and effective antidotes for cadmium poisoning^{[5]}
The structures of these compounds are various. As far as dithiocarbamate is concerned, it is easy to form dimmer, polymer with B,B metals, whose coordination number is 4 or 5^{[6,7]}. However, the metal ions in those compounds have not been coordinative saturation, they could form adducts with ligands containing N, P, and the coordination number became 5 or 6^{[8]}. When nitric heterocycle were introduced, the coordination number , the difference among SM bonds length and average SM bond length were increasing; the symmetry and stability of complexes were decreasing; the differences of the chemical environment among sulfur atoms in ligands have been obvious; IR spectra became more complicated. In recent years, the researchers have more and more interests in studying the vibration of CS, NC and CSS frame.
There are many studies reported on dithiocarbamate complexes in experimental, but seldom in theory. In recent work, we have studied Zn(SCNBu by quantum chemistry method^{[9]}. In this paper, zinc complex with dibenzyldithiocarbamate Zn(SCNBz and adduct with pyridine Zn(SCNBzPy (Bz = benzyl, Py = pyridine) have been performed by density functional theory at B3LYP/631G* level. Through contrasting between the energies of frontier molecular orbital, charge distribution and components of HOMO orbital, the authors found that after introduced pyridine, the molecular symmetry had been destroyed, and the ZnS bond length became longer for the weakening of the ZnSCN conjugation. The pyridine in the molecular form had tendency to loose. The calculated IR spectra of complexes , which was similar to actual, have been interpreted.
1 EXPERIMENTAL AND CALCULATE METHOD
1.1 Experimental
The complexes have been synthesized according to reference 10.
The Xray crystal data was collected on an CCD area detector with graphitemonochromated Mo Kα radiation (= 0.071073nm) at 295(2)K, using the/2scan technique. All of the calculations were performed using the SHELXTL system of computer programs. The IR spectra were recorded on PerkinElmer l7300 FTIR spectrometer in 4000400cm^{1} region using KBr pellets.
1.2 Calculating method
Molecular structures are derived from single crystal Xray diffraction^{[10]}. The vibrational frequency have been calculated by B3LYP/631G* level of density function theory. All the calculations were performed using the Gaussian03^{[11]} programs package.
2 RESULT AND DISCUSSION
Complex is orthorhombic with space group Pbcn. Complex is triclinic with space group P1. Fig 1 shows the molecular structure of complexes, and table1 shows the selected bond length and bond angle of complexes.
Complexes 1([Zn(SCNBz]) Complexes 2([Zn(SCNBzPy])
Fig.1 The structures of complexes 1 and 2
Table 1The selected bond length(nm) and bond angle(°)
Zn(SCNBz  Zn1S1 0.23343(8)  Zn1S2 0.23636(8)  C1S1 0.1730(3)  C1S2 0.1726(3) 
C1N1 0.1329(3)  C9N1 0.1473(3)  C2N1 0.1479(3)  
S1C1S2 117.45(15)  S1C1N1 121.30(19)  N1C1S2 121.26(19)  
C1N1C9 122.4(2)  C1N1C2 122.8(2)  C2N1C9 114.9(2)  
Zn(SCNBzPy  Zn1N1 0.2044(4)  Zn1S1 0.26183(15)  Zn1S2 0.23662(15)  
Zn1S3 0.23644(14)  Zn1S1 0.25371(14)  C6S1 0.1713(5)  
C6S2 0.1705(5)  C21S3 0.1716(5)  C21S4 0.1712(5)  
C6N2 0.1340(6)  C14N2 0.1449(8)  C7N2 0.1471(8)  
C21N3 0.1334(6)  C29N3 0.1473(6)  C22N3 0.1475(6)  
S1C6S2 118.8(3)  N2C6S2 121.2(4)  S1C6N2 120.0(4)  
S3C21S4 118.2(3)  N3C21S4 121.4(3)  S3C21N3 120.3(4)  
C7N2C14 115.7(5)  C7N2C6 121.5(5)  C6N2C14 123.0(5)  
C29N3C22 114.3(3)  C21N3C22 123.8(4)  C29N3C21 121.7(4) 
2.1 The stability of complexes
Energies of frontier molecular orbital in complexes 1 and 2 were illustrated in Table 2. The energies of occupational molecular orbital of complexes are all negative, and HOMO energies of complexes are lower, 0.215a.u.(complex 1) and 0.177a.u.(complex 2). It demonstrated that the complexes 1 and 2 are too stable to lose electron. The_{HOMOLUMO}(0.186a.u.) of complex is higher than_{HOMOLUMO}(0.123a.u.) of complex , which indicated that the complex had higher reactivity than complex
Table 2 Energies of frontier molecular orbital in complexes 1 and 2 (a.u.)
Complexes  Complexes  
151a  0.250  172a  0.236 
152a  0.250  173a  0.229 
153a  0.245  174a  0.225 
154a  0.245  175a  0.222 
155a  0.227  176a  0.210 
156a  0.223  177a  0.205 
157a  0.222  178a  0.197 
158a(HOMO)  0.215  179a(HOMO)  0.177 
159a(LOMO)  0.029  180a(LOMO)  0.054 
_{HOMOLUMO}  0.186  _{HOMOLUMO}  0.123 
2.2 Natural charges distribution of atoms in the complexes
Table 3 shows the natural charges distribution of atoms in the complexes. It is indicated from Table 3:
1. Obvious covalent character: The valence of Zn atoms in two complexes are both +2, but the natural charges of Zn atoms, all less than +1, are 0.481 and 0.564, respectively. It demonstrated that obvious covalent bond have been formed between Zn atom and ligand.
2. Natural charges distribution of SCNC motion in dithiocarbamate exhibits polarity alternation: Under the effect of polar atoms S, N, the natural charges distribution of SCNC motion in dithiocarbamate exhibits polarity alternation along (S,S)CN(C,C). And, in complex 2, the C atoms in pyridine also exhibit polarity alternation along N1C1C2C3C4C5.
3. Natural charges distribution incline to equilibration: The absolute values of all atoms’natural charges are below 1. It indicated that natural charges distribution of complexes incline to equilibration because it exhibits polarity alternation.
When pyridine was introduced, the natural charge of Zn atom has been more positive; natural charges of ligands(DTC) have been more negative; and natural charge of pyridine in complex was positive(see Table 3). It demonstrated that electrons had transferred from Zn to DTC; the covalent character of ZnS has been decreased, and weakened; ZnS bond length has been extended, meanwhile CS has shortened and strengthened. That the natural charge of pyridine in complex is positive indicates that combination between pyridine and Zn is not so stable, and the pyridine in complex would lose under the circumstance^{[11]}. In addition, the introduced pyridine made the natural charges of S atoms in DTC different, which increased the asymmetry of DTC.
Table3Natural charges of some atoms or groups in complexes
Complex 1  Zn1: 0.481, S1: 0.176, S2: 0.172, C1: 0.041, N1:0.363, C2: 0.230, C9: 0.183, DTC: 0.24 
Complex 2  Zn1: 0.564, S1: 0.274, S2: 0.186, C6: 0.025, N2:0.372, C7: 0.252, C14: 0.170, DTC: 0.392 S3: 0.186, S4: 0.258, C21: 0.028, N3:0.350, N1: 0.506, C1: 0.133, C2: 0.164, C3: 0.089, 
2.3 The bond character in complexes
The square sums of all kinds of atom orbital coefficients have been added, and been normalized. The results are the contribution of atom orbital in HOMO orbital or components of HOMO orbital(see Table 4). Fig 2 shows HOMO orbital of complexes 1 and 2.
HOMO orbital of complexes are π bond which was composed of multitudinous(p) orbital of S and little(p) orbital of N, C(see Table 4). HOMO orbital of complex is a little distorted antibonding orbital, and its energy is 0.17701a.u.. HOMO orbital of complex is torsional antibonding orbital. Its energy(0.21516 a.u.) is lower than complex because of partly overlapping between S atoms and Zn atom(see Fig 2).
The orbital population of HOMO orbital in complex is higher than complex , which imaged that the symmetry of complex was lower than complex . The(p) orbital of N, C of HOMO in complex is lower than complex , and(p) orbital of S of HOMO in complex is higher than complex . It indicated that the introduced pyridine had destroyed conjugate of ZnSCN, made electrons congregate on S atoms, which agreed with natural charges distribution. From crystal xray analysis, ZnS,CN bond length in complex is longer than complex , and SC is shortened and strengthened.
Zn(SCNBz_{2 } Zn(SCNBzPy
Fig. 2 HOMO orbital of the complexes
Table4The components of HOMO orbital in complexes calculated by B3LYP
Atom orbital  Zn  
Complex  <0.01  2.33  4.76  0.02  79.17  <0.01  6.99  1.18  4.96  0.47 
Complex  0.09  0.16  4.16  0.79  88.01  0.36  0.50  1.12  3.39  1.04 
2.4 The effect of SZn by introducing nitric heterocycle
Bond Valence Sum Analysis (BVS) is applied to complexes of zinc and cadmium dithiocarb amates to estimate the effective valences of the metal ions from the bond lengths reported from their crystal structures. The valence vij of a bond between two atoms i and j is defined so that the sum of all the valences from a given atom i with valence Vi obeys Vi = Vij, v_{ij} = exp[(R_{ij}d_{ij})/B]. Here,‘B’is taken to be a universal constant equal to 0.37. d_{ij} is bond lengths. The parameter Rij (R_{ZnS}=2.08, R_{ZnN}= 1.77)is the bond valence parameter. The effective valence of metal atoms in dithiocarbamate complexes with zinc approach their formal oxidation states(+2), and the effective valence of metal atoms in parent dithiocarbamate complexes are greater than adducts^{[12]}. The effective valence of metal atoms in complex and complex are 1.94 and 1.92, respectively. It is accordance with this principle. According to the calculate method of the effective valence of metal atoms, we can predicate SM bond would be extended in adducts because that the coordinate number was greater in adducts than in parent dithiocarbamate complexes, and the effective valence of metal atoms was lower in adducts than in parent dithiocarbamate complexes.
The steric effect of ligands containing N have more effect to SZn in complexes. Since NZn bond length is shorter than SZn bond length, the ligands containing N are more approach than dithiocarbamate. For the steric effect of ligands containing N, the conjugate of MSCN would be destroyed, and the coordinative circumstance of S atoms in ligands would be difference. It will extend SZn, and destroy primary dimmer, polymer structure(see Table 5). In xanthate complexes which is similar to dithiocarbamate complexes, this situations will be occurred. Adding pyridine and 1,10 phenanthroline(phen) to polymer [Cd(SCOCn)(average SCd:0.2551nm) will acquire two monomers, [(Py)Cd(SCOCn)^{[13]} (average SCd:0.2687nm) and [(phen)Cd(SCOCn)^{[14]} (average SCd:0.2676nm), respectively.
Table 5Average SZn bond length in Zinc complexes with dithiocarbamate and their nitric heterocycle adducts (nm)
Complexes  Coordination number  Average SZn bond length  Reference 
[Zn(SCNEt  0.2383  15  
[Zn(SCNBu  0.2361  16  
[Zn(SCNBz  0.2349  10  
– [{O(CHNH}Zn(SCNEt  0.2471  17  
– [{O(CHNH}Zn(SCNEt  0.2468  17  
[(phen)Zn(SCNBu  0.2523  18  
[PyZn(SCNBz  0.2472  10  
[(4,4′Py)Zn(SCNBz  0.2464  19 
Table 6Mainly calculated and experimental IR vibration peak value of complex 1(cm^{1}
Calculated  Experimental  
CN  1549.1(1489)vw 1544.64(1485)vs 
1489s 1485vs 
1224.49(1177)m 1199(1152)m 
1157m  
CS  1027.5(987.7)m 1026.55(986.8)m 
987.8m 
values in parentheses have been multiplied correction factor: 0.9613
Fig. 3The calculated and experimental IR spectra of complex 1
(The calculated frequency will be multiplied by correction factor: 0.9613)
2.5 IR spectra
Fig 3 illustrates the calculated and experimental IR spectra of complex . Table 6 shows the mainly calculated and experimental IR vibration peak value of complex . Fig 4 illustrates the calculated and experimental IR spectra of complex . Table 7 shows the mainly calculated and experimental IR vibration peak value of complex
From crystal xray analysis, it is known that complex has higher symmetry than complex . Two ligands in complex have little difference. The CN vibration appears in 1549.1 (1489)cm^{1} and 1544.64 (1485) cm^{1}(value in parentheses have been multiplied by correction factor: 0.9613^{[20]}), and the experimental value is 1489 cm^{1} and 1485 cm^{1}. It demonstrates that CN have double bond character, which is in accordance with existing reports. The vibration of appears in 1224.49 (1177)cm^{1} and 1199 (1152) cm^{1}, and the experimental value is 1157 cm^{1}, which reveals obvious single bond character. It indicates that the electrons have only delocalized in SCN. The CS vibration has little split, appears in 1027.5 (987.7)cm^{1} and 1026.55 (986.8)cm^{1}, and experimental value is only one: 987.8cm^{1}. It indicates that the chemical circumstances of S atoms in ligands have little difference.
The symmetry of complex is lower. Two dithiocarbamate ligands have greater difference, and the IR spectra is more complicated. The CN vibration appears three peaks, 1554.54 (1494) cm^{1}, 1531.68 (1472) cm^{1}, 1498.57 (1441) cm^{1}, and experimental value is 1494 cm^{1}, 1470 cm^{1}, 1450cm^{1}. In two SCNC frame of complex , the asymmetry of A ( C6S1: 0.1713nm, C6S2: 0.1705nm, C14N2:0.1449nm, C7N2:0.1471nm ) is higher than B ( C21S3: 0.1716nm, C21S4:0.1712nm, C22N3:0.1475nm, C29N3:0.1473nm ) and complex ( C1S1: 0.1730nm, C1S2:0.1726nm, C9N1:0.1473m、C2N10.1479 ). And the polarization of A( S1: 0.274, S2: 0.186, C6: 0.025, N2:0.372, C7: 0.252, C14: 0.170) is also higher than B ( S3: 0.186, S4: 0.258, C21: 0.028, N3:0.350, C22: 0.212, C29: 0.239) and complex (S1: 0.176, S2: 0.172, C1: 0.041, N1:0.363, C2: 0.230, C9: 0.183). These higher asymmetry and polarization induce split in CN vibration of A. The vibration also splits, which appears in 1330.39 (1279)cm^{1} and 1221.49 (1174) cm^{1}, and experimental value is 1274cm^{1} and 1158cm^{1}. It indicates that the electrons have greater delocalized area in A, which induces C14N2 (0.1449nm) was closer than other three CN (0.1471nm, 0.1475nm, 0.1473nm ) and splits the vibration.The CS vibration has also three peaks, 1058.79 (1018) cm^{1}, 1040.01 (999.8)cm^{1}, 1025.42 (985.7)cm^{1}，and the experimental value is 1028 cm^{1}, 998cm^{1}, 985cm^{1}. It demonstrates that the chemical circumstances of S atoms are not the same. From crystal xray analysis data, the CS bond length could be considered as three groups, 0.1716nm, 0.1705nm, 0.1712nm and 0.1713nm; from natural charges distribution, they could also be considered as three groups, 0.2740.2580.186.
Table 7Mainly calculated and experimental IR vibration peak value of complex 2(cm^{1}
Calculated  Experimental  
CN  1554.54(1494)s 1531.68(1472) s 1498.57(1441)s 
1494s 1470s 1450s 
1330.39(1279)m 1221.49(1174)m 
1274w 1158m 

CS  1058.79(1018)m 1040.01(999.8)m 1025.42(985.7)m 
1028m 998m 985m 
valuse in parentheses have been multiplied by correction factor: 0.9613
Fig 4The calculated and experimental IR spectra of complex 2
(The calculated frequency will be multiplied by correction factor: 0.9613)
CONCLUSION
Introduced nitric hetercycle have increased the coordinate number of zinc complexes with dithiocarbamate, destroyed the conjugate of ZnSCN, weakened the ZnS, extended ZnS, CN and strengthened CS. The complexes are more stable because the occupational molecular orbital of complexes are all negative and lower. It indicates that zinc complexes with dithiocarbamate could react with nitric heterocycle to form adducts. In complex , the introduced pyridine has destroyed the symmetry of complex, increased the asymmetry and polarization of SCNC in ligands, induced greater difference among S atoms. The IR spectra of complex is more complicated, and the vibrations of CNCS and have all split.