Preparation and Spectral Properties of Mixed-Ligand Complexes of VO ( IV ) , Ni ( II ) , Zn ( II ) , Pd ( II ) , Cd ( II ) and Pb ( II ) with Dimethylglyoxime

A number of mixed-ligand complexes of the general formula [M(D)(G)] where D=dimethylglyoximato monoanion, G=N-acetylglycinato and M=VO(IV), Ni(II), Zn(II), Pd(II), Cd(II) and Pb(II) were prepared. Each complex was characterized by elemental analysis, determination of metal, infrared spectra, electronic spectra, (H and C) NMR spectra, conductivity and magnetic moments. All these complexes were not soluble in some of the organic solvent but highly soluble in dimethylformamide. The conductivity data showed the non-electrolytic nature of the complexes. The electronic spectra exhibited absorption bands in the visible region caused by the d-d electronic transition such as VO(IV), Ni(II) and Pd(II). The IR and ( H, C) NMR spectra which have indicate that the dimethylglyoxime was coordinated with the metal ions through the N and O atoms of the oxime group and Nacetylglycine was coordinated with metal ions through the N atom and terminal carboxyl oxygen atom.


Introduction
The dimethylglyoxime derivatives have received considerable attention from both of the chemical and biological scientists.It is stimulated the reactions of vitamin B 12 and vitamin-B 12 model chemistry [1][2][3] .Moreover, the dioximes are capable of coordinating through N, N or N, O sites of the oxime groups.Thus, some of the dioximes derivatives exhibited significant anticarcinogenic activity and antitumor agents 1 .The most famous complex among other complexes is the bright red Ni(dmgH) 2 .This planer complex is very poorly soluble and is used for the gravimetric determination of nickel 4,5 .There is lot of work have been done like trans-bis(dimethylglyoximato) with metal complexes [6][7][8] .Furthermore, the IR spectra of some dimethylglyoxime complexes show the formation of the strong metal-N covalent bonds and the presence of intra molecular O-H-O bonds 9 .
An amino acid like N-acetylglycine has been taken special attention in environment as it contains a peptide bond.The complexing ability towards the transition metal ions has not been extensively investigated 10 .Knowing this, the mixed ligand complexes of N-acetylglycine and dimethylglyoxime with some metal ions were prepared and investigated by IR, electronic and 1 H-and 13 C NMR spectra and magnetic measurements.

Experimental
All chemicals were obtained from commercial sources and were used without further purifications.VOSO UV-Visible spectra were measured in dimethylformamide (DMF) using a Shimadzu UV-Visible recorder spectrophotometer UV-160.The I.R spectra in the range of 4000-400 cm -1 were recorded as potassium bromide disc on a Shimadzu FTIR-8300 Fourier Transform infrared spectrophotometer.Determinations for metals were carried out using gravimetric methods.Elemental analysis (C, H, N) was performed on a perkin Elmer B-240 Elemental Analyzer. 1 H NMR and 13 C NMR spectra were recorded using the JEOL JNM-ECP 400 Spectrometer in DMSO-d 6 , relative to the internal standard tetramethylsilane (TMS).Conductivity measurements were carried out at 25 o C in DMF using a Philips Pw-9526 digital conductivity meter.Melting points were determined using a stuart-melting point apparatus and finally the magnetic susceptibility measurements were obtained using a magnetic susceptibility balance, model MsB-MK1.

General procedure for synthesis
An ethanolic KOH solution (12 mL) of dimethylglyoxime (D) (0.17-0.42 g) and an ethanolic KOH solution (10 mL) of N-acetylglycine (G) (0.24-0.42 g) were added respectively to an aqueous solution of metal salts, after constant stirring on water bath.The product precipitates immediately, filtered off and washed three times and recrystallized with 1:3 ethanol: water mixture.Then, it was dried in an oven at 60 o C.

Results and Discussion
The prepared complexes were found to be solids, insoluble in some of the organic solvents such as aceton, acetonitrile and chloroform but completely soluble in dimethylformamide.The lower value of molar conductivity in DMF indicates the nonelectrolyte behavior of this complexes [11][12][13] .All the compounds reported in this work are presented in Table 1 along with their analytical data, melting points and conductivity values at room temperature.

Infrared spectra
The infrared spectra of the dimethylglyoxime exhibited absorption bands at 3400, 2931, 1570, 1141 and 756 cm -1 which are attributed to ν(OH), ν(C-H) aliphatic, ν(C=N), ν(N-O) and ν(C=N-O) respectively.In addition, on the complexes the bands of ν(C=N) and ν(C=N-O) were shifted to the lower frequencies by 43-6 and 16-6 cm -1 .This type of coordination is usual in the complexes, as the ligand forms six-member chelate ring by coordinating with metal ions through the N and O atoms 1,3,14 .The infrared spectrum of N-acetylglycine shows absorption band at 3380 cm -1 which belongs to the ν(NH).On complexes formation, this band was shifted to the lower frequencies by 159-125 cm -1 .Moreover, it is noted that both of the ν(COO) asymmetric and symmetric bands of the ligand (G) appeared at 1740 and 1370 cm -1 .On complexes formation these two bands were shifted to the lower frequencies by 90-43 and 10-5 cm -1 respectively.The ν(NH) and ν(COO) bands of the ligand (G) after coordination excluding the coordination of these groups with metal ions were observed 10,15 .Thus, the spectra of the complexes appear as weak bands between 408-470 and 470-520 cm -1 which are due to the ν(M-N) and ν(M-O) respectively.This is indicated that the dimethylglyoxime and N-acetylglycine as a bidentate and are coordinated with the metal ions through the N and O atoms.So, the VO(IV) complex exhibited strong and sharp band at 979 cm -1 which is attributed to the ν(V=O).All the data are listed in Table 2.

H NMR spectra
The 1 H NMR spectra show single signal at 1.90 ppm which is attributed to the methyl groups of the dimethylglyoxime ligand.This peak undergoes a downfield shifted in the complexes by 0.30-0.40ppm.In general, downfield shifts observed on chelation may be due to the decrease in electron density which is caused by ligand interaction with the metal ions 2,16 .The δ due to the OH proton in ligand D appears at 11.20 ppm.This signal is shifted downfield in the complexes which appeared between 11.36-11.42ppm.Upon coordination of the metal ions with the nitrogen atom of the C=N group and the oxygen of the C=N-O 2,17,18 .
The N-acetylglycine shows single signal at 2.08 ppm which is attributed to the methyl group.Moreover, the spectrum of the ligand appeared two singlet signals at 6.35 and 4.03 ppm which are caused by the NH and CH 2 groups respectively.In complexes the signal due to the NH group was shifted downfield by 15-20 ppm.Nevertheless, the signal of the CH 2 group was shifted upfield by 0.64-0.23 ppm 19 .Table 3 summarizes the 1 H NMR data for the free ligands and their complexes in DMSO-d 6 .The δ due to oxime CH 3 carbon in dimethylglyoxime exhibited at 9 ppm.This signal was shifted downfield between 9.30-9.61ppm on coordination.Moreover, the δ for imine carbon exhibited at 154 ppm in free ligand of D. On the complexes this signal was shifted up field by 0.9-0.55ppm [1][2][3] .
The 13 C NMR spectra of N-acetylglycine show signals at 179.5, 176.70, 45.81 and 24.45 ppm due to the C1, C3, C2 and C4 respectively.The signal of C1 and C2 were shifted downfield by 0.5-2.2 and 0.19-1.09ppm respectively.All these signals have indicated that the dimethylglyoxime and N-acetylglycine were coordinated with the metal ions through the N and O atoms 18,20 .All the data of 13 C NMR for the ligands and their complexes were listed in Table 4. Table 4. 13 C NMR data (δ, ppm) of the free ligands and their complexes in DMSO-d 6

Magnetic susceptibility measurements
The magnetic moment for the prepared complex of VO(IV) ion was found to be 1.46 B.M within the expected value for one electron.The diamagnetic for the Ni(II) and Pd(II) complexes may be due to the low spin square planer.The complexes of Zn(II), Cd(II) and Pb(II) are also diamagnetic as expected from their electron configuration 21 .All the data and remarks are found in Table 5.

Electronic spectral studies
The electronic spectra of the dimethylglyoxime shows the absorption bands at 266 and 285 nm which are attributed to π→π * and n→π * transitions respectively.Thus, the electronic spectra of the N-acetylglycine exhibited electronic transitions of π→π * and n→π * at 251 and 347 nm respectively.
The electronic spectra of the VO(IV) complex reveals bands at 555 and 916, 982 nm which are attributed to the electronic transition of 2 B 2 → 2 B 1 and 2 B 2 → 2 E respectively.All these transitions corresponds to VO(IV) complex as a square pyramid 22 .The electronic spectra of the Ni(II) and Pd(II) complexes with an electronic configuration of d 8 shows new absorption bands in the visible region at 445 and 514 nm respectively which these bands are attributed to the electronic transition 1 A 1 g→ 1 A 2 g and the bands at 571 and 915, 980 nm respectively are caused by the electronic transition of 1 A 1 g→ 1 B 1 g.These assignments refers that the Ni(II) and Pd(II) complexes are square planer 22 .
Finally, the electronic configuration of Zn(II), Cd(II) and Pb(II) complexes was d 10 which confirms the absence of any d-d electronic transitions.Nevertheless, the absorption bands in their spectra were suffered from red and blue shift with hyper chromic effects 23,24 .All the absorption bands were fully assigned in Table 5.

Conclusion
A new mixed-ligand complexes of the general formula [M(D)(G)] where D = dimethyl glyoximato monoanion, G = N-acetylglycinato and M = VO(IV), Ni(II), Zn(II), Pd(II), Cd(II) and Pb(II) were prepared and based on the presented results, the following conclusion can be drawn: 1.All the complexes are insoluble in water and most of organic solvents but soluble in DMF and DMSO-d 6 , as well as all of them are non-electrolyte.2. The structure for the complexes based on UV-Vis, IR, ( 1 H, 13 C) NMR spectroscopy was proposed which is indicated that the dimethylglyoxime and N-acetylglycine were coordinated with the metal ions through the N and O atoms.3. It can be suggested that the geometries of the prepared complexes are as fellows: a. Square pyramid geometry for the VO(II) complex (Figure 1).b. Square planer geometry for the Ni(II) and Pd(II) complexes ( Figure 2).c.Tetrahedral geometry for the Zn(II) and Cd(II) complexes.d.Irregular tetrahedral for the Pb(II) complex.

Table 1 .
Analytical and physical data of the prepared complexes

Table 2 .
The infrared spectral data (cm -1 ) of the free ligands and their complexes

Table 3 .
1H NMR chemical shifts (δ, ppm) of the free ligands and their complexes

Table 5 .
The electronic spectra data and magnetic moments for the compounds