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Synthesis and characterization of N, N′-bis (isatin) diamino zirconium (IV) complexes

N, N′-bis (isatin) diamine schiff base ligands were synthesized by the condensation reaction of Isatin with various diamine (ethane-1,2-diamine, propane-1,3-diamine and hexane-1,6-diamine) in 2:1 molar ratios. These ligands were used to prepare Zr (IV) complexes. Prepared ligands and complexes were characterized by using conductance measurement, FT-IR, UV-Visible and 1HNMR spectroscopy. The presence of FT-IR band for azomethine group supports the formation of ligand. Vibrational bands for Zr←N and Zr←O in complexes signify the coordination through O and N sites of ligands. 1HNMR peak for NH moiety in ligand gets almost disappeared in complex reveals tautomerism of NH with nearby carbonyl oxygen due to the effect of complexation. The absence of peak above 500 nm in the electronic spectra indicates d0 system of zirconium in complexes. The elemental analytical data was analogous to theoretical composition of ligands and complexes. The molar conductance values obtained for the complexes represent their non-electrolytic character.

Therefore, designing of new complexes of diamino isatin based Schiff base ligand still possess considerable research value. In the present work, we would like to synthesize zirconium(IV) complex of N, N′-bis (isatin) diamine schiff base ligands, where diamines are ethane-1,2-diamine, propane-1,3-diamine and hexane-1,6-diamine. As a bioactive element zirconium complexes have clinical importance. The isotope 89 Zr has been applied to the tracking and quantification of molecular antibodies with positron emission tomography (PET) camera (Rij et al., 2011). Zirconium-containing compounds are used in many biomedical applications including dental implants and other restorative practices, knee and hip replacements, and middle-ear ossicular chain reconstruction (Lee et al., 2010). Although, well defined experimental study on zirconium complexes stated above, N, N′-bis (isatin) diamine ligands with zirconium have not yet been done. In this context, the synthesis of N, N′-bis (isatin) diamino zirconium (IV) complex [ZrL n Cl 2 ] is considered as a new attempt.

Preparation of N, N′-bis (isatin) diamine ligands
N, N′-bis (isatin) diamine ligands were prepared through the condensation reactions of isatin and diamines as in the reported procedure Khalifa and Hassaan,1995). Isatin (2 mmol) was dissolved in 25 cm 3 hot methanol with continuous stirring and allowed to cool. Then 1 mmol diamine (ethane-1,2-diamine, propane-1,3-diamine and hexane-1,6-diamine) was introduced in the prepared methanolic solution of isatin. After that, the whole mixture was refluxed for three hours. Finally, the precipitate was obtained inside the reaction vessel. There after the precipitate was filtered off, washed with methanol and dried over silica gel in vacuum desiccators. The reaction for ligand preparation is shown in Scheme A.

Analysis
Melting points, an important property of the ligands as well as of the complexes were performed by an Electro thermal Melting Point Apparatus of Stuart, model SMP 10, UK. The elemental analysis of ligands and complexes were done by CHEMMOL V1.0 analyzer and metal analysis by Shimadzu AA-7000, AAS using GFA-7000. Conductometer model HANNA (HI 2300) was used in conductivity measurements of prepared samples. N, N-dimethyl-formamide (DMF) was used as solvent for conductivity measurements. The FT-IR spectra were run on KBr pellet and recorded with FT-IR 8400S Shimadzu spectrophotometer in the range 4000-400 cm -1 at 2 cm -1 resolution and 30 times scanning. 1 H-NMR spectra of samples in DMSO solvent were run using BRUKER 400 MHz NMR spectrometer. Electronic absorption spectra were recorded on single beam Optizen UV Spectrophotometer in DMF solution using 1cm cell. Table I shows the physical data of the prepared ligands and complexes.

Elemental analysis
The percentage of elements of Ligands (L 1 , L 2 and L 3 ) and their corresponding Zr(IV) complexes are shown in Table I. The experimental results are comparable to the calculated values for the ligands (L 1 , L 2 and L 3 ) and their respective Zr (IV) complexes. These indicate almost similar elemental composition of synthesized ligands and complexes to the expected structure.

Molar conductance
The electrolytic nature of the complexes can be obtained from their molar conductance (Λ) measurement. The molar conductance values of the prepared zirconium (IV) complexes of the N, N′-bis (isatin) diamine schiff base ligands found very low which is depicted in Table I. The low molar conductivity of all complexes suggest their non-electrolytic character (Uddin et al. 2012). This revealed that all ligands were covalently bonded with zirconium.

FT-IR Spectra
The infrared studies help to demonstrate the functional groups of the ligands and bonded in the complexes. FTIR spectra of the prepared N, N′-bis(isatin) diamine Schiff base iigands and their zirconium (IV) complexes are shown in Fig.  1 and 2, respectively. The spectra of the ligands should contain stretching bands for the azomethine group ((>C=Ngroup), C-N and C=O bonds at 1625-1582 cm -1 (Singh et al., 2012), 1020-1250 cm -1 (Uddin et al., 2012) and 1730-1750 cm -1 (Konstantinovic et al., 2015) regions, respectively. The vibrational frequency at 3150-3300 cm -1 (Ade et al., 2012) and 1460 cm -1 (Konstantinović et al., 2015) correspond stretching and bending vibration of N-H bond. The broad IR band at 3400-3500 cm -1 (Ade et al., 2012;Legzdins et al.,1989) arises due to the stretching vibration of O-H in water molecule. In the present experiment, we observed vibrational band at 1608-1610 cm -1 for the synthesized N, N′-bis (isatin) diamine ligands, which can be assigned for the azomethine group (>C=N-). This supports the formation of ligands through the condensation of isatin and diamine. The IR band at 1741-1751cm -1 refered to the stretching vibration in C=O bond of the ligands. The FT-IR band for stretching and bending vibration of N-H bond was observed at 3234-3267 cm -1 and 1463 cm -1 , respectively. The peak at 1203-1234 cm -1 indicated the presence of the C-N both in prepared ligands and complexes. The broad band observed at 3448-3454 cm -1 relating to O-H stretching frequencies for moisture.
Where x = 0, 1 and 4 Scheme A. Possible synthetic route of N, N′-bis (isatin) diamine ligand (L n ; n =1, 2 and 3)   Jiang et al., 2017). The characteristic vibrational band of azomethine group (>C=N-) was observed at 1620-1624cm -1 in zirconium complex which was shifted to the higher frequency compared to ligand. This shifting can be explained due the participation of azomethine nitrogen in the coordination with Ziconium in the complex. Therefore, the FT-IR spectra of complexes support the formation of [ZrL n Cl 2 ] complexes through the formation of coordination bonds with O, N donor. FT-IR spectral data for ligands and complexes are published in Table II  and Table III, respectively.

H NMR Spectra
1 H NMR analysis of N, N′-bis (isatin)-1, 3-diaminopropane ligand (L 2 ) and N, N′-bis (isatin)-1, 3-diaminopropane zirconium(IV) complex [ZrL 2 Cl 2 ] were recorded using DMSO as solvent. 1 H NMR of the prepared ligand and complex are shown in Fig. 3 and 4, respectively. 1 H NMR spectral studies of Schiff base ligand usually give signals at δ = 6.47-7.67 ppm corresponding to aromatic protons (m, 7H, Ar-H) and at δ = 9.0 -10ppm due to isatin moiety NH Ade et al., 2012). According to    the proposed structure of selective ligand and complex, there are two types of -CH 2 groups in both ligand and complex that comes from the diamine. First one is two terminal -CH 2 and second one is middle -CH 2 group. For middle -CH 2 group multiplate was obtained at δ = 2.240-2.336 ppm with the integrated value 1.8 and the terminal -CH 2 protons assigned from triplet at the range δ= 4.133-4.166 ppm. There are two terminals -CH 2 group in the ligand therefore, the integrated value for these protons was 4.64. The presence of these peaks proves that the aliphatic chain is present in the ligand that bridges two isatin moiety and support the formation of ligand. The phenyl protons were observed at the chemical shift δ=6.863-7.864 ppm for ligand and in δ= 6.910-7.613 for the complex. The obtained integrated values of protons on phenyl part for ligand were 2.18, 2.29, 2.55 and 2.00 and that for complex were 2.00, 2.21, 2.80 and 2.26, respectively which indicate the presence of two phenyl groups in both the ligand and complex. The peaks for these two phenyl groups with eight integrated protons in the complex as like as ligand indicate the existence of ligand in the complex. The peak obtained in δ= 10.815-10.884 ppm for ligand was assigned for NH proton of isatin moiety. The N-H protonic peak in the complex gets almost disappeared through downfield shifting to δ= 11.051 ppm with respect to the ligand and indicated the lactam-lactim equilibrium Jiang et al. 2017). Therefore, the 1 H NMR also support the FT-IR result.

Electronic spectra
The N, N′-bis (isatin) diamine ligands and their Zr (IV) complexes are soluble in DMSO and DMF but insoluble in methanol, ethanol, and CCl 4 . Therefore, the electronic spectra of Schiff bases and its zirconium complexes were recorded in DMF. The UV spectra of the ligands and their complexes are shown in the Fig. 5 and 6, respectively. The electronic spectra of Schiff base ligand exhibit bands at 230, 290 due to π-π* transition and 380 nm n-π* transitions (Kriza and Parnau, 2001). In the UV spectra of ligands, bands at 240, 260 and 295 nm can be assigned due to the π-π*

Fig. 6. UV-visible Spectra of N, N′-bis (isatin) diamino zirconium (IV) complexes
ZrL 3 Cl 2 transitions within the aromatic ring which remain almost unchanged in the spectra of zirconium complexes. Another band at 385-415 nm is due to the n-π* transitions within the >C=N-group. The d-d electronic transition generally shows band around 550nm but pure d-d origin are not expected in the zirconium (IV) complexes having 4d 0 configuration (Jafarpour et al., 2013). In the UV spectra of present complexes, there was no absorption band found beyond 500 nm. This revealed the absence of d-d electronic transition over the visible region. Thus the electronic spectral data clearly confirmed that the complexes are genuine zirconium (IV). The complexes are, however, colored only through their absorptions tailing in the form of ultraviolet. Electronic spectral data are shown in Table IV and V for the prepared complexes.

Conclusion
The N, N′-bis (isatin) diamino Schiff base ligands were prepared by the usual condensation of isatin and diamine like ethane-1, 2-diamine, propane-1,3-diamine and hexane-1,6-diamine in 1:2 molar ratio. Zirconium complexes of these prepared ligands were synthesized by the reaction of the ligands and zirconium (IV) oxychloride octahydrate (ZrOCl 2 .8H 2 O). The prepared ligand and complexes were found to be of the type L n and ZrL n Cl 2 (where, n = 1, 2, 3). The analytical data indicated that the complexes have 1:1 (metal:ligand) stoichiometry. Molar conductance value indicated the non-electrolytic nature of the prepared complexes. Azomethine ῡC=N band in the FT-IR spectra indicating the successful formation of ligands. The shifting of ῡC=N band to the higher frequency in complexes indicated the influence of coordination of ligand with zirconium. The disappearance of ῡN-H and δN-H bands and de-intensification of ῡC=O indicated the lactam-lactim equilibrium. The appearance of ῡZr-O and ῡZr←N modes indicated the formation of metal-ligand coordination compound In 1 H NMR spectra the peaks for eight integrated protons of two phenyl groups and protons for methylene groups of bridging diamines in both the ligands and complexes pointed out their existence. The peak for NH proton at δ= 10.815-10.884 ppm of ligand gets almost disappeared in the complex indicated deprotonation through tautomerism due to the formation of coordinate bond and support the FTIR result. In the electronic spectra we observed absorption bands in the range below 500 nm. The absence of peak above 500 nm suggested that there is no electron in the d orbital of zirconium metal. It reveled the formation of complexes where zirconium exists in +4 oxidation state. The elemental analysis showed good agreement of experimental and theoretical percentage of C, H, and N in both ligands and their complexes. On the basis of overall analysis in this research, the formation of N, N′-bis (isatin) diamino zirconium (IV) complex (ZrL n Cl 2 ) with octahedral geometry could be expected. However, without crystal structure determination, it is difficult to put forward the exact geometry of ZrL n Cl 2 complexes. Therefore, further study is required to illustrate the precise structure of the prepared complexes and the mode of reaction.