https://doi.org/10.53656/nat2021-6.03

2021/6, стр. 586 - 605

SEMIEMPIRICAL QUANTUM CHEMICAL AND AB INTIO STUDY OF Co(II), Ni(II),Cu(II) AND Zn(II) COMPLEXES WITH THE AZO DYE DERIVED FROM 4-AMINO ANTIPYRINE AND 2,4-DIHYDROXY ACETOPHEONE

Резюме: Semi-empirical quantum chemical calculation was made to study the nucleophilicity of the ligand and to study the mode of bonding between the ligand and the metal ions. The natural atomic charge at different atomic sites of the ligand has been calculated along with the electrostatic potential map to predict the reactive sites for electrophilic and nucleophilic attack. The theoretical spectral data such as IR, NMR and electronic have been calculated and compared with the experimentally generated data.

Ключови думи: Semiempirical study; Chem 3D Ultra; Natural atomic charge; Molecular polarisability

From the Research Laboratories

Introduction

Molecular modelling uses theoretical approaches and computational techniques to study molecular systems ranging from small molecular units to large biological molecules (Cornado & Merlin 2003). Molecular modelling provides information of geometrical parameters such as bond lengths, dihedral angles and bond angles along with the spectroscopic data with respect to vibration, electronic and NMR (Leach 2009). In addition to this, Chemical reactivity sites of the ligand and its metal complexes can also be predicted from the natural atomic charge and electrostatic potential map (Ramachandran et al. 2008). The azo dye derived from 4-aminoantipyrine and 2,4-dihydroxyacetophenone as given in fig 1 and its metal complexes as given in fig 4 have been synthesised, characterised and reported (Chaulia 2016). In this study, attempts have been made to predict the reactive sites of the ligand and metal complexes by computing the natural atomic charge and electrostatic potential map. The spectral data such as IR, electronic and NMR of the ligand are calculated and compared with the experimental spectral data.

Computational Details

The computational calculations were made using Ab Intio and semi empirical methods with Chem 3D Ultra programme (Chem3D Ultra Molecular modelling and Analysis, 2003). The geometry of the ligand and its metal complexes were drawn by using the programme Chem draw present in the same Chem 3D Ultra programme. The geometry of the ligand and its complexes were optimised using molecular mechanics and low lying structures were again optimised using semi empirical quantum chemical methods.

Results and Discussion

The computational study of the ligand and its metal complexes were undertaken and compared with the experimental results.

Natural atomic charge

The natural atomic charges at different atomic sites of the ligand were calculated and presented in the table 1 and fig 2. The natural atomic charge plays important role in determining the electronic properties of a compound such as dipole moment, molecular polarisability and hyperpolarizability (Bejler et al. 1990). The region of highest electron density reflects the potential sites of electrophilic attack. The highest electron densities are concentrated over nitrogen and oxygen atoms, this fact is also supported by the molecular electrostatic potential study in fig 3. Hence, these atoms have the ability to coordinate with the metal ions.

Molecular electrostatic potential

The Electrostatic potential map gives an idea about the charge distribution and charge related properties of a molecules (Politzer & Murray 1991). The MEP of the investigated compound indicates red colour is concentrated around nitrogen and oxygen atoms and blue colour around some carbon and hydrogen atoms. This observation suggests that the nitrogen and oxygen atoms are rich source of electrons, and the metal ions is coordinating with the ligand through the azo nitrogen and oxygen atoms of the ligand as given in the Fig.3

IR spectra

The IR spectra of the ligand and its metal complexes were recorded and the experimental spectral data were compared with the computationally generated spectral data and presented in the fig 5, 8 and table 2 and 3. The IR spectrum of the ligand gives a band at 1270 cm-1 corresponding (C-O) vibration but the AM1 (Dewar et al. 1985 ) predicts it at 1262 cm-1 and the PM3 (Stewart 1989)predicts it at 1257 cm-1. The experimental frequency for the vibration of (N=N) appeared at 1493 cm-1 but the AM1 predicts it at 1479 cm-1 and the PM3 predicts it at 1491 cm-1. The computational study indicates the vibrational frequency for (C=O) is at 1708 cm-1 according to AM1 and at 1642 cm-1 according to PM3 but experimental study indicates it at 1639 cm-1. The correlation coefficient of experimental and computational vibration frequency calculated using AM1 and PM3 is 0.999 as given in fig 6 and 7.

The IR spectrum of metal complex shows a band at ~1219 cm-1 but in the free ligand, it was at 1270 cm-1 and missing of peak at ~3434 cm-1 in the metal complexes that indicates deprotonation of -OH group and formation of bond by the metal atom or ion with the oxygen atom of the -OH group. However, the computational study predicts it at 1262 cm-1 according to AM1 force field and at 1283 PM3 force field. The spectrum of the complex also shows peak at ~1454 cm-1 but it was at 1493 cm-1 in the free ligand indicating formation of bond with one of the nitrogen atoms of the azo group(-N=N-) with the metal atom or ion. The computational generated spectrum indicates this peak at 1492 cm-1 and 1490 cm-1 corresponding to AM1 and PM3 force fields. The (C=O) stretching vibrational frequency of the free ligand is at 1639 cm-1 but IR spectrum of the complex shows at 1610 cm-1 that suggests formation of bond of carbonyl oxygen with the metal ion or atom. The computer-generated spectrum data indicates the vibrational frequency at 1647 cm-1 according to AM1force field but at 1644 cm-1 according to PM3 force field. The bonding between the metal ion and oxygen atom of the (-OH) group is confirmed by the appearance of peak at 536 cm-1 in the IR spectrum of the complex7 but according to AM1 force field it is at 561 cm-1 but PM3 predicts it at 581 cm-1. The bonding between the metal ion and nitrogen atom of the (-N=N-) group is confirmed by the appearance of peak at 536 cm-1 in the IR spectrum of the complex but according to AM1 force field it is at 556 cm-1 but PM3 predicts it at 522 cm-1. (Sirichote et al. 1998). The correlation coefficient of experimental and computational vibration frequency calculated using AM1 and PM3 is ~ 0.997 as given in fig 9 and 10.

Electronic spectra

The semi empirical quantum chemical method find uses in calculating excited states hence to predict UV spectra of chemical compounds effectively. The single point energy calculations were carried out at semi empirical level to predict UV spectra. The computational electronic spectral bands of the ligand and its Co(II), Ni(II), Cu(II) complexes were calculated. The spectral bands of the investigated compounds were calculated by using the ZINDO programme (Zerner 1991).

The ligand and its metal complexes were also characterised by the Electronic spectra. The absorption bands are observed in the UV-Visible region of the electromagnetic spectrum due to transition of electrons between two electronic energy levels when energy difference between two electronic energy levels matches the energy of the incident photon1.

Energy of the incident radiation

n

The semi empirical quantum chemical method find uses in calculating excited states hence to predict UV spectra of chemical compounds effectively. The single point energy calculations were carried out at semi empirical level to predict UV spectra. The computational electronic spectral bands of the ligand and its Co(II), Ni(II), Cu(II) complexes were calculated and given in table. The spectral bands of the investigated compounds were calculated by using the ZINDO programme (Zerner 1991).

The computational study of the Co(II)complex predicts spectral bands at 12987 cm, 16393 cm, 22727 cm corresponding to 4T1g(F) 4T2g(F), 4T1g(F) 4T2g(F)and 4T1g(F) 4T2g(P). The theoretical spectral bands were compared with the experimental spectral bands which were found at 14492 cm-1, 20449 cm-1 and 25575 cm-1 respectively as given in table 4. Both experimental and computational values were compared, and the correlation coefficient was found to be 0.954 given in fig 11.

Table 4. Experimental and calculated wavelength of the ligand and its metal complexes

Ligand 275 377 n& $

245 318 [Co2LCl 2(H2O)6] 12987 14492 4T1g(F) " 4T2g(F)

16393 20449 4T1g(F) " 4A2g(F)

22727 25575 4T1g(F) " 4T2g(P)

[Ni2LCl2 (H2 O)6] 13157 14084 3A 2g(F) "3T 2g(F)

16528 18552 3A2g(F) " 3T1g(F)

23255 24271 3A2g(F) " 3T1g(P)

[Cu2LCl2(H2O)2] 15350 13642 2Eg " 2T2g

Similarly, Ni(II) complex predicts spectral bands at 13157 cm-1, 16528 cm-1 and 23255 cm-1 due to 3A2g(F) 3T2g(F), 3A2g(F) 3T 1g(F) and 3A2g(F) 3T1g(P). The computational study indicated it at 14084 cm-1, 18552 cm-1 and 24271 cm-1. A good correlation was obtained between the experimental and calculated data (R2= 0.954) given in fig 12 and percentage variation is less than 5.

The electronic parameters of the Co(II) metal complex were calculated by using the following relations and presented in the table no 5

Similarly, the electronic parameters of Ni(II) metal complex were also calculated by using the following relations and presented in the table no 5

It has been seen that the electronic parameters such as B, β and ʋ2/ʋ1 calculated from the computational values for Co and Ni as given in table 6 gives good correlation with the electronic parameters calculated from experimental its values as compared to [Co2LCl2(H2O)6] metal complex.

Nuclear magnetic spectra

The Nuclear magnetic resonance spectroscopy has been used for structure elucidations of various complex molecules. The use of experimental and computational data offers powerful techniques to interpret and predict the structure of bulky molecules (Torda & Gunsteren 1992). The gauge independent atomic orbital method has been used for calculating the NMR chemical shifts (Wolinski et al. 1990)

The 1H NMR spectra of the ligand as given in fig 13 and its Zn(II) given in fig 14 complex were recorded experimentally separately. The computational chemical shifts values of the ligand and metal complexes are also collected as given in fig 15 and 16 and these chemical shifts values ( were compared with the computational ( values. The correlation coefficient of both the values was found to be 0.828 as given in fig 17.

The 1H NMR computational data gathered for the ligand are given below

The 1H NMR computational data obtained for the Zn(II) complex are given below

The 13C NMR computational study of the ligand and Zn(II) complex was undertaken to study the nature of bonding between metal ion and donor atoms of the ligand as given in the fig 18 and 19.