Quantum Chemical Study on Molecular and Electronic Structures of Methyl and Methoxyl Substituted Cu ( II ) and Ni ( II ) Benzoic Acid Hydrazides Ions

The electronic structures of model methyl and methoxyl substituted benzoic acid hydrazides of Ni(II) and Cu(II) complexes have been studied both at semi empirical level (PM3). The ortho-methoxyl is relatively stable which may be due to the formation of hydrogen bonds between methoxyl oxygen and hydrogen on hydrazide (CH30---HNNH; 1.817Å for Cu(II) and 1.806~1.839Å for Ni(II)). The change in torsion in the complexes affect π-electrons delocalization (complexes containing π-electrons system) and consequently affect the band gap which is a measure of electronic properties that control the reactivity of the complexes. The curves for ortho and para methoxyl substituted Cu(II) complexes intercept at 50 and 135-144, this could suggest that both complex ions can co-exist and react in very similar ways in solution under certain conditions.


Introduction
The -CONHNH 2 moiety of hydrazides have attracted considerable interest in the literature partly because of their coordinating ability to chelate a variety of transition metal ions [1][2] and their importance of organic chemistry since they are included in retro-bispeptides [3][4][5] which are used to design enzyme substrate mimics and potential inhibitors and in aliphatic polyamides [6][7][8][9] which display structures different to the extended conventional ones.However the number of studies about molecular and electronic structures of the hydrazides moiety is scarce 10 .There has been investigation into conformational preferences of hydrazide functional groups in both gas phase and aqueous solution using ab-initio quantum mechanical calculations 11 .Recently a number of tin complexes were studied and in any case they seem to manifest efficient anticancer activity 12,13 .The advantage of their use as antitumour medicine lies in their lower costs and possibly in a greater effectiveness.However, in general, a satisfactory understanding of the mode of action of the drug requires a thorough study of its interaction with the relevant biomolecules or their model systems 14 .
Hydrazides generally have also shown a lot of promising biological activity, which could easily lead to their adaptation as a drug.In this paper, the electronic structures and stabilities of substituted Copper(II) and Nickel(II) benzoic acid hydrazides ions were studied using semi empirical method (PM3) (Figure 1).

Computational details
The unsubstituted and substituted Cu[C 6 H 7 CON 2 ] 2 2+ and Ni[C 6 H 7 CON 2 ] 3 2+ complexes were modeled using Spartan 04 essential 15 .The structures were fully optimized 16 at the PM3.PM3 method is a computational efficient technique that gives conformation and heats of formation in qualitative good agreement with trend in results of more accurate methods 17 .

Stability and geometry
Table 1 shows the heats of formation of the two series of Cu(II) and Ni(II) benzoic acid hydrazides ions.The stability of the Cu(II) complex ions increases in the order ortho < meta < para for methyl substituted.However, in methoxyl substituted Cu(II) complex ions, the stability increases in the order meta < ortho< para (the ortho methoxyl substituted is more stable than the meta substituted).In Ni(II) hydrazide ions, meta-methyl substituted is most stable and ortho-methoxyl substituted is energetically preferred, it is more stable than paramethoxyl substituted.The relative stability of methoxyl may be due to the formation of Hydrogen bonds between methoxyl oxygen and hydrogen on hydrazide (CH 3 0---HNNH; 1.817Å for Cu(II) and 1.806~1.839Åfor Ni(II)) as shown in Figure 2. The trends in stability of these series of complex ions coincide with that of HOMO-LUMO gaps (∆); the higher the stability the larger the ∆ gaps.Table 2 shows selected bond lengths and bond angles of the complexes calculated at PM3 level.The bond lengths of Cu(II) hydrazides are more or less affected with methyl substitution but methoxyl substitution affects bond lengths by shortened Cu-O and lengthened Cu-N especially for ortho-and para-substitutions.Both methyl and methoxyl substitutions affect copper-hydrazides bond angles (<CuN1N2 and CuN3N4) as compared to un-substituted.In case of Ni(II) hydrazides, greater distortion is noticed in both methyl (except m-CH 3 ) and methoxyl substitutions as reflected in bond lengths and bond angles (<NiN1N3, NiN2N4 and NiN5N6).

Torsion potentials
The Torsion potential curves for Ni[C 6 H 7 CON 2 ] 3

2+
and Cu[C 6 H 7 CON 2 ] 2 2+ and their substituted derivatives associated with the rotation of the one of CNNC in different directions are shown in Figures 3, 4, 5 and 6.The energy barrier high for the un-substituted and methyl substituted Ni[C 6 H 7 CON 2 ] 3 2+ (Figure 3) is less than 1.0 kcal/mol (almost flat).This shows that position (ortho, meta or para) of the methyl substituent is the major determining factor to the stability of the complexes rather than the methyl; this may be due to internal rotation restriction of the complex ions.In the methoxyl substituted Ni complexes, the energy barrier experienced is not similar to that of methyl-substituted, except in ortho-methoxyl position.The energy barrier experienced in m-OCH 3 substituted is about 4.0 kcal/mol, therefore there would be a greater internal rotation in m-OCH 3 substituted as compared to other methoxyl positions, which may have substantial influence on the chemical reactivity of the complex (Figure 4)., the energy barrier experienced on torsion is similar for methyl substituted except in ortho methyl substituted in which barrier height experienced is about 5.0 kcal/mol (Figure 5).The torsion curves of methoxyl substituted Cu(II) complex ions are quite similar to that of methoxyl substituted Ni(II) complex ions (Figure 6).It is very interesting to note that the curves for ortho and para methoxyl substituted Cu(II) complexes intercept at 50 0 and 135 0 -144 0 , this could suggest that both complex ions can co-exist and react in very similar ways in solution under certain conditions.In general, methoxyl group contribution more to the geometry distortion than methyl substituted in both Ni(II) and Cu(II) complexes ions.as a function of torsion angles.As expected, the change in torsion in the complexes affect π-electrons delocalization (complexes containing π-electrons system) and consequently affect the ∆ gap which is a measure of electronic properties of the complexes.It is observes that changes in ∆ become prominent with increase in torsion angles.

Evolution of HOMO and LUMO
, there is a decrease in ∆ as the torsion angles become larger until 100 0 for the un-substituted and substituted except for m-CH 3 and p-OCH 3 substituted in which the ∆ increases (Figure 7 and 8).The increment in torsion increased the electronic donating effect to the π-electrons delocalization of the benzoic rings except in m-CH 3 and p-OCH 3 substituted.2+ in that there is a decrease in ∆ gap as the torsion angles become larger in un-substituted and ortho-methyl substituted (Figure 9).In Figure 10, methoxyl substituent has the same effect on the π-electrons delocalization of benzoic rings in the Cu(II) complexes.Calculated Mulliken charges on copper and Nickel atoms are -0.494~ -0.351 and -0.974 ~ -1.061 respectively.The charge on copper atom increases on addition of methyl and methoxyl substituents, the effect is more pronounced with methoxyl substituent especially on meta and para positions.However, o-OCH 3 substituent has little effect on atomic charge of copper which may due to formation of hydrogen bond.The atomic charge on nickel reduces slightly except o-OCH 3 substituted which increased. .

Conclusion
In order to investigate the electronic effects and geometrical distortions of methyl and methoxyl substituted Cu(II) and Ni(II) hydrazides ions, we have utilized the PM3 method.The relative stability of methoxyl substituted may be due to the formation of Hydrogen bonds between methoxyl oxygen and hydrogen on hydrazide (CH 3 0---HNNH; 1.817Å for Cu(II) and 1.806~1.839Åfor Ni(II)).Methoxyl substitution has more noticeable effect on the complex ions than methyl substituted which may be due to internal rotations in methoxyl substituted ones.The curves for ortho and para methoxyl substituted Cu(II) complexes intercept at 50 0 and 135 0 -144 0 , this could suggest that both complex ions can co-exist and react in very similar ways in solution under certain conditions.In addition, both methyl and methoxyl substituents affect molecular stability of Cu(II) and Ni(II) hydrazides complex ions which is good indication that electronic properties control chemical reactivity.

Figure 7 ,
Figure 7, 8, 9 and 10 show the evolution of the HOMO-LUMO energy difference (∆) of the two series of substituted Ni[C 6 H 7 CON 2 ] 3 2+ and Cu[C 6 H 7 CON 2 ] 2 2+as a function of torsion angles.As expected, the change in torsion in the complexes affect π-electrons delocalization (complexes containing π-electrons system) and consequently affect the ∆ gap which is a measure of electronic properties of the complexes.It is observes that changes in ∆ become prominent with increase in torsion angles.