**Acknowledgements**

The nature of the ligand in inner coordination sphere is expected to play an important role in the binding of the metal complexes to biomolecules and in their cytotoxic effect. We have made an attempt to the mechanism of substitution between DNA constituent 5'-GMP and square-pyramidal [ZnCl2(terpy)] complex

*The proposed reaction pathways for the reaction of [ZnCl2(terpy)] complex and glutathione [13].*

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

*NMR spectra of the reaction between [ZnCl2(terpy)] and 5'-GMP at 295 K, pD 4.5 in D2O after*

H NMR method [14]. The reaction reached completion for less than 1 min, which was visible in NMR tube. The color of the solution after addition of 5'-GMP

during several weeks, but changes in spectrum have not been observed (**Figure 6**). The singlet of H8 proton of the coordinated 5'-GMP appeared at 8.70 ppm, while the doublet of the H1' proton was at 5.97 ppm. Corresponding to terpy ligand in the spectrum, six proton patterns have been observed. The doublet of 6,6<sup>00</sup> appeared at

As mentioned before, [ZnCl2(terpy)] reacted immediately with DNA constituent; the rate of nucleophilic substitution reaction is controlled by strong *π*-acceptor

pyridine; 4,4<sup>00</sup> protons appeared in the range 8.49–8.42 ppm, the triplets of 4<sup>0</sup> proton from middle pyridine ring; and 5,5<sup>00</sup> protons were at 8.19 and 7.71 ppm, respectively. The doublet of 3,3<sup>00</sup> protons seems to be at the same position as H8 proton of 5'-GMP (the signal was broadened) (**Figure 8**) [17]. The protons in the aromatic region of the spectra all correspond to signals from known terpy ligand

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between three pyridine rings causes a decrease in electronic density on the zinc center due additional formation of *π*-back bonding and makes it more electrophilic and more reactive. The final product [ZnCl(terpy)(*N7*-GMP)] is also characterized by DFT calculation in combination with experimental NMR technique. The results are in good agreement; it confirmed coordination via N7 donor (intermediate

In order to confirm the geometry around the coordinated center, structural index τ [21] was calculated for both [ZnCl(terpy)(*N7*-GMP)] and [ZnCl(en) (*N7*-GMP)] complexes. For five-coordinated Zn(II) complex, the structural index τ5=(β � α)/60° (α and β are the two largest angles around the central atom) [21], which represents the relative amount of trigonality (square-pyramid, τ5 = 0; trigonal-bipyramid, τ5 = 1) is 0.32. The coordination geometry around zinc ion could be best described as somewhat between square-pyramidal and trigonalbipyramidal, more like leaned toward distorted square-pyramidal geometry. Reported structures of three ZnN3S2 complexes, namely, [Zn(terpy)(iPrO)2PS2]2 [22], [Zn(BMIP)] [23], and [Zn(BMAP)] [24], have similar position of the donor atoms in the ligands resulting in distorted trigonal-bipyramidal geometry of

H NMR spectra were recorded after 24, 48 h, and

,2″-terpyridine. The electronic communication

,5<sup>0</sup> protons, from middle

by <sup>1</sup>

**Figure 7.**

**Figure 8.**

*1 minute [14].*

in molar ratio 1:1 turns white. <sup>1</sup>

8.95 ppm, the multiplet formed by covered signals of 3<sup>0</sup>

coordinated to metal ions as is expected [19, 20].

complex with minor distortion due to the bulky groups.

ability of the tridentate chelate 2,2<sup>0</sup>

Lewis base) [14].

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The author gratefully acknowledges the financial support from the State University of Novi Pazar and the Ministry of Education, Science and Technological Development, Republic of Serbia (Projects No. 172011).
