*3.3. UV-Vis and IR-Spectral Studies*

The UV-Vis spectra of the water-soluble products of reaction (1) were recorded in the UV-visible region (Figure 7 and Table 4). The spectra for the investigated copper(II) complexes displayed bands at 610 nm and 661nm, assigned to 2B1g→2Eg and 2Eg → 2A1g*d-d* transitions. According to the authors of [39–41], this indicates that the investigated complexes weremononuclear complexes with four-coordinate square planar geometry.

**Figure 7.** UV-Vis spectra of complex I (red line) and hexamers Cu-[I-Cu]6 (green line) in water.


**Table 4.** UV-Vis spectra of the studied compounds (I, II, Cu-[I-Cu]6, and Cu-[II-Cu]6).

The Fourier Transform Infrared (FTIR) spectra of the metal complexes were recorded in KBr discs over the range of 4000–400 cm<sup>−</sup>1. The data of the IR studies (Table 5 and Figure 8) of the corresponding samples provide valuable information on how axial complexes I and II bind to Cu2+ during the formation of chelate complexes. Based on the analysis of the spectra of the reaction (1) products, it can be concluded that the amino and carboxyl groups were simultaneously involved in the chelate complex formation. The IR spectra of the oligomers now have new bands caused by the bending vibrations of the bonds formed due to the coordination with Cu2+. The frequency ranges expected for these vibrations are well known [42]. In addition to the vibrations of the amino and carboxyl groups, the processes of chelation were also confirmed by the vibrations of the N-M and O-M bonds.

**Table 5.** Relevant IR bands for the compounds I and Cu-[I-Cu]6.


**Figure 8.** IR spectra of complex I (blue line) and Cu-[I-Cu]6 (red line) in KBr discs.

The IR spectra of the aminoacid fragments with a bipolar structure contained characteristic bands of the NH3+-group corresponding to symmetric stretching (in the region of 3200–3400 cm<sup>−</sup>1) and bending (in the region of 1550–1600 cm<sup>−</sup>1) vibrations. In the chelate complexes, the stretching vibrations of the bound NH2 group were shifted to longer wavelengths. Such a decrease in the frequency and increase in the intensity of the amino-group stretching vibrations can be interpreted by coordination interactions between the metal cation and the nitrogen atom of the amino-group, which increased the dipole moment value. Also characteristic of chelation is the band at 1160 cm<sup>−</sup>1, which was related to the deformation vibrations of the NH2 group but was not observed in the bipolar compound.

A vibration band typical of the free carboxylate anion appeared at 1607 cm−<sup>1</sup> and 1384 cm<sup>−</sup>1. The carboxyl group transition to the non ionized state caused this band to disappear, and the vibration appeared in the longer wavelength region as ν(C=O) in the carboxyl group. For the investigated complexes, the COO−asymmetric stretching frequencies were shifted to lower values compared with those of the ligand. The bands in the region of 480 cm−<sup>1</sup> indicate the formation of a Cu–O bond and further confirm the ligand coordination to the central metal ion via the oxygen atom of the carboxylate group [42]. Hypsochromic shifts were observed for the –NH2 frequencies during coordination. This indicates bond elongation during the coordination, therefore suggesting probable square planar geometry of the complexes. The new bands in the spectra of the complexes at 535–552 cm−<sup>1</sup> were assigned to the (M–N) stretching frequency. The participation of the lone pairs of electrons on the N atom of the amino group in the ligand in the coordination was confirmed by these band frequencies [43].

#### *3.4. EPR Studies*

The conclusions about the planar-square structure of the obtained Cu(II) complexes based on the results of the IR spectra were additionally confirmed by EPR spectroscopy data [44–47]. In the EPR spectra of the studied compounds (Figure 9) at room temperature, the hyperfine lines from the magnetic interaction of the unpaired electron spin with the copper atom nuclear spin were well resolved. The isotropic EPR spectra are described by a symmetric spin Hamiltonian and had four hyperfine lines of equidistant components of different intensities and widths for nuclear spin projections, which is explained by the McConnell relaxation mechanism [47]. The spectra were a superposition of the spectra from the 63Cu nuclei, with the trans-N2O2 coordination environment of the Cu(II) ion.

**Figure 9.** Powder Electron Paramagnetic Resonance (EPR) spectrum of Cu-[I-Cu]6 (**a**) and [I-Cu]n (**b**).

The presence of two five-membered metallocycles in complex compounds, regardless of the nature of the coordinated atoms, led to a planar conformation. The coordination center in oligomers based on II increased the electron-donating properties of the nitrogen and oxygen atoms. These conclusions were confirmed by the calculated parameters of the EPR spectra. The EPR parameters for the Cu- [I-Cu]6 hexamer with tyrazine fragments (L1), had the following values: g = 2.119, acu = 89.6 E, α<sup>2</sup> = 0.81, whereas for the oligomer with aminoresorcinol ligands, these values were within the following range: g = 2.108, aCu = 95.84 E, α<sup>2</sup> = 0.89. The αparameter calculated from the isotropic EPR parameters using Formula (2) [48]:

$$\alpha^2 = \frac{1}{0.43} \left( \frac{\kappa\_{Cu}}{0.036} + \text{g} - 2 \right) + 0.02 \tag{2}$$

characterizes the degree of covalence of the copper-ligand bond. If the oligomer based on II had α<sup>2</sup> = 0.89, then the oligomer based on I was somewhat lower (0.81).
