*3.1. Synthesis and Structure*

It is well known that when amino acids, such as some other polydentate ligands, interact with *d*-metal cations, they form stable compounds with one or two chelate rings [28]. The higher stability of such compounds is the result of each polydentate ligand binding to the complexing cation by at least two bonds (-M-O, M ← NH2 or M ← NH). The products of the amino acid interaction with *d*-metal cations can be mono- and bis-ligand particles. In the latter case, bicyclic chelating of the copper cations occurs with formation of 4-coordinate square planar geometry of the coordination center [32,33].

Depending on the self-assembly conditions, the products of the interaction of *bis*-axial complexes I and II with the Cu2+ cations can be both porphyrin dimers ([I-Cu-I] and [II-Cu-II]) and oligomers ([In-Cun±1] and [IIn-Cun±1]) consisting of several porphyrin fragments and copper cations (Figure 2).

$$\mathrm{NSnP(L)\_2} + (\mathrm{n} \pm 1)\mathrm{Cu}^{2+} \stackrel{t}{\rightarrow} [\mathrm{SnP(L)\_2}]\_n - [\mathrm{Cu}]\_{n \pm 1} \tag{1}$$

**Figure 2.** Proposed structures of products of the of the Sn(IV)-porphyrin axial complexes I and II interaction with Cu2+ cations.

The structures of *bis*-axial complexes I and II and products of their self-assembly (porphyrin dimers (I-Cu-I and II-Cu-II)), obtained by simultaneous interaction of Cu2+ with the hydroxy and amino groups of axial ligands belonging to two different porphyrinate molecules, were optimized by the DFT method with the CAM-B3LYP hybrid functional and 3–21 basis set. The data obtained are shown in Figure 3 and Table 1.

**Table 1.** Geometric parameters of the studied compounds obtained by quantum-chemical calculations using the Density-functional Theory DFT/CAM-B3LYP hybrid functional and 3–21 g basis.


As seen from Figure 3 and Table 1, complexes I and II had similar Sn-O and Sn-N bond lengths. A distinctive feature of I wasthe presence of additional points of binding between the axial ligands and the porphyrin macrocycle due to the formation of intramolecular hydrogen bonds, which could potentially prevent the formation of oligomeric and polymer structures. The inclination angle of the axial ligand aromatic part of the axial ligand to the porphyrin plane in complex I was 41◦, whereas in complex II, it was 50◦.

The formation of dimeric structures increased the inclination angle of the ligand phenolate fragment relative to the porphyrin plane, probably due to the repulsion of the aromatic fragments from each other. The functional groups involved in the chelation with Cu2+ werelocated in the dimeric structures at the maximum possible distance from the porphyrin plane. Obviously, in the case of a two-center interaction of the axial fragments with Cu2+, such a structure is the most favorable energetically. In the case of I-Cu-I, the formation of a chelate bond between the tyrosine and the copper cation destroys the hydrogen bonds between the tyrosine and sulfophenyl moieties.

A significant increase in the Sn-O-L angle can be observed in the II-Cu-II structure optimized by quantum chemical calculations. This increase is associated with the fact that the amino group of the diaminohydroquinone fragment approached the pyrrole nitrogen atom of the porphyrin macrocycle. Since there can be a significant electrostatic interaction between the porphyrinate nitrogen atom and the amino group protons, such a structure distortion can be energetically favorable.

Since the axial ligands in complexes I and II were of different sizes, the distance between the porphyrin fragments in the I-Cu-I and II-Cu-II dimers differed significantly and amounted to 21.3 and 17.6 Å, respectively. At the same time, the porphyrin fragments in the dimeric systems were almost parallel to each other (Figure 3). The structures of the porphyrin oligomers linked through Cu2+ werenot optimized. However, based on the data about the dimeric structures, it can be assumed that the longer porphyrin oligomers were almost linear, and the porphyrin polymers consisted of fragments similar to those shown in Figure 3.

I-Cu-I II-Cu-II

**Figure 3.** Structures of the dimers I-Cu-I and II-Cu-II optimized by the DFT/CAM-B3LYP hybrid functional and 3–21 g basis.

According to the experimental data, the result of these self-assembly of Sn(IV) porphyrinates (I-II) in the presence of Cu2+ in aqueous solutions depends on the concentration ratio of the starting reagents, reaction time, and temperature. Table 2 shows the empirical formula, molecular weight, and elemental analysis data of the reaction (1) products at different concentrations of the starting compounds. Oligomerization was achieved by heating compounds I and II for several hours at 90 ◦C. The self-assembly of the porphyrinate fragments was monitored by changes in the UV-Vis spectra.

The self-assembly of the porphyrinate macrocycles into larger aggregates led to a decrease in their solubility. The larger the oligomer, the lower its solubility. Upon reaching a certain size, the resulting oligomers precipitated. The proportions of soluble and insoluble self-assembly products in the studied systems are also presented in Table 2.

An analysis of the molecular weights of the substances presented in Table 2 shows that the soluble products of the interaction of I or II with Cu2+ at an equivalent quantitative ratio of the reagents were mainly porphyrin dimers (I-Cu-I and II-Cu-II). Under the conditions of a five-fold excess of copper cations and prolonged heating of the reaction mixture (up to 24 h), oligomers with a large number of macrocycles were formed. The maximum number of porphyrin fragments in soluble oligomers didnot exceed six. Chain oligomers with more than six Sn(IV)-porphyrin units (polymers) precipitated during reaction (1). The formation of porphyrin oligomers and polymers through strong bis-chelate binding with the formation of a flat coordination center (Figure 2) was confirmed by UV-vis, IR, 1H NMR, EPR spectroscopy, and thermogravimetric analysis. The composition of oligomeric chains was estimated from the data of elemental analysis, mass spectrometry, and 2D NMR.


**Table 2.** Empirical formula, molecular weight, and elemental analysis data of the reaction (1) products with the ratio of reagents (1:1 and 1:5).

Soluble (a) and insoluble (b) products of the reaction (1).

The mass spectrometry confirmed the formation of dimeric forms of complexes I and II in the products of reaction (1). In addition to the peaks with m/z 1406.01 and 1470.51, corresponding to the [I-H]<sup>−</sup> and ([I-Cu]-H)<sup>−</sup> ions, the mass spectrum of the product of the complex I interaction with Cu2+ (Figure 4) at a 1:1 molar ratio of the reagents hada peak with m/z 2877.03 corresponding to the [I-Cu-I] dimer. It was not possible to confirm the formation of larger (containing six macrocyclic fragments)porphyrin oligomers by the mass spectrometry method, which was probably due to the oligomer instability in the conditions of the mass spectral studies of the samples. Similar behavior wasobserved in the mass spectra of the products of the complex II interaction with Cu2+ at 1:1 and 1:5 molar ratios of the reagents.

**Figure 4.** Mass spectrum of the I-Cu-I.
