Octahedral Oxo-Bridged Tri-Nickel(II) Complex with 1,3,5-Tris(2-hydroxyphenyl)-2,4-diazapenta-1,3-diene

Abstract

An octahedral nickel(II) complex of 1,3,5-tris(2-hydroxyphenyl)-2,4-diazapenta-1,3-diene was obtained and characterized by X-ray crystallography. The organic ligand was obtained by the reaction of 2-hydroxybenzaldehyde with aqueous ammonia solution.

1. Introduction

Complexes of salicylidene imines with various metals, such as copper, nickel, cobalt, iron, and uranium, are known and widely used [1]. These complexes are examined for applications in various fields, such as catalysis [2,3], electrochemistry [4], and medical chemistry [5,6]. The key bis-iminium ligand, 1,3,5-tris(2-hydroxyphenyl)-2,4-diazapentadiene, can be easily obtained using the reaction of aromatic aldehyde with ammonia [7]. This compound is a close analog of the well-known salen ligand but contains a short methylene bridge between two imine nitrogens. This shortness, combined with the rigidity of the structure, suggests that this compound could serve as a ligand for the formation of unusual complexes with metals of different types of possible coordination, such as nickel. Complexes with iron and manganese [8] and cobalt [9] are described for this type of ligand or its close analogs. However, in the case of nickel(II), the complex was not characterized by the single XRD method, and its structure remained not precisely proven [10].

2. Results

The reaction of aromatic aldehydes with ammonia can lead to different products, depending on both the nature of the aromatic aldehyde and the reaction conditions. In the case of electron-deficient derivatives, the reaction more often leads to imidazolines [11], triazines [12], or hexamine derivatives [13]. For electron-sufficient benzaldehyde derivatives, the reaction more often leads to the diazapentadiene (1) derivative, often called hydrobenzamide. The latter, in turn, under the action of the base, can lead to trisarylimidazolines with various properties, such as corrosion inhibitors [14], inhibitors of protein–protein interactions, or precursors for diamines [15,16].

In the case of salicylic aldehyde, the reaction with an aqueous solution of ammonia leads to the corresponding hydrobenzamide 1 (Scheme 1). The obtained product is a yellow solid that can be easily isolated by filtration and is stable in air. Ligand 1 was characterized by NMR ¹H, ¹³C, and IR. It should be noted that the configuration of the double bonds of the resulting 2,4-diazapenta-1,4-diene (hydrobenzamide) is E,E due to the steric repulsion of the aryl fragments.

For the subsequent reaction with nickel(II) acetate, compound 1 was dried in a vacuum (1 h at 60 °C). The absence of water at the stage of obtaining the complex is very important, since traces of moisture in the presence of metal salts lead to the hydrolysis of imine and the formation of salicylate complexes [17]. Pyridine was added to the reaction mixture for basicity and as an auxiliary ligand. Anhydrous ethyl alcohol was used as a solvent. When the solution had settled, green crystals were obtained and then analyzed using the X-ray crystallography method (Figure 1).

For iminium complexes of nickel(II), the structure of a flat square is described in the literature, as well as an octahedral structure [17,18,19]. In this work, the structure obtained is the slightly distorted octahedral structure (Figure 1). The following features can be noted in the structure of the obtained complex. The nickel ion, in all cases, has an octahedral environment. For the Ni1 and Ni3 ions, the auxiliary ligands are pyridine molecules (two for each); Ni2 (in the center) is coordinated only by ligand molecules: 4 positions are occupied by oxygen atoms of the phenolic fragment, and two by iminium fragments. All iminium and phenolic fragments of both ligands participate in the coordination of nickel ions. The following brief conclusions can be made from the analysis of bond lengths. The distance between nickel (Ni1) and pyridine (N6) is greater than that of nickel (Ni1) with the iminium nitrogen atom (N1) (2.111 A vs. 2.041 A). At the same time, the distance between the central nickel atom (Ni2) and the iminium fragment is slightly greater than in the case of Ni1 (2.071 A vs. 2.041 A). The oxygen atoms involved in the coordination of two nickel ions have a bond greater than 2.060 A and, in some cases, even 2.202 A (for Ni1 and O4). At the same time, the oxygen atoms involved in the coordination of one Ni cation (O1 and O6) are at a distance of up to 2 A (1.996 A and 1.994 A, respectively).

The resulting complex does not have a high stability in air under normal conditions, as was also confirmed by thermogravimetric analysis. It was found that the complex loses four pyridine molecules when heated to 207 °C (Figure S7). This mass loss is consistent with the results of X-ray crystallography data and literature data for this type of complex [10]. Measurements in the UV range have shown that the nickel complex with this ligand has a maximum absorption peak at 386 nm (Figure S8).

3. Materials and Methods

Salicylic aldehyde was distilled in a vacuum. Pyridine was distilled over potassium hydroxide, and ethanol was dried via azeotropic distillation with benzene. Nickel acetate(II) was dried under a vacuum to a greenish-blue color before synthesis.

¹H and ¹³C NMR spectra for ligand 1 were recorded at 298 K on a Bruker Avance 300 spectrometer with operating frequencies of 400 and 100 MHz, respectively, and calibrated using residual DMSO-d5 (δH = 2.50 ppm) and DMSO-d6 (δC = 39.5 ppm). NMR data were presented as follows: chemical shift (δ ppm), multiplicity (s = singlet, m = multiplet, br. s. = broad singlet), the coupling constant (J) in Hertz (Hz), and integration.

Single X-ray diffraction data for compound 2 were collected at 100 K using a Bruker D8 QUEST, single-crystal X-ray diffractometer with a Photon III detector. The structure was solved by dual-space methods (SHELXT). All calculations were performed using the Olex2 interface [20]. UV and IR spectra were recorded on Jasco V-770 and Nicolet iS5 FT-IR (ATR method, angle of incidence of 45°) spectrometers, correspondently. UV measurements were performed as follows: a solution of 12.7 mL of ethanol and 4.7 mL of pyridine was prepared; 0.3 g of nickel(II) acetate and 1.2 g of ligand (1) were added. The resulting solution was placed in a cuvette, and UV measurements were performed. Thermo-gravimetry was performed on a NETZSCH STA 409PC/PG Thermal Analyzer (NETZSCH in Selb, Gebrüder-Netzsch-Straße, Germany).

3.1. Synthesis of 1,3,5-Tris(2-hydroxyphenyl)-2,4-diazapentadiene (1)

Salicylic aldehyde (20.0 g, 0.17 mol) and ammonium chloride (3.0 g, 5.6 mmol) were suspended in an aqueous ammonia solution (100 mL, 25%) in a beaker (250 mL) and left overnight. The resulting precipitate of the hydrobenzamide derivative 1 was filtered and left in the air to dryness (18.0 g, 95%). NMR ¹H (DMSO-d6, 400 MHz): δ 6.36 (s, 1H, NCHN), 6.85–6.95 (m, 6H, ArH), 7.18 (td, J = 8.3, 1.8 Hz, 1H, ArH), 7.33–7.39 (m, 3H, ArH), 7.55 (dd, J = 8.4, 1.7 Hz, 2H, ArH), 8.80 (s, 2H, ArCHN), 13.17 (br. s., 3H,ArOH). NMR ¹³C (DMSO-d6, 101 MHz): δ 84.1, 115.7, 116.5, 118.7, 118.9, 119.3, 126.6, 127.4, 129.3, 132.1, 133.0, 154.7, 160.3, 165.2. IR, cm⁻¹ (1400–1700 cm⁻¹ region): 1633, 1616, 1602, 1549, 1478, 1446, and 1415.

3.2. Synthesis of Ni₃L₂Py₄ (2)

Ni(OAc)₂ (0.1 g, 0.6 mmol) was dissolved in 3 mL of a pyridine/ethanol mixture (1:3 by volume). Anhydrous ligand (dried in a vacuum) (1) (0.4 g, 1.2 mmol) was dissolved in 3 mL of the same pyridine/ethanol mixture and added to a nickel acetate solution. The resulting emerald-green solution was left in a closed flask in a dark, cold place (−15 °C) until emerald-green crystals fell out (120 mg, 51%). IR, cm⁻¹ (1400–1700 cm⁻¹ region): 1618, 1533, 1467, 1447, 1419.

A summary of crystallographic data for the single-crystal experiment is available from CCDC; deposition number 2404114. The data can be obtained free of charge via www.ccdc.cam.ac.uk/structures (accessed on 24 December 2024).

4. Conclusions

In summary, we synthesized and characterized the new octahedral Ni(II) complex with two hydrobenzamide ligands and four pyridine molecules as auxiliary ligands. Its composition was confirmed by an X-ray diffraction method, which clearly showed the slightly distorted octahedral conformation.