New 5-Chloro-Maleonitrile-Salen Ligand and Relative Co(II) Complex

Abstract

Salen ligands and relative metal complexes are widely used in many fields, such as catalysis, sensing, optical and electronic materials. The introduction of a wide range of substituents in different positions, both in the salen scaffold and in the diamine bridge, allows the tuning of chemical, catalytic and spectroscopic properties. In this contribution, we report the synthesis and characterization of a new salen ligand and relative Co(II) complex bearing a maleonitrile bridge and two chlorine atoms in the salen backbone.

1. Introduction

The term Salen is an acronym widely employed to indicate a class of organic molecules which present a structure deriving from N,N’-bis(salicylidene)ethylenediamine (Scheme 1).

Most preparative and catalytic studies on this family of compounds have been dedicated to salen ligands with C2 symmetry, according to the procedure developed by Jacobsen and reported in Scheme 1 [1].

Common diamines used for the aforementioned synthesis are 1,2-cyclohexyldiamine, ethylenediamine, 1,2-diphenylethylenediamine and maleonitrile diamine [2,3,4]. The substituents on the aromatic rings can be different, although the most used have t-butyl groups in positions 3 and 5, and hydroxymethyls or halo substituents in position 5. Salen ligands may also be asymmetrical when they display different substituents on the two aromatic rings, leading to several advantages [5,6,7,8,9,10,11].

Furthermore, because of the complexation between metal ions and salen ligands, the imine groups increase their stability in acidic conditions since they are generally susceptible to acid-catalyzed hydrolysis in the presence of water.

For several years, salen ligands have been considered essential for catalysis, either symmetrical or asymmetrical [12,13,14,15,16,17], and sensing [18,19,20,21,22]. In particular, asymmetrical catalysis, also known as chiral synthesis, facilitates the formation of a specific stereoisomer during a chemical reaction. This is important in the field of pharmaceutical synthesis since enantiomers or diastereomers of the same molecule often have different biological activities.

Here, we report the synthesis and characterization of a new salen ligand 1 (Figure 1), having a malonitrile bridge and two chlorine atoms in 5-5′ positions, and its relative Co(II) metal complex 2 (Figure 1). These molecules can be used as precursors to obtain Lewis Acid receptors, thus used for sensoristic applications due to the intriguing fluorescence properties of the malonitrile and cobalt-based compounds. In addition, the presence of chlorine atoms in 5-5′ positions lead to the possibility of further functionalization [23].

2. Results

Salen ligand 1 was obtained through the reaction of 5-chloro-salicylaldehyde and 2,3-diaminomaleonitrile in absolute ethanol (Scheme 2).

The reaction was performed overnight through reflux. The conversion of the starting aldehyde was monitored by TLC analysis. After the disappearance of the aldehyde, the precipitate was collected by vacuum filtration. The desired compound was characterized by ¹H NMR, ¹³C NMR, ESI-MS and FT-IR analyses. In particular, the ¹H NMR spectrum showed the presence of three signals at 8.20 and 6.94 ppm (doublets) and at 7.33 ppm (doublet of doublets) relative to the aromatic protons (Figure 2, light blue, green and red circles, respectively). In addition, the diagnostic signal due to the formation of the desired compound is at 8.50 ppm (singlet), and it is relative to the proton of the imine group (yellow circle in Figure 2). Finally, the singlet signal at 10.62 ppm is ascribed to the phenolic proton (blue circle in Figure 2).

The ¹³C NMR spectrum confirmed the structure of the molecule by the presence of nine different carbon signals (see Supplementary Materials). Moreover, the ESI-MS spectrum of 1 in methanol showed the presence of a signal at m/z 408.1, relative to [M+H]⁺ (see Supplementary Materials). The UV-vis spectrum of 1 in CH₃CN showed a band centred at 379 nm, with ε = 20,698, calculated using four solutions at four different concentrations (1 × 10⁻⁵, 2 × 10⁻⁵, 3 × 10⁻⁵ and 4 × 10⁻⁵ mol/L, Figure 3). Upon excitation at 390 nm, an emission band with a maximum at 561 nm can be observed (Figure 3).

Salen ligand 1 was employed for the formation of a Co(II) complex 2 (Scheme 3). The reaction was performed at room temperature under a nitrogen atmosphere in the presence of Co(II) acetate until the formation of a precipitate. The conversion of the starting reagent was monitored by TLC analysis. The precipitate was collected by vacuum filtration. Due to the paramagnetic nature of Co(II), it is not possible to carry out NMR experiments [4,24,25]. The desired complex was characterized by ESI-MS analysis, showing a signal at m/z 440.9, relative to [M]⁻ (see Supplementary Materials). Furthermore, FT-IR spectrum of 2 shows, in the region 3000–3500 cm⁻¹, a broader band in respect to the ligand 1, probably due to the absence of OH groups and the introduction of cobalt metal ion.

3. Materials and Methods

The NMR experiments were carried out at 27 °C on a Varian UNITY Inova 500 MHz spectrometer (¹H NMR at 499.88 MHz and ¹³C NMR at 125.7 MHz) equipped with a pulse-filed gradient module (Z axis) and a tunable 5 mm Varian inverse detection probe (ID-PFG). The ESI spectrum was acquired on an API 2000-ABSciex (positive and negative ion-mode for 1 and 2, respectively). A JASCO V-560 UV-vis spectrophotometer, equipped with a 1 cm path-length cell, was used for UV-vis measurements (a resolution of 0.1 nm). Luminescence measurements were carried out using a Cary Eclipse fluorescence spectrophotometer at room temperature. The emission was recorded at 90° with respect to the exciting line beam, using 5:5 slit widths for all measurements. All chemicals were of reagent grade and were used without further purification.

3.1. Synthesis of 1

0.720 g (4.64 × 10⁻³ mol) of 5-chloro-salicylaldehyde were transferred into a 250 mL flask and solubilized with 100 mL of absolute ethanol. Then, 0.250 g (2.32 × 10⁻³ mol) of 2,3-diaminomaleonitrile were added to the solution. The reaction was stirred at reflux for 19 h. Conversion was monitored by TLC analysis (silica gel: n-hexane/EtOAc 8:2), and the disappearance of the starting reagent was observed. The precipitate was collected by vacuum filtration on Millipore, and it was washed with absolute ethanol. The product was dried on air (yield 45%). ¹H NMR (500 MHz, DMSO-d₆): δ 10.62 (s, 2H, OH), 8.50 (s, 2H, CH=N), 8.20 (d, J = 2.4 Hz, 2H, Ar-H), 7.33 (dd, J = 3.0 Hz, J = 9.1 Hz, 2H, Ar-H), 6.94 (d, J = 8.5 Hz, 2H, Ar-H) ppm. ¹³C NMR (125 MHz, DMSO-d₆) δ 103.46, 114.31, 118.64, 123.67, 124.08, 127.23, 132.82, 150.35, 157.16 ppm. ESI-MS: m/z 408.1 [M+Na]⁺. FT-IR: ν(OH) 3410 cm⁻¹; ν(Ar-C-H) 3195 cm⁻¹; ν(C≡N) 2247 cm⁻¹; ν(C=N) 1637 cm⁻¹; ν(C=C) 1553 cm⁻¹. Anal. Calcd. For C₁₈H₁₀Cl₂N₄O₂: C, 56.13; H, 2.62; N, 14.55. Found: C, 56.09; H, 2.58; N, 14.53.

3.2. Synthesis of 2

A total of 0.150 g (3.9 × 10⁻⁴ mol) of 1 were transferred into a 25 mL flask and solubilized with 2 mL of absolute ethanol. The reaction was stirred at reflux under N2 atmosphere. Then, 0.107 g (4.3 × 10⁻⁴ mol) of Cobalt(II) acetate tetrahydrate were added to the solution. Once the precipitate was formed, the reaction was monitored by TLC analysis (silica gel: n-hexane/EtOAc 6:4). The precipitate was filtered under vacuum on Millipore and washed with 1 mL of absolute ethanol. The product was dried on air (yield 94%). ESI-MS: m/z 440.9 [M]⁻. FT-IR: ν(C≡N) 2247 cm⁻¹; ν(Ar-C-H) 3424 cm⁻¹; ν(C≡N) 2247 cm⁻¹; ν(C=N) 1637 cm⁻¹; ν(C=C) 1553 cm⁻¹; g (Co-O) 550 cm⁻¹; g (Co-N) 494 cm⁻¹. Anal. Calcd. For C₁₈H₈Cl₂CoN₄O₂: C, 48.90; H, 1.82; N, 12.67. Found: C, 48.85; H, 1.78; N, 12.62.

4. Conclusions

Two new maleonitrile salen compounds, the salen ligand and its relative Co(II) complex, having chlorine atoms in 5-5′ positions, have been synthetized and characterized. Further spectroscopic studies on the metal-salen complex 2 are in progress. These compounds can have applications in catalysis as well as sensing applications.