Unveiling the Synthesis and Photophysical Properties of 2,2′-[Iminobis(2,1-ethanediyliminomethylene)]diphenol and Its Zinc Complexes ^†

Abstract

Background: In recent years, the uses of in situ reaction monitoring have expanded. ReactIR is one of the in situ reaction monitoring techniques used to understand chemical reactions. The reductive amination of Schiff base formed in situ, by treating salicylaldehyde and diethylene triamine using sodium borohydride in methanol, was completed within 28 min and monitored by ReactIR. Materials and Methods: Zinc complexes of diethylene triamine, Schiff base, and their reductive amination products were synthesized using 1:1, 1:2, and 1:3 mole ratios of ligand–zinc chloride at room temperature. Results and conclusion: The formation of the product was confirmed using IR spectra. All these complexes showed photoluminescence in the blue region.

1. Introduction

In modern science, chemists do not use traditional synthesis protocols for the design and synthesis of desired products. The cost-effective and quick synthesis of molecules with minimal impurities poses the biggest challenge. When the actual reaction takes place, the energy and temperature change, and whether or not the intermediate formed without changing the reaction conditions or in a given short time period, prove very difficult using traditional methods. In some cases, after the completion of the reaction, the impurity profile, or, in some cases, the desired product, undergoes degradation and forms new unexpected products. Therefore, various new methodologies and techniques have been developed to detect and analyze chemical reactions such as HPLC. This gives quantitative information regarding the reaction components at any time, but the only condition is that the reaction is sampled and then analyzed. The chemist must wait for sample analysis, which is time-consuming and, therefore, during the waiting period, there may be certain changes in the reaction composition.

In recent years, the uses of in situ reaction monitoring have expanded. ReactIR is one of the in situ reaction monitoring techniques used to understand chemical reactions. It takes the spectra of all chemical species present in the reaction mixture and translates them in a quantitative manner so that chemists may easily predict the participation of a catalyst or solvent in the reaction, changes in the concentration of reactants, new peaks for intermediate and products, etc. This helps in predicting the mechanism, pathway, and kinetic determination. It is also used to predict the exact stoichiometry of reactants and catalysts (if used), the reaction time, the external addition rate of the reactant or catalyst, the thermal and energetic properties, etc., of the reaction, which helps in designing a cost-effective reaction. The numbers of reactions are monitored and their reaction steps, mechanism, kinetics, and thermal properties are determined using ReactIR techniques; the catalytic double carbonylation of epoxides to succinic anhydride (Scheme 1) [1] and the palladium-catalyzed cross-coupling reaction of haloalkyne and terminal alkynes are also determined using a phosphine-olefin ligand (Scheme 2) [2].

Two methods are most commonly used for direct reductive amination, and these differ from reducing agents. The first method is catalytic hydrogenation with metals such as platinum, palladium, nickel, etc.; the catalyst [3,4] used for the reduction is economical and the reductive amination method is effective, particularly in large-scale reactions. But such reactions may give a mixture of products and low yields depending on the molar ratio and the structure of the reactant [5]. The second method utilizes hydride-reducing agents, particularly sodium borohydride or its derivatives, such as sodium cyanoborohydride (NaBH₃CN) or triacetoxyborohydride, for the reduction [6,7]. Sodium cyanoborohydride is successfully used because of its stability in relatively strong acidic conditions (~pH = 3) and higher stability in hydroxylic solvents such as methanol. It has different selectivities at different pH scales [8]. At pH = 3–4, it reduces ketones, but this effect slows at higher pH. At pH 6–8, the more basic imines are protonated preferentially and reduce faster than aldehydes and ketones [8]. The other hydride-reducing reagents reported for the reductive amination are boron-pyridine (BH₃-Py) [9], Ti(OiPr)₄/NaBH₃CN [9], borohydride exchange resin [9], Zn/AcOH [9], NaBH₄/Mg(ClO4)2 [9], Zn(BH₄)₂/ZnCl₂ [9d], etc. Some reports of electrochemical reductive amination have been also reported [10].

In most of the metal complexes or metal-doped organic compounds, metal ions with d⁰ to d¹⁰ configurations act as sensitizers for the organic phosphor. The electrons present in the incomplete filled d-orbitals of metal ions were significantly integrated with the π and π* orbitals of the organic ligand in the complexes. The singlet and triplet states of the organic ligands were very short-lived and, therefore, the quantum yields of triplet state formation are very low. Zinc complexes are important as light-emitting materials [11,12,13,14]; they also have tunable electronic properties [15,16], ternary complexes that act as electronic transporters, and electroluminescent materials [17].

2. Results and Discussion

2.1. Reaction Monitoring Study

The conversion of the reductive amination product from diethylene triamine (2) is a ‘two-step reaction’; however, the authors wished to reduce this into a single synthetic step or continuous steps without the need to isolate the product in order to save time. This conversion was carried out in methanol in the presence of a catalytic amount of acetic acid at room temperature. The ATR probe of the instrument was inserted into the reaction mixture which monitored the reaction without analyzing the sample separately.

After the addition of salicylaldehyde (1) into diethylene triamine (2), the initial rate of the reaction was very high; most of the aldehyde (1) and amine (2) was converted into Schiff base intermediate (3), which was confirmed by monitoring the reaction at an amine wavenumber (2) at 1233, salicylaldehyde at 828, and intermediate (3) at 1024, as shown in Figure 1 and Figure 2. The reaction was completed within 18 min and we then added a reducing agent. After the addition of NaBH₄, the initial rate of reduction was very slow, but it increased sharply after 4–5 min and the intermediate (3) was almost reduced within 10 min after the addition of borohydride. The formation and reduction of the intermediate (3) were also confirmed via the real-time monitoring of three-dimensional MIR spectral data collected with an in situ ATR probe at periodic intervals throughout the reaction. The absorption intensity at 1024 cm⁻¹ was initially visible after the addition of salicylaldehyde and started to disappear once the reduction started after the addition of borohydride.

Thermal Study

This study suggested that the formation of Schiff base (3) is an exothermic reaction; the temperature of the reaction initially increases sharply, after the addition of salicylaldehyde (1), to nearly 45 °C with a sharp decrease in the concentration of diethylenetriamine (2) and increase in the amount of Schiff base (3). The maximum content of the diethylenetriamine (2) reacted within 3 to 4 min and the reaction was nearly complete within 18 min. After the completion of the reaction, the in situ reduction of the imine bond was achieved using sodium borohydride at a slightly acidic pH (6 to 6.5; by adding acetic acid). After the addition of NaBH₄, the temperature of the reaction mixture increased by 4–5 °C with the initiation of the reaction. This was confirmed by the monitoring signal intensity at a different amine wavenumber at 1233, aldehyde (1) at 828, intermediate (3) at 1024, and a final reductive animation product (4) at 1259 cm⁻¹.

2.2. Synthesis of 2,2′-[Iminobis(2,1-ethanediyliminomethylene)]diphenol (4)

Dissolve 0.5 moles of salicylaldehyde (1) in 50 mL methanol, add 0.25 mole of diethylenetriamine (2), and slowly and constantly stir so that solution turns to a yellowish-brown and the temperature rises (Figure 3, Scheme 3). Add the catalytic amount of acetic acid and stir the solution at room temperature for 20 min so that it becomes yellow. Confirm the formation of the Schiff base (3) by monitoring TLC. Add cold distilled water slowly into the reaction mixture so that it turns turbid and an orange-yellow color and a viscous liquid starts to separate. Continue the stirring, adding 0.30 mole of anhydrous sodium borohydride in a batch-wise manner. After the complete addition, stir the reaction mixture for 15 min, pour into an ice-cold saturated sodium bicarbonate solution, stir, and filter the 2,2′-[iminobis(2,1-ethanediyliminomethylene)]diphenol (4) and the recrystallized product from the aqueous alcohol (Figure 4).

Colour–Faint Yellow solid; Yield–91%; M.P. 122–124 °C.

FT-IR (cm⁻¹): 3286, 3044, 2846, 2780, 1632, 1611, 1587, 1485, 1472, 1377, 1335, 1263, 1244, 1197, 1147, 1101, 1032, 974, 950, 779, 749.

NMR (CDCl₃, 61 MHz): 8.41 (bs, 2H, 2OH); 7.69–6.52 (m, 8H, Ar-H); 4.18–2.51 (m, 12H, -CH₂-N).

CMR (CDCl₃, 60 MHz): 50.11, 77.06, 116.24, 117.70, 118.68, 119.36, 131, 132.42.

The broad stretching vibrations of polar –NH and –OH show a broad peak in the range 3300–3000 cm⁻¹. The aromatic and aliphatic –C-H stretching vibrations are in the range of 2950–2780 cm⁻¹. The new amine peak was observed at 1463 and 1244 cm⁻¹, confirming the reduction of the imine bond.

2.3. Synthesis of Metal Complexes (5a-c) of Diethylene Triamine (2)

Dissolve 0.01 mole of diethylenetriamine (2) in absolute alcohol and add a stoichiometrical amount (0.01, 0.02, and 0.03 moles) of zinc (II) chloride while stirring. Stir the reaction mixture for 5–6 h at room temperature. Filter the solid Zn complex (5a-c) (Scheme 4) and wash with cold absolute alcohol. The stoichiometric complex (1:3 ratio) was not separated as a stable solid that absorbs moisture and liquefies due to the formation of hydrochloride or to the presence of excess unreacted zinc chloride. Add the gel or viscous solid into the saturated NaHCO₃ and stir for a few minutes until the solid becomes free and then filter. Wash the solid with cold ethanol.

2.4. Synthesis of Zn-Complex (6a-b) of Schiff Base (3)

Dissolve 0.5 moles of salicylaldehyde (1) in 50 mL ethanol and add 0.25 mmol of diethylenetriamine (2), stirring slowly and constantly so that solution turns to a yellowish brown with the rise in temperature. Add the catalytic amount of acetic acid and continue the stirring at room temperature for 20 min so that it becomes yellow. Add 0.25/0.50 mmole of zinc (II) chloride in a single lot while stirring. Initially, the color of the solution becomes dark and, after some time, the solid and gel (6a-b) become separated. Dissolve the gel/viscous solid in saturated NaHCO₃ and stir for a few minutes. Filter the solid complex (6a-b) and wash with cold ethanol (Scheme 5).

2.5. The Metal Complex of Amine and Its Salts

Dissolve 0.25 mmol of amine (4) and 0.25/0.50 mmol of zinc (II) chloride in ethanol and stir for 5–6 h. The complexes become separated into a gel and viscous form. Dissolve the gel/viscous complex in saturated NaHCO₃ and stir for a few minutes. Filter the solid complex (7a-b) and wash with cold ethanol (Scheme 6).

Sodium salt complexes were synthesized by repeating the same protocol in the presence of 0.50 mmol of sodium hydroxide. First, prepare the sodium salt of amine (8) and then add a stoichiometric amount of zinc chloride and stir for 3 h. Filter the complex (7a-b) and wash with water.

The FT-IR spectra were measured using the Bruker Tensor II model with platinum ATR, while the fluorescence study was carried out using a Spectrofluorometer (model number RF5301). The conductance of all metal complexes was measured using a digital conductivity meter with a cell constant of 1 cm⁻¹.

The FTIR spectra of all complexes were recorded using the ATR technique, 5c (1:3 mole ratio), and 7b (1:2 mole ratio; in both cases) showing a broad signal of nearly 3500 cm⁻¹ and indicating the presence of water, which was maybe part of the complexion or crystallization. Other complexes do not contain any water ligand or crystallization water.

The ionic character of the Zn-complex was determined by measuring the conductance in the DMSO. The chlorine ion, as part of the complexion (co-ordination sphere) or as associated with the ion complex, may be predicted from the ionic conductance of the complex. The 9a complex synthesized from Na-salt of (4), with a 1:1 mole ratio of the ligand and ZnCl₂, has the least conductance, indicating that it was the least ionic. All the complexes formed using a 1:1 mole ratio of ligand and ZnCl₂ had higher conductance than those synthesized using a 1:2 mole ratio. The conductance of the 9a complex is less compared to those synthesized using amine, indicating that the –O⁻ and –OH of the ligands are part of the coordination sphere. The 9a complex does not contain water in the coordination sphere, which is confirmed via IR spectra.

2.6. Photophysical Properties of Zn(II) Complexes

The photophysical properties of Zn(II) complexes of diethylenetriamine (2), the Schiff base (3), and the reductively aminated product (4) have been recorded using a Spectrofluorometer (model number RF5301). A Xe laser lamp was used for the excitation and emission spectra; samples were scanned in the range of 220 nm to 800 nm. For the fluorescence study of the metal complexes, dimethylformamide is used as a solvent and reference material. The absorption of light was due to the electronic excitations from bonding (π, n) to non-bonding orbitals (π*).

• Study Parameters:
  Solution concentration: 4–5 mg per 10 mL;
  Emission wavelength (for excitation): 400 nm;
  Slit width: 3–5 nm;
  Scan speed: Super;
  Sensitivity: High.
• Effect of Solvent on Fluorescence Properties of 6a:

The absorption, excitation, and emission of the compounds depended on the substitution pattern, solvent, temperature, pH, etc. The electronic spectra were measured in solvents of different polarities and in terms of the hydrogen bond donor/acceptor ability; it was found that the position, shape and intensity of the emission spectra were usually affected by solvents. This phenomenon was referred to as solvatochromism [18]. These changes are due to solute–solvent intermolecular forces such as specific electrostatic interactions (hydrogen bonding or electron donor/acceptor interaction) and or non-specific electrostatic interactions [18,19]. The effect of the solvent was studied by measuring the excitation and emission spectra of the compound in non-polar, polar aprotic, and polar solvents. The excitation maxima, emission maxima, and quantum efficiency are shown in the above table.

The excitation and emission spectra of 6a were recorded in different solvents in the wavelength range of 220–800 nm. The excitation spectra were due to electronic transitions from HOMO → LUMO or due to electronic transitions such as π → π* (strong) or n → π* (weak). For all solvents (except DMF), the excitation of the electrons takes place in the same range and emission shows a blue shift for both excitation wavelengths, except for polar solvents such as ethanol and methanol, at 400 nm. The stoke shift was unusually low for 400 nm and the excitation wavelength indicates that both S₀ and S₁ states do not have intra molecular charge transfer characteristics, as with other excitation wavelengths (near around 360 nm). The second excitation maxima were blue-shifted for all other solvents in comparison to benzene, indicating that the molecule was polar and showing specific electrostatic interactions, such as H-bonding or dipole-dipole interactions, leading to the net stabilization of the ground state of the 6a and to a hypsochromic shift in the spectrum. The excitation second maxima of 6a shows a longer shift in ethanol and methanol because of its higher ability to form H-bonding and solubility; water does not show a marginally longer shift because of the lessened solubility of the 6a (Figure 5 and Figure 6). The 6a shows a narrow band shift for the excitation at 400 nm but this was higher for excitation at nearly 360 nm. This observation indicated that triplet state stabilization was more for the excitation at, neatly, 360 nm than at 400 nm. The stoke shift was highest in DMF, which may be due to the charge transfer transitions.

C.

Photophysical properties of Zn(II) complexes (5a-c) (SI: Figure 7 and Figure 8):

D.

Photophysical properties of Zn(II) complexes (6a-b) (SI: Figure 9 and Figure 10):

E.

Photophysical properties of Zn(II) complexes (7a-b) (SI: Figure 11 and Figure 12):

F.

Photophysical properties of Zn(II)–SATAA complexes: (SI: Figure 13 and Figure 14):

The luminescence properties of Zn(II) complexes originate from organic ligands rather than from LMCT because the d-shell of the central ion is filled [19]. A survey of the literature suggests that zinc complexes of salicylaldehyde Schiff bases and their derivatives emit light in the blue, green, and red regions. All Zn(II) complexes were excited at 220–800 nm to determine the excitation wavelengths of electrons; higher excitation wavelengths were used to determine the emission. All the excitation and emission spectra of all the synthesized complexes with various combinations of ligand and zinc chloride were recorded and are shown in the above figures. The complexes showed good laminating properties in solid as well as in solution states. Since there was no d–d transition in the zinc complexes, the emission of light was due to the relaxation from a higher energy level to lower energy level via intra-ligand transitions. All complexes, except those synthesized from the sodium salt of amine, showed emission in the visible region; this may be due to the changing of the donor atom from OH to O⁻. The PL properties were changed by changing the mole ratio of the ligand and metal as well as the nature of the donating atom. A stronger and low-energy emission was observed for the 6a-b complexes. All the complexes showed redshift with an increase in the emission intensity.

G.

Combined study of photophysical properties of (4) and Zn(II) complexes (Figure 15, Figure 16, Figure 17 and Figure 18):

The quantum efficiency of the metal complexes was broadly calculated using the following equation. The result was more than one in all complexes except 5b and 9a-b.

Amine (4) showed strong excitation at 400 nm but weak emission (401 nm) at 400 nm in comparison to the excitation at 359 nm. The Zn complexes (7a-b) showed strong excitation at 313 nm. The quantum efficiency of complex 5b, formed from diethylene triamine (2) at 401 nm (light of excitation), was the least compared to other complexes. Also, the quantum efficiency of 9a-b was less than 100% compared to other complexes. It was highest for complexes (6a-b) formed from the Schiff base (3). The nature of the complex ions of 7a-b and 9a-b was different due to their synthesis under different reaction conditions (neutral and alkaline), which were confirmed via both conductance measurement and emission spectra.

3. Conclusions

A Schiff base (3) of diethylene triamine (2) and salicylaldehyde (1) was synthesized at room temperature in methanol under slightly acidic conditions and the reaction was monitored. The results suggest that the reaction was completed within 18 min and that it is an exothermic reaction. The Schiff base (3) was reduced in situ using sodium borohydride at room temperature, which subsequently completed within 8–10 min. It was also an exothermic reaction. These results indicate that there was no need for heating or refluxing the reaction mixture, as was reported in the literature. A series of zinc–metal ion complexes were prepared from diethylene triamine (2), Schiff base (3), reduced amine (4), and its sodium salt (8) at room temperature in methanol/ethanol. The conductance of the solutions of all complexes in DMSO (50 mg per 100 mL) was measured and was more than zero, indicating that chlorine was not a part of the coordination sphere or ion. These are ionic complexes. All the synthesized complexes emitted blue light, expect 9a-b synthesized from 8.

Zinc complexes of diethylene triamine (2), Schiff base (3), and reduced amine (4) have received particular attention due to their effective optical properties. These complexes display an efficient emission of blue light under a UV light (310–313 nm) source. These complexes emit blue light luminescence (405–450 nm) that could be efficiently used for the generation of white light in various optoelectronic applications. The conductance data suggested that 9a-b complexes have the least conductance due to their lessened ionic character.