Nitromethylene N-heterocycles as water soluble corrosion inhibitors

Abstract

Using a green methodology, we performed a double vinylic substitution on 1,1-bis(methylsulfanyl)-2-nitroethylene, synthesizing six 2-nitromethylene heterocyclic compounds. These compounds were evaluated as corrosion inhibitors, and in silico predictions were made to assess their toxicity risks. The synthesized compounds are water soluble and exhibit strong corrosion inhibition activity, effectively protecting metal against mass loss. Furthermore, the in silico toxicity predictions indicate that these compounds do not present significant toxicity risks, positioning them as innovative, environmentally friendly alternatives for organic corrosion inhibitors.

Key words
Ketene dithioacetals; nitromethylene heterocycles; synthesis; corrosion inhibitors; water soluble; in silico predictions

INTRODUCTION

Corrosion is a natural phenomenon that leads to significant economic consequences. Steel structures are particularly vulnerable to severe damage caused by electrochemical reactions from environmental factors (Hansson 2011, Silva et al. 2021).

Organic corrosion inhibitors can be highly effective and offer economic benefits. However, most commercially available inhibitors are toxic, highlighting the need for safer, environmentally friendly alternatives (Umoren et al. 2018, Silva et al. 2021).

In many industries, hydrochloric or sulfuric acids are used at concentrations of up to 20% in pickling processes to remove scale, with organic inhibitors employed to prevent corrosion of carbon steel (Sastri 2010, Silvério et al. 2024).

The most effective inhibitors are heterocyclic organic compounds containing nitrogen, oxygen, or sulfur atoms. These compounds typically have conjugated π double or triple bonds that increase the electron density of the molecule, allowing them to donate electron pairs and facilitate adsorption onto the metal surface (Al-Moubaraki & Awaji 2020, Bidi et al. 2020).

N-heterocyclic compounds are cyclic structures that contain atoms other than carbon in their backbone. These compounds possess valuable properties for both life sciences and materials science. Ketene dithioacetals are important building blocks in organic synthesis, often utilized to create heterocyclic derivatives through a unique reaction step (Huang et al. 2020, Xu et al. 2019, Baliza et al. 2024).

The International Union of Pure and Applied Chemistry (IUPAC) defines Green Chemistry as the invention, design, and application of chemical products and processes that minimize or eliminate the use and production of harmful substances (IUPAC 2020). Sustainable chemistry involves the design, manufacture, and use of environmentally benign chemical products and processes that prevent pollution, reduce or eliminate hazardous waste, and mitigate risks to human health and the environment (OECD 1999). In 2017, IUPAC established the International Committee on Green Chemistry for Sustainable Development (ICGCSD) to promote advancements in the field of green and sustainable chemistry (IUPAC 2020). To produce environmentally friendly organic corrosion inhibitors, several strategies can be employed, such as the use of renewable solvents and reducing energy consumption by using microwaves to assist in the synthesis process.

In our research, we have been synthesizing heterocyclic compounds to investigate their physicochemical properties. Using a green methodology, we employed ethanol as the solvent and microwave irradiation to efficiently promote double vinylic substitution in 1,1-bis(methylsulfanyl)-2-nitroethylene, aiming to produce non-toxic 2-nitromethylene heterocyclic compounds (Sangi et al. 2019, Baliza et al. 2024). The synthesized compounds demonstrated the ability to inhibit the oxidation of SAE 1020 carbon steel, a material commonly used in mortar reinforcement and small machine parts. Despite its excellent surface properties, including high ductility, hardness, and wear resistance, SAE 1020 is highly susceptible to severe corrosion (Gomes et al. 2005).

MATERIAIS AND METHODS

Chemicals and equipments

All chemicals were purchased from commercial sources and used without further purification. 1,1-bis(methylsulfanyl)-2-nitroethylene, ethylenediamine, 1,3-diaminopropane, 3-aminopropanol, 2-aminoethanol, 1,3-aminopropan-2-ol, and 1,4-diaminobutane were obtained in pure form from Sigma-Aldrich. Ethanol (AR grade) was purchased from Dinâmica.

Reactions assisted by microwave irradiation were carried out using an Anton Paar Monowave 300 device. Thin-layer chromatography analyses were performed on commercial aluminum plates (0.2 mm layer of silica gel) from Macherey-Nagel, with visualizations under ultraviolet light (254 nm).

Melting points were determined using a Gehaka PF1500 Farma instrument. Infrared spectra were obtained using a Bruker FT-IR Vertex 70 spectrophotometer in Attenuated Total Reflectance (ATR) mode. Mass spectrometry was performed on a Shimadzu GCMS-QP2010 Plus. Nuclear magnetic resonance (NMR) analyses were conducted using a Bruker Avance DRX 400 MHz and a Varian VNMRS 500 MHz spectrometer.

General procedure for the synthesis of 2-nitromethylene heterocycles

In a microwave-compatible glass vessel, 1,1-bis(methylsulfanyl)-2-nitroethylene (1) (165 mg, 1 mmol) and the nucleophiles (1 mmol) were dissolved in ethanol (3 mL). The mixture was irradiated with microwaves for 30 minutes, maintaining a temperature of 110 °C with constant stirring.

At the end of the reaction, the solvent was evaporated, and the products described below were obtained in pure form after recrystallization. All compounds were recrystallized, except for 2-(nitromethylene)-1,3-oxazinane (4), which required purification by column chromatography.

The data from infrared spectroscopy, mass spectrometry, and nuclear magnetic resonance spectra of hydrogen (1H NMR) and carbon (13C NMR), used to identify the synthesized compounds, are as follows.

2-(nitromethylene)imidazolidine (2) (Figure S1-S4): Yield 98%. Melting point: 175-176 °C (decomposition). 1H NMR (400 MHz, MeOD): δ 6.57 (s, 1H); 3.75 (s, 4H). 13C NMR (100 MHz, MeOD): δ 162.53; 98.13; 44.63. IR (ATR) (v_max/cm-1): 3210, 3100, 1597, 1554, 1192, 762. MS (m/z, (%)): 129(37); 83(43); 54(100).

2-(nitromethylene)oxazolidine (3) (Figure S5-S8): Yield 95%. Melting point: 134-136 °C (decomposition). 1H NMR (400 MHz, DMSO-d6): δ 6.53 (s, 1H); 3.56-3.14 (m, 4H). 13C NMR (100 MHz, DMSO-d6): δ 156.75; 97.69; 59.92; 44.71. IR (ATR) (v_max/cm-1): 3266, 3146, 1621, 1583, 1211, 1054, 1009. MS (m/z, (%)): 130(0); 84(39); 56(92); 54(100).

2-(nitromethylene)-1,3-oxazinane (4) (Figure S9-S12): Yields 64%. 1H NMR (CDCl₃, 400 MHz) δ 2.16 - 2.22 (m, 2H), 3.59 (t, 2H, J 6.02 Hz), 4.40 (t, 2H, J 5.40 Hz), 6.51 (s, 1H); 13C NMR (CDCl₃, 100 MHz) δ 20.51, 37.50, 66.40, 101.07, 163.67.

2-(nitromethylene)hexahydropyrimidine (5) (Figure S13-S15): Yield 90%. 1H NMR (MeOD, 400 MHz) δ 1.97 (quint, 2H, J 5.83 Hz), 3.41 (t, 4H, J 5.83 Hz), 6.48 (s, 1H); 13C NMR (MeOD, 100 MHz) δ 154.40, 37.87, 18.86. GC-MS (70 eV) m/z (%): 143 (M+, 100), 116 (23), 91 (13), 64 (18).

2-(nitromethylene)hexahydropyrimidin-5-ol (6) (Figure S16-S20): Yield 76%. Melting point: 221-224 °C (decomposition). 1H NMR (DMSO-d6, 400 MHz): δ 8.90-8.74 (m, 2H); 6.28 (s, 1H); 5.31 (d, J = 2.5 Hz, 1H); 4.06-3.98 (m, 1H); 3.39-3.32 (m, 2H); 3.20-3.10 (m, 2H). 13C NMR (DMSO-d6, 100 MHz): δ 154.23; 98.47; 58.38; 44.57. IR (ATR) (v_max/cm-1): 3220, 3134, 1603, 1498, 1186, 723.

2-(nitromethylene)-1,3-diazepine (7) (Figure S21-S24): Yield 98%. 1H NMR (500 MHz, DMSO) δ 8.83 (s, 2H), 6.39 (s, 1H), 3.71- 3.52 - (m, 4H), 1.71 – 1.65 (m, 4H); 13C NMR (126 MHz, DMSO) δ 162.81, 100.49, 44.62, 27.42.

Physicochemical properties calculation

Since certain physicochemical properties can be correlated with the inhibition capacity of a compound, Density Functional Theory (DFT) was used to optimize the structures of compounds 2-7. The calculations were performed in vacuum at the B3LYP/6-311G++(d,p) level of theory using the Gaussian 09W program (Frisch et al. 2010).

The energy values of the Highest Occupied Molecular Orbital (E_HOMO) and the Lowest Unoccupied Molecular Orbital (E_LUMO) were obtained from the optimized geometries. These parameters are related to the ionization potential (I) and electron affinity (A) as described by equations 1 and 2 (Obot & Obi-Egbedi 2010).

$$I=-E_{HOMO}$$ (1)

$$A=-E_{LUMO}$$ (2)

Other parameters related to inhibition efficiency, such as hardness (η), softness (σ) and the fraction of electrons transferred (ΔN) can be calculated from the ionization energy and electron affinity, as shown in equations 3 to 5 (Quattrociocchi et al. 2020).

$$\eta =(I–A)/2$$ (3)

$$\sigma =1/\eta$$ (4)

$$\mathrm{\Delta }N=(I+A)/2(I–A)$$ (5)

Procedure of mass loss evaluation

Plates of SAE 1020 steel, composed of C (0.18 - 0.23%), Mn (0.3 - 0.6%), P (0.03%), and S (0.05%), with dimensions of 2.5 x 2.5 x 0.13 cm, were polished using different grades of emery paper (150, 220, 320, 400, and 600), followed by rinsing with deionized (Milli-Q) water.

A solution of HCl (1 mol L⁻¹) was prepared and used as a control, while solutions containing HCl (1 mol L⁻¹) and a defined concentration (2 mmol L⁻¹) of inhibitor were prepared using deionized (Milli-Q) water. The SAE steel plates were immersed in these electrolytes for 4 hours.

The gravimetric method was used to assess the corrosion resistance by weighing the steel plates before and after pickling in an aqueous HCl solution. The mass loss (w) was calculated as the difference between the average initial (m_i ) and final (m_f ) masses of each plate (equation 6). This data was then used to calculate the inhibition efficiency (IE) according to equation 7.

$$w=m_f-m_i$$ (6)

$$IE(\mathrm{\%})=[(w^0-w)/w^0]\times 100$$ (7)

where w₀ and w are the mass losses in the absence and in the presence of the inhibitor, respectively.

Adsorption Isotherms

Mass loss values allow the calculation of the fraction of molecules adsorbed on the metal surface (θ), as described by equation 8.

​$$\theta =(w^0-w)/w^0$$ (8)

The fraction of adsorbed molecules (θ) enables the study of the inhibitor adsorption process on the metal surface, using adsorption isotherm models such as the Langmuir model (equation 9) (Langmuir 1918).

$$(C/\theta )=(1/K_{ads})+C$$ (9)

where C is the concentration of the inhibitor, θ the degree of surface coverage or fraction of adsorbed molecules and Kadsis the adsorption constant.

The adsorption type can be determined by calculating the standard Gibbs energy of adsorption (ΔG°_ads, equation 10) (Mamudu et al. 2023)

$$\Delta G{°}_{ads}=-RT\ln (C_{water}{K}_{ads})$$ (10)

where R is the universal gas constant (8.314 J mol-1 K-1) and T is the absolute temperature and C _water is the molar water concentration of 55.5.

In silico toxicity evaluation

The toxicity of compounds 2-7 was estimated in silico using two models: OSIRIS Property Explorer (Sander 2001), which utilizes the Registry of Toxic Effects of Chemical Substances database (Actelion Pharmaceuticals Ltd., USA), and Toxtree, which employs a decision tree approach to estimate toxic hazards (Patlewicz et al. 2008). The Benigni/Bossa rules for mutagenicity and carcinogenicity were applied in the predictions (Benigni et al. 2013).

RESULTS AND DISCUSSION

The presence of the nitro group in 1,1-bis(methylsulfanyl)-2-nitroethylene (1), an electron-withdrawing group conjugated to the double bond, makes the carbon atoms in the methylsulfanyl groups electrophilic centers, enabling double vinylic substitution and the one-step synthesis of heterocycles. By exploiting this reactivity, we used ethylenediamine, 1,3-diaminopropane, 3-aminopropanol, 2-aminoethanol, 1,3-aminopropan-2-ol, and 1,4-diaminobutane as nucleophiles, resulting in the production of six 2-nitromethylene heterocyclic derivatives (2-7), as shown in Figure 1.

Figure 1
Synthesis of nitromethylene heterocycles (2-7) with respective yields.

Analysis of the 1H NMR spectra revealed that all heterocycles derived from 1,1-bis(methylsulfanyl)-2-nitroethylene (1) exhibited a signal in the range of 6.2-6.6 ppm, integrated to 1H, corresponding to the vinylic C-H bonded to the nitro group. Additionally, compounds 2-7 did not show signals around 2.7 ppm, which would be expected for the methylsulfanyl groups, confirming that these groups were eliminated

The calculated physicochemical properties for compounds 2-7 are presented in Table I.

The difference between the energies of the HOMO and LUMO orbitals, known as the Gap energy (ΔE), is an important indicator of molecular stability. A smaller ΔE suggests greater efficiency of the inhibitor, as the energy required to remove an electron from the last occupied orbital is lower (Obot & Obi-Egbedi 2010). This requirement is important because the adsorption of a heterocyclic compound onto a metal surface occurs via donor–acceptor interactions between the electrons of the inhibitor and the vacant d-orbitals of the metal atoms (Obot et al. 2015). The results (Table I) indicate that compound 7 is expected to have the best performance as a corrosion inhibitor, due to its significantly lower ΔE compared to the other compounds.

Table I
Physicochemical properties calculated for the studied compounds.

Inhibitor	EHOMO (eV)a	ELUMO (eV)b	ΔE (eV)c	ηd	σe	ΔNf
2	-6.50	-1.84	4.66	2.33	0.43	0.89
3	-6.79	-2.09	4.71	2.35	0.43	0.94
4	-6.43	-1.73	4.69	2.35	0.43	0.87
5	-6.12	-1.46	4.67	2.33	0.43	0.81
6	-6.24	-1.55	4.69	2.35	0.43	0.83
7	-6.50	-1.91	4.59	2.29	0.44	0.92

• ^aHighest occupied molecular orbital (E_HOMO); ^bLowest unoccupied molecular orbital (E_LUMO); ^cGap energy (ΔE); ^dHardness (η); ^eSoftness (σ); ^fFraction of electrons transferred (ΔN).

Metallic surfaces are considered ‘soft’ due to their extensive electron cloud. Therefore, ‘harder’ molecules are not typically effective inhibitors (Obot & Obi-Egbedi 2010). The analysis of these properties also suggests that compound 7 should act as a better inhibitor. The fraction of electrons transferred allows for the evaluation of electron flow from the molecule with the lowest electronegativity (the inhibitor) to the one with the highest electronegativity (the metallic surface). Thus, the greater the fraction of electrons transferred, the better the inhibition potential of the molecule (Silva et al. 2021). Compounds 3 and 7 exhibited a higher fraction of electrons transferred.

Compounds 2-7 exhibited solubility in water, which facilitated their evaluation as corrosion inhibitors. After a 4-hour exposure under conditions that promote metal oxidation, it was observed that all 2-nitromethylene heterocycles inhibited corrosion. The inhibition efficiency, based on mass loss tests, ranged from 24% to 87% at a 2 mM concentration, with 2-(nitromethylene)-1,3-diazepine (7) showing the highest inhibition efficiency (Figure 2). This result aligns with the calculated physicochemical properties (Table I) related to the inhibition capacity. Compound 3, which also displayed a high fraction of electrons transferred, showed the second-best inhibition efficiency.

Figure 2
Inhibition efficiency on loss mass tests to compounds 2-7 at 2 mM.

Thiourea, thiourea derivatives and N-heterocyclic compounds as pyrimidine and triazoles are examples of organic corrosion inhibitors widely known and commercially applied (Ozcan et al. 2004, Jiang et al. 2024). Comparing our results with some recently published works allow us to observe that 2-nitromethylene heterocycles 3, 6 and 7 are on par with some commercial corrosion inhibitors (Huong et al. 2019, Hrimla et al. 2021). However, using exactly the same protocol, we also did mass loss tests for thiourea (2 mmol L⁻¹), and the inhibition efficiency was only 66,4 ± 1.1%, demonstrating that 2-nitromethylene heterocycles are indeed promising corrosion inhibitors.

Since the heterocyclic compound 7 exhibited the best inhibition activity, we decided to evaluate its performance at different concentrations. We found that the inhibition activity changed very little with concentration, and even when using significantly less inhibitor, the efficiency remained almost the same (Figure 3).

Figure 3
Performance on loss mass tests to compound 7 varying its concentration.

The fraction of adsorbed molecules (θ) was used to study the adsorption of the inhibitor on the metal surface and to assess whether the Langmuir model is suitable for describing the process. The adsorption isotherm was obtained by fitting the experimental data to the model in equation 9 (Figure 4).

Figure 4
Langmuir isotherm for adsorption of inhibitor 7 on SAE 1020 steel at 25 °C.

The Langmuir model describes adsorption as a process occurring in a monolayer, making it finite and limited. Additionally, the model assumes that the adsorption sites are homogeneous and equivalent, with no lateral interactions between the adsorbate molecules. Each site can hold only one adsorbed molecule (Langmuir, 1918).

The experimental data were well fitted to the Langmuir adsorption isotherm model (Figure 4), with a regression coefficient very close to 1 (R² = 0.9997). This strong fit suggests the formation of a monolayer of inhibitor 7 on the surface of SAE 1020 steel.

The standard free energy of adsorption was calculated using equation 10. Based on the standard adsorption energy, the adsorption mechanism of the inhibitor on the metal can be identified. Values of ΔG°_ads > -20 kJ mol⁻¹ indicate physisorption, -20 kJ mol⁻¹ > ΔG°_ads > -40 kJ mol⁻¹ suggest a mixed mechanism (physisorption and chemisorption), and ΔG°_ads ≤ -40 kJ mol⁻¹ indicates chemisorption (Fu et al., 2024).

The value of ΔG°_ads, calculated using the K_ads obtained from the Langmuir model, was -33.323 kJ mol⁻¹. This negative value indicates that the adsorption process is spontaneous. Since the value falls between -20 kJ mol⁻¹ and -40 kJ mol⁻¹, the interaction type between inhibitor 7 and the SAE 1020 steel surface is likely a mixture of physisorption and chemisorption.

Regarding toxicity, the OSIRIS results indicate that compounds 2-7 do not present mutagenic or tumorigenic risks, nor do they show irritant or reproductive effects. The Toxtree evaluation returned negative results for both genotoxic and non-genotoxic carcinogenicity. Additionally, it indicated that all the studied compounds pose no alerts for Salmonella typhimurium mutagenicity.

The results, showing no toxicity risks and the maintenance of inhibition efficiency even at low concentrations, make compound 7 a promising candidate for an environmentally friendly corrosion inhibitor.

CONCLUSIONS

Using a green methodology for double vinylic substitution in 1,1-bis-(methylsulfanyl)-2-nitromethylene, we synthesized six 2-nitromethylene heterocyclic compounds with yields ranging from 64% to 95%.

Compounds 2-7 are water soluble and demonstrate corrosion inhibition efficiency in the range of 24–87% in mass loss tests using a 2 mM inhibitor concentration. Compound 7 exhibited the best inhibition efficiency, which aligns with the previously calculated physicochemical properties related to its inhibition capacity. The interaction type between this inhibitor and the SAE 1020 steel surface is likely a combination of physisorption and chemisorption. Its efficiency varied little (72–88%) across a broad concentration range (0.25–5 mM). This stable performance, coupled with the fact that all the studied compounds are predicted to pose no toxicity risks according to in silico predictions, and are water soluble and synthesized through a green methodology, positions compounds 2-7 as innovative candidates for potential organic, eco-friendly corrosion inhibitors.

Acknowledgements

The authors acknowledge the financial support of FAPERJ and CNPq. This research was funded by grants from the Fundação de Amparo à Pesquisa do Estado do Rio de Janeiro - FAPERJ (grants E-26/210.460/2024) and from Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (grant number 309695/2022-8).

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SUPPLEMENTARY MATERIAL

Figures S1-S24.

Publication Dates

• Publication in this collection
  09 June 2025
• Date of issue
  2025

History

• Received
  31 July 2024
• Accepted
  21 Jan 2025