INTRODUCTION:

The synthetic adaptability, varied stereochemistry, and extensive coordination versatility of main group IV elements, notably silicon, in complexes with nitrogen, oxygen, and sulfur donor ligands have generated significant interest. Silicon (IV) ions has garnered significance over the past decades, primarily due to their diverse coordination behaviors and wide-ranging applications in biological activities, including antibacterial, antifungal, anticarcinogenic, antifertility, nematicidal, tuberculostatic, acaricidal, insecticidal, anticancer, and pharmacological activities.^1-6

The chemistry of Schiff bases upon complexation with Additionally, Silicon and its complexes play crucial roles across various sectors, including enhancing strength, elasticity, and water impermeability. They contribute to the growth of epithelial and connective tissues, hair, and bone, and find applications in biological engineering, as well as in industrial, agricultural, and food production contexts.^7,8Amino acid-substituted Schiff base metal complexes have garnered significant attention due to their ease of synthesis, stability, versatility, flexibility, rapid reaction kinetics, moderate electron-donating capabilities, chelating properties, and extensive applications across diverse fields. Keeping this in view we have synthesized ethyl silicon (IV) Schiff base complexes and investigated their spectral (IR, ¹H-NMR, ¹³C-NMR, and ²⁹Si-NMR) analysis, antibacterial activity, and pharmacological activities.

MATERIALS AND METHODS:

All chemicals used in the present study were sourced from Sigma Aldrich. Elemental analyses were conducted using a Thermo Scientific (FLASH 2000) CHN elemental analyzer at SAIF Punjab University, while silicon was determined gravimetrically as SiO₂. Infrared spectra were recorded using an RZX (Perkin Elmer) spectrophotometer, Model SAIF Punjab University, covering the wave number range from 4000 to 400 cm^-1. A Bruker Advance neo 500 MHz NMR spectrometer at SAIF Punjab University was employed to acquire the multinuclear (¹H, ¹³C, and ²⁹Si NMR) spectra. Chemical shifts (δ) were referenced to TMS and reported in parts per million (ppm). A UV-visible spectrophotometer was employed to measure absorbance for assessing pharmacological activity at GBPUAT, Pantnagar, Uttarakhand.

Synthesis of Schiff base (LH₂) (1):

Schiff base was synthesized through the reaction of 2-hydroxy-1-naphthaldehyde (2.24g, 13mmol) with a hot aqueous solution (25 ml) of alanine (1.16g, 13mmol) dissolved in ethanol (50ml) (Scheme 1).⁹ After the completion of the addition, the solution was refluxed for 3 to 4 hours in a round bottom flask. After standing overnight, the polycrystalline precipitate was produced. Following multiple washes in aqueous ethanol, the substance underwent vacuum drying to attain purification.

Scheme 1. Synthesis of Schiff base

Synthesis of ethyl silicon (IV) Schiff base complexes (1a, 1b, 1c):

For the preparation of complexes, a calculated amount of the triethoxy(ethyl)silane and Schiff base (1) were dissolved separately in 30ml of benzene and mixed with constant stirring.¹⁰ The mixture was stirred magnetically for 20-22hours using a CaCl₂ guard tube. Excess solvent was removed under distillation, and the compound was finally dried. The crystalline solids were separated and purified by re-crystallization from benzene.

Antibacterial activity:

The antibacterial activity of the samples was tested against the selected bacterial strains using the agar well diffusion method.^11-1⁶ The bacteria strains used were B. subtilis, S. abony, S. aureus and E. coli. Using stock solutions of (10mg/ml) four other concentrations (5mg/ml, 2.5mg/ml, 1mg/ml, and 0.5mg/ml) of the samples were prepared by dissolving them in DMSO followed by through shaking in a vortex shaker. Each bacterium was grown in nutrient broth for 24hours at 37±2⁰C. After 24hours B. subtilis, S. abony, S. aureus were evenly spread with the help of sterilized cotton swabs on Nutrient Agar plates while for E. coli, MacConkey Agar plates were used for better visualization of zones. Using a cork borer, wells were created in each plate, and samples of various concentrations were added to each well. The plates were then incubated for another 24hours at 37±2⁰C. Streptomycin was used as standard while DMSO was used as the negative control. The complete testing was performed under aseptic conditions. The experiments were performed in triplicates. The zone of inhibition was determined using the formula provided below:

Zone of Inhibition = (Diameter of the clear zone) - (Diameter of the well)

Percentage Inhibition was calculated using the formula mentioned below:

Percentage Inhibition = [Zone of Inhibition of sample / Zone of Inhibition by Standard] × 100

Pharmacological activities:

Formula to determine the % inhibition of pharmacological activities:

Percentage Inhibition (IC₅₀) = [1- (sample/control)] × 100

Antioxidant activity:

To assess the antioxidant properties of Schiff base and its ethyl silicon (IV) complexes, three in-vitro assays were used: DPPH, H₂O₂radical scavenging, and metal chelating activity.

DPPH radical scavenging activity:

In DPPH radical scavenging activity, various amounts (5μg/ml - 25μg/ml) of the Schiff base and ethyl silicon (IV) complexes were mixed into five milliliters of MeOH solution containing four milligrams of DPPH. BHT was utilized as an antioxidant standard at equivalent concentrations to those of the samples. A UV-visible spectrophotometer was utilized to measure the absorbance in triplicate at 517nm, after incubation for 30 minutes in darkness at room temperature.^17-21

H₂O₂ radical scavenging activity:

In H₂O₂ radical scavenging activity, 0.6ml of 40mM H₂O₂dissolved in phosphate buffered saline (PBS) (pH 7.4) was combined with 0.4ml of an 80% MeOH solution, which included samples or BHT at varying concentrations (5μg/ml-25μg/ml). At room temperature, the solution was incubated for ten minutes. BHT (5μg/ml-25μg/ml) was used as standard in this study.²²

Metal chelating activity:

In metal chelating activity, the absorbance was measured at 560nm. For this, 0.1ml of 2mM FeCl₂•4H₂O, 0.2ml of 5mM ferrozine, and 4.7ml of MeOH were mixed into samples (5μg/ml-25μg/ml), followed by incubation for ten minutes at ambient temperature. After mixing the solution thoroughly and subsequently incubating it for thirty minutes at ambient temperature, the absorbance at 562nm was measured using a UV-visible spectrophotometer. As a standard, ascorbic acid was employed.^23,24

In-vitro protein denaturation activity (Anti-inflammatory activity):

The experimental setup comprised samples at different concentrations (5μg/ml-25μg/ml) with 200μl of fresh albumin protein. Subsequently, 2.8ml of PBS (pH 6.4) was mixed to the solution mixture to reach a total quantity of 5ml. The resulting mixture was heated for five minutes up to 70 degrees Celsius after being incubated for fifteen minutes at 37 degrees Celsius. A UV-visible spectrophotometer was used to detect the absorbance at 660nm after the reaction mixture had cooled. Diclofenac sodium was used as standard.²⁵

In vitro α-amylase inhibition activity (Anti-diabetic activity):

The standard medication acarbose and varying amounts of the samples (5μg/ml–25μg/ml) were introduced into a mixture containing 200μl of α-amylase solution, along with 100μl of 2mM PBS (pH 6.9). Followed by incubation for twenty minutes.²⁶ Afterward, 100μl of a 1% starch solution was introduced into the sample solution. Following five-minute incubation at 37°C, 500μl of 3,5-Dinitrosalicylic acid reagent was mixed into the mixture, which was then subjected to boiling in a water bath for five minutes. The absorbance was measured at 540nm.

Statistical data analysis:

The analysis was performed three times and the mean of triplicate values was calculated using MS-Excel. All the results were presented as mean±standard deviation (SD).

RESULT DISCUSSION:

Reactions of triethoxy(ethyl)silane with Schiff base were conducted in 1:1, 1:2, and 1:3 molar ratios in benzene. The reaction resulted in the release of ethanol due to acidic hydrogen (s) from Schiff base (1) and the ethoxy group(s) from ethyl silicon alkoxides, yielding (1a, 1b, 1c) (Figure 1). All the newly synthesized ethyl silicon (IV) Schiff base complexes are colored solids, soluble in most of the common organic solvents. The results of elemental analysis data were tabulated in Table 1.

(1a)

(1b)

(1c)

Figure 1: Proposed structures of ethyl silicon (IV) Schiff base complexes (1a-1c)

Table 1: Elemental analysis, color, and melting points of Schiff base and its ethyl silicon (IV) complexes

Schiff base and complexes	Color	M.P. (⁰C)	Calculated (found)%				Mol. Wt.gmol^-1
Schiff base and complexes	Color	M.P. (⁰C)	C	H	N	Si	Mol. Wt.gmol^-1
C₁₄H₁₃NO₃ (1)	Ethnic brown	82	69.12 (68.82)	5.39 (5.01)	5.76 (5.15)	-	243.255
C₁₈H₂₁NO₄Si (1a)	Buttercup- N	85	62.95 (62.51)	6.16 (6.01)	4.08 (3.91)	8.18 (8.01)	343.449
C₃₂H₃₄N₂O₇Si (1b)	Bush land	89	65.51 (65.24)	5.84 (5.71)	4.77 (4.55)	4.79 (4.35)	586.708
C₄₄H₄₁N₃O₉Si (1c)	Peanut Butter	95	67.42 (67.11)	5.27 (5.03)	5.36 (5.11)	3.58 (3.41)	783.898

Table 2: ¹H-NMR spectra of Schiff base and its ethyl silicon (IV) complexes

Schiff base and complexes	-HC=N	-CH₃	- C₂H₅Si	Phenoic –OH	Aromatic proton
C₁₄H₁₃NO₃ (1)	10.82 (s)	1.62 (d)	-	12.17	7.19-8.86 (d,dd,m)
C₁₈H₂₁NO₄Si (1a)	10.84 (s)	1.63 (d)	0.86 (d,t)	-	7.20-8.84 (d,dd,m)
C₃₂H₃₄N₂O₇Si (1b)	10.86 (s)	1.65 (d)	0.86 (d,t)	12.10	7.21-8.85 (d,dd,m)
C₄₄H₄₁N₃O₉Si (1c)	10.87 (s)	1.67 (d)	0.86 (d,t)	12.13	7.25-8.94 (d,dd,m)

¹H-NMR spectra:

In the case of Schiff base, the signal assigned to phenolic –OH proton is observed at a δ value of 12.17ppm, which disappears in the spectra of the silicon complexes 1a.²⁷A sharp singlet proton signal of the -CH=N group is observed at δ10.82ppm, which appears at δ10.84-10.87ppm in the spectra of complexes due to the coordination of the azomethine nitrogen to the metal atom.²⁸ The results of ¹H-NMR spectral data were tabulated in Table 2.

¹³C-NMR spectra:

The signals due to the carbon atom attached to the carboxylic acid group in the Schiff base appear at δ192.93ppm.²⁹ A signal of the azomethine group appears at δ163.88ppm. However, these occur at δ163.85±0.2ppm in the spectra of ethyl silicon (IV) Schiff base complexes.^30,31 The significant shift in the resonance of the carbon atom bonded to nitrogen suggests that the azomethine nitrogen has participated in coordination.³² The results of ¹³C-NMR spectral data were tabulated in Table 3.

Infrared spectra:

In the Schiff base, the peak observed at 1592.32 cm^-1, attributed to the υ C=N,³³ is found to be shifted to higher wave numbers in the complexes, suggesting coordination of the azomethine nitrogen with the silicon ion.³⁴ The observed broadband at 3430.84cm^-1 and 3369.74cm^-1 was attributed to the υ(OH)³⁵and υCOOH groups.³⁶ These stretching frequencies support the formation of the Schiff base. The complexes show two sharp bands observed at 1622.24-1637.04cm^-1 which are assigned to the υ_asy(COO) and 1311.63-1382.06cm^-1, which are assigned to the υ_sym(COO) stretching modes.³⁷ The amino acid carboxyl group coordinates in a monodentate manner, as indicated by the variation in frequency (Δv) between the two types of stretching modes. The results of IR spectral data were tabulated in Table 4.

²⁹Si-NMR spectra:

To confirm the geometry of the complexes, ²⁹Si-NMR spectra of the complexes was recorded (Figure. 2). The value of 𝛿 ²⁹Si in the spectra reflects the coordination number of the nucleus in the corresponding silicon complexes.¹ In general, ²⁹Si chemical shift moves to lower frequency with increasing coordination number of the nuclei. The ²⁹Si-NMR spectra showed one sharp singlet in each complex indicating the formation of a single species. The type of R group that is bonded to the silicon atom determines the value of the chemical shift in the ²⁹Si NMR spectra; an alkyl group has a greater value.³⁸ When R= ethyl, spectra revealed a single sharp singlet at δ -95.58ppm, δ -108.25ppm, and δ -123.85ppm, respectively, which amply illustrates the penta, hexa, and hepta-coordinated environments around the silicon atom (Figure 2).^{39, 40}

Table 3: ¹³C-NMR spectra of Schiff base and its ethyl silicon (IV) complexes

Schiff base and complexes	-HC=N	-COOH	-C=O	-COO^-	-C-OH	Aromatic carbon
C₁₄H₁₃NO₃ (1)	163.88	192.93	-	-	159.96	112.24-138.27
C₁₈H₂₁NO₄Si (1a)	163.84	-	174.25	171.58	-	112.25-138.23
C₃₂H₃₄N₂O₇Si (1b)	163.85	-	174.29	171.59	159.93	112.24-138.29
C₄₄H₄₁N₃O₉Si (1c)	163.87	-	174.28	171.57	159.95	112.24-138.25

Table 4: IR spectra a (4000–400 cm^-l) of Schiff base and its ethyl silicon (IV) complexes

Schiff base and complexes	υ (OH)	υ (C=N)	υ C=O	υ_asy (COO)	υ_sym (COO)	υ_asy (Si-O)	υ_sym (Si-O)	υ Ar-CH	υ (Si←N)
C₁₄H₁₃NO₃ (1)	3430.84	1592.32	1723.25	1622.24	1382.06	-	-	3094.09	-
C₁₈H₂₁NO₄Si (1a)	-	1618.01	1728.30	1637.04	1311.63	862.82	621.13	3077.33	477.98
C₃₂H₃₄N₂O₇Si (1b)	3431.94	1618.11	1727.11	1635.14	1362.11	864.33	625.14	3084.35	478.54
C₄₄H₄₁N₃O₉Si (1c)	3432.45	1618.34	1723.25	1635.57	1374.35	863.54	628.37	3093.51	479.49

Antibacterial activity:

Based on the findings, it can be said that each sample prevented the growth of the four bacteria that were chosen. It was discovered that the zone of inhibition was dose-dependent in each sample, meaning that it grew as the concentration rose. The bacteria B. subtilis, 1c, exhibited a maximum zone of inhibition of 24mm and a percent inhibition of 80% at a dosage of 10mg/ml. In contrast, 1b and 1a displayed comparable results, with a zone of 21mm and 20mm, and a percent inhibition of 70% and 66%. When compared to other bacteria, E. coli samples displayed the least amount of zone of inhibition at 10mg/ml, but the % inhibition was at its highest. 1c showed a zone of inhibition of 11mm and percent inhibition of 91.67%, while 1a and 1b were found to be 83% and 75%. For S. aureus, the maximum zone of inhibition was found in 1b and 1c at 10mg/ml with a percent inhibition of 69.69%, while for 1a, it was found to be 60%. In the case of S. abony, the maximum inhibition was found by 1c followed by 1b and 1a showing percent inhibition of 50%, 48%, and 46% respectively 10mg/ml (table 5).

Pharmacological activities:

Antioxidant activity: The antioxidant activity of Schiff base and its ethyl silicon (IV) complexes, using three assays: DPPH radical scavenging activity, H₂O₂ radical scavenging activity, and metal chelating activity. The results were displayed as mean±SD of three experimental data.

DPPH radical scavenging activity: The DPPH scavenging activity of Schiff base and its ethyl silicon (IV) complexes was assessed against the standard antioxidant, BHT, which has an IC₅₀ value of 8.56±0.02μg/ml. Samples 1c and 1b exhibited strong antioxidant activity closest to the BHT. The IC₅₀ values for the various samples were arranged in the following sequence: BHT (8.56±0.02)>(1c) (15.19±0.02)>(1b) (15.23±0.005) > (1a) (15.53±0.005) > (1) (19.87±0.01) (Table 6).

H₂O₂ radical scavenging activity:

The results of the activity indicated that the ethyl silicon (IV) Schiff base complexes from 1b and 1c exhibited a less H₂O₂ scavenging compared to BHT. The IC₅₀ values for the various samples were arranged in the following sequence: BHT (10.46±0.02)>(1c) (17.21±0.05)>(1b) (17.24±0.05)>(1a) (17.26±0.08) > (1) (23.57±0.01) (Table 6).

Metal chelating activity:

As the concentrations of the Schiff base and its ethyl silicon (IV) complexes increased, so did their chelation activity. The results of the activity indicated that the ethyl silicon (IV) Schiff base complexes from 1b and 1c exhibited a less metal chelating effect compared to ascorbic acid. The IC₅₀ values for the various samples were arranged in the following sequence: ascorbic acid (15.64±0.02) > (1c) (17.18±0.01) > (1b) (17.21±0.01) > (1a) (17.23±0.01) > (1) (24.07±0.05) (Table 6).

In-vitro protein denaturation activity (Anti-inflammatory activity):

In comparison to other complexes, 1c exhibited higher potency. However, against diclofenac sodium (IC₅₀=7.26±0.04μg/ml), Schiff base and all ethyl silicon (IV) complexes showed lesser activity. The sequence of samples in inhibiting protein denaturation, accompanied by their IC50 values, was as follows: diclofenac sodium (7.26±0.04) > (1c) (12.20±0.01) > (1b) (12.43±0.02) > (1a) (12.58±0.01) > (1) (16.76±0.02) (Table 6).

In vitro α-amylase inhibition activity (Anti-diabetic activity): The sample demonstrated notable α-amylase inhibitory activity, though to a lower degree compared to the acarbose. Results obtained from performing the activity revealed that Schiff base (1) exhibited less anti-diabetic activity. Ethyl silicon (IV) Schiff base complexes 1a, 1b, and 1c exhibited more inhibition, closer to the reference acarbose. The IC₅₀ values for the various samples were arranged in the following sequence: acarbose (7.17±0.02) > (1c) (11.23±0.01) > (1b) (11.62±0.02)>(1a) (11.82±0.01)>(1) (15.68±0.01) (Table 6).

The conclusion of the pharmacological activities is that the IC₅₀ values of ethyl silicon (IV) Schiff base complexes (1a-1c) are nearest to the standard value of IC₅₀ so they exhibited good antioxidant, anti-inflammatory, and anti-diabetic activites as compared to Schiff base (1). The pharmacological activity increases with alkyl groups as well as the coordinating environment of silicon.^41-43

Table 5: Antibacterial activity of ethyl silicon (IV) Schiff base complexes

Complexes	zone of inhibition at 10mg/ml
Complexes	*E. coli* (10mg/ml / %)	*B. subtilis* (10mg/ml / %)	*S. abony* (10mg/ml / %)	*S. aureus* (10mg/ml / %)
C₁₈H₂₁NO₄Si (1a)	10mm (83%)	20mm (66%)	12mm (46%)	20mm (60%)
C₃₂H₃₄N₂O₇Si (1b)	9mm (75%)	21mm (70%)	12mm (48%)	23mm (69.69%)
C₄₄H₄₁N₃O₉Si (1c)	11mm (91.67%)	24mm (80%)	12.5mm (50%)	23mm (69.69%)

Table 6: Pharmacological activities of Schiff base and its ethyl silicon (IV) complexes in terms of IC₅₀(μg/ml±SD)

Schiff base, complexes, and standard	Antioxidant			Anti-inflammatory	Anti-diabetic
Schiff base, complexes, and standard	DPPH radical	H₂O₂radical	Metal chelating	Protein denauration	α-amylase inhibitory
C₁₄H₁₃NO₃ (1)	19.87±0.01	23.57±0.01	24.07±0.05	16.76±0.02	15.68±0.01
C₁₈H₂₁NO₄Si (1a)	15.53±0.005	17.26±0.08	17.23±0.01	12.58±0.01	11.82±0.01
C₃₂H₃₄N₂O₇Si (1b)	15.23±0.005	17.24±0.05	17.21±0.01	12.43±0.02	11.62±0.02
C₄₄H₄₁N₃O₉Si (1c)	15.19±0.02	17.21±0.05	17.18±0.01	12.20±0.01	11.23±0.01
BHT	8.56±0.02	10.46±0.02	-	-	-
Ascorbic acid	-	-	15.64±0.02	-	-
Diclofenac sodium	-	-	-	7.26±0.04	-
Acarbose	-	-	-	-	7.17±0.02

CONCLUSION:

Ethyl silicon (IV) Schiff base complexes were successfully synthesized and characterized by the spectroscopic investigations. It was established that the Schiff base acts as bidentate and coordinated through imine nitrogen and carboxylate oxygen to the silicon atoms. Trigonal bipyramidal, octahedral, and pentagonal bipyramidal geometries have been proposed for ethyl silicon (IV) complexes with the help of different spectral studies like IR, ¹H, ¹³C, and ²⁹Si NMR. In antibacterial activity, new ethyl silicon (IV) Schiff base complexes inhibited the growth of all the four selected bacteria. In all the complexes the zone of inhibition was found to be dose dependent, i.e. with the increase in the concentration the zone of inhibition increased. The maximum zone of inhibition for the bacterium B. subtilis, S. Aureus, E. Coli, and S. abony, 1c showed maximum zone of inhibition with (80%), (69.69%), (91.67%), and (50%), percent inhibition at 10mg/ml. Interestingly in our present pharmacological investigations, it was concluded that the ethyl silicon (IV) Schiff base complexes (1a-1c) showed better pharmacological activity than the Schiff base (LH₂) (1). The pharmacological activity increases with the number of electron-donating groups, as well as the coordinating environment of silicon.

ACKNOWLEDGEMENT:

We are highly thankful to department of chemistry, S.S.J. Campus Almora; Department of Chemistry, College of Basic Sciences and Humanities, GBPUAT, Pantnagar for providing us laboratory for research work; Spectral facilities supported by SAIF Chandigarh, Punjab University and Department of Chemistry, Indian Institute of Technology Roorkee; Department of Botany, School of Basic and Applied Science, Sri Guru Ram Rai University, Dehradun for antibacterial activity.

CONFLICT OF INTEREST:

The authors have no relevant financial or non-financial interests to disclose.

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Received on 10.07.2024 Revised on 20.11.2024

Accepted on 15.02.2025 Published on 05.09.2025

Available online from September 08, 2025

Research J. Pharmacy and Technology. 2025;18(9):4248-4254.

DOI: 10.52711/0974-360X.2025.00610

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