Methyl Silicon (IV) Schiff Base Complexes: Synthesis, Coordination Behavior and Their Pharmacologically Significance as Antioxidant, Anti-Inflammatory and Anti-Diabetic Agents

Introduction

The design of Schiff base is widely recognized as versatile ligand due to its easy synthesis and diverse applications in synthesizing metal complexes. Among these, Schiff base amino acid complexes have acquired attention for their inorganic significance and potential physiological and pharmacological activities^{1, 2}. The combination of Schiff bases derived from o-hydroxy aromatic aldehydes with amino acids holds relevance to various biological processes like transamination, racemization, and carboxylation, highlighting their importance ³. Organosilicon (IV) complexes are renowned for their diverse pharmacological effects, including anticarcinogenic, antibacterial, antifungal, tuberculostatic, insecticidal, and acaricidal properties^4-7. These properties highlight their potential for various therapeutic applications, making them subjects of continued research and exploration in medicinal chemistry. Keeping this in view, we have synthesized new methyl silicon (IV) complexes and evaluated their pharmacological activities.

Materials and Method

All chemicals used in the present work were acquired from Sigma Aldrich. Elemental analyses were obtained using Thermo Scientific (FLASH 2000) CHN elemental analyzer at SAIF Punjab University, while silicon was determined gravimetrically as SiO₂. A Bruker Advance neo 500 MHz NMR spectrometer at SAIF Punjab University was utilized to acquire the multinuclear (¹H and ¹³C NMR) spectra. Using TMS as the internal reference, the chemical shift (δ) was measured and expressed in parts per million. Infrared data were obtained using an RZX (Perkin Elmer) spectrophotometer, Model SAIF Punjab University, considering the wave numbers from 4000 to 400cm⁻¹. A UV-visible spectrophotometer was used to measure absorbance to determine pharmacological activity at GBPUAT, Pantnagar, Uttarakhand.

Preparation of ligand (LH₂) (1)

The ligand 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 (50 ml) (Scheme 1)⁸. 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 ligand. Click here to View Scheme

 

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

For the preparation of complexes, a calculated amount of the trimethoxymethylsilane and ligand (1) were dissolved separately in 30ml of benzene and mixed with constant stirring. The mixture was stirred magnetically for 20-22 hours 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.

Pharmacological activities

Formula to calculate percentage inhibition of pharmacological activities:

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

Evaluation of antioxidant activity

To assess the antioxidant properties of ligand and methyl silicon (IV) Schiff base complexes, three in-vitro assays were used: metal chelating, DPPH, and H₂O₂ radical scavenging activity. In metal chelating activity, the absorbance was measured at 560 nm. For this, 0.1 ml of 2 mM FeCl₂•4H₂O, 0.2 ml of 5 mM ferrozine, and 4.7 ml of MeOH were mixed into samples (5 μg/ml – 25 μg/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 562 nm was measured using a UV-visible spectrophotometer. As a standard, ascorbic acid was employed⁹.

In DPPH radical scavenging activity, various amounts (5 μg/ml – 25 μg/ml) of the ligand and methyl silicon (IV) Schiff base 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 temperature¹⁰.

In H₂O₂ radical scavenging activity, 0.6ml of 40mM H₂O₂ dissolved in 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 study¹¹.

Evaluation of 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 standard¹².

Evaluation of 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 minutes¹³. Afterward, 100μl of a 1% starch solution was introduced into the sample solution. Following five-minute incubation at 37°C, 500μl of DNSA 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.

Result Discussion

Reactions of trimethoxymethylsilane with ligand were conducted in 1:1, 1:2, and 1:3 molar ratios in benzene. The reaction resulted in the release of methanol due to acidic hydrogen(s) from ligand (1) and the methoxy group(s) from methyl silicon alkoxides, yielding (1a, 1b, 1c) (Figure 1). All the newly synthesized methyl 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.

Figure 1: Proposed structures of methyl silicon (IV) Schiff base complexes (1a-1c). Click here to View Figure

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

Ligand and complexes	Color	M.P. (0C)	Calculated (found)%				Mol. Wt. gmol-1
			C	H	N	Si
C14H13NO3 (1)	Ethnic brown	82	69.12 (68.82)	5.39 (5.01)	5.76 (5.15)	–	243.255
C16H17NO4Si (1a)	Peanut Butter	84	60.93 (60.25)	5.43 (5.13)	4.44 (4.02)	8.90 (8.45)	315.396
C30H30N2O7Si (1b)	Machaccino	88	64.51 (64.14)	5.41 (5.05)	5.01 (4.81)	5.02 (4.75)	558.655
C43H39N3O9Si (1c)	Sunderbans	92	67.08 (66.84)	5.10 (4.91)	5.46 (5.03)	3.66 (3.15)	769.871

 

¹H NMR spectra

In the case of ligand, 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 1a¹⁴. A sharp singlet proton signal of the -CH=N group is observed at δ10.82ppm, which appears at δ10.83-10.98ppm in the spectra of complexes due to the coordination of the azomethine nitrogen to the metal atom¹⁵. The results of ¹H-NMR spectral data were tabulated in Table 2.

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

Ligand and Complexes	-HC=N	-CH3	-CH3Si	Phenoic –OH	aromatic proton	DMSO
C14H13NO3 (1)	10.82(s)	1.62(d)	–	12.17 (s)	7.19-8.86(d,t,m)	2.50(m)
C16H17NO4Si (1a)	10.83(s)	1.63(d)	1.01(s)	–	6.23-8.35(d,t,m)	2.50(m)
C30H30N2O7Si (1b)	10.85(s)	1.62(d)	1.13(s)	12.09(s)	6.24-8.34(d,t,m)	2.50(m)
C43H39N3O9Si (1c)	10.98(s)	1.62(d)	1.10(s)	12.11(s)	6.21-8.35(d,t,m)	2.50(m)

 

¹³C- NMR spectra

The signals due to the carbon atom attached to the carboxylic acid group in the ligand appear at δ192.93ppm¹⁶. A signal of the azomethine group appears at δ163.88ppm. However, these occur at δ163.30±0.5ppm in the spectra of methyl silicon (IV) Schiff base complexes^{17, 18}. The significant shift in the resonance of the carbon atom bonded to nitrogen suggests that the azomethine nitrogen has participated in coordination¹⁹. The results of ¹³C-NMR spectral data were tabulated in Table 3.

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

Ligand and Complexes	-HC=N	-COOH	-C=O	-COO–	-OCH3	-C-OH	Aromatic carbon	DMSO
C14H13NO3 (1)	163.92	192.93	–	–	–	–	112.24-138.27	39.50
C16H17NO4Si (1a)	163.21	–	175.15	170.80	52.85	–	112.17-138.26	39.50
C30H30N2O7Si (1b)	163.25	–	175.02	170.74	52.84	159.59	112.21-138.28	39.50
C43H39N3O9Si (1c)	163.35	–	175.11	170.79	–	159.65	112.25-137.25	39.50

 

Infrared spectra

In the ligand, the peak observed at 1592.32 cm^-1, attributed to the υ C=N²⁰, is found to be shifted to higher wave numbers in the complexes, suggesting coordination of the azomethine nitrogen with the silicon ion²¹. The observed broadband at 3430.84cm^-1 and 3369.74-2865.93cm^-1 was attributed to the υ(OH)²² and υCOOH groups²³. These stretching frequencies support the formation of the ligand. The complexes show two sharp bands observed at 1622.24-1646.81cm^-1 which are assigned to the υ_asy(COO) and 1310.94-1382.06cm^-1, which are assigned to the υ_sym(COO) stretching modes²⁴. 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.

Table 4: IR spectra of ligand and methyl silicon (IV) Schiff base complexes

Ligand and complexes	υ Ar-CH	υ  (OH)	υ C=O	υ (C=N)	υasy (COO)	υsym (COO)	υasy (Si-O)	υsym (Si-O)	υ (Si←N)
C14H13NO3 (1)	3094.09	3430.84	1723.25	1592.32	1622.24	1382.06	–	–	–
C16H17NO4Si (1a)	3077.83	–	–	1594.53	1646.81	1368.23	838.96	653.66	531.21
C30H30N2O7Si (1b)	3075.75	3433.15	1720.10	1594.54	1633.27	1315.17	841.11	652.13	532.19
C43H39N3O9Si (1c)	3074.87	3432.48	1730.41	1594.52	1624.25	1310.94	840.01	650.27	533.01

 

Pharmacological activities

Evaluation of Antioxidant activity

As the concentrations of the ligand and methyl silicon (IV) Schiff base complexes increased, so did their chelation activity. The results of the activity indicated that the methyl silicon (IV) 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 (15.63±0.02) > (1c) (18.18±0.01) > (1b) (18.20±0.01) > (1a) (18.24±0.01) > (1) (24.07±0.05) (Table 5).

The DPPH scavenging activity of ligand and methyl silicon (IV) Schiff base 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) (17.75±0.02) > (1b) (17.91±0.02) > (1a) (18.17±0.01) > (1) (19.87±0.01) (Table 5).

The results of the activity indicated that the methyl 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) (19.66±0.02) > (1b) (19.72±0.01) > (1a) (20.06±0.01) > (1) (23.57±0.01) (Table 5).

Evaluation of Anti-inflammatory activity

In comparison to other samples, sample 1c exhibited higher potency. However, against diclofenac sodium (IC₅₀=7.26±0.04μg/ml), ligand and all complexes showed lesser activity. The sequence of samples in inhibiting protein denaturation, accompanied by their IC50 values, was as follows: standard (7.26±0.04) > (1c) (14.95±0.06) > (1b) (15.10±0.01) > (1a) (15.22±0.01) > (1) (16.76±0.02) (Table 5).

Evaluation of 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 ligand (1) exhibited less anti-diabetic activity. Methyl 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: standard (7.17±0.02) > (1c) (13.75±0.01) > (1b) (13.93±0.01) > (1a) (13.98±0.02) > (1) (15.68±0.01) (Table 7).

Table 5: Pharmacological activities of ligand and methyl silicon (IV) Schiff base complexes in terms of IC₅₀ (μg/ml±SD)

Ligand, complexes, and standard	Antioxidant			Anti-inflammatory	Anti-diabetic
	Metal Chelating	DPPH  radical	H2O2 radical	Protein denauration	α-amylase inhibitory
C14H13NO3 (1)	24.07±0.05	19.87±0.01	23.57±0.01	16.76±0.02	15.68±0.01
C16H17NO4Si (1a)	18.24±0.01	18.17±0.01	20.06±0.01	15.22±0.01	13.98±0.02
C30H30N2O7Si (1b)	18.20±0.01	17.91±0.02	19.72±0.01	15.10±0.01	13.93±0.01
C43H39N3O9Si (1c)	18.18±0.01	17.75±0.02	19.66±0.02	14.95±0.06	13.75±0.01
BHT	–	8.56±0.02	10.46±0.02	–	–
Ascorbic acid	15.63±0.02	–	–	–	–
Diclofenac sodium	–	–	–	7.26±0.04	–
Acarbose	–	–	–	–	7.17±0.02

 

Conclusion

Methyl silicon (IV) Schiff base complexes were successfully synthesized and characterized by the spectroscopic investigations. It was established that the ligand 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 methyl silicon (IV) complexes with the help of different spectral studies like IR, ¹H, and ¹³C NMR. Interestingly in our present pharmacological investigations, it was concluded that the methyl silicon (IV) Schiff base complexes (1a-1c) showed better pharmacological activity than the ligand (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, CBSH, Govind Ballabh Pant University, Pantnagar for granting us access to their laboratory facilities for our research work. Spectral facilities are supported by SAIF Chandigarh, Punjab University.

Conflict of Interest

There is no conflict of interest.

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