Synthesis,characterization and in vitro antibacterial activity of di- and tri n-butyltin(IV) complexes of 3-amino-4 -chloridobenzohydroxamic acid

Abstract

New di-and tri- n-butyl tin(IV) hydroxamate complexes of composition [n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂](I) and [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)] (II) have been synthesized by the reactions of n-Bu₂SnCl₂ and n-Bu₃SnCl with potassium 3-amino-4-chlorido benzohydroxamate(KHL) in predetermined metal:ligand 1:2 and 1:1 molar ratios respectively in MeOH+THF solvent medium and characterized by physiochemical, spectroscopic (IR,¹ H and ¹³ C NMR) studies and mass spectrometry. The bidentate nature of hydroxamate ligand involving bonding through carbonyl and hydroxamic oxygen atoms (O,O coordination) has been inferred from IR spectra. The distorted octahedral and trigonal bipyramidal geometry around tin for (I) and (II) respectively have tentatively been proposed. The electrochemical behavior of complexes studied by cyclic voltammetric technique has shown two electron tin metal centered reductions. The in vitro antibacterial activity of (I) and (II) assayed against pathogenic gram -ve bacteria E.coli, P.aeruginosa; gram +ve bacteria S.aureus, B.cereus by MIC method has revealed these to be promising antibacterial agents relative to parent ligand.

Keywords

Di- and tri n-butyltin(IV) complexes 3 -amino 4 -chlorido benzohydroxamic acid

1. Introduction

Hydroxamic acids constitute medically important class of organic bio-ligands [1–3] in which hydroxylamine (NH₂OH) is inserted into carboxylic acid group forming RC(O)NHOH. The biological importance of hydroxamic acids is reflected in their metal organic derivatives to exert a subtle influence on medicinal applications [4–7]. Hydroxamic acids (HAs) have gathered considerable importance as supporting ligands in coordination chemistry and biology because of their potential as therapeutics agents. The hydroxamic acids display different tautomeric forms and flexible bonding modes [8]. Numerous spectral studies have shown that hydroxamic acids preferably form metal complexes through the hydroxamide form rather than the hydroximic form [9].

Structure of potassium 3-amino -4-chloridobenzohydroxamate(KHL).

The chemistry of organotin (IV) compounds has been reported to display numerous potential applications in healthcare as anti-inflammatory, antituberculosis, anti-leishmaniasis and antibacterial agents [10–13]. The nature and number of organic groups attached to tin affect the properties of complexes especially the biological activity. As a part of our continuous interest in coordination chemistry of organotin(IV) hydroxamates [14–18] we report, herein the synthesis and characterization of di-and tri n-butyl tin(IV) complexes of a di-substituted hyroxamate ligand potassium 3-amino-4-chloridobenzohydroxamate containing electron donating as well as electron withdrawing substituents. The hydroxamic acids with electron donating substituent form more stable complexes by increasing the localized negative charge on oxygen atoms while those with electron withdrawing substituents exhibit higher biological activity [19]. Compared to numerous reports on transition metal hydroxamates [20–23] scattered reports describe tin and organotin(IV) hydroxamates [24, 25]. The newly synthesized complexes have been screened for in vitro antibacterial activity against some pathogenic bacteria, E.coli, P.Aeruginosa Bacillus cereus & Staphylococcus aureus

2. Experimental

2.1. Materials and physical measurements

All the solvents and chemicals were of reagent grade. The potassium 3-amino-4-chlorido-benzohydroxamate was synthesised by reported method [26]. The tin content in complexes was determined as tin dioxide by treating the complex with conc. H₂SO₄ (2 Volume) and conc. HNO₃ (3 Volume) [27]. The chloride ion is determined by Volhards method [28]. The carbon, hydrogen and nitrogen analyses were obtained on Eager 300 NCH System Elemental Analyzer. IR spectra were recorded as KBr pellets on Nicolet-5700 FTIR spectrophotometer. The pellets were prepared in a dry box to avoid the action of moisture. ¹ H and ¹³ C NMR spectra of complexes were recorded on BRUKER AVANCE II 400 spectrometer using TMS as an internal standard and DMSO (deuterated) as solvent. Electrospray ionization mass spectral measurements of complexes were recorded in chloroform(deuterated) with WATERS,Q-TOF MICROMASS(ESI-MS) mass spectrometer. The cyclic voltammetric experiment was performed on Autolab Potentiostat 128 N electrochemical analyzer in single compartmental cell of volume 10–15 mL containing a three-electrode system comprising of a Pt-disk working electrode, Pt-wire as auxiliary electrode and Ag/AgCl as reference electrode. The supporting electrolyte was 0.4 M KNO₃ in milli-Q water and methanol-H₂O (5:95) electrolyte system.

2.2. Synthesis

Synthesis of [n- Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂](I): To a solution of n-Bu₂SnCl₂ (1 mol, 3.03 g) in distilled THF(10 ml) was added methanolic solution(10 ml) of bimolar amount of potassium 3-amino-4-chloridobenzohydroxamate (4.8 g, 2 mol). The reaction mixture was refluxed for 24 h where upon the formation of a white solid (KCl) was observed. It was filtered off. The addition of petroleum ether to filtrate gave white solid. It was recrystallized from absolute ethanol. (Yield: 5.14 g, 85%). For [n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂][C₂₂H₃₃N₂O₂ClSn] [605]Anal.Calcd. (%): Sn,19.70 C, 45.36 H, 4.5 N, 9.8 Cl, 5.2 Found, (%): Sn, 19.44 C, 45.80 H, 5.01 N, 9.71 Cl, 5.44 ∧_m (Methanol): 0.72 Scm²mol⁻¹.

Synthesis of n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO): To methanolic solution of n-Bu₃SnCl (3.26 g, 1 mol) in THF(10 ml) was added methanolic solution (10 ml) of equimolar amount of potassium 3-amino-4-chloridobenzohydroxamate (2.24 g,1 mol). The reaction mixture was stirred for 6 h to ensure the completion of the reaction. The white solid formed during the course of reaction was filtered and identified as KCl. To the filtrate petroleum ether was added and recrystallised from 1:1 mixture of methanol and diethylether. A light brown solid was obtained. (Yield: 3.76 g, 78%). For [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)] [C₁₉H₃₃N₂O₂ClSn] [475] Anal. Calcd. (%): Sn, 25.02 C, 47.94 H, 6.94 N, 5.8 Cl, 7.4; Found, (%): Sn, 25.52 C, 47.44 H, 7.01 N, 5.45,Cl, 7.52; ∧_m (Methanol): 0.66 Scm²mol⁻¹.

2.3. Antibacterial Activity

The ligand potassium 3-amino-4-chloridobenzohydroxamate and organotin(IV) complexes [n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂](I), [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)](II) were screened in vitro for their antibacterial activity on selected Gram positive bacteria S. aureus, B.cereus and Gram-negative bacteria Escherichia coli and P.aeruginosa at different concentrations in DMSO solution employing the standard Minimum Inhibitory Concentrations (MIC) method as recommended by National Committee for Clinical Laboratory Standard (NCCLS). MIC is the lowest concentration of the antimicrobial agents that prevents the development of visible growth after overnight incubation The commercial antibiotic tetracyclin was used as standard for the comparison of results.

The in vitro anti-bacterial activitiy of organotin(IV) hydroxamates through MIC assay are assayed in a 96-well micro-titre plate (tissue culture grade) by two fold serial dilution method using oxidation reduction colorimetric indicator dye Resazurin, for the determination of drug resistance and minimal inhibitory concentration of test complexes against different microorganisms. The dye, blue in its oxidized state turns pink when reduced by viable cells indicating the growth of bacteria. The lowest concentration of test complex that prevented this colour change was considered as MIC according to CLSI M07-A9 [29]. A stock solution of test ligand and complexes was prepared in DMSO (500 μg/mL) for two fold serial dilution. For MIC assay of each test complex; a row of twelve wells was used out of which last two wells were taken as control (no test complex added). The 100 μL of the Muller-Hinton broth was placed in each of the ten wells except the first well which contained 200 μL of broth and 500 μg / mL of the test complex. From the first well (containing test complex), 100 μL broth was withdrawn with a sterile tip, and same was added to the 100 μL of the broth in the 2^nd well. The contents were mixed four times. In this way a range of two-fold serial dilution were prepared (500–7.81 μg / mL). Resazurin prepared as 0.02 % weight/volume in distilled water sterilized by filtration and stored at 4°C for one week was added to each well. The broth in each of the wells was inoculated with 20 μL of the bacterial culture and the contents were mixed by ten clockwise and ten anticlockwise rotations on a flat surface. Thereafter, the plate was incubated at 35°C for 24 h in case of bacteria. To evaluate the role of solvent in biological screening if any, separate studies were carried out with DMSO which did not show any activity. The results were compared with standard anti-bacterial drug tetracycline. All the experiments were carried out in triplicate [30].

3. Results and discussion

The reactions of n-Bu₂SnCl₂ and n-Bu₃SnCl with potassium 3-amino-4-chlorido-benzohydroxamate (1:2 and 1:1) molar ratio respectively in MeOH/THF under reflux afforded white and light brown complexes in agreement with elemental analyses according to the equations: Scheme 1.

Scheme 1.

Synthesis of complexes.

The complexes are stable in air and soluble in MeOH, CHCl₃ and DMSO. The molar conductance values of the millimolar solutions of complexes in methanol (0.72 and 0.66 Scm²mol⁻¹) are indicative of their non-electrolytic nature [31].

3.1. Infrared Spectra

The formation of complexes has been ascertained from a comparison of their IR spectra with that of free ligand recorded in 4000–200 cm⁻¹ region. The uncoordinated potassium 3-NH₂-4-Cl benzohydroxamate ligand has shown two distinct ν(N-H) vibrations at 3481 and 3362 cm⁻¹ due to asymmetric and symmetric N-H vibrations of amino substituent and at 3219 cm⁻¹ attributed to characteristic ν(N-H) mode of hydroxamate group. The absorption bands at 1626 cm⁻¹ have been assigned to ν(C = O) mode and at 1303 cm⁻¹ and 965cm⁻¹ to ν(C-N) and ν(N-O) modes respectively.

The [n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂](I) displayed bands at 3450,3362 and 3212 cm⁻¹ and [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)](II) at 3470,3363 and 3215cm⁻¹ due ν_asym(NH₂), ν_sym(NH₂) and ν(N-H) modes of amino substituent and hydroxamate group respectively (Table 1). The absorption bands due to ν(C = O) at 1623 and 1624 cm⁻¹ and ν(C-N) at 1301 and 1301 cm⁻¹ modes have appeared nearly at the same wavenumber as in free ligand respectively suggesting weak bonding through carbonyl group. An explanation to these observations may be offered as that in the presence of both the electron donating and electron withdrawing substituents at the phenyl ring, the carbonyl double bond character is not weakened upon chelate ring formation.

Table 1.
IR spectral data of di-and tri-n-butylltin(IV)hydroxamates.

KHL I II Assignments

3481 3450 3470 νN-H (Amino gp) asymmetric

3362 3362 3363 νN-H (Amino gp) symmetric

3219 3212 3215 νN-H (Amide-I)

1626 1623 1624 νC=O (Amide-II)

1303 1301 1301 νC-N (Amide-III)

965 986 986 νN-O

– 480 482 νSn-O

KHL	I	II	Assignments
3481	3450	3470	νN-H (Amino gp) asymmetric
3362	3362	3363	νN-H (Amino gp) symmetric
3219	3212	3215	νN-H (Amide-I)
1626	1623	1624	νC=O (Amide-II)
1303	1301	1301	νC-N (Amide-III)
965	986	986	νN-O
–	480	482	νSn-O

The absorption band due to ν(N-O) mode has been observed to shift to higher wave number relative to free ligand occurred at 986 and 986cm⁻¹ indicating bonding through hydroxamic oxygen atom of NHO^- group upon complexation. The bidentate nature of the ligand involving bonding through carbonyl and hydroxamic oxygen atoms (O,O) of CONHO group has thus been indicated. The new bands observed at 480 and 482 cm⁻¹ in (I) and (II) respectively may be assigned to νSn-O mode based upon previous reports [18, 32].

3.2. ¹ H NMR spectra

A comparison of room temperature ¹ H NMR spectra of (I) and (II) with that of potassium 3-amino-4-chlorido-benzohydroxamate recorded in DMSO has further supported their formation. The ligand exhibited signals due to aromatic protons at δ 7.07–7.47 range, –NH₂ substituent at aromatic ring at δ 6.98–7.0 and –CONH at δ 8.08 ppm. Complexes [n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂] (I) and [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)] (II) exhibited respective resonances due to aromatic protons, NH₂ substituent and–CONH at δ 7.22–7.48;7.14–7.19 and 8.03; δ 7.24–7.47;7.15–7.17 and 8.0 ppm respectively. The signals due to aromatic protons have undergone moderate downfield shifts suggesting deshielding upon complexation. The resonances due to n-Bu group attached to tin metal have occurred in δ 0.88–1.70 ppm range in complexes (Table 2).

Table 2.
¹ H NMR data of di- and tri-n-butyltin(IV) hydroxamates.

Ligand/Complex -CON-H Aromatic protons C-NH₂ R-Sn

C₆H₃(3-NH₂)(4-Cl)CONHOK 8.08 7.07–7.47 6.98–7.0 —-

[n-Bu₂Sn[C₆H₃(3-NH₂)(4-Cl)CONHO)₂] (I) 8.03 7.22–7.48 7.14–7.19 0.88–1.70

[n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)](II) 8.0 7.24–7.47 7.15–7.17 0.90–1.70

Ligand/Complex	-CON-H	Aromatic protons	C-NH₂	R-Sn
C₆H₃(3-NH₂)(4-Cl)CONHOK	8.08	7.07–7.47	6.98–7.0	—-
[n-Bu₂Sn[C₆H₃(3-NH₂)(4-Cl)CONHO)₂] (I)	8.03	7.22–7.48	7.14–7.19	0.88–1.70
[n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)](II)	8.0	7.24–7.47	7.15–7.17	0.90–1.70

3.3. ¹³ C NMR Spectra

The ¹³ C NMR spectral pattern of (I) and (II) in [(CD₃)₂SO] is supportive of their composition in consistent with ¹ H NMR spectra. The free potassium 3-amino-4-chlorido-benzohydroxamate exhibited resonances due to aromatic ring carbons, C-NH₂ and CO at δ 114.19–143.26, 144.56 and 166.26 respectively. Complex [n-Bu₂Sn[C₆H₃(3NH₂)(4Cl)CONHO)₂](I) exhibited the respective resonances and due to Bu-Sn at δ 115.92–129.09, 144.43, 166.20 and 13.65–25.78 respectively. Complex [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)](II) displayed these resonances at δ 115.92–129.01, 144.32, 166.13 and 13.82–29.00 respectively. The small upfield shift in the position of carbonyl signal on complexation is indicative of coordination through carbonyl oxygen to tin metal (Table 3).

Table 3.
¹³ C NMR data of di- and tri-n-butyltin (IV)hydroxamates.

Ligand/Complex CO Aromatic carbons C-NH₂ R-Sn

C₆H₃(3-NH2)(4-Cl)CONHOK 166.26 114.19–143.26 144.56 ——-

[n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂] (I) 166.20 115.92–129.09 144.43 13.65–25.78

[n-Bu₃Sn[C₆H₃(3-NH₂)(4-Cl)CONHO)](II) 166.13 115.92–129.01 144.32 13.82–29.00

Ligand/Complex	CO	Aromatic carbons	C-NH₂	R-Sn
C₆H₃(3-NH2)(4-Cl)CONHOK	166.26	114.19–143.26	144.56	——-
[n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂] (I)	166.20	115.92–129.09	144.43	13.65–25.78
[n-Bu₃Sn[C₆H₃(3-NH₂)(4-Cl)CONHO)](II)	166.13	115.92–129.01	144.32	13.82–29.00

For di- and triorganotin(IV) compounds, empirical relationships between ^1J(¹¹⁹Sn,¹³ C) and the corresponding C-Sn-C angle θ have been proposed by Lockhart et al. [33, 34]. The C-Sn-C bond angle (θ) can be calculated by following Lockhart and Manders equation:

|^{1} J | = 11.4 θ - 875

(1)

(where θ= the C-Sn-C angle (deg) and ¹J is measured in Hz).

In tetra-coordinated organotin compounds (θ ≤ 112°) ¹Js are predicted to be smaller than about 400 Hz; for penta-coordinated tin (θ= 115–130°), ¹Js fall in the 450–670 Hz range and for hexacoordinated tin (θ≥135°) ¹Js generally larger than 670 Hz [35]. The tin-carbon J coupling, (¹J(¹¹⁹Sn, ¹³ C)(¹J), have been reported to depend on the C-Sn-C angle for tetra-, penta- and hexa-coordinated di- and tri-alkyltin(IV) compounds [36, 37].

From above equation, for complex (I) (¹J = 710 Hz) and (II) (¹J = 520 Hz) C-Sn-C bond angle (θ) have been calculated to be 139°03^′ and 122°36^′ respectively which are lying in the range of reported C-Sn-C angles for hexa and penta coordinate complexes [38, 39] indicating distorted octahedral (hexa-coordinate) and trigonal bi-pyramidal (penta-coordinate) geometry around tin in respective complexes.

3.4. Mass spectra

The ESI-MS of [n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂]/ [n-Bu₂Sn L₂](I) (mass = 652) displayed the most intense peak at m/z(%) 652 (100) corresponding to [n-Bu₂ Sn₂(L)₂+ 2Na]⁺ The fragment ions at m/z(%) 513(7.25), 457(6.17) 419(43.39),404(6.48), 305(12.53) 262(4.48) and 186(3.86) in (I) corresponded to [n-Bu Sn(L)₂-Cl]⁺, [Sn(L)₂-Cl]⁺, [n-Bu₂ Sn(L)]⁺, [n-Bu₂ Sn(L)-NH₂]⁺, [Sn(L)]⁺, [L + 2K⁺+[H]⁺]⁺and [L +[H]⁺]⁺ respectively. Complex [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)]/ [n-Bu₃SnL](II) (mass = 476) at m/z(%) 419(100) corresponding to [n-Bu₂ Sn(L)]⁺ formed by the removal of one butyl group from the complex. The fragmaent ion in (II) at m/z(%) 457(23.46), 379(5.78),305(78.99), 276(10.57), 235(9.21), 220(8.70),186(3.81) 162(6.05) corresponded to [n-Bu₂ Sn(L) +K⁺]⁺, [n-Bu₃ Sn(L) +NH₂]⁺, [Sn(L)]⁺, [Sn(L)-CO]⁺, [n-Bu₂ Sn-H⁺]⁺, [n-Bu₂ Sn + H+-Me]⁺, [LH], [n-BuSn-Me]⁺ respectively (Table 4, 5). The interpretation of mass spectra is based on earlier reports of ESI-mass spectra of organotin compounds and all the m/z values are related to the ¹²⁰Sn isotope. The presence or absence of tin atom in different ions have been observed on the basis of 10 characteristic natural tin isotopes with the most abundant ¹²⁰Sn isotope [40].

Table 4.
Mass spectral data of [n-Bu₂Sn (C₆H₃(3-NH₂)(4-Cl)CONHO)₂](I).

Fragment ions m/z(%)

[n-Bu₂Sn(L)₂ + 2 Na⁺]⁺ 652(100) B.P.

[n-BuSn(L)₂-Cl]⁺ 513(7.25)

[Sn(L)₂-Cl]⁺ 457(6.17)

[n-Bu₂ Sn(L)]⁺ 419(43.39)

[n-Bu₂ Sn(L)-NH₂]⁺ 404(6.48)

[Sn(L)]⁺ 305(12.53)

[L + 2K⁺+H⁺]⁺ 262(4.48)

[L + H]⁺ 186(3.86)

Fragment ions	m/z(%)
[n-Bu₂Sn(L)₂ + 2 Na⁺]⁺	652(100) B.P.
[n-BuSn(L)₂-Cl]⁺	513(7.25)
[Sn(L)₂-Cl]⁺	457(6.17)
[n-Bu₂ Sn(L)]⁺	419(43.39)
[n-Bu₂ Sn(L)-NH₂]⁺	404(6.48)
[Sn(L)]⁺	305(12.53)
[L + 2K⁺+H⁺]⁺	262(4.48)
[L + H]⁺	186(3.86)

Table 5.

Mass spectral data of [n-Bu₃Sn[C₆H₃(3-NH₂)(4-Cl)CONHO] (II).

Fragment ions	m/z(%)
[n-Bu₃Sn(L) +Na⁺]⁺	499(5.59)
[n-Bu₂Sn(L) +K⁺]⁺	457(23.46)
[n-Bu₂Sn(L)]⁺	419 B.P.(100)
[n-Bu₃ Sn-Cl]	379(5.78)
[Sn(L)]⁺	305(78.99)
[Sn(L)-CO]⁺	276(10.57)
[n-Bu₂Sn-H⁺]⁺	235(9.21)
[n-Bu₂Sn+H⁺-CH₃]⁺	220(8.70)
[HL]	186(3.81)
[n-BuSn-Me]⁺	162(6.0)

3.5. Electrochemical Studies

The cyclic voltammetric measurements were carried out using 2 mM solution at room temperature in the forward scan in which potential is swept negatively from starting point to switching point and then scan direction was reversed in potential range –2.0 to +2.0 V with a single scan rate of 100mVS⁻¹ [15]. The solution was degassed with N₂ prior to use and kept under N₂ atmosphere throughout the experiment. The cyclic voltammogram of potassium 3-amino-4-chloridobenzohydroxamate displayed one feeble Epc^cath and one moderate Epa^oxd peak at –0.4858 and –0.6176 V respectively ascribed to quasi-reversible redox couple. Complex [n-Bu₂Sn[C₆H₃(3-NH₂) (4-Cl)CONHO)₂](I) exhibited irreversible two electron reductions at –0.3735 and –0.8520 V with no anodic peak indicative of two sequential one electron reductions. The first step may be ascribed to be centred on the hydroxamate ligand followed by the second reduction with Sn-O bond cleavage. Complex [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)](II) showed a feeble reduction at –0.4199 V.

3.6. Antibacterial activity

Of biologically active metallopharmaceuticals, organotin(IV) compounds exhibit biological potential depending upon the nature of the organic ligand (the easily dissociable ligand facilitates the transportation of complex across the cell membrane) and the nature and number of organic groups attached to tin [24]. The biological applications of organotin(IV) complexes derived from a variety of ligands have been reported [44–48]. Organotin(IV) hydroxamates have widely been investigated for their cytotoxicity [44–48] while reports on their antimicrobial activities are rather scarce [49, 50]. Hence, in vitro antimicrobial potential of new organotin(IV) hydroxamates towards Gram-positive bacteria Bacillus cereus and Staphylococcus aureus, Gram-negative bacteria P. aeruginosa and Escherichia coli has been studied by the MIC method recommended by National Committee for Clinical Laboratory Standards (NCCLS). The commercial antibiotic tetracyclin have been used as standard drugs for comparison of results.

Over the last few years, the microbial resistance to various antibiotics has become a serious healthcare issue. The trend of biological activity has been reported as dimethyltin > dibutyl tin > diphenyl tin [18]. The uncoordinated potassium 3-amino-4-chlorido benzohydroxamate inhibited the gram +ve bacteria (B.cereus and S.aureus) at MIC 62.5 μg/mL and gram –ve bacteria (E.coli and P.aeruginosa) at MIC 250 and 125 μg/mL respectively. Complex [n-Bu₂Sn[C₆H₃(3-NH₂)(4-Cl)CONHO]₂](I) exhibited appreciable inhibitory effect towards studied bacteria at MIC 15.62–62.5 μg/mL range. The B. cereus and P.aeruginosa are prominently inhibited at MIC 15.62 μg/mL. Complex [n-Bu₃Sn[C₆H₃(3-NH₂)(4-Cl)CONHO](II) has also shown enhanced inhibitory activity against all the bacteria at MIC 31.25 –125 μg/mL. Of the two complexes, [n-Bu₂Sn[C₆H₃(3-NH₂)(4-Cl)CONHO]₂] (I) has shown the pronounced growth inhibiting activity against the test bacteria than (II) (Table 6). The enhanced antibacterial activity of complexes may be ascribed to an efficient diffusion across the cell membrane [51–54] and hydrophobicity of hydroxamate ligand in consonance with Overtone’s concept and Tweedy’s chelation theory [55, 56] whereby the polarity of the central metal ion gets reduced by sharing its positive charge with the ligand upon complexation. This inhibits the activity of essential enzymes for microbial growth and their reproduction by blocking their interaction with DNA.

Table 6.
Antibacterial activity data of di- and tri n-butyltin(IV) hydroxamates (μg/mL).

Ligand/ Complex S. aureus B. cereus P. aeruginosa E. coli

Gram +ve Gram –ve

C₆H₃(3-NH₂)(4-Cl)CONH-OK 62.50 ± 0.8 62.50 ± 0.8 125 ± 0.8 250 ± 0.8

[n-Bu₂Sn(C₆H₃ (3-NH₂)(4-Cl)CONH-O₂] _(I) 31.25 ± 0.35 15.62 ± 0.30 15.62 ± 0.30 62.50 ± 0.8

[n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONH-O])](II) 31.25 ± 0.35 31.25 ± 0.35 31.25 ± 0.35 125 ± 0.80

Tetracycline 15.62 ± 0.05 3.90 ± 0.15 7.81 ± 0.20 15.62 ± 0.25

Bu₂SnCl₂ 250 ± 0.30 500 ± 0.30 500 ± 0.80 500 ± 0.35

Bu₃SnCl 250 ± 0.30 250 ± 0.80 250 ± 0.35 500 ± 0.35

DMSO 125 ± 0.05 125 ± 0.05 125 ± 0.05 250 ± 0.30

Ligand/ Complex	S. aureus	B. cereus	P. aeruginosa	E. coli
C₆H₃(3-NH₂)(4-Cl)CONH-OK	62.50 ± 0.8	62.50 ± 0.8	125 ± 0.8	250 ± 0.8
[n-Bu₂Sn(C₆H₃ (3-NH₂)(4-Cl)CONH-O₂] _(I)	31.25 ± 0.35	15.62 ± 0.30	15.62 ± 0.30	62.50 ± 0.8
[n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONH-O])](II)	31.25 ± 0.35	31.25 ± 0.35	31.25 ± 0.35	125 ± 0.80
Tetracycline	15.62 ± 0.05	3.90 ± 0.15	7.81 ± 0.20	15.62 ± 0.25
Bu₂SnCl₂	250 ± 0.30	500 ± 0.30	500 ± 0.80	500 ± 0.35
Bu₃SnCl	250 ± 0.30	250 ± 0.80	250 ± 0.35	500 ± 0.35
DMSO	125 ± 0.05	125 ± 0.05	125 ± 0.05	250 ± 0.30

The standard drug tetracycline inhibiting test bacteria at MIC 3.90–15.62μg/mL range is more effective against B. cereus and P. aeruginosa at 3.90 μg/mL and 7.81 μg/mL respectively.

4. Conclusion

The new di-and tri-n-butyl tin (IV) complexes of substituted benzohydroxamic acid (3-amino-4-chloridobenzohydroxamate) of stoichiometric composition [n-Bu₂Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)₂](I) and [n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONHO)](II) have been synthesized and in conformity with elemental analysis characterized by IR,¹ H and ¹³ C NMR specta and mass spectrometry. The IR spectra has suggested chelation of ligand through hydroxamic and carbonyl oxygen atoms (O, O) coordination. A distorted octahedral geometry around tin in (I) and trigonal bipyramidal in (II) has tentatively been proposed. The complexes are redox active undergoing an overall two electron metal centred reductions. The in vitro antibacterial activity assay of (I) and (II) on selected Gram +ve bacteria S. aureus, B. Cereus and Gram-ve bacteria E. coli& P.aeruginosa by MIC method has shown promising antibacterial activity than the parent ligand offering a great potential as new antibacterial agents.

Footnotes

Acknowledgments

Authors thank Sophisticated Analytical Instrument Facility (SAIF) Panjab University Chandigarh for recording IR, ¹ H, ¹³ C NMR spectra, Indian Institute of Technology Mandi (H.P.) for mass spectra and Department of Biotechnology Himachal Pradesh University Shimla for laboratory facilities for antibacterial study.

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Ligand/ Complex	S. aureus	B. cereus	P. aeruginosa	E. coli
	Gram +ve		Gram –ve
C₆H₃(3-NH₂)(4-Cl)CONH-OK	62.50 ± 0.8	62.50 ± 0.8	125 ± 0.8	250 ± 0.8
[n-Bu₂Sn(C₆H₃ (3-NH₂)(4-Cl)CONH-O₂] _(I)	31.25 ± 0.35	15.62 ± 0.30	15.62 ± 0.30	62.50 ± 0.8
[n-Bu₃Sn(C₆H₃(3-NH₂)(4-Cl)CONH-O])](II)	31.25 ± 0.35	31.25 ± 0.35	31.25 ± 0.35	125 ± 0.80
Tetracycline	15.62 ± 0.05	3.90 ± 0.15	7.81 ± 0.20	15.62 ± 0.25
Bu₂SnCl₂	250 ± 0.30	500 ± 0.30	500 ± 0.80	500 ± 0.35
Bu₃SnCl	250 ± 0.30	250 ± 0.80	250 ± 0.35	500 ± 0.35
DMSO	125 ± 0.05	125 ± 0.05	125 ± 0.05	250 ± 0.30

Synthesis,characterization and in vitro antibacterial activity of di- and tri n-butyltin(IV) complexes of 3-amino-4 -chloridobenzohydroxamic acid

Abstract

Keywords

1. Introduction

2. Experimental

2.1. Materials and physical measurements

2.2. Synthesis

2.3. Antibacterial Activity

3. Results and discussion

Table 4. Mass spectral data of [n-Bu2Sn (C6H3(3-NH2)(4-Cl)CONHO)2](I). Fragment ions m/z(%) [n-Bu2Sn(L)2 + 2 Na+]+ 652(100) B.P. [n-BuSn(L)2-Cl]+ 513(7.25) [Sn(L)2-Cl]+ 457(6.17) [n-Bu2 Sn(L)]+ 419(43.39) [n-Bu2 Sn(L)-NH2]+ 404(6.48) [Sn(L)]+ 305(12.53) [L + 2K++H+]+ 262(4.48) [L + H]+ 186(3.86)

3.6. Antibacterial activity

Footnotes

Acknowledgments

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