Mixed ligand complexes of dioxouranium (VI) of the type [UO2(Q)(L)·2H2O] have been synthesized using 8-hydroxyquinoline (HQ) as a primary ligand and amino acids (HL) such as L-threonine, L-tryptophan, and L-isoleucine as secondary ligands. The metal complexes have been characterized by elemental analysis, electrical conductance, magnetic susceptibility measurements, and spectral and thermal studies. The electrical conductance studies of the complexes indicate their nonelectrolytic nature. Magnetic susceptibility measurements revealed diamagnetic nature of the complexes. Electronic absorption spectra of the complexes show intraligand and charge transfer transitions, respectively. Bonding of the metal ion through N- and O-donor atoms of the ligands is revealed by IR studies, and the chemical environment of the protons is confirmed by NMR studies. The thermal analysis data of the complexes indicate the presence of coordinated water molecules. The agar cup and tube dilution methods have been used to study the antibacterial activity of the complexes against the pathogenic bacteria
It is well known that mixed ligand ternary complexes of some metals play an important role in the activation of enzymes [
This paper reports the synthesis, characterization, and antibacterial studies of mixed ligand dioxouranium(VI) complexes prepared with 8-hydroxyquinoline as a primary ligand and amino acids such as L-threonine, L-tryptophan, and L-isoleucine as secondary ligands. These complexes have been screened for their antibacterial properties against the pathogenic bacteria
Analytical grade uranyl nitrate hexahydrate was used as such without further purification. L-threonine, L-tryptophan, L-isoleucine, and 8-hydroxyquinoline were obtained from S.D. Fine Chemicals, Mumbai. Solvents like ethanol and dimethylformamide and laboratory grade chemicals whenever used were distilled and purified according to standard procedures [
Mixed ligand dioxouranium(VI) complexes were prepared from uranyl nitrate hexahydrate, 8-hydroxyquinoline (HQ) as a primary ligand, and different amino acids such as L-threonine, L-tryptophan, and L-isoleucine as secondary ligands.
To an aqueous solution (10 mL) of uranyl nitrate hexahydrate (502 mg, 1 mmol), ethanolic solution (10 mL) of 8-hydroxyquinoline (145 mg, 1 mmol) was added. The mixture was stirred and kept in a boiling water bath for 10 min. To this hot solution, an aqueous solution (10 mL) of amino acid (1 mmol) was added with constant stirring. The mixture (1 : 1 : 1 molar proportion) was again heated in a water bath for 10 min till the temperature reached to 50°C. The complexes were precipitated by raising the pH of the reaction mixture by adding diluted ammonia solution. The mixture was cooled, and solid complex obtained was filtered and washed with water followed by ethanol. The complexes thus prepared were dried under vacuum and were used for further studies.
The complexes were analyzed for C, H, and N contents on Thermo Finnigan Elemental Analyzer Model no. FLASH EA 1112 Series at the Department of Chemistry, I.I.T., Mumbai. Metal content was estimated gravimetrically by standard procedure [
In the agar cup method, a single compound can be tested against a number of organisms or a given organism against different concentrations of the same compound. The method was found suitable for semisolid or liquid samples and was used in the present work. In the agar cup method, a plate of sterile nutrient agar with the desired test strain was poured to a height of about 5 mm and allowed to solidify, and a single cup of about 8 mm diameter was cut from the center of the plate with a sterile cork borer. Thereafter, the cup was filled with the sample solution (1000
The test compound (10 mg) was dissolved in dimethylsulphoxide (10 mL) so as to prepare a stock solution of concentration 1000
The test compounds were subjected to
Bacterial inoculums were prepared in sterilized Mueller Hinton broth and incubated for 4 h at 37°C. This was dispersed (5 mL) in each borosilicate test tube (
The lowest concentration which showed no visible growth was noted as minimum inhibitory concentration (MIC).
The synthesis of mixed ligand uranyl complexes may be represented as follows:
All the complexes are coloured, nonhygroscopic, thermally stable solids (Table
Colour, decomposition temperature, and pH of the uranyl complexes.
Sr. no. | Complex | Colour | Decomposition temperature(°C) | pH |
---|---|---|---|---|
(1) | [UO2(Q)(Thr) | Light Brown | 265 | 7.00 |
(2) | [UO2(Q)(Try) | Light Brown | 257 | 7.00 |
(3) | [UO2(Q)(Iso) | Light Brown | 250 | 7.00 |
Where Q represents the deprotonated primary ligand 8-hydroxyquinoline, Thr, Try, and Iso represent deprotonated secondary ligands L-threonine, L-tryptophan, and L-isoleucine, respectively.
The elemental analysis data (Table
Empirical Formula, Molecular Weight, Elemental Analysis Data and Molar Conductance of Uranyl Complexes.
Sr. no. | Complex | Empirical formula | Molecular weight | Elemental analysis Found (Calculated) | Molar conductance | |||
%M | %C | %H | %N | |||||
(1) | [UO2(Q)(Thr) | UC13H18N2O8 | 568.31 | 41.81 (41.88) | 27.40 (27.45) | 3.15 (3.17) | 4.91 (4.93) | 0.001 |
(2) | [UO2(Q)(Try) | UC20H21N3O7 | 653.42 | 36.43 (36.43) | 36.72 (36.73) | 3.20 (3.21) | 6.41 (6.43) | 0.001 |
(3) | [UO2(Q)(Iso) | UC15H22N2O7 | 580.37 | 41.00 (41.01) | 31.01 (31.01) | 3.76 (3.79) | 4.81 (4.82) | 0.002 |
The magnetic moment (Table
Magnetic susceptibility data of uranyl complexes (−10−6 c.g.s. units).
Sr. no. | Complex | |||
---|---|---|---|---|
(1) | [UO2(Q)(Thr) | 0.6844 | 389.00 | Diamagnetic |
(2) | [UO2(Q)(Try) | 0.9499 | 620.69 | Diamagnetic |
(3) | [UO2(Q)(Iso) | 0.9855 | 571.99 | Diamagnetic |
The electronic spectra of the metal complexes in DMF were recorded in the UV-visible region. The spectra show three transitions in the range 36364–36765 cm−1, 29762–30303 cm−1, and 25253–26316 cm−1 ascribed to
The FT-IR spectra of the metal complexes were recorded for KBr discs over the range 4000–400 cm−1. On the basis of the reported infrared spectra of amino acids, 8-hydroxyquinoline, and its metal complexes [
A broad band was observed in the region between 3460 and 3431 cm−1 due to asymmetric and symmetric O–H stretching modes and a band in the range 1600–1585 cm−1 due to H–O–H bending vibrations indicating the presence of coordinated water molecules further confirmed by thermal studies.
The
Broad bands at 3040 and 2960 cm−1 due to N–H (asymmetric) and N–H (symmetric) vibrations of free amino acid moiety are shifted to higher wave numbers, in the range 3177–3140 cm−1 and 3050–3025 cm−1, respectively, in the spectra of metal complexes, suggesting coordination of the amino group through nitrogen with the metal ion.
The
An important feature of infrared spectra of the metal complexes with 8-HQ is the absence of band ~3440 cm−1 due to the O–H stretching vibration of the free O–H group of HQ [
The FT-IR spectra of the uranyl complexes show no absorption bands near 1352 cm−1 where ionic nitrate is known to absorb [
The bands at 898–889 cm−1 and 821–820 cm−1 were assigned to
Some new bands of weak intensity observed in the regions around 604 cm−1 and 486 cm−1 may be ascribed to the M–O and M–N vibrations, respectively [
1H NMR spectra of complexes in DMSO exhibits a singlet at
In case of complex with L-threonine it shows doublet at d 1.26 ppm (
The complex with L-tryptophan shows doublet at
The complex with L-isoleucine shows triplet at
The TG and DTA studies of the uranyl complexes have been recorded in the nitrogen atmosphere at the constant heating rate of 10°C per minute.
The TG of the uranyl complexes shows that they are thermally quite stable to varying degree. The complexes show gradual loss in weight due to decomposition by fragmentation with increasing temperature as presented in Table
Thermal data of uranyl complexes.
Sr. no. | Complex | Decomposition temperature (°C) | Temperature range (°C) | % Weight loss | Decomposition product | |
Found | Calculated | |||||
(1) | [UO2(Q)(Thr) | 250 | 110–190 | 06.00 | 06.04 | [UO2(Q)(Lys)] |
250–360 | 24.20 | 24.35 | [UO2(Q)] | |||
425–550 | 24.10 | 24.19 | [UO2] | |||
(2) | [UO2(Q)(Try) | 260 | 110–190 | 06.00 | 06.18 | [UO2(Q)(Asp)] |
260–360 | 22.52 | 22.67 | [UO2(Q)] | |||
430–520 | 24.51 | 24.73 | [UO2] | |||
(3) | [UO2(Q)(Iso) | 230 | 110–160 | 06.31 | 06.31 | [UO2(Q)(Cys)] |
230–380 | 21.02 | 21.04 | [UO2(Q)] | |||
460–585 | 25.00 | 25.25 | [UO2] |
The DTA of the complexes displays an endothermic peak in the range 110–190°C which indicates the presence of coordinated water molecules. As the temperature is raised, the DTA curve shows a small exotherm in the range 250–445°C and a broad exotherm in the range 450–690°C attributed to decomposition of amino acid and 8-hydroxyquinoline moieties present in the complexes, respectively. The formation of a broad exotherm is possibly due to simultaneous decomposition of ligand moieties and their subsequent oxidation to gaseous products like CO2 and H2O and so forth [
Like most of the metal organic complexes, these complexes also decompose to a fine powder of metal oxide, that is, UO2. The constant weight plateau in TG after 700°C indicates completion of the reaction. The UO2 form was confirmed by X-ray diffraction pattern of the decomposed product [
On the basis of the physicochemical studies, the bonding and structure for the uranyl complexes may be represented as shown in Figure
Proposed structures and bonding for the uranyl complexes.
All the metal complexes were screened against
The studies based on agar cup method revealed that the complexes are more active against
Antibacterial activity (mm) of uranyl complexes by agar cup method.
Sr. no. | Complex | Test | |||
(1) | [UO2(Q)(Thr) | 12 | 12 | 20 | 26 |
(2) | [UO2(Q)(Try) | 12 | 14 | 23 | 18 |
(3) | [UO2(Q)(Iso) | 13 | 12 | 26 | 25 |
(4) | Tetracycline | 30 | 25 | 26 | 26 |
The minimum inhibitory concentration (MIC) of ligand and the metal salts ranges between 50 and 110
MIC data of uranyl complexes.
Sr. no. | Complex | MIC ( | |||
(1) | [UO2(Q)(Thr) | 20 | 20 | 5 | 10 |
(2) | [UO2(Q)(Try) | 20 | 25 | 10 | 10 |
(3) | [UO2(Q)(Iso) | 20 | 20 | 10 | 5 |
(4) | UO2(NO3)2 | 50 | 50 | 100 | 100 |
(5) | 8-hydroxyquinoline | 50 | 50 | 110 | 100 |
(6) | Tetracycline | 1.5 | 2.0 | 1.5 | 2.5 |
The results show that, as compared to the activity of metal salts and free ligand, the metal complexes show higher activity. The activity of metal complexes is enhanced due to chelation. The chelation reduces considerably the polarity of the metal ions in the complexes, which in turn increases the hydrophobic character of the chelate and thus enables its permeation through the lipid layer of microorganisms [
Based on the above results, the following conclusions may be drown.
The higher decomposition temperatures of the complexes indicate a strong metal-ligand bond, and electrical conductance studies show nonelectrolytic nature of the complexes, respectively. Magnetic studies indicate diamagnetic nature of the complexes. Electronic absorption spectra of the complexes show intraligand and charge transfer transitions, respectively. IR spectra show bonding of the metal ion through N- and O- donor atoms of the two ligands. 1H NMR study reveals the chemical environment of protons and presence of water molecules in the complexes. Thermal analysis confirms the presence of coordinated water molecules.
On the basis of the above results, coordination number eight is proposed for uranyl complexes.
The antibacterial study shows that complexes are found to be more active against
Compared to standard antibacterial compound, tetracycline, the complexes show moderate activity against the selected strains of microorganisms.
The authors are grateful to Dr. S. T. Gadade, a Principal in Changu Kana Thakur Arts, Commerce and Science College, New Panvel and a Member of the Management Council, University of Mumbai for providing the laboratory and library facilities.