INTRODUCTION:

Salicylaldehyde is a basic precursor to a diversity chelating agent, some of which are commercially significant, salicylaldehyde is an ordinary highly functionalized arene that has often been utilized as a precursor to still other chemical. Salicylaldehyde is converted to chelating ligands by condensation with amines.

Schiff bases are ketone- or aldehyde-like compounds in which the carbonyl group is substituted by an imine or azomethine group¹. Schiff bases are resourceful ligands prepared from the condensation of an amino compound with carbonyl compounds^2-5 and were first described by Hugo Schiff in 1864. Formation of Schiff base usually takes place under acid or base catalysis or with heat. The Schiff bases are typically crystalline solids, which are weakly basic but at least some form insoluble salts with strong acids³. Nowadays, Schiff bases are used as intermediates for the synthesis of amino acids or as ligands for synthesis of metal complexes having a series of diverse structures³.

Schiff bases are the most extensively used organic compounds ⁶for industrial purposes and also display a comprehensive range of biological activities¹^,^7-9.

Schiff base ligands and their metal complexes are very essential as catalysts in various biological systems, polymers and dyes, water splitting, medicinal and pharmaceutical fields²^,^10-13. They encompassvarious therapeutically effective applications in the field of medicinal chemistry¹⁴. Their application in birth control, food packages and as an O₂detector is likewise outlined ¹³. Schiff’s bases metal complexes also used in quantitative analysis as an analytical chemical reagents and/or separation reagentsand synthetic applications in the field of inorganicand organic chemistry¹⁴. Moreover, Schiff bases have been expose to show a broad range of biological activities, including antifungal, antibacterial, antimalarial, antiproliferative, anti-inflammatory, antiviral, and antipyretic properties¹^,⁶.

In this study, we present the evaluation of the antibacterial activity and thermal behaviour of Cu(II), Co(II), Ni(II) and Mn(II) complexes with a ONO Schiff base ligand derived from the condensation of o-aminophenol with salicylaldehyde. The synthesis and characterization of all the metal complexes of the Schiff base ligand is also presented.

EXPERIMENTAL:

Chemicals and Instrumentation:

All the chemicals were of reagent grade (supplied by Sigma-Aldrich or Merck) and used as supplied.

FT-IR spectra were obtained on a FT-IR spectrophotometer [JASCO, FT-IR/4100 Japan], using KBr pellets as the standard reference. ESI-MS spectra were recorded with an Agilent Technologies MSD SL Trap mass spectrometer with ESI source coupled with an 1100 Series HPLC system. The UV/Visible spectra were obtained from PerkinElmer Lambda 25 spectrophotometer. Microanalyses for CHN were performed using a Vario EL cube [Germany elements (Elemental) analysis system]. The melting points of synthesized compounds were determined using a digital melting point apparatus (METTLER TOLEDO). A Sherwood Scientific MX Gouy magnetic susceptibility apparatus was used to determine the magnetic moments of the metal complexes. The thermogravimetric analysis was carried out using a thermobalance of the type Mettler Toledo STARe System.

Synthesis of Ligand C₁₃H₁₁NO₂ (SB):

Schiff base (SB) ligand was prepared by ethanolic solution of 2-aminophenol (10mmol) in a round bottomed flax which was magnetically stirred and followed by dropwise addition of salicylaldehyde (10mmol). The solution was mixed and refluxed for 3-4 hours at 90^⁰C. On cooling off reddish orange colored product was formed which was washed with ethanol and acetone and collected by filtration and dried in vacuum desiccator over anhydrous CaCl₂. The purity of ligand was checked by TLC using different solvents. The product was found to be soluble in DMSO, DMF, Acetonitrile (hot), and hot methanol.

General Methods of Synthesis of Schiff Base (SB) Metal Complexes:

A hot ethanolic solution (20mL) of metal [Cu(II)/Co(II)/Ni(II)/Mn(II)] acetate (1mmol) was added slowly into a solution of Schiff base (SB) ligand (1mmol) in 20mL of hot ethanol with constant stirring. Then the reaction mixture was refluxed and heated for 3h at 90°C. After cooling, the solid product was removed by filtration, washed several times with ethanol and dried the products in a desiccator over anhydrous CaCl₂ under vacuum. The purity of the formed products were checked using TLC.

Scheme 2 Synthesis pathway of Schiff base (SB) metal complexes [M = Cu(II), Co(II), Ni(II) and Mn(II)]

Metal Content Estimation:

A known quantity of metal complex was put into a conical flask whose weight was known. Then, concentrated H₂SO₄ (500mL) was added. It was fumed until dry and the process was repeated. Concentrated HNO₃ (500mL) and HClO₄ (500mL) were then added and the mixture was further fumed until dry. The process of adding acids and fuming was continued until there was no black material. 100mL distilled water was added to dissolve the residue. Finally, the weight of the metal was estimated complexometrically using EDTA (Ethylenediamine tetra acetic acid)¹⁵ and gravimetrically using DMG (Dimethylglyoxime)¹⁶.

Antibacterial Activity Study:

The Schiff base ligand and its metal complexes were screened for their in vitro antibacterial activity against Bacillus cereus (gram-positive) and Escherichia coli (gram-negative). The qualitative antibacterial activity testing of the synthesized compounds were evaluated using the disc diffusion technique¹⁷^,¹⁸.

Each test organism was inoculated onto a nutrient agar plate and incubated at 37⁰C for 24 hrs to get the primary culture. Numerous discrete colonies were picked from the culture to make a bacterial suspension (10mL) in a test tube using saline water. The turbidity of the suspension was compared with 0.5 McFarland standard to obtain 10⁶-10⁸ CFUs. The bacterial suspension (0.1 mL) was inoculated onto Mueller Hinton plate and the sterile discs that have been impregnated with the test compounds were firmly placed on it. The assay was inoculated at 37⁰C for 16 hrs and the zone of inhibition was measured as millimeters diameter. Ampicillin and dimethylsulphoxide (DMSO) were used as standard antibacterial drug and control solvent respectively.

RESULT AND DISCUSSION:

The synthesized compounds were characterized by melting point, conductivity and magnetic susceptibility measurements, Infra-Red (IR), Ultraviolet-visible (UV-vis) and Electrospray ionisation mass (ESI-MS) spectroscopy and thermal analysis ¹⁹.

Melting Point:

Melting point values gives an approximate idea about the nature of the compounds²⁰. The melting point or decomposition temperatures of all the synthesized compounds were observed in a digital melting point apparatus (METTLER TOLEDO). It could not measure the melting points above the temperature 300°C.

Fig. 1 (a) 3D structure, (b) Highest Occupied Molecular Orbital (HOMO) and (c) Lowest Unoccupied Molecular Orbital (LUMO) of Cu(II) complex.

Table 1 Physical characteristics and analytical data of SB (C₁₃H₁₁NO₂) and its metal complexes.

Compound/ Empirical Formula	Formula Weight (g/mol)	Color	Yield (%)	Melting point/ Decomposition Temperature (°C)	Conductivity (ohm^-1cm²mol^-1)
Ligand, SB C₁₃H₁₁NO₂	213.23	Reddish - Orange	85	182-185	1
[CuSB(H₂O)] (C₁₃H₁₁NO₃)Cu	292.77	Deep Green	72	>300	3
[CoSB(H₂O)] (C₁₃H₁₁NO₃)Co	288.16	Greenish-Brown	81	>300	2
[NiSB(H₂O)] (C₁₃H₁₁NO₃)Ni	287.92	Reddish- Brown	70	>300	1
[MnSB(H₂O)]H₂O [(C₁₃H₁₁NO₃)Mn]H₂O	284.16	Deep Yellow	65	>300	3

Conductivity Measurements:

The conductivity values for the synthesized compounds in DMSO solvent (1.0×10^-3M) are presented in Table 1. Conductivity measurements have commonly been used to know the nature of the coordination compounds.In order to determine whether the coordination compound is ionic or non-ionic, conductance measurements play an important role²¹. Molar conductance values of all the synthesized compoundswere inthe range 1-3 ohm^-1cm^-2mol^-1, very low conductance indicates the non-electrolyticnature of the complexes.

Magnetic Measurements:

The magneto chemistry is one of the most valuable methods in the investigation of transition metal complexes²²^,²³. The measurement of magnetic properties can provide information of the electronic structure, oxidation state of metal ion and in some cases stereochemistry and nature of bonding in metal complexes.

The observed effective magnetic moment values of the complexes of at room temperature are given in Table 2. Figgissuggested normal μ_eff value greater than 1.9 B.M. for tetrahedral and less than 1.9 BM for square planar or pseudo octahedral Cu(II) complexes²⁴.

The observed magnetic moment of Cu(II) complex in the present investigation were found 1.86 B.M. at room temperature corresponding to one unpaired electron, indicative of square planar geometry and paramagnetic in nature.

The Co (II) ion having d⁷ configuration, forms good number of complexes in several stereochemical types. Most of them were found to have either octahedral or tetrahedral geometry. Nevertheless, quite a good number of complexes of Co(II) with low spin square planar and five coordinated geometries were reported. Co(II) complexes with three unpaired electrons may be either octahedral or tetrahedral ²⁵. Cobalt (II) forms generally tetrahedral complexes than any other transition metal ion. This is because cobalt (II) ion has a d⁷configuration and for this configuration the ligand field stabilization energies favor the tetrahedral configuration relative to the octahedral configuration. In the present work Co(II) complex shows magnetic moment 2.14 B.M. at room temperature as given in Table 2 which supports the low spin tetrahedral geometry of the complex.

The Ni(II) has 3d⁸ configuration and is widely studied in coordination complexes. The key structural types of Ni(II) are octahedral, tetrahedral and square planar. Strong ligand fields pair up the unpaired electrons of Ni(II) complexes giving rise to formation of diamagnetic square planar or trigonal bipyramidal complexes. The color of the complexes themselves gives an indication of the geometry in many instances, octahedral Ni(II) complexes and sometimes tetrahedral complexes are blue purple and green whereas square planar complexes are red, yellow or brown ²⁶. The magnetic moment value (1.05 B.M) and the color of the complex is the indication of square planar geometry with diamagnetic nature.

The Mn(II) has d⁵ configuration. The divalent state of manganese is the most stable and key oxidation state. It forms wide-ranging series of salts with all common anions. The most common stereochemical state of Mn(II) in its complex is octahedral. However, the complexes of other geometries such as tetrahedral, square planer, 5-coordinate and 7-coordinate have been reported ²⁷. Thus the magnetic moment value is 5.09 B. M, demonstrates that the Mn(II) complex is paramagnetic and has high spin tetrahedral geometry.

Infrared Spectral Study:

IR spectroscopy is used for the structural investigation of metal complexes. It is recommended that when metal ion combines with the ligand to form complex, its vibrational spectra expected to change. The change in the vibration can be related to molecular symmetry or with the change in the individual frequency. The metal complex spectra is compared with ligand spectra.

The spectrum of the ligand showed a strong absorption band at 1636 cm^-1 due to the azomethine ν(C═N) stretching frequency of the free ligand indicating that condensation is occurred between the CHO moiety of salicylaldehyde and ‒NH₂ moiety of 2-aminophenol²⁸. The observed band at 3446 cm^-1 which may be assigned to the ν(OH) of hydroxyl group²⁹. The absence of bands at 1735 and 3420 cm^-1 due to carbonyl ʋ(C═O) and ν(NH₂) stretching vibration indicating that aldehyde and amino moiety of the starting reagents converted into the azomethine moiety (Fig. 2).

The coordinated water molecule associated with the copper complex which is assigned to ν(OH) band at 3435 cm^-1²⁹. On complexation, the strong band observed in the IR spectra of the free ligand at 1636 cm^-1due to ν(C═N) shifted to lower frequency (1634 cm^-1) confirming the coordination of azomethine nitrogen atom to the Cu(II) ion³⁰^,³¹. Another two new bands appeared at 710 and 538 cm^-1 were assigned to Cu─O and Cu─Nbond frequency respectively and it was the conclusive evidence for the bonding of azomethine nitrogen and phenolic oxygen of the Schiff base ligand (SB) to copper ion³²^,³³ (Fig.2).

The band observed at 3447 cm^-1is assigned to ν(OH) which was attributed to coordinated water molecule in the cobalt complex. The strong band observed at 1636 cm^-1in the spectrum of free liganddue toν(C═N) shifted to lower frequency (1633 cm^-1) when coordinated to Co(II) ion. This is the most characteristic feature for the participation of azomethine nitrogen atom in coordination. The bands at 745 and 566 cm^-1 were assigned to Co─O and Co─N bond stretching which supports coordination via azomethine nitrogen atom and phenolic oxygen atom to cobalt ion (Fig.2).

The band appeared at 3436 cm^-1was due to coordinated water molecule in the nickel complex. The coordination of azomethine nitrogen atom to Ni(II) ion was confirmed by the appearance of a peak at the lower frequency region at 1615 cm^-1than the ligand.The peaks at 746 and 521cm^-1 were assigned to Ni─O and Ni─N bond stretching and itwas thestrong evidence for the bonding of azomethine nitrogen and phenolic oxygen of the Schiff base ligand to nickel ion (Fig. 2).

The broad band appeared at 3450 cm^-1 supportsthe coordination of water molecule to Mn(II) ion. In the spectra of free ligand, the strong band found at 1636 cm^-1due to ν(C═N) bond stretching was shifted to lower frequency region by 21 cm^-1 on complexation confirming the coordination of azomethine nitrogen atom to Mn(II). Newly formed another two bands appeared at 746 and 514 cm^-1 were assigned to Mn─O and Mn─Nbond stretching respectively and itwas the indication of coordination via azomethine nitrogen and phenolic oxygen to manganese ion (Fig. 2).

UV-vis Spectra:

The structure of metal complexes can be predicted from interpretation of their electronic absorption spectra and comparing them with the electronic absorption spectra of corresponding ligands and earlier reported studies.

The UV-vis spectrum of the ligand (SB) and its Cu (II), Co (II), Ni (II) and Mn (II) complexes were recorded in DMSO solvent at room temperature.

The UV-vis spectrum of the ligand displayed two high intensity absorption bands a 261nm and 299nm which assigned to n→π^*and π→π^*transition due to ˃C=N and ˃C=O groups³⁴. These transitions were observed in the spectrum of the metal complexes but they shifted to longer or lower wavelength and thus it confirmed the formation of metal-ligand complexes.

In the UV-vis spectra, copper complex exhibits intense ligand to metal charge transfer transitions (LMCT) at 259nm. A less intense absorption band due to d→d transitions observed at 441nm, suggesting a square-planar geometry of Cu(II) ion³⁵.

On the basis of simplest model three spin-allowed bands are expected in tetrahedral cobalt (II) complexes. UV-Vis spectra of cobalt complex were obtained from solid sample using diffuse reflectance technique. The compound shows a broad shoulder at 437nm is due to 𝑑→𝑑transition. Absorption at 306nm is assigned to charge transfer from the nonbonding orbital of phenolic oxygen to half-filled d orbitals of cobalt(II)³⁶. Absorption at 263nm is associated with 𝜋→𝜋^∗or 𝑛→𝑛∗transitions of the ligand³⁷.

The electronic spectrum of the nickel complex shows three bands at 260nm, 303nm and 445nm, assigned to π→π^*, charge transfer (CT) and d→d transitions suggesting the square-planar geometry of Ni(II) ion³⁴.

The electronic spectra of Mn(II) complex displayed three bands at 258nm, 298nm and 451nm, assigned to π→π^* charge transfer (CT) and d→d transitions confirming tetrahedral geometry of Mn(II) ion³⁸.

Table 4. Magnetic moments and Electronic spectral data of the ligand C₁₃H₁₁NO₂ (SB) and its metal complexes.

Ligand / Metal Complexes

λ_max

Wavenumber

cm^-1

µ_eff

B.M.

Assignment

C₁₃H₁₁NO₂ (SB)

261

299

38314

33444

π→π^*

n→π^*

[(C₁₃H₁₁NO₃)Cu]

259

441

38610

22675

1.86

LMCT

d→d

[(C₁₃H₁₁NO₃)Co]

263

306

437

38022

32679

22883

2.14

π→π^*

C.T

d-d

[(C₁₃H₁₁NO₃)Ni]

260

303

445

38461

33000

22471

1.05

π→π^*

C.T

d→d

[(C₁₃H₁₁NO₃)Mn]H₂O

258

298

451

38759

33557

22172

5.09

π→π^*

C.T

d→d

Fig. 3 UV-vis spectra of synthesized compounds.

Fig. 4ESI-MS Spectra of (a) Ligand, C₁₃H₁₁NO₂ (SB) (b) Copper complex, [(C₁₃H₁₁NO₃)Cu] (c) Cobalt complex, [(C₁₃H₁₁NO₃)Co] (d) Nickel complex, [(C₁₃H₁₁NO₃)Ni] and (e) Manganese complex, [(C₁₃H₁₁NO₃)Mn] complex.

Electrospray Ionization Mass Spectrometry (ESI-MS):

ESI-MS analysis enables us to directly determine the complexes formed in the solution ³⁹. ESI-MS can provide important information concerning the structure, stoichiometry, and metal oxidation state of dissolved metal complexes⁴⁰.

The ESI-Mass spectra of the ligand and complexes are presented in Fig. 4. The obtained m/z values are similar to the formula weight (Table 1 and 3) which further supports the proposed structure of the synthesized compounds.

Thermogravimetric analysis:

From thermal analysis, the properties, nature of intermediates and final products of the thermal decomposition of coordination compounds can be obtained⁴¹. From TGA curves, the mass loss was calculated for the different steps and compared with those theoretically calculated for the suggested formulae based on the results of elemental analyses and FTIR and ESI-MS together with the molar conductance measurements. The found and calculated mass losses, relative residues and temperature observed in each step of TGA/DTA curves are given in Table 5. An exothermic peak found in the DTA curves for all the complexes confirmed the data obtained from the TGA curves.

Thermal analysis of metal complexes were analyzed under nitrogen atmosphere and heating rate was linearly increased at 10^°C min^-1 and the weight loss was checkedfrom the ambient temperature to 800^°C. TGA and DTA clearly showed that the decomposition of the complexes proceed in two or three steps. The presence of coordinated water molecule to the complexes is confirmed by the thermal analysis.

The TGA and DTA curve of Cu(II) complex shown in Fig. 5indicated the decomposition of the complex into two steps. In the first step, coordinated water and organic ligand moiety were eliminated at the temperature range 300-440°C (calculated 72.85%, experimental 71.85%). In the second step above the 440°C temperature the complex was decomposed and removed as CuO (calculated 27.17 %, experimental 29.95%)⁴².

The TGA results of the cobalt complex (Fig. 5) showed that the complex decomposed mainly in two steps. The first step appeared within the temperature range 280-420°C was due to the elimination of coordinated water molecule along with the major parts of ligand (calculated 74.02%, experimental 73.73%)⁴³. Above 420ºC the complex was removed as CoO (calculated 26.01%, experimental 30.47%).

The TGA curve for Ni (II)complex (Fig. 5) displays two stages of mass loss. The first stage is at 410-600°C, and exhibits a mass loss of 72.60%, corresponding to the loss of C₁₃H₉NO and coordinated H₂O molecule (calc. 74.04%). The final stage occurs above 600°C and here the complex removed as NiO.

The TGA and DTA curve of Mn(II) complex exhibited three steps decomposition (Fig. 5). In the first step one molecule of hydrated water was lost at the temperature range 130-180°C (calculated 5.96%, experimental 7.70%). The loss of water molecule in this temperature range indicates that the water moleculewas lattice type⁴⁴. In the second step coordinated water and organic ligand moiety were eliminated at the temperature range 380-510°C (calculated 65.26%, experimental 65.5%). In the final step above the 510°C temperature the complex removed as MnO (calculated 25.95 %, experimental 27.20%).

Fig. 5 TGA and DTA curve for the metal complexes.

Table 5 Thermal data of Cu(II),Co(II),Ni(II) and Mn(II) complexes of Schiff Base ligand C₁₃H₁₁NO₂ (SB).

Complexes

Steps

Temperature

Range (°C)

DTA

Peak (°C)

TGA mass (%)

Calc/found

Assignment

[(C₁₃H₁₁NO₃)Cu]

1^st

2^nd

300-440

>440

425

72.85/71.85

27.17/29.95

C₁₃H₉NO+H₂O

CuO

[(C₁₃H₁₁NO₃)Co]

1^st

2^nd

280-420

>420

390

74.02/73.73

26.01/30.47

C₁₃H₉NO+H₂O

CoO

[(C₁₃H₁₁NO₃)Ni]

1^st

2^nd

410-600

>600

570

74.04/72.60

25.95/28.10

C₁₃H₉NO+H₂O

NiO

[(C₁₃H₁₁NO₃)Mn]H₂O

1^st

2^nd

3^rd

130-180

380-510

>510

150

440

5.96/7.70

65.26/65.5

28.78/27.2

H₂O

C₁₃H₉N+H₂O

MnO

Antibacterial Activity Studies:

The prepared ligand and its complexes were screened for their antibacterial activity against Bacillus cereus (gram-positive) and Escherichia coli (gram-negative). The standard drug ampicillin were also tested for its antibacterial activity at the same concentration and conditions similar to that of the tested compounds concentration. The antimicrobial results are presented in Table 6. From the antibacterial studies, it is inferred that the Schiff base ligand and its complexes under investigation exhibited moderate activity against both the organisms. From the obtained results, it can be observed that the complexes showed greater activity as compared to the Schiff base. The improved activities of the metal complexes as compared to the ligand can be explained on the basis of chelation theory⁴⁵. This theory explains that a decrease in the polarizability of the metal could enhance the lipophilicity of the complexes, which leads to a breakdown of the permeability of the cells, resulting in interference with normal cell processes⁴⁶. This indicates that the chelation tends to make the Schiff base act as more powerful and potent antimicrobial agents, thus inhibiting the growth of bacteria more than the parent Schiff base⁴⁷^,⁴⁸.

Table 6 Antibacterial activities of the compounds.

Compounds	Inhibition zone diameter (mm)
Compounds	*Bacillus cereus* (Gram-positive)	*Escherichia coli* (Gram-negative)
Control: DMSO	0	0
Ampicillin	25	20
Ligand	10	13
Cu-Complex	20	15
Co-Complex	21	16
Ni-Complex	15	14
Mn-Complex	17	15

CONCLUSION:

The Schiff base (SB) ligand derived from 2-aminophenol and salicylaldehyde and its metal complexes with Cu(II), Co(II), Ni(II) and Mn(II) ions were synthesized and characterized by elemental and thermal analyses, spectral, and magnetic data. The IR, and UV-Vis spectra revealed that the ligand coordinate in tridentate modes to metal ions forming square planar and tetrahedral complexes. ESI-MS spectral data are in agreement with the elemental analyses, confirming the compositions and purities of the metal complexes. All the synthesized complexes exhibited moderate activity against bacteria (Bacillus cereus and Escherichia coli), thus giving a new thrust of such type of compoundsin the field of metallo-drugs (Bio-Inorganic Chemistry). Metallation increased the bacterial activity compared with the free ligand.

CONFLICT OF INTEREST:

The authors declare no conflict of interest.

ACKNOWLEDGEMENT:

This paper was funded by the Faculty of Science, Rajshahi University, Rajshahi-6205, Bangladesh. The authors, therefore, acknowledge with thanks Faculty of Science, Rajshahi University for technical and financial support.

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Received on 23.04.2020 Modified on 18.05.2020

Asian J. Research Chem. 2020; 13(4):265-274.

DOI: 10.5958/0974-4150.2020.00052.8

Complexes	Length of the sample (l) in cm	Weight of the sample (m) in g	Susceptibility of the empty tube, R_o	Susceptibility of the sample with tube, R	µ_eff in B.M.	No of unpaired electron
[CuSB(H₂O)]	1.5	0.0195	-003	+027	1.86	1
[CoSB(H₂O)] (C₁₃H₁₁NO₃)Co	1.8	0.0115	-007	+023	2.14	1
[NiSB(H₂O)] (C₁₃H₁₁NO₃)Ni	1.5	0.0680	-005	-001	1.05	Dia
[MnSB(H₂O)]H₂O [(C₁₃H₁₁NO₃)Mn]H₂O	1.8	0.0218	-006	+194	5.09	4

Ligand / Metal Complexes	IR /cm^-1				ESI-MS
Ligand / Metal Complexes	ν (O-H)	ν (C═N)	ν (M-O)	ν (M-N)	ESI-MS
C₁₃H₁₁NO₂ (SB)	3446	1636	-	-	213.2054
[(C₁₃H₁₁NO₃)Cu]	3435	1634	710	538	292.7590
[(C₁₃H₁₁NO₃)Co]	3447	1633	747	566	288.1417
[(C₁₃H₁₁NO₃)Ni]	3436	1615	746	521	287.9216
[(C₁₃H₁₁NO₃)Mn]H₂O	3450	1615	746	514	284.1656