Bioinorganic Chemistry and Applications

Volume 2013 (2013), Article ID 261731, 13 pages

http://dx.doi.org/10.1155/2013/261731

Research Article

Spectroscopic, Thermal, Electrochemical, and Antimicrobial Studies of Mononuclear Manganese(II) Ditolyldithiophosphates

Ruchi Khajuria, Atiya Syed, Sandeep Kumar, and Sushil K. Pandey

Department of Chemistry, University of Jammu, Baba Saheb Ambedkar Road, Jammu and Kashmir 180 006 , India

Received 29 April 2013; Accepted 19 June 2013

Academic Editor: Imre Sovago

Copyright © 2013 Ruchi Khajuria et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

New complexes of manganese(II) corresponding to [Mn] and [Mn.nL] (CH₃C₆H₄ and p-Cl-m-CH₃C₆H₃; , L = N₂C₁₂H₈, N₂C₁₀H₈; , L = NC₅H₅, P(C₆H₅)₃) have been synthesized and characterized by microelemental analyses (C, H, and N), magnetic susceptibility, molar conductance, thermogravimetric, cyclic voltammetry, and spectral analyses including ESI mass spectrometry, IR, and UV-visible. The presence of a four-and-six coordinated Mn atoms has been established in the complexes and adducts, respectively. Antimicrobial screening of the complexes against gram negative bacteria E. coli, K. pneumonia, and P. aeruginosa and fungus S. rolfsii has shown potential bioactivity.

1. Introduction

Manganese has attracted the attention of several researchers owing to the fascinating physical and biochemical characteristics. The utility of manganese is quite diversified such as an alloying agent for aluminium, in dry cell batteries (carbon-zinc Leclanche type), oxidizing agent, and so forth [1]. In nature, manganese is present at the active site of a wide range of enzymes such as catalase and ribonucleotide reductase and participates in a variety of biological reactions [2]. Manganese(II) in its coordination compounds exhibits marked preference for hard donor atoms such as oxygen and nitrogen. Manganese(II) complexes containing soft donor atoms are scanty in number as compared with those of other transition metals, presumably due to difficulties encountered in preparing and preserving such complexes [3]. The first evidence for manganese binding to sulfur donor group in metalloprotein is tryptophan-modified Mn(III) containing acid phosphatise [4]. As compared to the other transition metals, manganese forms complexes with 1,1-dithio and 1,2-dithiolene ligands such as [Mn(SPh)₄]²⁻, [MnCl(SPh)₃]²⁻, [Mn₄(SPh)₁₀]²⁻, [Mn(edt)₂]²⁻, [Mn₂(edt)₄]²⁻ (edt = ethane-1,2-dithiolate), and [Mn₂(S₂-O-xyl)₂X₂]²⁻ (X = PhS⁻, Et₂NCS₂⁻; X₂ = S₂-O-xyl²⁻) [5].

The high efficacy of the ligands containing sulfur atoms has been well established in metal chelation therapy and has high potency as fungicides and pesticides [6]. Among sulfur donor ligands, O,O′-dialkyl and alkylene dithiophosphates have been used for producing potential coordination compounds with most of the metals [7–10]. These soft donor ligands are versatile ligands which show both monodentate [11, 12]and bidentate [13–18] behaviour and form complexes with various metal ions, mainly transition metals [16–18]. Various dithiophosphate derivatives find extensive applications in agriculture [19], industries [20, 21], analytical studies [22], and tribology [23–25]. The versatility of applications of dithiophosphates and their ability to combine with metal ions into a variety of coordination complexes have led to a large interest in their structure and the factors governing the properties of dithiophosphate complexes. The best known complexes of manganese with sulfur ligands are dithiophosphinates [26, 27] and the dithiocarbamates [28]. A perusal of the literature reveals no report on the manganese(II) ditolyldithiophosphates. In continuation of our earlier work on ditolyldithiophosphates derivatives, we report in this paper the synthesis and characterization of manganese(II) ditolyldithiophosphates and their adducts with nitrogen and phosphorus donor bases like 1,10-phenanthroline, N₂C₁₂H₈, 2,2′-bipyridyl, N₂C₁₀H₈, pyridine, NC₅H₅, and triphenylphosphine, P(C₆H₅)₃. The electrochemistry of manganese complexes has also been investigated. The complexes were also screened for their antimicrobial activities against some bacteria and fungus.

2. Experimental

2.1. Materials and Methods

Stringent precautions were taken to exclude moisture. Moisture was carefully excluded throughout the experimental manipulations by using standard Schlenk techniques. Ortho-, meta-, and parahydroxytoluene (cresols) were distilled before use. Manganese dichloride tetrahydrate (Thomas baker) was used as received. Sodium salts of O,O′-(o-, m-, p- and p-Cl-m-ditolyl)dithiophosphoric acids were prepared by the literature method [29]. The solvents used, benzene, acetone, dichloromethane, hexane, and ethanol were purified and dried by standard methods before use.

2.2. Physical Measurements

Chlorine was estimated volumetrically by Volhard’s method [30]. Manganese was estimated gravimetrically as Mn₂P₂O₇ [30]. IR spectra were recorded in KBr pellets in the range of 4000–200 cm⁻¹ with a Perkin Elmer-spectrum RX1 FT-IR spectrophotometer and Shimadzu FT-IR-8400S. The electronic spectra of the complexes were recorded in a range of 200–800 nm on a T90 + UV/VIS spectrophotometer using a pair of matched quartz cells of 10 mm path length at an ambient temperature. The mass spectra were recorded on VG-70-S mass spectrophotometer. The room temperature magnetic susceptibility measurements have been carried out by vibrating sample magnetometer (VSM). Molar conductance of the complexes was determined by using digital conductivity meter Century CC 601. The thermograms were analyzed by using Perkin Elmer, diamond TG/DTA instrument. Recrystallized alumina sample holder was used with the heating rate of 20°C per minute. The thermograms were recorded in the temperature range from 30°C to 1000°C. The experiment was carried out under a flow rate of 50 mL per minute of nitrogen atmosphere. The cyclic voltammograms were recorded on Autolab. The potential is applied between the reference electrode (Ag/AgCl) and the working electrode (Gold electrode), and the current is measured between the working electrode and the counter electrode (platinum wire). For all the measurements, 0.1 M phosphate buffer solution (pH = 7) was used. Elemental analyses (C, H, and N) were carried out on Vario EL III and CHNS-932 Leco Elemental analyser; their results were found to be in good agreement (±0.3%) with the calculated values.

2.3. Synthesis of [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] (1)

An Acetone solution (30 mL) of MnCl₂.4H₂O (0.30 g, 1.51 mmol) was added dropwise to acetone solution of (o-CH₃C₆H₄O)₂PS₂Na (1.00 g, 3.00 mmol) with constant stirring and then refluxed for 4 h. The color of contents changed to colorless with the precipitation of sodium chloride. The precipitates of sodium chloride were filtered off by using a funnel fitted with G-4 disc. Excess of solvent from the filtrate was evaporated in vacuo which resulted in the formation of the complex [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] (1) as white solid in 95% yield. The complexes 2–4 were prepared by similar procedure.

2.4. Synthesis of [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·N₂C₁₂H₈] (5)

A mixture of MnCl₂.4H₂O (0.30 g, 1.51 mmol) and N₂C₁₂H₈ (0.27 g, 1.49 mmol) in water-ethanol medium (30 mL) was added to the aqueous solution of (o-CH₃C₆H₄O)₂PS₂Na (1.00 g, 3.01 mmol) in a dropwise manner through a dropping funnel. Immediately, the yellow colored precipitates were formed which were separated by filteration using funnel fitted with G-4 disc. The product [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·N₂C₁₂H₈] (5) was obtained as yellow solid in 94% yield. Adducts numbered 6–16 were isolated by similar route.

2.5. Synthesis of [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·2P(C₆H₅)₃] (17)

Acetone solution (30 mL) of (o-CH₃C₆H₄O)₂PS₂Na (1.00 g, 3.01 mmol) was added to a acetone solution (10 mL) of MnCl₂.4H₂O (0.30 g, 1.50 mmol) with constant stirring at room temperature followed by the addition of the solution of P(C₆H₅)₃ (0.79 g, 3.01 mmol) in acetone (10 mL). The reaction mixture was refluxed for 4 h. The precipitated sodium chloride was removed by filtration using a funnel fitted with G-4 disc. The desired product [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·2P(C₆H₅)₃] (17) was obtained in 90% yield as white solid. Complexes numbered 18–20 were prepared by this method.

2.6. Biological Activity

2.6.1. Antibacterial

Test samples were prepared in different concentrations (100, 150, 200, and 250 ppm) in DMSO. Agar medium (20 mL) was poured into each petri plate and left to solidify. The plates were then swabbed with broth cultures of the respective microorganisms (E. coli, K. pneumonia, and P. aeruginosa) and kept for 15 min for adsorption to take place. Using a punch, ≈6 mm diameter, wells were bored in the seeded agar plates and 100 μL of the DMSO solution of each test compound was added into the wells. DMSO was used as the control for all the test compounds as it exhibited no effect on the organism tested, and Ciprofloxacin was used as the standard drug. After holding the plates at room temperature for 2 h to allow diffusion of the compounds into the agar, the plates were incubated at 37°C for 24 h. The antibacterial activity was determined by measuring the diameter of the inhibition zone. The entire tests were made in triplicates, and the mean of the diameter of zone of inhibition was calculated.

2.6.2. Antifungal

Potato dextrose medium (PDA) was prepared in a flask and sterilized. 100 μL of each sample was added to the PDA medium and poured into each sterilized Petri plate. Mycelial discs taken from the standard culture (S. rolfsii) of fungi were grown on PDA medium for 7 days. These cultures were used for aseptic inoculation in the sterilized Petri dish. Standard cultures, inoculated at °C, were used as control. The efficacy of each sample was determined by measuring the radial fungal growth. The radial growth of the colony was measured in two directions at right angle to each other, and the average of two replicates was recorded in each case. Data were expressed as percent inhibition over the control from the size of the colonies. The percent inhibition was calculated using the formulae: % inhibition = (()/C) × 100, where C is the diameter of the fungus colony in the control plate after 96 h incubation and T is the diameter of the fungus colony in the tested plate after the same incubation period. Both antibacterial and antifungal activities were tested in the Bio-assay lab, Department of Chemistry, University of Jammu, Jammu.

3. Results and Discussion

The reactions of manganese dichloride (tetrahydrate) with sodium salt of O,O′-(o-, m-, p- and p-Cl-m-ditolyl)dithiophosphates in refluxed acetone with constant stirring in 1 : 2 molar stoichiometry yielded the complexes corresponding to [{(ArO)₂PS₂}₂Mn] (1–4) (Ar = o-, m-, p-CH₃C₆H₄ and p-Cl-m-CH₃C₆H₃). The adducts were prepared by the reaction MnCl₂·4H₂O, [(o-, m-, p-CH₃C₆H₄O or p-Cl-m-CH₃C₆H₃O)₂PS₂Na] and donor base in 1 : 2 : 1 and 1 : 2 : 2 molar stoichiometry for bidentate (N₂C₁₂H₈ (5–8) and N₂C₁₀H₈ (9–12)) and monodentate donor ligand, (NC₅H₅ (13–16) and P(C₆H₅)₃ (17–20)), respectively (Scheme 1). It is pertinent to mention that adducts of manganese ditolyldithiophosphates with nitrogen donors were prepared in water-ethanol medium, while adducts with triphenylphosphine were prepared in acetone. This choice of solvent medium was made on the basis of better solvation of triphenylphosphine in acetone in comparison to water-ethanol medium.

Scheme 1: Reactions showing synthesis of O,O′-(o-, m-, p- and p-Cl-m-ditolyl) dithiophosphato complexes of Mn(II) and their adducts with nitrogen and phosphorus donor ligands.

All these complexes are soluble in common organic solvents and insoluble in solvents like n-hexane and carbon tetrachloride. The complexes (1–4) obtained are white solids, whereas adducts with nitrogen donors are yellow (5–16), and that of phosphorus donor are white (17–20) solids. The synthetic and analytical data of the complexes (1–20) are summarized in Table 1.

Table 1: Synthetic and analytical data for the Mn(II) ditolyldithiophosphates.

S. no.
Complex
Physical state*  

Yield (%)
m.p. (°C)
Molar conductance

(Ω⁻¹ cm² mol⁻¹)
Analysis found (calcd.) (in %)

C
H
S
N
Cl
Mn

1.
[{(o-CH₃C₆H₄O)₂PS₂}₂Mn]
White

(95)
55
4.62
49.89

(49.92)
4.17

(4.19)
19.03

(19.04)
—
—
8.13

(8.16)

2.
[{(m-CH₃C₆H₄O)₂PS₂}₂Mn]
White

(94)
56
5.62
49.89

(49.92)
4.18

(4.19)
19.02

(19.04)
—
—
8.12

(8.16)

3.
[{(p-CH₃C₆H₄O)₂PS₂}₂Mn]
White

(98)
55
5.30
49.90

(49.92)
4.18

(4.19)
19.02

(19.04)
—
—
8.13

(8.16)

4.
[{(p-Cl-m-CH₃C₆H₃O)₂PS₂}₂Mn]
White

(97)
57
6.42
41.42

(41.44)
2.96

(2.98)
15.79

(15.81)
—
17.46

(17.48)
6.75

(6.77)

5.
[{(o-CH₃C₆H₄O)₂PS₂}₂Mn · N₂C₁₂H₈]
Yellow

(94)
61
7.63
56.23

(56.26)
4.24

(4.25)
15.00

(15.02)
3.26

(3.28)
—
6.40

(6.43)

6.
[{(m-CH₃C₆H₄O)₂PS₂}₂Mn · N₂C₁₂H₈]
Yellow

(92)
63
7.63
56.24

(56.26)
4.22

(4.25)
15.00

(15.02)
3.25

(3.28)
—
6.41

(6.43)

7.
[{(p-CH₃C₆H₄O)₂PS₂}₂Mn · N₂C₁₂H₈]
Yellow

(97)
62
6.54
56.24

(56.26)
4.22

(4.25)
14.99

(15.02)
3.27

(3.28)
—
6.40

(6.43)

8.
[{(p-Cl-m-CH₃C₆H₃O)₂PS₂}₂Mn · N₂C₁₂H₈]
Yellow

(98)
63
9.81
48.43

(48.45)
3.22

(3.25)
12.90

(12.93)
3.22

(3.25)
14.28

(14.30)
5.51

(5.54)

9.
[{(o-CH₃C₆H₄O)₂PS₂}₂Mn · N₂C₁₀H₈]
Yellow

(96)
60
4.36
54.99

(55.00)
4.35

(4.37)
15.43

(15.46)
3.37

(3.38)
—
6.60

(6.62)

10.
[{(m-CH₃C₆H₄O)₂PS₂}₂Mn · N₂C₁₀H₈]
Yellow

(91)
61
7.63
54.97

(55.00)
4.34

(4.37)
15.44

(15.46)
3.36

(3.38)
—
6.59

(6.62)

11.
[{(p-CH₃C₆H₄O)₂PS₂}₂Mn · N₂C₁₀H₈]
Yellow

(97)
60
5.45
54.97

(55.00)
4.36

(4.37)
15.43

(15.46)
3.37

(3.38)
—
6.59

(6.62)

12.
[{(p-Cl-m-CH₃C₆H₃O)₂PS₂}₂Mn · N₂C₁₀H₈]
Yellow

(98)
62
5.45
47.15

(47.17)
3.27

(3.30)
13.23

(13.26)
2.89

(2.90)
14.63

(14.66)
5.66

(5.68)

13.
[{(o-CH₃C₆H₄O)₂PS₂}₂Mn · 2NC₅H₅]
Pale yellow

(92)
56
5.45
54.84

(54.87)
4.57

(4.60)
15.41

(15.42)
3.34

(3.37)
—
6.58

(6.60)

14.
[{(m-CH₃C₆H₄O)₂PS₂}₂Mn · 2NC₅H₅]
Pale yellow

(90)
57
6.54
54.85

(54.87)
4.58

(4.60)
15.40

(15.42)
3.35

(3.37)
—
6.59

(6.60)

15.
[{(p-CH₃C₆H₄O)₂PS₂}₂Mn · 2NC₅H₅]
Pale yellow

(95)
56
4.36
54.86

(54.87)
4.57

(4.60)
15.40

(15.42)
3.36

(3.37)
—
6.57

(6.60)

16.
[{(p-Cl-m-CH₃C₆H₃O)₂PS₂}₂Mn · 2NC₅H₅]
Pale yellow

(96)
58
4.36
47.06

(47.07)
3.50

(3.53)
13.20

(13.23)
2.87

(2.89)
14.60

(14.63)
5.66

(5.67)

17.
[{(o-CH₃C₆H₄O)₂PS₂}₂Mn · 2P(C₆H₅)₃]
White

(90)
64
6.54
64.13

(64.15)
4.85

(4.88)
10.68

(10.70)
—
—
4.56

(4.58)

18.
[{(m-CH₃C₆H₄O)₂PS₂}₂Mn · 2P(C₆H₅)₃]
White

(90)
63
6.54
64.13

(64.15)
4.85

(4.88)
10.67

(10.70)
—
—
4.56

(4.58)

19.
[{(p-CH₃C₆H₄O)₂PS₂}₂Mn · 2P(C₆H₅)₃]
White

(94)
67
5.45
64.12

(64.15)
4.86

(4.88)
10.68

(10.70)
—
—
4.57

(4.58)

20.
[{(p-Cl-m-CH₃C₆H₃O)₂PS₂}₂Mn · 2P(C₆H₅)₃]
White

(96)
62
6.54
57.52

(57.54)
4.05

(4.07)
9.58

(9.60)
—
10.59

(10.61)
4.10

(4.11)

*: solid.

3.1. IR Spectra

IR spectral assignments of these complexes (1–20) were made on the basis of relevant literature reports [26–28, 31, 32]. The comparison of IR spectra of these complexes with starting materials has also shown significant and characteristic changes and also shifting of bands. Two strong intensity bands were observed in the region 1160.7–1080.0 cm⁻¹ and 960.2–896.8 cm⁻¹, which may be ascribed to the ν(P)−O−C and νP−O−(C) vibrations, respectively. The bands for νP=S and νP−S were observed in the regions 696.2–651.8 cm⁻¹ and 640.3–541.9 cm⁻¹. The bands due to νP=S and νP−S vibrations have depicted a shift of 10–30 cm⁻¹ towards the lower frequency region in comparison to the parent dithiophosphate ligands. The observation of two closely spaced bands arising from v(PS₂) vibrations is entirely typical for bidentate chelating dithiophosphates units. This shift of νP−S vibrations is, perhaps, due to bidentate mode of bonding by dithiophosphates ligands. The presence of a band in the region 291.0–214.3 cm⁻¹, attributed to νMn−S, is indicative of the formation of manganese-sulfur bond. This is supported by the literature survey [26]. Cavell et al. have reported similar structure IR frequency ranges for Mn-S bond which is in agreement with our observations. IR spectra have also revealed the presence of bands of νMn-N and νMn-P in the regions 486.0–422.3 cm⁻¹ and 384.9–360.2 cm⁻¹, respectively [31, 32]. The relevant IR spectral data of these complexes are given in Table 2.

Table 2: IR spectral data for Mn(II) ditolyldithiophosphates (cm⁻¹).

S. no.
v(P)–O–C
vP–O–(C)
vP=S
vP–S
vMn–S
vMn–P
vMn–N

1.
1159.2, s
923.4, s
686.3, s
546.8, m
235.6, w
—
—

2.
1109.1, s
919.2, s
686.4, s
581.4, m
249.5, w
—
—

3.
1103.2, s
925.7, s
696.0, s
561.7, m
247.3, w
—
—

4.
1140.3, s
957.0, s
684.8, s
607.6, m
246.8, w
—
—

5.
1143.7, s
900.7, s
682.7, s
551.6, m
252.4, w
—
445.5, w

6.
1121.4, s
903.6, s
675.2, s
586.4, m
273.6, w
—
453.6, w

7.
1112.7, s
898.7, s
692.4, s
565.4, m
254.9, w
—
476.3, w

8.
1149.5, s
954.7, s
688.5, s
640.3, m
263.4, w
—
445.5, w

9.
1157.1, s
906.4, s
669.2, s
552.4, m
273.4, w
—
472.6, w

10.
1110.9, s
900.3, s
671.1, s
566.4, m
261.2, w
—
428.1, w

11.
1101.2, s
896.8, s
694.3, s
560.1, m
254.9, w
—
464.8, w

12.
1151.4, s
954.7, s
651.8, s
609.4, m
255.7, w
—
484.1, w

13.
1108.9, s
910.3, s
671.1, s
559.3, m
232.7, w
—
441.6, w

14.
1080.0, s
919.9, s
696.2, s
590.1, m
245.7, w
—
486.0, w

15.
1103.2, s
910.3, s
694.3, s
592.4, m
250.0, w
—
422.3, w

16.
1159.3, s
960.2, s
683.6, s
601.3, m
246.3, w
—
442.6,w

17.
1105.1, s
908.4, s
673.7, s
541.9, m
214.3, w
384.9, w
—

18.
1103.2, s
925.4, s
692.4, s
587.6, m
242.4, w
379.4, w
—

19.
1123.2, s
902.6, s
692.4, s
557.4, m
291.0, w
361.1, w
—

20.
1160.7, s
957.4, s
680.1, s
603.5, m
282.4, w
360.2, w
—

s: strong; m: medium; w: weak.

3.2. Mass Spectra

The mass spectra of a few representative complexes (1, 5, 8, 9, 11, 14, 16, 17 and 19) have exhibited the presence of molecular ion peak. In addition to the molecular ion peak, several other peaks were also observed, which are corresponding to the fragmented species after the consecutive removal of different groups. The occurrence of molecular ion peak in the complexes is supporting the monomeric nature of the complexes. Furthermore, the adducts (8 and 16) that contain multiple chlorines (i.e., two chlorine atoms) also show isotopic peaks. Based on the presence of the peaks in the mass spectra of some of the representative complexes, the various fragments have been given in Table 3.

Table 3: Mass spectral data of the Mn(II) ditolyldithiophosphates.

S. no.
M. Wt.
, relative intensities of the ions and assignment

1.
673.6
M⁺ 673.6 (10) [{(o-CH₃C₆H₄O)₂PS₂}₂Mn]⁺;

M⁺ 364.3 (14) [{(o-CH₃C₆H₄O)₂PS₂}Mn]⁺;

M⁺ 277.1 (45) [Mn(S₂PO)₂]⁺;

M⁺ 107.1 (59) [o-CH₃C₆H₄O]⁻

5.
853.8
M⁺ 853.8 (12) [{(p-CH₃C₆H₄O)₂PS₂}₂Mn · C₁₂H₈N₂]⁺;

M⁺ 544.5 (18) [{(p-CH₃C₆H₄O)₂PS₂}Mn · C₁₂H₈N₂]⁺;

M⁺ 309.3 (56) [(p-CH₃C₆H₄O)₂PS₂]⁻;

M⁺ 218.2 (67) [(p-CH₃C₆H₄O)P(O)S₂]⁻;

M⁺ 107.1 (70) [p-CH₃C₆H₄O]⁻

8.
991.6
M⁺† 991.6 (15), 993.6 (9), 995.6 (5) [{(p-Cl-m-CH₃C₆H₃O)₂PS₂}₂Mn · C₁₂H₈N₂]⁺  

M⁺† 788.4 (17), 790.4 (11), 792.4 (9) [{(p-Cl-m-CH₃C₆H₃O)₂PS₂}Mn · C₁₂H₈N₂]⁺;

M⁺† 433.1 (21), 433.3 (14), 433.5 (10) [{(p-Cl-m-CH₃C₆H₃O)₂PS₂}Mn]

M⁺ 277.2 (48) [(C₆H₃O)₂PS₂]⁻;

M⁺ 201.2 (56) [m-CH₃C₆H₃OPS₂]⁻;

M⁺† 141.5 (60), 143.5 (32), 145.5 (17) [p-Cl-m-CH₃C₆H₃O]⁻;

9.
829.8
M⁺ 829.8 (7) [{(o-CH₃C₆H₄O)₂PS₂}₂Mn · C₁₀H₈N₂]⁺;

M⁺ 520.4 (98) [{(o-CH₃C₆H₄O)₂PS₂}Mn · C₁₀H₈N₂]⁺;

M⁺ 364.3 (14) [{(o-CH₃C₆H₄O)₂PS₂}Mn]⁺;

M⁺ 277.1 (7) [Mn(S₂PO)₂]⁺;

M⁺ 107.1 (20) [o-CH₃C₆H₄O]⁻;

11.
829.8
M⁺ 829.8 (8) [{(p-CH₃C₆H₄O)₂PS₂}₂Mn · C₁₀H₈N₂]⁺;

M⁺ 520.4 (98) [{(p-CH₃C₆H₄O)₂PS₂}Mn · C₁₀H₈N₂]⁺;

M⁺ 364.3 (14) [{(p-CH₃C₆H₄O)₂PS₂}Mn]⁺;

M⁺ 277.1 (7) [Mn(S₂PO)₂]⁺;

M⁺ 107.1 (20) [p-CH₃C₆H₄O]⁻;

14.
831.8
M⁺ 831.8 (7) [{(m-CH₃C₆H₄O)₂PS₂}₂Mn · 2C₅H₅N]⁺;

M⁺ 364.3 (98) [{(m-CH₃C₆H₄O)₂PS₂}Mn]⁺;

M⁺ 309.3 (40) [(m-CH₃C₆H₄O)₂PS₂]⁻;

M⁺ 202.5 (40) [m-CH₃C₆H₄OPS₂]⁻;

M⁺ 277.1 (7) [Mn(S₂PO)₂]⁺;

M⁺ 107.1 (20) [m-CH₃C₆H₄O]⁻

16.
969.6
M⁺† 969.6 (12) 971.6 (8), 973.6 (3) [{(p-Cl-m-CH₃C₆H₃O)₂PS₂}₂Mn · 2C₅H₅N]⁺;

M⁺† 512.2 (18), 514.2 (11), 516.2 (6) [(p-Cl-m-CH₃C₆H₃O)₂PS₂Mn · C₅H₅N]⁺;M⁺ 277.2 (45) [(C₆H₃O)₂PS₂]⁻;

M⁺ 201.2 (56) [m-CH₃C₆H₃OPS₂]⁻;

M⁺† 141.5 (65), 143.5 (34), 145.5 (19) [p-Cl-m-CH₃C₆H₃O]⁻;

M⁺ 106.1 (91) [m-CH₃C₆H₃O]⁻;

17.
1198.2
M⁺ 1198.2 (6) [{(o-CH₃C₆H₄O)₂PS₂}₂Mn · 2P(C₆H₅)₃]⁺;

M⁺ 364.3 (10) [{(o-CH₃C₆H₄O)₂PS₂}Mn]⁺;

M⁺ 202.2 (18) [o-CH₃C₆H₄OPS₂]⁻;

M⁺ 277.1 (7) [Mn(S₂PO)₂]⁺;

M⁺ 107.1 (20) [o-CH₃C₆H₄O]⁻;

19.
1198.2
M⁺ 1198.2 (8) [{(p-CH₃C₆H₄O)₂PS₂}₂Mn · 2P(C₆H₅)₃]⁺;

M⁺ 364.3 (10) [{(p-CH₃C₆H₄O)₂PS₂}Mn]⁺;

M⁺ 278.7 (98) [(p-C₆H₄O)₂PS₂]⁻;

M⁺ 202.2 (18) [p-CH₃C₆H₄OPS₂]⁻;

M⁺ 107.1 (20) [p-CH₃C₆H₄O]⁻;

isotopic peaks.

3.3. Electronic Spectra

The intense electronic spectra of the complexes (1–20) show absorption in the ranges of 339–396 nm, 431–479 nm, and 510–573 nm, which may be ascribed to ⁶ → ⁴Eg (⁴D), ⁶A1g → ⁴Eg (⁴G) and ⁶A1g → ⁴T1g (⁴G) and are assigned to d-d transition [33]. The transition in these complexes indicated that the manganese is with the ground term ⁶S which do not split and transformed to the ⁶A1g state. The transition in these complexes is both spin forbidden and orbital forbidden, due to this, the absorption bands are extremely weak. Other weak intensity absorption bands in the region 247–296 nm are observed in adducts (5–20) which may be ascribed to the transition between the metal to ligand (M→L) charge transfer spectra [34]. This signifies that there is another moiety present in these adducts that is, donor atoms of 1,10-phenanthroline, 2,2′-bipyridyl, pyridine, and triphenylphosphine. The tentative assignments of the important bands for the complexes (1–4) and adducts (5–20) have been made and summarized in Table 4.

Table 4: Magnetic moment and electronic spectral data for the Mn(II) ditolyldithiophosphates.

S. no.

(nm)
Assignments

1.
—
347
6A1g → (⁴D)

431
6A1g → (⁴G)

510
6A1g → (⁴G)

2.
—
339
6A1g → (⁴D)

435
6A1g → (⁴G)

512
6A1g → (⁴G)

3.
—
344
6A1g → (⁴D)

433
6A1g → (⁴G)

517
6A1g → (⁴G)

4.
5.90
341
6A1g → (⁴D)

432
6A1g → (⁴G)

514
6A1g → (⁴G)

5.
—
370
6A1g → (⁴D)

449
6A1g → (⁴G)

536
6A1g → (⁴G)

269, 286
Charge transfer

6.
5.60
365
6A1g → (⁴D)

452
6A1g → (⁴G)

542
6A1g → (⁴G)

271, 285
Charge transfer

7.
—
373
6A1g → (⁴D)

456
6A1g → (⁴G)

538
6A1g → (⁴G)

264, 286
Charge transfer

8.
—
364
6A1g → (⁴D)

465
6A1g → (⁴G)

532
6A1g → (⁴G)

264, 284
Charge transfer

9.
5.68
363
6A1g → (⁴D)

450
6A1g → (⁴G)

540
6A1g → (⁴G)

271, 283
Charge transfer

10.
—
361
6A1g → (⁴D)

446
6A1g → (⁴G)

536
6A1g → (⁴G)

266, 285
Charge transfer

11.
—
359
6A1g → (⁴D)

445
6A1g → (⁴G)

533
6A1g → (⁴G)

270, 282
Charge transfer

12.
—
370
6A1g → (⁴D)

453
6A1g → (⁴G)

539
6A1g → (⁴G)

269, 280
Charge transfer

13.
—
396
6A1g → (⁴D)

479
6A1g → (⁴G)

542
6A1g → (⁴G)

273, 294
Charge transfer

14.
—
390
6A1g → (⁴D)

472
6A1g → (⁴G)

573
6A1g → (⁴G)

270, 296
Charge transfer

15.
—
393
6A1g → (⁴D)

478
6A1g → (⁴G)

549
6A1g → (⁴G)

273, 294
Charge transfer

16.
5.65
390
6A1g → (⁴D)

470
6A1g → (⁴G)

565
6A1g → (⁴G)

277, 295
Charge transfer

17.
—
352
6A1g → (⁴D)

435
6A1g → (⁴G)

524
6A1g → (⁴G)

253, 276
Charge transfer

18.
—
356
6A1g → (⁴D)

447
6A1g → (⁴G)

521
6A1g → (⁴G)

247, 274
Charge transfer

19.
5.76
353
6A1g → (⁴D)

432
6A1g → (⁴G)

525
6A1g → (⁴G)

251, 273
Charge transfer

20.
—
356
6A1g → (⁴D)

442
6A1g → (⁴G)

528
6A1g → (⁴G)

249, 275
Charge transfer

3.4. Magnetic Susceptibility

As a consequence of the weak field nature of the dithiophosphate ligand, effective magnetic moment values of all the manganese(II) complexes are high and appear in the range of 5.60–5.90 B.M. Magnetic susceptibility measurements show that these complexes are paramagnetic [26, 27]. The relevant magnetic moment values of these complexes are tabulated in Table 4.

3.5. Molar Conductance

The mononuclear complexes are dissolved in DMSO, and the molar conductance of 10-3 M solutions at 30°C was measured. It is concluded from the results that the complexes have a molar conductance in the range of 4.36–9.81 Ω⁻¹cm2 mol⁻¹, which indicates the nonionic nature of these complexes [6]. Hence, these complexes have nonelectrolytic properties in DMSO. The molar conductance value of the complexes is given in Table 1.

3.6. Thermogravimetric Analysis

The thermal behaviour of the complex [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] (1) displayed a thermolysis step that covers a temperature range from 150 to 986°C. The main weight loss occurs in the steeply descending segment of the TG curve (Figure 1); the DTG curve shows the increasing rate of the weight loss up to 986.71°C. This weight loss of 58.58% at 151.7°C is due to the formation of bis(dithiometaphosphato) manganese(II) [Mn(S₂PO)₂], (the calculated weight loss is 58.86%) as an intermediate product, which agrees with thermogravimetric data for dithiophosphates. The weight of the final residue is 21.38% of the initial weight of the sample; this corresponds to the formation of MnSO₄ contaminated with other thermolysis products (the calculated value is 22.42%) at 986.71°C. The thermogravimetric analysis of the adduct [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·N₂C₁₂H₈] (5) (Figure 2) exhibited loss of the donor ligand resulting in the formation of the intermediate compound [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] (the calculated weight loss is 21.85%) at 68.99°C. The rate of the weight loss begins to increase steeply at 303.92°C. The weight loss at this point is due to the formation of [Mn(S₂PO)₂] (the calculated weight loss is 67.85%). The weight of the final residue is 14.14% of the initial weight of the sample; this corresponds to the formation of MnS contaminated with other thermolysis products (the calculated value is 10.09%) at 788.99°C.

Figure 1: TGA curve of complex [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] (1).

Figure 2: TGA curve of [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·N₂C₁₂H₈] (5).

The DSC curve for the complex [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] shows an endotherm in the region of stability of [Mn(S₂PO)₂] near 151.7°C. Two endotherms are observed for the adduct [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·N₂C₁₂H₈], one sharp endotherm (extrapolated to 68.99°C) corresponds to the weight loss leading to the formation of [{(o-CH₃C₆H₄O)₂PS₂}₂Mn], and a less intense endotherm appears in the region of formation of [Mn(S₂PO)₂].

3.7. Cyclic Voltammetry

The complex [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·2P(C₆H₅)₃] (17) is electroactive with respect to the metal center. The cyclic voltammogram (Figure 3) of the complex in the potential range of +2.5 to −1.5 V at a scan rate of 100 mV/s exhibited two redox processes; each reduction is associated with a single-electron transfer process. Two well-defined one-electron cyclic responses were observed as discussed in Table 5. The first reduction peak was observed at  V with the cathodic peak current, × 10⁻⁵ A, and a corresponding oxidation peak at  V with the anodic peak current, × 10⁻⁵ A. The second redox process was related with the reduction peak at  V with the cathodic peak current, × 10⁻⁵ A, with a corresponding oxidation peak at  V with the anodic peak current, × 10⁻⁵ A.

Table 5: Cyclic voltammetric analysis data of complex [{(o-CH₃C₆H₄O)₂PS₂}₂Mn · 2P(C₆H₅)₃] (17).

Index
Peak position
Peak height
Base start
Base end
Peak (1/2)

1
1.8784

1.3754
2.2739
0.29762

2
0.73334

0.64056
0.91644
−0.07277

3
−1.127

−1.4835
−0.46295
0.25486

4
−0.46539

−0.7193
−0.16754
−0.17766

Figure 3: Cyclic voltammetric curve of complex [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·2P(C₆H₅)₃] (17).

3.8. Antimicrobial Screening

3.8.1. Antibacterial

The bacterial species under investigation were the strains of three human pathogenic gram negative bacteria: Escherichia coli, Klebsiella pneumonia, and Pseudomonas aeruginosa. The free ligands, MnCl₂·4H₂O, and a few complexes were screened for their in vitro antibacterial study by well diffusion method [35]. The results achieved by these studies have been enlisted in Table 6. The antibacterial data reveals that MnCl₂·4H₂O in solution shows prominent inhibition capacities while the free ligands did not possess antibacterial properties against the bacterial species under study. The results also confirmed that the antimicrobial activity of the metal chelates shows more inhibitory effects than the parent ligand. Furthermore, the [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·N₂C₁₂H₈] (5) exhibited enhanced activity than the [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] (1). On chelation, the polarity of the metal ion will reduce to a greater extent due to the overlap of the ligand orbital and partial sharing of the positive charge of the metal ion with donor groups [36]. Further, it increases the delocalization of π-electrons around the whole chelate ring and enhances the penetration of the complexes into lipid membranes and blocking of the metal binding sites in the enzymes of microorganisms. These complexes also disturb the respiration process of the cell and thus block the synthesis of proteins, which restricts further growth of the organisms. It has also been observed that concentration plays a vital role in increasing the degree of inhibition. The inhibition is directly proportional to the concentration. The bacterial growth inhibitory capacity of ligand and its complexes with manganese(II) have also been illustrated in Figures 4(a)–4(d).

Table 6: Antibacterial activities of the Mn(II) ditolyldithiophosphates (zone of inhibition, in mm).

Compound
Concentration (ppm)
Bacterial species

Escherichia coli
Klebsiella pneumonia
Pseudomonas aeruginosa

Ciprofloxacin

(standard drug)
50
1.4
2.5
2.6

100
1.8
2.7
2.9

150
1.9
2.8
3.0

200
2.1
2.9
3.5

MnCl₂ · 4H₂O

(metal salt)
50
2.4
2.6
2.9

100
2.6
2.8
3.0

150
2.9
3.0
3.4

200
3.0
3.1
3.6

Free ligand

(o-CH₃C₆H₄O)₂PS₂Na
50
0.0
0.0
0.0

100
0.0
0.0
0.0

150
0.0
0.0
0.0

200
0.0
0.0
0.0

(1)
50
1.7
1.7
1.8

100
1.9
1.9
2.1

150
2.1
2.1
2.6

200
2.3
2.5
3.2

(5)
50
1.6
2.1
2.5

100
1.9
2.3
2.6

150
2.3
2.5
2.8

200
2.5
2.8
3.0

(a)

(b)

(c)

(d)

Figure 4: Comparative results of antibacterial screening data for: (a) Ciprofloxacin (drug), (b) MnCl₂·4H₂O, (c) [{(o-CH₃C₆H₄O)₂PS₂}₂Mn] (1), and (d) [{(o-CH₃C₆H₄O)₂PS₂}₂Mn·N₂C₁₂H₈] (5).

3.8.2. Antifungal

The antifungal activity of ligands and a few representative metal complexes were evaluated by the poisoned food technique [35] against plant pathogenic strain, S. rolfsii. The antifungal screening data are given in Table 7, which shows that the colony diameter of the fungus decreases as the concentration of the complex increases; that is, all the complexes inhibited the growth of fungus significantly. This shows a linear relationship between concentration and percent inhibition. The increase in antimicrobial activity is due to faster diffusion of metal complexes as a whole through the cell membrane or due to combined activity effect of the metal and the ligand. It is evident from the antifungal screening data that adducts of nitrogen and phosphorus donor ligands (8, 9, 15, and 18) are more potent than the parent complex 1. The chelation theory accounts for the increased activity of the metal complexes [36]. The results of fungi-toxicity analysis have been illustrated in Figure 5. Both antibacterial and antifungal activities were tested in the Bio-assay lab, Department of Chemistry, University of Jammu, Jammu.

Table 7: In vitro evaluation of Mn(II) ditolyldithiophosphates against Sclerotium rolfsii.

S. no.
Concentration

(ppm)
Colony diameter

(T)

(in mm)
% inhibition

Free ligand

(o-CH₃C₆H₄O)₂PS₂Na
50
36
10

100
24
40

150
19
52.5

200
15
62.5

1
50
41
18

100
10
80

150
0
100

200
0
100

8
50
15
70

100
0
100

150
0
100

200
0
100

9
50
25
50

100
0
100

150
0
100

200
0
100

15
50
10
80

100
10
80

150
0
100

200
0
100

18
50
10
80

100
0
100

150
0
100

200
0
100

Mean colony diameter of control (C) = 50 mm.

Figure 5: Comparative results of antifungal screening data.

3.9. Structural Features

The outcome of the above results confirms the formation of the manganese(II) ditolyldithiophosphate complexes as indicated from elemental analyses, magnetic susceptibility, molar conductance, thermogravimetric, cyclic voltammetry, and spectral analyses including ESI mass spectrometry, IR, and UV-visible. The shiftings of νP=S and νP−S bands (10–30 cm⁻¹) in comparison to the parent dithiophosphate ligands are quite diagnostic to propose bidentate mode of bonding of dithio moiety with manganese. Further, these bands have smaller gap between each other compared to the parent dithio ligand, which is also supporting the bidentate mode of attachment. The mass spectra support the respective molecular masses and fragmentation pattern as well as monomeric nature of these complexes. The UV-visible and magnetic susceptibility analysis confirmed the high spin nature of manganese atom in these complexes.

Attempts to obtain suitable crystals for the X-ray diffraction study have so far been unsuccessful. From the above data, plausible-four and six-folded geometries may be suggested for these complexes and adducts (Figures 6, 7, and 8).

Figure 6: Proposed square planar structure of bis-O,O′-(o-, m-, p- and p-Cl-m- ditolyl)dithiophosphates of manganese(II) (1–4); X = H, CH₃, or Cl.

Figure 7: Proposed octahedral structure of manganese(II) complexes with 1,10-phenanthroline (5–8) and 2,2′-bipyridyl (9–12); X = H, CH₃, or Cl.

Figure 8: Proposed octahedral structure of complexes of manganese(II) with pyridine (13–16) and triphenylphosphine (17–20); X = H, CH₃, or Cl.

4. Conclusion

We have synthesized and characterized a series of twenty new O,O′-(o-, m-, p-, p-Cl-m-ditolyl) dithiophosphate derivatives of manganese. In conjunction with the literature reports and observations based on elemental analysis, IR, mass spectral studies, and thermogravimetric analysis, bidentate chelation by the ligand to the manganese(II) ion may be postulated in these complexes. The melting points were found to be consistent with the decomposition pattern in the thermogravimetric analyses. The cyclic voltammetric analysis predicted the two electron reduction of the metal. The complexes are found to have higher biological activities as compared to the respective ligand and the parent drug.

Acknowledgments

The authours gratefully acknowledge the financial assistance from the University Grants Commission, New Delhi. they are grateful to Sophisticated Analytical Instrumentation Facility, Panjab University, Chandigarh, and Indian Institute of Integrative Medicine, Jammu, for providing spectral facilities.

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