Reactions of MoCl5 with 4-Methylpyridine, 2-Methylpyridine and 1-Methylimidazole in Tetrahydrofuran

Introduction

Pyridine derivatives have a number of biological activities, like antimycobacterial¹, antimicrobial^{2, 3}, analgesic⁴, anticonvulsant⁵, antitumoral⁶, antimalarial⁷, antiparkinsonian⁸, cytotoxic⁹, pesticidal¹⁰, antidiabetic¹¹, antibacterial¹², inhibitory¹³ and receptor¹⁴ antagonists. Electron donating groups¹⁵ in pyridine derivatives increase the biological activity of transition metal-pyridine derivative compounds.

4-Methylpyridine

4-Methylpyridine¹⁶ is used as a precursor for the preparation of a number of heterocyclic compounds. It is used for the preparation of many compounds of medicinal interest, like ‘isoniazid’ used as antituberculosis drug¹⁷.

2-Methylpyridine

2-Methylpyridine¹⁸ acts as an intermediate for the manufacture of drugs, like picoplatin, amprolium, encainide and dimethindene. It is used as a precursor for the preparation of 2-vinylpyridine. It is used to manufacture nitrapyrin¹⁹ which controls loss of NH₃ from fertilizers. It is also used to prepare herbicide piclorand.

1-Methylimidazole

Azoles²⁰ are widely used in antibiotics and antifungal agents, because cytochrome P450 sterol 14α-demethylase of the pathogens are inhibited by them. Azole action mechanism involves,

Coordination of imidazole non-protonated nitrogen and enzyme heme-iron

π-cation interaction

Hydrogen binding

Only a few antifungal agents are available: azoles, allylamines, polyenes, fluoropyrimidines and thiocarbamates²¹. Among these agents, azoles are the most important because of their high therapeutic index. Effectiveness of the azole drug depends on the strength of the coordinate bond²².

Imidazole drugs have many applications^23-35 as antifungal, 20-carboxypeptidase inhibitors, hemeoxygenase inhibitors, anticancer, anti-inflammatory, antibacterial, antimalarial, antitubercular, antiviral agents. They are also anticoagulants, antidiabetic HETE (20-Hydroxy-5, 8, 11, 14-eicosatetraenoic acid) synthase inhibitors, β-lactamase inhibitors and antiaging agents.

Metal chelates drugs are more effective than ligands themselves^36-41.

Aim of Investigation

MoCl₅ reactions with imides, diaminoalkanes, aromatic azoles, thiols like 4-phenylimidazole-2-thiol, 2-thiazoline-2-thiol, alkylpyridines and mercaptopyridine-N-oxide have been reported^42-51 by the author.

Since applications of transition metal compounds with pyridine and imidazole derivatives are widening very fast, so MoCl₅ compounds of 4-methylpyridine, 2-methylpyridine and 1-methylimidazole have been prepared and investigated. These complexes were studied with elemental analysis, FT-IR, LC-MS, FT-¹H NMR, FT-¹³C NMR and microbiological studies.

Materials and Methods

4-Methylpyridine, 2-methylpyridine, 1-methylimidazole and MoCl₅ used were of Sigma-Aldrich

Reactants and products being prone to oxidation by air/moisture, so all reactions were carried out and products handled in vacuum line in dry nitrogen atmosphere.

4-Methylpyridine/2-methylpyridine/1-methylimidazole was added to the dropping funnel and dissolved in 10 ml dry THF. It was added dropwise to MoCl₅ solution in THF with continuous agitation in 100 ml B-14 flask. Contents were stirred for 7-8 h. Filtration was carried out. Products from both the residue and filtrate were isolated and studied.

Molybdenum was estimated using 8-hydroxyquinoline gravimetrically⁵². Chlorine was estimated using silver nitrate gravimetrically⁵². Other elements were determined quantitatively by Thermo Finnigan Elemental Analyser. Proton/¹³C nuclear magnetic resonance spectra were recorded in DMSO-d₆ with Multinuclear Brucker Avance-II 400 NMR spectrometer. Vibrational peaks have been studied using Perkin-Elmer 400 FTIR Spectrometer.LC-MS studies have been carried out in the span 0 – 1100 m/z. P. U. Chandigarh, India provided the instrumental support.

Molybdenum compounds activity against bacteria and fungi was investigated by ‘Agar Well Diffusion Assay’ method. Various strains used are: fungi Candida albicans and Aspergillus niger, Gram positive bacteria Staphylococcus aureus and Gram negative bacteria E. coli. The Microbial Type Culture Collection and Gene Bank, Chandigarh, India cultures were used. Amoxicillin and ketoconazole for bacteria and virus, respectively were used as standard drugs for comparison. Studies were performed at Indo Soviet Friendship College of Pharmacy, Moga, Punjab, India.

Results and Discussions

Elemental Studies

Table 1: (observed (theoretical) percentage)

Compounds	Chlorine	Molybdenum	Hydrogen	Carbon	Nitrogen
MoO2Cl(C6H7N), [1] (Light brown/256.5)	13.74 (13.84)	36.96 (37.42)	03.13 (02.72)	27.65 (28.07)	04.85 (05.45)
Mo2O2Cl5(C6H7N)2(C4H8O)2, [2] (Black/731.5)	23.57 (24.26)	25.53 (26.24)	04.00 (04.10)	31.93 (32.80)	03.16 (03.82)
Mo4O4Cl4(C6H7N)3(C4H8O)2, [3] (Grey/1013.0)	13.23 (14.01)	37.11 (37.90)	03.82 (03.65)	30.09 (30.79)	03.27 (04.14)
Mo2O4Cl4(C4H6N2)2(C4H8O), [4] (Coffee brown/634.0)	21.78 (22.39)	29.67 (30.28)	03.09 (03.15)	21.92 (22.71)	07.13 (08.83)

FT-IR Spectra

Ring υ(C-H) of 4-methylpyridine^{50, 53-60} occurs at 3072 cm^-1 and the corresponding str. in ¹ has been noted at 3090 cm^-1. Ring υ(C=N) and torsion show upward shift, but ring δ(C-H) shows downward shift, showing Mo(dπ)→N(pπ). 976 cm^-1 peak suggests the presence of terminal Mo=O^{45, 61, 62} in [1]. υ(Mo=O) decreases on N→Mo coordination⁶³. Coordinate bond and Mo=O are trans to each other. Molybdenum abstracts^{64, 65} oxygen from THF to form Mo=O (Table-2).

Ring υ(C-H) has not been noticed in ². It has been observed that ring υ(C=N) and torsion show upward shift, but ring δ(C-H) has decreased, as a result of Mo(dπ)→N(pπ). 975 cm^-1 str. of Mo=O^{45, 61, 62} group at terminal position is observed. υ(Mo=O) decreases on N→Mo coordinate bond⁶³ formation. Coordinate bond is trans to Mo=O. Molybdenum abstracts^{64, 65} oxygen from THF forming Mo=O (Table-2).

Table 2: (FT-IR absorptions in cm-1 )

Assignments	4-Methylpyridine50, 53-60	[1]	[2]
Ring υ(C-H)	3072, 3034	3403 b, 3090 sh	3399 s
Methyl υ(C-H)	2990, 2927	2951 sh
Ring υ(C=N)	1611, 1564	1640 s	1640 s
Methyl δasym(C-H)	1499	1507 m	1508 w
Ring δ(C-H)	1458, 1445, 1418, 1365, 1194	1445 sh, 1316 w
Methyl δsym(C-H)	1383	1380 w	1381 w
Ring υ(C-N)	1227	1256 w
υ(C-CH3)	1210	1203 w	1204 sh
Methyl ρ(C-H)	1114	1099 w	1111 sh
Ring breathing	1072	1039 w
Ring out of plane bending	994, 877	870 sh
Ring ꞷ(C-H), ꞷ(C-C), ꞷ(C-N)	730	794 m, 734 m	791 w, 735 w
Ring torsion	524	619 sh , 566 w, 521 w	570 w
δ(C-CH3)	486	476 w	476 w
Terminal Mo=O45, 61, 62 str.		976 s	975 m

Ring υ(C-H) in 2-methylpyridine^{58, 59, 66, 67} is noted at 3065 cm^-1, but [3] does not show this vibration. υ(C=N) in [3] has been noted at 1632 cm^-1 showing an upward shift⁴⁷ by 36 cm^-1 and torsion was observed at 571 cm^-1 showing an upward shift⁴⁷ by 24 cm^-1. This upshift is because of Mo(dπ)→N(pπ). υ(Mo=O) at 979 cm^-1 in [3] has been recorded because of Mo=O^{45,61, 62} group at terminal position. υ(Mo=O) drops N→Mo coordination⁶³. Coordinate bond and Mo=O are trans to each other. Molybdenum abstracts^{64, 65} oxygen from THF to form Mo=O (Table-3).

Table 3:  (FT-IR absorptions in cm⁻¹)

Assignments	2-Methylpyridine58, 59, 66, 67	[3]
Ring υ(C-H)	3135 m, 3087 m, 3065 m	3379 s
Methyl υ(C-H)	2960 m
Ring υ(C=N)	1596 vs	1632 s
Methyl δasym(C-H)	1463 s
Ring δ(C-H)	1475 s, 1148 m,	1477 sh
Methyl δsym(C-H)	1375 w	1404
Ring υ(C-N)	1297 s,	1297 w
υ(C-CH3)	1235 m
Methyl ρ(C-H)	1075
Ring breathing	1059 s	1050 w
Ring ꞷ(C-H), ꞷ(C-C), ꞷ(C-N)	750 vs, 730 m	762 s
Ring ꞷ(C-C)	630 m
Ring torsion	547 w	571 sh
δ(C-CH3)	471
Terminal Mo=O45, 61, 62 str.		979 s

 

Unprotonated nitrogen of 1-methylimidazole⁶⁸ coordinates with molybdenum. Ring υ(C=C) has been noted at 1553 cm^-1 showing an increase of 36 cm^-1. Ring υ(N-C) is located at 1446 cm^-1 showing an increase of 39 cm^-1. This increase in wave numbers on N→Mo coordination is caused by,

Inductive effect on ligand-molybdenum cation coordination,

d_π-p_π back bonding which changes the electron density on the ligand.

It causes electron density to increase in the ring which increases N→Mo coordinate bond strength. 978 cm^-1 str. of Mo=O^{45, 61, 62} group at terminal position is observed. υ(Mo=O)

It causes electron density to increase in the ring which increases N→Mo coordinate bond strength. 978 cm^-1 str. of Mo=O^{45, 61, 62} group at terminal position is observed. υ(Mo=O) decreases on N→Mo coordinate bond⁶³ formation. Coordinate bond is trans to Mo=O. Molybdenum abstracts^{64, 65} oxygen from THF forming Mo=O (Table-4).

Table 4: (FT-IR absorptions in cm⁻¹)

Assignments	1-Methylimidazole68	[4]
Ring υ(C-H)	3017 m, 2955 w	3398 s
Ring υ(C=N)		1630 s
Ring υ(C=C)	1519 vs	1553 sh
Ring υ(C-N)	1409 m	1446 w
Ring δ(C-H)	1107 m, 1087 m, 1035 vw	1086 w
Ring ꞷ(C-H), Ring τ(C-H)	814 s, 774 s	740 s
Ring τ(C-H)	640 s	624 m
ꞷ(N-H), Ring twisting	525	569 m
Terminal Mo=O45, 61, 62str.		978 s

 

FT-¹H NMR Spectra

Compounds were dissolved in DMSO-d₆ to take spectrum. DMSO-d₆⁶⁹residual peak appears at 2.5 ppm.THF⁶⁹ solution in DMSO-d₆ shows CH₂ peak at 1.7 ppm and O-CH₂ peak at 3.6 ppm. ↑ and ↓ signify upfield/downfield shift.

When spectra of 4-methylpyridine^{50, 58, 59, 70} and ¹ are compared, it is observed that absorptions show deshielding due to flow of π-electron density towards molybdenum cation on N→Mo coordinate bond formation (Fig. 1, Table-5).

Figure 1: FT-1H NMR of MoO2Cl(C6H7N), [1] Click here to View figure

4-Methylpyridine^{50, 58, 59, 70} and ² spectra on comparison show that absorptions go downfield in ². This results from flow of π-electrons towards molybdenum on coordination (Fig. 2, Table-5).

Figure 2: FT-1H NMR of Mo2O2Cl5(C6H7N)2(C4H8O)2, [2] Click here to View figure

Table 5:  (¹H-NMR absorptions in ppm)

Assignments	4-Methylpyridine50, 58, 59, 70 in CDCl3	[1]	[2]
H-C2 & H-C6 (Ortho),	8.4 2H d	8.8 ↓	8.7 ↓
H-C3 & H-C5 (Meta),	7.1 2H d	7.8 ↓	7.9 ↓
CH3 attached to C4.	2.3 3H s	2.5 ↓	2.5 ↓
Residual69 DMSO-d6		2.5	2.5
THF69 O-CH2			3.5
THF69 CH2			1.7

2-Methylpyridine^{58, 59, 70}  and [3] spectra reveal that all of the protons resonances occur at lower field due to delocalisation of  ring π-electron density when ligand coordinates with molybdenum through nitrogen (Fig. 3, Table-6).

Figure 3: FT-1H NMR of Mo4O4Cl4(C6H7N)3(C4H8O)2, [3] Click here to View figure

Table 6: (¹H-NMR absorptions in ppm)

Assignments	2-Methylpyridine58, 59, 70 in CDCl3	[3]
CH3 attached to C2,	2.5 3H s	2.8 ↓
H-C5 (Meta),	7.0 1H t	7.9 ↓
H-C3 (Meta),	7.1 1H d	7.9 ↓
H-C6 (Ortho),	8.4 1H d	8.8 ↓
H-C4 (Para).	7.5 1H t	8.5 ↓
Residual69 DMSO-d6		2.5
THF69 O-CH2		3.6
THF69 CH2		1.8

[4] spectrum shows that all the protons of the 1-methylimidazole^{67, 68, 71} have deshielded on account of flow of π-electrons towards molybdenum on coordination (Fig. 4, Table-7).

Figure 4: FT-1H NMR of Mo2O4Cl4(C4H6N2)2(C4H8O), [4] Click here to View figure

Table 7: (¹H-NMR absorptions in ppm)

Assignments	1-Methylimidazole67, 68, 71 in D2O	[4]
H-C4	7.0 1H	7.6 ↓
H-C5	7.0 1H	7.6 ↓
H-C2 (between two N)	7.5 1H	9.1 ↓
CH3 attached to N	3.6 3H	3.8 ↓
Residual69 DMSO-d6		2.5
THF69 O-CH2		3.6
THF69 CH2

FT-¹³C NMR Spectra

Compounds were dissolved in DMSO-d₆ to take spectrum. DMSO-d₆⁶⁹residual peak occurs at 39.52±0.06 ppm.↑ and ↓ represent upfield/downfield shift.

Spectrum of [2] reveals that all carbons of 4-methylpyridine⁷² show resonances at lower field except meta carbons on ligand-metal coordination (Fig. 5, Table-8).

Figure 5: 13C-NMR of Mo2O2Cl5(C6H7N)2(C4H8O)2, [5] Click here to View figure

Table 8: (¹³C NMR absorptions in ppm)

Assignments	4-Methylpyridine72 in CDCl3	[2]
C-2 C-6 (ortho)	149.6	157.6
C-3 C-5 (meta)	146.9	139.8
C-4 (para)	124.6	127.5
CH3 attached to C-4	20.8	22.1
Residual69 DMSO-d6		39.4

 

Microbiological Activity

Potency of molybdenum compounds synthesized against Gram-positive bacteria Staphylococcus aureus, Gram-negative bacteria E. coli, fungi Candida albicans and Aspergillus niger has been investigated. Amoxicillin and ketoconazole have been used as reference drugs against bacteria and fungi, respectively. Efficiency of molybdenum compounds have been tested by ‘Zone of inhibition’ method⁷³. Potency of molybdenum compounds has been found to be good to control bacteria and fungi (Table-9). It is to be noted that,

Compounds 1, 2, 3 and 4 are more potent against E. coli than amoxicillin.

Compound 4 is more potent against C. albicans than ketoconazole.

Table 9: (MicrobiologicalStudy)

Compound (100 µg/ml)	Zone of inhibition73 (mm)
	S. aureus	E. coli	C. albicans	A. niger
Reference Drug	24.99	17.85	21.37	27.88
[1]	21.36	21.58	19.57	19.63
[2]	21.28	23.14	19.87	18.54
[3]	22.31	21.56	18.38	19.88
[4]	19.87	21.32	22.17	21.38

 

Mass (LC-MS) Spectra

Masses of the most abundant isotopes have been used to calculate theoretical m/z values⁷⁴ of the ions (Tables-10, 11),

Table 10: LC-MS Ionization Click here to View table

Table 11: (m/z values of ions)

Comp.	Ion	Calculated74	Observed	Relative  abundance
[1]	[C6H7N]+	93.05	94.06	100%
[2]	[MoO2Cl(C6H7N)]+	257.92	256.18	58%
	[MoO2Cl3(C6H7N)]+	327.85	328.25	3%
	[MoO2Cl3(C6H7N)]2+	163.92	166.14	2%
	[MoO2Cl3]+	234.80	238.21	4%
	[C6H7N]+	93.05	94.06	100%
[3]	[MoO2Cl(C6H7N)]+	257.92	256.15	92%
	[MoO2Cl3(C6H7N)]+	327.85	328.25	34%
	[MoO2Cl3(C6H7N)]2+	163.92	166.14	25%
	[C6H7N]+	93.05	94.06	100%
[4]	[MoOCl]+	148.86	149.05	1%
	[MoOCl2(C4H8O)]+	255.89	256.19	8%
	[MoOCl2(C4H8O)(C4H6N2)]+	337.94	338.39	1%
	[C4H6N2]+	82.05	83.06	100%

 

Conclusion

There is an increase in υ(C=N) and torsion and decrease in ring δ(C-H) due to Mo(dπ)→N(pπ) in [1]. 976 cm^-1 peak reveals that Mo=O is in terminal position. υ(Mo=O) decreases on N→Mo coordination. Mo=O and the coordinate bond are trans to each other. Molybdenum abstracts oxygen from THF to form Mo=O. Presence of N→Mo coordinate bond is further supported by the ¹H NMR in which all absorptions move to lower field, due to flow of π-electron density towards molybdenum. [1] is very much active against microbes. Ions detected in LC-MS suggest the formula presented.

υ(C=N) and torsion show upward shift and ring δ(C-H) shows downward shift owing to Mo(dπ)→N(pπ) [2]. Mo=O is a terminal group is inferred by in the observation of 975 cm^-1 peak. N→Mo coordination leads to decrease of υ(Mo=O). Coordinate bond and Mo=O are trans to each other. Molybdenum abstracts oxygen from THF forming Mo=O. N→Mo coordinate bond is further supported by the fact that all ¹H NMR absorptions show deshielding, because of depletion of ring π-electron density. Deshielding is further supported by ¹³C NMR in which absorptions move downfield. Compound is very much potent against microbes. LC-MS fragments noted are in tune with the formula suggested.

υ(C=N) shows an upward shift by 36 cm^-1 and torsion shows an upward shift by 24 cm^-1 in [3] metal to ligand back bonding. υ(Mo=O) at 980 cm^-1 due to refers to Mo=O group in terminal position. υ(Mo=O) declines on coordinate bond formation. Mo=O and the coordinate bond are trans to each other. Molybdenum abstracts oxygen from THF leading to formation of Mo=O. N→Mo coordination is further evident from ¹H NMR which shows that all the protons have deshielded on account of decrease in ring π-electron density. Compound can control microorganisms. Fragments observed in mass spectrum conform to the formula.

υ(C=C)shows an increase of 36 cm^-1 and υ(N-C) also shows an increase of 39 cm^-1 in [4]. This increase is due to inductive effect on coordination of ligand with molybdenum cation and d_π-p_π interactions. Increase in electron density increases the coordinate bond strength. υ(Mo=O) is observed at 978 cm^-1 indicating Mo=O being terminal one. N→Mo coordination is further supported by ¹H NMR which shows that all the protons have deshielded. Compound has been found to be potent against microbes. Ions observed in LC-MS agree with the formula derived.

Acknowledgement

We are thankful to P. U. Chandigarh, India for extending the instrumental help for characterizing the samples.

Conflict of Interest

There is no conflict of interest among the authors.

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