Synthesis and Characterization of 2-[2-Aryltelluroethyl] -3-Methyl Pyridines (Te,N) and 2-[2-(3-Methyl Pyridoethyltelluro) Ethyl]-3-Methyl Pyridine (N,Te,N)

Introduction

There has been a recent interest in the ligand chemistry of tellurium^1,2 but despite this growth of activity the literature contains few reports of bi-or multidentate ligands containing tellurium. Incorporation of the ditelluride group into a cyclic system may be expressed to enhance the reactivity at the Te-Te bond as a result of strain involving both entropic and enthalpic effect³. Here we report synthesis and characterization of 2-(2-aryltelluroethyl)-3-methyl pyridines (L) [C₁₄H₁₅NTe], where Ar = Phenyl (La) [C₁₄H₁₅NTe], 4-methylphenyl (Lb) [C₁₅H₁₇NTe], 4-methoxyphenyl (Lc) [C₁₅H₁₇ONTe], 4-ethoxyphenyl (Ld) [C₁₆H₁₉ONTe], and 2-[2-(3-methyl pyridoethyltelluro) ethyl]-3-methyl pyridine (L’) [C₁₆H₂₀ N₂Te].

Material and Methods

Tellurium tetrachloride, aryl tellurium trichlorides, diarylditellurides, diphenyldiselenides, 2-(2-chloroethyl) pyridine are the main starting materials used in this work. The methods of conductance and magnetic moment measurement are given. The instruments and techniques used to record IR and NMR [¹H, ¹³C {¹H}]. Chemicals were obtained from BDH, Aldrich, Strem and Sigma are used without purification. Organic solvents (BDH, Merck and Glaxo) were used after purification and drying (whenever required) by standard methods^4-5.

Experimental

2-(2-chloroethyl)  pyridine and diarylditellurides (aryl= phenyl, 4-methylphenyl, 4-methoxyphenyl, 4-ethoxyphenyl) prepared by the literature methods^6-7. The detailed procedure for the synthesis of L and L’ are given below.

Synthesis of 2-[2-aryltelluroethyl]-3-methyl pyridines (La-d)

To the hot ethanolic (25cm³) solution of appropriate ditelluride (3 mmol), solution of sodium boro hydride (1.0 gm, 10 cm³ of 10% NaOH), added slowly in nitrogen atmosphere, till colourless solution of sodium aryl telluride formed. To this 2-(2-chloroethyl)-3-methyl pyridine (6 mmol dissolved in 5 cm³ ethanol) was added drop wise with vigorous stirring and the resulting solution further refluxed for one hour. The ligand (La-Ld) thus formed were extracted into chloroform (250 cm³). After removing the excess of chloroform, liquid form of the compound separated out, which was purified by column chromatography, using silica column and chloroform: hexane (5: 95; 10: 90) as elutent.

The high boiling liquids L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] are pale yellow to red in colour, soluble in nonpolar and polar organic solvents. On exposure to air they change their colour from yellow to dark red. The various physical and spectroscopic properties of L and L’ are discussed below:

Conductance and molecular weight IR spectra measurements

The molar conductance (/\_m) values of ligand L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] (Table 1)

determined in acetonitrile and nitro methane (1mM), reveals their nonelectrolytic behaviour. Molecular weights (Table 1) of L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] determined in chloroform have been found close to the molecular formulations arrived at by analytical data.

Conductance and molecular weight IR spectra measurements

The molar conductance (/\_m) values of ligand L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] (Table 1)

The important IR bands along with their assignments and are presented in table 2. The assignments of the bands have been made on the basis of earlier reports^8,10. The IR spectra of L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] show characteristic band at 410 cm^-1 due to δ(C-N) vibration (out of plane ring deformation) ⁽⁸⁾. Two medium to low intensity υ [Te-C (aromatic)] bands appear between 240-260 and 280-295 in the IR spectra of L [C₁₄H₁₅NTe] concurring with earlier observations⁹. A band of medium to strong intensity at around 480 cm^-1 in the IR spectra of L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] attributed to υ[Te-C(aliphatic)] vibration⁽⁹⁾.

Table 1: Physical properties of ligands La-Ld and L’

Compound	Color (Liquid)	Yield %	Bpt. (°C)	Solubility		Mol.wt. (Calcd.)	Te% (Calcd.)In CHCl3	(/\M) In Ch3cn/C6h5no2 Ohm-1cm2mol-1
				Good	Moderate
La [C14H15NTe]	Pale yellow	85	172	Hexane,	_	310 (324.6)	38.20 (39.3)	16.30/2.6
Lb [C15H17NTe]	Yellow	82	180	Hexane,CHCl3CH3 CN	_	303 (338.6)	37.50 (37.68)	14.2/3.4
Lc [C15H17ONTe]	Orange yellow	85	178	CHCl3,CH3CN	Hexane	325 (354.6)	35.93 (35.98)	15.2/2.9
Ld [C16H19ONTe]	Pale yellow	90	184	CHCl3,CH3CN	Hexane	375 (368.6)	34.0 (34.61)	15,6/2.8
L’ [C16H20 N2Te]	Red	90	136	Hexane,CHCl3CH3 CN	_	325 (367.6)	33,60 (34.71)	14.0/2.6

 

For 1:1 electrolyte /\m in acetonitrile = 100-160 and in nitrobenzene = 20-30 ohm-1cm2mol-1

La = C₆H₅TeCH₂CH₂–₂-(C₅H₃NCH₃) Ld = ₄-C₂H₅OC₆H₄TeCH₂CH₂–₂-(C₅H₃NCH₃)
Lb = ₄-CH₃C₆H₄TeCH₂CH₂–₂-(C₅H₃NCH₃) L’ = [(C₅H₃N.CH₃)-₂-CH₂CH₂]₂Te
Lc = ₄-CH₃OC₆H₄TeCH₂CH₂–₂-(C₅H₃NCH₃)

Table 2: Important ir bands (Cm^-1) of L and L’

Ligands	υ{Te-C(aliphatic)}	υ{Te-C(aromatic)}	δ (C-N)
La [C14H15NTe]	480	262,292	410
Lb [C15H17NTe]	482	248,288	408
Lc [C15H17ONTe]	480	250,290	410
Ld [C16H19ONTe]	484	256,298	408
L’ [C16H20 N2Te]	497	–	410

 

Table 3. ¹H and ¹²⁵Te {¹H} NMR data for 2-(2-chloroethyl) pyridine, La-Ld, and L’ in CdCl₃ at25°c (δ, ppm)

Compound	H6 (d)	Aryl and pyridyl protons (m)	H7(m) and H8	H (R) 1H	125Te{1H}
(C5H4N)-2-(CH2)2Cl	8.45	7.20(H3,H5);7.58(H4)	3.9t,3.2t	–	–
La [C14H15NTe]	8.56	7.00-7.60	3.2	–	–
Lb [C15H17NTe]	8.51	6.95-7.70	3.2	2.38s	–
Lc [C15H17ONTe]	8.5	6.70-7.70	3.2	2.75s	481
Ld [C16H19ONTe]	8.55	6.70-7.70	3.2	3.97q, 1.40t	–
L’ [C16H20 N2Te]	8.55	7.39(H3,H5);7.58(H4)	3.3	–	167

 

La = 2-(2-Aryltelluroethyl)-3-methyl pyridine(s = singlet; d = doublet; t = triplet; q = quartet; m =multiplet)

Lb = 2-[2-(4-methylaryl) Telluroethyl]-3-methyl pyridine

Lc = 2-[2-(4-methoxyaryl) Telluroethyl]-3-methyl pyridine

Ld = 2-[2-(4-ethoxyaryl) Telluroethyl]-3-methyl pyridine

L’ = 2-[2-(3-methyl pyridoethyltelluro) ethyl]-3-methyl pyridine

Table 4: ¹³C {¹H} NMR chemical shifts for ligands La-Ld and L’ IN CDCl₃ (δ, ppm)

Ligand	C7	C8	Aryl and Pyridyl	C(R)
La*	40	6.5	137.8(C2), 121.4(C ), 136.2(C ), 122.7(C ), 149.5(C ),	–
			112.6(C9), 138.3(C10), 129.0(C11), 127.3(C12)
Lb	40.1	3.2	134.0(C2), 117.7(C3), 135.0(C4), 119.0(C5), 149.5(C6),	17.6
			105.6(C9), 132.5(C10), 126.5(C11), 121.3(C12)
Lc	40.3	6.8	140.0(C2), 121.3(C3), 136.3(C4), 122.7(C5), 149.5(C6),	55.1
			101.2(C9), 141.0(C10), 115.1(C11), 160.1(C12)
Ld	40.2	6.8	140.0(C2), 121.4(C3), 136.4(C4), 122.2(C5), 149.5(C6),	63.5,14.9
			101.2(C9), 141.1(C10), 115.8(C11), 159.3(C12)
L’*	40.3	2.2	136.5(C2), 120.2(C3), 135.6(C4), 122.3(C5), 148.2(C6)	–

* = Not Observed.

La = 2-(2-Aryltelluroethyl)-3-methyl pyridine [C₁₄H₁₅NTe]

Lb = 2-[2-(4-methylaryl) Telluroethyl]-3-methyl pyridine [C₁₅H₁₇NTe]

Lc = 2-[2-(4-methoxyaryl) Telluroethyl]-3-methyl pyridine [C₁₅H₁₇ONTe]

Ld = 2-[2-(4-ethoxyaryl) Telluroethyl]-3-methyl pyridine [C₁₆H₁₉ONTe]

L’ = 2-[2-(3-methyl pyridoethyltelluro) ethyl]-3-methyl pyridine [C₁₆H₂₀ N₂Te]

¹H and ¹²⁵Te{¹H} NMR spectra

The ¹H NMR spectra of Ligands 2-(2-aryltelluroethyl)-3-methyl pyridines (L) and 2-[2-(3-methyl pyridoethyltelluro) ethyl]-3-methyl pyridine (L’) were recorded in CDCl₃ ^(11-13). The various chemical shifts along with their assignments are given in table 3.

The ¹H NMR spectra of L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] show multiplet at ~δ,3.2for H₇ and H₈ rather than two triplets (centered at δ, 3.9 and 3.2 ppm respectively) as observed in the precursor, 2-(2-chloroethyl)-3-methyl pyridine. The up field shift (0.7ppm) of H₈ in the ligand with respect to their precursor may be attributed to replacement of chlorine atom by tellurium, an atom of much lower electro negativity. This shielding of H₈ protons result in their merger with H₇ signals^14-16.

The atomic ring protons in La [C₁₄H₁₅NTe] appear as multiplet and merged with the pyridine ring protons between δ 7.00-7.60 ppm. The aromatic ring protons of Lb-Ld [C₁₅H₁₇NTe], [C₁₅H₁₇ONTe], [C₁₆H₁₉ONTe], do not appear as two doublets as expected for a 1, 4-disubstituted benzene ring having substituents of different electro negativities⁽¹⁹⁾ but merge with the pyridine ring protons and appear as a multiplet between δ 6.70-7.70 ppm.

In L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] the proton linked to the carbon ortho to nitrogen (H₆) appears most downfield as compare to other ring protons and appears at δ, 8.5ppm as a doublet.

The nitrogen being electronegative in nature deshields the adjacent carbon most, and consequently the proton linked to it, is deshielded most^17,18.

In L’, [C₁₆H₂₀ N₂Te] H₄ appear at δ, 7.25ppm, and H₃ and H₅ appear at ä, 7.08 ppm as multiplet as expected for ortho substituted alkyl pyridine²⁰.

The  ¹²⁵Te  {¹H}  NMR  spectra  of  Lc [C₁₅H₁₇ONTe] and L’ [C₁₆H₂₀ N₂Te] show a sharp singlet at ä, 481 and 167 ppm respectively which are in concordance with the values reported for asymmetric alkyl aryltellurides and symmetric dialkyltelluride^(21-23).

¹³C {¹H} NMR spectra

The assignment of the signals (Table 4) in the ¹³C {¹H} NMR spectra of L [C₁₄H₁₅NTe] and L’ [C₁₆H₂₀ N₂Te] recorded in CDCl₃ have been made on the basis of literature reports on the related compounds and addivity principle. The C8 appears between δ, 2.2 and 6.8 ppm and C₇ around δ, 40.0 ppm. The C₈ have been found to be shielded because of the presence of a lone pair of electrons on the less electronegative tellurium atom in the ligand concurring with the earlier reports²⁷ on such compounds.

This observation supports the shielding of CH₂Te protons and consequently their merger with the C₇ protons in ¹H NMR spectra. The phenyl ring carbon linked to tellurium (i.e. C₉) appears around δ, 100 ppm in the ¹³C {¹H} NMR spectra of L [C₁₄H₁₅NTe] because it experiences greatest shielding among the phenyl carbons due to lone pair of electrons of the tellurium²⁸. The carbons of the pyridine ring appear as expected for the ortho substituted alkyl pyridine. The carbon ortho to nitrogen (C₆) appears at δ, 149.5 ppm, most downfield as compared to the other carbons of the ring. This is because of the greater electro negativity of the nitrogen which deshields C₆ most; consequently it appears at most downfield^29-31.

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