INTRODUCTION:
Electrochemical synthesis of metal alkoxides, phenoxides, glycolates and thiolates by the oxidation of sacrificial anodes in the solutions of alcohols, phenols, aldehydes, ketones, thiols, dithiols etc. have been reported in literature1-6. A close study of technique reveals that this technique provides a direct and single step synthetic route and the reactions proceed with high current efficiencies. In continuation to our efforts to explore the use of the electrochemical technique as a synthetic tool, we report in the present communication the electrochemical synthesis of organozinc halides and their coordination compounds with 1,10-phenanthroline and 2,2'-bipyridyl.
Materials and Methods
Reagents and solvent: Acetonitrile was dried over phosphorus pentoxide and then fractionally distilled and was used as solvent in all these reactions. Tetrabutylammonium chloride was crystallized from conductivity water, dried under vacuum at 100°C and was used as supporting electrolyte. Dicyclopentadiene was refluxed for eight hours to obtain cyclopentadiene. Other organic compounds were used as procured.
Organozinc /Organozinc halides:
Electrolysis of the solutions of cyclopentadiene or alkyl/aryl halide (0.02 mole) and tetrabutylammonium chloride (0.003 mole) in acetonitrile (250 mL) was carried out at zinc anode and platinum cathode in a H-type pyrex glass cell. The solution was stirred thoroughly during the progress of electrolysis with the help of a magnetic stirrer. The electrolytic cell can be represented as:
RX or Cp +
Zn(+) Bu4NCl + CH3CN Pt (-)
Where;
Zn(+) and Pt(-) represent zinc anode and platinum cathode respectively.
RX is alkyl/aryl halide and Cp is cyclopentadiene.
The solid products started separating in the anode compartment during the progress of electrolysis. After ten hours of electrolysis, the solid products from anode compartment were filtered, washed with hot acetonitrile and dry ether and then dried under vacuum.
Coordination Compounds of Organozinc /Organozinc halides:
Coordination compounds of these Organozinc /Organozinc halides were synthesized by electrolyzing the solution of cyclopentadiene or alkyl/aryl halide, supporting electrolyte and 0.005 mole of the ligand in acetonitrile essentially by the same method as detailed above.
Zinc contents in the products were determined volumetrically by oxine method7and halide contents were determined volumetrically by Mohr’s titration method7. Microanalyses for carbon and hydrogen in these products have also been conducted.
Infrared spectra of these products have been scanned in potassium bromide pellets by using Perkin-Elmer spectrophotometer, RXI, in the region of 4000 – 450 cm-1.
Current efficiencies (gram equivalent of metal dissolved per faraday of electricity) of all these products were determined by electrolysing the above solutions in the similar manner at a constant current of 20 mA for one hour. The ratio of experimental and theoretical zinc contents gives the current efficiency.
RESULTS AND DISCUSSION:
Organozinc halides:
Solid products isolated from the anode compartment are insoluble in different organic solvents such as dimethyl sulphoxide, N,N-dimethyl formamide, benzene, carbon tetrachloride, methanol, etc. These products do not melt upto 300°C.
Analytical data of the products are summarised in Table – I. The data correspond to 1:1 stoichiometry of zinc and alkyl halide while 2:1 stoichiometry of cyclopentadiene and zinc.
Infrared spectra of these products have also been recorded in the range of 4000 to 450 cm-1. The characterstic bands in the products appear in the region of 503 – 492 cm-1, while in case of zinc-cyclopentadiene system the characteristic absorption bands appear at 3060.1 cm-1, 1428.6 cm-1, 797.2 cm-1 and 492.5 cm-1.
Survey of literature reveals8-11 that metal – halogen bands appear in the range of 350 to 250 cm-1 and the absorption bands due to ν(Zn – C) appear12-14 in the region of 560 – 480 cm-1. The absorption bands appearing in the region of 503 – 492 cm-1 in all the products may thus be assigned to ν(Zn – C) stretching vibrations. Infrared spectral data thus suppliments the analytical data and the product of alkyl/aryl halides can be assigned the molecular formula of RZnX. In case of cyclopentadiene system, the absorption band appearing at 492.5 cm-1 may be assigned to ν(Zn – C) stretching vibrations and the intensive bands observed at 797.2 and 3060.1 cm-1 may be assigned ν(C – H) deformation vibrations of cyclopentadienyl ring15-16 and sharp absorption band at 1428.6 cm-1 may be assigned to ν(C – C) of Cp ring.
Survey of literature also reveals that Zn(Cp)2 exist as polymer17. Analytical and infrared data reveals that the present product of cyclopentadiene may be polymeric in nature.
Coordination Compounds of Organozinc /Organozinc halides:
These compounds are insoluble in commonly used organic solvents and do not melt upto 300ºC. But the colour change has been observed.
Analytical data of the products are summarised in Table – III. The data correspond to 1:1:1 stoichiometry of zinc, alkyl halide and ligand, while 2:1:1 stoichiometry of cyclopentadiene, zinc and ligand.
Infrared spectra of these products have also been recorded in the range of 4000 to 450 cm-1. The bands present in the region of 515 – 500 cm-1 may be assigned to ν(Zn – C) stretching modes12-14. The shift of these bands towards higher region as compared to those in the parent organozinc compounds indicates the coordination of ligand molecules. The attachment of the ligand molecule is also confirmed by the appearance of absorption bands due to ligand molecules (ν(C…C) and ν(C … N) bands due to ligand molecules18) in the region of 1630 – 1420 cm-1. These bands shift towards the lower region as compared to those in pure ligand molecules. In addition to these bands infrared spectra of cyclopentadiene + ligand systems also show ν(C – H) deformation vibrations.
Current efficiencies of all these systems are very high even more than one in all the cases (given in Table – II and IV) except in case of Cp systems. The unusually high current efficiencies can be explained on the basis of the following mechanism:
At inert cathode:
The halide ions formed at cathode migrate to the anode compartment under the influence of applied potential and undergo there following sequence of reactions:
At sacrificial anode:
TablE -I: Electrolysis Characteristics, Analytical and other Related Data of Electrolytic Products of Alkyl Halides/Chlorobenzene and Cyclopentadiene at Zinc Anode
|
System |
Potential applied (v) |
Electricity passed (Coulombs) |
Product |
Colour |
Elemental analysisFound (Calc.) % |
|||
|
Zn |
C |
H |
X |
|||||
|
Bromoethane |
25 |
680 |
C2H5ZnBr |
Light brown |
36.2 (37.5) |
11.0 (13.7) |
1.8 (2.9) |
43.4 (45.9) |
|
I-Bromopropane |
25 |
680 |
C3H7ZnBr |
White |
33.1 (34.7) |
17.9 (19.1) |
2.8 (3.7) |
41.3 (42.4) |
|
I-Chlorobutane |
25 |
680 |
C4H9ZnCl |
Brown |
39.0 (41.4) |
28.1 (30.4) |
4.6 (5.7) |
20.1 (22.5) |
|
Chlorobenzene |
30 |
680 |
C6H5ZnCl |
Light brown |
34.9 (36.8) |
36.7 (40.6) |
2.2 (2.8) |
18.0 (20.0) |
|
Cyclopentadiene |
30 |
720 |
(C5H5)2Zn |
Light brown |
33.5 (32.9) |
61.4 (59.1) |
5.1 (4.3) |
- |
Table - II: Current Efficiencies of Electrochemical Reactions of Alkyl Halides/ Chlorobenzene at Different Intervals of Time
System |
Time (Hours) |
Current efficiency (Gram equivalent/faraday) |
|
Bromoethane |
0.5 1.0 2.0 3.0 |
2.30 1.50 1.20 0.63 |
|
1-Bromopropane |
0.5 1.0 2.0 3.0 |
2.70 1.50 1.30 0.69 |
|
1-Chlorobutane |
0.5 1.0 2.0 3.0 |
2.00 1.40 1.00 0.64 |
|
Chlorobenzene |
0.5 1.0 2.0 3.0 |
1.30 1.10 1.00 0.66 |
Table - III: Electrolysis Characteristics, Analytical and other Related Data of Electrolytic Products of Alkyl Halides/ Chlorobenzene and Cyclopentadiene + Ligand systems
System |
Potential applied (V) |
Electricity passed (Coulombs) |
Product |
Colour |
Elemental analysis Found (Calc.) % |
|||
|
Zn |
C |
H |
X |
|||||
|
1-Bromoethane + 1,10-Phenanthroline |
25 |
680 |
C2H5ZnBrC12H8N2 |
Pinkish white |
22.0 (18.5) |
42.1 (47.4) |
3.3 (3.7) |
19.5 (22.6) |
|
1-Bromopropane + 1,10-Phenanthroline |
25 |
680 |
C3H7ZnBrC12H8N2 |
Pinkish white |
17.1 (17.8) |
45.4 (48.9) |
3.2 (4.1) |
21.7 (21.7) |
|
1-Chlorobutane + 1,10-Phenanthroline |
25 |
680 |
C4H9ZnClC12H8N2 |
Brown |
18.1 (19.4) |
54.6 (56.8) |
3.7 (5.0) |
9.1 (10.5) |
|
Chlorobenzene + 1,10-Phenanthroline |
25 |
680 |
C6H5ZnClC10H8N2 |
Dark brown |
17.1 (18.3) |
55.9 (60.4) |
3.4 (3.6) |
8.5 (9.9) |
|
Cyclopentadiene + 1,10-Phenanthroline |
25 |
720 |
(C5H5)2ZnC12H8N2 |
Dark brown |
17.4 (17.0) |
70.3 (68.1) |
3.8 (3.9) |
- |
|
Bromoethane + 2,2'-Bipyridyl |
25 |
680 |
C2H5ZnBrC10H8N2 |
Pinkish white |
19.0 (19.8) |
41.0 (43.6) |
3.4 (3.9) |
22.5 (24.2) |
|
1-Bromopropane + 2,2'-Bipyridyl |
25 |
680 |
C3H7ZnBrC10H8N2 |
Pinkish white |
18.1 (19.0) |
43.5 (45.3) |
3.9 (4.4) |
20.0 (23.2) |
|
1-Chlorobutane + 2,2'-Bipyridyl |
25 |
680 |
C4H9ZnClC10H8N2 |
Light brown |
19.9 (20.8) |
49.3 (53.5) |
4.7 (5.4) |
9.1 (11.3) |
|
Chlorobenzene + 2,2'-Bipyridyl |
25 |
680 |
C6H5ZnClC10H8N2 |
Dark pink |
17.8 (19.6) |
55.4 (57.5) |
3.5 (3.9) |
10.8 (10.6) |
|
Cyclopentadiene + 2,2'-Bipyridyl |
25 |
720 |
(C5H5)2ZnC10H8N2 |
Dark pink |
18.6 (17.9) |
68.3 (66.0) |
5.1 (4.0) |
- |
TablE - IV: Current efficiencies of the electrochemical reactions of Alkyl halides/Chlorobenzene + ligand systems at zinc anode at different intervals of time
|
System |
Time (Hours) |
Current efficiency (Gram equivalent/Faraday) |
|
Bromoethane + 1,10-Phenanthroline
|
0.5 1.0 2.0 |
3.10 2.00 1.00 |
|
1-Bromopropane + 1,10-Phenanthroline
|
0.5 1.0 2.0 |
2.00 1.10 0.98 |
|
1-Chlorobutane + 1,10-Phenanthroline
|
0.5 1.0 2.0 |
3.10 2.10 0.80 |
|
Chlorobenzene + 1,10-Phenanthroline
|
0.5 1.0 2.0 |
2.40 1.40 0.98 |
|
Bromoethane + 2,2'-Bipyridyl
|
0.5 1.0 2.0 |
2.80 1.50 1.10 |
|
1-Bromopropane + 2,2'-Bipyridyl
|
0.5 1.0 2.0 |
2.50 1.90 0.98 |
|
1-Chlorobutane + 2,2'-Bipyridyl
|
0.5 1.0 2.0 |
3.10 1.68 1.00 |
|
Chlorobenzene + 2,2'-Bipyridyl
|
0.5 1.0 2.0 |
2.10 1.00 0.67 |