| dc.description.abstract | Hydrogen fluoride is one of the most inexpensive fluorinating reagents, used for fluorinations, but its use generally requires work under pressure due to its low boiling point. To overcome this difficulty, it has been found that when hydrogen fluoride is complexed with pyridine it forms a stable solution of pyridinium poly(hydrogen fluoride) C?H?NH?(HF)?F?, PPHF.
This pyridinium poly(hydrogen fluoride) reagent has been widely used in organic chemistry for fluorination reactions, but it is scarcely used for inorganic fluorinations except for the solitary use of this reagent for the preparation of sulphuryl chlorofluoride from sulphuryl chloride. Recently, in this laboratory it has been shown that this reagent can be used for inorganic fluorinations with equally good advantage for the preparation of a variety of inorganic fluorides and complex fluorides of both metals and non-metals. Hence to further explore the synthetic utility of this reagent in inorganic chemistry, the present work has been undertaken. The results of the present study indicate that PPHF could be considered as a general purpose fluorinating reagent for inorganic compounds.
The solvents required for the present investigation have been purified by standard procedures. Some of the special reagents used in the present study are prepared following the methods reported in the literature. Pyridinium poly(hydrogen fluoride) reagent has been prepared (in different compositions) by the method described by Olah et al. by adding liquid hydrogen fluoride to well-cooled pyridine at -70° to -80°C with constant stirring. Though PPHF [70% w/w of HF and 30% w/w pyridine] has been mainly used for organic fluorination reactions, a dilute solution of PPHF with 40% w/w of HF and 60% w/w pyridine is found to be more suitable in the present study for inorganic fluorinations.
Standard analytical methods have been adopted for the estimation of pyridine, nitrogen, fluorine, zirconium, hafnium, antimony, aluminium and other elements present in the various compounds synthesized presently.
A variety of physico-chemical techniques such as infrared and nuclear magnetic resonance (¹H, ¹?F) spectroscopy, X-ray powder diffraction, EDAX and SEM have been employed for the identification and characterisation of the compounds prepared in the present study.
Metal powders have been found to react directly with pyridinium poly(hydrogen fluoride) at room temperature, producing metal fluorides and pyridinium complexes of the metal fluorides in their most stable oxidation states. Generally the metal powders are unreactive towards strong fluorinating reagents such as fluorine, halogen fluorides and anhydrous hydrogen fluorides as they form a protective (passive) layer of the metal fluoride on the surface, which prevents further reaction. To enhance the reactivity of metal powders with these fluorinating reagents the reactions have to be carried out at higher temperatures. This difficulty has been overcome in the present investigation, where the metal powders react with PPHF at room temperature to give a single product in good yields and purity. In addition this procedure has several advantages compared to the use of other fluorinating reagents which give rise to mixtures of metal fluorides in different oxidation states and in low yields and purity. The extent of reaction of the metal powders depends on the particle size and intimate contact brought about by vigorous shaking or stirring.
The following table gives the metal powders reacted with PPHF and products obtained:
Manganese ? Manganese(II) fluoride
Cobalt ? Cobalt(II) fluoride
Nickel ? Nickel(II) fluoride
Copper ? Copper(II) fluoride dihydrate
Zinc ? Zinc(II) fluoride
Cadmium ? Cadmium(II) fluoride
Bismuth ? Bismuth(III) fluoride
Chromium ? Pyridinium hexafluorochromate
Iron ? Pyridinium hexafluoroferrate
Zirconium ? Pyridinium pentafluorozirconate
Aluminium ? Pyridinium hexafluoroaluminate
Silicon ? Pyridinium hexafluorosilicate
Pyridinium poly(hydrogen fluoride) reacts with zirconium(IV) oxide, hafnium(IV) oxide, antimony(III) chloride, bismuth(III) oxide and manganese(II) chloride to yield the respective pyridinium fluorometallates and binary fluorides. The overall reactions can be represented as:
ZrO? + C?H?NH?(HF)?F? ? C?H?NHZrF? + 2 H?O
HfO? + C?H?NH?(HF)?F? ? C?H?NHHfF? + 2 H?O
SbCl? + C?H?NH?(HF)?F? ? (C?H?NH)?SbF? + 3 HCl
Bi?O? + C?H?NH?(HF)?F? ? 2 BiF? + 3 H?O
MnCl? + C?H?NH?(HF)?F? ? MnF? + 2 HCl
All the product compounds have been obtained in good yields and purity in a convenient one-pot reaction. This eliminates the need for high temperature and further processing of products, as is common with other fluorinating reagents.
The rare earth oxides such as lanthanum oxide, neodymium oxide, samarium oxide, europium oxide, dysprosium oxide and yttrium oxide react with pyridinium poly(hydrogen fluoride) at room temperature to form the trifluorides of the corresponding lanthanides.
The interesting features are:
The reactions of lanthanum and yttrium oxide with PPHF have been found to be highly exothermic, hence these reactions are carried out at low temperature (-30°C).
This method of obtaining the trifluorides of lanthanides is advantageous over the other methods which need high temperatures for the reaction and also the products obtained presently are highly pure and therefore do not need further processing as only one type of the fluoride is obtained. A new complex fluoride of aluminium, pyridinium hexafluoroaluminate,
(C?H?NH)?AlF?, has been prepared in very good yields and high purity by the reaction of pyridinium poly(hydrogen fluoride) with aluminium(III) compounds, such as aluminium trichloride, aluminium trihydroxide and aluminium trioxide. Earlier reports are not available in the literature regarding the synthesis of pyridinium hexafluoroaluminate. This compound has been prepared and characterised for the first time in the present study. NMR-(¹H, and ¹?F) and IR-spectroscopic in the region 200 to 4000 cm?¹ have been recorded and reported in the present study. Analysis for both aluminium and pyridine confirms the composition and purity of the compound pyridinium hexafluoroaluminate.
Pure samples of pyridinium hexafluoroaluminate have been used as a convenient starting material for the preparation of ammonium, sodium, potassium, rubidium and cesium hexafluoroaluminates by cation exchange reactions using the corresponding metal chlorides/hydroxides. Similar methods have been standardised using pyridinium hexafluoroaluminate as the precursor for the preparation of magnesium, calcium, strontium, barium, nickel, manganese and cobalt hexafluoroaluminates.
As has been highlighted in the introductory chapter, the conventional methods used for the preparation of compounds involve the handling of anhydrous reagents, tedious and time consuming. The present method is very simple, convenient and good yields are of high purity.
Polytetrafluoroethylene (PTFE) is an industrially important polymer, as has been highlighted in the introductory chapter. It is resistant to attack by highly corrosive chemicals such as aqua regia, hydrofluoric acid, fuming sulphuric acid, nitric acid etc., due to its unusual and remarkable combination of properties. During the present investigation it has been observed that this polymer undergoes partial reduction (cleavage of C-F bonds and formation of C-H bonds) with PPHF in presence of metals (chromium/manganese) at room temperature giving a hydrogenated new polymer, PTFE(H) in powder form. The new polymer has an overall chemical composition ---(CHF)?--- i.e., a C:H:F ratio 1:1:1 and differs considerably from polyvinylidene fluoride which has a similar ratio indicating a discontinuity in cleavage of C-F bonds. Thus, PPHF acts as a reducing medium in the presence of metal powders (chromium/manganese). This reducing property stems from either the presence of the atomic hydrogen generated or hydrogen adsorbed on the metal surface.
The SEM photographs of PTFE (treated bit) show some change in surface structure after reaction. Hence it was thought that the surface which has undergone some modification may bind the metal when electroplated more firmly than the surface before treatment, since these metal plated sheets play a very important role in industry. Hence electroplating of copper has been carried out on the PTFE bit after reaction. The electroplated PTFE bit before and after reaction has been tested using the tape peel-off test and by taking the SEM photographs of the surface. From these examinations we could observe that there is a moderate improvement in the adhesion of metal to the surface of a PTFE bit after treatment.
The fluorinating capacity of PPHF has been extended for the preparation of methyl fluorosilanes. Pyridinium poly(hydrogen fluoride) reacts with trimethyl chlorosilane, dimethyl dichlorosilane and methyl trichlorosilane at room temperature to give trimethyl fluorosilane, dimethyl difluorosilane and methyl trifluorosilane respectively. The conventional methods, as have been mentioned in the introductory chapter, need either high temperature, or mixtures of products are obtained in low yields which require prolonged periods for their purification. The present method is more elegant, the products obtained are very pure, no mixtures of products are obtained and the yields are also high as well as this is a one-step reaction, making it all the more acceptable.
In conclusion all the above results indicate that PPHF could be used as a versatile and convenient fluorinating reagent for the preparation of metal fluorides and complex metal fluoride salts, at room temperature. The advantage of using this reagent over hydrogen fluoride is that it eliminates the reactions to be carried out at high pressures and high temperatures. Most of the reactions are one-step reactions, giving pure products (without any mixtures) in good yields. Very fine particles of the reactants react very fast and thus there is saving on time. The fluorinated products obtained are very stable and easily isolable. The pyridinium fluorometallates isolated play a very important role as precursors for the synthesis of other alkali and alkaline earth metal fluoro complexes by cation exchange reactions (e.g., several metal fluoroaluminates have been isolated in pure form as the displaced pyridine can easily be washed away by chloroform). In addition moisture-sensitive fluorides and complex fluorides (e.g., pyridinium fluorocomplexes) can be isolated by adopting a dry nitrogen cover during processing and storing.
The ace in the study is however the cleavage of the C-F bond in PTFE and formation of a C-H bond (without C-C bond cleavage) thus retaining the polymeric character of PTFE with added advantages of a smoother surface. This has led to a denser deposit of copper when electroplated. | |
