PREPARATION OF TETRAHYDROXOARGENTATE (III) ION AND STUDY OF ITS REACTIONS WITH CERTAIN AROMATIC AMINES

Abstract

Transition metal ions and their coordination compounds are well known to play an important catalytic role in different homogeneous and heterogeneous redox reactions. The unusual oxidation state species of these metal ions are often being assigned to explain the catalysis. The mechanism of the catalytic action involving stable oxidation state couples viz., Co(II) / Co(III), T1(I) / Tl(III), Fe(II)/ Fe(III), Pt(II) / Pt(IV) etc. have been studied widely. But the reactions of redox couples containing short-lived species have been poorly investigated because of the problems encountered in their generation and stabilization. Moreover, their production generally involve sophisticated instrumentation like pulse radiolysis, flash photolysis etc. which further restricts these investigations to the place of their availability. Silver(I) is a well known catalyst for a variety of redox 2- reactions involving Ce(IV), SO etc. as the oxidising agents. Its higher oxidation state species i.e., Ag(II) and Ag(III) are often being invoked to be responsible for the catalytic action. To work out the mechanism of these systems, the reactions of silver(II) species with a variety of organics have been studied in detail using pulse radiolysis and stopped-flow techniques. However, the reaction of silver(III) species, which is generally produced by disproportionation of silver(II) species Ag(II) + Ag(II) *» Ag(III) + Ag(I) could not be investigated extensively since the complexatlon and oxidation of the substrates occurs at shorter time scale by silver(II) species itself. The study of silver!III) species could become possible by the tetrahydroxoargentate(III) Ion ([ Ag (OH) 1 ) which is produced in the metastable state by the anodic oxidation of silver metal in highly basic medium. Its reactions with few organlcs have been examined during last few years and exhibit quite diverse and complex mechanisms. The mechanism involves the formation of both four as well as five-coordinate intermediates as precursors prior to the occurrence of redox reaction. Further complexity has also been noted in the mode of reduction of silver(III) centre. Reduction of [Ag (OH) ] has been found to take place in simultaneous two-electrons as well as stepwise one-electron process. In view of this complex behaviour a systematic investigation on the reactions of certain aromatic amines with the tetrahydroxoargentate(111) ion has been undertaken. Unlike In the past, emphasis on the isolation, characterization and quantitation of the products has been laid which may be helpful in the elucidation of the reaction mechanism. The present thesis comprises five chapters. The first chapter includes the background of the work being done in this area and defines the objectives of the present investigation. The second chapter describes experimental details viz.^ materials, equipments, techniques and methods used. It also incorporates the preparation and characterization of the tetrahydroxoargentate(111) ion.

The studies of the reaction of [Ag (OH).]" with aniline are Included in the third chapter. TLC and GC techniques have been used for the separation and qualitative identification of different components of the products. GC-MS has been employed for the characterization of these components. Azobenzene and p-benzoquinone have been identified as the main products of this reaction. The yields of these compounds have been determined by using GC technique. The effect of concentration of aniline on the yield of products has also been investigated. An increase In aniline enhances the yield of azobenzene but does not affect appreciably the amount of p-benzoquinone. The stolchlometry of the reaction is found to vary with a change in the concentration of aniline. At low [aniline] -4 IS 5 x 10 M) it was 1 : 1 and changed to 1:2 at higher concentrations of aniline (fe 1 x 10~3 M) for [Ag (OH) ]" : aniline. The kinetics of the early events are analysed by stopped-flow technique. It consists of four klnetlcally distinguishable processes. The first step is assigned as the aquation path which leads to the formation of [Ag (Oliy H.,0) ] species. This step Is followed by nucleophilic displacement of water to form monoanilino sllverdlD-complex which has higher absorption than silver(III) species. This species then decays in two consecutive steps. The first decay involves further uptake of ligand to form bis (anilino) silverdlD-complex and is followed by its decomposition in the subsequent step to eventually produce azobenzene and silver(O). The nature of the intermediates formed is confirmed by carrying out this reaction in the presence of radical scavenger. ill The fourth chapter presents the studies on the reactions of 2,6-, 2,4-, 3,4- DMA and N,N- DMA with the tetrahydroxoargentate( III) ion. These reactions are carried out to verify the findings of [Ag (OH).] and aniline reaction and to arrive at a general mechanism. Interestingly, the products of the reaction of [Ag (OH) ] with 2,6-DMA are 2,6-dlmethyl-p-benzoqulnonR and 2,6, 2'6'-tetramethylazobenzeno which are similar to those of reaction of silver(III) and aniline. In the case of 2,4- and 3,4-DMA one product each I.e., the 2,2",4,4'-and 3,3',4,4'- dimethylazobenzene are formed selectively. The early kinetics of these reactions also demonstrate a very similar behaviour to those observed with [Ag (OH) ] and aniline reaction and comprises four steps. The effect of acrylonitrlle on these reactions is also studied to find out the nature of the intermediates. The fifth chapter gives a brief account of general behaviour of [Ag (OH) ] species. Mechanistic observations made with the earlier studied reactions have been discussed. It incorporates a summary of the results of the present Investigation. On the basis of these data a general mechanism for the studied reactions has been concluded.