Spectrophotometric determination of the stability constant of vanadium(v)-p-chloro-N-phenyl-p-chlorobenzo- hydroxamic acid complex

The stability constant of the complex vanadium(v) p-chloro-N-Phenyl-p-chlorobenzohydroxamic acid was determined spectrophotometrically. The ligand p-chloro-N-phenyl-p-chloro-benzohydroxamic acid abbreviated in this paper as PCNPPCBHA was synthesized by the coupling of equimolar solutions of p-chlorobenzoyl chloride and freshly prepared p-chloro-ß-phenyl hydroxylamine; in which the later was prepared by the reduction of p-chloronitrobenzene with zinc powder. The pH at maximum extraction of the ligand PCNPPCBHA to the metal vanadium(v) and the ratio of the ligand: metal , were determined.


Introduction
Hydroxamic acid have been known since 1869 with the discovery of oxalohydroxamic acid by Lossen (Rappoport & Liebman 2009) .Dispite this , researches on these compounds were lacking until the1980,after which an enormous amount of information has accumulated with respect to their biomedical applications, synthesis, determination of their metal complexes (Mamion.et al . 2004) , their pharmaceutical (Musser et al .1990) and industrial applications(Odake et al.1991) , so the chemistry of hydroxamic acid has received considerable attention (Miller 1989),particularly important is their role as siderphores, as well as a model system for natural siderphores (Matzanke et al. 1989). The ability of hydroxamic acids to form stable transition metal complexes is the basis of their usefulness as an analytical reagent; this is the reason why hydroxamic acids have been used as chelating agents for determination of metals in samples of environmental and industrial importance (Rasuljan et al .1996) . The colored and stable solutions of these metal complexes often provide spectrophotometric methods for determination of several metal ions. Hydroxamic acids having the bidentate functional grouping fulfill the basic requirements of complex formation with metal ions and , therefore form an important family of chelating agents . The complex formation usually takes place with the replacement of the hydroxylamine hydrogen by the metal ion and ring closure through the carbon oxygen as shown below:

Experimental
All chemicals used were AnalaR grade. All solutions were prepared according to the usual analytical procedures. using distilled water. A series of buffer solutions ranging from pH 1.0 to pH 7.0 were prepared from a mixture of 0.2 M hydrochloric acid and 0.2 M potassium chloride in different proportions ; and also a mixture of 0.2M acetic acid and 0.2M sodium acetate in different proportions. (Vogel 1951) p-chloro-ß-phenyl hydroxylamine was prepared by reduction of 0.4 mole p-chloronitrobenzene with 0.9 mole zinc powder in present of 0.4 mole of ammonium chloride at temperature of 55 o C .The products were recrystallized from petroleum benzene. The ligand PCNPPCBHA was prepared by the coupling of 0.1 mole p-chlorobenzoyle chloride (dissolved in diethyl ether) and 0.1 mole of freshly prepared p-chloro-ß-phenyl hydroxylamine (dissolved in diethyl ether) at temperature of 0 O C and in present of 0.2 mole sodium hydrogen carbonate. The product was recrystallized from a mixture of benzene and petroleum ether (1:1). The solutions were transferred to a series of 100 cm 3 seperatory funnels; few drops of 0.001M potassium permanganate were added to each till the solution became faint pink color. 25 cm 3 of 0.02M PCNPPCBHA in chloroform were added to each seperatory funnel and the mixture were shacked gently for two minutes ; the layers were allowed to separate; (I) The organic layer ( lower phase) and (II) The aqueous layer (upper phase). The aqueous layers were taken for further analysis to determine the pH at maximum extraction.

Analysis of the aqueous layer by spectrophotometric
procedure (Abdalla, 1980, Ryan 1960): The standard ((calibration curve)) for vanadium(v) and the sample ((aqueous layer)) were carried out at the same time for spectrophotometric procedure. Results are shown in Table (6) and (7) and represented graphically in figure (1).

Results and discussion
The synthesized ligand p-chloro-N-Phenyl-p-chloro benzohydroxamic acid ( PCNPPCBHA) was identified by its melting point table (1) ; infra -red spectroscopy table (2) appendix (A) ; ultra-violet absorption in 95% ethyl alcohol table (3) appendix (B); elemental analysis of its nitrogen (Abdalla, 1992) and chlorine ( Openshow1995) contents table (4) and table(5) and its reactions towards acidic solutions of vanadium(v) and iron (III) to give respectively ; violet and blood-red colored complexes in the chloroform layers.       0.091 7 * pH at maximum extraction implies the lowest value of the of the absorbance i.e. at pH = 3.00  Table (8) and represented graphically in figure (2). The blank used was chloroform.

Synthesis of the Ligand:
The ligand PCNPPCBHA was prepared by a coupling reaction which took place under ether process (Usha & Tandon 1967 ) which is favored over the aqueous process (Tandon & Bhattacharya 1962) due to the relative purity of the product obtained. The use of sodium hydrogen carbonate in the coupling reaction is to keep the coupling medium slightly alkaline ; also it serves to neutralize hydrochloric acid evolved throughout the reaction. The infra-red spectroscopy for the ligand reveals the absorbance of the most important characteristic functional groups (( Table (2)appendix (A) )) such as O-H at 3185 cm -1 , the lower shift of O-H instead of 3190 cm -1 (Agrawal 1967) is due to the intra molecular hydrogen bonding of the type -OH…C=O. The ultra-violet spectra of the ligand reveal two absorption maxima (λ max ) when dissolved in 95% ethyl alcohol; The first band absorbs at 233 nm referring to the absorption due to the benzene ring and the other band absorbs at 205 nm referring to the absorption due π ‫ــــ‬ π * transition of the carbonyl group (C=O). The substitution of the chloride at the para-position of the two phenyl rings to the parent acid decreases the absorption band due to the carbonyl π ‫ــــ‬ π * transition when compared with the parent acid alone (Bowie et al .1969). p-Chloro-ß -phenyl hydroxylamine which is needed for the synthesis of the ligand is an intermediate and of low stability which decompose upon staying for a long time; so it must be freshly prepared and recrystallized for the coupling reaction.
Calculation of the stability constant for Vanadium (V) PCNPPCBHA complex (Korthoff 1963): The formation (stability) constant can be determined by job's method.