PHYSICO-CHEMICAL STUDIES ON SOME MIXED LIGAND COMPLEXES OF CHROMIUM

Abstract

A perusal of the literature cited in chapter I reveals that extensive studies have been carried out on alarge variety of carboxylato cobalt(IIl) complexes. They exhibit remarkable stability in aqueous solutions and undergo reduction through different mechanistic paths when subjected to reduction by Cr(ll). It has been observed that minor changes in the structure of the organic moiety results in asubstantial change in the rate of reduction of these complexes. These results have been interpreted in terms of the role of the organic structural units as electron mediators. In contrast to this, very little work has been done with other metal ions which form inert complexes, especially chromium(lll). It may be further observed that although homogeneous reduction studies with cobalt(III) complexes have played avery important role in elucidating the mechanism of inner- and outer-sphere reduction, the analogous heterogeneous reduction has received very little attention. Studies with the substitution inert chromium(lll) complexes provide very suitable systems for investigating the mechanistic reduction patterns in heterogeneous reduction at the mercury electrode. It is thus of interest to investigate whether the several variations of reduction patterns encountered in the case of cobalt(UI) complexes extend to the analogous chromium(lir) complexes and elucidate the mechanism of their heterogeneous reduction. For this purpose three series of ligands, viz. .amino acids, their schiff trase derivatives with o-vanillin and salicylaldehyde -127- and schiff bases of o-vanillin, salicylaldehyde, and benzaldehyde with c-, ^ and p-aminobenzoic acids were selected to obtain mixed ligand complexes of pentaammine chromium(lll). The results of these investigations are summarised below: I. preparation.and characterisation of amino acid mESmSS3^mSHEi commevL substituted (References and Tables vide Chapter II). The schiff bases of glycine, alanine, aspartic acid and glutamic acid with salicylaldehyde and o-vanillin were prepared by the method pf Dietrich16 and. those" of o-,. mc^prm»m^m$U acids with.bertZaidehyde,:,salioylaldehyde and o-vanillin by the method of Roe and Montgomery17. For the preparation of the amino acid substituted pentaammine chromium(HI) complexes, the method of Sykes11 for the preparation of the glycinato complex,was suitably modified so as to give higher yields employing lesser amounts of the amino acids. In a typical preparation, 20 mM of the amino acid was heated at 50 °C for 30 minutes with 10 mM chloropentaammine chromium(lll) keeping the pH of the solution at 3.0 with HC104. Precipitation was affected with ice cold 70'/. HC104 and the desired product extracted from the precipitate with methyl alcohol and purified by crystallisation. A similar method was employed for the preparation of the complexes with the schiff bases obtained from the amino acids. In the case of aminobenzoic acids and their schiff bases, 100 mM of the ligand was mixed with a solution of 10 mM -128- of chloropentaammine chromium(lll) chloride in 100 mL of water. The pH was then carefully adjusted to between 4-5 with Li2C03. After heating at 50°C for 30 min, the insoluble matter was filtered off and the precipitation affected by adding 10 mL of saturated ice cold NaCl04 solution. The desired complex was isolated from the precipitate with methyl alcohol and purified by crystallisation. Aj3jino_acid §__bsjyjtute^^omglexeji Chemical analysis of the amino acid substituted chromium(lll) pentaammines (Table 2.1) shows that they correspond to the formula [CrO^)^ (ClO^ where AA is the amino acid molecule. The charge on the complex ion was confirmed by the molar conductance values (Table 2.1) of these complexes which lie between 335-355 ohm"1 cm2. The visible absorption data of the amino acid substituted pentaammines (Table 2.2)shows two bands in the range 363-370 nm and 485-497 nm. These two bands are due to the transitions (t^)3 -» (t2g)2(eg) (F) and <t2g)3-> (%)^h respectively. The wavelengths of these bands are sensitive to the nature of the coordinating ligands according to the normal spectrochemical series28'29 and the band in 485-497 nm region gives directly the crystal-field parameter 10 Dq. Thus, it can be concluded that the position of the unidentate amino acids in the normal spectrochemical series lies in between that of H20 and CI" near CH3COO" and the spectrochemical order for the amino acids can be written as: asparagine < valine ~isoleucine ^ alanine < glycine^ leucine /-^arginine. -129- The infrared spectra of amino acid substituted chromium(lll) pentaammines shows binding of the amino acid through the carboxylic group since a new band appears around 1400 cm"1 followed by a more pronounced band at 1600 cm'1 (Table 2.2). The former is due to the symmetrical stretching vibrations of the coordinated carboxyl group while the latter can be attributed to overlapping of the asymmetric vibration of the carboxyl group and antisymmetric NH3 deformation mode. The presence of these two bands coupled with the absence of a peak at 1700 cm due to carbonyl absorption of the carboxyl group of the amino acid provide the necessary evidence33'34 to confirm that these amino acids are bound to chromium through the carboxyl group. Schiff base (derived from amino acids) substituted complexes The formation of the schiff base complexes can be represented by the following equation: 2+ 2+ |_Cr(NH3)5cri + NaOOC-R-N=CHR'->C (NH3)5CrOOC-R-N=CHR3 +NaCl. Molar conductance values of these complexes which lie in the range 245-260 ohm""1 cm2 (Table 2.3) confirm the formula [Cr(NH3)5SB] (C104)2, where SB represents the schiff base ion. The complexes gave X in the range 520-530 nm and max J 372-380 nm (Table 2.4). According to the position of these bands, the spectrochemical order for these schiff bases can be written as V-G< V-Gly <V-Al^S-Gly< V-A < S-Ala-S-A <S-G