SYNTHESIS, SPECTROSCOPIC CHARACTERISATION, AND KINETIC PARAMETER STUDY OF LABILE CHROMIUM COMPLEXES USING BENZOIC ACID AS A LIGAND

Ethanol was used as the solvent in the process of producing chromium benzoic acid complexes by combining CrO 3 (chromium trioxide) and benzoic acid. Its spectrum characteristics were evaluated by the use of elemental analysis, FAB-Mass Spectroscopy, 1 HNMR spectroscopy, FTIR Spectroscopy, UV-Vis Spectroscopy


INTRODUCTION
It is generally known that chromium (VI) is a carcinogen as well as a mutagen, but it is also effective to minimize it to a lower oxidation state using a broad range of organic reductants. As good complexing agents for hypervalent chrome as salicylic acid or other ligands with two oxygens, these ligands have the capacity to create five-membered rings around the metal ion, which may help stabilize chrome's oxidation states. Salicylic acid is one example of such a ligand. 1 Cr (III) complexes' reactivity is characterized by very slow ligand substitution processes, which results in exceptional configurational stability. 2 Under some conditions, chromium ligand substitution reactions have the potential to proceed quickly and might offer a mechanism for the process. 3 Many additional systems, including Cr (EDTA) (H2O) and porphyrin complexes, show comparable kinetic behavior to the Cr (III)-Schiff's bases combination with nicotinic acid. In the Thermal analysis approaches DSC (differential scanning calorimetry) is of particular importance. The simplicity with which samples may be prepared for DSC analysis sets it apart from competing methods. Furthermore, the activation conditions are simply adjustable. There are several advantages to using the DSC method for thermodynamic and kinetic measurements concurrently. The DSC technique may be used for all reactions since practically all processes generate or consume heat. 4 Chromium (III) was shown to have an important role in maintaining proper glucose, lipid, and protein metabolism in humans. 5 The production of a chromium (III)-organic acid complex is an exciting area of research because of the compounds' involvement in the hazardous Cr (VI) being reduced to Cr (III) 6 chromium (III)-organic acid complexes have also been discovered to be effective catalysts under certain circumstances. [7][8][9] Chromium (III)-organic acid complexes have been also used extensively in various industrial processes such as tanning, dyeing, hardening photographic emulsion, etc.

RESULTS AND DISCUSSION
Elemental (C and H %) and ICP-OES (Cr %) Analysis of Complexes Analysis of metal complexes is reported in Table-2 using ICP-OES and C&H data. The compounds produced have been strongly colored and insoluble in water and ordinary organic solvents, however DMSO at room temperature may dissolve these complexes. It was discovered that the amount of coordinating benligands in the complexes rose proportionately as Cr: acid molar ratio raised. In the Cr/benzoic acid-water (Cr/Hben-water) system, the chromium concentration also rose correspondingly, indicating that there was an increase in the metal complexes' degree of polymerization.

UV-VIS Analysis/ Interpretation of Metal Complexes
This has been stated 11 that for the solution, which was generated by dissolving active Cr(III)-hydroxide in an ethanolic solution of Hben the electronic spectra show absorption maxima at 436 along with 587 nm. The two maxima are located in the vicinity of the d-d transitions of octahedral chromium complexes, namely the 4 A2g → 4 T1g and 4 A2g→ 4 T2g d-d transitions. It has also been hypothesized that such solutions include a combination of low oligomeric Cr(III) complexes, which exist as anionic and ion partners. There are two bands in the UV visible spectrum of all complexes that may be recognized as the 4 A2g → 4 T1g and 4 A2g→ 4 T2g d-d transitions of the octahedral complexes. At 587 and 429 nm, in aqueous solution chromium shows bands that are indicative of oligomeric species, most likely hexamer, tetramer, or trimer complexes in Cr/benzoic acid solutions (Cr/Hben).

Characterization of Complexes by Fourier Transform Infrared Spectrophotometry (FTIR)
We used the Perkin Elmer spectrum-2000, FTIR (Fourier Transform Infrared) spectrometer with an auto mode to record the infrared spectra of solid materials from the 4000-400cm -1 range on the KBr pellets. Table-3 represents the FTIR spectra of benzoic acid and its Cr/Hben complexes.

HNMR Studies of Cr/ Hben Complexes
Proton peaks from distinct groups in the complexes could not be differentiated from one another, according to the experimental data. NMR spectra of complexes were marginally different from those of their ligands. Coordination causes a downfield shift in the spectrum of the aromatic ring's protons, which display a peak around 6.86-8.02 ppm when compared to the free ligand's spectrum.   A half-molecular water loss may be associated with the molecular weight differential. energy. It may due to polymeric (BA1-tetramer), (BA2-pentamer), (and BA3-hexamer) in nature. In all cases Cr/Hben-TAA complexes second step decomposition associated with the exothermic process and heat flow take place at 363.57, 345.55, and 313.1ºC respectively. An endothermic reaction with low activation energy indicates the complexes' sturdiness in the first stage. Following the first and second order of reaction, decomposition occurs. In most cases, the first stage of the breakdown process is connected with an endothermic activity, while the second step is often related to an exothermic process. As the scanning only went up to 450 °C, it was unable to capture the last step change of the Cr/Hben complexes, which occur naturally as tetramers, pentamers, and hexamers. Table-8 contains an illustration of the related kinetic parameters.

Lability of the Chromium (III) Complexes from DSC Results
According to reports, a complex is said to be inert if the activation energy (Ea) of both the process that forms it and the reaction that breaks it down is high then, the compound is produced, and it slowly decomposed. The energy required for the production and breakdown of a labile complex is quite low. Because of this, the equilibria of creation and breakdown of the complex are quickly formed (at 300 Kelvin, reactions with Ea. less than 25 KJ/mol are fast). 15 This was also noticed that the covalent coordinate bond with an energy of 20-80 Kcal/mol shows high lability with stability with the first-row metal-ligand bond, with the second row it shows medium lability and high stability, while the third row shows low lability and high stability. These findings were based on observations of the first, second, and third rows of metal-ligand bonds. 16 There are a few additional systems, including porphyrin complexes, that exhibit a labile kinetic behavior that is comparable to that of nicotinic acid and Cr (III) -Schiff's base complexes, which has led to the observation of a quick reaction rate between the two. Most reports indicate that the water molecules in the inner sphere of chromium (III) octahedral complexes cannot be substituted by other ligands, but the Aqua (ethylenediaminetriacetatoacetic acid) chromium complexes with five coordinates of EDTA and one molecule of water, show suddenly rapid substitution rates with numerous anionic ligands. It has been suggested that the strain present in the complexes may be responsible for this rapid reaction rate. 17 group. It was found that complexes had wide absorptions in the region of 3206-3308 cm -1 , which suggested the existence of a coordinated water molecule. Sharp and more defined IR spectra may be due to the polymeric nature of these complexes, based on Cr-OH-Cr bridges. NMR of Cr/Hben, deprotonation of -COOH group was noticed. The peak at 6.86 to 8.02 ppm is due to the aromatic ring proton. When the spectrum of the complex is compared with the spectrum of the free ligand a downfield shift caused by coordination was verified. The complexes'-Mass molecular formula generated might be anticipated based on the findings of this study. Cr/benzoic acid-water (Cr/Hben-water) complexes show very high molecular weights, which may be due to the polymeric nature of complexes.  8] From the DSC thermograms of the complexes, kinetic parameters-lnK0Ea (activation energy), H(enthalpy), and order of reaction (n) of decomposition could be calculated. In all cases of Cr/Hben-water complexes, the first and second step decomposition which were endothermic gave a high value of activation energy. It may due to polymeric BA1)-tetramer, BA2-pentamer, BA3-hexamer) in nature. Lability prediction from DSC of Cr/Hben complexes indicated that all water-based complexes were labile to some extent.

CONCLUSION
Complexes were formed with CrO3 and benzoic acid in an aqueous medium, the complexes formed were different and with a varying molar ratio of the reactants. Polymeric-like, tetramer, pentameric, and hexameric complexes were formed. The complexes' lability and kinetics energy may be anticipated from DSC. A few of the complexes produced were found to be labile to some degree.