SYNTHESIS, SPECTRAL CHARACTERIZATION AND POWDER X-RAY DIFFRACTION ANALYSIS OF INDOLINIUM DICHROMATE AS A CHROMIUM(VI)-BASED OXIDIZING AGENT

Abstract

An efficient, mild  and stable chromium(VI)-based reagent, indolinium dichromate (InDC), was synthesized through the controlled reaction of indole with chromium trioxide (CrO3) under optimized thermal conditions. The structural, electronic and physical properties of the synthesized complex were thoroughly established using complementary analytical techniques. FT-IR spectroscopy confirmed the formation of the organic-inorganic hybrid complex, exhibiting characteristic broad bands for the indolinium cation N–H stretching at ~3000–3250 cm-1 alongside sharp stretching modes for the dichromate anion Cr–O and Cr=O at 700–950 cm-1. UV-Vis spectroscopy displayed distinct electronic absorption peaks, including high-energy π π * transitions of the indolinium aromatic core ~275 nm and characteristic Ligand-to-Metal Charge Transfer LMCT ,O Cr(d) bands at ~350 nm and ~430–450 nm. Solution-state 1Hand 13C NMR spectroscopy further verified the structural integrity, protonation state, and distinct chemical shifts of the indolinium cation in DMSO-d. Powder X-ray diffraction (PXRD) analysis demonstrated the crystalline nature of the complex, exhibiting sharp, well defined diffraction reflections across 10°≤ 2θ≤ 60°. Interplanar d-spacing values calculated via Bragg’s law (nλ = 2dsinθ) established phase purity and single-phase characteristics of the material. Additionally, coupled thermogravimetric and differential thermal analysis (TGA/DTA) alongside elemental analysis (CHN) verified the stoichiometry and demonstrated excellent thermal stability. The synthetic utility of InDC was successfully demonstrated in organic synthesis, showing high efficiency and selectivity for the oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones under mild conditions, without over-oxidation to carboxylic acids. Overall, indolinium dichromate represents a robust, well-characterized, and highly selective oxidant for modern chemical transformations.