Preparation of Chromium(III) Phthalate and Chromium(III) Pyromellitate via Chromium(VI) Oxide

Reduction of CrO 3 at room temperature by ethanol in the presence of phthalic acid (H 2 pht) and pyromellitic acid (H 4 pyr) yields solutions that contain monomeric and low oligomeric (dimeric) Cr(III) complexes. A solution with an H 4 pyr/Cr mol ratio of 1.5:1 is indefinitely stable toward precipitation, whereas the corresponding solution having a H 2 pht/Cr mol ratio of 3: I gives a precipitate in the course of ≈5 weeks; the difference in stability can be attributed to differences of acidity in the two systems. The solid from the H 2 pht/Cr solution contains both the Hpht − and OH − species and is probably a trimeric OH − -bridged Cr(III) complex. The solid obtained on evaporation of the stable H 4 pyr/Cr solution does not contain OH − species, its polymeric (trimeric) character being based on Cr-H 2 pyr-Cr bridges. If the reaction in the H 2 pht/Cr system is carried out under reflux, precipitation occurs immediately; the solid exhibits a higher degree of condensation than that obtained at room temperature.