The reactivity of coordinated and uncoordinated chromate ions in the quasi-intramolecular redox reactions of [pentaamminechromatocobalt(iii)] chloride and chromate resulting in tuneable cobalt-chromium spinel oxide catalysts.

Abstract

The synthesis routes, crystal and molecular structures, and controlled thermal decomposition reactions of two [pentamminecobalt(iii) compounds having a coordinated chromate ion, [Co(NH₃)₅CrO₄]Cl (compound 1) and [Co(NH₃)₅CrO₄]₂CrO₄ (compound 2) were studied experimentally. Quantum chemical optimizations and QTAIM calculations were performed for compounds 1 and 2, and the selected Bond Critical Points (BCPs) properties were discussed. The extended hydrogen-bond network between the coordinated chromate ions and ammonia ligands acts as reactive redox reaction centres, and both compounds show solid-phase quasi-intramolecular redox reactions with the formation of Co[Co_0.5Cr_1.5O_3.75] and Co[Co_0.2Cr_1.8O_3.90] nanosized regular spinels with nominal Co : Cr = 1 : 1 and 2 : 3 molar ratios, respectively. The tetrahedral positions of these spinels are filled with Co^II ions. The octahedral positions of 2 : 3 and 1 : 1 spinels prepared in air are filled Cr^III and mainly with Co^III ions. The 2 : 3 spinel prepared under N₂ shows a similar distribution of ions, whereas the 1 : 1 spinel prepared under N₂ contains only Co^II ions at T-4, and Cr^III, Co^III, and Co^II ions at the OC-6 positions, resulting in a strongly defective structure. The position of the chromate ions (inner-sphere coordinated or outer-sphere non-coordinated) plays an important role in the quasi-intramolecular redox reactions and thermal decomposition characteristics of compounds 1 and 2. The inner-sphere coordinated chromate ions of both compounds took part in redox reactions with their own coordinated ammonia only after the loss of the fifth ammonia, in an endothermic ligand-releasing and an exothermic redox reaction mediated by the outer-sphere chromate ion for compounds 1 and 2, respectively. The cobalt(iii) content of both complexes is also reduced completely to cobalt(ii). These spinels were proven to be active catalysts in the visible-light-driven photocatalytic oxidation of rhodamine B with H₂O₂.