Comparative Study on the Catalytic Activity of Molecular [Co ₄ ^III O ₄ ] Heterocubanes in Alcohol Oxidation Reactions in Solution

A comparative study on the catalytic oxidation of 2-propanol (i-PrOH), benzyl alcohol (BnOH), and cinnamyl alcohol (CnOH) in solution under moderate reaction conditions (65–150 °C) using two tetranuclear CoIII-oxo complexes [Co4O4(O2CR)4(py)4] (R = Me 1, Et 2, py = pyridine) and three different oxidants (O2, H2O2, tert-butyl hydroperoxide (TBHP)) is reported. TBHP is the most active oxidant, resulting in turnover frequency (TOF) values ranging from 40 to 100 h–1. The slightly larger carboxylate ligand in complex 2 favors aldehyde selectivity but slightly decreases the conversion rates as was confirmed by GC analyses. Mechanistic investigations (in situ 1H NMR, EPR) confirm the active participation of transiently formed radicals in these reactions. Using quantum-chemical calculations, a detailed mechanistic understanding of the catalytic reactions was gained that fully agrees with the experimental observations. Two interrelated catalytic cycles were identified: one for the oxidant activation and one for the alcohol oxidation. Catalytic TBHP oxidation reactions are initiated via Haber-Weiss activation of TBHP, resulting in the formation of tert-butyl peroxyl radicals (t-BuOO•), and simultaneous reduction of CoIII to CoII.