Comparative study of the catalytic thermodynamic barriers for two homologous Mn- and Fe-non-heme oxidation catalysts

[Display omitted] •Comparative catalytic, spectroscopic study of homologous non-heme Mn- and Fe-oxidation catalysts.•EPR and Low-Temp. UV–vis allows detection of transient intermediate species and determination of thermodynamic barriers.•FeIIIOOH and MnIVO, rate-limiting intermediates respectively, face Ea=91kJ/mole and 55kJ/mole respectively. Two sets of homologous Mn- and Fe-catalysts, [MnIILCl2], [FeIILCl2] and [MnIIL(OAc)2], [FeIIL(OAc)2] have been synthesized. A detailed comparative study of their catalytic oxidative performance with H2O2, in tandem with EPR and Low-Temperature UV–vis spectroscopies has been carried out. The [Metal-L(OAc)2] and [Metal-LCl2] catalysts did not show any difference in their catalytic behavior i.e. there is no effect of the labile ligands on the studied catalysis. It is found that the Mn-catalysts consistently outcompeted the homologous Fe-catalysts i.e. TOFs (Mn)=162 vs. TOFs (Fe)=16. We found that the Fe-catalyst faces a significantly higher activation barrier than the Mn-catalyst i.e. Ea(FeIIL(OAC)2)=91KJ/mol≫Ea(MnIIL(OAC)2)=55kJ/mole, while the free-energy difference, ΔG(FeIIL(OAC)2)∼ΔG(MnIIL(OAC)2)∼−145kJ/mole, did not make difference. Taken altogether the present data clarify that the main thermodynamic barrier, ultimately determining the overall catalytic performance, of these homologous Mn- and Fe-catalysts is the activation energy for the transient intermediates i.e. MnII to MnIVO for the Mn-catalysts and FeII to FeIIIOOH for the Fe-catalysts. A unified/consistent catalytic thermodymanic concept is discussed, that bears relevance to the catalytic behavior of many non-heme Mn- vs. Fe-oxidation catalysts.