Co(salen) (salen = N,N′-bis(salicylidene)-1,2-ethylenediamine) complexes have been extensively studied as promising candidates for modeling biological O2 binding and transport. However, the reactivity of such molecular models often deviates from the biological function. Previous work has shown that they form the binuclear μ-peroxo complexes in contrast to the biologically relevant superoxo complex, oxyhemoglobin, a ferric electrostatically stabilized mononuclear superoxide. Analogous to nature, employing electrostatic interactions as charged residues or local electric fields to control enzymatic selectivity and reactivity, alkali and alkaline earth cations are studied to investigate speciation of O2binding by cobalt. Our studies demonstrate that a crown-appended Co(salen) complex Co(salencrown)Mn+ (Mn+ = Na+, K+, Ca2+, Ba2+) changes its O2 binding behavior depending on the identity of the secondary metal cation, kinetically trapping a stable complex in some cases. Complexes with monovalent cations in the crown yield mostly superoxo species after O2 addition, monitored by low-temperature UV-vis spectroscopy, whereas complexes with divalent cations or without a secondary metal yield [Co(salen)]2(μ-peroxo) species. These observations were further examined by electron paramagnetic resonance (EPR) spectroscopy, resonance Raman spectroscopy, and Density Functional Theory (DFT) calculations, providing evidence of electrostatic interactions with the secondary metal cations. Kinetic analysis gives further insight into the mechanism for [Co(salen)]2(μ-peroxo) formation.
Demonstrating controlled speciation of O2 adducts by electrostatic fields further encouraged the study of kinetically trapped Co(salencrown)M+(superoxo) complexes for electrophilic reactivity of similar intermediates found for enzymes. Preliminary electrophilic reactivity studies of [Co(salencrown)Na+]2(μ-peroxo) species suggest that it mediates oxygen atom transfer towards triphenyl phosphine in a catalytic manner. Hydrogen atom transfer and proton transfer reactivity are under investigation as well. This work unveils changes in cobalt O2 speciation in response to controlled electrostatic modifications similar to triggering selective reaction mechanism pathways for O2 bound species as found for enzymes that might further result into distinct reactivity.