Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base

Abstract

The catalytic reduction of dioxygen (O₂) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O₂ reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(^p-tbudhbpy)Cl (1) was used as a control complex and Mn(^nPrdhbpy)Cl (2) contains a pendent –OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N′-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H₂O₂ and 2 is quantitative for H₂O₂, with O₂ binding to the reduced Mn(II) center being the rate-determining step. Upon addition of the conjugate base, N,N′-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H₂O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O–O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H₂O₂ at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.