Influence of nitro substituents on the redox, electronic, and proton reduction catalytic behavior of phenolate-based [NO]-type cobalt() complexes

We report on the synthesis, redox, electronic, and catalytic behavior of two new cobalt( iii ) complexes, namely [Co III (L 1 )MeOH] ( 1 ) and [Co III (L 2 )MeOH] ( 2 ). These species contain nitro-rich, phenolate-based pentadentate ligands and present dramatically distinct properties associated with the position in which the -NO 2 substituents are installed. Species 1 displays nitro-substituted phenolates, and exhibits irreversible redox response and negligible catalytic activity, whereas 2 has fuctionalized phenylene moieties, shows much improved redox reversibility and catalytic proton reduction activity at low overpotentials. A concerted experimental and theoretical approach sheds some light on these drastic differences. The electronic and redox properties of two new nitro-substituted cobalt species are investigated towards proton reduction.