Oxygen activation by mononuclear Mn, Co, and Ni centers in biology and synthetic complexes.

The active sites of metalloenzymes that catalyze O-dependent reactions generally contain iron or copper ions. However, several enzymes are capable of activating O at manganese or nickel centers instead, and a handful of dioxygenases exhibit activity when substituted with cobalt. This minireview summarizes the catalytic properties of oxygenases and oxidases with mononuclear Mn, Co, or Ni active sites, including oxalate-degrading oxidases, catechol dioxygenases, and quercetin dioxygenase. In addition, recent developments in the O reactivity of synthetic Mn, Co, or Ni complexes are described, with an emphasis on the nature of reactive intermediates featuring superoxo-, peroxo-, or oxo-ligands. Collectively, the biochemical and synthetic studies discussed herein reveal the possibilities and limitations of O activation at these three "overlooked" metals.