Bimetallic Reactivity. One-Site Addition Two-Metal Oxidation Reaction of Dioxygen with a Bimetallic Dicobalt(II) Complex Bearing Five- and Six-Coordinate Sites

The di-Co2+ complex, [Co2+(μ-OH)(oxapyme)Co2+(H2O)]+, contains an unsymmetrical binucleating
ligand (oxapyme) which provides five- and six-coordinate metal sites when a hydroxide bridge is introduced.
This complex absorbs 1 equiv of O2 irreversibly in solution, producing an unstable di-Co3+ oxygenated
product. The oxygenated product has been studied at low temperatures, where its electronic absorption
and 1H NMR spectra were recorded. It is probable that the oxygenation reaction involves a one-site addition
two-metal oxidation reaction to produce an end-on-bonded peroxide ligand at the available coordination
site, giving the complex [Co3+(μ-OH)(oxapyme)Co3+(η1-O2)]+. Addition of 1 equiv of HClO4 to this oxygenation
product gives a stable peroxide complex, [Co3+(μ,η1:η2-O2)(oxapyme)Co3+]2+, where one of the oxygen
atoms bridges the two metals and is sideways bonded to one of the metals. The formation of this stable
complex involves expulsion of the OH- bridge. Addition of NO2- to the sideways-bonded peroxide complex
leads to the formation of another stable complex, [Co3+(μ,η1:η1-O2)(oxapyme)Co3+(NO2)]+, where the
peroxide forms a classic di-end-on bridge to the two metals. Both of these complexes have been fully
characterized. Addition of acid to this second stable dioxygen complex leads to the release of HNO2 and
the formation of the μ,η1:η2 sideways-bonded peroxide complex.