Copper–Dioxygen Reactivity Using Dinucleating Ligands with Activated Methylene or Ketone Function

Copper(I)–dioxygen reactivity studies utilizing new binucleating ligands are described. When catalytic quantities of either CuI or CuII salts were introduced to LH2, which is a dinucleating ligand with two tridentate donors connected via a methylene group at a bis(2-pyridyl)methane junction, dioxygen addition caused complete oxygenation to give bis(2-pyridyl)ketone product L=O. Isolated dicopper(I) complex [CuI2(LH2)]2+ (1) also reacted with excess O2 to give L=O. Additional observations were consistent with a mechanism that does not involve “oxygen-activation” in this process. Ketone hydration and gem-diol(ate) copper(II)-coordination occurred by addition of copper(II)–perchlorate to L=O, giving [CuII2{L(OH)(O−)}Cu2II(−OClO3)]2+ (4), with a bridging alkoxide moiety (X-ray). Oxygenation of [CuI2(L=O)(CH3CN)2]2+ (2) (2/O2 = 0.5, i.e., Cu/O2 = 4:1 manometry), gave [CuII2{L(O−)2}(−OClO3)]+ (5), where both oxygen atoms of the doubly deprotonated gem-diolate bind separate copper(II) ions; mass spectrometric data showed that one of the two oxygen atoms of the gem-diolate L(O−)2 came from O2. An oxo–dicopper(II) species is suggested as an important intermediate, which is supported by the observation that reaction of 2 with NO or iodosylbenzene also produced 5 in high yields. Complexes 4 and 5 are acid–base conjugate pairs and can be readily interconverted using Et3N or HClO4(aq) reagents.