Synthesis and Reactivity of a (μ-1,1-Hydroperoxo)(μ-hydroxo)dicopper(II) Complex:  Ligand Hydroxylation by a Bridging Hydroperoxo Ligand

A new tetradentate tripodal ligand (L3) containing sterically bulky imidazolyl groups was synthesized, where L3 is tris(1-methyl-2-phenyl-4-imidazolylmethyl)amine. Reaction of a bis(μ-hydroxo)dicopper(II) complex, [Cu₂(L3)₂(OH)₂]²⁺ (1), with H₂O₂ in acetonitrile at −40 °C generated a (μ-1,1-hydroperoxo)dicopper(II) complex [Cu₂(L3)₂(OOH)(OH)]²⁺ (2), which was characterized by various physicochemical measurements including X-ray crystallography. The crystal structure of 2 revealed that the complex cation has a Cu₂(μ-1,1-OOH)(μ-OH) core and each copper has a square pyramidal structure having an N₃O₂ donor set with a weak ligation of a tertiary amine nitrogen in the apex. Consequently, one pendant arm of L3 in 2 is free from coordination, which produces a hydrophobic cavity around the Cu₂(μ-1,1-OOH)(μ-OH) core. The hydrophobic cavity is preserved by hydrogen bondings between the hydroperoxide and the imidazole nitrogen of an uncoordinated pendant arm in one side and the hydroxide and the imidazole nitrogen of an uncoordinated pendant arm in the other side. The hydrophobic cavity significantly suppresses the H/D and ¹⁶O/¹⁸O exchange reactions in 2 compared to that in 1 and stabilizes the Cu₂(μ-1,1-OOH)(μ-OH) core against decomposition. Decomposition of 2 in acetonitrile at 0 °C proceeded mainly via disproportionation of the hydroperoxo ligand and reduction of 2 to [Cu(L3)]⁺ by hydroperoxo ligand. In contrast, decomposition of a solid sample of 2 at 60 °C gave a complex having a hydroxylated ligand [Cu₂(L3)(L3-OH)(OH)₂]²⁺ (2-(L3-OH)) as a main product, where L3-OH is an oxidized ligand in which one of the methylene groups of the pendant arms is hydroxylated. ESI-TOF/MS measurement showed that complex 2-(L3-OH) is stable in acetonitrile at −40 °C, whereas warming 2-(L3-OH) at room temperature resulted in the N-dealkylation from L3-OH to give an N-dealkylated ligand, bis(1-methyl-2-phenyl-4-imidazolylmethyl)amine (L2) in ∼80% yield based on 2, and 1-methyl-2-phenyl-4-formylimidazole (Phim-CHO). Isotope labeling experiments confirmed that the oxygen atom in both L3-OH and Phim-CHO come from OOH. This aliphatic hydroxylation performed by 2 is in marked contrast to the arene hydroxylation reported for some (μ-1,1-hydroperoxo)dicopper(II) complexes with a xylyl linker.