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

Abstract

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(mu-hydroxo)dicopper(II) complex, [Cu2(L3)2(OH)2]2+ (1), with H2O2 in acetonitrile at -40 degrees C generated a (mu-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(mu-1,1-OOH)(mu-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(mu-1,1-OOH)(mu-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(mu-1,1-OOH)(mu-OH) core against decomposition. Decomposition of 2 in acetonitrile at 0 degrees 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 degrees 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 degrees 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 approximately 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 (mu-1,1-hydroperoxo)dicopper(II) complexes with a xylyl linker.

Exogenous Nitrile Substrate Hydroxylation by a New Dicopper-Hydroperoxide Complex

“…As mentioned, Suzuki et al have shown their Cu II 2 -OOH species to effect intramolecular-coordinated ligand Ar-CH 2 NR 2 methylene hydroxylation and subsequent N-dealkylation. 4 Although recent experiments and calculations suggest that a Cu-superoxide may be the preferred active species in D M and PHM, 2f-h both Suzuki's 4 and the present study argue that a Cu I n /O 2 -derived Cu II 2 -OOH moiety could initiate useful substrate hydroxylation reactions in biological or chemical systems. Further studies to delineate Cu II -O 2 -vs Cu II -OOH reactivity are in progress.…”

“…8 Elimination of water and oxygen rebound would lead to a coordinated (and deprotonated) R-hydroxynitrile (b). This could de-ligate from copper (via protonation) 4,17 or directly eliminate to give the aldehyde, leaving a cyanide complex (c), which dimerizes to the observed tetranuclear product 5.…”

Modeling tyrosinase activity. Effect of ligand topology on aromatic ring hydroxylation: An overview

“…Decomposition of a solid sample of this complex at 60°C gave, after stripping of copper(II) ion, two ligands (intact tetradentate along with a tridentate counterpart) and a hydroxylated ligand (Fig. 12) [121].…”

Stabilization of the Hydroperoxido Ligand: A 1κO,2κO′ Dimetallic Coordination Mode

“…[9] Moreover, in these three compounds the OOH moiety was found to be coordinated in a m-1,1-OOH fashion, while complex 3 represents the first example of a m-1,2-OOH hydroperoxido ligand, that is, each oxygen atom is bonded to only one metal center.…”