Reactions of Copper(II)-H₂O₂ Adducts Supported by Tridentate Bis(2-pyridylmethyl)amine Ligands: Sensitivity to Solvent and Variations in Ligand Substitution

Kunishita, A.; Scanlon, J. D.; Ishimaru, H.; Honda, K.; Ogura, T.; Suzuki, M.; Cramer, C. J.; Itoh, S.
Inorg. Chem. 2008, 47, 8222.

The copper(II) complexes 1^H and 1^Ar(X), supported by the N,N-di(2-pyridylmethyl)benzylamine tridentate ligand (L^H) or its derivatives having m-substituted phenyl group at the 6-position of pyridine donor groups (L^Ar(X)), have been prepared, and their reactivity toward H₂O₂ has been examined in detail at low temperature. Both copper(II) complexes exhibited a novel reactivity in acetone, giving 2-hydroxy-2-hydroperoxypropane (HHPP) adducts 2^H and 2^Ar(X), respectively. From 2^Ar(X), an efficient aromatic ligand hydroxylation took place to give phenolate-copper(II) complexes 4^Ar(X). Detailed spectroscopic and kinetic analyses have revealed that the reaction proceeds via an electrophilic aromatic substitution mechanism involving copper(II)-carbocation intermediates 3^Ar(X). Theoretical studies at the density functional theory (DFT) level have strongly implicated conjugate acid/base catalysis in the O-O bond cleavage and C-O bond formation steps that take the peroxo intermediate 2^Ar(X) to the carbocation intermediate 3^Ar(X). In contrast to the 2^Ar(X) cases, the HHPP-adduct 2^H reacted to give a copper(II)-acetate complex [Cu^II(L^H)(OAc)](ClO₄) (6^H), in which one of the oxygen atoms of the acetate co-ligand originated from H₂O₂. In this case, a mechanism involving a Baeyer-Villiger type 1,2-methyl shift from the HHPP-adduct and subsequent ester hydrolysis has been proposed on the basis of DFT calculations; conjugate acid/base catalysis is implicated in the 1,2-methyl shift process as well. In propionitrile, both 1^H and 1^Ar(X) afforded simple copper(II)-hydroperoxo complexes LCu^II-OOH in the reaction with H₂O₂, demonstrating the significant solvent effect on the reaction between copper(II) complexes and H₂O₂.

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