Reactions of copper(II)-H2O2 adducts supported by tridentate bis(2-pyridylmethyl)amine ligands: sensitivity to solvent and variations in ligand substitution.
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.
Kunishita
Atsushi
A
Department of Chemistry, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan.
Scanlon
Joseph D
JD
Ishimaru
Hirohito
H
Honda
Kaoru
K
Ogura
Takashi
T
Suzuki
Masatatsu
M
Cramer
Christopher J
CJ
Itoh
Shinobu
S
D000096:Acetone / Q000737:chemistry; D000588:Amines / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D006861:Hydrogen Peroxide / Q000737:chemistry*; D008024:Ligands; D009570:Nitriles / Q000737:chemistry; D011407:Propane / Q000031:analogs & derivatives / Q000737:chemistry; D011789:Quantum Theory; D012680:Sensitivity and Specificity; D012997:Solvents / Q000737:chemistry*
D000588:Amines; D008024:Ligands; D009570:Nitriles; D012997:Solvents; D000096:Acetone; D003300:Copper; D006861:Hydrogen Peroxide; C005557:propionitrile; D011407:Propane