A new heme-peroxo-copper complex structural type with μ-η2:η2 peroxo ligation has been generated utilizing a heterobinucleating ligand with bis(2-(2-pyridyl)ethyl)amine tridentate chelate for copper. Oxygenation of [(2L)FeIICuI]+ (1) at -80 °C in CH2Cl2/6%EtCN, 1 (λmax, 426, 530 nm) produces [(2L)FeIII-(O22-)-CuII)]+ (3) (λmax, 419, 488, 544, 575 nm). Stopped-flow kinetic/spectroscopic probing reveals that a superoxo complex, [(2L)FeIII-(O2-)⋯CuI(NCEt)]+ (2) (λmax = 544 nm), initially forms, k1 = 5.23 ± 0.09 × 104 M-1 s-1 (-105 °C). Subsequent intramolecular reaction of the copper(I) ion in 2 occurs with k2 = 2.74 ± 0.04 × 101 s-1 (-105 °C), producing 3. Resonance Raman spectroscopy (rR) confirms the peroxo assignment for 3; ν(O-O) = 747 cm-1 (Δ(18O2) = -40 cm-1). In an 16O-18O mixed isotope experiment a single band is observed at 730 cm-1. The low ν(O-O) value and the absence of a splitting of the 730 cm-1 band are indicative of a symmetrical binding of the peroxide group in a side-on μ-η2:η2 geometry. This conclusion is supported by X-ray absorption spectroscopy on 3. Copper K-edge EXAFS indicates a five-coordinate metal center: 2 N, 2.028(7) Å; 2 O, 1.898(7) Å; 1 N, 2.171(12) Å. An outer-sphere Fe scatterer is found at 3.62(1) Å. The iron center K-edge EXAFS fits to either a five- or six-coordinate metal center: 4 N(pyrrole), ∼2.1 Å; 1,2 O, ∼1.9 Å. A preedge feature (Fe(1s) → Fe(3d) transition) at 7113.2(2) eV resembles that obtained for a η2-peroxo ferric heme complex, being weaker and at ∼1.5 eV lower energy than those found in five-coordinate (P)FeIII-X (in C4v symmetry) complexes. Arguments based on rR properties of relevant peroxo compounds also effectively point to the copper(II) ion in 3 as being side-on bound, leading to the very low O-O stretching frequency observed in comparison to those of heme-peroxo species or heme-peroxo-copper complexes with a tetradentate copper chelate. These investigations derive from interest in establishing relevant and/or fundamental O2 chemistry at heme-copper centers, in relation to heme-copper oxidase active-site chemistry.
|Original language||English (US)|
|Number of pages||2|
|Journal||Journal of the American Chemical Society|
|State||Published - Oct 13 2004|
ASJC Scopus subject areas
- Colloid and Surface Chemistry