A study using X-ray absorption and emission spectroscopy of dioxygen-binding xerogels incorporating cyclam units complexed with copper salts

Abstract

X-ray absorption spectroscopy was used to elucidate how hybgrid xerogels complexed with CuCl2 could reversibly bind molecular dioxygen. Difference EXAFS analyses at the Cu K-edge suggest that dioxygen could bridge two Cu atoms in a μ-η1:η1 peroxo-like conformation with unequal Cu⋯O distances. Only the short distance (R Cu-O1 = 1.86±0.01Å) was unambiguously determined and looks typical of a CuII site. The Cu⋯Cu internuclear distances would be rather long: RCu-Cu ≈ 4.0 Å (3.9 Å) for the oxygenated (oxygen-free) xerogels. Cl K-edge EXAFS spectra revealed the pre-existence in the oxygen-free xerogels of CuI sites with short Cl-Cu bonds (2.11 ±0.03 Å). Pentacoordinate CuII sites with a longer Cl-Cu bond (2.45±0.03 Å) were also identified. Another signal at a further distance (2.73±0.03 Å) suggests that the Cl ions could bridge mixed-valence {CuICuII} sites. In oxygenated xerogels complexed with CuCl2, ca. 75 % of the Cu atoms would be singly bound to dioxygen while the signature of the pentacoordinate CuII sites would totally vanish. Substituting CuBr2 for CuCl2 yields xerogels with a poor capability to bind dioxygen. This is remarkably correlated with a decrease of the signatures at RCu-O1 = 1.86 Å and RCu-Cu = 4.0 Å. EXAFS spectra of the oxygen-free xerogel complexed with CuBr2 again exhibit signatures assigned to short (2.30 Å)/long (2.49 Å) Cu-Br bonds as expected for mixedvalence {CuICuII} systems. Inactive xerogels prepared with metallated cyclams seem to undergo a structural change during hydrolysis and polycondensation. Deconvoluted XANES spectra of the active xerogels exhibit a strong pre-edge resonance which is characteristic of mixed-valence CuI sites. This interpretation was supported by ab initio XANES simulations carried out beyond the muffin-tin approximation. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.