6533b7d5fe1ef96bd1265158

RESEARCH PRODUCT

A Study Using X-ray Absorption and Emission Spectroscopy of Dioxygen-Binding Xerogels Incorporating Cyclam Units Complexed with Copper Salts.

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X-ray absorption spectroscopy was used to elucidate how hybrid 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 (RCu–O1 = 1.86 ± 0.01 A) was unambiguously determined and looks typical of a CuII site. The Cu···Cu internuclear distances would be rather long: RCu–Cu ≈ 4.0 A (3.9 A) 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 A). Pentacoordinate CuII sites with a longer Cl–Cu bond (2.45 ± 0.03 A) were also identified. Another signal at a further distance (2.73 ± 0.03 A) suggests that the Cl ions could bridge mixed-valence {CuI,CuII} 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 correlatedwith a decrease of the signatures at RCu–O1 = 1.86 A and RCu–Cu 4.0 A. EXAFS spectra of the oxygen-free xerogel complexed with CuBr2 again exhibit signatures assigned to short (2.30 A)/long (2.49 A) Cu–Br bonds as expected for mixed-valence {CuI,CuII} 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)