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

Abstract

AbstractX‐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 Å) 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 {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 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 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)