A Systematic K-edge X-ray Absorption Spectroscopic Study of Cu(III) Sites

Abstract

Highly oxidized metals are constituents of oxidants, reactive intermediates, and materials with interesting conductive and magnetic properties. High-energy spectroscopies have played an important role in identifying and describing the bonding character of highly oxidized metals in these materials. A systematic study of Cu(III) K-edge X-ray absorption spectra was carried out to identify analytically useful signatures of Cu(III) in the K-edge, and to elucidate bonding descriptions for Cu(III)-containing complexes. K-edges for six Cu(III) complexes and their same-ligand Cu(II) counterparts are compared. Edges for the Cu(III) species generally appear at higher energies than their Cu(II) counterparts, though energy shifts between most individual edge features vary. However, for all Cu(III) compounds studied, the 1s f 3d transition in the preedge energy range exhibits a distinct, 2 eV shift to higher energy, relative to the known and relatively unvarying energy of the 1s f 3d transition in Cu(II) species. This energy shift provides a direct means of distinguishing Cu(III) from Cu(II). The K-edge for a complex containing Cu(II) coordinated to a 1e --oxidized ligand (phenoxyl) does not show such a change in the 1s f 3d transition energy. The analytical potential of the Cu K-edge was tested with good success using a mixed-valent trinuclear species. Cu(III) is detectable using the K-edge. The limitations of the K-edge as a Cu(III) analytical probe are discussed. An analysis applied to the 1s f 4p and 1s f 4p + shakedown transitions in the edge for a {Cu II 2 (µ-OH) 2 } 2+ dimer, using a configurational interaction (CI) model, predicted ∼75% d-character in the ground state. A similar analysis of the K-edge for {Cu III 2 (µ-O) 2 } 2+ indicates that the Cu in this complex has far more covalent bonds with the oxo bridging ligands (dcharacter ∼60%).

Absence of Mn-Centered Oxidation in the S2→ S3Transition: Implications for the Mechanism of Photosynthetic Water Oxidation

“…17,18,43,45,46,[74][75][76][77][78][79][80] Figure 3 shows the 2nd derivatives of the pure S-state spectra from Figure 2A compared to the 2nd derivatives for each flash sample. The spectra of the individual flash samples, shown in Figure 3A, demonstrate the reproducibility achieved in this experiment.…”

Co and Cu co-doped ZnO epitaxial films—A magnetically soft nano-composite

“…At an intermediate Ar:O 2 ratio of 10:2.5, the shape of the XANES is strongly altered indicating the presence of Cu 2þ . 22 The spectral shape of the small XLD signal is similar to the one observed at the Co K-edge which may be taken as an indication that a small fraction of the Cu 2þ could substitute for Zn. Further increasing the oxygen partial pressure, the XANES remains unaffected but the shape of the XLD changes completely which we take as an indication that CuO forms as a secondary phase.…”

“…At low oxygen partial pressures (10:0.5 to 10:1.5), the appearance of the XANES is purely metallic (Cu 0 ) and can be well compared with reference spectra from the literature. 22 Accordingly, the XLD is zero which is typical for cubic metals as well as hexagonal like Co due to the low crystal field strength stemming from the screening of the free carriers. At an intermediate Ar:O 2 ratio of 10:2.5, the shape of the XANES is strongly altered indicating the presence of Cu 2þ .…”

Nanostructured Copper Oxide on Silica–Zirconia Mixed Oxides by Chemical Implantation

“…[57,61,62] The XANES spectrum of CuO has the following features: in the pre-edge region there is a low-intensity 1s!3d transition at about 8979 eV, which is typical of Cu II compounds. [63,64] In addition, there are two peaks along the rising edge between the pre-edge region and the absorption maximum. [62,63] The first peak at about 8986 eV is attributed to a 1s!4p transition with shakedown contributions and the second peak at about 8993 eV is assigned to a normal 1s!4p transition.…”