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%).

Recently a Cu(III)2 bis-μ-oxo dimer ligated by peralkylated amines has been found to interconvert with the side-on peroxo-bridged, μ-η2:η2 isomer. The Cu(III)2(μ-O)2 dimer exhibits two intense charge transfer (CT) features in the near-UV region of the electronic absorption spectrum. Laser excitation into the lower-energy CT absorption band at 25 000 cm-1 results in intense resonance enhancement of the Raman peaks at 609 and 118 cm-1 which profile this band and give overtone and combination progressions. The combined application of a normal coordinate analysis of the Raman features and a time-dependent Heller theory analysis of the electronic absorption spectrum and resonance Raman profiles provide the excited-state geometry. As this transition corresponds to an oxo-to-Cu(III) CT, this excited state is formally an oxyl−Cu(II) species. Density functional calculations correlated to these data (including the excited-state geometry and the relative CT intensities) allow for an unambiguous assignment of the observed charge-transfer transitions. This assignment shows that one of the CT features involves the same orbital origin as a corresponding transition in the side-on peroxo dimer, while the new, low-energy band (∼25 000 cm-1) only observed for the bis-μ-oxo species corresponds to an oxo σu* to Cu(III) d xy CT transition which is present when the O−O bond is cleaved. This study provides electronic structural insight into the relationship between the bis-μ-oxo and side-on peroxo-bridged Cu species and their relative reactivities.

The reaction of metal complexes with dioxygen (O2) generally proceeds in 1:1, 21, or 41 (metal:O2) stoichiometry. A discrete, structurally characterized 31 product is presented. This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O2 bond scission and intriguing structural, spectroscopic, and redox properties. The relevance of this synthetic complex to the reduction of O2 at the trinuclear active sites of multicopper oxidases is discussed.

Copper in its infrequently observed Cu(III) oxidation state has gained attention recently, in particular as a proposed constituent of O 2 -cleaving binuclear and trinuclear complexes. 1,2 A few discrete molecular species generally accepted as containing Cu(III) are known. 3 Assignment of the formal Cu(III) oxidation state in these compounds is based on a variety of spectroscopic, electrochemical, and structural evidence. In particular, their very short Cu-ligand bond distances relative to bond lengths in their Cu(II) counterparts suggest a higher oxidation state. However, a direct and unambiguous probe of the metal oxidation state is notably absent. 4 Metal K-edge X-ray absorption spectroscopy (XAS), because of its ability to probe directly the core-electronic environment of the absorbing metal, offers a means of explicitly examining the oxidation state of the copper. A specific probe of the metal oxidation state is particularly relevant, given the occurrence of ligand-centered oxidation in some Cu(II) systems. 5 More generally, defining differences between compounds with similar atomic compositions but differing electronic distributions, is potentially relevant to understanding variations in their reactivities. In particular, the (µ-η 2 :η 2 -peroxo)dicopper(II), 1b,6 bis-µ-oxyl dicopper(II), and bis-µ-oxo dicopper(III) 1 cores are all formally equivalent, though the latter two may be largely indistinguishable by most spectroscopic methods. 7 Here we report on a Cu K-edge XAS study of two binuclear copper complexes [(L ME ) 2 Cu 2 O 2 ] 2+ (1) and [(L TEED ) 2 Cu 2 O 2 ] 2+ , 1,9 (2) (Figure 1) showing that these complexes are best formally described as Cu(III) 2 (µ-O) 2 and giving the first direct spectroscopic evidence of stabilized Cu(III) in a Cu 2 O 2 rhomb.The Cu(III) valence state for complex 1 may be proposed on the basis of its remarkably short, crystallographically determined metal-ligand bond lengths. 10 Cu K-edge EXAFS analysis 11 confirms these bond distances as well as the notably short Cu-Cu distance of 2.73 Å for 1. 13,14 EXAFS analysis of 2 gives similar metrical details (Cu-O ) 1.80 Å, Cu-N ) 1.92 Å, Cu-Cu ) 2.75 Å). 14 Verification of these structures by EXAFS provides an internal check against possible decomposition in these thermally sensitive molecules. Analogous Cu-ligand and Cu-Cu distances are 0.1-0.2 Å longer in the 2e -, 2H + reduced bis-µ-hydroxo dicopper(II) forms of 1 and 2 (i.e., 3 and 4, respectively). 14,15 The Cu-Cu separation is ∼0.05 Å longer in related, recently reported Cu 2 O 2 complexes (e.g., [(Bn 3 -TACN) 2 Cu 2 O 2 ] 2+ ). 1b,16 For comparison with 1-4, two accepted Cu(III) speciessthe N 3 O tripeptide chelate complex Cu(III)-(H -2 Aib 3 ) 3a and the solid state material KCuO 2 17 shave been