Cu K-Edge XAS Study of the [Cu₂(μ-O)₂] Core:  Direct Experimental Evidence for the Presence of Cu(III)

Abstract

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

A Paramagnetic Copper(III) Complex Containing an Octahedral CuIIIS6 Coordination Polyhedron

“…The Cu K-edge energy in 2 is about 0.7 eV higher than that in 1 in accord with a metal-centered one-electron oxidation of 1 to 2. [8] Furthermore, the analysis of the Co and Cu K-edge EXAFS spectra (extended X-ray absorption fine structure) of 1 and 2 shows that oxidation of 1 to 2 does not affect the Co ± N and Co ± S bond lengths (2.02(1) and 2.239(6) , respectively); they are in excellent agreement with the crystallographically determined values for 1-(BPh 4 ) 2´6 CH 3 CN. Even the Cu ± S distances in 1 and 2 appear to be identical (but see the comment in ref.…”

“…In contrast, the spectrum of 2 is typical for a triplet ground state (S 1) with small zero-field splitting. In summary, we have shown that six bridging thiolato ligands are capable of stabilizing a high-spin d 8 (%): C 52.2, H 6.1, N 4.7, S 10.7, Co 6.6, Cu 3.5; found: C 51.9, H 6.1, N 4.5, S 10.9, Co 6.5, Cu 3.5.…”

Experimental and Theoretical Investigations of the Existence of CuII, CuIII, and CuIV in Copper Corrolato Complexes

“…Avery weak preedge peak is detected at 8980.5 eV,w hile am ore intense feature is observed in the edge at 8986.5 eV.W hereas the position of the first 1s!3d transition is an unambiguous diagnostic of Cu III ,t he more intense 1s!4p + LMCT shakedown transition is more ligand-dependent and can only support the conclusion drawn from the weak quadrupole transition. [17] Thec yclic voltammograms and the differential pulse voltammograms of complexes 1, 2,a nd 3 were measured in CH 2 Cl 2 /0.1m TBAP (see Figure S1 and Table S2). Each of the three copper complexes displayed one reversible oxidation and one reversible reduction step.T he oxidation couple was (2), and À0.64(90) (3)v ersus ferrocene/ferrocenium.…”

Electronic π-Delocalization Boosts Catalytic Water Oxidation by Cu(II) Molecular Catalysts Heterogenized on Graphene Sheets

“…Cu K-edge XANES were generally characterized by two peaks along the rising maximum edge, namely the 1s  4p main transition along with a 1s  (4p+shakedown) transition, assigned as the 1s  4p transition with concurrent ligand to metal charge transfer (LMCT), as illustrated in Figure 2A and S14. 8 The XANES spectrum of 2 -relative to 2 2-shows a clear edge energy shift of 1.5 eV at around half height and 0.65 normalized absorption, reflecting the higher ionization energy required for ejecting a core 1s electron from a more positively charged ion. The distinct metal-centered oxidation of 2 -vs. 2 2-is shown by the 1.5 eV energy shift in the pre-edge energy range from 8979.4 to 8980.9 eV, corresponding to the 1s  3d electronic transition ( Figure 2A, S16, Table S1).…”