Magnetic, vibrational, and optical techniques are combined with density functional calculations to elucidate the electronic structure of the diamagnetic mononuclear side-on CuII-superoxo complex. The electronic nature of its lowest singlet/triplet states and the ground-state diamagnetism are explored. The triplet state is found to involve the interaction between the Cu xy and the superoxide pi v * orbitals, which are orthogonal to each other. The singlet ground state involves the interaction between the Cu xy and the in-plane superoxide pi v * orbitals, which have a large overlap and thus strong bonding. The ground-state singlet/triplet states are therefore fundamentally different in orbital origin and not appropriately described by an exchange model. The ground-state singlet is highly delocalized with no spin polarization.

Recently a Cu(III)(2) bis-mu-oxo dimer ligated by peralkylated amines has been found to interconvert with the side-on peroxo-bridged, mu-eta(2):eta(2) isomer. The Cu(III)(2)(mu-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(LII) 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 (similar to 25 000 cm(-1)) only observed for the bis-mu-oxo species corresponds to an oxo sigma(u)* 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-mu-oxo and side-on peroxo-bridged Cu species and their relative reactivities.

The four-electron reduction of dioxygen to water by trinuclear copper clusters is of great biological significance. Recently we reported the crystal structure of a trinuclear model complex in which the three coppers provide the four electrons necessary to fully reduce dioxygen, generating two mu(3)-oxo bridges. This complex is best described as a localized, mixed-valence Cu(II,II,III) system which has C-2v effective symmetry. The magnetic properties of this trinuclear cluster have been investigated by MCD and SQUID magnetic susceptibility. The two Cu(II) ions are found to be ferromagnetically coupled with a triplet/singlet splitting of 14 cm(-1). Density functional calculations reproduce these geometric, electronic, and magnetic properties of the trinuclear cluster and provide insight into their origin. Since the trinuclear copper complex has a 3+ charge, the Cu3O2 core is one electron too oxidized to permit each atom to be in a preferred oxidation state (2+ for Cu and 2- for O). The extra hole in this highly oxidized Cu3O2 cluster is found to be localized on one Cu, which is therefore a Cu(III) ion, rather than on an O ligand (which would then be an oxyl) due to the strong stabilization of the oxo valence orbitals which derives from bridging to the Cu(II) centers. The communication between the coppers is weak, as it involves superexchange through the oxo bridges which provide nearly orthogonal orbital pathways between the copper ions. This leads to a ferromagnetic interaction between the two Cu(II) ions and weak electronic coupling between the Cu(III) and the Cu(II) ions. In the idealized D-3h high symmetry Limit which would be the favored geometry in the case of complete electronic delocalization, the triplet ground state is orbitally degenerate and subject to a large Jahn-Teller distortion [E' x e'] toward the observed C-2v structure. This combination of a large Jahn-Teller distortion and weak electronic coupling leads to localization of the Cu(III) on one metal center.

Spectroscopic studies of a &mgr;-1,1-hydroperoxo-bridged copper dimer are combined with SCF-Xalpha-SW molecular orbital calculations to describe the vibrational and electronic structure of the hydroperoxo-copper complex and compare it to that of previously studied peroxo-copper species. Four vibrational modes of the Cu(2)OOH unit in the resonance Raman and infrared spectra are assigned on the basis of isotope shifts: nu(O-O) = 892 cm(-)(1), nu(as)(Cu-O) = 506 cm(-)(1), nu(s)(Cu-O) = 322 cm(-)(1), and nu(O-H) = 3495 cm(-)(1). The 892 cm(-)(1) O-O stretch of the &mgr;-1,1-hydroperoxo-bridged copper dimer is 89 cm(-)(1) higher than that of the unprotonated complex. Resonance Raman profiles of the 892 cm(-)(1) O-O stretch are used to assign an electronic absorption band at 25 200 cm(-)(1) (epsilon = 6700 M(-)(1) cm(-)(1)) to a hydroperoxide pi-to-Cu charge transfer (CT) transition. This band is approximately 5000 cm(-)(1) higher in energy than the corresponding transition in the unprotonated complex. The pi-to-Cu CT transition intensity defines the degree of hydroperoxide-to-copper charge donation, which is lower than in the unprotonated complex due to the increased electronegativity of the peroxide with protonation. The lower Cu-O covalency of this hydroperoxo-copper complex shows that the high O-O stretching frequency is not due to increased pi-to-Cu charge donation but rather reflects the direct effect of protonation on intra-peroxide bonding. Density functional calculations are used to describe changes in intra-peroxide and Cu-O bonding upon protonation of the peroxo-copper complex and to relate these changes to changes in reactivity.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTIrreversible Reduction of Dioxygen by Simple Peralkylated Diamine−Copper(I) Complexes: Characterization and Thermal Stability of a [Cu2(μ-O)2]2+ CoreViswanath Mahadevan, Zhiguo Hou, Adam P. Cole, David E. Root, Tapan K. Lal, Edward I. Solomon, and T. D. P. StackView Author Information Department of Chemistry, Stanford University Stanford, California 94305 Cite this: J. Am. Chem. Soc. 1997, 119, 49, 11996–11997Publication Date (Web):December 10, 1997Publication History Received22 August 1997Published online10 December 1997Published inissue 1 December 1997https://pubs.acs.org/doi/10.1021/ja972946nhttps://doi.org/10.1021/ja972946nrapid-communicationACS PublicationsCopyright © 1997 American Chemical SocietyRequest reuse permissionsArticle Views1563Altmetric-Citations198LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-AlertscloseSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Alkyls,Ligands,Organic reactions,Reactivity,Substituents Get e-Alerts

The reaction of metal complexes with dioxygen (O 2 ) generally proceeds in 1:1, 2:1, or 4:1 (metal:O 2 ) stoichiometry. A discrete, structurally characterized 3:1 product is presented. This mixed-valence trinuclear copper cluster, which contains copper in the highly oxidized trivalent oxidation state, exhibits O 2 bond scission and intriguing structural, spectroscopic, and redox properties. The relevance of this synthetic complex to the reduction of O 2 at the trinuclear active sites of multicopper oxidases is discussed.

We have seen from the previous discussion that absorption spectral studies in the ligand field region probe the energy splittings of the d orbitals and that this relates to the geometry of the metal center. The energies and intensities of ligand-to-metal charge transfer transitions sensitively probe bonding interactions of the ligand with the metal center. Charge transfer transitions can be used both qualitatively to observe ligand binding to a metal center, owing to the requirement of orbital overlap for significant charge transfer intensity, and quantitatively to define the electron donor ability of that ligand and experimentally evaluate the results of electronic structure calculations. Studies of the intensities of peaks at the ligand K edge can define the covalent interaction of the ligand with the metal valence orbitals, whereas copper K-edge spectroscopy is a powerful probe of metal ion oxidation state and the ligand field geometry of d10 cuprous sites that are inaccessible through other spectroscopic methods. Absorption spectral studies in all regions are strongly complemented by CD, variable temperature MCD, and single-crystal polarized absorption spectroscopies, which should also be pursued whenever possible to obtain detailed electronic structural insight of relevance to catalysis.

Spectrosopic studies on two side-on mu-eta2:eta2 peroxide-bridged cupric dimers, [Cu(HB(3,5-R2pz)3)]2(O2), where (HB(3,5-R2pz)3) is a tris(pyrazolyl)borate ligand and R = i-Pr or Ph, are presented and compared to our previous studies of end-on bound(eta1) peroxide-coppper monomer and bridged dimer (trans-mu-1,2) complexes. Two transitions at 350 nm (epsilon = 26 000 M-1 cm-1) and 538 nm (epsilon = 2000 M-1 cm-1) are assigned as peroxide-to-copper charge-transfer transitions. A method has been developed to quantitate electron density donation from the ligand to the metal center using the intensity of the ligand-to-metal charge-transfer transitions. It is found that the side-on bridging peroxide donates significantly more electron density from the peroxide pi* orbitals to the coppers than does,peroxide in end-on bound monomer and dimer complexes. The amount of charge donation is proportional to the number of sigma bonds between the peroxide and the coppers (side-on bridged dimer = 4, end-on bridged dimer = 2, end-on monomer = 1). The increased charge donation results in a less negative peroxide in the side-on complex and should in principle produce a stronger O-O bond. Four vibrational modes are assigned in the resonance Raman and infrared spectra on the basis of isotopic shifts at 763 (rR, 723 cm-1 with O-18(2)), 331 (IR, 321 cm-1 with O-18(2)), 284 (rR, no shift with O-18(2)), and 572 cm-1 (based on an overtone at 1144 cm-1 in rR, 1098 cm-1 with O-18(2)) in the R = Ph complex, with corresponding peaks at 749, 285, and 1055 cm-1 in the resonance Raman spectra of the R = i-Pr complex. A normal coordinate analysis shows that the oxygen-oxygen force constant, k(O-O), is smaller in the side-on bridged complex (2.4 mdyn/angstrom) than in the end-on monomer (2.9 mdyn/angstrom) or end-on bridged dimer (3.1 mdyn/angstrom), and thus the O-O bond is weaker in the side-on complex than in the end-on, despite its greater peroxide charge donation. This is direct evidence for the pi acceptor ability of the peroxide in this side-on bridged structure, which involves some peroxide sigma* character mixing into the highest energy occupied molecular orbital, as predicted by Xalpha calculations [Ross, P. K.; Solomon, E. I. J. Am. Chem. Soc. 1991, 113, 3246-3259]. Using the correlation of charge-transfer intensities to ligand charge donation, the charge-transfer intensity of oxyhemocyanin indicates that it likely has four copper-peroxide bonds and thus a side-on peroxide bridging structure. The electronic structure of this side-on bridging geometry in oxyhemocyanin explains its unique spectroscopic features, including the high intensity and energy of the 345-nm absorption band, the low O-O stretching frequency, and the lack of a symmetric Cu-O stretch in the expected energy range of the resonance Raman spectrum. This electronic structure also provides insight into the mechanisms of oxygen binding and activation in hemocyanin and tyrosine.