Combined Spectroscopic and Theoretical Evidence for a Persistent End‐On Copper Superoxo Complex

Schatz, Markus; Raab, Volker; Foxon, Simon P.; Brehm, G.; Schneider, Siegfried; Reiher, Markus; Holthausen, Max C.; Sundermeyer, Jörg; Schindler, Siegfried

doi:10.1002/anie.200454125

Cited by 173 publications

(157 citation statements)

References 49 publications

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“…3, 4, 5, and 6 unrestricted DFT level, the nitrogen atom of the triazacyclononane ring originally axially coordinated to copper decoordinates (although the nitrogen lone pair remains directed towards the copper atom); subsequent full optimization leads to a structure that maintains the side-on coordination mode for O 2 , 1 6 u (g 2 ), although the orientation of the O 2 fragment is rotated compared with that in 1 6 r (g 2 ). It is noteworthy that Schatz et al [26] observed exactly the same phenomenon in their DFT studies of 5: in order to go from g 1 to g 2 coordination, one nitrogen atom of the tris(tetramethylguanidino)tren ligand had to be permitted to decoordinate. Key geometric details for all structures of 6 are provided in Table 3.…”

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confidence: 66%

“…With such rare examples of isotopomer peak splitting, an important question is whether such peak splitting or the absence thereof (i.e., the negative result) is sufficient to differentiate between g 1 and g 2 coordination modes. Schatz et al [26] recently faced this exact question for the case of a 1:1 Cu-O 2 adduct with a tris(tetramethylguanidino)tren ligand 5. The RR spectrum showed only a single absorption for the two mixed-label isotopomers, which might be interpreted to imply a side-on coordination mode.…”

Section: Introductionmentioning

confidence: 99%

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How useful are vibrational frequencies of isotopomeric O2 fragments for assessing local symmetry? Some simple systems and the vexing case of a galactose oxidase model

et al. 2005

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confidence: 66%

Section: Introductionmentioning

confidence: 99%

How useful are vibrational frequencies of isotopomeric O2 fragments for assessing local symmetry? Some simple systems and the vexing case of a galactose oxidase model

et al. 2005

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“…Such species have been found in the reactions of O 2 with heme proteins and models (6,7) as well as with one copper enzyme (8) and several synthetic copper and nickel complexes (9)(10)(11)(12)(13)(14)(15)(16). Such species have also been invoked in the mechanisms for nonheme iron enzymes, but only indirect evidence has been obtained to date (39)(40)(41).…”

Section: Discussionmentioning

confidence: 99%

“…Crystallographic evidence for an analogous formulation has also recently been reported for a copper enzyme peptidylglycine ␣-hydroxylating monooxygenase (PHM) (8). Furthermore, synthetic copper(I) and nickel(I) complexes react with O 2 to give rise to copper and nickel superoxo species (9)(10)(11)(12)(13)(14)(15)(16) 2 was prepared by using the same procedure in the presence of 10 eq of D 2 O. All manipulations with complex 1 and solutions thereof were carried out under inert atmosphere by using a glovebox filled with nitrogen.…”

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confidence: 99%

Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

Shan

Que

2005

Proc. Natl. Acad. Sci. U.S.A.

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The reaction of [Fe2(-OH)2(6-Me3-TPA)2] 2؉ (1) [6-Me3-TPA, Tris(6-methyl-2-pyridylmethyl)amine] with O2 in CH2Cl2 at ؊80°C gives rise to two new intermediates, 2 and 3, before the formation of previously characterized [Fe2(O)(O2)(6-Me3-TPA)2] 2؉ (4) that allow the oxygenation reaction to be monitored one electron-transfer step at a time. Raman evidence assigns 2 and 3 as a diiron-superoxo species and a diiron-peroxo species, respectively. Intermediate 2 exhibits its (O-O) at 1,310 cm ؊1 with a ؊71-cm ؊1 18 O isotope shift. A doublet peak pattern for the 16 O 18 O isotopomer of 2 in mixedisotope Raman experiments strongly suggests that the superoxide ligand of 2 is bound end-on. This first example of a nonheme iron-superoxo intermediate exhibits the highest frequency (O-O) yet observed for a biomimetic metal-dioxygen adduct. The bound superoxide of 2, unlike the bound peroxide of 4, is readily reduced by 2,4-di-tert-butylphenol via a proton-coupled electron-transfer mechanism, emphasizing that metal-superoxo species may serve as oxidants in oxygen activation mechanisms of metalloenzymes. The discovery of intermediates 2 and 3 allows us to dissect the initial steps of dioxygen binding at a diiron center leading to its activation for substrate oxidation.nonheme diiron enzymes ͉ oxygen activation ͉ superoxo intermediate T he first step in the oxygen activation mechanisms of metalloenzymes is typically the binding of dioxygen, resulting in electron transfer from metal to O 2 to form a metal-superoxo species (1-5). This conversion is best exemplified by heme proteins and synthetic iron porphyrin complexes where there is strong crystallographic and spectroscopic evidence for an Fe III -( 1 -O 2•Ϫ ) formulation (6, 7). Crystallographic evidence for an analogous formulation has also recently been reported for a copper enzyme peptidylglycine ␣-hydroxylating monooxygenase (PHM) (8). Furthermore, synthetic copper(I) and nickel(I) complexes react with O 2 to give rise to copper and nickel superoxo species (9)(10)(11)(12)(13)(14)(15)(16) 2 was prepared by using the same procedure in the presence of 10 eq of D 2 O. All manipulations with complex 1 and solutions thereof were carried out under inert atmosphere by using a glovebox filled with nitrogen. Dichloromethane was distilled from CaH 2 under nitrogen. 2,4-di-tert-butylphenol (DTBP) was purchased from Aldrich and used as received. Saturated solutions of O 2 in CH 2 Cl 2 for the kinetic studies were prepared by bubbling the dried O 2 gas through the solvent for 10 min at room temperature. The concentration of O 2 in this saturated solution was accepted to be as reported in the literature (25) Sample Preparation. For UV-visible experiments, an anaerobic solution of 1⅐(OTf) 2 (3.3 mg) in dry CH 2 Cl 2 (3 ml) was introduced into a 1-cm-path-length cuvette and then cooled to 193 K in a Unisoku cryogenic cell holder attached to a HewlettPackard 8453A diode array spectrophotometer. O 2 gas was bubbled through the solution for 5 min, and the formation of 2 was complete within 1 h....

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“…Compound 2 also has been established by X-ray crystallography to bind O 2 end-on, 32 and its ground electronic state has been confirmed to be a triplet. 35 Structure 2 was originally assigned, incorrectly, to have a singlet ground state, 20 based in part on density functional calculations, which nicely illustrates the significant challenges that LCuO 2 systems can pose for computation. In the case of compound 3, no crystal structure has been obtained, but resonance Raman data support an end-on binding mode; the electronic ground state has not been determined.…”

Section: Introductionmentioning

confidence: 99%

Stereoelectronic Effects on Molecular Geometries and State-Energy Splittings of Ligated Monocopper Dioxygen Complexes

et al. 2008

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The relative energies of side-on versus end-on binding of molecular oxygen to a supported Cu(I) species, and the singlet versus triplet nature of the ground electronic state, are sensitive to the nature of the supporting ligands and, in particular, depend upon their geometric arrangement relative to the O 2 binding site. Highly correlated ab initio and density functional theory electronic structure calculations demonstrate that optimal overlap (and oxidative charge transfer) occurs for the side-on geometry, and this is promoted by ligands that raise the energy, thereby enhancing resonance, of the filled Cu d xz orbital that hybridizes with the in-plane π* orbital of O 2 . Conversely, ligands that raise the energy of the filled Cu d z 2 orbital foster a preference for end-on binding as this is the only mode that permits good overlap with the in-plane O 2 π*. Because the overlap of Cu d z 2 with O 2 π* is reduced as compared to the overlap of Cu d xz with the same O 2 orbital, the resonance is also reduced, leading to generally more stable triplet states relative to singlets in the end-on geometry as compared to the side-on geometry, where singlet ground states become more easily accessible once ligands are stronger donors. Biradical Cu(II)-O 2 superoxide character in the electronic structure of the supported complexes leads to significant challenges for accurate quantum chemical calculations that are best addressed by exploiting the spin-purified M06L local density functional, single-reference completely renormalized coupled-cluster theory, or multireference second-order perturbation theory, all of which provide predictions that are qualitatively and quantitatively consistent with one another.

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Combined Spectroscopic and Theoretical Evidence for a Persistent End‐On Copper Superoxo Complex

Cited by 173 publications

References 49 publications

How useful are vibrational frequencies of isotopomeric O2 fragments for assessing local symmetry? Some simple systems and the vexing case of a galactose oxidase model

How useful are vibrational frequencies of isotopomeric O2 fragments for assessing local symmetry? Some simple systems and the vexing case of a galactose oxidase model

Intermediates in the oxygenation of a nonheme diiron(II) complex, including the first evidence for a bound superoxo species

Stereoelectronic Effects on Molecular Geometries and State-Energy Splittings of Ligated Monocopper Dioxygen Complexes

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