Snapshots of Dioxygen Activation by Copper:  The Structure of a 1:1 Cu/O<sub>2</sub> Adduct and Its Use in Syntheses of Asymmetric Bis(μ-oxo) Complexes

Aboelella, Nermeen W.; Lewis, Elizabeth; Reynolds, Anne; Brennessel, William W.; Cramer, Christopher J.; Tolman, William B.

doi:10.1021/ja027164v

Cited by 184 publications

(182 citation statements)

References 15 publications

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“…As summarized in Scheme 1 and Table 1, our theoretical models (labeled with a t to facilitate discussion distinguishing them from the experimentally characterized systems) include all substituents and thus are exact replicas for the cases 2 (11), 3 (K. Qin (14,15), and 7 (26). In the case of 1 (10), the theoretical model simplifies the 5-isopropyl groups on the pyrazole rings for computational efficiency, and for the same reason the phenyl rings pendant on the porphinato ligands in 8 (27) Value is an estimate from experimental data, for anion well separated from any counterion.…”

Section: Resultsmentioning

confidence: 99%

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Variable character of O—O and M—O bonding in side-on (η ² ) 1:1 metal complexes of O ₂

Cramer

Tolman

Theopold

et al. 2003

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

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324

319

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The structures and the OOO and MOO bonding characters of a series of reported side-on ( 2 ) 1:1 metal complexes of O2 are analyzed by using density functional theory calculations. Comparison of the calculated and experimental systems with respect to OOO bond distance, OOO stretching frequency, and OOO and MOO bond orders provides new insights into subtle influences relevant to O2 activation processes in biology and catalysis. The degree of charge transfer from the generally electronrich metals to the dioxygen fragment is found to be variable, such that there are species well described as superoxides, others well described as peroxides, and several cases having intermediate character. Increased charge transfer to dioxygen takes place via overlap of the metal dxy orbital with the in-plane * orbital of O2 and results in increased MOO bond orders and decreased OOO bond orders. Comparison of theory and experiment over the full range of compounds studied suggests that reevaluation of the OOO bond lengths determined from certain x-ray crystal structures is warranted; in one instance, an x-ray crystal structure redetermination was performed at low temperature, confirming the theoretical prediction. Librational motion of the coordinated O2 is identified as a basis for significant underestimation of the OOO distance at high temperature. With the longstanding goal of understanding in detail how dioxygen binds and is activated at metal centers in biological and catalytic systems, great effort has been expended to characterize the structures, physicochemical properties, and reactivity of metal-dioxygen complexes (1-8). As the initially formed species in most oxidation processes, 1:1 metal-O 2 adducts are of special interest, particularly in view of their postulated involvement in enzymes that react with dioxygen at an isolated monometallic active site, such as Cu in dopamine ␤-monooxygenase or galactose oxidase (9). Two binding modes have been identified in 1:1 metal-O 2 complexes, end-on ( 1 ) and side-on ( 2 ). These adducts have been further defined as superoxo or peroxo complexes, primarily on the basis of x-ray structural data (OOO bond distance) and vibrational spectroscopy (OOO stretching frequency, OO ) (1-4). Thus, compounds with an OOO bond length of Ϸ1.4-1.5 Å and OO between Ϸ800 and 930 cm Ϫ1 are designated as peroxides, whereas those with OOO Ϸ1.2-1.3 Å and OO Ϸ1,050-1,200 cm Ϫ1 fall into the superoxide category. On this basis, and keeping in mind the caveat that it is often difficult to assess experimentally the actual amount of charge transferred to dioxygen on complexation, the majority of the known 2 1:1 compounds are generally agreed to be best described as peroxo complexes (1,5,8).Reports of , Tris(3,5-dimethylpyrazol-1-yl)hydroborate) (13) (Scheme 1). In the course of the investigation of 6, for which intermediate character of the dioxygen unit was found (14, 15), we noted puzzling incongruities among the data reported for these complexes (Table 1). For example, whereas 1 and 2 (as reported in ref. 11, d...

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Section: Resultsmentioning

confidence: 99%

“…In the course of the investigation of 6, for which intermediate character of the dioxygen unit was found (14,15), we noted puzzling incongruities among the data reported for these complexes (Table 1). For example, whereas 1 and 2 (as reported in ref.…”

mentioning

confidence: 85%

Variable character of O—O and M—O bonding in side-on (η ² ) 1:1 metal complexes of O ₂

Cramer

Tolman

Theopold

et al. 2003

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

Self Cite

324

319

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“…The inaccessible oxidation state is also the case for the mononuclear Cu III -peroxide species (Table 1, row F), which was recently synthesized with an exceptionally strong electron-donating ligand (35). Therefore, to form the 1-e Ϫ reduced superoxide level species Cu M II -superoxo, which from the above model would be the reactive species in H-atom abstraction and not proceed further to a thermodynamically favored 2-e Ϫ reduced peroxo species [at pH ϭ 7, E°(O 2 ͞H 2 O 2 ) ϭ 0.28 V, E°(O 2 ͞O 2-) ϭ Ϫ0.33 V versus normal hydrogen electrode] (36), the two Cu sites have to be nonelectronically coupled.…”

Section: Correlation Of Electronicmentioning

confidence: 91%

O ₂ activation by binuclear Cu sites: Noncoupled versus exchange coupled reaction mechanisms

Chen

Solomon

2004

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

160

146

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Binuclear Cu proteins play vital roles in O2 binding and activation in biology and can be classified into coupled and noncoupled binuclear sites based on the magnetic interaction between the two Cu centers. Coupled binuclear Cu proteins include hemocyanin, tyrosinase, and catechol oxidase. These proteins have two Cu centers strongly magnetically coupled through direct bridging ligands that provide a mechanism for the 2-electron reduction of O2 to a -2 : 2 side-on peroxide bridged Cu II 2 (O 2 2؊ ) species. This side-on bridged peroxo-Cu 2 II species is activated for electrophilic attack on the phenolic ring of substrates. Noncoupled binuclear Cu proteins include peptidylglycine ␣-hydroxylating monooxygenase and dopamine ␤-monooxygenase. These proteins have binuclear Cu active sites that are distant, that exhibit no exchange interaction, and that activate O2 at a single Cu center to generate a reactive Cu II ͞O2 species for H-atom abstraction from the C-H bond of substrates. O2 intermediates in the coupled binuclear Cu enzymes can be trapped and studied spectroscopically. Possible intermediates in noncoupled binuclear Cu proteins can be defined through correlation to mononuclear Cu II ͞O2 model complexes. The different intermediates in these two classes of binuclear Cu proteins exhibit different reactivities that correlate with their different electronic structures and exchange coupling interactions between the binuclear Cu centers. These studies provide insight into the role of exchange coupling between the Cu centers in their reaction mechanisms.

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“…Their electronic structures have been assigned to be singlet ground states having a dominant LCu(II)O 2 (-) character based on the O-O bond length in 4a from X-ray crystallography, 9,11 the O-O stretching frequency (which has been shown 8,11 to be well correlated by Badger's rule 37 with the O-O bond length) from resonance Raman spectra for 4a and 4b, 9,12,31 and K-and L 3 -edge X-ray absorption spectroscopy (XAS) for 4b. 12,31 Compounds 5b 10 and 6 23 also have been determined by X-ray crystallography to bind O 2 side-on, but the singlet ground states for these species have been assigned to be dominated by LCu(III)O 2 (2-) character based on O-O bond lengths, stretching frequencies (available also for 5a 38 and essentially identical to that for 5b), and K-and L 3 -edge XAS for 5a. 31 Thus, over an array of different ligands, it is evidently possible to access the full range of Cu(I), Cu(II), or Cu(III) oxidation states, both O 2 coordination motifs, and either singlet or triplet ground states.…”

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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Snapshots of Dioxygen Activation by Copper: The Structure of a 1:1 Cu/O₂ Adduct and Its Use in Syntheses of Asymmetric Bis(μ-oxo) Complexes

Cited by 184 publications

References 15 publications

Variable character of O—O and M—O bonding in side-on (η ² ) 1:1 metal complexes of O ₂

Variable character of O—O and M—O bonding in side-on (η ² ) 1:1 metal complexes of O ₂

O ₂ activation by binuclear Cu sites: Noncoupled versus exchange coupled reaction mechanisms

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

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Snapshots of Dioxygen Activation by Copper: The Structure of a 1:1 Cu/O2 Adduct and Its Use in Syntheses of Asymmetric Bis(μ-oxo) Complexes

Cited by 184 publications

References 15 publications

Variable character of O—O and M—O bonding in side-on (η 2 ) 1:1 metal complexes of O 2

Variable character of O—O and M—O bonding in side-on (η 2 ) 1:1 metal complexes of O 2

O 2 activation by binuclear Cu sites: Noncoupled versus exchange coupled reaction mechanisms

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

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Product

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Snapshots of Dioxygen Activation by Copper: The Structure of a 1:1 Cu/O₂ Adduct and Its Use in Syntheses of Asymmetric Bis(μ-oxo) Complexes

Variable character of O—O and M—O bonding in side-on (η ² ) 1:1 metal complexes of O ₂

Variable character of O—O and M—O bonding in side-on (η ² ) 1:1 metal complexes of O ₂

O ₂ activation by binuclear Cu sites: Noncoupled versus exchange coupled reaction mechanisms