Dioxygen Activation at a Single Copper Site:  Structure, Bonding, and Mechanism of Formation of 1:1 Cu−O₂ Adducts

Abstract

To evaluate the fundamental process of O(2) activation at a single copper site that occurs in biological and catalytic systems, a detailed study of O(2) binding to Cu(I) complexes of beta-diketiminate ligands L (L(1) = backbone Me; L(2) = backbone tBu) by X-ray crystallography, X-ray absorption spectroscopy (XAS), cryogenic stopped-flow kinetics, and theoretical calculations was performed. Using synchrotron radiation, an X-ray diffraction data set for L(2)CuO(2) was acquired, which led to structural parameters in close agreement to theoretical predictions. Significant Cu(III)-peroxo character for the complex was corroborated by XAS. On the basis of stopped-flow kinetics data and theoretical calculations for the oxygenation of L(1)Cu(RCN) (R = alkyl, aryl) in THF and THF/RCN mixtures between 193 and 233 K, a dual pathway mechanism is proposed involving (a) rate-determining solvolysis of RCN by THF followed by rapid oxygenation of L(1)Cu(THF) and (b) direct, bimolecular oxygenation of L(1)Cu(RCN) via an associative process.

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

“…The prediction of a singlet ground state is consistent with experiment. The large difference in relative energetics between CASPT2 and DFT, with the former providing agreement with experiment for the correct ground state while the latter fails to do so, is consistent with past experience regarding Cu-O 2 adducts [28,29]. Analysis of the results for 9 suggests that deficiencies in basis-set and active-space size may lead to errors as large as 4-5 kcal mol À1 in predicted relative energies for 7; however, the qualitative ordering of the relative energies noted before are invariant to errors of this magnitude.…”

“…On the basis of computed relative energies and all of the frequency data, Schatz et al concluded that the coordination mode was end-on. However, interpretation of density functional results in such systems can be complicated by the varying degrees of multiconfigurational character that 1:1 Cu-O 2 adducts may exhibit both intrinsically and as a function of coordination motif [27][28][29]. This variation can lead to significant errors in computed energies and, in principle, vibrational force constants.…”

“…Note that the predicted relative energies of the end-on and side-on structures are substantially different with the two different functionals, primarily because B3LYP predicts the triplet state to be stabler than the singlet state by roughly twice as much as the pure DFT mPWPW91 functional. We have noted elsewhere the tendency for B3LYP to be in error with respect to the ground-state spin for monocopper dioxo species because of multireference character in the singlet state(s) [28,29], and this is yet another example.…”

The restricted active space followed by second-order perturbation theory method: Theory and application to the study of CuO2 and Cu2O2 systems

“…In both of these cases, the resulting compounds with a variety of ligands can have singlet ground states that exhibit substantial biradical character because of the spin separation associated either with two d 9 Cu͑II͒ ions or one such ion and a superoxide radical anion. [40][41][42][43][44][45][46][47][48][49][50][51][52] Wave function theories restricted to a single determinant are poorly suited to the description of such species since singlet biradicals are intrinsically two determinantal. 53,54 Moreover, even when oxidation states more likely to be characterized as closed shell in nature are considered, e.g., ͓LCu͑III͔͒ 2 ͓O͑2−͔͒ 2 , computational studies have found that large contributions from dynamical correlation effects influence relative isomer energetics.…”

Singlet-triplet gaps in large multireference systems: Spin-flip-driven alternatives for bioinorganic modeling

“…47,92 There is, however, no alternative to using DFT for geometry optimizations and it has always been considered that using restricted DFT and even using unrestricted DFT would provide useable geometries for the systems investigated here or their analogs. 35,57,93,94 We will take advantage of the appealing possibility of SF-DFT to optimize such adducts. In order to explore this point, we here have considered the ͓Cu͑tren͒͑O 2 ͔͒ + adduct in its two possible coordination modes, end-on and side-on.…”