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

Abstract

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...

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

“…4 B and C). This strong interaction leads to a covalently delocalized singlet ground state for the side-on Cu II -superoxo species with no spin polarization (i.e., no net antiparallel spin localization on the Cu II and the superoxide ligand), arguing against an antiferromagnetic exchange-coupled description (or biradical) previously used for the diamagnetism of Cu II -superoxo species (21)(22)(23) (Fig. 4B versus Fig.…”

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.…”

“…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. [40][41][42][47][48][49]51,52 In this section, we apply the RASPT2 method to two problems previously studied in considerable detail using a wide range of theoretical models ͑see Fig. 2͒.…”

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

“…Moreover, covalent character in the Cu-O bonding can lead to electronic structures best viewed as intermediate between pairs of formal oxidation states. 8,11,16,35 Scheme 1 illustrates those cases for which significant structural and spectral data are available. The first case, 1, is an end-on precatalytic complex of O 2 in the Cu B site of peptidylglycine R-hydroxylating monooxygenase (PHM).…”