Molecular Cu^II‐O‐Cu^II Complexes: Still Waters Run Deep

Abstract

Research on O2 activation at ligated Cu(I) is fueled by its biological relevance and the quest for efficient oxidation catalysts. A rarely observed reaction is the formation of a Cu(II) -O-Cu(II) species, which is more special than it appears at first sight: a single oxo ligand between two Cu(II) centers experiences considerable electron density, and this makes the corresponding complexes reactive and difficult to access. Hence, only a small number of these compounds have been synthesized and characterized unequivocally to date, and as biological relevance was not apparent, they remained unappreciated. However, recently they moved into the spotlight, when Cu(II) -O-Cu(II) cores were proposed as the active species in the challenging oxidation of methane to methanol at the surface of a Cu-grafted zeolite and in the active center of the copper enzyme particulate methane monooxygenase. This Minireview provides an overview of these systems with a special focus on their reactivity and spectroscopic features.

Copper–Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity

“…The relatively few synthetic complexes with ( μ -oxo)dicopper(II) cores have been reviewed recently. 29 Thus, herein, we only briefly survey selected examples. In early work, complexes 84 , 29,366 85 , 367 86 , 368–370 and 87 218,371 supported by mononucleating ligands were prepared (Figure 64), with their formulations indicated by spectroscopy and their accessibility from multiple routes described (cf.…”

“…70–76 Other dicopper species have also been suggested (Figure 1d), in large part stimulated by the identification by X-ray crystallography and EXAFS of a dicopper site in the enzyme. 29,77–80 These species include triplet 75 or mixed-valent Cu(II)Cu(III) 72 variants of the bis( μ -oxo)dicopper core, a ( μ -oxo)dicopper(II) unit akin to what has been proposed in Cu-doped zeolite catalysts (see section 1.2), 81–83 and dicopper units that incorporate a copper(II)-oxyl moiety. 77 Alternative hypotheses of mono- 84 and tricopper catalytic sites in pMMO have also been advanced, and proposals of additional tricopper reactive intermediates such as that shown in Figure 1d have been made.…”

Transition Metal Complexes and the Activation of Dioxygen

“…As described in a recent review [358], synthetic complexes with this core are relatively rare, in part because of their tendency to react readily with water to yield bis(μ-hydroxo)dicopper(II) complexes [359][360][361][362]. In a recent resurgence of interest in these species, ligands 110 and 111 ( Figure 52) have been used to prepare and characterize species proposed to contain (μ-oxo)dicopper(II) cores [363,364].…”

Reactions Catalysed by a Binuclear Copper Complex: Aerobic Cross Dehydrogenative Coupling of N‐Aryl Tetrahydroisoquinolines

“…For example, the R 2 protein of ribonucleotide reductase (RNR‐R 2 ) and the hydroxylase component of soluble methane monooxygenase (sMMO) have carboxylate‐bridged non‐heme diiron centres –. Dicopper proteins are particularly prominent and feature in a number of important biological reactions that involve oxygen . Well‐known examples include haemocyanin, tyrosinase and catechol oxidase, whereby hemocyanin acts as an oxygen transporter, and the other two enzymes catalyse the oxidation of phenolic compounds [Eqs.…”