Organometallic Cu^III species have been invoked in many copper-catalyzed reactions, but until recently, experimental evidences were lacking. The advent of new spectroscopic techniques and the characterization of simple and stable catalyst models have been key to gain clear mechanistic insights into these important reactions. In this perspective, we will discuss these discoveries and the corresponding mechanistic proposals that derive from important reactions such as nucleophilic organocuprate chemistry for C–C bond formation, C–heteroatom cross-coupling reactions (Ullmann condensation reactions and halide-exchange processes) and direct C–H bond functionalizations catalyzed by copper.

Oxo-transfer chemistry mediated by iron underpins many biological processes and today is emerging as synthetically very important for the catalytic oxidation of C-H and C=C moieties that are hard to activate conventionally. Despite the vast amount of research in this area, experimental characterization of the reactive species under catalytic conditions is very limited, although a Fe(V)=O moiety was postulated. Here we show, using variable-temperature mass spectrometry, the generation of a Fe(V)=O species within a synthetic non-haem complex at -40 °C and its reaction with an olefin. Also, with isotopic labelling we were able both to follow oxygen-atom transfer from H(2)O(2)/H(2)O through Fe(V)=O to the products and to probe the reactivity as a function of temperature. This study pioneers the implementation of variable-temperature mass spectrometry to investigate reactive intermediates.

A series of aryl–copper(III)-halide complexes have been synthesized and characterized by NMR and UV-visible spectroscopy, cyclic voltammetry and X-ray crystallography. These complexes closely resemble elusive intermediates often invoked in catalytic reactions, such as Ullmann–Goldberg cross-coupling reactions, and their preparation has enabled direct observation and preliminary characterization of aryl halide reductive elimination from Cu^III and oxidative addition to Cu^I centers. In situ spectroscopic studies ( ¹H NMR, UV-visible) of a Cu-catalyzed C–N coupling reaction provides definitive evidence for the involvement of an aryl-copper(III)-halide intermediate in the catalytic mechanism. These results provide the first direct observation of the Cu^I /Cu^III redox steps relevant to Ullmann-type coupling reactions.