Observation of Fe(V)5O using variable- temperature mass spectrometry and its enzyme- like C–H and C5C oxidation reactions

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 C5C 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)5O moiety was postulated. Here we show, using variable-temperature mass spectrometry, the generation of a Fe(V)5O species within a synthetic non-haem complex at 240 8C and its reaction with an olefin. Also, with isotopic labelling we were able both to follow oxygen-atom transfer from H2O2/H2O through Fe(V)5O 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. F or small-molecule activation processes, iron is the element of choice, selected by nature to perform a number of chemically challenging oxidative processes with high precision and reaction rates. Iron-based enzymes, such as cytochrome P450 (ref. 1) and Rieske dioxygenases2, use O2 to catalyse highly selective C–H and CC oxidation reactions, key steps in the metabolic synthesis o