Diverse reactivity of a cationic N-heterocyclic phosphenium complex towards anionic substrates – substitution vs. reduction

Abstract

A cationic N-heterocyclic phosphenium (NHP) iron tetracarbonyl complex was synthesised from the free cation and its behaviour towards various anionic reactants studied. Reactions with fluoride, chloride, and hydride sources proceeded under attachment of the anion at phosphorus to yield Fe(CO)₄-complexes of neutral diazaphospholenes, while bromide and iodide reacted under addition of the anion at the metal and decarbonylation to yield NHP iron halides. Reactions with amides and organometallics were unselective. At room temperature, predominantly reduction of the cationic complex to yield a spectroscopically detectable Fe-centred radical and its deactivation products was observable. At −78 °C, CH-metalation at the heterocycle was preferred, as evidenced by the structural characterisation of a neutral borane-adduct of the metalation product of a modified NHP complex. The dimer of the Fe-centred radical formed also in reactions of chloro- and bromo-diazaphospholenes with Fe₂(CO)₉, which proceed not only by complexation of the P-donors as expected, but involve also oxidative addition steps and single electron transfer processes in which excess iron complex acts as the reductant. The title complex and the products isolated in the reaction studies were characterised by spectroscopic data and in many cases by XRD studies. Computational studies were employed to analyse the differing reactivity of the cationic NHP complex towards light and heavy halide ions, and to help in the assignment of the radical intermediate observed. The more diverse reactivity of the cationic NHP complexes compared to their neutral analogues is attributed to their higher electrophilicity.