The First Structural Characterisation of a Group 2 Metal Alkylperoxide Complex: Comments on the Cleavage of Dioxygen by Magnesium Alkyl Complexes

Abstract

A new high‐yield synthesis of [(PhCH₂)₂Mg(thf)₂] and [{(PhCH₂)CH₃Mg(thf)}₂] via benzylpotassium has allowed a simple entry into benzylmagnesium coordination chemistry. The syntheses and X‐ray crystal structures of both [(η²‐Me₂NCH₂CH₂NMe₂)Mg(CH₂Ph)₂] and [η²‐HC{C(CH₃)NAr′}₂Mg(CH₂Ph)(thf)] (Ar′=2,6‐diisopropylphenyl) are reported. The latter β‐diketiminate complex reacts with dioxygen to provide a 1:2 mixture of dimeric benzylperoxo and benzyloxo complexes. The benzylperoxo complex [{η²‐HC{C(CH₃)NAr′}₂Mg(μ‐η²:η¹‐OOCH₂Ph)}₂] is the first example of a structurally characterised Group 2 metal–alkylperoxo complex and contains the benzylperoxo ligands in an unusual μ‐η²:η¹‐coordination mode, linking the two five‐coordinate magnesium centres. The OO separation in the benzylperoxo ligands is 1.44(2) Å. Reaction of the benzylperoxo/benzyloxo complex mixture with further [η²‐HC{C(CH₃)NAr′}₂Mg(CH₂Ph)(thf)] results in complete conversion of the benzylperoxo species into the benzyloxo complex. This reaction, therefore, establishes the cleavage of dioxygen by this system as a two‐step process that involves initial oxygen insertion into the MgCH₂Ph bond followed by OO/MgC σ‐bond metathesis of the resulting benzylperoxo ligand with a second MgCH₂Ph bond. The formation of a 1:2 mixture of the benzylperoxo and benzyloxo species indicates that the rate of the insertion is faster than that of the metathesis, and this is shown to be consistent with a radical mechanism for the insertion process.