The first structural characterisation of a group 2 metal alkylperoxide complex: Comments on the cleavage of dioxygen by magnesium alkyl complexes

A new high-yield synthesis of [(PhCH2)(2)Mg(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 [(eta(2)-Me2NCH2CH2NMe2)Mg(CH2Ph)(2)] and [eta(2)-HC{C(CH3)NAr'}(2)Mg(CH2Ph)(thf)] (Ar' = 2,6-diisopropylphenyl) are reported. The latter beta-diketiminate complex reacts with dioxygen to provide a 1:2 mixture of dimeric benzylperoxo and benzyloxo complexes. The benzylperoxo complex [{eta(2)-HC{C(CH3)NAr' }(2)Mg(mu-eta(2):eta(1)-OOCH2Ph)}(2)] is the first example of a structurally characterised Group 2 metal-alkylperoxo complex and contains the benzylperoxo ligands in an unusual mu-eta(2):eta(1)-coordination mode, linking the two five-coordinate magnesium centres. The O-O separation in the benzylperoxo ligands is 1.44(2) Angstrom. Reaction of the benzylperoxo/benzyloxo complex mixture with further [eta(2)-HC{C(CH3)NAr'}(2)Mg(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 sigma-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.