Magnesium aryloxides: synthesis, structure, solution behavior and magnesiate ion formation

Abstract

The heteroleptic magnesium complexes [{Ar′OMgBu}₂] 1 and [{Ar′OMgN(i-Pr)₂}₂] 2, where OAr′ = 2,6-di-tert-butylphenoxy, have been prepared and found to be dimeric in the solid state, with tri-coordinate metal centers. Complex 1 utilizes the aryloxide anions as bridging groups whereas the amido anions connect the metals in 2. Addition of THF or TMEDA to hydrocarbon solutions containing 2 results in disproportionation and the exclusive precipitation of the homoleptic, solvated, complexes [Mg(OAr′)₂·2THF] 3 or [Mg(OAr′)₂·TMEDA] 4. Both 3 and 4 are monomeric in the solid state with tetra-coordinate magnesium centers. Solution NMR spectroscopic studies of 1 and 2 reveal that disproportionation to the homoleptic complexes is promoted in THF-d₈ but that the main component still appears to be the heteroleptic species. Dissolution of the unsolvated dimeric complex [Mg(OAr′)₂] 5 in THF-d₈ results in partial formation of the magnesiate complex [Ar′OMg]⁺[(Ar′O)₃Mg]⁻10, along with the monomer 3. In contrast, no magnesiate is formed on dissolution of 3 in THF-d₈, indicating that magnesiate formation most likely proceeds via unsymmetrical cleavage of the dimer. Ab initio calculations (HF/6-31G*) have been used to investigate the possible structures of the magnesiate species.