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

The heteroleptic magnesium complexes [{Ar'OMgBu}(2)] 1 and [{Ar'OMgN(i-Pr)(2)}(2)] 2, where OAr' = 2,6-di-tertbutylphenoxy, 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-d(8) but that the main component still appears to be the heteroleptic species. Dissolution of the unsolvated dimeric complex [Mg(OAr')(2)] 5 in THF-d(8) results in partial formation of the magnesiate complex [Ar'OMg](+)[(Ar'O)(3)Mg](-)10, along with the monomer 3. In contrast, no magnesiate is formed on dissolution of 3 in THF-d(8), 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.