β-Diketiminate-Stabilized Magnesium(I) Dimers and Magnesium(II) Hydride Complexes: Synthesis, Characterization, Adduct Formation, and Reactivity Studies

Article abstract of DOI:10.1002/chem.200902425

The preparation and characterization of a series of magnesium(II) iodide complexes incorporating β-diketiminate ligands of varying steric bulk and denticity, namely, [(ArNC-Me)₂CH]- (Ar = phenyl, (PhNacnac), mesityl (MesNacnac), or 2,6-diisopropyl-phenyl (Dipp, DippNacnac)), [(DippNC-tBu) ₂CH]- (tBuNacnac), and [(DippNC-Me)(Me₂NCH ₂CH₂NCMe)CH]- (DmedaNacnac) are reported. The complexes [(PhNacnac)MgI(OEt₂)], [(MesNacnac)MgI(OEt₂)], [(DmedaNacnac)MgI(OEt₂)], [(MesNacnac)MgI-(thf)], [(DippNacnac)MgI(thf)], [(tBuNac-nac)MgI], and [(tBuNacnac)MgI-(DMAP)] (DMAP = 4-dimethylamino-pyridine) were shown to be monomeric by X-ray crystallography. In addition, the related β-diketiminato beryllium and calcium iodide complexes, [(MesNacnac)BeI] and [{(DippNacnac)-CaI(OEt₂)} ₂] were prepared and crystallographically characterized. The reductions of all metal(II) iodide complexes by using various reagents were attempted. In two cases these reactions led to the magnesium(I) dimers, [(MesNacnac)MgMg(MesNacnac)] and [(tBuNacnac)MgMg(tBuNacnac)]. The reduction of a 1:1 mixture of [(DippNacnac)MgI(OEt₂)] and [(MesNacnac)MgI(OEt ₂)] with potassium gave a low yield of the crystallo-graphically characterized complex [(DippNacnac)Mg(μ-H)(μ-I)Mg(MesNacnac)]. All attempts to form beryllium(I) or calcium(I) dimers by reductions of [(MesNacnac)BeI], [{(DippNacnac)CaI(OEt₂)}₂], or [{(tBuNacnac)CaI(thf)}₂] have so far been unsuccessful. The further reactivity of the magnesium(I) complexes [(MesNacnac)MgMg(MesNacnac)] and [(tBuNacnac)MgMg(tBuNacnac)] towards a variety of Lewis bases and unsaturated organic substrates was explored. These studies led to the complexes[(MesNacnac) Mg(L)Mg(L)(MesNacnac)](L = THF or DMAP), [(MesNacnac)Mg-(μ-AdN ₆Ad)Mg(MesNacnac)] (Ad = 1- adamantyl), [(tBuNacnac)Mg(μ-Ad-N ₆Ad)Mg(tBuNacnac)], and [(MesNacnac)Mg(μ-tBu₂N ₂C₂O₂)Mg- (MesNacnac)] and revealed that, in general, the reactivity of the magnesium(I) dimers is inversely proportional to their steric bulk. The preparation and characterization of [(tBuNacnac)Mg(μ- H)₂Mg(tBuNacnac)] has shown the compound to have different structural and physical properties to [(tBuNacnac)-MgMg(tBuNacnac)]. Treatment of the former with DMAP has given [(tBuNacnac)Mg(H)(DMAP)], the X-ray crystal structure of which disclosed it to be the first structurally authenticated terminal magnesium hydride complex. Although attempts to prepare [(MesNacnac)Mg(μ-H) ₂Mg(MesNacnac)] were not successful, a neutron diffraction study of the corresponding magnesium(I) complex, [(MesNacnac)Mg-Mg(MesNacnac)] confirmed that the compound is devoid of hydride ligands.