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

The preparation and characterization of a series of magnesium(II) iodide complexes incorporating β‐diketiminate ligands of varying steric bulk and denticity, namely, [(ArNCMe) 2 CH] − (Ar=phenyl, ( Ph Nacnac), mesityl ( Mes Nacnac), or 2,6‐diisopropylphenyl (Dipp, Dipp Nacnac)), [(DippNC t Bu) 2 CH] − ( t Bu Nacnac), and [(DippNCMe)(Me 2 NCH 2 CH 2 NCMe)CH] − ( Dmeda Nacnac) are reported. The complexes [( Ph Nacnac)MgI(OEt 2 )], [( Mes Nacnac)MgI(OEt 2 )], [( Dmeda Nacnac)MgI(OEt 2 )], [( Mes Nacnac)MgI(thf)], [( Dipp Nacnac)MgI(thf)], [( t Bu Nacnac)MgI], and [( t Bu Nacnac)MgI(DMAP)] (DMAP=4‐dimethylaminopyridine) were shown to be monomeric by X‐ray crystallography. In addition, the related β‐diketiminato beryllium and calcium iodide complexes, [( Mes Nacnac)BeI] and [{( Dipp Nacnac)CaI(OEt 2 )} 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, [( Mes Nacnac)MgMg( Mes Nacnac)] and [( t Bu Nacnac)MgMg( t Bu Nacnac)]. The reduction of a 1:1 mixture of [( Dipp Nacnac)MgI(OEt 2 )] and [( Mes Nacnac)MgI(OEt 2 )] with potassium gave a low yield of the crystallographically characterized complex [( Dipp Nacnac)Mg(μ‐H)(μ‐I)Mg( Mes Nacnac)]. All attempts to form beryllium(I) or calcium(I) dimers by reductions of [( Mes Nacnac)BeI], [{( Dipp Nacnac)CaI(OEt 2 )} 2 ], or [{( t Bu Nacnac)CaI(thf)} 2 ] have so far been unsuccessful. The further reactivity of the magnesium(I) complexes [( Mes Nacnac)MgMg( Mes Nacnac)] and [( t Bu Nacnac)MgMg( t Bu Nacnac)] towards a variety of Lewis bases and unsaturated organic substrates was explored. These studies led to the complexes [( Mes Nacnac)Mg(L)Mg(L)( Mes Nacnac)] (L=THF or DMAP), [( Mes Nacnac)Mg(μ‐AdN 6 Ad)Mg( Mes Nacnac)] (Ad=1‐adamantyl), [( t Bu Nacnac)Mg(μ‐AdN 6 Ad)Mg( t Bu Nacnac)], and [( Mes Nacnac)Mg(μ‐ t Bu 2 N 2 C 2 O 2 )Mg( Mes Nacnac)] and revealed that, in general, the reactivity of the magnesium(I) dimers is inversely proportional to their steric bulk. The preparation and characterization of [( t Bu Nacnac)Mg(μ‐H) 2 Mg( t Bu Nacnac)] has shown the compound to have different structural and physical properties to [( t Bu Nacnac)MgMg( t Bu Nacnac)]. Treatment of the former with DMAP has given [( t Bu Nacnac)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 [( Mes Nacnac)Mg(μ‐H) 2 Mg( Mes Nacnac)] were not successful, a neutron diffraction study of the corresponding magnesium(I) complex, [( Mes Nacnac)MgMg( Mes Nacnac)] confirmed that the compound is devoid of hydride ligands.