Stickstoffverbindungen von elementen der dritten hauptgruppe mit intra- und intermolekularen donor-akzeptor-bindungen. IV. Polycyclische alumina-, lithia-, magnesia- und sodia-silazane

Abstract

The Lewis-acidic elements Li, Na, Mg, and Al have been successfully incorporated into cyclic structures, which have the Me2Si(NtBu)2 unit as common molecular fragment. X-ray structure analyses of [Me2Si(NtBu)2Mg·THF]2 (II) and of [Me2Si(N(tBu)AlMe2)(N(tBu)MgI)]2 (X) show the compounds to be dimeric. In the case of II, two four-membered SiN2Mg cycles interact via NMg bonds, to form a third N2Mg2 cycle with point symmetry Ci(1). The magnesium atom is four-coordinated with three different MgN bonds (199.3(4), 211.2(4), and 218.8(4) pm) and one MgO bond (204.9(6) pm) from the coordination by tetrahydrofuran. In X the dimer arises from IMg interaction, again forming a centrosymmetric Mg2I2 rectangle. The magnesium atom is further coordinated by two nitrogen atoms, with MgN bond lengths of 225.9(6) and 229.9(6) pm and a methyl group at a distance of 261(1) pm. The most remarkable feature is a N(Mg,Al,Si)N trigonal bipyramid, which originates from intramolecular two-centre and donor-acceptor bonds (AlN 187.7(6) and 188.6(6) pm, SiN 180.6(5), and 181.0(6) pm). The three neighbouring elements of the periodic table Mg, Al, and Si are thus bonded to a common nitrogen atom, which has a t-butyl group as an additional ligand. Derivatives of X have been found to exist as monomers, and the following molecules have been synthesized: Me2Si(N(tBu)AlMe2)(N(tBu)MgR) R = Me, Bz, tBu, Ph. A "precursor" to these compounds, the lithium derivative Me2Si(N(tBu)AlMe2)(N(t)Li), and the corresponding Na compound have been isolated. From 1H NMR spectroscopy a trigonal bipyramidal N(Li,Al,Si)N structure was again deduced as the skeleton of the molecule. © 1987.