Magnesium(II) Complexes of the dpp-BIAN Radical-Anion: Synthesis, Molecular Structure, and Catalytic Activity in Lactide Polymerization

Reaction of (dpp-BIAN)Mg(THF)(3) (1) {dpp-BIAN = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene} with one molar equivalent of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) proceeds with oxidation of the dpp-BIAN dianion in 1 to the radical-anion and affords the (dpp-BIAN)Mg(TEMPO)(thf) (2) complex. The reaction of dpp-BIAN with an excess amount of magnesium and 0.5 molar equivalents of I-2 in Et2O gives (dpp-BIAN)Mgl(Et2O)(n), which then reacts in situ with (Me3Si)(2)NK to produce (dpp-BIAN)Mg[N(SiMe3)(2)](Et2O) (3). Solvent-free magnesium amide (dpp-BIAN)Mg[N(SiMe3)(2)] (4) was synthesized by treating equimolar amounts of MgI2, dpp-BIAN, and sodium in toluene at reflux followed by treatment with (Me3Si)(2)NK. Deprotonation of diphenylacetonitrile with [(dpp-BIAN)Mg(mu-Me)](2) (5) produces keteniminate complex (dpp-BIAN)Mg(N=C=CPh2)(Et2O) (6). Complexes 2-4 and 6 were characterized by ESR and IR spectroscopy; their molecular structures were determined by single-crystal X-ray analysis. Compounds 1-3 catalyze the ring-opening polymerization of L- and rac-lactides to give high molecular weight isotactic and stereoblock polylactides correspondingly. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)