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 Et 2 O gives (dpp‐BIAN)MgI(Et 2 O) n , which then reacts in situ with (Me 3 Si) 2 NK to produce (dpp‐BIAN)Mg[N(SiMe 3 ) 2 ](Et 2 O) ( 3 ). Solvent‐free magnesium amide (dpp‐BIAN)Mg[N(SiMe 3 ) 2 ] ( 4 ) was synthesized by treating equimolar amounts of MgI 2 , dpp‐BIAN, and sodium in toluene at reflux followed by treatment with (Me 3 Si) 2 NK. Deprotonation of diphenylacetonitrile with [(dpp‐BIAN)Mg(μ‐Me)] 2 ( 5 ) produces keteniminate complex (dpp‐BIAN)Mg(N=C=CPh 2 )(Et 2 O) ( 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.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)