Binuclear magnesium, calcium and zinc complexes based on bis(salicylaidimine) ligands with rigid bridges

Four dinucleating ligands based on sterically protected salicylaldimine units bridged by a variety of rigid groups have been prepared: 1,4-bis[(bis(3,5-di-tert-butylsalicylidene)amino]benzene (PARA-H(2)), 1,3-bis[3benzene (META-H(2)), 2,6-bispyridine (PUR-H(2)) and 1,4-bis[(3,5-di-tert-butylsalicylidene)amino]-2,3,5,6-tetramethylbenzene (Me(4)PARA-H(2)). Reaction with (Me(3)Si)(2)N](2)Mg center dot(thf)(2) gave three new heteroleptic dinuclear magnesium complexes: PARA-[MgN(SiMe(3))(2)center dot thf](2), META-[MgN(SiMe(3))(2)center dot thf](2) and PYR-[MgN(SiMe(3))(2)center dot thf](2). The complex PARA-[MgN(SiMe(3))(2)center dot thf](2) was characterized by X-ray diffraction. Reactions with [MgN(SiMe(3))(2)center dot thf](2) gave mixtures of homoleptic and heteroleptic species. The homolepic and heterolepic calcium complexes [PARA-Ca center dot(thf)(2)](2) and PARA-[CaN(SiMe(3))(2)center dot thf](2) could be isolated and were structurally charcterized. These xomplexes are not stable in solution. Attempted syntheses of Me(4)PARA-[CaN(SiMe(3))(2)center dot thf](2) gave insoluble coordination polymers of Me(4)PARA-Ca. Reactions with [(Me(3)Si(2))N](2)Zn gave precipitates of homoleptic dinuclear zinc complexes of which PYR-Zn could be structurally characterized as a cyclic tetramer. The dinuclear magnesium complexes PARA-[MgN(SiMe(3))(2)center dot thf](2), META-[MgN(SiMe(3))(2)center dot thf](2) and PYR-[MgN(SiMe(3))(2)center dot thf](2) are neither active in the polymerization of epoxides nor in the copolymerization of epoxide/CO(2) (cyclohexane or propylene oxide) but intitate the polymerization of rac-lactide. Yields of isolated polylactide strongly depend on the bridging unit and the solvent. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).