Magnesium and zinc complexes of a potentially tridentate β-diketiminate ligand

Abstract

The synthesis of the unsymmetrically substituted β-diketimine, 2-(2-methoxyphenylimino)-4-(2,6-diisopropylphenylamido)pent-2-ene, (BDI-2)H, is described and its complexation chemistry with magnesium and zinc is explored. Emphasis is placed on the preparation of alkoxide and amide derivatives for the ring-opening polymerisation of lactide; their behaviour as polymerisation initiators is compared to analogous compounds supported by the N,N′-bis(2,6-diisopropylphenyl) β-diketiminate ligand, BDI-1. (BDI-2)H reacts with Me₂Mg to give the bis(chelate) complex, (BDI-2)₂Mg, 3. Magnesium alkyls supported by BDI-2 may be prepared by increasing the size of the alkyl group. Hence, lithiation of (BDI-2)H affords [(BDI-2)Li]₂, 4; its subsequent treatment with ⁱPrMgCl produces (BDI-2)MgⁱPr, 5. Aminolysis of complex 5 using ⁱPr₂NH yields the amide complex, (BDI-2)MgNⁱPr₂, 6. Zn(NTMS₂)₂ and ZnEt₂ react with (BDI-2)H to give (BDI-2)Zn(NTMS₂), 7, and (BDI-2)ZnEt, 8, respectively. The former is converted into the siloxide complex,(BDI-2)Zn(OSiPh₃), 9, upon reaction with Ph₃SiOH. The chloride derivative, (BDI-2)ZnCl, 10, has also been prepared via the reaction of ZnCl₂ with 4. Crystallographic analysis of compounds 3, 4, 7 and 8 reveals that the potential for (BDI-2) to bind in a tridentate manner is only realised with the more electrophilic metals Li and Mg. Compared to their (BDI-1) counterparts, complexes 6, 7 and 9 are more active, but less well-controlled, initiators for the ring-opening polymerisation of rac-lactide, a consequence of the diminished steric protection afforded by (BDI-2) relative to (BDI-1).