Chemistry of BDI*M(2+) complexes (M = Mg, Zn) and their role in lactidepolymerization where BDI* is the anion derived from methylenebis(C-tBu, N-2,6-diisopropylphenyl)imineBDI*H

Abstract

The bulky diimine CH₂{C(^tBu)N-2,6-Prⁱ₂C₆H₃}₂, BDI*H, is shown to exist in two isomers trans-trans, 1a and cis-trans, 1b that interconvert slowly on the NMR time-scale. Treatment of the diimine with ⁿBuLi in hexane proceeds slowly to give the lithium β-diketoiminate LiBDI*, 2, which upon hydrolysis yields the eneimine tautomer of BDI*H which has been characterized by X-ray studies in three rotamers, 3a, 3b and 3c. Dialkylmetal compounds (M = Mg, R = Bu; M = Zn, R = Et) react with either 1 or 3 in hydrocarbon and ether solvents to give BDI*Mg(ⁿBu)THF, 4, and BDI*ZnEt, 5, which have been structurally characterized. M(N(SiMe₃)₂)₂ compounds where M = Mg, Zn or Ca failed to react with either 1 or 3 in hydrocarbon solvents. The reactions between ZnCl₂ and 2 yield BDI*ZnCl, 6, and a further reaction with LiNMe₂ yields BDI*ZnNMe₂, 7. The compounds 5 and 6 are shown to contain trigonal planar, M(2+) ions. Compounds 4, and 7 initiate ring-opening polymerization, ROP, of rac-lactide to give atactic polylactide, PLA. The rate of ROP depends on the metal M = Mg > Zn and is slower than that observed for the related β-diketoiminate complexes where the bulky-^tBu group is replaced by Me.