Magnesium, calcium and zinc [N 2 N'] heteroscorpionate complexes

Abstract

A new family of sterically demanding N 2 N' heteroscorpionate pro-ligands (HC( t Bu 2 pz) 2 SiMe 2 N(H)R (R = i Pr, t Bu, Ph, Xyl)) has been prepared via a straightforward modular synthetic route. An extensive study into the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes supported by both 3,5- t Bu and 3,5-Me substituted N 2 N' ligand families has been conducted. Attempted deprotonation of the pro-ligands with n BuLi afforded the corresponding lithium salts Li{HC( t Bu 2 pz) 2 SiMe 2 NR} (R = i Pr (1), t Bu (2), Ph (3) and Xyl (4)) but air- and thermal-sensitivity limited the yields of these potentially useful precursors; only the sterically encumbered ligand system allowed clean reactivity. Magnesium methyl complexes Mg{HC( t Bu 2 pz) 2 SiMe 2 NR}Me (R = i Pr (5) and R = Ph (6)) were prepared using an excess of the Grignard reagent MeMgCl. Magnesium butyl complexes were synthesised in good yields using the dialkyl precursor Mg n Bu 2 to afford Mg{HC(R' 2 pz) 2 SiMe 2 NR} n Bu (R' = Me; R = i Pr (7), t Bu (8), Ad (9), Ph (10). R' = t Bu; R = i Pr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R' 2 pz) 2 SiMe 2 NR}{N(SiHMe 2 ) 2 } (R' = Me; R = i Pr (13), t Bu (14), Ph (15). R' = t Bu; R = Ph (16)) or Mg{HC(R' 2 pz) 2 SiMe 2 NR}{N(SiMe 3 ) 2 } (R' = Me; R = i Pr (17), t Bu (18), Ph (19). R' = t Bu; R = Ph (20)), and Ca{HC(R' 2 pz) 2 SiMe 2 NR}{N(SiMe 2 ) 2 } (L) (R' = Me; L = thf; R = i Pr (21), t Bu (22), Ph (23). R' = t Bu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R' 2 pz) 2 SiMe 2 NR}Me (R' = Me; R = i Pr (25), t Bu (26), Ph (27). R' = t Bu; R = Ph (28)) were prepared by reaction of the N 2 N' heteroscorpionate pro-ligands with ZnMe 2 . In preliminary studies, magnesium amide complexes 16 and 20 were evaluated as initiators for the ring-opening polymerisation (ROP) of ϵ-caprolactone (ϵ-CL) and rac-lactide (rac-LA). Although the overall polymerisation control was poor, 16 and 20 were found to be active initiators.