Magnesium, calcium and zinc [N2N′] heteroscorpionate complexes

Abstract

A new family of sterically demanding N₂N′ heteroscorpionate pro-ligands (HC(^tBu₂pz)₂SiMe₂N(H)R (R = ⁱPr, ^tBu, 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-^tBu and 3,5-Me substituted N₂N′ ligand families has been conducted. Attempted deprotonation of the pro-ligands with ⁿBuLi afforded the corresponding lithium salts Li{HC(^tBu₂pz)₂SiMe₂NR} (R = ⁱPr (1), ^tBu (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(^tBu₂pz)₂SiMe₂NR}Me (R = ⁱPr (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 MgⁿBu₂ to afford Mg{HC(R′₂pz)₂SiMe₂NR}ⁿBu (R′ = Me; R = ⁱPr (7), ^tBu (8), Ad (9), Ph (10). R′ = ^tBu; R = ⁱPr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R′₂pz)₂SiMe₂NR}{N(SiHMe₂)₂} (R′ = Me; R = ⁱPr (13), ^tBu (14), Ph (15). R′ = ^tBu; R = Ph (16)) or Mg{HC(R′₂pz)₂SiMe₂NR}{N(SiMe₃)₂} (R′ = Me; R = ⁱPr (17), ^tBu (18), Ph (19). R′ = ^tBu; R = Ph (20)), and Ca{HC(R′₂pz)₂SiMe₂NR}{N(SiMe₂)₂} (L) (R′ = Me; L = thf; R = ⁱPr (21), ^tBu (22), Ph (23). R′ = ^tBu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R′₂pz)₂SiMe₂NR}Me (R′ = Me; R = ⁱPr (25), ^tBu (26), Ph (27). R′ = ^tBu; R = Ph (28)) were prepared by reaction of the N₂N′ heteroscorpionate pro-ligands with ZnMe₂. 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.