Heteroscorpionate Magnesium Alkyls Bearing Unprecedented Apical σ‑C(sp3)–Mg Bonds: Heteroselective Ring-Opening Polymerization of rac-Lactide

The previously described reaction of the low sterically hindered heteroscorpionate lithium acetamidinates [Li­(κ3-pbpamd)­(THF)] and [Li­(κ3-tbpamd)­(THF)] with a series of commercially available Grignard reagents RMgCl in an equimolecular ratio yielded the magnesium monoalkyls [Mg­(R)­(κ3-NNN)] (NNN = pbpamd, R = CH2SiMe3, Et (1), Bn (2); NNN = tbpamd, R = CH2SiMe3, Et (3), Bn (4)). However, subsequent reaction of these monoalkyls [Mg­(R)­(κ3-NNN)] with two additional equivalents of the same RMgCl in tetrahydrofuran gave rise to dinuclear dialkyls of the type [RMg­(κ3-N,N,N;κ2-C,N)­MgR­(thf)] (κ3-N,N,N;κ2-C,N = pbpamd–, R = CH2SiMe3 (5), Et (6); κ3-N,N,N;κ2-C,N = tbpamd–, R = CH2SiMe3 (7), Et (8)). Furthermore, when the reaction was carried out in a mixture of tetrahydrofuran/dioxane with the same stoichiometry, a new family of tetranuclear tetraalkyl magnesium complexes [{RMg­(κ3-N,N,N;κ2-C,N)­MgR}2{μ-O,O-(C4H8)}] (κ3-N,N,N;κ2-C,N = pbpamd–, R = CH2SiMe3 (9), Et (10), Bn (11); κ3-N,N,N;κ2-C,N = tbpamd–, R = CH2SiMe3 (12), Et (13), Bn (14)) was obtained. In both families, an apical methine C–H activation process on the heteroscorpionate takes place. The single-crystal X-ray structures of 4, 8, 9, and 12 confirm the nuclearity of each family, with 4-coordinative arrangements for all magnesium atoms. More importantly, the presence in the di- and tetranuclear complexes of unprecedented apical carbanions with a direct σ-C­(sp3)–Mg covalent bond, and as a result, the existence of stereogenic magnesium centers, have been unambiguously confirmed. Interestingly, the dinuclear dialkyls 5 and 7, as well as the tetranuclear tetraalkyls 9, 10, and 12, can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and lactides. Lactide (LA) polymerizations afforded polylactide (PLA) materials with medium molecular weights in only a few minutes even at 20 °C for l-LA and in a few hours at 50 °C for rac-LA propagations. More importantly, microstructural analysis of the poly­(rac-lactide) materials revealed that the tetranuclear tetra-alkyl 12 exerts enhanced levels of heteroselectivity on the PLAs under mild conditions, with Ps values up to 0.78.