Heteroscorpionate Magnesium Alkyls Bearing Unprecedented Apical σ‑C(sp³)–Mg Bonds: Heteroselective Ring-Opening Polymerization of <i>rac</i>-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 (<b>1</b>), Bn (<b>2</b>); NNN = tbpamd, R = CH₂SiMe₃, Et (<b>3</b>), Bn (<b>4</b>)). 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-<i>N</i>,<i>N</i>,<i>N</i>;κ²<i>-C</i>,<i>N</i>)­MgR­(thf)] (κ³-<i>N</i>,<i>N</i>,<i>N</i>;κ²<i>-C</i>,<i>N</i> = pbpamd^–, R = CH₂SiMe₃ (<b>5</b>), Et (<b>6</b>); κ³-<i>N</i>,<i>N</i>,<i>N</i>;κ²<i>-C</i>,<i>N</i> = tbpamd^–, R = CH₂SiMe₃ (<b>7</b>), Et (<b>8</b>)). 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-<i>N</i>,<i>N</i>,<i>N</i>;κ²<i>-C</i>,<i>N</i>)­MgR}₂{μ-<i>O</i>,<i>O</i>-(C₄H₈)}] (κ³-<i>N</i>,<i>N</i>,<i>N</i>;κ²<i>-C</i>,<i>N</i> = pbpamd^–, R = CH₂SiMe₃ (<b>9</b>), Et (<b>10</b>), Bn (<b>11</b>); κ³-<i>N</i>,<i>N</i>,<i>N</i>;κ²<i>-C</i>,<i>N</i> = tbpamd^–, R = CH₂SiMe₃ (<b>12</b>), Et (<b>13</b>), Bn (<b>14</b>)) was obtained. In both families, an apical methine C–H activation process on the heteroscorpionate takes place. The single-crystal X-ray structures of <b>4</b>, <b>8</b>, <b>9</b>, and <b>12</b> 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 <b>5</b> and <b>7</b>, as well as the tetranuclear tetraalkyls <b>9</b>, <b>10</b>, and <b>12</b>, 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 <i>rac</i>-LA propagations. More importantly, microstructural analysis of the poly­(<i>rac</i>-lactide) materials revealed that the tetranuclear tetra-alkyl <b>12</b> exerts enhanced levels of heteroselectivity on the PLAs under mild conditions, with <i>P</i>_s values up to 0.78