Heteroscorpionate Rare‐Earth Metal Zwitterionic Complexes: Syntheses, Characterization, and Heteroselective Catalysis on the Ring‐Opening Polymerization of rac‐Lactide
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Heteroscorpionate Rare‐Earth Metal Zwitterionic Complexes: Syntheses, Characterization, and Heteroselective Catalysis on the Ring‐Opening Polymerization of rac‐Lactide
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<jats:title>Abstract</jats:title><jats:p>Novel neutral phosphine‐modified heteroscorpionate ligand (3,5‐Me<jats:sub>2</jats:sub>Pz)<jats:sub>2</jats:sub>CHPPh<jats:sub>2</jats:sub> (<jats:bold>1</jats:bold>) and its derivatives oxophosphine (<jats:bold>2</jats:bold>) and iminophosphine (<jats:bold>3</jats:bold>) heteroscorpionates were synthesized for the first time. These neutral heteroscorpionate ligands displayed unique chemistry towards rare‐earth metal tris(alkyl)s [Ln(CH<jats:sub>2</jats:sub>SiMe<jats:sub>3</jats:sub>)<jats:sub>3</jats:sub>(thf)<jats:sub>2</jats:sub>] (Ln=Y, Lu, Sc). The reaction between compound <jats:bold>1</jats:bold> and [Ln(CH<jats:sub>2</jats:sub>SiMe<jats:sub>3</jats:sub>)<jats:sub>3</jats:sub>(thf)<jats:sub>2</jats:sub>] afforded heteroscorpionate rare‐earth metal trialkyl adduct complexes <jats:bold>4 a</jats:bold>–<jats:bold>c</jats:bold>. Compounds <jats:bold>2</jats:bold> and <jats:bold>3</jats:bold> were treated with [Ln(CH<jats:sub>2</jats:sub>SiMe<jats:sub>3</jats:sub>)<jats:sub>3</jats:sub>(thf)<jats:sub>2</jats:sub>] to give the unprecedented zwitterionic heteroscorpionate rare‐earth metal dialkyls <jats:bold>5</jats:bold> and <jats:bold>6</jats:bold>, respectively. In the process, the heteroscorpionates transferred to the carbanions by means of methine CH bond cleavage that was attributed to the presence of the electron‐withdrawing groups. In addition the ligand and central metal showed a concerted effect on both the catalytic activity and specific selectivity of complexes <jats:bold>4</jats:bold>–<jats:bold>6</jats:bold> for the ring‐opening polymerization (ROP) of <jats:italic>rac</jats:italic>‐lactide (<jats:italic>rac</jats:italic>‐LA). All the adduct complexes <jats:bold>4</jats:bold> were nonselective and gave atactic polylactide (PLA), probably due to the dissociation of ligand <jats:bold>1</jats:bold> from the active metal center during the polymerization. Strikingly, zwitterionic complexes <jats:bold>5</jats:bold> catalyzed rapid ROP of <jats:italic>rac</jats:italic>‐LA to produce PLAs with heterotacticity up to 0.87. However, the zwitterionic complexes <jats:bold>6</jats:bold> were less active and less selective than <jats:bold>5</jats:bold>, which might be on account of the stronger coordination of the tetradentate ligand. Complexes <jats:bold>5</jats:bold> represent rare examples of the selective ROP of <jats:italic>rac</jats:italic>‐LA mediated by rare‐earth metal complexes supported by non‐bisphenolate ligands.</jats:p>