Halogenated β‐diketiminato magnesium complexes: Preparation, characterization, and catalysis for ring‐opening polymerization of aliphatic lactones

The halogenated β‐diketiminato magnesium butyl complexes LXMgBu(THF) (X = Cl (1), Br (2)) and allyl complexes LXMg(allyl)(THF) (X = Cl (3), Br (4)) (LCl = ((2,6‐Cl2C6H3)N═C(Me)CH═C(Me)N(2,6‐Cl2C6H3)), LBr = ((2,6‐Br2C6H3)N═C(Me)CH═C(Me)N(2,6‐Br2C6H3)) were prepared and characterized. The treatment of butyl complex 2 with (S)‐1‐phenylethanol gave the corresponding alkoxide complex [LBrMgOCHMePh]2 (5), the dimeric structure of which was confirmed by single‐crystal x‐ray diffraction. When bulky diphenylmethanol was employed to react with Complex 2, THF‐ligated monomeric complex LBrMgOCHPh2(THF) (6) was afforded. The catalytic performances of these magnesium complexes for ring‐opening polymerization of ε‐caprolactone (ε‐CL), δ‐valerolactone (δ‐VL), and γ‐butyrolactone (γ‐BL) were evaluated. β‐Diketiminato ligands bearing alkyl groups (Me, Et, and iPr) on the N‐aryl moieties have been widely investigated in the organometallic chemistry and the ring‐opening polymerization (ROP) of cyclic esters, while halogenated β‐diketiminate ligands have been far less unexplored. In this contribution, a series of halogenated β‐diketiminato magnesium complexes with various initiating groups have been synthesized and characterized, among them a dimeric alkoxide complex was confirmed with single‐crystal x‐ray diffraction. These complexes could efficiently initiate the ROP of ε‐CL and δ‐VL at ambient temperature, where the alkoxide complexes (MgO bond) exert better control than the hydrocarbyl complexes (MgC bond). Moreover, these complexes could catalyze the ROP of nonstrained γ‐BL at −40 °C.