NHC Bis-Phenolate Aluminum Chelates: Synthesis, Structure, and Use in Lactide and Trimethylene Carbonate Polymerization

Abstract

A novel family of Al­(III) complexes supported by a tridentate, dianionic N-heterocyclic carbene bis-phenolate ligand ((OCO)^2–) was prepared via various synthetic routes, and the derived compounds were all structurally characterized. The methane elimination reaction of the protio ligand <i><i>N,N</i></i>′-bis­(2-hydroxy-3,5-di-<i>tert</i>-butylphenyl)-4,5-dihydroimidazolium chloride (<b>1</b>·H₃Cl) with AlMe₃ quantitatively led to the formation of the bis-phenolate imidazolinium Al zwitterion (<b>1</b>·H)­Al­(Me)­(Cl) (<b>2</b>), whose formulation was established by X-ray diffraction studies. The deprotonation of species <b>2</b> with 1 equiv of lithium diisopropylamide (LDA) proceeded with the elimination of LiCl to afford the Al-NHC methyl derivative [(OCO)­AlMe]₂ (<b>3</b>), which was isolated as a dimer, as confirmed by X-ray diffraction studies. Alternatively, compound <b>3</b> may be accessed via a salt metathesis route involving the reaction of the NHC bis-phenolate Li salt <b>1</b>·Li₂, generated in situ via reaction of <b>1</b>·H₃Cl with 3 equiv of ^<i>n</i>BuLi (−40 °C, THF), with 1 equiv of MeAlCl₂. The serendipitous hydrolysis of compound <b>3</b> allowed the X-ray characterization of the Al-oxo dinuclear species [(OCO)­Al-O-Al-(OCO)] (<b>3</b>′), in which both Al­(III) centers adopt a distorted-trigonal-monopyramidal geometry. The reaction of the salt <b>1</b>·H₃Cl with Al­(O<i>i</i>Pr)₃ afforded the corresponding bis-phenolate imidazolinium Al zwitterion (<b>1</b>·H)­Al­(O<i>i</i>Pr)­(Cl) (<b>4</b>), which incorporates a four-coordinate tetrahedral Al center effectively κ²<i>O,O</i>-chelated by the two phenolate moieties of the OCO^2– ligand. Compound <b>4</b> may be readily converted to the Al-NHC alkoxide derivative [(OCO)­AlO<i>i</i>Pr]₂ (<b>5</b>) upon reaction with 1 equiv of LDA. Alternatively, the alcoholysis of the Al-NHC methyl species <b>3</b> with <i>i</i>PrOH also permitted access to the derived Al alkoxide <b>5</b> and proceeds via the formation of the kinetic product (<b>1</b>·H)­Al­(O<i>i</i>Pr)­(Me) (<b>6</b>) that may readily eliminate methane upon heating to produce species <b>5</b>. The Al alkoxide species <b>5</b> was shown to efficiently polymerize <i>rac</i>-lactide and trimethylene carbonate in a highly controlled manner for the production of narrow disperse materials. The observed catalytic performances are in the range of the majority of those for group 13 metal based ROP catalysts developed thus far, and all data support the noninvolvement of the NHC moiety in these polymerization reactions