Discrete Cationic Zinc and Magnesium Complexes for Dual Organic/Organometallic‐Catalyzed Ring‐Opening Polymerization of Trimethylene Carbonate

Abstract

We describe herein an original approach for the efficient immortal ring-opening polymerization (iROP) of trimethylene carbonate (TMC) under mild conditions using dual-catalyst systems combining a discrete cationic metal complex with a tertiary amine. A series of new zinc and magnesium cationic complexes of the type [{NNO}M](+) [anion](-) ({NNO}(-) = 2,4-di-tert-butyl-6-{[(2'-dimethylaminoethyl)methylamino]methyl}phenolate; M = Zn, [anion](-) = [B(C(6)F(5))(4)](-) (2), [H(2)N-{B(C(6)F(5))(3)}(2)](-) (3), and [EtB(C(6)F(5))(3)](-) (4); M = Mg, [anion](-) = [H(2)N{B(C(6)F(5))(3)}(2)](-) (7)) have been prepared from the corresponding neutral compounds [{NNO}ZnEt] (1) and [{NNO}-Mg(nBu)] (6). Compounds 2-4 and 7 exist as free ion pairs, as revealed by (1)H, (13)C, (19)F, and (11) B NMR spectroscopy in THF solution, and an X-ray crystallographic analysis of the bis(THF) adduct of compound 7, 7⋅(THF)(2). The neutral complexes 1 and 6, in combination with one equivalent or an excess of benzyl alcohol (BnOH), initiate the rapid iROP of TMC, in bulk or in toluene solution, at 45-60 °C (turnover frequency, TOF, up to 25-30,000 mol(TMC)⋅mol(Zn)⋅h(-1) for 1 and 220-240,000 mol(TMC)⋅mol(Mg)⋅h(-1) for 6), to afford H-PTMC-OBn with controlled macromolecular features. ROP reactions mediated by the cationic systems 2/BnOH and 7/BnOH proceeded much more slowly (TOF up to 500 and 3000 mol(TMC)⋅mol(Zn or Mg)⋅h(-1) at 110 °C) than those based on the parent neutral compounds 1/BnOH and 6/BnOH, respectively. Use of original dual organic/organometallic catalyst systems, obtained by adding 0.2-5 equiv of a tertiary amine such as NEt(3) to zinc cationic complexes [{NNO}Zn](+) [anion](-) (2-4), promoted high activities (TOF up to 18,300 mol(TMC)⋅mol(Zn)⋅h(-1) at 45 °C) giving H-PTMC-OBn with good control over the M(n) and M(w)/M(n) values. Variation of the nature of the anion in 2-4 did not significantly affect the performance of these catalyst systems. On the other hand, the dual magnesium-based catalyst system 7/NEt(3) proved to be poorly effective.

Computational Studies on the Selective Polymerization of Lactide Catalyzed by Bifunctional Yttrium NHC Catalyst

“…These organocatalysts often exhibit higher functional group tolerance [27] than metal-based complexes, and inherently work under milder conditions. Inspired by above features, bifunctional catalysts formed by combining metal and organic catalysis, which may result in a combination of their respective advantages, have been demonstrated to be a very powerful strategy in organic synthesis; these dual systems have attracted considerable interest in the last few years [28][29][30][31][32]. To exploit the potential of NHC-ZnR2 complexes, different approaches have been followed in which the synthesis and characterization of NHC-ZnR2 are carried out, and these complexes have then been used in ROP [33].…”

“…In addition, Guillaume and Carpentier used analogous discrete cationic zinc and magnesium complexes for dual organic/organometallic-catalyzed ROP of trimethylene carbonate. The zinc cationic compounds are highly active catalysts for the ROP of TMC in the presence of an exogenous alcohol and addition of a small amount of tertiary amine [32]. However, in this content, bifunctional rare earth metal catalysts had been rarely explored for polymer synthesis.…”

Synthesis and Single Crystal X‐ray Structures of Cationic Zinc β‐Diketiminate Complexes

“…Very recently, cationic zinc complexes containing both threefold and fourfold coordinated zinc atoms were successfully used in polymerization reactions. [16f, g, 19–23] Threefold coordinated cationic complexes of type VII are active in lactide polymerization and trimethylene carbonate polymerization 16g. In addition, the base‐stabilized complex VIII was found to polymerize ϵ‐caprolactone without the need of additional initiators such as alcohols 23…”

β‐Diketiminato–Alkaline Earth Cationic Complexes: Synthesis, Structures, Lactide Polymerization and Unusual Oxidative Reactivity of the Ancillary Ligand

“…[10b] It was only very recently that our group has described the syntheses of {BDI iPr }AeN(SiMe 2 H) 2 (THF) n (Ae = Ca, n = 1; Sr, n = 2; Ba, n = 2) compounds, which are a family of kinetically stable complexes that feature internal β-agostic Ae···H-Si interactions. Recent achievements [14] with discrete cations include the cyclohydroamination of aminoalkenes catalyzed by (aminotroponiminato)Zn complexes, [15] the synthesis of allylcalcium cations, [16] and the ROP of cyclic esters promoted by Zn-Mg [17] and Ca-Ba [18] cationic complexes bearing neutral (bis)phosphinimine [17a-17e] or monoanionic amino ether phenolate ancillary ligands. [13] By contrast, occurrences of discrete Zn, Mg and Ae cationic complexes remain rather scarce.…”