Discrete Cationic Complexes for Ring-Opening Polymerization

Review pubs.acs.org/CR

Discrete Cationic Complexes for Ring-Opening Polymerization Catalysis of Cyclic Esters and Epoxides Yann Sarazin* and Jean-François Carpentier* Institut des Sciences Chimiques de Rennes, Organometallics, Materials and Catalysis Laboratories, UMR 6226 CNRS, Université de Rennes 1, Campus de Beaulieu, F-35042 Rennes Cedex, France 4.1.1. Cations with or without a Reactive Nucleophilic Group 4.1.2. Living vs Immortal ROP of Cyclic Esters and Carbonates 4.2. Aluminum Cations in the ROP of Cyclic Esters 4.3. Zinc Cations in the ROP of Cyclic Esters/ Carbonates 4.4. Alkaline-Earth Cations in the ROP of Cyclic Esters/Carbonates 4.5. Rare-Earth and Actinide Cations 4.5.1. Cationic Complexes of Trivalent Rare Earths 4.5.2. Cationic Complexes of Divalent Rare Earths 4.5.3. Cationic Complexes of Actinides 4.6. Cations of Group 4 Metals 4.7. Cations of Late Transition Metals 4.8. Tin Cations 4.9. Concluding Comments on the ROP of Cyclic Esters and Related Monomers 5. Outlook Author Information Corresponding Authors Notes Biographies Acknowledgments References

CONTENTS 1. Introduction 1.1. Background 1.2. Scope 2. General Considerations 2.1. Foreword and Definitions 2.2. Synthetic Routes to Discrete Coordinatively Unsaturated Cations Used in ROP Catalysis 2.2.1. Salt Elimination Using Alkaline Salts 2.2.2. Halide Abstraction with a Lewis-Acidic Metal Halide 2.2.3. Metal−Halide Bond Dissociation upon Addition of a Chelating Ligand 2.2.4. Alkyl Abstraction with B(C6F5)3 2.2.5. Alkyl Abstraction with Fluorinated Borate or Aluminate Trityl Salts 2.2.6. Alkyl/Amido Elimination with Brønsted Acid Salts of Bulky Fluorinated Anions 2.2.7. One-Step Alkane/Amine Elimination with Acidified Proteo Ligands 2.3. On the Importance of the Associated Anion in Soluble Cationic Metal Complexes 3. Ring-Opening Polymerization of Epoxides 3.1. Mechanistic Considerations 3.2. Cationic Metal Catalysts for the ROP of Epoxides 3.2.1. Aluminum Cations 3.2.2. Zinc and Related Alkaline-Earth Cations 3.2.3. Cations of Metals Other Than Al/Zn/Mg 3.3. Concluding Comments on the ROP of Epoxides 4. Ring-Opening Polymerization of Lactides and Related Monomers 4.1. Structural, Mechanistic, and Analytical Considerations © 2015 American Chemical Society

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1. INTRODUCTION

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1.1. Background

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Interest in ring-opening polymerization (ROP) reactions has gained tremendous momentum in the course of the past two decades.1 While the polymerization of olefins for the production of both commodity and specialty polymers once attracted most of the attention both from industry and academia, the limited long-term availability of fossil resources together with the instability of their prices and growing awareness toward environmental issues has led many to realize that other products (monomers and polymers) need to be made accessible and attractive to the largest community. Although much older, the ROP of strained epoxides such as cyclohexene oxide (CHO) and propylene oxide (PO) for the production of aliphatic polyethers became the subject of intense scrutiny in the late 1980s. Poly(propylene oxide) (PPO) is in particular a

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DOI: 10.1021/acs.chemrev.5b00033 Chem. Rev. 2015, 115, 3564−3614

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Figure 1. Structural diversity and associated ROP mechanisms.

activator such as methylaluminoxane or a powerful molecular Lewis or Brønsted acid,8 researchers have sought to increase the catalytic performance of ROP catalytic systems by having recourse to discrete coordinatively unsaturated cationic complexes associated with weakly coordinating anions (WCA). The formula for such separated ion pairs is given by [{LXn}Metm(Nu)m−n−1]+·[WCA]− (Figure 1). The common reasoning is that the positive charge should result in increased electrophilicity of the metal center and hence higher affinity for the heteroatom-containing monomers, greater ability to activate the bound monomer toward nucleophilic attack from a reactive nucleophile, and overall enhanced performances during ROP catalysis which often proceeds according to so-called Lewisacid-mediated “activated monomer” processes but which can also sometimes obey a more traditional coordination−insertion mechanism. Note that only the electrophilic activation of the monomer is taken into consideration here, and this must be mitigated, as it depends on the nature of the rate-limiting step in these processes which may vary from one case to another (vide infra). For instance, the nucleophilic character of the initiating/propagating moiety may also come into play in the case of cationic metal centers bearing a reactive nucleophilic group (Type I, see section 4), and the overall influence of the metal electrophilicity will therefore depend on the prevailing factor. The first discrete, coordinatively unsaturated cations used in ROP catalysis date back to the mid-1990s; they mostly involved aluminum complexes for the ROP of epoxides. In the two decades that have since elapsed, the synthetic efforts have expanded to other metals, with the applications gradually shifting from the ROP of epoxides to that of cyclic esters.

main component for the preparation of polyurethanes, and its use as a bulk commodity material is widespread.2 As a result of the societal and environmental concerns already mentioned, the impetus has more recently switched from the simple ROP of epoxides to the ring-opening copolymerization with carbon dioxide, an attractive feedstock.3 Comparatively, thorough investigations in the polymerization of cyclic esters such as ε-caprolactone (CL), lactide (LA), and other related monomers, e.g., glycolide, β-butyrolactone, and trimethylene carbonate (TMC), were initiated later. Yet following the earliest studies, a succession of key contributions in the late 1990s sparked fantastic interest;4 the growing number of review articles5 and textbooks1,6 covering the development of metal-based catalytic systems for the ROP of cyclic esters is a testimony to the vitality of the field. Polylactides and related aliphatic polyesters and polycarbonates are biocompatible and biodegradable. They are used as bulk commodity materials with applications in the packaging and textile industries and for specialty applications in the biomedical (excipient for controlled drug delivery, surgical implants) and IT fields.6 Metal complexes have been at the forefront of catalyst development for the ROP of epoxides and that of cyclic esters and carbonates, even if organocatalysis has also emerged as an alternative.7 Initial efforts for homogeneous ROP catalysis chiefly focused on the design of charge-neutral discrete complexes of formulas {LXn}Met(Nu)m, where {LXn} is an ancillary ligand which can be mono-, di-, or trianionic (n = 1−3), Met is a main-group or transition metal with an oxidation number between +2 and +4, and Nu is a nucleophile, typically alkyl, amido, alkoxide, or halide (m = 1−3 typically) (Figure 1). The development of these metal-based catalytic systems has abundantly been reviewed in recent years, both for epoxide and for cyclic ester polymerizations.5,6 With these complexes, ROP catalysis most often follows a coordination−insertion mechanism, although the earliest systems agreed to a simpler anionic polymerization. Cases of cationic ring-opening polymerization of epoxides (sometimes referred to as “CROP”) using such catalysts have also long been known. Detailed accounts of stereoselective polymerizations already exist, as, for instance, may be the case for CHO, PO, and LA, and interested readers are referred to these many review articles.3,5 While the search for well-defined complexes enabling stereoselective ROP reactions remains one (if not the main) of the challenges in this area, charge-neutral ROP catalytic systems are often hampered by limited activity and productivity (respectively, measured by the turnover frequencies, TOF, and turnover numbers, TON). By analogy with the development of soluble group 4 precatalysts for olefin polymerization, for which it has long been revealed that the active species consist of cationic metal−alkyl complexes generated by treatment with an

1.2. Scope

The present review covers the literature between 1995 and 2014 relating to the development of well-defined cationic complexes of all metals across the Periodic Table used to promote the ring-opening (co)polymerizations of epoxides (CHO, PO) and cyclic esters (LA, CL) and carbonates (TMC) of main academic and industrial interest (Figure 2). Occurrences of metal cations for the polymerization of less usual heterocycles (e.g., oxazolines, δ-valerolactone) are also included for the sake of completion.

Figure 2. Main monomers covered in this literature survey. 3565


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In the majority of cases, a single reactive nucleophilic group is initially attached to the metal center, and only one polymer chain is eventually generated per metal center: this grossly corresponds to a “regular living” polymerization. Because in these reactions the metal complex intervenes at each stage (i.e., each monomer insertion) of the ROP process, it is a catalyst (or precatalyst if it is first modified prior to the first monomer insertion, e.g., as a result of alcoholysis of a Met−alkyl bond as often described in the literature5). In such cases, only a very small number of polymer chains (as low as 1) are produced per metal atom, whereas the terminology “catalyst” is sometimes unduly associated with the production of very large numbers of macromolecules for each metallic center. However, the term “catalyst” should not be related to the number of polymer chains produced, since, essentially, the catalytic action relates to the conversion of the substrate (here a monomer). The situation and nomenclature may differ for cationic complexes, since fundamentally different mechanisms are at work. In section 3 it can be seen that discrete cationic metal complexes act as Lewis acids that trigger the cationic ROP of epoxides. Although there is usually limited, if any at all, control over the number of polymer chains generated per metal atom, each catalytically active metal species usually generates a large number of macromolecules and beyond the first ring opening it retains its structure during catalysis. It will be referred to as a catalyst. Section 4 shows that the ROP of cyclic esters promoted by metal cations usually follows yet another scenario (Figure 3). With rare exceptions, the metal cation does not bear any reactive nucleophile required for the opening of the heterocyclic monomer, and therefore, it requires the mandatory addition of an exogenous initiating agent. In the “activated monomer” mechanism or in a so-called “immortal living” ringopening polymerization (iROP) with fast and reversible transfer between growing and dormant macromolecules, the number of polymer chains produced is preset by the number of added exogenous nucleophiles, which is most typically a protic agent, e.g., an alcohol. (For ease of reading, all of the following

Regarding the metal complexes of interest, the intention is to focus on well-defined ion pairs [{LXn}Metm(Nu)m−n−1]+· [WCA]− (m ≥ 1) where the coordinatively unsaturated cationic metal complex is paired with a weakly coordinating anion such as BPh4− or voluminous fluorinated anions such as the ubiquitous B(C6F5)4− and the likes: [B{3,5-(CF3)2-C6H3}4]−, [H2N{B(C6F5)3}2]−, and [Al{OCH(CF3)2}4]−. Relevant exceptions include complexes where anions such as [AlCl4]−, [GaCl4]−, [MeB(C6F5)3]−, Cl−, or [PF6]− are loose from the cationic metal atom. With the exception of pertinent examples, the survey does not include metal salts (e.g., triflates, carboxylates) and borohydrides in which the metallic cation is tightly bound to the anion: although these compounds do mediate very competently ROP reactions, they form a different class of compounds from those targeted here. The ambition is to summarize how the need for improved catalytic behavior (activity, control over the macromolecular features) has led to catalyst design of increasing originality, complexity, and efficacy.

2. GENERAL CONSIDERATIONS 2.1. Foreword and Definitions

Most of the existing ROP literature is rather ambiguous about terminology in the labeling of the role of the metal complexes. The terms “initiator”, “catalyst”, and “precatalyst” are often randomly employed either as synonyms or without clear purpose and consideration of what defines them. In the following, a distinction is made between these terms (Figure 3), and although it is not the purpose to impose these choices beyond the scope of this survey, it seems both simple and reasonable. In traditional “coordination−insertion” and anionic ROP catalysis involving charge-neutral complexes such as {LXn}Metm(Nu)m−n or Metm(Nu)m, the reactive nucleophile Nu which will open the first incoming monomer is covalently bound to the metal; its number is definite and normally ranges from one to the valence (m) of the metal at most. The initial metal complex sees its structure being modified to some extent during catalysis (at least one growing polymer chain replaces Nu and remains attached to the metal center at all times).

Figure 3. Distinguishing (pre)catalysts and initiators in ROP catalytic systems. 3566


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immortal living ROP reactions will be more simply called “immortal ROP” reactions, abbreviated as iROP. By opposition, regular living ring-opening polymerizations (without exogenous agent) will be simply called “living ROP”. This is also in agreement with common literature practice.) This nucleophile can be in much larger excess (dozens if not hundreds of equivalents per metal atom) with respect to the metal. The metal complex, which maintains its identity during the whole duration of the polymerization, behaves as a true catalyst generating a large number of polymer chains per metal center; the external nucleophile, often referred to as a (chain-)transfer agent in these polymerizations where it is found in excess, is the actual initiating group and therefore constitutes an initiator. For the very limited set of known cationic complexes which contain reactive nucleophiles, the addition of an external initiator is not required, and these compounds are therefore catalysts. In these cases where the metal complex first reacts with one or more of the components of the catalytic system prior to the first catalytic turnover, it is called a precatalyst, but this seldom happens with metal cations such as those reviewed here.

Scheme 1. Example of Salt Elimination Using Alkaline Tetraphenylborate Salt9

2.2.3. Metal−Halide Bond Dissociation upon Addition of a Chelating Ligand. In some cases, the addition of a polydentate chelating ligand to a metal−halide has yielded discrete solvent-free cations. Although conceptually simple, this method has been hampered by the limited choice of suitable metal−halide starting materials and by the fact that the anion in the resulting ion pair remains fairly nucleophilic. An example by Atwood and co-workers involves the reaction of AlMe2Br with PMDETA (N,N,N′,N″,N″-pentamethyldiethylenetriamine, Scheme 3).11 2.2.4. Alkyl Abstraction with B(C6F5)3. The method consisting in removing a methyl (or higher alkyl) group from a metal center by treatment with Massey’s ubiquitous perfluorinated tris(pentafluorophenyl)borane B(C6F5)312 is extremely efficient (Scheme 4). Abundantly utilized to produce (post)metallocenium in the early stages of homogeneous olefin polymerization catalyzed by group 4 metals8 it has naturally also been used to generate cationic species of all metals for ROP catalysis. In the case of methyl abstraction, the counterion is [MeB(C6F5)3]−, and this fairly coordinating anion usually remains close to the metal center, sometimes in fact generating zwitterion-like compounds (“inner sphere ion pairs”). An archetypical example in aluminum chemistry was Milione’s reaction of a dimethylaluminum complex supported by a monoanionic heteroscorpionate {bpzmp}− ligand, which leads to the [{bpzmp}AlMe]+ cation as [MeB(C6F5)3]− salt (6a, Scheme 4).13 2.2.5. Alkyl Abstraction with Fluorinated Borate or Aluminate Trityl Salts. This method generates cationic ROP metal catalysts by treatment of a metal−alkyl precursor with salts of the trityl cation [Ph3C]+ associated with a WCA, e.g., [B(C6F5)4]−, [B{3,5-(CF3)2C6H3}4]−, [H2N{B(C6F5)3}2]−, or less commonly [Al{OCH(CF3)2}4]−. It is again a procedure imported from the field of olefin polymerization catalysis where it has been used to generate (post)metallocenium cations very effectively. This is particularly obvious from Hayakawa’s early account of the polymerization of 7-membered cyclic ester (CL) and carbonate using [Cp2ZrMe]+·[B(C6F5)4]− (7; Cp = cyclopentadienyl) obtained by methyl abstraction upon treatment of Cp2ZrMe2 with [Ph3C]+·[B(C6F5)4]−.14 In the case of alkyl metal complexes possessing β-hydrogen atoms (e.g., isobutyl, ethyl), a β-H abstraction process typically occurs, generating the desired ion pair together with an alkene (isobutene, ethylene) byproduct. For instance, Milione’s heteroscorpionate {bpzmp}Al(iBu)2 reacts with [Ph3C]+·[B(C6F5)4]− to produce the [{bpzmp}Al(iBu)]+·[B(C6F5)4]− (6b) ion pair along with the byproduct Ph3CH and isobutene (Scheme 5, top).13 The more common methyl complexes, which do not possess any

2.2. Synthetic Routes to Discrete Coordinatively Unsaturated Cations Used in ROP Catalysis

Many synthetic procedures have been devised along the years in order to generate well-defined metal-containing ion pairs of the type [{LXn}Metm(Nu)m−n−1]+·[WCA]− used to promote ROP reactions. Even if some are so specific that they do not enter any family of synthetic protocols, the majority of them can be classified into one of the following seven groups of reactions. (1) Salt elimination using alkaline salts (2) Halide abstraction with a metal halide (3) Metal−halide bond dissociation upon addition of a chelating ligand (4) Alkyl abstraction with B(C6F5)3 (5) Alkyl abstraction with fluorinated borate or aluminate trityl salts (6) Alkyl/amido elimination with acid salts of fluorinated bulky anions (7) One-step alkyl/amido elimination with acidified proteo ligands The specifics of each synthesis will be mentioned in the relevant sections of this review, but the main features of these families of synthetic routes are presented and illustrated below. 2.2.1. Salt Elimination Using Alkaline Salts. This trivial method involves the reaction of a charge-neutral metal halide complex with an alkaline (most usually lithium or sodium) salt, such as, for example, LiBPh4, NaBPh4, or LiB(C6F5)4, and generates the desired cation along with a metal halide as byproduct. It is, for instance, the route that was first implemented by Atwood, one of the pioneers in this field, to synthesize discrete aluminum catalysts for the ROP of propylene oxide (Scheme 1).9 2.2.2. Halide Abstraction with a Lewis-Acidic Metal Halide. This method is far less common than the previous one. A Lewis acid like AlCl3 or GaCl3 is employed to remove a chloride from a charge-neutral starting material, thus generating a cationic complex associated with a counterion of the type [AlCl4]− or [GaCl4]− (Scheme 2). The halide atom abstracted from the initial complex is not truly eliminated but is now a component of the “ate” counterion. This was implemented by Atwood and co-workers to prepare five-coordinate, solvent-free salen-type aluminum complexes associated with [GaCl4]−.10 3567


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Scheme 2. Example of Halide Abstraction with a Lewis-Acidic Metal Halide10

[B(C6F5)4]−15 to give the resulting salts [(Et2O)3Met(X)]+· [B(C6F5)4]− (Met = Zn, X = Et, 8a; N(SiMe3)2, 8b; Met = Mg, X = nBu, 9a; N(SiMe3)2, 9b) after elimination of alkane and hexamethyldisilazane, respectively (Scheme 6).16 Besides, upon treatment with Bochmann’s acid [H(OEt2)2]+·[H2N{B(C6F5)3}2]− 17 the heteroleptic calcium complex {LO}Ca(N(SiMe3)2) (where {LO}− = 2-{[bis(2-methoxyethyl)amino]methyl}-4,6-di-tert-butylphenolate) supported by a stabilizing aminoether−phenolate ligand yielded [{LO}Ca]+·[H2N{B(C6F5)3}2]− (10).18 This last method, although effective, presents the drawback of first requiring the synthesis of the chargeneutral heteroleptic precursors {LXn}Metm(Nu)m−n; this can be circumvented by using a protic form of the ligand and a simple homoleptic metal precursor (vide infra). 2.2.7. One-Step Alkane/Amine Elimination with Acidified Proteo Ligands. The foregoing developments, with their synthetic advantages and drawbacks, culminated in a convenient one-step protocol for the synthesis of [{LXn}Metm(Nu)m−n−1]+·[WCA]− ion pair where the metal is stabilized by a bulky ligand, consisting in the reaction of Metm(Nu)m (Nu = alkyl, amido) with an acidified proteo ligand of the type [{LXn}H]+·[WCA]−. In this reaction, 1 equiv of alkane/amine (HX) is eliminated and the ligand {LXn}−, which is usually bi- or tridentate and contains N/O/P heteroatoms, binds to the newly produced metallic cation. This was first illustrated by Bochmann and co-workers, who showed that the reaction of ZnX2 (X = Me, Et, N(SiMe3)2) with [{DAD}H]+·[B(C6F5)4]−, where DAD is the bulky diazadiene (MeCNC6H3(iPr)2-2,6)2, afforded cleanly the zinc cation [{DAD}ZnX]+ as the [B(C6F5)4]− borate salt (11a−c, Scheme 7).19 In a further evolution, highly electrophilic complexes of divalent main-group metals [{LX}Met]+·[WCA]− devoid of a

Scheme 3. Example of Metal−Halide Bond Dissociation upon Addition of a Chelating Ligand11

Scheme 4. Example of Alkyl Abstraction with B(C6F5)3 in Aluminum Precursors13

β-hydrogen atom, neatly yield the cationic metal complex upon release of Ph3CMe (Scheme 5, bottom). 2.2.6. Alkyl/Amido Elimination with Brønsted Acid Salts of Bulky Fluorinated Anions. The reaction of heteroleptic metal complexes {LXn}Metm(Nu)m−n bearing an ancillary ligand or even in some cases the simple Metm(Nu)m (Nu = alkyl, N(SiMe3)2) with strong Brønsted acids such as [H(OEt2)2]+·[WCA]− or [HNMe2Ph]+·[WCA]− is a very clean and efficient way to generate the corresponding metallic salts [{LXn}Metm(Nu)m−n−1]+·[WCA]− and [Metm(Nu)m−1(D)x]+· [WCA]− (D = donor solvent, e.g., THF, Et2O), respectively. For instance, Bochmann and co-workers showed that Met(alk)2 and Met[N(SiMe3)2]2 reacted with Jutzi’s acid [H(OEt2)2]+·

Scheme 5. Examples of Alkyl Abstraction with the Borate Salt of the Trityl Cation13,14

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Scheme 6. Example of Alkyl/Amido Elimination with Brønsted Acid Salts of Fluorinated WCAs16,18

Scheme 7. One-Step Alkane/Amine Elimination with Acidified Proteo Ligands18−20

2.3. On the Importance of the Associated Anion in Soluble Cationic Metal Complexes

reactive nucleophilic group were accessed by treatment of Met(Nu)2 with the doubly acidic proteo ligands [{LX}HH]+· [WCA]−, where {LX}− can, for instance, be a multidentate aminoether−phenolate, as shown by Sarazin and Carpentier in their preparation of [{LONO4}Met]+·[H2N{B(C6F5)3}2]− (Met = Zn and Mg−Ba, 12−16; Scheme 7).18,20 The acidified proteo ligands required for these syntheses were conveniently obtained in excellent yields upon addition of a [H(OEt2)2]+ salt to the proteo ligand precursor.

The design of cationic metal complexes for ROP catalysis cannot be dissociated from the concept of weakly coordinating anions (WCAs). Indeed, regardless of the heterocyclic monomer under study, the efficiency of the catalytic system is connected to the ability of the metal center to bind and activate the monomer toward the nucleophilic attack leading to the opening of the heterocycle. Obviously, the structural features of 3569


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and binds to coordinatively unsaturated metal centers (vide supra), thereby reducing the affinity of the cationic metal for the incoming monomer. The latter [H2N{B(C6F5)3}2]− grandly facilitates crystallization processes, and it has led to the XRD structural identification of a number of main-group metal cations. This anion, the largest of those used in this context of ROP catalysis, has been shown to be virtually noncoordinating.25 It owes most of its chemical robustness to a pattern of stabilizing H···F hydrogen interactions between the NH2 hydrogen atoms and the many surrounding F atoms.

a coordinatively unsaturated cationic metal complex, the extent of its electrophilicity, and its ability to bind and activate heterocyclic monomers (which can be seen as transient coordinating coligands) depend intimately on the nature of its counterion. The synthetic, structural, and reactivity features of the main WCAs have already been reviewed in some detail.8,21 The choice of a WCA for polymerization catalysis is usually driven by a simple consideration: the least coordinating WCA produces the most reactive catalytic systems, because the electrophilicity of the metal ion is then at its highest. The ion pair may however be unstable, affecting the catalytic productivity. Whereas the earliest well-defined cationic metal complexes used in the context of ROP catalysis were paired to relatively nucleophilic halides, tetraphenylborate, or triflate anions, highly electrophilic complexes presenting enhanced reactivity and overall better ROP catalytic performances were next obtained upon combination with WCAs, notably [MeB(C6F5)3]− and [B(C6F5)4]−. This very much followed the trend already established in olefin polymerization catalysis, where cationic group 4 metal complexes associated with these borates yielded homogeneous catalysts of remarkable efficacy.8,22 A selection of the most representative ones of these anions is depicted in Figure 4; their common features include chemical

3. RING-OPENING POLYMERIZATION OF EPOXIDES The first cationic metal compounds used for ROP catalysis were reported for aluminum in the mid 1990s and dealt with the polymerization of cyclohexene oxide (CHO) and propylene oxide (PO). With a few exceptions, most of the well-defined cationic metal complexes used in this purpose are those of aluminum (mainly) and zinc. Interested readers should also take note of Atwood’s and Dagorne’s reviews which survey the synthesis and several applications of discrete cationic compounds of aluminum and related group 13 metals.26 3.1. Mechanistic Considerations

The polymerization of epoxides mediated by charge-neutral metal complexes typically proceeds via coordination−insertion or anionic mechanisms. It leads to the formation of polyethers with fairly well controlled macromolecular features and preponderantly regioregular “head-to-tail” enchainment of consecutive monomer units.27 By contrast, although the specifics of the associated mechanisms have not been investigated in detail, cationic metal complexes are thought to catalyze the polymerization of PO and CHO following a Lewis-acid-mediated cationic ring-opening mechanism. The main mechanistic features (Scheme 8) of these cationic ROP reactions are assumed to consecutively involve (i) the binding of the epoxide onto the electrophilic metal cation followed by (ii) opening of the epoxide, leading to the formation of an unstable carbocation that will be stabilized by surrounding monomer molecules, and (iii) chain propagation by iterative addition of an epoxide monomer unit to the growing polymer chain. In this scenario, the active cationic site is located at the end of the polymer chain opposite to the now charge-neutral metal atom; hence, this scenario is often alluded to as the “activated chain-end” (ACE) mechanism. Other than triggering the polymerization by binding the first monomer of each polymer chain and catalyzing its ring opening, the metal exerts little or no influence over the polymerization process. These Lewis-acidic polymerizations do not afford regioregular enchainment of monomers in the polymer chain; polymers that are characterized by high contents of “head-to-head” and “tail-to-tail” defect diads distributed randomly along the chain in addition to the “head-to-tail” diads expected for a well-controlled insertion process are produced28 (Scheme 8). No example of stereoselective metal-catalyzed cationic ROP has been reported to date, be it for CHO or PO. These cationic ROP of CHO can take place even at very low temperature (down to −78 °C in some cases16) owing to the very strained nature of this monomer. Also, these Lewis-acid-catalyzed reactions often yield polyethers with low molecular weights and large molecular weight distributions, with Mw/Mn values characteristically in the range 1.5−4.0; the polymers are usually accompanied by a large fraction of volatile materials. However, in some cases, the presence

Figure 4. Representative weakly coordinating anions used as counterion in the context of ROP catalysis mediated by cationic metal complexes.

and redox inertness and delocalization of the negative charge over as large a volume as possible in order to reduce their propensity to bind to the coordinatively unsaturated metal cations. While Schulz and co-workers employed Krossing’s perfluorinated aluminate [Al{OC(CF3)3}4]− 23 as a counterion to obtain their zinc cations,24 mostly three main non-nucleophilic fluorinated anions have been used in the context of ROP catalysis: the early [MeB(C6F5)3]− and [B(C6F5)4]− and Bochmann’s [H2N{B(C6F5)3}2]−. All are large anions with van der Waals volumes of 277, 349, and 538 Å3, respectively.22f Massey’s12 B(C6F5)3 is commercially available and can also be easily made, and starting from this borane, [Ph3C]+ and [H(OEt2)2]+ salts of [B(C6F5)4]− and [H2N{B(C6F5)3}2]− require limited synthetic efforts.15,17 The tetraarylborate [B(C6F5)4]−, which can be considered spherical, does not possess a permanent dipolar moment; this tends to make crystallization processes of salts of this anion rather tedious. By contrast, both [MeB(C6F5)3]− and [H2N{B(C6F5)3}2]− do have a permanent dipolar moment. The former actually features a distinct polarity 3570


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Scheme 8. Activated Chain-End ROP of Propylene Oxide (PO) Catalyzed by a Lewis Acid Metal Cation and Regiochemistry in the Lewis Acid Cationic Ring-Opening Polymerization of PO

Scheme 9. Synthesis of Seminal Salen-Type Aluminum Cations by Atwood and Co-Workers9

six-coordinate complexes paired with [BPh4]− and Cl− anions and stabilized by two molecules of methanol, as in [{SalenR,X)}Al(MeOH)2]+·[BPh4]− (R = X = H, 1a; R = H, X = Cl, 2a; R = Me, X = H, 3a), or two molecules of water, as in [{SalenR,X)}Al(H2O)2]+·[Cl]− (R = X = H, 1b; R = H, X = Cl, 2b; R = Me, X = H, 3b). Compounds 1a−3a were prepared by salt metathesis with NaBPh4 in methanol, whereas the H2O adducts 1b−3b were obtained upon displacement of chloride by dissolution of the appropriate starting materials in water (Scheme 9).

of unreacted monomer combined with a coordinating anion stabilizing the activated chain end has been shown to lead to high molecular weight, narrowly disperse PPO.29 These will be discussed in the following. 3.2. Cationic Metal Catalysts for the ROP of Epoxides

3.2.1. Aluminum Cations. The very first case of discrete aluminum cations used to catalyze the ROP of PO was reported in 1995 by Atwood and co-workers.9 They prepared salen-type 3571


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Scheme 10. Bulky, Soluble Salen−Aluminum Cations Reported by Atwood and Co-Workers30

The solubility of these six complexes in nonpolar organic solvents was low. While the H2O-bound cations 1b−3b were inert toward PO, the MeOH-bound 1a−3a catalyzed the polymerization of PO at room temperature in neat monomer in 12 h, affording low molecular weight materials (Mn = ca. 750− 1000 g·mol−1). Yields or conversions were not reported. In a continuation of their seminal work, Atwood and co-workers prepared Salen−aluminum cations of the type [{Salen(tBu)4}Al(base)2]+·[A]−, where the coordinated base was again MeOH or H2O and the anion was chosen from Cl− (17a−b), [BPh4]− (18a), or TsO− (19a) (Scheme 10).30 The presence of the tBu groups in the ortho and para positions of the aromatic ring guaranteed that these congested six-coordinate complexes were soluble in common organic solvents. As previously found, the MeOH adduct with a loose chloride counterion 17a catalyzed the oligomerization of neat PO to give oily materials (Mn = ca. 400 g·mol−1, Mw/Mn = 1.4−1.5). Its congeneric H2O adduct 17b and the neutral starting precursor did not show any reactivity. No data were provided for the complexes with less nucleophilic anions, 18a and 19a. The authors first postulated a mechanism where two PO molecules replace the bound MeOH in 1a−3a and related compounds to give the putative species [{SalenR,X)}Al(PO)2]+· [Cl]−.31 This was assumed to be followed by nucleophilic attack by Cl− onto one of the bound PO molecules to afford a charge-neutral aluminum−alkoxide species, followed by further insertions of PO units in the Al−oxygen bond. In this scenario, the growing polymer chain remains attached to the aluminum atom, and beyond the initial insertion it presents all the characteristics of a traditional coordination−insertion ROP mechanism. This hypothesis was however not demonstrated. Quite the contrary, the same group used the related sixcoordinate Salen [{Salen(tBu)4}Al(THF)2]+·[BPh4]− (20) and Salpen [{Salpen(tBu)4}Al(THF)2]+·[BPh4]− (21) salts (obtained by salt elimination with NaBPh4 in THF, Scheme 11) having two metal-bound THF molecules to show that the propagating species most likely involved a stabilized carbocation,29 with features corresponding to an ACE cationic mechanism. (The Salen or Salpen nomenclature used here is that taken from the literature and relates to these families of ligands that, respectively, contain 1,2-ethylenediamine or 1,3-propylenediamine in their backbone as the linker between two salicylaldehyde moieties.) Compounds 20 and 21, free of coordinated protic solvent, polymerized PO (neat or in CH2Cl2) to very high molecular weight PPO (Mn = ca. 400 000 and 180 000 g·mol−1, respectively) while maintaining fairly narrow molecular weight distributions (Mw/Mn = 1.3 and 1.2, respectively) typical of controlled

Scheme 11. Atwood’s Synthesis of THF-Stabilized Salen and Salpen Cationic Al Complexes29

ROP reactions. They assumed that these narrow dispersities resulted from stabilization of the carbocationic active site by unreacted monomer as well as contact with the anion. The molecular weights did not however grow linearly with monomer conversion, thus excluding a living mechanism. The geometry about the metal ions in 20 and 21 was expected to differ from one another, since the Salen and Salpen ligands that were used are known to, respectively, encourage square pyramidal and trigonal bipyramidal arrangements about the metal in fivecoordinated aluminum complexes. Yet, no clear relationship could be drawn between structural features and catalytic activity in these cations. Computational methods (MNDO/d PES technique) were implemented to assess the mechanism. They showed that coordination of PO onto a solvent-free metal cation [{Salen)}Al]+ was favorable (ΔH = −9.0 kcal·mol−1), and ring opening of the monomer from that point proceeded with an activation barrier of 26.7 kcal·mol−1, only partly compensated by the cleavage of the strained C−O bond (ca. 6.5 kcal·mol−1). These computations supported the existence of a cationic ROP mechanism, and the activation energy for each monomer insertion allegedly decreased as the propagating active species was more remote from the metal atom (Figure 5).29 The flexible and unusual Salen-H2 and Salpen-H2 proteo ligands {SalenN3H}H2 and {SalpenN3H}H2 incorporating a secondary amine in the ligand backbone were also employed successfully.32 The two soluble compounds [{SalenN3H}Al(THF)]+·[AlMe2Cl2]− (22) and [{SalpenN3H}Al(THF)]+· [AlMe2Cl2]− (23) containing six-coordinate aluminum cations, with coordinated secondary amine and hence only one additional bound THF molecule, were obtained in a one-pot process involving double protonolysis and halide redistribution (Scheme 12). These compounds yielded low molecular weight 3572


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Figure 5. Computed initial steps of the cationic ROP of propylene oxide with a Salen−Al putative cation.29

Scheme 12. Atwood’s Salen-Type Aluminum Cations with Secondary Amine in the Backbone32

PPO (Mn = ca. 700 g·mol−1, Mw/Mn = 1.5) under unspecified conditions. The structurally remarkable and solvent-free bimetallic dications in [{Salpen(tBu) 4 } 2 Al 2 ] 2+ ·2[GaCl 4 ]− (4) and [{Salophen(tBu)3}2Al2]2+·2[GaCl4]− (24), where each metal atom lies in a five-coordinate environment, were prepared by halide displacement from the neutral starting materials using GaCl3 (Scheme 13).10 The dimeric structure of these cations was established by XRD crystallography and 27Al NMR spectroscopy. The flexible nature of the propylene backbone in the Salpen derivative 4 allowed the formation of the Ophenolate-bridged bimetallic cation through minimization of steric repulsion between the bulky ortho tBu substituents. On the other hand, the Salophen (a specific Salen-type ligand with an o-phenyl linker) framework in 24 was much more rigid, which prevented dimerization in this straightforward fashion. Instead, the high Lewis acidity of the solvent-free cation induced an unusual Friedel−Crafts dealkylation process, with expulsion of the two ortho tBu groups (likely under the form of isobutene) which would otherwise prevent the formation of a dimer. Compounds 4 and 24 polymerized neat PO to low molecular weights PPO (Mn = 2000−3000 g·mol−1, Mw/Mn ≈ 1.6) in 12 h at room temperature (yields unspecified), but they were otherwise inactive at −78 °C.10

Original O,O-dianionic, tetradentate ligands successfully led to the preparation of five-coordinate aluminum cations using B(C6F5)3 as a methyl-abstracting reagent. The fluorinated dialkoxide−diimino [{O(CF3)2NEtNO(CF3)2}Al(OEt2)]+·[MeB(C6F5)3]− (25) was isolated in 76% yield by Dagorne and Carpentier (Scheme 14).33 The Al−Et2O adduct exhibits a cationic metal atom resting in a distorted trigonal bipyramidal environment (Cs symmetry) without contact with the counterion. Compound 25 quantitatively converted 200 equiv of PO (1 h at 20 °C, 6 h at 0 °C, and 18 h at −78 °C; reactions in CH2Cl2) but only produced very low molecular weight materials (Mn = 300−510 g·mol−1, Mw/Mn = 1.1−1.3) independently of the reaction conditions. Ishii reported in 2014 on the dinuclear dication [{OSSO(tBu)4}2Al2]2+ stabilized by two Okuda-type34 {OSSO}2− mixed soft−hard dithiabis(phenolate) ligands.35 Hence, [{OSSO(tBu)4}2Al2]2+·2[MeB(C6F5)3]− (26) was prepared in 51% yield by dealkylation of {OSSO(tBu)4}2AlMe (Scheme 14). It catalyzed the ROP of PO at 25 °C in CH2Cl2 (44% yield, unspecified [PO]0/[Al] ratio) to yield extremely narrow-disperse atactic PPO (Mn = 2500 g· mol−1, Mw/Mn = 1.04). Atwood and co-workers also synthesized several solvent-free five-coordinate cationic aluminum complexes for ROP purposes. The simplest ones, [{PMDETA}AlMe2]+·[Br]− (5a; via metal−halide bond dissociation) and [{PMDETA}3573


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Scheme 13. Atwood’s Solvent-Free, Salpen- and Salophen-Supported Aluminum Dications10

Scheme 14. Five-Coordinate Aluminum Cations Incorporating {O(CF3)2NEtNO(CF3)2}2− (25, Carpentier/Dagorne)33 and {OSSO}2− (26, Ishii)35 Dianionic Tetradentate Ligands

AlMe2]+·[Me2AlCl2]− (5b, a case of redistribution upon addition of a Lewis base), were isolated by adding the triamine PMDETA to AlMe2Br (generated in situ) or AlMe2Cl, respectively (Scheme 15).11 These salts containing identical cations highlighted the role of the anion in ROP catalysis: stirring neat PO with 5b at room temperature for 24 h afforded PO oligomers (Mn = 530 g·mol−1) as glassy solids, whereas 5a with its more nucleophilic anion only gave minute amounts of very low molecular weight oligomers. Bertrand and co-workers used related (diamine)amido frameworks to prepare the first chiral four-coordinate aluminum cations, which however could not be structurally characterized.36 The compounds [(RHNCH2CH2)NR′(CH2CH2NR)Al(Cl)]+·[AlCl4]− (R = R′ = SiMe3, 27; R = SiMe3, R′ = Me, 28; R = iPr, R′ = Me, 29) were obtained upon quaternization of the neutral chloro precursors

Scheme 15. Five-Coordinate PMDETA−Aluminum Cations by Atwood and Co-Workers.11

{(RHNCH2CH2)NR′(CH2CH2NR)}AlCl with HCl, followed by treatment with 1 equiv of AlCl3 (Scheme 16). These otherwise remarkable complexes proved to be poorly active catalysts for the ROP of neat PO, taking 48 h to convert 2−35% of 16 equiv of monomer at 30 °C or up to 58% at 80 °C. Although narrowly disperse (Mw/Mn = 1.2−1.3), the 3574


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[{2-(CH2N(C4H8NMe))-4-Me-6-CPh3-C6H2O}AlMe]+·[MeB(C6F5)3]− (31d) was obtained in identical fashion. Although none of these cations could be structurally characterized by diffraction methods, solution NMR data were consistent with binding of the two side arm heteroatoms onto the cationic metal, resulting in four-coordinate environments. The stability of these cations in CD2Cl2 solutions was found to decrease as the steric bulk in the ortho position increased: Ph (31a−b, no decomposition) ≫ tBu (31c, t1/2 = 16 h) > CPh3 (31d, t1/2 = 12 h). On the other hand, the combination of a morpholino tether with a sterically encumbering tBu or CPh3 group in ortho positions of the phenolate moiety led to the dinuclear monocations [{2(CH2N(C4H8O))-6-tBu-C6H3O}AlMe2·{2-(CH2N(C4H8O))-6tBu-C6H3O}AlMe]+ and [{2-(CH2N(C4H8O))-4-Me-6-CPh3C6H2O}AlMe2·{2-(CH2N(C4H8O))-4-Me-6-CPh3-C6H2O}AlMe]+ as the [MeB(C6F5)3]− salts (32a and 32b, respectively; Scheme 18); these formulations were proposed on the basis of NMR data.40 In the presence of 1 equiv of THF, the expected monometallic THF adducts [{2-(CH2N(C4H8O))-6-tBuC6H3O}AlMe(THF)]+·[MeB(C6F5)3]− (33a) and [{2-(CH2N(C4H8O))-4-Me-6-CPh3-C6H3O}AlMe(THF)]+·[MeB(C6F5)3]− (33b) were nonetheless generated with both ligands. Compounds 31a, 31b, 32b, and 33a polymerized PO (100 equiv, [PO]0 = 1.0 M) at room temperature in CH2Cl2, achieving conversions in the range 80−95% after 30 min. The resulting PPOs featured comparable properties: all were atactic, with typically low molecular weights and molecular weight distributions (Mn = 2500−4000 g·mol−1, Mw/Mn = 1.4−1.7). End-group analysis (NMR, MALDI-ToF MS) was not helpful, but the authors concluded that the polymerization proceeded according to a cationic mechanism. Dagorne and co-workers also employed P,O-bidentate phosphine−phenolate ligands mixing both hard and soft heteroatoms, a combination otherwise seldom exploited in the chemistry of electrophilic cations, to prepare a series of four-coordinate aluminum cations.41 Hence, the dissociated salts [{κ 2 -O,P-(2-PPh 2 -4-R′-6-R-C 6 H 2 O)}AlMe(THF)] + · [MeB(C6F5)3]− (R′ = H, R = Ph, 34a; R′ = Me, R = tBu, 34b) were obtained in good yields by methyl abstraction with B(C6F5)3 in the presence of THF (Scheme 19). The coordinating solvent was necessary to yield stable compounds as was the presence of a bulky ortho substituent (R = Ph, tBu); no clean compound could be obtained for the smaller R = Me. The cations [{κ2-O,P-(2-PPh2-4-R′-6-R-C6H2O)}2Al]+·[MeB(C6F5)3]− (R′ = H, R = Ph, 35a; R′ = Me, R = tBu, 35b) bearing two phosphine−phenolates were synthesized in a similar manner.

Scheme 16. Bertrand’s N,N′,N″-Supported Chiral FourCoordinate Aluminum Cations36

resulting polymers were of low molecular weights (Mn = 600− 2200 g·mol−1). Dagorne and co-workers showed that aluminum complexes supported by monoanionic O,N-bidentate amino−phenolates constitute potent ROP catalysts.37,38 They synthesized the dinuclear cationic species [{2-(CH2L)-6-R-C6H3O}AlMe2· {2-(CH2L)-6-R-C6H3O}AlMe]+ (R = Ph, L = NMe2, 30a; R = tBu, L = NMe2, 30b; L = NC4H8, 30c; L = NC5H10, 30d) as [MeB(C6F5)3]− salts by methyl abstraction with B(C6F5)3 from the corresponding neutral precursors {2-(CH2L)-6-RC6H3O}AlMe2 (Scheme 17). These cations contain two elements of chirality, as established by XRD studies and solution NMR spectroscopy: one resulted from the stereogenic tetrahedral cationic Al center and the other from the configurationally frozen metallacycle incorporating the AlMe2 fragment. As a consequence, 30a−d formed as a pair of diastereoisomers in varying proportions. While the cation in 30a bearing a Ph group in the ortho position proved inefficient, compounds 30b−d catalyzed very rapidly the ROP of PO under mild conditions (15 min at room temperature in toluene), converting 50−60% of 200 equiv of monomer per dinuclear monocation to atactic PPO. All polymers prepared at this temperature had the same macromolecular features (Mn = 2500−3000 g·mol−1, Mw/Mn = 1.5−1.6) regardless of the identity of the catalyst, but a medium molecular weight PPO was obtained with 30b in 1 h at 0 °C (yield = 70%, Mn = 9000 g·mol−1, Mw/Mn = 1.7).37a The three-coordinate derivatives [{2-(CH2NMe2)-4-Me-6-CPh3-C6H2O}AlMe]+· [WCA]− ([WCA]− = [MeB(C6F5)3]− or [B(C6F5)4]−) were also prepared, but these mononuclear compounds (as a result of steric congestion) were only used to mediate the ROP of cyclic esters (vide infra).39 In a subsequent effort,40 the same group reported that the reaction of 1 equiv of B(C6F5)3 with the neutral {2-(CH2N(C4H8L))-6-R-C6H3O}AlMe2 containing a tridentate monoanionic phenolate ligand generated [{2-(CH2N(C4H8L))-6-RC6H3O}AlMe]+·[MeB(C6F5)3]− (R = Ph, L = O, 31a; L = NMe, 31b; R = tBu, L = NMe, 31c) (Scheme 18). The congested

Scheme 17. Dagorne’s Dinuclear Aluminum Monocations Bearing Amino−Phenolate Ligands37a

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Scheme 18. Use of Potentially Tridentate Amino−Phenolate Ligands for the Synthesis of Cationic Aluminum Species by Dagorne and Co-Workers40

Mn = 7400 g·mol−1, Mw/Mn = 1.7. Compounds 35a and 35b bearing each two ancillary O,P-bidentate ligands proved less active but more robust than 34a and 34b, converting, respectively, 87% and 92% of the monomer (100 equiv, [PO]0 = 1.0 M in CH2Cl2) after 12 h, to give medium molecular weight, monodisperse atactic PPOs (Mn = 7800 g·mol−1, Mw/Mn = 1.7 and Mn = 8700 g·mol−1, Mw/Mn = 1.6, respectively).41 Some other isolated examples of well-defined aluminum cations tested for the ROP of epoxides have been reported, but investigations with these were not pursued in detail (Scheme 20). Jordan and co-workers mentioned that the aminotroponiminate-containing [{ATI iPr2 }Al(iBu)] + ·[B(C6F5)4]− (36), prepared by alkyl abstraction with the trityl borate salt (Scheme 20), converted a 500-fold excess of PO to atactic PPO with a good activity (TOF = 240 molPO· molAl−1·h−1).42 The formally three-coordinate metal atom in this complex is extremely electrophilic; the molecular structure of its congener [{ATIiPr2}AlEt (ClPh)]+·[B(C6F5)4]−, recrystallized from chlorobenzene, very unusually showed Al···F contacts with the [B(C6F5)4]− neighboring anion together with a Al−ClPh dative bond to the cocrystallized solvent molecule.42 Reed, Sen, and collaborators synthesized a unique “ligandfree” cation [Et2Al]+ as the halocarborane salts [AlEt2]+· [CB11H6X6]− (X = Cl, 37a; Br, 37b).43 The molecular structure of 37b showed the presence of bidentate interactions (two sets of Al···Cl contacts) between the anion (otherwise known to be virtually noncoordinating) and the highly electron-deficient metal atom (Scheme 20). Derivative 37a was said to be an extremely active catalyst for the quantitative, fast, and exothermic ROP of CHO carried out in chlorobenzene at room temperature (Mn = 7600 g·mol−1, Mw/Mn = 1.5).

Scheme 19. Dagorne’s Phosphine−Phenolates FourCoordinate Aluminum Cations41

The polymerization of PO (100 equiv, CH2Cl2, room temperature) catalyzed by 34a,b proceeded much along the same lines as those described for the aforementioned 31−33.40 This raised the question of the role of the ligand framework in these cationic ROP catalyzed by Lewis acids. The conversion reached 52−61% in 15 min, and the resulting atactic PPO again featured low molecular weights and monomodal distributions diagnostic of these systems (Mn = 3100−3500 g·mol−1, Mw/Mn = 1.4−1.7).41 Conversion was not improved with longer reaction times, suggesting fast deactivation of the catalyst. The situation somewhat improved by carrying the reaction at 0 °C under otherwise identical conditions, for instance, with 34b: yield = 79%, 3576


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copolymerizations, the authors were able to show that the affinity of aluminum for PO was much greater with the TTP ligand than for the Salen one; this was in line with the greater donating ability of Salen compared to that of TTP. 3.2.2. Zinc and Related Alkaline-Earth Cations. The synthesis of well-defined zinc−alkyl cations was achieved early in the 1990s, notably following the addition of heteroatomcontaining macrocycles (e.g., crown ethers or cryptands) to ZnR2 in the presence of AlR3 or tetraphenylcyclopentadiene or after protonolysis of ZnEt2 with [Bz3TACH]+·[PF6]− (Bz3TAC = 1,3,5-Bn3-1,3,5-triazacyclohexane).46 However, it is not until 200219 that, following their initial efforts for the fabrication of stable discrete zinc−alkyl cations paired with WCAs such as [(Et2O)3ZnR]+·[B(C6F5)4]−,47 Bochmann and co-workers successfully prepared a series of DAD-supported zinc cations (DAD is the diazadiene (MeCNC6H3(iPr)2-2,6)2) that could be used as catalysts for the ROP of epoxides. The compounds [{DAD}ZnX]+·[B(C6F5)4]− (X = Me, Et, or N(SiMe3)2, 11a−c) and [{DAD}ZnMe]+·[MeB(C6F5)3]− (11d), the first cases of zinc cations in a three-coordinate environment in solution as well as in the solid state,48 were obtained in good to excellent yields after standard alkyl/amido abstraction procedures (Scheme 21).19,49 Compound 11a proved an excellent catalyst for the ROP of CHO and PO.19 The polymerization of CHO ([CHO]0/[Zn] = 10 000:1) was extremely exothermic and had to take place at 0 °C in diluted toluene solutions to be controlled. The molecular weights of PCHOs and PPOs were high (Mw = 53 000−177 000 g·mol−1), and they increased regularly with time over the whole duration of the reaction (5 min, conversion up to 54% with corresponding TOFs around 3000 to 9000 kgpol·molZn−1·h−1); however the resulting PCHOs exhibited large polydispersities (Mw/Mn = 1.5−4.1). The polymerization of neat PO ([PO]0/[Zn] = 10 000:1) was slower and took place at 23 °C. Conversion reached up to 73%; it increased with time over 72 h, albeit not linearly. Medium to high molecular weight PPOs with broad molecular weight

Scheme 20. Highly Electrophilic Three-Coordinate (36, Jordan)42 and Formally Two-Coordinate (37a,b, Reed/Sen)43 Aluminum Cations for ROP of Epoxides

Saunders Baugh and co-workers tried to implement in-situ combinations of four-coordinate {LX}AlMe2 neutral complexes supported by N,N (e.g., amidinates) or N,O (e.g., oxypyridines, salicylaldimines) bidentate monoanionic ligands {LX}− with B(C6F5)3 for the polymerization of PO; some moderate catalytic activity was indeed observed for these systems, but it could not be unambiguously distinguished from the baseline activity of the borane alone.44 To conclude this section on aluminum cations, one should take note of the work of Chen and Chisholm, who studied the binding of PO to a family of in-situ-generated [{LXn}Al]+ model cations (as well as their Cr(III) and Co(III) analogues) in the gas phase by electrospray tandem mass spectrometry. The dianionic ligand {LXn} was either tetraphenylporphyrin (TTP) or a chiral hindered salen with a cyclohexylene backbone linker (Figure 6).45 Although of limited relevance in the context of the present survey since these studies pertained to overall charge-neutral {LXn}−aluminum chlorides for PO/CO2

Figure 6. Binding of propylene oxide to porphyrin and salen aluminum cations.45 3577


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Scheme 21. Synthesis of the Discrete DAD-Supported Zinc Cations 11a−d with an ORTEP Rendering of the Three-Coordinate Cation in 11a19,49

Scheme 22. Reactions of Zinc Complexes Containing η2-Coordinated Substituted Cp with B(C6F5)350

distributions (Mn = 29 000−38 000 g·mol−1, Mw/Mn = 1.9− 6.4) were obtained, but these features could not be controlled with reaction time and monomer conversion. These observations pointed at an ACE cationic ROP mechanism. The same research group also reported a family of original cyclopentadienyl-supported cationic zinc ROP catalysts, although in this case the addition of TMEDA was required to yield identifiable compounds.50 Hence, the reaction of {CpMes}ZnEt (where {CpMes} = 3,5-dimethylbenzyl-substituted cyclopentadienyl is η2-coordinated to zinc) with B(C6F5)3 yielded {CpMes}Zn(C6F5) and a mixture of B(Et)x(C6F5)3−x species (x = 1−3) as a result of ligand scrambling.51 Yet, treatment of the TMEDA adduct {CpMes}ZnEt(TMEDA) with B(C6F5)3 on an NMR scale in toluene-d8 afforded the ion pair [{CpMes}Zn(TMEDA)]+·[EtB(C6F5)3]− (38) after clean ethyl abstraction (Scheme 22). The analogous reaction starting from {CpPyr}ZnEt(TMEDA) was not controlled ({CpPyr}H = cycloC4H4NSiMe2C5H5) and proceeded with β-H, ethyl, and even cyclopentadienyl abstraction to give a complicated mixture of ionic compounds. The in-situ-generated 38 was inactive toward the polymerization of PO, but it catalyzed competently the ROP of CHO ([CHO]0/[Zn] = 2000:1, quantitative) to high molecular weight PCHO (Mw = 102 000 g·mol−1, Mw/Mn = 1.6)

within 5 min at 0 °C. Under the same conditions, the ill-defined mixture resulting from the system {CpPyr}ZnEt(TMEDA)/ B(C6F5)3 produced a much lower molecular weight PCHO with a very broad polydispersity (Mw = 60 000 g·mol−1, Mw/Mn = 6.1).50 It was subsequently shown that the incorporation of a bulky ancillary ligand, as in 11a−d or 38, was not compulsory to generate stable zinc, and even magnesium, cations that could catalyze ROP reactions. Having originally synthesized the cationic zinc−alkyl [(Et2O)3ZnEt]+·[B(C6F5)4]− (8a) and its ZnMe analogue by treatment of ZnR2 (R = Me, Et) with Jutzi’s acid [H(OEt2)2]+·[B(C6F5)4]− in Et2O,47 Bochmann and coworkers reported on the synthesis of the zinc [(Et2O)3Zn(N(SiMe3)2)]+·[B(C6F5)4]− (8b) and magnesium [(Et2O)3Mg(R)]+·[B(C6F5)4]− (R = nBu, 9a; N(SiMe3)2, 9b) cations produced in identical fashion (see Scheme 6) and which polymerized CHO and PO (and even ε-caprolactone).16 Compound 9b represented a unique case of structurally characterized four-coordinate magnesium cation (Figure 7), and with 9a it constituted a valuable addition to the set of available well-defined Mg cations as part of a stable separated ion pair.52 The zinc and magnesium compounds 8a,b and 9a,b polymerized CHO (5000 equiv vs the metal) diluted in 40 volumes of toluene at 0 °C. Under identical experimental conditions the 3578


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Figure 8. Neutral zinc ROP precatalysts stabilized by nitrogencontaining ancillary ligands.53 Figure 7. ORTEP rendering of the cation [(Et2O)3Mg(N(SiMe3)2)]+ in the molecular structure of 9b.16

related adduct precursor (41) (Figure 8).53 No cationic complex could be isolated and structurally authenticated, but once activated in situ by B(C6F5)3 in toluene, 39a−d polymerized CHO ([CHO]0/[Zn] = 1000:1) with good activities (TOF up to 60 000 molCHO·molZn−1·h−1) to high molecular weight polymers (Mn up to 37 000 g·mol−1) but with the customary broad polydispersities (Mw/Mn = 2.9−8.2). These binary systems also catalyzed the ROP of propylene oxide ([PO]0/[Zn] = 1000:1) in toluene at 30 °C; however, the isolated yields were only in the region 7−25% after 30 min, and the polymers presented decent molecular weights but very broadly distributed (Mw = 16 000−20 000 g·mol−1, Mw/Mn = 3.2−8.7). For the bis(oxazoline) compounds, the catalytic efficiency varied systematically according to 39c > 39b > 39a ≈ 39d; the presence of chiral centers in these cations bore no impact on stereoselectivity, and no mention was made of the tacticity of the resulting polyethers. The catalytic behaviors of 40 and 41 toward CHO and PO were grossly comparable with those of the bis(oxazoline) compounds; overall, no clear influence of the ligand framework other than stabilization against ligand redistribution could be discerned. 3.2.3. Cations of Metals Other Than Al/Zn/Mg. Only a handful of cases of well-defined metal cations built around metals other than aluminum or zinc and the very comparable magnesium are known to catalyze the ROP of epoxides (Figure 9). The dinuclear zwitterionic titanoxane [{Cp}{η5C5H4B(C6F5)3}Ti]2O (42) (which does not fully qualify as a separated ion pair) catalyzed the exothermic ROP of PO in temperature ranges between −30 and +20 °C, but it only afforded liquid oligomers.54 The XRD characterized [{Cp}2Zr(OiPr)(HOiPr)·(Et2O)]+·[H2N{B(C6F5)3}2]− (43)55 catalyzed the polymerization of neat PO at 20 °C ([PO]0/[Zr] = 3400:1; conversion in the range 30−70% within 15−90 min; TOF = 24−69 molPO·molzr−1·h−1) to afford low molecular weight polymers: Mw = 2200−4000 g·mol−1, Mw/Mn = 2.0−2.4. 1H and 13 C NMR data, coupled with the detection of low molecular weight oligomers by GPC, were indicative of a cationic ACE mechanism. Compound 43 also mediated the ROP of CHO in 25 volumes of toluene, affording quantitative conversion of 470 equiv of monomer within 5 min (Mw = 31 000 g·mol−1, Mw/Mn = 3.2). Two cationic gallium(III) compounds have been shown to polymerize epoxides. Wehmschulte and co-workers prepared [{2,6-Mes2-C6H3}2Ga]+·[Li{Al[OCH(CF3)2]4}2]− (44; Mes = 2,4,6-Me3C6H2) by salt elimination upon treatment of {2,6Mes2-C6H3}2GaCl with 2 equiv of Li{Al[OCH(CF3)2]4}2; the

catalytic productivity decreased substantially according to 8a (yield = 64%) > 8b (27%) > 9a (13%) > 9b (4%) after 30 min, that is, zinc yielded more efficient catalytic systems than magnesium, and the alkyl-substituted cations were more efficient than their amido derivatives. Since these polymerizations likely proceeded via a cationic ACE mechanism, these observations were thought to reflect the stability rather than the intrinsic catalytic ability of the compounds themselves. Reasonably high molecular weight PCHOs were obtained (Mw = 80 000−104 000 g·mol−1) with molecular weight distributions in the range 2.1−3.0. The activity and macromolecular weight features could not be simply extrapolated in obvious fashion to the properties (Lewis acidity, monomer affinity) and composition of these cations. The most efficient of these, the organozinc 8a, afforded very high molecular weight PCHO (Mw up to 380 000 g·mol−1, Mw/Mn = 2.2−3.8) during polymerizations carried out at low to very low temperatures (from 0 to −78 °C). The efficiency of this complex (up to 58 800 kgCHO·molZn−1·h−1 was achieved, i.e., complete conversion of 5000 equiv of monomer in 5 volumes of toluene within 30 s) outclassed any of those reported before for the ROP of CHO. The polymerization of neat PO (20 000 equiv per metal atom) at room temperature followed essentially the same lines (albeit with much lower productivity), that is, 8a (yield = 58% after 24 h) > 8b (11%) > 9a (9%) > 9b (1%). Typically for such cations, low molecular weight polymers with large molecular weight distributions were obtained (Mw = 1000−3000 g·mol−1, Mw/Mn = 1.8−3.6). Interestingly, the authors provided evidence for the occurrence of a poorly controlled cationic ACE mechanism, and this is despite the presence of reactive groups (Et, N(SiMe3)2) which could potentially have acted as reactive nucleophile toward these epoxides in a coordination−insertion ROP pathway. Hence, analysis of the regiochemistry by NMR spectroscopy (about equal contents of H−T, H−H, and T−T diads, see Scheme 8) and GC-MS analysis showed that the analyzed PPOs contained up to 6.5% of volatile organic compounds.16 Very few reports of metal cations (be it with zinc or other metals) for the ROP of epoxides were released after this contribution, perhaps because it emerged that it was difficult to control reaction rates and architectures of the resulting polyethers with these catalytic systems. In 2011, Le Roux and co-workers reported on neutral three-coordinate zinc complexes supported by chiral bis(oxazoline) ligands (39a−d), bis(pyridinyl)-functionalized N-heterocyclic carbene (40) or a 3579


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Figure 9. Group 4, 13, and 14 catalysts and precatalysts for the ROP of epoxides.54−58

unusual unsolvated, nearly linear (Ci−Ga−Ci′ = 175.69(7)°) two-coordinate cation polymerized CHO (1000 equiv, 1 M in C6H5Cl, 70 °C) in an exothermic reaction.56 Dagorne and coworkers’ [{BOXMe2}GaMe(PhNMe2)x]+·[MeB(C6F5)3]− (x = 0, 45a; x = 1, 45b) supported by a bis(oxazolinato) ligand {BOXMe2}− partly oligomerized 200 equiv of PO at room to low temperature (from +20 to −20 °C), but the highly sensitive nature of these cations led to the production of oligomers featuring broad or multimodal molecular weight distributions as a result of rapid catalyst decomposition. The PhNMe2 adduct 45a (conversion = 85% in 15 min; Mn = 339 g·mol−1, multimodal) was found to be more productive than the donor-free 45b (40% in 5 min, Mn = 448 g·mol−1, Mw/Mn = 2.4), perhaps because of its greater robustness.57 The organotin(IV) neutral complexes (2,4,6-iPr3-C6H2)2Sn(CHCH2)2 (46) and (2,4,6-iPr3-C6H2)2Sn(Br)(Bn) (47) reacted in situ with the cationizing reagents [Et3Si]+·[B(C6F5)4]− and AgSbF6, respectively, to generate metal cations that polymerized PO. The reaction also proceeded without regio- and stereoselectivity according to a cationic mechanism.58

catalyst that was used; and (iv) under these conditions there was little hope to be able to control, let alone to fine tune, the parameters (reaction rates, macromolecular features, polymer architecture) of these polymerizations unless the active site could be maintained in the immediate vicinity of the metal atom. In particular, the possibility to be able to achieve copolymerization of epoxides and carbon dioxide with these cationic systems appeared distant. Perhaps because these polymerizations of epoxides promoted by well-defined Lewisacidic metal cations overall offered little promise, many teams gradually shifted their attention toward the ROP of cyclic esters.

4. RING-OPENING POLYMERIZATION OF LACTIDES AND RELATED MONOMERS Around 2002−2005 it became apparent that cations offered limited scope in the ROP of epoxides; many research groups considered instead using them to catalyze the polymerization of cyclic esters. This coincided with a streak of encouraging results in the metal-catalyzed ROP of lactide,4,5 which for quite some time had been considered a difficult cyclic ester to polymerize in a controlled manner owing to its limited ring strain and relative acidity of the hydrogen atoms in α positions to the carbonyl groups. This obviously also coincided with a surge and fashionable, societal, technological, and scientific interest for conversion of renewable resources into (bio)degradable polymer materials. Hayakawa and co-workers used the discrete zirconocenium [Cp2ZrMe]+·[B(C6F5)4]− to catalyze the ROP of sevenmembered cyclic esters and carbonatesε-caprolactone (CL) and 1,3-dioxepan-2-one (tetramethylene carbonate), respectively.14 This was swiftly followed by Wu et al.’s work, who used the same compound to catalyze the ROP of the spiro monomer 1,5,7,11-tetraoxaspiro[5,5]undecane.59 Considering that the first cations used in the ROP of epoxides were by a long margin aluminum ones,9 it is perhaps surprising that it is not until 2002 that Saunders Baugh investigated in some detail their performances in the polymerization of CL,44 even if Bertrand had previously made a brief attempt at the polymerization of

3.3. Concluding Comments on the ROP of Epoxides

In the wake of Atwood’s pioneering work on aluminum complexes bearing Salen-type ligands in the 1990s it emerges that the vast majority of well-defined cations utilized to catalyze the ROP of CHO and PO involved this very metal. Bochmann subsequently devised many zinc cations in the 2000s, while monoanionic phenolates were tested by Dagorne with aluminum. However, despite substantial synthetic efforts it transpired that (i) ancillary ligands did not bring particularly added value compared to simple organozinc systems; (ii) even if the cation contained a potentially reactive group, all these polymerizations very probably proceeded according to an activated chain-end mechanism with the propagating site located in the polymer chain at the end opposite to the metal (that is, if the growing polymer chain was still bound to the metal); (iii) the produced polyethers very much presented the same macromolecular (low molecular weights with broad distributions) and architectural features (especially so in the case of PO, the monomer of highest interest) irrespective of the 3580


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Figure 10. Examples of cations with (types I and II) or without (type III) potentially reactive nucleophilic group for ROP catalysis.18,60 For type III catalysts, the ancillary ligands (e.g., aminoether−phenolate in the cases of 12−16) are usually inert and not considered as reactive nucleophiles.

cases, the presence of excess initiator also acting as a chaintransfer agent is needed if immortal ROP reactions are sought (vide supra). Compounds of type III devoid of any potentially reactive nucleophile, [{LXn}Met]+·[WCA]−, are much more widespread than compounds of type I. This is seen, for instance, in the aminoether−phenolate complexes of divalent metals [{LONO4}Met]+·[H2N{B(C6F5)3}2]− (Met = Zn and Mg−Ba, 12−16, see Scheme 7 and Figure 10).18,20 With such compounds, the addition of one equivalent (for a living ROP reaction) or excess equivalents (for an immortal ROP) of exogenous nucleophile is mandatory to achieve controlled ROP reactions; otherwise, no activity or, in the most favorable circumstances, uncontrollable Lewis-acid polymerizations are observed. The catalytic system, which can mediate living or immortal polymerizations, combines two components: the metal catalyst and the nucleophilic initiator, the latter also acting as a chain-transfer agent if found in excess vs the metal during an iROP reaction. 4.1.2. Living vs Immortal ROP of Cyclic Esters and Carbonates. Some clarifications must be given to distinguish between catalyzed regular living and immortal living ROP processes (Scheme 23). In a living polymerization, each metal generates a single polymer chain and all polymer chains are of equal length. The rare known catalysts of type I can initiate these reactions on their own and yield only one polymer chain per metal atom. On the other hand, compounds of types II and III require the addition of 1 equiv (and only 1!) of protic nucleophile, the initiator. Experimental indicators for a living system include (i) a very narrow distribution of molecular weights for the resulting materials (Mw/Mn < 1.10), (ii) a linear dependence of the polymer molecular weight on the monomer-to-metal ratio at a given monomer conversion, and (iii) a linear increase of molecular weight with monomer conversion. To actually fulfill the criteria for a living ROP, a system must in essence feature 100% initiation efficiency, that is, the molecular weight is defined by the (converted) monomer-to-catalyst ratio or the monomer-to-initiator one if an exogenous initiator is used. Immortal ROP reactions (iROP) are a specific type of living ROP, relying on chain transfer between growing and dormant macromolecules. The concept of iROP, originally introduced

racemic lactide (rac-LA) with his chiral four-coordinate aluminum cations.36 In fact, the utilization of aluminum cations for the ROP of cyclic esters and carbonates never quite met the success it encountered for the ROP of epoxides. Zinc and related alkaline-earth cations have offered in this case much greater prospects, notably in terms of catalytic activity. Following timid beginnings when it was thought that metal cations mediated the ROP of cyclic esters and carbonates according to a Lewis-acid catalysis, some contributions highlighting the role of nucleophiles (present in the cation or added as exogenous agents) in these systems led to the design of highperformance catalytic systems that, in addition, allowed for extremely well-controlled reactions. The importance of analytical methods (NMR spectroscopy, MALDI-ToF mass spectrometry) for the analysis of the polymer end groups was instrumental in these developments. 4.1. Structural, Mechanistic, and Analytical Considerations

4.1.1. Cations with or without a Reactive Nucleophilic Group. The range of available cationic complexes for ROP catalysis can be divided into three main categories: those containing a nucleophilic group reactive toward the incoming cyclic monomer (type I), those that contain a nucleophilic group inert toward the monomer (type II), and those that do not incorporate any potential nucleophile apart from ancillaries (type III), see Figure 10. Compounds of types I and II are typically of the general formula [{LXn}Metm(Nu)m−n−1]+·[WCA−], where {LXn} is an ancillary ligand, Nu is a nucleophile (alkoxide, amido, alkyl), WCA is a weakly coordinating anion, and m > 1. Representative examples are, for instance, Hayes’ [{bPPI(PiPP)2}Zn(O-Me + − L -lactate)] ·[B(3,5-(CF 3 ) 2 -C 6 H 3 ) 4 ] (48, type I) and (PiPP)2 + [{bPPI }ZnMe] ·[B(3,5-(CF3)2-C6H3)4]− (49, type II), two compounds where the metal cation is supported by a bulky bis(phosphinimino) (bPPI) ligand (Figure 10).60 The reactive nucleophile (such methyl O-lactate in 48) in complexes of type I triggers the polymerization of cyclic esters. The presence of an initiator (alcohol, amine, etc.) is not required to observe ROP catalytic activity, and these commonly give rise to living polymerizations. In the type II cases where the nucleophile is inert toward the monomer (as, for instance, in 49), addition of an initiator is necessary to polymerize the monomer.60 In both 3581


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Scheme 23. Living and Immortal ROP Processes with Metal Cations of Types I−III

by Inoue for the ROP of epoxides mediated by aluminum− porphyrin complexes,61 has become a main feature of the ROP of cyclic esters. iROP reactions are performed upon addition of an excess of a protic nucleophile, in most cases an alcohol, behaving as an exogenous initiator as well as a chain-transfer agent, and the cationic complex acts as a genuine catalyst. When the transfer between growing and dormant macromolecules is fast and reversible, the number of polymer chains generated per metal atom is defined by the [protic nucleophile]0/[metal]0 ratio, while the chain length is set by the [monomer]converted/[protic nucleophile]0 ratio. In the case of cations of type III (by far the most common ones), the terminology immortal ROP for such process is widely accepted (Scheme 23); it will be used here on the basis of the practice used elsewhere. Cationic metal complexes have proved particularly efficient for controlled immortal ROP processes, and many a group have endeavored in recent years to develop such binary catalyst systems. The main features of these mechanisms have been reviewed elsewhere and shall not be discussed here in detail.5i Suffice it to say that some advantages of these reactions include (i) a great number of polymer chains can be produced per metal atom, thereby optimizing the catalyst productivity and limiting at the same time the issue of metal (and ligand) contamination in the final polymer material, (ii) for an effective iROP, the macromolecular features of a

polymer are extremely well controlled (independent of partial catalyst deactivation) and one can tailor their molecular weight in a predictable fashion, and (iii) excellent opportunities for end-group functionalization are provided if a difunctional protic nucleophile (e.g., propargylic alcohol) is used as the initiator/ chain-transfer agent, giving access to a variety of original polymer materials which can then be utilized for macromolecular engineering purposes. In addition to the foregoing, ROP reactions are said to be “controlled” (or to give a good control of the macromolecular parameters) if they meet simultaneously two criteria. First, the experimental polymer molecular weight (established by gel permeation chromatography (Mn,GPC) or by NMR (Mn,NMR) must match that calculated (Mn,calcd) on the basis of the monomer-to-catalyst (for living) or monomer-to-initiator (for immortal) ratio weighted by monomer conversion. (In ROP catalysis, one often finds it convenient to use number-average molecular weights, Mn, when alluding to the molecular weight of the polymers. This habit, useful in kinetics and to assess initiator efficiency, perhaps also stems from the fact that the polydispersity indexes, Mw/Mn, are often close to 1.0, the ideal case for living and immortal polymerizations). Second, the molecular weight distribution of the polymers must be narrow, typically Mw/Mn < 1.2. Analysis of the polymer end groups helps further delineate the level of control over the polymerization 3582


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(Mn = 1500 g·mol−1, Mw/Mn = 1.2); no mention was made of the tacticity or composition of the polymer. Longer chains were obtained upon polymerization of 500 equiv of rac-LA, while the polymer remained fairly monodisperse (yield = 85%, Mn = 18 500 g·mol−1, Mw/Mn = 1.4). The authors surmised that the reaction of PO with 28 generated a putative aluminum− alkoxide species 28′ upon insertion of PO into the Al−Cl bond and that this new species then polymerized rac-LA (Scheme 25). However, no indication supporting this scenario was provided.36 Saunders Baugh prepared in-situ combinations of aluminum complexes bearing N,N′- (50 and 51) or N,O-bidentate (52 and 53) monoanionic ligands with 1 equiv of B(C6F5)3 or [Ph3C]+·[B(C6F5)4]− to polymerize ε-caprolactone (Scheme 26).44 The reactions were carried out for 4 h at 25 °C, with 1.0 M solutions of CL either in toluene or in THF, and [CL]0/[Al] = 100:1. The neutral compounds 50−53 did not initiate the polymerization. The amidinate compounds 50 and 51 proved effective upon cationization, yielding PCL with low molecular weights (Mn = 2000−8000 g·mol−1) and rather broad polydispersities (Mw/Mn = 1.5−2.2). The molecular weights were much lower than those expected on the basis of monomer-tometal ratio and monomer conversion, and this was attributed to deleterious transesterification phenomena. The ROP reactions were slower in THF than in toluene, presumably because of competitive THF coordination onto the active species in the former case. For the N,O-ligated 52 and 53, no product (with B(C6F5)3) or alcohol-soluble materials (with the trityl borate) were obtained, which however highlighted the influence of the nature of the counterion in these polymerizations. The possibility of a cationic mechanism was not excluded, but the authors considered a coordination−insertion pathway more likely. The available experimental data were not sufficient to give a more assertive conclusion. Lewiński and co-workers carried out the in-situ synthesis of the four-coordinated aluminum cations [{Me2N∧N∧O}AlMe]+· [X]− (X− = AlCl4−, 54a; [BPh4]−, 55a) stabilized by a tridentate (amino−imino)phenolate Schiff base by chloride abstraction in the neutral precursor.63 Compound 55a was in rapid exchange with [{Me2N∧N∧O}AlPh]+·[MeBPh3]− (56a) due to an unusual solution Me/Ph exchange process between the anionic moieties, as evidenced by 27Al and 11B NMR spectroscopy. Reaction of 55b and 56b with dry O2 gave the corresponding alkoxy and phenoxy species [{Me2N∧N∧O}Al(OR)]+·[X]− (R = Me, X− = [BPh4]−, 55b; R = Ph, X− = [MeBPh3]−, 56b) according to NMR spectroscopy (Scheme 27). These compounds could not be structurally characterized, but 56b crystallized from THF with two bound solvent molecules, [{Me2N∧N∧O}Al(OPh)(THF)2]+·[MeBPh3]−. The unresolved, in-situ-generated mixture of 55a and 56a polymerized CL ([CL]0/[Al] = 50:1) in dichloromethane at 40 °C nearly quantitatively in 60 min. MALDI-ToF MS analyses showed that two types of PCL were produced, with one hydroxyl and one methoxy/phenoxycarbonyl termini, respectively; both were monodisperse (Mw/Mn = 1.3). By comparison, [{Me2N∧N∧O}Al(OMe)]+·[AlCl4]− (54b), supposed to have been generated by oxygenation of 54a, also yielded PCL under the same conditions, although in this case a single population (with hydroxyl and methoxycarbonyl end groups) was detected by MALDI-ToF MS (Mw/Mn = 1.2). These observations were consistent with a coordination−insertion mechanism taking place by insertion of CL units into the Al−OMe or Al−OPh bonds, and these compounds therefore belonged to the family of catalysts of type I.63

parameters for both living and immortal ROP reactions. This analysis, performed by combining 1H NMR spectroscopy (13C and 2D NMR techniques may also be required in specific cases) and MALDI-ToF mass spectrometry, aims at authenticating the identity of termini capping the polymer chains. It is essential in the case of iROP, since it is only through the combination of GPC and end-group analyses that one demonstrates the efficiency of iROP processes involving the initiator: initiation of polymer chains and high rate and reversibility of the chain transfer between growing macromolecules and dormant ones (Scheme 23).5i,18,62 The followings will deal with well-defined cations for the polymerizations of ε-caprolactone (CL), enantiopure L-lactide (L-LA, industrially produced on large scales6), racemic lactide (rac-LA, the equimolar mixture of D-LA and L-LA), trimethylene carbonate (TMC), and δ-valerolactone (VL). The monomers and corresponding polymers are depicted in Scheme 24, without tackling the details of the stereochemistry of the ROP of racemic lactide. Scheme 24. Main Cyclic Esters/Carbonates and Their Corresponding Aliphatic Polyesters/Polycarbonates

4.2. Aluminum Cations in the ROP of Cyclic Esters

Regardless of their many uses for the ROP of epoxides, aluminum cationic complexes have been relatively little tested in the polymerization of cyclic esters and not at all for carbonates. In addition, the view is clouded by a limited understanding of the associated ROP mechanisms. Bertrand and co-workers reported that their chiral fourcoordinate aluminum−chloride cations [(RHNCH2CH2)NR′(CH2CH2NR)AlCl]+·[AlCl4]− (R = R′ = SiMe3, 27; R = SiMe3, R′ = Me, 28; R = iPr, R′ = Me, 29) did not on their own polymerize rac-LA at 80 °C.36 However, upon addition of 20 equiv of PO vs Al, 28 polymerized 50 equiv of rac-LA in 5 days (yield = 46%) to give low molecular weight PLA 3583


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Scheme 25. Bertrand’s Proposed Pathway for the ROP of rac-LA Mediated by 28/PO36

Scheme 26. Saunders Baugh’s N,N′- and N,O-Ligated Precatalysts for ε-Caprolactone ROP44

Scheme 27. Lewiński’s

N∧N∧O-Supported Alkyl-, Alkoxy-, and Aryloxyaluminum Cations63

Me2

The heteroscorpionate-containing [{bpzmp}AlMe]+·[MeB(C6F5)3]− (6a, see Scheme 4) that was shown to polymerize PO also initiated the ROP of CL in toluene solutions.13 Complete conversion of 50−150 equiv of monomer occurred within 2 h at 50 °C, although experimental molecular weights higher than their theoretical values suggested that only a fraction (ca. 30%) of the metal complex initiated the formation of a polymer chain. NMR investigations on 1:1 mixtures of CL and 6a led the authors to conclude that the polymerization followed a coordination−insertion mechanism with a slow (ca. 24 h) first insertion of CL in the Al−Ophenolate bond (initially designed to be an ancillary moiety), followed by faster (ca. 1 h) second monomer insertion in the newly formed Al−Oalkoxide bond. The Al−CH3 bond in this unusual type I catalyst therefore remained inert toward the monomer during this process (Scheme 28). In a much detailed study, Dagorne and co-workers investigated the reactivity and catalytic performances of their aluminum cations supported by O,N amino−phenolate ligands sterically encumbered in the ortho position.39 The salt

[{2-(CH2NMe2)-4-Me-6-CPh3-C6H2O}Al(iBu)(PhBr)]+·[B(C6F5)4]− (57a), prepared from {2-(CH2NMe2)-4-Me-6-CPh3C6H2O}Al(iBu)2 via alkyl abstraction with trityl salts in bromobenzene, interestingly did not polymerize CL or LA, even at 100 °C in PhBr. However, it reacted with 1 equiv of CL in C6D5Br to give the stable CL adduct, [{2-(CH2NMe2)-4-Me-6CPh3-C6H2O}Al(iBu)(CL)]+ (57b). The congeneric [{2(CH 2 NMe 2 )-4-Me-6-CPh 3 -C 6 H 2 O}AlMe(CL)] + ·[MeB(C6F5)3]− (58b), which was crystallographically authenticated, was obtained in a one-pot process by reacting {2-(CH2NMe2)-4Me-6-CPh3-C6H2O}AlMe2, B(C6F5)3, and CL in dichloromethane (Scheme 29). There was no interaction between the metallic cation and the counterion in either of these salts according to solution NMR and available XRD data. The bound CL molecule in 57b and 58b did not undergo ring opening. The molecular structure of the four-coordinate 58b was seen as a model of the putative [{2-(CH2NMe2)-4-Me-6-CPh3C6H2O}Al(OiPr)(CL)]+·[B(C6F5)4]− (59b), reasonably assumed by the authors to be a reactive intermediate during the ROP of CL catalyzed by [{2-(CH2NMe2)-4-Me-6-CPh33584


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Scheme 28. Milione’s Stepwise Insertion of CL (1 + 1 equiv vs Al) into Al−O Bonds in Four-Coordinate Aluminum Cation Supported by a Reactive Heteroscorpionate Ligand13

Scheme 29. Dagorne’s Stable Four-Coordinate Aluminum−CL Cations Bearing a Bidentate O,N Amino−Phenolate Ligand39

C6H2O}Al(OiPr)(THF)]+·[B(C6F5)4]− (59a).39 Several synthetic routes were devised, and many aspects of the reactivity of these cations were explored. The structurally characterized four-coordinate THF adduct Al−isopropoxide 59a was best obtained by elimination of isobutene upon heating the otherwise kinetically inert [η1-{6-(CH2NHMe2)-2-CPh3-4-MeC6H2O}Al(iBu)(OiPr)(THF)]+·[B(C6F5)4]− having an ammonium side arm. This last cation was itself obtained most conveniently by treatment of [{6-(CH2NMe2)-2-CPh3-4-MeC6H2O}Al(iBu)(THF)]+·[B(C6F5)4]− (57c) with iPrOH in CH2Cl2 (Scheme 30). The cation [{2-(CH 2 NMe 2 )-4-Me-6-CPh 3 -C 6 H 2 O}Al(OiPr)(THF)]+ in 59a was a very effective type I catalyst for the polymerization of CL in dichloromethane at 45 °C, fully converting 120 equiv of monomer within 15 min to monomodal PCL with good control (t = 3 min, yield = 46%, Mn,calcd = 6200 g·mol−1, Mn,GPC = 4800 g·mol−1, Mw/Mn = 1.3; t = 15 min, yield = 99%, Mn,calcd = 13 700 g·mol−1, Mn,GPC = 15 300 g·mol−1, Mw/Mn = 1.4). On the other hand, 59a did not polymerize racor L-LA in PhBr, even at 100 °C. It still reacted with 1 equiv of L-LA to afford the monoinsertion aluminum-O-lactate product [{2-(CH2NMe2)-4-Me-6-CPh3-C6H2O}Al(isopropyl O-lactate)(THF)]+·[B(C6F5)4]− (60), a most interesting compound featuring a five-coordinate aluminum atom (Scheme 30), with a five-membered metallacycle formed upon coordination of the Ocarbonyl atom onto the metal. This highly stable Al−O-lactate cation was unable to polymerize L-LA at 100 °C, but it did readily produce PCL by ROP of CL (75% conversion of 100 equiv in 60 min) at 40 °C, albeit a little more slowly than its parent 59a. The formation of a five-membered chelate in 60 and related cations in the presence of lactide is key to the stability

and reactivity of these species, but further information (e.g., DFT computations) was not available.39 Cations supported by O,P phosphine−phenolate ligands showed comparable behavior toward CL and LA.41 The salts [{κ 2 -O,P-(2-PPh 2 -4-R′-6-R-C 6 H 2 O)}AlMe(THF)] + ·[MeB(C6F5)3]− (R′ = H, R = Ph, 34a; R′ = Me, R = tBu, 34b) and [{κ 2 -O,P-(2-PPh 2 -4-R′-6-R-C 6 H 2 O)} 2 Al(THF)] + ·[MeB(C6F5)3]− (R′ = H, R = Ph, 35a; R′ = Me, R = tBu, 35b) (see Scheme 19) were not able to polymerize rac-LA (100 equiv) after 12 h in toluene at 75 °C. However, they all afforded PCL in toluene ([CL]0 = 1.0 M) at 75 °C, reaching ca. 95% conversion of 100 equiv of CL after 2 h. The resulting PCLs all exhibited comparable properties (Mn,GPC = 32 000−38 000 g·mol−1, Mw/Mn = 1.2−1.3), but they were considerably higher than their calculated value (Mn,calcd = 11 400 g·mol−1, based on only one growing chain per Al atom at any given time), suggesting that only 30% of the metal atoms initiated the formation of a polymer chain. End-group analysis (1H and 31P NMR spectroscopy, MALDI-ToF MS) showed the presence of phosphinoxy− phenolates as only termini, proving that the first ring-opening event occurred by insertion of CL in the Al−Ophenolate bond. Overall, the four-coordinate salts with O,P bidentate ligands in 34a−b and 35a−b were effective type I catalysts (albeit with a reactive nucleophile initially designed as an ancillary ligand) for the ROP of CL but less so than the related ones with O,N amino−phenolates, e.g., 59a.39,41 By comparison, compounds 31b and 33a,b (see Scheme 18) where the aluminum cation is supported by a tridentate O,N,E-phenolate ligand (E = O, NMe) displayed poor performance.40 They required 12 h to convert 100 equiv of CL in dichloromethane at 38 °C to give PCLs with multimodal and relatively broad molecular weight distributions 3585


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Scheme 30. Synthesis and Reactivity of a Four-Coordinate Amino-Phenolate Al−Isopropoxide Cation (59a) with an ORTEP Rendering of the Cation in the Product of L-LA Monoinsertion (60) in the Al−Oisopropoxide Bond (Dagorne and Co-Workers)39

Scheme 31. O,S-Bidentate Thioether−Phenolate in a Four-Coordinate Aluminum−Alkyl Cation Stabilized by an Additional Lewis Base64

(Mw/Mn = 1.6−2.0), suggesting that several catalytically active species were at work; they were also unable to polymerize lactides. Related to these compounds, a four-coordinate aluminum− alkyl cation with a bidentate thioether−phenolate ligand, [{2,4(tBu) 2 -6-CH 2 SPh-C 6 H 2 O}AlMe(THF)] + ·[MeB(C 6 F 5 ) 3 ] − (61), was prepared by methyl abstraction in {2,4-(tBu)2-6CH2SPh−C6H2O}AlMe2 with B(C6F5)3 in the presence of equimolar amounts of THF (Scheme 31).64 The addition of THF was required to yield the ion pair, as otherwise ligand exchange occurred between the borane and the starting material to give {2,4-(tBu)2-6-CH2SPh-C6H2O}AlMe(C6F5). The addition of 1 equiv of CL to 61 (which could not be isolated) led to a dynamic process in CD2Cl2 solutions, whereby THF and CL molecules exchanged rapidly and reversibly. This reactivity of 61 toward cyclic esters was not questioned further. Otero, Lara-Sánchez, and co-workers reported the preparation of aluminum−alkyl cations ligated by N-phenyl-2,2-bis(3,5dimethylpyrazol-1-yl)thioacetamide ({pbptam}−), a tridentate monoanionic thioacetamidate ligand.65 Of relevance to ROP catalysis, the neutral {κ2-pbptam}AlMe2 reacted with B(C6F5)3 or [Ph3C]+·[B(C6F5)4]− to give the corresponding dissociated ion pairs [{κ3-pbptam}AlMe]+·[WCA]− (Scheme 32;

Scheme 32. Thioacetamide Heteroscorpionate-Ligated Aluminum−Alkyl Compounds65

[WCA]− = [MeB(C6F5)3]−, 62; WCA− = [B(C6F5)4]−, 63), which could however not be isolated. In doing so, NMR spectroscopy revealed that the binding mode of the ancillary ligand onto the metal changed from κ2 to κ3, with coordination of the two pyrazolyl moieties in the resulting four-coordinate cationic metal center. Although they did not show any ability to polymerize lactides, in-situ-prepared 62 and 63 competently polymerized CL in diluted toluene solutions, converting, respectively, 40% and 50% of the 500 equiv of CL per metal atom in 45 min at 70 °C. That the same cation with different counterions gave 3586


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different outcomes, with the less coordinating anion [B(C6F5)4]− giving the higher conversion, again highlighted the role of the anion in these catalytic systems. Importantly, these compounds were more active than their charge-neutral dialkyl parent which polymerized CL with a controlled coordination−insertion ROP mechanism. The large discrepancy between calculated and measured molecular weights together with the somewhat broad distributions (for 62, Mn,calcd = 23 000 g·mol−1, Mn,GPC = 57 000 g·mol−1, Mw/Mn = 1.6; for 63, Mn,calcd = 28 500 g·mol−1, Mn,GPC = 62 000 g·mol−1, Mw/Mn = 1.6−1.7) incited the authors to assume a cationic ROP mechanism. End-group analysis for these polymers was not given.

imidazolium salts to ZnEt2 (0.5 equiv) or more conveniently by heating equimolar amounts of the imidazolium salt with the neutral {MesNHCPyr }ZnEt(I) (Scheme 33). Cationic complex 64 and its neutral precursor {MesNHCPyr}ZnEt(I) both polymerized rapidly rac-LA in the melt (140 °C) to give slightly heterotactic-enriched PLA with values of Pr around 0.6.67 The cationic system was less active than its neutral parent, but it afforded better control. Hence, 64 converted 73% of 200 equiv of rac-LA after 5 min to give polymers with rather a broad polydispersity indexes (ca. 1.8− 2.5) but with generally good agreement between calculated and observed molecular weights (Mn = 6100−12 000 g·mol−1), suggesting that a single active species was operating. Salt 64 was however somewhat sensitive to experimental conditions and perhaps susceptible to release free NHC or imidazolium salt. Le Roux’s related bis(pyridinyl)-functionalized NHC complex {PyrNHCPyr}ZnEt(I) (40, see Figure 8), which polymerized epoxides upon activation with borane/borate activators, was not tested in the ROP of cyclic esters.53 Dagorne and co-workers reported that [{MesNHCMes}ZnMe(THF)2]+·[MeB(C6F5)3]− (65) and [{DippNHCDipp}ZnMe(THF)]+·[MeB(C6F5)3]− (66) containing THF coligands could be prepared as weakly associated ion pairs (Scheme 34). However, both of these zinc−methyl cations were unable to polymerize β-butyrolactone (BL) or rac-LA.68 NMR experiments showed that they decomposed in the presence of 5 equiv of either of these cyclic esters to release free imidazolium salts with unidentified zinc species. The group of Hayes explored in detail the synthesis and ROP catalytic performances of coordinatively unsaturated zinc cations supported by a variety of neutral mono- and bis(phosphinimino) dibenzofuran (dibenzofuran = dbf) ligands (Scheme 35).69 The effects of the reactive nucleophile bound to the Zn ion and the number of phosphinimino tether, steric and electronic properties, and presence of P-stereogenic centers in the ligand were assessed.60,69−75 The cations were prepared by one-step alkane elimination in zinc−dialkyls with the appropriate protonated ligand. In a first contribution, the mono(phosphinimine) dibenzofuran {NDippPPh2-dbf} led to the high-yield syntheses of [{NDippPPh2(dbf)}ZnEt]+·[B(C6F5)4]− (67a) and [{NDipp PPh2(dbf)}ZnEt]+·[CF3SO3]− (67b) (Figure 11).70 NMR spectroscopy indicated that the borate anion in 67a was dissociated from the zinc cation. On the other hand, XRD crystallography revealed that the triflate counterion was coordinated to the metal atom in 67b, which crystallized as a 2:1 mixture of two components presenting major structural differences, in particular with respect to the binding of the Odbf atom onto the metal (d(Zn−O(dbf)) = 2.60(1) and 2.08(2) Å). Compounds 67a and 67b both polymerized rac-LA at 100 °C in a 1:1 mixture of C6D6 and C6D5Br with [rac-LA]0 = 1.0 M. Whereas 67a only required 6 h to achieve 90% conversion of 100 equiv of the monomer, 67b took 9 h to convert 85%: this underscored very clearly the role of the anion in such catalysts, with the least coordinating anionexpectedlygiving the highest reaction rates. However, the resulting PLA displayed bimodal molecular weight distributions with lower than expected molecular weights, all of which point to transesterification and possibly other side reactions. With the hope of achieving some stereoselectivity in the ROP of rac-LA, Hayes and co-workers next designed chiral mono(phosphinimine) ligand scaffolds {NDippPPhMe(dbf)} and {NMesPPhMe(dbf)} bearing stereogenic centers at the

4.3. Zinc Cations in the ROP of Cyclic Esters/Carbonates

In a reversal of situation compared to the ROP of epoxides with well-defined metal cations, zinc has been the subject of many more investigations than aluminum, or in fact any other metal, in the context of cyclic esters/carbonates polymerizations. While the first studies for Zn dates back to 2002−2004, almost all of the many available reports were released in the past decade. They dealt mostly with the ROP of lactides, perhaps highlighting the preponderance taken by this monomer in recent years as a benchmark over other polymerizable heterocycles. The seminal accounts of the ROP of cyclic esters with discrete zinc cations resulted from the works of Bochmann’s group. They transferred their compounds originally designed to polymerize epoxides to the polymerization of ε-caprolactone. The diazadiene compound [{DAD}ZnMe]+·[MeB(C6F5)3]− (11a, see Scheme 21), a compound of type I or II, polymerized CL (1000 equiv) at 60 °C. It converted 13% of the monomer in 60 min (TOF = 129 molCL·molZn−1·h−1) to give a narrowly disperse PCL of relatively high molecular weight (Mw = 40 000 g·mol−1 and Mw/Mn = 1.1).19a The dissociated ion pair [{CpMes}Zn(TMEDA)]+·[EtB(C6F5)3]− (38, see Scheme 22) polymerized CL ([CL]0/[Zn] = 1000:1) in toluene at 65 °C.50 The yield was modest after only 3 min (8%), but the activity of this salt (TOF = 1600 molCL·molZn−1·h−1) was substantially higher than that of its charge-neutral zinc−ethyl congener. Besides, the resulting polymer already exhibited high molecular weight and a narrow polydispersity (Mn,GPC = 38 300 g·mol−1 and Mw/Mn = 1.2). Evidently, the experimentally observed molecular weight was much higher than its theoretical value calculated on the basis of 100% initiation efficiency (Mn,calcd = 9100 g·mol−1), indicating that only a fraction of the metal atoms (ca. 25%) actually initially formed polymer chains. This group also reported that, whereas the salts [(Et2O)3ZnX]+· [B(C6F5)4]− (X = Et, 8a; N(SiMe3)2, 8b, see Scheme 6) could not polymerize L-LA, they rapidly polymerized CL (1000−6000 equiv, TOF up to 2630 molCL·molZn−1·h−1) at room temperature and at 50 °C, giving high molecular weight PCL with a rather broad distribution (Mn up to 23 900 g·mol−1, Mw/Mn = 2.3); end-group analysis was however not possible.16 These catalysts contained a potentially reactive nucleophile group (types I/II), but it was not possible to determine the nature of the ROP mechanisms. Attempts to use side-functionalized N-heterocyclic carbenes (NHC) in zinc cations to polymerize cyclic esters/carbonates were also carried out early on. In 2005, Tolman, Hillmyer, and co-workers prepared [{MesNHCPyr }2ZnI]+·[I]− (64) featuring a zinc cation ligated by two sterically encumbered bidentate NHC-pyridinyl moieties {MesNHCPyr } and a remote iodide counterion.66 This compound was obtained by addition of the 3587


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Scheme 33. Synthesis of Zinc−Iodide Cation Stabilized by a Bulky NHC-Pyridinyl (Tolman/Hillmyer)66

Scheme 34. Dagorne’s {mesNHCmes}- and {DippNHCDipp}-Stabilized Zinc−Methyl Cations68

Scheme 35. Neutral Mono- and Bis(phosphinimino)dibenzofuran Ligands Used by Hayes and Co-Workers for the Preparation of Zinc−Alkyl Cations60,69−75

Figure 11. Hayes’ [{NDippPPh2(dbf)}ZnEt]+·[X]− ([X]− = [B(C6F5)4] −, 67a; [CF3SO3] −, 67b) with an ORTEP rendering of the minor component in 67b showing the Zn−Odbf bond (2.08(2) Å).70

phosphorus atom. The corresponding zinc cations [{Narene PPhMe(dbf)}ZnX]+·[B(C6F5)4]− (arene = 2,6-iPr2C6H3, X = Et, 68; X = Me (O)-lactate, 69; arene = 2,4,6-Me3C6H2, X = Et, 70; X = Me (O)-lactate, 71) were obtained by reaction of ZnEt2 or

ZnEt(Me (O)-lactate) with the protonated ligands (Scheme 36).71 Attempts to prepare analogues with the [BPh4]− anion failed and gave decomposition products or ZnPh2 species resulting from ligand (alkyl/aryl) scrambling. 3588


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Scheme 36. Hayes’ [{NarenePPhMe(dbf)}Zn(X)]+·[B(C6F5)4]− (68−71) with a Chiral Mono-Phosphinimine Ligand71

Figure 12. Hayes’s bis(phosphinimine) dibenzofuran ligand for the synthesis of zinc cations for ROP catalysis.72

(O)-lactate derivative [{(NmesPPh2)2(dbf)}Zn(Me (O)-(S)lactate)]+·[B(C6F5)4]− (74) proved a competent catalyst.72 Due to synthetic constraints, 74 could not be isolated pure and was contaminated by ca. 15% of [{(NmesPPh2)2(dbf)}ZnEt]+· [B(C6F5)4]−, which was however inert toward lactides under the selected polymerizations conditions. The 85:15 mixture that was used was therefore representative of the catalytic activity of 74 alone. It converted 90% of 50 equiv of rac-LA under mild conditions (60 °C, [rac-LA]0 = 0.25 M) within 3.5 h to atactic PLA. NMR monitoring of small-scale polymerization reactions (showing initiation by the (O)-lactate moiety) and end-group analyses were fully compatible with a coordination−insertion mechanism. Although plagued by transesterification reactions, this type I catalyst afforded narrowly disperse PLAs (Mw/Mn = 1.2−1.3), yet with molecular weights (Mn,GPC ≈ 18 000 g·mol−1) three times larger than calculated. With the less congested bis(phosphinimine) ligand {(Np‑iPPPPh2)2(dbf)}, the dissociated ion pair [{(Np‑iPP P Ph2 ) 2 (dbf)}Zn(Me (O)-(S)-lactate)] + ·[B(3,5-(CF 3 ) 2 C6H3)4]− (48) could be obtained cleanly, and its molecular structure was established (Figure 13).60 The zinc ion rested in a five-coordinated environment upon coordination of the Nimine atoms and Odbf, Olactate, and Ocarbonyl atoms and with formation of a five-membered metallacycle with the lactate; the decrease of steric congestion compared to 74 induced main structural modifications leading to greater stability. Although its fourcoordinate zinc−methyl analogue was inert toward rac-LA, 48 was highly efficient for the ROP of this monomer even at room temperature in CD2Cl2. It converted 90% of 200 equiv of monomer within 50 min to give slightly heterotactic-enriched PLA (Pr = 0.63). The rate of polymerization followed partial first order in both monomer and catalyst concentrations, with kobs = 8.65(4) × 10−4 s−1 and kp = 0.17(1) M−1·s−1. The activation parameters ΔH‡ = 11.2(1) kcal·mol−1 and ΔS‡ = −35.1(1) cal·K−1·mol−1 were comparable with values reported

The zinc−ethyl compound 68 proved to be a very poor catalyst for the ROP of rac-LA ([rac-LA]0/[Zn] = 100:1, [racLA]0 = 0.5 M in bromobenzene) at 100 °C, converting only 9% of the monomer after 3 h. By contrast, its Me (O)-lactate congener 69 was much more efficient under the same conditions, converting 90% of the monomer after 9 h. It also converted 69% of 400 equiv of rac-LA (conditions unspecified) and was able to convert two sequential batches of 200 equiv of monomer. The molecular weights of the PLAs produced by 69 were only slightly lower than expected but exhibited rather broad polydispersities (Mn,calcd = 21 300−39 800 g·mol−1, Mn,GPC = 17 300−35 700 g·mol−1, Mw/Mn = 1.8−2.0), possibly an indication of deleterious transesterifications processes. Analysis (MALDI-ToF MS, NMR spectroscopy) of low molecular weight PLAs produced by 69 showed the presence of two fractions, the main one consisting of cyclic macromolecules, while the minor fraction corresponded to H−(OCHMeCO)n−OMe·Na linear oligomers; this suggested that 69 acted as a type I catalyst. The PLAs produced by 69 under these conditions were all essentially atactic. Following identical synthetic procedures, related compounds where the metal ion is supported by neutral bis(phosphinimine) ligands were also synthesized by the same group (Figure 12).72 Overall, they proved able to accelerate the reaction rates and improve the control over the parameters of the ROP of rac-LA compared to mono(phosphinimine)-ligated cations. The zinc−methyl [{(NmesPPh2)2(dbf)}ZnMe]+· [WCA]− ([WCA]− = [B(C6F5)4]−, 72a; [BPh4] −, 72b) and zinc−acetate [{(NmesPPh2)2(dbf)}ZnOAc]+·[BPh4]− (73) were structurally authenticated. They exhibited three- and four-coordinate zinc atoms, respectively, with κ2 binding of the ligand via the two nitrogen atoms; the oxygen atom in the backbone did not coordinate to the metal. They did not polymerize rac-LA, even under forcing conditions, which is perhaps unsurprising owing to the poorly nucleophilic character of the Me and OAc groups. On the other hand, their methyl 3589


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Figure 13. Hayes’ bis(phosphinimine) zinc−lactate cationic catalyst for the living heteroselective ROP of rac-LA with an ORTEP rendering of the cation in 48.60,73

Figure 14. Ligands modification in Hayes’ zinc cations bearing bis(phosphinimino) ligands.74,75

The authors concluded that the substantial activity difference (75 < 48 ≈ 76, all being catalysts of type I) between the fourcoordinate metal in 75 and the five-coordinate, less electrophilic metal in 48 and 76 expressed chiefly the beneficial influence of (i) the reduced steric constraints when o-Me substituents are not present and (ii) the stabilizing effect of the then possible Zn−Odbf interactions in 48 and 76. This may thus be one of these few cases where an increase of the electrophilicity of the metal center is not beneficial to overall catalytic activity. Further modifications of the ligand backbone, where alkyl substituents replaced aryl ones on the nitrogen and phosphorus atoms, brought moderate improvement of the reaction rates, as in the lactate-substituted compound [{(NBnPEt2)2(dbf)}Zn(Me (O)-(S)-lactate)]+·[B(C6F4)4]− (77) (Figure 14).74 This salt polymerized 90% of 100−400 equiv of rac-LA at 25 °C in CH2Cl2 ([rac-LA]0 = 1.0 M), with first-order kinetics in [racLA]0 and an observed rate constant kobs = 1.88(1) × 10−3 s−1 somewhat greater than that measured for 48. By opposition with 48, compound 77 produced only atactic PLAs. The effect of the change of weakly coordinating anion in 77 was however not measured. This type I catalyst proceeded according to a coordination−insertion mechanism, with initial insertion of lactide in the Zn−Olactate bond, as demonstrated by MALDIToF MS, and featured all the characteristics of a living system. Nonetheless, the systems relying on 48 and 75−77 induced transesterification side reactions, generating a fraction of short PLA chains and inducing discrepancies between calculated and measured molecular weights at high monomer feeds.

with other systems for a ROP coordination−insertion mechanism. Indeed, end-group analysis together with NMR monitoring of the initial stages of an oligomerization reaction were also consistent with this initial insertion of rac-LA in the Zn−Olactate bond. For the ROP of 100−400 equiv of rac-LA, the observed molecular weights matched their expected values, and the heterotactic-biased PLAs displayed narrow molecular weight distributions (Mw/Mn = 1.1−1.3), which suggested a controlled, living polymerization (yet discrepancies and broadening of the polydispersity occurred at lower metal loading). The living nature of the system was corroborated by a doublefeed experiment resulting in polymer chain extension. The presence of an ortho Me substituent on the Naromatic ring decreased dramatically the catalytic activity in [{(No‑MeP P Ph2 ) 2 (dbf)}Zn(Me (O)-(S)-lactate)] + ·[B(3,5-(CF 3 ) 2 C6H3)4]− (75) and [{(NPhPPh2)2(dbf)}Zn(Me (O)-(S)lactate)]+·[B(3,5-(CF3)2-C6H3)4]− (76),73 two salts directly related to 48 (Figure 13). The more sterically congested 75 features a four-coordinate Zn atom in the molecular solid state, without interaction between the Zn and the Odbf atoms; by contrast, the metal is five- coordinate in the less sterically encumbered 76. Both polymerized rac-LA ([rac-LA]0/[Zn] = 200, [rac-LA]0 = 1.0 M) in chlorinated solvents with good control of the macromolecular parameters (Mw/Mn ≈ 1.4); while 76 was active at 25 °C (kobs = 5.11(3) × 10−4 s−1, CH2Cl2), 75 required running the polymerizations at 60 °C in chloroform (kobs = 3.65(2) × 10−4 s−1). The resulting PLAs all exhibited modest levels of heterotacticity (yet appreciable for zinc cations), with Pr values in the range 0.60−0.70. 3590


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Scheme 37. Dagorne’s Bis(aryl)BIAN-Supported Zn−Methyl Cations for the iROP of ε-Caprolactone76

Attempts to use bis(phosphinimino) ligands bearing P-stereogenic centers did not improve on the level of tacticity obtained with 48. A series of Zn−methyl cations using racemic (78−81a) or meso (80b−81b) versions of diastereomerically pure ligands were synthesized (Figure 14).75 These compounds polymerized rac-LA (200−1000 equiv) in CH2Cl2 at 40 °C, requiring 2−4 h to achieve complete monomer conversion to give slightly heterotactic-biased PLAs (Pr = 0.52−0.63). The molecular weights of the polymers were about two times larger than the calculated ones, and their distribution was rather broad (Mw/Mn = 1.7−2.6); this is possibly a result of the poorly electrophilic nature of the alkyl in these cations and slow initiation. The identity of the ligand bore little influence on the outcome of the polymerizations. The Zn−lactate analogues of 78−80a and 80b−81b could not be synthesized. Dagorne and co-workers reported the synthesis of stable Zn−methyl cations supported by a bulky bis(aryl)BIAN α-diimine ligand, [{BIANAr2}ZnMe(THF)]+·[MeB(C6F5)3]− (Ar = mesityl, 82; 2,6-iPr2-C6H3, 83), by methyl abstraction in the isolable dimethylzinc precursors {BIANAr2}ZnMe2 with B(C6F5)3.76 By themselves, 82 and 83 constituted poor catalysts for the ROP of ε-caprolactone with slow rates and poor control of the macromolecular parameters (Scheme 37). They proved much more competent as type II precatalysts upon addition of excess BnOH (or menthol) as an initiator for the iROP of CL with high monomer loadings. ROP reactions were performed at 60 °C in THF (with [CL]0 = 1.0 M) or in neat monomer and [CL]0/[Zn] = 300−5000 and [OH]0/[Zn] = 3−50. Under these conditions, the reactions were complete within 2−6 h (TOF up to 2500 molCL·molZn−1·h−1) and afforded reasonable control over the formation of PCL chains: Mn,GPC ≈ Mn,calcd ≈ 10 000−48 000 g·mol−1, Mw/Mn = 1.1−1.3. The less congested 82 proved more efficient than 83, the latter requiring 6 h to convert 300 equiv of CL in THF ([CL]0/ [OH]0/[Zn]0 = 300:3:1) where 82 only took 2 h; this observation was not discussed. The ROP mechanism was not detailed either, but MALDI-ToF MS analyses showed the presence of the BnO−C(O)− end group, resulting from initiation by nucleophilic attack of BnO−.76 In the last 5 years, a variety of type III zinc cations have been used in ROP catalysis, with mitigated results until they were combined with an exogenous nucleophile. Carpentier and coworkers first reported the synthesis of the ion pair [{NpyrazNNpyraz}ZnEt]+·[EtB(C6F5)3]− (84) with a presumably three-coordinate zinc atom supported by a bis(pyrazolyl)amido ligand (Figure 15).77 This compound polymerized 100 equiv of rac-lactide in CH2Cl2 at 20 °C (30 h, unoptimized reaction time) but with very poor control, giving a broadly disperse PLA

Figure 15. Zinc cations with low coordination numbers and devoid of “X−” reactive nucleophile.24,77

of molecular weight much higher than expected (Mn,calcd = 14 000 g·mol−1, Mn,GPC = 111 000 g·mol−1, Mw/Mn = 2.0). Similar polymerization results were reported by Schulz and coworkers for the ROP of rac-LA with the very coordinatively unsaturated, base-free cation [{BDIMe3}Zn]+ (85), where the metal is ligated by a bidentate β-diketiminate ligand and was obtained as the salt of the [Al{OC(CF3)3}4]− anion by salt elimination (Figure 15).24a In the melt (160 °C), compound 85 gave high molecular weight PLA upon ROP of 200 equiv of monomer, without control over the macromolecular features (Mn = 69 000 g·mol−1, Mw/Mn = 1.7). Full conversion was reached within 8 min. Although its structure was not established, the NMR data for 85 suggested the presence of interactions between the metal and one of the two mesityl moieties. The polymerization performances were enhanced with less electron-poor cations stabilized by tridentate β-ketiminate ligands having hemilabile −(CH2)nNMe2 side arms. The compound [{ONC2NMe2}Zn]+·[Al{OC(CF3)3}4]− (86) and its derivative with a longer side arm [{ONC3NMe2}Zn]+· [Al{OC(CF3)3}4]− (87) were prepared in high yields (Figure 15).24b Both fully and rapidly polymerized (in 12 and 22 min, respectively) 200 equiv of rac-LA in the melt at 160 °C (but not in the range 20−60 °C) to essentially atactic PLAs (Pr = 0.55 and 0.54, respectively). The control over the molecular weights and their distribution was equally good (Mn,calcd ≈ 28 000 g·mol−1, Mn,GPC ≈ 26 000 g·mol−1, Mw/Mn = 1.3). Despite these promising results, these zinc cationic systems were not investigated in more detail, perhaps because unlike their charge-neutral parents they were unable to polymerize racLA at room temperature. Carpentier, Sarazin, and co-workers reported the synthesis of an aminoether−phenolate cationic complex, [{LONO4}Zn]+· [H2N{B(C6F5)3}2]− (12), obtained by alkane elimination with the acidified aminoether−phenol (see Scheme 7). 18,20 This compound (Figure 16) polymerized L-LA (1000 equiv) at 100 °C in toluene. Without initiator, the reaction was poorly 3591


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similar aptitudes toward the stabilization of electron-deficient, oxophilic metal complexes and that the reactivity of these complexes could often be compared.78 All heteroatoms in the aminoether side arm were coordinated to the zinc atom in 88, making it a six-coordinate center. The initial working hypothesis made by the authors was that these iROP reactions catalyzed by 12 (or 88) and an alcohol followed a simple activated monomer mechanism, where the (macro)alcohol attacked the monomer bound onto the Lewis-acidic metal cation (Scheme 38). A dual organic/organometallic approach for the catalyzed ROP of lactides and trimethylene carbonate (TMC), combining a type III cationic zinc complex containing a diaminophenolate tridentate ligand {LON2}− and a tertiary amine, was reported by the groups of Carpentier, Guillaume, and Bourissou.79,80 When paired with 1 equiv of neo-PentOH or 1 equiv of the bulky tertiary amine pentamethylpiperidine (PMP), the complex [{LON2}Zn]+·[B(C6F5)4]− (89) did not polymerize rac-LA ([rac-LA]0/[Zn] = 200:1) at 50 °C in toluene, even after 24 h. On the other hand, the ternary system 89/neo-PentOH/PMP performed very well and in a controlled manner. It converted quantitatively the monomer in 3 h at 25 °C in CH2Cl2 or at 50 °C in toluene (Scheme 39).79 The polymerization was slower in THF and required 24 h to fully convert the monomer at 50 °C, hence highlighting important solvent effects. The best reaction rates were obtained in CH2Cl2 with ca. 2.0 equiv of alcohol and ca. 1.0 equiv of tertiary amine per metal atom, and the catalytic system tolerated up to 500 equiv of monomer per zinc atom. In all cases, the reactions were well controlled, with linear relationships between polymer molecular weights and [monomer]0/ [alcohol] ratios, narrow polydispersities (Mw/Mn < 1.3), and Mn,GPC = Mn,NMR ≈ Mn,calcd. The reaction rates were very sensitive to the basicity of the amine. For [rac-LA]0/[neoPentOH]0/[89]0/[amine]0 = 200:2:1:1, the rates increased dramatically with pKas according to PhNMe2 (pKa = 5.1, yield = 3% in 144 h) < collidine (pKa = 7.5, yield = 56% in 168 h) < NEt3 (pKa = 10.7, yield = 90% in 8 h) ≈ PMP (pKa = 11.2, yield = 92% in 3 h). No epimerization took place in the ROP of

Figure 16. Related zinc cationic complexes containing aminoether− phenolate (12) and −fluoroalkoxide (88) ligands with an ORTEP rendering of the cation in 88 (Carpentier/Sarazin).18,20

controlled and incomplete after 3 h (yield = 65%, Mn,calcd = 93 600 g·mol−1, Mn,GPC = 49 400 g·mol−1, Mw/Mn = 1.7). However, in combination with excess iPrOH (5−50 equiv vs Zn) as an initiator/chain-transfer agent, the binary system 12/iPrOH catalyzed the reactions efficiently, achieving full conversion (TOF ≈ 62 molLA·molZn−1·h−1) and good control over the ROP parameters (Mn,calcd ≈ Mn,GPC, Mw/Mn = 1.1− 1.4). Benzylamine could also be used instead of iPrOH as the initiator. The kinetics established with iPrOH displayed firstorder dependence on [L-LA]0, with kobs = 6.83(3) × 10−5 s−1 at 100 °C. End-group analysis showed that all chains were capped by −CH(CH3)OH and −C(O)OCH(CH3)2 termini. At constant monomer loading and full monomer conversion, the molecular weight of the resulting P-L-LAs decreased linearly with increasing initiator contents. In a continuation of this work, the same authors showed that the related dissociated ion pair [{RNO4O(CF3)2}Zn]+·[H2N{B(C6F5)3}2]− (88) bearing a fluorinated aminoether−alkoxide ligand {RNO4O(CF3)2}− instead of the aminoether−phenolate in 12 (Figure 16) was also competent in the ROP of L-LA. Excellent control and good activities (Mn,calcd = Mn,GPC, Mw/Mn = 1.07, TOF ≈ 90 molL‑LA·molZn−1·h−1) were achieved under identical conditions.18 It had previously been shown that such phenolate and fluorinated alkoxide ligand scaffolds displayed

Scheme 38. Activated Monomer Mechanism Initially Proposed for the iROP of L-LA Catalyzed by the Binary Catalyst 12/ ROH18,20

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Scheme 39. Dual Organic/Organometallic ROP Catalysis with a Zinc Diamino−Phenolate Cation, a Tertiary Amine, and an Alcohol79,80

Figure 17. Herres-Pawlis’ air-stable zinc cationic complexes supported by neutral guanidines.5r,83−87

whereas the polymerization of rac-LA only produced atactic PLA. It was postulated that a dual organic/organometallic activation81 process took place, with the electrophilic zinc ion activating the bound monomer, while the amine activated the nucleophile (ROH) via hydrogen bonding (Scheme 39).79 The same cationic zinc complex 89 (and its magnesium analogue) also quantitatively polymerized 100 equiv of TMC under mild conditions (45 °C) upon association with a tertiary amine (0.2−5.0 equiv) and benzyl alcohol within 5 min.80 The ROP reactions were faster in bulk TMC than in 1.0 M toluene solutions, and TOF values as high as 18 300 molTMC·molZn−1·h−1 were recorded. They were well controlled, affording PTMCs with polydispersities in the range 1.5−1.9 (broad but quite characteristic for this monomer) and Mn,GPC ≈ Mn,calcd. The rate of monomer consumption was zero order in monomer concentration. Whether the cation [{LON2}Zn]+ was paired with the anion [B(C6F5)4]− (as in 89) or its derivatives [EtB(C6F5)3]− or [H2N{B(C6F5)3}2]− bore no incidence of catalytic activity. Although less pronounced than for lactides, the reaction rates showed a dependence of the amine, as ROP kinetics increased with amine basicity (NEt3 < N(iPr)2Et < PMP). It was also found that excessive amounts of amine vs the metal (ca. 10 equiv) led to a substantial drop of catalytic activity, presumably because at high amine concentrations, binding onto the metal cation became competitive with monomer coordination. Interestingly, the “simple” binary system 89/BnOH (i.e., without tertiary amine) was also able to catalyze the controlled ROP of TMC but under more forcing conditions (110 °C) and was less efficient (especially less controlled) than the ternary system involving addition of an amine. When the charge-neutral complex {LON2}ZnEt was also considered for comparison,82 the relative order of efficacy in the iROP of TMC performed in the presence of alcohol as a initiator/chain-transfer agent was [{LON2}Zn]+ < {LON2}ZnEt ≈ [{LON2}Zn]+/NR3.

Herres-Pawlis and co-workers developed a remarkable class of air-stable zinc cationic complexes supported by neutral N-donor ligands (guanidines)5r capable of catalyzing the ROP of rac-LA. These were claimed to be operative under industrially relevant conditions using commercial-grade monomer and without exogenous initiator. In a first report,83 they showed that the bis-ligated, dissociated salt [{DMEGe}2Zn]2+·2[CF3SO3]− (90), obtained by displacement of the triflate anions upon coordination of 2 equiv of ligand and where DMEGe is an aliphatic guanidine (Figure 17), polymerized 500−1000 equiv of monomer under aerobic conditions in the melt (135−160 °C) in 24−48 h to give PLAs with high weight-average molecular weights (Mw,GPC = 20 000−40 000 g·mol−1) and Mw/Mn = 1.6− 1.9. The optimal results were achieved at 150 °C. The anion was important, as the nondissociated salts {DMEGe}ZnX2 (X = Cl, O(CO)CH3) showed lower catalytic efficacy. The resulting PLAs were atactic.5r It was shown that in these cationic zinc complexes and in related charge-neutral ones containing imidazolin-2-imine ligands84 the activity increased with charge separation and as the calculated positive and negative Mulliken charges increased on zinc and on the Nimine atom in the ligand, respectively. Presumably the main feature in these systems was the enhanced Lewis acidity of the metallic center, leading to greater ability to bind and activate the monomer toward nucleophilic attack. The use of the neutral guanidine−pyridine hybrid ligands {TMGqu} and {DMEGqu} in [{TMGqu}Zn(CH3SO3)]+· [CH3SO3]− (91),85 [{TMGqu}Zn(CF3SO3)]+·[CF3SO3]− (92) and [{DMEGqu}Zn(CF3SO3)]+·[CF3SO3]− (93) yielded ROP catalysts of excellent value (Figure 17).86 Loose anion binding in 92 and 93 (d(Zn−O) = 2.684(3) and 2.452(7) Å, Figure 18) possessing six-coordinate zinc cations (with the coordinated triflate binding in κ2-fashion) afforded particularly active ROP catalysts. Complexes 92 and 93 quantitatively polymerized 500−1000 equiv of melted rac-LA at 130 or preferably 150 °C in 24−48 h to produce high molecular weight atactic PLAs, with Mn

L-LA,

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of catalysts. The reactions were not perfectly controlled as the polydispersity gradually increased from 1.2 to 2.2, but polymer molecular weights still increased linearly with monomer conversion. UV−vis and fluorescence measurements indicated that the ligand acted as an initiating group and remained bound to the polymer chain after workup. DFT computations (B3LYP, 6-31G(d) and 6-311 g+(d) basis sets) proposed that the neutral guanidine moiety, which is strongly nucleophilic, initiated the polymerization by an endothermic (ΔH = +13.8 kcal·mol−1) nucleophilic ring-opening attack on the lactide molecule coordinated onto the zinc cation (Scheme 40). Subsequent chain propagation occurred by exothermic (ΔH = −16.2 kcal·mol−1) insertions of the monomer into the newly created Zn+−Oalkoxide bond. The calculated highest transition state of 15.5 kcal·mol−1 matched well that determined experimentally (ΔH‡ = 18.8(1) kcal·mol−1). These features corresponded to a coordination−insertion mechanism. It was demonstrated that catalytic activity increased with the Lewis acidity of the metal cation and the nucleophilic character of the N-donor ligand (Scheme 40). Figure 18. ORTEP rendering of Herres-Pawlis’ six-coordinate [{DMEGqu}Zn(CF3SO3)]+·[CF3SO3]− (92).86

4.4. Alkaline-Earth Cations in the ROP of Cyclic Esters/Carbonates

reaching up to 88 000 g·mol−1 and polydispersities that were somewhat broad (Mw/Mn ≈ 2.0). These single-components catalysts were able to work under aerobic atmosphere and did not mind the presence of moisture and other impurities found in the technical-grade monomer. By contrast, the anion bound more tightly to the metal in 91 (d(Zn−O) = 2.103(1) Å), with the result of lowering the catalytic performances with [racLA]0/[Zn] = 500:1; yield = 33% in 48 h at 150 °C, Mn = 17 500 g·mol−1, Mw/Mn = 1.6.85 A subsequent investigation revealed the mechanism of the ROP of LA mediated by 92 and 93 under these experimental conditions.87 With the TMGqu-stabilized complex 92, the kinetic rate law r = kp·[92]·[rac-LA] was established and the activation parameters ΔH‡ = 18.8(1) kcal·mol−1 and ΔS‡ = −7.9(1) cal·K−1·mol−1 were determined by Eyring analysis. Although of the same order of magnitude as those reported elsewhere,5 these values were relatively high, in line with the high temperatures required to catalyze rac-LA with this family

Compared with zinc catalysts, there are only a few reports on alkaline-earth cationic complexes for ROP catalysis. This is mostly because of the synthetic difficulties associated with these electropositive elements, in particular, the larger elements calcium, strontium, and barium.88 Although magnesium belongs to the family of group 2 metals, its reactivity is dramatically different from that of its heavier congeners. In fact, the reactivity and structural features of complexes of the ion Mg2+ have often been compared to those of the Zn2+ analogues, because these two ions exhibit almost identical ionic radii (r) and electrostatic surface potentials (ESP).89 The ESP, defined as the ratio between electric charge and surface of the cation (ESP = q/4πr2), is a convenient way to quantify the relative hardness/softness of metal ions. Some of the pertinent physical properties of zinc and alkaline-earth metals are collated in Table 1. Because magnesium is less electron rich than zinc, chargeneutral magnesium complexes often display (much) greater reactivity than their Zn counterparts, at least in the field of

Scheme 40. Schematic Representation of the Initiation Step of the Coordination−Insertion Mechanism for the ROP of Lactide with 92 (on the basis of DFT computations)87

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dissociated ion pair [{LON2}Mg]+·[H2N{B(C6F5)3}2]− (95), which was related to the zinc compound [{LON2}Zn]+· [B(C6F5)4]− (89).80 The Mg species crystallized from THF as the [{LON2}Mg(THF)2]+·[H2N{B(C6F5)3}2]− adduct (95· (THF)2), which was structurally authenticated (Figure 19). In the presence of up to 10 equiv of BnOH, 95 catalyzed the iROP of neat TMC at 110 °C, converting quantitatively 500− 1000 equiv of monomer within 10 min. The immortal polymerizations were well controlled and took place without decarboxylation side reactions. Under identical experimental conditions, the magnesium cationic complex 95 proved ca. 1 order of magnitude more active than the congeneric zinc compound 89. Representative results are given in Table 2. On the other hand, 95 was inefficient in the ROP of TMC performed in the additional presence of a tertiary amine (NEt3), i.e., under these conditions where the ternary zincbased system 89/BnOH/NEt3 proved particularly effective in the dual organic/organometallic catalysis approach (vide supra). The aminoether−phenolate and aminoether−fluoroalkoxide ligand platforms enabled the preparation of the discrete cationic complexes [{LO N O 4 }Mg] + ·[H 2 N{B(C 6 F 5 ) 3 } 2 ] − (13) [{RNO4O(CF3)2}Mg]+·[H2N{B(C6F5)3}2]− (96), which were crystallographically identified.18,20 In stark contrast with their zinc analogues (12 and 88, respectively), they constituted poor if not entirely inactive catalysts for the ROP of L-LA even at 100 °C ([L-LA]0 = 2.0 M in toluene, [L-LA]0/[Mg]0/[BnOH]0 = 1000:1:10). Well-defined calcium, strontium, and barium cations have generated type III catalysts of great efficacy for the ROP of lactides and, to a lesser extent, trimethylene carbonate. The solvent-free cations [{LO NO4 }Met] + ·[H 2 N{B(C 6 F 5 ) 3 } 2 ] − (Met = Ca, 14; Sr, 15; Ba, 16; see Scheme 7) supported by the monoanionic, hexadentate aminoether−phenolate {LONO4}− described by Sarazin and Carpentier catalyzed the fast and controlled iROP of L-LA.18,20 These complexes were able to tolerate up to 50 equiv of alcohol initiator and up to 5000 equiv of monomer. This ability was attributed to the stabilizing nature

Table 1. Some Physical Properties of Zinc and AlkalineEarth Metallic Ions effective ionic radius for CN = 6 [Å]

electrostatic surface potential [e· nm−2]

first ionization enthalpy [kcal·mol−1]

second ionization enthalpy [kcal·mol−1]

0.74 0.72 1.00 1.18 1.35

29.1 30.7 15.9 11.4 8.7

216 176 141 131 120

414 346 273 254 230

Zn2+ Mg2+ Ca2+ Sr2+ Ba2+

ring-opening polymerization.5g,i In spite of these considerations, magnesium cations (Figure 19) have on the whole proved of limited utility as catalysts in the ROP of cyclic esters. Unlike the zinc compounds 8a and 8b, Bochmann’s magnesium cationic complexes [(Et2O)3MgX]+·[B(C6F5)4]− (X = nBu, 9a; N(SiMe3)2, 9b) did not polymerize ε-caprolactone; this was attributed to the extreme sensitivity of these unshielded species.16 The first cationic magnesium catalysts useful for the ROP of cyclic esters were Hayes’ [{(NmesPPh2)2(dbf)}Mg(nBu)]+·[WCA]− ([WCA]− = [B(C6F5)4]−, 94a; [BPh4]−, 94b),90 the direct analogues of the zinc compounds 72a and 72b (see Figure 12). The magnesium cationic complexes 94a and 94b exhibited very comparable activity in the ROP of CL at 23 °C in C6D6 ([CL]0/[Mg] = 130, [Mg]0 = 4.0 mM). They both converted 90−95% of the monomer in 4 min, without marked difference between the two anions. The kinetic rate law showed first-order dependence in monomer concentration. The resulting PCLs exhibited high molecular weights (Mn up to 130 000 g·mol−1, Mw/Mn = 1.5−1.6) and did not contain any detectable nBu end group; this latter observation suggested that these poorly controlled polymerizations proceeded via a cationic ACE mechanism. Comparative data of the respective catalytic performances of the zinc (72a−b) and magnesium (94a,b) cations were not available. As aforementioned, Carpentier, Guillaume, and Bourissou reported the synthesis of the diamino−phenolate magnesium

Figure 19. Magnesium cationic complexes as catalysts in the ROP of cyclic esters/carbonates.18,20,80,90

Table 2. iROP of Neat TMC at 110 °C Catalyzed by the Congeneric Cationic Complexes 89 (Zn) or 95 (Mg) in the Presence of BnOHa

a

catalyst

[TMC]0/[cat]0/[OH]0

t [min]

conv. [%]

Mn,calcd [g·mol−1]

Mn,GPC [g·mol−1]

Mw/Mn

TOF

89 95 95 95

500:1:5 500:1:5 500:1:10 1000:1:5

60 10 10 10

99 94 100 90

10 200 9700 5200 18 500

14 500 9800 5500 17 400

1.55 1.28 1.38 1.47

495 2820 3000 5400

TOF values are expressed in molTMC·molmetal−1·h−1.80 3595


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P-L-LA was detected. End-group analysis confirmed that the termini consisted of the expected −CH(CH3)OH and BnOC(O)CH(CH3)−. These observations were indicative of well-controlled iROP processes. Kinetic studies indicated a first-order dependence on monomer concentration. The apparent rate constants for iPrOH and BnOH ([L-LA]0/[14]0/[OH]0 = 1000:1:10, [L-LA]0 = 2.0 M, 60 °C) were kobs,iPrOH = 5.83(2) × 10−5 s−1 and kobs,BnOH = 4.72(1) × 10−5 s−1, respectively. From this study on [{LONO4}Met]+·[H2N{B(C6F5)3}2]− cationic ROP catalysts built on alkaline earths the activity trend Mg (13) ≪ Zn (12) < Ca (14 < Sr (15) < Ba (16) clearly emerged, that is, for alkaline earths it increased with the size, electropositivity, and polarizability of the metal.18 The aminoether−fluoroalkoxides [{RNO4O(CF3)2}Met]+· [H2N{B(C6F5)3}2]− (Met = Ca, 97; Sr, 98; Ba, 99) were obtained in high yields and structurally characterized.18 They formed Oalkoxide-bridged dicationic complexes (97)2−(99)2 in the molecular solid state, with all heteroatoms in the macrocyclic tether of the ligands binding to the metal atoms. In addition, the electron deficiency in these cationic complexes was such that the metallic centers required further stabilization by means of strong Met···F−C so-called secondary interactions91 with fluorine atoms from CF3 groups in the ligand framework (Figure 21).

Figure 20. ORTEP rendering of the cationic fragment in (14)2; ortho and para tBu groups omitted for clarity.20

of the ancillary ligand which contained as many as four O atoms in the heterocyclic side arm, providing a particularly effective protection to the oxophilic metallic centers. The calcium complex 14 was recrystallized as the dimeric [{LONO4}2Ca2]2+· 2[H2N{B(C6F5)3}2]− ((14)2), with two Ophenolate-bridged sevencoordinate Ca atoms (Figure 20). The iROP of L-LA was catalyzed by 14−16 at 30−60 °C upon addition of 5−50 equiv of iPrOH or BnOH as an initiator/chain-transfer agent (Table 3).18 The agreement between experimental and theoretical molecular weights was generally satisfactory except for the overly active Sr and Ba systems, and the molecular weight distribution was narrow (Mw/Mn = 1.06−1.3). The binary catalytic system 14−16/ iPrOH therefore proceeded in the expected fashion for the ROP of L-LA, that is, according to an activated monomer mechanism. The Sr and Ba analogues 15 and 16 yielded highly active binary catalysts in the presence of iPrOH, allowing rapid conversion of the monomer even at 30 °C with TOFs reaching 6000 molL‑LA·molBa−1·h−1. However, the observed molecular weights did not perfectly match their calculated values presumably because of transesterification side reactions. The Ca derivative 14 offered the best compromise between activity and level of control. Full conversion of up to 3000 equiv of L-LA was reached in 24 h at 60 °C. The control of the ROP parameters was excellent (Mn,GPC ≈ Mn,calcd; Mw/Mn = 1.06−1.12). There was no influence of the contents in BnOH on the catalytic activity in the concentration range examined, and at full conversion the molecular weights varied linearly with the [L-LA]0/[OH]0 ratio. The identity of the initiator had little influence on the activity and none on the molecular weight features. No epimerization of the optically active centers of

Figure 21. Calcium, strontium, and barium aminoether−fluoroalkoxides [{RNO4O(CF3)2}Met]+·[H2N{B(C6F5)3}2]− with an ORTEP rendering of the dication in (98)2 showing Sr···F interactions in the molecular solid state.18

The metal−fluorine distances in these strong interactions (ca. 25−40 kcal·mol−1 according to DFT computations) were well below the sum of the van der Waals radii for the

Table 3. iROP of L-LA Catalyzed by [{LONO4}Met]+·[H2N{B(C6F5)3}2]− + ROH Binary Systemsa catalyst

ROH

[L-LA]0/[cat.]0/[OH]0

Tre [°C]

t [h]

yield [%]

12 13 14 14 14 14 14 15 16 16

iPrOH iPrOH iPrOH BnOH BnOH BnOH BnOH iPrOH iPrOH iPrOH

1000:1:10 1000:1:10 1000:1:10 1000:1:10 3000:1:10 1000:1:20 1000:1:50 1000:1:10 1000:1:10 1000:1:10

100 100 60 60 60 60 60 30 100 30

16 3 11 24 24 8 8 1 0.05 1

99 traces 90 97 96 78 83 48 30 34

TOF [h−1]


Mn,GPC [g·mol−1]

Mw/Mn

62

14 300

13 300

1.25

82 40 120 97 104 480 6000 340

13 000 14 000 41 600 5700 2500 7000 4400 5000

12 900 12 800 30 000 5800 2400 14 000 11 000 13 000

1.07 1.10 1.06 1.07 1.12 1.20 1.21 1.27

Polymerizations carried out in toluene with [L-LA]0 = 2.0 M. TOF values expressed in molL‑LA·molmetal−1·h−1. Mn,calcd = [L-LA]0/[ROH]0 × yield ×144.13 + MROH, with MBnOH = 108 g·mol−1 and MiPrOH = 60 g·mol−1.18,20 a

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Table 4. iROP of L-LA Catalyzed by [{RNO4O(CF3)2}Met]+·[H2N{B(C6F5)3}2]− + BnOH Binary Systemsa catalyst

[L-LA]0/[cat.]0/[OH]0

Tre [°C]

t [h]

yield [%]

TOF [h−1]


Mn,GPC [g·mol−1]

Mw/Mn

97 98 99 98 98 98 98 98

1000:1:10 1000:1:10 1000:1:10 1000:1:10 1000:1:5 1000:1:20 1000:1:50 2000:1:5

100 100 100 100 100 100 100 100

3 3 3 6 1.5 1.5 1.5 24

47 74 50 89 34 57 75 85

157 247 167 148 227 380 500 71

6900 10 800 7300 12 900 9900 4200 2300 49 100

6700 9900 6800 12 100 9300 4100 2400 35 000

1.12 1.17 1.16 1.30 1.09 1.10 1.10 1.41

f

Polymerizations carried out in toluene with [L-LA]0 = 2.0 M. TOF values expressed in molL‑LA·molmetal−1·h−1. Mn,calcd = [L-LA]0/[ROH]0 × yield ×144.13 + MROH, with MBnOH = 108 g·mol−1.18

a

ΔS‡ = −7.6(2.0) cal·K−1·mol−1 were calculated by Eyring analysis in the temperature range 85−100 °C. The apparent rate constants for the iROP of L-LA catalyzed by addition of BnOH to [{RNO4O(CF3)2}Met]+·[H2N{B(C6F5)3}2]−, 88 (for Zn) and 96−99, were determined by NMR monitoring of monomer conversion in toluene-d8 under identical experimental conditions. The Mg derivative 96 exhibited hardly any catalytic activity, while by contrast kobs of 0.0001, 0.0004, 0.0013, and 0.0014 s−1 were calculated for 88, 97, 98, and 99, respectively, at 100 °C. The catalytic activity hence followed Mg (96) ≪ Zn (88) < Ca (97) < Sr (98) ≈ Ba (99), that is, it increased with the ionic radius of the metal, in agreement with the trend seen with 12−16 containing the aminoether−phenolate ligand {LONO4}−. In a subsequent study, Sarazin and co-workers focused on the most effective of the metals, barium, where the metal cation was supported by related phenolate or fluoroalkoxide ligands (Figure 22).92 Combined with BnOH (10 equiv), the cationic complex [{LONO2}Ba]+·[H2N{B(C6F5)3}2]− (100) supported by a potentially tetradentate aminoether−phenolate ligand displayed high activity in the ROP of L-LA even at 30 °C ([L-LA]0/ [100]0/[BnOH]0 = 1000:1:10, [L-LA]0 = 2.0 M in CH2Cl2), achieving complete conversion of the monomer within 15 min but with poor control of the parameters (Mn,calcd = 12 600 g· mol−1, Mn,GPC = 8300 g·mol−1, Mw/Mn = 1.8−1.9; TOF = 3840 molL‑LA·molBa−1·h−1). Regarding the influence of the ligand framework (the anion [H2N{B(C6F5)3}2]− was used in all cases), under comparable conditions, the catalytic activity decreased when the overall stabilizing ability of the ligand increased: {RNO4O(CF3)2}− (as in 99, TOF = 77 molL‑LA·molBa−1·h−1 at 100 °C) < {LONO4}− (16, TOF = 1820 molL‑LA·molBa−1·h−1 at 30 °C) < {LONO2}− (100, TOF = 3840 molL‑LA·molBa−1·h−1 at 30 °C). The best compromise between activity and control of the parameters was obtained with 16. This catalyst could operate with high control at 0−30 °C in the presence of 10−100 equiv of BnOH (or other initiators, e.g., benzylamine, 1,3-propanediol or alkoxyamines) as an initiator/chaintransfer agent and 1000−5000 equiv of monomer (Table 5).92 The polymerization of rac-LA catalyzed by 16/BnOH proceeded along the same lines but only afforded atactic PLA. The exact mechanism of the iROP catalyzed by these systems was later revealed, following a combined DFT and experimental investigation by Carpentier, Sarazin, and co-workers. These authors had initially been unable to reconcile the rate law expressed in eq 1 with the classical activated monomer mechanism described in Scheme 38. With the added knowledge that the excess alcohol (BnOH, iPrOH) necessary for iROP catalysis could displace ancillary phenolate or fluoroalkoxide ligands from the metal cation under polymerization conditions,93

considered elements. For each metal atom, one found one Ca···F interaction in 97 (d(Ca−F) = 2.664(3)−2.681(4) Å), one Sr···F interaction in 98 (d(Sr−F) = 2.741(6)−2.859(7) Å), and two Ba···F interactions in 99 (d(Ba−F) = 2.935(3)− 2.992(3) Å). There was no Met···F interactions with the counterion, and like their Zn and Mg analogues, the three compounds 97−99 existed in solution and in the solid state as dissociated ion pairs. Although less active than the aminoether−phenolate complexes 14−16, which was attributed to the stabilizing effect of intramolecular Met···F interactions, effective iROP binary catalytic systems were generated upon addition of BnOH to the solvent-free fluorinated complexes 97−99 (Table 4).18 Polymerization occurred at 100 °C with [L-LA]0/[M+]0/ [ROH]0 = 1000:1:10. General features of the systems 97− 99/BnOH included (i) relatively low catalytic activity (compared to 14−16) requiring a high polymerization temperature with TOFs in the range 100−500 molL‑LA·molmetal−1·h−1, (ii) good control over the polymerization (Mn,calcd ≈ Mn,GPC and Mw/Mn = 1.1−1.2), and (iii) the absence of epimerization of the chiral centers in the P-L-LAs. Importantly, the combination of 97 and BnOH proved totally unable to catalyze the ROP of TMC under conditions where the polymerization of L-LA occurred rapidly. At 100 °C, the catalytic system 98/BnOH based on strontium polymerized up to 2000 equiv of L-LA in controlled fashion, with a linear increase of molecular weights with conversion. The molecular weight distribution remained narrow until 80% conversion, when broadening occurred as a result of transesterification reactions. Up to 50 equiv of BnOH could be used without detrimental effect to the catalytic activity or to the control: the activity increased regularly with alcohol contents, indicating kinetic dependence on [BnOH]0, whereas molecular weights decreased linearly with increasing [BnOH]0. End-group analyses showed that the polymers possessed −CH(CH3)OH and BnOC(O)CH(CH3)− termini, confirming the likelihood of a controlled activated monomer mechanism. NMR monitoring of the reaction of L-LA and 1 equiv of BnOH in the presence of 0.1 equiv of 98 showed formation of the ring-opened product BnOC(O)CH(CH3)OC(O)CH(CH3)OH after 1 h in CD2Cl2 at 25 °C. Moreover, the reaction of L-LA, 0.5 equiv of BnOH, and 0.05 equiv of 98 in toluene-d8 at 100 °C gave the doubleinsertion product BnOC(O)CH(CH3)[O−C(O)−CH(CH3)]2OC(O)CH(CH3)OH within 10 min. The system 98/BnOH was also used for kinetic studies performed by 1H NMR spectroscopy at 100 °C in toluene-d8, and the kinetic rate law r = kp·[L-LA]1.0·[98]1.0·[BnOH]1.0 (1) was established. The activation parameters ΔH‡ = 14.8(5) kcal·mol−1 and 3597


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Figure 22. Comparative activity and polydispersity data for the iROP of L-LA catalyzed by a variety of discrete barium cationic complexes. Polymerization in CH2Cl2 at 30 °C (for 16 and 100) or in toluene at 100 °C (for 99) [L-LA]0:[Ba]0:[BnOH]0 = 1000:1:10, [L-LA]0 = 2.0 M. TOF values expressed in molL‑LA·molmetal−1·h−1.92

Table 5. iROP of L-LA Catalyzed by Barium Cationic Complexes 16, 99, and 100 Combined with BnOHa

a

catalyst

[L-LA]0/[Ba]0/[BnOH]0

Tre [°C]

t [h]

yield [%]

TOF [h−1]


Mn,GPC [g·mol−1]

Mw/Mn

100 99 16 16 16 16 16

1000:1:10 1000:1:10 1000:1:10 1000:1:10 1000:1:50 2500:1:100 5000:1:100

30 100 30 0 30 30 30

0.25 6.5 0.5 6.5 0.5 5.5 24

96 50 91 99 97 97 95

3840 77 1820 152 1940 441 198

12 600 7300 13 300 14 300 2900 3600 7000

8300 5600 10 200 8 500 2600 3000 5300

1.85 1.23 1.33 1.08 1.15 1.12 1.08

Polymerizations carried out in CH2Cl2 (for 16 and 100) or toluene (for 99) with [L-LA]0 = 2.0 M.92

with the release of fluoroalcohol (step B), and acyl cleavage with recapture of the proton regenerates the initial metal species while releasing the ring-opened (macro)molecule (P)-L-LA−OH (step C). In this concerted 6-center mechanism, the fluoroalkoxide is not a spectator ligand but participates actively to the catalytic cycle through activation of the nucleophile. This ligand-assisted activated monomer mechanism can be compared to the dual activated mechanism described for 89 associated to an alcohol and an amine (see Scheme 39).79,80 It is also very reminiscent of the operative mechanism of some metalloenzymes, for instance that of the zinc-dependent deacetylase LpxC.94b Computations at the DFT (B3PW91) level for the strontium cationic complexes 15 and 98 confirmed the viability of this scenario (Figure 23).94a The corresponding calculated free energies of activation (19.0−22.0 kcal·mol−1) were commensurate with the experimental values. Calculations for the two Sr cations 15 and 98 (ΔG‡ = 22.0 and 19.0 kcal·mol−1 for the ratelimiting step, respectively) agreed with the trend {RNO4O(CF3)2}− < {LONO4}− established experimentally for the impact of the ligand on catalytic activity in these systems.18 Carpentier, Sarazin, and co-workers synthesized fivecoordinate β-diketiminate cationic complexes of calcium and strontium [{BDIDipp2}Met(pyridine)3]+·[H2N{B(C6F5)3}2]− (Met = Ca, 101; Sr, 102) stabilized by three molecules of pyridine (Figure 24).95 There was no contact between the cations and their counterion. Cations supported by the otherwise highly versatile {BDIDipp2}− ligand generally proved to be highly reactive but rather unstable. The presence of the strongly basic pyridine in 101 and 102 was mandatory, as with THF or worse in the absence of solvent coligands the complexes decomposed

Scheme 41. Proposed Ligand-Assisted Activated Monomer Mechanism for the iROP of L-LA Catalyzed by 97−99/ BnOH94a

they proposed the concept of ligand-assisted activated monomer mechanism (Scheme 41).94a In this scenario, the monomer is classically activated by coordination onto the cationic metal atom, while the nucleophile (the initiator BnOH for the first ring opening or the growing macro-alcohol after this) is activated through hydrogen bonding with the fluoroalkoxide (step A). Addition of RO− onto the activated carbonyl occurs concomitantly 3598


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co-workers in 2011 (Scheme 42).96 Both compounds were structurally characterized by XRD crystallography and exhibited six-coordinated calcium atoms. The simple [(BH4)Ca(THF)5]+·[BPh4]− (103) initiated the rapid ROP of CL, fully converting 200 equiv within 2 min at room temperature to yield the α,ω-dihydroxytelechelic PCLs that are characteristic of ROP reactions initiated by tetrahydroborate catalysts.97 Complex 103 initiated the living ROP of rac-LA with good control (except for slightly broad polydispersity, ca. 1.4, due to transesterification processes) and end-group fidelity, but rather slowly, the conversion of monomer taking up to 3−5 h at 70 °C in THF to reach completion. The resulting PLAs were essentially atactic. The ROP of L-LA occurred in a similar manner and without epimerization of the chiral centers. The Tpm-supported [{Tpm}Ca(BH4)(THF)5]+·[BPh4]− (104) showed enhanced activity compared to 103, which was attributed to the ability of the charge-neutral Tpm ancillary ligand to prevent the formation of multinuclear or chain-chelated catalyst resting state. Thus, 104 converted 250 equiv of rac-LA in 2−4 h at room temperature to give atactic PLAs with experimental molecular weights slightly higher than their calculated values and with fairly narrow molecular weight distributions (Mw/Mn = 1.2−1.4). Although not cationic per se, very interestingly the related zwitterionic tris(pyrazolyl)borate complex {TptBu,Me}Ca(BH4)(THF) (105) was an extremely active catalyst (converting 200 equiv of monomer in 5 min at 0 or even −20 °C) for the controlled and heteroselective ROP of rac-LA, yielding PLAs with Pr values as high as 0.88−0.90, a feast not equaled with any other alkalineearth ROP catalyst.

Figure 23. Computed reaction pathway for the iROP of L-LA promoted by 15 or 98 via the ligand-assisted activated monomer mechanism depicted in Scheme 41. MeOH was used as a model of the external nucleophile.94a

4.5. Rare-Earth and Actinide Cations Figure 24. Five-coordinate [{BDIDipp2}Met(pyridine)3]+·[H2N{B(C6F5)3}2]− complexes 101 and 102 with an ORTEP rendering of the cation in 102 (Sarazin/Carpentier).95

Examples of discrete lanthanide (and even more so actinide) cationic complexes used in the catalysis of ROP reactions are very seldom. Most involve the metals in their most common trivalent oxidation state, with only one case of divalent rareearth cations reported at the time of writing. Such paucity perhaps owes much to the extreme electrophilicity and air and moisture sensitivity of the compounds under consideration. 4.5.1. Cationic Complexes of Trivalent Rare Earths. Lewis-acidic rare-earth salts, such as triflates, are known to promote the controlled living/immortal ROP of cyclic esters in the presence of a protic nucleophile; of note is the work of Nomura,98 but such species do not qualify as discrete cationic species since the anion is firmly bound to the metal atom and are therefore beyond the scope of this survey. In 2007, Shen and co-workers reported that the phenolate cationic complex [{2,6-tBu 2 -4-Me-C 6H 2 O}Sm(DME) 2 ]+· [BPh4]− (106), obtained by one-electron oxidation of [{2,6tBu2-4-Me-C6H2O}Sm(THF)3 with the silver salt AgBPh4, was a very fast catalyst for the polymerization of CL (Figure 25).99 A 1000 equiv amount of monomer was converted quantitatively

rapidly in solution. In addition, it was not possible to synthesize the corresponding barium complex. Upon association with benzyl alcohol, 101 and 102 catalyzed the rapid iROP of L-LA ([L-LA]0/[Met]0/[OH]0 = 1000:1:10) at 30 °C in toluene. The reactions were controlled and proceeded with good end-group fidelity. The strontium complex 102 (yield = 82% in 10 min, TOF = 4920 molL‑LA·molmetal−1·h−1; Mn,calcd = 11 900 g·mol−1, Mn,GPC = 10 400 g·mol−1, Mw/Mn = 1.2) was much more active than its calcium counterpart (yield = 66% in 180 min, TOF = 220 molL‑LA·molmetal−1·h−1; Mn,calcd = 9600 g·mol−1, Mn,GPC = 7300 g·mol−1, Mw/Mn = 1.3), making it one of the best catalysts in terms of activity combined with good control for the ROP of L-LA. The preparation of two calcium tetrahydroborate cations, [(BH4)Ca(THF)5]+·[BPh4]− (103), and the more sterically shielded [{Tpm}Ca((THF)5]+·[BPh4]− (104, where Tpm = tris(pyrazolyl)methane) was reported by Mountford and

Scheme 42. Mountford’s Calcium Tetrahydroborate Cations for the ROP of Cyclic Esters96

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Figure 25. Trivalent rare-earth discrete cations for the catalysis of the ROP of cyclic esters: Shen (106)99 and Okuda (107−110 and 111a,b).100,101

within 3 min at room temperature in five volumes of toluene, and as much as 3000 equiv were polymerized in 15 min. The reactions, however, proceeded with limited control to give high monomodal molecular weight PCLs (Mn = 50 000−120 000 g· mol−1) that exhibited broad polydispersities (Mw/Mn = 1.8− 2.1). The polymer molecular weights generally increased linearly with monomer conversion. The ROP was proposed to follow a coordination−insertion mechanism, with insertion of CL into the Sm−Ophenolate bond via acyl cleavage. Okuda and co-workers synthesized in good yields the monocationic bis(tetraborohydride) lanthanide complexes [(BH4)2Ln(THF)5]+·[BPh4]− (Figure 25; Ln = Y, 107; La, 108; Nd, 109; Sm, 110) by treatment of the tris(borohydride) precursors with [Et3NH]+·[BPh4]−.100 These dissociated ion pairs are the lanthanide analogues of the calcium [(BH4)Ca(THF)5]+·[BPh4]− (103). They were characterized by X-ray diffraction, and all featured seven-coordinate metal atoms with two η3-bound tetrahydroborate ligands in axial positions and all THF in equatorial sites (Figure 26). Complexes 107−110

to be little differences between the other three complexes, which produced ca. 3.3−4.5 polymer chains of medium molecular weights (Mn = 13 000−17 400 g·mol−1) per metal atom. Related to this study, Maron, Okuda, and co-workers examined the mechanisms of the ROP of CL mediated by the yttrium cation [(X)YMe(THF)5]+ (X = BH4, 111a; NMe2, 111b) using DFT computations;101 this cation had previously been synthesized as the [BPh4]− salt by reaction of [YMe(THF)6]2+·2[BPh4]− with NaBH4.102 They showed that with the tetrahydroborate 111a, initiation (first insertion) of the ROP reaction could take place on both the methyl and the borohydride side with comparable activation barriers (ΔG‡ = +24.5 and +27.6 kcal·mol−1, respectively) to give ring-opened products of comparable stability (ΔG° = −39.3 and −33.2 kcal· mol−1, respectively). This was consistent with experimental data, from which determination of the reactive group in 111a[CH3]− vs [BH4]−proved impossible. A beneficial trans effect was highlighted upon replacement of [BH4]− in 111a by the more donating [NMe2]− in 111b. The activation barrier then dropped to a mere +14.2 kcal·mol−1 with the amido group, rendering the initial insertion of CL in the Y−CH3 bond kinetically much more facile, while the thermodynamics were hardly impacted (ΔG° = −38.6 kcal·mol−1 with 111b). Mountford and co-workers showed that the yttrium− isopropoxide dicationic complex [{Tpm}Y(OiPr)(THF)3]2+· 2[BPh4]− (112, Figure 27), prepared by treatment of

Figure 26. ORTEP rendering of the cation in [(BH4)2Nd(THF)5]+· [BPh4]− (109) showing the H atoms of the tetraborohydride moieties.100 Figure 27. Mountford’s dicationic catalyst [{Tpm}Y(OiPr)(THF)3]2+· 2[BPh4]− for the ROP of rac-LA.103

constituted remarkably efficient catalysts for the ROP of CL. They polymerized 500 equiv of monomer at 19 °C within 30 s to give PCLs with relatively narrow molecular weight distributions (Mw/Mn = 1.2−1.4). Except for the samarium complex 110 which gave comparatively low molecular weight PCL with ca. 7.0 polymer chains per metal atom, there seemed

[Y(CH2SiMe3)(THF)5]2+·2[BPh4]− with the ligand Tpm and isopropanol, was an excellent catalyst for the controlled iROP of rac-LA, proceeding according to an activated monomer mechanism.103 The yttrium complex 112 can to some extent be 3600


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The molecular structure of 116 featured an Oalkoxide-bridged dimeric structure, with eight-coordinate YbII atoms due to the presence of a strong intramolecular Yb···F interaction about each metal (d(Yb−F) = 2.726(9) and 2.711(7) Å). These complexes exist as dissociated ion pairs without interaction with the anion, and the fluorine contacts in 116 were with fluorine atoms from the ligand backbone. In the presence of BnOH (10−40 equiv), the aminoether− fluoroalkoxides 116 (at 100 °C) and 117 (at 60 °C) catalyzed very competently the controlled iROP of L-LA (1000− 2000 equiv) in toluene (Table 6); chloroform could also be used with an identical effect. The resulting polymers were monodisperse, and end-group fidelity was attested by MALDIToF MS and NMR analyses. Except for one case (vide infra), no indication was found that oxidation of the complexes to trivalent rare-earth compounds occurred under polymerization conditions. The aminoether−phenolate europium(II) complex 115 proved particularly efficient, but interestingly, its ytterbium(II) analogue 114 oxidized and was totally inactive; this possibly highlights the important influence of the different redox ability between the two metals (Yb3+/Yb2+, E° = −1.05 V; Eu3+/Eu2+, E° = −0.35 V). Importantly, under identical conditions, the cationic divalent rare-earth fluoroalkoxides 116 (YbII) and 117 (EuII) were superior by at least an order of magnitude to their direct Ca (97) and Sr (98) congeners. This was evident in particular from the 1H NMR kinetic monitoring of reactions carried out in CDCl3 at 60 °C with 116 (kobs = 21.3 × 10−5 s−1; Ea = +18.6 kcal·mol−1) and 97 (kobs = 2.60 × 10−5 s−1). Interestingly, both divalent ytterbium complexes 114 (TOF = 5280 molTMC·molYb−1·h−1) and 116 (TOF = 7200 molTMC·molYb−1·h−1) were able to polymerize TMC at 30 °C in toluene ([TMC]0/[YbII]0/[BnOH]0 = 1000:1:10) in a controlled fashion (i.e., no oxidation/ decomposition of 114 was detected), whereas under these or more forcing conditions the calcium complex 97 was totally inactive.94a The mechanism of these ROP reactions catalyzed by divalent ytterbium and europium cationic complexes was proposed to follow the same ligand-assisted activated monomer mechanism as that described for alkaline-earth compounds (see Scheme 41 and Figure 23). It is likely that the variations observed in the reactivity of these families of alkaline- and rareearth complexes can be related to the different bonding situation, with lower ionicity for ytterbium(II) and europium(II) compared to calcium and strontium.104 4.5.3. Cationic Complexes of Actinides. The single example of a cationic actinide complex used in ROP catalysis was a uranium(IV) compound of formula [U(NEt2)3]+·[BPh4]− (118), as reported by Eisen and co-workers (Figure 29).105 This complex, first prepared by reaction of U(NEt2)4 and

related to the calcium complex 104, although they differ by the nature of their respective reactive nucleophiles. Be it with 0 (living) or 5 (immortal) equiv of iPrOH per metal atom, 112 mediated the controlled ROP of rac-LA (100 equiv) in THF at 70 °C. Chain growth proceeded linearly with monomer conversion (which followed first-order kinetics in monomer concentration). Benzylamine (5 equiv) could also be used as an effective chain-transfer agent. End-group fidelity, showing the presence of iPrO− (with added iPrOH) and mostly BnNH− (with added BnNH2) termini, was established by MALDI-ToF MS. All PLLAs produced by 112 and iPrOH or BnNH2 were atactic. Interestingly, in the same publication, the authors showed that the zwitterionic [{ONNMe2O}Y{ONNMe2OH}] (113; {ONNMe2O}H2 = Me2NCH2CH2N(CH2-2-OH-3,5-CH2tBu2)2) with a negatively charged yttrium atom bearing two bis(phenolate) ligands generated highly heterotactic PLA (Pr up to 0.93) under very mild conditions (20 min, room-temperature THF).103 4.5.2. Cationic Complexes of Divalent Rare Earths. Examples of discrete cationic complexes of divalent rare earths are confined to the aminoether−phenolate-supported [{LONO4}Met]+·[H2N{B(C6F5)3}2]− (Met = YbII, 114; EuII, 115) and their aminoether−fluoroalkoxo analogues [{RNO4O(CF3)2}Met]+·[H2N{B(C6F5)3}2]− (Met = YbII, 116; EuII, 117) (Figure 28).94a Since YbII and EuII (1.02 and 1.17 Å,

Figure 28. Discrete cationic ROP catalysts of divalent ytterbium and europium (Sarazin/Carpentier).94a

respectively, for CN = 6) have ionic radii almost identical to those of the Ca2+ and Sr2+ ions (1.00 and 1.18 Å), the divalent rare-earth cationic complexes 114−115 and 116−117 can be seen as the direct congeners of the alkaline earths 14−15 and 97−98, respectively. All are d0, highly electrophilic complexes. Unlike the strongly paramagnetic Eu complexes 115 and 117 (4f76s0), the YbII compounds 114 and 116 are diamagnetic (4f146s0) and NMR active.

Table 6. iROP of L-LA Catalyzed by 114−117 + BnOH Binary Systemsa catalyst

[L-LA]0/[cat.]0/[OH]0

Tre [°C]

t [min]

yield [%]

TOF [h−1]


Mn,GPC [g·mol−1]

Mw/Mn

114 115 116 116 116 116 117

1000:1:10 1000:1:10 1000:1:10 2000:1:10 2000:1:20 2000:1:40 1000:1:10

100 60 100 100 100 100 60

180 30 60 180 180 180 180

0 94 90 96 97 99 74

0 1880 900 384 392 396 247

13 600 13 000 27 800 14 100 7200 10 800

13 500 12 500 23 800 12 200 7900 10 000

1.04 1.13 1.03 1.03 1.05 1.05

Polymerizations carried out in toluene with [L-LA]0 = 2.0 M. TOF values expressed in molL‑LA·molmetal−1·h−1. Mn,calcd = [L-LA]0/[BnOH]0 × yield ×144.13 + MBnOH, with MBnOH = 108 g·mol−1.94a

a

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Figure 29. Eisen’s uranium(IV)−tris(amide) cationic catalyst for the ROP of CL and L-LA.105

[HNEt3]+·[BPh4]−,106 proved an effective catalyst for the controlled living ROP of cyclic esters. In THF, 600 equiv of CL was quantitatively converted within 15 min at 25 °C or within 5 min at 70 °C to give narrowly dispersed PCLs (Mw/Mn = 1.2, Mn = 57 000 and 59 000 g·mol−1, respectively). Nearstoichiometric studies suggested the existence of an activated monomer ROP mechanism. Although slower, the ROP of L-LA (600 equiv) in THF was also well controlled, with partial conversion of the monomer at 25 (yield = 60% after 18 h; Mw/Mn = 1.1, Mn = 39 000 g·mol−1) or 70 °C (yield = 76% after 45 min; Mw/Mn = 1.2, Mn = 43 000 g·mol−1); the kinetic rate law was zeroth order in [L-LA].

Figure 30. ROP of CL catalyzed by discrete zirconium cations (Hayakawa).108

PCLs (Mw/Mn = 1.06−1.3); however, the reactions were slow, and the initiation efficiency, measured as the ratio between calculated and observed molecular weights, was limited. Yet it increased with steric bulk of the ligands, probably because in these cases the formation of catalytically inactive bimetallic compounds was increasingly prohibited.109 The best catalytic system based on 121 could polymerize CL with good control (Mw/Mn < 1.13, Mn,calcd ≈ Mn,GPC), and the molecular weight of the resulting PCLs increased with monomer conversion. It also proved efficient for the controlled living polymerization of δ-valerolactone (VL, 100 equiv; Mw/Mn = 1.2−1.3) and DOP (100 equiv; Mw/Mn = 1.2) as well as the copolymerizations of these monomers, but it was inactive toward γ-butyrolactone, TMC, and a 13-membered cyclic lactone. The reactions mediated by these catalytic systems were said to proceed according to a cationic ACE mechanism with O-alkyl bond cleavage. Hadjichristidis and co-workers subsequently highlighted that the multiple combinations deriving from the use of Cp2ZrMe2, Ind2ZrMe2 (Ind = indenyl), or {C5H4tBu}2ZrMe2 with B(C6F5)3, [Ph3C]+·[B(C6F5)4]−, or [HNMe2Ph]+·[B(C6F5)4]− as activator generated active zirconocenium catalysts for the controlled ROP of CL and VL.110 For instance, nearquantitative conversion of CL (100 equiv) was achieved with Cp2ZrMe2/B(C6F5)3 within 3−24 h at 25 °C, depending on the concentrations on the reactants (reaction rates increased with the concentrations), and narrowly dispersed PCLs were obtained, Mw/Mn < 1.10. Depending on the choice of zirconocene complex and activator, the produced PCLs typically featured molecular weights in the range 10 000−38 000 g·mol−1 with monomodal distributions. The polymerization of VL proceeded along the same lines (yield > 90% after 24 h at 25 °C), affording polymers of slightly lower molecular weights (Mn = 12 000−18 000 g·mol−1) with broader polydispersities (Mw/Mn = 1.1−1.3). Block copolymers of CL or VL with methyl methacrylate (MMA) were obtained by sequential polymerization, starting with the polymerization of MMA. The same group also polymerized substituted oxazolines using Cp2ZrMe2 or the hafnocene {C5H4tBu}2HfMe2 in combination with B(C6F5)3 or [HNMe2Ph]+·[B(C6F5)4]−.111 The compounds [Cp2ZrMe]+·[X]− ([X]− = [MeB(C6F5)3]−, 122; [B(C6F5)4]−, 7) and [{C5H4tBu}2HfMe]+·[B(C6F5)4]− (123) polymerized 2-Me-oxazoline and 2-Ph-oxazoline at 120 °C over 24 h in acetonitrile to give low molecular weight poly(oxazoline)s with narrow polydispersities (Mn,GPC = 1000− 6000 g·mol−1 and Mw/Mn =1.1−1.2). These ROP reactions, which were assumed to proceed according to a cationic ACE

4.6. Cations of Group 4 Metals

Group 4 metal cations have been employed very early to promote the ROP of cyclic esters and related monomers, but overall their use has been fairly little spread. Hayakawa and co-workers first utilized the well-known [Cp2ZrMe]+·[B(C6F5)4]− (7) to initiate the controlled, living ROP of CL (affording PCL with narrow polydispersities, Mw/ Mn = 1.06−1.3)107 or cyclic carbonates such as 1,3-dioxepan-2one (DOP) (Scheme 43).14 Due to the limited solubility of Scheme 43. Hayakawa’s Polymerization of DOP Initiated by [Cp2ZrMe]+·[B(C6F5)4]− 14

DOP and solvent contingencies, a mixture of toluene and γ-butyrolactone (an inert 5-membered lactone) was used as the reaction medium. The polymerizations were carried out at room temperature, with [DOP]0/[7]0 = 100:1. The kinetics were grossly first order in DOP concentration, and the molecular weights increased linearly with monomer conversion to give monodisperse PDOPs (Mw/Mn = 1.1−1.2, Mn up to 8500 g·mol−1) as colorless solids. The preparation of a diblock copolymer and CL and DOP was achieved by well-controlled sequential polymerizations of DOP (100 equiv; Mw/Mn = 1.2, Mn = 8500 g·mol−1 for the resulting PDPO prepolymer) followed by CL (100 equiv). The final material featured the expected macromolecular features, Mw/Mn = 1.2, Mn = 16 000 g·mol−1. The mechanism of these polymerizations was presumed to be a cationic ACE. Building on these initial results, Hayakawa and co-workers showed that the zirconocenium complexes 7 and 119−121, featuring varying levels of steric protection, initiated the ROP of CL and related monomers (Figure 30).108 The quantitative polymerization of CL (100 equiv) in toluene at room temperature or 60 °C afforded monodisperse 3602


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per Ti atom) in toluene at 60−75 °C.113 The chloro compound 125 was somewhat more active than the bromo derivative 126: monomer conversion was in the range 11−61% for 125 and 6−22% for 126 after 8 days. Both afforded high molecular weight monodisperse PCLs (Mn = 43 000−112 000 g·mol−1 and Mw/Mn = 1.3−1.6).

Scheme 44. Cationic Activated Chain End (ACE) ROP of Oxazolines Mediated by Group 4 Cations (Pitsikalis)111

4.7. Cations of Late Transition Metals

Only a handful of discrete cationic complexes of late transition metals, mainly based on silver and ruthenium, have been mentioned to catalyze the ROP of cyclic esters. Ghosh, Sunoj, and co-workers reported a range of bis(Nheterocyclic carbene) (NHC) silver complexes paired with a dissociated chloride anion that could catalyze the ROP of L-LA. All were structurally characterized. The first one, [{1-isopropyl3-(N-phenylacetamido)imidazol-2-ylidene}2Ag]+·[Cl]− (127) bearing a functionalized side arm, was prepared by stoichiometric reaction of Ag2O with the imidazolium salt (Figure 32).114 This compound, which was found to be stable up to 180 °C by TGA, polymerized melted L-LA ([L-LA]0/[Ag]0 = 100:1) in 4 h at 120−180 °C, to give low molecular weight P-L-LAs with reasonably narrow polydispersities (Mn = 4100− 10 400 g·mol−1, Mw/Mn = 1.2−1.5). The molecular weights decreased with increasing temperatures, which was attributed to thermal depolymerization115 and increased rates of transesterification reactions. The reactions could be carried out with 50−1000 equiv of monomer per metal atom, but partial conversions only were obtained at high loadings, while the molecular weights of the polymers did not increase linearly with the degree of polymerization, suggesting limited control over the reaction parameters. End-group analyses (NMR, MALDIToF MS) demonstrated the presence of NHC moieties at the end of the polymer chains. This could have resulted from initial monomer insertion in at least one of the two Ag−NHC bonds or from release of minute amounts of NHC acting as the organocatalyst at high temperature (NHCs are known to competently catalyze ROP reactions7). The complex [{1-mesityl-3-(N-phenylacetamido)imidazol-2ylidene}2Ag]+·[Cl]− (128) was obtained following an identical protocol and was also stable up to 180 °C.116 It polymerized 50−300 equiv of L-LA in the melt at 160 °C to high conversions (75−92%) within 4 h. Compared to 127, the ROP reactions catalyzed by the more sterically congested 128 were more controlled; the resulting P-L-LAs featured narrower molecular weight distributions (Mw/Mn = 1.2−1.3), whereas the molecular weights (Mn = 3000−7600 g·mol−1) increased regularly with monomer conversion, and no sign of thermal depolymerization could be detected. Overall, the presence of the functionalized side arm in 127 and 128 seemed to have little influence on the catalytic ability. Using the simpler [{1,3diisopropyl-imidazol-2-ylidene}2Ag]+·[Cl]− (129) as a model compound (Figure 32), computational studies were carried out

mechanism (Scheme 44), were fairly well controlled but not living. There was very little difference in the catalytic activity of the congeneric Zr and Hf compounds 7 and 123. On the other hand, the anion [MeB(C6F5)3]− in 122 led to lower catalytic activity and produced lower molecular weight polymers compared to the less nucleophilic anion [B(C6F5)4]− in 7. It was found that 2-Me-oxazoline polymerized faster than 2-Ph-oxazoline. Wu and co-workers reported that [Cp2ZrMe]+·[MeB(C6F5)3]− (122) catalyzed the ROP of 1,5,7,11-tetraoxaspiro[5,5]undecane (50 equiv) in benzene (Scheme 45).59 The reaction was very slow at room temperature, but it was complete within 2 h at 55 °C. Polymers with low polydispersity (Mw/Mn ≤ 1.2) were obtained. The reaction was first order in monomer and catalyst concentrations when [monomer]0 was kept reasonably low (0.19 M), but an initial rate independent of [monomer] was found at higher concentrations ([monomer]0 ≥ 0.57 M). It was proposed that the rate-limiting step consisted of the reaction of the catalytically active species, i.e., the cationic chain end, with the monomer. Cationic titanium complexes have seldom been employed for the ROP of cyclic esters. Shur, Rosenthal, and co-workers showed that the dinuclear zwitterionic titanoxanes [Cp{η5C 5 H 4 B(C 6 F 5 ) 3 }Ti] 2 O (42) and [{η 5 -iPrC 5 H 4 }{η 5 -1,3iPrC5H3B(C6F5)3}Ti]2O (124),112 which each contain two positively charged titanium(IV) atoms (Figure 31), catalyzed the ROP of CL (1000−6000 equiv per dinuclear zwitterion) in toluene (with [CL]0 = 2.0 M) or better in bulk monomer at 60 °C to afford very high molecular weights PCLs (83 000− 538 000 g·mol−1, polydispersities not specified).54 However, the reactions were very slow, requiring 8−10 days to reach near complete conversion of the monomer. Moreover, the monometallic titanium(IV) halides Cp{η5-C5H4B(C6F5)3}Ti(X) (X = Cl, 125; Br, 126; Figure 31), obtained by oxidation of Cp{η5-C5H4B(C6F5)3}Ti with CCl4 or BrCH2CH2Br, also partially polymerized large amounts of CL (1000−6000 equiv

Scheme 45. Wu’s ROP of 1,5,7,11-Tetraoxaspiro[5,5]undecane Initiated by [Cp2ZrMe]+·[MeB(C6F5)3]− 59

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Figure 31. Zwitterionic titanium(IV) complexes with positively charged metal atoms for the ROP of CL with an ORTEP rendering of 125 (Shur/ Rosenthal/Baumann).54,112,113

Figure 32. Silver−NHC cationic complexes for the ROP of lactides (Sunoj/Ghosh).114,116,117

molecular weight distributions (Mw/Mn = 1.1−1.3). Initiation efficiency was in the range 38−74%, with formation of ca. 1.9− 3.7 polymer chains per metal atom instead of the expected five. It was found that the control was even better if the reaction was run at 120 °C for 30 min and then at 50 or 70 °C for the rest of the 24 h. No reaction took place at 50 or 70 °C without the preliminary short time interval at 120 °C. Under identical conditions, the catalytic activity was found to increase according to 131 < 132 < 130, but these observations were not rationalized with respect to the nature of the arene ligand. End-group analysis confirmed the presence of the −CH(CH3)OH and iPrOC(O)CH(CH3)− termini expected for a controlled iROP, and the living nature of the reactions was confirmed by a double-feed sequential experiment using L-lactide for the chain extension. On the basis of a combination of NMR, MALDI-ToF MS, and DFT investigations, it was postulated that the reactions proceeded according to an activated monomer mechanism with acyl cleavage. Coordination of the incoming monomer onto the cationic RuII center entailed change of hapticity of the coordinated arene, with transient and reversible change from η6- to η4-coordination of the arene onto the metal center (Scheme 46). Of particular interest, albeit sluggish, these complexes were extremely robust and even resisted quenching of the iROP reaction with the customary excess MeOH.118

to investigate the feasibility of a metal-mediated ROP catalysis with these silver cations, that is with insertion of L-LA in the Ag−NHC bond, looking at the first two insertion steps.117 Both the initiation (ΔG‡ = +42.0 kcal·mol−1, ΔG° = +38.7 kcal·mol−1) and the first propagation (ΔG‡ = +31.5 kcal·mol−1, ΔG° = +13.4 kcal·mol−1) were found to be thermodynamically prohibited, while the insertion step also looked kinetically unrealistic. It was, however, not stipulated what more plausible mechanism, other than the release of free NHC in the reaction media discarded on account of the thermogravimetric analyses on 127 and 128, could be at work with these systems. Garcia and co-workers implemented cationic ruthenium(II)− arene complexes [Cp{η6-arene}Ru]+·[PF6]− (Cp = {η5-C5H5}; arene = 2-phenylpyridine, 130; dibenzosuberone, 131; toluene, 132) that proved capable of catalyzing both the living and the immortal ROP of CL (Figure 33).118

Figure 33. Garcia’s cationic ruthenium(II)−arene catalysts for the ROP of ε-caprolactone.118

4.8. Tin Cations

The polymerization of 100 equiv of CL at 120 °C for 24 h without added initiator afforded only partial conversion (50−60%) with Mn,calcd ≈ Mn,GPC = 5000−7000 g·mol−1 and Mw/Mn = 1.3−1.6, independently of the catalyst that was used. In the additional presence of 5 equiv of iPrOH acting as an initiator/chain-transfer agent (Table 7), 130−132 behaved as type III iROP catalysts. The iROP reactions at 120 °C (24 h) were moderately controlled, generating PCLs with narrower

Considering that under the form of the very versatile and robust SnII(2-ethyl-hexanoate)2, tin is the main choice of metal to catalyze the polymerization of cyclic esters in industry,1,6 it is surprising that only very few and short accounts of well-defined tin cationic ROP catalysts have been released. Pappalardo and co-workers mentioned the ROP of CL with the stable tin(IV) cations obtained upon treatment of the 3604


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Table 7. iROP of CL with Ruthenium(II) Catalysts 130−132 and iPrOH; [CL]0/[Ru]0/[iPrOH]0 = 100:1:5118

a

catalyst

Tre [°C]

t [h]

conv. [%]


Mn,GPC [g·mol−1]

Mw/Mn

Mn,NMR [g·mol−1]

init. eff.a [%]

chains per Ru

130 130 131 132 132 132

120 50b 120 120 50b 70b

24 23.5b 25 24 23.5b 23.5b

96 72 60 77 43 86

2200 1650 1400 1800 1000 1950

5000 2300 1900 4800 1700 2700

1.2 1.1 1.3 1.3 1.1 1.1

5600 1750 1800 5400 1600 2800

44 72 74 38 57 72

2.2 3.6 3.7 1.9 2.9 3.6

Initiation efficiency = (Mn,calcd/Mn,GPC) × 100. bReaction time =0.5 h at 120 °C, followed by 23.5 h at the indicated temperature.118

Scheme 46. Proposed Activated Monomer Mechanism for the ROP of CL Catalyzed by 131/ROH Involving Reversible Change of Hapticity from η6- to η4-Coordination for the Arene Ligand118

Table 8. ROP of CL (13.5 mmol) in Toluene (12 mL) Using the Tin(IV) Precatalysts 46 and 47 (60 μmol)119 precatalyst (60 μmol)

activator (60 μmol)

46 47

[Et3Si] ·[B(C6F5)4] AgSbF6 +

iPrOH

Tre [°C]

t [h]

conv. [%]

Mn,GPC [g·mol−1]

Mw/Mn

Mn,NMR [g·mol−1]

60 μmol

25 110

24 24

69 64

8000 2400

1.8 1.8

9600 3200

−

charge-neutral precatalysts (2,4,6-iPr3-C6H2)2Sn(CHCH2)2 (46) and (2,4,6-iPr3-C6H2)2Sn(Br)(Bn) (47) with [Et3Si]+· [B(C6F5)4]− and AgSbF6 (see Figure 9)58 and later detailed that CL polymerized in toluene (12 mL, 60 μmol of metal).119 The reactions were slow, requiring 24 h to partially convert 13.5 mmol of monomer (Table 8). [(2,4,6-iPr3-C6H2)2Sn(CHCH2)]+·[B(C6F5)4]− was active at 25 °C (yield = 69%) but afforded PCL with a broad polydispersity (Mw/Mn = 1.8, Mn,GPC = 8000 g·mol−1). End-group analysis demonstrated the presence of −CH2CH2OH and −COOH chain ends the latter possibly as a consequence of the presence of adventitious water acting as an initiator. The catalyst [(2,4,6-iPr3-C6H2)2Sn(Br)]+· [SbF6]− was used in association with 1 equiv of iPrOH but overall proved less effective, requiring heating to 110 °C to afford 64% conversion (Mw/Mn = 1.8, Mn,GPC = 2400 g·mol−1); the possible importance of the nature of the anion was not discussed. In this case, (CH3)2CHOC(O)− and −CH2CH2OH termini were identified, and it was concluded that the reactions followed an activated monomer mechanism. Carpentier and Sarazin reported that in combination with iPrOH, the aminoether−phenolate tin(II) cationic complex [{LONO4}Sn]+·[H2N{B(C6F5)3}2]− (133), a type III catalyst, promoted the iROP of L-LA in toluene at 100 °C, with [L-LA]0/[Sn]0/[iPrOH]0 = 1000:1:10 (Figure 34).93 Complete conversion required 6 h to give monodisperse polymers with good control over the macromolecular features

Figure 34. Sarazin and Carpentier’s tin(II) [{LONO4}Sn]+·[H2N{B(C6F5)3}2]− with its spectroscopic data.93

(Mn,calcd ≈ Mn,GPC = 7000−16 100 g·mol−1, Mw/Mn = 1.08− 1.3; TOF ≈ 1390 molL‑LA·molSn−1·h−1). Although fairly competent, the tin(II) complex 133 was therefore at least one, if not two, order of magnitude less active that its zinc (12) and Ca−Ba (14−16) analogues. This was in line with observations made for families of charge-neutral complexes based on these different metals.5 Although structural data for 133 were not available, the 119Sn NMR and 119Sn Mössbauer spectroscopic data revealed that not all heteroatoms were coordinated onto the metal center, which most probably existed in a 3-coordinate environment. 3605


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Table 9. Polymerization of Lactides Catalyzed by Cationic Metal Complexesa catalyst aluminum 28 zinc 12 48 64 67a 67b 69 74 75d 76e 77f 81a 81b 84 85 86 87 88g 89 91 92 93 magnesium 13 calcium 14i 97j 101 103 104 strontium 15 98k 102 barium 16


Mn,GPC [g·mol−1]

[LA]0/[Cat]0/[Nu]0

Tre [°C]

t [min]

yield [%]

500:1:20b

80

600 × 60

85

n.a.

18 500

1.4

36

1000:1:50 200:1:0 200:1:0 100:1:0 100:1:0 400:1: 0 50:1:0 200:1:0 200:1:0 400:1:0 1000:1:0 1000:1:0 100:1:0 200:1:0 200:1:0 200:1:0 1000:1:10 500:1:5h 500:1:0 1000:1:0 1000:1:0

100 25 140 100 100 100 60 60 25 25 40 40 20 160 160 160 100 50 150 150 150

16 × 60 50c 5 6 × 60 9 × 60 9 × 60 3.5 × 60 n.a. n.a. 60 4 × 60 4 × 60 30 × 60 8 12 22 3 × 60 3 × 60 48 × 60 24 × 60 24 × 60

97 90 73 90 85 69 90 n.a. n.a. 92 90 95 98 100 97 97 27 95 33 94 80

2900 26 000 21 000 n.a. n.a. 39 800 6500 n.a. n.a. 53 100 129 000 137 000 14 100 28 800 28 000 28 000 4000 13 800 23 800 135 000 115 000

3200 29 000 12 000 n.a. n.a. 27 100 18 000 n.a. n.a. 19 700 56 300 208 000 111 000 69 000 33 100 36 200 4100 12 600 17 500 66 500 67 000

1.1−1.2 1.1−1.3 1.8 multimodal multimodal 1.9 1.2−1.3 n.a. n.a. 1.6 1.7 1.9 2.0 1.7 1.3 1.3 1.1 1.3 1.6 2.3 2.3

18 60 66 70 70 71 72 73 73 74 75 75 77 24a 24b 24b 18 79 85 86 86

500:1:10

100

5 × 60

28

2100

2500

1.1

20

3000:1:10 1000:1:50 1000:1:10 1000:1:10 200:1:0 250:1:0

60 60 100 30 70 23

24 × 60 8 × 60 3 × 60 3 × 60 5 × 60 2 × 60

96 83 47 66 100 91

41 600 2500 6900 9600 35 000 32 800

30 000 2400 6700 7300 35 000 43 200

1.1 1.1 1.1 1.3 n.a. 1.2−1.4

18 18 18 95 96 96

1000:1:10 1000:1:10 2000:1:5 1000:1:10

30 100 100 30

60 3 × 60 24 × 60 10

48 74 85 82

7000 10 800 49 100 11 900

14 000 9900 35 000 10 400

1.2 1.2 1.4 1.2

18 18 18 95

1000:1:10 5000:1:100 1000:1:10 1000:1:10

100 30 100 30

3 24 × 60 3 × 60 15

30 95 50 96

4400 7000 7300 12 600

11 000 5300 6800 8300

1.2 1.1 1.2 1.8

18 92 18 92

70

12 × 60

88

2100

2200

n.a.

103

100 60 100 100 60

3 × 60 30 60 3 × 60 3 × 60

0 94 90 99 74

13 600 13 000 7200 10 800

13 500 12 500 7900 10 000

1.04 1.1 1.05 1.05

94a 94a 94a 94a 94a

70

45

76

n.a.

43 000

1.2

105

160 160

4 × 60 8 × 60

98 98

14 100 35 300

8700 7600

1.4 1.3

114 116

100

5 × 60

94

13 500

16 100

1.3

93

99l 100 yttrium 112 100:1:5m divalent rare earths Yb(II) and Eu(II) 114 1000:1:10 115 1000:1:10 116 1000:1:10 2000:1:40 117 1000:1:10 uranium(IV) 118 600:1:0 silver 127 100:1:0 128 250:1:0 tin(II) 133 1000:1:10

Mw/Mn

ref

n.a. = not applicable. bNu = PO. ckobs = 8.65(4) × 10−4 s−1. dkobs = 3.65(2) × 10−4 s−1. ekobs = 5.11(3) × 10−4 s−1. fkobs = 1.88(1) × 10−3 s−1. gkobs = 1.0 × 10−4 s−1. hWith 1 equiv of tertiary amine, i.e., [LA]0/[Cat]0/[Nu]0/[NR3]0 = 500:1:5:1. ikobs,iPrOH = 5.83(2) × 10−5 s−1 and kobs,BnOH = 4.72(1) × 10−5 s−1. jkobs = 4.0 × 10−4 s−1. kkobs = 13.0 × 10−4 s−1. lkobs = 14.0 × 10−4 s−1. mNu = iPrOH or BnNH2. a

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Table 10. Polymerization of ε-Caprolactone Catalyzed by Cationic Metal Complexesa catalyst

[CL]0/[cat.]0/[Nu]0

aluminum 6a 150:1:0 31b 100:1:0 33b 100:1:0 34a 100:1:0 35b 100:1:0 55b 50:1:0 59b 120:1:0 62 500:1:0 63 500:1:0 zinc 8a 1000:1:0 8b 6000:1:0 11a 1000:1:0 38 1000:1:0 82 5000:1:50 83 300:1:3 magnesium 94a 130:1:0 94b 130:1:0 calcium 103 200:1:0 trivalent rare earths 106 3000:1:0 107 500:1:0 108 500:1:0 109 500:1:0 110 500:1:0 uranium(IV) 118 600:1:0 zirconium 7 100:1:0 121 100:1:0 titanium 42 4000:1:0 125 6000:1:0 126 6000:1:0 ruthenium(II) 130 100:1:0 100:1:5 131 100:1:0 100:1:5 132 100:1:0 100:1:5 tin(IV) 46 225:1:0 47 225:1:1 a

Tre [°C]

t [min]

yield [%]


Mn,GPC [g·mol−1]

Mw/Mn

ref

50 38 38 75 75 40 40 70 70

2 × 60 12 × 60 12 × 60 2 × 60 2 × 60 60 15 45 45

98 100 100 95 95 95 99 40 50

n.a. n.a. n.a. 11 400 11 400 n.a. 13 700 23 000 28 500

n.a. n.a. n.a. 37 700 37 400 n.a. 15 300 57 000 62 000

n.a. multimodal multimodal 1.3 1.3 1.3 1.4 1.6 1.6−1.7

13 40 40 41 41 63 39 65 65

22 50 60 65 60 60

120 × 60 20 × 60 60 3 2 × 60 6 × 60

93 15 13 8 100 95

n.a. 103 000 14 800 9100 11 400 10 800

n.a. 23 900 40 000 38 300 13 600 12 300

n.a. 2.3 1.1 1.2 1.1 1.1

16 16 19a 50 76 76

23 23

4 4

95 90

n.a. n.a.

n.a. n.a.

n.a. 1.5−1.6

90 90

23

2

100

n.a.

n.a.

n.a.

96

20 19 19 19 19

15 0.5 0.5 0.5 0.5

97 100 100 100 100

332 000 57 000 57 000 57 000 57 000

120 000 17 400 12 800 14 800 8200

1.9 1.3 1.4 1.2 1.3

99 100 100 100 100

70

5

100

n.a.

59 000

1.2

105

25 25

2 × 60 22 × 60

100 100

11 400 11 400

36 000 10 000

1.1 1.3

108 108

60 60 60

192 × 60 192 × 60 192 × 60

88 11 6

n.a. 75 300 41 100

n.a.b 49 300 55 100

n.a. 1.4 1.3

54 113 113

120 120 120 120 120 120

24 24 24 24 24 24

× × × × × ×

60 60 60 60 60 60

56 96 62 60 60 77

6400 2200 7000 1400 6800 1800

4900 5000 6000 1900 5900 4800

1.3 1.2 1.6 1.3 1.4 1.3

118 118 118 118 118 118

25 110

24 × 60 24 × 60

69 64

17 700 16 400

8000 2400

1.8 1.8

119 119

n.a. = not applicable. bMolecular mass quoted as 365 000 g·mol−1.

4.9. Concluding Comments on the ROP of Cyclic Esters and Related Monomers

They are reasonably active and offer promising leads in terms of control of the stereoselectivity of the polymerization of rac-LA. With this respect, Mountford’s calcium tetrahydroborate cation also appears to convey very strong potential. Unprecedented catalytic activities have been achieved during immortal polymerizations, and this is in fact true also for charge-neutral catalysts. From the point-of-view of catalytic activity and reactions rates, it emerges clearly that cationic complexes built on electrophilic metal, e.g., zinc, rare and alkaline earths, present the best performances. Currently, alkalineearth cationic complexes of Ca−Ba are the most active ones,

A large range of well-defined cations is now available to catalyze the controlled ring-opening polymerization of cyclic esters, carbonates, and even oxazolines; comparative data of the key features of these catalysts in the ROP of the main monomers of interest are collated for convenience in Tables 9 (lactides) and 10 (ε-caprolactone). Hayes’ phosphinimine zinc−lactate cations, e.g., 75, are attractive in the sense that as type I complexes they can catalyze living as well as immortal ROP reactions with or without recourse to exogenous nucleophiles. 3607


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although the observed activity trend (Mg ≪ Ca < Sr < Ba, inverse to the Lewis acidity of these elements) has not yet been clearly rationalized. However, this needs to be mitigated, since these complexes are usually very sensitive to impurities, air, and moisture, and it so far has proved difficult to convert thousands of equivalents of monomer with such complexes. With this respect, Herres-Pawlis’ very robust zinc guanidinate complexes certainly stand out, and they clearly present great scope, and more efforts should surely be paid in this very area in the years to come. However, the greatest challenge still facing synthetic coordination and polymer chemists certainly resides in achieving the isospecific polymerization of rac-LA. Because of the societal and economical relevance of entirely isotactic polylactides, this area should attract considerable attention in the future. Only a handful of stereoselective cationic catalysts have been developed to date, and this is exclusively for the production of heterotactic PLA, a material of lesser significance. The controlled and stereospecific ROP of lactide with cations, which by far and large follow an activated monomer mechanism where the metal chain is not permanently bound to the metal, is a feast by no mean easy to consider let alone to achieve, but there certainly is still considerable mileage in this domain. One can, for instance, refer to the highly heterotactic ROP of rac-LA catalyzed by InCl3/BnOH/NEt3 for inspiration.120 Rational and principled ligand design influencing the interaction between the bound monomer and the cationic metal atom will be key to these investigations, which should also be extended to the preparation of cationic complexes with metal that have perhaps been overlooked so far, e.g., indium and group 14 elements.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

Yann Sarazin, born in 1977, is a CNRS Research Fellow. He studied chemistry at the University of Bordeaux for 4 years, before graduating from the University of Lille. In 2000, he moved to Norwich, U.K., where he received his Ph.D. degree in 2004 in the group of Pr. Manfred Bochmann in the area of homogeneous catalysis for olefin and ring-opening polymerizations. He stayed in the same group for another 3 years as a postdoctoral research associate to investigate the stability and reactivity of heavy main-group cations (Hg, Sn, Tl). In 2007, he returned to France, as a postdoctoral fellow sponsored by Total Petrochemicals in the group of Pr. Carpentier. He was recruited by the CNRS in 2008 on a project dedicated to metal-mediated catalysis for the utilization of biomass. His current research interests include the coordination chemistry of main-group metals and homogeneous catalysis of polymerization and fine chemical reactions using main-group metal complexes, and he has published over 50 papers in these fields.

5. OUTLOOK Many discrete cationic complexes have been designed and exhibited increasingly impressive catalytic performances for the ROP of epoxides and cyclic esters. The current state-of-the-art suggests that major breakthroughs cannot be foreseen in the homopolymerization of epoxides, due to the nature of the associated mechanisms, and as a consequence, interest in the use of metal cations to polymerize epoxides has dwindled. Perhaps one way to reinvigorate the field consists in achieving the copolymerization with carbon dioxide to produce polycarbonates, but for now it is hard to see how to reconcile the characteristic cationic activated (carbocationic) chain-end mechanism with the efficient use of CO2. Current interest is therefore chiefly focusing of the ROP of lactides, and one can anticipate this situation is unlikely to change in the coming years. High catalytic activities can now be achieved at minimal expense, and this is no longer a brake to catalyst development in this area. Instead, future efforts should focus on developing robust, nontoxic catalysts allowing the conversion of dozens of thousands of equivalents of monomer. Above all, achieving the isospecific ROP of racemic lactide and understanding the principles that govern it represents some sort of a stalemate, hampering further developments and industrial applications. Limited success has been achieved with highly active chargeneutral complexes of oxophilic metals, even if some positive signs are now starting to emerge.121 It seems to the authors that in view of the activated monomer mechanism that is often associated with discrete metal cations, this is clearly an area where the use of rationally devised cationic complexes could bring original breakthroughs.

Jean-François Carpentier is 48. He graduated from the Chemical Engineering School of Lille, France, in 1989 and received his Ph.D. degree in Molecular Catalysis from the University of Lille in 1992 under the guidance of Pr. André Mortreux. After a postdoctoral appointment at the French Nuclear Agency in Tours, he returned to Lille in 1993 to take up a CNRS research fellow position, working on late transition metal catalysis (Ru, Rh, Ni, Pd, Pt) for C−C and C−H bond (asymmetric) formation processes. In 1997, he spent 1 year as a 3608


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2002, 124, 1316−1326. (g) Nomura, N.; Ishii, R.; Akakura, M.; Aoi, K. Stereoselective Ring-Opening Polymerization of Racemic Lactide Using Aluminum-Achiral Ligand Complexes: Exploration of a ChainEnd Control Mechanism. J. Am. Chem. Soc. 2002, 124, 5938−5939. (5) (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Polymerization of Lactide and Related Cyclic Esters by Discrete Metal Complexes. J. Chem. Soc., Dalton Trans. 2001, 2215−2224. (b) Stridsberg, K. M.; Ryner, M.; Albertsson, A.-C. Controlled RingOpening Polymerization: Polymers with Designed Macromolecular Architecture. Adv. Polym. Sci. 2002, 157, 41−65. (c) Coates, G. W. Polymerization Catalysis at the Millennium: Frontiers in Stereoselective, Metal-Catalyzed Polymerization. J. Chem. Soc., Dalton Trans. 2002, 467−475. (d) Dechy-Cabaret, O.; Martín-Vaca, B.; Bourissou, D. Controlled Ring-Opening Polymerization of Lactide and Glycolide. Chem. Rev. 2004, 104, 6147−6176. (e) Wu, J.; Yu, T.-L.; Chen, C.-T.; Lin, C.-C. Recent Developments in Main Group Metal Complexes Catalyzed/Initiated Polymerization of Lactides and Related Cyclic Esters. Coord. Chem. Rev. 2006, 250, 602−626. (f) Williams, C. K.; Hillmyer, M. A. Polymers from Renewable Resources: A Perspective for a Special Issue of Polymer Reviews. Polym. Rev. 2008, 48, 1−10. (g) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Complexes of Mg, Ca and Zn as Homogeneous Catalysts for Lactide Polymerization. Dalton Trans. 2009, 4832−4846. (h) Kricheldorf, H. R. Syntheses of Biodegradable and Biocompatible Polymers by Means of Bismuth Catalysts. Chem. Rev. 2009, 109, 5579−5594. (i) Ajellal, N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.; Hélou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Metal-Catalyzed Immortal RingOpening Polymerization of Lactones, Lactides and Cyclic Carbonates. Dalton Trans. 2010, 39, 8363−8376. (j) Carpentier, J.-F. Discrete Metal Catalysts for Stereoselective Ring-Opening Polymerization of Chiral Racemic β-Lactones. Macromol. Rapid Commun. 2010, 31, 1696−1705. (k) Thomas, C. M. Stereocontrolled Ring-Opening Polymerization of Cyclic Esters: Synthesis of New Polyester Microstructures. Chem. Soc. Rev. 2010, 39, 165−173. (l) Stanford, M. J.; Dove, A. P. Stereocontrolled Ring-Opening Polymerisation of Lactide. Chem. Soc. Rev. 2010, 39, 486−494. (m) Sutar, A. K.; Maharana, T.; Dutta, S.; Chen, C.-T.; Lin, C.-C. Ring-Opening Polymerization by Lithium Catalysts: an Overview. Chem. Soc. Rev. 2010, 39, 1724−1746. (n) Arbaoui, A.; Redshaw, C. Metal Catalysts for ε-Caprolactone Polymerisation. Polym. Chem. 2010, 1, 801−826. (o) Buffet, J.-C.; Okuda, J. Initiators for the Stereoselective RingOpening Polymerization of meso-Lactide. Polym. Chem. 2011, 2, 2758−2763. (p) Dijkstra, P. J.; Du, H.; Feijen, J. Single Site Catalysts for Stereoselective Ring-Opening Polymerization of Lactides. Polym. Chem. 2011, 2, 520−527. (q) Guillaume, S. M.; Carpentier, J.-F. Recent Advances in Metallo/Organo-Catalyzed Immortal RingOpening Polymerization of Cyclic Carbonates. Catal. Sci. Technol. 2012, 2, 898−906. (r) dos Santos Vieira, I.; Herres-Pawlis, S. Lactide Polymerisation with Complexes of Neutral N-Donors − New Strategies for Robust Catalysts. Eur. J. Inorg. Chem. 2012, 765−774. (s) Lecomte, P.; Jérôme, C. Recent Developments in Ring-Opening Polymerization of Lactones. Adv. Polym. Sci. 2012, 245, 173−218. (t) Dagorne, S.; Normand, M.; Kirillov, E.; Carpentier, J.-F. Gallium and Indium Complexes for Ring-Opening Polymerization of Cyclic Ethers, Esters and Carbonates. Coord. Chem. Rev. 2013, 257, 1869− 1886. (u) Sauer, A.; Kapelski, A.; Fliedel, C.; Dagorne, S.; Kol, M.; Okuda, J. Structurally Well-Defined Group 4 Metal Complexes as Initiators for the Ring-Opening Polymerization of Lactide Monomers. Dalton Trans. 2013, 42, 9007−9023. (6) Poly(lactic acid): Synthesis, Structures, Properties, Processing and Applications; Auras, R.; Lim, L.-T.; Selke, S. E. M.; Tsuji, H.; Eds.; John Wiley and Sons Inc.: Hoboken, NJ, 2010. (7) For reviews on organo-catalyzed ROP, see: (a) Matsumura, S. Enzymatic Synthesis of Polyesters via Ring-Opening Polymerization. Adv. Polym. Sci. 2006, 194, 95−132. (b) Kamber, N. E.; Jeong, W.; Waymouth, R. M.; Pratt, R. C.; Lohmeijer, B. G. G.; Hedrick, J. L. Organocatalytic Ring-Opening Polymerization. Chem. Rev. 2007, 107, 5813−5840. (c) Bourissou, D.; Moebs-Sanchez, S.; Martín-Vaca, B. Recent Advances in the Controlled Preparation of Poly(α-hydroxy

research associate with Pr. Richard F. Jordan at the University of Iowa, working on group 4 metal d0-olefin complexes. In 2001, he moved to the University of Rennes as a Full Professor. His current research interests lie in the organometallic chemistry of oxophilic elements (groups 2−4, 12−14) and their use in catalysis for polymer materials engineering and fine chemicals synthesis. He is coauthor of more than 250 publications and 45 patent families. He is/was a member of several Editorial Boards (ChemistryA European Journal, Current Inorganic Chemistry, European Journal of Inorganic Chemistry, Organometallics, Polymers), and he has been Editor of Catalysis Communications since 2012. In 2005, he was elected a member of the Institut Universitaire de France. In 2014, he was awarded the Silver CNRS medal and the prix Germaine & André Lequeux from the French Academy of Sciences.

ACKNOWLEDGMENTS We thank Total Raffinage-Chimie, The Institut Universitaire de France (Fellowship to J.-F. C.), ANR (CP2D-08-01, BioPolyCat), and the European Union (Marie Curie Fellowship FP7-People-2010-IIF, ChemCatSusDe) for subsidizing our research in the field of cation-mediated ring-opening polymerization catalysis. We are also most grateful to the excellent students who have carried out this research in our laboratories over the years with great commitment, skills, and enthusiasm: Dr. Valentin Poirier, Dr. Pierre Brignou, Dr. Bo Liu, Dr. Lingfang Wang, and Dr. Nicolas Maudoux. REFERENCES (1) In Handbook of Ring-Opening Polymerization; Dubois, P.; Coulembier, O.; Raquez, J.-M.; Eds.; Wiley-VCH: Weinheim, 2009. (2) Ultree, A. J. Encyclopedia of Polymer Science and Engineering; John Wiley and Sons: New York, 1986; Vol. 6, pp 733−755. (b) Bolick, J. E.; Jensen, A. W. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John Wiley and Sons: New York, 1993; Vol. 10, pp 624−638. (3) For recent reviews, see: (a) Kember, M. R.; Buchard, A.; Williams, C. K. Catalysts for CO2/Epoxide Copolymerisation. Chem. Commun. 2011, 47, 141−163. (b) Darensbourg, D. J.; Wilson, S. J. What’s New with CO2? Recent Advances in its Copolymerization with Oxiranes. Green Chem. 2012, 14, 2665−2671. (c) Brocas, A.-L.; Mantzaridis, C.; Tunc, D.; Carlotti, S. Polyether Synthesis: From Activated or Metal-Free Anionic Ring-Opening Polymerization of Epoxides to Functionalization. Prog. Polym. Sci. 2013, 38, 845−873. (d) Childers, M. I.; Longo, J. M.; Van Zee, N. J.; LaPointe, A. M.; Coates, G. W. Stereoselective Epoxide Polymerization and Copolymerization. Chem. Rev. 2014, 114, 8129−8152. (e) Taherimehr, M.; Pescarmona, P. P. Green Polycarbonates Prepared by the Copolymerization of CO2 with Epoxides. J. Appl. Polym. Sci.; DOI: 10.1002/ app.41141. (4) (a) Leborgne, A.; Vincens, V.; Joulgard, M.; Spassky, N. RingOpening Oligomerization Reactions Using Aluminium Complexes of Schiff’s Bases as Initiators. Makromol. Chem., Macromol. Symp. 1993, 73, 37−46. (b) Wisniewski, M.; Le Borgne, A.; Spassky, N. Synthesis and Properties of (D)- and (L)-Lactide Stereocopolymers Using the System Achiral Schiff’s Base/Aluminium Methoxide as Initiator. Macromol. Chem. Phys. 1997, 198, 1227−1238. (c) Ovitt, T. M.; Coates, G. W. Stereoselective Ring-Opening Polymerization of mesoLactide: Synthesis of Syndiotactic Poly(lactic acid). J. Am. Chem. Soc. 1999, 121, 4072−4073. (d) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. Single-Site Catalysts for Ring-Opening Polymerization: Synthesis of Heterotactic Poly(lactic acid) from rac-Lactide. J. Am. Chem. Soc. 1999, 121, 11583−11584. (e) Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. Polymerization of Lactide with Zinc and Magnesium β-Diiminate Complexes: Stereocontrol and Mechanism. J. Am. Chem. Soc. 2001, 123, 3229−3238. (f) Ovitt, T. M.; Coates, G. W. Stereochemistry of Lactide Polymerization with Chiral Catalysts: New Opportunities for Stereocontrol Using Polymer Exchange Mechanisms. J. Am. Chem. Soc. 3609


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Discrete Cationic Complexes for Ring-Opening Polymerization

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