Rare-Earth Complexes Supported by Tripodal Tetradentate Bis

Review pubs.acs.org/Organometallics

Rare-Earth Complexes Supported by Tripodal Tetradentate Bis(phenolate) Ligands: A Privileged Class of Catalysts for RingOpening Polymerization of Cyclic Esters Jean-François Carpentier*

Downloaded by STOCKHOLM UNIV on August 24, 2015 | http://pubs.acs.org Publication Date (Web): August 24, 2015 | doi: 10.1021/acs.organomet.5b00540

Institut des Sciences Chimiques de Rennes, Organometallics: Materials and Catalysis Laboratories, UMR 6226 CNRS, Université de Rennes 1, F-35042 Rennes Cedex, France ABSTRACT: Tripodal dianionic diamino- or amino-alkoxy-bis(phenolate) ligands {ONXOR1,R2} (X = NR2, OR) constitute a privileged class of ligands which has found much interest in combination with group 3 and group 4 metals for generating highly efficient polymerization catalysts. This account article describes the structural variety of and synthetic routes toward trivalent rare-earth complexes incorporating such {ONXOR1,R2} ancillaries along with reactive amido, alkyl, alkoxide, amidinate, guanidinate, halide, and borohydride groups. The chemistry of related divalent rare-earth complexes is also included. This class of Ln{ONXOR1,R2}(R) complexes features outstanding performance in the ring-opening polymerization (ROP) of cyclic esters. Examples of their high reactivity, which has allowed enlarging the scope of ROP reactions to monomers that are difficult to ring-open, and of their ability to control reactions are provided. A particular emphasis is given on their propensity to fine-tune the stereoselectivity of ROP reactions involving chiral cyclic esters, by modifying the ortho substituents on the phenolate rings. The associated mechanistic issues which have evidenced steric and much less common electronic interactions are discussed.

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weights, dispersity, end-group fidelity, regio- and stereoselectivity, and even monomer sequence.5,6 Thousands of metal-based catalysts have been developed over the past three decades for promoting ROP reactions of cyclic esters. Many of those catalysts operate via a so-called “coordination−insertion” mechanism and combine a highly electrophilic, Lewis acidic metal center to activate the cyclic ester monomer via coordination of its carbonyl group, and a nucleophilic, anionic ligand moiety−typically an alkoxide, an amide or an alkyl group−to induce the ring-opening (Scheme 1).6 As is customary of molecular catalysis, additional ancillary ligands are key elements in ROP catalysis to fine tune the sterics and electronics of the metal complex and to control eventually the polymerization process. However, these ancillaries are not alone in playing this role. Indeed, conversely to catalysis for fine chemicals synthesiswhere the molecular reaction product is evolved from the metal coordination sphere after each catalytic cyclein polymerization catalysis following a coordination− insertion mechanism, the growing macromolecular chain remains coordinated onto the metal center after each monomer insertion (Scheme 1); hence, it constitutes another genuine component which affects the properties of the catalytically active, propagating organometallic species. This turns out to be of fundamental importance for achieving stereocontrol of the ROP when a (pro)chiral cyclic ester is used. In fact, since the growing macromolecular chain contains stereogenic centers, notably those of the last and penultimate monomer units

INTRODUCTION Development of effective procedures in organic and macromolecular synthesis using metal-based catalysis has long relied on privileged classes of ligands. Such ligands feature a readily accessible, highly versatile modular structure that enables the formation of ligand libraries and easy fine tuning for a specific catalytic reaction. Among the most popular examples, Salen-1 and Binap-type2 ligands are probably some of those that may come first to mind, with exceptional performance in a plethora of catalytic applications, ranging from fine chemicals to polymer synthesis. Ethylene- or silylene-bridged bis(indenyl) (“EBI” or “SBI”, respectively)3 and phenoxy-imine frameworks4 are other examples of modular ligand classes extensively used in group 4 metallocenes and postmetallocenes, which have undergone tremendous developments for preparing a wide variety of tailormade polyolefin materials, many of which can be prepared in a stereoregular manner. A current highly topical field in polymerization catalysis is the ring-opening of cyclic esters to produce aliphatic polyesters, a very valuable class of polymers with attractive thermomechanical properties combined with good biocompatibility/ degradability.5 The inherent 100% atom economy of ringopening polymerization (ROP) and the accessibility of a number of suitable cyclic esters from renewable resources (biomass), with lactide at the forefront, make this process eligible for green chemistry concepts. In addition, as opposed to polycondensation between diacids and diols, ROPespecially when performed with organometallic catalystsis arguably the most effective route to access polyesters with a very fine degree of control over the macromolecular parameters: molecular © XXXX American Chemical Society

Received: June 21, 2015

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DOI: 10.1021/acs.organomet.5b00540 Organometallics XXXX, XXX, XXX−XXX

Review

Organometallics Scheme 1. “Coordination−Insertion” ROP Mechanism Illustrated with Lactide as Monomer and a Mononuclear Catalyst Initiator5,6

their use in ROP catalysis, especially in stereoselective polymerization; to date, more than 50 papers have been published on this very topic. This paper provides a short but comprehensive account of these developments, highlighting the diversity of prepared complexes, hereafter abbreviated Ln{ONXOR1,R2},13 and their synthetic routes, reactivity, and use in ROP catalysis. A particular emphasis is given to the stereoselective polymerization of racemic cyclic esters and the associated mechanistic issues which have evidenced steric and much less common electronic interactions.


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PREPARATION AND MAIN STRUCTURAL FEATURES OF TRIVALENT AND DIVALENT RARE-EARTH Ln{ONXOR1,R2} COMPLEXES A large variety of rare-earth Ln{ONXOR1,R2} complexes have been prepared; to date, more than 130 examples are known, and many of them have been structurally characterized by X-ray diffraction. Targeting applications in ROP catalysis, most synthetic efforts have focused on complexes incorporating a trivalent rare-earth center combined with a nucleophilic anionic “reactive” ligand (vide supra). Trivalent Rare-Earth Complexes. Amido complexes of the type [Ln{ONXOR1,R2}(NR2) (donor)n]m offer an ideal compromise between ease of preparation, stability for isolation/ handling, and reactivity as a catalyst (the amido acting as the initiating group) or a catalyst precursor (the amido being transformed in situ in an alkoxide; vide infra). They constitute the largest subclass developed in this chemistry, with many variations in terms of substituents (R1 and R2) on the phenolate rings, fourth pendant donor (X), amido group, and rare-earth metal.10−12,14−25 They are most conveniently prepared by amine elimination, via the protonolysis of two bulky amido ligands in the homoleptic precursors Ln(N(SiRMe2)2)3(THF)n (R = H, Me) by the acidic bis(phenol) {ONXOR1,R2}H2 proligands (Scheme 3).11 This is actually the technique that was employed by Kerton and co-workers in 2004 for the fast construction of libraries of complexes, by a combination of six [Ln{N(SiMe3)2}3] reagents (Ln = Y, La, Pr, Sm, Gd, Yb) and eight tripodal {ONXOR1,R2}H2 proligands (among others), in a search for the best combination in ROP catalysis (vide infra).25 Among Ln{ONXOR1,R2}(NR2) (donor)n species (Scheme 3), the small Sc and Yb centers (ionic radii for six-coordinated centers 0.745 and 0.868 Å, respectively)26 lead systematically to mononuclear five-coordinated, THF-free complexes.18,20 For other rare-earth elements, mononuclear six-coordinated complexes with one coordinated THF molecule are usually obtained; the metal center then lies in a distorted-octahedral environment, with a meridional coordination of the ONO ligand atoms, the amido group being located trans to the nitrogen bridge atom, and the fourth X donor group and THF molecule occupying the axial sites. For the large La (ionic radius for the six-coordinated center 1.032 Å),26 a dinuclear structure with μ-bridging O(phenolates) and six-coordinated metal centers has been most often observed (Scheme 3).15 Such a peculiar dimeric structure was also established for Y{ON(OMe)OCl,Cl}(N(SiHMe2)2), despite the small Y center, possibly as a result of limited crowding provided by the chloro substituents.23 Alternatively, a few amido complexes have been prepared by salt metathesis from chloro or borohydride precursors (vide infra) (Scheme 4);16,24 the yields were just slightly lower than those from the amine elimination route. Alkyl and aryl complexes Ln{ONXOR1,R2}(R) (donor)n were also accessed efficiently via alkane elimination (Scheme 5) or

incorporated, it can provide the necessary chiral elements to control the (pro)chirality of new incoming monomer units. This so-called “chain-end stereocontrol mechanism” (CEM) actually enables highly diastereoselective polymerizations with ROP (pre)catalysts that, initially, do not contain any chirality element: in particular, no chiral ligand.6 Multidentate bis(phenolate) ligands incorporating an amino bridge have met particular success in polymerization catalysis along the above principles. The corresponding bis(phenol) proligands were reported as early as in the 1950s by Burke et al., who prepared them via straightforward double Mannich condensation reactions from formaldehyde, the substituted phenols, and functional primary amines (Scheme 2).7 They Scheme 2. Potentially Tetradentate Amino-Bis(phenol) Proligands {ONXOR1,R2}H213 and Their Usual Mannich Synthesis7

were first essentially used in combination with molybdenum and copper(II) for fundamental coordination chemistry.8 From the 2000s, as pioneered and popularized by the group of Kol,9 these tripodal tetradentate diamino- or amino-alkoxy-bis(phenolate) ligands have been used in association with group 4 metals for highly effective living, stereoselective polymerization of α-olefins. The preparation of related rare-earth complexes rapidly followed on in 2002,10 while the first report on their use in cyclic ester ROP catalysis first appeared in 2003.11,12 Since then, intense research efforts have been focused on the development of this class of rare-earth complexes and B


Review

Organometallics


Scheme 3. Amine Elimination Route toward Amido Complexes of the Type {Ln{ONXOR1,R2}(NR2)(donor)n}m

Scheme 4. Salt Metathesis Route toward Amido Complexes of the Type Ln{ONXOR1,R2}(NR2)(donor)n

Scheme 5. Alkane Elimination Route toward Alkyl Complexes of the Type Ln{ONXOR1,R2}(R)(THF)

salt metathesis (Scheme 6) reactions.10,11,19,24,27−29 As is usual in alkyl rare-earth chemistry, only complexes based on small metal centers (Y, Sc, Er, Yb, Lu) were readily prepared. Yao, Shen, et al. were able to isolate infrequent examples of simple methyl complexes.24 All of those alkyl complexes featured a sixcoordinated metal center in a distorted-octahedral environment, with either a THF molecule or an extra donor (i.e., amino) group embedded in the alkyl/aryl moiety, as reported by Marques, Martins, et al. (Scheme 6).19 The latter authors isolated the unusual complex Li2(μ4-O)[Y(ON(NMe2)OtBu,tBu)(CH2SiMe3)]2 from the salt metathesis reaction between LiCH2SiMe3 and the chloro precursor (Scheme 7); although this could not be evidenced, the origin of the μ4oxygen bridge was presumed to be THF rather than adventitious moisture, possibly via formation of an ate complex stabilized by (Li(THF)n)+.19 Because the active propagating species in ROP of cyclic esters is a metal alkoxide (Scheme 1), access to these types of

complexes, namely Ln{ONXOR1,R2}(OR)(donor)n in the present case, has been long highly desired. Such complexes are indeed, in principle, ideal catalyst precursors. However, the chemistry of rare-earth alkoxides is much trickier than that of the corresponding amides and alkyls. This is essentially due to the tendency of the highly oxophilic rare-earth elements to generate oxo species, which often collapse as multinuclear clusters and generate mixtures of compounds. Moreover, the valence of oxygen allows only one alkyl/aryl group in the alkoxide/aryloxide, which may prove, in some instances, not bulky enough to prevent aggregation (dimerization) through double or triple μ-O bridging; this is another important complication factor in this chemistry. As a consequence, alkoxides are rarely isolated and are most often generated in C


Review

Organometallics Scheme 6. Salt Metathesis Route toward Alkyl and Aryl Complexes of the Type Ln{ONXOR1,R2}(R)(donor)n

Scheme 8. Alcoholysis Route toward Alkoxide/Aryloxide/ Thiophenoxide Complexes of the Type Ln{ONXOR1,R2}(OR)(donor)n


Scheme 7. Unusual Alkyl Complex Featuring a μ4-Oxygen Bridge Isolated from Salt Metathesis19

Scheme 9. Alcoholysis Route toward Bridging Bis(aryloxide) Complexes of the Type {(donor)nLn{ON(NMe2)OtBu2}(OR-/)}2

situ or just prior to ROP catalysis, upon addition of an alcohol (phenol)30 onto an amido or alkyl precursor, such as those described above. This is also the case for Ln{ONXOR1,R2}(OR)(donor)n complexes, although a significant number of aryloxides, arguably simpler to prepare due to their lower basicity/donor ability (and hence lower tendency to aggregate), could be isolated and structurally characterized, not only in solution but also in the solid state (Scheme 8).31−36 All but one of the complexes structurally characterized were mononuclear in the solid state, with one or two coordinated THF molecules. In the absence of a THF donor, a dinuclear structure with μO(aryloxide) bridges was observed (Scheme 8).36 The same type of dinuclear structure occurred in a partially hydrolyzed yttrium μ-hydroxy-μ-aryloxide complex.34 Dinuclear complexes with a bridging diaryloxy moiety were purposely prepared, and successfully isolated and characterized, by the use of hydroquinone or a diol (instead of a mono-ol) (Scheme 9);37 those complexes proved useful as diinitiators in ROP catalysis (vide infra). Attempts to access analogous trinuclear complexes with triols led to mixtures of compounds.37 Borohydride (Scheme 10)16,38 and chloride (Scheme 11)10,16,19,25,39 Ln{ONXOR1,R2} complexes were prepared following regular salt metathesis routes, using the disodium salts Na2{ONXOR1,R2}. The solid-state structure of THF-free borohydride complexes remained unsure for a while: initially assumed to be μ-BH4 bridged dimers (II-BH4), they are actually likely μ-O(phenoxide) bridged dimers (I-BH4), as established by Mountford and co-workers, at least for the

samarium compound containing a 2-pyridyl donor moiety.16 These bridging structures are analogous to those observed in amido La{ONXOR1,R2} complexes (Scheme 3). Upon addition of (and recrystallization from) a donor solvent such as THF or pyridine, they lead to monomeric six-coordinated complexes (III-BH4). When the dilithium salts Li2{ONXOR1,R2} were used, ate complexes of the type Nd{ON(pyridyl)OtBu,Me}(BH4)(μ2-BH4)Li(THF)2, containing a bridging borohydride and phenolate, were isolated.38 Chloride Ln{ONXOR1,R2} complexes followed the same trend: small metal centers (Sc, Y, Ho, Er, Yb) afford sixD


Review

Organometallics


Scheme 10. Salt Metathesis Route toward Borohydride Complexes of the Type {Ln{ONXOR1,R2}(BH4)(donor)n}m

Scheme 11. Salt Metathesis Route toward Chloride Complexes of the Type {Ln{ONXOR1,R2}(Cl)(donor)n}m

Scheme 12. Salt Metathesis Route toward Amidinate and Guanidinate Complexes of the Type Ln{ONXOR1,R2}((R4)C(NR3)2)(donor)n

coordinated monomeric structures with one donor solvent molecule, and the larger elements (La, Sm, Nd) induce dimeric structures with μ-Cl bridging atoms, containing one or no donor solvent molecules coordinated per metal center. With a chelating donor solvent such as DME, a monomeric sevencoordinated structure was observed even for the larger Sm.39 Amidinate and guanidinate complexes Ln{ONXOR1,R2}((R4)C(NR3)2), although not as efficient ROP initiators as the amido derivatives (vide infra), were readily prepared by salt metathesis approaches from the corresponding chlorides and alkali amidinates/guanidinates (Scheme 12).27,40,41 All of them

feature monomeric structures in the solid state, with solventfree, six-coordinated metal centers. Interestingly, Marques, Martins, et al. showed that reaction of an aryl complex with excess acetonitrile proceeded with the insertion/coupling of two CH3CN molecules and the C6H4CH2NMe2 group to produce a β-diketiminate complex (Scheme 13).19 The reaction was proposed to proceed through a ketimide intermediate (spectroscopically identified), which would undergo an imine− enamine tautomerism through a 1,3-hydrogen shift to generate a nucleophilic methylene center; subsequent intramolecular E


Review

Organometallics


Scheme 13. Formation of a β-Diketiminate Complex through Double Insertion/Coupling of Acetonitrile in an ArylY{ON(NMe2)OtBu2} Precursor19

family of ligandwas prepared by alcoholysis of Ce(OtBu)4 with 2 equiv of the proligand.46 Divalent Rare-Earth Complexes. The chemistry of divalent rare-earth elements with the {ONXOR1,R2} tripodal ligands has been also largely explored. Delbridge and Shen have shown that alcoholysis of bis(bulky amide) rare-earth precursors with 2 equiv of proligands {ONXOR1,R2}H2 yield μ-O(phenolate) bridged dimeric complexes void of any donor solvent when the reactions were carried out in hydrocarbons (Scheme 16).18,47−49 When these reactions were performed starting from TMEDA-bis(amide) precursors, or when the final product was eventually recrystallized from THF, mononuclear six-coordinated complexes containing TMEDA or THF were obtained (Scheme 16).36,49−51 [Ln{ON(NEt2)OtPe2}] complexes (Ln = Sm, Yb; tPe = tert-pentyl) (Scheme 16) may be a notable exception, since according to NMR spectroscopy (no single-crystal X-ray diffraction data were available), they seem to remain monomeric in hydrocarbon solutions, despite the absence of coordinated donor solvent; those two compounds actually displayed greater air sensitivity than the aggregated compounds.49 The dinuclear divalent rare-earth complexes {Ln{ONXOR1,R2}}2 can be readily treated with easily reduced alcohols (phenols) to form heteroleptic alkoxide (aryloxide) compounds where the rare-earth center has been oxidized to the +III state (Scheme 17).18,47 Simple complexes of the type “Ln{ONXOR1,R2}(OR)” are assumed to form in these reactions; such a nature of the isolated products was corroborated by good elemental analysis data. However, reinvestigations allowed the isolation of more complicated ate compounds that retain in the coordination sphere some alkalimetal alkoxide/hydroxide and also free phenol for some of them (Scheme 17).47 Shen et al. showed that the reactions of Yb{ON(NMe2)OtBu,tBu}(THF)2 with phenylacetylene or phenyl isocyanate proceeded also via single electron transfer and eventual oxidation of the Yb center (Scheme 18).50 The resulting monomeric alkynide complex Yb{ON(NMe2)OtBu,tBu}(C CPh)(DME) was isolated by recrystallization from DME. In [Yb{ON(NMe2)OtBu,tBu}(OCNPh) (THF)]2(THF)4, the dianionic oxamide ligand bridging in a μ,η4 fashion two Yb(III) atoms results from the reductive coupling of two phenyl isocyanate molecules. On the other hand, diisopropylcarbodiimide acts as a two-electron oxidant to provide the carbene-type complex [Yb{ON(NMe2)OtBu,tBu}](μ-NiPrCNiPr) with, formally, two Yb−N single bonds and a three-center, two-electron bridge bond extending over the two metal atoms and the carbon atom of carbodiimide (Scheme 18).52 Subsequent reaction of this carbene-type species with excess PhNCO afforded the dinuclear ytterbium complex [Yb{ON(NMe2)OtBu,tBu}(OCN(Ph))]2(NPh). It was proposed that the reaction proceeds via insertion of PhNCO into the two Yb−N bonds, with formation of an intermediate complex featuring a free carbene moiety, which would react immediately with 2 equiv of PhNCO to yield an intermediate imido-bridged complex; upon further insertion of two molecules of isocyanate into the two Yb−N bonds, this complex would produce the final isolated product. The proposed intermediates could not be trapped.

nucleophilic attack on the MeCN nitrile carbon followed by a second γ-H shift may thus lead to the observed β-diketiminate. Homoleptic complexes of the type [Ln{ONXOR1,R2}2]z (z = 0, 1−) are, in principle, somewhat ill-defined for ROP catalysis, because of the conflict between ancillary and active roles of the ligands. Two types of such complexes have been prepared, incidentally or purposely. Mountford et al. reported that the reaction of a rare-earth tris(amide) with a bis(phenol) proligand, regardless of reactant stoichiometry, in aromatic hydrocarbons, i.e. in the absence of any donor solvent such as THF, systematically yielded the zwitterionic products Ln{ON(NMe2)OtBu2}(HON(NMe2)OtBu2) (Scheme 14).16,42,43 Salt Scheme 14. Formation of Homoleptic Zwitterionic Ln{ON(NMe2)OtBu2}(HON(NMe2)OtBu2) complexes

metathesis reactions of rare-earth trichlorides with 2 equiv of alkali-metal salts of the ligands in a donor solvent (THF, DME) yielded the corresponding ate complexes in which the alkali metal remains coordinated to the O(phenolate) (Scheme 15).44,45 The same product was formed even from the divalent YbI2(THF)2;18 in this case, the bis(phenol) ligand was deprotonated in situ with KH and it was assumed that incomplete deprotonation caused the oxidation of Yb(II) to Yb(III).18 Addition of 18-crown-6 ether to the ate complexes allowed abstraction of the alkali metal to generate a loose ion pair.45 Also, the charge-neutral complex Ce{ON(NMe2)OtBu2}2the only example of a Ce(IV) derivative with this F


Review

Organometallics Scheme 15. Formation of Homoleptic Ate Complexes


Scheme 16. Amine Elimination Route toward Divalent Rare-Earth Complexes of the Type {Ln{ONXOR1,R2}(donor)n}m

Scheme 17. One-Electron-Oxidation Reactions of Divalent Rare-Earth Complexes of the Type {Ln{ONXOR1,R2}(donor)n}m

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APPLICATION OF RARE-EARTH Ln{ONXOR1,R2} COMPLEXES IN ROP CATALYSIS General Features and Abilities of Trivalent Rare-Earth Ln{ONXOR1,R2} Complexes. Although some trivalent rareearth Ln{ONXOR1,R2} complexes have been used to promote a few other reactions such as hydrophosphonylation of aldehydes,41 group transfer polymerizations of vinylic mono-

mers (2-vinylpyridine, 2-isopropenyl-2-oxazoline, vinylphosphonates, N,N-dimethylacrylamide),53 or more recently styrene and trans-1,4-isoprene (co)polymerizations,54 this class of complexes has been developed almost exclusively for catalyzing/initiating cyclic ester ROP reactions. Our group first reported in 2003 that the prototypic amide complex Y{ON(OMe)OtBu,tBu}(N(SiHMe2)2)(THF) catalyzes slugG


Review

Organometallics


Scheme 18. One- and Two-Electron-Oxidation Reactions of the Divalent Complex Yb{ON(NMe2)OtBu,tBu}(THF)250,52

nitrogen (more generally, X) donor in the ligand was also shown to be crucial, as secondary amines led only to low conversions, possibly due to competing N−H activation processes. (iii) Yttrium and midsized rare-earths such as samarium showed increased activity in comparison with the larger rare-earths such as lanthanum and praseodymium and a smaller rare-earth such as ytterbium. Extensive complementary investigations carried out by different research groups over the next decade on isolated Ln{ONXOR1,R2} complexes confirmed the general pertinence of the above preliminary conclusions.14,17,18,23−25,28,36,37 Although some peculiarities depend on the nature of the solvent and, more notably, of the cyclic ester (vide infra), the most effective complexes in this class are in fact almost always those based on lower midsized metal centers (Y, Lu), with bulky substituents on the phenolate rings (tBu, tPe, CMe2Ph, ...) and a non-NH-containing X donor (OMe, NR2, etc.). The only premature conclusion to be noted in Kerton’s early study was about systems based on ligands containing pendant pyridyl donors, which in fact do not feature performance significantly lower than that of those based on other tertiary amine X groups. This X donor group actually proved to be a sensitive parameter, as in the series of Y{ONXOtBu,tBu} complexes, higher activities and heterotacticities in the ROP of rac-LA were generally observed with the CH2 CH 2NMe2 moiety in comparison to CH2CH2OMe.14,28 NMR studies suggested that this X group (OMe, NR2) is not involved in dynamic phenomena: i.e., it remains coordinated onto the metal center throughout ROP catalysis.17,28,31 Cui and co-workers noted that the bridge between the nitrogen atom and X donor was crucial in governing the catalytic performance via tuning the geometry of the metal center: the more flexible trimethylene bridge resulted in a less distorted tetragonal bipyramid, while the short

gishly the polymerization of methyl methacrylate to give isotactic-rich PMMA but is actually very active for the ROP of ε-caprolactone (CL): 400 equiv of this monomer was quantitatively polymerized within 1 min at room temperature to afford poly(ε-caprolactone) (PCL) with number-average molecular weights (Mn) close to the expected values, calculated from the monomer-to-yttrium ratio, yet with broad dispersities (Mw/Mn = 2.4−4.1).11 Right away after these preliminary results, we reported that this amido complex and its carbyl (−CH2SiMe3) analogue initiate the ROP of racemic lactide (rac-LA), equally fast in terms of activity in toluene and THF solvents, in a controlled manner, to give heterotactic-rich polylactic acid (vide infra, Scheme 20).12 These polymerizations were in fact living, as the Mn values increased linearly with monomer conversion, matching very closely the relationship Mn = conv(rac-LA) × [rac-LA]0/[Y] × M(LA) (M(LA) = 144 g mol−1), and the dispersities were this time quite narrow (Mw/Mn = 1.2−1.3). Soon after, in 2004, Kerton and co-workers used a combinatorial approach for determining the best combination between six [Ln{N(SiMe3)2}3] reagents (Ln = Y, La, Pr, Sm, Gd, Yb) and eight tripodal {ONXOR1,R2}H2 proligands to catalyze/initiate the ROP of CL.25 This high-throughput approach allowed the authors to come early to three valuable conclusions. (i) The steric demand of the ligand has a significant effect on the polymerization process; i.e., ligands containing bulky substituents such as tert-butyl or tert-pentyl groups at ortho phenolic positions afforded species capable of performing controlled ROP of CL, whereas less bulky groups such as methyl were not effective. If there was no substituent in the aromatic ring, the complexes were often inactive. Such steric demand on the aromatic rings was presumed to be essential to prevent metal complex dimerization,55 protect the active site, and prevent termination reactions. (ii) The type of H


Review

Organometallics

Figure 1. Scope of monomers efficiently polymerizable by Ln{ONXOR1,R2} complexes.


Scheme 19. Representation of the Elementary Processes in an Immortal ROP of Cyclic Esters Mediated by {ONXOR1,R2}Ln− Nu/ROHa

a

The active species {ONXOR1,R2}Ln(OR) (generated upon alcoholysis of the precursor) propagate by incorporating monomer units and exchange reversibly with dormant free alcohol molecules that are progressively converted into macro-ols.

in terms of stereocontrol (vide infra). The Ln{ONXOR1,R2} complexes have also served in valuable macromolecular engineering using other reactive cyclic esters, such as trimethylene carbonate (TMC) and substituted derivatives,62 or 1,4-dioxan-2-one (p-dioxanone, PDO).40 Alkoxide complexes Ln{ONXOR1,R2}(OR), although difficult to isolate, are most conveniently generated in situ upon addition of an alcohol onto an amide or alkyl precursor (vide supra). One peculiarity is when, instead of 1 equiv per amido, an excess of alcohol is used. Under these so-called “immortal” conditions, degenerative transfer between the growing macromolecular chain (an alkoxide-Ln{ONXOR1,R2} species (Scheme 1), and free excess alcohol (the conjugated Bronsted acid) can take place.30 If this exchange is fast and reversible, as many polymer chains as alcohol molecules initially introduced can be grown (Scheme 19).30 This allows multiplication of the catalyst efficiency, producing dozens or hundreds of well-controlled macromolecular chains per metal center, in contrast to a simple regular living polymerization where a single macromolecule (per metal center) is produced. One essential condition for achieving such an immortal ROP process is the stability of the active species in the presence of a large excess of alcohol; this can be very problematic for the highly oxophilic metals used to

ethylene and the rigid pyconyl generated a more twisted octahedral geometry.28 As is customary of cyclic ester ROP reactions, the most versatile and effective subclass of complexes, irrespective of the nature of the {ONXOR1,R2} ancillary ligand, turned out to be amides and alkoxides; those are highly nucleophilic groups that allow, generally, a complete and fast (vs propagation) initiation and hence achievement of good control over the molecular weights (matching of experimental and calculated Mn values, narrow dispersities). The high reactivity of the alkoxide and amide Ln{ONXOR1,R2} complexes has been profitable for expanding the range of ROP reactions from the simple, benchmark monomers (CL, LA) to the much more difficult to ring-open lactones and cyclic carbonates such as morpholinediones,56 β-butyrolactone,32,57 other β-lactones58 and more recently β-malolactonates,21 and fused five-membered carbonates such as trans-cyclohexenecarbonate (Figure 1).59 For the latter demanding monomers, Ln{ONXOR1,R2} complexes arguably belong to the very best ROP catalysts developed so far; they are comparable in terms of activity with β-diketiminate and diaminophenolate zinc complexes, [(BDI iPr )Zn{N(SiMe3)2}] and [(NNO)ZnEt], respectively developed by Coates60 and Hillmyer and Tollman61 but are actually superior I


Review

Organometallics


Scheme 20. Influence of Substituents and Metal Centers in the Heteroselective ROP of Racemic Lactide Promoted by Amido and Alkyl Ln{ON(OMe)OR1,R2} Complexes12,14,23

armed heterotactic PLA with excellent end-hydroxy fidelity.64 Then, the in situ addition of an aluminum-{salen} alkyl precursor to the reaction medium allowed incorporation of the remaining rac-LA monomer, but with isospecific selectivity; this eventually afforded a three-armed architecture with isotactic− heterotactic stereoblock (hard−soft) PLA segments, controlled molecular weight, and very narrow dispersity (Mw/Mn < 1.08).64 It must be noted that in regular (i.e., non-immortal) living ROP reactions, the functionality of the terminal groups in the polymer is controlled by the nature of the active nucleophilic group in the {ONXOR1,R2}Ln−Nu precursor/active species. Hence, amido, alkoxy, and alkyl precursors generate polyesters with respectively amide, alkoxycarbonyl, and alkyl ketone terminal groups at one terminus of the macromolecules;65 the other terminus is an hydroxy group formed upon terminal hydrolysis (quenching of the ROP reaction) of the propagating alkoxy-Ln{ONXOR1,R2} species. Borohydride Ln{ONXOR1,R2}(BH4) type precursors studied by the group of Mountford,16 as observed for other rare-earth borohydrides, generate polyesters having either an aldehyde terminus or a hydroxy terminus if further reduction takes place. Often, of these two pathways, the one giving α,ω-dihydroxy-terminated polymers was the most favored, as determined experimentally and corroborated with DFT computations.16b Stereoselective Polymerizations. As mentioned above on several occasions, in addition to their high activity and control over the molecular weights, another very valuable property of Ln{ONXOR1,R2} complexes is undoubtedly their capability to control finely the stereochemistry of ROP reactions starting from racemic mixtures of cyclic esters. Our group first reported the formation of heterotactic-rich PLA in the ROP of rac-LA using Y{ON(OMe)OtBu,tBu}(N(SiHMe2)2)(THF).12 In line with Kerton’s observations,25 further investigations revealed that manipulating the nature of the metal center and of the substituents on the phenolate rings allowed access to PLAs with quite different levels of

catalyze ROP, and the stabilization provided by the ancillary ligands is a key element to avoid formation of ill-defined aggregated species which result in catalyst decay: i.e., loss of activity and/or control over the polymerization.5,6,30 As a matter of fact, tripodal {ONXOR1,R2} ligands proved particularly well suited for stabilizing rare-earth elements and achieving very effective immortal ROP reactions. Our group provided the first such examples, demonstrating that Y{ON(OMe)OR1,R2}(N(SiHMe2)2) complexes are highly active catalysts for ROP of rac-LA in the presence of up to 50 equiv of isopropyl alcohol or allyl alcohol vs Y; the reactions were highly stereoselective (vide infra), as much as those without any alcohol, evidencing that the nature of the active species, or at least its operative mode, is not affected by the presence of excess alcohol.32 Some convincing examples but with more limited amounts of alcohols were also given for the syndioselective ROP of β-butyrolactone (BL) into poly(3hydroxybutyrate) (PHB).32 After those proofs of concept, just by tuning appropriately the nature of the added alcohol, immortal ROP reactions promoted by Ln{ONXOR1,R2} complexes have been employed for macromolecular engineering applications in the preparation of polymers with functional end-groups. Hence, the Cui group has prepared polyesters (PLAs, PHBs) with aggregationinduced emission (AIE) characteristics using tetraphenylethenes containing one, two, or four hydroxyl groups as multiple chain-transfer agents (up to 100 equiv. vs Lu{ON(NMe2)OtBu2}(CH2SiMe3) (THF)).29 This group has also used the immortal approach to prepare a variety of fluorescent dye labeled ω-hydroxylated polyesters from CL, LA, and BL, including some with a stereocontrolled backbone (syndiotactic PHB, heterotactic PLA),63 taking advantage of the high stereocontrol ability of the latter lutetium precatalyst (vide infra). Another very interesting application developed by this group consisted of the use of triethanolamine as a transfer agent (9−27 equiv vs Y) to prepare, employing this time Y{ON(NMe2)OtBu2}(CH2SiMe3)(THF) as catalyst precursor, threeJ


Review

Organometallics


Scheme 21. Influence of Substituents in the Syndioselective ROP of Racemic β-Butyrolactone Promoted by Amido and Alkyl Y{ONXOR1,R2} Complexes22,23,57

syndiotactic PHB (Scheme 21).22,23,57 The first noticeable peculiarity, in contrast to the ROP of rac-LA, is that higher syndiotacticities were obtained in toluene (or dichloromethane) than in THF (e.g., with R1 = R2 = CMe2Ph, Pr = 0.94 vs 0.83, respectively); this is a trend confirmed by different studies but which has not been rationalized so far, even in the more recent mechanistic investigations.17 Even more unexpected was the influence of R1 substituents. If cumyl and, even more, trityl substituents proved to generate highly syndioselective systems, in line with trends observed in ROP of rac-LA, the bulky aliphatic CMe2tBu led to poor results. Also, introducing an electron-withdrawing CF3 group on a remote (para) position of the cumyl substituent induced a significant drop in the Pr value. Those observations suggested not only that stereocontrol in ROP of rac-BL is governed by steric parameters but also that electronic considerations may play an important role as well. We proposed a possible explanation for those apparent electronic effects, implying C−H···π interactions67 between the acidic methylene moiety of the alkoxy-butyrate group in the growing polymer chain and aryl rings present in R1 substituents. DFT computations performed on putative intermediates indeed indicated that such interactions may stabilize the active species by 5−10 kcal mol−1 and freeze the growing macromolecular chain in a specific conformation, enhancing stereocontrol (Figure 2);23 no such interactions were computed for the equivalent Y{ON(OMe)OCMe2Ph,Me}(OCH(Me)CO2CH(Me)CO2R) species, i.e. the likely intermediates in the ROP of LA, as anticipated from the absence of acidic hydrogen in the O-lactyllactate moiety. Several experimental observations came in support of such C−H···π interactions. (a) In the solid -state

heterotacticity (Pr values in the range 0.56−0.9666) (Scheme 20).14,23 First, it turned out that the bulkier the ortho substituent, the higher the heterotacticity, with values very close to a perfectly heterotactic PLA for R1 = CMe2tBu, CPh3 (trityl). The quite similar Pr values obtained for the last two substituents, and the absence or very limited influence of para substituents (R2), indicated that the reaction is under purely steric control; this is consistent with the usual hypothesis for a chain-end stereocontrol mechanism (CEM) in which crowding in the coordination sphere of the active metal center allows an optimal “transfer of the chiral information” from the growing macromolecular chain. Also consistent with this reasoning is the influence of the metal center: the smaller the metal center (provided it is large enough to allow good reactivity: i.e., Y and Lu but not Sc), the higher the heterotacticity (Scheme 19). Cui et al. reported additional results with yttrium and lutetium alkyl complexes based on {ONXOR1,R2} ligands with pendant X amino donor groups that confirmed those trends.28 The aminofunctionalized {ON(NR2)OR1,R2} systems proved more stereoselective than those with a methoxy donor group ({ON(OMe)OR1,R2}), enabling the achievement of Pr values in the range 0.94−0.97 only with tert-butyl substituents on the phenolate rings;28 this possibly arises from a higher steric contribution of NR2, arguably bulkier than the OMe group. Another important factor in these ROP reactions of rac-LA is the solvent. High heterotacticities were observed systematically using THF. With toluene or dichloromethane, much lower stereoselectivities (e.g., Pr = 0.65 in toluene vs 0.90 in THF for Y{ON(OMe)OtBu,tBu}(N(SiHMe2)2)(THF))12,14 and sometimes much slower reactions28 were observed. Interestingly enough, highly stereoselective reactions were also observed in the ROP of racemic β-lactones, but with noticeable differences with regard to the stereocontrol parameters. The most common cyclic ester in this series is βbutyrolactone (BL), which leads to poly(3-hydroxybutyrate) (PHB). PHB is a polymer naturally found in some microorganisms, where it serves as energy storage; such natural PHB is purely isotactic. ROP of rac-BL can lead to isotactic, syndiotactic, and atactic PHB. As anticipated from the ability of some Ln{ONXOR1,R2} complexes to enchain R,R/S,S-lactide units with opposite configuration, we first evidenced that the ROP of rac-BL with yttrium catalysts of this class proceeds rapidly (TOF up to 12000 h−1 at 25 °C; TON up to 4000 mol of BL (mol of Y)−1), in a living manner (100% initiation efficiency, Mw/Mn < 1.15), and indeed produces highly

Figure 2. Schematic representation of two DFT-optimized model intermediates in the ROP of BL mediated by Y{ON(OMe)OCMe2Ph,Me}(OR) species, showing C−H···π interactions (relative computed energies; P stands for the polymeryl chain).23 K


Review

Organometallics


Scheme 22. Influence of Substituents in the Syndioselective ROP of Racemic Benzyl and Allyl β-Malolactonates Promoted by in Situ Generated Y{ONXOR1,R2}(OiPr) Complexes21

Scheme 23. Highly Alternating Copolymerization of 50:50 Mixtures of Different MLAR Monomers with Opposite Configurations using a Syndioselective Y{ON(OMe)OCl,Cl}(OiPr) Catalyst21

−122 (Sm), −41 (Tb), −107 (Y), −161 (Lu)). In the DFT calculations, a strong preference was noted for propagation involving a κ2-coordinated polymer chain end and the monomer to be attacked, namely κ3-coordinated C−O dissociation transition states. With such sterically crowded TS, syndio propagation was determined to be energetically preferred over iso propagation due to minimization of steric repulsion between the ligand tert-butyl group and the BL methyl group.17 Our group also recently investigated the ROP of βmalolactonates.21 Alkyl β-malolactonates (MLAR, Figure 1) are monomers that, upon ROP, lead to poly(β-malic acid) derivatives (PMLAR), which are PHAs of high interest because of the potentially accessible pendant functional carboxylate moieties. Although a variety of catalysts/initiators have been disclosed for the ROP of such derivatives, none of them has shown stereoselectivity toward racemic monomers. In line with their performance in ROP of the related BL, we have shown that yttrium catalysts Y{ONXOR1,R2}(OiPr) are highly active and, for some of them, also syndioselective for the controlled (1.03 < Mw/Mn < 1.64) ROP of rac-MLAR derivatives (R = Bn = benzyl, All = allyl) (Scheme 22).21 Surprisingly, high syndioselectivity (Pr > 0.95) was observed for the ligand with o,p-dichloro substituents on the phenolate ring:, i.e., {ON(OMe)OCl,Cl}. “Regular” catalyst systems with bulky aryl/alkyl substituents that proved syndioselective or heteroselective toward the ROP of rac-BL and rac-LA, respectively, afforded only syndiotactic-biased/-enriched PMLAR (Pr = 0.68−0.87). It is noteworthy that, in sharp contrast, this [Y{ON(OMe)OCl,Cl}(OiPr)] catalyst is essentially nonstereoselective in the ROP of rac-LA (Pr = 0.56) and rac-BL (Pr = 0.42−0.45).23

structure of the amido precursor Y{ON(OMe)OCMe2Ph,CMe2Ph}(N(SiHMe2)2)(THF), the two phenyl rings of the cumyl substituents lie parallel to the amido group with close H···π contacts (∼3.0 Å), instead of being rejected out of the coordination sphere to minimize steric repulsions.23 (b) In the NOESY 1H−1H NMR spectrum of the corresponding isopropoxide species Y{ON(OMe)OCMe2Ph,CMe2Ph}(OiPr)(THF) (generated upon alcoholysis of the former amido species), CH(OiPr)···aryl(cumyl) close contacts were also evidenced.23 (c) Despite the fact that species of the type Y{ON(OMe)OCMe2Ph,R}(OiPr)(THF) (i.e., similar to those computed in Figure 2) could never be generated cleanly, close analogues based on tridentate ONO pyridine-bisphenolate ligands were prepared and structurally characterized; hence, in {ONO CMe2Ph,Me}Al((R)-OCH(Me)CH 2CO 2 Me), the two cumyl substituents point in the direction of the βalkoxybutyrate moiety, with short contacts (∼2.6−2.8 Å) between the phenyl π system and one methylene C−H of the butyrate moiety.68 Recently, Rieger and co-workers studied the kinetics, determining the enthalpies and entropies of the ratedetermining steps through temperature-dependent in situ IR measurements and performed DFT computations on the ROP of BL initiated by the mononuclear55 Ln{ON(OMe)OtBu2}(N(SiHMe2)2)(THF) (Ln = Sm, Tb, Y, Lu).17 They found that the ΔH⧧ and ΔS⧧ values are strongly affected by the ionic radius of the metal center, without any monotonous correlation. The ΔG⧧(T) values, determined by the Gibbs− Helmholtz equation, accounted for the varying influence of temperature onto the respective activities of these four catalysts, due to different entropic contributions (ΔS⧧ (J K−1 mol−1) = L



Organometallics

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Obviously, considerations different from the steric and electronic ones mentioned above for the ROP of rac-LA and rac-BL are at work in the ROP of of β-malolactonates. However, their exact nature still remains unknown; it is unclear if this might be related to the dinuclear nature of this peculiar catalyst precursor.55 The ROP was extended to mixtures of chemically different βmalolactonates with opposite absolute configuration to access chemically alternating copolymers. This original strategy was initially implemented by Coates and co-workers in the ROP of nonfunctional β-lactones using an yttrium complex supported by a nontripodal tetradentate bis(phenolate) ligand.69 Hence, the copolymerization of 50:50 mixtures of (R)-MLAAll and (S)MLABn proceeded with the same control as that observed for the ROP of these racemic monomers: yttrium catalysts bearing aryl/alkyl substituents afforded copolymers with 52−75% of alternation, while the dichloro-substituted catalyst gave a very high control of alternation (>95%) (Scheme 23).21 Obviously, this strategy offers many possible variations in terms of substrates and pendant functionalities, and it should lead in the future to new classes of original functionalized alternating copolymers.

Review

AUTHOR INFORMATION

Corresponding Author

*E-mail for J.-F.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The group members whose names appear in the following references are gratefully acknowledged for their essential contribution to the work achieved in our laboratories. This research was financially supported in part by the CNRS (ATIPE Program), the French Ministry of Higher Education (MESR), Institut Universitaire de France (JFC Fellowship, 2005−2009), Agence Nationale de la Recherche (grant no. ANR-06-BLAN-0213), Region Bretagne (grants PolyBio, MaPolBio), CAPES-COFECUB (joined action Ph55607_2007-2010), and CAPES-CNRS (joined action PICS05923).

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REFERENCES

(1) (a) Bellemin-Laponnaz, S.; Dagorne, S. Coordination Chemistry and Applications of Salen, Salan and Salalen Metal Complexes. In Metal Phenolates-Patai’s Chemistry of Functional Groups; Wiley: Weinheim, Germany, 2013. (b) Larrow, J. F.; Jacobsen, E. N. Top. Organomet. Chem. 2004, 6, 123−152. (c) McGarrigle, E. M.; Gilheany, D. G. Chem. Rev. 2005, 105, 1563−1602. (d) Atwood, D. A.; Harvey, M. J. Chem. Rev. 2001, 101, 37−52. (e) Darensbourg, D. J. Chem. Rev. 2007, 107, 2388−2410. (2) (a) Miyashita, A.; Yasuda, A.; Takaya, H.; Toriumi, K.; Ito, T.; Souchi, T.; Noyori, R. J. Am. Chem. Soc. 1980, 102, 7932−7934. (b) Berthod, M.; Mignani, G.; Woodward, G.; Lemaire, M. Chem. Rev. 2005, 105, 1801−1836. (3) (a) Brintzinger, H. H.; Fischer, D.; Mülhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143−1170. (b) Coates, G. W. Chem. Rev. 2000, 100, 1223−1252. (c) Resconi, L.; Cavallo, L.; Fait, A.; Piemontes, F. Chem. Rev. 2000, 100, 1253−1346. (4) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev. 2011, 111, 2363−2449. (5) Handbook of Ring-Opening Polymerization; Dubois, P., Coulembier, O., Raquez, J.-M., Eds.; Wiley: Weinheim, Germany, 2009. (6) For leading reviews, see: (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Dalton Trans. 2001, 2215−2224. (b) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D. Chem. Rev. 2004, 104, 6147−6176. (c) Thomas, C. M. Chem. Soc. Rev. 2010, 39, 165−173. (d) Dijkstra, P. J.; Du, H.; Feijen, J. Polym. Chem. 2011, 2, 520−527. (e) Buchard, A.; Bakewell, C. M.; Weiner, J.; Williams, C. K. Top. Organomet. Chem. 2012, 39, 175−224. (f) Sauer, A.; Kapelski, A.; Fliedel, C.; Dagorne, S.; Kol, M.; Okuda, J. Dalton Trans. 2013, 42, 9007−9023. (g) Guillaume, S. M.; Kirillov, E.; Sarazin, Y.; Carpentier, J.-F. Chem. - Eur. J. 2015, 21, 7988−8003. (7) (a) Burke, W. J.; Smith, R. P.; Weatherbee, C. J. Am. Chem. Soc. 1952, 74, 602−605. (b) Burke, W. J.; Kolbezen, M. J.; Stephens, C. W. J. Am. Chem. Soc. 1952, 74, 3601−3605. (c) Burke, W. J.; Glennie, E. L. M.; Weatherbee, C. J. Org. Chem. 1964, 29, 909−912. (8) (a) Hinshaw, C. J.; Peng, G.; Singh, R.; Spence, J. T.; Enemark, J. H.; Bruck, M.; Wexler, P. A. Inorg. Chem. 1989, 28, 4483−4491. (b) Uma, R.; Viswanathan, R.; Palaniandavar, M.; Lakshminarayanan, M. J. Chem. Soc., Dalton Trans. 1994, 1219−1926. (9) (a) Tshuva, E. Y.; Versano, M.; Goldberg, I.; Kol, M.; Weitman, H.; Goldschmidt, Z. Inorg. Chem. Commun. 1999, 2, 371−373. (b) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Weitman, H.; Goldschmidt, Z. Chem. Commun. 2000, 379−380. (c) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Inorg. Chem. 2001, 40, 4263−4270. (d) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2001, 20, 3017−3028. (e) Tshuva, E. Y.; Groysman,

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CONCLUDING REMARKS The above examples demonstrate the outstanding performance of trivalent rare-earth complexes bearing tripodal dianionic {ONXOR1,R2} ligands in the ring-opening polymerization of cyclic esters (divalent complexes feature essentially similar performance, after likely oxidation of the Ln(II) metal center70). Those complexes, which are fairly easy to prepare in a variety of forms (amides, alkyls, alkoxides, amidinates, borohydrides, ...), feature a very high reactivity which has allowed enlarging the scope of ROP reactions to quite demanding, usually difficult to ring-open monomers. Most of the polymerizations proceeded with a quite good level of control over the molecular weights and a good end-group fidelity. Undoubtedly the most salient feature of these catalysts is their propensity to fine-tune the stereoselectivity of ROP reactions involving chiral monomers. Modifying the ortho substituents on the phenolate rings induces significant alteration of the syndio-/heteroselectivity toward monomers such as lactide, β-lactones, and β-malolactonates. The large range of monomers that can be polymerized in a controlled and stereoselective manner with this class of complexes offers opportunities to combine them, to produce new materials with original microstructures. In particular, access to new alternated polyesters can be envisioned by enlarging the scope of chiral monomers beyond the current functional and nonfunctional βlactones. The exact stereocontrol mechanisms at work in those polymerizations appear to be different according to the monomer involved. While the ROP of lactide seems to obey simply steric considerations, the case of β-lactones is more complicated, with apparent subtle electronic effects. There is here a need to combine further experimental and theoretical investigations around second-sphere weak interactions. The possibility to achieve immortal ROP with these Ln{ONXOR1,R2} complexes in the presence of excess alcohols as chain-transfer agents, still retaining their stereocontrol abilities, has led to a series of valuable applications in macromolecular engineering. Clearly, one can expect more original developments in this direction. M


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Organometallics

(35) Yuan, F.; Zhou, Y.; Li, L.; Zhu, X. Inorg. Chim. Acta 2013, 408, 33−38. (36) Yang, S.; Nie, K.; Zhang, Y.; Xue, M.; Yao, Y.; Shen, Q. Inorg. Chem. 2014, 53, 105−115. (37) Nie, K.; Feng, T.; Song, F.-K.; Zhang, Y.; Sun, H.-M.; Yuan, D.; Yao, Y.-M.; Shen, Q. Sci. China: Chem. 2014, 57, 1106−1116. (38) Bonnet, F.; Cowley, A. R.; Mountford, P. Inorg. Chem. 2005, 44, 9046−9055. (39) Willans, C. E.; Sinenkov, M. A.; Fukin, G. K.; Sheridan, K.; Lynam, J. M.; Trifonov, A. A.; Kerton, F. M. Dalton Trans. 2008, 3592−3598. (40) Zeng, T.; Wang, Y.; Shen, Q.; Yao, Y.; Luo, Y.; Cui, D. Organometallics 2014, 33, 6803−6811. (41) Zeng, T.; Qian, Y.; Wang, Y.; Yao, Y.; Shen, Q. J. Organomet. Chem. 2015, 779, 14−20. (42) Clark, L.; Cushion, M. G.; Dyer, H. E.; Schwarz, A. D.; Duchateau, R.; Mountford, P. Chem. Commun. 2010, 46, 273−275. (43) Li, Q.; Nie, K.; Xu, B.; Yao, Y.; Zhang, Y.; Shen, Q. Polyhedron 2012, 31, 58−63. (44) Ma, M.; Xu, X.; Yao, Y.; Zhang, Y.; Shen, Q. J. Mol. Struct. 2005, 740, 69−74. (45) Liang, Z.; Ni, X.; Li, X.; Shen, Z. Dalton Trans. 2012, 41, 2812− 2819. (46) Li, L.; Yuan, F.; Li, T.; Zhou, Y.; Zhang, M. Inorg. Chim. Acta 2013, 397, 69−74. (47) Binda, P. I.; Delbridge, E. E.; Dugaha, D. T.; Skelton, B. W.; White, A. H. Z. Anorg. Allg. Chem. 2008, 634, 325−334. (48) Guo, H.; Zhou, H.; Yao, Y.; Zhang, Y.; Shen, Q. Dalton Trans. 2007, 3555−3561. (49) Dugah, D. T.; Skelton, B. W.; Delbridge, E. E. Dalton Trans. 2009, 1436−1445. (50) Zhou, H. H.; Guo, H.; Yao, Y.; Zhou, L.; Sun, H.; Sheng, H.; Zhang, Y.; Shen, Q. Inorg. Chem. 2007, 46, 958−964. (51) Yang, S.; Du, Z.; Zhang, Y.; Shen, Q. Chem. Commun. 2012, 48, 9780−9782. (52) Du, Z.; Zhou, H.; Yao, H.; Zhang, Y.; Yao, Y.; Shen, Q. Chem. Commun. 2011, 47, 3595−3597. (53) Altenbuchner, P. T.; Soller, B. S.; Kissling, S.; Bachmann, T.; Kronast, A.; Vagin, S. I.; Rieger, B. Macromolecules 2014, 47, 7742− 7749. (54) Bonnet, F.; Dyer, H. E.; El Kinani, Y.; Dietz, C.; Roussel, P.; Bria, M.; Visseaux, M.; Zinck, P.; Mountford, P. Dalton Trans. 2015, 44, 12312−12325. (55) In fact, as shown in Schemes 3−7 and as mentioned above, the vast majority of the amido and alkyl trivalent lanthanide complexes of this class are mononuclear in the solid state, as established by X-ray diffraction studies. The corresponding alkoxide-type precursor and active species (Scheme 8) are also assumed to be mononuclear, and in some cases, NMR studies were consistent with such a mononuclear structure in solution (in THF-d8).31 Accordingly, Rieger and coworkers also considered in their recent computational studies only the mononuclear core [Y{ON(OMe)OtBu,tBu}].17 Cases where the existence of a dinuclear species was established (Scheme 3) correspond to some complexes of the large lanthanum, with μbridging O (phenolates) and six-coordinated metal centers, or the yttrium complex Y{ON(OMe)OCl,Cl}(N(SiHMe2)2), with μ-bridging Cl atoms, possibly as a result of limited crowding provided by the chloro substituents.23 (56) Castro, P. M.; Zhao, G.; Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Chem. Commun. 2006, 4509−4511. (57) Amgoune, A.; Thomas, C. M.; Ilinca, S.; Roisnel, T.; Carpentier, J.-F. Angew. Chem., Int. Ed. 2006, 45, 2782−2784. (58) (a) Ajellal, N.; Thomas, C. M.; Carpentier, J.-F. J. Polym. Sci., Part A: Polym. Chem. 2009, 47, 3177−3189. (b) Ajellal, N.; Thomas, C. M.; Aubry, T.; Grohens, Y.; Carpentier, J.-F. New J. Chem. 2011, 35, 876−880. (59) Guérin, W.; Diallo, A.; Kirillov, E.; Helou, M.; Slawinski, M.; Brusson, J.-M.; Carpentier, J.-F.; Guillaume, S. M. Macromolecules 2014, 47, 4230−4235.

S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2002, 21, 662−670. (10) Skinner, M. E. G.; Tyrrell, B. R.; Ward, B. D.; Mountford, P. J. Organomet. Chem. 2002, 647, 145−150. (11) Cai, C.-X.; Toupet, L.; Carpentier, J.-F. J. Organomet. Chem. 2003, 683, 131−136. (12) Cai, C.-X.; Amgoune, A.; Lehmann, C. W.; Carpentier, J.-F. Chem. Commun. 2004, 330−331. (13) In {ONXOR1,R2}, a dianionic tripodal potentially tetradentate ligand, X stands for the pendant donor group (e.g., X = OMe, NMe2, 2-pyridyl, etc.) and R1 and R2 stand respectively for the ortho and para substituents on the two phenolate rings. While R1 and R2 can be different, so far only symmetric ligands, i.e. having two identical phenolate groups, have been reported to our knowledge. (14) Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F. Chem. - Eur. J. 2006, 12, 169−179. (15) Deng, L.-Q.; Zhou, Y.-X.; Tao, X.; Wang, Y.-L.; Hua, Q.-S.; Jin, P.; Shen, Y.-Z. J. Organomet. Chem. 2014, 749, 356−363. (16) (a) Bonnet, F.; Cowley, A. R.; Mountford, P. Inorg. Chem. 2005, 44, 9046−9055. (b) Dyer, H. E.; Huijser, S.; Susperregui, S.; Bonnet, F.; Schwarz, A. D.; Duchateau, R.; Maron, L.; Mountford, P. Organometallics 2010, 29, 3602−3621. (17) Altenbuchner, P. T.; Kronast, A.; Kissling, S.; Vagin, S. I.; Herdtweck, E.; Pöthig, A.; Deglmann, P.; Loos, R.; Rieger, B. Chem. Eur. J. 2015, DOI: 10.1002/chem.201501156. (18) Delbridge, E. E.; Dugah, D. T.; Nelson, C. R.; Skeltonb, B. W.; White, A. H. Dalton Trans. 2007, 143−153. (19) Barroso, S.; Cui, J.; Carretas, J. M.; Cruz, A.; Santos, I. C.; Duarte, M. T.; Telo, J. P.; Marques, N.; Martins, A. M. Organometallics 2009, 28, 3449−3458. (20) Chapurina, Y.; Klitzke, J.; de, L.; Casagrande, O., Jr.; Awada, M.; Dorcet, V.; Kirillov, E.; Carpentier, J.-F. Dalton Trans. 2014, 43, 14322−14333. (21) Jaffredo, C. G.; Chapurina, Y.; Guillaume, S. M.; Carpentier, J.F. Angew. Chem., Int. Ed. 2014, 53, 2687−2691. (22) Ajellal, N.; Bouyahyi, M.; Amgoune, A.; Thomas, C. M.; Bondon, A.; Pillin, I.; Grohens, Y.; Carpentier, J.-F. Macromolecules 2009, 42, 987−993. (23) Bouyahyi, M.; Ajellal, N.; Kirillov, E.; Thomas, C. M.; Carpentier, J.-F. Chem. - Eur. J. 2011, 17, 1872−1883. (24) Yao, Y.; Ma, M.; Xu, X.; Zhang, Y.; Shen, Q.; Wong, W.-T. Organometallics 2005, 24, 4014−4020. (25) Kerton, F. M.; Whitwood, A. C.; Willans, C. E. Dalton Trans. 2004, 2237−2244. (26) Shannon, R. D. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, A32, 751. (27) Boyd, C. L.; Toupance, T.; Tyrrell, B. R.; Ward, B. D.; Wilson, C.; Cowley, A. R.; Mountford, P. Organometallics 2005, 24, 309−330. (28) Liu, X.; Shang, X.; Tang, T.; Hu, N.; Pei, F.; Cui, D.; Chen, X.; Jing, X. Organometallics 2007, 26, 2747−2757. (29) Zhao, W.; Li, C.; Liu, B.; Wang, L.; Li, P.; Wang, Y.; Wu, C.; Yao, C.; Tang, T.; Liu, X.; Cui, D. Macromolecules 2014, 47, 5586− 5594. (30) Addition of more than 1 equiv of alcohol onto a catalyst precursor enables achievement of degenerative chain transfer, also referred to as immortal ring-opening polymerization (iROP), as pioneered by Inoue:. Asano, S.; Aida, T.; Inoue, S. J. Chem. Soc., Chem. Commun. 1985, 1148−1149. Aida, T.; Inoue, S. Acc. Chem. Res. 1996, 29, 39−48. For a leading review, see:. Ajellal, N.; Carpentier, J.-F.; Guillaume, C.; Guillaume, S. M.; Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans. 2010, 39, 8363−8376. (31) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Pure Appl. Chem. 2007, 79, 2013−2030. (32) Amgoune, A.; Thomas, C. M.; Carpentier, J.-F. Macromol. Rapid Commun. 2007, 28, 693−697. (33) Nie, K.; Gu, X.; Yao, Y.; Zhang, Y.; Shen, Q. Dalton Trans. 2010, 39, 6832−6840. (34) Nie, K.; Fang, L.; Yao, Y.; Zhang, Y.; Shen, Q.; Wang, Y. Inorg. Chem. 2012, 51, 11133−11143. N


Review


Organometallics (60) (BDIiPr)− = CH(CMeNC6H3-2,6-iPr2)2: Chamberlain, B. M.; Cheng, M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am. Chem. Soc. 2001, 123, 3229−3238. (61) (NNO)− = 2,4-di-tert-butyl-6-{[(2′-dimethylaminoethyl)methylamino]methyl}phenolate: Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V. G.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125, 11350−11359. (62) Zhao, W.; Wang, Y.; Liu, X.; Cui, D. Chem. Commun. 2012, 48, 4588−4590. (63) Zhao, W.; Wang, Y.; Liu, X.; Cui, D. Chem. Commun. 2012, 48, 4483−4485. (64) Zhao, W.; Wang, Y.; Liu, X.; Chen, X.; Cui, D. Chem. - Asian J. 2012, 7, 2403−2410. (65) Of course, along linear macromolecules, macrocyclic polyesters can be formed due to intramolecular “back-biting” reactions; their extent depends on the reaction conditions and the nature of the monomer and catalyst. See for instance ref 40. (66) Pr is the probability of racemic enchainment (identical with Ps, the probability of syndiotactic enchainment) of a monomer unit in the macromolecular chain (i.e., enchainment of stereocenters with opposite absolute configuration). Conversely, Pm is the probability of meso enchainment (identical with Pi, the probability of isotactic enchainment) of a monomer unit in the macromolecular chain (i.e., enchainment of stereocenters with identical absolute configuration). In the ROP of rac-LA, a monomer which contains two stereocenters per monomer unit (R,R for D-LA and S,S for L-LA), a racemic enchainment leads to a so-called heterotactic polymer (...R,R/S,S/R,R/S,S/R,R/S,S/ ...) with 0.5 (heterotactic-biased) > Pr > 1 (perfectly heterotactic) (i.e., 0 < Pm < 0.5). For a perfectly isotactic polymer (...R,R/R,R/R,R/... or··· S,S/S,S/S,S/...), Pm = 1.0 (i.e., Pr = 0), while Pm = Pr = 0.5 indicates an essentially atactic polymer. (67) Marshall, E. L.; Gibson, V. C.; Rzepa, H. S. J. Am. Chem. Soc. 2005, 127, 6048−6051. (68) (a) Klitzke, J. S.; Roisnel, T.; Kirillov, E.; de, L.; Casagrande, O., Jr; Carpentier, J.-F. Organometallics 2014, 33, 309−321. (b) Klitzke, J. S.; Roisnel, T.; Kirillov, E.; Casagrande, O., Jr.; Carpentier, J.-F. Organometallics 2014, 33, 5693−5707. (69) Kramer, J. W.; Treitler, D. S.; Dunn, E. W.; Castro, P. M.; Roisnel, T.; Thomas, C. M.; Coates, G. W. J. Am. Chem. Soc. 2009, 131, 16042−16044. (70) As investigated by Delbridge and Shen, many divalent rare-earth Ln{ONXOR1,R2} complexes were also found to be active precursors in the ROP of cyclic esters such as caprolactone (CL),18,49,50 lactide (LA),36,49,51 and trimethylenecarbonate (TMC): Zhao, B.; Hu, X. L.; Lu, C. R. J. Appl. Polym. Sci. 2011, 120, 2693−2698. In the ROP of rac-LA, YbII{ONXOtBu,Me}(TMEDA) complexes (X = 2-tetrahydrofuryl, 2-furyl) yielded highly heterotactic PLA (Pr = 0.82−0.99), and an important influence of the X donor on the stereoselectivity was noticed, just as for the analogous trivalent Yb and Y partners.36,51 Those ROP reactions proceeded often very quickly, comparable to those initiated by equivalent trivalent Ln{ONXOR1,R2}(R) complexes (R = amido, alkoxide, alkyl), with the general trend Yb(II) ≪ Sm(II), possibly related to the lower oxidation potential of Sm related to Yb; the degree of control over the molecular weights were, however, somewhat lower, probably due to the transformation of the divalent precursor into the real active species. Details on the initial step of the polymerization reaction are, however, not yet clear. It is assumed that those divalent complexes are rapidly oxidized by the cyclic ester (which is concomitantly reduced), to produce a “Ln(III)-acyl(ringopened monomer)” species. So far, this has been supported only by indirect evidence (rapid discoloration of the precursor, end-group analysis of the produced polymer), as the actual initiating species could not be isolated.

O


Rare-Earth Complexes Supported by Tripodal Tetradentate Bis

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