Heteroscorpionate Magnesium Alkyls Bearing Unprecedented Apical σ-C(sp3)–Mg Bonds: Heteroselective Ring-Opening Polymerization of rac-Lactide

Metal Complexes in the Synthesis of Biodegradable Polymers: Achievements and Prospects

This review describes recent advances in the synthesis of homopolymers of lactide and related cyclic esters via ring-opening polymerization (ROP) in the presence of metal complexes based on group 1, 2, 4, 12, 13 and 14 metals. Particular attention is paid to the influence of the initiator structure on the properties of the obtaining homo- and copolymers. Also, a separate chapter is devoted to the study of metal complexes in the synthesis of copolymers of lactide and lactones. This review highlights the efforts made over the last ten years or so, and shows how main-group metals have received increasing attention in the field of the polymerization of lactide and related cyclic esters.

Cooperative Initiation in a Dinuclear Indium Complex for CO2 Epoxide Copolymerization

Dinuclear indium complexes have been synthesized and characterized. These include neutral and cationic indium complexes supported by a Schiff base ligand bearing a binaphthol linker. The new compounds were investigated for alternating copolymerization of CO₂ and cyclohexene oxide. In particular, the neutral indium chloride complex (±)-[(O_NapN_iN)InCl₂]₂ (4) showed high conversion of cyclohexene oxide and selectivity for poly(cyclohexene carbonate) formation without cocatalysts at 80 °C under various CO₂ pressures (2-30 bar). Importantly, the reactivity of the dinuclear indium chloride complex 4 is drastically different from that of the mononuclear indium chloride complex (±)-(NN_iO_tBu)InCl₂ (5), suggesting a cooperative initiation mechanism involving the two indium centers in 4.

Alkali and Alkaline Earth Metal Complexes as Versatile Catalysts for Ring-Opening Polymerization of Cyclic Esters

Biodegradable polyesters such as poly(ϵ-caprolactone) (PCL) and poly(lactic acid) (PLA) have been considered for use in several areas, such as drug delivery devices, sutures, tissue engineering, and GBR membranes, due to its bio-renewability, biodegradability, and biocompatibility. Several synthetic techniques for the preparation of polyesters have been reported in the literature, amongst which the ring-opening polymerization (ROP) of cyclic esters is the most efficient. A convenient approach to access iso-selective PLAs is polymerization of racemic lactide (rac-LA), which shows excellent stereoregularity without the need for costly chiral auxiliaries or ligands. In this personal account, we review a series of methods that have been practiced to the synthesis of biodegradable polyesters from various cyclic monomers using alkali and alkaline earth metal complexes as efficient catalysts.

Cooperative Heterometallic Catalysts for Lactide Ring-Opening Polymerization: Combining Aluminum with Divalent Metals

While homometallic (salen)Al catalysts display excellent performance in lactide ring-opening polymerization (ROP), heterometallic (salen)Al complexes have yet to be reported. Herein, we describe four heterobimetallic (salen)Al catalysts and show that the choice of the heterometal is key. Cooperative Al/Mg and Al/Zn combinations improved the catalyst activity by a factor of up to 11 compared to the mono-Al analogue, whereas the mono-Mg and mono-Zn analogues were completely inactive. In contrast, Al/Li and Al/Ca heterocombinations stunted the polymerization rate. Kinetic and computational studies suggest that Al/Mg and Al/Zn cooperativity arises from the close intermetallic proximity facilitating chloride bridging (thus enhancing initiation), which promotes a rigid square pyramidal geometry around the Al center and further increases the available monomer coordination sites. This work also translates the use of ab initio molecular dynamics calculations to ROP, introducing a useful method of investigating catalyst flexibility and revealing that ligand strain and molecular rigidity can enhance heterometallic catalyst performance.

Make or break: Mg(ii)- and Zn(ii)-catalen complexes for PLA production and recycling of commodity polyesters

A series of Mg(ii) and Zn(ii) catalen complexes have been prepared for PLA formation and recycling.

Isoselective Polymerization of rac-Lactide by Highly Active Sequential {ONNN} Magnesium Complexes

The coordination chemistry and the activities in the ring-opening polymerization catalysis of racemic lactide (LA) of magnesium complexes of a series of {ONNN}-type sequential monoanionic ligands are described. All ligands include pyridyl and substituted-phenolate as peripheral groups. The ligands bearing either chiral or meso-bipyrrolidine cores led to single diastereomeric complexes, whereas the ligands bearing a diaminoethane core led to diastereomer mixtures. All {ONNN}Mg-X complexes [X=Cl, HMDS (hexamethyldisilazide)] led to highly active and isoselective catalysts. The complexes bearing the chiral bipyrrolidine core exhibited the highest activities (full consumption of 5000 equiv. of rac-LA at RT within 5 min) and highest isoselectivities (P_m =0.91), as well as a living character. The complexes of the meso-bipyrrolidine based ligands were almost as active and slightly less stereoselective, while those of the diaminoethane based ligands exhibited reduced activities and isoselectivities.

Syntheses, structures and catalytic activity of tetranuclear Mg complexes in the ROP of cyclic esters under industrially relevant conditions

Tetranuclear magnesium imino(phenolate) complexes Mg₄(L^1–4)₄ are excellent catalysts for the ROP of bulk rac-lactide and ε-caprolactone under industrially relevant conditions.

Structure-Solubility Relationship of 1,4-Dioxane Complexes of Di(hydrocarbyl)magnesium

Systematic variation of the 1,4‐dioxane (dx) concentration during the precipitation of sparingly soluble [MgBr₂(dx)₂] from ethereal Grignard solutions of RMgBr has allowed the structural investigation of crystallized [R₂Mg(dx)_n] (n=1, 1.5, 2, and 3), which form during this dioxane method, depending on the bulkiness of R. The numbering of the complexes explored in this study is based on the number n of dioxane molecules per magnesium atom, followed by the substituent R; an apostrophe denotes coordination polymers. The following derivatives were studied by X‐ray crystal‐structure determination and NMR spectroscopy: n=1: [Me₂Mg(μ‐dx)]_∞ (1′‐Me) and [nPr₂Mg(μ‐dx)]_∞ (1′‐nPr); n=1.5: [{iPr₂Mg(dx)}₂(μ‐dx)] (1.5‐iPr), [{oTol₂Mg(dx)}₂(μ‐dx)] (1.5‐oTol), and [(Me₃Si‐C≡C)₂Mg(dx)_1.5]_∞ (1.5′‐C₂SiMe₃); n=2: [tBu₂Mg(dx)₂] (2‐tBu) and [oTol₂Mg(dx)₂] (2‐oTol); n=3: [Ph₂Mg(dx)₃] (3‐Ph). In the structure types 1′, 1.5, and 2, the magnesium atom exhibits the coordination number 4, whereas pentacoordinate metal atoms are observed in types 3 and 1.5′. The structure type 2′ is realized for [(Ph‐C≡C)₂Mg(dx)₂]_∞ (2′‐C₂Ph), [MgCl₂(dx)₂]_∞ (2′‐Cl), and [MgBr₂(dx)₂]_∞ (2′‐Br) with hexacoordinate metal atoms. The solubility of the dioxane adducts in common organic solvents strongly depends on the degree of aggregation with the solubility decreasing from molecular to strand to layer structures.

Magnesium, calcium and zinc [N2N'] heteroscorpionate complexes

A new family of sterically demanding N2N' heteroscorpionate pro-ligands (HC(tBu2pz)2SiMe2N(H)R (R = iPr, tBu, Ph, Xyl)) has been prepared via a straightforward modular synthetic route. An extensive study into the synthesis and characterisation of lithium, magnesium, calcium and zinc complexes supported by both 3,5-tBu and 3,5-Me substituted N2N' ligand families has been conducted. Attempted deprotonation of the pro-ligands with nBuLi afforded the corresponding lithium salts Li{HC(tBu2pz)2SiMe2NR} (R = iPr (1), tBu (2), Ph (3) and Xyl (4)) but air- and thermal-sensitivity limited the yields of these potentially useful precursors; only the sterically encumbered ligand system allowed clean reactivity. Magnesium methyl complexes Mg{HC(tBu2pz)2SiMe2NR}Me (R = iPr (5) and R = Ph (6)) were prepared using an excess of the Grignard reagent MeMgCl. Magnesium butyl complexes were synthesised in good yields using the dialkyl precursor MgnBu2 to afford Mg{HC(R'2pz)2SiMe2NR}nBu (R' = Me; R = iPr (7), tBu (8), Ad (9), Ph (10). R' = tBu; R = iPr (11), Ph (12)). Protonolylsis reactions were used to synthesise magnesium and calcium amide complexes Mg{HC(R'2pz)2SiMe2NR}{N(SiHMe2)2} (R' = Me; R = iPr (13), tBu (14), Ph (15). R' = tBu; R = Ph (16)) or Mg{HC(R'2pz)2SiMe2NR}{N(SiMe3)2} (R' = Me; R = iPr (17), tBu (18), Ph (19). R' = tBu; R = Ph (20)), and Ca{HC(R'2pz)2SiMe2NR}{N(SiMe2)2} (L) (R' = Me; L = thf; R = iPr (21), tBu (22), Ph (23). R' = tBu; L = none; R = Ph (24). Zinc methyl complexes Zn{HC(R'2pz)2SiMe2NR}Me (R' = Me; R = iPr (25), tBu (26), Ph (27). R' = tBu; R = Ph (28)) were prepared by reaction of the N2N' heteroscorpionate pro-ligands with ZnMe2. In preliminary studies, magnesium amide complexes 16 and 20 were evaluated as initiators for the ring-opening polymerisation (ROP) of ε-caprolactone (ε-CL) and rac-lactide (rac-LA). Although the overall polymerisation control was poor, 16 and 20 were found to be active initiators.

Efficient CO2 fixation into cyclic carbonates catalyzed by NNO-scorpionate zinc complexes

Chiral bifunctional and bicomponent NNO-scorpionate zinc-based catalysts have been developed for the fixation of CO₂ into cyclic carbonates with broad substrate scope and functional group tolerance under mild and solvent-free conditions.

Alkali metal complex-mediated ring-opening polymerization of rac-LA, ε-caprolactone, and δ-valerolactone

Catalytic ring opening polymerization (ROP) of rac-lactide, ε-caprolactone, and δ-valerolactone using alkali metal (Li, Na, K) complexes as competent catalysts are reported.

Multiple coordination modes of a new ditopic bis(pyrazolyl)methane-based ligand

A new ditopic ligand, N-(2,2-bis(pyrazolyl)ethyl)-2,2-bis(pyrazolyl)acetamide ((pz)₂CH-C(O)-NH-CH₂-CH(pz)₂, L4Pz, pz = pyrazolyl ring), comprising two bis(pyrazolyl)methane donor groups linked via an amide bridge, has been prepared from the reaction of HOOCCH(pz)₂ and H₂NCH₂CH(pz)₂. The ligand coordinates to various metallic salts (i.e. AgO₃SCF₃, PdCl₂, Re(CO)₅Br, and Fe(BF₄)₂), in either a κ²-μ-κ² or a κ³-μ-κ² fashion, depending on the coordination preferences of the metallic center. These compounds were characterized by NMR, UV-Vis and IR spectroscopy, and in solid state by single crystal X-ray diffraction. In the case of silver(i), a mono-dimensional coordination polymer was obtained, while the others were found to be discrete complexes. The synthesis and characterization of a heterobimetallic complex is also described. In solid state, all compounds are associated into supramolecular architectures via hydrogen bonding and pyrazolyl embrace interactions.

Organo-Aluminum and Zinc Acetamidinates: Preparation, Coordination Ability, and Ring-Opening Polymerization Processes of Cyclic Esters

The reaction of the highly sterically demanding NNN'-heteroscorpionate protioligands pbp^tamd-H, tbp^tamd-H, and phbp^tamd-H (a) and the low sterically hindered analogs pbpamd-H, tbpamd-H, and phbpamd-H (b), with 1 equiv of AlR₃ (R = Me, Et) proceed in high yields to give two families of complexes: the mononuclear dialkyl aluminum bidentate-acetamidinates [AlR₂(κ²- N' N')] (κ²- N' N' = pbp^tamd, R = Me 1, Et 2; tbp^tamd, R = Me 3, Et 4; phbp^tamd, R = Me 5, Et 6) and the monodentate-acetamidinates [AlR₂(κ²- NN')] (κ²- NN' = tbpamd, R = Me 7; phbpamd, R = Me 8, Et 9). In complexes 7-9, the presence of two possible CH-NH tautomers as low extended π-N-C-N'(sp²)-Al and high extended π-HN-C₂-N'(sp²)-Al complexes, respectively, could be identified. Moreover, the reaction of aluminum dimethyls 7 and 8 with ZnMe₂ afforded the isolation of the more stable scorpionate zinc monoalkyls [Zn(Me)(κ³- NNN')] ( NNN' = tbpamd 10 and phbpamd 11), through a very unusual ligand exchange process, involving a zinc-to-aluminum transmetalation of an alkyl group. The X-ray crystal structures of 1, 3, 7, and 8, as well as that of 11, confirmed unambiguously the different κ²-arrangements proposed for bi- or monodentate acetamidinate dialkyls 1-6 and 7-9, respectively, the presence of NH tautomer in 7 and 8, and a κ³- NNN' coordination in monoalkyl 11. Density functional theory calculations were used to explore the three different favored κ²-arrangements found in acetamidinate aluminum dialkyls 1-9, the relative stability of both CH-NH tautomers, and the ligand transfer reaction leading to the formation of κ³- NNN' zinc monoalkyls 10 and 11. Interestingly, dialkyls 1, 5, 7, and 8 can act as highly efficient single-component living initiators for the ring-opening polymerization of ε-caprolactone and rac-lactide in mild conditions after hours. These initiators efficiently mediated the immortal polymerization in the presence of excess of benzyl alcohol (up to 20 equiv), as evidenced by the narrow dispersity values and the good agreement between the experimental M_n values and monomer/benzyl alcohol ratios. In addition, the most sterically hindered initiator, 5, exhibits enhanced levels of heteroselectivity on the produced PLAs, reaching P_s values up to 0.70.

Versatile organoaluminium catalysts based on heteroscorpionate ligands for the preparation of polyesters

A series of alkyl aluminium complexes based on heteroscorpionate ligands were designed as catalysts for the ring-opening polymerisation of cyclic esters and ring-opening copolymerisation of epoxides and anhydrides. Treatment of AlX3 (X = Me, Et) with ligands bpzbeH [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide], bpzteH [bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-bpzmmH [(R,R)-bpzmm = (1R)-1-{(1R)-6,6-dimethyl-bicyclo[3.1.1]-2-hepten-2-yl}-2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide] for 2 hours at 0 °C afforded the mononuclear dialkyl aluminium complexes [AlMe2{κ2-bpzbe}] (1), [AlEt2{κ2-bpzbe}] (2), [AlMe2{κ2-(R,R)-bpzmm}] (3) and [AlEt2{κ2-(R,R)-bpzmm}] (4), and the dinuclear dialkyl complexes [AlMe2{κ2-bpzte}]2 (5) and [AlEt2{κ2-bpzte}]2 (6). The molecular structures of the new complexes were determined by spectroscopic methods and confirmed by X-ray crystallography. The alkyl-containing aluminium complexes can act as highly efficient single-component initiators for the ring-opening polymerisation of ε-caprolactone and l-lactide and for the ring-opening copolymerisation of cyclohexene oxide and phthalic anhydride to give a range of biodegradable polyesters.

Highly active Mg(ii) and Zn(ii) complexes for the ring opening polymerisation of lactide

Simple, highly active, Zn(ii) and Mg(ii) complexes for the industrial ring opening polymerisation (ROP) of lactide.

Isoselective mechanism of the ring-opening polymerization of rac-lactide catalyzed by chiral potassium binolates

A chain end control mechanism for the isoselective ring-opening polymerization of rac-lactide catalyzed by potassium complexes was proven via three different methods.

Preparation of zirconium and hafnium complexes containing chiral N atoms from asymmetric tertiary amine ligands, and their catalytic properties for polymerization of rac-lactide

Preparation of Zr and Hf complexes containing chiral N atoms, and their use in polymerization of rac-lactide.

Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

The synthesis of the first alkoxide-bridged indium complex supported by a chiral dinucleating ligand platform (1), along with its zinc analogue (2), is reported. Both complexes are synthesized in a one-pot reaction starting from a chiral dinucleating bis(diamino)phenolate ligand platform, sodium ethoxide, and respective metal salts. The dinucleating indium analogue (7) based on an achiral ligand backbone is also reported. Indium complexes bearing either the chiral or achiral ligand catalyze the ring-opening polymerization of racemic lactide (rac-LA) to afford highly heterotactic poly(lactic acid) (PLA; Pr > 0.85). The indium complex bearing an achiral ligand affords essentially atactic PLA from meso-LA. The role of the dinucleating ligand structure in catalyst synthesis and polymerization activity is discussed.

Synthesis of Sodium Complexes Supported with NNO-Tridentate Schiff Base Ligands and Their Applications in the Ring-Opening Polymerization of L-Lactide

A series of sodium complexes bearing NNO-tridentate Schiff base ligands with an N-pendant arm were synthesized and used as catalysts for the ring-opening polymerization of L-lactide (L-LA). Electronic effects of ancillary ligands coordinated by sodium complexes substantially influence the catalysis, and ligands with electron-donating groups increase the catalytic activity of the sodium complexes for catalyzing L-LA polymerization. In particular, a sodium complex bearing a 4-methoxy group has the highest activity with conversion up to 95% within 30 s at 0 °C and a low polydispersity index of 1.13, whereas the 4-bromo group showed the poorest performance with regard to the catalytic rate of L-LA polymerization in the presence of benzyl alcohol (BnOH). (1)H NMR pulsed-gradient spin-echo diffusion experiments and single-crystal X-ray analyses showed that sodium complexes [L(H)Na(THF)]2 and [L(4-Cl)Na(THF)]2 were dinuclear species in both solution and the solid state. The kinetic results indicated a first-order dependence on each of [[L(4-Cl)Na]2], [l-LA], and [BnOH].

Controlling the stereoselectivity of rac-LA polymerization by chiral recognition induced the formation of homochiral dimeric metal alkoxides

Chiral recognition of monomeric Me₂MOR units resulting in the formation of homochiral dimeric species [Me₂M(μ-OR)]₂(M = Ga, In), leads to heteroselective catalysts for the ring opening polymerization ofrac-lactide (rac-LA).

Catalytic behaviour in the ring-opening polymerisation of organoaluminiums supported by bulky heteroscorpionate ligands

A series of alkyl organoaluminium complexes based on bulky heteroscorpionate ligands were designed as catalysts for the ring-opening polymerisation of cyclic esters. Thus, the treatment of AlX3 (X = Me, Et) with bulky acetamide or thioacetamide heteroscorpionate ligands nbptamH (1) [nbptamH = N-naphthyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide], fbpamH (2) [fbpamH = N-fluorenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide], ptbptamH (3) [ptbptamH = N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide], ntbptamH (4) [ntbptamH = N-naphthyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide], ptbpamH (5) [ptbpamH = N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)acetamide] and (S)-mtbpamH (6) [(S)-mtbpamH = (S)-(−)-N-α-methylbenzyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)acetamide] for 1 hour at 0 °C afforded the dialkyl aluminium complexes [AlX2{κ(2)-nbptam}] (X = Me 7, Et 8), [AlX2{κ(2)-fbpam}] (X = Me 9, Et 10), [AlX2{κ(2)-ptbptam}] (X = Me 11, Et 12), [AlX2{κ(2)-ntbptam}] (X = Me 13, Et 14), [AlX2{κ2(-)ptbpam}] (X = Me 15, Et 16) and [AlX2{κ(2)-(S)-mtbpam}] (X = Me 17, Et 18). The structures of the complexes were determined by spectroscopic methods and the X-ray crystal structure of 14 was also established. The alkyl-containing aluminium complexes 7–18 can act as efficient single-component initiators for the ring-opening polymerisation of ε-caprolactone and rac-lactide. The polymerisations are living, as evidenced by the narrow polydispersities of the isolated polymers and the linear nature of the number average molecular weight versus conversion plot. Finally, a comparative study of ring-opening polymerisation for new bulky heteroscorpionate aluminium initiators and the less congested aluminium analogues is reported.

Iso-selective ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium naphthalenolates

Two crown ether complexes of sodium and potassium naphthalenolates were synthesized and entirely characterized. The two complexes can iso-selectively catalyze the ring-opening polymerization (ROP) of rac-lactide at room temperature and afford polylactides with desired molecular weights and narrow PDIs; the best isotacticity (Pm) achieved was 0.73.