Highly isoselective and active ring-opening polymerization of rac-lactide to obtain stereocomplexed polylactide is a long-standing challenge. In this contribution, zinc catalysts with good isoselectivities and high activities (Pm = 0.87, TOF = 3312 h–1 at 25 °C; Pm = 0.92, TOF = 117 h–1 at −20 °C, toluene) for rac-lactide polymerization are reported. These catalysts are coordinated by a well-designed chiral oxazolinyl or achiral benzoxazolyl aminophenolate ligand. Both types of zinc complexes afforded multiblock isotactic PLAs, proving to be formed via a chain-end control mechanism regardless of the existence of a chiral group or not. Preliminary studies suggested that the dihedral angle between the phenoxy and oxazolinyl/benzoxazolyl planes in these complexes reflects the steric bulkiness of the ligand and has a close relation with the isoselectivity of the complex, which might serve as a guide in obtaining new isoselective catalysts of similar types. Furthermore, unprecedented combination of excellent activity (TOF up to 44 000 h–1) and sufficient isoselectivity (Pm = 0.80) could be achieved under solvent-free immortal conditions with catalyst loadings as low as 0.005 mol % (vs monomer), which shows the potential of industrial application.

A series of potassium complexes bearing monoanionic tetradentate amino-phenolate ligands, [LK]2 (L = {(2-R1)C6H4CH2N[(CH2)2R2]CH2(4-R4-6-R3)C6H2O-}, R1 = NMe2, R2 = NEt2, R3 = CPh3, R4 = Me (1); R1 = R2 = NEt2, R3 = CPh3, R4 = Me (2); R1 = NMe2, R2 = NEt2, R3 = R4 = cumyl (4); R1 = R2 = OMe, R3 = tBu, R4 = Me (6); L = (2-NMe2)C6H4CH2N[[CH2-(S)-1-butylpyrrolidinyl]CH2(4-Me-6-CPh3)C6H2O-] (3)), have been synthesized via reactions of KN(SiMe3)2 and 1 equiv. of the corresponding aminophenols. The solid-state structures of typical complexes 4 and 6 are determined via X-ray diffraction studies, which reveal the dinuclear nature of these complexes. By contrast, DOSY measurements of 1, 4 and 6 suggest that these complexes are monomeric in solution. It is noteworthy that the coordination chemistry of these potassium complexes is versatile, which is closely related to the nature of the ortho-substituent of the phenolate ring, as indicated by the results of the corresponding spectroscopic studies. In the presence of iPrOH, 1–4 and 6 could initiate the polymerization of 500 equiv. of rac-lactide to achieve high monomer conversions within several minutes but afford atactic PLAs with slightly isotactic-enriched microstructures (Pm = 0.58–0.60). Experimental results also demonstrated that a bulky trityl substituent at the ortho-position of the phenolate ring of the ligand framework is beneficial for the enhancement of the activities of these potassium complexes.

•Mild reaction conditions, convenient manipulation.•Moisture tolerance.•Wide substrates scope.CuF2·2H2O/4,7-diphenyl-1,10-phenanthroline was found to be a highly efficient catalyst for the nucleophilic alkynylation of cyclopropyl trifluoromethyl ketone. The corresponding trifluoromethyl-substituted tertiary propargyl alcohols were synthesized in moderate to excellent yields under mild conditions. The catalyst system could also be applied to aromatic trifluoromethyl ketone as well as trifluoropyruvate.Download high-res image (103KB)Download full-size image

NTrifluoromethyl and cyclopropyl substituted 2-isoxazolines were synthesized via a DBU-promoted domino reaction of β-trifluoromethyl-/β-cyclopropyl-substituted enones with hydroxylamine. The domino reaction consists of a Michael addition and the followed cyclization. A wide range of 3-substituted 5-cyclopropyl-5- trifluoromethyl-2-isoxazolines were obtained in good to excellent yields under mild reaction conditions. The method could also apply to other trifluoromethyl-substituted enones.Download high-res image (85KB)Download full-size imageTrifluoromethyl and cyclopropyl substituted 2-isoxazolines were synthesized via a DBU-promoted domino reaction of β-trifluoromethyl-/β-cyclopropyl-substituted enones with hydroxylamine. A wide range of 3-substituted 5-cyclopropyl-5- trifluoromethyl-2-isoxazolines were obtained in good to excellent yields under mild reaction conditions.

A Cs2CO3-catalyzed alkylation reaction of indoles with trifluoromethyl ketones was presented. Both alicyclic and aromatic trifluoromethyl ketones as well as various substituted indoles are compatible with the methodology. Good to excellent yields of the corresponding trifluoromethyl substituted tertiary alcohols 2,2,2-tritrifluoro-1-(1H-indol-3-yl)-ethan-1-ols were acquired as the sole products.Download high-res image (78KB)Download full-size image

A series of tridentate chiral aminophenol proligands and corresponding zinc complexes, LZnX (L = (S)-2-{[(1-R4-2-pyrrolidinyl)CH2N(R3)-]CH2}-6-R1-4-R2-C6H2O, X = N(SiMe3)2, R3 = nBu, R4 = Bn: R1 = R2 = Cl (1), R1 = R2 = Me (2), R1 = R2 = tBu (3); X = N(SiMe3)2, R1 = trityl, R2 = Me: R3 = n-octyl, R4 = Bn (4), R3 = Bn, R4 = Bn (5), R3 = nBu, R4 = naphthalen-1-ylmethyl (6), R3 = nBu, R4 = iPr (7); R1 = R2 = cumyl, R3 = Et, R4 = Bn: X = N(SiMe3)2 (8), X = OtBu (9), X = Et (10), X = Cl (11)), have been synthesized. Complexes 4, 6, and 11 were obtained as enantiopure products (4 and 6 as enantiopure a; 11 as enantiopure b), while complexes 1–3, 5, and 7–10 as a pair of diastereomers, but in different ratios, which have been proved by X-ray diffraction and NMR spectroscopic studies. When exposed to the ring-opening polymerization of rac-lactide, most of these complexes can effectively produce PLAs with narrow polydispersities, desirable molecular weights, and moderate to high isotacticities. The structure–selectivity relationships, including the relationships of structure–synthesis diastereoselectivity and structure–polymerization stereoselectivity, have been further investigated. Consistent trends of diastereoselectivity and stereoselectivity are observed with the variations of the R1 group at the ortho-position of the phenolate ring and the R3 group in the pyrrolidinyl moiety. The decrease of the steric bulkiness of the R4 group on the central amine has less influence on the diastereoselectivity, but leads to considerable loss of the stereoselectivity, whereas the decrease of the steric bulkiness of the X group results in a reverse of the diastereoselectivity, but shows no influence on the stereoselectivity. There is probably no direct relationship between the diastereoselectivity in complex synthesis and the stereoselectivity of the complex toward the ROP of rac-LA. The stereocontrol of these complexes might largely rely on the substituents in the ligand framework rather than their diastereomer ratios.

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6′-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe3 with various amounts of the proligands L1−4H2 (6,6′-[(6,6′-dimethyl-[1,1′-biphenyl]-2,2′-diyl)bis(nitrylomethilidyne)]-bis(2-R1-4-R2-phenol): L1H2, R1 = R2 = Me; L2H2, R1 = tBu, R2 = Me; L3H2, R1 = R2 = cumyl; L4H2, R1 = Br, R2 = tBu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [L1−3AlMe (1–3), L1−4Al2Me4 (4–7)]. Aluminum alkoxide complexes L2AlOiPr (8), L2AlOBn (9), and α-alkoxy ester complexes L2Al(OCMe2CO2Me) (10), L2Al[(S)-OCHMeCO2Me] (11) were prepared via in situ alcoholysis of the parent aluminum methyl complex 2 with the corresponding alcohols. The molecular structures of mononuclear complexes 1–3, dinuclear complex 6, alkoxide complexes 8–9 and α-alkoxy ester complexes 10–11 were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69–70 ppm and 25–41 ppm were observed in the 27Al NMR spectra of complexes 10 and 11, indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex 11 is also a rare example of an O-lactate model complex that mimics the first insertion of L-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes 1–3 in the presence of iPrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03–1.05) and 65–73% isotacticities. Particularly, complex 8 showed an “immortal” behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.

A series of magnesium silylamido complexes supported by chiral aminophenolate ligands were synthesized, and their catalytic behaviors toward the ring-opening polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of Mg[N(SiMe3)2]2 with chiral tridentate aminophenols L1–10H [L = (S)-{[(1-R4, 2-pyrrolidinyl)CH2N(R3)-]}CH2-(6-R1-4-R2)-C6H2O−] in a 1:1 molar ratio in toluene gave the chiral aminophenolate magnesium silylamido complexes L1–10MgN(SiMe3)2 as enantiopure compounds or as pairs of diastereomers. As determined by X-ray analysis, structures a and b of diastereomer mixtures adopt the configuration of SCSNRNRMg-a and SCSNSNSMg-b, respectively. It was found that most of these magnesium complexes served as highly active initiators for the ring-opening polymerization of rac-LA. Besides, the stereoselectivity of chiral magnesium complexes gradually changed from isoselective-biased (Pm = 0.67) to heteroselective-enriched (Pr = 0.81) via a stepwise adjustment of the substituents of the ligand framework. It was noted that, as the rare reported active magnesium initiator for the ROP of rac-BBL, complexes 3–9 can efficiently initiate the ring-opening polymerization of rac-BBL in a controlled manner, giving moderate syndiotactic (Ps up to 0.73) PHBs.

The mono- and dinuclear salen aluminum complexes L1–3AlMe (1, 2, 5), L2AlOiPr (3), L2AlOBn (4) and L1–3Al2Me4 (6–8) (L = 6,6′-[(6,6′-dimethyl-[1,1′-biphenyl]-2,2′-diyl)bis(nitrylomethilidyne)]-bis(2-R1-4-R2-phenol): L1, R1 = R2 = Me; L2, R1 = tBu, R2 = Me; L3, R1 = R2 = cumyl) with a racemic 6,6′-dimethylbiphenyl bridge were tested as initiators for the copolymerization of L-LA and ε-CL. In the presence of alcohol, the mononuclear aluminum complexes catalyzed the controlled copolymerization in the melt at 110 °C, producing practically random copolymers with narrow polydispersity indices (Đ = 1.03–1.07). Notably, the component of the copolymer produced by complex 3 strictly correlated with the feed ratio of the monomers at different stages of the copolymerization. In a 50/50 feed ratio of L-LA/ε-CL, the reactivity ratios of the two monomers are rLA = 1.17, rCL = 0.80. The average lengths of the lactidyl and caproyl sequences in the resultant copolymer are LLA = 1.91, LCL = 1.93. Even at 180 °C, the copolymerization initiated by complex 3 was still well-controlled, producing random copolymers with narrow dispersity indices (Đ = 1.07–1.09). Compared with the mononuclear counterparts, the dinuclear aluminum methyl complexes enable a few fold boost in activity because of the lower steric hindrance around the aluminum centers, but gave tapered copolymers with broadened Đ values (Đ = 1.12–1.22).

The conventional etherification uses toxic or hardly degradable alkylating agents. An eco-friendly tandem etherification/aromatization is presented to prepare estrone 3-secondary ethers from easily available dienone 1. Three marketed 3-etherified estrogen drugs were synthesized with the method from commercial available starting material.

A series of magnesium silylamido complexes supported by tetradentate aminophenolate ligands, LMgN(SiMe3)2 (L = {(2-R1)Ar-CH2N[(CH2)2R2]CH2-(4-R4-6-R3)C6H2O−}, R1 = NMe2, R2 = NiPr2, R3 = R4 = Cl (1), R3 = R4 = cumyl (2); R1 = NMe2, R2 = NEt2, R3 = R4 = cumyl (3), R3 = CPh3, R4 = Me (4); R1 = NEt2, R2 = NEt2, R3 = CPh3, R4 = Me (5); R1 = NMe2, R2 = (S)-1-butylpyrrolidine-2-yl, R3 = R4 = cumyl (6), R3 = CPh3, R4 = Me (7)), were synthesized and fully characterized, and their catalytic behaviors for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) were explored. The monomeric nature and versatile coordination modes of these complexes in the solid state were further confirmed by X-ray diffraction studies on complexes 1, 3 and 7. In complex 1, the N,N-diisopropylamino group on the alkylamino side arm does not coordinate to the metal centre, whilst the remaining three donors of the aminophenolate ligand and the silylamido group bond with the magnesium centre. By contrast, in complexes 3 and 7, the arylamino moiety does not coordinate to the metal centre, while the alkylamino side arm coordinates. It is worth noting that the introduction of the N,N-dimethyl amino group to the pendant benzyl group realises the diastereoselective synthesis of multichiral aminophenolate magnesium complexes, which has not been achieved by the corresponding tridentate aminophenolate ligand. It was found that most of these magnesium complexes serve as highly active initiators for the ROP of rac-LA. The coordination pattern of the aminophenolate ligand exerted a considerable influence on the activity and stereoselectivity of the corresponding complex toward the polymerization of rac-LA, leading to the production of moderate to high heterotactic biased PLAs. The enantiopure complex 7 showed the highest heteroselectivity among these magnesium complexes (Pr = 0.80 in toluene, Pr = 0.82 in THF).Two different coordination modes were observed during the synthesis of some new magnesium complexes. The chiral alkylamino coordinated 7, as an enantiopure compound, showed the highest heteroselectivity among these magnesium complexes (Pr = 0.80 in toluene, Pr = 0.82 in THF).

A series of monomeric zinc silylamido complexes bearing [NNNO]-type tetradentate aminophenolate ligands, LZnN(SiMe3)2 [L = {(2-R1)ArCH2N[(CH2)2R2]CH2(4-R4-6-R3)C6H2O−}, R1 = NMe2, R2 = NiPr2, R3 = R4 = Cl (1), R3 = R4 = cumyl (3); R1 = NMe2, R2 = NEt2, R3 = R4 = cumyl (2), R3 = CPh3, R4 = Me (4); R1 = NEt2, R2 = NEt2, R3 = CPh3, R4 = Me (5); R1 = NMe2, R2 = (S)-1-butylpyrrolidin-2-yl, R3 = R4 = cumyl (6), R3 = CPh3, R4 = Me (7)], have been synthesized via reactions of Zn[N(SiMe3)2]2 and 1 equiv of the corresponding aminophenols. The monomeric nature and versatile coordination patterns of these complexes in the solid state were further confirmed by X-ray diffraction studies on complexes 2, 3, 5, and 7. In complex 3, the N,N-diisopropylamino group on the pendant side arm does not coordinate to the metal center; only the remaining three donors of the aminophenolate ligand and the silylamido group interact with the zinc center. By contrast, in complexes 2, 5, and 7, the amino group of the aryl moiety does not coordinate to the metal center, while the amino group on the pendant side arm coordinates. At room temperature, the above-mentioned structural features of these complexes are retained in solution, as confirmed by 1H NMR spectroscopy. Complexes 1–7 proved to be efficient initiators for the ring-opening polymerization of rac-lactide (rac-LA) at ambient temperature, and the polymerizations were better controlled in the presence of 2-propanol. The coordination pattern of the aminophenolate ligand exerted a significant influence on the stereoselectivity of the corresponding complex toward the polymerization of rac-LA, leading to the production of heterotactic biased polylactides (PLAs) by complexes 1 and 3 (Pm = 0.40–0.46) and moderately to highly isotactic PLAs by complexes 2 and 4–7 (Pm = 0.70–0.81). Detailed kinetic investigations revealed a first-order dependence on the monomer concentration for all complexes and different orders in the initiator concentration ranging from 1.78 to 1.81. The nature of the solvent as well as the molar ratio of the zinc complex and 2-propanol also displayed certain influence on the order of rac-LA polymerization in the initiator concentration. Factional orders of 1.80, 1.38, and 1.11 were obtained by using complex 5/iPrOH (1:1) in toluene and tetrahydrofuran and complex 5/iPrOH (1:2) in toluene, respectively. On the basis of DOSY and 1H and 13C NMR studies of zinc alkoxide model complexes “LZn(OCMe2COOMe)” as well as the fractional orders of 1.78–1.81 in the initiator concentration, activation/insertion processes likely involving more than one monomeric active species were then hypothesized.

A series of racemic 2-[(2′-(dimethylamino)binaphthyl-2-ylimino)methyl]-4-R2-6-R1-phenols (L1H–L6H) were reacted with 1 equiv. of Mg[N(SiMe3)2]2 or Zn[N(SiMe3)2]2 to provide three heteroleptic magnesium silylamido complexes [(L4–6)MgN(SiMe3)2] (4a: R1 = R2 = cumyl; 5a: R1 = CPh3, R2 = Me; 6a: R1 = CPh3, R2 = tBu) and one heteroleptic zinc silylamido complex [(L4)ZnN(SiMe3)2] (4b: R1 = R2 = cumyl). The molecular structures of complexes 4a, 4b and a homolepic zinc complex (L5)2Zn (5b′) were further confirmed by X-ray diffraction studies. In the solid state, the heteroleptic magnesium complex 4a has a four-coordinated metal core chelated by three donor atoms of the ligand and one bis(trimethylsilyl)amido group; however, the heteroleptic zinc complex 4b has a three-coordinated metal core chelated by two donor atoms of the ligand and one bis(trimethylsilyl)amido group, where the NMe2 group of the iminophenolate ligand is not coordinated to the zinc center. Magnesium complexes 4a–6a efficiently initiated the ring-opening polymerization (ROP) of rac-lactide in the presence/absence of 2-propanol, affording atactic polylactides in toluene and heterotactic bias polylactides in THF. Zinc complex 4b exhibited the highest stereoselectivity for the ROP of rac-lactide, affording substantially heterotactic polylactides in both solvents (Pr = 0.80–0.84). The kinetic studies of the polymerization revealed that the rate of polymerization is first-order in both monomer and catalyst concentrations. All these complexes were also very active in the polymerization of racemic α-methyltrimethylene carbonate (α-MeTMC). Detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen–acyl bond and zinc complex 4b gave the highest regioselectivity (Xreg = 0.98).

A series of racemic 2-[(2′-(dimethylamino)biphenyl-2-ylimino)methyl]-4-R2-6-R1-phenols (L1H–L4H) were reacted with {Mg[N(SiMe3)2]2}2 to provide four heteroleptic magnesium complexes (L1–4)MgN(SiMe3)2·(THF)n (1, R1 = tBu, R2 = Me, n = 1; 2, R1 = R2 = CMe2Ph, n = 0; 3, R1 = CPh3, R2 = tBu, n = 1; 4, R1 = Br, R2 = tBu, n = 0), which have been fully characterized. X-ray structural determination shows that complex 1 possesses a monomeric structure, but complex 4 is dimeric with C2-symmetry where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. The heteroleptic complexes 1–4 efficiently initiate the ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding magnesium complex. Microstructure analyses of the resulting PLAs revealed that Pr values range from 0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC, detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen–acyl bond (Xreg = 0.65–0.86).

A series of mononuclear aluminum dimethyl complexes bearing bidentate N-[2-(1-piperidinyl)benzyl]anilino or N-(2-morpholinobenzyl)anilino ligands were synthesized via reactions of the corresponding proligands with trimethylaluminum upon heating, while at ambient temperature two trimethylaluminum adducts with neutral N-[2-(1-piperidinyl)benzyl]aniline proligands were obtained. These complexes were well characterized by NMR spectroscopy, elemental analysis and occasionally by EI-HRMS. The molecular structures of the typical trimethylaluminum adduct 2b and aluminum dimethyl complex 3b were further confirmed by X-ray diffraction studies. All the aluminum dimethyl complexes could effectively initiate the ring-opening polymerization (ROP) of rac-lactide in a well-controlled manner to afford PLAs with narrow molecular weight distributions (PDI = 1.06–1.18). The polymer samples obtained are systematically end-capped with the bidentate ancillary ligands as characterized by 1H NMR and ESI-TOF mass spectroscopy. Moreover, the introduction of substituent(s) at the ortho-position(s) of the anilino moiety in the ligand results in an obvious decrease in the catalytic activity of the corresponding aluminum complex, and complexes with meta-chloro substituted anilino units show higher activities likely due to the enhanced electrophilicity of the metal centers induced by the anilino groups.

A series of titanium trichloride complexes 1a–4a, 7a–10a ligated with claw-type tetradentate aminophenolate ligands were synthesized from the direct reaction of TiCl4(THF)2 with 1 equiv. of the corresponding aminophenol in the presence of triethylamine. For comparison purposes, titanium isopropoxide complexes 5e–8e were also synthesized via the reaction of Ti(OiPr)4 and 1 equiv. of the proligand. Similar reactions of ZrCl4(THF)2 with the corresponding aminophenol ligands in the presence of triethylamine only allowed the isolation of zirconium complex 8b. The X-ray diffraction studies reveal that titanium trichloride complexes 2a, 9a and titanium triisopropoxide complex 5e all possess a distorted octahedral geometry with the tetradentate aminophenolate ligand in cis-O, N, N chelating mode, where the methoxy group of the aryl unit does not coordinate with the metal center in the solid state. Upon activation with MMAO, these titanium and zirconium(IV) complexes exhibited moderate to high catalytic activities for ethylene polymerization at 30–120 °C, producing high-molecular-weight polyethylenes with broad distributions (Mw/Mn = 10.2–34.8). The activities of titanium trichloride complexes are significantly higher than those of titanium isopropoxide and zirconium trichloride complexes at high temperatures. The highest activity of 15456 kg (mol-Ti h)−1 could be achieved by titanium trichloride complex 8a with bromo groups on both ortho- and para-positions of the phenolate ring of the ligand at 120 °C.

Mono(ketiminate) aluminum dimethyl complexes 1a–5a and bis(ketiminate) aluminum methyl complex 2b were synthesized via the reactions of AlMe3 with various N-aryl substituted ketimine compounds in a 1:1 or 1:2 molar ratio, respectively. The reaction of Al(OiPr)3 with the ketimine compound L2H resulted in a bis(ketiminate) aluminum isopropoxide complex 2c. All the complexes were characterized by 1H and 13C{1H} NMR spectroscopy, either EA or HRMS analysis. X-ray diffraction measurement of complexes 2a and 3a confirmed that in the solid state both 2a and 3a exist as a tetrahedron structure with the aluminum atom center surrounded by the oxygen and nitrogen donors of the bis-chelating ketiminate ligand as well as two methyl groups. Besides, a triligated aluminum complex 3 (Al(L3)3) with a six-coordinated metal core and a dinuclear aluminum isopropoxide complex 6d [(L6)2Al(μ-OiPr)2Al(OiPr)2] with C2-symmetry were isolated and characterized by X-ray diffraction studies. All aluminum methyl complexes behaved as active initiators in the ring-opening polymerization (ROP) of rac-lactide (rac-LA), affording isotactic-enriched polylactides (PLAs) with high molecular weights and broad molecular weight distributions at 70 °C. The steric and electronic characters of the ancillary ligands show significant effects both on the activity and the stereoselectivity. The introduction of an electron-withdrawing group on the para-position of N-aryl ring of the ligand resulted in an increase of the catalytic activity. However, the electronic character of ancillary ligands showed a neglectable effect on the ROP of ε-CL initiated by these aluminum complexes. For both monomers, bis(ketiminate) aluminum complex 2b was found to be more active than the corresponding mono(ketiminate) complex 2a of the same ligand. Aluminum isopropoxide complex 2c exerted better control for the polymerizations of both monomers, providing polymers with narrower molecular weight distributions and smaller molecular weights which are comparable to the theoretical valuesGraphical abstractSeveral aluminum methyl and isopropoxide complexes bearing ketiminate ligands were synthesized, and characterized in selected cases with X-ray diffraction studies. These aluminum complexes proved to be active initiators for the ROP of rac-lactide and ε-caprolactone, affording polymers with high molecular weighs and broad distributions. Moderate isoselectivities were also achieved in the ROP of rac-lactide.

A series of ethylene-bridged C1-symmetric ansa-(3-R-indenyl)(fluorenyl) zirconocene complexes 3a–i (R = 2-[2-(4-methylphenyl)propyl], 3a; R = 2-[2-(3,5-dimethylphenyl)propyl], 3b; R = 2-(2-benzylpropyl), 3c; R = 2-methylbenzyl, 3d; R = 2-(2-cyclohexylpropyl), 3e; R = 2-[2-(1-cyclohexenyl)propyl], 3f; R = 2-(2-n-butylpropyl), 3g; R = cyclohexyl, 3h; R = iPr, 3i) were synthesized by a salt metathesis method and characterized by NMR spectroscopy, elemental analysis (or HRMS) and X-ray diffraction (3e and 3h). Upon activation with methylaluminoxane, most of these zirconocene complexes exhibited sufficient catalytic activities up to 2.5 × 105 g C6/(mol-Zr·h) and high selectivities up to 99% toward propylene dimerization, affording 2-methyl-1-pentene as the major isomer which was confirmed by gas chromatography. Remarkably, the selectivity and activity of complexes 3a–i were significantly influenced by the structural features of the substituent on the 3-position of indenyl ring: a pendant aryl or alkyl group linked by a quaternary carbon bridge provided the complex with high selectivities in the range of 89.9–99.0% for 2-methyl-1-pentene and low to moderate catalytic activities; the lack of a quaternary carbon bridge within the substituent would lead to mainly polypropylenes of low molecular weight. The steric hindrance around the active metal center induced by the pendant group might be responsible for the catalytic dimerization behavior, and the presumed mechanism was discussed. In addition, for complexes 3h and 3i, the selectivity for propylene dimerization could also be enhanced with the increase of reaction temperature. Noticeably, most of these ansa-zirconocene complexes exhibit excellent thermal stability at 100 °C, which is important with regard to industrial application.A series of ethylene-bridged C1-symmetric ansa-(3-R-indenyl)(fluorenyl) zirconocene complexes were synthesized and characterized. Upon activation with MAO, most of them showed moderate catalytic activities up to 2.25 × 105 g C6/(mol-Zr·h) and at the same time high selectivity up to 99% toward propylene dimerization, affording 2-methyl-1-pentene as the major isomer.

The preparation and characterization of several zinc and magnesium complexes bearing chiral aminophenolate ligands, [(S)- or (R)-L]ZnN(SiMe3)2 (1, 2), [(S)- or (R)-L]ZnOtBu (3, 4) and [(S)- or (R)-L]MgN(SiMe3)2 (5, 6) ((S)- or (R)-L = (S)- or (R)-2-{N-benzyl-N-[(1-nbutyl-2-pyrrolidinyl)methyl]}aminomethyl-4-methyl-6-tritylphenolate), have been reported. While the X-ray diffraction studies as well as the NMR spectroscopic data revealed that zinc and magnesium silylamido complexes possess similar structures both in the solid state and in solution, their catalytic performance toward rac-LA polymerization exhibited significant difference. The replacement of zinc center by magnesium realized an interesting stereoselectivity switch from isoselective (Pm = 0.80) to heteroselective (Pr = 0.81). The reactions of enantiopure zinc complexes 1 and 2 with (S)-methyl lactate or (R)-tert-butyl lactate resulted in the target lactate complexes 7–9 as a mixture of two diastereomers, but in different ratios. Two typical zinc lactate isomers 8α and 9β/9β′ were isolated and further characterized by the X-ray diffraction method to have pentacoordinated geometry where the coordination of lactate carbonyl group displays some hemilabile nature. The molecular structures of 8α and 9β/9β′ indicated that the specific configuration of lactate moiety in combination with the aminophenolate ligand of given chirality would favor the formation of one of the two diastereomers in each case. Based on the structures of zinc lactate model complexes and the active propagating species, as well as the preferential enchainment of one specific lactide monomer in the polymerization, a cooperation of enantiomorphic-site control and chain-end control mechanisms involving three types of active species was proposed for the formation of stereoblock PLAs from rac-LA by zinc initiators. Results of apparent rate constants for rac-, d-, and l-LA polymerizations by magnesium initiators are consistent with a chain-end control mechanism. The 13C NMR spectra of magnesium lactate complexes proved that the bonding of carbonyl group to magnesium center is stronger than that in zinc active species. The coordination number of magnesium species can expand easily to six in the presence of coordinative molecules, suggesting that the heteroselective magnesium active species have a hexacoordinated core structure. The difference of coordination geometry between zinc and magnesium active species should be responsible for the stereoselectivity switch in the polymerization of rac-LA.

An enantiopure zinc complex supported by an aminophenolate ligand with multiple stereogenic centers has been diastereoselectively synthesized via the variation of the ortho-substituent of a phenoxy moiety and the N-alkyl group of a chiral pyrrolidinyl ring in the ligand framework, which displays high isoselectivity in the polymerization of rac-lactide.

A series of racemic 2-[(2′-methoxybiphenyl-2-ylimino)methyl]-4-R2-6-R1-phenols (L1H-L8H) were reacted with {Mg[N(SiMe3)2]2}2 and Ca[N(SiMe3)2]2·2THF (THF = tetrahydrofuran), respectively, to provide nine heteroleptic magnesium complexes L1–8MgN(SiMe3)2 [R1 = iPr, R2 = H (1a); R1 = tBu, R2 = Me (2a and 2a·THF); R1 = R2 = tBu (3a); R1 = R2 = CMe2Ph (4a); R1 = CPh3, R2 = tBu (5a); R1 = 1-piperidinylmethyl, R2 = tBu (6a); R1 = Cl, R2 = tBu (7a); R1 = Br, R2 = tBu (8a)], two homoleptic calcium complexes (L2,5)2Ca (2b and 5b), and one heteroleptic calcium complex [(L4)CaN(SiMe3)2·THF] (4b), which have been fully characterized. In the solid state, magnesium complexes 2a and 6a are isostructural, and each possesses a monomeric structure, while magnesium complexes 7a and 8a are dimeric, where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. Although bearing the same iminophenoloate ligand, the molecular structures of complexes 2a and 2a·THF are different from each other. In complex 2a·THF, the coordination of one molecule of THF to the magnesium atom leads to dissociation of the methoxy group of the ligand from the metal center. The homoleptic calcium complex 2b has a six-coordinate metal core ligated by all six donor atoms of two iminophenolate ligands. The heteroleptic magnesium complexes 1a–8a and calcium complex 4b proved to be efficient initiators for the ring-opening polymerization of rac-lactide at ambient temperature in THF or at 70 °C in toluene, and the polymerizations were better controlled in the presence of 2-propanol. The introduction of a bulky ortho substituent on the phenoxy unit of the ligand resulted in an increase of the catalytic activity of the corresponding metal complex. Microstructure analysis of the resultant poly(rac-lactide) samples via homonuclear-decoupled 1H NMR spectroscopy revealed Pr values ranging from 0.60 to 0.81, which closely depended on the employed catalyst and polymerization conditions.

A series of magnesium silylamido complexes bearing salan-like multidentate aminophenolate ligands, [(L1–4)MgN(SiMe3)2] (1–4) and [(L1)MgN(SiHMe2)2] (1′) (L1–4 = –OAr1N(Me)(CH2)nN(Me)Ar2, n = 2 or 3), have been synthesized via the reaction of Mg[N(SiMe3)2]2 or Mg[N(SiHMe2)2] with 1 equiv. of the corresponding aminophenol. The monomeric nature of the magnesium complexes 1, 2, 3 and the main stoichiometric structure of 1′ in the solid state were further confirmed by X-ray diffraction studies, where the four donors of the ligand and the silylamido group coordinated with the magnesium center constructing a distorted trigonal bipyramidal coordination geometry at the metal center. The existence of isomers in complexes 1′ and 4 was verified by the variable temperature 1H NMR spectroscopy, and the non-coordination of OMe group with the central magnesium in complex 4 was deduced from the comparison of the proton chemical shifts in complex 4 and its proligand. All these complexes efficiently initiated the ring-opening polymerization of rac-lactide with the presence/absence of isopropanol, affording isotactic bias polylactides with relatively broad molecular weight distributions (Pm = 0.59–0.70, Mw/Mn = 1.28–2.21) in toluene at r.t. Declining the steric hindrance of the initiating group from N(SiMe3)2 to N(SiHMe2)2 increased the catalytic activity considerably. The steric and electronic characteristics of the ancillary ligands showed some influence on the polymerization performance of the corresponding magnesium complexes. The introduction of bulky ortho-substituents on the phenoxy moiety resulted in some changes both in activity and stereoselectivity, and either the elongation of the amine bridge from NCH2CH2N to NCH2CH2CH2N or the introduction of chloro-substituent on the phenoxy moiety led to a significant decline of the catalytic activity.

A series of novel eight-coordinate dichloride zirconium complexes with the general formula L2ZrCl2 supported by two tridentate [ONN] ligands have been synthesized by the reaction of ZrCl4·2THF with 2 equiv of the corresponding ligands (tridentate phenoxy-imine ligands (L1H–L4H) or tridentate phenoxy-amine ligands (L5H and L6H)). These complexes were characterized by NMR and elemental analyses. The molecular structures of representative zirconium complexes C2 and C6 were confirmed by single-crystal X-ray diffraction and revealed that the metal center is eight-coordinated by two tridentate [ONN] ligands and two chlorides in a distorted-square-antiprismatic geometry. When C1–C6 were activated by modified methylaluminoxane (MMAO), the resultant catalysts displayed notable thermal stability and high activities toward ethylene polymerization. The ligand substituents, the metal coordination environment, and the reaction conditions had a profound effect on the polymerization. The catalytic activity increases consistently with increasing polymerization temperature, and the highest activity of 1.04 × 106 g of PE (mol of Zr)−1 h–1 was achieved in o-xylene at 140 °C. A catalytic lifetime of nearly 5 h was observed for the C5/MMAO catalyst system at 140 °C.

By combining the influence of excess alcohol, temperature and monomer-to-initiator ratios in the feed, dinuclear salan aluminum complexes LRAl2Me4 exhibited a high degree of control towards the copolymerization of L-LA and ε-CL, producing blocky, gradient, tapered and random copolymers.

A series of zinc silylamido complexes bearing claw-type multidentate aminophenolate ligands, [LZnN(SiMe3)2] (L = –OAr1-CH2N[(CH2)nNR2]CH2Ar2, n = 2 or 3; R = Me or Et (1a–3a, 5a, 7a and 8a); L = –OC6H2-4,6-tBu2-2-CH2N[(CH2)2OMe]2 (9a)), have been synthesized via the reaction of Zn[N(SiMe3)2]2 and 1 equiv. of corresponding aminophenol. The reaction of Zn[N(SiMe3)2]2 with the proligand L6H (2-{N-(2-methoxybenzyl)-N-[3-(N′,N′-dimethylamino)propyl]aminomethyl}-4-methyl-6-tritylphenol) resulted in the formation of bisphenolate zinc complex 6 regardless of the stoichiometric ratio of the two starting materials. Complex 4b with an initiating group of 3-tert-butyl-2-methoxy-5- methylbenzyloxy was obtained and further studied via the X-ray diffraction method to be monomeric. Zinc ethyl complex 2c was also prepared from the reaction of ZnEt2 and 1 equiv. of proligand L2H as the representative complex with an alkyl initiating group. All zinc silylamido complexes efficiently initiated the ring-opening polymerization of rac-lactide in the presence or absence of isopropanol at ambient temperature. The steric and electronic characteristics of the ancillary ligands significantly influenced the polymerization performance of the corresponding zinc complexes. The introduction of bulky ortho- substituents on the phenoxy moiety resulted in an apparent decrease of catalytic activity while a slightly enhanced isotactic selectivity. Meanwhile, the elongation of the pendant amine arm to three-carbon-atom linkage led to significant decline of the catalytic activity in the absence of isopropanol. The zinc ethyl complex 2c was not such an efficient initiator as the silylamido ones, but the alkoxy complex 4b gave an obviously faster and better controlled polymerization when compared to the zinc silylamido complexes.

Monomeric zinc silylamido and ethyl complexes bearing tetradentate aminophenolato ligands [(DNNO)ZnR] (D = NMe2, OMe; R = N(SiMe3)2 (1–5, 8), Et (6, 7, 9, 10)), were isolated from the reaction of Zn[N(SiMe3)2]2 or ZnEt2 and one equivalent of aminophenols {aryl-CH2N[(CH2)2NMe2]CH2-phenol} in moderate to high yields. The monomeric nature of these complexes was further confirmed by X-ray diffraction studies of silylamido complexes 1, 3 and ethyl complexes 7, 9, 10. The methoxy or N,N-dimethylamino group of the aryl unit does not coordinate with the metal center in the solid state, only the remaining three donors of the ligand and silylamido or ethyl group interact with zinc center constructing a distorted tetrahedral coordination geometry at the metal center. All these zinc complexes efficiently initiated the ring-opening polymerization of rac-lactide, and the polymerization runs were better controlled in the presence of isopropanol, giving atactic PLAs end-capped with isopropyl ester and hydroxyl groups. The structure of the ancillary ligands showed some influence on the catalytic activity and selectivity of the corresponding zinc complexes. The introduction of bulky ortho-substituents on the phenoxy unit resulted in a decrease of the polymerization rate, whereas the isotactic dyad selectivity in the ROP of rac-lactide was enhanced.

A series of aluminium alkyl complexes {PhC(NR′)(NR′′)}AlR2 (4a–n, R′ = 2,6-iPr2C6H3, 2,6-Me2C6H3; R′′ = aryl groups with various ortho-, para- or meta-substituents, tert-butyl; R = methyl, ethyl) bearing non-symmetrically N-substituted benzamidinate ligands were synthesized via the reaction of trialkylaluminium and the corresponding benzamidine proligands. Complex 5 bearing symmetric amidinate ligand was also obtained for comparison purposes. The X-ray diffraction studies of complexes 4b, 4c and 5 show in each case a distorted tetrahedral geometry around the aluminium center. All the amidinate aluminium complexes were found to catalyze the ring-opening polymerization (ROP) of rac-lactide with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-withdrawing substituents at the ortho-positions of N-phenyl ring of the ligands resulted in an obvious increase in catalytic activity. Complex 4b showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the ortho-chloro substituents on the phenyl ring. The existence of ortho-substituents of small steric bulkiness is also beneficial for the increase of activity of these catalysts. However, further increase of steric hindrance of the ligands by introducing bulky ortho-substituents onto the phenyl moieties resulted in a decrease of activity and an increase in the isotactic bias of the obtained polylactides. The broad molecular weight distributions (PDI = 1.13–2.02) of the polymer samples indicated that the ROP of rac-lactide initiated by these complexes was not well-controlled.

Three new mixed salicylaldiminato cyclopentadienyl zirconium complexes Cp′[2-But-6-(C6H11NCH)C6H3O]ZrCl2 (Cp′ = nBuC5H4 (3a), tBuC5H4 (3b) and Me4C5H (3c)) were prepared and the structure of complex 3c was confirmed by X-ray diffraction analysis. All of the three complexes showed high activities for ethylene homopolymerization with the activation of methylaluminoxane, and 3a showed the highest activity up to 1.15 × 106 g PE/mol Zr h for ethylene homopolymerization at 70 °C. The 13C NMR spectrum showed that the obtained polymer is linear polyethylene. Complexes 3a–c also catalysed ethylene/1-hexene copolymerization with high activities from 6.29 × 105 to 12.3 × 105 g Copolymer/mol Zr h and with 0.89–1.39% 1-hexene incorporation level. In addition, the influence of the substituted alkyl on Cp on the catalytic behavior of corresponding zirconium complexes was discussed.Three new mixed salicylaldiminato cyclopentadienyl zirconium complexes Cp′[2-But-6-(C6H11NCH)C6H3O]ZrCl2 (Cp′ = nBuC5H4 (3a), tBuC5H4 (3b) and Me4C5H (3c)) were prepared and showed high activities for ethylene polymerization and ethylene/1-hexene copolymerization with the activation of MAO.

Zirconium and hafnium complexes bearing new 1,2-ethanediyl- or 1,3-propanediyl-linked bis(β-diketiminate) ligands, [{CnH2n-(BDIAr)2}MCl2] (Ar = 2,6-Me2-C6H3, 2,6-Cl2-C6H3, 2,6-iPr2-C6H3; M = Zr, n = 2 (4a–c), n = 3 (5a–c); M = Hf, n = 2 (6b)), were synthesized via the reaction of MCl4·2THF and one equivalent of dilithium salt of the corresponding ligand. Distorted trigonal prismatic and octahedral coordination geometries as well as C1-symmetric structures are found for zirconium complexes 4c and 5c in the solid state. Variable temperature 1H NMR spectra indicated the fluxional nature of 4a and 5a in solution. Upon activation with methylaluminoxane (MAO), all these complexes except hafnium complex 6b displayed moderate catalytic activities for ethylene polymerization. 1,2-Ethanediyl-linked complexes 4a and 4b are generally more active than their 1,3-propanediyl-linked analogues. The substituents at the ortho-positions of the phenyl rings have different effect on the catalytic activities of 1,2-ethanediyl-linked series or 1,3-propanediyl-linked series. It is noteworthy that even at a low Al/Zr molar ratio of 500, the catalytic activities of these zirconium complexes could be retained. Polyethylenes with broad molecular weight distributions (MWD = 15.3–20.3) were produced, which might result from the fluxional character of the zirconium complexes. The linear structure of obtained polyethylenes was further determined by 13C NMR spectroscopy and DSC analysis.

A series of aluminium alkyl complexes (BDI)AlEt2 (3a–m) bearing symmetrical or unsymmetrical β-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the corresponding β-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral geometry. Attempt to synthesize β-diketiminate aluminium alkoxide complexes by the reactions of monochloride complex “(BDI-2a)AlMeCl” (4) with alkali salts of 2-propanol gave unexpectedly an aluminoxane [(BDI-2a)AlMe]2(μ-O) (7) as characterized by X-ray diffraction methods. Complexes 3a–m and [(2,6-iPr2C6H3NCMe)2HC]AlEt2 (8) were found to catalyze the ring-opening polymerization (ROP) of ε-caprolactone with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-donating substituents at the para-positions of the aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a decrease in the catalytic activity. Although the viscosity average molecular weights (Mη) of the obtained poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of ε-caprolactone initiated by complexes 3a–m and 8 were not well-controlled, as judged from the broad molecular weight distributions (PDI = 1.66–3.74, Mw/Mn) of the obtained polymers and the nonlinear relationship of molecular weight versus monomer conversion.

A series of zinc silylamido complexes bearing claw-type multidentate aminophenolate ligands, [LZnN(SiMe3)2] (L = –OAr1-CH2N[(CH2)nNR2]CH2Ar2, n = 2 or 3; R = Me or Et (1a–3a, 5a, 7a and 8a); L = –OC6H2-4,6-tBu2-2-CH2N[(CH2)2OMe]2 (9a)), have been synthesized via the reaction of Zn[N(SiMe3)2]2 and 1 equiv. of corresponding aminophenol. The reaction of Zn[N(SiMe3)2]2 with the proligand L6H (2-{N-(2-methoxybenzyl)-N-[3-(N′,N′-dimethylamino)propyl]aminomethyl}-4-methyl-6-tritylphenol) resulted in the formation of bisphenolate zinc complex 6 regardless of the stoichiometric ratio of the two starting materials. Complex 4b with an initiating group of 3-tert-butyl-2-methoxy-5- methylbenzyloxy was obtained and further studied via the X-ray diffraction method to be monomeric. Zinc ethyl complex 2c was also prepared from the reaction of ZnEt2 and 1 equiv. of proligand L2H as the representative complex with an alkyl initiating group. All zinc silylamido complexes efficiently initiated the ring-opening polymerization of rac-lactide in the presence or absence of isopropanol at ambient temperature. The steric and electronic characteristics of the ancillary ligands significantly influenced the polymerization performance of the corresponding zinc complexes. The introduction of bulky ortho- substituents on the phenoxy moiety resulted in an apparent decrease of catalytic activity while a slightly enhanced isotactic selectivity. Meanwhile, the elongation of the pendant amine arm to three-carbon-atom linkage led to significant decline of the catalytic activity in the absence of isopropanol. The zinc ethyl complex 2c was not such an efficient initiator as the silylamido ones, but the alkoxy complex 4b gave an obviously faster and better controlled polymerization when compared to the zinc silylamido complexes.

A series of racemic 2-[(2′-(dimethylamino)binaphthyl-2-ylimino)methyl]-4-R2-6-R1-phenols (L1H–L6H) were reacted with 1 equiv. of Mg[N(SiMe3)2]2 or Zn[N(SiMe3)2]2 to provide three heteroleptic magnesium silylamido complexes [(L4–6)MgN(SiMe3)2] (4a: R1 = R2 = cumyl; 5a: R1 = CPh3, R2 = Me; 6a: R1 = CPh3, R2 = tBu) and one heteroleptic zinc silylamido complex [(L4)ZnN(SiMe3)2] (4b: R1 = R2 = cumyl). The molecular structures of complexes 4a, 4b and a homolepic zinc complex (L5)2Zn (5b′) were further confirmed by X-ray diffraction studies. In the solid state, the heteroleptic magnesium complex 4a has a four-coordinated metal core chelated by three donor atoms of the ligand and one bis(trimethylsilyl)amido group; however, the heteroleptic zinc complex 4b has a three-coordinated metal core chelated by two donor atoms of the ligand and one bis(trimethylsilyl)amido group, where the NMe2 group of the iminophenolate ligand is not coordinated to the zinc center. Magnesium complexes 4a–6a efficiently initiated the ring-opening polymerization (ROP) of rac-lactide in the presence/absence of 2-propanol, affording atactic polylactides in toluene and heterotactic bias polylactides in THF. Zinc complex 4b exhibited the highest stereoselectivity for the ROP of rac-lactide, affording substantially heterotactic polylactides in both solvents (Pr = 0.80–0.84). The kinetic studies of the polymerization revealed that the rate of polymerization is first-order in both monomer and catalyst concentrations. All these complexes were also very active in the polymerization of racemic α-methyltrimethylene carbonate (α-MeTMC). Detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen–acyl bond and zinc complex 4b gave the highest regioselectivity (Xreg = 0.98).

A series of potassium complexes bearing monoanionic tetradentate amino-phenolate ligands, [LK]2 (L = {(2-R1)C6H4CH2N[(CH2)2R2]CH2(4-R4-6-R3)C6H2O-}, R1 = NMe2, R2 = NEt2, R3 = CPh3, R4 = Me (1); R1 = R2 = NEt2, R3 = CPh3, R4 = Me (2); R1 = NMe2, R2 = NEt2, R3 = R4 = cumyl (4); R1 = R2 = OMe, R3 = tBu, R4 = Me (6); L = (2-NMe2)C6H4CH2N[[CH2-(S)-1-butylpyrrolidinyl]CH2(4-Me-6-CPh3)C6H2O-] (3)), have been synthesized via reactions of KN(SiMe3)2 and 1 equiv. of the corresponding aminophenols. The solid-state structures of typical complexes 4 and 6 are determined via X-ray diffraction studies, which reveal the dinuclear nature of these complexes. By contrast, DOSY measurements of 1, 4 and 6 suggest that these complexes are monomeric in solution. It is noteworthy that the coordination chemistry of these potassium complexes is versatile, which is closely related to the nature of the ortho-substituent of the phenolate ring, as indicated by the results of the corresponding spectroscopic studies. In the presence of iPrOH, 1–4 and 6 could initiate the polymerization of 500 equiv. of rac-lactide to achieve high monomer conversions within several minutes but afford atactic PLAs with slightly isotactic-enriched microstructures (Pm = 0.58–0.60). Experimental results also demonstrated that a bulky trityl substituent at the ortho-position of the phenolate ring of the ligand framework is beneficial for the enhancement of the activities of these potassium complexes.

A series of magnesium silylamido complexes supported by chiral aminophenolate ligands were synthesized, and their catalytic behaviors toward the ring-opening polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of Mg[N(SiMe3)2]2 with chiral tridentate aminophenols L1–10H [L = (S)-{[(1-R4, 2-pyrrolidinyl)CH2N(R3)-]}CH2-(6-R1-4-R2)-C6H2O−] in a 1:1 molar ratio in toluene gave the chiral aminophenolate magnesium silylamido complexes L1–10MgN(SiMe3)2 as enantiopure compounds or as pairs of diastereomers. As determined by X-ray analysis, structures a and b of diastereomer mixtures adopt the configuration of SCSNRNRMg-a and SCSNSNSMg-b, respectively. It was found that most of these magnesium complexes served as highly active initiators for the ring-opening polymerization of rac-LA. Besides, the stereoselectivity of chiral magnesium complexes gradually changed from isoselective-biased (Pm = 0.67) to heteroselective-enriched (Pr = 0.81) via a stepwise adjustment of the substituents of the ligand framework. It was noted that, as the rare reported active magnesium initiator for the ROP of rac-BBL, complexes 3–9 can efficiently initiate the ring-opening polymerization of rac-BBL in a controlled manner, giving moderate syndiotactic (Ps up to 0.73) PHBs.

A series of aluminium alkyl complexes (BDI)AlEt2 (3a–m) bearing symmetrical or unsymmetrical β-diketiminate ligand (BDI) frameworks were obtained from the reaction of triethyl aluminium and the corresponding β-diketimine. The monomeric structure of the aluminium complex 3k was confirmed by an X-ray diffraction study, which shows that the aluminium center is coordinated by both of the nitrogen donors of the chelating diketiminate ligand and the two ethyl groups in a distorted tetrahedral geometry. Attempt to synthesize β-diketiminate aluminium alkoxide complexes by the reactions of monochloride complex “(BDI-2a)AlMeCl” (4) with alkali salts of 2-propanol gave unexpectedly an aluminoxane [(BDI-2a)AlMe]2(μ-O) (7) as characterized by X-ray diffraction methods. Complexes 3a–m and [(2,6-iPr2C6H3NCMe)2HC]AlEt2 (8) were found to catalyze the ring-opening polymerization (ROP) of ε-caprolactone with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-donating substituents at the para-positions of the aryl rings in the ligand resulted in an apparent decrease in catalytic activity. Complex 3h showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the meta-trifluoromethyl substituents on the aryl rings. The increase of steric hindrance of the ligand by introducing ortho-substituents onto the phenyl moieties also resulted in a decrease in the catalytic activity. Although the viscosity average molecular weights (Mη) of the obtained poly(caprolactone)s increased with the enhancement of monomer conversion, the ROPs of ε-caprolactone initiated by complexes 3a–m and 8 were not well-controlled, as judged from the broad molecular weight distributions (PDI = 1.66–3.74, Mw/Mn) of the obtained polymers and the nonlinear relationship of molecular weight versus monomer conversion.

A series of titanium trichloride complexes 1a–4a, 7a–10a ligated with claw-type tetradentate aminophenolate ligands were synthesized from the direct reaction of TiCl4(THF)2 with 1 equiv. of the corresponding aminophenol in the presence of triethylamine. For comparison purposes, titanium isopropoxide complexes 5e–8e were also synthesized via the reaction of Ti(OiPr)4 and 1 equiv. of the proligand. Similar reactions of ZrCl4(THF)2 with the corresponding aminophenol ligands in the presence of triethylamine only allowed the isolation of zirconium complex 8b. The X-ray diffraction studies reveal that titanium trichloride complexes 2a, 9a and titanium triisopropoxide complex 5e all possess a distorted octahedral geometry with the tetradentate aminophenolate ligand in cis-O, N, N chelating mode, where the methoxy group of the aryl unit does not coordinate with the metal center in the solid state. Upon activation with MMAO, these titanium and zirconium(IV) complexes exhibited moderate to high catalytic activities for ethylene polymerization at 30–120 °C, producing high-molecular-weight polyethylenes with broad distributions (Mw/Mn = 10.2–34.8). The activities of titanium trichloride complexes are significantly higher than those of titanium isopropoxide and zirconium trichloride complexes at high temperatures. The highest activity of 15456 kg (mol-Ti h)−1 could be achieved by titanium trichloride complex 8a with bromo groups on both ortho- and para-positions of the phenolate ring of the ligand at 120 °C.

A series of mononuclear aluminum dimethyl complexes bearing bidentate N-[2-(1-piperidinyl)benzyl]anilino or N-(2-morpholinobenzyl)anilino ligands were synthesized via reactions of the corresponding proligands with trimethylaluminum upon heating, while at ambient temperature two trimethylaluminum adducts with neutral N-[2-(1-piperidinyl)benzyl]aniline proligands were obtained. These complexes were well characterized by NMR spectroscopy, elemental analysis and occasionally by EI-HRMS. The molecular structures of the typical trimethylaluminum adduct 2b and aluminum dimethyl complex 3b were further confirmed by X-ray diffraction studies. All the aluminum dimethyl complexes could effectively initiate the ring-opening polymerization (ROP) of rac-lactide in a well-controlled manner to afford PLAs with narrow molecular weight distributions (PDI = 1.06–1.18). The polymer samples obtained are systematically end-capped with the bidentate ancillary ligands as characterized by 1H NMR and ESI-TOF mass spectroscopy. Moreover, the introduction of substituent(s) at the ortho-position(s) of the anilino moiety in the ligand results in an obvious decrease in the catalytic activity of the corresponding aluminum complex, and complexes with meta-chloro substituted anilino units show higher activities likely due to the enhanced electrophilicity of the metal centers induced by the anilino groups.

By combining the influence of excess alcohol, temperature and monomer-to-initiator ratios in the feed, dinuclear salan aluminum complexes LRAl2Me4 exhibited a high degree of control towards the copolymerization of L-LA and ε-CL, producing blocky, gradient, tapered and random copolymers.

An enantiopure zinc complex supported by an aminophenolate ligand with multiple stereogenic centers has been diastereoselectively synthesized via the variation of the ortho-substituent of a phenoxy moiety and the N-alkyl group of a chiral pyrrolidinyl ring in the ligand framework, which displays high isoselectivity in the polymerization of rac-lactide.

A series of aluminium alkyl complexes {PhC(NR′)(NR′′)}AlR2 (4a–n, R′ = 2,6-iPr2C6H3, 2,6-Me2C6H3; R′′ = aryl groups with various ortho-, para- or meta-substituents, tert-butyl; R = methyl, ethyl) bearing non-symmetrically N-substituted benzamidinate ligands were synthesized via the reaction of trialkylaluminium and the corresponding benzamidine proligands. Complex 5 bearing symmetric amidinate ligand was also obtained for comparison purposes. The X-ray diffraction studies of complexes 4b, 4c and 5 show in each case a distorted tetrahedral geometry around the aluminium center. All the amidinate aluminium complexes were found to catalyze the ring-opening polymerization (ROP) of rac-lactide with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-withdrawing substituents at the ortho-positions of N-phenyl ring of the ligands resulted in an obvious increase in catalytic activity. Complex 4b showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the ortho-chloro substituents on the phenyl ring. The existence of ortho-substituents of small steric bulkiness is also beneficial for the increase of activity of these catalysts. However, further increase of steric hindrance of the ligands by introducing bulky ortho-substituents onto the phenyl moieties resulted in a decrease of activity and an increase in the isotactic bias of the obtained polylactides. The broad molecular weight distributions (PDI = 1.13–2.02) of the polymer samples indicated that the ROP of rac-lactide initiated by these complexes was not well-controlled.

Monomeric zinc silylamido and ethyl complexes bearing tetradentate aminophenolato ligands [(DNNO)ZnR] (D = NMe2, OMe; R = N(SiMe3)2 (1–5, 8), Et (6, 7, 9, 10)), were isolated from the reaction of Zn[N(SiMe3)2]2 or ZnEt2 and one equivalent of aminophenols {aryl-CH2N[(CH2)2NMe2]CH2-phenol} in moderate to high yields. The monomeric nature of these complexes was further confirmed by X-ray diffraction studies of silylamido complexes 1, 3 and ethyl complexes 7, 9, 10. The methoxy or N,N-dimethylamino group of the aryl unit does not coordinate with the metal center in the solid state, only the remaining three donors of the ligand and silylamido or ethyl group interact with zinc center constructing a distorted tetrahedral coordination geometry at the metal center. All these zinc complexes efficiently initiated the ring-opening polymerization of rac-lactide, and the polymerization runs were better controlled in the presence of isopropanol, giving atactic PLAs end-capped with isopropyl ester and hydroxyl groups. The structure of the ancillary ligands showed some influence on the catalytic activity and selectivity of the corresponding zinc complexes. The introduction of bulky ortho-substituents on the phenoxy unit resulted in a decrease of the polymerization rate, whereas the isotactic dyad selectivity in the ROP of rac-lactide was enhanced.

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6′-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe3 with various amounts of the proligands L1−4H2 (6,6′-[(6,6′-dimethyl-[1,1′-biphenyl]-2,2′-diyl)bis(nitrylomethilidyne)]-bis(2-R1-4-R2-phenol): L1H2, R1 = R2 = Me; L2H2, R1 = tBu, R2 = Me; L3H2, R1 = R2 = cumyl; L4H2, R1 = Br, R2 = tBu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [L1−3AlMe (1–3), L1−4Al2Me4 (4–7)]. Aluminum alkoxide complexes L2AlOiPr (8), L2AlOBn (9), and α-alkoxy ester complexes L2Al(OCMe2CO2Me) (10), L2Al[(S)-OCHMeCO2Me] (11) were prepared via in situ alcoholysis of the parent aluminum methyl complex 2 with the corresponding alcohols. The molecular structures of mononuclear complexes 1–3, dinuclear complex 6, alkoxide complexes 8–9 and α-alkoxy ester complexes 10–11 were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69–70 ppm and 25–41 ppm were observed in the 27Al NMR spectra of complexes 10 and 11, indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex 11 is also a rare example of an O-lactate model complex that mimics the first insertion of L-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes 1–3 in the presence of iPrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03–1.05) and 65–73% isotacticities. Particularly, complex 8 showed an “immortal” behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.

Zirconium and hafnium complexes bearing new 1,2-ethanediyl- or 1,3-propanediyl-linked bis(β-diketiminate) ligands, [{CnH2n-(BDIAr)2}MCl2] (Ar = 2,6-Me2-C6H3, 2,6-Cl2-C6H3, 2,6-iPr2-C6H3; M = Zr, n = 2 (4a–c), n = 3 (5a–c); M = Hf, n = 2 (6b)), were synthesized via the reaction of MCl4·2THF and one equivalent of dilithium salt of the corresponding ligand. Distorted trigonal prismatic and octahedral coordination geometries as well as C1-symmetric structures are found for zirconium complexes 4c and 5c in the solid state. Variable temperature 1H NMR spectra indicated the fluxional nature of 4a and 5a in solution. Upon activation with methylaluminoxane (MAO), all these complexes except hafnium complex 6b displayed moderate catalytic activities for ethylene polymerization. 1,2-Ethanediyl-linked complexes 4a and 4b are generally more active than their 1,3-propanediyl-linked analogues. The substituents at the ortho-positions of the phenyl rings have different effect on the catalytic activities of 1,2-ethanediyl-linked series or 1,3-propanediyl-linked series. It is noteworthy that even at a low Al/Zr molar ratio of 500, the catalytic activities of these zirconium complexes could be retained. Polyethylenes with broad molecular weight distributions (MWD = 15.3–20.3) were produced, which might result from the fluxional character of the zirconium complexes. The linear structure of obtained polyethylenes was further determined by 13C NMR spectroscopy and DSC analysis.

A series of magnesium silylamido complexes bearing salan-like multidentate aminophenolate ligands, [(L1–4)MgN(SiMe3)2] (1–4) and [(L1)MgN(SiHMe2)2] (1′) (L1–4 = –OAr1N(Me)(CH2)nN(Me)Ar2, n = 2 or 3), have been synthesized via the reaction of Mg[N(SiMe3)2]2 or Mg[N(SiHMe2)2] with 1 equiv. of the corresponding aminophenol. The monomeric nature of the magnesium complexes 1, 2, 3 and the main stoichiometric structure of 1′ in the solid state were further confirmed by X-ray diffraction studies, where the four donors of the ligand and the silylamido group coordinated with the magnesium center constructing a distorted trigonal bipyramidal coordination geometry at the metal center. The existence of isomers in complexes 1′ and 4 was verified by the variable temperature 1H NMR spectroscopy, and the non-coordination of OMe group with the central magnesium in complex 4 was deduced from the comparison of the proton chemical shifts in complex 4 and its proligand. All these complexes efficiently initiated the ring-opening polymerization of rac-lactide with the presence/absence of isopropanol, affording isotactic bias polylactides with relatively broad molecular weight distributions (Pm = 0.59–0.70, Mw/Mn = 1.28–2.21) in toluene at r.t. Declining the steric hindrance of the initiating group from N(SiMe3)2 to N(SiHMe2)2 increased the catalytic activity considerably. The steric and electronic characteristics of the ancillary ligands showed some influence on the polymerization performance of the corresponding magnesium complexes. The introduction of bulky ortho-substituents on the phenoxy moiety resulted in some changes both in activity and stereoselectivity, and either the elongation of the amine bridge from NCH2CH2N to NCH2CH2CH2N or the introduction of chloro-substituent on the phenoxy moiety led to a significant decline of the catalytic activity.

A series of racemic 2-[(2′-(dimethylamino)biphenyl-2-ylimino)methyl]-4-R2-6-R1-phenols (L1H–L4H) were reacted with {Mg[N(SiMe3)2]2}2 to provide four heteroleptic magnesium complexes (L1–4)MgN(SiMe3)2·(THF)n (1, R1 = tBu, R2 = Me, n = 1; 2, R1 = R2 = CMe2Ph, n = 0; 3, R1 = CPh3, R2 = tBu, n = 1; 4, R1 = Br, R2 = tBu, n = 0), which have been fully characterized. X-ray structural determination shows that complex 1 possesses a monomeric structure, but complex 4 is dimeric with C2-symmetry where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. The heteroleptic complexes 1–4 efficiently initiate the ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding magnesium complex. Microstructure analyses of the resulting PLAs revealed that Pr values range from 0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC, detailed analyses using 1H and 13C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen–acyl bond (Xreg = 0.65–0.86).