Search

Alkyl aluminum complexes of chiral salan ligands assembled around the 2,2'-bipyrrolidine core form as single diastereomers that have identical configurations of the N donors. Active catalysts for the polymerization of lactide were formed upon the addition of benzyl alcohol. Polymeryl exchange between enantiomorphous aluminum species had a dramatic effect on the tacticity of the poly(lactic acid) (PLA) in the polymerization of racemic lactide (rac-LA): The enantiomerically pure catalyst of the nonsubstituted salan ligand led to isotactic PLA, and the racemic catalyst exhibited lower stereocontrol. The enantiomerically pure catalyst of the chloro-substituted salan ligand led to PLA with a slight tendency toward heterotacticity, whereas the racemic catalyst led to PLA of almost perfect heterotacticity following an insertion/auto-inhibition/exchange mechanism.

Here, we report a simple one-step access to new rigid N,O-calixarene ligands which is based on copper-catalyzed amination at the lower rim. We also present the coordination properties of these ligands with some main group and transition metals leading to new complexes with superior catalytic activity, in several organic transformations, compared with calixarene metal complexes reported in the literature.

Sequential tetradentate dianionic thio-imine diphenolate ligands featuring an ortho-phenylene core and their zirconium complexes are described for the first time. Ligands that include different combinations of bulky-alkyl groups and halo groups on the two phenol arms were prepared by a substitution/condensation reaction sequence. An unexpected fac-fac wrapping mode was found in the solid state for the ligands in the octahedral [{ONSO}Zr(O(t)Bu)2] complexes. The complexes were all fluxional, and the barrier for enantiomer interconversion was found to depend on the phenolate substituents. The complexes were found to catalyze the polymerization of rac-lactide to poly(lactic acid) in solution with polymer tacticities varying from heterotactic to atactic which showed correlation to the nature of phenolate substituents but not to the degree of complex fluxionality.

Aluminum complexes of enantiomerically pure aminomethylpyrrolidine-based salalen ligands and their application in the stereoselective polymerization of lactide are described. Poly(lactic acid) featuring the new gradient isotactic multiblock microstructure was synthesized by isoselective catalysts, which operate by a combination of enantiomorphic-site and chain-end control mechanisms.

The coordination chemistry of chiral tetradentate dianionic ligands of the diamine-diphenolato and diamine-diolato families having the (R,R)-2,2′-bipyrrolidine core around ZnII was investigated. Reactions with diethylzinc led to complexes of the type [{ONNO}Zn] for most ligands and to bridging dinuclear complexes of the type [μ-Lig4(ZnEt)2] for one of the diamine-diolato ligands. Reactions with bis(hexamethyldisilazide)zinc led to complete conversions. Most complexes were obtained as mononuclear complexes. The least bulky Salan ligand, Lig3, led to an equilibrium between mononuclear and dinuclear complexes in noncoordinating solvents. All ligands were found to wrap around the tetrahedral zinc with very high diastereoselectivities supporting predetermined chiral induction from the bipyrrolidine core to the helical ligand wrapping. Molecular structures determined by single-crystal X-ray diffraction for two complexes of the type [{ONNO}Zn] substantiated the predicted Δ wrapping of the (R,R)-based ligands. In contrast, representative Salan and diamine-diolato ligands assembled around the trans-1,2-diaminocyclohexane core led to diastereomer mixtures of the corresponding complexes.

The salen complex catalyst (SAA) in combination with Bu4NBr as co-catalyst shows high catalytic activity in reactions of epoxides (I) and (IV) with carbon dioxide to give the corresponding carbonates.

The salen complex catalyst (SAA) in combination with Bu4NBr as co-catalyst shows high catalytic activity in reactions of epoxides (I) and (IV) with carbon dioxide to give the corresponding carbonates.

Three dinuclear aluminum alkyl complexes of the general formula LAl2Me4, where L are salen ligands with an alkyl backbone of different lengths between the nitrogen atoms (1,3-propylene (1), 1,5-pentylene (2) and 1,12-dodecaylene (3)), have been prepared through alkane elimination reactions between each ligand and two equivalents of AlMe3. The related hemi-salen aluminum complex 4 was prepared by an analogous reaction between a phenoxy–imine ligand and a single equivalent of AlMe3. The activities of these aluminum complexes in the ring-opening polymerization (ROP) of rac-lactide and of several epoxides have been investigated and compared. The dinuclear complex 1, bearing the salen ligand with the shortest alkyl bridge, was the most active in the ROP of LA producing isotactic enriched PLA. Otherwise, the other complexes (2 and 3), in which the metal centers are remote, produced atactic PLA with inferior activity. Analogous differences in terms of activity emerged in the ROP of epoxides. The comparison of the catalytic behavior of the dinuclear complexes as well as their mononuclear counterparts suggests the cooperation between the two aluminum metal centers of the dinuclear species in which these are close enough.

Yttrium and aluminium complexes of two dithiodiolate ligands that feature different bridges (CF3)2C(OH)CH2SRSCH2C(OH)(CF3)2 (L(1)-H2, R = CH2CH2 and L(2)-H2, R = C6H4) were synthesized in good yields by reacting tris(silylamide)yttrium or trimethylaluminium with one equivalent of the proligand. All complexes were characterized by NMR and elemental analysis, and single-crystal X-ray structural analysis was also performed for one of the yttrium complexes. The catalytic activities of the four complexes in the ring-opening polymerization of ε-caprolactone and rac-lactide have been investigated. Furthermore, DOSY experiment and DFT calculations have been carried out to determine the structure of the isopropoxo derivative of the complex L(2)Y amide.

Aluminum complexes of non-chiral-salalen ligands were investigated in the catalysis of ring-opening polymerization (ROP) of lactide and e-caprolactone and in their copolymerization. The aluminum-salalen complexes were found to polymerize all varieties of lactide, namely: l-, d-, rac- and meso-lactide, and showed moderate productivities. rac-LA gave rise to isotactic polylactides (with Pm up to 72%), while meso-LA gave rise to heterotactic polylactides (with a Pm of 79%). An experiment was designed for distinguishing between chain-end control and enantiomorphic-site control combined with polymeryl exchange for the isotactic stereoblock microstructure observed for the PLA produced from rac-LA; it gave strong evidence for polymeryl exchange between propagating species. Finally, this class of catalysts promoted the copolymerization of e-caprolactone and lactides. In particular, compound allowed controlled random copolymerization of e-caprolactone and l-lactide.

Based on two well-established ligand systems, the aminopyridinato (Ap) and the phenoxyimine (FI) ligand systems, new Ap-FI hybrid ligands were developed. Four different Ap-FI hybrid ligands were synthesized through a simple condensation reaction and fully characterized. The reaction of hafnium tetrabenzyl with all four Ap-FI hybrid ligands exclusively led to mono(Ap-FI) complexes of the type [(Ap-FI)HfBn2 ]. The ligands acted as tetradentate dianionic chelates. Upon activation with tris(pentafluorophenyl)borane, the hafnium-dibenzyl complexes led to highly active catalysts for the polymerization of 1-hexene. Ultrahigh molecular weights and extremely narrow polydispersities support the living nature of this polymerization process. A possible deactivation product of the hafnium catalysts was characterized by single-crystal X-ray analysis and is discussed. The coordination modes of these new ligands were studied with the help of model titanium complexes. The reaction of titanium(IV) isopropoxide with ligand 1 led to a mono(Ap-FI) complex, which showed the desired fac-mer coordination mode. Titanium (IV) isopropoxide reacted with ligand 4 to give a complex of the type [(ApH-FI)2 Ti(OiPr)2 ], which featured the ligand in its monoanionic form. The two titanium complexes were characterized by X-ray crystal-structure analysis.

The activity of dibenzylzirconium and dibenzylhafnium Salalen complexes in polymerisation of propylene with MAO as a cocatalyst is described. Three Salalen ligand precursors combining a bulky alkyl group (1-adamantyl) on the imine-side phenol and electron withdrawing halo groups of different sizes on the amine-side phenol were explored. All metal complexes were obtained as single diastereomers. An X-ray crystallographic structure of a hafnium complex of an additional ligand carrying the combination of tert-butyl and chloro substituted phenolates, 4-Hf, revealed a fac–mer wrapping of the Salalen ligand around the metal centre. All complexes led to active catalysts in propylene polymerisation and to isotactic polypropylene of high regioregularity. The zirconium complexes led to polypropylene having molecular weights of Mw = 132 000–200 000 and isotacticities of [mmmm] = 65.7–75.0%. The hafnium complexes led to polypropylene of higher molecular weights of Mw = 375 000–520 000 and higher stereoregularities of [mmmm] = 80.6–89.3%, the highest isotacticity obtained with 3-Hf.