Zinc and cobalt complexes based on tripodal ligands: synthesis, structure and reactivity toward lactide

Ring-opening polymerization of lactides and ε-caprolactone catalyzed by Zn(II) aryl carboxylate complexes supported by 4-pyridinyl schiff base ligands

Synthesis and catalytic studies of aryl carboxylate Zn (II) complexes is reported. Reaction of substituted (E)-N-phenyl-1-(pyridin-4-yl)methanimine with a methanolic solution of Zn(CH₃COO)₂ and substituted aryl carboxylate co-ligands gave heteroleptic Zn(II) complexes; [Zn(C₆H₅COO)₂(L1)]₂ (1), [Zn(C₇H₇COO)₂(L1)]₂ (2), [Zn (4-F-C₆H₄COO)₂(L1)]₂ (3), [Zn(C₆H₅COO)₂(L2)]₂ (4), [Zn(C₇H₇COO)₂(L2)]₂ (5), [Zn (4-F-C₆H₄COO)₂(L2)]₂ (6), [Zn(C₆H₅COO)₂(L3)]₂ (7), [Zn(C₇H₇COO)₂(L3)]₂ (8), [Zn (4-F-C₆H₄COO)₂(L3)]₂ (9). The molecular structures of complexes 1 and 4 are dinuclear with the zinc atom in complex 1 adopting a distorted trigonal bipyramidal geometry in a bi-metallacycle while complex 4 is square pyramidal where all four benzoate ligands bridge the zinc metals in a paddle wheel arrangement. All complexes successfully initiated mass/bulk ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) and lactides (LAs) monomers with or without alcohol co-initiators at elevated temperatures. Complexes 1, 4 and 6 containing the unsubstituted benzoate co-ligands were the most active in their triad; with complex 4 being the most active (k _app) of 0.3450 h^-1. The physicochemical properties of the polymerization products of _l-lactide and rac-lactide in toluene revealed melting temperatures (T_m) between 116.58 °C and 188.03 °C, and decomposition temperatures between 278.78 °C and 331.32 °C suggestive of an isotactic PLA with a metal capped end.

Immortal Polymerization of LA: the Influence of Steric Effect, Electron Effect and pKa for Chain Transfer Agents

The “immortal” ring-opening polymerization (iROP) of L-LA, catalyzed by ligand-free Ca[N(SiMe3)2]2(THF)2 in combination with different chain transfer agents (alcohols, phenols and PhCH2NH2) was systematically investigated for the first time. When...

A Single Catalyst for Promoting Reverse Processes: Synthesis and Chemical Degradation of Polylactide

A simple zinc catalyst showing high activity for both the synthesis of polylactide, a biodegradable polymer produced from renewable feedstock, and its degradation was described. In the ring-opening polymerization of lactides, the zinc catalyst showed one of the highest activities reported in the literature for reactions carried out in solution at room temperature. This excellent performance was preserved even when the process was performed under industrial conditions: at high temperature, in the absence of solvent, and by using a low catalyst loading with unpurified monomers. The same complex revealed high efficiency also in depolymerization of polylactide by alcoholysis, a process that occurred efficiently at room temperature and in the absence of solvent, conditions that reduce costs and guarantee low environmental impact.

Efficient Bulky Organo-Zinc Scorpionates for the Stereoselective Production of Poly(rac-lactide)s

The direct reaction of the highly sterically demanding acetamidinate-based NNN'-scorpionate protioligand Hphbp^tamd [Hphbp^tamd = N,N'-di-p-tolylbis(3,5-di-tertbutylpyrazole-1-yl)acetamidine] with one equiv. of ZnMe₂ proceeds in high yield to the mononuclear alkyl zinc complex [ZnMe(κ³-phbp^tamd)] (1). Alternatively, the treatment of the corresponding lithium precursor [Li(phbp^tamd)(THF)] with ZnCl₂ yielded the halide complex [ZnCl(κ³-phbptamd)] (2). The X-ray crystal structure of 1 confirmed unambiguously a mononuclear entity in these complexes, with the zinc centre arranged with a pseudotetrahedral environment and the scorpionate ligand in a κ³-coordination mode. Interestingly, the inexpensive, low-toxic and easily prepared complexes 1 and 2 resulted in highly efficient catalysts for the ring-opening polymerisation of lactides, a sustainable bio-resourced process industrially demanded. Thus, complex 1 behaved as a single-component robust initiator for the living and immortal ROP of rac-lactide under very mild conditions after a few hours, reaching a TOF value up to 5520 h^-1 under bulk conditions. Preliminary kinetic studies revealed apparent zero-order dependence on monomer concentration in the absence of a cocatalyst. The PLA materials produced exhibited narrow dispersity values, good agreement between the experimental M_n values and monomer/benzyl alcohol ratios, as well as enhanced levels of heteroselectivity, reaching P_s values up to 0.74.

Towards a selective synthetic route for cobalt amino acid complexes and their application in ring opening polymerization of rac-lactide

Catalysts based on cobalt amino acids and 2,2 bipyridine (bipy) present an attractive and cost-effective alternative as ring opening polymerization catalysts, yet this system remains underexplored despite the advantageous coordination properties of amino acids and bipy as ligands combined with the variety of accessible oxidation states and coordination geometries of cobalt. Here, metal complexes of type [Co(aa)₂(bipy)] with amino acids (aa: glycine, leucine and threonine) as ligands are reported. The complexes were characterized spectroscopically (IR, UV-vis and ¹H, ¹³C NMR for diamagnetic species), and by MS spectrometry and elemental analysis. The data reveal that the 2,2 bipyridine acts as a neutral bidentate donor coordinating to the metal ion through two nitrogen atoms and the amino acid acts as a bidentate ligand coordinating through the carboxylate and amino group forming a stable five membered ring and a pseudo-octahedral geometry around the Co center. The activity of the complexes for the ring opening polymerization (ROP) of rac-lactide is presented. The complexes are effective initiators for the ROP of rac-lactide (K_obs = 9.05 × 10⁻⁴ s⁻¹) at 100 : 1 [rac-lactide] : [catalyst] 1 M overall concentration of lactide in toluene at 403 K.

Catalysts based on Co, amino acids, and 2,2-bipyridine present an attractive and economic alternative in ring opening polymerization, and possess advantageous ligand coordination properties combined with a variety of accessible oxidation states and coordination geometries.

Architecture and synthesis of P,N-heterocyclic phosphine ligands

Diverse P,N-phosphine ligands reported to date have performed exceptionally well as auxiliary ligands in organometallic catalysis. Phosphines bearing 2-pyridyl moieties prominently feature in literature as compared to phosphines with five-membered N-heterocycles. This discussion seeks to paint a broad picture and consolidate different synthetic protocols and techniques for N-heterocyclic phosphine motifs. The introduction provides an account of P,N-phosphine ligands, and their structural and coordination benefits from combining heteroatoms with different basicity in one ligand. The body discusses the synthetic protocols which focus on P–C, P–N-bond formation, substrate and nucleophile types and different N-heterocycle construction strategies. Selected references are given in relation to the applications of the ligands.

Highly efficient metal(iii) porphyrin and salen complexes for the polymerization of rac-lactide under ambient conditions

Metal(iii) complexes supported by porphyrin and salen ligands were highly efficient for rac-lactide polymerization at room temperature giving isotactic-enriched PLA.

Bis(2-pyridylmethyl)amine-functionalized alizarin: an efficient and simple colorimetric sensor for fluoride and a fluorescence turn-on sensor for Al3+ in an organic solution

Bis(2-pyridylmethyl)amine-functionalized alizarin (H₂L) can visibly sense F⁻ and fluorescence-turn-on sense Al³⁺ sensitively in organic solvents. The presence of NiCl₂ increases the chromic sensitivity to F⁻.

Dinuclear iminophenoxide copper complexes in rac-lactide polymerisation

Dinuclear bis(R'-(R''-iminomethyl)phenoxide) copper complexes L2Cu2(μ-OR)2 were prepared from the reaction of copper methoxide with ROH and LH (ROH = dimethylaminoethanol or pyridylmethanol, R' = H, 4,6-tBu, 1,3-Cl, R'' = benzyl, cyclohexyl, diphenylmethyl and 2,6-dimethylphenyl). Preparation was complicated by formation of homoleptic L2Cu and only 9 of the 24 possible combinations could be prepared. All complexes were characterized by single crystal X-ray diffraction studies and crystallized as dinuclear penta-coordinated complexes. Homoleptic complexes L2Cu were inactive in lactide polymerization at room temperature. Most heteroleptic complexes showed modest to good activities with full conversion in less than 6 h at room temperature. Complexes with R' = H showed poor molecular weight control, complexes with R' = Cl were inactive in polymerization. In pyridylmethoxide-containing complexes, only one alkoxide initiated chain growth. All complexes produced atactic polymer.

Divergent [{ONNN}Mg-Cl] complexes in highly active and living lactide polymerization

Chloro-magnesium complexes of divergent tetradentate-monoanionic {ONNN}-type ligands which formed as either mononuclear or dinuclear complexes are reported. These complexes catalyze the ring-opening polymerization of lactide with high activity and in a living manner, and enable the synthesis of well-defined stereo-diblock copolymers with Mn > 200 kDa, as well as stereo-n-block copolymers (n = tri, tetra) with high molecular weights by sequential monomer addition. Thermal and crystallographic characterizations revealed that even the very high molecular weight PLA copolymers crystallized in a stereocomplex phase, and that their degradation temperature was as high as 354 °C.

Potassium complexes supported by monoanionic tetradentate amino-phenolate ligands: synthesis, structure and catalysis in the ring-opening polymerization of rac-lactide

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 (P_m = 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.

Mechanistic Aspects of the Polymerization of Lactide Using a Highly Efficient Aluminum(III) Catalytic System

We report here a unique example of an in situ generated aluminum initiator stabilized by a C₂-symmetric salen ligand which shows a hitherto unknown high activity for the ROP of rac-lactide at room temperature. Using a simple and robust catalyst system, which is prepared from a salen complex and an onium salt, this convenient route employs readily available reagents that afford polylactide in good yields with narrow polydispersity indices, without the need for time-consuming and expensive processes that are typically required for catalyst preparation and purification. In line with the experimental evidence, DFT studies reveal that initiation and propagation proceed via an external alkoxide attack on the coordinated monomer.

Mono- and binuclear chiral N,N,O-scorpionate zinc alkyls as efficient initiators for the ROP of rac-lactide

Activity enhancement in the ROP ofrac-LA through the cooperative effect of two remote zinc metals in N,N,O-scorpionate binuclear trisalkyls.

Steric and chelating ring concerns on the l-lactide polymerization by asymmetric β-diketiminato zinc complexes

Asymmetric β-diketiminato zinc complexes were investigated in l-lactide polymerization. The catalytic rate depended heavily on the N-aryl substituents.

Gallium and indium complexes containing the bis(imino)phenoxide ligand: synthesis, structural characterization and polymerization studies

A series of gallium and indium complexes containing the bis(imino)phenolate ligand framework were synthesized and completely characterized with different spectroscopic techniques.

Synthesis and structures of bis-ligated zinc complexes supported by tridentate ketoimines that initiate L-lactide polymerization

Eight bis-ligated, homoleptic, zinc complexes were synthesized through the reaction of NNO Schiff base ketoimines bearing varying substituents with diethyl zinc in an inert atmosphere glovebox at room temperature and isolated in 62-95% yield. The complexes were characterized with (1)H, (13)C, and (19)F nuclear magnetic resonance spectroscopy, absorbance spectroscopy, high resolution mass spectrometry, elemental analysis, and single crystal X-ray crystallography. The complexes were shown to adopt distorted octahedral coordination geometry around zinc. The (1)H and (19)F NMR spectra of complexes 1-7 showed stable zinc coordination at 300 K while the effect of steric encumbrance and two trifluoromethyl groups in complex 8 was investigated with variable temperature NMR. The bis-ligated zinc complexes were effective initiators for the ring opening polymerization of L-lactide into poly-L-lactic acid (PLLA). With [L-lac]/[Zn complex] = 50, the bis-ligated zinc complexes yielded percentage conversion of 14-98% with polymerization times varying from 15-1440 min, where the longest reaction times were required when two trifluoromethyl groups were present. The addition of 4-fluorophenol co-catalyst resulted in up to a 5-fold increase in the percentage conversion in toluene solution and up to a 14-fold increase in bulk melt polymerization with reductions in the poly-dispersity index values for the isolated PLLA. Addition of 4-fluorophenol to complex 1 was studied with (1)H and (19)F NMR and appeared to yield an in situ generated zinc alkoxide complex.

Zinc-Catalyzed Highly Isoselective Ring Opening Polymerization of rac-Lactide

A family of chiral zinc amido-oxazolinate complexes are shown to be highly active and isoselective initiators for the ring-opening polymerization (ROP) of rac-lactide, yielding isotactic stereoblock polylactides (PLA) with P_m up to 0.91. This represents the highest isoselectivity observed with zinc-based catalysts for ROP of rac-lactide.

Enantiopure N,N,O-scorpionate zinc amide and chloride complexes as efficient initiators for the heteroselective ROP of cyclic esters

Amide-containing zinc heteroscorpionates can act as single-component initiators for the well-controlled ROP of rac-LA to produce heterotactic-enriched materials.