Bis(3,5-dimethylpyrazole) copper(II) and zinc(II) complexes as efficient initiators for the ring opening polymerization of ε-caprolactone and d,l-lactide

Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination-Insertion

We report three niobium-based initiators for the catalytic ring-opening polymerization (ROP) of ε-caprolactone, exhibiting good activity and molecular weight control. In particular, we have prepared on the gram-scale and fully characterized a monometallic cationic alkoxo-Nb(V) ε-caprolactone adduct representing, to the best of our knowledge, an unprecedented example of a metal complex with an intact lactone monomer and a functional ROP-initiating group simultaneously coordinated at the metal center. At 80 °C, all three systems initiate the immortal solution-state ROP of ε-caprolactone via a coordination-insertion mechanism, which has been confirmed through experimental studies, and is supported by computational data. Natural bond orbital calculations further indicate that polymerization may necessitate isomerization about the metal center between the alkoxide chain and the coordinated monomer. The observations made in this work are expected to inform mechanistic understanding both of amine tris(phenolate)-supported metal alkoxide ROP initiators, including various highly stereoselective systems for the polymerization of lactides and of coordination-insertion-type ROP protocols more broadly.

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.

Influence of the Lewis basicity hardness of crystallization solvents on the coordination sphere of the complex [Co(3,5-dinitrobenzoate-O,O′)2]: crystallographic and theoretical analysis

Crystal growth of [Co(DNB)2] in different solvents is a thermodynamic process favored by ΔErecrys (kcal mol−1) where mono/trinuclear molecules are obtained. Closeness of HOMO orbitals of solvents to the SOMOs orbitals of Co(ii) ions guide the process.

Ring opening polymerization of d,l-lactide and ε-caprolactone catalysed by (pyrazol-1-yl)copper(ii) carboxylate complexes

1,2-Bis{(3,5-dimethylpyrazol-1-yl)methyl}benzene (L) reacts with [Cu(OAc)₂] and C₆H₅COOH, 4-OH-C₆H₄COOH, 2-Cl-C₆H₄COOH and (3,5-NO₂)₂-C₆H₃COOH to afford the copper complexes [Cu₂(C₆H₅COO)₄(L)₂] (1), [Cu₂(4-OH-C₆H₄COO)₄(L)₂] (2), [Cu₂(2-Cl-C₆H₄COO)₄(L)₂]_n (3) and [Cu{(3,5-NO₂)₂-C₆H₃COO}₂L]_n (4) which are characterised by IR, mass spectrometry, elemental analyses, and X-ray crystallography. The structural data revealed two geometries that are adopted by the complexes: (i) paddle wheel in 1, 2·7H₂O, 3 and (ii) regular chains in 3 and 4. Magnetic studies show strong antiferromagnetic couplings in the paddle wheel complexes and a weak antiferromagnetic coupling in the monometallic chain one. Catalysis studies performed with these complexes (1–4) showed that they initiate ring opening polymerization (ROP) of ε-caprolactone (ε-CL) under solvent-free conditions and d,l-lactide in toluene at elevated temperatures. Polycaprolactone (PCL) and poly(d,l-lactide) (PLA) obtained from the polymerization reactions are of low molecular weights (858 for PCL and 602 Da for PLA for initiator 1) and polydispersity indices (typically 2.16 for PCL and 1.64 for PLA with 1 as the initiator). End group analysis of the polymers, determined by MALDI-ToF MS, indicates that the polymers have benzoate, hydroxyl, methoxy and cyclic end groups.

We report the synthesis, structure and complete characterization of four new pyrazolyl carboxylate-based copper(ii) complexes that catalyze the ring opening polymerization of ε-caprolactone under solvent-free conditions and of d,l-lactide in toluene.

Crystal structure of (E)-4-((4-chlorophenylimino)methyl)pyridinium 3,5-dinitrobenzoate, C19H13ClN4O6

C19H13ClN4O6, monoclinic, P21/n (no. 14), a = 4.8032(1) Å, b = 19.5545(5) Å, c = 19.6915(6) Å, β = 90.640(1)°, V = 1849.39(8) Å3, Z = 4, R gt(F) = 0.0369, wR ref(F 2) = 0.1011, T = 100(2) K.

Undiscovered Potential: Ge Catalysts for Lactide Polymerization

Polylactide (PLA) is a high potential bioplastic that can replace oil-based plastics in a number of applications. To date, in spite of its known toxicity, a tin catalyst is used on industrial scale which should be replaced by a benign catalyst in the long run. Germanium is known to be unharmful while having similar properties as tin. Only few germylene catalysts are known so far and none has shown the potential for industrial application. We herein present Ge complexes in combination with zinc and copper, which show amazingly high polymerization activities for lactide in bulk at 150 °C. By systematical variation of the complex structure, proven by single-crystal XRD and DFT calculations, structure-property relationships are found regarding the polymerization activity. Even in the presence of zinc and copper, germanium acts as the active site for polymerizing probably through the coordination-insertion mechanism to high molar mass polymers.

Replacing sulfonate by carboxylate: application of pyridyliminocarboxylato copper(II) complexes in rac-lactide polymerization and Chan–Evans–Lam coupling

Copper(II) complexes carrying pyridylmethyleneaminobenzoate or –propanoate ligands, LCuX, were prepared in one-pot reactions from pyridinecarboxaldehyde, aminobenzoic acid or β-alanine, and CuX2 (X = Cl, NO3, OAc, or OTf). All complexes were characterized by single-crystal X-ray diffraction studies and formed either dimers, tetramers, or coordination polymers. Attempted preparation of the respective alkoxide complexes, LCu(OR), was unsuccessful, but use of LCuX/NaOMe mixtures in rac-lactide polymerization indicated under some conditions coordination–insertion polymerization via a copper alkoxide as the mechanism. The complexes performed poorly in rac-lactide polymerization, showing low activities (12 h to completion at 140 °C), low to moderate heterotacticity (Pr = 0.6–0.8), and poor polymer molecular weight control (intramolecular transesterification). They were competent catalysts for Chan–Evans–Lam couplings with phenylboronic acid, without any indication of side reactions such as deboration or aryl homocoupling. The complexes were active in undried methanol, without addition of base, ligand, or molecular sieves. Aniline, n-octylamine, and cyclohexylamine were coupled quantitatively under identical reaction conditions. There is only little influence of the anion on activities (less than a factor of 2) but a strong influence on induction periods. The complexes were not active in CEL coupling with alcohols, phenols, or alkylboronic acids.

Polymers of ε-Caprolactone Using New Copper(II) and Zinc(II) Complexes as Initiators: Synthesis, Characterization and X-Ray Crystal Structures

Five of six new Zn(II) and Cu(II) complexes were active in the ring-opening polymerization (ROP) of ε-caprolactone (CL) under solvent-free conditions, producing polycaprolactones (PCLs) of high crystallinity with molecular weights between 22,900 and 38,700 g mol⁻¹ and decomposition temperatures above 260 °C. ¹H NMR analysis demonstrated that the PCLs obtained were mainly linear, having hydroxymethylene groups at the chain ends. The results obtained indicated a significant improvement in terms of the ratio of monomer:initiator compared to related Cu(II) and Zn(II) complexes. In addition, the structures of the complexes 1 and 4 were determined by single-crystal X-ray diffraction. The synthesis and full characterization of all complexes are described in this paper.

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.

Synthesis, characterization and evaluation of bulky bis(pyrazolyl)palladium complexes in Suzuki–Miyaura cross-coupling reactions

Pyrazole-containing compounds have been used in recent times as ligands to stabilize metal complexes used as pre-catalysts in cross-coupling reactions. With various substituents at various positions in the pyrazole ring, the overall electrophilic and steric properties of the metal complexes can be fine-tuned. Herein, we report the synthesis of bulky pyrazole-based ligands by condensation of methyl 4-(bromomethyl)benzoate or benzyl bromide with various substituted pyrazole compounds. These ligands were utilised in the synthesis of bis(pyrazolyl)palladium(ii) complexes. The complexes' catalytic activity in Suzuki–Miyaura cross-coupling reactions was evaluated. Phenyl bearing pre-catalyst 7, at a catalyst loading of 0.33 mol%, successfully converted 98% of bromobenzene and phenylboronic acid to biphenyl in 4 h at 140 °C, while the tertiary butyl bearing pre-catalyst 8 converted up to 81% of the same substrates to biphenyl. An increase in conversion was seen for all pre-catalysts when an electron-withdrawing substituent was present on the aryl halide substrate, and the opposite was observed when the electron-withdrawing group was present on the phenyl boronic acid.

Pyrazole-containing compounds have been used in recent times as ligands to stabilize metal complexes used as pre-catalysts in cross-coupling reactions.

Ring-Opening Polymerization Reactions of ε-Caprolactone and Lactides Initiated by (Benzimidazolylmethyl)amino Magnesium(II) Alkoxides

Reactions of (benzimidazolylmethyl)amine ligands N-((1H-benzo[s]imidazol-2-yl)methyl)-2,6-dimethylaniline (L1), N-((1H-benzo[d]imidazol-2-yl)methyl)-2,6-diisopropylaniline (L2), and N-((1H-benzo[d]imidazol-2-yl)methyl)-2,4,6-trimethylaniline (L3) with Mg(nBu)2 in the presence of either benzyl alcohol (BnOH) or tert-butyl alcohol (t-BuOH) afforded the respective MgII alkoxides [Mg(L1)(OBn)]2 (1), [(Mg(L2)(OBn)]2 (2), [Mg(L3)(OBn)]2 (3), [Mg(L2)(t-BuO)]2 (4). Complexes 1–4 formed efficient catalysts for the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL), d,l-lactide (d,l-LA) and l-lactide (l-LA) at 110°C. The catalytic activities of these complexes in the ROP reactions were influenced by the steric effect of the ligands. Kinetic studies showed pseudo-first-order dependency on monomer. Polycaprolactone and polylactides of moderate weight-average molecular weights of 15285 and 5200 g mol−1 and fairly narrow polydispersity indexes from 1.24 to 1.58 were produced.

Coordination Polymers Containing 1,3-Phenylenebis-((1H-1,2,4-triazol-1-yl)methanone) Ligand: Synthesis and ε-Caprolactone Polymerization Behavior

The reaction of isophthaloyl dichloride with 1H-1,2,4-triazole afforded the new ligand 1,3-phenylenebis(1,2,4-triazole-1-yl)methanone (1). A series of Co(II), Cu(II), Zn(II) and Ni(II) complexes were synthesized using 1 and then characterized by melting point analysis, elemental analysis, theoretical calculations, thermogravimetric analysis, X-ray powder diffraction, nuclear magnetic resonance, infrared and Raman spectroscopy. Experimental and computational studies predict the formation of coordination polymers (CPs). The cobalt and copper CPs and zinc(II) complex were found to be good initiators for the ring-opening polymerization of ε-caprolactone (CL) under solvent-free conditions. ¹H-NMR analysis showed that the obtained polymers of CL were mainly linear and had terminal hydroxymethylene groups. Differential scanning calorimetry showed that the obtained polycaprolactones had high crystallinity, and TGA showed that they had decomposition temperatures above 400 °C. These results provide insight and guidance for the design of metal complexes with potential applications in the polymerization of CL.

Copper(II) complexes with imino phenoxide ligands: synthesis, characterization, and their application as catalysts for the ring-opening polymerization of rac-lactide

ABSTRACT

Four new copper complexes based on bidentate imino phenoxide ligands were synthesized and characterized by IR, UV-Vis spectroscopy, ESI mass spectrometry, single crystal X-ray diffraction, and electrochemistry. The crystal structures revealed that the copper(II) atoms are surrounded by phenolate oxygen and imine nitrogen atoms of two ligands in a distorted square-planar geometry. The existence of ligand-centered, as well as Cu(II)-centered quasi-reversible and reversible redox reactions are observed in the cyclic voltammetry experiments of all the complexes. All complexes are able to catalyze the ring-opening polymerization of rac-lactide yielding polymers with moderate molecular weights and moderately broad molecular weight distributions.

GRAPHICAL ABSTRACT

Crystal structure of 1,1′-(butane-1,4-diyl)bis(5-methyl-1H-pyrazole-3-carbaldehyde), C14H18N4O2

C14H18N4O2, monoclinic, P21/c (no. 14), a = 4.4260(9) Å, b = 14.502(4) Å, c = 11.455(3) Å, β = 96.319(5)°, V = 730.8(3) Å3, Z = 2, Rgt(F) = 0.0712, wRref(F2) = 0.1946, T = 296(2) K.

Novel pyrazolylphosphite– and pyrazolylphosphinite–ruthenium(ii) complexes as catalysts for hydrogenation of acetophenone

New pyrazolylphosphite– and pyrazolylphosphinite–ruthenium(ii) complexes that are effective hydrogenation catalysts for acetophenone.

Zn(ii) and Cu(ii) formamidine complexes: structural, kinetics and polymer tacticity studies in the ring-opening polymerization of ε-caprolactone and lactides

The flexibility of ligands and their coordination chemistry with metal centers provided effective catalysts in the ring opening polymerization of cyclic esters.