Chlorides of group 4 metals containing the bis(imino)phenoxide ligand scaffold were synthesized from a direct reaction between the respective ligand and metal chlorides MCl4 (M=Ti, Zr and Hf). These complexes were thoroughly characterized using various spectroscopic methods and single crystal X-ray diffraction studies. They are potent catalysts for the polymerization of rac-lactide (rac-LA), L-lactide (L–LA), ϵ-caprolactone (ϵ-CL), epoxides such as, rac-cyclohexene oxide (rac-CHO), rac-propylene oxide (rac-PO), rac-styrene oxide (rac-SO) and ethylene, resulting in polymers with high number average molecular weights (Mn) and narrow molecular weight distributions (MWDs). The kinetic and mechanistic studies associated with the polymerization of lactide, ϵ-CL and CHO has been investigated in details. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Debashis Chakraborty

Eswara Rao Chokkapu

Venkatachalam Ramkumar

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

ChemistrySelect

The synthesis of homoleptic salalen ligand supported group 4 complexes and their applications in the ring-opening polymerization (ROP) of ε-caprolactone (ɛ-CL) and lactide (LA) are described. All these complexes of type M(L)2 (LH = (E)-N-(2-(3,5-dibromo-2-hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide: M = Ti (1), M = Zr (4), M = Hf (7); LH = (E)-N-(2-(2-hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide: M = Ti (2), M = Zr (5), M = Hf (8); LH = (E)-N-(2-(3,5-di-t-butyl-2-hydroxybenzylideneamino)phenyl)-4-methylbenzenesulfonamide: M = Ti (3), M = Zr (6), M = Hf (9)) are characterized by various spectroscopic techniques. The single crystal X-ray diffraction analysis confirmed that the metal centres in complexes 6 and 8 adopt distorted dodecahedral geometry. The self-assembly of the complexes in solid state through the molecular interactions demonstrated that the supramolecular network forms molecular channels which act as a host lattice for the assembly of toluene solvent molecules. Complexes (1–9) are effective catalysts for the ROP of ɛ-CL and LA and produced high molecular weight (Mn) polycaprolactone (PCL) and poly (lactic acid) (PLA) respectively with narrow molecular weight distributions (MWDs) in the range of 1.17–1.28. PLA produced from the polymerization of rac-LA is atactic. However, better control over the polymerization process was achieved in the presence of toluene as solvent at 80 °C. All these polymerizations follow first-order kinetics with respect to monomer concentration. MALDI-TOF and NMR spectral analysis suggested that the polymerization proceeds through the coordination-insertion mechanism. © 2018

Journal of Organometallic Chemistry

Group 4 metal complexes containing the salalen ligands: Synthesis, structural characterization and studies on the ROP of cyclic esters

A series of novel mononuclear (1a-7a and 1b-6b) as well as tetranuclear (8a and 9a) niobium (Nb) and tantalum (Ta) complexes of benzotriazole or benzoxazole phenoxide pro-ligands bearing different substituents at the ortho and para positions of the phenol rings were synthesized and characterized. The reaction of NbCl5 or TaCl5 with one equivalent of benzotriazole or benzoxazole phenoxide pro-ligands (L1H-L6H) in dry toluene or chloroform produced the corresponding chloride (1a-6a and 1b-6b) and ethoxy (7a) mononuclear Nb and Ta complexes in high yields. Furthermore, from the mononuclear Nb complexes (1a or 4a), a new structural form of tetrameric niobium complexes (8a and 9a) was synthesized through a controlled hydrolysis reaction. The molecular structures of complexes 1b, 4b, 7a, 8a and 9a were unambiguously confirmed by single crystal X-ray diffraction analyses. Furthermore, all these complexes (1a-9a and 1b-6b) were tested as catalysts for the ring-opening polymerisation (ROP) of glycidol to synthesize hyperbranched polyglycerols (HPG) by using 1,1,1-tris(hydroxymethyl)propane (TMP) as an initiator. The degree of branching (DB) observed was 0.30-0.54, which is an indication of hyperbranched structures. In particular, for the niobium complex with electron-withdrawing substituents on the benzoxazole phenoxide pro-ligand (5a), we achieved superior behavior for the ROP of glycidol in terms of activity, control of molecular weight (Mn) and molecular weight distributions (MWDs) (92% of glycidol to HPG, Mn = 10.52 kg mol-1, MWDs &lt;1.33, DB = 0.53 and Tg = -57 °C). A highly hydrophilic nature was observed for the synthesized HPG polymer by water contact angle measurement (20° to 35°). © 2017 The Royal Society of Chemistry.

Dalton Transactions

Nb and Ta benzotriazole or benzoxazole phenoxide complexes as catalysts for the ring-opening polymerization of glycidol to synthesize hyperbranched polyglycerols

A series of homoleptic iminophenoxide and bis(imino)phenoxide supported Zr and Hf complexes 1–10 has been synthesized and characterized by NMR, mass spectrometry and elemental analysis. Structural analysis of the selected complexes by single crystal X-ray diffraction method revealed that the complexes are monomeric in their solid state with distorted dodecahedral geometries. These complexes are found to be potential single-site catalysts for ring-opening polymerization (ROP) of lactide (LA) and ϵ-caprolactone (ϵ-CL) and afforded controlled molecular weight (Mn) polyesters with narrow molecular weight distributions (MWDs) of 1.12-1.28. Kinetic studies revealed that the polymerization rate have shown to be first order dependency on lactide. The low molecular weight oligomer analysis imparted that the polymerization proceeds through the coordination-insertion mechanism. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Homoleptic Zr and Hf Complexes of Imino/Bis(imino)phenoxide Scaffolds: Synthesis, Structural Characterization and Their Catalytic Activity in the ROP of Cyclic Esters

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: [Figure not available: see fulltext.] © 2016, The Author(s).

Fulltext

Monatshefte fur Chemie

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

The stoichiometric reaction of the aminophenol ligands L1-L3 with NbCl5 or TaCl5 afforded a series of metal chloride complexes 1-6. These were characterized by 1H, 13C NMR, mass spectroscopy and elemental analysis. Complex 2 crystallized as ethoxy substituted complex 2a upon attempting crystallization from a solvent mixture of chloroform and ethanol. The molecular structure confirms the zwitterionic nature of the complex where the protonated nitrogen atom of the ligand backbone is neutralised by a negative charge on the metal center. Moderate electrostatic hydrogen bonding interactions between the NH+ donor moieties and acceptor chlorine atoms stabilise these complexes. The complexes show moderate activities towards the ring opening polymerization (ROP) of lactides (rac-LA, l-LA) with good control over the polymerization parameters. All the complexes produced heterotactic enriched poly(lactic acid) (PLA). Low molecular weight oligomers prepared and analysed by MALDI-TOF mass and 1H NMR spectroscopy revealed that the ligand initiates the ROP. The complexes were optimized at the B3LYP/LANL2DZ level and the frontier orbitals were studied in order to calculate the trend in reactivity order theoretically. © 2016 The Royal Society of Chemistry.

RSC Advances

Zwitterionic niobium and tantalum complexes with bidentate aminophenol scaffolds: Synthesis, structural characterization and use in the ring opening polymerization of lactides

A complete library of new alkoxides containing group 4 metals and the imino phenol ligand were synthesized in high yields and purity. These compounds showed catalytic activity towards the polymerization of l-lactide (l-LA), rac-lactide (rac-LA), ε-caprolactone (CL), δ-valerolactone (VL), rac-β-butyrolactone (rac-BL), ethylene and propylene. The zirconium catalysts were found to yield better polymerization results in comparison to the titanium and hafnium analogues. The number average molecular weight (M n) values of the polymers are very high for LA and CL with controlled molecular weight distributions (MWDs). Analyses of MALDI-TOF and 1H NMR spectra of low Mn oligomers reveal that the ligand initiates the ring-opening polymerization (ROP). For ethylene and propylene polymerization, we have achieved moderate to good activity using MAO as a co-catalyst. © 2013 The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

New Journal of Chemistry

Imino phenoxide complexes of group 4 metals: Synthesis, structural characterization and polymerization studies

Cu(OAc)2 was assessed to be a good catalyst for the bulk ring opening polymerization of lactides. These polymerizations are highly controlled leading to the formation of polymer with expected number average molecular weights and narrow molecular weight distribution. This method may be used to synthesize polymers with different end terminal groups. The over all process is green, since these polymers being biodegradable and the system is independent of solvents. © 2011 Elsevier B.V. All rights reserved.

Journal of Molecular Catalysis A: Chemical

Copper acetate catalyzed bulk ring opening polymerization of lactides

A large variety of aryloxy compounds and benzyloxy derivatives of Hf(IV) were synthesized from Hf(O-tBu)4 using the alcoholysis route. These compounds were completely characterized using various spectroscopic and analytical methods along with single crystal X-ray diffraction studies. Multinuclear NMR studies prove high degree of fluxional behavior of these compounds in solution. They adopt a dimeric structure in the solid-state as seen from X-ray diffraction studies on a few of them. These compounds are powerful catalysts for the ring-opening polymerization of ε-caprolactone (CL) and δ-valerolactone (VL) resulting in high number average molecular weight (Mn) polymers and controlled molecular weight distributions (MWDs). Significant control in the polymerization was achieved with these compounds as initiators in comparison to their titanium and zirconium analogues. Kinetic studies reveal that the rates of such polymerizations are in general slower than the corresponding titanium and zirconium derivatives. 1H NMR and MALDI-TOF studies of low molecular weight oligomers suggest that these polymerizations proceed by the activated monomer mechanism. © 2010 Elsevier Ltd.

Polymer

Aryloxy and benzyloxy compounds of hafnium: Synthesis, structural characterization and studies on solvent-free ring-opening polymerization of ε-caprolactone and δ-valerolactone

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ISSN	23656549
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