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Issue 8, 2017

Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization

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Abstract

The preparation of heterodinuclear complexes, especially those comprising early-late transition metals coordinated by a simple or symmetrical ancillary ligand, represents a fundamental challenge and an opportunity to prepare catalysts benefitting from synergic properties. Here, two new mixed titanium(IV)–zinc(II) complexes, [LTi(OiPr)2ZnEt] and [LTi(OiPr)2ZnPh], both coordinated by a diphenolate tetra(amine) macrocyclic ligand (L), are prepared. The synthesis benefits from the discovery that reaction of the ligand with a single equivalent of titanium tetrakis(iso-propoxide) allows the efficient formation of a mono-Ti(IV) complex, [LTi(OiPr)2]. All new complexes are characterized by a combination of single crystal X-ray diffraction, multinuclear NMR spectroscopy and mass spectrometry techniques. The two heterobimetallic complexes, [LTi(OiPr)2ZnEt] and [LTi(OiPr)2ZnPh], feature trianionic coordination by the macrocyclic ligand and bridging alkoxide groups coordinate to both the different metal centres. The heterodinuclear catalysts are compared to the mono-titanium analogue, [LTi(OiPr)2], in various polymerization reactions. In the alternating copolymerizations of carbon dioxide and cyclohexene oxide, the mono-titanium complex is totally inactive whilst the heterodinuclear complexes show moderate activity (TOF = 3 h−1); it should be noted the activity is measured using just 1 bar pressure of carbon dioxide. In the ring opening polymerization of lactide and ε-caprolactone, the mono-Ti(IV) complex is totally inactive whilst the heterodinuclear complexes show moderate-high activities, qualified by comparison to other known titanium polymerization catalysts (L-lactide, kobs = 11 × 10−4 s−1 at 70 °C, 1 M in [lactide]) and ε-caprolactone (kobs = 5 × 10−4 s−1 at 70 °C, 0.9 M in [ε-caprolactone]).

Graphical abstract: Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization

Supplementary files

Article information


Submitted
02 Nov 2016
Accepted
12 Jan 2017
First published
03 Feb 2017

This article is Open Access

Dalton Trans., 2017,46, 2532-2541
Article type
Paper

Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization

J. A. Garden, A. J. P. White and C. K. Williams, Dalton Trans., 2017, 46, 2532 DOI: 10.1039/C6DT04193K

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

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