Heterodinuclear titanium/zinc catalysis: synthesis, characterization and activity for CO2/epoxide copolymerization and cyclic ester polymerization.
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 × 10-4s-1at 70 °C, 1 M in [lactide]) and ε-caprolactone (kobs= 5 × 10-4s-1at 70 °C, 0.9 M in [ε-caprolactone]).
EastCHEM School of Chemistry, Joseph Black Building, University of Edinburgh, David Brewster Road, Edinburgh EH9 3FJ, UK.
Jennifer A
http://orcid.org/0000-0002-9027-4931
JA
Garden
Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.
Andrew J P
AJP
White
Chemistry Research Laboratory, 12 Mansfield Road, University of Oxford, Oxford, OX1 3TA, UK. williams@chem.ox.ac.uk.
Charlotte K
http://orcid.org/0000-0002-0734-1575
CK
Williams