Titanium, zinc and alkaline-earth metal complexes supported by bulky O,N,N,O-multidentate ligands: syntheses, characterisation and activity in cyclic esterpolymerisation

Abstract

The reactions of the bulky amino-bis(phenol) ligand Me₂NCH₂CH₂N{CH₂-3,5-Bu^t₂-C₆H₂OH-2}₂ (1-H₂) with Zn[N(SiMe₃)₂]₂ (4), [Mg{N(SiMe₃)₂}₂]₂ (5) and Ca[N(SiMe₃)₂]₂(THF)₂ (6) yield the complexes 1-Zn, 1-Mg and 1-Ca in good yields. The X-ray structure of 1-Ca showed the complex to be dimeric, with calcium in a distorted octahedral coordination geometry. Five of the positions are occupied by an N₂O₃ donor set, while the sixth is taken up by an intramolecular close contact to an o-Bu^t substituent, a rare case of a Ca⋯H–C agostic interaction (Ca⋯H distances of 2.37 and 2.41 Å). Another sterically hindered calcium complex, Ca{2-Bu^t-6-(C₆F₅NCH)C₆H₃O}₂(THF)₂·(C₇H₈)_2/3 (7), was prepared by reaction of 6 with the iminophenol 2-Bu^t-6-(C₆F₅NCH)C₆H₃OH (3-H). According to the crystal structure, 7 is monomeric and octahedral, with trans THF ligands. The complex Ti[N{CH₂-3-Bu^t-5-Me-C₆H₂O-2}₂{CH₂CH₂NMe₂}](OPrⁱ)₂ (2-Ti) was prepared by treatment of Ti(OPrⁱ₄) with the new amino-bis(phenol) Me₂NCH₂CH₂N{CH₂-3-Bu^t-5-Me-C₆H₂OH-2}₂ (2-H₂). The reduction of 2-Ti with sodium amalgam gave the titanium(III) salt Ti[N{CH₂-3-Bu^t-5-Me-C₆H₂O-2}₂{CH₂CH₂NMe₂}](OPrⁱ)₂·Na(THF)₂ (8). A comparison of the X-ray structures of 2-Ti and 8 showed that the additional electron in 8 significantly reduced the intensity of the π-bonding from the oxygen atoms of the isopropoxide groups to titanium. 1-Ca and 8 were active initiators for the ring-opening polymerisation of ε-caprolactone (up to 97% conversion of 200 equivalents in 2 hours) and yielded polymers with narrow molecular weight distributions.