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

The reactions of the bulky amino-bis(phenol) ligand Me2NCH2CH2N{CH2-3,5-But2-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-But substituent, a rare case of a Ca⋯H–C agostic interaction (Ca⋯H distances of 2.37 and 2.41 A). Another sterically hindered calcium complex, Ca{2-But-6-(C6F5NCH)C6H3O}2(THF)2·(C7H8)2/3 (7), was prepared by reaction of 6 with the iminophenol 2-But-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-But-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-But-5-Me-C6H2OH-2}2 (2-H2). The reduction of 2-Ti with sodium amalgam gave the titanium(III) salt Ti[N{CH2-3-But-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 e-caprolactone (up to 97% conversion of 200 equivalents in 2 hours) and yielded polymers with narrow molecular weight distributions.