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-Bu-2(t)-C6H2OH-2}(2) (1-H-2) 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 center dot center dot center dot H-C agostic interaction (Ca center dot center dot center dot H distances of 2.37 and 2.41 angstrom). Another sterically hindered calcium complex,Ca{2-Bu-t-6-(C6F5N=CH)C6H3O}(2)(THF)(2)center dot(C7H8)(2/3) (7), was prepared by reaction of 6 with the iminophenol 2-Bu-t-6-(C6F5N=CH)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(OPr4i) with the new amino-bis(phenol) Me2NCH2CH2N{CH2-3-Bu-t-5-Me-C6H2OH-2}(2) (2-H-2). The reduction of 2-Ti with sodium amalgam gave the titanium(III) salt Ti[N{CH2-3-Bu-t-5-Me-C6H2O-2}(2){CH(2)CH(2)NMe2}](OPri)(2)center dot 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 pi-bonding from the oxygen atoms of the isopropoxide groups to titanium. 1-Ca and 8 were active initiators for the ring-opening polymerisation of epsilon-caprolactone (up to 97% conversion of 200 equivalents in 2 hours) and yielded polymers with narrow molecular weight distributions.