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

摘要

The reactions of the bulky amino-bis(phenol) ligand Me_2NCH_2CH_2N{CH_2-3,5-Bu~t_2-C_6H_2OH-2}_2 (1-H_2) with Zn[N(SiMe_3)_2]_2 (4),[Mg{N(SiMe_3)_2}_2]_2 (5) and Ca[N(SiMe_3)_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 N_2O_3 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 A).Another sterically hindered calcium complex,Ca{2-Bu~t-6-(C_6F_5N=CH)C_6H_3O}_2(THF)_2 centre dot (C_7H_8)_(2/3) (7),was prepared by reaction of 6 with the iminophenol 2-Bu~t-6-(C_6F_5N=CH)C_6H_3OH (3-H).According to the crystal structure,7 is monomeric and octahedral,with trans THF ligands.The complex Ti[N{CH_2-3-Bu~t-5-Me-C_6H_2O-2}_2{CH_2CH_2NMe_2}](OPr~i)_2 (2-Ti) was prepared by treatment of Ti(OPr~i_4) with the new amino-bis(phenol) Me_2NCH_2CH_2N{CH_2-3-Bu~t-5-Me-C_6H_2OH-2}_2 (2-H_2).The reduction of 2-Ti with sodium amalgam gave the titanium(III) salt Ti[N{CH_2-3-Bu~t-5-Me-C_6H_2O-2}_2{CH_2CH_2NMe_2}](OPr~i)_2 centre 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.