Synthesis, structures and ring-opening polymerization studies of new zinc chloride and amide complexes supported by amidinate heteroscorpionate ligands

Abstract

The reaction of the heteroscorpionate lithium salts [Li(pbpamd)(THF)] [pbpamd = N,N'-diisopropylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] and [Li(tbpamd)(THF)] [tbpamd = N-ethyl-N'-tert-butylbis(3,5-dimethylpyrazol-1-yl)acetamidinate] with 1 equivalent of ZnCl2 in THF affords very high yields of the neutral heteroscorpionate chloride zinc complexes [ZnCl(NNN)] (NNN = pbpamd 1 and tbpamd 2). Compound 1 was used as a convenient starting material for the synthesis of aromatic amide zinc compounds [Zn(NHAr)(pbpamd)], where NHAr = 4-methylphenylamide (NH-4-MeC6H4) 3, 2,4,6-trimethylphenylamide (NH-2,4,6-Me3C6H2) 4 and 2,6-diethylphenylamide (NH-2,6-Et2C6H3) 5, by the reaction of the corresponding aromatic primary amide lithium salts. Alternatively, aliphatic amide derivatives [Zn(NR2)(pbpamd)] (R = SiMe3 6, SiHMe2 7 and iPr 8) were cleanly prepared by reacting the amidine-heteroscorpionate compound Hpbpamd with the corresponding bis(amide) zinc complexes [Zn(NR2)2] (R = SiMe3, SiHMe2 and iPr). The single-crystal X-ray structures of complexes 2, 3 and 6 confirm a 4-coordinate arrangement in all cases, with the zinc metal surrounded in a distorted tetrahedral geometry and the heteroscorpionate ligands arranged in a kappa3 coordination mode. Whereas aliphatic amide heteroscorpionates 6-8 can act as efficient single-component initiators for the ring-opening polymerization of epsilon-caprolactone at room temperature, aromatic amide derivatives were not capable of yielding polymers even at high temperature. Epsilon-caprolactone is polymerized within minutes to give medium-high molecular weight polymers under mild conditions and with narrow polydispersities (M(w)/M(n) = 1.26). Polymer end group analysis shows that the polymerization mediated by aliphatic amide zinc complexes is initiated by amide transfer to the monomer.