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 = SiMe₃6, SiHMe₂7 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 κ³ coordination mode. Whereas aliphatic amide heteroscorpionates 6–8 can act as efficient single-component initiators for the ring-opening polymerization of ε-caprolactone at room temperature, aromatic amide derivatives were not capable of yielding polymers even at high temperature. ε-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.