Sterically variable dizinc complexes bearing bis(iminopyridyl)phenolate ligands: synthesis, structures and reactivity studies

Abstract

A series of chiral dizinc complexes of the type [(2,6-{ArNC(Me)C₅H₃N}₂C₆H₃O)Zn₂(µ-Cl)Cl₂] [Ar = 2,6-i-Pr₂C₆H₃ (1a), 2,6-Me₂C₆H₃ (1b), 2,4,6-Me₃-C₆H₂ (1c), 2,4-Me₂C₆H₃ (1d)] can be conveniently prepared in good yield by the template reaction of 2,6-{OC(Me)C₅H₃N}₂C₆H₃OH with an excess of the corresponding aniline and two equivalents of zinc dichloride in n-BuOH at elevated temperature. Alternatively, the pro-ligands, 2,6-{(ArNC(Me)C₅H₃N}₂C₆H₃OH [Ar = 2,6-i-Pr₂C₆H₃ (L1-H), 2,6-Me₂C₆H₃ (L2-H), 2,4,6-Me₃C₆H₂ (L3-H), 2,4-Me₂C₆H₃ (L4-H)], can be isolated and then treated with two equivalents of zinc dichloride to afford 1a–1d. Interaction of 1a with two equivalents of NaOAc in the presence of TlBF₄ gives the diacetate-bridged salt [(L1)Zn₂(µ-OAc)₂](BF₄) (2) while with Nadbm (dbm = dibenzoylmethanato) the bis(dbm)-chelated salt [(L1)Zn₂(dbm)₂](BF₄) (3) is obtained. Hydrolysis occurs on reaction of 1a with TlOEt to furnish [(L1)Zn₂(µ-OH)Cl₂] (4) as the only isolable product. Conversely, reaction of 1a with Tlhp (hp = 2-pyridonate) affords the neutral bis(pyridonate)-bridged trimetallic complex [(L1)Zn₃(µ-hp)₂Cl₃] (5) as the major product along with 4 as the minor one. Complex 4 and mixtures of 4/5 act as modest activators for the ring-opening polymerisation of ε-caprolactone. Single crystal X-ray diffraction studies have been performed on 1b, 1c, 2, 3, 4 and 5 reveal Zn⋯Zn separations in the range: 3.069(4)–4.649(6) Å.