Synthesis, structure and ROP capability of a tetra(amidine)bis(zirconium) calix[8]arene complex

Abstract

The solvate {Zr[NHC(Me)NMe2]2}2L•7.25MeCN (1•7.25MeCN) has been obtained on interaction of [Zr(NMe2)4] with p-tert-butylcalix [8]areneH8 (LH8) and subsequent work-up in acetonitrile. The X-ray crystal structure of the solvate reveals a tetra(amidine) complex with two distorted octahedral zirconium centres, each of which is bound by two cis amidine ligands, which result from acetonitrile insertion into the amide bonds; the coordination sphere at each metal is completed by the fully deprotonated calix[8]arene. Preliminary screening of tetra(amidine) complex 1, LH8 and [Zr(NMe2)4], in the presence of benzyl alcohol (BnOH) for 1 and [Zr(NMe2)4], for the ring opening polymerization (ROP) of ε-caprolactone and ẟ-valerolactone reveals the formation of low molecular weight polycaprolactone (PCL) (Mn ≤ 2280 Da) and high molecular weight polyvalerolactone (PVL) (Mn ≤ 23430 Da), respectively. In both cases, kinetics revealed superior activity for 1 versus [Zr(NMe2)4] and LH8. Both cyclic and linear (H/OBn) end groups polymers were formed.