Thorium 2-pyridylamidinates: synthesis, structure and catalytic activity towards the cyclo-oligomerization of ε-caprolactone

Abstract

Salt metathesis of ThCl₄·3THF with N,N′-bis(trimethylsilyl)-2-pyridyl-lithium-TMEDA (1) yields bis(N,N′-bis(trimethylsilyl)-2-pyridylamidinate) thorium-chloride (μ-Cl)₂Li(TMEDA) (2) (60% isolated yield) and tris(N,N′-bis(trimethylsilyl)-2-pyridylamidinate) thorium monochloride (3) (10% isolated yield). The latter compound is the first crystallographically characterized tris(amidinate) thorium complex. The bis pyridyl amidinate thorium (2) displays a unique reactivity towards the dual-site cyclo-oligomerization of ε-caprolactone, which produces two fractions of macrocyclic oligo-esters with extremely narrow (1.01–1.06) polydispersity. A mechanism for the dual-site oligomerization reaction is proposed based on kinetic, poisoning, and ¹H NMR studies.