Anilido-Oxazoline-Ligated Iron Alkoxide Complexes for Living Ring-Opening Polymerization of Cyclic Esters with Controllability

Anilido-oxazoline-ligated iron complexes, including bis(anilido-oxazolinate) iron(II), mononuclear iron(II) alkyl and aryloxide, as well as the dinuclear analogues, were synthesized, and their catalytic performance on ring-opening polymerization (ROP) has been studied. Transmetalation of FeCl₂(THF)_1.5 with in situ-generated anilido-oxazolinate lithium afforded the bis(anilido-oxazolinate) iron complexes <b>1</b> and <b>2</b>. Half-sandwich anilido-oxazolinate iron trimethylsilylalkyl complexes <b>3</b> and <b>4</b> could be synthesized in good yields via taking pyridine as an L-type ligand. Treatment of <b>3</b> with benzyl alcohol and 4-phenoxyphenol, respectively, generated the dimeric alkoxide or aryloxide complexes <b>5</b> and <b>6</b>, whereas the reaction with 2,4,6-trimethylphenol and 2,6-di-<i>tert</i>-butyl-4-methylphenol yielded the mononuclear aryloxide complexes <b>7</b> and <b>8</b>, respectively. The iron alkoxide and aryloxide complexes were active single component catalysts for the ROP of ε-caprolactone (CL). Remarkably, the dinuclear complex <b>5</b> exhibited excellent controllability, livingness, and high initiation efficiency for ROP of CL. ROP of CL derivatives by <b>5</b> produced the corresponding polycyclic esters with good controllability, and the well-defined block copolymers could be generated by sequentially feeding different monomers. The chain initiation and propagation processes were investigated by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and kinetics analysis. In addition, a computational study was conducted to rationalize the mechanism and synergistic effect of the alkoxide-bridged bimetallic iron centers.