N-(2-(Diphenylphosphino)ethyl)-2-alkyl-5,6,7,8-tetrahydro-quinolin-8-amines iron(ii) complexes: structural diversity and the ring opening polymerization of ε-caprolactone

Abstract

A series of N-(2-(diphenylphosphino)ethyl)-2-alkyl-5,6,7,8-tetrahydroquinolin-8-amines was prepared and used in individually reacting with iron chloride under nitrogen atmosphere to form their iron(II) complexes Fe1–Fe6. All compounds were characterized using FT-IR spectroscopy and elemental analyses, the organic compounds were confirmed with NMR measurements, and the iron complexes were submitted to single-crystal X-ray diffraction, revealing Fe1, Fe2, Fe4, Fe5, and Fe6 as either mono- or di-nuclear forms. Forming a binary system in situ with two equivalents of LiCH₂SiMe₃, all iron complexes Fe1–Fe6 efficiently initiated the ring opening polymerization of ε-caprolactone, achieving the TOF up to 8.8 × 10³ h⁻¹. More importantly, the resultant polycaprolactone (PCL) possessed high molecular weights with the M_n range of 9.21–24.3 × 10⁴ g mol⁻¹, being a rare case of the iron(II) catalyst in producing PCL with such high molecular weight. The ¹H NMR and MALDI-TOF investigations demonstrated that the PCLs were linear features capped with a methoxy group or CH₂SiMe₃ or cyclic structure that varied with the molar ratio of [ε-CL]/Fe.