Synthesis and structure of a ferric complex of 2,6-di(1H-pyrazol-3-yl)pyridine and its excellent performance in the redox-controlled living ring-opening polymerization of ε-caprolactone

Abstract

The reaction of FeCl₃ with a pincer ligand, 2,6-di(1H-pyrazol-3-yl)pyridine (bppyH₂), produced a mononuclear Fe(III) complex [Fe(bppyH₂)Cl₃] (1), which could be reduced to the corresponding Fe(II) dichloride complex [Fe(bppyH₂)Cl₂] (2) by suitable reducing agents such as Cp₂Co or Fe powder. 1 and 2 exhibited a reversible transformation from each other with appropriate redox reagents. 1 could be utilized as a pre-catalyst to initiate the ring-opening polymerization of ε-caprolactone in the presence of alcohol but 2 did not work. The 1/alcohol system displayed characteristics of a well-controlled polymerization with the resulting poly(ε-caprolactone) having low molecular weight distributions, a linear tendency of molecular weight evolution with conversion, and polymer growth observed for the sequential additions of ε-caprolactone monomer to the polymerization reaction. The polymerization was completely turned off by the in situ reduction of the catalytic Fe center via Cp₂Co and then turned back upon the addition of [Cp₂Fe]PF₆. The rate of polymerization was modified by switching in situ between the Fe(III) and Fe(II) species.