FeCl2·4H2O-Mediated Conversion of the CF3 Group into a Series of Esters: A Strategy for the Synthesis of FeII Complexes with In Situ-Formed Ester-Containing Ligands

A practical strategy for the preparation of a series of iron(II) complexes has been developed. This methodology features in situ esterification of the CF₃ group on the backbone of the PIP-CF₃ ligand (HPIP = 3-(pyridin-2-yl)imidazo[1,5-a]pyridine, PIP-CF₃ = 3-(pyridin-2-yl)-1-(trifluoromethyl)imidazo[1,5-a]pyridine) by a wide range of alcohols. Treatment of FeCl₂·4H₂O with the PIP-CF₃ ligand in EtOH under solvothermal conditions leads to the formation of complexes [Fe(PIP-COOEt)₂Cl₂] (1), [Fe₂(PIP-COOEt)₂Cl₄] (2), and [Fe(PIP-COOEt)Cl₂] (2') (PIP-COOEt = ethyl 3-(pyridin-2-yl)imidazo[1,5-a]pyridine-1-carboxylate). EtOH serves as a solvent and is also involved in the esterification of the CF₃ group. The esterification protocol features a broad substrate scope. The CF₃ moiety of the PIP-CF₃ ligand could be esterified by a wide range of alcohol substrates. Compounds [Fe(PIP-COOⁿPr)₂Cl₂] (3), [Fe₂(PIP-COOⁿPr)₂Cl₄] (4), [Fe₂(PIP-COOⁱPr)₂Cl₄] (5), and [Fe(PIP-CF₃)₂Cl₂]·ⁱPrOH (6·ⁱPrOH) were isolated, and their structures were characterized. The mechanism for the esterification of the CF₃ group was proposed by examining the conditions for the esterification transformations.