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.