FeCl₂·4H₂O‑MediatedConversion of the CF₃ Group into a Series of Esters: AStrategy for the Synthesis of Fe^II Complexes with In Situ-FormedEster-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-<i>a</i>]pyridine, PIP–CF₃ = 3-(pyridin-2-yl)-1-(trifluoromethyl)imidazo[1,5-<i>a</i>]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] (<b>1</b>), [Fe₂(PIP–COOEt)₂Cl₄] (<b>2</b>), and [Fe(PIP–COOEt)Cl₂] (<b>2′</b>) (PIP–COOEt = ethyl 3-(pyridin-2-yl)imidazo[1,5-<i>a</i>]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–COO^<i>n</i>Pr)₂Cl₂] (<b>3</b>), [Fe₂(PIP–COO^<i>n</i>Pr)₂Cl₄] (<b>4</b>), [Fe₂(PIP–COO^<i>i</i>Pr)₂Cl₄] (<b>5</b>), and [Fe(PIP–CF₃)₂Cl₂]·^<i>i</i>PrOH (<b>6</b>·^<i>i</i>PrOH) 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.