Finally Stable and Homoleptic: Isolation and Characterization of {Cu[Fe(CO)₅]₂}⁺ Stabilized with Perfluoroalkoxyaluminate Anion, [Al(OC(CF₃)₃)₄]⁻

Abstract

Developments in the chemistry of weakly coordinating anions enabled isolation of numerous unique metal complexes with unusual ligands. An important example is the family of metal‐Fe(CO)5 complexes. In the current paper we present synthesis and thorough characterization of the first truly homoleptic {Cu[Fe(CO)5]2}+ cation obtained as a salt of weakly coordinating [Al(ORF)4]− (RF=C(CF3)3) anion. TGA/DSC/MS study show that its decomposition becomes noticeable only above 110 °C, thus it can be stored as powder in air‐free conditions for months. The crystal structure of {Cu[Fe(CO)5]2}+ shows strong asymmetry of the cation and very short Cu‐CO bonds in comparison to analogous {M[Fe(CO)5]2}+ where M=Ag or Au. Characterization is complemented with analysis of vibrational spectra and extensive DFT calculations which give insight into the energetics of Cu+‐Fe(CO)5 systems. Our results show that {Cu[Fe(CO)5]2}+ is homoleptic only as salt of [Al(ORF)4]−. Furthermore, in the presence of additional, even weakly basic ligands, the Cu+‐Fe(CO)5 bond strength decreases what may contribute to the complex's instability in liquid SO2 or in the presence of [SbF6]−. These conclusions point at the key role of selection of proper anion and solvent in stabilization of these types of complexes.