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

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.