Tetrakis(ferrocenecarbonitrile) Copper(I) Complexes

The homoleptic tetrakis(ferrocenecarbonitrile) copper(I) complexes [Cu(NCFc)4]A [A = BF4 (3a), A = SO3CF3 (3b)] were succefully prepared by treatment of copper with the copper(II) salt [CuA2]center dot xH2O [A = BF4, x = 5 (1a); A = SO3CF3, x = 0 (1b)] in presence of cyanoferrocene (2). For 3a the tetrahedral arrangement of the four ferrocenecarbonitrile ligands at the copper atom was proven by single X-ray diffraction studies. The FcCN units are weakly bonded to copper, which is reflected by a complete ligand exchange upon addition of MeCN forming [Cu(NCMe)4]BF4 (4). Electrochemical measurements of 3a revealed that all four ferrocenyl units are oxidized in a close potential range and the individual processes are not resolved in the cylic voltammogram. Due to the decrease of electron density at the ferrocenyl termini, the redox event is merely shifted to 456 mV (for comparison, free 2 has E degrees = 436 mV). For 3a intervalence charge transfer (IVCT) absorption between FeII/FeIII and/or CuI/FeIII could not be observed in the in situ UV/Vis/NIR spectra in any oxidation state. This alluded that no electronic interaction between the Fc/Fc+ termini over the -CN-CuI-NC- connectivities takes place. Nevertheless, upon oxidation a ligand-to-metal charge transfer (LMCT) band of the ferrocenium groups at 643 nm is visuable accompanied with a shift in the d-d transition absorptions of the central iron atoms.