(η4-Tetraarylcyclobutadiene)(η5-formylcyclopentadienyl)cobalt(I) complexes: Facilities to finetune the electron-donating capability in dipolar organometallics

Five different (η4-tetraarylcyclobutadiene)(η5-formylcyclopentadienyl)cobalt(I) complexes (1a–1e) were synthesized in reasonable yields in a one-pot reaction of CoCl(PPh3)3, formylcyclopentadienyl sodium and the appropriate diarylethyne. The aryl groups of the ethyne were modified by various para-substituents X (X = Cl, H, Me, OMe, NMe2), which were intended to alter the redox potentials of the synthesized cobalt sandwich complexes. A cyclic voltammetry study revealed a linear dependence of the first oxidation potential to the Hammett parameter σp. X-ray structure analyses performed for two complexes (X = Me and NMe2) demonstrate only subtle changes in the solid state structure despite the large differences in electrochemical properties. A theoretical analysis by the density functional theory method has been performed on the geometries and electronic structures of the complex (η4-cyclobutadiene)(η5-cyclopentadienyl)Co(I), its cation and dication.