Synthesis and properties of new dinuclear organoiron(II) hydrazones combining the potent electron-donating [-(η5-C₅ H₄)FeCp] fragment with [CpFe(η6-arene)-]+-type acceptors

The stereoselective condensation reaction between organometallic hydrazine precursors [CpFe(η⁶-p-RC₆ H₄-NHNH₂)]⁺PF₆^- (R = H, (1)⁺PF₆^-; Me, (2)⁺ PF₆^-; MeO, (3)⁺PF₆ ^-; Cl, (4)⁺PF₆^-) and (E)-(2-ferrocenylvinyl)-methyl ketone, [(E)-[CpFe(η ⁵-C₅H₄)-CH=CH-CO-Me] (5), allowed the synthesis of a family of four new push-pull-type homodinuclear hydrazones complexes formulated as (E,E)-[CpFe(η⁶-p-RC₆ H₄)-NHN=CMe-CH=CH-(η⁵-C₅ H₄)FeCp]⁺PF₆^- (R=H, (6) ⁺PF₆^-; Me, (7)⁺PF₆ ^-; MeO, (8)⁺PF₆^-; Cl, (9) ⁺PF₆^-). These compounds were fully characterized by a combination of elemental analysis and spectroscopic techniques (¹H-NMR, IR and UV-Vis), and electrochemical studies in order to prove the π-donor-acceptor interactions between the two metallocenic termini through the asymmetric elongated π-conjugated spacer. Cyclic voltammetry studies reveal an electrochemically reversible one-electron oxidation and an irreversible one-electron reduction step for all the hydrazone derivatives. The redox potentials clearly indicate that the oxidation occurs at the ferrocenyl unit, and the reduction is localized on the mixed sandwich entity. In the electronic absorption spectra, two intense bands are observed in the visible region which undergo positive solvatochromic shifts. The origin of the high-energy absorption band is assumed to be an intraligand charge-transfer (ILCT) transition and the low-energy absorption band is assigned to a metal to ligand charge-transfer (MLCT) transition. Both ILCT and MLCT are shifted to lower energy with increasing spacer length.