Homodimetallic iron(II) hydrazones: syntheses, spectroscopic, electrochemical, and theoretical investigations. X-Ray crystal structure of both syn- and anti- rotamers of [(η5-Cp)Fe(η6-C6H5)–NHNC(Me)–(η5-C5H4) Fe(η5-Cp)]+PF6−

Two homogeneous series of homodimetallic iron(II) hydrazone complexes of general formula [CpFe(η6-p-RC6H4)–NHNCH–(η5-C5H4)FeCp]+PF6− (Cp=η5-C5H5): R=H, [5]+PF6−; Me, [6]+PF6−; MeO, [7]+PF6−; Cl, [8]+PF6−; and [CpFe(η6-p-RC6H4)–NHNCMe–(η5-C5H4)FeCp]+PF6−: R=H, [9]+PF6−; Me, [10]+PF6−; MeO, [11]+PF6−, have been prepared. These hydrazones were stereoselectively obtained as their trans isomers about the NC double bond, by reaction of the corresponding organometallic hydrazines [CpFe(η6-p-RC6H4NHNH2)]+PF6−, R=H, [1]+PF6−; Me, [2]+PF6−; MeO, [3]+PF6−; and Cl, [4]+PF6−, with formylferrocene, CpFe(η5-C5H4–CHO), and acetylferrocene, CpFe(η5-C5H4–CO-Me), respectively. All the new compounds were characterized by elemental analysis and IR, UV-Vis and 1H NMR spectroscopies. For compound [9]+PF6−, the activation energy ΔG# for the hindered rotations of the mixed sandwich [CpFe(η6-C6H5–)]+ (41.8±0.9 and 41.2±3.7 kJ mol−1) and the ferrocenyl unit (40.0±3.2 kJ mol−1) about the –NH–NCMe– hydrazone backbone have been determined by variable temperature 1H NMR spectroscopic studies. The electrochemical behavior of the dinuclear hydrazones was explored by cyclic voltammetry, and features the role of the cationic [CpFe(η6-arene)]+ moiety as an electron acceptor. The crystalline and molecular structure of [9]+PF6− was determined by single crystal X-ray crystallographic analysis. The structure shows the presence of both syn and anti rotamers in the asymmetric unit. The analysis of the electronic structure of [9]+ through DFT calculations indicate a strong perturbation of the metal centers by the hydrazone bridge as well as some metal-metal interaction through the bridge.