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)-NHN=C (Me)-(η⁵-C₅H₄) Fe(η5-Cp)]⁺PF₆^-

Two homogeneous series of homodimetallic iron(II) hydrazone complexes of general formula [CpFe(η⁶-p- RC₆H₄)-NHN=CH-(η⁵-C₅H ₄)FeCp]⁺PF₆^- (Cp = η⁵-C₅H₅): R = H, [5]⁺PF₆^-; Me, [6]⁺PF₆^-; MeO, [7]⁺PF₆^-; Cl, [8]⁺PF₆^-; and [CpFe(η⁶-p-RC₆H₄)-NHN=CMe-(η ⁵-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 N=C double bond, by reaction of the corresponding organometallic hydrazines [CpFe(η6-p-RC₆H₄NHNH₂)]⁺PF ₆^-, 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-N=CMe- 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)]¹ 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.