Organo-iron benzaldehyde-hydrazone complexes. Synthesis, characterization, electrochemical, and structural studies

A series of new monocationic organo-iron(II) benzaldehyde-hydrazone complexes of general formula [CpFe(η⁶-o-RC₆H₄-NHN=CH-C₆ H₄-p-R′)]⁺PF₆^- (Cp = η⁵-C₅H₅;R,R′ = H, H 5; H, Me 6; H, MeO 7; H, NMe₂ 8; Me, Me 9; Me, MeO 10; Me, NMe₂ 11; MeO, Me 12; MeO, MeO 13; MeO, NMe₂ 14; Cl, Me 15; Cl, MeO 16; Cl, NMe₂ 17) have been synthesized. These mononuclear hydrazones were stereoselectively obtained as their trans-isomers about the N=C double bond, by reaction of the corresponding organometallic hydrazine precursors [CpFe(η⁶-o-RC₆H₄- NHNH₂)]⁺PF₆^- (R= H, 1; Me, 2; MeO, 3; Cl, 4) with p-substituted benzaldehydes p-R′C₆H₄ CHO (R′ = H, Me, MeO, NMe₂) in refluxing ethanol. The N-methyl derivatives formulated as [CpFe{η⁶-C₆H₅-N(Me)N=Ch-C₆ H₄-p-R′}] ⁺PF₆^- (R′ = Me, 18; MeO, 19; NMe₂, 20) are readily prepared by deprotonation of complexes 6-8 with n-butyllithium and subsequent alkylation with idomethane in THF at -70°C. All these complexes have been fully characterized by elemental analysis, IR, UV-vis, and ¹H NMR spectroscopies, and mass spectrometry for compounds 10, 12, and 16. Complexes 6 and 19 have also been characterized by single-crystal X-ray diffraction analysis. One of the more salient features of these structures is the long Fe-C_ipso bond distances and the slight cyclohexadienyl character at the coordinated C₆ ring with a folding angle of 6.2° and 7.4°, respectively. Moreover, the two phenyl rings of each compound are almost coplanar with a dihedral angle of 4.9° for 6 and 6.2° for 19. Cyclic voltammetry of the benzaldehyde-hydrazone complexes 5-8, 11, 14, and 17 in DMF shows an irreversible reduction wave at ca. -1.92 V vs Ag/AgCl, depending on the nature of both R and p-R′ substituents, corresponding to the reduction of an in situ generated zwitterionic Fe(II) intermediate. The irreversible reduction waves of the cationic N-methylated hydrazones 18-20 are observed in the expected range -1.35 to -1.47 V vs Ag/AgCl, either in DMF or in acetonitrile.