Abstract

A series of new monocationic organo-iron(II) benzaldehyde-hydrazone complexes of general formula [CpFe(?6-o-RC6H4-NHN=CH-C6 H4-p-R?)]+PF6- (Cp = ?5-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(?6-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{?6-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-Cipso 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.