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(eta(6)-o-RC6H4-NHN=CH-C6H4-p-R')]+PF6- (Cp = eta(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(eta(6)-o-RC6H4-NHNH2)]+PF6- (R = H, 1; Me, 2; MeO, 3; Cl, 4) with p-substituted benzaldehydes p-R'C6H4CHO (R' = H, Me, MeO, NMe2) in refluxing ethanol. The N-methyl derivatives formulated as [CpFe{eta(6)-C6H5-N(Me)N=CH-C6H4-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 iodomethane in THF at -70 degreesC. All these complexes have been fully characterized by elemental analysis, IR, LTV-vis, and H-1 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 C-6 ring with a folding angle of 6.2degrees and 7.4degrees, respectively. Moreover, the two phenyl rings of each compound are almost coplanar with a dihedral angle of 4.9degrees for 6 and 6.2degrees 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.