Organometallic iron(II) complexes containing p-substituted acetophenone-arylhydrazone ligands

A series of twelve new organometallic acetophenone-hydrazone complexes of general formula [(η⁵-Cp)Fe(η⁶-o-RC₆H₄)- NHN=CMe-C₆H₄-p-R']+PF₆^- (Cp=C₅H₅; R,R'=H,Me, [5]⁺ PF₆^-; H,MeO, [6]⁺ PF₆^-; H,NMe₂, [7]⁺ PF₆^-; Me, Me, [8]⁺ PF₆^-; Me,MeO, [9]⁺ PF₆^-; Me,NMe₂, [10]⁺ PF₆^-; MeO,Me, [11]⁺ PF₆^-; MeO,MeO, [12]⁺ PF₆^-; MeO,NMe₂, [13]⁺ PF₆^-; Cl,Me, [14]⁺ PF₆^-; Cl,MeO, [15]⁺PF₆^-; Cl,NMe₂, [16]⁺PF₆^-) has been prepared by reaction between their corresponding organometallic hydrazine precursors [(η⁵-Cp)Fe(η⁶-o-RC₆H₄)- NHNH₂]⁺PF₆^- (R=H, [1]⁺PF₆^-; Me, [2]⁺PF₆^-; MeO, [3]⁺PF₆^-; Cl, [4]⁺PF₆^-, and aromatic ketones of the type MeC(=O)C₆H₄-p-R' (R'-Me, MeO, NMe₂) in refluxing ethanol containing concentrated acetic acid. These mononuclear hydrazones were stereoselectively obtained as their trans-isomers about the N=C double bond, and were characterized by elemental analysis or high resolution mass spectrometry, I.R., UV-Vis, and ¹H NMR spectroscopies and, in the case of complex [11]⁺PF₆^- (R,R'=MeO,Me), by single-crystal X-ray diffraction analysis. The more salient features of this structure are: (i) the concomitant lengthening of the Fe-C_ipso and the shortening of the C_ipso-N bond distances leading to a slight iminium-cyclohexadienyl character of the coordinated C₆ ring, (ii) the intramolecular hydrogen bonding between the benzylic N-H group and the oxygen atom of the ortho-methoxo-substituent, with the N(1)⋯O(1) separation of ca. 2.627(4) Å, and (iii) the slight deviation of coplanarity between the coordinated and the free phenyl rings (dihedral angle=15.60(0.25)°). Finally, the cyclic voltammetry studies reveal an irreversible one-electron reduction step for compounds [5-7]⁺PF₆^-, and the redox potentials clearly indicate that the reduction occurs at the cationic electron acceptor mixed sandwich unit.