Preparation, characterization and electrochemical and X-ray structural studies of new conjugated 1,1′-ferrocenediyl-ended [CpFe-arylhydrazone] ⁺ salts

A series of new conjugated bimetallic ferrocenyl 1,1′-bis- substituted compounds of the type (E)-[CpFe(η⁶-p-RC ₆H₄)NHN=CH(η⁵-C₅H ₄)Fe(η⁵-C₅H₄)-CH=CHC ₆H₄-p-R′]⁺PF₆^- (Cp = η⁵-C₅H₅; R, R′ = H, NO ₂, 11; Me, NO₂, 12; MeO, NO₂, 13; Cl, NO ₂, 14; Me, CN, 15; Me, Me, 16), with end-capped (E)-ethenylaryl and [CpFe(arylhydrazone)]⁺ substituents, have been prepared by the condensation reaction of 1,1′-(p-R′-arylethenyl) ferrocenecarboxaldehyde (R′ = Me, 4; NO₂, 5; CN, 6) with the organometallic hydrazine precursors [CpFe(η⁶-p-RC ₆H₄NHNH₂)]⁺PF₆ ^- (R = H, 7; Me, 8; MeO, 9; Cl, 10). In the trimetallic series, {[CpFe(η⁶-p-RC₆H₄)NHN=CH(η ⁵-C₅H₄)]₂Fe}²⁺[PF ₆^-]₂ (R = H, 17; Me, 18; MeO, 19, Cl, 20), which results from the condensation of two equivalents of the same organometallic hydrazine precursor (7-10) with 1,1′- ferrocenedicarboxaldehyde, the ferrocenediyl core symmetrically links two cationic mixed-sandwich units. These ten hydrazones (11-20) were stereoselectively obtained as their trans isomers about the N=C double bond. All the new compounds were thoroughly characterized by a combination of elemental analysis, spectroscopic techniques (¹H NMR, IR and UV-Vis) and electrochemical studies in order to prove electronic interaction between the donating and accepting units through the π-conjugated system. A representative example of each series has also been characterized by single crystal X-ray diffraction analysis. The bimetallic complex 16⁺ adopts an anti conformation with the two iron atoms on opposite faces of the dinucleating hydrazonato ligand, whereas the trinuclear complex 19²⁺ adopts a syn conformation with an Fe-Fe-Fe angle of 180°. Other salient features of these structures are the long Fe-C_ipso bond distances and the slight cyclohexadienyl character at the coordinated C₆ ring, with a folding angle of 7.4° and 7.0° for 16⁺ and 19 ²⁺, respectively.