Organometallic-inorganic conjugated unsymmetrical Schiff-base hybrids. Synthesis, characterization, electrochemistry and X-ray crystal structures of functionalized trinuclear iron-nickel-ruthenium dipolar chromophores

The synthesis of neutral dinuclear iron-nickel unsymmetrical Schiff base complexes 3 and 4 was achieved via a template reaction involving equimolar amounts of alkyl or aryl "half-unit" precursors, respectively, Fc-C(O)CH=C(CH ₃)N(H)R (1: R = CH ₂CH ₂NH ₂; 2: R = o-C ₆H ₄NH ₂; Fc = CpFe(η ⁵-C ₅H ₄); Cp = η ⁵- C ₅H ₅), 5-bromosalicylaldehyde and nickel(II) acetate tetrahydrate in a refluxing CH ₂Cl ₂/MeOH (1:1) mixture. The ionic trinuclear unsymmetrical complex 5 was prepared by reacting its dinuclear precursor 3 with the arenophile source, [Cp*Ru(NCCH ₃) ₃]PF ₆ (Cp* = η ⁵-C ₅(CH ₃) ₅), in refluxing CH ₂Cl ₂ for 2 h, whereas the trinuclear species 6 was formed upon regioselective π-complexation of the 5-bromosalicylidene ring of 4 by [Cp*Ru] ⁺ at room temperature overnight. All the new compounds were adequately characterized by analytical and spectroscopic techniques and, in addition, the crystal and molecular structures of the "half-unit" 1, the binuclear complex 4 and its hemisolvate adduct 4 • 0.5CH ₃OH, the trinuclear Schiff base compound 5 • 2(CH ₃) ₂CO, and the mixed sandwich metalloligand 7 have been determined by X-ray crystallography. Both organometallic-inorganic hybrids 5 and 6 contain the neutral electron-releasing ferrocenyl group, and the cationic electron-withdrawing ruthenium mixed sandwich, linked through the unsymmetrical tetradentate Schiff base complex {Ni(ONNO)}. UV-vis, ¹H and ¹³C NMR as well as electrochemical data clearly indicate a mutual donor-acceptor electronic influence between the organometallic termini. Furthermore, X-ray crystal structure analysis of 5 • 2(CH ₃) ₂CO reveals the partial delocalization of bonding electron density throughout the dinucleating nickel Schiff base ligand.