Functionalized ferrocenes: Unique properties based on electronic communication between amino group of the ligand and Fe center

Abstract

Recent results of our studies on preparation and properties of the ferrocene derivatives with amine-containing ligands are reviewed. We established a route to 2-aza[3]ferrocenophanes with an N-aryl or N-alkyl substituent by using RuCl 2(PPh 3) 3 as catalyst for condensation of 1,1′-bis(hydroxymethyl)ferrocene with primary amines. The obtained ferrocenophanes show reversible redox of the Fe center. At higher potentials, quasi-reversible or irreversible electrochemical oxidation of amino groups of the ligand is observed for most of the 2-aza[3]ferrocenophanes. The initially formed one-electron-oxidized species undergo rapid and reversible intramolecular electron transfer between the Fe center and the nitrogen atom of the ligand. The 2-aza[3]ferrocenophanes with N-aryl group bonded to trans-aminoazobenzene group undergo photo-induced isomerization to its cis isomer, giving an equilibrated mixture under photostationary state. Electrochemical oxidation of the Fe(II) center shifts the equilibrium to formation of the trans-azobenzene even under photoirradiation. Thus, thermodynamics of the isomerization of azobenzene group is regulated by changing the oxidation state of the molecule. Protonation of ferrocene derivatives with aminomethyl group on the cyclopentadienyl ligand forms the dialkylammonium group which forms a complex with dibenzo[24]crown-8 (DB24C8) with interlocked structures. The pseudorotaxanes obtained were characterized by X-ray crystallography and mass spectrometry. Electrochemical oxidation of the ferrocene with aminomethyl group at a cyclopentadienyl ligand in the presence of 1-hydroxy-2,2,6,6-tetramethylpiperidine (TEMPOH) leads to formation of the dialkylammonium group which produces the pseudorotaxane instantly with added DB24C8.