Synthesis, Characterization and Complexation Studies of a Cage Ligand Bearing a Tris(bipyridyl)iron(II) Capping Group

Abstract

Reaction between 5-(4-amino-2-thiabutyl)-5-methyl-3,7-dithianonane-1,9-diamine (N3S3) and 5-methyl-2,2-bipyridine-5-carbaldehyde and subsequent reduction of the resulting imine with sodium borohydride results in a potentially ditopic ligand (L). Treatment of L with one equivalent of an iron(II) salt led to the monoprotonated complex [Fe(HL)]³⁺, isolated as the hexafluorophosphate salt. The presence of characteristic bands for the tris(bipyridyl)iron(II) chromophore in the UV/vis spectrum indicated that the iron(II) atom is coordinated octahedrally by the three bipyridyl (bipy) groups. The [Fe(bipy)₃] moiety encloses a cavity composed of the N3S3 portion of the ditopic ligand. The mononuclear and monomeric nature of the complex [Fe(HL)]³⁺ has been established also by accurate mass analysis. [Fe(HL)]³⁺ displays reduced stability to base compared with the complex [Fe(bipy)₃]²⁺. In aqueous solution [Fe(HL)]³⁺ exhibits irreversible electrochemical behaviour with an oxidation wave ca. 60 mV to more positive potential than [Fe(bipy)₃]²⁺. Investigations of the interaction of [Fe(L)]²⁺ with copper(II), iron(II), and mercury(II) using mass spectroscopic and potentiometric methods suggested that where complexation occurred, fewer than six of the N3S3 cavity donors were involved. The high affinity of the complex [Fe(L)]²⁺ for protons is one reason suggested to contribute to the reluctance to coordinate a second metal ion.