Self-assembly of a diferrous triple-stranded helicate with bis(2,2'-bipyridine) ligands: thermodynamic and kinetic intermediates

The protonation and iron(II) coordination properties of a bis(2,2'-bipyridine) ligand L were investigated in methanol. The protonated forms showed allosteric effects due to the flexibility of the strand. Speciation studies of the corresponding ferrous complexes were carried out as a function of pH and iron(II) concentrations. A combination of electrospray mass spectroscopy, potentiometry, and spectrophotometry allowed the determination in solution of three ferrous complexes, one mononuclear (L2Fe2+) and two dinuclear (L2Fe2(4+) and L3Fe2(4+)) species. Their structure was deduced from the metal spin state and confirmed by 1H NMR measurements and molecular modeling. The dissociation process of the triple-stranded diferrous helicate L3Fe2(4+) by OH- revealed two rate-limiting steps. The former leads to the formation of a monoferrous triple-stranded compound via a classical mechanism, which involves hydroxy-ferrous complexes. A similar process was observed in the latter step for the release of the ferrous cation from the mononuclear intermediate. Taking into account the structural, thermodynamic, and kinetic features provided by the present study, we could propose a self-assembling mechanism of the triple-stranded diferrous helicate.