Synthesis of five generations of redox-stable pentamethylamidoferrocenyl dendrimers and comparison of amidoferrocenyl- and pentamethylamidoferrocenyl dendrimers as electrochemical exoreceptors for the selective recognition of H2PO4-, HSO4-, and adenosine 5′-triphosphate (ATP) anions:: Stereoelectronic and hydrophobic roles of cyclopentadienyl permethylation

A family of five metallodendrimers with pentamethylamidoferrocenyl termini were synthesized from the DSM dendrimers G(n)-DAB-dend(NH2)(x) (x = 4, 8, 16, 32, 64) and characterized by standard techniques, including prominent molecular peaks (broad for x = 64) in their MALDI-TOF mass spectra. Oxidation of G(n)-DAB-dend-(NHCOFc*)(x) (Fc* = C5H4FeCp*, Cp* = eta(5)-C5Me5) with SbCl5 in CH2Cl2 yields the stable 17-electron pentamethylferrocenium analogue, which can be characterized by ESR and Mossbauer spectroscopy and reduced back to the initial Fell dendrimer, the cycle being carried out without decomposition. The cyclic voltammograms (CVs) of all dendrimers, recorded in CH2Cl2 or DMF, show a fully reversible ferrocenyl wave without adsorption. They are much cleaner than those of the parent ferrocenyl analogues previously synthesized and studied by Cuadrado et al. These properties allow much easier recognition and titration of H2PO4- and ATP(2-) by CV with the permethylated series than with the parent series. On the other hand, permethylation reduces the difference between the potentials recorded before and after titration. This is not crucial for H2PO4- and ATP(2-), but it is for HSO4-, because of the weak interaction in this case. Thus recognition and titration in CH2Cl2 proceeds best with the parent series, and a positive dendritic effect is revealed by the appearance of a new wave whose difference in potential relative to the initial wave increases with increasing generation number. In DMF, recognition and titration are only possible with the permethylated series and are subject to a dramatic dendritic effect. Indeed, the titration is followed by only a shift of the initial wave with G, and by the appearance of a new wave with G(2) and G(3). In conclusion, the permethylated dendrimers allow excellent recognition and titration of the oxoanions by CV due to the stereoelectronic stabilization of the 17-electron form and their hydrophobic effect. The magnitude of the recognition and positive dendritic effects is very sensitive to the dendrimer structure and to the nature of the solvent. The recognition is of the strong-interaction type (square scheme) between these dendrimers and ATP(2-) with a stoichiometry of 0.5 equiv ATP(2-) per ferrocenyl branch.