Effective recognition of H 2 PO 4 2 by a new series of dendrimers functionalized with ferrocenyl-urea termini Beatriz Alonso, a Carmen M. Casado, a Isabel Cuadrado,* a Moisés Morán a and Angel E. Kaifer* b a Departamento de Química Inorgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain. E-mail: isabel.cuadrado@uam.es; Fax: 34 91-397-4833; Tel: 34 91-397-4834 b Center for Supramolecular Science and Department of Chemistry, University of Miami, Coral Gables, FL 33124-0431, USA. E-mail: akaifer@miami.edu; Fax: 305 444-1777; Tel: 305 284-3468 Received (in Cambridge, UK) 11th June 2002, Accepted 3rd July 2002 First published as an Advance Article on the web 15th July 2002 A new series of dendrimers with poly(propylene imine) backbones and 4, 8, 16, or 32 peripheral ferrocenyl-urea groups were prepared and characterized; their voltammetric behavior in DMSO solution was very sensitive to the presence of hydrogenphosphate anions at submillimolar concentration levels. Anion recognition and sensing has received considerable attention in the last twenty years. 1 In this context, dendrimers offer an interesting molecular framework for the design and construction of anion recognition sites. The extensive work of Astruc and coworkers has made use of ferrocenyl-amide- terminated dendrimers for the electrochemical recognition of anions. 2 This group has recently extended their voltammetric anion sensing work to gold nanoparticles capped with mono- layers containing terminal ferrocenyl-amide groups. 3 We have also investigated ferrocenyl-terminated dendrimers as anion receptors. 4 The groups of Vögtle 5 and Meijer 6 have recently reported the functionalization of dendrimers with surface urea residues for molecular recognition. Based on this body of work, we decided to combine in a dendrimer framework the redox activity of ferrocenyl groups with the anion binding ability of urea residues. In this work, we report the synthesis, character- ization and preliminary electrochemical data corresponding to the ferrocenyl-urea-terminated DiAminoButane-based poly- (propylene imine) dendrimers 1–4 (see Scheme 1). The voltammetric behavior of these new dendrimers in DMSO solution is very sensitive to the presence of low concentrations of hydrogenphosphate anion. Ferrocenecarboxylic acid was first converted to isocyanato- ferrocene (5), via its acyl azide derivative, using modified literature procedures. 7 Isocyanate 5 was found to be relatively unstable and was stored at temperatures below 5 °C. Reaction of the DAB dendrimers with excess 5 in CH 2 Cl 2 produced the desired dendrimers in moderate yields (30–50%). Dendrimers 1–4 were fully characterized by 1 H and 13 C NMR spectrosco- pies and FAB or MALDI-TOF mass spectrometry.† Den- drimers 2–4 gave rise to intense molecular peaks in their MALDI-TOF mass spectra and 1 did so in its FAB mass spectrum, thus providing unequivocal evidence for the comple- tion of the surface functionalization reaction. The electrochemical behavior of dendrimers 1–4 is domi- nated by the reversible one-electron oxidation of the ferrocenyl groups. In cyclic voltammetric (CV) experiments in DMSO solution—also containing 0.1 M TBAPF 6 as supporting electro- lyte—a single reversible oxidation wave was observed for each dendrimer. This finding reveals that there is no electronic communication between the multiple ferrocenyl centers in one dendrimer. The observed half-wave potentials (E 1/2 ) are all in the range 364 to 400 mV vs. Ag/AgCl, and they tend to increase with dendrimer generation, a reflection of the enhanced steric congestion on the surface of the larger dendrimers. Electrochemical anion sensing experiments were carried out using square wave voltammetry (SWV) because this technique has a lower detection limit than CV. Therefore, experiments at lower concentrations of dendrimers and anions are feasible with SWV. Fig. 1 shows the SWV response of dendrimer 1 (50 mM) in 0.1 M TBAPF 6 –DMSO. The observed peak potential corresponds to a E 1/2 value of 364 mV vs. Ag/AgCl, identical to Scheme 1 Fig. 1 Square-wave voltammograms of 1 (5 3 10 25 M) recorded in DMSO containing 0.1 M TBAPF 6 using a glassy-carbon working electrode, in the presence of various anions X 2 added as their TBA + salts. Scan rate: 100 mV s 21 . This journal is © The Royal Society of Chemistry 2002 1778 CHEM. COMMUN. , 2002, 1778–1779 DOI: 10.1039/b205619d