DOI: 10.1002/chem.200601757 ACHTUNGTRENNUNG[3.3]Ferrocenophanes with Guanidine Bridging Units as Multisignalling Receptor Molecules for Selective Recognition of Anions, Cations, and Amino Acids Francisco Otón, [a] Arturo Espinosa, [a] Alberto Tµrraga,* [a] Carmen Ramírez de Arellano, [b] and Pedro Molina* [a] Introduction The development of chemosensors capable of recognizing and sensing anions and cations is one of the most challeng- ing fields from the viewpoint of organic and supramolecular chemistry. These multisite ligands, which are able to bind a single heteroditopic guest, or simultaneously bind to a non- ACHTUNGTRENNUNGidentical guest, have been achieved by integrating cationic and anionic guest binding sites in a single molecule. [1] The guanidine function, due to its amphoteric nature, has a rich history in biological [2] and artificial receptors. [3] The guanidinium group within a variety of molecular architec- tures forms strong noncovalent interactions with anionic groups through hydrogen-bonding and charge-pairing inter- actions. In addition, deprotonated guanidines (guanidinates) have the potential to develop into valuable ancillary ligands in coordination and organometallic chemistry, [4] although the straightforward coordination of neutral guanidines to metal centers remains comparatively underdeveloped, [5] and metal-guanidinyl complexes are barely known and unex- plored. [6] Ferrocene-based ligands have been found to be useful for incorporating redox functions into supramolecular com- plexes that bind and allow the electrochemical sensing of cations, [7] anions, [8] and neutral molecules [9] by a change in the oxidation potential of the ferrocene. Despite the rich chemistry of guanidines, as the binding site, and ferrocene, Abstract: The synthesis, electrochemi- cal, and optical properties of a new [3.3]ferrocenophane framework in which two ferrocene subunits, with sim- ilar electronic environments, are linked through two substituted guanidine moi- eties, are reported. The receptors 4–7 have been prepared in good yields by the reaction of bis(carbodiimide) 3 with primary amines. This architecture is exceptionally “tunable” because a variety of “legs” may be appended to the basic [3.3]ferrocenophane scaffold to give a wide range of signaling units. These receptors show remarkable ion- sensing properties, due to the presence of a redox active unit (ferrocene), and an amphoteric binding site (guanidine). In this nitrogen-rich structural motif the guanidine bridges act as multipoint binding sites for anions, cations, and amino acids. Sensing of anions takes place both by unprecedented redox-ra- tiometric measurements (F  , Cl  , AcO  , NO 3  , HSO 4  ,H 2 PO 4  , and HP 2 O 7 3 ), and colorimetric change (F  , AcO  ,H 2 PO 4  , and HP 2 O 7 3 ). Sensing and discrimination of amino acids takes place by redox-ratiometric meas- urements, whereas the recognition of metal cations (Zn 2 + , Ni 2 + , and Cd 2 + ) is achieved either by electrochemical or fluorescence measurements. Moreover, the reported receptors display splitting of the oxidation wave of the Fe II /Fe III redox couple, and form the mixed-va- lence species 4 + C-7 + C by electrochemi- cal partial oxidation which, interesting- ly, show intervalence charge-transfer transitions associated to the appear- ance of absorption bands in the near infrared spectral region. Keywords: amino acids · ferrocene · guanidine · mixed-valence com- pounds · molecular recognition [a] F. Otón, Dr. A. Espinosa, Prof. A. Tµrraga, Prof. P. Molina Universidad de Murcia Departamento de Química Orgµnica, Facultad de Química Campus de Espinardo 30100 Murcia (Spain) Fax: (+ 34)968-364-149 E-mail: atarraga@um.es pmolina@um.es [b] Dr. C. R. de Arellano Universidad de Valencia Departamento de Química Orgµnica, Facultad de Farmacia Campus de Burjasot, 46100 Valencia (Spain) Supporting information for this article is available on the WWW under http://www.chemeurj.org/ or from the author. # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Chem. Eur. J. 2007, 13, 5742 – 5752 5742