The Influence of the Amide Linkage in the Fe super(III)-Binding Properties of Catechol-Modified Rosamine Derivatives

• Published in: Chemistry : a European journal 2015-10, Vol.21 (44), p.15692-15704
• Main Authors: Queiros, Carla, Leite, Andreia, G.M. Couto, Maria, Cunha-Silva, Luis, Barone, Giampaolo, deCastro, Baltazar, Rangel, Maria, M.N. Silva, Andre, M.G. Silva, Ana
• Format: Article
• Language: English
• Subjects: Amides; Catechol; Chelating; Coordination compounds; Fluorescence; Ligands; Linkages; Mathematical analysis
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201502093

The two new fluorescent ligands RosCat1 and RosCat2 contain catechol receptors connected to rosamine platforms through an amide linkage and were synthesized by using microwave-assisted coupling reactions of carboxyl- or amine-substituted rosamines with the corresponding catechol units and subsequent deprotection. RosCat1 possesses a reverse amide, whereas RosCat2 has the usual oriented amide bond (HNCO vs. CONH, respectively). The ligands were characterized by means of NMR spectroscopy, mass-spectrometry, and DFT calculations and X-ray crystallography studies for RosCat1. The influence of the amide linkage on the photophysical properties of the fluorescent ligands was assessed in different solvents and showed a higher fluorescence quantum yield for RosCat1. The coordination chemistry of these ligands with a Fe super(III) center has been rationalized by mass-spectrometric analysis and semiempirical calculations. Octahedral Fe super(III) complexes were obtained by the chelation of three RosCat1 or RosCat2 ligands. Interestingly, the unconventional amide connectivity in RosCat1 imposes the formation of an eight-membered ring on the chelate complex through a "salicylate-type" mode of coordination. Showing a preference: The coordination chemistry of two catechol-modified rosamines containing differently oriented amide linkages with Fe super(III) centers is rationalized by using mass spectrometry and semiempirical calculations (see figure). A "salicylate-type" mode of coordination is shown to take place and the unconventional amide imposes the formation of an eight-membered ring on the chelate complex.