Biological activity of Fe(iii) aquo-complexes towards ferric chelate reductase (FCR)Electronic supplementary information (ESI) available: Titration curves, UV-vis spectra, spectroscopic and MS data, cyclic voltammograms of all compounds studied, Cartesian coordinates (in ) and total energies (in au, non corrected zero-point vibrational energies included). See DOI: 10.1039/c2ob06754d

In this study we have obtained experimental evidence that confirms the high activity of aquo complexes III and IV towards the enzyme FCR, responsible for the reduction of Fe( iii ) to Fe( ii ) in the process of iron acquisition by plants. The in vivo FCR assays in roots of stressed cucumber plants have shown a higher efficiency of the family of complexes III and a striking structureactivity relationship with the nature of the substituent placed in a phenyl group far away from the metal center. The results obtained in this work demonstrate that all the aquo compounds tested interact efficiently with the enzyme FCR and hence constitute a new concept of iron chelates that could be of great use in agronomy. Fe( iii )- aquo complexes derived from phenol polyaminocarboxylic acids are highly efficient towards the enzyme ferric chelate reductase (FCR) and show interesting structureactivity properties in the enzymatic reduction process.