This work was financially supported by the Ministerio de Ciencia e Innovación of Spain (MICINN, grant nos. CTQ2007-61048/BQU and CTQ2010-16790). M.G.S. and R.D.C. thank the MEC of Spain for FPU predoctoral fellowships. We thank also Prof. P. J. Campos and Dr. R. Alonso (University of La Rioja) for help with X-ray analysis and Prof. J. M. Costa and A. M. Coto (University of Oviedo) for assistance with fluorescence studies.