1. College of Chemical Engineering; Hebei United University; Tangshan 063009 P.R. China
2. Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry and Materials Science; Northwest University; Xi'an 710069 P. R. China
3. Department of Inorganic and Physical Chemistry; Ghent University; Ghent B-9000 Belgium