1. Department of Chemistry, Hanoi University of Sciences, 19 Le Thanh Tong, Hanoi, Vietnam
2. Present address: Institute of Chemistry and Biochemistry, Freie Universität Berlin, Fabeckstr. 34/36, 14195 Berlin, Germany
3. Instituto de Química de São Carlos, Universidade de São Paulo, 13566‐590 São Carlos – SP, Brazil
4. Institute of Chemistry and Biochemistry, Freie Universität Berlin, Fabeckstr. 34/36, 14195 Berlin, Germany, Fax: +49‐30‐838‐52676