1. Department of Chemistry, University of Khartoum, P.O. Box 321, Khartoum 11115,
Sudan
2. INM−Leibniz Institute for New Materials, Campus D2 2, 66123 Saarbruecken,
Germany
3. Institute of Inorganic
Chemistry, Saarland University, 66041,
Saarbruecken, Germany
4. University of Applied Science, Amerikastraße 1, 66482 Zweibruecken, Germany