1. Chemistry Department, Moscow State University B‐234 Vorob'evy Gory, 119899 Moscow, Russia
2. Department of Chemistry, University of Durham, Science Laboratories South Road, Durham, DH1 3LE, England
3. Fachbereich Chemie, Philipps‐Universität Marburg, Hans‐Meerwein‐Straße, D‐35032 Marburg/Lahn, Germany