1. Kekulé Institute for Organic Chemistry and Biochemistry, Bonn University, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany, X-ray Analysis Department, Institute for Inorganic Chemistry, Bonn University, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany, and Institute for Organic Chemistry, Johannes Gutenberg-University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany