1. Contribution from the Laboratoire de Chimie Structurale Organique, CNRS UMR 8074, ICMMO, Bât. 410, Université de Paris-Sud, 91405 Orsay, France, the Université Paris VII-Denis Diderot, Département de Chimie, 2 Place Jussieu, 75251 Paris Cedex 05, France, and the University of Southampton, Department of Chemistry, Highfield Southampton SO17 1BJ, United Kingdom