1. Contribution from the Schuit Institute of Catalysis and Department of Polymer Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Crystal Structure Center, Chemical Physics, Material Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, and Chemistry Department, University of Ottawa, K1N 6N5 Ottawa, Ontario, Canada