1. Contribution from the Laboratoire de Chimie Inorganique et Bioinorganique, Ecole Polytechnique Fédérale de Lausanne, EPFL-BCH, CH-1015 Lausanne, Switzerland, National High Magnetic Field Laboratory, Center for Interdisciplinary Magnetic Resonance, Florida State University, Tallahassee, Florida, and Research Department, Guerbet, 95943 Roissy Cdg Cedex, France