Auto-ionization in lutetium iodide complexes: effect of the Iioic radius on lanthanide-iodide binding

Reaction of lutetium metal with 1.5 equiv of elemental iodine in 2-propanol leads to the isolation of LuI(3)(HO(i)Pr)(4) (1). An X-ray crystal structure reveals an ionic structure with well-separated LuI(2)(HO(i)Pr)(4)] cations and I] anions. Dissolution of 1 in pyridine generates the unusual alkoxide species LuI(O(i)Pr)(py)(5)]I] (2) with the elimination of HI. An X-ray crystal structure of 2 confirmed the ionic nature of the compound, with the cationic portion of the complex exhibiting a seven-coordinated lutetium center with trans-disposed iodo and alkoxide ligands and five pyridine molecules equally displaced within the equatorial plane. Exposure of 2 to iodotrimethylsilane yields the expected triiodide species LuI(2)(py)(5)]I] (3), which may also be prepared by refluxing commercially available LuI(3) in THF, followed by crystallization from a THF/pyridine mixture. The solid-state structure of 3 is similar to that of 2, with the alkoxide ligand having been replaced by an iodide. The formation of ionic structures 1-3 as opposed to the higher-coordinated neutral species may be traced to the small lutetium center and the presence of relatively strong Lewis bases within the coordination sphere of the metal.