[Hydrotris(3-isopropyl-5-methylpyrazolyl)borato]iodocobalt(II): Unusual Purification by "Inverse Recrystallization"

In the search for homogeneous oxidation catalysts, the authors are investigating sterically hindered tris(pyrazolyl) borate complexes. For example, hydrotris(3-tert-butyl-5-methylpyrazolyl)borate (Tp{prime}) stabilizes the unusual side-on bonded superoxo-cobalt complex Tp{prime}Co{sup II}(O{sub 2}). During study of this complex there arose the need to prepare an analogous molecule with isopropyl groups in place of the tert-butyl substituents of the pyrazolyl rings. As a starting material for this chemistry, the authors prepared and structurally characterized Tp{double_prime}Co{sup II}I (Tp{double_prime} = hydrotris(3-isopropyl-5-methylpyrazoly)borate). The separation of the latter from its regioisomers takes advantage of a rather unusual solubility effect, termed {open_quotes}inverse recrystallization{close_quotes}. The potential utility of this phenomenon in related systems prompted this note.