Discrimination of Octahedral versus Trigonal Bipyramidal Coordination Geometries of Homogeneous Ti-IV, V-V, and Mo-VI Amino Triphenolate Complexes through Nitroxyl Radical Units

A new stable radical-containing ligand and the corresponding V-V, Ti-IV, and Mo-VI complexes have been synthesized and studied by electron paramagnetic resonance (EPR) spectroscopy. To achieve this goal, a novel strategy for the post-derivatization of a triphenolamine (TPA) ligand has been developed. This has allowed the effective embedding of three TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) radicals in the corresponding metal complexes. In these systems, dipolar interactions among the organic radicals are strongly dependent on the geometry assumed in solution: a vertical bar Delta m(s)vertical bar = 2 transition is observed when the geometry of the complexes is octahedral, whereas no signal is detected with trigonal bipyramidal geometries.