Moybdenum Tris-dithiolene Compounds Having Unusual Magnetic Character

By Laura R. Snyder

Molybdenum dithiolene complexes have been well studied because of their variable coordination geometries and because a Mo-dithiolene unit is present in the molybdenum co-factor, the catalytic site of the oxo-molybdenum enzymes. We have discovered an unexpected magnetic behavior in Mo(IV) complexes containing three dithiolene ligands. The tris-dithiolene compound [Et₄N]₂Mo(S₂PEQO)₃ has dithiolene chelates substituted by quinoxaline and phenyl groups and is produced through a multi-step synthesis, beginning with 2-quinoxalinol as a precursor for the ligand substituent. The synthesis is culminated by a reaction of [Et₄N]₂MoS₉ with phenylethynylquinoxaline (PEQO) to yield [Et₄N]₂Mo(S₂PEQO)₃ as a dark blue compound. In contrast to the typical diamagnetic behavior of Mo(IV)tris-dithiolenes, previous results have shown [Et₄N]₂Mo(S₂PEQO)₃ to have unusually high magnetic susceptibility corresponding to 5 unpaired electrons. We are probing the structural parameters that favor this “hyper-paramagnetic” behavior by synthesizing other derivatives. For example, a similar compound having 2,4-difluorophenyl and quinoxalyl dithiolene substituents also possess paramagnetic also possess paramagnetic properties. Using UV/vis spectroscopy, color changes are monitored as the molybdenum centers are oxidized from Mo(IV) to Mo(VI).