Redox Noninnocence in First-Row Transition Metal Complexes of a π-Extended β-Diketiminate

The synthesis and characterization of a series of complexes featuring a π-extended benzo[<i>f,g</i>]tetracene β-diketiminate (BT-BDI) ligand is presented. The homoleptic iron(II), cobalt(II), nickel(II), and zinc(II) complexes are synthesized by reaction of the metal dichlorides or dibromides with the potassium salt of the ligand. The resulting complexes were characterized by nuclear magnetic resonance, and infrared and ultraviolet-visible spectroscopy. An X-ray crystal structure determination confirmed the distorted tetrahedral geometry and homoleptic nature of the Ni(^ClBT-BDI)₂ complex. The redox properties of the nickel and zinc complexes were studied in depth using cyclic voltammetry, differential pulse voltammetry, and X-band electron paramagnetic resonance spectroscopy. These studies revealed that the highly redox-active ligand allows for the formation of a monoradical [Zn(BT-BDI)₂]^<b>·</b>- and diradical [Zn(BT-BDI)₂]^2- complex. The diradical has a singlet ground state and triplet excited state, and the radicals were found to be delocalized through the benzo[<i>f,g</i>]tetracene backbone. For the Ni complex, it was found that reduction by one electron results in the reduction of the metal to its monovalent state, while reduction by a second electron results in the reduction of the ligand.