Formazanate Ir(III)/Pt(II) complexes and their derivatives
Formazanates, the monoanionic form of formazans with a 1,2,4,5-tetraazapentadienyl (NNCNN) core, have become a well-known redox-active ligand class in recent years and the two additional nitrogen atoms in the backbone provide formazanates with more accessible redox properties and stronger absorption, due to the more stabilized LUMOs, compared with structural analogue β-diketiminates and thus, greatly expand the redox chemistry of formazanates. The massive attention on the coordination chemistry of formazanates is because of their ligand-based redox process, which may facilitate multielectron redox behaviors, bond activations and excited-state charge separation. A variety of formazanate complexes of many main group metal and first- and second-row transition metals have been established, highlighting the chelating behaviors of formazanate ligands as well as their unique magnetic and photoluminescent properties. Our group focuses on the coordination chemistry of formazanate with third row transition metals and their derivatives, exhibiting strong absorption in visible region as well as multiple accessible redox potenials. We found a new route for preparation of homoleptic azo-iminate Pt(II) complexes via hydrogenative cleavage of formazans, involving the proton-coupled electrons transfer to the formazanate, with two ligand radicals paired in the delocalized HOMO. Further research indicated that the irreversible first oxidation is accompanied with protonation. A new class of bis-cyclometalated Ir(III) formazanate complexes were also prepared with different configurations and in most cases the “open form” were major products. However, different coordination modes minimally impact the orbital energies, with similar absorption and redox behaviors for both isomers. In addition, a new route for the preparation of azo-triazolide Ir(III) complexes was found via cyclization of 3-cyanodiarylformazanate ligands when we tried to increase the steric profile of the substituents. These rearranged complexes share similar orbital structures with formazanate congeners but have a blue-shift in absorption due to the major transition being (HOMO−1)→LUMO. We further expanded the coordination chemistry of formazanates and a series of dinuclear formazanate Ir(III)/Pt(II) complexes were obtained with different coordination modes. Ir (III) and Pt(II) displayed different preferences on coordinating sites of 4-pyridylformazanates. Some dinuclear compounds showed distinctly red-shifted absorption and a smaller HOMO–LUMO gap compared to others.