Developing Synthetic Methods for Redox-active and Luminescent Cyclometalated Complexes with Third-row Transition Metals

Formazans are a class of conjugated organic N-chelating ligands with an established coordination chemistry with many transition and main group metals. The formazans and the complexes derived from them generally have notable visible absorption coupled with accessible reduction potentials, making them a good candidate for accessing new coordination complexes with intriguing electronic properties. Transition metal complexes of formazan ligands with numerous p-block metals and several first-row transition metals have been reported, but their potential as redox-active chelating ligands with heavier transition metal remain underexplored. In Chapters 2 and 3, a series of di/triarylformazanate cyclometalated complexes with platinum (II) and iridium (III) is described, where unique redox and photophysical properties are observed by combining third-row transition metals with formazanate ligands. X-ray crystallography reveals the molecular structure of several Pt/Ir formazanate complexes. An intense visible π → π* absorption is observed in both types of metal formazanate complexes; and solvent-dependent electronic absorption spectra reveal some charge-transfer character in the HOMO → LUMO transition in Pt complexes. The formazanate-based reduction potential shifts anodically in presence of electron withdrawing group and the opposite effect is observed for electron donors, establishing that the redox potentials of both the Pt/Ir formazanate complexes could be controlled by altering the substituents in the formazan backbone. The easy synthetic routes to prepare these metal complexes provides opportunities to expand the coordination chemistry of this ligand class to other third row metals, to discover new binding modes and structural motifs involving formazanate ligands and heavy transition metals; and to demonstrate the effect of metal d-orbital mixing on the formazanate-derived redox and optical properties. In Chapter 3, a suite of red-emitting iridium complexes featuring two different cyclometalated ligands and a variety of ancillary ligands are described, featuring the effect of strongly π-donating ancillary ligand on electrochemical and photophysical properties. Electron-donating substituents can augment the radiative rate constant (kr) and photoluminescent quantum yield, and/or shift the luminescence to the near-IR region. The work described here motivate further pursuit of the more ancillary ligand modification along with computational studies to control and optimize the structural properties to design better red phosphors.