Cyclometallated Iridium and Platinum Complexes with Noninnocent Ligands

The electronic properties of the cyclometalated (C∧N) complexes of iridium and platinum metals with a catechol
ligand have been studied experimentally and computationally. The synthesis and characterization of (p-tolylpyridine)Ir(3,5-di-tert-butylcatechol) (abbreviated Ir-sq) and (2,4-diflorophenylpyridine)Pt(3,5-di-tert-butylcatechol) (abbreviated
Pt-sq) are reported along with their structural, spectral, and electrochemical properties. Reaction of the 3,5-di-tert-butylcatechol (DTBCat) ligand with the prepared cyclometalated metal complex was carried out in air in the presence
of a base. The resulting complexes are air stable and are paramagnetic with the unpaired electron residing mainly
on the catechol ligand. The bond lengths obtained from X-ray structure analysis and the theoretical results suggest
the semiquinone form of the catechol ligand. Low-energy, intense (∼103 M-1 cm-1) transitions are observed in the
visible to near-infrared region (600−700 nm) of the absorption spectra of the metal complexes. Electrochemically,
the complexes exhibit a reversible reduction of the semiquinone form to the catechol form of the ligand and an
irreversible oxidation to the unstable quinone form of the ligand. The noninnocent catechol ligand plays a significant
role in the electronic properties of the metal complexes. Density functional theory (DFT) and time-dependent density
functional theory (TD-DFT) calculations on the two open-shell molecules provide the ground-state and excited-state energies of the molecular orbitals involved in the observed low-energy transitions. The spin density in the two
complexes resides mainly on the catechol ligand. The intense transition arises from excitation of the β electron
from a HOMO-n (n = 1 or 2 here) to the LUMO, rather than from the excitation of the unpaired α electron.