Cyclometallated Iridium and Platinum Complexes with Noninnocent Ligands
datasetposted on 14.05.2007, 00:00 by Bhavna Hirani, Jian Li, Peter I. Djurovich, Muhammed Yousufuddin, Jonas Oxgaard, Petter Persson, Scott R. Wilson, Robert Bau, William A. Goddard, Mark E. Thompson
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.