IRIDIUM

RICHARD EISENBERG , UNIVERSITY OF ROCHESTER

The lab was dark--intentionally so--except for the eerie purplish glow of a handheld dark light and the bright red emis-sion of an unlabeled sample vial that belonged to a student no longer there. The year was 1980, and I had just heard a stimulating lecture from Allen J. Bard about electrochemiluminescence using Ru(bpy)₃²⁺ and oxalate. When asked what to look for in trying to extend this to other systems, he said, "First, find a good luminescent complex."

Hence, the search--a poor man's high-throughput screening done by opening up every sample drawer in the lab and walking through the darkened lab with a handheld UV light. The red light was impressive, but its origin was a mystery. However, within several days, the compound was identified as an iridium(I) carbonyl complex with triphenylphosphine and the anionic ligand maleonitriledithiolate (mnt). Derivatives with other phosphines, phosphites, and even isocyanides followed quickly, most showing a distinctive red luminescence that was tunable by changing the ligand.

	NIGHT-LIGHT A sample of (NBu4)[Ir(CO)(PPh3)(mnt)] under room light (left) and under a handheld ultraviolet light.

I have been fascinated by iridium since I first learned of the complex reported by Lauri Vaska that bears his name, IrCl(CO)(PPh₃)₂, nearly 40 years ago. This remarkable compound forms adducts with small molecules like O₂ and SO₂ and activates others like H₂, methyl iodide, and silanes by oxidative addition. The importance of this chemistry, which is described in every modern inorganic and organometallic chemistry textbook, relates to the controlled breaking of bonds for substrate activation in catalysis. The key to this process for Vaska's complex and its derivatives comes from the aesthetically pleasing electronic structure of square-planar complexes and the diversity of its frontier orbitals--a vacant p orbital and filled d orbitals for and synergic bonding with a substrate and a filled d_{z 2} orbital that can serve as an electron-pair donor to an addend.

Not all square-planar complexes can do oxidative addition chemistry, but iridium(I) complexes like IrCl(CO)(PPh₃)₂ are particularly adept at it. As in the study of any chemistry, the more one looks, the more there is to ask. For oxidative addition to square-planar Ir(I) systems, the reaction's stereoselectivity is a question that we have probed with H₂ as substrate, using, in part, sensitive nuclear magnetic resonance methods based on parahydrogen.

Iridium(I) complexes have also been in the forefront of research addressing one of chemistry's holy grails: the activation of stable, unactivated carbon-hydrogen bonds. The seminal studies of Robert G. Bergman and William A. G. Graham 20 years ago featured iridium complexes containing the pentamethylcyclopentadienyl ligand and were followed with investigations using other Ir(I) systems having electronically related tridentate ligands. Different lines of investigation on the same problem, first by Robert H. Crabtree and more recently by Craig M. Jensen, William C. Kaska, and Alan S. Goldman, led to success with complexes having different ligands and geometries, but all unified in containing iridium. Iridium complexes are generally not as good as rhodium analogs for homogeneous catalysis because they form more stable oxidative addition products, but the Cativa system for acetic acid synthesis developed by BP employs Ir(I) and Ir(III) carbonyl iodides.

Numerous variations of Vaska's complex have been made--different halides, different phosphines, substitution of CO, and change from trans phosphines to cis--and all exhibit to differing extent the extraordinary oxidative addition chemistry shown by IrCl(CO)(PPh₃)₂. The mysterious complex that luminesced red was first synthesized as an anionic derivative, but its beautiful photoemission moved us in another direction. Again, iridium did not disappoint. Coordination of mnt to Ir(I) introduced a charge-transfer excited state, and variation of the other ligands in the complex led to subtle tuning of the emission energy.

The luminescence properties of other iridium systems have garnered great attention. Ru(bpy)₃²⁺ is arguably the most extensively studied metal complex luminophore. Yet the "isoelectronic" Ir(III) system made by Richard J. Watts containing orthometallated phenylpyridine as the chelate has been found by Stephen R. Forrest and Mark E. Thompson to be a highly efficient emitter of green light in prototypes of flat-panel displays based on electroluminescence. This phenomenon serves as the basis of OLEDs (organic light-emitting diodes), but here the iridium plays a key role in giving emission from a triplet excited state that leads to greatly increased efficiency. Extensive work has shown that the emission color can be tuned by ligand variation or substitution, and studies suggest that these Ir(III) systems may have important applications in emerging display technologies.

From oxidative addition, bond activation, and catalysis to electronic structure and luminescence, the allure of iridium is powerful and seductive. Ensconced between osmium and platinum, the element's compounds possess properties and reactivity that continue to draw me to the joys of iridium.

Richard Eisenberg is the Tracy Harris Professor of Chemistry at the University of Rochester. He is editor-in-chief of Inorganic Chemistry and is the 2003 recipient of the ACS Award for Distinguished Service in the Advancement of Inorganic Chemistry.

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