IT'S ELEMENTAL!
gands 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, RICHARD EISENBERG, UNIVERSITY OF ROCHESTER but the Cativa system for acetic acid synthesis developed by BP employs Ir(I) and tance of this chemistry, which is described HE LAB WAS DARK— INTENTIONIr(III) carbonyl iodides. in every modern inorganic and organoally so—except for the eerie purNumerous variations of Vaska's commetallic chemistry textbook, relates to the plish glow of a handheld dark plex have been made—different halides, difcontrolled breaking of bonds for substrate light and the bright red emisferent phosphines, substitution of CO, and activation in catalysis. The key to this prosion of an unlabeled sample vial change from trans phosphines to cis—and that belonged to a student no longer there. all exhibit to differing extent the extraordiThe year was 1980, and I had just heard a nary oxidative addition chemistry shown stimulating lecture from Allen J. Bard by IrCl(CO)(PPri3)2. The mysterious comabout electrochemiluminescence using plex that luminesced red was first syntheRu(bpy)32+ and oxalate. W h e n asked what sized as an anionic derivative, but its beauto look for in trying to extend this to othtiful photoemission moved us in another er systems, he said, "First, find a good ludirection. Again, iridium did not disappoint. minescent complex." Coordination of mnt to Ir(I) introduced a NIGHT-LIGHTAsampleof Hence, the search—a poor man's highcharge-transfer excited state, and variation throughput screening done by opening up (NBuJ[lr(CO)(PPh3)|mnt)] under room of the other ligands in the complex led to every sample drawer in the lab and walklight (left) and under a handheld subde tuning of the emission energy ing through the darkened lab with a handultraviolet light. held UVlight. The red light was impressive, The luminescence properties of other but its origin was a mystery However, withiridium systems have garnered great attencess forVaska's complex and its derivatives tion. Ru(bpy)32+ is arguably the most exin several days, the compound was identi- comes from the aesthetically pleasing electensively studied metal complex lufied as an iridium(I) carbonyl complex with tronic structure of square-planar complextriphenylphosphine and the anionic ligand minophore. Ifet the "isoelectronic" Ir(III) es and the diversity ofits frontier orbitals— maleonitriledithiolate (mnt). Derivatives system made by Richard J. Watts containa vacant p orbital and filled d orbitals for with other phosphines, phosphites, and a and IT synergic bonding with a substrate ing orthometatlated phenylpyridine as the even isocyanides followed quickly, most and a filled dz2 orbital that can serve as an chelate has been found by Stephen R. Forshowing a distinctive red luminescence rest and Mark E. Thompson to be a highly electron-pair donor to an addend. that was tunable by changing the ligand. efficient emitter of green light in protoNot all square-planar complexes can do I have been fascinated by iridium since I types of flat-panel displays based on elecoxidative addition chemistry but iridium(I) first learned of the complex reported troluminescence. This phenomenon serves complexes like IrClCCC^CPPh^ are particas the basis of OLEDs (organic light-emitby Lauri Vaska that bears his name, ularly adept at it. As in the study of any ting diodes), but here the iridium IrCl(CO)(PPh3)2, nearly 40 years ago. This chemistry, the more one looks, plays a key role in giving emission remarkable compound forms adducts with the more there is to ask. For oxsmall molecules like 0 2 and S 0 2 and actifrom a triplet excited state that idative addition to square-planar vates others like H 2 , methyl iodide, and Ir(I) systems, the reaction's steleads to greatly increased effireoselectivity is a question that ciency Extensive work has shown silanes by oxidative addition. The imporwe have probed with H 2 as subthat the emission color can be strate, using, in part, sensitive nutuned by ligand variation or subIRIDIUM AT A GLANCE clear magnetic resonance methstitution, and studies suggest that THIS ELEMENT ods based on parahydrogen. these Ir(III) systems may have Name: From the Latin iris, rainbow. BROUGHT important applications in emergIridium salts are highly colored. Iridium© complexes have alTO YOU BY ing display technologies. Atomic Mass: 192.22. so been in the forefront of reSTREM History: Discovered with osmium by From oxidative addition, search addressing one of chemCHEMICALS English chemist Smithson Tennant in bond activation, and catalysis to istry's holy grails: the activation 1803 in the residue left when crude electronic structure and luminescence, the of stable, unactivated carbon-hydrogen platinum is dissolved by aqua regia. bonds. The seminal studies of Robert G. allure of iridium is powerful and seductive. Occurrence: Found in platinum ores Bergman and William A. G. Graham 2 0 Ensconced between osmium and platinum, and as a by-product of mining nickel. years ago featured iridium complexes conthe element's compounds possess properAppearance: Silvery white metal. taining the pentamethylcyclopentadienyl ties and reactivity that continue to draw Behavior: The pure metal is very brittle ligand and were followed with investigame to the joys of iridium. and difficult to machine, but it is also tions using other Ir(I) systems having electhe most corrosion-resistant metal tronically related tridentate ligands. DifRichard Eisenberg is the Tracy Harris Proknown. ferent lines of investigation on the same fessor of Chemistry at the University of Rochesproblem, first by Robert H. Crabtree and ter. He is editor-in-chief ofInorganic ChemUses: Primarily used as a hardening istry and is the 2003 recipient of the ACS more recently by Craig M.Jensen, William agent for platinum. It is also used in C. Kaska, and Alan S. Goldman, led to suc- Award for Distinguished Service in the Adhelicopter spark plugs. vancement ofInorganic Chemistry. cess with complexes having different li-
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