he Englishman William Hyde Wollaston discovered palladium in 1803 in South African crude platinum ore. Palladium derives from the Greek name Pallas, a name associated in Greek mythology with Athena, the goddess of wisdom. Apparently, Wollaston was stimulated to use this name by the asteroid Pallas, one of the brightest of asteroids, which was discovered at about the same time.
Palladium is a silver-white metal that does not corrode in air even at high temperatures. It is the least dense of the group 10 metals, becoming soft and ductile when annealed. Palladium has been called the "amazing soaking sponge" for its ability at room temperature to absorb up to 900 times its own volume of hydrogen, a property that can be exploited to purify hydrogen or activate it for chemical reaction.
Although automobile catalytic converters use less than one-fifth of an ounce of palladium each, the largest commercial use of this metal (60% of demand in 1999, according to the United Nations Conference on Trade & Development) is in the auto industry for reducing exhaust emissions of hydrocarbons, carbon monoxide, and oxides of nitrogen. Palladium's high resistance to corrosion leads to it being employed in the electronics sector and in the formulation of dental alloys, uses that constituted approximately one-third of world demand in 1999. In the former sector, palladium is used in the production of multilayer ceramic capacitors, components of mobile telephones, personal computers, and numerous auto and home electronics. Palladium has long been used in jewelry, either alone or mixed with gold to produce "white gold."
Palladium's remarkable catalytic properties are responsible for the widespread use of this metal in the chemical industry. Palladium is employed in the production of bulk chemicals such as purified terephthalic acid (a component of artificial fibers) and nitric acid for use in fertilizers. In the specialty chemicals area, palladium catalysis has had a remarkable impact, fundamentally changing during the past 30 years how exploratory drug candidates are synthesized in the pharmaceutical and biotechnology industries and to some extent how fine chemicals and pharmaceuticals are manufactured.
Finely divided palladium has long been employed in catalytic hydrogenations, where molecular hydrogen is added across carbon-carbon or carbon-hetero multiple bonds. The modern era of organopalladium chemistry can be traced to 1960, when the Wacker process for producing acetaldehyde by air oxidation of ethylene using a PdCl2/CuCl2 catalyst was introduced. Five years later, it was shown that allyl and alkene complexes of PdCl2 react with carbon nucleophiles to form new CC s-bonds. In subsequent years, the use of palladium catalysis to fashion CC bonds has become a staple in the synthesis of fine chemicals. Of particular importance are cross-coupling reactions in which two organic fragments are reliably joined by CC bond formation. I venture to guess that it would be a rare month (likely week) when a medicinal chemist involved in discovery research does not employ some palladium-catalyzed coupling reaction.
Many other reactions of organic compounds are catalyzed by palladium, such as carbonylations, hydrosilylations, and molecular rearrangements, the latter reaction first attracting my research group to this area in the late 1970s. The use of palladium catalysis in the synthesis of fine chemicals is certain to continue to grow, stimulated by ongoing developments such as carbon-heteroatom cross-coupling reactions and broadly useful asymmetric processes for fashioning carbon stereocenters of chiral molecules in a single configuration.
Larry Overman is a distinguished professor of chemistry at the University of California, Irvine. He is the 2003 recipient of the ACS Arthur C. Cope Award. He trained as a chemist at the beginning of the modern era of palladium chemistry, starting graduate school at the University of Wisconsin, Madison, in the same year, 1965, that palladium-promoted formation of carbon-carbon bonds was first described.
Chemical & Engineering News
Copyright © 2003 American Chemical Society
|PALLADIUM AT A GLANCE
|Name: Named for the asteroid Pallas.
|Atomic mass: 106.42.
|History: Discovered in 1803 by English chemist William Hyde Wollaston while analyzing samples of platinum ore.
|Occurrence: Rare. Palladium tends to appear with deposits of platinum, nickel, copper, silver, and gold.
|Appearance: Silvery white, soft metal.
|Behavior: Highly resistant to corrosion.
|Uses: Used as a catalyst in several industrial processes and in catalytic converters. Palladium is also used in electrical contacts, dental crowns, and jewelry.