Rhodium is renowned forits catalytic activity, and hydroquinones are renowned for reversible proton- and electron-transfer reactions in chemical and biological systems. Putting the two together in the form of a rhodium quinone complex is turning out to be a powerful new combination for chemistry professor Dwight A. Sweigart and his research group at Brown University.
Rhodium quinones have "marvelous properties," Sweigart says. They turn out to be fast and efficient catalysts. And the quinone ligand allows for formation of extended metal-organometallic frameworks with potential applications as functional materials.
"The secret is rhodium," Sweigart observes. "It's the superman of elements."
Last fall, Sweigart, Seung Uk Son (then a visiting professor from Sungkyunkwan University in Suwon, South Korea), and their colleagues reported the synthesis of a cationic rhodium cyclooctadiene complex with a 1,4-hydroquinone ligand (J. Am. Chem. Soc. 2005, 127, 12238). The ensuing reaction chemistry of the rhodium quinones has landed Sweigart's research on the covers of Dalton Transactions, Chemical Communications, and Angewandte Chemie International Edition in recent months.
An interesting property of rhodium quinones is that an anionic charge is necessary for the complex to be catalytically active, Sweigart points out. This feature requires the OH substituents of the hydroquinone to be deprotonated, a reversible process that can be carried out stepwise with a metal alkoxide such as potassium tert-butoxide, he explains.
Deprotonation of one OH group forms the neutral semiquinone (one OH and one keto group), and deprotonation of the second OH group forms the anionic quinone (two keto groups) supported by a metal counterion. These steps are accompanied by electron transfers to the rhodium atom and alter the hapticity, or the number of atoms in the quinone ligand that coordinate to rhodium, from six to five to four.
Rhodium and other metal hydroquinones aren't new, Sweigart notes, but they are uncommon. His group originally developed its metal-quinone chemistry some 10 years ago by coordinating hydroquinones to manganese tricarbonyl, Mn(CO)3. The researchers have shown that the manganese semiquinone and quinone complexes can form one-, two-, and three-dimensional frameworks that, like the rhodium analogs, have potential applications as nanomaterials (Dalton Trans. 2006, 2385).
In some examples, the frameworks are held together by hydrogen bonds, while in other examples, the quinone complexes serve as "organometalloligands" by binding to other metals through the oxygen atoms. The latter adds a new twist to metal-organic frameworks, which typically are linked by pyridine- or carboxylate-based spacer groups, Sweigart says.
Given the interesting manganese chemistry, the Brown researchers decided to explore other metals and zeroed in on rhodium because of its prominence as a catalyst. Besides Sweigart and Son, other initial members of the research team who worked on the rhodium quinone chemistry are former graduate student Jeffrey A. Reingold, now at Interpharm Inc., Hauppauge, N.Y.; graduate student Sang Bok Kim; and Brown emeritus chemistry professor Gene B. Carpenter.
The first reaction carried out by the researchers was the 1,2-addition of an arylboronic acid, RB(OH)2, to an aryl aldehyde, RCHO, to form a diaryl alcohol under mild reaction conditions. When optimized, the rhodium quinone catalyst provides better than 90% isolated yields for the reaction.
Sweigart and his colleagues subsequently have had success with the 1,4-addition of arylboronic acids to 2-cyclohexen-1-one and other unsaturated acceptor molecules. These experiments, which are about to be published, were carried out in conjunction with Brown chemistry professor William C. Trenkle and graduate students Julia L. Barkin and Marcus D. Faust Jr. The researchers found that in situ generation of the catalyst is possible, which makes the process even more efficient.
These addition reactions, known as Miyaura-Hayashi coupling, constitute an important method for linking two molecules together by new carbon-carbon bond formation. The Miyaura-Hayashi reaction uses a rhodium catalyst and is similar to the well-known Suzuki-Miyaura coupling, which uses a palladium catalyst to couple an aryl halide to an organoboron compound, Trenkle explains. Both types of coupling reactions are accelerated by a base, such as potassium hydroxide, which binds to the electrophilic boron atom to facilitate aryl transfer.
The anionic rhodium quinone appears to operate in an "intriguing multifunctional manner," Sweigart says. One quinone oxygen binds the boron atom, while the other quinone oxygen binds to the metal counterion of the catalyst and indirectly activates the aldehyde or cyclohexenone substrate. Organization of this supramolecular intermediate is unique, Sweigart believes, and "offers opportunities to improve product yields and stereoselectivities compared with standard catalytic systems." For example, the cyclooctadiene ligand could be replaced by a chiral ligand, he says.
One of the promising derivative developments of rhodium quinones is the ability of the complex to coordinate other metals. In one example, soon to be published, Sweigart, Son, and graduate students Kang Hyun Park and Kwonho Jang discovered that deprotonation of the rhodium hydroquinone complex with Al[OCH(CH3)2]3 in tetrahydrofuran solvent results in the rapid formation of 400-nm catalytic nanospheres.
Because the nanospheres are formed directly from a rhodium-aluminum polymeric framework, solid catalyst supports such as silica or a polymer, typically used for heterogeneous catalysis, aren't required, Sweigart points out. The materials appear to be "the first self-supported organometallic nanocatalysts," he says, and could open new possibilities for making more stable, selective, and recyclable catalysts.
The researchers have found the insoluble, amorphous nanospheres to have high activity for stereoselective polymerization of phenylacetylene to form cis-poly(phenylacetylene). Polyacetylenes are an important class of polymers used in optoelectronics, and the cis isomer is needed for optimal performance in some applications, Sweigart notes.
A related development is the formation of lithium alkoxide aggregates coupled by rhodium quinones, which could have potential as organolithium-catalyst reagents (Angew. Chem. Int. Ed. 2005, 44, 7710). Deprotonation of the rhodium hydroquinone complex by excess lithium tert-butoxide in tetrahydrofuran solvent, left standing over a period of weeks, slowly deposits crystals of a novel lithium cubane macromolecule, Sweigart notes. Two Li4O4 cubes are bridged by a pair of parallel rhodium complexes, with one quinone oxygen atom from each rhodium complex serving as one of the corners of each cubane unit.
Lithium cubanes with Li2O2 or Li4O4 units that share oxygen atoms are "fairly common" and are useful reagents in organic synthesis, Sweigart says. But the dimeric rhodium assembly is "an unprecedented example of lithium cubanes bridged by a transition-metal complex."
In a different vein, Sweigart and his team took advantage of the OH groups of the hydroquinone ligand on the cationic rhodium complex and the fluorine atoms of BF4- counterions to form a "charge-assisted" hydrogen-bonded framework (Chem. Commun. 2006, 708). They first traded the cyclooctadiene ligand of the rhodium complex for two triphenylphosphite ligands, P(OC6H5)3, to add some steric bulk to the system.
The charges on the rhodium complex and the counterion contribute to the hydrogen bonding, Sweigart says, and help form a supramolecular framework featuring channels lined entirely by hydrophobic phenyl groups. The material should be amenable to guest-host applications, such as serving as a model for hydrogen storage, he believes, which is a prospect the group is currently checking out.
"Sweigart's group has become a leader in the development of the coordination chemistry of quinone ligands with transition-metal ions," comments University of Colorado chemistry professor Cortlandt G. Pierpont, who has extensively studied transition-metal quinone chemistry. He cites the various facets of manganese and rhodium quinone chemistry as being significant contributions. In particular, the potential for chiral catalysis with the rhodium complex "should be an important future direction."
Sweigart and his colleagues are in the process of patenting the complexes and some applications through the university. While it's still early in the process, there's some initial industry interest in licensing or otherwise further developing the rhodium quinones, Sweigart relates.