MAIN GROUPING | May 10, 2004 Issue - Vol. 82 Issue 19 | Chemical & Engineering News
Volume 82 Issue 19 | pp. 39-42
Issue Date: May 10, 2004

MAIN GROUPING

Chemists gather to paint a 21st-century picture of the resurgent field of main-group chemistry
Department: Science & Technology

Main-group chemistry has cycled between periods of prosperity and recession over the years. One reason for this is that the synthesis and characterization of main-group compounds--those of elements in groups 1 and 2 and groups 13 to 18, carbon excepted--are often difficult and the outcomes are sometimes unpredictable. Another reason is that much of main-group chemistry has had limited potential for broad applications until recently.

Today, main-group chemistry is having a revival, spurred by new developments in medical diagnostics and therapies, materials science, catalysis, and environmental remediation. The chemistry behind these topics and others was discussed at the recent American Chemical Society national meeting in Anaheim, Calif., during a symposium sponsored by the Division of Inorganic Chemistry titled "Modern Aspects of Main-Group Chemistry." The symposium brought together an international, acclaimed group of chemists who presented 33 lectures to illustrate what main-group chemistry has to offer at the beginning of the 21st century.

"The past 20 years have seen a resurgence in main-group chemistry, from fundamental breakthroughs involving multiple bonding in silicon, phosphorus, and heavier elements to applications in nanotechnology," noted symposium co-organizer Richard A. Kemp, a chemistry professor at the University of New Mexico and a staff member at Sandia National Laboratories.

"Main-group compounds are now used in the synthesis of nano- and mesoporous materials via sol-gel methods and in the preparation of chemically modified single-walled nanotubes," Kemp added. "Ceramics and dense materials are almost all made using main-group oxides, carbides, nitrides, or borides." He touted inorganic semiconductors, coatings, catalysts, and inorganic polymers such as polyphosphazenes, noting that "they are all important aspects of modern main-group chemistry."

Informally dubbed "CowleyFest," the symposium was held in honor of the 70th birthday of prominent main-group chemist Alan H. Cowley, who holds the Robert A. Welch Chair in Chemistry at the University of Texas, Austin. "Rarely has there been an opportunity to see such an impressive main grouping of chemists in one symposium," commented co-organizer Michael Lattman, a chemistry professor at Southern Methodist University. "The primary reason we were able to assemble this gathering of scientists is because of our honoree."

In Europe, main-group chemistry has enjoyed a long and rich history, particularly in France, Germany, and the U.K., Cowley told C&EN. In North America, main-group chemistry had its first heyday in the 1960s, triggered in part by Neil Bartlett, now an emeritus professor at the University of California, Berkeley, when he published a paper describing XePtF6, the first noble-gas compound. A number of groups successfully pursued fluorine, boron, and phosphorus chemistry at the time, Cowley added.

Main-group chemistry's current wave of prosperity stems in part from the discovery of multiple bonding between heavier main-group elements, which started in the mid-1970s, Cowley said. Double and triple bonds between most main-group elements have now been formed, he noted.

"Some of the many other exciting areas today include the use of aluminum(I) and gallium(I) halides to form a rich harvest of fascinating clusters," Cowley added. "Strong interest continues in the use of carboranes, particularly for making novel assemblies. Low-coordination-number phosphorus compounds are also beginning to be used to form novel polymers. And cyclopentadienyl indium compounds are useful sources of nanoparticles."

In his talk opening the symposium, Cowley focused on main-group analogs of carbenes, a general theme that has threaded through much of the chemistry conducted in his lab over the years. After reviewing some of his group's past research accomplishments, he turned to current work on the synthesis of the first triple-decker cation complexes containing tin, lead, or indium.

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Credit: COURTESY OF ALAN COWLEY
8219inandout
 
Credit: COURTESY OF ALAN COWLEY

THESE REACTIONS involve adding a positively charged metal fragment--a single metal atom (M) coordinated to a cyclopentadienyl (Cp) ring--to a neutral metallocene. The result is a Cp-M-Cp-M-Cp triple-decker cation. The cation is countered by a [Ga(C6F5)4] or [B(C6F5)4] anion. The research has been carried out by graduate students Jamie N. Jones and John D. Gorden (now at UC Berkeley) and postdocs Charles L. B. Macdonald (now at the University of Windsor, in Ontario), and Andreas Voigt (now at Blacklight Power).

"The idea was to explore donor-acceptor chemistry of main-group metallocenes, such as stannocene, with Lewis acids," Cowley explained. "That's not what happened, of course. Instead, what we got were these triple-decker cations." Triple-decker anions with main-group metals were previously known.

The structures of the triple-decker cations are interesting, Cowley added. The central Cp ring serves as a bridge between two M-Cp units. The terminal Cp ligands are canted from the linear center of the sandwich, in a cis geometry. The main-group triple-decker anions have trans geometry, he noted.

The UT Austin researchers also have synthesized the first inverse sandwich main-group cation, where two indium atoms are terminally bonded to a single Cp ring. Cowley's group is continuing to investigate the possibility of making sandwich compounds with additional layers and mixed-metal combinations.

"Polyhedral Boranes in the Nanoworld" was the title selected by chemistry professor M. Frederick Hawthorne of the University of California, Los Angeles, for his talk in Anaheim. The title reflects the latest research in Hawthorne's lab, where boranes (B12H122–) and carboranes (C2B10H12) are used to create "nanoesque" structures. These polyhedral cage compounds can serve as building blocks for self-assembled liposomes, microrods, or dendrimer-like arrays for drug delivery or medical diagnostics, he noted, while metallacarborane nanomachines can function as molecular valves or switches.

"During the past 40 years, the closed-cage boranes and carboranes have been shown to be aromatic and highly stable compounds with unique redox and magnetic properties," Hawthorne said. "The ability to readily functionalize these compounds makes them especially amenable to nanoscale applications."

The application that Hawthorne is currently most excited about is work by his group and coworkers at UCLA and Hebrew University of Jerusalem, in Israel, to control stepwise rotary motion in a single-molecule nickel carborane complex [Science, 303, 1849 (2004)]. The complex consists of a nickel atom between two open-cage dicarbollide ligands (C2B9H112–). The nickel atom is coordinated to the C2B3 pentagonal open face of each ligand, and it serves as the "axle" for the oscillatory motion.

Hawthorne described how interconversion of cis and trans configurations of the complex results in a rotation as the nickel center is reduced and oxidized. The oxidized Ni(IV) compound has a cis configuration in which the carbon pairs of the dicarbollide ligands are aligned on the same side of the molecule, he explained. The reduced Ni(III) compound has a trans configuration in which the ligand carbon pairs face in opposite directions. As an aside, he added that Ni(IV) is rarely encountered, and the carborane complex--first synthesized in his lab in 1970--was the first Ni(IV) organometallic compound.

The cis-trans interconversion can be accomplished electrochemically, by chemical redox reactions, or by excitation of nearby photoelectron donors, Hawthorne noted. The ligands rotate about 140° relative to each other, and the direction of the rotation can be controlled by interligand repulsion when methyl or longer alkyl groups are substituted for hydrogen atoms on the carborane carbons.

The rotary motion arises from molecular orbital stabilization, he said. The calculated minimum energy of the highest occupied molecular orbital occurs for the oxidized cis structure, and the molecule rotates to the trans structure on reduction to stabilize the antibonding lowest unoccupied molecular orbital as an electron enters.

The goal for the metallacarboranes, he said, is to mimic biological motors such as bacterial flagella, which are driven by ATP synthase hydrolysis in cells. But rather than relying on chemical energy, the idea is to use light or electrical energy to power them.

"There are many possible applications for this system," Hawthorne said. "If one of the carborane ligands were fixed to a surface to prevent its rotation, a complete machine could be synthesized. The free rotor with a substituent could be used on command to block or unblock pores or reactive sites of catalysts or enzymes." If two or more individual rotors with the same rotational properties are linked along the rotational axis, he noted, the resulting extended rotor assembly could deliver sequential rotation by successive oxidation or reduction of the various nickel centers.

DESIGNING MOLECULES as precursors for chemical vapor deposition (CVD) of thin films was the subject presented by senior lecturer Claire J. Carmalt of University College London. Carmalt's group has synthesized a variety of precursors to make transition-metal binary materials such as TiP and NbS2, and ternary compounds such as MgAl2O4. But for the symposium in Anaheim, she focused on preparation of aluminum and gallium nitrides, sulfides, and oxides.

Main-group thin films have chemical- and wear-resistant properties, she said, and are now used in a variety of semiconductor electronic applications, such as light-emitting diodes. "Key to preparing the films is designing appropriate precursors and selecting optimum CVD conditions," Carmalt explained. "Different CVD procedures operate at different temperatures and can lead to films with different morphologies and thus different properties."

Films can be derived from dual-source or single-source precursors, she added. For dual-source precursors, the starting materials are mixed in the reactor and the targeted material assembles and adsorbs on a glass substrate. Single-source precursors, on the other hand, already have the elements of interest bonded together in a single molecule that is thermally decomposed in the CVD process.

Single-source precursors tend to be easier to handle and provide cleaner and more efficient film deposition at lower temperatures, Carmalt said. Some single-source precursors prepared by students Simon J. King, John D. Mileham, and Erum Sabir in her group include dimeric gallium and aluminum silylamido complexes. One example is [Cl2Ga{µ-NHSi(CH3)3}]2, which contains a –Ga–N–Ga–N– ring at its core. It was synthesized by reacting GaCl3 with HN[Si(CH3)3]2 in dichloromethane at room temperature. The silylamido compounds decompose by stepwise release of ClSi(CH3)3 and HCl to make GaN or AlN films.

Carmalt also discussed the synthesis of precursors to Ga2O3 films. One example is the dimeric [Ga(CH2CH3)2{OCH2CH2N(CH3)2}]2, which is synthesized by reacting Ga(CH2CH3)3 with the bidentate ligand HOCH2CH2N(CH3)2.

Gallium oxide is an electrical insulator at room temperature, she noted, but it becomes semiconducting above 400 °C. That makes it an ideal material for high-temperature thin-film gas sensors. Carmalt and graduate student Russell Binions, in collaboration with University College London chemistry professor Ivan P. Parkin, have demonstrated that a film prepared by dual-source atmospheric-pressure CVD of GaCl3 and CH3OH can function as a simple sensor.

At 450 °C, the researchers observed a rapid, measurable increase and decrease in the electrical resistance of the Ga2O3 film as a stream of ethanol vapor is toggled on and off. The increase is discernible for ethanol concentrations down to about 1 ppm, Carmalt pointed out.

"Sensor devices are often required to work in remote or hostile environments and run off a battery," she commented. "A maximum response temperature of 450 °C is considerably lower than for other sensor films, and so it would require less energy." The sensor results will be published in an upcoming issue of Chemistry of Materials.

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Credit: COURTESY OF FREDERICK HAWTHORNE
8218motivepower
 
Credit: COURTESY OF FREDERICK HAWTHORNE

ON THE RADICAL FRONT, chemistry professor Guy Bertrand discussed work in his group on stable group 13–15 diradicals and tetraradicals. Bertrand holds a joint appointment at Paul Sabatier University, in Toulouse, France, and at the University of California, Riverside. At UC Riverside, he serves as director of the UCR-CNRS Joint Research Chemistry Laboratory.

Diradicals are molecules that have one less bond than is needed to accommodate the number of available valence electrons. The two leftover, unbonded electrons occupy two molecular orbitals that have the same or nearly the same energy, and the electron spins can be antiparallel (singlet state) or parallel (triplet state).

Obtaining experimental data on singlet diradicals such as cyclobutane-1,3-diyls or cyclopentane-1,3-diyls is difficult, Bertrand noted. The reason for this is that diradicals are extremely short-lived species--the free electrons have a strong tendency to form a bond. But by replacing the ring carbons with alternating phosphorus and boron atoms, his group has prepared substituted B2P2 diradical rings that are stable indefinitely at room temperature.

In the base diradical studied by Bertrand's group, the phosphorus atoms are substituted with two isopropyl groups and the boron atoms--where the free electrons reside--are each substituted with a single tert-butyl group. The diradical can be prepared in moderate yield by reaction of lithium diisopropylphosphide with 1,2-dichloro-1,2-di(tert-butyl)diborane.

The steric interaction between the bulky substituents on phosphorus forces the ring to take on a planar geometry, Bertrand explained. Electrostatic repulsion of the negative charges on the boron atoms, combined with the long B–B bond distance across the ring center, serves to prevent any significant bonding interaction, he said, hence the double radical.

But Bertrand and coworkers have found that sequentially replacing the substituents with less sterically demanding phenyl or substituted phenyl groups leads to an increasingly nonplanar B2P2 core and a shorter B–B distance. With all phenyl substituents, a B–B bond forms and the compound fully converts to a neutral bicyclic "butterfly" structure [Angew. Chem. Int. Ed., 43, 585 (2004)]. The reverse process of stretching the B–B bond of the bicyclic form until it snaps to form the singlet diradical inspired Bertrand and his coworkers to call these compounds "bond-stretch isomers."

Other work under way in Bertrand's lab involves polyradicals in which two B2P2 diradicals are linked by a phenylene bridge between two boron atoms. When the antiferromagnetic p-phenylene linker was used, a stable tetraradical was obtained. In contrast, the group observed that when the ferromagnetic m-phenylene linker was used, a bis(butterfly) structure was formed. The researchers are currently investigating the nature of the communication between the two diradical sites.

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METAMORPHOSIS
An isopropyl-substituted singlet diradical (left) has a planar B2P2 core (B = blue, P = red) and is stable indefinitely at room temperature. Upon sequentially replacing the isopropyl groups with phenyl groups, the planar core becomes distorted and the distance between boron atoms, where the free electrons reside, shortens. By the time all the substituents are replaced with phenyl groups, the boron atoms are close enough to bond, nullifying the diradical, and the neutral bicyclic butterfly structure (right) is born.
Credit: COURTESY OF GUY BERTRAND
8219metamorphosis
 
METAMORPHOSIS
An isopropyl-substituted singlet diradical (left) has a planar B2P2 core (B = blue, P = red) and is stable indefinitely at room temperature. Upon sequentially replacing the isopropyl groups with phenyl groups, the planar core becomes distorted and the distance between boron atoms, where the free electrons reside, shortens. By the time all the substituents are replaced with phenyl groups, the boron atoms are close enough to bond, nullifying the diradical, and the neutral bicyclic butterfly structure (right) is born.
Credit: COURTESY OF GUY BERTRAND

Catenation of singlet diradicals via suitable linkers is expected to lead to antiferromagnetic polymers, Bertrand noted. In these compounds, the half-filled electron bands would confer metal-like conduction without the doping that is typically needed for main-group semiconductors. Triplet diradicals also might serve as building blocks for the construction of nonconducting polymers with ferromagnetic properties, he added. Other possible uses for the compounds include serving as radical scavengers or as initiators for radical reactions involving atom transfer or olefin polymerizations.

The main-group symposium in Anaheim ended with a "Texas dinner" in Cowley's honor attended by more than 100 people. Cowley was roasted during a slide show by UT Austin chemistry professor Richard A. Jones, a longtime friend and collaborator. Norman Hackerman, former UT Austin department chairman and university president and now chairman of the scientific advisory board of the Welch Foundation, recounted how he lured Cowley away from London to join the faculty at Texas in 1962. And more than three dozen of Cowley's former students and postdocs briefly stood and noted their experiences with their "godfather."


MORE ON THIS STORY

MAIN GROUPING
Chemists gather to paint a 21st-century picture of the resurgent field of main-group chemistry

LOOKING BACK
Alan Cowley Has Long Been A 'Main' Player

 
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