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SCIENCE
But now, a stable molecule has been found that boasts a significantly lower ionization energy, with an onset at 3.51 eV. This molecule and its relatives represent a new class of powerful reductants that could lead to novel chemical transformations and behavior [Science, 298, 1971 (2002)]. The discovery grew out of research conducted over the past few years by chemistry professor F. Albert Cotton and adjunct professor Carlos A. Murillo at Texas A&M University. The research involves dimetal paddlewheel complexes, in which a metal-metal multiple bond forms the axis of a wheel with four blades (anionic ligands bridging the two metal atoms). A few years ago, Cotton, Murillo, and coworkers found that a promising new bridging ligand known as hpp (the anion of a hexahydropyrimidopyrimidine) is better than other ligands at stabilizing dimetal units in higher oxidation states. In other words, the strongly basic hpp ligand makes it much easier to oxidize a species such as the quadruply bonded Mo24+ unit. In fact, merely dissolving Mo2(hpp)4 in a chlorinated solvent such as CH2Cl2 leads to formation of the Mo25+ compound--that is, Mo2(hpp)4Cl--in essentially quantitative yield. With the help of an additional oxidizing agent, the dimetal unit can be further oxidized to Mo26+, which is highly unusual, Murillo tells C&EN. Since ditungsten compounds are easier to oxidize than their dimolybdenum analogs, the Texas re- searchers also prepared W2(hpp)4. This compound, they report in Science, "is so easy to oxidize that it can be taken directly by chloroalkanes to W2(hpp)4Cl2."
The experiments, together with theoretical modeling performed by chemist Jiande Gu of the Shanghai Institute of Materia Medica, in China, suggest that during ionization, an electron is removed from the Chemistry professor Malcolm H. Chisholm of Ohio State University calls the finding "surprising" and "quite remarkable." He believes the hpp complexes have potential as "essentially noncoordinating reducing agents" that might react via a different mechanism than many conventional reductants. Cotton and Murillo have shown that the dimetal hpp complexes readily transfer electrons to electron acceptors such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and buckminsterfullerene to yield the corresponding anion or dianion. Their studies suggest that by chemically modifying the hpp ligand, the researchers may be able to produce molecules with even lower ionization energies. |
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Cotton PHOTO BY BILL CRAWFORD
bonding orbital of the metal-metal quadruple bond. A "strong interaction of this orbital with a filled orbital on the hpp ligands largely accounts for the low ionization energies," the researchers explain.
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