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OH
(CH^C-i :^NCH2^JC(CH3)3
CH3
CH3
ILTRANOX 236 Antioxidant 4,4'-thio-bis(2-t-biir\'l-S-mcthylphcnol) CH, CH3
Science USDA plant physiologist Horace G. Cutler, who chaired the session on control of pests with natural products, concedes that natural products, per se, have some disad vantages for pest control, especially from the commercial viewpoint. They can't be patented, they tend to be so specific that they have lim ited markets, and they're just as hard to register as anything else. Never theless, they make fine "templates" that can lead to the development of industrially useful products; for example, pyrethrins have led to a host of synthetic pyrethroid insecti cides. Cutler asserts that Japan is far ahead of the U.S. in the field of natural product chemistry. Within 10 years, he says, the Japanese will
have developed "highly specific, biodegradable, ecologically superi or" insecticides based on natural products—and the U.S. will be importing them. One clue to Japanese ascendancy in the field can be gained just by looking at their papers, according to Cutler. Typically, he says, there are three authors—one from a uni versity, one from industry, and one from government. That sort of com bination is seldom found in U.S. papers, he adds, and it points up a weakness in our system, in which "bits of information seem to exist in a vacuum." Cutler calls for an end to such parochialism, and for greater efforts—including the pro vision of funds—to integrate the disciplines. D
HO-/ Ο V s / o \ θ Η C(CHj)3
QCHj),
ILTRANOX 256 Antioxidant Polymeric sterically hindered phenol OH r OH η
CH,
L
CH,
J
n
ILTRANOX 254 Antioxidant Polymeric 2,2,4-trimethyl-l. 2-dihydroquinoline CH,
"hT Η
i-CH3 CH,
ILTRANOX 226 Antioxidant 2,6 di-t-bur\l-4-meth\iphenol OH C(CH,)3
(CHj)3C
CH,
WESTON® TNPP TrisNonylphenyl Phosphite EBS WAX Ν,Ν'-ethylenebisstearamide
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Rhenium species may aid organometallic studies Metal oxides increasingly are being used in industry and academia as reagents and catalysts to make or ganic compounds via selective trans formations. As useful as these ox ides are, though, scientists still don't completely understand how they perform their synthetic feats. Recently, researchers have creat ed two unusual rhenium complexes that may open up new avenues for studying how oxo species interact w i t h organic molecules. What's more, these new complexes have intrigued inorganic chemists and ex tended the limits of what they thought possible. Both discoveries are firsts. At Du Pont's experimental station in Wilmington, Del., visiting scientist James M. Mayer has synthesized oxoiodobis(2-butyne)rhenium(III), "the first well-characterized low-valent oxo complex." And at Johann Wolf gang Goethe University in Frank furt, West Germany, a trio of chem ists has been surprised by the un expected appearance in their reaction flask of a half-sandwich rhenium species that sports three oxo ligands. Complexes carrying t e r m i n a l , multiply bonded oxo groups have been characterized for many transi tion metals, including r h e n i u m . However, these complexes, until now, have been restricted to d°, d 1 , and d 2 electron configurations and
therefore to the highest oxidation states of these elements. So, Mayer knew he had a rare bird when he characterized his oxo compound as a rhenium(III) species, which has a d 4 configuration [/. Am. Chem. Soc, 106,3878(1984)]. Mayer's complex can be prepared from any one of several similar rhe nium precursors, though the reac tions all involve reduction of rhenium(V) to rhenium(III) with con comitant oxidation of a coordinated ligand.
Mayer: diamagnetic oxo complex
The molecule's x-ray crystal structure, which was worked out by Mayer's Du Pont colleague Thomas H. Tulip, shows a p e n t a g o n a l pyramid arrangement with the oxygen atom at the apex. Alternatively, the shape of the molecule can be described as a slightly distorted tetrahedron in which the midpoints of the alkyne ligands occupy two vertexes. The oxo complex is diamagnetic and "appears to be a rare example of a low-spin tetrahedral d 4 molecule," according to Mayer and Tulip. Furthermore, they note, the complex "is remarkably inert considering its unusual oxidation state and structure." The stability of rhenium(V) oxo complexes and tungsten(IV) oxo acetylene complexes would lead one to believe that a rhenium(III) oxo species would be relatively easy to oxidize, but such is not the case with Mayer's complex. It survives treatment, at ambient temperatures, with water, oxygen, iodine, hydrogen, and methyl iodide, as well as carbon monoxide and ethylene. To be sure, other low-valent oxo transition-metal complexes have been studied and discussed in the literature. For example, John T. Groves at the University of Michigan and his coworkers have implicated an oxoiron(IV) (d 4 ) porphyrin complex as the active species in the monooxygenase cytochrome P-450. However, none of these other complexes have been so well characterized as the Du Pont find. The reason: These other complexes are highly reactive, in sharp contrast to Mayer's complex. The unusual stability of Mayer's complex apparently is due to its electronic structure, which has been calculated by David L. Thorn, also at Du Pont. The tetrahedral geometry, as opposed to the octahedral one more commonly found in this kind of molecule, leads to a different— and stabilizing—splitting of the d orbitals, Mayer explains. The synthesis of this stable, lowvalent oxo species opens up new vistas for research. Previously, when chemists had access only to highvalent complexes, "it was hard to make generalizations" about how
the oxo group affects the structure and reactivity of oxo complexes, Mayer notes. But now, with this and presumably forthcoming examples of low-valent complexes, it likely will be easier to discern the chemical pattern. In September, Mayer will move to the University of Washington, where he will continue his research as an assistant professor. No less eye-opening is the West German discovery, made by chemis-
try professors Wolfgang A. Herrmann and Hans Bock and postdoctoral associate Ricardo Serrano, with support from the Fonds der Chemischen Industrie, the Deutsche Forschungsgemeinschaft, and the Spanish Ministry of Education. They serendipitously found a route to a unique half-sandwich complex in which a rhenium(VII) center of d° configuration carries a 7r-bonded p e n t a m e t h y l c y c l o p e n t a d i e n y l ligand and three oxo groups [Angew.
7 Complete Structure Determination Now Faster, Easier.
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27
Science
Open-air oxidation yields trioxorhenium complex ChL
Re
CH,
CH3
H3C HXC ^ C H 3
hv, THF
3
H3C^A^CH3 H3C ^ CH3
+0, ^ CO, CO
H 3 C ^ ^ CH, H,C'^-CH,
°1 b
THF = tetrahydrofuran
Chem. Int. Ed. Engl., 23, 383 (1984)]. Structurally, it seems to resemble methyltrioxorhenium (CH 3 Re0 3 ), a stable compound first reported by another group a few years ago. The half-sandwich compound, which is air-stable, was made by irradiating pentamethylcyclopentadienylrhenium tricarbonyl in tetrahydrofuran (THF) with a mercury high-pressure lamp in the presence of oxygen. The West German chemists established that in the photochemical first step a molecule of
THF replaces one of the carbonyl ligands. This intermediate t h e n reacts with oxygen, eliminating carbon monoxide and carbon dioxide, to give the trioxo species. The appearance of a mononuclear trioxo complex was entirely unexpected, the researchers admit. The THF-containing intermediate behaves quite differently from the corresponding manganese analog. The differences in the chemistries of the r h e n i u m and manganese complexes, they note, can be as-
cribed to the "considerably greater stability" of the rhenium-oxygen bond and rhenium's "much higher oxophilicity." One of the remarkable aspects of the formation of the trioxorhenium complex is that it involves the oxidation of a d 6 rhenium(I) center to a d° rhenium(VII) center. It's also surprising, adds Du Pont's Mayer, that "as good an oxidant as rhenium(VII) must be, it doesn't chew up the organic ligand." According to Mayer, any molecule that contains organic and oxo groups attached to a metal atom potentially could serve as a model for heterogeneous catalysis by metal oxides. Thus, the synthesis of these unprecedented organometallic oxo complexes is helping to lay the groundwork for understanding practical catalytic chemistry. It also is giving chemists "a sense of what can be made," he adds, "and I think that's very exciting." Ron Dagani, Washington
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