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Scorpionate ligand

From Biocrawler, the free encyclopedia.

Scorpionate ligands, are representatives of tridentate ligands that are more precisely members of the polypyrazolylborate ligands. These ligands first appeared in journals in 1966 from the then little-known DuPont chemist, Swiatoslaw Trofimenko. Trofimenko called this discovery "a new and fertile field of remakable scope" (JACS 88(8), 1842). The term "scorpionate ligand" specifically refers to tris(pyrazolyl)borates, a BH unit bound to three pyrazolate anions; the "pincers" of the compound refer to two of the pyrazolyl groups (C₃H₄N₂) that can bind a metal, where the third pyrazolyl group, the tail, can move forward to "sting" the metal, holding it more firmly in place via tridentate coordination. The R group attached to the boron center is usually hydrogen but can also be phenyl and alkyl groups (see alkane). When the the "stinger" is the same as the "pincers," then the compound is considered homoscorpionate, but when the stinger is different from the pincers, it is considered to be heteroscorpionate.

The pinch and sting of the Scorpionate Ligands

These compounds are usually synthesized by reacting pyrazole with alkali-metal borohydrides, such as sodium borohydride NaBH₄, under reflux. H₂ is evolved as the borohydride is sequentially coverted first to pyrazolylborate [H₃B(C₃N₂H₃)], then to dipyrazolylborate [H₂B(C₃N₂H₃)₂], and finally to tris(pyrazolyl)borate [HB(C₃N₂H₃)₃]. Bulky pyrazolyl borates can be prepared from 3,5-disubstituted pyrazoles, such as the dimethyl derivative. These bulky pyrazolyl borates have proven especially valuable in the preparation of catalysts and models for enzyme active sites. Utilizing scorpionate ligands in the syntheses of metal catalysts may allow simpler and more accurate methods to be developed. ligands allow for good shielding of the bound metal while strong sigma bonds between the nitrogens and the metal stabilize the metal; these attributes help scorpionate compounds with creating highly symmetrical supramolecular silver complexes and olefin polymerization (with the compound hydrotris(pyrazolyl)borate Mn). Since Trofimenko's initial work in the field, many other chemists to continue exploring the possibilities of scorpionate ligand alternatives, such as:

replacing the nitrogen atoms with sulfur alternatives so the ligands act more similar to that of thiourea compounds¹;
utilizing pyrrole, imidazole, or indole compounds in place of the pyrazole rings²;
the possibility of tripodal heptadentate ligands such as N₄O₃ from the ligand tris[6-((2-N,N-diethylcarbamoyl)pyridyl)methyl]amine³;
changing the ligands to alter the type of encapsulation needed to metals;
for very different applications, "heteroscorpionate ligands" have been examined of hybrid scorpionate/cyclopentadienyl-lithium compounds such as [Li(2,2-bis(3,5-dimethyl pyrazol-1-yl)1,1-diphenylethylcyclopentadienyl(THF)] which catalyzes olefin polymerization.