Scorpionate Ligands, also known as Tridentate Ligands or more precisely named polypyrazolylborate ligands, first appeared in journals in 1966 from a little-known chemist, Swiatoslaw Trofimenko. With incomparable ability to play active roles in not only coordination chemistry and catalysis, but enzyme modelling as well, Trofimenko called this discovery "a new and fertile field of remakable scope" (JACS 88(8), 1842). These amazing new compounds were simply derivatives of boryl-pyrazole compounds that had amazing capability of binding metals. The term "Scorpionate Ligand" specifically refers to tris(pyrazolyl)borates, a boron bound to three pyrazoles; the "pincers" of the compound refer to the nitrogen hetero atoms from two of the pyrazole groups (C3H4N2) which can bind a metal, where the third pyrazole group, the tail, can move forward to "sting" the metal, holding it more firmly in place. The R group attached to the boron center, is varied with such ligands as hydrogen, benzene, 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. .
These compounds were initially created by reacting pyrazole with alkali-metal borohydrides, such as sodium borohydride NaBH4, under reflux. The bulky pyrazole ligands allow for good shielding of the bound metal while strong sigma bonds between the nitrogens and the metal stabilize the metal; these attribtues help scorpionate compounds with creating highly symmetrical supramolecular silver complexes and olefin polymerization (with the compound hydrotris(pyrazolyl)borate Mn). Stable in air and water, Trofimenko had stumbled upon a branch of chemistry that did indeed leave endless options for chemistry. Since his initial work in the field, Trofimenko has been joined by many other devoted 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]amine that can make multiple bonds to the metal³;
- changing the ligands to alter the type of encapsulation needed to metals;
- for very different applications, "heterscorpionate 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 polymerization.
External Links
C&E News PINCH AND STING: THE SCORPIONATES [1]
References
Trofimenko's original paper: JACS 88(8) 1842-1844
Trofimenko, Swiatoslaw. Scorpionates: Polypyrazolylborate Ligands and Their Coordination Chemistry. World Scientific Publishing Company, 1999.
C&EN Aug. 28 (1967) pg. 72
Experimentation examples with Scorpionate Ligands:
1. Inorg. Chem. 43(26), 8212-8214
2. Chem. Rev. 102, 1851-1896
3. Inorg Chem. 42(24), 7978-7989
4. JACS 126, 1330-1331
Inorg. Chem. 44(4), 846-848
Organometallics 23, 1200-1202