Scorpionate ligand

The term Scorpionate ligand refers to a tridentate ligand which would bind to a metal in a fac manner. The most popular class of scorpionates are the tris(pyrazolyl)hydroborates or Tp ligands. These were also the first to become popular. 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".^[1]

The term scorpionate comes from the fact that the ligand can bind a metal with two donor sites like the pincers of a scorpion, the third and final donor site reachs over the plane formed by the metal and the other two donor atoms to bind to the metal. The binding can be thought of as being like a scorpion grabbing the metal with two pincers before stinging it.

While many scorpionate ligands are of the Tp class, many other scorpionate ligands are known. For example the Tm and tripodal phosphine classes have an equally good claim to be scorpionate ligands. Many of the scorpionate ligand have a central boron atom which bears a total of four groups, but it is possible to create ligands which use other central atoms.

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Overview

Homoscorpionates vs. Heteroscorpionates

Trofimenko's initial work in the field was with the homoscorpionates where three pyrazoyl groups are attached to a boron. Since this work a range of ligands where more than one type of metal binding group is attached to the central atom have been reported, these are the heteroscorpionates.

Many other chemists to continue exploring the possibilities of scorpionate ligand alternatives, such as:

• 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;
• Sulfur donor groups such as those found in the Tm ligand.
• 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.

Isolobal

Since the work by Wilkinson and others on ferrocene a vast amount of work has been done on cyclopentadienyl complexes. It was soon understood by many organometallic chemists that a Cp ligand is isolobal to Tp. As many insights into chemistry can be obtained by the study of a series of closely related compounds (where only one feature is changed) a great deal of organometallic chemistry has been done using Tp (and more recently Tm) as a co-ligand on the metal.

The Tp, Tm, trithia-9-crown-3 (a sulfur version of a small crown ether) and cyclopentadienyl (Cp) ligands related ligands and form related complexes. These ligands donate the same number of electrons to the metal, and the donor atoms are arranged in a fac manner covering a face of a polyhedron.

The Tp and Tm ligands are isolobal with the Cp ligands, here is the structure of the Cp manganese tricarbonyl complex. This compound like the others shown in this section can be formed by the reaction of [MnX(CO)₅] with the anionic ligands, in the case of the thio crown ether a better synthesis would be to react [Mn(CO)₅(MeCN)].OTf with the crown in a non coordinating solvent such as dichloromethane.

Below is shown the structure of the tricarbonyl manganese complex of trithia-9-crown-3

While the geometry of the Tp ligands do not allow the formation of simple borane complexes with the metals, the geometry of the Tm ligands (and sometimes their bidentate versions Bm) are such that with late transition metals such as osmium and platinum it is possible to turn the Tm ligand inside out to form a borane to which the metal forms a dative bond.

Here is the manganese complex of Tm with (again three carbonyls).

Tp class

The trispyrazolylborate ligand is often known as Tp to many organometallic chemists, using different pyrazoles a range of different ligands can be formed. In this article we will group all the trispyrazolylborates together.

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 converted 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).

For more details on this topic, see Tp ligands.

Tm class

By replacing the nitrogen donor of a Tp ligand atoms with sulfur atoms, a class of ligands known as Tm can be made. These are related to the thioureas.¹; Several research groups including Tony Hill's group [1]have been working on this ligand class. To form NaTm {Na⁺ HB(mt)₃^-), Methimazole and sodium borohydride are heated together.

Coordination chemistry with Ruthenium, rhodium, osmium, Molybdenum, Tungsten and other metals has been reported.

For more details please see the main article at Tm ligands.

Tripodal phosphines

A range of tripodal phosphines such as HC(CH₂PR₂)₃, N(CH₂CH₂PPh₂)₃ and P(CH₂CH₂PMe₂)₃ have been reviewed in Cotton and Wilkinson (ISBN 0-471-84997-9, 5th Ed) on page 436.

Imines formed from tris-(2-aminoethyl)amine

The tetra amine (tris-(2-aminoethyl) amine) can be reacted with 2-hydroxybenzaldehyde to form a ligand which can bind with three oxygens and three nitrogens to a metal.

See also

• Pincer ligands

References

• S. Trofimenko, Scorpionates: Polypyrazolylborate Ligands and Their Coordination Chemistry. World Scientific Publishing Company, 1999.
• Chemical & Engineering News, Aug. 28 1967, pg. 72.

Further reading

Examples of Scorpionate ligands

1. Inorganic Chemistry, 43(26), 8212-8214.
2. Chemical Reviews, 102, 1851-1896.
3. Inorganic Chemistry, 42(24), 7978-7989.
4. Journal of the American Chemical Society, 126, 1330-1331.
5. Inorganic Chemistry, 44(4), 846-848.
6. Organometallics, 23, 1200-1202.