Tp ligand

In organometallic chemistry, the trispyrazolylborate ligand, often known as Tp, is a scorpionate ligand. By using different pyrazoles, a range of different ligands can be formed.

The Tp ligands 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).

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Manganese

As commented above, [MnBr(CO)₅] reacts with KTp to form [MnTp(CO)₃]. This is a compound which is interesting because examples of it with many of the scorpionates (Cp, Tp, Tm and the thia-crown) are known. These compounds are interesting because by using solution state IR spectrscopy that it is possible to use the carbonyl stretching frequencies as a measure of the electron density on the metal, the electron density in turn is controlled by the ligands.

Solid state infra-red spectrscopy can be complicated by the existence of more than one crystal form (A good example being the ruthenium complex [RuHCl(CO)(PPh₃)₃] which has a pink and a yellow form which have different IR spectra).

Copper

While the first row transition metals from chromium through to nickel form plenty of both homoleptic and mixed ligand carbonyl complexes (such as nickel carbonyl, iron carbonyl and chromium carbonyl), it is important to note that few copper carbonyls are known: If copper was to form a homoleptic carbonyl 18VE it would be Cu₂(CO)₇. It is however possible by reacting a copper (I) salt with a Tp ligand and then with carbon monoxide to form a copper (I) carbonyl complexes. [CuTp(CO)] is an 18VE complex. For a recent example see.

S. Imai, K. Fujisawa, T. Kobayashi, N. Shirasawa, H. Fujii, T. Yoshimura, N. Kitajima, and Y. Moro-oka, Inorganic Chemistry, 1998, 37, 3066.

Gold

Gold complexes similar to the copper (I) Tp carbonyls have been reported, in addition an isocyanide complex has been reported. In this paper the normal Tp ligand was replaced with a fluorinated tris(pyrazolyl)borate ligand {hydrotris(3,5-bis(trifluoromethyl)pyrazolyl)borate)}.

H. V. Rasika Dias and W. Jin, Inorganic Chemistry', 1996, 35, 3687.

Molybdenum

An example of a metal which has a wide range of Tp complexes is molybdenum, a great wealth of different complexes of this metal with Tp ligands are known.

It is possible to react molybdenum hexacarbonyl with acetonitrile to form molybdenum triscarbonyl trisacetonitrile ([Mo(CO)₃(MeCN)₃]) with KTp to form the anionic complex [MoTp(CO)₃]^-. This anion can be crystallised using a tetraalkyl ammonium cation, or it can be reacted with a range of electrophilic reagents.

Here is picture of the tetraethyl ammonium salt of [MoTp(CO)₃]^-.

• Proton, it is possible by reacting [MoTp(CO)₃]^- with a bronsted acid to form [MoTp(H)(CO)₃]. This is a metal hydride.

• Tin electrophiles, by reacting the [MoTp(CO)₃]^- anion with triphenyl tin chloride or trimethyl tin chloride compounds with molybdenum-tin bonds can be formed. In the case of the triphenyl tin compound this provides a method of forming an air stable crystaline compound which is an synthon for the [MoTp(CO)₃]^- anion.

• If the [Mo(CO)₃(MeCN)₃] is reacted with 1 equiv of bromine or iodine then [MoX₂(CO)₃(MeCN)₂] will be formed, this can be reacted with KTp to form [MoTp(X)(CO)₃].

• If the [Mo(CO)₃(MeCN)₃] is reacted with allyl chloride before the addition of the KTp, then [MoTp(Allyl)(CO)₂] will be formed.[1]

• If the [MoTp(CO)₃]^- anion is treated with isoamyl nitrite (or another alkyl nitrite ester) then a nitrosyl complex ([[MoTp(CO)₂(NO)]] is formed. This is one of many examples of molybdenum nitrosyls.[2]

Here is a picture of a typical mixed nitrosyl carbonyl complex. Note that the carbonly and nitrosyl ligands are disordered for clarity of expression: a randomly-chosen ligand has been designated as the nitrosyl.

Ruthenium

A range of ruthenium complexes of Tp are known, [3],[4],[5].