Potassium tris(3,5-dimethyl-1-pyrazolyl)borate, abbreviated KTp*, is the potassium salt of the anion HB((CH
3)
2C
3N
2H)
3. Tp*
− is a
tripodal ligand
Tripodal ligands are tridentate, tri- and tetradentate ligands. They are popular in research in the areas of coordination chemistry and homogeneous catalysis. Because the ligands are polydentate, they do not readily dissociate from the metal centre ...
that binds to a metal in a facial manner, more specifically a
Scorpionate ligand
In coordination chemistry, a scorpionate ligand is a tridentate (three-donor-site) ligand that binds to a central atom in a ''fac'' manner. The most popular class of scorpionates are the hydrotris(pyrazolyl)borates or Tp ligands. These were al ...
.
KTp* is a white crystalline solid that is soluble in polar solvents, including water and several alcohols.
Synthesis
KTp* is synthesized in a manner similar to that of
KTp by the reaction of
potassium borohydride
Borohydride refers to the anion , which is also called tetrahydroborate or more commonly tetrahydrobiopterin, and its salts. Borohydride or hydroborate is also the term used for compounds containing , where ''n'' is an integer from 0 to 3, for ex ...
and
3,5-dimethylpyrazole
3,5-Dimethylpyrazole is an organic compound with the formula (CH3C)2CHN2H. It is one of several isomeric derivatives of pyrazole that contain two methyl substituents. The compound is unsymmetrical but the corresponding conjugate acid (pyrazolium) ...
. Hydrogen gas is evolved as each of the pyrazole reacts at the boron. The rate of B-N bond formation becomes more difficult with each successive 3,5-dimethylpyrazolyl due to the increase in steric hindrance around the boron:
:3 Me
2C
3N
2H
2 + KBH
4 → KHB(Me
2C
3N
2H)
3 + 3 H
2
The required dimethylpyrazole is obtained by condensation of hydrazine and
acetylacetone
Acetylacetone is an organic compound with the chemical formula . It is classified as a 1,3-diketone. It exists in equilibrium with a tautomer . The mixture is a colorless liquid. These tautomers interconvert so rapidly under most conditions that ...
.
Role as ligand
The active binding sites in Tp*
− are the three nitrogen centers that are not bonded to the boron. Although more weakly binding than cyclopentadienyl ligands, Tp*
− is still a tightly coordinating. The benefit of Tp*
− over its sister compound Tp
− is the addition of the methyl groups on the pyrazolyl rings, which increases the steric hindrance of the ligand enough that only one Tp*
− can bind to a metal. This leaves the remaining coordination sites available for catalysis.
[{{cite journal , last1 = Trofimenko , first1 = S , year = 2004 , title = Scorpionates: genesis, milestones, prognosis , journal = Polyhedron , volume = 23 , issue = 2–3, pages = 197–203 , doi = 10.1016/j.poly.2003.11.013 ]
:
References
Organoboron compounds
Tripodal ligands
Potassium compounds