Potassium tris(3,5-dimethyl-1-pyrazolyl)borate

Potassium tris(3,5-dimethyl-1-pyrazolyl)borate, abbreviated KTp*, is the potassium salt of the anion HB((CH3)2C3N2H)3. Tp*− is a tripodal ligand that binds to a metal in a facial manner, more specifically a Scorpionate ligand.[1] KTp* is a white crystalline solid that is soluble in polar solvents, including water and several alcohols.

Potassium tris(3,5-dimethyl-1-pyrazolyl)borate
Names
IUPAC name
Potassium tri(3,5-dimethyl-1-pyrazolyl)borohydride
Other names
Tp* ligand
Identifiers
ECHA InfoCard 100.203.488
Properties
C15H22BKN6
Molar mass 336.28 gmol−1
Appearance White solid
Melting point 292 to 301 °C (558 to 574 °F; 565 to 574 K)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Synthesis

KTp* is synthesized in a manner similar to that of KTp by the reaction of potassium borohydride and 3,5-dimethylpyrazole. 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:[2]

3 Me2C3N2H2 + KBH4 → KHB(Me2C3N2H)3 + 3 H2

The required dimethylpyrazole is obtained by condensation of hydrazine and acetylacetone.

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.[3]

Structure of generic Tp*M complex illustrating the steric protection afforded by the methyl groups.
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References
  1. Trofimenko, Swiatoslaw (1999). Scorpionates: Polypyrazolylborate Ligands and Their Coordination Chemistry. World Scientific. ISBN 978-1860941726.
  2. Trofimenko, S. (2002). "CARBONYL HALIDE TUNGSTEN(II) COMPLEXES OF TRIS(3,5-DIMETHYLPYRAZOLYL)HYDROBORATE". Inorganic Syntheses. 33: 220–221. doi:10.1002/0471224502.ch4.
  3. Trofimenko, S (2004). "Scorpionates: genesis, milestones, prognosis". Polyhedron. 23 (2–3): 197–203. doi:10.1016/j.poly.2003.11.013.
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