Potassium tris(3,5-dimethyl-1-pyrazolyl)borate
# 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 $HB((CH_3)_2C_3N_2H)_3.$ $Tp*^−$ is a tripodal ligand that binds to a metal in a facial manner, more specifically a Scorpionate ligand. 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 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:

    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.

## 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.

    Structure of generic Tp*M complex illustrating the steric protection afforded by the methyl groups.
