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

Potassium tris(3,5-dimethyl-1-pyrazolyl)borate
Names
	IUPAC name Potassium tri(3,5-dimethyl-1-pyrazolyl)borohydride
	Other names Tp* ligand
Identifiers
CAS Number	17567-17-8;
3D model (JSmol)	Interactive image;
ChemSpider	2007;
ECHA InfoCard	100.203.488
EC Number	678-433-5;
PubChem CID	23663216;
CompTox Dashboard (EPA)	DTXSID60635418 ;
	InChI InChI=1S/C15H22BN6.K/c1-10-7-13(4)20(17-10)16(21-14(5)8-11(2)18-21)22-15(6)9-12(3)19-22;/h7-9,16H,1-6H3;/q-1;+1 Key: NTWZGFNSHCFHIJ-UHFFFAOYSA-N;
	SMILES [BH-](n1c(cc(n1)C)C)(n2c(cc(n2)C)C)n3c(cc(n3)C)C.[K+];
Properties
Chemical formula	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

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.

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.

References

^ Trofimenko, Swiatoslaw (1999). Scorpionates: Polypyrazolylborate Ligands and Their Coordination Chemistry. World Scientific. ISBN 978-1860941726.
^ Trofimenko, S. (2002). "Compounds of General Interest". Inorganic Syntheses. Inorganic Syntheses. Vol. 33. pp. 220–221. doi:10.1002/0471224502.ch4. ISBN 9780471208259.
^ Trofimenko, S (2004). "Scorpionates: genesis, milestones, prognosis". Polyhedron. 23 (2–3): 197–203. doi:10.1016/j.poly.2003.11.013.

[trofimenko2-1] Trofimenko, Swiatoslaw (1999). Scorpionates: Polypyrazolylborate Ligands and Their Coordination Chemistry. World Scientific. ISBN 978-1860941726.

[2] Trofimenko, S. (2002). "Compounds of General Interest". Inorganic Syntheses. Inorganic Syntheses. Vol. 33. pp. 220–221. doi:10.1002/0471224502.ch4. ISBN 9780471208259.

[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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