Potassium tris(3,5-dimethyl-1-pyrazolyl)borate
{{Chembox
|ImageFile = KTp*.png
| ImageClass = skin-invert-image
|IUPACName = Potassium tri(3,5-dimethyl-1-pyrazolyl)borohydride
|OtherNames = Tp* ligand
|Section1={{Chembox Identifiers
|CASNo = 17567-17-8
|ChemSpiderID = 2007
|EINECS = 678-433-5
|PubChem = 23663216
|SMILES = [BH-](n1c(cc(n1)C)C)(n2c(cc(n2)C)C)n3c(cc(n3)C)C.[K+]
|StdInChI = 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
|StdInChIKey=NTWZGFNSHCFHIJ-UHFFFAOYSA-N
}}
|Section2={{Chembox Properties
|Formula = C15H22BKN6
|MolarMass = 336.28 gmol−1
|Appearance = White solid
|MeltingPtC = 292 to 301
}}
}}
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.{{cite book | last1 = Trofimenko | first1 = Swiatoslaw | publisher = World Scientific | title =Scorpionates: Polypyrazolylborate Ligands and Their Coordination Chemistry | year = 1999 |isbn=978-1860941726}} 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:{{cite book | last1 = Trofimenko | first1 = S. | title = Inorganic Syntheses | chapter = Compounds of General Interest | year = 2002 | series = Inorganic Syntheses | volume = 33 | pages = 220–221 | doi = 10.1002/0471224502.ch4 | isbn = 9780471208259 }}
: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.{{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 }}
