In inorganic chemistry, the trispyrazolylborate ligand, abbreviated Tp−, is an anionic tridentate and tripodal ligand. Trispyrazolylborate refers specifically to the anion−, but the term trispyrazolylborate refers to derivatives substituted at on the pyrazolyl rings. This family of compounds are sometimes called scorpionate ligands.
Tp ligands
As suggested by the resonance structures, the nitrogen centers that are not bonded to boron are basic. These centers bind to three adjacent sites of a metal such that the simple adducts have C3v symmetry. The facial bonding mode is reminiscent of cyclopentadienyl ligands, although the ligand field stabilization energy of Tp− is weaker as indicated by the fact that Fe2 is a spin-crossover complex whereas ferrocene is low-spin. The Tp ligands are usually prepared from the reaction of pyrazole with potassium borohydride: Intermediates include the monopyrazolylborate and the bispyrazolylborate. KTp is a colourless solid that soluble in polar solvents.
Substituted tris(pyrazolylborate)s
Condensation of 3-substituted pyrazoles with borohydride affords the corresponding substituted Tp derivatives. The substituent forces boron to the less hindered nitrogen center. Thus 3-phenylpyrazole gives HB3]−, abbreviated −, wherein the phenylsubstituents project away from the metal. Analogously 3-isopropylpyrazole gives HB3]-, abbreviated −. 3,5-Dimethylpyrazole gives the hexamethylated ligand−, sometimes called Tp*−. Because pyrazoles are readily prepared from 1,3-diketones, a large number of substituted Tp complexes are possible. Derivatives are known with perfluorinated, chiral, and functional substituents.
Examples
Illustrative of the synthetic routes to TpR− complexes, MnBr5 and KTp react as follows: Electronically related compounds are known, such as CpMn3 and +. The labile acetonitrile complex Mo33 reacts with KTp to give the anion −, which can be crystallised as its tetraethylammonium salt : Protonation, allylation, and nitrosylation of this salt gives the corresponding neutral hydride, allyl, and nitrosyl derivatives. The inductive effect of substituents on the pyrazolyl groups is illustrated by the values of νCO for TpCF3CuCO vs TpMeCuCO. Although of no practical value, trispyrazolylborate compounds have been applied to a variety of themes. In bioinorganic chemistry, some of the first crystallizable copper dioxygen complexes were obtained using this ligand platform, including examples of the Cu2 bonding mode. Models for hemerythrin, an enzyme with a diiron active site, and xanthine oxidase, a molybdoenzyme, have been examined. In such model complexes, the Tp− simulates the coordination environment provided by three imidazole ligands in proteins. In organometallic chemistry, Tp*Rh2 and related complexes participate in C-H activationreactions. Derivatives of Grignard reagents can be generated, such as TpiBuMgCH3.
