Pyrazoles polypyrazolylborates

Pyrazolate as a ligand can exhibit various coordination modes, inter alia, monodentate, exo-bidentate, and endo-bidentate. Several review articles have been published in the field.556-559 The use of pyrazole in the modeling of biological systems was also reviewed.560,561 The most common application of pyrazole ligands is to use its exo-bidentate coordination mode linking two metal centers that may be identical or different (compare Section 6.3.4.12.6). Polypyrazolylborates and related ligands are covered in the subsequent section. 

On the addition of anhydrous CrBr2 dissolved in warm ethanol to a warm solution of the [NEt4]+ salt of the appropriate polypyrazolylborate (17) in the same solvent, crystals of the complexes [Cr H B(pz)4 2] (n = 0, 1 or 2 appear) (Table 19).115 These are moderately air-stable, and the ethyl derivative [Cr Et2B(pz)2 2]2, which was obtained from [Cr2(02CMe2)4] and Na Et2B(pz)2 in THF on which hexane was layered to induce crystallization, is stable to air.116 An indazole (a 4,5-substituted pyrazole) derivative [Cr(H2B 4,5-(5-N02benzo)pz 2)2] is also known (Table 19). The isolation of [Cr HB(pz)3 2] and [Cr B(pz)4 2] from aqueous solution117 could not be repeated.115... 

More recently, imidazole- and pyrazole-type ligands with a phosphorus atom as the bridge have been reported,97,98 as depicted in formulae (46)-(48). Also rings with imidazolines (49)," or bridged by a —C—OH group (SO),100 have been reported. Boron-bridged polypyrazolylborates and related systems are dealt with in Chapter 13.6. 

The synthesis of Tp ligands that contain different substituents on the 3- and 5-positions on the pyrazole have been reported, For example, Graham and co-workers reported the synthesis of tris[3-(trifluoromethyl)-5-methyl]pyrazole borate. However, the most commonly used polypyrazolylborate ligands are substituted symmetrically at the 3- and 5-position. The unsymmetrical systems can undergo shifts of boron between the two nitrogens, and this shift scrambles the 3- and 5- positions. ... 

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