Pyrazabole ligand,

The area of organoboron polymers containing borazine and its derivatives is covered in Chapter 5 of this book by Miele and co-workers. Miele and Bernard also describe the utilization of these polymers in ceramics, fibers, and so on, in Chapter 3 of this book. In this section, the utilization of polymers containing borazine or in some cases the bicyclic boron ligand, 9-BBN, for the production of SiC or Si/C/B fibers is briefly described. Recent advances in polypyrazolylborate or pyrazabole-containing polymers and other boron ring system-derived polymers also have been briefly described. 

Over 2000 papers have appeared on the chemistry of polypyrazolylborate/ pyrazabole ring systems, but there have only been a few examples of polymers containing these ligands.112... 

Of commercial interest are benzo- and other fused aromatic 1,2,3-diazaborine derivatives which have exhibited good antibacterial activity against a variety of microorganisms (155—157). The reaction of pyrazole or C-substituted pyrazoles with boranes yields the pyrazabole system, a class of exceptionally stable compounds. More than 70 species in this system have been reported and the subject comprehensively reviewed (158). These compounds have been used as ligands in transition-metal complexes (159). 

The presence of sufficient pyrazolate ion (pz) is necessary to convert the R2B(pzx) species quickly to the [R2B(pzx)2], otherwise the R2B(pzx) species will dimerize to the stable pyrazabole R2B (/r-pzx)2BR2 (2), which are not readily convertible to scorpionate ligands.34 35... 

The bidentate ligands (RR C(pz)2) react with R"2BX compounds, where X is a leaving group, to form boronium cations [(RR C(pz)2)BR"2]+ isosteric with pyrazaboles and isolable as PF6 salts.202... 

A,7V-polyazolylmethanes can be prepared by reaction of azoles with methylene chloride in an autoclave at 150 °C (Scheme ll).20 At 200 °C this reaction leads to 4,4 -dipyrazolylmethane, which upon reaction with boranes gives a pyrazaboles polymer. Some derivatives have been prepared from the reaction of potassium salts of the azole and methylene iodide. The ligand (tetrakis-(l-pyrazolyl)methane was prepared analogously. In selected cases a basic medium was employed.21... 

Organoboron polymers containing transition metals (Ni, Pd, Pt) and pyrazaboles were obtained by Cu-catalyzed coupling polymerization. The Ni- and Pt-containing polymers showed MLCT (metal-to-ligand charge transfer) absorption.78... 

Pyrazaboles of the general types represented by (47) and (48) have been under study for a number of years, and the syntheses of numerous derivatives have been reviewed. The compounds are relatively stable, and a significant body of C-atom organic substitution chemistry has been developed. In addition, the anionic character of (47) has made these species particularly attractive ligands for both metal and nonmetal cations. That topic is summarized in Section 10 of this article. 

---