Pyrazole ligand applications

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. 

Since the choice is difficult due to the huge amount of examples, the selection is based either on aesthetic criteria (77) and (78), or on important applications. In a review, Synthetic Models for Binuclear Copper Proteins, Sorrell reports his previous results with pyrazole ligands which mimics histidyl-imidazole <89T3>. 

Poly(phenylenepyrazoles), 5, 300 Polypyrazoles, 5, 300 N-substituted, 5, 300 Polypyrazolines, 5, 300 Poly(pyrazol-l-yl) borates as ligands, 5, 225, 235 Polypyrroles applications, 4, 376 Polypyrrole tetrafluoroborate conductors, 1, 355 Polysaccharides as pharmaceuticals, 1, 152 Poly-2,5-selenienylenes applications, 4, 971 Polysilacyclopentanes, 1, 609 Polysufides macrocyclic... 

Stmcturally novel pyrazole derivatives include the propellene 2,3,4,5,6-pentakis(pyrazol-r-yl)pyridine 1 and the corresponding 3 ,5 -dimethylpyrazole derivative 2<96T11075>. Poly(pyrazol-l-ylmethyl)benzenes, such as 3, have been prepared as multidentate ligands <95AJC1587>. Solid phase synthesis of structurally diverse 1-phenylpyrazolones was reported, with application to combinatorial synthesis <96SL667>. 

Many classes of pyrazole-based ligands have been developed in the second half of the last century, which have been employed in the coordination chemistry of main group, transition, and lanthanide metal ions and, in the last decade, biological or catalytic applications have been reported [3]. 

In recent years, catalytic applications of half-sandwich metal complexes have been extensively developed. A series of (p-cymene)Ru(II) complexes containing pyrazole-based iV-heterocyclic carbine (pyrazoUn-3-ylidene) ligands (Fig. 21.13a and b) showed excellent catalytic y3-alkylation of secondary alcohols with primary alcohols and the dimerization of phenylacetylene [30]. 

In conclusion, many developments in the last decades on the coordination chemistry of half-sandwich Ru(II), Rh(III), and Ir(III) derivatives with pyrazole-based ligands were proposed, and also interesting applications in catalysis and biochemistry reported. However, there is a vast area of coordination chemistry that is waiting to be further explored and developed, and many of the complexes yet prepared can be further tested to check their catalytic or biological properties. 

For comprehensive reviews on the application of pyrazoles as ligands in coordination chemistry, see (a) Ojwach SO, Darkwa J (2010) Pyrazole tmd (pyrazol-l-yl)metal complexes as carbon-carbon coupling catalysts. J Inoig Chim Acta 363 1947-1964 (b) Halcrow MA (2009) Pyrazoles and pyrazoUdes-flexible synthons in self-assembly. Dtilton Trans 2059-2073... 

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