Pyrazole based ligands

As discussed above, the active site of catechol oxidase comprises two copper ions, each of which surrounded by three nitrogen donor atoms from histidine residues. To model the active site of this enzyme, we have designed and synthesized new macrocyclic pyrazole-based ligands [22]py4pz (9,22-bis(2-pyridylmethyl)-l,4,9,14,... 

Mussinu J-M, Ruiu S, Mule AC, Pau A, Carai MAM, Loriga G, Murineddu G, Pinna GA (2003) Tricyclic pyrazoles. Part 1 synthesis and biological evaluation of novel 1,4-dihydroindeno[l,2-c]pyrazol-based ligands for CBI and CB2 cannabinoid receptors. Bioorg Med Chem 11 251-263... 

Complexes with pyrazole-based ligands are a frequent subject of chemical investigations aimed at understanding the relationship between the structure and activity of the active site of metallo-proteins. The metal ion in biological systems is often coordinated to one or more imidazole groups, which are part of histidine fragments of the proteins. A thermoanalytical and structural study of several copper complexes with pyrazole substitutes has been reported [156]. 

AND RUTHENIUM(II) COMPLEXES WITH ANCILLARY PYRAZOLE-BASED LIGANDS... 

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]. 

A large number of half-sandwich complexes of Ru(II), Ru(III), Rh(III), and Ir(III) with pyrazole-based ligands with pyridine, pyridazine, and pyrimidine moieties has been recently reported by several authors (Fig. 21.17) [39]. 

Figure 21.17 Pyrazole-based ligands with pyridine, pyridazine, and pyrimidine moieties.

In conclusion to this chapter, we have to mention the coordination chemistry of half-sandwich Rh(in), Ir(III), and Ru(II) acceptors with a particular type of pyrazole-based ligands, built on the pyrazolyl fragment by inserting a carbonyl functionality in 5-position of the heterocyclic ring and an acyl moiety in 4-position, named 4-acyl-5-pyrazolones (HQ) (Fig. 21.24a) [57]. 

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. 

The nucleophile in p-lactam hydrolysis by biomimetic Zn(II) complexes has been reported previously to be either a bridging hydroxide or a terminal water molecule. A bridging hydroxide was ruled out as nucleophile [26]. It is unlikely that this is the nucleophile in the reported complex as the two Cd(II) ions are separated by 4.162 A and thus too far away to establish a single hydroxide bridge between the two metal centers. Crystal structures with pyrazolate based ligands reported by Meyer et al. support this proposal [47, 48]. There are a number of findings that led to the proposal that the metal bound alcohol is the nucleophile (Fig. 5.22) ... 

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