Metal pyrazolates Group 11 metals

This latter formulation may give access to metal derivatives in which such a di(l-pyrazolyl)borate unit rves as a chiral ligand, i.e., by employing a C-substituted pyrazole as donor molecule. There exists only one report in the literature on forming poly(l-pyrazolyl)borate ions containing two different pyrazole groups, which were obtained in quite laborious manner... 

Transition metal complexes containing the pyrazolate group as anionic noncoordinating ligand, although rare, have been reported (5). They are undoubtedly of lesser importance and will not be further discussed. 

Azoles can form stable compounds in which metallic and metalloid atoms are linked to nitrogen. For example, pyrazoles and imidazoles Af-substituted by B, Si, P and Hg groups are made in this way. Imidazoles with a free NH group can be Af-trimethylsilylated and Af-cyanated (with cyanogen bromide). Imidazoles of low basicity can be Af-nitrated. 

Pyrazoles are weak acids unless they carry powerful electron-withdrawing groups (Section 4.04.2.1.3(v)). They form metallic salts which are readily hydrolyzed by water (Section 4.04.2.1.3(vi)). 

It is neeessary to emphasize that the direet amination of the methyl group at position 5 of pyrazoles is impossible. Neither 1,3,5-tiimethyl- nor4-ethynyl-l,3,5-trimethylpyrazole undergoes sueh transformations under the reaetion eonditions and starting materials are reeovered nearly quantitatively. Moreover, 4-bromo-ethynyl-l,3,5-trlmethyl- and 4-iodoethynyl-l,3,5-trimethylpyrazole with sodium amide in ammonia exehange the halogen for metal almost quantitatively and in this respeet are similar to phenylehloroaeetylene (Seheme 102). 

The halogen migration is completely suppressed by halogen-metal exchange when the chloroethynyl group is in position 5 of the pyrazole ring. The concentrations of 3-pyrazolyl and 4-pyrazolyl anions are probably small, and they cannot compete with NH2 anions for chlorine bonded to the acetylenic carbon. 

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