Pyrazolate complexes Group

Bonati has classified the pyrazole complexes into two groups compounds containing neutral pyrazoles (HPz), called 2-monohaptopyrazoles since it is the N-2 pyridinic nitrogen lone pair which confers on them the ligand properties and compounds containing pyrazole anions (Pz) which can act as monodentate or, more often, as exobidentate ligands (72CRV497). 

It is now widely accepted that the nuclearity of Group 11(IB) univalent metal binary pyrazolate complexes is strongly affected by the nature of the substituents on the heterocyclic ligand. In some cases, depending on the preparative method, complexes having different nuclearities have been selectively obtained. 

Binary pyrazolate complexes containing divalent metals bridged by two pz groups are known for many transition metals. The increasing interest for this class of complexes stems from the expectation that they may provide useful insights in the field of magnetostructural correlations as well as in multimetal centered catalysis. 

The treatment with gaseous HC1 of the cationic nitrosyl-pyrazolate complex l(cod)(NO)Ir(/i-pz)2Ir(cod)](BF4), 69 (obtained from [Ir(cod)(/i-pz)]2 and NOBF4) led to the isolation of [(cod)(Cl)Ir(/t-pz)2(jt-NO)Ir(Cl)(cod)](BF4), 70 (90). Complex 70 has been characterized by X-ray crystal structure determination (90). The reaction affording complex 70 has been described as an unprecedented two-center oxidative addition as a result of which a bent, terminal NO group takes up a bridging position. 

The interest for pyrazolate complexes containing the pz group as a mono-dentate ligand arises because of their recognized usefulness in the synthesis of dinuclear and trinuclear heterometallic derivatives. The mononuclear complex... 

Several studies were carried out in the last decade to test the catalytic activity of mononuclear pyrazole complexes as well as dinuclear and polynuclear species containing bridging pyrazolate groups. Particularly investigated were the pyrazolate complexes in which the M( -pz ) M (Type A) or the M(/x-pz )(ft-X)M" (Type B) framework is present. 

Scheme 4 shows in a general manner cyclocondensations considered to involve reaction mechanisms in which nucleophilic heteroatoms condense with electrophilic carbonyl groups in a 1,3-relationship to each other. The standard method of preparation of pyrazoles involves such condensations (see Chapter 4.04). With hydrazine itself the question of regiospecificity in the condensation does not occur. However, with a monosubstituted hydrazine such as methylhydrazine and 4,4-dimethoxybutan-2-one (105) two products were obtained the 1,3-dimethylpyrazole (106) and the 1,5-dimethylpyrazole (107). Although Scheme 4 represents this type of reaction as a relatively straightforward process, it is considerably more complex and an appreciable effort has been expended on its study (77BSF1163). Details of these reactions and the possible variations of the procedure may be found in Chapter 4.04. 

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