Pyrazolate complexes

Despite the weak basicity of isoxazoles, complexes of the parent methyl and phenyl derivatives with numerous metal ions such as copper, zinc, cobalt, etc. have been described (79AHC(25) 147). Many transition metal cations form complexes with Imidazoles the coordination number is four to six (70AHC(12)103). The chemistry of pyrazole complexes has been especially well studied and coordination compounds are known with thlazoles and 1,2,4-triazoles. Tetrazole anions also form good ligands for heavy metals (77AHC(21)323). 

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). 

Studies on metal-pyrazole complexes in solution are few. The enthalpy and entropy of association of Co(II), Ni(II), Cu(II) and Zn(II) with pyrazole in aqueous solution have been determined by direct calorimetry (81MI40406). The nature of the nitrogen atom, pyridinic or pyrrolic, involved in the coordination with the metal cannot be determined from the available thermodynamic data. However, other experiments in solution (Section 4.04.1.3.3(i)) prove conclusively that only the N-2 atom has coordinating capabilities. 

Complex 270 is made by the metal-transfer method starting from tiie bis (phosphino)pyrazole complex of nickel (85IC2334). 

Aqueous solutions of dimetiiylgold(III) nitrate easily react with pyrazole and 3,5-dimethylpyrazole to form the pyrazolate complexes 271 (R = H, Me) [85 JOM (295)401]. However, 3,5-diphenylpyrazole gives the complex [Me2Au(3,5-Ph2pzH)2], where the ligand is not deprotonated. To obtain the 3,5-diphenylpyra-zolate complex, dimethylgold(III) iodide must be reacted with silver diphenylpyra-zolate. 

Kim, S.J., Kang, S.H., Park, K.-M., Kim, H Zin, W.-C., Choi, M.-G. and Kim, K. (1998) Trinuclear Gold(l) Pyrazolate Complexes Exhibiting Hexagonal Columnar Mesophases with Only Three Side Chains. Chemistry of Materials, 10, 1889-1893. 

Other tri-coordinate silver(I) complexes are the bridging pyrazolate complexes such as [Ir(775-C5Me5)(pz)3Ag(PPh3)], [Ir(//5-C3Mc5)(pz)3 Ag(PPh3) 2]BF4, and [Ir(j 5-C5Me5)(PPh3)-... 

Table 40 Nitrobenzene reduction rates for rhodium pyrazolate complexes in different solvents160 ...

Pyrazole complexes of type [RuCl2(pz)(dmso-5)3] and [RuCl2(pz)2(dmso-5)2] have been synthesized and characterized spectroscopically and crystallographically. The complexes react with a range of species including CO, NO, and... 

Martens, C.F., Klein Gebbink, R.J.M., Feiters, M.C. and Nolte, R.J.M. (1994) Shape-selective oxidation of benzylic alcohols by a receptor functionalized with a dicopper(ll) pyrazole complex. J. Am. Chem. Soc., 116, 5667-5670. 

Hexakis imidazole and pyrazole complexes of nickel(II) can be easily obtained by the direct reaction of the hydrated nickel(II) salt with the appropriate ligand using ethanol as solvent and stoichiometric amounts of the reactants.898 899 Analogous complexes can be obtained with pyrazoles and imidazoles having substituents in positions not adjacent to the donor site. 

Copper(I) pyrazolate complex 8 was reported to show two phosphorescent properties depending on the cooling procedures to an ordered columnar phase from an isotropic phase by Aida and coworkers (Fig. 10) [34]. Compound 8 shows yellow phosphorescence after slow cooling with deliberate aging. On the other hand, red phosphorescence is observed by rapid cooling. A rewritable phosphorescent paper was prepared by using compound 8. 

Rate data for the iodination of pyrazole in aqueous solution showed the reaction to be first-order in both iodine and heterocycle and an inverse first-order [H+] dependence was found over the pH range 5.96-6.74 (64JA2857). A kinetic study of the aqueous iodination of pyrazole coordinated to Ni2+ showed the coordinated ligand to react more rapidly, and a [H+] dependence that differed from that of the free ligand (82JA2460). However, the results of this study should be viewed with caution, as the presence of several nickel-pyrazole complexes in solution necessarily leads to uncertainties about the exact nature of the reactive species. 

Table 15. Structurally characterized lanthanide pyrazole complexes...

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