Metal pyrazolates

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. 

Sequential functionalization of pyrazole-l-oxides via regioselective metallation led to the synthesis of 3,4,5-trisub-stituted-l-hydroxypyrazoles <2002JOC3904>. 3-Acylated-2-(4-methoxybenzyl)-2//-pyrazole 1-oxides were formed by the reaction between a 3-magnesium 2//-pyrazole-l-oxide and acid chlorides <2002J(P1)428>. 3-Arylated-l-hydroxypyrazoles were synthesized from 3-metallated-pyrazole 1-oxides <2001JOC8654>. The reaction between hexafluorobenzene and the anion of 1-hydroxypyrazole affords a mixture of the products of bis-, tetrakis-, and hexakis-substitution <2004ARK100>. In the case of hexakis(bromomethyl)benzene, its reaction with 1-hydroxy-pyrazole leads to the hexakis-substituted product. 

List of Structurally Characterized Binary Metal Pyrazolates... 

The metal-pyrazolate saga has been known since the pioneering work of Buchner in the nineteenth century. In the last few years, a definitive characterization of several binary complexes has been possible due to the advent of the recent method of ab initio structure determination from X-ray powder diffraction data. In this respect, it is likely that a number of new molecules and/or crystalline phases, which have so far escaped a complete characterization, will be structurally analyzed, thus confirming the variability of the coordination modes and stoichiometries attributable to simple pyrazolato complexes. 

The bis(pyrazolyl)silanes Bps (Scheme 24) were readily prepared from the reaction of Me2SiCl2 and two molar equivalents of the corresponding alkali metal pyrazolates.262 Both Bps and BpsMe2 ligands were isolated in excellent yield. They are stable in dry air for at least six months and soluble in aliphatic and aromatic hydrocarbons, acetonitrile, dichloromethane and ethers. However, they slowly decompose in acetone or in the presence of solvents with acidic protons such as water and alcohols, an important difference with respect to the analogous poly(pyrazolyl)methane ligands. Both bidentate ligands have been used to prepare zinc(II) derivatives. 

The [3+2] cycloaddition of Fischer alkynyl carbene complex 1 with diazomethane was first reported in 1973 (Scheme 5.1) [9]. The reaction initially generates a pyrazole carbene complex 2 as an intermediate, followed by the nucleophilic addition of the second diazomethane to the carbene center, and the denitrogenation occurs to give the A-metalated pyrazole 3. When trimethylsilyldiazo-methane (TMSCHN2) is used instead of diazomethane, the reaction of the initially formed [3+2] cycloadduct with TMSCHN2 was suppressed to give the pyrazole carbene... 

---