Pyrazole metallation

P. Bonati, Pyrazole Metallic Derivatives, Chim. Ind. (Roma) 62, 323 (1980). 

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). 

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. 

Pyrazole and its C-methyl derivatives acting as 2-monohaptopyrazoles in a neutral or slightly acidic medium give M(HPz) X, complexes where M is a transition metal, X is the counterion and m is the valence of the transition metal, usually 2. The number of pyrazole molecules, n, for a given metal depends on the nature of X and on the steric effects of the pyrazole substituents, especially those at position 3. Complexes of 3(5)-methylpyrazole with salts of a number of divalent metals involve the less hindered tautomer, the 5-methylpyrazole (209). With pyrazole and 4- or 5-monosubstituted pyrazoles M(HPz)6X2... 

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. 

In the preceding parts of Section 4.04.2.1.3 the electrophilic attack on pyrazolic nitrogen with the concomitant formation of different classes of N—R bond has been examined N—H (iv, v), N—metal (vi), N—C(sp ) (vii, viii, xi), N—C(sp ) (be, x, xi), N—SO2R (x), N—halogen (xii), N—O (xiii) and N—-N (xiv). In this last part the reaction with other Lewis acids leading to the formation of pyrazole N—metalloid bonds will be discussed, and the study of their reactivity will be dealt with in Section 4.04.2.3.lO(viii). 

Pyrazoles, and some indazoles, substituted on the nitrogen by B, Al, Ga, In, Si, Ge, Sn, P and Hg are known. Poly(pyrazol-1 -yl)borates have been studied by Trofimenko (72CRV497) who found that they were excellent ligands (Section 4.04.2.1.3(vi)). The parent ligands (282), (283) and (284) are available by the reaction of an alkali metal borohydride with pyrazole, the extent of substitution depending on the reaction temperature (Scheme 22). 

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)). 

In different seetions of this ehapter, pyrazoles and indazoles C-linked to a metal or a metalloid have been deseribed or they will be deseribed in the preparative seetions, ineluding lithio derivatives (Seetion 4.04.2.1.7), pyrazolylmagnesium reagents (Seetion 4.04.2.3.7(iii)), ehloromereury derivatives (Seetion 4.04.2.1.4(vii)) and silylpyrazoles (Section 4.04.3.1.2(ii)). All these compounds are useful intermediates and some of their most characteristic reactions will be dijcussed here. 

The Knorr pyrazole synthesis has been extensively utilized in the preparation of a number of pyrazoles as metal chelators, photographic dyes, herbicides, and biologically active... 

The above value for R = H corresponds to an intermolecular process, either assisted by the solvent (in solution) or by other NH-pyrazole molecules (in solution and in the solid state) while metal migrations are probably intramolecular (the bigger the metal, the easier) and those of COR correspond (for R = NHR ) to a dissociation-recombination mechanism. Minkin [quoted in 96MI(15)339] suggests that a tautomeric process should... 

BrMn(CO)5] reacts with pyrazolate ion to yield the charged complex 38 (69JA5410). Here the metals are bridged by three pyrazolate ligands (71IC1372). 

Pyrazole and [Os3(CO)io(AN)2] form two isomeric cluster complexes 90 and 91 (82IC634 84POL1175), corresponding to metallation of both basic nitrogen atoms (the major product having the symmetrical structure 90) and C,N-metallation... 

The cyclometallated palladium and platinum derivatives of trimesityl phosphine or arsine react with pyrazole or 3,5-dimethylpyrazole to form metal chelates 243 (E = P, As M = Pd, Pt R = H, Me) having the trans configuration (81TMC24). 

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

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