Metal pyrazolates coordination chemistry

N-heterocyclic compounds containing six-membered rings (pyridine and analogues) behave as excellent -acceptors and in turn they provide a rather soft site for metal ion coordination. The 7r-excessive five-membered pyrazole is a poorer 7r-acceptor and a better 7r-donor and it acts as relatively hard donor site. Inclusion of six- and five-membered N-heterocycles like pyridine and pyrazole in one ligand system leads to very attractive coordination chemistry with variations of the electronic properties.555 The insertion of a spacer (e.g., methylene groups) between two heterocycles, which breaks any electronic communication, makes the coordination properties even more manifold. 

The coordination chemistry of Hpz and pz has been thoroughly examined by Trofimenko (5, 6) in his notable reviews dealing with the coordination chemistry of pyrazole-derived ligands. The pyrazolate ligand pz is known to exhibit three coordination modes on binding metal centers (Fig. 1). 

Many classes of pyrazole-based ligands have been developed in the second half of the last century, which have been employed in the coordination chemistry of main group, transition, and lanthanide metal ions and, in the last decade, biological or catalytic applications have been reported [3]. 

For comprehensive reviews on the application of pyrazoles as ligands in coordination chemistry, see (a) Ojwach SO, Darkwa J (2010) Pyrazole tmd (pyrazol-l-yl)metal complexes as carbon-carbon coupling catalysts. J Inoig Chim Acta 363 1947-1964 (b) Halcrow MA (2009) Pyrazoles and pyrazoUdes-flexible synthons in self-assembly. Dtilton Trans 2059-2073... 

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). 

Finally, pyrazole binding to manganese(l) has been achieved by photoinduced substitution of CO, and the adjacent N4-coordination pocket can accommodate a second metal ion (Equation 12). The heterodinuclear MnZn complex 55 was characterized crystallographically and its redox chemistry investigated by spectroelectrochemical methods <2002JOMII3, 2005IC3863>. 

The compound UCp3(C3H3N2) (C3H3N2 = pyrazolate anion) was synthesized (30) with the expectation of forming a dimeric organo-uranium derivative. This was based on the known coordination behavior of the ligand in transition metal chemistry where it is almost exclusively exo-bidentate. The structure of the molecule is shown in Fig. 10 and reveals a novel n -endo-bidentate bonding mode for the... 

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