Metal pyrazole complexes

Bis(pyrazolyl)borate copper complex 902 has been employed as a catalyst in homogeneous and heterogeneous styrene epoxidation reactions 2000CC1653 . Pyrazole palladacycles 903 have proved to be stable and efficient catalysts for Heck vinylations of aryl iodides 2000CG2053 . A hemilabile pyrazolyl-functionalized N-heterocyclic carbene complex of palladium(ll) such as 904 has been found to be an excellent catalyst for Heck and Suzuki crosscoupling reactions in ionic liquids 2006JOC426 . 

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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. 

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). 

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

Dipolar cycloaddition reactions are most commonly applied for the synthesis of five-membered heterocyclic compounds.86 87 [3+2] cycloaddition reactions of transition-metal propargyl complexes have been reviewed.88 Addition of diazomethane to carbene complexes (CO)5Cr= C(OEt)R results in cleavage of the M = C bond with formation of enol ethers H2C = C(OEt)R,3 89 but (l-alkynyl)carbene complexes undergo 1,3-dipolar cycloaddition reactions at the M = C as well as at the C=C bond. Compound lb (M = W, R = Ph) affords a mixture of pyrazole derivatives 61 and 62 with 1 eq diazomethane,90 but compound 62 is obtained as sole... 

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. 

In these complexes, identical or different metal centers are, generally, connected by two pyrazolate ligands coordinated in an exo-bidentate fashion. In most cases, the M(/t-pz )2M core exhibits a boat conformation. Doubly bridged pyrazolate complexes are usually dimeric however, in the last years some homo-(MMM) and heterotrinuclear (MM M) species containing the metallocycles M(n-pz )2M(ji-pz )2M and M(/t-pz )2M (/i-pz )2M were reported. 

This procedure used involves the reaction of a ruthenium(II) mononuclear pyrazole complex with a second metal acac, as indicated in Eq. 6... 

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. 

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