Three pyrazole-3-carboxylic acid complexes as efficient solvent-free heterogeneous catalysts for C=C bond formation

Three pyrazole-3-carboxylic acid (H(2)pca) complexes, Cr(Hpca)(3)center dot 2H(2)O (1), Ni-2(pca)(2)(H2O)(6)center dot 2H(2)O (2) and [Pb-2(Hpca)(4)](n) (3), have been synthesized and fully characterized. The single crystal X-ray diffraction indicate that the mononuclear Cr(III) cation in complex 1 and binuclear Ni(II) cations in complex 2 are coordinated by pyrazole-3-carboxylic acid ligands and water molecules in a distorted octahedral geometry and the binuclear Pb(II) cations in complex 3 presents a distorted seven-coordinated {PbN2O5} pentagonal-bipyramid geometry. Notably, these complexes as effective heterogeneous catalysts show efficient catalytic activities for Knoevenagel condensation (yield up to 97%) and the catalyst can be recycled without losing activity. (C) 2017 Published by Elsevier B.V.