Three transition metal complexes with uncoordinated carboxyl groups: synthesis, structures, and luminescence properties

Abstract

A new ligand 1-carboxymethyl-5-methyl-1H-pyrazole-4-carboxylic acid (H2L1) and its isomer 1-carboxymethyl-3-methyl-1H-pyrazole-4-carboxylic acid (H2L2) have been synthesized and characterized by IR and 1H NMR. Three complexes of these ligands, [M(HL1)2(H2O)2] (1: M = Zn; 2: M = Cu) and [Cd(HL2)2(H2O)2] (3) were synthesized by the reactions of H2L1 or H2L2 with the corresponding M(NO3)2·6H2O. The complexes were characterized by physico-chemical and spectroscopic methods, single-crystal X-ray diffraction, and thermogravimetric analysis. Complexes 1 and 2 are isostructural, while complex 3 crystallizes in the triclinic system. The M(II) atom in each of the complexes has a distorted octahedral geometry involving two oxygen and two nitrogen atoms from two anionic (HL1)− or (HL2)− ligands and two oxygen atoms from water ligands. Adjacent monomeric components are connected by hydrogen bonds to form a 3D supramolecular network. The free ligands and complexes 1 and 3 show dual-emissive luminescence in the solid state at room temperature. The luminescence intensity of solid complex 1 is weakened in the presence of NH3. Two new isomers, H2L1 and H2L2, and three complexes, [Zn(HL1)2(H2O)2], [Cu(HL1)2(H2O)2], and [Cd(HL2)2(H2O)2] have been synthesized, which display dual-emissive luminescence in the solid state.