A rare pentanuclear cadmium(II) complex and two new mononuclear zinc(II) complexes of pyrazole derived ditopic ligands – Synthesis, crystal structures and spectral studies

Pyrazole based ditopic ligand HL1 ((Z)-N′-(5-methyl-1H-pyrazole-3-carbonyl) picolinohydrazonamide) reacts with Cd(ClO4)2·6H2O and Zn(ClO4)2·6H2O to form a self assembled pentanuclear Cd(II) complex [Cd5(L1)6](ClO4)4 (1) and a mononuclear Zn(II) complex [Zn(HL1)2](ClO4)2 (2). On the other hand, HL2 ((Z)-N′-(5-methyl-1H-pyrazole-3-carbonyl) pyrazine-2-carbohydrazonamide) also forms a mononuclear [Zn(HL2)](ClO4)2(MeOH)2 (3) with Zn(ClO4)2.6H2O. All these complexes have been characterized crystallographically and spectroscopically. The Cd(II) complex (1) has a pentanuclear core with trigonal bipyramidal arrangement of Cd(II) atoms, where, the axial metal centers (Cd2 and Cd5) have N3O3 chromophore and the equatorial centers (Cd1, Cd3 and Cd4) have N4O2 chromophore with distorted octahedral arrangement around the metal centres. In 2 and 3, the NNO donor ligand molecules in their ketoimine form produce distorted octahedral environment around the Zn(II) centre with N4O2 chromophore utilizing pyridine/pyrazine N, one azomethine N and one carbohydrazide O atoms. Here, the pyrazole ring N atoms remain silent in coordination process. On the other hand, in 1, the same ligand HL1 coordinates in its deprotonated imineol form and functions as N3O donor, while, the pyrazole N atoms take part in coordination process. Ligands HL1 and HL2 are fluorescence silent but complexes 1, 2 and 3 show considerable fluorescence. The fluorescence intensity of 1 is higher than that of 2 and 3.