Complexes of a functionally modified pyrazole derived ligand - Mononuclear zinc(II), dinuclear nickel(II) and a rare pentanuclear cadmium(II) complex with a TBP core and their photoluminescence studies

The pyrazole based ditopic ligand PzCAP (HL, 5-methyl-N'-(1-(pyridine-2-yl)ethylidene)-1H-pyrazole-3 carbohydrazide) reacts with Ni(ClO4)(2)center dot 6H(2)O, Zn(ClO4)(2)center dot 6H(2)O and Cd(ClO4)(2)center dot 6H(2)O to form a new alkoxide bridged binuclear Ni(II) complex [Ni-2(PzCAP)(3)](ClO4)(3)center dot 6(H2O) (1), a mononuclear Zn(II) complex [Zn(PzCAP)(2)](ClO4)(H2O) (2) and a self assembled pentanuclear Cd(II) complex [Cd-5(PzCAP)(6)](ClO4)(4)(H-2-O) (3). These complexes have been characterized crystallographically and spectroscopically. In complex 1, the three ligand molecules display variable denticity towards the Ni(II) centers, one ligand behaves as tetradentate and two others behave as tridentate ligands to create distorted octahedral environments around the two Ni(II) centers, with N5O and N4O2 chromophores. In 2, the same ligand, using keto and enolic oxygen atoms, produces a distorted octahedral environment around the Zn(II) center with a N4O2 chromophore. Complex 3 has a pentanuclear core with a trigonal bipyramidal arrangement of Cd(II) atoms, where the axial metal centers (Cd2 and Cd4) have N3O3 chromophores and the equatorial centers (Cd1, Cd3 and Cd5) have N4O2 chromophores, with distorted octahedral arrangements around the metal centers. Interestingly, the pyrazole unit remains inactive to coordination for complex 2, whereas this unit takes part in coordination for 1 and 3. The ligand HL, complex 2 and complex 3 show fluorescence, while in 1 the fluorescence is substantially quenched compared to the ligand (HL) and its Zn and Cd complexes. As the ligand is capable of using different binding modes, according to the demands of the complex, the emission properties also change accordingly. (C) 2012 Elsevier Ltd. All rights reserved.