Nickel(II) and copper(II) Schiff base complexes bearing benzo-15-crown-5 functionalities as probes for spectroscopic recognition of lanthanide ions

Abstract

A combination of host guest and coordination chemistry in the design of a spectroscopic sensor for lanthanide and aluminium ions is described. Nickel(II) and copper(II) complexes with a N 2O Schiff base ligand and bearing two benzo-15-crown-5 moieties as flexible arms have been prepared and characterised by FAB-MS, UV–Vis and FTIR spectroscopies and, depending on the transition metal, by 1H NMR or EPR. Recognition studies of alkaline, alkaline-earth, aluminium and lanthanide ions have been attempted in dmf by UV–Vis spectroscopy. Charge transfer and intraligand electronic bands of the Ni(II) and Cu(II) complexes show significant changes only in the presence of the M 3+ ions used, lanthanides (Er 3+, Gd 3+ and Pr 3+) and Al 3+. Spectral changes induced by increasing M 3+ concentrations have allowed the calculation of stability constants associated with the coordination of one trivalent ion in each crown ether moiety, corresponding to the formation of 1:2 adducts (metal complex:M 3+), with values of log β 21 in the range 8.2–9.5. No apparent correlation is perceptible between log β 21 values and the transition metal, but a cavity effect was detected for each metal complex since log β 21 values increased as lanthanide radii increased.