Synthesis and structural characterization of lanthanide(III) nitrate complexes of a tetraiminodiphenol macrocycle in the solid state and in solution

Abstract

The lanthanide(III) complexes [Ln(LH₂)(NO₃)₃] 1-11 (La-Er), 15 (Y) and [Ln(LH₂)(NO₃)₂(H₂O)](NO₃) 12-14 (Tm-Lu) of the tetraiminodiphenolate macrocycle L₂- have been prepared by the transmetallation reaction between [Pb(LH₂)(NO₃)₂] and Ln(NO₃)3·nH₂O. In these compounds, the uncoordinated imino nitrogens are protonated and are hydrogen bonded to the phenolate oxygens. The X-ray crystal structures of the La (1), Ho (10) and Lu (14) compounds have been determined. Compounds 1 and 10, in which all the three nitrates are bound in bidentate fashion, are isostructural with distorted bicapped square antiprism geometry for the metal centre. In [Lu(LH₂)(NO₃)₂(H₂O)](NO₃) 14, of the two metal bound nitrates one is bidentate and the other is unidentate, while the metal centre obtains a distorted square antiprism coordination environment. Proton NMR spectra of the paramagnetic lanthanide complexes have been studied in detail. Contributions of contact and pseudo-contact shifts to the lanthanide induced isotropic shifts (LIS) of the macrocycle protons have been separated and good agreement has been obtained between the calculated LIS values and the experimentally observed values. Analysis of the NMR data has led us to conclude that all the complexes in dimethyl sulfoxide solution attain similar configurations. The absorption and emission spectral characteristic of several compounds have been investigated. The complexes of samarium (5) and europium (6) on photoexcitation at 400 nm exhibit well-resolved luminescence spectra at 77 K both in the solid state and a methanol-ethanol (1 : 4) glassy matrix. For the terbium (8) and dysprosium (9) complexes, however, the observed luminescence peaks are less resolved and weak in intensity.