Mono- and bimetallic lanthanide(III) phenolic cryptates obtained by template reaction: solid state structure, photophysical properties and relaxivity

Abstract

We report here a structural and photophysical study of lanthanide monometallic complexes with the macrobicyclic axial phenolic cryptand N[(CH₂)₂NCH–R–CHN(CH₂)₂]₃N (R = m-C₆H₂OH-2-Me-5) L as well as of bimetallic complexes with its de-protonated form (L − 3H)³⁻. The X-ray crystal structure of [DyL(NO₃)](NO₃)₂·2CH₃CN· 0.5H₂O shows the metal ion being asymmetrically positioned into the macrobicyclic cavity and bonded to seven donor atoms of L and two oxygen atoms of a bidentate nitrate ion. The X-ray crystal structure of the bimetallic cryptate, [Dy₂(L − 3H)(NO₃)₂](NO₃)·3H₂O·MeOH, confirms that both Dy(III) ions are held into the cavity of the cryptand at a very short distance from each other, 3.4840(4) Å. High resolution laser-excited emission spectra of the crystalline monometallic Eu(III) cryptate point to the presence of a single site with low symmetry for the metal, while lifetime measurements in H₂O and D₂O solutions allowed us to estimate the number of bound water molecules, q = 1. Both mono- and binuclear Yb(III) cryptates display strong emission upon excitation through the ligand electronic levels, the spectrum of the binuclear complex being consistent with the presence of two Yb(III) ions in different co-ordination environments. Proton NMRD profiles for the mononuclear Gd(III) complex have been measured in the temperature range 5–37 °C and show that the relaxivity is mainly limited by fast rotation; this compound is stable towards acid-catalysed decomposition in aqueous solution only at pH > 5.5.