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

We report here a structural and photophysical study of lanthanide monometal lic complexes with the macrobicyclic axial phenolic cryptand N[(CH2)(2)N=CH -R-CH=N(CH2)(2)](3)N (R=m-C6H2OH-2-Me-5) L as well as of bimetallic complex es with its de-protonated form (L-3H)(3-). The X-ray crystal structure of [ DyL(NO3)](NO3)(2). 2CH(3)CN . 0.5H(2)O shows the metal ion being asymmetric ally positioned into the macrobicyclic cavity and bonded to seven donor ato ms of L and two oxygen atoms of a bidentate nitrate ion. The X-ray crystal structure of the bimetallic cryptate, [Dy-2(L-3H)(NO3)(2)](NO3). 3H(2)O . M eOH, confirms that both Dy(III) ions are held into the cavity of the crypta nd at a very short distance from each other, 3.4840(4) Angstrom. High resol ution 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 liga nd electronic levels, the spectrum of the binuclear complex being consisten t with the presence of two Yb(III) ions in different co-ordination environm ents. Proton NMRD profiles for the mononuclear Gd(III) complex have been me asured in the temperature range 5-37 degrees C and show that the relaxivity is mainly limited by fast rotation; this compound is stable towards acid-c atalysed decomposition in aqueous solution only at pH > 5.5.