Mixed-ligand lanthanide complexes supported by ditopic bis(imino-methyl)-phenol/calix[4]arene macrocycles: synthesis, structures, and luminescence properties of [Ln2(L2)(MeOH)2] (Ln = La, Eu, Tb, Yb)

The lanthanide binding ability of a macrocyclic ligand H6L2 comprising two bis(iminomethyl)phenol and two calix[4]arene units has been studied. H6L2 is a ditopic ligand which provides dinuclear neutral complexes of composition [Ln(2)(L-2)(MeOH)(2)] (Ln = La (1), Eu (2), Tb (3), and Yb (4)) in very good yield. X-ray crystal structure analyses for 2 and 3 show that (L-2)(6-) accommodates two seven coordinated lanthanide ions in a distorted monocapped trigonal prismatic/octahedral coordination environment. UV-vis spectroscopic titrations performed with La3+, Eu3+, Tb3+ and Yb3+ ions in mixed MeOH/CH2Cl2 solution (I = 0.01 M NBu4PF6) reveal that a 2 : 1 (metal : ligand) stoichiometry is present in solution, with log K-11 and K-21 values ranging from 5.25 to 6.64. The ratio alpha = K-11/K-21 of the stepwise formation constants for the mononuclear (L-2 + M = ML2, log K-11) and the dinuclear complexes (ML2 + M = M2L2, log K-21) was found to be invariably smaller than unity indicating that the binding of the first Ln(3+) ion augments the binding of the second Ln(3+) ion. The present complexes are less luminescent than other seven-coordinated Eu and Tb complexes, which can be traced to vibrational relaxation of excited Eu-III and Tb-III states by the coligated MeOH and H2O molecules and/or low-lying ligand-to-metal charge-transfer (LMCT) states.