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

The lanthanide binding ability of a macrocyclic ligand HL comprising two bis(iminomethyl)phenol and two calix[4]arene units has been studied. HL is a ditopic ligand which provides dinuclear neutral complexes of composition [Ln(L)(MeOH)] (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) accommodates two seven coordinated lanthanide ions in a distorted monocapped trigonal prismatic/octahedral coordination environment. UV-vis spectroscopic titrations performed with La, Eu, Tb and Yb ions in mixed MeOH/CHCl solution (I = 0.01 M NBuPF) reveal that a 2 : 1 (metal : ligand) stoichiometry is present in solution, with log K and K values ranging from 5.25 to 6.64. The ratio α = K/K of the stepwise formation constants for the mononuclear (L + M = ML, log K) and the dinuclear complexes (ML + M = ML, log K) was found to be invariably smaller than unity indicating that the binding of the first Ln ion augments the binding of the second Ln 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 and Tb states by the coligated MeOH and HO molecules and/or low-lying ligand-to-metal charge-transfer (LMCT) states.