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 H 6 L 2 comprising two bis(iminomethyl)phenol and two calix[4]arene units has been studied. H 6 L 2 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 La 3+ , Eu 3+ , Tb 3+ and Yb 3+ ions in mixed MeOH/CH 2 Cl 2 solution ( I = 0.01 M NBu 4 PF 6 ) 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 α = K 11 / K 21 of the stepwise formation constants for the mononuclear (L 2 + M = ML 2 , log K 11 ) and the dinuclear complexes (ML 2 + M = M 2 L 2 , 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 H 2 O molecules and/or low-lying ligand-to-metal charge-transfer (LMCT) states. H 6 L 2 was found to be an effective dinucleating ligand for various Ln 3+ ions forming luminescent, mixed-ligand complexes of composition [Ln 2 (L 2 )(MeOH) 2 ] with one exchangeable coligand per seven-coordinated Ln site.