Eu(iii) and Tb(iii) complexes of 6-fold coordinating ligands showing high affinity for the hydrogen carbonate ion: a spectroscopic and thermodynamic study

Abstract

In the present contribution, four classes of Ln(III) complexes (Ln = Eu and Tb) have been synthesized and characterized in aqueous solution. They differ by charge, Ln(bpcd)⁺ [bpcd²⁻ = N,N′-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate] and Ln(bQcd)⁺ (bQcd²⁻ = N,N′-bis(2-quinolinmethyl)-trans-1,2-diaminocyclohexane N,N′-diacetate) being positively charged and Ln(PyC3A) (PyC3A³⁻ = N-picolyl-N,N′,N′-trans-l,2-cyclohexylenediaminetriacetate) and Ln(QC3A) (QC3A³⁻ = N-quinolyl-N,N′,N′-trans-l,2-cyclohexylenediaminetriacetate) being neutral. Combined DFT, spectrophotometric and potentiometric studies reveal the presence, under physiological conditions (pH 7.4), of a couple of equally and highly stable isomers differing by the stereochemistry of the ligands (trans-N,N and trans-O,O for bpcd²⁻ and bQcd²⁻; trans-O,O and trans-N,O for PyC3A³⁻ and QC3A³⁻). Their high log β values (9.97 < log β < 15.68), the presence of an efficient antenna effect and the strong increase of the Ln(III) luminescence intensity as a function of the hydrogen carbonate concentration in physiological solution, render these complexes as very promising optical probes for a selective detection of HCO₃⁻in cellulo or in extracellular fluid. This particularly applies to the cationic Eu(bpcd)⁺, Tb(bpcd)⁺ and Eu(bQcd)⁺ complexes, which are capable of guesting up to two hydrogen carbonate anions in the inner coordination sphere of the metal ion, so that they show an unprecedented affinity towards HCO₃⁻ (log K for the formation of the adduct in the 4.6–5.9 range).