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

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) + [bpcd 2− = N , N ′-bis(2-pyridylmethyl)- trans -1,2-diaminocyclohexane N , N ′-diacetate] and Ln(bQcd) + (bQcd 2− = N , N ′-bis(2-quinolinmethyl)- trans -1,2-diaminocyclohexane N , N ′-diacetate) being positively charged and Ln(PyC3A) ( PyC3A 3− = N -picolyl- N , N ′, N ′- trans -l,2-cyclohexylenediaminetriacetate) and Ln(QC3A) ( QC3A 3− = 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 bpcd 2− and bQcd 2− ; trans -O,O and trans -N,O for PyC3A 3− and QC3A 3− ). 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 HCO 3 − 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 HCO 3 − (log K for the formation of the adduct in the 4.6-5.9 range). The unprecedented affinity of Eu(bpcd) + and Eu(bQcd) + towards HCO 3 − renders these complexes as high-performing systems for the optical assessment of metabolic acidosis.