Rigidified Derivative of the Non-macrocyclic Ligand H₄OCTAPA for Stable Lanthanide(III) Complexation

The stability constants of lanthanide complexes with the potentially octadentate ligand CHXOCTAPA^4-, which contains a rigid 1,2-diaminocyclohexane scaffold functionalized with two acetate and two picolinate pendant arms, reveal the formation of stable complexes [log K_LaL = 17.82(1) and log K_YbL = 19.65(1)]. Luminescence studies on the Eu³⁺ and Tb³⁺ analogues evidenced rather high emission quantum yields of 3.4 and 11%, respectively. The emission lifetimes recorded in H₂O and D₂O solutions indicate the presence of a water molecule coordinated to the metal ion. ¹H nuclear magnetic relaxation dispersion profiles and ¹⁷O NMR chemical shift and relaxation measurements point to a rather low water exchange rate of the coordinated water molecule (k_ex²⁹⁸ = 1.58 × 10⁶ s^-1) and relatively high relaxivities of 5.6 and 4.5 mM^-1 s^-1 at 20 MHz and 25 and 37 °C, respectively. Density functional theory calculations and analysis of the paramagnetic shifts induced by Yb³⁺ indicate that the complexes adopt an unprecedented cis geometry with the two picolinate groups situated on the same side of the coordination sphere. Dissociation kinetics experiments were conducted by investigating the exchange reactions of LuL occurring with Cu²⁺. The results confirmed the beneficial effect of the rigid cyclohexyl group on the inertness of the Lu³⁺ complex. Complex dissociation occurs following proton- and metal-assisted pathways. The latter is relatively efficient at neutral pH, thanks to the formation of a heterodinuclear hydroxo complex.