Mn ²⁺ Complex of a Fluorinated CDTA‐Derivative: Stability, Inertness, and Paramagnetic ¹ H, ¹⁷ O, and ¹⁹ F Relaxation Properties

L2 is a bisamide derivative of cyclohexyl‐diamine tetraacetate (CDTA) containing one trifluoromethyl‐piperidine‐ and one dimethyl‐amide. While the amide functions are stable at pH 7, at pH 4.35 hydrolysis occurs after 1 day, and it is faster for the amide involving the piperidine. pH potentiometry showed lower basicity for L2 than for CDTA , and similar stability for the two complexes (pMn = 8.28 vs. 8.68 for MnL2 and MnCDTA , respectively). The replacement of carboxylates by amides results in two orders of magnitude higher kinetic inertness for MnL2 ( t 1/2 = 1134 hours vs. 12 hours; pH 7.4). 17 O NMR confirmed one inner sphere water in MnL2 and fast exchange ( k ex 298 = 2.8 × 10 7 s −1 ). Its proton relaxivity is remarkable for a small Mn 2+ chelate ( r 1 = 4.59 mM −1 s −1 , 25 °C, 20 MHz). In the complex, 19 F relaxation is strongly accelerated with respect to free L2 , with more important effect on longitudinal (130‐fold), than on transverse relaxation (25–70‐fold). A Mn─F distance of ∼8 Å was estimated from the 19 F relaxation rates. MnL2 provides good signal‐to‐noise ratios on 19 F phantom UTE MR images. These data show that fluorinated, monohydrated Mn 2+ complexes can offer both 1 H and 19 F MRI detection.