A Bisamide Derivative of [Mn(1,4‐DO2A)] – Solution Thermodynamic, Kinetic, and NMR Relaxometric Studies

Abstract

Abstract A new 1,4‐DO2A‐bisamide [1,4‐DO2AM; 1,4‐DO2A = 2,2′‐(1,4,7,10‐tetraazacyclododecane‐1,4‐diyl)diacetic acid] ligand was synthesized, and its Mn II complex was prepared and investigated in detail by solution thermodynamic, kinetic, and 1 H and 17 O NMR relaxometric measurements. Although the stability constant of [Mn(1,4‐DO2AM)] 2+ (log K MnL = 12.64) is 2.5 log K units lower than that of [Mn(1,4‐DO2A)], its kinetic inertness, investigated through its transmetallation reactions with Zn 2+ ions, is much higher. In particular, the dissociation half‐life ( t 1/2 ) of [Mn(1,4‐DO2AM)] 2+ is ten times higher than that of [Mn(1,4‐DO2A)] ( t 1/2 = 556 vs. 49 h, pH 7.4). The longer rotational correlation time ( τ R = 53 ps) for [Mn(1,4‐DO2AM)] 2+ is mainly responsible for the slightly higher (+20 %) relaxivity ( r 1 = 2.5 m M –1 s –1 , 20 MHz, 25 °C) with respect to that of [Mn(1,4‐DO2A)]. Notably, the water exchange rate of the coordinated water molecule is one order of magnitude slower than that of [Mn(1,4‐DO2A)] ( 298 k ex = 115 ± 6 × 10 6 s –1 ). Finally, the dynamic properties of [Zn(1,4‐DO2AM)] were investigated by NMR spectroscopy, and the decoordination and arm‐rotation processes of the acetamide pendant arms were followed.