Gadolinium(III) 1,2-Hydroxypyridonate-Based Complexes: Toward MRI Contrast Agents of High Relaxivity¹

Prospective gadolinium(III) MRI contrast agent precursors [Gd-TREN-1,2-HOPO] (1) {TREN-1,2-HOPO = tris[(1-hydroxy-2-oxo-1,2-dihydropyridine-6-carboxamido)ethyl]amine} and [Gd-TREN-bis(Me-3,2-HOPO)-1,2-HOPO] (2) have been synthesized and characterized by relaxometric measurements. The water proton relaxivity values of 1 and 2 (20 MHz and 25 °C) are 9.5 and 9.3 mM-1s-1, respectively, suggesting the presence of two coordinated water molecules. The molecular structure of [1·DMF]2 was obtained and reveals a similar eight-coordinate geometry to [Gd-TREN-Me-3,2-HOPO·2H2O] ([3·2H2O]). A shape analysis of the coordination polyhedron of 1 reveals that this geometry is best described as a bicapped trigonal prism, poised to accommodate an additional donor atom to give a tricapped trigonal prismatic intermediate. This geometry supports the model that formation of a tris-aquo intermediate for 1 enables fast and associative water exchange.