Non-Macrocyclic Schiff Base Complexes of Iron(II) as ParaCEST Agents for MRI

Nearly two decades of research efforts have been devoted to paramagnetic chemical exchange saturation transfer (paraCEST) to produce image contrast in magnetic resonance imaging. Less than twenty iron-based paraCEST agents have been reported so far, the majority of which are ligated by macrocyclic chelates. In the work presented here three isostructural tripodal ligands sharing a common tris-(aza-butenyl)amine foundation are explored to further diversify iron-based paraCEST agents. One of the complexes provides contrast with a CEST effect of 33 % for a 10 mm sample, a paramagnetic shift (Delta omega) of 85 ppm from bulk H2O, and exhibits some stability to biologically relevant ions.