Low-Spin Fe(III) Macrocyclic Complexes of Imidazole-Appended 1,4,7-Triazacyclononane as Paramagnetic Probes.

Two macrocyclic complexes of 1,4,7-triazacyclononane (TACN), one with N-methyl imidazole pendants, [Fe(Mim)], and one with unsubstituted NH imidazole pendants, [Fe(Tim)], were prepared with a view toward biomedical imaging applications. These low-spin Fe complexes produce moderately paramagnetically shifted and relatively sharp H NMR resonances for paraSHIFT and paraCEST applications. The [Fe(Tim)] complex undergoes pH-dependent changes in NMR spectra in solution that are consistent with the consecutive deprotonation of all three imidazole pendant groups at high pH values.