Exploring Inner-Sphere Water Interactions of Fe(II) and Co(II) Complexes of 12-Membered Macrocycles To Develop CEST MRI Probes

Several paramagnetic Co­(II) and Fe­(II) macrocyclic complexes were prepared with the goal of introducing a bound water ligand to produce paramagnetically shifted water ¹H resonances and for paramagnetic chemical exchange saturation transfer (paraCEST) applications. Three 12-membered macrocycles with amide pendent groups including 1,7-bis­(carbamoylmethyl)-1,4,7,10-tetraazacyclodocane (DCMC), 4,7,10-tris­(carbamoylmethyl)-,4,7,10-triaza-12-crown-ether (N3OA), and 4,10-bis­(carbamoylmethyl)-4,10-diaza-12-crown-ether (NODA) were prepared and their Co­(II) complexes were characterized in the solid state and in solution. The crystal structure of [Co­(DCMC)]­Br₂ featured a six-coordinated Co­(II) center with distorted octahedral geometry, while [Co­(NODA)­(OH₂)]­Cl₂ and [Co­(N3OA)]­(NO₃)₂ were seven-coordinated. The analogous Fe­(II) complexes of NODA and NO3A were successfully prepared, but the complex of DCMC oxidized rapidly to the Fe­(III) form. Similarly, [Fe­(NODA)]²⁺ oxidized over several days, forming crystals of the Fe­(III) complex isolated as the μ-O bridged dimer. Magnetic susceptibility values and paramagnetic NMR spectra of the Fe­(II) complexes of NODA and N3OA, as well as Co­(II) complexes of DCMC, NODA, and N3OA, were consistent with high spin complexes. CEST peaks ranging from 60 ppm to 70 ppm, attributed to NH groups of the amide pendents, were identified. Variable-temperature ¹⁷O NMR spectra of Co­(II) and Fe­(II) NODA complexes were consistent with rapid exchange of the water ligand with bulk water. Notably, the Co­(II) and Fe­(II) complexes presented here produced substantial paramagnetic shifts of bulk water ¹H resonances, independent of having an inner-sphere water.