Inner-Sphere and Outer-Sphere Water Interactions in Co(II) paraCEST Agents

High-spin Co(II) complexes are promising for development as paraCEST agents (paraCEST = paramagnetic chemical exchange saturation transfer) for magnetic resonance imaging applications. The first examples of Co(II) paraCEST agents with bound water ligands are presented here. Four Co(II) macrocyclic complexes based on 1,4,7-triazacyclononane and containing either pendent alcohol or pendent amide groups were prepared. Two of the macrocycles encapsulate the Co(II) and contain no water ligands as shown by X-ray crystallographic studies, and two complexes have macrocycles with only five ligand donor groups to leave an open coordination site for bound water. The ionization of alcohol, water, or amide groups in the complexes was characterized by using pH potentiometry. These data show that one of the complexes has a readily deprotonated group with a pK_a close to 6, which is assigned as an alcohol pendent. Amide pendents deprotonate at high pH (>8), and the water ligands of the Co(II) complexes are not deprotonated at neutral pH. All complexes produce CEST peaks through either alcohol OH or amide NH proton exchange. The water ligands exchange too rapidly to produce a CEST effect as shown by variableerature ¹⁷O NMR spectroscopy studies. The complexes with available coordination sites for inner-sphere water ligands produce large paramagnetic shifts and broadening of the ¹⁷O resonances of bulk water, whereas the encapsulated complexes show much less shifting and broadening of ¹⁷O resonances. All complexes produce substantial paramagnetic shifts of the ¹H resonances of bulk water, which is promising for the development of supramolecular CEST agents.