Co(II) complexes of tetraazamacrocycles appended with amide or hydroxypropyl groups as paraCEST agents.

Co(II) complexes of 1,4,7,10-tetraazacyclododecane (CYCLEN) or 1,4,8,11-tetraazacyclotetradecane (CYCLAM) with 2-hydroxypropyl or carbamoylmethyl (amide) pendants are studied with the goal of developing paramagnetic chemical exchange saturation transfer (paraCEST) agents. Single-crystal X-ray diffraction studies show that two of the coordination cations with hexadentate ligands, [Co(DHP)] and [Co(BABC)], form six-coordinate complexes; whereas two CYCLEN-based complexes with potentially octadentate ligands, [Co(THP)] and [Co(HPAC)], are seven-coordinate with only three of the four pendant groups bound to the metal center. H NMR spectra of these complexes suggest that the six-coordinate complexes are present as a single isomer in aqueous solution.