Octahedral Coordination Cages of Co(II) and Ni(II): Enhanced Probe Signal Through Symmetry

The solid-state structures and solution properties of octahedral M₆L₄-type metal–organic cages with acylhydrazone linkages containing paramagnetic Co(II), Ni(II), or diamagnetic Zn(II) and their respective mononuclear analogs are presented toward development of paramagnetic shift probes for magnetic resonance spectroscopy (MRS) imaging. Each metal center is six-coordinate and has two methylpyridine groups serving to maintain the high-spin divalent center for Co(II) or Ni(II). The Co(II) cage and mononuclear complex showed sharp ¹H NMR resonances at the clinically relevant magnetic field strength of 1.4 T (60 MHz) or at 9.4 (400 MHz) and 11.7 T (500 MHz), whereas the Ni(II) cage and complex showed broader proton resonances. ¹H NMR studies at 1.4 T showed that the methyl protons of the Co(II) coordination cage could be detected at 8-fold lower concentration per molecule than the mononuclear analog due to the high symmetry and rigidity of the cage. Moreover, the fully deprotonated coordination cages were substantially more inert toward trans-metalation than were the simple monomeric complexes. Cationic tetraalkylammonium or tetraethylphosphonium guests, including the metabolites acetylcholine and choline, were encapsulated within the Co(II) cage. These guests produced a shift in the cage proton resonances, which highlights the potential for mapping small molecules by MRS imaging.