Paramagnetic d‐Block Metal Organic Cages as MRI or Chemical Shift Probes

Paramagnetic d‐block metal organic cages (MOCs) show promise for the development of magnetic resonance imaging (MRI) contrast agents and magnetic resonance spectroscopy imaging (MRSI) probes. MOC‐based MRI probes increase the relaxation rates of water protons with predominant examples containing high‐spin Fe(III). MOC T 1 MRI probes show enhanced signal based, in part, on slower reorientation times attributed to rigidly linked paramagnetic metal centers. Paramagnetic shift (parashift) probes for MRSI are highly symmetrical MOCs that produce intense paramagnetically shifted proton resonances for improved signal and generally contain high‐spin Co(II) or Ni(II). The host–guest properties of MOCs are of interest for the delivery of drugs and for the development of responsive imaging probes to sense small molecules as guests. New MOCs with improved water solubility and stability would facilitate new applications in biomedical imaging.