Preparation of a Cobalt(II) Cage: An Undergraduate Laboratory Experiment That Produces a ParaSHIFT Agent for Magnetic Resonance Spectroscopy.

Laboratory experiments that demonstrate the effect of paramagnetic complexes on chemical shifts and relaxation times of protons are a useful way to introduce magnetic resonance spectroscopy (MRS) probes or magnetic resonance imaging (MRI) contrast agents. In this undergraduate inorganic chemistry experiment, a paramagnetic Co(II) cage complex is prepared by reduction of a classical Co(III) cage complex. The Co(II) cage behaves as a paramagnetic shift agent (paraSHIFT) and produces relatively sharp and highly dispersed proton resonances. Comparison of the proton NMR spectra of the diamagnetic and paramagnetic cage complexes provides opportunities for students to consider coordination complex symmetry and the effect of unpaired electrons on the chemical shifts of proton resonances. Oxidation and spin state changes of transition metal complexes, Co(III)/Co(II), are also illustrated in this experiment.