Cobalt(II) scorpionates possess distinct electronic and magnetic properties in comparison to other first-row transition metal complexes in this ligand class. One unique property is significantly downfield paramagnetically shifted 11B NMR signals. The utilization of a heteroscorpionate ligand introduces asymmetry to metal scorpionates, producing cis and trans isomers. Cobalt(II) hydrobenzotriazolyl-bis(3,5-dimethylpyrazolyl) borate, Co(Tp’)2, was synthesized by a direct reaction of anhydrous cobalt(II) sulfate and KTp’ ligand in methanol with cis and trans isomers separated by column chromatography. While previous 11B NMR data show most cobalt(II) homoscorpionates with a signal hovering around 270 ppm, trans-Co(Tp’)2 and cis-Co(Tp’)2 differ with signals at 140 ppm and 230 ppm, respectively. Variable temperature 11B NMR of cis- and trans-Co(Tp’)2 suggests the complexes maintain the same spin state within a 200K to 350K range exhibiting Curie-Weiss behavior. This substantial difference between cis and trans isomers allows for new EPR measurements where traditional cobalt(II) homoscorpionates have been well studied. Reversible redox activity has also been observed in both Co(Tp’)2 isomers with an evident color change from yellow to orange upon oxidation to cobalt(III). Redox activity can be observed via 11B NMR with the replacement of the paramagnetic cobalt(II) peaks at 140 ppm or 230 ppm to a common diamagnetic cobalt(III) peak at -8 ppm. The cobalt(III) diamagnetism allows for the investigation of cis- and trans-[Co(Tp’)2]+ via 1H NMR where the inequivalence of pyrazole-methyl groups of Tp’ are revealed in cis-[Co(Tp’)2]+ versus the equivalent methyl groups in trans-[Co(Tp’)2]+. This fundamental understanding of cobalt(II) and cobalt(III) in various scorpionate ligand environments should allow for the fine tuning of magnetic properties in scaffolded molecular magnets in nano-scale devices and candidates for thermometry applications.

Magnetic and redox properties of a cobalt(II) heteroscorpionate

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