High-spin cobalt(II) typically exhibits a large magnetic anisotropy and a sizable zero field splitting as a direct result of its extensive spin-orbit coupling. Scorpionate ligands have provided a method to create varying magnetic and electronic properties of first-row transition metal complexes with simple synthetic alterations directly upon the pyrazole rings or completely substituting a pyrazole for another heterocycle such as 1H-benzotriazole. Cobalt(II) scorpionates have shown to possess distinct magnetic and electronic properties as seen by their significantly downfield paramagnetically shifted ¹¹B NMR signals ( ~270 ppm) in comparison to other first-row transition metal complexes in this ligand class. By utilizing the heteroscorpionate ligand, hydrobenzotriazolyl-bis(3,5-dimethylpyrazolyl) borate (Tp’), asymmetry is introduced in these metal scorpionate complexes resulting in cis and trans isomers. cis- and trans-Co(Tp’)₂ were synthesized and separated via flash column chromatography. ¹¹B NMR data show cis- and trans-Co(Tp’)₂ differ from homoscorpionate analogs with signals of 230 ppm and 140 ppm, respectively. Paramagnetic NMR studies indicate both cis- and trans-Co(Tp’)₂ maintain the same spin state in a 200K to 350K range exhibiting Curie-Weiss behavior and magnetic moments consistent with that of high-spin cobalt(II). EPR measurements and theoretical calculations (ORCA) of Co(Tp’)₂ indicate trans-Co(Tp’)₂ to be in a more rhombic electronic environment than cis-Co(Tp’)₂ with both possessing a negative zero field splitting value indicating potential M_s = 3/2 ground states. This fundamental understanding of cobalt(II) magnetic and electronic properties in various scorpionate ligand environments should allow for the fine tuning of these properties for scaffolded molecular magnets and nano-scale device applications.

David Tierney

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