Effect of Substituted Terpyridine Ligands on Magnetic and CO ₂ Photoreduction Properties of High Spin Co(II)(tpy)Br ₂

A series of monoligated cobalt terpyridine complexes, Co(R-tpy)Br₂ (<b>Co1</b>-<b>3</b>), were synthesized and characterized via elemental analysis, thermogravimetric analysis, and spectroscopic techniques. The structure of <b>Co1</b> was determined by single-crystal X-ray diffraction (SCXRD), showing a distorted square pyramidal geometry for Co(II). Magnetic susceptibility studies at different temperatures indicated high-spin Co(II) states, and zero-field splitting parameters (|D|>30) determined by computational techniques showed values similar to those reported for Co(tpy)Cl₂, suggesting high magnetic anisotropy, which is dependent on the ligands. <b>Co2</b> and <b>Co3</b> gradually converted to bis-ligated cobalt complexes in solution, indicating greater instability of penta-coordinated Co compared to octahedral Co, confirmed by SCXRD of an oxidized <b>Co(III)2b</b> complex, formed under crystallization conditions. The complexes showed minimal electrochemical activity toward CO₂ reduction in acetonitrile (<i>i</i>_cat/<i>i</i>_p 0.4-1.2), while <b>Co1</b>-<b>3</b> were active in CO₂ photoreduction with an organic photosensitizer (TON_CO ∼ 34-57). Introducing ferrocene substituents on the terpyridine ligand architecture impeded the catalytic activity, and the mechanistic insights were obtained from IR spectroelectrochemistry and <i>in situ</i> UV-vis spectroscopy. In summary, the CO₂ photoreduction activity of <b>Co1-3</b> was dependent on the substituents on the tpy ligands.