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)Br 2 ( Co1 – 3 ), were synthesized and characterized via elemental analysis, thermogravimetric analysis, and spectroscopic techniques. The structure of Co1 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)Cl 2, suggesting high magnetic anisotropy, which is dependent on the ligands. Co2 and Co3 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 Co(III)2b complex, formed under crystallization conditions. The complexes showed minimal electrochemical activity toward CO 2 reduction in acetonitrile ( i cat / i p 0.4–1.2), while Co1 – 3 were active in CO 2 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 in situ UV–vis spectroscopy. In summary, the CO 2 photoreduction activity of Co1–3 was dependent on the substituents on the tpy ligands.