Iron(III)‐Terpyridine Complexes for CO 2 Photoreduction: Effects of Ligand Substitution and Coordination Environment

ABSTRACT Developing light‐driven pathways for recycling CO 2 into value‐added chemicals is a promising approach for decarbonizing the economy and addressing global warming. A suitable catalyst is required to drive CO 2 conversion with high product selectivity and good energy efficiency. In this work, a series of mono ( Fe1 − 4 ) and bis‐ligated ( Fe5 − 8 ) terpyridine (tpy) complexes of Fe(III) were synthesized to act as catalysts for CO 2 photoreduction and were characterized using various analytical techniques. The mono‐tpy‐ligated complexes ( Fe1 − 3 ) without 6,6′′‐substitutions, when dissolved in acetonitrile, were converted into their bis‐tpy‐ligated counterparts ( Fe5 − 7 ), as studied by their absorption spectra. The binding constants of the Fe(III) ion with terpyridine ligands were determined, and ferrocene‐appended terpyridine (Fctpy) showed the highest binding ability (∼10 9 M −2 ) to form a bis‐tpy‐ligated homoleptic complex. Infrared spectroelectrochemical analysis showed that Fe1 and Fe5 formed Fe I −CO (∼1879 cm −1 ) and Fe II −CO (∼1940 cm −1 ) species under electrochemical conditions. The photoreduction of CO 2 by Fe1 − 8 was studied under homogeneous conditions with 1,2,3,5‐tetrakis(carbazol‐9‐yl)‐4,6‐dicyanobenzene (4‐CzIPN) as a photosensitizer and triethanolamine (TEOA) as a sacrificial electron donor, with Fe1 exhibiting the highest catalyst turnover number for CO (TON CO ) of 230 after 4 h of irradiation ( λ max = 440 nm). This study sheds light on the effects of substituents on the ligand framework and coordination environment of iron complexes on CO 2 photoreduction.