Photocatalytic CO2 reduction to CO by Cobalt(II) Pyridinyl-1,3,5-Triazine-Diamine complexes

The development of molecular photocatalytic systems for CO₂ reduction is a continuous challenge for chemists. Herein, the photocatalytic reductions of CO₂ by [Co^II(L1)(η¹-ONO₂)₂] (1) (L1 = 6,6'-(pyridine-2,6-diyl)bis(1,3,5-triazine-2,4-diamine)) and [Co^II(L2)(H₂O)₂]²⁺ (2) (L2 = 6,6'-([2,2'-bipyridine]-6,6'-diyl)bis(1,3,5-triazine-2,4-diamine)) have been investigated. With Ir(ppy)₃ as the photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as the sacrificial reductant in DMF/triethylamine solution under irradiation by white light-emitting diode (λ > 420 nm), CO was selectively produced with a turnover number (TON) of 36 and 89 for 1 and 2, respectively. Based on the electrochemical studies, the triply reduced [Co⁰(L2•)]ˉ species from 2 is found to be responsible for the activation of CO₂ with a large rate constant of k = 506 M⁻¹ s⁻¹. However, the strong CO binding constant of Co^I(L2)-CO adduct (K = 5.01 × 10⁶) and the slow CO release from Co⁰(L2)-CO adduct limit the catalytic efficiency.