Exploring the Full Potential of Photocatalytic Carbon Dioxide Reduction Using a Dinuclear Re₂Cl₂ Complex Assisted by Various Photosensitizers

Abstract

Photosensitizing units have already been applied to enable light-driven catalytic reduction of CO 2 with mononuclear rhenium complexes. However, dinuclear catalytic systems that are able to activate CO 2 in a cooperative bimetallic fashion have only rarely been combined with photosensitizers. We here present detailed studies on the influence of additional photosensitizers on the catalytic performance of a dirhenium complex (Re 2 Cl 2 ) and present correlations with spectroscopic measurements, which shed light on the reaction mechanism. The use of [Ir(dFppy) 3 ] (Ir, dFppy = 2-(4,6-difluorophenyl)pyridine)) resulted in considerably faster CO 2 to CO transformation than [Cu-(xant)(bcp)]PF 6 (Cu, xant = xantphos, bcp = bathocuproine). Emission quenching studies, transient absorption as well as IR spectroscopy provide information about the electron transfer paths of the intermolecular systems. It turned out that formation of double reduced species [Re 2 Cl 2 ] 2À along with an intermediate with a ReÀ Re bond ([ReRe]) can be taken as an indication of multi-electron storage capacity. Furthermore, under catalytic conditions a CO 2 -bridged intermediate was identified.

Sensitized Photocatalytic CO2 Reduction With Earth Abundant 3d Metal Complexes Possessing Dipicolyl-Triazacyclononane Derivatives

“…The photocatalytic activity of the prepared complexes was determined using Ir(dFppy) 3 and triethylamine (TEA) as sacrificial electron donor. In earlier studies we successfully applied the following conditions ( Giereth et al, 2021 ): 5 × 10 −5 M catalyst concentration in 5 ml DMF with 5 × 10 −5 M Ir as photosensitizer (PS) and 5 Vol% TEA as sacrificial electron donor (SR). The catalysis mixture is placed in front of a 200 W Hg-lamp equipped with a 400 nm low-wavelength cut-off filter to exclude UV light.…”

“…The catalysis mixture is placed in front of a 200 W Hg-lamp equipped with a 400 nm low-wavelength cut-off filter to exclude UV light. Ir was selected because of its ability to still absorb solar photons in the region of 400–460 nm ( Becker et al, 2020 ) and its potentially well suited (excited state) redox properties ( Table 1 ) ( Koike and Akita, 2014 ; Lee and Han, 2020 ; Giereth et al, 2021 ). For this work we just focused on the gaseous reduction products formed and did not investigate the formation of liquid products.…”

“…To investigate the influence of the type of PS, catalytic experiments were performed using Cu . This PS was chosen due to its similar absorption and redox features as compared to Ir ( Table 1 ) ( Giereth et al, 2021 ). Unexpectedly, the combination of Cu and Co-L N3 shows only a TON CO of 13 after 24 h of illumination, i.e., only a seventh part compared to the reaction using Ir .…”

“…Under the catalytically relevant reaction conditions, 5·10 −5 M Co-L N3 was present, and given a rate constant of 1.1·10 9 M −1 s −1 for oxidative Ir luminescence quenching, this leads to a pseudo first-order rate constant of 5.5·10 4 s −1 for photoinduced electron transfer from Ir to Co-L N3 . Given an inherent excited-state decay of 5·10 5 s −1 (lifetime of Ir of about 2 μs) ( Giereth et al, 2021 ) this implies that 11 in 100 photo-excitations (5.5·10 4 s −1 /5·10 5 s −1 = 0.11) will lead to electron transfer from Ir to Co-L N3 under the catalytically relevant conditions with 5·10 -5 M Co-L N3 .…”

“…Unexpectedly, the combination of Cu and Co-L N3 shows only a TON CO of 13 after 24 h of illumination, i.e., only a seventh part compared to the reaction using Ir. The much lower overall TON CO can be attributed to the rather fast photodecomposition of Cu (Giereth et al, 2021). In the first 2 hours of illumination, Cu actually shows a better performance than Ir (Supplementary Figure S25; TON CO of 9 vs. 3).…”