Elucidation of Cooperativity in CO₂ Reduction Using a Xanthene-Bridged Bimetallic Rhenium(I) Complex

Abstract

Mechanistic studies on dinuclear complexes that can activate CO 2 are rare. Based on the investigations done for the mononuclear compound (bpy)Re(CO) 3 Cl (bpy-Re, with bpy = 2,2′-bipyridine), many reports favor a mononuclear catalytic cycle, while the possibility of a dinuclear catalytic species is discussed in the literature in only a few cases. Here, we report the synthesis and characterization of a homobimetallic rhenium(I) compound, in which two (bipyridine)Re(CO) 3 Cl fragments are brought into close vicinity by attaching them to a xanthene backbone. First, photocatalytic investigations show a significant increase of the catalytic performance compared to the mononuclear parent compound. Second, spectroelectrochemical experiments demonstrate the remarkable fast formation of an intermediate with a Re−Re bond that forms upon reduction of the starting compound, but which is not able to activate CO 2 . Third, spectroscopic investigations under (photo)catalytic conditions were performed to shed light on the crucial intermediates emerging in the reaction cycle. The assignment of these intermediates is assisted by extensive density functional theory calculations. As a result, the enhanced photocatalytic activity is reasoned by inhibition of deactivation channels and a cooperative reaction mechanism, in which one metal center functions as a photosensitizer to assist the second, catalytically active, metal.

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

“…Compound Re 2 Cl 2 was synthesized according to an already published method. [20] As outlined in the introduction, the goal of the recent studies is to achieve an efficient and fast formation of higher reduced species of Re 2 Cl 2 , i. e. [Re 2 Cl 2 ] nÀ . Since the prerequisite for the formation of doubly-reduced Re 2 Cl 2 by an external photosensitizer is a) a high reductive power in the ground-state (c.f.…”

“…Initial studies by us revealed that the accumulation of two electrons on Re 2 Cl 2 by means of photoinduced electron transfer reactions from sacrificial electron donors is hampered by a strong electronic communication between the two metal fragments. [20] However, if the system was reduced twice (e. g. by [a] Ir 200 200 1 154 4 [a] Ir 200 400 1 131 5 [a] Ir 400 200 1 134 6 [a] Ir 400 400 1 193 7 [a] Ir 1 eq. Ir, 5 % TEA; blue: 1 eq.…”

“…Reductive bond cleavage within this intermediate can therefore be exploited to access up to 4-fold reduced species, as has been evidenced by (spectro-)electrochemical studies. [20] In order to investigate the reason for the observed catalytic results presented above, mechanistic studies focusing on the electron transfer pathways after light excitation were performed. The addition of an external photosensitizer certainly complicates mechanistic studies and adds complexity to the investigations because multiple reaction pathways are conceivable (Scheme 2).…”

“…[19] We therefore synthesized and thoroughly characterized a new xanthene-bridged dinuclear Re 2 Cl 2 complex, where special attention was given to the spatial orientation of the two Re(bpy)(CO) 3 Cl units. [20] Two isomers, cis and trans, were synthesized and only latter (shown in Scheme 1) investigated in more detail due to possible interconversion of the cis to the trans isomer. Although increased photo-as well as electrocatalytic activity compared to mononuclear bpy-Re was observed, the full potential of Re 2 Cl 2 (always referring to the trans isomer in the following) in photocatalysis could not fully be exploited yet.…”

“…Initial studies revealed that the accumulation of two electrons on Re 2 Cl 2 by photoinduced electron transfer reactions from sacrificial electron donors is hampered by a strong electronic communication between the two metal fragments. [20] However, if the system was reduced twice (e. g. by electrochemical reduction), a cooperative behavior was observed, as evidenced by a considerably lowered onset potential for electrocatalysis.…”