The bimetallic N4-macrocyclic complex ([CuI(Xantphos)FeII(Mabiq)(OTf)2]) is active in the photocatalytic reduction of CO2, even in the absence of a photosensitizer.1 In contrast, the monometallic ([FeII(Mabiq)(MeCN)2]OTf) requires the addition of a Ruthenium photosensitizer. The differing behavior inspired us to examine the photophysics and -chemistry of these two compounds, using a combination of ultrafast transient absorption spectroscopy and spectroelectrochemistry.2
Both compounds display short excited-state lifetimes in the range of 10-20 ps. However, marked differences in the photoreduction in the presence of BIH as sacrificial electron donor were observed. For both complexes, two consecutive reductions are observed which are significantly faster for the CuFe-Mabiq complex than for the Fe-Mabiq complexes (6.5 min vs. 130 min). The two-electron reduced form of the monometallic complex also acquires a proton from the BIH⦁+, while the bimetallic complex remains unprotonated. Moreover, the three-electron reduced form is accessible for the bimetallic complex, yielding a formal CuIFe-1 complex. These fast transformations are likely the decisive factor, making the CuFe-Mabiq complex active in CO2 reduction without the requirement of a photosensitizer. We are currently exploring the effect of different metals on the redox properties and photochemistry of Fe-Mabiq complexes.