Synthesis and Electrochemical Study of Re(I) and Mo(0) Complexes with polypiridyl ligands for Photo- and Electrocatalytic Reduction of CO2

In recent years, the photo- and electrochemical conversion of CO2 to higher-energy products has been improved by employing transition metal coordination compounds with polypyridyl ligands (e.g. Ru, Os, Re, Mn [1,2]), in the form of molecular or supramolecular organometallic catalysts. In this perspective, our research has mainly focused on the synthesis of a series of novel Re(I)-carbonyl diimine complexes, to be efficiently used as catalysts in both photo- and electrocatalysis for CO2 reduction. In more detail, this new class of compounds is characterized by the presence of common polypyridyl ligands covalently attached to the highly fluorescent PNI cromophore (N-(diimine)-4-(1- piperidinyl) naphthalene-1,8-dicarboximide), which revealed to be able to provide a huge excited state lifetime enhancement in a Re(I) charge-transfer complex [3]. This feature may play a key role in the process of catalytic CO2 reduction (especially in photocatalysis). The spectroscopic characterization (optical and NMR) of the new samples was followed by the study of their electrochemical behaviour either in inert atmosphere and in presence of CO2, which was finally compared with that of Re(CO)3(bipy)Cl, taken as a reference compound [4]. Photocatalytic measurements are under progress. On the other hand, looking at a practical use of these systems, the usage of quite abundant first and second row transition metals instead of rare ones is very attractive. For example, to our knowledge there are no reports about the use of tetracarbonyl Mo(0) complexes for the same purpose. This reason lead us to synthesize also a series of [Mo(CO)4(L)] complexes (L = 2,2’-bipyridyl and derivatives), testing them for electrochemical reduction of carbon dioxide. Our first results are quite promising, showing certain activity and selectivity in reducing CO2 to CO.