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A significant number of molecular catalysts have been developed for electrochemical CO 2 reduction with high efficiency and selectivity; however, testing of these electrocatalysts in an application-ready system is lacking. Here, we present an example of a nonaqueous flow cell electrolyzer with [Ni(cyclam)] 2+ as the homogeneous electrocatalyst for CO 2 reduction. Using ferrocene as a sacrificial electron donor and ammonium salts as both electrolyte and proton donor, efficient catalytic CO 2 reduction is achieved. The nonaqueous design shows high selectivity for the reduction of CO 2 to CO (>80%) and achieves high current densities with a graphite felt working electrode (up to 50 mA·cm -2 with 0.5 M proton donor in MeCN solution), producing >40 mL·h -1 of CO. The choice of a molecular electrocatalyst, solvent, and proton donor are the key factors for achieving high activity with an efficient flow electrolyzer and the eventual development of a viable continuous process.

The interligand charge dynamics of the lowest singlet metal-to-ligand charge-transfer states ( 1 MLCT S 1 states) of a series of quadruply bonded trans-Mo 2 (NN) 2 (O 2 C-X) 2 paddlewheel compounds are investigated, where NN is a π-accepting phenylpropiolamidinate ligand and O 2 C-X (X = Me, t Bu, T i PB, or CF 3 ) is an auxiliary carboxylate ligand. The compounds show strong light absorption in the visible region due to MLCT transitions from the Mo 2 center to the NN ligands. The transferred electron density was followed by femtosecond time-resolved infrared (fs-TRIR) spectroscopy with vibrational reporters such as the ethynyl groups on the NN ligands. The observed fs-TRIR spectra show that these compounds have asymmetric 1 MLCT S 1 excited states where the transferred electron mainly resides on a single NN ligand. The presence of interligand electron transfer (ILET) is suggested to explain the shape of the ν(C≡C) bands and the influence of auxiliary ligands and solvents on the interligand electronic coupling. The ILET in the 1 MLCT S 1 state is shown to be sensitive to the functional groups on the auxiliary ligands while being less responsive to changes in solvents.

Four photophysically interesting dimolybdenum paddlewheel compounds are synthesized and characterized: I and II contain amide ligand (N,3-diphenyl-2-propynamide), and III and IV contain thioamide ligand (N,3-diphenyl-2-propynethioamide). I and III are trans-Mo 2 L 2 (O 2 C-T i PB) 2 -type compounds, and II and IV are Mo 2 L 4 -type compounds, where O 2 C-T i PB is 2,4,6-triisopropylbenzoate. I-IV display strong light absorption due to metal to ligand charge transfer (MLCT) transitions from molybdenum to the amide/thioamide ligands. Charge transfer dynamics in the MLCT excited states of I-IV have been examined using femtosecond transient absorption (fs-TA) spectroscopy and femtosecond time-resolved infrared (fs-TRIR) spectroscopy. The asymmetric amide/thioamide ligands show two forms of regioarrangements in the paddlewheel compounds. Analyses of the ν(C≡C) bands in the fs-TRIR spectra of I and II show similar electron density distribution over ligands in their 1 MLCT S 1 states where only two amide ligands are involved and the transferred electron is mainly localized on one of them. The fs-TRIR spectra of III and IV, however, show different charge distribution patterns where the transferred electron is fully delocalized over two thioamide ligands in III and partially delocalized in IV. Fast interligand electron transfer (ILET) was recognized as the explanation for the various charge distribution patterns, and ILET was shown to be influenced by both the ligands and the ligand arrangements.