Recent reports show [FeFe] hydrogenase mimics are active for the electrochemical reduction of CO2 to formate (HCOO−). Herein, the electrochemical reduction of CO2 with the [FeFe] hydrogenase mimic [Fe2(μ‐pdt)(CO)6, 1, where pdt = propane‐1,3‐dithiolate] in acetonitrile is reported. In the presence of the weak acid, methanol (MeOH), 1 reduces CO2 to both CO (Faradaic Efficiency maximum [FEmax] of 16 ± 6%) and HCOO− (FEmax = 20%) and produces H2 (FEmax = 56 ± 4%). Without added MeOH, 1 reacts with adventitious water to form H2 (FEmax = 85 ± 1%), HCOO− (FEmax = 7.8%), and CO (FEmax = 7 ± 3%) with CO32− being detected by infrared spectroscopy.  Product formation is potential dependent: more negative potentials increases selectivity for HCOO− over CO. The first reduction of 1 forms a pdt‐bridged dimer, 2. However, the reduction of 2 at the potentials required for electrochemical CO2 reduction leads to two new species. Using density functional theory, and infrared spectroelectrochemistry (IR‐SEC), these structures are identified to be [Fe(CO)4]2− (3) and a trinuclear Fe3 species (4). While these species can reduce CO2 to CO and HCOO−, the predominant formation of H2 reveals kinetic issues in  CO2 reduction. The work offers to consider alternate competing mechanistic pathways and explains the lack of product selectivity when using hydrogen evolution reaction catalyst for CO2 reduction to HCOO−.

We describe the synthesis and characterization of a versatile platform for gold functionalization, based on self-assembled monolayers (SAMs) of distal-pyridine-functionalized N-heterocyclic carbenes (NHC) derived from bis(NHC) Au(I) complexes. The SAMs are characterized using polarization-modulation infrared reflectance-absorption spectroscopy, surface-enhanced Raman spectroscopy, and X-ray photoelectron spectroscopy. The binding mode is examined computationally using density functional theory, including calculations of vibrational spectra and direct comparisons to the experimental spectroscopic signatures of the monolayers. Our joint computational and experimental analyses provide structural information about the SAM binding geometries under ambient conditions. Additionally, we examine the reactivity of the pyridine-functionalized SAMs toward H₂SO₄ and W(CO)₅(THF) and verify the preservation of the introduced functionality at the interface. Our results demonstrate the versatility of N-heterocyclic carbenes as robust platforms for on-surface acid-base and ligand exchange reactions.

In this work, we investigate trion dynamics occurring at the heterojunction between organometallic molecules and a monolayer transition metal dichalcogenide (TMD) with transient electronic sum frequency generation (tr-ESFG) spectroscopy. By pumping at 2.4 eV with laser pulses, we have observed an ultrafast hole transfer, succeeded by the emergence of charge-transfer trions. This observation is facilitated by the cancellation of ground state bleach and stimulated emission signals due to their opposite phases, making tr-ESFG especially sensitive to the trion formation dynamics. The presence of charge-transfer trion at molecular functionalized TMD monolayers suggests the potential for engineering the local electronic structures and dynamics of specific locations on TMDs and offers a potential for transferring unique electronic attributes of TMD to the molecular layers.

Abstract: In this work, we investigate trion dynamics occurring at the heterojunction between organometallic molecules and a monolayer transition metal dichalcogenide (TMD) with transient electronic sum frequency generation (tr‐ESFG) spectroscopy. By pumping at 2.4 eV with laser pulses, we have observed an ultrafast hole transfer, succeeded by the emergence of charge‐transfer trions. This observation is facilitated by the cancellation of ground state bleach and stimulated emission signals due to their opposite phases, making tr‐ESFG especially sensitive to the trion formation dynamics. The presence of charge‐transfer trion at molecular functionalized TMD monolayers suggests the potential for engineering the local electronic structures and dynamics of specific locations on TMDs and offers a potential for transferring unique electronic attributes of TMD to the molecular layers.