Unraveling Bifurcating Pathways for CO and HCOOH Formation: Insights from Stopped-Flow FTIR Spectroscopy of a Second-Sphere Modified Mn Catalyst

Abstract

Manganese bipyridine tricarbonyl complexes show high efficiency and selectivity in electrochemical CO₂ reduction (e-CO₂RR) to CO. Efforts to shift selectivity toward HCOOH have been made by introducing second-sphere hydroxyl or amine functional groups and using amines or proton-coupled electron transfer (PCET) mediators. However, the direct spectroscopic evidence for the bifurcation pathways leading to CO and HCOOH remained elusive. Using stopped-flow mixing with decamethyl cobaltocene reductant and time-resolved infrared (TRIR) spectroscopy, we identified, for the first time, the key intermediates in this bifurcation pathway for an Mn complex with second-sphere hydroxyl groups in real time under catalytic conditions. The measured rate constants align with reported TOF values from electrochemical studies, validating the relevance of the results to e-CO₂RR conditions. Our findings reveal that HCOOH production involves proton transfer from hydroxyl groups to the doubly reduced Mn center, forming the Mn-hydride intermediate, followed by CO₂ insertion, leading to the Mn-formate intermediate. However, the inability of the resulting phenolate to rebind protons from weak acids like water leads to rapid catalyst degradation, limiting sustained catalysis. This work provides mechanistic insights and paves the way for designing molecular catalysts with enhanced selectivity and stability for HCOOH production during e-CO₂RR.

1

(A) The proposed mechanistic cycle of e-CO₂RR catalyzed by Mn­(bpy-R)­(CO)₃X complex; the intermediate at the bifurcation point of the CO and formate pathways, [Mn­(bpy-R)­(CO)₃]⁻, 2 ^– , is highlighted in a purple box. (B) Chemical structures of the complexes discussed in this paper.

2

Cyclic voltammograms of complex 5 (1 mM) in acetonitrile under argon (black) and CO₂ (brown) atmosphere. Scan rate: 100 mV/s, working electrode: glassy carbon, counter electrode: Pt. Here, tetra-butyl ammonium hexafluoro phosphate was used as a supporting electrolyte.

3

(A) TRIR difference spectra of 0.5 mM of complex 5, showing spectral changes upon adding 2.6 mM CoCp₂* in CO₂-saturated anhydrous acetonitrile. (B) A zoomed-in section highlighting the appearance and disappearance of various intermediate species. In these figures, the first spectrum was taken as the background and was subtracted from the rest of the data. Therefore, the decay of the species present at the initial time (within the time resolution of the instrument) and the formation of new species are represented as negative bands and positive bands, respectively. (C) FTIR spectra showing the vibrational features of complex 5 (in black), the species present at the initial time (0 s, in red), and at 23.15 s (in sky blue) during the same stopped-flow TRIR experiment as (A). Note that the FTIR spectrum of complex 5 (in black) was obtained from a regular FTIR measurement.

4

Kinetic traces of (A) 2 ^– (purple), 6-H ⁺ (orange), and (B) 2H–H ⁺ (turquoise) during the reaction of 0.5 mM 5 and 2.6 mM CoCp₂* in CO₂-saturated anhydrous acetonitrile. (B) Chemical structures of the intermediates, 2 ^– , 6-H ⁺ , and 2H–H ⁺ . The rate constant values are mean values obtained from at least three individual experiments; standard deviations are given in the text.

5

Kinetic traces of (A) 4 (red), (B) CO (blue), and (C) 5-H ⁺ (green) during the reaction between 0.5 mM 5 and 2.6 mM CoCp₂* in CO₂-saturated anhydrous acetonitrile. (D) Chemical structures of the intermediates, 4, and 5-H ⁺ . The rate constant values are mean values obtained from at least three individual experiments; standard deviations are given in the text.

6

(A) TRIR difference spectra of 0.5 mM of complex 5 in acetonitrile solution showing the spectral changes upon adding 2.6 mM CoCp₂* in CO₂-saturated acetonitrile containing 5% H₂O. (B) Zoomed-in section of (A). Kinetic traces of (C) 2 ^– (purple), 6-H ⁺ (orange), and (D) 2H–H ⁺ (turquoise). In these figures, the decay of the species present at the initial time (within the time resolution of the instrument) and the formation of new species are represented as negative bands and positive bands, respectively. The rate constant values are mean values obtained from at least three individual experiments, standard deviations are given in the text.

7

DFT optimized (b3lyp/def2-tzvp with GD3BJ and CPCM/MeCN) geometry of (A) [Mn­(dhbpy)­(CO)₃]⁻ species (2 ^– ) and (B) [Mn­(dhbpy)­(CO)₃]⁻ species (2 ^– ) in the presence of a nearby H₂O molecule. The H-bonding distance from one of the pendant O–Hs to the reduced Mn­(–I) center is shown here.