Catalyst Protonation Changes the Mechanism of Electrochemical Hydride Transfer to CO2

Abstract

It is well-known that addition of a cationic functional group to a molecule lowers the necessary applied potential for an electron transfer (ET) event. This report studies the effect of a proton (a cation) on the mechanism of electrochemically driven hydride transfer (HT) catalysis. Protonated, air-stable [HFe₄N(triethyl phosphine (PEt₃))₄(CO)₈] (H4) was synthesized by reaction of PEt₃ with [Fe₄N(CO)₁₂]^- (A ^-) in tetrahydrofuran, with addition of benzoic acid to the reaction mixture. The reduction potential of H4 is -1.70 V vs SCE which is 350 mV anodic of the reduction potential for 4 ^-. Reactivity studies are consistent with HT to CO₂ or to H⁺ (carbonic acid), as the chemical event following ET, when the electrocatalysis is performed under 1 atm of CO₂ or N₂, respectively. Taken together, the chemical and electrochemical studies of mechanism suggest an ECEC mechanism for the reduction of CO₂ to formate or H⁺ to H₂, promoted by H4. This stands in contrast to an ET, two chemical steps, followed by an ET (ECCE) mechanism that is promoted by the less electron rich catalyst A ^-, since A ^- must be reduced to A ^2- before HA ^- can be accessed.

Chart 1. Compound numbering system used in the text. The cation in each case is Et₄N⁺. Compounds A^–, HA, 1^–, 2^–, and 4^– have been previously reported.^,

Scheme 1. Proposed Mechanisms for Formate (HCO₂^–) Formation under 1 atm CO₂ (Left) by A^–, 1^–, and 2^– Following an ECCE Mechanism; and (Right) by H4 Following an ECEC Mechanism1

The catalytic cycle for both schemes starts at the top.

Scheme 2. Synthesis of H4 and Reaction Chemistry with Brønsted Acids and CO₂

Figure 1

Solid-state structure of H4. Green, blue, pink, gray, and red ellipsoids, and white sphere represent Fe, N, P, C, O, and H atoms, respectively. Ellipsoids shown at 50%; H atoms omitted except for hydride.

Figure 2

(Left) CV’s of 0.1 M Bu₄NBF₄ MeCN solution under 1 atm N₂ at 0.1 V s^–1 (gray); with added 0.1 mM H4 (black); and with the scan direction reversed at −1.5 V (dotted). (Right) Normalized DPV’s of H4 (blue) and of 4^–, 2^–, 1^–, and A^– Glassy carbon (GC) working electrode.

Figure 3

CV’s of 0.1 mM H4 in 0.1 M Bu₄NBF₄ MeCN: (left) under N₂ (black), in MeCN/H₂O (95:5) under N₂ (blue), and under 1 atm CO₂ in MeCN/H₂O (95:5) (red) at 0.1 V s^–1; (right) variable scan rate data collected under 1 atm CO₂ from 0.1–1 V s^–1 suggests HT following ET. Insets: Plot of E_P vs scan rate (υ). GC working electrode.

Figure 4

Correlation between E_p vs ν_CO; for A^–, 1^–, 2^–, 4^– (black), and H4, and HA (blue). E_p values obtained from DPV experiments (Figure 2 right and ref (13)).