Chapter 4. Fe-Mediated Nitrogen Fixation with a Metallocene Mediator: Exploring
6.2 E Electrochemical Behavior in the Presence of a Hydrogen Atom Acceptor
To explore the possibility of electrocatalytic PCET, we sought out a model substrate for which thermodynamics favored a PCET pathway. DFT calculations suggest that acetophenone is an ideal substrate. The calculated O–H bond strength of the radical intermediate is well-matched to that of our catalyst (BDFEO–H = 38.7 kcal mol−1, ΔG(PCET)
= 0 kcal mol−1), while both the ET to form the ketyl anion (ΔG(ET) = +26 kcal mol−1) and PT to form the oxonium (ΔG(PT) = +19 kcal mol−1) are significantly uphill ( Figure 6.9).
These calculated energies for electron transfer and proton transfer are consistent with the known experimental reduction potential and pKa of acetophenone.34,35 We were particularly interested in this substrate because recently it has been demonstrated that photochemical PCET strategies are a means of selectively forming pinacol products at potentials much more mild than traditional, ET routes.36,37
Figure 6.9: DFT calculated thermodynamics of PT, ET, and PCET for the reaction of acetophenone with the [(Cp)Co(CpNH)]+.
To test the suitability of acetophenone as a hydrogen-atom acceptor, we added 12 equiv of the ketone to the aforedescribed solution containing 1 mM [(Cp)Co(CpNH)][OTf], 100 mM [4-CNPhNH3][OTf], and 200 mM [TBA][PF6]. Cyclic voltammograms at 100 mV/s after the addition of the ketone lead to the observation of enhanced current (180% increase) and the complete loss of reversibility indicating a catalytic reaction. Control CV’s without [(Cp)Co(CpN)][OTf] do reveal slightly enhanced current densities relative to the background HER reaction but significantly less than the catalytic reaction at the relevant potential. To interrogate the product of these CV experiments, we performed CPE experiments.
Optimization of the CPE conditions ultimately led us to the use of a one-compartment cell with tosic acid (pKa of 8.45 in MeCN),38 and a glassy-carbon counter electrode. The one- compartment cell was necessary to avoid diffusion of the acetophenone out of the working compartment. The tosic acid was necessary due to undesirable anodic reactivity of the conjugate base of anilinium acids. Lastly, although reasonable results could be obtained with a Pt mesh counter electrode (35.7% FE for pinacol, 38.4% FE for H2, Table 4.1, Entry 1)
substantially higher turnover number (10.5 vs 41.6) could be achieved with a glassy- carbon counter electrode (Table 6.1). This improved performance with a glassy carbon counter electrode could be due to favorable adsorption of the ketone on the Pt electrode, as no ketone was recovered in this experiment despite the lower yield of pinacol.
Table 6.1. Results from CPE experiments on a DME solution of 200 mM [TBA][PF6], 100 mM tosic acid, 50 mM acetophenone at −1.45 V vs Ag+/0
Under our optimized conditions (200 mM [TBA][PF6], 100 mM tosic acid, 50 mM acetophenone, 1 mM [(Cp)Co(CpN)][OTf], BDD working electrode, and glassy carbon counter electrode) a CPE experiment at −1.45 V for ~53 hours with stirring results in the passage of 71.6 Coulombs (Figure 6.10). Analysis of the headspace by gas chromatography revealed the formation of H2 (45.0% FE). Analysis of the solution phase products by comparison of their GC-MS and GC-FID traces to authentic standards after work-up led to
Catalyst
Counter Electrode
Pinacol Yield
Ketone Recovery
Pinacol TON (FE)
H2 Yield
(FE) Q (C) 1 mM
[(Cp)Co(CpN)][OTf]
Pt 22% 5% 11
(36%)
12%
(38%) 21
1 mM
[(Cp)Co(CpN)][OTf]
Glassy Carbon
83% 11%
42 (39%)
48%
(45%)
72
1 mM [Cp2Co][PF6] Glassy Carbon
6% 0% 3
(3%)
66%
(62%)
72
none
Glassy Carbon
10% 0%
N/A (47%)
1%
(8%)
8
the identification of 83.2% yield for pinacol (39.0% FE) and 10.8% recovery of the ketone (Table 6.1, Entry 2). No 1-phenylethanol or other reduction products were identified.
Differential pulse voltammograms (DPV’s) taken of the bulk solution with a fresh electrode after the CPE experiment reveals redox features assignable to both the reduction and oxidation of [(Cp)Co(CpN)][OTf]. Furthermore, x-ray photoelectron spectroscopy measurements of the BDD electrode surface after the CPE experiment does not reveal the presence of any cobalt. These observations are consistent with catalyst stability under the CPE conditions.
Figure 6.10: Time course of the current (I) and charge (Q) in a CPE experiment at −1.45 Vvs Ag+/0 of a 200 mM [TBA][PF6], 100 mM tosic acid, 1 mM [(Cp)Co(CpN)][OTf] DME solution at a BDD working electrode with a glassy carbon counter electrode, and Ag wire reference electrode.
A CPE experiment for the same amount of time in the absence of our catalyst (Table 4.1, Entry 3) reveals minimal pinacol formation (10.4% yield) and H2 formation (1.0% yield).
The low conversion is consistent with the minimal currents observed in the CV experiments.
Intriguingly, no ketone is observed suggesting that the ketone may become adsorbed or degraded over long-times in the absence of a catalytic reaction promoting its conversion. A
CPE experiment performed with 1 mM [Cp2Co][PF6] (Table 6.1, Entry 4) instead of our base-appended cobalt catalyst, revealed slightly enhanced rates for HER (65.8% yield and 62.0% Fe for H2) and a slight deterioration relative to the background for pinacol formation (5.6%). This is consistent with our hypothesis that the protonated Cp2Co species is not the predominant reactive species under these conditions.
To mechanistically interrogate the [(Cp)Co(CpN)]2+ mediated catalysis, we returned to performing cyclic voltammograms. For mechanistic experiments, we continued to use [4-
CNPhNH3][OTf], as the similar pKa of [(Cp)Co(CpNH)][OTf]2 and tosic acid (8.6 and 8.45 in MeCN respectively) complicates the analysis due to incomplete protonation of the Co catalyst. Titrating acetophenone into a DME solution of 200 mM [TBA][PF6], 100 mM [4-
CNPhNH3][OTf], and 1 mM [(Cp)Co(CpN)][OTf] results in S-shaped CV’s with increasing current density (representative CV shown in Figure 6.11).39 Plotting the plateau current observed after subtraction of the background current against the concentration of acetophenone reveals a first-order dependence on ketone (Figure 6.11).
Figure 6.11: (left) Cyclic voltammograms of 1 mM [(Cp)Co(CpN)][OTf] at 10 mV/s in the presence of 100 mM [4-CNPhNH3][OTf] and the presence (red) and absence (blue) of 50 mM acetophenone. (right) Plot of the dependence of the plateau current on the ketone concentration with a line of best fit.
Titration of [(Cp)Co(CpN)][OTf] into a DME solution of 100 mM [4-CnPhNH3][OTf], 50 mM acetophenone, and 200 mM [TBA][PF6] led to observation of a first order dependence on Co concentration. Although these data indicate a rate-limiting reaction between Co and acetophenone, we find that CV’s of a DME solution of 1 mM [(Cp)Co(CpN)][OTf] and 50 mM acetophenone in the absence of acid demonstrates no evidence of an interaction between these two species. In contrast, CV’s of [(Cp)Co(CpNH)][OTf]2 at 100 mV/s in the presence of acetophenone (increasing from 0 to 50 equiv) leads to observation of an increasing amount of the product of H-atom transfer, [(Cp)Co(CpN)][OTf] (Figure 6.12), behavior which is not observed in the absence of ketone.
Thus, we conclude that the cumulative evidence support a mechanism involving a rate limiting reaction between [(Cp)Co(CpNH)][OTf] and acetophenone. We suggest that this reaction is a concerted proton-electron transfer (CPET).
Figure 6.12: (left) Plateau current dependence on the concentration of [(Cp)Co(CpN)][OTf]
in the reaction with 100 mM [4-CNPhNH3][OTf] and 50 mM acetophenone in a 200 mM [TBA][PF6] solution of DME at 100 mV/s. (right) Repeated cyclic voltammograms in 200 mM [TBA][PF6] DME solution of [(Cp)Co(CpNH)][OTf]2 at 100 mV/s with increasing equivalents of acetophenone (0 to 50 mM).