Dissection of Electronic Substituent Effects in MultielectronâMultistep

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Dissection of Electronic Substituent Effects in MultielectronMultistep Molecular Catalysis. Electrochemical COto-CO Conversion Catalyzed by Iron Porphyrins 2

Iban Azcarate, Cyrille Costentin, Marc Robert, and Jean-Michel Savéant J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09947 • Publication Date (Web): 15 Nov 2016 Downloaded from http://pubs.acs.org on November 18, 2016

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The Journal of Physical Chemistry

Dissection of Electronic Substituent Effects in MultielectronMultistep Molecular Catalysis. Electrochemical CO2-to-CO Conversion Catalyzed by Iron Porphyrins Iban Azcarate, Cyrille Costentin*, Marc Robert* and Jean-Michel Savéant* Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire, Unité Mixte de Recherche Université - CNRS N° 7591, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France. [email protected], [email protected], [email protected] 33(0)157278796

, 33(0)157278790

, 33(0)157278795

ABSTRACT. Redox pairs of transition metal complexes are often involved in small molecule activation in response to modern energy challenges as well as in other areas of electrocatalysis. Within such family of molecular electrocatalysts, ligand substitution is a means of varying catalytic efficiency, best gauged through catalytic Tafel plots relating overpotential and turnover frequency. In practice, efficient molecular catalysis involves multielectron-multistep processes. It is in this framework that we discuss through-structure inductive substituent effects. What is the best choice for the reference thermodynamic index, how the global substituent effect may be expressed as a function of this index and how it may be dissected into individual effects assigned to each of the reaction steps are challenging questions that are addressed and resolved here for the first time. The discussion is illustrated by the effect of successive phenyl perfluoration and of ortho-ortho’-methoxy substitution of the FeI/0 tetraphenylporphyrin catalysts of the CO2-to-CO electrochemical conversion. Consequences on the relative position of the catalytic Tafel plots are also examined. This analysis of through-structure electronic effects is a necessary preliminary to the investigation of substituent through-space effects (electrostatic, H-bonding) because, albeit of different nature, they may occur simultaneously. Investigation of these two aspects of substituent effects and of the rules that emerge thereof pave the way to future imaginative design of catalysts for the CO2-to-CO-conversion and also for any other molecular catalytic reactions. Introduction Molecular catalysis of electrochemical reactions currently attracts a lot of attention, notably concerning the transformation of small molecules in response to issues raised by modern energy challenges 1,2,3,4,5,6,7,8 (oxidation

9,10,11,12,13,14,15,16,17

and of carbon dioxide.

and reduction

18,19,20,21,22,23

28,32,33,34,35,36,37,38,39,40

of water, reduction of dioxygen

24,25,26,27,28,29,30,31

These reactions implement redox pairs, mostly involving

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transition metal complexes, whose electrode-generated reduced or oxidized form is the active catalyst molecule. The production of the target molecules is coupled with electron transfer regeneration of the active catalyst, directly at the electrode or through a series of steps that ultimately involves the electrode. 41

Analysis of substituent effects is relatively straightforward when catalysis consists of a single outersphere electron transfer between the electrode and the catalyst and between the electron transferactivated form of the latter and the substrate as pictured in figure 1. In practice catalysis rather involves

−

e−

cat ox

electrode

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

cat red

substrate

product

solution

Fig. 1. Single electron transfer molecular catalysis scheme. multistep-multi-electron processes. Many such catalytic reactions are (for reductions) push-pull reactions 42

(for oxidation “push” and “pull” are reversed) in which the first step (or series of steps) pushes

electrons into the substrate and the second step (or series of steps) pulls out electrons with the help of a Brönsted or Lewis acid species 43,44,45,46 (viz. basic species for oxidations) and of bond-breaking reactions up to the formation of the products and regeneration of the catalyst as in the case of, e.g. , the catalyzed CO2-to-CO electrochemical conversion as represented in Scheme 1 (the quantitative aspects involving the parameters shown in the yellow rectangle will be discussed later on). This reaction scheme exemplifies cases where the role of the acid is both to stabilize the primary intermediate by means of H-bonding and to provide the required protons. Generally speaking, the cleavage reaction that regenerates the catalyst may involve several successive elementary steps. In all these cases, the stepwise versus concerted character of the elementary events should also be documented.

47,48,49

It follows that mechanism analysis

is a necessary task when investigating substituents effects. Two broad categories of substituent effects


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may be envisioned: through-structure (electronic, inductive or resonant) effects and through-space effects. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

In the latter case, the substituent exerts a direct influence (e.g., through electrostatic charge interactions)

0 Ecat 0 (1) K1 ∆G1

k2

0 (2) K2 ∆G2

k3

0 (3) K3 ∆G3

0 (4) K4 ∆G4

(

0 0 0 ∆Gtot = 2 F Ecat − ECO

kcat

2 /CO

= ∆G10 + ∆G20 + ∆G30 + ∆G40

) (2)

Scheme 1. Mechanism of the CO2-to-CO reaction catalyzed by electrogenerated iron(0) porphyrins. Capital Kn s, lower case kn s, and ∆Gn0 are the equilibrium constants, rate constants and standard free energies of the subscript reactions respectively.

on reaction intermediates, but may well exert an electronic effect simultaneously. This is one reason that we focus, in the present contribution, on a detailed analysis of through-structure electronic effects. It renders possible to further address through-space electrostatic substituent effects on sound backgrounds. Another reason for analyzing these through-structure electronic substituent effects is that it provides the opportunity to emphasize the interest of catalytic Tafel plots for benchmarking catalyst in a rational way. Even if pretty obvious for some time, the need for a lucid benchmarking of catalysts in front of their rapidly growing number has been the object of a recent strong multi-authored statement.

50

Comparison

of catalytic Tafel plots (CTP) is one such rational way for benchmarking molecular catalysts. First introduced a few years ago, 51,52,53,54 CTPs correlate the turnover frequency, TOF , to the overpotential, η.

(

)

0 - E (+ for reductions, - for oxidations, E is the electrode potential at The latter is defined as η = ± Et,r

0 which the electrolysis is carried out, Et,r is the standard potential of the target reaction, i.e., the couple


3


substrate 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

+

0 E t0, r = E CO

co-reactants/

/CO,DMF 2

products.

In

the

case

= -0.74 V vs. SHE (see SI).

of

55,56

the

CO2-to-CO

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conversion

catalysis,

TOF is defined as TOF = N product / Nactive cat ,

where Nproduct is the number of moles of product generated per unit of time and Nactive cat the maximal number of moles of active catalyst present in the reaction-diffusion layer and not in the whole volume of the solution. The notion of reaction-diffusion layer is central in the analysis of fast electrochemical catalytic processes. This layer is the portion of space adjacent to the electrode surface in which all of catalysis takes place. The reaction-diffusion layer is the result of a steady-state state established by mutual compensation of the diffusion of the catalyst and the reaction it undergoes.

41

The fact that

catalysis involves the catalyst molecules located in the reaction-diffusion layer rather than in the whole volume of the solution ensures that the so-defined benchmarking is independent from cell particularities such as the electrolyte volume as should be the case for a rational benchmarking protocol.

38,45

Figure 2

shows examples of CTPs in which have been represented, for simplicity, catalytic processes in which the initial electron transfer step is reversible and so fast as to obey the Nernst law, thus log TOF

η

Fig. 2. Sketches of crossing and non-crossing catalytic Tafel plots (see text). The black, red and magenta non-crossing curves illustrate the case where there is a linear correlation between 0 entailed with a -1 correlation coefficient. ( RT ln10 / F ) log TOFmax and Ecat leading to: TOF =

0 (where Ecat

TOFmax

(1)  F  F  0 0  1 + exp  E − Ecat  × exp  − η  RT t ,r   RT  is the standard potential of the catalyst couple and TOFmax is the maximal catalytic rate

(

)

constant asymptotically reached for large overpotentials, noting however that different CTPs with different slopes and locations would arise with other mechanisms and kinetics). In all cases, ACS Paragon Plus Environment

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representative points of the good catalysts are located on the top left of the TOF-η diagram and on the 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

bottom right corner for the least efficient catalysts. CTPs also allow compromising between rapidity of product formation and energy costs. Besides CO2-to-CO conversion, reported for H2-evolution molecular catalysts.

57,58

38,51

CTPs have been recently

As seen in reference 57, electron withdrawing

substituents are favorable in terms of η and unfavorable in terms of TOFmax (and vice versa for electrondonating substituents). This is what we intuitively expect if we content ourselves with an analysis based exclusively on the stability of the initial adduct: electron-withdrawing groups favorably shift the standard potential of the catalyst toward positive values, but in doing so, decrease the basicity (Brönsted or Lewis) of the reduced form of the catalyst and therefore its reactivity in the product formation process. Symmetrical considerations apply to oxidation processes. This approach is at best grossly qualitative and may even be in error. A much deeper analysis is needed, detailing the effect of substituents on each step of the catalytic reaction. An additional pending question is the possible correlation that may exist between these elementary substituent effects, resulting or not in crossings of the CTPs as sketched in figure 2. 59,60,61

An interesting limiting case is shown in figure 2 (black, red and magenta curves) in which a linear

0 would exist and be endowed with a coefficient correlation between ( RT ln10 / F ) log TOFmax and Ecat

close to -1 resulting in non-crossing CTPs. We discuss these points in the following sections, taking as illustrative example catalysis of CO2-to-CO conversion by Fe0 porphyrins shown in Chart 1 with variable amounts of phenol acting as cosubstrate.


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Chart 1. Molecular structure of the substituted iron porphyrin catalysts. In the first of the three substituted molecules the phenyl groups of tetraphenylporphyrin (TPP) were replaced successively by one, two and four perfluorinated phenyl rings so as to obtain a progressive increase of electron-withdrawing effect from the iron porphyrin molecule. In the fourth molecule, the eight ortho, ortho’ methoxy substituents of the phenyl rings are intended to oppositely provide a strong electron-donating effect. As pictured in Chart 1, the phenyl rings are not coplanar with the porphyrin ring. The consequence is that the expected through-structure substituents effects are of inductive rather than of resonant nature. Results Figure 3 shows a catalytic cyclic voltammetric response typical of the series of porphyrins in Chart 1,


6

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


i (µ A)

50 FeI + e-

45

Fe0

Fe 0 + CO2+ 2PhOH

40

kcat 35

FeI ICO + 2PhO- + H2O

30 Fe0

25

2 FeI + CO

20 15 10 5 0 -5

i (µ A)

2

FeI + e-

FeII + e-

Fe0

FeI

1 0 -1

FeII CO

FeI + CO - e-

E (V vs.SHE) -2 -0.4

-0.6

-0.8

-1

-1.2

-1.4

-1.6

Fig. 3. Cyclic voltammetry of 1 mM FeF10TPP at a Hg working electrode in DMF + 0.1 M NBu4PF6 + 0.1 M H2O under argon (blue) and under 1 atm. CO2 in the presence of 1 M phenol (red) at 0.1 V/s. taking FeF10TPP as example. In the absence of CO2, two reversible waves are seen corresponding successively to the FeII/I and FeI/0 redox couples, from the second of which the standard potential 0 , can be determined (see Table 1). Under 1 atm. CO and in the presence characterizing the catalyst, E cat 2

of phenol, a very large catalytic current appears at the level of the FeI/0 couple. Another interesting feature appears on the CV represented in Figure 3, namely the significant negative shift of the FeI oxidation wave after the scan has been reversed past the cathodic catalytic wave. This observation is indicative of the formation of CO in large amounts (see references 38, 51 and the change of the FeII/I CV response of the same iron porphyrins upon addition of CO as shown in the Supporting Information (SI)). Confirmation and quantitation of CO formation was then obtained from preparative scale electrolysis experiments (see SI for experimental details), whose typical results are summarized in Figure 4. CO is formed in practically 90% Faradaic yield with almost negligible production of dihydrogen.


7

The Journal of Physical Chemistry a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

140

b Faradaic yield (%)

Faradaic yield (%)

120

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140 120

100

100

80

80

60

60

40

40

20

20

0

0

1

2

3

4

0

Time (hours)

Time (hours)

-20

0

1

2

3

4

5

-20 6

Fig. 4. Faradaic yields of the production of CO (red dots) and H2 (blue dots) upon preparative-scale electrolysis in DMF + 0.1 M NBu4PF6 in the presence of 1 atm. CO2 of: a: 0.5 mM FeF10TPP + 1 M PhOH at -1.26 V vs. SHE (Hg working electrode, current density 0.2 mA/cm2); b: 1 mM FeMeO8TPP + 0.2 M PhOH + 0.1 M H2O at -1.66 V vs. SHE (glassy carbon working electrode, current density 0.17 mA/cm2). Comparing the way in which catalysis varies with phenol concentration for each iron porphyrin of Chart 1 then allows for gauging the effect of substituents in a more precise manner. This is shown in figure 5 in which series of catalytic responses obtained at various PhOH concentrations have been gathered. The corresponding information concerning FeTPP was obtained from previous studies. 38,51 With the exception of FeMeO8TPP, the CV responses were closely similar on glassy carbon (GC) and mercury electrode as tested for many conditions, indicating that mercury does not interfere in the catalytic process we are interested in. For these three iron porphyrins, the Hg electrode rather than the GC electrode was systematically used because it simplifies the determination of the catalytic rate constants when the scan rate had to be raised to minimize the secondary phenomena (see below). The situation is different with FeMeO8TPP where the CVs responses on Hg show some instability upon raising the phenol concentration, leading us to use the GC-electrode results in this case. We also note with the later porphyrin, that sizable catalytic currents are observed at high phenol concentrations in the absence of CO2 (see Figure S2 in the SI), corresponding presumably to H2 evolution-reduction of PhOH. It follows that the analysis of the data obtained at [PhOH] > 0.2 M cannot be used reliably to analyze the kinetics of the CO2-to-CO conversion in this PhOH concentration range. The bottom Figure 5 clearly indicates the formation of CO in each case. It indeed shows that the FeI/FeII reoxidation anodic wave that results from the presence of the CO that has been formed along the ACS Paragon Plus Environment

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CO2 catalytic reduction wave increases together with phenol concentration. This is also what happens 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with FeMeO8TPP below the aforementioned limit of 0.2 M. In order to definitely establish this point, a FeF10TPP

FeF20TPP -0.3 -0.5 -0.7 -0.9 -1.1 -1.3 -1.5 25

-0.4 -0.6 -0.8

-1 -1.2 -1.4 -1.6

55

i

140

i0p

130

45

20

15

40

110

35

100

i

i

i0p

i0p

70 60 50

90 80

25

70

20

60

10

40 30

50

15

0

40

10

30

5

20

20 10

10

0

0 -5

E (V vs.SHE)

-5 2

-0.6 -0.8 -1 -1.2 -1.4 -1.6 -1.8 -2 -2.2 80

120

30

5

FeMeO8TPP

-0.5 -0.7 -0.9 -1.1 -1.3 -1.5 -1.7 150

i 50

i0p

FeF5TPP

E (V vs.SHE) -10

0

-10

E (V vs.SHE)

-20

E (V vs.SHE) -10

i

i

i

i

i0p

i0p

i0p

i0p

2

2

2

1

1

1

1

0

0

0

0

-1

-1

-1

-1

E (V vs.SHE) -2

E (V vs.SHE) -2

-0.3 -0.5 -0.7 -0.9 -1.1 -1.3 -1.5

E (V vs.SHE) -2

-0.4 -0.6 -0.8

-1 -1.2 -1.4 -1.6

E (V vs.SHE) -2

-0.5 -0.7 -0.9 -1.1 -1.3 -1.5 -1.7

-0.6 -0.8 -1 -1.2 -1.4 -1.6 -1.8 -2 -2.2

Fig. 5. Cyclic voltammetry of the substituted iron porphyrin of Chart 1 (conc.: 1 mM) in DMF + 0.1 M NBu4PF6 + 0.1 M H2O, at 0.1 V/s under argon (black) and under 1 atm. CO2 in the presence of PhOH, conc. (M): 0.005 (pink), 0.01 (blue), 0.02 (red), 0.05 (green), 0.1 (yellow), 0.2 (purple), 0.5 (orange), 1.0 (light blue), 1.5 (light green), 2.0 (magenta), 2.5 (grey), 3.0 (brown), 3.5 (white), 4.0 (cyan). The bottom figures are a blow-up of the upper figures showing the formation of CO on the reverse scan (see text). The current axis is normalized toward the peak current i 0p of a one-electron reversible wave at the same concentration at same scan rate (0.1 V/s) as can be obtained from the FeII/I wave. Hg working electrode in the first three cases, GC working electrode in the last. preparative-scale experiment was carried out at this phenol concentration. The results displayed in figure 4b do confirm that CO is practically the sole product of electrolysis with negligible production of dihydrogen. Perusal of figure 5 points to an increase of the catalytic current when going from FeF20TPP to FeF10TPP, FeF5TPP and FeMeO8TPP for the same phenol concentration. A precise determination of the


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catalysis rate constant in each case, according to the reaction scheme in top figure 3, is however required 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

to accurately gauge the substituent effects. For modest catalytic enhancements of the catalytic current ( i p / i 0p ≤ 2 ), as encountered for the less reactive iron porphyrins in the series and low PhOH concentrations, we used working curves i p / i 0p to the kinetic parameter

( RT / F ) ( kcat / v)

62

relating

in the framework of the reaction scheme of figure 3

(top), to obtain the value of kcat (see SI for details). For larger values of kcat , pure kinetic conditions are achieved by mutual compensation of catalyst diffusion and reaction. The current-potential response is expected to take an S shape (see examples in figure 5):62,63 i=

i pl

(

)

 F 0  1 + exp  E − Ecat   RT

0 , i pl = FSCcat Dcat 2 kcat

0 : concentration of catalyst, D : diffusion coefficient ( ipl : plateau current, S: electrode surface area, Ccat cat

of catalyst) from which the rate constant kcat is obtained. In doing so, it is convenient to divide the current i by the normalizing factor: 0 i0p = FS × 0.446 × Ccat Dcat

Fv RT

(v: scan rate), so as to avoid a separate determination of S and Dcat : i pl i0p

= 2.24

2kcat RT v F


10

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig. 6. Elimination of the secondary phenomena by raising the scan rate (see text). Cyclic voltammetry of the substituted iron porphyrins of Chart 1 (conc.: 1 mM) in DMF + 0.1 M NBu4PF6 + 0.1 M H2O under 1 atm. CO2 in the presence of PhOH, conc. (M), from top to bottom: FeF20: 2.5, 3, 3.5, 4; FeF10: 1, 2, 3; FeF5: 1, 2, 3; FeMeO8: 0.005, 0.01, 0.02, 0.05, 0.1, 0.2. The scan rates are indicated at the bottom of the figure. The values of the rate constants thus obtained are summarized in Table 1. Hg working electrode in the first three cases, GC working electrode in the last. However, when catalysis becomes stronger upon increasing phenol concentration, the CV responses may take back a peak shape as can be seen in figure 5. As analyzed in detail elsewhere,

51

this is due to

the interference of secondary phenomena such as substrate or co-substrate consumption, inhibition by product and possibly other phenomena that all increase with the charge passed. One way of fighting the interference of such phenomena is to raise the scan rate, and thereby, decrease the charge passed, to obtain an S-shaped CV and derive the rate constant from the plateau current.

51

Examples are given in


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figure 6. By joining to these results (Table 1) those previously obtained with FeTPP, 53 the comparative 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

variations of the catalytic rate constant with phenol concentration for all five iron porphyrins are as shown in Figure 7. Table 1. Catalytic rate constants kcat (s-1) and standard potential characteristics Porphyrin

FeF20TPP FeF10TPP

0 Ecat

FeF5TPP

FeMeO8TPP

-1.118

-1.276

-1.365

-1.725

0.378

0.536

0.625

0.985

(V vs. SHE)

0 c Et0,r − Ecat

[PhOH] d (M)

Number between parenthesis: scan rate in V/s at which the determination of kcat was done

0.00

0

1.0 (0.1)a

2.0 (0.1)b

319 (1) b

0.005

-

-

7.2 (0.1) b

1770 (0.8) b

0.01

-

2.6 (0.1) a

16.1 (0.1) b

2390 (0.8) b

0.02

-

3.8 (0.1) a

38.3 (0.1) b

3170 (0.7) b

0.05

-

6.06 (0.1) b

144 (0.1) b

3610 (0.5) b

0.1

0.71 (0.1) a

20.9 (0.1) b

477 (0.1) b

4330 (0.75) b

0.2

-

74.1 (0.1) b

1240 (0.1) b

5230 (1) b

0.5

2.6 (0.1) a

343 (0.1) b

4330 (0.1) b

-

1

7.25 (0.1) a

944 (0.5) b

7480 (0.25) b

-

-

-

-

14 (0.1)

b

2

32 (0.1)

b

2.5

78.6 (0.5) b

1.5

1730 (1)

b

-

8500 (1)

b

-

-

149(0.5)

b

3.5

224 (0.5)

b

-

-

-

4

344 (0.5) b

-

-

-

3

2540 (3)

b

-

6520 (5)

b

-

0 a: using the working curve method. b: from the plateau current .c: E t , r = 0.74 V vs. SHE. 64. d : In the presence of 0.1 M H2O.

6

log kcat (s-1 )

5 4

FeMeO8TPP

3 2 FeTPP

1 FeF5TPP

0 FeF10TPP

FeF20TPP

log [ PhOH ] (M)

-1 0

1

2

3

4


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig.7. Comparative variations of the catalytic rate constant with phenol concentration for the porphyrins of Chart 1 (blue: FeMeO8TPP; black: FeTPP; green: FeF5TPP; red: FeF10TPP; magenta: FeF20TPP in presence of 0.1 M H2O. The straight lines indicate the order in PhOH is successively 2 and 1 as [PhOH] increases. Discussion As general trends emerging from Table 1 and in figure 7, we note the variation of the catalyst standard 0 potential toward positive values upon passing from FeTPP ( E ca = -1.428 V vs. SHE) to FeF5TPP, t

FeF10TPP, and, FeF20TPP reflecting the increasing electron-withdrawing effect of the substituents in the series. Conversely, the eight methoxy groups in ortho, ortho’ of the four phenyls of TPP exert a strong electron 0 in the negative direction. A sound donating effect, that translates into a 340 mV shift of the Ecat

comparison between the various substituted iron porphyrins as catalysts of the CO2-to-CO electrochemical conversion requires establishing a catalytic Tafel plot in each case. Since the catalytic rate constant, kcat, is a function of phenol concentration, catalytic Tafel plots may be drawn for each value of this concentration. Comparison between the five iron porphyrins implies that we select a common value for [PhOH] where the catalytic response could be recorded and analyzed in each case. The catalytic Tafel plots thus derived from the information gathered in Table 1 (and from reference 53 for FeTPP) were established for [PhOH] = 0.1 and 1 M by application of equation (1) and of: TOF =k max cat

They are represented in figure 8. 6

6

logTOF

logTOF

4

4

2

2

0

0

-2

-2

-4

-4

-6

-6

[PhOH] = 0.1M

-8 -10

[PhOH] = 1M

-8

η

-10

η

-12

-12 0

0.2

0.4

0.6

0.8

1

0

0.2

0.4

0.6

0.8

1

1.2

Fig. 8. Comparative catalytic Tafel plots for all five iron porphyrins of Chart 1 (blue: FeMeO8TPP; black: FeTPP; green: FeF5TPP red: FeF10TPP; magenta: FeF20TPP) at two phenol concentrations. ACS Paragon Plus Environment

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The fact that two conflicting trends are brought about by electron-withdrawing or electron-donating 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

substitution clearly appears when comparing the catalytic Tafel plots at the two phenol concentrations. Crossings are seen in the two diagrams of figure 8. On the one hand, electron-withdrawing substituents entail a positive shift of the catalyst standard potential − a favorable factor in terms of overpotential. On the other hand, the same inductive effect tends to decrease the overall rate constant of the catalytic reaction and hence TOFmax. One reason for this is that electron-withdrawing substituents are expected to decrease the electron density on the iron atom, and therefore its capability to combine with CO2 in the first step of the catalytic process (Scheme 1). Symmetrical effects are expected for electron-donating substituents. A compensating linear correlation may well relate these opposing substituent effects. If the correlation 0 exists and if this correlation coefficient is equal coefficient between ( RT ln10 / F ) log TOFmax and Ecat

to -1, non-crossing CTPs should be observed as sketched in figure 2. The CTPs for FeTPP, FeF5TPP and FeF10TPP at [PhOH] = 0.1 M (respectively the black, green and red curves on figure 8, left) are close to this situation. Further analysis of substituent effect correlations should however take into account that the initial adduct –forming step is associated with the interference of the acid AH both as an H-bond promoter and a proton donor according to the mechanism depicted in Scheme 1, previously established in the case of FeTPP (for AH = PhOH as here but also for other acids). 53 FeMeO8TPP appears to behave in a slightly different manner, giving rise to a large catalytic constant in the absence of phenol (Table 1). This indicates that, because of the strong electron-donating effect of the eight ortho, ortho’ methoxy groups on the phenyls of TPP, an important electronic charge is present on the iron at the Fe0 stage and on the oxygens of CO2 within the initial iron-CO2 adduct. H2O is thus sufficient to vigorously trigger the CO2-to-CO conversion. The same inductive effect is also responsible for the catalysis, by the iron(0) complex, of the reduction of phenol into H2 as can been seen in figure S2 of the SI, which was already the reason for limiting the investigation of the role of PhOH to a maximal concentration of 0.2 M. ACS Paragon Plus Environment

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The variations of the catalytic rate constant with [PhOH] in the fluorine substituted series (figure 7) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

show two domains corresponding to a second and first order in PhOH, respectively. The transition from the first domain to the second can be interpreted in the framework of Scheme 1, as the passage of the kinetic control of the catalytic sequence by K1K2k3 to K1k2 as the concentration of phenol increases.

53

Indeed, the acceleration of reaction (3), when [PhOH] increases, outweighs back reaction (2) so as to render forward reaction (2) rate-determining. This also explains why increasing phenol concentrations are required to pass from the 2nd to the 1st order regime when the electron-withdrawing effect increases, making reaction (3) less and less rapid and therefore less and less competitive with back reaction (2). This effect is maximal with FeF20TPP where only the second order kinetic domain is accessible. 0 Further analyses of the possible correlation of kcat with Ecat were restricted to the three FeTPP,

FeF5TPP and FeF10TPP porphyrins because the data are available in the two reaction order regimes for these compounds and also because the data for FeF20TPP were less precise in line with the fact that catalysis is slow in this case. Figure 9 gathers the corresponding pertinent data. The linear correlations displayed in figure 9 may be analyzed by introduction of correlation coefficients 0 (β s) of the pertinent thermodynamic and kinetic parameters with Ecat taken as a quantitative index of the

electron-withdrawing or electron-donating substituent effect. The correlation coefficients β s are thus introduced for the variations of the driving forces,- ∆G0 , with 0 Ecat , ( ∆G0 : standard free energy of the reaction of interest) and the correlation coefficients γ s for the 0.4 0.35

RT ln10 log kcat F

0.3 0.25 0.2 0.15 0.1 0.05

0 Ecat (V vs. SHE)

0 -1.1 -1.2 -1.3 -1.4 -1.5 -1.6


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Fig. 9. Correlation between the catalytic rate constant and the catalyst standard potential in the [PhOH] first order (red dots, [PhOH] = 1 M) and second order (blue dots, [PhOH] = 0.02 M) regimes. From left to right: FeF10TPP, FeF5TPP and FeTPP.

variations of the driving forces with the pK of the acid. Expressing the driving forces by means of the 0 standard chemical potentials of the various species, µ species , allows to easily tracing back the

0 contributions of the catalyst standard potential, Ecat and the pK of the acid co-catalyst (phenol here, but

also different acids as in reference 51c) to the equilibrium constants: 0 0 µFe(0) + µCO − µ -0 ∆G10 RT ln10 2 Fe(I)CO2 − 0 log K1 = − = = − β1Ecat + C1 F F F

∆G20 RT ln10 log K 2 = − = F F ∆G30 RT ln10 log K3 = − = F F

∆G40 RT ln10 log K 4 = − = F F (6)

µ -0

Fe(I)CO2−

0 + µAH − µ -0

Fe(I)CO 2−LHA

F

µ -0

Fe(I)CO2−LHA

0 0 + µAH − µFe(II)CO − 2µ 0 A

F

µ 02−

Fe(0)

0 = − β 2 Ecat − γ 2 pK + C2

0 0 + µFe(II)CO − µCO − 2µ -0

Fe(I)

F

0 = − β3 Ecat − γ 3 pK + C3

(3)

(4)

(5)

0 0 0 = EFe(II)CO/Fe(I) +CO − Ecat = − β 4 Ecat + C4

(The C s are the intercepts of the correlation lines, whose values need not being known for the present analysis). Combining equations (3) – (6) leads to:

(

0 0 2 Ecat − ECO

2 /CO+H2O

) = (β + β 1

2 + β3 + β4

0 + ( γ 2 + γ 3 ) pK − ( C1 + C2 + C3 + C4 ) ) Ecat

and therefore to: β1 + β2 + β3 + β4 = 2 , γ2 +γ3 = 2

(7)

Passing now to kinetics, the correlation between the catalytic rate constant in the second order kinetic 0 regime, K1K2 k3 and Ecat may be expressed as:

∆G10 + α2,3 ∆G20 + ∆G30  RT ln10   = − β + α β + β  E0 − 2α pK + C + α C + C log K1K2k3 = − 2,3 1 2,3 ( 2 3)  1 2,3 ( 2 3 )  cat F F (8) ACS Paragon Plus Environment

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where α2,3 is the coefficient that relates the overall rate constant K2k3 to the driving force of the reaction 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(2 + 3). 2α2,3 = 0.35

(9)

α2,3 is obtained from a previously established correlation between K1K2 k3 and the pK in the case of FeTPP using trifluoroethanol, phenol and acetic acid as the co-catalyst. 53 Another relationship:

β1 +α2,3 ( β2 + β3) = β1 +α2,3 ( 2− β4 −β1) = 0.853

(10)

is derived from the slope of the blue line in figure 9. 0 0 The standard potential E Fe(II)CO/Fe(I) + CO is practically insensitive to the variation of Ecat , indicating

that:

β4 ≈ 0.0

(11)

The next relationship is obtained from the slope of the correlation between the catalytic rate constant in 0 kin kin the first order kinetic regime and Ecat corresponding to the red line in figure 9: β1 + β2 = 0.58 ( β2 is 0 the correlation coefficient of the rate of the H-bonding reaction (2) with Ecat ). Combination of equations

(8) – (11) finally provides the magnitudes of the inductive effect on the various step of Scheme 1, taking 0 as index the standard potential of the catalyst, Ecat :

Formation of the initial Fe0 – CO2 adduct: β 1 = 0.61 kin H-bonding stabilization by one PhOH molecule: β2 = − 0.02 ( ≈ 0)

Protonation of the H-bonded adduct by a second PhOH molecule: β 3 =1.33 Reductive cleavage of the FeIICO adduct producing CO and regenerating the catalyst: β 4 ≈ 0 emphasizing the idea that the changes in electronic density, brought about by the substituents, essentially result in a change of the Lewis basicity Fe0 atom and of the Brönsted basicity of the oxygens in the initial iron-CO2 adduct rather than having a marked influence on H-bonding. ACS Paragon Plus Environment

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Conclusion 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Catalysis of the electrochemical CO2-to-CO conversion by the FeI/0 porphyrin couple in the presence of an acid-cosubstrate provides a tutorial example of the analysis of through-structure electronic effects entailed by introduction of electron-withdrawing and electron-donating substituents in the catalyst molecule. Estimation of these effects was carried out by comparing the electrochemical behavior of irontetraphenylporphyrin itself with a series of four different substituted iron-tetraphenylporphyrins. Electronwithdrawing electronic effects were obtained by replacing phenyl rings in iron-tetraphenylporphyrin by one, two, and four perfluorophenyl rings, successively. Substitution of the four phenyl rings by ortho, ortho’- dimethoxy phenyls gave access to the effect of electron-donation. Comparison between the various iron porphyrins in the series actually involves observing the way in which the catalytic rate constant varies with the concentration of an added Brönsted acid acting as H-bond provider and proton donor – phenol in the present investigation. These electronic substitution effects involve both the standard potential of the catalyst couple and the rate of the catalytic reaction. This is the reason that the best way of gauging substituents electronic effects is to compare the ensuing catalytic Tafel plots (CTP) relating the turnover frequency to the overpotential as determined for each FeI/0 porphyrin couple in the series. It thus appears that the substituent electronic effects, which are exerted through the catalyst molecular structure, entail two opposite tendencies: electron-withdrawing substitution pushes the catalyst standard potential in the positive direction by way of stabilizing the Fe0 member of the couple− a favorable effect in terms of overpotential−, and, at the same time, decreases the catalytic rate constant. These effects appear at a given phenol concentration and may or may not lead to crossings of the CTPs. The mutual position of the CTPs depends on the exact correlation that may relate the catalytic rate constant (and henceforth the value of TOFmax) to the catalyst standard potential taken as an index of the substituent electronic effect. This global correlation is entailed with a negative coefficient as manifestation of the abovementioned contradictory trends. This may lead to CTP crossing but also to CTPs having a common, or almost common, oblique section at low overpotential as featured by examples in figures 1 and 8-left. It is worth noting that, in such ACS Paragon Plus Environment

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circumstances, the overbalance of the two opposite effects of substitution does not make it globally 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

worthwhile: the slight positive shift in potential is more than largely offset by a strong decrease of TOFmax. Dissection of the overall inductive effect in the FeTPP-FeF5TPP-FeF10TPP series allowed the assignment of a correlation coefficient to each of the four successive steps of the reaction scheme. The result is that the most important effects are exerted on the formation of the initial Fe0 – CO2 adduct − a typical Lewis acid-base reaction−, and on the protonation of the H-bonded Fe0 – CO2 adduct rather than on the H-bonding stabilization of this initial adduct. This is the first application of a method able to fully analyze through-structure substituent effects in multi-step catalytic reactions, which can be extended to any other catalyst of any other electrochemical reaction. It provides a robust background for investigating through-space substituent effects, which may more specifically directed toward the modification of the reactivity of intermediates and thereof lead to very significant improvements of catalytic efficiency both in terms of overpotential and turnover frequency.

Acknowledgments Partial financial support from the SATT IDF Innov (project 098) is gratefully acknowledged.

Supporting Information Experimental details. FeII/I couple under CO for the five porphyrins of Chart 1. Determination of 0

ECO

2 /CO

according to the solvent and to the acids present.

References and Notes

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24. Jasinski, R.: A New Fuel Cell Cathode Catalyst. Nature 1964, 201, 1212-1213. 25. Kobayashi, N.; Nevin, W. A.: Electrocatalytic Reduction of Oxygen Using Water-Soluble Iron and Cobalt Phthalocyanines and Porphyrins. Appl. Organometal. Chem. 1996, 10, 579-590. 26. Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C.: Electrode catalysis of the four-electron reduction of oxygen to water by dicobalt face-to-face porphyrins. J. Am. Chem. Soc. 1980, 102, 6027-36. 27. Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L.: Functional Analogues of Cytochrome C Oxidase, Myoglobin, and Hemoglobin. Chem. Rev. 2004, 104, 561-588. 28. Savéant, J.-M.: Molecular Catalysis of Electrochemical Reactions. Mechanistic Aspects. Chem. Rev. 2008, 108, 2348-2378. 29. Gewirth, A. A.; Thorum, M. S.: Electroreduction of Dioxygen for Fuel-Cell Applications: Materials and Challenges. Inorg. Chem. 2010, 49, 3557-3566. 30. Chatterjee, S.; Sengupta, K.; Samanta, S.; Das, P. K.; Dey, A.: Concerted Proton–Electron Transfer in Electrocatalytic O2 Reduction by Iron Porphyrin Complexes: Axial Ligands Tuning H/D Isotope Effect. Inorg. Chem. 2015, 54, 2383-2392. 31. Sengupta, K.; Chatterjee, S.; Dey, A.: Catalytic H2O2 Disproportionation and Electrocatalytic O2 Reduction by a Functional Mimic of Heme Catalase: Direct Observation of Compound 0 and Compound I in Situ. ACS Catal. 2016, 1382-1388. 32. Collin, J. P.; Sauvage, J. P.: Electrochemical reduction of carbon dioxide mediated by molecular catalysts. Coord. Chem. Rev. 1989, 93, 245-268. 33. Costamagna, J.; Ferraudi, G.; Canales, J.; Vargas, J.: Carbon dioxide activation by aza-macrocyclic complexes. Coord. Chem. Rev. 1996, 148, 221-248. ACS Paragon Plus Environment

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34. Tanaka, K.; Ooyama, D.: Multi-electron reduction of CO2 via Ru---CO2, ---C(O)OH, ---CO, --CHO, and ---CH2OH species. Coord. Chem. Rev. 2002, 226, 211-218. 35. Windle, C. D.; Perutz, R. N.: Advances in molecular photocatalytic and electrocatalytic CO2 reduction. Coord. Chem. Rev. 2012, 256, 2562-2570. 36. Izumi, Y.: Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond. Coord. Chem. Rev. 2013, 257, 171-186. 37. Oh, Y.; Hu, X.: Organic molecules as mediators and catalysts for photocatalytic and electrocatalytic CO2 reduction. Chem. Soc. Rev. 2013, 42, 2253-2261. 38. Costentin, C.; Robert, M.; Savéant, J.-M.: Catalysis of the electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42, 2423-2436. 39. Qiao, J.; Liu, Y.; Hong, F.; Zhang, J.: A review of catalysts for the electroreduction of carbon dioxide to produce low-carbon fuels. Chem. Soc. Rev. 2014, 43, 631-675. 40. Mondal, B.; Rana, A.; Sen, P.; Dey, A.: Intermediates involved in the 2e–/2H+ reduction of CO2 to CO by iron(0) porphyrins. J. Am. Chem. Soc. 2015, 137, 11214-11217. 41. Savéant, J.-M. Elements of molecular and biomolecular electrochemistry: an electrochemical approach to electron transfer chemistry; John Wiley & Sons: Hoboken, NJ, 2006, chap. 4. 42. Bhugun, I.; Lexa, D.; Saveant, J. M.: Catalysis of the electrochemical reduction of carbon dioxide by iron(0) porphyrins: Synergistic effect of weak Brönsted acids. J. Am. Chem. Soc. 1996, 118, 17691776. 43. Early reports have emphasized the role of Bronsted acids, 42,44 which will be an essential item of the following discussion of substituent effects. The role of Lewis acid has similarly been investigated in early 45

and recent reports. 46 ACS Paragon Plus Environment

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44. Wong, K.-Y.; Chung, W.-H.; Lau, C.-P.: The effect of weak Bronsted acids on the electrocatalytic reduction of carbon dioxide by a rhenium tricarbonyl bipyridyl complex. J. Electroanal. Chem. 1998, 453, 161-169. 45. Bhugun, I.; Lexa, D.; Saveant, J. M. J. Phys. Chem. 1996, 100, 19981. 46. Sampson, M. D.; Nguyen, A. D.; Grice, K. A.; Moore, C. E.; Rheingold, A. L.; Kubiak, C. P.: Manganese Catalysts with Bulky Bipyridine Ligands for the Electrocatalytic Reduction of Carbon Dioxide: Eliminating Dimerization and Altering Catalysis. J. Am. Chem. Soc. 2014, 136, 5460-5471. 47. Savéant, J.-M.: Electron transfer, bond breaking, and bond formation. Acc. Chem. Res. 1993, 26, 455-461. 48. Costentin, C.; Robert, M.; Savéant, J.-M.: Concerted proton-electron transfers: electrochemical and related approaches. Acc. Chem. Res. 2010, 43, 1019-1029. 49. Costentin, C.; Robert, M.; Savéant, J.-M.; Tard, C.: Breaking bonds with electrons and protons. Models and examples. Acc. Chem. Res. 2014, 47, 271-280. 50. Bligaard, T.; Bullock, R. M.; Campbell, C. T.; Chen, J. G.; Gates, B. C.; Gorte, R. J.; Jones, C. W.; Jones, W. D.; Kitchin, J. R.; Scott, S. L.: Toward Benchmarking in Catalysis Science: Best Practices, Challenges, and Opportunities. ACS Catal. 2016, 2590-2602. 51. Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M.: Turnover numbers, turnover frequencies and overpotential in molecular catalysis of electrochemical reactions. Cyclic voltammetry and preparativescale electrolysis. J. Am. Chem. Soc. 2012, 134, 11235-11242.; 2012, 134, 19949. 52. Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M.: A local proton source enhances CO2 electroreduction to CO by a molecular Fe catalyst. Science 2012, 338, 90-94.


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53. Costentin, C.; Drouet, S.; Passard, G.; Robert, M.; Savéant, J.-M.: Proton-coupled electron transfer cleavage of heavy-atom bonds in electrocatalytic processes. Cleavage of a C–O bond in the catalyzed electrochemical reduction of CO2. J. Am. Chem. Soc. 2013, 135, 9023-9031. 54. Costentin, C.; Passard, G.; Robert, M.; Savéant, J.-M.: Ultraefficient homogeneous catalyst for the CO2-to-CO electrochemical conversion. Proc. Natl. Acad. Sci. U.S.A. 2014, 111, 14990-14994. 0 55. It should be emphasized that η is, through Et,r , a thermodynamic parameter referred to a standard

state. Despite the naming of the correlation TOF- η correlation according to Tafel, the present definition of η differs from the definition classically used for H2-evolution

56

where the overpotential is referred to

an equilibrium potential rather than to a standard potential. 56. Azzam, A. M.; Bockris, J. O. M.; Conway, B. E.; Rosenberg, H.: Some aspects of the measurement of hydrogen overpotential. T. Faraday Soc. 1950, 46, 918-927. 57. Artero, V.; Savéant, J.-M.: Toward the rational benchmarking of homogeneous H2-evolving catalysts. Energy Environ. Sci. 2014, 7, 3808-3814. 58. Elgrishi, N.; Chambers, M. B.; Fontecave, M.: Turning it off! Disfavouring hydrogen evolution to enhance selectivity for CO production during homogeneous CO2 reduction by cobalt-terpyridine complexes. Chem. Sci. 2015, 6, 2522-2531. 59. An apparently similar correlation has been mentioned in the catalysis of CO2 reduction by a series of palladium complexes containing tridentate ligands. 60,61 However the potential chosen as the abscissa of the correlation has no definite thermodynamical meaning and the determination of the TOF is problematic for two reasons: (i) the extraction of the catalytic rate constant from the cyclic voltammetric responses does not take into account the existence of secondary phenomena and their elimination,

51


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resulting in “constants” that are not actually constant but vary with the scan rate. (ii) The definition of the TOF appropriate for homogenous catalysis 51 is not taken into consideration. 60. Steffey, B. D.; Miedaner, A.; Maciejewski-Farmer, M. L.; Bernatis, P. R.; Herring, A. M.; Allured, V. S.; Carperos, V.; DuBois, D. L.: Synthesis and Characterization of Palladium Complexes Containing Tridentate Ligands with PXP (X = C, N, O, S, As) Donor Sets and Their Evaluation as Electrochemical CO2 Reduction Catalysts. Organometallics 1994, 13, 4844-4855. 61. Bernatis, P. R.; Miedaner, A.; Haltiwanger, R. C.; DuBois, D. L.: Exclusion of Six-Coordinate Intermediates in the Electrochemical Reduction of CO2 Catalyzed by [Pd(triphosphine)(CH3CN)](BF4)2 Complexes. Organometallics 1994, 13, 4835-4843. 62. Savéant, J.-M. Elements of molecular and biomolecular electrochemistry: an electrochemical approach to electron transfer chemistry; John Wiley & Sons: Hoboken, NJ, 2006. 63. Savéant, J.-M.; Vianello, E. In Advances in Polarography; 1 ed.; Longmuir, I. S., Ed.; Pergamon Press: Cambridge, U. K., 1959; Vol. 1, p 367. 64. See SI for a corrected evaluation of the CO2/CO standard potential.

TOC Graphic TurnOverFrequency

Overpotential


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Dissection of Electronic Substituent Effects in MultielectronâMultistep

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