Theoretical and Experimental Reactivity Predictors for the

Subscriber access provided by BUFFALO STATE

C: Surfaces, Interfaces, Porous Materials, and Catalysis

Theoretical and Experimental Reactivity Predictors for the Electrocatalytic Activity of Copper Phenanthroline Derivatives for the Reduction of Dioxygen Ricardo Venegas, Karina Muñoz-Becerra, Luis A. Lemus, Alejandro Toro-Labbé, Jose H. Zagal, and Francisco Javier Recio J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03200 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 21, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 60 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

The Journal of Physical Chemistry

Theoretical and Experimental Reactivity Predictors for the Electrocatalytic Activity of Copper Phenanthroline Derivatives for the Reduction of Dioxygen Ricardo Venegasa,b, Karina Muñoz-Becerraa,b, Luis Lemusc, Alejandro Toro-Labbéd, Jose H. Zagalc*, Francisco J. Recioa,b*.

aFacultad

de Química y de Farmacia, Departamento de Química Inorgánica, Pontificia

Universidad Católica de Chile, Macul, Santiago, Chile.

bCentro

de Nanotecnología y Materiales Avanzados, CIEN-UC, Pontificia Universidad

Católica de Chile, Santiago, Chile.

cFacultad

de Química y Biología, Departamento de Química de los Materiales,

Universidad de Santiago de Chile, Santiago, Chile.

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 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

dFacultad

Page 2 of 60

de Química y de Farmacia, departamento de Química-Física, Pontificia

Universidad Católica de Chile, Macul, Santiago, Chile.

*Corresponding authors: [email protected] , [email protected].

ABSTRACT.

We have systematically studied the catalytic activity of a series of substituted bisphenanthroline Cu complexes adsorbed on glassy carbon electrodes for the oxygen reduction reaction (ORR) in aqueous media. The aim of this work is to test reactivity descriptors for ORR previously proposed for MN4 complexes. Since the active site is the Cu(I) center the foot of the wave for O2 reduction on electrodes modified with Cu bisphenanthrolines appears at potentials very close to the [Cu(II)phen2]++ad + e-  [Cu(I)phen2]+ad redox process. Generally, the catalytic activity as (logi)E versus the E°Cu(II)/(I) redox potentials follows a linear correlation with a slope close to +0.120V/decade, where the activity increases as the E°Cu(II)/(I) becomes more positive. This indicates that electron-withdrawing substituents on the ligand favor the reaction by


Page 3 of 60 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


shifting the E°Cu(II)/(I) to more positive values, making the Cu center more noble. This shift in the E°Cu(II)/(I) is essentially an electronic effect. However another way to stabilize the Cu(I) state is by placing groups on the ligand that hinder the [Cu(II)square-planar]++ad/[Cu(I) + tetrahedral] ad

change. We have found that this steric effect seems to be more prominent that

the electronic effects caused by electron-withdrawing groups and the increase in the catalytic activity for ORR is more pronounced.

1. INTRODUCTION.

Fuel cells are very promising devices that deliver electrical energy from the oxidation of chemical fuels. Since O2 is abundant in the atmosphere the cathodic reaction generally involves the reduction of O2. As ORR is a multi-electron process its kinetic is very slow on most electrode materials1-11. To proceed at acceptable rates at low overpotentials that are necessary for fuel cell performance it requires the presence of catalysts on the electrode surface, since the multi-electron ORR is an inner sphere process that requires the binding of O2 reduction intermediates to active sites present on a conductive surface. Further, these catalysts need to promote the transfer of 4 electrons 3 ACS Paragon Plus Environment


Page 4 of 60

per O2 molecule to deliver the most energy out of the reaction. Pt containing electrodes are the most active known materials and promote the 4-e reduction of O2 but Pt is expensive and scarce11. This is one of the factors that has prevented the wide spread use of fuel cells. Many groups around the world have focused their attention on developing less-costly materials that are known as non-precious metal catalysts (NPMC) for the O2 reduction reaction (ORR)12. In this respect many metal complexes have been examined, specially MN4 macrocyclic compounds. The electrocatalytic activity of MN4 macrocyclic complexes for the reduction of O2 (ORR) has been studied for many years, as part of the search for non-precious metal catalysts for ORR, as alternative catalytic materials to replace Pt-containing catalysts in alkaline fuel cells1-5. The eminent advent of electric vehicles has stimulated this research even further3,

5, 9, 10.

Metal-nitrogen-carbon

pyrolyzed materials, which are claimed to possess a variety of active sites included MN4 centers in their structure, are highly promising because of their high activity and stability in acid media11-16. On the other hand, intact MN4 macrocyclic complexes have served as excellent models for identifying the reactivity predictors that can serve as guidelines to develop more active non-precious metal catalytic materials. For example, the 4 ACS Paragon Plus Environment

Page 5 of 60 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


electrocatalytic activity for ORR of MN4-macrocyclic metalloporphyrins (MP) and metallophthalocyanines (MPc) (M = Fe, Co and Mn) is linked to four main reactivity descriptors: (a) the M(III)/M(II) formal potential of the complex, (b) M-O2 binding energies, (c) d-electron-occupancy of metallic center, and (d) donor–acceptor intermolecular hardness4, 5, 17. These descriptors are dominated by the ability of the ligands to modulate the electronic structure of the metal center and its ability to bind an extra-planar ligand like O2 or any reacting molecule. This is true not only for ORR but for many other electrochemical reactions catalyzed by MN4 complexes as shown in several papers1,1825.

As expected, electron-withdrawing substituents located on the ligand (P or Pc), remove

electron density from the metal center and shift the M(III)/M(II) formal potential to more positive values. This makes the metal center “more noble” or more Pt-like and harder to oxidize. As a consequence this enhances the ORR catalytic activity, while the opposite occurs with electron-donating substituents1,4. There is also a linear correlation between the M(III)/M(II) formal potential and the M-O2 binding energies (obtained theoretically), and this can explains why the redox potential is a reactivity predictor since binding energies of ORR intermediates to the active sites are well known reactivity predictors for 5 ACS Paragon Plus Environment


Page 6 of 60

ORR activity (and for other energy conversion reactions) for many electrode materials, especially metals and alloys4,

26, 27.

Plots of activity expressed as (log i)E versus the

M(III)/(II) formal potential and versus the M-O2 binding energy for ORR give the wellknown volcano correlations generally observed for metals and alloys4. The volcano correlation (log i)E vs. M-O2 binding energy has two legs, one for the strong binding catalysts and another for weak binding catalysts. MnN4 and FeN4 macrocyclic complexes appear on the strong binding leg of the volcano correlation so the increase in activity when the M(III)/(II) is shifted to more positive values agrees with a decrease in the M-O2 binding energy4, 17. This explains the high activity of Mn, Fe and Co MN4-based catalysts and the poor or low ORR activity of Cu, Ni and Zn MN4 complexes. For example, Cu(II)phthalocyanine (CuPc) shows practically no activity at all and less than that of the pyrolytic graphite support used for the measurements. The low activity of CuPc can be attributed to several factors: no frontier orbitals with metal-d character, no redox activity centered on the Cu center and it has a flat rigid geometry1, 28-31. For copper complexes the catalytically active redox couple for ORR is Cu(II)/Cu(I) and this process involves a geometry change from square-planar Cu(II) to tetrahedral Cu(I). This process is then 6 ACS Paragon Plus Environment

Page 7 of 60 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


highly hindered in Cu(II)phthalocyanine due to its planar structural rigidity. Further, the frontier orbitals in CuPc have ligand character so redox processes occur on the ligand and not on the metal center1. On the other hand, Cu complexes that have flexible structures and can undergo Cu(II)/Cu(I) redox processes do show activity for ORR as pointed out above32. Chidsey and coworkers33 have compared the activity for ORR of several Cu mono-phenanthrolines expressed as the kinetic current at a constant potential (-0.150 V vs NHE) plotted versus the Cu(II)/Cu(I) formal potential, showing a decrease in activity as the formal potential becomes more positive. This trend is the opposite to what is found in MN4 complexes as metal porphyrins and phthalocyanines for ORR, where the catalytic activity increases when the formal potential of the catalysts become more positive3, 4, 9, 10.

On the other hand, several efforts have been focused in the chemical isolation of Cu(I) complexes containing Cu(I) centers like those present in bioavailable Cu-based enzymes and proteins as copper monooxygenase (e.g. laccase). This enzyme reduces dioxygen by a four-electron mechanism, or as hemocyanins that react reversibly with



Page 8 of 60

molecular oxygen and catalyze its reduction at potentials very close to that of the O2/H2O reversible potential34-36. It is well-known that the Cu(I) bis-phenanthroline complexes in organic media is oxidized by O2 to form Cu(II) bis-phenanthroline in homogeneous media and the rates of the reaction are very sensitive to the nature of substituents on the ligand37, 38.

The formal potential of the complexes and their photophysical and electrochemistry

properties are also dependent of the pKa of the ligands in these kind of complexes39-44. However, the catalytic activity of Cu(I) complexes for ORR in aqueous media, when they are confined on an electrode surface, have been difficult to study due to the existing dynamic equilibria between the bis-phenanthroline complex ([Cu(I)(phen)2]+) and the mono-phenanthroline complex ([Cu(I)(phen)X2]+, X = solvent coordinating ligand)44-46. Some authors have pointed out that mono-phenanthroline complexes are more catalytic than the bis-phenanthroline complexes33,43.45-47, this might be related to favorable changes in the binding energy of Cu-O2 for mono compared to bis-phenanthroline complexes but to the best of our knowledge no theoretical calculations of this energies have been reported. The coordination geometry is strongly dependent of the oxidation state of the copper center, for four-coordinated Cu(I) a pseudo-tetrahedral configuration 8 ACS Paragon Plus Environment

Page 9 of 60 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


is more stable, and for Cu(II) a square-planar configuration is more favorable42, 48. Thus, the Cu(II)/Cu(I) redox process for the studied species is accompanied by a geometric flattening of the Cu(I) center. This has been reported for example, for [Cu(I)(phen)2]+ species which in the excited state studied in solution, are formally described bearing a photo-oxidized square-planar Cu(II) center42. Therefore, in the oxidation Cu(I)  Cu(II) process the incorporation of substituents on the positions 2 and 9 of the ligand induces a steric hindrance over the geometric flattening that is expected to accompany the oxidation state change, increasing consequently the stabilization of the tetrahedral-Cu(I) conformation42,

45.

Additionally, several efforts have been dedicated to increase the

durability and selectivity of copper-based complexes to the 4-electrons ORR, demonstrating the importance of the nuclearity of this kind of catalysts47,49. They have improved the selectivity over mononuclear Cu-phen complexes by its covalent attachment to the electrode surface, inducing an arrangement that allows to anchor the oxygen molecule between two units of the catalysts, demonstrating that the chemical order is 2 with respect to the Cu centers, promoting thus the 4-electron ORR pathway. This nuclearity dependency was also proved by Thorseth et al.50 in which a mononuclear Cu 9 ACS Paragon Plus Environment


Page 10 of 60

complex dimerizes on the electrode surface in the presence of ORR, showing the lowest overpotential at pH 1. Furthermore, a recent study of Thiyagarajan et al.51 in the intent to mimic the multi-copper oxidases Cu active centers, demonstrate that a trinuclear copper species allows an efficient 4-electron ORR, obtaining the lowest overpotential for synthetic Cu-based complexes at pH 7.

In this work, we have tested the Cu(II)/(I) formal potential as a reactivity predictor for a series of Cu bis-phenanthrolines confined on graphite surfaces for ORR. We have tested a series of complexes where we have shifted the Cu(II)/(I) redox potentials to more positive values using two approaches: (i) placing substituents on the phenanthroline ligand having different electron-withdrawing power (ii) by stabilizing the tetrahedral Cu(I) geometry by using substituents on the ligand that will hinder the Cu(I)tetrahedral/Cu(II)square-

planar

conversion. Based on the electrocatalytic parameters and theoretical calculation, a

mechanism is proposed.

2. EXPERIMENTAL

2.1. Synthesis and characterization of Cu(I) complexes. 10 ACS Paragon Plus Environment

Page 11 of 60 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


A series of 8 substituted homoleptic copper (I) 1,10-phenanthrolines (Table 1) was synthesized from the Cu(I) salt, [Cu(I)(CH3CN)4]ClO4, and the ligands listed in table S.1.

a) Ligands: Ligands V 3-Br-phen and VI 3,8-Br2-phen were prepared following a simple one-step synthesis reported by Tzalis et al.52 and the ligand VII 2-Cl-phen was synthesized according to a very known procedure reported in literature53. The other 6 ligands were acquired from Sigma-Aldrich and used without any purification. The synthesized ligands V, VI and VII were characterized by 1H-NMR (S.I.).

b) Complexes: The substituted Cu(I) 1,10-phenanthroline derivatives synthesis was achieve modifying a general procedure from literature54: [Cu(I)(CH3CN)4]ClO4 (0.5mmol) precursor was dissolved in 50mL of CH2Cl2 under argon atmosphere resulting in a colorless solution. Parallel, a solution of the substituted-phenanthroline ligand (1.0mmol) was prepared in 50mL of CH2Cl2 and added by dropwise to Cu(I) precursor solution, a change to yellow – red color was observed. The mixture was stirred for 2 hours under inert atmosphere, a red (brown) precipitate was formed. The solvent was removed on a rotary evaporator obtaining the complex as a red-brown solid depending on the



Page 12 of 60

substituents in the ligand. The complexes were recrystallized in CH2Cl2 by solvent evaporation (or by diffusion with diethyl ether) for mass spectrometry and elemental analysis characterization. In order to avoid the formation of Cu(I) monophenanthroline complexes, the 2:1 molar ratio between the ligand and the Cu(I) salt precursor was controlled.

2.2. Electrochemical measurements.

a) Preparation of Cu(I) complex – carbon powder ink and electrode modification: For glassy carbon (GC) electrode modification, complex-carbon powder inks were prepared according to the following procedure: To a solution of 4 mL of isopropanol and 1 mL of Milli-Q water, 0.004 g of Vulcan XC-72 carbon black powder and 0.001 g of Cu(I) complex were added, the mixture was sonicated for 10 minutes to form dispersion. 20 µL of Nafion (5 wt % in alcohols, Sigma-Aldrich) were added to the mixture and it was sonicated for 10 more minutes. The GC electrode surface was modified with 10µL of the carbon black ink for electrochemical experiments and dried under nitrogen slow flow.


Page 13 of 60 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


b) Electrochemical measurements: The electrochemical experiments were performed with a BAS I EPSILON (Bioanalytical Systems) potentiostat in a conventional electrochemical cell with a Pt wire as counter electrode (20 mm x 0.5 mm) and a saturated calomel electrode as reference (SCE). The electrochemical characterization of the redox couple Cu(II)/Cu(I) is based on the results obtained by cyclic voltammetry (CV) on glassy carbon electrodes (GC, PINE Instruments®) modified with the catalytic inks of the synthesized Cu(I)-1,10-phenanthroline derivatives (working electrodes). The supporting electrolyte (0.10 M of NaClO4•H2O and 0.04 M of acid acetic/sodium acetate buffer adjusted to pH 5.2, prepared with Milli-Q water) was purged with argon to avoid the oxygen reduction interference in the faradaic processes of the redox couple Cu(II)/Cu(I). In order to study the ORR kinetic on modified electrodes, polarization curves were performed using a rotating ring-disk electrode consisting in a GC disk as working electrode (geometrical area of 0.196 cm2) and a Pt ring (geometrical area of 0.093 cm2) that was held at a potential where the hydrogen peroxide detection is limited by diffusional regime. Polarization curves were carried out in an oxygen saturated at 25ºC electrolyte



Page 14 of 60

varying the rotation rate to calculate the number of electrons transferred during the oxygen reduction.

2.3. Computational Details.

The theoretical calculations were carried out by means of Gaussian09 code55. The structural optimizations to obtain the electronic structure of the complexes were done at the PBE0/Def2TZV level56,

57.

In all cases the LANL2DZ58 functional that incorporates

relativistic effects was used for copper atoms. Frequency calculations were carried out for all the studied complexes in order to probe the energy minimum of the optimizations. The accuracy of the level of theory used for the optimization geometries was probed in base of the metric parameters of crystallographic structures of [Cu(I)(phen)2]+ and [Cu(I)(2,9-Me2-phen)2]+ compared with the optimized structures, which were found in good agreement. Reaction energy profiles of the dioxygen adsorption were carried out by means of the Intrinsic Reaction Coordinate (IRC) procedure59, 60, which requires as input structure an optimized transition state (characterized by one imaginary frequency) at the M06-2X/6-31G(d,p)61

– 63

calculation level. The evaluation of the natural bond order


Page 15 of 60 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


through the Mayer bond analysis64, 65 and the natural charge analysis66 were obtained by the point by point single point calculation of the IRC analysis, at the same calculation level. The geometry optimization of the [Cu(I)-O2-] one-electron reduced adducts were carried out also at the PBE0/Def2TZV level, which spin density surface were represented using the ChemCraft software67.

3. RESULTS AND DISCUSSION

Two series of substituted homoleptic copper 1,10-phenanthrolines were synthesized from a Cu(I) salt, [Cu(I)(CH3CN)4]ClO4, and the ligands listed in table 1: a) The complexes of the electronic series were synthesized with ligands I to V (S.I.), where the substituents of the ligand are located in the positions far from the C-N bonds (Scheme 1a), and the steric series complexes were synthesized with ligands VI to VIII (S. I.), where the substituents are located in the positions 2 and 9 of the ligand, next to the C-N bonds (Scheme 1b) (For more details, see S. I.).

Scheme 1. Scheme of the substituted copper(I)-1,10-phenanthrolines a) electronic series and b) steric series. 15 ACS Paragon Plus Environment


Page 16 of 60

Table 1. Description of the complexes studied in this work.

Position number 2

3

4

5

CH CH3 3

6

7

8

CH

CH3

Abbreviation

9 (1)

3,4,7,8-Me4-phen

(2)

4,7-Me2-phen

(3)

Phen

(4)

5-Cl-phen

(5)

3,8-Br2-phen

(6)

2-Cl-phen

(7)

2,9-Me2-4,7-Ph2-phen

(8)

2,9-Me2-phen

3

CH3

CH 3

Cl Br

Br

Cl CH3

Ph

Ph

CH 3

CH3

CH 3


Page 17 of 60 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


Figure 1a illustrates a series of cyclic voltammograms obtained with a GC electrode modified with a series of Cu complexes before and after the oxygen reduction reaction. Some complexes exhibit changes in the CV profile after polarization in the presence of O2 (depicted in blue) indicating some chemical change in the adsorbed complexes. The complexes that are not stable under ORR polarization condition correspond to the steric series (2,9-substituted complexes: (6),(7) and (8)). Figure 1a also shows the foot of the wave for ORR in red dashed lines. The very visible current peak in the absence of O2, before the polarization test, can be assigned to the Cu(II)/(I) process that is not completely reversible (separated anodic and cathodic peaks). The faradaic process attributed to the Cu(I)  Cu(II) process involves a conformational change, from Cu(I) tetrahedral to Cu(II) square-planar. As expected, the Cu(II)/(I) formal potential is gradually shifted to more positive values as the electron-withdrawing power of the ligands increases, as observed in figure 1a. This is a result of the lowering of the energy of the frontier orbitals with metal character so it becomes more difficult to remove an electron from that orbital. The Cu(II)/(I) redox potential can be tuned to a desired value if a complex can be synthetized with the appropriate substituents33, 68. However, this electronic effect 17 ACS Paragon Plus Environment


Page 18 of 60

is not observed for the steric series because the potential of the Cu(II)/(I) transition changes when the Cu(II)(square-planar)  Cu(I)(tetrahedral) conformational change becomes hindered by bulky groups placed on positions 2 & 9, where changes in the electronwithdrawing power unfollow this trend (the most electro withdrawing substituent present the lowest formal potential of 2,9-substituent complexes), and for these complexes the formal potential seem to be linked with the steric hindrance effect of substituents place in 2,9 position of phenanthroline ligand, hindering the tetrahedral  square-planar conformation change. To overcome this structural impediment in 2,9 substituted complexes, an extra energy is required for the conformational change, this extra energy is detected experimentally as an increase on the formal potential of these complexes, compared with the complexes where the electronic effect of the substituent is predominant.


Page 19 of 60 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


Figure 1. (a) Electrochemical characterization of the substituted [Cu(I)(phen)2]ClO4 derivatives at pH 5.2 under N2 (black lines), showing the polarization curves obtained in saturated O2 media at pH 5.2 (red dashed lines). (Amplification) Details of the cyclic voltammetry of the steric series before (gray line) and after (blue line) the ORR. (b)



Page 20 of 60

Potential energy profile constructed in function of the torsional angle () for complexes of the electronic series ((1), (3) and (5)) and the steric series ((6) and (8)).

Since this extra energy is dependent to the conformational change, the energy necessary for the tetrahedral  square-planar change has been estimated by a computational analysis as the energy needed for the internal rotational motion of the phenanthroline ligands in five representative compounds (three in the electronic series and two in the steric one). DFT calculations69 at the PBE0/Def2TZV level56 were used to calculate the energy needed to rotate from tetrahedral (90º) to square–planar (0º or 180º) conformation. The potential function was build up performing single point calculations varying the angle every 5° degree between the two planes of the phenanthroline ligands leading to the potential curves in which the energy is displayed against the torsional angle, α, (figure 1b). It can be observed that the electronic series (compounds (1), (3) and (5)) present broader potential functions indicating that the change required a low energy, whereas in the case of the steric series (compounds (6) and (8)) the potential function are narrower thus indicating the steric hindrance operates to prevent the conformational


Page 21 of 60 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


change (Cu(II)/(I)) and the energy needed to reach this change is higher that the stabilization energy of the Cu-N bond. This theoretical analysis allows to explain the increase of the experimental formal potential observed in the steric series. The compound presenting the most narrowed torsional potential function corresponds to the complex (8) [Cu(I)(2,9-Me2-phen)2]+, that has the most positive formal potential determined experimentally by cyclic voltammetry.

Figure 1a also illustrates the voltammetry profile, after the cathodic polarization, in N2 saturated media (blue line). As mentioned above, for complexes belonging to the steric series the initial faradaic process attributed to Cu(II)/(I), total or partially disappear after the polarization curve in presence of oxygen, and new faradaic processes are detected. These changes could be associated to the interaction of Cu(I)-oxygen, since the electronic configuration of the Cu(I) center (3d10), has not crystalline field stabilization and the ligands can be exchange easily. The steric hindrance generated by the 2,9 substituents for the coordination of a fifth ligand (oxygen), induce the discoordination of a



Page 22 of 60

phenanthroline moiety allowing the interaction between the Cu(I) center with the O2 molecule70.

Polarization curves for the oxygen reduction reaction (ORR) were carried out in oxygen saturated media (figure 2), and the kinetic part (corrected by limiting current) is represented as red dashed lines in fig 1a. As can be observed, the onset potential of the electronic series is linked with the formal potential of the catalyst, however, for the steric series the onset potential is not linked with the initial complex but with the new faradaic process detected by cyclic voltammetry after the polarization as can be observed in the zoom of fig 1a. Table 2 summarize the electrochemistry parameters of the Cu-phen complexes.


Page 23 of 60 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


Figure 2. a) Polarization curves for the reduction of O2 catalyzed by Cu(I)-1,10phenanthroline derivatives adsorbed on carbon black ink and deposited over a glassy carbon electrode surface. b) Rotating disk polarization curves for ORR and Koutecky-



Page 24 of 60

Levich plots of the inverse of the plateau currents of top: complex (1) bottom: complex (8). c) Tafel plots for all complexes studied in this work.

Table 2. On-set potential, formal potential, number of transferred electrons and Tafel slope values of the modified electrodes with the substituted [Cu(I)(phen)2]ClO4 derivatives.

Complex

On-set potential

Eºcat (Cu(II)/(I)

(V vs NHE)

Nº of

Tafel slope.

electrons transferred .

(1) [Cu(I)(3,4,7,8-Me4-phen)2]ClO4

0.239

0.168

3.8 ± 0.2

-0.11 ± 0.02

(2) [Cu(I)(4,7-Me2-phen)2]ClO4

0.250

0.170

3.7 ± 0.3

-0.09 ± 0.01

(3) [Cu(I)(phen)2]ClO4

0.276

0.240

3.1 ± 0.2

-0.123

±

0.007 (4) [Cu(I)(5-Cl-phen)2]ClO4

0.293

0.277

3.1 ± 0.3

-0.122

±

0.009 (5) [Cu(I)(3,8-Br2-phen)2]ClO4

0.305

0.329

3.8 ± 0.3

-0.12 ± 0.02

(6) [Cu(I)(2-Cl-phen)2]ClO4

0.115

0,054

2.8 ± 0.4

-0.126

±

0.005


Page 25 of 60 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


(7)

[Cu(I)(2,9-Me2-4,7-Ph2- 0.384

0,401

2.9 ± 0.3

phen)2]ClO4

-0.120

±

0.009

(8) [Cu(I)(2,9-Me2-phen)2]ClO4

0.415

0.452

2.7 ± 0.3

-0.12 ± 0.01

The number of electrons (n) transferred per O2 molecule can be estimated using the Koutecky-Levich (K-L) equation (eq 1), and the results are shown in Figure 2b.

i(E)-1 = iK(E)-1 + iL-1

eq. 1.

where i(E) is the measured current at the potential E, iK(E) is the kinetic current at E and

iL is the Levich current:

iL = -0.62 n F A (DO )2/3 ω1/2 υ-1/6 C*O 2

eq.2.

2

n is the number of electrons transferred, F is the Faraday constant, A = 0.196 cm2 is the macroscopic geometric area of the electrode surface, DO = 1.8x10-5 cm2 s-1 is the 2

diffusion coefficient of O2 in aqueous solutions, ω is the angular rotation rate, υ = 0.009 cm2 s-1 is the kinematic viscosity of the solution, and C*O = 0.3 mM is the concentration 2

of O2 in the oxygen-saturated electrolyte33. 25 ACS Paragon Plus Environment


Page 26 of 60

The K-L plots shows that n varies from complex to complex but generally give values higher than 2, showing that O2 reduction proceeds by mixed mechanisms, via 2 e- to give peroxide and via 4 e- to give H2O. The complexes belonging to the electronic series in general give higher values of n, and approaching the value n = 4, indicating that little peroxide is formed and that they split the O-O bond. However, the number of electrons for steric series are much lower than 4 and more H2O2 is produced during the oxygen reduction. Further, during the reduction some radical oxygen species (ROS) such as OH• can be formed, which can degrade the substrate70, 71. The H2O2 and radical production also could contribute to the degradation of the initial complex as detected by cyclic voltammetry.

Some insights of the ET mechanism can be obtained from the Tafel plots illustrated in figure 2c. The Tafel plots were constructed from experimental rotating disk currents corrected by mass transport. As can be observed, the lines are nearly parallel with slopes close to -0.120 V, indicating that the ET mechanism does not change when changing the complexes. This value suggests that the rate limiting step is the first electron transfer


Page 27 of 60 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


concerted with the binding of the dioxygen on Cu(I) center. The ligand has been omitted for simplicity:

Mechanism (a):

Cu(I) + O2 + e-  Cu(I)O-O- (r.d.s)

eq. 3.

An alternative sequence also agrees with the Tafel slope and involves a chemical step, i.e. the formation of the adduct before a one electron transfer step as follows:

Mechanism (b):

Cu(I) + O2  Cu(I)O-O  Cu(II)-O-O-

eq. 4.

Cu(I)O-O  Cu(II)-O-O- + e-  Cu(I)O-O- (r.d.s)

eq. 5.

However mechanism (b) is unlikely since the first step (eq. 4) implies the formation of an adduct where O2 is stabilized and this will make next step more difficult, increasing the activation energy compared to free O2 and this is not catalytic. In mechanism (b), the adduct could exist in two resonant forms. Steps occurring after the rate determining steps



Page 28 of 60

are highly speculative but it can be suggested that for those catalysts where n is ca. 4, the peroxide formed after the transfer of a second electron remains attached to the Cu center until it is released after the transfer of 2 subsequent electrons to give water. In contrast, 2e- catalysts release peroxide after the transfer of a second electron. According to the experimental results, the complexes studied in this work probably operate via 4eand 2e- parallels pathways. Considering that the first step is rate determining, the current density could be expressed as the product of two contributions, a term containing the adsorption of dioxygen on Cu center and other considering the electrode potential. So as in any electrocatalytic inner sphere process, the kinetic is affected by two driving forces: the energy of adsorption and the applied potential both affecting the activation energy.

i = n Fk Cu(I) (pO2)(1-) exp-(Gºad/RT) exp (-F(E-Eeq)/RT)

eq. 6.

n is the total number of electrons, F has the usual meaning, k is a potential independent rate constant, pO2 is the oxygen partial pressure, E-Eeq is the overpotential for the rate determining step (mechanism a), Cu(I) is the surface concentration, and (1-)


Page 29 of 60 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


is the fraction of Cu(I) free site. From equation 6, the catalytic activity (log i) can be expressed at constant potential and extrapolated from the Tafel slope determined experimentally. In this study, a constant potential of 0 V has been chosen, as all the compounds are in the Cu(I) state, and therefore Cu(I) = total concentration of the complex on the surface. Assuming that the coverage of dioxygen adsorbed on the active sites obeys a Langmuir isotherm the fraction of the occupied active sites by O2 can be expressed as:

= (pO2) exp (Gºad/RT)/(1+(pO2) exp (Gºad/RT)).

eq. 7.

As found previously for ORR on MN4 macrocyclic compounds, there is a correlation between the formal potential and the adsorption standard free energy as:

Gºad = FEº’ + const.

eq. 8.

Where E°’ is the formal potential of the Cu(II)/(I) couple so considering this and combining eq. 6 and 7, the catalytic activity could be expressed as:

i = n F K exp ( F Eº’/RT)

eq. 9.



Page 30 of 60

Where:

K= k (O2) exp (- nFEcte/RT)

eq. 10.

Equation 9 predicts a slope lni/E° of RT/ F of a lni versus E° Cu(II)/(I) plot.

Figure 3a shows correlations of catalytic activity of the different complexes. As it can be appreciated from the different CV profiles illustrated in Figure 1a all complexes exhibit different coverages, according to the electrical charge integrated under the peaks after subtracting the capacitive base line. We assume that the reaction is first order in active sites, so it is possible to normalize the currents dividing their values by the surface concentration of each catalyst. For the specific case of the steric series in the representation (log i vs Eº`), the formal potential was determined after the polarization tests as they change during the polarization. If one plots the initial values for these catalysts they show no correlation with the rest of the data (S.I. fig S1). There is a linear relationship (blue line) between the catalytic activity and Eº’

Cu(II)/(I)

as predicted by

equation 9. The catalytic activity increases as the formal potential goes more positive, indicating that the formal potential of the Cu complexes is also a reactivity index for


Page 31 of 60 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


oxygen reduction reaction. The same trend is observed for MN4 macrocyclic complexes independently of the substrate3,

4, 9, 10.

The slope of this relation is close to +0.120

V/decade or RT/F indicating that the Brönsted coefficient is close to 0.5, assuming that the formal potential is directly related to Gºad.



Page 32 of 60

Figure 3. Experimental and theoretical electrocatalytic trends: (a) correlation of the catalytic activity, expressed by log(i/), versus the formal potential of the complexes (Eº’Cu(II)/Cu(I)), (b) correlation of the catalytic activity, expressed by log(i/), versus the calculated oxygen binding energies Eb, (c) correlation of the catalytic activity, expressed by log(i/), versus the reaction energy (Erx).

The mechanism proposed implies that the rate determining step is the first electron transfer concerted with O2 adsorption and the formation of the adduct Cu(I)-O-O. By theoretical calculations, the stability of this adduct considering the spin density distribution of the adduct, after the dioxygen adsorption on the complexes was estimated. These calculations included complexes belonging to the electronic and steric series (SI, Fig. S2


Page 33 of 60 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


and Table S6). The structures were optimized at its minimum in energy, assuming the first electron transfer to Cu(I)-O-O [Cu(I)(2-Cl-phen)2-O2] structure of the steric series clearly showed a ligand detachment from the Cu(I) coordination sphere, with Cu-N distances ranging from 2.1 to 3.1 Å (Figure 4a, Table S6), as was found experimentally by cyclic voltammetry after the ORR, where the initial faradic process disappears partially or totally. However, the Cu-N distances obtained for the optimized structures of the electronic series do not exceed 2.3 Å, evidencing that for the electronic series the complexes can adopt the 5th coordination with oxygen without de-coordination of the phenanthroline ligands. Based on the experimental and theoretical results, the steric series can promote the oxygen reduction reaction only with the de-coordination of the phenanthroline ligand, as proposed for the catalytic ORR in homogeneous media.



Page 34 of 60


Page 35 of 60 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


Figure 4. (a) Optimized structures of Cu(I)-O2- adducts for complexes 3 and 6 of the electronic and steric series, respectively, showing the detachment of one ligand in 6. The purple lobes represent the spin density of the complexes due to the transfer of one electron (charge 0, multiplicity 2). Values of bond distances and spin density are listed in Table S6; (b) Intrinsic Reaction Coordinate (IRC) profile of complex 3; (c) bond order and (d) natural atomic charges for the Cu and oxygen atoms of 3 through the IRC of Figure 4b.

As already mentioned, the Cu(I)-O-O binding energy also plays a key role in the ORR activity. By DFT calculations69, a theoretical analysis of the binding energies associated with the dioxygen adsorption on the Cu(I) active site was performed. The calculations were focused only on the electronic series where the structure of the catalytic complexes is known and any change is detected after the ORR. The adduct O2 can be assumed to be in two resonant structures Cu(I)-O2  Cu(II)-O2-, however, under polarization conditions where the potential is more negative than the Cu(II)/(I) redox potential we can assume the Cu(I)-O2 is the active structure. The binding energies,



Page 36 of 60

calculated by the energy difference between the Cu(I)-O2-transition-state (Cu(I)-O2-TS) adduct and the isolated Cu(I)-phen derivatives and O2 species (Eb = E(Cu(I)-O2-TS)-{E(Cu(I)-

phen-derivative)

+ E(O2)}), have been represented vs. the catalytic activity of the catalysts in

Figure 3b. The representation shows a negative trend between them, following eq. 6 and eq. 8, where the catalytic activity increases when the Cu(I)-O2-TS becomes more stable (more negative adsorption energies). These results indicate that according to the Sabatier principle72 the catalytic activity of Cu(I)-phen derivatives are located on the weak binding side of an incomplete volcano correlation26, and hence, hypothetically the highest catalytic activity will be reached when the adsorption energy decrease to a point of the maximum in a volcano correlation.

To evaluate in a deeper insight the O2 adsorption process occurring as first step of the ORR, a theoretical mechanistic evaluation was carried out by means of the intrinsic reaction coordinate (IRC)60 procedure at the M06-2X/6-31G(d,p) level of calculation, which requires a stable Cu(I)-O2-TS as starting point structure (see SI Figure S3 and Computational Details). To the best of our knowledge, the IRC protocol has never been


Page 37 of 60 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


used before for this purpose. The IRC profiles were calculated for three representative complexes of the electronic series. In Figure 4b the IRC energy profile of the O2 binding pathway for complex 3 is presented. The profile shows that the Cu(I)-O2 adduct formation have an exothermic character, that goes down until reach a [Cu(I)(phen)2)]+-O2 species is formed in which the O2 molecule adopts the end-on configuration. Comparing the associated reaction energies, ΔErx, for 2, 3 and 5 complexes with their catalytic activity, a linear correlation is found where the activity increases when the reaction energy is less negative. This indicates that for complex with high activity, the energy required to extract the Cu(I)-O2 adduct from the potential well, reached during the O2 adsorption, will be lower. The link between the experimental ORR activities and the ΔErx indicates that, from a thermodynamic point of view, ΔErx is also a suitable new ORR reactivity descriptor. By the determination of the bond orders (Figure 4c) and the Cu(I) (Cu1) and O2 (O46-O47) atomic natural charges (Figure 4d) along reaction coordinate, ξ, during the formation of the (Cu(I)-O-O) adduct, the decrease in the bond order for di-oxygen, as expected, is concomitant with the formation of the Cu(I)-O-O. The study of the natural charge indicates an increase of the positive charge of copper due to the polarization once the ET is reached 37 ACS Paragon Plus Environment


Page 38 of 60

agrees with a negative polarization of the di-oxygen molecule. Both analysis indicates that the Cu(I)-O-O adduct is electronically and structurally facilitated to the subsequent electron transfer step in ORR, in agreement with the proposed mechanism.

4. CONCLUSIONS.

We have found that reactivity descriptors found previously for MN4 macrocyclic complexes, which have rather flat molecular structures also apply to Cu-phenanthrolines where the molecular structure changes from Cu(II)(square-planar) to Cu(I)(tetrahedral) upon reduction of the metal center to the catalytically active state Cu(I). In this case the reactivity predictor is the Cu(II)/(I) redox couple. In contrast for MN4 macrocyclics where the active state is M(II) and the reactivity predictor is the M(III)/(II) redox potential. In this work we have modulated the Cu(II)/(I) redox potential by using substituents possessing different electron withdrawing properties or by placing bulky groups on the phenanthroline ligand that can hinder the Cu(II)(square-planar) to Cu(I)(tetrahedral) conformational change necessary to generate the Cu(I) active site and this is reflected in a shift of the Cu(II) + e Cu(I) process to more positive potentials. Another interesting finding is that all Cu-


Page 39 of 60 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


phenanthrolines, the electronic series and the steric series group in one single linear correlation when comparing catalytic activities as (log i)E versus the Cu(II)/Cu(I) redox potential emphasizing once more that the redox potential seems to be an universal reactivity predictor for molecular catalysts, regardless of their molecular structure or the factors that shift the formal potential in any direction. As found for MN4 macrocyclic molecular catalysts, there is a linear correlation between the M-O2 binding energy and the formal potential of the catalysts so both parameters are reactivity predictors.

Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxxxx.

Details of experimental procedures and characterization of the studied complexes, Graphic of correlation of the catalytic activity of both series of complexes, Table of selected metric parameters for reported crystalline structures of [Cu(I)(phen)2]+ derivatives, and Table of selected structural parameters for the optimized structures by DFT methods. Table with energy values obtained theoretically for rotated conformations 39 ACS Paragon Plus Environment


Page 40 of 60

of calculated complexes, selected parameters of the IRC theoretical study, graphic representation of the energy surfaces of calculated IRC pathways, and graphical representation of the spin densities of Cu(I)-O2- adducts for both series of complexes, accompanied with a table including selected metric values of the optimized Cu(I)-O2adducts and the computed Mulliken spin density values.

AUTHOR INFORMATION

Corresponding Author *Dr. José H. Zagal: [email protected]

*Dr. F. Javier Recio: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors have contributed equally. Acknowledgments

This work was supported by Comisión Nacional de Investigación Científica y Tecnológica (CONICYT) – Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT)


Page 41 of 60 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


postdoctoral projects 3170330 and 3180509 and FONDECYT regular projects 1161117, 1181037 and 1181072. Abbreviations ORR, oxygen reduction reaction; NPMC, non-precious metal catalysts; MP, metalloporphyrins; MPc, metallophthalocyanines; IRC, Intrinsic Reaction Coordinate.

REFERENCES

(1) Zagal, J. H.; Griveau, S.; Silva, J. F.; Nyokong, T.; Bedioui, F. Metallophthalocyaninebased molecular materials as catalysts for electrochemical reactions. Coord. Chem. Rev. 2010, 254, 2755 – 2791.

(2) Masa, J.; Ozoemena, K.; Schuhmann, W.; Zagal, J. H. Oxygen reduction reaction using N4-metallomacrocyclic catalysts: fundamentals on rational catalyst design. J.

Porphyr. Phthalocya. 2012, 16, 761 – 784.

(3) Zagal, J. H.; Recio, F. J.; Gutierrez, C. A.; Zuñiga, C.; Páez, M. A.; Caro, C. A. Towards a unified way of comparing the electrocatalytic activity MN4 macrocyclic metal catalysts



Page 42 of 60

for O2 reduction on the basis of the reversible potential of the reaction. Electrochem.

Commun. 2014, 41, 24 – 26.

(4) Zagal, J. H.; Koper, M. T. M. Reactivity descriptors for the activity of molecular MN4 catalysts for the oxygen reduction reaction. Angew. Chem. Int. Ed. 2016, 55, 2 – 14.

(5) Yamazaki; S. I. Metalloporphyrins and related metallomacrocycles as electrocatalysts for use in polymer electrolyte fuel cells and water electrolyzers. Coord. Chem. Rev. 2018,

373, 148 – 166.

(6) Guo, S.; Zhang, S.; Sun, S. Tuning nanoparticle catalysis for the oxygen reduction reaction. Angew. Chem. Int. Ed. 2013, 52, 8526−8544.

(7) Nie, Y.; Li, L.; Wei, Z. Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction. Chem. Soc. Rev. 2015, 44, 2168−2201.

(8) Wang, Y. J.; Zhao, N.; Fang, B.; Li, H.; Bi, X. T.; Wang, H. Carbon-supported Pt-based alloy electrocatalysts for the oxygen reduction reaction in polymer electrolyte membrane


Page 43 of 60 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


fuel cells: particle size, shape, and composition manipulation and their impact to activity.

Chem. Rev. 2015, 115, 3433−3467.

(9) Venegas, R.; Recio, F. J.; Zuñiga, C.; Viera, M.; Oyarzún, M. P.; Silva, N.; Neira, K.; Marco, J. F.; Zagal, J. H.; Tasca, F. Comparison of the catalytic activity for O2 reduction of Fe and Co MN4 adsorbed on graphite electrodes and on carbon nanotubes. Phys.

Chem. Chem. Phys. 2017, 19, 20441 – 20450.

(10) Venegas, R.; Recio, F. J.; Riquelme, J.; Neira, K.; Marco, J. F.; Ponce, I.; Zagal, J. H.; Tasca, F. Biomimetic reduction of O2 in an acid medium on iron phthalocyanines axially coordinated to pyridine anchored on carbon nanotubes. J. Mater. Chem. A, 2017,

5, 12054 – 12059.

(11) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt. Science 2011, 332, 443 – 447.



Page 44 of 60

(12) Proietti, E.; Jaouen, F.; Lefèvre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J. P. Iron-based cathode catalyst with enhanced power density in polymer electrolyte membrane fuel cells. Nat. Commun. 2011, 2, 416.

(13) Serov, A.; Artyushkova, K.; Niangar, E.; Wang, C.; Dale, N.; Jaouen, F.; Sougrati, M. T.; Jia, Q.; Mukerjee, S.; Atanassov, P. Nano-structured non-platinum catalysts for automotive fuel cell application. Nano Energy 2015, 16, 293 – 300.

(14) Jia, Q.; Ramaswamy, N.; Tylus, U.; Strickland, K.; Li, J.; Serov, A.; Artyushkova, K.; Atanassov, P.; Anibal, J.; et al. Spectroscopic insights into the nature of active sites in iron–nitrogen–carbon electrocatalysts for oxygen reduction in acid. Nano Energy 2016,

29, 65 – 82.

(15) Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J.; Jia, Q.; Stamatin, S.; Harrington, G. F.; Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F. Identification of catalytic sites in cobalt-nitrogen-carbon materials for the oxygen reduction reaction. Nat. Commun. 2017, 8, 957.


Page 45 of 60 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


(16) Workman, M. J.; Serov, A.; Tsui, L. K.; Atanassov, P.; Artyushkova, K. Fe–N–C catalyst graphitic layer structure and fuel cell performance. ACS Energy Lett. 2017, 2, 1489 − 1493.

(17) Singh, K.; Razmjooei, F.; Yu, J. S. Active sites and factors influencing them for efficient oxygen reduction reaction in metal-N coordinated pyrolyzed and non-pyrolyzed catalysts: a review. J. Mater. Chem. A, 2017, 5, 20095 – 20119.

(18) Gulppi, M. A.; Recio, F. J.; Tasca, F.; Ochoa, G.; Silva, J. F.; Pavez, J.; Zagal, J. H. Optimizing the reactivity of surface confined cobalt N4-macrocyclics for the electrocatalytic oxidation of l-cysteine by tuning the Co(II)/(I) formal potential of the catalyst. Electrochim. Acta. 2014, 126, 37 – 41.

(19) Gutierrez, C.A.; Silva, J. F.; Recio, F. J.; Griveau, S.; Bedioui, Caro, C.A.; Zagal, J. H. In search of the best iron N4-Macrocyclic catalysts adsorbed on graphite electrodes and on multi-walled carbon nanotubes for the oxidation of l-cysteine by adjusting the Fe(II)/(I) formal potential of the complex. Electrocatalysis 2014, 5, 426 – 437.



Page 46 of 60

(20) Ochoa, G.; Gutierrez, C.; Ponce, I.; Silva J. F.; Paez, M.; Pavez, J.; Zagal, J. H. Reactivity trends of surface-confined Co-tetraphenyl porphyrins and vitamin B12 for the oxidation of 2-aminoethanethiol: comparison with Co-phthalocyanines and oxidation of other thiols. J. Electroanal. Chem. 2010, 639, 88 – 94.

(21) Recio, F. J.; Cañete, P.; Tasca, F.; Linares-Flores, C.; Zagal, J. H. Tuning the Fe(II)/(I) formal potential of the FeN4 catalysts adsorbed on graphite electrodes to the reversible potential of the reaction for maximum activity: hydrazine oxidation.

Electrochem. Comm. 2013, 30, 34 – 37.

(22) Recio, F. J.; Gutierrez, C. A.; Venegas, R.; Linares-Flores, C.; Caro, C. A.; Zagal, J. H. Optimization of the electrocatalytic activity of MN4-macrocyclics adsorbed on graphite electrodes for the electrochemical oxidation of L-cysteine by tuning the M(II)/(I) formal potential of the catalyst: an overview. Electrochim. Acta. 2014, 140, 482 – 488.

(23) Tasca, F.; Recio, F. J.; Venegas, R.; Geraldo, D. A.; Sancy, M.; Zagal, J. H. Linear versus volcano correlations for the electrocatalytic oxidation of hydrazine on graphite


Page 47 of 60 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


electrodes modified with MN4 macrocyclic complexes. Electrochim. Acta. 2014, 140, 314 – 319.

(24) Zúñiga, C.; Tasca, F.; Calderon, S.; Farías, D.; Recio, F. J.; Zagal, J. H. Reactivity indexes for the electrocatalytic oxidation of hydrogen peroxide promoted by several ligand-substituted and unsubstituted Co phthalocyanines adsorbed on graphite. J.

Electroanal. Chem. 2016, 765, 22 – 29.

(25) Silva, N.; Calderon, S.; Paez, M. A.; Oyarzun, M. P.; Koper, M. T. M.; Zagal, J. H. Probing the Fen +/Fe(n − 1)+ redox potential of Fe phthalocyanines and Fe porphyrins as a reactivity descriptor in the electrochemical oxidation of cysteamine. J. Electroanal. Chem. 2018, 819, 502 – 510.

(26) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. Origin of the overpotential for oxygen reduction at a fuel-cell cathode. J. Phys. Chem. B 2004, 108, 17886 – 17892.



Page 48 of 60

(27) Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T.F. Combining theory and experiment in electrocatalysis: insights into materials design.

Science 2017, 355, 146.

(28) Alt, H.; Binder, H.; Sandstede, G. Mechanism of the electrocatalytic reduction of oxygen on metal chelates. J. Catal. 1973, 28, 8-19.

(29) Randin, J. P. Interpretation of the relative electrochemical activity of various metal phthalocyanines for the oxygen reduction reaction. Electrochim. Acta, 1974, 19, 2, 83-85.

(30) Hipps, K. W.; Lu, X.; Wang, X. D.; Mazur, U. Metal d-orbital occupation-dependent images in the scanning tunneling microscopy of metal phthalocyanines. J. Phys. Chem. 1996, 100, 11207–11210.

(31) Reis, R. M.; Valim, R. B.; Rocha, R. S.; Lima, A. S.; Castro, P. S.; Bertotti, M.; Lanza, M. R. V. The use of copper and cobalt phthalocyanines as electrocatalysts for the oxygen reduction reaction in acid medium. Electrochim. Acta 2014, 139, 1–6.


Page 49 of 60 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


(32) Thorseth, M. A.; Tornow, C. E.; Tse, E. C. M.; Gewirth, A. A. Cu complexes that catalyze the oxygen reduction reaction. Coord. Chem. Rev. 2013, 257, 130 – 139.

(33) McCrory, C. C. L.: Ottenwaelder, X.; Stack, T. D. P.; Chidsey, C. E. D. Kinetic and mechanistic studies of the electrocatalytic reduction of O2 to H2O with mononuclear Cu complexes of substituted 1,10-phenanthrolines. J. Phys. Chem. A 2007, 111, 1264112650.

(34) Solomon, E. I.; Agustine, A. J.; Yoon, J. O2 Reduction to H2O by the multicopper oxidases. Dalton Trans. 2008, 30, 3921 – 3932.

(35) Solomon, E. I.; Sundaram, U.; Machonkin, T. Multicopper oxidases and oxygenases.

Chem. Rev. 1996, 96, 2563-2605.

(36) Solomon, E.; Heppner, D. E.; Johnston, E. M.; Ginsbach, J. W.; Cirera, J.; Qayyum, M.; Kieber-Emmons, M. T.; Kjaergaard, C. H.; Hadt, R. G.; Tian, L. Copper active sites in biology. Chem. Rev. 2014, 114, 3659−3853.



Page 50 of 60

(37) Zagal, J.; Spodine, E.; Zamudio, W. Oxidation of bis(phenanthroline)copper(I) complexes by molecular oxygen in nitromethane solution. J. Chem. Soc., Dalton, 1974, 85 – 87.

(38) Crumbliss, A. L.; Gestaut, L. J. Electronic effects on the rate of oxidation of substituted phenanthroline complexes of copper(I) by molecular oxygen.

J. Coord.

Chem., 1976, 5, 109 – 11.

(39) Ichinaga, A. K.; Kirchhoff, J. R.; McMillin, D. R.; Dietrich-Buchecker, C. O.; Marnot, P. A.; Sauvage, J. P. Charge-transfer absorption and emission of Cu(NN)2+ systems.

Inorg. Chem. 1987, 26, 4290 – 4292.

(40) Hua, L.; Iwamura, M.; Takeuchi, S.; Tahara, T. The substituent effect on the MLCT excited state dynamics of Cu(I) complexes studied by femtosecond time-resolved absorption and observation of coherent nuclear wavepacket motion. Phys. Chem. Chem.

Phys. 2015, 17, 2067 – 2077.


Page 51 of 60 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


(41) Zou, L.-Y.; Ma, M.-S.; Zhang, Z.-L.; Li, H., Cheng, Y.-X.; Ren, A.-M. Theoretical study on photophysical property of cuprous bis-phenanthroline coordination complexes. Org.

Electron. 2012, 13, 2627-2638.

(42) Lavie-Cambot, A.; Cantuel, M.; Leydet, Y.; Jonusauskas, G.; Bassani, D. M.; McClenaghan,

N.

D.;

Improving

the

photophysical

properties

of

copper(I)

bis(phenanthroline) complexes. Coord. Chem. Rev. 2008, 252, 2572-2584.

(43) Lei, Y.; Anson, F. C. Dynamics of the coordination equilibria in solutions containing copper(II), copper(I), and 2,9-dimethyl-1,10-phenanthroline and their effect on the reduction of O2 by Cu(I). Inorg. Chem. 1995, 34, 1083-1089.

(44) Pilo, M. I.; Manca, G.; Zoroddu, M. A.; Seeber, R. Electrochemical properties of copper complexes with unsubstituted and substituted 1,10-o-phenanthrolines in N,Ndimethylformamide solvent. Inorg. Chim. Acta 1991, 180, 225-230.



Page 52 of 60

(45) Zhang, J.; Anson, F. C. Electrocatalysts for the reduction of O2 and H2O2 based on complexes of Cu(II) with the strongly adsorbing 2,9-dimethyl-1,10-phenanthroline ligand.

Electrochim. Acta 1993, 38, 2423-2429.

(46) Lei, Y.; Anson, F. C. Mechanistic aspects of the electroreduction of dioxygen as catalyzed by copper-phenanthroline complexes adsorbed on graphite electrodes. Inorg.

Chem. 1994, 33, 5003-5009.

(47) McCrory, C. C. L.; Devadoss, A.; Ottenwaelder, X.; Lowe, R. D.; Stack, T. D. P.; Chidsey, C. E. D. Electrocatalytic O2 reduction by covalently immobilized mononuclear copper (I) complexes: Evidence for a binuclear Cu2O2 intermediate. J. Am. Chem. Soc. 2011, 133, 3696 – 3699.

(48) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Structural and functional aspects of metal sites in biology. Chem. Rev. 1996, 96, 2239−2314.


Page 53 of 60 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


(49) Tse, E. C. M.; Schilter, D.; Gray, D. L.; Rauchfuss, T. B.; Gewirth, A. A. Multicopper models for the Lacasse active site: Effect of nuclearity on electrocatalytic oxygen reduction. Inorg. Chem. 2014, 53, 8505 – 8516.

(50) Thorseth, M. A.; Letko, C. S.; Rauchfuss, T. B.; Gewirth, A. A. Dioxygen and hydrogen perodixe reduction with hemocyanin model complexes. Inorg. Chem. 2011, 50, 6158 – 6162.

(51) Thiyagarajan, N.; Janmanchi, D.; Tsai, Y. F.; Wanna, W. H.; Ramu, R.; Chan, S. I.; Zen, J. M.; Yu, S. S. F. A carbon electrode functionalized by a tricopper cluster complex: overcoming overpotential and production of hydrogen peroxide in the oxygen reduction reaction. Angew. Chem. 2018, 130, 3674 – 3678.

(52) Tzalis, D.; Tor, Y.; Salvatorre, F.; Siegel, S. J. Simple one-step synthesis of 3-bromoand 3,8-dibromo-1,10-phenanthroline: Fundamental building blocks in the design of metal chelates. Tetrahedron Lett. 1995, 36, 3489 – 3490.



Page 54 of 60

(53) Lewis, J.; O’Donghue, T. D. Chemistry of polydentate ligands. Part 5. Complexes of 2,9-di-hydrazino-derivatives of 1,10-phenanthroline. Dependence of co-ordination number of a ligand on the anion present. J. Chem. Soc. Dalton 1980, 0, 736 – 742.

(54) Pallenberg, A. J.; Koenig, K. S.; Barnhart, D. M. Synthesis and characterization of some copper(I) phenanthroline complexes. Inorg. Chem. 1995, 34, 2833 – 2840.

(55) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C. et al.

Gaussian 03, Revision D.2; Gaussian, Inc.: Pittsburgh, PA, 2003.

(56) Adamo, C.; Barone, V. Toward reliable density functional methods without adjustable parameters: The PBE0 model. J. Chem. Phys. 1999, 110, 6158 – 6170.

(57) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy.

Phys. Chem. Chem. Phys. 2005, 7, 3297 – 3305.


Page 55 of 60 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


(58) Hay, P. J.; Wadt, W. R. Ab initio effective core potentials for molecular calculations. Potentials for K to Au including the outermost core orbitals. J. Chem. Phys. 1985, 82, 299 – 310.

(59) Fukui, K. Formulation of the reaction coordinate. J. Phys. Chem. 1970, 74, 4161 − 1463.

(60) Fukui, K. The path of chemical reactions - the IRC approach. Acc. Chem. Res. 1981,

14, 363 – 368.

(61) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Account 2008, 120, 215 – 241.

(62) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.; Shirley, W. A.; Mantzaris, J. A complete basis set model chemistry. I. The total energies of closed‐shell atoms and hydrides of the first‐row elements. J. Chem. Phys. 1988, 89, 2193 – 2218.



Page 56 of 60

(63) Peterson, G. A.; Al-Laham, M. A. A complete basis set model chemistry. II. Open‐shell systems and the total energies of the first‐row atoms. J. Chem. Phys. 1991,

94, 6081 – 6090.

(64) Mayer, I. Charge, bond order and valence in the AB initio SCF theory. Chem. Phys.

Lett. 1983, 97, 270 – 274.

(65) Mayer, I. Bond order and valence: Relations to Mulliken's population analysis. Int. J.

Quantum Chem. 1984, 26, 151 – 154.

(66) Reed, A. E.; Weinstock, R. B.; Weinhold, F. Natural population analysis. J. Chem.

Phys. 1985, 83, 735 – 746.

(67) Zhurko, G. A.; Zhurko, D. A. Chemcraft, version 1.8, 2005; Graphical software for visualization of quantum chemistry computations. http://www.chemcraftprog.com

(68) Sanna, G.; Pilo, M. I.; Zoroddu, M. A.; Seeber, R.; Mosca, S. Electrochemical and spectroelectrochemical study of copper complexes with 1,10-phenanthrolines. Inorg.

Chim. Acta. 1993, 208, 153 – 158.


Page 57 of 60 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


(69) Parr, R.; Yang, W. T. Density Functional Theory of Atoms and Molecules; Oxford University Press: Oxford, UK, 1989.

(70) Romo, A. I. B.; Abreu, D. S.; Paulo, T. F.; Carepo, M. S. P.; Sousa, E. H. S.; Lemus, L.; Aliaga, C.; Batista, A. A.; NAscimiento, O. R.; Abruña, H. D. et al. Hydroxyl radical generation

and

DNA

nuclease

activity:

a

mechanistic

study

based

on

a

surface‐immobilized copper thioether clip‐phen derivative. Chem. Eur. J. 2016, 22, 10081 – 10089.

(71) Elwell, C. E.; Gagnon, N. L.; Neisen, B. D.; Dhar, D.; Spaeth, A. D.; Yee, G. M.; Tolman, W. B. Copper-oxygen complexes revisited: structure, spectroscopy, and reactivity. Chem. Rev. 2017, 117, 2059 – 2107.

(72) Chorkendorff, I.; Niemantsverdriet, J. W. Concepts of Modern Catalysis and Kinetics, 2nd ed.; Wiley-VCH Verlag GmbH & Co KGaA: Weinheim, Germany, 2007.

FIGURE CAPTIONS Figure 1. (a) Electrochemical characterization of the substituted [Cu(I)(phen)2]ClO4 derivatives at pH 5.2 under N2 (black lines), showing the polarization curves obtained in 57 ACS Paragon Plus Environment


Page 58 of 60

saturated O2 media at pH 5.2 (red dashed lines). (Amplification) Details of the cyclic voltammetry of the steric series before (gray line) and after (blue line) the ORR. (b) Potential energy profile constructed in function of the torsional angle () for complexes of the electronic series ((1), (3) and (5)) and the steric series ((6) and (8)).

Figure 2. a) Polarization curves for the reduction of O2 catalyzed by Cu(I)-1,10phenanthroline derivatives adsorbed on carbon black ink and deposited over a glassy carbon electrode surface. b) Rotating polarization curves for ORR and Koutecky-Levich plots of the inverse of the plateau currents of up: complex (1) bot. complex (8). c) Tafel plots for all complexes used in this work.

Figure 3. Experimental and theoretical electrocatalytic trends: (a) correlation of the catalytic activity, expressed by log(i/), versus the formal potential of the complexes (Eº’Cu(II)/Cu(I)), (b) correlation of the catalytic activity, expressed by log(i/), versus the calculated oxygen binding energies Eb, (c) correlation of the catalytic activity, expressed by log(i/), versus the reaction energy (Erx).


Page 59 of 60 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


Figure 4. (a) Optimized structures of Cu(I)-O2- adducts for complexes 3 and 6 of the electronic and steric series, respectively, showing the detachment of one ligand in 6. The purple lobes represent the spin density of the complexes due to the transfer of one electron (charge 0, multiplicity 2). Values of bond distances and spin density are listed in Table S6; (b) Intrinsic Reaction Coordinate (IRC) profile of complex 3; (c) bond order and (d) natural atomic charges for the Cu and oxygen atoms of 3 through the IRC of Figure 4b.



Page 60 of 60

TOC Graphic


Theoretical and Experimental Reactivity Predictors for the

Recommend Documents