Electrocatalytic Reduction of Dioxygen to Hydrogen Peroxide by a Molecular Manganese Complex with a Bipyridine-Containing Schiff Base Ligand

Article abstract of DOI:10.1021/jacs.7b09027

The synthesis and electrocatalytic reduction of dioxygen by a molecular manganese(III) complex with a tetradentate dianionic bipyridine-based ligand is reported. Electrochemical characterization indicates a Nernstian dependence on the added proton source for the reduction of Mn(III) to Mn(II). The resultant species is competent for the reduction of dioxygen to H₂O₂ with 81 ± 4% Faradaic efficiency. Mechanistic studies suggest that the catalytically active species has been generated through the interaction of the added proton donor and the parent Mn complex, resulting in the protonation of a coordinated phenolate moiety following the single-electron reduction, generating a neutral species with a vacant coordination site at the metal center. As a consequence, the active catalyst has a pendent proton source in close proximity to the active site for subsequent intramolecular reactions.

Full text of DOI:10.1021/jacs.7b09027

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Article
Electrocatalytic Reduction of Dioxygen to Hydrogen Peroxide by a Molecular
Manganese Complex with a Bipyridine-Containing Schiff Base Ligand
Shelby Hooe, Arnold L. Rheingold, and Charles W. Machan
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09027 • Publication Date (Web): 08 Dec 2017
Downloaded from http://pubs.acs.org on December 8, 2017
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Electrocatalytic Reduction of Dioxygen to Hydrogen Peroxide by a Molecular
Manganese Complex with a BipyridineꢀContaining Schiff Base Ligand
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Shelby L. Hooea, Arnold L. Rheingoldb, Charles W. Machana,*
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a – Department of Chemistry, University of Virginia, McCormick Road PO Box 400319,
Charlottesville, VA 22904ꢀ4319
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b – Department of Chemistry & Biochemistry, University of California, San Diego, La Jolꢀ
la, CA 92093ꢀ0358.
KEYWORDS (Word Style “BG_Keywords”). If you are submitting your paper to a journal
that requires keywords, provide significant keywords to aid the reader in literature reꢀ
trieval.
ABSTRACT: The synthesis and electrocatalytic reduction of dioxygen by a molecular
manganese (III) complex with a tetradentate dianionic bipyridineꢀbased ligand is reportꢀ
ed. Electrochemical characterization suggests that upon reduction to Mn(II), the coordiꢀ
nated phenolate moieties of the ligand are protonated with a Nernstian dependence on
the added proton source, allowing protons and electrons to be incorporated into the
overall complex with retention of a vacant axial coordination site. The resultant species
is competent for the reduction of dioxygen to H2O2 with 81±4% Faradaic efficiency.
Mechanistic studies suggest that the catalytically active species has been generated
through phenol acting as a proton donor to the Mn complex following a singleꢀelectron
reduction of the parent species, generating a neutral species with a vacant coordination
site at the metal center. As a consequence, the active catalyst has a pendent proton
source in close proximity to the active site for subsequent intramolecular reactions.
INTRODUCTION
The reduction of dioxygen (O2) to water (H2O) or hydrogen peroxide (H2O2) has imꢀ
portance in biological systems, oxidative functionalization reactions, and fuel cells.1ꢀ3
Heterogeneous Pt electrodes are among the most efficient at catalyzing the oxygen reꢀ
duction reaction (ORR), but natural metalloenzymes commonly use firstꢀrow transition
metals to achieve analogous reactivity. Of the abiotic molecular species studied for the
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ORR, manganese (Mn) appears infrequently.4ꢀ15 The molecular Mn complexes which
have been shown to electrocatalytically reduce O2 have either been porphyrins or porꢀ
phyrinꢀderivatives and were examined under aqueous conditions while adsorbed to the
electrode.4ꢀ10, 15 It is conspicuous that molecular Mn species can have profound thermal
reactivity with O2,1, 16ꢀ22 yet so few active homogeneous electrocatalytic systems have
been identified.
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The activity of Feꢀ, Coꢀ, and Mnꢀbased salens with O2 is wellꢀestablished and even
used as a deliberate component in their synthesis.23ꢀ26 Salicaldehydeꢀderived imines are
notꢀideal for use in ORR given their inherent instability: the condensation reaction to
produce Schiff base salenꢀtype ligands is in an equilibrium with H2O, which can be a
solvent, substrate, and product for the ORR. To address this deficiency, we were interꢀ
ested in a Schiff base ligand platform derived from 2,2′ꢀbipyridine (bpy) obtained
through synthetic modification at the 6,6′ꢀpositions with 2ꢀhydroxybenzene moieties.27ꢀ28
This ligand platform accesses a similar coordination environment to traditional salenꢀ
type ligands without relying on chemically/electrochemically reactive imine bonds as
structural elements. To the best of our knowledge, this report represents the first homoꢀ
geneous molecular Mn electrocatalyst for the ORR.
RESULTS
Synthesis and Characterization
Previous syntheses of bpyꢀ or 1,10ꢀphenanthrolineꢀbased ligands symmetrically
functionalized with 2ꢀhydroxybenzene moieties relied on lowꢀyielding dearomatization
reactions with alkyllithium reagents.28ꢀ30 A significant improvement in overall reaction
yield can be achieved through the use of a Suzukiꢀtype crossꢀcoupling reaction.27, 31
Beginning from 2,4ꢀdiꢀtertꢀbutylꢀphenol, bromination in CH2Cl2 with Br2 enabled the isoꢀ
lation of 2ꢀbromoꢀ4,6ꢀdiꢀtertꢀbutylꢀphenol in near quantitative yields. Treatment of the
brominated intermediate with nꢀbutyllithium, followed by B(OMe)3, and a final acidic
aqueous workꢀup was used to isolate the corresponding boronic acid. After a Pdꢀ
catalyzed crossꢀcoupling reaction with 6,6′ꢀdibromoꢀ2,2′ꢀbipyridine, 6,6′ꢀdi(3,5ꢀdiꢀtertꢀ
butylꢀ2ꢀhydroxybenzene)ꢀ2,2′ꢀbipyridine (tbudhbpy(H)2) could be obtained. Metalation of
(tbudhbpy(H)2) to generate Mn(tbudhbpy)Cl 1 was achieved after 2 h under refluxing conꢀ
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ditions in MeOH with manganese(II) acetate tetrahydrate, sparging with compressed air,
and precipitation with saturated brine solution.
UVꢀvis (Figure S1) and NMR spectroscopies (Figures S2ꢀS3), as well as ESIꢀMS
(Figure S4) and microanalysis are consistent with the proposed structure of the ligand
and the Mn complex shown in Figure 1. Evan’s method measurements in acetonitrile
(MeCN) exhibited a of ꢁeff = 4.82±0.03, consistent with a highꢀspin d4 Mn(III) complex
(Table S1).32ꢀ33 Single crystals suitable for Xꢀray diffraction studies were grown by layerꢀ
ing hexanes over a solution of 1 in THF. The molecular coordinates obtained from these
Xꢀray data is also consistent with the proposed structure; Mn is fully chelated in a tetraꢀ
dentate fashion by tbudhbpy with a chloride ligand in an axial coordination site and an
overall distorted square pyramidal geometry at the metal center (Figure 1). The UVꢀvis
spectrum of 1 (Figure S1) in MeCN exhibits charge transfer bands in the visible region.
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FIGURE 1. Molecular structure of Mn(tbudhbpy)Cl 1, obtained from singleꢀcrystal Xꢀray
diffraction studies. Blue = N, red = O, gray = C, green = Cl, purple = Mn; thermal ellipꢀ
soids at 50%, H atoms and a disordered occluded THF molecule omitted for clarity.
Computational Studies on Mn(tbudhbpy)Cl 1
To better understand the electronic structure of 1, DFT studies were conducted on a
model compound with hydrogen atoms instead of tertꢀbutyl groups to supplement experꢀ
imental results. These studies predicted a similar geometry to that obtained by diffracꢀ
tion methods. The HOMO and LUMO orbitals are predicted to be primarily composed of
unoccupied Mn δ orbitals and phenolate based π orbitals, respectively, by KohnꢀSham
orbital projections (Figure S5). TDꢀDFT results suggest that the bands with λmax = 345,
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448, and 538 nm all have some component of ligandꢀtoꢀmetal charge transfer involving
the HOMOꢀbased π and LUMOꢀbased Mn δ orbital energy levels (Figure 2).34ꢀ38 Qualiꢀ
tatively, these data are consistent with the electronꢀrich phenolate groups contributing to
all transitions, but the contribution of the metal center is predicted to increases at lower
wavelengths. This is visualized as difference density plots showing the ground state
(red) and excited singlet state (blue) for selected transitions in Figure 2. A simulated
UVꢀVis spectrum of the model compound shows general agreement with the experiꢀ
mental data, suggesting these calculations are a reasonable approximation of the elecꢀ
tronic structure.
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Figure 2. Overlay of normalized experimental data of Mn(tbudhbpy)Cl 1 in MeCN (red)
and predicted UVꢀvis spectrum from TDꢀDFT calculations on computational model
Mn(dhbpy)Cl (blue; 2000 cm–1 broadening) showing qualitative agreement. The vertical
black lines show the individual states and normalized intensities calculated in the visible
region. Difference densities plots visualizing the shift in electron density (red initial state,
blue final state) for the transitions predicted to be of greatest intensities near the experꢀ
imental λmax = 345, 448, and 538 nm are shown. B3LYP/G, CPCM(Acetonitrile), def2ꢀ
QZVPP, 2S+1 = 5; ORCA 4.0.1.
Electrochemical Studies with NonꢀBuffered Proton Sources
Cyclic voltammetry (CV) experiments were performed on 1 in a solution of 0.1 M tetꢀ
rabutylammonium hexafluorophosphate (TBAPF6) in MeCN (Figures 3, S6). At potenꢀ
tials negative of the ferrocene/ferricinium (Fc/Fc+) reduction potential measured under
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experimental conditions, a single quasiꢀreversible feature is observed before ꢀ1.0 V.
This wave is assigned to a MnIII/II reduction with E1/2 = ꢀ0.63 V vs Fc/Fc+ (∆Ep = 81 mV;
∆Ep(Fc) = 71 mV). Variableꢀscan rate studies show that the MnIII/II reduction is diffusion
controlled at this reduction feature (Figures S6ꢀS7).
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FIGURE 3. CVs of Mn(tbudhbpy)Cl 1, obtained under Ar saturation conditions (black), Ar
with 0.5 M PhOH (red), and 0.5 M with O2 saturation. Conditions: 1 mM analyte; glassy
carbon working electrode, Pt wire counter electrode, Ag/AgCl pseudoreference elecꢀ
trode; scan rate 100 mV/s; referenced to internal ferrocene standard.
This reduction feature (E1/2 = ꢀ0.63 V vs Fc/Fc+) shows a protonꢀdependent voltage
response; titrating phenol (PhOH) into solution shows a shift to more positive potentials
(Figure 4). This Nernstian response was analyzed by plotting the E1/2 against the log of
PhOH concentration, which established a 1 [PhOH]/1 eꢀ relationship through a slope of
70 mV/decade of PhOH concentration (Figure S8).39ꢀ42. Since the homogeneous elecꢀ
tron transfer reaction involving 1 is fast enough to remain reversible under these condiꢀ
tions, we assign this response to an electrochemically driven proton transfer reaction.
Given that the formation of Mn–H species under these conditions would be unexpected,
this is likely a proton transfer from a PhOH donor to a Mnꢀbound O atom acceptor to
generate [Mn(tbudhbpy(H))Cl] 2.
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FIGURE 4. CVs of Mn(tbudhbpy)Cl 1, obtained under Ar saturation conditions with variaꢀ
ble PhOH concentration. Conditions: 1 mM analyte; glassy carbon working electrode, Pt
wire counter electrode, Ag/AgCl pseudoreference electrode; scan rate 100 mV/s; referꢀ
enced to internal ferrocene standard.
Titration of tetrabutylammonium chloride (TBACl) into a solution of 1 under Ar satuꢀ
ration in MeCN shows a minimal shift in the MnIII/II reduction (Figures S9ꢀS12). With a
fixed concentration of PhOH, titration of TBACl into solution under Ar saturation supꢀ
presses the shift to positive potentials, but not completely, implying that Clꢀ loss is part
of the equilibrium following reduction but slow relative to the reductionꢀinduced protonaꢀ
tion reaction (Figure 5). ∆Ep remains in a range consistent with a oneꢀelectron process
at all concentrations of added PhOH under these conditions when compared to Fc/Fc+
(Table S2).
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FIGURE 5. CVs of Mn(tbudhbpy)Cl 1, obtained under Ar saturation conditions comparing
the effects of fixed PhOH concentration and increasing TBACl concentration. Condiꢀ
tions: 1 mM analyte; glassy carbon working electrode, Pt wire counter electrode,
Ag/AgCl pseudoreference electrode; 100 mV/s scan rate; referenced to internal ferroꢀ
cene standard.
If these studies are instead conducted in the absence of PhOH under O2 saturation,
a slight change in the current response at the MnIII/II reduction feature occurs, suggestꢀ
ing a very slow binding interaction with O2 under these conditions (Figure S13). Control
experiments show that O2 reduction by the glassy carbon electrode occurs at potentials
~650 mV more negative (Figure S14). If CV experiments are conducted with PhOH and
O2, however, a significant increase in current occurs at the MnIII/II reduction feature conꢀ
sistent with a catalytic response. Variable concentration studies under O2 saturation (6.3
mM in MeCN43) established that the observed current reaches a peak at 0.5 M PhOH
(Figures 3 and S15). Before this saturation is reached, a logꢀlog plot comparing the
added PhOH concentration with observed current density suggests a firstꢀorder deꢀ
pendence (Figure S16). Similar studies with variable O2 concentration with 0.5 M PhOH
suggest a firstꢀorder dependence on O2 (Figures S17 and S18). Variable catalyst conꢀ
centration studies with O2 saturation and 0.11 M PhOH are consistent with a firstꢀorder
dependence (Figures S19 and S20). Given that product analysis by rotating ringꢀdisk
electrode (RRDE) experiments under these conditions suggest that the overall reaction
is a 2H+/2eꢀ reduction to H2O2, these data suggest a unimolecular activation of O2 and a
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rateꢀlimiting protonation/reduction step, vide infra.13 Titration of TBACl into an MeCN soꢀ
lution with fixed PhOH concentration under O2 saturation results in a suppression of
catalytic current (Figures S21ꢀS22), suggesting that Cl loss occurs upon O2 binding and
activation. Lastly, the abstraction of Clꢀ from 1 with thallium(I) trifluoromethansulfonate
[Tl(OTf)] results in a diminished Nernstian shift under Ar with PhOH titration, but compaꢀ
rable activity in the presence of O2 (Figures S23ꢀS26).
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In order to quantify the overall efficiency of the catalytic reaction, it was necessary to
repeat CV analysis with 2,2,2ꢀtrifluoroethanol (TFE) as a proton source. As has been
reported previously,44 oxidation of the phenolate anions resulting from proton consumpꢀ
tion in situ is observable on the CV timescale (Figure S27). Repeating the kinetic experꢀ
iments described above with TFE in the presence of O2 established that a reaction with
analogous kinetic parameters to the PhOHꢀbased studies was occurring (Figures S28ꢀ
S41). These data are in qualitative agreement with the predicted lower pKa for TFE
(predicted pKa(MeCN) = 35.445; predicted value ignores homoconjugation) in compariꢀ
son to PhOH (experimental pKa (MeCN) = 29.1446). At low concentrations of TFE, O2
binding without catalytic current enhancement occurred on the CV timescale (Figure
S32). Similar experimental effects were observed when H2O, predicted to have an even
higher pKa than TFE, was used as the sacrificial proton source (Figure S42).
RRDE experiments were performed with Mn(tbudhbpy)Cl 1 and TFE to determine the
current efficiency of O2 reduction to H2O2. At concentrations of 0.25 mM 1 with 0.6 M
TFE under O2 saturation, efficiencies of 81±4% are observed (Figures S43ꢀS54). Using
rotating disk electrode experiments, diffusion coefficients for the electroactive species at
the MnIII/II feature were determined under all experimental conditions: Ar only 3.41 x 10ꢀ6
cm2s–1; Ar and 0.5 M PhOH 3.78 x 10ꢀ6 cm2s–1; O2 and 0.5 M PhOH 4.21 x 10ꢀ6 cm2s–1
(Figures S55ꢀS63). These values do not suggest that a Mn–[ꢁ2ꢀO–O]–Mn dimer is
forming, consistent with the kinetic data on catalyst concentration detailed above.
Electrochemical Studies with Buffered Proton Sources
In an attempt to understand the proton donorꢀproton acceptor interaction further,
buffered proton sources were analyzed by CV (Figures S64). The absence of a trend
from these data is ascribed to side phenomena not related to proton activity, such as
ligand exchange reactions with the anionic bases (Figure S65ꢀ68). In an attempt to inꢀ
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terrogate the Nernstian relationship of proton donor and 1 further under buffered condiꢀ
tions, different ratios of tetrabutylammonium phenoxide and phenol were monitored by
CV (Figure 6). When the ratio shifted towards higher PhOH concentration, a plot of E1/2
against the log of the [PhO–]/[PhOH] exhibited a slope of 91 mV/decade (Figure 7). Tiꢀ
trating an excess of phenoxide under these conditions exhibited a slope of 54
mV/decade (Figures S69ꢀS70). Although deviations from a Nernstian 1 substrate/1 e–
relationship are observed, the observed relationship is consistent with the concentration
of proton donor relating to the reversible reduction of 1. Similar behavior is observed
when the Clꢀabstracted compound Mn(tbudhbpy)[OTf] is used (Figure S71ꢀS74).
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FIGURE 6. CVs of Mn(tbudhbpy)Cl 1, obtained under Ar saturation conditions showing
the the effects of fixed PhO– concentration with increasing PhOH concentration on E1/2.
Conditions: 1 mM Mn(tbudhbpy)Cl 1; glassy carbon working electrode, glassy carbon
counter electrode, Ag/AgCl pseudoreference electrode; 100 mV/s scan rate; referenced
to internal ferrocene standard.
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FIGURE 7. Plot of E1/2 for Mn(tbudhbpy)Cl 1 against the log of the ratio of [PhO–]/[PhOH]
showing the the effects of fixed PhO– concentration with increasing PhOH concentration
on E1/2. Conditions: 1 mM Mn(tbudhbpy)Cl 1; glassy carbon working electrode, glassy
carbon counter electrode, Ag/AgCl pseudoreference electrode; 100 mV/s scan rate; refꢀ
erenced to internal ferrocene standard.
In order to analyze this system using the footꢀofꢀtheꢀwave analysis (FOWA47), seꢀ
lected experiments were repeated using the buffered pentafluorophenol (F5PhOH) proꢀ
ton source, which would have a nonꢀaqueous pH = pKa = 20.11.48 F5PhOH was used
as a proton source under buffered conditions because unsubstituted PhOH showed a
minimal response, presumably due to the higher nonꢀaqueous buffered pKa(MeCN)
(29.14; Figure S75). When the buffered proton source (F5PhOH/F5[PhO–]TBA+) was
titrated into solution, maximal current was reached at a concentration of 11 mM (Figꢀ
ures 8 and S76ꢀS78). These data were used to determine apparent catalytic rates (kobs)
and maximal turnover frequencies (TOFmax) through FOWA (Figures S79ꢀS80).13ꢀ14, 47
For F5PhOH/F5PhO–TBA+, TOFmax (kobsCsubstrate) = 3.13 x 104 s–1 and kobs = 4.52 x 108
M–1s–1 ([Buffered Acid] = 11 mM; [O2] = 6.3 mM). Using similar approximations to those
of Mayer et al.,13 thirdꢀorder rate constants (kcat) can be approximated by dividing these
kobs values by [catalyst] = 1 x 10–3 M to fully express the rate of dioxygen consumption
within the reaction diffusion layer, ꢀd[O2]/dt, if kobs = kcat[catalyst]. A simulated catalytic
Tafel plot49 comparing these catalysts to previous literature reports on catalysts which
electrocatalytically produce H2O2 is shown below in Figure 9.10, 50ꢀ53 When these buffꢀ
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ered acid studies are repeated with [Mn(tbudhbpy)][OTf], kobs decreases to 1.62 x 106 M–
1s–1 as determined by comparable FOWA analysis (Figure S81ꢀS83).
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FIGURE 8. CVs of Mn(tbudhbpy)Cl 1, obtained under Ar saturation conditions with 11
mM buffered F5PhOH under Ar (black) and O2 saturation (6.3 mM, red); aprotic condiꢀ
tions under Ar saturation are shown for reference (blue dotted line). Conditions: 1 mM
analyte; glassy carbon working electrode, Pt wire counter electrode, Ag/AgCl pseuꢀ
doreference electrode; scan rate 100 mV/s; referenced to internal ferrocene standard.
FIGURE 9. Simulated catalytic Tafel plots comparing Mn(tbudhbpy)Cl 1 to literature valꢀ
ues for O2 to H2O2 reduction by molecular species. The overpotential in V (η) refers to
the difference between catalyst E1/2 and the respective standard potentials E0 for
O2/H2O2 under the experimental conditions reported (see Methods Section); TMPP =
5,10,15,20ꢀtetrakis(Nꢀmethylꢀpyridyl)porphyrin, * indicates an electrode surfaceꢀ
adsorbed catalyst.10, 50ꢀ53
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PotentialꢀpKa Analysis
Using the method of Dempsey et al.41, further understanding of the proton donor to
the observed redox potential was obtained through a potentialꢀpKa relationship. Using
1:1 mixtures of acid with 1, a potentialꢀpKa diagram was constructed to establish the
relationship of E1/2 to the acid pKa (Figures 10 and S84). This diagram obviates the
need to define proton activity in a nonꢀaqueous system, where ion pairs from dissociatꢀ
ed acid are expected to exhibit drastically different properties than under aqueous conꢀ
ditions where solvation by water molecules can stabilize dissociation. Two distinct reꢀ
gions are observed in this diagram, a horizontal region from 29.14ꢀ20.11 pKa and a
slope of 32 mV/pKa from 20.11ꢀ12.65 pKa. Deviation from a Nernstian 1 pKa/1 e– relaꢀ
tionship is observed, but the electrochemical response of 1 demonstrates a relationship
between acid strength in MeCN and the thermodynamic reduction potential consistent
with a proton donorꢀproton acceptor interaction.
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FIGURE 10. A potentialꢀpKa diagram for acids from pKa = 12.65 to 29.14. Conditions:
Ar saturation; 0.5 mM Mn(tbudhbpy)Cl 1, 0.5 mM acid (Figures S91); glassy carbon
working electrode, glassy carbon counter electrode, Ag/AgCl pseudoreference elecꢀ
trode; scan rate 100 mV/s; referenced to internal ferrocene standard. pKa(MeCN): Pheꢀ
nol = 29.14; 4ꢀCF3ꢀPhOH = 25.54; acetic acid = 23.51; perflouoroꢀtertꢀbutanol = 20.55;
4ꢀBrꢀbenzoic acid = 20.3; F5PhOH = 20.11; Cl5PhOH = 18.02; 2,6ꢀdichloroꢀbenzoic acid
= 17.6; salicylic acid = 16.7; 4ꢀCF3ꢀF4PhOH = 16.62; trifluoroacetic acid = 12.65.48, 54ꢀ55
Spectrochemical Studies
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As a supplement to the electrochemical studies detailed above, a series of spectroꢀ
chemical studies were undertaken to better understand the electrocatalytic activity of 1
toward O2 reduction. Using potassium intercalated graphite (KC8), chemical reductions
were performed on a solution of 1 in tetrahydrofuran (THF; 2.3 mM). A distinct color
change was observed and quantified by UVꢀvis absorbance spectroscopy; new λmax
were observed at 242, 324, and 365 nm (Figure 11). Evan’s method measurements in
THF and MeCN are consistent with a highꢀspin d5 configuration Table S1. Upon expoꢀ
sure to air, the sample appeared to revert to the color of the starting material, suggestꢀ
ing that a rapid reaction with oxygen had taken place; a charge transfer band consistent
with the starting material is observed at λmax 347 nm when the color change is quantified
by UVꢀvis spectroscopy (Figure S85). In order to better understand this reaction, a seꢀ
ries of stoppedꢀflow experiments were used to determine relative rates of reaction beꢀ
tween [Mn(tbudhbpy)]0 and O2.
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FIGURE 11. UVꢀvis spectra of the as prepared chemically reduced [Mn(tbudhbpy)]0 beꢀ
fore and after the introduction of atmospheric air. Conditions: 2.3 mM starting concenꢀ
tration of 1 in THF; Omnicell, 0.2 mm spacer.
Removing the solvent from the as prepared THF solution of the chemically reduced
[Mn(tbudhbpy)]0 produced a yellow orange solid that was weighed and dissolved in anꢀ
hydrous degassed MeCN to prepare an exact concentration prior to stoppedꢀflow experꢀ
iments. The reaction of the reduced Mn complex [Mn(tbudhbpy)]0 with O2 was examined
by monitoring the charge transfer bands at λmax 345 nm. Fitting the resulting data from a
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singleꢀmixing experiment with O2 saturated MeCN suggested a kobsCsubstrate of 3.17 s–1
at 20.4°C under nominally aprotic conditions (Figure S86).13 An air exposure experiꢀ
ment with this MeCN solution was consistent with the product oxidized species being
analogous to [Mn(tbudhbpy)]1+ (Figure S87).
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Subsequent experiments were conducted on the reactions between the likely prodꢀ
•–
uct of this initial innerꢀsphere redox reaction, O2 , and the starting material 1. An MeCN
solution (5 mM) of solubilized potassium superoxide, KO2, was prepared by combining
equimolar amounts of 18ꢀcrownꢀ6 and KO2. Relative to the previous reaction, the suꢀ
peroxide radical reacts more readily with the Mn(III) center under these conditions, with
a kobsCsubstrate of 24.6 s–1 at 20.8°C (Figure S88). Kinetic and thermodynamic parameꢀ
ters of this reaction could be obtained through Van’t Hoff and Eyring analyses across
temperatures between 11.2 and 20.8°C (Figures S88ꢀS91 and Table S4). These
showed that the reaction between superoxide and 1 is both favorable in the forward diꢀ
rection (∆G298K = ꢀ30.7 kcal/mol) and has a low activation barrier (Ea(298K) = 12.0
kcal/mol). These data suggest that upon O2 activation through an innerꢀsphere reducꢀ
tion event, subsequent thermal steps are possible.
Isolating relevant substrate adduct species proved difficult, however. This can be
explained in part by the utility of structural analogues of 1 as effective superoxide disꢀ
mutase, catalase and peroxidase mimics.27 This class of bipyridineꢀderived Schiff base
Mn(III) complexes functions as both antioxidant and free radical scavengers; they are
•ꢀ
robust catalysts for converting O2 rapidly to H2O2 in protic environments.27 These propꢀ
erties are also the most plausible explanation for the incomplete mass balance, due to
effective peroxidase mimicry, which consumes hydrogen peroxide in the presence of
proton source. Unfortunately, any competitive superoxide dismutase activity would be
indistinguishable from a formal twoꢀelectron electrodeꢀdriven process in terms of current
efficiency. Given the kinetic experiments detailed above, however, the electrochemical
mechanism appears to be the dominant contributor to the observed catalytic response.
Lastly, a qualitative activity test in MeCN with added H2O2 showed significant O2 evoluꢀ
tion from H2O2 disproportionation over 2 hours, suggesting a viable thermal pathway exꢀ
ists to explain the incomplete mass balance observed by RRDE (Figure S92).
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DISCUSSION
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Based on these data, we can propose a mechanism for how the catalytic reaction is
taking place (Scheme 1). Starting from 1, a 1 PhOH/1 e– step generates 2 (i), as deterꢀ
mined by the slope of plot constructed from the changes in E1/2 versus the log of PhOH
concentration under Ar saturation (70 mV/decade of concentration) and a potentialꢀpKa
diagram demonstrating a Nernstian relationship between E1/2 and pKa(MeCN) between
12.65 and 20.11 (31 mV/pKa(MeCN)). We propose that species 2 reacts rapidly with O2
in a unimolecular fashion, resulting in the formation of the superoxide species 3, based
on the electrochemical and spectrochemical data above (ii). At this point, we believe
that the rateꢀdetermining step of the reaction occurs, the reduction and intramolecular
protonation of the superoxide adduct to generate a bound hydroperoxo species 4 (iii).
Due to the presumed increase in effective concentration of proximal proton sources
through ligand protonation, we propose that this proceeds via an intramolecular proton
transfer reaction. DFT results qualitatively agree with this: the model of compound 3,
[Mn(dhbpy(H))(η1ꢀO2)]+, has an O–O distance of 1.207 Å, consistent with minimal bond
activation.56 If the computational model of compound 3 is reduced by a single electron
to [Mn(dhbpy(H))(η1ꢀO2)] 3a, the O–O bond length increases to 1.330 Å, suggestive of a
superoxide species.56 Finally, in the model of 4, [Mn(dhbpy)(η1ꢀO2H)], (N.B. this is the
presumptive product of an intramolecular proton transfer from the ligand to η1ꢀO2 in 3a;
∆G0 = ꢀ20 kcal/mol) the O–O bond length is 1.461 Å, consistent with a metalꢀbound hyꢀ
droperoxo.56
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From product analysis by RRDE it is implied that the catalytic cycle primarily ends
here with a final intramolecular protonation and H2O2 loss (81±4%), returning to 2 with
further reduction in the presence of proton sources (iv). The incomplete mass balance
implies that either the direct 4H+/4e– reduction to H2O occurs slowly relative to the initial
catalytic reaction, or that 1 is capable of catalyzing the disproportionation of two equivaꢀ
lents of H2O2 to 2 equiv of H2O and one equiv of O2. Given the spectrochemical results
and literature precedent,27 as well as our peroxidase activity test in MeCN, we favor the
latter interpretation (vi). As noted above, Cl– loss occurs slowly in the presence of proꢀ
ton sources under Ar saturation conditions, implying an offꢀcycle equilibrium exists (v).
Interestingly, FOWA analysis and PhOH titration studies suggest that the Clꢀabstracted
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species has lower activity and a diminished Nernstian response. We interpret this as Cl–
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coordination helping to minimize charge with protonation in species 2.
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Scheme 1. Proposed catalytic cycle involving a cooperative metalꢀligand redoxꢀbased
protonation reaction.
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Cooperative metalꢀligand activation of O2 has been observed previously.16, 57 The
catalyst 1 reported here, however, exhibits Nernstian behavior with added proton
sources that is assigned to a protonation reaction on the ligand framework upon a
change in oxidation state at the metal center. However, this type of multiꢀsite electron
and proton transfer exhibits greater similarity to mechanisms proposed for hydrogen
evolution reactions with dithioleneꢀ and cobaloximeꢀbased electrocatalysts.58ꢀ62 Mnꢀ
based catechol complexes which invoke a mechanism invoking ligand O atom and subꢀ
strate H atom interactions have also been reported for similar O2/H2O2 catalytic activity
when using hydroxylamine as a proton and electron source.63ꢀ64 This cooperative reꢀ
sponse in the presence of substrate is reminiscent of biological multisite electron and
proton transfer mechanisms involving tyrosine residues.65 We rationalize this in a qualtiꢀ
tative way through the change in d orbital occupancy upon reduction, in the d4 highꢀspin
ground state, vacant d orbitals of appropriate π symmetry and energy are present to inꢀ
teract with the lone pairs of the O atoms of the ligand backbone. This is reflected in
electronic structure predicted by the DFT and TDꢀDFT calculations detailed above. Upꢀ
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on reduction, a change in d orbital occupancy will diminish this interaction, increasing
the basicity of the O atoms of the ligand. DFT calculations on the one electronꢀreduced
model complex, [Mn(dhbpy)Cl]–, predict that the HOMO is primarily metalꢀbased and the
HOMOꢀ1 exhibits antiꢀbonding interactions between Mn and the O atoms (Figure S93).
A comparison of Mulliken charges on the O atoms between Mn(dhbpy)Cl and
[Mn(dhbpy)Cl]– is also consistent with increased basicity; the relative negative charges
shift from ꢀ0.56 to ꢀ0.65 upon singleꢀelectron reduction (Tables S9 and S10).
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Mn(salen) complexes have been previously studied as superoxide dismutase mimics
(MnSOD).66 Although the ligand framework discussed here does not reconstruct the acꢀ
tive site of MnSOD, the observed catalytic activity runs an ORR through all the proꢀ
posed intermediates of the enzymatic cycle. MnSOD catalyzes either reaction (1) or (2)
to the right as written when presented with a superoxide radical. In this catalyst system,
the superoxide radical is instead formed by electrochemically produced Mn(II), which
can be understood as the reverse of reaction (1). The electrode maintains the supply of
Mn(II) in solution, which drives reaction (2) in the forward direction as written.
CONCLUSIONS
These data suggest that Mn(tbudhbpy)Cl is an active and selective molecular catalyst
for the reduction of O2 to H2O2. Notably, the catalytically active species has been genꢀ
erated through a one electron/two proton reaction, yet the complex retains a vacant coꢀ
ordination site at the metal center. As a consequence, the active catalyst has pendent
proton sources in close proximity to the active site where substrate activation occurs.
The change in metal oxidation state corresponding to substrate activation may favor
rapid intramolecular proton transfer from the ligand, if it is assumed the reversibility of
the redoxꢀdriven ligand protonation reaction observed under Ar saturation occurs under
catalytic conditions. Experiments examining this behavior in greater detail with a wider
substrate scope and spectrochemical methods are currently underway.13ꢀ14
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ASSOCIATED CONTENT
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Supporting Information. Additional CV, UVꢀvis, NMR, StoppedꢀFlow, and RDE/RRDE
data is included in the SI. This material is available free of charge via the Internet at
http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
* ꢀ machan@virginia.edu
Author Contributions
The manuscript was written through contributions of all authors. All authors have given
approval to the final version of the manuscript.
Funding Sources
University of Virginia.
ACKNOWLEDGMENT
The authors thank the University of Virginia for generous funding and Dr. Bradley A.
McKeown for helpful discussion.
Materials and Methods
General
All chemicals and solvents (ACS or HPLC grade) were commercially available and used
as received unless otherwise indicated. For all airꢀsensitive reactions and electrochemiꢀ
cal experiments, solvents were obtained as anhydrous and airꢀfree from a PPT Glass
Contour Solvent Purification System. Gas cylinders were obtained from Praxair (Ar as
5.0; O2 as 4.0) and passed through molecular sieves or Drierite prior to use. Gas mixing
for variable concentration experiments was accomplished using a gas proportioning roꢀ
tameter from Omega Engineering. NMR spectra were obtained on either a Varian 600
MHz or 500 MHz instrument and referenced to the residual solvent signal. UVꢀvis abꢀ
sorbance spectra were recorded on a Cary 60 from Agilent. GC experiments were perꢀ
formed using an Agilent 7890B Gas Chromatograph equipped with a thermal conductiviꢀ
ty detector (TCD) using an Agilent J&W Select Permanent Gases/CO2 column.
Stoppedꢀflow data were collected as singleꢀmixing experiments on a CSFꢀ61DX2
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StoppedꢀFlow System from HiꢀTech Scientific. Temperature was controlled using a
VWR Refrigerated Circulator with an ethylene glycol and water mixture. HRMS data
were obtained by the Mass Spectrometry Lab at the University of Illinois at Urbanaꢀ
Champaign and elemental analyses were performed by Midwest Laboratory. Unless
otherwise noted, buffered acids were prepared by combining 2:1 ratios of acid to tetꢀ
rabutylammonium hydroxide solutions (40%) in water or methanol, followed by extenꢀ
sive drying under vacuum. Analyses are summarized in the Supporting Information.
Synthetic Procedures.
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6,6′ꢀdi(3,5ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxybenzene)ꢀ2,2′ꢀbipyridine, tbudhbpy(H)2.
A Schlenk flask (200 mL) with stir bar was charged with 2,4ꢀdiꢀtertꢀbutyl phenol (20.0
g, 97 mmol) and dry degassed CH2Cl2 (100 mL) under N2 atmosphere. The resulting
mixture was placed in a dry ice/acetone bath for 10 minutes. After this time, Br2 (5.12
mL, 16.0 g, 0.10 mol) was added slowly via syringe. After this addition, the mixture was
brought to room temperature and allowed to stir over the course of 2 h. At this time, the
reaction was quenched with a saturated aqueous solution of NaHCO3 (1 x 50 mL) and
extracted before separating the organic and aqueous phases. The organic layer was
washed with brine (1 x 50 mL) and the organic and aqueous phases separated. The
combined organic phases were dried over MgSO4 for 10 minutes before being filtered
and the condensed under vacuum by rotary evaporator. The resulting yellowꢀorange
solid was dissolved again in CH2Cl2 (100 mL) and basic alumina was added to the mixꢀ
ture (10 g) before the suspension was stirred vigorously for 10 m. This suspension was
filtered through halfꢀinch layers of silica gel over diatomaceous earth and washed with
CH2Cl2 (2x75 mL) before being condensed under vacuum to reveal a pale yellowꢀ
1
orange solid. Yield (isolated) 27.1 g, 98% yield. H NMR (d2ꢀCD2Cl2, 600 MHz): δ 7.35
(d, 1H, ArH), 7.27 (d, 1H, ArH), 1.41 (s, 9H, ꢀC(CH3)3) 1.29 (s, 9H, ꢀC(CH3)3). Elemental
Analysis for C14H21BrO Calc’d: C 58.95, H 7.42, N 0; Found: C 52.37, H 6.92, N 0.17.
ESIꢀMS (m/z): Calc’d. 284.08 Found: 284.08.
A Schlenk flask (500 mL) with stir bar was charged with 2ꢀbromoꢀ4,6ꢀditertꢀbutylꢀ
phenol (5 g, 0.0175 mol) and dry degassed diethyl ether (200 mL). The resulting soluꢀ
tion was cooled to ꢀ78°C (dry ice/acetone bath) under N2. nꢀButyllithium (2.6 M in hexꢀ
anes) was added slowly via syringe (16.1 mL, 0.042 mol), after which the solution alꢀ
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lowed to warm to room temperature with vigorous stirring for 2 hours. After 2 h the soluꢀ
tion was returned to ꢀ78°C, at which point trimethyl borate was added rapidly by syringe
(3.15 mL, 0.0282 mmol) with vigorous stirring and the mixture left for 10 minutes before
the ice bath was removed and the mixture left stirring overnight under N2 (16 h). After
this time, the flask was cooled to 0°C before quenching with HCl (25 mL, 2 M) and
opened to air for 10 minutes. The suspension was extracted and the aqueous and orꢀ
ganic layers separated. The aqueous layer was washed with CH2Cl2 (3x50 mL) after
which all organic fractions were combined. The resulting solution was dried with MgSO4
for 10 minutes, before the mixture was filtered to remove solid and the solvent was reꢀ
moved under reduced pressure to yield a yellow/orange oil, (3,5ꢀditertꢀbutylꢀ2ꢀhydroxyꢀ
phenyl)boronic acid, which was used without further purification. A roundꢀbottom flask
(500 mL) was charged with a stir bar and placed in a heating bath at room temperature.
A suspension of 6,6'ꢀdibromobipyridine (2.0 g, 6.37 mmol), toluene (250 mL), the unpuꢀ
rified (3,5ꢀditertꢀbutylꢀ2ꢀhydroxyꢀphenyl)boronic acid oil, MeOH (30 mL), 2 M Na2CO3
(60 mL), and Pd(PPh3)4 (5.0 mol. cat. %, 376 mg). The mixture was brought to reflux for
72 h under N2 and then allowed to cool to room temperature and the layers were sepaꢀ
rated. The organic layer was washed with brine (1 x 50 mL) and the aqueous layer with
dichloromethane (3 x 50 mL). The organic fractions were combined and dried with
MgSO4 for 10 minutes. The suspension was filtered and concentrate to dryness under
reduced pressure. The yellow solid was recrystallized from toluene (10 mL) and washed
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with minimal hexanes (5 mL). Isolated yield: 2.28 g, 63%. H NMR (d2ꢀCD2Cl2, 600
MHz): δ 8.10 (m, 6H, ArH), 7.76 (d, 2H, ArH), 7.45 (d, 2H, ArH), 1.51 (s, 18H, ꢀC(CH3)3),
1.38 (s, 18H, ꢀC(CH3)3), 13C[1H] (d2ꢀCD2Cl2, 600 MHz) NMR δ 159.52 (ArC), 157.09
(ArC), 152.41 (ArC), 140.85 (ArC), 139.69 (ArC), 138.05 (ArC), 127.11 (ArC), 121.90
(ArC), 121.17 (ArC), 119.30 (ArC), 118.52 (ArC), 35.81 (tbuC), 34.88 (tbuC), 31.91
(tbuC), 29.92 (tbuC). Elemental analysis for C38H48N2O2: calc’d: C: 80.81, H: 8.57 N:
4.96 found: C: 81.01 H: 9.11 N: 5.10. MSMS (m/z): calc’d: 565.37 found: 565.3789
Mn(κ4ꢀN,N′,O,O′ꢀ(6,6′ꢀdi(3,5ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxybenzene)ꢀ2,2′ꢀbipyridine))Cl,
Mn(tbudhbpy)Cl, 1.
A pearꢀshaped roundꢀbottom flask (100 mL) was charged with a stir bar, methanol
(50 mL) and tbudhbpy(H)2 (250 mg, 443 mmol). The resulting suspension was brought to
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reflux in air before Mn(OAc)2•4H2O (114 mg, 0.465 mmol) was added and the mixture
refluxed for 1 h. During this time, the mixture became dark brown in color and was imꢀ
mediately quenched with saturated NaCl (50 mL). The brown suspension was filtered
and washed with H2O (2x5mL) before being dissolved in CH2Cl2, dried with MgSO4, and
condensed under reduced pressure. The isolated brown solid was then dissolved in
MeCN (25 mL) and passed through a syringe filter with repeated washing (3x25 mL) beꢀ
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fore being condensed under reduced pressure. Yield, 72 mg 25%. H NMR (d2ꢀCD2Cl2,
600 MHz): δ 8.12 (br m, 2H, ArH), 8.10 (br m, 2H, ArH), 8.07 (br m, 2H, ArH), 7.74 (d,
2H, ArH), 7.44 (d, 2H, ArH), 1.50 (s, 18H, ꢀC(CH3)3), 1.37 (s, 18H, ꢀC(CH3)3). Elemental
Analysis for C38H46ClMnN2O2 Calc’d: C 69.88, H 7.10, N 4.29; Found: C 68.75, H 7.35,
N 4.28. ESIꢀMS (m/z) [M–Cl]+: Calc’d. 617.2940. Found: 617.2964.
Mn(κ4ꢀN,N′,O,O′ꢀ(6,6′ꢀdi(3,5ꢀdiꢀtertꢀbutylꢀ2ꢀhydroxybenzene)ꢀ2,2′ꢀ
bipyridine))[CF3SO3].
A pearꢀshaped roundꢀbottom flask (50 mL) was charged with a stir bar under N2 atꢀ
mospheric conditions. Thallium trifluoromethanesulfonate (17.6 mg, 0.0498 mmol),
Mn(tbudhbpy)Cl 1 (31 mg, 0.0475 mmol), and CH2Cl2 (10 mL) were added to the flask
under an N2 stream. The resulting suspension was left stirring under nitrogen for a 12 h.
After this time, the reaction flask was sonicated until a dark cloudy solution was obꢀ
served. The solution was then filtered through a porous glass frit which contained a halfꢀ
inch of Celite. The Celite in the frit was then washed with additional CH2Cl2 (1x7 mL).
The product was then condensed under reduced pressure until a dark oil was obtained
and left in a vacuum desiccator for 24 hours to obtain the solid dark redꢀbrown product.
1
Isolated yield, 21 mg, 57.7%. H NMR (d2ꢀCD2Cl2, 600 MHz): δ 8.12 (br m, 6H, ArH),
7.76 (d, 2H, ArH), 7.45 (d, 2H, ArH), 1.38 (s, 18H, ꢀC(CH3)3), 1.28 (s, 18H, ꢀC(CH3)3).
ESIꢀMS (m/z) [M–CF3SO3]+: Calc’d. 617.2940. Found: 617.2922.
Electrochemistry
All electroanalytical experiments were performed using a Metrohm Autolab
PGSTAT302N potentiostat. Glassy carbon working (⌀ = 3 mm) and nonꢀaqueous silꢀ
ver/silver chloride pseudoreference electrodes behind PTFE tips were obtained from CH
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Instruments. The pseudoreference electrodes were obtained by depositing chloride on
bare silver wire in 10% HCl at oxidizing potentials and stored in a 0.1 M tetrabuꢀ
tylammonium hexafluorophosphate/acetonitrile solution in the dark prior to use. The
counter electrode was Pt wire (Alfa Aesar, 99.95%, ⌀ = 0.5 mm) or a glassy carbon rod
(⌀ = 3 mm) as indicated for individual experiments. All CV experiments were performed
in a modified scintillation vial (20 mL volume) as a singleꢀchamber cell with a cap modiꢀ
fied with ports for all electrodes and a sparging needle. Tetrabutylammonium hexafluorꢀ
ophosphate was purified by recrystallization from ethanol and dried in a vacuum oven
before being stored in a desiccator. All data were referenced to an internal ferrocene
standard (ferrocenium/ferrocene reduction potential under stated conditions) unless
otherwise specified. Rotating disk and rotating ringꢀdisk electrode experiments were
conducted using a Metrohm rotator with a Metrohm electrode consisting of a glassy
carbon disk (⌀ = 5 mm) and a Pt ring.
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StoppedꢀFlow Spectrochemical Methods.
Stoppedꢀflow spectrochemical kinetics studies were performed with a CSFꢀ61DX2
StoppedꢀFlow System from HiꢀTech Scientific with monitoring at a single wavelength
using Kinetic Studio Software. All data fits were performed within the Kinetic Studio 4.0
Software Suite. Prior to experiments, dried and degassed MeCN was passed through all
syringes and the cell block before reagents were loaded. In a typical experiment, SGE
airꢀtight syringes with Luer lock valves would be charged with known concentrations of
reagent under airꢀfree conditions in a glovebox or immediately prepared and loaded usꢀ
ing solvent from the solvent purification system. All concentrations are reported as syꢀ
ringe concentrations, which halve upon mixing in the cell block during an experiment.
–
The Mn(tbudhpbpy)Cl, [Mn(tbudhpbpy)]0, O2, and [K(18ꢀcrownꢀ6)+][O2 ] MeCN solutions
were prepared immediately prior to use. All data fits were taken from fiveꢀrun averages
using a secondꢀorder exponential with a linear component. For all experiments, pseudoꢀ
first order conditions were used: reagents were in ~100ꢀfold excess of [Mn].
Computational Methods.
DFT calculations were performed on the Rivanna HighꢀPerformance Computing
Cluster at the University of Virginia using ORCA 4.0.1.38 Geometry optimizations were
performed unrestricted with the B3LYP/G67ꢀ71 functional and def2ꢀTZVP35, 72 basis set
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with the RIJCOSX approximation73, D3BJ dispersion correction36ꢀ37, and CPCM34 to
model the N,NꢀDMF solvent. Numerical frequency calculations at the same level of theꢀ
ory were performed to validate the optimized geometries as minima on the potential enꢀ
ergy surface and to generate thermochemical data. TDꢀDFT calculations on the verified
minimum were performed with def2ꢀQZ2P basis sets35 with nroots = 50, and maxdim =
10.
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RDE Experiments
Conditions: performed under both argon and O2 saturation conditions, 0.1 M
TBAPF6, 0.5 mM analyte, MeCN (50 mL), 0.5 M PhOH, glassy carbon disk electrode (5
mm diameter), Pt ring electrode, Pt wire counter electrode, Ag/AgCl pseudoreference
electrode; scan rate 0.01 V/s.
The solution was sparged with argon until saturation was achieved and confirmed by
CV. Then the analyte (0.5 mM) and 0.5 M PhOH were dissolved in the solution. A
standard CV was taken of the solution to confirm the potential window to be used for the
experiment (0 to ꢀ0.65 V). Linear Sweep Voltammograms (LSVs) were obtained for varꢀ
ious rotation rates (between 200 and 2400 rpm) under the described conditions. Beꢀ
tween each LSV, the electrode was polished on alumina and sonicated in ethanol. The
cleaning procedure for the electrode was standardized by taking repeated LSVs at the
same rotation rates to confirm repeated scans at the same rotation rate were exact
overlays of one another. This same procedure was completed for O2 saturation condiꢀ
tions, which were achieved by sparging the solution with O2 for approximately 15
minutes.
Levich and KouteckyꢀLevich theory was applied to these experiments to calculate
the diffusion coefficients for each of the conditions under which the experiment was run,
argon and O2 saturation. The slope of the Levich plot was obtained by plotting the
square root of the angular rotation rate versus the limiting current at ꢀ0.55 V vs Ag/AgCl
for both argon and O2. The slope of these Levich plots were then used to calculate the
diffusion coefficients under each of the experimental conditions, argon and O2 saturaꢀ
tion, using the following equation:
ꢑ
ꢃꢄꢅꢆꢇ
0.620ꢈꢉꢊνꢋꢌ/ꢍꢎꢏ
ꢁ
ꢀ = ꢂ
∗ꢐ
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Where
D
is the diffusion coefficient, slope is the slope from the Levich plot, n is the
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number of electrons,
F is Faraday’s constant, A is the area of the electrode, ꢒ is the kinꢀ
ematic viscosity of the solvent (0.0044 cm2/s for MeCN) and C*0 is the concentration of
O2 at saturation in acetonitrile.
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RRDE Collection Efficiency % Determination
Conditions: argon saturation, 0.1 M TBAPF6, 0.25 mM Ferrocene, MeCN (50 mL),
glassy carbon disk electrode (5 mm), Pt ring electrode, Pt wire counter electrode,
Ag/AgCl pseudoreference electrode; scan rate 0.01 V/s.
To determine the collection efficiency of the rotating ring disk electrode (RRDE), an
argon saturated acetonitrile solution (50 mL) of ferrocene (0.25 mM) was prepared with
TBAPF6 (0.1 M) as the supporting electrolyte. A standard CV was taken prior to the adꢀ
dition of ferrocene (0.25 mM) to ensure no air was present in the solution. Then a
standard CV of ferrocene was obtained to determine the potential window (0.8 to 0 V vs
Ag/AgCl) to use for determining the collection efficiency of the RRDE instrument. Once
the potential window for ferrocene was verified by CV, the ring electrode potential was
set at 0 V and LSVs for various rotation rates between 200 and 2400 rpm were obꢀ
tained.
To calculate the collection efficiency of the RRDE, the ratio of the ring current to the
disk current at each rotation rate was used to determine Nempirical at each rotation rate.
The Nempirical value at reach rotation rate was then multiplied by a factor of 100 for deꢀ
termine the collection efficiency % at each rotation rate (~27%). A plot of the angular
rotation rate versus the collection efficiency % verified that the collection efficiency was
independent of rotation rate. Plots of rotation rate versus ring current and rotation rate
versus disk current confirmed that the current produced was dependent on rotation rate.
ꢕꢙꢚꢛꢜ ꢝꢞꢙꢙꢟꢝꢠꢟꢡ
ꢓꢅꢄꢄꢇꢓꢔꢕꢅꢈ ꢇꢖꢖꢕꢇꢈꢓꢗ ꢅꢖ ꢉꢇꢘꢘꢅꢓꢇꢈꢇ =
ꢕꢡꢚꢢꢣ ꢝꢞꢙꢙꢟꢝꢠꢟꢡ
RRDE Experiments (adapted13)
Conditions: performed under both argon and O2 saturation conditions, 0.1 M
TBAPF6, 0.25 mM analyte, MeCN (50 mL), glassy carbon disk electrode (5 mm diameꢀ
ter), Pt ring electrode, Pt wire counter electrode, Ag/AgCl pseudoreference electrode;
scan rate 0.01 V/s.
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The solution was sparged with argon until saturation was achieved and confirmed by
CV. The analyte (0.25 mM) was dissolved in solution and TFE (0.45 M and 0.6 M) was
added via syringe. A standard CV was taken of the solution to confirm the potential winꢀ
dow to be used for the experiment (0 to ꢀ0.65 V). The platinum ring was set to 1.2 V and
the glassy carbon disk was set to 0 V. LSVs were obtained for various rotation rates
(between 200 and 2400 rpm) under the described conditions. Between each LSV, the
electrode was polished on alumina and sonicated in ethanol. The cleaning procedure for
the electrode was standardized by taking repeated LSVs at the same rotation rates to
confirm repeated scans at the same rotation rate were exact overlays of one another.
This same procedure was repeated for O2 saturation conditions, which were achieved
by sparging the solution with O2 for approximately 15 minutes.
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The difference between the amount of current produced at the disk under O2 saturaꢀ
tion conditions and the amount of current produced at the disk under argon saturation
conditions was taken as the corrected disk current for O2 saturation conditions (Idisk corꢀ
rected) for each rotation rate. The difference between the amount of current produced at
the ring under O2 saturation conditions and the amount of current produced at the ring
under argon saturation conditions was taken as the corrected ring current for O2 saturaꢀ
tion conditions (Iring corrected) for each rotation rate.
To calculate the H2O2%, the Idisk corrected was multiplied by the corresponding Nempirical
value for the specific rotation rate to determine the maximum amount of ring current for
H2O2 production (Iring max). The ratio of Iring corrected to Iring max was multiplied by a factor of
100 to determine the H2O2% generated at the ring across all measured rotation rates.
Efficiencies of 82±3% and 81±4% were obtained for 0.45 M TFE and 0.6 M TFE, reꢀ
spectively.
ꢚ
ꢧꢨꢩꢪ ꢫꢬꢧꢧꢭꢫꢮꢭꢯ
%ꢤꢥꢦꢥ = (100) ꢚ
(ꢝꢞꢲꢲꢟꢝꢠꢚꢞꢛ ꢟꢳꢳꢚꢝꢟꢛꢝꢴ)
ꢯꢨꢰꢱ ꢫꢬꢧꢧꢭꢫꢮꢭꢯ
FootꢀofꢀtheꢀWave Analysis (FOWA) and Catalytic Tafel Plots (adapted13)
For FOWA CVs were compared under Ar and O2 saturation with the acid concentraꢀ
tions where maximum current was achieved for the buffered F5PhOH/F5PhO–TBA+ acid.
This plots ic/ip, where ic is the catalytic current and ip is the Faradaic current (this normalꢀ
izes the plot with respect to electrode area, catalyst concentration, diffusion coefficient,
ꢵ
and scan rate), on the yꢀaxis against an xꢀaxis with 1/(1+exp(f(EꢀE0)), where f = ꢶꢷ, E is
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the applied potential, and E0 is the E1/2 of the catalyst reduction where catalysis occurs.
From the slope of this plot, TOFmax (TOFmax = kobsCsubstrate; kobs = kcat[catalyst]) can be
determined according to the following relationship:
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ꢻꢼ
ꢺ
2.24ꢈꢝꢹꢠ
ꢽꢞꢾꢢꢎꢢꢿꢾꢢꢠꢙꢹꢠꢟ
ꢕꢝ
ꢉ_
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=
ꢉ
ꢕꢸ
ꢏ
(
)
ꣂ
1 + exp ꣀ
ꣁ − ꣁ
ꢻꢼ
From RRDE analysis, 81±4% efficiency for H2O2, a value of ꢈꢝꢹꢠ = 2.38 could be calꢀ
culated if the remaining current was assumed to be the 4H+/4e– reduction of O2 to 2
H2O. For these studies σ = 1 was used with the assumption that all electron transfer
steps come directly from the electrode, setting this expression as the lower limit for
TOFmax. The concentration for O2 was assumed to be 6.3 mM at saturation.43 The acids
were assumed to be first order based on kinetic data for the rateꢀdetermining step. The
ꢶꢷ
remaining value,
= 0.256796 s, could be calculated using constants under standard
ꢵ_
conditions and ν = 0.1 Vs–1. Fits were obtained to ~0.98, to represent a reasonable lowꢀ
er bound.
Catalytic Tafel plots for the catalysts reported here were simulated using the TOFmax
values determined from the FOWA. For the standard electrode potential of O2 reduction
to H2O2 in MeCN, a value of 0.356 V vs Fc/Fc+ was used, under the assumption that an
excess of protons exists, based on a reported value of 1.0 V vs NHE (corrected for
Fc/Fc+ = 0.644 V vs NHE).49 For Mn(tbudhbpy)Cl 1, E0: E1/2(11 mM buffered F5PhOH) = ꢀ
0.672 V vs Fc/Fc+. For the Mn(F5ꢀPh)corrole,10 the authors reported an O2 concentraꢀ
tion of 0.26 mM and H2O was assumed to be the proton source at pH = 7 with a conꢀ
centration of 55.5 M. The catalytic wave was assumed to begin at the reported reducꢀ
tion with E1/2 = ꢀ0.25 V vs Ag/AgCl (Ag/AgCl = 0.205 V vs NHE).74 A value of 0.281 V vs
NHE was used for the thermodynamic potential of O2 reduction to H2O2 under aqueous
conditions at pH = 7; 0.635 V vs NHE for pH = 1.2
Peroxidase Mimic Activity Test
Mn(tbudhbpy)Cl (15.6 mg) was dissolved in dry acetonitrile (20 mL) in a scintillation
vial (1.2 mM solution) capped with a septum and sealed with electrical tape. The headꢀ
space was purged with nitrogen and a vent needle for 15 minutes. Then three “preꢀ
injections” (1 mL) were taken from the solution headspace with an airtight syringe beꢀ
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fore any H2O2 was added. Aqueous H2O2 was then injected into the solution (0.015 mL
injection; 9.8 M solution in water) of H2O2 was injected into the solution. Right after this
addition of H2O2, a GC injection (1 mL) was taken of the headspace. After two additional
injections over ~60 minutes, an additional aliquot of H2O2 (0.015 mL injection; 9.8 M soꢀ
lution in water) was injected into the solution and two subsequent GC injections were
taken over ~60 minutes. These conditions were repeated without Mn(tbudhbpy)Cl for a
control reaction. All the data were processed by subtracting the peak area associated
with the largest peak area value observed for the three “preꢀinjections” from each of the
experimental injections to correct for environmental contamination; the retention time of
O2 was verified with calibration injections.
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