Supporting Information

Transcription

1 Supporting Information Kinetic and Mechanistic Characterization of Low-Overpotential, H2O2-Selective Reduction of O2 Catalyzed by N2O2-Ligated Cobalt Complexes Yu-Heng Wang, Zachary K. Goldsmith, Patrick E. Schneider, Colin W. Anson, James B. Gerken, Soumya Ghosh, Sharon Hammes-Schiffer, * and Shannon S. Stahl * Department of Chemistry, University of Wisconsin Madison, Madison, Wisconsin 53706, United States Department of Chemistry, Yale University, New Haven, Connecticut 06520, United States Table of Contents I. General Considerations 2 II. Synthesis of Ligands and Co Complexes III. EPR Experiments and Simulations 4 IV. Clark Electrode Measurement for Assessment of O 2 Binding to Co Complex 1 5 V. Substrate Dependence for O 2 Reduction 6 VI. Comparison of Two Chemical Reductants 9 VII. Kinetic Isotope Effects 9 VIII. Electrochemical Experiments 10 IX. Eyring Analysis 12 X. Computational Details 13 XI. Structures and Calculated Free Energies of All Species 16 XII. References 61 S1

2 I. General Considerations All commercially available reagents were used as received, except where otherwise noted. UV- Vis spectra were recorded on a Agilent Cary 60 spectrometer. EPR spectra were recorded on a Bruker EleXsys E500 spectrometer at 9.3 GHz (X-band) and 100 khz modulation; microwave power: 10 mw, modulation amplitude: 10 G, T: 110 K. Elemental analyses were provided by Robertson Microlit Laboratories, Ledgewood, NJ, USA. II. Synthesis of Ligands and Co Complexes 1-5 A. General synthetic considerations Cobalt complexes 2 and 4 were purchased from Sigma-Aldrich and used as received, and, therefore, ligands L2, and L4 were not independently prepared. Ligands L1, L3, L5 and cobalt complexes 1, 3, 5 were synthesized according to the literature procedures. 1 Characterization methods include 1 H and 13 C NMR, ESI-MS (electrospray ionization mass spectrometry) or MALDI-MS (matrix-assisted laser desorption), CV (cyclic voltammetry), UV-visible spectroscopy, and EA (elemental analysis). Spectra of new compounds are provided in section XI below. B. Characterization of known ligands (L1, L3, L5) Ligand L1 was synthesized according to a literature procedure. 1 Yield: 1.4 g (87%). MALDI-MS (m/z): [L1 + H] + calculated for C 21H 24N 2O 8: ; found H NMR (500 MHz, DMSOd 6): δ (s, 1H), (d, J = 12.6 Hz, 1H), (d, J = 12.6 Hz, 1H), 8.36 (d, J = 12.6 Hz, 1H), 8.22 (d, J = 12.8 Hz, 1H), 7.87 (m, 2H) 7.65 (d, J = 8.5 Hz, 1H), 4.16 (q, J = 6.9 Hz, 4H), 2.44 (s, 3H), 2.43 (s, 3H), 1.26 (t, J = 7.2 Hz, 3H), 1.24 (t, J = 7.0 Hz, 3H); 13 C NMR (500 MHz, DMSO-d 6): δ , , , , , , , , , , , , , , , , 59.67, 59.51, 30.72, 14.30, Ligand L3 was synthesized according to a literature procedure. 1 Yield: 466 mg, (82 %). 1 H NMR (500 MHz, CDCl 3): δ (s, 2H), 8.66 (s, 2H), 7.42 (d, J = 2.4 Hz, 2H), 7.20 (d, J = 2.5 Hz, 2H), 7.03 (s, 2H), 2.33 (s, 6H), 1.43 (s, 9H), 1.32 (s, 9H), 2.44 (s, 3H), 2.43 (s, 3H), 1.26 (t, J = 7.2 Hz, 3H), 1.24 (t, J = 7.0 Hz, 3H); 13 C NMR (500 MHz, CDCl 3): δ , , , , , , , , , 35.11, 34.16, 31.48, 29.45, S2

3 Ligand L5 was synthesized according to a literature procedure. 1 Yield: 362 mg, (78 %). 1 H NMR (500 MHz, CDCl 3): δ (s, 2H), 8.90 (s, 2H), 7.03 (s, 2H), 6.12 (d, J = 2.2 Hz, 2H), 5.82 (d, J = 2.2 Hz, 2H), 3.82 (s, 6H), 3.80 (s, 6H), 2.31 (s, 6H); 13 C NMR (500 MHz, CDCl 3): δ , , , , , , , , 94.40, 89.52, 55.57, 55.45, D. Characterization of known cobalt complexes (1, 3, 5) Cobalt complex 1 was synthesized according to a literature procedure. 1 Yield: 0.95 g (65%). MALDI-MS (m/z) calculated for C 21H 22CoN 2O 8 ([1] + ): ; found: Elemental analysis (%) for C 21H 22CoN 2O 8: calculated: C, 51.54; H, 4.53; N, 5.72; found: C, 51.43; H, 4.55; N, E 1/2 in MeOH: 0.31 V vs Fc* +/0. l max = 354 nm (e max = M -1 cm -1 ) and 304 nm (e max = M -1 cm -1 ). Cobalt complex 3 was synthesized according to a literature procedure. 1 Yield: 111 mg, (71 %). ESI-MS (m/z) calculated for C 38H 50CoN 2O 2 ([3] + ): ; found: Elemental analysis (%) for C 38H 50CoN 2O 2: calculated: C, 72.94; H, 8.05; N, 4.48; found: C, 72.97; H, 8.43; N, E 1/2 in MeOH: 0.18 V vs Fc* +/0. UV-Visible (MeOH): l max = 396 nm (e max = M -1 cm -1 ), 314 nm (e max = M -1 cm -1 ). Cobalt complex 5 was synthesized according to a literature procedure. 1 Yield: 128 mg, (49 %). ESI-MS (m/z) calculated for C 26H 26CoN 2O 8 ([5] + ): ; found: Elemental analysis (%) for C 26H 26CoN 2O 6: calculated: C, 59.89; H, 5.03; N, 5.37; found: C, 59.98; H, 5.31; N, E 1/2 in MeOH: V vs Fc* +/0. UV-Visible (MeOH): l max = 374 nm (e max = M -1 cm -1 ). S3

4 III. EPR Experiments and Simulations EPR parameters for all experiments: X-band, microwave frequency ca. 9.3 GHz, modulation 100 khz; 10 mw microwave power 10 mw, modulation amplitude 10 G, 110 K. EPR spectra were simulated using XSophe software from Bruker company. 2 A. EPR spectrum of Co complex 1 under N 2 To generate the spectrum for Co complex 1 (Co II ) in MeOH under N 2, Co 1 was dissolved in N 2- saturated MeOH in a glove box to prepare a 1 mm solution of MeOH. A 0.3 ml N 2-saturated MeOH solution of 1 mm Co 1 was transferred to a quartz EPR tube and the tube capped with a septum. The sample was removed from the glove box and frozen in liquid nitrogen. The experimental EPR spectrum was recorded at 110 K. The simulated EPR spectrum indicates that Co 1 exists in the low-spin state by comparing simulated g-factor and hyperfine constants to literature values (cf. Figure 2). 3 B. EPR spectrum of Co complex 1 under O 2 To generate the Co III -superoxide species, the above Co 1 (Co II ) solution was vigorously sparged with O 2 for 1 min under room temperature. The sample was then quickly capped with a septum and frozen in liquid N 2. The experimental EPR spectrum was recorded at 110 K. The simulated EPR spectrum indicates that Co III -superoxide is generated under aerobic conditions comparing simulated g-factor and hyperfine constants to literature values (cf. Figure 3a). 3 C. The reversible binding of O 2 to Co complex 1 To regenerate the Co complex 1 (Co II ) in MeOH, the above Co III -superoxide solution was vigorously sparged with N 2 for 1 min under room temperature. The sample was then quickly capped with a septum and frozen in liquid N 2. The experimental EPR spectrum was recorded at 110 K. The EPR spectrum of Co II was observed again, indicating that O 2 binding to Co complex 1 is reversible (cf. Figure 3b). Table S1. A summary of g-values and hyperfine constants (A) for Co II and Co III -O 2 species. These are the simulated parameters for EPR spectra in the main manuscript (Figure 2 and 3a). Co species g x g y g z A x (G) A y (G) A z (G) (MeOH)Co II (MeOH)Co III -O S4

5 IV. Clark Electrode Measurement for Assessment of O 2 Binding to Co Complex 1 (a) A Clark-type electrode (model YSI 5331) was interfaced to a 9 ml volume home-built cell as shown in Figure S1. Dissolved oxygen concentrations were measured by polarizing the Pt electrode at -800 mv vs. Ag/AgCl (saturated KCl), and the concentrations of dissolved oxygen in N 2-, air-, and O 2-saturated MeOH were measured to create a calibration curve, as shown in Figure S2. (b) The steady-state current decreases upon adding 0.25 mm 1, consistent with O 2 binding to 1, which will decrease the dissolved [O 2] in MeOH. The amount of O 2 determined from the calibration curve shows that the molar ratio of Co: O 2 is (1.1 ± 0.1):1. This outcome supports a mononuclear Co-O 2 adduct as the resting state under the catalytic conditions. The addition of 1 is limited to no lower than 0.25 mm because of the sensitivity of the Clark electrode, although the concentrations of catalyst concentrations for O 2 reduction studies in this work are typically between 0.02 to 25 µm (Figure S3). Figure S1. A homemade YSI 5331A Clark-type electrode. Figure S2. A calibration curve of [O 2] in N 2-, air-, and O 2-saturated MeOH. The current was recorded using a YSI 5331A Clark-type electrode at room temperature. Figure S3. The current decrease at 400 s corresponding to oxygen depletion upon adding 0.25 mm 1 into O 2-saturated MeOH at 298 K. The molar ratio of Co: O 2 is calculated to be (1.1 ± 0.1):1. S5

6 V. Substrate Dependence for O 2 Reduction Experimental protocols to analyze the kinetic contribution of individual reaction components [1] A 0.3 ml N 2-saturated MeOH solution of cobalt complex 1 (50, 100, 200, 300, 400, and 500 µm) and AcOH (4.2 µl) were rapidly added to a 2.7 ml O 2-saturated MeOH solution containing 1 mm Fc*. The reaction mixture was vigorously shaken for 30 sec, and the absorbance of Fc* + was monitored at 780 nm by UV-visible spectroscopy at ambient temperature (cf. Figure 5 in the manuscript for a representative collection of UV-vis data and reaction time course). Initial rate of Fc* + formation (R init), in units of mm s -1, were obtained by fitting UV-visible time-course data with linear regression during the first 100 sec of the reaction. These data provide the basis for the plot of R init vs. [1] in Figure 6a of the manuscript. Figure S4. Time profiles of absorbance at 780 nm due to formation of Fc* + in the two-electron reduction of O 2 ( M) 4 by Fc* ( M) with various concentrations of 1 (5, 10, 20, 30, 40, and 50 µm)) in the presence of AcOH ( M) in O 2-saturated MeOH at 298 K. [AcOH] A 0.3 ml N 2-saturated MeOH solution of cobalt complex 1 (250 µm) and AcOH (0.9, 3.4, 5.2, 6.9, and 8.6 µl) were rapidly added into a 2.7 ml O 2-saturated MeOH solution containing 1 mm Fc*. The reaction mixture was vigorously shaken for 30 sec and the absorbance of Fc* + was monitored at 780 nm by UV-visible spectroscopy at ambient temperature. Initial rate of Fc* + formation (R init), in units of mm s -1, were obtained by fitting UV-visible time-course data with linear regression during the first 100 sec of the reaction. These data provide the basis for the plot of R init vs. [AcOH] in Figure 6b of the manuscript. S6

7 Figure S5. Time profiles of absorbance at 780 nm due to formation of Fc* + in the two-electron reduction of O 2 ( M) by Fc* ( M) with 1 ( M) in the presence of various concentrations of AcOH (5, 20, 30, 40, and 50 mm) in MeOH at 298 K. [Fc*] A 0.3 ml N 2-saturated MeOH solution of cobalt complex 1 (125 µm) and a AcOH (1.7 µl) were rapidly added into a 2.7 ml O 2-saturated MeOH solution containing Fc* (0.28, 0.42, 0.56, and 1.1 mm). Then the reaction mixture was vigorously shaken for 30 sec and the absorbance of Fc* + was monitored at 780 nm by UV-visible spectroscopy at ambient temperature. Initial rate of Fc* + formation (R init), in units of mm s -1, were obtained by fitting UV-visible time-course data with linear regression during the first 300 sec of the reaction. These data provide the basis for the plot of R init vs. [Fc*] in Figure 6c of the manuscript. Figure S6. Time profiles of absorbance at 780 nm due to formation of Fc* + in the two-electron reduction of O 2 ( M) by various concentrations of Fc* (0.25, 0.375, 0.5, and 1.0 mm) with 1 ( M) in the presence of AcOH ( M) in MeOH at 298 K. S7

8 [O 2] (1) A 0.3 ml O 2-saturated MeOH solution of cobalt complex 1 (125 µm) and a 1.7 µl AcOH were rapidly added into an 2.7 ml O 2-saturated MeOH solution of 0.56 mm Fc*. Then the reaction mixture was vigorously shaken for 30 sec and the absorbance of Fc* + was monitored at 780 nm by UV-visible spectroscopy at ambient temperature ([O 2]» 10 mm). (2) A 0.3 ml air-saturated MeOH solution of cobalt complex 1 (125 µm) and a 1.7 µl AcOH were rapidly added into a 2.7 ml air-saturated MeOH solution containing 0.56 mm Fc*. Then the reaction mixture was vigorously shaken for 30 sec and the absorbance of Fc* + was monitored at 780 nm by UV-visible spectroscopy at ambient temperature ([O 2]» 2 mm). (3) A 1.5 ml O 2-saturated MeOH solution containing 0.5 mm Fc* and 125 µm cobalt complex 1 was mixed with a 1.5 ml N 2-saturated MeOH solution containing 0.5 mm Fc* and 125 µm cobalt complex 1. A 1.7 µl AcOH was rapidly added into the above mixed solution (3 ml), then the reaction mixture was vigorously shaken for 30 sec and the absorbance was monitored at 780 nm by UV-visible spectroscopy at ambient temperature ([O 2]» 5 mm). (4) Initial rate of Fc* + formation (R init), in units of mm s -1, were obtained by fitting UV-visible time-course data with linear regression during the first 300 sec of the reaction. These data provide the basis for the plot of R init vs. [O 2] in Figure 6d of the manuscript Figure S7. Time profiles of absorbance at 780 nm due to formation of Fc* + in the two-electron reduction of various concentrations of O 2 (2 mm (air-satur), 5 mm (N 2-satur + O 2-satur), and 10 mm (O 2-satur)) by Fc* ( M) with 1 ( M) in the presence of AcOH ( M) in MeOH at 298 K. S8

9 VI. Comparison of Two Chemical Reductants (a) A 0.3 ml O 2-saturated MeOH solution of cobalt complex 1 (50 µm), and 0.9 µl AcOH were rapidly added to a 2.7 ml O 2-saturated MeOH solution containing 0.56 mm (CpMe 5) 2Fe (or (CpMe 4) 2Fe). The reaction mixture was vigorously shaken for 30 sec, and the absorbance of Fc* + at 780 nm (or (CpMe 4) 2Fe + at 750 nm) was monitored by UV-visible spectroscopy at ambient temperature. Concentrations of each substrate in the reactions: 0.5 mm (CpMe 5) 2Fe (or (CpMe 4) 2Fe), 5 µm Co, 5 mm AcOH, and 10 mm O 2 ((CpMe 5) 2Fe = Fc*). (b) Time course data for the formation of (CpMe 5) 2Fe + and (CpMe 4) 2Fe + were monitored by UVvisible spectroscopy at ambient temperature as shown in Figure S8. The data points were fitted with linear regression to calculate the initial rates of formation of (CpMe 5) 2Fe + and (CpMe 4) 2Fe +. The result suggests that electron transfer is not involved in the rate-limiting step. Figure S8. Plots of (CpMe 4) 2Fe + (blue trace) and (CpMe 5) 2Fe + (red trace) versus time for the twoelectron reduction of O 2 ( M) with 1 ( M) in the presence of AcOH ( M) in O 2-saturated MeOH at 298 K. The electron transfer steps of the catalytic O 2 reduction were investigated using different chemical reductants, and their nearly identical initial rates indicate that electron transfer is not involved in the rate-limiting step. VII. Kinetic Isotope Effects (a) A 0.3 ml O 2-saturated MeOH (or MeOD) solution of cobalt complex 1 (50 µm) and 0.9 µl AcOH (or AcOD) was rapidly added to a 2.7 ml O 2-saturated MeOH solution containing 0.56 mm Fc*. The reaction mixture was vigorously shaken for 30 sec, and the absorbance of Fc* + was monitored at 780 nm by UV-visible spectroscopy at ambient temperature. Concentrations of each substrate in the reactions: 0.5 mm Fc*, 5 µm Co, 5 mm AcOH (or AcOD), and 10 mm O 2. (b) Time course data for the formation of Fc* + in hydrogen (AcOH/MeOH) and deuterium (AcOD/MeOD) environments were monitored at 780 nm by UV-visible spectroscopy at ambient temperature as shown in Figure 7 in the manuscript. The data points were fitted with linear regression to calculate the initial rates of O 2 reduction, k H and k D, respectively. The kinetic isotope effect (KIE) is defined as the ratio of rate constants, k H/k D, which is calculated to be 2.7. A KIE value of 2.7 indicates that protonation is involved in the rate-limiting step. S9

10 VIII. Electrochemical Experiments A. General considerations All cyclic voltammograms (CV) were performed with a CH Instrument 600E Potentiostat, and all differential pulse voltammograms (DPV) were conducted on a Pine WaveNow potentiostat. The supporting electrolyte for all electrochemical experiments was 0.1 M tetrabutylammonium perchlorate ([NBu 4][ClO 4]). The three-electrode setup for all cyclic voltammogram (CV) measurements included a glassy carbon (GC) working electrode (3.0 mm diameter), a platinum (Pt) wire counter electrode, and a 0.01 M Ag/AgNO 3 non-aqueous reference electrode. The halfwave potentials of cobalt complexes are referenced to the decamethylferrocenium/decamethylferrocene redox couple (Fc* +/0 ), and Fe* +/0 is V relative to Fc +/0 in MeOH solution. B. Cyclic voltammograms of ferrocene derivatives (Cp 2Fe, (CpMe 4) 2Fe), and (CpMe 5) 2Fe The half-wave potentials of ferrocene derivatives were recorded in MeOH solutions at ambient temperature anaerobically. The half-wave potentials of (CpMe 4) 2Fe +/0 and (CpMe 5) 2Fe +/0 are negative than the E 1/2(Co III/II ) of 1, which can act as chemical reductants to conduct chemical O 2 reduction (Cp 2Fe = Fc, (CpMe 5) 2Fe = Fc*). Figure S9. Cyclic voltammograms of N 2-saturated MeOH solutions of Cp 2Fe (black trace, M), (CpMe 4) 2Fe (blue trace, M), (CpMe 5) 2Fe (green trace, M), and 1 with 0.15 M AcOH (red trace, M) at 298 K, respectively. The sweep rate was 100 mv s 1. Supporting electrolyte: 0.1 M [NBu 4][ClO 4]. (CpMe 4) 2Fe +/0 is -0.4 V vs. Cp 2Fe +/0, and (CpMe 5) 2Fe +/0 is -0.5 V vs. Cp 2Fe +/0. C. Differential pulse voltammetry (DPV) studies (a) All DPVs were conducted on a Pine WaveNow potentiostat with a pulse amplitude of 50 mv, a pulse width of 0.05 s, a pulse period of 0.5 s, and an increment of 0.5 mv. 5 The three-electrode setup for all differential pulse voltammetric measurements included a glassy carbon (GC) working electrode (3.0 mm diameter), a platinum (Pt) wire counter electrode, and a 0.01 M Ag/AgNO 3 non-aqueous reference electrode. (b) For the DPV studies of 1 under anaerobic conditions, three electrodes were immersed into the electrochemical cell containing 50 µm 1, 0 or 10 mm AcOH, and 0.1 M [NBu 4ClO 4] in 10 ml MeOH. DPV were started by scanning in the positive (anodic) direction under 1 atm N 2. S10

11 2 i (µa) 1 (0 mm AcOH) (10 mm AcOH) scan E (V vs. Fc* +/0 ) Figure S10. DPV of 1 under anaerobic conditions (1 atm N 2). Solid trace: 50 µm 1 in MeOH in the absence of AcOH, anodic peak potential: 0.31 V; dashed trace: 50 µm 1 in MeOH in the presence of 10 mm AcOH, anodic peak potential: 0.31 V. The result indicates that the anodic peak potential of 1 under anaerobic conditions is not affected by AcOH. (c) For the DPV studies of 1 under aerobic conditions in the absence of AcOH, three electrodes were immersed into the electrochemical cell containing 50 µm 1, 0 or 10 mm AcOH, and 0.1 M [NBu 4ClO 4] in 10 ml MeOH. DPV were started by scanning in the negative (cathodic) direction under 1 atm O 2 (cf. Figure 8a, black trace). (d) For the DPV studies of 1 under aerobic conditions at various concentrations of AcOH, three electrodes were immersed into the electrochemical cell containing 50 µm 1, and 0.1 M [NBu 4ClO 4] in 10 ml MeOH. DPV were started by scanning in the negative (cathodic) direction under 1 atm O 2. AcOH: 10, 30, and 50 mm (cf. Figure 8a, red, green, and blue traces). (e) For the DPV studies of 1 under catalytic conditions at various concentrations of AcOH, three electrodes were immersed into the electrochemical cell containing 50 µm 1, 1 mm Fc*, and 0.1 M [NBu 4ClO 4] in 10 ml MeOH. DPV were started by scanning in the negative (cathodic) direction under 1 atm O 2. AcOH: 10, 25, and 50 mm (cf. Figure 8b, red, green, and blue traces). S11

12 IX. Eyring Analysis (a) UV-Vis spectra were recorded using a Agilent Cary 60 spectrometer equipped with a 2 mm path length optical fiber probe. A 6 ml sample vial equipped with a stir bar was filled with 2.7 ml air-saturated MeOH solution containing 1 mm Fc*, followed by adding 0.3 ml air-saturated MeOH solution of cobalt complex 1 (400 µm). The 3 ml mixed solution and the optical fiber probe were allowed to equilibrate for 5 min at desired temperatures (0, 20, 30, 40, and 50 C). Upon reaching the desired temperature, a 4.3 µl AcOH was rapidly injected into the 3 ml mixed solution. The reaction mixture was vigorously stirred for 30 sec, and the absorbance of Fc* + was monitored at 780 nm by UV-visible spectroscopy. Concentrations of each substrate in the reactions: 0.9 mm Fc*, 40 µm Co, 25 mm AcOH and 2 mm O 2. (b) Initial rate of Fc* + formation (R init), in units of mm s -1, were obtained by fitting UV-visible time-course data with linear regression during the first 50 sec of the reaction. These data provide the basis for the Eyring plot in Figure S11c. (a) (b) T R 1/T init k (K) (M s -1 ) (M -1 s -1 ) ln(k/t) (c) Figure S11. (a), (b) The rates of O 2 reduction were studies at various temperature to construct the Eyring plot. Time courses of formation of Fc* + were followed by UV-visible spectroscopy at 780 nm. Reaction conditions: 0.9 mm Fc*, 40 µm Co, 25 mm AcOH and 2 mm O 2, T: 273, 293, 303, 313, and 323 K. (c) Eyring plot of O 2 reduction catalyzed by 1 between 0 ºC to 50 ºC. DH = 4.9 kcal/mol, DS = -31 cal/mol K, and DG = 14 kcal/mol S12

13 X. Computational Details (a) Density functional theory (DFT) calculations were performed using the BP86 functional. 6 The 6-31G** 7 basis set was utilized for all non-metal atoms with diffuse basis functions (6-31+G**) added for O atoms not part of the N 2O 2 ligand scaffold (e.g., the atoms composing O 2 before and after binding to the Co complex). The LANL2DZ basis set 8 was used for Co. All calculations were performed using Gaussian Geometry optimizations (with the exceptions of O 2 and methanol) were performed in the solution phase (methanol solvent) using the SMD implicit solvation model. 10 (b) Vibrational frequency calculations were performed to determine the zero-point energy and entropic contributions for all reaction free energies at K. Each minimum reported was confirmed to contain no imaginary frequencies. Rotational contributions to entropy were excluded from the free energies of all species (except gaseous dioxygen) as is recommended for association/dissociation reactions. 11 In order to account for the experimental conditions, namely 1 atm partial pressure of O 2 and a methanol solvent concentration of 25 M, the following standard state corrections were employed: (1) The free energy of gas-phase O 2 was calculated at 1 atm partial pressure. Then, to account for the change in concentration from 1 atm to 1 M, a correction based on the ideal gas law was applied: RTln(24.5) = 1.89 kcal/mol. (2) The free energy of methanol was calculated at 1 atm partial pressure, and a correction to account for the change in concentration from 1 atm to 25 M in the gas phase using the ideal gas law, RTln( ) = 3.79 kcal/mol, was applied. Furthermore, the solvation free energy of methanol in methanol, 4.84 kcal/mol, as obtained from the literature, 12 was introduced to the standard state corrected molar gas-phase free energy to obtain the molar solution-phase free energy. (3) For all other species considered, molar solution phase free energies were directly calculated in Gaussian 09. (c) All redox potentials and pk a values were computed relative to a specified reference reaction, and the property for the reference reaction was set to the experimentally measured value. Thus, all quantities were shifted by a constant corresponding to the difference between the experimental and calculated value for the reference reactions. Most mechanistic reductions were referenced to the experimental E 1/2 for 1; EPT1 was referenced to the DPV peak potential under catalytic conditions because it is a ligand-centered rather than Co-centered reduction. S13

14 Figure S12. Dynamic equilibrium between two mononuclear cobalt complexes (1) and a µ- peroxo bridged cobalt dimer can likely be switched over an order of magnitude change in [Co]. The dimerization is a nearly isoergic reaction (DG = 1.54 kcal/mol). The µ-peroxo bridged cobalt dimer is mechanistically accessible but not involved in the proposed mechanism of ORR based on the observed rate law and Clark electrode studies. L = MeOH (solvent). Figure S13. A concerted electron-proton transfer (EPT1) mechanism (1ab 1ac, DG = kcal/mol) is proposed to generate the cobalt(iii) hydroperoxide species (Co III (OOH), 1d). The concerted mechanism avoids the high free-energy intermediate corresponding to proton transfer, and ET1 cannot be accessed because it is cathodic of the half-wave potential of decamethylferrocene (Fc*). The redox potential associated with concerted EPT1 is anodic of the half-wave potential of Fc* and thus is accessible. Because kinetic studies show that the reaction rate is zero order in [Fc*], the first protonation cannot contribute to the turnover-limiting step if it is intrinsically coupled to a reduction by Fc*. Moreover, the concerted mechanism for EPT1 is consistent with the DPV experiments showing that the rate-limiting proton transfer must follow formation of Co III (OOH). The redox potential for EPT1 is set to the experimental value as determined by DPV given that it cannot be appropriately referenced by the Co-centered Co III/II process. S14

15 Figure S14. Protonation on the proximal oxygen of 1b and the subsequent single-electron reduction of 1b-H + generates the cobalt(ii) hydrogen peroxide adduct species (1c). ET2' or EPT2 cannot be accessed under catalytic conditions, because the reduction potentials of ET2' and EPT2 are cathodic of the half-wave potential of Fc* by 280 mv and 740 mv, respectively. Figure S15. Reduction of the Co III (OOH) (1b) by Fc* under catalytic conditions is unlikely because the reduction potential (ET2 ) is cathodic of the half-wave potential of Fc* +/0 by 740 mv (1b 1b - ), whereas reduction of O 2 to H 2O by Fc* via the EPT2' pathway is thermodynamically favorable. This finding implies that the kinetic barrier must inhibit the EPT2' pathway, which is unsurprising given the complexity of EPT2'. Specifically, the EPT2' process entails a concerted electron-proton transfer coupled to O-O bond breaking (1b 1d). S15

16 Figure S16. Protonation of the distal oxygen of Co III (OOH) (1b) is more thermodynamically unfavorable than protonation of the proximal oxygen, which likely leads to selectivity for 2e - /2H + reduction of O 2 to H 2O 2 instead of H 2O. XI. Structures and Calculated Free Energies of All Species Format: Species number, name, or description Calculated solution phase Gibbs free energy (Hartree) Cartesian coordinates MeOH C H H H O H O O O H 2O O H O H H 2O O H S16

17 H AcOH C O O H C H H H AcO C O O C H H H Co O O N C C C C N C H C C C C C C H H H H S17

18 O C C H H H C H H H C O O C O O C C H H H H H C C H H H H H C C O O H H H H H [Co III ](MeOH) Co O O N C S18

19 C C C N C H C C C C C C H H H H O C C H H H C H H H C O O C O O C C H H H H H C C H H H H H C S19

20 C O O H H H H H O C H H H H a Co O O N C C C C N C H C C C C C C H H H H O C C H H H C H H S20

21 H C O O C O O C C H H H H H C C H H H H H C C O O H H H H H O O a Co O O N C C C C N C H C S21

22 C C C C C H H H H C C H H H C H H H C O O C O O C C H H H H H C C H H H H H C O O H O O a-H S22

23 Co O O N C C C C N C H C C C C C C H H H H O C C H H H C H H H C O O C O O C C H H H H H C C H S23

24 H H H H C C O O H H H H H O O H b Co O O N C C C C N C H C C C C C C H H H H O C C H H H C S24

25 H H H C O O C O O C C H H H H H C C H H H H H C C O O H H H H H O O H b Co O O N C C C C N S25

26 C H C C C C C C H H H H C C H H H C H H H C O O C O O C C H H H H H C C H H H H H C O O H O O S26

27 H b-H Co O O N C C C C N C H C C C C C C H H H H O C C H H H C H H H C O O C O O C C H H H H S27