Supporting Information. Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential

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1 Supporting Information Oxygen Reduction Catalysis at a Dicobalt Center: The Relationship of Faradaic Efficiency to Overpotential Guillaume Passard, Andrew M. Ullman, Casey N. Brodsky and Daniel G. Nocera* Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02183, United States dnocera@fas.harvard.edu S1

2 Table of Contents Page A. Experimental Methods... S3 A.1. Electrochemistry... S3 A.2. Synthesis... S3 B. Cyclic Voltammetric Experiments... S5 C. RRDE Experiments and Tabulation of Data... S9 C.1. Compound 1 in MeCN in the presence of PhOH, AcOH, ClAcOH and TFA... S9 S12 C.2. Compound 1 in DMF in the presence of AcOH and TFA... S13 S14 C.3. Compound 1 in H 2 O at different ph... S15 C.4. Compound 2 in MeCN in the presence of PhOH and AcOH... S16 S17 C.5. Compound 2 in H 2 O at different ph... S18 C.6. Tabulation of average ORR faradaic yields for H 2 O production... S19 D. Determination of ORR Thermodynamic Standard Potentials... S20 E. Plots of average ORR faradaic yields for H 2 O production in MeCN and H 2 O by 1 and 2... S23 S24 S2

3 A. Experimental Methods A.1. Electrochemistry Cyclic voltammograms (CV) and RRDE (rotating ring disk electrode) experiments were recorded using a CH Instruments potentiostat. The studied compound was dissolved in a solution containing a supporting electrolyte, 0.1 M n-bu 4 NPF 6 (purchased from Sigma Aldrich, used without any purification and stored under vacuum) in organic solvent or a 0.1 M phosphate buffer (purchased from Sigma Aldrich and used without further purification) solution. A three-electrode cell configuration was used where the counter electrode is a platinum wire, the reference electrode is a saturated calomel electrode (SCE) separated from the solution by a frit containing a solution of supporting electrolyte, and the working electrode is in glassy carbon carefully polished before each measurement. The polishing procedure was performed for 2 min on felt using different diamond pastes subsequently of 15, 6, 3 and 1 µm. Ethanol was used as a lubricant and to remove the diamond paste from the electrode. Before using the electrode, it was briefly sonicated in ethanol and dried with a stream of compressed air. Between each recorded CV, the polishing procedure was repeated using 1 µm paste. The same procedure was employed for the rotating ring disk electrode (RRDE). The RRDE and the rotator (MSRX) were purchased from Pine instruments. The ohmic drop was carefully compensated in all electrochemical experiments by using the positive feedback compensation implemented in the instrument. A.2. Synthesis Preparation of dipyridylethane naphthyridine (DPEN). Two literature methods 1,2 were combined in the following adaptation. n-butyllithium (2.5 M in hexanes, 14.0 ml, 35.0 mmol, 4.60 equiv) was added dropwise to a stirring solution of 2-ethylpyridine (4.67 ml, 40.8 mmol, 5.37 equiv) in 100 ml of THF at 78 C. The resulting deep red solution was stirred at 78 C for 30 min. The flask was then removed from the cooling bath, and 2- fluoropyridine (1.51 ml, 17.5 mmol, 2.30 equiv) was immediately added dropwise. The mixture was stirred for 1 h as it slowly warmed to room temperature, and then it was placed in an oil bath to reflux for 1 h under an atmosphere of N 2. The flask was then removed from the oil bath and cooled to room temperature, ~30 min. Under positive N 2 pressure, solid 2,7-dichloro-1,8-naphthyridine (1.51 g, 7.59 mmol, 1.00 equiv) was added to the stirring mixture, and the flask was returned to the oil bath to reflux for 16 h under an atmosphere of N 2. After cooling to room temperature, the reaction mixture was quenched with 100 ml of water, concentrated to remove the majority of the THF, and extracted with ml of CH 2 Cl 2. Organics were combined, dried over MgSO 4, filtered, and concentrated. The residual material was recrystallized from 200 ml of hot MeCN to give 2.84 g of (1) Davenport, T. C.; Tilley, T. D. Angew. Chem. Int. Ed. 2011, 50, (2) Ünal, E. A.; Wiedemann, D.; Seiffert, J.; Boyd, J. P.; Grohmann, A. Tet. Lett. 2012, 3, 54. S3

4 DPEN. The filtrate from the first crystallization could be concentrated and redissolved in 100 ml of hot MeCN to yield a second batch (0.39 g) of crystals of DPEN. Total yield of 3.24 g, 6.55 mmol, 86.3 %. Characterization data for this material ( 1 H NMR, ESI-MS) were in accordance with the literature. 1 Preparation of [Co 2 (OH) 2 DPEN(μ-1,3-OC(NH)CH 3 )](PF 6 ) 3 (1). Co(NO 3 ) 2 (0.2 M aq. solution, 14.2 ml, 2.84 mmol) was added to a solution of DPEN (702 mg, 1.42 mmol) in 5 ml of acetone. After stirring at room temperature for 20 min, H 2 O 2 (9.79 M aq. solution, 290 μl, 2.84 mmol) was added, and the reaction mixture was stirred for 16 h at room temperature. The mixture was then concentrated at 60 C to a volume of 3 ml to which one drop of 10% HNO 3 was added. Then 50 ml of acetone was quickly added, resulting in the precipitation of a pink solid. The solid was collected on 5 μm nylon filter paper and washed with 20 ml 20:1 acetone:water, then 20 ml of acetone. The pink solid (879 mg) was then dissolved in H 2 O (44 ml) and to this solution was added a solution of KPF 6 (1.05 g in 40 ml of H 2 O). The ph of the mixture was raised to 5.5 by the controlled addition of 0.5 M KOH, causing the formation of a pink precipitate. The precipitate was collected on 5 μm nylon filter paper and washed with 80 ml diethyl ether. The solid was then dissolved in 20 ml of MeCN with heating and stirred for 2 h at 60 C. The solution was concentrated to near saturation and vapor diffusion of Et 2 O into MeCN solution at room temperature yielded orange/red crystals (1.36 g, 1.19 mmol, 84 %). 1 H NMR (CD 3 CN, δ): 8.98 (2H, t), 8.86 (2H, d), 8.79 (2H, d), 8.63 (1 H, d), 8.55 (1H, d), 8.20 (4H, dd), 8.15 (4H, d), 7.72 (2H, t), 7.68 (2H, t), 7.42 (1H, s), 3.09 (3H, s), 3.05 (3H, s), 2.73 (3H, s). HR-ESI-MS (m/z): [1 (H +, 3PF 6 )] 2+ calcd (C 34 H 31 Co 2 N 7 O 3 ) ; found Elemental analysis (C 34 H 32 Co 2 F 18 N 7 O 3 P 3 ): calcd C 35.84, H 2.83, N 8.61; found C 35.69, H 2.65, N Preparation of [Co 2 (OH) 2 DPEN(μ-1,3-OAc)](PF 6 ) 3 (2). Solid DPEN ligand (200 mg, mmol) and Co(OAc) 2 (201 mg, mmol) were added to 20 ml of 4:1 acetone:h 2 O and stirred at room temperature until dissolved (20 min), to create a deep red solution. To this solution was added 9.79 M aqueous H 2 O 2 (0.206 ml, 2.02 mmol). The solution, which quickly turned darker red, was stirred at room temperature for 20 h. Volatiles were removed in vacuo and KPF 6 ( mg, 2.02 mmol) was added to the remaining aqueous solution to yield a dark red precipitate, which was collected by on 5 μm nylon filter paper and washed with H 2 O and Et 2 O. Vapor diffusion of Et 2 O into a concentrated solution in MeCN at 5 C yielded dark red crystals (188 mg, mmol, 40.8 %). 1 H NMR (CD 3 CN, δ): 9.02 (2H, d), 8.82 (4H, d), 8.62 (2H, d), 8.30 (4H, t), 8.25 (4H, d), 7.81 (4H, t), 3.10 (6H, s), 2.72 (3H, s). HR-ESI-MS (m/z): [2 (H +, 3PF 6 )] 2+ calculated (C 31 H 30 C 2 N 6 O 4 ) ; found Elemental analysis (C 34 H 31 Co 2 F 18 N 6 O 4 P 3 ): calcd C 35.81, H 2.74, N 7.37; found C 35.66, H 2.71, N S4

5 B. Cyclic Voltammetric Experiments Figure S1. Cyclic voltammograms of 0.5 mm solution of (a) 1 ( ) and 2 ( ) and free DPEN ( ) in MeCN containing 0.1 M n-bu 4 NPF 6 under Ar. Working electrode: glassy carbon (diameter 3 mm). Scan rate: 0.1 V s 1. S5

6 Figure S2. Cyclic voltammetry of 1 (0.5 mm) in MeCN (0.1 M n-bu 4 NPF 6 ) under O 2 and in the presence of increasing concentrations of AcOH: 0 ( ), 1, 2, 5 10, 20, 50, 100, 200 mm ( ); v = 0.1 V s 1. Reverse scans are removed for clarity. The 50, 100 and 200 mm traces reflect acidolysis of the dicobalt complex, which occurs at higher acid concentrations. S6

7 Figure S3. Cyclic voltammogram of 1 (0.5 mm) in MeCN ( ) and DMF ( ) (0.1 M n- Bu 4 NPF 6 ) under O 2 and in the presence of 20 mm TFA; v = 0.1 V s 1. S7

8 Figure S4. 1 ( ) and 2 ( ) (0.5 mm) in MeCN (0.1 M n-bu 4 NPF 6 ) under O 2 (saturated solution, 8.1 mm) and in the presence of AcOH (50 mm). (a) CV on glassy carbon (diameter = 3 mm) at 0.1 V s 1. (b) RRDE traces. Rotation rate: 500 rpm. Scan rate: 0.02 V s 1. Ring potential: 1.17 V. S8

9 C. RRDE Experiments and Tabulation of Data C.1. Compound 1 in MeCN in the presence of PhOH, AcOH, ClAcOH and TFA Figure S5. (a) RRDE data of 1 (0.5 mm) in MeCN (0.1 M n-bu 4 PF 6 ) with [PhOH] = 10 mm under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b) Faradaic efficiency in H 2 O 2 obtained by application of eq 1 at the different rotation rates. S9

10 Figure S6. RRDE data of 1 (0.5 mm) in MeCN (0.1 M n-bu 4 PF 6 ) with (a) [AcOH] = 10 mm and (c) [AcOH] = 50 mm under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b) and (d) Faradaic efficiency in H 2 O 2 obtained by application of eq 1 at the different rotation rates for data in (a) and (c), respectively. S10

11 Figure S7. RRDE data of 1 (0.5 mm) in MeCN (0.1 M n-bu 4 PF 6 ) with (a) [ClAcOH] = 10 mm, (c) [ClAcOH] = 20 mm and (e) [ClAcOH] = 50 mm under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b), (d) and (f) Faradaic efficiency in H 2 O 2 obtained by application of eq 1 at the different rotation rates for data in (a), (c) and (e), respectively. S11

12 Figure S8. RRDE data of 1 (0.5 mm) in MeCN (0.1 M n-bu 4 PF 6 ) with (a) [TFA] = 10 mm and (c) [TFA] = 20 mm under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b) and (d) Faradaic yield in H 2 O 2 obtained by application of eq 1 at different rotation rates for data in (a) and (c), respectively. S12

13 C.2. Compound 1 in DMF in the presence of PhOH and TFA Figure S9. RRDE data of 1 (0.5 mm) in DMF (0.1 M n-bu 4 PF 6 ) with (a) [AcOH] = 10 mm and (c) [AcOH] = 50 mm under under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b) and (d) Faradaic yield in H 2 O 2 obtained by application of eq 1 at different rotation rates for data in (a) and (c), respectively. S13

14 Figure S10. RRDE data of 1 (0.5 mm) in DMF (0.1 M n-bu 4 PF 6 ) with (a) [TFA] = 10 mm and (c) [TFA] = 20 mm under under under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b) and (d) Faradaic yield in H 2 O 2 obtained by application of eq 1 at different rotation rates for data in (a) and (c), respectively. S14

15 Figure S11. RRDE data of 1 (0.5 mm) in H 2 O (0.1 M phosphate buffer) at ph (a) 7, (c) 8, (e) 9, (g) 11 and (i) 12 under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b), (d), (f), (h) and (i) Faradaic yield in H 2 O 2 obtained by application of eq 1 at different rotation rates for data in (a), (c), (e), (g) and (i), respectively. S15

16 Figure S12. RRDE data of 2 (0.5 mm) in MeCN (0.1 M n-bu 4 PF 6 ) with (a) [PhOH] = 10 mm under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b) Faradaic yield in H 2 O 2 obtained by application of eq 1 at different rotation rates for data in (a). S16

17 Figure S13. RRDE data of 2 (0.5 mm) in DMF (0.1 M n-bu 4 PF 6 ) with (a) [AcOH] = 10 mm and (c) [AcOH] = 50 mm under O 2 (saturated solution, 8.1 mm) at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b) and (d) Faradaic yield in H 2 O 2 obtained by application of eq 1 at different rotation rates for data in (a) and (c), respectively. S17

18 Figure S14. RRDE data of 2 (0.5 mm) in H 2 O (0.1 M phosphate buffer) at ph (a) 8, (c) 9, (e) 11 and (g) 12 under O 2 (saturated solution, 8.1 mm at different rotation rates: 100 ( ), 250 ( ), 500 ( ), 750 ( ) and 1000 rpm ( ). Scan rate: 0.02 V s 1. Ring potential: 1.17 V. Disk current is positive; ring current is negative. (b), (d), (f) and (h) Faradaic yield in H 2 O 2 obtained by application of eq 1 at different rotation rates for data in (a), (c), (e) and (g), respectively. S18

19 Table S1. Faradaic yields in H 2 O of 1 and 2 extracted from RRDE experiments (Figures S5 S14). Catalyst Solvent Acid or ph [Acid] / mm Faradaic yield in H 2 O / % Average yield in H 2 O / % PhOH ± 11 AcOH ± 6 MeCN ClAcOH ± DMF TFA AcOH TFA ± 3 77 ± 7 97 ± 1 2 H 2 O (0.1 M phosphate buffer) MeCN H 2 O (0.1 M phosphate buffer) PhOH ± 7 AcOH ± S19

20 D. Determination of ORR Thermodynamic Standard Potentials The determination of the standard potential for O 2 to water in organic solvent (S) and in the presence of acid (HA) is akin to the approach developed by Costentin et al. to determine the standard reduction potential of CO 2 to CO in nonaqueous solution. 3 Conversion of O 2 into H 2 O in solvent S in the presence of acid HA is described by the equilibrium reaction: O ( ) HA ( ) 2 H O ( ) +4 A ( ) (S1) In practice, the standard potential is known for this reaction in aqueous solution (i.e., HA = H 2 O). Eq S1 can be referenced to the standard potential in water via the following thermodynamic cycle: Scheme S1. Thermodynamic cycle used to determine the thermodynamic standard potential of O 2 reduction to H 2 O in organic solvent. This cycle yields the following:,, =,,,, (S2) which describes the dependence of the standard potential of the O 2 /H 2 O couple in organic solvent on the strength of an exogenous acid. The standard potential of the O 2 /2H 2 O redox couple in water is 1.23 V vs. NHE. 4 The values of 5, and 6, in MeCN and DMF are known and reproduced in Table S2. (3) Costentin, C.; Drouet, S.; Robert, M.; Savéant, J.-M. Science 2012, 338, 90. (4) Sawyer, D. T. Oxygen Chemistry; Oxford University Press: New York, (5) Marcus, Y. Ion Properties; Marcel Dekker: New York, (6) Costentin, C.; Evans, D. H.; Robert, M.; Savéant, J.-M.; Singh, P. S. J. Am. Chem. Soc. 2005, 127, Singh, P. S.; Evans, D. H. J. Phys. Chem. B 2006, 110, 637. S20

21 Table S2. Free energies for proton solvation used to evaluate eq S2. MeCN DMF, ( ) , ( ) Different values of, (Table S3) will change the value of,,. In this work, we use the median values of, that were previously published (refs 7 9). We note that the general trends presented in Figure 4 of the text are maintained for either set (refs 7 9 or ref 10) of measurements. Table S3. Free energies for proton solvation (, ) in ev. CH 3 CN DMF Ref The overall potential is further modified by the interliquid junction potentials for solvent, E J,S which has previously been determined for MeCN and DMF by Costentin and al: 3, = V and, = V Substituting these values in eq S2, with the addition of E J,S, yields the following (eqs 3 and 4 in the main text):,,,, = = p, V vs. NHE p, V vs. NHE (S3) (S4) (7) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997, p (8) Cox, B. G. Acids and Bases; Oxford University Press, 2013, p 30. (9) Marcus, Y. Pure & Appl. Chem. 1983, 55, 977 (10) Pegis, M. L.; Roberts, J. A. S.; Wasylenko, D. J.; Mader, E. A.; Appel, A. M.; Mayer, J. M. Inorg. Chem. 2015, 54, S21

22 Application of these equations leads to the values in Table 1 of the main text. These equations are in agreement with the observation that the ratio H 2 O/H 2 O 2 does not change with concentration of acid in the solution, but only with the strength of the proton donor. S22

23 E. Plots of average ORR faradaic yields for H 2 O production in MeCN and H 2 O by 1 and 2 Figure S15. Comparison of the average ORR faradaic yields for H 2 O production listed Table S1 by 1 ( ) and 2 ( ) in MeCN. S23

24 Figure S16. Comparison of the average ORR faradaic yields for H 2 O production listed Table S1 by 1 ( ) and 2 ( ) in H 2 O. The two lines are issued from a linear fit. These two lines are closed showing that the bridge has no effect on the faradaic efficiency. S24