b Depertment of Chemistry, Texas A & M University, College Station, TX-77843, USA.

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Electrocatalytic O2 Reduction by [Fe-Fe]-Hydrogenase Active Site Models

Subal Dey,a_{Atanu Rana,} a_{Danielle Crouthers,}b_{Biswajit Mondal,}a_{Pradip Kumar Das,} a

Marcetta Y. Darensbourg b_{and Abhishek Dey}* a a

Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata, India – 700032. E-mail- icad@iacs.res.in

b_{Depertment of Chemistry, Texas A & M University, College Station, TX-77843, USA.}

E-mail- marcetta@chem.tamu. edu

General Procedures.

Complex 1 and 3 was synthesized using the standard Schlenk technique as reported

previously (ACS Catalysis 2013, 3, 429-436 and Angew Chem Int. Ed. 2001 40 1768,

Angew Chem Int. Ed. 1999, 38, 3178-3180). Anaerobic experiments were performed

either within MBRAUN glovebox or double jacket electrochemical cell from PINE instruments. Edge Plane Graphite (EPG) discs were purchased from Pine Instruments. All the electrochemical data were collected in normal phosphate buffer with defined pHs, adjusted by adding phosphoric acid in 0.1 N Na2HPO4 with KPF6 supporting electrolyte,

except 1 N H2SO4 solution.

Electrochemical Measurements.

The CV are recorded on a CH Instrument bipotentiostat model 720D. The aqueous aqueous cyclic voltamogram was recorded with the same instrument and same software in anaerobic pH 7 buffer with the complex A adsorbed EPG discs (5 mm outer diameter) held within a shaft (Pine Instruments, AFE6MB). An anaerobic environment was

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surface is washed with CHCl3 thoroughly and sonicated in ethanol to remove any loosely

bound catalyst on the EPG surface and washed with triple distilled water.

Rotating Disc Electrochemistry.

The RDE/RRDE measurements are performed on a CHI 700D bipotentiostat. An EPG disc is used as the working electrode which is mounted inside of a Pt ring assembly (Pine Instrument, AFE6RIP) which is mounted at the tip of a shaft (Pine Instruments, AFE6MB) which in turn is fitted to a MRS rotator. A water jacketed electrochemical cell (Pine Instrument, RRPG138) is obtained where the rotor was inserted through a taper plug assembly (AC01TPA6M) which allows free rotation of the vertical rotor while maintaining an airtight seal. An aqueous Ag/AgCl reference (Pine instruments, RREF0021) and Pt counter (AFCTR5) electrodes are attached to the cell through airtight joints. The graphite surface is cleaned by polishing it uniformly on a Silicon carbide grinding paper followed by sonication in triple distilled water. The complex is then physiabsorbed on the disc as described above.

Measurement of PROS

The PROS represent the ratio of the catalytic O2 reduction current and the H2O2 oxidation

current observed in the Pt ring. The H2O2 produced due to incomplete O2 reduction is

radially defused out to the Pt ring where it is oxidized back to O2 producing a oxidation

current (i.e. current having an opposite sign that the O2 reduction current). The collection

efficiency (~20-22%) is estimated before every new set of experiments using the ferrocyanide/ferricyanide redox system.

Controlled Potential Coulometry and Faradaic Yield.

The controlled potential bulk coulometry experiments are performed in the same water jacket electrochemical cell that is used for the LSV. These experiments are performed in degassed 1 N H2SO4 solutions at −0.5 V vs NHE for 8 h. The shaft bearing the working

electrode is rotated at 900−1100 rpm to disperse H2 bubbles formed into the bulk

solution. During electrolysis the H2 generated was collected through an outlet ofthe cell

and collected using an inverted buret setup. The ratio of the charge dispersed and the moles of H+ reduced in the process (estimated from the volume of H2 collected) is

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Figure S1: Bulk electrolysis with 1 on EPG in a 0.5M H2SO4 solution at -0.6 V vs NHE

using an EPG electrode bearing 1 under anoxic (blue) and oxic (green) conditions.

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Figure S3: A) FTIR data before (blue) and after (red) between 1700-3300 cm-1, B) FTIR

data before (blue) and after (red) between 700-1500 cm-1 and C) XPS data in the S2p

ionization region A 1750 2250 2750 3250 wavenumbers Before ORR After ORR B 700 900 1100 1300 1500 wavenumbers After ORR Before ORR C 157 162 167 172 177

Binding energy (eV)

C o unt s RS- RSO2 -RSO3

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Figure S5: Rotating ring disk electrochemistry (RRDE) data for complex 1 at 50 mvs-1

scan rate at various pHs. (A) Working electrode current at EPG electrode. (B) Corresponding platinum current.

-4 -3 -2 -1 0 -0.1 0.1 0.3 I (μ A) -0.5 -0.3 V vs NHE 1 N H2SO4 pH 1.19 pH 2.04 pH 3.04 pH 4.19 pH 4.72 pH 5.22 pH 6.22 pH 6.72 pH 7.92 0 4 8 12 16 20 24 28 -0.5 -0.3 -0.1 0.1 0.3 0.5 V vs NHE I (μ A) 1 N H2SO4 pH 1.19 pH 2.04 pH 3.04 pH 4.19 pH 4.72 pH 5.22 pH 6.22 pH 6.72 pH 7.92 A B -3 -2 -1 0 -0.5 -0.3 -0.1 0.1 0.3 0.5 V vs NHE I ( μ A) 1 N H2SO4 pH 1.19 pH 2.04 pH 3.04 pH 4.19 pH 4.72 pH 5.22 pH 6.22 pH 6.72 pH 7.92 0 -0.5 -0.3 -0.1 0.1 0.3 0.5 5 10 15 20 25 30 35 V vs NHE I ( μ A) 1 N H2SO4 pH 1.19 pH 2.04 pH 3.07 pH 4.19 pH 4.72 pH 5.22 pH 6.22 pH 6.72 pH 7.92 B A -4 -3 -2 -1 0 -0.5 -0.3 -0.1 0.1 0.3 0.5 V vs NHE I (μ A) 0 5 10 15 20 25 30 -0.5 -0.3 -0.1 0.1 0.3 0.5 V vs NHE I (μ A) 1N H2SO4 pH 1.19 1 N H2SO4 pH 1.19 pH 2.04 pH 3.04 pH 4.19 pH 4.72 pH 5.22 pH 6.22 pH 6.72 pH 7.92 pH 2.04 pH 3.04 pH 4.19 pH 4.72 pH 5.22 pH 6.22 pH 6.72 pH 7.92 A _B

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Figure S8: H/D isotope effect on PROS production by complex 1 at pH 7. The pH of the

D2O buffer is corrected appropriately.

Figure S9: The geometry of the complex resulting from O-O cleavage of the protonated

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0.2

0.5

0.8

1.1 1700

1800

1900

2000

2100

Wavenumber (cm-1) % Tr a ns m it ta nc e Sample Sample+H2O2

Figure S10: FTIR data of a CH3CN solution of complex 1 before (green) and after 1

hour (red) of incubation with equimolar H2O2.

Full Reference 21:

Gaussian 03, Revision C.02, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc., Wallingford CT, 2004.

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Optimized coordinates for the complex 2. Fe -1.06843100 4.67530900 6.60081700 S -0.43604900 3.67978000 8.60944200 S 0.78561600 6.04328300 6.94111900 N 0.71259900 5.97589600 9.74674300 C -1.95694300 3.22314200 6.22204500 O 1.51821000 0.92957000 7.08566000 O -0.99388000 5.37664800 3.76540500 C -0.99190700 5.07876600 4.90589500 O 3.73243200 4.03417900 9.17791400 C -0.37371100 5.01999300 9.89296200 H -1.37775400 5.50989500 9.88246800 H -0.24164900 4.50018400 10.85872300 C 0.95569900 6.72307500 10.97971200 C 0.56410900 6.85400700 8.59702100 H 1.35004100 7.62793100 8.65633300 H -0.43388200 7.35485700 8.56752900 C 1.47014100 2.10547100 7.15050700 Fe 1.48193900 3.84281900 7.29178200 C 2.76128900 4.05283300 8.50921900 O -2.60722400 2.27375300 5.96673700

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Optimized coordinates for the complex 3. Fe -0.10948700 -0.12532000 0.21234300 S 2.21637100 -0.15248200 0.00791000 S 0.07970800 2.18991200 -0.03468000 C 2.64314300 2.40719100 1.26982600 C -0.11523700 -1.80832900 -0.24745000 O 2.12506900 -0.93502700 -3.58781300 C 0.15810700 1.74106600 -2.85618000 O 3.46451300 3.03890700 -2.55869400 C 2.90583300 0.90251700 1.36954700 H 2.48913400 0.49762700 2.31157200 H 3.99496300 0.71207400 1.36491000 H 3.08935300 2.79802600 0.33730400 C 1.16699000 2.80476600 1.33641800 H 1.07578600 3.90641800 1.31117800 H 0.71061600 2.44313600 2.27774200 C 1.79341900 -0.10600300 -2.81924300 Fe 1.35384000 1.17500400 -1.72058300 C 2.65404600 2.33117400 -2.06876200 C -0.31658600 -0.28021400 1.96589200 O -0.64425200 -0.55594000 3.06820900 O -0.61016700 2.15389800 -3.64816400 O -0.14619000 -2.95866800 -0.50183700 C -1.77605300 0.02063600 -0.28243000 O -2.91876900 0.09768800 -0.56024500 H 3.17405100 2.90664600 2.10942900

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Optimized coordinates for the complex 3-OOH. Fe 0.48019400 1.39001600 4.13557400 Fe 2.73942200 0.04365900 3.34331500 S 0.81494500 0.41196000 2.00127100 S 2.61870500 2.36369200 3.83420100 C 1.37981300 -0.08875500 4.88365000 O 1.31499700 -0.84633000 5.80543500 C -1.04285100 0.51589500 4.28492400 O -2.05160400 -0.05331300 4.40578300 C 0.38149600 2.13945200 5.72762900 O 0.28993200 2.65887600 6.76632000 C 4.02291600 -0.10664600 4.54667900 O 4.88986300 -0.20948600 5.31809400 C 2.54334600 -1.69211700 3.08410100 O 2.43745700 -2.83683300 2.89487800 C 3.90602300 0.22971100 1.97357200 O 4.72681700 0.23722100 1.14752200 C 1.56560100 3.10510500 1.24077300 C 1.28522700 1.68824600 0.73639400 H 2.14711100 1.27271300 0.18154000

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Optimized coordinates for the complex 2-OOH. Fe 0.48019400 1.39001600 4.13557400 Fe 2.73942200 0.04365900 3.34331500 S 0.81494500 0.41196000 2.00127100 S 2.61870500 2.36369200 3.83420100 C 1.37981300 -0.08875500 4.88365000 O 1.31499700 -0.84633000 5.80543500 C -1.04285100 0.51589500 4.28492400 O -2.05160400 -0.05331300 4.40578300 C 0.38149600 2.13945200 5.72762900 O 0.28993200 2.65887600 6.76632000 C 4.02291600 -0.10664600 4.54667900 O 4.88986300 -0.20948600 5.31809400 C 2.54334600 -1.69211700 3.08410100 O 2.43745700 -2.83683300 2.89487800 C 3.90602300 0.22971100 1.97357200 O 4.72681700 0.23722100 1.14752200 C 1.56560100 3.10510500 1.24077300 C 1.28522700 1.68824600 0.73639400 H 2.14711100 1.27271300 0.18154000 H 0.41833300 1.70358100 0.05225900 C 2.73962600 3.25810500 2.20996500 H 2.84604500 4.31795000 2.50222600 H 3.69410000 2.93647900 1.75310700 O -0.45064600 3.01220300 3.53040500 O -1.61533800 2.73137100 2.72613500 H -2.27289300 3.38868200 3.11034300 H 0.65470800 3.51993200 1.69859200 H 1.80375700 3.72083400 0.34901300

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Optimized coordinates for the complex 2H-OOH. Fe 0.54121300 1.38113200 4.10916700 Fe 2.76177600 0.04819500 3.34134800 S 0.87501000 0.36622600 1.99464400 S 2.66697500 2.35916400 3.78367600 C 1.33931400 -0.11884300 4.88875400 O 1.25496200 -0.91032100 5.76731500 C -1.05331700 0.61593000 4.25858100 O -2.10304100 0.13865900 4.37995000 C 0.42687300 2.17969700 5.68167200 O 0.32481300 2.72628900 6.70072300 C 4.02879100 -0.11718100 4.58166200 O 4.87779400 -0.23849800 5.36304400 C 2.59648000 -1.70618900 3.10319200 O 2.52430100 -2.85067800 2.92657000 C 3.94371600 0.21442900 1.99091300 O 4.71330000 0.27099300 1.12394700 N 1.40082200 3.07547400 1.38076400 C 1.23028400 1.70605300 0.78326000 H 2.12471100 1.45770600 0.18889800 H 0.34892700 1.74446600 0.12664300 C 2.67103700 3.23461700 2.16338700

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Table S1: Optimized geometries of the hydroperoxide species.

Distance (Å) Charge (q) Spin (s)

Fe-Fe Fe-O O-O C-S qFe qO sFe sO

xNR 2.72 1.96 1.44 1.94 0.42, 0.21 -0.34, -0.39 0.32, 0.28 0.22, 0.06 xNHR 2.7 2.01 1.47 1.84 0.39, 0.22 -0.40, -0.39 0.34, 0.33 0.1, 0.03 xCH2 2.75 1.96 1.44 1.86 0.41, 0.22 -0.34, -0.39 0.32, 0.28 0.23, 0.07