A cobalt bis(iminopyridine) complex is a highly active electrocatalyst for water reduction, with an estimated apparent second order rate constant k_(app) ≤ 10^7 M^(–1)s^(–1) over a range of buffer/salt concentrations. Scan rate dependence data are consistent with freely diffusing electroactive species over pH 4–9 at room temperature for each of two catalytic reduction events, one of which is believed to be ligand based. Faradaic H_2 yields up to 87 ± 10% measured in constant potential electrolyses (−1.4 V vs SCE) confirm high reactivity and high fidelity in a catalyst supported by the noninnocent bis(iminopyridine) ligand. A mechanism involving initial reduction of Co^(2+) and subsequent protonation is proposed

Gray, Harry B.

Peters, Jonas C.

Stubbert, Bryand D.

English

Caltech Authors - Main

S-1 
 
Rapid Water Reduction to H2 Catalyzed by a Cobalt Bis(iminopyridine) Complex 
 
Bryan D. Stubbert, Jonas C. Peters, and Harry B. Gray 
 
Supporting Information 
General Considerations …………………………………………………………………………………………………..  S-2 
Electrochemistry …………………………………………………………………………………………………..  S-2 
Synthesis of 1(PF6)2 …………………………………………………………………………………………………..  S-3 
Figure S1  …………………………………………………………………………………………………..  S-4 
Figure S2  …………………………………………………………………………………………………..  S-9 
Figure S3  …………………………………………………………………………………………………..  S-10 
Figure S4  …………………………………………………………………………………………………..  S-10 
Figure S5  …………………………………………………………………………………………………..  S-11 
Table S1  …………………………………………………………………………………………………..  S-12 
Complete Ref. 2 …………………………………………………………………………………………………..  S-12 
References  …………………………………………………………………………………………………..  S-12  
S-2 
 
General considerations. All manipulations were carried out in an MBraun glovebox or using 
standard Schlenk techniques under an atmosphere of N2 or Ar. Solvents were degassed and dried by 
thoroughly sparging with N2 gas followed by passage through an activated alumina column.  Unless 
otherwise noted, all reagents were purchased from commercial vendors and used without further 
purification. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc., degassed, 
and stored over 3A molecular sieves prior to use. Milli-Q water was used in the preparation of all 
aqueous solutions, which were rigorously degassed (3 × freeze-pump-thaw cycles) or extensively 
sparged (≥ 30 min) with Ar prior to contact with catalyst. NMR spectra were recorded on a Varian 
Mercury 300 MHz spectrometer and referenced according to the solvent residual peak.S1 Absorbance 
spectra were recorded on a Varian Cary Bio50 UV-Visible Spectrophotometer using degassed solutions 
in Teflon valve sealed quartz cells. Gas chromatograms for headspace analyses were recorded on an 
Agilent Technologies 7890A GC System, with peak areas calibrated for % H2 by volume. Volumetric 
Faradaic yields were measured by displacement of water, mineral oil, or Hg in a graduated buret 
secured in a horizontal position. High resolution mass spectra (ESI-MS) were obtained using a JEOL JMS-
600H magnetic sector mass spectrometer at the California Institute of Technology Mass Spectral Facility. 
Elemental analyses are reported as the average of duplicate runs carried out by Columbia Analytical 
Services, Tucson, AZ. [Co(CH3CN)6](PF6)2 was prepared in 82% yield according to the literature procedure 
for [Co(CH3CN)6](BF4)2,S2 substituting NO(PF6) for NO(BF4). 2-Acetylpyridine and 1,3-diaminopropane 
were dried by passage through a plug of neutral alumina and degassed (3 × freeze-pump-thaw cycles) 
prior to use and storage in the glovebox. 
Electrochemistry. Solutions for electrochemical analysis were prepared from 1(PF6)2 stored and 
weighed in the glovebox under an N2 atmosphere. A CH Instruments 660D or 600B Electrochemical 
Analyzer was used for aqueous and acetonitrile solutions, respectively. Aqueous measurements were 
obtained using a Bioanalytical Systems Controlled Growth Mercury Drop Electrode (BAS CGME) working 
electrode run in static mode with an average drop surface area of 0.013 ± 0.002 cm2 (calculated by 
determining the average weight of a single drop collected in each buffer medium)S3 and a commercially 
available saturated calomel reference electrode (SCE). Data is reported as the average of three runs 
obtained at a given set of experimental parameters using a fresh Hg drop for each run. In acetonitrile, a 
glassy carbon disk served as the working electrode (area = 0.07 cm2). A platinum wire was used as the 
auxiliary electrode in cyclic and linear sweep voltammograms, each performed in a single compartment 
cell in the glovebox (N2 atmosphere) or in aqueous solutions on the benchtop (sparged ≥ 30 min with Ar 
prior to and between runs, and blanketed with Ar during runs). The nonaqueous reference electrode 
S-3 
 
was Ag+/Ag, exhibiting a reversible Fc+/Fc couple centered at +0.14 V in CH3CN when added as an 
internal standard. Bulk controlled potential electrolysis experiments were performed in an N2 
atmosphere using a mercury pool working electrode (area = 0.8 or 1.8 cm2), aqueous AgCl/Ag reference 
electrode stored in a 3 M NaCl solution, and either a silver or platinum mesh auxiliary electrode in 
rapidly stirred solutions sealed in gastight dual compartment cells bearing Teflon valves and ground 
glass stopcocks amenable to GC analysis and/or volumetric displacement of headspace gases. 
Concentrations of 12+ in all media ranged from 0.1 to 1 mM, with 0.05 or 0.1 M aqueous buffer or 0.1 M 
tetra(n-butyl)ammonium hexafluorophosphate (TBAH).  Unless otherwise noted, electrodes were 
purchased from CH Instruments. The diffusion coefficient for 12+ was estimated to be 4 × 10-5 cm2s-1, 
determined from the slope of the jmax vs ν1/2 plot measured for a CH3CN solution containing 0.51 mM 12+ 
and 0.1 M TBAH (Figure S4) according to the Randles-Sevcik equation (eq S1).S3,S4 
𝑗 = 𝑛3/2𝐹𝐶0∗𝐷0𝑣1/2       (S1) 
Background voltammograms obtained prior to dissolution of 12+ were subtracted from all data with little 
effect (≤ 5% change) on data collected in CH3CN or from aqueous solutions of pH ≥ 4. A significant 
overcorrection results at -1.3 V vs SCE for solutions with pH ≤ 3, though this data is omitted from CV 
analyses given the apparent complex decomposition under these conditions. 
Synthesis of [Co(MePyN)2pn(H2O)2](PF6)2  (1(PF6)2). A 100 mL sidearm flask in the glovebox was 
charged with a stir bar and a solution of 2-acetylpyridine (0.245 g, 2.02 mmol) in acetonitrile (7 mL). To 
this colorless solution was added an acetonitrile solution (7 mL) of 1,3-diaminopropane (74.7 mg, 1.01 
mmol). After stirring for 15 minutes at room temperature, a pink solution of [Co(CH3CN)6](PF6)2 (0.600 g, 
1.01 mmol) in acetonitrile (10 mL) was added dropwise over 5 minutes while stirring rapidly. The red-
orange solution was allowed to stir overnight at ambient temperature before evacuating to dryness. The 
red-orange residue that remained was covered with Et2O (3 × 10 mL) and ground into a powder with a 
spatula before removing volatiles in vacuo. An orange powder was obtained in 93% yield (0.626 g, 0.94 
mmol). 1H NMR (300 MHz, 20 °C, CD3CN, δ) 90.2 (sharp), 86.5, 55.5, 44.6, 18.6, -8.2 (sharp), -56.1, -83.6 
ppm, all broad singlets except where noted. 19F NMR (282 MHz) -71.76 ppm (d, J = 706.8 Hz). ESI-MS 
(M+·) Calc. for C17H20N4Co: 339.1020. Found: 339.1016. Anal. Calcd. for C19H27N5CoF12O2P2: C, 32.31; H, 
3.85; N, 9.92. Found: C, 32.48; H, 3.47; N, 9.46.  
S-4 
 
Figure S1. Representative cyclic voltammograms of aqueous solutions of 12+ obtained at room 
temperature with a scan rate of 100 mV/s. Concentrations of 12+ and aqueous buffer are indicated for 
each pH. Current densities calculated for an average mercury drop area of 0.013 cm2. 
(a) pH 1.0, 0.34 mM 12+, 0.1 M KCl/HCl 
 
(b) pH 2.0, 0.33 mM 12+, 0.05 M NaPi 
 
 
 
S-5 
 
(c) pH 3.0, 0.32 mM 12+, 0.05 M sodium citrate 
  
(d) pH 4.1, 0.12 mM 12+, 0.05 M sodium citrate 
  
S-6 
 
(e) pH 5.0, 0.35 mM 12+, 0.05 M sodium citrate 
 
 
(f) pH 6.1, 0.32 mM 12+, 0.05 M sodium citrate 
 
 
 
 
 
S-7 
 
(g) pH 7.0, 0.30 mM 12+, 0.05 M NaPi 
 
(h) pH 8.0, 0.33 mM 12+, 0.05 M NaPi 
 
 
 
 
 
S-8 
 
(i) pH 9.0, 0.33 mM 12+, 0.05 M sodium borax 
 
 
  
S-9 
 
Figure S2. Overlayed plots of current density vs (scan rate)1/2 for selected reductions relevant to 
electrocatalysis at pH 4.1 and 7.0. CV data obtained in buffered aqueous media (0.05 M citrate or 
phosphate; Hg drop area = 0.013 cm2). 
(a) pH 4.1, 0.12 mM 12+, 0.05 M sodium citrate 
 
 
(b) pH 7.0, 0.30 mM 12+, 0.05 M sodium phosphate 
 
 
 
S-10 
 
Figure S3. Dependence of current density (mA/cm2) on the concentration of 12+ over the range 0.1 to 1 
mM in aqueous 0.05 M citrate buffer at pH 4.5. Additional data for 5 and 10 mM [Co] are shown but not 
included in the line fit. 
 
 
 
 
Figure S4. Overlayed plots of maximum current density vs (scan rate)1/2 for reductions relevant to 
electrocatalysis. CV data obtained in CH3CN solution with 0.51 mM 12+ and 0.1 M TBAH (Fc+/Fc observed 
at +0.14 V vs Ag+/Ag; GCE area = 0.07 cm2). 
 
  
S-11 
 
Figure S5. Aggregation plots of UV-visible absorbance spectra obtained for 0.1 to 5 mM solutions of 12+ 
in neutral H2O at room temperature in Teflon valve sealed quartz cells with pathlength 0.1 to 7.5 cm.  
(a) 230 nm 
 
(b) 275 nm 
 
 
(c) 310 nm 
 
(d) 450 nm 
 
 
 
  
S-12 
 
Table S1. Summary of potentials corresponding to the observed reduction waves shown in Figures 2 and 4. 
Electrolyte Medium Co3+/2+ Co2+/1+ Catalysis/ligand jmax 
Acetonitrile (vs Ag+/Ag) -0.34 V -0.86 V -1.15 V 
pH 4.1 (vs SCE) -0.67 V Not Observed -1.34 V 
pH 7.0 (vs SCE) -0.90 V -1.05 V -1.34 V 
Experimental conditions: acetonitrile (0.5 mM [Co2+], 0.1 M TBAH, N2 atmosphere), pH 4.1 (0.1 mM [Co
2+], 0.05 M sodium 
citrate, Ar sparge/blanket), pH 7.0 (0.3 mM [Co2+], 0.05 M sodium phosphate, Ar sparge/blanket). Nonaqueous Ag+/Ag 
pseudoreference electrode shows Fc+/Fc E°′ = +0.14 V. Aqueous experiments show pH dependent behavior for the irreversible 
wave assigned as Co3+/2+ (extrapolates to -0.38 V vs SCE at pH 0). The potentials shown here correspond to the local jmax for 
processes scanned cathodically and are not reversible reduction potentials. 
 
 
 
Complete text for reference 2: 
Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; 
Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; 
Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, 
K.; Nicholas, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; 
Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. Rev. 2001, 101, 953. 
 
 
 
 
References 
                                                          
(S1)  Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; 
Goldberg, K. I. Organometallics 2010, 29, 2176. 
(S2)  Heintz, R. A.; Smith, J. A.; Szalay, P. S.; Weisgerber, A.; Dunbar, K. R.; Beck, K.; Coucouvanis, D. Inorg. 
Synth. 2002, 33, 75 
(S3)  Laboratory Techniques in Electroanalytical Chemistry 2nd ed.; Kissinger, P. T.; Heineman, W. R., Ed.; Marcel 
Dekker, Inc: New York, 1996. 
(S4) Bard, A. J.; Faulkner., L. R. Electrochemical Methods: Fundamentals and Applications; 2nd ed.; John Wiley: 
New York, 2000.   


Rapid Water Reduction to H_2 Catalyzed by a Cobalt Bis(iminopyridine) Complex

An operating potential of 1.4 V vs SCE corresponds to overpotentials of 0.69

Current density calculated from data reported in the Supporting Information for reference 10.

Electrochemical Methods: Fundamentals and Applications, 2nd ed.;

H.Chemical Kinetics and Reaction Mechanisms, 2nd ed.;

Nature 2010, 464, 1329 and references therein.

Portions of this work were previously reported: (a)

Similar Co(L4) complexes have been reported: (a)

The term “purely aqueous media” refers to systems where water is the lone solvent (∼55 M), such as in these representative studies of cobalt electrocatalysts: (a)

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Rapid Water Reduction to H_2 Catalyzed by a Cobalt Bis(iminopyridine) Complex

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