We report the preparation of a dicobalt compound with two singly proton-bridged cobaloxime units linked by a central [BO_4] bridge. Reaction of a doubly proton-bridged cobaloxime complex with trimethyl borate afforded the compound in good yield. Single-crystal X-ray diffraction studies confirmed the bridging nature of the [BO_4] moiety. Using electrochemical methods, the dicobalt complex was found to be an electrocatalyst for proton reduction in acetonitrile solution. Notably, the overpotential for proton reduction (954 mV) was found to be higher than in the cases of two analogous single-site cobalt glyoximes under virtually identical conditions

Blakemore, James D.

Brunschwig, Bruce S.

Gray, Harry B.

Henling, Lawrence M.

Laga, Stephanie M.

Caltech Authors - Main

Supporting Information for: 
Catalysis of Proton Reduction by a  
[BO4]-bridged Dicobalt Glyoxime 
Stephanie M. Laga,a,b James D. Blakemore,a,b,c Lawrence M. Henling,a,b  
Bruce S. Brunschwig,b,* and Harry B. Graya,b,* 
a Division of Chemistry and Chemical Engineering, California Institute of Technology, 
Pasadena, California 91125, USA 
b Beckman Institute, California Institute of Technology, Pasadena, California 91125, USA 
c Resnick Sustainability Institute, California Institute of Technology, Pasadena, California 91125, 
USA 
* To whom correspondence should be addressed.  Email: bsb@caltech.edu (BSB); 
hbgray@caltech.edu (HBG). 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
	   SI-2 
Experimental Details 
All solvents were of commercial grade and dried over activated alumina using a Grubbs-type 
solvent purification system prior to use. All chemicals were from major commercial suppliers 
and used as received without further purification. 1H and 13C NMR spectra were collected on 
300, 400, or 500 MHz Varian spectrometers and referenced to the residual protio-solvent signal1 
in the case of 1H and 13C or the deuterium lock signal in the case of 11B. Chemical shifts (δ) are 
reported in units of ppm and coupling constants (J) are reported in Hz. Mass spectra were 
obtained with a PE SCIEX API 365 triple quadrupole spectrometer.  
Protonated dimethylformamide ([DMFH]+OTf–) was synthesized according to the method of 
Favier and Duñach. 2  CoIII(dmgH)2(MeCN)Cl was synthesized according to the published 
method.3  CoIII(dmgBF2)2(MeCN)Cl was synthesized according to the published method.4 
Caution! Trimethyl borate is moisture sensitive and should be handled under inert atmosphere. 
 
[BO4]-bridged dicobalt glyoxime (3) 
A three-neck 500-mL round-bottom flask containing 1 (1.0 g, 3.08 mmol) was fitted with a 
condenser, evacuated, and refilled with N2(g). MeCN (250 mL) was cannula transferred into the 
flask, and trimethyl borate (1.72 mL, 15.4 mmol) was then syringed into the solution. The 
mixture immediately changed from green to brown. Tetraethylammonium chloride (255 mg, 1.54 
mmol) was then added to the solution. After refluxing for 3 h, the solution was cooled to room 
temperature and reduced in volume to ~50 mL. The addition of diethyl ether resulted in 
precipitation of a silvery, gray solid that was isolated by filtration. Upon drying, the solid turns 
brown. The brown solid was washed with dicholoromethane, yielding a darker brown solid that 
can be recrystallized with MeCN/diethyl ether. Yield: 937 mg, 1.1 mmol, 70%. 1H NMR (400 
MHz, MeCN-d3) δ 18.39 (s, 2H), 3.17 (q, J = 7.3 Hz, 8H), 2.56 (s, 12H), 2.47 (s, 12H), 1.22 (t, J 
= 7.2 Hz, 12H). 13C{1H} NMR (101 MHz, MeCN-d3) δ 160.7, 150.1, 52.1, 13.7, 12.3, 6.7. 11B 
NMR (128 MHz, MeCN-d3) δ 7.41. ESI-MS (negative) m/z: found 727.1 m/z (3 – NEt4), 693.1 
(3 – NEt4 – Cl– – H+), 657.0 (3 – NEt4 – 2Cl– – 2H+), 621.0 (3 – NEt4 – 3Cl– – 3H+). 
Regarding the synthesis of dimer 3, we note that higher oligomers were not observed or 
isolated under the conditions used here despite the presence of excess trimethyl borate. We 
propose that the dimeric structure is relatively stable, considering the high yield obtained. 
Oligomeric complexes would presumably have higher negative charges (–1 per monomer) 
perhaps minimizing their formation by reduced stability.  
 
X-ray crystallography 
Single-crystal diffraction data were collected with a Bruker KAPPA APEXII X-ray 
diffractometer. CCDC entry 1013942 contains the supplementary crystallographic data for 
compound 3. These data can be obtained free of charge via 
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data 
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: 
deposit@ccdc.cam.ac.uk 
 
 
	   SI-3 
Electrochemistry 
Electrochemistry was performed in a nitrogen-filled glove box with 0.10 M tetra(n-
butylammonium) hexafluorophosphate (n-Bu4NPF6, Fluka electrochemical grade) in acetonitrile 
as the supporting electrolyte. Measurements were made with a Gamry Reference 600 
potentiostat/galvanostat using a standard three-electrode configuration.  
The working electrode was the basal plane of highly oriented pyrolytic graphite (HOPG) 
(GraphiteStore.com, Buffalo Grove, Ill.; surface area: 0.09 cm2), the counter electrode was a 
platinum wire, and a silver wire immersed in electrolyte served as a pseudo-reference electrode. 
The reference was separated from the working solution by a Vycor frit (Bioanalytical Systems, 
Inc.)   
Ferrocene (Strem Chemicals, 99%) was added to the electrolyte solution at the conclusion of 
each experiment (~1 mM); the midpoint potential of the ferrocenium/ferrocene couple (denoted 
as Fc+/0) served as an external standard for comparison of the recorded potentials.  
Concentrations of analyte for cyclic voltammetry were typically 0.5 mM.   
Controlled-potential electrolysis coupled to product detection was carried out with a standard 
three-electrode configuration in a custom two-compartment air-tight cell.  The working electrode 
consisted of a ca. 10 cm2 block of HOPG.  The reference and counter electrodes were as 
described above.  The volume of solution held by the cell in total was 70 mL, with ca. 100 mL of 
total headspace volume.   
For the bulk generation of hydrogen by electrolysis at –1.5 V, the concentration of dimer 3 was 
0.5 mM and the concentration of buffered acid (1:1 [DMFH]+:DMF) was 5 mM. Consistent with 
cyclic voltammetry as well as literature results, 5  some background current for hydrogen 
production was observed. However, this current was far below that measured for catalysis with 3. 
Homogeneity of catalysis with 3 was studied by checking the electrode surface for formation 
of Co particles following catalysis, as has been observed with some other glyoxime-type 
complexes under organic6 and aqueous7 conditions. Specifically, an electrode used for extensive 
catalysis with 3 was compared to a fresh, clean electrode by voltammetry in the presence of acid. 
We found the response of a clean carbon electrode in the presence of 10 mM 1:1 [DMFH]+:DMF 
to be virtually identical to a carbon electrode previously used for extensive catalysis experiments. 
Additionally, cyclic voltammograms of solutions containing 3 and excess acid were virtually 
identical before and after extensive electrolysis. 
 
Gas chromatography 
 
Gas analysis for the controlled-potential electrolyses was performed with an Agilent 7890A gas 
chromatograph with separate columns for analysis of hydrogen gas and nitrogen, oxygen, carbon 
dioxide, carbon monoxide, hydrogen disulfide, methane, ethane, ethylene, and ethyne. The 
instrument was calibrated with a standard gas mixture to obtain quantitative data. 
The air-tight electrochemical cell was prepared in a nitrogen-filled glovebox. A background 
experiment before electrolysis confirmed only nitrogen and minimal oxygen in the glovebox 
atmosphere. Following 1.25 hr of electrolysis, headspace gas was analyzed for dihydrogen 
content. No other reduction products were detected. 
 
 
	   SI-4 
 
Figure S1.  1H NMR spectrum of 3 in CD3CN. 
 
 
	  
Figure S2.  13C{1H} NMR spectrum of 3 in CD3CN. 
 
 
Figure S3.  11B NMR spectrum of 3 in CD3CN. 
	   SI-5 
 
Figure S4.  Mass spectrum of 3 in the negative-ion mode. 
 
 
Figure S5.  UV-visible absorption spectra of the BO4--bridged dimeric cobaloxime at 2.23 mM 
(left) and 0.0742 mM (right).   
 
 
0	  
100	  
200	  
300	  
400	  
500	  
300	   400	   500	   600	   700	   800	   900	  
Ex
tin
ct
io
n 
C
oe
ffi
ci
en
t, 
ε 
(M
-1
 c
m
-1
)  
Wavelength (nm) 
Dimer	  2.23	  mM	  
0	  
1000	  
2000	  
3000	  
4000	  
5000	  
300	   350	   400	   450	   500	  
Wavelength (nm) 
Dimer	  0.0742	  mM	  
	   SI-6 
 
Figure S6.  Full cyclic voltammogram of 3. An irreversible oxidation was observed at 
approximately –0.15 V vs Fc+/0 and is assigned to oxidation of Co(III) to Co(II). Near –1 V, 
reduction of Co(III) to Co(II) was observed. At more negative potentials, two closely spaced 
couples were observed near –1.2 and –1.3 V vs Fc+/0 and these are assigned to single-electron 
CoII/I  couples (see main text).  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
	   SI-7 
 
Figure S7. To better establish the number of electrons participating in the oxidation and 
reduction of 3, differential pulse voltammetry was carried out using the same electrode 
configuration as above (Figure S6). Two closely spaced, but resolved, signals were observed on 
an oxidative sweep for a sample of 5 mM of the dimer in a NBu4PF6 acetonitrile solution, 
implying that two unique processes were occurring. A reductive sweep on a 10 mM solution of 
ferrocenium produced a clean single peak with an area roughly equal to that of the two peak 
areas for 3. The 1:1 comparison implies that indeed there is an overall two-electron process 
occurring as the two metal centers in the complex are oxidized from CoI to CoII. 
 
 
 
 
 
 
 
 
 
 
 
	   SI-8 
 
Figure S8. Cyclic voltammetry of 1 with 5 mM 1:1 mixture of [DMFH]+ and DMF (black line) 
and reversible response in the absence of acid (gray line) 
 
 
Figure S9. Cyclic voltammetry of 2 with 5 mM 1:1 mixture of [DMFH]+ and DMF (black line) 
and reversible response in the absence of acid (gray line). 
 
 
 
	   SI-9 
 
Figure S10. Comparison of the cyclic voltammetric response of 3 in the presence of 5 mM 1:1 
[DMFH]+:DMF (black line), and the response of the bare electrode only with 5 mM acid (gray 
line). 
 
Figure S11. Comparison of steady-state currents flowing during electrolysis at -1.5 V. Response 
in the presence of dimer 3 and 5 mM 1:1 [DMFH]+:DMF (black line) and in the presence of only 
5 mM 1:1 [DMFH]+:DMF (gray line). 
 
	   SI-10 
 
Figure S12. A test for formation of Co particles on the electrode surface was carried out by 
comparison of cyclic voltammetry with acid before and after running catalytic experiments with 
dimer 3. Specifically, the response of a clean carbon electrode in the presence of 10 mM 1:1 
[DMFH]+:DMF (gray line) was virtually identical to a carbon electrode previously used for 
extensive catalysis experiments with 3 (black line).  
 
Figure S13. A test of the stability of 3 under electrocatalytic conditions was carried out by 
collecting a cyclic voltammogram of 3 (~0.5 mM) in the presence of excess 1:1 [DMFH]+:DMF 
(~10-3 M) before (gray line) and after (black line) electrolysis of the solution at –1.5 V for 1 h. 
Only minor differences were apparent, suggesting that 3 is stable under electrocatalytic 
conditions. 
 
	   SI-11 
X-ray Structural Data 
Table S1. Crystal data and structure refinement for the cobalt dimer 3. 
  
  
Chemical formula C26 H49 B Cl4 Co2 N10 O8 
Formula weight 900.22 
Crystallization solvent Acetonitrile, diethyl ether 
Crystal habit Blade 
Crystal size 0.59 x 0.27 x 0.07 mm3 
Crystal color Brown 
  
Data collection  
Type of diffractometer Bruker KAPPA APEXII 
Wavelength 0.71073 
Data collection temperature 100(2) K 
  
Crystal system Triclinic 
Space group P -1 
Cell dimensions  
 a = 12.5189(7) Å 
b = 12.7064(7) Å 
c = 13.0978(8) Å 
α = 109.121(2) ⁰ 
β =  102.053(2)	  ⁰ 
 γ  = 90.025(2) ⁰ 
Cell volume 1919.77(19) 
Z 2 
Density (calculated) 1.557 Mg/m3 
F(000) 932 
  
q range for data collection 1.668 to 43.747 ⁰ 
Index ranges -24≤h≤24   -24≤k≤24    -25≤l≤25 
Data collection scan type f  and ω scans 
Reflections collected  288113 
Absorption coefficient 1.201 
Max. and min. transmission 0.921 and 0.538 
  
Refinement  
Structure solution program XT-2014/1 (Sheldrick, 2012) 
Structure refinement program SHELXL-2014/2 (Sheldrick, 2014) 
Refinement method Full matrix least-squares on F2 
Data/restraints/parameters 29359 / 0 / 662 
Goodness of fit 1.284 
Final R indices[I > 2\s(I), 29359 
reflections] 
R1 = 0.0317, wR2 = 0.0716 
R indices (all data) R1 = 0.0533, wR2 = 0.0771 
Weighting scheme Calc 
 w=1/[σ2Fo2+(0.0300P)2] 
P=(Fo2+2Fc2)/3 
Max shift/error 0.004 
Average shift/error 0.000 
Largest diff. peak and hole 0.877 and -0.662 Å3 
  
 
	   SI-12 
Table S2.  Selected lengths (Å) and angles (⁰) for the cobalt dimer 3. 
    
    
Co(1)-Co(2) 6.316(2) Co(2)-N(8)-C(14) 116.50(5) 
Co(1)-Cl(1) 2.2314(4) Cl(1)-Co(1)-Cl(2) 176.42(2) 
Co(1)-Cl(2) 2.2343(4) Cl(1)-Co(1)-N(1) 89.44(2) 
Co(1)-N(1) 1.8867(7) Cl(1)-Co(1)-N(2) 89.42(2) 
Co(1)-N(2) 1.8856(6) Cl(1)-Co(1)-N(3) 88.36(2) 
Co(1)-N(3) 1.8903(6) Cl(1)-Co(1)-N(4) 90.67(2) 
Co(1)-N(4) 1.8831(7) Cl(2)-Co(1)-N(1) 88.24(2) 
Co(2)-Cl(1) 2.2258(3) Cl(2)-Co(1)-N(2) 92.93(2) 
Co(2)-Cl(2) 2.2352(3) Cl(2)-Co(1)-N(3) 89.32(2) 
Co(2)-N(1) 1.8770(7) Cl(2)-Co(1)-N(4) 91.70(2) 
Co(2)-N(2) 1.8878(7) N(1)-Co(1)-N(2) 81.62(3) 
Co(2)-N(3) 1.8832(7) N(1)-Co(1)-N(3) 98.89(3) 
Co(2)-N(4) 1.8829(8) N(1)-Co(1)-N(4) 178.94(3) 
O(1)-N(1) 1.3343(8) N(2)-Co(1)-N(3) 177.72(3) 
O(2)-N(2) 1.3673(7) N(2)-Co(1)-N(4) 97.32(3) 
O(3)-N(3) 1.3264(8) N(3)-Co(1)-N(4) 82.17(3) 
O(4)-N(4) 1.3662(7) Cl(3)-Co(2)-Cl(4) 178.15(2) 
O(5)-N(4) 1.3648(8) Cl(3)-Co(2)-N(5) 92.64(2) 
O(6)-N(6) 1.327(1) Cl(3)-Co(2)-N(6) 88.53(2) 
O(7)-N(7) 1.3629(9) Cl(3)-Co(2)-N(7) 93.27(2) 
O(8)-N(8) 1.3384(9) Cl(3)-Co(2)-N(8) 88.92(2) 
B(1)-O(2) 1.470(1) Cl(4)-Co(2)-N(5) 88.32(2) 
B(1)-O(4) 1.4870(8) Cl(4)-Co(2)-N(6) 90.04(2) 
B(1)-O(5) 1.4747(8) Cl(4)-Co(2)-N(7) 88.16(2) 
B(1)-O(7) 1.477(1) Cl(4)-Co(2)-N(8) 90.14(2) 
  N(5)-Co(2)-N(6) 82.15(3) 
Co(1)-B(1)-Co(2) 145.21(1) N(5)-Co(2)-N(7) 97.48(3) 
Co(1)-N(1)-O(1) 122.00(5) N(5)-Co(2)-N(8) 178.31(3) 
Co(1)-N(2)-O(2) 124.51(5) N(6)-Co(2)-N(7) 178.17(3) 
Co(1)-N(3)-O(3) 122.06(5) N(6)-Co(2)-N(8) 98.55(3) 
Co(1)-N(4)-O(4) 125.11(5) N(7)-Co(2)-N(8) 81.78(3) 
Co(2)-N(5)-O(5) 125.04(4) N(2)-O(2)-B(1) 117.22(5) 
Co(2)-N(6)-O(6) 122.34(5) N(4)-O(4)-B(1) 118.64(5) 
Co(2)-N(7)-O(7) 124.43(5) N(5)-O(5)-B(1) 117.89(5) 
Co(2)-N(8)-O(8) 122.22(5) N(7)-O(7)-B(1) 118.08(5) 
Co(1)-N(1)-C(1) 116.46(5) O(2)-B(1)-O(4) 117.76(6) 
Co(1)-N(2)-C(2) 116.83(5) O(2)-B(1)-O(5) 100.01(5) 
Co(1)-N(3)-C(5) 115.80(5) O(2)-B(1)-O(7) 104.22(5) 
Co(1)-N(4)-C(6) 116.15(5) O(4)-B(1)-O(5) 104.07(5) 
Co(2)-N(5)-C(9) 116.39(5) O(4)-B(1)-O(7) 113.11(6) 
Co(2)-N(6)-C(10) 115.86(5) O(5)-B(1)-O(7) 117.54(6) 
Co(2)-N(7)-C(13) 116.34(5) O(1)-N(1)-C(1) 121.53(7) 
    
 
	   SI-13 
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Catalysis of Proton Reduction by a [BO_4]-Bridged Dicobalt Glyoxime

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Catalysis of Proton Reduction by a [BO_4]-Bridged Dicobalt Glyoxime

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