Cobaloximes as functional models for hydrogenases. 2. Proton electroreduction catalyzed by difluoroborylbis(dimethylglyoximato)cobalt(II) complexes in organic media

Article abstract of DOI:10.1021/ic061625m

Cobaloximes are effective electrocatalysts for hydrogen evolution and thus functional models for hydrogenases. Among them, difluoroboryl-bridged complexes appear both to mediate proton electroreduction with low overpotentials and to be quite stable in acidic conditions. We report here a mechanistic study of [Co(dmgBF₂)₂L] (dmg^2- = dimethylglyoximato dianion; L = CH₃CN or N,N-dimethylformamide) catalyzed proton electroreduction in organic solvents. Depending on the applied potential and the strength of the acid used, three different pathways for hydrogen production were identified and a unified mechanistic scheme involving cobalt(II) or cobalt(III) hydride species is proposed. As far as working potential and turnover frequency are concerned, [Co(dmgBF₂)₂(CH ₃CN)₂], in the presence of p-cyanoanilinium cation in acetonitrile, is one of the best synthetic catalysts of the first-row transition-metal series for hydrogen evolution.

Full text of DOI:10.1021/ic061625m

Inorg. Chem. 2007, 46, 1817−1824
Cobaloximes as Functional Models for Hydrogenases. 2. Proton
Electroreduction Catalyzed by
Difluoroborylbis(dimethylglyoximato)cobalt(II) Complexes in Organic
Media
Carole Baffert, Vincent Artero,* and Marc Fontecave
Laboratoire de Chimie et Biologie des Me´taux, UniVersite´ Joseph Fourier, CNRS, UMR 5249, and
CEA, DSV/iRT SV/LCBM, 38054 Grenoble, France
Received August 29, 2006
Cobaloximes are effective electrocatalysts for hydrogen evolution and thus functional models for hydrogenases.
Among them, difluoroboryl-bridged complexes appear both to mediate proton electroreduction with low overpotentials
-
and to be quite stable in acidic conditions. We report here a mechanistic study of [Co(dmgBF2)2L] (dmg2
)
dimethylglyoximato dianion; L
) CH3CN or N,N-dimethylformamide) catalyzed proton electroreduction in organic
solvents. Depending on the applied potential and the strength of the acid used, three different pathways for hydrogen
production were identified and a unified mechanistic scheme involving cobalt(II) or cobalt(III) hydride species is
proposed. As far as working potential and turnover frequency are concerned, [Co(dmgBF2)2(CH3CN)2], in the presence
of p-cyanoanilinium cation in acetonitrile, is one of the best synthetic catalysts of the first-row transition-metal
series for hydrogen evolution.
Introduction
behavior for H2/H+ interconversion3 and appears thus
competitive as an efficient molecular catalyst with regard to
platinum for the hydrogen evolution reaction (her). Concur-
rently, the development of new homogeneous catalysts for
her based on cheap first-row transition metals has been a
long-time goal for inorganic chemists.4 The final goal lies
in an alternative to the use of platinum as the electrode for
this reaction by modifying a common electrode material with
a molecular complex able to catalyze the reaction, i.e., to
lower the activation potential for her. Significant achieve-
ments have been reported in the past years in the structural
modelization of hydrogenases,5 but the new biomimetic
models show activities still below those obtained with non-
biomimetic synthetic catalysts such as [Ni(PPh2NPh2)2(CH3-
CN)](BF4)2 (PPh2NPh ) 1,3,5,7-tetraphenyl-1,5-diaza-3,7-
diphosphacyclooctane),6 [Cp2Mo2(µ-S)2(η2-µ2-CH2S2)],7 or
[Cp*Rh(bipy)(OH2)]2+.8
In the perspective of a hydrogen economy, one major issue
concerns the availability of economically viable methods for
the production of hydrogen from renewable sources. The
reduction of protons is apparently a very simple reaction but,
unfortunately, except on platinum metal electrodes, suffers
from kinetic limitations. On most electrodes, the rate-
determining step is the heterogeneous electron transfer from
the electrode to a proton located in the solution, resulting in
the formation of a hydride ion stabilized at the surface. The
reaction of this hydride with a proton from the bulk is fast.
Hence, on common and inexpensive solid materials, hydro-
gen evolution is generally not observed at potentials near
equilibrium (-400 mV vs SHE at pH 7 in water) but requires
the application of an overpotential, also called an activation
potential.1 Hydrogenases are unique metalloenzymes that
catalyze the reversible H2/H+ interconversion.2 When ad-
sorbed on a glassy carbon electrode, [NiFe] hydrogenase
from Allochromatium Vinosum displays a Nernstian catalytic
(2) (a) Adams, M. W. W.; Mortenson, L. E.; Chen, J.-S. Biochim. Biophys.
Acta 1981, 594, 105. (b) Vignais, P. M.; Billoud, B.; Meyer. J. FEMS
Microbiol. ReV. 2001, 25, 455.
(3) (a) Jones, A. K.; Sillery, E.; Albracht, S. P. J.; Armstrong, F. A. Chem.
Commun. 2002, 866. (b) Pershad, H. R.; Duff, J. L. C.; Heering, H.
A.; Duin, E. C.; Albracht, S. P. J.; Armstrong, F. A. Biochemistry
1999, 38, 8992. (c) Le´ger, C.; Jones, A. K.; Roseboom, W.; Albracht,
S. P. J.; Armstrong, F. A. Biochemistry 2002, 41, 15736.
(4) (a) Artero, V.; Fontecave, M. Coord. Chem. ReV. 2005, 549, 1518.
(b) Koelle, U. New J. Chem. 1992, 16, 157.
* To whom correspondence should be addressed. E-mail: vartero@cea.fr.
Phone: int+ 4 38 78 91 06. Fax: int+ 4 38 78 91 24.
(1) The same occurs for hydrogen oxidation. This kinetic limitation thus
significantly reduces the energetic yield during a complete formation/
uptake cycle of hydrogen and is therefore economically limitating for
most industrial applications.
10.1021/ic061625m CCC: $37.00
Published on Web 02/02/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 5, 2007 1817

Baffert et al.
Chart 1. [Co(dmgBF2)2L2] (L ) H2O, CH3CN, or DMF)
of acids with different pKa’s and extending the electrochemi-
cal window toward the reduction region. Interestingly, three
different pathways for hydrogen production could be identi-
fied, depending on the strength of the acid used, and we
propose here a unified mechanistic scheme for hydrogen
production involving cobalt(II) or cobalt(III) hydride.
Experimental Section
Materials. Commercial DMF and acetonitrile were degassed by
bubbling nitrogen. The supporting electrolyte (n-Bu4N)BF4 was
prepared from (n-Bu4N)HSO4 and NaBF4 (Aldrich) and dried
overnight at 80 °C under vacuum. [Co(dmgBF2)2(OH2)2],13 [Co-
(dpgBF2)2(OH2)2],14 [CoH(dmgH)2PBu3],15 p-cyanoanilinium tet-
rafluoroborate,6 and triethylammonium tetrafluoroborate16 were
prepared as previously described. Trifluoroacetic acid (Janssen
Chimica), tetrafluoroboric acid diethyl ether complex (Fluka), and
Et3NHCl (Acros) were used as received.
Methods and Instrumentation. Electrochemical measurements
were carried out under nitrogen. A standard three-electrode con-
figuration was used consisting of a glassy carbon (3 mm in
diameter) or platinum (2 mm in diameter) disk as the working
electrode, an auxiliary platinum wire, and an Ag/AgCl/aqueous
AgClsat + KCl (3 mol‚L-1; hereafter named Ag/AgCl) reference
electrode closed by a Vicor frit and directly dipped into the solution.
In order to take the liquid junction potential between the aqueous
and nonaqueous solution into account, this electrode was calibrated
with the internal reference system Fc+/Fc, which was found at 0.46
V vs Ag/AgCl in CH3CN (0.53 V vs in DMF). The Fc+/Fc couple
(E0 ) 0.400 V vs SHE)17 can be used to quote potentials to SHE,
when needed.
Cobaloxime is another class of such catalysts. The first
report concerned the reduction of protons by MCl2 (M ) V,
Cr, Eu) at pH 2 in water catalyzed by [Co(dmgBF2)2(OH2)2]
[dmgBF2- ) (difluoroboryl)dimethylglyoximato anion; Chart
1].9 Lehn et al. also described the photoproduction of
hydrogen at pH 8 in N,N-dimethylformamide (DMF) cata-
lyzed by [Co(dmgH)2(OH2)2] in the presence of a photosen-
sitizer [Ru(bipy)3]2+ and sacrificial triethanolamine as the
electron donor.10 We recently reported on the activity of
various cobaloximes11 as electrocatalysts for the reduction
of a weak acid, Et3NH+, in DMF or 1,2-dichloroethane and
demonstrated that the active species is a cobalt(III) hydride
formed by protonation of the CoI complex. We also
concluded that [Co(dmgBF2)2(OH2)2] is a slow electrocatalyst
under these conditions even at a potential 350 mV more
negative than that required for CoI formation. Soon after,
this complex was shown to behave electrocatalytically at the
CoI redox state in the presence of stronger acids such as HBF4
or CF3COOH in acetonitrile.12 In order to determine whether
this difference in reactivity implies a distinct catalytic
mechanism or simply reflects kinetic limitations, we thus
decided to reinvestigate [Co(dmgBF2)2L]-catalyzed proton
electroreduction in both DMF and CH3CN, using a variety
Cyclic voltammograms were recorded on a EG&G PAR 273A
instrument. Solution concentrations were ca. 1 mM for the
cobaloxime and 0.1 M for the supporting electrolyte (n-Bu4N)BF4.
Electrodes were polished on a MD-Nap polishing pad with a 1 µm
monocrystalline diamond DP suspension.
(5) (a) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem.
Soc. 2001, 123, 9476. (b) Gloaguen, F.; Lawrence, J. D.; Rauchfuss,
T. B.; Be´nard, M.; Rohmer, M.-M. Inorg. Chem. 2002, 41, 6573. (c)
Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710. (d) Zhao,
X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.; Chiang,
C.-Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917. (e)
Georgakaki, I. P.; Miller, M. L.; Darensbourg, M. Y. Inorg. Chem.
2003, 42, 2489. (f) Chong, D.; Georgakaki, I. P.; Mejia-Rodriguez,
R.; Sanabria-Chinchilla, J.; Soriaga, M. P.; Darensbourg, M. Y. Dalton
Trans. 2003, 4158. (g) Meija-Rodriguez, R.; Chong, D.; Reibenspies,
J. H.; Soriaga, M. P.; Darensbourg, M. Y. J. Am. Chem. Soc. 2004,
126, 12004. (h) Borg, S. J.; Behrsing, T.; Best, S. P.; Razavet, M.;
Liu, X.; Pickett, C. J. J. Am. Chem. Soc. 2004, 126, 16988. (i) Tard,
C.; Liu, X.; Ibrahim, S. K.; Bruschi, M.; De Gioia, L.; Davies, S. C.;
Yang, X.; Wang, L.-S.; Sawers, G.; Pickett, C. J. Nature 2005, 433,
610. (j) Ott, S.; Kritikos, M.; Ackermark, B.; Sun, L.; Lomoth, R.
Angew. Chem., Int. Ed. 2004, 43, 1006. (k) Perra, A.; Davies, E. S.;
Hyde, J. R.; Wang, Q.; McMaster, J.; Schro¨der, M. Chem. Commun.
2006, 1103. (l) Oudart, Y.; Artero, V.; Pe´caut, J.; Fontecave, M. Inorg.
Chem. 2006, 45, 4334.
(6) Appel, A.; DuBois, D. L.; Rakowski DuBois, M. J. Am. Chem. Soc.
2005, 127, 12717.
(7) Wilson, A. D.; Newell, R. H.; McNevin, M. J.; Muckerman, J. T.;
Rakowski DuBois, M.; DuBois, D. L. J. Am. Chem. Soc. 2006, 128,
358.
(8) (a) Ko¨lle, U.; Gru¨tzel, M. Angew. Chem., Int. Ed. Engl. 1987, 26,
567. (b) Ko¨lle, U.; Kang, B.-S.; Infelta, P.; Comte, P.; Gra¨tzel, M.
Chem. Ber. 1989, 122, 1869. (c) Cosnier, S.; Deronzier, A.; Vla-
chopoulos, N. J. Chem. Soc., Chem. Commun. 1989, 1259.
(9) Connolly, P.; Espenson, J. H. Inorg. Chem. 1986, 25, 2684.
(10) Hawecker, J.; Lehn, J.-M.; Ziessel, R. NouV. J. Chim. 1983, 7, 271.
(11) Part 1 of the series: Razavet, M.; Artero, V.; Fontecave, M. Inorg.
Chem. 2005, 44, 4786.
Et3NHBF4 and p-cyanoanilinium tetrafluoroborate were added
as solids, trifluoroacetic acid, HBF4·Et2O (0.2 mol‚L-1 solution in
the given solvent), and Et3NHCl (0.05 mol‚L-1 solution in the given
solvent) by syringe. Voltammograms of the supporting electrolyte
in CH3CN and of CH3CN solutions of Et3NHCl, CF3COOH, and
p-cyanoanilinium tetrafluoroborate in the presence of the supporting
electrolyte are given as Supporting Information. Bulk electrolysis
and coulometry were carried out on a EG&G PAR 273A instrument
in acetonitrile, using a mercury pool cathode. The platinum grid +
carbon foam counter electrode was placed in a separated compart-
ment connected with a glass frit and filled with a 0.1 mol‚L-1
solution of (n-Bu4N)BF4 in degassed acetonitrile. A degassed
acetonitrile solution (10 mL) containing 0.1 mol‚L-1 (n-Bu4N)BF4
and 0.1 mol‚L-1 of the acid studied was first electrolyzed at the
given potential for 1 h. The catalyst was added as a solid to reach
a final concentration of 1 mmol‚L-1, and electrolysis was then
performed. Hydrogen was tested for purity using a Delsi Nermag
DN200 gas chromatograph equipped with a thermal conductivity
detector.
Cyclic voltammograms were simulated using DigiElch soft-
ware.18 A first simulation and best fitting was performed for [Co-
(dmgBF2)2(CH3CN)2] in CH3CN in the absence of added acid.
(13) Bakac, A.; Espenson, J. H. J. Am. Chem. Soc. 1984, 106, 5197.
(14) Tovrog, B. S.; Kitko, D. J.; Drago, R. S. J. Am. Chem. Soc. 1976, 98,
5144.
(15) Schrauzer, G. N.; Holland, R. J. J. Am. Chem. Soc. 1971, 93, 1505.
(16) Mohamed, K. S.; Padma, D. K. Indian J. Chem. 1988, 27A, 759.
(17) Koepp, H. M.; Wedt, H.; Strehlow, H. Z. Elektrochem. 1960, 64, 483.
(12) Hu, X.; Cossairt, B. M.; Brunschwig, B. S.; Lewis, N. S.; Peters, J.
C. Chem. Commun. 2005, 4723.
1818 Inorganic Chemistry, Vol. 46, No. 5, 2007

Cobaloximes as Functional Models for Hydrogenases
and rotating disk electrode (RDE) steady-state voltammo-
gram (see the Supporting Information) of [Co(dmgBF2)2L]
recorded on a glassy carbon electrode exhibit two well-
defined one-electron waves with chemically reversible
characteristics. The first process at -0.44 V vs Ag/AgCl or
-0.50 V vs SHE in acetonitrile (-0.51 V vs Ag/AgCl or
-0.64 V vs SHE in DMF) is generally assigned to the CoII/
CoI couple in the literature.9,12,19 This potential value is in
good agreement with that of -0.43V vs SHE previously
reported by Connolly and Espenson.9 The second process at
-1.55 V vs Ag/AgCl in acetonitrile (-1.44 V vs Ag/AgCl
in DMF) could correspond to either a metal or a ligand-
centered reduction. Spectroscopic characterization of the one-
electron- and two-electron-reduced species using UV-visible
spectroscopy and electron paramagnetic resonance proved
impossible because both compounds are unstable at room
temperature in CH3CN or DMF solution at the time scale of
electrolysis.
A one-electron oxidative process is furthermore observed
with Epa ) 0.95 V and Epc ) 0.26 V vs Ag/AgCl in CH3CN
(Epa ) 0.74 and Epc ) 0.30 V vs Ag/AgCl in DMF). These
waves are not clearly defined but reveal some chemical
reversibility. This process could be tentatively assigned to
the CoIII oxidation state by comparison with the electro-
chemical behavior of the parent cobaloxime [Co(dmgH)2-
pyCl].11
Cyclic Voltammograms of [Co(dmgBF2)2L] in the
Presence of Acids. Cyclic voltammograms of [Co(dmgBF2)2L]
were recorded over the potential range from +1.0 to -1.9
V vs Ag/AgCl in CH3CN and DMF in the presence of 1.5,
3, and 10 equiv of different acids:20 tetrafluoroboric acid
diethyl ether complex (pKa ) 0.1 in CH3CN),12 p-cyano-
anilinium tetrafluoroborate (pKa ) 7.6 in CH3CN),20b tri-
fluoroacetic acid (pKa ) 12.7 in CH3CN), and triethylam-
monium tetrafluoroborate (pKa ) 18.7 in CH3CN and 10.7
in DMF). The values of the calculated standard potentials21
and experimental electrocatalytic reduction potentials of these
acids on a glassy carbon electrode in the presence of a
cobaloxime as a catalyst or on a platinum electrode are given
in Table 1. These values will be commented upon in the
Discussion section. In the following, only the electrocatalytic
waves of the cyclic voltammograms will be considered even
though less intense signals following these waves are also
observed. These signals are also found in cyclic voltammo-
grams recorded on platinum or glassy carbon electrodes in
the absence of a cobalt catalyst. They may correspond to
the reduction of minor protonated solvent species, the exact
nature of which is not definitively known, and are not directly
related with the electrocatalyzed process under study.
(a) Strong-Acid Behavior. As reported by Peters et al.,12
the addition of a relatively strong acid such as p-cyano-
Figure 1. Cyclic voltammogram of [Co(dmgBF2)2(CH3CN)2] (1 × 10-3
mol‚L-1) recorded in a CH3CN solution of n-Bu4NBF4 (0.1 mol‚L-1) on a
glassy carbon electrode at 100 mV‚s-1 (potentials vs Ag/AgCl).
Parameters were refined until voltammograms recorded at 50, 100,
and 500 mV‚s-1 could be simulated using the same set of values.
The resulting parameters were kept unchanged for simulation of
voltammograms recorded in the presence of acid. The model was
then modified to include the catalytic steps and the new parameters
refined until one set of values could provide correct simulation of
the voltammograms recorded in the presence of various amounts
of added acid (1.5, 3, and 10 equiv) at different scan rates (50,
100, and 500 mV‚s-1).
Results
Stability of [Co(dmgBF2)2L] (L ) CH3CN, DMF, H2O)
in the Presence of Acids. The spectrum of [Co(dmgBF2)2-
(OH2)2] in CH3CN (DMF) displays an absorption maximum
at 420 (460) nm. In the noncoordinating solvent CHCl3, the
maximum absorbance is at λmax ) 440 nm. The addition of
a few microliters of CH3CN (DMF) to this solution leads to
a shift of this maximum absorbance to 420 (460) nm,
indicating displacement of the coordinated water molecules
by CH3CN (DMF). In the following, the complexes will then
be written as [Co(dmgBF2)2L] (L ) CH3CN or DMF), even
if it is not precisely known whether the complex in the
solution is penta- or hexacoordinated with one very labile
axial ligand.13
The stability of millimolar solutions of [Co(dmgBF2)2L]
in the presence of acids (30 equiv) was monitored spectro-
photometrically by following the decay of the maximum
absorbance. [Co(dmgBF2)2L] proved stable over days in the
presence of Et3NHBF4 or Et3NHCl. In the presence of CF3-
COOH, however, the complex decomposes with half-lives
of 2 days in CH3CN and 3 days in DMF approximately.
Degradation is even faster with p-cyanoanilinium tetrafluo-
roborate (t1/2 ) 15 h and 1 day, respectively). As reported
by Peters et al.,12 stronger acids such as HCl‚Et2O and HBF4‚
Et2O hydrolyze [Co(dmgBF2)2L] within a few minutes in
acetonitrile. After the reaction, the solution does not display
any absorption in the visible spectrum. The same behavior
is observed in DMF for HCl‚Et2O, but the hydrolysis is
slower in the case of HBF4‚Et2O (t1/2 ) 15 h). Further studies
are needed to characterize the degradation product.
Cyclic Voltammogram of [Co(dmgBF2)2L]. The cyclic
voltammogram (Figure 1 and the Supporting Information)
(19) Campbell, C. J.; Haddleton, D. M.; Rusling, J. F. Electrochem.
Commun. 1999, 1, 618.
(20) (a) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific Publications: Oxford, U.K., 1990. (b)
Edidin, R. T.; Sullivan, J. M.; Norton, J. R. J. Am. Chem. Soc. 1987,
109, 3945.
(21) Felton, G. A. N.; Glass, R. S.; Lichtenberger, D. L.; Evans, D. H.
Inorg. Chem. 2006, 45, 9181.
(18) (a) Rudolph, M. J. Electroanal. Chem. 2003, 543, 23. (b) Rudolph,
M. J. Electroanal. Chem. 2003, 558, 171. (c) Rudolph, M. J.
Electroanal. Chem. 2004, 571, 289. (d) Rudolph, M. J. Comput. Chem.
2005, 26, 619. (e) Rudolph, M. J. Comput. Chem. 2005, 26, 633. (f)
Rudolph, M. J. Comput. Chem. 2005, 26, 1193.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1819

Baffert et al.
Table 1. Hydrogen Evolution Potentials vs Ag/AgCl/KClaq 3 mol‚L-1 on Glassy Carbon in the Presence of Various Cobaloximes and Platinum for
Different Acids
standard
E
her on
cobaloxime-catalyzed
acid
solvent
potential21/V
platinum/V
E
her on glassy carbon/V
catalyst
E°(CoII/CoI)/V
ref
Et3NH+
CF3COOH
DMF
-0.78
-0.43
-0.13
-0.95
-0.43
-0.15
-0.35
-0.26
-0.34
-0.98
-0.43
-0.34
-0.52
-0.28
-0.52
[Co(dmgH)2pyCl]
[Co(dmgBF2)2L]
[Co(dmgBF2)2L]
[Co(dmgBF2)2L]
[Co(dpgBF2)2L] a
[Co(dmgBF2)2L]
-0.98
-0.44
-0.44
-0.51
-0.29
-0.51
11
CH3CN
CH3CN
DMF
CH3CN
DMF
12 and this work
this work
this work
12
p-cyanoanilinium
p-cyanoanilinium
HCl‚Et2O
-0.21
HBF4‚Et2O
this work
-
a dpgBF2 ) (difluoroboryl)diphenylglyoximato anion.
Figure 2. Cyclic voltammograms of [Co(dmgBF2)2(CH3CN)2] (1.10-3 mol‚L-1) recorded in a CH3CN solution of n-Bu4NBF4 (0.1 mol‚L-1) on a glassy
carbon electrode at 100 mV‚s-1 in the presence of (top to bottom) 0, 1.5, 3.0, and 10 equiv of (a) p-cyanoanilinium tetrafluoroborate, (b) CF3COOH, (c) and
Et3NHCl. (d) Simulated cyclic voltammograms of [Co(dmgBF2)2(CH3CN)2] (1 × 10-3 mol‚L-1) at 100 mV‚s-1 in the presence of 10 equiv of (A)
p-cyanoanilinium tetrafluoroborate, (B) CF3COOH, and (C) Et3NHCl (potentials vs Ag/AgCl).
anilinium tetrafluoroborate to a solution of [Co(dmgBF2)2L]
in CH3CN (or HBF4‚Et2O to a solution of [Co(dmgBF2)2L]
in DMF) triggers the appearance of a new irreversible
cathodic wave near the CoII/CoI response (Figure 2a). This
behavior is typical for a “total” catalysis for hydrogen
evolution:22 At low acid/catalyst concentration ratios, the
catalytic wave lies at more positive potentials (-0.344 V vs
Ag/AgCl) than that of the CoII/CoI couple. Increasing the
acid/catalyst concentration ratio raises the height of the new
wave and shifts it to more negative potentials while the CoII/
CoI reversible wave disappears. This behavior is similar to
that previously observed for proton reduction catalyzed by
[Co(dmgH)2(py)Cl] in DMF,11 which suggests that the
mechanism (Scheme 1, pathway i) involves a first reduction
step. The protonation of [CoI(dmgBF2)2L]- then produces a
cobalt(III) hydride species as an intermediate, which is
proposed in Scheme 1 to evolve hydrogen heterolytically
(see the Discussion section). The initial species is regenerated
by a last electron-transfer step.
(b) Weak-Acid Behavior. By contrast, the addition of a
weak acid such as Et3NHCl to a solution of [Co(dmgBF2)2L]
in CH3CN (Figure 2c) or DMF leads to the appearance of a
similar catalytic wave but located near the [Co(dmgBF2)2L]-/
[Co(dmgBF2)2L]2- process at -1.43 V vs Ag/AgCl. It should
be noted that around 20% of the current measured at the
(22) (a) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J.-M.; M’Halla,
F.; Save´ant, J.-M. J. Electroanal. Chem. 1980, 113, 19. (b) Save´ant,
J.-M.; Su, K. B. J. Electroanal. Chem. 1984, 171, 341.
1820 Inorganic Chemistry, Vol. 46, No. 5, 2007

Cobaloximes as Functional Models for Hydrogenases
Scheme 1. Catalytic Mechanisms for Electrocatalyzed Hydrogen
Evolution
Scheme 2. Mechanistic Scheme Used for Modeling of Cyclic
Voltammograms
1-4
Scheme 3. Definition of Acid Dissociation Constants pKa
Table 2. Thermodynamic Parameters Extracted from Simulation of the
Voltammograms (E/V vs Ag/AgCl)
[CoH(dmgBF2)2L]/[Co(dmgBF2)2L]-
[CoH(dmgBF2)2L]-/[Co(dmgBF2)2L]2-
[Co(dmgBF2)2L]+ + H2/[CoH(dmgBF2)2L]
[Co(dmgBF2)2L] + H2/[CoH(dmgBF2)2L]-
[CoH(dmgBF2)2L]/[CoH(dmgBF2)2L]-
pKa1 ) 13.3
pKa2 ) 23.0
pKa3 ) 10.5
pKa4 ) 36.3
E° ) -0.98 V
CNCH2COOH catalyzed by the rhodium porphyrin complex
[Rh(TPP)I(PPh3)].24
peak potential for 10 equiv of Et3NHCl in Figure 2c is due
to the direct reduction of Et3NH+ at the glassy carbon
electrode. This catalytic process may involve a heterolytic
mechanism (Scheme 1, pathway iii) as in the case of pathway
i but involving a cobalt(II) hydride intermediate instead of
the cobalt(III) hydride species. However, a homolytical
reaction of the cobalt(II) hydride intermediate cannot be
excluded.
Cyclic Voltammograms Modeling. In order to confirm
the proposed pathways for her, we have modeled the cyclic
voltammograms for the electrocatalyzed reduction of p-
cyanoanilinium tetrafluoroborate, CF3COOH, and Et3NHCl
in CH3CN using the DigiElch software.18
In a first approach, only the heterolytic pathways leading
to dihydrogen from both CoIII-H and CoII-H were input in
the modelization. Pathways i-iii used for modeling the
different catalytic behaviors form, in fact, two fused square
schemes as shown in Scheme 2. Whereas the electrochemical
parameters (E°1, E°2, and E°3) for [Co(dmgBF2)2L] could
be determined from voltammograms recorded in the absence
(c) Intermediate-Acid Behavior. The addition of acids
with intermediate strength, such as CF3COOH in CH3CN
(Figure 2b) or HBF4‚Et2O and p-cyanoanilinium tetrafluo-
roborate in DMF (see the Supporting Information),23 results
in another modification of the cyclic voltammograms of [Co-
(dmgBF2)2L] (Figure 2b): A catalytic behavior is still
observed near the CoII/CoI response but with lower current
intensity and thus a slower overall reaction rate. In addition,
a new irreversible cathodic wave appears near -1.0 V vs
Ag/AgCl, the height of which rises upon increasing acid
concentration. As far as the mechanism is concerned (Scheme
1, pathway ii), the presence of this new catalytic wave may
indicate that the cobalt(III) hydride involved as a catalytic
intermediate in pathway i had not totally reacted and could
be electrochemically reduced at this potential value (-1.0
V vs Ag/AgCl), producing the active species for the second
catalytic wave. This new intermediate was formally written
as cobalt(II) hydride, which can evolve hydrogen by parallel
heterolytic and hemolytic pathways. This mechanism paral-
lels that established by Save´ant et al. for the reduction of
of acid, the four thermodynamic acidity constants (pKa1-4
)
and the electrochemical potential E°4 of the [CoH(dmgBF2)2L]/
[CoH(dmgBF2)2L]- couple (Scheme 1, pathway ii) were
unknown. The presence of two square schemes in the
mechanism (Scheme 2) finally reduces to three the number
of independent constants, and we chose the three acid-base
couple pKa values [CoH(dmgBF2)2L]/[Co(dmgBF2)2L]- (pKa1),
[CoH(dmgBF2)2L]-/[Co(dmgBF2)2L]2- (pKa2), and [Co-
(dmgBF2)2L]+ + H2/[CoH(dmgBF2)2L] (pKa3) defined in
Scheme 3 using the classical formalism for acid dissociation
reactions. We succeeded in modeling all of the cyclic
voltammograms recorded at 50, 100, and 500 mV‚s-1 in the
presence of 1.5, 3, and 10 equiv of the three acids with one
set of thermodynamic constants (Table 2) and thus were able
to reproduce the three different electrocatalytic behaviors
(Figure 2d and the Supporting Information).25 The derived
potential for [CoH(dmgBF2)2L]/[CoH(dmgBF2)2L]- (-0.98
(23) DMF is indeed known to strongly solvate protons and thus to level
down the strength of the acids. Tremillon, B. Chemistry in non-aqueous
solVents; D. Reidel Publishing Co.: Dordrecht, The Netherlands, 1971.
(24) Grass, V.; Lexa, D.; Save´ant, J.-M. J. Am. Chem. Soc. 1997, 119,
7526.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1821

Baffert et al.
V vs Ag/AgCl in CH3CN) is in the same range as that
measured in DMF (-0.78 V vs Ag/AgCl) on an authentic
sample of [CoH(dmgH)2PBu3]/[CoH(dmgH)2PBu3]-.15 The
pKa values determined in CH3CN for [CoH(dmgBF2)2L]/[Co-
(dmgBF2)2L]- and [Co(dmgBF2)2L]+ + H2/[CoH(dmgBF2)2L]
are very similar to those calculated from our previously
published data:11 15.4 for [CoH(dmgH)2py]/[Co(dmgH)2py]-
and 16.3 for [Co(dmgH)2py]+ + H2/[CoH(dmgH)2py] in
DMF.
Further simulations of cyclic voltammograms were carried
out using a hemolytic model consisting of a bimolecular
reductive elimination generating dihydrogen from either two
CoIII-H or two CoII-H species.
In the first case, we found it impossible to simulate the
differences observed between strong and intermediate acid
conditions. In both cases, the sole catalytic wave observed
was the one at -0.35 V vs Ag/AgCl.
In the second case, simulated voltammograms were found
unchanged for values of the bimolecular rate constant khomo
larger than 105 mol-1‚L‚s-1. For comparison, a value of 1.7
× 104 mol-1‚L‚s-1 was determined for the rate constant of
homolytic hydrogen evolution from [HCo(dmgH)2PBu3].26
The introduction of both heterolytical and homolytical
pathways in the model resulted in similar cyclic voltammo-
grams.
Bulk Electrolysis Experiments for Hydrogen Evolution.
Bulk electrolysis for hydrogen evolution, coupled to hydro-
gen titration, using [Co(dmgBF2)2L] as the catalyst was
already reported in the presence of Et3NHBF4 (-0.9 V vs
Ag/AgCl in DMF, 4.7 TON‚h-1, 80 TON)11 and CF3COOH
(-0.72 V vs SCE, 20 TON‚h-1, 20 TON).12 In both cases,
the faradaic yield was almost quantitative. In order to sort
the various conditions (nature of the acid and applied
potential) regarding turnover frequencies, bulk electrolysis
experiments were undertaken using the same experimental
procedure. A mercury pool cathode was chosen in order to
minimize the reduction current for the uncatalyzed reduction
of Et3NH+ at -1.6 V vs Ag/AgCl.21 The evolved hydrogen
was characterized by GC analysis. Three values of applied
potentials (-0.5, -1.0, and -1.6 V vs Ag/AgCl) were
chosen, corresponding to the electrocatalytic potentials
observed in cyclic voltammograms. Figure 3 shows the
evolution of the total charge passed as a function of time
during 1 h of bulk electrolysis of solutions containing [Co-
(dmgBF2)2L] in the presence of 100 equiv of acid. Catalytic
reduction of Et3NHCl (Figure 3, traces c and e) was
significantly faster when the applied potential was set to -1.6
V vs Ag/AgCl, a value corresponding to that of the sole
catalytic wave observed by cyclic voltammetry. CF3COOH
was reduced with a higher current at -1.0 V as compared
to -0.5 V vs Ag/AgCl (compare Figure 3, traces b and d).
[Co(dmgBF2)2L] was much more active at -0.5 V vs Ag/
Figure 3. Coulometry for bulk electrolysis of a CH3CN solution (10 mL)
of various acids (0.1 mol‚L-1) and n-Bu4NBF4 (0.1 mol‚L-1) at a mercury
pool electrode in the presence of 1 × 10-3 mol‚L-1 [Co(dmgBF2)2(CH3-
CN)2] at different potentials: (a) p-cyanoanilinium tetrafluoroborate/-0.5
V, (b) CF3COOH/-1.0 V, (c) Et3NHCl/-1.6 V, (d) CF3COOH/-0.5 V,
and (e) Et3NHCl/-1.0 V vs Ag/AgCl [1 turnover (2e-/catalyst) corresponds
to 1.93 C].
AgCl toward p-cyanoanilinium tetrafluoroborate compared
to CF3COOH. The reduction of p-cyanoanilinium tetrafluo-
roborate was almost finished (46 TON, 92% conversion
based on coulometry) after 30 min at -0.5 V vs Ag/AgCl
corresponding to a turnover frequency of 92 TON‚h-1.
We have, furthermore, observed that an exhaustive elec-
trolysis of CH3CN solutions of [Co(dmgBF2)2L] at either
-0.5 or -1.6 V vs Ag/AgCl in the absence of acid results
in the loss of catalytic properties of the complex: when, in
a second step, acid is added to the solution and a potential
(-0.5, -1.0, or -1.6 V vs Ag/AgCl) is applied, no catalytic
current could be measured. In the same manner, the species
formed by acidic degradation of [Co(dmgBF2)2L] was found
to be catalytically inactive. This indicates that, because of
fast turnover cycling in the presence of acid, the reduced
catalyst is protected against decomposition.
Discussion
Hydrogen Evolution Mechanism. Metal-hydride species
are classical intermediates of the hydrogen evolution reaction.
Their catalytic implication was demonstrated in a great
number of catalytic systems such as those based on metal
porphyrins (M ) Fe,27 Co,28 and Rh24), cyclopentadienyl
complexes of cobalt29 and rhodium,8 [Ni(PPh2NPh2)2(CH3CN)]-
(BF4)2 (PPh2NPh ) 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphos-
phacyclooctane),6 or diiron mimicks of iron-only hydroge-
nases.5 Co-H species are commonly proposed as intermediates
in cobaloxime-catalyzed evolution of molecular hydro-
gen9,11,12 on the basis of the known reactivity of isolated
hydridocobaloximes.26
Most of the molecular complexes reported so far are active
for proton electroreduction under only one redox state. A
notable exception is given by the rhodium tetraphenylpor-
phyrin system in which all three [RhIIH(TPP)], [RhIIIH(TPP)-
(PPh3)], and [RhIIH(TPP)(PPh3)] species catalyze CNCH2-
(25) For these simulations, the new electrochemical kinetic parameters
(CoIII-H/CoII-H couple) were fixed at standard values and were not
best-fitted. In contrast, the chemical kinetic parameters that influence
both the potential (EC processes) and the heights of the electrocatalytic
waves were adjusted during simulation (see the Supporting Informa-
tion).
(27) Bhugun, I.; Lexa, D.; Save´ant, J.-M. J. Am. Chem. Soc. 1996, 118,
3982.
(28) Kellett, R. M.; Spiro, T. G. Inorg. Chem. 1985, 24, 2373, 2378.
(29) Koelle, U.; Ohst, S. Inorg. Chem. 1986, 25, 2689.
(26) Chao, T.-H.; Espenson, J. H. J. Am. Chem. Soc. 1978, 100, 129.
1822 Inorganic Chemistry, Vol. 46, No. 5, 2007

Cobaloximes as Functional Models for Hydrogenases
CO2H reduction whereas [RhIIIH(TPP)] does not.24 The
results described in this paper highlight three different
pathways for hydrogen evolution electrocatalyzed by the
same complex, [Co(dmgBF2)2L], in nonaqueous solvents,
depending on the strength of the acid used. They can be
described starting from the CoII state (Scheme 1) as fol-
lows: (i) an ECCE mechanism involving CoIII-H as the
active species, (ii) an ECEC mechanism involving both
CoIII-H and CoII-H as intermediates, and (iii) an EECC
mechanism involving CoII-H as the active species.
Hydrogen evolution proceeds via pathway i if the acid used
is strong enough to protonate both [CoI(dmgBF2)2L]- and
[CoIIIH(dmgBF2)2L]. With acids of lower strength, the
CoIII-H intermediate cannot be protonated and is thus further
reduced to produce the electrocatalytic CoII-H intermediate
(pathway ii). Finally, very weak acids such as Et3NH+ do
not protonate [CoI(dmgBF2)2L]- but are able to react with
the very nucleophilic [Co(dmgBF2)2L]2-, which mediates
hydrogen evolution (pathway iii) below -1.4 V vs Ag/AgCl.
The reactivity reported by Peters et al. using trifluoroacetic
acid occurs at the CoII/CoI process and thus proceeds via
pathway i.12 We could show that this acid is also reduced
via pathway ii at -0.97 V vs Ag/AgCl. This means that the
diffusion layer is not fully depleted in acid at the end of the
first catalytic wave and thus that this first process is not that
fast. Indeed, p-cyanoanilinium is reduced much more ef-
ficiently (i.e., with higher currents in similar conditions) via
pathway i.
We can, moreover, understand why, as we reported
previously, [Co(dmgBF2)2L] catalyzes Et3NH+ reduction
slowly in 1,2-dichloroethane at -0.9 V vs Ag/AgCl (and
not at higher potentials):11 A small signal assigned to [CoH-
(dmgBF2)2L]/[CoH(dmgBF2)2L]- is seen at -0.9 V vs Ag/
AgCl in the cyclic voltammogram of [Co(dmgBF2)2L] in the
presence of Et3NH+ (Figure 2). Thus, bulk electrolysis
produces [Co(dmgBF2)2L]-, which accumulates in the bulk
and is partially protonated to form [CoH(dmgBF2)2L]. The
latter is electrochemically reduced to yield [CoH(dmgBF2)2L]-.
Hydrogen evolution catalysis proceeds via pathway ii. With
the first protonation step being thermodynamically strongly
unfavored and acting as a bottleneck for the reaction, the
turnover frequency was too low compared to the cyclic
voltammetry time scale for the electrocatalytic wave to be
observed using this technique.
Metal hydride can evolve hydrogen through two main
mechanisms: (i) hydride protonation leading to the hetero-
lytic cleavage of the M-H bond and (ii) reductive elimina-
tion from two complexes through the homolytic cleavage of
the M-H bond. For example, the hydridocobaloxime [Co-
(dmgH)2(PBu3)H] evolves dihydrogen by parallel heterolytic
and homolytic mechanisms.26 We could demonstrate that the
CoIII-H intermediate (pathway i) mediates only heterolytic
dihydrogen production in the same manner as [CoH-
(dmgH)2py].11 In contrast, whether hydrogen evolution from
the CoII-H species (pathways ii and iii) is heterolytic and/
or hemolytic could not be experimentally determined.
However, the CoII-H bond should be stronger than the
CoIII-H bond and thus more resistant to reductive elimina-
tion. This general trend is illustrated by the fact that
ruthenium(III) porphyrin hydrides easily eliminate dihydro-
gen while the corresponding anionic ruthenium(II) hydrides
are resistant to reductive elimination.30
Activation Potential. The H2/2H+ + 2e- interconversion
is a two-electron process, and related reactions should involve
either a hydrogen radical or a hydride ion. On platinum,
palladium, and nickel surfaces, the reduction of protons
occurs with low overvoltages (also called activation potential)
through a “homolytic” mechanism because of stabilization
of adsorbed hydrogen and the reversible desorption of
dihydrogen.4b In contrast, these reactions always proceed in
solution via a “heterolytic” pathway involving a hydride ion
stabilized through some chemical interaction with the
catalysts.4 Coordination complexes are thus naturally suited
for such a catalysis because they combine both redox and
acid-base reactivities. However, by contrast with a metallic
material displaying a conduction band with an energy
continuum, these complexes can only provide electrons at
their redox potentials. Thus, it is not conceivable that a given
molecular compound could catalyze electroreduction of
protons near equilibrium over the entire pKa range.
The reactivity of cobaloximes with regard to acid reduction
can be tuned by synthetic modifications of the cobalt
coordination sphere. Thus, given an acid and knowing its
standard potential in the solvent under study,21 it is possible
to find a cobaloxime with comparable CoII/CoI redox
potential that catalyzes hydrogen evolution on a carbon
electrode in organic solvents with a small activation potential,
very close to that observed on platinum (Table 1, lines 1, 2,
and 5).
Even if cobaloximes combined with glassy carbon elec-
trodes have small overvoltages for the hydrogen evolution
reaction, it is desirable for technological applications to
design catalysts with higher CoII/CoI potentials.4a However,
this will result in a less nucleophilic metal center and, as a
consequence, stronger acid will be required for electrocata-
lytic activity.4,11 Unfortunately, under these conditions,
cobaloximes are significantly hydrolyzed and lose activity.
The {BF2}-bridged cobaloximes are known as the more
stable cobaloximes in acidic conditions,4b,9,13 but, neverthe-
less, [Co(dmgBF2)2L] is readily, albeit slowly, hydrolyzed
by acids such as p-cyanoanilinium (pKa ) 7.6 in CH3CN)
or H2SO4 (pKa ) 7.2 in CH3CN). The same is true for [Co-
(dpgBF2)2L] [dpgBF2- ) (difluoroboryl)diphenylglyoximato
anion], which decomposes in the presence of HBF4‚Et2O.12
The balance should thus be found between thermodynamics
and stability: for example, [Co(dmgBF2)2L] catalyzes hy-
drogen evolution more slowly but, because the catalyst is
not hydrolyzed, for a longer time in the presence of CF3-
COOH as compared to p-cyanoanilinium.
Up to last year and with the exception of a rhodium-based
electrocatalyst active at -0.55 V vs SCE,31 all of the first-
(30) Collman, J. P.; Hutchison, J. E.; Wagenknecht, P. S.; Lewis, N. S.;
Lopez, M. A.; Guilard, R. J. Am. Chem. Soc. 1990, 112, 8206.
Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E.; Lewis, N. S.;
Lopez, M. A.; Guilard, R.; L’Her, M.; Bothner-By, A. A.; Mishra, P.
K. J. Am. Chem. Soc. 1992, 114, 5654.
Inorganic Chemistry, Vol. 46, No. 5, 2007 1823

Baffert et al.
row transition-metal series electrocatalysts were active at
potentials below -0.90 V vs Ag/AgCl.11 Two other catalysts
active at quite positive potentials were recently described in
the litterature, both of them in the presence of strong acids:
[Ni(PPh2NPh2)2(CH3CN)](BF4)2 (PPh2NPh2 ) 1,3,5,7-tetraphen-
yl-1,5-diaza-3,7-diphosphacyclooctane) catalyzes the reduc-
tion of triflic acid (pKa ) 2.6 in CH3CN) at -0.40 vs Ag/
AgCl,7 and [Cp2Mo2(µ-S)2(η2-µ2-CH2S2)] mediates hydrogen
evolution from p-cyanoanilinium (pKa ) 7.6 in CH3CN) at
-0.24 vs Ag/AgCl.6 Bulk electrolysis experiments for proton
electroreduction by [Ni(PPh2NPh2)2(CH3CN)](BF4)2 and [Cp2-
Mo2(µ-S)2(η2-µ2-CH2S2)] were only carried out up to 6.5 and
10 turnovers, respectively. Despite the fact that the electro-
catalytic wave in cyclic voltammograms is observed to be
100 mV more negative than that of [Cp2Mo2(µ-S)2(η2-µ2-
CH2S2)] under the same conditions, [Co(dmgBF2)2(CH3CN)2]
proved to be much more active as a catalyst during bulk
electrolysis experiments at -0.5 V vs Ag/AgCl in CH3CN
because 46 turnovers were achieved within 30 min to be
compared to the 10 equiv of hydrogen produced by [Cp2-
Mo2(µ-S)2(η2-µ2-CH2S2)] within 3 h.
Conclusions
[Co(dmgBF2)2L] (L ) CH3CN or DMF) is currently one
of the most efficient catalysts for hydrogen evolution in a
nonaqueous solvent in terms of potential, turnover frequency,
and stability. Increasing the electrochemical potential is
essential in two perspectives: (i) aqueous application where
the -0.413 V vs SHE value for the apparent potential of
the H+/H2 couple at pH 7 would apply and (ii) interaction
with semiconductor photosensitive materials such as TiO2,
which cannot transfer electrons to the catalyst at potentials
below that of its conduction band (E1/2(CB) ) -0.11 + 0.059
× pH V vs SHE).4b We are currently investigating the
properties of [Co(dmgBF2)2(H2O)2] for proton electroreduc-
tion in aqueous conditions.
Acknowledgment. The authors acknowledge the Life
Science Division of the Commissariat a` l’Energie Atomique
for financial support within the Biohydrogen program.
Supporting Information Available: Experimental data and
simulations, including parameters, using DigiElch and cyclic
voltammograms of supporting electrolyte (blank experiment),
p-cyanoanilinium tetrafluoroborate, CF3COOH, and Et3NHCl. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(31) (a) Ko¨lle, U.; Gru¨tzel, M. Angew. Chem., Int. Ed. Engl. 1987, 26,
567. (b) Ko¨lle, U.; Kang, B.-S.; Infelta, P.; Comte, P.; Gra¨tzel, M.
Chem. Ber. 1989, 122, 1869. (c) Cosnier, S.; Deronzier, A.; Vla-
chopoulos, N. J. Chem. Soc., Chem. Commun. 1989, 1259.
IC061625M
1824 Inorganic Chemistry, Vol. 46, No. 5, 2007