Electrochemical Reduction of CO
2by M(CO)
4(bpy-type) (M=Mo, W)
Complexes: Catalytic Activity Improved by 2,2'-Dipyridylamine
Federico Franco,
[a]Claudio Cometto,
[a]Fabrizio Sordello,
[a]Claudio Minero,
[a]Luca Nencini,
[a]Jan
Fiedler,
[b]Roberto Gobetto,*
[a]Carlo Nervi*
[a][a] F. Franco, C. Cometto, F. Sordello, C. Minero, L. Nencini, R. Gobetto, C. Nervi
Department of Chemistry and NIS University of Torino
via P. Giuria 7, 10125 Torino, Italy
E-mail: roberto.gobetto@unito.it , carlo.nervi@unito.it [b] J. Fiedler
The J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague, Czech Republic
Supporting information for this article is given via a link at the end of the document.
Abstract: Tetracarbonyl complexes of low-valent Group VI transition
metals (Mo and W), containing diimine bidentate ligands, namely W(CO)4(4,6-diphenyl-2,2'-bipyridine) (1), W(CO)4 (6-(2,6-dimethoxy-phenyl)-4-phenyl-2,2'-bipyridine) (2), Mo(CO)4(2,2’-dipyridylamine) (3) and W(CO)4(2,2’-dipyridylamine) (4), were synthesized and
tested as homogeneous catalysts for CO2 electrochemical reduction
in non-aqueous media. Cyclic Voltammetry performed under CO2 atmosphere revealed a significant catalytic activity for these complexes in acetonitrile, and gas chromatographic measurements coupled to exhaustive electrolysis showed CO as the major reduction product. Mechanistic insights were obtained by IR-spectroelectrochemical measurements. The substantially different electrocatalytic performances obtained for the two classes of catalysts are compared and discussed.
Introduction
Despite some interesting advances that have been made over the last few years, an efficient electrocatalytic conversion of carbon dioxide into more valuable chemicals remains a challen-ge.[1] Critical issues like selectivity, stability of the active species
in solution, high operating overpotentials and intrinsic low
catalytic activity toward CO2 reduction are the main technological
barriers for the development of molecular catalysts suitable for
industrial applications.[2] The design of catalysts based on rare
precious transition metals represents a crucial economic
limitation to the widespread use of these systems.[3] New
effi-cient molecular catalysts have taken advantage of abundant and
cheaper transition metals.[4] In this perspective, the use of Group
VI transition metals (Cr, Mo, W) in electrocatalytic CO2 reduction
is particularly attractive, as suggested by many examples.[5]
However, unlike Group VII counterparts (e.g. ReI and MnI [6]), the
use of tetracarbonyl low-valence Group VI metal compounds with polypyridyl ligands as potential electrocatalysts for CO2
reduction remains an almost unexplored area. In particular, pentacarbonyl Mo- or W-based dianions have been reported to
undergo stoichiometric CO2 reduction to CO.[7] Moreover, some
[M(CO)4(bpy-R)] derivatives (M = Mo, W; bpy-R = 2,2’-bipyridine
with R = H, t-Bu)[8] were recently found to reduce CO
2 to CO in
acetonitrile with high faradic efficiencies. However, the main drawback in using these complexes is the negative reduction potentials required for the 2e catalytic process, resulting in a considerably lower catalytic activity when compared to that of Group VII metal complexes. The use of gold as a working electrode decreases the potential required for the catalytic
conversion of CO2 to CO by [M(CO)4(bpy)] (M = Mo, W, Cr), via
a 2e pathway involving a double-reduced tricarbonyl species.[9]
As already reported in the literature,[10] the introduction of phenyl
groups shifts the reduction potentials at less negative values, while the bulky substituents may have positive effects on the CO-evolving properties of the redox catalysts.[6b] In trying to
pursue these goals, ligands L1 and L2, and the corresponding
novel W(CO)4(bpy-R) complexes 1 and 2 were synthesized and
characterized (Scheme 1). The use of tungsten also contributes
to shift at a less negative potentials the reduction.[10] We also
synthesized Mo(CO)4(dpa) (3) and W(CO)4(dpa) (4) that bear the
alternative diimine ligand 2,2’-dipyridylamine (characterized by the presence of a bridging N-H group between the two distinct pyridyl rings), and explored their electrocatalytic properties for
CO2 reduction. Their unexpected and interesting behaviour are
the subject of this communication, as well as the comparison of the electrochemical behaviour with the more conventional complexes 1 and 2. To the best of our knowledge, the effect of this ligand on molecular electrocatalytic reduction of carbon dioxide has never been reported before, except for a Re(I) catalyst immobilized onto the electrode surface through a pyrrole-tagged 2,2’-dipyridylamine ligand.[11]
Scheme 1. Structures of the explored ligands and complexes.
Results and Discussion
Cyclic Voltammetries (CVs) of complexes 1-4 were performed on 1 mM acetonitrile solutions, without any addition of Brønsted acids. The reduction processes of all the studied compounds are centred at very negative potentials, where the direct reduction of
acids to H2 becomes relevant. The electrochemical behaviour of
1 and 2 under inert atmosphere is very similar to that reported
for other tetracarbonyl diimine complexes of Group VI transition metals, showing two successive 1e reduction steps, both mainly localized on the bipyridyl moiety (Fig. 1).[8-10]
(b)
Figure 1. CVs of a 1 mM acetonitrile solution of 1 (a) and 2 (b) at 100 mV s–1
under Ar (in black and red up to the 1st and 2nd reduction peaks) and CO 2
(blue).
The first reversible reduction (at E1/2 = –1.42 V and –1.48 V vs.
SCE for 1 and 2, respectively) that gives the corresponding
radical anions is centred on the diimine.[12] Moreover, for both
compounds a second irreversible reduction takes place at more
negative potentials (Ep = –2.04 V and –2.25 V vs. SCE for 1 and
2, respectively). The reduction potentials of 2 is more negative
than that of 1 likely because of the electron donating properties of the methoxy groups in 2. It seems reasonable to suppose that the observed decreased anodic reoxidation peak height after the second reduction step is probably due to a slow loss of axial CO, with formation of the pentacoordinated tricarbonyl species [M(CO)3(bpy-R)]2-.[8,10] In agreement with the previously reported
[M(CO)4(bpy-R)] (M = Mo, W; R = H, t-Bu) catalysts, the
complexes 1 and 2 under CO2-saturated conditions showed a
catalytic current on the 2nd reduction peak, but no evident effect
after the 1st reduction.[8] This strongly suggests that the dianion
species 12– and 22– are catalytically active towards CO
2 reduction
via a 2e mechanism. Both catalysts have similar activities, as indicated by the icat/ip values of 2.31 and 2.27 for 1 and 2,
respectively. These values are also comparable to those found for similar bpy-based Mo0 and W0 catalysts,[8] thus suggesting
that the phenyl groups introduced in 1 and 2 play a marginal role in both decreasing the reduction potentials and facilitating the CO release process.
(a)
(b)
Figure 2. IR-SEC of a 1 mM acetonitrile solution of 1 under Ar during the
reduction at the 1st (a) and at the 2nd (b) peaks.
IR spectroelectrochemical (IR-SEC) experiments of 1 under Ar confirm the reversibility of the first reduction process, showing the isosbestic transformation of the CO absorptions at 2006,
1885 and 1826 cm–1 of [W(CO)
4(L1)] into the corresponding
ones at 1985, 1854 and 1796 cm–1 of the radical anion
[W(CO)4(L1)].– (Fig. 2a, Table 1). During the second reduction,
the growth of a broad absorption band at about 1717 cm–1
indicates the formation of the tricarbonyl dianion species [W(CO)3(L1)]2–, similarly as previously reported (Fig. 2b).[9]
However, at a slightly higher CO absorption values than those of
the radical anion we observed a transient reduced species (whose role in the catalytic cycle under CO2 is presently
unknown), assigned[9] to the protonated two-electron reduced
species [W(CO)4(L1-H)]– (Fig. 2b).
Table 1. IR CO absorption of selected tri- and tetracarbonyl diimine W
complexes and their reduction products.
Complex νCO (cm–1) Solvent Ref.
[W(CO)4(bpy)] 2005, 1888, 1879, 1837 THF [9]
[W(CO)4(bpy-H)]– 1988, 1854, 1835, 1796 NMP [9]
[W(CO)3(bpy)]2– 1838, 1713, 1701 THF [9]
[W(CO)4(L1)] 2006, 1886(s), 1826 MeCN This work
[W(CO)4(L1)].– 1985, 1854(s), 1797 MeCN This work
[W(CO)4(L1-H)]– 1991, 1860, 1800 MeCN This work
[W(CO)3(L1)]2– 1846(sh), 1717 (br) MeCN This work
[W(CO)4(dpa)] 2010, 1885, 1865(sh), 1818 MeCN This work
[W(CO)4(dpa)].– 1996, 1868, 1843, 1795 MeCN This work When the solution was purged with CO2 the tricarbonyl
[W(CO)3(L1)]2– species, likely involved in the catalytic cycle for
CO2 reduction, could hardly be detected upon the second
electron reduction, whereas [W(CO)4(L1-H)]– is still visible in
IR-SEC (Fig. S1). Some new species, unseen under Ar, were also
detected during the catalytic CO2 reduction process.
Furthermore, due to the very negative potentials of the catalytic
wave for 1, the rate of the direct CO2 reduction process at the Pt
minigrid working electrode is not negligible. Evolution of gases worsen the quality of the recorded spectra and in the region from
1600 to 1700 cm–1 appear absorptions related to the formation of
bicarbonate and formate species.[9] Thus, IR-SEC data suggest
that in 1 the ability of phenyl groups to delocalize the double negative charge plays an important role in stabilizing the protonated reduced form, rather than affecting the kinetics of CO loss process. The net result is that no remarkable difference can be observed in the catalytic activity of [W(CO)4(L1)] in
comparison with the reference compound [W(CO)4(bpy)], as
observed in the voltammetric data.
Conversely, voltammetric response of the dpa-containing com-plexes under Ar revealed a single totally irreversible reduction peak, with Ep = –2.25 V and –2.12 V vs. SCE for 3 and 4,
respectively (Fig. 3a and 3b). Other transition metal complexes showed similar electrochemical behaviour, thus suggesting a
diimine localized reductions.[13] When the solutions of 3 and 4
were purged with CO2, the catalytic currents were considerably
higher than those observed after the 2nd reduction peak for 1 and
2 in the same conditions. This suggest that the 1e reduced
(OER) dpa-based species 3– and 4–, produced at approximately
the same potentials needed for the formation of 12– and 22-, may
interact with carbon dioxide, giving a faster catalytic response than that observed for 1 and 2. For this reason we focused our attention on the atypical electrochemical behaviour of the dpa-containing complexes, in particular 4.
(a)
(b)
Figure 3. CVs of a 1 mM acetonitrile solution of 3 (a) and 4 (b) at 100 mV s–1
under Ar (red) and CO2 (blue).
Although the origin of the electrochemical irreversibility is not completely understood, an electrochemical N-alkylation of the dpa ligand is known to occur via a cathodic cleavage of N-H bond after 1e reduction.[14] In our experimental conditions the
reduction of the free ligand dpa occurs at rather negative potential, near the solvent discharge. IR-SEC experiments showed that after the reduction of 4 in inert conditions (Fig. 4a), the N-H absorption (3327 cm–1) disappears, and a new
tetracarbonyl species is quantitatively formed, since the original
four CO at 2010, 1885, 1865(sh) and 1818 cm–1 are
isosbestically transformed into the corresponding CO at 1996,
1868, 1843 and 1795 cm–1. These findings indicate not only that
the 1e process in 3 and 4 is primarily localized over the dpa ligand, but also that no CO dissociation is observed after the electron transfer. Catalytic activity of the complex 4 can be
observed also from IR-SEC performed in presence of CO2 in the
solution. In lower concentrations of CO2 the complex can be
partially reduced (Fig. 4b), however most of the current passing at the potential of reduction peak of 4 is employed in the
catalyzed CO2 reduction. During electrolysis in larger
concentrations of CO2 (saturated solution) the original spectrum
(a)
(b)
Figure 4. IR-SEC during the reduction of a 1 mM acetonitrile solution of 4
under Ar (a) and under CO2 (b).
Figure 5. Optimized structures of Mo (up) and W (down) complexes. 3 (up
left), 4 (down left), their corresponding electron density difference between the neutral and the radical anion at the geometry of the neutral (center) and spin density of the optimized radical anions (right) 3– (up) and 4– (down).
To investigate the nature of the observed irreversibility, we performed DFT calculations on 3 and 4 and the corresponding radical anions 3– and 4–. The electron density differences
between radical anions (3– and 4–) and the corresponding
neutral forms (3 and 4), as well as the spin densities of the geometry optimized radical anions are localized on the dpa ligand (Fig. 5), suggesting that the reductions of 3 and 4 are essentially dpa-centred, in agreement with the previous observa-tions. It is also worth noting that, unlike what expected for common tetracarbonyl complexes of Group VI metals carrying α,α’-diimine ligands,[10] the [M(CO)
4(dpa)]•– radical anions are not
supposed to undergo CO loss because their SOMOs have M-CO bonding character, strengthening the M-M-CO bonds (Table 2). Since high catalytic overpotentials are the major drawbacks for such catalysts, complexes 1 and 4 were selected for bulk electrolysis, as representative members of the two classes of catalysts, because of their slightly less negative reduction potentials. The experiments were carried out under a constant
Ar or CO2 gas flow (20 ml min–1) and gas chromatography was
employed for gaseous product analysis. The potential of –2.2 V
vs. SCE, which is 100 mV less negative than that used for
analogue molecular catalysts,[8a] was applied to all the
electrocatalytic measurements, to benchmark different catalysts under the same experimental conditions.
Table 2. Metal-carbonyl bond lengths (in Å) of the DFT optimized geometry
structures of 3 and 4, and those of the corresponding optimized radical anions 3– and 4–.
Metal Complex Axial M-CO (Å) Equatorial M-CO (Å)
Mo 3 2.062, 2.056 1.977
Mo 3– 2.055, 2.052 1.968
W 4 2.049, 2.043 1.973
W 4– 2.044, 2.040 1.968
Figure 6. Evolution of current over time during bulk electrolysis performed at –
2.2 V vs. SCE on a 1 mM acetonitrile solution of: 1 under CO2 (black); 4 under
Bulk electrolysis of 1 mM acetonitrile solutions of complexes 1
and 4 in CO2-saturated solutions and under constant CO2 flow,
significantly differs (Fig. 6). Complex 1 showed a slowly decreasing current during the first hour of electrolysis, achieving a plateau with a stable current of about 2 mA (Fig. 6, black curve); gas-chromatographic measurements detected CO as the main product, with a 35% faradic efficiency and an overall TON
of ≈2 (after 60 min). Small quantities of H2 were also produced,
corresponding to a CO/H2 ratio of approximately 24. In the same
conditions, the dpa-based W0 complex 4 highlighted a
remarkably higher average electrocatalytic current of 7 mA, sustained for half an hour of electrolysis (Fig. 6, blue).
Figure 7. CO production against the charge passed during bulk electrolysis at
−2.2 V vs. SCE of a 1 mM solution of 4. The slope of about 0.27 corresponds to a 55% faradaic efficiency.
After one hour, a charge of 25.7 C passed, and a relevant CO production, with a 55% faradic efficiency (Fig. 7) was observed, with a TON of ≈8 (after 60 min). At the end of the second hour of electrolysis, the charge raised to around 37.5 C (TON ≈10), with
a rapid deactivation of the catalyst.[15] Hydrogen was also found
during the experiment with a CO/H2 ratio of about 45. A direct
comparison of the curves describing the evolution of CO produc-tion over time emphasizes the difference between the catalytic activities of the two compounds (Fig. 8). The overall mechanism
of the catalytic CO2 reduction to CO is still under study, and it
can be a proton- or not proton-assisted.[6c] We and others
already pointed out[6] that such a negative potentials could
induce Hofmann degradation of TBAPF6[16a] or CH3CN
deproto-nation,[16b,c] as proton source. Moreover, it is noteworthy that an
exhaustive electrolysis performed on a 1 mM solution of 4 under inert conditions (Fig. 6, red curve) led to much lower currents
than those observed in the presence of CO2, and, unlike what is
commonly reported for W-based tetracarbonyl catalysts with
bipyridyl derivatives,[8] no detectable CO was found by means of
gas chromatography. This suggests that the whole CO amount
produced during the electrocatalytic measurements on 4 in CO2
-saturated solutions is presumably formed by the catalytic reduction process, rather than coming from a partial degradation of the catalyst.
Figure 8. CO evolution during the first 75 minutes of electrolysis experiments
at –2.2 V vs. SCE of 1 mM acetonitrile solutions of W(CO)4(L1) (1) (red curve)
and W(CO)4dpa (4) (black curve). Sampling of CO was automatically made
every five minutes of analysis by gas chromatography.
Conclusions
In summary, this work represents the first report in which tetracarbonyl complexes of Group VI metals in low oxidation state with non-classical diimine ligands have been investigated as potential homogeneous electrocatalysts for CO2 reduction.
The electrocatalytic capability of Mo- and W-based tetracarbonyl catalysts with the 2,2’-dipyridylamine ligand in acetonitrile has been studied and compared with analogous tungsten compounds containing 4,6-disubstituted 2,2’-bipyridyl derivatives. Conversely to the latter, characterized by modest catalytic activities under CO2 by a double-reduced species, 3
and 4 showed the activation of an alternative 1e pathway at
potentials close to the 2nd reduction of similar complexes with
polypyridyl ligands, providing a higher CO2 reduction activity.
Preparative-scale electrolysis under CO2 atmosphere revealed
that [W(CO)4(dpa)], 4, is able to reduce CO2 to CO as the major
product, outperforming the more common bpy-based catalysts like [W(CO)4(L1)], 1, in terms of both CO production and
durability of the catalytically active species over time. Furthermore, bulk electrolysis as well as IR-SEC measurements carried out on 4 under Ar gave indications that the OER species interacting with the substrate is a tetracarbonyl intermediate, without any experimental evidence of CO dissociation. These results may lead to two different catalytic mechanisms: i) accordingly to the widespread idea that the metal centre is the
binding site for CO2, the [M(CO)4(bpy-R)] (M = Mo, W) complexes
selectively convert CO2 to CO via a 2e pathway, involving a
double-reduced species formed after a M-CO bond cleavage;[8-9]
ii) for [M(CO)4(dpa)] (M = Mo, W) catalysts, the presence of a
tetracarbonyl catalytically active species may suggest that the dpa ligand is non-innocent, and that the catalytic process to CO formation is mainly localized on the bidentate ligand, rather than on the electron-rich metal atom. In particular, the reduction of 3
and 4 is ligand-centred, so that the increased nucleophilicity of
the bridging N could favour an interaction with CO2. It has been
postulated that CO2 may undergoes insertion into pyridinium
ion[17-18], but the energy barrier of its insertion into the N-H bond
of a radical PyH group are too high.[18] These considerations
seem to indicate a non-innocent role of the dpa ligand in the present metal catalysis. Further studies are in progress to clarify the origin of the catalytic behaviour and to decrease the large overpotentials required for the reduction of the dpa-based catalysts. Their anomalous, but innovative features could open new ways to tune the electrocatalytic properties of low-cost Group VI metal-based catalysts.
Experimental Section
General synthetic procedure for tetracarbonyl complexes of Group VI transition metals. All the solvents, purchased from Aldrich and used
as received, were purged for 10 minutes with argon before use, whereas the precursors were purchased by Aldrich and employed without any further purification. According to the literature, synthesis of the complexes W(CO)4(N-N) was carried out by refluxing a mixture of
W(CO)6 and equimolar bidentate ligand in xylenes for 4 hours under
stirring and an inert atmosphere.[19] The reaction mixture was then cooled
to room temperature, the product was filtered on a silica gel column and washed with xylenes and petroleum ether. A similar method was employed for the synthesis of molybdenum tetracarbonyl complexes, by using toluene as solvent.
The syntheses of ligands L1 and L2 were performed by using a reported procedure.[20] NMR spectra were recorded on a JEOL EX 400
spectrometer (1H operating frequency 400 MHz) at 298 K and data were
treated by Jeol Delta Software. 1H and 13C chemical shifts are reported
relative to TMS (δ = 0) and referenced against solvent residual peaks. Elemental analyses (C, H, N) were performed in-house with a Fisons 1108 CHNS-O elemental analyzer. Common abbreviations are used: s, singlet; d, doublet; t, triplet; m, multiplet; td, triplet of doublets; dd, doublet of doublets; ddd, doublet of doublets of doublets.
(E)-3-phenyl-1-(pyridin-2-yl)prop-2-en-1-one (a). According to the
literature procedure, an emulsion of benzaldehyde (19 mmol) in cold distilled water (100 ml) was prepared in a flask and then an equimolar amount of 1-(pyridin-2-yl)ethanone was added under vigorous stirring.[21]
A solution 10% of NaOH in water (10ml) was then added dropwise and the mixture became pale yellow. The emulsion was stirred for 24 hours at 4°C and the yellow product was filtered and washed with water and methanol (yield 98%). 1H-NMR [400 MHz, (CD 3)2CO]: δ/ppm = 8.79 (d, J = 4.7 Hz, 1H), 8.38 (d, J = 16.1 Hz, 1H), 8.15 (d, J = 7.7 Hz, 1H), 8.04 (td, 1J = 7.6 Hz, 2J = 1.5 Hz, 1H), 7.89 (d, J = 16.1 Hz, 1H), 7.82 (m, 2H), 7.66 (ddd, 1J = 7.6 Hz, 2J = 5.0 Hz, 3J = 1.5 Hz, 1H), 7.49 (m, 3H). 1-(2-(2,6-dimethoxyphenyl)-2-oxoethyl)pyridinium iodide (b).
According to a slight modification of the literature procedure, 20 mmol of 1-(2,6-dimethoxyphenyl)ethanone and 24 mmol of iodine were refluxed for 3 hours in 50 ml of pyridine.[22] The reaction mixture was cooled to 0°
C and the yellow product was precipitated, filtered and washed with cold pyridine (yield 88%). 1H-NMR [400 MHz, (CD
3)2CO]: δ/ppm = 9.19 (d, J
= 6.6 Hz, 2H), 8.88 (t, J = 7.6 Hz, 1H), 8.40 (t, J = 6.8 Hz, 2H), 7.50 (t, J = 8.5 Hz, 1H), 6.81 (d, J = 8.5 Hz, 2H), 6.30 (s, 2H), 3.92 (s, 6H).
Synthesis of 1-(2-oxo-2-phenylethyl)pyridin-1-ium iodide (c). The
same procedure reported for b was used (yield 59%). NMR spectra were found to be consistent with the already published data.[23]
Synthesis of 4,6-diphenyl-2,2'-bipyridine (L1). According to a slight
modification of the literature procedure, a suspension of 1.5 mmol of a, 1.5 mmol of c and 10 mmol of ammonium acetate was refluxed in methanol (10 ml) for 6 hours.[24] The mixture was then cooled to room
temperature and dried on vacuum. The dark solid was dissolved in CH2Cl2 and washed with saturated NaCl solution. The organic phase was
then evaporated at low pressure and the solid residue was then purified by silica gel chromatography in order to get the pure product (yield 20%). NMR spectra were in agreement with published data.[25]
Synthesis of 6-(2,6-dimethoxyphenyl)-4-phenyl-2,2'-bipyridine (L2).
A suspension of 11 mmol of b, 11 mmol of a and 0.110 mol of ammonium acetate was refluxed in methanol (30 ml) for 6 hours. The mixture was then cooled to room temperature and the yellow product was filtered and washed with cold methanol (yield 83%).[20] 1H-NMR [400
MHz, (CD3)2CO]: δ/ppm = 8.71 (m, 2H), 8.47 (d, J = 8.2 Hz, 1H), 7.89 (m, 3H), 7.57 (m, 3H), 7.50 (t, J = 8.5 Hz, 1H), 7.40 (m, 2H), 6.81 (d, J = 8.2 Hz, 2H), 3.74 (s, 6H). 13C-NMR [100 MHz, (CD 3)2CO]: δ/ppm = 159.31, 157.27, 157.07, 156.08, 150.07, 149.42, 139.50, 137.95, 137.65, 130.63, 130.11, 129.87, 127.90, 124.80, 124.67, 121.83, 117.11, 105.41, 56.38 W(CO)4(L1) (1). 1H-NMR [400 MHz, (CD3)2CO]: δ/ppm = 9.32 (d, J = 6.1 Hz, 1H), 8.93 (m, 2H), 8.26 (t, J = 7.6 Hz, 1H), 8.12 (d,J = 7.6 Hz, 2H), 8.00 (s, 1H), 7.71-7.67 (m, 3H), 7.63-7.51 (m, 6H) . 13C-NMR [100 MHz, (CD3)2CO]: δ/ppm = 217.21, 210.81, 203.13, 165.14, 158.15, 158.11, 153.60, 150.53, 143.65, 138.86, 136.96, 131.23, 130.25, 129.32, 129.19, 128.48, 127.24, 125.42, 125.29, 120.44. Anal. Calcd. (%) for W1O4N2C26H16: C 51.68, H 2.67, N 4.64. Found: C 51.44, H 2.60, N 4.59. W(CO)4(L2) (2). 1H-NMR [400 MHz, (CD3)2CO]: δ/ppm = 9.34 (d, J = 5.6 Hz, 1H), 8.88 (m, 2H), 8.23 (td, 1J = 7.9 Hz, 2J = 1.4 Hz, 1H), 8.04 (dd, 1J = 8.2 Hz, 2J = 1.7 Hz, 2H), 7.79 (d, J = 1.7 Hz, 1H), 7.65 (m, 1H), 7.58 (m, 3H), 7.41 (t, 8.5 Hz, 1H), 6.81 (d, J = 8.5 Hz, 2H), 3.77 (s, 6H). 13 C-NMR [100 MHz, (CD3)2CO]: δ/ppm = 216.88, 210.55, 202.83, 160.50, 158.24, 158.05, 157.62, 153.49, 150.03, 138.69, 136.99, 131.98, 131.02, 130.23, 128.28, 127.02, 126.94, 124.94, 120.25, 105.21, 56.05. Anal. Calcd. (%) for W1O6N2C28H20: C 50.62, H 3.03, N 4.22. Found: C 50.42, H
3.00, N 4.20.
Mo(CO)4(dpa) (3). NMR spectra were consistent with the spectroscopic
data previously reported for this compound.[26] 1H-NMR [400 MHz,
(CD3)2CO]: δ/ppm = 9.58 (s, br. 1H), 8.64 (dd, 1J = 5.5 Hz, 2J = 1.8 Hz,
2H), 7.86 (td, 1J = 7.3 Hz, 2J = 1.7, 2H), 7.28 (d,J = 8.5 Hz, 2H), 7.06 (td, 1J = 5.9 Hz, 2J = 1.2, 2H).
W(CO)4(dpa) (4). 1H-NMR [400 MHz, (CD3)2CO]: δ/ppm = 9.77 (s, 1H),
8.74 (d, J = 5.9 Hz, 2H), 7.91 (t, J = 7.4 Hz, 2H), 7.36 (d, J = 8.5 Hz, 2H), 7.08 (t, J = 7.0 Hz, 2H). 13C-NMR [100 MHz, (CD
3)2CO]: δ/ppm = 214.05,
156.24, 153.99, 140.93, 119.14, 115.80.
Electrochemical experiments. All the solvents for electrochemical
experiments were freshly distilled. Cyclic voltammetry experiments were performed with a Metrohm Autolab 302N potentiostat. CH3CN solutions
of the complexes (1 mM) were used, with tetrabutylammonium hexa-fluorophosphate (TBAPF6) as supporting electrolyte (0.1 M). A
single-compartment cell was employed with a glassy carbon (GC) working electrode (Ø = 1 mm), a Pt counter electrode and an aqueous SCE as reference electrode. In our conditions the E1/2 of the redox couple Fc/Fc+
in CH3CN was 0.39 V. The inert conditions and the CO2-saturated
atmosphere were ensured by purging Ar and CO2 gases for 10 minutes
before each experiment, repectively. Spectro-electrochemistry was performed using the optically transparent thin layer electrode (OTTLE) cell.[27] IR spectra were recorded on Nicolet iS50 FT-IR spectrometer. The
Controlled-Potential Electrolysis (CPE) experiments were performed using a Metrohm Autolab 302N potentiostat. The measurements were carried out in a H-type cell with a a Pt wire as counter electrode in a bridge separated from the cathodic compartment by a glass frit. A glassy carbon plate as working electrode (A = 12.35 cm2) and an aqueous SCE
reference electrode were also used. CPE experiments were performed in CH3CN solutions (V = ≈10 ml), with catalyst concentration of 1 mM (≈10
µmol), without the addition of any external proton source. A potential value of -2.2 V vs. SCE was applied for all the experiments.
Analytical detection and quantification of the gaseous products. A
Smart Trak 100 (Sierra) flow controller C100L was used in the electrochemical experiments and a constant Ar or CO2 flow (20 ml/min)
was maintained during bulk electrolysis, whereas the produced gas sample was analyzed by means of an Agilent 490 Micro GC gas chromatograph equipped with a Molsieve 5 Å column and a CP-PoraPLOT U column. For the CO and H2 quantification a thermal
conductivity detector was used and the Molsieve 5 Å column was kept at a temperature of 90° C and at a pressure of 200 kPa. The carrier gas was argon. For the functioning and the calibration of the GC apparatus, Ar, He, N2, O2, CO2 and CO were obtained from Sapio, whereas H2 was
produced with a Claind HyGen hydrogen generator. The limits of detection for CO and H2 were 20 ppmv and 0.5 ppmv respectively. DFT Calculations. Gaussian 09 Rev. D.01 was employed for all
calculations.[28] Geometry optimization was performed with the B3LYP
functional including the conductor-like polarizable continuum model method (CPCM)[29] with acetonitrile as solvent. The LanL2DZ[30] basis
sets was used for Mo and W, and 6-31G(d,p) for the other atoms. Unrestricted open shell calculations were performed on the radical anions. The nature of all stationary points were confirmed by performing a normal-mode analysis.
Supporting Information: Additional details about IR-SEC and DFT
calculations.
Acknowledgements
The financial support from project PHOTORECARB (Progetti di Ateneo/CSP 2012, Call 03, Università di Torino & Compagnia Sanpaolo) is gratefully acknowledged. J.F. thanks the Grant Agency of the Czech Republic (grant 14-05180S) for the support.
Keywords: carbon dioxide • electrocatalysis • tungsten •
2,2’-dipyridylamine • molybdenum
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Entry for the Table of Contents (Please choose one layout)
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ARTICLE
Novel M(CO)4(dpa) (M=Mo, W;
dpa=2,2’-dipyridylamine) complexes were synthesized and tested for
electrochemical reduction of CO2.
Innovative and improved catalytic
activity towards CO2 reduction was
observed in acetonitrile homogeneous solution even in the absence of Brønsted acids.
Federico Franco, Claudio Cometto, Fabrizio Sordello, Claudio Minero, Luca Nencini, Jan Fiedler, Roberto Gobetto*, Carlo Nervi*
Page No. – Page No.
Electrochemical Reduction of CO2 by
M(CO)4(bpy-type) (M=Mo, W)
Complexes: Catalytic Activity Improved by 2,2'-Dipyridylamine
