Re(I) NHC Complexes for Electrocatalytic Conversion of CO2

Article abstract of DOI:10.1021/acs.inorgchem.6b00079

The modular construction of ligands around an N-heterocyclic carbene building block represents a flexible synthetic strategy for tuning the electronic properties of metal complexes. Herein, methylbenzimidazolium-pyridine and methylbenzimidazolium-pyrimidi

Full text of DOI:10.1021/acs.inorgchem.6b00079

Article
pubs.acs.org/IC
Re(I) NHC Complexes for Electrocatalytic Conversion of CO
2
†
‡
§
∥
†
Charles J. Stanton, III, Charles W. Machan, Jonathon E. Vandezande, Tong Jin, George F. Majetich,
§
‡
∥
,§
Henry F. Schaefer, III, Clifford P. Kubiak, Gonghu Li, and Jay Agarwal*
†
Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States
‡
Department of Chemistry and Biochemistry, University of California, San Diego, California 92093, United States
Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, United States
§
∥Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824, United States
*
S Supporting Information
ABSTRACT: The modular construction of ligands around an N-
heterocyclic carbene building block represents a flexible synthetic
strategy for tuning the electronic properties of metal complexes.
Herein, methylbenzimidazolium-pyridine and methylbenzimidazo-
lium-pyrimidine proligands are constructed in high yield using
recently established transition-metal-free techniques. Subsequent
chelation to ReCl(CO) furnishes ReCl(N-methyl-N′-2-pyridylben-
5
zimidazol-2-ylidine)(CO) and ReCl(N-methyl-N′-2-pyrimidylben-
3
zimidazol-2-ylidine)(CO) . These Re(I) NHC complexes are shown
3
to be capable of mediating the two-electron conversion of CO2
following one-electron reduction; the Faradaic efficiency for CO
formation is observed to be >60% with minor H and HCO H
2
2
production. Data from cyclic voltammetry is presented and
compared to well-studied ReCl(2,2′-bipyridine)(CO)3 and MnBr(2,2′-bipyridine)(CO)3 systems. Results from density
functional theory computations, infrared spectroelectrochemistry, and chemical reductions are also discussed.
INTRODUCTION
dazol-2-ylidine)(CO) (2). In either case, the bidendate, redox-
3
■
active ligand is constructed from an aryl halide (either
halopyridine or halopyrimidine), benzimidazole, and a
haloalkane. This modular synthesis permits myriad routes for
customization, e.g., benzimidazole may be exchanged for
imidazole or halopyrazine for halopyrimidine. Moreover, the
requisite transformations are transition-metal free and proceed
Rhenium(I)−tricarbonyl complexes bearing ligands with an N-
heterocyclic carbene (NHC) moiety were first reported in
1
1
992. Since then, tens of articles have appeared in the
2
−25
literature,
with the majority published in the last five years,
underscoring the rapid growth of this subfield. Recent work has
focused primarily on the photophysical properties of Re(I)
NHC complexes, specifically on modifications of the NHC-
containing ligand to modulate absorption and emission
3
0,44
in high yield.
These compelling qualities are in contrast to a
45−48
majority of bpy syntheses.
2
6
quantum yields. These investigations parallel the well-studied
Further comparison with bpy highlights additional advan-
tages for employing NHC-containing ligands: (i) they allow for
the possibility of π-backbonding via the low-lying empty p
functionalization of 2,2′-bipyridine (bpy) in ReCl(bpy)(CO) ,
research that dates back to the 1970s.
studies have examined Re(I) NHC compounds for catalysis, as
noted in a recent review by Herrmann and Ku
investigated the first use of Mn(I) NHC complexes for the
electrocatalytic reduction of CO to CO. The success of these
compounds for mediating CO conversion engenders questions
regarding the efficacy and electrochemical response of valence-
3
2
7,28
By comparison, few
49,50
orbital on the carbene carbon,
which assists in stabilizing
29
̈
hn. In 2014, we
low-valent metals, and (ii) the atoms adjacent to the
coordinating carbene can be substituted, such as with sulfur
30
49,51,52
2
or oxygen using benzothiazole or benzoxazole.
Cyclic
2
(
alkyl)(amino)carbenes (CAACs), where a carbon atom is
53,54
adjacent to the carbene carbon, may also be employed.
isoelectronic Re(I) species, especially given the well-established
Taken together, NHC-containing ligands provide an accom-
application of ReCl(bpy)(CO) for both the electrocata-
55,56
3
3
1−36
37−43
modating framework for steric and electronic tuning
that
lytic
and the photocatalytic
reduction of CO2.
has already been exploited to modulate the photophysical
properties of Re(I) NHC compounds and may permit the
Indeed, Delcamp and co-workers very recently reported
successful photocatalytic CO conversion using Re−pyridyl−
2
2
5
rational construction of CO conversion catalysts.
NHC complexes.
2
In the present research, two Re(I) NHC compounds are
examined: ReCl(N-methyl-N′-2-pyridylbenzimidazol-2-
Received: January 12, 2016
ylidine)(CO) (1) and ReCl(N-methyl-N′-2-pyrimidylbenzimi-
3
©
XXXX American Chemical Society
A
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Article
+
−
+
Figure 1. Synthetic scheme for [N-methyl-N′-2-pyridylbenzimidazolium] PF (L1, Y = C−H) and [N-methyl-N′-2-pyrimidylbenzimidazolium]
6
−
PF (L2, Y = N) with metalation procedure.
6
Figure 2. X-ray crystal structures of ReCl(N-methyl-N′-2-pyridylbenzimidazol-2-ylidine)(CO) (1, left) and ReCl(N-methyl-N′-2-pyrimidylbenzi-
3
midazol-2-ylidine)(CO) (2, right) with ellipsoids set at the 50% probability level.
3
30,63
Here we demonstrate that both 1 and 2 are capable of
previous work,
the synthesis of L1 and L2 involved the
mediating the conversion of CO to both CO and HCO H
under an applied voltage. Cyclic voltammetry data is presented
transition-metal-free and solvent-free procedure reported by
2
2
44
Hermann and Ku
up.
̈
hn, which is “green” and amenable to scale-
and the results compared to model compoundsReCl(bpy)-
5
7
1
(
CO) and MnBr(bpy)(CO) in order to understand the
Complexes 1 and 2 were characterized with H NMR,
3
3
13
1
nature of the voltammetric responses. Results from density
functional theory computations, infrared spectroelectrochemis-
try, and chemical reductions are also employed in this regard.
C{ H} NMR, UV−vis, and FTIR spectroscopies. Single
crystals suitable for X-ray analysis were grown from the vapor
diffusion of hexanes into a solution of 1 or 2 dissolved in DCM.
The resolved X-ray structures (shown in Figure 2) indicate that
the NHC-aryl ligands coordinate to the Re(I) center within the
equatorial plane via imine-type N atoms and carbene-type C
atoms, with bite angles of 74.0° and 74.5° for 1 and 2,
respectively (see SI, pp S9−S14). In each structure, the three
carbonyl ligands exist in a facial arrangement, as expected.
The absorbance spectra for 1 and 2 (see Figure 3) include
two predominant transitions: a π → π* band below 300 nm
and a lower energy band at 350 nm that was previously
assigned as an admixture of metal-to-ligand charge transfer
(MLCT) and ligand-to-ligand charge transfer (LLCT)
RESULTS AND DISCUSSION
Synthesis. Compounds 1 and 2 were constructed from
■
ReCl(CO) and the appropriate proligand, namely, [N-methyl-
5
+
−
N′-2-pyridylbenzimidazolium] PF (L1) or [N-methyl-N′-2-
6
+
−
pyrimidylbenzimidazolium] PF (L2); see Figure 1. To our
6
22
knowledge 2 has not been reported, but Chan et al., Li et
1
6
14
al., and Casson et al. (butyl variant) prepared 1 for
photophysical examination. The former two groups utilized
silver oxide to deprotonate the proligand and form the requisite
22
16
carbene, reporting yields of 61% and 23%, respectively.
Alternatively, Casson et al. utilized potassium carbonate in
refluxing toluene to achieve an 87% yield for the chelation
1
4,19
excitations.
Extinction coefficients for the MLCT/LLCT
−1
−1
band were measured to be 5430 and 6390 L mol cm for 1
and 2, respectively. Emission spectra maxima (Figure 3, inset)
for 1 and 2 differ by 55 nm (λmax = 505 and 560 nm,
respectively). The red-shifted emission for 2 is consistent with
1
4
step, and we observed an 89% yield using the same
procedure. A lower yield was seen for 2 (43%).
In two of the three reports listed above, L1 was synthesized
58−60
18,65−67
via Ullman-type coupling using copper,
while Casson et al.
the relative electron deficiency of the pyrimidyl unit.
utilized a transition-metal-free transformation with refluxing
Using density functional theory (see Theoretical Methods),
we find that the LUMO energy of 2 is 6.8 kcal mol lower than
6
1,62
−1
DMSO and potassium hydroxide.
Aligned with our
B
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Inorganic Chemistry
Article
(
∼14%) and orbitals localized on the carbonyl ligands (∼28%).
Conversely, the LUMOs are largely centered on the redox-
active, NHC-aryl ligand (>80%). Molecular orbital plots are
provided in the Supporting Information, pp S21 and S24.
Electrochemistry. Cyclic voltammetry results for 1 and 2
are presented in Figure 4A and 4B, respectively, along with data
obtained for the reference compounds ReCl(bpy)(CO)3
(
Figure 4C) and MnBr(bpy)(CO) (Figure 4D). All scans
3
were recorded under argon saturation in dry acetonitrile with
.1 M tetrabutylammonium perchlorate (TBAP) supporting
electrolyte. Compounds were loaded at a concentration of 1
0
−1
mM, and the scan rate was varied from 50 to 2000 mV s .
−1
Each figure also includes an inset, recorded at 1000 mV s , of
the voltammetric response when the switching potential is set
before the second reduction. A listing of the first and second
reduction potentials for the aforementioned species is shown in
Table 1.
Figure 3. UV−vis absorption and emission (inset) spectra of 1 (blue)
and 2 (green) recorded at 50 μM in CH CN.
3
For the parent compound, ReCl(bpy)(CO) (Figure 4C),
1
. This energy decrease is aligned with chemical intuition
3
sweeping to negative potentials at 100 mV s− yields two one-
1
regarding the electronegativity of the substituted nitrogen and
may partially explain the red-shifted emission. It also suggests
that the reduction potentials for 2 will occur at more positive
potentials than 1. Lo
for both 1 and 2 the HOMOs are comprised of the Re d
orbitals (∼40%), with contributions from the Cl p orbitals
electron-reduction waves with peak currents at −1.77 and
+
31,33,68
−2.04 V vs Fc/Fc . The first reduction is ligand based,
6
3
̈
wdin population analyses indicate that
with the added electron residing on bpy, furnishing [ReCl-
(bpy)(CO) ] . This process is quasireversible; the corre-
sponding oxidation appears at −1.70 V vs Fc/Fc . Adding an
•
−
3
+
Figure 4. Cyclic voltammograms of (A) ReCl(N-methyl-N′-2-pyridylbenzimidazol-2-ylidine)(CO) (1), (B) ReCl(N-methyl-N′-2-pyrimidylbenzi-
3
midazol-2-ylidine)(CO) (2), (C) ReCl(2,2′-bipyridine)(CO) , and (D) MnBr(2,2′-bipyridine)(CO) under Ar saturation in dry MeCN with 0.1 M
3
3
3
tetrabutylammonium perchlorate (TBAP) supporting electrolyte. Colors indicate scan rate: 50 (red), 100 (orange), 250 (green), 500 (blue), 1000
−1
−1
(
purple), and 2000 mV s (gray). Figure insets show the voltammetric response at 1000 mV s when the switching potential is set before the
second reduction.
C
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Table 1. Comparison of First and Second Reduction
Potentials (vs. Fc/Fc ) Measured in This Work
This tendency yields a much larger current density for dimer
+
a
+
oxidation in both 1 (at −0.36 V vs Fc/Fc ) and MnBr(bpy)-
+
(
(
CO) (at −0.58 V vs Fc/Fc ) compared to ReCl(bpy)(CO)
3
3
complex
first reduction
second reduction
+
at ca. −0.5 V vs Fc/Fc ). Similar to ReCl(bpy)(CO) , 1
3
1
2
−2.06 V
−1.92 V
−1.77 V
−1.63 V
−2.44 V
−2.32 V
−2.04 V
−1.84 V
+
displays a new oxidative feature around −1.6 V vs Fc/Fc at
faster scan rates; again, the inset plot confirms that this wave
corresponds to the oxidation of species formed at the second
electron reduction.
,
bc
ReCl(bpy)(CO)3
,
bc
MnBr(bpy)(CO)3
aRecorded under Ar at 100 mV s−1 in dry MeCN with 0.1 M
For complex 2 (Figure 4B) the two one-electron-reduction
b
tetrabutylammonium perchlorate (TBAP) supporting electrolyte. bpy
+
waves appear at −1.92 and −2.32 V vs Fc/Fc . At fast scan
c
=
2,2′-bipyridine. For original reports, see refs 42 (Re) and 57 (Mn).
rates, the pattern in peak current densities for these waves, the
first larger than the second, is closest to the current response
additional electron leads to dissociation of the axial chloride to
observed for MnBr(bpy)(CO) , where intermediary dimer
3
−
33
give [Re(bpy)(CO) ] . At faster scan rates, two additional
3
formation is the likely cause of current attenuation at the
second reduction. For 2, slow Re−Cl bond dissociation in the
singly reduced species is more likely the limiting component.
Two observations support this assertion: (i) the peak current of
waves appear on the return scan: a prominent feature at ca.
+
+
−
1.6 V vs Fc/Fc and a slight feature at ca. −0.5 V vs Fc/Fc .
The inset scan indicates that neither of these two waves are
present when the switching potential is set before the second
reduction, indicating that they are related to the formation of
the second reduction is only diminished at fast scan rates (at
−1
1
00 mV s the current densities are roughly equal), and (ii)
−
+
[
Re(bpy)(CO) ] . Indeed, the process at ca. −1.6 V vs Fc/Fc
3
adding tetrabutylammonium chloride causes the peak current
−
may be ascribed to one-electron oxidation of [Re(bpy)(CO) ]
−1
3
of the second reduction wave to decrease, even at 100 mV s
•
to the neutral radical, [Re(bpy)(CO) ] . A minimal quantity of
3
(
see SI, pp S29 and S30). For 1, current attenuation is not
the neutral radical undergoes dimerization to form [Re(bpy)-
6
9,70
+
observed since the computed Re−Cl BDE is lower for the
(
CO) ] ;
the slight feature at ca. −0.5 V vs Fc/Fc
3
2
−1
singly reduced species (−2.0 for 1 vs 3.0 kcal mol for 2).
corresponds to oxidation of this dimer.
Indeed, for related Re−bpy systems, installation of electron-
Compound 1 (Figure 4A) displays comparable voltammetric
withdrawing groups on bpy (−CF ) slows the rate of chloride
3
features to ReCl(bpy)(CO) on the negative sweep: at 100 mV
3
72
−
1
+
loss; the relative electron deficiency of the pyrimidyl unit in 2
s
two waves appear at −2.06 and −2.44 V vs Fc/Fc , a 300
may impart the same effect. On the return sweep, three
oxidation waves appear at fast scan rates: ca. −0.2, ca. −0.7 V,
and 400 mV shift to more negative voltages, respectively,
compared to the parent compound. Unlike ReCl(bpy)(CO)3,
the first reduction wave for 1 is not quasireversible, even up to
+
and ca. −1.5 V vs Fc/Fc . The inset plot indicates that only the
−1
feature at ca. −0.7 persists when the switching potential is set
2
000 mV s . This behavior resembles that observed for
before the second reduction.
MnBr(bpy)(CO) (Figure 4D), where the first reduction is
3
71
As noted in the Introduction, ReCl(bpy)(CO) is capable of
metal based and leads to the rapid dissociation of bromide.
3
mediating both photocatalytic and electrocatalytic CO2
Indeed, our computations predict that after one-electron
conversion, that is, the conversion of CO at the potential of
reduction the Re−Cl bond in 1 has a bond dissociation energy
2
−
1
−1
the first and second reduction, respectively. Given the promise
(
BDE) of −2.0 kcal mol , akin to the −1.4 kcal mol BDE for
•−
of recent work in employing silicon substrates for photo-
the Mn−Br bond in [MnBr(bpy)(CO) ] . These values are
3
3
9,73−76
−
1
substantially lower than the 13.1 kcal mol BDE for the Re−Cl
electrocatalysis,
and 2 to convert CO
we were interested in the ability of 1
at the first reduction potential. To that
2
•
−
bond in [ReCl(bpy)(CO) ] .
3
A consequence of preferred metal-based reduction versus
end, we examined the voltammetric response under argon and
ligand-based reduction is the increased proclivity for the five-
CO saturation with added Brønsted acid (0.5 M PhOH).
These results are displayed in Figure 5A and 5B.
2
2
•
coordinate neutral radical, e.g., [M(κ -L)(CO) ] , to dimerize.
3
Figure 5. Cyclic voltammograms of (A) ReCl(N-methyl-N′-2-pyridylbenzimidazol-2-ylidine)(CO)3 (1) and (B) ReCl(N-methyl-N′-2-
−1
pyrimidylbenzimidazol-2-ylidine)(CO) (2) under Ar and CO saturation in MeCN (0.5 M PhOH) at 100 mV s with 0.1 M tetrabutylammonium
3
2
perchlorate (TBAP) supporting electrolyte.
D
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For both species, the addition of phenol under argon shifts
the first reduction potential to more positive voltages: +70 mV
for 1 and +90 mV for 2 (compare orange and black traces).
This behavior suggests the interaction of protons with the one-
amounts of other liquid or gaseous products were observed by
GC or NMR.
Infrared Spectroelectrochemistry. To further character-
ize the structures of 1 and 2 under electrochemical conditions,
infrared spectra were collected at controlled potentials (see
Figure 6A and 6B) using established infrared spectroelec-
77
electron-reduced metal center, indicating that there is not
strict selectivity for CO (vide infra). After purging with CO to
2
2
79,80
saturation, both compounds exhibit a catalytic response. The
trochemistry techniques.
These results are discussed in the
ratio of the peak current between the catalyzed (i ) and the
following paragraphs. Note that “BI-Py” and “BI-Pyrm” are
used as abbreviations for the NHC-aryl ligands in compounds 1
and 2, respectively.
cat
uncatalyzed (i ) processes is 2.3 times for 1 and 1.7 times for 2,
p
noting that the turnover frequency is higher for 1, but ca. 250
mV more electromotive force is required.
For complex 1 (Figure 6A), three infrared transitions appear
On the preparative scale, 1 and 2 display at least 60%+
in the carbonyl region at the resting potential (black spectrum):
−1
−1
Faradaic efficiency for CO production at ca. −2.2 V vs Fc/Fc
2020, 1923, and 1894 cm . These bands are within 10 cm of
34
(see Table 2), which is just negative of the first reduction
the same transitions in ReCl(bpy)(CO)3 and are consistent
with a facial tricarbonyl arrangement. Adjusting the applied
+
Table 2. Results from Controlled Potential Electrolyses at ca.
potential to the first reduction (ca. −2.06 V vs Fc/Fc , red
+
a
−2.2 V vs Fc/Fc with Product Analysis
spectrum) initially yields two sets of carbonyl stretching
−1
frequencies: 2007, 1905, and 1879 cm , which are assigned
complex
Coulombs
FEH2
FECO
FEHCO2H
•
−1
to [Re(BI-Py)(CO) ] , and 1985, 1942, 1888, and 1849 cm ,
3
8
0−83
ReCl(bpy)(CO)3
33 C
32 C
34 C
0%
99%
60%
62%
0%
2%
4%
which are assigned
to [Re(BI-Py)(CO) ] . After holding
3 2
•
1
26%
17%
this potential, the bands ascribed to [Re(BI-Py)(CO)3]
2
diminish as this species is converted to the Re−Re dimer
product (green spectrum). Note that supplanting the applied
a
Compounds were loaded at a concentration of approximately 1 mM
in a ∼45 mL solution of MeCN with 0.1 M tetrabutylammonium
potential with 1.1 equiv of a chemical reductant (KC ) yields
8
hexafluorophosphate (TBAPF ) supporting electrolyte and 0.5 M
6
sharp infrared transitions corresponding to [Re(BI-Py)(CO)3]2
phenol. FE is the Faradaic efficiency. CO and H were quantified by
2
(
(
see SI, p S15). Finally, at the potential of the second reduction
ca. −2.44 V vs Fc/Fc , blue spectrum), the bands assigned to
1
GC; formic acid with H NMR (see Experimental Methods).
+
the dimer disappear concomitant with the appearance of
transitions at 1905 and 1803 cm− for [Re(BI-Py)(CO)
1
] .
−
potentials. Some H production (26% and 17%, respectively) is
3
2
observed along with HCO H, which is consistent with the
This anionic species is also obtained with chemical reduction
(see SI, p S15).
2
7
7,78
interaction of protons with the reduced metal center.
Note
that H is also detected as a product for some Mn(I) NHC
catalysts. Supplemental infrared spectroelectrochemical ex-
The spectra for complex 2 are shown in Figure 6B. At resting
2
6
3
potential (black spectrum), three carbonyl stretching frequen-
−1
periments suggest that H O is a coproduct of CO generation
cies appear at 2022, 1926, and 1899 cm . These bands are
2
within 5 cm− of the same transitions for 1 but shifted to higher
1
(see SI, pp S27 and S28; also see ref 64). No significant
Figure 6. Infrared spectra of (A) ReCl(N-methyl-N′-2-pyridylbenzimidazol-2-ylidine)(CO) (1) and (B) ReCl(N-methyl-N′-2-pyrimidylbenzimi-
3
dazol-2-ylidine)(CO) (2) at controlled potentials under N in dry MeCN with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF )
3
2
6
−1
supporting electrolyte. The x axis is energy (cm ), and the y axis is absorbance.
E
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energies, which is aligned with the decrease in electron density
on the metal center as a result of a more π-acidic NHC-aryl
ligand. At the potential of the first reduction (ca. −1.92 V vs
and recorded in a three-neck, single-compartment cell with Ace #7
joints. A 3 mm diameter glassy carbon disc (BASi) was utilized as the
working electrode. The counter electrode was a platinum wire, and the
reference electrode was a silver/silver chloride (Ag/AgCl) electrode
separated from solution by a Teflon tip. The supporting electrolyte
was composed of MeCN and 0.1 M tetrabutylammonium perchlorate
+
Fc/Fc , red spectrum), one set of bands appears (2002, 1898,
−1
•
and 1865 cm ), which are assigned to [Re(BI-Pyrm)(CO) ] .
3
For complexes 1 and 2 we assign the one-electron-reduced
species (red spectra) to the neutral, five-coordinate radical
rather than the radical anion containing chloride, [ReCl(BI-
(TBAP). Preparation of the electrolyte was as follows. First, HPLC-
grade MeCN was dried over activated 4 Å sieves for 48 h and then
stored under argon. TBAP was synthesized from the dropwise addition
of perchloric acid into an equimolar amount of tetrabutylammonium
bromide in water, then recrystallized three times from ethyl acetate/
hexanes before drying under vacuum. After loading the TBAP into dry
MeCN, the solution was passed over activated neutral alumina under
•−
Pyrm)(CO) ] . This assignment is based on the low BDE for
3
the Re−Cl bond in the reduced complexes, the irreversibility
seen by CV under Faradaic conditions, and the observed CO2
reduction activity at the first potential, which requires Cl loss.
Using 1.1 equiv of chemical reductant instead of an applied
potential furnishes no infrared transitions consistent with a
dimer product (see SI, p S16). Adjusting the potential to the
argon. The electrolyte was purged with either Ar or CO before CVs
2
were recorded and stirred between successive experiments. Voltam-
+
metry was referenced to an internal ferrocene standard (Fc/Fc ), and
compounds were loaded at a concentration of 1 mM.
+
second reduction (ca. −0.32 V vs Fc/Fc , blue spectrum) yields
Bulk Electrolysis. Bulk electrolyses were performed in a custom-
threaded glass jar from Chemglass fitted with a custom PEEK
(polyether ether ketone) top. The top contained ports for venting, the
counter electrode (Pt wire) via a fritted glass insert, the working
electrode (graphite rod), pseudoreference electrode (Ag/AgCl behind
CoralPor), and a septum for sampling the headspace. The system was
sealed with a combination of o-rings, Teflon tape, and electrical tape
and tested for airtightness before each run. For an individual run the
jar was charged with a known amount of catalyst, known amount of
PhOH (0.5 M), stir bar, and an electrolyte solution (0.1 M TBAPF6/
−1
transitions (1993, 1887, 1861 cm ) that are ascribed to
−
[
Re(BI-Pyrm)(CO) ] .
3
CONCLUSIONS
■
The iterative optimization of Re(I) electrocatalysts for CO2
conversion requires the development of ligands that are highly
amenable to synthetic modification. As a proof-of-concept, two
alkyl-NHC-aryl motifs based on benzimidazole coupled to
either pyridine or pyrimidine were utilized as bidentate ligands
within Re(I)−tricarbonyl complexes: ReCl(N-methyl-N′-2-
MeCN) before being sparged to saturation with CO . To prevent
2
polymerization of PhOH on the counter electrode, 0.1 M Fc was
added to the electrolyte solution as a sacrificial oxidant. After a run was
completed, the headspace was sampled via airtight syringe and
characterized by GC; Faradaic efficiencies were determined using a
calibration curve. A turnover for this system is based on two electron
equivalents being passed for every catalyst molecule in solution; for
every mole of catalyst two electrons are passed to achieve one
pyridylbenzimidazol-2-ylidine)(CO) (1) and ReCl(N-methyl-
3
N′-2-pyrimidylbenzimidazol-2-ylidine)(CO) (2). The in-
3
creased π-acidity of the pyrimidyl unit in 2 compared to the
pyridyl moiety in 1 yields a lower LUMO energy, inducing a
red-shifted emission (+55 nm) and modulating the first and
second reduction potentials to more positive voltages (by 140
and 120 mV, respectively).
1
turnover. Formic acid was detected by H NMR. Specifically, 5 mL of
the reactant solution was removed after electrolysis and combined with
The cyclic voltammogram of 1 shows similar features to
ReCl(bpy)(CO)3 when scanning to negative potentials,
namely, two sharp one-electron-reduction waves spaced by
hundreds of millivolts. Comparable features to MnBr(bpy)-
1 mL of D O before adding DCM to yield a biphasic solution (total
volume approximately 25 mL) that was examined with H NMR (400
MHz Varian; 128 scans). Formic acid standards were established by
2
1
preparing MeCN solutions (5 mL; 0.1 M TBAPF ) with known
6
concentrations, followed by the aforementioned workup. A linear fit of
integrated formic acid resonances was used to determine the
concentration in unknown samples.
(
CO)3 are observed on the return sweep, including a
substantial current density for dimer oxidation. For complex
, scanning to negative potentials yields two irreversible one-
2
+
Preparation of [N-Methyl-N′-2-pyridylbenzimidazolium]
electron reduction waves, the first displaying a significantly
−
PF . Under an atmosphere of argon, a flask was charged with
6
higher current density than the second, which resembles the
benzimidazole (10.00 g, 84.6 mmol), potassium carbonate (7.79 g,
56.4 mmol), and 2-bromopyridine (2.74 mL, 28.6 mmol). The
reaction mixture was stirred at 200 °C for 18 h and then allowed to
cool to room temperature. The resulting solution was diluted with
water (200 mL) and extracted with DCM (3 × 200 mL). The
combined organic extracts were washed with saturated Na CO (3 ×
response of MnBr(bpy)(CO) , although no metal−metal dimer
3
formation is observed. On the preparative scale, compounds 1
and 2 are capable of mediating CO reduction to CO after
2
loading only one electron (Faradaic efficiency > 60%). Minor
2
3
H (ca. 20%) and HCO H (<5%) production is also observed.
2
2
2
00 mL) and brine (200 mL) and then dried over anhydrous MgSO4
Future work will consider ligand modifications to tune the
product specificity.
before filtration to remove the drying agent. Concentration under
reduced pressure using a rotary evaporator gave N,N′-2-pyridylbenzi-
midazole as a light red oil (5.45 g, 96%). This intermediate (2.70 g,
EXPERIMENTAL METHODS
■
13.8 mmol) was then dissolved in 100 mL of CH CN under an argon
3
General. All reagents were obtained from commercial suppliers and
used as received unless otherwise noted. Reactions were performed
using standard Schlenk-line techniques under an atmosphere of argon.
All synthetic manipulations were performed under red light to
minimize photodecomposition. Toluene was dried prior to use by
atmosphere. Iodomethane (0.95 mL, 15.2 mmol) was added, and the
reaction mixture was stirred at reflux overnight (11 h). After cooling to
room temperature, the mixture was concentrated under reduced
pressure using a rotary evaporator to afford a yellow solid. Subsequent
recrystallization from CH CN/Et O gave a white solid, [N-methyl,N′-
3 2
1
+ −
distilling it from Na/benzophenone. H NMR spectra were recorded
2-pyridylbenzimidazolium] I , that was collected by vacuum filtration
(4.30 g, 92%). Finally, the iodo salt (1.00 g, 3.0 mmol) was dissolved
in 50 mL of water and stirred for 5 min before adding ammonium
hexafluorophosphate (0.58 g, 3.6 mmol). The reaction mixture was
stirred at room temperature for 30 min (a precipitate forms
immediately). The desired product, [N-methyl-N′-2-pyridylbenzimi-
on a Varian Mercury Plus 400 MHz spectrometer, and 13C NMR
spectra were recorded on an Agilent DD2 600 MHz spectrometer.
NMR chemical shifts were referenced to the residual protio signal of
the deuterated solvent. FTIR spectra were recorded on a
ThermoNicolet 6700 spectrophotometer running OMNIC software.
Cyclic Voltammetry. Cyclic voltammetry experiments were
performed using a CH Instruments 601E potentiostat (Austin, TX)
+
−
dazolium] PF , was collected as a white precipitate via vacuum
6
1
filtration and dried in vacuo (1.05 g, 99%). H NMR (DMSO-d , 20
6
F
DOI: 10.1021/acs.inorgchem.6b00079
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
8
8
°
C): δ 10.47 (1H, s), 8.79 (1H, d, J = 4 Hz), 8.49−8.47 (1H, m),
with Br and Cl (LANL08D). These basis sets include an effective
89
8
.30−8.28 (1H, m), 8.16−8.14 (1H, m), 8.03 (1H, d, J = 8 Hz), 7.82−
.77 (2H, m), 7.74−7.71 (1H, m), 4.20 (3H, s).
core potential to describe electrons close to the nucleus. Conversely,
the “light” atoms (H, C, N, and O) were described with the def2-
TZVP basis set. All computations included implicit solvent using the
Conductor Like Screening Model (COSMO) with the default values
7
+
90
Preparation of [N-Methyl-N′-2-pyrimidylbenzimidazolium]
−
91
PF . Under an atmosphere of argon, a flask was charged with
6
benzimidazole (5.00 g, 42.3 mmol), potassium carbonate (3.90 g, 28.2
mmol), and 2-chloropyrimidine (1.62 g, 14.1 mmol). The reaction
mixture was stirred at 200 °C for 18 h and then allowed to cool to
room temperature. The resulting solution was diluted with water (200
mL) and extracted with DCM (3 × 200 mL). The combined organic
extracts were washed with saturated Na CO (3 × 200 mL) and brine
for acetonitrile (ϵ = 36.6). Optimized structures were confirmed with
vibrational analysis, and Lowdin population analysis was employed
̈
to understand the nature of the optimized molecular orbitals.
9
2
ASSOCIATED CONTENT
Supporting Information
■
2
3
*
S
(
200 mL) and then dried over anhydrous MgSO before filtration to
4
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00079.
remove the drying agent. Concentration under reduced pressure using
a rotary evaporator gave N,N′-2-pyrimidylbenzimidazole as an off-
white solid (2.00 g, 72%). This intermediate (1.00 g, 5.1 mmol) was
then dissolved in 50 mL of CH CN under an argon atmosphere.
3
ReCl(N-methyl-N′-2-pyridylbenzimidazol-2-ylidine)-
Iodomethane (1.59 mL, 25.5 mmol) was added, and the reaction
mixture was stirred at reflux overnight (11 h). After cooling to room
temperature, the mixture was concentrated under reduced pressure
(
CO) (CIF)
3
ReCl(N-methyl-N′-2-pyrimidylbenzimidazol-2-ylidine)-
using a rotary evaporator. Subsequent recrystallization from CH CN/
(CO) (CIF)
3
3
Et O gave a pale-yellow solid, [N-methyl,N′-2-pyrimidylbenzimidazo-
Selected spectra, X-ray crystallography data, data from
chemical reductions, data from computations, data from
controlled-potential infrared spectroelectrochemistry,
and data from titrations with tetrabutylammonium
chloride (TBACl) (PDF)
2
+
−
lium] I , that was collected by vacuum filtration (1.60 g, 93%).
Finally, the iodo salt (1.50 g, 4.4 mmol) was dissolved in 100 mL of a
1
:1 H O/MeOH solution and stirred for 5 min before adding
2
ammonium hexafluorophosphate (0.87 g, 5.3 mmol). The reaction
mixture was stirred at room temperature for 30 min (a precipitate
forms immediately). The desired product, [N-methyl-N′-2-pyrimidyl-
+
−
benzimidazolium] PF , was collected as a white precipitate via
AUTHOR INFORMATION
Corresponding Author
E-mail: jagarwal@uga.edu.
6
■
1
vacuum filtration and dried in vacuo (1.35 g, 85%). H NMR (DMSO-
d , 20 °C): δ 10.72 (1H, s), 9.15 (2H, d, J = 4 Hz), 8.85−8.83 (1H,
6
*
m), 8.15−8.13 (1H, m), 7.86−7.78 (3H, m), 4.24 (3H, s).
Preparation of ReCl(N-methyl-N′-2-pyridylbenzimidazol-2-
Notes
+
−
ylidine)(CO)3 (1). [N-methyl-N′-2-pyridylbenzimidazolium] PF
The authors declare no competing financial interest.
6
(
0.40 g, 1.1 mmol), ReCl(CO) (0.51 g, 1.4 mmol) and potassium
5
carbonate (0.47 g, 3.4 mmol) were suspended in 25 mL of toluene and
sparged with argon for 30 min. The reaction mixture was then heated
to reflux under argon for 24 h. After cooling to room temperature,
ACKNOWLEDGMENTS
■
Research at the University of Georgia was supported by the
National Science Foundation (NSF) under grant CHE-
1361178. G.F.M. acknowledges the Donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research (Grant 54228-ND1). Work performed at the
University of New Hampshire was supported by the NSF under
grant CHE-1352437. C.W.M. and C.P.K. acknowledge support
for this work from the AFOSR through a Basic Research
Initiative (BRI) grant (FA9550-12-1-0414). We thank Dr.
Pingrong Wei for acquisition and refinement of X-ray
structures.
Et O (50 mL), hexanes (50 mL), and water (25 mL) were added
2
before stirring for 30 min to afford a pale yellow precipitate, the
product, which was isolated by vacuum filtration, washed with Et O (3
2
−1
×
25 mL), and dried in vacuo (0.52 g, 89%). IR νCO (ATR, cm ):
1
2
014 (s), 1883 (s). H NMR (DMSO-d , 20 °C): δ 8.97 (1H, d, J = 4
6
Hz), 8.64 (1H, d, J = 8 Hz), 8.48−8.46 (1H, m), 8.38−8.34 (1H, m),
1
3
7
(
1
3
.95−7.92 (1H, m), 7.65−7.54 (3H, m), 4.20 (3H, s). C NMR
DMSO-d , 25 °C): δ 201.46, 198.78, 197.88, 188.74, 153.92, 153.22,
42.60, 135.32, 130.32, 125.44, 125.41, 123.71, 113.98, 113.10, 112.74,
4.95.
6
Preparation of ReCl(N-methyl-N′-2-pyrimidylbenzimidazol-
+
2
-ylidine)(CO) (2). [N-methyl-N′-2-pyrimidylbenzimidazolium]
3
−
PF6 (0.40 g, 1.1 mmol), ReCl(CO)5 (0.51 g, 1.4 mmol), and
potassium carbonate (0.47 g, 3.4 mmol) were suspended in 25 mL of
toluene and sparged with argon for 30 min. The reaction mixture was
then heated to reflux under argon for 24 h. After cooling to room
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NOTE ADDED AFTER ASAP PUBLICATION
■
This paper was published on the Web on March 7, 2016.
Additional minor text corrections were implemented, and the
paper was reposted on March 8, 2016.
I
DOI: 10.1021/acs.inorgchem.6b00079
Inorg. Chem. XXXX, XXX, XXX−XXX