Positional effects of second-sphere amide pendants on electrochemical CO2 reduction catalyzed by iron porphyrins

Article abstract of DOI:10.1039/c7sc04682k

The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach to transforming this greenhouse gas into value-added carbon products with sustainable energy input. Inspired by natural bioinorganic systems that fe

Full text of DOI:10.1039/c7sc04682k

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Positional effects of second-sphere amide
pendants on electrochemical CO reduction
2
Cite this: DOI: 10.1039/c7sc04682k
catalyzed by iron porphyrins†
ab
ab
a
ab
Eva M. Nichols,‡ Jeffrey S. Derrick,‡ Sepand K. Nistanaki, Peter T. Smith
abcd
and Christopher J. Chang
*
The development of catalysts for electrochemical reduction of carbon dioxide offers an attractive approach
to transforming this greenhouse gas into value-added carbon products with sustainable energy input.
Inspired by natural bioinorganic systems that feature precisely positioned hydrogen-bond donors in the
secondary coordination sphere to direct chemical transformations occurring at redox-active metal
centers, we now report the design, synthesis, and characterization of
a
series of iron
tetraphenylporphyrin (Fe-TPP) derivatives bearing amide pendants at various positions at the periphery of
the metal core. Proper positioning of the amide pendants greatly affects the electrocatalytic activity for
carbon dioxide reduction to carbon monoxide. In particular, derivatives bearing proximal and distal
amide pendants on the ortho position of the phenyl ring exhibit significantly larger turnover frequencies
(TOF) compared to the analogous para-functionalized amide isomers or unfunctionalized Fe-TPP.
Analysis of TOF as a function of catalyst standard reduction potential enables first-sphere electronic
effects to be disentangled from second-sphere through-space interactions, suggesting that the ortho-
2
functionalized porphyrins can utilize the latter second-sphere property to promote CO reduction.
Received 30th October 2017
Accepted 14th February 2018
Indeed, the distally-functionalized ortho-amide isomer shows a significantly larger through-space
interaction than its proximal ortho-amide analogue. These data establish that proper positioning of
secondary coordination sphere groups is an effective design element for breaking electronic scaling
DOI: 10.1039/c7sc04682k
rsc.li/chemical-science
2
relationships that are often observed in electrochemical CO reduction.
background reduction of protons to hydrogen that can occur at
comparable potentials. Indeed, despite broad interest in elec-
Introduction
Environmental challenges associated with rising atmospheric trochemical CO
CO concentrations and the promise of CO
as a cheap and focused largely on a limited number of primary coordination
abundant C1 feedstock motivate the research into technologies sphere motifs, including metal complexes with porphyrins
2
reduction, molecular catalysis efforts have
2
2
4
–8
1
–3
9–15
16–23
24–26
to convert CO
2
into value-added chemical products. Electro- and aza-macrocycles,
bipyridine,
phosphine,
and
27–29
chemical reduction of CO
2
offers an attractive approach to meet carbene
ligands. In contrast, biological systems for CO
2
this goal using sustainable energy input, but the kinetic and redox catalysis, such as the carbon monoxide dehydrogenase
thermodynamic barriers to CO activation provide signicant (CODH) enzymes that catalyze the reversible interconversion of
2
chemical challenges, necessitating the development of catalysts CO
that can promote CO
2
and CO, modulate activity using precisely positioned
reduction selectively over the competing hydrogen-bond and proton-relay groups in the second coordi-
2
30
nation sphere. For example, second-sphere histidine and
lysine residues in Ni–Fe CODH are positioned to stabilize a CO -
metal adduct and assist in C–O bond cleavage (Scheme 1).
2
1–33
a
Department of Chemistry, University of California, Berkeley, CA 94720, USA. E-mail:
chrischang@berkeley.edu
3
b
The aforementioned bioinorganic systems provide inspira-
tion for the design of metal complexes bearing second-sphere
Department of Molecular and Cell Biology, University of California, Berkeley, CA groups to aid with activation or transformation of small mole-
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA
9
c
4720, USA
9
4720, USA
cule substrates. Notable advances in utilizing intramolecular
hydrogen-bond donors to promote oxygen binding/activation
include picket fence-, picnic basket-, and hangman porphy-
d
Howard Hughes Medical Institute, University of California, Berkeley, CA 94720, USA
†
Electronic supplementary information (ESI) available: Procedures for synthetic,
spectroscopic, and electrochemical experiments. CCDC 1582750. For ESI and
crystallographic data in CIF or other electronic format see DOI:
34–39
rins by Collman, Reed, C. K. Chang, and Nocera,
dicopper
40,41
42–45
complexes by Masuda,
and tripodal systems by Borovik.
1
0.1039/c7sc04682k
Second-sphere donors have also been shown to play important
‡
These authors contributed equally to this work.
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Scheme 1 The active site of Ni–Fe CO dehydrogenase, showing activated CO
2
in red and the nearby network of hydrogen bond donor amino
acid residues in blue (left), inspires the design of ortho-amide-functionalized Fe porphyrins examined in this work (right).
4
6,47
roles in structural mimics of alcohol dehydrogenase,
as well effects of an electron-decient aryl amide additive on CO
2
48,49
as in complexes for stoichiometric reduction of nitrite,
nitrate, and nitrogen. Furthermore, second-sphere pendants uoromethyl)phenyl]amide to Fe-TPP in dimethylformamide
have been incorporated into electrocatalyst scaffolds to facilitate (DMF) reveals a dose-dependent increase in catalytic activity
and O2 reduc- (Fig. 1), with the additive acting as a proton source as well as
and to stabilize adducts or promote C–O bond cleavage a potential hydrogen bond donor/acceptor. Foot-of-the-wave
With specic regard to CO reduction, analysis (FOWA), as described by Sav ´e ant, Costentin, Robert
elegant work on iron tetraphenylporphyrins by Sav ´e ant, Cost- and co-workers, can be used to extract kinetic information
entin, and Robert has explored the use of phenol-based from cyclic voltammetric data, especially in cases where
reduction catalyzed by Fe-TPP. Titration of 3,5-[bis(tri-
50
51
52–54
protonation steps relevant to H evolution
2
55,56
tion,
in CO reduction.
11,12,57–59
2
2
61
5
7,60
pendants for enhancing electrochemical CO
2
reduction.
a plateau current is not observed due to substrate consumption
Against this backdrop, we targeted the study of functionalized or catalyst inhibition. Using this approach, the rate of CO
2
porphyrin complexes that could yield insight into questions of reduction was examined as a function of amide concentration
optimal placement of second-sphere pendants on a conserved under pseudo-rst order conditions of excess CO , and the rate
2

rst-sphere metal core. In this report, we describe the synthesis was found to exhibit a rst-order dependence on the amide
and electrochemical study of positional isomers of iron additive (Fig. S1†). Cyclic voltammograms under a nitrogen
porphyrin CO reduction catalysts bearing pendant amides in the atmosphere show a negligible change between 50 equivalents of
2
secondary coordination sphere. Intermolecular addition of amide additive and Fe-TPP alone, indicating that background
a parent iron tetraphenylporphyrin catalyst, Fe-TPP, with an aryl processes such as proton reduction are likely to be minimal
amide additive results in a dose-dependent increase in CO
2
with this additive (Fig. S2†). With this pilot result in hand, we
reduction activity. Building upon this result, we designed and sought to explore the effects of introducing intramolecular
evaluated a series of positional isomers bearing intramolecular amide substituents onto the porphyrin scaffold in a spatially
amide pendants in the ortho or para positions of the meso aryl well-dened fashion.
ring, both proximal and distal to the porphyrin plane. Compar-
ison of the catalytic activities of these isomers establishes that
ortho orientation of the second-sphere groups is critical for an
enhancement in catalysis. Correlation between the turnover
I/0
frequency (TOF) and standard potential of the formal Fe couple
indicates that through-space interactions play
a role in
enhancing the rate of CO2 reduction for both ortho amide-
functionalized catalysts, with the distal donor exhibiting
a more signicant through-space component. In contrast, such
effects are largely absent in the para-functionalized congeners.
These data provide a starting point for more sophisticated design
of second-sphere functionalities as bioinspired design elements
2
for redox catalysis of CO and other small-molecule substrates.
Results and discussion
2
Bis(aryl)amide additive promotes CO reduction with Fe
porphyrin
Fig. 1 Titration of 3,5-[bis(trifluoromethyl)phenyl]amide to Fe-TPP
under CO showing current increases with increasing concentrations
secondary coordination sphere pendants, we examined the of amide. Conditions: 0.1 M TBAPF in DMF, 1 mM Fe-TPP.
As a starting point to explore the use of amides as functional
2
6
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Synthesis of covalently-modied porphyrins bearing
intramolecular amide groups
in interrogating the effect of distal group positioning given
a previous report showing that proximal second-sphere groups
are located too far away from the metal center and are less
effective at promoting oxygen binding at iron. The corre-
sponding para-1-amide and para-2-amide porphyrins were
targeted to serve as control compounds, where the amide NH
group at the para position of the meso aryl rings is not expected
to interact productively with CO or CO -derived intermediates.
2 2
The synthesis of all four functionalized porphyrin ligands is
shown in Scheme 2.
As only amide groups located at the ortho position of the
porphyrin meso aryl rings are expected to engage in intra-
molecular proton donor- or hydrogen bonding interactions with
36
2
CO -derived intermediates bound at the iron center, we envi-
sioned two positional isomers: ortho-1-amide, bearing the NH
proximal to the porphyrin ring, and ortho-2-amide, bearing the
NH distal to the porphyrin ring. Most porphyrin-based catalysts
bearing second-sphere groups, especially those evaluated for
Condensation of 2-nitrobenzaldehyde and benzaldehyde
with pyrrole results in the mono-(2-nitrophenyl)porphyrin
starting material 1, which can be subsequently reduced to the
2
CO reduction, feature these functionalities in the proximal
location for reasons of synthetic practicality. We were interested
Scheme 2 Synthesis of ortho- and para-functionalized tetraphenylporphyrin ligands ortho-1-amide, ortho-2-amide, para-1-amide, and para-
-amide.
2
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mono-(2-aminophenyl)porphyrin synthon 2. Reaction with 3,5- pentacoordinate Zn porphyrins, the Zn atom is displaced from
bis(triuoromethyl)phenylacyl chloride affords the proximal the porphyrin plane. As desired, the amide is oriented such that
target ortho-1-amide. The distal ortho congener was prepared by it may engage CO directly or act as a proton relay or hydrogen
2
condensation of 2-formylphenylacetic acid ethyl ester 4 (ob- bond donor with CO -bound intermediates during catalysis. We
2
tained in two steps from 2-indanone) and benzaldehyde with 5- have been unable to obtain X-ray quality crystals of Zn–ortho-2-
phenyldipyrromethane to afford porphyrin 5. Subsequent amide, perhaps due to enhanced exibility of the amide group
hydrolysis, followed by in situ generation of the acyl chloride caused by the presence of the methylene spacer.
and reaction with 3,5-bis(triuoromethyl)aniline affords ortho-
2
-amide. Porphyrin para-1-amide is obtained from mono-(4-
Cyclic voltammetry studies in dimethylformamide
aminophenyl)porphyrin 8, which is in turn prepared by regio-
selective para-nitration of tetraphenylporphyrin and subse- With these positional isomers in hand (Scheme 3), we next
quent reduction. A precursor to para-2-amide was accessed by evaluated their electrochemical characteristics under an inert
condensation of commercially available 4-formylphenylacetic atmosphere and under CO
acid methyl ester and benzaldehyde with 5-phenyldipyrro- functionalized iron porphyrins and parent Fe-TPP measured in
methane to afford porphyrin 9. Hydrolysis, followed by in situ DMF under argon atmosphere (0.1 M TBAPF , 1 mM catalyst)
generation of the acyl chloride and reaction with 3,5-bis(tri- show three distinct redox events corresponding to formal Fe
. Cyclic voltammograms (CVs) of the
2
6
III/II
,
II/I
I/0
I/0

uoromethyl)aniline, gave the desired ligand.
The iron complexes of the aforementioned porphyrin observed at a scan rate of 100 mV s under these conditions.
ligands were obtained by metalation with FeBr in anhydrous Scan rate-dependence studies show a linear correlation between
Fe , and Fe couples (Fig. S3†). Reversible Fe couples are
ꢀ
1
2
1
/2
tetrahydrofuran in the presence of 2,6-lutidine as a base. All peak current and (scan rate) , indicating that all the complexes
porphyrin ligands were characterized by H-, C-, and F-NMR are freely diffusing under non-catalytic conditions (Fig. S4†).
spectroscopy, and freebase and metalloporphyrins were addi- The Fe standard potentials, Ecat, are given in Table 1 for each
tionally characterized by UV-visible spectroscopy and ESI mass porphyrin complex. The Ecat potentials trend with the electron-
1
13
19
I/0
0
0
spectrometry (see ESI† for details).
withdrawing nature of the substituent, with ortho substituents
exerting a larger electronic effect than para substituents: the
proximal ortho amide is inductively electron-withdrawing, fol-
lowed by the proximal para amide and unsubstituted TPP. The
distal amides are rendered electron-donating by the methylene
substituent, with the ortho positional isomer being more
electron-rich than the para-substituted analogue.
Structural characterization of Zn–ortho-1-amide
We sought structural characterization of the amide-appended
porphyrins to conrm the expected connectivity. Owing to
a lack of success in obtaining suitable quality single crystals for
structural characterization of the iron complexes, the corre-
sponding zinc complexes were targeted. Porphyrin Zn–ortho-1-
amide was prepared by metalation of the ligand with Zn(OAc)2.
Crystals suitable for X-ray diffraction were grown by vapor
diffusion of water into a concentrated DMF solution. The solid-
state structure is shown in Fig. 2, where a DMF molecule
occupies the h axial coordination site. As is typical for
Under 1 atmosphere of CO
an acid source, catalytic responses indicative of CO
2
and in the presence of phenol as
reduction
2
are observed by cyclic voltammetry for all catalysts examined. In
the presence of 100 mM phenol, Fe–ortho-1-amide exhibits
a slightly more positive catalytic onset than Fe–ortho-2-amide,
Fig. 2 Solid-state structure of Zn–ortho-1-amide. Non-coordinated
solvent molecules and non-amide hydrogen atoms have been omitted Scheme 3 Positional isomers of amide-functionalized iron tetraphe-
for clarity. Thermal ellipsoids are shown at the 50% level.
nylporphyrins examined in this work.
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Table 1 Summary of electrochemical properties of iron porphyrin catalysts
0
a
+
b
ꢀ1
c
ꢀ2 ꢀ1
d
Catalyst
E
cat (V vs. Fc/Fc )
TOFmax (s
)
log(TOFmax
)
k
cat (M
s
)
FECO (%)
4
6
Fe–ortho-1-amide
Fe–ortho-2-amide
Fe–para-1-amide
Fe–para-2-amide
Fe-TPP
ꢀ2.12
ꢀ2.18
ꢀ2.15
ꢀ2.16
ꢀ2.15
ꢀ2.15
2.24 ꢂ 10
4.35
6.74
2.23
3.84
2.83
3.61
1.01 ꢂ 10
3.33 ꢂ 10
1.03 ꢂ 10
4.20 ꢂ 10
2.13 ꢂ 10
3.18 ꢂ 10
83 ꢃ 3
92 ꢃ 2
74 ꢃ 8
79 ꢃ 8
90 ꢃ 6
—
6
8
4
5
5
5
5.50 ꢂ 10
2
1.70 ꢂ 10
3
6.76 ꢂ 10
2
6.76 ꢂ 10
3
Fe-TPP + 50 mM amide
4.04 ꢂ 10
a
I/0
0
b
The standard reduction potential for the formal Fe couple, Ecat, is reported as an average over three independent experiments. TOFmax values
are reported in presence of 100 mM PhOH and 0.23 M CO
2
except for the last entry, which is in presence of 50 mM amide additive and 0.23 M CO
2
.
c
k
cat values are reported as an average over three sets of experimental conditions (different [PhOH], 0.23 M CO , in the regime where rate is linearly
2
d
dependent on [PhOH]). FE for CO is reported as an average over three CPE experiments, see ESI for details.
but both show signicantly higher catalytic responses than the non-linearity at higher phenol concentrations, possibly as
corresponding para-functionalized positional congeners or a result of catalyst inhibition or local depletion of CO . Fe–
2
unfunctionalized Fe-TPP (Fig. 3). In the presence of low phenol ortho-1-amide exhibits the next highest set of kobs values, fol-
concentrations (5 mM), Fe–ortho-2-amide shows a signicantly lowed by Fe–para-2-amide and nally Fe–para-1-amide.
larger peak current compared to all other catalysts and Maximum turnover frequencies (TOFmax) in the presence of
a comparable onset potential to that of Fe–ortho-1-amide 100 mM phenol are summarized in Table 1. Likewise, all four
(Fig. S5†). In contrast, at higher phenol concentrations of catalysts show rst-order dependence on CO
2
concentration in
250 mM, Fe–ortho-1-amide displays a larger peak current and the presence of excess (500 mM) phenol (Fig. S20†), as well as on
a more anodic onset potential than Fe–ortho-2-amide (Fig. S5†). catalyst concentration (Fig. S21†). Taken together, in the linear
These ndings suggest that the role of phenol may differ among regimes of [PhOH] and [CO ] where secondary phenomena are
2
the positional isomers, or that altered mechanisms of catalyst minimal, all four catalysts follow the rate law
inactivation are at play.
To gain further insight into CO
series of amide-functionalized porphyrins, the observed rate
ꢀ
d½CO
dt
2
ꢁ
¼ kobs½catꢁ ¼ kcat½PhOHꢁ½CO
2
ꢁ½catꢁ
2
reduction catalyzed by this
ꢀ
2
ꢀ1
ꢀ
1
where kcat (M
s ) is the intrinsic third-order rate constant.
constants (kobs ¼ TOFmax, s ) determined by FOWA were
Average values for kcat, determined at several phenol concen-
trations in the linear regime, are reported in Table 1. TOFmax
and kcat values for unfunctionalized Fe-TPP in the presence of
0 mM amide additive are also shown for comparison,
demonstrating that ortho pendant amide groups are necessary
examined as a function of phenol and CO concentration.
2
Under pseudo-rst order conditions, all four functionalized
catalysts display rst-order dependence on phenol concentra-
tion (Fig. 4, S6–13†). Fe–ortho-2-amide achieves the largest
observed rate constants (kobs) of all catalysts tested, but exhibits
5
to observe large enhancements in activity.
ꢀ
1
Fig. 3 Cyclic voltammograms of amide-functionalized porphyrins Fig. 4 Observed rate constants (s ) for catalytic CO
and unfunctionalized Fe-TPP under CO atmosphere. Conditions: a function of PhOH concentration for Fe–ortho-1-amide (blue dia-
mM catalyst, 100 mM phenol, 0.1 M TBAPF in DMF, saturated with monds), Fe–ortho-2-amide (red squares), Fe–para-1-amide (purple
(0.23 M); scan rate 100 mV s . i represents current under catalytic upward triangles), and Fe–para-2-amide (orange downward triangles).
2
reduction as
2
1
CO
6
ꢀ1
2
0
II/I
conditions; i
p
represents the cathodic peak height of the formal Fe
Conditions: 1 mM catalyst, 0.1 M TBAPF
is 100 mV s . kobs values were determined by FOWA.
6
2
in DMF; 0.23 M CO ; scan rate
ꢀ1
couple under inert atmosphere.
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For the purposes of catalyst benchmarking, it is illustrative relationship occurs when second-sphere interactions either
to examine the log(TOF) vs. overpotential (h) relationships as promote or inhibit catalysis, as has been previously docu-
61
64
indicated by a catalytic Tafel plot. More effective catalysts mented for electrostatic effects.
operate with higher TOFs at lower overpotentials and are dis- In order to benchmark the electronic scaling relationship,
played in the upper le portion of such plots. As shown in Fig. 5, Fe-TPP and two additional iron porphyrins with no second-
Fe–ortho-2-amide exhibits higher TOFs over all overpotential sphere inuences were investigated: Fe–para-(CF , the iron
values compared to Fe–ortho-1-amide, which in turn exhibits complex of meso-tetra(4-triuoromethylphenyl)porphyrin, and
higher TOFs compared to Fe-TPP and both para-substituted Fe–para-(OMe) the iron complex of meso-tetra(4-
3 4
)
4
,
porphyrins. The values at the plateau of such curves represent methoxyphenyl)porphyrin. The synthesis and characterization
log(TOFmax), the maximum turnover frequency achievable at of these two additional complexes are provided in the ESI.†
large overpotential (when the catalyst is fully present in its Cyclic voltammograms of Fe–para-(CF ) and Fe–para-(OMe)
3
4
4
reduced state).
The selectivity and stability of the aforementioned catalysts responses indicative of CO
were examined by controlled potential electrolysis (CPE). A was applied to determine TOFmax values under conditions
solution of catalyst in CO -saturated DMF electrolyte (0.1 M identical to those given above for the amide-functionalized
TBAPF , 0.5 mM catalyst, 0.5 M phenol as acid source) was porphyrins (Fig. S15, S19†). Controlled potential electrolysis
in the presence of phenol and CO2 show characteristic
2
reduction (Fig. S14, S18†). FOWA
2
6
+
electrolyzed at potentials between ꢀ2.1 and ꢀ2.2 V vs. Fc/Fc in experiments (Fig. S26†) show that CO is also a major product for
a homemade gas-tight electrolysis cell (see ESI† for details). Gas these two catalysts, with FECO of 70–74% (Table S2†). The
chromatographic measurements of the headspace reveal that similar product selectivity permits meaningful comparisons
all four amide-functionalized catalysts have high selectivity for with the amide-functionalized complexes. The electrochemical
CO, with faradaic efficiencies (FE) between 74% and 92% parameters for Fe–para-(CF
Tables 1, S2†). No hydrogen was observed for any catalyst under summarized in Table S1.† Together with Fe-TPP, these two
these conditions. Representative CPE traces are provided in catalysts exhibit the expected linear relationship between
3
)
4
and Fe–para-(OMe)
4
are
(
0
Fig. S25.†
Though the previous metrics indicate that Fe–ortho-2-amide
log(TOFmax) and Ecat (circles, Fig. 6).
To examine the contribution of through-space interactions
is a superior catalyst compared to Fe–ortho-1-amide, it is on catalysis by the ortho- and para-amide-substituted porphy-
0
important to stress that the difference in E values between rins, each complex was added to the plot of log(TOFmax) vs.
cat
0
cat
catalysts must be considered when attempting to resolve
E
shown in Fig. 6. Both Fe–ortho-1-amide and Fe–ortho-2-
questions of optimal second-sphere pendant placement. As amide lie signicantly above the line corresponding to
62,63
illustrated previously,
substituents on the porphyrin aryl rings modulate Ecat and tions enhance the rate of CO
consequently alter the driving force for electrochemical CO ortho-2-amide shows a much larger departure from the elec-
electron withdrawing- or donating electronic-only effects, conrming that second-sphere interac-
0
2
reduction for both complexes. Fe–
2
reduction. In such cases, a linear scaling relationship between tronic correlation line compared to Fe–ortho-1-amide,
0
log(TOFmax) and E is observed, where catalysts having more
cat
0
cat
negative E values exhibit larger TOFs. Deviation from this
0
Fig. 6 Correlation between log(TOFmax) and Ecat illustrating the
through-space interactions that promote catalysis in the case of Fe–
ortho-1-amide (blue diamond) and Fe–ortho-2-amide (red square).
Fig. 5 Catalytic Tafel plots for Fe–ortho-1-amide (blue diamond), Fe– Porphyrin catalysts with no through-space interactions are shown in
ortho-2-amide (red square), Fe–para-1-amide (purple upward black: Fe–para-(CF
triangle), Fe–para-2-amide (orange downward triangle), and Fe-TPP para-(OMe) (gray circle). Fe–para-2-amide (orange downward
black circle). Conditions: 0.1 M TBAPF in DMF; 0.23 M CO ; 100 mM triangle) and Fe–para-1-amide (purple upward triangle) show minimal
3 4
) (white circle), Fe-TPP (black circle), and Fe–
4
(
6
2
ꢀ1
PhOH; scan rate is 100 mV s . TOF ¼ kobs and was determined by through-space effects. TOFmax determined by FOWA in the presence
FOWA.
2
of 100 mM phenol and 0.23 M CO .
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suggesting that the extent of through-space stabilization of the
These results conrm, unsurprisingly, that second-sphere
iron-bound CO adduct is larger for the distal ortho isomer than donors must be located at the ortho-positions of the porphyrin
2
for the proximal congener. In contrast, the log(TOFmax) for Fe– scaffold to exhibit productive interactions with catalytically-
para-2-amide is very slightly higher, and that for Fe–para-1- relevant intermediates. More interestingly, we show that
amide is very slightly lower, than that predicted by the elec- a distally-located second-sphere amide group is more effective
tronic scaling relationship, suggesting that the para-substituted at breaking the electronic scaling relationship for CO
positional isomers do not exhibit notable through-space effects. tion than the proximal positional isomer due to enhanced
In order to gain further insight into the role of the ortho hydrogen bond stabilization of the Fe–CO intermediate.
amide pendants in promoting catalysis, we determined the Differences in pK of the four amide pendants are minimal,
2
reduc-
2
a
equilibrium constants for CO2 binding, KCO2, based on the indicating that their position with respect to the iron-bound
I/0
potential shi of the formal Fe couple under Ar and CO2. CO2 is more likely what governs differences in catalytic
These experiments were performed at fast scan rates (2–10 V activity. The primary kinetic isotope effects observed for all four
ꢀ
1
s
) in the absence of an added proton source to prevent amide-functionalized catalysts are in agreement with those
65
subsequent catalytic turnover (Fig. S35†). As shown in Table 2, measured with other proton sources or for other CO
2
reduc-
Fe-TPP and Fe–para-2-amide, which lack the ability to engage in tion catalysts, and indicate that proton transfer is involved in
productive through-space interactions, exhibit low KCO2 values the rate-determining step. Given the pK of the amide pendants,
2–4 M ). In contrast, Fe–ortho-1-amide and Fe–ortho-2-amide they are expected to act as proton relays in addition to hydrogen
22
a
ꢀ
1
(
ꢀ1
exhibit signicantly larger KCO2 values (14–17 M ), indicating bond donors. We therefore conclude that distal ortho posi-
that properly positioned amide pendants act to increase the tioning allows the pendant to adopt a conguration that is more
affinity for CO via hydrogen bonding interactions. Quantum ideally suited to both hydrogen bond stabilization and proton
2
chemical calculations on the Fe–CO
functionalized complexes reveal differences in the length of the
amide-CO hydrogen bond (see ESI† for details), with an H–O
distance of 1.61 A for Fe–ortho-1-amide and 1.45 A for Fe–ortho-
-amide (Fig. S39, S40†). The shorter H-bond distance for the In summary, we investigate the positional dependence of
latter complex is in agreement with the larger through-space secondary coordination sphere groups on the reactivity of
interaction seen electrochemically (Fig. 6). a conserved primary iron porphyrin core for electrochemical
The role of proton transfer during catalysis was examined by CO2 reduction. To this end, four positional isomers were
measuring the pK values of all four amide pendants, as well as synthesized, varying the location of the second-sphere amide
the H/D kinetic isotope effects. The pK values of all four amide group ortho- and para-, as well as proximal and distal, to the
pendants were determined spectrophotometrically (Fig. S36– porphyrin plane. Cyclic voltammetry and controlled potential
8†) in DMSO, as shown in Table 2. Importantly, the amide of electrolysis were used to fully characterize the electrochemical
Fe–ortho-2-amide is less acidic (pK
2
adducts of both ortho- transfer.
2
Conclusions
˚
˚
2
a
a
3
a
¼ 19.3 ꢃ 0.1) than that of behaviour of this family of functionalized porphyrins, including
Fe–ortho-1-amide (pK ¼ 18.7 ꢃ 0.2). Thus, the higher catalytic kinetic parameters, catalytic Tafel plots, and faradaic efficien-
a
activity of Fe–ortho-2-amide cannot be ascribed simply to cies for the product CO. Studying the correlation between
I/0
0
cat
a more acidic amide pendant. In addition, kinetic isotope log(TOFmax) and the Fe standard potential, E , reveals that
effects for all four positional isomers were measured using H
or D O as the proton source (Fig. S20–S27†). Normal primary H/ ortho-functionalized positional isomers, with the distally-
D kinetic isotope effects were observed for all four amide- located amide exhibiting a more signicant enhancement. In
functionalized catalysts (k /k between 1.47 and 1.88, see contrast, the para-functionalized congeners display no signi-
Table 2), suggesting that proton transfer is involved in the rate- cant through-space effects. These results demonstrate that ne-
2
O
through-space interactions enhance the rate of catalysis in both
2
H
D
determining step regardless of amide positioning.
tuning the location of second-sphere pendants is an effective
design strategy for the development of highly-active biomimetic
catalysts for CO reduction, and by extension, for a wider range
of chemical transformations.
2
Table 2 Summary of equilibrium CO
pK
2
binding constants (KCO2), amide
values, and kinetic isotope effects (KIE) for the iron porphyrin
a
catalysts
a
ꢀ1
b
Conflicts of interest
Catalyst
K
CO2 (M
)
Amide pK
a
KIE
c
There are no conicts to declare.
Fe-TPP
2.3
N/A
1.5–2.5
1.47
1.77
1.88
1.84
Fe–ortho-1-amide
Fe–ortho-2-amide
Fe–para-1-amide
Fe–para-2-amide
17.1
14.4
—
18.7 ꢃ 0.2
19.3 ꢃ 0.1
18.8 ꢃ 0.1
18.7 ꢃ 0.5
Acknowledgements
3.6
We thank DOE/LBNL Grant 101528-002 for funding this
The binding constant for Fe–para-1-amide could not be measured but research. C. J. C. is an Investigator with the Howard Hughes
a
ꢀ1 b
is expected to be between 2–4 M
measured using water as the proton source (see ESI for details).
Values reported for various proton sources, see ref. 65.
H
. KIE values represent the ratio of k /
Medical Institute and a CIFAR Senior Fellow. E. M. N. and P. T.
S. acknowledge the NSF for graduate research fellowships. S.
K. N. thanks the UC Berkeley College of Chemistry for a summer
k
D
c
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