Electrocatalytic CO2 Reduction to Formate with Molecular Fe(III) Complexes Containing Pendent Proton Relays

Article abstract of DOI:10.1021/acs.inorgchem.9b03341

Previously, we reported an iron(III) complex with 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(2,4-ditertbutyl-phenol) as a ligand (Fe(^tbudhbpy)Cl, 1) as catalytically competent for the electrochemical reduction of CO2 to formate (Faradaic

Full text of DOI:10.1021/acs.inorgchem.9b03341

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Electrocatalytic CO2 Reduction to Formate with Molecular Fe(III)
Complexes Containing Pendent Proton Relays
Asa W. Nichols, Shelby L. Hooe, Joseph S. Kuehner, Diane A. Dickie, and Charles W. Machan*
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ABSTRACT: Previously, we reported an iron(III) complex with 6,6′-([2,2′-
bipyridine]-6,6′-diyl)bis(2,4-ditertbutyl-phenol) as a ligand (Fe(tbudhbpy)Cl,
1) as catalytically competent for the electrochemical reduction of CO2 to
−
formate (Faradaic efficiency FEHCO = 68 4%). In mechanistic experiments,
2
an essential component was found to be a pre-equilibrium reaction involving
the association of the proton donor with the catalyst, which preceded proton
transfer to the Fe-bound O atoms upon reduction of the Fe center. Here, we
report the synthesis, structural characterization, and reactivity of two iron(III)
compounds with 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(2-methoxy-4-methyl-
phenol) (mecrebpy[H]2, Fe(mecrebpy)Cl, 2) and 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(4-(tert-butyl)benzene-1,2-diol) (tbucatbpy[H]4,
Fe(tbucatbpy), 3) as ligands, where pendent −OMe and −OH groups are poised to modify the protonation reaction involving the Fe-
bound O atoms. Differences in selectivity and activity for the electrocatalytic reduction of carbon dioxide (CO2) to formate
−
(HCO2 ) between complexes 1−3 were assessed via cyclic voltammetry and controlled potential electrolysis (CPE) experiments in
N,N-dimethylformamide. Mechanistic studies suggest that the O atoms in the secondary coordination sphere are important for
−
relaying the exogenous proton donor to the active site through a preconcentration effect, which leads to the JHCO (partial catalytic
2
−
2
current density for formate) increasing by 3.3-fold for 2 and 1.2-fold for 3 in comparison to the JHCO of 1. These results also suggest
that there is a difference in the strength of the interaction between the pendent functional groups and the sacrificial proton donor
between 2 and 3, resulting in quantifiable differences in catalytic activity and efficiency. CPE experiments demonstrate an increased
−
−
FEHCO = 85 2% for 2, whereas 3 had a lower FEHCO = 71 3%, with CO and H2 generated as co-products in each case to reach
2
2
mass balance. These results indicate that using secondary sphere moieties to modulate metal−ligand interactions and multisite
electron and proton transfer reactivity in the primary coordination sphere through reactant preconcentration can be a powerful
strategy for enhancing electrocatalytic activity and selectivity.
INTRODUCTION
■
The steadily increasing market share of electricity from
renewable sources continues to generate interest in developing
electrochemical transformations for storing energy in chemical
bonds.1 One compelling approach is to direct energy from
these sources toward electrochemical transformations of CO2,
a byproduct of fossil fuel usage and a greenhouse gas, into
useful fuel precursors and commodity chemicals.1,2 In
comparison to molecular electrocatalysts developed for the
reduction of CO2 to CO, relatively few catalysts have been
developed which selectively produce formate.3−5 Formate is
currently used as an organic hydride source in synthesis,6 an
Figure 1. Molecular electrocatalysts for CO2 reduction to formate
9
−
(HCO2 ) studied previously (1; Fe(tbudhbpy)Cl) and in this work
energy source in fuel cells,7 and an H2 storage material.8
Previously, we identified a molecular Fe(III) chloride
complex (Fe(tbudhbpy)Cl, 1; Figure 1) based upon a 6,6′-
di(3,5-ditert-butyl-2-hydroxybenzene)-2,2′-bipyridine
(2; Fe(mecrebpy)Cl and 3; Fe(tbucatbpy)).
Received: November 13, 2019
(tbudhbpy[H]2) ligand which produced formate (HCO2 ) with
−
a FE of 68 4% from CO2 in the presence of 0.5 M phenol
(PhOH) at an Ecat/2 of −2.45 V versus the ferrocenium/
ferrocene redox couple (Fc+/Fc).9 Mechanistic studies
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−
suggested that a noncatalytic electrochemical reaction was vital
to the observed reactivity: the Fe-bound oxygen atoms
underwent protonation upon the Fe(III)/(II) reduction, a
type of multisite electron and proton transfer (MS-EPT)
reaction.9−12 MS-EPT mechanisms involving ligand proto-
nation upon metal redox changes in the primary coordination
sphere are generally invoked for metal oxime and dithiolene
complexes which catalytically generate hydrogen (H2);13−17
similar mechanisms are underdeveloped for other substrates.3
MS-EPT has been explored broadly in photochemical and
chemical systems, including the photosynthetic oxidation of
tyrosine-Z,18 ruthenium-based model compounds for phenol
oxidation,19 and mixed TEMPOH/organic base/chemical
oxidant systems.20 Under electrochemical conditions, MS-
EPT has also been studied in the context of tyrosine oxidation
by [M(bpy)3]3+/2+ systems (M = Os, Ru, and Fe; bpy = 2,2′-
bipyridine).21
A successful strategy for improving the activity, selectivity,
and thermodynamic efficiency of molecular electrocatalysts
which facilitate the transformation of CO2 is the incorporation
of pendent proton shuttles in their secondary coordination
spheres.22−24 The utility of these functional groups holds for
many electrocatalytic transformations: examples exist for the
H2 evolution,25−27 oxygen reduction,28−30 and CO2 reduc-
tion22,31−34 reactions. In studies on molecular electrocatalysts
for CO2 reduction, the design emphasis is generally placed on
positioning the pendent relay such that it can interact with
CO2 bound to the active site of the complex to stabilize
intermediates, act as a proton (or proton donor) relay, and/or
provide access to new reaction pathways to lower the energy
input and increase catalytic rates.22,23,34−41
current density for HCO2 production; no significant increase
−
−
in FEHCO is observed (FEHCO of 3 = 71 3%). We propose
2
2
that the improvements in selectivity and activity for 2 arise
from beneficial interactions between the secondary sphere
moieties, exogenous proton donor, and inner-sphere Fe-bound
oxygen atom, which contribute to the preconcentration of the
proton donor and kinetic selectivity.
RESULTS
■
The isolation of the [mecrebpy(H)2] and [tbucatbpy(H)4]
ligands was accomplished through Pd-catalyzed cross-coupling
procedures analogous to the previously reported synthesis of
[
tbudhbpy(H)2] (see the Experimental Methods).9 The
corresponding Fe(III) complexes were obtained by refluxing
the respective ligands in ethanol in the presence of equimolar
FeCl3·6H2O with NaOAc as a proton scavenger. Single crystals
of sufficient quality for X-ray diffraction studies were obtained
for both 2 and 3 by the slow cooling of supersaturated
acetonitrile solutions from refluxing temperatures. The solid-
state structure of complex 2 is a dimeric species, with a single
oxygen atom from each ligand framework coordinated to an
axial position of a second Fe complex to create a hexadentate
coordination environment (Figure 2). Likewise, complex 3 also
Intrigued by our initial observations of the MS-EPT reaction
and its relationship to the catalytic response,9 we have
synthesized two new ligand frameworks containing secondary
sphere hydrogen bond-donor and -acceptor groups which are
instead oriented to interact with the Fe-bound O atoms that
are protonated upon reduction of the metal center (Figure 1;
complexes 2 and 3).37−40 By tuning the ability of the
secondary sphere moiety to direct the exogenous sacrificial
proton donor to the proton-sensitive Fe-bound O atom, an
−
increased current density for formate production (JHCO ) and
2
Figure 2. (A) Unit cell and (B) overall dimer structure for
Fe(mecrebpy)Cl 2; each equivalent of 2 is generated by symmetry
and not crystallographically distinct. C = gray, O = red, N = blue, Fe =
orange, and Cl = green. Thermal ellipsoids set to 50%. Occluded
acetonitrile molecules and hydrogen atoms omitted for clarity.
−
selectivity for HCO2 is achieved. Importantly, this demon-
strates that noncatalytic MS-EPT reactions can be optimized
via analogous strategies to those used for enhanced
protonation rates of activated substrate at the metal center.
Here, we report the synthesis, characterization, and
electrochemical behavior of two new Fe(III) complexes
containing pendent hydroxy or methoxy moieties oriented to
direct proton donors to the Fe-bound oxygen atoms of an
N2O2 coordination environment. These complexes contain the
ligands 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(2-methoxy-4-
methylphenol) (mecrebpy[H]2) for Fe(mecrebpy)Cl (2; Figure
1) and 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(4-(tert-butyl)-
benzene-1,2-diol) (tbucatbpy[H]4) for Fe(tbucatbpy) (3; Figure
1). Initial studies have shown complex 2 to be an excellent
exists as a dimeric species in the solid state; however, in this
case hexadentate coordination is instead facilitated by the
partial deprotonation of one of each of the pendent −OH
moieties per equivalent of 3 in the dimeric structure, replacing
the Cl− ligand for both Fe centers (Figure S1). Microanalysis
of as-synthesized 3 confirmed that complete chloride loss
occurs for the bulk material, with the charge for the Fe(III)
center being balanced by the partial deprotonation in the
secondary coordination sphere (3; Figure 1). In both cases, the
solution-phase behavior suggests that these are monomeric
species: UV−vis spectra of 2 and 3 obtained in N,N-
dimethylformamide (N,N-DMF) maintain linearity upon
dilution, which is consistent with the absence of an equilibrium
process (Figures S2 and S3). Diffusion coefficients for 2 and 3
determined by rotating disk electrode (RDE) methods using
the Levich equation are consistent with those determined for 1
(Table S1, Figures S4−S6).42 Finally, the magnetic moments
−
catalyst for the reduction of CO2 to HCO2 , with a 3.3-fold
enhancement in partial catalytic current density corresponding
to HCO2 production in comparison to that of 1. Complex 2
also shows an enhanced FEHCO (FEHCO 2 = 85 2%) in
−
−
−
2
2
9
−
comparison to the 68 4% FEHCO previously observed for 1
under analogous conditions. In contrast, 3 is only slightly more
active than 1 with a 1.2-fold enhancement in terms of catalytic
2
B
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determined by Evans’ method of 5.9 0.1 and 6.0 0.1 Bohr
magnetons obtained for 2 and 3, respectively, are consistent
with a monomeric high-spin S = 5/2 Fe(III) species in each
case.9
Cyclic voltammetry (CV) experiments conducted in 0.1 M
TBAPF6/N,N-DMF under Ar with each complex demon-
strated redox processes at similar potentials to 1.9 Complex 2
exhibits four total redox processes: one irreversible reduction
with Ep,a = −1.45 V vs Fc+/Fc, two quasi-reversible 1e−
reductions with E1/2 = −0.84 and −2.55 V vs Fc+/Fc, and
one reversible 1e− reduction with E1/2 = −1.93 V vs Fc+/Fc
(Figure 3, black, and S7, and Tables S2−S4). The irreversible
rate-dependent relationship is observed between the final two
reduction features for 3 (Figure S10). The reduction at −2.24
V vs Fc+/Fc produces larger current densities at slow scan rates
(<200 mV/s), but the reduction feature at −2.58 V vs Fc+/Fc
becomes relatively more prominent at faster scan rates. The
ratio between the two decreases asymptotically (Figure S10).
Variable scan rate experiments indicate the electrochemical
response of both 2 and 3 is in a diffusion-limited regime for the
first two reversible reductions (Figures S11 and S12,
respectively).43 CVs of the corresponding Zn analogues of 2
and 3 (Zn(mecrebpy) and Zn(tbucatbpy), respectively) show
redox features only at potentials similar to the final two
reductions for each, suggesting that the frontier orbital
configuration in highly reduced states incorporates contribu-
tions from the ligand manifold (Figures S13 and S14). As we
have shown previously, the initial Fe(III)/(II) reduction
feature is sensitive to the presence of a proton donor.9 For
complexes 2 (Figure S15) and 3 (Figure S16), distinct shifts to
positive potentials and the coalescence of the more positive
reduction features occur upon addition of phenol (PhOH) as a
proton donor. These changes are not conducive to the simple
equilibrium modeling used previously,9 which we attribute to
multiple competitive equilibria involving the pendent moieties,
the Fe-bound O atoms and PhOH at reducing potentials. In an
attempt to deconvolute these processes for the precatalytic
complexes, we also examined the interactions between PhOH
and both 2 and 3 using UV−vis spectroscopy; however, no
well-defined isosbestic points were observed in the relevant
concentration ranges (Figures S17 and S18).
In the presence of CO2 under aprotic conditions, 2 and 3
exhibit an increase in current, with ic/ip values of 5.5 and 13,
with Ep,c = −2.50 V and −2.37 V vs Fc+/Fc, respectively
(Figure 4, red).44 Upon the addition of (PhOH) under CO2
saturation conditions (0.23 M),45 each complex shows an
increase in current relative to aprotic conditions, consistent
with the electrocatalytic response observed previously for 1
(Figures 4, black, S19, and S20). Stepwise titrations of PhOH
show that current saturation is reached at 0.9 M PhOH for 2
with Ecat/2 = −2.33 V vs Fc+/Fc and at 0.5 M PhOH for 3 with
Ecat/2 = −2.33 V vs Fc+/Fc (100 mV/s scan rate; Figures S21
and S22).
Figure 3. Cyclic voltammograms of 0.5 mM Fe(mecrebpy)Cl 2
(black) and Fe(tbucatbpy) 3 (red) under an Ar atmosphere in 0.1 M
TBAPF6/N,N-DMF supporting electrolyte. The working electrode is
glassy carbon, the counter electrode is a glassy carbon rod, and AgCl/
Ag is the pseudoreference electrode. Referenced to Fc+/Fc from
internal decamethylferrocene (Cp2*Fe) standard; 100 mV/s scan rate.
reduction with Ep,a = −1.45 V vs Fc+/Fc disappears at fast scan
rates upon the addition of TBACl as a source of Cl− (Figure
S8), suggesting that it corresponds to the product of an
equilibrium reaction involving Cl− loss. Complex 3 exhibits
four total redox processes as well: two reversible, 1e−
reductions with E1/2 = −1.01 V and −1.65 V vs Fc+/Fc,
respectively, followed by two 1e− irreversible reductions with
E1/2 = −2.24 V and Ep,a = −2.58 V vs Fc+/Fc (Figures 3, red,
and S9; Tables S5 and S6). On the return sweep to oxidizing
potentials with 3, a feature with Ep,c = −1.39 V vs Fc+/Fc is
observed when the CV switching potential is ca. −2.7 V vs
Fc+/Fc, indicating that this CV feature is associated with the
final two reduction waves (Figure 3, red). Interestingly, a scan-
In our previous study, we noted that in the presence of CO2
and PhOH 1 showed the following: (A) decreased current
densities under increasing CO2 concentrations due to the
suppression of competitive HER activity, (B) saturation of the
catalytic current at a PhOH concentration of 0.2 M, and (C) a
Figure 4. CVs of (A) Fe(mecrebpy)Cl 2 and (B) Fe(tbucatbpy) 3 under Ar (black) and CO2 (red, blue, and green) saturation conditions. Blue and
green traces are in the presence of PhOH added as a proton source. Working electrode is glassy carbon, counter electrode is a glassy carbon rod,
AgCl/Ag is the pseudoreference electrode. Referenced to Fc+/Fc from internal Cp2*Fe standard; 100 mV/s scan rate.
C
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first-order dependence on catalyst concentration.46 For 2,
current saturation was observed above concentrations of 0.04
M CO2 in the presence of 1.0 M PhOH (Figure S23), and a
first-order concentration dependence on PhOH was observed
under CO2 saturation conditions (Figure S24). We again
observed electrocatalytic current saturation above 0.04 M CO2
for complex 3 (Figure S25); however, varied PhOH
concentrations under saturated CO2 give a slope of 0.15,
which we attribute to the role of the −OH relay (Figure S26).
A fractional reaction order for the added sacrificial proton
donor in a proton-dependent reaction suggests that transfer of
proton equivalents or PhOH from the relay to the active site is
extremely facile for 3 relative to that for 1 and 2. Plots
comparing the current response to the [catalyst] were linear
for both 2 and 3, suggesting the reaction is first-order with
respect to each, consistent with our prior study (Figures S27
and S28).46
dependent reduction reactions.47 Therefore, we reason that the
related shifts in reduction potential observed for complexes 2
and 3 which do not conform to our previous model (Figures
S15 and S16) represent multiple related pre-equilibria
involving PhOH. Importantly, we expect several differences
from 1 at applied potential based on the inclusion of the
pendent −OMe/−OH functional groups in the secondary
coordination sphere: As mentioned above for 2 and 3, the
possibility of multipoint hydrogen bonding interactions
between PhOH and the ligand framework (2[PhOH] and
3[PhOH]; Figure 5A,B) implies that a significant preconcen-
tration of the sacrificial proton donor relative to 1 is possible.
We note that these noncovalent adducts are expected to be in
equilibrium with the products of formal proton transfer to the
Fe-bound oxygen atom, 3(H+) and 2(H+) (Figure 5). For
complex 3, however, there is the added possibility of
homoconjugation-type interactions48 between the ligand
framework and phenol/phenolate (3(H+)[PhO−] versus
3[PhO−] in Figure 5B,C), which have previously been
implicated as possible determinants of both kinetic and
thermodynamic reaction parameters for electrochemical
reactions.11,49−52 We describe the observed difference in
catalytic response with respect to [PhOH] for 2 and 3 to
the ability of the −OH groups to form both hydrogen bond
donor and acceptor interactions, resulting in a greater
preconcentration of the sacrificial proton donor.
These results indicate that a different mechanism is
occurring for 2 and 3, in comparison to that for 1, which we
hypothesize is the result of a preconcentration of PhOH
through favorable noncovalent equilibrium interactions with
the O atoms in the secondary coordination sphere of the ligand
(Figure 5). In our previous studies on complex 1, we noted
Partial catalytic current densities for formate production
−
(JHCO ) were determined for each complex utilizing Faradaic
2
efficiencies from controlled potential electrolysis experiments
under the saturation conditions optimized from CV studies. At
0.5 M PhOH, 3 exhibits JHCO = 0.7 mA/cm2. At 1.0 M
−
2
PhOH, 2 exhibits an increased JHCO = 2.0 mA/cm2. By
−
2
treating the catalytic response observed for 1 with the same
−
method at 0.5 M concentration of PhOH, a JHCO = 0.6 mA/
2
cm2 is obtained. Under these conditions, 2 exhibits an increase
in current at the second reduction potential at −1.93 V vs Fc+/
Fc, as well as a new prewave feature at more reducing
potentials in the voltammetric response (Figure 4A). We
attribute these changes to the formation of intermediate
species containing a noncovalent assembly of the complex and
PhOH (or several PhOH equivalents) under reducing
conditions, e.g., [2][PhOH], Figure 5A, prior to the formation
of the active catalyst species capable of net hydride transfer to
CO2.
The catalytic efficiencies and selectivities of 2 and 3 for the
reduction of CO2 were determined by controlled potential
electrolysis (CPE) experiments at the peak catalytic potentials
for their respective PhOH saturation concentrations (Table 1,
entries 2 and 8). CPE at −2.50 V vs Fc+/Fc with 2 and 0.9 M
Figure 5. Diagram of select possible equilibria involving complexes 2
(A) and 3 (B and C) in the reduced state. In previous studies, formal
proton transfer to the O atom of the ligand was implicated for 1; the
effect of multipoint hydrogen-bonding interactions for 2 and 3 with
PhOH is observed in their concentration-dependent electrochemical
responses. Where n = 0 or −1 and m = 0, −1, or −2.
−
PhOH under CO2 saturation conditions produced HCO2 at
−
85 2% FEHCO with the remainder of current going to H2
2
(12 4%) and CO (5 1%) through 11.0 TON (22 electron
equivalents passed per catalyst molecule, Figure S29). CPE at
−2.60 V vs Fc+/Fc with 3 and 0.5 M PhOH under CO2
that electrochemical behavior consistent with the redox-
dependent protonation of the Fe-bound oxygen atoms
occurred; a quantifiable equilibrium response was obtained
for the Fe(III)/(II) reduction potential with respect to the
concentration of added PhOH.9 This experimentally observed
equilibrium potential shift requires an appreciable concen-
tration of a noncovalent assembly between complex 1 and
PhOH, “preloading” the complex for subsequent proton-
−
saturation conditions yielded 71
3% FEHCO , with the
2
remainder of current going to H2 (23 9%) and CO (0.8
0.1%) (Figure S30). Following these electrolysis experiments,
the working electrode was rinsed with ethanol and allowed to
air-dry before a second electrolysis experiment was conducted
where the putative molecular precatalyst was omitted under
otherwise identical conditions of proton donor concentration
D
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a
Table 1. Summary of CPE Data
b
b
b
−
trial
1
2
complex
atmosphere
[PhOH] (M)
Eapplied
TON
formate (HCO2
)
CO
H2
2
2
2
2
CO2
CO2
CO2
Ar
CO2
CO2
CO2
CO2
CO2
Ar
0.9
0.9
0
0.9
0.9
0.9
0.5
0.5
0
−2.5
−2.5
−2.5
−2.5
−2.5
−2.5
−2.6
−2.6
−2.45
−2.6
−2.6
−2.6
9.0
11.0
3.9
85
84
2
1
9
5
2
1
10
12
2
4
c
3
4
5
6
7
30
6
9.5
51
32
9
4
rinse 2
ND
ND
70
6
2
1
ND
2
60 10
3
3
3
3
9.4
9.8
3.6
8
3
28
23
4
9
d
8
71
0.8 0.1
9
17
3
10
11
12
7.5
0.5
0.5
12.9
48
ND
17
8
7
rinse 3
CO2
CO2
ND
ND
4
1
1.0 0.5
a
Conditions: 0.1 M TBAPF6/N,N-DMF supporting electrolyte with 0.5 M Fc as sacrificial oxidant. The working and counter electrodes were
b
c
d
graphite rods; the pseudoreference electrode was Ag/AgCl. Faradaic efficiency determined by 1H NMR. Duplicate of trial 1. Duplicate of trial 7
and CO2 saturation (Figures S29 and S30, blue). The rinse test
with both 2 and 3 indicated no HCO2 production (Table 1,
entries 5 and 11). Additional control experiments with no
DISCUSSION
■
−
−
In both 2 and 3, an enhanced JHCO in comparison to our
2
original report with 1 is observed under catalytic conditions.
Only complex 2 shows increased selectivity for HCO2 , which
added catalyst material and a freshly polished electrode
−
−
generated no HCO2 under either set of conditions (Table
−
suggests that under catalytic conditions, the production of H2
is suppressed in comparison to 1 and 3. We ascribe this in part
to differences in the efficiency of the transfer of proton donors
from the secondary coordination sphere relay mechanism and
to corresponding differences in the effective pKa of the
exogenous proton donor resulting in lower rates of putative
metal hydride protonation.57 To enhance an intrinsic
molecular electrocatalytic response, the combined forward
reaction rate constants of net proton transfer from the
exogenous proton through the relay and to the active site
during the electrocatalytic response must compare favorably to
that for the catalytic response in the absence of a relay
mechanism, where the proton donor interacts directly with the
Fe center (Figure 6).31 We propose that the existence of the
zwitterionic state of 3 in the secondary sphere as synthesized is
suggestive of possible deleterious effects on catalyst turnover
1, entries 6 and 12). To confirm that the observed HCO2
product originated from CO2 rather than N,N-DMF, isotopi-
cally labeled 13CO2 was utilized under identical conditions to
previous CPE experiments. 1H NMR spectra from these
experiments with 2 and 3 indicate that the observed HCO2− is
1
generated from 13CO2: A doublet reflecting H−13C coupling
(JCH= 133.1 Hz) is seen in the spectra where a singlet for
HCO2− is observed in electrolysis experiments using CO2 with
a normal isotopic abundance (Figures S31−S34).53
For comparison with the results discussed above, CO2
reduction activity was examined by CPE under aprotic
conditions with both 2 and 3 at −2.5 V vs Fc+/Fc (Figures
S35 and S36). Both 2 and 3 show appreciable production of
CO under these conditions (30 6% FECO for 2 and 17 3%
FECO for 3; Table 1, entries 3 and 9). This observation is in
stark contrast to 1, which only showed ∼1% FECO after 15 h
under these conditions.9 Comparable CPE experiments were
also conducted to examine the inherent HER activity in the
absence of CO2 (Figure S37 and 38). For 2, 51 9% FEH was
2
−
observed with no CO or HCO2 detected after 9.5 TON
(Table 1, entry 4). For 3, 48 8% FEH was observed with no
2
−
CO or HCO2 detected after 13 TON (Table 1, entry 10). A
CPE experiment with 3 in the absence of PhOH under Ar
saturation did not result in H2 production from the pendent
proton remaining on the ligand framework (Figure S39).
To assess the proposed mechanism of proton donor
preconcentration by the pendent functional groups for 2 and
3, CV experiments were repeated using 2,4,6-tritert-butyl-
phenol (ΔpKaDMSO = 0.2,54 pKa values are known to scale well
between DMSO and N,N-DMF55) as a sterically hindered
proton source (Figure S40).56 At concentrations of 2,4,6-
tritert-butyl-phenol comparable to that of PhOH at saturation
of the catalytic response, only 2 showed a catalytic increase in
current relative to the background electrode current (Figure
S40, green). This is consistent with the proposal that the
pendent methoxy group in 2 is more competent for facilitating
the net transfer of proton equivalents from the sacrificial donor
to the active site than the hydroxy/phenolate moieties present
in 3.31
Figure 6. Summary of the basic relay mechanism of the exogenous
proton donor. For boosting effects to be observable in an
electrocatalytic response, the forward reaction rate constants of the
relay need to be competitive with the rate of direct protonation.31 AH
and A− are PhOH and its conjugate base PhO−.
E
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a
Scheme 1. Synthesis of mecrebpy[H2]
a
(a) (i) 4,4,5,5-tetramethyl-1,3,2-dioxaborolane, RT, 10 m. (ii) 4,4,5,5-tetramethyl-2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1,3,2-
dioxaborolane, 4-tert-butyl-2-(4-tert-butyl-2-pyridyl)pyridine, Ir[(cod)(OMe)]2, cyclohexane, 90 °C 21 h. (b) Pd(PPh3)4, K3PO4, 1,4-dioxane,
130 °C, 72 h. (c) FeCl3·6H2O, NaOAc, EtOH, 78 °C, 12 h.
due to the partial or complete deprotonation of the pendent
−OH groups during the course of the reaction (Figure 5C).
The experimental observation of near-zero-order dependence
of catalytic current on [PhOH] for complex 3 (Figure S26)
indicates that a significant preconcentration of the proton
donor is occurring (Figure 6A). This preconcentration allows
the “preloaded” proton donor relay to accumulate in excess at
reducing potentials because of a mismatch with the rate of
electrocatalytic formate production. Deprotonation of the
ligand during the catalytic turnover of 3 will induce
noncovalent homoconjugation interactions with the sacrificial
proton donor which are much stronger than those possible for
the neutral methoxy groups in complex 2 for purely
electrostatic reasons (Figure 5).48,55,58 The effectiveness of
this preconcentration effect for 3 is likely also the origin of the
diminished activity relative to that of 2, since the same effects
are likely to cause steric hindrance at the active site, as well as
alter the barrier for net CO2 insertion into the putative metal
hydride intermediate (Figure 6B,C).
Future studies will attempt to deconvolute the relevant
equilibrium processes involving the proton donors and
pendent functional groups at applied potential. We note that
the propensity of Fe(III) ions to coordinate phenolate is well-
known and represents an as yet unquantified contribution to
the driving force for the association of the exogenous proton
donor with the pendent relay which will need to be considered.
These data implicate that both the proton-donor-shuttling
mechanism and the intermolecular protonation of the active
site result in the formation of a common metal hydride
intermediate, the net transfer of which to CO2 will result in
formate production (Figure 6A). The experimental observation
of identical Ecat/2 values for both 2 and 3 is consistent with the
existence of a comparable intermediate metal hydride for both
catalysts, suggesting that the electronic difference between the
two active sites is minimal. This indirectly indicates that the
activity difference between complexes 2 and 3 arises from a
difference in the kinetic parameters of proton donor loading
through the −OMe/−OH groups.57
donor and the two-electron reduction product of 2 (Figure
5A); the formation of new intermediate species is also
suggested by the proton dependence of the preceding
reduction features (Figure S15). Furthermore, the experimen-
tal results with a sterically hindered proton source demonstrate
catalytic current increases only for 2 (Figure S40). This
suggests facile transfer of proton donor equivalents from
solution through the relay to the active site for complex 2 in
comparison to 3. Consistent with our postulation that the
outer-sphere pendent Lewis basic −OMe site generates less
hindering noncovalent interactions with PhOH than those
−
implied for 3, the electrocatalytic response of 2 achieves JHCO
2
greater than that for 1 or 3. This putative kinetic preference in
the proton donor-shuttling mechanism also results in an
−
2
improvement in selectivity for 2 (FeHCO = 85
2%) in
−
−
2
comparison to those of 1 (FeHCO = 68 4%) and 3 (FeHCO
2
= 71 3%). While it cannot be explicitly discounted that the
hydricity or pKa values of the putative Fe hydride intermediates
for 2 and 3 are different, we emphasize again that there is no
observed difference between their Ecat/2, suggesting that the
proposed catalytically competent iron hydride intermediates
are analogous.57,59
CONCLUSIONS
■
We have reported the synthesis, characterization, and reactivity
for the electrocatalytic reduction of CO2 to HCO2− of two new
Fe(III) complexes with pendent functional groups capable of
preconcentrating the sacrificial proton donor PhOH. Enhance-
ment of the catalytic rate constant is observed for both methyl-
ether and hydroxy pendent functional groups. However,
enhanced selectivity was only observed for the methyl-ether
pendent functional group, suggesting that the strength of
interaction between the external proton donor and the pendent
relay site has a nuanced effect on reaction control. These
results further demonstrate that the precatalytic MS-EPT
mechanism we observed in our original report is vital to the
intrinsic catalytic properties and that its overall effect can be
modulated through noncovalent secondary sphere interactions
to optimize the catalytic response. This suggests that secondary
sphere moieties which do not interact directly with substrate
Consistent with this interpretation, a prewave feature
appears in the electrocatalytic response of 2, indicative of the
formation of a new intermediate species when the proton
donor is present, which we ascribe to adducts of the proton
F
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a
Scheme 2. Synthesis for Fe(tbucatbpy)
a
n
Reaction conditions: (a) NBS, N,N-DMF, N2, room temperature, 2 h. (b) (i) DIPEA, N2, CH2Cl2. (ii) MOMCl, 0 °C→RT, 5 h. (c) (i) BuLi,
−78 °C→RT, 2 h. (ii) B(OMe)3, −78 °C→RT, 16 h. (iii) 2 M HCl. (d) Pd(PPh3)4, Na2CO3, toluene, MeOH, H2O, 280 W microwave irradiation,
5 h. (e) HCl (gas), EtOAc. (f) FeCl3·6H2O, NaOAc, EtOH, 78 °C, 3 h.
cell with a cap modified with ports for all electrodes and a sparging
needle.
bound at the active site are a complementary way to modulate
activity and selectivity in this class of catalysts.
CPE experiments were performed in a H-Cell with a porous glass
frit separating the working and counter chambers. Prior to electrolysis,
a CV of ferrocene was obtained to reference the cell. CPE
experiments were run with graphite working and counter electrodes
(Electron Microscopy Sciences, type A, 3 mm diameter), a custom
silver/silver chloride pseudoreference electrode (BASi, nonaqueous
reference electrode kit), and ports for headspace sampling and gas
sparging via needles through septa. To generate an oxygen free
environment under the correct atmosphere, the H-Cell was degassed
for 8 min with the corresponding gas, with the exception of 13CO2-
labeled experiments. In this case, Ar was sparged for 8 min followed
by a 30 s sparge with 13CO2. The working chamber was separated
from the counter and pseudoreference electrodes by utilizing a porous
glass frit, and 0.05 M ferrocene was added to the counter chamber as a
sacrificial reductant. TBAPF6 was purified by recrystallization from
ethanol and dried in a vacuum oven before being stored in a
desiccator. All data were referenced to an internal decamethylferro-
cene (Cp*2Fe) standard (decamethylferricenium/decamethylferro-
cene reduction potential under stated conditions) which was
subsequently corrected relative to the relative E1/2 of ferrocene61
unless otherwise specified.
Synthesis of 2-Methoxy-4-methyl-6-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenol,62 II. In a glovebox, 2-methoxy-4-methyl-
phenol (2.5 mL, 19.7 mmol) and 4,4,5,5-tetramethyl-1,3,2-dioxabor-
olane (2.5 mL, 21.7 mmol) were combined and stirred for 10 m in a
250 mL pressure flask, during which time vigorous effervescence was
observed. Subsequently, 4,4,5,5-tetramethyl-2-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-1,3,2-dioxaborolane (3.51 g, 13.8 mmol), 4-
tert-butyl-2-(4-tert-butyl-2-pyridyl)pyridine (0.318 g, 6 mol %),
Ir[(cod)(OMe)]2 (0.392 g, 3 mol %), and 100 mL of dry cyclohexane
were added. The flask was capped and heated to 90 °C for 21 h.
When the flask cooled to room temperature after being removed from
the heat source, 10 g of silica gel was added, and the solvent was
removed under reduced pressure. A silica gel column was run using a
Combiflash purification system (ramp 0 to 10% EtOAc/Hexanes),
and the fraction which eluted after the starting material was isolated
(very little UV−vis absorbance is observed for the product). Solvent
EXPERIMENTAL AND METHODS
■
Materials and Methods. General. All chemicals and solvents
(ACS- or HPLC-grade) were commercially available and used as
received unless otherwise indicated. For all air-sensitive reactions and
electrochemical experiments, solvents were obtained as anhydrous
and air-free from a PPT Glass Contour Solvent Purification System.
Gas cylinders were obtained from Praxair (Ar as 5.0; CO2 as 4.0) and
passed through molecular sieves prior to use. 13CO2 was obtained
from Sigma (99.0% enriched, <3.0% 16O). NMR spectra were
obtained on a Varian 600 MHz or Bruker 800 MHz instrument and
referenced to the residual solvent signal. Microwave reactions were
carried out using an Anton-Parr Multiwave Pro SOLV, NXF-8
microwave reactor. Flash chromatography columns were run utilizing
Teledyne ISCO CombiFlashRf+. UV−vis absorbance spectra were
obtained on a Cary 60 from Agilent. Gas-phase products were
analyzed on an Agilent 7890B Gas Chromatograph with an Agilent
J&W Select Permanent Gases/CO2 column and thermal conductivity
detector; eluent retention times and product characterization were
determined by standard injections. Formate was quantified by a
previously reported NMR method.60 HRMS data were obtained by
the Mass Spectrometry Lab at the University of Illinois at Urbana−
Champaign or on an Agilent 6545 Q-TOF LC/MS at UVA and
elemental analyses were performed by Midwest Microlab. All
synthetic procedures are summarized in Schemes 1 and 2 above.
Electrochemistry. All electroanalytical experiments were performed
using a Metrohm Autolab PGSTAT302N or Biologic SP-50
potentiostats. Glassy carbon working (3 mm) and nonaqueous
silver/silver chloride pseudoreference electrodes behind PTFE tips
were obtained from CH Instruments. The pseudoreference electrodes
were obtained by depositing chloride on bare silver wire in 10% HCl
at oxidizing potentials and stored in a solution of 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) in N,N-DMF
prior to use. The counter electrode was a glassy carbon rod (Alfa
Aesar, 99.95%, 3 mm diameter). All CV experiments were performed
in a modified scintillation vial (20 mL volume) as a single-chamber
G
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was removed to yield 2.52 g of a pale-yellow solid. Yield: 48.4%.
Characterization was consistent with prior reports.62
and 3-bromo-5-tert-butyl-benzene-1,2-diol (8.67 g, 35.4 mmol) were
added. The headspace was flushed with nitrogen and diisopropylethyl-
amine (21.6 mL, 124.0 mmol) was added. A color change from brown
to maroon was observed. The round-bottomed flask was cooled to 0
°C using an ice bath. Methoxymethyl chloride (8.1 mL, 110 mmol)
was added dropwise with vigorous stirring. After the addition, the ice
bath was removed and the solution was allowed to stir for 5 h. The
reaction was quenched with aqueous NH4OH (2 N, 120 mL), the
organic layer was separated, and the aqueous layer was extracted using
diethyl ether (3 × 50 mL). The organic fractions were combined,
washed with brine (3 × 50 mL), dried over MgSO4, and condensed in
vacuo to yield a red oil. Yield 11.2 g, 94.9%. EI-MS (m/z) [M]+:
Calcd 332. Found: 332. 1H NMR (CDCl3, 600 MHz, Figure S49): δ
(ppm) 1.28 (s, 9H), 3.51 (s, 3H), 3.67 (s, 3H), 5.16 (s, 2H), 5.18
ppm (s, 2H) 7.10 (d, 1H), 7.21 (d, 1H). 13C{1H} NMR (CD2Cl2,
600 MHz, Figure S50): δ (ppm) 31.39, 34.91, 56.52, 58.08, 95.83,
98.97, 114.07, 117.42, 123.81, 142.04, 148.99, 150.64.
Bis(MOM)catbpy, IX. A 500 mL Schlenk flask with stir bar was
charged with 1-bromo-5-tert-butyl-2,3-bis(methoxymethoxy)benzene
(11.2 g, 33.6 mmol) and dry diethyl ether (100 mL). The resulting
solution was cooled to −78 °C under N2. n-Butyllithium (1.6 M in
hexanes) was added dropwise via syringe (23.1 mL, 36.9 mmol) still
under N2, after which the solution was allowed to warm to room
temperature with vigorous stirring. At this point the reaction mixture
was a brown suspension. After 2 h, the solution was returned to −78
°C. Trimethylborate was then added rapidly by syringe (3.95 mL,
35.3 mmol) with vigorous stirring and left for 10 min before the cold
bath was removed and the mixture left overnight under N2 (16 h).
After this time the reaction was quenched with deionized water (50
mL) and opened to air. The suspension was extracted, and the
aqueous and organic layers separated. The aqueous layer was carefully
neutralized to pH 7 with ∼0.1 M HCl and washed with diethyl ether
(3 × 50 mL), and all organic fractions were combined. The combined
layers were then washed with brine (3 × 50 mL). The resulting
solution was dried with MgSO4 for 10 min before the mixture was
filtered to remove solid, and the solvent was removed under reduced
pressure. The resulting dark orange oil was used as isolated without
further purification.
A microwave-assisted Suzuki−Miyaura coupling reaction was
performed with 6,6′-dibromo-2,2′-bipyridine and the crude boronic
acid generated above using an Anton Paar Multiwave Pro equipped
with Rotor 8NXF100. Two PTFE reaction tubes each were equipped
with a magnetic stir bar and charged with 6,6′-dibromo-2,2′-
bipyridine (1.0 g, 3.2 mmol) and Na2CO3 (2.0 g, 38 mmol).
Deionized water and methanol were degassed with Ar. The loaded
PTFE tubes, solvents, and crude boronic acid were pumped into a N2
glovebox. Pd(PPh3)4 (0.18 g, 0.16 mmol) was added to each tube.
The boronic acid was added to this suspension by diluting in toluene
(20 mL total volume) and splitting this stock solution equally
between the two PTFE vessels. Methanol (10 mL), deionized water
(10 mL), and toluene (30 mL) were added to the vessels. The vessels
were sealed and loaded into the microwave. The power was ramped to
280 W for 10 min and then held at this value for 5 h. After cooling to
room temperature, the tubes were combined, and the organic and
aqueous layers were separated. The aqueous layer was extracted with
CH2Cl2 (3 × 50 mL), and all organic fractions were combined, dried
with MgSO4, filtered to remove solids, and condensed under vacuum
to yield a crude red oil. This oil was left overnight at room
temperature, and an off-white crystalline solid formed overnight. This
solid was collected and washed with hexanes. Upon combination of
the hexanes wash and the initial fraction, more crystalline material was
produced. Characterization of each fraction was consistent with the
desired product. Yield 2.85 g, 67.7%. EI-MS (m/z) [M − H]+: Calcd
661.3489. Found: 661.3486. Theory (CHN): C: 69.07, H: 68.83, N:
7.32. Found: C: 68.83, H: 7.24, N: 4.02. 1H NMR (CDCl3, 600 MHz,
Figure S52): δ (ppm) 1.39 (s, 18H), 3.12 (s, 6H), 3.56 (s, 6H), 4.96
(s, 4H), 5.27 ppm (s, 4H), 7.27 (d, 2H), 7.55 (d, 2H)), 7.81−7.88
(m, 4H), 8.53 (d, 2H). 13C{1H} NMR (CDCl3, 600 MHz, Figure
S53): δ (ppm) 31.60, 34.91, 56.45, 57.23, 95.89, 99.37, 115.17,
mecrebpy(H)2, III. A 500 mL pressure flask with stir bar was charged
with 6,6′-dibromobipyridine (0.7 g, 2.23 mmol), K3PO4 (2.56 g, 12
mmol), 2-methoxy-4-methyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaboro-
lan-2-yl)phenol (1.77 g, 6.69 mmol), Pd(PPh3)4 (0.260 g, 0.22
mmol), and dioxane (250 mL). After this time, the mixture was
cooled to RT, diluted with brine (250 mL) and extracted with toluene
(6 × 50 mL) and CH2Cl2 (6 × 50 mL). The organic layers were
combined and dried with MgSO4, filtered, and dried under vacuum.
The yellow product elutes from a CH2Cl2/MeOH ramp of 0−5%
MeOH using a Combiflash purification system. Recrystallization of
the product fraction from CH2Cl2/hexanes yields pure product. Yield:
0.430 g, 45%. ESI-MS (m/z) [M + H]+: Calcd 429.181. Found:
429.181. CHN analysis: theory (CHN): C: 72.88, H: 5.65, N: 6.54.
1
Found: C: 72.59, H: 5.53, N: 6.32. H NMR (DMSO-d6, 600 MHz,
Figure S43): δ (ppm) 2.34 (s, 6H), 3.82 (s, 6H), 6.92 (s, 2H), 7.50 (s,
2H), 8.13 (d, 2H), 8.21 (t, 2H), 8.29 (d, 2H), 13.18 (s, 2H). 13C{1H}
NMR (DMSO-d6, 600 MHz, Figure S44): δ (ppm) 20.82, 55.79,
114.71, 119.11, 119.18, 119.25, 121.60, 127.17, 139.59, 146.37,
148.50, 151.86, 157.10.
Synthesis of Fe(mecrebpy)Cl, 2, IV. Ferric chloride hexahydrate
(Fe(Cl)3·6H2O, 0.088 g, 0.33 mmol), sodium acetate (NaOAc, 0.052
g, 0.64 mmol), and 6,6′-([2,2′-bipyridine]-6,6′-diyl)bis(2-methoxy-4-
methylphenol) (0.133 g, 0.310 mmol) were combined in a 100 mL
round-bottomed flask equipped with stir bar. EtOH (20 mL) was
added to the solids, and the solution was brought to reflux overnight
for 16 h. The resulting suspension was removed from heat, and the
dark red-black solid was collected by vacuum filtration and washed
with water (3 × 10 mL) and diethyl ether (3 × 5 mL). Crystals
suitable for X-ray diffraction were grown by slow cooling a boiling
acetonitrile solution saturated with 2. Yield: 0.120 g, 74.1%. UV−vis
in N,N-DMF: λmax: 354 nm (ε: 28 000 M−1 cm−1) and 473 nm (ε:
5300 M−1 cm−1). CHN analysis: theory (FeC26H22N2O4·0.5H2O): C:
60.31, H: 4.28, N: 5.41. Found: C: 59.15, H: 4.21, N: 5.09. ESI-MS
(m/z) [M + Na]+: Calcd 540.052. Found: 540.045. Magnetic
moment: 5.9 0.1 μB
Zn(mecrebpy). 6,6′-([2,2′-Bipyridine]-6,6′-diyl)bis(2-methoxy-4-
methylphenol) (0.61 g, 0.142 mmol), sodium acetate (23.9 mg,
0.29 mmol), zinc dichloride (0.020 g, 0.149 mmol), and absolute
ethanol (20 mL) were combined in a 100 mL round-bottomed flask
equipped with stir bar and condenser. The mixture was brought to
reflux (78 °C) overnight (16 h). The resultant yellow-orange
precipitate was collected via vacuum filtration, washed with DI
water (1 × 5 mL) and diethyl ether (1 × 5 mL). Yield: 21 mg, 30%.
CHN analysis: theory (ZnC26H22N2O4·3H2O): C: 57.21, H: 5.17, N:
1
5.13. Found: C: 56.90, H: 4.26, N: 5.05. H NMR (DMSO-d6, 600
MHz): δ (ppm) 2.25 (s, 6H), 3.76 (s, 6H), 6.65 (s, 2H), 7.10 (s, 2H),
8.01 (d, 2H), 8.13 (t, 2H), 8.31 (d, 2H). 13C{1H} NMR (DMSO-d6,
600 MHz): (L1) δ (ppm) 21.04, 55.10, 112.39, 117.82, 120.17,
123.24, 140.03, 147.76, 152.31, 156.88, 159.23.
3-Bromo-5-tert-butyl-benzene-1,2-diol,63 VI. 4-tert-Butyl catechol
(V) (10.0 g, 0.0602 mol) was dissolved in N,N-DMF (25 mL) in a
200 mL Schlenk round-bottomed flask with a stir bar under nitrogen.
A 2.0 M solution of N-bromosuccinimide in N,N-DMF (30.0 mL,
0.0662 mol) was added to the solution dropwise via syringe with
vigorous stirring. After 2 h, the solution was diluted with 100 mL of
ethyl acetate and washed with water (3 × 50 mL) and then with brine
(3 × 50 mL). The organic fraction was collected, dried over MgSO4,
and filtered, and the solvent was removed under reduced pressure to
yield a dark red oil. The oil can be used without further purification;
however, sublimation of the oil produces white X-ray quality crystals
of analytical purity. Yield: 13.2 g, 89.5%. ES-MS (m/z) [M]+: Calcd
244.0099. Found: 244.0098. 1H NMR (CDCl3, 600 MHz, Figure
S46): δ (ppm) 1.26 (s, 9H), 6.92 (d, 1H), 6.99 (d, 1H). 13C{1H}
NMR (CDCl3, 600 MHz, Figure S47): (L1) δ (ppm) 31.43, 34.55,
109.23, 112.69, 120.16, 137.98, 144.08, 145.78.
1-Bromo-5-tert-butyl-2,3-bis(methoxymethoxy)benzene, VII.
Dry dichloromethane (50 mL) was dispensed in an oven-dried
round-bottomed flask (250 mL). To this flask, an oven-dried stir bar
H
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119.45, 121.54, 125.57, 134.95, 136.76, 142.57, 147.85, 150.04,
155.96.
Accession Codes
CCDC 1938478−1938479 and 1940614 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
tbucatbpy(H)4, X. 6,6′-Bis(5-(tert-butyl)-2,3-bis(methoxymethoxy)-
phenyl)-2,2′-bipyridine (2.50 g, 3.78 mmol) was dissolved in minimal
EtOAc (∼100 mL) in a 250 mL round-bottomed flask equipped with
a stir bar. Excess anhydrous HCl was generated by adding
concentrated H2SO4 dropwise to solid NaCl in an airtight vessel
that vented through the EtOAc solution via a PTFE cannula transfer.
The residual HCl was quenched by venting through an outlet flask
containing saturated aqueous NaHCO3. The flask containing EtOAc
was then sealed and left to stir overnight. During the ∼9 h of stirring,
the EtOAc solution changed from colorless to yellow, and a solid
precipitated from the solution. After purging the EtOAc solution with
N2, saturated aqueous NaHCO3 was added to ensure neutralization.
The resultant solid was collected via vacuum filtration, washed with
water, and triturated with CHCl3 to yield the desired product as an
orange powder. Yield: 1.71 g, 93.3%. EI-MS (m/z) [M − H]+: Calcd
485.2440. Found: 485.2435. CHN analysis: theory (C30H32N2O4·
0.8CHCl3): C: 63.77, H: 5.70, N: 4.83. Found: C: 63.84, H: 5.69, N:
4.69. 1H NMR (DMSO-d6, 600 MHz, Figure S55): δ (ppm) 1.31 (s,
18H), 6.89 (s, 2H), 7.49 (s, 2H), 8.04−8.20 (m, 4H), 8.45 (s, 2H).
13C{1H} NMR (DMSO-d6, 600 MHz, Figure S56): δ (ppm) 31.33,
33.94, 113.88, 114.71, 118.78, 118.92, 121.44, 139.64, 141.06, 145.01,
145.86, 151.91, 157.70.
AUTHOR INFORMATION
■
Corresponding Author
Charles W. Machan − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904-4319, United States;
orcid.org/0000-0002-5182-1138; Email: machan@
virginia.edu
Authors
Asa W. Nichols − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904-4319, United States
Shelby L. Hooe − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904-4319, United States
Joseph S. Kuehner − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904-4319, United States
Diane A. Dickie − Department of Chemistry, University of
Virginia, Charlottesville, Virginia 22904-4319, United States;
orcid.org/0000-0003-0939-3309
Fe(tbucatbpy), 3. 6,6′-([2,2′-Bipyridine]-6,6′-diyl)bis(4-(tert-
butyl)benzene-1,2-diol) (0.150 g, 0.310 mmol), NaOAc (52.1 mg,
0.63 mmol), Fe(Cl)3·6H2O (0.088 g, 0.33 mmol), and absolute
ethanol (20 mL) were combined in a 100 mL round-bottomed flask
equipped with a stir bar and a condenser. The mixture was brought to
reflux (78 °C) overnight (16 h). The resulting black suspension was
removed from heat, and the volume was reduced to ∼10 mL in vacuo
before DI H2O (5 mL) was added, inducing the formation of a black
precipitate. The solid was collected by vacuum filtration and washed
with H2O (3 × 5 mL) and Et2O (3 × 5 mL). Yield: 0.119 g, 71%.
UV−vis in N,N-DMF: λmax: 290, 345, and 560 nm. ε: 31 000, 20 000,
and 3700 M−1 cm−1, respectively. EI-MS (m/z) [M-H]+: Calcd
535.1555. Found: 535.1553. CHNCl analysis: None. Theory
(FeC30H29N2O4·2H2O): C: 62.84, H: 5.80, N: 4.89, Cl: None.
Found: C: 63.08, H: 5.26, N: 4.92, Cl: None. Magnetic moment: 6.1
0.1 μB
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b03341
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the University of Virginia for generous
funding.
REFERENCES
■
Synthesis of Zn(tbucatbpy). 6,6′-([2,2′-Bipyridine]-6,6′-diyl)bis(4-
(tert-butyl)benzene-1,2-diol) (0.052 g, 0.108 mmol), sodium acetate
(18.1 mg, 0.22 mmol), zinc dichloride (0.015 g, 0.11 mmol), and
absolute ethanol (20 mL) were combined in a 100 mL round-
bottomed flask equipped with a stir bar and a condenser. The mixture
was brought to reflux (78 °C) overnight (16 h). After this time, a
yellow-orange precipitate formed and was collected via vacuum
filtration, washed with DI water (1 × 5 mL) and diethyl ether (1 × 5
mL). Yield: 39 mg, 68%. CHN analysis: theory: C: 65.76, H: 5.52, N:
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ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
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ip,c/ip,a data (PDF)
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