Photoinduced Generation of a Durable Thermal Proton Reduction Catalyst with in Situ Conversion of Mn(bpy)(CO)3Br to Mn(bpy)2Br2

Article abstract of DOI:10.1021/acs.inorgchem.0c00480

The conversion of protons to H2 is a critical reaction for the design of renewable fuel generating systems. Robust, earth-abundant, metal-based catalysts that can rapidly facilitate this reduction reaction are highly desirable. Mn(bpy)(CO)3Br generates an active catalyst for the proton reduction reaction upon photolysis at a high, directly observed H2 production rate of 1,300,000 turnovers per hour, with a low driving force for this reaction. Through the use of FcMe10 as an electron source, a proton source (triflic acid, 4-cyanoanilinium, or tosylic acid), and MeCN/H2O as solvent, the thermal reaction at room temperature was found to proceed until complete consumption of the electron source. No apparent loss in catalytic activity was observed to the probed limit of 10,000,000 turnovers of H2. Interestingly, a catalytically competent complex (Mn(bpy)2Br2), which could be isolated and characterized, formed upon photolysis of Mn(bpy)(CO)3Br in the presence of acid.

Full text of DOI:10.1021/acs.inorgchem.0c00480

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Article
Photoinduced Generation of a Durable Thermal Proton Reduction
Catalyst with in Situ Conversion of Mn(bpy)(CO)3Br to Mn(bpy)2Br2
Hunter Shirley, Sean Parkin, and Jared H. Delcamp*
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ABSTRACT: The conversion of protons to H2 is a critical
reaction for the design of renewable fuel generating systems.
Robust, earth-abundant, metal-based catalysts that can rapidly
facilitate this reduction reaction are highly desirable. Mn(bpy)-
(CO)3Br generates an active catalyst for the proton reduction
reaction upon photolysis at a high, directly observed H2
production rate of 1 300 000 turnovers per hour, with a low
driving force for this reaction. Through the use of FcMe10 as an
electron source, a proton source (triflic acid, 4-cyanoanilinium, or
tosylic acid), and MeCN/H2O as solvent, the thermal reaction at
room temperature was found to proceed until complete
consumption of the electron source. No apparent loss in catalytic
activity was observed to the probed limit of 10 000 000 turnovers
of H2. Interestingly, a catalytically competent complex (Mn(bpy)2Br2), which could be isolated and characterized, formed upon
photolysis of Mn(bpy)(CO)3Br in the presence of acid.
bromide complexes have been reported to dramatically change
reactivity toward protons or CO2 based on the addition of
water,23 and many of these complexes are reported under
conditions that rigorously exclude light with red-light
laboratory illumination being described in some literature
instances as a needed precaution.11,24,26−29 The use of dark
conditions is interesting, since the literature suggests that
discrete complexes are formed upon irradiation, which could
be catalytically active in reduction reactions.30 Whereas it can
be difficult to isolate and confirm the identity of photo-
generated compounds, the use of a photoinduced catalyst
generation approach is convenient in that a simple to prepare
starting complex can be transformed in situ to a new active
catalyst for evaluation in the proton reduction reaction.
To probe the effects of light on Mn(bpy)(CO)3Br driven
proton reduction reactions, a series of studies were performed
using nuclear magnetic resonance (NMR), crystallography,
infrared spectroscopy, electrochemical techniques, and thermal
catalysis with an SED under varied conditions (Figure 1). For
the thermal reduction reactions, a potential redox mediator,
decamethylferrocene (FcMe10), was selected as the sacrificial
INTRODUCTION
■
The efficient storage of energy in chemical bonds for use on
demand is a critical step forward in renewable fuel production.1
Proton reduction to H2 is a commonly pursued approach in
the search for a chemical fuel that can be derived from an
abundant resource such as water. If the hydrogen evolution
reaction is paired with water oxidation, only O2 is generated as
a stoichiometric waste.2 Ideally, these catalytic processes would
be driven by robust catalysts derived from earth-abundant
elements with fast rates of catalysis.3,4 The catalysts should also
durably operate with minimal driving force to avoid energetic
losses.3 Several reaction system designs coupling oxidative and
reductive processes are reported in the literature surrounding
electrochemical cells (EC) and photoelectrochemical cells
(PEC), including those employing redox mediators, which can
alleviate relative rate concerns of the anodic and cathodic
reactions.5−10 PEC and EC catalyst performances can be
efficiently probed through the study of half reactions using a
stoichiometric chemical electron source or sacrificial electron
donor (SED). This approach is common in the literature and
has led to the discovery of molecular catalysts used to drive
reductive reaction processes in EC and PEC systems.2,3,11−21
The use of an SED with reversible electron transfers (ETs) and
no rapid chemical transformation step following the electron
transfer step allows for the SED to potentially be used a redox
mediator in full PEC/EC systems.10
Received: February 14, 2020
Manganese bipyridyl complexes have recently been shown to
catalyze proton reduction reactions22 in addition to CO2
reduction reactions.23−25 Intriguingly, manganese tricarbonyl
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Figure 1. Photolytic CO dissociation from Mn(bpy)(CO)3Br (as
shown by GC) to give Mn(bpy)2Br2 in the presence of protons.
Mn(bpy)2Br2 is shown to facilitate the proton reduction reaction.
Figure 2. Absorption spectra of Mn complexes in MeCN before
photolysis (red), after photolysis (blue), after photolysis with addition
of TfOH (black), and after crystallization of Mn(bpy)2Br2 (green).
electron donor. This electron source was selected as a strongly
reducing electron donor (−0.51 V versus Fc+/Fc),14 which
leads to triflic acid in MeCN (2.6 pKa)31 resulting in an
estimated exothermic reaction via the Nernst equation (E = E°
− 0.059 pKa) at a free energy of −0.33 V when the standard
reduction potential of H+ in MeCN is taken at −0.03 V versus
Fc+/Fc (eq 1, Figure S1).32 The use of tosylic acid (8.6 pKa in
MeCN) results in an estimated slightly endothermic reaction
at a free energy of +0.03 V (eq 3).31,33,34 4-Cyanoanilinium (7
pKa in MeCN) was selected due to it being reasonably well-
behaved under anhydrous conditions with a comparable
intermediate free energy (−0.07 V) to the other acids in the
reaction being probed (eq 2).31,35 The reactions could be
driven at room temperature, and the use of proton and
electron sources without large driving forces for proton
reduction allows for the indirect probing of the activation
energy barrier of the catalyst in powering these reactions.
Given the dramatic effects water has shown in Mn catalyzed
CO2 reduction reactions, water in varying amounts is also
investigated.23
in the presence of coordinating MeCN. Upon irradiation of
Mn(bpy)(CO)3Br (red line), the absorption spectrum shows a
quenching of the absorption at 422 nm and growth of the blue-
shifted signal representative of the photolyzed, colorless
solution (blue line in Figure 2; Figure S2). Upon the
subsequent addition of TfOH, a signal at 370 nm arises,
which is a new photogenerated complex (Figure 2). Addition-
1
ally, H NMR analysis of photolyzed Mn(bpy)(CO)3Br in
CD3CN shows NMR signals in a broad shape with a high
signal-to-noise possibly due to the poor solubility of the
photolyzed complex in MeCN or the generation of a
paramagnetic complex with trace by-products generating the
observed signals (Figures S3 and S4). Addition of TfOH to the
NMR mixture after photolysis gives a by-product with sharp
signals in the aromatic region not attributed to either the pre-
photolyzed or post-photolyzed complex (Figure S5). No
hydride signal was evident in the 0 to −70 ppm range.
However, when TfOH, TsOH, or AcOH was added as an acid,
the known complex Mn(bpy)2Br2,37 could be obtained as X-
ray quality crystals by evaporating the volatile components and
taking the solids up in MeOH followed slow vapor diffusion of
diethyl ether to give the crystalline complex at the top of the
vial in high yield (>90% based on bpy, > 45% based on Mn).
The structure was confirmed by crystallography at 90 K
(Figure 1). Neither independently synthesized Mn(bpy)2Br2
2TfOH + 2FcMe10 → H2 + 2[FcMe10]+[TfO]−
(1)
ΔG = −0.33V
2[NC‐p‐C6H4NH3]+[BF ]− + 2FcMe10
4
→ H2 + 2NC‐p‐C6H4NH2 + 2[FcMe10]+[BF ]−
4
1
nor that formed in situ gives paramagnetic H NMR signals
(2)
between 200 ppm and −200 ppm (Figure S6).
ΔG = −0.07 V
Absorption spectroscopy with Mn(bpy)2Br2 (green line)
shows a matched absorption curve to the sample of
Mn(bpy)(CO)3Br exposed to light and TfOH (black dashed
line), indicating the complex is formed readily under the
reaction conditions in solution (Figure 2). It is not apparent
why the addition of an acid source is required to obtain the
Mn(bpy)2Br2 complex. Additionally, attenuated total reflec-
tance infrared spectroscopy (ATR-IR) confirms that the
collected crystals no longer have a CO related signal in the
2100−1650 cm−1 region (Figures S10 and S11). Consistent
with only one CO being observed extruding from the reaction
mixture via gas chromatography (GC), the IR spectrum of the
remaining material in the mother liquor shows numerous CO
signals with broadened features in the 2100−1650 cm−1
region, which may be due to either a number of species
being present or the formation of Mnx(CO)y-clusters (Figures
2TsOH + 2FcMe10 → H2 + 2[FcMe10]+[TsO]−
ΔG = +0.03V
(3)
RESULTS AND DISCUSSION
■
As expected based on literature precedent,30 the exposure of
Mn(bpy)(CO)3Br to ambient light in MeCN leads to the
quantitative loss of one CO ligand, as confirmed by GC in our
laboratories. During this time, the solution of the complex
changes from yellow to clear (Figures 2, S2 and S9, Table 1).
This color change is diagnostic since Mn(bpy)(CO)3Br is
known to dimerize to form [Mn(bpy)(CO)3]2 in non-
coordinating solvents, and [Mn(bpy)(CO)3]2 has significant
absorption spectrum features near 500, 650, and 850 nm.36
These features are not present during ambient light photolysis
B
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a
Table 1. Electrochemical and Optical Data for Mn(bpy)2Br2 and Mn(bpy)(CO)3Br
complex
λmax (nm)
λonset (nm)
Ered1 no H+ (V) peak | onset
Ered2 no H+ (V) peak | onset
Ered H+ (V) onset
icat/ip
Mn(bpy)(CO)3Br
photolyzed Mn(bpy)(CO)3Br
Mn(bpy)2Br2
422
302
371
485
325
375
 | −2.20
 | 
−1.25
−0.51
−0.55
13
17
23 (dark)
29 (light)
−1.79 | −1.65
−1.20 | −0.90
−1.97 | −1.85
 | 
a
All values are measured in MeCN. icat/ip is calculated from the CV curve where icat is the peak current with TfOH present and ip is the peak
current without TfOH, with Mn(bpy)(CO)3Br measured at −1.75 V, the photoinduced catalyst measured at −0.80 V, and Mn(bpy)2Br2 measured
at −1.20 V.
S10 and S11). This material is observed to absorb in the UV
region (<350 nm), suggesting loss of any possible metal−
ligand charge transfer bands due to loss of bipyridine (Figure
S2). The IR spectrum further confirms near complete loss of
bipyridine related features in the 1600−1400 cm−1 region. The
fate of the remaining Mn atom after two Mn(bpy)(CO)3Br
complexes are converted to Mn(bpy)2Br2 is not apparent. We
note that two CO molecules are observed in the headspace via
GC from two Mn(bpy)(CO)3Br complexes. This leaves four
CO groups and a Mn atom needed to balance the
stoichiometry of the transformation shown in Figure S12.
Subjecting the crystallized Mn(bpy)2Br2 to the chemical
reaction conditions found to be catalytically competent gave
H2 at a very similar rate to photolyzed Mn(bpy)(CO)3Br,
although it should be noted that the rate of this system is likely
SED solubility limited (see discussion below). Additionally,
Mn(bpy)(CO)3Br and Mn(bpy)2Br2 prepared via independent
methods gave the same aromatic NMR signals when the
complex was exposed to TfOH and the reducing reagent
FcMe10 (Figures S7 and S8). Although it cannot be confirmed
that the signals observed are not a by-product unrelated to
catalysis, it is plausible that the paramagnetic Mn(bpy)2Br2
formed in the reaction mixture can be reduced to a
diamagnetic Mn(I) compound. Notably, all components
necessary for catalytic H2 production are present once the
reducing reagent and TfOH are added, which suggests the
versus Fc+/Fc with the pKa of TfOH in MeCN taken at 2.6 and
estimated through the Nernst equation (see above). The
observed current enhancement is measured as an icat/ip value of
13 at the first reduction’s plateau (−1.75 V).28,38 The
photogenerated species shows a reduction potential onset
near −0.51 V versus Fc+/Fc in the presence of TfOH (middle:
black line). This corresponds to an overpotential of only 0.32
V and occurs 0.75 V more positive than that observed with
Mn(bpy)(CO)3Br under otherwise identical conditions
(middle: red line). Additionally, a large shift in reduction
potential onset (1.14 V) is observed when the CVs of
photolyzed Mn(bpy)(CO)3Br with (middle: black line) and
without (middle: blue line) TfOH are compared, which is
consistent with the formation of a relatively electron deficient
catalytically active complex upon the addition of TfOH. The
observed current enhancement is representative of a 17×
increase in current at the first reduction’s plateau (−0.80 V)
(Figure 3, see Figures S13−S16 for additional CV data).
Upon exposure of Mn(bpy)2(Br)2 to TfOH in the dark
(bottom: red line) or in the light (bottom: black line), a
current increase is observed (icat/ip of 23 and 29, respectively)
with a slight shoulder near the curve peak emerging, which is
not apparent from the curve with Mn(bpy)2(Br)2 in the
absence of TfOH (bottom: blue line) (Figure 3). No dramatic
difference is observed in the light or dark when TfOH is
present with respect to curve shape or current increase. The
slight shoulder observed near −1.1 V and the reduction wave
peak at −1.20 V with Mn(bpy)2(Br)2 match closely in
potential to the two shoulder features observed when
Mn(bpy)(CO)3Br is exposed to light and TfOH (bottom:
green line). The features of the Mn(bpy)(CO)3Br voltammo-
gram taken in the presence of light and TfOH (green line)
appear to be comprised of the catalytic reduction wave features
of the bipyridine free Mn material (gray line with TfOH,
purple line without TfOH) and Mn(bpy)2Br2.
To confirm the current enhancement observed via CV is due
to proton reduction, controlled potential electrolysis (CPE)
studies were carried out at the initial current plateau potential
observed via CV for photolyzed Mn(bpy)(CO)3Br in the
presence of TfOH with a glassy carbon working electrode,
glassy carbon counter electrode, and a silver wire reference
electrode. Molecular hydrogen is produced with a Faradaic
efficiency (FE) of 100% when applying a constant potential at
the first reduction (−0.80 V) of photolyzed Mn(bpy)(CO)3Br
(Figure 4, red line). However, at the same potential negligible
molecular hydrogen relative to the background reaction (blue
line) is produced when Mn(bpy)(CO)3Br is kept in the dark
(black line). A steady rate of charge passing is observed over a
2 h time period for photolyzed Mn(bpy)(CO)3Br with no
evidence of catalyst decomposition based on the lack of charge
passage rate changes.
1
observed H NMR signals may either be a by-product or the
resting state of the catalyst. Importantly, both complexes
1
appear to be doing the same chemistry given the same H
NMR spectrum is arrived at after proton and reducing reagent
are added.
Electrochemically, a significant change in the reduction
potential of Mn(bpy)(CO)3Br is observed after light exposure
in MeCN (Figure 3 top: green line (dark), blue line (light),
Table 1). A shift of 0.55 V toward a more positive potential is
observed with a first reduction potential peak of −1.79 V for
photolyzed Mn(bpy)(CO)3Br under argon (top: blue line).
Even under conditions that rigorously exclude light, small
amounts of the photolyzed product are present in the “dark”
with Mn(bpy)(CO)3Br. Mn(bpy)2Br2 shows a reduction wave
onset near −0.9 V with a reduction wave peak at −1.20 V (top:
black line). Introduction of a proton source (TfOH) leads to a
shift toward more positive potentials for the Mn(bpy)(CO)3Br
complex in the dark with an onset of −1.25 V, and a peak
potential observed at −2.24 V after an initial plateau near
−1.75 V (middle: green line (without TfOH), red line (with
TfOH)). The observed shift in electrochemical potential in the
dark suggests an initial chemical reaction has taken place
resulting in a new species with a more positive reduction
potential. The observed onset of reduction is estimated to be
an overpotential of 1.07 V as calculated from standard
reduction potential of protons in MeCN solvent at −0.18 V
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Figure 4. Charge passage over time graph for CPE experiments with
Mn(bpy)(CO)3Br (black, 1 mM) and photolyzed Mn(bpy)(CO)3Br
(red, 1 mM). The blue line is a background reaction with all
components kept constant minus catalyst.
durability with an SED that could be rendered catalytic in EC/
PEC systems.5−10 Mn(bpy)(CO)3Br was first dissolved in
MeCN and TfOH, then FcMe10 was added in the dark. After 4
h, no appreciable H2 was detected in the headspace (Figure 5,
Figure 5. TON vs time under identical reaction conditions (0.1 mM
catalyst, 0.19 M TfOH, 33.7 μmol of FcMe10, and 2% H2O), but with
varying light exposure parameters. The black curve shows a reaction
that was kept shielded from light for 4 h prior to being placed in front
a solar simulator at 1 sun intensity. The red curve shows a reaction
that was irradiated constantly. The green curve shows a reaction that
was prephotolyzed before TfOH addition and then placed in the dark.
Figure 3. Top: CVs of Mn(bpy)(CO)3Br in the light or dark and
Mn(bpy)2Br2, all without TfOH. Middle: Cyclic voltammograms for
Mn(bpy)(CO)3Br at 1 mM (μmol cat/mL MeCN) in the light or
dark and with or without 0.19 M TfOH present. Bottom: CVs with
Mn(bpy)2Br2, Mn(bpy)(CO)2Br, or the remaining Mn material after
crystallization under varied conditions. A scan rate of 100 mV/s is
used in all cases. The electrolyte is 0.1 M Bu4NPF6 in MeCN with
glassy carbon working, platinum counter, and Ag-wire reference. The
onset potentials in Table 1 are determined as shown by the purple
arrow example on the middle graph.
black line). Upon exposure to 1 sun intensity solar simulated
light for 5 min, catalysis immediately began, with no additional
irradiation needed and catalysis proceeded until a high yield
(85%) of H2 was observed with respect to the amount of
FcMe10 added assuming 2 FcMe10 SED molecules are required
per H2 molecule produced (Table 2, entry 1). This
corresponds to 29 turnover numbers (TONs) calculated via
the equation TON = (H2 mol/Mn-catalyst moles). A turnover
frequency (TOF) of 81 h−1 is observed over the initial 20 min
period of the reaction, according to the equation TOF =
TON/time. The reaction was found to follow a near identical
profile with respect to a TON versus time plot during
continuous irradiation (red line) or irradiation prior to acid
and SED addition (green line) indicating the reaction is
photoinitiated without any significant deactivation pathway,
which would require photons to access the active catalyst again
Thermal catalytic studies with an SED (FcMe10) were
undertaken having found the resulting complex from
irradiation of Mn(bpy)(CO)3Br to have desirable rates of
proton reduction and favorable reaction energetics via
electrochemical analysis. The use of a potential redox mediator
as an SED is attractive in allowing for the probing of catalyst
D
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Table 2. Reactions Performed with Photolyzed Mn(bpy)(CO)3Br or Mn(bpy)2Br2, FcMe10 (33.7 μmol), and 0.19 M TfOH (or
a
Replacement Acid) in MeCN (5 mL) for 4 h unless Otherwise Noted
b
c
entry
[catalyst]
0.1 mM
none
none
0.1 mM
0.1 mM
0.1 mM
0.1 mM
change
TON of H2
TOF (h−1
)
H2 yield
Benchmark Experiment and Controls
1
2
3
4
5
6
7
none
no catalyst
29
<1
<1
<1
<1
<1
<1
81
<1
<1
<1
<1
<1
<1
85%
0%
0%
0%
0%
0%
0%
no catalyst, + 2% H2O
no FcMe10
no TfOH
d
−FcMe10, + BIH
−FcMe10, + ferrocene
Acid Effects
e
8
0.1 mM
0.1 mM
Mn(bpy)2Br2 (0.1 mM)
0.1 mM
0.1 mM
−TfOH, + [NC-p-C6H4NH3]+
−TfOH, + TsOH
−TfOH, + TsOH
−TfOH, + TFA
−TfOH, + AcOH
H2O Effects
25
22
20
<1
<1
8
74%
65%
59%
0%
9
12
13
<1
<1
10
11
12
0%
13
14
15
0.1 mM
0.1 mM
0.1 mM
+2% H2O
+5% H2O
+10% H2O
34
10
5
24
13
11
quant.
29%
15%
Catalyst Durability Evaluation
+2% H2O
+2% H2O
+2% H2O
+2% H2O
+2% H2O
+2% H2O, 3× FcMe10
+2% H2O, 3× FcMe10
16
17
18
19
20
0.01 mM
1.0 μM
0.1 μM
0.01 μM
1.0 nM
340
3400
34,000
340,000
3,400,000
10,000,000
10,000,000
140
1,600
16,000
120,000
1,100,000
1,300,000
1,300,000
quant.
quant.
quant.
quant.
quant.
quant.
quant.
f
21
22
1.0 nM
Mn(bpy)2Br2 1.0 nM
f
a
b
TON vs. time plots are shown in Figures S17−22. TOF is calculated as an initial rate by taking a sample after the first 20 min of the reaction and
dividing the observed TON by the time elapsed. Note: the TOF value only establishes a lower limit for catalytic rate (see discussion below), and
the TON value for Mn(bpy)(CO)3Br is calculated with respect to the amount of starting complex rather than the amount of active catalyst formed.
% yield of H2 is calculated based on the number of H2 molecules observed divided by half the number of FcMe10 molecules present and
multiplied by 100%. 1,3-Dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial reductant. Reaction was monitored over 12
c
d
e
f
h. Reaction was monitored over 22 h.
(i.e., continuous illumination is not necessary, Figure 5).
Removal of the Mn catalyst, FeMe10, or TfOH from the
reaction mixture led to no appreciable H2 formation (Table 2,
entries 2−5; Figure S22). Exchange of the SED for either BIH
or ferrocene led to no H2 product formation, presumably due
to these SEDs being less reducing, resulting in endothermic
reactions with free energies of +0.08 V and +0.18 V,
respectively (Table 2, entries 6−7; Figure S22).12
Exchange of the TfOH proton source for acids with higher
pKas was evaluated with 4-cyanoanilinium tetrafluoroborate,
TsOH, trifluoroacetic acid (TFA), and acetic acid (AcOH) to
examine the effects of changing the free energy of the reaction
being catalyzed (Table 2, entries 8−12; Figure S20). The free
energies for the proton reduction reactions with FcMe10 are
estimated at −0.07 V, + 0.03 V, + 0.78 V, and +1.42 V for 4-
cyanoanilinium tetrafluoroborate, TsOH, TFA, and AcOH,
respectively. Notably, the rate of chemical reduction with 4-
cyanoanilinium is slowed relative that of TfOH (TOF of 8
versus 81 h−1, respectively); however, the overall TON value is
similar at 25 versus 29 for the reportedly more well-behaved 4-
cyanoanilinium (Table 2, entry 8).31 Interestingly, TsOH was
found to be a competent proton source for this reaction,
indicating the thermal energy present at room temperature is
sufficient to overcome any activation energy barriers and drive
this slightly endothermic reaction. A TON of 22 was observed
for a 65% H2 yield with respect to the SED. A slower TOF is
observed with TsOH and photolyzed Mn(bpy)(CO)3Br to
TfOH (12 versus 81 h−1). Similarly, crystallographically
characterized Mn(bpy)2Br2 was found to react TsOH for 20
TONs and a TOF of up to 13 h−1 (Table 2, entry 10; Figure
S21). As expected, the larger endothermic reactions with TFA
and AcOH gave no appreciable reactivity.
Water was explored as an additive for this reaction since
Mn(bpy)3(CO)3Br is known to undergo dramatic catalysis
changes in the presence of H2O.23 Addition of 2% H2O v/v
with MeCN led to an increase in observed H2 with a TON of
34 for a quantitative H2 yield (Table 2, entry 13; Figure S19).
The TOF of the reaction slowed by ∼4× upon water addition
for this reaction, although it is not clear if this is due to a
change in pKa of the proton source with the new solvent
medium changing the reaction energetics or some other role.
Further increasing the percent composition of H2O to 5% and
10% led to diminished TON values of 10 and 5 along with
decreased percent yield values of H2 at 29% to 15% and a
dramatic slowing of catalyst TOF by 10× (Table 2, entries 14
and 15, Figure S19). This loss in reactivity may be due to
changes in reaction energetics or additional loss of solubility of
FcMe10, which is already sparingly soluble in pure MeCN and
reacts into solution in dry MeCN over time. The solubility of
FcMe10 is likely rate limiting under these conditions.
With optimal conditions identified for the proton reduction
reaction, the catalyst durability was probed by decreasing the
E
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catalyst concentration and holding the remaining component
concentrations constant (Table 2, entries 16−20; Figure S17).
Interestingly, from 0.1 mM to 1.0 nM photolyzed Mn(bpy)-
(CO)3Br was found to quantitatively consume electrons from
FcMe10 to produce H2 with up to 3 400 000 TONs observed
with a TOF observed of >1 000 000 h−1. Given that complete
consumption of FcMe10 is turnover limiting, CPE was
attempted with FcMe10 to probe if an increase in H2
generation rate electrochemically could be observed with an
added redox shuttle that might have advantageous interfacial
electron transfer kinetics at an electrode surface relative to the
active catalyst; however, catalysis was found to slow during
electrolysis when FcMe10 was added due to electrode fouling as
a result of the limited solubility of FcMe10. The near linear
changes in TOF across the 5 orders of magnitude in catalyst
concentrations probed in the thermal reaction suggests that the
reaction rate is likely limited by the relative concentration of
the electron source in solution under optimized conditions.
The solubility limit of FcMe10 is estimated at 1 mg per 75 mL
of MeCN, and solid FeMe10 can be observed reacting into
solution over time. To assess the catalyst durability limits, a
large excess of FcMe10 was added, and a TON value of
∼10 000 000 was observed with a TOF value of 1 300 000 h−1
(Table 2, entry 21; Figure S18). The same values are observed
with Mn(bpy)2Br2 as the starting complex (Table 2, entry 22).
The leveling off of TOF after dramatically increasing the
amount of FcMe10 while holding catalyst concentration
constant further suggests the electron source solubility is
limiting catalytic rates. Thus, the reported TOF values can only
be used to establish a lower limit catalyst TOF under the
optimized conditions since the maximum rate cannot be
observed due to the limited solubility of FeMe10. Notably, the
TOF and TON values in Table 2 can be effectively doubled
when starting with Mn(bpy)(CO)3Br since Mn(bpy)2Br2 is
formed seeming cleanly under the reaction conditions in >90%
yield relative to bpy ligand and displays very similar reactivity.
Experimental details, additional spectroscopic informa-
tion, catalysis experiments (PDF)
Accession Codes
CCDC 2003559 contains the supplementary 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
Jared H. Delcamp − Department of Chemistry and
Biochemistry, University of Mississippi, University, Mississippi
38677, United States; orcid.org/0000-0001-5313-4078;
Email: delcamp@olemiss.edu
Authors
Hunter Shirley − Department of Chemistry and Biochemistry,
University of Mississippi, University, Mississippi 38677, United
States; orcid.org/0000-0002-6224-556X
Sean Parkin − Department of Chemistry, University of Kentucky,
Lexington, Kentucky 40506, United States; orcid.org/0000-
0001-5777-3918
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00480
Funding
This work is supported by NSF Award 1800281. The D8
Venture diffractometer at the University of Kentucky was
funded by the NSF (MRI CHE-1625732).
Notes
The authors declare no competing financial interest.
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CONCLUSION
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Mn(bpy)(CO)3Br was shown to photolyze to a new catalyst
for H+ reduction. Upon addition of H+, Mn(bpy)2(Br)2 could
be crystallized after photolysis of Mn(bpy)(CO)3Br, and upon
subjecting these crystals to the reaction conditions, similar
reactivity was observed for Mn(bpy)(CO)3Br in the same
reaction conditions. The photogenerated catalyst was found to
catalyze the reduction of protons at a lower potential and a
larger icat/ip ratio than Mn(bpy)(CO)3Br via cyclic voltamme-
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which powers the low overpotential, thermal reaction of TsOH
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00480.
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