Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
1 of 9
CHEMISTRY
Catalytic transfer hydrogenation of
N
2
to NH
3
via
a photoredox catalysis strategy
Christian M.
Johansen†, Emily A.
Boyd†, Jonas C.
Peters*
Inspired by momentum in applications of reductive photoredox catalysis to organic synthesis, photodriven trans-
fer hydrogenations toward deep (>2 e
−
) reductions of small molecules are attractive compared to using harsh
chemical reagents. Noteworthy in this context is the nitrogen reduction reaction (N
2
RR), where a synthetic photo-
catalyst system had yet to be developed. Noting that a reduced Hantzsch ester (HEH
2
) and related organic struc-
tures can behave as 2 e
−
/2 H
+
photoreductants, we show here that, when partnered with a suitable catalyst (Mo)
under blue light irradiation, HEH
2
facilitates delivery of successive H
2
equivalents for the 6 e
−
/6 H
+
catalytic reduc-
tion of N
2
to NH
3
; this catalysis is enhanced by addition of a photoredox catalyst (Ir). Reductions of additional
substrates (nitrate and acetylene) are also described.
INTRODUCTION
Multielectron reductive transformations of small-molecule sub-
strates (e.g., N
2
, CO
2
, and NO
3
−
) are challenging to mediate in ho-
mogeneous catalysis and most typically require considerable energy
input via harsh chemical reagents and/or conditions to be driven
forward. The nitrogen reduction reaction (N
2
RR) offers a case in
point; substantial progress has now been made in molecular catalyst
design, but substantial overpotentials are generally needed to ob-
serve the NH
3
product (
1
–
3
). For nitrogen reduction (N
2
R), kinetic
challenges also prevail for enzymatic and heterogeneous catalysis
that require substantial energy inputs, via adenosine triphosphate
hydrolysis for the former and high temperature and pressure or
electrochemical overpotential for the latter (
4
–
6
), despite a thermal-
ly favorable Gibbs free energy of formation,
G
f
(NH
3
) (Fig. 1A).
The organometallic catalysis field has pursued photochemical
strategies as a means of driving small-molecule reductions, with
considerable success being achieved for CO
2
reduction (CO
2
R; typ-
ically by 2 e
−
/2 H
+
) as the target transformation (
7
,
8
). These strat-
egies are still challenged by the widespread use of sacrificial donors
whose oxidation products are not readily recycled. While design
schemes are envisaged to someday couple photodriven CO
2
R catal-
ysis with water oxidation, photodriven transfer hydrogenation using
a suitable precatalyst offers an approach to reductive small-mole-
cule catalysis, especially if the net H
2
donor (subH
2
; Fig. 1B) derives
from a structure that can be efficiently recycled, for example, via
hydrogenation or electrochemically.
Reduced Hantzsch esters (HEH
2
; Fig. 1B) and chemically related
structures (e.g., reduced acridine and phenanthridine) have been
explored for thermally and photochemically driven reductive hy-
dride (H
−
; NADH-like) and H atom transfers in organic synthesis
(
9
). Moreover, they are highlighted for their chemical (and electro-
chemical) recyclability via net hydrogenation of the spent pyridine-
type oxidation product (
10
,
11
). Whereas the types of transformations
they participate in are most typically two-electron processes, they
are also tempting to explore for deeper multielectron reductions of
the type pursued in small-molecule reductive catalysis. Focusing on
N
2
R (
12
), we noted that despite long known and still debated studies of
photocatalytic nitrogen fixation using semiconductors (
13
–
15
), and
photodriven N
2
R mediated by nitrogenase coupled with CdS (
16
,
17
),
as yet, there were no examples of photochemically driven catalytic
N
2
R using well-defined molecular systems. Hence, photoinduced
N
2
R via transfer hydrogenation from a Hantzsch ester or related
donor, which requires the donors to engage in successive transfers
to mediate a deep 6 e
−
/6 H
+
reduction process, provides an excellent
test case of this strategy for small-molecule substrates.
Considering thermodynamic parameters relevant to the afore-
mentioned goals, in its ground state, the first C
─
H bond dissocia-
tion free energy (BDFE
C
─
H
) of HEH
2
is 62.3 kcal mol
−1
in MeCN at
25°C (all following thermochemical values are defined at these con-
ditions), which is not weak enough to bimolecularly liberate H
2
(
18
). Photoexcitation of HEH
2
, however, renders an excited state that
is highly reducing [
E
ox
for [HEH
2
]
*
is ~ −2.6 V versus ferrocenium/
ferrocene (Fc
+/0
)] (
19
,
20
). Photodriven [blue light-emitting diode
(LED)] reduction of
-bromoacetophenone to acetophenone by HEH
2
illustrates its capacity to deliver an H
2
equivalent (Fig. 1B) (
19
). For
a dark N
2
R reaction, we estimate the overpotential for reduction
of N
2
by HEH
2
to generate NH
3
as 1.8 kcal mol
−1
[
G
f
(NH
3
);
Fig. 1C]. Using light (blue LED), we show here that it is indeed pos-
sible to catalyze photoinduced transfer hydrogenation from HEH
2
to N
2
using Nishibayashi’s molybdenum precatalyst (Fig. 1C) (
21
)
at atmospheric pressure and 23°C.
The inclusion of an Ir photore-
dox catalyst (Fig. 1C) within this system, while not necessary for
turnover, enhances the yields and rates of NH
3
generation.
For our present catalysis system, we noted that a photoreduction step
from the excited state of HEH
2
, [HEH
2
]
*
, liberates the ground-state rad-
ical cation HEH
2
•+
, which is a sufficiently strong oxidant (
E
red
= 0.48 V
versus Fc
+/0
) to be deleterious to N
2
R (
18
). We therefore reasoned that
inclusion of a base to deprotonate HEH
2
•+
(p
K
a
~ −1) would be prudent
(
18
). However, the presence of a moderate Brønsted acid is typically re-
quired for chemically driven N
2
R, suggesting that a buffered system
might be needed. A collidine/collidinium [abbreviated as Col/[ColH]
+
;
Col (2,4,6-trimethylpyridine)] mixture was chosen as Col will readily
deprotonate HEH
2
•+
, while [ColH]
+
, with a p
K
a
of 15 in MeCN (
22
), has
been previously shown to be compatible with chemically driven N
2
R
using (PNP)MoBr
3
as a precatalyst {PNP [2,6-bis(di-
tert
-butylphosphi-
nomethyl)pyridine]} with (Cp
*
)
2
Co [
E
1/2
(Co
III/II
) = −1.91 V; Cp
*
(pentamethylcyclopentadienyl)] as the reductant (
21
,
23
).
AQ3
AQ4
Division of Chemistry and Chemical Engineering, California Institute of Technolo-
gy, Pasadena, CA 91125, USA.
*Corresponding author. Email: jpeters@caltech.edu
†These authors contributed equally to this work.
Copyright © 2022
The Authors, some
rights reserved;
exclusive licensee
American Association
for the Advancement
of Science. No claim to
original U.S. Government
Works. Distributed
under a Creative
Commons Attribution
License 4.0 (CC BY).
Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
2 of 9
RESULTS AND DISCUSSION
We find that [Mo]Br
3
(1 equiv at 2.3 mM) in the presence of 54
equiv each of HEH
2
, [ColH]OTf [OTf (triflate)], and Col in tetrahy-
drofuran (THF), under an N
2
atmosphere and blue LED irradiation
at 23°C for 12 hours, yields 9.5 ± 1 equiv of NH
3
/Mo (Fig. 2, entry 1).
Assuming that HEH
2
is a 2 e
−
donor in this process provides an NH
3
yield with respect to HEH
2
of ~25%. Use of
15
N
2
confirmed N
2
as
the source of the NH
3
produced (fig. S2). To cement this interpreta-
tion, using either
15
N-labeled HEH
2
or
15
N-labeled Col/[ColH]OTf
produced only
14
NH
3
. Analysis of the organic products following
catalysis revealed complete consumption of HEH
2
, with the fully
oxidized Hantzsch ester pyridine (HE) as the major organic by-
product, consistent with HEH
2
acting as a 2 e
−
/2 H
+
donor. We note
that the yield of HE is ~90%; similarly, ~10% of the initial buffer
loading is not recovered (fig. S7). In addition to HE and recovered
buffer, a complex mixture of organic species is observed follow-
ing catalysis. A major component of this mixture is generated inde-
pendently via irradiation of HEH
2
and buffer in the absence of
metal catalysts (fig. S8), possibly as a result of light-induced reduc-
tive coupling as has been previously observed upon irradiation of
HE in the presence of amine reductants (
24
). Another factor limit-
ing NH
3
selectivity per HEH
2
concerns background hydrogen evo-
lution under blue light irradiation (see fig. S10).
Higher yields of NH
3
per Mo center could be obtained by de-
creasing the [Mo]Br
3
loading (21.8 ± 0.8 equiv per Mo; entry 2), but
with a loss in the yield of NH
3
with respect to HEH
2
. The Mo cata-
lyst and irradiation were required to generate NH
3
, and yields were
substantially lower without the added buffer (entries 3 to 5). Attempts
to use catalytic amounts of Col/[ColH]OTf (5 equiv per [Mo]Br
3
)
substantially lowered the NH
3
yields (entry 6). The reaction run in
benzene instead of THF solvent remained catalytic but gave attenuated
yields (4.7 ± 0.1 equiv of NH
3
/Mo; entry 7), likely because of the
lower solubility of [ColH]OTf in benzene.
While future studies are needed to probe the mechanism of this
transformation, the fate of photoexcited [HEH
2
]
*
is likely key. Two
limiting scenarios to consider are the direct reduction of N
2
R
Fig. 1. Thermodynamics and strategies for hydrogenation of N
2
.
(
A
) Thermodynamics of hydrogenation of N
2
to NH
3
. (
B
) Schematic of an overall design for
light-driven transfer hydrogenation of N
2
, chemical structure of the Hantzsch ester used in this study (HEH
2
), and representative reduction of
-bromoacetophenone.
(
C
) Net stoichiometry and estimated driving force of transfer hydrogenation from HEH
2
to N
2
, forming NH
3
; photodriven (blue LED) process described in this study, in the
absence and presence of a photoredox catalyst. All thermochemical values are given in MeCN at 25°C with ferrocenium/ferrocene (Fc
+/0
) as the reference potential.
RT, room temperature.
Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
3 of 9
Fig. 2. Catalytic yields for photodriven transfer hydrogenation of N
2
to NH
3
, NO
3
−
to NH
3
, and acetylene to ethylene and ethane.
Reactions performed with
2.3 mM [Mo]Br
3
concentration, using a single 34-W Kessel H150 blue lamp unless otherwise noted. All yields reported are an average of at least two runs. All runs with
Ir used 2.3 mM photosensitizer loading unless otherwise noted.
a
3.6 mM [Mo]Br
3
.
b
3.6 mM [Ir]BAr
F
4
. [Ir], [Ir(ppy)
2
(dtbbpy)]
+
; ppy, 2-phenylpyridinyl; dtbbpy, 4,4′-di-
tert
-
butyl-
2,2′-bipyridine; BAr
F
4
, tetrakis(3,5-bis(trifluoromethyl)phenyl)borate; dF(CF
3
)ppy, 5-trifluoromethyl-2-(3,5-difluoro-phenyl)-pyridine;
p
-F(Me)ppy, 5-methyl-2-(5-fluoro-
phenyl)-
pyridine; PF
6
−
, hexafluorophosphate.
Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
4 of 9
intermediates by [HEH
2
]
*
(fig. S20) or the reduction of the [ColH]
OTf to [ColH]
•
radical, which then reacts with M(N
2
) (Fig. 3A) to
form an N
─
H bond via M(N
2
H). Pyridinyl radicals have been posited
as possible intermediates of N
2
R in thermally driven catalysis with
molecular systems (
25
). Increasing the buffer concentration to 216
equiv per Mo boosted the NH
3
yield to 20.3 ± 1.1 equiv of NH
3
/Mo
(entry 8). This observation points to a pathway whereby reduction
of [ColH]OTf by [HEH
2
]
*
dominates (Fig. 3A), consistent with the
high reactivity expected of [HEH
2
]
*
(
E
ox
~ −2.6 V; p
K
a
~ −20; BD-
FE
C
─
H
~ −8.5 kcal mol
−1
) and its short solution lifetime [0.419 ns in
dimethyl sulfoxide (DMSO) solvent at 25°C] (
18
,
20
). Accordingly,
steady-state fluorimetry studies show efficient quenching of [HEH
2
]
*
upon titrating in [ColH]OTf (fig. S11). Similar titrations of Col revealed
less-efficient quenching (fig. S12). However, as some NH
3
can be
detected under irradiation even in the absence of buffer (entry 5),
other photoinduced pathways for N
─
H bond formation via HEH
2
are accessible. The addition of 10 equiv of tetrabutylammonium bro-
mide (TBABr) had no effect on the NH
3
yield (entry 9), suggesting
that reductive Br
−
loss from the precatalyst is not a limiting factor.
Figure 3A provides a generalized mechanistic outline to help il-
lustrate how a photon might facilitate delivery of H
2
from HEH
2
to
M(N
2
), to first generate an M(NNH
2
) intermediate, and to ultimately
generate NH
3
via successive H
2
transfers. For simplicity, we show
only this one scenario in Fig. 3A but emphasize that other scenarios,
including the early generation and then reduction of a terminal ni-
tride intermediate (Mo
≡
N + HEH
2
→
Mo(NH
2
) + HE) (fig. S21),
are also very plausible (
26
). A recent study showed that a Mn
V
≡
N
can be photoreduced by 9,10-dihydroacridine to liberate NH
3
(
27
).
Limitations stemming from a short [HEH
2
]
*
excited-state life-
time and low-quantum yield (0.031) (
20
) for HEH
2
motivated us to
explore a photosensitizer to enhance this photodriven catalysis. To
test this idea, [Ir(ppy)
2
(dtbbpy)]BAr
F
4
([Ir]BAr
F
4
;
E
1/2
(Ir
III/II
) =
−1.90 V) was chosen as its reduction potential is close to that of
Cp
*
2
Co and hence should be compatible with N
2
R using [Mo]Br
3
(
21
,
28
).
Including [Ir]BAr
F
4
with [Mo]Br
3
(1 equiv, both at 2.3 mM), in
addition to 54 equiv each of HEH
2
and Col/[ColH]OTf in THF,
under an N
2
atmosphere and blue LED irradiation for 12 hours
at 23°C, yields 24 ± 4 equiv of NH
3
/Mo (entry 10). Assuming that
HEH
2
is a 2 e
−
/2 H
+
donor, these conditions correspond to an over-
all NH
3
yield of 67 ± 10% with respect to HEH
2
. Furthermore, in
the presence of the Ir photosensitizer, catalytic amounts of buffer
can be used, producing 15.8 ± 0.8 equiv of NH
3
/Mo (entry 11).
In addition to higher yields, the inclusion of [Ir]BAr
F
4
enhances the
photocatalytic rate; the catalysis is ~80% complete after 30 min (entry
12). By contrast, under Ir-free conditions, 2-hour reaction times
are required to achieve ~80% completion (entry 13). Comparing
this photodriven Mo-catalyzed N
2
R via HEH
2
with thermally driv-
en Mo-catalyzed N
2
R using (Cp
*
)
2
Co and [ColH]OTf as reported
by Nishibayashi, we find that the NH
3
yields with respect to reduc-
tant are quite similar (69% for the latter case) (
21
).
As in the Ir-free process, lowering the [Mo]Br
3
loading increased
the turnover for NH
3
with catalytic buffer (26.0 ± 0.4 equiv of
NH
3
/Mo; entry 14), but with decreased total yield. No NH
3
is pro-
duced without irradiation (entry 15), and the presence of [Mo]Br
3
and HEH
2
are likewise essential (entries 16 and 17). Similar to the
Ir-free reaction, HE was found to be the major organic product
(>80%), and complete consumption of HEH
2
was observed (fig. S4).
Solvent screening suggests that the reaction is most efficient when
all components are soluble (see table S5). By contrast, other catalytic
AQ7
Fig. 3. Possible scenarios for photodriven transfer hydrogenation from HEH
2
to N
2
mediated by a metal catalyst and buffer system (Col/[ColH]
+
).
(
A
) Scenario in
the absence of photoredox catalyst, in which [HEH
2
]* is oxidatively quenched by [ColH]
+
to generate [ColH]
•
. (
B
) Scenario with photoredox catalyst, in which [Ir
III
]
+
* is re-
ductively quenched by HEH
2
. Pathways involving N
≡
N bond cleavage to yield M
≡
N intermediates (not shown) are also plausible (fig. S21).
Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
5 of 9
N
2
R methods rely on low solubility of either the acid or the reduc-
tant to attenuate competing H
2
evolution, demonstrating an advan-
tage to using a terminal H atom source that is not competent for H
2
release in the ground state (
1
).
A range of candidate H
2
carriers, subH
2
, should be explored in
future studies to identify donors whose spent products can be recycled
efficiently, perhaps in situ, via hydrogenation with H
2
or electro-
chemically (2 e
−
/2 H
+
). In an initial survey, the Ir-photosensitizer
cocatalyst enables catalytic production of NH
3
under irradiation
with 9,10-dihydroacridine or 5,6-dihydrophenanthridine as the H
2
donor (6.4 ± 0.3 equiv of NH
3
/Mo and 4.6 ± 0.8 equiv of NH
3
/Mo,
respectively; entries 18 and 19). While noncatalytic, N
2
-to-NH
3
conversion is also achieved with [Ir]BAr
F
4
and the H
−
donor 1-benzyl-
1,4-dihydronicotinamide (1.2 ± 0.1 equiv of NH
3
/Mo; entry 20). In
the absence of [Ir]BAr
F
4
, none of these H
2
or H
−
carriers are compe-
tent for the photoinduced N
2
RR (see table S2). The reaction with
HEH
2
tolerates a 1:1 mixture of N
2
and H
2
(1 atm of total pressure,
14 ± 4 equiv of NH
3
/Mo; entry 21), indicating that the Mo catalyst
is not (at least irreversibly) poisoned by H
2
under these conditions,
important for considering downstream recycling of the spent donor.
In addition to varying the subH
2
, we have examined the effect of
varying the Ir-photosensitizer. [Ir(dF(CF
3
)ppy)
2
(dtbbpy)]PF
6
yielded
substantially less NH
3
(entry 22) than [Ir]PF
6
(entry 23) or [Ir]
BAr
F
4
(entries 10 and 12; Fig. 2). [Ir
II
(dF(CF
3
)ppy)
2
(dtbpy)] is also
less reducing (
E
1/2
(Ir
III/II
) = −1.75 V) (
29
), possibly pointing to a
redox-
based cutoff for photodriven N
2
R. Accordingly, [Ir(
p
-F(Me)
ppy)
2
(dtbbpy)]PF
6
(
E
1/2
(Ir
III/II
) = −1.88 V) restores the yields ob-
served in the parent system (entry 24) (
30
). However, Ir(ppy)
3
, despite
having the strongest reduction potential (
E
1/2
(Ir
III/II
) = −2.57 V),
gave attenuated NH
3
yields (entry 25) and therefore suggests that
multiple factors may be at play.
Figure 3B provides a working model to account for the role of
[Ir]BAr
F
4
. Upon excitation of [Ir
III
]
+
to [Ir
III
]
+
*
, reductive quenching
by HEH
2
would generate [Ir
II
], as has been established in related
reductions of organic substrates (Fig. 3B) (
9
). This proposed path-
way is consistent with the lack of enhancement observed with Ir(ppy)
3
,
with which reductive quenching by HEH
2
is very uphill [
E
1/2
(
*
Ir
III/II
) =
−0.08 V,
E
1/2
(HEH
2
0/+
) = 0.48 V] (
29
). The resulting radical cation
HEH
2
•+
is then deprotonated by Col, mitigating back-electron
transfer from [Ir
II
]. As noted above, [Ir
II
] is assumed to be suffi-
ciently reducing to generate an M(N
2
)
−
species from M(N
2
). The
former would then undergo protonation by [ColH]
+
to form an
N
─
H bond via M(N
2
H), which itself can be reduced further by dif-
fusing HEH
•
to generate M(NNH
2
). As noted for Fig. 3A, this series
of steps is plausible but is only one of several related scenarios that
may be viable (e.g., [Ir
II
] might be oxidized by [ColH]
+
instead of a
[Mo] species), and future mechanistic studies are needed.
In contrast to the Ir-free conditions, the system with the photo-
sensitizer remains catalytically competent even without added buffer,
albeit with an attenuation in turnover (7.4 ± 0.4 equiv of NH
3
/Mo;
entry 26). Presumably, under a Col/[ColH]
+
-free cycle, the liberated
radical cation HEH
2
•+
(formed via reductive quenching) can be con-
sumed via proton or H atom transfer with a [Mo]N
x
H
y
intermediate.
Having established photodriven transfer hydrogenation as a via-
ble strategy for N
2
R, we have begun to explore the deep reduction of
other substrates. While success here will ultimately be best realized
by exploring a broader array of transition metal catalysts, promising
early results with the [Mo]Br
3
catalyst discussed here include the
complete reduction of nitrate to ammonia (8 e
−
/9 H
+
) and acetylene
to ethylene (major product; 2 e
−
/2 H
+
) and ethane (minor product;
4 e
−
/4 H
+
). These transformations have been previously explored by
photochemical methods, including with semiconductors as for N
2
(
31
,
32
). Also of relevance is the photoinduced hydroalkylation of
alkynes using Hantzsch ester derivatives, although transfer hydro-
genation from HEH
2
to acetylene has not, to our knowledge, been
previously reported (
33
).
Reduction of [TBA]NO
3
with HEH
2
in the presence of buffer and
[Mo]Br
3
under blue LED irradiation and argon atmosphere yields
9.8 ± 1.2 equiv of NH
3
/Mo, representing a 73 ± 9% yield with respect
to HEH
2
(Fig. 2, entry 27). The reaction carried out with [TBA]-
15
NO
3
yielded
15
NH
3
(fig. S16), confirming NO
3
−
as the source of N
atoms. In contrast to N
2
R, addition of [Ir]BAr
F
4
did not enhance
catalysis (entry 28). Distinct from N
2
as the substrate where no
background reactivity is observed (entry 3), there is some back-
ground reactivity for NO
3
−
even in the absence of the Mo catalyst;
this reactivity is enhanced by the Ir photocatalyst (entries 29 and 30;
see section S5.5 for further discussion). Only trace NH
3
was detected
in the absence of light (entry 31).
The reduction of acetylene under the same conditions (HEH
2
,
Col/[ColH]OTf buffer, and [Mo]Br
3
under blue LED irradiation
and argon atmosphere) provides a mixture of ethylene and ethane
in a ~6:1 ratio and a total yield of 24 ± 5% with respect to HEH
2
(entry 32). Addition of [Ir]BAr
F
4
to this reaction marginally de-
creases the yield (entry 33). However, as in the NO
3
−
reduction re-
action, [Ir]BAr
F
4
enhances Mo-free reactivity (entries 34 and 35).
Again, no reduced products could be detected in the absence of
light (entry 36). In sum, each of these three substrates (N
2
, NO
3
−
,
and HCCH) illustrates the capacity of HEH
2
to deliver H
2
equiva-
lents via photodriven transfer hydrogenation.
To close, it is instructive to consider the thermodynamics of the
photodriven N
2
R system described here and its hypothetical dark
reaction (Fig. 1C). To do this, one can compare the BDFE
eff
(Fig. 4,
Eq. 1), a measure of the thermodynamics of H atom transfer from a
set of reagents, to the BDFE of H
2
(103.9 kcal mol
−1
) (
34
–
36
). The
difference between these values provides an overpotential for N
2
hydrogenation, expressed as
G
f
(NH
3
) (Eq. 2) (
37
). For the dark
reaction, the BDFE
eff
is the average of the first (C
─
H) and second
(N
─
H) BDFEs for HEH
2
and HEH
•
, respectively, correlating to a
very small overpotential [
G
f
(NH
3
) = 1.8 kcal mol
−1
] (
18
). NH
3
synthesis via transfer hydrogenation from HEH
2
to N
2
is therefore
F4
Fig. 4. Estimated BDFE
eff
values and corresponding
G
f
(NH
3
) for the trans-
formations of interest here.
Values are estimated using Eqs.
1 and 2.
Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
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RESEARCH ARTICLE
6 of 9
thermodynamically comparable to N
2
hydrogenation by the Haber-
Bosch process. Where the latter uses high temperature and pressure
to overcome the high kinetic barrier, the photodriven process de-
scribed here obtains excess driving force directly from visible light.
More specifically, under conditions that exclude the photosensitiz-
er, using the estimated excited-state reduction potential of [HEH
2
]
*
and the p
K
a
of [ColH]
+
to estimate BDFE
eff
, blue light affords access
to a large added driving force [
G
f
(NH
3
) = 123 kcal mol
−1
; Fig. 4]
to push the transfer hydrogenation forward. In the presence of
the Ir photosensitizer, a smaller but still considerable driving force
[
G
f
(NH
3
) = 68 kcal mol
−1
] is available. Regardless, the key point
is that light generates an overpotential from an otherwise unreac-
tive source of 2 e
−
/2 H
+
stored within HEH
2
that is sufficient to
perform, via successive transfers, a net 6 e
−
/6 H
+
reduction of N
2
in
the presence of an appropriate catalyst and cocatalyst buffer, with
an additional benefit gained from inclusion of a photoredox co-
catalyst. Important future goals for the work presented here in-
clude extensive mechanistic studies and studies aimed at in situ
recycling of the spent HE back to HEH
2
.
MATERIALS AND METHODS
Experimental design
To develop and study photodriven N
2
R, catalytic reactions were
performed, and their fixed-N products were quantified using a vari-
ety of reagents, (co)catalysts, and conditions. Additional spectro-
scopic experiments were conducted to gain mechanistic insight.
General considerations
All manipulations were carried out using standard Schlenk or
glovebox techniques under an N
2
atmosphere. Solvents were deox-
ygenated and dried by thoroughly sparging with N
2
followed by
passage through an activated alumina column in a solvent purifica-
tion system by SG Water USA LLC.
Nonhalogenated solvents were
tested with sodium benzophenone ketyl in THF to confirm the
absence of oxygen and water. Deuterated solvents were purchased
from Cambridge Isotope Laboratories Inc., degassed, and dried over
activated 3-Å molecular sieves before use.
Reagents
HEH
2
(
38
), (PNP)MoBr
3
(
21
), [ColH]OTf (
21
), [P
3
B
Fe]BAr
F
4
[P
3
B
(tris[2-(diisopropylphosphino)phenyl]borane)] (
39
), BTH
2
(
18
),
NaBAr
F
4
(
40
),
15
N-Col (
41
), phenH
2
(
42
), phenazH
2
(
43
), and
[TBA]
15
NO
3
(
44
) were prepared according to literature procedures.
Triflic acid, ethylacetoacetate, and 37% aqueous formaldehyde were
purchased from Sigma-Aldrich and used without further purification.
Ir(ppy)
3
, [Ir(ppy)
2
(dtbbpy)]PF
6
, [Ir(dF(CF
3
)ppy)
2
(dtbbpy)]PF
6
,
and [Ir(
p
-F(Me)ppy)
2
(dtbbpy)]PF
6
were purchased from Strem and
used without further purification. [TBA]NO
3
was purchased from
Alfa Aesar and was dissolved in THF and filtered over activated alumina
to dry and purify before use.
15
N
2
was obtained from Cambridge
Isotope Laboratories Inc. (lot number: I-25854/XZ732957).
15
NH
4
Cl
(99%
15
N, 98% purity) and Na
15
NO
3
(98%
15
N, 98% purity) was
purchased from Cambridge Isotope Laboratories Inc. and used
without further purification. Col was purchased from Sigma-Aldrich
and was distilled before use. 9,10-Dihydroacridine (98%) was pur-
chased from Combi Blocks and used without further purification.
1-benzyl-1,4-dihydronicotinamide was purchased from TCI and
used without further purification. Acetylene (99.6% purity) was
purchased from Matheson Gas. THF used in the experiments here
was stirred over Na/K (≥12 hours) and filtered over activated alumina
or vacuum-transferred before use unless otherwise stated. Photoin-
duced reactions were performed using Kessil 34-W 150 blue lamps.
Spectroscopy
Nuclear magnetic resonance spectroscopy
Nuclear magnetic resonance (NMR) measurements were recorded
with a Varian 400-MHz spectrometer.
1
H NMR chemical shifts are
reported in parts per million (ppm) relative to tetramethylsilane, us-
ing
1
H resonances from residual solvent as internal standards (
45
).
Ultraviolet-visible spectroscopy
Ultraviolet-visible (UV-vis) absorption spectroscopy measurements
were collected with a Cary 50 UV-vis spectrophotometer using a 1-cm
path length quartz cuvette. All samples had a blank sample back-
ground subtraction applied.
Electron paramagnetic resonance spectroscopy
All X-band continuous-wave electron paramagnetic resonance spectra
were obtained on a Bruker EMX spectrometer using a quartz liquid
nitrogen immersion dewar on solutions prepared as frozen glasses
in 2-MeTHF, unless otherwise noted.
Steady-state fluorimetry
Steady-state fluorimetry was performed in the Beckman Institute
Laser Resource Center (California Institute of Technology). Sam-
ples for luminescence measurements were prepared in dry THF and
transferred to a 1-cm path length–fused quartz cuvette sealed with a high-
vacuum Teflon valve (Kontes). Steady-state emission spectra were
collected on the Jobin S4 Yvon Spec Fluorolog-3-11 with a Hamamatsu
R928P photomultiplier tube detector with photon counting.
Standard NH
3
generation reaction procedure
All solvents are stirred with Na/K for ≥2 hours and filtered before
use. In a nitrogen-filled glovebox, the precatalysts ([Mo]Br
3
and/or
[Ir]BAr
F
4
) (2.3
mol) are weighed in individual vials. The precata-
lysts are then transferred quantitatively into a Schlenk tube using
THF, and the THF is evaporated to provide a thin film of precatalyst.
The tube is then charged with a stir bar, and the acid ([ColH]OTf)
and Hantzsch ester (HEH
2
) are added. The tube is cooled to 77 K in
a cold well. The base (Col) is dissolved in 1 ml of solvent. The 1-ml
solution of base and solvent is added to the cold tube to produce a
concentration of the precatalyst of 2.3 mM.
The temperature of the
system is allowed to equilibrate for 5 min, and then the tube is sealed
with a Teflon screw valve. The tube is passed out of the box into a
liquid N
2
bath and transported to a fume hood. For experiments run
at −78°C, the tube is then transferred to a dry ice/isopropanol bath
where it thaws and is allowed to stir under blue LED irradiation for a
minimum of 3 hours before warming. For experiments run at 23°C,
the tube is instead transferred to a water bath where it thaws and is
allowed to stir for 12 hours. To ensure reproducibility, all experi-
ments were conducted in 200-ml Schlenk tubes (50 mm outer diam-
eter) using 10-mm egg-shaped stir bars, and stirring was conducted
at ~600 rpm. Both the water bath and the dry ice/isopropanol bath
were contained in highly reflective dewars. The blue LED was placed
above the bath as close to the stirring reaction as possible.
NH
3
detection by optical methods
Reaction mixtures are cooled to 77 K and allowed to freeze. The
reaction vessel is then opened to atmosphere, and excess of a solution
of HCl (3 ml of a 2.0 M solution in Et
2
O; 6 mmol) is slowly added to
AQ8
AQ9
Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
7 of 9
the frozen solution over 1 to 2 min. This solution is allowed to
freeze, and then the headspace of the tube is evacuated and the tube
is sealed. The tube is then allowed to warm to room temperature (RT)
and stirred at RT for at least 10 min. Solvent is removed in vacuo,
and the solids are extracted with 1 M HCl(aq) and filtered to give a
total solution volume of 10 ml. A 5-ml aliquot is taken and washed
repeatedly with
n
-butanol to remove Hantzsch pyridine (HE) and
[ColH]
+
. After
n
-butanol washing, additional 1 M HCl(aq) is added
to give a final total volume of 5 ml. From these 5-ml solutions, a 100-
l
aliquot is analyzed for the presence of NH
3
(present as [NH
4
]Cl)
by the indophenol method. Quantification was performed with UV-vis
spectroscopy by analyzing the absorbance at 635 nm (
46
). When
specified, a further aliquot of this solution was analyzed for the
presence of N
2
H
4
(present as [N
2
H
5
]Cl) by a standard colorimet-
ric method (
47
). Quantification was performed with UV-vis spec-
troscopy by analyzing the absorbance at 458 nm.
NH
3
detection by
1
H NMR spectroscopy
Reaction mixtures are cooled to 77 K and allowed to freeze. The
reaction vessel is then opened to atmosphere, and an excess (with
respect to acid) solution of a NaO
t
Bu solution in MeOH (0.25 mM)
is slowly added to the frozen solution over 1 to 2 min. This solution
is allowed to freeze, and then the headspace of the tube is evacuated
and the tube is sealed. The tube is then allowed to warm to RT and
stirred at RT for at least 10 min. An additional Schlenk tube is
charged with HCl (3 ml of a 2.0 M solution in Et
2
O; 6 mmol) to
serve as a collection flask. The volatiles of the reaction mixture are
vacuum-transferred at RT into this collection flask. After completion
of the vacuum transfer, the collection flask is sealed and warmed to
RT.
Solvent is removed in vacuo, and the remaining residue is dis-
solved in 0.7 ml of DMSO-
d
6
containing 20 mM 1,3,5-trimethoxybenzene
as an internal standard. Integration of the
1
H NMR peak observed
for NH
4
+
is then integrated against the two peaks of trimethoxybenzene
to quantify the ammonium present. This
1
H NMR detection method
was also used to differentiate [
14
NH
4
]Cl and [
15
NH
4
]Cl produced
in the control reactions conducted with
15
N
2
,
15
N-Col/[ColH]OTf,
or
15
N-HEH
2
.
Standard [TBA]NO
3
reduction reaction procedure
Catalytic experiments for the reduction of [TBA]NO
3
were con-
ducted in a manner similar to the reduction of N
2
. The precatalysts,
solids, and stir bar are added in the same way, with [TBA]NO
3
in-
cluded with the other solids. The tube is cooled to 77 K in a cold
well, and the base (Col) is added using a micropipette. The tube is
then sealed and passed out of the glovebox without warming and
thoroughly degassed. Degassed THF solvent (1 ml) is vacuum-
transferred into the catalytic tube. The tube is allowed to warm
briefly and back-filled with argon. The reaction is then irradiated
with blue LED in a 23°C water bath as for the N
2
RR.
Standard acetylene reduction reaction procedure
Catalytic experiments for the reduction of acetylene were conducted
in a manner similar to the reduction of N
2
. The precatalysts, solids,
and stir bar are added in the same way. The tube is wrapped in alu-
minum foil, and Col and THF-
d
8
(0.7 ml) are added. The tube is
sealed, passed out of the glovebox, and degassed (three freeze-pump
thaw cycles). The desired volume of acetylene gas is added using
a calibrated bulb while the tube is cooled in liquid nitrogen.
The headspace of the tube is then backfilled to 1 atm with argon
while cooled in a dry ice/acetone bath. The tube is transferred
to a 23°C water bath and is irradiated with blue LED for the time
specified.
After 12 hours of irradiation, the volatiles of the reaction mixture
are vacuum-transferred into a J.
Young NMR tube of known volume
containing a known amount of 1,3,5-trimethoxybenzene. In the
1
H
NMR spectrum of the resulting sample, the peaks corresponding to
ethylene (5.36 ppm) and ethane (0.85 ppm) are distinguishable
when present (
45
). Integration to the internal standard provides the
yield of dissolved gases. Henry’s constant for each gas in THF (
48
)
was used to estimate their partial pressures in the headspace.
Synthetic details
15
N-labeled 2,6-dimethyl-3,5-dicarboethoxy-l,4-
dihydropyridine (
15
N-HEH
2
)
Adapted from (
38
), aqueous formaldehyde (37%, 78
l) and ethy-
lacetoacetate (280
l, 2.19 mmol) were placed in a 10-ml round-
bottom flask equipped with a stir bar and fitted with a reflux
condenser.
15
NH
4
Cl (305 mg, 5.7 mmol) in 1 ml of H
2
O was added
to a 1-ml aqueous solution of NaOH (228.3 mg, 5.7 mmol). The
resulting solution of
15
NH
4
OH was added to the flask through the
neck of the condenser. The condenser neck was rinsed into the flask
with 0.5 ml of ethanol. The reaction mixture was heated at reflux for
1.5 hours and then chilled in an ice bath. The resulting precipitate
was collected by filtration and washed with cold ethanol (~3 ml)
and Et
2
O to yield the title compound as a pale yellow powder (60 mg,
22% yield).
1
H NMR (400 MHz, DMSO-
d
6
)
8.28 ppm (d,
1
J
H,N
=
94.6 Hz, 1H), 4.05 ppm (q,
J
= 7.1 Hz, 4H), 3.11 ppm (s, 2H), 2.11 ppm
(d,
J
= 2.9 Hz, 6H), and 1.19 ppm (t,
J
= 7.1 Hz, 6H).
15
N-labeled 2,4,6-trimethylpyridinium triflate (
15
N-[ColH]OTf)
Identical procedure to what has previously been reported with un-
labeled Col was used (
21
).
1
H NMR (400 MHz, DMSO
-d
6
)
14.87 ppm
(broad s, 1H), 7.57 ppm (d,
3
J
H,N
= 2.8 Hz, 2H), 2.62 ppm (d,
3
J
H,N
=
2.9 Hz, 6H), and 2.49 ppm (s, 3H).
[Ir(ppy)
2
(dtbbpy)]BAr
F
4
([Ir]BAr
F
4
)
[Ir(ppy)
2
(dtbbpy)]PF
6
(100 mg, 0.11 mmol) and NaBAr
F
4
(92.2 mg,
0.10 mmol, 0.95 equiv) were stirred in 5 ml of Et
2
O at RT for 1 hour.
The solution was filtered through celite, layered with pentane, and
stored at −40°C overnight to yield the title compound as yellow
crystals (161 mg, 90% yield).
1
H NMR (400 MHz, MeCN-
d
3
)
8.48
ppm (s, 2H), 8.06 ppm (d, 2H,
J
= 8.2 Hz), 7.93 to 7.76 ppm (m, 6H),
7.74 to 7.64 ppm (m, 10H), 7.58 ppm (d,
J
= 5.8 Hz, 2H), 7.50 ppm
(dd,
J
= 5.9, 1.9 Hz, 2H), 7.03 ppm (t,
J
= 6.8 Hz, 2H), 6.91 ppm (t,
J
= 6.8 Hz, 2H), 6.28 ppm (d,
J
= 6.3 Hz, 2H), and 1.40 ppm (s, 18H).
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at https://science.org/doi/10.1126/
sciadv.ade3510
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Acknowledgments:
We thank the Dow Next Generation Educator Fund and Instrumentation
Grants for support of the NMR facility at Caltech. The Beckman Institute Laser Resource Center
and J.
R. Winkler are acknowledged for providing support with steady-state luminescence
experiments. We also thank the Resnick Sustainability Institute at Caltech for enabling
Johansen,
Sci. Adv.
8
, eade3510 (2022) 26 October 2022
SCIENCE ADVANCES
|
RESEARCH ARTICLE
9 of 9
facilities, including its Water and Environment Laboratory (WEL).
Funding:
This work was
supported by the National Institutes of Health (R01 GM-075757). E.A.B. acknowledges the
support of the National Science Foundation for a Graduate Research Fellowship under grant
no. DGE-1745301.
Author contributions:
Conceptualization: C.M.J. and J.C.P.
Methodology:
C.M.J., E.A.B., and J.C.P.
Investigation: C.M.J. and E.A.B.
Visualization: C.M.J., E.A.B., and
J.C.P.
Funding acquisition: E.A.B. and J.C.P.
Project administration: J.C.P.
Supervision:
J.C.P.
Writing—original draft: C.M.J. and E.A.B.
Writing—review and editing: C.M.J., E.A.B., and
J.C.P.
Competing interests:
The authors declare that they have no competing interests.
Data
and materials availability:
All data needed to evaluate the conclusions in the paper are
present in the paper and/or the Supplementary Materials.
Submitted 10 August 2022
Accepted 2 September 2022
Published 26 October 2022
10.1126/sciadv.ade3510