Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia

Article abstract of DOI:10.1021/jacs.0c03346

The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H₂ activation and hydrogen-atom transfer is described. The phenylimine-substituted rhodium complex (η⁵-C₅Me₅)Rh(^MePhI)H (^MePhI = N-methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η⁵-C₅Me₅)Rh(ppy)H (ppy = 2-phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh-H bond dissociation free energies of 51.8 kcal mol^-1 for (η⁵-C₅Me₅)Rh(^MePhI)H and 51.1 kcal mol^-1 for (η⁵-C₅Me₅)Rh(ppy)H. In the presence of 10 mol% of the phenylimine rhodium precatalyst and 4 atm of H₂ in THF, the manganese nitride (^tBuSalen)MnN underwent hydrogenation to liberate free ammonia with up to 6 total turnovers of NH₃ or 18 turnovers of H^? transfer. The phenylpyridine analogue proved inactive for ammonia synthesis under identical conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthesis by proton-coupled electron transfer.

Full text of DOI:10.1021/jacs.0c03346

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Article
Catalytic Hydrogenation of a Manganese(V) Nitride to Ammonia
Sangmin Kim, Hongyu Zhong, Yoonsu Park, Florian Loose, and Paul J. Chirik
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Apr 2020
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Catalytic Hydrogenation of a Manganese(V) Nitride to Ammo-
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Sangmin Kim, Hongyu Zhong, Yoonsu Park, Florian Loose, and Paul J. Chirik*
Department of Chemistry, Frick Laboratory, Princeton University, Princeton, New Jersey 08544, United States
ABSTRACT: The catalytic hydrogenation of a metal nitride to produce free ammonia using a rhodium hydride catalyst that promotes H2
activation and hydrogen atom transfer is described. The phenylimine-substituted rhodium complex, (η5-C5Me5)Rh(MePhI)H (MePhI = N-
methyl-1-phenylethan-1-imine) exhibited higher thermal stability compared to the previously reported (η5-C5Me5)Rh(ppy)H (ppy = 2-
phenylpyridine). DFT calculations established that the two rhodium complexes have comparable Rh–H bond dissociation free energies of
51.8 kcal mol-1 for (η5-C5Me5)Rh(MePhI)H and 51.1 kcal mol-1 for (η5-C5Me5)Rh(ppy)H. In the presence of 10 mol% of the phenylimine
rhodium precatalyst and 4 atm of H2 in THF, the manganese nitride, (tBuSalen)Mn≡N underwent hydrogenation to liberate free ammonia
with up to 6 total turnovers of NH3 or 18 turnovers of H·. The phenylpyridine analogue proved inactive for ammonia synthesis under identi-
cal conditions owing to competing deleterious hydride transfer chemistry. Subsequent studies showed that the use of a non-polar solvent such
as benzene suppressed formation of the cationic rhodium product resulting from the hydride transfer and enabled catalytic ammonia synthe-
sis by proton coupled electron transfer.
INTRODUCTION
mol-1, rendering the overall process spontaneous. Importantly, the
rhodium hydride was regenerated by addition of H2 enabling a
catalytic reaction.
The hydrogenation of molecular dinitrogen to ammonia is one
of the most important and impactful technological innovations in
human history.1-4 While the Haber-Bosch process relies on high
temperature and pressures to combine N2 and H2 to form NH3, it is
the most thermodynamically efficient method for ammonia synthe-
sis.5,6 Exploration of homogeneous catalysts for nitrogen fixation
has been motivated by realizing milder reaction conditions and
understanding of elementary steps en route to NH3. Several homo-
geneous ammonia synthesis catalysts are now known and while
turnover has been observed at ambient temperature and pressure,7-
18 the reliance on stoichiometric proton and electron sources incurs
large chemical overpotentials and poor atom economy.19
Hydrogenation of metal nitrides has been of long-standing inter-
est for the synthesis of ammonia following N2 cleavage (Scheme
1b).31-34 The challenge stems from the low N–H BDFEs of resulting
metal imido species and the strong M≡N bond, particularly with
those derived from N2 cleavage. Recently, our laboratory reported
the application of photodriven proton coupled electron transfer for
the synthesis of ammonia from the manganese nitride,
(tBuSalen)Mn≡N (MnN) using
a combination of 9,10-
dihydroacridine, a ruthenium photocatalyst and a proton source
(Scheme 1c).25,26 The computed, successive N–H BDFEs of 60, 84
and 85 kcal mol-1 make this specific metal nitride attractive for am-
monia formation. Because of the requirement for 9,10-
dihydroacridine as the stoichiometric hydrogen atom source and
light as the energy source, this reaction is not ideal for NH3 synthe-
sis in terms of chemical overpotential and overall poor atom econ-
omy. Here we describe a thermal, rhodium-catalyzed method for
the synthesis of NH3 from MnN using H2 as the terminal reductant
(Scheme 1c).
For homogeneous ammonia synthesis catalysts to operate near
thermodynamic potential, dihydrogen (H2) is the optimal stoichi-
ometric reductant.19-21 The challenge lies in the synthesis of weak
N–H bonds as many intermediates in the catalytic cycle range be-
tween 30 and 50 kcal mol-1, below the driving force for spontaneous
H2 formation.22 Proton coupled electron transfer (PCET) has
emerged as a promising strategy for the synthesis of these bonds
and examples relevant to ammonia synthesis from metal nitrides
include SmI2·xH2O,23,24 9,10-dihydroacridine,25,26 and TEMPO–
H.27 Our group demonstrated this concept using (η5-
C5Me5)Rh(ppy)H (1) (ppy = 2-phenylpyridine)28-30 as the hydro-
gen atom transfer catalyst for the synthesis of ammonia from a ti-
tanocene amide complex with H2 as the terminal reductant
(Scheme 1a).20,21 A key feature of the hydrogen transfer catalyst is a
M–H bond dissociation free energy (BDFE) higher than 48.6 kcal
mol-1 (ΔGfº(H·)) to promote H2 cleavage but also as close to this
value as possible to minimize the driving force for N–H bond for-
mation. In the titanocene amide example, the Rh–H BDFE of 52
kcal mol-1 in 1 was lower than the incipient N–H bonds of 61 kcal
Scheme 1. (a) Catalytic NH3 formation from a titano-
cene amide complex using H2 as the terminal reductant.
(b) Nitride pathway in catalytic N2 reduction via
PCET. (c) Catalytic NH3 formation from a manganese
nitride using H2 as the terminal reductant (this work).
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SiMe3
0.5 [(η5-C5Me5)RhCl2]2
SiMe3
5 mol% 1
4 atm H2
+
Cl
NaOAc
NH3
LiHBEt3
+
N
Rh
Rh
N
N
Rh
N
Ti
THF, 5 days
Ti
Cl
Cl
H
H
NH2
THF, 2 h
- BEt3
- LiCl
DCM, rt
24 h
- HOAc
- NaCl
BDFERh-H < BDFEN-H
• Use of H2 as the H source
SiMe3 • Relevant to only the last step
of NH3 formation
SiMe3
2, (η5-C5Me5)Rh(MePhI)H
N-methyl-1-phenylethan
(η5-C5Me5)Rh(MePhI)Cl
1
in N2 reduction pathway
-1-imine (MePhI)
(η5-C5Me5)Rh(ppy)H
38 %
7
(b)
(b)
8
9
H
H
H
1
H+/e-
H+/e-
H+/e-
N
/2 N2
N
N
M
NH3
M
M
M
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(c)
Previous work
Ru photocat.
Blue LED
• Photodriven PCET
• H source from
stoichiometric
9,10-dihydroacridine
acid additive
tBu
tBu
N
9,10-dihydroacridine
tBu
O
O
N
tBu
Mn
N
+ [Mn]red
NH3
10 mol%
N
This work
MnN
N1-Rh1 : 2.0751(19) Å
C1-Rh1 : 2.018(2) Å
C1-Rh1-N1 : 78.18(9)º
Rh
(
tBuSalen)MnN
H
• Thermal PCET
• H2 as
the H atom source
4 atm H2
THF, 5 days
(c)
RESULTS AND DISCUSSION
In exploring the thermal and photostability of 1, we recently re-
ported its conversion to multimetallic clusters that proved catalyti-
cally active for the hydrogenation of N-heteroarenes.35 Because C–
H reductive elimination from 1 was identified as the initial step in
formation of the multimetallic clusters,35 modifications to the L–X
chelate were explored to improve the stability of the rhodium
precatalyst. Initial efforts focused on replacement of the phenylpyr-
idine donor with a phenylimine, owing to the straightforward syn-
thesis and the strong σ-donating character of the imine.36-39 The N-
methyl imine derivative, (η5-C5Me5)Rh(MePhI)H (2) (MePhI = N-
methyl-1-phenylethan-1-imine) was prepared from the chloride
precursor, (η5-C5Me5)Rh(MePhI)Cl and LiHBEt3 (Scheme 2a).
The solid-state structures of (η5-C5Me5)Rh(MePhI)Cl and 2 were
determined by X-ray diffraction and the hydride of 2 was located
and refined (Scheme 2b, c).
N1-Rh1 : 2.070(4) Å
C11-Rh1 : 2.003(5) Å
C11-Rh1-N1 : 78.36(18)º
A cyclic imine analogue (η5-C5Me5)Rh(4H-ppy)H (3) (4H-ppy
= 2-phenyl-3,4,5,6-tetrahydropyridine) was isolated in small quan-
tities (6% yield) from exposure of 1 to 4 atm of H2 (Scheme 3a).
The majority of the remainder of the material in the hydrogenation
was the pentametallic rhodium cluster, (η5-C5Me5)4Rh5H7.35 The
solid-state structure of 3 was established by X-ray diffraction
(Scheme 3b) and confirmed the partial reduction of the pyridine
ring in the phenylpyridine chelate.
Scheme 2. (a) Synthesis of 2 with a readily available
and modular phenylimine ligand. (b) Solid-state struc-
ture of (η5-C5Me5)Rh(M ePhI)Cl as determined by X-ray
diffraction. (c) Solid-state structure of 2 determined
by X-ray diffraction. All structures at 30% probability
ellipsoids with H atoms except for hydride omitted for
clarity.
Scheme 3. (a) Synthesis of 3 by partial hydrogenation
of the pyridine ring from 1. (b) Solid-state structure of
3 as determined by X-ray diffraction at 30% probability
ellipsoids with H atoms except for hydride omitted for
clarity.
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Scheme 4. (a) Reductive elimination of phenylpyridine
from 1. (b) Thermal stability of 2 and deuterium in-
corporation into the phenyl ring in 2-d1.
(a)
(η5-C5Me5)4Rh5H7
49 %
4 atm H2
Rh
N
Rh
N
H
+
H
THF, 70 o
5 days
C
(ref. 35)
3, (η5-C5Me5)Rh(4H-ppy)H
N
N
Rh
Rh
Rh
heat
1
H
+
6 %
N
H
C-H Reductive
coupling
Ligand
dissociation
(b)
1
9
10
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(b)
Rh
N
Rh
Rh
N
N
H/D
H/D
H
H/D
no free
ligand
THF, 60 o
24 h
C
THF, 50 o
24 h
C
H/D
observed by 2H NMR
2
2-d1
Attempts to determine the Rh–H BDFE in 2 have been unsuc-
cessful. Treatment of a benzene-d6 solution of 2 with 1.05 equiva-
lents of TEMPO, a reagent successfully used for the determination
of the BDFE of the Rh–H in 1,29 produced several new rhodium
N1-Rh1 : 2.051(2) Å
C11-Rh1 : 2.019(3) Å
C11-Rh1-N1 : 78.55(10)º
1
products along with TEMPO–H that were detected by H NMR
spectroscopy. One of the rhodium products was EPR active with an
isotropic signal (giso = 2.084) consistent with formation of the 17-
electron organometallic product (4) expected from H-atom trans-
fer (Scheme 5a, b). Upon addition of H2 to this mixture, 2 was
cleanly regenerated. Although 4 was present in the solution, a clean
and reversible cyclic voltammogram was not obtained due to pres-
ence of other rhodium compounds in the mixture. Nevertheless,
the pKa of 2 was determined by titration with TBD (1,5,7-
Triazabicyclo[4.4.0]dec-5-ene, pKa = 21.0 in THF)40 and a value of
23.2 was measured in THF (Scheme 5c). Deprotonation of 1 by a
phosphazene base established a pKa of 30.3 for 1 in MeCN.29 Be-
cause the pKa of the phosphazene base in THF is 20.2, the expected
pKa value of 1 in THF is approximately 20.2, lower than that of 2.
The H NMR chemical shifts of the hydrides and the 103Rh–H
coupling constants in 1, 2 and 3 support slightly stronger Rh–H
bonds in 2 and 3 than 1 (Figure 1). The vibrational frequency of 2
1
(νRhH = 1967 cm-1, KBr pellet) by IR spectroscopy is also higher
than that of 1 (νRhH = 1934 cm-1, KBr pellet). However, the DFT
computed Rh–H BDFEs of 2 and 3 are 51.8 and 51.5 kcal mol-1,
respectively, and are indistinguishable from the value of 51.1 kcal
mol-1 computed for 1.
Rh
Rh
Rh
N
N
N
H
H
H
Scheme 5. (a) Treatment of 2 with 1.05 equivalents
TEMPO. (b) X-band EPR spectrum of (a) and the ex-
pected paramagnetic product 4. (c) pKa Measurement
of 2.
1
2
3
Chemical shift
(Rh-H, ppm, THF-d8)
-12.64
31.6
-13.45
33.1
-13.43
33.2
Coupling constant
(1JRh-H, Hz, THF-d8)
Rh-H BDFE
(DFT, kcal mol-1
51.1
51.8
51.5
,
gas phase)
Figure 1. Comparison of the chemical shifts and Rh–H coupling
constants of 1, 2 and 3 from 1H NMR spectroscopy in THF-d8 and
DFT-computed Rh–H BDFEs in gas phase (B3LYP/{6-
311+G**,LANL2TZ(-f)}//{6-31G**,LANL2DZ}).
The thermal stability of 2 was evaluated as reductive elimination
of phenylpyridine was previously identified as the first step in the
formation of the multimetallic clusters from 1 (Scheme 4a).35 Heat-
ing a THF solution of 2 at 60 ºC for 24 hours produced no evi-
dence for loss of the N-methyl-phenylimine chelate, supporting a
higher barrier for C–H reductive elimination (Scheme 4b). The
rhodium deuteride, 2-d1 was prepared and incorporation of the
isotopic label into the phenyl ring was observed after warming a
THF-d8 solution of the compound to 50 ºC for 24 hours. This ob-
servation demonstrates that C(sp2)–H reductive coupling occurs at
50 ºC but subsequent ligand dissociation has a higher barrier, likely
a result of the relatively strong σ-donation by the imine ligand.
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With a new rhodium hydride in hand, the catalytic performance
of 2 for the hydrogenation of MnN was examined (Table 1). With
5 mol% of 2 under 4 atm of H2 in THF, 22% of NH3 was obtained
after 5 days at 23 ºC (Table 1, entry 1). Notably, 1 did not yield any
NH3 under the same conditions (entry 2). The cyclic imine deriva-
tive, 3 produced NH3 in 21% yield almost identical to the results
with 2 (entry 3). Previous studies have established that addition of
L-type ligands to ammonia synthesis reactions with MnN increases
the yield of free NH3, likely by outcompeting ammonia coordina-
tion to the manganese.25,26 Repeating the hydrogenation of MnN
with 10 mol% of 2 at higher concentration and addition of two
equivalents of TMEDA (N,N,N',N'-tetramethylethylenediamine)
increased the yield of NH3 to 40% (entry 4). Again, 1 produced no
NH3 under these conditions (entry 5). Increasing the temperature
of the hydrogenation to 50 ºC improved the performance of 2 and
yielded 60% yield free NH3 after 5 days (entry 6). Although the
total turnovers based on the amount of free NH3 are only 6.0, turn-
overs by the number of PCET event (H·) are 18, comparable to the
19 turnovers reported for catalytic NH3 formation from the titano-
cene amide with 1 after 5 days.20,21 In the absence of a rhodium
complex, no NH3 was generated and demonstrating that TMEDA
is not the source of the hydrogen atom (entry 7).
Rh
two
Rh
N
unidentified
diamagnetic
Rh-H complexes
1.05 equiv TEMPO
N
H
+
2
+
C6D6, rt
1 h
- TEMPO-H
4
2
giso = 2.084
4 atm H2
C6D6, rt, 24 h
full conversion
(b)
9
10
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13
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19
20
21
22
23
24
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Rh
N
4
giso = 2.084
(c)
N
Table 1. Evaluation of catalytic MnN hydrogenation to
free NH3 with 1, 2 and 3.
[Rh-H]
2
pKa of 2 in THF = 23.2
pKa of 1 in MeCN = 30.3
pKa of 1 in THF ~ 20.2
[Rh]-
[Rh-H] : [Rh]- = 13 : 1
TBD-H+
+
+
N
N
H
THF-d8, rt
12 days
6.0 equiv TBD
(pKa = 21.0
in THF)
tBu
tBu
N
[Rh] cat.
4 atm H2
tBu
O
O
tBu
Mn
NH3
N
N
Repeating the experiment with two equivalents of TEMPO re-
sulted in isolation of red crystals and X-ray diffraction established
the formation of dimeric rhodium complex, 5 arising from loss of
two hydrogen atoms – one from the metal and the other from the
imine methyl group (Scheme 6). Identification of this product
indicates that the C–H bond of the α-methyl imine also has a lower
C–H BDFE (62.3 kcal mol-1 by DFT computation) than 65 kcal
mol-1.41
THF, rt
5 days
- [Mn]red
MnN
NH3
TON
TON
(H·)
entry
conditions
5 mol% 2
yield
(NH3)
(%)a
1b
2b
3b
22
< 1
21
4.4
-
13
-
5 mol% 1
5 mol% 3
Scheme 6. (a) Formation of 5 from double H-atom ab-
straction by TEMPO and DFT-computed C–H BDFE in
gas phase (B3LYP/{6-311+G**,LANL2TZ(-f)}//{6-
31G**,LANL2DZ}). (b) Solid-state structure of 5 at
30% probability ellipsoids with H atoms omitted for
clarity.
4.2
13
10 mol% 2, 2 equiv
TMEDA
4c
5c
6c
7c
40
< 1
60
4.0
-
12
-
10 mol% 1, 2 equiv
TMEDA
(a)
10 mol% 2, 2 equiv
TMEDA, 50 ºC
6.0
-
18
-
via
Rh
Rh
N
Rh
2 TEMPO
N
no Rh catalyst, 2 equiv
TMEDA
H
N
<1
C6H6, rt
24 h
- 2 TEMPO-H
N
Rh
H
0.5
aThe volatiles were vacuum transferred on to an HCl solution in
H
2
Et2O and the yield of NH4Cl was determined by 1H NMR spectros-
C-H BDFE:
62.3 kcal mol-1
5
b
(b)
copy using 1,2-dichloroethane as an internal standard. 0.20 M
concentration. c0.40 M concentration.
15
To confirm the MnN as the source of ammonia, the N-labeled
isotopologue, Mn15N was prepared (Scheme 7a). Under the hy-
drogenation conditions at room temperature (Table 1, entry 4), 31%
15
1
of NH3 was obtained as judged by H NMR spectroscopy. Treat-
ment of the non-volatile product with C2Cl6 produced the manga-
nese chloride (tBuSalen)MnCl as confirmed by comparison of the
UV-Vis spectrum to an authentic sample (Scheme 7b). This result
also demonstrates that the salen ligand remains intact under the
hydrogenation conditions with 2. The starting nitride MnN was
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2
3
4
5
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regenerated in 40% yield by treatment with aqueous NH3 followed
by bleach (Scheme 7b).
Scheme 7. (a)
tBuSalen)Mn≡15N (Mn15N) to 15NH3. (b) Chlorination
and oxidation of the non-volatile residue to regenerate
MnN.
1. NMR
No Rh-H signal
2
+
PCET
4 atm H2
2. EPR
Catalytic
hydrogenation
of
2
2 equiv
MnN
giso = 2.046
regenerated
THF, rt
30 min
THF, rt
5 days
(
3. IR
Reduced
nitride signal
(b)
(a)
1. NMR
10 mol% 2
4 atm H2
2 eq. TMEDA
tBu
tBu
15N
No Rh-H signal
1
+
non-PCET
tBu
O
O
tBu
4 atm H2
2. EPR
Mn
9
1
15NH3
No EPR signal
2 equiv
MnN
N
N
not regenerated
THF, rt
30 min
THF (0.04 M), rt
5 days
- [Mn]red
THF, rt
5 days
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
31 %
3. IR
Nitride remains
Mn15
N
(b)
(c)
aq. NH3
Clorox bleach
4 equiv C2Cl6
[Mn]red
reduced Mn
(
tBuSalen)MnCl
(
tBuSalen)MnN (MnN)
DCM/MeOH
rt, 24 h
THF, 60 ºC
48 h
detected by
UV-Vis
40 % NMR yield
from starting MnN
after hydrogenation
The difference between the hydrogenation performance of 1 and
2 is remarkable given their comparable BDFEs. To gain additional
insight, stoichiometric experiments were conducted whereby two
equivalents MnN were treated with one equivalent of either 1 or 2
(Scheme 8). With 2 the successful hydrogenation of the precatalyst
1
was supported by disappearance of the H NMR resonances of
both metal complexes and observation of a strong isotropic signal
(giso = 2.046) by EPR spectroscopy at 23 ºC, presumably corre-
sponding to the reduced [(Salen)Mn] (Scheme 8a).25 In addition,
the metal nitride stretch (νMnN = 1049 cm-1, KBr pellet)42 was not
observed in the infrared spectrum. The signal of Rh–H also disap-
1
peared in both the H NMR and IR spectra. It is likely that H-atom
transfer from the rhodium to the nitride occurs, resulting in the
disappearance of the manganese nitride stretch in the IR spectrum.
Addition of 4 atm H2 to this product regenerated 2 over the course
of 48 hours as confirmed by 1H NMR spectroscopy. In these exper-
iments, there was no spectroscopic evidence for the formation of
detectable quantities of 5.
In contrast, addition of 1 to two equivalents MnN generated a
1
red-colored solution within 30 min and analysis by H NMR spec-
troscopy established disappearance of the resonances of the metal
complexes including the Rh–H signal (Scheme 8b). Importantly,
no signal was observed by EPR spectroscopy and the nitride signal
remained in the IR spectrum. Addition of 4 atm H2 to the mixture
did not regenerate 1, establishing deactivation of the rhodium from
interaction with MnN.
Scheme 8. Stoichiometric reactions of MnN with (a) 2
and (b) 1 in THF and (c) stacked X-band spectra
measured at 0.02 M concentration.
The above deactivation process suggests that the Rh–H in 1 re-
acts by a pathway other than PCET such as proton or hydride
transfer (Scheme 9). Addition of 1 to two equivalents MnN in
acetonitrile yielded the cationic rhodium acetonitrile derivative,
[(η5-C5Me5)Rh(ppy)(MeCN)]+ (6),28 which was detected by IR
spectroscopy, ruling out the possibility of proton transfer. It was
previously reported that the cationic 6 was unreactive with H2 to
regenerate 1 in the absence of an appropriate base. 29 Because the
nitride signal remained in the IR spectrum, it is likely that the hy-
dride was transferred into the electrophilic imine backbone of the
salen ligand to produce an anionic manganese(V) complex.43-46 On
the other hand, the reaction of 2 and MnN did not yield the cati-
onic species, [(η5-C5Me5)Rh(MePhI)(MeCN)]+, which was inde-
pendently prepared and structurally characterized (SI S6, Figure
S12-S14).
Scheme 9. Stoichiometric reaction between MnN and 1
in acetonitrile to yield 6.
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AUTHOR INFORMATION
tBu
tBu
N
H- transfer
Rh
N
tBu
O
O
tBu
1
+
N
Mn
Corresponding Author
+
N
N
H
Me
2 equiv
MnN
MeCN, rt
18 h
H
*pchirik@princeton.edu
6
proposed structure of Mn(V)
observed by
IR spectroscopy
ORCID
Sangmin Kim: 0000-0002-7289-0693
Hongyu Zhong: 0000-0002-6892-482X
Yoonsu Park: 0000-0003-1511-0635
Florian Loose: 0000-0003-1643-4607
Paul J. Chirik: 0000-0001-8473-2898
To suppress the formation of 6, hydrogenation in a non-polar,
non-coordinating solvent was explored. The stoichiometric reac-
tion of 1 and two equivalents MnN in benzene produced an iso-
tropic EPR signal identical to the signal observed from mixing
MnN and 2 in THF (Scheme 10a). Under catalytic conditions,
25% free ammonia was observed with 10 mol% 1 for 5 days at 23
ºC in benzene (Scheme 10b). Although the number of catalytic
turnovers (2.5 for NH3, 7.5 for H·) is modest, these results demon-
strate that solvent selection can be used to suppress deleterious
hydride transfer pathways.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This research was supported by the U.S. Department of Energy, Office
of Science, Office of Basic Energy Sciences, Catalysis Science program,
under Award DE-SC0006498. S.K. acknowledges Samsung Scholar-
ship for financial support.
Scheme 10. (a) Stoichiometric reaction between MnN
and 1 in benzene. (b) Catalytic hydrogenation of MnN
using 1 as the precatalyst in benzene.
(a)
1. NMR
No Rh-H signal
PCET
1
+
REFERENCES
2. EPR
iso = 2.047
g
2 equiv
MnN
(1) MacKay, B. A.; Fryzuk, M. D. Dinitrogen Coordination Chemistry:
On the Biomimetic Borderlands. Chem. Rev. 2004, 104, 385-402.
(2) Gambarotta, S. Dinitrogen Fixation and Activation After 30 Years: A
Puzzle Still Unsolved. J. Organomet. Chem. 1995, 500, 117-126.
(3) Hager, T. in The Alchemy of Air, Three River Press, New York,
2008.
(4) Smil, V. in Enriching the Earth: Fritz Haber, Carl Bosch, and the
Transformation of World Food Production, MIT Press, Cambridge, MA,
2004.
benzene, rt
30 min
3. IR
Reduced
nitride signal
(b)
tBu
O
tBu
N
10 mol% 1
4 atm H2
tBu
O
tBu
NH3
25 %
Mn
N
N
benzene (0.02 M), rt
5 days
- [Mn]red
(5) Haber, F.; van Oordt, G. Über die Bildung von Ammoniak aus den
Elementen. Z. Anorg. Chem. 1905, 47, 42-44.
CONCLUSIONS
In conclusion, a rare hydrogenation of a metal nitride using well-
(6) Topham, S. A. in The History of the Catalytic Synthesis of Ammo-
nia BT - Catalysis: Science and Technology (Eds.: Anderson, J.R.; Boudart,
M), Springer, Berlin, Heidelberg, 1985, pp. 1-50.
(7) Yandulov, D. V.; Schrock, R. R. Catalytic Reduction of Dinitrogen to
Ammonia at a Single Molybdenum Center. Science 2003, 301, 76-78.
(8) Eizawa, A.; Nishibayashi, Y. Catalytic Nitrogen Fixation Using Mo-
lybdenum-Dinitrogen Complexes as Catalysts. Top. Organomet. Chem.
2017, 60, 153-170.
(9) Tanaka, H.; Yoshizawa, K. Computational Approach to Nitrogen
Fixation on Molybdenum-Dinitrogen Complexes. Top. Organomet. Chem.
2017, 60, 171-196.
(10) Kuriyama, S.; Nishibayashi, Y. Catalytic Transformations of Mo-
lecular Dinitrogen by Iron and Cobalt-Dinitrogen Complexes as Catalysts.
Top. Organomet. Chem. 2017, 60, 215-234.
(11) Tanaka, H.; Nishibayashi, Y.; Yoshizawa, K. Interplay between
Theory and Experiment for Ammonia Synthesis Catalyzed by Transition
Metal Complexes. Acc. Chem. Res. 2016, 49, 987-995.
(12) Nishibayashi, Y. Recent Progress in Transition-Metal-Catalyzed
Reduction of Molecular Dinitrogen under Ambient Reaction Conditions.
Inorg. Chem. 2015, 54, 9234-9247.
(13) MacLeod, K. C.; Holland, P. L. Recent Developments in the Ho-
mogeneous Reduction of Dinitrogen by Molybdenum and Iron. Nat Chem.
2013, 5, 559-565.
(14) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. A Molybdenum Com-
plex Bearing PNP-type Pincer Ligands Leads to the Catalytic Reduction of
Dinitrogen into Ammonia. Nat. Chem. 2011, 3, 120-125.
(15) Tanaka, H.; Arashiba, K.; Kuriyama, S.; Sasada, A.; Nakajima, K.;
Yoshizawa, K.; Nishibayashi, Y. Unique Behaviour of Dinitrogen-Bridged
Dimolybdenum Complexes Bearing Pincer Ligand Towards Catalytic
Formation of Ammonia. Nat. Commun. 2014, 5, 3737.
defined rhodium H2 activation and hydrogen atom transfer precata-
lysts is described. Although the bond dissociation free energies are
indistinguishable, the phenylimine (MePhI) variant of the rhodium
precatalyst was more efficient due to the propensity to promote
PCET over competing, deleterious hydride transfer pathways. With
the phenylpyridine (ppy) variant, this side reaction was suppressed
by use of non-polar, non-coordinating solvents. These findings
provide important fundamental understanding and ultimately de-
sign principles for H2 activation and proton coupled electron trans-
fer catalysts for catalytic ammonia synthesis as well as other poten-
tial applications.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
General considerations; preparation of transition metal
complexes; general procedures; spectroscopic and crystallo
graphic data
Crystallographic data for (η5-C5Me5)Rh(MePhI)Cl
Crystallographic data for 2
Crystallographic data for 3
Crystallographic data for 5
5
Crystallographic
data
for
[(
η
-
C5Me5)Rh(MePhI)(MeCN)][BArF24]
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1
2
3
4
5
6
7
8
(16) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.;
(33) Klopsch, I.; Kinauer, M.; Finger, M.; Würtele, C.; Schneider, S.
Conversion of Dinitrogen into Acetonitrile under Ambient Conditions.
Angew. Chem., Int. Ed. 2016, 55, 4786-4789.
(34) Bezdek, M. J.; Chirik, P. J. Expanding Boundaries: N2 Cleavage and
Functionalization Beyond Early Transition Metals. Angew. Chem., Int. Ed.
2016, 55, 7892-7896.
(35) Kim, S.; Loose, F.; Bezdek, M. J.; Wang, X.; Chirik, P. J. Hydro-
genation of N-Heteroarenes Using Rhodium Precatalysts: Reductive Elim-
ination Leads to Formation of Multimetallic Clusters. J. Am. Chem. Soc.
2019, 141, 17900-17908.
(36) Conifer, C. M.; Law, D. J.; Sunley, G. J.; Haynes, A.; Wells, J. R.;
White, A. J. P.; Britovsek, G. J. P. Dicarbonylrhodium(I) Complexes of
Bipyridine Ligands with Proximate H–Bonding Substituents and Their
Application in Methyl Acetate Carbonylation. Eur. J. Inorg. Chem. 2011,
3511-3522.
(37) Poel, H. V. D.; Koten, G. V.; Vrieze, K. Four-co-ordinate Rhodi-
um(I) Complexes [{Rh(CO)2Cl}n(α-diimine)] (n = 1, 2) with σ,σ-N,N'
Chelate or σ-N, σ-N' Bridge Bonded RN=C(R')C(R'')=NR Lgands. Influ-
ence of the Branching at Cα of R on the RhI–α-diimine Interaction. Inorg.
Chim. Acta 1981, 51, 253-262.
(38) Delgado-Laita, E.; Sanchez-Muñoyerro, E. Carbonyl Rhodium(I)
Complexes with α-Diimines Ligands. Polyhedron 1984, 3, 799-804.
(39) Simões, J. A. M.; Beauchamp, J. L. Transition Metal-Hydrogen and
Metal-Carbon Bond Strengths: The Keys to Catalysis. Chem. Rev. 1990,
90, 629-688.
(40) Tshepelevitsh, S.; Kütt, A.; Lõkov, M.; Kaljurand, I.; Saame, J.;
Heering, A.; Plieger, P. G.; Vianello, R.; Leito, I. On the Basicity of Organic
Bases in Different Media. Eur. J. Org. Chem. 2019, 6735-6748.
(41) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of Pro-
ton-Coupled Electron Transfer Reagents and its Implications. Chem. Rev.
2010, 110, 6961-7001.
(42) Clarke, R. M.; Storr, T. Tuning Electronic Structure to Control
Manganese Nitride Activation. J. Am. Chem. Soc. 2016, 138, 15299-
15302.
Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Catalytic Transformation of Dini-
trogen into Ammonia and Hydrazine by Iron-Dinitrogen Complexes Bear-
ing Pincer Ligand. Nat. Commun. 2016, 7, 12181.
(17) Eizawa, A.; Arashiba, K.; Tanaka, H.; Kuriyama, S.; Matsuo, Y.;
Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Remarkable Catalytic Activi-
ty of Dinitrogen-Bridged Dimolybdenum Complexes Bearing NHC-Based
PCP-Pincer Ligands Toward Nitrogen Fixation. Nat. Commun. 2017, 8.
14874.
(18) Anderson, J. S.; Rittle, J.; Peters, J. C. Catalytic Conversion of Ni-
trogen to Ammonia by an Iron Model Complex. Nature 2013, 501, 84-87.
(19) Bezdek, M. J.; Pappas, I.; Chirik, P. J. Determining and Under-
standing N–H Bond Strengths in Synthetic Nitrogen Fixation Cycles. Top.
Organomet. Chem. 2017, 60, 1-22.
(20) Pappas, I.; Chirik, P. J. Ammonia Synthesis by Hydrogenolysis of
Titanium-Nitrogen Bonds Using Proton Coupled Electron Transfer. J. Am.
Chem. Soc. 2015, 137, 3498-3501.
(21) Pappas, I.; Chirik, P. J. Catalytic Proton Coupled Electron Transfer
from Metal Hydrides to Titanocene Amides, Hydrazines and Imides: De-
termination of Thermodynamic Parameters Relevant to Nitrogen Fixation.
J. Am. Chem. Soc. 2016, 138, 13379-13389.
(22) Bezdek, M. J.; Chirik, P. J. A Fresh Approach to Synthesizing Am-
monia from Air and Water. Nature 2019, 568, 464-466.
(23) Ashida, Y.; Arashiba, K.; Nakajima, K.; Nishibayashi, Y. Molyb-
denum-Catalysed Ammonia Production with Samarium Diiodide and
Alcohols or Water. Nature 2019, 568, 536-540.
(24) Bruch, Q. J.; Connor, G. P.; Chen, C.; Holland, P. L.; Mayer, J. M.;
Hasanayn, F.; Miller, A. J. M. Dinitrogen Reduction to Ammonia at Rheni-
um Utilizing Light and Proton-Coupled Electron Transfer. J. Am. Chem.
Soc. 2019, 141, 20198-20208.
(25) Wang, D.; Loose, F.; Chirik, P. J.; Knowles, R. R. N–H Bond For-
mation in a Manganese(V) Nitride Yields Ammonia by Light-Driven Pro-
ton-Coupled Electron Transfer. J. Am. Chem. Soc. 2019, 141, 4795-4799.
(26) Loose, F.; Wang, D.; Tian, L.; Scholes, G. D.; Knowles, R. R.; Chi-
rik, P. J. Evaluation of Excited State Bond Weakening for Ammonia Synthe-
sis from a Manganese Nitride: Stepwise Proton Coupled Electron Transfer
is Preferred over Hydrogen Atom Transfer. Chem. Commun. 2019, 55,
5595-5598.
(27) Scepaniak, J. J.; Young, J. A.; Bontchev, R. P.; Smith, J. M. For-
mation of Ammonia from an Iron Nitrido Complex. Angew. Chem., Int.
Ed. 2009, 48, 3158-3160.
(28) Hu, Y.; Li, L.; Shaw, A. P.; Norton, J. R.; Sattler, W.; Rong, Y. Syn-
thesis, Electrochemistry, and Reactivity of New Iridium(III) and Rhodi-
um(III) Hydrides. Organometallics 2012, 31, 5058-5064.
(29) Hu, Y.; Norton, J. R. Kinetics and Thermodynamics of H-/H·/H+
Transfer from a Rhodium(III) Hydride. J. Am. Chem. Soc. 2014, 136,
5938-5948.
(30) Bezdek, M. J.; Chirik, P. J. Pyridine(diimine) Chelate Hydrogena-
tion in a Molybdenum Nitrido Ethylene Complex. Organometallics 2019,
38, 1682-1687.
(31) Rebreyend, C.; de Bruin, B. Photolytic N2 Splitting: A Road to Sus-
tainable NH3 Production? Angew. Chem., Int. Ed. 2015, 54, 42-44.
(32) Klopsch, I.; Yuzik-Klimova, E. Y.; Schneider, S. Functionalization
of N2 by Mid to Late Transition Metals via N–N Bond Cleavage. Top.
Organomet. Chem. 2017, 60, 71-112.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
(43) Chiu, S.-M.; Wong, T.-W.; Man, W.-L.; Wong, W.-T.; Peng, S.-M.;
Lau, T.-C. Facile Nucleophilic Addition to Salophen Coordinated to Nitri-
doosmium(VI). J. Am. Chem. Soc. 2001, 123, 12720-12721.
(44) Matsumoto, K.; Sawada, Y.; Saito, B.; Sakai, K.; Katsuki, T. Con-
struction of Pseudo-Heterochiral and Homochiral Di-µ-oxotitanium(Schiff
base) Dimers and Enantioselective Epoxidation Using Aqueous Hydrogen
Peroxide. Angew. Chem., Int. Ed. 2005, 44, 4935-4939.
(45) Ziegler, J. E.; Du, G.; Fanwick, P. E.; Abu-Omar, M. M. An Efficient
Method for the Preparation of Oxo Molybdenum Salalen Complexes and
Their Unusual Use as Hydrosilylation Catalysts. Inorg. Chem. 2009, 48,
11290-11296.
(46) Ji, J.; Chen, X.; Wang, C.-J.; Jia, A.-Q.; Zhang, Q.-F. Syntheses,
Structures and Electrochemical Properties of Ruthenium(II/III) Complex-
es with Tetradentate Schiff Base Ligands. J. Coord. Chem. 2019, 72, 480-
490.
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Catalytic ammonia formation from a metal nitride and H2
by a new thermally stable rhodium hydride catalyst via PCET
5
6
cat.
tBu
tBu
N
7
8
9
tBu
O
N
O
N
tBu
N
Rh
Mn
H
NH3
H2
+
TON (NH3) ~ 6
TON (H•) ~ 18
(η5-C5Me5)Rh(MePhI)H
(
tBuSalen)MnN
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