Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen Atom Transfer Reactions Using a Chromium P4 Macrocycle

Article abstract of DOI:10.1021/jacs.7b11132

We report the first discrete molecular Cr-based catalysts for the reduction of N₂. This study is focused on the reactivity of the Cr-N₂ complex, trans-[Cr(N₂)₂(P^Ph₄N^Bn₄)] (P₄Cr(N₂)₂), bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-P^Ph₄N^Bn₄ ligand is critical to preserve the structural integrity of the catalyst. P₄Cr(N₂)₂ was found to mediate the reduction of N₂ at room temperature and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N₂ to N(SiMe₃)₃ by Na and Me₃SiCl, affording up to 34 equiv N(SiMe₃)₃; (2) stoichiometric reduction of N₂ by protons and electrons (for example, the reaction of cobaltocene and collidinium triflate at room temperature afforded 1.9 equiv of NH₃, or at -78 °C afforded a mixture of NH₃ and N₂H₄); and (3) the first example of NH₃ formation from the reaction of a terminally bound N₂ ligand with a traditional H atom source, TEMPOH (2,2,6,6-tetramethylpiperidine-1-ol). We found that trans-[Cr(¹⁵N₂)₂(P^Ph₄N^Bn₄)] reacts with excess TEMPOH to afford 1.4 equiv of ¹⁵NH₃. Isotopic labeling studies using TEMPOD afforded ND₃ as the product of N₂ reduction, confirming that the H atoms are provided by TEMPOH.

Full text of DOI:10.1021/jacs.7b11132

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Article
Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net
Hydrogen Atom Transfer Reactions using a Chromium P4 Macrocycle
Alexander J. Kendall, Samantha I. Johnson, R. Morris Bullock, and Michael T. Mock
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11132 • Publication Date (Web): 31 Jan 2018
Downloaded from http://pubs.acs.org on January 31, 2018
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Journal of the American Chemical Society
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Catalytic Silylation of N2 and Synthesis of NH3 and N2H4 by Net Hydrogen
Atom Transfer Reactions using a Chromium P4 Macrocycle
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Alexander J. Kendall, Samantha I. Johnson, R. Morris Bullock, and Michael T. Mock*
Center for Molecular Electrocatalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA
ABSTRACT: We report the first discrete molecular Cr-based catalysts for the reduction of N2. This study is focused on the reactivity of the Cr-
N2 complex, trans-[Cr(N2)2(PPh4NBn4)], P4Cr(N2)2, bearing a 16-membered tetraphosphine macrocycle. The architecture of the [16]-PPh4NBn
4
ligand is critical to preserve the structural integrity of the catalyst. P4Cr(N2)2 was found to mediate the reduction of N2 at room temperature
and 1 atm pressure by three complementary reaction pathways: (1) Cr-catalyzed reduction of N2 to N(SiMe3)3 by Na and Me3SiCl, affording
up to 34 equiv N(SiMe3)3; (2) stoichiometric reduction of N2 by protons and electrons. For example, the reaction of cobaltocene and col-
lidinium triflate at room temperature afforded 1.9 equiv of NH3, or at -78 °C to afforded a mixture of NH3 and N2H4; (3) the first example of
NH3 formation from the reaction of a terminally bound N2 ligand with a traditional H atom source, TEMPOH, (2,2,6,6-tetramethyl-piperidine-
15
1-ol). We found that trans-[Cr(15N2)2(PPh4NBn4)] reacts with excess TEMPOH to afford 1.4 equiv of NH3. Isotopic labelling studies using
TEMPOD afforded ND3 as the product of N2 reduction, confirming the source of the H atoms are provided by TEMPOH.
describing the N2 silylation mechanism suggest that silyl radicals,10
generated in situ from Me3SiCl and Na, K, or KC8, react with a M-N2
INTRODUCTION
The development of catalysts for N2 reduction to NH3 is a vital
species to form N-Si bonds. The seminal 1972 report by Shiina re-
area of energy research to reduce the enormous infrastructure, en-
vealed CrCl3 to be the first transition metal salt to catalyze this reac-
ergy input, and CO2 emissions of the industrial Haber-Bosch process
that generates the critical supply of NH3 used in agriculture and in-
dustry.1 The emergence of NH3 as a promising energy carrier for H2
storage or use in direct NH3 fuel cells2 also motivates the investiga-
tion of small-scale processes for the synthesis of NH3 from N2. Ro-
bust molecular electrocatalysts could provide the necessary kinetic
selectivity for N2 reduction to NH3 over thermodynamically pre-
ferred H+ reduction to H2 when utilizing protons and electrons.3
Such advances may lead to small-scale, decentralized, CO2-free NH3
production facilities with protons and electrons derived from renew-
able resources.
Drawing inspiration from biological N2 fixation with protons and
electrons carried out by the multimetallic active sites of the nitrogen-
ase enzymes,4 rapid developments of well-defined synthetic com-
plexes based on Fe,5 Mo,6 and Co7 have recently emerged as catalysts
for N2 reduction to NH3 and N2H4 using Brønsted acids and chemi-
cal reductants such as metallocenes or KC8. While the N2 reduction
mechanism had commonly been thought to proceed through a series
of H+/e- transfer steps, Peters and co-workers recently proposed that
N-H bond forming reactions may follow proton-coupled electron
transfer (PCET) pathways through the formation of protonated me-
tallocenes.5d PCET pathways8 could invoke hydrogen atom transfer
(HAT) to M-N2 and M-NxHy intermediates en route to NH3 for-
mation. While PCET pathways using separate acids and reductants
have been demonstrated, NH3 formation from a M-N2 complex by
concerted delivery of H+/e- as a hydrogen atom (H•) from a hydro-
gen atom donor such as TEMPOH remains elusive.
Figure 1.
Top: Selected Group 6 complexes shown to catalyze the re-
duction of N2 to N(SiMe3)3. Bottom: N2 reduction reactions examined
in this work with P4Cr(N2)2
.
tion, forming 5.4 equiv of N2-derived tris(trimethylsilyl)amine,
N(SiMe3)3, using Li as the reducing agent.11
Since that early account, several homogeneous catalytic systems
using first-row transition metals such as Fe,10b,12 Co,12d,13 and V14 have
been reported; of the group 6 metals several molecular Mo0-N2 pre-
cursors and one W0-N2 complex bearing phosphine ligands catalyze
N2 silylation (Figure 1, top).10a,c,15 Even though solvated CrCl3 dis-
played notable reactivity to catalyze N2 reduction 45 years ago, only
one example of Cr-mediated N2 cleavage has been subsequently re-
ported.16 No discrete molecular Cr catalysts for N2 reduction are cur-
rently known. Notably, two reported attempts to utilize Cr with mul-
tidentate ligand platforms that afforded Mo-based N2 reduction cat-
alysts did not lead to Cr catalysts.15c,17 In both cases, the targeted Cr-
The reduction of N2 to silylamines is a complementary approach
for NH3 production, where NH3 can be attained by subsequent treat-
ment of the silylamine product with acid (Figure 1, eq 1).9 Studies
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Page 2 of 10
N2 complex was not attained. These examples underscore the chal-
loss at Cr0 or could not be attained (see Table S1, Supporting Infor-
mation (SI)).18c For example, our attempts to prepare a Cr0-N2 com-
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lenge of synthesizing stable synthetic Cr complexes for N2 reduction
and the divergence of chemical behavior Cr has compared with anal-
ogous well-studied congeners. Therefore, Cr complexes have the
potential to provide group 6 metal-N2 reduction chemistry that is
distinct from Mo and W.
plex with the tetradentate PPh4NPh ligand12c resulted in a thermally
2
sensitive Cr0(N2)2 speciesdespite the rigid, but distorted, planar and
meridonal P4 coordination environment. In a second example, the
complex Cr(N2)(dmpe)(PPh3NBn3), entry 3 in Table 1, is a remarka-
bly stable Cr-N2 complex formed in high yield when using dmpe
(Me2PCH2CH2PMe2) as the bidentate ligand. In contrast,
Cr(N2)(dmpm)(PPh3NBn3) could not be attained using dmpm
(Me2PCH2PMe2), a diphosphine with a smaller bite angle. While it
is not surprising that ligand chelate effect increases complex stabil-
ity,21 Cr0 seems far more sensitive to ligand bite angles than Mo, es-
pecially in the latter example where diphosphines with a single car-
bon atom in the backbone have been used extensively to support
P5Mo-N2 complexes.22 Consequently, an apparently critical core ge-
ometric parameter we have noted as a general trend to attain stable
Cr-N2 complexes is P-P ligand bite angles that are close to 90°, af-
fording an archetypal octahedral coordination environment for Cr.
In the present case, a main contributor to the high stability of the
complex is the P-P bite angles of the [16]-PPh4NBn4 ligand, 89.7° and
90.0°, giving Cr a nearly perfect octahedral geometry with the two
axial N2 ligands.
Our interest in Cr for N2 reduction originated with the discovery
of isolable Cr-N2 complexes containing PPhnNBnn ligands (n = 2, 3 or
4).18 In particular, trans-[Cr(15N2)2(PPh4NBn4)], P4Cr(15N2)2 bearing
15
a 16-membered macrocycle (Figure 1, bottom panel) affords N2-
9
+
+
derived 15N2H5 and 15NH4 upon reaction with triflic acid at -50
°C.18b Thus, P4Cr(N2)2, with the notable kinetic and thermodynamic
macrocyclic stability of a tetraphosphine macrocycle,19 inspired our
efforts to investigate Cr for catalytic N2 reduction. Herein we report
the first molecular Cr complexes for the catalytic conversion of N2 to
silylamines, (Figure 1, eq 1). Our studies focus on the reactivity of
P4Cr(N2)2 that affords up to 34 equiv of N(SiMe3)3 per Cr center.
The unique 16-membered phosphorus macrocycle is critical to pre-
serve the structural integrity of the catalyst, allowing the homogene-
ous complex to maintain its catalytic activity and to be recycled, pro-
ducing substantial catalytic formation of N(SiMe3)3 upon substrate
reloading. In this study, we establish the reactivity of P4Cr(N2)2 at
room temperature with protons and electrons, (Figure 1, eq 2) and
consider the role of PCET pathways in the production up to 1.9
equiv of NH3 or a mixture of NH3 and N2H4. Lastly, the reactivity of
P4Cr(15N2)2 with TEMPOH (2,2,6,6-tetramethyl-piperidine-1-ol),
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Inherent to the P4Cr(N2)2 structure are the contrasting steric en-
vironments above and below the P4Cr plane in which the N2 ligands
reside (Figure 2). One N2 ligand occupies a “pocket” formed by the
four phenyl substituents on P, and the opposing N2 ligand is in a
comparatively open face of the macrocycle. Accordingly, these con-
trasting environments impact the strength N2 binding to Cr, which
we believe contributes to catalytic reactivity. We evaluated the N2
binding affinities by DFT computational analysis with the B3LYP
functional and D2 dispersion corrections, and found the “pocket” N2
ligand exhibits a lower dissociation energy (17.2 kcal/mol) com-
pared to the N2 ligand in the open face (26.5 kcal/mol). As discussed
below, we propose that N2 dissociation is a required step before ca-
talysis; thus, based upon this computational assessment of the N2
binding affinities, the N2 functionalization occurs at the open face N2
ligand.
15
15
to form N2-derived NH3 (Figure 1, eq 3) is presented, providing
the first experimental evidence for NH3 formation directly from N2
using a traditional hydrogen atom donor.
RESULTS AND DISCUSSION
Structure, Stability, and N2 Binding of P4Cr(N2)2. The macrocy-
clic complex P4Cr(N2)2 was prepared using a modified synthetic pro-
cedure developed since our initial report,18b and was isolated as an
orange crystalline solid in 21% yield. In Figure 2, we recount the mo-
lecular structure of P4Cr(N2)2 that was reported in our prior study
from x-ray crystallography to illustrate the relationship between the
structure of P4Cr(N2)2 enforced by the all-syn-isomer of the [16]-
PPh4NBn4 ligand, and the high stability of the complex. We have noted
the difficulty in forming discrete Cr0-N2 complexes with chelating
Catalytic Reduction of N2 to N(SiMe3)3. The reaction of
P4Cr(N2)2 with 100 equivalents of Na and Me3SiCl at 1 atm N2 and
room temperature yielded 10.6 turnovers of N(SiMe3)3 (TON =
turnover number = N atoms/Cr). The N(SiMe3)3 that was pro-
duced was identified by GC/MS and then acidified to give NH4Cl
“pocket” N2
“open face” N2
1
that was quantified by H NMR spectroscopy, in which 64% of the
electrons went into reducing N2 (Table 1, entry 1). Higher TONs
were achieved by increasing the loading of Na and Me3SiCl up to 105
equivalents/Cr, yielding up to 21.2 TON in a single run. After a cat-
alytic run is complete, the mixture can be directly replenished (or fil-
tered and replenished) with fresh reagents, and P4Cr(N2)2 continues
to catalytically reduce N2 to N(SiMe3)3 with yields doubling from
17.1 TON (first loading) to 34.1 TON (combined total after second
loading). The observed TONs do not appear to scale linearly with
reagent concentration – likely caused by the heterogeneous nature
of Na and active radical species concentration in a constant flux (see
proposed mechanism below). Catalytic N2 reduction with homoge-
neous complexes is notoriously sensitive to reaction conditions to
achieve catalysis,5d especially the solvent.5a,b,14 Accordingly, we
screened a variety of experimental conditions in our catalytic N2 si-
lylation studies of P4Cr(N2)2 including, silane identity, reductant,
solvent, and temperature. The results of these catalytic trials are
listed in Tables S2-S5.
“open face” N2
“pocket” N2
Figure 2. Top and side views of the molecular structure of
P4Cr(N2)2 highlighting the contrasting steric environments around
the N2 ligands. Only the benzyl carbon atoms of NBn groups are
shown for clarity.
phosphine ligands compared to Mo and W analogues.20 Our own
attempts have yielded a handful of stable Cr0-N2 complexes in low to
moderate yields; and we found the intrinsic geometric constraints of
some bidentate and tetradentate phosphine ligands greatly impact
stability, i.e. the complexes are thermally sensitive toward N2 ligand
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Journal of the American Chemical Society
N2 product. This extensive test of Cr-based compounds more clearly
1
2
3
demonstrates the activity of Cr for N2 reduction, regardless of
whether the compounds have an N2 ligand. Similar catalytic activity
has been observed for Fe complexes that do not bind a N2 ligand at
room temperature.10b,12b
Table 1. Catalytic reduction of N2 to N(SiMe3)3 using Cr complexes.
4
For the Cr-complexes containing N2 ligands, there does not ap-
pear to be a correlation between N2 activation, as measured by the
νNN bands in the infrared spectrum, and catalyst TON under the con-
ditions in Table 1. The P4Cr(N2)2 complex was unique amongst this
group in that it produced the highest TONs, was recyclable, and ex-
ists as a molecular species during and after catalysis (see below). All
other chromium salts and complexes displayed rapid Cr0 precipita-
tion out of solution. For instance, reactions run with trans-
5
6
7
8
νNN
equiv
NH4Clb
10.6
Entry
1
Cr complexa
(cm-1)
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1918,
2072
(THF)
17.1c
21.2c,d
34.1e
31
[Cr(N2)2(dmpe)2] yielded free dmpe ligand by P NMR spectros-
P4Cr(N2)2
copy. Based on these observations, it is likely that the reduction and
oxidation of chromium, specifically Cr0 to CrI oxidation states repre-
sent a soft/hard transition24 and causes ligand lability. It is proposed
that once a chromium species is oxidized in the cycle for N2 reduc-
tion, ligand dissociation leads to metal aggregation and observable
precipitation. Thus, we infer that it is the Cr-ligand stability over re-
dox cycles, not the activation of N2, that leads to catalytic turnover.
Multidentate phosphine ligand strategies have been pursued for
N2 reduction by Tuczek and co-workers to prevent ligand loss at
high metal oxidation states of Mo.22a,b,25 For Cr, geometry optimized
multidentate ligand systems are imperative for mere stability. The
P4Cr(N2)2 complex is resilient to ligand dissociation; in fact, we have
not observed ligand loss as a pathway of catalyst deactivation in this
study. The P4Cr(N2)2 remains molecularly discrete and in solution
during the redox cycling necessary for catalytic turnover.26
To illustrate this point, we compared the catalytic reactivity of
P4Cr(N2)2 (Table 1, entry 1), to trans-[Cr(N2)2(dmpe)2] (Table 1,
entry 2), and cis-[Cr(N2)2(PPh2NBn2)2] (Table 1, entry 4). Trans-
[Cr(N2)2(dmpe)2] is most structurally similar to P4Cr(N2)2, while
cis-[Cr(N2)2(PPh2NBn2)2] is a structural isomer of P4Cr(N2)2. In reac-
tions performed with increased loading of silane and reductant, 105
equiv Na, 105 equiv. Me3SiCl (Table S8), both trans--
[Cr(N2)2(dmpe)2] and cis-[Cr(N2)2(PPh2NBn2)2] performed almost
identically to the results in Table 1, while P4Cr(N2)2 afforded almost
double the TON of N(SiMe3)3. In addition, trans-
[Cr(N2)2(dmpe)2] and cis-[Cr(N2)2(PPh2NBn2)2] could be not be re-
cycled to generate additional N(SiMe3)3 as illustrated with
P4Cr(N2)2. Because of the electronic and structural similarities of
these complexes, the striking divergence in reactivity is assigned to
the macrocyclic effect of the ligand – specifically the ability to main-
tain chromium as a molecular species during the redox cycling, N2
reduction and catalysis.
Mechanistic Considerations for Silylation Catalysis with
P4Cr(N2)2. To improve our understanding of the mechanism and
speciation of the reaction components formed during catalysis, the
catalytic reaction was examined after 8 h, before complete consump-
tion of the Na and Me3SiCl reagents at 16 h. Upon analysis of the
reaction mixture by GC-MS and 1H NMR spectroscopy, a better pic-
ture of the reaction profile emerged. In addition to the N(SiMe3)3
generated from N2 reduction, the only organic reaction products
were (Me3Si)2, trimethyl(4-(trimethylsilyl)butoxy)silane, and an in-
soluble polymer of THF (Figure 3). The formation of these organic
products, which have been reported previously,10c,15a,b supports the in
situ generation of SiMe3 radicals in solution, formed from the reac-
tion of Na with Me3SiCl. Consequently, the reaction of SiMe3 radi-
cals with THF, and the homocoupling reaction, represent significant
1932
2
3
4
5.2
6.2
4.8
(hexane23)
trans-[Cr(N2)2(dmpe)2]
1918
(THF18c)
Cr(N2)(dmpe)(PPh3NBn
)
3
1937,
2009
(THF18a)
cis-[Cr(N2)2(PPh2NBn2)2]
5
6
-
6.8
5.0
fac-[CrCl3(k3-
(P,P,N)PPh2NBn2)]
-
fac-[CrCl3(PPh3NBn3)]
Cr(CO)6
Cr(C6H6)(CO)3
Cr(C6H6)2
CrCl2(THF)
CrCp2
7
8
9
10
11f
12g
13
14
15
16
-
-
-
-
-
-
-
-
-
-
4.8
3.8
0.2
<0.1
4.4
1.9
0.5
2.5
<0.1
CrCp*2
CrCl3(THF)3
CrBr3(THF)3
Cr powder
None
<0.1
a[“Cr”] = 10-4 M, 23° C, 1 atm N2. All values reported are an
average of at least two trials. cSilylated glassware, 1.0 M Me3SiCl
(105 equivalents) and 105 equivalents of Na. dRun 72 hours. eRun
16 hours then refreshed with another 1.0 M Me3SiCl (105 equiv-
alents) and an equivalent amount Na and run an additional 16
hours. fCp = C5H5. gCp* = C5(CH3)5.
b
A variety of molecular chromium complexes and Cr-salts were ex-
amined to determine the generality of N2 reduction by Cr (Table 1).
Surprisingly, several of the chromium complexes that were tested ex-
hibited TONs comparable to the initial report by Shiina.11 In fact,
nine of the fourteen chromium salts or complexes yielded over two
TON, with all Cr entries yielding at least a trace amount of reduced
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Page 4 of 10
side reactions that reduce the concentration of SiMe3 radicals in so-
Fe0(N2)(P4PhNPh2).12c Intuitively, this suggested to us that dissocia-
1
2
3
4
5
6
7
8
lution, thus competing kinetically with N2 reduction process.
tion of one N2 ligand to a generate a putative 5-coordinate
“P4Cr0(N2)” complex is a prerequisite for catalysis. Hidai and co-
workers have proposed a similar initial step of dissociation of N2
from cis-[Mo(N2)2(PMe2Ph)4] prior to subsequent N2 reduction.27
organic
inorganic
Ph
Cl
Bn
Ph
N(SiMe3)3
+
N
P
NBn
P
CrII
Ph
P
N
Bn
Ph
P
SiMe3
N
Ph
In our previously described protonation mechanism of P4Cr(N2)2,
N2
Cr
Bn
1000 Na
1000 Me3SiCl
O
Bn
Ph
Me3Si
N
Cl
+
P
NBn
the dissociation of one N2 was determined by DFT calculations to
increase the proton affinity of the bound N2 to enable N-H bond for-
mation. The lability of N2 is also the likely cause of the previously
reported irreversible CrI/0 redox couple at slow scan rates by cyclic
voltammetry.18b
P
Ph
P
N
Bn
Ph
P
THF, 23 °C, 8h
+
Ph
N
N2
Bn
Ph
Bn
N
P
Me3Si SiMe3
N2
NBn
P
Ph
P
Cr
N
Bn
Ph
P
+
N
9
Bn
poly-THF
N2
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Based on the results of the catalytic trials, the independent reac-
tivity of P4Cr(N2)2 and P4CrII(Cl)2, and insights from related group
6 catalysts,10,15b,28 we propose a mechanism for catalytic reduction of
N2 to silylamines by P4Cr(N2)2 (Figure 5). The proposed mecha-
nism initiates with P4Cr(N2)2. (a) Upon mixing a background reac-
tion is established between the two Cr species, Me3SiCl, and Na. (b)
After the dissociation of the “in pocket” N2 ligand a 5-coordinate
P4Cr0(N2) complex enters the catalytic cycle. (c) Concomitant for-
mation of SiMe3 radicals are generated in situ from the reaction of
Na and Me3SiCl. The SiMe3 radicals are presumed to be the active
species in catalysis; however, the concentration of SiMe3 radicals
available to react with Cr-N2 can be affected by the rate of (Me3Si)2
formation and reactions with THF as shown above. (d) The SiMe3
radical reacts with the distal N atom of N2 initiating an oxidation
state change of Cr. DFT calculations predict this reaction is favora-
ble by 14 kcal/mol. The resistance of the “P4Cr” fragment to phos-
phine ligand dissociation at this stage is believed to be critical to pre-
vent Cr0 precipitation. (e) The silylated intermediates formed in
subsequent reaction steps have not been identified experimentally.
However, DFT calculations suggest the addition of the second
SiMe3 radical is thermodynamically favored to react at the distal ni-
trogen atom by 14.7 kcal/mol, forming a silylhydrazido intermedi-
ate. The lack of phosphine ligand dissociation of the macrocycle fa-
vors this intermediate over silyl radical addition at the proximal ni-
trogen atom. DFT predicts the addition of a third silyl radical leads
to N-N bond cleavage, generating N(SiMe3)3 and a P4Cr-N-SiMe3
species that undergoes further reactions with silyl radicals to pro-
duce the second equiv of N(SiMe3)3. A similar reaction mechanism
in a recent Mo-based N2 silylation catalyst was described by
Mézailles and co-workers and was supported by several isolated and
structurally characterized intermediates.10a On the basis of the bond-
ing description of reduced Cr-NxHy intermediates from H+ and e- ad-
ditions by DFT computations,18b,c the formation of a silylhydrazine
(Me3Si)2NN(SiMe3)2 product is plausible, however the steric bulk
of the SiMe3 groups and the necessary (but unlikely) dissociation of
a phosphine atom from Cr disfavors this N2 silylation pathway. (f)
Under reducing conditions P4Cr0 is regenerated, the coordination of
N2 to chromium completes the catalytic cycle.
Figure 3. Organic and inorganic reaction products identified after 8 h of
catalysis with P4Cr(N2)2 during the reduction of N2 to N(SiMe3)3. Or-
1
ganic products were identified by H NMR spectroscopy and GC-MS
1
analysis; inorganic products identified by H and 31P NMR spectros-
copy, and single crystal x-ray diffraction.
The identity of the inorganic reaction products was determined
by NMR spectroscopy. After reacting for 8 h, P4Cr(N2)2 was ob-
31
served in the reaction mixture by P NMR spectroscopy. In addi-
1
tion, a paramagnetic species in the H NMR spectrum that was iso-
lated as yellow crystals and identified by single crystal x-ray diffrac-
tion matched the previously reported complex trans-
[Cr(Cl)2(PPh4NBn4)] P4CrII(Cl)2 18b Most importantly, no free ligand
.
was observed in the reaction mixture by 31P NMR spectroscopy, in-
dicating the macrocycle remained intact. Independently, we con-
firmed that P4CrII(Cl)2 can be directly generated from the reaction
of P4Cr(N2)2 with Me3SiCl in THF (Figure 4). To further confirm
the identity of the isolated paramagnetic CrII species and to
Ph
N2
Ph
Cl
Bn
Bn
Ph
Ph
N
xs Me3SiCl
THF
N
P
P
NBn
P
NBn
P
Ph
P
CrII
Ph
P
Cr
N
Bn
Ph
P
N
Bn
Ph
P
xs Na0
THF
16h
N
N
Bn
Bn
N2
Cl
87% isolated
quant (NMR)
68% isolated
Figure 4. Independent verification of observed inorganic products
during catalytic reduction of N2 to N(SiMe3)3.
understand its reactivity under catalytic conditions, the isolated
P4CrII(Cl)2 was reacted with excess Na to cleanly yield P4Cr(N2)2
,
reaching full conversion after 16 h. The slow rate of reduction of
P4CrII(Cl)2 by Na metal to generate P4Cr(N2)2 is likely due to the
heterogeneous reduction conditions. Based on the independent re-
activity of these two complexes, it is likely that their individual con-
centrations are in constant flux during catalysis. Importantly, the
clean reduction to continuously regenerate P4Cr(N2)2 from
P4CrII(Cl)2 and Na allows the Cr complex to be recycled upon sub-
strate reloading.
To enhance catalytic TON, reactions were performed under in-
creased N2 pressure (90 atm). Unexpectedly, P4Cr(N2)2 consistently
failed to produce more than 4.1 TON using the same reaction con-
ditions that afforded 17.2 TON at 1 atm N2 (Table S7). The delete-
rious effect of N2 pressure on catalysis is surprising, as we anticipated
that increasing the concentration of dissolved N2 in solution would
enhance catalysis by kinetically favoring N2 binding during catalytic
turnover. Indeed, this result contrasts with the 4-fold increase in
TON we observed upon increasing the N2 pressure from 1 to 100
atm in the catalytic reduction of N2 to N(SiMe3)3 using
ACS Paragon Plus Environment

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Journal of the American Chemical Society
10
11
12
CoCp2
CoCp2
CoCp2
ColH[OTf]
ColH[OTf]
ColH[OTf]
pentane
PhF
0.3
0.5
0.1
0.1
0.2
0.4
1
2
3
4
5
6
7
8
N
N
(a)
Cl
Et2O
2 Me3SiCl
(b)
N2
b
aRun at 0.1 mM [Cr], 23 °C, 16 hours. Cp = C5H5, Cp* =
CrII
Cl
Cr0
N2
2 Na
2 N2
C5(CH3)5 cCol = 2,4,6-trimethylpyridine, OTf = trifluoro-
methanesulfonate. Equivalents of NH4 quantified using H NMR
spectroscopy. Equivalents of N2H5 quantified using p-dimethyla-
minobenzaldehyde test.30 fRun at 55° C. gRun at -78° C for 4 h, then
slowly warmed to 23 °C for 8 h.
N
N
d
+
1
e
+
(c)
N2
Me3Si
Me3SiCl + Na
Cr0
ΔG° = -14.0
kcal/mol
NaCl(s)
(d)
(f)
9
SiMe3
Cr0
and collidinium triflate (ColH[OTf]) as a proton source (Table 2,
entry 1). The reducing strength of CoCp2 is only 100 mV more neg-
ative than the quasi-reversible E1/2 value for the CrI/0 couple of
P4Cr(N2)2 at -1.22 V vs Cp2Fe0/+ in THF.18b Interestingly, the for-
10
11
12
13
14
15
16
17
18
19
20
21
22
Bn
N
N
P
NBn
P
Ph
P
N
Cr
(e)
N
Bn
Ph
P
=
Cr
N
CrI
Ph
Bn
Ph
5 Me3Si
+
mation of NH4 from N2 at room temperature displays a clear de-
2 N(SiMe3)3
pendence on the reduction potential of the metallocene. For exam-
ple, no reduced N2 products were observed at room temperature
with stronger reductants such as decamethylcobaltocene (CoCp*2)
(-1.98 V vs Cp2Fe0/+ in THF),5a or decamethylchromocene
(CrCp*2) (-1.47 V vs Cp2Fe0/+ in CH2Cl2). In addition, chromocene
(CrCp2) (-1.07 V vs Cp2Fe0/+ in CH3CN)31 was ineffective at afford-
ing reduced N2 products, although this may be due to its inability to
reduce Cr to the Cr0 oxidation state. At room temperature, it is pos-
sible that competing side reactions, such as H2 evolution,6c,32 be-
tween the stronger reducing agents and ColH[OTf] occur rapidly,
before productive N-H bond formation. This is particularly likely if
N2 dissociation from P4Cr(N2)2 is a prerequisite step before initiat-
ing reactivity at N2. Reactions with CoCp*2 and ColH[OTf] con-
+
N(SiMe3)3
SiMe3
(e)
SiMe3
Me3Si
SiMe3
N
N
N
N
N
2 Me3Si
N-N cleavage
Me3Si
CrII
CrI
CrII
ΔG° = -14.7
kcal/mol
Figure 5. Proposed catalytic cycle for P4Cr(N2)2 reducing N2 to
N(SiMe3)3. The proposed intermediates listed in the box for the
steps shown as (e) were obtained from DFT calculations; see SI for
details.
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31
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53
54
55
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59
60
Reduction of N2 to NH3 Using Protons and Electrons. In addition
to studying P4Cr(N2)2 reactivity with silyl radicals, we examined the
reaction of P4Cr(N2)2 with various sources of protons and electrons
for the reduction of N2 directly to NH3; the results are summarized
in Table 2. Protonated metallocenes to serve as PCET reagents or
effective H atom sources for N2 reduction5d may exhibit one-electron
radical-based reactivity with P4Cr(N2)2, similar to reactions with si-
lyl radicals. In our experiments, P4Cr(N2)2 was added to a freshly pre-
pared solutionof 40 equiv of acid and 30 equiv reductant in THF. In
reactions performed at room temperature P4Cr(N2)2 generated 1.9
+
ducted at -78 °C further support this hypothesis, as 1.3 equiv of NH4
was formed by initially lowering the reaction temperature (Table 2,
entry 8).
Though catalytic turnover was not observed with P4Cr(N2)2 using
the current combination of acid, counter anion, reductant, and sol-
+
vent, N2H5 was detected and quantified in several trials. Perhaps
most striking is the comparison between entries 1 and 7 in Table 2,
+
wherein N2H5 is observed when the reaction is initially conducted
at -78 °C before warming to room temperature for 8 h. These results
suggest that at lower temperatures an alternating N2 reduction path-
way33 is occurring at Cr, where the first two N-H bonds are formed
at the distal and proximal nitrogen atoms, respectively. The alternat-
ing N-H bond formation would eventually lead to the formation of
hydrazine, which was observed in several cases (Table 2). The ob-
servation of N2H4 as a product in the reaction implicates P4Cr-N2H4
as a possible reaction intermediate in the complete reduction of N2
to NH3. Although it is not conclusive that NH3 formation proceeds
+
equiv of NH4 using cobaltocene (CoCp2) as the reductant (-1.33 V
vs Cp2Fe0/+ in THF),29
+
+
Table 2. Direct synthesis of NH4 and N2H5 from P4Cr(N2)2, pro-
tons, and a reducing agent.
+
via N2H4 directly,34 we observed N2H5 at room temperature in en-
tries 5 and 12 using of the weaker acid Ph2NH2[OTf] or less polar
+
solvent, respectively.35 Because N2H5 is observed at low tempera-
+
ture in a reaction that yields exclusively NH4 at room temperature,
Reduct-
+
+d
+e
Entrya
Acidc
Solvent NH4 N2H5
and N2H5 is observed in several other reactions that also yield am-
antb
monia, it is likely that N2H4 is an intermediate in the mechanism of
reduction from N2 to NH3.
In a series of control experiments focusing on the separate reactiv-
ity of P4Cr(N2)2 with ColH[OTf] and CoCp2, we discovered that
P4Cr(N2)2 did not react with either of these reagents independently.
1
2
3
4
CoCp2
CoCp*2
CrCp2
CrCp*2
CoCp2
CoCp2
CoCp2
CoCp*2
CoCp2
ColH[OTf]
ColH[OTf]
ColH[OTf]
ColH[OTf]
Ph2NH2[OTf]
ColH[OTf]
ColH[OTf]
ColH[OTf]
ColH[OTf]
THF
THF
THF
THF
THF
THF
THF
THF
toluene
1.9
<0.1
<0.1 <0.1
<0.1 <0.1
<0.1 <0.1
0.7
0.7
0.6
1.3
5
0.4
<0.1
0.2
When 8 equiv of ColH[OTf] was mixed with 2 over several
P Cr(N )
4
2
6f
7g
8g
9
days in a sealed NMR tube, no observable reactivity was noted, as
determined by the absence of free collidine, absence of paramagnetic
features, no H2 formation, and unchanged 1H and 31P NMR spectra
of P4Cr(N2)2 (Figure S5). The stability of P4Cr(N2)2 in the presence
<0.1
<0.1 <0.1
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Journal of the American Chemical Society
Page 6 of 10
+
of ColH[OTf] was also investigated by in situ IR spectroscopy,
showing the vibrational frequency of the symmetric and asymmetric
νNN bands remain unchanged after acid addition (Figure S4). The
lack of reactivity between P4Cr(N2)2 and ColH[OTf] is surprising
because low-valent molecular N2 coordination complexes typically
exhibit a very basic metal center and are susceptible to protonation
at the metal to form metal hydrides, especially with ligand platforms
containing pendant amine groups.20a,b,36 Typically this pervasive H+
reduction event must be mitigated by low concentrations of acid or
insoluble acids for N2 reduction.5d,6a The long-term acid stability of
P4Cr(N2)2 toward ColH[OTf] must be due to poor kinetics for pro-
ton transfer since H2 formation is thermodynamically favora-
ble. P4Cr(N2)2 lacks accessible cis coordination sites to N2 which
would otherwise provide a more facile route to proton reduction and
H2 formation. Moreover, the four phenyl groups of the P4 ligand of-
fer steric protection from bulky acids such as ColH[OTf] from ef-
fectively transferring a proton to the face of the complex most likely
to have dissociated an N2 ligand.
P4Cr(15N2)2 affords 15NH4 observed by 1H NMR spectroscopy (Fig-
ure S9). In addition, we have established the origin of the hydrogen
atoms in the formation of ammonia from reduction of the terminally
bound N2 ligand by reacting P4Cr(N2)2 with excess TEMPOD in
protio THF. Treatment of P4Cr(N2)2 with 100 equiv TEMPOD at
room temperature affords ND3, which was identified as a broad sin-
glet at 0.65 ppm by 2H NMR spectroscopy (Figure S11). In an NMR
tube experiment, the reaction of P4Cr(N2)2 with excess TEMPOH
1
2
3
4
5
6
7
8
1
yields unidentified paramagnetic products by H NMR spectros-
copy, and no signals were observed in the P NMR spectrum. The
31
9
effort to identify the final Cr-containing product of this NH3 forming
reaction is ongoing; these observations suggest the P4N4 ligand has
remained intact and oxidation of P4Cr(N2)2 has occurred (Figure
S7). Since TEMPO radical was not observed as a product, it is plau-
sible that NH3 generation is accompanied by the concomitant for-
mation of Cr-O bonds,43 akin to the Fe-(TEMPO) product formed
in the reactions of the FeIV-nitride with TEMPOH by Smith and co-
workers.39a
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Addition of CoCp2 to a THF-d8 solution of ColH[OTf] and
P4Cr(N2)2 at room temperature led to an immediate reaction as evi-
dent by the appearance of free collidine, changing paramagnetic fea-
tures, and H2 in the 1H NMR spectrum.37 Because P4Cr(N2)2 was not
observed to react with CoCp2 or ColH[OTf] independently, but re-
Given that excess ColH[OTf] did not react independently with
P4Cr(N2)2, proton transfer from the weakly acidic TEMPOH (pKa
~ 41 in CH3CN)44 is not expected to be thermodynamically accessi-
ble because it is not sufficiently acidic (although the electron-rich
P4Cr(15N2)2 has been shown to react with HOTf to form 15NH4 and
+
+
+
acts (to yield NH4 ) when both reagents are present, either an inter-
15N2H5 ). Furthermore, based on the redox properties of TEMPOH
mediate species (between CoCp2 and ColH[OTf]) or ternary sys-
tem is required for N2 reduction. A ternary system would not be ki-
netically favorable given the dilute conditions. Alternatively, a pro-
tonated metallocene (CoCp2H[OTf]) or pyridinyl radicals38
(ColH.) from the reduction of pyridinium acids, are two plausible
intermediate species that could be generated in situ that exhibit
bond dissociation free energies (BDFEs) optimal for PCET or HAT
reactivity with coordinated N2. Because the proton source and elec-
tron source must both be present in solution for reactivity with
P4Cr(N2)2, a PCET pathway must be operating in the initial reduc-
tive steps from N2 to NH3. Based on the known BDFEs of
CoCp2H[OTf] and ColH., HAT is a plausible mechanism.5d
To further assess one-electron radical-based reactivity for the syn-
thesis of NH3 from N2 by hydrogen atom transfer pathways, we in-
vestigated the reaction of P4Cr(N2)2 with a traditional organic HAT
reagent 2,2,6,6-tetramethylpiperidin-1-ol (TEMPOH). Related re-
actions of TEMPOH with M-nitride complexes have been reported.
For example, in a study from Smith and co-workers, HAT steps were
proposed in the stoichiometric synthesis of NH3 from the reaction
of excess TEMPOH with a terminal iron(IV) nitride complex.39 Sim-
ilarly, Schneider and co-workers proposed HAT in the formation of
an Ir-NH2 complex from the reaction of an Ir-nitride complex with
excess TEMPOH.40 Lastly, Holland and co-workers formed NH3
from the reaction of 2,4,6-tri-tert-butylphenol with an N2-derived
tetrairon bis(nitride) complex.41 However, to our knowledge, NH3
formation from the reaction of TEMPOH with a terminally bound
N2 molecule is unprecedented.
(E1/2 = 0.71 V in CH3CN)45 electron transfer to P4Cr(N2)2 (CrI/0 = -
1.22 V vs Cp2Fe0/+ in THF; no reduction wave was observed for
P4Cr(N2)2 up to -2.5 V in THF) is also an unlikely initial step. While
the complete balance of products formed in this transformation is
not completely defined at this time, the reaction of TEMPOH with
P4Cr(N2)2 to form N-H bonds of NH3 shows the plausibility that
concerted hydrogen atom transfers are occuring directly with a ter-
minally bound N2 ligand. Because we have not yet identified the final
Cr-containing product, we cannot rigorously rule out N2 reduction
by heterolytic pathways. While the labelling studies have unambigu-
ously established 15N2 and TEMPOD as the sources of nitrogen and
hydrogen atoms, respectively, in the net hydrogen atom transfer re-
actions to form ammonia, this description of the overall reaction
does not require that the reaction proceed by a single-step HAT
mechanism.
CONCLUSION
We report the first molecular chromium complexes capable of
catalytic N2 reduction. These Cr complexes catalytically reduce N2
to silylamines at room temperature and pressure, with the
macrocycle containing complex P4Cr(N2)2 affording up to 34 equiv
of N(SiMe3)3 per Cr. P4Cr(N2)2 is also capable of stoichiometric
reduction of nitrogen with H+ and e- or with TEMPOH. Most Cr
species screened in this study showed some activity towards N2
reduction. The low TONs observed with almost all Cr species
studied can be explained by the inability of the Cr complexes to
remain in solution when undergoing redox chemistry necessary for
catalysis, with reactions typically resulting in Cr0(s) precipitating out
of solution (with observed free ligand in solution). The key
structural feature to achieving higher turnover and even recyclability
of a catalyst was a tetradentate macrocyclic ligand affording long
lifetimes in solution.
Treatment of P4Cr(N2)2 with 100 equiv of TEMPOH affords 1.4
equiv of free NH3, which was vacuum transferred directly out of the
1
reaction flask (without any additives), then quantified by H NMR
spectroscopy upon acidification of the NH3 gas in a separate vessel
(see SI for details).42 Hydrazine was not detected as a product in this
reaction, and the reaction of P4Cr(N2)2 with 87 equiv of TEMPO
radical produced no NH3 (Figure S10). Importantly, we confirmed
the ammonia that is generated originates from the reduction of the
dinitrogen ligands, as the reaction of excess TEMPOH with
+
+
Direct synthesis of NH4 and N2H5 from N2 was achieved, though
catalytic N2 reduction with protons and electrons was not observed
with the current scope of reagents examined in this study. Notably,
+
N2H5 was detected in several cases, suggesting that N2H4 is a
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Journal of the American Chemical Society
Me3SiCl was purified by refluxing overnight over CaH2 under N2, fol-
reduction intermediate and the Cr complex proceeds through an
lowed by an air-free fractional distillation yielding >99.9 % pure Me3SiCl by
1H NMR. Sodium sand was prepared by taking sodium metal (20 g) in do-
decane (250 mL) and refluxing with vigorous stirring under N2 (Caution!
Use a heating mantle and grease all joints thoroughly). Once the sodium liq-
uid dispersion forms a fine particulate, the stirring was halted, and the vessel
was slowly cooled back to room temperature, yielding a fine sodium sand.
The solid was collected by filtration on a frit in a glovebox, washed with THF
followed by pentane and dried under reduced pressure yielding ultra-fine so-
1
2
3
4
5
6
7
8
alternating N2 reduction pathway that diverges from analagous Mo-
and W-N2 reduction chemistry. P4Cr(N2)2 does not react directly
with the acid or the reductant used in these reactions. Rather, it is
very likely that an intermediate species is generated in situ from the
CoCp2 reductant and the ColH[OTf] acid that performs HAT to
P4Cr(N2)2, the details of which are currently under investigation. We
more clearly demonstrated the likelihood of HAT using TEMPOH
as a hydrogen atom source to produce free NH3 directly from N2.
In these cases, both independent electron transfer and proton
transfer are unlikely initial mechanistic pathways for N-H bond
formation due to thermodynamic or kinetic barriers, implying HAT
for the initial step. Isotopic labeling (e.g., 15N2 and TEMPOD)
unambigiously distinguish the sources of N and H for NH3
formation, further corroborating this interpretation.
Though some details of this reaction are not currently
understood, the proof of principle for a HAT mechanism for N2
reduction ot NH3 directly at room temperature and pressure has
been demonstrated. This work supports the notion that HAT can
have significant advantages over stepwise H+/e- pathways, and both
Cr complexes and HAT mechanisms will play a key role in
homogeneous N2 reduction in the future.
dium sand. PPh2NBn was prepared according to the literature preparation of
2
t
PPh2NtBu with the modification that BnNH2 was used instead of BuNH2.
2
9
Note that very slow addition of BnNH2 is recommended because the reac-
tion is exothermic.48
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Procedure for Cr catalyzed reduction of N2 to N(SiMe3)3. A solution of
Me3SiCl and reductant were stirred for 5 minutes in THF. To this mixture
was added the chromium complex as a THF solution. The mixture was
stirred for 8-72 hours. The reaction mixture was then filtered thorough
Celite and rinsed thoroughly with additional THF. The filtrate was acidified
with 1000 equivalents HCl in Et2O (1 M, 1.5 mL) and the solvent was evap-
orated, giving a solid. To the residue was added 0.500 mL of a stock solution
of 8.5 mM 1,3,5-trimethoxybenzene (TMB) in DMSO-d6. The resulting so-
1
lution was analyzed by H NMR spectroscopy with the relaxation delay set
to 10 s based on the longest T1 relaxation measurement of 1.4 s for the TMB
+
aromatic proton. Spectroscopy for the diagnostic NH4 peak at 7.29 ppm
(1:1:1 triplet, J = 50.9 Hz) and quantified versus TMB.
Procedure for reduction of N2 to NH4+ with P4Cr(N2)2 using protons and
electrons. 40 equivalents of solid acid were added to 30 equivalents of solid
reductant in a specialized vacuum transfer Schlenk flask (Figure S1). To this
mixture was added solvent followed by P4Cr(N2)2 (10 μL from a 10 mM
stock solution, 0.1 μmol delivery) in either THF or toluene. The vessel was
quickly sealed under 1 atm of N2 and stirred overnight at 23 °C. Following
the protocol described Ashley and co-workers,5a the mixture was quenched
with HCl etherate (500 equiv) and volatiles were removed under reduced
pressure. While frozen at -196 °C, 40% wt/wt KOH(aq) was added to the
solids. In the collection bulb attached to the reaction bulb, HCl etherate was
frozen as well. The apparatus was evacuated under a high vacuum and sealed.
The reaction bulb was warmed to room temperature for the vacuum transfer
of NH3 gas to the frozen acidified bulb. Upon warming, the acidified bulb
was thoroughly mixed, the solvent was removed under reduced pressure, and
1H NMR spectroscopic analysis as described above was used to quantify
NH4Cl. The reaction bulb was re-acidified with conc. HCl(aq) and tested
for hydrazinium using the procedure described by Ashley and co-workers5a
and the p-dimethylaminobenzaldehyde test.30
EXPERIMENTAL SECTION
All synthetic procedures were performed under an atmosphere of N2 us-
ing standard Schlenk or glovebox techniques. Reactions performed with 15N2
gas were subsequently handled in the glovebox under an atmosphere of ar-
gon. Unless described otherwise, all reagents were purchased from commer-
cial sources and were used as received. Protio solvents were dried by passage
through activated alumina columns in an Innovative Technology, Inc., Pure-
Solv solvent purification system and stored under N2 or argon until use. Vir-
gin glassware was used without surface modification. Acid washed glassware
was prepared by washing virgin glassware with 12.1 M HCl overnight at
room temperature. Silylated glassware was prepared by washing virgin glass-
ware with conc. HCl overnight at room temperature, then silylated following
the literature procedure using Me2SiHCl.46 All glassware was heated to 160
oC overnight before use.
All 1H, 13C, 15N, and 31P NMR spectra were collected in thin-walled NMR
tubes on a Varian Inova or NMR S 500 MHz spectrometer at 25 °C unless
otherwise indicated. 2H NMR spectra were recorded on a Varian NMR S 300
MHz spectrometer at 25 °C in non-deuterated THF. 1H and 13C NMR
chemical shifts are referenced to residual protio solvent resonances in the
deuterated solvent. 31P NMR chemical shifts are proton decoupled unless
otherwise noted and referenced to 85% H3PO4 (δ = 0) as an external refer-
ence. 15N NMR chemical shifts were referenced to CH315NO2 (δ = 0) as an
external reference.
Infrared spectra were recorded on a Thermo Scientific Nicolet iS10 FT-
IR spectrometer as a KBr pellet under a purge stream of nitrogen gas. In situ
IR experiments were performed in a nitrogen filled glovebox and recorded
on a Mettler-Toledo ReactIR 15 spectrometer equipped with a liquid nitro-
gen cooled MCT detector, connected to a 1.5 m AgX Fiber DS series (9.5
mm × 203 mm) probe with a silicon sensor. 15N2 (98%) gas and THF-d8 were
purchased from Cambridge Isotope Laboratories. THF- d8 was dried over
NaK and vacuum transferred before use. Rieke Magnesium powder was pur-
chased from Rieke Metals LLC and used as received. All chromium reagents
were purchased and used as received. A procedure for the synthesis of
P4Cr(N2)2 is described in the SI. Chromium complexes examined for silyla-
tion catalysis were prepared from literature procedures as described in the
SI. TEMPOH was purchased from Cambridge Isotope Labs, which was ad-
ditionally dissolved in pentane, filtered, and vacuum dried to ensure com-
plete removal of water. TEMPOD was synthesized using a modified prepa-
ration for TEMPOH, with acetone-d6 and D2O replacing the non-deutero
reagents.47
Procedure for reduction of N2 to NH4+ with P4Cr(N2)2 using TEMPOH.
100 equivalents of solid TEMPOH were added a specialized vacuum transfer
Schlenk flask (Figure S1). To this mixture was added THF followed by
P4Cr(N2)2 in THF (see above). The vessel was quickly sealed under 1 atm
of N2 and stirred overnight at 23 °C. In a collection bulb attached to the re-
action bulb, HCl etherate was frozen. The reaction bulb was also frozen. The
apparatus was evacuated under a high vacuum and sealed. The reaction bulb
was warmed to room temperature for the volatiles to vacuum transfer to the
frozen acidified bulb. Upon warming, the acidified bulb was thoroughly
mixed, solvent removed under reduced pressure, and 1H NMR spectral anal-
ysis as above was used to quantify NH4Cl. The reaction bulb was acidified
with conc. HCl(aq) and tested for hydrazinium using the procedure described
by Ashley and co-workers5a and the p-dimethylaminobenzaldehyde test.30
Procedure for the reduction of N2 to ND3 using TEMPOD. 100 equivalents
of solid TEMPOD was added to J. Young NMR tube. A solution of
P4Cr(N2)2 in protio THF was then added, and the tube was quickly sealed
and thoroughly mixed. The resulting orange-brown solution was analyzed
by 2H NMR spectroscopy. A diagnostic broad singlet resonance at 0.65 ppm
was identified as the free ND3 product.
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, computational details, quantification
methods, NMR spectra, and selected experiments are included in the
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J. Am. Chem. Soc. 2011, 133, 3498-3506.
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supporting information. The Supporting Information is available free of
charge on the ACS Publications website.
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AUTHOR INFORMATION
Corresponding Author
Piro, N. A.; Kassel, W. S.; Bullock, R. M.; Mock, M. T. J. Am. Chem. Soc. 2017
,
* michael.mock@pnnl.gov
139, 9291-9301. (d) Imayoshi, R.; Nakajima, K.; Takaya, J.; Iwasawa, N.;
Nishibayashi, Y. Eur. J. Inorg. Chem. 2017, 3769-3778.
(13) (a) Siedschlag, R. B.; Bernales, V.; Vogiatzis, K. D.; Planas, N.; Clouston, L.
J.; Bill, E.; Gagliardi, L.; Lu, C. C. J. Am. Chem. Soc. 2015, 137, 4638-4641. (b)
Imayoshi, R.; Tanaka, H.; Matsuo, Y.; Yuki, M.; Nakajima, K.; Yoshizawa, K.;
Nishibayashi, Y. Chem. Eur. J. 2015, 21, 8905-8909.
(14) Imayoshi, R.; Nakajima, K.; Nishibayashi, Y. Chem. Lett. 2017, 46, 466-468.
(15) (a) Liao, Q.; Saffon-Merceron, N.; Mézailles, N. Angew. Chem. Int. Ed.
2014, 53, 14206-14210. (b) Komori, K.; Oshita, H.; Mizobe, Y.; Hidai, M. J. Am.
Chem. Soc. 1989, 111, 1939-1940. (c) Kuriyama, S.; Arashiba, K.; Nakajima, K.;
Tanaka, H.; Yoshizawa, K.; Nishibayashi, Y. Eur. J. Inorg. Chem. 2016, 4856-
4861.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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This research was supported as part of the Center for Molecular Electro-
catalysis, an Energy Frontier Research Center funded by the U.S. De-
partment of Energy (DOE), Office of Science, Office of Basic Energy
Sciences. PNNL is operated by Battelle for the U.S. DOE. The authors
thank Dr. Geoffrey Chambers for the single crystal x-ray diffraction iden-
tification of P4CrII(Cl)2 and Dr. Eric Wiedner for helpful discussions.
(16) Vidyaratne, I.; Scott, J.; Gambarotta, S.; Budzelaar, P. H. M. Inorg. Chem.
2007, 46, 7040-7049.
(17) Smythe, N. C.; Schrock, R. R.; Müller, P.; Weare, W. W. Inorg. Chem. 2006
45, 7111-7118.
,
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