Multiple N-H and C-H Hydrogen Atom Abstractions through Coordination-Induced Bond Weakening at Fe-Amine Complexes

Article abstract of DOI:10.1021/acs.inorgchem.1c00923

We report the use of the reported Fe-phthalocyanine complex, PcFe (1; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine), to generate PcFe-amine complexes 1-(NH3)2, 1-(MeNH2)2, and 1-(Me2NH)2. Treatment of 1 or 1-(NH3)2 to an excess of the stable aryloxide radical, 2,4,6-tritert-butylphenoxyl radical (tBuArO?), under NH3 resulted in catalytic H atom abstraction (HAA) and C-N coupling to generate the product 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one (2) and tBuArOH. Exposing 1-(NH3)2 to an excess of the trityl (CPh3) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical (TrArO?), under NH3 did not lead to catalytic ammonia oxidation as previously reported in a related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N-H and β C-H in the alkylamine congeners, 1-(MeNH2)2 and 1-(Me2NH)2, led to multiple HAA events yielding the unsaturated cyanide complex, 1-(MeNH2)(CN), and imine complex, 1-(MeN═CH2)2, respectively. Subsequent C-N bond formation was also observed in the latter upon addition of a coordinating ligand. Detailed computational studies support an alternating mechanism involving sequential N-H and C-H HAA to generate these unsaturated products.

Full text of DOI:10.1021/acs.inorgchem.1c00923

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Article
Multiple N−H and C−H Hydrogen Atom Abstractions Through
Coordination-Induced Bond Weakening at Fe-Amine Complexes
Zongheng Wang, Samantha I. Johnson, Guang Wu, and Gabriel Ménard*
Cite This: Inorg. Chem. 2021, 60, 8242−8251
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ABSTRACT: We report the use of the reported Fe-phthalocyanine
complex, PcFe (1; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalo-
cyanine), to generate PcFe−amine complexes 1-(NH3)2, 1-
(MeNH2)2, and 1-(Me2NH)2. Treatment of 1 or 1-(NH3)2 to an
excess of the stable aryloxide radical, 2,4,6-tritert-butylphenoxyl
radical (tBuArO•), under NH3 resulted in catalytic H atom
abstraction (HAA) and C−N coupling to generate the product 4-
amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one (2) and tBuArOH. Exposing 1-(NH3)2 to an excess of the trityl (CPh3) variant, 2,6-
di-tert-butyl-4-tritylphenoxyl radical (TrArO•), under NH3 did not lead to catalytic ammonia oxidation as previously reported in a
related Ru-porphyrin complex. However, pronounced coordination-induced bond weakening of both α N−H and β C−H in the
alkylamine congeners, 1-(MeNH2)2 and 1-(Me2NH)2, led to multiple HAA events yielding the unsaturated cyanide complex, 1-
(MeNH2)(CN), and imine complex, 1-(MeNCH2)2, respectively. Subsequent C−N bond formation was also observed in the
latter upon addition of a coordinating ligand. Detailed computational studies support an alternating mechanism involving sequential
N−H and C−H HAA to generate these unsaturated products.
Mock, Bullock, and co-workers have utilized N−H bond
weakening to target H atom abstraction (HAA) at metal-
coordinated NH3.6,23,24 In a recent example, catalytic C−N
coupling versus AO was controlled by tuning the HAA agent
INTRODUCTION
■
Chemical energy vectors (H2, MeOH, and NH3) can play an
important role in stabilizing renewable energy (RE) grid
fluctuations and, when generated at the source (e.g., RE-
powered H2O electrolysis), can further be used for long-term
energy storage.1−3 Ammonia is an attractive RE storage
candidate;1,4 however, its end use requires a fundamental
understanding of the complex multielectron ammonia
oxidation (AO) reaction. Homogeneous AO catalysts can
provide important insight into these molecular-level trans-
formations, and several groups have recently reported
important contributions to this field. For instance, the groups
of Bullock,5 Mock,6 Nishibayashi,7 and Hamann and Smith8
have all reported Ru-based electro/chemical catalysts for AO.
For first-row metal complexes, we have reported a
pseudocatalytic “synthetic” cycle using a Mn complex,9
whereas Peters and co-workers have recently reported an Fe-
based electrocatalyst for AO.10 While the proposed AO
mechanisms vary in all of these reports, the underlying
challenge in converting NH3 to N2 lies in activating the strong
N−H bonds sufficiently to prompt subsequent N−N bond
formation (N−H bond dissociation free-energy (BDFEN−H) =
99.4 kcal/mol).11,12
from 2,4,6-tri-tert-butylphenoxyl radical (tBuArO•; BDFEO−H
=
76.7 kcal/mol)11 to the para-trityl variant, TrArO•, respectively,
using the porphyrin-based catalyst, (TMP)Ru(NH3)2, in the
presence of excess NH3 (TMP = tetramesitylporphyrin; Figure
1b).5
Our interest lies in developing new first-row catalysts for AO
and understanding the mechanistic details underlying N−H
activation and N−N bond forming pathways.9 In parallel with
Bullock and co-workers,5 we investigated the use of our
recently reported Fe−phthalocyanine (PcFe) complex (1),25
analogous to (TMP)Ru(NH3)2, and its potential use as an AO
catalyst (Figure 1c). While we discovered that 1 was a
competent C−N coupling catalyst, we found it was mostly
inactive for catalytic AO through HAA chemistry (Figure 1c,
left). However, in an effort to bolster our mechanistic
understanding of these transformations, we synthesized related
alkylamine complexes and exposed these to similar HAA
conditions. In these cases, we uncovered that pronounced
Coordination-induced bond weakening (CIBW) of NH3 or
other ligands bound to redox-active metal centers can result in
substantial decreases in E−H (E = C, N, and O) BDFEs with
effects seen at positions α13−19 or downstream (β, γ, etc.)20−22
of the metal center (Figure 1a). Bezdek and Chirik have
recently demonstrated substantial N−H CIBW at a Mo−NH3
complex leading to spontaneous H2 formation,18 whereas
Received: March 25, 2021
Published: May 20, 2021
https://doi.org/10.1021/acs.inorgchem.1c00923
© 2021 American Chemical Society
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CIBW led to multiple α N−H and β C−H HAA events,
generating unsaturated C−N bonds (Figure 1c, right).26−30 In
one case, subsequent C−N bond formation could be induced.
The details of these transformations are presented herein.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of PcFe−Amine
Precursors. The starting complex, 1, was synthesized
according to our previously reported procedure.25 Treatment
of 1 with excess NH3, MeNH2, or Me2NH afforded green
products 1-(NH3)2, 1-(MeNH2)2, or 1-(Me2NH)2, respec-
tively, in high yields (Figure 1c). Each new complex was
unambiguously identified by single crystal X-ray diffraction
(XRD) studies, with the solid-state structures shown in Figure
2a. The observed Fe−Namine bond distances of 2.017(4),
2.037(6), and 2.087(5) Å for 1-(NH3)2, 1-(MeNH2)2, and 1-
(Me2NH)2, respectively, are consistent with increasing amine
bulk and similar to previous reports.31−33 Pc ring distortions
are most pronounced for 1-(MeNH2)2 and may be due to
packing effects observed in the extended structure.25 The zero-
field 57Fe Mossbauer spectra of each complex were collected at
̈
90 K and revealed very similar quadrupole doublets with
isomer shift and quadrupole splitting (δ, |ΔEQ| (mm/s))
values of: 1-(NH3)2 (0.29, 1.14), 1-(MeNH2)2 (0.24, 0.87),
and 1-(Me2NH)2 (0.25, 0.99) (Figure 2b). While the δ values
are similar to previously reported axially substituted PcFe
species,34 their |ΔEQ| values are significantly reduced, similar
to what was previously observed in 1 and which is again
tentatively attributed to the electron donating EtO groups
(Figure 1c).25 We note also that the spectral parameters of the
amine-substituted complexes are all smaller than those in 1
(0.33, 1.37), which is attributed to the addition of axially
bound σ-donating ligands, resulting in a spin-state change from
S = 1 (1) to S = 0 (1-(NH3)2, 1-(MeNH2)2, and 1-
Figure 1. (a) Various reported examples of α, β, or γ E−H CIBW at
metal complexes (E = C, N, and O). Reported E−H (red) BDFEs are
shown. (b) Previously reported catalytic HAA for C−N coupling or
AO at (TMP)Ru(NH3)2. (c) This work featuring (left) catalytic C−
N coupling of NH3 with tBuArO• using 1-(NH3)2 through CIBW, and;
(right) multiple α N−H and β C−H HAA events enabled by CIBW
at alkylamine complexes, 1-(MeNH2)2 and 1-(Me2NH)2, using
tBuArO•.
(Me2NH)2).25,34,35 The H NMR spectra of each compound
1
revealed diamagnetic species with heavily shielded amine
resonances due to their axial coordination above the strong,
diatropic Pc ring current.36 As expected, this effect was more
pronounced for the closer N−H resonances than the further
C−H resonances (Figure 2c). Finally, the UV−vis absorption
Figure 2. (a) Solid-state molecular structures of 1-(NH3)2, 1-(MeNH2)2, and 1-(Me2NH)2. Ethoxy groups, cocrystallized solvent molecules, and
all H atoms, except N−H, have been omitted for clarity. Fe, orange; N, blue; C, black; H, gray. (b) Zero-field 57Fe Mossbauer spectra (90 K) of the
̈
1
corresponding complexes with isomer shift (δ) and quadrupole splitting (|ΔEQ|) values indicated. (c) Partial H NMR spectra (in C6D6) of the
corresponding complexes highlighting the strongly shielded amine N−H and C−H resonances.
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spectra displayed nearly identical prominent Q-band (HOMO-
(π) → LUMO(π*)) absorption peaks at 714 nm (ε = 47 432
M−1 cm−1), 716 nm (ε = 36 102 M−1 cm−1), and 713 nm (ε =
37 992 M−1 cm−1) for 1-(NH3)2, 1-(MeNH2)2, and 1-
(Me2NH)2, respectively (Figures S26−S28), all of which are
significantly blue-shifted relative to 1 (765 nm (ε = 41 868
M−1 cm−1)).25 In contrast to 1, the substituted complexes all
contained distinct absorptions in the 400−500 nm region
assigned to Fe → Pc metal-to-ligand charge transfer (MLCT)
bands previously observed in axially substituted low-spin PcFeII
complexes:37,38 1-(NH3)2 (433 nm), 1-(MeNH2)2 (438 nm),
and 1-(Me2NH)2 (436 nm) (Figures S26−S28).
N−H HAA at PcFe−NH3. In order to test possible AO
catalysis with 1 via HAA chemistry, we investigated the
reactivity of this compound with tBuArO• or TrArO• under an
atmosphere of NH3. Similar to the (TMP)Ru(NH3)2 system
reported by Bullock and co-workers,5 we observed that 1 was a
competent C−N coupling catalyst (Figure 1b,c (left side in
each)). For instance, treatment of 1 with 300 equiv of tBuArO•
under an atmosphere of NH3 in C6D6 revealed the formation
of tBuArOH and the reported species, 4-amino-2,4,6-tritert-
butylcyclohexa-2,5-dien-1-one (2; Figure 1c),5 as observed by
1H NMR spectroscopy after 24 h (Figure S10). The ratio of
tBuArOH to 2 was slightly elevated (ca. 1.5:1) and may be due
to adventitious water from the NH3 gas or decomposition of
tBuArO• (vide infra). A turnover number (TON) of 303 was
calculated using 0.05% of 1 after 27 h, similar to the reported
reactivity of (TMP)Ru(NH3)2.5 The headspace gas of a
reaction carried out under an 15NH3 atmosphere was analyzed
by GC-MS and revealed some isobutylene, attributed to the
decomposition of tBuArO•,39 but not 15N2 (Figure S33). A
control experiment involving the reaction of tBuArO•
(BDFEArO−H = 76.7 kcal/mol) with NH3 (BDFEN−H = 99.4
kcal/mol) in the absence of 1 revealed no reactivity, indicating
that significant N−H CIBW occurred upon coordination to
1.11 HAA from the isolated complex, 1-(NH3)2, also occurred
using tBuArO•. Treatment of 1-(NH3)2 with 6.1 equiv of
tBuArO• resulted in the formation of 2 equiv of 2, as well as an
excess of tBuArOH (3 equiv) as observed by 1H NMR
Figure 3. (top) Synthesis of 1-OArtBu. (bottom) Solid-state molecular
structure of 1-OArtBu. Ethoxy groups, cocrystallized solvent
molecules, and all H atoms have been omitted for clarity. Selected
bond lengths and angles: Fe−O1 1.836(6) Å, O1−C1 1.370(11) Å,
Fe1−O1−C1 177.3(7)°.
Under an atmosphere of 15NH3, 93 equiv of TrArO• was added
to 1 and stirred in benzene for 16 h. Analysis of the headspace
gas by GC-MS revealed only trace 15N2 (Figure S34). A
parallel concentrated reaction in a J. Young NMR tube
monitored after 48 h revealed no detectable 15N2 by 15N NMR
spectroscopy (Figure S14). These data suggest that 1 is at best
a very poor catalyst for AO, in contrast to the related
(TMP)Ru(NH3)2 system (Figure 1b,c).5 While a substantial
amount (∼70−90%) of TrArO• was converted to TrArOH, as
observed by 1H NMR spectroscopy and perhaps attributable to
HAA at NH3, a significant quantity of isobutylene was also
observed suggesting a competing TrArO• decomposition
pathway (Figure S13). Stoichiometric reactions of TrArO•
with 1-(NH3)2 were also inconclusive. In order to shed more
light on these transformations, we decided to investigate this
chemistry further using model EtOPcFe−alkylamine complexes.
N−H and C−H HAA at PcFe−Alkylamines. In an
attempt to simplify our system and reduce the total number of
N−H bonds in 1-(NH3)2, we synthesized the dimethylamine
variant, 1-(Me2NH)2 (vide supra), containing only two N−H
bonds. Exposure of this compound to 1.25 equiv of tBuArO• led
to the clean formation of two new products (ca. 60%) as
2
spectroscopy and confirmed by H NMR spectroscopy using
1-(ND3)2 (Figures S11 and S12). We further observed the
formation of a new, dark green, paramagnetic product. The
identity of this product was confirmed by independent
synthesis by reacting 1 with tBuArO• in dichloromethane
(DCM). The solid-state structure obtained by XRD studies
revealed the formation of the adduct, 1-OArtBu (Figure 3).
The complex featured a domed EtOPc ligand with a nearly
linear Fe1−O1−C1 bond (177.3(7)°)40 and an elongated
Fe1−O1 bond (1.836(6) Å).40−44 The C1−O1 bond
(1.370(11) Å) is average relative to reported Fe−OArtBu
species40,43−45 and longer than in free tBuArO• (1.246(2)
1
observed by H NMR spectroscopy, along with residual 1-
(Me2NH)2 (ca. 40%). The major new product contained a set
of resonances in the 1H NMR spectrum consistent with amine
(Me2NH) and imine (MeNCH2) coordination to Fe, with
each integrating in a 1:1 ratio (Figure S15). We assign this as
the putative intermediate, 1-(MeNCH2)(Me2NH), although
efforts to isolate this single product have failed (Scheme 1).
The minor (ca. 15%) of the two new products could be cleanly
formed by addition of 4 equiv of tBuArO• to 1-(Me2NH)2. A
singlet at −2.19 ppm (6H) correlating to a set of doublets (2H
each) at 2.18 and 0.71 ppm was observed and together
suggested N-methylmethanimine (MeNCH2) coordination
to Fe (Figure S8). The solid-state structure obtained by single-
crystal XRD studies indeed unambiguously confirmed the
Å).46 The zero-field 57Fe Mossbauer spectrum of 1-OArtBu (90
̈
K) revealed a δ = 0.26 mm/s and a narrow |ΔEQ| = 0.28 mm/s
(Figure S23), consistent with formal oxidation to FeIII.47 This
was further supported by the appearance of broadened,
1
paramagnetically shifted resonances in the H NMR spectrum
(Figure S7). Treatment of 1-OArtBu to an atmosphere of NH3
resulted in the formation of 1-(NH3)2, tBuArOH, and 2 in
similar ratios to above.
In order to circumvent the catalytic formation of 2, we
investigated AO using the bulkier congener, TrArO• (Figure
1b,c), bearing the trityl (CPh3) group in the para position.5
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Scheme 1
Scheme 2
specific mechanistic pathway for the specific conversion of 1-
(Me2NH)2 to 1-(MeNCH2)2. To do so, we first carried out
the control reaction of tBuArO• with Me2NH in the absence of
1. In contrast to the control reaction of tBuArO• with NH3, an
HAA reaction was observed upon mixing tBuArO• with
Me2NH. Analysis of the reaction mixture by 1H NMR
spectroscopy and GC-MS revealed the formation of tBuArOH
in addition to a second product assigned as the dimethylamine
analog of 2 (Figure 1c), 2-NMe2, in a 1.5:1 ratio (Figures S18
and S35). The observed reactivity was surprising given the
reported experimentally determined N−H and C−H bond
dissociation enthalpy values of related Et2 NH
(BDEN−H/C−H(avg) ≈ 87 kcal/mol) and calculated values for
Me2NH (BDEC−H = 91.7 kcal/mol; BDEN−H = 95.4 kcal/mol)
were significantly higher than the reported experimental value
for tBuArO−H (BDEO−H = 81.6 kcal/mol).50,51 Moreover,
previous kinetic studies revealed faster C−H than N−H HAA
rates with the alkyloxide radical, tBuO•, in contrast to our
observed results with tBuArO•.52 In addition, no further HAA
events were observed when exposing 2-NMe2 to excess
tBuArO•. These results highlight the important role of CIBW
in enabling multiple HAA events at Me2NH. We propose that
initial N−H HAA at the Fe-coordinated complex, 1-
(Me2NH)2, results in pronounced C−H bond weakening
due to available hyperconjugative interactions between the β
C−H σ bond and the newly formed semioccupied Fe−N π*
orbital (Scheme 3). This may enable the subsequent,
structure as the bis-imine product, 1-(MeNCH2)2 (Figures
1c and 4). The structure revealed distinctively shortened N1−
Figure 4. Solid-state molecular structure of 1-(MeNCH2)2. Ethoxy
groups, cocrystallized solvent molecules, and all H atoms, except
imine N−H and C−H, have been omitted for clarity. Selected bond
lengths: Fe1−N1 2.006(5) Å, N1−C1 1.324(9) Å, N1−C2 1.395(8)
Å.
C1 bonds (1.324(9) Å), relative to the N1−C2 bonds
(1.395(8) Å). However, we note that these bond differences
are not as pronounced as the only other crystallographically
characterized σ-coordinated N-methylmethanimine complex
(bound to Co) reported by Power and co-workers which
contained C−N and CN bond lengths of 1.451(3) and
1.262(3) Å, respectively.48 The smaller difference in 1-
(MeNCH2)2 may be due to crystallographic averaging of
the single and double bonds in the symmetry-generated XRD
structure. Given the pure imine σ coordination to Fe, and
similar Fe−Nimine (2.006(5) Å) to Fe−Namine (2.087(5) Å)
bond lengths in 1-(MeNCH2)2 and 1-(Me2NH)2, respec-
Scheme 3
̈
tively, the Mossbauer spectral parameters are also nearly
identical (δ,|ΔEQ| (mm/s)): 1-(MeNCH2)2 (0.25, 0.92)
and 1-(Me2NH)2 (0.25, 0.99) (Figures S24 and 2b). UV−vis
absorption features are also very similar for both (Figures S28
and S30).
thermodynamically favorable β C−H HAA reaction to
proceed, generating the observed intermediate, 1-(MeN
CH2)(Me2NH), and subsequently 1-(MeNCH2)2. This
hypothesis is supported by computational studies we carried
out (vide infra).
We observed that the MeNCH2 ligands were labile
toward high heat as observed by a thermogravimetric analysis
(TGA) of 1-(MeNCH2)2 (Figure S36) which revealed a
7.5% mass drop from 120 to 170 °C, in the range expected for
loss of both imine ligands (8.5%). Room-temperature
substitution was achieved by addition of 2.6 equiv of PMe3
to 1-(MeNCH2)2, which led to the formation of a new
We next explored similar HAA chemistry at the related
methylamine complex, 1-(MeNH2)2 (Figures 1c and 2a).
Exposure of this to 1, 2, or 4 equiv of tBuArO• led to the
increased production of tBuArOH with concurrent reduction in
the intensity of the resonances attributed to 1-(MeNH2)2 as
1
diamagnetic FePc complex, as observed by H and 31P NMR
spectroscopy (Figures S16 and S17). We assigned this new
species as the bis-trimethylphosphine species, 1-(PMe3)2.
Interestingly, release of MeNCH2 led to its spontaneous
cyclization via C−N bond formation to the 6-membered
1
observed by H NMR spectroscopy (Figure S19). While new
diamagnetic resonances were observed, the growth of these
was not commensurate with the quantity of tBuArO• added,
suggesting these were from an unknown byproduct and that
the major product formed was likely paramagnetic. Addition of
5 equiv of tBuArO• led to the complete disappearance of the
resonances for 1-(MeNH2)2. Following workup, a para-
1
product, 1,3,5-trimethyl-1,3,5-triazinane, as observed by H
NMR spectroscopy (Scheme 2 and Figure S16).49
While the oxidative dehydrogenation of coordinated amines
has been reported,26−30 we next wanted to elucidate the
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magnetic species was isolated and unambiguously identified by
single-crystal XRD studies as the cyanide product, 1-
(MeNH2)(CN) (Figure 5). The Fe1−C1 (1.962(6) Å) and
Figure 5. Solid-state molecular structure of 1-(MeNH2)(CN). Ethoxy
groups, cocrystallized solvent molecules, and all H atoms, except
amine N−H and C−H, have been omitted for clarity. Selected bond
lengths and angles: Fe1−C1 1.962(6) Å, C1−N1 1.155(7) Å, Fe1−
N2 2.060(5) Å, Fe1−C1−N1 173.8(5)°.
C1−N1 (1.155(7) Å) bond distances are similar to previous
reports,53,54 whereas the trans Fe1−N2 (2.060(5) Å) distance
is longer than in 1-(MeNH2)2 (2.037(6) Å), and expected due
to the trans-influence of the cyanide group. FT-IR spectros-
copy revealed a CN stretching frequency at 2102 cm−1,
similar to previously reported Fe-cyanide complexes (Figure
S32).55 The Fe Mossbauer spectrum of 1-(MeNH2)(CN)
57
̈
revealed contracted δ (0.16 mm/s) and |ΔEQ| (0.71 mm/s)
values relative to those of 1-(MeNH2)2 (0.24, 0.87,
respectively), suggesting formal oxidation to FeIII (Figures
S25 and 2b).47 This was further supported by the
disappearance of the MLCT band for the precursor, 1-
(MeNH2)2 (438 nm), observed in axially substituted low-spin
PcFeII complexes (Figure S31).37,38 We note, however, that
while these data support oxidation to a formal FeIII state,
significant Pc structural distortions observed in 1-(MeNH2)-
(CN) may also suggest some ligand-based contributions
(Figure 5).36
Figure 6. DFT-calculated free energy values (kcal/mol) in benzene
for the conversions of 1-(NH3)2 (a) and 1-(MeNH2)2 (b). BDFEs are
in red and ΔG values are in blue. All free energy values are calculated
relative to the tBuArO•/tBuArOH BDFE in benzene (76.7 kcal/mol).11
The most thermodynamically favorable pathways follow the green
arrows.
In contrast to Me2NH, the control reaction of tBuArO• with
MeNH2 revealed no reaction, similar to NH3, highlighting
again the role of CIBW. Similar to the conversion of 1-
(Me2NH)2 to 1-(MeNCH2)2, we propose that initial N−H
HAA at 1-(MeNH2)2 leads to significant C−H bond
weakening, due in part to hyperconjugative interactions
(Scheme 3), enabling multiple alternating N−H/C−H HAA
reactions to yield 1-(MeNH2)(CN). We note that addition of
excess tBuArO• to 1-(MeNH2)(CN) did not result in additional
reactivity suggesting a reduced CIBW effect at the higher
valent FeIII center. We next proceeded to evaluate the possible
common mechanisms at play leading to these multiple HAA
reactions at PcFe-alkylamines (1-(Me2NH)2 and 1-
(MeNH2)2) and contrast this to the reactions at 1-(NH3)2
using computational methods.
Mechanistic Studies Using DFT. In order to shed light
on the mechanistic pathways for the catalytic conversion of 1-
(NH3)2 to 2, as well as the multistep N−H and C−H HAA
reactions for the conversion of 1-(MeNH2)2 to 1-(MeNH2)-
(CN) (Figure 1c), free energy calculations were carried out on
the likely stepwise intermediate products (Figure 6). These
calculations were completed using the BP86 functional56,57
with single point energy calculations employing a triple-ζ basis
set. Zero-point energy and thermal corrections at 298.15 K
were also applied. While all low-, mid-, and high-spin
geometries were calculated, only the lowest energy state is
presented here. More details can be found in the Experimental
Section and in Table S2 (i.e., energies of all spin states).
The calculated BDFEs for the first two sequential N−H
HAA reactions at a single N site in 1-(NH3)2 (84.5 and 93.6
kcal/mol) were similar to the values in the related (TMP)-
Ru(NH3)2 system (81.7 and 92.9 kcal/mol) (Figures 1b and
6a). The corresponding ΔG of reaction for the initial HAA was
slightly higher in 1-(NH3)2 (7.8 kcal/mol) relative to
(TMP)Ru(NH3)2 (5.0 kcal/mol), yet the second HAA events
in each case were nearly identical (16.9 and 16.2 kcal/mol,
respectively). Interestingly, the competing C−N coupling step
of the intermediate 1-(NH3)(NH2) with tBuArO• was
significantly more exergonic (−15.4 kcal/mol) than in the
related intermediate (TMP)Ru(NH3)(NH2) (−0.9 kcal/
mol).5 This less favorable value in (TMP)Ru(NH3)(NH2) is
likely the result of the increased steric protection offered by the
flanking mesityl groups. This steric clash is also likely what
drives catalytic AO at (TMP)Ru(NH3)2 with the bulkier
TrArO•, something not observed in our sterically unprotected
compound, 1-(NH3)2 (Figure 1b,c). While these data
demonstrate a clear thermodynamic pathway to catalytic C−
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prior to use. Celite and 4 Å molecular sieves were dried at 250 °C
under dynamic vacuum (<0.1 Torr) for 24 h prior to use. Elemental
analyses were recorded at the University of California, Berkeley, using
a PerkinElmer 2400 Series II combustion analyzer. Hexamethylben-
zene and dimethylamine (2 M in THF) were purchased from Fisher
Scientific, ammonia gas, and methylamine (2 M in THF) from Sigma-
Aldrich and all were used without further purification. EtOPcFe,25
tBuArO•,46 and TrArO•58 were prepared following previous reports. All
other reagents were obtained from Sigma-Aldrich, Fisher Scientific, or
VWR and used without further purification.
N bond formation using 1-(NH3)2 to generate 2, the identity
of the Fe product following possible N−H HAA with TrArO•
(vida supra) remains unclear (Figure 1c).
Calculations carried out on the amine compound, 1-
(MeNH2)2, revealed a significantly weaker N−H versus C−
H bond with respective calculated BDFE values of 81.1 and
96.2 kcal/mol. These data support initial N−H HAA using
tBuArO• (Figure 6b). Calculations carried out on intermediate
A further highlight significant C−H CIBW resulting in a
drastic drop in its BDFE (50.9 kcal/mol) relative to the
residual N−H bond (89.3 kcal/mol), perhaps due to proposed
hyperconjugative interactions (Scheme 3). Subsequent N−H
HAA at imine intermediate B is significantly more facile than
the corresponding C−H HAA, similar in pattern to the
proposed reactivity in 1-(Me2NH)2 (Scheme 3). The resulting
favorable intermediate, C, can then undergo 2 facile C−H
HAA reactions through intermediate D to yield isocyano
intermediate E. The coordination mode for D features an end-
on −NCH group. A side-on coordinated −NCH was also
explored as a potential concerted route to 1-(MeNH2)(CN)
which would avoid the thermoneutral formation of E.
However, side-on configurations were higher in energy by
over 30 kcal/mol, requiring all H atoms to be removed before
isomerization could occur. Conversion of E to final product 1-
(MeNH2)(CN) was calculated to be exergonic by −9.9 kcal/
mol (Figure 6b).
Spectroscopic Measurements. NMR spectra were obtained on
an Agilent Technologies 400 MHz DD2, Varian Unity Inova 500
MHz, Bruker Avance NEO 500 MHz, or a Varian 600 MHz
spectrometer, and referenced to residual solvent. Chemical shifts (δ)
1
are recorded in ppm and the coupling constants are in Hz. H NMR
assignments are supported by 2D NMR experiments. J. Young airtight
adaptors were used for air- and water-sensitive compounds. UV−vis
spectra were collected on a Shimadzu UV-2401PC spectrophotom-
eter. All measurements were carried out on recrystallized product. All
stock solutions and dilutions were prepared by mass. ATR FT-IR
spectra were collected on a Bruker Alpha Platinum with an ATR
Quicksnap Sampling Module. All measurements were carried out on
57
̈
recrystallized product. Fe Mossbauer spectroscopy was carried out
using a SEECo Model W304 resonant gamma-ray spectrometer
(activity = 50 mCi 10%, 57Co/Rh source manufactured by Ritverc)
equipped with a Janis Research Model SVT-400 cryostat system. The
source line width was <0.12 mm/s for the innermost lines of a 25 μm
α-Fe foil standard. Isomer shifts are referenced to α-Fe foil at room
temperature. All 57Fe Mossbauer samples were prepared using 20−30
̈
Other pathways were also studied for the conversion of 1-
(MeNH2)2 to 1-(MeNH2)(CN); however, these were
ultimately deemed unlikely due to unfavorable thermodynamic
values. Together, these data outline an alternating mechanism
involving sequential N−H and C−H HAA pathways to form
the final cyanide product, 1-(MeNH2)(CN). We propose that
an analogous pathway is likely at play with 1-(Me2NH)2 but
that subsequent HAA reactions are arrested due to a lack of
N−H bonds, thereby resulting in the final bis-imine product,
1-(MeNCH2)2.
mg of powdered material and measured at 90 K unless otherwise
stated. Data was fitted using a custom Igor Pro (Wavemetrics) macro
package developed by the Betley group at Harvard University.
Mass Spectrometry. Mass spectra were collected on a Shimadzu
GC-2010 gas chromatograph coupled to a Shimadzu GCMS-QP2010
mass spectrometer. The instrument is equipped with a 30 m × 0.25
mm Agilent DB-1 column with a dimethylpolysiloxane stationary
phase (0.25 μm). Helium was used as the carrier gas.
X-ray Crystallography. Data was collected on a Bruker KAPPA
APEX II diffractometer equipped with an APEX II CCD detector
using a TRIUMPH monochromator with a Mo Kα X-ray source (α =
0.71073 Å). The crystals were mounted on a cryoloop under
Paratone-N oil, and all data were collected at 110 K using an Oxford
nitrogen gas cryostream system. A hemisphere of data was collected
using ω scans with 0.5° frame widths. Data collection and cell
parameter determination were conducted using the SMART program.
Integration of the data frames and final cell parameter refinement
were carried out using SAINT software. Absorption correction of the
data was carried out using SADABS. Structure determination was
done using direct or Patterson methods and difference Fourier
techniques. All hydrogen atom positions were idealized and rode on
the atom of attachment. Structure solution, refinement, graphics, and
creation of publication materials were carried out using SHELXTL or
OLEX.2
Density Functional Theory Calculations. DFT calculations
were carried out on a modified version of PcFe wherein ethoxide
groups were exchanged for methoxide groups to avoid issues with
rotation in the ethyl groups. All calculations employed the BP86
functional56,57 with Grimme’s D3 dispersion correction with Becke−
Johnson damping.59,60 This functional is known to give accurate
geometries in first row metal complexes.61 Geometry optimization
calculations were carried out using the Karlsruhe double-ζ def2-SVP
basis set.62 Subsequent frequency calculations at the same level of
theory were used to obtain zero-point energy (ZPE) and entropic and
enthalpic corrections at 298.15 K. These frequency calculations were
also used to validate that each point was indeed a minimum on the
potential energy surface. The final electronic energies were obtained
from a single-point calculation at the previous geometry using the
def2-TZVP basis set.62 Solvation in benzene was approximated using
the SMD solvation method.63 The electronic energy is combined with
ZPE, entropic/enthalpic contributions, and solvation energy to give
CONCLUSIONS
■
In summary, we have described the synthesis of a series of
PcFe−amine complexes, 1-(NH3)2, 1-(MeNH2)2, and 1-
(Me2NH)2, as well as their reactivity with aryloxide radicals,
tBuArO• and TrArO•. CIBW led to facile HAA at 1-(NH3)2 and
subsequent catalytic C−N coupling in the presence of NH3 to
generate 2. Significant CIBW at 1-(MeNH2)2 and 1-
(Me2NH)2 also led to multiple α N−H and β C−H HAA
reactions when treated to excess tBuArO• generating the
unsaturated products, 1-(MeNH2)(CN) and 1-(MeN
CH2)2, respectively, similar to previous studies.26−30 Detailed
computational studies supported an alternating mechanism
involving sequential N−H and C−H HAA to generate these
unsaturated products. These fundamental transformations may
enable the development of more complex new catalytic
substrate activation reactions through CIBW functionalization.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
under an atmosphere of dry, oxygen-free N2 within an MBraun
glovebox (MBRAUN UNIlab Pro SP Eco equipped with a −35 °C
freezer), or by standard Schlenk techniques. Pentane, hexanes,
benzene, Et2O, DCM, and THF (inhibitor-free) were dried and
degassed on an MBraun Solvent Purification System and stored over
activated 4 Å molecular sieves. All other solvents were degassed by
freeze−pump−thaw and stored on activated 4 Å molecular sieves
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the free energies of all complexes reported here. Bond dissociation
energies were referenced to 2,4,6-tritert-butyl phenoxyl radical using
the experimental BDFE in benzene.11 While all low-, mid-, and high-
spin geometries were calculated, only the lowest energy state is
presented here. The preferred spin state of each complex can be found
in the Supporting Information. All calculations were completed using
Orca 4.1.2.64
Synthesis of 1-(NH3)2. In the glovebox, 0.300 g (0.326 mmol, 1
equiv) of 1 was mixed in ca. 6 mL of benzene or THF in a 100 mL
Schlenk flask equipped with a magnetic stir bar. The mixture was
brought to the Schlenk line and degassed using three freeze−pump−
thaw cycles, followed by addition of ca. 1.5 atm of NH3 (excess). The
solution was allowed to stir for 1 h under NH3. All volatiles were
removed under reduced pressure, resulting in a green crude powder of
1-(NH3)2. The crude product was purified by dissolving in minimal
benzene or THF and layering with pentane. Yield: 0.24 g (78%).
Single crystals suitable for XRD studies were obtained by vapor
diffusion of Et2O over a saturated THF solution of 1-(NH3)2 at −35
°C. 1H NMR (500 MHz, C6D6): δ 7.42 (s, 8H, C6H2), 5.14 (q, J = 7.5
Hz, 16H, OCH2CH3), 1.82 (t, J = 7.4 Hz, 24H, OCH2CH3), −9.17
(s, 6H, NH3). 13C{1H} NMR (500 MHz, C6D6): δ 150.9, 147.6,
(500 MHz, C6D6): δ 17.10 (b, 8H), 6.07 (b, 16H), 2.51 (b, 24H).
̈
Mossbauer: δ = 0.26 mm/s, |ΔEQ| = 0.28 mm/s. UV−vis: λmax = 775
nm (ε = 96 154 M−1 cm−1).
Synthesis of 1-(MeNCH2)2. In the glovebox, 0.050 g (0.049
mmol, 1 equiv) of 1-(Me2NH)2 was dissolved in ca. 1.5 mL of
benzene in a vial equipped with a magnetic stir bar. To this, 0.053 g of
tBuArO• (0.203 mmol, 4.1 equiv) in ca. 1.5 mL of benzene was added
dropwise and the resulting green solution was stirred for 4 h. All
volatiles were removed under reduced pressure, resulting in a dark
green powder. The crude solid was transferred to a glass frit with
Celite and washed with pentane (3 × 3 mL), then extracted with ca. 5
mL of DCM. The DCM fraction was then brought to dryness under
reduced pressure, affording a dark green powder of 1-(MeNCH2)2.
The crude product was further purified by dissolving in minimal
DCM and layering with pentane. Yield: 0.0368 g (73.9%). Single
crystals suitable for XRD studies were obtained by vapor diffusion of
Et2O over a saturated DCM solution at −35 °C. 1H NMR (500 MHz,
C6D6): 7.41 (s, 8H, C6H2), 5.07 (q, J = 7.0 Hz, 16H, OCH2CH3),
2.19 (d, J = 13.1 Hz, 2H, CH3NCH2), 1.77 (t, J = 7.0 Hz, 24H,
OCH2CH3), 0.72 (d, J = 13.0 Hz, 2H, CH3NCH2), −2.19 (s, 6H,
CH3NCH2). 13C{1H} NMR (500 MHz, C6D6): δ 161.1(CH3NCH2),
̈
133.9, 118.0, 68.2, 15.9. Mossbauer: δ = 0.29 mm/s, |ΔEQ| = 1.14
̈
151.1, 147.2, 133.5, 117.9, 68.2, 45.4 (CH3NCH2), 15.8. Mossbauer:
mm/s. UV−vis: λmax = 714 nm (ε = 47 432 M−1 cm−1). Anal. Calcd
for C48H54FeN10O8: C, 60.38; H, 5.70; N, 14.67. Found: C, 60.11; H,
5.73; N, 14.46.
δ = 0.25 mm/s, |ΔEQ| = 0.92 mm/s. UV−vis: λmax = 706 nm (ε =
100,976 M−1 cm−1). Anal. Calcd for C52H58FeN10O8: C, 62.03; H,
5.81; N, 13.91. Found: C, 61.79; H, 5.57; N, 13.67.
Synthesis of 1-(MeNH2)2. In the glovebox, 0.500 g (0.543 mmol,
1 equiv) of 1 was mixed in ca. 4 mL of THF in a vial equipped with a
magnetic stir bar. To this, 1.0 mL of 2 M MeNH2 in THF solution
(2.00 mmol, 3.7 equiv) was added dropwise, and the resulting green
solution was stirred for 1 h. All volatiles were removed under reduced
pressure resulting in a dark green powder of 1-(MeNH2)2. Yield: 0.51
g (96%). Single crystals suitable for XRD studies were obtained by
vapor diffusion of Et2O over a saturated DCM solution of 1-
Synthesis of 1-(MeNH2)(CN). In the glovebox, 0.046 g (0.047
mmol, 1 equiv) of 1-(MeNH2)2 was dissolved in ca. 1.5 mL of
benzene in a vial equipped with a magnetic stir bar. To this, 0.0633 g
of tBuArO• (0.243 mmol, 5.1 equiv) in ca. 2 mL of benzene was added
dropwise. The resulting green solution was stirred for 3 h, and all
volatiles were removed under reduced pressure, resulting in a dark
blue green powder. The crude solid was transferred to a glass frit with
Celite and washed with pentane (3 × 3 mL), then extracted with ca. 5
mL of DCM. The DCM fraction was brought to dryness under
reduced pressure, affording a dark green powder of 1-(MeNH2)(CN).
The crude product was further purified by dissolving in minimal
DCM and layering with pentane. Yield: 39.6 mg (86.7%). Single
crystals suitable for XRD studies were obtained by vapor diffusion of
pentane onto a saturated THF solution at −35 °C. NMR spectra are
1
(MeNH2)2 at −35 °C. H NMR (500 MHz, C6D6): δ 7.41 (s, 8H,
C6H2), 5.12 (q, J = 7.0 Hz, 16H, OCH2CH3), 1.81 (t, J = 7.0 Hz,
24H, OCH2CH3), −3.54 (t, J = 6.6 Hz, 6H, CH3NH2), −8.58 (q, J =
7.0 Hz, 4H, CH3NH2). 13C{1H} NMR (500 MHz, C6D6): δ 151.01,
̈
147.5, 133.6, 117.7, 68.0, 25.6 (CH3NH2), 15.7. Mossbauer: δ = 0.24
mm/s, |ΔEQ| = 0.87 mm/s. UV−vis: λmax = 716 nm (ε = 36 102 M−1
cm−1). Anal. Calcd for C50H58FeN10O8: C, 61.10; H, 5.95; N, 14.25.
Found: C, 61.18; H, 5.71; N, 14.04.
̈
silent due to paramagnetism. Mossbauer: δ = 0.25 mm/s, |ΔEQ| =
0.92 mm/s. UV−vis: λmax = 763 nm (ε = 48 645 M−1 cm−1). Anal.
Calcd for C50H53FeN10O8: C, 61.41; H, 5.46; N, 14.32. Found: C,
60.68; H, 5.13; N, 14.05.
Synthesis of 1-(Me2NH)2. In the glovebox, 0.300 g (0.326 mmol,
1 equiv) of 1 was mixed in ca. 3 mL of THF in a vial equipped with a
magnetic stir bar. To this, 0.6 mL of 2 M Me2NH in THF solution
(1.20 mmol 3.7 equiv) was added dropwise, and the resulting green
solution was stirred for 1 h. All volatiles were removed under reduced
pressure, resulting in a dark green powder of 1-(Me2NH)2. Yield: 0.32
g (97%). Single crystals suitable for XRD studies were obtained by
vapor diffusion of Et2O over a saturated DCM solution of 1-
Catalytic C−N Coupling of tBuArO• with NH3. In the glovebox,
0.001 g (0.001 mmol, 1 equiv) of 1 was dissolved in ca. 0.2 mL of
C6D6 and transferred to a J. Young NMR tube. To this, a C6D6 (0.4
mL) solution of tBuArO• (0.085 g, 0.300 mmol, 300 equiv) was added.
The tube was sealed and connected to the Schlenk line. The solution
was degassed using three freeze−pump−thaw cycles, followed by
addition of ca. 1.5 atm of NH3. All volatiles were removed under
reduced pressure after 24 h. The crude solid was dissolved in ca. 3 mL
of pentane and insoluble material was filtered over Celite. The
pentane fraction was then brought to dryness under reduced pressure,
affording a mixture of tBuArOH and 2. The mixture was analyzed by
1H NMR spectroscopy (see the Supporting Information).
1
(Me2NH)2 at −35 °C. H NMR (400 MHz, C6D6): δ 7.40 (s, 8H,
C6H2), 5.09 (q, J = 7.0 Hz, 16H, OCH2CH3), 1.79 (t, J = 7.0 Hz,
24H, OCH2CH3), −3.47 (d, J = 6.2 Hz, 12H, (CH3)2NH), −8.28 (m,
2H, (CH3)2NH). 13C{1H} NMR (500 MHz, C6D6): δ 151.0, 148.0,
̈
133.9, 117.6, 68.1, 35.2 ((CH3)2NH), 15.8. Mossbauer: δ = 0.25 mm/
s, |ΔEQ| = 0.99 mm/s. UV−vis: λmax = 713 nm (ε = 37,992 M−1
cm−1). Anal. Calcd for C52H62FeN10O8: C, 61.78; H, 6.18; N, 13.85.
Found: C, 61.59; H, 5.91; N, 13.67.
Stoichiometric C−N Coupling of tBuArO• with 1-(NH3)2. In
the glovebox, 1-(NH3)2 (0.005 g, 0.005 mmol, 1 equiv) was dissolved
in ca. 0.2 mL of C6D6 and transferred to an NMR tube. To this, a
C6D6 (0.3 mL) solution of tBuArO• (0.0083 g, 0.032 mmol, 6.1 equiv)
and hexamethylbenzene (0.0017 g, 0.010 mmol, 2 equiv) was added.
The reaction was monitored by 1H NMR spectroscopy (see the
Supporting Information). The analogous study with 1-(ND3)2 in
C6H6 was also carried out and was monitored by 2H NMR
spectroscopy.
Synthesis of 1-OArtBu. In the glovebox, 0.050 g (0.0543 mmol, 1
equiv) of 1 was dissolved in ca. 1.5 mL of DCM in a vial equipped
with a magnetic stir bar. To this, tBuArO• (0.0155 g, 0.0594 mmol, 1.1
equiv) in ca. 1 mL of DCM was added dropwise and the resulting
green solution was stirred overnight. All volatiles were removed under
reduced pressure, resulting in a dark green powder. The crude mixture
was washed with pentane (3 × 3 mL) over Celite on a glass frit and
extracted with ca. 5 mL of DCM. The DCM fraction was brought to
dryness under reduced pressure, affording a dark green powder of 1-
OArtBu. The crude product was further purified by dissolving in
minimal DCM and layering with pentane. Yield: 0.0533 g (83%).
Single crystals suitable for XRD studies were obtained by vapor
diffusion of Et2O over a saturated DCM solution at −35 °C. 1H NMR
Attempted Catalytic Ammonia Oxidation with TrArO•. In the
glovebox, 0.002 g (0.002 mmol, 1 equiv) of 1 was dissolved in ca. 0.3
mL of C6D6 and transferred to a J. Young NMR tube. To this, a C6D6
(0.3 mL) solution of TrArO• (0.090 g, 0.200 mmol, 93 equiv) was
added. The tube was sealed and connected to the Schlenk line. The
solution was degassed using three freeze−pump−thaw cycles,
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followed by addition of ca. 1.5 atm of NH3 or 15NH3. The reactions
Office of Basic Energy Sciences. Pacific Northwest National
Laboratory (PNNL) is operated by Battelle for the DOE.
Calculations were carried out using the Constance Computing
Cluster as part of Research Computing at PNNL. Dr. Peter
Dunn (PNNL) is thanked for helpful discussions.
1
were monitored by H and 15N NMR spectroscopy, respectively (see
the Supporting Information).
Generation of 1,3,5-Trimethyl-1,3,5-triazinane and 1-
(PMe3)2 through C−N Bond Formation. In the glovebox, 1-
(MeNCH2)2 (0.019 g, 0.020 mmol, 1 equiv) was dissolved in ca.
0.5 mL of C6D6 and transferred to a J. Young NMR tube. To this,
0.005 mL of PMe3 (0.049 mmol, 2.6 equiv) was added and the tube
was sealed. The reaction was monitored by 1H and 31P NMR
spectroscopy (see the Supporting Information).
REFERENCES
■
(1) Service, R. F. Liquid sunshine. Science 2018, 361, 120−123.
(2) Giddey, S.; Badwal, S. P. S.; Munnings, C.; Dolan, M. Ammonia
as a Renewable Energy Transportation Media. ACS Sustainable Chem.
Eng. 2017, 5, 10231−10239.
Control Reaction of tBuArO• with Me2NH: Observation of 2-
NMe2. In the glovebox, tBuArO• (0.028 g, 0.1 mmol, 1 equiv) was
dissolved in ca. 0.4 mL of C6D6 and transferred to a J. Young NMR
tube. The tube was sealed and connected to the Schlenk line. The
solution was degassed using three freeze−pump−thaw cycles,
followed by addition of ca. 1.5 atm of Me2NH2. The reaction was
monitored by 1H NMR spectroscopy and GC-MS (see the
Supporting Information).
(3) Parkinson, G. Solar fuels could be Australia’s biggest energy
export. Renew Economy. https://reneweconomy.com.au/solar-fuels-
could-be-australias-biggest-energy-export-59042/ (accessed 12/8/
2020).
(4) Philibert, C. Producing ammonia and fertilizers: new
opportunities from renewables. Renewable Energy Division, Interna-
tional Energy Agency. https://www.ee.co.za/wp-content/uploads/
2017/06/Producing-ammonia-and-fertilizers-new-opportunities-from-
renewables.pdf (accessed 12/19/2020).
(5) Dunn, P. L.; Johnson, S. I.; Kaminsky, W.; Bullock, R. M.
Diversion of Catalytic C−N Bond Formation to Catalytic Oxidation
of NH3 through Modification of the Hydrogen Atom Abstractor. J.
Am. Chem. Soc. 2020, 142, 3361−3365.
(6) Bhattacharya, P.; Heiden, Z. M.; Chambers, G. M.; Johnson, S.
I.; Bullock, R. M.; Mock, M. T. Catalytic Ammonia Oxidation to
Dinitrogen by Hydrogen Atom Abstraction. Angew. Chem., Int. Ed.
2019, 58, 11618−11624.
(7) Nakajima, K.; Toda, H.; Sakata, K.; Nishibayashi, Y. Ruthenium-
catalysed oxidative conversion of ammonia into dinitrogen. Nat.
Chem. 2019, 11, 702−709.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00923.
■
sı
̈
NMR, Mossbauer, UV−vis, and FT-IR spectra; MS,
TGA, and DFT results (PDF)
Cartesian coordinates (XYZ)
Accession Codes
CCDC 2059222−2059227 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(8) Habibzadeh, F.; Miller, S. L.; Hamann, T. W.; Smith, M. R.
Homogeneous electrocatalytic oxidation of ammonia to N2 under
mild conditions. Proc. Natl. Acad. Sci. U. S. A. 2019, 116, 2849−2853.
(9) Keener, M.; Peterson, M.; Hernández Sánchez, R.; Oswald, V.
F.; Wu, G.; Ménard, G. Towards Catalytic Ammonia Oxidation to
Dinitrogen: A Synthetic Cycle by Using a Simple Manganese
Complex. Chem. - Eur. J. 2017, 23, 11479−11484.
AUTHOR INFORMATION
Corresponding Author
■
Gabriel Ménard − Department of Chemistry and
Biochemistry, University of California, Santa Barbara,
California 93106, United States; orcid.org/0000-0002-
2801-0863; Email: menard@chem.ucsb.edu
(10) Zott, M. D.; Garrido-Barros, P.; Peters, J. C. Electrocatalytic
Ammonia Oxidation Mediated by a Polypyridyl Iron Catalyst. ACS
Catal. 2019, 9, 10101−10108.
(11) Warren, J. J.; Tronic, T. A.; Mayer, J. M. Thermochemistry of
Proton-Coupled Electron Transfer Reagents and its Implications.
Chem. Rev. 2010, 110, 6961−7001.
Authors
Zongheng Wang − Department of Chemistry and
Biochemistry, University of California, Santa Barbara,
California 93106, United States
Samantha I. Johnson − Center for Molecular Electrocatalysis,
Pacific Northwest National Laboratory, Richland,
Washington 99352, United States; orcid.org/0000-0001-
6495-9892
Guang Wu − Department of Chemistry and Biochemistry,
University of California, Santa Barbara, California 93106,
United States
(12) Dunn, P. L.; Cook, B. J.; Johnson, S. I.; Appel, A. M.; Bullock,
R. M. Oxidation of Ammonia with Molecular Complexes. J. Am.
Chem. Soc. 2020, 142, 17845−17858.
(13) Spiegel, D. A.; Wiberg, K. B.; Schacherer, L. N.; Medeiros, M.
R.; Wood, J. L. Deoxygenation of Alcohols Employing Water as the
Hydrogen Atom Source. J. Am. Chem. Soc. 2005, 127, 12513−12515.
(14) Cuerva, J. M.; Campan
López, J. L.; Robles, R.; Cárdenas, D. J.; Bun
̃
a, A. G.; Justicia, J.; Rosales, A.; Oller-
uel, E.; Oltra, J. E. Water:
̃
The Ideal Hydrogen-Atom Source in Free-Radical Chemistry
Mediated by TiIII and Other Single-Electron-Transfer Metals?
Angew. Chem., Int. Ed. 2006, 45, 5522−5526.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00923
(15) Hulley, E. B.; Bonanno, J. B.; Wolczanski, P. T.; Cundari, T. R.;
Lobkovsky, E. B. Pnictogen-Hydride Activation by (silox)3Ta (silox =
tBu3SiO); Attempts to Circumvent the Constraints of Orbital
Symmetry in N2 Activation. Inorg. Chem. 2010, 49, 8524−8544.
Notes
(16) Paradas, M.; Campan
E.; Bunuel, E.; Justicia, J.; Cárdenas, D. J.; Cuerva, J. M.
̃
a, A. G.; Jiménez, T.; Robles, R.; Oltra, J.
The authors declare no competing financial interest.
̃
Understanding the Exceptional Hydrogen-Atom Donor Character-
istics of Water in TiIII-Mediated Free-Radical Chemistry. J. Am. Chem.
Soc. 2010, 132, 12748−12756.
(17) Fang, H.; Ling, Z.; Lang, K.; Brothers, P. J.; de Bruin, B.; Fu, X.
Germanium(III) corrole complex: reactivity and mechanistic studies
of visible-light promoted N−H bond activations. Chem. Sci. 2014, 5,
916−921.
ACKNOWLEDGMENTS
■
We thank the Hellman Family Faculty Fellowship and the
University of California, Santa Barbara, for financial support.
S.I.J. was supported as part of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded
by the U.S. Department of Energy (DOE), Office of Science,
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Article
(18) Bezdek, M. J.; Guo, S.; Chirik, P. J. Coordination-induced
weakening of ammonia, water, and hydrazine X−H bonds in a
molybdenum complex. Science 2016, 354, 730−733.
(37) Dale, B. W. Effect of axial ligands upon the electronic
absorption spectrum of phthalocyanineiron(II). Trans. Faraday Soc.
1969, 65, 331−339.
(38) Ough, E. A.; Stillman, M. J. Analysis of the absorption and
magnetic circular dichroism spectra of iron(II) phthalocyanine. Inorg.
Chem. 1994, 33, 573−583.
(19) Bezdek, M. J.; Chirik, P. J. Interconversion of Molybdenum
Imido and Amido Complexes by Proton-Coupled Electron Transfer.
Angew. Chem., Int. Ed. 2018, 57, 2224−2228.
(39) Hedoyatullah, M.; Thevenet, F. Oxydation d’Amines
(20) Estes, D. P.; Grills, D. C.; Norton, J. R. The Reaction of
Cobaloximes with Hydrogen: Products and Thermodynamics. J. Am.
Chem. Soc. 2014, 136, 17362−17365.
(21) Semproni, S. P.; Milsmann, C.; Chirik, P. J. Four-Coordinate
Cobalt Pincer Complexes: Electronic Structure Studies and Ligand
Modification by Homolytic and Heterolytic Pathways. J. Am. Chem.
Soc. 2014, 136, 9211−9224.
́
Aromatiques Primaires en Presence du Radical Tritertiobutyl-2,4,6
́
Phenoxyle. Bull. Soc. Chim. Belg. 1987, 96, 311−323.
(40) Muller, H.; Seidel, W.; Görls, H. Zur Chemie des
̈
Dimesityleisens. X. Mesityleisenkomplexe [FeMes(X)]2 mit zentraler
{Fe2(μ-Mes)2}-Einheit (Mes = C6H2-2,4,6-(CH3)3). Z. Anorg. Allg.
Chem. 1996, 622, 1968−1974.
(41) Ercolani, C.; Monacelli, F.; Dzugan, S.; Goedken, V. L.;
Pennesi, G.; Rossi, G. X-Ray crystal structure of μ-oxo-bis[(1-
methylimidazole)-phthalocyaninatoiron(III)] and comments on the
molecular structure and chemistry of oxo-bridged iron phthalocyani-
nate dimers. J. Chem. Soc., Dalton Trans. 1991, 1309−1315.
(42) Gorun, S. M.; Rathke, J. W.; Chen, M. J. Long-range solid-state
ordering and high geometric distortions induced in phthalocyanines
by small fluoroalkyl groups. Dalton Trans. 2009, 1095−1097.
(43) Walter, M. D.; White, P. S. Reactivity Studies on [Cp′FeI]2:
Monomeric Amido, Phenoxo, and Alkyl Complexes. Inorg. Chem.
2012, 51, 11860−11872.
(44) MacLeod, K. C.; McWilliams, S. F.; Mercado, B. Q.; Holland,
P. L. Stepwise N−H bond formation from N2-derived iron nitride,
imide and amide intermediates to ammonia. Chem. Sci. 2016, 7,
5736−5746.
(45) Bartlett, R. A.; Ellison, J. J.; Power, P. P.; Shoner, S. C.
Synthesis and characterization of the homoleptic aryloxides [M{O-
(2,4,6-tert-Bu3C6H2)}2]2 (M = manganese, iron), the adducts
[Mn(OCPh3)2(py)2] and [Fe(OCPh3)2(THF)2], and the mixed
complex [Fe{N(SiMe3)2}{.mu.-O(2,4,6-tert-Bu3C6H2)}]2: evidence
for primarily ionic metal-oxygen bonding. Inorg. Chem. 1991, 30,
2888−2894.
(46) Manner, V. W.; Markle, T. F.; Freudenthal, J. H.; Roth, J. P.;
Mayer, J. M. The first crystal structure of a monomeric phenoxyl
radical: 2,4,6-tri-tert-butylphenoxyl radical. Chem. Commun. 2008,
256−258.
(47) Mekhryakova, N. G.; Gulina, T. Y.; Li, V. D.; Bundina, N. I.;
Kaliya, O. L.; Luk’yanets, E. A. Reaction of Iron Phthalocyanine with
Oxygen: Critical Survey and New Data on the Structure of the
Forming PcFe(II) μ-Oxo Dimer. Russ. J. Gen. Chem. 2001, 71, 570−
590.
(22) Tarantino, K. T.; Miller, D. C.; Callon, T. A.; Knowles, R. R.
Bond-Weakening Catalysis: Conjugate Aminations Enabled by the
Soft Homolysis of Strong N−H Bonds. J. Am. Chem. Soc. 2015, 137,
6440−6443.
(23) Bhattacharya, P.; Heiden, Z. M.; Wiedner, E. S.; Raugei, S.;
Piro, N. A.; Kassel, W. S.; Bullock, R. M.; Mock, M. T. Ammonia
Oxidation by Abstraction of Three Hydrogen Atoms from a Mo−
NH3 Complex. J. Am. Chem. Soc. 2017, 139, 2916−2919.
(24) Cook, B. J.; Johnson, S. I.; Chambers, G. M.; Kaminsky, W.;
Bullock, R. M. Triple hydrogen atom abstraction from Mn−NH3
complexes results in cyclophosphazenium cations. Chem. Commun.
2019, 55, 14058−14061.
(25) Peterson, M.; Hunt, C.; Wang, Z.; Heinrich, S. E.; Wu, G.;
Ménard, G. Synthesis, characterization, and electrochemical proper-
ties of a first-row metal phthalocyanine series. Dalton Trans. 2020, 49,
16268−16277.
(26) Diamond, S. E.; Tom, G. M.; Taube, H. Ruthenium promoted
oxidation of amines. J. Am. Chem. Soc. 1975, 97, 2661−2664.
(27) Keene, F. R.; Salmon, D. J.; Meyer, T. J. Oxidation of primary
amines bound to bis(2,2′-bipyridine)ruthenium(II). J. Am. Chem. Soc.
1976, 98, 1884−1889.
(28) Chen, B.; Wang, L.; Gao, S. Recent Advances in Aerobic
Oxidation of Alcohols and Amines to Imines. ACS Catal. 2015, 5,
5851−5876.
(29) Clares, M. P.; Acosta-Rueda, L.; Castillo, C. E.; Blasco, S.;
Jiménez, H. R.; García-Espana, E.; Basallote, M. G. Iron(II)
̃
Complexes with Scorpiand-Like Macrocyclic Polyamines: Kinetico-
Mechanistic Aspects of Complex Formation and Oxidative Dehydro-
genation of Coordinated Amines. Inorg. Chem. 2017, 56, 4400−4412.
(30) Dutta, I.; Yadav, S.; Sarbajna, A.; De, S.; Hölscher, M.; Leitner,
W.; Bera, J. K. Double Dehydrogenation of Primary Amines to
Nitriles by a Ruthenium Complex Featuring Pyrazole Functionality. J.
Am. Chem. Soc. 2018, 140, 8662−8666.
(48) Ni, C.; Long, G. J.; Grandjean, F.; Power, P. P. Synthesis,
Characterization, and Magnetism of Divalent Aryl Transition-Metal
Complexes of the Simplest Dialkylamide, NMe2: Rare T-Shaped
Coordination at Chromium. Inorg. Chem. 2009, 48, 11594−11600.
(49) Guillemin, J. C.; Denis, J. M. Flash vacuum thermolysis of α-
aminonitriles and subsequent HCN removal on solid base, a ‘one line’
multistep sequence to reactive N-methyleneamines. J. Chem. Soc.,
Chem. Commun. 1985, 951−952.
(31) Tomyn, S.; Haukka, M.; Nedelkov, R. Diammine{N-[2-
(hydroxyimino)propionyl]-N′-[2-(oxidoimino)propionyl]propane-
1,3-diaminido-[kappa]4N,N′,N″,N‴}iron(III). Acta Crystallogr., Sect.
E: Struct. Rep. Online 2012, 68, m1568−m1569.
(32) McQuarters, A. B.; Goodrich, L. E.; Goodrich, C. M.; Lehnert,
N. Disproportionation of O-Benzylhydroxylamine Catalyzed by a
Ferric Bis-Picket Fence Porphyrin Complex. Z. Anorg. Allg. Chem.
2013, 639, 1520−1526.
(50) Lalevée, J.; Allonas, X.; Fouassier, J.-P. N−H and α(C−H)
Bond Dissociation Enthalpies of Aliphatic Amines. J. Am. Chem. Soc.
2002, 124, 9613−9621.
(33) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. Bis(imino)-
pyridine Ligand Deprotonation Promoted by a Transient Iron Amide.
Inorg. Chem. 2006, 45, 2−4.
(51) Mader, E. A.; Manner, V. W.; Markle, T. F.; Wu, A.; Franz, J.
A.; Mayer, J. M. Trends in Ground-State Entropies for Transition
Metal Based Hydrogen Atom Transfer Reactions. J. Am. Chem. Soc.
2009, 131, 4335−4345.
(34) Dale, B. W.; Williams, R. J. P.; Edwards, P. R.; Johnson, C. E.
̈
Mossbauer spectra of compounds containing iron(II) in strong-field
(52) Nazran, A. S.; Griller, D. Hydrogen abstraction from amines:
formation of aminyl vs. .alpha.-aminoalkyl radicals. J. Am. Chem. Soc.
1983, 105, 1970−1971.
tetragonal environments. Trans. Faraday Soc. 1968, 64, 620−629.
(35) Ouedraogo, G. V.; More, C.; Richard, Y.; Benlian, D. Charge-
transfer and Moessbauer spectra of axially substituted iron
phthalocyanines. Inorg. Chem. 1981, 20, 4387−4393.
(53) Rittle, J.; Peters, J. C. Proton-Coupled Reduction of an Iron
Cyanide Complex to Methane and Ammonia. Angew. Chem., Int. Ed.
2016, 55, 12262−12265.
(36) Hunt, C.; Peterson, M.; Anderson, C.; Chang, T.; Wu, G.;
Scheiner, S.; Ménard, G. Switchable Aromaticity in an Isostructural
Mn Phthalocyanine Series Isolated in Five Separate Redox States. J.
Am. Chem. Soc. 2019, 141, 2604−2613.
(54) Matsuda, M.; Asari, T.; Naito, T.; Inabe, T.; Hanasaki, N.;
Tajima, H. Structure and Physical Properties of Low-Dimensional
Molecular Conductors, [PXX][FeIII(Pc)(CN)2] and [PXX]-
8250
https://doi.org/10.1021/acs.inorgchem.1c00923
Inorg. Chem. 2021, 60, 8242−8251

Inorganic Chemistry
pubs.acs.org/IC
Article
[CoIII(Pc)(CN)2] (PXX = peri-xanthenoxanthene, Pc = phthalocya-
ninato). Bull. Chem. Soc. Jpn. 2003, 76, 1935−1940.
(55) Geiss, A.; Vahrenkamp, H. M(μ-CN)Fe(μ-CN)M‘ Chains with
Phthalocyanine Iron Centers: Preparation, Structures, and Isomer-
ization. Inorg. Chem. 2000, 39, 4029−4036.
(56) Becke, A. D. Density-functional exchange-energy approxima-
tion with correct asymptotic behavior. Phys. Rev. A: At., Mol., Opt.
Phys. 1988, 38, 3098−3100.
(57) Perdew, J. P. Density-functional approximation for the
correlation energy of the inhomogeneous electron gas. Phys. Rev. B:
Condens. Matter Mater. Phys. 1986, 33, 8822−8824.
(58) Teuber, H.-J.; Gross, H.-J. Beweis des monovalenten
Charakters von Oxidationsreaktionen mit Kalium-nitrosodisulfonat
̈
durch ESR-Messungen, insbesondere an diffundierenden Losungen.
Chem. Ber. 1975, 108, 2097−2106.
(59) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and
accurate ab initio parametrization of density functional dispersion
correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010,
132, 154104.
(60) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the damping
function in dispersion corrected density functional theory. J. Comput.
Chem. 2011, 32, 1456−1465.
(61) Jensen, K. P.; Roos, B. O.; Ryde, U. Performance of density
functionals for first row transition metal systems. J. Chem. Phys. 2007,
126, 014103.
(62) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence,
triple zeta valence and quadruple zeta valence quality for H to Rn:
Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7,
3297−3305.
(63) Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal
Solvation Model Based on Solute Electron Density and on a
Continuum Model of the Solvent Defined by the Bulk Dielectric
Constant and Atomic Surface Tensions. J. Phys. Chem. B 2009, 113,
6378−6396.
(64) Neese, F. Software update: the ORCA program system, version
4.0. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2018, 8, e1327.
8251
https://doi.org/10.1021/acs.inorgchem.1c00923
Inorg. Chem. 2021, 60, 8242−8251