Protonation studies of a tungsten dinitrogen complex supported by a diphosphine ligand containing a pendant amine

Article abstract of DOI:10.1021/om401127v

Treatment of trans-[W(N₂)₂(dppe)(P^EtN ^MeP^Et)] (dppe = Ph₂PCH₂CH ₂PPh₂; P^EtN^MeP^Et = Et₂PCH₂N(Me)CH₂PEt₂) with 3 equiv of tetrafluoroboric acid (HBF₄·Et₂O) at °78 °C generated the seven-coordinate tungsten hydride trans-[W(N ₂)₂(H)(dppe)(P^EtN^MeP ^Et)][BF₄]. At higher temperatures, protonation of a pendant amine is also observed, affording trans-[W(N₂) ₂(H)(dppe)(P^EtN^Me(H)P^Et)][BF ₄]₂, with formation of the hydrazido complex [W(NNH ₂)(dppe)(P^EtN^Me(H)P^Et)][BF ₄]₂ as a minor product. A similar product mixture was obtained using triflic acid (HOTf). The protonated products are thermally sensitive and do not persist at ambient temperature. Upon acid addition to the carbonyl analogue cis-[W(CO)₂(dppe)(P^EtN ^MeP^Et)], the seven-coordinate carbonyl hydride complex trans-[W(CO)₂(H)(dppe)(P^EtN^Me(H)P ^Et)][OTf]₂ was generated. A mixed diphosphine complex without the pendant amine in the ligand backbone, trans-[W(N₂) ₂(dppe)(depp)] (depp = Et₂P(CH₂) ₃PEt₂), was synthesized and treated with HOTf, selectively generating a hydrazido complex, [W(NNH₂)(OTf)(dppe)(depp)][OTf]. Computational analysis probed the proton affinity of three sites of protonation in these complexes: the metal, pendant amine, and N₂ ligand. Room-temperature reactions with 100 equiv of HOTf produced NH₄ ⁺ from reduction of the N₂ ligand (electrons come from W). The addition of 100 equiv of HOTf to trans-[W(N₂) ₂(dppe)(P^EtN^MeP^Et)] afforded 0.81 equiv of NH₄⁺, while 0.40 equiv of NH₄ ⁺ was formed upon treatment of trans-[W(N₂) ₂(dppe)(depp)] with HOTf, showing that the complexes containing proton relays produce more products of reduction of N₂.

Full text of DOI:10.1021/om401127v

Article
pubs.acs.org/Organometallics
Protonation Studies of a Tungsten Dinitrogen Complex Supported
by a Diphosphine Ligand Containing a Pendant Amine
Charles J. Weiss, Jonathan D. Egbert, Shentan Chen, Monte L. Helm, R. Morris Bullock,
and Michael T. Mock*
Center for Molecular Electrocatalysis, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington
99352, United States
S
* Supporting Information
ABSTRACT: Treatment of trans-[W(N2)2(dppe)-
(PEtNMePEt)] (dppe = Ph2PCH2CH2PPh2; PEtNMePEt
=
Et2PCH2N(Me)CH2PEt2) with 3 equiv of tetrafluoroboric
acid (HBF4·Et2O) at −78 °C generated the seven-coordinate
tungsten hydride trans-[W(N2)2(H)(dppe)(PEtNMePEt)][BF4].
At higher temperatures, protonation of a pendant amine is also
observed, affording trans-[W(N2)2(H)(dppe)(PEtNMe(H)-
PEt)][BF4]2, with formation of the hydrazido complex
[W(NNH2)(dppe)(PEtNMe(H)PEt)][BF4]2 as a minor prod-
uct. A similar product mixture was obtained using triflic acid
(HOTf). The protonated products are thermally sensitive and
do not persist at ambient temperature. Upon acid addition to
the carbonyl analogue cis-[W(CO)2(dppe)(PEtNMePEt)], the seven-coordinate carbonyl hydride complex trans-[W(CO)2(H)-
(dppe)(PEtNMe(H)PEt)][OTf]2 was generated. A mixed diphosphine complex without the pendant amine in the ligand
backbone, trans-[W(N2)2(dppe)(depp)] (depp = Et2P(CH2)3PEt2), was synthesized and treated with HOTf, selectively
generating a hydrazido complex, [W(NNH2)(OTf)(dppe)(depp)][OTf]. Computational analysis probed the proton affinity of
three sites of protonation in these complexes: the metal, pendant amine, and N2 ligand. Room-temperature reactions with 100
equiv of HOTf produced NH4+ from reduction of the N2 ligand (electrons come from W). The addition of 100 equiv of HOTf
to trans-[W(N2)2(dppe)(PEtNMePEt)] afforded 0.81 equiv of NH4 , while 0.40 equiv of NH4 was formed upon treatment of
trans-[W(N2)2(dppe)(depp)] with HOTf, showing that the complexes containing proton relays produce more products of
reduction of N2.
+
+
INTRODUCTION
■
Catalytic reduction of N2 to ammonia using molecular
complexes has been a longstanding goal that has challenged
chemists for more than four decades. The importance of this
transformation is evident when considering how the industrial
method for ammonia synthesis (Haber−Bosch process) has
accelerated production of NH3 as an agricultural feedstock,
resulting in dramatic human population growth in the last
century.1 In contrast with the energy-intensive conditions (high
temperatures and pressures) used in the Haber−Bosch process,
nitrogenase enzymes convert N2 to ammonia under ambient
conditions using a combination of protons and electrons at the
structurally complex iron−molybdenum cofactor (FeMoco)
active site2 shown in Figure 1.
Deciphering the mechanism of N2 reduction by nitrogenase
enzymes remains the subject of intense investigation.3 Synthetic
transition-metal dinitrogen complexes have served as means to
investigate N2 reactivity under mild conditions using protons
and electrons.4 Additionally, structural models of the proposed
N2 binding site in FeMoco5 and investigations of proposed
metal-bound intermediates6 along the N2 reduction pathway
Figure 1. Structure of the iron−molybdenum cofactor (FeMoco) of
nitrogenase.
have helped to understand this challenging chemical trans-
formation.
Three examples of molecular catalysts for N2 reduction with
protons and electrons have been reported. The groups of
Schrock7 and Nishibayashi8 developed Mo-based systems, and
recently an Fe complex capable of catalytically reducing N2 to
ammonia was reported from the group of Peters.9 Importantly,
each catalytic system operates optimally under a specific set of
reaction conditions to achieve the maximum number of catalyst
turnovers. Moreover, the steric and electronic features of the
Received: November 20, 2013
Published: April 28, 2014
© 2014 American Chemical Society
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supporting ligand scaffold play a decisive role in producing an
active catalyst that can convert N2 to ammonia. In this context,
a variety of innovative ligand design strategies have been
implemented to produce low-valent group 6 dinitrogen
complexes, M(N2)2(P−P)2 (M = Mo, W; P−P = diphosphine
ligand), one of the most extensively examined structure types
investigated for the reduction of N2 to ammonia.4b,10 For
example, Nishibayashi and co-workers reported the complexes
of the type M(N2)2(P−P)2, supported by bidentate metal-
locene-containing diphosphine ligands (M = Mo, W; P−P =
1,1′-bis(diethylphosphino)ferrocene (depf),11 1,1′-bis(diethyl-
phosphino)ruthenocene (depr),12 bis(diethylphosphino-
benzene)chromium (bepc)).13 Treatment of these dinitrogen
complexes with sulfuric acid in methanol resulted in the
formation of ammonia in good yield. This result is in contrast
with the acid reactivity of complexes bearing traditional
diphosphine ligands such as M(N2)2(dppe)2 (M = Mo, W),
which did not produce ammonia,14 further emphasizing the
importance of the supporting ligand in facilitating protonation
and reduction of the N2 ligand.
We recently reported a series of Mo and W−N2 complexes
supported by diphosphine ligands containing noncoordinating
pendant amine groups in the second coordination sphere.15
Introduction of a ligand scaffold containing the pendant bases
represented a strategy aimed at regulating proton movement to
facilitate the reduction of N2 to ammonia. Pendant amines have
been shown to have a large influence on the catalytic rates in
electrocatalytic reduction of protons to give H2.16 Herein we
focus our efforts on understanding the reactivity of the pendant
amine containing W−N2 complex trans-[W(N2)2(dppe)-
(PEtNMePEt)] upon treatment with protic reagents such as
HBF4·Et2O and HOTf. In this report, we describe the
protonated products formed in these reactions, illustrating
that the incorporation of this pendant methylamine group in
the ligand backbone results in protonated products that differ
from those of the complex bearing a traditional bidentate
diphosphine ligand containing a hydrocarbon backbone.
Furthermore, we demonstrate that treatment of the pendant
amine-containing complex with 100 equiv of triflic acid
produces twice the amount of ammonium than a complex
bearing a traditional diphosphine ligand.
addition, the effects on reactivity of the complex with acid due
to the incorporation of only a single pendant amine
functionality could be determined before a study on complexes
bearing two PNP ligands is performed.15
To provide a direct comparison of the reactivity of 1(N2)2, a
W−N2 analogue without a pendant amine, trans-[W-
(N2)2(dppe)(depp)] (2(N2)2), was prepared by the reduction
of W(dppe)Cl4 according to the reaction in eq 1. Here, the
diphosphine ligand depp was employed due to its similarities in
ring size, bite angle, and electronic characteristics to those of
PEtNMePEt. The 31P NMR spectrum in THF-d8 of the solution
after reduction indicated that multiple products were produced,
including 2(N2)2, cis-[W(N2)2(dppe)(depp)], and minor
products observed as singlets at δ 46.6 and −19.7 for trans-
[W(N2)2(dppe)2] and trans-[W(N2)2(depp)2], respectively.
We found that multiple extractions of the crude reaction
mixture with pentane are necessary to obtain a pure sample of
2(N2)2. The 31P NMR spectrum of 2(N2)2 in THF-d8 contains
two multiplets at δ 43.9 and −16.3 for the dppe and depp
ligands, respectively. Crystals suitable for X-ray diffraction were
grown at room temperature by slow evaporation of an Et2O/
THF mixture. The molecular structure of 2(N2)2 is shown in
Figure 2. Metric parameters for 2(N2)2 are comparable to those
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Tungsten N2
Complexes. The preparation of group 6 dinitrogen complexes
supported by one or two chelating phosphine ligands bearing
pendant amines in the second coordination sphere have been
reported for Cr,17 Mo,15,18 and W.15,18a To understand how a
pendant base in the second coordination sphere affects the
reactivity patterns of the complexes upon addition of acid, in
comparison to analogous systems bearing the conventional
hydrocarbon-based ligand backbones, the mixed diphosphine
complex trans-[W(N2)2(dppe)(PEtNMePEt)]15 (1(N2)2) was
Figure 2. Molecular structure of 2(N2)2. Thermal ellipsoids are drawn
at the 50% probability level. Only the ipso carbons of the phenyl
groups on the dppe ligand are shown. Hydrogen atoms are omitted for
clarity. Selected bond distances (Å) and angles (deg): P(1)−W(1) =
2.4459(9), P(2)−W(1) = 2.4551(9), P(3)−W(1) = 2.4399(9), P(4)−
W(1) = 2.4435(8), N(1)−W(1) = 2.033(3), N(3)−W(1) = 2.012(3),
N(1)−N(2) = 1.095(4), N(3)−N(4) = 1.104(4); P(1)−W(1)−P(2)
= 86.25(3), P(3)−W(1)−P(4) = 79.38(3), W(1)−N(1)−N(2) =
176.8(3), W(1)−N(3)−N(4) = 175.8(3), N(1)−W(1)−N(3) =
172.05(10).
19
prepared in THF by the reduction of W(dppe)Cl4 with
20
magnesium powder in the presence of 1 equiv of PEtNMePEt
under an N2 atmosphere (eq 1).
The spectroscopic and structural characterization of 1(N2)2
has been described in our earlier report.15 In this study, we
focus on the reactivity of 1(N2)2 because it contains electron-
donating ethyl groups on the PNP ligand, resulting in an
electron-rich metal center, thus enhancing the basicity of the
coordinated N2 ligands in comparison to analogues with
electron-withdrawing phenyl substituents on the PNP ligand. In
of related W(N2)2(diphosphine)2 complexes that have been
structurally characterized,21 including those containing PNP
ligands.15
Protonation Studies of trans-[W(N2)2(dppe)(PEtNMePEt)]
(1(N2)2) with HBF4·Et2O. The products generated upon the
addition of 3 equiv of HBF4·Et2O to a CD2Cl2 solution of
1(N2)2 were dependent on the temperature of the reaction. For
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Figure 3. Measured (top) and simulated (bottom) 31P{1H} NMR spectra of trans-[W(N2)2(H)(dppe)(PEtNMePEt)][BF4] ([3(N2)2]BF4) recorded
at −35 °C in CD2Cl2.
example, treatment of 1(N2)2 with 3 equiv of HBF4·Et2O at
−78 °C, when the reaction mixture was kept rigorously cold,
formed the seven-coordinate tungsten bis(dinitrogen) hydride
complex trans-[W(N2)2(H)(dppe)(PEtNMePEt)][BF4]
([3(N2)2]BF4) (eq 2). The 31P NMR spectrum recorded at
−35 °C contains four ddd resonances of equal intensity at δ
61.0, 34.7, −15.5, and −29.2 (Figure 3), indicating four
inequivalent 31P environments. The 31P{1H} coupling constants
and the cross-peaks in the 31P−31P COSY NMR spectrum (see
the Supporting Information) confirm that the four resonances
belong to the same spin system.
(PEt(15N)MePEt)][BF4] ([3(N2)2(15N)]BF4) contains a triplet
at δ −362 (JNP = 14 Hz) and does not exhibit 15N−H coupling
(typically ∼80 Hz)22,23 in the proton-coupled 15N NMR
spectrum. Seven-coordinate transition-metal hydride bis-
(dinitrogen) complexes have been reported for Mo and W
supported by traditional chelating phosphine ligands (such as
dppe) upon reaction with mineral acids such as HCl in
methanol or THF,14b,24 and additional details are described
below.
Warming [3(N2)2(15N)]BF4 to 5 °C for several minutes and
then returning the sample to −35 °C for data collection
afforded the seven-coordinate hydride complex with a
protonated pendant amine trans-[W(N2)2(H)(dppe)-
(PEt(15N)Me(H)PEt)][BF4]2 ([3(N2)2(15NH)][BF4]2). In this
complex, the presence of a protonated pendant amine is
evidenced by a doublet at δ −341 (JNH = 79 Hz) in the proton-
coupled 15N NMR spectrum. Furthermore, a cross peak in the
1H−15N HSQC NMR spectrum with a 15N chemical shift at δ
1
The tungsten hydride resonance in the H NMR spectrum
appears as an apparent triplet of triplets (actually a dddd) at δ
−3.76 (Figure 4). To confirm that [3(N2)2]BF4 was protonated
only at the metal center, the reaction was performed with a
15N-labeled amine in the PNP ligand, trans-[W(N2)2(dppe)-
(PEt(15N)MePEt)][BF4] (1(N2)2(15N)). The corresponding
15N{1H} NMR spectrum of trans-[W(N2)2(H)(dppe)-
1
−341 correlating to a H signal at δ 7.7 confirms the presence
of the 15N−H bond. In the 31P NMR spectrum, resonances
corresponding to the PNP ligand exhibit a significant shift
downfield by ∼13 ppm to δ −2.6 and −16.3 upon protonation
of the pendant amine. The hydride reasonance of
[3(N2)2(15NH)][BF4]2 shifts slightly upfield to δ −4.3 (Figure
4) and is resolved into its component dddd (JPP = 87.4, 75.4,
12.1, 8.9 Hz). A 15N{1H} NMR spectrum of the 15N2-labeled
isotopologue [3(15N2)2(NH)][BF4]2 exhibits resonances at δ
−80 and −43 for the proximal and distal 15N atoms of the 15N2
ligands, respectively. A cross-peak in the 1H−15N HMBC NMR
spectrum (15N, δ −80; H, δ −3.7) shows that the hydride
1
ligand is coupled to the proximal nitrogen atom of the
coordinated 15N2 ligand. [3(15N2)2(NH)][BF4]2 is unstable at
room temperature. Warming the sample to 25 °C resulted in
liberation of the dinitrogen ligands from the complex (a sharp
Figure 4. The hydride region of the 1H NMR spectrum of
[3(N2)2]BF4 (bottom) and [3(N2)2(NH)][BF4]2 (top) recorded at
−35 °C in CD2Cl2.
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singlet at δ −71 observed in the 15N NMR spectrum for
liberated 15N2) and the disappearance of the 31P NMR
resonances, forming unidentified paramagnetic products.
Treatment of [3(N2)2(15NH)][BF4]2 with 20 equiv of Et3N
at −35 °C deprotonates the pendant amine of the PNP ligand,
immediately regenerating [3(N2)2]BF4 (eq 2). When the
sample is warmed to 25 °C in the presence of 20 equiv of
Et3N, [3(N2)2]BF4 is deprotonated at the metal center to
regenerate 1(N2)2. However, 1(N2)2 is not stable in the
presence of Et3NH+, and over the course of ∼30 min, due to N2
loss, it converts to unidentified paramagnetic products on the
basis of the absence of 31P NMR signals. In contrast,
deprotonating [3(N2)2(NH)][BF4]2 with 100 equiv of Et3N,
followed by gradual warming to 25 °C, regenerates 1(N2)2 as
the only observed product by 31P NMR spectroscopy. In the
presence of 100 equiv of Et3N, 1(N2)2 is stable for ∼24 h at 25
°C. The large excess of base in solution clearly inhibits the
transfer of protons to 1(N2)2, which results in a thermally
unstable product. The instability of 1(15N2)2 toward Et3NH+
was examined independently, resulting in immediate liberation
of N2 ligands from tungsten (see the Supporting Information
for details).
protonation of the pendant amine in cis-[Fe(H)(N2)-
(PEtNMePEt)2]+.26 The addition of 20 equiv of Et3N
immediately deprotonates the pendant amine site in
[3(N2)2(NH)][BF4]2, resulting in a rapid shift of the 2017
cm−1 band back to 1995 cm−1. When the reaction mixture is
warmed to 21 °C, the starting bis(dinitrogen) complex 1(N2)2
is regenerated, indicated by the νNN band at 1925 cm−1. Over
the course of 40 min, 1(N2)2 slowly loses the bound N2 ligands
in the presence of Et3NH+, consistent with N2 loss and
conversion of 1(N2)2 into unidentified paramagnetic products
observed by NMR (vide supra). In an attempt to observe the
complex that is only protonated at the pendant amine, trans-
[W(N2)2(dppe)(PEtNMe(H)PEt)][BF4], in situ IR spectra were
collected at a rate of 5 scans/s. However, even at the faster rate
of data collection, this product was not detected in the in situ
IR spectrum. This observation suggests that the metal center is
protonated directly or that the rate of intramolecular proton
transfer from the hypothesized protonated pendant amine to
the metal center is too rapid to be observed under these
conditions.
In the reactions described above, [3(N2)2]BF4 was generated
when the reaction was kept rigorously cold during the initial
addition of HBF4·Et2O to 1(N2)2. Once [3(N2)2]BF4 is
formed, warming the solution to 5 °C afforded [3(N2)2(NH)]-
[BF4]2, where the pendant amine site of the PNP ligand is
protonated. In contrast to this stepwise protonation (as a result
of changing the reaction temperature), when a CD2Cl2 solution
of 1(N2)2 cooled to −78 °C is treated with 3 equiv of HBF4·
Et2O and the solution is warmed briefly27 during the initial
sample mixing, before inserting into the NMR probe
maintained at −35 °C, a different mixture of protonated
products is obtained, shown in eq 3. Under these conditions,
The protonation of 1(N2)2 was investigated further by in situ
IR spectroscopy by adding 3 equiv of HBF4·Et2O to 1(N2)2 in
CH2Cl2 at −40 °C. A plot of in situ IR spectra monitoring the
νNN bands of the starting material and products over the course
of this reaction is shown in Figure 5. Upon addition of acid, the
Figure 5. In situ IR plot recorded at −40 °C from the reaction of
1(N2)2 with 3 equiv of HBF4·Et2O in CH2Cl2, followed by the
addition of 20 equiv of Et3N. The total reaction time was 150 min.
Data were collected in 15 s increments. The IR plot was rendered with
each slice being approximately 200 s.
νNN band at 1925 cm−1 from the starting bis(dinitrogen)
complex 1(N2)2 immediately disappeared, and a new νNN band
appeared at 1995 cm−1, assigned to [3(N2)2]BF4. The increase
in the N2 vibrational frequency of 70 cm−1 upon protonation of
the metal center clearly demonstrates the effect of decreased
back-bonding to the dinitrogen ligand in the oxidized (formally
W(II)) metal center. This band persists for less than 30 s before
the next IR measurement is recorded and the appearance of a
new νNN band at 2017 cm−1 corresponding to the tungsten
hydride product with a protonated pendant amine,
[3(N2)2(NH)][BF4]2. The higher energy shift by 22 cm−1 of
the νNN band upon pendant amine protonation is primarily due
to the electrostatic effect of increasing the positive charge of the
molecule from a monocation to a dication. Similarly, a 25 cm−1
shift of the νNN band to higher energy was observed upon
protonation of the pendant amine site in trans-[Fe(H)(N2)-
(PEtNMePEt)(dmpm)]+ (dmpm = (CH3)2PCH2P(CH3)2),25
while a 16 cm−1 increase in the νNN band was observed upon
the 31P NMR spectral data indicated the formation of
[3(N2)2(NH)][BF4]2, which accounted for ∼85% of the total
phosphorus-containing products. A second protonated species
accounting for the remaining ∼15% of the total products was
identified as the hydrazido complex trans-[W(NNH2)(F)-
(dppe)(PEtNMe(H)PEt)][BF4]2 ([1(NNH2)(F)(NH)][BF4]2)
(see the Supporting Information). The 31P resonances assigned
to [1(NNH2)(F)(NH)][BF4]2 include a pair of doublet of
doublets at δ 35.4 and 10.9, due to 31P−31P and 31P−19F
coupling (∼40−50 Hz).22,23,28
A multiplet in the 19F NMR spectrum at δ −187 corresponds
1
to the tungsten fluoride resonance. The H−15N HSQC NMR
spectrum, using the 15N2-labeled complex [1(15N15NH2)(F)-
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Figure 6. 31P{1H} NMR spectrum recorded at −40 °C in CD2Cl2 after 6 equiv of HOTf was added to 1(15N2). Two major products observed were
observed: (black) trans-[W(15N2)2(H)(dppe)(PEtNMe(H)PEt)][OTf]2 ([3(15N2)2(NH)][OTf]2); (blue) trans-[W(15N15NH2)(OTf)(dppe)-
(PEtNMe(H)PEt)][OTf]2 ([1(15N15NH2)(OTf)(NH)][OTf]2). The identity of the minor product with resonances at δ 43.7 and 7.5 has not
been determined.
(NH)][BF4]2, exhibits a cross peak for the protonated distal
nitrogen atom of the hydrazido ligand with a 15N chemical shift
at δ −46 and a corresponding 1H resonance at δ 5.60.
Accordingly, in the 1H NMR spectrum, a doublet at δ 5.63 (JHN
= 80.0 Hz) is assigned as the protons of the hydrazido ligand.
This resonance is a broad singlet in the W−14N14NH2
isotopologue. Performing the reaction with a 15N-labeled
pendant amine, 1(N2)2(15N), upon generation of [1(NNH2)-
(F)(15NH)][BF4]2 two cross peaks are observed in the 1H−15N
scribed above. Factors that may influence the stability of the
protonated products include the identity of the metal center
and the electron-donating ability of the supporting phosphine
ligands; thus, it is difficult to conclusively identify the reasons
for the different stability of [Mo(NNH2)(F)(dppe)(pyN(H)-
P2)][BF4]2 in comparison to [1(NNH2)(F)(NH)][BF4]2.
While it is clear the pendant amine incorporated in the
backbone of the ligand results in reactivity patterns different
from those in the absence of the amine, the proximity of the
pendant aminopyridine group with respect to the metal center
and/or the hydrazido ligand and the basicity of the pendant
group are important factors that could influence the stability of
these products. In 1(N2)2, the pendant amine is more basic
than the distal nitrogen atom of the N2 ligand (vide infra),
indicating that the transfer of a proton from a protonated
pendant amine to the N2 ligand would be unfavorable. Because
intramolecular proton transfer could occur from a protonated
N2 intermediate (such as the hydrazido species) to the pendant
amine, the hydrazido complexes are not expected to form
without a protonated pendant amine.
1
HSQC NMR spectrum with H NMR signals at δ 10.37 and
10.55, with a 15N NMR chemical shift of δ −341. These signals
are assigned as two possible isomers of [1(NNH2)(F)(15NH)]-
[BF4]2 with the proton on the pendant amine positioned endo
or exo with respect to the hydrazido group. The products
generated in this reaction are thermally sensitive, and when the
sample is warmed above 10 °C, it slowly converted to
unidentified paramagnetic products. However, if the reaction
is maintained at −35 °C, addition of 20 equiv of Et3N converts
[1(NNH2)(F)NH)][BF4]2 to 1(N2)2. Further warming to
room temperature converts the remaining [3(N2)2]BF4 back to
1(N2)2 before eventual loss of 31P NMR signals. Precedent for
deprotonation of the hydrazido groups comes from tungsten
hydrazido complexes supported by diphosphine ligands such as
trans-[W(NNH2)(p-MeC6H4SO3)(dppe)2]+ (without pendant
amines) that have been deprotonated by Et3N24a,29 to
regenerate the parent dinitrogen complex.
In a related study examining the reactivity of a pendant
amine containing Mo bis(dinitrogen) complex with a strong
acid, Tuczek and co-workers treated trans-[Mo(N2)2(dppe)-
(pyNP2)]18a (pyNP2 = N,N-bis(diphenylphosphinomethyl)-2-
aminopyridine) with 3 equiv of HBF4·Et2O in CD2Cl2 at −15
°C, generating the hydrazido-containing product [Mo(NNH2)-
(F)(dppe)(pyN(H)P2)][BF4]2.18b In addition to protonation
of the N2 ligand, the pyridine group of the supporting pyNP2
ligand was protonated, as determined from X-ray crystallo-
graphic data. Notably, while acid addition was performed at
reduced temperature, the protonated product [Mo(NNH2)-
(F)(dppe)(pyN(H)P2)][BF4]2 is thermally stable, in contrast
to the thermally sensitive protonated tungsten products
[1(NNH2)(F)(NH)][BF4]2 and [3(N2)2(NH)][BF4]2 de-
Reactivity of trans-[W(N2)2(dppe)(PEtNMePEt)] and
trans-[W(N2)2(dppe)(depp)] with HOTf. In addition to the
protonation studies with HBF4·Et2O, the reactivity of 1(N2)2
with triflic acid (HOTf) was examined to determine if the
identity of the counterion would influence the distribution of
protonated products. A solution of 1(15N2)2 in CD2Cl2 was
treated with 6 equiv of HOTf at −78 °C before inserting into
an NMR probe maintained at −40 °C, as shown in eq 4. In this
reaction, despite conditions in which the NMR tube was kept
rigorously cold during sample mixing, integration of the 31P
NMR spectrum, shown in Figure 6, indicated ∼81% conversion
to trans-[W(15N2)2(H)(dppe)(PEtNMe(H)PEt)][OTf]2
([3(15N2)2(NH)][OTf]2) and 18% conversion to trans-[W-
(
1 5 N1 5 NH2 )(OTf)(dppe)(PE t NM e (H)PE t )][OTf]2
([1(15N15NH2)(OTf)(NH)][OTf]2). Multiplets at δ 63.7, 40.1,
−0.2, and −13.6 correspond to the seven-coordinate W
hydride, with their PNP ligand resonances shifted downfield,
indicative of a protonated pendant amine as described above. In
addition, multiplets at δ 35.4 and 2.7 (JPP = 127 Hz)
1
correspond to the hydrazido complex. The H NMR spectrum
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contains a dddd (apparent tt) at δ −4.32 for the W hydride
resonance in [3(15N2)2(NH)][OTf]2, while the hydrazido
resonance of [1(15N15NH2)(OTf)(NH)][OTf]2 appears as a
doublet at δ 6.54 (JNH = 89 Hz). The 15N chemical shifts of the
proximal and distal N atoms in [1(15N15NH2)(OTf)(NH)]-
[OTf]2 and [3(15N2)2(NH)][OTf]2 are identical with those of
complexes with the BF4− anion. Treatment of the products with
30 equiv of Et3N at −40 °C deprotonated the pendant amine,
forming [3(15N2)2]OTf. Accordingly, the 31P resonances for the
PNP ligand shift 11−13 ppm upfield as in the reactions with
HBF4·Et2O described above. The pendant amine and hydrazido
ligand in [1(15N15NH2)(OTf)(NH)][OTf]2 are also deproto-
nated, regenerating the starting dinitrogen complex 1(N2)2.
intensity are observed at 1792 and 1729 cm−1 in the IR
spectrum. These bands are shifted to lower frequency by
approximately 45 cm−1 in comparison to those in the 12CO
sample. Treatment of cis-[1(CO)2] with 3 equiv of 4-
bromoanilinium triflate, [4-BrPhNH3][OTf] (pKa = 9.4 in
acetonitrile30), at room temperature results in an immediate
color change from orange to yellow, as trans-[W(CO)2(H)-
(dppe)(PEt(15N)Me(H)PEt)][OTf]2 ([3(CO)2(15NH)][OTf]2)
is generated (eq 6). The IR spectrum (KBr) exhibits an intense
Warming the sample to room temperature results in loss of 31
NMR signals.
P
As noted above, complex 2(N2)2 was prepared as an
analogue to 1(N2)2, in which the depp ligand replaces the
pendant amine-containing PNP ligand. To provide a
comparison of the reactivity studies with acid, a solution of
2(N2)2 in CD2Cl2 was treated with 3 equiv of HOTf at −78 °C.
31P NMR spectral data collected at −40 °C shows conversion
to the hydrazido product trans-[W(NNH2)(OTf)(dppe)-
(depp)][OTf] ([2(NNH2)(OTf)][OTf]), (eq 5). Similarly,
antisymmetric νCO band at 1851 cm−1 and a weak symmetric
νCO band at 1957 cm−1, indicating a trans configuration of the
CO ligands. Similar isomerization from a cis-dicarbonyl to a
trans-dicarbonyl product upon protonation of the metal center
was noted in molybdenum complexes without pendant amine
containing ligands. For example, the conversion of cis-
[Mo(CO)2(dmpe)2] (dmpe = (CH3)2PCH2CH2P(CH3)2) to
trans-[Mo(H)(CO)2(dmpe)2]HCl2 has been reported upon
the addition of HCl.31 In the present case, NMR spectral data
corroborate the structural assignment from the IR data. A single
carbonyl resonance is observed in the 13C{1H} NMR spectrum
at δ 204.5 for the 13CO-labeled complex. The 1H NMR
spectrum identifies the tungsten hydride at δ −5.18. In
addition, the pendant amine of the seven-coordinate dicarbonyl
hydride complex is protonated, indicated by a doublet (JNH
=
the hydrazido product [2(NNH2)(F)][BF4] was formed upon
treatment of 2(N2)2 with HBF4·Et2O. In contrast to the
protonation studies with 1(N2)2, no W hydride product was
observed in these reactions, indicating that incorporation of the
pendant amine in the second coordination sphere has a
significant effect in promoting the protonation of the metal
center. In addition, [2(NNH2)(OTf)][OTf] is more thermally
robust than [1(NNH2)(OTf)(NH)][OTf]2 and does not
convert into paramagnetic product upon warming to 22 °C.
Reactivity of a W(CO)2 Analogue with [4-BrPhNH3]-
[OTf]. To compare the preferred sites of protonation with
1(N2)2 and to provide a comparison of NMR and IR
spectroscopic data, the tungsten dicarbonyl analogue W-
(CO)2(dppe)(PEt(15N)MePEt) was treated with acid. The
previously reported cis-dicarbonyl complex cis-[W-
(CO)2(dppe)(PEt(15N)MePEt)] (cis-[1(CO)2(15N)]) was syn-
thesized by stirring a solution of 1(N2)2 under an atmosphere
of CO.15 The 13CO-labeled complex cis-[W(13CO)2(dppe)-
(PEtNMePEt)] (cis-[1(13CO)2]) was prepared to provide addi-
tional spectroscopic information. For cis-[1(13CO)2], two
76 Hz) in the 15N NMR spectrum at δ −338. The 31P{1H}
NMR spectrum of [3(CO)2(15NH)]2+ contains four multiplets
at δ 60.7, 43.6, −11.6, −23.4, indicating four distinct
phosphorus environments. In a related study, the molecular
structure of the molybdenum dicarbonyl complex trans-
[Mo(H)(CO)2(dppe)2]+ was determined by X-ray diffrac-
tion.32 Although the hydride ligand was not observed in the
final difference map, the location of the hydride ligand was
inferred to be located between chelating phosphine ligands on
the P4 plane, on the basis of Mo−P bond angles. An analogous
seven-coordinate structure is expected in the present case.
Treatment of [3(13CO)2(15NH)][OTf]2 with 20 equiv of Et3N
at −40 °C formed a mixture of cis-[1(13CO)2] and trans-
[W(13CO)2(dppe)(PEtNMePEt)] (trans-[1(13CO)2]). When 10
equiv of Et3N is added at 21 °C, only cis-[1(13CO)2] is formed,
thus, cis/trans isomerization is slow at reduced temperatures.
Computational Analysis. Computational analysis employ-
ing density functional theory (DFT) based electronic structure
methods was used to augment the protonation studies of
1(N2)2 and 2(N2)2. An evaluation of the thermodynamically
preferred protonation sites in these molecules was performed in
order to (1) rationalize the products formed upon treatment of
carbonyl resonances are identified in the 13C{1H} NMR
13
CO
spectrum at δ 221.0 and 218.7, while two ν
bands of equal
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Figure 7. Computationally derived proton affinity values in kcal/mol for the protonation of 1(N2)2, 2(N2)2, and [3(N2)2]+.
with strong acids, (2) evaluate the effect of pendant amine
protonation on N2 binding, and (3) understand how to predict
and control reactivity patterns to aid in the future design of new
compounds. First, we examined the proton affinity of the
proton-accepting sites on 1(N2)2 (metal center, pendant amine,
and N2 ligand) in this computational assessment, shown in
Figure 7. The proton affinity values for the single protonation of
1(N2)2 in THF varied over a wide range. The electron-rich
metal center was determined to have the highest proton affinity
of 22.1 kcal/mol, followed by the pendant amine site of 10.4
kcal/mol. The proton affinity for the distal nitrogen of the
dinitrogen ligand was considerably lower at 2.0 kcal/mol. For
comparison, the proton affinity of the tungsten center and the
N2 ligand were determined for complex 2(N2)2. As expected for
complexes that are structurally and electronically similar, the
calculated proton affinity values were nearly identical with those
of 1(N2)2 at 22.4 and 2.0 kcal/mol for the tungsten center and
the dinitrogen ligand, respectively.
While the computational results indicate that the metal
center is the thermodynamically favored protonation site in
1(N2)2 and 2(N2)2, the reactivity studies discussed above show
that the formation of the seven-coordinate tungsten hydride
product is observed in reactions with 1(N2)2, but not with
2(N2)2. Furthermore, despite the ∼20 kcal/mol lower proton
affinity value of the N2 ligand versus the metal center, only
protonation at the N2 ligand is observed with 2(N2)2. The
disparity in reactivity between the two complexes could be
rationalized by the pendant amine and dinitrogen ligands being
kinetically favored protonation sites upon acid addition, as a
result of their location in the second coordination sphere. In
the case of 1(N2)2, initial protonation of the pendant amine
may ultimately result in rapid intramolecular nitrogen to metal
proton transfer to give the thermodynamically preferred
product. In reactions with 2(N2)2, the second protonation at
the N2 ligand, leading to the formation of a stable hydrazido
product, may occur before protonation at the thermodynami-
cally preferred metal center. No clear intermolecular pathway is
accessible to transfer the proton to the metal center before the
formation of the stable hydrazido product.33
the adduct) being faster at lower acid concentrations in
comparison to protonation of the N2 ligand by an additional 1
equiv of acid. This example demonstrates that a metal hydride
product for complexes such as 2(N2)2 can be generated under
certain experimental conditions, which are likely dependent on
the identity of the acid and the medium. However, under the
conditions employed in the present study with 2(N2)2, the W
hydride product was not formed, indicating that the direct
protonation of the metal center is not kinetically preferred.
A computational assessment of the proton-accepting sites of
the seven-coordinate hydride complex [3(N2)2]+ indicates a
substantial shift in the pendant amine proton affinity to −1.9
kcal/mol, while the dinitrogen ligand remains the least basic
site, with a proton affinity of −20.4 kcal/mol. The dramatic
shift in proton affinity values at these sites reflects the cationic
charge of the hydride complex as well as a less electron
donating metal center. Promoting additional protonation
events at the N2 ligand after the formation of the tungsten
hydride complex is clearly disfavored on the basis of the more
acidic nature of this site.
To investigate the W−N2 binding strength in 1(N2)2 and in
potential (but not spectroscopically observed) structures
containing a protonated pendant amine, the free energy for
the dissociation of dinitrogen from tungsten was evaluated
computationally. For 1(N2)2, an N2 dissociation free energy of
16.4 kcal/mol was predicted, indicating a moderately labile N2
ligand in this low-valent complex. Experimental observations
are in line with the predicted lability of the N2 ligand, as
substitution by CO (presumably through a dissociative
mechanism) is slow, requiring ∼37−72 h for complete
conversion to the tungsten dicarbonyl complex described
above.15 Upon protonation of the pendant amine, the N2
dissociation free energy of the cationic species was slightly
lower at 15.0 kcal/mol. The magnitude of the change in N2
dissociation energy in the protonated species is consistent with
our previously reported analysis for the protonation of a
pendant amine site in trans-[FeH(N2)(PEtNMePEt)(dmpm)]+.
An electrostatic effect (increased positive charge) was
determined to be the major contributing factor, resulting in a
decrease of N2 binding energy and an increase in the νNN
vibrational frequency. In the current study, IR spectroscopic
data did not indicate the formation of a discrete complex
formulated as trans-[W(N2)2(dppe)(PEtNMe(H)PEt)]+, but the
22 cm−1 shift between the hydride species 3(N2)2 and
[3(N2)2(NH)]2+ indicates the trend in νNN vibrational
frequency remains consistent between the Fe and W systems
upon protonation of the pendant amine. Most importantly,
however, is recognizing the protonation events that have the
greatest impact toward reduction of the N2 ligand. Initial
protonation of the pendant amine site lowers the proton affinity
of the N2 ligand and the N2 dissociation energy to the metal
It is important to note a related kinetic study by Henderson
that reported a mechanism by which hydrazido and metal−
hydrido complexes are formed upon addition of mineral acids
(HX = HCl, HBr, H2SO4) to trans-[M(N2)2(P−P)2] (M = Mo,
W; P−P = dppe, depe) in THF (depe = Et2PCH2CH2PEt2).
Formation of [W(N2)2(dppe)2]HX, an adduct between HX
and the metal complex, was suggested to be the initial product
formed in the reaction with acid. In reactions with HCl, low
acid concentrations favored the formation of the metal hydride
product. Higher concentrations of acid resulted in formation of
the hydrazido product. These results were rationalized by
intramolecular electrophilic attack of HX on the metal (within
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center. Thus, directing the initial protonation at the N2 ligand
represents an important objective. In addition, modifications to
the ligand design that include modulating the proximity of the
amine with respect to the N2 ligand and tuning the amine
proton affinity to be more closely matched to the N2 ligand
may help avoid potential undesired intramolecular proton
transfer and favor intramolecular transfer steps that lead to
reduced N2 products.
Protonolysis Reactions with Triflic Acid. To determine if
the incorporation of a pendant amine in the second
coordination sphere of the dinitrogen complex would result
+
in NH4 formation upon the addition of a significant excess of
triflic acid, experiments examining this reactivity were
performed on 1(N2)2 and 2(N2)2. Triflic acid was chosen
because oxygen-containing anions have been suggested to be
1
Figure 8. H NMR spectrum (top) from the addition of HOTf (100
equiv) to 1(N2)2 in CH2Cl2 at 21 °C, resulting in the formation of
+
33
NH4 as a 1:1:1 triplet (JHN = 51 Hz) at δ 7.06 in DMSO-d6.
important in facilitating the reduction of N2 and to avoid
Repeating the experiment with 1(15N2)2 affords a doublet (JHN = 72
coordination of fluoride ligand, which binds upon protonation
with HBF4·Et2O.34,22,35 Treatment of an orange solution of
1(N2)2 in CH2Cl2 at room temperature with 100 equiv of
HOTf resulted in a pale orange solution. After the mixture was
stirred for 20 h, the ammonium produced in the reaction was
quantified using the indophenol method (see the Experimental
Section for details). In reactions with 1(N2)2, 0.81 equiv of
ammonium was produced per metal center (eq 7). For
Hz) (bottom).
containing a hydrazido intermediate would exhibit a more
positive reduction potential, which could help promote
intermolecular electron transfer reactions to aid N2 reduc-
tion.25,18a Proton transfer to the N2 ligand could follow or occur
concomitantly with the electron transfer step. Although
formation of the seven-coordinate tungsten hydride complex
appears to be detrimental toward facilitating protonation and
reduction of the N2 ligand, in a related mechanistic study by
Henderson, the initial protonation of trans-[Mo(N2)2(depe)2]
with HCl in THF resulted in the formation of Mo(H)(N2)-
(Cl)(depe)2 upon loss of one N2 ligand.24b Although the initial
protonation did not occur at the N2 ligand, this study proposed
that subsequent formation of trans-[Mo(NNH2)(Cl)(depe)2]+
could proceed through the initially formed Mo−H complex by
an acid-catalyzed pathway. In addition, metal hydrides have
been shown to play an important role in a variety of systems for
N2 reduction to ammonia.36 Relevant examples include
intermediate states formed during catalytic turnover of the
nitrogenase enzyme, silica-bound transition-metal-containing
heterogeneous systems, and a variety of synthetic homogeneous
complexes.36 In the present case, preferential formation of the
W−H complex is apparent in reactions of 1(N2)2 with
stoichiometric amounts of acid. It is unclear at this time
whether the W−H product plays a role in the protonolysis
comparison, the addition of acid to an orange solution of
2(N2)2 afforded a green solution and resulted in the formation
of 0.40 equiv of ammonium per tungsten center. Thus, the
pendant amine containing complex 1(N2)2 produced twice as
much ammonium as 2(N2)2. Hydrazine was not detected as a
product in either reaction. Two equivalents of ammonium is
the theoretical yield without the addition of an external
reductant, with the six d electrons in W(0) being the limiting
reagent.
To confirm that the ammonium produced in this reaction
resulted from reduction of the metal-bound N2 ligand and not
from the degradation of the pendant amine or from
atmospheric N2, the reaction was performed with the 15N2-
labeled isotopologue 1(15N2)2. Accordingly, ammonium was
+
reactions, resulting in higher yields of NH4 in comparison to
those for complex 2(N2)2, where the formation of a W−H
product is not observed. Studies are underway to understand
the effect of the pendant amine and the transition-metal
+
1
observed in the H NMR spectrum as a doublet (15N: I = 1/2)
hydride complexes in promoting NH4 production.
at δ 7.06 (JHN = 72 Hz) instead of a 1:1:1 triplet (JHN = 51 Hz,
resulting from coupling to 14N (I = 1)), shown in Figure 8.
It is difficult to provide the exact reason for the higher yields
of ammonium produced from 1(N2)2. One possibility is that
the pendant amine group plays a key role in promoting the
formation of protonated N2 intermediates past the hydrazido
species, one of the most stable intermediates in the N2
reduction pathway. An examination of the products at the
end of the reactions with 2(N2)2 showed the presence of the
hydrazido complex [2(NNH2)(OTf)][OTf], indicating that a
portion of this material did not proceed past this intermediate,
thus failing to cleave the N−N bond. A hydrazido product was
not observed at the end of the reactions with 1(N2)2. In the
case of 1(N2)2, the greater positive charge (due to the
protonated pendant amine) associated with a complex
CONCLUSIONS
■
We examined the reactivity of a tungsten bis(dinitrogen)
complex supported by a PNP ligand containing a pendant
methyl amine in the ligand backbone as a strategy for regulating
proton movement in the second coordination sphere to
facilitate the reduction of N2 to ammonia. The reactivity of
1(N2)2 and a structural analogue containing a hydrocarbon
backbone, 2(N2)2, toward protic reagents such as HBF4·Et2O
and triflic acid at low temperature led to an understanding of
the reactivity patterns of these complexes. Our results
demonstrate that the incorporation of the pendant amine
group in the ligand backbone affords the seven-coordinate
tungsten hydride complex [3(N2)2(NH)]2+ as the major
product. Protonation of the N2 ligand formed the tungsten
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hydrazido complex [1(NNH2)(X)(NH)]2+ (X = F, OTf),
observed as a minor product. Analogous reactions with 2(N2)2
formed the W hydrazido complex [2(NNH2)(X)]+ (X = F,
OTf) without the formation of a tungsten hydride product.
Importantly, in protonolysis reactions with excess triflic acid,
the complex containing the PNP ligand produced twice the
amount of ammonium in comparison to the complex with the
depp ligand. These results indicate that the incorporation of a
basic pendant amine in the ligand backbone of the dinitrogen
complex significantly influences the formation of reduced N2
intermediates, such as a hydrazido species, that are commonly
generated as stable products in complexes such as 2(N2)2.
While the amine-containing complex 1(N2)2 clearly favors the
formation of the seven-coordinate hydride product in reactions
with stoichiometric amounts of acid, the role of the amine in
enhancing the formation of ammonia in the presence of excess
acid is unclear at this time. In light of this encouraging result,
future work will examine the reactivity of group 6 dinitrogen
complexes containing less basic pendant amine substituents to
more closely match the amine basicity to that of the dinitrogen
ligand. Moreover, an understanding of how to utilize the
pendant bases to control proton movement and delivery to the
N2 ligand to facilitate ammonia formation will be of great utility
toward the goal of developing electrocatalytic N2 reduction
systems in our laboratory.
The oil was dissolved in 20 mL of pentane and filtered through Celite,
and the solvent was removed under reduced pressure, affording a clear
1
oil. Yield: 89% (1.56 g, 7.08 mmol). The H and 31P NMR spectral
data of the product matched previously reported values.39
trans-[W(N2)2(H)(dppe)(PEtNMePEt)][BF4] ([3(N2)2]BF4). Neat
HBF4·Et2O (3.3 μL, 2.4 × 10−2 mmol) was injected into a yellow
CD2Cl2 solution of trans-[W(N2)2(dppe)(PEtNMePEt)] (1(N2)2; 6.6
mg, 7.6 × 10−3 mmol) at −78 °C, resulting in a pale yellow solution.
The NMR tube was mixed by inversion before promptly being
inserted into an NMR probe, maintained at −35 °C. The products
described below are formed depending on the temperature of the
reaction mixture during mixing. 1H NMR (CD2Cl2, −35 °C): δ 7.64−
7.35 (m, 20H, PPh2), 2.94 (s, 2H, PCH2N), 2.85 (s, 2H, PCH2N),
2.56−2.35 (m, 4H, PCH2CH2P), 2.33 (s, 3H, NCH3), 1.90 (m, 4H,
PCH2CH3), 1.47 and 1.36 (m, 4H, PCH2CH3), 0.93 (dt, 6H, Jd = 15.3
Hz, Jt = 7.3 Hz, PCH2CH3), 0.77 (dt, 6H, Jt = 14.0 Hz, Jt = 7.3 Hz,
PCH2CH3), −3.76 (dddd, 1H, JHP = 77.0 Hz, JHP = 77.0 Hz, JHP = 11.2
Hz, JHP = 11.2 Hz, W-H). 31P{1H} NMR (CD2Cl2, −35 °C): δ 61.0
(ddd with 183W satellites JPW = 196 Hz, 1P, JPP = 44, 33, and 17 Hz,
dppe), 34.7 (ddd with 183W satellites JPW = 230 Hz, 1P, JPP = 49, 33,
and 33 Hz, dppe), −15.5 (ddd with 183W satellites JPW = 170 Hz, 1P,
JPP = 58, 49, and 17 Hz, PEtNMePEt), −29.2 (ddd with 183W satellites
JPW = 208 Hz, 1P, JPP = 58, 44, and 33 Hz, PEtNMePEt). 15N NMR
(CD2Cl2, −35 °C): δ −362 (t, 1N, JNP = 14 Hz, NMe). IR (CH2Cl2,
−41 °C, νNN, cm−1): 1995 (s, asym). This protonated product is
thermally sensitive and does not persist at ambient temperature.
trans-[W(N2)2(H)(dppe)(PEtNMe(H)PEt)][BF4]2 ([3(N2)2(NH)]-
[BF4]2). 1H NMR (CD2Cl2, −35 °C): δ 7.7 (br, N(H)Me, from
1H−15N HSQC data with 15N-amine), −4.33 (dddd, 1H, JHP = 87.4
EXPERIMENTAL SECTION
Hz, JHP = 75.4 Hz, JHP = 12.1 Hz, JHP = 8.9 Hz, W-H). 31P{1H} NMR
(CD2Cl2, −35 °C): δ 59.8 (ddd with 183W satellites JPW = 198 Hz, 1P,
■
General Experimental Procedures. All synthetic procedures
were performed under dry N2 or argon using standard Schlenk
techniques or a glovebox. Protio solvents were purchased as
anhydrous, dried by passage through activated alumina columns in a
solvent purification system, and stored under N2 or argon until use.
Benzene-d6 and THF-d8 were dried over Na/K and degassed under
high vacuum (10−6 Torr) before being distilled and stored under N2
until use. CD2Cl2 was dried over CaH2. DMSO-d6 was used as
received. All reagents were purchased from commercial sources and
used as received. Triethylamine was degassed under vacuum and dried
over 3 Å molecular sieves before use. The 1H, 13C, 15N, and 31P NMR
spectra were collected on a 500 MHz spectrometer at ambient
JPP = 32, 21, and 47 Hz, dppe), 36.3 (ddd with 183W satellites JPW
=
228 Hz, 1P, JPP = 32, 47, and 32 Hz, dppe), −2.6 (ddd with 183W
satellites JPW = 277 Hz, 1P, JPP = 21, 47, and 59 Hz, PEtNMePEt), −16.3
(ddd with 183W satellites JPW = 222 Hz, 1P, JPP = 47, 32, and 59 Hz,
PEtNMePEt). 15N NMR (CD2Cl2, −35 °C, for 15N2 labeled versions): δ
−80 (s, 1N, W-NN), −43 (s, 1N, W-NN), −341 (d, 1N, JNH = 79
Hz, N(H)Me). IR (CH2Cl2, −40 °C, νNN, cm−1): 2017 (s, asym). This
protonated product is thermally sensitive and does not persist at
ambient temperature.
[W(NNH2)(F)(dppe)(PEtNMe(H)PEt)][BF4]2 ([1(NNH2)(F)(NH)]-
[BF4]2). Following the procedure above for the synthesis of trans-
[W(N2)2(H)(dppe)(PEtNMe(H)PEt)][BF4]2 from HBF4·Et2O, ∼15%
of the product formed is [W(NNH2)(F)(dppe)(PEtNMe(H)PEt)]-
[BF4]2. 1H NMR (CD2Cl2, −35 °C): δ 10.55 and 10.33 (br, N(H)Me,
from 1H−15N HSQC data with 15N-amine), 5.63 (bs, NNH2). 31P{1H}
NMR (CD2Cl2, −35 °C): δ 35.4 (dd, 1P, JPP = 144 Hz, JPF = 48 Hz),
10.8 (dd, 1P, JPP = 144 Hz, JPF = 44 Hz). 19F NMR (CD2Cl2, −35 °C):
δ −187 (m). This protonated product is thermally sensitive and does
not persist at ambient temperature.
temperature unless otherwise indicated. H and 13C NMR chemical
1
shifts are referenced against tetramethylsilane using internal solvent
resonances. 31P NMR chemical shifts are proton decoupled unless
otherwise noted and referenced to H3PO4 as an external reference. 15
N
NMR chemical shifts were referenced to CH315NO2 (δ 0). Phosphorus
coupling to the 183W nuclei was not included in the simulations of the
31P NMR spectra. Infrared spectra were recorded as a KBr pellet under
a purge stream of nitrogen gas. In situ IR experiments were recorded
on a Mettler-Toledo ReactIR 15 spectrometer equipped with a liquid
nitrogen cooled MCT detector, connected to a 1.5 m AgX Fiber DS
series (9.5 mm × 203 mm) probe with a silicon sensor. Experiments
were performed in a 5 or 10 mL two-neck pear-shaped flask under a
dinitrogen atmosphere. IR spectra were collected in intervals of 15 s in
the normal collection mode or in “rapid collect” mode at a rate of five
scans per second. In situ IR experiments typically used 5−10 mg of
sample in dissolved in ∼1.0−1.5 mL of CH2Cl2. The compounds
trans-[W(N2)2(dppe)(PEtNMePEt)],15 [4-BrPhNH3][OTf],37 and
[Et3NH][BPh4]38 were synthesized by previously reported procedures.
Elemental analysis was performed by Atlantic Microlabs, Norcross,
GA.
[W(15N15NH2)(F)(dppe)(PEtNMe(H)PEt)][BF4]2 ([1(15N15NH2)(F)-
1
(NH)][BF4]2). H NMR (CD2Cl2, −35 °C): δ 5.63 (d, JHN = 80 Hz,
NNH2). 15N{1H} NMR (CD2Cl2): δ −80.0 (s, 1N, W-NNH2), −45.7
(s, 1N, W−NNH2).
Neat HOTf (3.4 μL, 3.9 × 10−2 mmol) was injected into a ca. 1 mL
CD2Cl2 solution of trans-[W(15N2)2(dppe)(PEtNMePEt)] (5.5 mg, 6.3
× 10−3 mmol) at −78 °C, resulting in a pale yellow solution. After the
entire length of the NMR tube was cooled, the contents were mixed by
inversion before promptly inserting into a −40 °C NMR probe. A
mixture of products was observed, including W hydrazido, a seven-
coordinate W hydride with a protonated pendant amine, and a minor
product that is unassigned.
trans-[W(15N2)2(H)(dppe)(PEtNMe(H)PEt)][OTf]2 ([3(15N2)2(NH)]-
Et2P(CH2)3PEt2 (depp). A 250 mL Schlenk flask was charged with
1.5 g (17 mmol) of diethylphosphine dissolved in 20 mL pentane
followed by 10 mL of 1.6 M n-BuLi in hexanes. After 1 h, the solvent
was removed under reduced pressure, affording a white solid. The
solids were dissolved in 100 mL of THF and cooled to −78 °C before
adding 0.84 mL (1.7 g, 8.3 mmol) of 1,3-dichloropropane. The
reaction mixture was warmed to 21 °C and stirred for 90 min. The
solvent was removed under reduced pressure, affording a cloudy oil.
1
[OTf]2). H NMR (CD2Cl2, −40 °C): δ −4.32 (dddd, 1H, JHP = 85,
1
76, 12, 12 Hz, W-H); due to multiple overlapping H resonances for
the mixture of [3(15N2)2(NH)][OTf]2 and [1(15N15NH2)(OTf)-
1
(NH)][OTf]2, the resonances in the aliphatic region of the H NMR
spectrum were difficult to assign. 31P{1H} NMR (CD2Cl2, −40 °C): δ
63.7 (ddd, 1P, JPP = 47, 32, and 20 Hz, dppe), 43.6 (ddd, 1P, JPP = 70,
31, and 31 Hz, unassigned), 40.1 (ddd, 1P, JPP = 47, 32, and 31 Hz,
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dppe), 8.1 (ddd, 1P, JPP = 70, 57, and 12 Hz, unassigned), 0.2 (ddd,
1P, JPP = 59, 47, and 20 Hz, PEtNMe(H)PEt), −13.6 (ddd, 1P, JPP = 59,
47, and 31 Hz, PEtNMe(H)PEt). 15N{1H} NMR (CD2Cl2, −40 °C): δ
−44 (s, W−NN), −81 (s, W-NN). This protonated product is
thermally sensitive and does not persist at ambient temperature.
(CH3CH2P of depp), 18.4 (EtPCH2CH2CH2 of depp), 8.8 (CH3CH2P
of depp).
[W(NND2)(OTf)(dppe)(depp)][OTf]. This compound was pre-
pared by the same method as above with DOTf. IR (KBr, 21 °C,
NND2): 2506 (w, N-D), 2386 (w, N-D), 2325 (w) cm−1.
[W(NNH2)(F)(dppe)(depp)][BF4] ([2(NNH2)(F)][BF4]). Neat
HBF4·Et2O (1.5 μL, 1.1 × 10−2 mmol) was injected into a CD2Cl2
solution of trans-[W(N2)2(dppe)(depp)] (2.8 mg, 3.3 × 10−3 mmol)
at −78 °C. The NMR tube was mixed by inversion before promptly
trans-[W(N2 )2 (H)(dppe )(P E t 1 5 N )M e (H)PE t ][OTf]2
(
([3(15N2)2(15NH)][OTf]2). 15N{1H} NMR (CD2Cl2, −40 °C): δ −341
1
(s). H-coupled 15N NMR (CD2Cl2, −40 °C): δ −341 (d, JNH = 78
Hz).
1
trans-[W(15N15NH2)(OTf)(dppe)(PEtNMe(H)PEt)][OTf]2
([1(15N15NH2)(OTf)(NH)][OTf]2). 1H NMR (CD2Cl2, −40 °C): δ 6.54
(d, JHN = 89 Hz, 15N15NH2). 31P{1H} NMR (CD2Cl2, −40 °C): δ 35.4
(m, JPP = 127, dppe), 2.7 (m, JPP = 127 Hz, PNP). 15N{1H} NMR
(CD2Cl2, −40 °C): δ −47 (s, W−NNH2), −80 (s, 1N, W-NNH2).
This protonated product is thermally sensitive and does not persist at
ambient temperature.
being inserted into a NMR probe maintained at −40 °C. H NMR
(CD2Cl2, −40 °C): δ 5.24 (bs, NNH2). 31P{1H} NMR (CD2Cl2, −40
°C): δ 36.7 (dd, 2P, JPP = 143 Hz, JPF = 49 Hz), −2.1 (dd, 2P, JPP
=
−
143 Hz, JPF = 41 Hz). 19F NMR (CD2Cl2, −40 °C): δ −150 (bs, BF4 )
−185 (tt, JFP = 41 Hz, JFP = 49 Hz, W−F).
cis-[W(13CO)2(dppe)(PEtNMePEt)] (cis-[1(13CO)2]). A Teflon-
valved NMR tube was loaded with trans-[W(N2)2(dppe)(PEtNMePEt)]
(9.0 mg, 0.010 mmol) dissolved in ca. 1 mL THF-d8, affording an
orange solution. The tube was degassed and charged with 1 atm of
13CO and mixed by repeated inversion for 37 h, resulting in >98%
conversion to the desired product15 as a yellow solution (see the
Supporting Information). 13C{1H} NMR (THF-d8, 21 °C): δ (13CO)
221.0 (ddddd with 183W satellites JPW = 154 Hz, 1C, JCP = 21.0, 9.5,
6.0, 5.4 Hz, JCC = 4.2 Hz), 218.7 (ddddd with 183W satellites JPW = 146
Hz, 1C, JCP = 28.0, 8.0, 8.0, 5.5 Hz, JCC = 4.2 Hz). 31P{1H} NMR
(THF-d8, 21 °C): δ 46.8 (ddddd with 183W satellites JPW = 281 Hz, 1P,
JPP = 74.3, 18.5, 8.7 Hz, JPC = 8.0, 5.4 Hz), 39.8 (ddddd with 183W
satellites JPW = 219 Hz, 1P, JPP = 18.5, 18.5, 8.7 Hz, JPC = 28.0, 6.0 Hz),
−11.2 (ddddd with 183W satellites JPW = 274 Hz, 1P, JPP = 74.3, 27.9,
1
trans-[W(15N2)2(H)(dppe)(PEtNMePEt)][OTf] ([3(15N2)2][OTf]). H
NMR (CD2Cl2, −30 °C): δ 7.63−7.22 (m, 20H, PPh2), 2.94 (s, 2H,
PCH2N), 2.84 (s, 2H, PCH2N), 2.56−2.32 (m, 4H, PCH2CH2P), 2.34
(s, 3H, NCH3), 1.90 (m, 4H, PCH2N), 1.54−1.30 (m, 4H, PCH2N),
0.93 (dt, 6H, Jd = 15.2 Hz, Jt = 7.3 Hz, CH2CH3), 0.77 (dt, 6H, Jd =
13.8, Hz, Jt = 7.3 Hz, CH2CH3), −3.77 (tt, 1H, Jt = 77.3 Hz, Jt = 11.0
Hz, W−H). 31P{1H} NMR (CD2Cl2, −30 °C): δ 61.0 (ddd with 183
W
satellites JPW = 183 Hz, 1P, JPP = 45, 33, and 17 Hz, dppe), 34.7 (ddd
with 183W satellites JPW = 231 Hz, 1P, JPP = 49, 33, and 33 Hz, dppe),
−15.6 (ddd with 183W satellites JPW = 170 Hz, 1P, JPP = 59, 49, and 17
Hz, PEtNMeP), −29.3 (ddd with 183W satellites JPW = 215 Hz, 1P, JPP
=
59, 45, and 33 Hz, PEtNMeP). 15N{1H} NMR (CD2Cl2, −30 °C): δ
−47.3 (d, JNN = 4 Hz, 1N, W−NN), −75.4 (d, JNN = 2 Hz with JNW
= 49 Hz 183W satellites, 1N, W-NN). This protonated product is
thermally sensitive and does not persist at ambient temperature.
trans-[W(N2)2(dppe)(depp)] (2(N2)2). To a 100 mL round-
bottom flask with a large stir bar were placed W(dppe)Cl4 (0.320 g,
0.442 mmol), depp (0.102 g, 0.464 mmol), magnesium powder (1.0
g), and 50 mL of THF, and the mixture was stirred vigorously for 72 h
under an atmosphere of N2. The solution was filtered to remove Mg
solids and the solvent removed under reduced pressure. The product
was extracted with pentane (8 × 60 mL). The remaining orange-
brown solid (crude yield 0.26 g) was stirred in pentane (1 L) and
filtered, leaving behind a tan solid. The pentane was removed under
reduced pressure, affording a bright orange solid. This material was
recrystallized from a pentane/THF solution (10/1) at −20 °C and
dried under vacuum, resulting in a bright orange solid (31 mg, 8%). 1H
NMR (CD2Cl2): δ 7.47−7.25 (m, 20H, P(C6H5)), 2.14 (m, 4H,
Ph2PCH2), 1.94 (m, 4H, Et2P(CH2)CH2), 1.85 (m, 2H, Et2P(CH2)-
CH2), 1.83 (m, 4H, PCH2CH3), 1.63 (m, 4H, PCH2CH3), 0.69 (dt,
12H, PCH2CH3). 31P{1H} NMR (CD2Cl2): δ 44.6 (s (br), 2P, dppe),
−16.8 (s (br), 2P, depp). 31P{1H} NMR (THF-d8): δ 43.9 (m, 2P,
dppe), −16.3 (m, 2P, depp). IR (KBr, νNN (cm−1)): 1919 (s, asym),
1981 (w, asym). Anal. Calcd for C37H50N4P4W: C, 51.76; H, 5.87; N,
6.53. Found: C, 51.63; H, 5.78; N, 6.49.
18.5 Hz, JPC = 9.5, 5.5 Hz), −25.5 (ddddd with 183W satellites JPW
=
202 Hz, 1P, JPP = 27.9, 18.5, 18.5 Hz, JPC = 21.0, 8.0 Hz). IR (KBr,
νCO, cm−1): 1792 (s, sym), 1729 (s, asym).
trans-[W(13CO)2(dppe)(PEtNMePEt)] (trans-[1(13CO)2]). 13C{1H}
NMR (THF-d8, 21 °C): δ (13CO) 213.2 (bm with 183W satellites JPW
=
123 Hz, 31P{1H} NMR (THF-d8, 21 °C): δ 53.6 (m with 183W
satellites JPW = 287 Hz, 2P), −10.6 (m with 183W satellites JPW = 315
Hz, 2P). IR (KBr, νCO, cm−1): 1785 (s, asym).
trans-[W(CO)2(H)(dppe)(PEtNMe(H)PEt)][OTf]2 ([3(CO)2(NH)]-
[OTf]2). p-Bromoanilinium triflate (10.3 mg, 0.0320 mmol) was
added to the above solution of cis-[W(CO)2(dppe)(PEtNMePEt)] at 21
°C and agitated, affording a pale yellow solution. 31P{1H} NMR
(THF-d8, 21 °C): δ 60.7 (bm, 1P, J = 27 Hz), 43.6 (m with 183W
satellites JPW = 201 Hz, 1P, J = 30 Hz), −11.6 (bs, 1P), −23.4 (bs, 1P).
1H NMR (THF-d8, 21 °C): δ 7.74−6.95 (m, 20H, PPh2), 2.96 (s, 2H,
PCH2N), 2.94 (s, 2H, PCH2N), 2.67−2.54 (m, 2H, PCH2CH2P),
2.33−2.20 (m, 2H, PCH2CH2P), 2.37 (s, 3H, NCH3), 2.06−1.95 (m,
4H, PCH2CH3), 1.44−1.33 (m, 4H, PCH2CH3), 0.93 (dt, 6H, JHP
=
15.7 Hz, Jt = 7.2 Hz, PCH2CH3), 0.76 (dt, 6H, JHP = 14.4 Hz, Jt = 7.2
Hz, PCH2CH3), −5.18 (dddd, 1H, Jt = 78.0 Hz, Jt = 13.8 Hz, W-H).
IR (KBr, νCO, cm−1): 1957 (w, sym), 1851 (s, asym).
trans-[W(CO)2(H)(dppe)(PEt(15N)(H)MePEt)][OTf]2. 15N{1H}
NMR (THF-d8, 21 °C): δ −338 (s, 15NCH3). Proton-coupled 15N
NMR (THF-d8, 21 °C): δ −338 (d, JNH = 76 Hz, 15NCH3).
Regeneration of cis-[W(CO)2(dppe)(PEtNMePEt)] with Et3N. The
above solution containing [3(CO)2(NH)][OTf]2 was degassed and
placed under an atmosphere of 15N2 in a septum-capped NMR tube.
Triethylamine (14 μL, 0.10 mmol) was injected into the NMR tube at
room temperature, resulting in immediate darkening of the yellow
color (see the Supporting Information). The NMR spectra match
those of cis-[W(CO)2(dppe)(PEtNMePEt)].
[W(NNH2)(OTf)(dppe)(depp)][OTf] ([2(NNH2)(OTf)][OTf]). In a
septum-capped NMR tube, 6.2 mg (7.2 × 10−3 mmol) trans-
[W(N2)2(dppe)(depp)] was dissolved in 1 mL of CD2Cl2, cooled to
−78 °C, and treated with 2.0 μL (2.2 × 10−2 mmol) of HOTf. The
contents were mixed, and the tube was returned to the cold bath for an
additional 10 min before inserting the sample into an NMR
spectrometer that was precooled to −40 °C. NMR spectral data
collected at −40 °C are consistent with the formation of [2(NNH2)-
Protonolysis Procedure and Quantification of Ammonium.
A known amount of complex 1(N2)2 or 2(N2)2 was dissolved in 10
mL of dichloromethane, resuling in a orange solution. While the
solution was stirred, triflic acid (100 equiv) was added, resulting in a
pale orange solution for 1(N2)2 and a light green solution for 2(N2)2.
After the mixture was stirred vigorously for 20 h, it was frozen with
liquid nitrogen and a solution of KOtBu (200 equiv) in THF/MeOH
(2 mL/8 mL) was added. The solution was stirred vigorously while it
was warmed to room temperature. The volatiles were distilled under
vacuum into a flask submerged in liquid nitrogen which contained 4
mL of a 2 M aqueous HCl solution. The reaction mixtures were
1
(OTf)][OTf]; however, the H NMR resonances were broad at this
1
temperature. H NMR (CD2Cl2, 22 °C): δ 8.94 (bs, NNH2), 7.52−
7.38 (m, 20H, PPh2), 2.80 (m, 4H, Ph2PCH2CH2Ph2), 2.44 and 2.25
(m, 4H, Et2PCH2CH2CH2PEt2), 2.43 and 1.85 (m, 2H,
Et2PCH2CH2CH2PEt2), 2.00, 1.72, 1.85, 1.71 (m, 8H, CH3CH2P),
0.90 (m, 12H, CH3CH2P). 31P{1H} NMR (CD2Cl2, 22 °C): δ 31.5
(m, 2P, dppe), −14.1 (m, 2P, depp). 19F NMR (CD2Cl2, 22 °C): δ
−76 (s, W-OTf). 13C{1H} NMR (CD2Cl2, 22 °C): δ 133.4−128.9
(C6H5P of dppe), 31.4 (PhPCH2 of dppe), 23.5 (CH3CH2P of depp),
23.4 (CH3CH2P of depp), 20.4 (EtPCH2CH2CH2 of depp), 19.8
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analyzed for NH3 and N2H4 using the indophenol method40 and p-
(dimethylamino)benzaldehyde reagent,41 respectively. Table S1 in the
Supporting Information contains the compiled results of NH3
quantification. The following elemental analysis results were obtained
for the microcrystalline solid of 1(N2)2 used in the protonolysis
reactions for NH3 production: Anal. Calcd for C37H51N5P4W: C,
50.87; H, 5.88; N, 8.02. Found: C, 50.63; H, 5.98; N, 8.19.
Dr. Daniel L. DuBois and Dr. Roger Rousseau for helpful
discussions.
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For the H NMR experiments comparing the formation of NH4
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ASSOCIATED CONTENT
* Supporting Information
■
S
Figures, tables, and a CIF file giving NMR and IR spectral data,
crystallographic information, and Cartesian coordinates for
computed molecules. This material is available free of charge
via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail for M.T.M.: Michael.Mock@pnnl.gov.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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Commun. 2011, 47, 12212−12214. (b) Mock, M. T.; Chen, S.;
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This work was supported as part of the Center for Molecular
Electrocatalysis, an Energy Frontier Research Center funded by
the U.S. Department of Energy Office of Science, Office of
Basic Energy Sciences. Computational resources were provided
by the National Energy Research Scientific Computing Center
(NERSC) at Lawrence Berkeley National Laboratory. Pacific
Northwest National Laboratory is operated by Battelle for the
DOE. We also thank Dr. Molly O’Hagan and Dr. Elliott B.
Hulley for their assistance with NMR experiments. We thank
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