Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond Using a "bulky Hydrogen Atom" Surrogate: Analogies to PCET

Article abstract of DOI:10.1021/jacs.8b09063

Recent computational studies suggest that the phosphate support in the commercial vanadium phosphate oxide (VPO) catalyst may play a critical role in initiating butane C-H bond activation through a mechanism termed reduction-coupled oxo activation (ROA) similar to proton-coupled electron transfer (PCET); however, no experimental evidence exists to support this mechanism. Herein, we present molecular model compounds, (Ph₂N)₃V=N-P(O)Ar₂ (Ar = C₆F₅ (2a), Ph (2b)), which are reactive to both weak H atom donors and a Me₃Si^? (a "bulky hydrogen atom" surrogate) donor, 1,4-bis(trimethylsilyl)pyrazine. While the former reaction led to product decomposition, the latter resulted in the isolation of the reduced, silylated complexes (Ph₂N)₃V-N=P(OSiMe₃)Ar₂ (3a/b). Detailed analyses of possible reaction pathways, involving the isolation and full characterization of potential stepwise square-scheme intermediates, as well as the determination of minimum experimentally and computationally derived thermochemical values, are described. We find that stepwise electron transfer (ET) + silylium transfer (ST) or concerted EST mechanisms are most likely. This study provides the first experimental evidence supporting a ROA mechanism and may inform future studies in homogeneous or heterogeneous C-H activation chemistry, as well as open up a possible new avenue for main group/transition metal cooperative redox reactivity.

Full text of DOI:10.1021/jacs.8b09063

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Probing Hydrogen Atom Transfer at a Phosphorus(V) Oxide Bond
Using a “Bulky Hydrogen Atom” Surrogate: Analogies to PCET
Jiaxiang Chu,†,§ Timothy G. Carroll,†,§ Guang Wu,† Joshua Telser,‡ Roman Dobrovetsky,*,∥
,†
́
and Gabriel Menard*
†Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
‡Department of Biological, Chemical, and Physical Sciences, Roosevelt University, Chicago, Illinois 60605, United States
∥School of Chemistry, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 69978, Israel
S
* Supporting Information
ABSTRACT: Recent computational studies suggest that the phosphate
support in the commercial vanadium phosphate oxide (VPO) catalyst may
play a critical role in initiating butane C−H bond activation through a
mechanism termed reduction-coupled oxo activation (ROA) similar to
proton-coupled electron transfer (PCET); however, no experimental evidence
exists to support this mechanism. Herein, we present molecular model
compounds, (Ph2N)3VN−P(O)Ar2 (Ar = C6F5 (2a), Ph (2b)), which are
reactive to both weak H atom donors and a Me3Si• (a “bulky hydrogen atom”
surrogate) donor, 1,4-bis(trimethylsilyl)pyrazine. While the former reaction
led to product decomposition, the latter resulted in the isolation of the
reduced, silylated complexes (Ph2N)3V−NP(OSiMe3)Ar2 (3a/b). Detailed
analyses of possible reaction pathways, involving the isolation and full
characterization of potential stepwise square-scheme intermediates, as well as
the determination of minimum experimentally and computationally derived
thermochemical values, are described. We find that stepwise electron transfer (ET) + silylium transfer (ST) or concerted EST
mechanisms are most likely. This study provides the first experimental evidence supporting a ROA mechanism and may inform
future studies in homogeneous or heterogeneous C−H activation chemistry, as well as open up a possible new avenue for main
group/transition metal cooperative redox reactivity.
study invoked the PO4 support in any reactive step.4,9−13
3−
INTRODUCTION
■
However, recent computational studies by Goddard and co-
Catalyst support systems typically involve unreactive main
workers have found that PO43− may be involved in the key C−
group oxides, such as silica (SiO2), alumina (Al2O3), and
H activation chemistry. They proposed that inert C−H
cleavage at butane occurs at a terminal PVO bond, resulting
in its protonation with concurrent reduction of a neighboring
VV (Scheme 1a) as part of a mechanism coined reduction-
coupled oxo activation (ROA).14−16 It is worth noting the
similarities between the ROA and well-studied proton-coupled
electron transfer (PCET) reactions,17−19 particularly in
molecules involving well-separated metal and basic sites,
which can nonetheless engage in concerted PCET.20−22 In
contrast to PCET, the proposed ROA mechanism as applied to
VPO has yet to be supported by any experimental evidence.
To investigate the proposed noninnocent role of terminal
PVO bonds in VPO, we have recently reported the synthesis
and preliminary reactivity of homogeneous mono- and
multimetallic vanadium phosphinate complexes bearing VIII−
O−PVO linkages.23,24 These complexes were unreactive to
common H atom donors (HADs), perhaps a result of their
phosphate (PO43−), supporting the catalytically active metal
centers. Common supported commercial catalysts include
(support in parentheses) the Phillips Cr catalyst for ethylene
polymerization (SiO2); the Haber−Bosch Fe catalysts for N2
reduction to NH3 (Al2O3 and others); the hydrodesulfuriza-
tion catalysts for petroleum processing (Al2O3); and the
vanadium phosphate oxide (VPO) catalyst for the partial
oxidation of butane to maleic anhydride (PO43−).1−4 Supports
are typically assumed to act as high-surface platforms capable
of binding catalytically active centers, while playing little role in
the catalytic processes themselves. However, studies have
shown that supports may indeed be active in catalysis through
various mechanisms, such as spillover effects, strong metal−
support interactions, and more,5−8 thus requiring a rethinking
of the once accepted dogma.
For decades, the mechanism for the conversion of butane to
maleic anhydride using the VPO catalyst has been the subject
of extensive experimental studies. Key mechanistic steps,
including activation of the relatively inert butane C−H bonds,
were thought to occur solely at high-valent V-oxo centers. No
Received: August 22, 2018
© XXXX American Chemical Society
A
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Scheme 1. (a) Proposed ROA (PCET) Mechanism for the
C−H Activation of Butane at a PVO Bond in VPO; (b)
Present Work Highlighting the Reaction of Molecular VPO
Analogues (2a, 2b) with H• and TMS• Donors
Figure 1. Solid state XRD structure of 2a (C, black; N, blue; V,
purple; P, orange; O, red; F, violet). Phenyl groups (excluding ipso
carbons) are omitted for clarity.
(1.661(7) Å) relative to 1 (1.582(3) Å),28 both of which are
also nearly identical to the N1−P1 bonds in 2a (1.661(11) Å)
and 2b (1.674(7) Å). Combined with the near linear V1−N1−
P1 angles (2a: 175.9(7)°; 2b: 173.1(5)°), this suggests a
delocalized π manifold.
relatively low VIII oxidation states.10 Herein, we describe the
synthesis of high-valent VV complexes (2a, 2b) bearing VV
N−PVO linkages as molecular VPO models and further
probe their reactivity with both HADs and a trimethylsilyl
radical (Me3Si• = TMS•) donor (Scheme 1b). The latter
involved the formation of a reduced product with a silylated
PO bond, following a pathway analogous to ROA (Scheme
1a). As will be outlined, we describe TMS• as a “bulky
hydrogen atom”similar to the use of TMS+ as a “bulky
proton”25−27and together, these results describe the first
direct experimental evidence supporting the proposed ROA
mechanism.
Exposure of 2a and 2b to common HADs with various
element−H bond dissociation free energies (BDFEsolvent), such
as 2,4,6-tri-tert-butylphenol (O−H BDFEMeCN = 77.1 kcal/
mol), 1,4-cyclohexadiene (C−H BDFEtol = 72.6 kcal/mol),
and 2,2,6,6-tetramethylpiperidine-N-hydroxyl (TEMPO-H)
(O−H BDFEtol = 65.2 kcal/mol) in benzene resulted in no
1
appreciable reaction as determined by both H NMR and X-
band EPR spectroscopies of the reaction solutions.17,31 In
contrast, exposure of 2a to Cp(CO)3Cr−H (BDFEMeCN = 57.3
kcal/mol) (Cp = η5-C5H5)32−34 in C6D6 at room temperature
resulted in the quantitative appearance of the protonated
amide, Ph2NH, as well as a half-equivalent of the Cr dimer
1
product, [Cp(CO)3Cr]2, as observed by H NMR spectrosco-
py (Figure S29).35 The analogous reaction with the deuterium
RESULTS AND DISCUSSION
■
isotopomer, Cp(CO)3Cr−D,34,36 resulted in the formation of
The target complexes (2a, 2b) were synthesized by salt
metathesis of the reported28 vanadium nitride, (Ph2N)3V(μ-
N)Li(THF)3 (1), with diaryl phosphinic chlorides, (C6F5)2P-
(O)Cl29 or Ph2P(O)Cl, resulting in the isolation of 2a or 2b,
respectively, following purification (Scheme 2).
2
Ph2ND as observed by H NMR spectroscopy, yet we were
unable to quantify the exact amount (Figure S33). No reaction
was observed between any of these HADs and the silane-
substituted complex, (Ph2N)3VNSiMe3the precursor to
128 (Scheme 2)after several days at room temperature.
Together, these data may support a possible reaction pathway
involving the formal transfer of H• to the terminal PVO
bond with concurrent reduction of VV to VIV, following a
ROA/PCET pathway (Scheme 1).14−16 However, the putative
PO−H bond would be incompatible with the basic Ph2N−
ligands, resulting in an intramolecular acid−base reaction.
While the byproducts of this reaction remain unknown,
analysis of the reaction mixture by EPR spectroscopy revealed
diagnostic eight-line hyperfine patterns consistent with VIV
byproducts (Scheme 1b; Figure S30).
Due to the difficulty in determining the reactive site in 2a
with the Cp(CO)3Cr−H HAD, we explored whether TMS•
could be used as a “bulky hydrogen atom” surrogate,
generating more stable, isolable products. We used the
antiaromatic 1,4-bis(trimethylsilyl)pyrazine (TMS2-pyz) com-
pound as our TMS• source.37,38 Mashima and co-workers have
recently demonstrated the salt-free reduction of transition
metal chloride complexes with TMS2-pyz involving the formal
transfer of 2 equiv of TMS•, reduction of the metal centers,
and production of pyrazine and 2 equiv of TMSCl.39−41
Exposure of 2 equiv of 2a or 2b to an equivalent of TMS2-pyz
Scheme 2. Synthesis of Complexes 2a and 2b
Both VV complexes displayed significantly shifted, broad-
ened 51V NMR resonances at 117 ppm (2a) and −6 ppm (2b)
relative to 1 (−217 ppm), consistent with an increasingly
deshielded V center. In contrast, the 31P NMR resonances
followed the opposite trend, with broad signals (due to
coupling to the quadrupolar (I = 7/2) 51V) at −20 and 19
ppm, respectively, both shifted ∼20 ppm upfield relative to the
starting materials. Single crystals suitable for X-ray diffraction
(XRD)30 studies were grown from pentane/ether (2a, Figure
1) or ether (2b, Figure S45). Pertinent bond lengths and
angles are compiled in Tables S1 and S2. Notable are the
elongated V1−N1 bonds in 2a (1.662(11) Å) and 2b
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resulted in the formation of pyrazine and the disappearance of
from stepwise (ex. PT + ET) processes is performed by
comparing the thermochemical values of the stepwise ground
state free energy changes, ΔG°ET and ΔG°PT, to the reaction
free energy barrier, ΔG⧧. A concerted process is assumed when
ΔG⧧ is lower than both stepwise free energy change values.17,45
In order to elucidate the mechanism of the transformation
from 2 to 3, we employed a similar strategy of dissecting and
studying the individual steps of ET and silylium transfer (ST)
as part of a square scheme (Scheme 4).17
the diamagnetic resonances for 2a and 2b as observed by
1
multinuclear (51V, 31P, 19F, H) NMR spectroscopy. Single
crystals suitable for XRD studies were grown for both
reactions, and the solid state structures confirmed the
formation of the reduced, silylated products, (Ph2N)3VIV−
NP(OTMS)Ar2 (Scheme 3) (3a, Ar = C6F5, Figure 2; 3b,
Ar = Ph, Figure S48).
Scheme 3. Reaction of 2a or 2b with TMS2-pyz to Generate
3a or 3b
Scheme 4. Square Scheme for the Stepwise Conversion of 2
to 3 via ST + ET or ET + ST Pathways
To distinguish a concerted from a stepwise process, the
thermodynamic parameters ΔG°ST(2 → 2-Si+), ΔG°ET(2 →
2−), and ΔG⧧ are required (Scheme 4, eqs 1 and 2).17 We
sought to determine approximate values for these parameters
by combining experimental and computational results.
ΔG°ST = −RT ln(Krel)
= −1.37(kcal·mol−1)×[pK(2−Si+)
− pK(TMS2‐pyz)]
Figure 2. Solid state XRD structure of 3a (C, black; N, blue; V,
purple; P, orange; O, red; F, violet; Si, pink). Phenyl groups
(excluding ipso carbons) are omitted for clarity.
(1)
ΔG°ET = −FE°
In contrast to 2a, 3a contains a single V1−N1 (1.837(2) Å)
bond and a P1−N1 double bond (1.535(2) Å), with a
resulting bent V1−N1−P1 angle (145.90(13)°). The P1−O1
bond is also elongated (1.5637(16) Å) relative to 2a (1.484(8)
Å), consistent with more single-bond character.42 Similar bond
metric differences are observed between 2b and 3b (Tables S1
and S2). X-band EPR spectra in tetrahydrofuran (THF) were
collected for 3a and 3b, and both contain similar spectral
features, including expected eight-line splitting patterns at
room temperature due to hyperfine coupling of the d1 electron
to the 51V center (I = 7/2, ∼100% abundance). Modeling
these, as well as the anisotropic 100 K spectra, suggests that the
single unpaired electron is minimally delocalized and rests
primarily at the V center (Figures S10−13).43,44
= −23.06(kcal·mol−1·V−1)×[E°
(20/−) − E°(TMS2‐pyz+/0)]
(2)
Initial ST was first investigated (Scheme 4). Exposure of 2a
and 2b to 1 equiv of [TMS][OTf] resulted in a reaction only
with 2b (OTf = −OSO2CF3 = triflate).46 The new complex
(2b-Si+) was fully characterized and revealed a significantly
downfield shifted 51V resonance at 229 ppm vs −6 ppm for 2b.
A low-resolution solid state XRD structure (Figure S47)
revealed a near-linear V1−N1−P1 linkage in 2b-Si+
(174.3(10)°) similar to 2b (173.1(5)°, Table S2). The
unreactivity of 2a with an equivalent of [TMS][OTf] is likely
the result of its poor Lewis basicity (vide infra). However,
silylation was possible using the silylium cation, [Et3Si][B-
(C6F5)4].47 The new complex (2a-Si+) was fully characterized,
including by XRD studies, and revealed the expected PO−
SiEt3 linkage and corresponding [B(C6F5)4]− counteranion
(Figure S44). Bond metrics are also similar to those found in
2a (Table S1). Both complexes (2a/b-Si+) could be readily
The formal transfer of TMS• from TMS2-pyz to 2 generating
3 represents what we believe is the clearest example yet of
reactivity at a terminal PVO bond mimicking the proposed
ROA/PCET mechanism (Scheme 1). We next wanted to
probe the mechanism of this transformation. For PCET
processes, distinguishing concerted (ex. H• or H+/e− transfer)
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reduced with CoCp2 or CoCp*2 (Cp* = η5-C5Me5),
generating 3a′ (where R = Et) or 3b (Scheme 4), respectively,
and closing the ST + ET pathways. The formation of both
products was confirmed by EPR spectroscopy (Figures S27
and S28).
To obtain estimated ΔG°ST values (eq 1), relative pK values
for 2a-Si+ and 2b-Si+ were needed. We define pK as the extent
of TMS+ dissociation from the Lewis basic (LB) PO bonds
(eq 3), in analogy to Brønsted acids (Krel is the equilibrium
constant relative to an arbitrary LB′). Previously reported Krel
valuesdetermined by NMR spectroscopy and set against N-
methylpyridone (NMP) as LB′ (eq 3)for pyridine (pyr),
triphenylphosphine oxide (TPPO), and hexamethylphosphor-
amide (HMPA) were 10, 9.1 × 10−2, and 1.0 × 10−4,
respectively, in dichloromethane (DCM).48 The correspond-
ing pKDCM values for the LB-TMS+ cations are −1.0 (pyr),
1.04 (TPPO), and 4.00 (HMPA), consistent with an expected,
more highly Lewis basic HMPA vs TPPO and pyr.
Figure 3. DFT-calculated reaction coordinates for the stepwise
conversion of 2a/b to 3a/b via ST + ET or ET + ST pathways. Gibbs
free energies are given in kcal/mol relative to the starting materials.
For more information, see the Supporting Information.
Krel
LB‐TMS+ + LB′ HooI LB + LB′‐TMS+
(2a/b‐Si+)
(2a/b)
(3)
We performed similar competition reactions48 and moni-
tored each by NMR spectroscopy (31P, 51V). Exposing 2a or
2b in a 1:1 ratio to [TPPO-TMS][OTf]49 in DCM revealed
broadened 51V NMR resonances centered around 2a or 2b-Si+,
respectively (Figures S35 and S37), with corresponding 31P
NMR spectra matching these assignments (Figures S36 and
S38). Taken together, a relative Lewis basicity of 2a < TPPO <
2b can be established. To bracket an upper basicity limit for
2b, we exposed 2b-Si+ to HMPA. The NMR spectra (51V, 31P)
revealed resonances centered around those for 2b and
[HMPA-TMS]+ with the latter confirmed by in situ generation
of [HMPA-TMS][OTf] (Figures S39 and S40). We also note
that no reaction occurred between HMPA and TMS2-pyz, as
very downhill at −83.47 kcal/mol (2a) and −74.81 kcal/mol
(2b), resulting in overall ΔG°rxn values of −18.66 and −21.91
kcal/mol, respectively (Figure 3). Together, these data suggest
that a stepwise mechanism involving initial ST is unlikely
(Scheme 4).
The alternative stepwise process involving initial ET was also
probed experimentally and computationally (Scheme 4, Figure
3). Both 2a and 2b were analyzed by cyclic voltammetry (CV)
in DCM, and each revealed quasi-reversible VIV/VV couples at
E1/2 = −0.99 V and −1.26 V, respectively, versus the
ferrocene/ferrocenium (Fc/Fc+) couple (Figure S49). The
more oxidizing 2a is consistent with the enhanced electron-
withdrawing ability of the C6F5 substituents. Chemical
isolation of the reduced forms, 2a− and 2b−, was possible
using CoCp*2 (E1/2 = −1.94 V vs Fc/Fc+)56 (Scheme 4), and
both were structurally characterized (Figures S43 and S46).
Bond metrics for 2a− revealed a bent V1−N1−P1
(145.3(10)°) fragment similar to 3a (145.90(13)°; Figure 2)
and shortened P1−N1 (1.551(14) Å) and P1−O1 (1.455(12)
Å) bonds relative to 2a with similar trends observed in 2b−
(Table S2). Both complexes were analyzed by EPR spectros-
copy at room temperature and 100 K. The spectra displayed
similar simulation parameters to 3a and 3b (Figures S14−
S17). The stepwise ET + ST sequence was closed using
[TMS][OTf] (Scheme 4). Treatment of 2a− or 2b− with
[TMS][OTf] cleanly generated the product 3a or 3b,
respectively, following removal of the [Cp*2Co][OTf] by-
product. We note that, in contrast to 2a, 2a− reacts with
[TMS][OTf] (vide supra), consistent with an expected
increase in basicity from 2a → 2a− upon ET, similar to what
is commonly observed in stepwise ET + PT studies.45
observed by H and 31P NMR spectroscopy. To bracket an
1
upper basicity limit for 2a, we exposed 2a-Si+ to an equivalent
of pyr. In this case, 2a was regenerated (Figures S41 and S42).
Together, an overall basicity trend of 2a < pyr < TPPO < 2b <
HMPA < [TMS-pyz]− is assigned with corresponding
experimentally bracketed pKDCM values of 2a-Si+ < −1.0 and
1.04 < 2b-Si+ < 4.00.48 A conservative minimum experimental
ΔG°ST value of ∼7 kcal/mol was extracted based on the
bracketed relative pK values of < −1 for 2a-Si+ and >4 for
TMS2-pyz (eq 1). A similar approach could not be used for 2b-
Si+ due to the possible (yet unlikely) overlapping experimental
pK values for 2b-Si+ (1.04−4.00) and TMS2-pyz (>4).
We note that the above ΔG°ST is an extremely conservative
estimate due to very few published Krel values for TMS+
equilibria.48 In an attempt to extract more precise values, we
turned to density functional theory (DFT). Calculations were
performed using the uwB97XD method,50,51 with Ahlrichs’
def2-SVPP basis set,52 and with the relativistic effect of V,
which was accounted for by the Stuttgart−Dresden effective
core potential.53 We also used the polarizable continuum
model (PCM) to calculate the systems in DCM.54,55 Initial ST
involving 2a/b + TMS2-pyz → 2a/b-Si+ + TMS-pyz− was
calculated, and extracted ΔG°ST values were 64.81 kcal/mol
(2a, Figure 3, blue) and 52.90 kcal/mol (2b, Figure 3, orange),
much higher than the experimental minimum. These results
are consistent with the observed increased basicity from 2a to
2b, reflected by a calculated pK difference of ∼8.7 units (eq 1)
and resulting in a lower ΔG°ST. The following ET steps (2a/b-
Si+ + TMS-pyz− → 3a/b + TMS-pyz•) were calculated to be
Next, experimentally derived ΔG°ET values could be
determined based on the obtained E1/2 values for the 20/−
couples (eq 2). However, we noticed a discrepancy in the
reported E1/2 of the TMS2-pyz+/0 couple (E1/2 = −0.24 V vs
Fc/Fc+ in DCM).39 Using analogous conditions, an E1/2
=
−0.97 V vs Fc/Fc+ was obtained in our hands (Figure S50).
We further corroborated our value using a different reference,
Fe(η5-C5Me5)2 (Fc*), and obtained an E1/2 = −0.39 V vs Fc*/
Fc*+ (Figure S51), corresponding to approximately −0.95 V vs
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Fc/Fc+ (using an E1/2 = −0.56 V for Fc*/Fc*+ vs Fc/Fc+ in
DCM in our hands (Figure S52)).57 Thus, using our E1/2
values for 2a (−0.99 V), 2b (−1.26 V), and TMS2-pyz
(average of −0.96 V), we calculated experimental ΔG°ET
values of 0.7 and 6.9 kcal/mol, respectively, for the stepwise
ET steps with 2a and 2b (eq 2). The corresponding DFT-
calculated values are both slightly exergonic at −4.49 and
−1.76 kcal/mol for 2a and 2b, respectively (Figure 3). The
deviation from experimental values is likely a result of the PCM
solvation model, which provides only a dielectric continuum
and does not take into account the solvent’s molecular
interactions with the calculated molecules. Despite this, the
DFT results follow the same trends as the experimental values
wherein 2a is more prone to initial ET than 2b.
Eco equipped with a −38 °C freezer). Pentane, toluene, benzene,
diethyl ether, tetrahydrofuran, and dichloromethane were dried using
an MBraun solvent purification system. Benzene-d6, bromobenzene-
d5, and tetrahydrofuran-d8 were purchased from Aldrich and dried
over CaH2 for several days prior to distillation. All solvents were
degassed by freeze−pump−thaw and stored on activated 4 Å
molecular sieves prior to use. Ph2NH, nBuLi (1.6 M in hexanes),
i
VCl3(THF)3, Me3SiN3, Pr2NH, Ph2POCl, (C5Me5)2Co, (C5H5)2Co,
Me3SiCl, Me3SiOTf, Et3SiH, [Ph3C][B(C6F5)4], and D2 were
purchased from Aldrich, Strem, or other commercial vendors and
were used as received. (C6F5)2POCl,29 1,4-bis(trimethylsilyl)pyrazine
(TMS2-pyz),39 [Ph3P(OSiMe3)][OTf],49 CpCrH(CO)3,33 [CpCr-
(CO)3]2,35 and 128 were prepared according to literature procedures.
Elemental analyses (C, N, H) were performed at the University of
California, Berkeley, using a PerkinElmer 2400 Series II combustion
analyzer.
As noted earlier, in the PCET literature, concerted PET
processes are assumed when the reaction barrier (ΔG⧧) is
lower than the stepwise ground state free energy changes,
ΔG°ET and ΔG°PT (or ΔG°ST in our case).17,45 In an
analogous fashion, we attempted to obtain kinetic information
by UV−vis spectroscopy for the 2 → 3 transformations;
however, our efforts were hampered by several factors. First,
the reactions appeared to be very fast, even at the
concentrations used (10−5 M). Second, while stopped-flow
methods were attempted, the absorption spectra of 2a/3a and
2b/3b revealed no distinct isosbestic points amenable to clean
kinetic analyses (Figures S55, S58, S59). Moreover, monitoring
the reactions over extended periods of time also revealed the
emergence of Ph2NH (Figure S59), which we ascribe to the
introduction of adventitious water, further complicating our
kinetic analyses. While we are unable to obtain experimental
ΔG⧧ values, our results nonetheless suggest that the stepwise
ST + ET mechanism is highly unlikely (Figure 3), whereas
both stepwise ET + ST or near barrierless concerted EST
pathways are possible for the transformations of 2 → 3
(Scheme 4). The low experimental and computational ΔG°ET
values obtained would, however, render it difficult to
distinguish between the two.
Spectroscopic Analyses. NMR spectra were obtained on a
Varian Unity Inova 500 MHz or Agilent Technologies 400 MHz
spectrometer and referenced to residual solvent or externally (11B:
BF3·Et2O; 19F: CFCl3; 51V: VOCl3; 31P: 85% H3PO4). Chemical shifts
(δ) are recorded in ppm, and the coupling constants are in Hz. X-
band EPR spectra were collected on a Bruker EMX EPR spectrometer
equipped with an Oxford ESR 900 liquid helium cryostat. A
modulation frequency of 100 kHz was used for all EPR spectra, and
the data were plotted using SpinCount. EPR simulations used the
program QPOWA by Belford and co-workers, as modified by J.
Telser.59 UV−vis spectroscopy was performed using a Shimadzu UV-
2401PC spectrophotometer with quartz cuvettes equipped with
airtight J. Young adaptors.
X-ray Crystallography. Data were 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 =
0.710 73 Å). The crystals were mounted on a cryoloop with Paratone-
N oil, and all data were collected at 100(2) 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 performed 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 performed using SHELXTL or
OLEX2.
Electrochemical Analyses. Cyclic voltammetry was performed
on a CH Instruments 630E electrochemical analysis potentiostat,
equipped with a 3 mm diameter glassy carbon working electrode, a Ag
wire pseudoreference electrode, and a Pt counter electrode with
[Bu4N][PF6] (0.1 M) supporting electrolyte solution in CH2Cl2. The
glassy carbon working electrode was cleaned prior to each experiment
by polishing with 1, 0.3, and 0.05 mm alumina (CH Instruments) in
descending order, followed by sonication in distilled water for 2 min.
Background scans were conducted for each experiment in a solution
containing only electrolyte and were then subtracted from each
experiment. All voltammograms were referenced to the Fc/Fc+ redox
couple.
CONCLUSION
■
In summary, we have demonstrated a clear example of
reactivity at a terminal PVO bond mimicking the proposed
ROA/PCET mechanism for VPO catalysis (Scheme 1) using a
TMS• donor (TMS2-pyz) as a “bulky hydrogen atom”
surrogate. With the use of a square scheme and resulting
experimentally and computationally derived thermochemical
values, we have determined that a stepwise ST + ET
mechanism is highly unlikely; however, stepwise ET + ST or
concerted EST pathways are viable. Together, this study has
provided the first experimental evidence supporting the
cooperative metal/main group reactivity of a VVN−PVO
fragment with a TMS• source in direct analogy to overall H•
transfer proposed in the ROA mechanism (Scheme 1a). These
results may benefit future studies in homogeneous or
heterogeneous C−H activation chemistry58 and serve to
highlight the potential for main group/metal cooperative
redox reactivity over multiple bonds.20−22 This, as well as
possible applications of TMS• donors as bulky hydrogen atom
surrogates, is currently being investigated in our laboratory.
Synthesis of (Ph2N)3VNPO(C6F5)2 (2a). In the glovebox, a
solution of (C6F5)2POCl (208 mg, 0.5 mmol) in 3 mL of benzene was
added dropwise to a solution of complex 1 (415 mg, 0.5 mmol) in 10
mL of benzene and briefly shaken. The resulting black solution stood
at room temperature for 1 h. The LiCl precipitate was removed by
filtration over Celite using a fine-porosity filter, and the solvent was
then removed in vacuo. The black residue was washed with cold
pentane (2 × 5 mL) to afford a dark brown solid (400 mg, 0.42 mmol,
84% yield). Dark brown crystals suitable for XRD studies were grown
by slow vapor diffusion of pentane into a concentrated solution of 2a
EXPERIMENTAL SECTION
■
1
General Considerations. All manipulations were performed
under an atmosphere of dry, oxygen-free N2 or Ar by means of
standard Schlenk or glovebox techniques (MBraun UNIlab Pro SP
in diethyl ether at room temperature. H NMR (400 MHz, C6D6, 25
°C): δ = 6.84 (m, 24H; o-ArH and m-ArH), 6.69 (m, 6H; p-ArH). 13C
NMR (100 MHz, C6D6, 25 °C): δ = 154.1, 128.9, 126.0, 123.0
E
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Article
(NPh2). The signal-to-noise ratio was too low for properly identifying
any C6F5 13C resonance. 51V NMR (105 MHz, C6D6, 25 °C): δ =
117.3 (br). 31P NMR (162 MHz, C6D6, 25 °C): δ = −20.0 (br m).
19F NMR (376 MHz, C6D6, 25 °C): δ = −131.6 (br, 4F; o-C6F5),
−148.2 (br, 2F; p-C6F5), −160.3 (br, 4F; m-C6F5). Anal. Calcd for
C48H30F10N4OPV: C, 60.64; H, 3.18; N, 5.89. Found: C, 60.64; H,
3.29; N, 5.91.
stirbar at ambient temperature for 0.5 h. A separate solution of
trimethylsilyl trifluoromethanesulfonate (22.2 mg, 0.1 mmol) in
dichloromethane (1 mL) was added dropwise to the reaction solution.
The resulting dark red solution was again allowed to stir at ambient
temperature for another 0.5 h. The volatiles were removed in vacuo to
give a dark brown residue, which was washed with cold pentane (2 ×
2 mL) to afford a dark brown solid. The product was extracted into
ether (2 × 3 mL) and filtered over a pad of Celite, and then the
solvent was removed in vacuo. The resulting brown solid was washed
with cold pentane (2 mL) to give a fluffy brown powder (48 mg,
0.057 mmol, 57% yield). Anal. Calcd for C51H49N4OPSiV: C, 72.58;
H, 5.85; N, 6.64. Found: C, 71.47; H, 5.56; N, 6.60. Attempts to
obtain satisfactory elemental analysis consistently resulted in reduced
carbon percentages likely due to incomplete combustion.60
Synthesis of (Ph2N)3VNPOPh2 (2b). In the glovebox, a solution
of Ph2POCl (82.8 mg, 0.35 mmol) in 3 mL of THF was added
dropwise to a solution of complex 1 (290 mg, 0.35 mmol) in 10 mL of
THF and briefly shaken. The resulting black solution stood at room
temperature for 1 h before the solvent was removed in vacuo. The
black residue was extracted into benzene (8 mL) and filtered over a
pad of Celite. The filtrate solvent was removed in vacuo, and the
residue was recrystallized from ether at −35 °C over 24 h to afford a
dark brown solid (260 mg, 0.322 mmol, 92% yield). Dark brown
crystals suitable for XRD studies were grown by cooling a
concentrated solution of 2b in ether to −35 °C and standing
overnight. 1H NMR (400 MHz, C6D6, 25 °C): δ = 7.55 (m, 4H; ArH
Synthesis of [(Ph2N)3VNPO(C6F5)2][Cp*2Co] (2a−). In the
glovebox, a solution of Cp*2Co (25.3 mg, 0.0768 mmol) in 2 mL
of fluorobenzene was added dropwise to a solution of complex 2a (73
mg, 0.0768 mmol) in 3 mL of fluorobenzene. The resulting dark red
solution stood at room temperature for 4 h, and the volume was
reduced to about 0.5 mL in vacuo. Pentane (2 mL) was slowly added
dropwise to the concentrated solution to afford an orange-red
precipitate. The precipitate was filtered, washed with pentane (5 mL),
and dried in vacuo to afford an orange-red solid (92 mg, 0.0719 mmol,
94% yield). Dark brown needles suitable for XRD studies were grown
by slow diffusion of pentane into a concentrated solution of 2a− in
fluorobenzene at room temperature. Anal. Calcd for
C68H60CoF10N4OPV: C, 63.80; H, 4.72; N, 4.38. Found: C, 63.56;
H, 4.99; N, 4.13.
3
of Ph2PO), 7.07−6.97 (m, 6H; ArH of Ph2PO), 6.94 (d, JHH = 8.0
3
Hz, 12H; o-ArH of NPh2), 6.89 (t, JHH = 8.0 Hz, 12H; m-ArH of
3
NPh2), 6.77 (t, JHH = 7.2 Hz, 6H; p-ArH of NPh2). 13C NMR (100
MHz, C6D6, 25 °C): δ = 154.4 (Ph2N), 135.2 (d, JPC = 124.7 Hz;
Ph2P), 132.3 (s; Ph2P), 131.7 (d, JPC = 117.8 Hz; Ph2P), 129.1
(Ph2N), 125.3 (Ph2N), 123.6 (Ph2N). 51V NMR (105 MHz, C6D6, 25
°C): δ = −6.1 (br). 31P NMR (162 MHz, C6D6, 25 °C): δ = 19.9 (br
m). Anal. Calcd for C48H40N4OPV: C, 74.80; H, 5.23; N, 7.27.
Found: C, 74.77; H, 5.14; N, 7.28.
Synthesis of [(Ph2N)3VNPO(C6H5)2][Cp*2Co] (2b−). In the
glovebox, a solution of Cp*2Co (24.7 mg, 0.075 mmol) in 2 mL of
fluorobenzene was added dropwise to a solution of complex 2b (60.6
mg, 0.075 mmol) in 3 mL of fluorobenzene. The resulting dark red
solution stood at room temperature for 3 h, and the volume was
reduced to about 0.5 mL in vacuo. Pentane (2 mL) was slowly added
dropwise to the concentrated reaction solution to afford an orange-
red precipitate. The precipitate was filtered, washed with pentane (5
mL), and dried in vacuo to afford an orange-red solid (92 mg, 0.0719
mmol, 94% yield). Dark brown crystals suitable for XRD studies were
grown by slow diffusion of pentane into a concentrated solution of
2b− in THF at −35 °C (77 mg, 0.070 mmol, 93% yield). Anal. Calcd
for C68H70CoN4OPV: C, 74.24; H, 6.41; N, 5.09. Found: C, 73.87; H,
6.65; N, 4.94.
Synthesis of (Ph2N)3VNP(OSiMe3)(C6F5)2 (3a). Method 1. In
the glovebox, a solution of 2a (82 mg, 0.086 mmol) in cold toluene (3
mL, −35 °C) was added dropwise to a solution of TMS2-pyz (9.8 mg,
0.043 mmol) in cold toluene (2 mL, −35 °C), and the resulting black
solution was kept at −35 °C for 0.5 h. The volatiles were removed in
vacuo to give a dark brown solid, which was washed with cold pentane
(1 mL) to give a dark brown solid (80 mg, 91% yield). Dark red
crystals suitable for XRD studies were grown by slow vapor diffusion
of pentane into a concentrated solution of 3a in toluene at −35 °C
over several days. Anal. Calcd for C51H39F10OPSiV: C, 59.83; H, 3.84;
N, 5.47. Found: C, 59.75; H, 3.66; N, 5.45.
Method 2. In the glovebox, a solution of 2a (47.5 mg, 0.05 mmol)
in fluorobenzene (3 mL) was added dropwise to a solution of Cp*2Co
(16.5 mg, 0.05 mmol) in fluorobenzene (1 mL). The resulting dark
brown solution was stirred using a magnetic stirbar at ambient
temperature for 0.5 h. A separate solution of trimethylsilyl
trifluoromethanesulfonate (11.1 mg, 0.05 mmol) in fluorobenzene
(1 mL) was added dropwise to the reaction solution. The reaction
solution turned dark red and was allowed to stir at ambient
temperature for another 0.5 h. The volatiles were removed in vacuo,
and the dark brown solid mixture was washed with cold pentane (2 ×
2 mL) to afford a dark brown solid. The product was extracted into
benzene (2 × 3 mL) and filtered over a pad of Celite, and the solvent
was removed in vacuo. The resulting brown solid was washed with
cold pentane (2 mL) to give a dark brown powder (44 mg, 0.043
mmol, 86% yield). Anal. Calcd for C51H39F10OPSiV: C, 59.83; H,
3.84; N, 5.47. Found: C, 59.60; H, 4.00; N, 5.44.
Synthesis of (Ph2N)3VNP(OSiMe3)Ph2 (3b). Method 1. A
solution of 2b (81 mg, 0.1 mmol) in cold fluorobenzene (3 mL,
−35 °C) was mixed with a solution of TMS2-pyz (11.3 mg, 0.05
mmol) in cold fluorobenzene (1 mL, −35 °C), and the mixed
solution was kept at −35 °C for 0.5 h. The volatiles were removed in
vacuo to give a dark brown, greasy solid mixture, which was washed
with cold pentane (2 × 2 mL) to give a dark brown solid (45 mg,
0.053 mmol, 53% yield). Dark blue crystals suitable for XRD studies
were grown from a concentrated solution of 3b in pentane at −35 °C
over several days. Satisfactory elemental analysis could not be
obtained from this method due to the contamination of pyrazine.
Method 2. In the glovebox, a solution of 2b (81 mg, 0.1 mmol) in
dichloromethane (3 mL) was added dropwise to a solution of
Cp*2Co (33 mg, 0.1 mmol) in dichloromethane (1 mL). The
resulting dark brown solution was allowed to stir using a magnetic
Synthesis of [(Ph2N)3VNP(OSiEt3)(C6F5)2][B(C6F5)4] (2a-Si+).
In the glovebox, neat Et3SiH (1.5 mL, 9.39 mmol) was mixed with
[Ph3C][B(C6F5)4] (46 mg, 0.05 mmol). The reaction mixture was
allowed to stir for 12 h, and a white precipitate crashed out of
solution. The excess silane was decanted, and the white solid was
washed with pentane (2 × 4 mL) and dried in vacuo. A separate
solution of 2a (47 mg, 0.05 mmol) in benzene (3 mL) was added
dropwise to the white solid, and the resulting dark green solution was
stirred under ambient temperature for 0.5 h. The volatiles were
removed in vacuo and washed with pentane (2 mL × 2) to afford a
green solid (84 mg, 0.048 mmol, 96% yield). Dark green needles
suitable for XRD studies were grown by slow diffusion of pentane into
a concentrated solution of 2a-Si+ in dichloromethane at room
1
temperature. H NMR (400 MHz, C6D5Br, 25 °C): δ = 7.14−6.92
3
(m, 30H; Ph2NH), 0.80 (t, JHH = 7.2 Hz, 9H; SiCH2CH3), 0.58 (q,
3JHH = 7.2 Hz, 6H; SiCH2CH3). 51V NMR (105 MHz, C6D5Br, 25
°C): δ = 521.0 (d, J = 172.2 Hz). 31P NMR (162 MHz, C6D5Br, 25
°C): δ = −21 (br). 19F NMR (376 MHz, C6D5Br, 25 °C): δ = −129.5
(br, 4F; o-P(C6F5)2), −131.8 (br, 8F; o-B(C6F5)4), −138.9 (br, 2F; p-
P(C6F5)2), −155.3 (br, 4F; m-P(C6F5)2), −162.5 (br, 4F; p-
B(C6F5)4), −166.3 (br, 8F; m-B(C6F5)4). 13C NMR (100 MHz,
C6D5Br, 25 °C): δ = 153.8, 129.1, 128.6, 121.4 (NPh2), 6.1, 5.4
(SiEt3). The signal-to-noise ratio was too low for properly identifying
any C6F5 13C resonance. Anal. Calcd for C78H45B F30N4OPSiV: C,
53.69; H, 2.60; N, 3.21. Found: C, 53.46; H, 2.71; N, 3.27.
Synthesis of [(Ph2N)3VNP(OSiMe3)(C6H5)2][OTf] (2b-Si+). In
the glovebox, a solution of 2b (142 mg, 0.176 mmol) in DCM (4 mL)
was added dropwise to a solution of Me3SiOTf (39 mg, 0.176 mmol)
F
DOI: 10.1021/jacs.8b09063
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Journal of the American Chemical Society
Article
in DCM (1 mL). The resulting dark brown solution was allowed to
stir at ambient temperature for 10 min. The volatiles were removed in
vacuo, and the brown residue was washed with pentane (5 mL × 2) to
afford a dark brown solid (165 mg, 0.166 mmol, 94% yield). Dark
brown needles suitable for XRD studies were grown by slow diffusion
of pentane into a concentrated solution of 2b-Si+ in dichloromethane
at room temperature. 1H NMR (400 MHz, THF-d8, 25 °C): δ = 7.71
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(t, 3JHH = 7.8 Hz, 2H; p-H of POPh2), 7.51 (td, 3JHH = 7.8 Hz, 4JPH
=
3.6 Hz, 4H; m-H of POPh2), 7.28 (dd, 3JPH = 14.4 Hz, 3JHH = 7.2 Hz,
4H; o-H of POPh2), 7.17 (t, 3JHH = 7.8 Hz, 12H; m-H of NPh2), 7.11
(t, 3JHH = 7.8 Hz, 6H; p-H of NPh2), 6.70 (d, 3JHH = 7.8 Hz, 12H; o-H
of NPh2), 0.15 (s, 9H; SiMe3). 13C NMR (100 MHz, THF-d8, 25 °C):
δ = 154.5 (Ph2N), 135.1 (Ph2P), 132.6 (d, JPC = 12.7 Hz; Ph2P),
130.2 (d, JPC = 14.5 Hz; Ph2P), 129.7 (Ph2N), 127.5 (Ph2N), 122.9
(Ph2N), 0.8 (SiMe3). 51V NMR (105 MHz, THF-d8, 25 °C): δ =
229.8 (br). 31P NMR (162 MHz, THF-d8, 25 °C): δ = 22 (br). Anal.
Calcd for C52H49F3N4O4PSSiV: C, 62.89; H, 4.97; N, 5.64. Found: C,
62.79; H, 5.01; N, 5.84.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b09063.
■
(9) Zhanglin, Y.; Forissier, M.; Sneeden, R. P.; Vedrine, J. C.; Volta,
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V5+ in n-Butane Oxidation Catalyzed by Vanadium Phosphates.
Science 1997, 275, 191−193.
S
NMR, EPR, UV−vis, DFT, CV, competition results
(PDF)
Crystallographic data of 2a (CIF)
Crystallographic data of 2a− (CIF)
Crystallographic data of 2a−Si+ (CIF)
Crystallographic data of 2b (CIF)
Crystallographic data of 2b− (CIF)
Crystallographic data of 2b−Si+ (CIF)
Crystallographic data of 3a (CIF)
Crystallographic data of 3b (CIF)
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AUTHOR INFORMATION
■
Corresponding Authors
*rdobrove@post.tau.ac.il
*menard@chem.ucsb.edu
ORCID
Jiaxiang Chu: 0000-0002-2755-093X
Joshua Telser: 0000-0003-3307-2556
Roman Dobrovetsky: 0000-0002-6036-4709
́
Gabriel Menard: 0000-0002-2801-0863
Author Contributions
§J. Chu and T. G. Carroll contributed equally.
Notes
The authors declare no competing financial interest.
(18) Weinberg, D. R.; Gagliardi, C. J.; Hull, J. F.; Murphy, C. F.;
Kent, C. A.; Westlake, B. C.; Paul, A.; Ess, D. H.; McCafferty, D. G.;
Meyer, T. J. Proton-Coupled Electron Transfer. Chem. Rev. 2012, 112,
4016−4093.
ACKNOWLEDGMENTS
■
This paper is dedicated to Prof. Doug Stephan on the occasion
of his 65th birthday. We thank the US-Israel Binational Science
Foundation (Grant #2016241), the ACS Petroleum Research
Fund (#58693-DNI3), the University of California, Santa
Barbara, and Tel Aviv University for financial support.
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Moore, A. L. Proton-Coupled Electron Transfer in Artificial
Photosynthetic Systems. Acc. Chem. Res. 2018, 51, 445−453.
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of Iron−Porphyrin−Imidazole Complexes as Models for Histidine-
Ligated Heme Reactivity. J. Am. Chem. Soc. 2008, 130, 2774−2776.
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Concerted Proton−Electron Transfers in a Ruthenium Terpyridine-
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