Direct detection and reactivity of the short-lived phenyloxenium ion

Article abstract of DOI:10.1021/ja403370k

Photolysis of protonated phenylhydroxylamine was studied using product analysis, trapping experiments, and laser flash photolysis experiments (UV-vis and TR³ detection) ranging from the femtosecond to the microsecond time scale. We find that the excited state of the photoprecursor is followed by two species: a longer-lived transient (150 ns) that we assign to the phenoxy radical and a shorter-lived (3-20 ns) transient that we assign to the singlet phenyloxenium ion. Product studies from photolysis of this precursor show rearranged protonated o-/p-aminophenols and solvent water adducts (catechol, hydroquinone) and ammonium ion. The former products can be largely ascribed to radical recombination or ion recombination, while the latter are ascribed to solvent water addition to the phenyloxenium ion. The phenyloxenium ion is apparently too short-lived under these conditions to be trapped by external nucleophiles other than solvent, giving only trace amounts of o-/p-chloro adducts upon addition of chloride trap. Product studies upon thermolysis of this precursor give the same products as those generated from photolysis, with the difference being that the ortho adducts (o-aminophenol, hydroquinone) are formed in a higher ratio in comparison to the photolysis products.

Full text of DOI:10.1021/ja403370k

Article
pubs.acs.org/JACS
Direct Detection and Reactivity of the Short-Lived Phenyloxenium
Ion
Patrick J. Hanway,† Jiadan Xue,‡ Ujjal Bhattacharjee,† Maeia J. Milot,† Zhu Ruixue,‡ David Lee Phillips,‡
and Arthur H. Winter*,†
†Department of Chemistry, Iowa State University, 2101d Hach Hall, Ames, Iowa 50011, United States
‡Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
S
* Supporting Information
ABSTRACT: Photolysis of protonated phenylhydroxylamine was
studied using product analysis, trapping experiments, and laser flash
photolysis experiments (UV−vis and TR3 detection) ranging from the
femtosecond to the microsecond time scale. We find that the excited state
of the photoprecursor is followed by two species: a longer-lived transient
(150 ns) that we assign to the phenoxy radical and a shorter-lived (3−20
ns) transient that we assign to the singlet phenyloxenium ion. Product
studies from photolysis of this precursor show rearranged protonated o-/
p-aminophenols and solvent water adducts (catechol, hydroquinone) and
ammonium ion. The former products can be largely ascribed to radical recombination or ion recombination, while the latter are
ascribed to solvent water addition to the phenyloxenium ion. The phenyloxenium ion is apparently too short-lived under these
conditions to be trapped by external nucleophiles other than solvent, giving only trace amounts of o-/p-chloro adducts upon
addition of chloride trap. Product studies upon thermolysis of this precursor give the same products as those generated from
photolysis, with the difference being that the ortho adducts (o-aminophenol, hydroquinone) are formed in a higher ratio in
comparison to the photolysis products.
ions. Studies of related intermediates such as carbenes,22,23
INTRODUCTION
■
nitrenes,24−26 and nitrenium ions27−29 have benefited tremen-
Oxenium ions are reactive species with the formula R−O+. As
dously from the ability to generate these species photochemi-
cally from established precursors (e.g., aryl azides, diazo
isolectronic analogues of nitrenes, oxenium ions are strong
hypovalent electrophiles with a formally positive charge on an
compounds, etc.), permitting detailed spectroscopic studies of
electronegative oxygen. As we have detailed elsewhere,1,2 these
their reactivity and properties. In contrast, there are few
species have been proposed in a number of useful unpolung
photoprecursors to oxenium ions. Some reports suggest
synthetic transformations, such as electrochemical oxidations of
phenols and phenolates,3,4 alkane oxidations,5 and a wide
oxenium ions can be formed as part of intermediate mixtures
from multiphoton ionization studies or from mass spectral ion
variety of other phenolic oxidations and tautomerization
fragmentations,17,30−32 but recent attempts at developing
reactions.6−10 Many of these reactions convert feedstock
robust single-photon precursors to these species suitable for
phenols and alkanes into value-added products such as
mechanistic studies, by either photolysis or thermolysis, have
substituted phenols, oxidized alkanes, and cyclohexadienones
proven to be challenging.33−36
as well as petrochemicals such as poly(1,4-phenylene ether), a
high-value industrial thermoplastic.11−13 Like nitrenes and
However, in exciting recent studies reported by Novak and
Platz, a 4′-methyl-4-biphenylyloxenium ion was generated by
carbenes, oxenium ions are short-lived in solution but can
photolysis of the 4-(4-methylphenyl)-4-acetoxy cyclohexadie-
form stable complexes with transition metals, acting as
nonyl derivative (shown in Scheme 1).34,37 These studies were
interesting ligands.14 These species also have relevance to
significant because they demonstrated the direct observation of
astrochemistry, as aryloxenium ions are formed from exposing
phenols, anisoles, and nitrobenzenes to ionizing radiation,15−17
a discrete oxenium ion using laser photolysis. This photo-
precursor has the limitation that it requires a para substituent to
prevent tautomerization, making it unsuitable for the photo-
generation of the parent phenyloxenium ion.
and oxenium ions can persist in interstellar clouds and planet
atmospheres.18−20 Of biological relevance, some aryloxenium
ions are proposed to be reactive intermediates in iron-mediated
enzymatic oxidations of phenols into quinone compounds.21
However, it is of considerable interest to understand the
reactivity of the phenyloxenium ion, since this species is the
Given the diverse applicability of these species, it is surprising
parent aryloxenium ion and may allow benchmarking the
how very little is known about this class of reactive
intermediates.
A major roadblock that has stymied the detailed study of
oxenium ions is the lack of suitable photoprecursors to these
Received: April 4, 2013
© XXXX American Chemical Society
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Scheme 1. Previous Example of LFP of the 4′-Methyl-4-
biphenylyloxenium Ion38
reactivity of this class of reactive intermediate in comparison to
the related phenylnitrene, phenylnitrenium ion, and phenyl-
carbene. Thus, to generate the phenyloxenium ion, we required
a new photoprecursor. On the basis of a report of N-
arylhydrazinium ions generating arylnitrenium ions upon
photolysis,39 we considered that 3 might similarly generate
the phenyloxenium ion by photolysis, leading to O−N
heterolytic scission with ejection of neutral ammonia. A
potential advantage of this precursor (Scheme 2) is that it is
Figure 1. LFP spectra of the precursor 3 in acetonitrile.
vibrational cooling, wherein the vibrationally hot radical sheds
heat to solvent. The lifetime of this 400 nm transient is not
measurably reduced by an added azide trap, as would be
expected of a radical and not a cationic species. The other
transient is a broad absorption centered around 525 nm (after
vibrational cooling) that is short-lived (τ = 3−20 ns). We must
provide a range for the lifetime of this transient, since the decay
falls within the temporal “blind spot” between our femtosecond
and nanosecond LFP setupsthe decay is too slow to get a
proper fit employing our femtosecond system, but the transient
is gone within the detection limit (∼20 ns) of our nanosecond
setup. Unfortunately, having this transient decay within our
temporal blind spot makes kinetic studies of this band difficult.
However, on the basis of evidence described below, we assign
this band to the phenyloxenium ion. Thus, the likely lifetime of
this species is ∼5 ns, with an upper limit on the lifetime of 20
ns, which is the shortest time we can observe with our
nanosecond LFP setup.
As further confirmation that the 400 nm transient is the
phenoxy radical, we performed LFP experiments with time-
resolved resonance Raman (TR3) detection. The TR3 experi-
ments using an incident beam at 400 nm (Figure 2) show a
strong peak at 1490 cm−1, which correlates to reported values
for the phenoxy radical.41,43 Unfortunately, TR3 experiments
with an incident beam at 532 nm to observe the resonance
Raman spectrum of the putative oxenium ion gave no
measurable spectrum. Possibly, the extinction coefficients and
amounts of the presumed oxenium ion are too low to obtain a
measurable spectrum using our experimental setup.
Assignment of the 525 nm Transient to the Phenyl-
oxenium Ion. While we were unable to obtain a TR3 spectrum
of the 525 nm transient, a number of pieces of evidence suggest
that it is the phenyloxenium ion. First, the laser flash photolysis
spectra that we obtain are very similar to those seen for the
previously reported 4′-methyl-4-biphenylyloxenium ion.38
Photolysis of the acetyl precursor 1 led to two transients: a
long-lived transient at lower wavelength (360 nm) correspond-
ing to a biphenyloxy radical and a shorter-lived transient (170
ns, 460 nm) at higher wavelength corresponding to the 4′-
methyl-4-biphenylyloxenium ion.37,38 Our LFP studies give
very similar results, with a long-lived band at ∼400 nm (τ = 150
ns) that we can definitively assign to the phenoxy radical and a
short-lived weakly absorbing band at 525 nm (τ = 3−20 ns).
Scheme 2. Proposed Precursor for the Observation of the
Phenyloxenium Ion 4
positively charged rather than neutral, so that in principle the
oxenium ion could be generated in any solvent system and
would not be restricted to the ionizing solvents necessary for
efficient generation of ion pairs from neutral precursors.
On the basis of laser flash photolysis studies ranging from the
femtosecond to the microsecond time scale, product analysis,
trapping experiments, and computed spectroscopic signatures,
we find that photolysis of the protonated phenyl hydroxylamine
3 generates a short-lived intermediate consistent with the
phenyloxenium ion as well as a longer-lived phenoxy radical.
The putative phenyloxenium ion is too short-lived to be
significantly trapped by external nucleophiles other than
solvent. The major products from photolysis and thermolysis
of this precursor are phenol as well as the rearranged
protonated o-/p-aminophenol and water adducts (hydro-
quinone, catechol), with photolysis leading to para adducts as
the major products and thermolysis leading to an increased
preference for ortho adducts.
RESULTS AND DISCUSSION
■
Laser Flash Photolysis (LFP) of 3. Photolysis of 3 by LFP
was performed in acetonitrile and a mixture of acetonitrile and
water (9/1 and 1/1), using 266 nm excitation. The LFP spectra
can be seen in Figure 1. A very broad, short-lived transient is
observed immediately after the pulse (τ ≈ 5 ps), which we
attribute to the excited state of 3. Two longer-lived transients
are also observed at later times after the excitation pulse. The
first, having a set of absorptions at ∼400 nm and a weaker band
at 620 nm, can be assigned to the known spectrum of the
phenoxy radical (τ = 150 ns).40−42 The slight blue shift and
sharpening of the bands over ∼30 ps is characteristic of
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Figure 4. LFP spectra of precursor 3 in different solvents obtained at
∼500 ps.
Figure 2. ps-KTR3 spectra at 12 and 100 ps of the precursor 3 along
with computed spectra of the phenyloxenium ion (bottom) and the
phenoxy radical (top). Asterisks represent solvent subtraction artifacts.
from photolysis (and thermolysis) of 3 (described below) show
small amounts of water adducts that would be expected of a
cationic but not a radical intermediate.
Thus, the 525 nm transient absorption is in a location similar to
that for the 4′-methyl-4-biphenylyloxenium ion, but with a
shorter lifetime (τ = 120 ns for the 4′-methyl-4-biphenylyloxe-
nium ion in water), which would be expected of a less stabilized
species.
Second, previous studies have shown that TD-DFT can
sometimes provide reasonable estimates of UV−vis spectra for
related reactive intermediates.44−46 Thus, we computed the
absorption spectrum of the singlet phenyloxenium ion using
TD-B3LYP/6-311+G(2d,p), shown in Figure 3. The computed
Finally, alternative assignments including a triplet excited
3
state of 3 or the triplet oxenium ion 4 seem less likely. Our
DFT computations of the triplet excited state of 3 indicates that
it is a dissociative state, optimizing to ammonia and the triplet
3
phenyl oxenium ion 4. Further, our previous computational
study indicates that 4 has a closed-shell singlet ground state,
which is ca. 20 kcal/mol lower in energy than the lowest energy
triplet state, making it less likely to be the triplet phenyl-
1
3
oxenium ion. Finally, The triplet oxenium ion 4 is computed
by TD-DFT to have major absorptions at 346 and 789 nm, in
poor agreement with what we observe. Although in the absence
of a TR3 spectrum it is hard to be definitive, the sum of this
evidence leads us to assign this 525 nm band to the singlet
phenyloxenium ion.
Photochemical and Thermal Product Studies of 3.
Products of the singlet oxenium ion are anticipated to derive
from standard two-electron nucleophilic addition chemistry
rather than diradical-type chemistry that is often associated with
atom-centered triplets, such as the isoelectronic ground-state
triplet phenylnitrene. Product studies were performed in
different solvent systems, shown in Scheme 3. When the
precursor is photolyzed in solvents containing water, the
products are a mixture of protonated o- and p-aminophenol as
well as the water adduct hydroquinone. We also see the 1/1/1
triplet at 6 ppm characteristic of the 1H NMR of the
ammonium cation. ortho and para ring adducts are character-
istic of oxenium ions.33,35,47−51
Figure 3. Computed UV spectra (TD-B3LYP/6-311+G(2d,p)) of the
phenyloxenium ion (red) and the phenoxy radical (blue).
The rearranged products could derive either from recombi-
nation of the phenoxy radical and ammonia radical cation or
from recombination of ammonia with the phenyloxenium ion.
The water adduct, hydroquinone, likely derives from water
addition to the phenyloxenium ion. Phenol is also obtained in
solvents other than water (e.g., neat MeOH, CH3CN). Phenol
may derive from either the phenoxy radical via a hydrogen atom
abstraction process or from the phenyloxenium ion from a
hydride transfer process. Addition of chloride as a trap, either in
the form of added NaCl to solutions of 3 or from photolysis of
1
absorption band for 4 gives a single band in the visible region
of the UV−vis spectrum centered at 524 nm, in good
agreement with the experimental band (see Figure 1).
Third, this 525 nm absorption is quenched as the solvent is
changed to mixtures with increasing amounts of water (Figure
4). The band is largest in neat acetonitrile, smaller in 1/1
CH3CN/H2O, and negligible in 1/9 CH3CN/H2O. This
quenching behavior by nucleophilic water would be expected
of a highly electrophilic intermediate. Fourth, product studies
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Scheme 3. Product Studies on the Phenylhydroxylamine
Tetrafluoroborate Salt 3
CONCLUSIONS
■
a
Photolysis of phenylhydroxylamine salt 3 leads to the phenoxy
radical as well as a short-lived transient we assign to the
phenyloxenium ion. This photoprecursor has a number of
advantages, including a simple synthesis and the theoretical
possibility to generate other oxenium ions in nonionizing
solvents. A derivative of this precursor that cleanly generates
oxenium ions in a general way would be a major help in
studying the chemistry and properties of these important
species.
MATERIALS AND METHODS
■
LFP Experiments. fs-Transient Absorption (fs-TA) Experiments.
The fs-TA measurements were performed on the basis of a
commercial femtosecond Ti/sapphire regenerative amplifier laser
system and Helios Transient Absorption Spectrometer. A flow cell
was used in order to prevent the accumulation of photodecomposition
products. For the present experiments, the sample solution was excited
by a 267 nm pump beam (the third harmonic of 800 nm, the
regenerative amplifier fundamental) and probed by a white light
continuum produced from a one-dimensional movable CaF2 plate
pumped by the fundamental laser pulses (800 nm). The pump and
probe laser beam spot sizes at the sample were about 500 and 200 μm,
respectively. The detection signals (with and without 267 nm pump)
were focused into an optics fiber coupled to a multichannel
spectrometer with a CMOS sensor. The time delay between the
pump and probe pulse was controlled by an optical delay line, and the
instrument response time was ∼200 fs. The fs-TA experiments were
carried out for precursor 3 in neat MeCN and 1/1 H2O/MeCN with a
sample concentration of ∼1 μM.
a
1
Yields were determined by H NMR integration.
−
the chloride salt of 3 rather than the BF4 salt, leads to only
trace amounts of the o-/p-chlorophenol as detected by GC-MS.
It is possible that ejected ammonia may not undergo cage
escape prior to recombination. This result suggests that the
phenyloxenium ion is too short-lived to be trapped to a
significant extent by externally added nucleophiles other than
solvent under these conditions.
ps-Kerr-Gated-Time-Resolved Resonance Raman (ps-KTR3) Ex-
periments. Briefly, the Ti/sapphire regenerative amplifier laser system
was operated in picosecond mode with a 800 nm, ∼1 ps, and 1 kHz
output. Samples were pumped by a 267 nm pulse and probed with a
400 nm pulse. The 267 and 400 nm probe pulses were the third and
second harmonics of the regenerative amplifier fundamental,
respectively. The pump and probe pulse durations were ∼1.5 ps; the
pulse energies at the sample were 8−10 μJ. The pump and probe laser
beams were lightly focused onto a flowing liquid sample, and the
Raman scattering was collected via a backscattering configuration and
then focused into the Kerr medium (CS2 in a 1 mm thickness UV cell)
placed at the other focus point of the ellipse. The Kerr medium was
placed between a crossed polarizer pair with an extinction coefficient
of ∼104. The gating beam was polarized at 45° and focused to the Kerr
medium with an adjusted intensity to create, in effect, a half-waveplate
that rotates the polarization of the light from the sample, allowing it to
be transmitted through a Glan Taylor polarizer for the duration of the
induced anisotropy created by the femtosecond gating pulse. The
Raman light that passed through the second polarizer was focused into
a monochromator and detected by a liquid N2 cooled CCD. The
wavenumber shifts of the resonance Raman spectra were calibrated
using the known MeCN solvent Raman bands, and the spectra
presented were obtained from subtraction of an appropriately scaled
probe-before-pump spectrum from the corresponding pump−probe
spectrum. The ps-KTR3 measurements were also carried out for BDP
in 100% MeCN.
Product Studies. Product studies were performed with 5 mg of
precursor 3 dissolved in 3 mL of solvent. The sample was degassed
before beginning the photolysis, and the sample was irradiated with
254 nm UV light from a mercury vapor lamp for 1 h in a Rayonet
photoreactor. The solvent was then removed, and a 1H NMR
spectrum was obtained. Thermolysis studies were performed similarly
with 10 mg of precursor 3 and 10 mL of solvent refluxed for 1 h.
Computational Methods. For the computational studies, all of
the molecular geometries were optimized at the B3LYP/CC-pVTZ
level of theory.52−54 The stationary points were found to have zero
imaginary frequencies, and all energies contain a correction for the
zero-point energy. Transition states were found to have one imaginary
The thermolysis of 3 in water gives products similar to those
seen from photolysis. The major difference is that the
thermolysis products show an increasing preference for forming
the ortho adducts in comparison to what is found under the
photolysis conditions. Thus, larger amounts of ortho
ammonium phenol are formed as well as catechol along with
hydroquinone. One possibility for this increasing preference for
formation of the ortho adducts could be that the thermolysis
conditions are under thermodynamic control. We performed
DFT computations of the Wheland σ complex for addition of
water and ammonia to the ortho and para positions. For both
the protonated aminophenol and the dihydroxybenzene cases,
the ortho σ complex was found to be lower in energy than the
para σ complex. These lower energies could be attributed to the
ortho σ complexes having access to an internal hydrogen bond
(in the case of water addition to the ortho position of the
phenyloxenium ion, geometry optimization leads to a proton
transfer from the water to the aryl oxygen apparently without a
barrier). The transition states were also computed for return of
the ortho and para Wheland intermediates to the water-bound
oxenium ion. Barriers for this back-reaction were found to be
reasonably small (11 and 7 kcal/mol, respectively). However, it
is also important to consider that a low-barrier tautomerization
of the Wheland intermediate to the final aromatic product may
effectively render the reaction irreversible. An alternative
explanation is that the thermolysis leads to a larger partitioning
toward the oxenium ion rather than the radical, leading to more
solvent adducts and the differing product ratios. Our
preliminary product studies of a protonated hydroxylamine
precursor to the biphenylyl oxenium ion are suggestive that this
latter explanation is more likely to be the correct one.
D
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frequency that connected the starting material and product and
included a PCM solvent model for water. The UV spectra were
computed using TD-B3LYP/6-311+G(2d,p). The resonance Raman
spectra were found using B3LYP/6-31+G(d,p) with a 400 nm incident
beam, and all frequencies were scaled by 0.964.55 All computations
were done with Gaussian 09.56
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and figures giving additional LFP experimental
data, synthesis details, compound characterization data, NMR
product studies, and Cartesian coordinates and absolute
energies for computed structures. This material is available
free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
winter@iastate.edu
Notes
■
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
A.H.W. thanks the Petroleum Research Fund and Cottrell
Scholar Award from the Research Corporation for Scientific
Advancement for funding. D.L.P. acknowledges funding from
the Research Grants Council of Hong Kong (HKU 7048/11P)
and the University Grants Committee Special Equipment Grant
(SEG-HKU-07). Support from the University Grants Commit-
tee Areas of Excellence Scheme (AoE/P-03/08) is also
gratefully acknowledged.
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