Copper(i)-based oxidation of polycyclic aromatic hydrocarbons and product elucidation using vacuum ultraviolet spectroscopy and theoretical spectral calculations

Article abstract of DOI:10.1039/c8nj04740e

Copper(i) complexes supported by fluorinated 1,3,5-triazapentadienyl ligands have been used as catalysts for the oxidation of anthracene, naphthalene, and pyrene to the corresponding quinones, using H₂O₂ as an oxidant under mild conditions without an acid co-catalyst. Gas chromatography-vacuum ultraviolet spectroscopy (GC-VUV) combined with time-dependent density functional theory theoretical computations of absorption spectra was demonstrated as a new and useful tool-set for unknown determination in complex reaction mixtures, especially when standards are not available for spectral comparisons and product mixtures involve closely related isomers. The anthracene has been converted to 9,10-anthraquinone in quantitative yield using this copper catalyzed process. The oxidation of naphthalene afforded 1,4-naphthoquinone as the major product, and 1-naphthol and 2-naphthol as minor products. The pyrene oxidation resulted in 4,5-, 1,6-, and 1,8-pyrenequinones, among other products. The X-ray crystal structure of [N{(CF₃)C(C₆F₅)N}₂]CuNCCH₃ is also reported.

Full text of DOI:10.1039/c8nj04740e

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Copper(I)-based oxidation of polycyclic aromatic
hydrocarbons and product elucidation using
vacuum ultraviolet spectroscopy and theoretical
spectral calculations†
Cite this: New J. Chem., 2018,
42, 19442
a
Tharun T. Ponduru,
Changling Qiu,a James X. Mao,a Allegra Leghissa,a
Jonathan Smuts,b Kevin A. Schug *a and H. V. Rasika Dias
*
a
Copper(I) complexes supported by fluorinated 1,3,5-triazapentadienyl ligands have been used as catalysts
for the oxidation of anthracene, naphthalene, and pyrene to the corresponding quinones, using H2O2 as
an oxidant under mild conditions without an acid co-catalyst. Gas chromatography-vacuum ultraviolet
spectroscopy (GC-VUV) combined with time-dependent density functional theory theoretical
computations of absorption spectra was demonstrated as a new and useful tool-set for unknown
determination in complex reaction mixtures, especially when standards are not available for spectral
comparisons and product mixtures involve closely related isomers. The anthracene has been converted
to 9,10-anthraquinone in quantitative yield using this copper catalyzed process. The oxidation of
naphthalene afforded 1,4-naphthoquinone as the major product, and 1-naphthol and 2-naphthol as
minor products. The pyrene oxidation resulted in 4,5-, 1,6-, and 1,8-pyrenequinones, among other
products. The X-ray crystal structure of [N{(CF3)C(C6F5)N}2]CuNCCH3 is also reported.
Received 17th September 2018,
Accepted 30th October 2018
DOI: 10.1039/c8nj04740e
rsc.li/njc
synthesis of quinones via direct chemical modifications of
Introduction
PAHs.14–18 Nevertheless, oxidation of PAHs to quinones can be
Direct oxidation of polycyclic aromatic hydrocarbons (PAHs), achieved but often requires high temperatures, strong acids, and
such as naphthalene, anthracene, phenanthrene, and pyrene, is powerful oxidants. For example, anthracene can be oxidized to
one of the most important, yet challenging processes relevant 9,10-anthraquinone with CrO3 in acetic acid at 90 1C,19 while
to both biology and chemical industries.1–5 Such processes pyrene has been oxidized to 1,6- and 1,8-pyrenequinones by
have received considerable attention over many years since K2Cr2O7 in 4 M H2SO4 under reflux.20 Methods for the conversion
the oxidation of PAHs provides a path for their detoxification, of hydroxyl derivatives of PAHs (e.g., naphthols) to quinones are
and their oxidized products (e.g., quinones) are of significant also known.21–24
industrial, biochemical, and environmental importance.6–8
During the past few years, homogeneous transition-metal
For example, anthraquinone derivatives are important in the catalyzed routes have emerged as straightforward and environ-
industrial synthesis of hydrogen peroxide,9 naphthoquinones mentally friendly synthetic tools for the oxidation of PAHs to
are used in industry on a large scale as dye reagents,10,11 and are quinones.25–28 Transition metals, such as Ru, Rh, and Ir, have
important in vitamins of the K group,12 and pyrenequinones can been utilized as effective catalysts to facilitate these trans-
be used as photosensitizers in photodynamic therapy.13 The low formations.29–32 For example, Mu¨ller and Bobillier reported a
reactivity of aromatic C–H bonds and the low selectivity towards rhodium catalyst for the oxidation of anthracene to 9,10-anthra-
specific oxidized products, including the formation of hydroxy- quinone with a 91% yield using t-BuOOH as an oxidant.30
´
lated derivatives, are some of the difficulties encountered in the Recently, Orti, Perez and co-workers have reported the use of
an iron catalyst with H2O2 as an oxidant for the oxidation of
a Department of Chemistry and Biochemistry, The University of Texas at Arlington,
Arlington, Texas 76019, USA. E-mail: dias@uta.edu, kschug@uta.edu;
Tel: +1-817-272-3813, +1-817-272-3541
pyrene, naphthalene, and anthracene, which afforded corres-
ponding quinones in modest to good yields.8 This method,
however, requires acetic acid as part of the reaction medium.
Identification of products resulting from the oxidation
of PAHs can be challenging and quite time-consuming.
The separation of products using column chromatography for
analysis using traditional methods like NMR is a common but
b VUV Analytics, Inc, Cedar Park, Texas 78613, USA
† Electronic supplementary information (ESI) available: X-ray crystallographic
data (CIF), ORTEP diagrams, chromatograms, additional figures and details of
theoretical spectral calculations. CCDC 1867904. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c8nj04740e
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tedious process and often requires a large amount of organic with computationally-predicted spectra using time-dependent
solvents. Gas chromatography-mass spectrometry (GC-MS) is density functional theory for the screening and identification of
also an option for the identification of PAH oxidation products the resulting product mixtures. In this article, we report that
in a mixture, due to its broad detection capability for chemical copper(I) complexes supported by fluorinated 1,3,5-triazapenta-
compounds and the qualitative information afforded by mass dienyl ligands can be used as catalysts for the oxidation of
spectral data. However, GC-MS is not a fail-safe method, anthracene, naphthalene, and pyrene to the corresponding
because some of the oxidation products of PAHs are isomeric quinones, using H2O2 as an oxidant under mild conditions
and have similar fragmentation patterns, making them virtually without an acid co-catalyst (Scheme 1). The resulting quinone
indistinguishable by this approach.
products were analyzed by GC-VUV. The GC-VUV spectral
A new GC detector based on vacuum ultraviolet spectroscopy library and theoretical computations of VUV/UV absorption
(VUV),33,34 which measures full range absorption from 125–240 nm, spectra proved effective as complementary tools for product
has been demonstrated as a powerful detector for sensitive quali- identification.
tative and quantitative GC analysis of various molecules and
isomers, including polycyclic aromatic hydrocarbons (PAHs),33,35
Results and discussion
polychlorinated biphenyls (PCBs),36 fatty acids,37,38 terpenes,39 and
petroleum fuels.40–42 It has been shown to be highly complemen- We have utilized fluorinated supporting ligands in this work, as
tary to GC-MS, due to its exceptional capability for distinguishing they are some of the most robust ligand auxiliaries found in
isomeric species. A general schematic of the instrumentation and chemistry and often produce unusual chemistries typically not
operation principles of the gas chromatography-vacuum ultraviolet seen with hydrocarbon analogues.44–57 We have also had great
(GC-VUV) have been described in detail in these previous studies. success in utilizing such ligands in catalysis, including the
Further, VUV absorption spectra can be modeled using time- C(sp3)–H and C–Cl functionalization chemistry via carbene
dependent density functional theory (TD-DFT).35,43 The computa- insertion.58–64 Furthermore, 1,3,5-triazapentadienyl systems
tional prediction of spectra, especially those for which pure stan- can be tuned both sterically and electronically by changing
dards of compounds of interest are not readily available, may be substituents on the ligand backbone, thus providing a potential
advantageous in the support of relevant GC-VUV analytical work. means of controlling the catalytic activity and product
This is not a capability common to many other detections systems. distributions.65–69 Oxidation catalysts based on copper repre-
The VUV detector can be coupled to any standard GC system and, sent an attractive choice for investigations because, they are
together with computational prediction of spectra, could be of cheap and relatively non-toxic compared to the heavy metal
value in the analysis of the PAH oxidation products.
ions, and are relevant in many laboratory and biochemical
The aims of this work concern the development of an acid- oxidation processes involving arenes.6,7,70 Copper catalysed
free process for the oxidation of PAHs using earth abundant oxidation chemistry is also an area of research focus in our
catalysts, as well as to probe the utility of GC-VUV combined laboratory.71,72
Scheme 1 Oxidation of polyaromatic hydrocarbons to quinones using H2O2 and copper catalyst supported by 1,3,5-triazapentadienyl ligand.
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of anthracene to 9,10-anthraquinone was observed after stirring
for 2 h at 60 1C. The product was isolated in 87% yield after
chromatographic separation. The influence of the stoichiometry
on the catalytic reaction was investigated by varying the molar
ratio of anthracene to H2O2 while keeping the catalyst [N{(CF3)-
C(C6F5)N}2]CuNCCH3 at 4 mol%. As evident from the results
summarized in Table 1, both the anthracene conversion and
the isolated yield of 9,10-anthraquinone were reduced when
anthracene to H2O2 ratios of 1 : 1 and 1 : 10 were used under
similar conditions (giving isolated yields of 9,10-anthraquinone
at 28% and 56% yield, respectively). The [N{(C3F7)C(Dipp)N}2]-
CuNCCH3 also catalyzes oxidation of anthracene to 9,10-anthra-
quinone (entry 3 in Table 1) but at a lower yield. These results
indicate both the amount of oxidant H2O2 and the supporting
ligand have a significant effect on this oxidation process.
We have also compared the activity of [N{(CF3)C(C6F5)N}2]-
CuNCCH3 with the commercially available catalysts like CuBr
and Cu(CH3CN)4BF4, but they afforded only moderate yields of
9,10-anthraquinone (Table 1).
The starting material anthracene and the oxidized products
were analyzed by GC-VUV and the absorption spectrum for each
compound was recorded. As shown in Fig. 2, anthracene
exhibited unique and rich spectral features in the range of
125–240 nm. These features are attributed to the presence of
aromaticity (e.g. p - p* transitions) in the compounds. With a
spectral resolution of approximately 0.5 nm, the gas phase
absorption spectra show less spectral broadening and more
fine structure compared to those observed in solution phase UV
systems. Based on the chromatogram of oxidation of anthracene
(1 mmol) using 4 mol% of the catalyst [N{(CF3)C(C6F5)N}2]-
CuNCCH3 and 15 mmol of H2O2 in acetonitrile, only one
product, and the complete conversion of anthracene was
observed (Fig. S1, ESI†). The product was determined to be
9,10-anthraquinone (VUV spectrum illustrated in Fig. 2) by
searching against the VUV spectral library, and this result
was consistent with the identity of the chromatographically
purified product confirmed by 1H NMR spectroscopy. The
spectrum of 9,10-anthraquinone showed strong characteristic
absorption features in the range of 160–185 nm and 185–210 nm.
On demonstrating the capability of these copper(I) com-
plexes to catalyze the oxidation of anthracene and the potential
for GC-VUV, in combination with theoretical methods, for
assignment of reactants and products, we focused our attention
on other PAHs, like naphthalene and pyrene. In the presence of
[N{(CF3)C(C6F5)N}2]CuNCCH3 (4 mol%) and H2O2 (15 mmol),
naphthalene (1 mmol) produced the corresponding 1,4-naphtho-
quinone (NQ), although in a lower yield (19% isolated yield), as
Fig. 1 X-ray crystal structure (left) and ChemDraw sketch (right) of
[N{(CF3)C(C6F5)N}2]CuNCCH3.
The copper(I) catalyst [N{(CF3)C(C6F5)N}2]CuNCCH3 (Fig. 1)
used in this work was synthesized by treating the free ligand
[N{(CF3)C(C6F5)N}2]H73,74 with copper(I) oxide in acetonitrile.
It was isolated as a crystalline solid in 77% yield. Solid samples
and acetonitrile solutions of [N{(CF3)C(C6F5)N}2]CuNCCH3 can
be handled in air for short periods without any apparent signs
of decomposition. Suitable crystals for X-ray analysis were
obtained from a hexane solution of the complex stored at
ꢀ20 1C. The X-ray crystal structure of the acetonitrile adduct
[N{(CF3)C(C6F5)N}2]CuNCCH3 is illustrated in Fig. 1. The triaza-
pentadienyl ligand acts as a chelating, bidentate donor toward
copper(I). The copper center adopts a trigonal planar geometry.
The copper(I) acetonitrile complex [N{(C3F7)C(Dipp)N}2]CuNCCH3
(Dipp = 2,6-diisopropylphenyl) was also synthesized via a similar
route, as reported in our previous work.68 The triazapentadienyl
ligand in [N{(C3F7)C(Dipp)N}2]CuNCCH3 is relatively more
electron-rich and has sterically demanding iso-propyl substituents
on the 2- and 6-positions of the N-aryl groups, compared to the
supporting ligand in [N{(CF3)C(C6F5)N}2]CuNCCH3.
In a typical catalytic process, 4 mol% of the catalyst
[N{(CF3)C(C6F5)N}2]CuNCCH3, 1.0 mmol of PAH substrate,
and 15 mmol of H2O2 in acetonitrile were mixed in a round-
bottomed flask and refluxed at 60 1C under stirring. After 2 h,
the conversion was 100% by TLC and the resulting products
were purified by silica gel-column chromatography and analyzed
using NMR spectroscopy. In addition, the product mixtures
were analyzed using GC-VUV, as discussed below. Our initial
experiments were performed using anthracene as the substrate.
Remarkably, under these conditions a quantitative conversion
Table 1 Oxidation of anthracene (AN) using copper(I) catalyst and H2O2 (AQ = 9,10-anthraquinone)
Catalyst (mol%)
H2O2 (mmol)
Temp. (1C)
Time (h)
Conv. (%)
Isolated yield (%) AQ
[N{(CF3)C(C6F5)N}2]CuNCCH3 (4%)
[N{(CF3)C(C6F5)N}2]CuNCCH3 (4%)
[N{(CF3)C(C6F5)N}2]CuNCCH3 (4%)
[N{(C3F7)C(Dipp)N}2]CuNCCH3 (4%)
CuBr (4%)
1.0
10
15
15
15
60
60
60
60
60
60
2
2
2
2
2
2
39
65
100
66
57
64
28
56
87
54
42
52
Cu(CH3CN)4BF4 (4%)
15
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Fig. 2 VUV library (blue), experimental (orange) and theoretical (green) VUV spectra for anthracene and 9,10-anthraquinone. The theoretical spectra
were simulated with the full width at half maximum (FWHM) of 0.4 eV and computed data (excitation energies and corresponding oscillator strengths)
from TDDFT(PBE0/6-31+G**). All sum-squared residual (SSR) values43 between experimental and calculated spectra were less than 50.
well as a mixture of additional oxidation products. The conversion in Fig. 3. The VUV spectrum of the starting material also has
of naphthalene was rather high at about 64%. Although the yield been included for comparison and to show the agreement
of desired quinone was low and not as impressive as the outcome between experimental data and the computed spectrum. The
involving anthracene, the result was not unusual for a milder yield of 1,4-naphthoquinone was quantified using standard GC
homogeneous catalytic process involving a lighter metal. For calibration procedures and was found to be 21%. The result
comparison, a {Fe(bpmen)(OTf)2} (bpmen = N,N0-bis(2-pyridyl- was in very good agreement with the isolated yield (19%)
methyl)-N,N0-dimethyl-1,2-ethylenediamine) catalyzed process (see Table S1, ESI†).
involving 15 mol% catalyst, 3 eq. of H2O2, and 1 eq. of naph-
Two of the minor oxidation products were also identified as
thalene in the presence of acetic acid produced 1,4-naphtho- 1-naphthol and 2-naphthol from the GC-VUV spectra, as shown
quinone in 32% yield.8 The same Fe(II) system catalyzed the in Fig. 4. It is worth noting that as the level of the products
oxidation of anthracene to 9,10-anthraquinone in 85% yield becomes lower, the detection limits for the GC-VUV instrument
(cf. 87% isolated yield of [N{(CF3)C(C6F5)N}2]CuNCCH3 catalyzed become challenged and spectra become noisier. Such noise
process noted above). To examine the effect of the ligand on the could confound assignments based solely on computational
catalytic reaction with respect to naphthalene, we have used data. In the case of 1- and 2-naphthol, the availability of library
the catalyst [N{(C3F7)C(Dipp)N}2]CuNCCH3, which afforded the spectra and the ability to partially chromatographically resolve
desired 1,4-naphthoquinone (in 14% yield, Table S1, ESI†) along the isomers provided high confidence in the assignments.
with the other oxidized products.
Computational data of 2-naphthol also matched quite well to
Fig. S2 (ESI†) shows the chromatogram of the oxidation the experimental observations. However, computed 1-naphthol
product mixture of naphthalene. We mainly observed several spectrum did not agree completely with the observed data,
products and the unreacted naphthalene from the oxidation especially in the lower energy region. Further studies, perhaps
process, which is in good agreement with TLC on silica gel involving a larger set of related compounds, are needed to
chromatography. The desired product 1,4-naphthoquinone (the understand this anomaly.
peak with a retention time of 8.53 min) was observed as the
In case of the oxidation of pyrene (1 mmol) using
major product, based on the VUV analysis. For example, the [N{(CF3)C(C6F5)N}2]CuNCCH3 catalyst (4 mol%) and H2O2
observed and computed VUV spectra agree well, as illustrated (15 mmol), we have observed the formation of 4,5-, 1,6-, and
Fig. 3 VUV library (blue), experimental (orange) and theoretical (green) VUV spectra of naphthalene and 1,4-naphthoquinone. The theoretical spectra
were simulated with FWHM value of 0.4 eV and computed data (excitation energies and corresponding oscillator strengths) from TDDFT(PBE0/
6-31+G**).
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Fig. 4 VUV library (blue) and experimental (orange) VUV spectra of 1-naphthol and 2-naphthol.
1,8-pyrenequinones in 4%, 5%, and 7% isolated yields, respec- mediated method described here compares well to the much
tively (Scheme 1). These compounds were identified using stronger oxidation conditions reported earlier, as well as to the
1H NMR spectroscopy and by comparing to data from previous mild {Fe(bpmen)(OTf)2} catalyzed process reported by Perez
reports.8 In contrast to the oxidation of anthracene and and co-workers.8
naphthalene, [N{(C3F7)C(Dipp)N}2]CuNCCH3 showed a slightly
We observed a total of 7 peaks including pyrene from the
better catalytic activity for the oxidation of pyrene, obtaining oxidation of pyrene in the chromatogram (Fig. S3, ESI†). Using the
6%, 9%, and 12% isolated yields for 4,5-, 1,6-, and 1,8-pyrene- computational modelling of the observed spectra, we identified
quinones, respectively. However, in both cases, the conversion and assigned three of these pyrene oxidation products as 4,5-, 1,6-,
of pyrene to oxidized products was in the 40–50% range. Upon and 1,8-pyrenequinones (consistent with the identification
increasing the catalyst loading of [N{(C3F7)C(Dipp)N}2]CuNCCH3 based on NMR spectroscopic analysis), with SSRs of 76, 61,
to 6 mol%, we observed an increase in the yields (7%, 11%, and and 35 respectively (Fig. 5). As a reference, the SSR for pyrene is
16% isolated yields for 4,5-, 1,6-, and 1,8-pyrenequinones, 32 (Fig. 5).
respectively). Adding even more catalyst (10 mol%) did not
Note that to identify an analyte, standard spectral analysis
produce a better outcome. Although the isolated yields were normally requires the pre-existence of the pure analyte spectra
relatively modest in the case of pyrene, the mild copper in a searchable database library. Although the VUV library is
Fig. 5 Experimental (orange) and theoretical (green) VUV spectra for pyrene and three pyrene oxidation products. The theoretical spectra were
simulated with FWHM value of 0.4 eV and computed data (excitation energies and corresponding oscillator strengths) from TDDFT(PBE0/6-31+G**).
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developing quickly (B2000 compounds currently), there are Computed line spectra (consisting of excitation energies and
situations where the desired spectra of some molecules have oscillation strengths) were artificially broadened using Gaussian
not been measured and are therefore unavailable in the library. functions, to match the appearance of experimental spectra;
Some pure standards are simply unavailable for populating the improved methods to translate computational data into more
library. The 4,5-, 1,6- and 1,8-pyrenequinones represent three experimental-like data would likely improve the power of this
such examples. In these cases, we have explored the solution of GC-VUV-TDDFT combined strategy for reaction product identifi-
using reliable theoretical spectra generated by computational cation, among other applications.
chemistry (see ESI†) to substitute experimental spectra as noted
here for 4,5-, 1,6- and 1,8-pyrenequinones, and elsewhere for be utilized for the oxidation of anthracene selectively to
other analytes.43
9,10-anthraquinone in quantitative yield. The oxidation of
The copper catalysed process described in this paper can
The isolated yields of 4,5-, 1,6-, and 1,8-pyrenequinones are naphthalene affords 1,4-naphthoquinone as the major product.
consistent with the GC-VUV chromatogram intensity (or peak The yields are comparable to previous reports but the [N{(CF3)C-
height) of the corresponding peaks in the chromatogram. (C6F5)N}2]CuNCCH3 mediated process does not involve the use of
As it is quite time consuming, very tedious, and requires a fair acids. Two minor products were also identified as 1-naphthol and
amount of solvent to separate and isolate these oxidized 2-naphthol using GC-VUV spectra. The pyrene oxidation resulted
quinone products from silica gel-column chromatography, in 4,5-, 1,6-, and 1,8-pyrenequinones (with a combined yield of
the GC-VUV spectroscopic technique presented here in combi- B34%), among other products. A comparison of the observed
nation with computational modeling, is a worthy alternative for experimental VUV spectra of each oxidation product to the
product identification and sample screening. Other PAHs, like calculated spectra of possible and reasonable oxidation products
phenanthrene (1 mmol) did not produce the corresponding (derived by common sense of chemistry) allowed us to identify
9,10-phenanthrenequinone using [N{(CF3)C(C6F5)N}2]CuNCCH3 three of the unknown as 4,5-, 1,6-, and 1,8-pyrenequinones. This
catalyst (4 mol%) and H2O2 (15 mmol), according to the was further confirmed by NMR spectroscopic analysis of the
GC-VUV spectroscopic analysis.
purified products (separation of products of this complex mixture
is a very tedious process). We are presently probing other metal
catalysts/complexes for selective oxidation of PAH under mild
conditions, and the use of GC-VUV and theoretical computations
as a new tool-set for unknown determination. Each of these
Summary and conclusions
Selective oxidation of polycyclic aromatic hydrocarbons to obtain combined aspects presented in this initial study can be indepen-
specific products is an important and challenging process. Often dently improved to create a powerful workflow for advancing and
harsh conditions are required for oxidation, including acidic characterizing useful and complex reaction chemistries.
media, which result in product mixtures that are difficult to
separate or to identify. We have uncovered a relatively mild,
copper catalyzed process for the oxidation of anthracene,
naphthalene, and pyrene, using H2O2 as the oxidant without
Experimental procedures
General procedures
an acid co-catalyst.
All manipulations were carried out under an atmosphere of
Identification of unknowns from mixtures using analytical-
purified nitrogen using standard Schlenk techniques. Solvents
scale chromatographic techniques relies upon either retention
were purchased from commercial sources and purified by
time matching to a pure standard or the availability of a
conventional methods prior to use. Glassware was oven-dried
detector, which can deduce appropriate qualitative information.
at 150 1C overnight. The NMR spectra were recorded at
However, even for the latter (e.g., using a mass spectrometric
the room temperature on a JEOL Eclipse 500 spectrometer
detector), certain species may be largely indistinguishable
(1H: 500.16 MHz). Proton chemical shifts are reported in ppm
because of their isomeric nature. The products resulting from
and are referenced using the residual proton and carbon
oxidation of a PAH such as pyrene is a good example. Gas
chromatography-vacuum ultraviolet spectroscopy provides for
signals of the deuterated solvent (1H: CDCl3, d 7.26). 19F NMR
chemical shifts were referenced relative to external CFCl3.
high resolution spectroscopic analysis of separated compounds.
Elemental analyses were performed at Intertek USA, Whitehouse,
The shape of gas phase absorption spectra in the 125–240 nm
NJ. Deuterated solvents were purchased from Cambridge Isotope
wavelength range are highly sensitive to molecular structure and
Laboratories. The Cu2O was purchased from commercial sources.
atom connectivity, and thus provide a good way to distinguish
many isomeric species, which may be indistinguishable by
mass spectrometry. Here we have demonstrated the use of
[N{(CF3)C(C6F5)N}2]H.73,74 and [N{(C3F7)C(Dipp)N}2]CuNCCH3
68
were synthesized using published procedures.
experimental absorption spectra, combined with spectral library
matching, and computed absorption spectra for the analysis of
[N{(CF3)C(C6F5)N}2]CuNCCH3 (1)
products resulting from PAH oxidation. Based on the data in this [N{(CF3)C(C6F5)N}2]H (0.5 g, 0.93 mmol) and Cu2O (0.099 g,
and other work, where time-dependent density functional theory 0.69 mmol) were mixed in acetonitrile (ca. 15 mL) and refluxed
was explored to predict or match experimental data, it is apparent for 12 hours. The resulting solution was filtered through a bed
that there is still room to improve computational methods. of Celite. The filtrate was collected and the solvent was removed
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hexane and cooling to ꢀ20 1C. Isolated yield 69%. 1H NMR
(CDCl3, 500.16 MHz, 298 K): d 2.15 (s, 3H, NCCH3). 19F NMR
(CDCl3, 470.62 MHz, 298 K): d ꢀ67.54 (s, CF3), ꢀ149.26
(d, J = 21.9 Hz, O-C6F5), ꢀ160.74 (t, J = 22.0 Hz, p-C6F5),
ꢀ163.00 (t, J = 18.3 Hz, m-C6F5). Anal. calcd for C18H3CuF16N4:
C, 33.63; H, 0.47; N, 8.72. Found: C, 33.58; H, 0.45; N, 8.77.
General catalytic procedure for direct oxidation of polycyclic
aromatics into quinones
The oxidation reaction was performed in a 25 mL round
bottomed flask equipped with a reflux condenser and a mag-
netic stirrer bar. In a typical experiment, 1.5 mL (15 mmol)
of an aqueous solution of hydrogen peroxide (30% w/v) was
added to a solution of 25.7 mg (0.04 mmol) of [N{(CF3)-
C(C6F5)N}2]CuNCCH3 and 178.2 mg (1.0 mmol) of anthracene
in 6 mL of a mixture dichloromethane : acetonitrile (1 : 1, v/v).
The resulting solution was stirred for 2 hours at 60 1C, and then
cooled to room temperature. The combined organic layers were
dried over anhydrous Na2SO4, evaporated under reduced pressure
and purified by column chromatography (hexane : ethyl acetate
7 : 3) to obtain pure 9,10-anthraquinone with 100% conversion.
Isolated yield 87%.
X-ray crystallography
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Conflicts of interest
There are no conflicts to declare. Disclaimer: KAS is a member
of the scientific advisory board for VUV Analytics, Inc.
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Acknowledgements
This work was supported by the Robert A. Welch Foundation
(Grant Y-1289). Additional support was provided by VUV
Analytics, Inc.
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