Controlled Photocatalytic Hydrocarbon Oxidation by Uranyl Complexes

Article abstract of DOI:10.1002/cctc.201900037

Controlled, photocatalytic C?H bond activations are key reactions in the toolkits of the modern synthetic chemist. While it is known that the uranyl(VI) ion, [U^VIO₂]²⁺, the environmentally dominant form of uranium, is photoactive, most literature examines its luminescent properties, neglecting its potential synthetic utility for photocatalytic C?H bond cleavage. Here, we synthesise and fully characterise an air-stable and hydrocarbon-soluble uranyl phenanthroline complex, [U^VIO₂(NO₃)₂(Ph₂phen)], U^Ph2phen, and demonstrate that it can catalytically abstract hydrogen atoms from a variety of organic substrates under visible light irradiation. We show that the commercially available parent complex, uranyl nitrate ([U^VIO₂(NO₃)₂(OH₂)₂]?4H₂O; U^NO3), is also competent, but from electronic spectroscopy we attribute the higher rates and selectivity of U^Ph2phen to ligand-mediated electronic effects. Ketones are selectively formed over other oxygenated products (alcohols, etc.), and the catalytic oxidation of substrates containing a benzylic C?H position is particularly improved for U^Ph2phen. We also show uranyl-mediated photocatalytic C?C bond cleavage in a model lignin compound for the first time.

Full text of DOI:10.1002/cctc.201900037

Communications
DOI: 10.1002/cctc.201900037
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Controlled Photocatalytic Hydrocarbon Oxidation by Uranyl
Complexes
Polly L. Arnold,*[a] Jamie M. Purkis,[a] Ryte Rutkauskaite,[a] Daniel Kovacs,[b] Jason B. Love,[a]
and Jonathan Austin[c]
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or aromatic group (i.e. toluene[3]) by quenching of the organic
Controlled, photocatalytic CÀ H bond activations are key reac-
radicals with molecular oxygen.[3b,4] Aqueous solutions of uranyl
tions in the toolkits of the modern synthetic chemist. While it is
known that the uranyl(VI) ion, [UVIO2]2+, the environmentally
have also been studied in considerable detail for the photo-
catalytic destruction of organic pollutants.[5] In these cases,
dominant form of uranium, is photoactive, most literature
uranyl speciation varies substantially with pH or counter-anion
(i.e. nitrate and carbonate),[6] causing these reactions to be
unselective.[7] Recently, however, Sorensen and co-workers
reported the first use of photo-excited uranyl in the selective
fluorination of cycloalkanes in organic media (Scheme 1).[8]
Judicious choice of both organic solvent (CD3CN or acetone-D6)
and anion (nitrate vs. acetate) were crucial, and recent quantum
mechanical calculations highlighted the complex interplay of
singlet and triplet excited states in the reaction coordinate.[9]
Subsequently, Azam and co-workers used a chiral salen1 ligand
to saturate the uranyl equatorial coordination plane, resulting
in a photoactive complex that acts as a catalyst for the α-
cyanation of anilines (Scheme 1);[10] in this case, commercially
available uranyl acetate, [UVIO2(OAc)2(OH2)2], shows no activity.
It has also recently been shown that photoexcited
[UVIO2(CO3)3]4À in uranyl tricarbonate, the molecular analogue of
the naturally-occurring rutherfordine mineral, can oxidise bor-
ohydrides to boric acid under photolysis.[11]
examines its luminescent properties, neglecting its potential
synthetic utility for photocatalytic CÀ H bond cleavage. Here, we
synthesise and fully characterise an air-stable and hydrocarbon-
soluble uranyl phenanthroline complex, [UVIO2(NO3)2(Ph2phen)],
U
Ph2phen, and demonstrate that it can catalytically abstract
hydrogen atoms from a variety of organic substrates under
visible light irradiation. We show that the commercially
available
parent
complex,
uranyl
nitrate
([UVIO2(NO3)2(OH2)2]·4H2O; UNO3), is also competent, but from
electronic spectroscopy we attribute the higher rates and
selectivity of UPh2phen to ligand-mediated electronic effects.
Ketones are selectively formed over other oxygenated products
(alcohols, etc.), and the catalytic oxidation of substrates
containing a benzylic CÀ H position is particularly improved for
U
Ph2phen. We also show uranyl-mediated photocatalytic CÀ C
bond cleavage in a model lignin compound for the first time.
Due to their capability as ligands[12] and ubiquity in photo-
redox reactions,[13] we reasoned that phenanthroline ligands
would be excellent candidates for ligands in new photoactive
uranyl complexes; the simplest member of the series,
[UVIO2(NO3)2(phen)] (phen=phenanthroline), has been reported
previously, but not tested photocatalytically.[14] Here, we report
the synthesis and characterisation of a new uranyl phen
The photo-excited state of the uranyl ion, denoted [*UVIO2]2+, is
a highly oxidising (ca. +2.6 V, cf. F2) and long-lived (~ μs) motif,
and is accessible using visible and UV light (ca. 300–420 nm).[1]
!
The absorption of 420 nm light causes a weak U(5f) O(2p)
ligand-to-metal charge transfer (LMCT) which is thought to
*
form the highly reactive 5f1 uranyl(V) oxyl ion [O=UVÀ O ]+. This
excited state is readily quenched by contact with an organic
hydrocarbon, either by hydrogen atom abstraction (HAA) from
an aliphatic group to give a functionalised [O=UVÀ OH]2+ ion
and organic radical,[2] or electron transfer with an unsaturated
complex, [UVIO2(NO3)2(Ph2phen)] (UPh2phen
phenyl-1,10-phenanthroline), and demonstrate that it is a more
) (Ph2phen=4,7-di-
effective
photocatalyst
than
uranyl
nitrate
[UVIO2(NO3)2(OH2)2]·4H2O (UNO3) for the oxidation of a variety of
organic substrates.
The addition of one equivalent of Ph2phen to UNO3 in
acetonitrile solution yields bright yellow, air-stable UPh2phen in
94% yield (Scheme 2). Repeating the reaction with UNO3 and
phenanthroline (phen) or 2,2’-bipyridine (bipy) yields the known
phen adduct which is too insoluble to be useful for catalysis in
organic solutions,[i] or simply the protonated adduct [Hbi-
py]2[{UVIO2(NO3)2}2(μ-OH)2][15], respectively. The 1H NMR spectrum
of UPh2phen (Figure S2) reveals that the H atoms closest to the UVI
centre are deshielded by approx. 1.4 ppm and, therefore, that
the ligand is likely complexed in CH3CN. The absence of the H-
[a] Prof. P. L. Arnold, J. M. Purkis, R. Rutkauskaite, Prof. J. B. Love
EaStCHEM School of Chemistry, Joseph Black Building
University of Edinburgh
Edinburgh, EH9 3FJ (United Kingdom)
E-mail: Polly.Arnold@ed.ac.uk
[b] D. Kovacs
Ångströmslaboratoriet, Uppsala University
Lägerhyddsvägen 1, 752 37 Uppsala (Sweden)
[c] Dr. J. Austin
National Nuclear Laboratory
5th Floor, Chadwick House, Birchwood Park
Warrington WA3 6AE (United Kingdom)
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201900037
This manuscript is part of the Special Issue dedicated to the Women of
Catalysis.
1 H2L, salen, is 2,2’-((1E,1’E)-(cyclohexane1,2-diylbis(azanylylidene))bis(metha-
nylylidene))diphenol.
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Scheme 1. Photocatalytic fluorination of sp3 CÀ H bonds, and α-cyanation of anilines using a uranyl salen complex.
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Scheme 2. Synthesis of UPh2phen from UNo3 and Ph2phen in CH3CN.
bonding νO-H stretch at ca. 3300 cmÀ 1 in the infrared spectrum
(solid state) suggests the compound is anhydrous (i.e. no
coordinated water). The asymmetric νU=O stretch at 936 cmÀ 1
and diagnostic modes of bidentate[16] nitrato ligands at ca.
1280 cmÀ 1 for UPh2phen are similar to those in the parent
[UO2(NO3)2(phen)][14] (νU=O =942 cmÀ 1, νNO =1286 cmÀ 1) but dif-
ferent from UNO3 (νNO =1300, 1330 cmÀ 1), also suggesting
complexation (Figures S3 & S4).
ca. 420 nm for UPh2phen (21692 cmÀ 1) and UNO3 (21400 cmÀ 1), are
similar, suggesting the energy of this transition is minimally
affected by Ph2phen coordination.
The emission spectrum of UPh2phen has bands at 288 nm and
365 nm (Figure S12) and also contains a broad, featureless band
at ca. 520 nm from irradiation at either 288 or 365 nm. These
are tentatively assigned as ligand absorptions, and combined
with the uranyl absorption at ca. 420 nm contribute to the
broad emission profile centred at ca. 520 nm. While time-
dependent density functional theory (TD-DFT) on 5f ions with
open-shell, 5f1 excited states are non-trivial,[20] these observa-
tions suggest a degree of electronic mixing between metal and
The solid-state structure of UPh2phen (Figure S13) is also
similar to that of the parent [UVIO2(NO3)2(phen)]. The uranium
centre possesses a distorted 8-coordinate hexagonal bipyrami-
dal coordination geometry in which the short U=Oyl distances
(Oyl = the uranyl oxo group) (1.747(3)–1.756(3) Å) and the
ligand that modulates the luminescent properties of UPh2phen
.
°
essentially linear O=U=O angle (177.0(2) ) are consistent with
Because UPh2phen has this readily accessible, ligand-modified
excited state, contains no coordinated water, and is soluble in
organic solvents (in contrast to the previously reported
unsubstituted phen complex),[17] we have studied its capacity to
react photolytically with organic substrates in a controlled
manner (Scheme 3 and Table 1).
uranyl(VI) (Table S4).[17]
Pertinently, the presence of the phen ligand in UPh2phen
increases the peak intensity of the uranyl LMCT band in its
electronic absorption spectrum relative to UNO3 (Figures S8 and
S9), with ɛ425 increasing from 11 MÀ 1cmÀ 1 for UNO3 to ca.
65 MÀ 1cmÀ 1 in UPh2phen. UV-energy ligand absorptions for UPh2phen
are also bathochromically shifted (i.e. towards the visible;
π!π*, 270!285 nm; n!π*, 221!225 nm).[18] The emission
spectra of UPh2phen (Figures S10–12) show a broad featureless
band at ca. 520 nm, consistent with a reduction in symmetry at
the uranium centre, and in contrast to well-resolved fine
structure in the analogous emission profile of UNO3 [19]
.
The
Scheme 3. Conditions employed for substrate oxidation catalysed by UNo3
(10 mol%) and UPh2phen (5 mol%). R=hydrocarbyl.
Stokes shift for UPh2phen of 4645 cmÀ 1 is larger than that of UNO3
,
3820 cmÀ 1, consistent with a greater degree of structural
reorganisation in the excited-state for the more complicated
!
ligand. The E0-0 values2 for the weak U(5f) O(2p) LMCT band at
It was found that UPh2phen gives higher conversions than UNO3
for all substrates tested. For example, using UPh2phen, the
archetypal substrate 9,10-dihydroanthracene (DHA; D(CÀ H)=
326.4 kJmolÀ 1, pKa 30 in DMSO)[21] undergoes catalytic H-atom
abstraction to form anthracene in 89% yield (Table 1 entry 1,
2 zero-zero transition, E0-0, is the energy difference between ground vibra-
tional state of ground electronic state and ground vibrational state of first
electronically-excited state.
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Table 1. Comparison of UPh2phen and UNo3 as homogeneous photocatalysts for the controlled CÀ H bond cleavage of a range of substrates.[a]
1
2
3
Overall conversion[a]
[%]
Major (and minor) measurable product(s) of oxidation (Yield [%])
Entry
Substrate
UNO3
UPh2phen
UNO3 UPh2phen
4
5
6
7
8
9
DHA[d]
See Table 2
1
68[b]
89[b]
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Toluene
PhCH3
2
3
2[b]
35[c]
33[b]
Diphenylmethane
Ph2CH2
21[c]
Triphenylmethane
Ph3CH
4
14[b]
37[b]
5
Indane
47[b]
87[b]
6
7
Phthalan
99[b]
99[c]
Isochroman
35[b]
80[c]
8
9
2P1PE
19[c]
0
18[c]
0
DHA, control
–
–
[a] UNO3 10 mol% or UPh2phen 5 mol%, in CH3CN (5 mL) for 16 hours at 293 K with hν (420 nm); [b] GC-MS; [c] 1H NMR spectroscopy and GC-MS; [d] DHA is 9,10-
dihydroanthracene; [e] inferred (anthracene photo-dimerises under these conditions)
and see below), compared with 68% conversion using UNO3. The
most significant improvement in product conversion is for
toluene, with an increase from 2% to 35% upon changing from
UNO3 to UPh2phen as catalyst (Table 1, entry 2). Conversions of the
simple benzylic hydrocarbons Ph2CH2 and Ph3CH are both
increased, from 21% to 33%, and 14% to 37%, respectively
(Table 1. entries 3 & 4), and the most favoured products for
both catalysts are PhCHO (formed from toluene), Ph2CO (formed
from Ph2CH2) or Ph3COH (formed from Ph3CH). Conversion of
indane and isochroman roughly doubles on changing catalyst
from UNO3 to UPh2phen, increasing from 47% to 87% and 35% to
80%, respectively; these compounds are selectively oxidised at
the benzylic positions, with the ketones, rather than alcohols, as
the favoured products (Table 1, entries 5 & 7). In contrast,
conversion of phthalan (entry 6) is quantitative for both
catalysts and there is negligible change in product selectivity,
forming the lactone in excellent yields (70–80%). Control
reactions (Table 1, entry 9) in the absence of catalyst show no
conversion.
We were curious to see if the reaction scope could be
extended from catalytic CÀ H to CÀ C bond cleavage. Lignin
mimics that possess benzylic CÀ H bonds, such as 2-phenoxy-1-
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phenylethanol (2P1PE; Table 1, entry 8), have been reported to
undergo CÀ C bond cleavage rather than CÀ H bond
activation,[22] for example, when treated with [VVO2(acac)]
(acac=acetylacetonate) under photocatalytic conditions, yield-
ing benzaldehyde and benzoic acid with 54% conversion in
CH3CN.[23] Similarly, both UNO3 and UPh2phen catalyse photolytic
CÀ C bond cleavage, albeit at lower yields (19% and 18% by
(758.0274 Da)
and
[(UVIO2)2(μ-O2)(NO3)3(Ph2phen)]À
1
2
3
4
5
6
7
8
9
(1090.1711 Da) are found. Bands in the Raman spectrum at 838
and 849 cmÀ 1 (Figure S7) also compare well with the peroxo-
bridged complex, [{UVIO2(NO3)(py)2}2(μ-O2)]·py[24] which has a
,
symmetric μ-O2 stretch at 860 cmÀ 1. Other reported uranyl-
peroxo oligomers have Raman bands between 820 and
870 cmÀ 1.[25] This product could be formed as the result of a
photolytically-induced oxygen reduction, something observed
very recently in other uranyl complexes derived from
photoreactions.[26] Combustion analysis performed on this solid
(42.5% C, 2.7% H, 6.0% N) is also consistent with a formulation
of [(UVIO2)2(μ-O2)(NO3)2(Ph2phen)2].
2+
GC-MS, respectively; Table S8, entry 8), suggesting that *UVIO2
-mediated reactivity may be viable for lignin decomposition.
CÀ C bond activation with uranyl has been reported
previously.[3]
The catalytic oxidation of DHA was studied in more depth
to investigate other factors that influence the reactions: the two
complexes UNO3 and UPh2phen both form anthracene as the
oxidation product in low to excellent yields depending on the
conditions (Table 2). Using UNO3 at 0.5 mol% loading, the
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DHA consumption plateaus at roughly 6 or 7 hours with
U
Ph2phen, quicker than UNO3 at 9 hours (Figure S21). The addition
of equimolar anthracene at the start of the reaction also causes
conversion to drop from 53 to 27% (Table S6, entry 6; Table S7,
entry 7; Figure S24). As both anthracene and UPh2phen have
absorption bands at ca. 360 nm, we suggest there is compet-
itive photon absorption between UPh2phen and anthracene (λmax
356 nm) at the tailing edge of the lamp spectral output (Figure.
S1), reducing conversion when anthracene is present. This is
not observed for UNO3 as there are no absorption bands at ca.
360 nm.
Table 2. Comparison of conversions between UNO3 and UPh2phen at different
loadings and times.[a]
Entry
Catalyst loading [%]
T [h]
DHA conversion [%]
UNO3
UPh2phen
1
2
3
4
5
6
7
8
0.001
0.01
0.1
1
0.5
0.5
0.5
0.5
3
3
3
3
1
2
4
8
33
36
32
37
16
28
48
86
23
34
45
57
24
38
64
96
Reactions of either catalyst in benzonitrile solvent instead of
acetonitrile roughly halve the conversion (Table S6, entry 7;
Table S7, entry 8; Figure S23). For example, in a reaction at
0.5 mol% UPh2phen loading, switching solvent from CH3CN to
benzonitrile reduces the conversion from 53 to 37% after
3 hours. This is probably because the benzonitrile solvent can
quench the uranyl photoexcited state by forming an exciplex
through the aromatic π-systems that decays through non-
radiative processes.[3a,27] For UNO3, the role of water was also
[a] DHA (450 mg), CH3CN (50 mL), appropriate [cat.], 293 K with hν
1
(420 nm) over time. Analysis by H NMR spectroscopy at appropriate time
intervals.
examined; the addition of 100 eq. of water to the 5 mol% UNO3
/
oxidation of DHA is almost complete after 8 h with 86%
conversion (Table 2 and Figure S21), with further irradiation
resulting in photodegradation of products. Catalyst loadings of
DHA/CH3CN reaction mixture (Table S6, entry 12; Figure S22)
increases the initial rate of DHA consumption, which then tails
off over time with ca. 35% conversion of DHA observed after
3 h (i.e. conversion is initially faster with added water, but after
3 h is equivalent). UPh2phen is also hydrolytically stable, and
shows no sign of decomplexation in the presence of up to
20 eq. of water in CH3CN solution (Figure S52). It is possible that
the added water for the UNO3 reaction forms stabilising, hydro-
gen-bonding interactions with oxygen-derived radicals and ions
near the outer coordination sphere of the uranyl in the
intermediates and thus increases the initial rate of DHA
consumption.
U
NO3 between 0.001 and 25 mol% were further tested (Table S2;
Figures S15 & S16, S25–S39) and show the reaction is 0th order
in catalyst under these photolytic conditions, with conversions
of around 35% at all concentrations (Figure S20; Table S6,
entries 1–5, 8–10, 14 & 15); i.e. catalyst concentration has no
discernible effect on DHA conversion under these conditions.
No conversion occurs for samples stored in the dark, and there
is no change in DHA conversion in the presence of mercury
droplets (5 mol% UNO3), suggesting that the reaction does not
proceed heterogeneously (Table S6, entry 13).
Oxygen is necessary in this system for turnover and is likely
required to reoxidise the UV [UO2H]2+ ion that is first formed
from the H atom abstraction, Equation (1).
For UPh2phen increasing catalyst loading from 0.001%–1%
sees conversion increase from 23 to 57% within the first 3 hours
(Figures S18–S20, S44–S51; Table S7, entries 1–6, 8 (see also
entries 1-4 in Table 2)), in marked contrast to conversion
2 UVIO2 ! 2 ½UVO2H� ! UIV þ UVIO2
2þ
2þ
2þ
ð1Þ
employing UNO3. However, at higher loadings of UPh2phen
,
A reaction mixture containing 5 mol% UNO3 and DHA was
irradiated in the absence of oxygen, upon which a grey-black
precipitate (51 mg) is formed (Table S6, entry 11). The uranium-
containing (41.8% U, ICP-MS) precipitate contains no UVI uranyl
but water, nitrate, and organic material are present according
to FTIR spectroscopy and combustion analysis (see SI). This
precipitation of a yellow solid is commonly observed after
several minutes of photolysis in the presence of substrate (no
precipitate is observed upon photolysis of a solution of only
UPh2phen in CH3CN). This yellow precipitate is characterised as a
uranyl- and peroxo-containing oligomer, as ions in the mass
spectrum
that
correspond
to
[(UVIO2)2(μ-O2)(NO3)3]À
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precipitate becomes yellow on standing in air for 48 h and a
stretch that may be assigned to the asymmetric OUO stretch of
the uranyl ion becomes visible in FTIR spectra (Figure S5),
suggesting re-oxidation. This compound is probably an aggre-
gate of UIV, nitrate and oxidised substrate which is not re-
1
2
3
4
5
6
7
8
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generated into [UVIO2]2+
.
In summary, we have prepared and fully characterised a
new uranyl photocatalyst, UPh2phen, which shows higher con-
versions than UNO3 in the oxidation of selected substrates,
attributed to ligand-mediated electronic effects. Substrates with
benzylic CÀ H bonds are most readily oxidised, with toluene,
triphenylmethane, indane and isochroman showing the highest
improvements in oxidation whencatalysed by UPh2phen. We also
report preliminary results on the uranyl-mediated photocata-
lytic CÀ C bond cleavage in a model lignin compound for the
first time. We attribute the higher efficacy of UPh2phen to ligand-
tuning of the excited state and work to understand the
mechanistic implications of this is underway.
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We are grateful to Dr. Louise Natrajan (University of Manchester)
and Prof. Anita C. Jones (University of Edinburgh) for help with the
fluorescence measurements. We thank the Nuclear Decommission-
ing Authority for financial support and the UK National Nuclear
Laboratory for industrial supervision. This project has received
funding from the European Research Council (ERC) under the
European Union’s Horizon 2020 research and innovation pro-
gramme (grant agreement No 740311, PLA). We also thank the
University of Edinburgh and the EPSRC for funding under grants
EP/N022122/1 and EP/M010554/1. DK thanks Liljewalch travel
scholarships, Uppsala University, Sweden.
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The authors declare no conflict of interest.
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Keywords: Actinides · CÀ H bond activation · fluorescence
spectroscopy · photocatalysis · uranium
Manuscript received: January 5, 2019
Revised manuscript received: February 7, 2019
Version of record online: ■■■, ■■■■
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J. Su, E. Mitchell, G. Zhang, J. Li, Sci. China Chem. 2013, 56, 1671–1681.
ChemCatChem 2019, 11, 1–6
www.chemcatchem.org
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Communications
COMMUNICATIONS
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Prof. P. L. Arnold*, J. M. Purkis, R.
Rutkauskaite, D. Kovacs, Prof. J. B.
Love, Dr. J. Austin
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Controlled Photocatalytic Hydro-
carbon Oxidation by Uranyl
Complexes
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HATs off to U An air-stable, organic-
soluble uranyl-phenanthroline
complex, [UO2(NO3)2(Ph2phen)],
U
Ph2phen compared to the parent
uranyl nitrate (also tested here) to
ligand-mediated electronic effects.
The catalytic oxidation of substrates
containing a benzylic CÀ H position is
particularly improved for UPh2phen. We
also demonstrate uranyl-mediated
photocatalytic cleavage in a model
lignin compound for the first time.
U
Ph2phen (Ph2phen=4,7-diphenyl-1,10-
phenanthroline) demonstrates
catalytic hydrogen atom abstraction
from a variety of organic substrates
under visible-light irradiation. From
photophysical studies we attribute
the higher rates and selectivity of