Solution or solid-it doesn't matter: Visible light-induced CO release reactivity of zinc flavonolato complexes

Article abstract of DOI:10.1039/c6dt01709f

Two types of zinc flavonolato complexes ([(6-Ph₂TPA)Zn(flavonolato)]ClO₄ and Zn(flavonolato)₂) of four extended flavonols have been prepared, characterized, and evaluated for visible light-induced CO release reactivity. Zinc coordination of each flavonolato anion results in a red-shift of the lowest energy absorption feature and in some cases enhanced molar absorptivity relative to the free flavonol. The zinc-coordinated flavonolato ligands undergo visible light-induced CO release with enhanced reaction quantum yields relative to the neutral flavonols. Most notable is the discovery that both types of zinc flavonolato derivatives undergo similar visible light-induced CO release reactivity in solution and in the solid state. A solid film of a Zn(flavonolato)₂ derivative was evaluated as an in situ CO release agent for aerobic oxidative palladium-catalyzed alkoxycarbonylation to produce esters in ethanol. The CO release product was found to undergo ester alcolysis under the conditions of the carbonylation reaction.

Full text of DOI:10.1039/c6dt01709f

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Solution or solid – it doesn’t matter: visible light-
induced CO release reactivity of zinc flavonolato
complexes†
Cite this: DOI: 10.1039/c6dt01709f
Stacey N. Anderson, Michael T. Larson and Lisa M. Berreau*
2 4 2
Two types of zinc flavonolato complexes ([(6-Ph TPA)Zn(flavonolato)]ClO and Zn(flavonolato) ) of four
extended flavonols have been prepared, characterized, and evaluated for visible light-induced CO release
reactivity. Zinc coordination of each flavonolato anion results in a red-shift of the lowest energy absorp-
tion feature and in some cases enhanced molar absorptivity relative to the free flavonol. The zinc-
coordinated flavonolato ligands undergo visible light-induced CO release with enhanced reaction
quantum yields relative to the neutral flavonols. Most notable is the discovery that both types of zinc
flavonolato derivatives undergo similar visible light-induced CO release reactivity in solution and in the
Received 1st May 2016,
Accepted 23rd August 2016
2
solid state. A solid film of a Zn(flavonolato) derivative was evaluated as an in situ CO release agent for
DOI: 10.1039/c6dt01709f
aerobic oxidative palladium-catalyzed alkoxycarbonylation to produce esters in ethanol. The CO release
product was found to undergo ester alcolysis under the conditions of the carbonylation reaction.
www.rsc.org/dalton
To address issues of solubility and concerns regarding the
use of redox-active and heavy metals in MCCs, efforts to encap-
Introduction
Delivery of controlled amounts of carbon monoxide (CO) is of sulate these compounds in micelles, dendrimers, and solid
1
1,15–19
significant current interest both in biology and synthetic supports have been reported.
Efforts to develop metal-
2
0–23
chemistry. In humans, CO is produced endogenously via the free organic photoCORMs have also advanced.
enzyme-catalyzed breakdown of heme. Studies of the effects opment of organic photoCORMs holds significant promise as
The devel-
1
of CO have demonstrated its potential to produce a variety of standard approaches employed in medicinal chemistry for
2
4–26
beneficial health outcomes including anti-inflammatory, anti- tuning and targeting could be possible.
apoptotic, vasodilation, and anti-bacterial effects. To explore
2
,3
CO-releasing molecules are also of significant current inter-
the biology of CO and its possible uses as a therapeutic, CO- est for applications in synthetic organic chemistry. Due to the
releasing molecules (CORMs) have been developed for the health hazards of handling CO gas, efforts in recent years have
4
delivery of controlled amounts of CO. To date, the majority of focused on the development of molecules that release CO on
CORMs investigated have been metal carbonyl compounds demand for synthetic applications, such as palladium-cata-
2
7–32
(
(
MCCs), with the most extensively studied being [Ru lyzed carbonylation reactions.
The majority of the CO
5
CO)
3
(glycinate)Cl] (CORM-3). This compound coordinates to release compounds currently being used in synthetic organic
histidine residues of proteins and spontaneously releases CO applications either are used in an ex situ fashion or are used
6
–8
via ligand exchange.
enzyme-induced reactivity,
visible light-induced reactivity (photoCORMs).
Other MCC CORMs release CO via in situ but require the introduction of additional reagents,
9
,10
11
magnetic heating,
or via microwave irradiation, or heating to induce CO release. Ex situ
1
2–14
CORMs approaches require either the use of specialized two-chamber
that can be controlled in terms of their CO release reactivity glassware to separate the CO-generation process from the sub-
2
7–31
are especially attractive as they offer the opportunity for loca- strate reaction chamber,
or manual transfer of the gas (e.g.
3
2
lized release of CO at specific sites for biological studies or via a balloon). In situ sources for CO have also been pre-
therapeutic purposes.
viously reported. These include metal carbonyls (e.g. Mo(CO)6),
which can serve as in situ CO release agents for palladium-cata-
lyzed carbonylations, albeit not at a stoichiometric level with
3
3–43
respect to CO.
When metal carbonyls are used, additional
Department of Chemistry & Biochemistry, Utah State University, 0300 Old Main Hill,
Logan, UT 84322-0300, USA. E-mail: lisa.berreau@usu.edu
reagents, high temperatures and/or microwave assistance are
generally needed for CO release. It is also important to note
that low valent metal carbonyls typically cannot be used in situ
†
Electronic supplementary information (ESI) available: Spectra and additional
experimental details. See DOI: 10.1039/c6dt01709f
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for substrates containing functional groups that can be Table 1 Properties of 1–4
reduced (e.g. –NO derivatives) due to the reducing nature of
2
a
3
4–40,44
λmax (nm)
Aerobic
CO release
Anaerobic
CO release
the low valent metal complex.
Two chamber systems
−
1
b
c
Compound
ε (M cm−1)
Φ
4
5
46
can be used to address this issue. Formamides (including
4
7,48
49,50
1
2
3
4
409 (16 600)
442 (51 000)
478 (36 700)
544 (85 500)
Yes
Yes
Yes
Yes
0.007(3)
0.006(1)
d0.43(3)
No
No
No
Yes
N-formylsaccharin
), formates and formic acid
deriva-
5
1
52,53
tives, as well as CHCl
3
,
and diethylpyrocarbonate
have
been recently investigated as in situ CO release agents at near
stoichiometric levels. However, all of these CO release com-
pounds require either significant heat (formamides and
formic acids, diethylpyrocarbonate), or the presence of enthesis represent standard error. Quantum yield for CO release not
additional reagents (e.g. N-formylsaccharin, mild base or KF);
a
b
Most red-shifted absorption feature. Measured in acetonitrile in the
presence of air. Average of three independent trials; values in par-
c
d
reported due to concurrent aerobic and anaerobic CO release
reactions.
formates (mild base); formic acid (strongly acidic conditions);
3
CHCl (metal hydroxide) to induce CO release.
An area that has seen little development is that of solid
compounds capable of visible light-induced CO release. Such
compounds could offer the possibility of CO release materials
that could have tremendous advantages for both biological
and synthetic applications. Mascharak recently reported two
visible light-induced MCC photoCORMs that undergo solid-
of the CO-releasing moiety. In the studies reported herein, we
have used a supporting ligand structural template from our
laboratory that has been employed in studies of other metal
5
9–61
flavonolato compounds
to evaluate how zinc stabilization
of 1–4 affects CO release reactivity. Key findings include that
zinc coordination: (1) red shifts the absorption spectral fea-
tures of the compounds, with some being in the therapeutic
window (>650 nm), (2) significantly enhances the quantum
yield for visible light-induced CO release, and (3) enables
solid-state CO release activity that is identical to the reactivity
seen in solution. The discovery of visible light-induced solid-
state CO release reactivity is particularly significant as it
suggests that CO release materials based on a zinc flavonolato
motif could be developed. Building on this discovery, we
sought to design zinc flavonolato derivatives that are insoluble
in organic solvents, which could facilitate their use as in situ
light-driven CO release agents in synthetic organic processes.
Our experience with simple Zn(II) bis-flavonolato compounds
5
4
state CO release. However, the quantity of CO released and
the byproducts of this reactivity have not been reported.
Our laboratory is developing visible light-induced CO
release compounds based on the 3-hydroxy-4-flavone moiety
found in naturally occurring flavonols. Prior to our work,
metal flavonolato species were well known to undergo dioxy-
genase-type degradation to release CO via either enzyme cataly-
5
5
56
sis or thermal reactivity involving O
reports of light-driven CO release reactivity involving O
vation with either free or metal-coordinated flavonols were
2
activation. However,
2
acti-
5
7
scarce. We recently reported a new family of neutral flavonol
derivatives (1–4, Fig. 1) that undergo dioxygenase-type visible
light-induced quantitative CO release when dissolved in
5
8
(Zn(flavonolato) ) suggested that such compounds could
2
organic solvents or organic/aqueous mixtures. As shown in
Table 1, members of this class of compounds exhibit CO
release under aerobic and/or anaerobic conditions. These fla-
vonol derivatives are fluorescent and therefore trackable in
cells. Compound 1 has been demonstrated to penetrate cells
and then undergo CO release upon exposure to visible light.
The structural framework in 1–4 also offers the opportunity for
chelation to a metal center as a means of tuning the reactivity
5
9
exhibit the desired minimal solubility. Therefore derivatives
of this type using 1–4 were synthesized, characterized, and
evaluated as solid-state CO release agents. As outlined herein,
we have found that a member of this family can be used in
aerobic oxidative palladium-catalyzed carbonylation processes
that employ O as the sole oxidant and are performed under
2
6
2
very mild conditions. Overall, our results suggest that the
structural motif in 1–4 is a highly versatile CO release unit that
can be developed for various CO release applications.
Experimental section
General
Chemicals and reagents. All chemicals and reagents were
obtained from commercial sources and used as received
unless otherwise noted. The synthesis of the extended flavo-
5
8
nols 1–4 proceeded as previously reported. The 6-Ph
2
TPA
(
N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine)
6
3
ligand was prepared and purified as previously described.
Physical methods. Anaerobic procedures were performed in
a Vacuum Atmospheres glovebox under N . Solvents for glove-
2
Fig. 1 Structures of flavonol-based CO-releasing molecules.
box use were dried according to published methods and dis-
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6
4
tilled under N
2
.
UV-vis spectra were recorded at room temp- dichloromethane in the elemental analysis sample was con-
1
erature using a Hewlett-Packard 8453A diode array spectro- firmed through integration of the CH Cl resonance in the H
2
2
photometer. Fluorescence emission spectra were collected NMR spectrum.
using a Shimadzu RF-530XPC spectrometer in the range of [(6-Ph TPA)Zn(2 )]ClO
00–800 nm with the excitation wavelength corresponding to (CD CN, 300 MHz) δ 8.50 (d, J = 4.8 Hz, 1H), 8.31 (d, J = 9.3 Hz,
−
1
2
4
(6). Orange-red solid (93%). H NMR
4
3
the most red shifted absorption maximum of the compound. 2H), 8.22 (s, 1H), 8.09 (s, 1H), 8.05–7.93 (m, 2H), 7.85 (t, J = 7.8
Infrared spectra were recorded using a Shimadzu FTIR-8400 as Hz, 1H), 7.78 (t, J = 7.5 Hz, 2H), 7.64–7.45 (m, 4H), 7.44–7.34
KBr pellets. Water was present in the KBr used for all IR (m, 3H), 7.31–7.21 (m, 5H), 7.11 (t, J = 7.5 Hz, 2H), 7.01 (t, J =
1
samples. H NMR spectra (in ppm) are referenced to the 7.5 Hz, 4H), 6.80 (d, J = 9.0 Hz, 2H), 4.93 (d, J = 14.4 Hz, 2H),
residual solvent peaks in acetonitrile-d
3 5
or pyridine-d . J values 4.54 (d, J = 14.4 Hz, 2H), 4.37 (s, 2H), 3.51 (q, J = 7.2 Hz, 4H),
−
1
are given in Hz. Mass spectral data was collected at the Mass 1.24 (t, J = 7.2 Hz, 6H) ppm. FT-IR (KBr, cm ) 1088 (νClO4), 618
Spectrometry Facility, University of California, Riverside. (νClO4). UV-vis (CH
3
CN) nm (ε, M− cm ) 524 (80 000). ESI/
1
−1
Elemental analyses were performed by Atlantic Microlab, Inc., APCI-MS, m/z (relative intensity) 864.2887, calc. 864.2882 ([M −
+
Norcross, GA, or by Robertson Microlit Laboratories, ClO ] , 7%). Anal. Calc. C H N O ClZn·3H O: C, 62.42; H,
4
53 46
5
7
2
Ledgewood, NJ. Photochemical experiments were performed 5.14; N, 6.87. Found: C, 62.20; H, 4.65; N, 6.94. The presence
using a Luzchem photoreactor equipped with RPR-4190A, and quantity of water in the elemental analysis sample was
1
RPR-5750A, or Sylvania cool white lamps. Carbon monoxide confirmed through integration of the H O resonance in the H
2
quantification was performed by gas chromatography as pre- NMR spectrum.
6
1
−
1
viously described. Quantum yields were determined by pot-
2 4
[(6-Ph TPA)Zn(3 )]ClO (7). Dark red solid (93%). H NMR
assium ferrioxalate or potassium reineckate actinometry using (CD CN, 300 MHz) δ 8.80 (s, 1H), 8.47–8.39 (m, 2H), 8.38–8.30
3
6
5–67
an integrative analysis method.
The output measured (m, 2H), 8.13 (d, J = 9.0 Hz, 1H), 8.05 (d, J = 9.0 Hz, 1H), 7.85 (t
during the quantum yield determination for 5 was 1.46091 × J = 7.8 Hz, 1H), 7.79 (t, J = 7.8 Hz, 2H), 7.64 (t, J = 6.6 Hz, 1H),
1
7
1
0
photons per s when illuminated with RPR-4190A lamps 7.60–7.52 (m, 4H), 7.43 (t, J = 6.6 Hz, 5H), 7.28 (d, J = 6.9 Hz,
1
5
and for 6, 7, and 8 was 6.87824 × 10 photons per s when illu- 5H), 7.08 (t, J = 7.2 Hz, 2H), 6.99 (t, J = 7.2 Hz, 4H), 4.82 (bd, J
minated with white light lamps equipped with 546 nm cut off = 14.4 Hz, 2H), 4.49 (bd, J = 14.4 Hz, 2H), 4.41 (s, 2H) ppm.
filters.
Caution! Perchlorate salts of metal complexes with organic (ε, M cm ) 550 (9300), 396 (7200). ESI/APCI-MS, m/z (relative
ligands are potentially explosive. Only small amounts of intensity) 809.1930, calc. 809.1923 ([M − ClO
−1
FT-IR (KBr, cm ) 1086 (νClO4), 619 (νClO4). UV-vis (CH CN) nm
3
−1
−1
+
4
] , 25%). Anal.
material should be prepared, and these should be handled Calc. C H N O ClSZn·0.5H O·1CH Cl : C, 59.77; H, 4.01; N,
4
9
37
4
6
2
2
2
6
8
with great care.
5.58. Found: C, 59.29; H, 3.51; N, 5.57. The presence and quan-
tity of water and dichloromethane in the elemental analysis
Synthesis of 5–8
sample was confirmed through integration of the respective
1
In a N
2
-filled glovebox, a methanolic solution (2 mL) of Zn resonances in the H NMR spectrum.
−
1
(
ClO ·6H
4
)
2
2
O (0.0500 g, 0.134 mmol) was added to solid
[(6-Ph
2
TPA)Zn(4 )]ClO
4
(8). Dark blue solid (91%). H NMR
6
-Ph TPA (0.0594 g, 0.134 mmol) and the resulting mixture (CD CN, 300 MHz) δ 8.69 (s, 1H), 8.58 (d, J = 9.3 Hz, 2H), 8.43
2
3
was stirred until all of the chelate ligand dissolved. The solu- (d, J = 5.4 Hz, 1H), 8.21 (s, 1H), 8.10 (d, J = 8.1 Hz, 1H), 8.00 (d,
tion was then added to a methanolic solution (2 mL) contain- J = 8.1 Hz, 1H), 7.89–7.77 (m, 3H), 7.64–7.42 (m, 10H), 7.38 (t, J
ing 1, 2, 3, or 4 (0.134 mmol) and Me NOH·5H O (0.0243 g, = 7.2 Hz, 1H), 7.29 (d, J = 7.8 Hz, 1H), 7.24–7.17 (m, 2H),
4
2
0
.134 mmol). The mixture was allowed to stir for 4 hours at 7.16–7.07 (m, 4H), 6.86 (d, J = 9.3 Hz, 2H), 4.83 (d, J = 13.8 Hz,
ambient temperature. The solvent was then removed under 2H), 4.49 (d, J = 13.8 Hz, 2H), 4.41 (s, 2H), 3.54 (q, J = 7.2 Hz,
−
1
reduced pressure and the residual solid was dissolved in 4H), 1.23 (t, J = 7.2 Hz, 6H) ppm. FT-IR (KBr, cm ) 1084
2 2 3
Cl . The solution was then filtered through a Celite/glass- (νClO4), 622 (νClO4). UV-vis (CH
CN) nm (ε, M− cm−1) 600
1
CH
wool plug and the final product was precipitated by the (111 700). ESI/APCI-MS, m/z (relative intensity) 880.2659, calc.
+
addition of excess hexanes. Each solid product was dried 880.2658
([M
−
ClO ] ,
15%).
Anal.
Calc.
4
under reduced pressure.
C
53
H
46
N
5
O
6
ClSZn·H O: C, 63.67; H, 4.84; N, 7.00. Found: C,
2
−
1
[(6-Ph
2
TPA)Zn(1 )]ClO
4
(5). Orange solid (89%). H NMR 63.37; H, 4.57; N, 6.75. The presence and quantity of water in
(
2
CD CN, 300 MHz) δ 8.62 (d, J = 5.1 Hz, 1H), 8.27 (d, J = 6.0 Hz, the elemental analysis sample was confirmed through inte-
3
1
H), 8.21 (s, 1H), 8.14 (s, 1H), 8.07–7.99 (m, 2H), 7.96 (t, J = 7.5 gration of the H
2
O resonance in the H NMR spectrum.
Hz, 1H), 7.78 (t, J = 7.8 Hz, 2H), 7.67–7.35 (m, 9H), 7.33 (d, J =
General synthetic method for zinc bis(flavonolato) compounds
9–12)
7
1
.8 Hz, 2H), 7.18–7.09 (m, 4H), 7.06–6.85 (m, 6H) 4.92 (d, J =
4.7 Hz, 2H), 4.57 (d, J = 14.7 Hz, 2H), 4.43 (s, 2H) ppm. FTIR
(
−
1
(KBr, cm ) 1094 (νClO4), 622 (νClO4). UV-vis (CH
3
CN) nm (ε, A 500 mL round bottom flask, protected from light with alumi-
cm ) 480 (6300), 384 (1700). ESI/APCI-MS, m/z (relative num foil, was charged with the flavonol (21 mmol),
−
1
−1
M
+
intensity) 793.2152, calc. 793.2157 ([M − ClO
Calc. C49 ClZn·0.7CH Cl : C, 62.56; H, 4.06; N, 5.87. ing mixture was stirred at room temperature for 2 hours. Zinc
Found: C, 62.62; H, 4.26; N, 6.16. The presence and quantity of triflate (10.3 mmol) was then transferred to the flask using
4 4 2
] , 34%). Anal. Me NOH·5H O (21 mmol), and MeOH (50 mL) and the result-
H
37
N
4
O
7
2
2
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MeOH (15 mL). This mixture was stirred overnight after which Table 2 Absorption and CO release properties of 5–12
time the product was collected by filtration, washed with water
Solution
Solid-state
CO release
(eq. CO)
and dried in vacuo.
a
λ
max (nm)
CO release
d
−
1
Zn(1 )
2
·4H
2
O (9). 88% yield. Orange solid. H NMR (pyri-
ε (M cm−1)
−1
g
Compound
(eq. CO)
Φ
dine-d , 300 MHz) 9.35 (d, J = 7.8 Hz, 4H), 8.86 (s, 2H), 8.07 (s,
5
b
b,e
b,e
b,e
b,e
c, f
c, f
c, f
c, f
h
j
5
6
7
8
9
480 (6300)
0.94(4)
0.98(1)
0.92(3)
0.97(2)
1.86(2)
1.91(4)
1.89(3)
1.95(4)
0.651(2)i
0.96j
2
4
H), 7.89 (d, J = 8.4 Hz, 2H), 7.80–7.77 (m, 6H), 7.54–7.39 (m,
b
524 (80 000)
0.583(4)i
0.93j
−
1
b
H), 7.27 (t, J = 7.2 Hz, 2H). UV-vis (pyridine), nm (ε, M
550 (9300)
0.951(4)i
0.97j
b
−
1
600 (111 700)
0.947(7)
0.94
cm ) 483 (23 300), 375 (7600), 306 (25 700). UV-vis (pyridine)
c
k
nm (ε, M− cm ) 479 (23 300). LIFDI-MS, m/z (relative inten-
1
−1
479 (23 300)
1.77(4)
1.63(2)
c
k
10
525 (92 500)
+
c
sity) 638.0710, calc. 638.0702 ([M] , 100%). Anal. Calc. 11
564 (31 700)
No
No
c
1
2
609 (189 750)
C H O Zn·4H O: C, 64.10; H, 4.25. Found: C, 63.48; H, 4.08.
3
8
22
6
2
The presence and quantity of water present in the solid
a
b
Most red-shifted absorption feature. Compound dissolved in aceto-
c
sample was confirmed through integration of the water peak nitrile in the presence of O
2
.
Compound dissolved in pyridine in the
d
1
presence of O2. Determined using GC; average of three independent
measurements. CO quantification performed by GC after illumination
with 419 nm lamps (5) or white light lamps equipped with 546 nm cut
in the H NMR spectrum.
e
−
1
Zn(2 )
2
·2H
2
O (10). 86% yield. Red solid. H NMR (pyridine-
f
d
5
, 300 MHz) 9.37 (d, J = 8.7 Hz, 4H), 9.03 (s, 2H), 8.10 (s, 2H), off filters (6, 7, 8) for 24 h. Illuminated with white compact fluo-
rescent (CFL) bulbs for 48 h. Compounds 9 and 10 will also undergo
7
.91 (d, J = 8.1 Hz, 2H), 7.88 (d, J = 8.7 Hz, 2H), 7.46 (t, J = 8.7
g
CO release using blue CFL bulbs in 48 h. Average of three indepen-
Hz, 2H), 7.34 (t, J = 8.1 Hz, 2H), 6.94 (d, J = 8.7 Hz, 4H), 3.30
h
dent trials; values in parenthesis represent standard error.
q, J = 7.2 Hz, 8H), 1.08 (t, J = 7.2 Hz, 12H) ppm. UV-vis (pyri- obtained for solution (CH
Φ
(
3
CN) CO release via illumination using
i
−1
−1
419 nm lamps and potassium ferrioxalate actinometer. Φ obtained
for solution (CH CN) CO release via illumination with white light
lamps equipped with 546 nm cut off filters and potassium reineckate
dine) nm (ε, M cm ) 525 (92 500), 402 (17 600). LIFDI-MS,
m/z (relative intensity) 780.2163, calc. 780.2172 ([M] , 100%).
+
3
j
Anal. Calc. C46
H
40
N
2
O
6
Zn·2H
2
2
O: C, 67.52; H, 5.42; N, 3.42. actinometer. Powdered compound in the presence of O . CO quantifi-
cation performed by GC after illumination with 419 nm light (5) or
Found: C, 67.30; H, 5.29; N, 3.50. The presence and quantity of
water present in the solid sample was confirmed through inte-
white light lamps equipped with 546 nm cut off filters (6, 7, 8) for
k
4
8 h. Average CO release of two independent samples. Illuminated
1
gration of the water peak in the H NMR spectrum.
with white CFL bulbs for 4 days; average of three independent
measurements.
−
1
Zn(3 ) ·2H O (11). 92% yield. Purple solid. H NMR (pyri-
2
2
dine-d
5
, 300 MHz) 9.48 (d, J = 7.5 Hz, 4H), 9.45 (s, 2H), 8.26 (s,
H), 8.10 (d, J = 8.7 Hz, 2H), 8.01 (d, J = 8.4 Hz, 2H), 7.70 (t, J =
.5 Hz, 4H) 7.56–7.43 (m, 6H) ppm. UV-vis (pyridine) nm (ε,
2
7
−
1
−1
M
cm ) 564 (31 700), 405 (27 500). LIFDI-MS, m/z (relative 7.64–7.51 (m, 6H), 7.50–7.37 (m, 5H), 7.34–7.23 (m, 2H),
+
−1
intensity) 670.0259, calc. 670.0246 ([M] , 72%). Anal. Calc. 7.20–7.05 (m, 9H), 4.58–4.26 (m, 6H) ppm. FT-IR (KBr, cm
)
C H O S Zn·2H O: C, 64.45; H, 3.70. Found: C 64.21; H: 1724 (νCvO). ESI/APCI-MS, m/z (relative intensity), 797.2113,
3
8
22
4
2
2
+
3
.73. The presence and quantity of water present in the solid calc. 797.2201 ([M − ClO
4
] , 35%).
sample was confirmed through integration of the water peak
[(6-Ph
ate)]ClO (14). H NMR (CD CN, 300 MHz) δ 8.24 (d, J = 5.4 Hz,
4 3
2
TPA)Zn(3-((4-(diethylamino)benzoyl)oxy)-2-naphtho-
1
1
in the H NMR spectrum.
−
1
2 2
Zn(4 ) ·2H
O (12). 91% yield. Blue solid. H NMR (pyridine- 1H), 8.15–7.97 (m, 3H), 7.88–7.77 (m, 2H), 7.68 (t, J = 9.0 Hz,
, 300 MHz) 9.55 (d, J = 9.3 Hz, 4H), 9.44 (s, 2H), 8.16 (s, 2H), 4H), 7.61–7.50 (m, 4H), 7.49–7.39 (m, 3H), 7.34–7.24 (m, 2H),
5
d
8
4
6
.05 (d, J = 8.1 Hz, 2H), 7.95 (d, J = 8.7 Hz, 2H), 7.55–7.37 (m, 7.21–6.84 (m, 9H), 6.60 (d, J = 9.0 Hz, 2H), 4.65–4.27 (m, 6H),
H), 6.91 (d, J = 6.9 Hz, 4H), 3.33 (q, J = 6.9 Hz, 8H), 1.09 (t, J = 3.41 (q, J = 7.2 Hz, 4H), 1.15 (t, J = 7.2 Hz, 6H) ppm. FT-IR
−
1
−1
−1
.9 Hz, 12H) ppm. UV-vis (pyridine) nm (ε, M cm ) 609 (KBr, cm ) 1722 (νCvO). ESI/APCI-MS, m/z (relative intensity),
+
(
8
189 750). LIFDI-MS, m/z (relative intensity) 812.1711, calc. 868.2870, calc. 868.2836 ([M − ClO
4
] , 3%).
+
12.1715 ([M] , 100%). Anal. Calc. C46
H
40
N
2
O
4
S
2
Zn·2H
2
O: C,
[(6-Ph
2
TPA)Zn(3-((benzoyloxy)naphthalene)-2-carbothiolate)]
1
6
4.97; H, 5.22; N, 3.29. Found: C, 65.75; H, 5.08; N, 3.27. The ClO
4
(15). H NMR (CD CN, 300 MHz) 8.46 (d, J = 5.4 Hz, 1H),
3
presence and quantity of water present in the solid sample was 8.15–7.84 (m, 6H), 7.83–7.74 (m, 3H), 7.70 (t, J = 7.5 Hz, 1H),
1
confirmed through integration of the water peak in the
NMR spectrum.
H
7.65–7.32 (m, 13H), 7.27 (t, J = 7.8 Hz, 4H), 7.09 (d, J = 7.8 Hz,
3H), 4.60–4.16 (m, 6H) ppm. FT-IR (KBr, cm− ) 1742 (νCvO).
1
ESI/APCI-MS, m/z (relative intensity), 813.1889, calc. 813.1872
Photoinduced reactivity of 5–8 in CH CN
+
3
(
[M − ClO
[(6-Ph
light (5: 419 nm; 6, 7, 8: >546 nm) results in the release of one 2-carbothiolate)]ClO
equivalent of CO (Table 2) and the formation of the carboxy- J = 5.1 Hz, 1H), 8.00–7.88 (m, 4H), 7.85–7.75 (m, 2H), 7.65 (d, J
4
] , 100%).
Illumination of aerobic CH
3
CN solutions of 5–8 using visible
2
TPA)Zn(3-((4-diethylamino)benzoyl)oxy)naphthalene-
1
(16). H NMR (CD CN, 300 MHz) 8.46 (d,
3
4
lato derivatives [(6-Ph
were characterized by H NMR, IR, and ESI/MS.
2
1
TPA)Zn(carboxylato)]ClO
4
(13–16) which = 9.3 Hz, 2H), 7.61–7.52 (m, 4H), 7.48–7.35 (m, 7H), 7.25 (t, J =
7.8 Hz, 4H), 7.10 (d, J = 7.5 Hz, 4H), 6.68 (d, J = 9.3 Hz, 2H),
[
(6-Ph
2
TPA)Zn(((3-benzoyl)oxy)-2-naphthoate)]ClO
4
(13). 4.62–4.28 (m, 6H), 3.51 (q, J = 6.9 Hz, 4H), 1.25 (t, J = 6.9 Hz,
1
−1
H NMR (CD
3
CN, 300 MHz) δ 8.22 (d, J = 4.5 Hz, 1H), 6H) ppm. FT-IR (KBr, cm ) 1739 (νCvO). ESI/APCI-MS, m/z
+
8
.14–7.99 (m, 3H), 7.93–7.78 (m, 4H), 7.66 (d, J = 7.5 Hz, 2H), (relative intensity), 884.2641, calc. 884.2607 ([M − ClO ] , 50%).
4
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Photoinduced reactivity of 9–12 in pyridine
Solutions of 9–12 (∼0.01 mmol) in pyridine-d
tion for ∼5 minutes. Each NMR tube was then placed in a
photoreactor and illuminated with 419 nm (9 and 10) or white
light lamps (9–12) for 24 hours. Evaluation of each solution by
5
(∼0.7 mL) were
placed in NMR tubes under air. Each was then placed in a
Luzchem photoreactor equipped with 419 nm or white light
lamps and illuminated until the reaction was determined to be
1
H NMR indicated that no reaction had occurred.
CO release quantification of 5–12 in solution
1
complete by H NMR. The solvent was then removed under
5
0 mL round bottom flasks were loaded with 5–8 (ca. 10 mg)
dissolved in acetonitrile (5.0 mL). The flask was then sealed
with a septum, purged with O for 45 seconds, and illuminated
reduced pressure and an FT-IR spectrum was obtained.
Zinc bis((3-benzoyl)oxy)-2-naphthoate) (17). Beige solid. H
1
2
NMR (pyridine-d
5
, 300 MHz) δ 9.14 (s, 2H), 8.48 (d, J = 6.9 Hz,
with white light for 24 hours, which resulted in bleaching of
each solution to colorless. A sample (10 mL) of the headspace
gas was then analyzed by gas chromatography and the area of
the peak associated with CO applied to a calibration curve
created specifically for reactions done in acetonitrile. The
solvent was then removed under reduced pressure and the
4
H), 7.94–7.82 (m, 6H), 7.56–7.37 (m, 10H) ppm. FT-IR (KBr,
−
1
cm ) 1728 (νCvO).
Zinc bis(3-((4-(diethylamino)benzoyl)oxy)-2-naphthoate)
18). Beige solid. H NMR (pyridine-d , 300 MHz) δ 9.11 (s,
1
(
5
2
4
6
H), 8.47 (d, J = 7.2 Hz, 4H), 8.00–7.75 (m, 6H), 7.51–7.26 (m,
H), 6.71 (d, J = 7.2 Hz, 4H), 3.19 (q, J = 6.9 Hz, 8H), 0.99 (t, J =
.9 Hz, 12H) ppm. FT-IR (KBr, cm−1) 1714 (νCvO).
1
residual solid was analyzed by H NMR for completeness of
reaction.
Zinc bis(3-(benzoyloxy)naphthalene-2-carbothiolate) (19).
1
Beige solid. H NMR (pyridine-d , 300 MHz) δ 9.14 (s, 2H), 8.36
5
Photoreactivity of 5–8 as powdered solids
(d, J = 8.6, 4H), 7.97 (d, J = 8.1, 2H), 7.84 (d, J = 8.1, 2H), 7.79 (s,
−1
Compounds 5–8 (ca. 10 mg) were loaded into 50 mL round
bottom flasks under O . Exposure of solid 5–8 to white light
2
2H), 7.55–7.35 (m, 10H) ppm. FT-IR (KBr, cm ) 1739 (νCvO).
Zinc bis(3-((4-(diethylamino)benzoyl)oxy)naphthalene-2-car-
1
for 48 hours results in clean conversion to 13–16 as deter-
bothiolate) (20). Beige solid. H NMR (pyridine-d , 300 MHz) δ
9
5
1
mined by H NMR of the final product. Additionally, the color
.06 (s, 2H), 8.36 (d, J = 7.5, 4H), 8.00–7.65 (m, 6H), 7.57–7.29
of the solid compounds changed to beige (Fig. 3).
(m, 4H), 6.73 (d, J = 7.5, 4H), 3.26 (q, J = 6.9, 8H), 1.04 (t, J =
−
1
6
.9, 12H) ppm. FT-IR (KBr, cm ) 1713 (νCvO).
Photoreactivity of 9–12 as powdered solids
Compounds 9–12 were placed in 50 mL round bottom flasks
Dark control reactions for 5–8
as powdered solids. Each flask was then sealed with a septum,
Solutions of 5–8 (∼10 mM) in acetonitrile-d were prepared in
3
gently purged with O for 1 minute, and placed in a photo-
2
air under minimal red light and placed in NMR tubes. The
tubes were then covered with aluminum foil and illuminated
with white light for 24 hours. Evaluation of each solution by
reactor with 419 nm (9 and 10) or white light lamps (9–12) for
72 hours. The head space gas of the flask was then analyzed by
gas chromatography for the production of CO. Compounds 9
and 10 as powdered solids produced two equivalents of CO per
equivalent of compound when illuminated with 419 nm or
white light lamps. Compounds 11 and 12 did not produce CO
upon illumination with white light lamps. The residual solid
1
H NMR indicated that no reaction had occurred which
provides evidence that light is required for the O
chemistry leading to CO release.
2
activation
Dark control reactions for 9–12
5
in the flask was then dissolved in pyridine-d and the solution
Solutions of 9–12 (∼2 mM) in pyridine-d
5
were prepared in air
1
evaluated by H NMR, indicating 9 and 10 had fully converted
to the photoproducts 17 and 18, while no reaction occurred for
under minimal red light and each was placed in a NMR tube.
Each NMR tube was then covered in foil, placed in a photo-
reactor, and illuminated using 419 nm (9 and 10) or white
11 and 12.
light lamps (9–12) for 24 hours. Evaluation of each solution by Photoreactivity of 9 as a film deposit
1
H NMR indicated that no reaction had occurred.
Powdered compound 9 (52.6 μmol, 33.7 mg) was placed in a
round bottom flask and dissolved in pyridine (ca. 10 mL). The
solvent was then removed via rotary evaporator using the
highest rpm setting to deposit the compound on the vessel
walls as a film. The flask was sealed with a septum and placed
in a photoreactor with 419 nm or white light lamps and illumi-
nated for 48 hours. Complete conversion to 17 was determined
Anaerobic control reactions for 5–8
Solutions of 5–8 (∼10 mM) in acetonitrile-d were prepared in
3
2
a Vacuum Atmospheres glovebox under N and placed in NMR
tubes. The caps of the NMR tubes were wrapped securely with
parafilm and the tubes were illuminated with white light
1
1
lamps for 24 hours. Evaluation of the samples by H NMR
by H NMR.
indicated that no reaction had occurred which provides evi-
CO quantification for solid samples of 9 and 10
dence that O is required for CO release reactivity.
2
A GC calibration curve for the quantification of CO was created
using sodium bicarbonate as an inert solid in a 50 mL round
Anaerobic control reactions for 9–12
Solutions of 9–12 in pyridine-d
5
were prepared, each was bottom flask. Each flask was sealed with a septum, evacuated,
placed in a NMR tube, and N was bubbled through each solu- and varying volumes of O2 and CO were injected. The inte-
2
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grated area of the CO peak was plotted against the number of extended 3-hydroxy-4-pyrone based frameworks, which are
moles of CO calculated to be in the flask. Following a similar easily structurally modified to modulate spectroscopic and CO
set up procedure, followed by illumination of solid samples of release properties. The presence of the 3-hydroxy-4-pyrone unit
9
or 10 for 96 h, GC analysis was performed using 10 mL of also offers the possibility of metal coordination. The chelate
the head space gas to determine the amount of CO released.
ligand-supported 5–8 were prepared via the approach shown in
Scheme 1(a). Each was obtained in high yield as an analytically
pure solid following precipitation from hexanes. Although
Aerobic oxidative palladium-catalyzed carbonylation reactions
The general reaction conditions were as follows. Compound 9 X-ray quality crystals could not be obtained for any member of
1
was deposited as a film in a 50 mL round bottom flask as this family of compounds, the H NMR spectral features of 5–8
described above. The flask was then loaded with phenyl (Fig. S1, S6, S11, and S16†) suggest a mononuclear pseudo-
boronic ester (52.6 μmol, 10 mg), PdCl
2
(PPh
3
)
2
(2.63 μmol, octahedral Zn(II) center with the flavonolato ligand positioned
TPA
proof). The flask was then sealed with a septum and placed in ligand. Evidence for this type of structure comes from the H
an oil bath pre-heated to 40 °C. Each reaction mixture was illu- NMR features of the benzylic hydrogens, which are consistent
1
.8 mg), NEt
3
(105.2 μmol, 15 μL), and ethanol (5 mL, 190 between the phenyl-appended pyridyl donors of the 6-Ph
2
1
minated with two CFL blue bulbs set at an average distance of with C
cm from the flask for 24 hours. The flask was removed from in CH
the oil bath and allowed to cool to room temperature. The reac- with the proposed mononuclear formulation (Fig. S5, S10, S15,
s
(mirror plane) symmetry in the cation. When dissolved
5
3
CN, each compound exhibits a molecular ion consistent
−
tion mixture was then passed through a short silica plug (ca. and S20†). The outer sphere ClO
4
is evidenced in the solid
6
00 mg). 2 μL of the filtrate was then analyzed by GC-MS. The state IR spectra of the compounds (Fig. S2, S7, S12, and S17†).
remaining filtrate from the reactions was then brought to
dryness under reduced pressure and analyzed by H NMR.
Each compound 5–8 exhibits a ∼60–90 nm red-shift of the
lowest energy absorption feature relative to neutral 1–4 (Tables
1
1
and 2; Fig. S3, S8, S13, S18†). While 5 and 7 exhibit lower
molar absorptivity values for the lowest energy absorption
band than the neutral flavonols, those of the NEt -substituted
and 8 are enhanced relative to the neutral flavonols. In terms
GC-MS parameters
GC-MS spectral data was collected on
a
Shimadzu
2
6
GCMS-QP5000 equipped with an EC-5 column that is 5%
phenyl : 95% methylpolysiloxane with a fused silica coating.
The column is 30 m in length with a 0.25 μm coating and
of previously reported photoCORMs, the molar absorptivity
values of 6–8 at >500 nm exceed those exhibited by molecular
1
2
metal carbonyl photoCORMs that absorb above 500 nm.
Compounds 6 and 8 have molar absorptivity values at >500 nm
0
.25 mm diameter. The injection temperature was 185 °C with
an interface temperature of 250 °C. The column initial temp-
erature was 70 °C with a final temperature of 250 °C and a
ramp rate of 12 °C per minute. The column inlet pressure was
2
0–23
that exceed all organic photoCORMs,
including recently
2
0
reported BODIPY derivatives. Illumination into the lowest
energy absorption band of 5–8 produces a fluorescent emis-
sion with a Stokes shift of 30–72 nm (Fig. S4, S9, S14, and
−
1
4
3.5 kPa with a column flow of 0.8 mL min and a linear
−1
−1
velocity of 32.8 cm s for a total flow of 35.4 mL min
.
Evaluation of CO release reactivity of 9 (film deposit) in the
presence of ethanol
A film deposit of 9 in a 50 mL round bottom flask was gener-
ated as outlined above. To this flask was added ethanol
(5 mL). The flask was then sealed with a septum and placed in
an oil bath pre-heated to 40 °C. The flask was illuminated with
two CFL blue bulbs set at an average distance of 5 cm from the
flask for 24 hours. The flask was removed from the oil bath
and allowed to cool to room temperature. The ethanol solution
was passed through a short silica plug (ca. 600 mg). The filtrate
was then analyzed by GC-MS, which revealed the formation of
ethyl benzoate. The remaining filtrate was then brought to
1
dryness under reduced pressure and analyzed by H NMR.
Results
The development of CO-releasing structural motifs that can be
easily prepared and tuned via multiple strategies (e.g. organic
synthesis and/or metal coordination) offers advantages toward
the generation of compounds that can be applied in biology
and synthetic chemistry. Our approach focuses on the use of Scheme 1 Synthesis of (a) 5–8 and (b) 9–12.
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S19†). The sulfur-containing 7 and 8 exhibit smaller Stokes
shifts relative to the oxygen analogs.
Mixing of 1–4 with Zn(OTf)
base yielded precipitates of the analytical formulation [Zn
flavonolato) ]·nH O (9, n = 4; 10–12, n = 2; Scheme 1(b)). All
2
in MeOH in the presence of
(
2
2
exhibit poor solubility in common organic solvents except pyri-
dine. Each compound was characterized as a solid using
1
elemental analysis and FTIR. H NMR, LIFDI-MS, UV-vis and
emission spectra for 9–12 were obtained in pyridine (Fig. S21–
1
S36†). The H NMR features are consistent with equivalent fla-
vonolato ligands, suggesting possible trans water ligands
(Scheme 1(b)) in a pseudo-octahedral structure. As shown in
Fig. 2, the absorption features of 9–12 in pyridine (Table 2)
span the visible region from ∼400–650 nm. The NEt -contain-
2
ing derivatives 14 and 16 have high extinction coefficients.
These features are generally similar to those exhibited by 5–8
albeit with increased molar absorptivity values, which is con-
sistent with the presence of two flavonolato ligands.
Photoinduced O reactivity of 5–12
2
Illumination of CH CN solutions of 5–8 with visible light
3
under aerobic conditions results in the release of one equi-
valent of CO and the clean formation of a single zinc carboxyl-
ate compound (13–16; Scheme 2(a)) as determined by
Scheme 2 (a) Visible light induced CO release reactivity of 5–8 in
3
CH CN or the solid state. (b) Visible light induced CO release reactivity
1
1
H NMR, IR, and ESI-MS. The H NMR spectra of 13–16
of 9–12 in pyridine.
exhibit features that are consistent with mononuclear struc-
tures for the cationic component of each compound (Fig. S37,
S41, S44, S47†). Infrared and ESI/APCI mass spectral data also
support the proposed formulation of mononuclear carboxylato
compounds that have resulted from dioxygenase-type reactivity
and CO release (IR: Fig. S38, S42, S45, S48; ESI/APCI: Fig. S39,
S43, S46, S49†). The incorporation of two oxygen atoms from
O into the product is evidenced by the reaction of 5 in the
2
1
8
presence of
2
O , which yields 13 containing two labelled
oxygen atoms (ESI/APCI MS Fig. S40†). The reaction quantum
yields for CO release from 5–8 (Table 2) in CH CN significantly
3
exceed those exhibited by the neutral flavonols 1–4, metal
1
2
carbonyl photoCORMs, and all reported neutral organic
2
0–23
photoCORMs.
When dissolved in pyridine under aerobic conditions and
illuminated with white (9–12) or blue light (9 and 10), the Zn(II)
bis(flavonolato) compounds exhibit dioxygenase-type photo-
induced release of two equivalents of CO (Table 2). The zinc-
containing products (17–20, Scheme 2(b)) are bis-carboxylate
1
derivatives based on H NMR and IR spectral features
(
Fig. S50–S57†). Thus, both flavonolato ligands in 9–12
undergo light-induced CO release when dissolved in pyridine.
We note that Tran and Cohen previously reported light-
induced reactivity for a zinc bis(flavonothianoato) complex in
6
9
3
CHCl but did not report the nature of the products.
Solid-state photoinduced CO release reactivity of 5–8
Light-induced CO release compounds offer the possibility of
control over the temporal and spatial release of CO. To date,
compounds developed for biological and in situ synthetic
applications have focused on soluble species. However, solid-
state materials or compounds that release CO upon introduc-
tion of visible light could also be very useful in various appli-
cations, including in medicine and for CO release within
reaction mixtures. Visible and near infra-red light-driven
Fig. 2 Absorption spectral features of 9–12.
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NO-releasing solid-state materials (photoNORMs) have been
7
0–78
previously developed for biological applications.
A current
limitation in terms of the development of photoCORM
materials is the almost complete lack of availability of com-
pounds that exhibit solid state CO release reactivity upon illumi-
nation with visible light. Recently, Mascharak et al. reported two
Mn(I) tricarbonyl compounds that will release CO upon illumi-
nation of the crystals with microscope light. However, these
compounds were not characterized in terms of the amount of
5
4
CO released or the products remaining following CO release.
Notably, we have discovered that exposure of powders of
–8 to visible light in the presence of O for 48 h results in
5
2
quantitative CO release (Table 2). Analysis of the remaining
1
beige powders (Fig. 3) following light illumination via H NMR
3
in CD CN indicates the clean formation of 13–16. Thus, the
same reactivity is observed for 5–8 both in solution and in the
solid-state. It is important to note that the neutral flavonols
Scheme 3 (a) Aerobic palladium-catalyzed alkoxycarbonylation reac-
tions. (b) The relative yields of the products generated for R’’ = –NO in
2
could not be determined due to peak overlap in the GC-MS trace.
1
–4 are unreactive as solids upon extended illumination (one
week) with visible light. The zinc flavonolato derivatives (5–8)
are unreactive as solids under anaerobic conditions.
In terms of the zinc bis(flavonolato) compounds, only 9 flask was coated with one equivalent of 9 (up to 2 eq. CO) as a
and 10 exhibit quantitative CO release (2 eq.) reactivity upon film via rotary evaporation of a pyridine solution of the
illumination with white light under air. The solid thiocarbonyl complex. The boronic ester, NEt , and PdCl (PPh ) were then
3 2 3 2
derivatives did not exhibit CO release even after extensive added along with ethanol (5 mL). Each reaction mixture was
periods of illumination (1 week). These results demonstrate heated under air at 40 °C using an oil bath and was illumi-
that there are factors that influence solid-state CO release reac- nated with two blue CFL bulbs to initiate CO release. After
tivity within this family of compounds that remain to be deter- 24 h of heating and illumination, the products of the reactions
1
mined. However, it is noteworthy that the pairs of compounds were evaluated using GC-MS and H NMR. In the reactions of
5
/9 and 6/10 demonstrate that solid-state CO release reactivity 21a and 21b, the desired carbonylation ester products are gen-
can be retained while the structure (e.g. supporting chelate erated, along with phenol and biphenyl byproducts (Scheme 3;
ligand) can be tuned to adjust solubility.
Fig. S58 and S59†). Generally similar results were obtained for
the nitro derivative 21c albeit no biphenyl product was identi-
fied (Fig. S60†). The production of significant amounts phenol
and biphenyl byproducts indicates that the CO flux in the reac-
tion mixture needs to be further optimized to limit the for-
mation of these oxidation byproducts. However, this was not
pursued in these reactions due to the additional identification
of formation of ethyl benzoate in each reaction mixture. This
ester is generated via the breakdown of the flavonol-based CO
release product by ester alcoholysis (Scheme 4). Compound 9
releases ∼1 eq. of CO after 24 h of illumination of the de-
posited film (Fig. S61†). Quantification (via GC-MS) of the
ethyl benzoate generated in reaction mixtures shown in
Scheme 3(b) after 24 h indicates that ∼60–80% of the expected
CO release product (1 eq.) has undergone ethanolysis to
produce ethyl benzoate. Overall, these results indicate that
while the starting heterogeneous zinc bis(flavonolato) complex
is a viable source of CO for carbonylation reactivity, the ester
functionality in the CO release product is reactive with alco-
hols. Minimizing this background alcoholysis reactivity is the
subject of ongoing studies. Additionally, efforts to modify the
reaction conditions to enhance selectivity for the ester product
are in progress. We believe that such studies are justified by
the combination of the low temperature reaction conditions,
use of air as the sole oxidant, and the safe CO-handling
present in these reactions.
Initial evaluation of 9 as a CO source for oxidative
palladium-catalyzed carbonylation in ethanol
Building on the discovery of solid-state CO release reactivity
for 9, we performed initial studies to examine the possibility of
using this compound as an in situ heterogeneous visible light-
induced CO release agent for oxidative palladium-catalyzed
alkoxycarbonylation processes such as the reactions outlined
in Scheme 3(a). Liu et al. have previously reported that such
reactions can be performed using air : CO (ratio 3 : 1–5 : 1) to
give carbonylated products in >70% yield using various alco-
6
2
hols as the solvent. Our initial screening reactions focused
on the use of 9 at a near stoichiometric level in terms of avail-
able CO in reactions with boronic esters having various substi-
tuents (Scheme 3(b)). For each reaction, a 50 mL round bottom
Fig. 3 Color change observed for solid 5 upon illumination with white
2
light for 48 h at room temperature in the presence of O .
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nity for the development of metal flavonolato-based solid-state
CO-releasing materials that can be triggered using visible
light. As demonstrated herein, solid zinc bis(flavonolato)
derivatives can be used as in situ CO release compounds in oxi-
dative palladium-catalyzed carbonylation reactions albeit with
some degradation of the CO release product. Further studies
of the applications of these solids in carbonylation processes
are currently in progress.
Acknowledgements
The authors thank the National Science Foundation
(
CHE-1301092) and USTAR (Utah Science and Research
Technology Initiative) for funding. We thank Tatiana Soboleva
for technical assistance.
Scheme 4 Proposed ester aloholysis reactivity of CO release product.
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chemistry for divalent metal flavonolato compounds
Our discovery of light-induced CO release
6
0,61,67
has
spurred us to further evaluate the light-induced CO release
reactivity of flavonol and flavonolato species for applications
as photoCORMs. Construction of the extended 3-hydroxy-4-
pyrone derivatives 1–4 led to the discovery that unlike neutral
5
7
3
-hydroxyflavone, which undergoes multiple types of reac-
tions upon illumination with UV light, the neutral extended
flavonols undergo clean dioxygenase-type visible-light induced
5
8
CO release in the presence of O2. In this contribution we
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large absorption feature that extends into the therapeutic
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