Solid state generation of phenoxyl radicals through β-fragmentation from specifically designed diazenes. An ESR investigation

Article abstract of DOI:10.1016/j.tet.2016.02.010

Upon irradiation at room temperature, symmetrical diazene precursors enabled the formation of phenoxyl radicals through β-fragmentation reaction in the solid state. This traceless generation of phenoxyl radicals was investigated by ESR. This study showed that although the fragmentation of β-phenoxy radicals is a slow process in solution, it could be useful in solid state thanks to the absence of faster competitive pathways.

Full text of DOI:10.1016/j.tet.2016.02.010

Tetrahedron xxx (2016) 1e5
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Tetrahedron
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Solid state generation of phenoxyl radicals through
b-fragmentation
from specifically designed diazenes. An ESR investigation
Cyrielle Dol, Mich eꢀ le P. Bertrand, St eꢁ phane Gastaldi , Eric Besson
*
*
Aix-Marseille Universit ꢁe , CNRS, Institut de Chimie Radicalaire UMR 7273, 13397 Cedex 20, Marseille, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Upon irradiation at room temperature, symmetrical diazene precursors enabled the formation of phe-
noxyl radicals through -fragmentation reaction in the solid state. This traceless generation of phenoxyl
radicals was investigated by ESR. This study showed that although the fragmentation of -phenoxy
radicals is a slow process in solution, it could be useful in solid state thanks to the absence of faster
competitive pathways.
Received 9 December 2015
Received in revised form 29 January 2016
Accepted 3 February 2016
Available online xxx
b
b
Ó 2016 Elsevier Ltd. All rights reserved.
Keywords:
Phenoxyl radical
ESR
b
-fragmentation reaction
1
. Introduction
Many methodologies lead to the formation of phenoxyl radi-
cals.11 They can be generated by oxidation of the corresponding
12
13
14
Phenoxyl radicals are a very interesting class of radicals. They
are involved in a broad spectrum of domains ranging from chemical
biology or metal complexes to materials chemistry with appli-
cations in radical batteries or in organic magnetism. Most phe-
noxyl radicals are transient as illustrated by their extinction rate
constants.6 Their lifetime increases when ortho and para bulky
substituents are present, which decreases their reactivity by hin-
phenols with metal salts (K
3
Fe(CN)
6
,
FeCl
3
,
Ce(SO
4 2
) ,
15
16
17
18
Pb(OAc)
4
), metal oxides (PbO
2
,
Ag
2
O, MnO
2
), organic oxi-
1
2
3
19
20
4,22
dizing agent like quinones, enzymatic systems, dioxygen under
basic conditions and by electrochemistry.
abstraction by another radical,
also been described. Conversely, they can also be formed from 4-
halo-2,5-cyclohexadienones by using reductive metals (Hg, Ag, Cu,
4
5
21
Hydrogen atom
11b
typically an alkoxyl radical, has
23
7
24
dering the carbon atoms bearing the highest spin densities. From
Zn, Na, K). All these methods need a solvent and produce by-
a synthetic point of view, phenoxyl radicals are mainly involved in
dimerization or polymerization processes.
products incompatible with the purpose of our study.
8
Photochemistry might offer an alternative. The homolysis of
ArO-H or ArO-Alkyl bonds at 254 nm leads to phenoxyl radicals but
In previous studies, we have shown that the lifetime of a tran-
sient radical, like aryl sulfanyl radicals, can be dramatically in-
creased when their precursors either form an organic monolayer in
2
5
with low photochemical yields. Moreover, the energetic wave-
length needed to promote these cleavages might be the source of
other undesired homolysis. The photolytic rearrangement of an
aromatic nitro group at 330e380 nm is a milder way to produce
9
the pore of a nanostructured silica or are anchored in its frame-
10
work. We wanted to extend this concept to a more challenging
radical: the phenoxyl radical. Indeed, unlike phenyl sulfanyl radical
in which the spin density is centered on the sulfur atom, the spin
density in phenoxyl radical is largely delocalized on the carbon
atoms of the aromatic ring which results in an increase of the
number of propagation and termination reactions. To trigger the
generation of a radical in a nanostructured organic-inorganic hy-
brid silica a clean method was needed in order to avoid the for-
mation of other paramagnetic species and thus to facilitate the
study of the radical functionalized material.
2
6
a phenoxyl radical but the concomitant generation of a persistent
radical, i.e., nitrogen monoxide, precludes its use for the present
study.
For the traceless generation of arylsulfanyl radicals in the
framework of hybrid silicas, we have designed diazene based pre-
cursors which release the sulfur centered radicals via the b-frag-
mentation of an appropriately designed tertiary carbon-centered
radical formed by the decomposition of a diazene moiety upon ir-
10
radiation at 360 nm (Scheme 1).
We guessed that a similar strategy could be applied to the
generation of phenoxyl radicals. The -fragmentation of -oxygen
b
b
carbon-centered radicals is not a common route to release oxygen-
centered radicals, yet, a few exceptions have been reported in the
*
Corresponding authors. E-mail address: stephane.gastaldi@univ-amu.fr
(
S. Gastaldi).
http://dx.doi.org/10.1016/j.tet.2016.02.010
040-4020/Ó 2016 Elsevier Ltd. All rights reserved.
0
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2
C. Dol et al. / Tetrahedron xxx (2016) 1e5
Scheme 1. Arylsulfanyl radical generation through double fragmentation.
literature such as the ring opening of vinyl epoxides27 or the use of
fluoropyridoxyl radical as leaving group in the allylic alcohols based
2
8
radical allylation reactions recently devised by Zard. The design of
the latter originates from the observation of the unanticipated
fragmentation of
versible radical trapping in xanthate chemistry. The fragmenta-
tion of -oxygen carbon-centered radicals is also implied in the
Surzur-Teissier reaction and related rearrangements.
b-p-chlorophenoxyl radicals thanks to the re-
2
9
Scheme 3. Photolysis of diazene precursors in the presence of DMPO.
b
3
0
Herein, we report the behavior of diazene based precursors for
the generation of phenoxyl radicals in solution and in solid phase as
a study preliminary to the design of ordered silicas grafted with the
same radical precursors.
- Addition to DMPO that produces adducts 5aed.
-
b-Fragmentation that releases phenoxyl radicals 4aed and
subsequently gives rise to adducts 6aed.
Whatever the amount of DMPO, in all cases, the irradiation
enabled the characterization of spin adduct 5.
2
. Results and discussion
The hyperfine coupling constants of nitroxydes 5aed reported
Table 1 are in good agreement with the trapping of tertiary carbon
centered radicals, likely to be 3aed (Fig. 1). In the case of 2a,
The skeleton of the diazene precursors was readily prepared by
nucleophilic substitution of p-chloronitrobenzene with diol 1
Scheme 2). The p-nitro precursor 2a was subsequently reduced
into the corresponding p-amino diazene 2b, which in turn was
converted into the corresponding bis-acetylamino and ureido de-
rivatives 2c and 2d.
32
(
a competitive trapping by the nitro group is likely to explain the
33
presence of 7.
Table 1
ESR splitting constant of 5aed, 6aed and 7
DMPO
5aedb
6aedb
7
a
N
a
H
a
N
a
H
a
H
a
H
a
N
2
2
2
2
2
2
2
2
a
a
b
b
c
c
d
d
0.1 equiv
2 equiv
0.1 equiv
2 equiv
0.1 equiv
2 equiv
0.1 equiv
2 equiv
14.2
14.2
14.3
14.2
14.2
14.2
d
21.9
21.9
22.0
22.0
22.1
22.1
d
d
d
d
d
d
8.1
d
6.9
d
d
d
d
d
d
d
1.7
d
d
d
d
d
d
14.8
d
d
d
d
d
d
12.6
d
d
d
a
a
c
12.9
d
0.8
d
d
14.2
21.8
d
Scheme 2. Synthesis of diazene based precursors 2aed.
a
b
c
In toluene.
Hyperfine splitting constants (G) of adduct.
After turning off the irradiation.
The nitro group was converted into different amino-(2b),
imido-(2c) or urea-groups (2d) in order to mimic the linker de-
vised to anchor the diazene precursor to nanostructured silicas,
in view of the upcoming study of the impact of the structure of
hybrid materials on the release and the persistence of phenoxyl
radicals.
The fragmentation of diazenes 2aed was studied at room
temperature by ESR under different conditions, in solution (spin
trapping experiments) and in solid-state.
The lowering of DMPO concentration allowed radical 3 to evolve
6
via fragmentation. Consequently, the adduct of phenoxyl radical
34
was only detected with 2c in the presence of 10% of DMPO. The
behavior of 2d was slightly different. A complex signal was recor-
ded with a small amount of spin trap. After turning off the irradi-
ation, a simplification of the signal enabled the observation of 6d.
It can be underlined that when the same experiments were
performed without spin trap, a complex ESR signal was recorded
for substrate 2bed. This complexity was not unexpected consid-
2
.1. ESR fragmentation studies in solution
6
ering the fast rate of polymerization of phenoxyl radicals. In the
In a typical procedure, a solution of substrate (2aed) and 5,5-
case of 2a, only the characteristic triplet signal of an N-alkyloxy N-
aryl nitroxide of type 7 (g¼2.0058), resulting from the addition of
the tertiary carbon centered radical 3a to the nitro group, was
detected. In other words, the nitro group acted as spin trap agent.
These experiments showed that a fast trapping of 3 occurred. In
only two cases, the formation of phenoxyl radicals was evidenced
by spin adducts 6c and 6d. This observation performed in the
dimethyl-1-pyrroline N-oxide (DMPO, 0.1 or
2 equiv) was
ꢀ
5
degassed (10 mbar) in a 4 mm quartz-glass tube and radicals
were generated via the light-induced decomposition of the diazene
inside the spectrometer cavity (xenon lamp (200e800 nm) fitted
with a 360ꢁ10 nm filter). The experiments were performed in
benzene with the exception of 2d for which toluene was used for
the sake of solubility.
3
3
presence of 10% of DMPO confirmed the low rate of the b-frag-
The 360 nm irradiation of diazene 2aed enables the formation
mentation of radicals 3 in solution. This is in agreement with the
fragmentation rate estimated by Zard with a cyclobutylcarbinyl
radical probe for the release of fluoropyridoxyl radical (i.e.,
31
of tertiary carbon centered radicals 3aed. In the presence of
a spin trap agent, these intermediates can evolve through two main
pathways (Scheme 3):
3
4 ꢀ1 28a
10 e10 s ).
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C. Dol et al. / Tetrahedron xxx (2016) 1e5
3
Table 2
g Tensors for 4aed
a
g
z
g
y
g
x
g
iso
t
1/2
4
4
4
4
a
b
c
1.990
1.990
1.990
d
2.005
2.005
2.005
d
2.018
2.019
2.018
d
2.004
2.005
2.004
2.004
5.9 h
d
b
11 min
>8 j
d
a
g
iso¼(g
x
þg
y
þg
z
)/3.
b
Not determined.
In the case of radical 4b, an intense ESR signal was recorded
prior to irradiation. The increase of the signal was weak under ir-
radiation as well as its decrease after the end of the photolysis. This
phenomenon might be due to the interference of spontaneous or
photo-stimulated electron transfer reactions.
Under these conditions the tertiary carbon centered radicals
3
aed were not observed. Unlike the ESR experiment performed in
solution, only the phenoxyl radicals were detected. Although slow,
the fragmentation of the -oxygen carbon-centered radicals oc-
curred cleanly. The fact that the experiments were achieved under
vacuum does not seem to favour the fragmentation since the same
results were observed under argon.
Fig. 1. ESR spectrum of 5c at room temperature (irradiation of 2c in the presence of
3
2
b
DMPO (2 equiv)), the small triplet (a
N
¼14.8 G) fits DMPO degradation by product.
2
.2. ESR studies at solid state
Radical precursors 2 were designed to generate phenoxyl radi-
3. Conclusion
cals grafted to nanostructured silica in the solid state. Prior to this
study, in order to investigate the behavior of 2 under close condi-
tions, ESR experiments were directly performed on the solid pre-
cursors at room temperature.
This ESR study showed that diazene-based precursors do not
enable an efficient generation of phenoxyl radicals in solution.
However, the low rate of the -fragmentation step does not impede
ꢀ5
b
The powders (2aed) were degassed (10 mbar) in a 4 mm
the generation of phenoxyl radical in the solid state to be observed.
No faster competitive pathway occurs thanks to the absence of
diffusion.
quartz-glass tube and irradiated at 293 K with a xenon lamp
(
200e800 nm) fitted with a 360 nm filter.
In the absence of solvent, the direct ESR observation of the
This type of precursor is a candidate of choice to generate
phenoxyl radicals from hybrid materials since it is stable under the
conditions necessary to the silica synthesis. No co-reagent is
needed and consequently no solvent is required. The side products
fragmentation of diazene precursor 2aed showed the formation of
phenoxyl radicals 4aed. An anisotropic signal was recorded for all
precursors (Fig. 2) but 2d, for which an isotropic signal was
observed.
(
nitrogen and isobutene) resulting from the fragmentation diffuse
10
readily in the silica matrix and thus no other radical species is
generated.
It is reasonable to assume that this approach to produce aryl-
heteroatomic radicals will apply to heteroatoms other than oxygen
since the cleavage of the CeO bond is thermodynamically among
the most unfavorable fragmentation processes.
4
4
. Experimental section
.1. General information
All reactions were carried out in dry glassware using magnetic
stirring and a positive pressure of argon. Solvents are commercially
available, they were used as purchased without further purification.
THF was distilled over sodium benzophenone ketyl prior to use.
Anhydrous DMSO was purchased. Dry state adsorption conditions
and purification were performed on Macherey Nagel silica gel 60 A
Fig. 2. ESR. Spectrum of 4b powder at room temperature.
(70e230 mesh). Analytical thin layer chromatography was per-
The ESR parameters of these signals were in agreement with the
formed on pre-coated silica gel plates. Visualization was accom-
21b,35
formation of phenoxyl radicals.
g Factors are gathered in
plished by UV (254 nm), with p-anisaldehyde and with
1
13
Table 2. It must be outlined that the solid state generation has
a dramatic impact on the half-lives of these phenoxyl species. The
decay curves were fitted with exponential or multi-exponential
models, which did not allow to determine the mechanism in-
volved. The half-lifetimes were graphically estimated from the
decay curves. Half-lives ranging from 11 min to higher than 8 days
were registered. This is likely to correlate to the solid state and the
restricted mobility which, in anaerobic medium, should slow down
most decay pathways via dimerization.
phosphomolybdic acid in ethanol. H NMR, C NMR spectra were
recorded on 300 or 400 MHz spectrometer. Chemical shifts ( ) are
reported in ppm. Signals due to residual protonated solvent ( H
d
1
13
NMR) or to the solvent ( C NMR) served as the internal standard:
CDCl (7.27 ppm and 77.0 ppm), DMSO-d (2.50 ppm and
3
6
39.52 ppm). Multiplicity is indicated by one or more of the following
abbreviations: s (singlet), d (doublet), t (triplet), q (quartet), quint
(quintet), m (multiplet), br (broad). The lists of coupling constants (J)
reported in Hertz (Hz) correspond to the order of multiplicity
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4
C. Dol et al. / Tetrahedron xxx (2016) 1e5
assignment. APT was used for 13C spectra assignment. All melting
points were uncorrected and were recorded in open capillary tubes
using a melting point apparatus. EPR experiments were performed
with commercially available HPLC grade solvents and reactants that
were used as received. EPR experiments were performed on an
ELEXSYS Bruker instrument and the Bruker BVT 3000 set-up was
utilized to control the temperature. Irradiations were performed
with a Hamamatsu LC8 01A light source with a 360e370 nm filter.
EPR spectra were simulated using WinSim 2002 software.
(200 mg, 0.56 mmol, 1 equiv) in THF (3.5 mL) was added octyl iso-
cyanate (0.22 mL,1.23 mmol, 2.2 equiv). The mixture was stirred one
night at room temperature. After completion (TLC monitoring,
pentane/EtOAc: 50/50), the mixture was diluted with Et
residue was filtrated, washed with Et O and dried to give a pale rose
solid (267 mg, 0.40 mmol, 71%). Mp 172e173 C. H NMR (400 MHz,
DMSO-d
J¼9.0 Hz, 4H, ArH), 5.97 (t, J¼5.5 Hz, 2H, NH), 4.04 (s, 4H, OeCH
3.03 (pseudo q, J¼6.4 Hz, 4H, NHCH ), 1.38 (t, J¼6.2 Hz, 4H, CH CH
1.26 (br s, 20H, CH ), 1.16 (s, 12H, CquateCH
). C NMR (75 MHz, CDCl
þTFA d
129.0 (HCar), 127.5 (Car), 116.6 (HCar), 78.0 (CH
2.8 (HNCH ), 32.2 (CH ), 30.2 (CH ), 29.6 (CH
(CH ), 23.0 (CH ), 21.3 (CH
calcd for [MþH]
2
O. The
2
ꢂ
1
6
)
d
: 8.15 (br s, 2H, NH), 7.23 (d, J¼8.8 Hz, 4H, ArH), 6.75 (d,
2
),
),
2
2
3
Diol 1 was prepared according to a literature procedure.10
), 0.86 (t, J¼6.2 Hz, 6H,
: 158.2 (HNCONH),
eO), 71.8 (CquateN),
), 29.5 (CH ), 27.0
). HRMS (ESI): m/z:
2
3
13
CH
2
CH
3
3
1
) d
4
.2. Synthesis of compounds 2aed
2
4
2
2
2
2
2
4
.2.1. (E)-1,2-Bis(2-methyl-1-(4-nitrophenoxy)propan-2-yl)diazene
2
2
3
O
eCquat), 13.6 (CH
3
þ
þ
(2a). To a solution of 1 (217 mg, 1.25 mmol, 1 equiv) and p-nitro-
C
38
H
63
N
6
4
: 667.4905, found: 667.4903.
chlorobenzene (432 mg, 2.74 mmol, 2.2 equiv) in DMSO (2 mL)
under argon was slowly added NaOH (175 mg, 4.36 mmol,
4.3. ESR experiments
ꢂ
3
.5 equiv) at 0 C. The mixture was stirred one night at room
temperature. After completion (TLC monitoring, pentane/EtOAc:
0/20), the mixture was diluted with water and extracted three
times with CH Cl . Organic phase was washed with water and
brine, dried over MgSO and concentrated. The residue was taken
on Et O, stirred for 1 night, and filtrated to give 2a as a white crystal
345 mg, 0.83 mmol, 66%). Mp 148e149 C. H NMR (400 MHz,
CDCl
: 8.16 (d, J¼9.3, 4H, ArH), 6.90 (d, J¼9.3, 4H, ArH), 4.19 (s, 4H,
CH ), 1.27 (s, 12H, CH
4
.3.1. Spin-trapping experiments. In a 4 mm quartz-glass tube, 4 mg
8
of diazene precursor and DMPO (2 or 0.1 equiv) in 0.25 mL of
benzene were degassed with three freeze-pump-thaw cycles with
a 10 mbar vacuum pump. EPR spectra were recorded at room
temperature with the parameters: modulation amplitude¼0.5 G,
receiver gain¼99 dB, modulation frequency¼100 kHz,
power¼2 mW (20 dB), sweep width¼100 G, conversion
time¼29.30 ms, sweep time¼30 s, number of scans¼1.
2
2
ꢀ5
4
2
ꢂ
1
(
3
) d
1
3
2
3
). C NMR (75 MHz, CDCl
3
)
d
: 164.3 (CareO),
), 69.5 (CeN), 22.2
24 4 6
H N O : 417.1769 Da,
1
41.7 (CareN),126.0 (HCar),114.7 (HCar), 74.5 (CH
2
þ
(
CH
3
). HRMS (ESI): m/z: calcd for [MþH] C20
4.3.2. Solid state ESR experiments e direct observation. In solution:
found: 417.1768 Da.
in a 4 mm quartz-glass tube, 5 mg of diazene precursor in 0.25 mL
of benzene were degassed with three freeze-pump-thaw cycles
with a 10 mbar vacuum pump. EPR spectra were recorded at
0
4
.2.2. (E)-4,4 -((Diazene-1,2-diylbis(2-methylpropane-2,1-diyl))bi-
ꢀ5
s(oxy))dianiline (2b). To a solution of 2a (47 mg, 0.11 mmol,1 equiv)
room
amplitude¼0.5
temperature
G,
with
receiver
the
parameters:
gain¼99 dB,
modulation
modulation
in EtOH (1.5 mL) was added SnCl
2
$2H
2
O (248 mg, 1.1 mmol,
ꢂ
10 equiv). The resulting mixture was stirred one night at 60 C.
frequency¼100 kHz, power¼2 mW (20 dB), sweep width¼100 G,
conversion time¼29.30 ms, sweep time¼30 s, number of scans¼1.
Solid state: in a 4 mm quartz-glass tube, 5 mg of diazene pre-
cursor were degassed with three freeze-pump-thaw cycles with
After completion (TLC monitoring, pentane/EtOAc: 50/50), the
mixture was diluted with iced water and a solution saturated of
NaHCO
times with EtOAc, washed with brine, dried over MgSO
centrated. The residue was purified using silica gel column
3
was added until pH¼7e8. The mixture was extracted three
and con-
ꢀ5
4
a 10 mbar vacuum pump. EPR spectra were recorded with the
parameters: modulation amplitude¼2 G, receiver gain¼99 dB,
modulation frequency¼100 kHz, power¼0.2 mW (30 dB), sweep
width¼200 G, conversion time¼29.30 ms, sweep time¼30 s,
number of scans¼2.
(
0
(
1
30e50% EtOAc in pentane) to give 2b as a red solid (32 mg,
ꢂ
1
.09 mmol, 80%). Mp 98e99 C. H NMR (300 MHz, CDCl
3
)
d
: 6.72
), 1.23 (s,
: 143.43 (CareO), 140.0
d, J¼8.4, 4H, ArH), 6.61 (d, J¼8.4, 4H, ArH), 3.99 (s, 4H, CH
2
13
2H, CH
3
). C NMR (75 MHz, CDCl
3
)
d
The half-lifetimes were estimated after turning off the irradia-
tion by monitoring the decay of the signal intensity (determined
from the double integrated EPR signal). The decay curves were
fitted with exponential or multi-exponential models, which did not
allow the mechanism involved to be determined. Several termi-
nation processes occurred. The half-lifetimes were graphically es-
timated from the decay curves.
(
(
3
C
CH
areN), 116.5 (HCar), 116.2 (HCar), 76.7 (CH
2
), 69.6 (CeN), 22.2
). HRMS (ESI): m/z: calcd for [MþH]þ
C
H
20 29
N
4
O
þ
:
3
2
57.2285 Da, found: 357.2284 Da.
0
.2.3. (E)-N,N -(((Diazene-1,2-diylbis(2-methylpropane-2,1-diyl))bi-
4
s(oxy))bis(4,1-phenylene))bis(N-acetylacetamide) (2c). To a flask
containing 2b (100 mg, 0.28 mmol, 1 equiv) under argon was added
acetic anhydride (0.92 mL, 9.52 mmol, 34 equiv). The mixture was
Acknowledgements
ꢂ
stirred 5 days at 60 C. After completion (NMR monitoring), the
mixture was diluted with water and extracted three time with
The authors acknowledge the Agence Nationale de la Recherche
for funding (ANR-12-JS07-005).
CH
MgSO
column (40e60% AcOEt in pentane) to give 2c as a yellow solid
2
Cl
2 3
. Organic phase was washed with NaHCO , brine, dried over
4
and concentrated. The residue was purified using silica gel
ꢂ
1
References and notes
(
51 mg, 0.097 mmol, 35%). Mp 92e94 C. H NMR (300 MHz, CDCl )
3
d
: 7.01 (d, J¼9.0 Hz, 4H, ArH), 6.93 (d, J¼9.0 Hz, 4H, ArH), 4.11 (s, 4H,
1
. (a) Stubbe, J.; Nocera, D. G.; Yee, C. S.; Chang, M. C. Y. Chem. Rev. 2003, 103,
2167e2201; (b) Stubbe, J.; van der Donk, W. A. Chem. Rev. 1998, 98, 705e762; (c)
Pierre, J. L. Chem. Soc. Rev. 2000, 29, 251e257.
13
CH
CDCl ) d
3
2
), 2.27 (s, 12H, COCH
3
), 1.26 (s, 12H, CH
3
). C NMR (75 MHz,
: 173.5 (CO), 159.4 (CareO),132.1 (CareN),129.7 (HCar), 115.8
2. (a) Thomas, F. In Stable Radicals; Hicks, R. G., Ed.; Wiley: 2010; pp 281e316;
(
HCar), 74.4 (CH
2
3
), 69.4 (CeN), 27.1 (COCH ), 22.2 (CH
3
). HRMS (ESI):
Chap 8; (b) Shimazaki, Y.; Yamauchi, O. Indian J. Chem., Sect. A 2011, 50,
m/z: calcd for [MþH]þ
C
H
28 37
N
4
O
6
:
525.2708 Da, found:
383e394; (c) Shimazaki, Y. In The Chemistry of Metal Phenolates; Zabicky, J., Ed.;
5
25.2707 Da.
John Wiley & Sons: 2013; pp 593e668; Chap 10.
3
4
. J a€ hnert, T.; Hager, M. D.; Schubert, U. S. J. Mater. Chem. A 2014, 2, 15234e15251.
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