The quenching of fluorescence as an indicator of donor-strength in meso arylethynyl BODIPYs

Article abstract of DOI:10.1039/c3dt53056f

A series of meso arylethynyl BODIPYs (2a-2h) were designed and synthesized by the Pd-catalyzed Sonogashira cross-coupling reaction. The effects of the donor on the photophysical properties of the BODIPYs were explored. The DFT optimized structures and crystal structures show the planar orientation of the donor group with respect to the acceptor BODIPY, which favors a high degree of conjugation and induces strong donor-acceptor interactions. The quenching of fluorescence was correlated with the electron donating strength of the donor. The anthracene, pyrene and triphenylamine were found to have a stronger electron donating ability than the p-methoxyphenyl, phenanthrene, 1-naphthalene, biphenyl, and 2-naphthalene moieties. This was further supported by computational calculations and electrochemical analysis. The single crystal structures of BODIPYs 2d and 2e are reported, which show marvellous supramolecular structures.

Full text of DOI:10.1039/c3dt53056f

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The quenching of fluorescence as an indicator of
donor-strength in meso arylethynyl BODIPYs†
Cite this: Dalton Trans., 2014, 43,
4854
Rajneesh Misra,* Bhausaheb Dhokale, Thaksen Jadhav and Shaikh M. Mobin
A series of meso arylethynyl BODIPYs (2a–2h) were designed and synthesized by the Pd-catalyzed Sono-
gashira cross-coupling reaction. The effects of the donor on the photophysical properties of the BODIPYs
were explored. The DFT optimized structures and crystal structures show the planar orientation of the
donor group with respect to the acceptor BODIPY, which favors a high degree of conjugation and
induces strong donor–acceptor interactions. The quenching of fluorescence was correlated with the
electron donating strength of the donor. The anthracene, pyrene and triphenylamine were found to have
a stronger electron donating ability than the p-methoxyphenyl, phenanthrene, 1-naphthalene, biphenyl,
and 2-naphthalene moieties. This was further supported by computational calculations and electrochem-
ical analysis. The single crystal structures of BODIPYs 2d and 2e are reported, which show marvellous
supramolecular structures.
Received 29th October 2013,
Accepted 18th December 2013
DOI: 10.1039/c3dt53056f
www.rsc.org/dalton
Dehaen has introduced an efficient synthetic strategy for the
incorporation of the ethynyl linkage at the meso position.7
Introduction
In recent years BODIPY chemistry has gained momentum,
Our previous report on ferrocenyl BODIPYs has established
which is reflected from their share in most of the journals.1 that the meso alkynylated BODIPYs show superior electronic
BODIPY dyes distinguish themselves from others due to strong communication than the pyrrolic alkynylated BODIPYs.8 This
absorption, sharp fluorescence and high thermal stability.2 encouraged us to study the effect of various donors at the meso
These properties of the BODIPYs make them a potential candi- position of the BODIPY via an ethynyl linkage. In this report
date in organic electronics and photonics.3 Our group is inter- we have incorporated various donors at the meso position of
ested in the design of donor–acceptor molecular systems for the BODIPY via an ethynyl linkage and studied their structural,
photonic applications.4 The BODIPY unit acts as strong accep- electronic, photophysical, and electrochemical properties.
tor. Therefore the functionalisation of BODIPY dyes with
donors will result in donor–acceptor (D–A) type molecular
systems.5 Various strategies are reported in the literature for
Results and discussion
the functionalisation of the BODIPY dyes at the meso position
as well as the pyrrolic positions.6 In meso aryl functionalized
The meso-arylethynyl BODIPYs 2a–2h were synthesized by the
BODIPY, the meso substituent adopts an orthogonal orien-
tation with respect to the BODIPY core, which hinders the elec-
tronic communication. One can overcome this problem by
introducing the ethynyl linkage at the meso position, so that
the substituent and the BODIPY will be planar, with respect to
each other for a better electronic communication. Wim
palladium catalyzed Sonogashira cross-coupling reaction of
8-chloro BODIPY 1 with respective arylethynes (a–h) as shown
in Scheme 1. The 8-chloro BODIPY 1, was synthesized from
dipyrrylketone.7 The dipyrrylketone was reacted with POCl3 fol-
lowed by in situ deprotonation by base and complexation with
BF3 etherate, which resulted 8-chloro BODIPY 1 in a 59% yield.
The dipyrrylketone was synthesized by the condensation reac-
tion of thiophosgene and pyrrole, followed by oxidation with
H2O2 under basic condition.9 The arylethynes a–h were syn-
thesized from reported procedures.10
The Pd-catalyzed Sonogashira cross-coupling reaction of
8-chloro BODIPY 1, with the respective arylethynes at 0 °C
resulted BODIPYs 2a–2h in 44 to 77% yields. These reactions
were completed within half an hour due to the high reactivity
of 8-chloro BODIPY. The BODIPYs 2a–2h were well character-
ized by 1H NMR, 13C NMR and HRMS techniques. The
Department of Chemistry, Indian Institute of Technology Indore, Indore 452 017,
India. E-mail: rajneeshmisra@iiti.ac.in
†Electronic supplementary information (ESI) available: general experimental
methods, and copies of 1H, 13C NMR, and HRMS spectra of all new compounds,
crystallographic information files (CIFs) for compounds 2d and 2e, the thermal
and photophysical properties, the electrochemical properties, and the theoretical
calculations. CCDC number 967406 for 2d and 967407 for 3e. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c3dt53056f
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Scheme 1 The synthesis of meso-arylethynyl BODIPYs 2a–2h.
BODIPYs 2d and 2e were also characterized by the single
crystal X-ray diffraction technique.
Table 1 Photophysical and thermal properties of meso-arylethynyl
BODIPYs 2a–2h
The typical 1H NMR spectrum of the BODIPY derivatives
show two α pyrrolic protons as a singlet at ∼7.8 ppm, whereas
the β and β′ protons appear as doublets at ∼6.6 and ∼7.7 ppm.
The protons of the aryl substituent appear in the aromatic
region.
The thermal stability is one of the key criteria for 2d
optoelectronic applications. The thermal properties of the
meso-arylethynyl BODIPYs (2a–2h) were investigated using
thermogravimetric analysis (TGA) by heating the BODIPYs in a 2h
λS0→S1 ε × 104
BODIPY (nm)
(M−1 cm−1 a
λem
(nm) (cm−1
Stokes shift
Td
(°C)
b
d
c
)
)
ΦF
2a
2b
2c
538
540
543
544
545
546
548
553
4.4
552
472
—
0.23 414
e
e
e
5.0
3.8
4.9
3.5
4.4
4.5
4.6
—
—
379
558
559
561
563
495
493
523
554
0.18 392
0.19 386
0.19 357
0.20 484
504
0.02 365
2e
2f
2g
e
e
e
—
—
—
565
384
nitrogen atmosphere at the heating rate of 10 °C per minute
a Recorded at S0→S1. b Excited at λS0→S1
.
c Determined using
Rhodamine 6G as the standard (Φ
=
0.88, in ethanol). d The
and monitoring the weight loss against temperature (Fig. 1).
The TGA analysis shows that the meso-arylethynyl BODIPYs
were stable enough up to 350 °C. The decomposition tempera-
ture at 10% weight loss is shown in Table 1, indicating the
good thermal stability of the BODIPYs.
decomposition temperature at 10% weight loss, determined by TGA.
e The BODIPYs are non emissive.
Photophysical properties
The photograph of the BODIPYs 2a–2h in a dichloromethane
solvent in day light and in UV light is displayed in Fig. 2. The
coloring pattern of the BODIPYs reflects the D–A interaction.
The BODIPYs 2b, 2g and 2h are non-fluorescent with a dark
purple color indicating a strong D–A interaction, whereas the
BODIPYs 2a and 2c–2f are fluorescent with a yellow–red shade
which indicates a moderate D–A interaction.
The UV-vis absorption spectra of the meso-arylethynyl
BODIPYs 2a–2h were recorded in dichloromethane (Fig. 3),
and the corresponding data are shown in Table 1. The
BODIPYs 2a–2h show a strong absorption band between 538
and 553 nm which corresponds to the S0→S1 transition with a
high extinction coefficient, and a shoulder at a higher energy
region corresponding to the respective vibronic transition of
the BODIPY. The BODIPYs 2a–2h show a red shift of 35–50 nm
in the S0→S1 absorption band compared to 8-chloro BODIPY 1
(Table 1) due to enhanced conjugation. The red shift in the
S0→S1 absorption band follows the order of 2h > 2g > 2f > 2e >
2d > 2c > 2b > 2a. The BODIPY 2b shows a shoulder in the
Fig. 1 The TGA plot of the meso-arylethynyl BODIPYs 2a–2h.
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Fig. 2 The BODIPYs 2a–2h in dichloromethane at a concentration of 10−4 M (a) in day light and (b) in UV light.
Fig. 4 The emission spectra of the meso-arylethynyl BODIPYs 2a–2h,
recorded in dichloromethane.
Fig. 3 The normalized UV-vis absorption spectra of the meso-arylethy-
nyl BODIPYs 2a–2h recorded in dichloromethane, the inset shows the
enlarged view.
(Fig. 5). The potentials were referenced against Fc/Fc+ as per
IUPAC guidelines.13 The redox peaks were mostly irreversible
or quasi reversible. The BODIPYs 2a–2h show two oxidation
and two reduction waves corresponding to the formation of a
dication and dianion radical, respectively. The stronger the
D–A interaction, the better the delocalization of the electrons
from the donor aryl moiety to the acceptor BODIPY moiety,
showing the easier oxidation and more difficult reduction of
the BODIPYs.
The BODIPY 2b shows the easiest oxidation and the hardest
reduction indicating the pronounced delocalization of the elec-
tron density of the donor triphenylamine moiety on the accep-
tor BODIPY; on the other hand, the BODIPY 2c shows easiest
reduction and the hardest oxidation indicating the poor de-
localization of the electrons of the donor aryl on the acceptor
BODIPY. The other BODIPYs show redox peaks at intermediate
potentials. The redox properties suggest a strong D–A inter-
action in the BODIPYs 2b, 2g and 2h and moderate D–A inter-
actions in 2a and 2c–2f (Table 2).
S0→S1 absorption band at a low energy region, which can be
attributed to a charge transfer.11
The emission properties of the meso-arylethynyl BODIPYs
2a–2h were studied in dichloromethane (Fig. 4) and the corres-
ponding data are given in Table 1. The meso-arylethynyl
BODIPYs 2a, 2c–2f and 2h emit in the 520–650 nm region with
a Stokes shift ranging from 380–550 cm−1. The fluorescence
quantum yields of the BODIPYs 2a–2h were decreased drasti-
cally compared to 8-chloro BODIPY 1. The BODIPYs 2a and 2g
were non-emissive in nature. The quenching of the fluorescence
is may be due to the intramolecular energy transfer from the
aryl moiety to the BODIPY.12 The trend in the quenching of the
fluorescence can be correlated to the electron donating ability
of the aryl substituent. In addition the electron donating ability
lesser will be the quantum yield. From the quantum yield
values the electron donating ability of the aryl substituent
follows the order: 2b > 2g > 2h > 2d > 2c > 2e > 2f > 2a.
Electrochemical properties
Theoretical calculations
The electrochemical properties of the meso-arylethynyl
BODIPYs were investigated by the cyclic voltammetric and The electronic distribution of the meso-arylethynyl BODIPYs
differential pulse voltammetric (CV and DPV) techniques could be better understood by DFT calculations. The energy
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Fig. 5 The overlayed CV and DPV plots of the meso-arylethynyl BODIPYs 2b and 2c.
The crystal structure of the BODIPYs 2d and 2e are shown
in Fig. 7. The torsional angle between the BODIPY and aryl
moiety are shown in Table S2 (ESI†). The crystal structures
show the highly planar orientation of the aryl moiety and
BODIPY unit.
Table 2 The electrochemical properties of the meso-arylethynyl
BODIPYs 2a–2ha
E2
E1
E1
E2
BODIPY
oxidb
oxidb
redb
redb
2a
2b
2c
2d
2e
2f
1.28
1.13
1.35
1.28
1.28
1.32
1.22 (broad)
1.16 (broad)
1.07
0.65 (broad)
1.1
1.07
1.07
0.99
0.85
0.89
−1
−1.35
−1.29
−1.35
In the packing of BODIPY 2d, the π–π staking interaction
between the two BODIPY units forms a staircase like structure.
−1.01
−0.98
−0.99
−1.01
−1.01
−1
−1.28 Such a staircase like structure connects another stair case like
−1.37
structure in a perpendicular direction via a C(17)–H(17)⋯π
(pyrrolic) interaction. If flipped to view along the a-axis it
looks L-shaped, hiding behind the π-staked molecules. The
L-shaped structure further grows to form a complex 3-D struc-
tural motif (Fig. 8 and Fig. S3 (ESI†)).
Similarly in the crystal structure of 2e two molecules
connect to each other in a head to tail fashion via another
molecule by two mutual F(1)⋯H(9)–C(9) and a F(2)⋯H(18)–
C(18) interaction forming a 2D chain. Two such chains
connect to another chain in an anti-parallel fashion by face to
face π⋯π staking interactions between two BODIPY units
(Fig. 9a), these chains further cross link to other chains
forming a complex 3D packing diagram along the tilted c-axis.
In the packing diagram the molecules orient in such a way
that the aryl part of the BODIPY units is at the centre and sur-
rounded by the BODIPY moiety (Fig. 9b).
−1.32
2g
2h
−1.37
−1
−1.4
a The electrochemical analysis was performed in a 0.1 M solution of
Bu4NPF6 in dichloromethane at a 100 mV s−1 scan rate, versus Fc/Fc+.
b Irreversible wave.
minimized structures show the planar orientation of the
BODIPY core with respect to the meso aryl substituent, which
indicates a better electronic communication. The energy opti-
mized structures closely resemble the single crystal X-ray struc-
tures. The frontier molecular orbital plots are displayed in
Fig. 6. The LUMO in the meso-arylethynyl BODIPYs 2b, 2g and
2h is majorly contributed by the BODIPY moiety, and the
HOMO is contributed by the arylethynyl moiety, supporting
strong D–A interactions. In the meso-arylethynyl BODIPYs 2a,
2c–2f the HOMO is localized on the BODIPY unit and the
LUMO is distributed on the whole molecule but majorly on
the arylethynyl unit suggesting moderate D–A interactions.
Conclusion
In summary, we have reported the synthesis of meso-arylethy-
nyl BODIPYs. The photophysical and electrochemical pro-
Single crystal X-ray diffraction studies
The single crystals of the BODIPYs 2d and 2e were obtained by perties of the BODIPYs can be tuned by varying the strength of
the slow evaporation of a mixture of chloroform and hexane the donor. The DFT calculations and photophysical and elec-
solution (1 : 3 ratio). The BODIPYs 2d and 2e crystallize in the trochemical properties suggest strong donor–acceptor inter-
space groups P212121 and P2/c, respectively. The crystal struc- actions in the BODIPYs 2b, 2g and 2h, and moderate
ture and data refinement parameters and the distances and interactions in 2a, and 2c–2f. The quenching of fluorescence
angles of the intermolecular interactions in the crystal struc- can be used as a measure of the relative electron donating
tures are summarized in Tables S1 and S3, respectively (ESI†).
strength of the aryl substituent. The crystal structures show
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Fig. 6 The HOMO and LUMO frontier molecular orbitals of the meso-arylethynyl BODIPYs at the B3LYP/6-31G(d).
interesting supramolecular interactions. The NLO properties of
these BODIPYs are currently being studied.
Experimental section
General methods
The chemicals were used as received unless otherwise indi-
cated. All oxygen or moisture sensitive reactions were per-
formed under
a nitrogen/argon atmosphere using the
1
standard Schlenk method. H NMR (400 MHz) and 13C NMR
(100 MHz) spectra were recorded on a 400 MHz NMR instru-
1
ment using CDCl3 as the solvent. The H NMR chemical shifts
are reported in parts per million (ppm) relative to the solvent
residual peak (CDCl3, 7.26 ppm). The multiplicities are given
as: s (singlet), d (doublet), t (triplet), q (quartet), dd (doublet of
doublets), m (multiplet), and the coupling constants, J, are
given in Hz. The 13C NMR chemical shifts are reported relative
to the solvent residual peak (CDCl3, 77.36 ppm). The thermo-
gravimetric analyses were performed on the Thermal Analysis
system at the heating rate of 10 °C per minute under a nitro-
gen atmosphere. The UV-visible absorption spectra of all of
the compounds were recorded on UV-visible Spectrophoto-
meter in dichloromethane. The fluorescence spectra of all the
compounds were recorded on a fluorescence spectropho-
tometer in a dichloromethane solvent. Cyclic voltammograms
(CVs) and differential voltammograms (DPVs) were recorded
on an electrochemical analyzer using glassy carbon as the
Fig. 7 The single crystals of the BODIPYs 2d and 2e, front view and side
view.
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Fig. 8 The crystal packing of 2d.
working electrode, Pt wire as the counter electrode, and Satu-
13C NMR (CDCl3, 100 MHz, ppm): 150.8, 146.3, 142.6,
rated Calomel Electrode (SCE) as the reference electrode. 136.3, 134.5, 129.9, 128.4, 126.2, 125.1, 120.5, 118.00, 112.1,
HRMS was recorded on a mass spectrometer (ESI-TOF).
109.3, 85.6.
HRMS (ESI-TOF) m/z = calculated for C29H20BF2N3 + Na+ =
482.1616, measured 482.1615.
Synthesis and characterization
2c
Red crystalline solid. Yield: 64% (105 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 7.84 (s, 2H), 7.74 (d, J =
8 Hz, 2H), 7.69 (d, J = 8 Hz, 2H) 7.65 (m, 2H), 7.49 (m, 2H),
7.42 (m, 3H), 6.56 (d, J = 4 Hz, 2H).
A generalized procedure for the Sonogashira coupling reac-
tion. 8-Chloro BODIPY 1 (190 mg, 0.845 mmol) and the corres-
ponding arylethyne (0.845 mmol) were dissolved in THF–
triethylamine (10 : 1, v/v; 5 ml), and the mixture was cooled to
0 °C using an ice bath. The reaction mixture was purged with
argon, and Pd(PPh3)2Cl2 (29.6 mg, 5 mol%), and CuI (16 mg,
10 mol%) were added, followed by stirring at 0 °C for 30 min.
Upon the completion of the reaction, the mixture was evapor-
ated to dryness, and the crude product was dissolved in
CH2Cl2, chromatographed on silica (1 : 1; hexanes–CHCl3), and
recrystallized from a chloroform–hexane (1 : 3) mixture to give
2a–2h (yield 44–77%) as purple crystalline solids.
2a
13C NMR (CDCl3, 100 MHz, ppm): 144.0, 143.7, 139.8, 136.7,
133.4, 129.2, 128.5, 127.6, 127.5, 127.27, 119.7, 118.5, 106.1, 85.1.
HRMS (ESI-TOF) m/z
=
[M +
Na]+ calculated for
C23H15BF2N2 + Na+ = 391.1193, measured 391.1197.
2d
Red crystalline solid. Yield: 73% (110 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 8.23 (s, 2H), 7.88 (m,
5H), 7.65 (dd, J = 1.50 Hz and J = 8.54 Hz, 1H), 7.59 (m, 2H),
7.47 (d, J = 4 Hz, 4H), 6.58 (d, J = 4 Hz, 2H).
13C NMR (CDCl3, 100 MHz, ppm): 143.8, 136.7, 134.2,
134.1, 132.9, 129.2, 128.9, 128.4, 128.2, 128.1, 127.6, 127.4,
118.5, 118.2, 106.6, 84.7.
Yellow-red crystalline solid. Yield: 77% (110 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 7.81 (s, 2H), 7.61 (d, J =
8 Hz, 2H), 7.38 (d, J = 4 Hz, 2H), 6.96 (d, J = 8 Hz, 2H), 6.34 (d,
J = 4 Hz, 2H) 3.88 (s, 3H).
HRMS (ESI-TOF) m/z = calculated for C21H13BF2N2 + Na+ =
365.1036, measured 365.1039.
13C NMR (CDCl3, 100 MHz, ppm): 162.2, 143.1, 136.5,
135.0, 128.9, 128.3, 118.2, 114.8, 113.0, 107.7, 84.4, 55.7.
HRMS (ESI-TOF) m/z = calculated for C18H13BF2N2O + Na+ =
345.0984, measured 345.0988.
2e
Red crystalline solid. Yield: 65% (99 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 8.35 (dd, J = 8 Hz, 1H),
7.02 (d, J = 8 Hz, 1H), 7.94 (m, 2H), 7.87 (s, 2H), 7.71 (m, 1H)
7.62 (m, 1H), 7.56 (dd, J = 8 Hz, 1H), 7.51 (d, J = 4 Hz, 2H).
13C NMR (CDCl3, 100 MHz, ppm): 143.8, 136.7, 133.4,
133.3, 133.2, 132.0, 129.2, 129.0, 128.1, 127.5, 127.2, 125.6,
125.5, 118.64, 118.60, 104.4, 89.0.
2b
Dark purple crystalline solid. Yield: 44% (90 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 7.79 (s, 1H), 7.47 (d, J =
9 Hz, 2H), 7.34 (m, 6H), 7.17 (m, 6H), 7.01 (d, J = 8.8 Hz, 2H)
6.52 (d, J = 3 Hz, 2H).
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Fig. 9 The crystal packing of 2e along the tilted c-axis.
HRMS (ESI-TOF) m/z = calculated for C21H13BF2N2 + Na+ =
365.1036, measured 365.1036.
HRMS (ESI-TOF) m/z = calculated for C27H15BF2N2 + Na+ =
439.1193, measured 439.1194.
2f
2h
Red crystalline solid. Yield: 57% (99 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 8.73 (m, 1H), 8.69 (d, J =
Dark purple crystalline solid. Yield: 62% (108 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 8.62 (s, 1H), 8.56 (m,
8 Hz, 1H), 8.40 (m, 1H), 8.24 (s, 1H), 7.93 (d, J = 8 Hz, 1H), 2H), 8.09 (d, J = 8 Hz, 2H), 7.90 (s, 2H), 7.73 (m, 2H), 7.60 (m,
7.88 (s, 2H), 7.77 (m, 3H), 7.67 (m, 1H), 7.52 (d, J = 2 Hz, 2H), 4H), 6.65 (d, J = 4 Hz, 2H).
6.60 (d, J = 4 Hz, 2H).
13C NMR (CDCl3, 100 MHz, ppm): 143.4, 136.5, 134.1,
13C NMR (CDCl3, 100 MHz, ppm): 143.8, 136.8, 135.7, 132.0, 131.2, 129.5, 129.0, 128.5, 127.5, 126.3, 126.0, 118.6,
131.6, 130.8, 130.6, 130.3, 129.4, 129.3, 127.9, 127.9, 127.6, 114.5, 103.8, 95.9.
127.4, 126.5, 123.4, 123.0, 118.6, 117.6, 104.7, 88.6.
HRMS (ESI-TOF) m/z = calculated for C25H15BF2N2 + Na+ =
HRMS (ESI-TOF) m/z = calculated for C25H15BF2N2 + Na+ = 415.1193, measured 415.1198.
515.1193, measured 5151.1190.
2g
Dark purple crystalline solid. Yield: 47% (87 mg).
1H NMR (CDCl3, 400 MHz, ppm): δ 8.52 (d, J = 8 Hz, 2H),
8.22 (m, 6H), 8.08 (m, 2H), 7.88 (s, 2H), 7.54 (d, J = 4 Hz, 2H),
Acknowledgements
6.61 (d, J = 8 Hz, 2H).
RM thanks CSIR and DST, New Delhi for financial support. BD
and TJ thank CSIR and UGC, New Delhi for their fellowship.
We are thankful to Sophisticated Instrumentation Centre (SIC),
Single Crystal X-ray diffraction Facility, IIT Indore.
13C NMR (CDCl3, 100 MHz, ppm): 143.4, 136.6, 133.6,
133.5, 131.2, 131.1, 130.9, 130.1, 130.0, 128.9, 127.7, 127.3,
127.0, 126.9, 124.9, 124.8, 124.5, 124.1, 118.5, 114.9, 106.3.
4860 | Dalton Trans., 2014, 43, 4854–4861
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2013, 42, 4340–4342; (i) R. Sharma, P. Gautam,
S. M. Mobin and R. Misra, Dalton Trans., 2013, 42, 5539;
( j) R. Misra, P. Gautam, T. Jadhav and S. M. Mobin, J. Org.
Chem., 2013, 78, 4940–4948; (k) R. Sharma, R. Maragani,
S. M. Mobin and R. Misra, RSC Adv., 2013, 3, 5785;
(l) R. Misra, T. Jadhav and S. M. Mobin, Dalton Trans.,
2013, 42, 16614; (m) R. Maragani and R. Misra, Tetrahedron
Lett., 2013, 54, 5399–5402.
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