Reduction of 4-azidonaphthalimide with different phosphine ligands and exploration of their spectroscopic properties

Article abstract of DOI:10.1016/j.molstruc.2015.08.056

A convenient, high efficient method for the reduction of 4-azidonaphthalimide to 4-aminonaphthalimide (1) by using PMe₃ has been developed. Several 4-substituted 1,8-naphthalimide iminophosphoranes were also successfully synthesized. Their structures were characterized by NMR and MS analyses. The structures of compounds 2 and 3 were also confirmed by single crystal X-ray diffraction analysis. Their optoelectronic properties of these naphthalimides were investigated. The results indicated that their optical properties could be tuned by different phosphine ligands, which make them novel potential organic luminescent materials.

Full text of DOI:10.1016/j.molstruc.2015.08.056

Journal of Molecular Structure 1102 (2015) 197e202
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Reduction of 4-azidonaphthalimide with different phosphine ligands
and exploration of their spectroscopic properties
Shou De Xu, Cheng Hui Fang, Guang Xuan Tian, Yi Chen, Ye Hong Dou, Jun Feng Kou,
Xiang Hua Wu*
College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming 650500, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A convenient, high efficient method for the reduction of 4-azidonaphthalimide to 4-aminonaphthalimide
(1) by using PMe3 has been developed. Several 4-substituted 1,8-naphthalimide iminophosphoranes
were also successfully synthesized. Their structures were characterized by NMR and MS analyses. The
structures of compounds 2 and 3 were also confirmed by single crystal X-ray diffraction analysis. Their
optoelectronic properties of these naphthalimides were investigated. The results indicated that their
optical properties could be tuned by different phosphine ligands, which make them novel potential
organic luminescent materials.
Received 3 May 2015
Received in revised form
21 August 2015
Accepted 25 August 2015
Available online 1 September 2015
Keywords:
© 2015 Elsevier B.V. All rights reserved.
4-Azidonaphthalimide
Phosphine ligands
Reduction
4-Aminonaphthalimide
Iminophosphoranes
1. Introduction
this transformation in mind, several 4-substituted 1,8-
naphthalimide and 1,8-Naphthalic anhydride derivatives have
Recently, 4-amino-1,8-naphthalimides (1) have emerged as an
important class of organic fluorophores [1] as a consequence its
wide applications in DNA targeting binders [2], fluorescent sensors
[3], fluorescent cellular imaging agents [4] and supramolecular
chemistry [5]. Up to now, 4-amino-1,8-naphthalimides are usually
synthesized by several methods including: (i) the reduction of the
4-nitro-1,8-naphthalimides with SnCl2/HCl [6]; (iii) substitution of
the 4-bromo-1,8-naphthalimides with conc. ammonia and copper
power [7]; (iii) the reduction of the 4-azide-1,8-naphthalimides
with H2S [8]. Generally, the drawbacks of these methods are low-
yield, or transition-metal catalyzed, even using harmful gas. The
formation of primary amines is a cornerstone of organic synthesis.
The reduction of azides is commonly achieved through the Stau-
dinger procedure using PPh3. In this regard, we recently attempted
to form compound 1 from the 4-azidonaphthalimide compound (4)
using PPh3. However, we found the reduction of the azide 4 to
iminophosphorane 2 rather than the formation of the desired
amine 1. Wondrously, the desired amine 1 was obtained with PPh3
being replaced by PMe3 in high yield at room temperature. With
been designed and synthesized to investigate the reduction of the
azide compounds 4 and 7 with different phosphine ligands
including PPh3, PMe3, bis(1,3-diphenylphosphino)propane (dppp),
and 1,10-Bis(diphenylphosphino)ferrocene (dppf) in the paper. As
far as we know, a few studies on the photophysical properties of
iminophosphorane-naphthalimides have been reported. Therefore,
the electronic spectroscopy behavior of the corresponding reduc-
tion products has been also investigated.
2. Experimental section
2.1. General procedures
All manipulations were carried out at room temperature under a
nitrogen atmosphere using standard Schlenk techniques, unless
otherwise stated. All reagents were purchased from commercial
sources and were used without further purification. 1H, 13C, and 31
P
NMR spectra were collected on a Bruker 500 MHz magnetic reso-
nance spectrometer. 1H and 13C NMR chemical shifts are relative to
TMS, and 31P NMR chemical shifts are relative to 85% H3PO4. Mass
spectrometry was recorded with a Bruker amaZon SL spectrometer.
The UVeVis spectra were obtained using U-3010 spectrophotom-
eter. The fluorescence spectra were obtained with F-4500 FL
* Corresponding author.
E-mail address: chxhwu@sina.com (X.H. Wu).
http://dx.doi.org/10.1016/j.molstruc.2015.08.056
0022-2860/© 2015 Elsevier B.V. All rights reserved.

198
S.D. Xu et al. / Journal of Molecular Structure 1102 (2015) 197e202
spectrometer with a 1 cm standard quartz cell. The starting mate-
rials 4-azido -N-(n-butyl)naphthalimide (4) [9], and 4-azido-1,8-
naphthalic anhydride (7) [9] were prepared by the procedures
described in literature methods.
2.2. Synthesis of 4-amino-N-(n-butyl)naphthalimide 1
2.2.1. Method 1
A dichloromethane solution (10 mL) of PMe3 (1.0 mL, 1 M in THF,
1.0 mmol) was added dropwise to dichloromethane solution
(20 mL) of 4 (0.24 g, 1.0 mmol). Than the reaction mixture was
stirred at room temperature over night. After completion of the
reaction, solvent was removed under vacuum. The residue was
purified by column chromatography on silica gel (eluent: CH2Cl2/
CH3OH,100:1, V/V) to give a yellow solid. Yield: 0.25 g, 93%. 1H NMR
Scheme 2. Synthetic route of 4-amino-1,8- naphthalic anhydride and 1,8-naphthalic
anhydriden-based iminophosphorane.
(500 MHz, CDCl3)
d
8.59 (d, J ¼ 7.1 Hz, 1H, -PhH), 8.41 (d, J ¼ 8.1 Hz,
purified by column chromatography on silica gel (eluent: CH2Cl2/
CH3OH, 50:1, V/V) to give a yellow solid. Yield: 0.21 g, 40%. 1H NMR
1H, -PhH), 8.11 (d, J ¼ 8.3 Hz, 1H, -PhH), 7.64 (t, J ¼ 7.8 Hz, 1H, -PhH),
6.88 (d, J ¼ 8.1 Hz, 1H, -PhH), 4.99 (s, 2H, eNH2), 4.16 (t, J ¼ 7.4 Hz,
2H, eNCH2CH2e), 1.71 (dt, J ¼ 15.3, 7.6 Hz, 2H, eCH2CH2e),
1.49e1.40 (m, 2H, eCH2CH2e), 0.97 (t, J ¼ 7.4 Hz, 3H, eCH2CH3). MS
calcd for C16H17N2O2 [MþHþ]: 269.13, found: 269.1.
(500 MHz, CDCl3)
d
9.16 (d, J ¼ 8.2 Hz, 1H, ePhH), 8.57 (d, J ¼ 8.5 Hz,
1H, ePhH), 8.09 (d, J ¼ 8.3 Hz, 1H, ePhH), 7.87e7.74 (m, 6H, ePhH),
7.68e7.57 (m, 4H, ePhH), 7.55e7.46 (m, 6H, ePhH), 6.45 (d,
J ¼ 9.0 Hz, 1H, ePhH), 4.15 (t, J ¼ 7.4 Hz, 2H, eNCH2e), 1.69 (dt,
J ¼ 9.5, 4.7 Hz, 2H, eCH2CH2e), 1.42 (dq, J ¼ 14.8, 7.4 Hz, 2H,
eCH2CH2e), 0.95 (t, J ¼ 7.4 Hz, 3H, eCH2CH3). 13C NMR (125 MHz,
2.2.2. Method 2
The synthesis is similar to method 1, with PMe3 being replaced
by dppp (0.82 g, 2.0 mmol). The residue was purified by column
chromatography on silica gel (eluent: CH2Cl2/CH3OH, 100:1, V/V) to
give a yellow solid. Yield: 0.13 g, 50%. 1H NMR (500 MHz, DMSO)
CDCl3) d 165.23, 164.48, 157.48,133.32, 132.58, 132.51, 131.94,131.10,
129.44, 129.09, 128.99, 128.65, 123.96, 122.26, 115.07, 114.98, 109.95,
39.80, 30.36, 20.44, 13.89. 31P NMR (200 MHz, CDCl3)
d 8.90. MS
calcd for C35H30N2O2P [MþHþ]: 529.20, found: 529.2.
d
8.61 (d, J ¼ 8.4 Hz, 1H, ePhH), 8.42 (d, J ¼ 7.3 Hz, 1H, ePhH), 8.19
(d, J ¼ 8.4 Hz, 1H, ePhH), 7.65 (t, J ¼ 7.8 Hz, 1H, ePhH), 7.43 (s, 2H,
eNH2), 6.85 (d, J ¼ 8.4 Hz, 1H, ePhH), 4.01 (t, J ¼ 7.4 Hz, 2H,
eNCH2CH2e), 1.68e1.50 (m, 2H, eCH2CH2e), 1.42e1.26 (m, 2H,
eCH2CH2e), 0.92 (t, J ¼ 7.4 Hz, 3H, eCH2CH3).
2.4. Synthesis of 1,8-naphthalimide-based iminophosphorane 3
The synthesis is similar to 2, with PPh3 being replaced by dppf
(1.1 g, 2.0 mmol). The residue was purified by column chromatog-
raphy on silica gel (eluent: CH2Cl2/CH3OH, 30:1, V/V) to give a
brown-yellow solid. Yield: 0.75 g, 72%. 1H NMR (500 MHz, CDCl3)
2.3. Synthesis of 1,8-naphthalimide-based iminophosphorane 2
d
9.18 (d, J ¼ 8.2 Hz, 2H, ePhH), 8.63 (d, J ¼ 8.5 Hz, 2H, ePhH), 8.08
A dichloromethane solution (10 mL) of triphenylphosphine
(0.26 g, 1.0 mmol) was added dropwise to dichloromethane solu-
tion (20 mL) of 4 (0.24 g, 1.0 mmol). Than the reaction mixture was
stirred at room temperature over night. After completion of the
reaction, solvent was removed under vacuum. The residue was
(d, J ¼ 8.3 Hz, 2H, ePhH), 7.73e7.60 (m, 10H, ePhH), 7.56 (td, J ¼ 7.3,
1.2 Hz, 4H, ePhH), 7.49e7.40 (m, 8H, ePhH), 6.29 (d, J ¼ 8.4 Hz, 2H,
ePhH), 4.49 (s, 4H, eCpH), 4.20 (dd, J ¼ 8.2, 6.2 Hz, 8H, eCH2CH2e,
eCpH), 1.73 (dt, J ¼ 15.2, 7.6 Hz, 4H, eCH2CH2e), 1.52e1.42 (m, 4H,
eCH2CH2e), 0.99 (t, J ¼ 7.4 Hz, 6H, eCH2CH3). 13C NMR (125 MHz,
Scheme 1. Synthetic route of 4-amino-1,8-naphthalimide and 1,8-naphthalimide-based iminophosphoranes.

S.D. Xu et al. / Journal of Molecular Structure 1102 (2015) 197e202
199
CDCl3)
d
165.15, 164.40, 157.03, 133.29, 132.50, 131.90, 131.82, 131.31,
2.5. Synthesis of 4-amino-1,8-naphthalic anhydride 5
131.17, 130.58, 129.82, 129.42, 129.23, 128.98, 128.91, 128.81, 124.08,
122.48, 115.10, 114.99, 109.74, 74.39, 39.87, 30.39, 29.68, 20.46,
The synthesis is similar to 1. The residue was purified by column
chromatography on silica gel (eluent: CH2Cl2/CH3OH, 50:1, V/V) to
give a yellow solid. Yield: 0.17 g, 79%. 1H NMR (500 MHz, DMSO)
13.90. 31P NMR (200 MHz, CDCl3)
d 9.40. MS calcd for C66H56Fe-
N4O5P2Na [MþNaþ]: 1109.31, found: 1109.4.
Fig. 1. Molecular structure of compound 2. H-atoms are omitted for clarity.
Fig. 2. Molecular structure of compound 3. H-atoms are omitted for clarity.

200
S.D. Xu et al. / Journal of Molecular Structure 1102 (2015) 197e202
Table 1
anisotropically. The hydrogen atoms were placed in the ideal po-
sitions and refined as riding atoms. Further crystal data and details
of the data collection are summarized in Table S1. Selected bond
distances and angles were given in Table S2.
Photophysical properties of 1, 2, 3, 5, and 6 in different solvents.
Compound
Solvent
Absorptiona
Fluorescencea
labs (nm)
ε (10ꢁ4 Mꢁ1 cmꢁ1
)
lem (nm)
Fb
1
Toluene
CH2Cl2
THF
CH3CN
DMF
MeOH
DMSO
Toluene
CH2Cl2
THF
404
404
416
416
432
432
436
430
442
436
440
444
446
446
436
446
440
444
480
450
490
424
412
420
422
432
424
438
430
440
434
438
440
360
444
0.518
0.346
0.147
0.942
0.607
0.384
0.746
0.401
0.404
0.503
0.851
0.596
0.149
0.981
1.061
0.337
0.713
0.590
0.600
0.478
0.400
0.122
0.137
0.467
0.334
0.616
0.595
0.368
0.361
0.362
0.616
0.993
0.627
0.071
0.669
482
447
494
507
513
522
519
486
508
496
513
511
527
516
488
501
494
510
534
533
528
489
495
507
520
533
536
553
492
517
503
525
522
557
526
0.10
0.78
0.41
0.07
0.15
0.14
0.21
0.73
0.45
0.30
0.32
0.40
0.07
0.47
0.04
0.26
0.01
0.18
0.10
0.28
0.05
0.85
0.45
0.12
0.26
0.23
0.03
0.15
0.80
0.54
0.23
0.25
0.42
0.02
0.16
3. Results and discussion
3.1. Synthesis
Reduction of 4- azido-1,8-naphthalimide with different phos-
phine ligands is shown in Scheme 1. Initially, the iminophosphor-
ane 2 was obtained after the reduction of 4-azidonaphthalimide
compound (4) using PPh3 in 40% yield. Unfortunately, only 5%
hydrolyzation to the amine 1 was observed (Scheme 1). With this
transformation in mind, we attempted to use the phosphine ligand
PMe3. Wondrously, the desired amine 1 was obtained in 93% yield.
In the next moment, the corresponding iminophosphorane 3 was
formed using the bisphosphine ligand dppf in good yield. Notably,
the amine 1 was also achieved with dppf being replaced by
bisphosphine ligand dppp in moderate yield. The results above
showed that the corresponding amine was obtained from reduction
of azide 4 using PMe3 or dppp, while the iminophosphoranes were
acquired using PPh3 or dppf. This may be attributed to the phos-
phine ligand reduction activity. The same conclusion has been
further confirmed and supported by reduction of 4-azido-1,8-
naphthalic anhydride azide with phosphine ligands including
PMe3 and PPh3 (Scheme 2). Theses compounds were confirmed by
NMR and MS. The structures of compounds 2 and 3 were also
confirmed by single crystal X-ray diffraction analysis. The detailed
experimental procedures of these compounds are described in
Supplementary Data.
2
3
5
6
CH3CN
DMF
MeOH
DMSO
Toluene
CH2Cl2
THF
CH3CN
DMF
MeOH
DMSO
Toluene
CH2Cl2
THF
CH3CN
DMF
MeOH
DMSO
Toluene
CH2Cl2
THF
CH3CN
DMF
MeOH
DMSO
3.2. X-ray structures of 2 and 3
a
The absorption and emission values of all the complexes in different solvent and
As shown in Figs. 1 and 2, the molecular structure of 3 is
completely centrosymmetric configuration. And some C atoms in
the NC4H9 groups are disordered in compounds 2 and 3. From the
data in the Supporting Information, the NeP bond distances in
compounds 2 and 3 are 1.577(4) and 1.584(4) Å, respectively, which
suggests that these nitrogen-phosphorus bonds are double bond
and are shorter than that in the related compound Ph3P ¼ NPh
(1.603(3) Å) [12]. The PNC angles are 129.1(4) (2) and 127.9(4)ꢀ (3),
respectively, which are slightly smaller than that in compound
Ph3P ¼ NPh (130.4ꢀ) [12].
the emission wavelength was monitored at 367 nm at 298 K.
b
With quinine sulfate in sulfuric acid as quantum yield standard (
F
¼ 0.55).
d
8.69 (d, J ¼ 8.4 Hz, 1H, ePhH), 8.43 (d, J ¼ 7.3 Hz, 1H, ePhH), 8.18
(d, J ¼ 8.4 Hz, 1H, ePhH), 7.79 (s, 2H, eNH2), 7.69 (t, J ¼ 7.8 Hz, 1H,
ePhH), 6.88 (d, J ¼ 8.5 Hz, 1H, ePhH).
2.6. Synthesis of 1,8-naphthalic anhydriden-based
iminophosphorane 6
3.3. Photophysical properties
The synthesis is similar to 1. The residue was purified by column
chromatography on silica gel (eluent: CH2Cl2/CH3OH, 50:1, V/V) to
give a yellow solid. Yield: 0.43 g, 90%. 1H NMR (500 MHz, CDCl3)
The absorption and emission spectra of 1,8-naphthalimide de-
rivatives 1, 2, 3, 5, and 6 were measured in various solvent (Table 1
d
9.22 (d, J ¼ 8.3 Hz, 1H, ePhH), 8.54 (d, J ¼ 7.3 Hz, 1H, ePhH), 8.06
and Figs.
3 and 4, and S1eS9). Compared to 4-amino-1,8-
(d, J ¼ 8.4 Hz, 1H, ePhH), 7.86e7.76 (m, 6H, ePhH), 7.66e7.60 (m,
naphthalimide compound 1, of particular note is that the intro-
duction of the iminophosphoranes at the 4-position of the 1,8-
naphthalimide unit induces a certain extent red-shift in the posi-
tion of the longest wavelength band as demonstrated for the imi-
nophosphoranes 2, 3, and 6 as shown in Table 1 and Fig. 3, which is
similar to other 4-amidonaphthalimide derivatives [13,14]. The
broad absorption bands in the long wavelength region arise from
the polarization of chromophore naphthalimide system due to the
intramolecular charge transfer (ICT) interaction. As expected, the
introduction of iminophosphoranes at C-4 of the naphthalene ring
enhances ICT interaction in the molecule because of inductive and
conjugation effect. As compared with 4-amino-1,8-naphthalimide
compounds, the fluorescence spectrum of the iminophosphor-
anes showed relatively weak solvatochromic shift. Fluorescence
emission spectra of iminophosphorane 6 are strongly dependent on
4H, ePhH), 7.58e7.51 (m, 6H, ePhH), 6.44 (d, J ¼ 9.0 Hz, 1H, ePhH).
13C NMR (126 MHz, CDCl3)
d 161.87, 160.36, 134.69, 134.56, 132.93,
132.08, 131.67, 131.23, 128.95, 128.75, 128.51, 128.41, 127.67, 126.87,
123.52, 117.22, 114.49, 102.97. 31P NMR (200 MHz, CDCl3)
d 10.53.
2.7. Crystal structure determination for 2 and 3
Single crystals of compounds 2 and 3 suitable for X-ray analysis
were obtained by slow diffusion of hexane into solutions in
dichloromethane. Diffraction intensity data were collected on a
Nonius Kappa CCD diffractometer with Mo Ka radiation (0.71073 Å)
at room temperature (292 K). The structure was solved by direct
methods (SHELXS-97) [10] and refined by full matrix least squares
on F2 (SHELXL-97) [11]. All non-H atoms were refined

S.D. Xu et al. / Journal of Molecular Structure 1102 (2015) 197e202
201
0.6
0.4
0.2
0.0
1
2
3
5
6
1
2
3
5
6
1500
1000
500
0
350
400
450
500
550
400
450
500
550
600
650
Wavelength/nm
Wavelength/nm
Fig. 3. (a) UVeVis absorption and (b) fluorescence spectra of compounds 1, 2, 3, 5, and 6 in CH2Cl2.
the polarity of the solvent: the maximum emission wavelength is
red-shifted almost 65 nm from 492 nm in toluene to 557 nm in
MeOH, and the fluorescence quantum yield decreases more than 40
times from toluene (0.80) to MeOH (0.02). However, the
fluorescence quantum yields of 6 were lower than those of 2. By
comparison of the Fluorescence emission spectra of iminophos-
phoranes 2 and 3 (Table 1), relately large solvatochromic shift (ca.
45 nm) was observed in the case of 3 from 488 nm in toluene to
600
1.2
Toluene
CH2Cl2
Toluene
CH2Cl2
1.0
THF
THF
CH3CN
CH3CN
DMF
DMF
400
0.8
MeOH
DMSO
MeOH
DMSO
0.6
200
0.4
0.2
0.0
0
450
500
550
600
400
450
500
Wavelength/nm
Wavelength/nm
1.2
1.0
0.8
0.6
0.4
0.2
0.0
150
100
50
Toluene
CH2Cl2
Toluene
CH2Cl2
THF
CH3CN
THF
CH3CN
DMF
MeOH
DMSO
DMF
MeOH
DMSO
0
400
450
500
550
450
500
550
600
650
Wavelength/nm
Wavelength/nm
Fig. 4. (a) UVeVis absorption and (b) fluorescence spectra of compound 2 in different solvents. (c) UVeVis absorption and (d) fluorescence spectra of compound 3 in different
solvents.

202
S.D. Xu et al. / Journal of Molecular Structure 1102 (2015) 197e202
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more than 7 times from toluene (0.04) to MeOH (0.28) as can be
seen in Table 1 and Fig. 4. From the above, fluorescence sol-
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4. Conclusion
In summary, several 4-substituted 1,8-naphthalimide and 1,8-
Naphthalic anhydride iminophosphoranes were successfully syn-
thesized from the reduction of the corresponding azides with
different phosphine ligands. Notably, we developed a convenient,
high efficient method for the reduction of 4-azidonaphthalimide to
4-aminonaphthalimide by using PMe3. Optical studies indicated
the introduction of the different phosphine ligands have large ef-
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The authors acknowledge financial support from National Nat-
ural Science Foundation of China (Nos. 21002086), and the Natural
Science Foundation of Yunnan Province (No. 2010ZC071).
Appendix A. Supplementary data
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Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.molstruc.2015.08.056.
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