Synthesis, characterization, photoinduced isomerization, and spectroscopic properties of vinyl-1,8-naphthyridine derivatives and their copper(I) complexes

Article abstract of DOI:10.1021/ic100094y

A series of 1,8-naphthyridine derivatives containing vinyl, 2-(2-acetylamino-pyridine-6-ethylene)-4-methyl-7-acetylamino-1,8-naphthyridine (L¹), 2-(2-acetylamino-pyridine-6-ethylene)-1,8-naphthyridine (L ²), 2-(2-acetylamino-pyridinyl-6-ethylene)-4-methyl-7-hydroxyl-1,8- naphthyridine (L³), 2-(2-diacetylamino-pyridinyl-3-ethylene)-7- diacetylamino-1,8-naphthyridine (L⁴), and 7-(2-diacetylamino- pyridinyl-3-ethylene)-4′-acetyl-pyrrolo[1′,5′-a]-1, 8-naphthyridine (L⁵), as well as complexes [CuL¹(PCy ₃)](BF₄)₂ (1) (PCy₃ = tricyclohexylphosphine), [Cu₂L¹(PPh₃) ₄](BF₄)₂ (2) (PPh₃ = triphenylphosphine), [Cu₂L¹(dppm)](BF₄) ₂ (3) (dppm = bis(diphenylphosphino)methane), and [Cu ₂(L¹)(dcpm)][BF₄]₂ (4) (dcpm = bis(dicyclohexylphosphino)methane, were synthesized. All these compounds, except for L¹ and L², were characterized by single crystal X-ray diffraction analysis, and a comprehensive study of their spectroscopic properties involving experimental theoretical studies is presented. We found an intramolecular 1,3-hydrogen transfer during the formation of L³ and L⁴, which in the case of the latter plays an important role in the 1,5-dipolar cyclization of L⁵. The spectral changes that originate from an intramolecular charge transfer (ICT) in the form of a π_py→π*_napy transition can be tuned through acid/base-controlled switching for L¹-L³. A photoinduced isomerization for L¹-L³, 1, and 2 having flexible structures was observed under 365 nm light irradiation. Quantum chemical calculations revealed that the dinuclear complexes with structural asymmetry exhibit different metal-to-ligand charge-transfer transitions.

Full text of DOI:10.1021/ic100094y

4524 Inorg. Chem. 2010, 49, 4524–4533
DOI: 10.1021/ic100094y
Synthesis, Characterization, Photoinduced Isomerization, and Spectroscopic
Properties of Vinyl-1,8-naphthyridine Derivatives and Their Copper(I) Complexes
Wen-Fu Fu,*,†,‡ Lin-Fang Jia,† Wei-Hua Mu,‡ Xin Gan,‡ Jia-Bing Zhang,‡ Ping-Hua Liu,‡ Qian-Yong Cao,†
Gui-Ju Zhang,† Li Quan,† Xiao-Jun Lv,† and Quan-Qing Xu‡
†Key Laboratory of Photochemical Conversion and Optoelectronic Materials, HKU-CAS Joint Laboratory on
New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Peking 100190,
‡
China, and College of Chemistry and Chemical Engineering, Yunnan Normal University,
Kunming 650092, China
Received January 16, 2010
A series of1,8-naphthyridinederivativescontainingvinyl, 2-(2-acetylamino-pyridine-6-ethylene)-4-methyl-7-acetylamino-
1,8-naphthyridine(L1), 2-(2-acetylamino-pyridine-6-ethylene)-1,8-naphthyridine (L2), 2-(2-acetylamino-pyridinyl-6-
ethylene)-4-methyl-7-hydroxyl-1,8-naphthyridine (L3), 2-(2-diacetylamino-pyridinyl-3-ethylene)-7-diacetylamino-1,8-
naphthyridine (L4), and 7-(2-diacetylamino-pyridinyl-3-ethylene)-40-acetyl-pyrrolo[10,50-a]-1,8-naphthyridine (L5), as
well as complexes [CuL1(PCy3)](BF4)2 (1) (PCy3 = tricyclohexylphosphine), [Cu2L1(PPh3)4](BF4)2 (2) (PPh3 =
triphenylphosphine), [Cu2L1(dppm)](BF4)2 (3) (dppm = bis(diphenylphosphino)methane), and [Cu2(L1)(dcpm)][BF4]2
(4) (dcpm = bis(dicyclohexylphosphino)methane, were synthesized. All these compounds, except for L1 and L2, were
characterized by single crystal X-ray diffraction analysis, and a comprehensive study of their spectroscopic properties
involving experimental theoretical studies is presented. We found an intramolecular 1,3-hydrogen transfer during the
formation of L3 and L4, which in the case of the latter plays an important role in the 1,5-dipolar cyclization of L5. The
spectral changes that originate from an intramolecular charge transfer (ICT) in the form of a πpyfπ*napy transition can
be tuned through acid/base-controlled switching for L1-L3. A photoinduced isomerization for L1-L3, 1, and 2 having
flexible structures was observed under 365 nm light irradiation. Quantum chemical calculations revealed that the
dinuclear complexes with structural asymmetry exhibit different metal-to-ligand charge-transfer transitions.
Introduction
the N atoms in these compounds can effectively bind with the
nitrogenous bases of DNA through hydrogen bonding,
which allows for potential applications of the 1,8-naphthyr-
idyl derivatives in medical and biological fields.3-5 Addi-
tional research effort on luminescent naphthyridine deri-
vatives and their Zn(II), Cu(I) complexes has focused on
emissive ππ* or metal-to-ligand charge-transfer (MLCT)
excited states which can be tuned by hydrogen-bonding
sites leading to intra- or intermolecular interactions.6,7 The
luminescent spectroscopic changes induced through hydro-
gen transfer and multiple hydrogen-bonding interactions
have been used to produce luminescent sensors for biomedi-
cal applications.8 Furthermore, Co(II),9 Ru(II), and Ir(III)
A number of 1,8-naphthyridine derivatives have been
synthesized over the past few decades because of their
biocompatibility1 and their good fluorescence properties.2
Studies concerned with naphthyridine rings have shown that
*To whom correspondence should be addressed. E-mail: fuwf@mail.
ipc.ac.cn.
(1) (a) Eldrup, A. B.; Christensen, C.; Haaima, G.; Nielsen, P. E. J. Am.
Chem. Soc. 2002, 124, 3254–3262. (b) Nakatani, K.; Horie, S.; Saito, I. J. Am.
Chem. Soc. 2003, 125, 8972–8973.
(2) (a) Fang, J. M.; Selvi, S.; Liao, J. H.; Slanina, Z.; Chen, C. T.; Chou, P.
T. J. Am. Chem. Soc. 2004, 126, 3559–3566. (b) Hikishima, S.; Minakawa, N.
Angew. Chem., Int. Ed. 2005, 44, 596–598. (c) Katz, J. L.; Geller, B. J.; Foster, P.
D. Chem. Commun. 2007, 1026–1028.
(3) (a) He, C.; Lippard, S. J. J. Am. Chem. Soc. 2000, 122, 184–185.
(b) Smith, E. A.; Kyo, M.; Kumasawa, H.; Nakatani, K.; Saito, I.; Corn, R. M.
J. Am. Chem. Soc. 2002, 124, 6810–6811.
(4) (a) Kobori, A.; Nakatani, K. Bioorg. Med. Chem. 2008, 16, 10338–
10344. (b) Dohno, C.; Uno, S. N.; Sakai, S.; Oku, M.; Nakatani, K. Bioorg. Med.
Chem. 2009, 17, 2536–2543.
(6) (a) He, C.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Lippard, S. J.
Angew. Chem., Int. Ed. 2001, 40, 1484–1487. (b) Takei, F.; Suda, H.; Hagihara,
M.; Zhang, J.; Kobori, A.; Nakatani, K. Chem.;Eur. J. 2007, 13, 4452–4457.
(7) (a) He, C.; Lippard, S. J. Inorg. Chem. 2000, 39, 5225–5231. (b) Araki,
H.; Tsuge, K.; Sasaki, Y.; Ishizaka, S.; Kitamura, N. Inorg. Chem. 2007, 46,
10032–10034.
(5) (a) Eldrup, A. B.; Nielsen, B. B.; Haaima, G.; Rasmussen, H.;
Kastrup, J. S.; Christensen, C.; Nielsen, P. E. Eur. J. Org. Chem. 2001,
1781–1790. (b) Peng, T.; Nakatani, K. Angew. Chem., Int. Ed. 2005, 44, 7280–
7283. (c) Minakawa, N.; Ogata, S.; Takahashi, M.; Matsuda, A. J. Am. Chem.
Soc. 2009, 131, 1644–1645.
(8) (a) Dohno, C.; Uno, S. N.; Nakatani, K. J. Am. Chem. Soc. 2007, 129,
11898–11899. (b) de Greef, T. F. A.; Ligthart, G. B. W. L.; Lutz, M.; Spek, A. L.;
Meijer, E. W.; Sijbesma, R. P.; Dohno, C.; Uno, S. N.; Nakatani, K. J. Am. Chem.
Soc. 2008, 130, 5479–5486. (c) Kuykendall, D. W.; Anderson, C. A.; Zimmerman,
S. C. Org. Lett. 2009, 11, 61–64.
r
pubs.acs.org/IC
Published on Web 04/21/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4525
Scheme 1
complexes containing the 1,8-naphthyridine derivatives have
been synthesized, and their magnetic and catalytic properties
have been investigated.10 The 1,8-naphthyridyl compounds
with flexible structures have not yet been exploiteddespite the
large amount of reports on 1,8-naphthyridine derivatives and
their complexes.11,12 Previous study on luminescent copper(I)
and platinum(II) complexes with a flexible naphthyridine-
phosphine ligand demonstrated that this class of ligand
exhibits various intriguing coordination modes and bonding
properties.13 A flexible ligand bis(7-methyl-1,8-naphthyri-
dine-2-ylamino)methane with η4-chelating and η2-bridging
modes has been synthesized and reported. Its Zn(II) and
Cd(II) complexes display intense solution emissions. The
fluorescence of the ligand bound to Cd(II) was found to be
more pronounced than that with other cations, and this
property can thus be applied to Cd(II) ion recognition.14
Recently, we conducted a two-electron reduction of azo-1,8-
naphthyridine with anionic CH3COO- according to the
Kolbe reaction.15 Herein, we report on the synthesis and
characterization of a series of new compounds L1-L5 in
which a 1,8-naphthyridinyl segment and a pyridinyl group
are connected by a vinyl bridge (Scheme 1). We also propose
a mechanism for the synthesis of L5 and report on the syn-
thesis of one mononuclear complex [CuL1(PCy3)](BF4)2 (1)
and three dinuclear copper(I) complexes [Cu2L1(PPh3)4]-
(BF4)2 (2), [Cu2L1(dppm)](BF4)2 (3), and [Cu2(L1)(dcpm)]-
[BF4]2 (4). Time-dependent density functional theory (TD-
DFT) calculations reveal some interesting photochemical
and photophysical properties.
Experimental Section
(9) Chien, C. H.; Chang, J. C.; Yeh, C. Y.; Lee, G. H.; Fang, J. M.; Peng,
S. M. Dalton Trans. 2006, 2106–2113.
(10) (a) Nakajima, H.; Tanaka, K. Angew. Chem., Int. Ed. 1999, 38, 362–
363. (b) Zong, R.; Thummel, R. P. J. Am. Chem. Soc. 2005, 127, 12802–12803.
(c) Sinha, A.; Wahidur Rahaman, S. M.; Sarkar, M.; Saha, B.; Daw, P.; Bera, J. K.
Inorg. Chem. 2009, 48, 11114–11122.
General Comments. All the reactions were performed under a
nitrogen atmosphere. Reagent grade solvents were dried by
standard procedures and freshly distilled before use. The sol-
vents used for spectroscopic measurements were HPLC grade.
All other chemicals were obtained from commercial sources and
(11) Bera, J. K.; Sadhukhan, N.; Majumdar, M. Eur. J. Inorg. Chem.
2009, 4023–4038.
(12) Zuo, J. L.; Fu, W. F.; Che, C. M.; Cheung, K. K. Eur. J. Inorg. Chem.
2003, 255–262.
(14) Zhang, H. M.; Fu, W. F.; Wang, J.; Gan, X.; Xu, Q. Q.; Chi, S. M.
Dalton Trans. 2008, 6817–6824.
(13) Zhang, J. F.; Fu, W. F.; Gan, X.; Chen, J. H. Dalton Trans. 2008,
3093–3100.
(15) Fu, W. F.; Li, H. F. J.; Wang, D. H.; Zhou, L. J.; Li, L.; Gan, X.; Xu,
Q. Q.; Song, H. B. Chem. Commun. 2009, 5524–5526.

4526 Inorganic Chemistry, Vol. 49, No. 10, 2010
Fu et al.
1
used without further purification. H NMR spectra were re-
elemental analysis calcd (%) for C17H14N4O (290.3): C 70.33,
H 4.86, N 19.30; found: C 70.52, H 5.02, N 19.38.
corded on a Bruker 400 or 500 MHz AVANCE II spectrometer
at 298 K with chemical shifts (δ, ppm) relative to tetramethylsi-
lane (Me4Si) for 1H. Electrospray ionization mass spectrometry
(ESI-MS) data were obtained with an APEX II Model FT-ICR
mass spectrograph. Matrix-assisted laser desorption/ionization-
time of flight (MALDI-TOF) MS data were obtained on a
Bruker Biflex III Model MALDI-TOF mass spectrograph.
Elemental analyses were performed with an Elementar Vario
EL (Germany) instrument. IR data were recorded on a Varian
3100 FTIR spectrometer. UV-vis spectra were obtained using a
HITACHI U-3010 spectrophotometer. Corrected emission
spectra of solutions and solids were obtained on a HITACHI
F-4500 fluorescence spectrophotometer adapted to a right-angle
configuration at room temperature. The emission lifetimes of
solid or solution samples were determined using single photon
counting on a FL920 Spectrometer (Edinburgh).
2-(2-Acetylamino-pyridinyl-6-ethylene)-4-methyl-7-hydroxyl-
1,8-naphthyridine (L3). The compound was synthesized in the
same way as L1, from 2-amino-6-aldehydepyridine (0.61 g, 5.0
mmol) and 2,4-dimethyl-7-hydroxyl-1,8-naphthyridine (0.87 g,
5.0 mmol) that was prepared by a previouslydeveloped method.19
Yellow single crystals were obtained by diffusion of diethyl
ether into a dichloromethane solution. Yield: 0.593 g (37%); 1H
NMR (400 MHz, CDCl3): δ = 2.27 (s, 3H; COCH3), 2.50 (s, 3H;
4-Me of naphthyridyl ring), 6.67 (d, J = 9.6 Hz, 1H; CH), 6.70
(s, 1H; Naph-H), 7.14 (d, J = 7.4 Hz, 1H; Naph-H), 7.42 (d, J =
15.5 Hz, 1H; Py-H), 7.70 (m, 2H; Naph-H, Py-H), 7.86 (d, J =
9.6 Hz, 1H; CH), 8.17(d, J = 7.7 Hz, 1H; Py-H), 9.00 (s, 1H;
OH), 10.20 (s, 1H; NH); MS (EI): m/z (%): 319 (M-H)þ;
elemental analysis calcd (%) for C18H16N4O2 (320.4): C 67.49,
H 5.03, N 17.49; found: C 67.28, H 5.10, N 17.63.
2-Amino-6-aldehydepyridine and 2-Amino-3-aldehydepyridine.
The 2-amino-6-aldehydepyridine and 2-amino-3-aldehydepyri-
dine were synthesized according to an improved method follow-
ing reported procedures.16,17
2-(2-Diacetylamino-pyridinyl-3-ethylene)-7-diacetylamino-1,8-
naphthyridine (L4). 2,7-Dimethyl-1,8-naphthyridine (1.00 g, 6.3
mmol) was added to a solution of 2-amino-3-aldehydepyridine20
(1.00 g, 8.2 mmol) in 20 mL of normal acetic anhydride, and
then the mixture was heated at reflux for 10 h under nitrogen
atmosphere. The resulting solution was concentrated. The crude
product was purified by column chromatography over silica
gel column using chloroform and then 10% ethyl acetate/petro-
leum ether as eluents to give a red solid. Red crystals suitable
for X-ray diffraction were obtained by diffusion of diethyl ether
into a dichloromethane solution.Yield: 0.25 g (9.2%); m.p.:
2,4-Dimethyl-7-amino-1,8-naphthyridine.18 2,6-Diaminopyr-
idine (10.90 g, 0.1 mol) and acetyl acetone (10.00 g, 0.1 mol)
were mixed in 50 mL of acetic acid, and 1.5 mL of 96% sulfuric
acid was added. The mixture was refluxed with stirring for 24 h
under nitrogen atmosphere. The resulting solution was then
cooled to room temperature, after which 150 mL of sodium
hydroxide (6.67 M) solution was added slowly in ice bath. The
precipitate was isolated by filtration, and washed with cool
water (3 ꢀ 5 mL) and dried in vacuo. The crude product was
recrystallized from ethanol by slow evaporation to give a pale
yellow solid. Yield: 5.48 g (32%); m.p.: 220-221 °C; 1H NMR
(400 MHz, CDCl3): δ = 2.52 (s, 3H; CH3), 2.57 (s, 3H; CH3),
6.67 (d, J = 9.7 Hz, 1H; Naph-H), 6.89 (s, 1H; Naph-H), 7.90
(d, J = 9.7 Hz, 1H; Naph-H); MS (EI): m/z (%): 173 (Mþ).
2-(2-Acetylamino-pyridine-6-ethylene)-4-methyl-7-acetylamino-
1,8-naphthyridine (L1). 2,4-Dimethyl-7-amino-1,8-naphthyridine
(0.90 g, 5.2 mmol) and 2-amino-6-aldehydepyridine (0.61 g, 5.0
mmol) were mixed in 20 mL of acetic anhydride. The mixture was
refluxed with stirring for 24 h under nitrogen atomsphere. The
resulting solution was added in 100 mL of ice water while hot, and
then filtered after stirring for 1 h. The crude product was purified
by column chromatography over silica gel column using chloro-
form/ethyl acetate/ethanol (1/10/1) as eluent to give ligand L1
in the form of a yellow solid. Yield: 0.867 g (48%); 1H NMR (400
MHz, CDCl3): δ = 2.26 (s, 3H; COCH3), 2.29 (s, 3H; COCH3),
2.71 (s, 3H; 4-Me of naphthyridyl ring), 7.20 (d, J = 7.4 Hz, 1H;
Py-H), 7.38 (s, 1H; Naph-H), 7.72 (m, 2H; Naph-H, Py-H),
7.91(d, J = 15.56, 1H; Py-H), 8.07 (s, 1H; NH), 8.14(d, J = 5.6
Hz, 1H; Naph-H), 8.34 (d, J = 9 Hz, 1H; CH), 8.48 (d, J = 9 Hz,
1H; CH), 8.68 (br, 1H; NH); MS (EI): m/z (%): 361 (Mþ);
elemental analysis calcd (%) for C20H19N5O2 (361.4): C, 66.47;
H, 5.30; N, 19.38; found: C, 66.61; H, 5.21; N, 19.46.
1
197-198 °C. H NMR (500 MHz, CDCl3): δ=13.88 (s, 1H),
8.56(d, J = 3.4 Hz, 1H), 8.15 (d, J = 7.2 Hz, 1H), 7.63 (d, J = 7.8
Hz, 1H), 7.54 (d, J=15.9 Hz, 1H), 7.45 (dd, J=4.7 Hz, J = 7.7
Hz, 1H), 7.32 (dd, J =9.2 Hz, J =14.0 Hz, 1H), 7.14(m, 1H), 7.12
(m, 1H), 6.55 (d, J = 9.2 Hz, 1H), 5.36 (s, 1H), 2.36 (s, 6H), 2.19
(s, 3H); 13C NMR (125 MHz, CDCl3): δ = 195.4, 172.5, 155.2,
151.4, 150.8, 149.4, 148.7, 135.9, 135.4, 134.1, 132.6, 131.1, 126.9,
124.9, 123.6, 118.4, 117.3, 94.3, 29.3, 26.5; IR (KBr, νmax/cm-1):
1723, 1702, 1619; TOF MS (EI): m/z (%): 431 (MþH)þ; ele-
mental analysis calcd (%) for C24H22N4O4 (430.5): C 66.97, H
5.15, N 13.02; found: C 66.57, H 5.45, N 12.67.
7-(2-Diacetylamino-pyridinyl-3-ethylene)-40-acetyl-pyrrolo-
[10,50-a]-1,8-naphthyridine (L5). 2,7-Dimethyl-1,8-naphthyri-
dine (2.00 g, 12.6 mmol) was added to a solution of 2-amino-3-
aldehydepyridine (1.55 g, 12.7 mmol) in 30 mL of anhydrous
acetic anhydride, and then the mixture was heated at reflux for
24 h under nitrogen atmosphere. The resulting solution was
concentrated. The crude product was purified by column chro-
matography over silica gel column using 15% ethyl acetate/
petroleum ether as eluent to afford a brown solid, which was
recrystallized from CH2Cl2/diethyl ether to give a brown crystal.
Yield: 0.55 g (10.6%); m.p.: 226-227 °C; 1H NMR (500 MHz,
CDCl3): δ = 8.60 (m, 1H), 8.36 (d, J = 9.3 Hz, 1H), 8.28 (d, J =
3.0 Hz, 1H), 8.24 (d, J = 7.8 Hz, 1H), 8.03 (d, J = 8.0 Hz, 1H),
7.71 (d, J = 15.9 Hz, 1H), 7.49 (m, 1H), 7.45 (d, J = 8.0 Hz, 1H),
7.32 (d, J = 4.5 Hz, 1H), 7.30 (m, 1H), 7.19 (d, J = 3.1 Hz, 1H),
2.61 (s, 3H), 2.39 (s, 6H); 13C NMR (125 MHz, CDCl3): δ =
194.3, 172.5, 152.8, 150.9, 149.6, 143.0, 137.0, 135.4, 133.8, 132.3,
130.9, 126.5, 124.8, 122.5, 120.6, 120.3, 118.8, 117.8, 115.0, 113.6,
28.4, 26.5; IR (KBr, νmax/cm-1): 1721, 1707, 1643; TOF MS (EI):
m/z (%): 412 (Mþ); elemental analysis calcd (%) for C24H20N4O3
(412.4): C 69.89, H 4.89, N 13.58; found: C 69.53, H, 5.12, N
13.24.
2-(2-Acetylamino-pyridine-6-ethylene)-1,8-naphthyridine (L2).
The pale yellow compound was prepared following the same
procedure as L1 except that 2-methyl-1,8-naphthyridine was
used instead of 2,4-dimethyl-7-amino-1,8-naphthyridine. Yield:
(42%); 1H NMR (400 MHz, CDCl3): δ = 2.26 (s, 3H; COCH3),
7.23 (m, 1H; Naph-H), 7.46 (m, 1H; Py-H), 7.68 (d, J = 8.4 Hz,
1H; Py-H), 7.75 (t, J = 7.9 Hz, 1H; Naph-H), 7.85 (d, J = 15.6
Hz, 1H; CH), 7.98 (d, J = 15.6 Hz, 1H; CH), 8.08 (br, 1H; NH),
8.17 (m, 1H; Py-H), 8.19 (m, 1H; Naph-H), 8.20 (m, 1H; Naph-
H), 9,13 (m, 1H; Py-H); MS (EI): m/z (%): 289 (M-H)þ;
[Cu(L1)(PCy3)]BF4 (1). Ligand L1 (0.072 g, 0.2 mmol) and
Cu(CH3CN)4BF4 (0.126 g, 0.4 mmol) were mixed with stirring
in dichloromethane (25 mL) under a nitrogen atmosphere, and
the reaction was accompanied by production of a yellow pre-
cipitate. After stirring for 3 h at room temperature, upon
(16) He, C.; Lippard, S. Tetrahedron 2000, 56, 8245–8252.
(17) Liang, F. S.; Xie, Z. Y.; Wang, L. X. Tetrahedron Lett. 2002, 43,
3427–3430.
(18) Henry, R. A.; Hammond, P. R. J. Heterocyclic Chem. 1977, 14, 1109–
1114.
(19) Brown, E. V. J. Org. Chem. 1965, 30, 1607–1610.
(20) Majewicz, T. G.; Caluwe, P. J. Org. Chem. 1974, 39, 720–721.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4527
Table 1. Selected Crystallographic and Data Collection Parameters for L3-L5 and 1-4
L3
L4
L5
1
2
3
4
formula
C19H22
N4O4
370.41
293(2)
0.43 ꢀ
-
C24H22
N4O4
-
C24H20
N4O3
412.44
294(2)
0.22 ꢀ
-
C38H52BCuF4- C190.5H171.5B4Cu4- C47H45B2Cl4-
16Cl8.5N10O4.5P8
C46H64B2Cl2-
Cu2F8N5O2P2 Cu2F8N5O2P2
N5O2P
792.17
293(2)
F
formula weight
T [K]
crystal size [mm]
430.46
298(2)
0.56 ꢀ
3823.36
294(2)
1216.32
293(2)
1152.56
293(2)
0.18 ꢀ
0.26 ꢀ 0.24 ꢀ
0.28 ꢀ 0.22
0.16 ꢀ 0.16 ꢀ
0.35 ꢀ 0.31
monoclinic
P2(1)/n
7.109(2)
19.665(5)
13.658(4)
90.00
0.42 ꢀ 0.30
triclinic
P1
0.16 ꢀ 0.10
monoclinic
P2(1)/c
11.599(2)
22.941(4)
7.953(1)
90.00
0.16 ꢀ 0.08
triclinic
P1
0.20
triclinic
P1
ꢀ 0.16
0.14
triclinic
P1
crystal system
space group
triclinic
P1
˚
a [A]
8.1747(9)
10.968(2)
13.163(2)
72.119(2)
88.134(3)
73.414(2)
1074.5(3)
2
1.330
50.02
0.093
5595/3732
10.908(3)
12.499(3)
15.228(4)
74.782(4)
79.373(5)
77.383(5)
1937.1(9)
2
18.606(2)
20.521(2)
25.548(3)
87.966(2)
85.734(2)
88.467(2)
9718.7(19)
2
12.133(2)
14.930(3)
14.932(3)
80.844(3)
88.464(3)
85.300(3)
2661.2(8)
2
12.433(4)
13.295(4)
19.041(6)
98.253(6)
106.798(6)
100.550(6)
2896.9(17)
2
1.321
50
0.945
14998/10132
˚
b [A]
˚
c [A]
R [deg]
β [deg]
γ [deg]
103.784(6)
90.00
1854.4(9)
4
1.327
105.636(7)
90.00
2037.8(6)
4
1.344
50.34
0.091
11561/4253
3
˚
V [A ]
Z
F
calcd [g cm-3
]
1.358
52.76
1.307
50.00
1.518
52.82
2θmax [deg]
50.00
0.095
μ [mm-1
]
0.664
11161/7821
0.684
49939/34021
1.131
15515/10785
reflections collected/ 9523/3261
unique
parameters
Rint
goodness of fit
R1, wR2[all data]
R1, wR2[I > 2σ(I)]
residual electron
254
0.0465
1.204
298
0.0249
1.015
283
0.0329
1.013
509
0.0350
1.008
2201
0.0348
1.009
0.1668, 0.3249
0.0876, 0.2537
-0.922/1.233
729
0.0258
1.002
781
0.0345
1.019
0.1104, 0.2001 0.0925, 0.1462 0.0892, 0.1216 0.1162, 0.1290
0.0632, 0.1712 0.0477, 0.1150 0.0427, 0.0987 0.0515, 0.0987
-0.208/0.320
0.1189, 0.1587 0.1364/0.2382
0.0574, 0.1444 0.0691/0.1939
-0.513/0.791
-0.196/0.181
-0.203/0.155
-0.435/0.401
-0.383/0.901
-3
˚
density [e A
]
P
P
P
P
R1 = ||Fo| - |Fc||/ |Fo|. wR2 = { w(Fo2 - Fc2)2/ w(Fo2)2}1/2
.
addition of PCy3 (0.234 g, 0.8 mmol) the yellow suspension
dissolved gradually, the resulting solution was stirred for an-
other 2 h and then filtered. The filtration was concentrated
in vacuum, and the crude product was recrystallized from
CH2Cl2/diethyl ether to give a yellow solid. Yellow crystals were
obtained by diffusion of diethyl ether into a dichloromethane
solution. Yield: (39%). The yellow crystalline solid was char-
DMSO-d6): δ = 2.05 (s, 3H; CH3), 2.09 (s, 3H; CH3), 2.13 (s,
2H; CH2), 2.70 (s, 3H; CH3), 6.59 (t, 2H; Ph-H), 6.67 (d, 2H; Ph-
H), 6.84 (t, 2H; Ph-H), 6.90 (t, 2H; Ph-H), 6.96 (t, 2H; Ph-H),
7.03 (t, 2H; Ph-H), 7.10 (t, 2H; Ph-H), 7.18-7.43 (m, 12H;
Ar-H), 7.52 (t, 1H; Py-H), 7.65 (br, 2H; NH), 7.70 (d, 1H; Py-
H); elemental analysis calcd (%) for C45H41B2Cu2F8N5O2P2
(1046.49): C 51.65, H 3.95, N 6.69; found: C 51.85, H 4.22,
N 4.37.
1
acterized by H NMR, elemental analysis, and X-ray crystal-
lography. 1H NMR (400 MHz, CDCl3): δ=1.20-1.88 (m, 33H;
Cy), 2.16 (s, 3H; CH3), 2.20 (s, 3H; CH3), 2.23 (s, 3H; CH3), 7.10
(d, 1H; Py-H), 7.30 (s, 1H; Naph-H), 7.50-7.65 (m, 2H; Naph-
H, Py-H), 7.83-7.98 (m, 2H; Naph-H, Py-H), 8.03 (s, 1H; NH),
8.16 (d, 1H; CH), 8.32 (d, 1H; CH); elemental analysis calcd (%)
for C38H52BCuF4N5O2P (1792.18): C 57.61, H 6.62, N 8.84;
found: C 57.93, H 6.79, N 8.78.
[Cu2(L1)(PPh3)4](BF4)2 (2). Complex 2 was obtained by
reacting Cu(CH3CN)4BF4, L1 and PPh3 in a stoichiometric
ratio of 2:1:4 in dichloromethane and via the same procedure
as for 1. Yield: (58%). The crystalline solid was characterized by
1H NMR, elemental analysis and X-ray crystallography. 1H
NMR (400 MHz, CDCl3): δ = 2.08 (s, 3H; CH3), 2.11 (s, 3H;
CH3), 2.19 (s, 3H; CH3), 6.70 (d, 1H; Py-H), 7.08 (t, 26H; Ph-H),
7.15 (d, 24H; Ph-H), 7.25 (m, 13H; Naph-H, Ph-H), 7.45 (t, 1H;
Py-H), 7.57 (d, 2H; CH), 7.66 (br, 2H; NH), 7.76 (d, 1H; Py-H);
elemental analysis calcd (%) for C92H79B2Cu2F8N5O2P4
(powders, 1711.24): C 64.57, H 4.65, N 4.09; found: C 64.85,
H 4.92, N 4.37.
[Cu2(L1)(dppm)](BF4)2 (3). A mixture of L1 (0.072 g, 0.2
mmol), Cu(CH3CN)4BF4 (0.126 g, 0.4 mmol) in dichloro-
methane (30 mL) was stirred for 4 h under a nitrogen atmo-
sphere at room temperature, resulting in a yellow precipitate.
After adding dppm (0.154 g, 0.4 mmol) the precipitate was
dissolved slowly to give a yellow solution. The solvent was
removed in vacuo leaving an orange solid. Orange crystals of
3 suitable for an X-ray crystallograph structure determination
were obtained by vapor diffusion of diethyl ether into solution
of crude product in dichloromethane. Yield: (0.163 g, 78%). The
crystalline solid was characterized by 1H NMR, elemental
analysis, and X-ray crystallography. 1H NMR (400 MHz
[Cu2(L1)(dcpm)](BF4)2 (4). The orange crystals of complex 4
were obtained following the same procedure as 3 except that
dcpm was used, and Cu(CH3CN)4BF4, L1, and bis(dicyclohexyl-
phosphino)methane (dcpm) are in a stoichiometric ratio of
2:1:1. Yield: (62%). The yellow crystalline solid was character-
1
ized by H NMR, elemental analysis, and X-ray crystallogra-
phy. 1H NMR (400 MHz, CDCl3): δ = 1.18-2.02 (m, 44H;
dcpm-H), 2.12-2.21 (m, 9H; CH3), 3.02 (s, 2H; CH2), 7.06 (d,
1H; Py-H), 7.12 (s, 1H; Naph-H), 7.41 (t, 1H; Py-H), 7.58 (d, 1H;
Naph-H), 7.68 (d, 1H; Py-H), 8.04 (d, 1H; Naph-H), 8.14 (d, 1H;
CH), 8.20 (d, 1H; CH), elemental analysis calcd (%) for
C45H65B2Cu2F8N5O2P2 (1070.68): C, 50.48, H 6.12, N 6.52;
found: C 50.26, H 6.04, N 6.46.
X-ray Crystallography. Crystals suitable for X-ray structure
determination were obtained by the diethyl ether diffusion meth-
od. X-ray data were collected on a Bruker SMART X-ray
diffractometer using a graphite monochromator with Mo-KR
radiation (λ = 0.071073 nm) at roomtemperature. A summary of
the crystallographic parameters and data is given in Table 1. The
structures were solved by direct methods (SHELXL 97) and
refined by full-matrix least-squares methods on all F2 data.21,22
Metal atoms in each complex were located from the E-maps and
other non-hydrogen atoms were located in successive difference
Fourier syntheses. The hydrogen atoms of the ligands were
generated theoretically onto the specific atoms and refined iso-
tropically with fixed thermal factors.
(21) Sheldrick, G. M. SHELXS 97, Program for the Solution of Crystal
€
€
Structure; University of Gottingen: Gottingen, Germany, 1997.
(22) Sheldrick, G. M. SHELXL97, Program for the Refinement of Crystal
€
€
Structures; University of Gottingen: Gottingen, Germany, Germany, 1997.

4528 Inorganic Chemistry, Vol. 49, No. 10, 2010
Fu et al.
Scheme 2
Scheme 3
TD-DFT Calculations. All optimizations were carried out
with the Gaussian 03 program package23 using the 6-311þG*
basis set,24,25 with the hybrid density functional theory (B3LYP)
employed.26,27 Time-dependent density functional theory at the
TD-DFT(B3LYP/6-311þG*) level were also carried out with
the Gaussian 03 program to predict and verify the absorption
spectra of various species.
Results and Discussion
Synthesis and X-ray Characterization. Yields of the 1,8-
naphthyridine derivatives used as starting materials can
be improved by increasing the solution alkalinity. L1-L3
were synthesized by refluxing the corresponding 1,8-
naphthyridine derivatives and 2-amino-6-aldehydepyri-
dine in normal or anhydrous acetic anhydride (Scheme 2).
The condensed compounds were isolated in yields of
19-36%, and they are freely soluble in organic solvents
such as CH2Cl2, CH3CN, and MeOH.
The reaction of 2,7-dimethyl-1,8-naphthyridine with
2-amino-3-aldehydepyridine in normal acetic anhydride
under reflux yields an unexpected compound L4 contain-
ing a β-diketone group. In anhydrous acetic anhydride,
the subsequent reaction of L4 leads to the indolizine
derivatives of 1,8-naphthyridine (L5). A structural inves-
tigation of L4 indicates that intramolecular 1,3-hydrogen
transfer occurs during the formation of the compound, and
this plays an important role during the production of L5.
Several methods of synthesizing indolizine derivatives
of pyridine have been reported and reviewed in the litera-
ture.28 Our group was the first to report on the synthesis
of pyrrolo[10,50-a]-1,8-naphthyridinederivatives.29,30 Gene-
rally, a five-membered ring moiety is formed in the
indolizine framework through intramolecular or inter-
molecular condensation by 1,3-dipolar cycloaddition31,32
or 1,5-dipolar cyclization33,34 To investigate the mechan-
ism for the formation of L5, we chose L4 as the starting
reactant, anhydrous acetic anhydride as the solvent, and
L5 was obtained in 60% yield. The suggested mechanism
is shown in Scheme 3. L4 is in a tautomeric form of the
intermediates, L4(a) or L4(b), and undergoes ring closure
to L4(c) followed by a dehydration leading to L5. A
similar mechanism has previously been proposed by
Chichibabin and Stepanow,35 but no structural analysis
was reported.
(23) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr., T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A.
D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;
Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
L1 reacts with [Cu(CH3CN)4]BF4 and PCy3, PPh3,
dppm, or dcpm to afford the mono- or dinuclear
(29) Zhao, X. J.; Chen, Y.; Fu, W. F.; Zhang, J. B. Synth. Commun. 2007,
37, 2145–2152.
(30) Gueiffier, A.; Blache, Y.; Viols, H.; Chapat, J. P.; Dauphin, G.
J. Heterocyclic Chem. 1992, 29, 691–697.
(31) Scholtz, M. Ber. Dtsch. Chem. Ges. 1912, 45, 734–746.
(32) Kaloko, J., Jr.; Hayford, A. Org. Lett. 2005, 7, 4305–4308.
(33) Kakehi, A.; Suga, H.; Kako, T.; Fujii, T.; Tanaka, N.; Kobayashi, T.
Chem. Pharm. Bull. 2003, 51, 1246–1252.
(34) Østby, O. B.; Dalhus, B.; Gundersen, L. L.; Rise, F.; Bast, A.;
Haenen, G. R. M. M. Eur. J. Org. Chem. 2000, 3763–3770.
(35) Tschitschibabin, A. E.; Stepanow, F. N. Ber. Dtsch. Chem. Ges. 1929,
62, 1068–1075.
(24) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys.
1980, 72, 650–654.
(25) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. R.
J. Comput. Chem. 1983, 4, 294–301.
(26) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(27) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785–789.
(28) Borrows, E. T.; Holland, D. O. Chem. Rev. 1948, 42, 611–643.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4529
3
Table 2. Selected Bond Lengths [A] and Angles [deg] of L -L5 and 1-4
˚
L3
L4
C(1)-O(1)
C(7)-C(10)
C(10)-C(11)
1.236(3)
1.461(4)
1.327(4)
C(16)-N(4)-C(17)
C(7)-C(10)-C(11)
N(4)-C(17)-O(2)
130.3(3)
126.0(3)
121.9(3)
C(2)-C(14)
C(14)-C(15)
C(8)-C(9)
1.463(3)
1.326(3)
1.407(3)
C(2)-C(14)-C(15)
C(9)-C(10)-O(1)
C(16)-N(4)-C(23)
125.4(2)
121.7(2)
119.2(2)
L5
N(2)-C(9)
1.382(2)
1.395(3)
1.359(3)
1.420(3)
1.325(3)
C(1)-C(14)-C(15)
C(6)-C(11)-C(10)
C(9)-N(2)-C(5)
C(6)-N(2)-C(9)
C(9)-C(10)-C(11)
125.3(2)
106.2(2)
127.8(2)
109.2(2)
109.4(2)
C(6)-C(11)
C(9)-C(10)
C(10)-C(11)
C(14)-C(15)
1
2
Cu(1)-N(1)
Cu(1)-O(1)
Cu(1)-P(1)
1.984(3)
2.159(3)
2.175(1)
N(2)-Cu(1)-O(1)
N(1)-Cu(1)-P(1)
P(1)-Cu(1)-O(1)
84.4(1)
159.3(1)
112.6(1)
Cu(2)-N(4)
Cu(2)-O(2)
Cu(2)-P(3)
2.130(6)
2.117(6)
2.295(2)
N(2)-Cu(1)-O(1)
N(2)-Cu(1)-P(1)
P(1)-Cu(1)-P(2)
84.3(2)
118.1(2)
126.74(9)
3
4
Cu(1)-N(2)
Cu(1)-O(1)
Cu(1)-P(1)
1.984(6)
2.134(4)
2.169(1)
N(2)-Cu(1)-O(1)
N(2)-Cu(1)-P(1)
P(1)-Cu(1)-O(1)
86.8(2)
170.6(1)
102.6(1)
Cu(2)-N(4)
Cu(2)-O(2)
Cu(2)-P(2)
1.981(6)
2.07(2)
2.164(2)
N(4)-Cu(2)-O(2)
N(4)-Cu(2)-P(2)
P(2)-Cu(2)-O(2)
88.7(7)
155.1(2)
116.2(7)
Figure 1. Perspective views of with labeling scheme for L3-L5. The thermal ellipsoids are at 30% probability.
complexes [Cu(L1)PCy3]BF4 (1), [Cu2(L1)(PPh3)4](BF4)2
(2), [Cu2(L1)(dppm)](BF4)2 (3), and [Cu2(L1)(dcpm)]-
(BF4)2 (4). Crystals of L3-L5 and 1-4 suitable for
X-ray structural analysis were obtained by diethyl ether
diffusion, and their structures were determined by X-ray
diffraction analysis. The crystal data and details on data
collection and refinement are summarized in Table 1.
Selected bond lengths and angles are given in Table 2. The
groups around the CdC double bond adopt a trans-
configuration in the compounds. A structural investiga-
tion showed that the hydrogen atom on the hydroxyl of L3
transfers to an adjacent naphthyridine-N atom leading to
a keto configuration. A perspective view with a labeling
scheme is shown in Figure 1. The molecular naphthyr-
idine and pyridine rings in L3 are almost coplanar, and
the two molecules in the unit cell in parallel planes
are oriented head to tail. A π-π interaction with an
˚
average interplanar separation of 3.38 A was observed
in the crystal lattice. In the crystal structure of L4,
two methyl hydrogens at the 2-position of the naphthyr-
idine moiety are substituted by acetyl groups, and a
β-diketone is generated, which is accompanied by one
hydrogen atom transfer to an adjacent naphthyridine-N
atom and the -NH2 forms a diacetylamino group. The
hydrogen transfer results in the formation of a six-
membered ring, N1-C8-C9-C10-O1 H1, with an
3 3 3

4530 Inorganic Chemistry, Vol. 49, No. 10, 2010
Fu et al.
Figure 2. Perspective views with labeling scheme for 1-4. The thermal ellipsoids are at 30% probability. All hydrogen atoms and solvent molecules have
been omitted for clarity.
˚
intramolecular hydrogen-bond length of 1.84 A, and the
The bond angles at the copper atom vary from 82.8 to
126.74°. The crystal structures of3 and 4 show that ligand L1
bridges two Cu(I) metal centers through the acetylamino-
naphthyridine and -pyridine moieties. Addition of the
bridging ligands, dppm, or dcpm generates a bimetallic
13-membered ring. The steric effect of both dppm and
dcpm can be seen clearly by comparing the non-bonding
distances of C(12)-O(2) and C(10)-O(1) are 1.233 and
1.251 A, respectively (Figure 1). A perspective view of L
5
˚
is shown in Figure 1. In accordance with its aromaticity,
the pyrrolo[10,50-a]-1,8-naphthyridine moiety in the mo-
lecule has a planar geometry, and all bond lengths and
angles have values expected for aromatic compounds.
Complexes 1-4 exhibit different coordination modes
depending on the nature of the phosphine ligands used.
Their Oak Ridge thermal ellipsoid plot (ORTEP) dia-
grams along with the atom-numbering scheme are shown
in Figure 2. When a mixture of L1, [Cu(CH3CN)4]BF4,
and the bulky ligand PCy3 (1:2:4 molar ratio) in dichlor-
omethane was stirred at room temperature, complex 1
was obtained as a mononuclear complex leaving a free
acetylamino-pyridine moiety. The complex features
T-shaped CuNPO geometry around the Cu atom, and
the bond angles vary from 84.39 to 159.30° (Table 2).
Cu(I) Cu(I) intra-separation with the former being
˚
4.835 and latter 5.110 A (Figure 2). Each copper atom
3 3 3
has a slightly distorted planar trigonal geometry with bond
angles at the copper atoms varying from 85.8 to 170.63°.
Spectroscopic Properties of L1-L5 and 1-4. The
UV-vis absorption spectra of L1-L3 exhibit an intense
transition at λmax values ranging from 360 to 390 nm. In
comparison, the absorptions centered at <390 nm for
complexes 1-4 are similar to that of the ligand L1 but
have tails extending from 400 to about 500 nm (Table 3).
On the basis of a TD-DFT calculation (Figures 3 and 4),
the lower-energy absorption bands of L1-L3 are assigned
to an ICT πpyfπ*napy transition. As shown in Figure 3,
the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) for 1 are
mainly localized on the Cu(I) center and the naphthyr-
idine moiety, respectively. However, the calculated en-
ergy gap of 567 nm from the HOMO to the LUMO does
not correlate with the observed absorption spectrum. TD-
DFT calculation demonstrates that the oscillator strength
Compared with the copper(I) to naphthyridine-N bond
˚
length Cu(1)-N(1) of 1.984 A, the Cu(1) N(2) inter-
action distance is 2.748 A (Figure 2). In contrast, the
3 3 3
˚
coordination geometry of Cu in the binuclear complex
2 is best described as a distorted tetrahedron with each
copper(I) center coordinated to a naphthyridine- or pyr-
idine-nitrogen atom, an acetyl oxygen atom as well as two
PPh3 ligands (Figure 2). The average Cu-N, Cu-O, and
˚
Cu-P distances are 2.099, 2.186, and 2.269 A, respectively.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4531
Table 3. Spectroscopic Data of L1-L3 and 1, 3, and 4
compound
medium (T[K])
λ
abs[nm] (ε[dm3 mol-1 cm-1])
λem[nm]
φem
L1
CH3CN (298)
CH2Cl2 (298)
CH3OH (298)
solid (298)
363 (25200), 372 (25100)
365 (25500)
360 (25700), 373 (25900)
399
399, 415 (sh)
407
491
0.86
L2
L3
1
CH3CN (298)
CH2Cl2 (298)
CH3OH (298)
solid (298)
CH3CN (298)
CH2Cl2 (298)
CH3OH (298)
solid (298)
CH3CN (298)
CH2Cl2(298)
CH3OH (298)
solid (298)
358 (11500)
359 (16400)
359 (13500)
411
406
429
526
424
427
422
466
372 (13700), 389 (10440)
375 (12600), 393 (9812)
370 (13800), 388 (11200)
361 (25490), 374 (26500)
376 (29800)
360 (28890), 374 (29700)
400
395, 415 (sh)
410
0.6
3
CH3CN (298)
CH2Cl2(298)
CH3OH (298)
solid (298)
CH3CN (298)
CH2Cl2(298)
CH3OH (298)
solid (298)
364 (25300), 373 (25400)
377 (28100), 393 (27700)
375 (27400), 430(sh) (6486)
403
394 (sh), 416
410
452, 514
412
430
0.018
0.045
4
364 (24640), 373 (24970)
378 (25080), 395 (23100)
376 (25800), 425(sh) (3120)
428
469, 524
Figure 4. Plots of the relevant HOMO and LUMO for compounds cis-
and trans-L1.
from the Cu-naphthyridine and the Cu-pyridine moieties.
The excitation energy of the former (dCufπ*napy) transi-
tion is lower than that of the latter and corresponds to an
energy gap between HOMO and LUMO of 574.7 and
571.6 nm for 3 and 4, the MLCT excitation energies from
the dCufπ*py transition are 479 and 468 nm, respectively
(see Supporting Information). We therefore attribute the
observed lower-energy absorption to a dCufπ*py transi-
tion in nature. Compounds L1-L3 and complex 1 display
an intense solution emission with a λmax at 400-430 nm
upon excitation at 360 nm. The fluorescence emissions
originate from an intraligand excited state, and the
photophysical data are listed in Table 3.
Figure 3. Plots of the relevant HOMO and LUMO for compounds
L3-L5 and 1-4.
(f) values are 0.165 and 0.002 for the πpyfπ*napy transi-
tion and the lower-energy gap, respectively. Thus, it is
appropriate that the observed higher-energy absorption
is assigned to a πpy electron to π*napy orbital transition
(see the Supporting Information). The UV-vis absorp-
tion spectra of the dinuclear complexes 3 and 4 have a
shoulder peak at about 430 nm. A quantum chemical
calculation indicates that the asymmetrical complexes
may undergo two different MLCT transitions originating
The absorption spectra of L4 and L5 in dichloro-
methane show intense low-energy absorption bands at
454 and 420 nm with large extinction coefficients of 15443
and 14960 mol-1 dm3 cm-1, respectively (Figure 5). The
theoretical calculation reveals that the methyl-substituted
naphthyridine unit is electron rich and acts as an electron
donor in the molecules. The character of the LUMO is

4532 Inorganic Chemistry, Vol. 49, No. 10, 2010
Fu et al.
Figure 7. ChangesintheabsorptionspectraofL1 (5.0ꢀ 10-5 moldm-3
)
upon addition of various concentrations of HBF4 in CH3OH [HBF4]:
Figure 5. UV-vis absorption spectra of the compounds L4 and L5 in
dichloromethane.
0-6.2 ꢀ 10-5 mol dm-3. Inset: the addition of KOH in CH3OH solution
to the solution of the acid titration [OH-]: 0-7.0 ꢀ 10-5 mol dm-3
.
Acid/Base-Controlled Molecular Switching On the basis
of L1. Upon titration of L1 with HBF4 as a proton source
in CH3OH, the absorption at about 360-375 nm steadily
decreases and a concomitant absorption peak centered
at 395 nm appears together with an isosbestic point at
378 nm. To validate a tentative assignment of the peaks
in the absorption spectra, a geometry optimization was
performed at the B3LYP/6-311þG* level (with the single-
point energy and orbitals calculated at the TD-DFT-
(B3LYP/6-311þG*) level) to determine the binding
site of Hþ within L1. The calculated total energies of
the molecule in which Hþ was associated with O1, O2,
N3, N4, and N5 are -1198.478435, -1198.478574,
-1198.506804, -1198.519931, and -1198.532781 au,
respectively (Scheme 1 and Supporting Information).
The hydrogen ion associated with N5 on the naphthyrine
moiety involves the promotion of the πpyfπ*napy transi-
tion, leading to a longer wavelength absorption. This
process is fully reversible by adding a base such as
KOH in CH3OH to the solution of the acid titration
(Figure 7). In CH3OH, L1 exhibits an intensive emission
with a λmax at 407 nm upon excitation at 380 nm at room
temperature. The luminescence intensity was measured as
a function of HBF4 concentration and decreased with an
increase in the degree of protonated N atoms. The emis-
sion maximum was red-shifted from 407 to 440 nm with a
low-energy emission band that tails beyond 630 nm at
room temperature.
Photoinduced Isomerization. The photoinduced iso-
merization of L1-L3, 1, 3, and 4 was investigated at
different excitation wavelengths. By irradiating L1-L3
in CH2Cl2, CH3CN, or CH3OH at 365 nm, the intensity
of the absorption band around 370 nm decreased, but new
bands emerged at 315 and 420 nm. Their absorption
intensities developed as the irradiation times increased.
These processes are characterized by isosbestic points at
330 and 392 nm. During the irradiation the emission band
with a λmax at about 405 nm rapidly weakened upon
excitation at 330 nm. The spectral changes recorded
during irradiation at room temperature are displayed in
Figure 8. A comparison of the absorption spectra for
complexes 1, 3, and 4 demonstrates that the mononuclear
Figure 6. Solid-state emission spectra ofthe compounds L4 and L5 upon
excitation at 400 nm.
dominated by the pyridyl ring with a contribution from
the naphthyridine moiety. The calculated energy gaps
of L4 and L5 are consistent with the measured absorption
energies and are 2.68 and 2.74 eV, (463 and 453 nm,
Figure 3) respectively. The low-energy absorptions of L4
and L5 can be attributed to an ICT πnapyfπ*py transition
that is opposite to those of L1-L3.
Compound L4 has an intense red luminescence with a
λmax at 626 nm in the solid state at room temperature.
Compared to the emission of L4, the solid powder of L5
has a weaker photoluminescence with a λmax at 607 nm
(Figure 6). Upon excitation at 300 nm, compound L5
displays an emission peak maximum at 520 nm in di-
chloromethane at room temperature, and the emission
quantum yield is 0.58. Compound L4 exhibits both high-
and low-energy emissions with λmax values at 393 and 593
nm while the overall emission quantum yield is 0.01.36
The extended π-conjugated molecule with a rigid struc-
ture in L5 leads to an enhancement of the emission
intensity in solution.
(36) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4533
In contrast to 1, the spectral changes of 3 and 4 in the
presence of light are best described as resulting from a
structural distortion that causes a change in the natural
ICT state because of the introduction of the bridging
ligands, dppm, or dcpm, into the binuclear complexes to
increase their rigidity.
Conclusions
We synthesized five novel flexible 1,8-naphthyridine deri-
vatives containing vinyl and four mono- and dinuclear
complexes. TD-DFT calculations and structural character-
ization of the compounds showed that groups around the
CdC double bond adopt a stable trans-configuration. The
hydrogen atom on the hydroxyl of L3 can easily transfer to an
adjacent naphthyridine-N atom resulting in a keto config-
uration. A structural investigation of L4 revealed that two
hydrogen atoms on the methyl at the 2-position of the
naphthyridine or amino groups on pyridine can be substi-
tuted by acetyl groups in normal acetic anhydride and a 1,3-
hydrogen transfer from the -CH- group of the β-diketone
to an adjacent naphthyridine-N atom occurs during the
formation of the compound. This leads to the formation of
a pyrrolo[10,50-a]-1,8-naphthyridine derivative L5 with pro-
mising visible absorption and red luminescence by the 1,5-
dipolar cyclization of L4 in anhydrous acetic anhydride. The
calculations further demonstrated that the lower-energy
absorption bands of L1-L3 come from an intramolecular
πpyfπ*napy charge transfer transition whereas those of L4
and L5 had opposite natural excited states. The dinuclear
complexes exhibited two different MLCT characteristics that
originated from the Cu-naphthyridine and Cu-pyridine moi-
eties, and the excitation energy of the former is lower than
that of the latter. The absorption spectra of L1-L3 show an
acid/base-controlled molecular switching effect, and this
process is reversible. The flexible compounds L1-L3 and 1
can undergo photoinduced isomerization in going from their
trans- to cis-conformations after solution irradiation, result-
ing in a red shift of their absorption spectra. These results
provide new insights into the development of flexible 1,8-
naphthyridine derivatives and their copper(I) complexes for
potential application in medicine and materials science.
Figure 8. Emission spectra recorded during the irradiation of L1 solu-
tion in CH3OH at 365 nm upon excitation at 330 nm. Inset: changes in the
absorption spectra.
Figure 9. Absorption spectra recorded during the irradiation of com-
plex 1 solution in CH3OH at 365 nm. Inset: changes in the absorption
spectra during the irradiation of complex 3 at 380 nm.
complex 1 has similar spectral behavior to ligand L1
during irradiation at 365 nm, but in the case of 3 and 4
the new absorption bands were not observed. Moreover,
the absorption peaks of 3 and 4 at 375 nm decreased in
intensity (Figure 9). A geometry optimization was carried
out for L1 at the B3LYP/6-311þG* level to interpret
the spectral properties (Supporting Information). The
minimized energies for the cis- and trans-conformations
were -1198.125283 and -1198.135446 au, respectively.
The computational results showed that for the former
conformation, a transition at 353.5 nm (3.508 eV, f =
0.123) is possible although the calculated lower-energy gap
(ΔELUMO-HOMO) of 3.201 eV is smaller than the 3.293 eV
of the latter conformation. This indicates that a photo-
induced isomerization reaction occurs from the trans- to
cis-conformations after the irradiation of L1 in solution.
Acknowledgment. This work was financially supported by
the National Basic Research Program of China (973 Pro-
gram 2007CB613304) and the foundation (GJHZ200817)
for Bureau of International Cooperation of Chinese Acad-
emy of Science. We thank the Solar Energy Initiative of the
Knowledge Innovation Program of the Chinese Academy of
Sciences under Grant KGCX2-YW-393-2 and the National
Natural Science Foundation of China (NSFC Grant
20761006).
Supporting Information Available: X-ray crystallographic
1
files in CIF format for L3-5 and complexes 1-4; H NMR of
L1-5 and 13C NMR of L4 and L5; and computational details for
TD-DFT are provided. This material is available free of charge
via the Internet at http://pubs.acs.org.