Fluorescent tin(IV) complexes with schiff base ligands: Synthesis, structures, and fluorescence lifetime

Article abstract of DOI:10.1246/bcsj.20120150

Eleven Schiff base tin(IV) complexes of the type [SnCl₃(L)] (HL: Schiff base) were synthesized and structurally characterized by X-ray crystallography and ¹HNMR spectroscopy, and their properties were investigated. The Schiff bases a

Full text of DOI:10.1246/bcsj.20120150

1210 Bull. Chem. Soc. Jpn. Vol. 85, No. 11, 1210-1221 (2012)
© 2012 The Chemical Society of Japan
Fluorescent Tin(IV) Complexes with Schiff Base Ligands:
Synthesis, Structures, and Fluorescence Lifetime
Kentaro Takano,1 Masayuki Takahashi,1 Takaaki Fukushima,1 Makoto Takezaki,2
Toshihiro Tominaga,2 Haruo Akashi,3 Hideaki Takagi,4 and Takashi Shibahara*1
1Department of Chemistry, Okayama University of Science, Ridai-cho, Kita-ku, Okayama 700-0005
2Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Kita-ku, Okayama 700-0005
3Research Institute of Natural Sciences, Okayama University of Science, Ridai-cho, Kita-ku, Okayama 700-0005
4Department of International Conservation Studies for Cultural Properties, Kibi International University,
8 Igamachi, Takahashi, Okayama 716-8508
Received May 28, 2012; E-mail: shiba@chem.ous.ac.jp
Eleven Schiff base tin(IV) complexes of the type [SnCl3(L)] (HL: Schiff base) were synthesized and structurally
1
characterized by X-ray crystallography and H NMR spectroscopy, and their properties were investigated. The Schiff
bases are prepared from aldehydes with a benzene or a naphthalene ring and 8-aminoquinoline or its derivatives. The
complexes adopt geometries close to octahedral, with three chloride ions coordinated meridionally. Eight of the eleven tin
complexes show fluorescence in DMSO, and the emission range from bluish green to orange (-max: 492-545 nm). Two
solid tin complexes are fluorescent, although they are not in DMSO, and one tin complex is not fluorescent even in the
solid state. The fluorescence quantum yields (Φf) of the complexes depend very much on the ligands (Φf = 0.21-0.022).
The fluorescence lifetimes (¸ = 0.49-1.76 ns) were measured and the results will be discussed. DFT calculation (B3LYP/
Sn, LanL2DZ; others, 6-31G**) shows that the order of the magnitudes of HOMO-LUMO gaps is consonant with that of
the emission wavelengths (-max) of the complexes. All the cyclic voltammograms of the Schiff base ligands and the
complexes exhibited irreversible waves and these results suggest that the reduction products of the ligands and the
complexes were unstable under the time scale in CV measurements (0.1 V s¹1). Relatively good correlation was observed
between reduction potentials in DMSO and the energies of LUMO.
Fluorescent compounds have attracted much attention from
researchers because of their intrinsic interests and applications
such as organic electroluminescence1 and ability as tumor
markers.2 One of the Schiff bases, 2-hydroxy-1-naphthyl-
methylene-8-aminoquinoline (HL1, Chart 1), which is pre-
pared from 2-hydroxy-1-naphthaldehyde and 8-aminoquino-
line, has been used for the fluorometric measurement of Be(II),3
Cr(III),4 Mo(VI),5 Pb(II),6 and Hg(II),7 where interfering ions,
Al(III), Ga(III), In(III), Tl(III), Cu(II), have also been report-
ed,5,6 however, there has been no structural report of compound
composed of any one of the above metal ions and HL1 ligand
has been reported to date, while the X-ray structure of HL1
was reported recently.8 In contrast, the X-ray structures of
[VO2(L1)],9 [TcCl2(L1)],10 and [FeCl3(L1)]11 complexes with
the ligand HL1 were reported, but no description of fluores-
cence was noted. Many methods have been reported for the
fluorometric determination of vanadium(V);12 however, reports
describing both the fluorescence and X-ray structure are very
limited.
Recently we succeeded in determining the X-ray structure of
a fluorescent Schiff base tin(IV) complex with the HL1 ligand,
and we thought it worthwhile to prepare a series of tin(IV)
complexes with Schiff base ligands analogous to the ligand,
since there are many kinds of derivatives of aldehydes and
the combination of aldehydes and 8-aminoquinoline (and its
derivatives) will give many types of Schiff base ligands, which
will help investigate the tunability of the fluorescence wave-
length using the Schiff base tin(IV) complexes. So far, many
reports have appeared on fluorescence as described above, but
it is still difficult to predict the appearance of fluorescence and
their intensities of fluorescent compounds. We now report syn-
thesis and characterization of eleven novel Schiff base tin(IV)
CHO
N
H2O
H2N
+
+
N
N
OH
OH
2-hydroxy-1-
8-aminoquinoline
HL1
naphthaldehyde
Chart 1. Formation of HL1 ligand.
Published on the web November 1, 2012; doi:10.1246/bcsj.20120150

K. Takano et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012) 1211
N
N
N
N
N
N
N
N
OH
OH
OH
Me
HL2
HL3
HL1
Me
MeO
Cl
O2N
N
Me
N
N
N
OH
OH
OH
CH
OMe
CHO
HL5
HL4
HL4'
HO
N
N
N
N
N
N
N
OH
OH
OH
Cl
HL6
HL7
HL8
F
MeO
N
N
N
N
N
OH
OH
OH
OMe
HL10
HL11
HL9
Chart 2. Schiff bases. Note: The aldehyde group of HL4¤ changes to an acetal group on coordination to tin(IV) to give a4
(Figure S1).
complexes, [SnCl3(L1)] (1), [SnCl3(L2)] (2), [SnCl3(L3)] (3),
[SnCl3(L4)] (4), [SnCl3(L5)] (5), [SnCl3(L6)] (6), [SnCl3(L7)]
(7), [SnCl3(L8)] (8), [SnCl3(L9)] (9), [SnCl3(L10)] (10), and
[SnCl3(L11)] (11), where HL2, HL3, HL4, HL5, HL6, HL7,
HL8, HL9, HL10, and HL11 are also Schiff bases analogous
to HL1 (Chart 2 and Table 1). Complexes 1 through 7 and
complex 9 are fluorescent in DMSO, but complexes 8, 10,
and 11 are nonfluorescent. A complex of the formula [SnCl3-
(H2O)(L6)], which is close to [SnCl3(L6)] (6), has been
reported.13 The thermochromism of the former was reported,
but fluorescence was not referred to.
As for fluorescent non-Schiff base tin(IV) complexes, organo
tin(IV) complexes with 1-fluorenecarboxylic acid or 9-fluore-
necarboxylic acid (Chart S1) have been reported recently
together with their X-ray structures,14 and a fluorescent tin-
iridium mixed metal complex, [Ir2(SnCl)(CO)2Cl2(¯-dpma)2]-
[SnCl3], has also been reported.15 Incidentally, there are reports
on tin(IV) complexes with the 8-aminoquinoline ligand, which
is a the starting material of the Schiff base ligands, but there is
no reference to the fluorescence of the complex.16 However,
there has been a report of an electroluminescent complex of
diphenyltin(IV) complex with an 8-aminoquinoline ligand.17 A
preliminary report of portions of this work has appeared.18
Table 1. Designations of Ligands
Schiff base ligands
2-Hydroxy-1-naphthylmethylene-8-aminoquinoline
1-Hydroxy-2-naphthylmethylene-8-aminoquinoline
2-Hydroxy-1-naphthylmethylene-8-amino-2-methyl-
quinoline
3-Formyl-2-hydroxy-5-methylbenzylidene-8-amino-
quinoline
2-Hydroxy-3-dimethoxymethyl-5-methylbenzyli-
dene-8-aminoquinoline
5-Nitrosalicylidene-8-aminoquinoline
Salicylidene-8-aminoquinoline
3,5-Dichlorosalicylidene-8-aminoquinoline
5-Hydroxysalicylidene-8-aminoquinoline
5-Fluorosalicylidene-8-aminoquinoline
5-Methoxysalicylidene-8-aminoquinoline
5-Allyl-3-methoxysalicylidene-8-aminoquinoline
HL1
HL2
HL3
HL4¤
HL4
HL5
HL6
HL7
HL8
HL9
HL10
HL11
thaldehyde, 2-hydroxy-5-methylisophthalaldehyde, 5-nitro-
salicylaldehyde, salicylaldehyde, 3,5-dichlorosalicylaldehyde,
2,5-dihydroxybenzaldehyde, Tokyo Kasei; [Al(C9H6NO)3]
(Alq3) and DMSO-d6(99.9%), Aldrich; (n-C4H9)4NPF6, Merck.
Measurements. Electronic spectra were measured using
Hitachi U-2000 and -2800 double-beam spectrophotometers.
Fluorescent spectra were measured using a Hitachi F-2000
spectrophotometer. 1H NMR spectra were obtained with a
Bruker ARX-400R spectrometer. Elemental analyses were
determined with a Perkin-Elmer 240 NCH analyzer. Fluores-
Experimental
Materials and Reagents.
Chemicals were used as
purchased: SnCl4¢5H2O, DMSO, and DMF for fluorescence
analysis, Nacalai Tesque; 8-aminoquinoline, 8-amino-2-meth-
ylquinoline, 2-hydroxy-1-naphthaldehyde, 1-hydroxy-2-naph-

1212 Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012)
Fluorescent Tin(IV) Complexes
¹4
cence decays were measured using an apparatus including a
femtosecond laser system and streak camera.19 The second
harmonic of a femtosecond Ti:sapphire laser (Spectra-Physics,
Tsunami 3960, wavelength of pulse: ca. 800 nm, fwhm of the
pulse: ca. 80 fs) pumped by the SHG output of a Nd, YVO4 laser
(Spectra-Physics, Millennia-V) was used to excite the com-
plexes. The detected fluorescence was dispersed by a mono-
chromator and measured using a streak scope (Hamamatsu,
C4334). The fluorescence signals were averaged using a PC.
Fluorescence decay measurements were performed at 22 °C
for the complexes.
Syntheses. Syntheses of Schiff Base Ligands: HL1 was
prepared according to a modified literature procedure:5 2-
hydroxy-1-naphthaldehyde (540 mg, 3.0 © 10¹3 mol) and 8-
aminoquinoline (432 mg, 3.0 © 10¹3 mol) were added to meth-
anol (5 mL), which was heated at 50 °C for two hours. Allow-
ing the the solution to stand at 50 °C gave orange plate-like
crystals: yield 879 mg (98%). Anal. Found (calcd for C20H14-
N2O, MW: 298.34) C, 80.36 (80.52); H, 4.81 (4.73); N, 9.40
(9.39)%.
[SnCl3(L1)] (a1): To a solution of HL1 (29.8 mg, 1.0 © 10
mol) in acetonitrile (20 mL) at 60 °C was added a solution of
SnCl4¢5H2O (36.0 mg, 1.0 © 10¹4 mol) in acetonitrile (10 mL).
Keeping the mixture at 60 °C overnight gave orange crystals
of a1: yield 20.9 mg (40%). Anal. Found (calcd for C20H13-
Cl3N2OSn, MW: 522.40): C, 46.26 (45.98); H, 2.48 (2.51); N,
5.50 (5.36)%.
[SnCl3(L2)] (a2): To a solution of HL2¢0.3MeOH (10.12
mg, 3.3 © 10¹5 mol) in acetone (2 mL) was added a solution
of SnCl4¢5H2O (10.02 mg, 2.86 © 10¹5 mol) in acetone (2 mL).
Keeping the mixture at room temperature overnight gave
orange crystals of a2: yield 8.3 mg (56%). Anal. Found (calcd
for C20H13Cl3N2OSn, MW: 522.40): C, 46.04 (45.98); H, 2.67
(2.51); N, 5.26 (5.36)%.
[SnCl3(L3)] (a3): To a solution of HL3 (10.12 mg, 3.24 ©
10¹5 mol) in a mixture of acetonitrile (1 mL) and methanol
(1 mL) was added a solution of SnCl4¢5H2O (10.02 mg,
2.86 © 10¹5 mol) in acetonitrile (1 mL). Keeping the mixture
at room temperature overnight gave orange plate-like crystals
of a3: yield 7.1 mg (46%). Anal. Found (calcd for C21H15-
Cl3N2O3Sn, MW: 536.43): C, 47.03 (47.02); H, 2.86 (2.82); N,
5.21 (5.22)%.
Similar procedures to that for HL1 were used for the
syntheses of Schiff base ligands from HL2 through HL5, HL7,
HL8, and HL10 using equimolar amounts of aldehydes and
amines: 8-aminoquinoline was used except for HL3, where
8-amino-2-methylquinoline was used. Methanol was used as
solvent unless otherwise noted. HL6, HL9, and HL11 were not
isolated. The following products (yields) and analytical results
were obtained.
[SnCl3(L4)] (a4): HL4¤ (10.15 mg, 3.5 © 10¹5 mol) and
SnCl4¢5H2O (10.0 mg, 2.9 © 10¹5 mol) were dissolved in
methanol (3 mL). The resultant red mixture turned to an orange
solution after a few minutes of heating at 60 °C, which gave
yellow powder of a4: yield 8.3 mg (64%). Anal. Found (calcd
for C20H19Cl3N2O3Sn, MW: 560.45): C, 42.32 (42.86); H, 3.20
(3.42); N, 5.08 (5.00)%.
HL2¢0.3MeOH
(using
1-hydroxy-2-naphthaldehyde):
¹4
bright-orange plate-like crystals (61%). Anal. Found (calcd
for C20.3H15.2N2O1.3, MW: 307.95, MWHL2: 298.34): C, 79.54
(79.43); H, 4.96 (4.66); N, 9.26 (9.13)%. The crystals seemed
efflorescent.
HL3 (using 8-amino-2-methylquinoline): orange powder,
which was washed with ice-cooled ethyl acetate. For recrystal-
lization, the orange powder was recrystallized from toluene to
give block crystals (66%). Anal. Found (calcd for C21H16N2O,
MW: 312.36): C, 80.53 (80.75); H, 5.10 (5.16); N, 8.95
(8.97)%.
HL4¤ (using 2-hydroxy-5-methylisophthalaldehyde): red
powder (52%). Anal. Found (calcd for C18H14N2O2, MW:
290.32): C, 74.31 (74.47); H, 4.61 (4.86); N, 9.88 (9.65)%.
HL5 (using 5-nitrosalicylaldehyde): red-brown plate-like
crystals (71%). Anal. Found (calcd for C16H11N3O3, MW:
293.28): C, 65.58 (65.53); H, 3.76 (3.78); N, 14.29 (14.33)%.
Toluene was used as the solvent.
HL7¢MeOH (using 3,5-dichlorosalicylaldehyde): bright-red
plate-like crystals (95%). Anal. Found (calcd for C17H14-
N2O2Cl2, MW: 349.21, MWHL7: 317.17): C, 58.03 (58.47); H,
3.73 (4.04); N, 8.21 (8.02)%.
HL8 (using 2,5-dihydroxybenzaldehyde): yellow powder
(84%). Anal. Found (calcd for C16H12N2O2, MW: 265.29): C,
72.83 (72.99); H, 4.31 (4.21); N, 10.65 (10.64)%.
HL10 (using 2-hydroxy-5-methoxybenzaldehyde): red plate-
like crystals (58%). Anal. Found (calcd for C17H15N2O2, MW:
279.31): C, 73.37 (73.10); H, 5.05 (5.41); N, 10.07 (10.03)%.
Syntheses of Schiff Base Tin(IV) Complexes: The follow-
ing procedures were followed for the synthesis of the Schiff
base tin(IV) complexes:
[SnCl3(L5)] (a5): To a slurry of HL5 (50.11 mg, 1.71 © 10
mol) in methanol (20 mL) was added a solution of SnCl4¢5H2O
(10.02 mg, 2.86 © 10¹5 mol) in methanol (5 mL). Keeping
the mixture at room temperature overnight gave orange plate-
like crystals of a5: yield 54.8 mg (73%). Anal. Found (calcd
for C16H10Cl3N3O3Sn, MW: 517.34): C, 37.02 (37.15); H, 1.78
(1.95); N, 7.98 (8.12)%.
¹4
[SnCl3(L6)] (a6): 8-Aminoquinoline (20 mg, 1.4 © 10
mol), salicylaldehyde (20 ¯L, 1.6 © 10¹4 mol), and
SnCl4¢5H2O (50 mg, 1.4 © 10¹4 mol) were dissolved in
acetonitrile (5 mL). Leaving the solution at room tem-
perature overnight gave yellow plate-like crystals of a6: yield
19.2 mg (28%). Anal. Found (calcd for C16H11Cl3N2OSn,
MW: 472.34): C, 40.74 (40.68); H, 2.14 (2.35); N, 5.90
(5.93)%.
[SnCl3(L7)] (a7): To a solution of HL7¢MeOH (10.89 mg,
3.1 © 10¹5 mol) in a mixture of acetonitrile (4 mL) and meth-
anol (8 mL) was added a solution of SnCl4¢5H2O (10.29 mg,
2.9 © 10¹5 mol) in acetonitrile (1 mL). Keeping the mixture at
room temperature overnight gave yellow plate-like crystals
of a7: yield 59.54 mg (76%). Anal. Found (calcd for C16H9-
Cl5N2OSn, MW: 541.23): C, 35.54 (35.51); H, 1.61 (1.68); N,
5.09 (5.18)%.
¹5
[SnCl3(L8)] (a8): To a solution of HL8 (7.97 mg, 3.0 © 10
mol) in methanol (10 mL) was added a solution of SnCl4¢5H2O
(10.0 mg, 2.9 © 10¹5 mol) in methanol (1 mL). Keeping the
mixture at 50 °C overnight gave red needle-like crystals of
a8: yield 53.7 mg (73%). Anal. Found (calcd for C16H11-
Cl3N2O2Sn, MW: 488.34): C, 39.59 (39.35); H, 2.22 (2.27); N,
5.58 (5.74)%.

K. Takano et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012) 1213
[SnCl3(L9)] (a9yel and a9or): 8-Aminoquinoline (13.85 mg,
Measurements of Fluorescent Spectra of Schiff Base
Ligands and Schiff Base Tin(IV) Complexes. Alq3 (1.90
mg, 4.1 © 10¹6 mol) was dissolved in DMF (100 mL) and
diluted 4 times with DMF to give a standard solution (1.03 ©
10¹5 M; 1 M = 1 mol dm¹3).25 Solution samples: Schiff base
ligands HL1-HL5, HL7-HL8, and HL10 and tin complexes
a1-a11 were dissolved in DMSO-d6 to make a ca. 1.0 ©
10¹5 M solution. In addition a 1 © 10¹6 M solution of HL4¤
was also prepared for the measurement. Solid samples: Schiff
base ligands or tin complexes (ca. 1 mg) were immersed with
silicone oil (a few drops) on a filter paper (ADVANTEC 5C),
respectively.
¹5
9.6 © 10¹5 mol), 5-fluorosalicylaldehyde (13.45, 9.6 © 10
mol), and SnCl4¢5H2O (50 mg, 1.4 © 10¹5 mol) were dissolved
in a mixture of acetonitrile (12 mL) and water (1.25 mL).
Leaving the solution at room temperature overnight gave a
mixture of yellow (a9yel) and orange (a9or) crystals: yield
33.8 mg (73%). The two kinds of crystals were separated
manually: the amount of a9yel is somewhat larger than that of
a9or. The following analytical values were obtained: a9yel:
Anal. Found (calcd for C16H10Cl3FN2OSn, MW: 490.33): C,
39.05 (39.19); H, 1.93 (2.06); N, 5.64 (5.71)%; a9or: Anal.
Found (calcd for C16H10Cl3FN2OSn, MW: 490.33): C, 39.05
(39.19); H, 2.02 (2.06); N, 5.66 (5.71)%.
Electrochemical Measurement.
All electrochemical
[SnCl3(L10)] (a10): To a solution of HL10 (8.01 mg,
2.9 © 10¹5 mol) in methanol (10 mL) was added a solution
of SnCl4¢5H2O (9.6 mg, 2.7 © 10¹5 mol) in methanol (1 mL).
Keeping the mixture at 50 °C overnight gave orange needle-like
crystals of a10: yield 5.84 mg (40%). Anal. Found (calcd for
C17H13Cl3N2O2Sn, MW: 502.37): C, 40.67 (40.64); H, 2.48
(2.61); N, 5.36 (5.58)%.
[SnCl3(L11)] (a11): 8-Aminoquinoline (13.85 mg, 9.6 ©
10¹5 mol), 5-allyl-3-methoxysalicylaldehyde (18.25 mg, 9.6 ©
10¹5 mol), and SnCl4¢5H2O (50 mg, 1.4 © 10¹5 mol) were
dissolved in acetonitrile (5 mL). Leaving the solution at
room temperature overnight gave red needle-like crystals of
a10: yield 19.82 mg (40%). Anal. Found (calcd for C20H12-
Cl3N2O2Sn, MW: 542.43): C, 43.93 (44.28); H, 3.04 (3.15); N,
5.22 (5.16)%.
X-ray Structural Determinations of a1, a3, a5, a6, a7, a8,
a9yel, a9or, a10, and a11. Data were collected on a Rigaku/
MSC Mercury CCD diffractometer, using graphite monochro-
mated Mo K¡ radiation. Suitable crystals were attached to the
tip of a glass capillary using silicone grease and transferred
to the goniostat, where they were cooled at ¹180 °C for data
collection. The structures were solved using the Patterson
Method (DIRDIF99,20 a1, a3, a5, a8, a9yel, a9or, a10, and
a11) or the Direct Methods (SIR2004,21 a6; SHELXS97,22
a7), and all the remaining non-hydrogen atoms were located
from difference Fourier maps.23 The hydrogen atoms were also
located from difference Fourier maps, and all of the isotropic
thermal parameters of hydrogen atoms were constrained to
1.2Ueq to which they were attached. The program package
Crystal Structure24 was used, SHELXL being used for the
refinement. Crystallographic data have been deposited with
Cambridge Crystallographic Data Centre: Deposition number
CCDC-656122 for a1, -656123 for a3, -661505 for a5,
-882535 for a6, -882536 for a7, -882537 for a8, -882538
for a9or, -882539 for a9yel, -882540 for a10, and -882541
for a11. Copies of the data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge, CB2 1EZ, U.K.; Fax: +44 1223 336033; e-mail:
deposit@ccdc.cam.ac.uk).
measurements were performed under argon at room temper-
ature using a Bioanalytical Systems Inc. BAS-100B Electro-
chemical Analyzer with a scan rate of 0.1 V s¹1. Each solution
contained 0.5 mM sample and 0.1 M tetrabutylammonium
hexafluorophosphate as a supporting electrolyte in anhydrous
dimethyl sulfoxide (DMSO). A three electrode cell was
employed with a platinum disk working electrode, a platinum
wire auxiliary electrode, and a Ag/Ag+ reference electrode
(0.01 M AgPF6 in CH3CN). Ferrocene (Fc) was added as
an internal standard and potentials are referenced versus the
Fc+/Fc couple.
DFT Calculations. The DFT calculation with the hybrid
functional method (B3LYP) was carried out by using the
Gaussian 03 program package.26 We performed full structural
optimization (basis sets: Sn, LanL2DZ; others, 6-31G**)
starting from X-ray structures (a1, a3, a5, a6, a7, a8, a9yel,
a10, and a11) or from molecular mechanics using Chem3D
(a2 and a4).
Results and Discussion
Synthesis of Schiff Base Ligands and Schiff Base Tin
Complexes. Of eleven Schiff base ligands, HL1, HL2, HL3,
HL4¤, HL5, HL7, HL8, and HL10 were isolated and used for
the syntheses of tin(IV) complexes. HL6, HL9, and HL11
were not isolated. Eleven Schiff base tin(IV) complexes were
synthesized and the structures of a1, a3, a5, a6, a7, a8, a9yel,
a9or, a10, and a11 were determined by X-ray crystallography,
which will be discussed in the next section, and those of a2
and a4 were determined by 1H NMR spectroscopy (20 °C,
400 MHz, DMSO-d6) (Figure S1). As to HL4¤, the aldehyde
moiety remained unchanged, though prepared in methanol: on
the preparation of a4, the aldehyde moiety of the ligand
changed into acetal.
X-ray Structures of a1, a3, a5, a6, a7, a8, a9yel, a9or, a10,
and a11. ORTEP drawings of a1, a3, a5, a6, a7, a8, a9yel,
a9or, a10, and a11 are shown in Figure 1. The complexes
adopt geometries close to octahedral: three chloride ions
are coordinated meridionally, respectively. Crystallographic
data and atomic distances are shown in Table 2 and Table 3,
respectively. Of all the complexes Sn1-Cl1 and Sn1-Cl2
distances are clearly longer than those of Sn1-Cl3, probably
because of the trans influence of chloride ion. The dihedral
angle between the least-squares planes, quinoline ring and
benzene (or naphthalene ring), of each complex is also listed
in Table 3. There are some differences between the dimensions
of a9yel and a9or.
Measurements of 1H NMR Spectra of Schiff Base
Ligands and Schiff Base Tin(IV) Complexes. Solid samples
of Schiff base ligands HL1, HL2, HL3, HL4¤, HL5, HL7,
HL8, and HL10 and tin complexes a1-a11 were dissolved
1
in DMSO-d6, respectively, for the measurement of H NMR at
room temperature.

1214 Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012)
Fluorescent Tin(IV) Complexes
a1
a3
a5
a6
a7
a8
a9yel
a9or
a10
a11
Figure 1. ORTEP drawings of a1, a3, a5, a6, a7, a8, a9yel, a9or, a10, and a11 with the atom-numbering schemes at the 50%
probability level (see text).

K. Takano et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012) 1215

1216 Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012)
Fluorescent Tin(IV) Complexes
Regarding a8, strong intermolecular hydrogen bonds cause
dimerization of the molecules as shown in Figure 2. As is
expected in a10, the hydrogen bonds disappeared by the
introduction of an OCH3 group in place of the OH group in a8.
No such strong hydrogen bonds are present in any other
complex. Regarding a9yel, weak C-H£F and C-H£Cl inter-
actions are present, while in a9or, ³-³ stackings exist between
neighbor molecules (Figure 3). The dihedral angle of a3 is
larger than that of a1 but has the same framework, which is
probably due to the steric hindrance between the methyl group
and chloride ion. The angles of a8 and a9yel are also large
compared to others, probably due to hydrogen bonds with
the neighboring molecules. Incidentally, a complex [SnCl3-
(H2O)(L6)]13 having similar composition to that of a6 was
reported (Chart S2). The former was synthesized in ethanol,
and its structure was determined by infrared spectroscopy.
Electronic Spectra, Fluorescence Spectra, and Fluores-
cence Lifetimes of Schiff Base Ligands and Schiff Base
Tin(IV) Complexes. Photophysical data of Schiff bases and
Schiff base tin complexes in DMSO at room temperature are
summarized in Table 4. The data of the unisolated ligands
HL6, HL9, and HL11 are not included in the table. The ligand
HL5 is unstable in DMSO, and only the datum of the solid-
state emission is included. Electronic spectra of Schiff base
ligands are shown in Figure S2. The table and the spectra show
that the ligands HL1-HL7 have absorption peaks in the visible
region while HL8 does not; they also show that the spectra
of HL1-HL3, which have naphthalene moieties, have large
1
absorption peaks in the 450-550 nm region. H NMR spectra
indicate that HL1-HL7 are in the keto form in DMSO, while
HL8 is in the enol form. It has been reported that the keto
forms have absorption bands in the visible region, while the
enol forms do not.27 Schiff base ligands are not fluorescent
in DMSO, except HL4¤, and are fluorescent in solids. The
complexes a1-a7 are fluorescent both in solutions and solids.
Probably coordination of the ligands decreases their flexibility,
which reduces the vibrations of the ligands. The planarities and
fluorescence magnitudes of Schiff bases of the complexes have
no correlation.
Electronic spectra of a1-a11 are shown in Figures 4a
and 4b. Most of the Schiff base complexes have their character-
istic peaks at around 360 and 450 nm. The lowest excited singlet
(S1) and second excited singlet (S2) states for these ligands are
1
³³* in character because the molar extinction coefficients for
these ligands are large, 5000-27000 M¹1 cm¹1 at the absorption
peak wavelengths and tin(IV) complexes such as SnCl4 and
[SnCl6]2¹ do not show any absorption in near ultraviolet and
visible regions. In DMSO, the molar extinction coefficients
for these tin complexes are also large, 4800-18000 M¹1 cm¹1 at
the absorption peak wavelengths. Therefore, the S1 and S2 states
Figure 2. Partial packing diagram of a8 showing hydrogen
bonds. O-H, 0.91 ¡; H-Cl*; 2.27 ¡, O-H£Cl*: 175.4°;
symmetry operator: (*) ¹X + 1, ¹Y + 1, ¹Z + 1.
1
for these complexes will also be ³³* in character.
1) a9yel
2) a9or
Figure 3. Partial packing diagrams of a9yel and a9or.
Table 3. Selected Bonds Length (¡) and Dihedral Angles (°) for a1, a3, a5, a6, a7, a8, a9yel, a9or, a10, and a11
Sn1-Cl1
Sn1-Cl2
Sn1-Cl3
Sn1-N11
Sn1-N21
Sn1-O21
Dihedral ang.
a1
a3
a5
a6
a7
a8
a9yel
a9or
a10
a11
2.437(1)
2.4605(7)
2.4139(8)
2.4075(8)
2.407(1)
2.425(1)
2.4210(5)
2.384(1)
2.396(3)
2.407(1)
2.408(1)
2.4458(7)
2.4001(8)
2.4142(8)
2.420(1)
2.441(1)
2.3945(5)
2.421(1)
2.438(3)
2.405(1)
2.368(1)
2.4027(7)
2.3639(8)
2.3634(8)
2.367(1)
2.348(1)
2.3530(5)
2.357(1)
2.350(3)
2.366(1)
2.186(3)
2.290(2)
2.187(2)
2.201(2)
2.182(3)
2.187(2)
2.200(2)
2.195(2)
2.212(4)
2.202(2)
2.180(3)
2.193(2)
2.179(2)
2.181(2)
2.185(3)
2.192(2)
2.189(2)
2.191(2)
2.149(3)
2.170(2)
2.024(3)
2.069(2)
2.034(1)
2.019(2)
2.020(2)
2.003(2)
2.020(1)
2.014(2)
2.029(4)
2.016(2)
2.8
25.9
6.0
8.8
0.5
15.6
28.3
10.7
1.9
4.0

K. Takano et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012) 1217
Table 4. Photophysical Data of Schiff Bases HL1-HL3,
HL4¤, HL5, HL7-HL10 and Schiff Base Tin Complexes
a1-a11 in DMSO and in Solid State at Room Temperature
2
1
Absorbance
max/nm
(¾/M¹1 cm
Stokes
Emission
soln (QY)a),b)
Emission
in solid statec)
Compd
-
shift
¹1
¹1
)
/cm
HL1
HL2
HL3
HL4¤
457 (23100), no emission
481 (24400)
470 (24800), no emission
496 (23100)
457 (25600), no emission
481 (27000)
®
®
®
®
553
589
557
594
0
2
400
500
600
Wavelength/nm
(a)
350 (26400), 483
504 (7700)
d)
d)
HL5
HL7
HL8
®
531 (5290)
®
no emission
®
®
®
570
586
no emission
400sh (6620) no emission
HL10 355 (8100)
®
515 (0.21)
a1
a2
a3
a4
366 (9340),
453 (17700)
360 (16600), 545 (0.025)
470 (16200)
363 (9620),
450 (18000)
347 (15300), 538 (0.042)
435 (11700)
2660
2930
2580
4400
555
586
546
563
1
0
509 (0.13)
400
500
600
Wavelength/nm
(b)
a5
a6
402 (14900)
343 (15200), 518 (0.028)
421 (12900)
348 (14700), 540 (0.046)
429 (9700)
355 (17000), no emission
476 (8170)
360sh (8090), 535 (0.058)
430 (9050)
360sh (8090), 535 (0.058)
430 (9050)
351 (13400), no emission
464 (8430)
368 (17500), no emission
450 (4810)
492 (0.022)
4550
4450
523
536
Figure 4. (a) Electronic spectra of a1 ( ), a2 ( ), a3 ( ),
a4 ( ), a5 ( ), a6 ( ), a7 ( ), and a8 ( ) in DMSO.
(b) Electronic spectra of a9 ( ), a10 ( ), and a11 ( ) in
DMSO.
a7
4790
®
553
a8
no emission
540
a9yel
a9or
a10
a11
4560
4560
®
549
584
®
590
a) Emission -max/nm (quantum yield, QY) in a DMSO solu-
tion: 1.0 © 10¹5 mol L¹1, -ex = 390 nm. b) Standard: a DMF
500
600
solution of Alq3,25b
d) Unstable in DMSO.
-ex = 390 nm. c) Emission -max/nm.
Wavelength/nm
Figure 5. Emission spectra of a1 ( ), a2 ( ), a3 ( ), a4
), a5 ( ), a6 ( ), and a7 ( ) in DMSO at room
temperature (-ex = 390 nm).
Normalized fluorescent spectra of a1-a7 are shown in
Figure 5. Table 4 also contains fluorescent data of the ligands
and complexes in solutions and in solid states. Solid samples
show fluorescence at longer wavelengths than solution samples
(Figure S3). The electronic, fluorescence, and 1H NMR spectra
of a9yel and a9or in DMSO agree fully with each other.
The complexes a8, a10, and a11 do not show fluorescence in
DMSO. Especially a8 does not show fluorescence even in the
solid state, while a10 and a11 show fluorescence in the solid
states. The dimerization of a8 by hydrogen bonds is most
probably the cause of its nonfluorescence in solid, and the
interaction of a8, a10, and a11 with DMSO is likely the cause
of their nonfluorescence. The dimerized form of a8 may also be
present in DMSO.
(
The alteration of ligands led to fairly large changes of
fluorescence peak wavelength. This demonstrates that the
emission wavelength of tin(IV) complexes is tunable by
ligand modification.1c,28 The emission wavelengths (-max) of
the fluorenecarboxylato tin complexes are 355 and 390 nm,14
while that of the tin-iridium complex is 645 nm;15 the emission
wavelengths (492-545 nm) of the present Schiff base tin com-
plexes are between those of the two types of complexes.
The fluorescence quantum yields (Φf) of each complex were
determined by eq 1.29

1218 Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012)
Fluorescent Tin(IV) Complexes
Table 5. Fluorescence Lifetimes of Tin Complexes in
DMSO at Room Temperature
Table 6. HOMO-LUMO Gaps Obtained by DFT Calcula-
tions (B3LYP/6-31G**; Sn, LanL2DZ)
2
¹1
¹1
Compd
¸/ns
»
kf/ns
knr/ns
HOMO
/eV
LUMO
/eV
HOMO-LUMO
gap/eV
Compd
a1
a2
a3
a4
a6
a7
1.76
0.49
1.36
0.86
0.78
1.10
1.616
1.120
1.428
1.339
1.306
1.412
0.119
0.449
1.989
0.639
1.114
1.246
0.867
0.0510
0.0955
0.0488
0.0359
0.0418
a1
a2
a3
a4
a5
a6
a7
a8
a9
a10
a11
¹6.05
¹5.81
¹6.01
¹6.03
¹6.80
¹6.23
¹6.35
¹5.82
¹6.20
¹5.71
¹5.66
¹3.01
¹3.01
¹2.86
¹3.01
¹3.47
¹3.09
¹3.35
¹3.21
¹3.20
¹3.05
¹2.97
3.04
2.80
3.15
3.02
3.34
3.14
3.00
2.61
3.00
2.66
2.69
2
ꢀf
ꢀr
Ff Ar nf
Fr Af nr2
¼
ð1Þ
where F is the integrated fluorescence intensity, Φ is the
quantum yield, A is the absorbance, n is the refractive index for
solvent, and the subscripts f and r represent the complex sample
solutions and the reference solution, respectively. The Φr
value for reference complex, Alq3, in DMF is 0.11.25b After
the photoirradiation, the complexes are excited to the first
excited state. The first excited state decays to the ground state
by the radiative (fluorescent) and/or nonradiative (thermal)
relaxation processes. The fluorescence lifetimes of complexes
a1-a7 were measured. Complex a5 was not stable enough to
get the lifetime. The time-resolved fluorescence intensities are
described as the single exponential function (eq 2):
3.4
3.3
3.2
3.1
3
a5
y = 1.6685x - 0.8915
R² = 0.826
a6
a1
a3
a4
a9
a7
2.9
2.8
a2
2.7
2.2
F ¼ F0 expðꢀt=¸Þ
ð2Þ
2.3
2.4
2.5
2.6
where ¸ is the lifetime of the excited state. The fluorescent and
nonradiative rate constants are described as
emission energy /eV
Figure 6. Plots of emission wavelengths vs. HOMO-
LUMO gaps for a1-a7 and a9.
kf ¼ ꢀf=¸
knr ¼ ꢀnr=¸
ꢀf þ ꢀnr ¼ 1
ð3Þ
ð4Þ
ð5Þ
where k is the rate, Φ is the quantum yield, and the subscripts
f and nr represent the fluorescent and nonradiative relaxation
processes, respectively (eqs 3-5).
The fluorescence lifetimes (ca. 1.8-0.5 ns) of a1-a4, a6,
and a7 are relatively small and summarized in Table 5. Plots
of lifetime/ns vs. quantum yield, kf vs. quantum yield, and knr
vs. quantum yield are shown in Figures S4a, S4b, and S4c,
respectively. The table and figures indicate the following: 1)
Complexes a1 and a3 have relatively large quantum yields, and
their fluorescence rate constants kf’s are similar to each other,
while knr of a1 is less than that of a3. The introduction of a
methyl group to the ligand HL1 gives HL3, and the stretch-
ing vibration of the quinoline ring-methyl group will cause
deactivation of a3. 2) Complexes a4, a6, and a7 have relatively
small quantum yields, and their kf’s and knr’s are similar to each
other. 3) Complex a2 has small quantum yields, and the
magnitude of kf is similar to that of other complexes, but the
magnitude of knr is larger than that of any other complexes.
Probably, the orientation change of the naphthalene ring from
HL1 to HL2 causes loss of activated energy through vibration.
4) The Stokes shifts of a1, a2, and a3 which have naphthalene
rings are ca. 2700 cm¹1, while those of a4, a6, a7, and a9
which have benzene rings are ca. 4500 cm¹1. It is clear that
the fluorescence quantum yields and Stokes shifts have no
correlation ship.
HOMO
LUMO
Figure 7. Visualized frontier orbitals for a6.
DFT Calculation. DFT calculations show that the order of
the magnitudes of HOMO-LUMO gaps (Table 6) is consonant
with that of the emission wavelengths (-max) of the complexes
as shown in Figure 6. It is common knowledge that the fluo-
rescence wavelength depends on both excited and ground
states, however, the wavelength is predictable in the case of
these Schiff base complexes.
Frontier orbitals of all the complexes show that the main
components of HOMO’s are aldehyde moieties and those of
LUMO’s are quinoline moieties. The frontier orbitals of a1
(Figure S5) and a6 (Figure 7) are shown, for example, together
with energy diagram of a6 (Figure S6). The molecular orbital
coefficients of a6 indicate that the main atomic components of
HOMO are C23 and C25, and those of LUMO are C11, C13,
and C16: the atom numbering scheme of a6 employed here is
the same as that shown in Figure 1. Therefore the introduction
of either electron-donating or -withdrawing groups into some

K. Takano et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012) 1219
Table 7. Comparison of Cathodic Peak Potentials E1 for Reduction of Schiff Bases and Schiff Base-Tin(IV)
pc
Complexes, and Sums of para- and ortho-Substituted Hammett Parameters
E1pc/V
vs. Fc/Fc+
Compd
p-
·
o-
·
o
·p + ·o
p
HL1
HL2
HL3
HL4¤
HL7
HL8
a1
a2
a3
a4
a5
¹1.87
¹1.88
¹1.88
¹1.50
¹1.56
¹1.98
¹1.32
¹1.32
¹1.36
¹1.32
¹1.17
¹1.32
¹1.15
¹1.35
¹1.34
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
®
CH3
NO2
H
Cl
®
®
CH(OCH3)2
H
H
Cl
®
®
®
®
¹0.17
+0.78
0.00
+0.23
¹0.37
¹0.14
0.00
0.00
+0.37
0.00
¹0.24
+0.78
0.00
+0.60
¹0.37
¹0.38
a6
a7
a8
a11
OH
CH2CH=CH2
H
OCH3
of the atoms (C11, C13, C16, C23, and C25) will cause the
wavelength change of a6. The introduction of a nitro group
(a5) or chlorine atoms (a7) to a6 lowers both the energies of
HOMO and LUMO. The nitro group and chlorine atoms have
similar effects of lowering the HOMO of a6, but the nitro
group has larger effect of lowering the HOMO of a6 than the
chlorine atoms have. As a result, larger HOMO-LUMO gap
appears in the case of a5.
-1
a7
-1.2
a5
a11
Electrochemistry.
Cyclic voltammetry (CV) was used
to determine the reduction potentials for the ligands and
the complexes. For the purpose of an influence of ligand
modifications on the electrochemical properties, these measure-
ments were undertaken using a uniformed dimethyl sulfoxide
(DMSO) solution containing 0.1 M tetrabutylammonium hexa-
fluorophosphate (Bu4NPF6) with a scan rate of 0.1 V s¹1. All
cyclic voltammograms exhibited irreversible waves and these
results suggest that the reduction products of the ligands
and the complexes were unstable under a time scale in CV
measurements (Figures S7a, S7b, and S7c).
The cathodic peak potentials (E1pc) for reduction of the
Schiff bases HL1-HL3, HL4¤, HL7, and HL8 and Schiff base
tin(IV) complexes, and relevant Hammett parameters are listed
in Table 7. The HL5 ligand decomposed in DMSO and the
HL6 ligand was not isolated as a solid, and CV measurements
of them were not conducted. The chemical structures of HL1
and HL2 have an isomeric relation. Therefore, the reduction
potentials of these two ligands were quite similar. The chemical
structure of HL3 and HL1 differs with regard to the methyl
group. The substituent has little influence on the reduction
potential. The reduction potentials of Schiff base tin(IV) com-
plexes were more positive than these corresponding free Schiff
base ligands. DFT calculations for these complexes indicate
that all LUMO are distributed to the ligands and the reduc-
tions are considered to occur at the ligand moiety. These
results suggest that these enolates, which are generated by
the complexations, have more easily reduced moieties and the
differences among these five complexes are due to the influence
of the substituent groups.
a6
a8
-1.4
-1
0
1
σp
+
σo
Figure 8. Correlation between first reduction peak poten-
tials (E1pc) and sums of para- (·p) and ortho-substituted
(·o) Hammett parameters.
Correlation between Hammett Parameters and Electro-
chemistry. As for the complexes a5, a6, a7, a8, and a11 in
Table 7, the substituents are considered to be in a para- and
ortho-position with respect to the phenolic oxygen, and the
para ·p30- and ortho ·o31-substituted Hammett parameters are
as noted in Table 7. There is a good linear relationship between
the sum of the ·p and ·o values and the E1pc values (R2 = 0.93,
Figure 8) and the larger sum of the ·p and ·o values
corresponds the larger Epc value. Generally, substituents with
a plus · value are electron-withdrawing groups and the larger
· value renders the reduction easier.32-34 These electrochemical
results confirm that substituents, which are in the para- and
ortho-positions with respect to the phenolic oxygen of the
Schiff base ligands, affect the electronic structures of the
complexes.
Correlation between Theoretical Studies and Electro-
chemistry. In the case of the one-electron-reduction process,

1220 Bull. Chem. Soc. Jpn. Vol. 85, No. 11 (2012)
Fluorescent Tin(IV) Complexes
-2.8
References
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1
a11
-3
-3.2
-3.4
a1, a2, a4
a6
2
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9
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an electron flows from an electrode to a LUMO and the
standard potential, E0, is known to be related to a LUMO
energy level.35 Plots of first cathodic peak potentials of a series
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Summary
Eleven Schiff base tin(IV) complexes a1-a11 of the types
[SnCl3(L)] (HL: Schiff base) were synthesized and character-
ized especially for fluorescence. All except a8, a10, and a11 are
fluorescent in DMSO (-max/nm: 492-545 nm). The fluores-
cence quantum yields (Φf) of the complexes depend very much
on the ligands (Φf = 0.21-0.022). Of the solids, a10 and a11
are fluorescent, but a8, the ligand having an OH group, is not.
The fluorescence lifetimes (¸ = 0.49-1.76 ns) of a1-a4, a6,
and a7 are relatively small. The DFT calculation shows that the
order of the magnitudes of HOMO-LUMO gaps is consonant
with that of the emission wavelengths (-max) of the complexes,
and the emission wavelength is predictable in the case of these
Schiff base tin complexes. The cyclic voltammograms indicate
that reduction products of the complexes are unstable under
the time scale in CV measurements (0.1 V s¹1). A relatively
good correlation except for a8 was observed between reduction
potentials in DMSO and the energies of LUMO.
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We thank Profs. S. Toyota and H. Yamada of Okayama
University of Science for helpful discussions of the NMR
spectra.
Supporting Information
Additional spectroscopic, electrochemical, and DFT data and
line drawings of related compounds. This materital is available
free of charge on the Web at: http://www.csj.jp/journals/bcsj/.

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