Synthesis and properties of nitrogen-introduced phenylazomethine dendrimer

Article abstract of DOI:10.1080/00397911.2014.891746

The pyridine ring-introduced phenylazomethine dendrimer (PyDPA) was synthesized by a dehydration reaction using titanium(IV) chloride. The ultraviolet-visible absorption and the electrochemical study showed that the introduction of the pyridine ring produ

Full text of DOI:10.1080/00397911.2014.891746

This article was downloaded by: [Florida International University]
On: 02 September 2014, At: 08:53
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An
International Journal for Rapid
Communication of Synthetic Organic
Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/lsyc20
Synthesis and Properties of Nitrogen-
Introduced Phenylazomethine Dendrimer
Ken Albrecht a , Hideyuki Higashimura b & Kimihisa Yamamoto a
a Chemical Resources Laboratory , Tokyo Institute of Technology ,
Kanagawa , Japan
b Advanced Materials Research Laboratory , Sumitomo Chemical
Co. , Ibaraki , Japan
Accepted author version posted online: 23 Apr 2014.Published
online: 06 Jun 2014.
To cite this article: Ken Albrecht , Hideyuki Higashimura & Kimihisa Yamamoto (2014) Synthesis
and Properties of Nitrogen-Introduced Phenylazomethine Dendrimer, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 44:15, 2239-2247,
DOI: 10.1080/00397911.2014.891746
To link to this article: http://dx.doi.org/10.1080/00397911.2014.891746
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Synthetic Communications1, 44: 2239–2247, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.891746
SYNTHESIS AND PROPERTIES OF
NITROGEN-INTRODUCED PHENYLAZOMETHINE
DENDRIMER
Ken Albrecht,1 Hideyuki Higashimura,2 and
Kimihisa Yamamoto1
1Chemical Resources Laboratory, Tokyo Institute of Technology,
Kanagawa, Japan
2Advanced Materials Research Laboratory, Sumitomo Chemical Co.,
Ibaraki, Japan
GRAPHICAL ABSTRACT
Abstract The pyridine ring–introduced phenylazomethine dendrimer (PyDPA) was
synthesized by a dehydration reaction using titanium(IV) chloride. The ultraviolet–visible
absorption and the electrochemical study showed that the introduction of the pyridine ring
produces a smaller band gap by increasing the highest occupied molecular orbital level and
decreasing the lowest unoccupied molecular orbital level. A crystal of a PyDPA-PdCl2
complex was also prepared. Traditional phenylazomethine dendrimers cannot form a
complex with palladium, indicating that PyDPA can potentially coordinate with various
metal salts such as Co, Ni, Ru, and Mn and can be used for catalytic or electronic
applications.
Keywords Dendrimer; phenylazomethine
Received December 26, 2013.
Address correspondence to Kimihisa Yamamoto, Chemical Resources Laboratory, Tokyo Institute
of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8503, Japan. E-mail:
yamamoto@res.titech.ac.jp
Color versions of one or more of the figures in the article can be found online at www.
tandfonline.com/lsyc.
2239

2240
K. ALBRECHT, H. HIGASHIMURA, AND K. YAMAMOTO
INTRODUCTION
Dendrimers[1] are perfectly branched polymers that have unique structures with
several applications. Dendrimers can be constructed with several flexible to rigid
building blocks. The building blocks of most dendrimers do not have specific
functions; that is, the dendron is only used as a separator of the core and periphery.
However, some of the dendrimers that consist of functional building blocks
that are luminescent,[2] photochemically active,[3] electrochemically active,[4] or
coordinatable[5] are showing impressive performances. The phenylazomethine den-
drimer (DPA)[6] is one of the functional dendrimers that can be used as an electronic
material.[7] Furthermore, DPA can coordinate to Lewis acids in a radial stepwise fas-
hon that cannot be achieved with other dendritic ligands. This novel property allows
to control the number and position of the Lewis acids that are encapsulated in the
dendrimer.[8] Encapsulated metal salts can be directly used as a catalyst[9] or can
be reduced or oxidized to form size-controlled nanoparticles.[10] However, the tra-
ditional DPA has a limit in the numbers of the metal salts that can be coordinated;
for example, the coordination of palladium has not been yet achieved.
Pyridine derivatives are well known as Lewis bases, and various complexes that
include pyridine ligands are reported. Introducing a pyridine ring into the DPA
backbone can add a new coordination site that can coordinate with new metal salts.
Additionally, the introduction of nitrogen can lower the lowest unoccupied molecu-
lar orbital (LUMO) level of the molecule because of the high electron affinity, which
can be important as an electron-transporting material.[11] We now report the first
synthesis of a nitrogen (pyridine) introduced DPA (PyDPA) and the basic properties
that can be potentially used as a new dendritic ligand or electronic material.
RESULTS AND DISCUSSION
DPAs are synthesized by repeating the irreversible dehydration reaction of
amines and ketones to give imines, and the oxidation reaction reproduces the
ketone.[12] To check the applicability of this route to PyDPA, we have chose the
di-2-pyridyk ketone (BPyK) as the starting material and synthesized a simple model
compound PyG0 (Fig. 1).[13] The dehydration reaction using TiCl4 under the usual
conditions (1 eq. TiCl4 and 5 eq. DABCO) gave only an ca. 20% yield of the product.
The reaction was also performed with reduced TiCl4 (0.5 eq. TiCl4 and 5 eq. DABCO,
59% yield) and excess DABCO (1 eq. TiCl4 and 50 eq. DABCO, 65% yield). The
increase in the yield by changing the reaction conditions is suggesting that the poor
yield under the first condition is caused by the coordination of titanium to the pyri-
dine. Based on this finding, the dehydration reaction for the PyDPA was performed
with excess 1,4-diazabicyclo[2.2.2]octane (DABCO) in the following sections.
Figure 1. Synthesis of PyG0.

PHENYLAZOMETHINE DENDRIMER
2241
According to the literature method, the oxidation reaction of prePyG2on using
potassium permanganate was carried out (Fig. 2). However, the target dendron
PyG2on could not be obtained. The main product after the reaction was orange
and water soluble, which is presumably the N-oxide species. Therefore, another
route without the oxidation reaction was chosen to synthesize PyDPA dendrons
(Fig. 2).[6] This new route gives by-products because of the self-condensation reac-
tion of BAPK. However, by changing the solvent to acetonitrile to increase the solu-
bility of BAPK, the PyG2on could be obtained in ca. 20% yield. This dendron easily
formed hemiacetals because of the strong electron-withdrawing effect of the pyridine
moiety (the formation was confirmed by 1H and 13C NMR in deuterated methanol).
For purification, extra attention was paid to avoid using alcoholic solvents including
chloroform that contains ethanol as a stabilizer. Finally, the PyDPA dendrimers
were synthesized by the dehydration reaction of the dendrons and the p-phenylene-
1
diamine (Fig. 3). All compounds were identified by their H NMR, 13C NMR, and
matrix-assisted laser desorption=ionization time-of-flight (MALDI TOF) mass
spectra (MS).
The ultraviolet–visible (UV-vis) spectra of PyDPAs were measured in chloro-
form and compared to the corresponding DPAs (Fig. 4). The peaks and the absorp-
tion edge clearly show bathochromic shifts by the introduction of the nitrogen. This
is probably caused by the donor (imine)–acceptor (pyridine) interaction or the relax-
ation of the steric hindrance because of the reduced hydrogen atom by the nitrogen
introduction. In either case, this results in a more planar molecular structure with a
larger conjugation and this causes the bathochromic shift in the p-pꢀ and n-pꢀ
transition absorptions of the imine moiety. Notably, this shift indicates that the
highest occupied molecular orbital (HOMO)–LUMO gap has been reduced ca.
0.1 eV compared to the DPAs.
The electrochemical property of PyG2 was measured in benzonitrile and com-
pared with DPA G2 (Fig. 5). The first oxidation peak (region ‘‘a’’ in Fig. 5) has
shifted 0.1 eV lower and the reduction onset (region ‘‘c’’) has shifted 0.3 eV higher
compared to G2. The peak in region ‘‘b’’ did not appear when the measurement
Figure 2. Synthesis of nitrogen-introduced phenyl-azomethine dendron.

2242
K. ALBRECHT, H. HIGASHIMURA, AND K. YAMAMOTO
Figure 3. Synthesis of nitrogen-introduced phenyl-azomethine dendrimers.
Figure 4. UV-vis spectra of phenylazomethine dendrimers.

PHENYLAZOMETHINE DENDRIMER
2243
Figure 5. Cyclic voltammogram of DPA G2 and PyG2.
was started from 0 V and scanned to ꢁ2 V. This means that the introduction of the
nitrogen is both affecting the HOMO and LUMO levels of the dendrimer. The large
shift in the LUMO level can be attributed to the high electron affinity of the pyridine
ring. This decrease of the band gap was larger compared to the result of the UV-vis
˚
Figure 6. ORTEP diagrams of PyG0-PdCl2 complex with 50% probability. The bond radius is 0.06 A.

2244
K. ALBRECHT, H. HIGASHIMURA, AND K. YAMAMOTO
absorption. This reason might be caused by the different solvent nature and the
chemical structure change of the dendrimer by reduction, which is suggested by
the irreversible cyclic voltammogram.
A single crystal of the PyG0-PdCl2 complex was prepared by slow evaporation
of the acetonitrile solution, and X-ray single-crystal structural analysis has been
completed (Fig. 6). PyG0 was coordinated to PdCl2 with the imine nitrogens and
one of the pyridine nitrogen. It is known that this kind of coordination ligand can
coordinate to Pd,[14] Co,[15] Ni,[16] Ru,[17] and Mn[18] and can be used for catalytic[19]
or magnetic[20] applications. Coordination of these metals is not achieved with DPA
and show the diverse possibility of PyDPA as a ligand.
In conclusion, a new nitrogen-introduced phenyl-azomethine dendrimer
(PyDPA) was synthesized with the dehydration reaction using TiCl4 in the presence
of an excess amount of DABCO. The bathochromic shift in the UV-vis absorption
and the decrease in the HOMO-LUMO gap compared to DPA were observed.
This new dendrimer is a potential ligand for various metal salts. Its applications
are now under investigation.
EXPERIMENTAL
Dehydrated benzonitrile was purchased from Aldrich and used without further
purification. All other solvents and chemicals were purchased from Kanto Kagaku
Co., Ltd., and used without further purification (the solvent for the UV-vis absorp-
tion measurement was a spectroscopic grade). The silica gel for column chromato-
graphy was the neutral grade (Kanto Kagaku Co., Ltd.).
The NMR spectra were obtained using a JEOL JNM-GX400 (400 MHz)
with tetramethylsilane (TMS) as the internal standard (the spectra is shown in the
Supplementary Data). The HRMS (ESI-TOF-MS) data were obtained using a Bru-
ker micrOTOF II in the positive ion mode. Acetonitrile was used as the solvent. The
UV-vis spectra were recorded using a Shimadzu UV-3150 spectrometer with a quartz
cell having a 1 cm optical length. The electrochemical measurements were done by
a conventional three-electrode configuration using an ALS Chi660 electrochemical
analyzer. The scan rate was 0.1 V=s, the working electrode was a GC electrode,
the counter electrode was a Pt wire, and the reference electrode was Ag=Agþ.
The solvent was dehydrated benzonitrile and was purged for 15 min with nitrogen
before the measurement. The solvent concentration was 1 mM, and the supporting
electrolyte was 0.2 M tetra-n-butylammonium perchlorate.
Crystal Structure Determination
Crystal data of PyG0-PdCl2 complex: C17H13Cl2N3Pd, M ¼ 436.62, mono-
˚
˚
˚
clinic space group P21=n a ¼ 10.0299(4) A, b ¼ 15.1659(5) A, c ¼ 11.3270(5) A, b ¼
107.5204(16) , U ¼ 1643.05(11) A , Z ¼ 4, Dcalc ¼ 1.765 g=cm3, m ¼ 1.456 mmꢁ1
,
o
3
˚
crystal size of 0.25 ꢂ 0.25 ꢂ 0.15 mm, 15,759 reflections collected, S ¼ 1.010,
R ¼ 0.0235, and Rw ¼ 0.0515. Orange block crystals of PyG0-PdCl2 complex were
obtained by the slow evaporation of the acetonitrile solution of a 1: 1 mixture of
PyG0 and PdCl2. A suitable crystal was mounted in a cryoloop. The intensity data
were collected at 223 K by a Rigaku RAXIS-RAPID diffractometer with graphite

PHENYLAZOMETHINE DENDRIMER
2245
˚
monochromatized Mo Ka radiation (k ¼ 0.71075 A). The structure was solved by a
direct method (SIR92) on a Dell workstation using the program system Crystal-
Structure. The structure was refined on F2 by full-matrix least square methods using
3133 reflections within I > 2.0r(I). The atomic parameters are listed in the CIF file.
Synthesis
N-(Di(pyridin-2-yl)methylene)aniline (PyG0, general procedure for
dehydration reaction). Di-2-pyridyl ketone (BPyK, 205 mg, 1.11 mmol) and
DABCO (6071 mg, 54.1 mmol) were added to a three-necked flask. The flask was
evacuated and backfilled with nitrogen. Aniline (100 mg, 1.07 mmol) and chloroben-
zene (20 mL) were added and heated to 75 ꢃC. TiCl4 (204 mg, 1.08 mmol) dissolved in
5 mL of chlorobenzene was added dropwise, and the addition funnel was then rinsed
with 5 mL of chlorobenzene. The mixture was heated to 125 ꢃC and stirred for 1.5 h
under a nitrogen atmosphere. The reaction mixture was cooled to room temperature,
stirred for 2 h in air, and then filtered through celite. The filtrate was concentrated,
and the product was isolated by silica-gel (neutral) column chromatography
1
(EtOAc=MeOH ¼ 2: 1 with 2% Et3N). Yield: 65% (182 mg, 0.70 mmol). H NMR
(400 MHz, CDCl3, 25 ꢃC): d8.63–8.63 (2H, m), 8.21 (1H, d, J ¼ 7.8 Hz), 7.83 (1H,
td, J ¼ 7.7, 1.6 Hz), 7.54 (1H, td, J ¼ 7.7, 1.6 Hz), 7.35 (1H, ddd, J ¼ 7.4, 4.8,
1.1 Hz), 7.18–7.16 (3H, m), 7.05 (1H, d, J ¼ 7.8 Hz), 6.97 (1H, t, J ¼ 7.3 Hz), 6.77
13
(2H, dt, J ¼ 8.5, 1.6 Hz). C NMR (100 MHz, CDCl3, 25 ꢃC): 166.49, 156.66,
154.90, 150.20, 149.23, 149.13, 136.58, 135.77, 128.50, 124.69, 123.97, 123.54,
123.12, 120.77. HRMS (ESI-TOF): Calcd: 260.1182 ([M þ H]þ); found: 260.1178.
PyG2on. As per the general procedure for the dehydration reaction,
BAPK (299 mg, 1.40 mmol), BPyK (2582 mg, 14.2 mmol), and DABCO (7810 mg,
69.6 mmol) were dissolved in dry acetonitrile (240 mL). TiCl4 (531 mg, 2.80 mmol)
dissolved in 10 ml chlorobenzene was added and the mixture was stirred for 1.5 h
at 75 ꢃC. The product was isolated by silica-gel column chromatography (EtOAc=
Et3N ¼ 4: 1) and purified by preparative GPC (eluent: THF). Yield: 17% (130 mg,
1
0.24 mmol). H NMR (400 MHz, CDCl3, 22 ꢃC): d8.66 (1H, d, J ¼ 4.3 Hz), 8.59
(1H, d, J ¼ 4.5 Hz), 8.22 (1H, d, J ¼ 7.9 Hz), 8.19 (1H, d, J ¼ 2.5 Hz), 8.01 (1H, d,
J ¼ 8.3 Hz), 7.87 (1H, t, J ¼ 7.2 Hz), 7.62 (1H, t, J ¼ 7.4 Hz), 7.41 (1H, dd, J ¼ 7.1,
5.0 Hz), 7.28–7.21 (2H, m), 7.16 (1H, d, J ¼ 7.6 Hz). 13C NMR (100 MHz, CDCl3,
22 ꢃC): 190.42, 168.41, 155.41, 153.50, 149.55, 149.43, 149.34, 148.73, 141.18,
136.72, 136.17, 127.82, 126.19, 125.34, 124.73, 123.99, 123.81. HRMS (ESI-TOF):
Calcd: 547.1989 ([M þ H]þ); found: 547.2006.
PyG1. As per the general procedure for the dehydration reaction, BPyK
(379 mg, 2.06 mmol), p-phenylenediamine (102 mg, 0.943 mmol), and DABCO
(5230 mg, 46.6 mmol) were dissolved in chlorobenzene (50 mL). TiCl4 (351 mg,
1.85 mmol) was added and the mixture was stirred for 2 h at 125 ꢃC. The product
was isolated by silica-gel column chromatography (EtOAc=Et3N ¼ 4: 1). Yield:
1
38% (159 mg, 0.36 mmol). H NMR (400 MHz, CDCl3, 26 ꢃC): d8.61 (4H, d, J ¼
4.9 Hz), 8.13 (2H, d, J ¼ 8.3 Hz), 7.80 (2H, td, J ¼ 7.8, 1.5 Hz), 7.54 (2H, td, J ¼ 7.8,
1.5 Hz), 7.34–7.32 (2H, m), 7.20–7.19 (2H, m), 7.01 (2H, d, J ¼ 7.8 Hz), 6.58 (4H, s).
13C NMR (100 MHz, CDCl3, 26 ꢃC): 166.44, 156.56, 154.99, 149.29, 149.18, 146.60,

2246
K. ALBRECHT, H. HIGASHIMURA, AND K. YAMAMOTO
136.54, 135.77, 124.73, 124.68, 123.49, 123.07, 121.36. HRMS (ESI-TOF): Calcd.:
441.1822 ([M þ H]þ); found: 441.1816.
PyG2. As per the general procedure for the dehydration reaction, PyG2on
(289 mg, 0.529 mmol), p-phenylenediamine (27.8 mg, 0.257 mmol), and DABCO
(2905 mg, 25.9 mmol) were dissolved in chlorobenzene (50 mL). TiCl4 (98 mg,
0.52 mmol) was added and the mixture was stirred for 2 h at 125 ꢃC. The product
was isolated by silica-gel column chromatography (EtOAc=MeCN=Et3N ¼ 4: 2: 1),
and purified by preparative GPC (eluent: chloroform). Yield: 20% (60 mg,
1
0.051 mmol). H NMR (400 MHz, CDCl3, 23 ꢃC): d8.64–8.62 (6H, m), 8.56–8.55
(2H, m), 8.21 (4H, t, J ¼ 7.5 Hz), 8.11 (2H, dd, J ¼ 2.5, 0.7 Hz), 8.02 (2H,
dd, J¼ 2.4, 0.8 Hz), 7.88–7.80 (6H, m), 7.63–7.60 (4H, m), 7.39–7.34 (4H, m), 7.25–
7.19 (6H, m), 7.14 (2H, d, J¼ 7.6Hz), 7.02 (2H, d, J¼ 7.9 Hz), 6.98 (2H, dd, J¼ 8.3,
2.5 Hz), 6.78 (2H, dd, J¼ 8.3, 0.7 Hz), 6.39 (4H, s). 13C NMR (100 MHz, CDCl3,
23 ꢃC): 168.43, 168.09, 165.54, 155.69, 153.78, 153.72, 151.77, 150.13, 149.53, 149.41,
149.24, 149.12, 147.10, 146.40, 145.65, 141.42, 141.15, 136.59, 136.20, 136.07, 128.16,
127.78, 125.13, 125.09, 124.67, 124.62, 124.33, 123.87, 123.66, 123.57, 123.38, 121.32.
HRMS (ESI-TOF): Calcd.: 1165.4382 ([M þ H]þ); found: 1165.4364.
ACKNOWLEDGMENT
K. A. thanks Dr. Yousuke Ochi for helping with the X-ray single-crystal
structure analysis.
FUNDING
This work was supported in part by CREST program of the Japan Science and
Technology (JST) Agency, Grant-in-Aids for Scientific Research on Innovative
Areas ‘‘Coordination Programming’’ (area 2107, No. 21108009), and by JSPS
KAKENHI Grant Number 26410128.
SUPPLEMENTARY DATA
Supplemental data for this article can be accessed on the publisher’s website.
REFERENCES
1. (a) Astruc, D.; Boisselier, E.; Ornelas, C. Chem. Rev. 2010, 110, 1857; (b) Tomalia, D. A.
J. Nanopart. Res. 2009, 11, 1251; (c) Fischer, M.; Vogtle, F. Angew. Chem., Int. Ed. 1999,
¨
38, 884; (d) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665; (e)
Newkome, G. R.; Shreiner, C. Chem. Rev. 2010, 110, 6338; (f) Grayson, S. M.; Fre
J. M. J. Chem. Rev. 2001, 101, 3819.
´
chet,
2. Wang, P. W.; Lin, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S. Adv. Mater. 1996, 8, 237.
3. (a) Ma, C. Q.; Fonrodona, M.; Schikora, M. C.; Wienk, M. M.; Janssen, R. A. J.; Ba¨uerle,
P. Adv. Funct. Mater. 2008, 18, 3323; (b) Choi, M. S.; Aida, T.; Luo, H.; Araki, Y.; Ito, O.
Angew. Chem. Int. Ed. 2003, 42, 4060.
4. Albrecht, K.; Yamamoto, K. J. Am. Chem. Soc. 2009, 131, 2244.

PHENYLAZOMETHINE DENDRIMER
2247
5. Tomalia, D. A.; Naylor, A. M.; Goddard III, W. A. Angew. Chem., Int. Ed. Engl. 1990,
29, 138.
6. Higuchi, M.; Shiki, S.; Ariga, K.; Yamamoto, K. J. Am. Chem. Soc. 2001, 123, 4414.
7. (a) Cho, J. S.; Takanashi, K.; Higuchi, M.; Yamamoto, K. Synth. Met. 2005, 150, 79;
(b) Satoh, N.; Cho, J. S.; Higuchi, M.; Yamamoto, K. J. Am. Chem. Soc. 2003, 125, 8104.
8. Yamamoto, K.; Higuchi, M.; Shiki, S.; Tsuruta, M.; Chiba, H. Nature 2002, 415, 509.
9. Yamamoto, K.; Kawana, Y.; Tsuji, M.; Hayashi, M.; Imaoka, T. J. Am. Chem. Soc. 2007,
129, 9256.
10. (a) Yamamoto, K.; Imaoka, T.; Chun, W. J.; Enoki, O.; Katoh, H.; Takenaga, M.; Sonoi,
A. Nat. Chem. 2009, 1, 397; (b) Nakamula, I.; Yamanoi, Y.; Imaoka, T.; Yamamoto, K.;
Nishihara, H. Angew. Chem. Int. Ed. 2011, 50, 5830; (c) Satoh, N.; Nakashima, T.;
Kamikura, K.; Yamamoto, K. Nat. Nanotechnol. 2008, 3, 106.
11. Sasabe, H.; Kido, J. Chem. Mater. 2011, 23, 621.
12. Takanashi, K.; Chiba, H.; Higuchi, M.; Yamamoto, K. Org. Lett. 2004, 6, 1709.
13. Preut, H.; Kolev, T.; Bleckmann, P. Acta Cryst. C 1988, 44, 1864.
14. Yorke, J.; Dent, C.; Decken, A.; Xia, A. Inorg. Chem. Commun. 2010, 13, 54.
15. Barbaro, P.; Bianchini, C.; Giambastiani, G.; Rios, I. G.; Meli, A.; Oberhauser, W.;
Segarra, A. M.; Sorace, L.; Toti, A. Organometallics 2007, 26, 4639.
16. Suh, J.; Min, D. W. J. Org. Chem. 1991, 56, 5710.
17. Kim, D. H.; Kim, H. S.; Jo, W. K.; Ahn, H. G.; Kwak, C. H.; Lee, J. H.; Kim, D. H.;
Hua, R.; Chung, M. C. J. Nanosci. Nanotech. 2012, 12, 4317.
18. Sasi, S.; Sithambaresan, M.; Prathapachandra Kurup, M. R.; Fun, H.-K. Polyhedron
2010, 29, 2643.
19. Yorke, J.; Dent, C.; Decken, A.; Xia, A. Inorg. Chem. Commun. 2010, 13, 54.
20. Oso, Y.; Kanatsuki, D.; Saito, S.; Nogami, T.; Ishida, T. Chem. Lett. 2008, 37, 760.