Divergent route to the preparation of hybrid Pt-Fe 2,4,6-tris(4-ethynyl) phenyl-1,3,5-triazine metallodendrimers for nonlinear optics

Article abstract of DOI:10.1021/om300745v

The synthesis strategy for the preparation of novel platinum acetylide homometallic and heterobimetallic dendrimers (containing Fe as the other metal fragment) based on a 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine core (3) is reported. All the dendrimer generations (G0-G2) were synthesized under copper-free conditions following a divergent route. The G0-Pt dendrimer (4) was synthesized using the 1,3,5-triazine core (3) and cis-[Pt(PEt₃) ₂Cl₂] with a molar ratio of 1/4. The advantage of the current method is that different dendrimers can be prepared by following the same procedure with only changes in the molar ratios of the reactants involved. For instance, when 3 reacts with 4 in a 4/1 molar ratio, the G1 dendrimer 7 is afforded without the peripheral Pt moiety, but the G1 dendrimer with the peripheral Pt moiety (8) is formed when 3 reacts with 4 in a 1/3 molar ratio. On the other hand, the G2 dendrimer with a peripheral Pt moiety (9) is synthesized when 7 reacts with 4 in a 1/6 molar ratio. The heterobimetallic dendrimers were synthesized up to generation 1 by capping the corresponding Pt dendrimers with the ethynylferrocenyl group (EFC). The respective G0 (6)- and G1-capped (10) dendrimers were synthesized when EFC reacted with 4 and 8 in molar ratios of 9/1 and 18/1, respectively. Nonlinear optical (NLO) polarizabilities measured by hyper-Rayleigh scattering (HRS) have been evaluated for the core 3, for the G0 dendrimer 4, and for the G0 dendrimer capped with EFC (6). In spite of the fact that the stability of the higher generations in chloroform is too low to allow HRS measurements, the reported NLO results show a remarkable enhancement (plus 50%) upon capping the zero dendrimer generation (6), reflecting the importance of the introduction of electron donor organometallic capping groups in the hyperpolarizabilities of the resulting dendrimers.

Full text of DOI:10.1021/om300745v

Article
pubs.acs.org/Organometallics
Divergent Route to the Preparation of Hybrid Pt−Fe 2,4,6-Tris(4-
ethynyl)phenyl-1,3,5-triazine Metallodendrimers for Nonlinear
Optics
Swarup K. Maiti,† Manuel G. Jardim,† Joao Rodrigues,*,† Kari Rissanen,‡ Jochen Campo,§
̃
and Wim Wenseleers*,§
†
́
́
CQM-Centro de Quımica da Madeira, MMRG, Universidade da Madeira, Campus Universitario da Penteada, 9000-390 Funchal,
Portugal
‡Department of Chemistry, NanoScience Center, University of Jyvaskyla, P.O. Box 35, 40014 JYU, Finland
̈
̈
§Physics Department, University of Antwerp, Universiteitsplein 1, B2610, Antwerp, Belgium
S
* Supporting Information
ABSTRACT: The synthesis strategy for the preparation of novel platinum
acetylide homometallic and heterobimetallic dendrimers (containing Fe as the
other metal fragment) based on a 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine
core (3) is reported. All the dendrimer generations (G0−G2) were synthesized
under copper-free conditions following a divergent route. The G0-Pt
dendrimer (4) was synthesized using the 1,3,5-triazine core (3) and cis-
[Pt(PEt3)2Cl2] with a molar ratio of 1/4. The advantage of the current method
is that different dendrimers can be prepared by following the same procedure
with only changes in the molar ratios of the reactants involved. For instance,
when 3 reacts with 4 in a 4/1 molar ratio, the G1 dendrimer 7 is afforded
without the peripheral Pt moiety, but the G1 dendrimer with the peripheral Pt
moiety (8) is formed when 3 reacts with 4 in a 1/3 molar ratio. On the other hand, the G2 dendrimer with a peripheral Pt moiety
(9) is synthesized when 7 reacts with 4 in a 1/6 molar ratio. The heterobimetallic dendrimers were synthesized up to generation
1 by capping the corresponding Pt dendrimers with the ethynylferrocenyl group (EFC). The respective G0 (6)- and G1-capped
(10) dendrimers were synthesized when EFC reacted with 4 and 8 in molar ratios of 9/1 and 18/1, respectively. Nonlinear
optical (NLO) polarizabilities measured by hyper-Rayleigh scattering (HRS) have been evaluated for the core 3, for the G0
dendrimer 4, and for the G0 dendrimer capped with EFC (6). In spite of the fact that the stability of the higher generations in
chloroform is too low to allow HRS measurements, the reported NLO results show a remarkable enhancement (plus 50%) upon
capping the zero dendrimer generation (6), reflecting the importance of the introduction of electron donor organometallic
capping groups in the hyperpolarizabilities of the resulting dendrimers.
the highly polarizable π-electron system of a conjugated ligand
with the electron-donating or -accepting character of the metal
center(s). This allows for the formation of noncentrosymmetric
push−pull systems with high nonlinear polarizabilities (hyper-
polarizability β).2a−e Instead of the traditional dipolar design,
such an NLO-active complex can also have an octupolar
symmetry (e.g., an electron-accepting core symmetrically
surrounded by three conjugated arms with electron donors),
resulting in a vanishing dipole moment. This is more likely to
yield a favorable macroscopic arrangement and a better
transparency/hyperpolarizability trade-off.10 Suitably designed
dendrimers also offer the prospect of further enhanced NLO
responses by assembling multiple octupolar units in well-
defined orientations and by extending the conjugated
system.2e−i
INTRODUCTION
■
Recently, there has been tremendous interest in the synthesis
and development of well-defined functionalized nanoscopic
dendritic macromolecules with specific functions. The incor-
poration of transition metals into simple organic dendrimers
provides them with desirable functionalities. These new
materials1 offer an attractive strategy for enhancement and
modulation of material properties such as nonlinear optical
(NLO),2 liquid crystalline,3 luminescent,4 light-harvesting,5 and
electronic6 properties. Although many organometallic den-
drimers have been prepared so far, most of them contain
metallic species either only at the core7 or at the surface,8 owing
to their low stabilities and limited synthetic availability.
Dendrimers with transition-metal ions in every branching
point have so far been prepared9 less and therefore still remain
a topical challenge.
Organometallic conjugated compounds are particularly
interesting for NLO applications, such as electro-optic
modulators or frequency converters, because they combine
Received: August 3, 2012
Published: January 9, 2013
© 2013 American Chemical Society
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a
Scheme 1. Synthesis of Compounds 4−6
a
Legend: (a) 3 and cis-[PtCl2(PEt3)2] (1/4 molar ratio), toluene/piperidine (2/1, v/v), 111 °C, 48 h; (b) CHCl3/Et2NH, CuI, 60 °C, 20 h; (c)
toluene/diethylamine (1/1, v/v), CuI, 48 °C, 18 h; (d) CHCl3/Et2NH, CuI, 60 °C, 20 h. The shaded “teardrop” denotes PEt3.
Apart from the specific metal, the core group or building
block in an organometallic dendrimer plays a pivotal role in
regulating its characteristic properties. In terms of NLO
properties, octupolar organic compounds with a 1,3,5-triazine
core are expected to have larger first hyperpolarizabilities than
the analogous organic compounds with a 1,3,5-benzene core,2h
since the triazine core is more electron withdrawing and
possesses a larger nucleophilic susceptibility than the benzene
core.11 Thus, organometallic dendrimers with a 2,4,6-
trisubstituted 1,3,5-triazine core might also have interesting
physical properties: in particular, nonlinear optical properties.
Over the past decade, much attention has been focused on the
synthesis and characterization of purely organic dendrimers
containing a 1,3,5-triazine core.12 The symmetry and electronic
properties of the triazine core have made it a valued molecular
skeleton for exploring a wide range of interesting applications
such as antitumor agents,12a,13a molecular recognition,12c,e gene
therapy,13b OLEDs,13c and catalytic supports.13d However,
organometallic dendrimers with the 2,4,6-trisubstituted-1,3,5-
triazine core are extremely rare, to the best of our knowledge;
so far, only one example of a zero-generation dendrimer based
on the 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine core (3) with
Ru and Au has appeared in the literature14 and a few others
with additional N and S donors have been used to construct
supramolecular and luminescent coordination complexes.15,16
However, higher generation truly organometallic dendrimers
based on the 2,4,6-trisubstituted 1,3,5-triazine core are
uncommon,17,18 and to date they have never been prepared
with the 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine core at a
generation greater than 0. This could be due to the low
stability/solubility of the products or to severe obstacles in the
synthetic chemistry.18 Platinum−acetylide complexes have
attracted our attention, as they are not only thermally robust
and stable19 but also have the potential to provide new
nanoscopic organometallic materials with unique properties.20
The capping group plays an important role in the electronic
communication throughout the organometallic dendrimer as
well as in the NLO properties.2i Ethynylferrocene (EFC) is a
good choice, because it is a good electron reservoir and a redox-
active group.21
On the basis of our continued interest in the field of
synthesis and characterization of organometallic dendrimers,22
we present here the synthesis of novel platinum−acetylide
dendrimers up to the second generation (G2) with a 2,4,6-
tris(4-ethynyl)phenyl-1,3,5-triazine core using the divergent
strategy. Also, the zero (G0)- and first-generation (G1)
dendrimers were successfully capped with ethynylferrocene,
the so-called hybrid Pt−Fe 2,4,6-tris(4-ethynyl)phenyl-1,3,5-
triazine metallodendrimers. The second-order NLO properties
of the zero-generation dendrimers (capped and uncapped with
EFC) are reported.
RESULTS AND DISCUSSION
■
1. Synthesis. The G0 dendrimer 4 was prepared by treating
2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine (3) with cis-
[PtCl2(PEt3)2] in a 1/4 molar ratio using toluene and
piperidine as solvents (2/1, v/v) (Scheme 1). The reaction
was carried out at 111 °C for 48 h without using any Cu(I) salt
as a cocatalyst. The use of Cu salt leads to the formation of a
gelatinous precipitate, which is difficult to characterize due to
its utter insolubility.
The G1 dendrimer 7 without the peripheral metal moiety
was prepared using the triazine core (3) and the G0 dendrimer
4 at a molar ratio of 4/1. By a reaction protocol similar to that
used for the G0 dendrimer 4, when 3 and 4 were used at a
molar ratio of 1/3, another G1 dendrimer (8) with a peripheral
metal Pt moiety was generated. A similar reaction of G1
dendrimer 7 with G0 dendrimer 4 at a molar ratio of 1/6 led to
the successful formation of the G2 dendrimer 9 with a
peripheral metal Pt moiety (Scheme 2). Surprisingly, our
attempts to prepare 8 in significant yield starting from 7 using
cis-[PtCl2(PEt3)2] were less successful.
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a
Scheme 2. Synthesis of Compounds 7−10
a
Legend for 7−9 (toluene/piperidine (2/1, v/v), 111 °C, 48 h): (a) 3 and 4 (4/1 molar ratio); (b) 3 and 4 (1/3 molar ratio); (c) 7 and cis-
[PtCl2(PEt3)2] (1/9 molar ratio); (d) 7 and 4 (1/6 molar ratio). Legend for 10: (e) 8 and EFC (ethynylferrocene) (1/18 molar ratio), CHCl3/
Et2NH, CuI, 60 °C, 20 h. [Pt] = Pt1Cl; [Pt1] = trans-[Pt(PEt3)2].
The successful synthesis of G0−G2 generation dendrimers
following the divergent synthetic route tempted us to extend
this protocol toward the capping of the G0 dendrimer in an
effort to increase its NLO response. Therefore, the capping of
the G0 dendrimer was carried out with ethynylferrocene and
the successful preparation of the desired compound 6 was
accomplished by either (I) coupling the G0 dendrimer directly
with the capping group EFC or (II) coupling the platinated
EFC group23 (5) with compound 3 (Scheme 1). Method II was
cumbersome and gave lower yield after purification in
comparison to method I. Our successful results in capping
the G0 dendrimer encouraged us to extend the capping
reaction to the G1 dendrimer as well, in this case following
were capped by EFC (for further details see the Experimental
Section and the Supporting Information).
2. Characterization. 2.1. H and 31P NMR. The H NMR
spectrum of building block 3 showed two doublets at δ 8.72
and 7.69; the former signal is assignable to the ortho protons
(with respect to the triazine core) in the aryl groups and the
latter to the meta protons. In the zero-generation metal-
lodendrimer 4, the corresponding proton signals in the
aromatic region showed a slight upfield shift, arising at δ 8.60
1
1
2
and 7.40, respectively. The drifting of electrons from dz orbitals
on the Pt atom to the triazine core make the aromatic protons
more shielded in 4 in comparison to the triazine core 3. The
first (7 and 8)- and second-generation (9) dendrimers showed
1
a similar pattern for the aromatic protons in their H NMR
1
method I. The H NMR spectrum, though not well resolved,
proves that at least five or even six out of six positions of 10
spectra. The capped zero (6)- and first-generation (10)
1
dendrimers also showed a similar trend in their H NMR
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spectra. In compound 7, as the chemical and magnetic
surroundings around the internal and external aryl groups are
not identical, four different aromatic proton environments were
obtained instead of six because of coincidences; two distinct
doublets appeared for each corresponding ortho and meta
proton of the internal and external aryl groups, respectively.
11.7 (JPt, P = 2351 Hz) and 11.4 (JPt, P = 2354 Hz) in about a 1/
1 integral ratio. The latter and former signals are assigned to the
internal and the external phosphanes, respectively. The results
show that the EFC group has a relatively lower deshielding
effect than the −Cl group on the phosphane atoms. The
magnitude of the JPt, P value (∼2350−2375 ppm) in 31P NMR
spectra strongly supports a trans arrangement24 of the PEt3
group around the corresponding “Pt” atom.
1
Due to the influence of the two triazine cores, the H NMR
signals for the P(CH2CH3)3 protons appeared at higher δ
values (δ 2.24 and 1.30) in comparison with the corresponding
ethyl protons (δ 2.09 and 1.25) in 4. The presence of the
electronegative chloro group coordinated to the Pt atom in 4
made the P nucleus (in PEt3) more deshielded (δ 14.9) in
comparison to the P nucleus in 7 (δ 11.2) without the chloro
group. The six acetylenic protons in 7 were not observed due to
the presence of piperidine, which functioned as a base to
abstract these acidic protons. As a result, six piperidinium salt
molecules are visible in the form of two singlets at δ 3.14 (24H)
2.2. 13C NMR. The 13C NMR spectra are in good agreement
with the corresponding 1H NMR spectra for each compound. It
is noteworthy that upon metalation the acetylenic carbon atom
bearing the Pt moiety in compound 4 showed an 18 ppm
downfield shift. In compound 6, three responses appeared in
the region 111.2−102.0 ppm for three chemically and
magnetically different acetylenic “C” atoms, whereas four
distinct 13C responses in the region 70.2−67.0 ppm are
obtained for Cp C atoms. In compound 7, four 13C responses
for aromatic carbons bearing chemically different protons are
obtained in the region 144.2−130.9 ppm. This result well
supports the corresponding proton NMR spectrum of the aryl
protons. Compounds 8 and 9 both showed two distinct 13C
responses at around 16.6 and 14.6 ppm for the −CH2CH3
group (italics); the former is assignable to the inner and the
latter to the outer −CH2− carbon, respectively. Compound 10
showed three distinct responses at 70.1, 69.3, and 67.06 ppm
for the CH carbons of the Cp ring.
1
and 1.63 (36H) in the H NMR spectrum (for further details
see the Supporting Information).
As for 7, the 1H NMR spectrum of 8 showed four doublets at
the aromatic region for aryl protons. The proton signals for the
ethyl group were observed at δ 2.14 (m, 36H), 2.09 (m, 72H),
and 1.29 (m, 162H); the first signal is assignable to the
−CH2CH3 protons (italics) of the inner phosphane group,
whereas the second signal is attributed to the same protons but
of the corresponding ethyl groups linked to the outer
phosphane atoms, and the last signal corresponds to the
overall −CH2CH3 protons. These data are consistent with the
31P NMR spectrum of 8, which exhibited two singlets at δ 14.9
(JPt, P = 2380 Hz) and 11.2 (JPt, P = 2358 Hz) in a 2/1 integral
ratio, which correspond to the phosphane groups bound to the
six outer and three inner platinum atoms, respectively. Due to
the presence of electronegative chloro groups attached to the
terminal “Pt” atom, the terminal phosphorus atoms are more
deshielded in comparison to the inner phosphorus atoms.
Although the inner and outer phosphane groups have different
shielding effects, this difference is only visible on the adjacent
−CH2CH3 protons.
2.3. UV−Vis Spectroscopy. The UV−vis spectrum of 3 in
dichloromethane showed an intense band with λmax at 304 nm,
which can be attributed to the π → π* transition occurring
from the pheripheral aryl group to the central triazine core
(Figure 1). The Pt metallodendrimers 4, 8, and 9 each showed
Compound 9 exhibited a pattern similar to that of 8 in its 1H
and 31P NMR spectra. The number and position of aryl and
ethyl protons observed in the 1H NMR spectrum are in
agreement with the proposed structure of 9. Only a slight
deviation of integral value from that expected was observed for
−CH2CH3 protons (italics); signals for 90 protons at δ 2.33
and 162 protons at δ 2.09 were seen instead of 108 and 144
1
protons, respectively. The H NMR spectrum of first (8)- and
second-generation (9) metallodendrimers, in spite of being
large molecules with high molecular weights, indicates the
presence of a highly symmetric structure. The 31P NMR
spectrum of 9 showed two singlets with satellites at δ 14.9 (JPt, P
= 2378 Hz) and 11.2 (JPt, P = 2356 Hz) with an integral ratio of
about 16/9 instead of the expected 4/3. Apart from showing
the expected proton signals and ratios for the proposed
structure, compounds 7−9 also showed the presence of the
conjugated acid of piperidine, a byproduct of the reaction which
is extremely difficult to remove completely.
Figure 1. UV−vis spectra of compounds 3−10.
an intense band with λmax at 378, 382, and 378 nm, respectively,
which are longer than that of 3. The replacement of the
acetylenic protons of the triazine core by [Pt(PEt3)2Cl]
originates a bathochromic shift. The more ionic Pt−C bonds
in compounds 4, 8, and 9 (in comparison to the C−H bonds in
compound 3) make the π conjugation system more electron
rich and, thereby, stabilizes the excited state (LUMO) relative
to the ground state (HOMO), resulting in a smaller energy gap
(HOMO to LUMO) for electronic transition; as a result,
absorption occurs at a longer wavelength. It is noteworthy that
the increase of π conjugation on going from the zero- to the
1
The H NMR spectra of 6 and 9 showed the expected
number of proton signals and shifts. In the case of 10, the
integral value tallies with 48 EFC proton signals (although the
spectrum is not well resolved) instead of 54, indicating that at
least 5 or even 6 EFC groups are bound to the external Pt
atoms. The 31P NMR spectrum of 6 showed one signal at δ
11.2 (JPt, P = 2349 Hz), and that of 10 showed two signals at δ
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second-generation metallodendrimers does not have a sig-
nificant effect on the λmax value other than the gradual increase
of molar absorptivity (ε (M−1 cm−1) = 3485 × 102 (4), 1168 ×
103 (8), 1303 × 103 (9)). Compound 7 showed two absorption
bands with λmax at 284 and 398 nm which can be attributed to
the intraligand π → π* transition occurring in the external and
internal triazine cores, respectively. The Pt EFC compound 5
and the Pt metallodendrimers capped with EFC (6 and 10)
present an intense absorption band with λmax at 306, 384, and
392 nm, respectively; these results show a gradual increase in
the bathochromic shift upon replacement of the chloro group
in compounds 4 and 8 by ethynylferrocene (capped Pt
metallodendrimers 6 and 10).
2.4. Infrared Spectroscopy. The trans-chloroalkynylplatinum
metallodendrimers 4, 8, and 9 exhibit a CC stretching
frequency at relatively higher wavenumbers (i.e., 2113, 2095,
and 2111 cm−1, respectively) in comparison to compound 7,
where the chloro group is replaced by the acetylide group of the
triazine core. This evidence can be explained by suggesting that
the chloro group has relatively more electron withdrawing
capacity from the CC bond in comparison to the triazine
core, resulting in the higher wavenumber. Compounds 5, 6, and
10 showed similar trends. The CC stretching frequency in
compound 5 having a chloro group appeared at ν 2115 cm−1,
whereas the same group in 6 and 10, upon substitution of the
chloro group by ethynylferrocene, appeared at ν 2096 and 2093
cm−1, respectively.
(β) for the triazine core 3, the zero-generation dendrimer 4,
and the heterometallodendrimer 6 were also reported. We
found that it is possible to increase significantly the hyper-
polarizability by capping Pt metallodendrimers with ferrocene
units. This work not only introduces a novel approach to reach
target metallodendrimers by a short-cut route but also provides
a new pathway in the direction of more efficient NLO systems
based on hybrid metallodendrimers. Current work is underway
to extend the new reaction route to other metal fragments28
and to prepare more soluble and higher (hetero) metal-
lodendrimer generations.
EXPERIMENTAL SECTION
■
General Information. All the experiments were carried out under
dry nitrogen atmosphere by using standard vacuum line Schlenk
techniques. All solvents used for the reactions were distilled under a
nitrogen atmosphere and collected prior to use. Acetone was dried
over CaSO4 and distilled. Toluene and tetrahydrofuran were distilled
from sodium/benzophenone, whereas methylene chloride and chloro-
form were distilled from calcium hydride. Diethylamine and
triethylamine were distilled from potassium hydroxide. Usually, in
most of the cases, solvents were degassed using dry nitrogen
immediately prior to the reaction. Chemicals were purchased from
Sigma−Aldrich or Acros. The starting materials, compounds 2,4,6-[4-
BrC6H4]3-1,3,5-C3N3 (1),28 2,4,6-[4-(Me3SiCC)C6H4]31,3,5-C3N3
(2),29 and 2,4,6-[4-(HCC)C6H4]3-1,3,5-C3N3 (3),28 were prepared
by literature methods, and their purity was confirmed by NMR and
MS. The complex trans-[PtCl(CCC5H4FeC5H5)(PEt3)2] (5), which
structural characterization was not published, was prepared by
modifying a procedure reported by Vives et al.30 All of the new
3. NLO Studies. The NLO results obtained by hyper-
Raleigh scattering25 (HRS, with full correction for any spurious
multiphoton-induced fluorescence contributions26) for the
triazine core 3, the G0 dendrimer 4, and the G0 dendrimer
capped with EFC (6) show an increase of the NLO response,
from the core 3 to compound 6, yielding hyperpolarizabilities
(βxxx component, measured at 1550 nm) of 41 × 10−30 esu for
3, 59 × 10−30 esu for 4, and 91 × 10−30 esu for 6
1
compounds were characterized by H, 13C, and 31P NMR, UV−vis,
FTIR, and when possible, by EA and MS (in the case of compounds 7,
9 and 10, despite the variety of conditions tested, we have not
succeeded in locating the molecular ions in the mass spectra of these
metallodendrimers).
Physical Measurements. FTIR spectra were recorded with a
Nicolet Avatar 360 FTIR spectrometer, calibrated with polystyrene, in
KBr pellets; only specific bands are cited in the text. The UV−vis
spectra were recorded with a GBC-Cintra 40 UV−vis spectrometer
using a 1 cm optical path quartz cell with freshly prepared solutions
(ca. 2 × 10−4 M) in distilled methylene chloride. Elemental analyses
were carried out with a VariolEL instrument from Elementar
Analysensysteme. The mass spectra (TOF-MS) were recorded with
a Micromass LCT spectrometer. 1H, 13C{1H}, and 31P{1H} NMR
spectra were performed with a Bruker Avance II+ 400 at 299 K (probe
temperature). The chemical shifts (δ) are reported in ppm and
(corresponding to static values of 33 × 10−30, 42 × 10−30
,
and 65 × 10−30 esu, respectively), reflecting the importance of
the introduced electron-donating organometallic moieties.
Despite the use of the electron-accepting triazine core, the Pt
donor complex 4 still only yields relatively modest hyper-
polarizability (yet ∼50% higher than the free triazine core,
which itself performs comparably or even better than other
1,3,5-triazine derivatives27). However, the additional capping
with ferrocene units results in a further enhancement of β by
another ∼50% (while maintaining an excellent transparency
throughout the visible range: λmax 384 nm for 6), showing that
the two metals cooperatively result in a larger effective donor
strength at the periphery of the octupolar complex.
Unfortunately, under experimental conditions, higher gener-
ation dendrimers present limited stability in chloroform,
preventing the NLO measurements; therefore, we restricted
our NLO studies to the smaller dendrimer generation, G0
dendrimer 4, and the G0 dendrimer capped with EFC (6).
1
referenced to residual solvent peaks for H and 13C{1H} NMR. The
31P{1H} NMR is referenced to external 85% H3PO4 aqueous solution.
NLO Measurements. The molecular first hyperpolarizabilities β of
compounds 3, 4, and 6 were determined in dilute chloroform solution
by means of the hyper-Rayleigh scattering (HRS) technique,25 at the
fundamental wavelength of 1550 nm, far from resonance. Briefly, our
highly sensitive HRS setup (described in detail in ref 25) consists of a
Ti:sapphire regenerative amplifier (Spectra-Physics Spitfire), pumping
an optical parametric amplifier (Spectra-Physics OPA-800CP) tuned
to 1550 nm (pulse duration ∼2 ps, repetition rate 1 kHz, pulse energy
at the sample ∼20 μJ). A narrow spectral range around the second
harmonic wavelength is detected in parallel by means of an intensified
charge-coupled device (I-CCD) with red-sensitive photocathode,
coupled to a spectrograph, enabling fast and complete correction for
any multiphoton fluorescence background. Such a fluorescence
background was observed for compounds 3 and 4 (respectively ∼2.5
and 1.2 times the solution HRS signal after integration over the 12 nm
wide central region), underlining the necessity of this spectral analysis
of the scattered light, even for these weakly fluorescent compounds.
For both complexes 4 and 6 and the organic core 3, octupolar (D3h)
symmetry is assumed, so that the only non-zero components of the β
tensor are βxxx = −βxyy = −βyxy = −βyyx (with the x axis oriented along
CONCLUSIONS
■
In summary, we have prepared, by using a reaction strategy
under copper-free conditions coupled to a divergent synthetic
protocol, Pt metallodendrimers up to the second generation
using 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine as a core. In
addition, the synthesized metallodendrimers were capped up to
the first generation with the ethynylferrocenyl fragment, giving
the hybrid Pt−Fe 2,4,6-tris(4-ethynyl)phenyl-1,3,5-triazine
metallodendrimers 6 and 10. The first hyperpolarizabilities
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one of the three molecular arms).31,32 Chloroform itself was used as an
internal calibration standard (as described before),2c,26,33 adopting the
effective |βxxx| value of 0.49 × 10−30 esu as determined in ref 34. In this
way, the βxxx component is obtained from the equation
dissolved in degassed CHCl3 (50 mL). Then 0.130 g (0.6 mmol) of
ethynylferrocene complex (EFC) and 0.006 g of CuI (5 mol %), as
catalyst were added along with 0.5 mL of NHEt2. The solution was
heated to 60 °C for 20 h. Thereafter the solution was dried under
vacuum and chromatographed through silica gel using chloroform and
methanol (98/2−96/4) as eluents. The title compound was obtained
as a orange-red solid. Yield: 0.149 g (42%). δH (400 MHz; CDCl3):
4.21 (2H, s, C5H4−CC), 4.13 (5H, s, C5H5), 4.06 (2H, s, C5H4−
CC), 2.05 (12H, m, CH2CH3), 1.21 (18H, m, CH2CH3). δP (161.9
MHz, CDCl3) 14.46 (s, JPt,P = 2407 Hz)). δC (100.6 MHz; CDCl3):
96.0 (s, Cp−CC−Pt), 72.7 (s, C of Cp), 70.2, 69.6, and 67.1 (s, CH
of Cp and C5H4(Cp)), 14.7 (s, CH2CH3), 9.1 (s, CH2CH3). νmax
(KBr)/cm−1: 3093.20 (w), 2932.41 (w), 2875.87 (w), 2115.70 (m,
CC), 1451.68 (s), 1411.05 (m), 1252.04 (w), 1230.15 (w), 1104.82
(m), 1035.27 (s), 1003.85 (w), 922.88 (w), 813.15 (m), 768.81 (s),
733.38 (m), 522.58 (m), 486.81 (m). λmax (CH2Cl2)/nm (ε/dm3
mol−1 cm−1): 306 (28550) m/z (EI+): 676.10 ([M]+, calcd for
C24H40ClFeP2Pt 676.12), 699.11 ([M + Na]+, calcd for
C24H39ClFeP2PtNa 699.11).
Synthesis of Pt-G0 Dendrimer Capped with EFC (6). Method I. In
a well-dried Schlenk tube, 4 (0.089 g, 0.05 mmol) and
ethynylferrocene (0.098g, 0.45 mmol) were dissolved in degassed
CH3Cl (30 mL) and diethylamine (4.5 mL). After 10 min, CuI
(0.002g, 0.01 mmol, 20 mol %) was added and the mixture was heated
to 60 °C with stirring for 20 h. The solvent was removed on a rotavap,
and the crude compound was chromatographed on silica gel using
CHCl3 and MeOH (98/2−95/5) as eluents. After drying, the title
compound was obtained as an orange-red semisolid. Yield: 0.048 g
(42%).
Method II. In a well-dried Schlenk tube, 3 (0.038 g, 0.1 mmol) and
5 (0.236 g, 0.4 mmol) were dissolved in degassed toluene (4 mL) and
diethylamine (4 mL). After 10 min, CuI (0.004 g, 0.02 mmol, 20 mol
%) was added and the mixture was heated to 48 °C with stirring for 18
h. The solvent was removed on a rotavap, and the crude compound
was chromatographed on silica gel using CHCl3 and MeOH (98/2−
95/5) as eluents. After drying, the title compound was obtained as an
orange-red semisolid. Yield: 0.076 g (36%). δH (400 MHz; CDCl3):
8.61 (6H, d, J = 8 Hz, C6H4), 7.40 (6H, d, J = 8 Hz, C6H4), 4.23 (6H,
s, C5H4−CC), 4.14 (15H, s, C5H5), 4.06 (6H, s, C5H4−CC),
2.21 (36H, m, CH2CH3), 1.26 (54H, m, CH2CH3). δC (100.6 MHz;
CDCl3): 171.1 (s, C3N3), 132.2 (s, C of C6H4), 130.9 (s, CH of
C6H4), 128.8 (s, CH of C6H4), 128.6 (s, C of C6H4), 111.2 (s, CC−
Pt), 104.0 (s, Cp−CC−Pt), 102.0 (s, (triazine)CC−Pt), 70.2 (s,
CH of C5H4(Cp)), 69.4 (s, C of Cp), 68.1 (s, CH of C5H4(Cp)), 67.0
(s, C of C5H4(Cp)), 16.5 (s, CH2CH3), 8.4 (s, CH2CH3). δP (161.9
MHz, CDCl3): 11.2 (s, JPt,P = 2349 Hz). νmax (KBr)/cm−1: 3093.11
(w), 2962.24 (w), 2929.03 (w), 2874.16 (w), 2096.39 (m, CC),
1724.64 (w), 1597.66 (m), 1561.46 (m), 1501.2 (s), 1452.34 (w),
1356.60 (m), 1261.19 (w), 1173.45 (w), 1143.35 (w), 1103.89 (m),
1034.15 (m), 817.89 (m), 766.17 (m), 732.28 (w). λmax (CH2Cl2)/nm
(ε/dm3 mol−1 cm−1): 384 (23250). m/z (EI+): 2299.77 ([M + H]+,
calcd for C99H130N3P6Pt3Fe3 2300.57), 2321.75 ([M + Na]+, calcd for
C99H129N3P6Pt3Fe3Na 2322.55). Anal. Calcd for C99H129N3P6Pt3Fe3:
C, 51.70; H, 5.65; N, 1.83. Found: C, 51.94; H, 5.71; N, 1.52.
Preparation of Pt-G1 (without Metal End (7) and with Metal End
(8)) and Pt-G2 (with Metal End (9)) Metallodendrimer. To prepare
these compounds, the same procedure as for the preparation of
compound 4 was followed. Only the stoichiometric ratios were varied,
depending on the specific reactions, along with an additional
purification treatment.
(2ω)
(2ω)
N
N
S
− S
9
20
|βcomplex| =
|βeff,chloroform
|
chloroform solution
(2ω)
chloroform
xxx
xxx
S
complex
chloroform
(1)
The reported results for β are expressed in the B* convention.35 As
all of the investigated compounds absorb only at very short wavelength
(λmax 304, 378, and 384 nm for 3, 4, and 6, respectively), they were all
measured far from resonance, and the obtained hyperpolarizabilities
can be reliably extrapolated to the static limit simply using the
undamped two-level model.36
With the exception of higher dendrimer generations, all compounds
were found to be stable in solution, i.e. no decomposition was
observed in the linear absorption spectra; to limit effects of local
photodecomposition in the laser focus, the solutions were stirred
during the measurement to continuously refresh the exposed
molecules. No correction for reabsorption of the HRS light at 775
nm was necessary (as this reabsorption was completely negligible; see
Figure 1). The experimental error on β, excluding the systematic error
on the calibration value of chloroform, is estimated to be 5%.
Characterization of Compounds 1−3. 2,4,6-[4-BrC6H4]3-1,3,5-
C3N3 (1).29 Yield: 1.9 g (90%). δH (400 MHz; CDCl3; Me4Si): 8.60
(6H, d, J = 8.1 Hz, C6H4), 7.71 (6H, d, J = 8.1 Hz, C6H4). m/z (EI+):
545.00 ([M+], calcd for C21H12N3Br3 544.86).
2,4,6-[4-(Me3SiCC)C6H4]3-1,3,5-C3N3 (2).29 Yield: 2.2 g (66%).
δH (400 MHz; CDCl3): 8.67 (6H, d, J = 8.4 Hz, C6H4), 7.64 (6H, d, J
= 8.4, C6H4), 0.31 (27H, s, Si(CH3)3). m/z (EI+): 597.24 ([M+], calcd
for C36H39N3Si3 597.25).
2,4,6-[4-(HCC)C6H4]3-1,3,5-C3N3 (3).28 Yield: 0.21 g (94%). δH
(400 MHz; CDCl3): 8.72 (6H, d, J = 8.8 Hz, C6H4), 7.69 (6H, d, J =
8.4 Hz, C6H4), 3.28 (3H, s, CCH). δC (100.6 MHz; CDCl3): 172.2
(s, C3N3), 136.0 (s, C of C6H4), 132.4 (s, CH of C6H4), 128.8 (s, CH
of C6H4), 127.0 (s, C of C6H4), 84.0 (s, CCH), 80.0 (s, CCH).
νmax (KBr)/cm−1: 3290.20 (m (medium)), 3239.46 (m, CC),
1575.39 (s (strong), in-plane vibration of triazine ring), 1409.71 (w
(weak)), 1372.85 (s), 1359.10 (s), 869.25 (w), 816.23 (C−H
wagging) {658.98 (m) and 645.93 (m), out of plane ring vibration}.
λmax (CH2Cl2)/nm (ε/dm3 mol−1 cm−1): 304 (30650); m/z (EI+):
381.00 ([M+], calcd for C27H15N3 381.13).
Preparation and Analytical Data of Compounds 4−10.
Synthesis of 2,4,6-[4-(Cl(PEt3)2PtCC)C6H4]3-1,3,5-C3N3, Pt-G0
Dendrimer (4). In a well-dried Schlenk tube, 0.046 g (0.12 mmol)
of 3 and 0.271 g (0.54 mmol) of cis-[PtCl2(PEt3)2] were dissolved in
45 mL of degassed toluene/piperidine (2/1, v/v), and the solution was
refluxed (111 °C) for 48 h. The reaction solution was then vacuum-
dried. The crude compound was washed with acetone followed by
diethyl ether several times and filtered. After filtering, the title
compound was obtained as a light yellow solid. It was then further
purified through column chromatography on silica gel using benzene/
chloroform as eluents. After crystallization from a dichloromethane/
hexane solvent system, the title compound was obtained as light yellow
crystals. Yield: 0.119 g (56%). δH (400 MHz; CDCl3): 8.60 (6H, d, J =
8.4 Hz, C6H4), 7.40 (6H, d, J = 8.4 Hz, C6H4), 2.09 (36H, m,
CH2CH3) and 1.25 (54H, m, CH2CH3). δC (100.6 MHz; CDCl3):
171.1 (s, C3N3), 133.1 (s, C of C6H4), 130.9 (s, CH of C6H4), 128.6
(s, CH of C6H4), 102.5 (s, CC−Pt), 88.2 (s, CC−Pt), 14.5 (s,
CH2CH3) and 8.1 (s, CH2CH3). δP (161.9 MHz, CDCl3) 14.9 (s, JPt,P
= 2378 Hz). νmax (KBr)/cm−1: 2964.45 (m), 2876.10 (w), 2113.37 (s,
CC), 1598.57 (s), 1562.48 (s), 1507.27 (s), 1454.71 (w), 1405.03
(m), 1359.35 (s), 1257.8 (w), 1173.75 (m), 1144.89 (w), 1034.94
(m), 818.95 (s), 766.73 (s), 734.29 (m). λmax (CH2Cl2)/nm (ε/dm3
mol−1 cm−1): 378 (34850). m/z (EI+): 1779.59 ([M + H]+, calcd for
C63H103Cl3N3P6Pt3 1779.46). Anal. Calcd for C63H103Cl3N3P6Pt3: C,
42.53; H, 5.78; N, 2.36. Found: C, 43.02; H, 6.03; N, 3.24.
Additional Purification Treatment. Each dry compound was stirred
in a round-bottom flask in an acetone and methanol mixture for about
12 h. Most of the piperidinium salt mixed with the desired compound
passes to the methanolic solution. The solid part was filtered off on a
gouche filter and washed thoroughly with acetone and diethyl ether
and then vacuum-dried. These compounds are very difficult to purify
through column chromatography, as they get stuck near the baseline
unless a solvent with very high polarity is used for elution. As a result,
other undesired compounds also come out along with the desired one.
Therefore, precipitation is the only tool to purify the desired
Synthesis of trans-[PtCl(CCC5H4FeC5H5)(PEt3)2] (5).30 In a well-
dried Schlenk tube, 0.302 g (0.6 mmol) of trans-[PtCl2(PEt3)2] was
411
dx.doi.org/10.1021/om300745v | Organometallics 2013, 32, 406−414

Organometallics
Article
compounds. Initially these compounds were purified by the
precipitation method using dichloromethane and hexane as solvents.
Finally, several reprecipitations of each compound from the dichloro-
methane/hexane solvent system afforded an almost pure compound
(for further information regarding the purification process, see the
Supporting Information).
washed with acetone followed by diethyl ether several times.
Afterward, the compound was stirred in a round-bottom flask in an
acetone and methanol mixture for about 12 h. The solid part was
filtered off on a gouche filter and washed thoroughly with acetone and
diethyl ether and then vacuum-dried. The title compound was a
brown-red solid. Yield: 0.026 g (38%). δH (400 MHz; CDCl3): 8.59
(24H, m, C6H4), 7.41 (24H, m, C6H4), 4.23, 4.14, and 4.06 (appeared
as a broad band, 48H, Cp ring), 2.18 (108H, m, CH2CH3), 1.23
(162H, m, CH2CH3). δC (100.6 MHz; CDCl3): 171.1 (s, C3N3), 133.1
(s, C of C6H4), 130.9 (s, CH of C6H4), 128.6 (s, CH of C6H4), 111.2
(s, CC), 104.0 (s, CC), 70.1 (s, CH of C5H4(Cp)), 69.3 (s, C of
Cp), 67.1 (s, CH of C5H4(Cp)), 16.5 (s, CH2CH3), 8.2 (s, CH2CH3).
δP (161.9 MHz, CDCl3) 11.7 (s, JPt,P = 2351 Hz), 11.4 (s, JPt,P = 2354
Hz). νmax (KBr)/cm−1: 2950−2875 (3 weak peaks), 2093.63 (m, C
C), 1598.02 (m), 1562.14 (m), 1501.19 (s), 1404.65 (w), 1358.63
(m), 1175.00 (w), 1135.0 (w), 818.18 (m), 865.00 (w). λmax
(CH2Cl2)/nm (ε/dm3 mol−1 cm−1): 392 (52600). Anal. Calcd for
C288H372Fe6N12P18Pt9: C, 52.01; H, 5.64; N, 2.53. Found: C, 49.68; H,
5.37; N, 3.06 (although these results are outside the range viewed as
establishing analytical purity, they are provided to illustrate the best
values obtained to date).
Synthesis of Pt-G1 without Metal End (7). Starting from 3 (0.091
g, 0.24 mmol) and 4 (0.091 g, 0.06 mmol), the compound was
obtained as a light orange-yellow solid. Yield: 0.101 g (60%). δH (400
MHz; CDCl3): 8.62 (12H, d, J = 6 Hz, C6H4), 8.58 (12H, d, J = 8 Hz,
C6H4), 7.44 (12H, d, J = 5.2 Hz, C6H4), 7.31 (12H, d, J = 8.4 Hz,
C6H4), 2.24 (36H, m, CH2CH3), 1.30 (54H, m, CH2CH3). δC (100.6
MHz; CDCl3): 171.1 (s, C3N3), 171.0 (s, C3N3), 133.4 (s, C of C6H4),
133.0 (s, C of C6H4), 131.4 (s, CH of C6H4), 130.9 (s, CH of C6H4),
128.6 (s, C of C6H4), 111.2 (s, CC−Pt), 98.2 (s, CC, CC−Pt),
16.5 (s, CH2CH3), 8.4 (s, CH2CH3). δP (161.9 MHz, CDCl3): 11.2 (s,
JPt,P = 2358 Hz). νmax (KBr)/cm−1: 2948.26 (w), 2808.81 (w), 2513.00
(w), 2413.53 (w), 2092.27 (s, CC), 1632.33 (m), 1596.62 (m),
1563.64 (m), 1501.00 (s), 1359.47 (s), 1211.75 (m), 1143.84 (w),
1115.39 (m), 1032.12 (w), 933.58 (w), 818.23 (m), 766.82 (w). λmax
(CH2Cl2)/nm (ε/dm3 mol−1 cm−1): 398 (49690). Anal. Calcd for
C144H144N12P6Pt3: C, 61.47; H, 5.16; N, 5.97. Found: C, 58.43; H,
5.97; N, 6.75 (although these results are outside the range viewed as
establishing analytical purity, they are provided to illustrate the best
values obtained to date).
Synthesis of Pt-G1 Dendrimer with Metal End (8). Starting from 3
(0.015 g, 0.039 mmol) and 4 (0.210 g, 0.117 mmol), the compound
was obtained as a light yellow solid. Yield: 0.144 g (66%). δH (400
MHz; CDCl3): 8.62 (12H, m, C6H4), 8.60 (12H, m, C6H4), 7.44
(12H, d, J = 7.6 Hz, C6H4), 7.40 (12H, d, J = 7.4 Hz, C6H4), 2.24
(36H, m, inner CH2CH3), 2.09 (72H, m, outer CH2CH3), 1.29
(162H, m, CH2CH3). δC (100.6 MHz; CDCl3; Me4Si): 171.1 (s,
C3N3), 133.1 (s, C of C6H4), 130.9 (s, CH of C6H4), 128.6 (s, C and
CH of C6H4), 111.2 (s, CC−Pt), 98.2 (s, CC−Pt), 16.6 (s, inner
CH2CH3), 14.6 (s, outer CH2CH3), 8.2 (s, CH2CH3). δP (161.9 MHz,
CDCl3): 14.9 (s, JPt,P = 2380 Hz, outer P), 11.2 (s, JPt,P = 2358 Hz,
inner P). νmax (KBr)/cm−1: 2964.23 (w), 2932.79 (w), 2875.87 (w),
2095.02 (s, CC), 1633.08 (s), 1588.05 (s), 1563.06 (s), 1509.72 (s),
1404.21 (m), 1366.87 (s), 1240.83 (w), 1173.72 (m), 1144.38 (m),
1034.00 (s), 864.20 (w), 849.49 (w), 817.98 (s), 766.78 (m), 733.60
(m). λmax (CH2Cl2)/nm (ε/dm3 mol−1 cm−1): 382 (116800). m/z
(EI+): 5466.07 ([M − 4Cl]+, calcd for C216H318Cl2N12P18Pt9 5466.68).
Anal. Calcd for C216H318Cl6N12P18Pt9: C, 46.25; H, 5.71; N, 3.0.
Found: C, 46.65; H, 5.68; N, 3.35. Our attempts to prepare 8 from 7
using cis-[PtCl2(PEt3)2] in a molar ratio of 1/9 gave compound 8 but
with decreased yield.
Synthesis of Pt-G2 Dendrimer with Metal End (9). Starting from 7
(0.04 g, 0.014 mmol) and 4 (0.151 g, 0.0852 mmol), the compound
was obtained as a yellow solid. Yield: 0.103 g (54%). δH (400 MHz;
CDCl3): 8.60 (60H, d, J = 8.4 Hz, C6H4), 7.44 (24H, d, J = 6.8 Hz,
C6H4), 7.39 (d, J = 8.0 Hz, 24H, C6H4), 7.30 (12H, d, J = 8.4 Hz,
C6H4), 2.23 (90H, m, inner CH2CH3), 2.09 (162H, m, outer
CH2CH3), 1.23 (378H, m, CH2CH3). δC (100.6 MHz; CDCl3): 171.1
(s, C3N3), 133.1 (s, C of C6H4), 130.9 (s, CH of C6H4), 128.6 (s, CH
of C6H4), 123.3 (s, C of C6H4), 111.1 (s, CC−Pt), 102.5 (s, CC−
Pt), 88.1 (s, CC), 16.5 (s, inner CH2CH3), 14.6 (s, outer CH2CH3),
8.2 (s, CH2CH3). δP (161.9 MHz, CDCl3): 14.9 (s, JPt,P = 2378 Hz,
outer P), 11.2 (s, JPt,P = 2356 Hz, inner P). νmax (KBr)/cm−1: 2933.24
(w), 2963.98 (w), 2875.85 (w), 2111.41 (m, CC), 1632.32 (w),
1603.41 (s), 1597.65 (s), 1508.49 (s), 1453.25 (w), 1404.34 (m),
1363.87 (s), 1293.33 (w), 1242.85 (w), 1174.19 (m), 1144.61 (w),
1115.99 (w), 1034.72 (m), 850.86 (w), 818.47 (s), 766.85 (s), 733.79
(m). λmax (CH2Cl2)/nm (ε/dm3 mol−1 cm−1): 378 (130300).
Synthesis of Pt-G1 Dendrimer with Capping by EFC (10). In a
Schlenk tube, compound 8 (0.06 g, 0.01 mmol) and ethynylferrocene
(0.040 g, 0.18 mmol, 18 equiv with respect to Pt-G1 dendrimer (8))
were dissolved in a mixture of degassed chloroform (20 mL) and
diethylamine (3 mL). After 10 min, CuI (0.001 g, 0.005 mmol, 50 mol
%) was added and the mixture heated to 60 °C with stirring for 20 h.
The compound was dried after removing the solvent on a rotavap and
ASSOCIATED CONTENT
■
S
* Supporting Information
Text regarding the purification process of higher dendrimer
1
generations and figures giving H, 13C, and 31P NMR and MS
spectra of the compounds prepared in this paper. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: joaor@uma.pt (J.R.); wim.wenseleers@ua.ac.be
(W.W.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We greatly appreciated the financial support of Fundaca
■
̧
o para a
̃
Cien
̂
cia e a Tecnologia (FCT) for the purchase of NMR and
mass spectrometers with funds from POCI 2010 (FEDER) and
the Portuguese Government (Contracts PTNMR-REDE/
1517/RMN/2005 and RNEM-REDE/1508/REM/2005), as
well as for the research project PTDC/QUI/64202/2006 and
the pluriannual base funding of CQM (PEst-OE/QUI/
UI0674/2011). M.G.J. also thanks the FCT for a Ph.D. grant
(SFRH/BD/65036/2009). K.R. acknowledges support from
the Academy of Finland (project nos. 130629, 122350,
140718). Financial support from the Fund for Scientific
Research−Flanders (FWO; projects G.0129.07 and
G.0206.12) is gratefully acknowledged. J.C. is a postdoctoral
fellow of the FWO.
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