Non-conventional synthesis and photophysical studies of platinum(ii) complexes with methylene bridged 2,2′-dipyridylamine derivatives

Article abstract of DOI:10.1039/c8dt03912g

Methylene bridged 2,2′-dipyridylamine (dpa) derivatives and their metal complexes possess outstanding properties due to their inherent structural flexibility. Synthesis of such complexes typically involves derivatization of dpa followed by coordination on metals, and may not always be very efficient. In this work, an alternative synthetic approach, involving the derivatization step after-rather than prior to-coordination of dpa on metal center, is proposed and applied to synthesis of a number of platinum(ii) complexes with substituted benzyldi(2-pyridyl)amines. Comparison with the more conventional synthetic route reveals greater efficiency and versatility of the proposed approach. The obtained complexes are not luminescent in solution at room temperature, but display blue phosphorescence emission (ca. 415 nm) with the lifetimes of μs order in glassy matrix at 77 K, with additional green (ca. 485 nm) and relatively long living (τ = 3.7 ms) emission in the case of iodine substituted derivative.

Full text of DOI:10.1039/c8dt03912g

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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DOI: 10.1039/C8DT03912G
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ARTICLE
Non-conventional Synthesis and Photophysical Studies of
Platinum(II) Complexes with Methylene Bridged 2,2′-
Dipyridylamine Derivatives†
Received 00th January 20xx,
Accepted 00th January 20xx
Evgeny Bulatov and Matti Haukka*
DOI: 10.1039/x0xx00000x
Methylene bridged 2,2′-dipyridylamine (dpa) derivatives and their metal complexes possess outstanding properties due to
their inherent structural flexibility. Synthesis of such complexes typically involves derivatization of dpa followed by
coordination on metals, and may not always be very efficient. In this work, an alternative synthetic approach, involving the
derivatization step after – rather than prior to – coordination of dpa on metal center, is proposed and applied to synthesis
of a number of platinum(II) complexes with substituted benzyldi(2-pyridyl)amines. Comparison with the more conventional
synthetic route reveals greater efficiency and versatility of the proposed approach. The obtained complexes are not
luminescent in solution at room temperature, but display blue phosphorescence emission (ca. 415 nm) with the lifetimes of
µs order in glassy matrix at 77 K, with additional green (ca. 485 nm) and relatively long living (τ = 3.7 ms) emission in the
case of iodine substituted derivative.
www.rsc.org/
complicated synthesis, which often involves transition metal
catalyzed cross-coupling reactions (Scheme 1, left).
Non-conjugated derivatives, in which dpa moiety is connected
with an aliphatic carbon atom, represent another family of dpa
Introduction
Profound flexibility and chelating capabilities of 2,2′-
dipyridylamine (di(2-pyridyl)amine, dpa) have stimulated
excessive studies of its coordination chemistry,1 as well as
incorporation of this moiety as a metal binding center into more
complex organic materials.2 The latter is usually achieved by
connecting dpa with an organic fragment via the amino
nitrogen atom, and the resulting derivatives can be conjugated
or non-conjugated (Scheme 1). Such organic molecules can be
applicable in organic light emitting devices,3–6 while their ability
to bind various transition metals through the dpa moiety allows
their use as various luminescent sensors.7,8 Since dpa complexes
of platinum(II) and palladium(II) have been found to possess
cytotoxic activities comparable to or higher than cisplatin,9–11
complexes with dpa derivatives have been considered as
potential antitumor agents.12–15
based materials. The number of studies on alkyl-substituted
di(2-pyridyl)amines is rather limited,12,13 whereas methylene
bridged aromatic derivatives receive more attention recently
(Scheme 1, right). Inherent flexibility provided by the methylene
linker not only expands the range of possibilities for various
supramolecular motifs in solid state, as studied by L. Lindoy et
al.,22–24 but also allows for non-trivial reactivity25 and
luminescent properties26,27 of these compounds and their metal
complexes. Compared to their conjugated counterparts,
methylene bridged dpa derivatives are much easier to
synthesize from dpa and corresponding halides via nucleophilic
substitution reaction in presence of a base. Corresponding
metal complexes can be then obtained by coordinating the dpa
fragment on metals using conventional methods of
coordination chemistry.
Variety of conjugated derivatives and their metal complexes, in
which the amino nitrogen atom of dpa is directly connected
with an aromatic system, have been synthesized and studied by
the group of S. Wang6,16–18 and more recently by K.-J. Wei et
al.19–21 Despite their attractive luminescent properties, real life
application of such materials can be hindered by the
In this work we demonstrated, that the aforementioned
conventional synthesis of methylene bridged dpa derivatives
from dpa and corresponding halides does not always provide
sufficient yields, and proposed an alternative synthetic
Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä,
Finland. E-mail: matti.o.haukka@jyu.fi
† Electronic Supplementary Information (ESI) available: additional details and
discussion on syntheses and side products, photocyclization of 1, phosphorescence
excitation and variable temperature time resolved emission spectra of [7-10]∙Br2,
1H NMR spectra of all compounds, and crystallographic data. CCDC 1868987-
1868999. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x
Scheme 1 Conjugated and non-conjugated dpa derivatives and their syntheses.
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approach, which involves the derivatization step after – rather atoms instead of the amino nitrogen atom maVyiewtaAkrteiclepOlnalcinee
DOI: 10.1039/C8DT03912G
than prior to – coordination of dpa on metal center. This (Scheme 2b).
approach makes the nucleophilic substitution reaction easier Inspired by reports of various transition metal complexes
and more tolerant to various substituents in the halides, and containing deprotonated di(2-pyridyl)amide (dpa-H)- ligands,30–
33
allows more efficient synthesis of series of dpa based metal
we suggested that use of such complexes in the benzylation
complexes with various organic backbones. Efficiency and reaction (Scheme 2c) could suppress the aforementioned
versatility of the proposed synthetic approach were undesired side processes. Thus, use of a base would be
demonstrated on synthesis of series of substituted benzyldi(2- unnecessary in the reaction since the amino nitrogen atom of
pyridyl)amines and their platinum(II) complexes. In addition, dpa is already deprotonated. Therefore, deprotonation of the
the products were structurally characterized and their methylene group of the benzyl halide would be avoided. The
photophysical properties were investigated.
Results and discussion
1. Synthesis
pyridine nitrogen atoms would be protected from benzylation
by metal center. Moreover, such approach would allow more
efficient synthesis of complexes of a particular metal with
various non-conjugated dpa-derived ligands, which is
a
common research practice (for examples, see refs.22,23,25,27,29).
Using the proposed method, such series of complexes could be
easily obtained in one step from the (dpa-H)- containing
complex and corresponding halides. The approach of
derivatization of coordinated ligands has been applied, for
example, to synthesis of various bipyridine and terpyridine
complexes of ruthenium(II) and named as “Organic chemistry of
coordination compounds”34 or “Chemistry on the complex”.35
We decided to test the proposed approach on synthesis of
substituted benzyldi(2-pyridyl)amines 1-3 with substituents R =
H, I, and NO2 in ortho-position of the phenyl ring as
representatives of methylene bridged dpa derivatives with
various electronic properties of the substituents. Metal center
was chosen to be platinum(II), which is often used in
development of phosphorescent materials due to its strong
spin-orbit coupling;36 complexes of platinum(II) with dpa-based
ligands are also considered as potential anticancer agents.12–15
In addition, synthesis of [Pt(dpa-H)2]33 and methylation of a
structurally related bis(imidoylamidinate)platinum(II) complex
[Pt{NH=C(Ph)-NC(Ph)=NPh}2],37 which can be seen as prototype
for the dpa benzylation reaction, have been published
previously (Scheme 3).
As mentioned above, the methylene bridged dpa derivatives are
usually obtained by nucleophilic attack of the amino nitrogen
atom of dpa on benzyl halides in presence of a base (Scheme
2a). However, several side processes can diminish the reaction
yield; for example, the reported yields of benzyldi(2-
pyridyl)amine vary from as high as 82%28 to as low as 16%.22 As
we demonstrated in this work, the reaction becomes
particularly problematic, when an electron withdrawing group
is introduced in the phenyl ring (the same trend can also be
found in another recent publication29). In such case, the
methylene group can be easier deprotonated by the base due
to stabilization of the resulting anion, leading to undesired side
reactions. Additionally, benzylation of the pyridine nitrogen
Scheme 3 Known reactions of platinum(II) complexes, which inspired this
work: (a) synthesis of [Pt(dpa-H)2],23 (b) methylation of [Pt{NH=C(Ph)-
NC(Ph)=NPh}2].26
Scheme 2 Reaction between dpa and benzyl halides: (a) target reaction, (b)
undesired side reactions, (c) proposed reaction with (dpa-H)- complexes.
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The target benzyldi(2-pyridyl)amines 1-3 and platinum(II) drastically affect the yields, in contrast with the VcieownAvreticnletiOonnlinael
DOI: 10.1039/C8DT03912G
complexes 4-10 were synthesized using both conventional synthesis. Lesser reaction steps are also important for a
(benzylation prior to coordination) and alternative (benzylation practical synthesis. The conventional method required 9 steps
after coordination) approaches (Scheme 4). Below particular to synthesize complexes [7-9]∙(OTf)2 (3 reaction steps per
details of syntheses are discussed in detail and comparison complex), and many of them required individual approach due
between the two approaches is presented.
to variable properties (such as solubility and crystallizability) of
the derivatives. On the other hand, starting from [Pt(dpa-H)2],
the alternative method took only 3 steps to synthesize these
complexes as bromides using very similar reaction conditions
and workup. Moreover, any other derivative could be obtained
just in one simple step using this method, as demonstrated by
synthesis of [10]∙Br2.
These results demonstrate, that the alternative approach can
be very convenient in synthesis of platinum(II) complexes with
various methylene bridged dpa ligands. Moreover, even though
separate tests were not performed, we believe that this
approach can also be utilized for synthesis of platinum(II)
complexes with only one methylene bridged dpa ligand.
1.1 Conventional synthetic route. Benzyldi(2-pyridyl)amines 1
and 2 were synthesized by adapting published procedure22 via
nucleophilic attack of dpa on corresponding benzyl bromides in
presence of sodium hydroxide. However, synthesis of 3 using
the same technique failed due to enhanced methylene group
reactivity (for discussion and identification of side products, see
section S1 in the ESI†), and consequently two step synthesis38
implying water-free conditions and sodium hydride as a base
was performed.
The obtained compounds were then coordinated on
platinum(II) by adapting known procedures33 in two steps. First,
diiodide complexes 4-6 were obtained by reacting K2PtCl4 with
one equivalent of 1-3 and excess of KI in water. The homoleptic
complexes [7-9]∙(OTf)2 were then obtained by reacting the
diiodide complexes with another equivalent of the
corresponding ligand 1-3 in acetonitrile in presence of silver(I)
triflate.
1.2 Alternative synthetic route. Neutral complex [Pt(dpa-H)2]
was synthesized using published procedure.33 Reaction of this
complex with corresponding benzyl bromides afforded target
homoleptic complexes [7-9]∙Br2. Additionally, complex [10]∙Br2
containing 2-cyanobenzyldi(2-pyridyl)amine ligands was
synthesized with ease using this method.
Table 1 Yields of complexes 7-10 in the two synthetic approaches
Overall yield (yields of individual stages), %
Complex
Conventional synthesisa
3 (26 x 82 x 14)
13 (44 x 48 x 61)
0.3 (9 x 20 x 15)
–
Alternative synthesisb
37 (38 x 97)
7
8
24 (38 x 64)
9
32 (38 x 85)
10
30 (38 x 79)
a
Overall yields were obtained by multiplication of the individual yields of the
corresponding stages in the following order: dpa → ligands (1-3), ligands (1-3) →
diiodide complexes (4-6), and diiodide complexes (4-6) → homoleptic complexes
([7-9]∙(OTf)2).
Yields of the two syntheses are compared in Table 1. Clearly, the
alternative synthetic route provides higher overall yields of the
homoleptic complexes 7-10 due to lower number and higher
yields of the individual steps. In addition, introduction of the
electron withdrawing groups in the benzyl moiety does not
b Overall yields were obtained by multiplication of the yields of the corresponding
individual stages in the following order: dpa → [Pt(dpa-H)2], [Pt(dpa-H)2] →
homoleptic complexes ([7-10]∙Br2).
Scheme 4 Synthesis of benzyldi(2-pyridyl)amines 1-3 and their platinum(II) complexes 4-10: the conventional (top) and alternative (bottom) approaches.
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This suggestion is supported by the obtained side product
2-cyanobenzyldi(2-pyridyl)amine-dibromoplatinum(II) (S4) in
synthesis of [10]∙Br2 (see section S2 in the ESI† for details).
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DOI: 10.1039/C8DT03912G
2. Characterization of the products
1
In this section, crystal structures and general H NMR spectral
features of all the obtained benzyldi(2-pyridyl)amines 1-3 and
platinum(II) complexes 4-10 are discussed.
Benzyldi(2-pyridyl)amines 1 and 2 crystallize in monoclinic space
group P21/n, and 3 crystallizes in triclinic P-1 space group. In
crystal structure of 1 pyridyl rings are in cis-trans conformation,
whereas substituted derivatives 2 and 3 possess cis-cis
conformation (using notation from ref.1), and orientation of the
phenyl ring in 2 and 3 is such that the substituents are turned
outward from the pyridyl rings (Fig. 1). Halogen bond Npy...I
(d(Npy...I) = 3.29 Å, ∠(Npy...I–C) = 166°) in crystal structure of 2
accounts for formation of the halogen bonded dimers (Fig. 2).
1H NMR spectra of 1-3 possess a number of multiplet signals in
aromatic area (8.3 – 6.8 ppm) corresponding to the hydrogen
atoms of pyridyl and phenyl rings, and one singlet signal at 5.8
– 5.4 ppm corresponding to the methylene hydrogen atoms.
Diiodide complexes 4 and 6 crystallize in triclinic P-1 space
group, whereas 5 crystallizes in monoclinic space group P21/n.
In all complexes, the metal center is coordinated by the dpa
derived ligands via pyridine nitrogen atoms (d(Pt–N) = 2.03 –
2.06 Å, ∠(N–Pt–N) = 84.9 – 85.3°), forming six-membered
chelate rings in boat conformation, with 50.6 – 53.5° dihedral
angles between the pyridine planes. Such coordination is
common for platinum(II) complexes with methylene bridged
dpa derivatives.12,13 Two iodide ligands complete slightly
distorted square planar configuration of the platinum metal
centers with Pt–I bond lengths 2.58 – 2.60 Å and I–Pt–I angles
91.0 – 91.9°. In crystal structure of 5, the phenyl iodine atom I3
is oriented towards platinum center, with Pt...I distance of 3.94
Å. In 6, however, the nitro group is oriented away from platinum
center (Fig. 3).
Fig. 2 Halogen bonded dimers of 2.
position. The other aromatic and methylene proton signals of
the complexes resemble those of the corresponding ligands.
Homoleptic complexes 7-10 form crystals with different shapes
depending on counter ion. In order to obtain high quality
crystals, 7,8,10 were crystallized as triflates, whereas 9 was
crystallized as a bromide. [7,8]⸱(OTf)2 and [9]⸱Br2 crystallize
in monoclinic space group C2/c ([9]⸱Br2 was crystallized as
tetrahydrate), and [10]⸱(OTf)2 crystallizes in triclinic P-1 space
group as dihydrate. Complexes 7-10 are centrosymmetric in
solid state with platinum atoms at the inversion centers. Thus,
asymmetric units in 7-10 contain platinum center with one
ligand and one counter ion (and two and one water molecule in
[9]⸱Br2 and [10]⸱(OTf)2 accordingly). Platinum centers
possess square planar configuration with Pt–N bond lengths
2.01 – 2.02 Å and N–Pt–N angles 85.4 – 86.4°, and the six-
membered chelate rings adopt boat conformation (dihedral
angles between the pyridine planes 46.0 – 55.7°). Orientation of
the phenyl ring in [8]⸱(OTf)2 is disordered between two
positions: with iodine atom directed towards and away from
platinum center. Nitro group in [9]⸱Br2 and cyano group in
[10]⸱(OTf)2 are oriented towards platinum center with
d(O2...Pt) = 3.27 Å in the former and d(N4...Pt) = 3.39 Å in the
latter (Fig. 4).
1H NMR spectra of 4-6 show downfielded (9.4 – 9.5 ppm)
aromatic signal possessing characteristic broadened satellites
due to splitting from 195Pt nucleus (3JH-Pt = 48 – 64 Hz), which can
therefore be assigned to pyridine hydrogen atoms in 6th
Fig. 1 Crystal structures of 1-3.
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DOI: 10.1039/C8DT03912G
Fig. 3 Crystal structures of 4-6.
1H NMR spectra of 7-10 show that hydrogen atoms of the two upfield compared with the precursor diiodide complexes, and
ligands are pairwise magnetically equivalent in all complexes. the satellites due to splitting from 195Pt can barely be observed
Signal of the pyridine hydrogen atoms in 6th position is shifted among the other aromatic signals.
Fig. 4 Crystal structures of 7-10. Anions and solvent molecules are omitted for clarity.
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3. Optical spectroscopy
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DOI: 10.1039/C8DT03912G
Absorption spectra of 1-3 in methanol are presented in Fig. 5,
and their features are summarized in Table S1 in the ESI†. The
lowest energy absorption bands of the ligands can be assigned
to the π-π* transitions within dpa moiety.27 Studies of
photophysical properties of benzyldi(2-pyridyl)amines 1-3 are
complicated due to their ability to undergo photocyclization in
solution.39,40 While this process is slow enough to allow the
measurement of absorption spectra, strong luminescence of
the products and intermediates of the photocyclization
interferes with the emission spectra of 1-3 in solution (for
details, see section S3 in the ESI†). For this reason, studies of
luminescent properties of the ligands were not included in this
work. We have previously studied luminescent behavior of dpa
hydrochloride, in which protonation of the pyridine nitrogen
atoms suppresses the reaction of photocyclization in solution.41
Analogous studies on luminescence of ligands 1-3 and other
methylene bridged dpa derivatives are underway in our
research group.
Absorption and photoluminescence emission spectra of [7-
10]∙Br2 are presented in Fig. 6 and summarized in Table 2. The
lowest energy absorption bands of the complexes and the
ligands possess similar wavelengths (λmax ≈ 310 nm and 305 nm
accordingly) and extinction coefficients (ε ≈ (13–16)∙103 l∙mol-
1∙cm-1). The latter observation may indicate change in the
nature of the lowest electronic transition upon coordination of
the ligands to the platinum center. Complexes [7-10]∙Br2
contain two ligands per molecule, and in the case of pure
intraligand π-π* transitions approximately twice higher
absorptivity would be expected for the complexes compared to
the ligands.42 In addition, previously published computational
studies on platinum(II) complexes with dpa derived ligands
revealed a significant contribution of the metal center in the
frontier orbitals.15 Therefore, the low energy absorptions in [7-
10]∙Br2 are assigned to π-π* transitions within dpa moiety with
an additional metal-to ligand charge transfer contribution.‡
Solutions of [7-10]∙Br2 are not luminescent in methanol at room
temperature. Quenching of luminescence of platinum(II)
complexes is usually attributed to thermal population of
relatively low lying metal centred excited states, leading to
Fig. 6 Absorption at room temperature (solid lines) and phosphorescence emission
(dashed lines) spectra of [7-10]∙Br2 in methanol/ethanol solution (C = 20 μM) at 80
K. Emission spectra were measured under excitation at λex = 320 nm, delay 20 ns,
gate 10 μs (30 μs for [8]∙Br2).
Table
2 Absorption (methanol solution, RT) and phosphorescence emission
(methanol/ethanol matrix, 80 K, λex = 320 nm) properties of [7-10]∙Br2
ε 103,
Complex
λabs, nm
λem, nm
τ1, μs
τ2, μs
l∙mol-1∙cm-1
31.0
244
313
244
[7]∙Br2
407
417
78 ± 4
≤ 6b
200 ± 3
13.4
30.6
39 ± 1
3700 ±
60c
[8]∙Br2
312
13.3
470/500a
≤ 90b
243
308
240
310
35.8
13.8
37.6
14.9
[9]∙Br2
425
≤ 5b
45 ± 1
422/448/
480a
160 ±
20
[10]∙Br2
21 ± 1
a bands with multiple local maxima;
b lifetimes too short for reliable determination using the applied gate times;
c
measured at an equilibrium state, since the sample was excited by the laser at
100 Hz pulse repetition rate. Therefore, the real value may be slightly lower.
distortion of coordination geometry and consequent non-
radiative relaxation.43 In order to observe the emission,
solutions of the complexes in methanol/ethanol mixture (1:4,
v/v)44 were cooled down to 80 K to form semi-rigid transparent
glasses.
Emission properties of complexes [7-10]∙Br2 share some
common features. Thus, all complexes display emission bands
with λmax = 407–425 nm, and a good match was observed
between the absorption (Fig. 6) and excitation (Fig. S6 in the
ESI†) spectra. Analysis of the time-resolved spectra (see section
S4 in the ESI†) reveals biexponential decays of these emission
bands. In all cases, the initial fast decay is followed by the slower
one, characterized by lifetimes τ1 and τ2 accordingly in Table 2.
The obtained values allow assignment of the observed emission
to phosphorescence.
However, significant differences are apparent from the
emission spectra of [8]∙Br2, possessing additional shoulder at
λmax ≈ 475 nm, and [10]∙Br2, possessing additional peaks at λmax
= 448 and 480 nm. While in the latter case the emission
spectrum only slightly changes with time at 80 K (Fig. S7 in the
ESI†), the two emission bands of [8]∙Br2 at λmax = 417 and
Fig. 5 Absorption spectra of ligands 1-3 (C = 50 μM) in
methanol.
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470/500 nm possess strikingly different lifetimes of ca. 90 μs serve as an example of the proposed non-conventional
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and 3.7 ms accordingly. Time dependence of emission spectrum approach towards synthesis of meDtaOlI: 10c.o10m3p9/lCex8eDsT03w91i2tGh
of [8]∙Br2 is illustrated on Fig. 7 and Fig. S8 in the ESI†.
methylene bridged dpa derivatives. This approach involves
At higher temperatures small red shift of all of the emission derivatization as the last step, when the dpa moiety is already
peaks is accompanied by decrease of lifetimes and emission coordinated on metal center, therefore allowing efficient one-
intensity until complete disappearance at 140 – 150 K. step synthesis of multiple complexes, and at the same time
Temperature dependencies of the observed rate constants preventing undesired side reactions. The compounds obtained
reveal plateaus in most of cases at temperatures close to 80 K, in this work possess conformational flexibility due to the
indicating that the temperature-dependent non-radiative methylene bridge, which results in various orientations of the
processes are suppressed at this point.45
substituents in solid state. The complexes are not emissive in
In addition, [7,9]∙Br2 display an additional longer wavelength solution at room temperature, but display blue and green
emission band (λmax ≈ 500 nm) at temperatures of 120 K and phosphorescence in frozen methanol/ethanol matrix at 77 K
higher, which strengthens upon continuous irradiation of the with the lifetimes of μs order. The iodine substituted complex 8
samples (Fig. S23 in the ESI†). Similar spectral changes may also possesses additional green emission with the lifetime of about
take place for [8,10]∙Br2, being less apparent due to the broad 3.7 ms. Further studies on complex 8 and origin of its dual
emission bands of these complexes. The appearance of the new emission are underway.
emission bands can be tentatively attributed to the formation
of excimers in viscous methanol/ethanol solution at higher
temperatures, which is a common feature of platinum(II)
complexes.43
Experimental
Summarizing, the obtained complexes [7-10]∙Br2 demonstrate
All reagents and solvents for syntheses were purchased from
generally similar absorption and phosphorescence spectra with
Sigma-Aldrich, except for K2PtCl4 (Strem Chemicals and Alfa
excited state lifetimes of microsecond order, typical for
1
Aesar), and used as received without further purification. H
platinum(II) complexes.43 On the other hand, the exceptionally
NMR spectra were recorded using Bruker Avance III HD 300
long lifetime of the second emission band of [8]∙Br2 – a unique
MHz and Bruker Avance 400 MHz spectrometers. Chemical
feature of among the obtained complexes – is quite intriguing.
shifts are expressed in ppm using residual solvent peak as
This emission does not seem to originate either from excimer or
internal standard. All the obtained 1H NMR spectra are
from aggregation, since it is present in frozen matrix at 77 K, and
presented in section S5 in the ESI†. Elemental analyses (C, H, N)
does not depend on concentration (Figs. S11-18 in the ESI†).
were preformed using Elementar Vario EL III elemental
Thus, the additional green emission of [8]∙Br2 can be supposedly
analyzer. Electrospray ionization mass spectra (ESI MS) were
associated with a specific conformation of the complex in the
obtained with a Bruker micrOTOF or Agilent 6560 LC-IMMS TOF
excited state, stabilized by an intramolecular interaction
spectrometers equipped with an ESI source using methanol or
involving iodine atoms. Further studies of conformational
acetonitrile as solvents.
flexibility and its impact on photophysical properties of 8 are
currently being conducted in our research group.
Crystallography
Conclusions
The crystals of 1–10 and S2–4 were immersed in cryo-oil,
Reaction between substituted benzyl bromides and [Pt(dpa-H)2] mounted in a MiTeGen loop and measured at 120 K on a Rigaku
is demonstrated to be an efficient synthetic route towards Oxford Diffraction Supernova diffractometer or at 170 K on a
benzyldi(2-pyridyl)amine complexes of platinum(II), which can Bruker Axs KappaApexII diffractometer using Mo K ( =
0.71073) radiation. The CrysAlisPro46 or Denzo-Scalepack47
program packages were used for cell refinements and data
reductions.
Multi-scan/Gaussian/Analytical
absorption
correction (CrysAlisPro,46 SADABS48) was applied to the
intensities before structure solution. The structures were solved
by direct method using the SHELXT49 software. Structural
refinements were carried out using Olex250 graphical user
interface. Intrinsic disorder was modelled over two positions for
one of the nitro groups in 3 and for 2-iodobenzyl moiety in 5
and [8]∙(OTf)2. C-H and O-H hydrogen atoms were positioned
geometrically and constrained to ride on their parent atoms,
with C-H = 0.95-1.00 Å, O-H = 0.85 Å, and Uiso = 1.2-1.5Ueq
(parent atom), except for [10]∙(OTf)2, in which the H2O
hydrogen atoms were located from the difference Fourier and
refined isotropically. The crystallographic details are
summarized in section S6 in the ESI†.
Fig. 7 Phosphorescence excitation (dashed lines, monitored at emission
maximum) and emission (solid lines, monitored at excitation maximum)
spectra of [8]∙Br2 in methanol/ethanol glass under two sets of timing
parameters (77 K, C = 20 μM, d = delay, g = gate).
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Journal Name
from acetone to obtain colorless crystals of the product (68 mg,
View Article Online
26%). Elemental analysis (%): С, 77.6; Н,D5O.I4:;10N.1,01396/.C18.DCTa0l3c9. 1f2oGr
C17H15N3: С, 78.1; Н, 5.8; N, 16.1. H NMR (acetone-d6, 400
Optical spectroscopy
1
MHz): δ 8.29-8.25 (m, 2H), 7.63-7.57 (m, 2H), 7.38-7.34 (m, 2H),
7.27-7.20 (m, 4H), 7.16-7.12 (m, 1H), 6.92-6.87 (m, 2H), 5.53 (s,
2H, CH2). ESI MS (m/z): 262.14 (calcd for [M+H]+ 262.13), 545.26
(calcd for [2M+Na]+ 545.24). Crystals suitable for X-ray
diffraction analysis were obtained by slow evaporation of
solution in acetone.
HPLC grade methanol (J.T. Baker) and Uvasolv grade ethanol
(Sigma-Aldrich) were used as solvents for optical
measurements. Absorption spectra of 1-3 and [7-10]∙Br2 were
measured at ambient temperature in MeOH solution on Varian
Cary 100 UV-visible spectrophotometer using quartz cuvettes
with 1 cm path length in range of 220 – 700 nm. Steady state
phosphorescence excitation and emission spectra of [7-10]∙Br2
at 77 K in MeOH/EtOH (1:4, v/v) solution (presented in Figs. 7
and S6) were measured on Varian Cary Eclipse Fluorescence
spectrophotometer equipped with Oxford Optistat cryostat
using cryogenic 1 cm quartz luminescence cuvette (FireflySci).
Delay and gate times were adjusted in ranges of 0.1 – 1 ms and
0.6 – 5 ms accordingly using functions implemented in the
spectrophotometer.
Phosphorescence emission decays were measured using the
stroboscopic time resolution technique.51 Samples were excited
at λex = 320 nm with 5 ns pulses (100 Hz repetition rate) using
the diode pumped Q-switched Nd:YAG laser Ekspla NL230-100
with integrated optical parametric oscillator. The
phosphorescence emission was detected at 90° angle using
Oriel InstaSpec V ICCD detector with Acton SpectraPro sp-150i
Spectrograph equipped with 300 grooves/mm grating blazed at
500 nm. Stanford Research Systems digital delay generator
DG535 was used for controlling delay and gate timing
parameters. Temperature of the samples was controlled within
80 – 150 K range using Oxford Optistat cryostat with nitrogen as
static heat exchange gas. Sample solutions were placed in
cryogenic 1 cm quartz luminescence cuvette (FireflySci) and
deaerated by bubbling nitrogen gas for 10 minutes prior to
measurements. A mechanical shutter was used to block the
excitation beam between measurements to avoid
photoinduced changes in the samples. The obtained spectra
were processed using OriginPro software. The lifetime values
were determined from the slopes of the fitted linear ln(I) vs.
delay functions (see section S4 in the ESI†).
Synthesis of 2-iodobenzyldi(2-pyridyl)amine (2). Solution of 2-
iodobenzyl bromide (600 mg, 2 mmol) in DMF (3 ml) was added
to suspension of dpa (342 mg, 2 mmol) and sodium hydroxide
(240 mg, 6 mmol) in DMF (5 ml). The resulted mixture was
stirred at room temperature for 24 h to change color from
yellowish to dark-yellow. Remaining solids of sodium hydroxide
were filtered and solvent was evaporated on rotary evaporator.
Brown residue was dissolved in mixture of water (20 ml) and
chloroform (20 ml). Water phase was separated and extracted
with chloroform (3 x 20 ml). Combined organic extracts were
dried over Na2SO4, filtered and evaporated to volume of 20 ml.
Acetone (20 ml) was added and the solution was left overnight
at 5 °C for crystallization. Obtained yellowish crystals were
filtered, washed with acetone (2 x 2ml), and recrystallized from
acetone/chloroform (1:1, v/v) solution to obtain the product as
colorless crystals (345 mg, 44%). Elemental analysis (%): С, 52.6;
1
Н, 3.4; N, 10.9. Calcd for C17H14N3I: С, 52.7; Н, 3.6; N, 10.9. H
NMR (CDCl3, 300 MHz): δ 8.31-8.29 (m, 2H), 7.83 (dd, 1H, J = 1.0,
7.9 Hz), 7.58-7.50 (m, 2H), 7.25-7.15 (m, 4H), 6.93-6.83 (m, 3H),
5.41 (s, 2H, CH2). ESI MS (m/z): 387.99 (calcd for [M+H]+ 388.03).
Crystals suitable for X-ray diffraction analysis were obtained by
slow evaporation of solution in chloroform.
Synthesis of 2-nitrobenzyldi(2-pyridyl)amine (3). Synthesis was
conducted under nitrogen atmosphere using standard Schlenk
techniques similarly to published procedure.38 Sodium hydride
(290 mg, 60% in mineral oil, 7.25 mmol) was mixed with
pentane (20 ml) and resulted suspension stirred for 3 min.
When solids settled down most of the solvent was decanted to
remove the oil. The procedure was repeated once more and
remaining pentane was dried out under vacuum. Toluene (5 ml)
was added with stirring at -78 °C followed by solution of dpa
(860 mg, 5.03 mmol) in toluene (20 ml). The reaction mixture
was allowed to heat up to room temperature. During this time
formation of gas bubbles and cream colored precipitate was
observed. The mixture was stirred for 1 h at room temperature
(stirring was hampered due to formation of precipitate) and 18
h at 100 °C. After cooling to room temperature, solution of 2-
nitrobenzyl bromide (1.3 g, 6.02 mmol) in toluene (20 ml) was
added to the resulted gray suspension. The mixture was stirred
at room temperature for 30 min and then at 100 °C overnight.
After cooling to room temperature formed light brown
precipitate was filtered; brown filtrate was concentrated on
rotary evaporator. The obtained brown oil was dissolved in
chloroform, filtered from formed white precipitate (side
product S3, see section S1 in the ESI†), and solution subjected
Synthetic procedures
Synthesis of benzyldi(2-pyridyl)amine (1). The product was
synthesized according to published procedure.22 Solution of
benzyl bromide (190 mg, 1.1 mmol) in DMF (3 ml) was added to
suspension of dpa (171 mg, 1 mmol) and sodium hydroxide (160
mg, 4 mmol) in DMF (5 ml) at room temperature. The mixture
was stirred at 75 °C for 24 h to change color from yellowish to
brown-orange. The solvent was evaporated on rotary
evaporator and the residue was dissolved in mixture of water
(15 ml) and chloroform (15 ml). Water phase was separated and
extracted with chloroform (2 x 15 ml). The combined organic
extract was dried over Na2SO4, filtered and evaporated. The
residue was subjected to column chromatography purification
on silica gel using chloroform/methanol (5:1, v/v) mixture as
eluent. Product containing fractions were combined and
evaporated to give yellow powder, which was recrystallized
to
column
chromatography
purification
using
chloroform/diethyl ether (1:1, v/v) mixture as eluent. Product
containing fractions were combined and subjected to second
8 | J. Name., 2012, 00, 1-3
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column chromatography
ARTICLE
purification
using
ethyl combined and evaporated. The residue was washed with
View Article Online
DOI: 10.1039/C8DT03912G
acetate/hexane (1:5 → 3:5, v/v) mixture as eluent. Product chloroform (2 x 2 ml) and suspended in THF (3 ml). Dark solution
containing fractions were combined and slowly evaporated at was decanted and brown-yellow solids were washed with THF
room temperature to produce yellow crystals, which were (3 x 1 ml) and chloroform (2 x 1 ml) to give the product as yellow
recrystallized from ethyl acetate/pentane/diethyl ether (1:2:1, powder (23 mg, 20%). Elemental analysis (%): С, 26.8; Н, 1.8; N,
1
v/v) solution to get the product as yellowish crystals (137 mg, 7.4. Calcd for C17H14N4O2I2Pt: С, 27.0; Н, 1.8; N, 7.4. H NMR
9%). Elemental analysis (%): С, 66.3; Н, 4.3; N, 18.05. Calcd for (THF-d8, 300 MHz): δ 9.48-9.29 (m, 2H, broad sidebands due to
C17H14N4O2: С, 66.7; Н, 4.6; N, 18.3. 1H NMR (CDCl3, 300 MHz): δ 3JPt,H = 44.4 Hz), 9.02 (d, 1H, J = 7.7 Hz), 8.04 (dd, 1H, J = 1.1, 8.1
8.29-8.27 (m, 2H), 8.04 (dd, 1H, J = 1.3, 8.0 Hz), 7.60-7.50 (m, Hz), 8.00-7.90 (m, 2H), 7.71-7.62 (m, 1H), 7.56-7.40 (m, 2H),
3H), 7.48-7.41 (m, 1H), 7.36-7.29 (m, 1H), 7.24-7.19 (m, 2H), 7.19-7.10 (m, 1H), 5.81 (s, 2H, CH2). ESI MS (m/z): 668.78 (calcd
6.89-6.84 (m, 2H), 5.83 (s, 2H, CH2). ESI MS (m/z): 307.11 (calcd for [M-I+CH3CN]+ 669.00). Crystals suitable for X-ray diffraction
for [M+H]+ 307.12). Crystals suitable for X-ray diffraction analysis were obtained by slow evaporation of solution in THF.
analysis were obtained by slow evaporation of solution in ethyl General procedure for conventional synthesis of
acetate/pentane/diethyl ether (1:2:1, v/v).
bis(benzyldi(2-pyridyl)amine)platinum(II) triflate complexes
Synthesis of benzyldi(2-pyridyl)aminediiodoplatinum(II) (4). [7-9]∙(OTf)2. Suspension of precursor complex 4-6 (0.1 mmol)
Solution of K2PtCl4 (62 mg, 0.15 mmol) and KI (620 mg, 3.73 and corresponding ligand 1-3 (0.1 mmol) in acetonitrile (10 ml)
mmol) in water (10 ml) was stirred for 20 min. Solid 1 (45 mg, was stirred at 60 °C for 10 min. Solution of silver triflate (55 mg,
0.17 mmol) was added to the obtained dark-red solution and 0.21 mmol) in acetonitrile (5 ml) was added dropwise at this
the resulting suspension was stirred for 19 h at room temperature within 5 – 7 min. Resulted mixture was stirred in
temperature and then 30 min at 85 °C. Yellow precipitate was darkness at 75 °C for 16 h. Yellow precipitate of silver iodide was
filtered out and washed with chloroform (5 x 1 ml) to give the filtered off and the filtrate was partially evaporated on rotary
product as yellow powder (87.3 mg, 82%). Elemental analysis evaporator to volume of 2 ml and allowed to evaporate slowly
(%): С, 28.6; Н, 2.2; N, 6.1. Calcd for C17H15N3I2Pt: С, 28.8; Н, 2.1; at room temperature overnight. Resulted crystals were washed
1
N, 5.9. H NMR (THF-d8, 400 MHz): δ 9.45-9.29 (m, 2H, broad with THF (2 x 2 ml), recrystallized from methanol/acetonitrile
3
sidebands due to JPt,H = 45.0 Hz), 7.93-7.87 (m, 2H), 7.84-7.79 mixture (1:1, v/v), and washed again with THF (3 x 2 ml) to give
(m, 2H), 7.38-7.27 (m, 4H), 7.18 (m, 1H), 7.11-7.06 (m, 2H), 5.37 product as colorless crystals. Crystals of [7,8]∙(OTf)2 suitable for
(s, 2H, CH2). ESI MS (m/z): 624.00 (calcd for [M-I+CH3CN]+ X-ray diffraction analysis were obtained by slow evaporation of
624.02), 262.12 (calcd for [M-PtI2+H]+ 262.13). Crystals suitable solution in methanol. No high quality crystals could be obtained
for X-ray diffraction analysis were obtained by slow evaporation for [9]∙(OTf)2, and crystals of [9]∙Br2 were measured instead.
of solution in THF.
Bis(benzyldi(2-pyridyl)amine)platinum(II) triflate ([7]∙(OTf)2).
Synthesis
of
2-iodobenzyldi(2-pyridyl)amine- Yield: 14 mg, 14%. Elemental analysis (%): С, 42.0; Н, 2.7; N, 8.0.
diiodoplatinum(II) (5). Solution of K2PtCl4 (124 mg, 0.30 mmol) Calcd for C36H30N6O6F6S2Pt: С, 42.6; Н, 3.0; N, 8.3. 1H NMR
and KI (1.25 g, 7.5 mmol) in water/THF mixture (10 ml, 1:1, v/v) (CD3CN, 400 MHz): δ 8.14-8.08 (m, 4H), 7.85 (d, 4H, J = 7.1 Hz),
was stirred for 20 min. Solid 2 (120 mg, 0.31 mmol) was added 7.78-7.68 (m, 8H), 7.44-7.37 (m, 4H), 7.36-7.30 (m, 2H), 7.22-
to the obtained dark-red solution and the resulting suspension 7.17 (m, 4H), 5.51 (s, 4H, CH2). ESI MS (m/z): 866.13 (calcd for
was refluxed for 16 h at 85 °C. The solvent was partly [M-OTf]+ 866.17).
evaporated on rotary evaporator and yellow-orange powder Bis(2-iodobenzyldi(2-pyridyl)amine)platinum(II)
triflate
was filtered and washed with chloroform (5 x 3 ml) and THF (5 ([8]∙(OTf)2). Yield: 83 mg, 61%. Elemental analysis (%): С, 34.1;
x 3 ml) to give the product as yellow powder (120.4 mg, 48%). Н, 2.1; N, 6.75. Calcd for C36H28N6O6F6S2I2Pt: С, 34.1; Н, 2.2; N,
Elemental analysis (%): С, 24.3; Н, 1.7; N, 4.9. Calcd for 6.6. 1H NMR (CD3OD, 300 MHz): δ 8.28-8.20 (m, 4H), 7.98-7.88
C17H14N3I3Pt: С, 24.4; Н, 1.7; N, 5.0. 1H NMR (THF-d8, 300 MHz): (m, 8H), 7.77 (m, 4H), 7.50-7.42 (m, 2H), 7.32-7.25 (m, 4H), 7.15-
δ 9.49-9.29 (m, 2H, broad sidebands due to 3JPt,H = 46.3 Hz), 8.40 7.08 (m, 2H), 5.57 (s, 4H, CH2). ESI MS (m/z): 1117.92 (calcd for
(d, 1H, J = 7.9 Hz), 7.99-7.92 (m, 2H), 7.84 (dd, 1H, J = 1.4, 7.9 [M-OTf]+ 1117.96).
Hz), 7.70-7.30 (m, 3H), 7.16-7.08 (m, 2H), 6.99-6.91 (m, 1H), Bis(2-nitrobenzyldi(2-pyridyl)amine)platinum(II)
triflate
5.43 (s, 2H, CH2). ESI MS (m/z): 749.89 (calcd for [M-I+CH3CN]+ ([9]∙(OTf)2). Yield: 17 mg, 15%. Elemental analysis (%): С, 39.1;
749.91). Crystals suitable for X-ray diffraction analysis were Н, 2.6; N, 9.7. Calcd for C36H28N8O10F6S2Pt: С, 39.1; Н, 2.55; N,
obtained by slow evaporation of solution in THF.
Synthesis of
10.1. 1H NMR (CD3OD, 300 MHz): δ 8.30-8.22 (m, 4H), 8.04-7.92
2-nitrobenzyldi(2-pyridyl)amine- (m, 8H), 7.84-7.72 (m, 6H), 7.68-7.61 (m, 2H), 7.38-7.31 (m, 4H),
diiodoplatinum(II) (6). Solution of K2PtCl4 (61 mg, 0.15 mmol) 5.91 (s, 4H, CH2). ESI MS (m/z): 403.59 (calcd for [M-2OTf]2+
and KI (610 mg, 3.67 mmol) in water (10 ml) was stirred for 20 403.59).
min. Solution of 3 (45 mg, 0.15 mmol) in THF (4 ml) was added General procedure for alternative synthesis of bis(benzyldi(2-
to the obtained dark-red solution and the resulting suspension pyridyl)amine)platinum(II) bromide complexes [7-10]∙Br2.
was stirred for 20 h at 70 °C. The solvent was partly evaporated [Pt(dpa-H)2] (53 mg, 0.1 mmol, prepared according to
on rotary evaporator and brown precipitate was filtered and previously published procedure33 with 38% yield) and
washed with water (3 ml). Resulting solids were suspended in corresponding benzyl bromide (0.2 mmol) were suspended in
boiling chloroform (7 ml) and yellow solution was decanted. The acetonitrile (20 ml) and stirred at 75 °C for 24 h and refluxed for
procedure was repeated 5 times, the obtained solutions were 2 h. Solvent was evaporated on rotary evaporator, and yellow
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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10 A. K. Paul, T. S. Srivastava, S. J. Chavan, M. P. Chitnis, S. Desai
residue was washed with chloroform (2 x 2 ml) and diethyl ether
(2 x 2 ml). Resulting colorless solids were recrystallized from
methanol to afford product as colorless crystalline material
([7]∙Br2, 86 mg, 97%; [8]∙Br2, 72 mg, 64%; [9]∙Br2, 82 mg, 85%).
1H NMR spectra of [7-9]∙Br2 matched those presented above.
Crystals of [9]∙Br2 suitable for X-ray diffraction analysis were
obtained by slow evaporation of solution in methanol. In
synthesis of [10]∙Br2, additional side product S4 was isolated
and identified (see section S2 in the ESI†).
and K. K. Rao, J. Inorg. Biochem., 1996, 61, 179–V1ie9w6A.rticle Online
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Bis(2-cyanobenzyldi(2-pyridyl)amine)platinum(II)
bromide
([10]∙Br2). Yield: 78 mg, 79%. Elemental analysis (%): С, 46.2; Н,
3.2; N, 11.7. Calcd for C36H28N8Br2Pt: С, 46.6; Н, 3.0; N, 12.1. 1H
NMR (CD3OD, 300 MHz): δ 8.30-8.22 (m, 4H), 8.07-8.02 (m, 2H),
7.97-7.95 (m, 4H) 7.88-7.72 (m, 8H), 7.60-7.54 (m, 2H), 7.33-
7.27 (m, 2H), 5.75 (s, 4H, CH2). ESI MS (m/z): 846.12 (calcd for
[M-Br]+ 846.13), 383.60 (calcd for [M-2Br]2+ 383.60). Since only
poor quality thin needle crystals could be obtained from
[10]∙Br2, the reaction of anion exchange from bromide to
triflate was performed using silver(I) triflate in methanol.
Crystals of [10]∙(OTf)2 suitable for X-ray diffraction analysis were
obtained by slow evaporation of the resulted filtered solution.
22 B. Antonioli, D. J. Bray, J. K. Clegg, K. Gloe, K. Gloe, O. Kataeva,
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
E.B. and M.H. kindly acknowledge the financial support from the
Academy of Finland (Proj. no. 295581). The authors would also
like to acknowledge Elina Hautakangas for elemental analyses,
Dr. Elina Kalenius for mass spectrometry measurements, Dr.
Matti Tuikka for helping with X-ray analyses, and Dr. Pasi
myllyperkiö and Dr. Heikki Häkkänen for assisting with optical
spectroscopy measurements.
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‡ This assignment is also supported by the preliminary computational
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2
3
4
5
6
7
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