Synthesis, characterization, photophysics, and anion binding properties of platinum(II) acetylide complexes with urea group

Article abstract of DOI:10.1039/c5dt00307e

A new class of platinum(ii) acetylide complexes with urea group, [Pt(^tBu₃tpy)(CCC₆H₄-4-NHC(O)NHC₆H₄-4-R)](OTf) (^tBu₃tpy = 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine

Full text of DOI:10.1039/c5dt00307e

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Synthesis, characterization, photophysics, and
anion binding properties of platinum(II) acetylide
complexes with urea group†
Cite this: Dalton Trans., 2015, 44,
7785
Zhi-Hui Zhang, Jiewei Liu, Li-Qi Wan, Fang-Ru Jiang, Chi-Keung Lam, Bao-Hui Ye,
Zhengping Qiao and Hsiu-Yi Chao*
A new class of platinum(II) acetylide complexes with urea group, [Pt(tBu3tpy)(CuCC6H4-4-NHC(O)-
NHC6H4-4-R)](OTf) (tBu3tpy = 4,4’,4’’-tri-tert-butyl-2,2’:6’,2’’-terpyridine; R = H (3a), Cl (3b), CF3 (3c),
and NO2 (3d)), has been synthesized and characterized. The crystal structures of 3a, 3a·DMF·THF,
3b·CH3CN, and 3c·CH3CN have been determined by X-ray diffraction. Upon excitation at λ > 380 nm, the
solid samples of complexes 3a–3d show orange light at 298 K. The anion binding properties of com-
plexes 3a–3d have been studied by UV-vis titration experiments in CH3CN and DMSO. In general, the
log K values of 3a–3d with the same anion in CH3CN depend on the substituent R on the acetylide ligand
of 3a–3d and follow this order: R = NO2 (3d) > CF3 (3c) > Cl (3b) > H (3a). For the same complex with
Received 22nd January 2015,
Accepted 21st March 2015
different anions, the log K values are in the following order: F− > OAc− > Cl− > Br− ≈ HSO4 ≈ NO3 > I−,
which is in accordance with the decrease in the basicity of anions. Complex 3d with NO2 group shows a
dramatic colour change towards F− in DMSO, allowing the naked eye detection of F−.
−
−
DOI: 10.1039/c5dt00307e
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compounds have been reported as anion sensors due to their
specific hydrogen bonding interactions towards anions.
Introduction
Anion sensors have attracted increasing interest over the past Among them, urea-based receptors have been extensively
decade because of the crucial importance of anions and their studied owing to their unique geometry (two hydrogen bonds
potential applications in chemistry, biology as well as the towards anions) and their easy modification through organic
environment.1 Since the pioneering work of Park and synthesis.4
Simmons on the first anion sensor,2 a variety of artificial
While the early studies mainly focused on the organic
anion receptors have been designed and synthesized for the anion sensors,8 metal complexes based anion sensors have
stimulation of host–guest interactions within the supramolecu- also received immense attention. Metal complexes could have
lar chemistry. Thus, considerable effort has been devoted to some intrinsic properties, such as redox and luminescence,
the exploitation of selective anion sensors. An ideal anion and the metal center can directly interact with anions. Thus,
sensor consists of a receptor center for binding anions, which anion recognition and sensing by metal complexes are sup-
is covalently linked to a reporting center for the signaling of posed to extend the detection of anion binding. Iron(II),9
interactions.3 A sensing signal is generated once the receptor rhenium(I),10 ruthenium(II),11 iridium,12 gold(I),13 terbium(III),14
center binds anions through non-covalent interactions, and and gold(I)–copper(I)15 complexes with urea group have been
the binding of such species could produce changes in the reported as anion sensors. Amongst various metal complexes,
photophysical and redox properties of the reporter unit. a great deal of attention has been particularly given to the
Urea,4,5 thiourea,5 amide,6 and guanidinium7 based organic platinum(II) acetylide complexes because of their rich photo-
luminescence properties.16 The square planar geometry of
platinum(II) also allows the Pt⋯Pt interactions, which could
affect the properties of electronic excited states. In this regard,
platinum(II) acetylide complexes have been used as sensors
for VOC,17 pH,18 ions,19 temperature,18a,20 and mechanical
force.17c,21 In addition, the charge transfer absorption in the
visible region with the corresponding higher quantum yield
photoluminescence of platinum(II) acetylide complexes also
facilitates applications such as molecular recognition,22 photo-
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry
and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China.
E-mail: zhaoxy@mail.sysu.edu.cn; Fax: (+86)020-84112245; Tel: (+86)020-84110062
†Electronic supplementary information (ESI) available: Additional figures and
tables. CCDC 1028565 (3a), 1028514 (3a·DMF·THF), 1028592 (3b·CH3CN) and
1028516 (3c·CH3CN). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c5dt00307e
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Scheme 1 Synthetic route of platinum(II) acetylide complexes 3a–3d.
catalysts,23 energy conversion materials,24 electrolumines- butyl-2,2′:6′,2″-terpyridine) with 1a–1d in the presence of CuI
cence25 as well as broadband nonlinear transmission and diisopropylamine in dichloromethane gave 3a–3d, respect-
materials.26 However, to the best of our knowledge, the study ively. All the platinum(II) acetylide complexes 3a–3d were
1
of anion-sensing properties of platinum(II) acetylide complexes characterized by H NMR, IR as well as ESI-MS and gave satis-
has been less explored.27
factory elemental analyses. They are air-stable in the solid state
Our group has a long-term interest in the study of the at 298 K.
relationship between the structures and anion-binding pro-
In the 1H NMR spectra of the platinum(II) acetylide com-
perties of metal complexes.13a,13b To extend our study, in this plexes 3a–3d in DMSO-d6, the protons of N–H on the acetylide
research, we have synthesized a series of platinum(II) acetylide ligands of the complexes show resonance at ca. 8.65–9.46 ppm.
complexes with urea group [Pt(tBu3tpy)(CuCC6H4-4-NHC(O)- The chemical shifts are in the following order: 3a < 3b < 3c < 3d,
NHC6H5-R)](OTf) (tBu3tpy
=
4,4′,4″-tri-tert-butyl-2,2′:6′,2″- which is in accordance with the increasing electron-withdraw-
terpyridine; R = H (3a), Cl (3b), CF3 (3c), NO2 (3d); Scheme 1). ing ability of the substituent R on the acetylide ligand. In
Their photophysical and anion-binding properties have been addition, the chemical shifts at ca. 6.96–8.18 ppm are attribu-
investigated. We envisaged that if the photophysical properties ted to the resonance of the protons on the aromatic rings of
of these platinum(II) acetylide complexes could be changed the acetylide ligand. The chemical shifts at ca. 7.85–9.02 ppm
when they interact with anions through hydrogen bonds as well as 1.43 and 1.53 ppm are attributed to the resonance of
t
between the urea N–H(s) of the complexes and anions, they the protons on the Bu3tpy ligand. The IR spectra of 3a–3d
could be used as anion sensors. In addition, the effect of the reveal the bands at 3350–3433 and 1610–1641 cm–1, which are
substituent R of the acetylide ligand on the anion-binding the characteristic of ν(N–H) and ν(CvO) of the acetylide
ability of the complexes 3a–3d has also been examined. Inter- ligands, respectively.
estingly, complex 3d showed a significant colour change when
it interacted towards F− in DMSO, allowing the naked eye
Crystal structures of 3a, 3a·DMF·THF, 3b·CH3CN, and
3c·CH3CN
detection of F−.
The crystal structures of the platinum(II) acetylide complexes
3a, 3a·DMF·THF, 3b·CH3CN, and 3c·CH3CN have been deter-
mined by X-ray diffraction. Their crystallographic data as well
as selected bond distances and angles are listed in Table S1
(ESI†) and Table 1. Fig. 1 shows the perspective view of 3a (for
Results and discussion
Synthesis and characterization
Scheme 1 shows the synthetic routes of the acetylide ligands 3a·DMF·THF, 3b·CH3CN, and 3c·CH3CN, see Fig. S1–S3 (ESI†).
(1a–1d) and platinum(II) acetylide complexes (3a–3d). The acet- The platinum(II) coordination geometry in these complexes is
ylide ligands, HCuCC6H4-4-NHC(O)NHC6H4-4-R (R = H (1a), distorted square-planar. The Pt–N bond distances are in the
Cl (1b), CF3 (1c), NO2(1d)), were prepared by the reaction of range of 1.949(5)–2.029(5) Å, which are similar to those of
4-ethynylaniline with the corresponding isocyanate in a molar other related platinum(II) acetylide complexes.22c,27d,28 The
ratio of 1 : 1 in refluxing dichloromethane. Furthermore, the Pt1–C1 and C1uC2 bond distances are in the range of
reactions of [Pt(tBu3tpy)Cl](OTf) (2) (tBu3tpy = 4,4′,4″-tri-tert- 1.966(8)–1.992(8) and 1.206(10)–1.219(10) Å, respectively,
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butyl groups on the terpyridyl ligands, which prevent the
neighboring molecules from getting into close proximity with
each other. While viewed along the c axis, the platinum atoms
are almost superimposable to give a zigzag [Pt]n chain. The
individual molecules in the dimers are rotated with respect to
their neighbors, with a C–Pt–Pt–C torsion angle of 180° and a
partial stacking of the aromatic terpyridyl units. Unlike 3a, in
which the Pt metal center shows a strong propensity to form a
zigzag chains with shorter intermolecular Pt⋯Pt contacts, the
molecular packing in 3a·DMF·THF exhibits a long distance
between adjoining platinum atoms caused by the insertion of
solvent molecules into the lattice, allowing two types of Pt⋯Pt
distances to form a hexagon (Fig. 2(b)). The adjacent Pt⋯Pt
distance is 9.253 Å between the platinum atoms arranged in
the same order, while for those in the head-to-tail arrange-
ment, the nearest Pt⋯Pt distance is 9.841 Å. The view along
the c axis shows that the platinum atoms are arranged in a
honeycomb lattice. THF molecules are inserted into the lattice
through the three C–H⋯O hydrogen bonds (d(H⋯O1) =
2.667 Å, ∠(C–H⋯O1) = 171.25°; d(H⋯O2) = 2.528 Å, ∠(C–
H⋯O2) = 173.36°; d(H⋯O3) = 2.618 Å, ∠(C–H⋯O3) = 153.56°),
whereas DMF molecules form two C–H⋯O hydrogen bonds
with two hydrogen atoms of pyridine ring (d(H⋯O1) = 2.458 Å,
∠(C–H⋯O1) = 156.73°, d(H⋯O2) = 2.588 Å, ∠(C–H⋯O2) =
162.30°). It is interesting to note that the stacking varies in 3a
with/without solvates. This difference may indicate that the
aggregation of 3a is solvate-dependent.
Table 1 Selected bond distances (Å) and bond angles (°) of complexes
3a, 3a·DMF·THF, 3b·CH3CN, and 3c·CH3CN
3a
3a·DMF·THF 3b·CH3CN 3c·CH3CN
Pt⋯Pt (shortest) 5.804
9.253
4.931
5.139
Pt1–N1
2.027(6)
2.020(4)
1.949(5)
2.015(4)
1.966(8)
2.029(5)
1.973(6)
2.022(5)
1.992(8)
1.206(10)
1.225(7)
80.39(19)
80.41(17)
160.79(19) 160.87(13)
98.6(2)
179.0(2)
100.6(2)
177.0(5)
2.027(3)
1.960(3)
2.017(3)
1.971(5)
1.210(7)
1.208(6)
80.41(13)
80.46(13)
Pt1–N2
Pt1–N3
Pt1–C1
1.956(6)
2.026(6)
1.967(7)
C1–C2
C36–O1
1.219(10) 1.213(10)
1.154(13) 1.219(8)
N1–Pt1–N2
N2–Pt1–N3
N1–Pt1–N3
N1–Pt1–C1
N2–Pt1–C1
N3–Pt1–C1
Pt1–C1–C2
80.2(2)
80.6(2)
160.7(2)
100.6(3)
178.4(3)
98.6(3)
176.0(6)
80.08(17)
80.66(17)
160.63(18)
99.9(2)
179.6(2)
99.4(2)
98.82(15)
179.12(16)
100.32(15)
176.8(4)
177.3(7)
Fig. 2(c) shows the molecular packing of 3b·CH3CN, in
which CH3CN molecule is inserted into the lattice through the
C–H⋯N interaction between the hydrogen atom on the pyri-
dine ring and the nitrogen atom on the acetonitrile molecule
(d(H⋯N) = 2.655 Å, ∠(C–H⋯N) = 128.50°). When viewed along
the a axis, the adjacent Pt atoms form a head-to-tail dimeric
structure with the adjacent Pt⋯Pt distance alternating
between 4.931 and 10.001 Å, giving a zigzag [Pt]n chain with a
Pt–Pt–Pt angle of 159.16°. The shortest Pt⋯Pt distance is
4.931 Å, which is shorter than that of 3a, while the distance
between the Pt atoms of diverse adjacent dimeric unit is
longer (10.001 Å). This could be due to the insertion of CH3CN
molecule into the cavity because of which the distance
between neighboring dimeric units increases, while the Pt⋯Pt
distance in the dimeric Pt atoms decreases. The stacking
pattern of 3c·CH3CN has the same orientation as that of
3b·CH3CN (Fig. 2(d)). Both of them exist as sequential dimeric
units with no evidence of Pt⋯Pt contacts. The shortest Pt⋯Pt
distance of 3c·CH3CN is 5.139 Å, which is farther than that of
3b·CH3CN (4.931 Å). However, the distance between the Pt
atoms of nearby dimers decreases to 9.968 Å. Moreover, the
Pt–Pt–Pt angle of zigzag [Pt]n chain is 158.31° when viewed
from the c axis. Although the CH3CN molecules are found in
the crystal structure, there is no significant interaction
between the solvent molecule (CH3CN) and tert-butyl group.
The crystal structures of 3b·CH3CN and 3c·CH3CN are other
examples of solvent-dependent aggregation, which indicates
that the molecules tend to pack in the same aggregation in the
same solvent.
Fig. 1 Perspective view of 3a.
which are comparable to those of other related platinum(II)
acetylide complexes.27d,28 The N1–Pt1–N2, N2–Pt1–N3, and
N1–Pt1–N3 angles are ca. 80°, 80°, and 160°, respectively,
t
which could be due to the geometric constraints of Bu3tpy
ligand. Each pair of complex cations reveals a head-to-tail
stacking with the shortest Pt⋯Pt distance of ca. 4.931–9.841 Å.
In addition, there is a hydrogen bond interaction between the
N–H of the urea group on the acetylide ligand and the oxygen
atom of the triflate anion. The hydrogen bond parameters are
listed in Table S2 (ESI†).
Fig. 2 shows the stacking patterns of adjacent planar
platinum(II) moieties as well as the arrangement for complexes
3a, 3a·DMF·THF, 3b·CH3CN, and 3c·CH3CN. These platinum
(II) acetylide complexes with different acetylide ligands tend to
pack in diverse arrangements. Their adjacent platinum(II) moi-
eties display an antiparallel stacking pattern. Fig. 2(a) shows
the molecular packing of 3a, in which the adjacent Pt atoms
are in the zigzag arrangement with the Pt⋯Pt⋯Pt angle of
170.39° and the Pt⋯Pt distance alternating between 5.804 and
8.575 Å. In addition, no significant Pt⋯Pt or π⋯π interaction
is observed from the crystal packing because of the bulky tert-
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Fig. 2 Packing diagrams of (a) 3a, (b) 3a·DMF·THF, (c) 3b·CH3CN, and (d) 3c·CH3CN (left: stacking patterns of adjacent planar platinum(II) moieties;
right: arrangement of the molecules; counter-anions are omitted for clarity).
Electronic absorption and emission spectra of complexes 3a–3d Beer’s law (Fig. S4 and S5, ESI†), indicating that there is no
aggregation of 3a in DMSO in the concentration ranging from
The photophysical data for the acetylide ligands (1a–1d) and 10−5 to 10−3 mol dm−3. This result may be due to the steric
t
complexes (3a–3d) are summarized in Table S3 (ESI†) and effect from the bulky Bu3tpy ligand, preventing the aggrega-
Table 2, respectively. The electronic absorption spectra of 3a– tion of 3a.29
3c in acetonitrile show high-energy bands at 247–339 nm,
It is interesting to note that platinum(II) acetylide com-
which are assigned as the π → π* transitions of the tBu3tpy and plexes 3a–3d exhibit different colours in different solvents.
acetylide ligands (Fig. 3). For 3d in acetonitrile, the electronic Because 3a has the better solubility in solvents, it has been
absorption bands at 312, 328, and 339 nm could be due to the used to study the solvent effect on the electronic absorption
charge transfer transition from the amide to the NO2 group of spectrum. Fig. 4(a) shows the colours of 3a at a concentration
the acetylide ligand.13b The low-energy bands of 3a–3d in of 2.7 × 10−5 mol dm−3 in different solvents: light yellow
acetonitrile at 398–464 nm are ascribed to the admixture of (DMSO, CH3CN, and CH3OH), orange (CH3C(O)CH3), and pale
MLCT (dπ(Pt) → π*(tBu3tpy)) and LLCT (π(CuC-C6H4-NH-C(O)- red (CH2Cl2 and THF). Fig. 4(b) shows the electronic absorp-
NH-C6H4-4-R) → π*(tBu3tpy)) transitions. The ground-state tion spectra of 3a in 370–700 nm in various solvents. The low-
complex aggregation of 3a in DMSO has also been studied. energy absorption maximum is in the order: CH3OH (458 nm)
When the concentration of 3a in DMSO increases from 1 × < CH3CN (464 nm) < DMSO (472 nm) < CH3C(O)CH3 (476 nm)
10−5 to 1 × 10−3 mol dm−3, the absorbance at 510 nm obeys < THF (492 nm) < CH2Cl2 (498 nm) depending on the solvent
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Table 2 Photophysical data of complexes 3a–3d at 298 K
Complex
Medium
λabs/nm (ε/dm3 mol−1 cm−1
)
λem/nma (τ0/μs)
3a
DMSO
CH3CN
THF
CH2Cl2
CH3C(O)CH3
CH3OH
Solid
247 (sh, 43 150), 289 (61 480), 329 (sh, 16 180), 344 (14 300), 384 (sh, 4830), 407 (5290), 472 (7170)
247 (54 280), 259 (sh, 51 280), 285 (80 320), 323 (sh, 21 320), 383 (sh, 5240), 405 (5820), 464 (6940)
249 (41 440), 287 (64 960), 340 (sh, 14 740), 388 (sh, 4330), 412 (4880), 492 (5760)
231 (39 220), 251 (44 550), 287 (67 440), 338 (15 890), 411 (5330), 498 (5870)
328 (15 960), 339 (16 150), 389 (sh, 5130), 409 (5870), 476 (7610)
249 (45 070), 285 (73 670), 325 (sh, 18 630), 339 (17 520), 382 (sh, 4830), 409 (5500), 458 (5990)
616 (0.426)
3b
DMSO
CH3CN
THF
260 (sh, 40 160), 290 (69 350), 329 (sh, 19 880), 344 (16 690), 404 (sh, 4430), 469 (5640)
247 (39 105), 285 (68 455), 324 (sh, 17 720), 339 (15 530), 403 (sh, 4310), 460 (5160)
289 (41 830), 340 (sh, 3440), 413 (2670), 493 (3020)
CH3OH
CH2Cl2
Solid
DMSO
CH3CN
Solid
DMSO
CH3CN
Solid
286 (80 060), 326 (sh, 18 290), 339 (17 770), 395 (sh, 5350), 453 (6380)
287 (78 500), 339 (17 500), 412 (5240), 502 (5880)
607 (0.345)
601 (0.249)
599 (0.502)
3c
260 (sh, 41 430), 291 (65 140), 332 (sh, 15 990), 344 (14 180), 404 (sh, 4080), 469 (5610)
247 (43 850), 286 (72 810), 324 (sh, 17 950), 339 (16 670), 402 (sh, 4670), 459 (5700)
3d
260 (sh, 42 520), 285 (51 520), 332 (sh, 30 800), 346 (34 320), 468 (7480)
246 (39 105), 273 (sh, 65 760), 284 (45 850), 312 (28 780), 328 (32 720), 339 (35 280), 459 (4430)
a The emission of 3a–3d was too weak to be observed in these solvents.
Fig. 3 Electronic absorption spectra of 3a–3d in acetonitrile at 298 K.
polarity (Table S4, ESI†). However, there is no clear trend
between the absorption maximum and solvent polarity.
Similar trend can also been observed for 3b (Table S4 and
Fig. S6, ESI†). The solvent-dependent absorption bands indi-
cate their charge-transfer character in nature.
The excitation of complexes 3a–3d in the solid state at λ >
380 nm produces orange light. The emission maxima of 3a–3d
in the solid state at 298 K are in the following order: 616 nm
(3a) > 607 nm (3b) > 601 nm (3c) > 599 nm (3d), which is in
accordance with the increasing electron-withdrawing ability of
the substituent R on the acetylide ligand (R = H (3a) < Cl (3b) <
CF3 (3c) < NO2 (3d)) (Fig. 5). The lifetime of these emission
bands is 0.249–0.502 μs, indicating the nature of the triplet
excited state. The stronger electron-withdrawing acetylide
ligands could reduce the energy of dπ(Pt)- and π(acetylide
ligand)-based HOMO.26d Thus, the emission band at
599–616 nm for 3a–3d in the solid state is tentatively assigned
Fig. 4 (a) Colours of 3a (concentration = 2.7 × 10−5 mol dm−3) in
various solvents. (b) Electronic absorption spectra of 3a in 370–700 nm
in various solvents at 298 K.
to come from the mixture of 3MLCT (dπ(Pt) → π*(tBu3tpy)) and
3LLCT (π(acetylide ligand)
→
π*(tBu3tpy)) excited states.
However, in solutions, complexes 3a–3d show very weak or no
emission at 298 K.
Anion binding properties of complexes 3a–3d in CH3CN
The binding properties of 3a–3d in CH3CN towards anions,
such as F−, Cl−, Br−, I−, OAc−, NO3−, HSO4−, SO42−, and
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459 nm gradually decreases, while the new absorption bands
at 371 and 480 nm appear with the progressively increasing
intensity. Consequently, the colour of the solution (5 × 10−5
mol dm−3) changes from pale yellow to light red. Well-defined
isosbestic points are observed at 287, 301 and 358 nm, indicat-
ing that only two species coexist during the titration equili-
brium and suggesting the possible formation of anion-
complex adduct. The addition of other anions (Cl−, Br−, I−,
OAc−, NO3−, and HSO4−) into the CH3CN solution of 3d causes
similar (but smaller) spectral changes to those towards F−
(Fig. S7–S12, ESI†).
For non-nitro-derivatives 3a–3c, their UV-vis spectral
changes in CH3CN are similar to each other upon the addition
of anions studied in this study (Fig. S13–S32, ESI†). Fig. 7
demonstrates the UV-vis spectral changes of 3a upon the
addition of F− in CH3CN at 298 K. During the process, the
absorption bands at 405 and 464 nm gradually decrease, and a
new band at 485 nm appears. A well-defined isosbestic point is
observed at 473 nm. To determine the binding ratio between
the complex and anion, the Job’s plots have been made.
Fig. S33(a) (ESI†) shows the Job’s plots of complexes 3a–3d
with F− in CH3CN. The absorbance maximum appears at
ca. 0.5, indicating the 1 : 1 complex-anion formation. Using the
1 : 1 binding model and non-linear least-squares fitting, the
log K values of 3a–3d towards different anions were deter-
mined (Table 3). For complexes 3a–3d with the same anion,
Fig. 5 Emission spectra of 3a–3d in the solid state at 298 K.
−
H2PO4 (used their tetra-n-butylammonium salts), have been
investigated by the UV-vis titration experiments. Because the
precipitates were produced during the titration of 3a–3d with
2−
SO4
and H2PO4−, corresponding investigations were not
carried out. Fig. 6 shows the UV-vis spectral changes of 3d
upon the addition of F− in CH3CN at 298 K. The absorbance of
the absorption bands of 3d at 246, 273, 284, 312, 328, 339 and
Fig. 6 UV-vis spectral changes of 3d (5 × 10−5 mol dm−3) in CH3CN
upon addition of F−. Inset: A plot of the absorbance change at 250 nm
as a function of the concentration of F− and its theoretical fit for the 1 : 1
binding of complex 3d with F−.
Fig. 7 UV-vis spectral changes of 3a (1.2 × 10−4 mol dm−3) in CH3CN
upon the addition of F−. Inset: a plot of the absorbance change at
430 nm as a function of the concentration of F− and its theoretical fit for
the 1 : 1 binding of complex 3a with F−.
Table 3 Binding constants (log K) of 3a–3d with anions in CH3CNa
F−
OAc−
Cl−
Br−
I−
NO3
HSO4
−
−
Complex
b
3a
3b
3c
3d
4.52 0.14
4.88 0.18
5.07 0.51
5.53 0.11
4.34 0.16
4.48 0.21
4.96 0.13
5.22 0.21
3.35 0.02
3.51 0.03
3.80 0.06
3.85 0.08
2.53 0.03
2.75 0.07
2.92 0.05
3.03 0.04
2.31 0.06
2.82 0.13
2.90 0.14
2.91 0.09
2.63 0.08
2.92 0.05
3.02 0.13
3.09 0.05
2.34 0.15
2.48 0.06
2.60 0.05
a Binding constants were determined by 1 : 1 model using nonlinear fitting methods. b Spectral changes were not suitable for accurate
measurement of the binding constant.
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their log K values are in the following order: R = NO2 (3d) >
CF3 (3c) > Cl (3b) > H (3a), which is in accordance with the
decreasing electron-withdrawing ability of the substituent R on
the acetylide ligand of 3a–3d. This result could be rationalized
by the fact that the electron-withdrawing substituent R on the
acetylide ligand of 3a–3d would increase the acidity of urea
N–H(s) and strengthen the hydrogen bond interactions
between the urea N–H(s) of the complexes and anions. In
addition, for the same complex with different anions, the log K
values are in the following order: F− > OAc− > Cl− > Br−
≈
−
−
HSO4 ≈ NO3 > I−, which is in accordance with the decrease
in the basicity of the anions.
Anion binding abilities of complexes of 3a–3d towards F− in
DMSO
In our previous study on the anion sensing studies of gold(I)
acetylide complexes,13b the deprotonation process of the N–H
of urea group could be observed upon the addition of F− into
the DMSO solution of gold(I) acetylide complex with the nitro-
group. To examine this phenomenon in the platinum(II)
1
complex 3d, the UV-vis and H NMR titration experiments of
3d towards F− in DMSO and DMSO-d6 have been performed.
Fig. 8(a) and (b) show the UV-vis spectral changes of 3d in
DMSO upon the addition of F− at 298 K. When 0–9 equivalents
of F− are added, the absorbance of the absorption bands at
285, 332, as well as 346 nm gradually decreases, while the
absorbance at 379 and 489 nm increases. Well-defined isos-
bestic points are observed at 308 and 371 nm (Fig. 8(a)). After
more than 9 equivalents of F− are added, well-defined isos-
bestic points are observed at 296, 326 and 420 nm, and the
absorbance intensity of the band at 489 nm increases remark-
ably (Fig. 8(b)). Thus, the colour of the solution of 3d gradually
changes from yellow to red. We propose that two processes
occur upon the addition of F− into the DMSO solution of 3d.
The first one is the hydrogen bond interaction between the
urea N–H(s) of 3d and F−, which occurs at low concentrations
of F−. The second one is the deprotonation of the urea N–H of
3d when the concentration of F− is high. The appearance of
the low-energy absorption band at 489 nm is ascribed to the
deprotonation of the urea N–H of 3d (vide infra). In contrast,
upon the stepwise addition of F− into 3a–3c in DMSO
(Fig. S34–S36, ESI†), the colour changes from yellow to orange
and UV-vis spectral changes are similar to those of 3a–3c with
F− in CH3CN (Fig. S13, S19 and S26, ESI†, respectively). The
binding constants of 3a–3c with F− in DMSO are smaller than
those in CH3CN (Table S5, ESI†). This result could be rational-
ized by the fact that the DMSO solvent could compete with F−
to form hydrogen bonds with urea group of 3a–3c and reduce
the binding abilities of 3a–3c towards F−. The UV-vis spectral
changes of 3d towards OAc− in DMSO (Fig. S37, ESI†) are
different from those of 3d towards F− and are similar to those
of 3d towards OAc− in CH3CN; furthermore, the colour of the
solution changes from yellow to orange. Moreover, the
addition of other anions (Cl−, Br−, NO3−, and HSO4−) into the
DMSO solution of 3d causes a slight spectral change (Fig. S38–
S41, ESI†, respectively). For 3a–3c, the addition of OAc− can
Fig. 8 UV-vis spectral changes of 3d (3 × 10−5 mol dm−3) in DMSO
upon the addition of F−: (a) 0–9 equiv., (b) 9–13 equiv.; (c) a plot of the
absorbance change at 489 nm as a function of the concentration of F−.
cause a slight spectral change but the addition of Cl−, Br−,
−
NO3−, and HSO4 also produces a slight spectral change in
DMSO solution (Fig. S42–S55, ESI†).
The 1H NMR titrations experiments of 3d with F− and OAc−
have also been carried out. Fig. 9 shows the spectral changes
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protons (Ha and Hb) disappear and the signals of Hc–f show a
slight shift. After 4 equiv. of F− is added, a triplet centred at
16.10 ppm (J = 120 Hz), which is assigned as the signal of
HF2−, appears (Fig. S56, ESI†). The signals of Hc–f on the acety-
lide ligand exhibits a slight shift when the amount of F−
added is less than 2 equiv. Upon the addition of more than 2
equiv. of F−, Hc, Hd, and Hf show the dramatic upfield shift,
which could be due to the enhancement of shielding effect,
resulting from the increased electron density of aromatic ring
through the increased through-bond negative charge delocali-
zation that is caused by the deprotonation process of urea N–H
on the acetylide ligand.30d In 19F NMR spectra, when the
addition of F− into the DMSO-d6 solution of 3d is larger than 3
equiv., a doublet centred at −142.5 ppm (J = 129 Hz) is
−
observed, suggesting the formation of HF2 (Fig. S57, ESI†).
This result also supports the deprotonation process of the urea
N–H of 3d in DMSO-d6.
Fig. 9 1H NMR spectral changes of 3d upon the addition of OAc− in
DMSO-d6.
Conclusion
of 3d upon the addition of OAc− in DMSO-d6. An obvious
downfield shift of urea N–H (Ha and Hb) is found upon the
addition of OAc− from 0 to 2.5 equiv. Further addition after 3.0
equiv. does not cause any spectral changes. This result indi-
cates that there are hydrogen bond interactions between the
urea N–Hs of 3d and the acetate anion. The signals of Hc–g
undergo a slight upfield or downfield shift upon the addition
of OAc− from 0 to 4.0 equiv., which may be due to the through-
bond propagation or through-space effect, respectively.30
Fig. 10 shows the 1H NMR spectral changes of 3d upon the
addition of F− in DMSO-d6. Upon the addition of 0.2 equiv. of
F− into the DMSO-d6 solution of 3d, the signals of the urea
Platinum(II) acetylide complexes bearing urea group 3a–3d
have been synthesized and characterized. These complexes
show orange light emission in the solid state at 298 K, which
comes from the 3MLCT/3LLCT excited states. In CH3CN, the
anion-binding constant (log K) of this class of complexes
towards the same anion depends on the substituent R on the
acetylide ligand of 3a–3d: R = NO2 (3d) > CF3 (3c) > Cl (3b) > H
(3a). In addition, for the same complex with different anions,
the log K values follow this order: F− > OAc− > Cl− > Br−
≈
−
−
HSO4 ≈ NO3 > I−. In DMSO, 3d shows the selective colour
change towards F−, and this is ascribed to the deprotonation
of urea N–H of the acetylide ligand of 3d. The solvent-depen-
dent and selective colour change of 3d towards F− allows the
naked eye detection of F−.
Experimental
Materials and reagents
[Pt(tBu3tpy)Cl](OTf) (2) was synthesized according to the litera-
ture procedure.31 4-Ethynylaniline was obtained from Acros.
4-Chlorophenyl isocyanate, K2PtCl4, LiClO4 and LiSO3CF3 were
obtained from Energy Chemical. 4-Trifluoromethyl isocyanate,
and 4-nitrophenyl isocyanate were obtained from J&K. 4,4′,4″-Tri-
tert-butyl-2,2′:6′,2″-terpyridine was obtained from Sigma-Aldrich.
The solvents were dried and distilled, prior to use except that
those used for spectroscopic measurements were of spectro-
scopic grade. Other reagents were purchased from commercial
sources and used as received unless stated otherwise. All the
reactions were carried out under anhydrous and anaerobic con-
ditions using standard Schlenk techniques under nitrogen.
Synthesis
Fig. 10 1H NMR spectral changes of 3d upon the addition of F− in
DMSO-d6 at 6.90–9.85 ppm.
HCuCC6H4-4-NHC(O)NHC6H5 (1a). A mixture of 4-ethynyl-
aniline (300 mg, 2.56 mmol) and isocyanatobenzene (391 mg,
7792 | Dalton Trans., 2015, 44, 7785–7796
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2.54 mmol) in dichloromethane (30 mL) was refluxed for 24 h NH), 8.81 (s, 1H, NH), 8.71–8.70 (m, 4H, tpy), 7.90 (dd, J = 6
under N2. The yellow precipitate was collected and washed Hz, 2 Hz, 2H, tpy), 7.49–7.30 (m, 8H, aromatic ring), 1.52 (s,
with dichloromethane and diethyl ether. Yield: 555 mg, 80%. 9H, Bu), 1.43 (s, 18H, Bu). IR (KBr, cm−1): ν = 3356 (N–H),
1H NMR (300 MHz, DMSO-d6, 298 K) δ = 8.85 (s, 1H, NH), 8.70 1641 (CvO). ESI-MS: m/z = 866 [M − OTf]+. Anal. calcd for
(s, 1H, NH), 7.43 (t, J = 7.0 Hz, 4H, aromatic ring), 7.76–7.35 C43H45ClF3N5O4PtS: C, 50.86; H, 4.47; N, 6.90%. Found: C,
(m, 6H, aromatic ring), 7.26 (t, J = 8 Hz, 2H, aromatic ring), 50.87; H, 4.46; N, 6.86%.
t
t
6.96 (s, 1H, aromatic ring), 4.03 (s, 1H, HCuC). IR (KBr,
[Pt(tBu3tpy)(CuCC6H4-4-NHC(O)NHC6H4-4-CF3)](OTf) (3c).
cm−1): ν = 3293 (N–H), 1627 (CvO). Anal. calcd for C15H12N2O: The procedure was similar to that of complex 3a, except 1c was
C, 76.25; H, 5.12; N, 11.86%. Found: C, 76.55; H, 5.08; used instead of 1a. Yield: 55 mg, 63%. 1H NMR (300 MHz,
N, 11.67%.
DMSO-d6, 298 K) δ = 9.17 (s, 1H, NH), 8.94 (s, br, 3H, tpy +
HCuCC6H4-4-NHC(O)NHC6H4-4-Cl (1b). The procedure was NH), 8.69–8.65 (m, 4H, tpy), 7.87 (dd, J = 6 Hz, 2 Hz, 2H, tpy),
similar to that of complex 1a, except 4-chlorophenyl isocyanate 7.66–7.64 (m, 4H, aromatic ring), 7.45 (d, J = 8.9 Hz, 2H, aro-
was used instead of isocyanatobenzene. Yield: 540 mg, 78%. matic ring), 7.38 (d, J = 8.6 Hz, 2H, aromatic ring), 1.52 (s, 9H,
t
1H NMR (300 MHz, DMSO-d6, 298 K) δ = 8.90 (s, 1H, NH), 8.86 tBu), 1.43 (s, 18H, Bu). IR (KBr, cm−1): ν = 3433 (N–H), 1610
(s, 1H, NH), 7.47–7.38 (m, 4H, aromatic ring), 7.35–7.29 (CvO). ESI-MS: m/z = 900 [M − OTf]+. Anal. calcd for
(m, 4H, aromatic ring), 4.04 (s, 1H, HCuC). IR (KBr, cm−1): ν = C44H45F6N5O4PtS: C, 50.38; H, 4.32; N, 6.68%. Found: C, 50.40;
3281 (N–H), 1641 (CvO). Anal. calcd for C15H11ClN2O: C, H, 4.30; N, 6.67%.
66.55; H, 4.10; N, 13.09%. Found: C, 66.38; H, 4.12; N, 13.01%.
[Pt(tBu3tpy)(CuCC6H4-4-NHC(O)NHC6H4-4-NO2)](OTf) (3d).
HCuCC6H4-4-NHC(O)NHC6H4-4-CF3 (1c). The procedure The procedure was similar to that of complex 3a, except 1d was
was similar to that of complex 1a, except 4-trifluoromethyl iso- used instead of 1a. Yield: 50 mg, 70%. 1H NMR (300 MHz,
cyanate was used instead of isocyanatobenzene. Yield: 466 mg, DMSO-d6, 298 K) δ = 9.46 (s, 1H, NH), 9.01 (s, br, 3H, tpy +
72%. 1H NMR (300 MHz, DMSO-d6, 298 K) δ = 9.14 (s, 1H, NH), 8.71–8.69 (m, 4H, tpy), 8.18 (d, J = 9 Hz, 2H, aromatic
NH), 9.00 (s, 1H, NH), 7.62 (s, 4H, aromatic ring), 7.46 (d, J = ring), 7.89 (dd, J = 6 Hz, 2 Hz, 2H, tpy), 7.68 (d, J = 9 Hz, 2H,
9 Hz, 2H, aromatic ring), 7.38 (d, J = 9 Hz, 2H, aromatic ring), aromatic ring), 7.47–7.39 (m, 4H, aromatic ring), 1.53 (s, 9H,
t
4.05 (s, 1H, HCuC). IR (KBr, cm−1): ν = 3321 (N–H), 1647 tBu), 1.44 (s, 18H, Bu). IR (KBr, cm−1): ν = 3350 (N–H), 2054
(CvO). Anal. calcd for C16H11F3N2O: C, 63.16; H, 3.64; N, (CuC), 1620 (CvO). ESI-MS: m/z = 877 [M − OTf]+. Anal. calcd
9.21%. Found: C, 62.98; H, 3.63; N, 9.11%.
for C43H45F3N6O6PtS: C, 50.34; H, 4.42; N, 8.19%. Found: C,
HCuCC6H4-4-NHC(O)NHC6H4-4-NO2 (1d). The procedure 50.06; H, 4.48; N, 8.27%.
was similar to that of complex 1a, except 4-nitrophenyl iso-
Measurements and instrumentation
cyanate was used instead of isocyanatobenzene. Yield: 166 mg,
46%. 1H NMR (300 MHz, DMSO-d6, 298 K) δ = 9.46 (s, 1H, Electronic absorption spectra were measured on a PGENERAL
NH), 9.10 (s, 1H, NH), 8.18 (d, J = 9 Hz, 2H, aromatic ring), TU1901 UV-vis spectrophotometer. Emission spectra were
7.67 (d, J = 9 Hz, 2H, aromatic ring), 7.48 (d, J = 9 Hz, 2H, aro- obtained on
a FLSP920 fluorescence spectrophotometer.
matic ring), 7.40 (d, J = 8.6 Hz, 2H, aromatic ring), 4.06 (s, 1H, Chemical shifts (δ, ppm) were reported relative to tetramethyl-
HCuC). IR (KBr, cm−1): ν = 3290 (N–H), 1641 (CvO). Anal. silane for 1H NMR and NaF (δ = −122.4 ppm) for 19F NMR on a
calcd for C15H11N3O3: C, 64.05; H, 3.94; N, 14.94%. Found: C, Varian Mercury−Plus 300 spectrometer. Infrared spectra were
63.98; H, 3.93; N, 14.94%.
recorded from KBr pellets in the range 400–4000 cm−1 on a
(3a). A Bruker-EQUINOX 55 FT-IR spectrometer. Electrospray ioniza-
[Pt(tBu3tpy)(CuCC6H4-4-NHC(O)NHC6H5)](OTf)
mixture of 2 (70 mg, 0.088 mmol), 1a (24 mg, 0.089 mmol), tion (ESI) mass spectra were recorded on a LCQ DECA XP
CuI (5 mg, 0.026 mmol), and diisopropylamine (1 mL) in quadrupole ion trap mass spectrometer. Elemental analysis
dichloromethane (25 mL) was stirred for 18 h at room temp- was performed on an Elementar Vario EL elemental analyzer.
erature. The orange precipitate was collected and washed with
Crystal structure determination
diethyl ether. The product was recrystallized from acetonitrile.
Yield: 69 mg, 80%. 1H NMR (300 MHz, DMSO-d6, 298 K) δ = Crystals of 3a, 3b·CH3CN, and 3c·CH3CN were grown by the
8.89 (d, J = 7 Hz, 2H, tpy), 8.74 (s, 1H, NH), 8.66–8.65 (m, 5H, diffusion of diethyl ether into DMF–MeOH (v/v = 1), DMF–
tpy + NH), 7.85 (dd, J = 6 Hz, 2 Hz, 2H, tpy), 7.48–7.40 (m, 4H, CH3CN (v/v = 1), and DMF–CH3CN (v/v = 1) solutions, respecti-
aromatic ring), 7.42–7.35 (m, 2H, aromatic ring), 7.27 (t, J = 8 vely. Crystals of 3a·DMF·THF were obtained by layering diethyl
Hz, 2H, aromatic ring), 6.96 (t, J = 8 Hz, 1H, aromatic ring), ether onto DMF–THF (v/v = 1) solution. Selected single crystals
t
t
1.52 (s, 9H, Bu), 1.43 (s, 18H, Bu). IR (KBr, cm−1): ν = 3420 of 3b·CH3CN and 3c·CH3CN were used for data collection on a
(N–H), 1612 (CvO). ESI-MS: m/z = 832 [M − OTf]+. Anal. calcd Rigaku R-AXIS Spider IP with graphite monochromatized Mo-
for C43H46F3N5O4PtS: C, 52.65; H, 4.75; N, 7.14%. Found: C, Kα radiation (λ = 0.71073 Å) and Cu-Kα (λ = 1.54178 Å),
52.61; H, 4.76; N, 7.13%.
respectively, while 3a and 3a·DMF·THF were obtained by
[Pt(tBu3tpy)(CuCC6H4-4-NHC(O)NHC6H4-4-Cl)](OTf) (3b). Oxford Gemini S Ultra CCD area detector diffractometer with
The procedure was similar to that of complex 3a, except 1b was graphite monochromatized Mo-Kα radiation (λ = 0.71073 Å).
used instead of 1a. Yield: 45 mg, 59%. 1H NMR (300 MHz, An empirical absorption correction was applied using the
DMSO-d6, 298 K) δ = 9.02 (d, J = 7 Hz, 2H, tpy), 8.85 (s, 1H, PROCESS-AUTO (Rigaku, 1998) or CrysAlisPro (Version
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1.171.36.32) program. The structures were solved by direct
methods and refined by the full-matrix least-square method
based on F2 with the program SHELXL-2013 using the OLEX2
software package.32 In 3a, large regions of diffuse electron
density that could not be modeled (disordered solvents) were
removed from the refinement, using SQUEEZE function in
PLATON (see cif for details). CCDC 1028565, 1028514,
1028592, and 1028516 contain the supplementary crystallo-
graphic data for 3a, 3a·DMF·THF, 3b·CH3CN, and 3c·CH3CN,
respectively.
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For a typical UV-vis titration experiment of 3a–3d in CH3CN,
0.2 μL aliquots of a tetra-n-butylammonium salt (concen-
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dm−3) by a syringe, and the spectral changes were recorded by
a PGENERAL TU1901 UV-vis spectrophotometer at 298 K. The
UV-vis titration experiments in DMSO were the same as those
in CH3CN, expect the concentration of 3a–3d was changed to
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3 × 10−4 mol dm−3. For a typical H NMR titration experiment
of 3d in DMSO-d6, 0.5 μL aliquots of a tetra-n-butylammonium
salt ([F−] = 0.25 mol dm−3, [OAc−] = 0.3 mol dm−3 in DMSO-d6)
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1
3d in DMSO-d6 (9.7 × 10−4 mol dm−3) by a syringe, and the H
NMR and 19F NMR spectral changes were recorded by a Varian
Mercury-Plus 300 spectrometer at 298 K. The binding constant
log K values were determined by nonlinear fitting using
1 : 1 model.33 Job’s plots were obtained from a series of solu-
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keeping the total concentration (the complexes and anions)
constant. The maxima of the plots indicated the binding stoi-
chiometry of the complexes with anions.
Acknowledgements
We acknowledge financial support from the National Natural
Science Foundation of China (20971131 and J1103305), the
Natural Science Foundation of Guangdong Province
(S2012010010566), and Sun Yat-Sen University.
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7796 | Dalton Trans., 2015, 44, 7785–7796
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