Luminescence, electrochemistry and host-guest properties of dinuclear platinum(ii) terpyridyl complexes of sulfur-containing bridging ligands

Article abstract of DOI:10.1039/b821264c

A series of dinuclear platinum(ii) terpyridyl and terpyridyl-crown complexes with 2,2-dicyano-1,1-ethylenedithiolate (i-mnt), 1,3-benzenedithiolate (SC₆H₄S-1,3) and N,N-diethyldithiocarbamate (dtc) bridging ligands have been synthesi

Full text of DOI:10.1039/b821264c

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Luminescence, electrochemistry and host–guest properties of dinuclear platinum(II) terpyridyl
complexes of sulfur-containing bridging ligands
Rowena Pui-Ling Tang, Keith Man-Chung Wong, Nianyong Zhu and Vivian Wing-Wah Yam,
Dalton Trans., 2009, DOI: 10.1039/b821264c
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PAPER
www.rsc.org/dalton | Dalton Transactions
Luminescence, electrochemistry and host–guest properties of dinuclear
platinum(II) terpyridyl complexes of sulfur-containing bridging ligands†
Rowena Pui-Ling Tang, Keith Man-Chung Wong, Nianyong Zhu and Vivian Wing-Wah Yam*
Received 27th November 2008, Accepted 19th February 2009
First published as an Advance Article on the web 16th March 2009
DOI: 10.1039/b821264c
A series of dinuclear platinum(II) terpyridyl and terpyridyl-crown complexes with
2,2-dicyano-1,1-ethylenedithiolate (i-mnt), 1,3-benzenedithiolate (SC6H4S-1,3) and
N,N-diethyldithiocarbamate (dtc) bridging ligands have been synthesized and characterized. Their
photophysical and electrochemical properties, together with that of the related mononuclear
platinum(II) terpyridyl-crown complex and its crown-free analogue, have been studied. The ion-binding
properties of the terpyridyl-crown complexes have been determined by electronic absorption
spectroscopy and ESI-mass spectrometry. The X-ray crystal structures of
∫
[Pt(trpyC C-benzo-15-crown-5)Cl]PF6, [{Pt(trpy)}2(m-SC6H4S-1,3)](PF6)2 and
[{Pt(trpy)}2{m-(i-mnt)}](PF6)2 have also been determined.
from the nitrogen-donor bridging ligands, other related dinuclear
Introduction
platinum(II) terpyridyl systems using sulfur-containing bridg-
Square planar d8 metal complexes are well known to show a
pronounced tendency to aggregate, which allows metal–metal
and/or ligand–ligand interactions to occur.1–4 Novel features in
ing ligands, such as [{Pt(trpy)}2(m-dtc)]3+,5e and [{Pt(trpy)}2(m-
DMcT)]2+5f (dtc = N,N-diethyldithiocarbamate and DMcT =
2,5-dimercapto-1,3,4-thiadiazole), were also reported. Despite a
the electronic absorption and emission spectra of d8–d8 dinuclear
complexes have also attracted growing attention, and have led to
extensive investigations on the photophysics and photochemistry
of dinuclear d8–d8 metal complexes. Early works include [Rh2(1,3-
diisocyanopropane)4]2+,1a,b [Rh2(TMB)4]2+ (TMB7nbsp;= 2,5-
dimethyl-2,5-diisocyanohexane),1b [Rh2(TMB)2(dppm)2]2+,1c and
[Rh2(CNR)8]2+,1d in which the lowest-lying excited state was shown
to possess a relatively strong Rh–Rh bond. The photophysical
properties of dinuclear platinum(II) complexes have also been
studied. The most thoroughly investigated systems include the
infinite-chain [Pt(CN)4]2-2b and the luminescent dinuclear com-
plexes such as [Pt2(H2P2O5)4]4-2c that possess metal–metal bonded
(ds*)1(ps)1 excited states; the absorption and emission properties
of which have been studied by Gray, Roundhill and Che in detail.2c
Recently, there has also been a growing interest in the study
of platinum(II) polypyridine complexes owing to the intriguing
photophysical and spectroscopic properties reported of this class
compounds,3–7 in particular platinum(II) terpyridyl systems,4–7
in which metal–metal and/or ligand–ligand interactions have
been shown to play an important role in governing the spectro-
scopic properties of these complexes. Extension of the work to
dinuclear systems with well-defined metal–metal separation has
also been made. Examples include the studies of [{Pt(trpy)}2(m-
gua)]3+ (gua = guandine anion)5b and [{Pt(trpy)}2(m-X)]3+ (X =
pyrazole, azaindole, canavanine, diphenylformamidine),5a,c,d which
provided insights into the role of platinum–platinum separation in
governing the spectroscopic properties of these complexes. Apart
number of studies on these platinum(II) terpyridyl systems,4–7
relatively less was explored of the utilization of these systems for
chemosensing studies.6,7a,b,f–m The recent growth of interests in the
chemistry of crown ethers and other related inclusion compounds
has been prompted in part by the potentials of utilizing such
compounds in molecular recognition studies and the design of
molecular switches and probes. There are numerous excellent
reviews on transition metal complexes as ion and molecular
sensors,8 however, utilization of crown ether in dinuclear systems is
rare. Recently, we reported a series of novel crown ether-containing
dinuclear gold(I) complexes, which showed interesting lumines-
cence properties upon binding of potassium and caesium ions.9 As
an extension of our reported mononuclear crown ether-containing
platinum(II) terpyridyl complexes, [Pt(trpy)(S-benzo-15-crown-
5)]+,6a [Pt(trpy)(C C-benzo-15-crown-5)] , and [Pt(trpy)(C C-
N-phenylaza-15-crown-5)]+,7j attempts have been made to de-
sign and synthesize dinuclear d8–d8 platinum(II) terpyridyl and
terpyridyl-crown complexes containing various sulfur-containing
bridging ligands and to investigate the effect of metal–metal
interaction on the luminescence properties of these dinuclear
platinum(II) terpyridyl system. Exploration into the possibility
of utilizing these dinuclear crown ether-containing platinum(II)
complexes with metal–metal interactions for chemosensing work
has been made. Herein are reported the syntheses, photophysics,
electrochemistry and supramolecular ion-binding properties of a
series of dinuclear platinum(II) terpyridyl and terpyridyl-crown
complexes with 2,2-dicyano-1,1-ethylenedithiolate (i-mnt), 1,3-
benzenedithiolate (SC6H4S-1,3) and N,N-diethyldithiocarbamate
(dtc) bridging ligands. The syntheses and photophysical be-
haviour of the related mononuclear platinum(II) terpyridyl-
crown complex and its crown-free analogue have also been
studied.
+
7a,j
∫
∫
Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong SAR, People’s Republic of China. Fax: +852 2857 1586;
Tel: +852 2859 2153
† CCDC reference numbers 711464–711466. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b821264c
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3911–3922 | 3911
©

Synthesis of dinuclear platinum(II) terpyridyl complexes
Experimental
[{Pt(trpy)}2(l-SC6H4S-1,3)](PF6)2 (3). To a stirred solution of
[Pt(trpy)Cl]Cl·H2O (375 mg, 0.70 mmol) in methanol (40 mL) was
added 1,3-benzenedithiol (50 mg, 0.35 mmol) in the presence of
an excess of triethylamine (0.1 mL). The reaction mixture turned
from red to dark purple immediately and was stirred for 4 h at
room temperature. The resultant mixture was filtered and to the
filtrate was added a saturated solution of tetra-n-butylammonium
hexafluorophosphate in methanol. The product precipitated out
and was filtered, washed with methanol and air dried. Subsequent
recrystallization by diffusion of diethyl ether vapour into an
acetonitrile solution of the product gave 3 as dark purple crystals.
Yield: 1.35 g, 75%. 1H NMR (400 MHz, CD3CN, 298 K, relative
to Me4Si): d 6.91 (t, 1H, J = 7.8 Hz, –C6H4–), 7.21 (dd, 2H,
J = 1.9 Hz, 7.8 Hz, –C6H4–), 7.58 (m, 4H, trpy), 8.10 (m, 8H,
trpy), 8.21 (d, 4H, J = 8.1 Hz, trpy), 8.41 (s, 1H, –C6H4–), 8.49
(t, 2H, J = 8.1 Hz, trpy), 9.11 (d, 4H, J = 5.6 Hz, trpy). Positive
ESI-MS: m/z 1141 [M - PF6]+, 498 [M - 2PF6]2+. Anal. calcd for
C36H26N6Pt2S2P2F12: C 33.59, H 2.02, N 6.53. Found: C 33.61, H
1.99, N 6.57.
Reagents and materials
Dichloro(1,5-cyclooctadiene)platinum(II) and 2,2¢:6¢,2”-terpyri-
dine were obtained from Strem Chemicals Inc. 1,3-Benzen-
edithiol ((HS)2C6H4-1,3) was purchased from Aldrich Chemical
Co. [Pt(trpy)Cl]Cl·2H2O,10 trpyC C-benzo-15-crown-5, trpy-
11
∫
11
∫
C CC6H3(OCH3)2-3,4, potassium 2,2-dicyano-1,1-ethylenedi-
thiolate (K2(i-mnt)),12 diethylammonium 2,2-dicyano-1,1-ethyl-
enedithiolate ((Et2NH2)2(i-mnt)),12 and diethylammonium N,N-
diethyldithiocarbamate ((Et2NH2)dtc)12 were synthesized accord-
ing to the published procedures. Tetra-n-butylammonium hexaflu-
orophosphate (nBu4NPF6) (Aldrich, 98%) were purchased and
was recrystallized three times from hot ethanol prior to use.
Triethylamine was distilled over potassium hydroxide and stored
in the presence of potassium hydroxide prior to use. All other
reagents were of analytical grade and were used as received.
Synthesis of platinum(II) terpyridyl precursors
∫
[Pt(trpyC C-benzo-15-crown-5)Cl]X (X = Cl 1a; PF6 1b). To a
[{Pt(trpy)}2{l-(i-mnt)}](PF6)2 (4). To a stirred solution of
[Pt(trpy)Cl]Cl·H2O (100 mg, 0.19 mmol) in water (30 mL) was
added K2(i-mnt) (22 mg, 0.09 mmol) in water (10 mL) in a dropwise
manner. The orange solution turned to a dark red solution
immediately and was stirred for 2 h at room temperature. The
resultant mixture was filtered and to the filtrate was added a satu-
rated solution of tetra-n-butylammonium hexafluorophosphate in
methanol. The product was isolated, washed with methanol and
air dried. Subsequent recrystallization by diffusion of diethyl ether
vapour into an acetonitrile solution of the product gave 4 as dark
suspension of Pt(COD)Cl2 (100 mg, 0.27 mmol) in water-methanol
(1 : 1 v/v) (30 mL) was added trpyC C-benzo-15-crown-5 (153 mg,
∫
0.29 mmol). The reaction mixture was stirred at 50 ◦C for 1 h
during which the orange solution turned to a deep red solution.
After evaporation to dryness, red solids of 1a were obtained
and recrystallized by diffusion of diethyl ether vapour into an
acetonitrile solution of the complex. On the other hand, diffraction
quality crystals of 1b were collected after metathesis reaction
with tetra-n-butylammonium hexafluorophosphate in methanol,
followed by subsequent recrystallization by diffusion of diethyl
ether vapour into an acetonitrile solution of the product to give
1b as red crystals. Yield for 1b: 128 mg, 53%. 1H NMR (400 MHz,
(CD3)2SO, 298 K, relative to Me4Si): d 3.64 (m, 8H, –OCH2–),
3.81 (m, 4H, C6H3OCH2–), 4.13 (m, 4H, C6H3OCH2–), 7.09 (d,
1H, J = 8.3 Hz, –C6H3–), 7.19 (d, 1H, J = 1.7 Hz, –C6H3–), 7.28
(dd, 1H, J = 1.7 Hz, 8.3 Hz, –C6H3–), 7.95 (t, 2H, J = 6.2 Hz,
trpy), 8.51 (t, 2H, J = 7.7 Hz, trpy), 8.68 (d, 2H, J = 7.7 Hz,
1
red crystals. Yield: 400 mg, 82%. H NMR (400 MHz, CD3CN,
298 K, relative to Me4Si): d 7.57 (t, 4H, J = 7.7 Hz, trpy), 7.85 (d,
4H, J = 8.0 Hz, trpy), 7.92 (d, 4H, J = 8.2 Hz, trpy), 8.24 (t, 4H,
J = 7.7 Hz, trpy), 8.37 (t, 2H, J = 8.0 Hz, trpy), 8.47 (d, 4H, J =
-1
=
5.4 Hz, trpy). IR (Nujol mull on KBr disc, n/cm ): 1446 n(C C).
Positive ESI-MS: m/z 1141 [M - PF6]+, 498 [M - 2PF6]2+. Anal.
calcd for C34H22N8Pt2S2P2F12: C 31.72, H 1.71, N 8.71. Found: C
31.73, H 1.75, N 8.76.
trpy), 8.82 (s, 2H, trpy), 8.87 (d, 2H, J = 4.8 Hz, trpy). IR (Nujol
-1
∫
[{Pt(trpyC C-benzo-15-crown-5)}2{l-(i-mnt)}](PF6)2
(5).
∫
mull on KBr disc, n/cm ): 2120 n(C C). Positive ESI-MS: m/z
754 [M - PF6]+, 1653 [2M - PF6]+. Anal. calcd for C31H29N3O5-
PtClPF6: C 41.43, H 3.22, N 4.68. Found: C 41.48, H 3.26,
N 4.70.
The procedure was similar to that for complex 4 except
∫
[Pt(trpyC C-benzo-15-crown-5)Cl]Cl (1a) (100 mg, 0.13 mmol)
and (Et2NH2)2(i-mnt) (18 mg, 0.06 mmol) were used in place of
[Pt(trpy)Cl]Cl·H2O and K2(i-mnt), respectively, to give dark red
crystals of 5. Yield: 325 mg, 67%. 1H NMR (400 MHz, (CD3)2SO,
298 K, relative to Me4Si): d 3.67 (m, 16H, –OCH2–), 3.86 (m,
12H, –OCH2–, C6H3OCH2–), 4.15 (m, 4H, C6H3OCH2–), 6.96 (d,
2H, J = 8.2 Hz, –C6H3–), 7.00 (d, 2H, J = 1.3 Hz, –C6H3–), 7.19
(dd, 2H, J = 1.3 Hz, 8.2 Hz, –C6H3–), 7.81 (m, 4H, trpy), 8.41 (m,
∫
[Pt(trpyC CC6H3(OCH3)2-3,4)Cl]X (X = Cl 2a; PF6 2b).
The procedure was similar to that for complex 1 except
∫
trpyC CC6H3(OCH3)2-3,4 (114 mg, 0.29 mmol) was used in place
of trpyC C-benzo-15-crown-5 to give red crystals of 2a and 2b.
∫
Yield for 2b: 163 mg, 59%. 1H NMR (400 MHz, (CD3)2SO, 298 K,
relative to Me4Si): d 3.88 (s, 3H, –OCH3), 3.90 (s, 3H, –OCH3),
7.04 (d, 1H, J = 8.4 Hz, –C6H3–), 7.20 (d, 1H, J = 2.0 Hz, –C6H3–),
7.31 (dd, 1H, J = 2.0 Hz, 8.4 Hz, –C6H3–), 7.95 (t, 2H, J = 7.1 Hz,
trpy), 8.55 (t, 2H, J = 7.1 Hz, trpy), 8.69 (d, 2H, J = 8.0 Hz,
8H, trpy), 8.60 (d, 4H, J = 5.3 Hz, trpy), 8.63 (s, 4H, trpy). IR
-1
∫
=
(Nujol mull on KBr disc, n/cm ): 2201 n(C C), 1450 n(C C).
Positive ESI-MS: m/z 1722 [M - PF6]+, 789 [2M - 2PF6]2+. Anal.
calcd for C66H58N8O10Pt2S2P2F12: C 42.44, H 3.11, N 6.00. Found:
C 42.47, H 3.08, N 5.95.
trpy), 8.80 (s, 2H, trpy), 8.92 (d, 2H, J = 4.6 Hz, trpy). IR (Nujol
-1
∫
∫
mull on KBr disc, n/cm ): 2122 n(C C). Positive ESI-MS: m/z
[{Pt(trpyC CC6H3(OCH3)2-3,4)}2{l-(i-mnt)}](PF6)2 (6). The
623 [M - PF6]+, 1391 [2M - PF6]+. Anal. calcd for C25H19N3O2-
PtClPF6: C 39.06, H 2.47, N 5.47. Found: C 39.01, H 2.43,
N 5.49.
procedure was similar to that for complex
5
except
∫
[Pt(trpyC CC6H3(OCH3)2-3,4)Cl]Cl (2a) (86 mg, 0.13 mmol) was
∫
used in place of [Pt(trpyC C-benzo-15-crown-5)Cl]Cl to give dark
3912 | Dalton Trans., 2009, 3911–3922
This journal is The Royal Society of Chemistry 2009
©

red crystals of 6. Yield: 259 mg, 62%. 1H NMR (400 MHz,
(CD3)2SO, 298 K, relative to Me4Si): d 3.68 (s, 6H, –OCH3), 3.85
(s, 6H, –OCH3), 6.96 (d, 2H, J = 8.4 Hz, –C6H3–), 7.00 (d, 2H,
J = 1.4 Hz, –C6H3–), 7.17 (dd, 2H, J = 1.4 Hz, 8.4 Hz, –C6H3–),
7.78 (m, 4H, trpy), 8.39 (m, 8H, trpy), 8.59 (d, 4H, J = 5.4 Hz,
were degassed on a high-vacuum line in a two-compartment cell
consisting of a 10-ml Pyrex bulb and a 1-cm path length quartz
cuvette and sealed from the atmosphere by a Bibby Rotaflo HP6
Teflon stopper. The solutions were subject to no less than four
freeze–pump–thaw cycles.
trpy), 8.62 (s, 4H, trpy). IR (Nujol mull on KBr disc, n/cm-1): 2204
1H NMR (400 MHz) spectra were recorded on a Bruker DPX-
400 FT-NMR spectrometer at 298 K and chemical shifts are
reported relative to Me4Si. Positive ESI mass spectra were recorded
on a Finnigan LCQ mass spectrometer. Elemental analyses of the
new complexes were performed on a Carlo Erba 1106 elemental
analyzer at the Institute of Chemistry, Chinese Academy of
Sciences. Cyclic voltammetric measurements were performed by
using a CH Instruments, Inc. model CHI 620 Electrochemical
Analyzer, which was interfaced to a personal computer. The
electrolytic cell used was a conventional two-compartment cell.
Electrochemical measurements were performed in acetonitrile
solutions with 0.1 M nBu4NPF6 (TBAH) as supporting electrolyte
at room temperature. The reference electrode was a Ag/AgNO3
(0.1 M in acetonitrile) electrode and the working electrode was a
glassy carbon (Atomergic Chemetal V25) electrode with a piece
of platinum wire as counter electrode in a compartment which
is separated from the working electrode by a sintered glass frit.
The ferrocenium/ferrocene couple (FeCp2+/0) was used as the
internal reference.13a All solutions for electrochemical studies were
deaerated with pre-purified argon gas just before measurements.
Treatment of the electrode surfaces was as reported previously.13b
The electronic absorption spectral titration for binding constant
determination was performed with a Hewlett-Packard 8452A
diode array spectrophotometer at 25 ◦C, which was controlled by
a Lauda RM6 compact low-temperature thermostat. Supporting
electrolyte (0.1 mol dm-3 nBu4NPF6) was added to maintain a
constant ionic strength of the sample solution in order to avoid any
changes arising from a change in the ionic strength of the medium.
Binding constants for 1 : 1 complexation were determined by non-
linear least-squares fits to eqn (1), in which the derivations were
described previously.14
+
∫
=
n(C C), 1451 n(C C). Positive ESI-MS: m/z 1461 [M - PF6] , 658
[M - 2PF6]2+. Anal. calcd for C54H38N8O4Pt2S2P2F12: C 40.35, H
2.37, N 6.97. Found: C 40.37, H 2.38, N 6.94.
∫
[{Pt(trpyC C-benzo-15-crown-5)}2(l-dtc)](PF6)3 (7). The pro-
∫
cedure was similar to that for complex 4 except [Pt(trpyC C-
benzo-15-crown-5)Cl]Cl (1a) (100 mg, 0.13 mmol) and
(Et2NH2)dtc (14 mg, 0.06 mmol) were used in place of
[Pt(trpy)Cl]Cl·H2O and K2(i-mnt), respectively, to give dark red
crystals of 7. Yield: 331 mg, 63%. 1H NMR (400 MHz, CD3CN,
298 K, relative to Me4Si): d 1.61 (t, 3H, J = 7.2 Hz, –CH2CH3),
3.67 (m, 24H, –OCH2–), 3.85 (m, 4H, C6H3OCH2–), 4.06 (m, 4H,
C6H3OCH2–), 4.28 (q, 2H, J = 7.2 Hz, –CH2CH3), 6.59 (d, 2H,
J = 8.2 Hz, –C6H3–), 6.71 (d, 2H, J = 1.6 Hz, –C6H3–), 6.79
(dd, 2H, J = 1.6 Hz, 8.2 Hz, –C6H3–), 7.65 (t, 2H, J = 6.3 Hz,
trpy), 7.84 (d, 2H, J = 7.9 Hz, trpy), 7.92 (s, 2H, trpy), 8.25 (t,
2H, J = 7.9 Hz, trpy), 8.52 (d, 2H, J = 4.9 Hz, trpy). IR (Nujol
-1
∫
mull on KBr disc, n/cm ): 2196 n(C C). Positive ESI-MS: m/z
+
∫
867 [M - Pt(trpyC C-benzo-15-crown-5) - 3PF6] . Anal. calcd for
C67H68N7O10Pt2S2P3F18: C 39.82, H 3.37, N 4.85. Found: C 39.80,
H 3.33, N 4.81.
∫
[{Pt(trpyC CC6H3(OCH3)2-3,4)}2(l-dtc)](PF6)3
(8). The
5 except
procedure was similar to that for complex
∫
[Pt(trpyC CC6H3(OCH3)2-3,4)Cl]Cl (2a) (85 mg, 0.13 mmol)
∫
was used in place of [Pt(trpyC C-benzo-15-crown-5)Cl]Cl to
give dark red crystals of 8. Yield: 274 mg, 60%. 1H NMR
(400 MHz, CD3CN, 298 K, relative to Me4Si): d 1.62 (t, 3H, J =
7.1 Hz, –CH2CH3), 3.69 (s, 6H, –OCH3), 3.90 (s, 6H, –OCH3),
4.29 (q, 2H, J = 7.1 Hz, –CH2CH3), 6.87 (d, 2H, J = 8.2 Hz,
–C6H3–), 7.00 (d, 2H, J = 1.6 Hz, –C6H3–), 7.13 (dd, 2H, J =
1.6 Hz, 8.2 Hz, –C6H3–), 7.66 (t, 4H, J = 6.7 Hz, trpy), 7.95
(d, 4H, J = 7.9 Hz, trpy), 8.08 (s, 4H, trpy), 8.30 (t, 4H, J =
X lim - X 0
n+
1
È
Pt + M
]
˘
+
X = X 0
+
[
{
T
Î
˚
KS
7.9 Hz, trpy), 8.60 (d, 4H, J = 5.7 Hz, trpy). IR (Nujol mull
2 Pt
[
]
T
-1
∫
on KBr disc, n/cm ): 2190 n(C C). Positive ESI-MS: m/z 736
1
(1)
¸
2 Ô
˝
+
È
˘
˙
˚
∫
[M - Pt(trpyC CC6H3(OCH3)2-3,4) - 3PF6] . Anal. calcd for
Ê
ˆ2
n+
n+
1
È
˘
È
˘
M
-
Pt + M
+
- 4 Pt
[
]
[ ]
T Î
ÍÁ
Ë
˜
¯
T
Î
˚
˚
KS
C55H48N7O4Pt2S2P3F18: C 37.52, H 2.73, N 5.57. Found: C 37.50,
H 2.75, N 5.53.
Î
Ô
˛
where X0 and X are the absorbance of complex at a selected
wavelength in the absence and presence of the metal cation,
respectively, [Pt]T is the total concentration of the complex, [Mn+]
is the concentration of the metal cation Mn+, Xlim is the limiting
value of absorbance in the presence of excess metal ion and Ks is
the stability constant.
Physical measurements and instrumentation
UV-vis spectra were obtained on a Hewlett-Packard 8452A diode
array spectrophotometer, IR spectra on a Bio-Rad FTS-7 Fourier
transform infrared spectrophotometer (4000–400 cm-1), and
steady-state excitation and emission spectra on a Spex Fluorolog
111 spectrofluorometer. Solid-state photophysical studies were
carried out with solid samples contained in a quartz tube inside
a quartz-walled Dewar flask. Measurements of the butyronitrile
glasses or solid-state samples at 77 K were similarly conducted
with liquid nitrogen filled in the optical Dewar flask. Excited state
lifetimes were measured using a conventional laser system. The
excitation source was the 355-nm output (third harmonic, 8 ns)
of a Spectra-Physics Quanta-Ray Q-switched GCR-150 pulsed
Nd-YAG laser (10 Hz). All solutions for photophysical studies
Crystal structure determination
∫
Single-crystals of [Pt(trpyC C-benzo-15-crown-5)Cl]PF6 (1b),
[{Pt(trpy)}2(m-SC6H4S-1,3)](PF6)2 (3) and [{Pt(trpy)}2{m-(i-
mnt)}](PF6)2 (4) suitable for X-ray diffraction studies were grown
by vapour diffusion of diethyl ether into an acetonitrile solution
of the respective complexes. All the experimental details are given
in Table 1.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3911–3922 | 3913
©

Table 1 Crystal and structure determination data for 1b, 3 and 4
1b
3
4·4CH3CN
Empirical formula
Formula Weight
T/K
Crystal system
Space group
[C31H29ClN3O5PF6Pt]
899.08
301(2)
[C36H26N6S2P2F12Pt2]
1286.87
301
[(C34H22N8S2P2F12Pt2)·4CH3CN]
1451.04
301
Monoclinic
Monoclinic
Triclinic
¯
P21/n
P1 (no. 2)
a = 8.5245(9) A a = 105.638(7)
P21/c (no. 2)
◦
◦
◦
˚
˚
˚
˚
˚
˚
˚
Unit cell dimensions
a = 6.2690(13) A a = 90
a = 13.784(3) A a = 90
◦
◦
◦
◦
˚
˚
b = 14.713(3) A b = 95.08(3)
b = 13.522(2) A b = 95.371(7)
b = 22.003(2) A b = 108.09(1)
◦
◦
c = 34.398(7) A g = 90
c = 17.921(1) A g = 96.149(7)
c = 17.103(2) A g = 90
3
˚
V/A
3160.3(11)
Orange
4
1961.3(3)
Purple
4930(1)
Crystal colour
Z
Red
8
2784
1.955
0.30 ¥ 0.10 ¥ 0.07
0.71073
5.887
2
F(000)
1760
1.890
0.30 ¥ 0.15 ¥ 0.05
0.71073
4.657
1220
2.179
0.40 ¥ 0.15 ¥ 0.07
0.71073
7.380
Calculated density/g cm-3
Crystal size/mm3
˚
l/A
m/mm◦-1
2qmax
/
50.84
50
50
Index range
-7 ≤ h ≤ 6
-10 ≤ k ≤ 17
-39 ≤ l ≤ 39
7114
-10 ≤ h ≤ 10
0 ≤ k ≤ 16
-21 ≤ l ≤ 20
7208
0 ≤ h ≤ 16
0 ≤ k ≤ 26
-20 ≤ l ≤ 19
9320
Total number of reflections
Unique reflections
No. of data used in refinement
Completeness (%)
3657 [Rint = 0.0523]
6888 [Rint = 0.0280]
8927 [Rint = 0.0329]
3657
5288
6013
62.6
99.7
99.8
No. of parameters refined
Refinement method
433
541
639
Full-matrix least-squares on F2
0.913
Full-matrix least-squares on F2
1.21
Full-matrix least-squares on F2
1.58
Goodness-of-fit on F2
Final R indices
R1 = 0.0376, wR2 = 0.0860a
R = 0.024, wR = 0.033b
R = 0.034, wR = 0.041b
-3
-3
-3
˚
˚
˚
Largest diffraction peak and hole
0.542 and -0.822 e A
0.82, 0.58 e A
1.50, 1.10 e A
2
2
2
a w = 1/[s (Fo2)+(0.054P)2], where P is [2Fc2+Max(Fo2,0)]/3 with I > 2s(I). b w = 4Fo2/s2(Fo2), where s (Fo2) = [s (I) + (0.02Fo2)2] with I > 3s(I).
A crystal of 1b mounted on a glass fibre was used for data
collection at 28 ◦C on a MAR diffractometer with a 300 mm image
plate detector using graphite monochromatized MoKa radiation
one formula unit. In the least-squares refinement, all 60 non-H
atoms were refined anisotropically and 26 H atoms at calculated
positions with thermal parameters equal to 1.3 times that of the
attached C atoms were not refined. For 4, one crystallographic
asymmetric unit consisted of one formula unit. The F atoms of
one of the anion were disordered and were placed at 11 positions
with F(7), F(7¢), F(8), F(8¢), F(9), F(10), F(10¢), F(11), F(11¢),
F(12) and F(12¢) having occupation numbers of 0.8, 0.4, 0.4, 0.4,
0.8, 0.7, 0.5, 0.6, 0.4, 0.7 and 0.3, respectively. In the least-squares
refinement, 66 non-H atoms were refined anisotropically, the 11
disordered F atoms were refined isotropically and 34 H atoms at
calculated positions with thermal parameters equal to 1.3 times
that of the attached C atoms were not refined.
˚
(l = 0.71073 A). The images were interpreted and intensities
integrated using the program DENZO.15 The structure was solved
by direct methods employing SHELXS-9716 program on a PC. The
Pt, Cl and many non-H atoms were located according to the
direct methods. The positions of the other non-hydrogen atoms
were found after successful refinement by full-matrix least-squares
using program SHELXL-9717 on a PC. One crystallographic
-
asymmetric unit consisted of one formula unit, including one PF6
anion. In the final stage of least-squares refinement, all atoms
were refined anisotropically. H atoms were generated by program
SHELXL-97.17 The positions of H atoms were calculated based
on riding mode with thermal parameters equal to 1.2 times that
of the associated C atoms, and participated in the calculation of
final R-indices.
Results and discussion
Syntheses
Crystals of 3 and 4 mounted in a glass capillary were used
◦
∫
∫
for data collection at 28 C on a Rigaku AFC7R diffractometer
[Pt(trpyC C-benzo-15-crown-5)Cl]Cl (1a) and [Pt(trpyC
˚
with graphite monochromatized MoKa radiation (l = 0.71073 A)
CC6H3(OCH3)2-3,4)Cl]Cl (2a), which were prepared by
the modification of the literature reported procedures for
[Pt(trpy)Cl]Cl·H2O,13 together with [Pt(trpy)Cl]Cl·H2O, were
found to be good starting materials for the syntheses of
the dinuclear platinum(II) terpyridyl complexes. Reaction of
[Pt(trpy)Cl]Cl·H2O with 1,3-benzenedithiol in a 2 : 1 molar ratio
in methanol in the presence of triethylamine as the base afforded
3 as dark purple crystals. 4 was isolated as dark red crystals
by the reaction of [Pt(trpy)Cl]Cl·H2O with K2(i-mnt) in a 2 : 1
using w-2q scans with w-scan angle (0.73 + 0.35 tanq)◦ at a scan
speed of 8.0◦ min-1 (up to 6 scans for reflection with I < 15s(I)).
The space group was determined based on a statistical analysis
of intensity distribution and the successful refinement of the
structure solved by Patterson methods and expanded by Fourier
methods (PATTY)18 and refinement by full-matrix least squares
using the software package TeXsan19 on a Silicon Graphics Indy
computer. The crystallographic asymmetric unit of 3 consisted of
3914 | Dalton Trans., 2009, 3911–3922
This journal is
The Royal Society of Chemistry 2009
©

Scheme 1
molar ratio in water. The thiolate-bridged dinuclear platinum(II)
crown-ether containing terpyridyl complexes, 5 and 7, were
∫
synthesized by the reaction of [Pt(trpyC C-benzo-15-crown-
5)Cl]Cl with the corresponding salts of i-mnt and dtc in water
at room temperature, respectively. The diethylammonium salt of
i-mnt was used for the synthesis of 5 instead of K2(i-mnt) as in
the preparation of 4 to avoid the presence of K+ ions, which are
believed to bind to the crown moieties. Their crown-free analogues
6 and 8 were also successfully synthesized and characterized
in a similar manner. The schematic drawings of the newly
synthesized mononuclear precursor complexes and the dinuclear
platinum(II) terpyridyl and terpyridyl-crown complexes are
depicted in Scheme 1. The identities of all the newly synthesized
dinuclear platinum(II) complexes and platinum(II) precursor
complexes have been confirmed by satisfactory elemental
analyses, 1H HMR spectroscopy, IR spectroscopy, and ESI-mass
Fig.
1 Perspective drawing of the complex cation of
∫
[Pt(trpyC C-benzo-15-crown-5)Cl]PF6 1b with atomic numbering.
Hydrogen atoms have been omitted for clarity. Thermal ellipsoids were
shown at the 40% probability level.
∫
spectrometry. The crystal structures of [Pt(trpyC C-benzo-15-
5b
˚
crown-5)Cl]PF6 (1b), [{Pt(trpy)}2(m-SC6H4S-1,3)](PF6)2 (3) and
comparable to that of 2.302 A in the related [Pt(trpy)Cl]ClO4.
[{Pt(trpy)}2{m-(i-mnt)}](PF6)2 (4) have also been determined.
∫
The C(trpy)–C C unit is essentially linear with C(8)–C(16)–C(17)
bond angle of 177.4◦ and C(16)–C(17)–C(18) bond angle of 177.4◦
˚
∫
While the C C bond length of 1.173 A is typical of alkynes. The
phenyl ring of B15C-5 is essentially co-planar with respect to the
[Pt(trpy)] plane with the interplanar angle of 3.218◦, suggesting
X-Ray crystal structure determination
Fig. 1–3 show the perspective drawings of the complex cation
∫
∫
of [Pt(trpyC C-benzo-15-crown-5)Cl]PF6 (1b), [{Pt(trpy)}2(m-
that delocalization within the trpyC C-benzo-15-crown-5 ligand.
SC6H4S-1,3)](PF6)2 (3) and [{Pt(trpy)}2{m-(i-mnt)}](PF6)2 (4),
respectively. The crystal structure determination data are given in
Table 1 while the selected bond distances and bond angles are listed
in Table 2. The coordination geometry of all the complexes at the
platinum centre is distorted square planar, in which the distance
of the platinum to the inner nitrogen atom of the terpyridine
ligand is significantly shorter than those of the other two outer
nitrogen atoms. All the Pt–N distances and N–Pt–N bond angles
are comparable to those obtained in typical platinum(II) terpyridyl
Neither short Pt ◊ ◊ ◊ Pt contacts (the shortest Pt ◊ ◊ ◊ Pt distance =
˚
5.022 A) nor significant p–p interactions were observed in the
crystal lattices of 1b.
In complex 3, the bridging ligand, –SC6H4S–, links the
two platinum(II)-terpyridine co-ordination planes in an anti-
configuration with Pt(1)–S(1)–C(16) and Pt(2)–S(2)–C(20) bond
angles of 100.7◦ and 107.4◦, respectively, and a dihedral angle of
36.5◦ between the two coordination planes (Fig. 2). The bond
˚
˚
lengths of the Pt–S [Pt(1)–S(1), 2.316 A; Pt(2)–S(2), 2.303 A]
and S–C [S(1)–C(16), 1.781 A; S(1)–C(20), 1.774 A] bonds are
complexes.4a–e,5,6,7a–f,h,i,k The Pt–Cl distance of 2.308 A in 1b is
˚
˚
˚
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3911–3922 | 3915
©

◦
˚
Table 2 Selected bond distances (A) and bond angles ( ) with estimated
standard deviations (esds) in parentheses for 1b, 3 and 4
1b
Pt(1)–N(1)
2.01(1)
1.97(1)
2.03(1)
1.44(1)
81.8(4)
80.7(4)
162.4(3)
97.8(2)
179.6(3)
Pt(1)–Cl(1)
C(16)–C(17)
C(8)–C(16)
2.31(1)
1.17(1)
1.43(1)
Pt(1)–N(2)
Pt(1)–N(3)
C(17)–C(18)
N(1)–Pt(1)–N(2)
N(2)–Pt(1)–N(3)
N(1)–Pt(1)–N(3)
N(1)–Pt(1)–Cl(1)
N(2)–Pt(1)–Cl(1)
3
Pt(1)–S(1)
Pt(2)–S(2)
Pt(1)–N(1)
Pt(1)–N(2)
N(1)–Pt(1)–N(2)
N(2)–Pt(1)–N(3)
N(1)–Pt(1)–N(3)
N(4)–Pt(2)–N(5)
N(5)–Pt(2)–N(6)
N(4)–Pt(2)–N(6)
N(1)–Pt(1)–S(1)
4
N(3)–Pt(1)–Cl(1)
C(8)–C(16)–C(17)
C(16)–C(17)–C(18)
C(17)–C(18)–C(19)
C(17)–C(18)–C(23)
99.8(2)
178.4(13)
177.4(13)
119.1(9)
121.5(10)
2.316(1)
2.303(2)
2.034(4)
1.965(4)
80.8(2)
80.8(2)
161.5(2)
80.8(2)
Pt(1)–N(3)
Pt(2)–N(4)
Pt(2)–N(5)
Pt(2)–N(6)
N(2)–Pt(1)–S(1)
N(3)–Pt(1)–S(1)
N(4)–Pt(2)–S(2)
N(5)–Pt(2)–S(2)
N(6)–Pt(2)–S(2)
Pt(1)–S(1)–C(16)
Pt(2)–S(2)–C(20)
2.032(4)
2.024(5)
1.964(4)
2.021(5)
178.6(1)
100.3(1)
101.3(1)
177.5(1)
97.5(1)
80.3(2)
161.1(2)
98.1(1)
100.7(2)
107.4(2)
Fig.
2
Perspective drawing of the complex cation of
Pt(1)–S(1)
Pt(2)–S(2)
Pt(1)–N(1)
Pt(1)–N(2)
2.297(2)
2.314(2)
2.034(6)
1.955(6)
2.031(6)
80.6(3)
80.7(3)
161.1(3)
80.9(3)
80.1(3)
161.0(3)
97.6(2)
175.8(2)
100.9(2)
100.4(2)
Pt(2)–N(4)
Pt(2)–N(5)
Pt(2)–N(6)
Pt(1)–Pt(2)
2.035(7)
1.957(7)
2.033(7)
3.0882(8)
[{Pt(trpy)}2(m-SC6H4S-1,3)](PF6)2 3 with atomic numbering. Hydrogen
atoms have been omitted for clarity. Thermal ellipsoids were shown at the
40% probability level.
Pt(1)–N(3)
N(1)–Pt(1)–N(2)
N(2)–Pt(1)–N(3)
N(1)–Pt(1)–N(3)
N(4)–Pt(2)–N(5)
N(5)–Pt(2)–N(6)
N(4)–Pt(2)–N(6)
N(1)–Pt(1)–S(1)
N(2)–Pt(1)–S(1)
N(3)–Pt(1)–S(1)
N(4)–Pt(2)–S(2)
N(5)–Pt(2)–S(2)
N(6)–Pt(2)–S(2)
Pt(1)–S(1)–C(1)
Pt(2)–S(2)–C(1)
Pt(2)–Pt(1)–N(1)
Pt(2)–Pt(1)–N(2)
Pt(2)–Pt(1)–N(3)
Pt(2)–Pt(1)–N(4)
Pt(2)–Pt(1)–N(5)
Pt(2)–Pt(1)–N(6)
178.5(2)
98.5(2)
109.4(3)
109.8(2)
89.4(2)
100.8(2)
96.7(2)
96.3(2)
95.1(2)
86.3(2)
comparable to that previously reported for platinum(II)
terpyridyl3h and diimine20 complexes containing aromatic thiolate
ligands. Due to its anti-conformation, the two platinum(II)-
terpyridyl planes are directed away each other and no intramolec-
ular metal–metal or ligand–ligand interactions are observed. It is
interesting to note that the molecules exist in a dimeric structure
˚
with one short and one long Pt ◊ ◊ ◊ Pt distance of 3.753 A and
˚
4.375 A, respectively, between adjacent dinuclear molecules on
different sides of the Pt(trpy) moieties (Fig. 3). Such short Pt ◊ ◊ ◊ Pt
contacts together with the interplanar angle and distance of 0.0◦
and 3.5725 A, respectively, may suggest the possible existence of
angle and distance of 12.290◦ and 3.354 A. The intramolecular
˚
˚
˚
some weak intermolecular Pt ◊ ◊ ◊ Pt and p–p interactions between
the platinum-terpyridine moieties.
Pt(1) ◊ ◊ ◊ Pt(2) distance of 3.088 A is comparable to those found
5b
˚
in [{Pt(trpy)}2(m-gua)](ClO4)3 [3.09 and 3.07 A], [{Pt(trpy)}2(m-
5e
˚
=
Fig. 4(a) shows the perspective drawing of the complex cation
of 4, in which two platinum(II)-terpyridyl moieties are connected
by the i-mnt bridging ligand to form a molecular clip-like
structure. The two platinum-terpyridine co-ordination planes of
complex 4 are arranged in a syn-configuration with the interplanar
dtc)](PF6)3 [3.052 A], and [{Pt(trpy)}2(m-NHC( O)Me)](OTf)3
5g
˚
˚
˚
[3.119 A]. The Pt–S [Pt(1)–S(1), 2.298 A; Pt(2)–S(2), 2.314 A] and
˚
˚
S–C [S(1)–C(1), 1.733 A; S(2)–C(1), 1.734 A] bond lengths com-
pare well with that of (Et4N)2[Pt(i-mnt)2]21 but the S–C distances
are slightly shorter than that expected for a typical S–C single bond
˚
Fig. 3 (a) Dimeric structure and (b) crystal packing diagram of the complex cations of 3 showing alternating short and long Pt ◊ ◊ ◊ Pt distance of 3.753 A
˚
(black dotted line) and 4.375 A (red dotted line), respectively.
3916 | Dalton Trans., 2009, 3911–3922
This journal is
The Royal Society of Chemistry 2009
©

Fig. 4 (a) Perspective drawing of the complex cation of [{Pt(trpy)}2{m-(i-mnt)}](PF6)2 4 with atomic numbering. Hydrogen atoms have been omitted for
clarity. Thermal ellipsoids were shown at the 40% probability level. (b) Crystal packing showing the dimer-of-dimer structure of the complex cation of 4.
in a related complex, cis-(E)-[Pt(SPh)(PhSCSO)(PPh3)2] (Pt–S =
separation in a dimeric structure suggests the presence of some
intermolecular Pt ◊ ◊ ◊ Pt interactions between the neighbouring
metal centres. Similar one-dimensional infinite chain comprising
22
˚
˚
2.379 A and S–C = 1.791 A of the SPh moiety), indicating that
there are some electron delocalization within the i-mnt moiety that
is also commonly observed in other related platinum(II) diimine20
and phosphine23 complexes containing chelating mnt ligand. The
of dimer-of-dimer arrangement has also been reported in a related
5g
=
dinuclear complex, [{Pt(trpy)}2(m-NHC( O)Me)](OTf)3.
˚
C(1)–C(2) bond length of the i-mnt ligand is 1.37 A, typical of
=
a C C double bond. The crystal packing of 4 reveals that the
Electronic absorption spectroscopy
complex cations of 4 are arranged in a head-to-tail configuration
with respect to each other [N(5)–Pt(2)–Pt(2)–N(5) torsion angle,
180◦] and are extended along the a axis to form a one-dimensional
infinite chain with alternating “short” and “long” intermolecular
Pt ◊ ◊ ◊ Pt contacts with Pt ◊ ◊ ◊ Pt separations of 3.380 A and
4.986 A, respectively (Fig. 4(b)). The observation of short Pt ◊ ◊ ◊ Pt
The electronic absorption spectra of complexes 1–8 show intense
vibronic-structured bands at 274–348 nm with e in the order
of 104 dm3 mol-1 cm-1 and broad absorptions at 382–570 nm
in acetonitrile. Table 3 summarizes the electronic absorption
spectral data. The high-energy structured bands at 274–348 nm are
˚
˚
Table 3 Photophysical data for the terpyridyl-crown precursors and dinuclear platinum(II) terpyridyl complexes
Complex
Medium (T/K)
l
abs /nm (emax/dm3 mol-1 cm-1)
l
em/nm (toa/ms)
1b
CH3CN (298)
Solid (77)
290 (41 410), 322 (19 380), 340 (18 250), 430 (22 790)
630 (0.2)
630 (0.4)
585 (58.4), 660 (4.2)
640 (0.3)
650 (0.8)
575 (39.2), 670 (3.5)
Non-emissive
708 (<0.1)
Glassb (77)
CH3CN (298)
Solid (77)
2b
3
290 (40 045), 322 (18 860), 340 (17 865), 430 (21 845)
Glassb (77)
CH3CN (298)
Solid (298)
Solid (77)
274 (53 150), 314 (24 150), 332 (22 500), 348 (23 650), 382 sh (4150), 570 (1810)
707 (0.8)
Glassb (77)
CH3CN (298)
Solid (298)
Solid (77)
470 (26.3), 675 (4.9)
Non-emissive
725 (<0.1)
740 (1.0)
470 (11.8), 640 (4.7)
640 (<0.1)
745 (0.1)
545 (34.8), 670 (5.9)
640 (<0.1)
750 (0.2)
545 (35.3), 665 (7.8)
635 (<0.1)
735 (0.2)
540 (42.1), 685 (4.2)
645 (<0.1)
735 (0.2)
540 (39.9), 690 (5.3)
4
274 (38 370), 314 (23 200), 336 (29 320), 378 (18 300), 466 (4810)
Glassb (77)
CH3CN (298)
Solid (77)
5
6
7
8
292 (69 540), 340 sh (41 590), 380 (30 810), 430 (36 540), 486 sh (21 520)
290 (74 260), 340 sh (45 850), 378 (32 790), 432 (40 010), 488 sh (17 190)
290 (88 780), 340 (74 040), 414 (49 960), 470 (20 520)
Glassb (77)
CH3CN (298)
Solid (77)
Glassb (77)
CH3CN (298)
Solid (77)
Glassb (77)
CH3CN (298)
Solid (77)
292 (75 910), 340 (64 960), 416 (44 530), 470 (18 650)
Glassb (77)
a Luminescence lifetime values were measured with an uncertainty of 10%. b In butyronitrile glass.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3911–3922 | 3917
©

assigned as intraligand (IL) transitions of the terpyridine ligands
and/or sulfur-containing ligands, since similar absorptions have
also been observed in [Ir(trpy)2]3+24 and Zn(trpy)Cl2,25 and the
free terpyridine and sulfur-containing ligands. In view of the
electronic absorption study of [Pt(trpy)Cl]+ in acetonitrile, which
shows that the lowest energy absorption band occurs at ca. 400 nm
with an extinction coefficient of 103 dm3mol-1cm-1, the low-energy
absorption band at 430 nm of the mononuclear complexes 1 and 2
containing the terpyridine ligand substituted with ethynylbenzene
is assigned as an admixture of dp(Pt) → p*(trpy) metal-to-
ligand charge transfer (MLCT) transition and IL transition of
the terpyridyl ligand. It is worthwhile to note that the extinction
coefficient of this band (e ≥ 104 dm3mol-1cm-1) is much larger
than what one would have expected for a pure MLCT transition,
suggesting that this band contains some mixing of the more
intense IL transition of the terpyridyl ligand. The occurrence of
this IL transition at such low energy region is ascribed to the
lower p* orbital energy of the more conjugated terpyridyl ligand
as a result of the attachment of the substituted ethynylbenzene.
For complex 3 that contains the 1,3-benzenedithiolate bridge,
the low-energy broad absorption band at 570 nm is tentatively
assigned as a pp(SC6H4S-1,3) → p*(trpy) ligand-to-ligand charge
transfer (LLCT) transition. Similar assignments have also been
made for the low-energy absorption band at ca. 520–572 nm
in other related platinum(II) terpyridyl complexes containing
aromatic thiolates.3h,5f For the dinuclear complexes 4–8, there is an
additional lowest energy absorption band when compared to the
electronic absorption spectra of the corresponding mononuclear
chloro-counterparts. With reference to previous studies on din-
uclear platinum terpyridyl systems,5 such low-energy absorption
bands at 466–488 nm for complexes 4–8 are tentatively assigned
as ds*(Pt2) → p*(trpy) metal-metal-to-ligand charge transfer
(MMLCT) transition, probably with some mixing of a pp(mnt)
or pp(dtc) to p*(trpy) LLCT character. Although the energy
difference between the lowest energy absorption bands in 3 and 4
may be rationalized by the presence of two electron-withdrawing
cyano groups on the i-mnt bridging ligand, which renders it
less electron-rich than the 1,3-benzenedithiolate, as both the
1,3-benzenedithiolate and i-mnt bridging ligands of 3 and 4,
respectively, are dianionic ligands, it is believed that an assignment
of the low-energy absorption band of 4 as predominantly MMLCT
in character mixed with some LLCT transition is more appropriate
in light of the comparable absorption energy to other related
dinuclear platinum(II) terpyridyl complexes.5 In addition, the
presence of intramolecular Pt ◊ ◊ ◊ Pt contact revealed in the solid-
state structure of 4 is further supportive of the HOMO as
being ds* orbital in nature that results from the intramolecular
interaction between the two platinum metal centres, while the lack
of intramolecular Pt ◊ ◊ ◊ Pt contact in 3 due to its anti-conformation
eliminates the possibility of a MMLCT transition in solution state.
The lower absorption energies for complexes 5 and 6 that contain
substituted-terpyridine ligand than complex 4 are attributed to the
electron-withdrawing abilities of the ethynylbenzo-15-crown-5 and
3,4-dimethoxyphenylethynyl moieties on the terpyridine ligand.
The electronic absorption spectrum of 5 appears to be somewhat a
superimposition of the spectra of the mononuclear complex 1 and
the dinuclear complex 4 and Fig. 5 shows the electronic absorption
spectra of 1, 4 and 5 in acetonitrile at 298 K. Complexes 7 and 8,
similar to 5 and 6, are likely to show low-energy absorptions of
∫
Electronic absorption spectra of [Pt(trpyC C-benzo-15-
Fig.
5
crown-5)Cl]PF6
1
(
), [{Pt(trpy)}2{m-(i-mnt)}](PF6)2
4 ( ◊ ◊ ◊ ) and
∫
[{Pt(trpyC C-benzo-15-crown-5)}2{m-(i-mnt)}](PF6)2 5 (---) in acetoni-
trile at 298 K.
a MMLCT nature that mixes with a LLCT character in view of
the presence of short intramolecular Pt ◊ ◊ ◊ Pt contact in the related
[{Pt(trpy)}2(m-dtc)](PF6)3.5f The lower absorption energies of the
low-energy absorptions in 5 and 6 than the corresponding 7 and 8
are in line with the larger electron-donating property of i-mnt than
the dtc ligand,26 which would give rise to a lower energy LLCT
transition that mixes into the MMLCT transition. The relatively
large extinction coefficients for the low-energy bands in complexes
5–8 are also suggestive of a mixing of an IL and LLCT character
into the MMLCT band.
Emission properties
All the complexes 1–8 show luminescence properties in various
media and temperatures with the exception of 3 and 4, which
are non-emissive in acetonitrile at room temperature (Table 3).
In contrast to the related unsubstituted terpyridine complex,
[Pt(trpy)Cl]+, which is non-emissive in room-temperature fluid
solution, the mononuclear complexes, 1 and 2, exhibit intense
emission at 630–640 nm upon excitation at l > 400 nm in
acetonitrile at room temperature and such emission band is
independent of concentration in the range of 1 ¥ 10-6 to 1 ¥
10-4 M. Similar to the related complexes, [Pt(4¢-Ar-trpy)Cl]+ (4¢-
Ar-trpy = 4¢-aryl-substituted terpyridine derivative), which have
also been reported to show emission in fluid solution at room
temperature,3i the recovery of the luminescence property has been
similarly attributed to the mixing of p–p* (Ar-trpy) 3IL character
into the 3MLCT excited state, i.e. the emission of 1 and 2 in
acetonitrile is derived from excited states of an admixture of
dp(Pt) → p*(trpyC C-benzo-15-crown-5) 3MLCT and metal-
∫
3
∫
perturbed p→p* (trpyC C-benzo-15-crown-5) IL character. The
lack of luminescence of complexes 3 and 4 in acetonitrile at
room temperature is attributed to the absence of such a mixing
of 3IL character into the excited state. Other related dinu-
clear complexes, [{Pt(trpy)}2(m-L)]3+ (L = pyrazole,5c azaindole,5c
diphenylformamidine,5c arginine5c and dtc5f), were also found to
show no significant emission intensity at room-temperature in fluid
solution for the same reasons. The presence of a low-lying LLCT
state in 3 may also be responsible for its lack of emissive behaviour
in solution. The dinuclear complexes 5–8 exhibit broad emission
bands at 635–645 nm upon excitation at l > 400 nm in acetonitrile
at room temperature. Although the related dinuclear platinum(II)
3+5g
=
complexes, [{Pt(trpy)}2(m-NHC( O)Me)]
and [{Pt(trpy)}2(m-
guanidine)]3+,5b exhibit emission bands at 600 and 620 nm,
3918 | Dalton Trans., 2009, 3911–3922
This journal is
The Royal Society of Chemistry 2009
©

3
respectively, which are attributed to the MMLCT excited state,
In 77
K
butyronitrile glass of low concentrations
such an assignment is not preferred for the emission of 5–8 at
room-temperature in fluid solution since complex 4, with the same
bridging i-mnt ligand as 5 and 6, is found to be non-emissive. In
view of the fact that the emission bands of 5–8 occur at nearly
the same energy as that of 1 and 2, together with the close
resemblance of the excitation spectra of the mononuclear and
dinuclear complexes, the emission of 5–8 is suggested to be derived
(10-6 mol dm-3), complexes 3–8 all show highly structured emission
bands at 470–545 nm with vibrational progressional spacings of
ca. 1300 cm-1, typical of the aromatic vibrational mode of the
terpyridine ligand, and are tentatively assigned as derived from
states of 3p–p*(trpy) IL origin. Upon increasing the concentration,
all the complexes show, in addition to the high-energy vibronic-
structured emission, a broad emission band centred at 640–
690 nm, with an obvious preferential enhancement in the emission
intensity of this band upon increasing the concentration from 10-6
to 10-4 mol dm-3. As shown in Fig. 7 for 5, the excitation spectra
monitored at 540 nm and 685 nm appeared different, indicating
that the two emission bands are of different origins. Thus the broad
emission bands at ca. 640 – 690 nm that have comparable energy
to the low-energy emission in solution, are similarly assigned as
derived from triplet states of MMLCT character.
∫
from an excited state of predominantly dp(Pt) → p*(trpyC C-
3
∫
benzo-15-crown-5/trpyC CC6H3(OCH3)2-3,4) MLCT in nature,
∫
probably with some mixing of p–p* (trpyC C-benzo-15-crown-
5/trpyC CC6H3(OCH3)2-3,4) 3IL character. The emission and
∫
excitation spectra of 1 and 5 in acetonitrile at room temperature
are shown in Fig. 6. On the other hand, the emission bands of
complex 3 with the dithiolate bridge in various media at room
temperature or 77 K are assigned as derived from the excited state
of triplet 3MLCT/3LLCT character.3h
∫
Fig. 7 Emission spectra of [{Pt(trpyC C-benzo-15-crown-5)}2(m-dtc)]-
(PF6)3 7 at concentrations 1 ¥ 10-6
(
), 1 ¥ 10-5 (---), and 1 ¥ 10-4
∫
Fig.
6
Emission (a) and excitation (b) spectra of [Pt(trpyC C-
( ◊ ◊ ◊ ) mol dm-3 in butyronitrile glass at 77 K. Insets: excitation spectra
of monitored at 540 nm ( ) and 685 nm (---).
∫
benzo-15-crown-5)Cl]PF6
5)}2{m-(i-mnt)}](PF6)2 5 (---) in acetonitrile at 298 K.
1
(
)
and [{Pt(trpyC C-benzo-15-crown-
Although complexes 1–8 all showed solid-state emission at 77 K,
their emission origins are not the same, depending on the nuclear-
ities and the nature of the bridging ligands. For the mononuclear
complexes, 1 and 2, the emission bands at ca. 630–650 nm, which
are vibronically structured with vibrational progressional spacings
of ca. 1200 cm-1 that are typical of the aromatic stretch in the
ground state, are ascribed to phosphorescence derived from the
metal-perturbed 3IL excited state. The solid-state emission of
complexes 3 and 4 show a broad band at 708–725 nm at room
temperature that shows a red shift in emission energies to 750–
755 nm upon cooling to 77 K. This may be rationalized by the lat-
tice contraction at low temperatures, leading to a shortening of the
Pt ◊ ◊ ◊ Pt intermolecular separations and hence, increased Pt ◊ ◊ ◊ Pt
interactions. Complexes 5–8 also show a low-energy emission at
760–785 nm in the solid-state at low temperature. Such low energy
emissions in the solid-state for all the complexes are assigned
as derived from triplet states of MMLCT character, resulting
from the significant Pt ◊ ◊ ◊ Pt interaction in the solid-state. Similar
assignment has also been reported for other dinuclear platinum(II)
terpyridyl complexes, such as [{Pt(trpy)}2(m-X)](PF6)3 (X = pyra-
zole, azaindole, diphenylformamidine, canaverine, arginine)5c in
the solid states.
Electrochemical properties
The electrochemical properties of 3–8 have been studied by cyclic
voltammetry, while those of 1 and 2 were not studied due to
their limited solubility in acetonitrile. The cyclic voltammogram
of complex 3 in acetonitrile (0.1 M nBu4NPF6) shows two quasi-
reversible reduction couples at ca. -0.90 V and -1.50 V vs. SCE,
which are assigned as terpyridine-based ligand-centred reductions.
On the other hand, complexes 4–8 show, in addition to two quasi-
reversible reduction couples at ca. -1.45 V and -1.65 V, two less
negative reversible reduction couples at ca. -0.45 V and -0.73 V
vs. SCE for 5 and 6 and at ca. -0.35 and -0.63 V vs. SCE for
7 and 8. The electrochemical data of the dinuclear platinum(II)
terpyridyl complexes are summarized in Table 4. With reference
to previous studies on platinum(II) terpyridyl systems,4e,f,7a the
reduction couples at ca. -1.45 V and -1.65 V are tentatively
assigned as terpyridine-based reductions with some mixing of
the Pt(II) metal character, while the other two reduction couples
with less negative reduction potentials correspond to the metal-
perturbed i-mnt and dtc ligand-centred reductions as these two
ligands are reported to undergo reduction at ca. -0.40, -0.65 V and
-0.50,-0.80 V vs. SCE, respectively, in the literature.26 The more
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 3911–3922 | 3919
©

Table 4 Electrochemical data for dinuclear platinum(II) terpyridyl complexes in acetonitrile solution (0.1 M nBu4NPF6) at 298 Ka
Complex
Oxidation Epa/V vs. SCEb
Reduction E1/2/V vs. SCEc
[{Pt(trpy)}2(m-SC6H4S-1,3)](PF6)2 (3)
[{Pt(trpy)}2{m-(i-mnt)}](PF6)2 (4)
+1.02
+1.50
-0.90
-1.50
-0.40
-0.70
-1.44
-1.66
-0.45
-0.73
-1.45
-1.67
-0.44
-0.73
-1.45
-1.66
-0.35
-0.65
-1.48
-1.64
-0.35
-0.63
-1.44
-1.65
∫
[{Pt(trpyC C-benzo-15-crown-5)}2{m-(i-mnt)}](PF6)2 (5)
+1.46
+1.48
+1.50
+1.52
∫
[{Pt(trpyC CC6H3(OCH3)2-3,4)}2{m-(i-mnt)}](PF6)2 (6)
∫
[{Pt(trpyC C-benzo-15-crown-5)}2(m-dtc)](PF6)3 (7)
∫
[{Pt(trpyC CC6H3(OCH3)2-3,4)}2(m-dtc)](PF6)3 (8)
b
c
a Working electrode, glassy carbon; scan rate 100 mV s-1. Epa refers to the anodic peak potential for the irreversible oxidation waves. E1/2 = (Epa
+
Epc)/2; Epa and Epc are peak anodic and peak cathodic potentials, respectively.
negative reduction potentials for the terpyridine-based reductions
in complexes 4–8 than complex 3 are in line with the more difficult
reduction of the terpyridine ligand after the initial reductions
of the dtc and i-mnt ligands. An irreversible oxidation wave at
ca. +1.02 to +1.52 V is observed in all the dinuclear complexes
studied, tentatively assigned as a Pt(II) to Pt(III) oxidation. It is
interesting to note that complexes 5 and 6 are oxidized at a slightly
less positive potential than the corresponding complexes 7 and
8, in line with the poorer p-accepting ability and better electron-
richness of i-mnt than dtc, as reflected by the more negative i-mnt
ligand-centred reduction potentials than that of dtc. The poorer
p-accepting property and more electron-rich nature of the i-mnt
ligand would less stabilize the lower oxidation state, i.e. Pt(II),
leading to a less positive potential for Pt(II) → Pt(III) oxidation.
Cation-binding studies
∫
Fig.
8
Electronic absorption spectral traces of [{Pt(trpyC C-
benzo-15-crown-5)}2{m-(i-mnt)}](PF6)2 5 (2.5 ¥ 10-5 mol dm-3) in ace-
tonitrile (0.1 mol dm-3 nBu4NPF6) upon addition of KPF6 (path length =
1 cm). The insert shows a plot of absorbance vs. [K+] monitored at l =
480 nm (ꢀ) and its theoretical fit ( ).
The cation-binding behaviour of 5 and 7 have also been inves-
tigated by UV-vis absorption spectrophotometry. Upon addition
of alkali metal cations to an acetonitrile solution of dinuclear
platinum(II) terpyridyl-crown complexes 5 and 7, a UV-vis ab-
sorption spectral change was observed. Fig. 8 and 9 show the
UV-vis absorption spectral traces upon addition of potassium
hexafluorophosphate to a solution of 5 and 7, respectively, in which
well-defined isosbestic points were observed. Spectrochemical
recognition of guest metal ions is confirmed by the absence of
spectral changes in the electronic absorption spectra of the control
complexes 6 and 8. For the binding of KPF6, log KS values
of 3.45 ( 0.01) and 3.10 ( 0.01) were obtained for complexes 5
and 7, respectively, according to eqn (1). The 1 : 1 stoichiometry
for K+ ion-binding is evidenced by the close agreement of the
experimental data with the theoretical fit (Fig. 8 and 9 inserts for 5
and 7, respectively), and has further been confirmed by the method
of continuous variation, where a break point at a mole fraction of
0.5 is observed. Since the ionic diameter of the K+ ion is too large to
fit into the cavity of the benzo-15-crown-5, the 1 : 1 complexation
ratio may suggest that the K+ ion is sandwiched between the two
benzo-15-crown-5 units within the dinuclear platinum complex.
Similar binding model has also been observed in dinuclear gold(I)
complexes.9 The binding constant for K+ ion-binding of complex
7 is found to be smaller than that of complex 5. This may be
rationalized by the lower positive charge borne on complex 5,
which would give rise to a stronger ion-binding affinity for metal
ions and hence a larger binding constant. In case of Na+ ion-
binding studies, the lack of well-defined isosbestic points in the
UV-vis spectral traces and the absence of a satisfactory fit to eqn
(1) suggest that the binding of Na+ is not in a 1 : 1 stoichiometry,
3920 | Dalton Trans., 2009, 3911–3922
This journal is
The Royal Society of Chemistry 2009
©

Table 5 Ion clusters observed in the positive ESI-mass spectra of dinuclear platinum(II) terpyridyl-crown complexes with NaClO4, KPF6 and Ba(ClO4)2
Complex
Ion cluster, m/z
2+
∫
5
911 {[{Pt(trpyC C-benzo-15-crown-5)}2(m-(i-mnt))·2Na](ClO4)2}
+
∫
1907 {[{Pt(trpyC C-benzo-15-crown-5)}2(m-(i-mnt))·K](PF6)2}
+
∫
2013 {[{Pt(trpyC C-benzo-15-crown-5)}2(m-(i-mnt))·Ba](ClO4)3}
+
∫
7
990 {[Pt(trpyC C-benzo-15-crown-5)(m-dtc)·Na]ClO4}
+
∫
1051 {[Pt(trpyC C-benzo-15-crown-5)(m-dtc)·K]PF6}
+
∫
1203 {[Pt(trpyC C-benzo-15-crown-5)(m-dtc)·Ba](ClO4)2}
Kong. This work has been supported by the Strategic Research
Theme on Molecular Materials, the Faculty Development Fund
of The University of Hong Kong, and a National Natural Science
Foundation of China and the Research Grants Council of Hong
Kong Joint Research Scheme (NSFC-RGC project no. N_HKU
737/06) from the Research Grants Council of Hong Kong Special
Administrative Region, P. R. China. Dr K.-K. Cheung is gratefully
acknowledged for solving the crystal structures of 3 and 4. R. P.-
L. T. acknowledges the receipt of a postgraduate studentship,
administered by The University of Hong Kong.
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∫
Fig. 9 Electronic absorption spectral traces of [{Pt(trpyC C-benzo-
15-crown-5)}2(m-dtc)](PF6)3 7 (2.5 ¥ 10-5 mol dm-3) in acetonitrile
(0.1 mol dm-3 nBu4NPF6) upon addition of KPF6 (path length = 1 cm).
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(ꢀ) and its theoretical fit ( ).
but probably a mixture of 1 : 1 and 1 : 2 complex-to-sodium
ion binding. It is interesting to note that the binding model of
the dinuclear platinum(II) complexes is found to be completely
different from that of the mononuclear species, [Pt(trpy)(S-benzo-
15-crown-5)]PF6 and [Pt(trpy)(C C-benzo-15-crown-5)]PF6,
in which a 1 : 1 complexation stoichiometry is generally observed
6a
7a,j
∫
with Na+ ions instead of K+ ions.
The ion-binding studies have further been confirmed by positive
ESI-mass spectrometry. Table 5 summarizes the ion cluster peaks
for the ion-bound adducts of the dinuclear platinum(II) terpyridyl
crown ether-containing complexes, 5 and 7, and alkali metal
cations. A 1 : 1 adduct was observed for the binding of 5 with
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monomeric species {Pt(trpyC C-benzo-15-crown-5)(m-dtc)} was
+
∫
observed with the formation of 1 : 1 adducts of NaClO4, KPF6
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search Achievement Award Scheme from The University of Hong
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