Synthesis, photophysical properties, and biomolecular labeling studies of luminescent platinum(II)-terpyridyl alkynyl complexes

Article abstract of DOI:10.1021/om049898h

Three luminescent alkynylplatinum(II) terpyridyl complexes, [Pt( ^tBu₃tpy)(C≡C-C₆H_4- X-4)](OTf) (X = NH₂ 1, NCS 2, NHCOCH₂I 3), have been synthesized and characterized. Their electrochemical and photophysical properties have been studied, and the molecular structure of 2 has also been determined by X-ray crystallography. Complexes 2 and 3 exhibit strong luminescence in various media, the origin of which has been tentatively assigned as a dπ(Pt) → π*(^tBu₃tpy) ³MLCT excited state, with some mixing of a π(C≡CR) → π*(^tBu ₃tpy) ³LLCT character. Human serum albumin (HSA) has been labeled by the ready reaction of the isothiocyanate and iodoacetamide functional groups in 2 and 3, to afford the respective bioconjugates, 2-HSA and 3-HSA. Both bioconjugates in 50 mM Tris-Cl buffer (pH 7.4) are highly colored and exhibit luminescence in the visible region upon photoexcitation.

Full text of DOI:10.1021/om049898h

Organometallics 2004, 23, 3459-3465
3459
Syn th esis, P h otop h ysica l P r op er ties, a n d Biom olecu la r
La belin g Stu d ies of Lu m in escen t P la tin u m (II)-Ter p yr id yl
Alk yn yl Com p lexes
Keith Man-Chung Wong, Wing-Suen Tang, Ben Wai-Kin Chu,
Nianyong Zhu, and Vivian Wing-Wah Yam*
Open Laboratory of Chemical Biology of The Institute of Molecular Technology for Drug
Discovery and Synthesis, Centre for Carbon-Rich Molecular and Nano-Scale Metal-Based
Materials Research, Department of Chemistry, The University of Hong Kong, Pokfulam Road,
Hong Kong, People’s Republic of China
Received February 7, 2004
Three luminescent alkynylplatinum(II) terpyridyl complexes, [Pt(tBu3tpy)(CtC-C6H4-
X-4)](OTf) (X ) NH2 1, NCS 2, NHCOCH2I 3), have been synthesized and characterized.
Their electrochemical and photophysical properties have been studied, and the molecular
structure of 2 has also been determined by X-ray crystallography. Complexes 2 and 3 exhibit
strong luminescence in various media, the origin of which has been tentatively assigned as
a dπ(Pt) f π*(tBu3tpy) 3MLCT excited state, with some mixing of a π(CtCR) f π*(tBu3tpy)
3LLCT character. Human serum albumin (HSA) has been labeled by the ready reaction of
the isothiocyanate and iodoacetamide functional groups in 2 and 3, to afford the respective
bioconjugates, 2-HSA and 3-HSA. Both bioconjugates in 50 mM Tris-Cl buffer (pH 7.4) are
highly colored and exhibit luminescence in the visible region upon photoexcitation.
In tr od u ction
sively investigated in view of their rich photolumines-
cence properties.8-13 In addition, these complexes have
been shown to exhibit interesting spectroscopic and
photophysical behaviors associated with the occurrence
of Pt‚‚‚Pt and π‚‚‚π interactions.8-10 Apart from the
photophysical studies, platinum(II) complexes with
square-planar geometry have also attracted consider-
able attention as a result of their interesting biological
activities, such as antitumor cytotoxicity,14,15 DNA
Organic fluorescent biological labels have been ex-
tensively studied due to their use in fluorescence im-
munoassays,1 and numerous examples have been de-
veloped for biophysical applications and commercial
use.2 Since the short decay lifetime from such fluores-
cence immunoassays would limit many other biophysi-
cal measurements, the search for other luminescent
materials with longer lifetimes to serve as biomolecular
probes is urged. The accessibility of the use of gated
detection by having longer luminescence lifetime can
suppress any fluorescence interference from the biologi-
cal samples. Luminescent transition metal complex
systems, which commonly display phosphorescence in
the microsecond range, would be ideal candidates in this
respect. In addition, longer excitation wavelength, i.e.,
lower excitation energy, would be anticipated in the
process of detection, and thus the degree of photo-
degradation of biological samples would be minimized.
In view of this, establishment of transition metal
complexes, such as those containing ruthenium(II),3
rhenium(I),4 osmium(II),5 rhodium(III),6 and irid-
ium(III),7 for luminescent biological labeling has been
reported. Square-planar platinum(II) complexes, espe-
cially those with polypyridyl ligands, have been exten-
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Soc., Chem. Commun. 1992, 1369. (b) Yip, H. K.; Cheng, L. K.; Cheung,
K. K.; Che, C. M. J . Chem. Soc., Dalton Trans. 1993, 2933. (c) Bailey,
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Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591. (b) Bu¨chner,
R.; Cunningham, C. T.; Field, J . S.; Haines, R. J .; McMillin, D. R.;
Summerton, G. C. J . Chem. Soc., Dalton Trans. 1999, 711. (c) Tzeng,
B. C.; Fu, W. F.; Che, C. M.; Chao, H. Y.; Cheung, K. K.; Peng, S. M.
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* Corresponding author. Fax: (852) 2857-1586. E-mail: wwyam@
hku.hk.
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Press: San Diego, CA, 1996. (b) Kessler, C. Nonradioactive Labeling
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10.1021/om049898h CCC: $27.50 © 2004 American Chemical Society
Publication on Web 05/29/2004

3460 Organometallics, Vol. 23, No. 14, 2004
Wong et al.
(CSCl2), iodoacetic anhydride, and 4-aminophenylacetylene
were purchased from Aldrich Chemical Co. [Pt(tBu3tpy)-
(MeCN)](OTf)211e,g was prepared according to literature meth-
ods. Human serum albumin (HSA) fraction V was obtained
from Calbiochem and was used as received. All buffer compo-
nents were of molecular biology grade and used without
purification. All solvents were purified and distilled using
standard procedures before use. All other reagents were of
analytical grade and were used as received.
Ch a r t 1
Syn th esis. [P t(tBu 3tp y)(CtC-C6H4-NH2-4)](OTf) (1).
To a stirred solution of 4-aminophenylacetylene (50 mg, 0.43
mmol) in methanol (70 mL) was added sodium hydroxide (23
mg, 0.58 mmol). The resultant solution was stirred at room
temperature for 30 min. [Pt(tBu3tpy)(MeCN)](OTf)2 (300 mg,
0.39 mmol) was added to the reaction mixture and was then
heated under reflux for 12 h. The mixture was filtered, and
the solvent was evaporated to dryness. The product was
isolated by column chromatography on silica gel using di-
chloromethane-acetone (3:1 v/v) as eluent. Subsequent re-
crystallization by vapor diffusion of diethyl ether into a
dichloromethane solution of the product gave 1 as purple-red
intercalation,14,16 and protein binding behavior.17 To our
surprise, there has so far been no report on the utiliza-
tion of this class of platinum(II) complexes as lumines-
cent labeling reagents for biomolecules. Herein we
report the synthesis and characterization of three
luminescent alkynylplatinum(II) terpyridyl complexes,
[Pt(tBu3tpy)(CtC-C6H4-X-4)](OTf) (X ) NH2 1, NCS
2, NHCOCH2I 3) (Chart 1). Their electrochemical and
photophysical behaviors have been studied, and the
molecular structure of 2 has also been determined by
X-ray crystallography. In view of the ready reactivity
of the isothiocyanate and iodoacetamide functional
groups in complexes 2 and 3 with the primary amine
and sulfhydryl group, respectively, of biomolecules,
human serum albumin (HSA) has been labeled with
complexes 2 and 3. The luminescence properties of the
corresponding bioconjugates have been investigated in
order to demonstrate their role as potential luminescent
labels for biomolecules.
1
crystals. Yield: 200 mg (60%). H NMR (400 MHz, (CD3CN),
298 K, relative to Me4Si, δ/ppm): 1.46 (s, 18H, tert-butyl
protons), 1.54 (s, 9H, tert-butyl protons), 4.27 (s, 2H, amino
protons), 6.62 (d, 2H, J ) 9 Hz, phenyl protons), 7.22 (d, 2H,
J ) 9 Hz, phenyl protons), 7.72 (dd, 2H, J ) 2 Hz, 6 Hz,
terpyridyl protons), 8.26 (d, 2H, J ) 2 Hz, terpyridyl protons),
8.31 (s, 2H, terpyridyl protons), 9.02 (d with Pt satellite, 2H,
J ) 6 Hz, J Pt-H ) 46 Hz, terpyridyl protons). IR (KBr disk,
ν/cm-1): 2112(m) ν(CtC); 3365(m) ν(N-H). Positive FAB-
MS: m/z 712 [M - OTf]+. Anal. Calcd for C36H41N4F3SO3Pt‚
1/2CH2Cl2: C, 48.48; H, 4.54; N, 6.20. Found: C, 48.79; H,
4.66; N, 6.20.
[P t(tBu 3tp y)(CtC-C6H4-NCS-4)](OTf) (2). To a mixture
of 1 (100 mg, 84.4 µmol) and CaCO3 (34 mg, 339.7 µmol) in
dry acetone (8 mL) was added CSCl2 (13 µL, 170.5 µmol). After
stirring for 2 h in the dark under nitrogen at 0 °C, the
suspension was filtered and the filtrate evaporated to dryness
to yield an orange-yellow solid. Recrystallization from vapor
diffusion of diethyl ether into an acetone solution of the product
Exp er im en ta l Section
1
Ma ter ia ls a n d Rea gen ts. Dichloro(1,5-cyclooctadiene)-
platinum(II) was obtained from Strem Chemicals Inc. 4,4′,4′′-
Tri-tert-butyl-2,2′:6′,2′′-terpyridine (tBu3tpy), thiophosgene
afforded 2 as orange-yellow crystals. Yield: 60 mg (80%). H
NMR (400 MHz, (CD3CN), 298 K, relative to Me4Si, δ/ppm):
1.45 (s, 18H, tert-butyl protons), 1.53 (s, 9H, tert-butyl protons),
7.29 (d, 2H, J ) 8 Hz, phenyl protons), 7.47 (d, 2H, J ) 8 Hz,
phenyl protons), 7.70 (dd, 2H, J ) 2 Hz, 6 Hz, terpyridyl
protons), 8.27 (d, 2H, J ) 2 Hz, terpyridyl protons), 8.31 (s,
2H, terpyridyl protons), 8.96 (d with Pt satellite, 2H, J ) 6
Hz, J Pt-H ) 43 Hz, terpyridyl protons). IR (KBr disk, ν/cm-1):
2176(m) and 2118(m) ν(NCS). Positive FAB-MS: m/z 754 [M
- OTf]+. Anal. Calcd for C37H39N4F3S2O3Pt: C, 49.17; H, 4.32;
N, 6.20. Found: C, 49.09; H, 4.41; N, 6.01.
(11) (a) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem.
1994, 33, 722. (b) Arena, G.; Calogero, G.; Campagna, S.; Scolaro, L.
M.; Ricevuto, V.; Romeo, R. Inorg. Chem. 1998, 37, 2763. (c) Lai, S.
W.; Chan, M. C. W.; Cheung, K. K. Inorg. Chem. 1999, 38, 4267. (d)
Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Ko, C. C.; Cheung, K.
K. Inorg. Chem. 2001, 40, 571. (e) Yam, V. W. W.; Tang, R. P. L.;
Wong, K. M. C.; Cheung, K. K. Organometallics 2001, 20, 4476. (f)
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Yam, V. W. W.; Wong, K. M. C.; Zhu, N. Angew. Chem., Int. Ed. 2003,
42, 1400.
[P t(tBu 3tp y)(CtC-C6H4-NHCOCH2I-4)](OTf) (3). Com-
plex 1 (50 mg, 0.249 mmol) and iodoacetic anhydride (132 mg,
0.373 mmol) were stirred in acetonitrile (8 mL) at room
temperature in the dark under an inert atmosphere of nitrogen
for 24 h. The solution was evaporated to dryness to give an
orange-red solid. The product was purified by column chro-
matography on silica gel using dichloromethane-acetone (3:1
v/v) as eluent. Subsequent recrystallization by vapor diffusion
of diethyl ether into an acetone solution of the product gave 3
as red crystals. Yield: 154 mg (60%). 1H NMR (400 MHz,
(CD3CN), 298 K, relative to Me4Si, δ/ppm): 1.46 (s, 18H, tert-
butyl protons), 1.54 (s, 9H, tert-butyl protons), 3.85 (s, 2H,
methylene protons), 7.43 (d, 2H, J ) 9 Hz, phenyl protons),
7.54 (d, 2H, J ) 9 Hz, phenyl protons), 7.72 (dd, 2H, J ) 2 Hz,
6 Hz, terpyridyl protons), 8.27 (d, 2H, J ) 2 Hz, terpyridyl
protons), 8.32 (s, 2H, terpyridyl protons), 8.73 (s, 1H, amide
proton), 9.00 (d with Pt satellite, 2H, J ) 6 Hz, J Pt-H ) 45 Hz,
terpyridyl protons). IR (KBr disk, ν/cm-1): 2118(w) ν(CtC).
Positive FAB-MS: m/z 880 [M - OTf]+. Anal. Calcd for
(12) (a) Zuleta, J . A.; Chesta, C. A.; Eisenberg, R. J . Am. Chem. Soc.
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C. M.; Cheung, K. K.; Zhu, N. Chem. Eur. J . 2001, 7, 4180. (d) Lu, W.;
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Platinum(II)-Terpyridyl Alkynyl Complexes
Organometallics, Vol. 23, No. 14, 2004 3461
Ta ble 1. Cr ysta llogr a p h ic a n d Str u ctu r a l
Refin em en t Da ta for Com p lex 2
C
38H42N4SO5F3IPt‚(CH3)2CO: C, 44.61; H, 4.35; N, 5.08.
Found: C, 44.86; H, 4.27; N, 5.33.
La belin g of Biom olecu les. La belin g of Hu m a n Ser u m
Albu m in (HSA) w ith Com p lex 2. Complex 2 (1.3 mg, 1.67
µmol) in 20 µL of anhydrous DMSO was added to HSA (1.32
mg, 20 nmol) in 50 mM carbonate buffer (pH 9.0). The mixture
was incubated in the dark at 25 °C for 12 h. The solid residue
was removed by centrifugation. The supernatant was diluted
to 1 mL with 50 mM Tris-HCl (pH 7.4) and loaded onto a PD-
10 column (Pharmacia) that had been equilibrated with the
same buffer. The first band eluted out of the column with an
intense red color was collected, and the solution was concen-
trated with a YM-50 centricon (Amicon). The labeled protein
was further purified by HPLC equipped with a size-exclusion
column. The mobile phase was 50 mM Tris-HCl (pH 7.4) at a
empirical formula
fw
temperature
wavelength
cryst syst
C37H39F3N4O3PtS2
903.93
301(2) K
0.71073 Å
monoclinic
C2/c
space group
unit cell dimens
a ) 32.053(6) Å, b ) 10.973(2) Å,
c ) 21.495(4) Å; â ) 95.03(3)°
volume
Z
density (calcd)
abs coeff
F(000)
cryst size
data collection range
index ranges
7531(2) Å3
8
1.594 g cm-3
3.893 mm-1
3600
0.30 × 0.30 × 0.15 mm
1.96 to 25.35°
-38 e h e 38, -13 e k e 13,
-23 e l e 23
flow rate of 1 mL min-1
.
La belin g of Hu m a n Ser u m Albu m in (HSA) w ith Com -
p lex 3. The procedure was similar to that described for the
labeling of HSA with complex 2 except that 50 mM potassium
phosphate buffer (pH 7.0) was used instead.
no. of reflns collected
no. of indep reflns
completeness to θ ) 25.35°
absorp corr
22 215
6509 [R(int)a ) 0.0397]
94.3%
1
P h ysica l Mea su r em en ts a n d In str u m en ta tion . The H
none
full-matrix least-squares on F2
NMR spectra were recorded on a Bruker DPX-400 FT-NMR
spectrometer (400 MHz) in CD3CN at 298 K with chemical
shifts reported relative to Me4Si. Positive-ion FAB-mass
spectra were recorded on a Finnigan MAT95 mass spectrom-
eter. The UV/vis spectra were obtained on a Hewlett-Packard
8452A diode array spectrophotometer and IR spectra as KBr
disks on a Bio-Rad FTS-7 Fourier transform infrared spectro-
photometer (4000-400 cm-1). Elemental analyses of the new
complexes were performed on a Carlo Erba 1106 elemental
analyzer at the Institute of Chemistry, Chinese Academy of
Sciences. Steady-state excitation and emission spectra were
obtained 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 solid samples at 77 K were conducted using
a liquid nitrogen-filled optical Dewar flask. Excited-state
lifetimes of solution samples were measured using a conven-
tional laser system. The excitation source used 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).
Luminescence quantum yields were measured by the optical
dilute method reported by Demas and Crosby.18 A degassed
aqueous solution of [Ru(bpy)3]Cl2 (φ ) 0.042, excitation
wavelength at 436 nm) was used as the reference.19 All
solutions for photophysical studies 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. Cyclic voltammetric measurements were per-
formed by using a CH Instruments, Inc. model CHI 600A elec-
trochemical analyzer. The electrolytic cell used was a conven-
tional 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
electrode (CH Instruments, Inc.) with a platinum wire as the
counter electrode. The working electrode surface was first
polished with 1 mm alumina slurry (Linde) on a microcloth
(Buehler Co.). It was then rinsed with ultrapure deionized
water and sonicated in a beaker containing ultrapure water
for 5 min. The polishing and sonicating steps were repeated
twice, and then the working electrode was finally rinsed under
a stream of ultrapure deionized water. The ferrocenium/
ferrocene couple (FeCp2+/0) was used as the internal reference.
All solutions for electrochemical studies were deaerated with
prepurified argon gas just before measurements.
refinement method
no. of data/restraints/params 6509/0/436
goodness-of-fitbon F2
final R indices [I>2σ(I)]c
R indices (all data)
1.011
R1 ) 0.0349, wR2 ) 0.0934
R1 ) 0.0440, wR2 ) 0.0988
0.975 and -1.726 e Å-3
largest diff peak and hole
Rint ) ∑|Fo - Fo2(mean)|/∑[Fo2]. GooF ) {∑[w(Fo - Fc2)2]/
a
2
b
2
(n - p)}1/2, where n is the number of reflections and p is the total
number of parameters refined. The weighting scheme is w )
1/[σ2(Fo2) + (aP)2+ bP], where P is [2Fc2+ Max(Fo2,0)]/3. c R1 )
2
∑||Fo| - |Fc||/∑|Fo|, wR2 ) {∑[w(Fo - Fc2)2]/∑[w(Fc2)2]}1/2
.
X-r a y Cr ysta l Str u ctu r e Deter m in a tion . Single crystals
of 2 were obtained by vapor diffusion of diethyl ether into an
acetone solution of the complex. A crystal of dimensions 0.30
× 0.30 × 0.15 mm mounted in a glass capillary was used for
data collection at 301 K on a MAR diffractometer with a 300
mm image plate detector using graphite-monochromatized Mo
KR radiation (λ ) 0.71073 Å). Data collection was made with
2° oscillation of æ steps, 8 min exposure time, and a scanner
distance of 120 mm. A total of 82 images were collected. The
images were interpreted and intensities integrated using
DENZO.20 The structure was solved by direct methods employ-
ing SIR-97.21 The Pt and S atoms and most of the non-
hydrogen atoms were located according to direct methods and
successive least-squares Fourier cycles. The positions of other
non-hydrogen atoms were found after successful refinement
by full-matrix least-squares using SHELXL 97.22 All 6509
independent reflections (Rint equal to 0.0397, 5153 reflections
larger than 4σ(Fo)) from a total 22 215 reflections participated
in the full-matrix least-squares refinement against F2, where
2
Rint ) ∑|Fo - Fo2(mean)|/∑[Fo2]. These reflections were in the
range -38 e h e 38, -13 e k e 13, -23 e l e 23 with 2θmax
equal to 50.70°. One crystallographic asymmetric unit consists
of one formula unit. Hydrogen atoms were generated using
SHELXL 97 and their positions calculated based on the riding
mode with thermal parameters equal to 1.2 times that of
associated C atoms and participated in the calculation of the
(20) Written with the cooperation of the program authors Otwin-
owski, Z.; Minor, W.; Gewirth, D. DENZO: The HKL Manuals A
description of programs DENZO, XDISPLAYF, and SCALEPACK; Yale
University: New Haven, 1995.
(21) Sir97:
a new tool for crystal structure determination and
refinement: Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. J . Appl. Crystallogr. 1998, 32, 115.
(22) SHELXL97: Sheldrick, G. M. SHELXL97: Programs for
Crystal Structure Analysis (release 97-2); University of Go¨ttingen:
Go¨ttingen, Germany, 1997.
(23) (a) Blanton, C. B.; Murtaza, Z.; Shaver, R. J .; Rillema, D. P.
Inorg. Chem. 1992, 31, 3230. (b) von Zelewsky, A.; Gremaud, G. Helv.
Chim. Acta 1988, 84, 85.
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3462 Organometallics, Vol. 23, No. 14, 2004
Wong et al.
F igu r e 2. Ball-and-stick model showing the head-to-tail
arrangement of two complex cations of 2, from the view
along the y-axis.
F igu r e 1. Perspective drawing of the complex cation of 2
with atomic numbering scheme. Hydrogen atoms are
omitted for clarity. Thermal ellipsoids are drawn at 50%
probability level.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d Bon d
An gles (d eg) for Com p lex 2 w ith Estim a ted
Sta n d a r d Devia tion s (esd s) Given in P a r en th eses
final R-indices. Convergence ((∆/σ)max ) 0.001, av 0.001) for
436 variable parameters by full-matrix least-squares refine-
ment on F2 reaches R1 ) 0.0349 and wR2 ) 0.0934 with a
goodness-of-fit of 1.011. Crystallographic and structural refine-
ment data are given in Table 1.
Pt(1)-N(1)
Pt(1)-N(3)
C(1)-C(2)
N(4)-C(6)
S(1)-C(9)
2.012(4)
2.007(4)
1.154(7)
1.395(7)
1.573(7)
Pt(1)-N(2)
Pt(1)-C(1)
C(2)-C(3)
N(4)-C(9)
1.947(4)
1.988(5)
1.454(8)
1.167(7)
Resu lts a n d Discu ssion
N(1)-Pt(1)-N(2)
C(1)-Pt(1)-N(3)
80.89(15) N(2)-Pt(1)-N(3)
99.19(18) C(1)-Pt(1)-N(1)
80.24(16)
99.63(17)
Syn th esis a n d Ch a r a cter iza tion . Complex 1 was
prepared according to modification of a previously
reported procedure,11e,g involving the reaction of [Pt-
(tBu3tpy)(MeCN)](OTf)2 with 4-aminophenylacetylene in
the presence of sodium hydroxide in methanol under
reflux conditions. Complexes 2 and 3 were synthesized
from the reaction of complex 1 with SCCl2 and iodoacetic
anhydride, respectively, in acetone. The presence of
three tert-butyl groups on the terpyridyl ligand allows
for purification by column chromatography on silica gel,
and the dark purple band containing 1 and the orange
band containing 3 were collected as the desired prod-
ucts. The identities of complexes 1-3 have been con-
firmed by 1H NMR spectroscopy, FAB-mass spec-
trometry, IR spectroscopy, and satisfactory elemental
analyses. The crystal structure of 2 has also been
determined by X-ray crystallography. The IR spectra of
N(1)-Pt(1)-N(3) 161.03(16) N(2)-Pt(1)-C(1) 178.68(18)
Pt(1)-C(1)-C(2) 179.2(5)
C(6)-N(4)-C(9) 147.9(7)
C(1)-C(2)-C(3)
N(4)-C(9)-S(1)
177.6(6)
175.6(6)
90° and 180°. Similar to a related platinum-terpyridyl
complex, [Pt(tpy)(CtC-C6H5)]+,11e the small dihedral
angle (4.06°) between the platinum-terpyridyl plane and
the phenyl group of the alkynyl ligand indicates an
essentially coplanar arrangement of these two planes.
The isothiocyanate functional group shows a linear
arrangement about N(4)-C(9)-S(1) with a bond angle
of 175.6° and an exceptionally large C(6)-N(4)-C(9)
bond angle of 147.9°, which are commonly observed in
a related transition metal isothiocyanato system.4b As
a result of sp hybridization of the alkynyl carbon atoms,
an essentially linear arrangement about the alkynyl
group [Pt(1)-C(1)-C(2) 179.2°; C(1)-C(2)-C(3) 177.6°]
is observed. The bond lengths of Pt-N [Pt(1)-N(1) 2.012
Å; Pt(1)-N(2) 1.947 Å; Pt(1)-N(3) 2.007 Å] are com-
parable to that of other platinum(II)-terpyridyl sys-
tems,9a,b,10,11a,c-e,g while the Pt(1)-C(1) bond of 1.988 Å
and C(1)-C(2) of 1.154 Å are in the range typical of
platinum(II)-alkynyl complexes.10d,11e,g,13a,c,d It is inter-
esting to note that two nearby complex cations show a
head-to-tail conformation with a Pt‚‚‚Pt distance of
4.033 Å, as shown in Figure 2, to minimize the mutual
repulsion between the sterically bulky tri-tert-butylter-
pyridine ligands despite the fact that no significant
Pt‚‚‚Pt or π‚‚‚π interactions are observed.
1 and 3 show a weak band at 2112 and 2118 cm-1
,
respectively, typical of the ν(CtC) stretching frequency.
Due to the relatively weak intensity of this stretch
together with the overlapping of the much more intense
ν(NdCdS) stretch located at a similar frequency in
complex 2, no attempts were made to assign the ν(CtC)
stretch in 2. In the NMR spectra, the electron-with-
drawing effects of the carbonyl group in 3 resulted in
the downfield shift of the amide NH signal (δ 8.37 ppm)
compared to the corresponding NH2 signal of 1 (δ 4.27
ppm).
Cr ysta l Str u ctu r e Deter m in a tion . The perspective
drawing of the complex cation of 2 is shown in Figure
1. Selected bond distances and bond angles are tabu-
lated in Table 2. The platinum(II) center, coordinated
to a terpyridyl and an alkynyl group, adopts a distorted
square-planar geometry. Due to the steric demand of
the terpyridyl ligand, the N-Pt-N angles [N(1)-Pt(1)-
N(2) 80.89°; N(2)-Pt(1)-N(3) 80.24°; N(1)-Pt(1)-N(3)
161.03°] show deviations from the idealized values of
Electr och em istr y. The cyclic voltammograms of
complexes 1-3 in acetonitrile (0.1 M Bu4NPF6) show
n
two quasi-reversible reductions at -1.00 and -1.5 V (vs
SCE) and one irreversible oxidation at +0.71 to +1.41
V. Their electrochemical data are tabulated in Table 3.
The two reduction couples, which are independent of
the nature of substituents on the alkynyl ligand, are
assigned as the two successive reductions of the ter-

Platinum(II)-Terpyridyl Alkynyl Complexes
Organometallics, Vol. 23, No. 14, 2004 3463
Ta ble 3. Electr och em ica l Da ta for 1-3a
high-energy absorption bands, with extinction coef-
ficients (ꢀ) on the order of 104 dm3 mol-1 cm-1, are
assigned as intraligand (IL) π f π*(tBu3-tpy) transition
of the terpyridyl ligand. With reference to previous
spectroscopic work on related complexes, [{Pt(tBu3tpy)}2-
(CtC)n]2+ (n ) 1, 2, and 4) and [Pt(tpy)(CtCR)]+,10d,11d,g
the less intense low-energy absorption bands, with
extinction coefficients (ꢀ) on the order of 103 dm3 mol-1
cm-1, are assigned to dπ(Pt) f π*(tBu3tpy) metal-to-
ligand charge transfer (MLCT) transition, probably with
some mixing of π(CtCR) f π*(tBu3tpy) ligand-to-ligand
charge transfer (LLCT) transition, supported by elec-
trochemical studies. The different solution colors of the
complexes observed in acetonitrile (purple-blue in 1,
yellow in 2, and orange-red in 3) parallel the occurrence
of low-energy absorption bands at different wavelengths,
with 1 at 488 nm, 2 at 424 nm, and 3 at 448 nm (Figure
4). The energy trend of these absorption bands, 1 < 3
< 2, follows the degree of electron-richness of the
oxidation Epa/V
reduction E1/2/V
complex
vs SCEb
vs SCEc (∆Ep/mV)
1
+0.71
+1.41
+0.93
-1.02 (82)
-1.52 (64)
-0.99 (75)
-1.46 (72)
-1.00 (52)
-1.50 (54)
2
3
In acetonitrile solution with 0.1 M nBu4NPF6 (TBAH) as
a
supporting electrolyte at room temperature; scan rate 100 mV s-1
.
b
Epa refers to the anodic peak potential for the irreversible
c
oxidation waves. E1/2 ) (Epa + Epc)/2; Epa and Epc are peak anodic
and peak cathodic potentials, respectively.
substituent group on the alkynyl ligand, NH2
>
NHCOCH2I > NCS. Complex 1 with an amino group
absorbs at the lowest energy, while 2 with an isothio-
cyanate group absorbs at the highest energy, in ac-
cordance with the MLCT/LLCT admixture assignment
of the low-energy absorption band. The electron-donat-
ing NH2 group is the most electron-rich alkynyl ligand
and hence will raise the dπ(Pt) orbital energy to the
largest extent in 1. As a result, the HOMO-LUMO
energy gap of the MLCT transition would be the
smallest in 1, assuming the π*(tBu3tpy) energy is
insignificantly perturbed. On the other hand, the elec-
tron-withdrawing isothiocyanate group in 2 causes the
dπ(Pt) orbital to be the lowest lying in energy, giving
rise to the highest MLCT transition energy.
F igu r e 3. Cyclic voltammogram of complex 2.
pyridyl ligand. Similar observations have been reported
and similar assignments have also been made for other
related platinum(II)-terpyridyl systems.10d,11d The pres-
ence of three electron-donating tert-butyl groups on the
terpyridyl ligand did not seem to cause a significant
shift in the reduction potentials of complexes 1-3
relative to that of the unsubstituted terpyridyl analogue,
[Pt(tpy)(CtC-R)]+.11d In general, the oxidation wave
was found to occur at more positive potential for the
complexes with less electron-rich alkynyl ligands, such
that 2 (+1.41 V) > 3 (+0.93 V) > 1 (+0.71 V), in line
with the electron-richness of the substituent on the
alkynyl ligand, NCS < NHCOCH2I < NH2, resulting
in the energy of the dπ(Pt) orbital in the order 2 < 3 <
1. With reference to previous electrochemical studies of
related platinum(II) complexes,11d,13a,d,24 the irreversible
oxidation wave is ascribed to a Pt(II) to Pt(III) oxidation
process. Similar trends have also been reported in other
related alkynylplatinum(II)-terpyridyl complexes.11d It
is interesting to note that an additional quasi-reversible
redox couple appeared at ca. +0.06 V in 1 upon reverse
scanning toward cathodic potential after passing through
the irreversible oxidation wave, as shown in Figure 3.
In view of the fact that such a phenomenon is observed
only in complex 1, the additional quasi-reversible couple
resulting from an EC mechanism is suggested to be
derived from the decomposed products generated from
the irreversible oxidation of the amino group in 1.
UV-Vis Absor p tion . The electronic absorption spec-
tra of complexes 1-3 in acetonitrile solution display
high-energy absorption bands at 246-340 nm and low-
energy absorption bands at 400-488 nm. Table 4
summarizes their photophysical data, and Figure 4
shows the electronic absorption spectra of complexes
1-3 in acetonitrile solution at 298 K. In view of the sim-
ilar absorption bands observed in other platinum(II)-
terpyridyl systems,9-11 the intense vibronic-structured
P h otolu m in escen ce. Upon photoexcitation, com-
plexes 2 and 3 display intense emission bands at ca.
560-643 nm in fluid solution and in the solid state at
298 K and at 77 K, while complex 1 is only emissive at
77 K in glass medium. The photophysical data are
collected in Table 4. The large Stoke’s shift together with
the relatively long emission lifetime in the microsecond
range suggest that the emission originates from a spin-
forbidden triplet excited state. With reference to pre-
vious spectroscopic studies on related complexes,
[{Pt(tBu3tpy)}2(CtC)n]2+ (n ) 1, 2, and 4) and [Pt(tpy)-
(CtCR)]+,10d,11d,g together with consideration of the
close resemblance of the excitation band at ca. 433-
460 nm to the low-energy electronic absorption band at
ca. 440 nm in 2 and 3, an emission origin of a dπ(Pt) f
3
π*(tBu3tpy) MLCT excited state, with some mixing of
3
a π(CtCR) f π*(tBu3tpy) LLCT state, is tentatively
assigned. Figure 5 displays the excitation and emission
spectra of 2 and 3 in acetonitrile solution at 298 K.
Similar to the electronic absorption study, the observa-
tion of the emission band of 2 at higher energy than
that of 3 is in line with the lower electron-richness of
the isothiocyanate group than the iodoacetamide, which
would lower the dπ(Pt) orbital energy in 2, causing a
higher energy MLCT emission. The nonemissive behav-
ior of 1 in the solid state and in acetonitrile solution
may be ascribed to the quenching by photoinduced
electron transfer (PET), in which the electron is trans-
ferred from the electron-rich amino group to the plati-
num metal center to quench the emissive 3MLCT excited

3464 Organometallics, Vol. 23, No. 14, 2004
Ta ble 4. P h otop h ysica l Da ta for Com p lexes 1-3 a n d Biocon ju ga tes 2-HSA a n d 3-HSA
Wong et al.
emission
max/nm (τo/µs)
complex or
bioconjugate
absorption
medium (T [K])
λ
max/nm (ꢀmax/dm3 mol-1 cm-1
)
λ
Φlum
1
MeCN (298)
254 (42 680), 272 (47 310), 286 (53 935),
310 (23 515), 326 (19 220), 338 (16 935),
400 (3930), 488 (4920)
nonemissive
solid (298)
solid (77)
nonemissive
nonemissive
nPrCN (77)
MeCN (298)
590 (9.41) and 708a (1.55)
586 (0.56)
2
246 (44 520), 270 (37 920), 286 (46 300),
298 (51 620), 316 (53 340), 338 (22 610),
400 (7560), 424 (8305)
0.019
solid (298)
solid (77)
583 (0.48)
560, 600(6.40)
522 (14.71), 630a (2.59)
638 (0.49)
nPrCN (77)
MeCN (298)
3
250 (50 290), 270 (46 650), 286 (51 580),
308 (40 010), 326 (28 100), 340 (23 220),
404 (5960), 448 (7070)
7.2 × 10-4
solid (298)
solid (77)
638 (0.15)
643 (1.65)
nPrCN (77)
bufferb (298)
bufferb (298)
550 (13.3), 626a (2.71)
2-HSA
3-HSA
288, 320, 470
280, 334, 466
650
630
5.3 × 10-4
0.013
a
b
Excitation wavelength > 540 nm. 50 mM Tris-Cl buffer (pH 7.4).
F igu r e 6. Excitation spectra (a) monitored at 520 nm (s)
and at 630 nm (‚‚‚) and emission spectra (b) upon excitation
at 440 nm (s) and 520 nm (‚‚‚) of 3 in butyronitrile glass
at 77 K.
F igu r e 4. Electronic absorption spectra of 1 (s), 2 (‚‚‚),
and 3 (- - -) in acetonitrile solution at 298 K.
at 522-590 nm and a low-energy one at 626-708 nm
(Figure 6). In view of the large differences in the excited-
state lifetimes and the excitation spectra derived from
these two emission bands, they are suggested to be of
different origins. The high-energy emission band with
vibrational progressional spacings of ca. 1215 cm-1 is
assigned as intraligand phosphorescence of the ter-
pyridyl ligand, while the low-energy emission band is
assigned to be derived from the 3MLCT/3LLCT phos-
phorescence.
La belin g of Biom olecu le. In view of the ready
reaction of the isothiocyanate and the iodoacetamide
groups with the respective primary amine and sulf-
hydryl groups of biomolecules, complexes 2 and 3 are
designed to act as luminescent biolabeling agents via
the formation of a thiourea and thioether linkage,
respectively. In this report, the protein human serum
albumin (HSA) has been labeled with complexes 2 and
3 to afford the bioconjugates 2-HSA and 3-HSA (Scheme
1), respectively, and purified by size-exclusion chroma-
tography to remove any unreacted complexes. The
bioconjugates 2-HSA and 3-HSA display a low-energy
absorption band at 470 and 462 nm, respectively, in
their electronic absorption spectra, indicating the suc-
cessful attachment of the [Pt(tBu3tpy)(CtC-R)]+ moiety
F igu r e 5. Excitation (a) and emission (b) spectra of 2 (s)
and 3 (‚‚‚) in acetonitrile at 298 K.
state. Alternatively, the quenching of the 3MLCT excited
state may be attributed to the presence of an energeti-
3
cally accessible or lower-lying LLCT excited state, as
a result of a relatively higher-lying π(CtCR) orbital due
to the presence of the electron-donating amino substitu-
ent. Other related complexes, such as [Pt(tpy)(CtC-
C6H4-OMe)]+11d or [Pt(bpy)(CtC-C6H4-OMe)2],13a bear-
ing electron-donating substituents were reported to be
nonemissive or weakly emissive. It is noteworthy that
complexes 1-3 in 77 K glass display dual luminescence,
with a vibronic-structured high-energy emission band

Platinum(II)-Terpyridyl Alkynyl Complexes
Organometallics, Vol. 23, No. 14, 2004 3465
Sch em e 1
onto the protein. The low-energy absorptions of the
bioconjugates are characteristic of the alkynyl plati-
num(II)-terpyridyl moiety and are attributed to an
MLCT/LLCT admixture. Such absorptions are slightly
red-shifted with respect to the parent complexes 2 and
3, probably as a result of the perturbation associated
with the electronic as well as environmental effects of
the protein and the solution media. Upon photoexcita-
tion, the bioconjugates 2-HSA and 3-HSA in 50 mM
Tris-Cl buffer (pH 7.4) exhibit an emission band at 650
and 630 nm, respectively (Figure 7). Despite different
medium conditions, it is interesting to note that the
emission energy of 2-HSA (650 nm in buffer solution)
is much lower than that of its parent complex 2 (586
nm in acetonitrile), while similar emission energy is
observed for 3-HSA (630 nm in buffer solution) and 3
(638 nm in acetonitrile). On the basis of the spectro-
scopic studies of complexes 2 and 3, the emission origin
of 2-HSA and 3-HSA are assigned as derived from a
equipment. The higher luminescence quantum yield,
Φ
lum, of 3-HSA (0.013) than 2-HSA (5.3 × 10-4) suggests
that complex 3 could act as a potential luminescent
biolabeling agent owing to its higher emission intensity.
Despite a low luminescence quantum yield for 2-HSA,
the ability of the bioconjugate to emit with a different
color from its parent complex may find interesting
applications in luminescent biolabeling.
Con clu d in g Rem a r k s
A series of alkynylplatinum(II)-terpyridyl complexes,
[Pt(tBu3tpy)(CtC-C6H4-X-4)](OTf) (X ) NH2, NCS,
and NHCOCH2I), have been synthesized and character-
ized. The X-ray crystal structure of [Pt(tBu3tpy)(CtC-
C6H4-NCS-4)](OTf) has been determined. Electrochemi-
cal studies show that all the complexes display two
quasi-reversible reduction couples and one irreversible
oxidation wave in the cyclic voltammograms, corre-
sponding to the successive reduction of the terpyridine
ligand and a Pt(II) f Pt(III) metal-centered oxidation,
respectively. A low-energy absorption band at 400-488
nm is observed in their electronic absorption spectra,
assignable to a dπ(Pt) f π*(tBu3tpy) MLCT transition,
mixed with a π(CtCR) f π*(tBu3tpy) LLCT transition.
Upon photoexcitation, intense emission bands at 586-
708 nm are observed, which are believed to originate
from the excited states of 3MLCT/3LLCT character. The
complexes [Pt(tBu3-tpy)(CtC-C6H4-NCS-4)](OTf) and
[Pt(tBu3-tpy)(CtC-C6H4-NHCOCH2I-4)](OTf) are de-
signed to be utilized as luminescent labels, and human
serum albumin (HSA) has been labeled with these two
complexes to afford the corresponding bioconjugates.
Similar to their parent labels, both bioconjugates are
highly colored and exhibit luminescence in the visible
region upon photoexcitation. The bioconjugate resulting
from the isothiocyanate complex is found to emit with
a different luminescence color when compared to its
parent label, while the bioconjugate derived from the
iodoacetamide complex is found to emit with a higher
luminescence intensity.
3
dπ(Pt) f π*(tBu3tpy) MLCT excited state, with some
3
mixing of a π(CtCR) f π*(tBu3tpy) LLCT state. The
change in emission energy from 2 to 2-HSA is attributed
to the decrease in electron-withdrawing ability of the
thiourea group with respect to the isothiocyanate group,
giving rise to a higher dπ(Pt) orbital energy in 2-HSA
and hence a lower MLCT emission energy. The phe-
nomenon of the bioconjugate and its label exhibiting
different emission colors is quite unusual since very
similar emission energy is usually observed for the
bioconjugate and its label in other transition metal
luminescent biological labeling systems.3a,b,4b,d,5,7a,b,d The
higher emission energy observed in the bioconjugate
3-HSA relative to 2-HSA (Figure 7) is attributed to the
higher electron-withdrawing ability of the amide func-
tionality in 3-HSA than the thiourea group in 2-HSA.
The excited-state lifetimes of both 2-HSA and 3-HSA
in buffer solution are in the submicrosecond range (<0.1
µs) and cannot be determined with certainty with our
Ack n ow led gm en t. V.W.-W.Y. acknowledges sup-
port from the University Grants Committee Area of
Excellence Scheme and The University of Hong Kong
Foundation for Educational Development and Research
Limited. B.W.-K.C. acknowledges the receipt of a Uni-
versity Postdoctoral Fellowship and W.-S.T. the receipt
of a postgraduate studentship, both administered by
The University of Hong Kong. Helpful discussions and
assistance provided by Dr. Kenneth K.-W. Lo are
gratefully acknowledged.
Su ppor tin g In for m ation Available: Tables giving atomic
coordinates, anisotropic thermal parameters, bond lengths,
and bond angles for complex 2. This material is available free
of charge via the Internet at http://pubs.acs.org.
F igu r e 7. Emission spectra of 2-H SA (s) and 3-H SA
(‚‚‚) in 50 mM Tris-Cl (pH 7.4) buffer solution at 298 K.
The asterisk denotes the instrumental artifact.
OM049898H