Synthesis and characterization of platinum diimine bis(acetylide) coplexes containing easily derivatizable aryl acetylide ligands

Article abstract of DOI:10.1021/ic0263084

New Pt(II) diimine bis(acetylide) complexes where the diimine is a substituted bipyridine or phenanthroline and the aryl-acetylide is 4-ethynylbenzaldehyde have been prepared in good to excellent yields. Spectroscopic characterization supports a square planar coordination geometry with cis-alkynyl ligands, and the crystal structure of one of the complexes, Pt(phen)(C≡CC₆H₄CHO)₂ (1), confirms the assignment. The new diimine bis(acetylide) complexes exhibit an absorption band ca. 400 nm that corresponds to a Pt(d) → π* diimine charge transfer transition and are brightly emissive in fluid solution, with excited state lifetimes in the range 100-800 ns. Correlation of diimine substituent with λ_max for the 400 nm absorption band gives strong support to the MLCT assignment. Complex 1 undergoes electron transfer quenching, showing good Stern-Volmer behavior with a variety of oxidative and reductive quenchers. Quenching studies conducted with DNA nucleosides (A, T, C, G) were also investigated. Silyl-protected adenosine and guanosine were found to quench the luminescence of 1 better than similarly protected cytidine or thymidine. Since the former are the more easily oxidized bases, the results suggest that the Pt(II) diimine bis(acetylide) complexes are more powerful photooxidants than photoreductants with regard to electron transfer to DNA bases.

Full text of DOI:10.1021/ic0263084

Inorg. Chem. 2003, 42, 3772−3778
Synthesis and Characterization of Platinum Diimine Bis(acetylide)
Complexes Containing Easily Derivatizable Aryl Acetylide Ligands
Thaddeus J. Wadas, Rene J. Lachicotte, and Richard Eisenberg*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received December 27, 2002
New Pt(II) diimine bis(acetylide) complexes where the diimine is a substituted bipyridine or phenanthroline and the
arylacetylide is 4-ethynylbenzaldehyde have been prepared in good to excellent yields. Spectroscopic characterization
supports a square planar coordination geometry with cis-alkynyl ligands, and the crystal structure of one of the
complexes, Pt(phen)(CtCC H CHO)2 (1), confirms the assignment. The new diimine bis(acetylide) complexes
6
4
exhibit an absorption band ca. 400 nm that corresponds to a Pt(d) f π* diimine charge transfer transition and are
brightly emissive in fluid solution, with excited state lifetimes in the range 100−800 ns. Correlation of diimine
substituent with λmax for the 400 nm absorption band gives strong support to the MLCT assignment. Complex 1
undergoes electron transfer quenching, showing good Stern−Volmer behavior with a variety of oxidative and reductive
quenchers. Quenching studies conducted with DNA nucleosides (A, T, C, G) were also investigated. Silyl-protected
adenosine and guanosine were found to quench the luminescence of 1 better than similarly protected cytidine or
thymidine. Since the former are the more easily oxidized bases, the results suggest that the Pt(II) diimine bis-
(acetylide) complexes are more powerful photooxidants than photoreductants with regard to electron transfer to
DNA bases.
Introduction
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* Author to whom correspondence should be addressed. E-mail:
eisenberg@chem.rochester.edu.
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3772 Inorganic Chemistry, Vol. 42, No. 12, 2003
10.1021/ic0263084 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/20/2003

Platinum Diimine Bis(acetylide) Complexes
One possible set of chromophores that has shown itself
to be attractive is that of diimine-containing square planar
Pt(II) complexes where the diimine corresponds to 2,2′-
bipyridine, 1,10-phenanthroline or derivatives and analogues
thereof. These systems possess charge transfer excited states
that, depending on the anionic ligands, may be luminescent
in fluid solution.22-27 For these chromophores, the LUMO
is a π* orbital of the diimine ligand. Over the past decade,
we have focused on the synthesis, characterization, and
photophysical study of Pt(diimine)X2 complexes in which
X2 is either a dithiolate or bis(acetylide) ligands. For the
dithiolate systems, the HOMO has mixed metal and dithiolate
character,28-30 whereas for the bis(acetylide) complexes, the
LUMO is mainly localized on Pt. The latter complexes are
brightly emissive in fluid solution with emission quantum
yields as high as 14%, and at 77 K in butyronitrile glass,
they display resolved vibronic components.31 A study of the
effects of ligand variation on excited state energy has
Sonogashira-Hagihara coupling reactions35-38 with their
attendant problems of purification and catalyst metal ion
removal.21 Through the presence of an aldehyde substituent
on the arylacetylide ligand, we envisioned more facile and
better yielding coupling reactions between the chromophore
and the potential reductive quencher. In this paper, the first
step toward this goal is realized with the synthesis, charac-
terization, and study of Pt diimine complexes having the
aldehyde-containing acetylide p-CtCC6H4CHO-.
Platinum diimine complexes are also of interest for
possible binding with DNA. Intercalation of planar platinum
complexes containing terpyridyl ligands has been known
since 1974, while more recently, Che and co-workers have
described dppz complexes of platinum, where dppz )
dipyridophenazine, as possible intercalators into duplex
DNA.39 In these studies, the platinum intercalators are
cationic in order to impart both aqueous solubility and
Coulombic attraction to the negatively charged DNA back-
bone. For the dppz complexes, the luminescence of the Pt
complex appears to turn on upon intercalation.
The notion of intercalative binding coupled with electron
transfer quenching known for Pt(diimine)(CtCAr)2 com-
plexes suggested that these systems, if properly derivatized
for aqueous solubility and positive charge, may be of interest
for photochemical initiation of charge transfer through DNA.
In most of the studies reported to date of charge transfer
through DNA, the process involves hole injection into the
base pair stack of the DNA duplex.39-59 Since the platinum
diimine complexes appeared to be respectable photoreduc-
3
confirmed that the emissive state is MLCT in origin, as
originally proposed by Che.31-33,34 The MLCT assignment
is also supported by transient absorption and time-resolved
infrared spectroscopies where the shift of the ν(CtC) band
to higher frequency is consistent with charge transfer away
from Pt in the excited state.33 The Pt(diimine)(CtCAr)2
complexes undergo both oxidative and reductive quenching,
but the systems only exhibit long-term stability on continuous
irradiation under reductive quenching conditions.31,34
In order to incorporate the Pt(diimine)(CtCAr)2 chro-
mophore into multicomponent systems that have been
referred to as molecular photochemical devices (MPD’s), it
is necessary to have functional groups present on the ligands
for coupling reactions to create bridges and connections
between components. To date, only one triad having a Pt-
(diimine)(CtCAr)2 chromophore has been reported, and its
preparation was dependent on a number of Pd2+-catalyzed
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Inorganic Chemistry, Vol. 42, No. 12, 2003 3773

Wadas et al.
Pt(Clphen)(CtCC6H4CHO)2 (3). The procedure followed for
1 was used except that Pt(Clphen)Cl2 (0.100 g; 0.21 mmol) was
used. Yield: 50%. 1H NMR (DMSO-d6): δ 9.97 (2H, s), 9.86 (1H,
d, J ) 8 Hz), 9.76 (1H, d, J ) 3 Hz), 9.18 (1H, d, J ) 3 Hz), 9.01
(1H, d, J ) 9.0 Hz), 8.66 (1H, s), 8.38 (1H, dd, J ) 4.0 Hz, J )
4.0 Hz), 8.28 (1H, dd, J ) 5.2 Hz, J ) 8.00 Hz), 7.86 (4H, d, J )
8 Hz), 7.64 (4H, m, J ) 4 Hz). MS (FD): m/z (%) 668.0 ([M]+,
100). Anal. Calcd for C30H17ClN2O2Pt (Mr ) 668): C, 53.94; H,
2.57; N, 4.19. Found: C, 54.22; H, 2.23; N, 4.04.
tants, we became interested in whether intercalative Pt
systems could be used to initiate charge transfer by electron
injection into the base pair stack of the duplex. In order to
assess the feasibility of such a process, quenching studies
were conducted with the Pt(phen)(p-CtCC6H4CHO)2 (1)
complex using different silyl-protected nucleosides, as well
as other electron transfer quenchers. The present paper
describes the results of these studies.
Pt(bpy)(CtCC6H4CHO)2 (4). A Schlenk tube was charged with
Pt(bpy)Cl2 (0.100 g; 0.24 mmol), CuI (10 mg), 4-ethynylbenz-
aldehyde (0.3079 g; 2.3 mmol), and a solution of DMSO:
triethylamine (5:1 v/v). The resulting solution was stirred at 65 °C
for 20 h, the solvent was removed, water was added, and the
resulting precipitate was isolated by vacuum filtration. It was then
dissolved in a minimum amount of CH2Cl2 and recrystallized from
hexanes. An analytically pure sample was obtained by preparative
TLC (neutral alumina; 99.5% CH2Cl2/ 0.5% MeOH). Yield: 90%.
1H NMR (DMSO-d6): δ 9.95 (2H, s), 9.51 (2H, d, J ) 8 Hz),
8.70 (2H, d, J ) 8 Hz), 8.45 (2H, t, J ) 7 Hz), 7.93 (2H, t, J ) 8.0
Hz), 7.83 (4H, d, J ) 8 Hz), 7.57 (4H, d, J ) 12 Hz). MS (FD):
m/z (%) 609.0 ([M]+, 100). Anal. Calcd for C28H18N2O2Pt (Mr )
609): C, 55.17; H, 2.97; N, 4.59. Found: C, 54.93; H, 2.74; N,
4.25.
Experimental Section
Reagents. 1,10-Phenanthroline, 5-Cl-phenanthroline (Clphen),
5-Me-phenanthroline (Mephen), bipyridine (bpy), 4,4′-dimethylbi-
pyridine (dmbpy), 4,4′-diphenylbipyridine (dphbpy), and 4-trim-
ethylsilylethynylbenzaldehyde (all from Aldrich) and potassium
tetrachloroplatinate (Johnson Matthey) were used without further
purification. The diimine complexes Pt(Rphen)Cl2 and Pt(4,4′-R2′-
bpy)Cl2, where R is Me, H, Cl and R′ is Me, H, Ph, respectively,
were prepared according to a literature method.60 The acetylide
precursor 4-ethynylbenzaldehyde was obtained by TMS removal
following a literature method using K2CO3 in anhydrous methanol.61
The syntheses of the different Pt(diimine)(p-CtCC6H4CHO)2
complexes were performed under nitrogen with degassed solvents
following a procedure similar to that previously reported.31,36 The
DNA nucleosides adenosine, guanosine, cytidine, and thymidine
were protected by a standard silylation procedure,62 while all other
quenchers were purified prior to use.
Pt(dmbpy)(CtCC6H4CHO)2 (5). The procedure followed for
4 was used except that Pt(4,4′-Me2bpy)Cl2 (0.100 g; 0.22 mmol)
1
was employed. Yield: 55%. H NMR (DMSO-d6): δ 9.97 (2H,
s), 9.31 (2H, d, J ) 4 Hz), 8.59 (2H, s), 7.85 (4H, d, J ) 2 Hz),
7.76 (2H, d, J ) 4 Hz), 7.57 (4H, d, J ) 8 Hz), 2.53 (6H, s). MS
(FD): m/z (%) 637.0 ([M]+, 100). Anal. Calcd for C30H22N2O2Pt
(Mr ) 637): C, 56.51; H, 3.48; N, 4.39. Found: C, 56.14; H, 3.06;
N, 4.43.
Pt(phen)(CtCC6H4CHO)2 (1). A Schlenk tube was charged
with Pt(phen)Cl2 (0.100 g; 0.22 mmol), CuI (10 mg), 4-ethynyl-
benzaldehyde (0.33 g, 2.5 mmol), and 10 mL of DMSO:DMF:NEt3
(5:3:2 v/v/v). The resulting solution was stirred at 65 °C for 20 h.
The solvent was removed, water was added, and the organic
material was isolated by vacuum filtration. The solids were then
dissolved in CH2Cl2, reprecipitated by the addition of hexanes, and
isolated by vacuum filtration. An analytically pure sample was
obtained by preparative TLC on neutral alumina using CH2Cl2/
MeOH (99.5:0.5 v/v) and recrystallized from CH2Cl2/hexanes.
Yield: 90%. 1H NMR (DMSO-d6): δ 9.97 (2H, s), 9.76 (2H, d, J
) 4 Hz), 9.06 (2H, d, J ) 8 Hz), 8.31 (2H, s), 8.25 (dd, 2H, J )
5.2 Hz, J ) 5.2 Hz), 7.86 (4H, d, J ) 8 Hz), 7.63 (4H, d, J ) 8
Hz). MS (FD): m/z (%) 634.0 ([M]+, 100). Anal. Calcd for
C30H18N2O2Pt (Mr ) 634): C, 56.87; H, 2.86; N, 4.42. Found: C,
56.68; H, 2.84; N, 4.47.
Pt(dphbpy)(CtCC6H4CHO)2 (6). The procedure followed for
4 was used except that Pt(Ph2bpy)Cl2 (0.100 g; 0.17 mmol) was
employed. Yield: 45%. 1H NMR (DMSO-d6): δ 9.96 (2H, s), 9.53
(2H, d, J ) 8 Hz), 9.19 (2H, s), 8.30 (2H, dd, J ) 4 Hz, J ) 2
Hz), 8.14 (2H, t, J ) 8.0 Hz), 7.83 (4H, d, J ) 8 Hz), 7.85 (4H,
d, J ) 8 Hz), 7.63 (4H, d, J ) 8 Hz), 7.58 (4H, d, J ) 8 Hz). MS
(FD): m/z (%) 762.0 ([M]+, 100). Anal. Calcd for C40H26N2O2Pt
(Mr ) 762): C, 63.07; H, 3.44; N, 3.68. Found: C, 63.06; H, 3.11;
N, 3.74
1
Physical Measurements. H NMR spectra were recorded on
Bruker AMX-400 or Avance 400 spectrometers. Field desorption
mass spectrometry was performed by the Analytical Services
Division of the Kodak Research Laboratories, Rochester, NY, and
elemental analyses were provided by Quantitative Technologies,
Whitehouse, NJ. Absorption spectra were recorded on a Hitachi
U2000 spectrometer while luminescence spectra and Stern-Volmer
experiments were measured on a SPEX Fluorolog-2 spectropho-
tometer corrected for instrument response. Fluid solution emission
samples, as well as those used in the quenching studies, were
subjected to at least 4 freeze-pump-thaw cycles.
Lifetime data were collected on an excimer pumped dye laser
system (1-3 mJ/pulse) and fit to single exponential decays.63
Quenching studies were performed by luminescence intensity
measurements and fit to the modified Stern-Volmer equation:
Pt(Mephen)(CtCC6H4CHO)2 (2). The procedure followed for
1 was used except that Pt(Mephen)Cl2 (0.100 g; 0.21 mmol) was
used. Yield: 60%. 1H NMR (DMSO-d6): δ 9.97 (2H, s), 9.79 (1H,
d, J ) 5 Hz), 9.68 (1H, d, J ) 9 Hz), 9.11 (1H, d, J ) 2 Hz), 8.94
(1H, d, J ) 8 Hz), 8.29 (1H, dd, J ) 5.2 Hz, J ) 5.2 Hz), 8.21
(1H, dd, J ) 8.0 Hz, J ) 4.0 Hz), 8.11 (1H, s), 7.86 (4H, d, J )
8 Hz), 7.63 (4H, d, J ) 6 Hz). MS (FD): m/z (%) 647.0 ([M]+,
100). Anal. Calcd for C31H20N2O2Pt (Mr ) 647): C, 57.50; H, 3.11;
N, 4.33. Found: C, 57.53; H, 2.89; N, 4.11.
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Iem°/Iem ) 1 + τokq[Q]
where kq is the quenching rate constant, [Q] is the concentration of
quencher, τo is the excited state lifetime of the chromophore, and
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3774 Inorganic Chemistry, Vol. 42, No. 12, 2003

Platinum Diimine Bis(acetylide) Complexes
Scheme 1
Table 1. Crystallographic Data for Pt(phen)(CtCC6H4CHO)2‚CH2Cl2
Iem°/Iem is the ratio of emission intensities of the chromophore in
the absence and presence of quencher. The concentration of the Pt
chromophore was held constant at 3 × 10-5 M while the quencher
concentration varied from 1 × 10-4 to 8 × 10-3 M. All samples
were maintained in the absence of light until measurements were
taken.
emp formula C31H20Cl2N2O2Pt abs correction
SADABSb
fw
T, K
718.48
193(2)
transm range
F(000)
0.489-0.928
696
λ, Å
0.71073
triclinic
2θ range, deg
1.73-23.25
-21 e h e 19
-11 e k e 10
-12 e l e 13
cryst syst
limiting indices
Crystal Structure Determination of Pt(phen)(CtCC6H4CHO)2
(1). Crystals were grown by slow liquid diffusion of hexanes into
a concentrated solution of 1 in CH2Cl2. A yellow colored fragment
of approximate dimension 0.18 × 0.08 × 0.04 mm3 was cut from
a larger plate and mounted under Paratone-8277 on a glass fiber,
and it was immediately placed in a cold nitrogen stream at -80 °C
on the X-ray diffractometer. The X-ray intensity data were collected
on a standard Siemens SMART CCD area detector system equipped
with a normal focus Mo-target X-ray tube operated at 2.0 kW (50
kV, 40 mA). A total of 1321 frames of data (1.3 hemispheres) were
collected using a narrow frame method with scan widths of 0.3° in
ω and exposure times of 30 s/frame using a detector-to-crystal
distance of 5.09 cm (maximum 2θ angle of 56.5°). The total data
collection time was approximately 13 h. Frames were integrated
to a maximum 2θ angle of 46.5° with the Siemens SAINT program
to yield a total of 5988 reflections, of which 3746 were independent.
Unit cell, space group, data collection,64 and refinement parameters
are summarized in Table 1. The structure was solved using direct
methods and refined by full matrix least squares on F2. All non-
hydrogen atoms of the metal complex were refined with anisotropic
thermal parameters, while the non-hydrogen atoms of the solvent
were refined isotropically. The hydrogen atoms were included in
idealized positions.
space group P1h (No. 2)
no. of reflns collected 5988
Z
2
a, Åa
b, Åa
c, Åa
â, dega
V, Å3
10.4736(10)
10.8580(10)
11.8959(11)
96.221(2)
1334.3(2)
no. of data/restraints/ 3746/0/328
params
GOFc
1.017
0.0369, 0.0976
0.0382, 0.0987
R1, wR2 (I > 2σ)d
R1, wR2 (all data)d
F
calcd, g cm-3 1.788
µ, mm-1
5.490
a It has been noted that the integration program SAINT produces cell
constant errors that are unreasonably small, since systematic error is not
included. More reasonable errors might be estimated at 10 × the listed
value. b The SADABS program is based on the method of Blessing; see:
Blessing, R. H. Acta Crystallogr., Sect A 1995, 51, 33. c GOF ) [∑[w(Fo
2
2
- Fc )2]/(n - p)]1/2×e2 where n and p denote the number of data and
2
2
parameters. d R1 ) (||Fo| - |Fc||)∑|Fo|; wR2 ) [∑[w(Fo - Fc )2]/
2
2
2
∑[w(Fo )2]]1/2 where w ) 1/[σ2(Fo + aP)2 + bP] and P ) [(Max; 0,Fo )
2
+ 2Fc ]/3.
acetylide metathesis that has been used for synthesizing other
Pt alkynyl derivatives.21,31,36
Due to the relative insolubility of the Pt(diimine)Cl2
starting material, the reactions were run in solvent mixtures
of DMF, DMSO, and NEt3 that upon heating to 65 °C easily
solubilized all starting materials (Scheme 1). All of the
complexes were characterized by NMR, electronic absorp-
tion, and emission spectroscopies, mass spectrometry, and
elemental analyses. The spectroscopic data support the
expected structural assignment of a square planar Pt(II)
complex with a coordinated diimine and two cis acetylide
ligands. For example, in the case of Pt(bpy)(CtCC6H4CHO)2
Results and Discussion
Synthesis and Characterization. The synthetic strategy
for making the Pt(diimine)(CtCC6H4CHO)2 complexes 1-6
(Scheme 1) is based on the CuI-catalyzed chloride-to-
(64) Wavefunction, Inc., 18401 Von Karman, Suite 370, Irvine, CA 92612.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3775

Wadas et al.
CC6H4CH3)2,66 respectively. The Pt-C(acetylide) bond dis-
tances average 1.946(7) Å and agree well with the values of
1.948(3),31 1.947(17),66 2.01(3),67 2.026(9),68 2.013(1),69 and
1.998(4) Å,70 for Pt(phen)(CtCC6H4CH3)2, Pt(dbbpy)-
(CtCC6H4CH3)2, [cis-{Pt(C6F6)2(CtCSiMe3)2}Pd(η3-C3H5)]}-
, [(PPh3)2Pt(µ-η1-η2-CtCtBu)2Pd(η3-C3H5)]+, [{cis-Pt(C6F5)2-
(-CtCPh)2}Ag2]2-, and trans-Pt(2,2′:6′,2′′-terpyridine-4′-
ylethynyl)(PBu3)2+, respectively. Only the Pt-C distance of
2.17(2) Å found in [Pt2(µ-CdCHPh)(CtCPh)(PEt3)4]71 is
significantly longer than that found in the present structure.
Deviations from a square planar geometry are relatively
minor with a C-Pt-C angle of 89.8(3)° and a N-Pt-N
bond angle of 80.0(2)°, the latter resulting from the constraint
of the phenanthroline ligand. Finally, an examination of the
crystal packing (not shown) reveals that the closest inter-
molecular Pt‚‚‚Pt contact is greater than 7 Å, indicating the
absence of any interactions between neighboring metal
centers. This observation is in accord with other recently
published Pt(diimine) bis(acetylide) complexes.31
Figure 1. Molecular structure and atom-numbering scheme for Pt(phen)-
(CtCC6H4CHO)2 (1) (30% probability ellipsoids). Hydrogens and molecule
of CH2Cl2 omitted for clarity.
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1
Absorption and Emission Spectra. The absorption
spectra of all of the complexes were taken in acetonitrile
and are presented in Figure 2. All of the complexes exhibit
a band around 400 nm, which has been previously assigned
as the charge transfer transition corresponding to excitation
from a filled Pt(d) orbital to an unoccupied π* diimine orbital
(MLCT).32,33 Also seen are the high-energy bands (λ < 370
nm) that are assigned to diimine and acetylide-based intra-
ligand transitions. The transition comprising the 400 nm band
shifts to lower energy as the electron-withdrawing ability of
the diimine substituent increases (phen, Me < H < Cl; bpy,
Me < H < Ph), consistent with a lowering of the LUMO
energy and the resultant charge transfer to diimine transi-
tion.31
Pt(1)-C(22)
Pt(1)-C(13)
Pt(1)-N(1)
Pt(1)-N(2)
N(1)-C(12)
N(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(2)
1.943(7)
1.950(7)
2.072(6)
2.076(6)
1.331(9)
1.335(9)
1.402(10)
1.369(11)
1.408(10)
1.418(9)
N(2)-C(2)
1.346(9)
C(13)-C(14)
C(14)-C(15)
C(18)-C(21)
C(22)-C(23)
C(23)-C(24)
C(27)-C(30)
C(21)-O(1)
C(30)-O(2)
1.208(11)
1.425(10)
1.477(12)
1.212(10)
1.429(10)
1.482(11)
1.212(13)
1.200(11)
C(22)-Pt(1)-C(13)
C(22)-Pt(1)-N(1)
C(13)-Pt(1)-N(1)
C(22)-Pt(1)-N(2)
C(13)-Pt(1)-N(2)
89.8(3)
174.1(2)
95.8(3)
94.69(2)
174.3(2)
N(1)-Pt(1)-N(2)
Pt(1)-N(1)-C(12)
C(12)-N(1)-C(1)
Pt(1)-C(13)-C(14)
C(13)-C(14)-C(15)
O(1)-C(21)-C(18)
80.0(2)
129.3(5)
118.0(6)
177.3(6)
176.1(8)
125.3(10)
Figure 3 presents the normalized emission spectra of the
complexes in acetonitrile. All of the complexes are brightly
emissive in fluid solution and, with the exception of
Pt(dmbpy)(CtCC6H4CHO)2 (5), show a broad structureless
emission. The relative quantum yield for emission of 1 was
1
(4), the H NMR spectrum displays a strong singlet at 9.95
ppm indicating the presence of the aldehyde protons which
are located on the arylacetylide ligand. Four resonances
associated with a symmetrically substituted bipyridine ring
system (two doublets and two triplets) are located at 9.51,
8.70, 8.45, and 7.93 ppm, respectively. These resonances are
also shifted approximately 1 ppm downfield when compared
to the 1H NMR spectrum of uncoordinated bipyridine. Two
strong doublets at 7.83 and 7.57 ppm are also observed and
have been assigned to the ring protons associated with the
para-substituted arylacetylide. Finally, the integration pattern
of the entire spectrum is consistent with the 2:1 ratio of
acetylide:diimine ligands and is confirmed by mass spectral
data.
2+
measured using Ru(bpy)3 as a standard (φrel ) 0.06272)
and found to have a value (ca. 0.03) similar to what we
reported previously for other Pt diimine bis(acetylide)
systems.31 The influence of substituent variation on the
emission energies for the Pt(Rphen)(CtCC6H4CHO)2 and
Pt(R2bpy)(CtCC6H4CHO)2 is tabulated in Table 3, with the
magnitude of the observed shifts greater than that seen for
the ∼400 nm band in the absorption spectra. As the electron-
(65) Hazell, A.; Mukhopadhyay, A. Acta Crystallogr. 1980, B36, 1647-
1649.
(66) James, S. L.; Younus, M.; Raithby, P. R.; Lewis, J. J. Organomet.
Chem. 1997, 543, 233-235.
Molecular Structure of Pt(phen)CtCC6H4CHO)2 (1).
The molecular structure of 1 which has approximate C2
symmetry is shown in Figure 1 along with the numbering
scheme used. Important bond lengths and angles are given
in Table 2, while a complete tabulation of distances and
angles is found in the CIF deposited as Supporting Informa-
tion. The Pt-N distance of 2.076(6) Å compares well with
corresponding values found in other Pt phenanthroline
complexes such as 2.063(3), 2.033(6), and 2.040(5) Å in Pt-
(phen)2Cl2 ,65 Pt(phen)(CtCC6H4CH3)2,31 and Pt(dbbpy)(Ct
(67) Berenguer, J. R.; Fornies, J.; Lalinde, E.; Martinez, F. J. Organomet.
Chem. 1994, 470, C15-C18.
(68) Berenguer, J. R.; Fornies, J.; Lalinde, E.; Martinez, F. Organometallics
1996, 15, 4537-4546.
(69) Espinet, P.; Fornies, J.; Martinez, F.; Sotes, M.; Lalinde, E.; Moreno,
M. T.; Ruiz, A.; Welch, A. J. J. Organomet. Chem. 1991, 403, 253-
267.
(70) Harriman, A.; Hissler, M.; Ziessel, R.; DeCian, A.; Fisher, J. J. Chem.
Soc., Dalton Trans. 1995, 4067-4080.
(71) Afzal, D.; Lenhert, P. G.; Lukehart, C. M. J. Am. Chem. Soc. 1984,
106, 3050-3052.
(72) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024.
3776 Inorganic Chemistry, Vol. 42, No. 12, 2003

Platinum Diimine Bis(acetylide) Complexes
Figure 3. Normalized room temperature emission spectra of Pt(Rphen)-
(CtCC6H4CHO)2 series (A) and Pt(R2bpy)(CtCC6H4CHO)2 series (B) in
acetonitrile. Units are arbitrary.
Figure 2. Normalized absorption spectra of Pt(Rphen)(CtCC6H4CHO)2
series (A) and Pt(R2bpy)(CtCC6H4CHO)2 series (B) in acetonitrile at room
temperature. Units are arbitrary.
Table 3. Absorbance Maxima, Emission Maxima, and Excited State
Lifetimesa
withdrawing ability of the diimine substituent increases, the
emission band shifts to lower energy, consistent with a
mainly metal based HOMO that is affected little by sub-
stituent and a π* diimine LUMO that is highly substituent
dependent.
Lifetime measurements were recorded at dilute concentra-
tions for all of the complexes and are tabulated in Table 3.
All complexes have radiative lifetimes τo in the hundreds of
nanoseconds range, with 5 having the longest lifetime of 790
ns. This is noteworthy in light of the planned strategy of
using synthetically modified complexes for the preparation
of multicomponent systems for photoinduced charge separa-
tion through functional group coupling and the concern that
substituents used for these couplings may facilitate nonra-
diative decay and yield complexes having shorter-lived
excited states.
Quenching Studies. As with other reported Pt diimine
bis(acetylide) complexes,31 1 undergoes electron transfer
quenching with both donors and acceptors, and bimolecular
quenching of the excited state was studied quantitatively.
Analysis of the results gave linear Stern-Volmer plots of
Iemo/Iem vs [Q] for both reductive and oxidative quenchers
(Iemo and Iem are the emission intensities in the absence and
presence of quencher and [Q] is quencher concentration).
The results are summarized in Table 4. For the electron donor
quenchers phenothiazine, 10-methylphenothiazine, and N,N′-
dimethylaniline and the electron acceptor quenchers 4-ni-
compound
λ abs (nm)
λ em (nm) τo (ns)
Pt(Mephen)(CtCC6H4CHO)2 (2) 390 (22500)
515
548
554
508
546
558
360
340
54
790
124
98
Pt(phen)(CtCC6H4CHO)2 (1)
Pt(Clphen)(CtCC6H4CHO)2 (3)
Pt(dmbpy)(CtCC6H4CHO)2 (5)
Pt(bpy)(CtCC6H4CHO)2 (4)
Pt(dphbpy)(CtCC6H4CHO)2 (6)
392 (8950)
400 (12900)
375 (10500)
383 (13000)
395 (11500)
a Measured in acetonitrile at room temperature.
Table 4. Quenching Constants and Electrochemical Data for Oxidative
Quenchers, Reductive Quenchers, and Nucleosides
quencher
E1/2(red) (V)f
kq (Ms)-1 e
phenothiazinea
10-Me-phenothiazineb
N,N′-dimethylanilineb
nitrobenzeneb
0.83
0.97
1.05
1.3 × 1010
1.2 × 1010
1.2 × 1010
5.2 × 108
9.9 × 109
1.2 × 1010
0
-0.91
-0.62
-0.45
-2.18
-2.35
-2.52
<-2.76
4-nitrobenzaldehydeb
dinitrobenzeneb
thymidinec,d
cytidinec,d
9.5 × 107
6.4 × 108
5.9 × 108
adenosinec,d
guanosinec,d
a Kowert, B. A.; et al. J. Am. Chem. Soc. 1972, 94 (16), 5538-5550.
b Bock, C. R.; et al. J. Am. Chem. Soc. 1979, 101 (17), 4815-4824.
c TBDMS-protected. d Seidel, C. A. M.; et al. J. Phys. Chem. 1996, 100,
5541-5553. e Measured in acetonitrile. f As reduction potentials (V vs
NHE).
trobenzaldehyde and dinitrobenzene, the rate of quenching
was diffusion controlled with kq values of ∼1 × 1010 s-1.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3777

Wadas et al.
For nitrobenzene on the other hand, kq was determined to
be 5.2 × 108 s-1, consistent with the fact that it was the
poorest of the oxidative quenchers examined. However,
quenching by nitrobenzene was still 3 orders of magnitude
The rate of quenching by C is approximately 1 order of
magnitude slower than determined for either G or A, and 2
orders of magnitude slower than found for other oxidative
quenchers like dinitrobenzene (kq ≈ 1010). For T, no
quenching is observed, even at high quencher concentrations.
Given that C and T are the most easily reduced bases,46,77 it
can be concluded that quenching is not occurring through a
mechanism wherein the excited Pt chromophore transfers an
electron to the nucleoside. The results thus suggest that while
the Pt diimine bis(acetylide) complexes appear to be strong
photoreductants, they are not potent enough to effect electron
transfer to DNA upon irradiation. On the other hand, the Pt
diimine bis(acetylide) complexes are observed to be strong
enough photooxidants to transfer a hole to the more easily
oxidized bases for electron transfer through duplex DNA.
2+
faster than with Ru(bpy)3 (2.2 × 105),73 indicative of the
fact that 1 is a more powerful photoreductant. Only complex
1 was examined quantitatively in terms of a quenching study
because of its analytical purity and structural characterization,
but it should be representative of the other complexes
reported here.
The interest in electron transfer through DNA and the fact
that 1 was a strong photoreductant stimulated the notion of
using Pt diimine bis(acetylide) complexes as electron donors
to hybridized DNA. However, in order to assess the
feasibility of such a reaction prior to derivatization of 1 for
aqueous solubility and cationic charge, quenching studies
were conducted using the four different nucleosides adenos-
ine, thymidine, cytidine, and guanosine (A, T, C, G). Because
of the insolubility of the nucleosides, they were first
TBDMS-protected to render them more lipophilic and readily
soluble in acetonitrile for the quenching measurements. The
results are summarized in Table 4. Stern-Volmer behavior
is observed in every case except with T, for which no
quenching was observed. The fastest rates of quenching were
found for the nucleosides G and A, for which the quenching
rate constants were determined to be ca. 108 M-1 s-1. These
values are 2 orders of magnitude smaller than the nearly
diffusion controlled quenching rates reported for organic dyes
with DNA subunits.74 Since G and A are the most easily
oxidized bases in DNA,75 quenching by these bases most
likely occurs through a reductive quenching mechanism
wherein an electron is transferred from the nucleoside to the
excited platinum complex. The fact that the lowest triplet
state of any nucleoside is 3.21 eV76 whereas E00 for 1 is
2.55 eV rules against any energy transfer path as an
alternative means of quenching.
Conclusions
Several new Pt(diimine)(CtCC6H4CHO)2 complexes have
been synthesized with substituted phenanthroline and bipy-
ridine ligands. All of the complexes are brightly emissive in
fluid solution and have long-lived excited states, making
them potentially useful in the construction of multicomponent
systems for photoinduced charge separation. Pt(phen)-
(CtCC6H4CHO)2 was found to obey Stern-Volmer behav-
ior with a series of oxidative and reductive quenchers. In
studies with DNA nucleosides, the A and G nucleosides were
able to quench the luminescence of the Pt chromophore better
than C or T nucleosides. The results indicate that the Pt
diimine bis(acetylide) complexes may be more effective
photooxidants than photoreductants for initiating charge
transfer through the bases of duplex DNA.
Acknowledgment. This research was supported by the
Department of Energy, Division of Basic Chemical Sciences,
for which we are grateful. We also thank Dr. James E.
McGarrah for carrying out lifetime measurements.
Supporting Information Available: Complete crystallographic
information including crystal data and structure refinement param-
eters, atomic positional coordinates, calculated hydrogen atom
coordinates, isotropic and anisotropic displacement parameters, and
bond lengths and angles for 1 as a CIF. This material is available
free of charge via the Internet at http://pubs.acs.org.
(73) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten,
D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101,
4815-4824.
(74) Seidel, C. A. M.; Schulz, A.; Sauer, M. H. M. J. Phys. Chem. 1996,
100, 5541-5553.
(75) Steenken, S.; Jovanovic, S. V. J. Am. Chem. Soc. 1997, 119, 617-
IC0263084
618.
(76) Wood, P. D.; Redmond, R. W. J. Am. Chem. Soc. 1996, 118, 4256-
(77) Steenken, S.; Telo, J. P.; Novais, H. M.; Candeias, L. P. J. Am. Chem.
Soc. 1992, 114, 4701-4709.
4263.
3778 Inorganic Chemistry, Vol. 42, No. 12, 2003