Syntheses and structural characterization of luminescent platinum(II) complexes containing di-tert-butylbipyridine and new 1,1-dithiolate ligands

Article abstract of DOI:10.1021/ic001018d

Three new di-tert-butylbipyridine (dbbpy) complexes of platinum(II) (1-3) containing 1,1-dithiolate ligands have been synthesized and characterized. The 1,1-dithiolates are 2,2-diacetylethylene-1,1-dithiolate (S₂C=C(C(O)Me)₂) (1), 2-cyano-2-p-bromophenylethylene-1,1-dithiolate (S₂C=C(CN(CCO-p-C₆H₄Br)) (2), and p-bromophenyl-2-cyano-3,3-dithiolatoacrylate (S₂C=C(CN)(COO-p-C₆H₄Br)) (3). Complex 1 exhibits a solvatochromic charge-transfer absorption in the 430-488 nm region of the spectrum and a luminescence around 635 nm in ambient temperature CH₂Cl₂ solution. These observations are consistent with what has been seen previously in related Pt diimine 1,1-dithiolate complexes. The nature of the emissive state is assigned as a (mixed metal/dithiolate-to-diimine) charge transfer, while the solvatochromic absorption band corresponds to the singlet transition of similar orbital character. The other complexes also exhibit a low-energy solvatochromic absorption. The crystal structures of two of the complexes have been determined, representing the first time that Pt(diimine)(1,1-dithiolate) complexes have been crystallographically studied. The structures confirm the expected square planar coordination geometry with distortions in bond angles dictated by the constraints of the chelating ligands. The Pt-S and Pt-N bond lengths and S-Pt-S and N-Pt-N bond angles for the two structures are identical within experimental error (2.283(2) and 2.278(2) A; 2.053(6) and 2.050(8) A; 75.01(8)° and 75.40(8)°; 79.2(2)° and 79.0(2)°, respectively). Crystal data for 1: monoclinic, space group P2₁/n (No. 14), with a = 7.20480(10) A, b = 20.53880(10) A, c = 19.1072(2) A, β = 93.83°, V = A³, Z = 4, R1 = 3.34% (I > 2σ(I)), wR2 = 9.88% (I > 2σ(I)) for 3922 unique reflections. Crystal data for 2: monoclinic, space group P2₁/n (No. 14), with a = 15.0940(5) A, b = 9.5182(3) A, c = 20.4772(7) A, β = 111.151(1)°, V = A³, Z = 4, R1 = 4.07% (I > 2σ(I)), wR2 = 8.64% (I > 2σ(I)) for 3859 unique reflections.

Full text of DOI:10.1021/ic001018d

Inorg. Chem. 2001, 40, 1183-1188
1183
Syntheses and Structural Characterization of Luminescent Platinum(II) Complexes
Containing Di-tert-butylbipyridine and New 1,1-Dithiolate Ligands
Sonia Huertas,† Muriel Hissler, James E. McGarrah, Rene J. Lachicotte, and
Richard Eisenberg*
Department of Chemistry, University of Rochester, Rochester, New York, 14627-0216
ReceiVed September 7, 2000
Three new di-tert-butylbipyridine (dbbpy) complexes of platinum(II) (1-3) containing 1,1-dithiolate ligands have
been synthesized and characterized. The 1,1-dithiolates are 2,2-diacetylethylene-1,1-dithiolate (S2CdC(C(O)-
Me)2) (1), 2-cyano-2-p-bromophenylethylene-1,1-dithiolate (S2CdC(CN)(p-C6H4Br)) (2), and p-bromophenyl-
2-cyano-3,3-dithiolatoacrylate (S2CdC(CN)(COO-p-C6H4Br)) (3). Complex 1 exhibits a solvatochromic charge-
transfer absorption in the 430-488 nm region of the spectrum and a luminescence around 635 nm in ambient
temperature CH2Cl2 solution. These observations are consistent with what has been seen previously in related Pt
3
diimine 1,1-dithiolate complexes. The nature of the emissive state is assigned as a (mixed metal/dithiolate-to-
diimine) charge transfer, while the solvatochromic absorption band corresponds to the singlet transition of similar
orbital character. The other complexes also exhibit a low-energy solvatochromic absorption. The crystal structures
of two of the complexes have been determined, representing the first time that Pt(diimine)(1,1-dithiolate) complexes
have been crystallographically studied. The structures confirm the expected square planar coordination geometry
with distortions in bond angles dictated by the constraints of the chelating ligands. The Pt-S and Pt-N bond
lengths and S-Pt-S and N-Pt-N bond angles for the two structures are identical within experimental error
(2.283(2) and 2.278(2) Å; 2.053(6) and 2.050(8) Å; 75.01(8)° and 75.40(8)°; 79.2(2)° and 79.0(2)°, respectively).
Crystal data for 1: monoclinic, space group P21/n (No. 14), with a ) 7.20480(10) Å, b ) 20.53880(10) Å, c )
19.1072(2) Å, â ) 93.83°, V ) Å3, Z ) 4, R1 ) 3.34% (I > 2σ(I)), wR2 ) 9.88% (I > 2σ(I)) for 3922 unique
reflections. Crystal data for 2: monoclinic, space group P21/n (No. 14), with a ) 15.0940(5) Å, b ) 9.5182(3)
Å, c ) 20.4772(7) Å, â ) 111.151(1)°, V ) Å3, Z ) 4, R1 ) 4.07% (I > 2σ(I)), wR2 ) 8.64% (I > 2σ(I)) for
3859 unique reflections.
Introduction
in ambient temperature fluid solutions. The nature of this
luminescence has been the subject of several investigations that
Platinum diimine dithiolate complexes have been found to
have a number of interesting electronic structural properties that
have served to stimulate interest in these systems.1-14 Foremost
among them is the luminescence that these complexes exhibit
have led to its assignment as a charge transfer to diimine
phosphoresence from an orbital of mixed metal/sulfur/dithiolate
character.3,4,7,9 The complexes also show a solvatochromic
absorption that is similar to the emissive state in terms of orbital
character. Variation of substituents on both diimine and dithi-
olate ligands reveals that the excited-state properties of these
complexes, including energy, lifetime, and redox potentials, can
be systematically tuned.9 In general, complexes containing 1,1-
dithiolate ligands have both emission and solvatochromic bands
at higher energies than analogous 1,2-dithiolate systems. The
excited platinum diimine dithiolate complexes have also been
found to undergo self-quenching as solution concentration is
increased,11,15 most probably via Pt‚‚‚Pt interactions.
The directionality of the charge-transfer excited state in the
Pt diimine dithiolate complexes has made these systems of
particular interest in the development of multicomponent
assemblies for photoinduced charge separation and molecular
photochemical devices for light-driven, energy-storing reac-
tions.16,17 One approach to the linking of components in these
systems is through Pd-catalyzed coupling reactions involving
* To whom correspondence should be addressed. E-mail: eisenberg@
chem.rochester.edu. Fax: 716-273-3596.
† Current address: Departamento de Quimica Inorganica, Facultad de
Quimica, Universidad de Murcia, Apdo. 4021 30071-Murcia, Spain.
(1) Zuleta, J. A.; Chesta, C. A.; Eisenberg, R. J. Am. Chem. Soc. 1989,
111, 8916-8917.
(2) Zuleta, J. A.; Burberry, M. S.; Eisenberg, R. Coord. Chem. ReV. 1990,
97, 47-64.
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(10) Cummings, S. D.; Cheng, L.-T.; Eisenberg, R. Chem. Mater. 1997, 9,
440-450.
(11) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 19, 111620-
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(15) Connick, W. B.; Geiger, D. P.; Eisenberg, R. Inorg. Chem. 1999, 38,
3264-3265.
(16) Paw, W.; Cummings, S. D.; Mansour, M. A.; Connick, W. B.; Geiger,
D. K.; Eisenberg, R. Coord. Chem. ReV. 1998, 171, 125-150.
(17) Hissler, M.; McGarrah, J. E.; Connick, W. B.; Geiger, D. K.;
Cummings, S. D.; Risenberg, R. Coord. Chem. ReV., in press.
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(14) Base, K.; Grinstaff, M. W. Inorg. Chem. 1998, 37, 1432-1433.
10.1021/ic001018d CCC: $20.00 © 2001 American Chemical Society
Published on Web 02/08/2001

1184 Inorganic Chemistry, Vol. 40, No. 6, 2001
Huertas et al.
alkynes and aryl bromides,18-23 while a second has utilized
Schiff-base condensation reactions.
In the present paper, we describe three new Pt diimine
dithiolate complexes 1-3 and their structural and spectroscopic
characterization. The sulfur ligands in these complexes are of
The complex Pt(dbbpy)Cl2 was prepared following the method of
Rund by heating a suspension of dbbpy in an aqueous solution of 1
equiv of K2PtCl4 acidified with one drop of HCl until a yellow
precipitate formed (ca. 2 h).29 1H NMR (DMSO-d6): δ 9.34 (2 H, d, J
) 5.6 Hz), 8.58 (2H, s), 7.83 (2H, d, J ) 5.2 Hz), 1.41 (18 H, s).
Physical Measurements. 1H NMR spectra were recorded on a
Bruker AMX-400 spectrometer operating at 400 MHz. Chemical shifts
are referenced relative to TMS and were determined on the basis of
residual proton solvent resonances. Infrared spectra were recorded on
Mattson Galaxy 6020 FTIR and Nicolet Impact 400 spectrophotometers
as KBr pellets. Absorption spectra were recorded on a Hitachi U2000
UV-visible spectrophotometer. Steady-state emission measurements
were performed on a Spex Fluorolog-2 fluorescence spectrophotometer
equipped with a 450 W xenon lamp and Hamamatsu R929 photomul-
tiplier tube detector. Room-temperature measurements were made using
1 cm × 1 cm quartz fluorescence cells; solutions were freeze-pump-
thaw degassed four times and then placed under high-purity nitrogen.
Low-temperature emission spectra were recorded in 3:1 EtOH/MeOH
solvent glass formed in 4 mm diameter quartz EPR tubes placed in a
liquid nitrogen Dewar equipped with quartz windows. Emission was
collected at 90° from the excitation for both experiments. Cyclic
voltammetry experiments were carried out using an EG&G PAR 263A
potentiostat/galvanostat employing a three-electrode single-cell com-
partment. All samples were degassed with nitrogen or argon. A Pt
working electrode, a Pt auxiliary electrode, and a Ag/AgNO3/acetonitrile
reference electrode were used. For all of the measurements, the
ferrocene/ferrocenium couple at 0.40 V vs NHE was used to calibrate
the cell potential. Elemental analysis of complexes 1-3 was performed
by the Analytical Technology Division, Kodak Research Laboratories.
Syntheses of Pt(dbbpy)(dithiolate) Complexes. The Pt(dbbpy)-
(dithiolate) complexes were prepared by displacement of the two
chlorides in the Pt(dbbpy)Cl2 precursor with the appropriate 1,1-
dithiolate chelating ligand. The precursor to the dithiolate ligand used
to synthesize 3, i.e., 4-bromophenylcyanoacetate, was prepared by the
reaction of cyanoacetyl chloride and 4-bromophenol in dichloromethane
for 9 days. The reaction solution was concentrated using a rotary
evaporator, diethyl ether (60 mL) was added, and a cream-colored
precipitate formed. The precipitate was separated by filtration under a
nitrogen atmosphere and vacuum-dried. Yield: 13.10 g, (99%). p-Br-
the 1,1-dithiolate type and are formed by the reaction of active
methylene compounds with CS2 in the presence of base. Two
of the complexes have been characterized crystallographically,
making them the first Pt diimine 1,1-dithiolate systems to be
so analyzed. The dithiolate ligand in 1 was first reported in 1997,
and its coordination to Au and Tl(I) has been examined.24,25a
Mono-, di-, and tetranuclear Pt(II), Pd(II), and Ag(I) complexes
with the same ligand, including the Pt(II) and Pd(II) complexes
homologous to 1 with unsubstituted bpy, were previously
synthesized by one of us (S.H.) using the same method here
reported.25b The other two dithiolates have not been reported
previously, although the one in 3 represents simply a different
ester of the well-studied Rcda (alkyl 2-cyano-3,3-dithiolatoacry-
late) system.1-3,26,27 The dithiolate substituents in 1-3 provide
interesting opportunities for connection to donor components
in the construction of photoinduced charge separation devices.
1
C6H4OOCCH2CN. H NMR (MeOH-d4): δ 7.56 (d, 2 H, JHH ) 7.7
Hz), 7.12 (d, 2 H, JHH ) 7.7 Hz), 4.87 (s, 2 H, CH2).
Experimental Section
[Pt(dbbpy){S2CdC(C(O)Me)2}] (1). Pt(dbbpy)Cl2 (710.7 mg, 1.33
mmol) was added to a suspension of [Tl2{η2-(S, S′)-S2CdC(C(O)-
Me)2}] (775.4 mg, 1.33 mmol) in chloroform (70 mL). The mixture
was stirred at reflux temperature for 5 h and filtered, and the residue
was extracted with chloroform (3 × 30 mL). The combined filtrates
were concentrated using a rotary evaporator, diethyl ether (70 mL) was
added, and an orange precipitate formed. The precipitate was separated
by filtration and purified by recrystallization from dichloromethane/
Chemicals. Cyanoacetic acid, oxalyl chloride, 4-bromophenol,
4-bromophenylacetonitrile, and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
were used as received from Aldrich, and K2PtCl4 was used as received
from Alfa/AESAR Johnson-Matthey. Reagent grade CS2, acetone,
acetonitrile, diethyl ether (Et2O), and 2-propanol (ipa) were deoxygen-
ated with an N2 purge but otherwise used as received (Aldrich or Fisher
Chemical). Spectroscopic grade N,N-dimethylformamide (DMF), metha-
nol (MeOH), chloroform (CHCl3), and dichloromethane (CH2Cl2) were
used as received from Burdick and Jackson. The compounds 4,4′-di-
tert-butyl-2,2′-bipyridine (dbbpy),28 [Tl2{η2-(S,S′)-S2CdC{C(O)Me}2}],25
and cyanoacetyl chloride were prepared as described in the literature.
1
diethyl ether. Yield: 653 mg, (76.7%). H NMR (CDCl3): δ 8.31 (d,
2 H, JHH ) 7 Hz Hdbbpy), 7.99 (s, 2 H, Hdbbpy), 7.43 (d, 2 H, JHH
)
t
7 Hz, Hdbbpy), 2.36 (s, 6 H, CH3), 1.39 (s, 18 H, Bu). FT-IR (KBr)
ν (cm-1): 1701 (s, νCO), 1619 (s, νCdN), 1503 (s, νCdC). MS m/z 638
[M + H]+. Anal. Calcd for C24H30N2O2PtS2 (Mr ) 637.73 g mol-1):
C, 45.20; H, 4.74; N, 4.39. Found: C, 44.95; H, 4.61; N, 4.28.
[Pt(dbbpy){S2CdC(CN)(4-C6H4Br)}] (2). To a solution of 4-bro-
mophenylacetonitrile (103.9 mg, 0.53 mmol) in THF (15 mL), DBU
(0.16 mL, 1.06 mmol) was added under N2. The solution was stirred
at room temperature for 5 min, during which time its color changed
from colorless to pale-yellow. Carbon disulfide (0.032 mL, 0.53 mmol)
was then added, and the solution turned bright-yellow. The solution
was stirred for an additional 2 h and was then transferred dropwise via
cannula to a solution of Pt(dbbpy)Cl2 (282.8 mg, 0.53 mmol) in
dichloromethane (60 mL). The resulting solution turned red. The
solution was stirred at room temperature for 1 day, after which the
solvent was removed using a rotary evaporator. The crude precipitate
was chromatographed on silica gel using dichloromethane/methanol
(18) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627-630.
(19) Sonogashira, K.; Fujikura, Y.; Yatake, T.; Toyoshima, N.; Takahashi,
S.; Hagihara, N. J. Organomet. Chem. 1978, 145, 101-108.
(20) Connors, J. P. J.; Tzalis, D.; Dunnick, A. L.; Tor, Y. Inorg. Chem.
1998, 37, 1121-1123.
(21) Grosshenny, V.; Romero, F. M.; Ziessel, R. J. Org. Chem. 1997, 62,
1491-1500.
(22) Ziessel, R.; Suffert, J.; Youinou, M. T. J. Org. Chem. 1996, 61, 6535-
6546.
(23) Ziessel, R.; Suffert, J. Tetrahedron Lett. 1996, 37, 2011-2014.
(24) Vicente, J.; Chicote, M. T.; Gonzalez-Herrero, P.; Jones, P. G. Chem.
Commun. 1997, 2047-2048.
(25) Vicente, J.; Chicote, M. T.; Gonzalez-Herrero, P.; Jones, P. G.;
Humphrey, M. G.; Cifuentes, M. P.; Samoc, M.; Luther-Davies, B.
Inorg. Chem. 1999, 38, 5018-5026.
(26) Bevilacqua, J. M.; Zuleta, J. A.; Eisenberg, R. Inorg. Chem. 1994,
33, 258-266.
(28) Belser, P.; Von Zelewsky, A. HelV. Chim. Acta 1980, 63, 1675-
1703.
(29) Hodges, K. D.; Rund, J. V. Inorg. Chem. 1975, 14, 525-528.
(27) Bevilacqua, J. M.; Zuleta, J. A.; Eisenberg, R. Inorg. Chem. 1993,
32, 3689-3693.

Platinum(II) Complexes
Inorganic Chemistry, Vol. 40, No. 6, 2001 1185
Table 1. Crystallographic Data for 1 and 2
as the eluent. The red compound was further purified by recrystallization
from chloroform. Yield: 233.3 mg (60%). H NMR (CDCl3): δ 8.47
1
1
2
(pseudotriplet, 2 H, Hdbbpy), 7.97 (s, 2 H, Hdbbpy), 7.73 (d, 2 H, JHH
) 8.4 Hz, Ph), 7.51 (dd, 1 H, dbbpy, JHH ) 1.6 Hz, JHH ) 5.6 Hz),
7.47 (d, 2 H, JHH ) 1.6 Hz, J ) 6 Hz), 7.44 (dd, 1 H, dbbpy, J ) 8.4
chemical formula
fw
C24H34N2O4PtS2
673.74
C27H28BrN3PtS2
733.64
cryst syst
space group (No.)
Z
monoclinic
P21/n (No. 14)
4
monoclinic
P21/n (No. 14)
4
t
t
Hz, Ph), 1.55 (s, 9 H, Bu), 1.42 (s, 9 H, Bu). FT-IR (KBr) ν (cm-1):
2195 (s, νCtN), 1630, 1508 (s, νCdN, νCdC). MS m/z 734 [M + H]+.
Anal. Calcd for C27H28N3BrPtS2 (Mr ) 733.65 g mol-1): C, 44.26; H,
3.85; N, 5.74. Found: C, 44.41; H, 3.48; N, 5.84.
a,a Å
7.2048(1)a
20.5388(1)a
19.1072(2)a
93.830(1)a
2821.12(5)
1.586
15.0940(5)a
9.5182(3)a
20.4772(7)a
111.151(1)a
2743.7(2)
1.776
b,a Å
c,a Å
[Pt(dbbpy){S2CdC(CN)(C(O)-4-OC6H4Br)}] (3). To a solution of
4-bromophenylcyanoacetate (199.2 mg, 0.83 mmol) in THF (15 mL),
DBU (0.25 mL, 1.66 mmol) was added under N2. The solution was
stirred at room temperature for 5 min followed by the addition of CS2
(0.05 mL, 0.83 mmol). The solution, which turned yellow, was stirred
for an additional 2 h, after which it was transferred dropwise via cannula
to a solution of [PtCl2(dbbpy)] (443.5 mg, 0.83 mmol) in dichlo-
romethane (60 mL). The resulting solution turned red. The solution
was stirred at room temperature for 1 day, after which solvent was
removed using a rotary evaporator. The crude precipitate was chro-
matographed on silica gel using dichloromethane/methanol as the eluent.
The red compound was further purified by recrystallization from
chloroform. Yield: 374 mg (58%). 1H NMR (CDCl3): δ 8.29 (d, 1 H,
Hdbbpy), 8.24 (d, 1 H, Hdbbpy), 7.99 (s, 2 H, Hdbbpy), 7.48 (dd, 1 H,
â,a deg
vol, Å3
F
calcd, g cm-3
cryst dimens, mm3
temp, K
0.16 × 0.16 × 0.24 0.04 × 0.18 × 0.38
193(2)
0.710 73
3922
193(2)
0.710 73
2956
313
6.743
0.0407, 0.0864
0.0668, 0.0939
Mo radiation, Å
no. obsd data (I > 2σ(I))
no. params varied
288
5.15
µ, cm-1
2
R1(Fo), wR2(Fo ) (I > 2σ)b 0.0334, 0.0988
R1(Fo), wR2(Fo ) (all data)b 0.0428, 0.1030
2
a It has been noted that the integration program SAINT produces
cell constant errors that are unreasonably small because systematic error
is not included. More reasonable errors might be estimated at 10 times
the listed value. Structures were refined using SHELXTL: Structure
Analysis Program, version 5.04; Siemens Industrial Automation Inc.:
JHH ) 2 Hz, JHH ) 6.8 Hz, dbbpy), 7.44 (dd, 1 H, JHH ) 2 Hz, JHH
)
6 Hz, dbbpy), 7.40 (d, 2 H, JHH ) 8.8 Hz, Ph), 7.04 (d, 2 H, JHH ) 8.8
t
t
Hz, Ph), 1.50 (s, 9 H, Bu), 1.41 (s, 9 H, Bu). FT-IR (KBr) ν (cm-1):
2200 (s, νCtN), 1699, 1625, 1562, 1537 (s, νCdN, νCdC). MS m/z 779
[M + H]+. Anal. Calcd for C28H28N3O2PtBrS2 (Mr ) 777.66 g mol-1):
C, 43.25; H, 3.63; N, 5.40. Found: C, 43.37; H, 4.10; N, 5.14.
X-ray Structural Determination of 1. Crystals were grown by slow
evaporation of a CH2Cl2/ether solution of the complex in air. A yellow
needle of approximate dimensions 0.16 × 0.16 × 0.24 mm3 was
mounted under Paratone-8277 on a glass fiber and 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 molybdenum-
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
56.6°). The total data collection time was approximately 12 h. Frames
were integrated to a maximum 2θ angle of 46.3° with the Siemens
SAINT program to yield a total of 11 530 reflections, of which 3922
were independent and 3364 were above 2σ(I). Laue symmetry revealed
a monoclinic crystal system, and the final unit cell parameters (at -80
°C) were determined from the least-squares refinement of three-
dimensional centroids of 7702 reflections.
Madison, WI, 1995. b R1 ) (∑||Fo| - |Fc||)/∑|Fo|; wR2 ) [∑[w(Fo
2
- Fc2)2]/∑[w(Fo )2]1/2, where w ) 1/[σ2(Fo ) + (aP)2 + bP] and P )
2
2
2
[max(0,Fo ) + 2Fc2]/3.
Scheme 1
using direct methods and refined employing full-matrix least-squares
on F2 (Siemens, SHELXTL, version 5.04). For a Z value of 4, there is
one molecule in the asymmetric unit. All of the non-H atoms were
refined anisotropically, and hydrogen atoms were included in idealized
positions. The structure refined to a goodness of fit (GOF) of 1.043
and final residuals of R1 ) 4.07% (I > 2σ(I)), wR2 ) 8.64% (I >
2σ(I)). Crystallographic and selected data collection parameters for 2
are given in Table 1; a complete tabulation is provided in the Supporting
Information.
The space group was assigned as P21/n, and the structure was solved
using direct methods and refined employing full-matrix least-squares
on F2 (Siemens, SHELXTL, version 5.04). For a Z value of 4, there is
one molecule and two waters in the asymmetric unit. All of the non-H
atoms were refined anisotropically except the oxygen atoms of the water
molecules, and hydrogen atoms were included in idealized positions.
H atoms were neither found nor included for the water molecules. The
structure was refined to a goodness of fit (GOF) of 1.112 and final
residuals of R1 ) 3.34% (I > 2σ(I)), wR2 ) 9.88% (I > 2σ(I)).
Crystallographic and selected data collection parameters for 1 are given
in Table 1; a complete tabulation is provided in the Supporting
Information.
X-ray Structural Determination of 2. A red plate of approximate
dimensions 0.04 × 0.18 × 0.38 mm3 was mounted under Paratone-
8277 on a glass fiber and immediately placed in a cold nitrogen stream
at -80 °C on the X-ray diffractometer. The X-ray intensity data were
collected as for 1 with exposure times of 5 s/frame and total data
collection time of 12 h. Frames were integrated to a maximum 2θ angle
of 46.6° to yield a total of 11 994 reflections, of which 3859 were
independent and 2956 were above 2σ(I). Laue symmetry revealed a
monoclinic crystal system, and the final unit cell parameters (at -80
°C) were determined from the least-squares refinement of three-
dimensional centroids of 4242 reflections.
Results and Discussion
Syntheses and Characterization of Pt Di-tert-butylbi-
pyridine Dithiolates 1-3. As shown in Scheme 1, the three Pt
complexes described here are conveniently prepared by reaction
of Pt(dbbpy)Cl2 with the alkali metal salt of the respective 1,1-
ethylenedithiolate ligand. All complexes are isolated in moderate
yields (58-77%) as orange or bright-red powders. The com-
plexes are soluble in most organic solvents but insoluble in ether,
alcohol, pentane, and hexane. The complexes were characterized
by electronic, infrared, and 1H NMR spectroscopies and, in two
cases, by X-ray crystallography. All three complexes are air-
stable in the solid state and in degassed solutions at room
temperature.
The 1H NMR spectrum for complex 1 shows three resonances
at δ 8.31, 7.99, and 7.43 ppm, corresponding to the aromatic
protons of the dbbpy ligand, a resonance at δ 1.39 ppm
The space group was assigned as P21/n, and the structure was solved

1186 Inorganic Chemistry, Vol. 40, No. 6, 2001
Huertas et al.
Table 2. Selected Bond Lengths [Å] and Angles [deg] for 1
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-S(2)
Pt(1)-S(1)
S(1)-C(19)
S(2)-C(19)
O(1)-C(21)
2.050(6)
2.055(6)
2.281(2)
2.285(2)
1.757(9)
1.753(9)
1.239(14)
O(2)-C(23)
N(1)-C(1)
N(1)-C(5)
N(2)-C(10)
N(2)-C(6)
C(5)-C(6)
C(19)-C(20)
1.272(11)
1.347(9)
1.359(9)
1.341(9)
1.374(9)
1.484(10)
1.361(11)
N(1)-Pt(1)-N(2)
N(1)-Pt(1)-S(2)
N(2)-Pt(1)-S(2)
N(1)-Pt(1)-S(1)
N(2)-Pt(1)-S(1)
S(2)-Pt(1)-S(1)
C(19)-S(1)-Pt(1)
C(19)-S(2)-Pt(1)
C(1)-N(1)-C(5)
C(1)-N(1)-Pt(1)
C(5)-N(1)-Pt(1)
79.2(2)
175.7(2)
103.7(2)
102.3(2)
175.9(2)
75.01(8) N(2)-C(6)-C(5)
89.8(3)
90.1(3)
N(1)-C(1)-C(2)
N(1)-C(5)-C(4)
N(1)-C(5)-C(6)
C(4)-C(5)-C(6)
N(2)-C(6)-C(7)
122.6(7)
121.6(7)
114.4(6)
123.9(6)
121.2(7)
114.3(6)
122.7(7)
130.8(8)
124.3(7)
104.8(4)
127.3(9)
117.5(8)
115.2(8)
N(2)-C(10)-C(9)
C(20)-C(19)-S(2)
C(20)-C(19)-S(1)
S(2)-C(19)-S(1)
C(19)-C(20)-C(23)
C(19)-C(20)-C(21)
C(23)-C(20)-C(21)
118.2(6)
125.3(5)
116.3(5)
Figure 1. Perspective view of the molecular structure of Pt(dbbpy)-
{S2CdC(C(O)Me)2}, 1.
C(10)-N(2)-C(6) 117.7(6)
C(10)-N(2)-Pt(1) 126.6(5)
assignable to the dbbpy methyl protons, and a resonance at δ
2.36 ppm corresponding to the methyl protons of the dithiolate
ligand. Complexes 2 and 3 have 1H NMR spectra that are more
complicated because of the asymmetry of the dithiolate ligand,
leading to inequivalence of dbbpy protons that were equivalent
in 1. The dbbpy methyl resonances for 2 are observed at δ 1.42
(9 H) and 1.55 (9 H) ppm, while the corresponding resonances
for 3 are seen at δ 1.50 (9 H) and 1.41 (9 H) ppm. The aromatic
resonances for these complexes are listed in the Experimental
Section.
C(6)-N(2)-Pt(1)
115.5(5)
reported for other structurally characterized Pt(II) 1,1-dithiolate
complexes.5,27,30 The average Pt-N bond distances and N-Pt-N
bond angles in the two structures are statistically equivalent
(2.052(6) and 2.050(6) Å, and 79.2(2) and 79.0(3)°, respectively)
and agree closely with Pt-N distances and N-Pt-N bond
angles reported for other square planar Pt diimine complexes.
All other metrical parameters are similar to corresponding values
reported previously.5,11
Crystallographic Studies of 1 and 2. The square planar
geometry about platinum was confirmed by single-crystal X-ray
diffraction studies for complexes 1 and 2. Despite the increasing
interest in Pt diimine dithiolate complexes, these studies
represent the first ones done on 1,1-dithiolate systems. The
crystallographic and data collection parameters for both structure
determinations are shown in Table 1. ORTEP views of the
complexes are presented in Figures 1 and 2, while abbreviated
tables of bond distances and angles are given in Tables 2 and
3, respectively. Complete tabulations of refined positional and
thermal parameters and all bond distances and angles for both
structures are given in the Supporting Information.
Both structures exhibit the expected coordination geometry.
The constraints of the four-membered chelate rings lead to
S-Pt-S bond angles of 75.01(8)° and 75.40(8)° for 1 and 2,
respectively, with average Pt-S bond lengths of 2.283(3) Å
and 2.278(3) Å. The Pt-S distances are in the range of values
Visible-UV Spectroscopy of Complexes 1-3. Complex 1
exhibits four ultraviolet-visible absorption bands in fluid
solution at room temperature. In degassed acetonitrile, the two
highest energy bands at 233 and 290 nm (in acetonitrile) have
molar extinction coefficients ꢀ of 17 000 and 13 000 M-1 cm-1
and are assignable to ligand-based transitions. The third band
is seen at 347 nm (12 700 M-1 cm-1), while the fourth occurs
in the visible region of the spectrum at 430 nm. The former is
relatively insensitive to solvent, shifting from 347 nm in
acetonitrile to 357 nm (12 340 cm-1 M-1) in toluene, and is
assigned to a ligand pπ-pπ* transition. In contrast, the latter
absorption band at 430 nm (11 300 cm-1 M-1) is strongly
solvatochromic, similar to what has been reported for closely
related Pt diimine dithiolate complexes.1,3,7,9 The great solubility
of 1 in a variety of organic solvents allowed for a detailed
investigation of the solvatochromic shift of the 430 nm
transition, and the results are presented in Table 4 and Figure
Figure 2. Perspective view of the molecular structure of Pt(dbbpy){S2CdC(CN)(p-C6H4Br)}, 2.

Platinum(II) Complexes
Inorganic Chemistry, Vol. 40, No. 6, 2001 1187
Table 3. Selected Bond Lengths [Å] and Angles [deg] for 2
for a related Pt diimine 1,1-dithiolate complex support this
view.31 The degree of solvatochromism for 1 compares well
with that reported previously for Pt diimine toluenedithiolate
(tdt) complexes using an empirical scale devised in our
laboratory;9 a linear correlation exists between the lowest
absorption energies of Table 4 and the Cummings solvent
parameter with a linear regression agreement R2 of 0.957. The
direction of the observed shift in absorption energy with solvent
polarity is termed “negative” solvatochromism, in accord with
the assignment of this transition as a charge transfer from an
orbital of mixed metal/dithiolate character to a diimine π*
orbital.7,9 This assignment differs in degree from the LLCT
assignment proposed by Dance and Vogler for related complexes
in that the HOMO is thought to possess significant metal as
well as dithiolate character.32-34 Finally, it is noted that in
toluene, complex 1 does not rigorously follow Beer’s law at
concentrations greater than 0.10 mM; specifically, the 488 nm
band ceases to increase linearly with concentration, suggestive
of possible aggregation at higher concentrations. Such aggrega-
tion has been proposed for Pt diimine dithiolate complexes in
nonpolar solvents.31,35
Complexes 2 and 3 show absorption bands similar to 1 above
300 nm. In each case, the lower energy band exhibits substantial
solvatochromism whereas the higher energy band does not. For
complex 2, the two bands in chloroform are 492 and 357 nm,
while in acetonitrile, they are at 448 and 352 nm. For 3, the
bands appear at 428 and 349 nm in chloroform and at 404 and
342 nm in acetonitrile. The lower energy band for each complex
is given the same assignment as that for complex 1, namely, a
charge transfer from an orbital of mixed metal/dithiolate
character to a π*(diimine) acceptor function.
Electrochemical Results. The electrochemical behavior of
1-3 was determined by cyclic voltammetry in dichloromethane
with NBu4PF6 (0.20 M) as the electrolyte. Ferrocene (0.400 V
vs NHE) was used to calibrate the observed redox processes,
which are given vs NHE. For 1, a reversible reduction is seen
at -1.36 V (NHE) and a quasi-reversible oxidation is noted at
+1.05 V corresponding to the peak anodic current. A reversible
reduction is seen at -1.27 V for 2 and at -1.28 V for 3. As
seen for 1, the oxidations of 2 and 3 are quasi-reversible but 2
and 3 are more difficult to oxidize than 1 with peak anodic
currents observed at +1.43 and +1.47 V (NHE), respectively.
Both processes are typical for Pt(diimine)(dithiolate) complexes
in which reduction leads to placement of an electron in the π*
orbital of the diimine ligand and oxidation involves removal of
an electron from an orbital of mixed metal/sulfur/dithiolate
character.9 Specifically, the reduction potentials for 1-3 are in
the range of -1.27 to -1.40 V (NHE) observed for the dbbpy
1,1- and 1,2-dithiolate complexes reported earlier.9 While the
peak anodic potentials for 2 and 3 are more positive than for 1
and the previously reported analogues, these values should be
viewed with greater caution because of the irreversibility of the
oxidation in these systems.
Pt(1)-N(2)
Pt(1)-N(1)
Pt(1)-S(2)
Pt(1)-S(1)
Br(1)-C(24)
S(1)-C(19)
S(2)-C(19)
N(1)-C(5)
N(1)-C(1)
N(2)-C(10)
N(2)-C(6)
N(3)-C(27)
2.040(7)
2.060(6)
2.278(2)
2.279(2)
1.895(9)
1.724(9)
1.747(9)
1.330(10)
1.336(11)
1.342(10)
1.347(10)
1.157(12)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(9)-C(10)
C(19)-C(20)
C(20)-C(27)
C(20)-C(21)
1.377(12)
1.394(12)
1.397(11)
1.362(11)
1.471(11)
1.394(11)
1.371(11)
1.397(11)
1.361(11)
1.376(11)
1.408(13)
1.478(12)
N(2)-Pt(1)-N(1)
N(2)-Pt(1)-S(2)
N(1)-Pt(1)-S(2)
N(2)-Pt(1)-S(1)
N(1)-Pt(1)-S(1)
S(2)-Pt(1)-S(1)
C(19)-S(1)-Pt(1)
C(19)-S(2)-Pt(1)
C(5)-N(1)-C(1)
C(5)-N(1)-Pt(1)
C(1)-N(1)-Pt(1)
79.0(3)
101.8(2)
179.2(2)
176.6(2)
103.9(2)
75.40(8) N(2)-C(6)-C(5)
89.1(3)
88.6(3)
N(1)-C(1)-C(2)
N(1)-C(5)-C(4)
N(1)-C(5)-C(6)
C(4)-C(5)-C(6)
N(2)-C(6)-C(7)
122.4(8)
121.0(8)
114.8(7)
124.1(8)
120.8(7)
115.4(7)
122.3(8)
129.6(7)
123.5(7)
106.8(5)
N(2)-C(10)-C(9)
C(20)-C(19)-S(1)
C(20)-C(19)-S(2)
S(1)-C(19)-S(2)
119.1(7)
115.5(5)
125.4(6)
C(19)-C(20)-C(27) 115.8(8)
C(19)-C(20)-C(21) 127.8(8)
C(27)-C(20)-C(21) 116.4(8)
C(10)-N(2)-C(6) 118.6(7)
C(10)-N(2)-Pt(1) 126.2(6)
C(6)-N(2)-Pt(1)
115.2(5)
N(3)-C(27)-C(20)
179.3(13)
Table 4. Absorption Bands for Complex 1 as a Function of
Solventa
solvent
UV-vis λmax, nm
ꢀ, M-1 cm-1
toluene
488
357
306 (sh)
460
350
452
353
444
347
430
350
430
347
433
352
6 860
12 340
chloroform
10 580
14 560
10 460
13 900
8 120
11 000
16 620
19 200
11 260
12 680
17 680
18 580
dichloromethane
acetone
N,N-dimethylformamide
acetonitrile
dimethyl sulfoxide
a 5 × 10-5 M.
Emission Spectroscopic Results. Complex 1 is emissive in
fluid solution at ambient temperature. Irradiation into the
solvatochromic charge-transfer band gives rise to a broad
Figure 3. Lowest energy absorption bands for Pt(dbbpy){S2Cd
C(C(O)Me)2}, 1, as a function of solvent.
(30) Coucouvanis, D. Prog. Inorg. Chem. 1970, 11, 233.
(31) Vanhelmont, F. W. M.; Johnson, R. C.; Hupp, J. T. Inorg. Chem.
2000, 39, 1814-1816.
3. As solvent polarity decreases, the strongly allowed band at
430 nm in MeCN shifts to lower energy, consistent with the
notion of a highly polar ground state and a transition in which
the polarity of the complex is greatly reduced.10 Recent results
by Hupp and co-workers using transient dc photoconductivity
(32) Miller, T. R.; Dance, G. J. Am. Chem. Soc. 1973, 95, 6970.
(33) Benedix, R.; Hennig, H.; Kunkely, H.; Vogler, A. Chem. Phys. Lett.
1990, 175, 483-487.
(34) Vogler, A.; Kunkely, H. Comments Inorg. Chem. 1990, 9, 201-220.
(35) Crosby, G. A.; Kendrick, K. R. Coord. Chem. ReV. 1998, 171, 407-
417.

1188 Inorganic Chemistry, Vol. 40, No. 6, 2001
Huertas et al.
a broad band centered around 580 nm. This is similar to that
seen for Pt(dbbpy)(S2CdC(C(O)Me)2), 1. In addition, there is
a second weak emission feature at 490 nm, the basis of which
is uncertain at the present time. At 77 K in 3:1 EtOH/MeOH
glass, the emission is substantially greater in intensity with
evidence of a 1320 cm-1 vibronic progression and a significant
blue shift for the highest energy transition to ca. 495 nm. Similar
vibronic progressions have been observed for other Pt diimine
1,1-dithiolate and Pt diimine bis(acetylide) complexes.9,38 No
studies of the emission properties of 3 were conducted, but it
should be mentioned that very closely related analogues,
differing only in the nature of the ester substituent of the
dithiolate ligand, have been well studied in this regard.1-3,9
Figure 4. Emission spectra of 1 (2 × 10-5 M) at 77 K in 3:1 ethanol/
methanol with excitation at 355 nm (dashed) and at 430 nm (solid)
and at room temperature in acetonitrile (dot-dashed line), dichlo-
romethane (dotted line), and toluene (solid) with excitation in the lowest
energy absorption band.
Conclusions
The three complexes in the present study are members of
the class of Pt diimine dithiolates that have been extensively
studied over the past decade. They exhibit electronic properties
similar to what has been described previously in terms of a
solvatochromic charge-transfer absorption in the visible region
of the spectrum and a fluid solution emission (for 1 and 2) from
an excited state of similar orbital character. The excited-state
lifetime of 1 is consistent with the emissive state having spin-
forbidden character corresponding to 3(Pt(d)/S(p)-π*diimine). The
Pt diimine 1,1-dithiolates described here, as well as previously,
have higher energy CT absorptions and emissions than their
1,2-dithiolate analogues. The structures of two of the complexes
have been determined crystallographically, confirming the
anticipated square planar coordination distorted only by the
constraints of the chelate rings. The functionality of the different
dithiolates used for complexes 1-3 should make possible their
facile connection to potential donors and reductive quenchers
in the construction of dyads and multicomponent systems for
photoinduced charge transfer using well-established coupling
reactions.
emission that shows less sensitivity to solvent polarity in terms
of relative energy than the corresponding absorption. In CH2-
Cl2 the emission maximum occurs at 635 nm as seen in Figure
4. Relative quantum yields of emission for 1 were determined
using Ru(bpy)3(PF6)2 in MeCN (Φ ) 0.062) as the standard36
and were found to be 1.3 × 10-4 in MeCN and 0.011 in toluene.
The excitation spectrum measured with λem of 600 nm shows
bands that match closely with the absorption bands at 451 and
354 nm. Lifetime measurements reveal a significant dependence
of excited-state lifetime with solvent polarity. In MeCN and
CH2Cl2, excited-state lifetimes were measured to be 3.4 ( 0.1
ns and 11.0 ( 0.1 ns, respectively, by time-correlated single-
photon counting (TCSPC), while in toluene the lifetime was
determined to be 250 ( 30 ns using a previously described
nanosecond excimer system.37 This variation in excited-state
lifetime with solvent polarity has been seen for other Pt-
(diimine)(dithiolate) complexes and may relate to the coordinat-
ing ability of the solvent as well.7
Acknowledgment. We thank the Department of Energy,
Division of Chemical Sciences, for support of this research. S.H.
gratefully acknowledges a grant from the Spanish Ministerio
de Educacio´n y Cultura. We also thank Profs. M. T. Chicote
and J. Vicente for private communication of the synthesis of
2,2-diacetyl-1,1-dithiolene Pt(II), Pd(II), and Ag(I) complexes
and for a sample of the thalium salt used in the synthesis of 1.
The 77 K emission spectrum of 1 in 3:1 EtOH/MeOH glass
reveals a shift to higher energy for the emission band relative
to that seen in fluid solution at ambient temperature. The
rigidochromic shift leads to an observed emission maximum at
77 K of 535 nm. The profile of this emission is slightly broader
when generated with excitation at 355 nm vs that seen with
430 nm excitation.
Supporting Information Available: X-ray crystallographic files,
in CIF format, for complexes 1 and 2. This material is available free
of charge via the Internet at http://pubs.acs.org.
Emission studies of 2 were more limited in scope. Complex
2 is weakly emissive in acetonitrile at ambient temperature with
IC001018D
(36) Kober, E. M.; Marshall, J. L.; Dressick, W. J.; Sullivan, B. P.; Caspar,
J. V.; Meyer, T. J. Inorg. Chem. 1985, 24, 2755-2763.
(37) Chen, L.; Farahat, M. S.; Gaillard, E. R.; Farid, S.; Whitten, D. G. J.
Photochem. Photobiol. A 1996, 95, 21-25.
(38) Hissler, M.; Connick, W. B.; Geiger, D. K.; McGarrah, J. E.; Lipa,
D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem. 2000, 39, 447-457.