Probing d8-d8 interactions in luminescent mono- and binuclear cyclometalated platinum(II) complexes of 6-phenyl-2,2′-bipyridines

Article abstract of DOI:10.1021/ic990238s

A series of luminescent mono- and binuclear cyclometalated platinum(II) complexes, namely [Pt(L^1-6)Cl] (1a-6a; HL^1-6 = 4-(aryl)-6-phenyl-2,2′-bipyridine; aryl = H (1), phenyl (2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5),

Full text of DOI:10.1021/ic990238s

4046
Inorg. Chem. 1999, 38, 4046-4055
Probing d8-d8 Interactions in Luminescent Mono- and Binuclear Cyclometalated
Platinum(II) Complexes of 6-Phenyl-2,2′-bipyridines
Siu-Wai Lai,† Michael Chi-Wang Chan,† Tsz-Chun Cheung,† Shie-Ming Peng,‡ and
Chi-Ming Che*,†
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, and
Department of Chemistry, National Taiwan University, Taipei, Taiwan
ReceiVed February 26, 1999
A series of luminescent mono- and binuclear cyclometalated platinum(II) complexes, namely [Pt(L1-6)Cl] (1a-
6a; HL1-6 ) 4-(aryl)-6-phenyl-2,2′-bipyridine; aryl ) H (1), phenyl (2), 4-chlorophenyl (3), 4-tolyl (4),
4-methoxyphenyl (5), 3,4,5-trimethoxyphenyl (6)), [Pt(L1)E]+ (E ) py (7), PPh3 (8)), [Pt2(L1-6)2(µ-dppm)]2+
(1b-6b, dppm ) bis(diphenylphosphino)methane), [Pt2(L1)2(µ-pz)]+ (9, Hpz ) pyrazole), and [Pt2(L1)2(µ-dppCn)]2+
(dppCn ) bis(diphenylphosphino)propane (10, n ) 3) and -pentane (11, n ) 5)), were synthesized in order to
examine fluid- and solid-state oligomeric d8-d8 and ligand-ligand interactions. The molecular structures of
4b(ClO4)2 and 9(PF6) reveal intramolecular Pt-Pt distances of 3.245(1) and 3.612(2) Å, respectively. While
minimal metal-metal communication is expected for 9, weak π-π interactions are possible. All complexes
described in this work are emissive in fluid solution at room temperature. Negligible changes in emission energy
are detected by incorporating different aryl substituents into the 4-position of 6-phenyl-2,2′-bipyridine, and this
indicates little electronic delocalization between them. Self-quenching of the 3MLCT emission by the mononuclear
derivatives are observed in CH2Cl2 at 298 K, and a red shift in the emission energy is exhibited by complex 7 in
acetonitrile at 77 K. The fluid emissions of the µ-dppm species 1b-6b at λmax 652-662 nm appear at substantially
lower energies than their mononuclear counterparts and show dramatic solvatochromic effects. These emissions
are ascribed to 3[dσ*, π*] excited states. In contrast, the emission of 10 and 11, bearing long bridging diphosphine
3
ligands, are attributed to MLCT states of non-interacting [Pt(L1)] moieties. Significantly, the luminescence of
3
3
the µ-pyrazolate complex 9 displays transitional features which are reminiscent of both [dσ*, π*] and MLCT
excited states. Hence a relationship is observed between emission energy, the nature of the lowest energy excited
state, and metal-metal interactions. The excited-state redox potential [E(*Pt22+/Pt2+)] of 1b has been estimated
by electrochemical studies (1.61 V vs NHE) and by quenching experiments with aromatic hydrocarbons (1.63 V
vs NHE).
Introduction
since this class of molecules can mediate excited-state atom
transfer reactions and bond activation. In particular, the prolific
Luminescent coordinatively unsaturated metal complexes are
appealing from a photochemical perspective. While saturated
congeners such as [Ru(bpy)3]2+ (bpy ) 2,2′-bipyridine) are
restricted to outer-sphere interactions with substrates, these
chromophores allow inner-sphere electron-transfer reactions, and
applications for chemical sensing,1-6 solar energy conversion,
and photocatalysis7-9 have been developed. Investigations into
square planar d8 platinum(II) compounds have been prominent
excited-state chemistry of the binuclear derivative [Pt2(µ-
P2O5H2)4]4- has been demonstrated.10 The triplet (dσ*, pσ)
excited state, which is a manifestation of the d8-d8 interaction
between the diplatinum centers, is capable of C-H and
C-halogen bond cleavage and electron-transfer reactions.
The propensity for square planar d8 complexes to engage in
metal-metal interactions and form extended linear-chain solid-
state structures has been extensively studied.11 Unusual colors
and strong emission, as well as highly anisotropic properties,
are often the result of such stacking interactions. Platinum(II)
* Corresponding author. Fax: (852) 2857 1586. E-mail: cmche@hkucc.
hku.hk.
† The University of Hong Kong.
‡ National Taiwan University.
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10.1021/ic990238s CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/13/1999

d8-d8 Interactions in Platinum(II) Complexes
Inorganic Chemistry, Vol. 38, No. 18, 1999 4047
intermolecular interactions in Pt(II) polypyridine species.24-26
We became attracted to Pt(II) derivatives bearing cyclometalated
6-phenyl-2,2′-bipyridine (L1) and related ligands.3,27-30 Earlier
studies on bidentate analogues employing C-deprotonated 2-
phenylpyridine and 2-(2′-thienyl)pyridine revealed low-lying
metal-to-ligand charge transfer (MLCT) states with interesting
photochemical properties.31 We anticipated that the tridentate
ligand L1, which favors planar geometry upon cyclometalation,
would discourage a D2d distortion, while the extended π system
within L1 and the strongly σ-donating carbanion would increase
the energy difference between the ligand field (d-d) and the
MLCT states.
Herein is described the preparation and spectroscopic proper-
ties of a series of mono- and binuclear cyclometalated platinum-
(II) complexes derived from 6-phenyl-2,2′-bipyridine. They are
photoluminescent at room temperature in fluid solution. The
binuclear derivatives are envisaged as models for investigating
the photophysics and solid-state structures of cyclometalated
Pt(II) oligomers, and our objective was to characterize the [dσ*,
π*] excited state and d8-d8 interactions in detail. By systemati-
cally varying (1) the length of the bridging bidentate ligand and
(2) the electronic nature of substituents on 6-phenyl-2,2′-
bipyridine, we set out to modify the degree of metal-metal
and π-π interactions and to monitor the effects upon the nature
of the excited states.
Figure 1. Schematic molecular orbital diagram illustrating d8-d8 and
π-π interactions in binuclear platinum(II) polypyridine complexes.
R-diimine complexes display a variety of low-energy excited
states,12-14 including R-diimine intraligand (IL, π f π*)
transitions of both monomer and dimer and metal-to-ligand
charge transfer (MLCT). In addition, when two platinum(II)
R-diimine units are in close proximity so as to allow metal-
metal and ligand-ligand (π-π) contacts, a low-energy photo-
luminescence which is red-shifted from the 3MLCT emis-
sion of mononuclear species is typically observed. The elec-
tronic excited-state associated with this emission is denoted as
3[dσ*, π*] (the metal-metal-to-ligand charge transfer (MMLCT)
notation is also used in the literature, see Figure 112a). How-
ever, it should be noted that the Pt(II)-Pt(II) interaction in
the ground state is considerably weaker than a normal Pt-Pt
single bond. Although these R-diimine compounds exhibit
interesting photophysical properties, they are usually weak
emitters or even non-emissive in fluid solution. The tuning of
the excited-state properties of the related Pt(II) diimine dithio-
late derivatives have been documented.15 In addition, d8-d8
and π-π interactions are invoked in oligomerization16-19
and excimer formation20-22 of mononuclear platinum(II) spe-
cies in solution, which leads to changes in their photophysical
behavior.
Experimental Section
General Procedures. K2PtCl4 (Strem), bis(diphenylphosphino)-
methane, -propane, and -pentane (dppm, dppC3, and dppC5, respectively,
Aldrich), pyrazole (Hpz), and pyridine (py, Aldrich) were used as
received. HL1-6 (4-(aryl)-6-phenyl-2,2′-bipyridine; aryl ) H (1), phenyl
(2), 4-chlorophenyl (3), 4-tolyl (4), 4-methoxyphenyl (5), 3,4,5-
trimethoxyphenyl (6)) were prepared by literature methods.32 Syntheses
of [Pt(L1)Cl] (1a), [Pt2(L1)2(µ-dppm)](ClO4)2 (1b(ClO4)2), and [Pt(L1)-
PPh3]ClO4 (8(ClO4)) have been described previously.29 (Caution!
Perchlorate salts are potentially explosiVe and should be handled with
care and in small amounts.) Dichloromethane for photophysical studies
was washed with concentrated sulfuric acid, 10% sodium hydrogen
carbonate, and water, dried by calcium chloride, and distilled over
calcium hydride. Acetonitrile for photophysics was distilled over
potassium permanganate and calcium hydride. The other solvents used
were of analytical grade.
Square planar d8 complexes are proposed to be unstable with
respect to a D2d distortion, which is likely to result in non-
radiative decay. Many researchers have therefore diverted their
attention to the 2,2′:6′,2′′-terpyridine (tpy) ligand which shows
strong preference for planar geometry.16-19,22,23 Several binuclear
d8-d8 complexes of the type [Pt2(tpy)2(µ-L)]n+ (L ) bidentate
ligand) have been spectroscopically characterized to model
Physical Measurements and Instrumentation. Fast atom bombard-
ment (FAB) mass spectra were obtained on a Finnigan Mat 95 mass
spectrometer. Elemental analyses were performed by Butterworth
Laboratory, Teddington, U.K. 1H (in MHz, 300 or 500), 13C (126), 31
P
(202), and 195Pt (107) NMR measurements were performed on a Bruker
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(b) Houlding, V. H.; Miskowski, V. M. Coord. Chem. ReV. 1991, 111,
145. (c) Miskowski, V. M.; Houlding, V. H.; Che, C. M.; Wang, Y.
Inorg. Chem. 1993, 32, 2518.
(13) Che, C. M.; He, L. Y.; Poon, C. K.; Mak, T. C. W. Inorg. Chem.
1989, 28, 3081.
(14) (a) Zheng, G. Y.; Rillema, D. P.; DePriest, J.; Woods, C. Inorg. Chem.
1998, 37, 3588. (b) Zheng, G. Y.; Rillema, D. P. Inorg. Chem. 1998,
37, 1392.
(23) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem. 1994,
33, 722.
(24) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1993, 32,
369.
(25) Yip, H. K.; Che, C. M.; Zhou, Z. Y.; Mak, T. C. W. J. Chem. Soc.,
Chem Commun. 1992, 1369.
(26) Ratilla, E. M. A.; Scott, B. K.; Moxness, M. S.; Kostic, N. M. Inorg.
Chem. 1990, 29, 918.
(15) Paw, W.; Cummings, S. D.; Mansour, M. A.; Connick, W. B.; Geiger,
D. K.; Eisenberg, R. Coord. Chem. ReV. 1998, 171, 125.
(16) Arena, G.; Calogero, G.; Campagna, S.; Scolaro, L. M.; Ricevuto,
V.; Romeo, R. Inorg. Chem. 1998, 37, 2763.
(17) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem.
1996, 35, 4585.
(27) Chan, C. W.; Lai, T. F.; Che, C. M.; Peng, S. M. J. Am. Chem. Soc.
1993, 115, 11245.
(28) Chan, C. W.; Cheng, L. K.; Che, C. M. Coord. Chem. ReV. 1994,
132, 87.
(29) Cheung, T. C.; Cheung, K. K.; Peng, S. M.; Che, C. M. J. Chem.
Soc., Dalton Trans. 1996, 1645.
(18) Yip, H. K.; Cheng, L. K.; Cheung K. K.; Che, C. M. J. Chem. Soc.,
Dalton Trans. 1993, 2933.
(19) Jennette, K. W.; Gill, J. T.; Sadownick, J. A.; Lippard, S. J. J. Am.
Chem. Soc. 1976, 98, 6159.
(20) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625.
(21) Wan, K. T.; Che C. M.; Cho, K. C. J. Chem. Soc., Dalton Trans.
1991, 1077.
(22) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer,
W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591.
(30) (a) Tse, M. C.; Cheung, K. K.; Chan, M. C. W.; Che, C. M. Chem.
Commun. 1998, 2295. (b) Lai, S. W.; Chan, M. C. W.; Peng, S. M.;
Che, C. M. Angew. Chem., Int. Ed. 1999, 38, 669.
(31) (a) Chassot, L.; Mu¨ller, E.; von Zelewsky, A. Inorg. Chem. 1984, 23,
4249. (b) Maestri, M.; Sandrini, D.; Balzani, V.; Chassot, L.; Jolliet,
P.; von Zelewsky, A. Chem. Phys. Lett. 1985, 122, 375. (c) Sandrini,
D.; Maestri, M.; Balzani, V.; Chassot, L.; von Zelewsky, A. J. Am.
Chem. Soc. 1987, 109, 7720.
(32) Kro¨hnke, F. Synthesis 1976, 1.

4048 Inorganic Chemistry, Vol. 38, No. 18, 1999
Lai et al.
DRX 300 or 500 FT-NMR spectrometer with TMS (1H and 13C), H3-
PO4 (31P), and H2PtCl6 (195Pt) as references. UV-vis absorption spectra
were obtained on a Perkin-Elmer Lambda 19 UV-visible spectropho-
tometer.
d6): δ 20.8 (Me), 115.9, 116.5, 123.8, 124.1, 125.1, 127.3, 128.2, 129.7,
130.2, 133.2, 134.2, 140.3, 140. 4, 142.3, 147.0, 148.1, 150.4, 154.5,
156.8, 165.3.
5a. Yield: 0.21 g, 77%. MS (+ve FAB): m/z 568 (M+). Anal. Calcd
for C23H17N2OClPt: C, 48.64; H, 3.02; N, 4.93. Found: C, 48.85; H,
2.90; N, 4.87. 1H NMR (DMSO-d6, 300 MHz): δ 3.88 (s, 3H, OMe),
7.15 (d, 2H, J ) 6.6 Hz), 7.50-7.62 (m, 2H), 7.83 (d, 1H, J ) 7.6
Hz), 7.92 (t, 1H, J ) 6.4 Hz), 8.01 (d, 1H, J ) 8.7 Hz), 8.14 (d, 2H,
J ) 8.7 Hz), 8.25 (s, 1H), 8.35-8.40 (m, 1H), 8.50 (s, 1H), 8.76 (d,
1H, J ) 8.0 Hz), 8.92 (d, 1H, J ) 5.0 Hz). 13C{1H} NMR (DMSO-
d6): δ 56.2 (OMe), 115.4, 115.5, 118.9, 124.6, 128.0, 129.1, 129.4,
129.6, 130.0, 130.1, 141.1, 143.2, 148.0, 150.2, 150.9, 155.2, 157.7,
161.4, 162.1, 166.0.
Emission and Lifetime Measurements. Steady-state emission
spectra were recorded on a SPEX 1681 Fluorolog-2 series F111AI
spectrophotometer. Low-temperature (77 K) emission spectra for glasses
and solid-state samples were recorded in 5-mm diameter quartz tubes,
which were placed in a liquid-nitrogen Dewar equipped with quartz
windows. The emission spectra were corrected for monochromator and
photomultiplier efficiency and for xenon lamp stability.
Sample and standard solutions were degassed with at least three
freeze-pump-thaw cycles. The emission quantum yield was measured
by the method of Demas and Crosby33 with [Ru(bpy)3](PF6)2 in
degassed acetonitrile as the standard (Φr ) 0.062).
Emission lifetimes and flash-photolysis measurements were per-
formed with a Quanta Ray DCR-3 pulsed Nd:YAG laser system (pulse
output 355 nm, 8 ns). The emission signals were detected by a
Hamamatsu R928 photomultiplier tube and recorded on a Tektronix
model 2430 digital oscilloscope. Error limits are estimated: λ ((1 nm);
τ ((10%); φ ((10%).
Syntheses. [Pt(L2-6)Cl] (2a-6a; HL2-6 ) 4-(aryl)-6-phenyl-2,2′-
bipyridine; aryl ) phenyl (2), 4-chlorophenyl (3), 4-tolyl (4),
4-methoxyphenyl (5), 3,4,5-trimethoxyphenyl (6)). A modification
of Constable’s method was used.34 A mixture of K2PtCl4 (0.20 g, 0.48
mmol) and L2-6 (except L3, 0.48 mmol) in CH3CN/H2O (15/15 mL)
was refluxed for 18 h to give a deep red solution, which was evaporated
to dryness. The product was extracted with dichloromethane, and the
volume of extract was reduced to ∼5 mL. Addition of diethyl ether
yielded an orange solid which was recrystallized by vapor diffusion of
diethyl ether into an acetonitrile solution to afford a reddish-orange
crystalline solid.
2a. Yield: 0.22 g, 85%. MS (+ve FAB): m/z 538 (M+). Anal. Calcd
for C22H15N2ClPt: C, 49.12; H, 2.81; N, 5.21. Found: C, 48.98; H,
2.78; N, 5.16. 1H NMR (DMSO-d6, 300 MHz): δ 7.06-7.18 (m, 2H),
7.49-7.63 (m, 4H), 7.82 (d, 1H, J ) 6.2 Hz), 7.92 (t, 1H, J ) 6.4
Hz), 8.11-8.14 (m, 2H), 8.27 (s, 1H), 8.38 (t, 1H, J ) 8.5 Hz), 8.53
(s, 1H), 8.76 (d, 1H, J ) 8.1 Hz), 8.91 (d, 1H, J ) 4.4 Hz). 13C{1H}
NMR (DMSO-d6): δ 116.4, 117.0, 123.7, 124.1, 125.1, 127.5, 128.1,
129.0, 130.1, 130.2, 134.1, 136.2, 140.3, 142.3, 146.9, 148.0, 150.5,
154.5, 156.7, 165.3.
6a. Yield: 0.21 g, 69%. MS (+ve FAB): m/z 628 (M+). Anal. Calcd
for C25H21N2O3ClPt: C, 47.81; H, 3.37; N, 4.46. Found: C, 47.90; H,
3.25; N, 4.29. 1H NMR (DMSO-d6, 300 MHz): δ 3.76 (s, 3H, p-OMe),
3.98 (s, 6H, m-OMe), 7.11-7.17 (m, 2H), 7.34 (s, 2H), 7.51 (d, 1H, J
) 6.7 Hz), 7.85 (d, 1H, J ) 7.2 Hz), 7.92-7.96 (m, 1H), 8.23 (s, 1H),
8.40-8.44 (m, 2H), 8.74 (d, 1H, J ) 7.6 Hz), 8.92-8.94 (m, 1H).
13C{1H} NMR (DMSO-d6): δ 55.4 (m-OMe), 59.1 (p-OMe), 115.5,
116.0, 116.9, 122.8, 123.2, 124.3, 127.3, 129.3, 130.8, 132.6, 133.2,
138.8, 139.4, 145.9, 147.1, 149.8, 152.4, 153.3, 155.8, 164.2.
[Pt(L1)py]ClO4, 7(ClO4). A mixture of [Pt(L1)Cl] (0.25 g, 0.54
mmol) and excess pyridine (0.20 g, 2.70 mmol) in CH3CN/CH3OH
(20/20 mL) was stirred for 3 h at room temperature. Excess LiClO4
(0.2 g) was added to the resultant mixture, which was stirred for 5 h
and then filtered and evaporated to ∼5 mL. Addition of diethyl ether
yielded a yellow-brown solid which was filtered and washed with
diethyl ether. Recrystallization by vapor diffusion of diethyl ether into
an acetonitrile solution yielded 0.28 g (86%) of yellow crystals. MS
(+ve FAB): m/z 505 (M+). Anal. Calcd for C21H16N3O4ClPt: C, 41.70;
H, 2.67; N, 6.95. Found: C, 41.95; H, 2.55; N, 6.87. 1H NMR (DMSO-
d6, 300 MHz): δ 6.28 (d, 1H, J ) 7.5 Hz), 7.03-7.15 (m, 2H), 7.70-
7.85 (m, 4H), 8.07 (d, 2H, J ) 6.9 Hz), 8.15-8.43 (m, 4H), 8.58 (d,
1H, J ) 7.8 Hz), 9.09 (d, 2H, J ) 5.1 Hz). 13C{1H} NMR (DMSO-
d6): δ 120.3, 124.2, 125.0, 125.6, 126.2, 128.1, 129.2, 131.4, 132.4,
140.2, 141.4, 141.7, 142.3, 147.5, 149.4, 153.4, 155.0, 156.6, 165.5.
[Pt2(L2-6)2(µ-dppm)](ClO4)2, 2b-6b(ClO4)2. A mixture of [Pt-
(L2-6)Cl] (except L3, 0.32 mmol) and dppm (0.06 g, 0.16 mmol) in
CH3CN/CH3OH (15/15 mL) was stirred for 12 h under a nitrogen
atmosphere. The resultant solution was filtered and evaporated to ∼5
mL. Addition of excess aqueous LiClO4 afforded a bright orange solid,
which was filtered and washed with water and diethyl ether. Recrys-
tallization by vapor diffusion of diethyl ether into an acetonitrile solution
yielded an orangish-red crystalline solid.
3a. A mixture of K2PtCl4 (0.20 g, 0.48 mmol) and L3 (0.17 g, 0.49
mmol) in H2O/CH3CN (10/40 mL) was refluxed for 30 h. An orange
suspension was formed, and the reaction mixture was evaporated to
dryness. The solid residue was extracted using N,N-dimethylformamide
(DMF) (25 mL × 2), and the volume of the extract was reduced to ∼5
mL. An orange precipitate appeared upon addition of diethyl ether and
was filtered and washed with H2O and diethyl ether. Recrystallization
by vapor diffusion of diethyl ether into a DMF solution afforded
reddish-orange crystals. Yield: 0.19 g, 69%. MS (+ve FAB): m/z 572
(M+). Anal. Calcd for C22H14N2Cl2Pt: C, 46.17; H, 2.47; N, 4.89.
2b(ClO4)2. Yield: 0.20 g, 79%. MS (+ve FAB): m/z 1489 (M+
+
ClO4), 1389 (M+). Anal. Calcd for C69H52N4O8Pt2Cl2P2: C, 52.18; H,
1
3.30; N, 3.53. Found: C, 52.08; H, 3.27; N, 3.48. H NMR (DMSO-
d6, 300 MHz): δ 5.26 (broad t, 2H, 2J(PH) ) 12 Hz, PCH2P), 6.16 (s,
2H), 6.40-6.45 (m, 2H), 6.59-6.69 (m, 6H), 7.36-7.60 (m, 20H),
7.76-7.79 (m, 8H), 7.97-8.04 (m, 4H), 8.23 (s, 2H), 8.41-8.52 (m,
6H). 13C{1H} NMR (DMSO-d6): δ 19.4 (t, 1J(PC) ) 30.1 Hz, PCH2P),
116.7, 117.0, 124.4-134.9, 137.5, 140.0, 146.7, 150.7, 153.2, 153.4,
156.4, 162.3. 31P{1H} NMR (CD3CN): δ 19.36 (1J(PtP) ) 4114 Hz,
3J(PtP) ) 87 Hz). 195Pt NMR (CD3CN): δ -4095 (d, 1J(PPt) ) 4111
Hz).
1
Found: C, 46.10; H, 2.40; N, 4.78. H NMR (DMSO-d6, 300 MHz):
δ 7.10-7.16 (m, 2H), 7.51 (d, 1H, J ) 7.0 Hz), 7.69 (d, 2H, J ) 7.9
Hz), 7.83 (d, 1H, J ) 7.6 Hz), 7.93 (t, 1H, J ) 6.5 Hz), 8.18 (d, 2H,
J ) 8.0 Hz), 8.30 (s, 1H), 8.39 (t, 1H, J ) 7.7 Hz), 8.54 (s, 1H), 8.76
(d, 1H, J ) 8.0 Hz), 8.92 (d, 1H, J ) 4.9 Hz). 13C{1H} NMR (DMSO-
d6): δ 115.6, 116.4, 116.9, 123.8, 124.2, 125.3, 128.3, 129.1, 129.3,
130.3, 134.2, 140.4, 142.3, 146.2, 148.3, 149.2, 149.7, 154.5, 156.7,
165.8.
3b(ClO4)2. Dppm (0.08 g, 0.20 mmol) in CH3CN (10 mL) was added
dropwise to a solution of [Pt(L3)Cl] (0.23 g, 0.40 mmol) in CH3CN/
CH2Cl2 (10/10 mL) and stirred for 24 h under a nitrogen atmosphere
to afford a clear red-orange solution. After addition of methanolic
LiClO4 (0.2 g), the solution was further stirred for 2 h and then reduced
to ∼5 mL. Addition of diethyl ether gave a red-orange solid which
was filtered and washed with water and diethyl ether. Recrystallization
by vapor diffusion of diethyl ether into an acetonitrile solution yielded
an orangish-red crystalline solid. Yield: 0.23 g, 69%. MS (+ve
FAB): m/z 1558 (M+ + ClO4), 1458 (M+). Anal. Calcd for C69H50N4O8-
Pt2Cl4P2: C, 50.01; H, 3.04; N, 3.38. Found: C, 49.95; H, 2.95; N,
3.32. 1H NMR (DMSO-d6, 300 MHz): δ 5.22 (broad t, 2H, 2J(PH) )
12 Hz, PCH2P), 6.23-6.39 (m, 4H), 6.54-6.73 (m, 6H), 7.40-7.61
(m, 18H), 7.78-7.87 (m, 8H), 7.99-8.04 (m, 4H), 8.28-8.37 (m, 6H),
8.51 (d, 2H, J ) 8.1 Hz). 13C{1H} NMR (DMSO-d6): δ 20.3 (t, 1J(PC)
4a. Yield: 0.23 g, 86%. MS (+ve FAB): m/z 552 (M+). Anal. Calcd
for C23H17N2PtCl: C, 50.05; H, 3.10; N, 5.08. Found: C, 49.78; H,
1
3.02; N, 4.96. H NMR (DMSO-d6, 300 MHz): δ 2.41 (s, 3H, Me),
7.08-7.19 (m, 2H), 7.42 (d, 2H, J ) 7.9 Hz), 7.52 (d, 1H, J ) 7.2
Hz), 7.84 (d, 1H, J ) 6.6 Hz), 7.94 (t, 1H, J ) 6.4 Hz), 8.07 (d, 2H,
J ) 8.0 Hz), 8.28 (s, 1H), 8.39 (t, 1H, J ) 7.7 Hz), 8.53 (s, 1H), 8.78
(d, 1H, J ) 7.9 Hz), 8.93 (d, 1H, J ) 5.2 Hz). 13C{1H} NMR (DMSO-
(33) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(34) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A. J.
Chem. Soc., Chem. Commun. 1990, 513.

d8-d8 Interactions in Platinum(II) Complexes
Inorganic Chemistry, Vol. 38, No. 18, 1999 4049
) 32.1 Hz, PCH2P), 116.3, 116.9, 124.4-139.9, 146.6, 150.9, 151.7,
153.3, 156.5, 162.4. 31P{1H} NMR (CD3CN): δ 19.31 (1J(PtP) ) 4124
Table 1. Crystal Data
4b(ClO4)2‚5H2O
9(PF6)
3
1
Hz, J(PtP) ) 81 Hz). 195Pt NMR (CD3CN): δ -4087 (d, J(PPt) )
formula
fw
cryst system
space group
color
cryst size, mm
a, Å
b, Å
C71H66N4Cl2O13P2Pt2
1706.36
monoclinic
C2/c
C35H25N6PF6Pt2
1064.77
monoclinic
C2/c
4129 Hz).
4b(ClO4)2. Yield: 0.20 g, 77%. MS (+ve FAB): m/z 1517 (M+
+
ClO4), 1417 (M+). Anal. Calcd for C71H56N4O8Pt2Cl2P2: C, 52.76; H,
1
3.49; N, 3.47. Found: C, 52.70; H, 3.42; N, 3.45. H NMR (CD3CN,
orange
orange
2
500 MHz): δ 2.42 (s, 6H, Me), 4.85 (broad t, 2H, J(PH) ) 13 Hz,
0.35 × 0.35 × 0.35
31.338(8)
21.767(5)
27.515(6)
123.37(2)
15675(6)
8
1.447
37.68
6752
0.059, 0.058
1.75
0.20 × 0.30 × 0.40
15.339(2)
13.309(1)
16.264(5)
103.03(2)
3235(1)
PCH2P), 6.22 (s, 2H), 6.41 (t, 2H, J ) 6.9 Hz), 6.56 (t, 2H, J ) 6.1
Hz), 6.62 (t, 2H, J ) 7.3 Hz), 6.72-6.73 (m, 2H), 7.08 (d, 2H, J )
7.1 Hz), 7.18 (d, 4H, J ) 7.8 Hz), 7.41-7.57 (m, 16H), 7.59 (s, 2H),
7.70-7.87 (m, 8H), 8.02 (d, 2H, J ) 7.8 Hz), 8.25-8.33 (broad s,
c, Å
â, deg
1
4H). 13C{1H} NMR (CD3CN): δ 21.3 (t, J(PC) ) 31.4 Hz, PCH2P),
V, Å3
Z
4
21.5 (Me), 117.5, 117.7, 125.0-133.3, 139.1, 141.0, 142.9, 147.9,
152.4, 154.5, 155.0, 157.8, 163.8. 31P{1H} NMR (CD3CN): δ 19.40
(1J(PtP) ) 4111 Hz, 3J(PtP) ) 83 Hz). 195Pt NMR (CD3CN): δ -4091
Dc, g cm-3
µ, cm-1
F(000)
2.186
88.50
2008
1
(d, J(PPt) ) 4113 Hz).
R,a Rw
0.025, 0.027
1.48
-0.75, +1.50
b
5b(ClO4)2. Yield: 0.19 g, 72%. MS (+ve FAB): m/z 1549 (M+
+
GoFc
ClO4), 1449 (M+). Anal. Calcd for C71H56N4O10Pt2Cl2P2: C, 51.74; H,
3.42; N, 3.40. Found: C, 51.80; H, 3.46; N, 3.48. H NMR (CD3CN,
residual F, e Å-3
-0.57, +0.87
1
2 1/2
300 MHz): δ 3.88 (s, 6H, OMe), 4.84 (broad t, 2H, 2J(PH) ) 13 Hz,
PCH2P), 6.15 (s, 2H), 6.20-6.41 (m, 2H), 6.55-6.60 (m, 4H), 6.71-
6.80 (m, 2H), 6.88 (d, 4H, J ) 8.9 Hz), 7.07-7.14 (m, 2H), 7.42-
7.59 (m, 18H), 7.70-8.04 (m, 10H), 8.25-8.47 (broad s, 4H). 13C{1H}
a R ) Σ||Fo| - |Fc||/Σ|Fo|. b Rw ) [Σw(|Fo| - |Fc|)2/Σw|Fo| ]
.
c GoF
) [Σw(|Fo| - |Fc|)2/(n - p)]1/2
.
6H), 8.08 (t, 2H, J ) 8.0 Hz). 13C{1H} NMR (CD3CN): δ 26.1 (dd,
1
3
2
1J(PC) ) 37.2 Hz, J(PC) ) 15.6 Hz, PCH2), 26.9 (t, J(PC) ) 7.2
Hz, PCH2CH2), 120.9, 121.2, 125.4-138.6, 141.9, 143.7, 148.3, 152.2,
154.4, 158.9, 164.1. 31P{1H} NMR (CD3CN): δ 19.71 (1J(PtP) ) 3990
Hz).
NMR (CD3CN): δ 21.6 (t, J(PC) ) 31.3 Hz, PCH2P), 56.3 (OMe),
115.6, 116.5, 117.0, 125.0-135.4, 139.0, 140.9, 147.9, 152.3, 154.2,
154.4, 157.8, 163.2, 163.6. 31P{1H} NMR (CD3CN): δ 19.52 (1J(PtP)
3
) 4108 Hz, J(PtP) ) 85 Hz). 195Pt NMR (CD3CN): δ -4087 (d,
1J(PPt) ) 4110 Hz).
[Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2. The procedure for 1b(ClO4)2
was adopted using Pt(L1)Cl (0.18 g, 0.40 mmol) with bis(diphenylphos-
phino)pentane (0.09 g, 0.20 mmol) to afford 0.19 g (64%) of a yellow
crystalline solid. MS (+ve FAB): m/z 1393 (M+ + ClO4), 1293 (M+).
Anal. Calcd for C61H52N4O8Pt2Cl2P2: C, 49.10; H, 3.51; N, 3.75.
6b(ClO4)2. Yield: 0.21 g, 74%. MS (+ve FAB): m/z 1669 (M+
+
ClO4), 1569 (M+). Anal. Calcd for C75H64N4O14Pt2Cl2P2: C, 50.94; H,
3.65; N, 3.17. Found: C, 50.80; H, 3.52; N, 3.10. H NMR (CD3CN,
1
300 MHz): δ 3.79 (s, 6H, p-OMe), 3.82 (s, 12H, m-OMe), 4.85 (t,
2H, 2J(PH) ) 13.6 Hz, PCH2P), 6.25 (s, 2H), 6.36 (t, 2H, J ) 7.0 Hz),
6.50 (t, 2H, J ) 7.5 Hz), 6.69 (t, 2H, J ) 6.3 Hz), 6.79 (s, 6H), 7.32
(d, 2H, J ) 7.6 Hz), 7.44-7.94 (m, 22H), 8.15 (d, 2H, J ) 8.1 Hz),
1
Found: C, 48.96; H, 3.47; N, 3.70. H NMR (DMSO-d6, 500 MHz):
δ 1.44; 1.78; 2.84 (broad multiplets, 10H, P(CH2)5P), 6.48 (broad s,
2H), 6.68 (broad s, 2H), 6.98-7.06 (m, 4H), 7.25-7.52 (m, 18H),
7.69-7.81 (m, 8H), 8.21-8.36 (m, 6H), 8.52-8.55 (m, 2H). 13C{1H}
NMR (DMSO-d6): δ 24.1 (d, 1J(PC) ) 37.8 Hz, PCH2), 25.0 (s,
P(CH2)2CH2), 30.9-31.2 (m, PCH2CH2), 120.3, 124.7-136.8, 141.3,
142.9, 147.7, 150.9, 153.5, 158.0, 162.7. 31P{1H} NMR (CD3CN): δ
19.53 (1J(PtP) ) 3972 Hz).
1
8.28 (broad s, 4H). 13C{1H} NMR (DMSO-d6): δ 19.9 (t, J(PC) )
30.5 Hz, PCH2P), 56.1 (m-OMe), 59.9 (p-OMe), 105.3, 116.3, 116.9,
124.3-140.1, 146.7, 150.9, 153.0, 153.1, 153.3, 156.7, 162.2. 31P{1H}
NMR (CD3CN): δ 19.65 (1J(PtP) ) 4134 Hz, 3J(PtP) ) 78 Hz). 195Pt
1
NMR (CD3CN): δ -4083 (d, J(PPt) ) 4125 Hz).
[Pt2(L1)2(µ-pz)]X, 9(X) (X ) ClO4 and PF6). A mixture of [Pt-
(L1)Cl] (0.23 g, 0.50 mmol), pyrazole (Hpz) (0.02 g, 0.25 mmol), and
potassium tert-butoxide (0.03 g, 0.25 mmol) in CH3CN/CH3OH (20/
10 mL) was heated at 60 °C under a nitrogen atmosphere for 10 h.
The orange suspension gradually became a clear red-orange solution
and was allowed to cool to room temperature. A methanolic solution
of LiClO4 or NH4PF6 (1.5 mmol in 10 mL) was added, and the resultant
mixture was stirred for 1 h, after which the solvent was evaporated to
∼5 mL. Addition of diethyl ether yielded an orange solid which was
recrystallized by vapor diffusion of diethyl ether into an acetonitrile
solution. Yield: 0.14 g, 55% for 9(ClO4); 0.14 g, 53% for 9(PF6). Anal.
Calcd for 9(ClO4), C35H25N6O4Pt2Cl: C, 41.24; H, 2.47; N, 8.25. Found:
C, 41.18; H, 2.63; N, 8.42. Calcd for 9(PF6), C35H25N6Pt2PF6: C, 39.48;
H, 2.37; N, 7.89. Found: C, 39.42; H, 2.52; N, 8.05.
X-ray Crystallography. Crystals of 4b(ClO4)2‚5H2O and 9(PF6)
were obtained by vapor diffusion of diethyl ether into acetonitrile
solutions. Crystal data and details of data collection and refinement
are summarized in Table 1. The following data are listed in the order
4b(ClO4)2‚5H2O/9(PF6). A total of 10 201/2841 unique reflections were
collected at 298 K on a Nonius diffractometer (λ(Mo KR) ) 0.7107
Å, 2θmax ) 45/50°). The structure was solved by Patterson methods,
expanded using Fourier techniques, and refined by least-squares
treatment on F2 using the NRCVAX program: R ) 0.059/0.025, wR
) 0.058/0.027, GoF ) 1.75/1.48, for 3760/2257 absorption-corrected
(transmission 0.93-1.00/0.63-1.00) reflections with I > 2σ(I) and 847/
229 parameters. The pairs of atoms N(1)/C(16) and N(3)/C(39) for
4b(ClO4)2‚5H2O and N(1)/C(12) for 9(PF6) were differentiated by their
temperature factors; interchanging the respective C and N atoms resulted
in unreasonable temperature factors and/or higher R values. For 9(PF6),
the two halves of the cation are related by a 2-fold axis through C(18).
1
Data for 9(ClO4) and 9(PF6). MS (+ve FAB): m/z 919 (M+). H
NMR (DMSO-d6, 300 MHz): δ 6.78-6.80 (m, 4H), 6.88-7.12 (m,
5H), 7.48 (d, 2H, J ) 7.2 Hz), 7.83 (d, 2H, J ) 7.7 Hz), 7.93-8.09
(m, 8H), 8.25-8.36 (m, 4H). 13C{1H} NMR (DMSO-d6): δ 107.4,
119.2, 119.3, 123.4, 124.3, 125.0, 127.1, 130.5, 133.7, 140.4, 140.8,
141.1, 141.7, 146.4, 151.0, 154.3, 155.4, 165.1.
1
Primed atoms are located at (-x, y, /2-z).
Results and Discussion
[Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2. The procedure for 1b(ClO4)2
was adopted using Pt(L1)Cl (0.18 g, 0.38 mmol) and bis(diphenylphos-
phino)propane (0.08 g, 0.19 mmol) to yield 0.18 g (63%) of a yellow
crystalline solid. MS (+ve FAB): m/z 1365 (M+ + ClO4), 1265 (M+).
Anal. Calcd for C59H48N4O8Pt2Cl2P2: C, 48.40; H, 3.30; N, 3.83.
Synthesis and Characterization. The cyclometalating ligands
HL1-6 (Figure 2) are readily prepared by Kro¨hnke syntheses
using the appropriate enone and 2-pyridacylpyridinium iodide
in the presence of excess ammonium acetate.32 Complexes 1a-
6a are synthesized by modification of the procedure reported
by Constable and co-workers.34 The coordinated chloride ligand
in this series allows facile derivatization of the [Pt(L1-6)]
moieties. The cationic derivative [Pt(L1)py]+ (7) is afforded by
reaction of 1a with pyridine at room temperature.
1
Found: C, 48.25; H, 3.26; N, 3.76. H NMR (CD3CN, 500 MHz): δ
3.09-3.13 (m, 2H, PCH2CH2), 3.33-3.41 (m, 4H, PCH2), 6.25-6.33
(m, 4H), 6.61 (t, 2H, J ) 7.4 Hz), 6.74-6.75 (m, 2H), 6.86 (t, 2H, J
) 6.5 Hz), 7.04 (d, 2H, J ) 7.7 Hz), 7.35-7.38 (m, 8H), 7.43-7.47
(m, 4H), 7.58 (d, 2H, J ) 8.1 Hz), 7.72-7.76 (m, 8H), 7.84-7.97 (m,

4050 Inorganic Chemistry, Vol. 38, No. 18, 1999
Lai et al.
Figure 3. Perspective view of [Pt2(L4)2(µ-dppm)]2+ (4b, 30% prob-
ability ellipsoids).
Figure 2.
The binuclear complexes 1b-6b and 9-11 are conveniently
obtained in moderate to high yields by treatment of the
corresponding mononuclear precursor with the diphosphine or
pyrazole (for 9). The 31P{1H} NMR spectra of the dicationic
derivatives 1b-6b contain one signal with multiple 195Pt
satellites due to one- and three-bond couplings. These spectral
patterns indicate the presence of a Pt2(µ-dppm) unit with
Figure 4. Perspective view of [Pt2(L1)2(µ-pz)]+ (9, 40% probability
ellipsoids). The two halves of the cation are related by a 2-fold axis
through C(18).
1
chemically equivalent phosphorus atoms.35 In the H NMR
spectra, the dppm methylene group appears as a distinctive
triplet resonance. The positive FAB mass spectra for 1b-6b
contain signals corresponding to the (M+ + ClO4) fragment as
well as the molecular cation M+.
Table 2. Selected Bond Lengths (Å) and Angles (deg)
Complex 4b(ClO4)2‚5H2O
Crystal Structures and Pt-Pt Distances. The crystal
structures of complex 4b bearing the tolyl group and the
pyrazolate derivative 9 were determined by X-ray crystal-
lography (Figures 3 and 4 respectively, Table 2). Since the
structural parameters for 1b29 and 3b3a have previously been
elucidated, we can compare the effects of different substituents
(H: 1b; 4-chlorophenyl: 3b; 4-tolyl: 4b) on 6-phenyl-2,2′-
bipyridine upon the solid-state and in particular π-π stacking
interactions in these complexes. As in 1b and 3b, the platinum
centers in 4b reside in distorted square planar environments and
the molecular structure consists of two [Pt(L4)] units linked by
a dppm bridge. The adjacent [Pt(6-phenyl-2,2′-bipyridine)]
fragments are virtually parallel with a dihedral angle of 3.4°
(cf. 4.6° in 3b and 6.1° in 1b). The intramolecular Pt-Pt
contacts for 1b, 3b and 4b (3.270(1), 3.150(1), and 3.245(2)
Å, respectively) are slightly longer than those in the related
derivatives [Pt2(tpy)2(µ-L)]3+ (L ) anion of guanidine25 and
canavanine:26 mean 3.081 and 2.988 Å, respectively), but they
nevertheless fall within the range of intermetal distances (3.09-
3.50 Å) observed in monomeric Pt(II) extended linear-chain
structures.11a
Pt(1)-Pt(2)
Pt(1)-P(1)
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-C(16)
3.245(2)
2.251(8)
2.17(2)
1.99(2)
2.04(3)
Pt(2)-P(2)
Pt(2)-N(3)
Pt(2)-N(4)
Pt(2)-C(39)
P(2)-C(71)
2.241(8)
2.13(2)
2.01(2)
1.94(3)
1.79(3)
Pt(2)-Pt(1)-P(1)
Pt(2)-Pt(1)-N(1)
Pt(2)-Pt(1)-N(2)
Pt(2)-Pt(1)-C(16)
Pt(1)-Pt(2)-P(2)
Pt(1)-Pt(2)-N(3)
Pt(1)-Pt(2)-N(4)
Pt(1)-Pt(2)-C(39)
89.6(2)
94.1(5)
94.8(5)
88.4(7)
89.3(2)
93.4(6)
89.4(5)
90.2(8)
Pt(1)-P(1)-C(71)
P(1)-Pt(1)-N(1)
P(1)-Pt(1)-N(2)
P(1)-Pt(1)-C(16)
Pt(2)-P(2)-C(71)
P(2)-Pt(2)-N(3)
P(2)-Pt(2)-N(4)
P(2)-Pt(2)-C(39)
113.3(9)
104.0(5)
175.2(6)
101.2(7)
111.5(9)
109.8(6)
176.0(6)
96.9(8)
Complex 9(PF6)
Pt-N(1)
Pt-N(2)
Pt-N(3)
Pt-C(12)
2.113(5)
1.959(5)
2.009(4)
1.999(6)
N(3)-N(3’)
N(3)-C(17)
C(17)-C(18)
Pt-Pt’
1.368(8)
1.340(8)
1.384(9)
3.612(2)
N(1)-Pt-N(2)
N(1)-Pt-N(3)
N(1)-Pt-C(12)
79.3(2)
102.4(2)
161.4(2)
N(3)-Pt-C(12)
Pt-N(3)-N(3’)
N(3)-N(3’)-C(17)
96.2(2)
123.6(3)
108.3(5)
The distinct difference between these structures is the torsion
angle about the Pt-Pt axis, which is 44.6, 27.2, and 20.7° for
1b, 3b, and 4b (defined by the angle between the Pt(1)-
(35) Langrick, C. R.; McEwan, D. M.; Pringle, P. G.; Shaw, B. L. J. Chem.
Soc., Dalton Trans. 1983, 2487.

d8-d8 Interactions in Platinum(II) Complexes
Inorganic Chemistry, Vol. 38, No. 18, 1999 4051
Table 3. UV-Visible Spectral Data for Complexes 1a-6a, 7, and
8a
complex
λ )
max/nm (ꢀ/dm3 mol-1 cm-1
[Pt(L1)Cl], 1a
278 (19 600), 325 (9500), 360 (5000),
430 (1550), 510 (180)
[Pt(L2)Cl], 2a
[Pt(L3)Cl], 3a
[Pt(L4)Cl], 4a
[Pt(L5)Cl], 5a
[Pt(L6)Cl], 6a
285 (35 000), 334 (16 100), 369 (7600),
435 (3100), 519 (150)
288 (20 000), 332 (7800), 369 (4000),
434 (1950), 523 (260)
292 (34 000), 334 (17 800), 368 (7800),
438 (3500), 522 (200)
278 (21 800), 314 (18 700) 340 (17 100),
437 (3300), 522 (540)
278 (29 700), 336 (19 200), 418 (5000),
437 (5200), 525 (570)
259 (37 500), 335 (16 000) 350 (15 700),
422 (590), 498 (90)
266 (24 000), 336 (11 200), 350 (10 700),
425 (500)
[Pt(L1)py]ClO4,
7(ClO4)b
[Pt(L1)PPh3]ClO4,
8(ClO4)b
a Measured at room temperature in CH2Cl2 unless otherwise stated.
b In acetonitrile.
Figure 5. UV-vis absorption and emission (inset: λex 350 nm) spectra
of 6a in CH2Cl2 at 298 K.
Pt(2)-N(2) and Pt(1)-Pt(2)-N(4) planes), respectively. The
orientation in 1b optimizes the π-stacking interactions of the
aromatic ligand L1, since face-to-face overlap of π orbitals in
an eclipsed manner (i.e. torsion angle ca. 0°) introduces repulsive
forces.36 The reduced torsion angles in 3b and 4b are presumably
manifestations of further π-π interactions between the respec-
tive 4-chlorophenyl and tolyl substituents in achieving the most
stable conformation. In addition, these partially staggered
geometry essentially eliminate steric repulsion between the
chloro and methyl groups in 3b and 4b, respectively. We
therefore attribute the varying torsion angles in these structures
to the different geometrical demands for π-stacking interactions
by the cyclometalating ligands L1, L3 and L4.
Table 4. Solution Emission Data for Complexes 1a-6a, 7, and 8a
298 K data: λmax/nm;
τo/µs; φo; kq/M-1 s-1
77 K data:
max/nm
complex
λ
[Pt(L1)Cl], 1a
565; 0.51; 0.025; 3.1 × 109 535, 567 (max),
610, 600
[Pt(L2)Cl], 2a
[Pt(L3)Cl], 3a
[Pt(L4)Cl], 4a
[Pt(L5)Cl], 5a
[Pt(L6)Cl], 6a
[Pt(L1)py]ClO4,
7(ClO4)
564; 0.60; 0.052; 4.1 × 109 555 (max), 592
568; 0.52; 0.054; 2.4 × 109 572 (max), 595
562; 0.62; 0.064; 2.2 × 109 575 (max), 623
563; 0.72; 0.068; 2.9 × 109 578 (max), 613
566; 0.63; 0.057; 1.6 × 109 552 (max), 589
540; 1.47; 0.066; 1.5 × 109 522 (max), 554b
[Pt(L1)PPh3]ClO4, 535; 0.89; 0.062; 3.2 × 108 527 (max), 560b
8(ClO4)
The platinum centers in the structure of complex 9 are in
distorted square planar geometry, but unlike in the dppm
derivatives, the [Pt(L1)] moieties are not parallel due to the rigid
coordination mode of the pyrazolate linker (Figure 4). The
comparable bond lengths within the pz ring (range 1.340(8)-
1.384(9) Å) is consistent with the expected π-delocalization.
The resultant Pt-Pt distance of 3.612(2) Å is significantly longer
than that in [Pt2(tpy)2(µ-pz)](ClO4)3 (3.432(1) Å)24 and implies
negligible metal-metal communication, although weak π-π
interactions are possible. The effect of this and the general
consequences of longer metal-metal and π-π separations upon
the excited-state properties of these binuclear complexes will
be discussed later. Close intermolecular contacts are not
observed in the crystal lattice of 4b and 9.
a Complex concentration 1 × 10-5 M, in CH2Cl2 unless otherwise
stated; 1/τ ) kobs ) kq[complex] + ko. b In acetonitrile.
Table 5. Solid-State Emission Data for Complexes 1a-6a, 7, and 8
298 K data:
max/nm (τo/µs)
77 K data:
max/nm
complex
λ
λ
[Pt(L1)Cl], 1a
[Pt(L2)Cl], 2a
[Pt(L3)Cl], 3a
[Pt(L4)Cl], 4a
[Pt(L5)Cl], 5a
[Pt(L6)Cl], 6a
[Pt(L1)py]ClO4,
7(ClO4)
665 (0.40)
700
603 (max), 646 (0.66)
600 (max), 640 (0.27)
602 (max), 646 (0.51)
676 (0.34)
687 (0.25)
578 (max), 615 (1.80)
597 (max), 643, 702
594 (max), 643, 700
591 (max), 637, 700
716
722
572 (max), 611
[Pt(L1)PPh3]ClO4,
8(ClO4)
600 (2.41)
550 (max), 570
Absorption and Emission Spectroscopy. Mononuclear
Complexes. The UV-visible spectral data of the monomeric
complexes [Pt(L1-6)Cl] (1a-6a), [Pt(L1)py]+ (7), and [Pt(L1)-
PPh3]+ (8) are listed in Table 3. Their absorption spectra obey
Beer’s law in the concentration range 10-6-10-4 M. The optical
spectrum of 1a has been described previously.29 Similarly, the
high-energy region (λ < 370 nm) in the absorption spectra of
2a-6a, with 4-aryl substituents on the 6-phenyl-2,2′-bipyridine
4 and 5, respectively). With reference to earlier work, the
structureless emissions of complexes 2a-6a in CH2Cl2 at room
temperature are assigned as 3MLCT in nature (see Figure 5 for
6a). Variation of the emission maxima is small (range 562-
568 nm), and no trends are apparent for the different para-
substituents. Hence there appears to be limited electronic
communication between the 4-aryl group and the planar
6-phenyl-2,2′-bipyridine moiety. Minor solvatochromic effects
are observed for 2a-6a: for example, the room-temperature
emission of 2a shifts from 560 nm in acetonitrile to 567 nm in
dichloromethane. The blue-shifted emission maxima for the
cationic complexes 7 and 8 correlates with the greater charge
on the Pt(II) center, which is expected to increase the energy
1
ligand, is dominated by IL (π f π*) transitions, while the
moderately intense low-energy bands with λmax in the range
434-438 nm are assigned to 1MLCT (5d)Pt f π*(L) transitions
(see Figure 5 for 6a). The weak absorption tails at 519-525
nm (ꢀ < 600 dm3 mol-1 cm-1) are attributed to 3MLCT
transition.
The monomeric cyclometalated platinum(II) complexes stud-
ied in this work are emissive in solution and solid states (Tables
3
of the MLCT transition.
The orange to red colors of 1a-6a in the solid state are
noteworthy since these compounds absorb weakly at wave-
(36) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525.

4052 Inorganic Chemistry, Vol. 38, No. 18, 1999
Lai et al.
Table 6. UV-Visible Spectral Data for Complexes 1b-6b and 9-11 in Acetonitrile
complex
λ
max/nm (ꢀ/dm3 mol-1 cm-1
)
[Pt2(L1)2(µ-dppm)](ClO4)2, 1b(ClO4)2
[Pt2(L2)2(µ-dppm)](ClO4)2, 2b(ClO4)2
[Pt2(L3)2(µ-dppm)](ClO4)2, 3b(ClO4)2
[Pt2(L4)2(µ-dppm)](ClO4)2, 4b(ClO4)2
[Pt2(L5)2(µ-dppm)](ClO4)2, 5b(ClO4)2
[Pt2(L6)2(µ-dppm)](ClO4)2, 6b(ClO4)2
[Pt2(L1)2(µ-pz)](ClO4), 9(ClO4)
340 (16 500), 420 (3900), 478 (sh, 2550), 504 (sh, 1900)
286 (52 400), 316 (39 600), 421 (4000), 481 (sh, 2700), 511 (sh, 1800)
293 (47 900), 320 (41 200), 420 (3900), 488 (sh, 2600), 515 (sh, 1850)
292 (45 400), 325 (45 300), 420 (4500), 477 (sh, 2650), 510 (sh, 1700)
345 (46 200), 482 (2600), 510 (sh, 1750)
346 (38 700), 485 (2600), 522 (sh, 1450)
266 (34 800), 323 (13 300), 356 (9700), 409 (2500), 510 (sh, 250)
336 (17 000), 350 (15 200), 426 (900), 494 (150)
[Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2
[Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2
334 (18 800), 350 (17 500), 422 (850), 494 (120)
lengths longer than 450 nm in solution. Their intense colors
are attributed to the propensity of these square planar Pt(II)
species, like established diimine relatives, to undergo solid-state
intermolecular metal-metal and ligand-ligand interactions
which yield low-energy [dσ* f π*] transitions (Figure 1).12
Structurally determined cyclometalated analogues exhibit Pt-
Pt distances in the range 3.28-3.37 Å (for [Pt(L1)(CH3CN)]-
PF634 and [Pt(dpphen)(CH3CN)]ClO427 (Hdpphen ) 2,9-diphenyl-
1,10-phenanthroline), respectively) and π-π separations of
around 3.35 Å (for [Pt(L1)PPh3]ClO429). At room temperature,
microcrystalline samples of complexes 1a, 5a, and 6a emit with
λmax at 665, 676, and 687 nm, respectively (Table 5). Upon
cooling of the samples to 77 K, the bandwidths of the emissions
reduce and the emission maxima shift to 700, 716, and 722
nm, respectively. These data are comparable to those for the
binuclear analogues 1b, 5b, and 6b which display metal-metal
and/or ligand-ligand interactions (see later) and suggest that
Figure 6. Normalized emission spectra of 7 at different concentrations
in acetonitrile at 77 K (λex 350 nm).
3
the solid-state emissions of 1a, 5a, and 6a are [dσ*, π*] in
nature. The red-shift for these solid-state emissions upon
cooling can be rationalized by shortening of intermolecular Pt-
Pt and π-π separations in the crystal lattice, which re-
sults in 3[dσ*, π*] emissions of lower energy. In contrast,
complexes 2a-4a and 7 show a structured band at 298 K with
emission maxima at significantly higher energies (λmax 578-
The emissions of 1a-6a, 7, and 8 at 77 K in frozen
dichloromethane or acetonitrile have been studied (Table 4).
Except for 7, their emissions are insensitive to the complex
concentration in the range 10-6-10-3 M. They are highly
structured, with the most intense vibronic component being ν′
3
) 0 to ν′′ ) 0, which is characteristic of MLCT emissions.
3
603 nm) than the [dσ*, π*] emissions, while a blue shift is
The concentration-dependent emissive behavior of 7 in
acetonitrile at 77 K has been investigated (Figure 6). At low
concentrations (∼10-6 M), vibronic yellow emission with peak
detected at 77 K; 3MLCT excited states are therefore tentatively
assigned.
Self-Quenching and 77 K Frozen Emission. Like planar
aromatic systems, a small number of square planar platinum-
(II) complexes are known to display low-energy excimeric
emission in concentrated fluid solutions20-22 (eq 1 , NkN )
maxima at 522 and 554 nm (vibronic progression 1110 cm-1
)
is observed. Increasing the complex concentration yielded a new
red emission centered at 670 nm at the expense of the yellow
band. Indeed, at complex concentration > 10-3 M, the high-
energy emission is completely replaced by the low-energy band
at 670 nm. We note that the UV-vis absorption spectrum of 7
in acetonitrile at 298 K follows Beer’s law for concentration e
10-3 M; hence, no ground-state oligomerization is evident at
298 K. Emission from solid-state phases in frozen acetonitrile
is discounted because this occurs at λmax 572 and 611 nm at 77
K (Table 5). We suggest that, in frozen acetonitrile solution,
the complex cations of 7 form weakly interacting dimeric pairs,
and the 670 nm band can therefore be ascribed to a 3[dσ*, π*]
excited state.
Binuclear Complexes. Our earlier report on the parent
6-phenyl-2,2′-bipyridine complex [Pt2(L1)2(µ-dppm)]2+ (1b)
assigned the lowest energy absorption band at 420 nm in
acetonitrile to a singlet [dσ* f π*] transition, while the emission
at 652 nm was ascribed to a triplet [dσ*, π*] excited state (the
MMLCT notation was previously used).29 We now proceed to
examine the effects upon the [dσ*, π*] excited state of (A)
different aryl substituents at the 4-position of 6-phenyl-2,2′-
bipyridine and of (B) bidentate ligands with various bridging
lengths and geometry.
[Pt(NkN)(CN)2]* + [Pt(NkN)(CN)2] f
[Pt(NkN)(CN)2]2* (1)
4,7-diphenyl-1,10-phenanthroline, 4,4′-di-tert-butyl-2,2′-bipy-
ridine).
3
Self-quenching of the MLCT emission has been detected
for complexes 1a-6a, 7, and 8 at 298 K in CH2Cl2 (Table 4),
but no excimeric emission with complex concentration up to
10-3 M has been observed. In each case, a linear plot of 1/τ
against complex concentration was produced. The intrinsic
luminescence lifetimes τo range from 0.51 to 1.47 µs, and self-
quenching rate constants kq of 3.2 × 108 (for PPh3 complex 8)
to 4.1 × 109 dm3 mol-1 s-1 are observed. The emission quantum
yields φo of the substituted complexes 2a-6a range from 0.052
to 0.068 and are higher than that for 1a (φo 0.025). This is
reminiscent of studies on phenyl-substituted tpy complexes of
ruthenium, where the increased quantum yield was ascribed to
extended conjugation which stabilizes the emissive MLCT
excited state relative to a radiationless d-d transition.37
(A) Complexes with the µ-dppm Ligand: Effects of the
4-Aryl Substituent. The UV-vis absorption spectra of 2b-
(37) Hecker, C. R.; Gushurst, A. K. I.; McMillin, D. R. Inorg. Chem. 1991,
30, 538.

d8-d8 Interactions in Platinum(II) Complexes
Inorganic Chemistry, Vol. 38, No. 18, 1999 4053
corresponds with the absorption spectral bands, particularly in
the 480-510 nm region (Table 6). In conjunction with the
observed intramolecular Pt-Pt contacts in 3b and 4b, the
3
emissions of 2b-6b are ascribed to [dσ*, π*] excited states
such as for 1b. Self-quenching of the emissions in acetonitrile
is difficult to quantify accurately due to the relatively short
lifetimes. In dichloromethane, self-quenching rate constants of
1.2 × 108 and 2.0 × 108 dm3 mol-1 s-1 for 2b and 5b
respectively were estimated, which are comparable with that
for the mononuclear phosphine derivative 8 (3.2 × 108 dm3
mol-1 s-1).
The solvatochromic behavior of the 3[dσ*, π*] emissions of
1b(ClO4)2 and 5b(ClO4)2 has been studied (Table 8). The
respective emission energy, lifetime, and quantum yield are
highly sensitive to the solvent polarity but insensitive to the
complex concentration. For 5b(ClO4)2, the emission maximum
shifts by 689 cm-1 from dichloromethane to dimethylformamide
(DMF) while the luminescence lifetimes (τo) and quantum yields
(φo) change from 2.45 to 0.30 µs and 0.20 to less than 0.01,
respectively. This is ascribed to greater rates of non-radiative
decay in highly polar solvents such as DMF. Significantly, these
cyclometalated derivatives exhibit improved photophysical
properties compared to the tpy congeners, which show very
weak emission (φo in order of 10-4) in solution at room
temperature.24,25
Figure 7. UV-vis absorption and emission (inset: λex 350 nm, /
denotes instrumental artifact) spectra of 2b in acetonitrile at
298 K.
The solid-state emissions of 1b-6b (Table 9) are red-shifted
in energy as the temperature is lowered to 77 K. This may be
attributed to the shortening of Pt-Pt distances due to lattice
contraction. The emissions of 1b, 5b, and 6b are similar to
that for the mononuclear counterparts (see above), and we
propose that they have the same electronic origin, namely
3[dσ*, π*].
(B) Complexes with the 6-Phenyl-2,2′-bipyridine (L1)
Ligand: Effects of the Bridging Ligand. To influence the
distance and interaction between [Pt(L1)] moieties in a binuclear
molecule, the pyrazolate derivative 9 and the C3 and C5
diphosphine complexes 10 and 11, respectively, were prepared.
Consequences upon the excited-state properties can be probed
by comparisons with the dppm-bridged species 1b.
Figure 8. Time-resolved emission profile for 5b (0.01 mM in
acetonitrile) at 298 K (from top, 120, 200, 300, 400, 500, 600, 700,
800, 900, 1000 ns; λex 350 nm).
Because of longer carbon chains between the phosphorus
atoms, the intramolecular separations between the two [Pt(L1)]
units in 10 and 11 are expected to be greater than in 1b. This
is reflected in their absorption spectra, which closely resembles
that of the mononuclear phosphine congener [Pt(L1)PPh3]+ (8).
No moderately intense absorption is evident above 400 nm like
6b are generally similar to that of 1b (Table 6, see Figure 7 for
2b). All binuclear complexes in this work are photoluminescent
in solution (Table 7). Complexes 2b-6b exhibit structureless
emissions with peak maxima at 654-662 nm in acetonitrile at
room temperature, while at 77 K blue-shifted emissions are
observed at 633-644 nm. These are comparable to the emissive
behavior of 1b. Trends arising from the different nature of aryl
substituents are not apparent. The time-resolved emission
profiles for 2b-6b (Figure 8 for 5b) each show a mono-
exponential decay, thus implying a single emitting species is
present. The 77 K excitation spectrum of 5b in acetonitrile
(monitoring emission at 640 nm; see Supporting Information)
1
the [dσ* f π*] absorption band in 1b; hence, the [Pt(L1)]
fragments in 10 and 11 are proposed to behave as discrete non-
interacting moieties from a spectroscopic viewpoint. In contrast,
the absorption spectrum of complex 9 in acetonitrile at room
temperature displays a moderately intense band at 409 nm (ꢀ
) 2500 dm3 mol-1 cm-1). This low-energy band is absent in
complex 10 and the mononuclear pyridine derivative [Pt(L1)-
py]+ (7), but the intensity of its tail (>500 nm) is noticeably
Table 7. Emission Data for Complexes 1b-6b and 9-11 in Acetonitrile (Complex Concentration 5 × 10-5 M)
complex
298 K data: λmax/nm; τo/µs; φo
77 K data: λmax/nm
[Pt2(L1)2(µ-dppm)](ClO4)2, 1b(ClO4)2
[Pt2(L2)2(µ-dppm)](ClO4)2, 2b(ClO4)2
[Pt2(L3)2(µ-dppm)](ClO4)2, 3b(ClO4)2
[Pt2(L4)2(µ-dppm)](ClO4)2, 4b(ClO4)2
[Pt2(L5)2(µ-dppm)](ClO4)2, 5b(ClO4)2
[Pt2(L6)2(µ-dppm)](ClO4)2, 6b(ClO4)2
[Pt2(L1)2(µ-pz)](ClO4), 9(ClO4)
652; 0.14; 0.015
659; 0.23; 0.016
661; 0.19; 0.009
654; 0.29; 0.024
655; 0.40; 0.025
662; 0.60; 0.018
548; 0.2; 0.003
547; 1.9; 0.048
544; 0.8; 0.020
638
641
644
633
640
644
555 (max), 590, 651
530 (max), 561
532 (max), 565
[Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2
[Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2

4054 Inorganic Chemistry, Vol. 38, No. 18, 1999
Lai et al.
3
Table 8. Solvent Dependence of the [dσ*, π*] Emission of
1b(ClO4)2 and 5b(ClO4)2
λ
max/nm; τo/µs; φo
[Pt2(L1)2(µ-dppm)]2+
,
[Pt2(L5)2(µ-dppm)]2+
,
solvent
1b
5b
dichloromethane
chloroform
acetone
acetonitrile
methanol
640; 2.60; 0.18
643; 2.45; 0.17
657; 0.49; 0.026
652; 0.14; 0.015
654; 0.2; φo < 0.01
non-emissive
645; 2.45; 0.20
647; 2.10; 0.19
657; 0.85; 0.051
655; 0.40; 0.025
653; 0.30; φo < 0.01
675; 0.30; φo < 0.01
dimethylformamide
Table 9. Solid-State Emission Data for Complexes 1b-6b and
9-11
298 K data:
max/nm (τo/µs)
77 K data:
max/nm
complex
λ
λ
[Pt2(L1)2(µ-dppm)](ClO4)2, 1b(ClO4)2
[Pt2(L2)2(µ-dppm)](ClO4)2, 2b(ClO4)2
[Pt2(L3)2(µ-dppm)](ClO4)2, 3b(ClO4)2
[Pt2(L4)2(µ-dppm)](ClO4)2, 4b(ClO4)2
[Pt2(L5)2(µ-dppm)](ClO4)2, 5b(ClO4)2
[Pt2(L6)2(µ-dppm)](ClO4)2, 6b(ClO4)2
[Pt2(L1)2(µ-pz)](ClO4), 9(ClO4)
630 (1.6)
647 (1.6)
643 (1.2)
665 (1.5)
643 (1.4)
670 (1.7)
596 (max),
620 (1.0)
577
640
656
655
668
650
674
580 (max),
624, 685
547, 573
541, 572
Figure 9. Lowest energy UV-vis absorption bands of 1b, 9, and 10
in acetonitrile at 298 K.
[Pt2(L1)2(µ-dppC3)](ClO4)2, 10(ClO4)2
[Pt2(L1)2(µ-dppC5)](ClO4)2, 11(ClO4)2
579
smaller than that for complex 1b in the same spectral region
(Figure 9). For assignment purposes, this absorption band
exhibits characteristics which are transitional between 1MLCT
and 1[dσ* f π*]. This is in accordance with the relatively long
Pt-Pt distance in 9 (3.612(2) Å), which implies virtually no
metal-metal interaction, although the presence of intramolecular
π-π contacts is feasible.
The luminescence properties of 9-11 in acetonitrile solution
have been examined. Their respective emission energy is
independent of complex concentrations from 10-6 to 10-3 M.
The emission spectra of 10 and 11 in acetonitrile at room
temperature show a structureless band centered at ca. 545 nm.
At 77 K, the emissions blue-shift in energy and become
vibronically resolved with spacing of ca. 1100 cm-1. This
emissive behavior is similar to that for 8 and is evidently
Figure 10. Normalized emission spectra of 1b, 9, and 10 in acetonitrile
at 77 K (λex 350 nm).
3
(L1)2(µ-L)]n+, the 298 K solid-state emission maxima for L )
dppm (Pt-Pt ) 3.270(1) Å) and pyrazolate (Pt-Pt ) 3.612(2)
Å) appear at 630 and 596 nm, respectively. It is apparent that
as the (L1)Pt-Pt(L1) separation is increased by lengthening L,
the origin of the excited state changes from 3[dσ*, π*] to
3MLCT and the solid-state emission is displayed at higher
energies (<600 nm). From its transitional luminescent behavior,
the µ-pyrazolate derivative 9 apparently approaches the limit
for metal-metal interaction.
different from the [dσ*, π*] excited state of 1b, which emits
at lower energies without vibronic character. The solution
3
emissions of 10 and 11 are assigned as MLCT in nature.
The pyrazolate-bridged complex 9 emits in acetonitrile at
room temperature with a peak maximum at 548 nm. This
structureless emission is similar to the 3MLCT emissions of 10
and the mononuclear species 7, but the 77 K frozen acetonitrile
emission is strikingly different (Figure 10). A vibronically
structured band is evident with the most intense peak at 555
nm and shoulders at 590 and 651 nm. The room-temperature
emission of a microcrystalline sample of 9 at room temperature
is diffuse, but at 77 K, a well-resolved vibronic structure (peaks
at 580, 624, and 685 nm) is observed. While 9 displays
negligible Pt-Pt contacts at room temperature (as indicated by
the crystal structure), the intramolecular separation between [Pt-
(L1)] units may decrease at 77 K to afford weak π-π
interactions. These would give rise to the low-energy shoulders
in the solution- and solid-state emissions.
Photoredox Properties of [Pt2(L1)2(µ-dppm)](ClO4)2 (1b).
The electro- and photochemical properties of 1b have been
examined. The cyclic voltammogram of 1b reveals two quasi-
reversible one-electron reduction waves [(ipc/ipa) ≈ 1] at -0.54
and -0.77 V vs NHE with 0.1 M nBu4NPF6 in acetonitrile (eqs
2 and 3, respectively). The potentials are independent of scan
Pt22+ + e- f Pt2 E(Pt22+/Pt2 ) ) -0.54 V vs NHE (2)
+
+
Pt2+ + e- f Pt2 E(Pt2 /Pt2) ) -0.77 V vs NHE (3)
+
It is interesting to compare the solid-state emission spectra
of [Pt2(tpy)2(µ-L′)]3+ (L′ ) anionic N-N bridging ligand) at
298 K.24 For example, the emission maxima for L′ ) azaindolate
(Pt-Pt ) 3.13(2) Å) and pyrazolate (Pt-Pt ) 3.432(1) Å)
appear at 690 and 630 nm, respectively; hence, a blue shift is
evident with greater Pt-Pt separations. In our study on [Pt2-
+
+
E(*Pt22+/Pt2 ) ) E(Pt22+/Pt2 ) + E0-0
(4)
rates from 20 to 100 mV/s. The excited-state potential can be
estimated by eq 4 (Pt2 ) [Pt2(L1)2(µ-dppm)]).

d8-d8 Interactions in Platinum(II) Complexes
Inorganic Chemistry, Vol. 38, No. 18, 1999 4055
3
Table 10. Rate Constants for Reductive Quenching of the [dσ*, π*] Emission of 1b (Pt22+) by Organic Donors in Dichloromethane at 20 (
0.2 °C
organic quencher
E(D+/D°)a/V
kq/dm3 mol-1 s-1
kq′/dm3 mol-1 s-1
ln kq′ b
N,N,N′,N′-tetramethyl-p-phenylenediamine
N,N,N′,N′-tetramethylbenzidine
o-phenylenediamine
benzidine
phenothiazine
diethylaniline
diphenylamine
p-chloroaniline
2,4-dichloroaniline
1,4-dimethoxybenzene
1,2,3-trimethoxybenzene
0.35
0.67
0.75
0.79
0.86
0.94
1.07
1.31
1.46
1.58
1.66
1.08 × 1010
1.16 × 1010
4.80 × 109
6.21 × 109
6.65 × 109
4.61 × 109
1.24 × 109
6.18 × 108
6.02 × 106
1.20 × 106
2.06 × 105
2.35 × 1010
2.76 × 1010
6.32 × 109
9.01 × 109
9.96 × 109
5.99 × 109
1.31 × 109
6.38 × 108
6.02 × 106
1.20 × 106
2.06 × 105
23.88
24.04
22.57
22.92
23.02
22.51
21.00
20.27
15.61
14.00
12.24
a Reduction potential versus NHE. b 1/kq′ ) 1/kq - 1/kd, where kd is taken as 2 × 1010 dm3 mol-1 s-1
.
The 0-0 transition energy of Pt22+, E0-0, can be estimated
excited-state properties. The planar geometry of the mononuclear
derivatives facilitates face-to-face interactions, and self-quench-
ing of the emissions have been observed at room temperature
in dichloromethane.
from the overlap of the emission and excitation spectra, which
2+
occurs at 2.15 eV. Thus *Pt22+ is a strong oxidant with E(*Pt2
/
Pt2+) at 1.61 V vs NHE. The excited-state redox potential has
also been estimated through quenching studies with a series of
aromatic hydrocarbons in dichloromethane. The chosen aromatic
hydrocarbons have similar size and electronic structure but
different reduction potentials E(D+/D°). For each quencher, the
Stern-Volmer plot is linear over the range of quencher
concentrations. Analysis of the quenching rate data was
performed by the method of Meyer and co-workers.38 Quenching
rate constants and the reduction potentials of the quenchers are
summarized in Table 10. From the plot of ln kq′ vs E(D+/D°)
(D ) quencher; see Supporting Information), the excited-state
potential E(*Pt22+/Pt2+) is estimated to be 1.63 V vs NHE, which
is in close agreement with the value of 1.61 V calculated from
the spectroscopic and electrochemical data.
On the basis of the structural parameters and spectroscopic
assignments described in this work, it is proposed that metal-
3
metal interactions in this system will yield [dσ*, π*] excited
states which emit above 600 nm. As the Pt-Pt separation
lengthens, the energy of the [dσ*, π*] excited state increases
and the nature of the excited state accordingly switches to
MLCT with emissions below 600 nm. The transitional behavior
of [Pt2(L1)2(µ-pz)]+, from structural and photophysical perspec-
tives, is noteworthy and provides a “reference point” for future
investigations into d8-d8 interactions. Hence a correlation
between (1) the energy of the emission band, (2) the nature of
excited state, and (3) the degree of metal-metal interaction is
established.
Concluding Remarks
Acknowledgments. We are grateful for financial support
from The University of Hong Kong. The work described in this
paper was partially supported by a grant from the Research
Grants Council of the Hong Kong Special Administrative
Region, China [HKU 7298/99P].
The mono- and binuclear complexes reported in this study
exhibit luminescence with relatively large quantum yields in
solution at room temperature. We have shown that cyclometa-
lating tridentate ligands based on 6-phenyl-2,2′-bipyridine offer
favorable photophysical characteristics compared to 2,2′:6′,2′′-
terpyridine in square planar platinum(II) systems. Because of
the ease in modifying the bridging ligands, these complexes
represent a new class of luminophores with potentially tunable
Supporting Information Available: Listings of crystal data, atomic
coordinates, calculated coordinates, anisotropic displacement param-
eters, and bond lengths and angles for 4b(ClO4)2•5H2O and 9(PF6),
excitation spectrum of 5b in acetonitrile at 77 K, and a plot of ln kq′ vs
E(D+/D°). This material is available free of charge via the Internet at
http://pubs.acs.org.
(38) For details of data treatment, see: 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, and references therein.
IC990238S