Synthesis and structural characterization of a new vapochromic Pt(II) complex based on the 1-terpyridyl-2,3,4,5,6-pentaphenylbenzene (TPPPB) ligand

Article abstract of DOI:10.1021/ic700947g

A novel terpyridine ligand containing a pentaphenylphenyl moiety linked to the terpyridyl core (1-terpyridyl-2,3,4,5,6-pentaphenyl-benzene (TPPPB)) has been synthesized in good yield and reacted with Pt(DMSO)₂Cl ₂, to produce the cationic complex [Pt(TPPPB)Cl]Cl (5). 5 was studied structurally and spectroscopically. It is observed to be brightly luminescent in the solid state at room temperature and at 77 K, with no change in λ_em^max. The complex exhibits reversible vapochromic behavior upon exposure to methylene chloride vapors, changing color from red (5-R) to green (5-G). The shift to higher energy in the emission maximum from 654 to 514 nm is the largest vapochromic shift (140 nm) yet reported. The [Pt(TPPPB)Cl]Cl complex exhibits high selectivity for certain volatile organic compounds (VOCs) including methylene chloride, ethanol, ethyl acetate, and acetonitrile. The crystal structures of both the green and red forms have been determined by single-crystal X-ray diffraction. In both forms, the cationic Pt(II) complex possesses the anticipated square-planar coordination geometry that is distorted as a consequence of constraints from the terpyridyl binding. Analysis of the crystal packing of the green form (5-G) reveals the presence of non-interacting Pt...Pt separations with distances of 3.9092(9) and 4.5483(11) A and a zigzag arrangement between neighboring Pt(II) ions. The red form (5-R) contains complexes that are stacked with Pt...Pt separations of 3.2981(14) and 3.3427(14) A, indicative of metallophilic interaction. The change in the emitting state, as a consequence of the effect of the volatile organic compounds, results from a disruption of the d ⁸-d⁸ metallophilic interactions in the red form and its metal-metal-to-ligand charge transfer (MMLCT) excited state to a more-localized Pt(dπ)-to-tpy(π*) metal-to-ligand charge transfer (MLCT) excited state in the green form.

Full text of DOI:10.1021/ic700947g

Inorg. Chem. 2008, 47, 69−77
Synthesis and Structural Characterization of a New Vapochromic Pt(II)
Complex Based on the 1-Terpyridyl-2,3,4,5,6-pentaphenylbenzene
(TPPPB) Ligand
Pingwu Du, Jacob Schneider, William W. Brennessel, and Richard Eisenberg*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received May 16, 2007
A novel terpyridine ligand containing a pentaphenylphenyl moiety linked to the terpyridyl core (1-terpyridyl-2,3,4,5,6-
pentaphenyl-benzene (TPPPB)) has been synthesized in good yield and reacted with Pt(DMSO)2Cl2, to produce
the cationic complex [Pt(TPPPB)Cl]Cl (5). 5 was studied structurally and spectroscopically. It is observed to be
brightly luminescent in the solid state at room temperature and at 77 K, with no change in λemmax. The complex
exhibits reversible vapochromic behavior upon exposure to methylene chloride vapors, changing color from red
(5-R) to green (5-G). The shift to higher energy in the emission maximum from 654 to 514 nm is the largest
vapochromic shift (140 nm) yet reported. The [Pt(TPPPB)Cl]Cl complex exhibits high selectivity for certain volatile
organic compounds (VOCs) including methylene chloride, ethanol, ethyl acetate, and acetonitrile. The crystal structures
of both the green and red forms have been determined by single-crystal X-ray diffraction. In both forms, the cationic
Pt(II) complex possesses the anticipated square-planar coordination geometry that is distorted as a consequence
of constraints from the terpyridyl binding. Analysis of the crystal packing of the green form (5-G) reveals the
presence of non-interacting Pt‚‚‚Pt separations with distances of 3.9092(9) and 4.5483(11) Å and a zigzag
arrangement between neighboring Pt(II) ions. The red form (5-R) contains complexes that are stacked with Pt‚‚‚Pt
separations of 3.2981(14) and 3.3427(14) Å, indicative of metallophilic interaction. The change in the emitting
state, as a consequence of the effect of the volatile organic compounds, results from a disruption of the d8
metallophilic interactions in the red form and its metal metal-to-ligand charge transfer (MMLCT) excited state to a
more-localized Pt(d )-to-tpy( *) metal-to-ligand charge transfer (MLCT) excited state in the green form.
−
d8
−
π
π
Introduction
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Published on Web 12/05/2007
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 1, 2008 69

Du et al.
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[Pt(Me2bzimpy)Cl]X (Me2bzimpy ) 2,6-bis-(N-methylben-
zimidazol-2-yl)pyridine; X ) Cl, PF6), which gave a
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[Pt(Nttpy)Cl](PF6)2 (Nttpy ) 4′-(p-nicotinamide-N-meth-
ylphenyl-2,2′:6′,2′′-terpyridine) that showed a change in
absorption and emission energies upon exposure to methanol
and acetonitrile vapors. The structures of both forms were
determined from the same crystal before and after exposure
to MeOH vapor, confirming the change in Pt‚‚‚Pt separations
as key to the spectroscopic changes. A case of (4) can be
found in the recent report by Che et al.,44 who describe
binuclear Pt(II) systems [(R-C∧N∧C)2Pt2(µ-dppm)] (R) H,
Ph, BrC6H5, 3,5-F2C6H3) that have intramolecular Pt‚‚‚Pt
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diffusion of chloroform into one of these samples in the solid
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The present article reports the synthesis, structural char-
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reversible vapochromic response toward a limited number
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70 Inorganic Chemistry, Vol. 47, No. 1, 2008

New Vapochromic Pt(II) Complex
[Pt(TPPPB)Cl]Cl (5). 1-Terpyridyl-2,3,4,5,6-pentaphenyl-ben-
zene (TPPPB) (0.10 g, 0.15mmol) and Pt(DMSO)2Cl2 (0.061 g,
0.15mmol) were refluxed in 15 mL methanol for 24 h. The product
precipitated from solution upon the addition of 15 mL diethyl ether
and was isolated by filtration, followed by washing with cold
ethanol and diethyl ether to yield a red solid denoted as 5-R (0.116
structural characterization of both the red and green forms
of the title complex provide interesting details on the basis
of the vapochromic effect in this system. The results show
that the crystalline state possesses significant flexibility to
allow expansion and contraction of the lattice with resultant
slippage between neighboring stacked planar complexes that
affects the metallophilic interactions between them and
consequent emission behavior.
1
g, yield, 75%). H NMR (DMSO-d6): 8.78-8.72 (2H, d), 8.70-
8.60 (2H, d), 8.50 (2H,s), 8.10-8.00 (2H, t), 7.70-7.65 (2H, m),
7.60-7.45 (5H, m). 13C NMR (DMSO-d6) at 25 °C: δ 158.0, 155.8,
152.9, 152.0, 143.4, 142.6, 141.1, 139.7, 139.6, 139.4, 138.7, 136.6,
131.3, 131.0, 129.8, 127.8, 127.4, 127.3, 127.0, 126.9, 126.4, 125.5.
ESI-MS calcd for M+ 919.2, Found 920.2 (MH+). Anal. Calcd
(C51H35Cl2N3Pt): C, 64.09; H, 3.69; N, 4.40; Found: C. 63.96, H
3.85, N. 4.83.
Green crystals suitable for single-crystal X-ray diffraction were
grown by the slow evaporation of a concentrated solution of 5
dissolved in methylene chloride at ambient temperature. The green
form of 5 thus obtained is denoted as 5-G. Anal. Calcd (C51H35N3-
Cl2Pt‚CH2Cl2): C, 60.01; H, 3.58; N, 4.04. Found: C, 60.13; H,
3.55; N, 4.09.
Experimental Section
1
Characterization. H NMR (400.1 MHz) and 13C NMR (75
MHz) spectra were recorded on a Bruker Avance-400 spectrometer.
Mass determinations were carried out by electrospray ionization
mass spectrometry (ESI-MS) using a Hewlett-Packard Series 1100
mass spectrometer (Model A) equipped with a quadrupole mass
filter. Absorption spectra were recorded using a Hitachi U2000
scanning spectrophotometer (200-1100 nm), and elemental analy-
ses were determined by Desert Analytics.
Luminescence. Luminescence spectra were obtained using a
Spex Fluoromax-P fluorimeter corrected for the spectral sensitivity
of the photomultiplier tube and the spectral output of the lamp with
monochromators positioned for a 2 nm band-pass. Solution samples
were degassed by three freeze-pump-thaw cycles, and frozen glass
samples (1:4 (v/v) MeOH:EtOH) were prepared in NMR tubes
placed in a quartz-tipped immersion dewar filled with liquid
nitrogen. Solid-state emission samples were prepared as a 4%
mixture (w/w) of the red form of [Pt(TPPPB)Cl]Cl in a matrix of
finely ground KBr contained in resealable J. Young tubes and
purged with argon. Solvent vapors for vapochromism studies were
generated by vigorously bubbling argon through a sealed 100 mL
Erlenmeyer flask containing the anhydrous solvent, and the vapors
were transferred to the sample tube via cannula. Total exposure
time of the sample to different VOC vapors was 30 min, and the
removal of the vapors was accomplished by exposing the sample
to the air or gently heating the sample under vacuum.
X-ray Structural Determinations of the Green (5-G) and Red
(5-R) Forms of [Pt(TPPPB)Cl]Cl. A crystal of 5-G (0.50 × 0.08
× 0.02 mm3) was placed onto the tip of a 0.1 mm diameter glass
capillary tube or fiber and mounted on a Bruker SMART APEX II
CCD Platform diffractometer for data collection at 100.0(1) K. A
preliminary set of cell constants and an orientation matrix were
calculated from 296 reflections harvested from 3 sets of 20 frames.
These initial sets of frames were oriented such that orthogonal
wedges of reciprocal space were surveyed. Data collection was
carried out using Mo KR radiation (graphite monochromated) with
a frame time of 90 s and a detector distance of 4.98 cm. Crystals
of 5-G undergo solvent loss at ambient temperature, with color
changes from green to red (5-R), but the red crystals thus obtained
were unsuitable for crystallography.
Crystals of 5-R suitable for single-crystal X-ray diffraction were
grown by the slow diffusion of diethyl ether into a concentrated
solution of 5 dissolved in methanol at ambient temperature. A red
crystal (0.28 × 0.12 × 0.04 mm3) was placed onto the tip of a 0.1
mm diameter glass capillary tube or fiber and mounted on a Bruker
SMART APEX II CCD Platform diffractometer for data collection
at 100.0(1) K. A preliminary set of cell constants and an orientation
matrix were calculated from 578 reflections harvested from 3 sets
of 20 frames. These initial sets of frames were oriented such that
orthogonal wedges of reciprocal space were surveyed.
The space groups were assigned as C2/c and P2/n for 5-G and
5-R, respectively. Both structures were solved by direct methods
and refined employing full-matrix least-squares on F 2 (Bruker-
AXS, SHELXTL-NT, version 5.10). For 5-G, all of the non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. All of the hydrogen atoms were placed in ideal positions
and refined as riding atoms with relative isotropic displacement
parameters. Reflection contributions from highly disordered solvent
molecules (methanol and/or dichloromethane) were removed using
program PLATON, function SQUEEZE, which determined there
to be 812 electrons in a volume of 3099 Å removed. The final
full-matrix least-squares refinement converged to R1 ) 0.0557 (F 2,
I > 2σ(I)) and wR2 ) 0.1268 (F 2, all data).
For 5-R, only the platinum and chlorine atoms were refined with
anisotropic displacement parameters; all of the other atoms were
refined with isotropic displacement parameters. All of the hydrogen
atoms were placed in idealized positions and refined as riding atoms
with relative isotropic displacement parameters. The final full-matrix
least-squares refinement converged to R1 ) 0.1026 (F 2, I >
Syntheses of New Compounds 4-(Phenylethynyl)-2,2′:6′,2′′-
Terpyridine (2). A solution of 4′-[[(trifluoromethyl)sulfonyl]oxy]-
2,2′:6′,2′′-terpyridine45 (0.5 g, 1.31 mmol), Pd(PPh3)4 (0.75 g, 0.065
mmol), and phenylacetylene (0.17 g, 1.62 mmol) in toluene (10
mL) containing triethylamine (15 mL) was heated at 100 °C for
24 h under a dry dinitrogen atmosphere. The solution was allowed
to cool to room temperature, followed by the removal of the organic
solvents under reduced pressure. The crude product was recrystal-
lized from methanol, yielding an off-white solid (0.33 g, 74%). 1H
NMR (DMSO-d6): δ 8.78-8.72 (2H, d), 8.70-8.60 (2H, d), 8.50
(2H, s), 8.10-8.00 (2H, t), 7.70-7.65 (2H, m), 7.60-7.45 (5H,
m). ESI-MS calcd for M+ 333.1, found 334.1 (MH+).
1-Terpyridyl-2,3,4,5,6-pentaphenyl-benzene (TPPPB, 4). 4-(Phe-
nylethynyl)-2,2′:6′,2′′-terpyridine (0.20 g, 0.60mmol) and tetraphe-
nylcyclopentadienone (0.23 mg, 0.60mmol) were heated to reflux
in diphenyl ether (3 g) overnight. The dark-red solution was
evaporated under reduced pressure. The crude product was washed
1
by methanol to give an off-white solid (0.30 g, 69%). H NMR
(DMSO-d6): δ 8.63-8.60 (2H, d), 8.38-8.35 (2H, d), 8.00 (2H,
s), 7.88-7.84 (2H, t), 7.41-7.37 (2H, m), 7.00-6.75 (23H, m),
6.72-6.68 (2H, m). 13C NMR (DMSO-d6) at 25 °C: δ 156.1, 153.7,
150.0, 140.9, 140.5, 140.3, 140.0, 139.8, 136.4, 131.3, 131.2, 126.8,
126.6, 125.6, 125.3, 123.9, 123.3, 120.8. ESI-MS Calcd for M+
689.3, Found 690.3 (MH+).
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Inorganic Chemistry, Vol. 47, No. 1, 2008 71

Du et al.
Scheme 1
2σ(I)) and wR2 ) 0.2671 (F 2, all data). There are very-large
channels parallel to the b axis that contain methanol solvent
molecules and chloride counterions. These areas are highly
disordered to the extent that specific chloride sites could not be
found from the difference Fourier map. Reflection contributions
from species in the channels were removed using program PLATON,
function SQUEEZE, which determined there to be a total of 2372
electrons in a volume of 7707 Å per unit cell removed (six channels
per unit cell). The Cl- counterions that were removed are accounted
for in the chemical formula and calculations that derive therefrom.
The data were very weak due to the high degree of disorder. This
structural determination is intended for the demonstration of packing
and to provide an estimate of intermolecular stacking distances.
Figure 1. Top, absorption spectrum of 5 in CH2Cl2 (concentration: 5.0
× 10-5 M); bottom, diffuse reflectance spectra of 5-R (red) and 5-G (green)
in the solid state at ambient room temperature.
into a clear-orange solution, and the product (5) can be
isolated by simple filtration followed by the addition of a
large amount of ether into the flask to give a red precipitate
denoted as 5-R in >75% yield. As discussed later, it was
also observed that a green solid-state form of the complex,
denoted 5-G, could be obtained upon exposure of 5-R to
vapors of methylene chloride and several other VOCs.
Results and Discussion
Synthesis of New Compounds: Ligand TPPPB, 4, and
Complex [Pt(TPPPB)Cl]Cl, 5. The synthetic sequence
leading to the formation of the title complex 5 is outlined in
Scheme 1 and commences with the formation of 4-(phenyl-
ethynyl)-2,2′:6′,2′′-terpyridine through a Sonogoshira-cata-
lyzed coupling using Pd(PPh3)4 and the subsequent reaction
of that product with tetraphenylcyclopentadienone in reflux-
ing diphenyl ether overnight by an yne-diene Diels-Alder
reaction to give ligand 4, which has been prepared previously
by another procedure.46 The insolubility of terpyridine
derivative 1 in MeOH made its purification easy by continued
washing of the solid until it became off-white in color. All
of the reactions in the scheme involved no chromatography,
making the procedure straightforward and time-saving for
the synthesis of both ligand 4 and complex 5. Whereas
MeOH is a poor solvent for 4, it was still used as the solvent
for producing 5, with the latter having unexpectedly better
solubility. The reaction is finished when the suspension turns
5 is observed to be soluble, to differing extents, in a
number of organic solvents including MeOH, CH3CN, CH2-
Cl2, CHCl3, DMF, and DMSO and was characterized by 1H
NMR, 13C NMR, 2D-COSY, ESI-MS, and elemental analy-
sis. The spectroscopic data are consistent with a cationic Pt-
(II) complex having a square-planar geometry with three of
the four coordination sites occupied by the (pentaphenyl)-
phenyl-modified terpyridyl ligand and the fourth site oc-
cupied by chloride.
Absorption and Emission Spectroscopy. The UV-vis
absorption spectrum of 5 measured at room temperature in
methylene chloride is shown in Figure 1. It exhibits a broad
low-energy absorption between 370 and 460 nm with λmax
at 415 nm (ꢀ ) ∼7740 dm3 mol-1 cm-1), which corresponds
to the dπ(Pt)-π(terpy) metal-to-ligand charge transfer
(MLCT) transition based on previous assignments made for
(46) Ito, S.; Wehmeier, M.; Brand, D. J.; Ku¨bel, C.; Epsch, R.; Rabe, J.
P.; Mu¨llen, K. Chem.sEur. J. 2000, 6, 4327-4342.
72 Inorganic Chemistry, Vol. 47, No. 1, 2008

New Vapochromic Pt(II) Complex
Figure 3. Emission spectra of 5-R and 5-G at 298 K in the solid state
(4% in KBr).
Figure 2. Emission spectrum of 5 at RT in CH2Cl2 (blue line, concentra-
tion: 1.0 × 10-5 M) and at 77 K in EtOH/MeOH (4:1) (red line,
concentration: 1.0 × 10-5 M).
this class of complexes.12,47-49 The absorption bands in the
range of 280-370 nm correspond to spin-allowed intraligand
(π-π*) transitions (ꢀ > 2 × 104 dm3 mol-1 cm-1) with
maxima in the range of 335-340 nm. Also shown in Figure
1 are the diffuse reflectance spectra of the two solid-state
forms of the complex, 5-R and 5-G, at ambient room
temperature. The most-salient difference between these two
spectra is the broad absorption of 5-R that extends to nearly
600 nm, which accounts for the observed color difference.
Unlike [Pt(terpy)Cl]Cl,12,50 5 is emissive in solution at
room temperature as shown in Figure 2, but its emission is
not as intense as [Pt(ttpy)(CtC-p-tol)]+.51,52 The emission
spectrum was investigated further in methylene chloride at
room temperature. When the complex is excited at 420 nm,
the spectrum shows a broad emission band from 485 to 685
nm, with a maximum emission peak at 526 nm and a broad
shoulder around 560 nm, giving a luminescence quantum
yield (φ) of 0.0025 at λemmax, based on [Ru(bpy)3](PF6)2 (φ
) 0.062).53 On the basis of previous photophysical studies
of closely related [Pt(terpyridyl)(acetylide)]+ complexes, the
excited state that gives rise to the observed emission is a
triplet metal-to-ligand charge transfer (3MLCT) involving a
dπ metal orbital as the HOMO and a π*(TPPPB) orbital as
the LUMO. Upon cooling to 77 K in a 4:1 ethanol/methanol
glass, the emission becomes much more intense (Figure 2).
The structured emission between 475 and 750 nm contains
Figure 4. Emission spectra of 5-R at different temperatures in the solid
state (4% in KBr).
multiple components with maxima at 496, 509, and 536 nm
and also two shoulders at ca. 572 and 657 nm.
The two solid-state forms of 5 are brightly luminescent
in the solid state at room temperature (RT) (Figure 3).
Whereas the solid-state emission profile of 5-G is similar to
that of 5 in fluid solution, the emission spectrum of 5-R is
vastly different. The emission spectrum of 5-G excited at
420 nm extends from 480 to 720 nm with a λemmax centered
at 514 nm and two shoulders at ca. 550 and 591 nm that can
be ascribed to vibronic structuring. On the other hand,
excitation at 420 nm of a solid sample of 5-R at RT leads to
a broad emission centered at 654 nm, with a shoulder at 630
nm. The difference between emission maxima for 5-G and
5-R is substantial and can be attributed to a change in the
charge-transfer excited state between the two forms, as
discussed below. This difference also serves as the basis of
the vapochromism of this complex.
(47) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer,
W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591-4599.
(48) Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193-2200.
(49) Michalec, J. F.; Bejune, S. A.; McMillin, D. R. Inorg. Chem. 2000,
39, 2708-2709.
When the temperature is lowered to 77 K, the energies of
the emission maxima for both 5-R and 5-G remain un-
changed, whereas the emission intensities increase ap-
proximately 10-fold for 5-R and 12-fold for 5-G as a result
of lower rates of thermally activated nonradiative decay
(Figures 4 and 5). The vibronic structuring in the spectrum
of 5-G becomes more pronounced at low temperatures, with
(50) Buchner, R.; Field, J. S.; Haines, R. J.; Cunningham, C. T.; McMillin,
D. R. Inorg. Chem. 1997, 36, 3952-3956.
(51) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg.
Chem. 2002, 41, 5653-5655.
(52) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc.
2006, 128, 7726-7727.
(53) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D.;
Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620-6627.
Inorganic Chemistry, Vol. 47, No. 1, 2008 73

Du et al.
Table 1. Crystallographic, Data Collection, and Structure Refinement
Parameters for Both Forms of [Pt(TPPPB)Cl]Cl (5-G and 5-R)
form
green (5-G)
red (5-R)
formula
[PtCl(Ph5Ph-terpy)]
[Cl]‚CH2Cl2
C51 H35 Cl2 N3 Pt
[PtCl(Ph5Ph-terpy)]
[Cl]‚xCH3OH
C51 H35 Cl2 N3 Pt
955.81
emperical formula
fw
955.81
T (K)
100.0(1)
100.0(1)
wavelength (Å)
cryst syst
space group
Z
0.71073
monoclinic
C2/c
0.71073
monoclinic
P2/n
12
24
a (Å)
b (Å)
c (Å)
13.366(3)
45.731(10)
24.151(5)
90, 103.167(3), 90
14 374(5)
1.325
29.850(9)
21.834(7)
44.023(13)
90, 93.795(4), 90
28 628(15)
1.331
R, â, γ (deg)
V (Å3)
F
calcd (Mg/m3)
µ (mm-1
)
3.074
3.087
cryst size (mm3)
F(000)
0.50 × 0.08 × 0.02
0.28 × 0.12 × 0.04
Figure 5. Emission spectra of 5-G at different temperatures in the solid
state (4% in KBr).
5688
11 376
2θ range (deg)
1.63 to 27.10
-17 e h e 17
-58 e k e 58
-30 e l e 30
91 641
15 854 / 12 / 789
0.968
0.0557, 0.1141
0.1151, 0.1268
1.32 to 25.00
-35 e h e 35
-25 e k e 25
-52 e l e 52
249 515
50 150 / 2 / 1399
0.977
0.1026, 0.2334
0.1991, 0.2671
a separation between maxima of ca. 1273 cm-1, correspond-
ing to an aromatic vibrational mode of the terpyridyl
ligands.51,54
limiting indicies
no. of reflns collected
no. of data / rest / params
GOFa
The absence of a shift in the solid-state emission maxima
of 5-R and 5-G upon lowering the temperature stands in
contrast to what has been observed previously for related
systems.38,44,47,50 In [Pt(Nttpy)Cl],38 a red-shift upon cooling
of the solid-state sample was shown to correlate with lattice
contraction and the decrease of the Pt‚‚‚Pt separation,
whereas a similar red-shift for [(RC∧N∧C)Pt] seen by Che
and co-workers44 was ascribed to excimeric ππ* excited
states and increasing interactions at low temperatures. The
gold complex {Tl[Au(C6Cl5)2]}n37 has also been reported to
exhibit a shift to lower energy for the emission maximum
when the temperature is lowered from 298 to 77 K, with the
red-shift resulting from the shortened Au-Tl distance at low
temperatures. In the present case, the absence of red-shifts
in the emission energies for 5-R and 5-G as the temperature
is lowered relates to the bulkiness and propeller-like nature
of the pentaphenylphenyl moiety (vide infra) and the resultant
inability of the cationic complexes to stack more tightly with
reduced Pt‚‚‚Pt distances.
Vapochromic Behavior of [Pt(TPPPB)Cl]Cl, 5. Vapo-
chromism for 5 is seen upon the diffusion of methylene
chloride into solid [Pt(TPPPB)Cl]Cl, as the original red color
of the solid (5-R) changes to green (5-G). In going from
5-R to 5-G, the λemmax shifts from 654 to 514 nm, a difference
of 140 nm, corresponding to the largest vapochromic
response shift that, to our knowledge, has been seen in
systems of this type with VOCs. The conversion of 5-R to
5-G is entirely reversible, and the response is relatively rapid.
The time for the red-to-green color change upon exposure
to dichloromethane vapor is less than 1 min, whereas the
return to red emission after vapor exposure is stopped takes
several minutes in air at ambient temperature.
R1, wR2 (F 2, I > 2σ(I))b
R1, wR2 (F 2, all data)b
a GOF ) S ) [∑w(Fo - Fc )2/(m - n)]1/2, where m ) number of
reflections and n ) number of parameters. b R1 ) ∑||Fo| - |Fc||/∑|Fo|.
2
2
wR2 ) [∑w(Fo2 - Fc )2/∑w(Fo )2]1/2, where w ) 1/[σ2(Fo)2 + (aP) + bP]
2
2
1
2
2
and P ) /3 max(0, Fo ) + [2/3] Fc .
although the response times for these VOCs was distinctly
longer than that seen with CH2Cl2. The vapochromic response
of 5-R to each of these VOCs is shown in the Supporting
Information. They are all similar to what is seen with
dichloromethane in Figure 3 but with slight differences of
emission maximum (496, 506, and 498 nm for EtOH, MeCN,
and EtOAc, respectively) and modest differences in the
degree of vibronic structuring in the spectrum of the resultant
green form.
No detectable change of emission energy was found in
the presence of benzene, toluene, n-hexane, acetone, hexane,
diethyl ether, chloroform, petroleum ether, pentane, tetrahy-
drofuran, isopropyl alcohol, and methanol vapors. However,
when dichloromethane was mixed with these solvents and
5-R was brought into contact with the resultant vapors,
conversion to 5-G was observed, supporting the notion of
great selectivity for the sorption of dichloromethane in the
presence of other VOCs.
Crystal Structures of [Pt(TPPPB)Cl]Cl, 5. Crystal
structures of both the red and green forms of 5 have been
determined by X-ray diffraction, and crystallographic data
for both 5-R and 5-G are given in Table 1. Crystals of 5-R
were obtained by slow diffusion of diethyl ether into a
concentrated solution of the complex in methanol, whereas
those of 5-G were obtained by the slow evaporation of a
dichloromethane solution of 5. Because of poor diffracting
ability, possibly as the result of the extensive solvent and
anion disorder in the crystal, the structure determination of
5-R is unsatisfactory for a detailed metrical analysis, but it
does provide accurate connectivity and packing information
The vapochromism of 5 appears to be relatively selective.
The only other VOCs that evoked the response seen with
dichloromethane are ethanol, acetonitrile, and ethyl acetate,
(54) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476-4482.
74 Inorganic Chemistry, Vol. 47, No. 1, 2008

New Vapochromic Pt(II) Complex
Table 2. Selected Bond Lengths (Angstroms) and Angles (Degrees) for
Complex 5-G and 5-R
Form
5-G
5-R
Pt(1)-N(1)
Pt(1)-N(2)
Pt(1)-N(3)
Pt(1)-Cl(1)
N(2)-C(6)
N(3)-C(11)
N(3)-C(15)
N(1)-C(1)
N(1)-C(5)
C(1)-C(2)
C(8)-C(16)
C(5)-C(6)
C(10)-C(11)
C(6)-C(7)
C(9)-C(10)
C(11)-C(12)
2.031(7)
1.900(7)
2.024(7)
2.280(2)
1.374(10)
1.387(10)
1.350(10)
1.299(9)
1.355(9)
1.390(11)
1.514(10)
1.473(10)
1.500(11)
1.401(10)
1.355(10)
1.352(11)
2.004(15)
1.913(14)
2.023(15)
2.294(4)
1.36(2)
1.38(2)
1.32(2)
1.37(2)
1.37(2)
1.35(3)
1.51(2)
1.37(2)
1.46(2)
1.40(2)
1.39(2)
1.35(2)
Figure 6. The structure of the [PtCl(TPPPB)]+ cation in 5-G.
as well as valid bond distances and angles involving the
heavy atoms in the structure. Most importantly, Pt‚‚‚Pt
separations and the stacking of [Pt(TPPPB)Cl]+ cations in
5-R are accurately determined.
N(1)-Pt(1)-N(2)
N(2)-Pt(1)-N(3)
N(1)-Pt(1)-N(3)
N(1)-Pt(1)-Cl(1)
N(2)-Pt(1)-Cl(1)
N(3)-Pt(1)-Cl(1)
C(1)-N(1)-C(5)
C(1)-N(1)-Pt(1)
C(6)-N(2)-Pt(1)
C(10)-N(2)-Pt(1)
C(11)-N(3)-Pt(1)
C(15)-N(3)-Pt(1)
82.3(3)
79.6(3)
161.9(3)
98.6(2)
179.0(2)
99.5(2)
120.2(8)
126.5(6)
116.1(6)
122.5(6)
113.7(6)
127.4(6)
80.4(6)
82.1(6)
162.4(6)
98.8(4)
177.5(5)
98.6(4)
118.6(16)
130.2(13)
118.1(11)
118.0(11)
111.9(12)
130.7(14)
The molecular structure of cationic complex [Pt(TPPPB)-
Cl]+ obtained from the determination of 5-G is shown in
Figure 6. The distortion in the square-planar geometry
occasioned by the constraints of the terpyridyl ligand is
evidenced by a N(1)-Pt-N(3) trans angle of 161.9(3)° and
Pt-N distances that range from 1.900(7) to 2.031(7) Å, with
the central nitrogen having the shortest distance. The Pt-Cl
bond distance is 2.280(2) Å. The other bond distances and
angles are similar to corresponding metrical parameters
reported for other Pt(II) terpyridyl complexes.38,54-57,58 Table
2 lists important bond lengths and angles for both the green
(5-G) and red (5-R) crystalline forms.
molecules of methylene chloride cannot be found in the
crystal structure of 5-G, but it is reasonable to assume that
they reside in cavities between the stacks of the Pt(II) cationic
complexes. The presence of methylene chloride has been
confirmed in the 1H NMR spectrum of dissolved crystals of
5-G in DMSO-d6 that shows ca. 0.7 molecules of methylene
chloride per complex.
Figure 7, which shows the packing arrangements for both
5-G and 5-R, reveals that, for 5-G, the cationic Pt(II)
complexes are stacked in a head-to-tail orientation, with two
alternating distances between adjacent complexes. Despite
perpendicular distances of 3.33 and 3.40 Å between neigh-
boring terpyridyl ligands in 5-G, the minimal direct overlap
of terpyridyl carbon atoms when viewed down the stacking
direction (part B of Figure 7) indicates small-to-negligible
π-π interactions, whereas the Pt‚‚‚Pt distances of 3.9092-
(9) and 4.5483(11) Å and Pt‚‚‚Pt‚‚‚Pt angles of 153.4 and
167.5° give evidence of no significant metallophilic interac-
tions in the crystal. The distances and nonlinearity of the
cation stacking arrangement do not allow effective overlap
The cationic packing of 5-R is very different from that of
5-G. As illustrated in part C of Figure 7, the stacking of the
planar complexes in 5-R has an irregular spiral arrangement,
with each complex rotated by ca. 120° along the stacking
direction. The structure is unusual in having six crystallo-
graphically independent complexes in the asymmetric unit,
meaning that the asymmetric unit comprises two full turns
along the stacking (helical) direction. Of the independent
Pt‚‚‚Pt separations, four are beyond the range considered
indicative of metallophilic bonding but two are not. These
short Pt‚‚‚Pt separations are 3.30 and 3.34 Å, and the
arrangement of the complexes containing these Pt(II) ions
2
of nearest-neighbor platinum dz and pz orbitals that are the
basis of metallophilic interactions.10,12,17,32,57,59-67 Solvent
2
suggests that metallophilic bonding through dz and pz
overlaps is occurring. Part D of Figure 7 shows a view of
three complexes along the stacking direction - the first two
have a short Pt‚‚‚Pt separation with an eclipsing of the
platinum atoms, whereas the third is displaced from the
(55) Yang, Q.-Z.; Tong, Q.-X.; Wu, L.-Z.; Wu, Z.-X.; Zhang, L.-P.; Tung,
C.-H. Eur. J. Inorg. Chem. 2004, 1948-1954.
(56) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Ko, C.-C.; Cheung,
K.-K. Inorg. Chem. 2001, 40, 571-574.
(57) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N.-Y. J. Am. Chem. Soc. 2002,
124, 6506-6507.
(58) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Inorg. Chem.
1999, 38, 4262-4267.
(59) Roundhill, D. M.; Gray, H. B.; Che, C.-M. Acc. Chem. Res. 22, 55-
61.
(60) Yip, H. K.; Che, C. M.; Zhou, Z. Y.; Mak, T. C. W. J. Chem. Soc.,
Chem. Commun. 1992, 1369.
(61) Miskowski, V. M.; Houlding, V. H.; Che, C.-M.; Wang, Y. Inorg.
Chem. 1993, 32, 2518-2524.
(62) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1989, 28, 1529-
1533.
(63) Biedermann, J.; Gliemann, G.; Klement, U.; Range, K. J.; Zabel, M.
Inorg. Chem. 1990, 31, 4874.
(64) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1993, 32,
369-370.
(65) Chan, C.-W.; Lai, T. F.; Che, C.-M.; Peng, S.-M. J. Am. Chem. Soc.
1993, 115, 11245-11253.
(66) Liu, H. Q.; Peng, S. M.; Che, C.-M. J. Chem. Soc., Chem. Comm.
1995, 509-510.
(67) Liu, H. Q.; Cheung, T. C.; Che, C.-M. Chem. Comm. 1996, 1039-
1040.
Inorganic Chemistry, Vol. 47, No. 1, 2008 75

Du et al.
Figure 7. Comparison of the molecular stacking diagram in 5-G (from side, A; from top, B) and 5-R (from side, C; from top, D). Dashed lines show the
Pt‚‚‚Pt interactions in the 5-R structure. The pentaphenyl substituents have been removed for clarity in the structure.
stacking direction and has no attractive interactions with
neighboring complexes.
absent, leading to an excited-state emission that is essentially
the same as that seen in fluid solutions of 5 - notably,
3MLCT (dπ(Pt)-π*(TPPPB)) without perturbation from
metallophilic interactions. It is interesting to note that in 5-R
some of the complexes do not exhibit Pt‚‚‚Pt interactions,
suggesting the possibility of a dual emission from both
Basis of the Vapochromism of [Pt(TPPPB)Cl]Cl, 5. The
structural and crystal packing data for the two solid-state
forms of 5 provide a basis on which to interpret the observed
vapochromic behavior. For some of the complexes in 5-R,
metallophilic interactions lead to a change in the nature of
the HOMO and a consequent red-shift of the emission.
Specifically, the HOMO for the metallophilicly interacting
3
3MMLCT and MLCT excited states from 5-R. However,
this is not the case, and a possible explanation is that rapid,
efficient, and energetically favorable energy transfer occurs
3
3
2
complexes is the σ* combination of overlapping dz orbitals
in 5-R from MLCT to MMLCT, so that only the latter
emission is observed.
and is thus delocalized over two platinum centers. The energy
2
of this HOMO is greater than the energy of the dz orbital in
Electrochemistry of [Pt(TPPPB)Cl]Cl, 5. A cyclic vol-
tammogram of 5 is provided in the Supporting Information.
The complex in DMF shows two reversible reduction waves
at -0.59 and -1.18 V (estimated vs NHE using ferrocene
as an internal standard with E°(Fc+/0) ) +0.45 V vs SCE68
and SCE vs NHE ) +0.24 V, with all of the scans done at
100 mV per second) and no oxidations within the solvent
an isolated molecule of 5, leading to a red-shift of the excited-
state energy for the system. Because of the metal-metal
nature of the HOMO, the emissive excited state in 5-R is
3
designated as MMLCT.
Upon exposure to methylene chloride vapor, the packing
arrangement and Pt‚‚‚Pt interactions in 5-R are disrupted with
a consequent change in absorption and emission color. The
structure of 5-G shows that Pt‚‚‚Pt interactions are essentially
(68) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
76 Inorganic Chemistry, Vol. 47, No. 1, 2008

New Vapochromic Pt(II) Complex
Conclusions
The new vapochromic Pt(II) terpyridyl complex [Pt-
(TPPPB)Cl]Cl (5) has been synthesized, structurally char-
acterized, and studied spectroscopically. The complex ex-
hibits selective vapochromic behavior with VOCs including
dichloromethane, acetonitrile, ethanol, and ethyl acetate. The
magnitude of the vapochromic shift for this system is among
the largest reported for simple VOCs, changing from red
emission in the absence of selected VOCs to green in their
presence. For dichloromethane, the response time to signal
the presence of vapor through green emission is short (within
seconds), whereas the return to the red form indicative of
the absence of vapor occurs within a few minutes. The crystal
structures of both the red and the green forms of 5, the latter
obtained upon recrystallization from CH2Cl2 solution, have
been determined by X-ray diffraction studies, and analysis
of the packing confirms that the red form 5-R possesses
significant Pt‚‚‚Pt interactions with distances of 3.30 and 3.34
Å, whereas the green form 5-G contains Pt(II) ions that are
more-separated with Pt‚‚‚Pt distances of 3.9092(9) and
4.5483(11) Å and Pt‚‚‚Pt‚‚‚Pt angles of 153.4 and 167.5°.
The packing and spatial arrangements explain the distinct
color changes between the two forms and their resultant
photophysical properties. The green emission from 5-G arises
Figure 8. Thermogravimetric scans of 5-R and 5-G.
window. These results compare well with [Pt(tterpy)Cl]Cl
on the reductive side, where two reversible waves are found
at -0.59 and -1.13 V and are ascribed to two terpy ligand
reductions. A similar assignment can be made for the
reductions of 5, with modest differences between the
corresponding waves for 5 and [Pt(tterpy)Cl]Cl as the result
of relatively minor electronic effects from the pentaphe-
nylphenyl substituent in 5 to the p-tolyl group in [Pt(tterpy)-
Cl]Cl. The absence of an oxidation wave for 5 stands in
contrast to an irreversible oxidation process seen for [Pt-
(tterpy)Cl]Cl around +1.60 V (vs NHE).
3
from a MLCT excited state involving a HOMO of Pt(dπ)
character and a LUMO localized on the terpyridyl ligand,
whereas the red emission of 5-R results from
a
3MMLCT charge-transfer excited state having the same
LUMO but a different HOMO, as a consequence of metal-
lophilic bonding. The vapochromic behavior is highly
reversible and selective.
Thermogravimetric Study of the Vapochromic Process.
The thermal stability of 5 and the loss of the VOC sorbed
into it when vapochromism occurs was probed for dichlo-
romethane by thermogravimetric analysis (Figure 8). The
results indicate that, for 5-G, approximately 9% of the mass
is lost between 20 and 140 °C. Given that the molecular
weight of 5 is 956, the lost mass corresponds to ∼86 amu,
which would mean a stoichiometry of one molecule of
dichloromethane per molecule of complex if all of the lost
mass is assumed to be CH2Cl2. Independent assessment of
Acknowledgment. We wish to thank the Division of
Basic Sciences, U.S. Department of Energy for financial
support of this research (DE-FG02-90ER14125) and Dr.
Chunchang Zhao and Prof. Man-Kit Ng for their help with
the TGA measurements.
Supporting Information Available: Cyclic voltammograms of
1
the ratio of dichloromethane to complex was done by H
[Pt(TPPPB)Cl]Cl and [Pt(tterpy)Cl]Cl; characterization data of
1
compounds 2, 4 and 5 (LC-MS, H NMR, 13C NMR spectra and
NMR spectroscopy using a sample of the X-ray quality
crystals dissolved in DMSO-d6. This experiment yielded a
value of 0.7 for the dichloromethane/complex 5 ratio, which
is reasonably consistent with the TGA results for 5-G. A
sample of 5-R exhibits different TGA behavior, displaying
only a very minor (<2%) mass loss up to approximately
300 °C.
2D COSY spectra of 5); vapochromic response of compound 5-R
to ethanol, acetonitrile and ethyl acetate vapors. X-ray crystal-
lographic data of [Pt(TPPPB)Cl]Cl as 5-R and 5-G is available in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC700947G
Inorganic Chemistry, Vol. 47, No. 1, 2008 77