A comparison of isomers: Trans- And cis-dicyanobis(para- ethylisocyanobenzene)platinum

Article abstract of DOI:10.1021/om7008167

We report the synthesis and characterization of trans-Pt(p-CN-C ₆H₄-C₂H₅)₂(CN) ₂ and compare the properties to the previously reported cis isomer, cis-Pt(p-CN-C₆H₄-C₂H₅) ₂(CN)₂. These geometric isomers exhibit textbook differences in spectroscopic and vapochromic properties, especially in the solid-state.

Full text of DOI:10.1021/om7008167

Organometallics 2007, 26, 6243-6247
6243
A Comparison of Isomers: trans- and
cis-Dicyanobis(para-ethylisocyanobenzene)Platinum
Anthony G. Dylla,† Daron E. Janzen,‡ Marie K. Pomije,*,† and Kent R. Mann*,§
Department of Chemistry and Geology, Minnesota State UniVersity, Mankato, Mankato, Minnesota 56001,
and Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455
ReceiVed August 9, 2007
We report the synthesis and characterization of trans-Pt(p-CN-C6H4-C2H5)2(CN)2 and compare the
properties to the previously reported cis isomer, cis-Pt(p-CN-C6H4-C2H5)2(CN)2. These geometric isomers
exhibit textbook differences in spectroscopic and vapochromic properties, especially in the solid-state.
Introduction
Experimental Section
General Considerations and Materials. All solvents used were
ACS reagent grade and dried over molecular sieves prior to use.
[Pt(p-CN-C6H4-C2H5)4][Pt(CN)4] was prepared by a modification
of the method reported by Buss and Mann.1 The sample of cis-Pt-
(p-CN-C6H4-C2H5)2(CN)2 (cis-PtC2) was available from a previ-
ous study.1
trans-Pt(CN-p-C6H4-C2H5)2(CN)2 (trans-PtC2). A mixture of
[Pt(p-CN-C6H4-C2H5)4][Pt(CN)4] (0.201 g, 0.197 mmol) and
hydrocarbon-stabilized CHCl3 (25 mL, 0.31 mol) was stirred at
reflux under a nitrogen atmosphere for 5 h. The remaining solid
was removed by filtration, and the soluble product was purified by
chromatography (silica gel, 85:15 CH2Cl2/CH3CO2C2H5). The first
fraction was collected and rotary evaporated to afford a reddish-
purple powder (0.080 g, 0.157 mmol, 79.9%). mp 174 °C (dec).
1H NMR (300 MHz, rt, CD2Cl2): δ 7.54 (d, J ) 8.4 Hz, 2 H, Ph),
7.37 (d, J ) 8.4 Hz, 2 H, Ph), 2.74 (q, J ) 7.7 Hz, 2 H, CH2), 1.26
(t, J ) 7.7 Hz, 3 H, CH3). 13C NMR (75 MHz, rt, CD2Cl2): δ
149.9 (isocyanide; Ph), 129.9 (isocyanide; Ph), 127.7 (isocyanide;
We recently reported the synthesis and vapochromic sensing
capability of cis-Pt(p-CN-C6H4-C2H5)2(CN)2.1 This neutral
complex was formed from the thermal rearrangement of the
isomeric double-salt complex [Pt(p-CN-C6H4-C2H5)4][Pt-
(CN)4]. We are investigating these types of platinum complexes
due to their vapochromic properties,2 which have potential for
use in chemical sensors and other optoelectronic devices.3 Here
we report the synthesis and characterization of the trans isomer
of Pt(p-CN-C6H4-C2H5)2(CN)2 and compare it to the previ-
ously reported cis-Pt(p-CN-C6H4-C2H5)2(CN)2. This is the first
well-characterized trans isomer of the Pt(isocyanide)2(CN)2
family of compounds4 and presents a textbook example of the
differences in properties that geometric isomers can exhibit.
* Author to whom correspondence should be addressed. Email: marie.
pomije@mnsu.edu and mann@chem.umn.edu.
195Pt-13
C
Ph), 122.8 (isocyanide; Ph), 112.7 (m, J
) 490 Hz), 29.4
† Minnesota State University, Mankato.
(isocyanide; CH2), 15.4 (isocyanide; CH3). 195Pt NMR (64.5 MHz,
‡ The College of St. Catherine, St. Paul, MN.
§ University of Minnesota.
rt, CD2Cl2, external reference K2PtCl4 in D2O, δ -1624): δ -4698
(m, J
N ) 94 Hz). FTIR (ATR, ZnSe crystal): νCNR 2231 cm-1
(1) Buss, C. E.; Mann, K. R. J. Am. Chem. Soc. 2002, 124, 1031.
(2) (a) Nagel, C. C. U.S. Patent 4,834,909, 1989. (b) Mann, K. R.; Daws,
C. A.; Exstrom, C. L.; Janzen, D. E.; Pomije, M. U.S. Patent 5,766,952,
1998.
195Pt-14
(vs); νCN 2142 cm-1 (m). UV-vis (ATR, cubic zirconia crystal,
film cast from dichloromethane solution): λmax ) 568 nm. Emission
(thin film, λexc ) 397.6 nm) λmax ) 668 nm. HRFAB-MS:
(calculated) 532.1077; (found) 532.1066 (M + Na)+.
(3) Selected sensor references: (a) Drew, S. M.; Janzen, D. E.; Mann,
K. R. Anal. Chem. 2002, 74, 2547. (b) Albert, K. J.; Walt, D. R.; Gill, D.
S.; Pearce, T. C. Anal. Chem. 2001, 73, 2501. (c) Rakow, N. A.; Suslick,
K. S. Nature 2000, 406, 710. (d) Seker, F.; Meeker, K.; Kuech, T. F.; Ellis,
A. B. Chem. ReV. 2000, 100, 2505. (e) Dickinson, T. A.; White, J.; Kauer,
J. S.; Walt, D. R. Nature 1996, 382, 697. (f) Bohrer, F. I.; Sharoni, A.;
Colesniuc, C.; Park, J.; Schuller, I. K.; Kummel, A. C.; Trogler, W. C. J.
Am. Chem. Soc. 2007, 129, 5640. (g) Mansour, M. A.; Connick, W. B.;
Lachicotte, R. J.; Gysling, H. J.; Eisenberg, R. J. Am. Chem. Soc. 1998,
120, 1329. (h) Bailey, R. C.; Hupp, J. T. J . Am. Chem. Soc. 2002, 124,
6767. (i) Li, B.; Sauve´, G.; Iovu, M. C.; Jeffries-EL, M.; Zhang, R.; Cooper,
J.; Santhanam, S.; Schultz, L.; Revelli, J. C.; Kusne, A. G.; Kowalewski,
T.; Snyder, J. L.; Weiss, L. E.; Fedder, G. K.; McCullough, R. D.; Lambeth,
D. N. Nano Lett. 2006, 6, 1598. Selected electronic device applications
references: (j) Kunugi, Y.; Mann, K. R.; Miller, L. L.; Exstrom, C. L.
J. Am. Chem. Soc. 1998, 120, 589. (k) Kunugi, Y.; Miller, L. L.; Mann,
K. R.; Pomije, M. K. Chem. Mater. 1998, 10, 1487. (l) Kunugi, Y.;
Mann, K. R.; Miller, L. L.; Exstrom, C. L. U.S. Patent 6,160,267, 2000.
(m) Kunugi, Y.; Mann, K. R.; Miller, L. L.; Pomije, M. K. U.S. Patent
6,137,118, 2000.
(4) A thorough search of the Cambridge Structural Database (CSD
version 5.28, January 2007) reveals no examples of crystallographically
characterized trans-Pt(isocyanide)2(CN)2 structures. (a) trans-Pt(isocyanide)2-
(CN)2 isomers were reported based on KBr pellet IR spectra by Isci, H.;
Mason, W. R. Inorg. Chem. 1975, 14, 913. (b) trans-Pt(CNCH3)2(CN)2
isomer was detected by 195Pt NMR but not isolated or otherwise character-
ized in a report by Martellaro, P. J.; Hurst, S. K.; Larson, R.; Abbott, E.
H.; Peterson, E. S. Inorg. Chim. Acta, 2005, 358, 3377.
Infrared Spectroscopy. Infrared spectra were obtained by the
ATR (attenuated total reflectance) method using a Nicolet Magna-
FTIR System 550 spectrometer. The sample films were deposited
on a ZnSe trough HATR crystal purchased from PIKE Technolo-
gies. The films were cast from either CH2Cl2 or CHCl3 solutions.
Data were processed using OMNIC ESP v 4.1 software.
NMR Spectroscopy. NMR spectra were acquired on a Varian
Inova 300 MHz spectrometer operating at 64.457 MHz for 195Pt.
The 195Pt spectra were obtained at room temperature and externally
referenced to K2PtCl4 in D2O, δ -1624.0 ppm. A typical set of
parameters for the 195Pt experiments included: acquisition time )
0.164 s, relaxation delay ) 0.300 s, sweep width ) 400.0 kHz and
a 45° pulse. The 1H and 13C spectra were obtained at room
temperature using standard parameters and referenced to the residual
protonated solvent peaks (CH2Cl2 1H 5.32 ppm; 13C 54.0 ppm).
Visible Absorption Spectroscopy. Solid-state visible absorption
spectra were acquired at room temperature using a modification of
the apparatus reported by Drew, Mann, Marquardt, and Mann.5 Thin
(5) Drew, S. M.; Mann, J. E.; Marquardt, B. J.; Mann, K. R. Sens.
Actuators 2004, B97, 307.
10.1021/om7008167 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 11/07/2007

6244 Organometallics, Vol. 26, No. 25, 2007
Dylla et al.
films of the samples were cast from either CH2Cl2, CH2Cl2/CH3-
CO2C2H5 or CH2Cl2/THF solutions and were deposited on a custom-
built cubic zirconia trough HATR crystal. The spectrum was
collected using an Ocean Optics S2000 spectrophotometer and
corrected for ATR penetration depth effects.
collection was carried out using Mo KR radiation (graphite
monochromator) with a frame time of 10 s and a detector distance
of 4.971 cm. A randomly oriented region of reciprocal space was
surveyed to the extent of one sphere and to a resolution of 0.77 Å.
Four major sections of frames were collected with 0.30° steps in
ω at four different æ settings and a detector position of -28° in
2θ. The intensity data were corrected for absorption and decay
(SADABS).10 Final cell constants were calculated from the xyz
centroids of 899 strong reflections from the actual data collection
after integration (SAINT).11
The structure was solved using SIR-9712 and refined using
SHELXL-97.13 The space group Pccn was determined based on
systematic absences and intensity statistics. A direct-methods
solution was calculated that provided most non-hydrogen atoms
from the E-map. Full-matrix least-squares/difference Fourier cycles
were performed that located the remaining non-hydrogen atoms.
All non-hydrogen atoms were refined with anisotropic displacement
parameters. All hydrogen atoms were placed in ideal positions and
refined as riding atoms with relative isotropic displacement
parameters. The final full matrix least-squares refinement converged
to R1 ) 0.0407 and wR2 ) 0.1007 (F2, all data).
Emission Spectroscopy and Solid-State Quantum Yields. The
solid-state emission spectra were acquired using a modification of
the apparatus described previously.5 A bifurcated (six around one)
fiber optic probe with an angle tip was used to bring light into and
out of the sample. Spectra were corrected for detector response
with an Ocean Optics CCD spectrophotometer that was calibrated
with a standard light source. The raw intensity values at each
wavelength were multiplied by the appropriate correction factor as
determined with the standard lamp and then by λ3 so that the
resulting intensity units are proportional to photons/wavenumber.6
For vapochromic studies, a thin film of the sample was deposited
on the tip of a Delrin plug that was fitted into a sample compartment
outfitted to accommodate VOC saturated vapors to flow over the
film. A bifurcated fiber optic cable was used to excite the sample
with an LED at λ ) 397.6 nm and also to collect the spectrum
using an Ocean Optics S2000 spectrophotometer. Solid-state
quantum yields were obtained by a modification of the method
described by Wrighton et al.7 The diffuse reflectance was captured
from a “perfectly” scattering surface to give (after integration and
correction for instrument response) a quantity proportional to the
incident light intensity (I0, units of photons/wavenumber). The
scatterer was then replaced with the sample and the emission from
the sample (Iemit, the emitted light), and the remaining scattered
intensity from the light source (I, the reduced light source intensity)
were captured and corrected. These two measurements allow the
solid-state emission quantum yield to be calculated from:
Results and Discussion
The neutral trans-Pt(p-CN-C6H4-C2H5)2(CN)2 (trans-PtC2)
compound is synthesized in high yield by heating the isomeric
double salt complex to reflux in chloroform. An FTIR spec-
troscopy study of the column chromatography fractions obtained
from this reaction clearly illustrates that a minor byproduct
(<10%) of the synthesis is the cis isomer.1 The main product,
a reddish-purple solid, was characterized by single-crystal X-ray
1
diffraction, H, 13C, and 195Pt NMR, ATR-FTIR, solid UV-
Iemit
visible absorption, and emission and high-resolution FAB mass
spectroscopy methods. Solid samples of the trans isomer appear
stable in the laboratory environment for at least 1 year based
on infrared characterization;14 thus, no special precautions were
taken during these studies. Since we had samples of the
previously characterized cis complex and now the trans isomer
in hand, we set out to systematically compare first, their
interconversion, and then their solution and solid-state properties.
We had already used the solid-state isomeric transformation
of the double salt complexes of the form [Pt(CNR)4][Pt(CN)4]
by a bulk melting process1 to produce the isomeric cis-Pt(CNR)2-
(CN)2, so we set out to determine whether the interconversion
of the trans to cis neutral complex was possible. Heating a solid
sample of pure trans-PtC2 under an active argon atmosphere
to 200 °C in an oil bath for 20 min followed by slowly cooling
to room temperature gives a deep-red solid. During this experi-
ment, the solid never appears to melt but does become an
orange-red color at approximately 160 °C. An infrared spectrum
of the deep-red product confirms it is cis-PtC2.1 Conversion of
cis-PtC2 back to trans-PtC2 was never observed in our hands.
A comparison of the IR spectra of cis-PtC2 and trans-PtC2
is shown in Figure 1. As expected, the cis-PtC2 displays two
cyanide stretches at 2156 and 2150 cm-1 and two isocyanide
φ )
I0 - I
For the studies presented here, we have decreased a major source
of error in Wrighton’s method by replacing the powdered MgO
scatterer (which has a surface that is very difficult to reproduce)
with a 1 × 1 cm piece of Fluorilon scattering target.8 The Fluorilon
target is a NIST standard of fused powdered Teflon with a greater
than 99% scattering coefficient across the entire spectral region of
interest (350-1000 nm). The procedure consists of measuring the
diffuse reflectance of the Fluorilon target followed by rubbing the
freshly grown crystals into the pores of the target with a metal
spatula. The light (unabsorbed excitation beam and emission) from
the target with the sample embedded in the surface is then collected,
and the necessary calculations are carried out with an Excel
spreadsheet.
X-Ray Structure Determination. A single-crystal X-ray struc-
ture was obtained for trans-Pt(p-CN-C6H4-C2H5)2(CN)2.9 A red
crystal (approximate dimensions 0.5 × 0.08 × 0.03 mm3) was
placed onto the tip of a 0.1 mm diameter glass capillary and
mounted on a Bruker SMART Platform CCD diffractometer for a
data collection at 173(2) K. A preliminary set of cell constants was
calculated from reflections harvested from three sets of 20 frames.
These initial sets of frames were oriented such that orthogonal
wedges of reciprocal space were surveyed. This produced initial
orientation matrices determined from 77 reflections. The data
(10) (a) SADABS, v. 2.03; Sheldrick, G., 2002. (b) Blessing, R. Acta
Cryst. 1995, A51, 33.
(6) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
(7) Wrighton, M. S.; Ginley, D. S.; Morse, D. L. J. Phys. Chem. 1974,
78, 2229.
(8) Avian Technolgies: Wilmington, OH; http://www.aviantechnologi-
es.com/faq.html.
(11) SAINT, v. 6.2; Bruker Analytical X-ray Systems: Madison, WI,
2001.
(12) SIR97 - Altomare A.; Burla M. C.; Camalli M.; Cascarano G. L.;
Giacovazzo C.; Guagliardi A.; Moliterni A. G. G.; Polidori G.; Spagna R.
J. Appl. Cryst. 1999, 32, 115.
(9) Crystal data for trans-PtC2: C20H18N4Pt, orthorhombic, space group
Pccn, a ) 14.729(4) Å, b ) 20.074(5) Å, c ) 6.2507(16) Å, R ) â ) γ
(13) SHELXTL, v. 6.10; Bruker Analytical X-ray Systems: Madison,
WI, 2000.
(14) A film cast from a chloroform solution containing a 13 month-old
solid sample of trans-PtC2 (used previously to acquire the NMR data) was
analyzed by FTIR and found to contain only trans-PtC2.
) 90°, V ) 1848.2(8) Å3, Z ) 4, Dcalcd ) 1.831 mg/m3, m ) 7.601 mm-1
,
final R indices [I > 2σ(I)], R1 ) 0.0407, wR2 ) 0.0944, R indices (all
data), R1 ) 0.0616, wR2 ) 0.1007.

A Comparison of Isomers
Organometallics, Vol. 26, No. 25, 2007 6245
Figure 1. Solid-state ATR-IR spectra of cis-PtC2 (light line) and
trans-PtC2 (bold line).
Figure 2. 195Pt NMR spectra of cis-PtC2 (light line) and trans-
PtC2 (bold line) (64.5 MHz, 298 K, CD2Cl2).
stretches at 2230 and 2251 cm-1. This is consistent with the
cis arrangement (local C2V symmetry) of the cyanide ligands
and the isocyanide ligands, resulting in two IR active stretches
of each type. The trans-PtC2 (local D2h symmetry) displays
one cyanide stretch at 2142 cm-1 and one isocyanide stretch at
2231 cm-1. This is consistent with the trans arrangement of
the cyanide ligands and isocyanide ligands, resulting in one IR
active stretch of each type.
1
We also characterized trans-PtC2 in solution with H and
13C NMR spectroscopy. Not surprisingly, the respective spectra
of trans-PtC2 and cis-PtC2, acquired under identical conditions,
give nearly identical chemical shifts for the proton and carbon
nuclei.15 The most notable difference is in the observed coupling
between the 195Pt and the 13C nuclei of the cyanide ligand (trans-
195Pt-13 195Pt-13
PtC2: J C ) 490 Hz versus cis-PtC2: J C ) 560 Hz).
The similarity of the respective 1H and 13C NMR spectra is not
entirely surprising due to the remoteness of these nuclei from
the 195Pt. However, the 195Pt NMR spectra exhibit differences
because of the significant differences in the Pt-ligand bonding
in the two isomers. The 195Pt spectra in Figure 2 illustrate not
only different chemical shifts (trans-PtC2: δ ) -4698 ppm
versus cis-PtC2: δ ) -4682 ppm) but also different 195Pt-
Figure 3. ORTEP diagrams (50% ellipsoids) of (a) trans-PtC2
and (b) cis-PtC2.
14N coupling constants (trans-PtC2: J
) 94 Hz versus
195Pt-14
N
cis-PtC2: J
) 80 Hz).16
195Pt-14
N
Although the solution properties of the cis and trans isomers
show small differences in the ligand environment, they hint that
larger differences are present in the electronic environment at
the platinum center. These electronic differences at the metal
lead to large spectroscopic differences, as a consequence of the
structural and packing differences of the cis and trans isomers
in the solid state.
As can be seen from Figure 3a, the molecular structure consists
of a neutral square planar Pt atom coordinated to two cyanide
ligands and two p-CN-C6H4-C2H5 ligands. The ligands of the
same types are oriented in a trans fashion, which is unprec-
edented for compounds of the general type Pt(CNR)2(CN)2.4
The packing consists of columns of stacked planar neutral
molecules with only one Pt-Pt distance (3.1253(8) Å) within
a stack, which corresponds to half of the unit cell c-axis length.
Interestingly, a short intermolecular π-π distance of 3.36 Å
between the phenyl rings of independent platinum stacks is
present (Figure S1, Supporting Information). This intermolecular
π-π interaction between the phenyl rings occurs in sheets
perpendicular to the stacking axis and forms a second dimen-
sional interaction network throughout the structure (Figure S2,
Supporting Information). No solvent molecules or disorder are
present in the structure.
We were fortunate to obtain X-ray quality single crystals of
trans-PtC2 grown by evaporation of a dichloromethane solution.
(15) NMR data for cis-PtC2: 1H NMR (300 MHz, rt, CD2Cl2): δ 7.52
(d, J ) 8.4 Hz, 2 H, Ph), 7.36 (d, J ) 8.4 Hz, 2 H, Ph), 2.74 (q, J ) 7.6
Hz, 2 H, CH2), 1.25 (J ) 7.6 Hz, 3 H, CH3). 13C NMR (75 MHz, rt, CD2-
Cl2): δ 149.5 (isocyanide; Ph), 129.8 (isocyanide; Ph), 127.6 (isocyanide;
195Pt-13
Ph), 123.3 (isocyanide; Ph), 110.7 (m, J
C ) 560 Hz, cyanide ligand),
29.3 (isocyanide; CH2), 15.4 (isocyanide; CH3). 195Pt NMR (64.5 MHz, rt,
CD2Cl2, external reference K2PtCl4 in D2O, δ -1624): δ -4682 (m,
195Pt-14
J
) 80 Hz).
N
(16) A 195Pt-14N 2J coupling constant for the compound cis-Pt(CN)2-
A structural comparison between trans-PtC2 and cis-PtC2
(CNCH3)2 is reported as 80 Hz. Martellaro, P. J.; Hurst, S. K.; Larson, R.;
Abbott, E. H.; Peterson, E. S. Inorg. Chim. Acta, 2005, 358, 3377.
illustrates many interesting differences. As noted previously,1

6246 Organometallics, Vol. 26, No. 25, 2007
Dylla et al.
Figure 4. Solid-state absorbance spectra of cis-PtC2 (light line)
and trans-PtC2 (bold line) (298 K, ATR corrected).
Figure 5. Solid-state emission spectra of cis-PtC2 (light line) and
trans-PtC2 (bold line) (298 K, λexc ) 397.6 nm, corrected for
detector response).
we also have two crystal structures of solvates of the orange
morph of cis-PtC2, cis-PtC2‚0.5(toluene), and cis-PtC2‚
x(hexanes). In both cis isomer structures, the cis-PtC2 molecules
stack in one-dimensional columns. However, the cis-PtC2
structures have two different Pt-Pt distances alternating between
a “long” and “short” distance. For cis-PtC2‚0.5(toluene), these
distances are 3.281 and 3.300 Å with an average of 3.2905(2)
Å, whereas for cis-PtC2‚x(hexanes), these distances are 3.278
and 3.296 Å with an average of 3.287(2) Å. Notably, all of
these Pt-Pt distances are longer than the 3.1253(8) Å distance
in trans-PtC2. The 3.1253(8) Å distance in trans-PtC2 is one
of the shortest that we have characterized in our vapochromic
materials.1,17
The significant differences in the Pt-Pt distances give rise
to large electronic differences in these solid-state materials. The
solid-state absorption spectra of the cis and trans isomers are
shown in Figure 4. The cis isomer shows a strong absorption
at 476 nm, whereas the trans isomer absorbs at 568 nm,
consistent with a considerably shorter Pt-Pt distance in the trans
isomer.18 Both compounds also exhibit strong solid-state
luminescence at room temperature (Figure 5). Consistent with
the absorption spectra, the cis isomer emits at higher energy
(601 nm) than the trans isomer (647 nm). We also measured
the absolute solid-state luminescence quantum yield for the cis
and trans isomers to be 0.45 and 0.21, respectively, under a
nitrogen atmosphere.
followed by recasting rather than a”true” vapochromic response.
We concluded from these experiments that trans-PtC2 does not
exhibit vapoluminescent behavior. This conclusion is in direct
contrast to cis-PtC2, which exhibits vapoluminescent behavior
with a variety of aromatic VOCs as well as ethanol.1
These large isomeric differences in solid-state vapolumines-
cent behavior led us to examine the packing differences of the
cis and trans isomers more carefully. A unique feature of trans-
PtC2 is the presence of the intermolecular π-π interactions
between isocyanide ligands of different stacks; neither cis-PtC2‚
0.5(toluene) nor cis-PtC2‚x(hexanes) have interactions of this
type. The short intermolecular π-π interactions between the
isocyanide ligands give rise to a second-dimensional ordering
of the Pt-Pt stacks for trans-PtC2. We believe this combination
of packing forces (Pt-Pt stacking as well as intermolecular π-π
interactions) gives rise to a relatively close-packed structure.
Comparison of the overall packing efficiency of each using the
Kitaigorodskii packing coefficients19 illustrates that trans-PtC2
(0.678) packs significantly more efficiently than cis-PtC2
(0.620).
A comparison of space-filling representations for the packing
of these structures is shown in Figure 6. This figure illustrates
that the geometric isomerization leads to large differences in
the molecular shape, which in turn results in completely different
packing. In the trans-PtC2 packing view shown in Figure 6a,
the lack of voids or channels is clearly evident, whereas in the
cis-PtC2 packing view in shown Figure 6b, the presence of the
channels is equally apparent. We believe that the voids and
channels found in cis-PtC2 and in other vapochromic com-
pounds play a fundamental role in facilitating the vapochromic
response. We also believe the two-dimensional packing motif
is a strong factor contributing to the lack of vapochromic
behavior of trans-PtC2.
To assess the vapochromic potential of trans-PtC2, we
conducted several series of emission experiments exposing films
of trans-PtC2 to various vapor-phase VOCs such as acetone,
chloroform, methanol, and toluene. In each experiment, the
emission spectra show large, irreversible changes in the intensity
and the crude shape of the emission band. However, the spectra
also clearly show that λmax is barely affected by VOC exposure.
Further, we were not able to reproduce the original film
spectrum intensity after repeated exposure to VOCs. These
observations are consistent with partial dissolution of the film
In summary, we have synthesized and characterized the trans
isomer of Pt(p-CN-C6H4-C2H5)2(CN)2 and compared it with
the cis analog. Our comparisons illustrate how the geometric
isomerization leads to minor changes in the ligand environment
(17) (a) Buss, C. E.; Anderson, C. E.; Pomije, M. K.; Lutz, C. M.; Britton,
D.; Mann, K. R. J. Am. Chem. Soc. 1998, 120, 7783. (b) Buss, C. E. Ph.D.
Dissertation, University of Minnesota, Minneapolis, MN, 2002. (c) Ander-
son, C. E. MS Dissertation, University of Minnesota, Minneapolis, MN
1997.
(18) Gliemann, G.; Yersin, H. Struct. Bonding 1985, 62, 87 and references
therein.
(19) Kitaigorodskii, A. I. Organic Chemical Crystallography; Consultants
Bureau Enterprises: New York, 1961; Vol 1A. The packing coefficient
for the cis-PtC2 structure was calculated with the toluene solvate molecules
removed from the structure.

A Comparison of Isomers
Organometallics, Vol. 26, No. 25, 2007 6247
The efficient packing and short π-π interactions between
isocyanide ligands of trans-PtC2 precludes channels or voids
for VOC sorption and disfavors reorganization of the structure
to accommodate VOCs with concomitant modulation of the Pt-
Pt interactions.17a
Acknowledgment. A.G.D. and M.K.P. acknowledge the
National Science Foundation Research Site for Educators in
Chemistry Program administered by the University of Min-
nesota, Department of Chemistry for funding and support. D.E.J.
acknowledges the University of Minnesota, Department of
Chemistry for financial support through a Block Grant Fellow-
ship. M.K.P. also acknowledges a Faculty Research Grant award
from Minnesota State University, Mankato. M.K.P. also thanks
the NMR facility at the University of Minnesota and Dr. Letitia
Yao for her help with the 195Pt NMR spectroscopy. This work
was supported by the Center for Analytical Chemistry at the
University of Washington, the Initiative for Renewable Energy
and the Environment at the University of Minnesota, and the
Materials Research Science & Engineering Center at the
University of Minnesota. We acknowledge the X-ray Crystal-
lographic facility of the Chemistry Department at the University
of Minnesota, including Dr. Victor G. Young, Jr. and William
W. Brennessel.
Figure 6. Solid-state packing diagrams of (a) trans-PtC2 and (b)
cis-PtC2 (view parallel to the Pt-Pt stacking axis).
Supporting Information Available: Additional crystallographic
figures for trans-PtC2 and crystallographic information. This
material is available free of charge via the Internet at http:
//pubs.acs.org.
in solution but results in large changes in the solid-state packing.
The solid-state packing differences influence the solid-state
electronic properties, which are dominated by intermolecular
interactions. We believe that trans-PtC2 does not exhibit
vapochromic behavior due to a unique combination of factors.
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