Intermolecular interactions and electronic properties in phosphino-(oligothiophene) palladium(II) and platinum(II) complexes

Article abstract of DOI:10.1139/V07-033

A series of Pt(II) and Pd(II) complexes containing diphenylphosphino- substituted oligothiophene ligands ranging from 1 to 3 thiophene rings in length have been prepared. Crystal structures of four of these complexes were determined via single X-ray crystal diffraction and the solid-state packing arrangements found to vary with both the metal and the thiophene-containing ligand. In some cases, π-stacking between thiophene rings are found for the oligothiophene ligands. Solution and solid-state absorption spectra of these complexes are reported.

Full text of DOI:10.1139/V07-033

383
Intermolecular interactions and electronic
properties in phosphino-(oligothiophene)
palladium(II) and platinum(II) complexes
Tracey L. Stott, Michael O. Wolf, and Brian O. Patrick
Abstract: A series of Pt(II) and Pd(II) complexes containing diphenylphosphino-substituted oligothiophene ligands
ranging from 1 to 3 thiophene rings in length have been prepared. Crystal structures of four of these complexes were
determined via single X-ray crystal diffraction and the solid-state packing arrangements found to vary with both the
metal and the thiophene-containing ligand. In some cases, π-stacking between thiophene rings are found for the
oligothiophene ligands. Solution and solid-state absorption spectra of these complexes are reported.
Key words: oligothiophenes, metal complexes, structural properties, electronic spectra.
Résumé : On a préparé une série de complexes du Pt(II) et du Pd(II) contenant des ligands oligothiophènes substitués
par des groupes diphénylphosphino dans lesquels le nombre de noyaux thiophènes varie de un à trois. On a déterminé
les structures cristallines de quatre de ces complexes par diffraction des rayons X par un cristal unique et on a observé
que les arrangements de l’empilement à l’état solide varient en fonction de la nature du métal ainsi que du ligand
contenant le thiophène. Dans quelques cas, on a observé un empilement π pour les ligands oligothiophènes. On a
aussi mesuré les spectres d’absorption en solution et à l’état solide de ces complexes.
Mots-clés : oligothiophènes, complexes métalliques, propriétés structurales, spectres électroniques.
[Traduit par la Rédaction] Stott et al. 391
Introduction
proaches; for example, our group has recently reported the
use of pendant Ru(II) complexes to control the coplanarity
of rings in a terthiophene derivative (14, 15). In the present
work, we have selected palladium(II)- and platinum(II)-
bis(phosphine) complexes to further explore these effects.
These were chosen because they are readily synthesized, are
stable in air, crystallize easily (16), and exist in both cis- and
trans-substituted square planar geometries (16–22). Our
group has previously reported the synthesis and character-
ization of several β-substituted phosphinothiophene palla-
dium(II) complexes (23–25). The metal was found to affect
both the electronic properties and the structure of the
oligothiophene. Au(I) complexes of the α-phosphino-
thiophene ligands used here have also been reported (26).
Metal centers may create quenching sites within the crys-
tal (27, 28) thus limiting the immediate utility of these mate-
rials for application in optoelectronic devices; however, these
studies still allow exploration of solid-state effects. The crys-
tal structures of a series of phosphinothiophene palla-
dium(II) and platinum(II) complexes are reported in this
paper. The effects of the metal geometry and the length of
the thiophene ligand on the structure and electronic spectra
are explored. Gray and co-workers (29) recently reported the
third-order nonlinear optical behavior of one of the Pd com-
Conjugated oligomers are of significant interest for appli-
cation in electroluminescent devices (1), solar cells (2), and
field-effect transistors based on molecular materials (3).
Solid-state packing and intermolecular interactions are
known to influence the emission properties (4) and charge
mobility (5) of these materials. In π-conjugated oligomers
such as oligothiophenes, π-stacking (6, 7) has been shown to
influence the charge mobility as well as the excitation and
emission energy of the material (4, 8). The degree of conju-
gation may also be influenced by solid-state packing, and
unsubstituted oligothiophenes adopt a planar, fully conju-
gated arrangement in the solid state (5).
The solid-state packing of conjugated oligomers, such as
oligothiophenes, can be influenced by the use of different or-
ganic substituents (5, 9–12) or by a combination of electron-
rich and electron-poor groups (13). A novel approach in con-
trolling the solid-state packing is through the use of coordi-
nation chemistry where the conjugated groups are positioned
as ligands on metal complexes giving a defined local geo-
metrical arrangement, which may predispose the molecule
towards specific packing arrangements. Intramolecular ge-
ometry may be controlled with coordination chemistry ap-
Received 19 January 2007. Accepted 27 March 2007. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on
18 May 2007.
T.L. Stott, M.O. Wolf,1 and B.O. Patrick. Department of Chemistry, University of British Columbia, Vancouver, BC V6T1Z1,
Canada.
1Corresponding author (e-mail: mwolf@chem.ubc.ca).
Can. J. Chem. 85: 383–391 (2007)
doi:10.1139/V07-033
© 2007 NRC Canada

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Can. J. Chem. Vol. 85, 2007
pounds (Pd(PT3)2Cl2) and one of the Pt compounds
(Pt(PT3)2Cl2) described here; however, the solid-state struc-
tures of these complexes have not been reported.
1.2 Hz, 1H), 7.18 (dd, J = 1.2, 3.5 Hz, 2H), 6.98 (dd, J =
5.0, 3.6 Hz, 1H). 31P{1H} NMR (CDCl3) δ: 14.0 (s), 24.2
(s).
Pd(PT3)2Cl2 (3)
Experimental
Results obtained by using a different synthesis were previ-
ously reported by our group (24). PdCl2 (0.021 g,
0.12 mmol) was dissolved in an HCl–H2O solution
(0.2:3 v/v). This was added dropwise to a solution of 0.10 g
(0.23 mmol) of PT3 in THF, and a yellow precipitate began
to form almost immediately. The yellow slurry was stirred
for 1 h, and the solvent was then removed via rotary evapo-
ration. The resulting sticky material was dissolved in
CH2Cl2, filtered, and precipitated in hexanes. The product, a
bright yellow powder, was recrystallized in hexanes–CH2Cl2.
Yield 86%. Anal. calcd. for C418H34Cl2P2PdS6: C 55.30, H
3.29; found: C 55.06, H 3.29. H NMR (CDCl3) δ: 7.70–
7.63 (m, 4H), 7.46–7.38 (m, 6H), 7.28–7.20 (m, 6H, in-
cludes CHCl3), 7.18 (d, J = 3.6 Hz, 1H), 7.15 (d, J = 3.6 Hz,
1H), 7.08 (d, J = 3.6 Hz, 1H), 7.04 (d, J = 3.6 Hz, 1H), 6.99
(dd, J = 5.2, 3.6 Hz, 1H). 31P{1H} NMR (CD2Cl2) δ: 14.3
(s), 24.4 (s).
General
1H and 31P{1H} NMR experiments were performed on ei-
ther a Bruker AV-300 or a Bruker AV-400 spectrometer.
Spectra were referenced to a residual solvent (1H) or exter-
nal 85% H3PO4 (31P). Isomer ratios were determined from
31P NMR spectra obtained with a delay time of 2 s. Elec-
tronic spectra were obtained on a Cary 5000 in HPLC grade
CH2Cl2. Solid-state absorption spectra were obtained by
drop casting the compound from a CH2Cl2 solution onto a
quartz slide. Microanalyses were performed at UBC.
Electrospray mass spectra were obtained by infusion of
10 µmol/L of a methanol solution at -10 µL/min into the
electrospray source of a Bruker Esquire ion trap mass spec-
trometer. The spectra were acquired in positive ion mode.
MALDI-TOF mass spectra were obtained on a Bruker Biflex
IV mass spectrometer. Samples were prepared from di-
chloromethane in a dithranol matrix. PdCl2 and K2PtCl4
were purchased from Strem and used as received. THF was
dried over Na and benzophenone and was distilled before
use. CH2Cl2 was dried by passing over an alumina column.
Acetonitrile was dried over 3 Å molecular sieves and was
degassed by sparging with N2 for 20 min. No special precau-
tions for the exclusion of oxygen were taken during synthe-
sis. The phosphine ligands 2-diphenylphosphinothiophene
(PT), 2-diphenylphosphino-5,2′-bithiophene (PT2) and 2-
diphenylphosphino-5,2′:5′,2′′-terthiophene (PT3) were pre-
pared as described previously (30, 31).
General procedure for the synthesis of Pt(PTn)2Cl2
complexes
The appropriate ligand was dissolved in 50 mL of CH2Cl2
and stirred with a solution of 0.5 equiv. of K2PtCl4 in 10 mL
of H2O. The reaction was monitored both by the disappear-
ance of the red color in the H2O layer due to K2PtCl4 and by
the disappearance of the ligand via TLC. When the reaction
was complete the layers were separated, the organic layer
was dried over MgSO4 and gravity filtered, and the solvent
was removed via rotary evaporation. A minimal amount of
CH2Cl2 was added to dissolve the resulting solid, which was
then precipitated in 300 mL hexanes. The resulting powder
was collected via suction filtration and either dried under
vacuum or recrystallized from CH2Cl2–hexanes to yield an
analytically pure sample.
Synthesis
Pd(PT)2Cl2 (1)
PdCl2 (0.033 g, 0.19 mmol) was dissolved in an HCl–H2O
solution (0.2:3 v/v), which was then added dropwise to a so-
lution of 0.10 g (0.39 mmol) PT dissolved in an ethanol–
acetonitrile mixture (5:4 v/v). A yellow precipitate formed
immediately, and the yellow slurry was stirred for 1 h. The
solvent was then removed via rotary evaporation, and the re-
sulting sticky material was dissolved in CH2Cl2 and acetone,
then filtered, and a yellow powder was precipitated in hex-
anes. The product was recrystallized in hexanes–CH2Cl2,
yielding an analytically pure product. Yield 80%. Anal.
calcd. for C32H26Cl2P2PdS2: C 53.83, H 3.67; found: C
Pt(PT)2Cl2 (4).
Yield 67%. Anal. calcd. for C32H26Cl2P2PtS2: C 47.89, H
1
3.27; found: C 47.78, H 3.57. H NMR (CDCl3) δ: 7.65–
7.59 (m, 2H), 7.47–7.41 (m, 4H), 7.32–7.28 (m, 2H), 7.17–
7.13 (m, 4H), 7.04–7.01 (m, 1H). 31P{1H} NMR (CDCl3) δ:
5.6 (s), 1JP–Pt = 3680 Hz. MS (MALDI-TOF) m/z: 766 (MW-
Cl).
Pt(PT2)2Cl2 (5)
1
Yield 73%. Anal. calcd. for C40H30Cl2P2PtS4: C 49.69, H
53.67, H 3.67. H NMR (CDCl3) δ: 7.80 (m, 1H), 7.69–7.62
1
(m, 5H), 7.46–7.34 (m, 6H), 7.13 (m, 1H). 31P{1H} NMR
3.13; found: C 49.72, H 3.39. H NMR (CDCl3) δ: 7.58–
7.53 (m, 4H), 7.43–7.40 (dd, J = 7.7, 3.8 Hz, 1H), 7.36–7.32
(m, 2H), 7.24–7.19 (m, 6H, includes CHCl3), 7.07 (dd, J =
3.6, 0.9 Hz, 1H), 7.01 (dd, J = 3.6, 1.1 Hz, 1H), 6.97 (dd,
J = 5.1, 3.7 Hz, 1H). 31P{1H} NMR (CDCl3) δ: 5.1 (s),
1JP–Pt = 3690 Hz. MS (ESI) m/z: 931 (MW-Cl).
(CDCl3) δ: 13.6 (s), 23.9 (s).
Pd(PT2)2Cl2 (2)
PdCl2 (0.025 g, 0.14 mmol) and PT2 (0.10 g, 0.29 mmol)
were mixed together in a 1:2 mixture of CH3CN–CH2Cl2 for
3 h. The solvent was removed, and the resulting yellow powder
was dissolved in CH2Cl2, gravity filtered, and precipitated
with hexanes. The product was recrystallized in hexanes–
CH2Cl2. Yield 40%. Anal. calcd. for C40H30Cl2P2PdS4: C
54.71, H 3.44; found: C 54.31, H 3.58. 1H NMR (CD2Cl2) δ:
7.73–7.66 (m, 5H), 7.47–7.38 (m, 6H), 7.31 (dd, J = 5.0,
Pt(PT3)2Cl2 (6).
Previously reported by Gray and co-workers (29). Yield
81%. Anal. calcd. for C48H34Cl2P2PtS6: C 50.97, H 3.03;
1
found: C 50.61, H 3.27. H NMR (CD2Cl2) δ: 7.68–7.57 (m,
8H), 7.51–7.38 (m, 6H), 7.32–7.19 (m, 11H), 7.15 (dd, J =
© 2007 NRC Canada

Stott et al.
385
3.51, 1.1 Hz, 2H), 7.07–6.98 (m, 7H). 31P{1H} NMR
(CD2Cl2) δ: 4.9 (s), JP–Pt = 3700 Hz. MS (ESI) m/z: 1095
(MW-Cl).
Scheme 1.
1
X-ray Crystallography
Suitable crystals of 2, 3, 5, and 6 were mounted in oil on
a glass fiber, and the data for each compound were collected
at 173(1) K. The structures were solved using direct meth-
ods (32, 33) and refined using SHELXL-97 (34). All mea-
surements were made on a Bruker X8 APEX diffractometer
with graphite monochromated Mo Kα radiation. For each of
the four structures, one terminal thiophene ring was disor-
dered in two orientations. In each case, the disordered frag-
ments were modeled using the simple restraints (SADI or
SAME) found in SHELXL.
multiscan technique (SADABS) (36). The data were cor-
rected for Lorentz and polarization effects. All non-
hydrogen atoms were refined anisotropically. All hydrogen
atoms were included in calculated positions but not refined.
The thiophene ring containing S (6) was disordered and
modeled in two orientations with the populations of the ma-
jor and minor fragments refined to an approximate 2:1 ratio.
The data for 2 were collected to a maximum 2θ value of
55.8°. Data were collected in a series of φ and ω scans in
0.50° oscillations with 10.0 s exposures. Data were collected
and integrated using the Bruker SAINT (35) software pack-
age and were corrected for absorption effects using the
multiscan technique (SADABS) (36). The data were cor-
rected for Lorentz and polarization effects. The material
crystallizes with one half-molecule residing on an inversion
center. One thiophene ring in the asymmetric unit is disor-
dered and was modeled in two orientations. The atoms of the
major disordered fragment were refined anisotropically,
while the atoms of the minor fragment (with a relative popu-
lation of approximately 0.23) were refined isotropically. Re-
straints were used to ensure the disordered thiophene
fragments had reasonable geometries. All other non-
hydrogen atoms were refined anisotropically. All hydrogen
atoms were included in calculated positions but not refined.
The data for 3 were collected to a maximum 2θ value of
55.6°. Data were collected in a series of φ and ω scans in
0.50° oscillations with 10.0 s exposures. Data were collected
and integrated using the Bruker SAINT (35) software pack-
age and were corrected for absorption effects using the
multiscan technique (SADABS) (36). The data were cor-
rected for Lorentz and polarization effects. All non-
hydrogen atoms were refined anisotropically, while all hy-
drogen atoms were included in calculated positions but not
refined. The terminal thiophene ring containing S(3) was
disordered and modeled in two orientations with relative
populations of 0.77 and 0.23 for the major and minor frag-
ments, respectively.
The data for 5 were collected to a maximum 2θ value of
50.2°. Data were collected in a series of φ and ω scans in
0.50° oscillations with 20.0 s exposures. Data were collected
and integrated using the Bruker SAINT (35) software pack-
age, were corrected for absorption effects using the
multiscan technique (SADABS) (36), and were corrected for
Lorentz and polarization effects. One thiophene ring is dis-
ordered in two orientations. All atoms except the two sulfur
fragments and one carbon were refined anisotropically. All
hydrogen atoms were included in calculated positions but
not refined.
The data for 6 were collected to a maximum 2θ value of
52.8°. Data were collected in a series of φ and ω scans in
0.50° oscillations with 12.0 s exposures. Data were collected
and integrated using the Bruker SAINT (35) software pack-
age. Data were corrected for absorption effects using the
Results and discussion
Synthesis of complexes
The palladium complexes 1–3 were synthesized by one of
two methods. The ligand and PdCl2 were either stirred to-
gether in a mixture of CH2Cl2 and CH3CN, or PdCl2 was
dissolved in dilute HCl and subsequently added dropwise to
a solution of the ligand in an appropriate solvent
(Scheme 1). The crude products were crystallized in
CH2Cl2–hexanes to yield analytically pure materials in rea-
sonable yields. Complex 1 has been reported previously (37,
38), and the synthesis and structure of 3 was previously
communicated by our group (39). Palladium bis(phosphine)
complexes of this type are known to exhibit cis–trans equi-
libria in solution (20), which can be observed by 31P NMR
spectroscopy. The 31P NMR signals for the trans complexes
are generally found at a lower chemical shift than those of
the cis isomer (40–45). The 31P NMR spectra of 1–3 in
CD2Cl2, CDCl3, and DMSO indicate that although both cis
and trans isomers exist in solution, the trans isomer predom-
inates in these solvents (as determined by integration of 31P
NMR spectra).
As a single species is desirable for solution characteriza-
tion, we also chose to synthesize the more inert Pt (16, 46)
analogs 4–6. These were prepared by stirring together a so-
lution of K2PtCl4 in H2O and the appropriate ligand dis-
solved in CH2Cl2. The progress of the reaction was
monitored by the disappearance of the dark red color of
[PtCl4]– in the water layer. The crude product obtained was
either crystallized in CH2Cl2–hexanes or dried under vac-
uum. Complex 4 has been previously synthesized (37, 38,
47), and the synthesis of 6 was reported by Gray and co-
workers (29).
The 195Pt satellites in the 31P NMR spectra of 4–6 may be
used to determine the geometry at the metal (18, 48). Com-
plexes 4 (δ 6), 5(δ 5), and 6 (δ 6) have 195Pt–31P coupling
constants of 3681, 3685, and 3700 Hz, respectively, which
indicate that these are cis-substituted complexes. A second
less intense peak was observed in the 31P NMR spectra of
analytically pure 6 at δ 11.5. This peak is assigned to the
trans isomer, expected to appear downfield to the signal
from the cis isomer (48). However, because of the low inten-
sity, no 195Pt satellites are observed for this signal. It may be
© 2007 NRC Canada

386
Can. J. Chem. Vol. 85, 2007
Table 1. Selected crystal structure data for 2, 3, 5, and 6.
Compound
Formula
2
3
5
6
C40H30Cl2P2S4Pd
C48H34P2S6Cl2Pd
C40H30P2S4Cl2Pt
C48H34S6P2PtCl2
Colour, habit
Dimension (mm)
T (K)
Yellow, plate
0.15 × 0.05 × 0.02
173
Orange, rod
0.25 × 0.10 × 0.10
173
Colorless, prism
0.25 × 0.20 × 0.08
173
Yellow-orange, plate
0.25 × 0.25 × 0.10
173
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c (#14)
9.489(1)
17.637(2)
11.2772(9)
90.0
100.667(4)
90.0
1854.7(3)
2
Monoclinic
P21/n (#14)
11.8648(8)
20.875(1)
18.270(1)
90.0
98.054(4)
90.0
4480.4(4)
4
Monoclinic
P21/c (#14)
14.7543(5)
14.7758(2)
17.5571(6)
90.0
107.868(1)
90.0
3642.9(2)
4
Monoclinic
P21/n (#14)
11.749(1)
29.282(2)
12.609(1)
90
93.704(3)
90
4328.9(6)
4
α (°)
β (°)
γ (°)
V (Å3)
Z
Dcalc (g cm–3)
1.572
1.545
1.763
1.735
Unique data
4056
9.86
0.098
0.093
10 054
9.20
0.033
0.074
6425
43.46
0.064
0.078
8887
37.65
0.041
0.094
µ (Mo Kα) (cm–1)
R(F)a (I > 2σ(I))
Rw(F2)b (all data)
aR = Σ||Fo| – |Fc|| /Σ|Fo|
2
2
bRw = (Σ(Fo – Fc2)2/Σw(Fo )2)1/2
that the increased steric bulk of the terthienyl group results
in the formation of some trans isomer.
Fig. 1. Molecular structures of complexes (a) 2, (b) 3, (c) 5, and
(d) 6.
Molecular solid-state structures
Four of the complexes have been characterized by single
crystal X-ray diffraction and selected data is shown in Ta-
ble 1.2 Complex 3 was reported in a preliminary communi-
cation; however, comparisons are included here. The
molecular structures of the four complexes are shown in
Fig. 1.
All four complexes 2, 3, 5, and 6 are close to a square pla-
nar geometry at the metal center. The largest distortion is
observed in 5 where one of the Cl–Pt–P angles is
170.85(5)°. In the two Pd complexes the ligands are ar-
ranged in trans geometry, whereas the Pt complexes both ex-
hibit cis geometry at the metal. The metal–Cl, metal–P, and
P–C bond lengths are similar to those observed for other
complexes of this type. The bond lengths in the thiophene
rings range from 1.666(8) to 1.752(7) Å for the S–C bonds
and from 1.332(8) to 1.519(10) Å for the C–C bonds. The
arrangement and coplanarity of adjacent thiophene rings can
be assessed by the S–C–C–S torsion angle. A syn coplanar
arrangement is indicated by an S–C–C–S angle of 0°, while
an anti coplanar arrangment gives an angle of 180°. Unsub-
stituted thiophene oligomers typically crystallize in a nearly
completely anti coplanar arrangement (5). In all the struc-
tures here, adjacent thiophene rings are not completely co-
planar (S–C–C–S angle is between 7.6(7)° and 175.40(14)°),
and both syn and anti arrangements are found, sometimes
within the same molecule. Table 2 summarizes these angles.
The largest deviation from coplanarity between adjacent
rings is found in 6 where rings containing S2 and S3 have an
interannular angle of approximately 40°.
2 Supplementary data for this article are available on the journal Web site (http://canjchem.nrc.ca) or may be purchased from the Depository
of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 5158. For more
information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 252088, 626550, 626551, and 626552
contain the crystallographic data for this manuscript. These data can be obtained, free of charge, via http://www.ccdc.cam.ac.uk/conts/re-
trieving.html (Or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or
deposit@ccdc.cam.ac.uk).
© 2007 NRC Canada

Stott et al.
387
Fig. 2. Packing diagrams of 2 showing (a) the arrangement of
Table 2. S–C–C–S torsion angles for complexes 2, 3, 5, and 6.
bithienyl and phenyl groups with the hydrogen atoms omitted
and (b) looking down the c axis, with the hydrogen atoms omit-
ted.
Complex
Bonds
Torsion angle (°)
10(2)
24.4(3)
2
3
3
3
3
5
5
6
6
6
6
S1–C4–C5–S2
S1–C4–C5–S2
S2–C8–C9–S3
S4–C28–C29–S5
S5–C32–C33–S6
S1–C4–C5–S2
S3–C24–C25–S4
S1–C4–C5–S2
S2–C8–C9–S3
S4–C28–C29–S5
S5–C32–C33–S6
162.21(16)
169.15(14)
175.40(14)
14.1(8)
153.8(4)
22.9(7)
140.4(4)
7.6(7)
8.3(8)
Solid-state packing of Pd complexes
Complex 2 crystallizes in the P21/n point group. The
bithienyl groups are arranged on opposite sides of the plane
passing through the Pd, P, and Cl atoms (Fig. 2a). The
bithiophene groups are arranged in interlocking stacks with
adjacent bithienyl groups twisted relative to one another by
approximately 90°. When the structure is examined looking
down the c axis (Fig. 2b), it is apparent that the phenyl
groups form a tube surrounding the bithienyl stacks, separat-
ing the stacks from each other. No π-stacking of thiophene
rings is observed in this structure. Short S–S interactions are
believed to be important in facilitating charge-transfer in the
oligothiophenes (5), however here the shortest intermole-
cular S–S distance is ~5.6 Å, precluding any interactions be-
tween sulfur atoms.
Complex 3 was previously reported but the solid-state
packing is discussed here for comparative purposes. This
complex crystallizes in the P21/n point group. In contrast to
2, the terthienyl groups are oriented on the same side of the
metal plane in this case. In the crystal lattice, the arrange-
ment of the terthienyl groups may be examined by consider-
ing the crystallographically distinct terthienyl groups A and
B (Fig. 3). Pairs of terthienyl groups A are arranged with
interplanar distances, for the inner thiophene rings of the
terthienyl groups, within the range of π-stacking (3.65 Å)
(13, 49). The π orbital overlap between aromatic groups is
greatest if they are arranged in a stack perpendicular to the
molecular plane. The two A groups are slightly slipped rela-
tive to one another, with the slip greater along the short axis
of the groups. The degree of slip along the long and short
axis may be quantified via pitch and roll angles, as discussed
for organic conjugated oligomers by Curtis et al. (13). The
interactions between the other pair of coplanar terthienyl
groups (B and B) are much weaker, since these groups are
slipped much more substantially (large roll angle) relative to
each other despite having little slip along the long axis
(small pitch angle). Pairs of groups A and B are arranged
face-to-edge, similar to the motif found in herringbone
stacks of organic oligomers (13). The shortest S–S distance
in the lattice is 4.13 Å, again too long for any significant S–
S interactions in this structure (5).
Fig. 3. Packing diagram for 3, viewed down the long molecular
axis of the terthienyl groups. Phenyl groups and hydrogen atoms
are omitted for clarity.
complex 3. The bithienyl groups in 5 are also arranged on
the same side of the plane containing the metal, and weak
intermolecular π-stacking (3.76 Å) between the outer thienyl
Solid-state packing of Pt complexes
Complex 5 crystallizes in the P21/c point group. The
structure shows some similarities to that of the Pd terthienyl
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Can. J. Chem. Vol. 85, 2007
Fig. 4. Packing diagram of 5 viewed (a) down the long molecu-
lar axis of the bithienyl groups and (b) down the short molecular
axis of bithienyl groups A. Phenyl groups and hydrogen atoms
are omitted for clarity.
Fig. 5. Packing diagram of 6. Phenyl groups and hydrogen atoms
are omitted for clarity.
Fig. 6. Intramolecular stacking between phenyl ring and inner
thiophene ring in group B of 6. Hydrogen atoms are omitted for
clarity.
rings in group B on adjacent molecules is observed
(Fig. 4a). However, these groups show substantial slip along
the long molecular axis (large pitch angle) relative to what is
observed in 3 (Fig. 4b). The A groups are rather far apart in
the structure and do not interact. Some intramolecular π-
stacking of phenyl groups is observed (not shown), with a
plane-to-plane distance between the centroids of the rings of
3.63 Å.
Fig. 7. Solution absorption spectra of palladium and platinum
complexes 1–6.
Complex 6 crystallizes in the P21/n point group. The
terthienyl groups (A) in this structure are arranged in an ap-
proximately coplanar fashion to each other with an
intermolecular plane-to-plane distance (between the cen-
troids of the rings) of 3.45 Å (Fig. 5). The rings are slightly
slipped along both the long and short molecular axis. The
other terthienyl groups (B) are arranged so that the edges of
the pair of A groups point towards the plane of B with the A
and B groups twisted 90° relative to each other. Thus in
Fig. 5 the view is down the long axes of the two A groups,
with the long axis of the B groups perpendicular to these.
Intramolecular π-stacking is also observed between thienyl
and phenyl rings (Fig. 6), with a plane-to-plane distance be-
tween the centroids of the rings of 3.52 Å.
Absorption spectra
The solution and solid-state absorption spectra of com-
plexes 1–6 are shown in Figs. 7 and 8 respectively, and the
data is summarized in Table 3. Although both cis and trans
isomers are present in solutions of the Pd complexes 1–3,
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Stott et al.
389
Table 3. Solution and solid-state absorption data for 1–6.
Complex
λmax (nm), εmax (mol/L cm)–1
λmax (nm) (solid state)
1
2
3
4
5
6
267 (sh), 1.4 × 104; 296 (sh), 5.7 × 103; 352, 1.9 ×104
264 (sh), 1.8 × 104; 322, 3.7 × 104; 374 (sh), 2.3 × 104
264 (sh), 2.2 × 104; 372, 5.5 × 104; 415 (sh), 3.9 × 104
269 (sh), 1.6 × 104; 277 (sh), 1.2 × 104
270 (sh), 1.7 × 104; 277 (sh), 1.5 × 104; 331, 3.6 × 104
269 (sh), 2.1 × 104; 277 (sh), 1.6 × 104; 377, 5.8 × 104
269 (sh), 355
328, 384 (sh)
380, 428 (sh)
250 (sh)
250 (sh), 340, 364 (sh)
270 (sh), 396
Fig. 9. Solution absorption spectra of palladium complexes 1, 2,
and 3 in CH3CN (dotted line), CH2Cl2 (broken line), and toluene
(solid line).
Fig. 8. Solid-state absorption spectra of palladium complexes 1–6.
the amount of cis is low in CH2Cl2 (ca. 5% as determined by
integration of the 31P NMR spectrum) and should not signif-
icantly affect the spectra. In the solution spectrum of 1, there
are shoulders between 250 and 300 nm and a distinct peak at
352 nm. The higher energy bands are assigned to n→π* and
π→π* transitions of the ligands and are not substantially
shifted from the spectra of the uncoordinated PT (31). The
band at 352 nm is not seen in the spectra of PT or in the
analogous gold (26) or platinum (4) complexes. A band at
comparable energy and molar absorptivity is seen in the
spectra of other dichlorobis(phosphine) palladium com-
plexes (40, 50–55) and is usually assigned to a LMCT tran-
sition (Cl→Pd). The energy of charge transfer bands
typically change with the polarity of the solvent (56); how-
ever, for 1 no significant shift was observed for this band
when comparing the spectra in toluene, CH2Cl2, and CH3CN
(Fig. 9). In the solid state the band does shift slightly (3 nm).
Two overlapping bands can be seen in both the solution
(322 nm and 374 nm) and solid-state (328 and 384 nm) ab-
sorption spectra of 2. Presumably, one of these peaks is due
to a π→π* transition in the bithienyl group and the other to
a Cl→Pd charge transfer band, similar to that seen in the
spectrum of 1. The higher energy band is assigned as the
π→π* transition, as it is very similar in energy to PT2
(λmax = 333 nm) (31). The Pd center appears to have little
effect on the absorbance of the thiophene group. The lower
energy band is assigned to the LMCT transition, which is
shifted by 22 nm when compared with the spectrum of 1.
This indicates that the phosphinothiophene ligand has some
influence on the LMCT transition. Although the shift is sur-
prisingly large, it is known that there is a significant contri-
bution from triphenylphosphine to the LMCT band of trans-
dichlorobis(triphenylphosphine) palladium (II), as deter-
mined by resonance Raman spectroscopy (51). When the so-
lution spectra for 2 taken in toluene, CH2Cl2, and CH3CN
are compared, (Fig. 9) a shift in energy and a change in the
intensity of the lower energy band are observed.
The solution spectrum of 3 consists of a broad band at
372 nm with a shoulder at 415 nm. The peak is assigned to a
terthiophene-based π→π* transition. As in the spectrum of
2, it is very similar in energy to the ligand (λ max = 374 nm),
indicating little influence of the Pd center on the conjugated
moiety. This has also been noted by Gray and co-workers
(29). The solid-state spectrum of 3 appears much the same
as the solution spectrum, but the peak broadens and shifts to
lower energy (380 and 428 nm). These effects may be due to
the π-stacking of the terthienyl groups observed in the crys-
tal structure or to the increased planarity of the terthienyl
groups in the solid state (57–60). As was seen in the β-sub-
stituted phosphinothiophene palladium(II) complexes previ-
ously reported (24), no distinct LMCT band is observed
here. It is likely that this band overlaps with the terthienyl
π→ π * transition, which would also indicate some phos-
phinothiophene character in the LMCT. There appears to be
a small decrease in the energy of the band, as the polarity of
the solvent is increased and the shoulder at 415 nm becomes
more prominent (Fig. 9). This may be due to conformational
effects or, if the LMCT overlaps with the terthienyl π→ π *
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Can. J. Chem. Vol. 85, 2007
9. A. Facchetti, M. Mushrush, M.-H. Yoon, G.R. Hutchison,
M.A. Ratner, and T.J. Marks. J. Am. Chem. Soc. 126, 13859
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transition, it may be solvatochromism of the LMCT transi-
tion.
The spectra of platinum complexes 4–6 are dominated by
the π→ π * transitions of the oligothiophene moieties. The
spectra of these complexes in solution and in the solid state
are very similar to those of the corresponding ligands, indi-
cating little electronic effect of the metal center on the
thienyl moiety. The shoulders in the spectra of 5 and 6 have
been assigned to conformational effects (57, 60).
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Conclusions
13. M.D. Curtis, J. Cao, and J.W. Kampf. J. Am. Chem. Soc. 126,
4318 (2004).
The series of palladium (II) and platinum (II) phos-
phinothiophene complexes reported here show interesting
variation in their solid-state crystal structures. Although both
cis and trans species are observed in the 31P NMR spectra of
the palladium complexes 1–3, the isolated crystals of both 2
and 3 exhibit trans geometry exclusively. The platinum com-
plexes 4–5 exist in solution only as the cis isomer, as evi-
denced by their respective 31P–195Pt coupling constants,
while 6 appears to have a small amount of the trans isomer
present. Only cis isomers are seen in the solid state.
We have focused on the intermolecular interactions be-
tween the oligothiophene groups and have found that in con-
trast to most organic oligothiophenes that pack in a
herringbone fashion (5), these metal complexes adopt a vari-
ety of structures in the solid state depending on both the
metal and the oligothiophene ligand. Interestingly, we find
similarities in the structures of 3 and 5, both of which pack
with the oligothiophene groups interlocked and on the same
side of the metal plane. The overlap between the terthienyl
groups is greater in 3 and it is tempting to speculate that if
the bithienyl groups in 5 were extended by one ring, the
overlap would improve to give a structure similar to 3. How-
ever, this is not observed and the Pt terthienyl complex 6 ex-
hibits quite a different structure, although also one in which
π-stacking between terthienyl groups is observed.
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We thank the Natural Sciences and Engineering Council
of Canada for funding of this work. TLS thanks the Walter
C. Sumner Foundation for a fellowship.
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