A new series of luminescent phosphine stabilised platinum ethynyl complexes

Article abstract of DOI:10.1039/b505484b

A series of cis-platinum ethynyl complexes with the general formula cis-[Pt(dppe)(C=CR)₂] (dppe = 1,2-bis(diphenylphosphino)ethane; R = C₆H₄-p-NO₂ 1, C₆H ₄-p-CH₃ 2, C₆H₄-p-C=CH 3 and C ₆H₄-p-C₆H₄-p-C≡CH 4) have been prepared by the coupling reaction of cis-[Pt(dppe)Cl₂] with two equivalents of the appropriate alkyne. The new complexes have been fully characterized by spectroscopic techniques, and the cis square planar arrangement at the platinum centre has been confirmed by single-crystal X-ray diffraction studies of complexes 1, 2 and 4. The absorption spectra of the complexes 1-4 are dominated by a π → π* band that contains some platinum (n + 1) p orbital character. The position of the band is dependent on the electron donating or withdrawing properties of the ethynyl substituents, R. Complex 1 displays a triplet emission in the green, at room temperature, while complexes 2-4, display singlet emissions in the blue. Again, the difference can be attributed to the nature of the R substituents. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b505484b

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F U L L P A P E R
A new series of luminescent phosphine stabilised platinum
ethynyl complexes†
Ramkrishna Saha,a Md. Abdul Qaium,a Dipen Debnath,a Muhammad Younus,*a
Nazia Chawdhury,b Nasim Sultana,c Gabriele Kociok-Ko¨hn,d Li-ling Ooi,d Paul R. Raithby*d
and Masashi Kijimae
a Department of Chemistry, Shahjalal University of Science & Technology, Sylhet, 3114,
Bangladesh. E-mail: myounus-che@sust.edu
b Department of Physics, Shahjalal University of Science & Technology, Sylhet, 3114,
Bangladesh
c Bangladesh Council for Scientific and Industrial Research, Dhaka, 1205, Bangladesh
d Department of Chemistry, University of Bath, Bath, UK BA2 7AY.
E-mail: p.r.raithby@bath.ac.uk
e Institute of Material Science, University of Tsukuba, Tsukuba, Ibaraki, 305-8573, Japan
Received 19th April 2005, Accepted 30th June 2005
First published as an Advance Article on the web 12th July 2005
A series of cis-platinum ethynyl complexes with the general formula cis–[Pt(dppe)(C≡CR)2] (dppe =
1,2-bis(diphenylphosphino)ethane; R = C6H4-p-NO2 1, C6H4-p-CH3 2, C6H4-p-C≡CH 3 and C6H4-p-C6H4-p-C≡CH
4) have been prepared by the coupling reaction of cis-[Pt(dppe)Cl2] with two equivalents of the appropriate alkyne.
The new complexes have been fully characterized by spectroscopic techniques, and the cis square planar arrangement
at the platinum centre has been confirmed by single-crystal X-ray diffraction studies of complexes 1, 2 and 4. The
absorption spectra of the complexes 1–4 are dominated by a p → p* band that contains some platinum (n + 1) p
orbital character. The position of the band is dependent on the electron donating or withdrawing properties of the
ethynyl substituents, R. Complex 1 displays a triplet emission in the green, at room temperature, while complexes
2–4, display singlet emissions in the blue. Again, the difference can be attributed to the nature of the R substituents.
the diimine-stabilised platinum ethynyl complexes have shown
Introduction
considerable promise as white15 and red16 OLED materials.
There is considerable current interest in transition metal r-
The opto-electronic properties of these materials stimulated our
bonded ethynyl complexes and polymers because of their
interest in investigating the electronic properties of the chelating
material and opto-electronic properties.1,2 Platinum r-bonded
phosphine-stabilised cis-platinum bis-ethynyl and dienyl com-
ethynyl complexes were first prepared by Hagihara and co-
plexes. In this paper, for the first time, we describe the synthesis
workers in 1977,3 and since then a plethora of new materials
and systematic study of the electronic properties of a series
that contain Group 8,4 Group 9,5 Group 106 and other
of complexes with the general formula cis-[Pt(dppe)(C≡CR)2]
(dppe = 1,2-bis(diphenylphosphino)ethane; R = C6H4-p-NO2
1, C6H4-p-CH3 2, C6H4-p-C≡CH 3 and C6H4-p-C6H4-p-C≡CH
4). We have been able to correlate the electronic properties of the
substituent groups on the alkyne with the solution absorption
and emission spectra of the complexes.
transition metals7 have been prepared and their physical and
electronic properties investigated. For the Group 10 systems
the majority of the investigations have been centred on trans-
platinum phosphine complexes. For example, trans-[PtL2Cl2]
(L = monoalkyl phosphine) with two equivalents of mono-
terminal alkynes (HC≡CR) gives trans-[PtL2(C≡CR)2] (R = H,
Me, Bu, etc.),8 while with one equivalent of diterminal alkynes
and poly-ynes (HC≡CRC≡CH) gives linear rigid rod poly-
mers [–PtL2–C≡CRC≡C–]n (R = aromatic or heteroaromatic
group).3 We, and others, have prepared and investigated the
electronic properties of a wide range of these rigid rod polymers
and their molecular precursors, and have shown that the more
electron withdrawing is the aromatic or heteroaromatic spacer
group, R, the lower is the band gap between the valence and
conductance bands in the polymers.9 Until recently cis-platinum
ethynyl complexes have been less well studied, partly because
the cis isomer is thermodynamically unstable with respect to
the trans isomer in solution, particularly when monophosphine
groups are present as the auxiliary ligands.8 For example, the
reaction of cis-[Pt(PBu3)2Cl2] with PhC≡CH below −20 ◦C
gives the cis-[Pt(PBu3)2(C≡CPh)2] product, while above this
temperature a 50 : 50 mixture of the cis and trans isomers are
obtained.8 The thermodynamic instability has been overcome
by using chelating phosphine10–12 or diimine groups,13,14 and
Results and discussion
Synthesis and characterisation
The synthesis of the cis-[Pt(dppe)(C≡CR)2] (dppe = 1,2-
bis(diphenylphosphino)ethane; R = C6H4-p-NO2 1, C6H4-
p-CH3 2, C6H4-p-C≡CH 3 and C6H4-p-C6H4-p-C≡CH 4)
complexes were initially carried out by an adaptation of
the Sonogashira coupling route (Scheme 1)8 using cis-
[Pt(dppe)Cl2] with two equivalents of HC≡CR, in diethylamine,
† This paper is dedicated to the memory of Ian Rothwell, an outstanding
organometallic chemist.
Scheme 1
2 7 6 0
D a l t o n T r a n s . , 2 0 0 5 , 2 7 6 0 – 2 7 6 5
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

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Table 1 Spectroscopic characterisations of the new complexes 1–4
IR
d 31P NMR
(CDCl3)
(CH2Cl2)/cm−1
d 1H NMR (CDCl3)
d 13C NMR (CDCl3)
FAB +ve; m/z
886 [M+ + H]
[Pt(dppe)(C≡C-
C6H4-NO2)2] 1
2116(s),
2096(w).
2.4 (m, 4H, P(CH2)2P), 7.15 and 28.77 (P(CH2)2P), 111.72
7.96 (dd, 8H, ≡C-C6H4-NO2),
7.37–7.90 (m, 20H, PPh)
2.2 (s, 6H, -C6H4-CH3), 2.4 (m,
4H, P(CH2)2P), 6.8 and 7.00
(dd, 8H, p-C6H4-p), 7.35–7.90
(m, 20H, PPh).
2.4 (m, 4H, P(CH2)2P), 2.95 (s,
2H, C≡C-H), 7.0 and 7.2 (dd,
43.69 (JPt–P
2275 Hz)
=
=
(C≡C), 123–145 (PPh and
≡CPh)
[Pt(dppe)(C≡C-
C6H4-CH3)2] 2
2115(m)
21.69 (-C6H4-CH3),
125.74–135.14 (PPh and ≡CPh)
42.16 (JPt–P
2280 Hz)
823 [M]+
[Pt(dppe)(C≡C-
2104(s),
2115(w),
84.6 (C≡CH), 107,112.1,112.6
(C≡C), 129–134 (PPh and
≡CPh)
42.6 (JPt–P
2283 Hz)
=
843 [M]+
C6H4-p-C≡CH)2] 3
3295(s), 3299(w) 8H, ≡C-C6H4-CCH), 7.35–7.95
(m, 20H, PPh)
[Pt(dppe)(C≡C-
C6H4-p-C6H4-p-
C≡CH)2] 4
2105(s)
2.45 (m, 4H, P(CH2)2P), 3.1 (s,
2H, C≡C-H), 7.2 and 7.35 (dd,
16H,
77.61, 84.14 (C≡CH), 121–142
(PPh and ≡C-C6H4-C6H4-C≡)
42.5 (JPt–P
2282 Hz)
=
996 [M+ + H]
Pt-C≡C-C6H4-C6H4-CCH),
7.4–8.0 (m, 28H, PPh and
-C6H4-CCH)
with CuI catalyst. The reaction of cis-[Pt(dppe)Cl2] with
p-nitrophenyleneacetylene, O2N-p-C6H4-C≡CH, afforded cis-
[Pt(dppe)(C≡C-C6H4-p-NO2)2], 1, in 70% yield. However, a
similar reaction where p-tolylacetylene, H3C-p-C6H4-C≡CH,
was used instead of O2N-p-C6H4-C≡CH, gave only a 26% yield.
An improved synthesis using a mixture of the more solublizing
diisopropylamine and dichloromethane (1 : 3), instead of
diethylamine, gave a high yield (86%). Complexes 3 and 4 were
synthesized similarly by using a mixture of diisopropylamine
and dichloromethane. All the reactions were carried out under
nitrogen atmosphere, at room temperature, for 24 h.
complexes in the expected region. Similar coupling of the ortho
and meta phenyl ring carbons bonded to phosphorus have been
reported for the complex trans-[Pt(PPh3)2(C≡C–p-C6H4-C6H4-
p-C≡CH)(C≡CC6H4-p-NO2)].17
For all four complexes the 31P NMR spectra contained signals
with the expected satellites due to coupling with the 195Pt nuclei
at d 43.6 [J(Pt–P) = 2275 Hz] for complex 1, d 42.16 [J(Pt–P)
=
2280 Hz] for complex 2, d 42.6 [J(Pt–P) = 2283 Hz] for complex
3, d 42.5 [J(Pt–P) = 2282 Hz] for complex 4. The chemical shift in
the 31P NMR spectrum of cis-[Pt(dppe)Cl2] is d 42.64 ppm with
a coupling constant of 3627 Hz. The coupling constants for the
complexes 1, 2, 3 and 4 are much lower than for [Pt(dppe)Cl2].
These lower values are consistent with a cis-square planar
complex but reflect the greater trans influence of the alkynyl
ligands compared to the chlorides.10,11,18,19
The complexes 1–4 were characterized by the IR, 1H, 13C{H},
31P{H}NMR, positive ion FAB-MS, and elemental analysis
(Table 1). The molecular and crystal structures of complex 1,
2 and 4 were determined by single crystal X-ray crystallography.
In the IR spectra the m(C≡C) stretching frequency is diag-
nostic of the characterization of the metal ethynyl complexes
and is useful in following the course of reactions. In the case
of mono alkynes, the absence of terminal C≡CH confirms the
completion of the reaction. For the cis-alkynyl complexes two
m(C≡C) stretches are expected and this is observed for complexes
1 and 3 with pairs of signals at 2116, 2096 cm−1 and 2115,
2104 cm−1, respectively. In addition, for complex 3 the expected
pair of signals for the m(C–H) stretches are observed at 3299 and
3295 cm−1. However, only one broad signal was observed for the
m(C≡C) stretches in complexes 2 and 4, and only one signal for
the m(C–H) stretch is observed for 4. Similar spectral features of
single absorption peak for both m(C≡C) and m(C–H) have been
observed for the complex cis-[PtL2(C≡C–C≡CH)2] (L = PEt3,
dppe and dppp).10
The molecular formula for the complexes were also established
by the intense molecular ion peaks in the positive ion FAB
mass spectra observed at m/z 886 [M+ + H] for 1, at m/z 823
[M]+ for 2, at m/z 843 [M]+ for 3, and at m/z 996 [M+ + H]
for 4. Fragmentation patterns showing the loss of the ethynyl
fragments were also observed.
Crystallography
Suitable single crystals of complexes cis-[Pt(dppe)(C≡CC6H4-
p-NO2)2] 1, cis-[Pt(dppe)(C≡CC6H4-p-CH3)2]
2 and cis-
[Pt(dppe)(C≡CC6H4-p-C6H4-p-C≡CH)2] 4 were grown from
CH2Cl2/hexane solutions and the crystal and molecular struc-
tures obtained in order to confirm the molecular geometries
and to investigate any correlation of structural trends with the
electronic properties of the materials. The molecular structure
of complexes 1, 2 and 4 are shown in Figs. 1–3, respectively,
1
In the H NMR spectra of the complexes 3 and 4 singlet
signals at d 2.95 and 3.1 confirmed the presence of the C≡C-
H groups. For all the complexes, the expected peaks at d
2.4–2.5 were observed for the methylenic protons of the dppe
group, P(CH2)2P. The complex multiplets in the range of d 7.3–
8 indicate the presence of diphosphine aromatic protons. All
the complexes showed the expected (AAꢀBBꢀ) pattern for the
p-disubstituted aromatic protons.
In the 13C NMR spectrum of complex 3 the signals at d
84.6, 107, 112.1 and 112.6 indicate the presence of a C≡C unit.
However, it was not possible to detect all the signals for the C≡C
group for complexes 1, 2 and 4 because of the expected weak
resonance and because of the relatively low solubility of these
complexes. The P(CH2)2P carbon signal is observed at d 28.77
only for 1. The signals for the aromatic carbons appear at d 120–
145. For complex 2, two triplets at d 133.98 and 129.08 with
J ∼ 5 Hz were observed because of the coupling of ortho and
meta carbons, respectively, of the phenyl rings with phosphorus
nuclei. Similar spectral features were observed for all the other
Fig. 1 The molecular structure of cis-[Pt(dppe)(C≡CC6H4-p-NO2)2] 1
showing the atom numbering scheme.
D a l t o n T r a n s . , 2 0 0 5 , 2 7 6 0 – 2 7 6 5
2 7 6 1

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◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complexes 1, 2
and 4
1
2
4
Pt(1)–C(1)
Pt(1)–C(9)
C(1)–C(2)
C(2)–C(3)
C(9)–C(10)
C(10)–C(11)
Pt(1)–P(1)
Pt(1)–P(2)
P(1)–C(17)
P(2)–C(18)
C(17)–C(18)
2.022(8)
2.024(8)
1.197(12)
1.433(12)
1.190(12)
1.433(11)
2.274(2)
2.269(2)
1.838(8)
1.848(8)
1.522(11)
2.011(3)
2.015(4)
1.203(5)
1.451(5)
1.214(5)
1.445(5)
2.2644(9)
2.2645(9)
1.846(4)
1.835(4)
1.529(5
2.027(5)
2.020(5)*
1.175(7)
1.424(7)
1.193(7)*
1.435(7)*
2.2716(13)
2.2675(12)*
1.842(5)
1.842(5)*
1.514(10)ꢀ
Fig. 2 The molecular structure of cis-[Pt(dppe)(C≡CC6H4-p-CH3)2] 2
showing the atom numbering scheme.
C(1)–Pt(1)–C(9)
C(1)–Pt(1)–P(1)
C(9)–Pt(1)–P(1)
C(1)–Pt(1)–P(2)
C(9)–Pt(1)–P(2)
P(1)–Pt(1)–P(2)
Pt(1)–C(1)–C(2)
C(1)–C(2)–C(3)
Pt(1)–C(9)–C(10)
C(9)–C(10)–C(11)
96.7(3)
90.4(2)
172.6(2)
175.1(2)
86.9(3)
85.89(8)
169.9(7)
170.3(9)
166.6(8)
179.1(9)
95.91(15)
87.29(10)
175.86(11)
172.78(10)
91.00(11)
85.72(3)
168.0(3)
179.1(4)
167.4(3)
173.7(4)
94.0(3)
90.81(14)
174.89(14)ꢀ
174.89(14)ꢀ
90.81(14)
84.47(7)ꢀ
171.9(4)
175.8(5)
170.9(4)*
175.3(6)*
linearity with the two alkyne units showing slight deviation from
linearity with an average Pt–C≡C angle of 167.7◦ and an average
C≡C–C angle of 176.4◦. The two phenyl rings on the alkyne
ligands make angles of 62.7◦ and 58.2◦ with the PtP2C2 square
plane. These angles are more similar to each other compared
to the values found in 1. The chelating dppe ligand again
adopts the gauche conformation with a P–CH2–CH2–P torsion
angle of 50.7◦. Despite the difference in the electron donating
properties of NO2, in 1, and CH3, in 2, the bond parameters in
2 (Table 2) are very similar to those in 1. The trend is that the
* Structure 4 contains two independent half molecules, the “*” values
are quoted for the independent bond parameters in the second molecule.
ꢀThe final atom in the sequence is related to the others by the symmetry
operation 0.5 − x, y, 1.5 − z.
˚
Pt–C(alkynyl) distances in 2 are ca. 0.01 A shorter than in 1,
˚
and the Pt–P distances in 2 are also ca. 0.004 A shorter than
those in 1, however, these distances are barely significant at the
3r significance level.
In the crystal structure of 4 each of the two independent
half molecules of the platinum complex sit on crystallographic
two-fold axes which, in each case, pass through the platinum
atom and the mid point of the C–C bond of the P–CH2–CH2–
P unit. One of the independent molecules is shown in Fig. 3
while selected bond parameters are presented in Table 2. As in
1 and 2 the platinum atoms adopt a square planar coordination
geometry and the alkyne groups show some deviation from
linearity with average Pt–C≡C and C≡C–C angles of 171.4◦ and
175.6◦, respectively. The arene ring of the biphenyl group, in each
half molecule, makes an angle of 63.26◦ (molecule 1) and 53.49◦
(molecule 2) with the PtP2C2 square plane. The two arene rings
in the biphenyl groups make angles of 37.38◦ (molecule 1) and
30.55◦ (molecule 2) with each other as is expected to minimise
H · · · H short contacts between the adjacent rings. As in 1 and
2 the P–CH2–CH2–P unit of the chelating phosphine adopts the
gauche conformation with torsion angles of −53.7◦ (molecule
1) and −50.5◦ (molecule 2). The bond parameters around the
platinum centres in 4 are in closer agreement with those observed
in 1 than those in 2 but at the 3r level of significance these
and selected structural parameters are presented in Table 2.
Complexes 1 and 2 both crystallise in the monoclinic space group
P21/n with one independent molecule in the asymmetric unit.
Since the cell dimensions for 1 and 2 are also similar it appears
that the replacement of the para-NO2 group in 1 by the para-
CH3 group in 2 has little influence on the crystal packing in
the two materials. In 2 there are no intermolecular interactions
that are less than the sum of the van der Waals radii and in 1
˚
there is only a weak contact of 2.466 A between O(1) of one
of the nitro groups and a phenyl proton, H(23), related by the
symmetry operation 1 − x, 1 − y, −z. Complex 4 crystallises
in the monoclinic space group P2/n with two independent
but structurally similar half molecules in the asymmetric unit
together with half a molecule of hexane solvent. Again, there are
no significant intermolecular interactions. The shortest contact
in the crystal structure involving the alkyne H-atoms is a
weak C≡C–H · · · p(arene ring) interaction between the alkynylic
hydrogen, H(26ꢀ) in the second independent half molecule and
a carbon, C(35ꢀ), in a adjacent ring, related by the symmetry
˚
operation 1 − x, 3 − y, 1 − z, at a distance of 2.875 A.
The crystal structure of 1 confirms that the platinum(II) centre
adopts the square planar cis-configuration (Fig. 1), with the
˚
greatest deviation from the PtP2C2 plane being 0.038 A for Pt(1).
The two alkyne units show slight deviation from linearity with an
average Pt–C≡C angle of 168.3◦ and an average C≡C–C angle
of 174.7◦. The two phenyl rings of the two alkyne ligands make
dihedral angles of 86.2◦ {C(3)–C(8)} and 60.4◦ {C(11)–C(16)},
respectively, with the PtP2C2 square plane. The two NO2 groups
are planar and almost coplanar with the phenyl rings to which
they are bonded; the N(1)O(1)O(2) plane makes a dihedral angle
of 2.4◦ with the C(3)–C(8) ring plane, and the N(2)O(3)O(4)
plane makes an angle of 12.4◦ with the C(11)–C(16) ring plane.
The chelating phosphine adopts a gauche configuration with a
P(1)–C(17)–C(18)–P(2) torsion angle of −49.9◦. The molecular
parameters are similar to those found in related cis-phosphine
substituted platinum alkynes.10,20
Fig. 3 The molecular structure of one of the two independent molecules
in the asymmetric unit of cis-[Pt(dppe)(C≡CC6H4-p-C6H4-p-C≡CH)2]
4. The molecule lies on a crystallographic two-fold axis and the two
halves of the molecule are related by the symmetry operation 0.5 − x, y,
1.5 − z.
The molecular structure of 2 (Fig. 2) is closely related to
that of 1. The platinum(II) centre is again square planar with
˚
a maximum deviation from the PtP2C2 plane of 0.034 A for
Pt(1). The two alkyne groups again show some deviation from
2 7 6 2
D a l t o n T r a n s . , 2 0 0 5 , 2 7 6 0 – 2 7 6 5

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Table 3 Spectral absorption data for complexes 1–4 recorded in
CH2Cl2 at room temperature
Complex
kmax/nm
Extinction coefficient (e)/dm3 mol−1 cm−1
1
2
3
4
365
321
328
334
63,333
28,340
69,632
102,509
differences are hardly meaningful, and all are within the range
reported for related square planar complexes of platinum(II) that
contain a chelating phosphine group.10
Absorption and emission spectroscopy
For all four complexes the lowest energy absorption bands in
the UV/visible spectra, in dichloromethane solution, at room
temperature, occurred in the range 318–360 nm (Fig. 4 and
Table 3). The lowest energy band, in each case, was assigned to a
predominantly p(C≡C) → p*(C≡C) transition by comparison
with related systems.14,21 but can be considered to have some
LMCT character because of the mixing of the p* orbital of the
ligand with the platinum (n + 1) p orbitals.22 A comparison
of the spectra (Fig. 4) shows that there is a red shift in the
energy of the lowest energy band in the order 2 → 3 → 4 → 1
as the electron donating properties of the substituent R group
decrease. Thus, the absence of an extended chromophore such as
bipyridine or terpyridine means that MLCT effects are less in the
chelating phosphine stabilized systems compared to the related
diimine complexes,14 however, the complexes do remain sensitive
to the nature of the substituents on the ethynyl ligands. In addi-
tion, a comparison of these cis-chelating phosphine complexes
(Table 3) with comparable trans-diphosphine platinum(II) alky-
nyls9 indicates that there is a blue shift in the absorption spectra
of the cis-complexes compared to the trans-complexes, consis-
tent with a lack of conjugation through the metal centres in the
cis-series.
Fig. 5 The emission spectra of complexes 1–4, in CH2Cl2, at room
temperature. For complexes 2, 3 and 4 the excitation was in the range
320–350 nm while that for 1 was at 450 nm.
suggests that this emission does not originate from the observed
absorption band, and it may be that the emission is from a
triplet excited state stabilised by the strong spin–orbit coupling
induced by the platinum centre,9,24 as has been observed in
some platinum(II) alkyl monomers.25 Some related platinum-
containing complexes show no fluorescence band but only a
phosphorescence band around 2.7 ev.2,26
Conclusions
The new complexes cis-[Pt(dppe)(C≡CR)2] (dppe = 1,2-
bis(diphenylphosphino)ethane; R = C6H4-p-NO2 1, C6H4-p-
CH3 2, C6H4-p-C≡CH 3 and C6H4-p-C6H4-p-C≡CH 4) have
been prepared by the coupling reaction of cis-[Pt(dppe)Cl2]
with two equivalents of the appropriate alkyne, and have been
characterised spectroscopically. The crystal structures of 1, 2
and 4 have been determined and it has been shown that the
complexes adopt a square planar coordination geometry around
the platinum(II) centre. There are no significant differences
in the bonding parameters within the series and no strong
intermolecular interactions. The UV/visible absorption and
emission spectra, in solution, at room temperature, do show
significant differences across the series that can be correlated
to the differences in the nature of the R substituents. In this
series of compounds, the lowest band gap is associated with
complex 1 that has an electron withdrawing NO2 group. The
magnitude of the band gap of this type of system is governed
by the p-electron conjugation. Electron withdrawing groups,
that increase the electron density along the conjugated path,
decrease the band gap.9,27 In complexes 1–4 the absorption
spectra are dominated by p(C≡C) → p*(C≡C) transitions
and the higher lying orbitals include some platinum (n + 1)
p orbital character. There is a red shift in the energy of the
lowest energy band in the order 2 → 3 → 4 → 1 as the electron
donating properties of the substituent R group increase. Thus,
the absence of an extended chromophore such as bipyridine or
terpyridine means that MLCT effects are less in the chelating
phosphine stabilized systems compared to the related diimine
complexes. The emission spectra of complexes 2, 3 and 4
display one fluorescence emission band in the blue region of the
electromagnetic spectrum. However, for 1, a weak, broad band
in the green region of the spectrum, centred at approximately
565 nm, attributable to a phosphorescence emission, is observed.
Here, the triplet excited state is stabilised by the strong spin–orbit
coupling induced by the platinum centre.
Fig. 4 The UV/visible absorption spectra of complexes 1–4, in CH2Cl2,
at room temperature.
The room temperature photoluminescence spectra, in
dichloromethane, for complexes 1–4 are shown in Fig. 5.
Complexes 2, 3 and 4 display one emission band in the
blue region of the electromagnetic spectrum. The feature is
attributable to emission from a singlet excited state (fluo-
rescence) because of the small energy shift (Stokes’ shift)
between the absorption and emission bands; typical values
for 2-methoxy-5-[(2ꢀ-ethylhexyl)oxy] derivatives of PPV (PPV =
poly(pheylenevinylene) and MEH-PPV (MEH-PPV = poly(2ꢀ-
methoxy-5-(2ꢀ-ethylhexoxy)-1,4-pheylenevinylene) are in the
range 0.2–0.3 eV.23 No phosphorescence is observed for any
of these materials at room temperature. However, for 1, a weak,
broad emission band in the green region of the spectrum, centred
at approximately 565 nm, is observed. For this complex the
absorption band is observed at 360 nm (3.43 eV) whereas the
emission is at 565 nm (2.19 eV). The large Stokesꢀ shift (1.24 eV)
Experimental
All the reactions were performed under an inert atmosphere us-
ing standard Schlenk techniques, but no special precautions were
taken to exclude oxygen during work-up. Platinum(II) dichloride
D a l t o n T r a n s . , 2 0 0 5 , 2 7 6 0 – 2 7 6 5
2 7 6 3

View Article Online
Table 4 Crystallographic data for structures 1, 2 and 4a
Identification code
1
2
4
Empirical formula
Formula weight
Space group
C42H32N2O4P2Pt
885.73
P21/n
8.4730(1)
14.0020(2)
30.4920(6)
91.167(1)
3616.79(10)
4
C44H38P2Pt
823.77
P21/n
8.6180(1)
13.9790(2)
29.9090(4)
91.853(1)
3601.28(8)
4
C61H49P2Pt
1039.03
P2/n
16.3404(2)
8.9013(1)
32.8872(15)
95.902(1)
4578.11(11)
4
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
D(calc)/Mg m−3
1.627
4.014
1.519
4.015
1.450
3.056
l/mm−1
F(000)
1752
49363
1640
24838
2092
31321
Reflections collected
Independent reflections
Goodness-of-fit on F2
Final R indices [I > 2r(I)]: R1, wR2
R indices (all data): R1, wR2
6288 [R(int) = 0.1457]
10344 [R(int) = 0.0724]
10684 [R(int) = 0.0610]
1.165
1.038
1.080
0.0426, 0.1124
0.0743, 0.1585
1.275 and −2.472
0.0404, 0.1015
0.0515, 0.1095
2.681 and −3.873
0.0422, 0.1018
0.0669, 0.1300
1.384 and −3.442
−3
˚
Largest diff. peak and hole/e A
a
˚
Common parameters: wavelength, k = 0.71073 A; temperature, T = 150(2) K; space group = monoclinic; absorption correction = semi empirical.
was obtained from Johnson Matthey, and CuI, Pd(OAc)2, Ph3P
and HC≡C-C6H4-CH3 were purchased from Sigma-Aldrich.
Solvents were pre-dried from appropriate drying agents and
freshly distilled before use.28 Grade II neutral alumina and
Kieselgel 60 (70–230) were used for column chromatography.
IR spectra were recorded as CH2Cl2 solutions, in a NaCl
cell, on a Nicolet Nexus FTIR spectrometer. All the NMR
C42H32O4N2P2Pt: C, 56.95; H, 3.64; N, 3.16. Found: C, 57.2;
H, 3.64; N, 3.21%.
Synthesis of [Pt(dppe)(C≡C-C6H4-CH3)2] 2
Similarly, [Pt(dppe)Cl2] (0.3329 g, 0.05 mmol), HC≡C-C6H4-
CH3 (0.228 g, 2 mmol) and CuI (15 mg) were stirred in
CH2Cl2/diisopropylamine (3 : 1) for 24 h. Similar work-up
gave a pale yellow solid in 86% yield (0.350 g). Anal. Calc.
for C44H38P2Pt: C, 64.15; H, 4.64. Found: C, 63.9; H, 4.69%.
1
samples were prepared as CDCl3 solutions and standard H
1
1
NMR, 13C{ H} NMR and 31P{ H} NMR spectra were recorded
on a Bruker NMR spectrometer at 300.22 MHz, 75.50 MHz
and 121.53 MHz respectively. H NMR and 13C{ H} NMR
1
1
Synthesis of [Pt(dppe)(C≡C-C6H4-p-C≡CH)2] 3
were referenced to solvent resonances and 31P{ H} NMR was
1
referenced to external trimethyl phosphate. Mass spectra were
recorded on Autospec Mass spectrometer by the fast atom bom-
bardment (FAB) technique. Photoluminescence (PL) spectra for
all the complexes were taken from 10−4 M dichloromethane
solution. The photoluminescence spectra of the solutions were
measured from a 10 mm quartz cuvette at room temperature
using a PerkinElmer LS55 Luminescence spectrometer. The
spectrometer has a range from 200 nm to 800 nm for excitation.
Optical excitation was provided by a Xenon discharge lamp,
spectrally resolved by a Monk-Gillieson type monochromator.
The PL spectra of 2–4 were taken with excitation from the
320–350 nm light beams. The excitation lines were chosen such
that the absorption was enough to excite the polymers. The PL
spectrum of 1 was obtained with excitation at 450 nm light beam.
The PL signals were passed through a second monochromator
and the corrected emission data were collected using a red-
sensitive (400–900 nm) R928 photomultiplier. Microanalyses
were performed at the Department of Chemistry, University
of Bath, UK.
The procedure of 1 was followed, but using HC≡C-C6H4-p-
C≡CH, and this afforded 3 as a pale yellow solid in 58% yield
(0. 370 g). Anal. Calc. for C46H34P2Pt·0.5C6H14: C, 66.4; H, 4.51.
Found: C, 66.6; H, 4.33%.
Synthesis of [Pt(dppe)(C≡C-C6H4-p-C6H4-p-C≡CH)2] 4
The same procedure as for 1 was followed but using HC≡C-
C6H4-p-C6H4-p–C≡CH to give 4 as off white solid in 50% yield
(0.375 g). Anal. Calc. for C58H42P2Pt·0.5C6H14: C, 70.6; H, 4.6.
Found: C, 70.4; H, 4.44%.
Crystallography
For crystal data, see Table 4.
Data collection and reduction. The crystals of 1, 2 and 4
were mounted in inert oil on glass fibres. Data were measured
˚
using Mo-Ka radiation (k = 0.71073 A) with a Nonius Kappa
CCD diffractometer equipped with an Oxford Cryostream low-
temperature attachment.
cis-[(dppe)PtCl2],29
HC≡C-C6H4-NO2,30
HC≡C-C6H4-
C≡CH30 and HC≡C-C6H4-C6H4-C≡CH30 were prepared by
Structure solution and refinement. Structures were solved
by direct methods (SHELXS-86)31 and subjected to full-
matrix least-squares refinement on F2 (SHELXL-97).32 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
were included using rigid methyl groups or a riding model.
CCDC reference numbers 269323–269325.
literature methods.
Synthesis of [Pt(dppe)(C≡C-C6H4-NO2)2] 1
To
a mixture of CH2Cl2/diisopropylamine (3 : 1) cis-
[(dppe)PtCl2] (0.300 g, 0.45 mmol), HC≡C-C6H4-NO2 (0.198 g,
1.35 mmol) and CuI (17 mg) were sequentially added and the
mixture was stirred for 24 h at room temperature under a N2
atmosphere. The solvent was removed under reduced pressure.
The residue was washed 3 times with hot n-hexane (50 ml ×
3) to remove excess ligand. The precipitate was dissolved in a
minimum volume of CH2Cl2 and applied to a short alumina
column (5 inches). After removal of CH2Cl2 the bright yellow
solid was isolated in 69% yield (0.275 g). Anal. Calc. for
See http://dx.doi.org/10.1039/b505484b for crystallographic
data in CIF or other electronic format.
Acknowledgements
We are grateful to the Leverhulme Trust, the Japanese Society for
the Promotion of Science and the Royal Society for financial sup-
port (to M. Y.), Shahjalal University for study leave (to M. Y.),
the University of Bath for a studentship (to L.-L. O.), and
2 7 6 4
D a l t o n T r a n s . , 2 0 0 5 , 2 7 6 0 – 2 7 6 5

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the EPSRC for funding to purchase the X-ray diffractometer.
The generous loan of platinum salts by Johnson Matthey plc is
gratefully acknowledged.
10 M. I. Bruce, K. Costuas, J.-F. Halet, B. C. Hall, P. J. Low, B. K.
Nicholson, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton
Trans., 2002, 383.
11 S. M. Al-Qaisi, K. J. Galat, M. Chai, D. G. Ray, P. L. Rinaldi, C. A.
Tessier and W. J. Youngs, J. Am. Chem. Soc., 1998, 120, 12149; J. A.
Whiteford, C. V. Lu and P. J. Stang, J. Am. Chem. Soc., 1997, 119,
2524.
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D a l t o n T r a n s . , 2 0 0 5 , 2 7 6 0 – 2 7 6 5
2 7 6 5