Temperature dependence of the crystal structure and luminescence properties of [Pt{4′-(o-ClC6H4)trpy}Cl]SbF6 (trpy = 2,2′:6′,2″-terpyridine)

Article abstract of DOI:10.1039/b209754k

The synthesis and characterisation of 4′-(2?-chlorophenyl)-2, 2′:6′,2″-terpyridine [4′-(o-ClC₆H ₄)trpy] and [Pt{4′-(o-ClC₆H₄)trpy}Cl] SbF₆ are described. An X-ray crystal structure determination of the salt reveals columns of cations and anions with the cations stacked parallel and head-to-tail. At 295 K the Pt ... Pt distances alternate between 3.374(1) and 3.513(1) A while the perpendicular separation between the mean planes comprising the platinum and three bonded nitrogen atoms remains constant at 3.35 A. As evidenced by crystal structure determinations at 260, 240 and 180 K, the shorter Pt ...Pt distances become systematically shorter and the longer Pt ... Pt distances become systematically longer as the temperature of the crystal is lowered. Structural evidence is presented that shows that this trend is a consequence of the more closely separated platinum atoms slipping into line with respect to a perpendicular to the above planes; at the same time the more widely separated platinum atoms fall out-of-line as the temperature is lowered. Emission spectra measured on a polycrystalline sample of the salt at 40 K intervals from 280 to 80 K comprise a single, asymmetric and featureless band [λ_max^em = 609 nm at 280 K] that systematically increases in intensity as the temperature is lowered. There is a slight blue shift in the wavelength of the emission maximum as the temperature is lowered to 240 K but, below this temperature, the emission maximum gradually and systematically shifts to the red [λ_max^em = 642 nm at 80 K]. This behaviour is rationalised in terms of ³MMLCT emission, the energy of which being determined by subtle changes in the extent of d _z2-d_z2 orbital overlap between adjacent platinum atoms separated by the shorter distance on one hand, and between platinum atoms separated by the longer distance on the other. The Royal Society of Chemistry 2003.

Full text of DOI:10.1039/b209754k

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Temperature dependence of the crystal structure and luminescence
properties of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6 (trpy ؍ 2,2Ј:6Ј,2Љ-
terpyridine)†
John S. Field,*a Jan-Andre Gertenbach,a Raymond J. Haines,a Lesibana P. Ledwaba,a
Ndoda T. Mashapa,a David R. McMillin,b Orde Q. Munroa and Grant C. Summertona
a School of Chemical and Physical Sciences, University of Natal, Private Bag X01,
Pietermaritzburg 3201, South Africa. E-mail: fieldj@nu.ac.za
b Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393, USA.
E-mail: mcmillin@purdue.edu
Received 4th October 2002, Accepted 9th January 2003
First published as an Advance Article on the web 11th February 2003
The synthesis and characterisation of 4Ј-(2ٞ-chlorophenyl)-2,2Ј:6Ј,2Љ-terpyridine [4Ј-(o-ClC6H4)trpy] and
[Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6 are described. An X-ray crystal structure determination of the salt reveals columns
of cations and anions with the cations stacked parallel and head-to-tail. At 295 K the Pt ؒ ؒ ؒ Pt distances alternate
between 3.374(1) and 3.513(1) Å while the perpendicular separation between the mean planes comprising the
platinum and three bonded nitrogen atoms remains constant at 3.35 Å. As evidenced by crystal structure
determinations at 260, 240 and 180 K, the shorter Pt ؒ ؒ ؒ Pt distances become systematically shorter and the longer
Pt ؒ ؒ ؒ Pt distances become systematically longer as the temperature of the crystal is lowered. Structural evidence is
presented that shows that this trend is a consequence of the more closely separated platinum atoms slipping into line
with respect to a perpendicular to the above planes; at the same time the more widely separated platinum atoms fall
out-of-line as the temperature is lowered. Emission spectra measured on a polycrystalline sample of the salt at 40 K
intervals from 280 to 80 K comprise a single, asymmetric and featureless band [λmaxem = 609 nm at 280 K] that
systematically increases in intensity as the temperature is lowered. There is a slight blue shift in the wavelength of the
emission maximum as the temperature is lowered to 240 K but, below this temperature, the emission maximum
gradually and systematically shifts to the red [λmaxem = 642 nm at 80 K]. This behaviour is rationalised in terms of
3
2
2
MMLCT emission, the energy of which being determined by subtle changes in the extent of dz Ϫdz orbital overlap
between adjacent platinum atoms separated by the shorter distance on one hand, and between platinum atoms
separated by the longer distance on the other.
[Pt(4Ј-(Ph)trpy)Cl]BF4ؒCH3CN salt has the cations stacked as
Introduction
tetramers in the solid state.10 Nevertheless, this salt exhibits
3MMLCT emission with a red-shift in the emission maximum
on cooling.10 The triflate 6 and orange perchlorate7 salts of the
[Pt(trpy)Cl]ϩ cation exhibit dimeric structures, i.e., where the
cations are stacked as metal–metal bonded pairs in an infinite
π-stack. Interestingly, whereas a red shift in the emission maxi-
mum of 25 nm is observed on cooling the triflate salt from room
temperature to 77 K,6,9 there is very slight blue shift of 5 nm in
the emission energy (λmax) when the perchlorate salt is cooled
from 300 to 77 K.7 A fourth example is the tetrafluoroborate
salt of the [Pt{4Ј-(o-CH3C6H4)trpy}Cl]ϩ cation, a yellow com-
pound that has alternating Pt ؒ ؒ ؒ Pt distances that are too long
Polypyridyl ligand complexes of platinum() have received con-
siderable attention insofar as their luminescence properties are
concerned.1–10 Of interest here are those materials that have
planar chromophores stacked in the solid state with significant
electronic interactions between them. Well-studied examples
are the red α-diimine complexes [PtCl2(bpy)]2,8 (bpy = 2,2Ј-bi-
pyridine), [Pt(CN)2(bpy)]11,12 and [Pt(CN)2(i-biq)]13 (i-biq =
3,3Ј-biisoquinoline). These three compounds have in common
linear chain structures in the solid state i.e., platinum atoms that
are equally spaced along an infinite stack, and emission that is
3
assigned as MMLCT [dσ*(Pt)–π*(bpy/i-biq)]. A distinguish-
ing feature of the emission is that it is red-shifted when the
temperature of the sample is lowered. This is explained by
the decrease in the Pt–Pt distance that occurs on cooling, as
well as by a linear relationship between the emission energy
and the inverse of the cube of the Pt–Pt distance of the type
originally determined by Gliemann et al., for the tetra-
cyanoplatinates.8,13,14 The red terpyridyl ligand complex
[Pt{4Ј-(o-CH3C6H4)trpy}Cl]SbF6 also possesses a linear chain
structure and similarly exhibits 3MMLCT emission that is
red-shifted as the temperature of the sample is lowered.15
2
2
to support dz Ϫdz interactions between the platinum atoms
and that, for this reason, exhibits excimeric π–π* rather than
3MMLCT emission.15
We report here the temperature dependence of the crystal
structure and solid state emission spectrum of [Pt{4Ј-
(o-ClC6H4)trpy}Cl]SbF6. A very small but discernible blue shift
in λmax is initially observed on cooling the sample from room
temperature to 240 K but, at temperatures below 240 K, there is
a systematic shift to the red in the emission maximum as the
temperature is lowered. It is in an attempt to gain a better
understanding of the temperature dependence of the solid
emission by this salt that we have determined the crystal
structure of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6 over a range of
temperatures. Specifically we wished to establish how the
intermolecular Pt–Pt distances change on cooling the sample
since, as noted above, these are linked to the emission energy.
We note that changes in intermolecular Pt–Pt distances and
hence the emission can also be induced by changing the solvent,
both in solution16 and in the solid state.17
On the other hand all other known terpyridyl ligand com-
plexes of platinum() that have a chloro-group as the fourth
ligand and for which crystal structures have been reported
do not have linear chain structures. For example, the red
† Electronic supplementary information (ESI) available: Fig. S1: Unit
cell contents of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6 viewed down the
[a]-axis and Fig. S2: View of the cation stack in [Pt{4Ј-(o-ClC6H4)trpy}-
Cl]SbF6. See http://www.rsc.org/suppdata/dt/b2/b209754k/
D a l t o n T r a n s . , 2 0 0 3 , 1 1 7 6 – 1 1 8 0
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 3
1176

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are the details of how the cations are stacked within a column.
Results and discussion
We first note that successive cations are related by centres of
inversion that repeat at half-intervals along the [a]-axis. This
ensures that the cation planes, specifically the mean planes
through the platinum and three bonded nitrogen atoms, are
parallel and that the cations stack head-to-tail (Fig. S2). Sec-
ondly, the platinum atoms zigzag about the [a]-axis as indicated
by a Pt ؒ ؒ ؒ Pt ؒ ؒ ؒ Pt angle of 167.1(1)Њ and, thirdly, the [a]-axis
is at an angle of ca. 16Њ to the normal to the cation planes. With
regard to the Pt ؒ ؒ ؒ Pt distances, these alternate in length along
the stack, adopting values of 3.374(1) and 3.513(1) Å in the 295
K structure. The perpendicular separation between successive
mean planes defined by the Pt, N(1), N(2) and N(3) atoms (the
interplanar spacing) remains constant along the stack with a
value of 3.35 Å at 295 K. Thus, the difference in the Pt ؒ ؒ ؒ Pt
distances is a result of the platinum atoms of successive pairs of
cations being slipped to different extents with respect to a line
drawn perpendicular to the platinum coordination plane. Fig. 2
illustrates the two different cation–cation interactions along the
stack taking a view perpendicular to the mean plane through
the platinum atom and the nitrogen atoms bonded to it. Note
that the direction of the o-ClC6H4 moiety twist about the C(8)–
C(16) bond is such that the chlorines point between the cations
making up the more widely separated pair (Fig. 2B) whereas
they point away from the adjacent cation for the more closely
separated pair (Fig. 2A).
The method of Kröhnke,18 suitably modified,15 was employed
for the synthesis of 4Ј-(2ٞ-chlorophenyl)-2,2Ј:6Ј,2Љ-terpyridine
[4Ј-(o-ClC6H4)trpy]. Full details of the synthetic procedures as
well as of the characterisation data for the chalcone intermedi-
ate and for the ligand itself are given in the Experimental
section. The preparation of the complex follows procedures
previously used in our laboratories.9,10,15 Thus treatment of a
refluxing acetonitrile solution of [Pt(PhCN)2Cl2] with one
equivalent of AgSbF6 afforded a yellow solution and a white
precipitate of silver chloride. After removal of the silver chlor-
ide one equivalent of the ligand was added to afford the desired
product as an analytically pure and air-stable orange crystalline
solid. Full characterisation data are given in the Experimental
section.
Crystal structure of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6 at 295 K
A perspective view of the cation is given in Fig. 1 with a
selection of interatomic distances and angles being listed in the
caption. The coordination geometry of the platinum atom is
irregular square-planar, deviations from the idealised geometry
being evident in the N(1)–Pt–N(2) and N(2)–Pt–N(3) angles of
81.1(3) and 80.8(3)Њ, respectively. This is due to the geometric
constraints imposed by the tridentate ligand and is typical of
terpyridyl ligand complexes of platinum().6,7,9,10,19 Also char-
acteristic of a coordinated terpyridyl ligand is that the platinum
to bridgehead nitrogen [N(2)] distance of 1.936(6) Å is signifi-
cantly shorter than the two platinum to outer nitrogen distances
of 2.013(6) [N(1)] and 2.018(6) Å [N(3)], respectively.6,7,9,10,19
The o-ClC6H4 moiety is twisted about the C(8)–C(16) bond, as
reflected by a dihedral angle of 54.5(2)Њ between the mean plane
through the non-hydrogen atoms of the 4Ј-substituent on one
hand and the mean plane through the non-hydrogen atoms of
the terpyridyl moiety on the other. This twist is a consequence
of steric interactions between the chlorine atom and a hydrogen
atom of the central pyridine ring that is also ortho with respect
to the interannular bond. All other structural features of the
isolated cation are entirely as expected.
Fig. 2 ORTEP diagrams (20% thermal ellipsoids) showing the two
types of cation–cation interactions along the cation stacks in [Pt{4Ј-
(o-ClC6H4)trpy}Cl]SbF6 viewed perpendicular to the mean planes
through the platinum atom and the three nitrogen atoms bonded to
it: A, Pt ؒ ؒ ؒ Pt = 3.374(1) Å (at 295 K); B, Pt ؒ ؒ ؒ Pt = 3.513(1) Å
(at 295 K).
Though the Pt ؒ ؒ ؒ Pt distances alternate along the stack and
the shorter of the distances of 3.374(1) Å certainly allows for a
Fig. 1 ORTEP view (40% thermal ellipsoids) of the cation in [Pt{4Ј-
(o-ClC6H4)trpy}Cl]SbF6. Selected interatomic distances (Å) and angles
(Њ): Pt–Cl(1) 2.230(2), Pt–N(1) 2.013(6), Pt–N(2) 1.936(4), Pt–N(3)
2.018(6); Cl(1)–Pt–N(1) 99.2(2), Cl(1)–Pt–N(2) 179.7(2), Cl(1)–Pt–N(3)
98.9(2), N(1)–Pt–N(2) 81.1(2), N(1)–Pt–N(3) 161.9(2), N(2)–Pt–N(3)
80.8(3).
2
2
significant dz Ϫdz interaction, the cation stacking is not strictly
classified as that of a dimeric structure as reported for the tri-
flate 6 and orange perchlorate7 salts of the [Pt(trpy)Cl]ϩ cation.
The reason is that the longer distance of 3.513(1) Å is close to
the value of ca. 3.5 Å, usually taken to be the upper limit for a
2
2
The crystal structure of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6 con-
sists of separate columns of cations and anions that are aligned
along the [a]-axis of the unit cell (Fig. S1). Of particular interest
significant dz Ϫdz interaction between two platinum atoms,20
and so cannot be ignored. The cation stacking is thus probably
best described as intermediate between that of a linear chain
D a l t o n T r a n s . , 2 0 0 3 , 1 1 7 6 – 1 1 8 0
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a
Table 1 Crystal and refinement data for [Pt{4(o-ClC6H4)trpy}Cl]SbF6
295 K
260 K
240 K
180 K
a/Å
b/Å
c/Å
6.844(1)
18.965(2)
18.333(3)
90.05(1)
2379.6(6)
2.261
6.821(1)
18.889(4)
18.302(3)
90.05(1)
2358.0(7)
2.282
6.818(1)
18.863(5)
18.276(4)
90.0(2)
2350.5(9)
2.289
6.787(2)
18.804(11)
18.221(8)
91.38(4)
2325(2)
2.315
β/Њ
V/Å3
Dc/g cmϪ3
Total no. of reflns. measured
Independent reflns.
Rint
5764
4174
0.016
3290
0.037
0.049, 0.117
308, 0
0.002
2.19 [Pt]
Ϫ1.52 [N(2)]
5879
4132
0.041
3233
0.042
0.056, 0.129
308, 0
0.001
2.43 [Pt]
Ϫ1.80 [N(2)]
8909
4116
0.029
3474
0.041
0.051, 0.139
308, 0
0.013
2.60 [Cl(2)]
Ϫ2.18 [Cl(2)]
5568
4066
0.021
3583
0.057
0.063, 0.189
308, 0
0.002
4.62 [Cl(2)]
Ϫ4.67 [Cl(2)]
Reflections observed [I >2σ(I )]
R1 b [I > 2σ(I )]
R1 b, wR2 c (all data)
No. of refined parameters, restraints
(Shift/esd)max
Max. ∆ρ/e ÅϪ3
Min. ∆ρ/e ÅϪ3
a Crystal data common to all temperatures: formula (Mw), C21H14Cl2F6N3PtSb (810.09); crystal system, monoclinic; space group, P21/n (no. 14);
2
2
Z = 4; F(000) = 1512; µ = 7.29 mmϪ1; refinement on F 2. b R1 = Σ||Fo| Ϫ |Fc||/Σ|Fo|. c wR2 = [Σ[w(Fo2 Ϫ Fc2)2]/Σ[w(Fo )2]]1/2 where w = 1/[σ2(Fo ) ϩ (0.1P)2]
and P = (Fo2 ϩ 2Fc2)/3.
structure and that of a dimeric structure. Indeed, it is the first
example of a structure that represents the interface between the
two extremes of a linear chain structure on one hand and a
truly dimeric structure on the other. As discussed below, this
has consequences for the temperature dependent emission
behaviour of the compound.
Photophysical and low-temperature crystal structure studies
The absorption spectrum of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6
measured in acetonitrile is very similar to those recorded in the
same solvent for salts of the closely-related [Pt(trpy)Cl]ϩ and
[Pt{4Ј-(RC6H4)trpy)}Cl]ϩ (R = H, CH3 and CF3) chromo-
phores.6,7,10 Accordingly, the same assignments are made. The
strong band at 284 nm and the vibrationally structured band
1
with maxima in the range 300–360 nm are attributed to (π–π)
transitions of the substituted terpyridyl ligand, while the two
less intense and poorly resolved peaks at wavelengths greater
than 380 nm are assigned to Pt(5d)
Fig. 3 Solid state emission spectra of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6
trpy(π*) (1MLCT)
recorded at 40 K intervals over the range 80 K (a) to 280 K (b): λex = 330
transitions. Data are given in the Experimental section. The
compound does not give rise to detectable emission when dis-
solved in degassed acetonitrile and irradiated with light of
330 nm wavelength at room temperature. Quenching of the
fluid emission is probably due to coordination of the donor
solvent to the platinum atom in an axial position.21 Unfortu-
nately, because of solubility problems, it was not possible to
measure emission spectra in a non-coordinating solvent such as
dichloromethane.
nm.
distances due to unit cell contraction as the temperature of the
crystal is lowered.8,11 One exception is the orange salt
[Pt(trpy)Cl]ClO4 that has been shown to exhibit a very slight
blue shift of 5 nm in the emission maximum (λmax) when the salt
is cooled from 300 to 77 K.7
In order to try and understand the temperature depend-
ence of the solid emission by [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6
we have also determined the crystal structure of the com-
pound at 260, 240 and 180 K. Crystal and refinement data for
the room temperature and three low temperature structures
are given in Table 1. Two observations follow from the data in
Table 1. Firstly, there is no phase change on cooling. Secondly,
there is a systematic decrease in the unit cell volume with an
approximately equal decrease of about 0.7 % in the lengths of
each of the unit cell axes over the full temperature range i.e.,
the contraction of the unit cell is essentially isotropic. Struc-
tural results pertinent to the following discussion are listed in
Table 2. Insofar as the Pt ؒ ؒ ؒ Pt distances are concerned these
exhibit the following trend on cooling: the shorter distance
becomes systematically shorter while the longer distance
becomes systematically longer. With regard to the inter-
planar spacing, this shows a very small reduction as the tem-
perature of the crystal is lowered from 295 to 180 K. We
conclude: as the temperature is lowered the closer platinum
atoms slip into line with respect to a perpendicular to the stack-
ing planes, and so get closer together, while the more distant
platinum atoms slip out-of-line, and so get further apart. This is
Emission spectra measured over a range of temperatures on a
microcrystalline sample of the [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6
salt are shown in Fig. 3. A single asymmetric band that increases
in intensity and decreases in width as the temperature is lowered
is observed. The lack of vibrational structure, the relative
narrowness of the band (fwhm = 1595 and 1219 cmϪ1 at 280 and
80 K, respectively) and the lifetime (259 and 2150 ns at 295 and
77 K respectively) are typical of 3MMLCT emission. This
assignment is also consistent with the presence of platinum
2
2
dz Ϫdz interactions in the crystal (vide supra). Of particular
interest is that there is a slight but discernible blue shift of ca. 10
nm in the emission maximum when the temperature is lowered
from 280 to 240 K. Thereafter the wavelength of the emission
maximum gradually and systematically increases from a value
of 619 nm at 240 K to 642 nm at 80 K. The initial blue shift is
unexpected. In nearly all studies of the temperature dependence
3
of solid MMLCT emission by bipyridyl and terpyridyl ligand
complexes of platinum() a red shift in the emission maximum
is observed when the temperature is lowered.1,6,8,11,13 This red
shift has been associated with a shortening of the Pt ؒ ؒ ؒ Pt
D a l t o n T r a n s . , 2 0 0 3 , 1 1 7 6 – 1 1 8 0
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Table 2 Structural data for [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6
295 K 260 K 240 K
Experimental
180 K
Materials
Reagents for the ligand synthesis were obtained and used as
follows: The 2-acetylpyridine (Fluka) and 2-chlorobenzalde-
hyde (Merck) were obtained commercially and used without
further purification while the N-{1-(2Ј-pyridyl)-1-oxo-2-ethyl-
}pyridinium iodide was synthesised by the method of
Kröhnke.18 All manipulations for the complex salt syntheses
were performed under an atmosphere of nitrogen using stand-
ard Schlenk tube techniques. The acetonitrile used as the
solvent and for the purposes of crystal growth was purified by
the method of Carlsen and Anderson.24 The dichloro-
bis(benzonitrile)platinum() (Strem) and the silver salt AgSbF6
(Fluka) were used without further purification.
Pt–Pt/Å
3.374(1) 3.352(1) 3.344(1) 3.311(1)
3.513(1) 3.523(1) 3.533(1) 3.550(1)
167.1(1) 165.7(1) 164.0(1) 163.1(1)
Pt–Pt–Pt/Њ
Interplanar spacinga/Å
3.35
3.35
0.40
1.05
3.33
3.34
0.36
1.13
3.33
3.32
0.27
1.20
3.31
3.30
0.00
1.32
Lateral offsetb/Å
a The interplanar spacing is defined as the averaged perpendicular dis-
tance between adjacent mean planes calculated through Pt, N1, N2 and
N3. The first value is the distance between planes linked by the shorter
Pt–Pt distance, the second is the distance between planes linked by the
longer Pt–Pt distance. It is unlikely that any small differences between
the two values are significant. b The lateral offset was calculated as the
length of the third side in a right-angled triangle with a hypotenuse
equal to the Pt–Pt distance and the second side equal to the interplanar
spacing as defined above.
Physical measurements and instrumentation
Microanalyses for %C, H and N were performed by Galbraith
Laboratories Inc., Knoxville, Tennessee, USA. Melting points
were recorded on a Kofler hot stage apparatus and are un-
confirmed by an analysis of how the lateral offset between two
adjacent platinum atoms changes on cooling. The data
show a systematic decrease in the lateral offset for the closer
platinum atoms and a corresponding increase in the lateral
offset for the more distant platinum atoms as the temperature is
lowered.
1
corrected. H NMR (200 MHz) spectra were recorded on a
Varian Gemini 200 spectrometer at 25 ЊC with chemical shifts
referenced to SiMe4. IR spectra were recorded as KBr discs on a
Shimadzu FTIR-4300 spectrometer. Mass spectra were
obtained on a Hewlett Packard GCMS using electron impact
(EI) ionisation. UV-vis absorption spectra were recorded at
22 ЊC using a Shimadzu UV-2101PC scanning spectrophoto-
meter. Emission spectra were recorded on a SLM-Amico SPF
500C fluorometer at 22 ЊC unless otherwise stated. Deoxygen-
ated spectroscopic grade solvents were used for both the
absorption and emission measurements. Solid-state emission
spectra were recorded on microcrystalline samples. For the
variable-temperature emission measurements the cryostat was
an Oxford Instruments DN1704 liquid-nitrogen-cooled system
complete with an Oxford Instruments temperature controller.
The excitation wavelength was 330 nm, with the scattered light
removed by a 400 nm long-wave-pass filter. The 337 nm line
from a nitrogen laser served as the excitation source for the
lifetime measurements, with a 337 nm band pass filter used to
remove stray light from the beam. Lifetime data were analysed
as described previously.25
How do the above changes in the Pt ؒ ؒ ؒ Pt distances relate to
the observed shifts in the emission maximum brought about by
cooling the sample? Given that there is an initial blue shift in
the emission maximum the implication is that there is an initial
widening of the band gap when the temperature is lowered from
room temperature to ca. 240 K. We suggest that this increase
in the band gap is the result of a weakening of the already
2
2
weak (but finite) dz Ϫdz interactions between the more distant
platinum atoms that occurs when the longer Pt ؒ ؒ ؒ Pt distance
2
2
gets longer. Below 240 K this second dz Ϫdz interaction plays
no role in determining the energies of the highest occupied and
lowest unoccupied orbitals, and the analysis becomes one of a
series of molecular metal–metal bonded dimers. As has been
well-established for platinum dimers that exhibit 3MMLCT
emission, a shortening of the distance between the two metal
atoms is expected to result in a closing of the HOMO–LUMO
gap and a red-shift in the emission maximum.1,8,11 As noted
above and illustrated in Fig. 3, this is indeed the observed trend
below 240 K.
Syntheses
2-Chloro-1-{3-(2-pyridyl)-3-oxopropenyl}benzene. A solution
of 2-chlorobenzaldehyde (2.81 g, 20 mmol) in absolute ethanol
(100 mL) was cooled to 0 ЊC and 2-acetylpyridine (2.42 g,
20 mmol) added. Aqueous sodium hydroxide (20 mL,
1.0 M) was then added dropwise and the reaction mixture
stirred at 0 ЊC for 3 h. This resulted in the separation of
a light yellow precipitate that was collected by filtration, washed
with ethanol and dried in vacuo. Yield: 4.79 g, 98%. Anal.
Calc. for C14H10ClNO: C 69.0; H 4.1; N 5.8. Found: C 68.9; H
4.1; N 5.8%. MS(EI) m/z: 244, Mϩ. IR (KBr, cmϪ1): ν(CO)
1675s.
Concluding remarks
Connick et al. have noted that a major contribution to the
stability of linear chain structures of polypyridyl ligand com-
plexes of platinum() is the platinum dz Ϫdz interaction that
extends perpendicular to the stack.22 A shortening of the
Pt ؒ ؒ ؒ Pt distances brought about by cooling the sample is
2
2
2
2
expected to lead to an increase in the dz Ϫdz interactions and a
concomitant further stabilisation of the linear chain structure.
(We note in parentheses that this will only be true provided the
2
dz -orbitals remain suitably oriented for overlap.) It is therefore
of interest to note that when crystals of [Pt{4Ј-(o-ClC6H4)-
trpy}Cl]SbF6 are cooled the Pt ؒ ؒ ؒ Pt distances change in such
a way that a linear chain structure is not formed; instead a
dimeric structure is adopted at lower temperatures. It follows
that the dimeric structure is the preferred one at lower temper-
atures. This is consistent with Peierls theorem that states that
one-dimensional structures with equal separations of the
repeating units are not intrinsically stable and that, as the tem-
perature of the material is lowered, distortions are anticipated
that will lead to the formation of molecular units along the
stack.23 In this case the distortions take the form of the plat-
inum atoms slipping into different positions with respect to a
line perpendicular to the cation stack.
4Ј-(o-Chlorophenyl)-2,2Ј:6Ј,2Љ-terpyridine. N-{1-(2Ј-pyridyl)-
1-oxo-2-ethyl}pyridinium iodide (0.68 g, 2.2 mmol) and
ammonium acetate (10 g, excess) were added to a suspension
of 2-chloro-1-{3-(2-pyridyl)-3-oxopropenyl}benzene (0.49 g,
2.0 mmol) in absolute ethanol (8 mL) and the mixture heated at
reflux for 1.5 h. An off-white solid precipitated on cooling. This
was collected by filtration, washed with 50% aqueous ethanol
and dried in vacuo. Recrystallisation from ethanol afforded
colourless crystals of the ligand. Yield: 0.38 g, 55%. Mp: 158–
163 ЊC. Anal. Calc. for C21H14ClN3: C 73.4; H 4.1; N 12.2.
Found: C 73.4; H 4.2; N 12.2%. MS(EI) m/z: 343, Mϩ. 1H NMR
(CDCl3): δ 8.71 (m, 2 H, H6,6Љ); 8.68 (m, 2 H, H3,3Љ); 8.57 (s, 2 H,
H3Ј,5Ј); 7.84 (m, 2 H, H4Ј,4Љ); 7.51 (m, 4 H, C6H4); 7.35 (m, 2 H,
D a l t o n T r a n s . , 2 0 0 3 , 1 1 7 6 – 1 1 8 0
1179

View Article Online
H5,5Љ). UV-vis (20 µM in CH3CN): λmax/nm (ε/MϪ1 cmϪ1): 303
(sh, 1.6 × 104); 277 (3.0 × 104); 246 (3.5 × 104).
CCDC reference numbers 194846–194849.
See http://www.rsc.org/suppdata/dt/b2/b209754k/ for crystal-
lographic data in CIF or other electronic format.
[Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6. A suspension of [Pt(PhCN)2-
Cl2] (0.10 g, 0.21 mmol) in acetonitrile (10 mL) was treated with
an equimolar amount of AgSbF6 (0.073 g) dissolved in
acetonitrile (5 mL). The reaction mixture was heated under
reflux for 16 h, the AgCl precipitate removed by filtration and
one equivalent of 4Ј-(o-chlorophenyl)-2,2Ј:6Ј,2Љ-terpyridine
(0.051 g) added to the filtrate. The reaction mixture was heated
under reflux for an additional 24 h after which the volume was
reduced in vacuo, the solution cooled to room temperature
and allowed to stand for a further 24 h. This resulted in the
precipitation of [Pt{4Ј-(o-ClC6H4)trpy}Cl]SbF6. The precipi-
tate was washed with cold acetonitrile (ca. 5 mL) and diethyl
ether (ca. 10 mL) and dried in vacuo to afford an analytically
pure microcrystalline product. Yield and colour: 0.16 g, 91%,
orange. Anal. Calc. for C21H14Cl2F6N3PtSb: C 31.1; H 1.9;
Acknowledgements
We acknowledge financial support from the University of Natal
and the South African National Research Foundation. Thanks
also go to the United States National Research Foundation for
funding through grant CHE 97-26435.
References
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N 5.2. Found: C 31.0; H 1.6; N 5.3%. IR (KBr, cmϪ1):
Ϫ
ν[4Ј-(o-ClC6H4)trpy]: 1610s, 1479m, 1418m, 1033m; ν(SbF6
)
)
658vs. 1H NMR (DMSO-d6): δ 8.89 (2H, d, 3JHH 5.5 Hz, H6,6Љ
8.86 (2H, s, H3Ј,5Ј) 8.71 (2H, d,3JHH 8.2 Hz, H3,3Љ) 8.50 (2H,
t,3JHH 7.8 Hz H4,4Љ) 7.94 (2H, t,3JHH 6.6 Hz H5,5Љ) 7.65–7.81 (4H,
m, phenyl CH). 13C NMR (DMSO-d6): δ 158.2 (2C, s, terpy
quat. C) 154.2 (2C, s, terpy quat. C) 151.9 (1C, s, quat. C) 151.3
(2C, s, C6,6Љ) 142.7 (2C, s, C4,4Љ) 135.8 (1C, s, quat. C) 131.8 (1C,
s, phenyl CH) 131.3 (1C, s, phenyl CH) 130.9 (1C, s, quat C)
130.3 (1C, s, phenyl CH) 129.3 (2C, s, C5,5Љ) 128.1 (1C, s, phenyl
CH) 126.0 (2C, s, C4,4Љ) 125.2 (2C, s, C3Ј,5Ј). UV–vis (20 µM in
CH3CN): λmax/nm (ε/MϪ1 cmϪ1) 284 (3.5 × 104); 307 (1.7 × 104);
316 (1.6 × 104); 333 (1.8 × 104); 351 (8.0 × 103); 383 (4.1 × 103);
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Crystal structure determinations
Orange needle-shaped crystals of [Pt{4Ј-(o-ClC6H4)trpy}Cl]-
SbF6 were grown by slow evaporation at room temperature of
a saturated solution of the compound in acetonitrile. Crystal
data and details of the crystallographic study are reported in
Table 1. Intensity data were obtained on an Enraf-Nonius
CAD4 diffractometer, using graphite monochromated Mo-Kα
radiation and the ω–2θ scan technique. Unit cell parameters
were obtained by least squares fitting of 25 reflections moni-
tored in the range 3 < θ < 12Њ while the diffraction data were
collected in the range 2 < θ < 23Њ. For the low-temperature
measurements the crystal was cooled in a stream of nitrogen
supplied by a Bruker LT3 cryostat. Corrections for Lorentz,
polarisation, and absorption (χ scans of 9 reflections) effects
were applied. The intensities of three standard reflections
showed no variations greater than those predicted by counting
statistics. The structures were solved by Patterson and Fourier
methods and refined by full-matrix least-squares using
SHELXS-9726 with all non-hydrogen atoms assigned aniso-
tropic temperature factors and with the hydrogen atoms (in
calculated positions) assigned a single overall isotropic temper-
ature factor. The temperature factor for Cl(2) is exceptionally
high (Ueq = 0.308 Å2) with a difference Fourier indicating that
this chlorine is disordered between two positions, one on each
side of the plane of the phenyl ring. However, the disordered
model was not used in the refinement as it brought no signifi-
cant improvement to the R-factors nor did it add in any way to
the interpretation of the stacking of the cations. The crystal
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D a l t o n T r a n s . , 2 0 0 3 , 1 1 7 6 – 1 1 8 0
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