Non-covalent interactions between cations in the crystal structure of [Pt{4′-(p-tolyl)trpy}Cl]SbF6, where trpy is 2,2′:6′,2″-terpyridine, underpin the salt's complex solid-state luminescence spectrum

Article abstract of DOI:10.1016/j.ica.2011.03.043

The synthesis and characterization of [Pt{4′-(p-tolyl)trpy}Cl] SbF₆ is described where trpy is 2,2′:6′,2″- terpyridine. A single crystal X-ray structure determination at 100 K shows that the cations are stacked in columns that comprise cations arranged in a staircase motif. Successive cations within a column are linked by π(trpy)-π(phenyl) stabilizing interactions; and each cation in one column is linked to a cation in an adjacent column by a weakly stabilizing Pt?Pt interaction. The Pt?Pt distance is 3.434(1) . The metrics governing non-covalent interactions between [Pt{4′-(aryl)trpy}Cl]⁺ cations have been analyzed for the present structure and related structures in the CSD (Cambridge Structural Database). Cation dimers cluster into three distinct groups based on their lateral shifts and, to a lesser extent, the angular parameters governing their relative displacements; the dominant grouping exhibits Pt?Pt and π(trpy)-π(trpy) stabilizing interactions. An emission spectrum recorded at 77 K on a solid sample of the compound is best interpreted as arising from the decay of three photoexcited states: a ³MLCT (MLCT = metal-to-ligand charge transfer) state; a ³MMLCT (MMLCT = metal-metal-to-ligand charge transfer) state, and an excimeric ³π-π* state.

Full text of DOI:10.1016/j.ica.2011.03.043

Inorganica Chimica Acta 374 (2011) 197–204
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Non-covalent interactions between cations in the crystal structure
of [Pt{40-(p-tolyl)trpy}Cl]SbF6, where trpy is 2,20:60,200-terpyridine,
underpin the salt’s complex solid-state luminescence spectrum
⇑
John S. Field, Colin R. Wilson, Orde Q. Munro
School of Chemistry, University of KwaZulu-Natal, Private Bag X01, Pietermaritzburg 3201, South Africa
a r t i c l e i n f o
a b s t r a c t
The synthesis and characterization of [Pt{40-(p-tolyl)trpy}Cl]SbF6 is described where trpy is 2,20:60,200-ter-
pyridine. A single crystal X-ray structure determination at 100 K shows that the cations are stacked in
columns that comprise cations arranged in a staircase motif. Successive cations within a column are
Article history:
Available online 22 March 2011
Dedicated to our good friend and colleague,
Wolfgang Kaim, in recognition of his
considerable contributions to Coordination
Chemistry.
linked by p(trpy)–p(phenyl) stabilizing interactions; and each cation in one column is linked to a cation
in an adjacent column by a weakly stabilizing PtꢀꢀꢀPt interaction. The PtꢀꢀꢀPt distance is 3.434(1) Å. The
metrics governing non-covalent interactions between [Pt{40-(aryl)trpy}Cl]+ cations have been analyzed
for the present structure and related structures in the CSD (Cambridge Structural Database). Cation
dimers cluster into three distinct groups based on their lateral shifts and, to a lesser extent, the angular
Keywords:
Platinum(II) complexes
Terpyridine
X-ray structure
Luminescence
parameters governing their relative displacements; the dominant grouping exhibits PtꢀꢀꢀPt and
p(trpy)–
p
(trpy) stabilizing interactions. An emission spectrum recorded at 77 K on a solid sample of the com-
pound is best interpreted as arising from the decay of three photoexcited states: a 3MLCT (MLCT = me-
tal-to-ligand charge transfer) state; a 3MMLCT (MMLCT = metal–metal-to-ligand charge transfer) state,
p-Stack
and an excimeric 3p p⁄ state.
–
MMLCT
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
solid state [1,4–13]. However, the precise role is crystal structure
dependent, and we wished to elucidate the relationship between
the novel crystal structure for the title compound and its emission
properties in the solid state.
The [Pt{40-(p-tolyl)trpy}Cl]+ cation (trpy = 2,20:60,200-terpyridine)
was first reported as the perchlorate salt by Yip and co-workers in
1993 [1]. The complex exhibits moderately strong 3MLCT
(MLCT = metal-to-ligand charge transfer) emission in degassed
2. Experimental
acetonitrile (103
u = 0.7, kem(max) = 561 nm, s = 2.4 ls) [1]. These
properties make the complex useful as a photocatalyst for the pro-
duction of hydrogen from Hantzsch 1,4-dihydropyridines [2] and
water [3]. Of interest here however, are the photophysical proper-
ties of the complex in the solid state, in particular of the hexafluo-
roantimonate(V) salt, [Pt{40-(p-tolyl)trpy}Cl]SbF6 (Scheme 1). Our
interest in the hexafluoroantimonate(V) salt was stimulated by
an X-ray diffraction study, which showed that the cations stack
in an unusual ‘‘double staircase’’ motif, with both intermolecular
The 40-(p-tolyl)trpy ligand was synthesised following the gen-
eral method described by Kröhnke [14] and given in detail in Ref.
[5] for closely-related ligands. The [Pt(PhCN)2Cl2] starting complex
and the AgSbF6 salt were obtained from Strem Chemicals Inc. and
used without further purification. The acetonitrile for the complex
synthesis and crystal growth was of the Chromosolv™ HPLC grade
from Aldrich and used as received. The diethyl ether was from
Saarchem and of the uniLAB™ grade and also used as received.
p–p
and PtꢀꢀꢀPt stabilizing interactions present in the crystal. Previ-
ous work has shown that intermolecular interactions of this type
can play an important role in determining the photophysical prop-
erties of ionic terpyridyl ligand complexes of platinum(II) in the
2.1. Synthesis and characterization of [Pt{40-(p-tolyl)trpy}Cl]SbF6
A suspension of AgSbF6 (0.138 g, 0.40 mmol) and [Pt(PhCN)2Cl2]
(0.217 g, 0.46 mmol, 15% molar excess) in acetonitrile (30 mL) was
refluxed under dry nitrogen for 24 h and then cooled to room tem-
perature. The AgCl precipitate was separated by cannula transfer of
the solution though a frit, and a concomitant transfer of the filtrate
⇑
Corresponding author. Tel.: +27 33 260 5270; fax: +27 33 260 5009.
E-mail addresses: fieldj@ukzn.ac.za (J.S. Field), munroo@ukzn.ac.za (O.Q. Mun-
ro).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.03.043

198
J.S. Field et al. / Inorganica Chimica Acta 374 (2011) 197–204
Table 1
Crystal data, data collection and refinement details for [Pt{40-(p-tolyl)trpy}Cl]SbF6.
Empirical formula
Mr (g molꢁ1
C22H17ClF6N3PtSb
789.68
)
Crystal size (mm)
T (K)
0.05 ꢂ 0.20 ꢂ 0.40
100
k (Å)
0.71073
monoclinic
P21/c
7.6320(5)
21.907(2)
13.081(1)
94.935(5)
2179.0(3)
4
2.407
7.844
1480
2–32
Crystal system
Space group
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Z
Dcalc (g cmꢁ3
)
l
(mmꢁ1
F(0 0 0)
h Range (°)
Reflections collected (independent)
)
Scheme 1. Structure of the luminescent salt [Pt{40-(p-tolyl)trpy}Cl]SbF6.
19 831 (7009)
6401
Observed reflections [I > 2r(I)]
Rint
0.0478
Numbers refined parameters (restraints)
Final R1 [I > 2 (I)]
308 (0)
0.0504
0.1532
3.6 (Pt), ꢁ5.3 (Pt)
r
to a dry flask containing a 5% excess of 40-(p-tolyl)trpy (0.155 g,
0.48 mmol). This mixture was refluxed for a further 24 h. After
cooling, diethyl ether was added dropwise causing precipitation
of the crude product as a yellow solid that was dried in vacuo.
Re-crystallization was by means of slow vapor diffusion of diethyl
ether into a concentrated solution of the product in acetonitrile.
Final wR2 (all data)
Maximum, Minimum
D
q
(e Åꢁ3
)
2.3. Crystal structure determination and refinement
Yield: 0.201 mg (63%). Anal. Calc. for
C22H17ClF6N3PtSb
X-ray intensity data were collected with an Oxford Diffraction
Xcalibur 2 CCD 4-circle diffractometer linked to an Oxford Cryostat
(FW = 789.68 g molꢁ1): C, 33.46; H, 2.17; N, 5.32. Found: C,
33.53; H, 2.13; N, 5.28%. IR (KBr, cmꢁ1):
2917w, 1603s, 1551m, 1476s, 1400m, 1246s;
m
[40-(p-tolyl)trpy]
System. The data were collected at 100 K using Mo K
a radiation
m
[SbF6ꢁ] 649vs. 1H
(2.0 kW, 0.71073 Å), 0.75° frame widths, 20 s exposures at a crys-
tal-to-detector distance of 50 mm, and omega 2-theta scans of
ꢁ54.00–60.60° or ꢁ54.00–60.75° at h = 30°. The data were reduced
with the program CRYSALIS RED [16] (Version 170) using outlier rejec-
tion, scan speed scaling, as well as standard Lorentz and polariza-
tion correction factors. The structures were solved with SHELX-97
[17] using Patterson methods with all non-hydrogen atoms refined
anisotropically with SHELX-97 [17] (using WINGX [18] as an interface).
Hydrogen atoms were geometrically constrained (C–H = 1.05 Å)
using the appropriate AFIX command and refined isotropically
with Uiso fixed at 1.20 times the equivalent isotropic temperature
factor for a parent sp2 carbon atom, and 1.5 times for a parent
sp3 carbon atom. Plots were obtained with ORTEP3 for Windows
[19] and MERCURY 2.3 [20]. Lattice constants, refinement details
and final discrepancy indices are given in Table 1.
0
0
00
NMR (DMSO-d6): d 8.74 (2H, s, H3 ,5 ), 8.67 (2H, d, H3,3 ), 8.59
00
00
000,6000
(2H, d, H6,6 ), 8.38 (2H, t, H4,4 ), 8.00 (2H, d, H2 ), 7.77 (2H, t,
00
000,5000
H5,5 ), 7.43 (2H, d, H3 ), 2.44 (3H, s, CH3). UV–Vis (CH2Cl2)
kmax/nm: 284, 337 (p–
p⁄), 397, 417 (MLCT). Emission (CH2Cl2,
kex = 390 nm) kmax/nm: 545 (0–0), 580 (0–1).
2.2. Physical measurements
Elemental analyses were performed in the Elemental Micro-
analysis unit of the Department of Chemistry at the University of
Durham, UK. FTIR spectra were measured using a Perkin Elmer
Spectrum One spectrometer, and the samples prepared as KBr pel-
lets. The 1H (500 MHz) spectra were recorded on a Bruker Avance
III spectrometer with chemical shifts referenced to solvent protons.
The instrument used for the measurement of emission spectra was
a Photon Technologies Int. (PTI) fluorescence spectrometer con-
trolled by PTI’s Felix32Ó Version 1.1 software [15]. The solid sam-
ples used for the emission measurements were finely ground and
transferred to an NMR tube that served as the sample holder; for
the purposes of the 77 K measurements the NMR tube was inserted
in a PTI-supplied quartz cold finger filled with liquid nitrogen.
Steady state emission spectra were recorded using PTI’s Xeno-
Flash™ 300 Hz pulsed light source and gated emission scans with
3. Results and discussion
The synthesis of the perchlorate salt of the [Pt{40-(p-tolyl)tr-
py}Cl]+ cation has been reported [1]. Here we report the synthesis
of the hexafluoroantimonate(V) salt using methods developed in
our laboratories—details of the synthesis and characterization are
given in Section 2.
a delay of 95
ls, an integration window time of 100
ls and 50
3.1. Crystal structure of [Pt{40-(p-tolyl)trpy}Cl]SbF6
pulses per channel (shots). Detection was by means of PTI’s Model
814 Analog/Photon-Counting Photomultiplier Detector. The excita-
tion wavelength was 420 nm with the scattered light being re-
moved by means of a suitable wavelength band-pass filter. Note
that because the emission intensity was so weak (even at 77 K) it
was necessary to correct the measured spectrum for background
due to the glass of the NMR tube. For the lifetime measurements,
the excitation source was again the Xenon flash lamp with the
emission decay captured by the Photomultiplier Detector at a
wavelength corresponding to the peak of interest, and analyzed
by the FeliX32™ software [15].
Single crystals were grown by slow vapor diffusion of diethyl
ether into a concentrated solution of the complex in acetontrile.
Fig. 1 gives a perspective view of the cation with selected inter-
atomic distances and angles included in the caption. The coordina-
tion geometry at the platinum atom is irregular square-planar, as
evidenced by N1–Pt–N2 and N2–Pt–N3 angles of 81.1(2)° and
81.2(3)°, respectively; as well as by a ‘trans’ N1–Pt–N3 angle of
162.3(2)°. This is typical of terpyridyl ligand complexes of plati-
num(II) and arises from geometric constraints imposed by the rigid
tridentate ligand [1,4–13,21–23]. Also typical is that the platinum

J.S. Field et al. / Inorganica Chimica Acta 374 (2011) 197–204
199
view it is easily seen that the cations and anions occupy separate
columns parallel to the a-axis of the unit cell. Note that the col-
umns of cations stack, one after the other, along lines parallel to
the vertical axis of the diagram, i.e. there are rows of cation col-
umns perpendicular to the unit cell b-axis. Furthermore, successive
ꢁ
rows of cation columns are separated by SbF6 anions along the b-
axis. From this description of the crystal packing it becomes clear
that in order to understand potential cation–cation interactions,
we need to investigate close contacts between cations within a col-
umn, as well as close contacts between cations from neighboring
columns along a row. To this end, Fig. 3 depicts a side-view of four
cation columns belonging to one row of columns. With this view it
becomes apparent that successive cations within a column are lat-
erally offset from each other, such that they take on the appearance
of steps in a staircase. Close inspection of Fig. 3 also suggests that
there is a close contact between a Pt atom in the first staircase and
a Pt atom in the second staircase, i.e. a step in the first staircase is
linked to a (lower) step in the second staircase (Fig. S1, Supporting
Information, offers an enhanced view of these interactions). The
third and fourth staircases are similarly linked. (We examine the
PtꢀꢀꢀPt interaction in detail below: see Fig. 4b). Note that there is
no evidence for short contacts between the second and third stair-
Fig. 1. MERCURY 2.3 [20] view of the cation in [Pt{40 -(p-tolyl)trpy}Cl]SbF6 drawn with
50% thermal ellipsoid probabilities. Hydrogen atoms are drawn as spheres of
arbitrary radius. Selected bond lengths (Å) and angles (°) are: Pt–N1, 2.007(5); Pt–
N2, 1.931(5); Pt—N3, 2.027(5); Pt—Cl, 2.297(2); N1—Pt—N2, 81.1(2); N1–Pt–N3,
162.3(2); N1–Pt–Cl, 98.3(2); N2–Pt–N3, 81.2(2); N2–Pt–Cl, 178.7(2); N3–Pt–Cl,
99.4(2).
to bridgehead nitrogen distance of 1.931(5) Å is shorter than the
distances from the Pt atom to the outer nitrogen atoms of
2.007(5) and 2.027(5) Å [1,4–13,21–23]. The Pt–Cl distance of
2.297(2) Å fits with Pt–Cl distances reported for other terpyridyl li-
gand complexes of platinum where the co-ligand is a chloride ion
[1,4–6,9,10]. Of particular interest is that the p-tolyl group twists
slightly out of the plane of the trpy moiety, as evidenced by a dihe-
dral angle of only 9.3° between mean planes through the non-H
atoms of the trpy moiety on one hand, and those of the phenyl ring
on the other. The cation as a whole is consequently approximately
planar, with a maximum deviation for any one non-H atom from a
mean plane drawn through all the non-H atoms comprising the
cation of 0.16 Å (Cl). In fact, it is because the trpy moiety has been
extended by the addition of a co-planar p-tolyl group that intermo-
cases, specifically no contacts that allow for
p
–
p
or PtꢀꢀꢀPt interac-
tions. Thus, a more descriptive term for the cation packing would
be that they stack in rows of inversion paired ‘‘double staircases’’
perpendicular to the b-axis of the unit cell.
We now examine the precise nature of the intermolecular inter-
actions within a staircase and between staircases. To this end,
views perpendicular to the cation 25-atom mean planes (calcu-
lated using all aryl ring atoms and the Pt(II) ion of a cation to define
its mean plane) are shown in Fig. 4: in part (a) for two adjacent cat-
ions (steps) in one staircase and in part (b) for cations from adja-
cent staircases that are linked by the short PtꢀꢀꢀPt contact. In
lecular p(trpy)–p(phenyl) interactions are made possible; without
these stabilizing interactions, the novel crystal structure could not
Fig. 4a the two cations are laterally offset such that p(trpy)–p(phe-
nyl) overlap occurs at the edges of an outer pyridine ring of one
cation and the phenyl group of the other cation; the non-interact-
ing Pt atoms are separated by 7.6320(6) Å, giving an indication of
the magnitude of the lateral offset. As noted by Hunter and Saun-
exist.
The overall packing of the [Pt{40-(p-tolyl)trpy}Cl]+ cations and
ꢁ
SbF6 anions is illustrated in Fig. 2, which shows a projection of
the unit cell contents down the a-axis of the unit cell. With this
ders [24] and by Janiak [25], this has as a result that p–r attractive
forces dominate between the two aromatic rings: thus, this is a sta-
bilizing interaction. Consistent with this is that the perpendicular
distance (interplanar spacing) between the cation 25-atom mean
planes is 3.394(1) Å, a value that is well below the upper distance
limit of 3.8 Å for a stabilizing
p–p interaction between adjacent
aromatic species [24,25]. Important to note is that this interaction
Fig. 2. View of the packing of cations and anions in [Pt{40 -(p-tolyl)trpy}Cl]SbF6
drawn as a projection approximately down the a-axis of the unit cell. Hydrogen
atoms have been omitted for clarity; non-C atoms are rendered as spheres; C atoms
are located at bond intersections and have been rendered as cylinders along with all
other bonds.
Fig. 3. Side-view of two adjacent cation ‘‘double staircases’’: see Section 3.1. Note
that successive cations within a ‘‘staircase’’ are linked by a unit cell translation
along the a-axis. Hydrogen atoms have been omitted for clarity; non-C atoms are
rendered as spheres; C atoms are located at bond intersections and have been
rendered as cylinders along with all other bonds.

200
J.S. Field et al. / Inorganica Chimica Acta 374 (2011) 197–204
would not be possible unless the p-tolyl group were present and
approximately co-planar with the trpy moiety: it is the key inter-
action on which the staircase motif is built. In Fig. 4b we see that
the Pt atoms of the two linked cations (an inversion pair) are nearly
eclipsed. This fact, as well as the PtꢀꢀꢀPt distance of 3.434(1) Å, sug-
gest that there is a stabilizing interaction between the two Pt
atoms [5,9,10,26,27]. However, the interaction is likely a weak
one because the PtꢀꢀꢀPt distance is near the upper distance limit
of ꢃ3.5 Å usually taken to indicate a finite d2z ðPtÞ—dz2ðPtÞ orbital
interaction [28]. Also apparent from Fig. 4b is that there is
Pt(trpy) mean planes). In short, straightforward lateral shifts ap-
plied to a perfect linear-chain structure with a single infinitely
repeating PtꢀꢀꢀPt interaction distance, e.g., as found in [PtCl2(bpy)]
[28a], where bpy = 2,20-bipyridine, would be insufficient to gener-
ate the rather more complex array of dimers observed for the
[Pt{40-(p-tolyl)trpy}Cl]+ cations in this system. Finally, it is not
unreasonable to suggest that although the stabilizing
p–p and
PtꢀꢀꢀPt intermolecular interactions seen here are energetically weak
compared to the ionic interactions in the crystal lattice, they play a
crucial role in aligning and discretely spacing the chromophores in
the solid-state; these more subtle interactions will, in fact, be the
primary determinants of the emission properties of the polycrys-
talline material (vide infra).
p
(trpy)–
rings of the two cations. This overlap geometry also supports a sta-
bilizing interaction, as does the interplanar spacing (25-atom
mean planes) of 3.268(1) Å [24,25]. A more detailed diagram of
these interactions, both within the -stacked dimer of Fig. 4b
p(trpy) overlap at the leading edges of the outer pyridine
p–p
p
3.2. Analysis of the structures of [Pt{40-(aryl)trpy}Cl]+ cation pairs/
stacks in the Cambridge Structural Database (CSD)
and within the linear chain to which the dimer belongs in the crys-
tal lattice, is shown in Supporting Information (Fig. S2). Each dimer
is clearly related to the next dimer in the array by an inversion
operation as well as inequivalent translations (a large lateral shift
and shorter perpendicular shift) in two orthogonal directions in the
lattice. These symmetry operations result in a linear chain of dis-
crete dimers related by a significant lateral shift (13.79 Å) and an
interdimer (or dimerꢀꢀꢀdimer) perpendicular displacement of
3.753(1) Å (based upon the separation between the 19-atom
To examine the precise metrical parameters between symme-
try-related [Pt{40-(aryl)trpy}Cl]+ cations when paired in dimers or
stacks more generally than those discussed above for [Pt{40-(p-
Fig. 5. Top: interaction between two [Pt(trpy)Cl]+ cations (taken and simplified
from the inversion pair illustrated in Fig. 4b). Metrical parameters that define the
interaction geometry are indicated. The mean plane separation (MPS) is the
perpendicular distance between the two 19-atom mean planes of the cations
(excluding the chloride ions). The x-direction is defined as lying along the Pt1—N10
bond vector. The y-axis direction is slightly off (3°) the direction defined by the
Pt1—C5 vector (being located between this vector and the Pt1—N1 bond vector).
The Pt2ꢀꢀꢀPt1ꢀꢀꢀC5 angle is within 0.2° of the Pt2ꢀꢀꢀPt1—(y-axis) angle and may be
used as a convenient approximation of this angle. Bottom left: Illustration of the
lateral shift (LS) direction when a dimer is viewed perpendicular to the xy plane.
The top Pt(II) chelate is drawn with thick bonds; the top Pt(II) ion may be located
anywhere on the circumference of a circle in a plane above the lower Pt(II) ion.
Bottom right: relationship between the LS, metal-to-metal perpendicular displace-
ment (MMPD), and Pt1ꢀꢀꢀPt2 vector. Triangulation from the Pt1ꢀꢀꢀPt2 vector gives the
x- and y-components of the LS vector; its scalar magnitude is calculated from the
Fig. 4. Views of nearest neighbor cations: (a) within a staircase and (b) from two
different staircases; see Section 3.1. Top and side views are perpendicular and
parallel to the cation mean planes, respectively. Carbon and hydrogen atoms have
been rendered as cylinders at bond intersects and termini, respectively. All other
atoms are rendered as spheres.
Pythagorean relationship LS = {LS(x)2 + LS(y)2}0.5
.

J.S. Field et al. / Inorganica Chimica Acta 374 (2011) 197–204
201
Table 2
Metrical parameters for crystallographically characterized dimers in the CSD [29] involving [Pt{40 -(aryl)trpy}Cl]+ cations, where the aryl group is a substituted or unsubstituted
phenyl ring and the monomers are related by a crystallographic symmetry element (e.g., inversion).a
CSD Ref. codeb Type MPS (Å) Pt1ꢀꢀꢀPt2 (Å)
v
(°)
w
(°) LS(x) (Å) LS(y) (Å) %x displ.c %y displ.c LS(MPS) (Å) LS (Å) MMPD (Å) Direction (°)d
FAZJOG_b
LABJOO_b
FAZJOG_a
XIPNEP
VUYNEJ
SAHHIT
TW, Fig. 4b
LABJOO_a
XIPNIT
LUNLIP_b
XIPNOZ_b
FAZJUM_b
XIPNOZ_a
FAZJUM_a
SIYPIA
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HH
HT
3.325
3.366
3.373
3.322
3.447
3.430
3.333
3.280
3.408
3.317
3.453
3.390
3.475
3.358
3.336
3.511
3.258
3.241
3.301
3.352
3.364
3.368
3.410
3.424
3.434
3.523
3.573
3.608
3.629
3.661
3.695
4.038
4.584
4.611
7.130
7.632
94.4
91.1
84.8
91.5
84.3
82.1
91.5
97.1
83.2
94.4
87.9
81.6
87.1
70.2
70.1
59.7
32.4
38.0
81.4
84.5
89.6
82.8
81.7
82.0
77.0
71.9
73.7
69.9
72.6
66.7
70.4
66.9
55.6
67.2
75.5
66.3
ꢁ0.25
ꢁ0.07
0.30
0.49
0.32
0.02
0.42
0.49
0.48
0.77
1.09
1.00
1.24
1.08
1.45
1.24
1.59
2.59
1.79
1.79
3.07
1.8
0.5
2.2
0.6
2.4
3.4
0.6
3.1
3.0
2.0
0.9
3.9
1.3
9.8
11.2
16.7
43.1
43.1
2.2
1.5
0.1
1.9
2.2
2.1
3.5
4.9
4.5
5.6
4.9
6.5
5.6
7.1
11.6
8.0
8.0
13.8
0.55
0.33
0.31
0.43
0.60
0.67
0.78
1.18
1.09
1.27
1.09
1.55
1.25
2.09
3.02
2.94
6.28
6.76
3.25
3.34
3.35
3.34
3.36
3.36
3.35
3.32
3.40
3.38
3.46
3.32
3.48
3.45
3.45
3.55
3.38
3.55
117.1
101.6
4.4
ꢁ0.09
0.55
101.6
55.3
45.6
96.5
111.8
67.0
102.4
83.1
69.7
81.5
49.3
59.0
37.5
16.5
27.0
0.34
0.47
ꢁ0.09
ꢁ0.44
0.42
0.83
1.29
1.07
1.42
1.12
1.38
1.26
2.24
3.14
2.99
6.34
6.91
ꢁ0.27
0.13
0.54
0.19
1.37
1.56
2.33
6.02
6.02
XULDOY
LUNLIP_a
TW, Fig. 4a
HT
a
Abbreviations: HH, head-to-head; HT, head-to-tail; MPS, mean plane separation (measured as the distance between parallel 19-atom mean planes passing through the Pt
atom and trpy ligand of each cation);
v
, P2ꢀꢀꢀPt1—N10 angle;
w, P2ꢀꢀꢀPt1ꢀꢀꢀC5 angle; LS(x) and LS(y) are the x- and y-components of the lateral shift (LS) vector, respectively,
with the x-direction defined as the Pt—N10 bond vector; LS(MPS), lateral shift calculated from the MPS and Pt1ꢀꢀꢀPt2 distance; LS, lateral shift calculated trigonometrically from
LS(x) and LS(y); MMPD, metal-to-metal perpendicular displacement; TW, this work.
b
Crystallographically unique dimers are distinguished by their CSD reference codes and a lower case label where two unique dimers exist.
c
The % displacement reflects the magnitude of the x- and y-components of the LS vector relative to the x- and y-dimensions of a standard [Pt(trpy)Cl]+ cation, namely 6.98
and 11.15 Å, respectively.
d
The LS vector direction (angle) relative to the Pt–N10 bond vector (i.e. the x-direction).
Table 3
Descriptive statistics for the 18 crystallographically characterized dimers listed in Table 2 involving [Pt{40-(aryl)trpy}Cl]+ cations, where the aryl group is a substituted or
unsubstituted phenyl ring.a
MPS (Å) Pt1ꢀꢀꢀPt2 (Å)
v
(°)
w (°) LS(x) (Å) LS(y) (Å) %x displ. %y displ. LS(MPS) (Å) LS (Å) MMPD (Å) Direction (°)
Mean
SEM
SD
3.368
0.018
0.075
0.006
4.074
0.298
1.266
1.602
0.311
3.301
7.632
18
79.0
4.4
18.6
346.1 70.8
0.2
32.4
97.1
18
74.2
2.0
8.4
1.03
0.46
1.95
3.80
1.90
ꢁ0.44
6.02
18
1.16
0.19
0.80
0.64
0.69
0.02
3.07
18
8.3
3.2
13.4
178.8
1.6
0.5
43.1
18
5.2
0.8
3.6
12.8
0.7
0.1
13.8
18
2.35
0.57
2.05
4.21
0.87
0.55
6.91
13
1.79
0.45
1.90
3.60
1.06
0.31
6.76
18
3.39
0.02
0.08
0.01
0.02
3.25
3.55
18
68.2
8.0
33.8
1141.1
0.5
4.4
117.1
18
Variance
Coefficient of Variance 0.022
Minimum
Maximum
N
0.1
3.241
3.511
18
55.6
89.6
18
a
For abbreviations and metrical parameter descriptors, refer to the footnote to Table 2 and Fig. 5.
tolyl)trpy}Cl]+, we have analyzed the available data in the CSD [29]
using the geometrical framework and parameters defined in Fig. 5.
The results of this analysis are summarized in Tables 2 and 3. The
PtꢀꢀꢀPt distances for 18 independent observations range from 3.30
to 7.63 Å; the mean metal-to-metal distance for this class of cat-
ions is towards the short end of the range at 4.1(1.3) Å (Table 3).
The mean plane separation (MPS) averages 3.37(8) Å and is not
highly variable (as evidenced by the small standard deviation,
SD). The lateral shifts (LS) calculated trigonometrically from the
Pt atom coordinates span a broad range (0.31–6.76 Å); the mean
(1.79 Å) and SD (1.90 Å) reflect this, but do not portray the fact that
cation pairs in this class of luminescent salts tend to cluster
according to the type of dimer formed and consequently their char-
acteristic LS (see below; Fig. 6).
It is noteworthy that a LS cannot be calculated from the PtꢀꢀꢀPt
distance and MPS when the PtꢀꢀꢀPt interaction is very strong and
the distance separating the metal ions is shorter than the MPS
(e.g., when the trpy conformation is non-planar or, more specifi-
cally, concave with respect to the intradimer space) since this cor-
responds to a non-Pythagorean triangle. (Several dimers have no
entry for the LS(MPS) value in Table 2 because of the foregoing
condition.) In such cases, use of the x- and y-components of the
LS vector to determine its direction and magnitude is mandatory
and the plots in Fig. 6 have been made with LS values calculated
in this manner. This method also allows calculation of the MMPD
(metal-to-metal perpendicular displacement). It should be noted
that the MMPD and MPS values can only be equivalent in the ideal
case of perfectly planar [Pt(trpy)Cl]+ cations making up a dimer.
The MMPD values average 3.39(8) Å and are thus comparable in
magnitude and range (though never identical) to the MPS values
for this class of non-covalent dimers.
An important question to answer is whether or not the lateral
shift for these dimers takes on a preferred orientation or direction
relative to a particular molecular frame of reference. Calculation of
the orientation of the LS vector relative to the x-axis is trigonomet-
rically straightforward from the x- and y-components of the LS vec-
tor (Table 2). From Table 3, the LS vector exhibits a mean
orientation of 68(34)° relative to the Pt–N10 bond vector (x-direc-
tion). This suggests that, on average, the preferred displacement
for this class of cations is 22° off the y-axis and over the first che-
late ring within the trpy ligand (i.e. along a line connecting the
Pt(II) ion to the midpoint of the N1–C2 bond of the first pyridine
ring). The SD of the mean LS vector orientation is large because
the cationꢀꢀꢀcation lateral shift direction is not confined to a partic-
ular region over the trpy ligand, but tends to cluster into three
groups, from virtually over the x-axis to almost directly over the

202
J.S. Field et al. / Inorganica Chimica Acta 374 (2011) 197–204
an outlier in the sense that the LS is very short, but the LS vector
displacement is along the x- rather than the y-axis (open diamond
in Fig. 6a). As noted above and elsewhere [28], intermolecular
interactions between cations falling into this group are dominated
by a finite d2z ðPtÞ—d2z ðPtÞ orbital interaction (MM) as well as
p(trpy)–p(trpy) interactions. It is interesting to note that even with
a lateral shift as large as 1.2–1.5 Å, this type of intermolecular
interaction seemingly still holds presumably because the 5d2z orbi-
tals are diffuse and a laterally offset head-on overlap (which, fun-
damentally, means that the interaction takes on more p-character)
is not altogether unfavorable. (ii) A second group (Group II) of more
loosely interacting head-to-head dimers is discernible from Fig. 6a
(though with significantly fewer examples at present) and is char-
acterized by LS values of 2–3 Å and LS vector orientations of be-
tween 30° and 60°. For these dimers, the LS is most likely too
large to permit any significant side-on d2z ðPtÞ—dz2ðPtÞ orbital inter-
actions such that the key attractive forces between the platinum-
bound trpy groups are
p–r electrostatic interactions [24,25], i.e.
p
(trpy)– (trpy) overlap. (iii) A third group of cations (Group III)
p
interact with the reverse geometry, namely head-to-tail, and in
both examples form linear chains of cations stabilized by the over-
lap of the 4-aryl group with a flanking pyridine ring of the neigh-
boring trpy ligand. For [Pt{40-(p-tolyl)trpy}Cl]SbF6, the distance
between the centroids of the overlapping rings measures
3.594(2) Å. The long lateral shifts observed for the present salt
and
chloro-(2,20:60,200-terpyridyl-40-azobenzene)-platinum(II)
hexafluorophosphate (CSD structure LUNLIP) [30], the only other
example available at present in this group, are fully consistent with
p(aryl)–p(trpy) interactions as the dominant stabilizing intermo-
lecular interaction. The LS vector orientations for the present struc-
ture and LUNLIP are the most acute of all the complexes listed in
Table 2, falling between 15° and 30°, and indicate that long lateral
shifts preferentially occur along the x-axis direction of the chelate.
Although the data plotted in Fig. 6a are sparse in Groups II and
III and quite scattered in Group I, the linear trend is not altogether
unconvincing nor contrary to expectation. Indeed, the intercept is
90° within one standard deviation of the estimate and suggests
that at very short lateral shifts for this class of dimers or stacks,
the Pt(II) ion of one chelate is positioned slightly over the y-axis
of the reference Pt(II) chelate with an appropriate MMPD. Interest-
ingly, the straight line cuts the abscissa just short of a LS value of
8 Å. This suggests that dimers stabilized by
p(aryl)–p(trpy) inter-
actions have a LS displacement limit of about 8 Å and that this
would be achieved when the Pt(II) ion of one chelate is positioned
directly over the x-axis of the reference cation at an appropriate
MPS. More specifically, such a geometry would involve overlap of
the Pt(II) ion of one chelate with the 40-(aryl) ring of the juxtaposed
ligand and correspond to a finite and electrostatically favorable
Fig. 6. (a) Plot of the lateral shift (LS) direction against the magnitude of the LS for
the available crystallographically delineated [Pt{40-(aryl)trpy}Cl]+ cation dimers/
stacks. Groups I–III reflect the dominant type of intermolecular interaction
stabilizing the dimers. The dashed line illustrates the trend in the data as a linear
least-squares fit, f(x) = 94(7)–12(3) x (correlation coefficient = 0.74), without the
outlying point (open diamond). The inset shows a polar plot of the same data over
the full orientation range of the LS vector (0–180° due to the point group symmetry
Pt(II)ꢀꢀꢀ
p(aryl) interaction. Note that the distance from the Pt(II)
of the dimers). (b) Polar plot of the displacement angles
v and w (Fig. 5) for dimers
ion to the centroid of the 40-(aryl) ring of [Pt{40-(p-tolyl)tr-
py}Cl]SbF6 is 7.575(2) Å. A LS limit of around 8 Å (Fig. 6a) would
position the Pt(II) ion of one chelate over the reference cation at
an appropriate MPS and just beyond the centroid of the 40-(aryl)
ring along the line connecting this point to the para-C atom of
comprising [Pt{40-(aryl)trpy}Cl]+ cations.
y-axis in the coordinate system used to quantify the dimer geom-
etry. As discussed below, orientational clustering of the LS vector
depends on the type of intermolecular interactions stabilizing
[Pt{40-(aryl)trpy}Cl]+ cation pairs.
the aryl ring. Although cation-p stabilizing interactions are well
known for aromatic compounds and are indeed important in defin-
ing the tertiary and quaternary structures of proteins [31], an
example of such a dimer with [Pt{40-(aryl)trpy}Cl]+ cations has
Fig. 6a shows both conventional and polar plots of the LS vector
orientation against the magnitude of the LS. Despite the small size
of the data set at present, several noteworthy points may be made
from inspection of this plot: (i) the most common type of dimer
formed by this class of cations is a tight head-to-head dimer with
lateral shifts ranging from ꢃ0.3 to ꢃ1.55 Å and PtꢀꢀꢀPt distances
ranging from 3.30 to 3.70 Å (Group I). The LS vectors for this class
of dimers are mainly oriented between 45° and 120° (in the region
encompassing the y-axis as depicted in Fig. 5, or roughly in the
direction of the Pt–N1 bond). One salt (CSD code FAZJOG [9]) is
yet to be observed. However, Pt(II)ꢀꢀꢀ
p(aryl) interactions could be
favoured for these chelates if non-bulky electron-donating groups
occupied the meta-positions as well as the para-position on the 40-
aryl substituent of the trpy ligand—a situation that might permit
substantial electrostatic attraction between the Pt(II) ion and the
p
-electron cloud of the 40-(aryl) ring.
Finally, Fig. 6b shows a polar plot of the intradimer angles
v and
w
. The values of v span the range 30–115° while the w-values are

J.S. Field et al. / Inorganica Chimica Acta 374 (2011) 197–204
203
confined to a narrower range of just over 30° from 56° to 90°. The
broad range of -values is consistent with more facile cation slip-
page within a dimer or -stack along the x-axis direction (Pt–
N10 vector, Fig. 5). This phenomenon is also evident from the statis-
tics in Table 3, which show that the -angle has the highest vari-
sion; and 3.87(4) and 11.4(3)
650 nm emission a good fit with a single-exponential decay curve
was obtained, affording a lifetime of 4.832(5) s. The complexity of
l
s for the 610 nm emission. For the
v
p
–
p
p
l
the emission spectrum, as well as the biphasic nature of the inten-
sity decay curve for the first two peaks, clearly indicate that emis-
sion from more than one excited state is being recorded in the solid
state. To try and identify these separate excited states we have de-
convoluted the spectrum in Fig. 7 into its Gaussian components.
This is also shown in Fig. 7, along with the experimental and calcu-
lated spectra. The first Gaussian component (colored blue) maxi-
mizes at 554 nm, a close fit with the emission maximum of
545 nm measured for the complex in dichloromethane solution.
On this basis we assign the 550 nm peak to emission from a 3MLCT
excited state that has its origins in monomer excitation. The sec-
ond Gaussian peak (colored green) maximizes at 604 nm, and is
relatively narrow with a full-width-at-half maximum value of
1373(55) cmꢁ1; it is red-shifted by ꢃ60 nm from the 3MLCT peak,
v
ance of all the direct parameters used to measure the interaction
geometry. In real terms, however, 72% of the complexes analyzed
fall into Group I and have
v-values spread over a much narrower,
more orthogonal range of 81–90°. The Group II and III cation di-
mers without PtꢀꢀꢀPt interactions are in the minority, but are clearly
responsible for the high variance of
v. One other noteworthy
observation is that only 33% of the cation pairs exhibit obtuse val-
ues of . In other words, slippage in a negative x-direction (along
v
the Pt–Cl bond vector) is generally less common than a lateral shift
in the opposite direction.
3.3. Solid state photophysics of [Pt{40-(p-tolyl)trpy}Cl]SbF6
both indications that the peak has its origins in a dr⁄ p⁄ excita-
–
tion, i.e. this emission is from a 3MMLCT (metal–metal-to-ligand
charge transfer) excited state [1,5,6,8,9,11,27,32–35]. Consistent
with this assignment is the observation in the crystal structure of
a PtꢀꢀꢀPt distance that is short enough (3.434(1) Å, Fig. 4) to support
a stabilizing d2z ðPtÞ—dz2ðPtÞ orbital interaction. In fact, this band is,
both in terms of energy and profile, typical of the kind of solid
emission exhibited by planar complexes of platinum(II) that have
The absorption and emission spectra of the [Pt{40-(p-tolyl)tr-
py}Cl]+ cation have been previously recorded in acetonitrile [1].
We report in Section 2 the absorption and emission spectra of
the cation measured in dichloromethane. Dichloromethane was
chosen because it is a non-coordinating solvent with a lower
dielectric constant than acetonitrile and, on this basis, should af-
ford an emission spectrum that more closely resembles one mea-
sured for the cation in the solid state. The spectrum fits the
profile for emission from a 3MLCT state, the same assignment made
by Yip and co-workers [1]. The emission maximum (for the 0–0
transition) occurs at 545 nm and there is a shoulder at 580 nm,
easily assigned to the 0–1 transition. These data help in the inter-
pretation of the emission spectrum recorded on an analytically
pure micro-crystalline powder sample of [Pt{40-(p-tolyl)tr-
py}Cl]SbF6 (Fig. 7). Note that it was necessary to record the spec-
trum at 77 K in order to obtain a sufficiently intense signal; in
fact, even at 77 K, some difficulty was experienced in obtaining a
satisfactory signal to background ratio.
crystal structures containing Pt2 dimers: see [Pt(trpy)Cl]CF3SO3
max
(k
¼ 625 nm at 77 K) [1] and [(2,6-di-(20-naphthyl)-4-pyri-
em
max
em
dine)Pt2{
l
-bis(diphenylphosphino)methane}]
(k
¼ 602 nm)
and its solvates [36]. Two further Gaussian bands are necessary
to fit the experimental spectrum. One (solid red line) is broad,
and maximizes at 645 nm, while the other (broken red line) is even
broader, relatively weak and maximizes at the longer wavelength
of 673 nm. Assignment of these two peaks is less straightforward.
However, we suspect that they both derive from a single emitting
state, in view of the single lifetime measured for the 650 nm peak,
and that the ‘‘extra’’ peak at 673 nm is simply a vibrational compo-
nent. We speculate that this emission derives from an excimeric
The spectrum shown in Fig. 7 displays three main features.
There is a relatively sharp and intense peak at 550 nm, a broader
and somewhat more intense peak at 610 nm; and a third that
exhibits as a pronounced shoulder at ꢃ650 nm. We have measured
excited state lifetimes for the emissions at 550, 610 and 650 nm.
For the first two peaks the intensity decay curve was biphasic,
3p
–
p⁄ excited state. Consistent with this assignment is the broad-
ness and relatively low energy of the emission, and that there are,
indeed, (aryl)– (trpy) interactions in the crystal (vide supra). Sim-
p
p
ilar excimeric emission has been recorded in the solid state for
other complexes of platinum(II) with aromatic ligands
[4,5,28a,37]. Furthermore, from close inspection of the PtꢀꢀꢀPt inter-
actions in the solid-state structure of [Pt{40-(p-tolyl)trpy}Cl]SbF6
(Fig. S1, Supporting Information), we find that there are no addi-
tional PtꢀꢀꢀPt interactions shorter than 3.434(1) Å that could yield
a second, lower-energy 3MMLCT state P 650 nm in the emission
spectrum of the solid.
affording lifetimes of 1.46(1) and 7.59(7) ls for the 550 nm emis-
4. Concluding remarks
This work illustrates the idea that the photo-excited state prop-
erties of a material are dependent on the environment of the chro-
mophore in the solid state, i.e. on the crystal structure of the
material and, more specifically, on the nature of the intermolecular
interactions that link the chromophores (arrays of [Pt{40-(p-to-
lyl)trpy}Cl]+ cation pairs or dimers in the present case). Interest-
ingly, X-ray photocrystallographic analysis is now used to
quantify the relationship between environment and the nature of
the emitting state, the best characterized examples being of gold(I)
phosphine complexes [38]. These studies show directly the effect
of crystal structure on the structural changes that occur on photo-
excitation of the material, and hence on the emission properties of
the material [38]. As far as luminescent platinum terpyridine com-
plexes are concerned, there has not been the same detailed inves-
Fig. 7. Emission spectrum recorded on a microcrystalline sample of [Pt{40 -(p-
tolyl)trpy}Cl]SbF6 at 77 K: kex = 420 nm. The emission envelope has been corrected
for background glass emission. Also shown are the Gaussian bands obtained by de-
convolution of the spectrum, and the spectrum calculated using these bands
(r2 = 0.9994, fit std. error 21.8).

204
J.S. Field et al. / Inorganica Chimica Acta 374 (2011) 197–204
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there is a clear relationship between crystal structure and photo-
physical properties, usually between the strength of a stabilizing
PtꢀꢀꢀPt interaction (measured by the PtꢀꢀꢀPt distance) and the emis-
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The authors thank the South African National Research Founda-
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thanks also go to Judith Magee of the Department of Chemistry,
Durham University, UK for the elemental analyses.
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CCDC 802514 contains the supplementary crystallographic data
for [Pt{40-(p-tolyl)trpy}Cl]SbF6. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Fig. S1 (close-up view of
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and Fig. S2 (metrical parameters for a linear chain of laterally-
shifted dimers). Supplementary data associated with this article
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