Light-emitting cyclopalladated complexes of 6-phenyl-2,2′-bipyridines with Hydrogen-bonding functionality

Article abstract of DOI:10.1021/om020204j

The tridentate ligand 4-carboxy-6-phenyl-2,2′-bipyridine (HL1) was used to prepare the cyclometalated Pd(II) complex [Pd(L1)Cl] (1). Similar ligands were effective in the formation of [Pd(L2)Cl] (2) and [Pd(L3)Cl] (3) (where HL2 = 4-carboxyphenyl-6-phenyl-2,2′-bipyridine and HL3 = 4-hydroxyphenyl-6-phenyl-2,2′-bipyridine). The crystal structure of [Pd(L1)DCl]· H₂O has been determined by X-ray diffraction, revealing a variegated array of H-bonding motifs and π-stacking. The [Pd(L1)Cl] molecules are associated in dimers with an intradimer Pd?Pd distance of 3.27 A?, while dimers are separated by interdimer Pd?Pd contacts of 5.41 A?. All complexes 1-3 exhibited intense luminescence at 77 K, with lifetimes in the 10-200 μs range. The luminescence is assigned to metal-perturbed triplet ligand-centered (LC) excited states for 1 and 3 (partially mixed with an upper-lying metal-to-ligand charge transfer (MLCT) level) and to an excimeric excited state for 2. Room-temperature solid-state luminescence was also revealed by 1 (emission energy and lifetime of 650 nm and 1 μs, respectively), which has been assigned to oligomeric species. Differences in the photophysical properties of 1-3 are addressed with reference to the chemical structure and electronic properties of their polypyridine ligands.

Full text of DOI:10.1021/om020204j

Organometallics 2002, 21, 3511-3518
3511
Ligh t-Em ittin g Cyclop a lla d a ted Com p lexes of
6-P h en yl-2,2′-bip yr id in es w ith Hyd r ogen -Bon d in g
F u n ction a lity
Francesco Neve,*,† Alessandra Crispini,† Cinzia Di Pietro,‡ and
Sebastiano Campagna*,‡
Dipartimento di Chimica, Universita` della Calabria, I-87030 Arcavacata di Rende (CS), Italy,
and Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica,
Universita` di Messina, Via Sperone 31, I-98166 Messina, Italy
Received March 13, 2002
The tridentate ligand 4-carboxy-6-phenyl-2,2′-bipyridine (HL1) was used to prepare the
cyclometalated Pd(II) complex [Pd(L1)Cl] (1). Similar ligands were effective in the formation
of [Pd(L2)Cl] (2) and [Pd(L3)Cl] (3) (where HL2 ) 4-carboxyphenyl-6-phenyl-2,2′-bipyridine
and HL3 ) 4-hydroxyphenyl-6-phenyl-2,2′-bipyridine). The crystal structure of [Pd(L1)Cl]‚
H2O has been determined by X-ray diffraction, revealing a variegated array of H-bonding
motifs and π-stacking. The [Pd(L1)Cl] molecules are associated in dimers with an intradimer
Pd‚‚‚Pd distance of 3.27 Å, while dimers are separated by interdimer Pd‚‚‚Pd contacts of
5.41 Å. All complexes 1-3 exhibited intense luminescence at 77 K, with lifetimes in the
10-200 µs range. The luminescence is assigned to metal-perturbed triplet ligand-centered
(LC) excited states for 1 and 3 (partially mixed with an upper-lying metal-to-ligand charge
transfer (MLCT) level) and to an excimeric excited state for 2. Room-temperature solid-
state luminescence was also revealed by 1 (emission energy and lifetime of 650 nm and 1
µs, respectively), which has been assigned to oligomeric species. Differences in the
photophysical properties of 1-3 are addressed with reference to the chemical structure and
electronic properties of their polypyridine ligands.
In tr od u ction
application in organic light-emitting devices (OLED),
thus participating in the revived interest for cyclo-
palladated complexes as potential candidates for optical
and electroluminescent devices.11-15
Luminescent Pd(II) complexes are exceedingly rare
if compared to analogous Pt(II) luminophores.1 With few
notable exceptions,2,3 most systems emit only at low
temperature and with low efficiency.4-7 It has also been
recognized that when incorporated in polynuclear com-
plexes, Pd(II) centers may lead to partial quenching of
luminescence.7b,c Despite these unfavorable premises,
several photoactive Pd(II) complexes have been recently
the subject of accurate photophysical studies.8-10 In
particular, cyclometalated species9 were designed for
We have recently started an investigation on the
luminescence properties of cyclopalladated species which
also have mesogenic behavior.16-18 All the species under
study were of [Pd(L)X] type, where HL are substituted
6-phenyl-2,2′-bipyridine ligands. In this respect, our
studies were somewhat complementary to those of Che
and co-workers on mononuclear and dinuclear Pt(II)
complexes19 (and only very recently on Pd(II) ana-
* To whom correspondence should be addressed. E-mail: f.neve@
unical.it or photochem@chem.unime.it.
(8) Lai, S.-W.; Cheung, T.-C.; Chan, M. C. W.; Cheung, K.-K.; Peng,
S.-M.; Che, C.-M. Inorg. Chem. 2000, 39, 255.
(9) (a) Wu, Q.; Hook, A.; Wang, S. Angew. Chem., Int. Ed. 2000, 39,
3933. (b) Song, D.; Wu, Q.; Hook, A.; Kozin, I.; Wang, S. Organo-
metallics 2001, 20, 4683.
(10) (a) Yam, V. W.-W.; Zhang, L.; Tao, C.-H.; Wong, K. M.-C.;
Cheung, K.-K. J . Chem. Soc., Dalton Trans. 2001, 1111. (b) Yam, V.
W.-W.; Tao, C.-H.; Zhang, L.; Wong, K. M.-C.; Cheung, K.-K. Organo-
metallics 2001, 20, 453.
† Universita` della Calabria.
‡ Universita` di Messina.
(1) (a) Maestri, M.; Balzani, V.; Deuschel-Cornioley, C.; von Zelews-
ky, A. Adv. Photochem. 1992, 17, 1. (b) Paw, W.; Cummings, S. D.;
Mansour, M. A.; Connick, W. B.; Geiger, D. K.; Eisenberg, R. Coord.
Chem. Rev. 1998, 171, 125.
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Kutal, C. J . Am. Chem. Soc. 1985, 107, 8153.
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Lett. 1997, 267, 341.
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S. T. Adv. Mater. 1995, 7, 659.
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Chem., Int. Ed. 1998, 37, 1255.
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Soc. 2001, 123, 5598.
(16) Neve, F.; Ghedini, M.; Crispini, A. Chem. Commun. 1996, 2463.
(17) Neve, F.; Crispini, A.; Campagna, S. Inorg. Chem. 1997, 36,
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von Zelewsky, A.; Deuschel-Cornioley, C.; J olliet, P. Helv. Chim. Acta
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10.1021/om020204j CCC: $22.00 © 2002 American Chemical Society
Publication on Web 07/25/2002

3512 Organometallics, Vol. 21, No. 17, 2002
Neve et al.
Ch a r t 1
Single crystals of compounds 1-3 are very difficult
to grow. We succeeded for 1 but only after several
painstaking attempts. Yellow-orange crystals of 1 were
grown by very slow evaporation from a diluted acetone
solution. 1 crystallized as a monohydrate, and the role
of water in the crystal packing of 1 will become more
clear in the next section. The water uptake seems to be
a spontaneous process and, more importantly, charac-
teristic of both the yellow-orange crystals grown from
acetone and the yellow microcrystalline solid formed
from the reaction mixture. In both cases, cocrystallized
water may come from residual water in the solvent.
Although indirectly, inclusion of water in the micro-
crystalline solid was tested by infrared spectroscopy. In
addition to a very broad band in the region of O-H
stretching (3600-2500 cm-1), the IR spectrum (KBr
pellet) exhibited a strong absorption band at 1707 cm-1
(with a shoulder at ∼1730 cm-1) that can be assigned
to the CdO stretching of the carboxylic acid group of
the coordinated L1 ligand. Upon heating the KBr pellet
at 160 °C in a vacuum for 2 h and collecting the
spectrum immediately after, the IR spectrum revealed
a new strong ν(CdO) absorption band (with two close
maxima at 1752 and 1738 cm-1) in addition to the
original one at 1705 cm-1. Interestingly, the initial IR
spectrum was restored barely leaving the pellet in air
for 1 h. The substantial variation of the ν(CdO) value
upon heating 1 is a likely hint for the presence of water
molecules strongly associated with the carboxylic acid
group in the molecule, a fact that is also indirectly
confirmed by the resistance to complete water loss under
the experimental conditions.23 Thus, the microcrystal-
line 1 may contain water, which is partially lost on
heating and which is readsorbed from the air when the
sample is cooled back to room temperature. Similar
changes in the infrared spectrum were also noted for
the crystalline yellow-orange form of 1. In both cases,
no color changes were observed during the solvent
release-uptake cycle. Finally, it must be noted that a
thermal treatment similar to that of 1 gave no notable
changes in the IR spectrum of complex 2, leaving the
strong ν(CdO) absorption band (featuring two maxima
at 1684 and 1697 cm-1) unaffected.
logues8) containing similar ligands. While luminescence
in a glass matrix at 77 K was always observed, we found
that our cyclopalladated species were emissive also in
the solid state at room temperature.17,18 Interpretation
of the photophysical data however led us to suggest that
“isolated molecules” were responsible for the solid-state
emission process from these mononuclear compounds.
To improve the emissive properties of the organopalla-
dates, we have chosen functionalized 6-phenyl-2,2′-
bipyridine ligands (HLn (n ) 1-3), Chart 1) that may
promote intermolecular contacts through hydrogen bond-
ing and/or π-stacking. The photophysical properties of
the resulting cyclometalated [Pd(Ln)Cl] complexes are
now presented and discussed, also taking advantage of
the solid-state characterization of one of them.
Resu lts a n d Discu ssion
The ligands used in this study were prepared through
classical Kro¨hnke-type20 syntheses. The reactions were
single-step (HL1) or multistep18,21 processes (HL2 and
HL3). All the preparations led to the desired product
in good overall yield (50-70%), an outcome not always
achieved for oligopyridines prepared with this general
method.22 The metalated derivatives [Pd(L1)Cl] (1), [Pd-
(L2)Cl] (2), and [Pd(L3)Cl] (3) were obtained as yellow
microcrystalline solids by reaction of [Pd(PhCN)2Cl2]
and the HL ligands in a methanol/benzene mixture. Due
mainly to the different solubility properties of the
ligands, while the formation of both 1 and 2 required
reflux conditions, 3 was obtained at room temperature.
The use of [Pd(PhCN)2Cl2] as a precursor for cyclopal-
ladatated products containing a chloride ligand has
proved somewhat superior with respect to other re-
agents such as [PdCl4]2- alkali salts or Pd(OAc)2, as
shorter reaction times and milder conditions are usually
required. Apart from DMSO, common organic solvents
are not good solvents for compounds 1-3. Compound 1
is also slightly soluble in CH2Cl2 and acetone.
Cr ysta l Str u ctu r e of 1. The structure of 1 has been
determined by X-ray crystallography. The molecule is
nearly flat (except for the carboxy group) with the
palladium atom in a distorted square-planar coordina-
tion goemetry (Figure 1). Metal-ligand distances are
similar to those observed for other Pd(II) complexes of
cyclometalating 6-phenyl-2,2′-bipyridine ligands.8,16,24
The bite angle value of the chelating L1 ligand (N(1)-
Pd-C(12), 160.0°) is also typical for the above systems
and compares well tothat ofrelated N∧N∧N,25 N∧N∧C,26
(19) (a) Cheung, T.-C.; Cheung, K.-K.; Peng, S.-M.; Che, C.-M. J .
Chem. Soc., Dalton Trans. 1996, 1645. (b) Liu, H.-Q.; Cheung, T.-C.;
Che, C.-M. Chem. Commun. 1996, 1039. (c) Wu, L.-Z.; Cheung, T.-C.;
Che, C.-M.; Cheung, K.-K.; Lam, M. H. W. Chem. Commun. 1998, 1127.
(d) Tse, M.-C.; Cheung, K.-K.; Chan, M. C. W.; Cheung, K.-K.; Che,
C.-M. Chem. Commun. 1998, 2295. (e) Lai, S.-W.; Chan, M. C.-W.;
Cheung, T.-C.; Che, C.-M. Organometallics 1999, 18, 3327. (f) Lai, S.-
W.; Chan, M. C.-W.; Cheung, T.-C.; Peng, S.-M.; Che, C.-M. Inorg.
Chem. 1999, 38, 4046. (g) Wong, K. H.; Chan, M. C. W.; Che, C.-M.
Chem. Eur. J . 1999, 5, 2845. (h) Lu, W.; Chan, M. C. W.; Cheung, K.-
K.; Che, C.-M. Organometallics 2001, 20, 2477. (i) Lai, S.-W.; Lam,
H.-W.; Lu, W.; Cheung, K.-K.; Che, C.-M. Organometallics 2002, 21,
226. (j) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Zhu, N.; Lee, S.-T.;
Che, C.-M. Chem. Commun. 2002, 206.
(23) More severe conditions (heating at 180 °C for 72 h) resulted in
a slightly different behavior, since complete recovery of the initial
spectrum was only obtained after waiting a much longer time. Indeed,
the presence of carboxyl acid dimers in the solid state (that would
account for equally strongly H-bonded acid groups and therefore IR
stretching bands of similar magnitude) cannot be completely discarded.
In the latter case, a prediction of the actual role of water in the crystal
packing is very difficult.
(20) Kro¨hnke, F. Synthesis 1976, 1.
(21) Neve, F.; Crispini, A.; Campagna, S.; Serroni, S. Inorg. Chem.
1999, 38, 2250.
(22) Alternative procedures based on a highly competitive solvent-
less approach have been recently reviewed: Cave, G. W. V.; Raston,
C. L.; Scott, J . L. Chem. Commun. 2001, 2159.
(24) Constable, E. C.; Henney, R. P. G.; Leese, T. A.; Tocher, D. A.
J . Chem. Soc., Chem. Commun. 1990, 513.
(25) (a) Ramdeehul, S.; Barloy, L.; Osborn, J . A.; De Cian, A.;
Fischer, J . Organometallics 1996, 15, 5442. (b) Barloy, L.; Gauvin, R.
M.; Osborn, J . A.; Sizun, C.; Graff, R.; Kyritsakas, N. Eur. J . Inorg.
Chem. 2001, 1699.

Light-Emitting Cyclopalladated Complexes
Organometallics, Vol. 21, No. 17, 2002 3513
F igu r e 1. Perspective view of 1 with thermal ellipsoids
drawn at the 50% probability level. Selected bond lengths
(Å) and angles (deg): Pd-N(1), 2.150(3); Pd-N(2),
1.964(3); Pd-C(12), 1.997(4); Pd-Cl, 2.334(1); C(11)-O(1),
1.311(4); C(11)-O(2), 1.210(5); N(1)-Pd-N(2), 78.6(1);
N(2)-Pd-C(12), 81.4(1); N(1)-Pd-C(12), 160.0(1); N(2)-
Pd-Cl, 177.5(1); O(1)-C(11)-O(2), 124.3(3).
N∧C∧N,27 or C∧N∧C28 tridentate ligands with similar
geometric constraints.
The crystal structure of 1 consists of inversion-related
dimers with a Pd‚‚‚Pd intradimer contact of 3.27 Å
(Figure 2a), a value slightly larger than the sum of their
van der Waals radii.29 The molecules within the dimer
are arranged in a head-to-tail fashion. The Pd-Pd
vector is essentially orthogonal to the coordination plane
(1° angle with the plane normal), suggesting a weakly
attractive character for the M‚‚‚M interaction. The latter
also shows a small degree of pyramidalization (R )
91.0°) according to Aullon et al.30a In fact, intermolecular
d8-d8 short contacts are less common for mononuclear
square-planar palladium complexes,30b which testifies
to the lower ability of Pd(II) species to electronically
interact in the solid state with respect to the corre-
sponding Pt(II) analogues.31
Intermolecular contact also occurs through aromatic
π-π stacking within the dimer. The stacking interaction
(best seen in Figure 2a) involves both the external
pyridine and the metalated phenyl ring of the L1 ligand
in each molecule, giving rise to offset arene-pyridine
dimeric π-stacks. The slipped geometry of the aromatic
interaction (interplanar distance 3.34 Å) is characterized
by a centroid-centroid distance of 3.73 Å, interplanar
F igu r e 2. (a) Diagram showing the slipped face-to-face
π-π interaction in the dimeric unit of 1. (b) Stacking
arrangement of Pd2 dimers along the a axis with alterna-
tion of short (solid lines) and long (dashed lines) Pd-Pd
contacts.
angle of 2.8°, displacement angle of 27°, and horizontal
slip of 1.66 Å. Ring slippage leads to C-C or C-N
contacts of 3.41-3.62 Å, that is, within the range
expected for fairly strong aromatic interactions with
parallel displaced arrangement of the rings.32 Although
less effective, aromatic stacking extends beyond the
dimeric units. The Pd2 dimers stack along the a axis
(Figure 2b), bringing the interplanar distance between
two consecutive dimers to 3.45 Å. The interdimer
stacking is of pyridine-pyridine type,32b and it occurs
through the central pyridine ring of the L1 ligand. The
weaker interaction is slipped, parallel displaced (cen-
troid-centroid distance, 4.63 Å, displacement angle of
(26) (a) Butler, I. R. Organometallics 1992, 11, 74. (b) Isaac, C. J .;
Price, C.; Horrocks, B. R.; Houlton, A.; Elsegood, M. R. J .; Clegg, W.
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C. Organometallics 1999, 18, 3337.
(28) (a) Newman, C. P.; Cave, G. W. V.; Wong, M.; Errington, W.;
Alcock, N. W.; Rourke, J . P. J . Chem. Soc., Dalton Trans. 2001, 2678.
(b) Lu, W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-M. Organometallics
2001, 20, 2477. (c) Peris, E.; Loch, J . A.; Mata, J .; Crabtree, R. H. Chem.
Commun. 2001, 201.
(29) Bondi, A. J . J . Chem. Phys. 1964, 68, 441.
(32) (a) Hunter, C. A.; Sanders, J . K. M. J . Am. Chem. Soc. 1990,
112, 5525. (b) J aniak, C. J . J . Chem. Soc., Dalton Trans. 2000, 3885.
(c) Hunter, C. A.; Lawson, K. R.; Perkins, J .; Urch, C. J . J . Chem. Soc.,
Perkin Trans. 2 2001, 651. (d) Tsuzuki, S.; Honda, K.; Uchimaru, T.;
Mikami, M.; Tanabe, K. J . Am. Chem. Soc. 2002, 124, 104.
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111, 145.

3514 Organometallics, Vol. 21, No. 17, 2002
Neve et al.
Ch a r t 2
4
in Chart 2). Within the centrosymmetric R4 (12) ring,
short38,39 O-H‚‚‚O bonds (H‚‚‚O, 1.76 Å; O‚‚‚O, 2.58 Å)
alternate with longer ones (H‚‚‚O, 2.15 Å; O‚‚‚O, 2.92
Å). Intermolecular C-H‚‚‚O interactions (with H‚‚‚O
contacts in the range 2.4-2.6 Å) were also observed that
involve the oxygen atoms of both the water molecule
and the carboxylic acid group.
Since the inclusion of water increases the number of
hydrogen bond donors in the system, a further inter-
molecular interaction must be considered. The water
molecule is also H-bonded to the chloride ligand of an
adjacent molecule (Figure 3a) through a relatively
short40 O-H‚‚‚Cl-Pd contact (H‚‚‚Cl, 2.26 Å). This
interaction leads to the formation of infinite organome-
tallic chains (although with interleaving water mol-
ecules) running parallel to the [101] direction. This
supramolecular motif is reminiscent of the infinite
organometallic chains formed through intermolecular
H‚‚‚Cl-Pt hydrogen bonds in Pt(II) pincers functional-
ized with hydrogen bond donors.41
F igu r e 3. (a) H-bonding pattern revealed by 1‚H2O.
Hydrogen bond lengths (Å) and angles (deg) (with sym-
metry codes): O1-H1B‚‚‚O3 [H‚‚‚O 1.76, O‚‚‚O 2.581(4),
O-H‚‚‚O 176], O3-H1W‚‚‚O2A(3+x, 1-y, 2+z) [H‚‚‚O
2.15(4), O‚‚‚O 2.918(4), O-H‚‚‚O 138], O3-H2W‚‚‚ClB(1+x,
y, 1+z) [H‚‚‚Cl 2.26(3), O‚‚‚Cl 3.176(4), O-H‚‚‚Cl 161]. (b)
View emphasizing the lamellar arrangement of the crystal
packing. Dashed lines represent the short Pd-Pd contacts.
Since all different aspects of the crystal structure of
1 have been considered individually, it is perhaps also
useful to draw an overall picture. Molecules self-
assemble in the solid state using all types of assembling
functions available (i.e., hydrogen bond donors and
acceptors, aromatic π-stacking, coordinatively unsatur-
ated metal centers) including solvent inclusion. Thus
all interactions cooperate in the stabilization of stacking.
As a result, a lamellar-type structure is formed (Figure
3b), with in-plane intermolecular H-bonding and with
tight intermolecular contacts (between metals, between
aromatics, and again H-bonding) holding the lamellae
together. As will be discussed later, intermolecular
interactions seem to show a relevant effect on the solid-
state luminescence properties of complex 1.
42°, and horizontal slip of 3.1 Å), leading to shortest
contacts in the range 3.52-4.00 Å. The corresponding
interdimer Pd‚‚‚Pd distance is 5.41 Å. Intra- and inter-
dimer Pd-Pd contacts thus alternate along a zigzag
chain (Pd-Pd(a)-Pd, 131°). In this regard, the packing
behavior of 1 resembles that of [Pt(L)(MeCN)]PF6 (HL
) 6-phenyl-2,2′-bipyridine) (intradimer and interdimer
distances of 3.28 and 4.59 Å, respectively)24 and [Pt-
(terpy)Cl]ClO4 (terpy ) 2,2′:6′,2′′-terpyridine) (intradimer
and interdimer distances of 3.27 and 4.20 Å, respec-
tively)33 more than that of [Pd(L)Cl]5 and [Pd(L4)Cl]
(HL4 ) 4-(4′-dodecyloxyphenyl)-6-phenyl-2,2′-bipyri-
dine),16 which revealed no short Pd-Pd contacts. Taking
into account both the metal-metal and the aromatic
π-π stacking, the crystal packing of 1 can be described
in terms of discrete M-M units within an infinite
π-stacking. Going back to the role of cocrystallized
water, this brings into evidence the importance of
hydrogen bonding in the supramolecular organization
of 1. Since the cyclometalated L1 ligand bears a single
carboxylic acid group, the latter is expected to partici-
pate in intermolecular hydrogen bonding with a car-
boxylic acid group from a neighboring molecule. Pairing
indeed occurs although mediated by water molecules
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Crystallogr. 1990, B46, 256.
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(38) J effrey, G. A. An Introduction to Hydrogen Bonding; Oxford
University Press: New York, 1997.
(39) Vishweshar, P.; Nangia, A.; Lynch, V. M. Chem. Commun. 2001,
179.
(40) (a) Steiner, T. Acta Crystallogr. 1998, B54, 456. (b) Brammer,
L.; Bruton, E. A.; Sherwood, P. Cryst. Growth Des. 2001, 1, 277.
(41) (a) J ames, S. L.; Verspui, G.; Spek, A. L.; van Koten, G. Chem.
Commun. 1996, 1309. (b) Davies, P. J .; Veldman, N.; Grove, D. M.;
Spek, A. L.; van Koten, G. Angew. Chem., Int. Ed. Engl. 1996, 35, 1959.
(Figure 3a), affording an R4 (12) cyclic motif34-36 (I in
4
Chart 2) instead of the more familiar R2 (8) ring37 (II
2
(33) Bailey, J . A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.;
Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591.

Light-Emitting Cyclopalladated Complexes
Organometallics, Vol. 21, No. 17, 2002 3515
Ta ble 2. Cr ysta llogr a p h ic Da ta for Com p lex 1
1‚H2O
empirical formula
fw
C17H13ClN2O3Pd
453.14
cryst syst
triclinic
space group
a, Å
P1h
7.9364(16)
b, Å
9.4593(19)
c, Å
11.247(2)
R, deg
111.05(3)
94.59(3)
98.40(3)
771.5(3)
â, deg
γ, deg
V, Å3
Z
2
D(calcd), g/cm3
µ, mm-1
1.873
1.394
298
temp, K
wavelength, Å
F(000)
0.71073
432
scan type
θ-2θ
transm factors
no. measd reflns
no. unique reflns
no. of reflns with I > 2σ(I)
no. of refined params
R indicesa,b
R indices (all data)
goodness-of-fit
0.45-0.61
2634
2435 [R(int) ) 0.0288]
2328
223
R1 ) 0.0286, wR2 ) 0.0762
R1 ) 0.0299, wR2 ) 0.0771
1.11
F igu r e 4. Absorption spectra in DMSO solution of 1 (top),
2 (middle), and 3 (bottom).
Ta ble 1. Sp ectr oscop ic a n d P h otop h ysica l Da ta
for Com p lexes 1-3
absorptiona luminescence
R1 ) ∑(|Fo| - |Fc|)/∑|Fo|. wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2
.
a
b
λ
max, nm
λ
max, nm
(τ, µs)
compound
(ꢀ, M-1 cm-1
)
1, [Pd(L1)Cl] 278 (21 100)
326 (12 100)
2, [Pd(L2)Cl] 295 (31 200)
332 (17 600)
3, [Pd(L3)Cl] 280 (20 700)
320 (25 700)
510 (40)
650 (1)
610 (15)
glass matrix,b 77 K
solid, 298 K
glass matrix,b 77 K
495 (170)
glass matrix,b 77 K
368 (13 600)
a
b
In DMSO. DMF/toluene (1:1 v/v).
Absor p tion Sp ectr a a n d Lu m in escen ce P r op er -
ties. The absorption spectra of complexes 1-3 in DMSO
solution (Figure 4, Table 1) are dominated by intense
bands in the visible region (ꢀ in the range 104-105 M-1
cm-1), which can be attributed to spin-allowed metal-
perturbed ligand-centered (LC) transitions by compari-
son with the spectra of Pd(II) complexes with similar
polypyridine ligands.16-18 At longer wavelengths, less
intense shoulders are present, which can receive con-
tribution from spin-allowed metal-to-ligand charge-
transfer (MLCT) transitions. The absorption spectra of
the compounds in dichloromethane are qualitatively
similar to those in DMSO, although the poor solubility
in the chlorinated solvent inhibited a more detailed
analysis of the spectra, including the calculation of the
molar absorption coefficients. None of the new com-
plexes 1-3 exhibit appreciable luminescence in fluid
solution at room temperature in any of the solvents
employed (dichloromethane, DMF, DMSO).42 On the
contrary, all of them exhibit strong luminescence in a
DMF/toluene (1:1, v/v) rigid matrix at 77 K, with
monoexponential lifetimes in the 10-5-10-3 s time
range (see Table 1). Interestingly, the emission spec-
trum of 2 is unstructured, red-shifted, and shorter-lived
compared to those of 1 and 3 (Figure 5). Microcrystalline
F igu r e 5. Luminescence spectra of 1-3 in a DMF/toluene
(1:1 v/v) rigid matrix at 77 K.
F igu r e 6. Luminescence spectrum of 1 in the solid state
at room temperature.
samples of 1 also exhibit an unstructured room-tem-
perature emission spectrum peaking at 650 nm (Figure
6), significantly red-shifted relative to the emission in
a rigid matrix at low temperature. In all the media, the
luminescence spectra are independent of the excitation
wavelength.
Room-temperature luminescence of mononuclear
Pd(II) complexes is very rare,2,3 usually because of the
presence of low-lying metal-centered (MC) excited states
which deactivate the potentially luminescent metal-to-
ligand charge transfer (MLCT) and ligand-centered (LC)
(42) Actually, a very weak emission around 500 nm was detected
for all the species in DMSO and dichloromethane. However, the
emission output was at the limit of the sensitivity of our equipment
and therefore was not considered.

3516 Organometallics, Vol. 21, No. 17, 2002
Neve et al.
levels through thermally activated surface-crossing
processes. In fact, MC states have very distorted geom-
etries compared to those of the ground state, so that
the Franck-Condon factors for radiationless decay are
extremely high and make the radiative processes un-
competitive.43 This problem (common to several transi-
tion metal complexes) has proved to be very important
for Pd(II) species, so that the cyclometalated approach
alone, although useful for several Pt(II) species,4,9,17,19
has not been sufficient to favor room-temperature
luminescence for Pd(II) compounds.5,8,9,16-18 The lack of
luminescence for 1-3 at room temperature in fluid
solution is therefore not surprising.
either excimeric contribution to the emission is possible.
Yet, it should be considered that the luminescence
experiments were carried out on the yellow microcrys-
talline form of 1; thus we cannot exclude that the
arrangement of the molecules in the microcrystalline
sample is somewhat different from that obtained from
the crystallographic study. Hence, X-ray data alone
cannot support a definitive attribution. Furthermore,
this is the first time that an excimeric emission at room
temperature is reported for Pd(II) complexes, ruling out
the possibility of any direct comparison. In conclusion,
we cannot safely assign the luminescence of 1 in the
solid state either to a specific state (MMLCT or ππ*) or
to a mixture of both.
The luminescence data of a microcrystalline sample
of 1 (in particular the emission energy; see Table 1 and
Figure 6) directly suggest that the emission from this
species in the solid state can be attributed to excimeric
species. Excimer formation (particularly in the solid
state) for square-planar metal complexes has been
reported to be quite effective in generating room-
temperature luminescence.19,33,44 Excimeric excited states
are indeed substantially red-shifted from MLCT or LC
states related to “isolated” molecules, so that the energy
gap between the excimeric state and the deactivating
MC states increases (as a result, the thermally activated
processes involving nonemissive MC levels are inhib-
ited). The reported excimeric emission from square-
planar complexes is of two types: (i) charge transfer
emission arising from excited states exhibiting substan-
tial metal-metal interaction (metal-metal to ligand
charge transfer, MMLCT, sometimes designed as dσ*f
π*(ligand) charge transfer excited states, where the
donor orbital is derived from direct metal-metal inter-
action) and (ii) ππ* emission from excited states derived
from stacking interactions between the aromatic moi-
eties of the coordinated ligands.45,46 In some cases,
oligomerization can also occur in the ground state, and
in these cases the term “excimer” should be considered
in a broader sense.45 Since the emission spectra are
usually unstructured, and also the emission energies
are expected to be similar, a clear distinction between
these two types of contribution to the emission is not a
trivial task from an experimental viewpoint. X-ray
single-crystal data, which indicate both the formation
of dimers with short intradimer Pd‚‚‚Pd contacts and
extended π-stacking, would suggest that in principle
Unlike 1, compounds 2 and 3 do not exhibit room-
temperature emission in the solid state, probably be-
cause of a significantly different crystal packing. Major
differences in the solid-state supramolecular organiza-
tion must be expected as a result of several factors.
These include a different number of aromatic rings in
the cyclometalated ligand (L1 vs L2 and L3), the uneven
efficiency of H-bonding groups (L1 and L2 vs L3), and
the inclusion of solvent in the crystal lattice. A further
reason may be electronic in nature. In this regard, it is
useful to recall that the polypyridine ligand L1 has
orbitals (both π and π*) lower in energy than those of
L2 and L3, and it is also a much more electron-
withdrawing ligand than both L2 and L3, as suggested
by the photophysical data reported for cyclometalated
Ir(III) complexes based on similar ligands.46 These two
factors may have consequences relevant to dimer or
excimer formation: aromatic interactions can be af-
fected by the energies of the interacting orbitals, and
the different electron density on the palladium centers
(lower for 1 compared to 2 and 3) can influence metal-
metal interactions.
The picture is quite different at 77 K in rigid matrix,
where the thermally activated process involving the MC
states cannot occur. Luminescence of 1 and 3 can be
3
safely attributed to metal-perturbed LC states on the
basis of the structured spectra, energies, and lifetimes
(Table 1, Figure 5).4-6,8 However, the lifetime of 1 is
probably too short to be attributed to a pure (although
metal-perturbed) LC state. Most likely the emitting
level is mixed with a close-lying, upper MLCT, which
influences the excited-state dynamics by decreasing the
luminescence lifetime.47 A similar mixing is less effec-
tive for 3, because the MLCT state is expected to lie at
higher energy than the analogous level in 1.
The 77 K emission of 2 (Table 1, Figure 5) is very
different from those of 1 and 3. Because of the emission
energy, spectrum, and lifetime, it is attributed to ππ*
excimeric states. Such an attribution is also based on
the agreement with the photophysical properties re-
cently reported for a series of dimeric Pd(II) complexes,8
where excimer formation is favored by structural con-
formation and excimeric emission is observed at 77 K.
Interestingly, no emission features at higher energy are
present in the emission spectrum of 2 even changing
the concentration, thus suggesting that the equilibrium
between “isolated” single molecules and the excimers
(43) Crosby, G. R. Acc. Chem. Res. 1975, 8, 231.
(44) Kunkely, H.; Vogler, A. J . Am. Chem. Soc. 1990, 112, 5625.
(45) In principle, an excimer is formed by the combination of a
ground-state molecule with an excited-state molecule, a process quite
common for aromatic systems (for example, see: Klessinger, M.; Michl,
J . Excited States and Photochemistry of Organic Molecules; VCH: New
York, 1995). The excimer luminescence is expected to be at longer
wavelengths than the monomer luminescence, and the associated
emission band is commonly broad and without vibrational structure
owing to a transition into the unbound ground state. As an excited-
state process, excimer formation also competes with other deactivating
processes of the initially formed excited state of the monomer. It should
be noted that while this definition fully fits the ππ* luminescence
derived from interactions between aromatic moieties (one of them being
in its own excited state), the term “excimeric emission” is not formally
correct for a MMLCT excited state, for which a metal-metal interac-
tion in the ground state is usually considered. The latter would be more
appropriately described as the excited state of a dimeric (or oligomeric)
discrete system. However, since in the literature MMLCT emission is
very often also called excimeric emission, we decided to use this term
in this larger meaning also here, although formal differences should
not be forgotten.
(47) The influence of higher-energy excited states on the dynamics
of emitting states by mixing between the states, with minimal
perturbation on the emission energy, is well known. For example, see
ref 6.
(46) Neve, F.; Crispini, A.; Loiseau, F.; Campagna, S. J . Chem. Soc.,
Dalton Trans. 2000, 1399.

Light-Emitting Cyclopalladated Complexes
Organometallics, Vol. 21, No. 17, 2002 3517
syntheses of the ligands 4-(4′-carboxyphenyl)-6-phenyl-2,2′-
bipyridine (HL2)17 and 4-(4′-hydroxyphenyl)-6-phenyl-2,2′-
bipyridine (HL3)18 were reported previously. The preparation
of ligand 4-carboxy-6-phenyl-2,2′-bipyridine (HL1) was recently
reported by our group,46 although the synthetic details were
not given at that time.
strongly favors excimer formation under these condi-
tions, at least within the studied range of concentration
(10-5-10-4 M). The differences in the low-temperature
luminescence properties along this series of complexes
warrant some additional comment. Extension of the
aromatic framework (although not a rigid one) obviously
may be an important factor in the formation of the
excimer, so that it is not surprising that complex 1 does
not show excimeric emission. In contrast, the absence
of excimeric emission for 3 calls for a different reason.
This might be related to the nature of the substituent
on the phenyl ring not involved in metal coordination.
We suggest that for complex 2 a key role could be played
by hydrogen bonding between carboxyl groups of dif-
ferent molecules (leading to carboxyl dimers at low
temperature), which would assist the intermolecular
π-π association responsible for the excimeric emission.
4-Ca r boxy-6-p h en yl-2,2′-b ip yr id in e (HL1). A stirred
mixture of 3-benzoylacrylic acid (580 mg, 3.29 mmol), N-(2-
pyridacyl)pyridinium iodide (1.03 g, 3.29 mmol), and a 10-fold
excess of ammonium acetate (2.54 g) in methanol (10 mL) was
refluxed for 8 h. After the resulting red-black solution was
cooled to ambient temperature it was diluted with water (100
mL) and acidified with concentrated HCl to pH ≈ 2. The crude
solid formed was filtered off and recrystallized from methanol
and water to yield the product as small gray needles in a 76%
yield. Mp: 255 °C. IR (KBr, cm-1): ν(COO) 1709. 1H NMR
(CD3OD, 300 MHz): δ 8.99 (d, J ) 1.2 Hz, 1H, central
pyridine), 8.88 (br d, J ) 5 Hz, 1H, external pyridine), 8.81
(d, J ) 8.0 Hz, 1H, external pyridine), 8.58 (d, J ) 1.2 Hz, 1H,
central pyridine), 8.42 (m, 2H, phenyl), 8.19 (ddd, J ) 7.9, 7.5,
1.8 Hz, 1H, external pyridine), 7.76-7.65 (m, 4H, external
pyridine + phenyl). Anal. Calcd for C17H12N2O2: C, 73.90; H,
4.38; N, 10.24. Found: C, 73.47; H, 4.08; N, 10.51.
Con clu sion
In summary, we have exploited the well-known ability
of 6-phenyl-2,2′-bipyridine ligands to afford cyclopalla-
dated products.16,24 Such cyclometalating ability, coupled
to the presence of hydrogen-bond donor (COOH or OH)
and acceptor (M-Cl bond) groups on opposite sides of
the molecule, enables the potential formation of several
intermolecular contacts. While complete structural char-
acterization was not achieved for all complexes, it is
apparent that complex 1 fulfills our best expectations
relative to the maximization of metal-metal, ligand-
ligand, and H-bonding interactions.
Luminescence of complexes 1-3 at 77 K has variable
character with respect to the different ligands, being
related to excited states of different nature. Solid-state
luminescence at room temperature was revealed by 1,
which has been ascribed to an excimeric emission. Since
this is the first time this behavior has been proved for
a Pd(II) organometallic complex, we are confident that
future improvements may allow the consideration of
thermally and chemically stable Pd(II) organometallics
for light-emitting devices.
[P d (L1)Cl] (1). A solution of [Pd(PhCN)2Cl2] (82 mg, 0.215
mmol) in benzene (4 mL) was added to a suspension of HL1
(60 mg, 0.215 mmol) in methanol (5 mL). The mixture was
then refluxed for 7 h, affording a yellow microcrystalline
precipitate. The solid was collected by filtration, washed with
methanol and diethyl ether, and vacuum-dried. Yield: 79 mg
1
(87%); mp > 350 °C. IR (KBr, cm-1): ν(COO) 1707. H NMR
(DMSO-d6, 300 MHz): δ 8.74 (d, J ) 8.0 Hz, 1H, external
pyridine), 8.71 (br d, J ) 5 Hz, 1H, external pyridine), 8.59 (s,
1H, central pyridine), 8.35 (s, 1H, central pyridine), 8.34 (br
t, 1H, external pyridine), 7.89 (m, 1H, external pyridine), 7.81
(br m, 1H, metalated phenyl), 7.60 (br m, 1H, metalated
phenyl), 7.24-7.17 (m, 2H, metalated phenyl). Anal. Calcd for
C17H11ClN2O2Pd‚H2O: C, 46.92; H, 3.01; N, 6.72. Found: C,
47.33; H, 2.64; N, 6.39.
[P d (L2)Cl] (2). The procedure was similar to that adopted
for 1 (reflux time, 4 h). The bright yellow microcrystalline
product was obtained in 88% yield. Mp > 350 °C. IR (KBr,
1
cm-1): ν(COO) 1684, 1697. H NMR (DMSO-d6, 300 MHz): δ
8.82 (d, J ) 8.0 Hz, 1H, external pyridine), 8.74 (br d, J ) 5
Hz, 1H, external pyridine), 8.69 (s, 1H, central pyridine), 8.48
(s, 1H, central pyridine), 8.38 (br t, 1H, external pyridine), 8.32
(d, J ) 8.5 Hz, 2H, phenyl), 8.23 (d, J ) 8.5 Hz, 2H, phenyl),
7.95 (dd, J ) 7.2, 1.7 Hz, 1H, metalated phenyl), 7.90 (m, 1H,
external pyridine), 7.61 (dd, J ) 7.2, 1.6 Hz, 1H metalated
phenyl), 7.26-7.17 (m, 2H, metalated phenyl). Anal. Calcd for
Exp er im en ta l Section
Gen er a l Meth od s. All reactions were carried out under a
nitrogen atmosphere. 1H NMR spectra were recorded at room
temperature on a Bruker AC 300 spectrometer with tetram-
ethylsilane as internal standard. FT-IR spectra were recorded
on a Perkin-Elmer 2000 spectrophotometer for KBr pellets.
Elemental analyses were performed using a Perkin-Elmer
2400 microanalyzer. Absorption spectra were recorded with a
Kontron Uvikon 860 spetrophotometer, and luminescence
spectra were performed with a J obin Yvon-Spex Fluoromax 2
fluorimeter equipped with a Hamamatsu R3896 photomulti-
plier. The spectra were corrected for phototube response by
using a standard lamp. For the luminescence lifetimes at room
temperature, an Edinburgh OB900 time-correlated single-
photon-counting spectrometer was employed (nitrogen dis-
charge, pulse width, 4 ns). Luminescence lifetimes at 77 K (µs
time scale) were measured by a Perkin-Elmer 5B spetro-
fluorimeter with variable delay times (typically between 10
µs and 2 ms) and fixed gate times (1 ms). In all cases, the
decays were strictly monoexponential.
C
23H15ClN2O2Pd: C, 56.01; H, 3.07; N, 5.68. Found: C, 55.62;
H, 3.05; N, 5.58.
[P d (L3)Cl] (3). A golden yellow solution was obtained upon
addition of a solution of [Pd(PhCN)2Cl2] (71 mg, 0.185 mmol)
in benzene (4 mL) to a stirred suspension of HL3 (60 mg, 0.185
mmol) in methanol (4 mL). Stirring of the solution at room
temperature resulted in the formation of a yellow precipitate
within 1 h. The microcrystalline solid was filtered off, washed
with diethyl ether, and vacuum-dried. Yield: 82 mg (95%). Mp
1
> 350 °C. H NMR (DMSO-d6, 300 MHz): δ 10.0 (s, 1H, OH),
8.62 (d, 1H, J ) 8.0 Hz, external pyridine), 8.53 (br d, J ) 4.9
Hz, 1H, external pyridine), 8.37 (s, 1H, central pyridine), 8.17
(ddd, J ) 7.8, 7.1, 1.5 Hz, 1H, external pyridine), 8.13 (br s,
1H central pyridine), 7.91 (d, J ) 8.7 Hz, 2H, phenyl), 7.72
(dd, J ) 7.2, 1.7 Hz, 1H, metalated phenyl), 7.68 (m, 1H,
external pyridine), 7.41 (dd, J ) 7.2, 1.6 Hz, 1H, metalated
phenyl), 7.02-6.93 (m, 2H, metalated phenyl), 6.86 (d, J )
8.7 Hz, 2H, phenyl). Anal. Calcd for C22H15ClN2OPd: C, 56.80;
H, 3.25; N, 6.02. Found: C, 56.40; H, 3.16; N, 6.41.
Ma ter ia ls. Ammonium acetate (Lancaster), 3-benzoylacryl-
ic acid (Aldrich), and reagent-grade solvents were used as
received. N-(2-Pyridacyl)pyridinium iodide48 and [Pd(PhCN)2-
Cl2]49 were prepared according to literature procedures. The
(48) Treffert-Ziemelis, S. M.; Golus, J .; Strommen, D.; Kincaid, J .
R. Inorg. Chem. 1993, 32, 3890.

3518 Organometallics, Vol. 21, No. 17, 2002
Neve et al.
X-r a y Cr ysta llogr a p h ic Stu d y of 1. Single-crystal dif-
fraction data for complex 1 were recorded on a Siemens R3m/V
diffractometer using graphite-monochromated Mo KR radia-
tion (λ ) 0.71073 Å). Lorentz and polarization corrections were
applied to the data. An empirical absorption correction based
on Ψ scans was also applied. Crystallographic data and data
collection parameters are summarized in Table 2.
included in the refinement at fixed O-H and H‚‚‚H distances
(0.96 and 1.48 Å). The carboxylic hydrogen atom was revealed
in difference Fourier maps, and it was treated as a riding atom
with an O-H distance of 0.82 Å. The remaining hydrogen
atoms were included as idealized atoms riding on the respec-
tive carbon atoms with C-H bond lengths appropriate to the
carbon atom hybridization.
The structure solution (direct methods) and full-matrix
least-squares refinements based on F2 were perfomed with
SHELXS/L programs of the SHELXTL-NT software package
(Version 5.10).50 The weighting scheme used in the last
refinement cycle was w-1 ) [σ2(Fo)2 + (aP)2 + bP] where P )
Ack n ow led gm en t. Financial support from the ital-
ian Ministero dell’Istruzione, dell’Universita` e della
Ricerca (MIUR), is gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: Details of crystal
structure solution and refinement, complete lists of atomic
coordinates, bond lengths and angles, anisotropic thermal
parameters, torsion angles, and hydrogen bond parameters.
This material is available free of charge via the Internet at
http://pubs.acs.org.
2
[2Fc + Max(Fo2,0)]/3, with a ) 0.071 and b ) 0.82. All non-
hydrogen atoms were refined anisotropically. The observed
hydrogen atoms of the cocrystallized water molecule were
(49) Karash, M. S.; Seyler, R. C.; Mayo, F. R. J . Am. Chem. Soc.
1938, 60, 882.
(50) SHELXTL-NT crystal structure analysis package; Bruker AXS
Inc.: Madison, WI, 1999; Version 5.10.
OM020204J