Cyclometalated platinum(II) complexes of pyrazole-based, N∧C∧N-coordinating, terdentate ligands: The contrasting influence of pyrazolyl and pyridyl rings on luminescence

Article abstract of DOI:10.1021/ic8014157

1,3-Bis(1-pyrazolyl)-5-methyl-benzene, HL², undergoes cyclometalation at the C² position upon reaction with K ₂PtCl₄, to generate an N∧C∧N-coordinated complex, PtL²Cl. This compound is luminescent in degassed solution at 298 K, emitting in the blue region of the spectrum on the microsecond time scale (λ_max = 453 nm, τ = 4.0 μs, Φ_lum = 0.02, in CH₂Cl₂). Compared to the analogous complex Pt(dpyb)Cl that incorporates pyridyl rather than pyrazole rings {dpybH = 1,3-di(2-pyridyl)-benzene}, the excited state is displaced to higher energy by 1700 cm^-1. This effect is rationalized in terms of the poorer π-acceptor nature of pyrazolyl compared to pyridyl rings, leading to destabilization of the lowest unoccupied molecular orbital, which is largely localized on the heteroaromatic rings in both cases. Cyclic voltammetry and density functional theory calculations reinforce this interpretation, and suggest that the lowest-energy excited state is probably best described as heavily mixed π_L/d_Pt/p_Cl → pi;*_L (IL/MLCT/LLCT) in character. 5-Aryl-substituted analogues of HL² are accessible in three steps from 1,3,5-tribromobenzene by Pd-catalyzed cross-coupling with aryl boronic acids, followed by copper-catalyzed bromo-iodo exchange, and subsequent amination with pyrazole under relatively mild conditions also catalyzed by copper. The corresponding Pt(II) complexes display red-shifted and more intense luminescence compared to PtL²Cl. Ligands incorporating one pyrazole and one pyridyl ring are also accessible; for example, 1-(1-pyrazolyl)-3-(2-pyridyl)benzene, HL ⁶. Their complexes are highly luminescent in solution; for example, for PtL⁶Cl, λ_max = 487 nm, τ = 6.9 μs, Φ_lum = 0.55, in dilute solution in CH₂Cl₂. At elevated concentrations, PtL⁶Cl displays an additional excimeric emission band that is substantially blue-shifted compared to that displayed by Pt(dpyb)Cl (bands centered at 645 and 695 nm, respectively), indicating that the presence of the pyrazole ring destabilizes the excimer. The introduction of a methyl substituent into the central aryl ring of such complexes is sufficient to eliminate the excimer emission.

Full text of DOI:10.1021/ic8014157

Inorg. Chem. 2008, 47, 11129-11142
Cyclometalated Platinum(II) Complexes of Pyrazole-Based,
N∧C∧N-Coordinating, Terdentate Ligands: the Contrasting Influence of
Pyrazolyl and Pyridyl Rings on Luminescence
Ste´phanie Develay, Octavia Blackburn, Amber L. Thompson, and J. A. Gareth Williams*
Department of Chemistry, UniVersity of Durham, Durham, DH1 3LE, U.K.
Received July 29, 2008
1,3-Bis(1-pyrazolyl)-5-methyl-benzene, HL2, undergoes cyclometalation at the C2 position upon reaction with K2PtCl4,
to generate an N∧C∧N-coordinated complex, PtL2Cl. This compound is luminescent in degassed solution at 298 K,
emitting in the blue region of the spectrum on the microsecond time scale (λmax ) 453 nm, τ ) 4.0 µs, Φlum
)
0.02, in CH2Cl2). Compared to the analogous complex Pt(dpyb)Cl that incorporates pyridyl rather than pyrazole
rings {dpybH ) 1,3-di(2-pyridyl)-benzene}, the excited state is displaced to higher energy by 1700 cm-1. This
effect is rationalized in terms of the poorer π-acceptor nature of pyrazolyl compared to pyridyl rings, leading to
destabilization of the lowest unoccupied molecular orbital, which is largely localized on the heteroaromatic rings in
both cases. Cyclic voltammetry and density functional theory calculations reinforce this interpretation, and suggest
that the lowest-energy excited state is probably best described as heavily mixed πL/dPt/pCl f π*L (IL/MLCT/LLCT)
in character. 5-Aryl-substituted analogues of HL2 are accessible in three steps from 1,3,5-tribromobenzene by
Pd-catalyzed cross-coupling with aryl boronic acids, followed by copper-catalyzed bromo-iodo exchange, and
subsequent amination with pyrazole under relatively mild conditions also catalyzed by copper. The corresponding
Pt(II) complexes display red-shifted and more intense luminescence compared to PtL2Cl. Ligands incorporating
one pyrazole and one pyridyl ring are also accessible; for example, 1-(1-pyrazolyl)-3-(2-pyridyl)benzene, HL6. Their
complexes are highly luminescent in solution; for example, for PtL6Cl, λmax ) 487 nm, τ ) 6.9 µs, Φlum ) 0.55,
in dilute solution in CH2Cl2. At elevated concentrations, PtL6Cl displays an additional excimeric emission band that
is substantially blue-shifted compared to that displayed by Pt(dpyb)Cl (bands centered at 645 and 695 nm,
respectively), indicating that the presence of the pyrazole ring destabilizes the excimer. The introduction of a
methyl substituent into the central aryl ring of such complexes is sufficient to eliminate the excimer emission.
Introduction
thermally accessible to the emissive state. It is the thermally
activated population of such distorted states that accounts
for the lack of room temperature emission in simple
complexes like Pt(bpy)Cl2 and [Pt(tpy)Cl]+.1,2 The key to
raising the energy of the d-d states is to introduce strong-
field ligands or co-ligands. For example, a wide range of
luminescent complexes have been obtained by using acetylide
co-ligands in complexes of the general type Pt(N∧N)-
(sCtCR)2 and [Pt(N∧N∧N)(sCtCR)]+ (where N∧N and
N∧N∧N represent di- and tri-imine ligands such as bipyridine
and terpyridine),1d,e,3 while cyclometalating ligands such as
2-phenylpyridine and 6-phenyl-2,2′-bipyridine have a ben-
eficial effect on luminescence quantum yields compared to
Over the past 15 years, several families of platinum(II)
complexes have been developed that display photolumines-
cence in solution at room temperature.1 A strategy for
promoting emission over non-radiative decay has emerged,
namely to ensure that d-d excited states involving the
2
2
population of the strongly antibonding dx -y orbital are not
* To whom correspondence should be addressed. E-mail: j.a.g.williams@
durham.ac.uk.
(1) For recent reviews, see for example: (a) McMillin, D. R.; Moore, J. J.
Coord. Chem. ReV. 2002, 229, 113. (b) Lai, S. W.; Che, C. M. Top.
Curr. Chem. 2004, 241, 27. (c) Ma, B.; Djurovich, P.; Thompson,
M. E. Coord. Chem. ReV. 2005, 249, 1501. (d) Castellano, F. N.;
Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro, M. L.; Rajapakse,
N. Coord. Chem. ReV., 2006, 250, 1819. (e) Wong, K. M. C.; Yam,
V. W. W. Coord. Chem. ReV. 2007, 251, 2477. (f) Williams, J. A. G.
Top. Curr. Chem. 2007, 281, 205.
(2) Barigelletti, F.; Sandrini, D.; Maestri, M.; Balzani, V.; von Zelewsky,
A.; Chasot, L.; Jolliet, P.; Maeder, U. Inorg. Chem. 1988, 27, 3644.
10.1021/ic8014157 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 23, 2008 11129
Published on Web 11/01/2008

Develay et al.
Scheme 1. Generic Structures of the Pyrazole-Containing Complexes
(II and III) and Those Based on 1,3-Dipyridylbenzene (I)
complexes with their non-cyclometalated cousins bpy and
tpy.4,5 Potential applications range from chromophores for
promoting photoinduced charge separation for artifical pho-
tosynthesis,6 to triplet emitters in organic light-emitting
devices (OLEDs).7,8
Among metal complexes, a feature essentially unique to
square planar d8 systems is their coordinative unsaturation,
which allows them to engage in intermolecular interactions
that involve either metal orbitals orthogonal to the plane of
the molecule or overlap of cofacial ligand orbitals. Such
interactions include the formation of excimers, which may
themselves be emissive. If the excimer and monomer
emissions between them span the bulk of the visible
spectrum, this effect can provide a means of obtaining single-
dopant white-light OLEDs.7a,d,8
We have been exploring the chemistry and applications
of a family of brightly emissive cyclometalated platinum(II)
compounds incorporating N∧C∧N-coordinating terdentate
ligands based on 1,3-di(2-pyridyl)benzene (dpybH) (Scheme
1, generic structure I). As previously reported in this journal,
these complexes emit from triplet states with high lumines-
cence quantum yields, {e.g., for R ) H, Φlum ) 0.6},9 and
they function efficiently in OLEDs.10 The emission wave-
length is tunable over a wide range by varying the substituent
R, without compromising the quantum yield (λmax from 481
nm for R ) CO2Me to 588 nm for -C6H4-NMe2 in CH2Cl2
solution).11 At elevated concentrations in solution, the
complexes form excimers that emit with unusually high
efficiency at around 700 nm (Φlum ∼ 0.3). Neat thin films
display emission uniquely in this region, leading to high-
performance near-IR OLEDs,12 while the combination of
monomer and excimer emission when doped at appropriate
concentrations in polymer matrixes has led to particularly
efficient white-light emitting devices.13 In this contribution,
we describe our studies into the synthesis and luminescence
properties of a family of compounds, structurally related to
Pt(dpyb)Cl, but comprising either one or two pyrazoles (pyz)
in place of the pyridine (py) rings (Scheme 1, generic
structure II). Given the poorer π-acceptor nature of the
pyrazole compared to pyridine ring, we reasoned that the
lowest unoccupied molecular orbital (LUMO) energy in such
complexes would be raised relative to that in complexes of
type-I structure, potentially offering a route to triplet emission
in the challenging blue region of the spectrum. We also
sought to determine how the switch from py to pyz rings
would affect intermolecular interactions, particularly excimer
formation. The influence of aryl substituents at position R
is considered, and asymmetric systems incorporating one pyz
and one py ring (Scheme 1, generic structure III) have been
explored.
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Overview of Binding Modes of Azole-Containing
Ligands to Platinum(II). Most of the complexes of pyra-
zole-containing bidentate ligands reported that are lumines-
cent involve C-linked, 3-substituted pyrazoles: metal com-
plexation is often accompanied by deprotonation at N2,
leading to a strong ligand field.14,15 For example, 3-(2-
pyridyl)-pyrazole can be considered an analogue of the
cyclometalating ligand ppyH, if it binds through coordination
mode (ii) (Scheme 2,a). Similarly, 1,2,4-triazoles16 and
tetrazoles17 bind as anionic ligands through N-deprotonation.
Chi and co-workers have pioneered the use of such ligands
(8) For recent reviews of Pt complexes applied to OLEDs, see: (a) Xiang,
H.-F.; Lai, S.-W.; Lai, P. T.; Che, C.-M. Phosphorescent platinum(II)
materials for OLED applications. In Highly efficient OLEDs with
phosphorescent materials; H., Yersin, Ed.; Wiley-VCH: Weinheim,
2008. (b) Williams, J. A. G.; Develay, S.; Rochester, D. L.; Murphy,
L. Coord. Chem. ReV. [Online early access]. DOI: 10.1016/
j.ccr.2008.03.014. Published Online: 2008; in press.
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Williams, J. A. G. Inorg. Chem. 2005, 44, 9690.
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J. Appl. Phys. Lett. 2007, 90, 023506. (b) Cocchi, M.; Kalinowski, J.;
Virgili, D.; Williams, J. A. G. Appl. Phys. Lett. 2008, 92, 113302.
(13) (a) Cocchi, M.; Kalinowski, J.; Virgili, D.; Fattori, V.; Develay, S.;
Williams, J. A. G. Appl. Phys. Lett. 2007, 90, 163508. (b) Virgili, D.;
Cocchi, M.; Fattori, V.; Sabatini, S.; Kalinowski, J.; Williams, J. A. G.
Chem. Phys. Lett. 2006, 433, 145. (c) Kalinowski, J.; Cocchi, M.;
Virgili, D.; Fattori, V.; Williams, J. A. G. AdV. Mater. 2007, 19, 4000.
(14) For a recent review of pyridyl azolates in the preparation of luminescent
complexes, see: (a) Chi, Y.; Chou, P. T. Chem. Soc. ReV. 2007, 36,
1421.
(9) Williams, J. A. G.; Beeby, A.; Davies, E. S.; Weinstein, J. A.; Wilson,
C. Inorg. Chem. 2003, 42, 8609.
(10) (a) Sotoyama, W.; Satoh, T.; Sawatari, N.; Inoue, H. Appl. Phys. Lett.
2005, 86, 153505. (b) Cocchi, M.; Virgili, D.; Fattori, V.; Rochester,
D. L.; Williams, J. A. G. AdV. Funct. Mater. 2007, 17, 285. (c)
Kalinowski, J.; Cocchi, M.; Virgili, D.; Fattori, V.; Williams, J. A. G.
Chem. Phys. Lett. 2006, 432, 110. (d) Chem. Phys. Lett., 2007, 447,
279.
11130 Inorganic Chemistry, Vol. 47, No. 23, 2008

Cyclometalated Platinum(II) Complexes
Scheme 2. (a) Tautomeric Forms of the C-Linked Compound
3-(2-Pyridyl)-pyrazole, Which Can Coordinate to a Metal M Either As a
Neutral Ligand Leaving N1 Protonated (i), or As an Anionic Ligand
through Deprotonation (ii). (b) An example of an N-Linked
1-(Aryl)pyrazole. (c) Pt Complexes of the N-Linked, Terdentate Ligand
1,3-Bis(1-pyrazolyl)pyridine (bpp) Have Been Investigated by Connick
et al.
binding mode (i), Scheme 2a. However, treatment with base
leads to deprotonation and formation of a dimer, [Pt(µ-
phppz)]2, in which N1 becomes coordinated to a second
platinum(II) ion with displacement of the chloride co-ligand.
Meanwhile, Connick and co-workers have explored Pt(II)
complexes of N-linked, 2,6-bis(1-pyrazolyl)pyridine (bpp,
Scheme 2,c),22 a terpyridine analogue that has been widely
explored with other metal ions:23 emission was only observed
at 77 K for the complex [Pt(bpp)Ph](PF6). Earlier this year,
and toward the concluding stages of the work presented
herein, Connick also reported on structural aspects of the
Pt(II) complex of 1,3-bis(1-pyrazolyl)benzene (Scheme 1:
structure II, R ) H), the results of which relate directly to
our studies, and which are discussed in that context below.24
Results and Discussion
Synthesis of Ligands. The complexes studied are shown
in Scheme 3, and the strategies employed in the synthesis of
the requisite N-linked ligands are summarized in Scheme 4.
The formation of aryl-nitrogen bonds is typically catalyzed by
copper(I) salts. A wide range of conditions and catalytic species
have been explored since the original reports of Ullmann and
Goldberg at the beginning of the last century.25 Our initial
preparation of the parent ligand 1,3-bis(1-pyrazolyl)benzene HL1
was carried out by a modified Ullmann coupling of 1,3-
diiodobenzene with an excess of pyrazole, in which the addition
of 1,10-phenanthroline as a ligand for Cu(I) facilitates reaction
under milder conditions.26 However, yields were limited to
around 20% using this method. Buchwald et al. have recently
described how the combination of CuI with chelating aliphatic
amines, such as 1,2-diaminocyclohexane, allows N-arylation
of a range of N-heterocycles including pyrazoles under mild
conditions.27 This methodology proved superior for the synthesis
of HL1, giving yields of up to 80% (Scheme 4,a). The singly
reacted compound 1-iodo-3-(1-pyrazolyl)benzene, pre-6, was
isolated as a side product.
When applied to the synthesis of HL2 from 1,3-dibromo-
5-methyl-benzene, on the other hand, these conditions led
to poor conversion, probably because of the deactivating
influence of the methyl group and the lower reactivity of
bromo compared to iodo substrates. The latter problem could
be overcome by first carrying out a bromo-iodo exchange
reaction using NaI as the source of iodide, catalyzed by CuI
in the presence of N,N-dimethyl-1,2-ethylenediamine. Buch-
wald et al. reported that this catalyst system can offer near-
quantitative conversion of monobrominated aromatics to their
iodo analogues.28 In the present instance of a dibrominated
in the design of several luminescent Pt(II) complexes,15c,18
while a range of Os(II) and Ir(III) complexes have also been
reported.14,15a,b,d,16 N-linked 1-arylpyrazoles that cyclomet-
allate through the aryl ring19 (Scheme 2,b) featured in some
of the earliest examples of room-temperature-emissive Pt(II)
complexes; for example, Pt(thpy)(ppz) {thpyH ) 2-(2-
thienyl)pyridine; ppzH ) 1-phenylpyrazole}.20
There has been less investigation into the utility of
terdentate pyrazole-containing ligands. Recently, Lam and
co-workers described the intriguing properties of the Pt(II)
complex of 6-phenyl-2-(3-pyrazolyl)pyridine (HphppzH).21
This C-linked pyrazole ligand binds in a cyclometalating,
monoanionic, C∧N∧N-coordinating fashion to give Pt(php-
pzH)Cl, that is, without deprotonation of the pyrazolesthe
proton remains on the noncoordinated N1 position, as in
(15) (a) Wu, P.-C.; Yu, J.-K.; Song, Y.-H.; Chi, Y.; Chou, P.-T.; Peng,
S.-M.; Lee, G.-H. Organometallics 2003, 22, 4938. (b) Yang, C.-H.;
Li, S.-W.; Chi, Y.; Cheng, Y.-M.; Yeh, Y.-S.; Chou, P.-T.; Lee, G.-
H.; Wang, C.-H.; Shu, C.-F. Inorg. Chem. 2005, 44, 7770. (c) Chang,
S.-Y.; Kavitha, J.; Li, S.-W.; Hsu, C.-S.; Chi, Y.; Yeh, Y.-S.; Chou,
P.-T.; Lee, G.-H.; Carty, A. J.; Tao, Y.-T.; Chien, C.-H. Inorg. Chem.
2006, 45, 137. (d) Yang, C.-H.; Cheng, Y.-M.; Chi, Y.; Hsu, C.-J.;
Fang, F.-C.; Wong, K.-T.; Chou, P.-T.; Chang, C.-H.; Tsai, M.-H.;
Wu, C.-C. Angew. Chem., Int. Ed. 2007, 46, 2418.
(16) (a) Coppo, P.; Plummer, E. A.; De Cola, L. Chem. Commun. 2004,
1774. (b) Orselli, E.; Kottas, G. S.; Konradsson, A. E.; Coppo, P.;
Fro˘hlich, R.; De Cola, L.; van Dijken, A.; Bu¨chel, M.; Bo¨rner, H.
Inorg. Chem. 2007, 46, 11082.
(17) Stagni, S.; Orselli, E.; Palazzi, A.; De Cola, L.; Zacchini, S.; Femoni,
C.; Marcaccio, M.; Paolucci, F.; Zanarini, S. Inorg. Chem. 2007, 46,
9126.
(18) (a) Chang, S.-Y.; Kavitha, J.; Hung, J.-Y.; Chi, Y.; Cheng, Y.-M.; Li,
E. Y.; Chou, P.-T.; Lee, G.-H.; Carty, A. J. Inorg. Chem. 2007, 46,
7064. (b) Chang, S.-Y.; Chen, J.-L.; Chi, Y.; Cheng, Y.-M.; Lee, G.-
H.; Jiang, C.-M.; Chou, P.-T. Inorg. Chem. 2007, 46, 11202.
(19) Nonoyama, M.; Takayanagi, H. Transition Met. Chem. 1976, 1, 10.
(20) Maestri, M.; Sandrini, D.; Von Zelewsky, A.; Deuschel-Cornioley,
C. Inorg. Chem. 1991, 30, 2476.
(21) (a) Koo, C.-K.; Lam, B.; Leung, S.-K.; Lam, M.H.-W.; Wong, W.-Y.
J. Am. Chem. Soc. 2006, 128, 16434. (b) Koo, C.-K.; Ho, Y.-M.;
Chow, C.-F.; Lam, M.H.-W.; Lau, T.-C.; Wong, W.-Y. Inorg. Chem.
2007, 46, 3603.
(22) Willison, S. A.; Jude, H.; Antonelli, R. M.; Rennekamp, J. M.; Eckert,
N. A.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2004, 43,
2548.
(23) For a review, see: (a) Halcrow, M. A. Coord. Chem. ReV. 2005, 249,
2880.
(24) Willison, S. A.; Krause, J. A.; Connick, W. B. Inorg. Chem. 2008,
47, 1258.
(25) For a review, see: (a) Lindley, J. Tetrahedron 1984, 40, 1433.
(26) Goodbrand, H. B.; Hu, N.-X. J. Org. Chem. 1999, 64, 670.
(27) (a) Klapars, A.; Antilla, J. C.; Huang, X.; Buchwald, S. L. J. Am.
Chem. Soc. 2001, 123, 7727. (b) Antilla, J. C.; Baskin, J. M.; Barder,
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Inorganic Chemistry, Vol. 47, No. 23, 2008 11131

Develay et al.
Scheme 3. Structures of the Platinum(II) Complexes Reported
graphic purification to remove undesired disubstituted mate-
rial and unreacted tbb, was followed by copper-catalyzed
halogen exchange and subsequent bis-amination with pyra-
zole as for HL2.
The C-linked, 3-pyrazolyl isomer of HL1, namely 1,3-di(3-
pyrazolyl)benzene, HL9, in which the pyrazole rings are
connected to the central aryl ring via C3 rather than N1, was
prepared from 1,3-diacetyl benzene by condensation of its
bis(dimethylaminochalcone) derivative, pre-9, with hydrazine
(Scheme 5).29 Bis-N-arylation of HL9 to generate derivatives
HL10 and HL11 was accomplished under the same copper-
catalyzed conditions as those used in the synthesis of HL2-7
.
Synthesis and Characterization of Platinum(II) Com-
plexes. Attempts to prepare the N∧C∧N-coordinated plati-
num(II) complex of HL1, PtL1Cl, in acetic acid led to
mixtures of products, only one of which were we able to
1
isolate cleanly. The H NMR of this compound confirmed
that cyclometalation had occurred, but the symmetry of the
ligand had been lost, with the two pyrazoles in different
environments. We speculated that this might be due to the
Pt(II) ion cyclometalating at position C4 of the ligand rather
than C2: such competitive binding to give the kinetic product
has been observed previously for dpyb with Pd(II)30 and
Ir(III).31 We reasoned that it might be possible to inhibit
such binding through the steric influence of a substituent
introduced at position C5 of the central aryl ring, and thus
abandoned further studies with HL1 in favor of C5-substituted
ligands. Meanwhile, Connick and co-workers had also been
investigating the coordination chemistry of HL1 with Pt(II)
and, toward the end of our experimental work, they reported
that the product formed from HL1 and K2PtCl4 in acetic acid
after long reaction times is, in fact, a trimer, [Pt(µ-L1)Cl]3,
the structure of which they confirmed conclusively by
crystallography.24 The ligand is metallated at position C4 in
an N∧C fashion involving one of the two pyrazole rings,
with the second pyz bound to a neighboring platinum(II) ion.
They were also able to isolate the monomeric complex
[PtL1Cl] that had eluded us, by using shorter reaction times
and lower concentrations of platinum(II) salts.24
aromatic, we isolated a mixture of the desired compound
1,3-di-iodo-5-methyl-benzene, the singly reacted compound
1-iodo-3-bromo-5-methyl-benzene, and the starting material,
1
in the mole ratio 47:41:12 by H NMR after 3 days. This
The strategy of introducing substituents at position C5
of the ligand proved successful in directing metalation to
C2, even for the sterically undemanding methyl group.
Thus, the reaction of HL2 with K2PtCl4 in acetic acid led
to precipitation of PtL2Cl, analytically pure after a simple
washing sequence. No evidence was found for the
formation of C4-metalated products. The aryl-substituted
complexes PtL3Cl and PtL4Cl were obtained similarly,
although the more electron-rich amino derivative HL5
failed to provide a tractable complex. Similarly, HL2M
carrying methyl substituents in the pyrazole rings gave
ratio was remarkably consistent between runs, and the
proportion of starting material was not significantly increased
by using longer reaction times (5 days) or by addition of
fresh catalyst, suggesting that an equilibrium is reached.
Subsequent copper-catalyzed reaction of the crude mixture
with pyrazole led to a readily separable mixture of 1,3-bis(1-
pyrazolyl)-5-methyl-benzene (HL2) and 1-bromo-3-(1-pyra-
zolyl)-5-methyl-benzene, pre-7 (Scheme 4,b).
The mixed py-pyz ligands HL6 and HL7 were obtained
from the singly substituted side products pre-6 and pre-7
respectively, by means of a palladium-catalyzed Stille cross-
coupling reaction with 2-tri(n-butyl)stannylpyridine, as used
in the synthesis of dpybH and its derivatives.11 The use of
3,5-dimethylpyrazole led to HL8 in the same way.
(29) Pleier, A. K.; Glas, H.; Grosche, M.; Sirsch, P.; Thiel, W. R. Synthesis
2001, 1, 55.
(30) Ca´rdenas, D. J.; Echavarren, A. M.; Ram´ıre de Arellano, M. C.
Organometallics 1999, 18, 3337.
(31) (a) Wilkinson, A. J.; Goeta, A.; Foster, C. E.; Williams, J. A. G. Inorg.
Chem. 2004, 43, 6513. (b) Wilkinson, A. J.; Puschmann, H.; Howard,
J. A. K.; Foster, C. E.; Williams, J. A. G. Inorg. Chem. 2006, 45,
8685.
The 5-aryl-substituted ligands HL3-5 were prepared in
three steps from 1,3,5-tribromobenzene, tbb (Scheme 4,c).
Palladium-catalyzed Suzuki cross-coupling of tbb with 1
equiv of the appropriate arylboronic acid, and chromato-
11132 Inorganic Chemistry, Vol. 47, No. 23, 2008

Cyclometalated Platinum(II) Complexes
Scheme 4. Synthetic Strategies Employed in the Preparation of the N-Linked Pyrazole-Containing Ligands HL2-8
an unseparable mixture of products, perhaps associated
with the lack of good π-accepting units within the ligand
to satisfy the electron density at the metal.
of the order of 13 Hz), H4 (∼ 8 Hz) and H4pz (not
quantitatively resolvable at the field strengths employed
but evident from the peak shape).
Apart from the loss of the H2 resonance (atom number-
ing system of Scheme 4), the main systematic changes in
The Pt(II) complexes of the asymmetric py-pyz ligands,
HL6-8, were prepared in the same way. In this case, even
the unsubstituted ligand HL6 gave the N∧C2∧N-coordinated
product, with no evidence of competitive binding at C4 or
1
the H NMR spectra of the ligands upon coordination to
Pt(II) in a N∧C∧N manner were found to be as follows:
(i) a shift in the H3pz resonance to higher frequency (∆δ
0.3 ppm); (ii) a shift in the H4 resonances to lower
frequency (∆δ -0.6 ppm); and (iii) the appearance of 195Pt
(I ) 1/2) satellites for H3pz and H5pz (coupling constants
1
C6. The changes in the H NMR spectra for the two halves
of the ligands were directly comparable to those observed
in the respective bis-pyrazolyl and bis-pyridyl ligands. The
value of 3J(195Pt) is markedly larger for H6py than for H3pz
Inorganic Chemistry, Vol. 47, No. 23, 2008 11133

Develay et al.
(∼ 44 and 13 Hz respectively), in line with the smaller H-H
coupling constants observed in pyrazole rings compared to
pyridines.32
functional theory (DFT) calculations (see below) confirm that
the LUMO is largely localized on the pyrazole ring, while a
similar trend to more negative potentials was observed by
Connick et al. in going from [Pt(tpy)Cl]+ to [Pt(bpp)Cl]+,
{bpp is the bis(1-pyrazolyl) analogue of tpy)}.22 The
interpretation is further supported by the behavior of the
mixed py-pyz complexes, PtL6-8Cl, which display a pseudo-
reversible wave between -2.3 and -2.4 V, attributable to
reduction localized largely on the pyridyl ring, followed by
an irreversible reduction around -2.9 V associated with the
pyrazole.
Oxidations are irreversible for all of the compounds, as
typically observed for platinum(II) complexes. The peak
potential of the first oxidation wave of PtL2Cl (0.35 V) is a
little lower than that of Pt(dpyb)Cl (0.43 V), which would
be consistent with a more electron-rich complex. The
introduction of the tolyl substituent shifts the peak potential
anodically to 0.56 V, while the methoxyphenyl-substituted
complex lies between the two, at 0.43 V. The trend can
perhaps be rationalized in terms of the opposing effects of
σ-withdrawal and π-donation of aryl groups.35 In terms of
the σ-bonding framework, simple aryl substituents are
electron-withdrawing relative to methyl, which would tend
to lead to higher oxidation potentials for aryl derivatives, as
observed. However, the effect can be partially offset by
donation through the π-bonding framework, the influence
of which will be increased by electron-donating substituents
in the aryl ring; for example, the methoxy group in PtL4Cl.
The relative magnitudes of the σ-withdrawing and π-donating
effects could then lead to the observed order in Epox: PtL3Cl
> PtL4Cl > PtL2Cl.
Photophysical Properties. Absorption Properties. The
bis-pyrazolyl complexes PtL2-4Cl, which are off-white and
scarcely colored to the eye, display strong absorption bands
around 260 nm in dichloromethane solution at room tem-
perature because of π-π* transitions within the ligands (Table
1 and Figure 2). The extinction coefficients of these bands
for the aryl-substituted complexes PtL3-4Cl are substantially
increased compared to that of the methyl-substituted complex
PtL2Cl because of the more extended conjugation arising
from the pendant ring. A set of bands observed at longer
wavelengths in the region 315-370 nm, having no coun-
terparts in the uncoordinated ligands, may reasonably be
attributed to charge-transfer transitions that involve the metal,
and/or the chloride co-ligand, and/or the cyclometalated aryl
ring in which the electron density will be raised as compared
to the free ligand. In fact, the bands in this region resemble
in form those observed for Pt(dpyb)Cl and derivatives in the
near-UV/visible region, but displaced toward the blue by
about 50 nm (Figure 2). Such a displacement would be
anticipated to arise for charge-transfer transitions that involve
the pyrazole rings as acceptors, owing to the significantly
poorer π-acceptor nature of pyrazole compared to pyridine.
A crystal of complex PtL7Cl, obtained by slow evaporation
of an acetonitrile solution, was analyzed by X-ray diffraction,
confirming the expected N∧C∧N-coordination (Figure 1). The
Pt-C bond length of 1.917(7) Å is essentially identical to
that found in the Pt(II) complexes of dipyridylbenzenes
(average for 10 complexes for which data are available is
1.908 Å9,11,30,33) and also in Connick’s N∧C∧N-coordinated
bis-pyrazole complex PtL1Cl {1.924(5) Å 24}. As we have
noted previously,9 such values are significantly less than
those of around 2.04 Å observed in N∧N∧C-coordinated
complexes. The two Pt-N bond lengths are not significantly
different from one another (values are given in the caption
to Figure 1), and both they and the Pt-Cl are essentially
identical to the values found in Pt(dpyb)Cl and deriva-
tives,9,11,30,33 and in PtL1Cl.24 The N-Pt-N angle of
160.7(2) is similar to that found in the dpyb complexes
(which vary between 161.1(2) and 161.6(2) Å for the 10
complexes for which data are available), but larger than the
value of 158.9(1)° for PtL1Cl reported by Connick, who
speculated that the strain associated with this rather small
angle might account for the tendency of that complex to
convert to the trimer.24
We also investigated the C-linked bis-pyrazole ligand, 1,3-
di(3-pyrazolyl)benzene HL9, an isomer of HL1. A number
of attempts to obtain the N∧C∧N-coordinated Pt(II) complex
of this ligand (e.g., K2PtCl4 in CH3CO2H at reflux;
Pt(DMSO)2Cl2 in MeOH at reflux) failed to give tractable
products. Unlike HL1, tautomerism in this ligand will lead
to the co-existence of three different forms (N1H/N1H; N1H/
N2H; N2H/N2H, cf. Scheme 2,a), which may potentially lead
to more binding modes, perhaps again exacerbated by
metalation at C4/C6. To block tautomerism and direct
coordination to the desired N2, we also prepared the
N-arylated ligands HL10 and HL11, but again, there was no
evidence of formation of the N∧C∧N-coordinated complexes
1
with Pt(II) by H NMR spectroscopy. Mass spectrometry
showed signals for HL10 bound as a unidentate ligand {e.g.
Pt(HL9)(DMSO)Cl2}.
Electrochemistry of the Complexes. The complexes were
investigated by cyclic voltammetry in acetonitrile solution,
in the presence of Bu4NBF4 (0.1 M) as the supporting
electrolyte; data are reported in Table 1 relative to ferrocene
under the same conditions (Eox1/2 ) 0.40 V vs SCE34). The
bis-pyrazole complexes PtL2-4Cl each display an irrreversible
reduction wave in the region between -2.5 and -2.8 V. The
cathodic shift relative to Pt(dpyb)Cl and its derivatives, for
which pseudo-reversible reduction occurs around -2.0 V,
is consistent with the notion that reduction occurs on the
heteroaromatic ring, and with the established view that
pyrazole is a weaker π-acceptor than pyridine. Density-
(32) (a) Claramunt, R. M.; Sanz, D.; Santa Mar´ıa, M. D.; Jime´nez, J. A.;
Jimeno, M. L.; Elguero, J. Heterocycles 1998, 47, 301. (b) Eche´varr´ıa,
A.; Elguero, J.; Meutermans, W. J. Heterocycl. Chem. 1993, 30, 957.
(33) Rochester, D. L. Ph. D. Thesis, University of Durham, Durham, U.K.,
2007.
(35) Similar effects have been observed in 4′-aryl-substituted terpyridyl
Pt(II) complexes and substituted bipyridyl Ru(II) complexes: (a)
Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193. (b) Damrauer, N. H.; Boussie, T. R.; Devenney,
M.; McCusker, J. K. J. Am. Chem. Soc. 1997, 119, 8253.
(34) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
11134 Inorganic Chemistry, Vol. 47, No. 23, 2008

Cyclometalated Platinum(II) Complexes
Scheme 5. Synthesis of the C-Linked Pyrazolyl Ligands HL9-11
Table 1. Ground State UV-visible Absorption and Electrochemical Data of the Platinum Complexes in Solution at 298 K
b
complex
absorbancea
λ
max/nm (ε/M-1 cm-1
)
Epox/Vb
Epred/V (∆E /mV)
c
Pt(dpyb)Cl
PtL2Cl
332 (6510), 380 (8690), 401 (7010), 454 (270), 485 (240)
246 (42000), 262 (38300), 284 (17100), 317 (7140), 326 (8340), 347
(5150), 360 (4750)
0.43
0.35
-2.03
-2.76
PtL3Cl
PtL4Cl
PtL6Cl
PtL7Cl
PtL8Cl
237 (27700), 266 (75800), 320 (8190), 329 (9390), 343 (7380), 365
(4460)
268 (70100), 285 (48900), 320 (9200), 330 (10700), 345 (8550), 369
(4510)
242 (39500), 265 (31400), 286 (21500), 298 (19100), 321 (6570), 356
(7220), 396 (2870), 446 (110), 478 (50)
242 (34600), 267 (23000), 299 (14700), 323 (5670), 345 (4190), 360
(5060), 405 (2240), 488 (35)
0.56
0.43
0.26
0.22
0.23
-2.50
-2.65
-2.37 (90) -2.99
-2.36 (80) -2.89
-2.39 (90) -2.93
242 (28900), 266 (27000), 295 (15800), 317 (7770), 354 (5150), 400
(2170), 493 (30)
a In solution in CH2Cl2. b In solution in MeCN, using Bu4NPF6 (0.1 M) as the supporting electrolyte. Peak potentials are given for the irreversible waves.
For the pseudo-reversible reductions, values refer to E1/2 and the peak-to-peak separation is given in parentheses. Values refer to a scan rate of 300 mV s-1
and are reported relative to Fc+|Fc (E1/2 ) 0.40 V vs SCE). c Data from ref 11.
ox
DFT calculations have been carried out for the complexes
in the gas-phase using B3LYP (see Supporting Information):
they strongly support this hypothesis. Thus, the electron
density in the highest occupied molecular orbital (HOMO)
is indeed seen to be localized predominantly on the metal,
the chloride, and the central aryl ring of the terdentate ligand,
while the LUMO is localized almost exclusively on the
terdentate ligand, particularly on the pyrazole rings (Figure
3). Similar conclusions have been drawn previously for
Pt(dpyb)Cl and simple derivatives, where the LUMO is
localized on the pyridine rings36 (frontier orbitals of other
complexes are provided in the Supporting Information).
In these bis-pyrazole complexes, there is no evidence of
any lower energy bands arising from the direct SfT
transition, in contrast to Pt(dpyb)Cl and derivatives, where
weak bands (ε ∼ 200) around 485 nm arise from direct
excitation to the triplet state facilitated by spin-coupling
associated with the platinum(II) ion.
The asymmetric py-pyz complexes, PtL6-8Cl, display
features of both the bis-pyrazole and bis-pyridyl complexes.
Thus, they absorb out to longer wavelengths than PtL2-4Cl,
with maxima around 400 nm, comparable to Pt(dpyb)Cl
albeit with lower extinction coefficients (Figure 4). In fact,
there is qualitatively reasonable agreement between the
absorption spectrum of the 4-methyl-substituted complex
PtL7Cl and that of the summation of the spectra of the
Figure 1. Asymmetric unit of PtL7Cl in the crystal at 120 K, with the
ellipsoids depicted at the 50% probability level. Hydrogen atoms and
disorder of the L7 ligand are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Pt(1)-C(11) 1.917(7), Pt(1)-N(1) 2.040(6), Pt(1)-N(3)
2.023(6),Pt(1)-Cl(1)2.400(2);C(11)-Pt(1)-Cl(1)178.5(2),N(1)-Pt(1)-N(3)
160.7(2), C(11)-Pt(1)-N(1) 80.1(3), C(11)-Pt(1)-N(3) 80.7(3).
Figure 2. UV-visible absorption spectra of the bis-pyrazolyl complexes
PtL2Cl, PtL3Cl, and PtL4Cl in dichloromethane solution at 298 K, together
with that of Pt(dpyb)Cl for comparison.
Inorganic Chemistry, Vol. 47, No. 23, 2008 11135

Develay et al.
40 nm; λmax ) 453 nm (Figure 5). The change from pyridine
to pyrazole rings increases the triplet excited-state energy
by 1700 cm-1. The result vindicates the proposition at the
outset that the poorer π-acceptor character of pyrazole
compared to pyridine should lead to an increase in the energy
of the LUMO and hence of the excited state. Somewhat
related results have been observed in the chemistry of Pt(II)
and Ir(III) with bidentate ligands, where some of the most
blue-emitting complexes are those which incorporate azole
rings.15,16,18 We also note that the complex is only weakly
solvatochromic (overlaid spectra in different solvents are
provided in the Supporting Information). This would be
consistent with the qualitative picture from the frontier
orbitals of a symmetrical redistribution of the electron density
“outwards” from the principal axis of the molecule, which
would have only a relatively small influence on the dipole
moment.
Figure 3. Frontier orbital plots of PtL2Cl and PtL6Cl obtained by DFT.
Corresponding plots for the other complexes are provided in the Supporting
Information.
The higher energy of the emission relative to the bis-
pyridyl complexes is accompanied by a reduction in the
luminescence quantum yield: Φlum ) 0.02 compared to 0.60
for Pt(dpyb)Cl. Some insight into the origin of this decrease
in efficiency can be obtained from the radiative and non-
radiative rate constants, which may be estimated from the
measured values of quantum yield and lifetime (τ ) 4.0 µs
at infinite dilution) if it is assumed that the emissive excited
state is formed with unitary efficiency upon excitation. This
analysis reveals that the inferior quantum yield is the
combined result of a substantially lower radiative rate
constant {kr ) 5000 s-1 for PtL2Cl compared to 83000 s-1
for Pt(dpyb)Cl} and increased rate of non-radiative decay
{knr values are 25 × 104 and 5.6 × 104 s-1, respectively}
(Table 2). The lower radiative rate constant could be
accounted for in terms of a lower contribution of metal
character to the excited state, in line with the more electron-
rich nature of the pyrazole ligand system which will lead to
more ligand-centered character. The greater susceptibility of
the excited state to non-radiative decay may be associated,
in part, with the N∧C∧N-coordination in the bis-pyrazole
system being less rigid than in the bis-pyridines because of
the less appropriate bite angle, and thus to distortion of the
molecule upon formation of the excited state. This notion is
supported by two experimental observations: (i) the relative
intensities of the v′′ ) 1 and v′′ ) 2 vibrational bands to
the v′′ ) 0 in PtL2Cl are higher than in Pt(dpyb)Cl, consistent
with a greater displacement of the excited-state potential
energy surface relative to the ground state; and (ii) there is
a pronounced blue-shift of 1200 cm-1, and an increase in
the lifetime by a factor of 3, upon going from room
temperature solution to a rigid glass at 77 K (Figure 6),
whereas the spectra and lifetimes of the bis-pyridyl series
of complexes are essentially unaffected by temperature.11 It
is also possible that the higher energy of the excited state
brings it sufficiently close to higher-lying metal-centered
states to open up thermally activated population of such states
as a decay pathway.37
Figure 4. UV-visible absorption spectra of the asymmetric py-pyz
complexes PtL6Cl, PtL7Cl, and PtL8Cl, together with that of Pt(dpyb)Cl
for comparison. The weak, low-energy bands around 480 nm are shown
on an arbitrary expanded scale.
corresponding bis-pyrazole {PtL2Cl} and bis-pyridyl {Pt(Me-
dpyb)Cl} complexes, weighted 50% each (superimposed
spectra are shown in the Supporting Information). That the
lowest-energy spin-allowed absorption bands in these asym-
metric complexes specifically involve the pyridyl groups is
also supported by DFT calculations. They reveal that the
LUMO is now asymmetrically distributed, being localized
on the pyridyl ring, with essentially no contribution from
the pyrazole (Figure 3). On the other hand, the HOMO again
comprises significant contributions from the metal, the
chloride co-ligand, and the metalated aryl ring.
The asymmetric complexes also display weak bands
around 480 nm (ε ∼ 50), sharp and well-resolved for PtL6Cl
but less so for the methyl-substituted complexes PtL7Cl and
PtL8Cl, attributable to the SfT excitation as seen in the bis-
pyridyl systems (Figure 4, expansion).9
Emission Properties. All of the new complexes reported
here are luminescent in dichloromethane solution at room
temperature. Data are summarized in Table 2.
(i) Bis-pyrazole Complex PtL2Cl. The methyl-substituted
bis-pyrazole complex PtL2Cl displays a highly vibrationally
structured emission spectrum, similar in profile to that of
Pt(dpyb)Cl, but shifted substantially to the blue by almost
Only at low concentrations (<5 × 10-6 M) is the well-
defined, structured emission spectrum of Figure 5 observed
for PtL2Cl. As the concentration increases, a broad band
11136 Inorganic Chemistry, Vol. 47, No. 23, 2008

Cyclometalated Platinum(II) Complexes
Table 2. Luminescence Data for the Platinum Complexes in CH2Cl2 Solution at 298 K except Where Stated Otherwise
emission at 77 Kg
c d
,
τ0/µs
emissiona
max/nm
degassed
(aerated)
kQ
108 M-1 s-1 103 s-1
/
kr /
∑knr
/
kQ
/
SQ
O2
b
c
e
e
f
complex
λ
Φlum
104 s-1
108 M-1 s-1
λ
max/nm
τ/µs
Pt(dpyb)Cl 491, 524, 562 excimer: 695
0.60 7.2 (0.5)
453, 482, 514, aggregate: 555 0.02 4.0 (0.24)
53
23
4.0
5.7
6.6
6.5
2.0
83
5.0
7.1
5.5
93
53
36
5.6
25
8.5
1.8
45
38
7.6
10
487, 521, 558
430, 460, 489
480, 512, 548
482, 515, 552
480, 517, 551, 594(sh)
494, 531, 567, 614
498, 536, 573, 623
6.1
11
39
PtL2Cl
PtL3Cl
PtL4Cl
PtL6Cl
PtL7Cl
PtL8Cl
491, 520
507, 535
487, 522, 560, excimer: 645
503, 537, 577(sh)
509, 541, 587(sh)
0.15 21 (0.10)
0.12 22 (0.12)
0.64 6.9 (0.55)
0.53 10.0 (0.43)
0.32 8.8 (0.29)
4.1
4.0
5.2
4.7
7.8
41
8.8
10.5
9.7
15
a Emission maxima following excitation into the lowest-energy spin-allowed absorption band. b Luminescence quantum yield measured using quinine
sulfate in 1 M H2SO4 (aq) as the standard. c τ0 is the lifetime at infinite dilution and kQ the bimolecular self-quenching rate constant, determined from the
SQ
y-intercept and gradient, respectively, of a plot of τ-1 versus concentration. d λex ) 374 nm. e Radiative (kr) and non-radiative (∑knr) rate constants calculated
from τ and φ values at 298 K. f Bimolecular rate constant for quenching by O2, estimated from τ values in degassed and aerated solutions. g In diethyl
ether-isopentane-ethanol, 2:2:1 by volume.
Figure 5. Emission spectra of the bis-pyrazole complexes PtL2-4Cl in dilute
Figure 7. Concentration dependence of the emission spectrum of PtL2Cl
solution in degassed dichloromethane (2 × 10-6 M) at 298 K (λex ) 360
nm, band-passes of 2.0 nm). The dotted line shows the emission spectrum
of Pt(dpyb)Cl under the same conditions for comparison.
in CH2Cl2 at 298 K: spectra at 2 × 10-6 M (blue) and 2 × 10-5 M (red)
showing the appearance of an additional new band at low energy.
appearance of a low energy band around 700 nm has been
attributed to the formation of an excimer, formed by the
diffusion-controlled encounter of an excited-state complex
with the complex in its ground state. In the present case of
PtL2Cl, at least two experimental observations suggest that
the process involves a ground-state interaction of the
molecules rather than an excimer. First, the profile of the
excitation spectrum begins to deviate from that of the
absorption spectrum, even at concentrations where the optical
density is sufficiently low for inner-filter effects to be
minimal. This suggests a change in speciation in solution.
Second, the grow-in of the new, low-energy emission was
too fast to be resolved on the nanosecond time scale (using
equipment with a response function ∼ 0.5 ns). This contrasts
with the excimer emission displayed by Pt(dpyb)Cl, whose
growth is characterized by a much slower time-constant of
the order of 1 µs, reflecting the diffusion-controlled nature
of excimer formation. These observations imply that the
species responsible for the lower-energy band in PtL2Cl is
already present in solution prior to excitation, presumably
through ground-state aggregation. It is also noteworthy that
in the solid state (powder), PtL2Cl displays bright green
photoluminescence with an emission maximum around 530
nm, very different from that of the dilute solution, and again
suggesting the influence of ground-state intermolecular
interactions on the excited-state properties of this compound.
Figure 6. Solid lines: Emission spectra of PtL2Cl (blue) and PtL6Cl (red)
at 77 K in a glass of diethyl ether-isopentane-ethanol (2:2:1 by volume).
Dotted lines: the corresponding spectra in CH2Cl2 at 298 K for comparison.
The arrows highlight the direction and magnitude of the blue-shifts upon
cooling.
centered around 555 nm begins to appear (Figure 7). The
lifetime (τobs) of the 453 nm band decreases concomitantly,
initially following a Stern-Volmer type equation of the form
τobs ) τ0 + kQSQ [Pt] (where τ0 is the emission rate constant
SQ
at infinite dilution and kQ is the apparent rate constant of
self-quenching) but, at higher concentrations, the emission
decay ceases to be exponential. In the Pt(dpyb)Cl series, the
Inorganic Chemistry, Vol. 47, No. 23, 2008 11137

Develay et al.
(ii) Aryl-Substituted Bis-pyrazole Complexes PtL3-4Cl.
The 4-aryl-substituted complexes, PtL3Cl and PtL4Cl, display
emission spectra that are red-shifted relative to that of PtL2Cl
by approximately 40 and 50 nm respectively (in CH2Cl2
solution at room temperature; Figure 5). The trend of
decreasing excited-state energies on going from PtL2Cl
through PtL3Cl to PtL4Cl can be attributed to the influence
of the pendant aryl groups in extending the conjugation,
through their π-donating and σ-withdrawing nature which
can serve to raise the HOMO and lower the LUMO
respectively; such an effect, predominantly via the HOMO,
has been observed previously for the 4-aryl-substituted
derivatives of Pt(dpyb)Cl, where the emission energy de-
creased as a function of the electron-donating power of the
substituent.11 Although the spectra display clear vibrational
structure, with the 0-0 band again being the most intense,
the structure is somewhat less well-defined than for PtL2Cl,
and the constituent bands are broader. This difference in
profile probably arises from the thermal distribution of
conformers having different torsion angles between the
N∧C∧N framework and the aryl pendant, each with slightly
different excited-state energies. For example, a similar trend
has been observed in the π-π* states of terpyridyl iridium(III)
complexes upon aryl substitution.38
Figure 8. Solid lines: Emission spectra of PtL3Cl (blue) and PtL4Cl (red)
at 77 K in a glass of diethyl ether-isopentane-ethanol (2:2:1 by volume).
Dotted lines: the corresponding spectra in CH2Cl2 at 298K for comparison.
The arrows highlight the direction and magnitude of the blue-shifts upon
cooling.
At 77 K in an EPA glass, the spectra of PtL3Cl and PtL4Cl
both shift to the blue, but the shift is larger for the latter,
such that the two compounds have essentially identical
spectra under these conditions (Figure 8). The torsional
rearrangement necessary to bring the pendant aryl group from
the preferred geometry in the ground state (typical dihedral
angles for biaryls are 30° or more) to the conformation
required to maximize the conjugation in the excited state
(approximately coplanar with the N∧C∧N plane), will be
inhibited in the rigid glass, leading to the similar behavior
for both complexes.
The luminescence lifetimes of PtL3-4Cl are long, around
20 µs in degassed CH2Cl2 at 298 K (Table 2), compared to
4 µs for PtL2Cl under the same conditions. The quantum
yields are also superior to that of PtL2Cl by almost an order
of magnitude (Table 2). The data suggest that the improve-
ment stems from a reduction in the non-radiative decay, ∑knr,
whereas the radiative rate constant, kr, is not significantly
affected. At 77 K, the lifetimes increase to 40 µs. Such values
are an order of magnitude longer than those of the archetypal
MLCT emitters {e.g. for [Ru(bpy)3]2+, τ ) 3.8 µs at 77 K39},
and the kr values at least an order of magnitude smaller, in
Figure 9. Emission spectra of the py-pyz complexes PtL6Cl (blue), PtL7Cl
(green), and PtL8Cl (red) in dilute solution in degassed dichloromethane
(10-5 M) at 298 K (λex ) 400 nm, band-passes of 2.0 nm). The dotted and
dashed lines are the emission spectra of Pt(dpyb)Cl and PtL2Cl under the
same conditions for comparison.
line with the notion that a more predominantly ligand-
centered description of the excited state is appropriate.
The aryl-substituted complexes are less sensitive to self-
SQ
quenching at elevated concentrations than PtL2Cl (kQ are
given in Table 2). This lower propensity to quenching is
likely to arise from the additional steric hindrance to
bimolecular interactions arising from the pendant aryl group,
an effect that has also been observed in the aryl-substituted
derivatives of Pt(dpyb)Cl.11 Unlike PtL2Cl, there is no change
in the emission profile at high concentrations, suggesting
either that aggregates or excimers do not form in this case
or that they are non-emissive.
(iii) Asymmetric Pyridyl-Pyrazole Complexes PtL6-8Cl.
The emission spectra of the mixed py-pyz complexes
PtL6-8Cl in dilute solution are similar to that of Pt(dpyb)Cl
(Figure 9). The emission maxima of the unsubstituted
complex PtL6Cl are almost identical to those of Pt(dpyb)Cl,
and the luminescence quantum yields (0.64 and 0.60) and
lifetimes (6.9 and 7.2 µs, Table 2) of the two complexes are
the same within the uncertainty of the measurements. This
(36) Sotoyama, W.; Satoh, T.; Sato, H.; Matsuura, A.; Sawatari, N. J. Phys.
Chem. A 2005, 109, 9760.
(37) Note added prior to publication: While this manuscript was under
review, Haga et al. reported on iridium(III) complexes with closely
related 1,3-bis(3-methylpyrazole)aryl ligands. The very low emission
quantum yields of these complexes was shown to be due to the
presence of a low-lying d-d state, through which thermally-activated
non-radiative decay can occur. Yang, L.; Okuda, F.; Kobayashi, K.;
Nozaki, K.; Tanabe, Y.; Ishii, Y.; Haga, M. Inorg. Chem. 2008, 47,
7154.
(38) (a) Collin, J.-P.; Dixon, I. M.; Sauvage, J.-P.; Williams, J. A. G.;
Barigelletti, F.; Flamigni, L. J. Am. Chem. Soc. 1999, 121, 5009. (b)
Goodall, W.; Batsanov, A. S.; Howard, J. A. K.; Williams, J. A. G.
Dalton Trans. 2004, 623. (c) Williams, J. A. G.; Wilkinson, A. J.;
Whittle, V. L. Dalton Trans. 2008, 2081.
(39) Lytle, F. E.; Hercules, D. M. J. Am. Chem. Soc. 1969, 91, 253.
11138 Inorganic Chemistry, Vol. 47, No. 23, 2008

Cyclometalated Platinum(II) Complexes
Conclusions
In summary, the copper-diamine catalyzed, Buchwald-type
cross-coupling of pyrazoles with di-iodo aromatics provides
access to potentially N∧C∧N-coordinating 1,3-bis(1-pyra-
zolyl)benzenes under mild conditions. Dibromoaromatics
may be employed, preferably if they are first subjected to
bromo-iodo exchange, which can be accomplished under
similarly mild copper catalysis. Mixed pyridyl-pyrazole
ligands are also accessible by reaction of the monopyrazole-
substituted side products with 2-pyridylstannanes.
Cyclometalation of platinum(II) to 1,3-bis(1-pyrazolyl)-
benzenes can be directed toward the C2 position of the ligand,
leading exclusively to N∧C∧N-coordinated complexes, by the
presence of methyl or aryl substituents at C5, which inhibits
competitive metalation at the adjacent C4 position. No such
competition is observed in the case of the mixed ligand
1-(1-pyrazolyl)-3-(pyridyl)benzene.
Figure 10. Influence of concentration on the emission spectrum of PtL6Cl.
Spectra in degassed CH2Cl2 at 10-5 M (blue), showing exclusively monomer
emission, and at 3 × 10-4 M (red), in which the monomer is accompanied
by an excimer band centered at 650 nm. The spectrum of a concentrated
solution of Pt(dpyb)Cl is also shown for comparison of the excimer energies.
Several features of interest distinguish the emission
properties of these complexes from those of structurally
related dipyridylbenzenes. First, the poorer π-acceptor nature
of the pyrazole compared to pyridine ring shifts the LUMO
in the complex PtL2Cl to higher energy compared to
Pt(dpyb)Cl, leading to emission in the blue region. Blue
triplet emitters remain an important target in the development
of phosphorescent OLEDs. Second, aryl substituents at the
central 4-position of the aryl ring counteract this effect in
the excited state, but not in the ground state: the emission is
red-shifted whereas the absorption is scarcely affected. These
complexes are also much less sensitive to concentration-
induced self-quenching. Third, while the complexes of the
mixed ligands seem at first sight to behave much like
Pt(dpyb)Cl in dilute solution, the presence of the pyrazole
ring exerts a significant effect on the intermolecular interac-
tions. Thus, the excimeric excited state in PtL6Cl is signifi-
cantly destabilized. Inspection of the resulting spectrum
indicates broad and relatively uniform coverage over the
range from 480 to 700 nm, of potential interest for the
generation of white light. It contrasts with Pt(dpyb)Cl, for
which emission in the 550-600 nm region is minimal, and
where significant energy is lost in the near-IR region.
close similarity is consistent with the notion that the frontier
orbitals in these complexes have essentially no contribution
from the pyrazole ring, the LUMO being largely localized
on the pyridine ring and the HOMO involving primarily the
central aryl ring, the metal and the chloride ligand, as
discussed earlier. The degree of solvatochromism is again
minimal (plots are provided in the Supporting Information).
The introduction of a methyl group at position 4 of the
cyclometallating ring (PtL7Cl) red-shifts the emission by 16
nm, similar to the effect in the corresponding derivative of
Pt(dpyb)Cl, while a further small shift of 6 nm arises in
PtL8Cl from the methyl groups in the pyrazole ring.
Despite the close similarity between the emission of
PtL6-8Cl and Pt(dpyb)Cl in dilute solution, the presence of
the pyrazole does exert an influence on the behavior at higher
concentrations. From the kQSQ values, the propensity for self-
quenching is reduced by almost an order of magnitude (Table
2). In the case of the unsubstituted complex, PtL6Cl, an
excimer band is observed, but which is dramatically blue-
shifted compared with that displayed by Pt(dpyb)Cl (Figure
10). Evidently, the presence of the electron-rich pyrazole ring
destabilizes the excimer, despite having no significant effect
on the monomeric excited state. Excimer formation involves
an electron donor-acceptor interaction between excited and
ground-state molecules, and the increased electron density
in the pyrazole ring presumably makes the ground state a
poorer π-acceptor in its interactions with the excited-state
molecule. Consistent with this explanation is the observation
that the most electron-rich, dimethyl-pyrazolyl system,
PtL8Cl, is the least sensitive complex to self-quenching, based
on the magnitude of kQSQ (Table 2). The result contrasts with
the previously observed insensitiVity of the excimer energy
in the Pt(dpyb)Cl-based systems to substituents at the
4-position of the cyclometallating ring. No excimer band
could be detected in the case of PtL7Cl or PtL8Cl, highlight-
ing the subtlety of the intermolecular interactions in these
systems.
Experimental Section
1H and 13C NMR spectra, including NOESY and COSY, were
recorded on a Varian 200, 300, or 500 MHz instruments. Chemical
shifts (δ) are in ppm, referenced to residual protio-solvent
resonances, and coupling constants are in Hertz. Electrospray
ionization mass spectra were acquired on a time-of-flight Micromass
LCT spectrometer; electron ionization (EI) and chemical ionization
(CI) spectra were recorded at the EPSRC National Mass Spec-
trometry Service Centre. All solvents used in preparative work were
at least Analar grade, and water was purified using the Purite
system. Solvents used for optical spectroscopy were HPLC grade.
1,3-Bis(1-pyrazolyl)benzene HL1. A mixture of di-iodobenzene
(500 mg, 1.5 mmol), pyrazole (310 mg, 4.5 mmol), potassium
carbonate (1.25 g, 9.0 mmol), copper(I) iodide (11 mg, 60 µmol),
and trans-1,2-diaminocyclohexane (68 mg, 0.6 mmol) in dioxane
(5 mL) was degassed, and then heated at reflux with vigorous
stirring under a nitrogen atmosphere for 24 h. After cooling to room
temperature, the solution was filtered and the solvent was removed
Inorganic Chemistry, Vol. 47, No. 23, 2008 11139

Develay et al.
from the filtrate under reduced pressure. The residue was purified
by chromatography on silica gel using a gradient elution. The
monoaminated compound 1-iodo-3-(1-pyrazolyl)-benzene (pre-6)
eluted first at 80% hexane/20% diethyl ether, followed by HL1, a
pale yellow oil, at 55% hexane/45% diethyl ether (255 mg, 81%).
HL3. From 1,3-dibromo-5-(4-methoxyphenyl)-benzene (240 mg,
0.74 mmol), giving HL4 (55 mg, 25% over two steps), eluting at
1
70% hexane/30% diethyl ether. H NMR (CDCl3, 200 MHz): δ
8.07 (2H, d, 3J ) 2.5, H5pz), 8.02 (1H, t, 4J ) 2.0, H2), 7.84 (2H,
4
3
3
d, J ) 2.0, H4), 7.77 (2H, d, J ) 1.5, H3pz), 7.60 (2H, d, J )
1
4
8.0, Hb), 7.29 (2H, d, J ) 8.0, Ha), 6.52 (2H, dd, J ) 2.5, 1.5,
H4pz), 2.42 (3H, s, CH3). MS (ES+): m/z 301 (M + H+).
HL4. From 1,3-dibromo-5-(4-methoxyphenyl)-benzene (300 mg,
0.88 mmol), giving HL4 (48 mg, 15% over two steps), eluting at
3
3
Data for pre-6: H NMR (CDCl3, 200 MHz): δ 8.07 (1H, t, J )
2.0, H2), 7.85 (1H, d, 3J ) 2.5, H5pz), 7.70 (1H, d, 3J ) 1.5, H3pz),
3
7.60 (2H, overlapping m, H4 and H6), 7.12 (1H, t, J ) 8.0, H5),
6.43 (1H, dd, 3J ) 2.5, 3J ) 2.0, H4pz). MS (ES+): m/z 271.0 (M
+ H+). Data for HL1: 1H NMR (CDCl3, 500 MHz): δ 8.12 (1H, t,
1
70% hexane/30% diethyl ether. H NMR (CDCl3, 300 MHz): δ
3
3
4J 2.0, H2), 8.03 (2H, d, J ) 2.5, H5pz), 7.76 (2H, d, J ) 1.5,
H3pz), 7.65 (2H, dd, 3J ) 8.0, 4J ) 2.0, H4), 7.54 (1H, t, 3J ) 8.0,
H5), 6.51 (2H, m, H4pz). 13C NMR (CDCl3, 500 MHz): 141.7
(C3pz), 141.4 (C1), 130.7 (C5), 127.2 (C5pz), 116.8 (C4), 110.2 (C2),
108.3 (C4pz). MS (EI+): m/z 210 (M+).
8.04 (2H, d, 3J ) 2.5, H5pz), 7.97 (1H, t, 4J ) 2.0, H2), 7.80 (2H,
4
3
3
d, J ) 2.0, H4), 7.76 (2H, d, J ) 1.5, H3pz), 7.61 (2H, d, J )
9.0, Hb), 6.98 (2H, d, J ) 9.0, Ha), 6.52 (2H, appears as t, J )
2.0, H4pz), 3.84 (3H, s, CH3). MS (ES+): m/z 317 (M + H+).
HL5. From 1,3-dibromo-5-(4-(dimethylamino)phenyl)-benzene
(255 mg, 0.72 mmol), giving HL5 (33 mg, 14% over two steps),
3
3
1,3-Bis(1-pyrazolyl)-5-methyl-benzene HL2. Step 1. A mixture
of 1,3-dibromo-5-methyl-benzene (680 mg, 2.7 mmol), sodium
iodide (1.6 g, 10.9 mmol), copper(I) iodide (52 mg, 0.27 mmol),
and N,N′-dimethylethylenediamine (48 mg, 0.54 mmol) in toluene
(3 mL) was degassed, and then heated at reflux with vigorous
stirring for 48 h. After cooling, the solvent was removed under
reduced pressure, and the residue was taken up into a mixture of
dichloromethane (50 mL) and aqueous sodium hydroxide (1 M,
30 mL). The organic phase was separated, washed with water, dried
over anhydrous MgSO4, and evaporated to dryness, to give a
mixture of 1,3-di-iodo-5-methylbenzene, 1-iodo-3-bromo-5-meth-
ylbenzene, and the starting material dibromotoluene. The com-
pounds are not readily separable by chromatography: it proves more
straighforward to use the crude mixture directly in Step 2. 1H NMR
for the di-iodo product (CDCl3, 300 MHz): δ 7.85 (1H, s, H2),
7.48 (2H, s, H4), 2.25 (3H, s, CH3).
1
eluting at 60% hexane/40% diethylether. H NMR (CDCl3, 200
MHz): δ 8.06 (2H, d, J ) 2.5, H5pz), 7.94 (1H, t, J ) 2.0, H2),
3
4
7.83 (2H, d, 4J ) 2.0, H4), 7.77 (2H, d, 3J ) 1.5, H3pz), 7.62 (2H,
3
3
d, J ) 9.0, Hb), 6.81 (2H, d, J ) 9.0, Ha), 6.52 (2H, appears as
t,3J ) 2.0, H4pz), 3.03 (6H, s, NMe2). MS (ES+): m/z 330 (M+H+).
Mixed Pyridyl-Pyrazole Ligands HL6-8. HL6. The precursor
1-bromo-3-(1-pyrazolyl)-benzene, pre-6, (130 mg, 0.48 mmol),
2-tributylstannylpyridine (266 mg, 0.72 mmol), Pd(PPh3)2Cl2 (6 mg,
0.014 mmol), and LiCl (64 mg, 1.5 mmol) were combined in
toluene (5 mL) and degassed by freeze-pump-thaw (5 cycles).
The mixture was heated at reflux with stirring for 24 h. After cooling
to room temperature, a saturated aqueous solution of KF (10 mL)
was added and stirring continued for 1 h. The insoluble residue
formed was removed by filtration and washed with toluene. The
combined organic solution was evaporated to dryness under reduced
pressure, giving a brown residue, which was taken up into
dichloromethane (75 mL) and washed with NaHCO3(aq) (1 M, 3 ×
50 mL). The organic layer was separated, dried over anhydrous
potassium carbonate, and the solvent removed under reduced
pressure. The brown residue was purified by chromatography on
silica gel, gradient elution from hexane to 70% hexane/30% diethyl
ether, giving HL6 as a colorless oil (48 mg, 45%).
Step 2. The crude mixture from Step 1 was subject to the copper-
catalyzed amination with pyrazole, as described for HL1. After
workup, the crude mixture was purified by column chromatography
on silica using a gradient elution. The side product 1-bromo-3-(1-
pyrazolyl)-5-methyl-benzene (pre-7) eluted first at 80% hexane/
20% diethyl ether, followed by 1,3-bis(1-pyrazolyl)-5-methyl-
benzene, HL2 at 60% hexane/40% diethyl ether (80%). Data for
pre-7: 1H NMR (CDCl3, 300 MHz): δ 7.89 (1H, d, 3J ) 2.5, H5pz),
3
1H NMR (CDCl3, 500 MHz): δ 8.70 (1H, d, J ) 5.0, H6py),
3
7.71 (1H, d, J ) 1.5, H3pz), 7.67, 7.48, 7.24 (each 1H, s, H2, H4
8.37 (1H, t, 4J ) 2.0, H2), 8.05 (1H, d, 3J ) 2.0, H5pz), 7.90 (1H,
d, 3J ) 7.5, H4 or H6), 7.77 (4H, overlapping m, H3pz, H3py, H4py,
and H6), 6.47 (1H, m, H4pz), 2.39 (3H, s, CH3). ES (MS+) 237/
239 (M + H+ 79/81Br). Data for HL2: 1H NMR (CDCl3, 300 MHz):
δ 7.99 (2H, d,3J ) 2.5, H5pz), 7.85 (1H, t, 4J ) 2.0, H2), 7.73 (2H,
d, 3J ) 1.5, H3pz), 7.46 (2H, t, 4J ) 2.0, H4), 6.48 (2H, m, H4pz),
2.47 (3H, s, CH3). MS (ES+): m/z 225 (M + H+), 247 (M + Na+).
13C NMR (CDCl3, 126 MHz): δ 141.5 (C3pz), 141.2 (Cq), 141.1
(Cq), 127.2 (C5pz), 117.7 (C4), 108.1 (C4pz), 107.4 (C2), 21.9 (CH3).
Anal. Calcd for C13H12N4: C, 69.6; H, 5.5; N, 25.0%. Found: C,
69.6; H, 5.4; N, 25.0%.
3
3
and H6 or H4), 7.55 (1H, t, J ) 7.5, H5), 7.26 (1H, dd, J ) 7.0,
3
3J ) 5.0, H5py), 6.49 (dd, J ) 2.5,3J ) 1.5, H4pz).
13C{1H} NMR (CDCl3;): δ 156.6 (C2py), 150.0 (C6py), 141.4
(C5pz), 141.0, 140.9, 137.1 (C4py), 130.1 (C5py), 127.2 (C3pz),
125.0 (C2′), 122.9, 120.9 (C3py), 119.9, 117.8, 107.9 (C4pz). MS
(ES+): m/z 222 [M + H+], 244 [M + Na+]. HRMS (ES+): m/z
222.0998 [M + H+]; calcd for C14H12N3, 222.1001.
HL7 and HL8, both colorless oils, were prepared similarly: HL7
from pre-7, eluting at 75% hexane/25% diethyl ether in a yield of
74%; HL8 from pre-8, eluting at 70% hexane/30% diethyl ether,
in 35% yield.
Data for pre-8 and HL2M, the 3,5-dimethylpyrazolyl analogues
of pre-7 and HL2, are provided in the Supporting Information.
1,3-Bis(1-pyrazolyl)-5-p-substituted-aryl-benzenes HL3-HL5.
Step 1. The preparation of the 1,3-dibromo-5-aryl-benzenes was
accomplished by cross-coupling with the appropriate aryl boronic
acid using the procedure described previously.11
Step 2. The 1,3-dibromo-5-aryl-benzene was subject to copper-
catalyzed bromo-iodo exchange using the procedure described
above for the synthesis of HL2, step 1. The crude product, consisting
primarily of the di-iodinated aromatic, was used directly in the next
step without purification at this stage.
Step 3. The crude 1,3-di-iodo-5-aryl-benzene was subject to
copper-catalyzed amination with pyrazole using the procedure
described above for step 2 of the synthesis of HL2.
Data for HL7. H NMR (CDCl3, 300 MHz): δ 8.70 (1H, d, J
1
3
) 5.0, H6py), 8.10 (1H, s, H2), 8.04 (1H, d, J ) 2.5, H5pz), 7.77
3
(4H, m, H3pz, H3py, H4py, and H6 or H4), 7.63 (1H, s, H4 or H6),
7.27 (1H, m overlapping CHCl3, H5py), 6.48 (1H, m, H4pz), 2.50
(3H, s). M + H+: cal 235.28 mes 236.1. HRMS (ES+) m/z:
236.1190 [M+H+]; calcd for C15H14N3, 236.1188.
Data for HL8. H NMR (CDCl3, 500 MHz): δ 8.69 (1H, m,
1
H6py), 7.84 (1H, s, H6), 7.80 (1H, s, H2), 7.74 (2H, m, H3 and
H4py), 7.32 (1H, s, H4), 7.26 (1H, m, H5py), 6.00 (1H, s, H4pz),
2.48 (3H, s, CH3aryl), 2.34 (3H, s, CH3 pz), 2.31 (3H, s, CH33pz).
5
13C{1H} NMR (CDCl3): δ 156.9 (C2py), 149.9 (C6py), 149.2 (C3pz),
11140 Inorganic Chemistry, Vol. 47, No. 23, 2008

Cyclometalated Platinum(II) Complexes
3
3
140.5 (C3), 139.7 (C5pz), 137.0 (C4py), 126.8 (C6), 126.2 (C4), 122.7
(C5py), 121.0 (C3py), 120.7 (C2), 107.1 (C4pz), 21.7 (aryl-CH3),
H5pz), 7.51 (1H, t, J ) 7.5, H5), 6.93 (2H, d, J ) 2.5, H4pz).
13C{1H} NMR (CDCl3, 126 MHz): δ 154.5 (C3pz), 133.7 (C5pz),
132.9 (C1), 129.5 (C5), 126.6 (C4), 124.0 (C2), 106.0 (C4pz). 19F
NMR (CDCl3, 282 MHz): δ -119.1 (4F, m, F2), 146.0 (4F, dd,
F3). MS (ES+): m/z 507 [M - 2I- + 3H+], 633 [M - I- + 2H+],
759 [M + H+], 781 [M + Na+]. HRMS (ES+) m/z: 758.87967 [M
+ H+], calcd for C24H9N4F8127I2 758.87835; 780.86126 [M + Na+],
calcd for C24H8N4F8127I2Na 780.86030. A large proportion of the
monoarylated product (143 mg) was also recovered on increasing
the eluent polarity to 1% methanol/99% CH2Cl2 (see Supporting
Information for data).
13.8 (CH33pz), 12.7 (CH3 pz). MS (ES+) m/z: 264 [M + H+], 527
5
[2M + H+]. HRMS (ES+) m/z: 264.14932 [M + H+]; calcd for
C17H18N3, 264.14952; 286.13157 [M + Na+] calcd for C17H17N3Na,
286.13147.
1,3-Bis(3-pyrazolyl)benzene HL9. Step 1. 1,3-Diacetylbenzene
(0.50 g, 3.1 mmol) and N,N-dimethylformamide dimethylacetal
(DMF-DMA) (1.75 g, 14.7 mmol) in toluene (2 mL) were refluxed
for 20 h, to give a yellow precipitate which was filtered off and
washed with ether to give the bis-dimethylamino chalcone, pre-9,
1
Platinum(II) Complexes. The platinum(II) complexes PtL2-4Cl
and PtL6-7Cl were prepared by reaction of the respective ligand
HLn with K2PtCl4 (1.1 equiv) in acetic acid (typically 3 mL per 50
mg of ligand). The mixture was first degassed by freeze-pump-thaw
(3 cycles), and then heated at reflux with stirring under nitrogen
for 3 d. The resulting precipitates, pale colored for PtL2-4Cl, yellow
for PtL6-7Cl, were separated by centrifugation, and washed suc-
cessively with water, ethanol, and diethyl ether (3 × 10 mL of
each). The resulting residue was then extracted into CH2Cl2, traces
of insoluble residue removed by filtration, and the solvent evapo-
rated under reduced pressure to give the complexes. In the case of
PtL8Cl, the complexation was carried out by mixing degassed
solutions of HL8 in MeCN and K2PtCl4 (1.2 equiv) in H2O (3:1
ratio by volume), and heating the solution at reflux under nitrogen
for 3 d. Work-up was similar to that for the complexes prepared in
acetic acid, although no final extraction was required in this case.
Yields were typically in the range 40-60%.
as a yellow solid (0.60 g, 71%). H NMR (CDCl3, 500 MHz): δ
3
4
8.40 (1H, s, H2), 7.99 (2H, dd, J ) 7.5, J ) 1.5, H4), 7.82 (2H,
d, 3J ) 12.5, CHNMe2), 7.46 (1H, t, 3J ) 8.0, H5), 5.78 (2H, d, 3J
) 12.5, COCHd), 2.94, 3.15 (12H, 2s, NMe2). 13C{1H} NMR
(CDCl3): δ 188.6 (CdO), 154.7 (CHNMe2), 140.7 (C1 and C3),
130.2 (C4 and C6), 128.3 (C3), 126.7 (C5), 92.6 (COCHd), 45.3,
37.6 (2CH3). MS (ES+) m/z: 273.3 ([M + H+], 100), 295.2 ([M
+ Na+], 87), 544.9 ([2M + H+], 20), 566.9 ([2M + Na+], 53).
Mp ) 172-176 °C.
Step 2. Compound pre-9 (0.90 g, 3.3 mmol) and hydrazine
monohydrate (1.98 g, 39.6 mmol) were dissolved in ethanol (20
mL) and refluxed for 7 h. The solution was evaporated to yield an
orange oil which was redissolved in ethanol and dried over MgSO4.
Evaporation yielded a white solid which was recrystallized from
1
EtOAc to give HL9 as a colorless solid (0.45 g, 65%). H NMR
(d6-acetone, 300 MHz): δ 12.30 (2H, br, NH), 8.37 (1H, s, H2),
7.80 (2H, d, 3J ) 7.8, H4), 7.74 (2H, d, 3J ) 1.8, H5pz), 7.45 (1H,
3
3
t, J ) 7.8, H5), 6.78 (2H, d, J ) 1.8, H4pz). 13C{1H} NMR
(CDCl3): δ 129.1 (C2), 124.6 (C4 and C6), 122.5 (C5), 102.2 (C4pz).
MS (ES+) m/z: 211 [M + H+], 421 [2M + H+]. Mp ) 120-122
°C.
PtL2Cl. H NMR (CDCl3, 200 MHz): δ 8.02 (4H, overlapping
1
m with satellites, H3pz and H5pz), 6.86 (2H, s with satellites,
4J(115Pt) ) 8, H3), 6.62 (2H, t with satellites, 3J ) 2.5, 4J(115Pt) )
9, H4pz), 2.39 (3H, s, CH3). MS (ES+): m/z 453.0 [M+]. Anal. Calcd
for C13H11ClN4Pt: C, 34.4; H, 2.4; N, 12.4%. Found: C, 33.8; H,
2.7; N, 12.1%. HRMS (EI): m/z 452.0290, calcd for
C13H1135ClN4194Pt 452.0293.
1,3-Bis(3-pyrazolyl-1-phenyl)benzene HL10. Ligand HL9 (0.25
g, 1.2 mmol), iodobenzene (0.73 g, 3.6 mmol), CuI (6.8 mg, 0.012
mmol), K2CO3 (0.67 g, 4.8 mmol) and trans-1,2-diaminocyclo-
hexane (41 mg, 0.36 mmol) were taken in dioxane (8 mL). The
mixture was degassed three times and refluxed under nitrogen for
24 h. The solution turned from blue/green to green/yellow. The
solvent was evaporated and the residue taken into CH2Cl2 and
washed with water. The organic layer was dried over MgSO4, and
evaporated to yield a yellow oil (288 mg). The crude product was
purified by silica column chromatography, gradient elution from
hexane to 30% diethyl ether/70% hexane, to afford HL10 as a white
PtL3Cl. 1H NMR (CDCl3, 300 MHz): δ 8.04 (2H, d with
satellites, 3J ) 3.0, 4J(115Pt) ) 13, H3pz), 7.97 (2H, dd with
3
3
3
satellites, J ) 2.5, J(115Pt) ) 10, H5pz), 7.48 (2H, d, J ) 8.0,
Hb), 7.27 (2H, d, 3J ) 8.0, Ha), 7.14 (2H, s with satellites, 4J(115Pt)
) 7, H3), 6.60 (2H, dd, J ) 3.0, J ) 2.5, H4pz), 2.40 (s, 3H,
CH3). Anal. Calcd for C19H15ClN4Pt: C, 43.1; H, 2.9; N, 10.6%.
Found: C, 43.4; H, 3.0; N, 10.1%. MS (ES+, carrier solvent )
CH3CN): m/z 535 [MCl- + CH3CN].
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1
PtL4Cl. H NMR (CDCl3, 200 MHz): δ 8.02 (2H, d, J ) 3.0,
4J(115Pt) ) 16, H3pz), 7.91 (2H, d with satellites, 3J ) 2.5, 3J(115Pt)
) 13, H5pz), 7.51 (2H, d, 3J ) 9.0, Hb), 7.08 (2H, s with satellites,
4J(115Pt) ) 8, H3), 6.98 (2H, d, 3J ) 9.0, Ha), 6.56 (1H, t, 3J ) 3.0,
4J ) 2.5, H4pz), 3.87 (3H, s, CH3). Anal. Calcd for C19H15ClN4OPt:
C, 41.8; H, 2.8; N, 10.3%. Found: C, 42.2; H, 3.0; N, 10.3%. MS
(ES+, carrier solvent ) CH3CN): m/z 551 [M - Cl- + CH3CN].
HRMS (EI): m/z 544.0557, calcd for C19H1535ClN4O194Pt, 544.0556.
PtL6Cl. C14H10ClN3Pt 1H NMR (CDCl3, 300 MHz): δ 9.27 (1H,
d with satellites, 3J ) 5.5, 3J(115Pt) ) 44, H6py), 8.02 (1H, d with
1
3
solid (80 mg, 19%). H NMR (CDCl3, 500 MHz): δ 8.43 (1H, s,
H2), 7.99 (2H, d, 3J ) 2.5, H5pz), 7.92 (2H, dd, 3J ) 8 2J ) 2, H4
3
3
and H6), 7.81 (4H, d, J ) 8.5, H2ph), 7.51 (1H, t, J ) 8.0, H5),
7.49 (4H, t, J ) 8.5, H3ph), 7.30 (2H, t, J ) 8.5, H4ph), 6.88
3
3
(2H, d, J ) 2.5, H4pz). 13C {1H} NMR (CDCl3, 126 MHz): δ
3
153.1 (C1ph), 140.5 (C3pz), 133.7 (C1 and C3), 129.7 (C3ph), 129.3
(C5), 128.3 (C5pz), 126.6 (C4ph), 125.8 (C4 and C6), 123.5 (C2),
119.4 (C4 and C2ph). MS (ES+): m/z 363 [M + H+], 385 [M +
Na+]. HRMS (ES+): m/z 363.16046 [M + H+], calcd for C24H19N4
363.16042; 385.14235 [M + Na+], calcd for C24H18N4Na 385.14237.
A comparable quantity of the monophenylated product (82 mg)
was also recovered when the polarity was increased to 100% diethyl
ether (see Supporting Information for data).
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3
satellites, J ) 2.5, J(115Pt) ) 13, H3pz), 7.99 (1H, d, J ) 2.0,
H5pz), 7.94 (1H, td, 3J ) 8.0, 4J ) 1.5), 7.67 (1H, d with satellites,
3J ) 7.5, J(115Pt) ) 13, H3py), 7.30 (2H, overlapping m, H5py
4
and H5), 7.19 (1H, t, 3J ) 8.0, H4), 7.09 (1H, d, 3J ) 8.0, 4J(115Pt)
) 11, H3), 6.62 (1H, t, 3J ) 2.5, H4pz). Anal. Calcd for
C14H10ClN3Pt: C, 37.3; H, 2.2; N, 9.3%. Found: C, 37.5; H, 2.3;
N, 9.0%. MS (ES+, carrier solvent ) MeOH): m/z 447 [M - Cl-
+ CH3OH]+.
1,3-Bis(3-pyrazolyl-1-(4-iodotetrafluorophenyl))benzene HL11.
This compound was prepared from HL9 (0.15 g, 0.71 mmol) by
the same procedure as that used for HL10, using iodopentafluo-
robenzene (0.629 mg, 2.1 mmol) in place of iodobenzene. The
product eluted at 90% hexane/10% diethyl ether, to give a colorless
solid (43 mg, 8%). 1H NMR (CDCl3, 500 MHz): δ 8.32 (1H, t, 4J
PtL7Cl. 1H NMR (CDCl3, 300 MHz): δ 9.31 (1H, d with
satellites, 3J ) 5.0, 3J(115Pt) ) 44, H6py), 8.03 and 8.00 (each 1H,
3
4
) 1.5, H2), 7.89 (2H, dd, J ) 7.5, J ) 1.5, H4), 7.76 (2H, m,
Inorganic Chemistry, Vol. 47, No. 23, 2008 11141

Develay et al.
3
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4
d, J ) 2.5, H3pz and H5pz), 7.94 (1H, td, J ) 8.0, J ) 1.5,
H4py), 7.67 (1H, d with satellites, 3J ) 7.5, 4J(115Pt) ) 13, H3py),
7.30 (1H, m overlapping with CHCl3, H5py), 7.18 (1H, s, H5), 6.97
were measured using a Jobin Yvon FluoroMax-2 spectrofluorimeter,
fitted with a red-sensitive Hamamatsu R928 photomultiplier tube;
the spectra shown are corrected for the wavelength dependence of
the detector, and the quoted emission maxima refer to the values
after correction. Samples for emission measurements were contained
within quartz cuvettes of 1 cm path length modified with appropriate
glassware to allow connection to a high-vacuum line. Degassing
was achieved via a minimum of three freeze-pump-thaw cycles
while connected to the vacuum manifold; final vapor pressure at
77 K was <5 × 10-2 mbar, as monitored using a Pirani gauge.
Luminescence quantum yields were determined by the method of
continuous dilution, using quinine sulfate in 1 M H2SO4 as the
standard (φ ) 0.546 42); estimated uncertainty in φ is ( 20% or
better.
The luminescence lifetimes of the complexes were measured by
time-correlated single-photon counting, following excitation at 374.0
nm with an EPL-375 pulsed-diode laser. The emitted light was
detected at 90° using a Peltier-cooled R928 PMT after passage
through a monochromator. The estimated uncertainty in the quoted
lifetimes is ( 10% or better. Lifetimes at 77 K in excess of 10 µs
were measured by multichannel scaling following excitation with
a µs-pulsed xenon lamp; an excitation wavelength of 374 nm (band-
pass 5 nm) was selected with a monochromator. Bimolecular rate
constants for quenching by molecular oxygen, kQ, were determined
from the lifetimes in degassed and air-equilibrated solution, taking
(1H, s with satellites, J(115Pt) ) 11, H3), 6.63 (1H, t, J ) 2.5,
H4pz), 2.38 (3H, s, CH3). Anal. Calcd for C19H15ClN4OPt: C, 38.8;
H, 2.6; N, 9.0%. Found: C, 38.5; H, 3.0; N, 8.5%. MS (ES+, carrier
solvent ) MeOH): m/z 461 [M - Cl- + CH3OH]+.
4
4
PtL8Cl. H NMR (CDCl3, 500 MHz): δ 9.51 (1H, d, J ) 6.0,
1
3
3J(195Pt) ) 40, H6py), 7.89 (1H, td, 3J ) 7.0, 4J ) 1.5, H4py), 7.61
(1H, d, J ) 8.0, H3py), 7.28 (1H, td, J ) 6.5, J ) 1.0, H5py),
7.08 (1H, s, H4), 6.99 (1H, s, H6), 6.07 (1H, s, H4pz), 2.73, 2.71
(each 3H, s, CH3pz), 2.34 (3H, s, CH3aryl). 13C{1H} NMR (CDCl3):
δ 167.3 (C5py), 155.1 (C5pz), 151.1 (C6py), 142.0 (C2), 141.6
(C3pz), 138.9 (C4py), 133.4 (C5), 123.0 (C2py), 120.4 (C4), 119.3
(C3py), 113.7 (C6), 108.8 (C4pz), 22.6 (CH3aryl), 14.7 (CH3pz), 13.8
(CH3pz). MS (EI) m/z: 456 [M - Cl]+, 491 [M+]. HRMS (EI) m/z:
491.0652 [M+], calcd for C17H1635ClN3194Pt, 491.0661.
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Crystallography. A yellow single crystal of PtL6Cl (0.10 × 0.06
× 0.06 mm) was analyzed at 120(2) K on a Bruker 1000 CCD
diffractometer using graphite monochromated Mo KR radiation (λ
) 0.71073 Å). Structure solution was carried out using SHELXS-
9740 and refined by full matrix least-squares in SHELXL-97.41 All
non-hydrogen atoms were refined with anisotropic displacement
parameters. Hydrogen atoms were positioned geometrically with
isotropic displacement parameters fixed to ride on the parent atom
(aromatic C-H 0.94 Å, Uiso ) 1.2 × Ueq(C); methyl C-H 0.98 Å,
Uiso ) 1.5 × Ueq(C)). The asymmetric unit consisted of two
molecules. The L6 ligand in the second molecule was disordered
over two orientations, depending which side the pyrazole ring is
on; the orientations were modeled at occupancies of 60:40 and the
geometry was constrained to be the same as that of the ordered
ligand in the asymmetric unit. Crystal data: M ) 464.82, monoclinic
space group P21/n, a ) 16.8287(13), b ) 8.8822(7), c )
18.3338(14) Å, ꢀ ) 92.968(1), V ) 2736.8(4) Å3, Z ) 8, Dc )
2.256 Mg/m3, µ ) 10.441 mm-1, F(000) ) 1744, 25639 reflections
collected (1.60 e θ e 26.37°), 5586 independent reflections (Rint
) 0.0618) were used for structure refinement, final R1 ) 0.0405
[F2 > 2σ(F)], wR2 ) 0.0624 [F2 > 2σ(F)], R1 ) 0.0619 (all data),
wR2 ) 0.0666 (all data), GOF (F2) ) 1.148, largest peak, hole )
1.072, -0.932 e Å-3. Crystallographic data in CIF format is
provided as Supporting Information.
the concentration of oxygen in CH2Cl2 at 0.21 atm O2 to be 2.2
43
mmol dm-3
.
Acknowledgment. We thank EPSRC (EP/D500265/1) and
the University of Durham for financial support, Professor
J.A.K. Howard for access to crystallography facilities, Dr.
H. Sparkes for assistance, and the EPSRC National Mass
Spectrometry Service Centre for providing EI and CI spectra
of platinum(II) complexes.
Supporting Information Available: Further details of synthesis
and characterization of ligands and intermediates; frontier orbital
plots of the complexes, obtained by DFT calculations; absorption
and emission spectra of the complexes in different solvents overlaid;
crystallographic data for PtL7Cl in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
Photophysical Measurements. Absorption spectra were mea-
sured on a Biotek Instruments XS spectrometer, using quartz
cuvettes of 1 cm path length. Steady-state luminescence spectra
IC8014157
(40) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(41) Sheldrick, G. M. Computer Program for Crystal Structure Refinement;
University of Go¨ttingen: Go¨ttingen, Germany.
(42) Meech, S.; Phillips, D. J. Photochem. 1983, 23, 193.
(43) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993.
11142 Inorganic Chemistry, Vol. 47, No. 23, 2008