Spectroscopic and electrochemical properties of cyclopalladated complexes of 2,3-diphenylquinoxaline and 2,2′,3,3′-tetraphenyl-6,6′- biquinoline

Article abstract of DOI:10.1134/S1070363208040257

Comparative study of cyclopalladated ethylenediamine complexes of 2,3-diphenylquinoxaline (Hdphqx) [Pd(dphqx)En]ClO₄ and 2,2′,3,3′-tetraphenylbiquinoline (H₂tphbq) [(PdEn) ₂(μ-tphbq)](ClO₄)₂, and the free heterocyclic ligands was performed by means of ¹H NMR spectroscopy, electronic absorption and emission spectroscopy, and cyclic voltammetry. It was shown that cyclopalladation gives rise to a long-wave absorption band in the visible spectrum, a batochromic shift of the vibrationally structured phosphorescence band, and an anodic shift of the ligand-centered reduction potential of the complexes com-pared to free ligands.

Full text of DOI:10.1134/S1070363208040257

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 4, pp. 668–672. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © T.A. Tkacheva, O.A. Farus, M.P. Puzyk, K.P. Balashev, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 4,
pp. 686–690.
Spectroscopic and Electrochemical Properties
of Cyclopalladated Complexes
of 2,3-Diphenylquinoxaline and 2,2',3,3'-Tetraphenyl-6,6'-
Biquinoline
T. A. Tkacheva, O. A. Farus, M. P. Puzyk, and K. P. Balashev
Herzen Russian State Pedagogical University,
nab. r. Moiki 48, St. Petersburg, 191186 Russia
e-mail: balashevk@mail.ru
Abstract—Comparative study of cyclopalladated ethylenediamine complexes of 2,3-diphenylquinoxaline
(Hdphqx) [Pd(dphqx)En]ClO4 and 2,2',3,3'-tetraphenylbiquinoline (H2tphbq) [(PdEn)2(μ-tphbq)](ClO4)2, and
1
the free heterocyclic ligands was performed by means of H NMR spectroscopy, electronic absorption and
emission spectroscopy, and cyclic voltammetry. It was shown that cyclopalladation gives rise to a long-wave
absorption band in the visible spectrum, a batochromic shift of the vibrationally structured phosphorescence
band, and an anodic shift of the ligand-centered reduction potential of the complexes com-pared to free ligands.
DOI: 10.1134/S1070363208040257
Cyclometalated square-planar complexes of
platimum metals have long-lived electronically excited
states and characteristically undergo reversible one-
electron electrochemical reduction. For this reason,
they are promising candidates for the design of
photoactive molecularly organized metal complex
systems with directed charge and energy transfer [1–
3]. Special attention is attached to coordinately
unsaturated “ligand complexes” capable of coor-
dinating additional metal complex fragments to form
complexes [Pd(dphqx)En]ClO4 and [(PdEn)2(μ-tphbq)]·
(ClO4)2.
Heterocyclic ligands prepared by the reaction of
benzil with phenylenediamine and biphenyl-3,3',4,4'-
tetramine undergo facile cyclopalladation when
reacted with an ethanolic solution of Li2[PdCl4] to give
poorly soluble complexes [Pd(dphqx)(μ-Cl)]2 and
[(PdCl2)2(μ-tphbq)]. Subsequent substitution of
chloride ligands with ethylenediamine and precipita-
tion of the reaction products as perchlorates leads to
isolation of the complexes [Pd(dphqx)En]ClO4 I and
[(PdEn)2(μ-tphbq)](ClO4)2 II.
molecularly organized systems with
spatially
separated photoinduced charges [4, 5].
It was previously shown [6–10] that cyclo-
metalated complexes of the general formula [M(C^N)
En]2+ [M = Pd(II), Pt(II), Au(III); (C^N)– are depro-
tonated forms of phenyl- and benzo-substituted imines
and diazines; En is ethylenediamine] are convenient
model compounds for tracing the effect of the
heterocyclic ligand (C^N)– on the spectroscopic and
electrochemical properties of the complexes as com-
pared to the free compound H(C^N).
1
The H NMR spectra provide evidence for the
composition and structure of the resulting complexes.
The coordination-induced chemical shifts of protons
(CIS = δc – δl) of the ligands, that reflect the changes
in their structure, produced by complex formation, are
listed in Table 1.
Comparison of the 1H NMR spectra of [Pd(dphqx)En]+
and [(PdEn)2(μ-tphbq)]2+ shows that cyclopalladation
similarly affects the chemical shifts of ligand protons.
In the dimeric complex [(PdEn)2(μ-tphdq)]2+, magnetic
equivalence of protons in both quinoxaline constituents
of the cyclometalated ligand, as well as in both
In the present work we present the results of a
comparative study of properties of the free 2,3-di-
phenylquinoxaline (Hdphqx) and 2,2',3,3'-tetraphenyl-
6,6'-biquinoline (H2tphbq) and their cyclopalladated
668

SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES
669
p
m
m
o
NH2
Pd
o
m
o
p
8
5
H2N
o
6'
5'
3'
7
m
N
N
C
N
6'
4'
8
6
N
5'
4'
7
N
C
C
Pd
Pd
5
N
3'
H2N
δ
γ
H2N
NH2
NH2
α
α
δ
γ
β
β
[Pd(dphqx)En]+
[(PdEn)2(μ-tphbq)]2+
ethylenediamine ligands is observed. The strongest
cyclopalladation-affected is the H5 shift (CIS =
–0.43 ppm), whereas the H7 and H8 shifts change only
slightly (CIS < 0.1). This result suggests selective
formation of a steric isomer of [(PdEn)2(μ-tphbq)]2–, in
which palladium is coordinated to nitrogen atoms of
2,2',3,3'-tetraphenyl-6,6'-biquinoline, nearest just to the
H5 protons. It was previously shown [6] that the steric
interaction of the H6' and H0 protons of the palladated
and uncoordinated phenyl rings in [Pd(dphqx)]– leads
to their ortogonal location (the angle between the ring
planes is 61°). This causes strong shielding of the H6'
and H5' protons, resulting from the anisotropic effect of
the circular current of the uncoordinated phenyl ring of
the ligand. The analogous effect in [(PdEn)2(μ-tphbq)]2+
can be also related to the ortogonal location of the
palladated and uncoordinated phenyl rings of the
ligand. In agreement with the different trans-effects of
the diazine and carbanionic part of the cyclometalated
ligands, both complexes exhibit magnetic nonequi-
valence of protons in the ethylenediamine ligand.
plexes are classed, in accordance with the prevailing
character of orbitals [11], as metal- and ligand-
centered. Therewith, provided the Koopmans theorem
is fulfilled [12], optical and redox orbitals of com-
plexes are similar in nature. The results of out study of
the optical and electrochemical properties of the
heterocyclic ligands and their cyclopalladated
complexes are summarized in Table 2.
The reduction voltammorgam of Hdphqx shows a
reversible one-electron wave assigned to electron
transfer on π* orbitals mostly localized in the quin-
oxaline part of the ligand. In the case of H2tphbq, two
reversible one-electron waves related to consecutive
electron transfer on π* orbitals of the two quinoxaline
groups of the ligand (Fig. 1). The difference between
the half-wave potentials (ΔE1/2 = 0.3 V) agrees with
the expected weak interaction between π orbitals of the
two σ-bonded free rotating quinoxaline parts of the
ligand.
The reduction voltammorgam of [Pd(dphqx)En]+,
too, shows a reversible one-electron reduction wave
attributed to ligand-centered electron transfer on p
orbitals of the quinoxaline part [Pd(dphqx)] of the
In terms of the localized MO model, the optical
and electrochemical properties of mixed-ligand com-
Table 1. Coordination-induced shifts (CIS = δc – δl, ppm) of ligand protons in DMSO solutions of complexes
Proton no.
Complex
[Pd(dphqx)En]ClO4
5
6
7
8
3'
4'
5'
6'
o
m
p
–0.14
0.05
0.00
0.01
–0.14
–0.29
–0.54
–0.81
0.2
0.2
0.2
α
β
γ
δ
4.18
0.29
0.22
3.16
[(PdEn)2(μ-tphbq)] (ClO4)2 –0.43
–
0.01
0.07
–0.12
–0.38
–0.56
–0.85
0.2
0.2
0.2
α
β
γ
δ
4.21
0.33
0.25
3.22
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TKACHEVA et al.
Table 2. Optical and electrochemical parameters of ligands and cyclopalladated complexes
Absorptiona
λmax, nm (ε × 10–3 l mol–1 cm–1)
Phosphorescenceb
Reduction
–E1/2,V (ΔE, mV)d
2.06 (60)
Compound
Hdphqx
λmax, nm (τ, μs)c
247 sh (33.6), 279 sh (11.1), 347 (8.6)e
270 (68.8), 290 sh (58.5), 382 (55.2)e
483
545
H2tphbq
1.65 (70)
1.94 (80)
1.48 (80)
[Pd(dphqx)En]ClO4
210 (27.3), 240 sh (26.9), 287 (9.5), 423 (2.1)f
584 (70)
[(PdEn)2(μ-tphbq](ClO4)2
240 (63.7), 273 (62.5), 290 (61.4), 380 (40.0), 440 sh
(~14), 464 (16.8)e
585 sh (90)
1.31g
a
d
293 K. b 77 K, DMF–toluene (1 : 1). c Position of the short-wave phosphorescence maximum and the decay time. Half-wave potential
e
f
g
against Fe+/Fe and the difference in the oxidation and reduction current peak potentials, DMF. CH2Cl2. CH3OH. Current peak
potential at the potential sweep rate 100 mV s–1
metal complex fragment. The broadened reduction
wave observed for [(PdEn)2(μ-tphdq)]2+ at –1.3 V is
evidently caused by overlap of two waves associated
with consecutive electron transfer on π* orbitals of the
two quinoxaline parts of the [Pd2(μ-tphdq)] fragment.
The anodic shift of the half-wave potential {0.6 V for
[Pd(dphqx)En]+} and peak current {~0.5 V for [(PdEn)2·
(μ-tphbq)]2+} reflect a decrease in the energy of
quinoxaline π orbitals, induced by cyclopalladation.
The oxidation voltammograms of the complexes have
a typical [6] irreversible character and are on the
electrochemical stability border of the solvent due to
formation of highly reactive Pd(III) complexes that
undergo subsequent fast chemical reactions. The
oxidation peak current potentials depend on the
potential sweep rate, which makes impossible
assessment of the effect of cyclopalladation on the
HOMO energy.
The absorption spectra of the free heterocyclic
ligands show strong vibrationally structured bands
(Fig. 2) related to spin-allowed π–π* optical transi-
tions. In agreement with the electrochemical data
showing that the energy of the free π* orbital of the
quinoxaline part of the ligand in H2tphbq is lower
compared to Hdphqx, a batochromic shift of the long-
wave optical transition (0.3 eV) is observed.
Photoexcitation of frozen (77 K) glassy solutions of
the ligands causes their vibrationally structured
phosphorescence resulting from spin-forbidden π–π*
optical transitions.
Comparison of the electronic absorption spectra of
the complexes and free ligands (Figs. 2, 3) shows that,
along with the expected batochromic shift of intra π–π *
optical transitions (ΔE < 1 kK) localized in the
heterocyclic ligands, cyclopalladation gives rise to
0.4
3
4
0.000
–0.003
–0.006
–0.009
1
0.2
0.0
2
–0.2
–0.4
–0.012
–2000
–1500
–1000
–500
–1600 –1400 –1200 –1000 –800
Е, eV
Е, eV
Fig. 1. Reduction voltammograms of (1) Hdphqx, (2) [Pd(dphqx)En]ClO4, (3) H2tphbq, and (4) [(PdEn)2(μ-tphbq)](ClO4)2 in DMF.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 4 2008

SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES
671
(a)
(a)
5
2.5
2.0
1.5
1.0
0.5
0.0
4
2
3
2
1
2
1
1
250 300 350 400 450 500 550
0
300
400
500
600 700
λ, nm
λ, nm
(b)
(b)
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2
1
2
1
250
350
450 550
650
750
λ, nm
250
300
350
400 450
500
λ, nm
Fig. 2. Absorption and luminescence spectra of (a) Hdphqx
and (b) H2tphbq.
Fig. 3. Absorption and luminescence spectra of (a) [Pd
(dphqx)En]ClO4 and (b) [(PdEn)2(μ-tphbq)](ClO4)2.
new low-energy bands related to spin-allowed charge-
transfer optical transitions from the HOMO to LUMO
(π*) in the [Pd(dphqx)] and [Pd2(μ-tphbq)] metal
complex fragments. The spectral and kinetic character-
istics of the vibrationally structured low-temperature
luminescence of the complexes are determined by the
spin-forbidden irradiative decay of the photoexcitation
energy from the lowest electronically excited state of
the metal complex fragments. This is confirmed by the
exponential luminescence decay kinetics and by the
independence of the spectrum on the excitation
wavelength (λ 250–400 nm). The strong temperature
quenching characteristic of spin-forbidden optical
transitions localized in the cyclopalladated metal
complex fragments [6–10] leads, together with the
effective spin-orbital interaction characteristic of
palladium complexes, leads to the absence of both
phosphorescence and fluorescence on photoexcitation
of the complexes in liquid (293 K) solutions.
The presence of long-lived electronically excited
states and the ligand-centered character of the
reduction of the complexes, together with their
coordinational unsaturation, allows the cyclometalated
“ligand complexes” on the basis of 2,3-diphenyl-
quinoxaline and 2,2',3,3'-tetraphenyl-6,6'-biquinoline
as possible structural components of photosensitive
molecularly organized systems with directed charge
and energy transfer.
EXPERIMENTAL
1
The H NMR spectra were taken on a Bruker AC-
200F spectrometer. The electronic absorption spectra
were obtained on an SF-2000 spectrophotometer at
293 K. Luminescence studies were carried out on an
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 4 2008

672
TKACHEVA et al.
SDL-2 spectrometer and a KSVU-1 device with pulse
laser excitation (LGI-21, λ 337 nm) in glassy 1 : 1
DMF–toluene matrices at 77 K and in acetonitrile
solutions at 293 K [13]. The cyclic voltammograms
were obtained on a modified SVA-1B device at 293 K
in a three-electrode cell with separated compartments
for the operating (Pt), auxiliary (Pt), and reference
(Ag) electrodes, in presence of an 0.1 M [N(C4H9)4]
ClO4 solution in DMF [14]. The half-wave potentials
are presented against the ferrocenium–ferrocene redox
system.
7.3), 7.01 (2H4', 8.0, 7.3), 6.83 (2H5', 8.0, 7.3) 6.71
(2H6', 8.0), 5.52 (4Hα), 4.53 (4Hδ), 2.81 (4Hβ), 2.73
(4Hγ).
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1
2,3-Diphenylquinoxaline. Yield 90%. H NMR
spectrum (DMSO-d6), δ, ppm (3J, Hz): 8.16 (H5,8, 3.6,
6.5), 7.89 (H6,7, 3.6, 6.5), 7.48 (4Ho), 7.37 (6Hm,p).
1
2,2'3,3'-Tetraphenylbiquinoline. Yield 70%. H
NMR spectrum (DMSO-d6), δ, ppm (3J, Hz): 8.71
(2H5), 8.33 (2H8, 8.7), 8.26 (2H7, 8.7), 7.54 (8Ho), 7.40
(12Hm,p).
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Ethylenediamine[(2-phenyl)(3-phenyl-2-ido)quin-
oxalin]palladium(II) perchlorate [Pd(dphqz)En]
1
ClO4. Yield 80%. H NMR spectrum (DMSO-d6), δ,
ppm (3J, Hz): 8.17 (H8, 8.0), 8.02 (H5, 8.0), 7.94 (H6,
5.8, 8.0), 7.89 (H7, 5.8, 8.0), 7.62-7.71 (Hm,p), 7.71 (Ho,
8.0), 7.23 (H3' 7.3), 7.08 (H4' 7.3), 6.82 (H5', 7.3, 8.0),
6.67 (H6', 8.0), 5.49 (2Hα), 4.47 (2Hδ), 2.77 (2Hβ), 2.70
(2Hγ).
Bis[ethylenediaminopalladium(II)]{μ-(2,2'-diphe-
nyl)[3,3'-di(phenyl-2-ido)]-6,6'-biquinoline} [(PdEn)2 ·
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1
(μ-tphbq)](ClO4)2. Yield 60%. H NMR spectrum
(DMSO-d6),δ, ppm (3J, Hz): 8.38 (2H7, 8.7), 8.33 (H8,
8.7), 8.28 (2H5), 7.73 (4Ho), 7.56 (6Hm,p), 7.23 (2H3',
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 4 2008