Spectroscopic and electrochemical properties of cyclopalladated complexes derived from 2,3-diphenylpyrazine

Article abstract of DOI:10.1134/S1070363207080191

A comparative study of complexes [Pd(dphpz)(N'N)]PF₆ [dphpz ^- is the deprotonated form of 2,3-diphenylpyrazine; (N'N) is ethylenediamine (En), 2,2'-bipyridine (bpy), o-phenanthroline (phen), dipyrido[a,c]phenazine (dppz), 6,7-dicyanopyrido[f,h]quinoxaline (dicnq)] was made, using ¹H NMR, electronic absorption, and emission spectroscopy, and also cyclic voltammetry. Steric interaction of the dphpz^- phenyl rings leads to significant proton shielding in the carbanionic moiety of the cyclometallated ligand. Introduction of heterocyclic diimines instead of ethylenediamine decreases the desheilding of the dphpz^- protons adjacent to the coordination center. Irrespective of the nature of the N'N ligands, the cyclopalladated complexes are characterized by specific parameters of photo-and electrostimulated electron transfer processes involving the Pd(dphpz) orbitals, namely, by the long-wave absorption band with λ_max 395±6 nm and ε (2.2±1.2) × 10³ 1 mol^-1 cm^-1, the vibrationally structured low-temperature (77 K) luminescence resulting from the spinforbidden optical transfer from the excited to the ground state of the complex (energy E ⁰⁰ 19.27±0.07 kK, lifetime τ 160±30 μs), and the one-electron electroreduction wave with E ₁-(2.0±0.1)V. For the [Pd(dphpz)?(N'N)]⁺ complexes containing diazine derivatives of phenanthroline (dppz, dicnq), the degradation of the photoexcitation energy from two electronically excited states can occur as isolated process with successive transfer of electrons to the π orbitals localized on the remote moieties: [Pd(dphpz)] and diazine fragments of the N'N ligands.

Full text of DOI:10.1134/S1070363207080191

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 8, pp. 1410 1415.
Pleiades Publishing, Ltd., 2007.
Original Russian Text O.A. Farus, K.P. Balashev, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 8, pp. 1332 1338.
Spectroscopic and Electrochemical Properties
of Cyclopalladated Complexes
Derived from 2,3-Diphenylpyrazine
O. A. Farus and K. P. Balashev
Herzen Russian State Pedagogical University,
nab. r. Moiki 48, St. Petersburg, 191186 Russia
e-mail: balashevk@mail.ru
Received October 18, 2006
Abstract A comparative study of complexes [Pd(dphpz)(N^N)]PF6 [dphpz is the deprotonated form of
2,3-diphenylpyrazine; (N^N) is ethylenediamine (En), 2,2 -bipyridine (bpy), o-phenanthroline (phen), dipyrido-
[a,c]phenazine (dppz), 6,7-dicyanopyrido[f,h]quinoxaline (dicnq)] was made, using 1H NMR, electronic
absorption, and emission spectroscopy, and also cyclic voltammetry. Steric interaction of the dphpz phenyl
rings leads to significant proton shielding in the carbanionic moiety of the cyclometallated ligand. Introduction
of heterocyclic diimines instead of ethylenediamine decreases the deshielding of the dphpz protons adjacent
to the coordination center. Irrespective of the nature of the N^N ligands, the cyclopalladated complexes are
characterized by specific parameters of photo- and electrostimulated electron transfer processes involving the
Pd(dphpz) orbitals, namely, by the long-wave absorption band with
395 6 nm and (2.2 1.2)
max
103 l mol 1 cm , the vibrationally structured low-temperature (77 K) luminescence resulting from the spin-
forbidden optical transfer from the excited to the ground state of the complex (energy E 00 19.27 0.07 kK,
lifetime 160 30 s), and the one-electron electroreduction wave with E1 (2.0 0.1)V. For the [Pd(dphpz)
(N^N)]+ complexes containing diazine derivatives of phenanthroline (dppz, dicnq), the degradation of the
photoexcitation energy from two electronically excited states can occur as isolated process with successive
transfer of electrons to the orbitals localized on the remote moieties: [Pd(dphpz)] and diazine fragments of
the N^N ligands.
1
DOI: 10.1134/S1070363207080191
The development of photo- and electroactive
molecularly organized metal complexes capable of
directed transfer and accumulation of charge or energy
on the reaction center due to the definite steric and
structural organization of the individual components
stimulates studies of coordination-unsaturated com-
plexes with variable optical and electrochemical
parameters as structural components of such systems
[1, 2].
This paper deals with a comparative study of the
spectroscopic and electrochemical properties of the
mixed-ligand
complexes
[Pd(dphpz)(N^N)]PF6
[(dphpz) is the deprotonated form of 2,3-diphenyl-
pyrazine; (N^N) is ethylenediamine (En), 2,2 -bi-
pyridine (bpy), o-phenanthroline (phen), dipyrido-
[a,c]phenazine (dppz), or 6,7-dicyanopyrido[f,h]-
quinoxaline (dichq)] (see scheme).
1
The H NMR spectra of the complexes confirm the
mononuclear structure and the mode of the ligand
coordination in the complexes [Pd(dphpz)(N^N)]. The
coordination-induced chemical shifts of protons in
It was shown previously that the cyclopalladated
ethylenediamine complexes based on deprotonated
forms of diphenyl- and dibenzo-substituted 1,4-di-
azines have a long-lived electronically excited state
responsible for their luminescence. They are also
characterized by a quasireversible one-electron elec-
troreduction process [3, 4]. Introduction of hetero-
cyclic diimine ligands with the low-energy delocalized
* orbitals instead of the aliphatic ethylenediamine
expands the possibility of modification of the optical
and electrochemical parameters of such complexes.
ligands [
=
l] reflecting the character of
c
changes in their chemical shifs in the course of com-
plex formation, resulting from the donor acceptor
ligand metal and anizotropic interactions, are listed
in Table 1.
The mononuclear structure of the complexes is
characterized by almost orthogonal arrangement of
the noncoordinated phenyl ring of the dphpz ligand
1410

SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES
1411
m
p
o
C
m
o
6
5
N
N3
5
4
N
C
6
3
dphpz
C
N
C
N
C
N
N
N
N
NH2
NH2
Pd
Pd
Pd
N
[Pd(dphpz)(phen)]+
[Pd(dphpz)En]+
[Pd(dphpz)(bpy)]+
N
N
N
N
C
N
N
C
N
N
CN
CN
Pd
Pd
N
N
[Pd(dphpz)(dppz)]+
[Pd(dphpz)(dicnq)]+
relative to the plane of complex [5]. This leads to
strong and practically constant shielding of the H6
Table 1. Coordination-induced chemical shifts (
l) of
c
ligand protons in complexes [Pd(dphpz)(N^N)]+
and H5 protons (
0.9 and 0.7 ppm, respectively),
resulting from the anisotropic effect of the ring current.
(DMSO-d6 solutions)
/
The
value of the H3 , H6, and H
ligand protons
(N^N)
En
bpy
phen
dppz dicnq
nearest to palladium is determined mainly by the
efficiency of the donor-acceptor ligand metal inter-
action. The presence of ethylenediamine, a typical
donor, in the inner sphere is accompanied by shielding
H5
0.00
0.26
0.3
0.4
0.6
0.01
0.13
0.4
0.5
0.7
1.0
0.2
0.45
0.37
0.46
0.20
0.00
0.09
0.4
0.4
0.7
0.9
0.2
0.27
0.43
0.54
0.38
0.00
0.04
0.3
0.5
0.7
1.0
0.2
0.19
0.26
0.26
0.00
0.04
0.4
0.5
0.7
H6
H3
of the H3 and H6 dphpz protons and deshielding of
H4
/
the H
En proton. Difference in the trans effect of
H5
the carbanionic and pyrazine moieties of the cyclo-
metallated ligand leads to the magnetic nonequivalence
of the H and H ethylenediamine protons. At the
same time, weakening of the donor acceptor (N^N)
Pd interaction resulting from a decrease in the
-donor properties on introduction of heterocyclic
diimines instead of the aliphatic ethylenediamine
leads to a decrease in the deshielding and to magnetic
equivalence of the H and H diimine protons, as well
as to a decrease in the deshielding of the H5 and H3
dphpz protons.
H6
0.8
0.2
0.9
0.2
Ho,m,p
H /H
H /H
H /H
H /H
H /H
2.67/1.84
1.32/1.30
1.10
0.48
0.17
0.15
0.1
phen > dpq > dppz
dicnq series is accompanied
by a regular decrease in the deshielding of the H
diimine protons and in the shielding of the H6 protons
from the pyrazine moiety of the dphpz anion.
A decrease in the basicity of diimines in the Bpy >
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

1412
FARUS, BALASHEV
Table 2. Parameters of the electronic absorption spectra and reduction voltammograms of the ligands and
[Pd(dphpz)(N^N)]+ complexes
Reduction,
a
1
3
Absorption, max, nm ( 10 , l mol 1 cm )
E1/2, V ( E, mV)b
Compound
(
*)
(
*)
2.29c
*
*
Ph/d
(dphpz)
(N N)
Hdphpz
bpy
phen
dppz
207 (25.8), 219 (15.8) sh, 280 (7.3), 318 (4.9)
235 (12.7), 243 (19.5), 283 (16.3)
2.52c
2.55 (60)
2.51 (60)
1.59 (60)
2.49 (60)
1.09 (70)
2.10c
226 (47.3), 267 (27.7) 268 (27.3), 363 (10.4), 378 (7.9) b
dicnq
265 (43.6), 307 (23.1), 364 (7.4) shb
2.57c
[Pd(dphpz)En]+
[Pd(dphpz)bpy]+
[Pd(dphpz)phen]+
207 (29.5), 232 (12.5) sh, 279 (7.6), 338 (3.6)
209 (28.9), 254 (7.5), 304 (5.9), 312 (6.1)
208 (28.9), 230 (12.9) sh, 264 (6.2) sh, 275 (7.2), 324
(2.7)
206 (27.7), 241 (10.6), 270 (16.7), 295 (7.9) sh,
359 (4.6), 363 (4.4), 377 (4.9)
388 (2.1)
388 (0.9)
399 (4.1)
1.85 (70)
1.95c
1.27c
1.30c
2.05c
[Pd(dphpz)dppz]+
[Pd(dphpz)dicnq]+
400 (2.2)
399 (1.6)
2.00 (70) 1.61 (90)
206 (28.1), 266 (6.9), 273 (1.6)
2.05c
1.16 (80)
1.59c
a
b
c
1
CH OH.
DMF.
Potential of the irreversible reduction wave at a potential sweep rate of 50 mV s
.
3
The electro- and photostimulated processes in the
on particular ligand. This fact allows absorption band
to be assigned to the spin-allowed optical transition
between the HOMO and LUMO of complexes loc-
alized mainly on the metal complex fragment
Pd(dphpz). Quantum-chemical calculations of the
platinum cyclometallated complexes derived from
phenyl-substituted imines [8] show that the HOMO
mixed-ligand complexes within the framework of the
model of localized molecular orbitals [6] are classed
as metal- or ligand-centered depending on the character
of the participating orbitals. Provided that the
Koopmans theorem is valid [7], the orbital nature of
the redox and spectroscopic LUMO and HOMO of
the complexes is observed. Contrary to the reversible
character of the ligand-centered electroreduction
processes for the complexes with the heterocyclic
ligands, involving the electron transfer to the de-
localized * orbitals, the metal-centered electroreduc-
tion of Pd(II) complexes is irreversible. This is due to
the high rate of the subsequent chemical reactions of
Pd(I) complexes, the primary products of the one-
electron electroreduction. The optical and electro-
chemical properties of the ligands and the cyclopal-
ladated complexes derived from them are given in
Table 2.
and LUMO of complexes have
symmetry. The
LUMO is localized mainly on the * orbital of the
imine moiety of cyclometallated ligands, whereas the
HOMO has mixed
phenyl metal character.
Within the frameworkPho/df the model of localized
molecular orbitals, this band may be classed as the
spin-allowed transfer of the
* type.
ph/d
Voltammograms of electroreduction of the free
ligands are characterized by one-electron reduction
waves caused by consecutive transfer of electron to
the unoccupied * orbitals. The specific features of
the electronic structure of the 1,4-diazine derivatives
of o-phenanthroline (dppz, dicnq) are determined by
the presence of two heterocycles, phenanthroline and
diazine, and hence, of the corresponding predom-
inantly localized *(phen) and *(diaz) orbitals.
Weak electronic interaction between them leads to
essentially isolated character of the corresponding
electro- and photostimulated electron transfer proc-
esses [9, 10]. The reduction voltammograms of
Comparison of electronic absorption spectra of the
free ligands and the related complexes shows that,
along with superposition of the overlapping intra-
ligand spin-allowed absorption bands in the UV range
(
< 380 nm), the [Pd(dphpz)(N^N)]+ complexes
exhibit a long-wave absorption band (Fig. 1). Its loca-
tion and the extinction coefficient [ 395 6 nm,
max
1
(2.2 1.2) 103 l mol 1 cm ] only slightly depend
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES
o-phenanthroline as well as of its 1,4-diazine deriva-
1413
tives contain one-electron waves with the practically
constant half-wave (phen, dppz) or peak current
(dicnq) potential (E = 2.53 0.03 V). They were
assigned to the electron transfer to the *(phen) redox
orbitals of the ligands. At the same time, the diazine
derivatives of o-phenanthrolinethe exhibit additional
anodically shifted reversible reduction waves (Table 2)
caused by the electron transfer to the lowest *(diaz)
redox orbitals of the ligands [9, 10].
20
15
10
5
3
1
2
In accordance with data of electronic absorption
spectroscopy, the reduction voltammograms of all the
mononuclear complexes are characterised by a one-
electron wave (Fig. 2) with virtually the same potential
(E = 2.0 0.1 V), which allows its assignment to the
electron transfer to the *(diaz) orbitals of the
Pd(dphpz) fragment. The anodic shift of the wave
potential of the cyclopalladated complexes as com-
pared to the free Hdphpz ligand ( E 0.3 V) agrees
with the expected decrease in energy of the *(diaz)
orbital of the ligand resulting from the donor acceptor
ligand metal interaction.
250
300
350
400 , nm
+
Fig. 1. Absorption spectra: (1) Hdphp, (2) [Pd(dphp)En] ,
and (3) [Pd(dphpz)dppz] .
+
(a)
The presence of heterocyclic chelating ligands
(N^N) with their own unoccupied low-energy * or-
bitals in the inner sphere of the complexes [Pd(dphpz)
(N^N)]+ leads to appearance of additional reversible
one-electron reduction waves. Comparison of the
voltammograms of reduction of the 1,4-diazine de-
rivatives of o-phenanthroline (dppz, dicnq) and com-
plexes [Pd(dphpz)(N^N)]+ derived from them allows
the additional electroreduction waves of complexes to
be assigned to the electron transfer to the *(diaz)
orbitals of the (N^N) ligands. Thus, in accordance
with the remoteness of the diazine moiety of the
ligands from the coordination center, the complex
formation does not noticeably affect the isolated
character of the * orbitals localized on the diazine
and phenanthroline moieties of the dipyridodiazine
lidands. The reduction voltammograms of the com-
plexes [Pd(dphpz)(bpy)]+ and [Pd(dphpz)(phen)]+ also
contain an additional one-electron reduction wave (Er
1.27 and 1.30 V, respectively) which may be
caused by the electron transfer to the * orbitals of the
diimine ligand.
2.0
1.6
1.2
E, V
(b)
2.0
1.6
E, V
Photoexcitation of complexes in a frozen glassy
matrix (77 K, DMF toluene, 1:1) leads to intense
luminescence (Fig. 3). The spectral and kinetic lumi-
nescence parameters of the complexes [Pd(dphpz)
(N^N)]+ [(N^N) = En, bpy, phen] (Table 3) are es-
sentially independent of the nature of the ligands. The
luminescence spectra are characterized by the vibra-
tional progression with the energy of the purely elec-
tronic transition (E00 19.27 0.07 kK) and the
+
Fig. 2. Cyclic voltammograms: (a) [Pd(dphp)En] and
(b) [Pd(phpz)(dppz)] .
+
vibrational frequency ( 1.00 0.07 kK) correspond-
ing to the C=N vibration of the cyclometallated
(dphpz) ligand [11]. The kinetics of the luminescence
decay is exponential with the lifetime of the elec-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

1414
FARUS, BALASHEV
Table 3. Luminescence characteristics of complexes [Pd(dphpz))(N^N)]+a
Pd(dphpz)
Pd (N^N)
max, nm
Compound
max, nm
,
s
[[Pd(dphpz)En]+
[[Pd(dphpz)bpy]+
[[Pd(dphpz)phen]+
[[Pd(dphpz)dppz]+
[Pd(dphpz)dicnq]+
517 sh, 623 sh, 542, 573 sh
521, 541 sh, 551, 570 sh
519, 547
190
140
140
c
516, 546b
546, b 585
440, 475 sh
c
519, 549
a
b
c
77 K, DMF toluene, 1 : 1.
Superposition of two bands.
Nonexponential kinetics of luminescence decay.
tronically excited states responsible for the lumine-
scence ( 160 30 s) typical of cyclopalladated
complexes. The whole set of the spectral and kinetic
luminescence parameters of these complexes allows
the luminescence to be considered as a spin-forbidden
radiative process of degradation of the photoexcita-
tion energy from the electronically excited state
localized on the metal complex fragment Pd(dphpz).
The low-temperature luminescence of the com-
plexes [Pd(dphpz)(N^N)]+ with the diazine derivatives
of phenanthroline has the nonexponential decay ki-
netics. The vibronically structured luminescence spec-
tra (Fig. 3) are determined by the superposition of the
vibronic transitions from two electronically excited
states. The parameters of these transitions are close to
those of the radiative transitions from the state loc-
alized on the Pd(dbpz) fragment and the diazine part
of free (N^N) ligands [10]. Although the energy
transfer to the lowest electronically excited state is
thermodynamically favorable, weak electronic inter-
action of the excited states localized on the spatially
separated diazine part of the (N^N) ligand and
Pd(dphpz) fragment leads to low efficiency of the
energy transfer between them. This effect determines
the isolated character of the energy degradation from
two electronically excited states and leads to low-
(a)
1
2
500
550
600
(b)
650
700
, nm
temperature
multiluminescence
of
complexes
1
[Pd(dphpz)(N^N)]+ with the diazine derivatives of
o-phenanthroline. Strong temperature quenching of
the luminescence, characteristic of optical transitions
localized on the Pd(dphpz) fragment as well as on the
diazine part of (N^N) ligands, leads to its absence in
liquid solutions of all the complexes at 293 K.
3
2
The whole set of the data obtained shows that the
coordination-unsaturated cyclopalladated complexes
derived from 2,3-diphenylpyrazine have the fixed and
predictable parameters determining their optical and
electrochemical properties in the ground and electron-
ically excited states. Therefore, they can be success-
fully used as photo- and electroactive components of
molecularly organized metal complex systems.
500
600
700 , nm
Fig. 3. Luminescence spectra of complexes:
(a) 1) [Pd(dphpz)En] and (2) [Pd(dphpz)phen] ;
+
+
+
(b)
(1)
[Ph(dphpz)(dppz)] ,
+
(2)
dppz,
and
(3) [Pd(dphpz)(dppz)] dppz.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007

SPECTROSCOPIC AND ELECTROCHEMICAL PROPERTIES
1415
/
EXPERIMENTAL
6.42 m (H6 ); Pd(dppz): 9.61 d (2H , 8.0), 9.24 d
(2H , 4.6), 8.42 d.d (2H , 4.6), 8.10 m (2H / ),
/
/
1
The H NMR spectra were taken on a Bruker AC-
200F spectrometer. The electronic absorption spectra
were recorded on an SF-200 spectrophotometer. The
luminescence studies were carried out on a KSVU-1
device with a pulse laser photoexcitation (LGI-21,
337 nm) at 77 K in a glassy DMF-toluene (1:1)
matrix and at 298 K in DMF solutions [12]. The
parameters of cyclic voltammograms were determined
on a modified SVA-1B installation at 298 K in a
three-electrode cell with the separated compartments
of the working (Pt), auxiliary (Pt), and reference (Ag)
electrodes in the presence of 0.1 M [N(C4H9)4]BF4 in
DMF solutions [13]. The half-wave and current peak
potential and are given vs. the ferricenium/ferrocene
redox system.
/
7.38 d.d (2H , 5.1, 8.0).
[2-(3-Phenyl-2-ido)phenylpyrazine](6,7-dicyano-
dipyrido[f,h]quinoxaline)palladium(II) hexaflu-
orophosphate. Yield 84%. 1H NMR spectrum
(DMSO-d6), , ppm (3JHH, Hz): Pd(dphpz): 8.71 s
(H5), 8.67 s (H6), 7.66 m (H3 ), 7.57 m (5Ho,m,p), 6.82
m (H4 ), 6.62 m (H5 ), 6.37 m (H6 ); Pd(dicnq): 9.69 d
/
/
/
(2H 8.0), 9.54 d (H , 5.1), 8.33 d.d (2H , 5.3,
8.0).
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1
85%. H NMR spectrum (DMSO-d6), , ppm (3JHH
,
Hz): Pd(dphpz): 8.71 s (H5), 8.45 d (H6, 3.0), 7.55 m
(5Ho,m,p), 7.08 d (H3 , 6.9), 7.01 t (H4 , 6.9), 6.74 t
(H5 , 6.9), 6.53 d (H6 , 6.9); PdEn: 5.29 s (H ), 4.46 s
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1
90%. H NMR spectrum (DMSO-d6), , ppm (3JHH
,
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7.57 m (5Ho,m,p), 6.87 m (H4 ), 6.75 m (H5 ), 6.40 m
(H6 ); Pd(bpy): 9.14 d (2H , 5.1), 8.59 d (2H
,
/
/
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8.0), 8.37 d.d (2H / , 7.3, 8.0), 7.82 d.d (2H , 5.1,
/
7.3).
[2-(3-Phenyl-2-ido)phenylpyrazine](o-phenan-
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1
76%. H NMR spectrum (DMSO-d6), , ppm (3JHH
,
12. Vasil’ev, V.V., Balashev, K.P., and Shagisultano-
Hz): Pd(dphpz): 8.71 s (H5), 8.60 s (H6), 7.74 m (H3 ),
7.56 m (5Ho,m,p), 6.98 m (H4 ), 6.67 m (H5 ), 6.43 m
(H6 ); Pd(phen): 9.35 d (2H , 5.1), 8.98 d (2H / ,
va, G.A., Opt. Spektrosk., 1983, vol. 54, no. 4, p. 876.
13. Kotlyar, V.S. and Balashev, K.P., Elektrokhimiya,
/
1996, vol. 32, no. 11, p. 1358.
/
/
8.0), 8.29 s (2H ), 8.14 d.d (2H , 5.1, 8.0).
14. Che, C.-M., Yang, M., Wong, K.-H., Chan, H.-L., and
[2-(3-Phenyl-2-ido)phenylpyrazine](dipyrido-
Lam, W., Chem. Eur. J., 1999, vol. 5, no. 11, p. 3350.
[a,c]phenazine)palladium(II) hexafluorophosphate.
15. Arounaguiri, S. and Maiya, B.G., Inorg. Chem., 1999,
1
Yield 89%. H NMR spectrum (DMSO-d6), , ppm
vol. 38, no. 5, p. 842.
(3JHH, Hz): Pd(dphpz): 8.71 s (H5), 8.67 s (H6), 7.72
16. Dickenson, J.E. and Summers, L.A., Aust. J. Chem.,
m (H3 ), 7.56 m (5Ho,m,p), 6.87 m (H5 ), 6.66 m (H5 ),
1970, vol. 23, no. 6, p. 1023.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 8 2007