Cyclopalladated ethylenediamine complexes on the basis of 4-phenylpyrimidine and 4,6-diphenylpyrimidine

Article abstract of DOI:10.1134/S1070363208060236

Mono-and binuclear cyclopalladated complexes [Pd(ppm)En]ClO₄, [Pd(Hdphpm)En]ClO₄, and [(PdEn)₂(μ-dphpm)]ClO ₄ (ppm^- is the deprotonated form of 4-phenylpyridine, Hdphpm^- and dphpm^2- are the mono-and bisdeprotonated forms of 4,6-diphenylpyrimidine, En is 1,2-diaminoethane) were prepared and characterized by means of ¹H NMR and electronic absorption and emission spectroscopy, and cyclic voltammetry. It was shown that cyclopalladation leads to a bathochromic shift of intraligand absorption and phosphorescence bands, appearance of new absorption band associated with metal-to-ligand charge transfer, and an anodic potential shift of the ligand-centered electroreduction Hppm ≈ H₂dphpm < [Pd(ppm)En]⁺ ≈ [Pd(Hdphpm)En]⁺ < [(PdEn) ₂(μ-dphpm)]²⁺.

Full text of DOI:10.1134/S1070363208060236

ISSN 1070-3632, Russian Journal of General Chemistry, 2008, Vol. 78, No. 6, pp. 1236–1240. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © E.V. Ivanova, M.V. Puzyk, K.P. Balashev, 2008, published in Zhurnal Obshchei Khimii, 2008, Vol. 78, No. 6, pp. 1008–1012.
Cyclopalladated Ethylenediamine Complexes on the Basis
of 4-Phenylpyrimidine and 4,6-Diphenylpyrimidine
E. V. Ivanova, M. V. Puzyk, and K. P. Balashev
Hertzen Russian State Pedagogical University,
nab. r. Moiki 48, St. Petersburg, 191186 Russia
e-mail: balashevk@mail.ru
Received December 17, 2007
Abstract—Mono- and binuclear cyclopalladated complexes [Pd(ppm)En]ClO4, [Pd(Hdphpm)En]ClO4, and
[(PdEn)2(μ-dphpm)]ClO4 (ppm– is the deprotonated form of 4-phenylpyridine, Hdphpm– and dphpm2– are the
mono- and bisdeprotonated forms of 4,6-diphenylpyrimidine, En is 1,2-diaminoethane) were prepared and
characterized by means of 1H NMR and electronic absorption and emission spectroscopy, and cyclic
voltammetry. It was shown that cyclopalladation leads to a bathochromic shift of intraligand absorption and
phosphorescence bands, appearance of new absorption band associated with metal-to-ligand charge transfer,
and an anodic potential shift of the ligand-centered electroreduction Hppm ≈ H2dphpm < [Pd(ppm)En]+ ≈
[Pd(Hdphpm)En]+ < [(PdEn)2(μ-dphpm)]2+.
DOI: 10.1134/S1070363208060236
Cyclopalladated complexes with heterocyclic
ligands are promising compounds for creation of
photoactive molecularly organized metal complex
systems and devices on their basis [1, 2]. Of special
interest are polydentate bridging ligands capable of
forming polynuclear complexes. Contrary to the
previously studied complexes of 2,3-diphenyl-sub-
stituted 1,4-diazines, where steric interactions of phe-
nyl rings predetermines preferential formation of
mononuclear cyclopalladated complexes [3-5], using
4,6-diphenylpyrimidine one can expect formation not
only of mono-, but also of binuclear complexes.
The present work deals with the synthesis,
identification, and comparative spectroscopic and
electrochemical study of mono- and biscyclopalladated
complexes on the basis of 4-phenylpyrimidine and 4,6-
diphenylpyrimidine, [Pd(ppm)En]ClO4, [Pd(Hdphpm)·
En]ClO4, and [(PdEn)2(μ-dphpm)](ClO4)2 (see
scheme).
4'
5'
p
m
3'
6'
o
m
6
5
6'
5'
3'
o
5
6'
5'
3'
5
6'
5'
3'
4'
H2N
N
α
'
Pd
4'
4'
N
N
C
N
β
2
Pd
N
C
N
C
NH
2
2
'
2α
β
NH2
'
H2N
Pd
Pd
α
α
'
H2N
NH2
'
H2N
NH2
'
α
α
β
α
'
α
'
β
β
β
β
β
[(PdEn)2(μ-dphpm)]2+
[Pd(ppm)En]+
[Pd(Hdphpm)En]+
Mono- and biscyclopalladated complexes were
obtained by the reaction of phenyl-substituted
pyrimidine ligands with one or two equiv of Li2
[PdCl4], followed by substitutionof the bridging
chloride ligands in [Pd(ppm)(μ-Cl)]2, [Pd(Hdphpm)(μ-
Cl)]2 and Pd(μ-dphpm)(μ-Cl)]n by ethylenediamine.
1236

CYCLOPALLADATED ETHYLENEDIAMINE COMPLEXES
1237
Table 1. Coordination-induced chemical shifts (Δδ = δc – δl,
ppm) of cyclopalladated complexes in CD3CN
The cationic complexes obtained are stable in the solid
state and in various solvents (CH3CN, DMSO, DMF).
[Pd(ppm)En]+ [Pd(Hdphpm)En]+ [(PdEn)2(μ-dphpm)]2+
The 1H NMR spectra of the complexes confirm the
structure and coordination mode of the ligands. The
coordination-induced chemical shifts of ligand protons
(Δδ = δc – δl) reflect the trend in variation of the
chemical shifts of the latter due to donor–acceptor
interactions with the metal (Table 1) and show that, in
response to the σ-donor effect of the latter, complex
formation produces a significant and practically con-
stant deshielding of ethylenediamine protons [Δδ(Hα) =
(3.01 0.06), Δδ(H)β = (2.37 0.08) ppm] both in
mono- and in binuclear complexes. The different trans
effects of the carbanionic and pyrimidine parts of the
cyclometallated ligands are responsible for the
magnetic nonequivalence of the ethylenediamine
protons Hα and Hβ. The monocyclopalladated complex
[Pd(Hdphpm)En]ClO4 complex contains a free and a
coordinated phenyl substituents in 4,6-diphenyl-
pyrimidine; in agreement with this, the proton
chemical shifts in the former ring change
insignificantly (Δδ < 0.04 ppm), whereas those in the
latter ring change like in [Ph(ppm)En]ClO4. The H2
protons of the pyrimidine part of the cyclometallated
ligands in [Pd(ppm)En]ClO4 and [Pd(Hdphpm)En]·
ClO4, which are closest to palladium, too, undergo
deshielding [Δδ(H2) –0.41 ppm].
H
2
5
6
–0.41
0.04
–0.06
–0.52
–0.32
–0.34
–0.31
–
–0.41
–0.06
–
–0.90
–0.24
–
3'
–0.52
–0.3
–0.3
–0.32
0.02
0.04
0.04
0.04
0.02
3.01
2.45
0.34
–0.52
–0.24
–0.24
–0.36
–
4'
5'
6'
2''
3''
4''
5''
6''
α
–
–0.52
–0.24
–0.24
–0.36
3.06
2.37
0.37
–
–
–
2.95
2.29
0.48
β
γ
electron transfer on the π* orbitals mainly localized on
the pyrimidine part of the ligand. Cyclopalladation
induces a regular anodic shift of the electroreduction
potential in the series: Hppm ≈ H2dphpm < [Pd(ppm)·
En]+ ≈ [Pd(Hdphpm)En]+ < [(PdEn)2(μ-dphpm)]2+.
Unlike Hppm, H2dphpm, and [Pd(ppm)En]+, which
undergo irreversible electroreduction, the reduction
voltammograms of [Pd(Hdphpm)En]+ and [(PdEn)2(μ-
dphpm)]2+ are reversible (Fig. 1). Consequently, the
one-electron reduction products of these two
complexes are relatively stable. The anodic shift of the
half-wave reduction potential (E1/2) of dimeric com-
plexes as compared to monomeric shows that the
energy of the LUMO localized mainly on the
pyrimidine part of the cylcometallated ligand in these
complexes decreases by 0.33 eV.
The binuclear symmetric structure of [(PdEn)2(μ-
dphpm)](ClO4)2 predetermines magnetic equivalence
both of two coordinated phenyl rings of 4,6-
diphenylpyrimidine and of two ethylenediamine
ligands; therewith, the coordination-induced chemical
shifts of protons are close to those in the mononuclear
complexes [Pd(ppm)En]ClO4 and [Pd(Hdphpm)En]·
ClO4. At the same time, biscyclopalladation much
enhances proton shielding in the pyrimidine part of the
cyclopalladated ligand [Δδ(H2) –0.90 and Δδ (H5)
–0.24 ppm].
The irreversible waves of electrooxidation of
complexes are attributed to the metal-centered
character of the process, which is explained by a high
rate of subsequent chemical reactions of primary
electrooxidation products: highly reactive Pd(III)
complexes.
The localized molecular orbitals model [7] treats
electrochemical and optical properties of mixed-ligand
complexes as metal- and ligands centers, taking into
account preferred localization of participating orbitals.
Therewith, provided the Koopmans theorem [7] is
fulfilled, the orbital nature of redox and spectroscopic
LUMOs and HOMOs of complexes is similar.
Comparison of the electronic absorption spectra of
free ligands and cyclopalladated complexes shows
(Fig. 2) that, along with the batochromic shift of
intraligand π–π* optical transitions in phenyl-
substituted pyrimidine ligands, cyclopalladation gives
rise to a new long-wave spin-allowed absorption band
(Table 2) corresponding to the optical transition
The voltammograms of one-electron electro-
reduction of cyclopalladated complexes and
heterocyclic ligands (Table 2) give evidence showing
that the process is ligand-centered and involves
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 6 2008

1238
IVANOVA et al.
ε ×103 l mol–1 cm–1
1
35
30
25
20
15
10
5
2
1
2
–2
–1.5
–1
–0.5
0
E, V
0
35
250
300
350
400
450
Fig. 1. Reduction voltammograms: (1) [Pd(Hdphpm)En]
ClO4 and (2) [(PdEn)2(μ-dphpm)](ClO4)2.
λ, nm
30
25
20
15
10
5
3
between the metal-centered HOMO (dPd) and ligand-
centered LUMO(π*) of complexes (d–π* charge
transfer). In agreement with the electrochemical
characteristics, the mononuclear [Pd(Hphpm)En]+ and
[Pd(ppm)En]+ complexes have practically constant
parameters of the new absorption band (λ ~354 nm, ε
~5 × 103 mol–1 cm–1), whereas the formation of the bi-
nuclear [(Pd(Hdphpm)En]2+ complex induces a bato-
chromic shift and enhancement of the band (λ 389 nm,
ε 11 × 103 mol–1 cm–1). The energy of the HOMO–
LUMO optical transition in [(PdEn)2(μ-dphpm)]2+ in
lower by 0.31 eV than in [Pd(Hdphpm)En]+, which
agrees well with the decrease of 0.33 eV for the
LUMO energy, established by electrochemical data.
4
5
0
250
300
350
400
450
λ, nm
Fig. 2. Electronic absorption spectra: (1) Hppm, (2) [Pd
(ppm)En]ClO4, (3) H2dphpm, (4) [Pd(Hdphpm)En]ClO4,
and (5) [(PdEn)2 (μ-dphpm)]ClO4)2.
the typical fluorescence extinguishment time (τ~10–4 s)
and the relatively small bathochromic shift of the
luminescence spectra of mononuclear complexes as
compared to free pyrimidine ligands [Δλ (27 4) nm].
Contrary to a 0.31-eV decrease in the energy of the
spin-allowed transition between the metal-centered
HOMO and ligand-centered LUMO in [(PdEn)2(μ-
dphpm)2+ as compared to [Pd(Hdphpm)En]+, the
The vibrationally structured (ν 1.4 kK) low tem-
perature (77 K) luminescence spectra of cyclo-
palladated complexes (Fig. 3) are attributed to spin-
forbidden intraligand optical transitions between
LUMO and HOMO, localized mainly on the
cyclometallated ligand. This is confirmed (Table 2) by
Table 2. Optical and electrochemical parameters of cyclopalladated complexes
Absorptionа
Luminescenceb
Voltammogram
Compound
λmax, nm (ε × 103, mol–1 cm–1)
λmax, nm (τ, μs)
–E1/2red, Vc Ер, Vd
Hppm
215 (18), 253 (24), 271 (34)
434, 465, 491, 512
423, 452, 478, 508 sh
2.40e
2.45e
2.06e
1.85
–
H2dphpm
248 (33), 284 (26), 300 sh (21.6)
–
[Pd(ppm)En]+
[Pd(Hdphpm)En]+
[(PdEn)2(μ-dphpm)]2+
222 (21), 265 (14), 304 sh (6.3), 312 (6.4), 353 (3.8) 457, 489, 517, 551 sh, 613 (140)
0.31
0.15
0.05
223 (19.6), 227 sh (18), 327 (13), 355 (6)
454, 490, 515, 550 sh (70)
220 (23.6), 292 (19), 326 (9.5), 343 (9.1), 389 (11)
478, 512, 542 (43)
1.52
a
Acetonitrile, 293 K. b DMF–toluene 1 : 1, 77 K. c DMF, 293 K. d .Acetonitrile, 293 K. e Peak current potential at the sweep rate 100 mV s–1.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 78 No. 6 2008

CYCLOPALLADATED ETHYLENEDIAMINE COMPLEXES
1239
energy of spin-forbidden transitions responsible for
12
10
8
phosphorescence decreases by only 0.14 eV. The
difference in the orbital nature of the low-energy spin-
allowed (d–π*) and spin-forbidden (π–π*) optical
transitions responsible for the low-wave absorption
and phosphorescence of cyclopalladated complexes is
explained by the fact that the singlet–triplet splitting
energy of intraligand (π–π*) transitions is larger
compared to (d–π*) charge-transfer transitions [8].
1
2
6
4
2
The absence of luminescence of cyclopalladated
complexes in solutions at room temperature suggests
an effective thermally-activated process nonradiative
degradation of photoexcitation energy, probably
involving population of a high-energy metal-centered
state of the 3(d–d*) type.
0
450
500
550
600
650
16
14
12
10
8
λ, nm
4
3
5
EXPERIMENTAL
6
4
1
The H NMR spectra were taken on a Bruker AC-
2
200F spectrometer. The electronic absorption spectra
were obtained on an SF-2001 spectrophotometer at
298 K. The luminescence parameters were measured at
77K in frozen glassy 1:1 DMF–toluene solutions on a
KSVU-1 device with pulse photoexcitation [LGI-21
nitrogen laser (λ 337 nm, τ 10 ns) ] [11]. The oxidation
and reduction voltammograms were obtained at 293 K
in DMF and acetonitrile solutions on a computerized
SVA-1B device in a three-electrode cell with separated
working (Pt), auxiliary (C), and reference (Ag)
electrode compartments in the presence of 0.1 M [N
(C4H9)4]ClO4 at the potential sweep rate 100 mV s–1,
according to the procedure in [12]. The potentials are
presented against the ferrocenium–ferrocene redox
system.
0
450
500
550
600
650
λ, nm
Fig. 3. Luminescence spectra: (1) Hppm, (2) [Pd(ppm)En]·
ClO4, (3) H2dphpm, (4) [Pd(Hdphpm)En](ClO4)2, and
(5) [(PdEn)2(μ-dphpm)](ClO4)2.
[(4-Phenyl-3-ido)pyrimidin](ethylenediamine)-
palladium(II) perchlorate. Yield 60%. 1H NMR
spectrum (CD3CN), δ, ppm (J, Hz): 8.84 d (H5, JHH
3
5.5), 8.82 s (H2), 7.85 d (H4, JHH 5.5), 7.78 m (H6'),
1
7.26 t.d (H4', 3JHH 7.3, 4JHH 1.4), 7.24 t.d (H5', 3JHH 7.3,
4JHH 1.4), 7.06 m (H3'), 4.10 s (2Hα), 3.44 s (2Hβ), 3.05
s (4Hγ).
[(4-Phenyl-3-ido)(6-phenyl)pyrimidin](ethylenedi-
amine)palladium(II) perchlorate. Yield 40%. 1H
NMR spectrum (DMSO-d6), δ, ppm (J, Hz): 8.85 s
4,6-Diphenylpyrimidine was prepared by the
reaction of dibenzoylmethane with formamide
according to the procedure in [9]. Yield 35%, 1H NMR
(H2), 8.32 s (H5), 8.31 m (2Ho), 7.98 d (H6', JHH 4.4),
3
3
spectrum, δ, ppm (J, Hz): 9.26 d (H2, JHH 1.5), 8.38 s
7.28 m (2H4',5'), 7.08 m (H3'), 4.16 s (2Hα), 3.50 s
(2Hβ), 2.91 s (4Hγ).
(H5), 8.30 d (4Ho, 3JHH 6.5), 7.60 m (6Ho,m).
Cyclopalladated complexes were prepared by a
common procedure, involving reaction of a methanolic
suspension of prepared [Pd(ppm)(μ-Cl)]2, [Pd·
(Hdphpm)(μ-Cl)]2, and [Pd(μ-dphpm)(μ-Cl)]n [10]
with an equivalent amount of ethylenediamine. The
solution obtained was filtered, and the resulting
complex was precipitated by treatment with saturated
methanolic NaClO4. The precipitate was filtered off,
washed with methanol and ether, and dried in a
vacuum.
[(μ-4,6-Diphenyl-3,3'-ido)pyrimidin](bisethylenedi-
1
amine)dipalladium(II) perchlorate. Yield 80%. H
NMR spectrum (CD3CN), δ, ppm (J, Hz): 8.36 s (H2),
8.14 s (H5), 7.93 d (2H6', 3JHH 8.7), 7.34 m (4H4',5'), 7.08 d
(H3', 3JHH 8.7), 4.16 s (4Hα), 3.50 s (4Hβ), 2.91 s (8Hγ).
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Fundation for Basic Research.
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1240
IVANOVA et al.
6. De Armond, M. and Carlin, C., Coord. Chem. Rev.,
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