A novel redox-active conjugated palladium homobimetallic complex

Article abstract of DOI:10.1002/1099-0682(200103)2001:3<651::AID-EJIC651>3.0.CO;2-6

The π-conjugated molecule N,N′-bis(4′-dimethylaminophenyl)-1,4-benzoquinone diimine (L²) was incorporated into the palladium(II) complex 1 bearing the N,N′-bis(2-phenylethyl)-2,6-pyridinedicarboxamide ligand, to afford the redox-active conjugat

Full text of DOI:10.1002/1099-0682(200103)2001:3<651::AID-EJIC651>3.0.CO;2-6

FULL PAPER
A Novel Redox-Active Conjugated Palladium Homobimetallic Complex
Toshiyuki Moriuchi,[a] Seiji Bandoh,[a] Manabu Miyaishi,[a] and Toshikazu Hirao*[a]
Keywords: Coordination modes / Molecular dynamics / N ligands / Palladium / Redox chemistry
The π-conjugated molecule N,NЈ-bis(4Ј-dimethylamino-
configuration is enthalpically more favorable than the anti
phenyl)-1,4-benzoquinone diimine (L2) was incorporated into configuration in CD2Cl2 by 1.0 kcal/mol, but entropically less
the palladium(II) complex 1 bearing the N,NЈ-bis(2-phenyl-
ethyl)-2,6-pyridinedicarboxamide ligand, to afford the redox-
active conjugated palladium(II) homobimetallic complex 2.
An X-ray crystal structure determination of 2 reveals that two
palladium complex units are bridged by the quinone diimine
moiety to form the C2-symmetrical 2:1 complex with an anti
configuration, and that the bridging π-conjugated spacer
moieties are aligned along a straight line almost parallel to
the a axis, to form the columns of the π-conjugated molecules
in the molecular packing. Variable temperature 1H NMR
favorable by 4.5 cal/mol from the van’t Hoff plot. The redox
function of the quinone diimine moiety is modulated by com-
plexation with the palladium complex 1. The conjugated
complex 2 shows three separate redox waves assignable to
the successive one-electron reduction of the quinone diimine
moiety and one-electron oxidation process of the two ter-
minal dimethylamino groups. Chemical reduction of 2 in
THF with CoCp2 resulted in the appearance of ESR signals
with weak 105Pd coupling centered around g = 2.0041. The
added electrons are considered to be delocalized over the
studies of the conjugated complex 2 indicate that the syn PdII-quinone diimine d-π* system in the complex.
Introduction
Results and Discussion
The construction of a conjugated complex system was
demonstrated by use of a redox-active π-conjugated mole-
cule, N,NЈ-bis(4Ј-dimethylaminophenyl)-1,4-benzoquinone
diimine (L2).[5] Complexation with a palladium complex
bearing a tridentate ligand, which has one open coordina-
tion site, is expected to provide the bimetallic conjugated
complex. In this context, the palladium(II) complex
[(L1)Pd(MeCN)] (1) bearing one interchangeable coordina-
tion site was prepared by treatment of Pd(OAc)2 with the
N-heterocyclic tridentate podand ligand, N,NЈ-bis(2-phenyl-
ethyl)-2,6-pyridinedicarboxamide (L1H2),[6] in acetonitrile.
The X-ray crystal structure of 1[7] indicates that the open
coordination site is occupied by an ancillary acetonitrile,
presenting the complex 1 with one labile ligand.[8] Treat-
ment of the palladium(II) complex 1 with L2 in acetonitrile
led to the formation of the 2:1 complex [(L1)Pd(L2)Pd(L1)]
(2, Scheme 1).
Quinone diimines can exist in three redox forms: the
reduced phenylenediamide dianions, the partially reduced
semiquinone diimine radical anions, and the oxidized neutral
quinone diimines. The combination of this redox behavior
and complexation with transition metals provides an effi-
cient redox system. o-Benzoquinone diimines have attracted
much attention as a redox-active compound in this con-
text.[1] However, transition metal complexes with p-benzo-
quinone diimine ligands have been investigated in only a
few cases and the coordination behavior of p-benzoquinone
diimine has hitherto remained unexplored.[2] In a previous
paper, the emeraldine base form of polyanilines as π-con-
jugated polymers, which contains a p-benzoquinone diimine
unit, has been revealed to coordinate to transition metals,
affording conjugated complex systems.[3] The polymer com-
plex can effectively serve as an oxidation catalyst,[4] in
which the quinone diimine moiety is considered to contrib-
ute to a reversible redox process in the catalytic cycle of a
transition metal. These results prompted us to investigate
the redox properties of the transition metal complexes with
p-benzoquinone diimines and to characterize the complexes
structurally. We herein report the synthesis of the palla-
dium(II) homobimetallic complex with a bridging π-con-
jugated p-benzoquinone diimine.
The structure of the conjugated complex 2 was confirmed
by X-ray crystallography. The solid-state molecular struc-
ture reveals that the two [(L1)Pd] units are bridged by the
quinone diimine moiety of L2 to form the C2-symmetrical
˚
2:1 complex anti-2 with a PdϪPd separation 8.17 A, as de-
picted in Figure 1. Crystallographic data for anti-2 can be
found in Table 1, with selected bond lengths and bond
angles listed Table 2. Each nitrogen atom of the quinone
diimine moiety of L2 is deviated from the least-squares
plane of C(37)ϪC(38)ϪC(39*)ϪC(37*)ϪC(38*)ϪC(39) by
4.7° in the same direction, probably due to coordination to
a palladium atom which is deviated from the plane by 10.7°.
One phenyl ring of the podand moiety of [(L1)Pd] is ori-
ented in a near face-to-face arrangement at a distance of ca.
[a]
Department of Applied Chemistry, Faculty of Engineering,
Osaka University, Yamada-oka, Suita,
Osaka 565Ϫ0871, Japan
1
˚
3.9 A with the phenyl ring of another [(L )Pd], suggesting a
E-mail: hirao@ap.chem.eng.osaka-u.ac.jp
weak π-stacking interaction. This interaction might cause
Eur. J. Inorg. Chem. 2001, 651Ϫ657
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ1948/01/0303Ϫ0651 $ 17.50ϩ.50/0
651

T. Moriuchi, S. Bandoh, M. Miyaishi, T. Hirao
FULL PAPER
Scheme 1
Figure 1. (a) Top view and (b) side view of the X-ray crystal structure of anti-2 (40% probability ellipsoids; hydrogen atoms are omitted
for clarity)
652
Eur. J. Inorg. Chem. 2001, 651Ϫ657

A Novel Redox-Active Conjugated Palladium Homobimetallic Complex
FULL PAPER
Table 1. Crystallographic data for anti-2
Empirical formula
Formula mass
Crystal system
Space group
C68H66N10O4Pd2
1300.13
monoclinic
C2/c (No. 15)
15.858(4)
23.955(8)
18.994(3)
112.13(2)
6683(2)
4
˚
a [A]
˚
b [A]
˚
c [A]
β [°]
3
˚
V [A ]
Z
Dcalcd. [g cmϪ3
]
1.292
µ(Mo-Kα) [cmϪ1
T [°C]
]
5.91
23
˚
λ(Mo-Kα) [A]
0.71069
0.081
R[a]
Rw
[b]
0.0112
[a]
[b]
R ϭ (|Fo| Ϫ |Fc|)/|Fo|. Ϫ Rw ϭ [w(|Fo| Ϫ |Fc|)2/w|Fo|2]1/2
.
˚
Table 2. Selected bond lengths [A] and bond angles [deg] for anti-2
Bond lengths
PdϪN(1)
1.93(2)
2.02(1)
2.03(2)
2.05(1)
1.42(2)
1.35(2)
C(37)ϪC(38)
C(37)ϪC(39)
C(38)ϪC(39*)
C(31)ϪC(36)
C(31)ϪC(32)
1.40(2)
1.45(2)
1.33(2)
1.39(2)
1.40(2)
PdϪN(2)
PdϪN(3)
PdϪN(4)
N(4)ϪC(31)
N(4)ϪC(37)
Bond angles
N(1)ϪPdϪN(2)
N(1)ϪPdϪN(3)
N(1)ϪPdϪN(4)
N(2)ϪPdϪN(3)
N(2)ϪPdϪN(4)
80.1(7)
N(3)ϪPdϪN(4)
PdϪN(4)ϪC(31)
PdϪN(4)ϪC(37)
C(31)ϪN(4)ϪC(37)
98.5(6)
116(1)
122(1)
120(1)
80.7(7)
179.0(6)
160.8(6)
100.7(6)
Figure 2. (a) Projection down the c axis and (b) projection down
the a axis of the molecular packing of anti-2
the above-mentioned deviation of the palladium atoms. Due
Variable temperature 1H NMR studies of the conjugated
to π-conjugation of L2, the orientation of each phenylene complex 2 indicated interesting molecular dynamics in solu-
ring should be within a limited range of locations parallel tion (Figure 3). As the temperature was lowered, the peaks
to the quinone diimine moiety. The steric interaction be- of the syn conformer syn-2 appeared and increased gradu-
tween the hydrogen atom at C(32) and the hydrogen atom ally. It should be noted that the conjugated complex 2 pre-
at C(38), however, causes the phenylene ring of L2 to rotate fers the anti configuration at temperatures above 220 K and
away from this orientation, resulting in a conformation with the syn configuration below this temperature. The equilib-
a dihedral angle of 50.1° between the phenylene and qui- rium constant Keq between anti-2 and syn-2 was calculated
none planes. Each phenylene ring of L2 has an opposite from variable temperature 1H NMR spectra (Scheme 2).
dihedral angle with respect to the quinone plane, causing a The temperature dependence of Keq is used to construct the
propeller twist of 79.8° between the planes of the two phen- van’t Hoff plot of lnKeq vs. T Ϫ1 (Figure 4). The syn config-
ylene rings. The least-squares plane of N(1)Ϫ uration is enthalpically more favorable than the anti config-
N(2)ϪN(3)ϪPd is at an angle of 109.7° instead of 90° from uration in CD2Cl2 by 1.0 kcal/mol, but entropically less fa-
the plane of C(31)ϪN(4)ϪC(37) as a result of the twist of vorable by 4.5 cal/mol from the van’t Hoff plot.
the π-conjugated plane of L2. Another interesting feature is
The interesting redox properties of 2 were disclosed by
that the bridging π-conjugated molecules L2 are aligned cyclic voltammetry. The conjugated complex 2 in CH2Cl2
along a straight line almost parallel to the a axis in the shows three separate redox waves (E1/2 ϭ Ϫ1.49 V, Ϫ0.85 V,
molecular packing (Figure 2a). The coordination planes of and 0.20 V vs. Fc/Fcϩ) as depicted in Figure 5. The waves at
palladium composed of the pyridyl and two amide moieties Ϫ1.49 V and Ϫ0.85 V are assigned to the successive one-
occupy nearly the bc plane (Figure 2b).
electron reduction of the quinone diimine moiety to give
Eur. J. Inorg. Chem. 2001, 651Ϫ657
653

T. Moriuchi, S. Bandoh, M. Miyaishi, T. Hirao
FULL PAPER
Ϫ1
Figure 4. Plot of lnKeq vs. T
for 2 in CD2Cl2
Figure 5. Cyclic voltammogram of 2 (1.0 ϫ 10Ϫ3 ) in CH2Cl2 (0.1
 nBu4NClO4) at a glassy carbon working electrode with scan
rate ϭ 100 mV/s under Ar
added electrons are considered to be delocalized over the
PdII-quinone diimine d-π* system. Compared with the un-
complexed one, the complexed quinone diimine becomes
stabilized as an electron sink. Accordingly, the redox prop-
1
Figure 3. Variable temperature H NMR spectrum of 2 in CD2Cl2
the corresponding reduced species. This result is in sharp erties of the quinone diimine moiety are modulated by com-
contrast to the redox behavior of L2 in CH2Cl2, in which plexation with the palladium complex 1, affording a multi-
an irreversible reduction wave is observed at Ϫ1.67 V. Gen- redox complex system. Furthermore, the most positive an-
erally, the generated radical anion appears to be unstable, odic peak with twice height (E1/2 ϭ 0.20 V) is attributable
although this depends on the availability of a proton to a one-electron oxidation process of the two terminal di-
source.[9] In the case of reduction of the complex 2, the methylamino groups.[10] A substantial positive shift of this
Scheme 2. Numbering of carbon atoms of the conjugated complex 2; the left palladium complex moiety in each isomer is moved
for labelling
654
Eur. J. Inorg. Chem. 2001, 651Ϫ657

A Novel Redox-Active Conjugated Palladium Homobimetallic Complex
FULL PAPER
Figure 6. ESR spectrum of 2•Ϫ in THF at 290 K
Figure 7. Electronic spectra of 2 (Ϫ) and 2•Ϫ (Ϫ Ϫ) (2.0 ϫ 10Ϫ5
) in THF
as illustrated in Figure 7. This broad absorption, the assign-
ment of which is not yet clear, might be a charge-transfer
transition or an internal transition with significant Pd con-
tent.
Scheme 3. Schematic representation of the redox behavior of the
conjugated complex 2
oxidation wave compared with the free quinone diimine L2
(E1/2 ϭ Ϫ0.08 V) is consistent with the coordination of L2
to palladium. The redox behavior of the conjugated com-
plex 2 is depicted schematically in Scheme 3.
Conclusion
Two palladium atoms were demonstrated to be bridged
by the quinone diimine moiety of the π-conjugated mole-
cule L2 to afford the novel redox-active conjugated homobi-
metallic complex system, in which the complexed quinone
diimine becomes stabilized as an electron sink. The syn con-
figuration of the conjugated complex 2 is enthalpically
more favorable than the anti one in CD2Cl2, but entropic-
ally less favorable. These results are related to the conjug-
ated complexes with polyanilines. Bimetallic complexes
composed of π-conjugated bridging spacers and terminal
redox-active transition metals have currently received much
attention as functional materials.[11] From this point of
view, the potential use of the present bimetallic complex
characterized by the redox-active π-conjugated bridging
spacer appears to be promising.
To gain insight into the redox properties of the conju-
gated complex 2, the redox behavior was investigated spec-
troscopically. The chemical reduction of 2 in THF with
CoCp2 resulted in the appearance of the ESR signals cent-
ered around g ϭ 2.0041, with hyperfine coupling to two
equivalent nitrogen nuclei and four equivalent protons and
weak coupling to the 105Pd nucleus (natural abundance
22.2%, I ϭ 5/2) as shown in Figure 6, indicating that the
unpaired electron is located mostly on the quinone diimine
moiety. Some delocalization onto the metal is revealed by
the weak satellite lines due to 105Pd coupling. The ESR sig-
nal could be approximately simulated by assuming the fol-
lowing parameters: AN ϭ 6.6 G; AH ϭ 1.6 G; APd ϭ 3.4 G.
The electronic spectrum of a solution of 2 in THF ex-
hibits a strong broad absorption around 806 nm (log ε ϭ
4.82), probably assignable to a low-energy charge-transfer
transition with significant Pd content (Figure 7). The gener-
ated semiquinone diimine complex 2•Ϫ could be character-
Experimental Section
General Methods: All reagents and solvents were purchased from
ized by a broad absorption around 931 nm (log ε ϭ 4.16) commercial sources and were further purified by the standard
Eur. J. Inorg. Chem. 2001, 651Ϫ657 655

T. Moriuchi, S. Bandoh, M. Miyaishi, T. Hirao
FULL PAPER
methods, if necessary. Melting points were determined on a Yanagi-
auxiliary electrode (BAS), and an Ag/AgCl (0.01 ) reference elec-
moto Micromelting Point Apparatus and were uncorrected. trode (BAS) at 100 mV/s scan rate. Potentials are given vs. Fc/Fcϩ.
Infrared spectra were obtained with a PerkinϪElmer Model 1605
X-ray Crystal Structure Determination of anti-2: A dark green crys-
tal of the conjugated palladium complex anti-2 with approximate
dimensions of 0.20 ϫ 0.20 ϫ 0.30 mm was mounted on a glass
FT-IR. 1H NMR spectra were recorded on a JEOL JNM-GSX-
400 (400 MHz) spectrometer with tetramethylsilane as an internal
standard. Mass spectra were run on a JEOL JMS-DX303HF mass
fiber. The measurement was made on a Rigaku AFC5R diffracto-
spectrometer. Electronic spectra were obtained using a Hitachi U-
meter with graphite monochromated Mo-Kα radiation and a rotat-
ing anode generator. The cell constants and an orientation matrix
3500 spectrophotometer.
N,NЈ-Bis(2-phenylethyl)-2,6-pyridinedicarboxamide (L1H2) was
prepared according to the method reported in a previous paper.[6b]
N,NЈ-Bis(4Ј-dimethylaminophenyl)-1,4-benzoquinone diimine (L2)
was prepared by the literature method.[5]
for data collection were obtained from a least-squares refinement
using the setting angles of 25 carefully centered reflections in the
range 22.81 Ͻ 2θ Ͻ 27.12°. The data were collected at a temper-
ature of 23 Ϯ 1 °C using the ω-2θ scan technique to a maximum
2θ value of 55.1°. Totals of 8202 independent reflections were ob-
tained and 7912 were unique. The structure was solved by heavy-
atom Patterson methods and expanded using Fourier techniques.
The non-hydrogen atoms were refined anisotropically. The final
cycle of full-matrix least-squares refinement was based on 2875 ob-
served reflections [I Ͼ 3σ(I)] and 379 variable parameters. R ϭ
0.081, Rw ϭ 0.112. Crystallographic details are given in Table 1.
Crystallographic data (excluding structure factors) for the structure
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication
no. CCDC-152467 for anti-2. Copies of the data can be obtained
free of charge on application to CCDC. 12 Union Road, Cam-
bridge CB2 1EZ, UK [Fax: (internat.) ϩ44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
Preparation of [(L1)Pd(MeCN)] (1): A mixture of L1H2 (18.7 mg,
0.05 mmol) and Pd(OAc)2 (11.2 mg, 0.05 mmol) in acetonitrile
(5 mL) was stirred under argon at room temperature for 2 h. After
evaporation of the solvent, the palladium complex 1 was isolated
in 96% yield (24.9 mg) as yellow crystals by recrystallization from
acetonitrile. M.p. 165Ϫ167 °C (uncorrected). Ϫ IR (KBr): ν˜ ϭ
1590, 1388 cmϪ1. Ϫ 1H NMR (400 MHz, CD3CN, 298 K): δ ϭ
2.78 (t, J ϭ 7.3 Hz, 4 H), 3.40 (t, J ϭ 7.3 Hz, 4 H), 7.19Ϫ7.33 (m,
10 H), 7.56 (d, J ϭ 7.7 Hz, 2 H), 8.01 (t, J ϭ 7.7 Hz, 1 H). Ϫ MS
(FAB): m/z ϭ 478 [(M Ϫ CH3CN)ϩ ϩ 1]. Ϫ C25H24N4O2Pd
(518.91): calcd. C 57.87, H 4.66, N 10.80; found C 57.83, H 4.75,
N 10.76.
Preparation of [(L1)Pd(L2)Pd(L1)] (2): A mixture of 1 (20.8 mg,
0.04 mmol) and L2 (6.9 mg, 0.02 mmol) was stirred in acetonitrile
(20 mL) under argon at room temperature for 2 h. After evapora-
tion of the solution, the conjugated palladium complex 2 was iso-
lated in 73% yield as dark green crystals by recrystallization from
chloroform/hexane. M.p. 227Ϫ229 °C (decomp.). Ϫ IR (KBr): ν˜ ϭ
1588, 1359, 1161 cmϪ1. Ϫ UV/Vis (THF): λmax (log ε) ϭ 806 (4.82)
Acknowledgments
We thank Professor M. Haga at Chuo University for helpful discus-
sions. We are indebted to Professor Y. Wei for the preparation of
L2. This work was financially supported in part by a Grant-in-
Aid for Scientific Research on Priority Areas from the Ministry of
Education, Science, and Culture, Japan. Thanks are also due to the
Analytical Center, Faculty of Engineering, Osaka University for
the use of their facilities.
1
nm. Ϫ H NMR (400 MHz, CD2Cl2, 220 K, anti/syn ϭ 1:1): δ ϭ
2.15Ϫ2.30 (m, 8 H, syn and anti), 2.38Ϫ2.50 (m, 8 H, syn and anti),
2.76Ϫ2.90 (m, 8 H, syn and anti), 3.00Ϫ3.10 (m, 8 H, syn and anti),
3.13 (s, 12 H, anti), 3.20 (s, 12 H, syn), 6.75Ϫ6.92 (m, 24 H, syn
and anti), 6.97Ϫ7.10 (m, 24 H, syn and anti), 7.38 (s, 2 H, syn),
7.62 (dd, J ϭ 10.3, 2.2 Hz, 2 H, anti), 7.70 (d, J ϭ 8.1 Hz, 4 H,
syn), 7.76 (d, J ϭ 8.1 Hz, 4 H, anti), 7.95 (d, J ϭ 8.8 Hz, 4 H, syn),
7.99 (d, J ϭ 8.8 Hz, 4 H, anti), 8.10 (t, J ϭ 8.1 Hz, 2 H, syn), 8.15
(t, J ϭ 8.1 Hz, 2 H, anti), 8.95 (dd, J ϭ 10.3, 2.2 Hz, 2 H, anti),
9.08 (s, 2 H, syn). Ϫ MS (FAB): m/z ϭ 1301 [Mϩ ϩ 1]. Ϫ
C68H66N10O4Pd2 (1300.18): calcd. C 62.82, H 5.12, N 10.77; found
C 62.50, H 5.11, N 10.60.
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A Novel Redox-Active Conjugated Palladium Homobimetallic Complex
FULL PAPER
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