A new approach to novel homobimetallic palladium complexes

Article abstract of DOI:10.1016/j.inoche.2004.05.023

Mononuclear trans-(PPh₃)₂Pd(I)(C₆H ₄-4-I) (3), which is accessible by the oxidative addition of C ₆H₄-1,4-I₂ (2) to (Ph₃P) ₄Pd (1), reacts with [AgOTf] (4) in a 1:1 molar ratio to give trans-(PPh₃)₂Pd(C₆H₄-4-I)(OTf) (5). Complex 5 affords on treatment with 0.5 equivalents of 4,4′- dicyanobiphenyl (6) homobimetallic trans-[(PPh₃)₂(C ₆H₄-4-I)Pd ← N≡C-C₆H ₄]₂(OTf)₂ (7). The latter species rearranges to give trans-[(PPh₃)₂(C₆H₄-4-PPh ₃)Pd ← N≡C-C₆H₄]₂(OTf) ₄ (8) and [(PPh₃)(C₆H₄-4-PPh ₃)Pd(μ-I)]₂(OTf)₂ (9), respectively. A possible mechanism for the formation of 8 and 9 will be presented. The solid-state structure of 8 is reported. In 8 two [trans-(PPh₃)(C ₆H₄-4-PPh₃)Pd]²⁺ fragments are bridged by the 4,4′-dicyanobiphenyl unit.

Full text of DOI:10.1016/j.inoche.2004.05.023

Inorganic Chemistry Communications 7 (2004) 1006–1009
www.elsevier.com/locate/inoche
A new approach to novel homobimetallic palladium complexes
*
D. Taher, B. Walfort, H. Lang
Institut fu¨r Chemie, Fakulta¨t fu¨r Naturwissenschaften, Lehrstuhl Anorganische Chemie, Technische Universita¨t Chemnitz,
Straße der Nationen 62, D-09111 Chemnitz, Germany
Received 29 April 2004; accepted 20 May 2004
Available online 10 August 2004
Abstract
Mononuclear trans-(PPh3)2Pd(I)(C6H4-4-I) (3), which is accessible by the oxidative addition of C6H4-1,4-I2 (2) to (Ph3P)4Pd (1),
reacts with [AgOTf] (4) in a 1:1 molar ratio to give trans-(PPh3)2Pd(C6H4-4-I)(OTf) (5). Complex 5 affords on treatment with 0.5
equivalents of 4,40-dicyanobiphenyl (6) homobimetallic trans-[(PPh3)2(C6H4-4-I)Pd N„CAC6H4]2(OTf)2 (7). The latter species
rearranges to give trans-[(PPh3)2(C6H4-4-PPh3)Pd N„CAC6H4]2(OTf)4 (8) and [(PPh3)(C6H4-4-PPh3)Pd(l-I)]2(OTf)2 (9), respec-
tively. A possible mechanism for the formation of 8 and 9 will be presented. The solid-state structure of 8 is reported. In 8 two [trans-
(PPh3)(C6H4-4-PPh3)Pd]2+ fragments are bridged by the 4,40-dicyanobiphenyl unit.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: Palladium; Oxidative addition; Phosphane; Homobimetallic; 4,40-dicyanobiphenyl; X-ray structure
Recently, homo- and heterobimetallic complexes in
which the transition metal atoms are spanned by p-
conjugated electron-transferring/-transmitting organic
and/or inorganic building blocks, possessing metals of
groups 8–10, were synthesized in order to use them
as model compounds to study, for example, intra-
and/or intermolecular electron-transfer [1–5]. The elec-
tronic communication between the participating transi-
tion metals proceeds via the p-conjugated bridging
units.
Monolayer assemblies provide excellent model sys-
tems for the study of interfacial processes. For example,
electron-transfer benefits from the orderliness provided
by self-assembling monolayers (SAMs) [6]. Self-assem-
bled monolayers of redox-active (thiol-substituted) tran-
sition metal complexes/materials onto gold and other
surfaces have been the topic of recent research [6a–b,
7], since the application of such systems and modified
surfaces ranges from fundamental studies, concerning
the structure of the layers and the thermodynamics
and kinetics of adsorption, to applications, including
sensors as well as molecular and electronic devices and
catalysis [8]. It was found that, in the case of the metal
complexes, they retain their redox-active responses at
potentials very close to those of non-adsorbed analogs
in homogeneous solutions [6a]. Although, the immobili-
zation of transition metal complexes onto electrode sur-
faces has been partly studied.
In this respect, it would be of interest to study the
interfacial electronic transfer between two surfaces
which are bridged by homobimetallic organometallic
species [9,10]. An entry into the area of molecular elec-
tronics would be given.
We here describe the synthesis of a rigid-rod struc-
tured homobimetallic palladium complex in which the
transition metals are linked by a p-conjugated organic
unit.
*
Corresponding author. Tel.: +49 371 531 1200; fax: +49 371 531
1833.
E-mail address: heinrich.lang@chemie.tu-chemnitz.de (H. Lang).
1387-7003/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2004.05.023

D. Taher et al. / Inorganic Chemistry Communications 7 (2004) 1006–1009
1007
A possibility to prepare a molecular wire type mole-
cule is given by, for example, the oxidative addition of
C6H4-1,4-I2 to Pd(PPh3)4, treatment of the obtained
trans-(PPh3)2Pd(I)(C6H4-4-I) species with [AgOTf] and
replacement of the weakly bound TfOꢀ in trans-
(PPh3)2Pd(C6H4-4-I)(OTf) by stronger Lewis-bases,
such as dicyano p-conjugated organic species. In this re-
spect, we reacted Pd(PPh3)4 (1) with equimolar
amounts of diiodobenzene (2) in toluene at 25 °C
(Eq. (1))
crystals of 8 are produced as the major product (Eq.
1
(3)). Concentration of the supernatant solution gives
crystals of 9 (Eq. (3))
2+
PPh3
2 OTf -
Ph3P
I
I
hν, 12 h
7
Pd
Pd
CH Cl / hexane
2
2
PPh3
Ph3P
9
4+
4 OTf -
PPh3
Pd
PPh3
Pd
PPh3
Ph3P
N
C
C
N
PPh3
+
o
25 C, 3 h
PPh3
PPh3
+
I
I
I
Pd
I
Pd(PPh3)4
8
-2 PPh
PPh3
3
ð3Þ
The formation of 8 and 9 can most probably be ex-
1
2
3
ð1Þ
plained as follows. Complex 7 decomposes in solution
by precipitation of palladium and formation of iodo-
benzene, triphenylphosphane and 4,40-dicyanobiphenyl
as detected by GC–MS studies. Presumably the Lewis-
base PPh3 undergoes a nucleophilic substitution reac-
tion at the activated iodo-carbon atom of 7 to give 8.
The thus formed iodide can attack the square-planar
coordinated palladium(II) ion in 8 to give 9.
Monometallic trans-(PPh3)2Pd(C6H4-I)(I) (3) precipi-
tates during the course of the reaction. Complex 3 is ob-
tained in 92% yield and in high purity as a colorless
solid.
Treatment of 3 with [AgOTf] in a 1:1 molar ratio in
dichloromethane/toluene mixtures led to the in situ
formation of the corresponding palladium-triflate com-
plex 5 (Eq. (2)). On addition of 0.5 equivalents of
4,40-dicyanobiphenyl (6) to 5 homobimetallic trans-
[(PPh3)2(C6H4-4-I)Pd N„CAC6H4]2(OTf)2 (7) is
produced (Eq. (2)). In this reaction the OTfꢀ is replaced
by the better p-acid 6
Complexes 3 and 7–9 were fully characterized by
elemental analysis, IR and NMR (1H, 31P{1H})
spectroscopy.
In the IR spectra of 7–9 the triflate counter-ion gives
rise to one very characteristic absorption at 1265 cmꢀ1
(mSO), which is consistent with the non-coordinating
character of TfOꢀ. The C„N stretching vibration of
6, 7 and 8 is a most informative spectroscopic tool for
monitoring the progress of the reaction of 3 with 6
and the rearrangement of 7–9 (vide supra). The mC„N
band of 6 is thereby shifted to higher wavenumbers (7,
2260 cmꢀ1; 9, 2280 cmꢀ1) as compared to that of non-
coordinated 6 (mC„N = 2228 cmꢀ1). This indicates the
dative bonding of the C„N groups to a palladium(II)
ion. This observation resembles the data found for other
PPh3
[AgOTf] (4)
I
Pd
I
o
25 C, 30 min
PPh3
-AgI
3
PPh3
(6)
C N
0.5N C
0.5
I
Pd OTf
PPh3
o
25 C, 1 h
5
2+
2 OTf -
PPh3
Pd
PPh3
I
N
C
C
N
Pd
I
PPh3
PPh3
1
Synthesis of 8. Compound 7 is dissolved in 5 ml of dichloro-
7
methane. 30 ml of toluene are added as second layer. The two layers
are mixed overnight whereby 8 crystallizes. The solvents are removed
by filtration and the crystals are dried in oil-pump vacuum. Complex 8
is obtained in form of colorless crystals (0.08 g, 0.031 mmol, 27% based
on 7).
ð2Þ
Complex 7 can be precipitated from the reaction
medium by slow addition of n-hexane. An off-white,
air-, moisture- and temperature-sensitive residue is
formed. While solid 7 is stable in the dark for weeks,
it appeared that on exposure to light it decomposes dur-
ing hours to unknown materials.
In an attempt to grow single crystals of 7 from satu-
rated dichloromethane solutions at 25 °C, we observed
that the color of the solution changes to orange and pal-
ladium precipitates. After filtration through a pad of
Celite and addition of n-hexane to the filtrate colorless
M.p.: 146 °C (dec.). Anal. Found: C, 58.69; H, 3.87; N, 0.92;
138H106F12N2O12P6Pd2S4 Æ CH2Cl2 (3027.24): Calc.: C, 59.11; H,
C
3.85; N, 0.99%. IR (KBr): 3065 (m), 2907 (s), 2255 (m), 1553 (m),
1432 (s), 1269 (vs), 1024 (s), 1154 (s), 1030 (s), 801 (s), 796 (s), 695 (s)
cmꢀ1. 1H NMR (250.123 MHz, 25 °C, CDCl3): d = 6.34 (d, 2H, J = 7.5
4
Hz, JHP = 3.7 Hz, meta-H/C6H4PPh3), 6.64 (d, 2H, J = 7.3 Hz, meta-
H/C6H4CN), 6.73 (d, 2H, J = 7.5 Hz, ortho-H/C6H4PPh3), 7.26–7.68
(m, 45H, PPh3), 7.70 (d, 2H, J = 7.3 Hz, ortho-H/C6H4CN). 13C{1H}
NMR (62.860 MHz, 25 °C, CDCl3): d = 117.0 (CN), 127.9–137 (Ph,
C6H4). 31P{1H} NMR (101.202 MHz, 25 °C, CDCl3): d = 21.86 (s,
PPh3), 21.86 (s, C6H4PPh3+).

1008
D. Taher et al. / Inorganic Chemistry Communications 7 (2004) 1006–1009
Fig. 1. XP drawing of 8 at 50% probability level (the hydrogen atoms, the two toluene molecules and the four TfOꢀ ions are omitted for clarity).
˚
Symmetry generated atoms are indicated with the suffix A. Selected interatomic distances (A): Pd1–N1 2.082(5), Pd1–P1 2.334(2), Pd1–P2 2.338(2),
Pd1–C44 1.993(5), N1–C43 1.142(8), C62–P3 1.802(8); selected angles (°): P1–Pd1–P2 173.03(6), N1–Pd1–C44 173.5(3), Pd1–N1–C43 174.1(7), N1–
C43–C37 176.1(9).
transition metal complexes, i.e., {[(dppp)Pd][C6H4-1,4-
(CN)2]2[OTf]2}4 (2280 cmꢀ1) [11].
Acknowledgements
1
The H NMR spectra of 3 and 7–9 show sharp and
We are grateful to Degussa AG/Hanau for a gener-
ous gift of PdCl2.
well-resolved resonance signals for the organic units
C6H4I, C6H4PPh3 and C6H4–C6H4.
In the 31P{1H}NMR spectra of 3 and 7–9 one (3, 7)
or two (8, 9) resonance signals can be detected at ca.
21 ppm for the PPh3 groups datively bound to palla-
dium and at ca. 27 ppm for the C6H4PPhþ3 units.
Single crystals of 8 suitable for X-ray structure anal-
ysis could be obtained by diffusion of toluene into a
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