'Double redox-activity' in azobenzene-quinonoid palladium(ii) complexes: A combined structural, electrochemical and spectroscopic study

Article abstract of DOI:10.1039/c0dt00944j

Reactions of [(az_-H)Pd(μ-Cl)₂Pd(az_-H)] (az = azobenzene) with the zwitterionic, p-benzoquinonemonoimine-type ligands 4-(n-butylamino)-6(n-butylimino)-3-oxocyclohexa-1,4-dien-1-olate (Q¹) or 4-(isopropylamino)-6(

Full text of DOI:10.1039/c0dt00944j

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PAPER
‘Double redox-activity’ in azobenzene-quinonoid palladium(II) complexes:
a combined structural, electrochemical and spectroscopic study†
Naina Deibel,a David Schweinfurth,a Ralph Huebner,a Pierre Braunstein*b and Biprajit Sarkar*a
Received 2nd August 2010, Accepted 12th October 2010
DOI: 10.1039/c0dt00944j
Reactions of [(az-H)Pd(m-Cl)2Pd(az-H)] (az = azobenzene) with the zwitterionic,
p-benzoquinonemonoimine-type ligands 4-(n-butylamino)-6(n-butylimino)-3-oxocyclohexa-1,4-
dien-1-olate (Q1) or 4-(isopropylamino)-6(isopropylimino)-3-oxocyclohexa-1,4-dien-
1-olate) (Q2) in the presence of a base leads to the formation of the mononuclear complexes
[(az-H)Pd(Q1-H)] (1) and [(az-H)Pd(Q2-H)] (2) respectively. Structural characterization of 2 shows an
almost square planar coordination geometry around the Pd(II) centre, a short Pd–C bond, a slight
elongation of the N N double bond of the az-H ligand and localization of the double bonds within the
Q2 ligand. Additionally, intermolecular N–H–O interactions exist between the uncoordinated N–H
-H
and O groups of two different molecules. Cyclic voltammetry of the complexes reveals an irreversible
oxidation and two reversible reduction processes. A combination of electrochemical and UV-vis-NIR
and EPR spectroelectrochemical studies are used to show that both coordinated ligands participate
successively in the redox processes, thus revealing their non-innocent character.
Introduction
Azobenzene (az) is an interesting molecule owing to its ability
to show photo- or redox-induced isomerism in its free as well
as metal coordinated forms.1–3 One of the first studies on C–H
activation by metal complexes was done with Pd(II) and Pt(II)
complexes of azobenzene and resulted in C, N-cyclometallation.4,5
Scheme 1 A zwitterionic quinonoid ligand.
Azo-ligands have been well established as non-innocent ligands in
coordination chemistry.6,7 Most of the metal complexes studied
in that regard contain either phenylazopyridine (pap)8–11 or 2,2¢-
azobispyridine (abpy)12,13 as ligands because of the increased
stability brought about by the presence of one or more pyri-
dine rings. The use of azobenzene as a non-innocent ligand
has been rarely seen or explored. Quinonoid compounds, in
contrast represent a class of ligands which have been recognized
for their non-innocent character.14,15 Not only are ligands such
as dioxolene16,17 and dithiolene18–20 non-innocent but also the
potentially bridging ligand 1,4-dihydroxy benzoquinone21,22 and
various substituted derivatives thereof.23–25 In this context, we have
investigated ligands of form Q (Scheme 1) which are zwitterionic,
benzoquinonemonoimine-type compounds containing two delo-
calized but mutually isolated 6p systems.26–29
These ligands have shown interesting properties in supramolec-
ular chemistry,29 homogenous catalysis,30,31 mediators for “metal–
metal coupling”32,33 and most recently as non-innocent ligands
in combination with copper.34,35 Cu(I) complexes with poten-
tially bridging quinonoid ligands are rare36 and we recently
reported on mono-34 and dinuclear complexes of such ligands.35
The para-isomers of these ligands have also been popularized
in recent years.37–40 Herein we present two metal complexes,
[(az-H)Pd(Q1-H)] (1) and [(az-H)Pd(Q2-H)] (2), which combine the cy-
clometallated form of azobenzene together with the mono depro-
tonated form of the ligands, 4-(n-butylamino)-6(n-butylimino)-
3-oxocyclohexa-1,4-dien-1-olate (Q1) and 4-(isopropylamino)-
6(isopropylimino)-3-oxocyclohexa-1,4-dien-1-olate) (Q2) respec-
aInstitut fu¨r Anorganische Chemie, Universita¨t Stuttgart, Pfaffenwaldring
55, D-70550, Stuttgart, Germany. E-mail: sarkar@iac.uni-stuttgart.de
bLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177
CNRS), Universite´ de Strasbourg, 4 rue Blaise Pascal, F-67081, Strasbourg
Cedex, France. E-mail: braunstein@unistra.fr; Fax: (+)33368851322;
Tel: (+)33368851308
1
tively. Results obtained from H-NMR spectroscopy, elemental
analyses and X-ray crystallography are used for the formulation
of the products. A combination of electrochemical and UV-vis-
NIR and EPR spectroelectrochemical methods are applied to
establish the non-innocent nature of both these ligands in the
aforementioned complexes.
† Electronic supplementary information (ESI) available: Cyclic voltam-
mogram of 1, changes in the UV-vis-NIR spectra of 2 during reduction
processes, EPR spectrum of 2∑-. CCDC reference numbers 784390. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt00944j
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Table 1 Crystallographic Details
Results and discussion
Chemical formula
Mr
Crystal system, space group
C48H52N8O4Pd2
1017.78
Monoclinic, C2/c
150
Syntheses and crystal structure of 2
The compounds 1 and 2 were synthesized by reacting [(az-H)Pd(m-
Cl)2Pd(az-H)] with the ligands Q1 or Q2, respectively, which were
deprotonated by the addition of a base prior to the addition of the
metal precursor (Scheme 2). The reactions proceeded smoothly at
room temperature and analytically pure deep brown solids were
isolated in reasonable yields after recrystallisation. The purity of
T/K
˚
a, b, c/A
23.6552(13), 25.0045(11), 17.5305(11)
b (◦)
120.109(8)
8970.0(8)
8
1.507
4160
Mo-Ka
0.855
0.32 ¥ 0.27 ¥ 0.23
35155
3
˚
V/A
Z
Density/g cm-3
F000
1
the complexes was established by using H-NMR spectroscopy
and elemental analyses.
Radiation type
m/mm-1
Crystal size/mm
meas. refl.
indep. refl.
7895
6396
0.116
0.040, 0.092, 1.035
1.63, -0.661
obsvd. [I > 2s(I)] refl.
Rint
R[F2 > 2s(F2)], wR(F2), S
-3
˚
Drmax, Drmin/e A
◦
˚
Table 2 Selected bond lengths (A) and bond angles ( ) of one of the two
molecules in the asymmetric unit of 2 and of Q2
Bond Lengths
2
Q2
Scheme 2 Synthesis of the complexes.
Pd1–O1
Pd1–C13
Pd1–N3
Pd1–N1
C1–O1
C3–O2
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C1
C6–N1
C4–N2
N3–N4
N3–C19
N4–C18
Bond Angles
O1–Pd1–N1
N1–Pd1–C13
C13–Pd1–N3
N3–Pd1–O1
N3–Pd1–N1
C13–Pd1–O1
2.058(2)
1.995(3)
2.018(3)
2.078(3)
1.286(4)
1.236(4)
1.363(5)
1.404(5)
1.519(5)
1.371(5)
1.411(5)
1.509(5)
1.317(5)
1.331(5)
1.269(4)
1.437(4)
1.398(5)
Compound 2 was crystallized by slow evaporation of a
dichloromethane solution of the complex layered with n-hexane
(1/4) at ambient temperature. It crystallises in the monoclinic
C2/c space group. An ORTEP diagram of 2 is shown in Fig. 1,
crystallographic details are presented in Table 1 and selected bond
lengths and angles in are given in Table 2.
1.251(2)
1.252(2)
1.398(2)
1.401(2)
1.533(2)
1.393(2)
1.395(2)
1.538(2)
1.321(2)
1.322(2)
78.8(1)
107.7(1)
78.5(1)
95.2(1)
172.1(1)
173.5(1)
˚
(trans influence). The Pd1–N3 distance of 2.018(3) A is shorter
˚
than the Pd1–N1 distance of 2.078(3) A. The p-acceptor character
Fig. 1 ORTEP view of 2. Ellipsoids are drawn at 30% probability.
Asymmetric unit contains two molecules. Hydrogen atoms are omitted
for clarity.
of the azo ligand results in back donation from the Pd(II) centre
and accounts for the shortening of the Pd1–N3 bond.13 Such an
effect is absent in the imino type N1 donor and hence the Pd1–N1
˚
The Pd centre is in a slightly distorted square planar envi-
ronment, being coordinated by the C- and azo N atoms of the
cyclometallated az-H ligand and by oxygen and nitrogen atoms of
the Q2-H ligand. The distortion is imposed by the chelating nature
of the two ligands and accordingly the O1–Pd1–N1 and C13–Pd1–
N3 angles are smaller (78.8(1) and 78.5(1)◦, respectively) than
the N3–Pd1–O1 and C13–Pd1–N1 angles (95.2(1) and 107.7(1)◦,
distance is longer. The N3–N4 distance of 1.269(4) A is slightly
˚
longer than a typical N N double bond distance of 1.25 A in an
azo compound. This elongation is related to back donation from
the Pd(II) centre into the p* orbital of the az-H ligand.6,13 In keeping
with this the N4–C18 distance of 1.398(5) A is shorter than the N3–
C19 distance of 1.437(4) A. Averaging out of bond distances within
the chelate ring is a typical feature of metal complexes containing
azo ligands.13 The uncoordinated phenyl ring is twisted with
respect to the rest of the molecule. The dihedral angle between the
plane of the uncoordinated phenyl ring and the mean coordination
plane of the Pd centre is 41.2◦. The phenyl ring is possibly twisted
˚
˚
˚
respectively). The Pd1–C13 distance at 1.995(3) A is relatively
short as would be expected for a s-bond between a C- atom
of a phenyl ring and a Pd(II) centre. Accordingly the Pd1–O1
˚
(2.058(2) A) distance which is trans to Pd1–C13 is relatively long
432 | Dalton Trans., 2011, 40, 431–436
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Table 3 Electrochemical dataa
due to packing effects. Analysis of the bond lengths within the
Q2-H ligand reveals that the C1–O1 distance at 1.286(4) A is longer
˚
Compound
Epa(ox)b
E
1/2(red1)
E1/2(red2)
˚
than the C3–O2 distance of 1.236(4) A. Accordingly, the C1–
˚
C2 distance of 1.363(5) A is shorter than the C2–C3 distance
Q1,c
Q2,c
1
0.91
0.89
0.58
0.80
-1.64d
-1.62d
-1.44
-1.56
-2.24d
-2.19d
-1.75
-1.85
of 1.404(5). For comparison the free ligand Q2 has distances of
1.251(2) and 1.252(2) A for C1–O1 and C3–O2 respectively and
1.398(2) and 1.401(2) A, for C1–C2 and C2–C3 respectively.34 The
˚
2
˚
a Electrochemical potentials from cyclic voltammetry in CH2Cl2/0.1 M
Bu4NPF6 at 295 K. The Fc0/Fc+ couple was used as an internal standard.
b Anodic peak potential for irreversible oxidation. c From ref. 31 d Cathodic
peak potential for irreversible reduction.
˚
C6–N1 and C4–N2 distances in 2 are 1.317(5) and 1.331(5) A,
respectively and the C5–C6 and C4–C5 distances are 1.411(5) and
˚
1.371(5) A, respectively. For comparison, relevant bond distances
within the free ligand Q2 are C6–N1, 1.321(2); C4–N2, 1.322(2);
˚
C5–C6, 1.395(2); and C4–C5, 1.393(2) A. These results show that
ligands in the complexes 1 and 2. In contrast to the oxidation
process, the complexes 1 and 2 showed a fully reversible one-
electron first reduction process at -1.44 and -1.56 V, respectively
as well as a quasi-reversible second reduction process at -1.75 and
-1.85 V, respectively (Fig. 3 and S1† and Table 3). It should be
noted here that the reduction processes for the free ligands were
not reversible.34 Azo-based ligands are known to undergo two
reversible one-electron reduction processes particularly in their
metal-complexed form, as extensively studied with ligands such
as phenylazopyridine (pap) or 2,2¢-azobispyridine (abpy).11,13 The
first reduction potential of metal-bound azo ligands is generally
at a lower negative potential and the difference between the two
azo-based reduction processes is usually about 1 V.11 Our own
studies on the ligands Q1 and Q2 and their metal complexes
have shown that such ligands are usually reduced at rather
high negative potentials owing to the +I effects of the alkyl
substituents and the difference between the two Q based reduction
processes is usually around 600 mV (Table 3).34,35 The difference
between the two reduction processes for 1 and 2 are 310 mV
and 290 mV, respectively. In view of this small difference and the
trends in the reduction potentials of az and Q ligands mentioned
above, we assign the first reduction process to the az-H ligand
resulting in the formation of species of the type [(az-H)∑-Pd(Q-H)]∑-
and the second reduction process to the Q1 or Q2 ligands
whereas the double bonds in the “upper” and “lower” parts of
the molecule are delocalized in the free ligand Q2,34 these bonds
become more localized on coordination to one metal centre.
Such metal-induced localization of the double bonds has been
observed previously for related systems.30,34 The C1–C6 and C3–C4
˚
distances of 1.509(5) and 1.519(5) A, respectively for 2 and 1.538(2)
2
˚
and 1.533(2) A, respectively for Q remain authentic single bonds
in both cases. Intermolecular interactions are observed in the solid
state which result in the formation of pseudo-dimers (Fig. 2). The
N–H and O parts of the uncoordinated side of Q2 of one of
-H
the molecules forms intermolecular hydrogen bonding with the
O and N–H part, respectively of a second molecule. The N–H
˚
˚
distance is 0.730 A and the H–O distance is 2.312 A. The N–H–
O angle is 158.3◦. Additionally, a C–H–p interaction is observed
between the C–H part of the coordinated phenyl ring of the az-H
ligand of one molecule and the central ring of the Q2 ligand of
-H
another molecule. This C–H–p interaction holds together two of
the pseudo-dimers formed through the aforementioned N–H–O
interactions (Fig. 2) thus forming an infinite layer. The C–H–p
˚
distance is 3.205 A and the angle between the C–H group and the
centroid of the Q2 ligand is 110.9◦
-H
-H
-H
Fig. 2 Packing of the molecules of 2 in crystal showing hydrogen bonding
and C–H–p interaction.
Cyclic voltammetry
Cyclic voltammetric measurements were carried out on the
complexes in order to investigate their redox properties. The
complexes 1 and 2 show irreversible oxidation processes at 0.58
and 0.80 V, respectively in CH2Cl2/0.1 M Bu4NPF6 vs. Fc0/Fc+.
The reversibility of the processes did not improve upon lowering
the temperature or varying the scan rate. Such processes with
similar behavior were also observed in the case of the free ligands
Q1 and Q2 (Table 3).34 In view of these similarities and the fact
that normally Pd(II) centres are usually resistant to reversible one-
electron oxidation at reasonable potentials, we tentatively assign
the oxidation process to the oxidation of the quinonoid ligands
in the complexes. Its irreversibility has most likely to do with
the N–H proton on the uncoordinated side of Q1 and Q2
Fig. 3 Cyclic voltammogram of 2 in CH2Cl2/0.1 M Bu4NPF6 at
295 K. Scan rate 100 mV s-1. The Fc0/Fc+ couple was used as an internal
standard.
-H
-H
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Table 4 UV-vis-NIR spectroelectrochemical dataa
max/nm (e/M-1cm-1)
transfer (ILCT, Q-H based). Confidence for the LLCT assignment
comes from the ease of oxidation of the ligands Q-H. The free
ligands Q1 and Q2 also show absorptions in a similar region.34 The
bands at 480 nm for 1 and 485 nm for 2 are assigned to a metal
to ligand charge transfer (MLCT, Pd → az-H) and those at 407 for
1 and 2 are assigned to a second MLCT (Pd → Q-H) transition.
Further higher energy bands are assigned to intra ligand charge
transfer (ILCT) transitions based on the az-H or Q-H ligands.
Changes in the absorption patterns of 1 and 2 during the reduction
processes were followed by using an optically transparent thin layer
electrochemical (OTTLE) cell. On one electron reduction to 1∑- or
2∑- the original ILCT and MLCT bands in the visible region show
a bathochromic shift and gain slightly in intensity (Fig. 4 and S2†
and Table 4). In addition, a new broad band appears in the NIR
region at 1435 and 1415 nm for 1∑- and 2∑-, respectively. On one-
electron reduction the az-H-based lowest unoccupied molecular
orbital (LUMO) of the starting complex will become the singly
occupied molecular orbital (SOMO). This new transition is then
assigned to a LLCT transition from SOMO(az-H) → LUMO(Q-H)
in the one-electron reduced complexes (Fig. 4 and Scheme 3).
These new LLCT bands are rather broad, the full width at half
height (Dn1/2) being 4075 and 4230 cm-1 for 1∑- and 2∑-, respectively.
The broadness probably originates from the reorganization in the
system that would have to take place as a result of this transition
between two different ligands. On further one-electron reduction
to 12- or 22-, the NIR band at around 1400 nm disappears and
a new band emerges at 898 or 908 nm for 12- or 22-, respectively.
These are tentatively assigned to new MLCT transitions from Pd
→ az-H. Further MLCT and ILCT transitions are seen at higher
energies.
l
1
555(1200), 480sh, 407(19100), 360(20900)
1∑-
12-
2
1435(1700), 630sh, 553(3100), 397(20200), 362(18800)
898(4600), 600sh, 487(5100), 382(15900), 337(13200)
560(820), 485sh, 407(17500), 359(20500)
1415(1650), 650sh, 557(3500), 397(19700), 360sh
908(3400), 605sh, 490(4800), 386(12500), 329(14700)
2∑-
22-
a From OTTLE spectroelectrochemistry in CH2Cl2/0.1 M Bu4NPF6.
leading to the formation of [(az-H)∑-Pd(Q-H)∑-]2-. In order to
further verify this interpretation, we carried out UV-vis-NIR and
EPR spectroelectrochemical measurements on these systems. The
oxidation as well as the reduction potentials of the two complexes
is rather similar as would be expected for similar electronic effects
of the n-butyl and isopropyl substituents.
UV-vis-NIR and EPR spectroelectrochemistry
The complexes 1 and 2 show bands in the visible as well as
ultraviolet regions in their native state in CH2Cl2/0.1 M Bu4NPF6
(Fig. 4 and S2† and Table 4). The lowest energy band at 555 nm for
1 and 560 nm for 2 is tentatively assigned to a mixture of ligand to
ligand charge transfer (LLCT, Q-H → az-H) and intra ligand charge
The in situ generated one-electron reduced species 1∑- and 2∑-
in CH2Cl2/0.1 M Bu4NPF6 were probed with EPR spectroscopy
to locate the site of electron transfer. The isotropic spectra for
1∑- and 2∑- at 295 K are centred at g values of 1.998 and 1.999,
respectively (Fig. 5 and S3†). The resolution of the spectra was not
ideal and hence only limits for the hyperfine coupling constants
can be found from simulations. The spectra could be simulated by
considering two different 14N (I = 1) couplings of around 4.5 G and
6 G. Additionally, 105Pd (I = 5/2, nat. abundance = 22.2%) satellites
Fig. 4 Changes in the UV-vis-NIR spectrum of 1 during the reduc-
tion processes. From OTTLE spectroelectrochemistry in CH2Cl2/0.1 M
Bu4NPF6.
Fig. 5 EPR spectrum of in situ generated 1∑- in CH2Cl2/0.1 M Bu4NPF6
at 295 K together with simulation.
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Scheme 3 Qualitative target orbitals for UV-vis-NIR transitions. Changes in orbital energies resulting from redox processes are not shown in this scheme.
of around 5 G can be seen at both extremities of the spectra.
Thus these data point to a generation of a species of the form
[(az-H)∑-Pd(Q-H)]∑- on one-electron reduction. Further one-electron
reduction to 12- or 22- leads to the spin-coupled diamagnetic singlet
diradical of the form [(az-H)∑-Pd(Q-H)∑-]2- as seen by its EPR silence.
The ferrocene/ferrocenium (Fc/Fc+) couple served as internal
reference. Elemental analysis was performed on a Perkin Elmer
Analyser 240.
Synthesis
[(az-H)Pd(Q1-H)], 1. A mixture of Q1 (50 mg, 0.2 mmol) and
KOt-Bu (24 mg, 0.2 mmol) in tetrahydrofuran (15 mL) was stirred
for 8 h at room temperature. The solvent was removed in vacuo. The
complex [Pd(m-Cl)(az-H)]2 (72 mg, 0.2 mmol) and dichloromethane
(15 mL) were added to the orange precipitate. The reaction mixture
was stirred for 3 h at room temperature. The colour of the solution
changed to deep brown. The solvent was removed in vacuo and
the product was extracted with n-hexane (20 mL) and filtered.
The removal of the solvent of the filtrate in vacuo afforded the
product as a brown solid. Yield: 56 mg (38%). 1H NMR (250 MHz,
CD3CN): d = 0.95 (m, 6H, n-butyl), 1.38 (m, 4H, n-butyl), 1.65 (m,
4H, n-butyl), 3.24 (q, 2H, J = 7.3 Hz, n-butyl), 3.77 (broad s, 2H, n-
butyl), 5.18 (s, 1H, N–C C–H ring proton, Q1-H), 5.43 (s, 1H, O–
Conclusion
In summary, we have reported here on the synthesis of two new
complexes 1 and 2. Structural characterization of 2 showed fea-
tures typical for metal bound az-H ligands and mono-coordinated
Q-H ligands. A combination of electrochemical, UV-vis-NIR and
EPR spectroelectrochemical studies have been used to show the
stepwise reduction of the two different ligands leading first to
a metal bound azo radical, [(az-H)∑-Pd(Q-H)]∑- and then to a
diamagnetic singlet diradical species, [(az-H)∑-Pd(Q-H)∑-]2-. These
complexes thus show double redox activity involving both coordi-
nated ligands.
C
C–H ring proton, Q1-H), 6.72 (broad s, 1H, NH), 7.37 (m, 3H,
Experimental section
azobenzene), 7.54 (m, 3H, azobenzene), 7.90 (m, 2H, azobenzene),
8.02 (m, 1H, azobenzene). Anal. Calc. for C26H30N4O2Pd: C, 58.16;
H, 5.63; N, 10.43. Found: C, 58.47; H, 5.58; N, 10.48.
General considerations
The ligands Q1 and Q2 and the precursor complex [Pd(m-Cl)(az-H)]2
were prepared according to reported procedures.4,34 All other
reagents are commercially available and were used as received.
All solvents were dried and distilled using common techniques
[(az-H)Pd(Q2-H)], 2. A mixture of Q2 (46 mg, 0.2 mmol) and
KOt-Bu (24 mg, 0.2 mmol) in tetrahydrofuran (15 mL) was stirred
for 8 h at room temperature. The solvent was removed in vacuo. The
complex [Pd(m-Cl)(az-H)]2 (72 mg, 0.2 mmol) and dichloromethane
(15 mL) were added to the yellow precipitate. The reaction mixture
was stirred for 3 h at room temperature. The colour of the solution
changed to deep brown. The solution was filtered and n-hexane
(10 mL) was added to precipitate the product. The precipitate
was filtered, washed with n-hexane and dried in vacuo. Yield:
1
unless otherwise mentioned. H NMR spectra were recorded at
250.13 MHz on a Brucker AC250 instrument. EPR spectra in
the X band were recorded with a Bruker System EMX. UV-
Vis-NIR absorption spectra were recorded on a J&M TIDAS
spectrophotometer. Cyclic voltammetry was carried out in 0.1 M
Bu4NPF6 solution using a three-electrode configuration (glassy
carbon working electrode, Pt counter electrode, Ag wire as pseu-
doreference) and PAR 273 potentiostat and function generator.
1
56 mg (55%). H NMR (250 MHz, CD3CN): d = 1.24 (s, 3H,
iso-propyl), 1.27 (s, 3H, iso-propyl), 1.51 (s, 3H, iso-propyl), 1.54
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(s, 3H, iso-propyl), 3.72 (q, 1H, 3J = 6.9 Hz, iso-propyl), 4.37 (q,
1H, 3J = 6.9 Hz, iso-propyl), 5.21 (s, 1H, N–C C–H ring proton,
Q2-H), 5.47 (s, 1H, O–C C–H ring proton, Q2-H), 6.49 (s, 1H,
NH), 7.36 (m, 2H, azobenzene), 7.54 (m, 4H, azobenzene), 7.91
(m, 2H, azobenzene), 8.02 (m, 1H, azobenzene). Anal. Calc. for
C24H26N4O2Pd: C, 56.64; H, 5.15; N, 11.01. Found: C, 56.58; H,
5.40; N, 11.20.
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Single crystals of 2 were grown by slow evaporation of a 1 : 4
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crystal X-ray diffractometer. The structures were solved and
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SHELX-97 program.41 The absorption correction was done by
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28 Q. Z. Yang, O. Siri and P. Braunstein, Chem. Commun., 2005, 2660.
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BS is indebted to the Baden–Wu¨rttemberg Stiftung for financial
support of this work through the Elite Program for Postdocs. Denis
Bubrin is kindly acknowledged for help with crystal structure
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32 F. A. Cotton, J.-Y. Jin, Z. Li, C. A. Murillo and J. H. Reibenspies,
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