Reversible switching of the coordination modes of a pyridine-functionalized quinonoid zwitterion; Its Di- and tetranuclear palladium complexes

Article abstract of DOI:10.1021/ic500194y

The coordination chemistry of a new functional quinonoid zwitterion (E)-3-oxo-4-((2-(pyridin-2-yl)ethyl)amino)-6-((2-(pyridin-2-yl)ethyl)iminio) cyclohexa-1,4-dienolate (2, H₂L), in which a CH₂CH ₂ spacer connects the N substituents of the quinonoid core with a pyridine group, was explored in Pd(II) chemistry. Different coordination modes have been observed, depending on the experimental conditions and the reagents. The reaction of H₂L with [Pd(μ-Cl)(dmba)]₂ (dmba = o-C₆H₄CH₂NMe₂-C,N) afforded the dinuclear complex [{PdCl(dmba)}₂(H₂L)] (3) in which H ₂L acts as a N_Py,N_Py bidentate ligand. Deprotonation of this complex with NaH resulted in the formation of the dinuclear complex [{Pd(dmba)}₂(μ-L)] (4) in which a shift of the Pd(II) centers from the N_Py sites to the N,O donor sites of the zwitterion core has occurred, resulting in a N₂O₂ tetradentate behavior of ligand L. Reaction of 4 with HCl regenerates 3 quantitatively. Chloride abstraction from 3 with AgOTf (OTf = trifluoromethanesulfonate) resulted in loss of one of the two dmba ligands and formation of an unusual tetranuclear Pd(II) complex, [{Pd(dmba)}(μ-L)Pd] ₂(OTf)₂ (5), in which two dinuclear entities have dimerized, one pyridine donor group from each dimer forming a bridge with the other dinuclear entity. This results in a N₂, O_2, N _Py, N_Py hexadentate behavior for the ligand L. Complexes 3 and 4 constitute an unprecedented reversible, switchable system where deprotonation or protonation promotes the reversible migration of the [Pd(dmba)]⁺ moieties, from the N_Py sites in 3, to the N,O donor sites of the quinonoid core in 4, respectively. This switch modifies the extent of φ-delocalization involving the potentially antiaromatic quinonoid moiety and is accompanied by a significant color change, from red in 3 to green in 4. The presence of uncoordinated pyridine donor groups in 4 allowed the use of this complex for the preparation of the neutral tetranuclear complex [{Pd(dmba)}₂(μ-L){PdCl(dmba)}₂] (6) in which 4 acts as a N_Py,N_Py-bidentate metalloligand toward two PdCl(dmba) moieties. Halide abstraction from 6 afforded the monocationic, tetranuclear complex [{Pd(dmba)}₂(μ-L){Pd(dmba)}₂(μ-Cl)]PF ₆ (7) in which the two Pd(dmba) moieties are connected by ligand L and a bridging chloride. By Cl/PF₆ anion metathesis, it was possible to switch quantitatively from complex 6 to 7 and vice versa. All new compounds were unambiguously characterized by IR, NMR, and mass spectroscopy. Single-crystal X-ray diffraction is also available for molecules 2-5 and 7.

Full text of DOI:10.1021/ic500194y

Article
pubs.acs.org/IC
Reversible Switching of the Coordination Modes of a Pyridine-
Functionalized Quinonoid Zwitterion; Its Di- and Tetranuclear
Palladium Complexes†
Alessio Ghisolfi, Audrey Waldvogel, Lucie Routaboul, and Pierre Braunstein*
Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite
F-67081 Strasbourg Cedex, France
́
de Strasbourg, 4 rue Blaise Pascal,
S
* Supporting Information
ABSTRACT: The coordination chemistry of a new functional
quinonoid zwitterion (E)-3-oxo-4-((2-(pyridin-2-yl)ethyl)-
amino)-6-((2-(pyridin-2-yl)ethyl)iminio)cyclohexa-1,4-dieno-
late (2, H2L), in which a CH2CH2 spacer connects the N
substituents of the quinonoid core with a pyridine group, was
explored in Pd(II) chemistry. Different coordination modes
have been observed, depending on the experimental
conditions and the reagents. The reaction of H2L with
[Pd(μ-Cl)(dmba)]2 (dmba = o-C6H4CH2NMe2-C,N) afforded the dinuclear complex [{PdCl(dmba)}2(H2L)] (3) in which H2L
acts as a NPy,NPy bidentate ligand. Deprotonation of this complex with NaH resulted in the formation of the dinuclear complex
[{Pd(dmba)}2(μ-L)] (4) in which a shift of the Pd(II) centers from the NPy sites to the N,O donor sites of the zwitterion core
has occurred, resulting in a N2O2 tetradentate behavior of ligand L. Reaction of 4 with HCl regenerates 3 quantitatively. Chloride
abstraction from 3 with AgOTf (OTf = trifluoromethanesulfonate) resulted in loss of one of the two dmba ligands and formation
of an unusual tetranuclear Pd(II) complex, [{Pd(dmba)}(μ-L)Pd]2(OTf)2 (5), in which two dinuclear entities have dimerized,
one pyridine donor group from each dimer forming a bridge with the other dinuclear entity. This results in a N2, O2, NPy, NPy
hexadentate behavior for the ligand L. Complexes 3 and 4 constitute an unprecedented reversible, switchable system where
deprotonation or protonation promotes the reversible migration of the [Pd(dmba)]+ moieties, from the NPy sites in 3, to the
N,O donor sites of the quinonoid core in 4, respectively. This switch modifies the extent of π-delocalization involving the
potentially antiaromatic quinonoid moiety and is accompanied by a significant color change, from red in 3 to green in 4. The
presence of uncoordinated pyridine donor groups in 4 allowed the use of this complex for the preparation of the neutral
tetranuclear complex [{Pd(dmba)}2(μ-L){PdCl(dmba)}2] (6) in which 4 acts as a NPy,NPy-bidentate metalloligand toward two
PdCl(dmba) moieties. Halide abstraction from 6 afforded the monocationic, tetranuclear complex [{Pd(dmba)}2(μ-
L){Pd(dmba)}2(μ-Cl)]PF6 (7) in which the two Pd(dmba) moieties are connected by ligand L and a bridging chloride. By
Cl/PF6 anion metathesis, it was possible to switch quantitatively from complex 6 to 7 and vice versa. All new compounds were
unambiguously characterized by IR, NMR, and mass spectroscopy. Single-crystal X-ray diffraction is also available for molecules
2−5 and 7.
Scheme 1. Transamination Reactions from the “Parent”
Zwitterion 1
INTRODUCTION
■
Quinonoid molecules have long been of special interest in
coordination chemistry as O-donor and/or π-donor ligands.1
Additional functionalities can be incorporated into these
molecules, which enhance and diversify their coordination
properties. This has become particularly notable with
quinonoid molecules possessing N-donor groups of the
enamino- or imino-type in ortho-position to the O-donor
atoms. During the last 10 years, our group has investigated the
properties of a versatile family of potentially antiaromatic2
zwitterionic benzoquinonemonoimines derived from the
“parent” zwitterion 4-(amino)-6-(iminio)-3-oxocyclohexa-1,4-
dien-1-olate (1) (Scheme 1).3 These molecules contain two
electronically delocalized but not mutually conjugated six-π-
electron systems, chemically connected by two C−C single
bonds.2 They readily form one-dimensional (1D) supra-
molecular associations in the solid state by virtue of NH···O
intermolecular interactions.4 Because of their ability as donor
ligands to easily involve their delocalized electronic π systems
with metal centers, significant synergistic effects may be
anticipated in their metal complexes. Thus, such zwitterions
have been used for the synthesis of functional, mixed-valence5
and redox-active multinuclear complexes,6 of homogeneous
Received: January 25, 2014
Published: May 12, 2014
© 2014 American Chemical Society
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precatalysts for the oligomerization of ethylene,6b,7 and of
complexes with potential applications in optical recording.8
These organic zwitterions can also be deposited on various
metal or semiconductor surfaces and on graphene, giving rise to
the occurrence of interesting physical properties.9 Although
unprecedented in quinonoid chemistry, a simple transamina-
tion reaction allows the functionalization of 1 without affecting
the quinonoid core and thus allows the fine-tuning or
amplification of its chemical properties (Scheme 1).4a,10 Here
we describe our first results on the coordination chemistry of
the new polyfunctional zwitterionic quinonoid, potentially
hexadentate N2, O2, NPy, NPy ligand, bis(2-(pyridin-2-yl)ethyl)
N-substituted zwitterion 2 (H2L) (Scheme 2). We will show
Figure 1. Crystalmaker view of 2 (H2L) in 2·CH2Cl2. Thermal
ellipsoids are drawn at the 50% probability level. Only the NH protons
are shown.
Scheme 2. Bis(2-(pyridin-2-yl)ethyl) N-substituted
Zwitterion (2, H2L)
of 1.530(5) Å correspond to typical single bonds. In the solid
state, the N1H−O2 and N3H−O1 moieties are involved in
intermolecular hydrogen bonding interactions (N1···O2 =
2.862(4) Å, N3···O1 = 2.923(4) Å), resulting in a head-to-tail,
zigzaglike 1D supramolecular network (Figure 2) in which a
stabilizing π−π stacking between the quinonoid rings of two
different chains occurs, with a C1···C4 separation of 3.437(5)
Å. If ligand 2 reacted with metal centers solely as a bis-pyridine-
type NPy,NPy ligand, the zwitterionic core would not be affected
and would retain its strong dipolar nature. This could have a
beneficial effect in subsequent reactivity studies since it has
been shown recently that the addition of an ionic liquid-type
zwitterion to a palladium complex improved its catalytic
performance in ethylene methoxycarbonylation.19
When 2 was treated in CH2Cl2 with 1 equiv of [Pd(μ-
Cl)(dmba)]2 (dmba = o-C6H4CH2NMe2-C,N), the neutral
dinuclear complex [{PdCl(dmba)}2(H2L)] (3) was obtained
quantitatively (yield: 92%) (Scheme 3). The pyridine donors
induce the cleavage of the chloride bridges in the dinuclear
metal precursor and coordinate the two PdCl(dmba) moieties
in a slightly distorted, square-planar fashion. Only the pyridine
moieties are involved in metal coordination, and they adopt a
cis arrangement and a perpendicular orientation with respect to
the dmba aryl ring, as shown by an X-ray diffraction study of
single crystals of 3 (Figure 3). Selected bond lengths are listed
in Table 1 (for crystal data and selected bond angles see
Supporting Information, Tables S1 and S2, respectively). The
coordination of the Pd centers by the pyridines leads to Pd1−
N2 and Pd2−N4 distances of 2.036(7) and 2.038(7) Å,
respectively. As anticipated, the bonding parameters within the
quinonoid core, in particular the C−O, C−C, and C−N
distances, remain unaffected in comparison with 2 (Table 1).
Complex 3 presents, in the solid state, the same head-to-tail,
zigzag, supramolecular arrangement already displayed in the
free zwitterion, owing to the formation of hydrogen bonding
intermolecular interactions between the N1H−O2 and the
N3H−O1 moieties (see Figure 4). Their distances of 2.783(9)
Å for N1H···O2 and 2.830(9) Å for N3H···O1 are slightly
shorter than they are in 2. The resulting linear array is
decorated with dangling PdCl(dmba) moieties coordinated by
the pyridines, which prevent the formation of the π−π stacking
interaction observed in 2. Fourier transform-far-infrared (FT-
far-IR) analysis of 3 showed a typical ν(Pd−Cl) stretching
vibration at 296 cm−1.20
how the interactions between this multidentate ligand and
Pd(II) centers can be fine-tuned, by changing the experimental
conditions, to result in an unexpected reversible switching of
the donor sites. Coordination switching can be triggered, for
example, by redox processes,11 deprotonation and/or halide
abstraction,12 reaction with the solvent,13 and thermal14 or
photochemical15 activation. Because of the ease with which the
coordination environment of the Pd(II) center can be modified,
our observations may have relevance to in situ transformations
occurring, for example, during Pd-catalyzed cross-coupling
reactions16 and surface functionalization.17
RESULTS AND DISCUSSION
■
The new functional zwitterion 2 was obtained as a brown solid,
in good yield (78%), from the reaction of the “parent”
zwitterion 1 with 2 equiv of 2-(pyridin-2-yl)ethanamine in
refluxing ethanol, following an established transamination
procedure.4a,10 Like its precursor, this zwitterion presents a
planar, potentially antiaromatic core,2 constituted by a 12-π-
electron system divided into two delocalized 6-π-electron
subunits, mutually connected by two C−C single bonds.2 Each
nitrogen group is functionalized by a dangling ortho-pyridine,
and the CH2CH2 spacer between them offers a much better
flexibility and solubility when compared to the derivative with a
single CH2 spacer.18 The latter was indeed found to be only
poorly soluble in polar solvents (e.g., MeOH, DMSO, water)
and afforded insoluble complexes not suitable for our purpose.
The molecular structure of 2·CH2Cl2 was determined by X-ray
crystallography (Figure 1).
Compound 2 crystallizes in the orthorhombic space group
Pna21, and the molecule presents a symmetry axis passing
through the carbon atoms C1 and C4. Selected bond lengths
are listed in Table 1 (for crystal data and selected bond angles,
see Supporting Information, Tables S1 and S2, respectively).
The structure of 2 confirms its zwitterionic character, with a
fully delocalized π system within the O1−C3−C4−C5−O2 and
N1−C2−C1−C6−N3 moieties: the C...O, C...N, and C...C bond
lengths are very similar, and the C2−C3 and C5−C6 distances
Solution NMR analyses of 3 revealed a temperature-
dependent dynamic behavior. In particular, room-temperature
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Table 1. Selected Bond Distances (Å) in 2−5 and 7
2·CH2Cl2
3
4
5
7·CH2Cl2
O1−C3
C3−C4
C4−C5
C2−O5
C2−C3
C5−C6
N1−C2
C1−C2
C1−C6
N3−C6
Pd1−N4
Pd1−Cl1
Pd1−N5
Pd1−C21
Pd1−N3
Pd1−O2
Pd1−N2
Pd2−N2
Pd2−Cl2
Pd2−C30
Pd2−N6
N1−Pd2
O1−Pd2
Pd3−N4
Pd3−C39
Pd3−N7
Pd3−Cl1
N2−Pd4
Pd4−C48
Pd4−N8
Pd4−Cl1
1.250(4)
1.391(4)
1.395(4)
1.248(4)
1.530(5)
1.530(5)
1.309(4)
1.394(4)
1.400(4)
1.315(4)
1.230(1)
1.390(1)
1.390(1)
1.260(1)
1.560(1)
1.510(1)
1.270(1)
1.410(1)
1.390(1)
1.310(1)
2.038(7)
2.431(3)
2.078(7)
1.870(1)
1.273(5)
1.384(5)
1.382(5)
1.273(4)
1.509(6)
1.502(6)
1.322(5)
1.400(5)
1.393(5)
1.332(5)
1.268(8)
1.414(9)
1.363(9)
1.307(8)
1.498(9)
1.497(9)
1.325(8)
1.417(9)
1.405(9)
1.320(8)
2.038(6)
1.268(8)
1.360(1)
1.390(1)
1.278(9)
1.520(1)
1.540(1)
1.341(9)
1.400(1)
1.390(1)
1.314(9)
2.067(3)
1.979(4)
2.027(3)
2.103(3)
2.081(6)
1.981(7)
2.033(6)
2.115(4)
1.969(6)
1.998(5)
2.033(5)
2.036(7)
2.412(3)
1.970(1)
2.068(7)
1.995(4)
2.067(3)
2.049(3)
2.101(3)
1.972(9)
2.074(6)
2.026(6)
2.142(5)
1.991(9)
2.083(6)
2.031(6)
2.147(6)
2.033(1)
1.964(1)
2.064(1)
2.380(2)
2.030(1)
1.978(9)
2.068(1)
2.435(2)
Figure 2. Crystalmaker views of the supramolecular array generated by 2 in 2·CH2Cl2, in the solid state. (A) Top view. (B) Interchain π−π stacking
view. (C) Side view. Color coding: nitrogen, blue; oxygen, red; hydrogen, pink.
rotating frame nuclear Overhauser effect spectroscopy
(ROESY) (see Supporting Information, Figure S8) indicated
a chemical exchange between two conformers involving the
hydrogen atoms on each CH2 group of the CH2CH2 spacers
linking the quinonoid nitrogen atoms and the pyridyl rings.
This interconversion was slowed down by lowering the
temperature and suppressed at 263 K (see Supporting
conformers. Therefore, 13C, 1H/13C heteronuclear single
quantum correlation (HSQC), and 1H correlation spectroscopy
(COSY) spectra were also recorded at this temperature and
allowed the complete discrimination of the signals belonging to
each conformer (named 3a and 3b in the Experimental
Section) (see Supporting Information, Figures S1−S7). Both
conformers show the typical chemical shifts of the zwitterionic
quinonoid core7b and a signal for the ortho H7 hydrogen of the
dmba phenyl (see Experimental Section, Scheme 9) at 5.61
1
Information, Figure S9), where integration of the H NMR
signals at this temperature indicated a 1:1 ratio between the two
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nonequivalent orientations of the metal centers are made
possible by the high flexibility of the CH2CH2 spacers, resulting
from easy rotation about their C−N and C−C bonds.
Addition of 2 equiv of NaH to a solution of complex 3 in
CH2Cl2 led to deprotonation and loss of the chloride ligands,
with formation of the neutral complex [{Pd(dmba)}2(μ-L)]
(4) (Scheme 5). We shall see below that this transformation is
fully reversible.
Scheme 3. Synthesis of the Dipalladium Complex 3
The green complex 4 was isolated in good yield (72%), and
crystals suitable for X-ray diffraction were obtained by
stratification of a toluene solution of the complex with n-
pentane. Selected bond lengths are listed in Table 1 (for crystal
data and selected bond angles see Supporting Information,
Tables S1 and S2, respectively). Complex 4 crystallizes in the
Pbca orthorhombic space group, and the molecule possesses a
C2 symmetry axis passing through C1 and C4. The structure of
4 depicted in Figure 5 shows that the deprotonation of 3 has
formally resulted in the migration of the two cationic
[Pd(dmba)]+ moieties from the pyridine to the quinonoid
site, which now acts as a bis(N,O)-chelating, bridging ligand
connected to two free dangling pyridines. As will be shown
below, these latter can act as donors for additional metal
centers.
Both metal centers in 4 display a distorted square planar
coordination geometry. The values of the bond distances within
the O1−C3−C4−C5−O2 and N1−C2−C1−C6−N3 moieties
indicate notable bond equalization, which is a clear indication
that the electronic delocalization within these π systems results
in bond orders intermediate between one and two. The C2−C3
and C5−C6 distances of 1.509(6) and 1.502(6) Å, respectively,
confirm the lack of electronic conjugation between the two π
Figure 3. Crystalmaker view of 3. Thermal ellipsoids are drawn at the
50% probability level. Only the NH protons are shown.
ppm. Such a low value is typical for a dmba phenyl ring cis and
orthogonal to a pyridine ring.21 An equilibrium between
possible conformers 3a and 3b is shown in Scheme 4, where
1
systems. The pattern of the H and 13C NMR signals of 4 is
consistent with the high molecular symmetry found in the solid
Figure 4. Crystalmaker view of the supramolecular array generated by 3 in the solid state. (A) Top view. (B) Side view. Color coding: nitrogen, blue;
oxygen, red; hydrogen, pink; palladium, magenta; chlorine, green.
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a
Scheme 4. Two Possible Conformers, 3a and 3b, Present in Solution
a
Conformer 3a is taken arbitrarily as corresponding to the structure of the complex in the solid state.
Interestingly, the deprotonation-induced conversion of 3 in 4
is reversible, and reaction of the latter complex with 2 equiv of
DMF·HCl23 allowed quantitative recovery of complex 3
(Scheme 5). Complexes 3 and 4 thus define an unprecedented
switchable system, in which deprotonation or protonation of
the complex allows a reversible change of the donor atoms and
of the position of the metal centers on the functional ligand
(Scheme 5). Accordingly, the spatial extension of the π
delocalization involving the quinonoid ring is modified.
Preliminary cyclic voltammetric measurements were carried
out in CH2Cl2 on complex 4, but unfortunately most processes
were irreversible, and the sample underwent decomposition
during analysis (see Supporting Information, Figures S10−
S14), even upon increasing the scan rate.
The discovery of this tunable metal migration prompted us
to investigate the reactivity of 3 toward simple halide
abstraction (without deprotonation). We thus reacted complex
3 with 2 equiv of AgOTf in MeOH. Rapid precipitation of AgCl
occurred with formation of a red intermediate that was not
isolated and is likely to be a solvento species where MeOH
coordinates the metal in place of Cl. The color of the solution
progressively changed from red to green, and the dicationic
complex [{Pd(dmba)}(μ-L)Pd]2(OTf)2 (5) was isolated as a
green solid in 68% yield (Scheme 6).
Scheme 5. Deprotonation−Reprotonation-Triggered
Transformations Involving 3 and 4 Leading to a Reversible,
Switchable System
state. The chemical shifts of the N...C and O...C carbon nuclei at
188.30 and 164.87 ppm, respectively, support the presence of
the delocalized π systems.7b All the data are consistent with the
fact that deprotonation of 3 has led to a new neutral dinuclear
complex 4 in which a direct interaction occurs between the Pd
centers and the quinonoid core. This results in an extension of
the two delocalized π systems that now involve the quinonoid
moiety and the metal centers. The associated modification in
the electronic structure is accompanied by a color change from
red, for 3, to green, for 4.7b The ultraviolet−visible (UV−vis)
spectra of complexes 3 and 4 are shown in Figure 6.
Compound 3 displays a broad absorption band at 518 nm,
which is assigned to intraquinone transitions of the zwitterionic
part.5c The spectrum of complex 4 exhibits an absorption band
and a shoulder at 430 and 454 nm, respectively, consistent with
a ligand-to-metal charge transfer transition (LMCT), and a
broad absorption band at 628 nm, assigned to intraquinone
transitions.5c The pronounced red shift of the intraquinone
transition absorptions between 3 (518 nm) and 4 (628 nm) is
consistent with an increase of the delocalization of the
conjugated π system.22
Green crystals of 5 suitable for X-ray diffraction analysis were
obtained by stratification of an acetone solution of the complex
with n-pentane. The structure of 5 is shown in Figure 7, and
selected bond lengths are listed in Table 1 (for crystal data and
selected bond angles see Supporting Information, Tables S1
and S2, respectively). Both metal centers present a distorted
square-planar coordination geometry. The structural parame-
ters within the quinonoid core are still consistent with the
presence of two, nonconjugated, delocalized π systems, and the
two Pd centers are involved in the electronic delocalization.
1
This is also confirmed by H and 13C NMR spectroscopy, in
Scheme 6. Synthesis of the Tetranuclear Pd(II) Complex 5
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Scheme 7. Synthesis of the Tetranuclear Pd(II) Complexes 6 and 7
Scheme 8. Overview of the Synthetic Transformations Involving Ligand 2
particular by the 13C chemical shifts of the O2...C5 and O1...C3
carbons at 189.63 and 189.86 ppm, respectively, and by the
signals of the C6...N2 and C2...N1 carbons at 165.94 and 165.04
ppm, respectively.7b The green color of the complex is very
similar to that of 4 and appears typical for the electronic
delocalization involving two metal centers and the quinonoid
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core acts again as a bis(N,O)-chelating, bridging ligand for two
Pd centers, which then complete their coordination spheres in
two different ways. While Pd2 retains its chelating dmba ligand
and displays a coordination environment similar to that in
complex 4, Pd1 has lost its dmba chelate and completes its
coordination sphere with two inequivalent pyridines: one
originates from the ligand itself (intramolecular Py2), while the
other is shared by the other half of the dimer (intermolecular
Py1) and becomes part of a bridge between the two dinuclear
moieties. The resulting double positive charge is balanced by
two triflate anions.
Scheme 9. Atom Numbering Scheme Used for NMR
Description
a
Because of its two uncoordinated pyridines, complex 4 could
still react with metal centers and act as a pyridinic NPy,NPy-
bidentate metalloligand.25 Thus, the reaction of 4 with [Pd(μ-
Cl)(dmba)]2 in a L/Pd ratio of 1:2 afforded the neutral
tetranuclear complex [{Pd(dmba)}2(μ-L){PdCl(dmba)}2] (6)
(Scheme 7) in 71% yield. NMR data of 6 in solution suggested
that this reaction did not affect the quinonoid part with respect
to the precursor metalloligand 4, while the two pyridines
coordinate the Pd centers in a similar way to what is observed
in complex 3. In particular, 6 displayed only one conformer
constituted by two inequivalent pyridinic arms, each bearing a
PdCl(dmba) moiety. The chemical shift of the ortho-H of the
dmba phenyl (H7, see Experimental Section and Scheme 9) at
5.73 ppm confirmed the cis and orthogonal orientation of the
pyridine rings with respect to the dmba phenyls (see above).
Furthermore, in the far-IR spectrum, the ν(Pd−Cl) stretching
vibration of 299 cm−1 is consistent with the value found in
complex 3 (295 cm−1).
a
In case of complex 5, the two non-equivalent pyridines (Py1 and
Py2) are named as in Scheme 6.
The reaction of 6 with 1 equiv of TlPF6 resulted in the
abstraction of one chloride ligand and the formation of the
monocationic tetranuclear complex [{Pd(dmba)}2(μ-L){Pd-
(dmba)}2(μ-Cl)]PF6 (7) (Scheme 7) in 83% yield.
Figure 5. Crystalmaker view of 4 (H atoms not shown). Thermal
ellipsoids are drawn at the 50% probability level.
As established by an X-ray diffraction on single crystals
obtained by layering a CH2Cl2 solution of the complex with
pentane, the assembling ligand L in 7 acts as a N2, O2, NPy, NPy
hexadentate donor where the dinuclear quinonoid moiety of
the precursor complex 4 remains unaffected, while the two
pyridines are involved in coordination to two additional metal
centers from the cationic [(dmba)Pd(μ-Cl)Pd(dmba)]+ moiety
(Figure 8). The resulting positive charge is balanced by one
−
PF6 anion. All the metal centers present a distorted square
planar coordination geometry. Selected bond lengths for 7 are
listed in Table 1 (for crystal data and selected bond angles see
Supporting Information, Tables S1 and S2, respectively). As
confirmed by the X-ray diffraction analysis, the two pyridine
rings are orthogonal and cis to the dmba aryls, as found for
complex 3 (see above). This is consistent with the NMR data in
solution (δ of H7 (ortho H of the dmba phenyl) = 5.35 ppm,
see Experimental Section and Scheme 9). The average Pd−(μ-
Cl) distance of 2.407 Å is similar to values found in the
literature for a related complex (average 2.443 Å).26 The
presence of a bridging μ-Cl ligand between the Pd centers
results in a shift of the ν(Pd−Cl) stretching vibration, from 299
cm−1 for complex 6 to 246 cm−1 for 7. The latter value is typical
for a ν[Pd(μ-Cl)] vibration involving a chloride ligand in trans
position to a σ-bonded carbon, and this absorption disappears
upon LiBr metathesis.27 Furthermore, as depicted in Scheme 7,
it was possible via Cl/PF6 anion exchange to switch
quantitatively from complex 6 to 7 and vice versa. Complex 7
could also be obtained (in lower yield: 57%) by the one-pot
reaction of 4 with [Pd(μ-Cl)(dmba)]2 and TlPF6 in 1:1:1 ratio.
By analogy with the formation of 7, we reacted complex 3 with
Figure 6. UV−vis absorption spectra of compounds 3 (c = 1.1 × 10−3
M) and 4 (c = 1.1 × 10−5 M) in CH2Cl2 at room temperature.
core, as observed previously with Ni(II), Pd(II), and Pt(II)
dinuclear complexes.5a,7b,24
This new complex results from an interesting transformation
of 3 upon chloride abstraction: despite the lack of an external
base, spontaneous deprotonation occurred by elimination of
one equivalent of (H2dmba)OTf, which resulted in the
formation of a dicationic, tetranuclear dimeric complex, formed
by two dipalladium moieties mutually connected by two
pyridine arms. Within each dinuclear moiety, the quinonoid
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Figure 7. Views of the crystal structure of 5. The OTf− anions were omitted for clarity. (A) and (C) Perspective views. (B) Simplified perspective
model, dmba ligands omitted for clarity. (D) Dinuclear fragment constitutive of 5, thermal ellipsoids are drawn at the 50% probability level. Color
coding: nitrogen, blue; oxygen, red; palladium, magenta.
(complex 4) or N2, O2, NPy, NPy hexadentate (complexes 5, 6,
and 7) ligand, respectively.
Complexes 3 and 4 constitute a novel reversible switchable
system, triggered by protonation or deprotonation, which
allows the selection of the chemoselective binding of the
cationic moiety [Pd(dmba)]+ on the ligand and, consequently,
the modification of the extent of π delocalization in the
quinonoid π system. Simple dehalogenation of 3 led to a
spontaneous deprotonation of the complex by elimination of 1
equiv of (H2dmba)OTf and formation of the dicationic,
dimeric, tetranuclear quinonoid complex 5, where the ligand
acts as N2, O2, NPy, NPy hexadentate. The presence of
uncoordinated pyridine donor groups in 4 opens the possibility
of using this complex as a metalloligand, and this was
demonstrated by the isolation of the neutral tetranuclear
complex 6 and the monocationic, tetranuclear complex 7, in
which complex 4 act as a NPy, NPy pyridinic bidentate
−
Figure 8. Crystalmaker view of 7 in 7·CH2Cl2. The PF6 anion is
metalloligand and, respectively, coordinates two PdCl(dmba)
omitted for clarity. Thermal ellipsoids are drawn at the 50% probability
fragments (6) and chelates the [(dmba)Pd(μ-Cl)Pd(dmba)]+
level.
moiety (7). Furthermore, complexes 3 and 4 represent
potential candidates for deposition and anchoring on surfaces,
and this will be examined in future work.
only 1 equiv AgOTf in CH2Cl2 or MeOH. Although a reaction
took place (1H and IR monitoring), we could not characterize
the species formed, and far-IR data indicated the presence of
terminal chloride(s).
EXPERIMENTAL SECTION
General Procedure. All operations were carried out by using
standard Schlenk techniques under an inert atmosphere. Solvents were
■
purified and dried under a nitrogen atmosphere by using conventional
methods. CD2Cl2 and CDCl3 were dried over 4 Å molecular sieves,
CONCLUSIONS
■
degassed by performing freeze−pump−thaw cycles, and stored under
The synthesis and coordination chemistry of the new
zwitterionic, multidentate ligand 2 (H2L) have further
demonstrated the tunability of this class of quinonoid-type
ligands. Its reactivity with various Pd(II) complexes has led to
di- and tetranuclear complexes, and the corresponding
transformations are summarized in Scheme 8. Modulation of
the ligand denticity and the chemoselectivity of its coordination
led to various situations in which 2 acts as a NPy, NPy bidentate
ligand, as in complex 3, and L acts as a N2O2 tetradentate
1
an argon atmosphere. H and 13C NMR spectra were recorded at
room temperature, unless specified, on Bruker AVANCE 400, 500, or
600 spectrometers and referenced to the residual solvent resonance.
Assignments are based on 1H, 1H COSY, 1H/13C HSQC, ROESY, and
13C NMR experiments. Chemical shifts (δ) are given in ppm, coupling
constants are in Hz, and the atom numbering used is shown in Scheme
9. IR spectra were recorded within the region of 4000−100 cm−1 on a
Nicolet 6700 FTIR spectrometer (ATR mode, SMART ORBIT
accessory, Diamond crystal). Elemental analysis was performed by the
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“Service de Microanalyses,” Universite
́
de Strasbourg, or by the
NCH2CH2Py, 3a+3b), 4.08−3.96 (m, 8H, NCH2Ph, 3a+3b), 2.97,
2.95, and 2.93 (s, 12H, 6H, 6H, NCH3, 3a+3b) ppm. 13C{1H} NMR
(CDCl3, 150.9 MHz, 263 K) δ: 172.79 (s, C...O, 3a), 172.75 (s, C...O,
3b), 158.51 (s, C1Py, 3a), 158.49 (s, C1Py, 3b), 156.76 (s, N...C, 3a),
156.72 (s, N...C, 3b), 153.29 (s, C5Py, 3a), 153.25 (s, C5Py, 3b),
147.63 (s, C6Ph, 3a), 147.61 (s, C6Ph, 3b), 146.64 (s, C11Ph, 3a+3b),
138.40 (s, C3Py, 3a), 138.38 (s, C3Py, 3b), 131.27 (s, C7Ph, 3a),
131.23 (s, C7Ph, 3b), 127.01 (s, C2Py, 3a), 126.95 (s, C2Py, 3b),
125.63 (s, C9Ph, 3a+3b), 124.97 (s, C8Ph, 3a+3b), 123.75 (s, C4Py,
3a), 123.73 (s, C4Py, 3b), 122.07 (s, C10Ph, 3a+3b), 99.14 (s, O...C...C,
3a), 99.03 (s, O...C...C, 3b), 82.68 (s, N...C...C, 3a), 82.64 (s, N...C...C,
3b), 73.93 (s, NCH2Ph, 3a), 73.92 (s, NCH2Ph, 3b), 52.93 and 52.79
(s, N(CH3)2, 3a), 52.91 and 52.76 (s, N(CH3)2, 3a), 42.20 (s,
NCH2CH2Py, 3a), 42.10 (s, NCH2CH2Py, 3b), 40.75 (s,
NCH2CH2Py, 3a), 40.69 (s, NCH2CH2Py, 3b) ppm. MS (ESI): m/z
= 865.13 [M − Cl]+.
“Service Central d’Analyse,” USR-59/CNRS, Solaize. Electrospray
mass spectroscopy was performed on a microTOF (Bruker Daltonics,
Bremen, Germany) instrument by using a flow of nitrogen gas as a
drying agent and nebulizing gas. Matrix-assisted laser desorption-
ionization time-of-flight mass spectrometry (MALDI-TOF-MS)
spectra were acquired on a TOF mass spectrometer (MALDI-TOF-
TOF Autoflex II TOF-TOF, Bruker Daltonics, Bremen, Germany)
equipped with a nitrogen laser (λ = 337 nm). An external multipoint
calibration was carried out before each measurement. Scan
accumulation and data processing were performed with FlexAnalysis
3.0 software. Matrix solutions were freshly prepared: α-cyano-4-
hydroxycinnamic acid (CHCA) was dissolved to saturation in a H2O/
CH3CN/HCOOH (50/50, 1%) solution, and Dithranol was dissolved
in tetrahydrofuran (THF) to obtain a 20 mg/mL solution. Typically,
0.5 μL of a mixture containing the sample solution and the matrix (1/
1) was deposited on the stainless steel plate. The UV−vis spectra were
recorded on an Analytic Jena Specord 205 spectrophotometer, using
optically transparent glass cells. The complex [Pd(μ-Cl)(dmba)]2 was
synthesized according to the literature.28 Other chemicals were
commercially available and were used as received.
Synthesis of Complex 4. Solid NaH (0.012 g, 0.51 mmol) was
added to a solution of 3 (0.230 g, 0.25 mmol) in 15 mL of CH2Cl2.
The reaction mixture was stirred for 2 h, during which it changed color
from red to bright green. After filtration, the volatiles were removed
under vacuum, and the green solid obtained was redissolved in
toluene. Addition of n-pentane to this solution led to the precipitation
of a dark green solid of 4, which was filtered and washed with n-
pentane. Green crystals suitable for X-ray diffraction were grown by
stratification of a solution of 4 in toluene with n-pentane. Yield: 0.151
g, 0.18 mmol (72%). Anal. Calcd for C38H42N6O2Pd2·H2O (845.63):
C, 53.97; H, 5.24; N, 9.93. Found: C, 54.03; H, 5.29; N, 9.53%. FTIR:
selected νmax(solid)/cm−1: 3043vw, 2970vw, 2915w, 2801vw, 1589m,
1570w, 1493vs, 1439w, 1402w, 1361w, 1292s, 1266vw, 1203w, 1180w,
1148w, 1104vw, 1075w, 1043w, 1021w, 990w, 969vw, 926w, 905w,
863mw, 848mw, 827mw, 773m, 742s, 697vw, 659w, 635w, 615m,
567w, 534m, 520s, 505vw, 494vw, 479vw, 466vw, 457w, 442w, 427w,
402m, 365s, 340vw, 311vw, 277vw, 246vw, 222vw, 172vs, 141vw,
128vw, 121w, 106w. 1H NMR (CD2Cl2, 400 MHz) δ: 8.40 (br-d, 2H,
3JH,H = 4.5 Hz, H5Py), 7.50 (dt, 2H, 3JH,H = 7.6, 4JH,H = 1.8 Hz, H3Py),
Synthesis of Zwitterion 2. To a dispersion of 1 (0.607 g, 4.40
mmol) in 10 mL of EtOH was added 2-(pyridin-2-yl)ethanamine
(0.950 g, 7.91 mmol). The reaction mixture was heated to reflux for 4
h and then cooled to room temperature. Volatiles were removed under
vacuum, the dark purple solid obtained was dissolved in CH2Cl2, and
the solution was filtered through Celite. Addition of n-pentane to the
filtrate led to the precipitation of a brown powder of 2. Red crystals
suitable for X-ray diffraction were grown by slow diffusion of n-
pentane into a solution of 2 in CH2Cl2. Yield: 1.204 g, 3.45 mmol
(78%). Anal. Calcd for C20H20N4O2 (348.40): C, 68.95; H, 5.79; N,
16.08. Found: C, 68.60; H, 5.80; N, 15.74%. FTIR: νmax(solid)/cm−1:
3184 ms, 3047vw, 3015vw, 1644w, 1535vs, 1477s, 1461w, 1432s,
1387w, 1356m, 1311w, 1285m, 1261w, 1205w, 1186w, 1148mw,
1103w, 1050w, 998m, 887vw, 848w, 821vw, 767vs, 746vs, 724vs,
1
3
7.35−7.34 (m, 2H, H8Ph), 7.19 (d, 2H, JH,H = 7.6 Hz, H2Py), 7.05−
684vw, 631mw. H NMR (CD2Cl2, 400 MHz) δ: 8.90 (br, 2H, NH),
8.69 (br-d, 2H, 3JH,H = 4.6 Hz, H5Py), 7.77 (dt, 2H, 3JH,H = 7.6, 4JH,H
=
7.03 (m, 2H, H4Py), 6.98 (m, 4H, H9,7Ph), 6.95 (m, 2H, H10Ph), 5.50
(s, 1H, N...C...CH), 5.41 (s, 1H, O...C...CH), 3.90 (m, 6H, NCH2Ph +
NCH2CH2Py), 3.14 (m, 4H, NCH2CH2Py), 2.78 (s, 12H, NCH3)
ppm. 13C{1H} NMR (CD2Cl2, 75.5 MHz) δ: 188.30 (s, O...C), 164.87
(s, N...C), 159.88 (s, C1Py), 149.35 (s, C5Py), 148.06 (s, C11Ph),
147.37 (s, C6Ph), 135.92 (s, C3Py), 133.34 (s, C8Ph), 125.37 (s,
C7Ph), 124.02 (s, C9Ph), 123.31 (s, C2Py), 121.65 (s, C4Py), 121.01
(C10Ph), 102.23 (s, O...C...C), 85.41 (s, N...C...C), 72.62 (s, NCH2Ph),
51.29 (s, NCH3), 50.79 (s, NCH2CH2Py), 37.4 (s, NCH2CH2Py)
ppm. MS (ESI): m/z = 829.15 [M + H]+.
1.9 Hz, H3Py), 7.34−7.31 (m, 4H, H2,4Py), 5.39 (s, 1H, N...C...CH),
3
5.32 (s, 1H, O...C...CH), 3.94 (t, 4H, JH,H = 6.7 Hz, HNCH2CH2Py),
3
3.29 (t, 4H, JH,H = 6.7 Hz, HNCH2CH2Py) ppm.13C{1H} NMR
(CD2Cl2, 75.5 MHz) δ: 172.51 (s, C...O), 158.12 (s, C1Py), 157.01 (s,
C...N), 149.94 (s, C5Py), 137.15 (s, C3Py), 123.88 (s, C2Py), 122.41 (s,
C4Py), 98.01 (s, O...C...C), 81.37 (s, HN...C...C), 42.79 (s,
HNCH2CH2Py), 36.29 (s, HNCH2CH2Py) ppm.
Synthesis of Complex 3. To a solution of 2 (0.098 g, 0.28 mmol) in
10 mL of CH2Cl2 was added [Pd(μ-Cl)(dmba)]2 (0.155 g, 0.28
mmol). The reaction mixture was stirred for 4 h. Addition of n-
pentane to this solution led to the precipitation of a red solid of 3 that
was filtered and washed with THF. Red crystals suitable for X-ray
diffraction were grown by slow diffusion of n-pentane into a solution of
3 in CH2Cl2. Yield: 0.232 g, 0.26 mmol (92%). Anal. Calcd for
C38H44Cl2N6O2Pd2 (900.49): C: 50.68; H: 4.92; N: 9.33. Found: C,
50.75; H, 5.20; N, 8.84%. FTIR: νmax(solid)/cm−1: 3168w, 3050w,
2975w, 2910vw, 2886vw, 1534vs, 1476m, 1449w, 1437m, 1352w,
1313vw, 1289w, 1260vw, 1207vw, 1178vw, 1107w, 1065vw, 1046vw,
1018mw, 983w, 968mw, 930vw, 901vw, 866mw, 850mw, 798vw,
769vw, 738vs, 658vw, 518m, 479vw, 445w, 423mw, 398m, 362s,
334vw, 296s (ν(Pd−Cl)), 226vs, 151vw, 126s, 108vw. An equilibrium
is present in solution between two conformers 3a and 3b in ca. 1:1
ratio (see text), which can be discriminated by low temperature
Synthesis of Complex 5. To a dispersion of 3 (0.230 g, 0.25 mmol)
in 15 mL of MeOH was added solid AgOTf (0.131 g, 0.51 mmol).
Precipitation of AgCl started rapidly. The reaction mixture was stirred
for 2 h and then filtered; the filtrate was allowed to stand overnight at
room temperature. During this time, the color of the solution changed
from red to bright green, and a green crystalline solid of 5 precipitated.
The product was isolated by filtration, dissolved in the minimum
amount of CH2Cl2, precipitated by addition of n-pentane, and
collected by filtration. Green crystals suitable for X-ray diffraction were
grown by stratification of a solution of 5 in acetone with n-pentane.
Yield: 0.282 g, 0.17 mmol (68%). Anal. Calcd for C60H60F6-
N10O10Pd4S2·CH2Cl2 (1769.93): C, 41.39; H, 3.53; N, 7.91. Found:
C, 41.25; H, 3.58; N, 8.04%. FTIR: selected νmax(solid)/cm−1: 3361w,
2972vw, 2941vw, 2901vw, 1610w, 1581vw, 1548m, 1497vw, 1484mw,
1440mw, 1413w, 1371w, 1340w, 1261vs, 1225w, 1159m, 1140m,
1113w, 1073w, 1029s, 967w, 908vw, 890vw, 873vw, 823mw, 804w,
788w, 769m, 756w, 720vw, 660vw, 636s, 584s, 572s, 559s, 532w,
512vs, 488w, 458mw, 437 ms, 390m, 350m, 334w, 324w, 315w, 280w,
265m, 255w, 247vw, 227w, 208 ms, 172m, 158vw, 151w, 140vw,
1
1
1
combined H, 13C, H COSY, and HSQC NMR analysis. H NMR
(CDCl3, 500 MHz, 263 K) δ: 9.01 (d, 2H, 3JH,H = 5.7 Hz, H5Py of 3a),
8.97 (d, 2H, JH,H = 5.7 Hz, H5Py, 3b), 8.53 (t, 2H, JH,H = 6.8 Hz,
3
3
3
NH, 3a), 8.43 (t, 2H, JH,H = 6.8 Hz, NH, 3b), 7.80−7.75 (m, 4H,
3JH,H = 7.8 Hz, 4JH,H = 1.5 Hz, H3Py, 3a+3b), 7.56−7.52 (m, 4H, H2Py,
3a+3b), 7.33−7.30 (m, 2H, H4Py, 3b), 7.27−7.25 (m, 2H, H4Py, 3a),
6.99−6.95 (m, 8H, H8,10Ph, 3a+3b), 6.70−6.67 (m, 4H, H9Ph, 3a
+3b), 5.87 (s, 1H, N...C...CH, 3a), 5.64 (s, 1H, N...C...CH, 3b), 5.61 (d,
4H, 3JH,H = 7.6 Hz, H7Ph, 3a+3b), 5.38 (s, 1H, O...C...CH, 3a), 5.37 (s,
1H, O...C...CH, 3b), 4.37−4.08 and 3.60−3.52 (m, 12H + 4H,
1
132vw, 125vw, 121vw, 105ms. H NMR (CD2Cl2, 500 MHz) δ: 8.88
(br d, 1H, 3JH,H = 5.6 Hz, H5Py1), 7.91 (dt, 1H, 3JH,H = 7.6, 4JH,H = 1.4
3
Hz, H4Py2), 7.72 (d, 1H, JH,H = 7.6 Hz, H2Py2), 7.55−7.52 (m, 1H,
3
4
H4Py1), 7.45 (dt, 1H, JH,H = 7.8, JH,H = 1.4 Hz, H3Py1), 7.20 (br-d,
1H, 3JH,H = 7.6 Hz, H5Py2), 7.16−7.14 (m, 1H, H8Ph), 7.10−7.08 (m,
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3H, H3Py2 + H7,8Ph), 6.99−6.97 (m, 1H, H10Ph), 6.93 (d, 1H, 3JH,H
=
359m, 342vw, 327vw, 314w, 302w, 289vw, 280mw, 266w, 246s
(ν(Pd−μ-Cl)), 225vs, 209vw, 202w, 177m, 158w, 151s, 140w, 133w,
7.8 Hz, H2Py1), 5.72 (s, 1H, N...C...CH), 5.16 (s, 1H, O...C...CH), 4.96
2
3
1
3
(dt, 1H, JH,H = 14.2 JH,H = 4.5 Hz, NCH2CHHPy1), 4.47 (dt, 1H,
121m, 104m. H NMR (CD2Cl2, 500 MHz) δ: 8.62 (dd, 2H, JH,H
=
2JH,H = 12.8 and 3.6 Hz, NCH2CHHPy2), 4.24 (br-d, 1H, 2JH,H = 12.8
Hz,, NCH2CHHPy2), 3.95 (d, 1H, A part of an AB system, JH,H
4.7 Hz, 4JH,H = 1.5 Hz, H5Py), 7.79 (dt, 2H, 3JH,H = 7.7, 4JH,H = 1.5 Hz,
H3Py), 7.47 (d, 2H, 3JH,H = 7.7 Hz, H2Py), 7.28 (overlapping ddd, 2H,
3JH,H = 7.7, 4.7 Hz, 4JH,H = 1.5 Hz, H4Py), 7.15 (d, 2H, 3JH,H = 7.6 Hz,
H7PhPd(O,N)), 7.03−6.99 and 6.96−6.93 (m, 4H + 6H, H9,10PhPd-
(μ-Cl) + H8,9,10PhPd(O,N)), 6.58−6.54 (m, 2H, H8PhPd(μ-Cl)), 5.35
2
=
13.5 Hz, NCHPh), 3.71−3.63 (m, 2H, NCHHCH2Py1−2), 3.36 (d,
2
1H, JH,H = 12.8 Hz, NCHHCH2Py1), 3.31 (d, 1H, B part of an AB
2
2
system, JH,H = 13.5 Hz, NCHPh), 3.17 (d, 1H, JH,H = 12.5 Hz,
2
NCHHCH2Py2), 2.96 (br-d, 1H, JH,H = 14.2 Hz, NCH2CHHPy1),
(d, 2H, JH,H = 7.8 Hz, H7PhPd(μ-Cl)), 5.28 (s, 1H, O...C...CH), 4.64
3
2.68 (s, 3H, N(CH3)CH3), 2.43 (s, 3H, N(CH3)CH3) ppm. 13C{1H}
NMR (CD2Cl2, 125.7 MHz) δ: 189.63 (s, (dmba)PdO...C), 189.86 (s,
(Py)2PdO...C), 165.94 (s, (dmba)PdN...C), 165.04 (s, (Py)2PdN...C),
161.26 (s, C1Py2), 159.22 (s, C1Py1), 150.48 (s, C5Py2 + C6Ph),
150.23 (s, C5Py1), 147.91 (s, C11Ph), 141.08 (s, C3Py2), 139.17 (s,
C3Py1), 133.33 (s, C8Ph), 128.32 (s, C2Py1), 128.00 (s, C2Py2),
126.41 (s, C4Py2), 124.66 (s, C4Py1), 125.10 (s, C7Ph), 124.66 (s,
C9Ph), 122.56 (s, C10Ph), 105.08 (s, O...C...C), 85.84 (s, N...C...C),
72.81 (s, NCH2Ph), 52.00 and 50.52 (s, N(CH3)2), 48.42 (s,
NCH2CH2Py2), 45.34 (s, NCH2CH2Py2), 41.85 (s, NCH2CH2Py1),
37.79 (s, NCH2CH2Py1) ppm. MS (ESI): m/z = 693.53 [M]2+.
Synthesis of Complex 6. To a solution of 4 (0.190 g, 0.21 mmol) in
10 mL of CH2Cl2 was added [Pd(μ-Cl)(dmba)]2 (0.116 g, 0.21
mmol). The reaction mixture was stirred for 4 h. Addition of n-
pentane to this solution led to the precipitation of a green solid of 6
that was filtered and washed with n-pentane. Yield 0.206 g, 0.15 mmol
(71%). Anal. Calcd for: C56H66Cl2N8O2Pd4·CH2Cl2 (1464.71): C,
46.74; H, 4.68; N, 7.30. Found: C, 46.76; H, 5.11; N, 7.65%. FTIR:
selected νmax(solid)/cm−1: 3044vw, 2969vw, 2909vw, 2886vw,
2856vw, 1502vs, 1441w, 1400w, 1295s, 1201w, 1179vw, 1156vw,
1106w, 1045w, 1022m, 987mw, 969w, 891vw, 863mw, 847m, 832vw,
771w, 738vs, 520vs, 477vw, 443w, 425s, 365vs, 299s (Pd−Cl), 270vw,
260vw, 226vs, 219vs, 192m, 185vw, 175vs, 157w, 142vw, 128vs,
122vw, 106s. 1H NMR (CD2Cl2, 500 MHz) δ: 8.81 and 8.77 (br-d, 1H
(s, 1H, N...C...CH), 4.10 and 3.88 (4H, AB system, JH,H = 14.2 Hz,
2
2
NCHPhPd(μ-Cl)), 3.99 and 3.93 (4H, AB system, JH,H = 13.0 Hz,
NCHPhPd(O,N)), the signals for the NCH2CH2Py protons overlap
with the signals in the 4.12−3.81 range, 2.94 and 2.90 (s, 6H + 6H,
N(CH3)2Pd(μ-Cl)), 2.75 (s, 12H, N(CH3)2Pd(O,N)) ppm. 13C{1H}
NMR (CD2Cl2, 125.7 MHz) δ: 187.92 (s, O...C), 165.19 (s, N...C),
161.43 (s, C1Py), 152.19 (s, C5Py), 148.89 (s, C11PhPd(O,N)), 147.08
(s, C6PhPd(O, N)), 146.67 (s, C11PhPd(μ-Cl)), 143.63 (s, C6PhPd(μ-
Cl)), 138.38 (s, C3Py), 132.39 (s, C7PhPd(O,N)), 131.02 (s,
C7PhPd(μ-Cl)), 127.93 (s, C2Py), 125.70, 125.18, 124.42, 123.22,
and 122.28 (s, C8,9,10PhPd(O,N)+C9,10PhPd(μ-Cl)), 125.39 (s,
C8PhPd(μ-Cl)), 101.58 (s, O...C...C), 86.00 (s, N...C...C), 73.00 (s,
NCH2Pd(μ-Cl)), 72.56 (s, NCH2Pd(O,N)), 52.86 and 52.57 (s,
N(CH3)2Pd(μ-Cl)), 51.15 and 51.08 (s, N(CH3)2Pd(O,N)), 49.38 (s,
NCH2CH2Py), 42.40 (s, NCH2CH2Py) ppm. MS (ESI): m/z = 829.15
[4 + H]+, 516.97 [(dmba)Pd(μ-Cl)Pd(dmba)]+. MALDI-TOF-MS:
m/z = 1068.14 [4·Pd(dmba)]+, 829.12 [4 + H]+.
X-ray Data Collection, Structure Solution, and Refinement
for All Compounds. Suitable crystals for the X-ray diffraction
analysis of all compounds were obtained as described above. The
intensity data for 2 and 3 were collected on a Nonius Kappa CCD
diffractometer (graphite monochromated Mo Kα radiation, λ = 0.710
73 Å) at 173(2) K.22 Data for 4−7 were collected on a Bruker APEX-
II Kappa CCD (triumph monochromated Mo Kα radiation, λ = 0.710
73 Å). Crystallographic and experimental details for the structures are
summarized in Supporting Information, Table S1. The structures were
solved by direct methods (SHELXS-97) and refined by full-matrix
least-squares procedures (based on F2, SHELXL-97)23 with aniso-
tropic thermal parameters for all the non-hydrogen atoms. The
hydrogen atoms were introduced into the geometrically calculated
positions (SHELXL-97 procedures) and refined riding on the
corresponding parent atoms. Except for complex 2 where the NH
protons were located from Fourier difference maps and refined
isotropically, for 3, 4, 5, and 7 semiempirical absorption correction was
applied using the MULTISCAN-ABS in PLATON24 or SAD-ABS in
APEX-II.25 For complex 3 a SQUEEZE procedure24 was applied, and
the residual electron density was assigned to five molecules of
disordered pentane. In 5, there is half a molecule in the asymmetric
unit, and a SQUEEZE procedure was also applied; the residual
electron density was assigned to one molecule of acetone. In 7 a
SQUEEZE procedure was also applied, and the residual electron
density was assigned to one molecule of CH2Cl2. The thermal ellipsoid
of Cl1 is flattened, but the nature of this atom was confirmed by other
analytical methods.
3
3
+1H, JH,H = 5.4 Hz, H5Py), 7.68 (t, 2H, JH,H = 7.9 H3Py), 7.33 and
7.30 (d, 1H+1H, JH,H = 7.9 Hz, H2Py), 7.19−7.13 (overlapping
3
multiplet, 4H, H4Py + H7PhPd(O,N)), 7.01−6.91 (m, 10H,
H
8−10PhPd(O,N) + H9−10PhPd(Cl)), 6.50 and 6.44 (t, 1H + 1H,
3
3JH,H = 7.5 Hz, H8PhPd(Cl)), 5.73 (d, 2H, JH,H = 7.5 Hz,
H7PhPd(Cl)), 5.66 (s, 1H, N...C...CH), 5.21 (s, 1H, O...C...CH),
3.99−3.84 and 3.80−3−66 (overlapping multiplets, 8H + 8H,
NCH2CH2Py + NCH2PhPd(O,N) + NCH2PhPd(Cl)), 2.84, 2.77,
2.73, 2.71, and 2.69 (s, 6H + 3H + 3H + 6H + 6H, N(CH3)2Pd(Cl) +
N(CH3)2Pd(N,O)) ppm. 13C{1H} NMR (CD2Cl2, 125.7 MHz) δ:
188.23 (s, O...C), 164.64 (s, N...C), 161.19 (s, C1Py), 152.19 (s, C5Py),
152.53 and 152.48 (s, C5Py), 148.63 (s, C11PhPd(O,N)), 147.93 (s,
C6PhPd(O,N)), 147.80 (s, C6PhPd(Cl)), 137.31 and 137.20 (s, C3Py),
133.27 and 133.18 (s, C7PhPd(O,N)), 132.01 and 131.82 (s,
C7PhPd(Cl)), 125.66 and 125.55 (s, C8PhPd(Cl)), 125.10 (s, C2Py),
121.84 (C4Py), 125.20, 125.16, 124.21, 124.07, 124.03, 123.99, 122.54,
122.24, 121.26, and 121.18 (s, C8−10PhPd(O,N) + C9−10PhPd(Cl)),
101.78 (s, O...C...C), 87.12 (s, N...C...C), 73.79 (s, NCH2Pd(Cl)), 72.30
(s, NCH2Pd(O,N)), 52.54, 52.45, 52.18, and 52.09 (s, N(CH3)2Pd-
(Cl)), 50.93 (br s, N(CH3)2Pd(O,N)), 48.01 and 47.89 (s,
NCH2CH2Py), 42.21 and 42.14 (s, NCH2CH2Py) ppm. MS (ESI):
m/z = 865.12 [M + H − (Pd(dmba)) − PdCl(dmba)]+, 588.01 [M +
H − (Pd(dmba)) − (PdCl(dmba))2]+, 516.19 [(dmba)Pd(μ-Cl)Pd-
(dmba)]+.
ASSOCIATED CONTENT
Synthesis of Complex 7. To a solution of 6 (0.290 g, 0.21 mmol) in
10 mL of CH2Cl2, was added TlPF6 (0.073 g, 0.21 mmol). The
reaction mixture was stirred for 4 h at room temperature. After
filtration, addition of n-pentane led to the precipitation of a green solid
of 7, which was filtered and washed with n-pentane. Green crystals
suitable for X-ray diffraction were grown by stratification of a solution
of 7 in CH2Cl2 with n-pentane. Yield: 0.259 g, 0.17 mmol (83%). Anal.
Calcd for: C56H66ClN8O2Pd4F6P (1489.29): C, 45.16; H, 4.67; N,
7.52. Found: C, 45.32; H, 4.64; N, 7.59%. FTIR: selected νmax(solid)/
cm−1: 3049vw, 2974vw, 2914brw, 1604w, 1580vw, 1509s, 1485vw,
1468vw, 1450mw, 1403mw, 1288m, 1206vw, 1181vw, 1161vw,
1110vw, 1064vw, 1044mw, 1023mw, 987mw, 969w, 834vs, 776w,
734s, 659w, 579vw, 555vs, 518m, 469mw, 455vw, 438vw, 420m, 398w,
■
S
* Supporting Information
Crystallographic data, including selected bond angles and CIF
files, NMR characterization of conformers 3a and 3b, and cyclic
voltammetry for complex 4. This material is available free of
charge via the Internet at http://pubs.acs.org. Crystallographic
information files (CIF) of the compounds 2·CH2Cl2, 3−5, and
7·CH2Cl2 were deposited with the CCDC, 12 Union Road,
Cambridge, CB2 1EZ, U.K., and can be obtained on request
free of charge, by quoting the publication citation and
deposition numbers 982040−982044.
5524
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Inorganic Chemistry
Article
(f) Braunstein, P.; Bubrin, D.; Sarkar, B. Inorg. Chem. 2009, 48, 2534.
(g) Deibel, N.; Hohloch, S.; Sommer, M. G.; Schweinfurth, D.; Ehret,
F.; Braunstein, P.; Sarkar, B. Organometallics 2013, 32, 7366.
(7) (a) Yang, Q.-Z.; Kermagoret, A.; Agostinho, M.; Siri, O.;
Braunstein, P. Organometallics 2006, 25, 5518. (b) Taquet, J.-P.; Siri,
O.; Braunstein, P.; Welter, R. Inorg. Chem. 2004, 43, 6944.
(8) Braunstein, P.; Siri, O.; Steffanut, P.; Winter, M.; Yang, Q. C. R.
Chim. 2006, 9, 1493.
(9) (a) Simpson, S.; Kunkel, D. A.; Hooper, J.; Nitz, J.; Dowben, P.
A.; Routaboul, L.; Braunstein, P.; Doudin, B.; Enders, A.; Zurek, E. J.
Phys. Chem. C 2013, 117, 16406. (b) Dowben, P. A.; Kunkel, D. A.;
Enders, A.; Rosa, L. G.; Routaboul, L.; Doudin, B.; Braunstein, P. Top.
Catal. 2013, 56, 1096. (c) Routaboul, L.; Braunstein, P.; Xiao, J.;
AUTHOR INFORMATION
Corresponding Author
*E-mail: braunstein@unistra.fr. Fax: +33 368 851 322.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the CNRS, the Minister
■
́
Zhang, Z.; Dowben, P. A.; Dalmas, G.; Da Costa, V.; Felix, O.; Decher,
̀
e de la Recherche
G.; Rosa, L. G.; Doudin, B. J. Am. Chem. Soc. 2012, 134, 8494.
(d) Rosa, L. G.; Velev, J.; Zhang, Z.; Alvira, J.; Vega, O.; Diaz, G.;
Routaboul, L.; Braunstein, P.; Doudin, B.; Losovyj, Y. B.; Dowben, P.
A. Phys. Status Solidi B 2012, 249, 1571. (e) Kong, L.; Perez Medina,
(Paris), and the DFH/UFA (International Research Training
Group 532-GRK532, Ph.D. grant to A.G.) for funding. We
thank Drs. L. Karmazin-Brelot and C. Bailly, Service de
Radiocrystallographie, Institut de Chimie (UMR 7177 CNRS-
UdS), for the X-ray diffraction studies and the UdS NMR
service for helpful suggestions. We are grateful to Prof. B.
Sarkar (F.U. Berlin) for the electrochemical experiments.
́ ́
G. J.; Colon Santana, J. A.; Wong, F.; Bonilla, M.; Colon Amill, D. A.;
Rosa, L. G.; Routaboul, L.; Braunstein, P.; Doudin, B.; Lee, C.-M.;
Choi, J.; Xiao, J.; Dowben, P. A. Carbon 2012, 50, 1981. (f) Xiao, J.;
Zhang, Z.; Wu, D.; Routaboul, L.; Braunstein, P.; Doudin, B.; Losovyj,
Y. B.; Kizilkaya, O.; Rosa, L. G.; Borca, C. N.; Gruverman, A.;
Dowben, P. A. Phys. Chem. Chem. Phys. 2010, 12, 10329. (g) Kong, L.;
Routaboul, L.; Braunstein, P.; Park, H.-G.; Choi, J.; Cordova, J. P. C.;
Vega, E.; Rosa, L. G.; Doudin, B.; Dowben, P. A. RSC Adv. 2013, 3,
10956. (h) Fang, Y.; Nguyen, P.; Ivasenko, O.; Aviles, M. P.; Kebede,
E.; Askari, M. S.; Ottenwaelder, X.; Ziener, U.; Siri, O.; Cuccia, L. A.
Chem. Commun. 2011, 47, 11255.
DEDICATION
■
†Dedicated to Prof. Barry Lever, Distinguished Research
Professor Emeritus at York University in Toronto, for his
numerous and outstanding contributions to inorganic chem-
istry.
(10) (a) Yang, Q.-Z.; Siri, O.; Braunstein, P. Chem. Commun. 2005, 5,
2660. (b) Tamboura, F. B.; Cazin, C. S. J.; Pattacini, R.; Braunstein, P.
Eur. J. Org. Chem. 2009, 2009, 3340.
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