Dinuclear palladium(III) complexes with a single unsupported bridging halide ligand: Reversible formation from mononuclear palladium(II) or palladium(IV) precursors

Article abstract of DOI:10.1002/anie.201100928

Stable Pd^III: Dinuclear Pd^III complexes of the tridentate ligand trimethyltriazacyclononane (Me₃tacn) were obtained by one-electron oxidation of mononuclear Pd^II precursors. Further oxidation led reversibly to m

Full text of DOI:10.1002/anie.201100928

Communications
DOI: 10.1002/anie.201100928
Dinuclear Palladium(III) Complexes
Dinuclear Palladium(III) Complexes with a Single Unsupported
Bridging Halide Ligand: Reversible Formation from Mononuclear
Palladium(II) or Palladium(IV) Precursors**
Julia R. Khusnutdinova, Nigam P. Rath, and Liviu M. Mirica*
Along with the well-known involvement of Pd0 and PdII
oxidation states in a large number of palladium-catalyzed
reactions,[1] recent reports have proposed the intermediacy of
less-common PdIV and PdIII oxidation states in several
chemical transformations.[2] Among these systems, dinuclear
and mononuclear PdIII complexes have been shown to act as
the reaction of Me3tacn with [(MeCN)2PdIIX2].[12] The X-ray
structure analysis of 1a reveals a square-planar arrangement
of two Clꢀ ions and two N atoms around Pd, while the third N
atom of Me3tacn is not in close proximity to the metal center
(Figure 1a),[13] in contrast to the five-coordinate geometry
active intermediates in both two- and one-electron oxidative
[3–6]
ꢀ
ꢀ
C H functionalization and C C bond formation reactions.
For example, dinuclear organometallic PdIII complexes stabi-
[3,7]
ꢀ
lized by a Pd Pd bond have been recently reported
and
shown to be a catalytically competent alternative to mono-
nuclear PdIV species in carbon–heteroatom bond-formation
reactions.[3,4] In this context, we have recently shown that
mononuclear PdIII complexes stabilized by a tetradentate
diazapyridinophane ligand can be isolated and have shown
[6]
ꢀ
that they exhibit C C bond-formation reactivity. Continu-
ing our work in the study of high-valent Pd complexes, we
report herein novel cationic dinuclear PdIII and mononuclear
PdIV complexes supported by a common tridentate nitrogen-
donor ligand, N,N’,N’’-trimethyl-1,4,7-triazacyclononane
(Me3tacn). Moreover, we provide evidence for the involve-
ment of a PdIII species in the Kharasch addition reaction and
confirm the ability of Pd to catalyze one-electron radical
reactions. In addition, the reported PdIII systems are the first
group 10 d7–d7 dinuclear complexes bridged by a single
unsupported halide ligand and represent a model of the
III
delocalized PdIII X Pd electronic structure that has been
ꢀ ꢀ
ꢀ
ꢀ ꢀ ꢀ ꢀ
proposed to exist in some Pd X Pd X one-dimensional
(1D) chains.[8]
Figure 1. ORTEP representation (ellipsoids set at 50% probability) of
a) 1a, b) the cation of 2a, and c) the cation of 3a. Selected bond
distances [ꢀ] and angles [8]: 1a: Pd1–N1 2.0736(5), Pd1–N2 2.0878(5),
Pd1–Cl1 2.30651(18), Pd1–Cl2 2.32102(17). 2a: Pd1–N1 2.094(2),
Pd1–N2 2.105(2), Pd1–N3 2.273(2), Pd1-Cl2 2.3179(7), Pd1-Cl3
2.3224(7), Pd1-Cl1 2.4801(2), Cl1–Pd1i 2.4801(2), Pd1–Pd1i 4.931;
Pd1-Cl1-Pd1i 167.47(4). 3a: Pd1–N3 2.080(8), Pd1–N1 2.0806(16),
Pd1–N2 2.093(8), Pd1–Cl1 2.302, Pd1–Cl2 2.308, Pd1–Cl3 2.308.
While triazacyclonane (tacn)[9,10] and other tacn deriva-
tives[11] have been employed in the synthesis of Pd complexes,
only one complex, [(Me3tacn)PdII(MeCN)2](PF6)2, has been
reported for Me3tacn.[10] We have synthesized the PdII
complexes [(Me3tacn)PdIIX2] (X = Cl 1a, X = Br 1b) through
[*] Dr. J. R. Khusnutdinova, Prof. Dr. L. M. Mirica
Department of Chemistry, Washington University
St. Louis, MO 63130 (USA)
observed for [(Me3tacn)PdII(MeCN)2](PF6)2.[10] Cyclic vol-
tammetry (CV) of both 1a and 1b in MeCN/Bu4NPF6 reveals
two closely-spaced waves at 0.05–0.17 V versus Fc+/Fc and
additional waves at higher potentials, suggestive of oxida-
tively induced chemical reactions.[12,14]
Fax: (+1)314-935-4481
E-mail: mirica@wustl.edu
Dr. N. P. Rath
Department of Chemistry and Biochemistry
University of Missouri–St. Louis (USA)
Controlled potential electrolysis (CPE) at 0.3–0.4 V for
both 1a and 1b leads to formation of a dark purple species in
60–70% yields of isolated product after passing a charge
corresponding to a one-electron oxidation. X-ray-quality
crystals of the electrooxidation products obtained from
MeCN/Et2O reveal the formation of dinuclear complexes
[**] We thank the Department of Chemistry at Washington University
and American Chemical Society Petroleum Research Fund (49914-
DNI3) for support. We also thank Prof. T. Daniel P. Stack for a gift of
the ligand Me3tacn.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100928.
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[(Me3tacn)PdIIIX2(m-X)PdIIIX2(Me3tacn)](PF6) (X = Cl 2a,
X = Br 2b), in which a single halide ion bridges the two Pd
centers (Scheme 1 and Figure 1b; Supporting Information,
ing Information, Figure S31).[12] This observation suggests
that the use of alternate exogenous ions can lead to PdIII
complexes with bridging ligands and altered electronic
properties.[18] Complexes 2a–c are structural models of the
[8]
ꢀ ꢀ ꢀ ꢀ ꢀ
dinuclear unit found in M X M X 1D extended chains
in the average-valent Mott–Hubbard (MH)[8,19] state MIII
ꢀ ꢀ
X
MIII (that is, a Robin–Day class III state).[20] The bridging
halide ligand in 2a–c is located at the midpoint between the
two metal centers,[21] while the short Pd···Pd distances (2a
4.931 ꢀ, 2b 5.133 ꢀ, 2c 5.031 ꢀ, Figure 1b; Supporting
Information, Figure S31)[22] suggest an intimate orbital over-
lap between Pd and the bridging halide that is supported by
the observed strong antiferromagnetic coupling between the
unpaired dz2 electrons of the two PdIII centers,[23] which is
typical for a delocalized MH state.[8,19b]
The UV/Vis spectra of complexes 2a–c in MeCN reveal at
least three intense absorption bands at 535–570 nm, 360–
410 nm, and 260–280 nm, respectively (Table 1 and
Figure 2).[24] Density functional theory (DFT) and time-
Scheme 1. Electrochemical synthesis of dinuclear PdIII complexes and
their interconversion to mononuclear PdII and PdIV complexes.
Figure S31).[13,15] Each metal center has a distorted octahedral
geometry with two N atoms and the two terminal halides in
the equatorial plane, while the third N atom of Me3tacn and
the bridging halide occupy the axial positions. The coordina-
tion geometry of Pd atoms and the overall charge of the dimer
confirm the presence of PdIII centers. To the best of our
knowledge, complexes 2a and 2b are the first dinuclear PdIII
[3,7]
ꢀ
complexes that are not stabilized by a Pd Pd bond.
Furthermore, this result suggests that stabilization of the
PdIII oxidation state can also be achieved by a tridendate
N-ligand and does not require a rigid tetradentate ligand.[6]
The formation of dinuclear PdIII complexes 2a and 2b
from the mononuclear precursors 1a and 1b is intriguing. The
circa 65% yield of product suggests that the bridging halide
ion comes from another molecule of the PdII precursor.[16]
Indeed, addition of 0.5 equiv of external halide leads to a
simpler CV that shows only two closely spaced oxidative
waves (Table 1; Supporting Information Figures S1–S4), and
CPE at 0.3–0.4 V leads to formation of dark purple complexes
2a and 2b in higher yields (ca. 80%).[12,17] Interestingly, when
Brꢀ was added to a solution of 1a, the mixed halide complex
[(Me3tacn)PdIIICl2(m-Br)PdIIICl2(Me3tacn)](PF6) (2c) formed
(Scheme 1), as confirmed by X-ray crystallography (Support-
Figure 2. UV/Vis spectra of 1a (a), 2a (c), and 3a (g) in
MeCN. Inset: CV of 1a in the presence of 0.5 equiv Clꢀ (g) and
1 equiv Clꢀ (c) in 0.1m Bu4NPF6/MeCN.
dependent DFT (TD-DFT) calculations were employed in
the assignment of these transitions. For 2a, the 534 nm band
exhibits an uncommonly large extinction coefficient (e =
21000 Lmolꢀ1 cmꢀ1) and is assigned to an intermetallic Pd-
to-Pd charge transfer (MMCT)[25] transition that is strongly
mixed with a m-Cl-to-Pd CT transition (LMCT).[26] A similar
assignment has been proposed for the low-energy transitions
Table 1: Spectroscopic properties of dinuclear PdIII complexes 2a–c.
II/III
III/IV
1
1
Complex E = , E =
UV/Vis (MeCN)
in PdIII X Pd X 1D chains. TD-DFT calculations sup-
port such an assignment by revealing a large oscillator
strength for the HOMO to LUMO + 1 MMCT transition,
where the HOMO exhibits s-bonding Pd-m-Cl character and
the LUMO + 1 has antibonding Pd-m-Cl character (Figure 3).
Furthermore, the higher-energy bands can be assigned to a
combination of bridging and terminal halide-to-Pd LMCT
bands (for example, HOMOꢀ10 and HOMOꢀ12 to
LUMO + 1, respectively; Figure 3), as suggested previously[8]
and supported by TD-DFT calculations.[12] As expected, the
replacement of Clꢀ with Brꢀ ligands for 2a to 2c to 2b leads to
III
[8]
2
2
ꢀ ꢀ
ꢀ
[mV][a]
l [nm] (e [Lmolꢀ1 cmꢀ1])
2a
55, 163
534 (21000), 449 (sh, 4900), 360 (6100),
260 (43000)
2b
2c
45, 171
570 (25000), 411 (8900), 273 (47000)
546 (17000), 378 (6800), 262 (41000)
65, 174[b]
[a] Potentials vs. Fc+/Fc are measured by differential pulse voltammetry
(DPV) for solutions of 1a or 1b in the presence of 1 equiv of Clꢀ or Brꢀ,
respectively, in 0.1m Bu4NPF6/MeCN. Complexes 2a–c show two
oxidation waves at similar potentials (see the Supporting Information).
[b] Potentials are measured by DPV for a solution of 1a in the presence of
1 equiv of Brꢀ.
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Communications
ambient conditions than either the corresponding PdIV species
or a PdII/PdIV mixture that is equivalent to a mixed-valence
PdIV-X-PdII species. This result is in contrast to the use of
extreme conditions[8] or Pd-to-Ni substitution[19c,d] required
III
for the generation of the PdIII X Pd state in 1D chains.
ꢀ ꢀ
Furthermore, the interconversion between dinuclear PdIII and
mononuclear PdIV species parallels the proposed involvement
ꢀ
of analogous intermediates in Pd-catalyzed C H oxidative
functionalization reactions and suggests that for a given
ligand environment both types of intermediates can be
present.[3–6]
The relatively low oxidation potentials of complexes 1a,b
have prompted us to investigate their reactivity in one-
electron redox reactions, such as the Kharasch addition of
polyhaloalkanes to alkenes.[28] Both 1a and 1b exhibit
catalytic activity in the addition of CCl3Br to a number of
alkenes (methyl methacrylate, methyl acrylate, styrene,
norbornene, cyclopentene) to give selectively the 1:1 addition
product in good to high yields at 658C under N2 (Supporting
Information, Chart S1, Table S1).[12] Kinetic studies of the
CCl3Br addition to methyl methacrylate (MMA) catalyzed by
1a reveal a first-order dependence on 1a and MMA concen-
tration and a saturation behavior with respect to CCl3Br
concentration, thus suggesting a mechanism that involves a
reversible electron transfer/halogen transfer from CCl3Br to
PdII to form a PdIII species and a Cl3CC radical that
subsequently reacts with the alkene and leads to the
addition-product formation (Scheme 2).[28] While no PdIII
Figure 3. DFT-calculated (UB3LYP/CEP-31G) molecular orbitals (MOs)
of 2a that are proposed to be involved in the observed UV/Vis
absorption bands. The calculated atomic contributions are listed for
each MO (Pd1 and Pd2 represent the left and the right metal center,
respectively, m-Cl is the bridging Clꢀ ligand, Cl refers to the terminal
Clꢀ ligands, and N to the N atoms).[12]
lower energies of all transitions and thus supports the halide
contributions to the corresponding MOs.[19b]
CV studies show that addition of one equivalent of halide
to 1a and 1b leads to an increase of the peak current
corresponding to the second oxidation wave (Figure 2, inset).
CPE of 1a at about 0.5 V versus Fc+/Fc in the presence of
1 equiv of Clꢀ led to the appearance of an intense band at
534 nm associated with the dinuclear species 2a, followed by
its complete disappearance to give a yellow solution after a
two-electron oxidation (Figure 2).[12] While the yellow species
is unstable at RT, it leads to formation of yellow crystals at
ꢀ208C in MeCN/Et2O. The crystal structure reveals a cationic
mononuclear PdIV complex, [(Me3tacn)PdIVCl3](PF6) (3a;
Scheme 1); a similar complex [(Me3tacn)PdIVBr3](PF6) (3b)
is obtained upon CPE of 1b in the presence of 1 equiv of Brꢀ,
both 3a and 3b exhibiting a pseudo-octahedral geometry
around the Pd center (Figure 1c; Supporting Information,
Figure S38).[12,27] The PdII/PdIII/PdIV interconversions are
reversible: electrochemical reduction of the mononuclear
species 3a (or 3b) at ꢀ0.3 V versus Fc/Fc+ occurs via the
intermediate dinuclear complex 2a (or 2b) to eventually
produce the mononuclear species 1a (or 1b). Interestingly,
complexes 3a,b are unstable and slowly decay to the
corresponding PdIII species 2a,b; moreover, a rapid reaction
occurs when 1 equiv of the PdII complex 1a (or 1b) is added to
a solution of 3a (or 3b) to generate the dinuclear PdIII
complex 2a (or 2b) in quantitative yield. These studies
suggest that the PdIII complexes 2a,b are more stable under
Scheme 2. Proposed mechanism for the Kharasch reaction catalyzed
by 1a,b and the formation of the dinuclear PdIII complexes 2a,b.
intermediate was observed under optimal catalytic conditions,
the dinuclear complex 2a was detected by UV/Vis at RT, both
in absence and in presence of MMA (Supporting Information,
Figure S24).[29] Moreover, a higher yield of 2a (or 2b) was
obtained when 1a (or 1b) was reacted with CCl3Br in
presence of O2, a known radical trap that can react with the
Cl3CC radical[30] and thus drive the formation of the dinuclear
PdIII species. As the catalytic reaction rate shows a first-order
dependence in catalyst concentration, and the complex 2a
was not observed under optimal catalytic conditions, the
formation of the dinuclear PdIII complex is most likely a
means of stabilizing the transient mononuclear PdIII inter-
mediate (Scheme 2).[12] Moreover, the observed chemical
oxidation of 1a,b to form 2a,b suggests that the polyhaloal-
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201, 61; e) A. A. Sobanov, A. N. Vedernikov, G. Dyker, B. N.
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[10] A. J. Blake, A. J. Holder, Y. V. Roberts, M. Schrꢂder, J. Chem.
Soc. Chem. Commun. 1993, 260.
kane acts as both the oxidant and the halogen-atom donor
and thus parallels the electrosynthesis of complexes 2a,b
through electrochemical oxidation in presence of an external
halide (Scheme 1).[31]
In conclusion, we have presented herein the synthesis and
characterization of stable dinuclear PdIII complexes formed
by oxidation of mononuclear PdII precursors. Effective
stabilization of the PdIII oxidation state was accomplished
[11] a) K. Wieghardt, E. Schoeffmann, B. Nuber, J. Weiss, Inorg.
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Inorg. Chem. 1995, 34, 6449; c) A. J. Blake, I. A. Fallis, A.
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[12] See the Supporting Information.
[13] CCDC 809190 (1a), 809186 (2a, T= 100 K), 809187 (2a, T=
293 K), 809183 (2b), 809185 (2c), 809188 (3a), 809189 (3b),
and 809184 ([(Me3tacn)PdIICl(MeCN)]ClO4) contain the sup-
plementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] Coulometry measurements confirm that both oxidations corre-
spond to one electron processes (see the Supporting Informa-
tion).
using the common tridentate ligand Me3tacn and halide ions
III
and does not require the formation of a PdIII Pd bond.
ꢀ
Moreover, these dinuclear PdIII complexes seem to be more
stable than the corresponding mononuclear PdIV (and PdIII)
complexes under an analogous ligand environment. These
results support our hypothesis that PdIII complexes are more
common than previously anticipated and can play important
roles in various Pd-catalyzed reactions. Furthermore, we
suggest that under a given ligand environment, the involve-
ment of either PdIII or PdIV species as reactive intermediates
cannot be unambiguously ruled out or confirmed, given their
[15] The presence of dimers 2a and 2b in solution was confirmed by
ESI-MS and the lack of an EPR signal for the corresponding
solutions at 77 K or RT.
facile interconversion. Moreover, the reported dinuclear PdIII
complexes are models of the delocalized PdIII X Pd
III
[16] The presence of [(Me3tacn)PdCl(MeCN)]+ in solution after one-
electron oxidation of 1a was detected by CV and confirmed by
independent synthesis (see the Supporting Information). If
(Me3tacn)PdCl(MeCN)+ is the only halide abstraction product,
then the theoretical yield of 2a is 67%.
ꢀ ꢀ
ꢀ ꢀ ꢀ
electronic structure found in Group 10 M X M X 1D
extended chains. Current research efforts are aimed at
characterizing the reactivity and electronic properties of
these unique dinuclear systems.
[17] The high extinction coefficients of 2a,b enable the unique
visualization of mass-transport processes at the disk electrode
during CV of 1a,b (Supporting Information, Figure S15).
[18] J. Terheijden, G. van Koten, D. M. Grove, K. Vrieze, A. L. Spek,
J. Chem. Soc. Dalton Trans. 1987, 1359.
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Received: February 7, 2011
Published online: May 9, 2011
Keywords: chain compounds · dinuclear complexes ·
.
Kharasch reaction · N ligands · palladium
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