Anionic Palladium(0) and Palladium(II) Ate Complexes

Article abstract of DOI:10.1002/anie.201707362

Palladium ate complexes are frequently invoked as important intermediates in Heck and cross-coupling reactions, but so far have largely eluded characterization at the molecular level. Here, we use electrospray-ionization mass spectrometry, electrical cond

Full text of DOI:10.1002/anie.201707362

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Accepted Article
Title: Anionic Palladium(0) and Palladium(II) Ate Complexes
Authors: Marlene Kolter, Katharina Böck, Konstantin Karaghiosoff, and
Konrad Koszinowski
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10.1002/anie.201707362
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Anionic Palladium(0) and Palladium(II) Ate Complexes
Marlene Kolter,[a] Katharina Böck,[b] Konstantin Karaghiosoff,[b] and Konrad Koszinowski*[a]
Abstract: Palladium ate complexes are frequently invoked as
important intermediates in Heck and cross-coupling reactions, but so
far have largely eluded a characterization at the molecular level.
Here, we use electrospray-ionization mass spectrometry, electrical
conductivity measurements, and NMR spectroscopy to show that the
resulting from oxidative additions.[13] Unlike Jutand and Amatore,
they suggested the formation of square-planar complexes, in
which the aryl halide coordinates the Pd center via its halogen
atom (Scheme 1).[13]
electron-poor
catalyst
[L3Pd]
(L = tris[3,5-bis(trifluoromethyl)-
phenyl]phosphine) readily reacts with Br− ions to afford the anionic,
zero-valent ate complex [L3PdBr]−. In contrast, more electron-rich Pd
catalysts display lower tendencies toward the formation of ate
complexes. Combining [L3Pd] with LiI and an aryl iodide substrate
ArI results in the observation of the PdII ate complex [L2Pd(Ar)I2]−.
Palladium catalysis forms a cornerstone of modern organic
synthesis. Among the different classes of Pd-catalyzed
transformations, Heck and cross-coupling reactions are of
particular importance and are used in countless applications.[1]
In many cases, the presence of salts (or ionic liquids) MX leads
to a boost in reactivity.[2] Jutand, Amatore, and coworkers were
the first to propose a detailed mechanistic rationalization of the
beneficial effect of these additives.[ 3 ] Using electrochemical
methods and 31P NMR spectroscopy, these authors were able to
detect Pd ate complexes, which result from the addition of the
anion X− to the Pd catalyst.[3,4] These anionic and nucleophilic
zero-valent Pd complexes[5] supposedly have a high reactivity
toward aryl halides and readily undergo oxidative addition.
According to Jutand and Amatore, the resulting intermediates
correspond to penta-coordinate PdII ate complexes.
(Scheme 1).[3] After dissociation of the anion, transmetallation,
and reductive elimination, the catalytic cycle is closed.
Scheme 1. Catalytic cycle of Pd-mediated cross-coupling reactions involving
ate complexes (X = halogen). For the intermediate resulting from the reaction
of the zero-valent catalyst with the substrate ArX, both a pentagonal and a
square-planar coordination geometry have been suggested.[3,13]
Here, we characterize Pd0 and PdII ate complexes by a
combination of electrospray-ionization (ESI) mass spectrometry,
electrical conductivity measurements, and multi-nuclear NMR
spectroscopy. 31P NMR spectroscopy is frequently employed for
the analysis of Pd phosphine complexes.[3a,b,4a,c,5] The former
two methods selectively detect charged species and, thus, are
particularly well-suited to address the present problem.[14]
We started with the hypothesis that electron-poor phosphines
facilitate the formation of anionic Pd0 ate complexes and chose
[L3Pd] as model system (see Chart). This complex is known as
an active catalyst in Heck and cross-coupling reactions.[15,16]
Although the role of anionic Pd ate complexes in cross-
6
,
7
]
coupling reactions is widely accepted,[2i,n,q,
the
characterization of these intermediates is far from complete.
Apart from the early suggestions of Jutand and Amatore, only a
few studies reported experimental evidence in support of Pd0 ate
complexes.[5- 10] This scarcity of experimental data presumably
reflects the relatively low stability of Pd0 ate complexes with
standard phosphine ligands. Quantum-chemical calculations on
the model complexes [(R3P)2PdX]− (R = Me, Ph; X = Cl, Br, I,
OAc) indeed predicted Pd−X− binding energies of only 40 –
90 kJ mol−1.[11, 12] Goossen, Thiel, and coworkers also applied
theoretical methods to investigate the PdII ate complexes
[a]
[b]
M. Kolter, Prof. K. Koszinowski
Institut für Organische und Biomolekulare Chemie
Universität Göttingen
Tammannstr. 2, 37077 Göttingen, Germany
E-mail: konrad.koszinowski@chemie.uni-goettingen.de
K. Böck, Prof. K. Karaghiosoff
Department Chemie
Ludwig-Maximilians-Universität München
Butenandtstr. 5-13, 81377 München, Germany
Negative-ion mode ESI of solutions of [L3Pd] and LiBr (5 eq.) in
tetrahydrofuran (THF) gave strong signals of the ate complexes
[LnPdBr]−, n = 2 and 3, [L3Pd2Br3]−, and [L4Pd2Br]− (Figure 1).[17]
Except for [L3Pd2Br3]− with an average oxidation state of I, these
complexes contained zero-valent palladium. In all cases, the
given assignments fully agreed with the measured exact m/z
ratios and isotope patterns (Table S1 and Figures S1 – S4,
Supporting Information).
Supporting information for this article is given via a link at the end of
the document.
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but sharply increased when LiBr was added in a titration
experiment (Figure 2, left, and Figures S31 – S33). Pure LiBr in
THF showed a very low conductivity, as is already known from
the literature.[ 19 ] Control experiments demonstrated that the
addition of LiBr to a solution of L did not increase the
conductivity (Figure S34) and, thus, confirmed that the ligand
does not interact with bromide anions to a significant extent. The
increase in conductivity observed upon mixing [L3Pd] and LiBr
reflects the formation of a new species that spontaneously
affords free ions and, thus, must correspond to the Pd0 ate
complexes Li+[LnPdBr]−, eq. (2) with n = 2 and 3. Raising the
overall concentration while keeping the ratio between [L3Pd] and
LiBr constant resulted in an almost linear increase of the
conductivity (Figure 2, right, and Figure S35). This behavior
Figure 1. Negative-ion mode ESI mass spectrum of a solution of [L3Pd] (3 mM)
and LiBr (15 mM) in THF.
suggests that the Li+[LnPdBr]− complexes have
a strong
tendency to dissociate into free ions even at relatively high
concentrations.
The mononuclear complexes [L2PdBr]− and [L3PdBr]− were also
observed when LiBr was replaced for NBu4Br (Figure S5). In
contrast, the Br− adduct [LBr]− showed a much lower signal
intensity when solutions of the phosphine (without Pd) were
treated with LiBr (Figure S6). This finding demonstrates that Br−
does not strongly bind to L and, thus, suggests that it indeed
coordinates directly to the metal center in the Pd-containing
complexes. Upon the addition of LiCl, LiI, or Li(OAc) to [L3Pd],[18]
the complexes [LnPdCl]−, [LnPdI]−, n = 2 and 3, and [L3Pd(OAc)]−,
respectively, were observed (Figures S7 – S14). Solutions of
[L3Pd] without any additive gave only weak signal intensities
(Figure S15).
Li+[LnPdBr]−
[LnPdBr]− + Li+(solv)
(2)
To examine the effect of the electronic properties of the
phosphine on the stability of the Pd ate complexes, we treated
solutions of [L3Pd] and LiBr with 3 equiv. of LPh1 and LPh2 (see
Chart above), respectively. These experiments afforded mixed
complexes, but clearly showed that the less electron-poor
phosphines LPh1 and LPh2 had lower affinities toward the anionic
Pd ate complexes than L, in line with our initial hypothesis
(Figures S16 – S20). The even more electron-rich complex
[(PPh3)4Pd] failed to give any detectable ate complexes in the
presence of LiBr.
Figure 2. Specific electrical conductivities (298 K) of (left) a solution of [L3Pd]
(c0 = 0.5 mM (white), 1.0 mM (grey), 2.0 mM (black)) in THF with varying
amounts of LiBr and of (right) a 1:1-mixture of [L3Pd] and LiBr in THF at
different concentrations.
Gas-phase fragmentation of mass-selected [L3PdBr]− led to
sequential losses of phosphine ligands, eq. (1) with n = 1 and 2
(Figure S21). Analogous fragmentation reactions were also
observed for the other Pd ate complexes (Figures S22 – S24).
Energy-dependent fragmentation experiments found the mixed
complexes to lose LPh1 and LPh2 much more easily than L
(Figures S25 – S30), thus confirming the higher affinity of the
latter toward the anionic Pd0 center.
Other lithium halides and (Bu4N)(OAc) also increased the
electrical conductivity of solutions of [L3Pd] (Figures S36 – S39),
whereas LiClO4 with its weakly coordinating perchlorate anion
had no effect (Figure S40). The addition of LiBr to a solution of
[Pd(PPh3)4] had only a small effect (Figures S41 and S42), thus
again indicating a lower tendency of this more electron-rich
complex to add Br− anions and form ate complexes.
We went on to investigate solutions of [L3Pd] in THF by
31P NMR spectroscopy. [L3Pd] displayed a sharp signal at
26.6 ppm at 198 K, which was shifted to 20.4 ppm upon the
addition of LiBr (Figure 3, top). We assign the shifted signal to
the Pd ate complexes Li+[LnPdBr]−, n = 2 and 3. Furthermore,
the signal of the free ligand appeared at −3.7 ppm. Its small
magnitude indicated that the association/dissociation equilibrium
between [L3PdBr]− and [L2PdBr]− lies largely on the side of the
tetra-coordinate complex, in accordance with the predominance
of this species in the ESI mass spectra. Replacing LiBr by
NBu4Br led to similar results (Figure S43). In another experiment,
we determined the ratio between free [L3Pd] and the
corresponding ate complexes [LnPdBr]− for 1:1 mixtures of [L3Pd]
[L3PdBr]−
→
[L3−nPdBr]−
+
n L
(1)
The facile loss of the phosphine, but not of the Br− anion,
highlights the considerable strength of the interaction between
the metal center and the halide. At the same time, it implies that
the complexes in solution presumably can also easily switch
between different coordination states. Only the less coordinated
complex should be prone to undergo oxidative addition in cross-
coupling reactions.[14g]
Next, we turned to electrical conductivity measurements. The
conductivity of a solution of [L3Pd] in THF was negligibly small,
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and LiBr at different concentrations. A comparison of the
measured data with simulations suggests that the formation of
the ate complexes is characterized by an effective association
constant of 1 ≤ Kass ≤ 10 μM−1 (Figure 3, bottom, Figure S44,
Table S2). This finding once more points to the high tendency of
[L3Pd] to form ate complexes.
Figure 4. Negative-ion mode ESI mass spectrum of a solution of [L3Pd]
(3.0 mM), LiI (15.0 mM), and ArI (15 mM) in THF after a reaction time of 45 min.
This reactivity is consistent with a penta- coordinate structure of
the parent ion (Scheme 2), but does not seem well compatible
with the alternatively suggested square-planar isomer, which
should eliminate ArI instead. The same fragmentation behavior
was found for the analogous complexes [L2,Pd,Ar,Br2]− and
[L2,Pd,Ar,Br,I]− (Figures S57 – S60).
Scheme 2. Gas-phase fragmentation of [L2Pd(Ar)I2−].
Figure 3. Top: 31P NMR spectra of a solution of [L3Pd] (c = 2 mM) in THF at
198 K in the absence (black) and in the presence (red) of 1 eq. of LiBr.
Bottom: Normalized fractions of the ate complexes [LnPdBr]−Li+, n = 2 and 3,
(red symbols, error bars correspond to one standard deviation) and [L3Pd]
In conclusion, we have used a combination of ESI mass
spectrometry, electrical conductivity measurements, and NMR
spectroscopy to investigate the tendency of Pd species to form
ate complexes. Pd0 ligated by the electron-poor ligand L adds
Br− anions much more readily than Pd0 bound to more electron-
rich phosphines. The resulting ate complex [L3PdBr]− easily
loses one ligand L to give a tri-coordinate complex, which
supposedly corresponds to the catalytically active intermediate
in cross-coupling reactions. Upon the addition of the aryl iodide
ArI, the newly formed ion [L2Pd(Ar)I2]− is detected by ESI mass
spectrometry. Gas-phase fragmentation experiments provide
first experimental evidence that this complex adopts a penta-
coordinate structure. The present results also open the door to
future ion-mobility measurements, which promise to probe the
geometry of Pd0 and PdII ate complexes in full detail.
−1
(black symbols) together with simulations for Kass = 0.1 µM (dashed lines),
Kass = 1 µM−1 (solid lines), and Kass = 10 µM−1 (dotted lines).
In accordance with an earlier report of Negishi and
coworkers,[5] the addition of LiBr also resulted in a shift of the
31PNMR signal of [Pd(PPh3)4] and, thus, indicated an interaction
between these two components (Figure S45). However, even at
low temperatures the low intensity and extreme broadness of
this signal prohibited a quantitative analysis. We also used
19F NMR spectroscopy to analyze solutions of [L3Pd]. Again, the
addition of NBu4Br led to a broadening and shift of the 19F signal,
as one would expect for the formation of [L3PdBr]− (Figure S46).
Finally, we added ethyl 4-iodo-benzoate (ArI) to a solution of
[L3Pd]/LiI and analyzed the resulting mixture by ESI mass
spectrometry (Figure 4). The observed base peak [L2,Pd,Ar,I2]−
corresponds to the above-mentioned intermediate, whose
structure has been discussed controversially (Scheme 1).[ 20 ]
Time-dependent measurements showed its gradual build-up at
the expense of reactant [L3Pd] and permitted the determination
of a reaction half-life of approx. 15 min (Figures S47 and S48).
Analogous species were also found in solutions of [L3Pd]/LiBr
and ArI (Figures S49 – S53), whereas the related aryl bromide
ArBr did not afford any reaction intermediates (Figure S54).
Gas-phase fragmentation of the mass-selected complex
[L2,Pd,Ar,I2]− resulted only in the loss of the phosphine ligands
(Figures S55 and S56).
Experimental Section
General. To exclude moisture and oxygen, standard Schlenk techniques
were applied in all cases. Lithium halides were repeatedly heated to
573 K in vacuo and then stored under inert atmosphere. THF was dried
over sodium/benzophenone and freshly distilled before use. THF-d8
(eurisotop, ≥99.5%) was distilled and degassed by the freeze-pump-thaw
method before use. The phosphines LPh1 and LPh2 were synthesized in
analogy to the synthesis of L (see Supporting Information).[16]
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ESI mass spectrometry. Sample solutions were injected into the ESI
source of a micrOTOF-Q II instrument (Bruker Daltonik)[21] at a typical
flow rate of 8 μL min−1. The ESI source was operated with a voltage of
3500 V and N2 as nebulizer gas (flow rate of 5 L min−1) and drying gas
(0.7 bar backing pressure, temperature of 333 K). In simple MS1
experiments, all generated ions with 50 ≤ m/z ≤ 3000 were allowed to
pass the quadrupole mass filter of the instrument. In MS2 fragmentation
experiments, ions of interest were mass-selected in the quadrupole-mass
filter, accelerated to a kinetic energy ELAB, and allowed to collide with N2
gas. Residual parent ions and resulting fragment ions were then detected
after passage through the TOF analyzer. Typically, accuracies of the
measured m/z ratios of 25 ppm were achieved (external calibration with a
mixture of CF3COOH and phosphazenes in H2O/CH3CN). Theoretical
exact m/z ratios and isotope patterns were calculated with the
DataAnalysis software package (Bruker Daltonik).
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Electrical conductivity measurements. Electrical conductivity
measurements were performed with a SevenMulti instrument (Mettler
Toledo) and a stainless steel-electrode cell (InLab741, Mettler Toledo,
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Acknowledgements
We thank Professor Herbert Mayr for his generous support and
gratefully acknowledge funding from the CaSuS (Catalysis for
Sustainable Synthesis) program (scholarship for M. K.).
Keywords: cross-coupling • mass spectrometry • palladium •
reactive intermediates • salt effects
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Entry for the Table of Contents
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M. Kolter, K. Böck, K. Karaghiosoff, and
K. Koszinowski*
Page No. – Page No.
Anionic Palladium(0) and
Palladium(II) Ate Complexes
Ate anions analyzed: Palladium ate complexes are supposed to act as central
intermediates in Heck and cross-coupling reactions, but so far have escaped an in-
depth characterization. These complexes can be stabilized by ligation with an
electron-poor phosphine, thus enabling their analysis by a combination of ESI mass
spectrometry, electrical conductivity measurements, and multi-nuclear NMR
spectroscopy.
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