A robust and efficient Pd3 cluster catalyst for the suzuki reaction and its odd mechanism

Article abstract of DOI:10.1021/acscatal.6b02527

The palladium-catalyzed Suzuki-Miyaura coupling reaction is one of the most versatile and powerful tools for constructing synthetically useful unsymmetrical aryl-aryl bonds. In designing a Pd cluster as a candidate for efficient catalysis and mechanistic investigations, it was envisaged to study a case intermediate between, although very different from, the "classic" Pd(0)Ln and Pd nanoparticle families of catalysts. In this work, the cluster [Pd₃Cl(PPh₂)₂(PPh₃)₃]+[SbF₆]- (abbreviated Pd3Cl) was synthesized and fully characterized as a remarkably robust framework that is stable up to 170 °C and fully air-stable. Pd3Cl was found to catalyze the Suzuki-Miyaura C-C crosscoupling of a variety of aryl bromides and arylboronic acids under ambient aerobic conditions. The reaction proceeds while keeping the integrity of the cluster framework all along the catalytic cycle via the intermediate Pd3Ar, as evidenced by mass spectrometry and quick X-ray absorption fine structure. In the absence of the substrate under the reaction conditions, the Pd3OH species was detected by mass spectrometry, which strongly favors the "oxo-Pd" pathway for the transmetalation step involving substitution of the Cl ligand by OH followed by binding of the OH ligand with the arylboronic acid. The kinetics of the Suzuki- Miyaura reaction shows a lack of an induction period, consistent with the lack of cluster dissociation. This study may provide new perspectives for the catalytic mechanisms of C-C cross-coupling reactions catalyzed by metal clusters.

Full text of DOI:10.1021/acscatal.6b02527

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Article
A Remarkably Robust and Efficient Pd3 Cluster Catalyst
for the Suzuki Reaction and its Odd Mechanism
Fangyu Fu, Ji Xiang, Hao Cheng, Longjiu Cheng, Hanbao Chong,
Shuxin Wang, Peng Li, Shiqiang Wei, Manzhou Zhu, and Yadong Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02527 • Publication Date (Web): 24 Jan 2017
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A Remarkably Robust and Efficient Pd3 Cluster Catalyst for the
Suzuki Reaction and its Odd Mechanism
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Fangyu Fu,a,§ Ji Xiang,a,§ Hao Cheng,b LongjiuCheng,a Hanbao Chong,a Shuxin
Wang,a Peng Li,*,a Shiqiang Wei,*,b Manzhou Zhu,*,a Yadong Lic
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a Department of Chemistry and Center for Atomic Engineering of Advanced Materials,
Anhui University, Hefei, Anhui Province 230601, P. R. China
b National Synchrotron Radiation Laboratory, University of Science and Technology
of China, Hefei 230029, P. R. China.
c Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China
Abstract
The palladiumꢀcatalyzed SuzukiꢀMiyaura coupling reaction is one of the most
versatile and powerful tools for constructing the synthetically useful unsymmetrical
arylꢀaryl bonds. In designing a robust Pd cluster as candidate for efficient catalysis
and mechanistic investigations, it was envisaged to study a case intermediate
betweenꢀ, although very different from the “classic” Pd(0)Ln and Pd nanoparticle
families of catalysts. Herein, the cluster [Pd3Cl(PPh2)2(PPh3)3]+ [SbF6]ꢀ (abbreviated
Pd3Cl) was synthesized and fully characterized as a remarkably robust framework
stable up to the 170°C and fully air stable. Pd3Cl was found to be an excellent catalyst
for the SuzukiꢀMiyaura cross CꢀC coupling of a variety of aryl bromides and aryl
boronic acid under ambient aerobic conditions. The reaction proceeds by keeping the
integrity of the cluster frame all along the catalytic cycle via the intermediate Pd3Ar
evidenced by mass spectrometry, and quick Xꢀray absorption fine structure. In the
absence of the substrate in the reaction condition the Pd3OH species was detected by
mass spectrometry, which strongly favors the “oxoꢀPd” pathway of the
transmetallation step involving substitution of the Cl ligand by OH followed by
binding of the OH ligand with the arylboronic acid. The kinetics of the
SuzukiꢀMiyaura reaction shows the lack of induction period consistent with the lack
of cluster dissociation. This study may provide new perspectives for the catalytic
mechanisms of cross CꢀC coupling reactions catalyzed by metal clusters.
Keywords: metal cluster; catalysis; SuzukiꢀMiyaura reaction; mechanism of
Pdꢀcatalyzed reaction; crossꢀCꢀC coupling
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Introduction
The palladiumꢀcatalyzed SuzukiꢀMiyaura reaction1,2 is among the most efficient
and useful cross carbonꢀcarbon coupling reactions in organic synthesis, because it
allows the synthesis of a large variety of organic compounds from readily accessible
reactants under mild conditions.3,4 In particular unsymmetrical arylꢀaryl coupling
using the SuzukiꢀMiyaura reactions between aryl bromide and arylboronic acids leads
to the biaryl scaffold that has received increased attention as a privileged structure by
the pharmaceutical industry. This motif has shown activity across a wide range of
therapeutic classes including antifungal, antiꢀinflammatory, antirheumatic, antitumor,
and antihypertensive agents.5 Here we present a new approach of this reaction using a
robust cationic Pd3 cluster catalyst that bridges the areas of wellꢀdefined mononuclear
palladium complexes[1ꢀ7] and palladium nanoparticles[8ꢀ13] that have so far both
provided efficient catalysts for this reaction. The interest of this work is the handling
and use of an air stable, robust catalyst that therefore in the same time can be
subjected to specific mechanistic investigations some of which are presented herein.
The catalytic cycle of the SuzukiꢀMiyaura reaction is well known to be mainly
composed of oxidative addition, transmetallation and reductive elimination with
classic molecular catalysts of the type Pd(0)L2 usually generated in situ from
preꢀcatalysts consisting of a Pd(II) salt and a phosphine (L).1,14ꢀ27 The transmetallation
step of this mechanism has been much debated, however (vide infra).14ꢀ27 The
reaction mechanism is more elusive with palladium nanoparticles, although it has
been considered that a related mechanism also proceeds at the Pd atom level
subsequent to leaching rather than at the nanoparticle surface.8,13 Indeed, it has been
disclosed that even subꢀppm Pd nanoparticles stabilized by dendrimers were effective
for the Suzuki reaction.12,13 In such cases the surface structure of nanoparticles is hard
to probe and reaction intermediates are not detectable. The atomically precise
nanoclusters that are a recently developed field modelize nanoparticles and have
therefore stimulated tremendous interest regarding their catalytic implications.24ꢀ26 For
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z
instance our group, for the first time, employed Au25 (z = ꢀ1,0,+1) nanoclusters as
electron transfer catalysts to induce an intramolecular cascade reaction in liquid
phase.[27] Various Pd clusters have also been generated to investigate the correlation
between their structures and chemical properties.[32ꢀ41] Corma’s group found that CꢀC
coupling reactions catalyzed by various palladium sources including a salt, a complex
and nanoparticles proceeded at high rates when threeꢀ or fourꢀatom Pd clusters were
formed. Such small Pd clusters were suggested to be the active catalytic species in the
CꢀC crossꢀcoupling reactions. 42
Here we report the very facile catalysis of the SuzukiꢀMiyaura reaction under
ambient and aerobic condition using the robust cluster [Pd3Cl(PPh2)2(PPh3)3]+ [SbF6]ꢀ
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ꢀ
noted in short Pd3Cl. The synthesis of the cationic cluster with the BF4 counter anion
has previously been reported by Dixon’s group, but the Xꢀray crystal structure was
not reported for this complex.43 The formation of a triangular Pd3 cluster containing
PPh2 bridges resulting from Pꢀphenyl bond cleavage in PPh3 had first been suggested
earlier by Coulson. The compound had been formulated as a neutral complex
[Pd3(PPh3)3(PPh2)3Cl2], and the exact molecular formula had not been disclosed.44
We have devised a new method to prepare Pd3Cl (Scheme 1) and conducted
SuzukiꢀMiyaura coupling with a variety of substrates using the Pd3Cl catalyst. Our
finding is accompanied by studies using mass spectrometry, NMR and optical
spectroscopy, quick Xꢀray absorption fine structure, and density functional theory
calculation (DFT). The crystal structures of the catalyst before and after the reaction
are disclosed and kinetic studies were also performed. Altogether these investigations
show that this cluster that is unusually robust is an excellent catalyst and allow
suggesting particular mechanistic features that significantly differ from those
generally ascertained using the classic palladium chloride/phosphine catalysts.
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Results
Crystal and electronic structure of Pd3X (X = Cl or Br)
The new standard procedure for the synthesis of Pd3Cl follows Scheme 1 (see the
experimental section).
+
Ph
Ph
Ph
Ph
Cl-
P
Ph
Ph
Ph
Ph
P
Pd
NaBH4
P
PdCl2 + 2 PPh3
Ph
Ph
Ph
Pd
Pd
H2O, HCl,
THF
P
P
Ph
Ph
Cl
NaSbF6
EtOH
+
Ph
P
Ph
Ph
P
Ph
Ph
-
SbF6
Pd
Ph
P
Ph
Pd
Pd
Ph
Ph
Ph
P
P
Cl
Ph
Ph
Ph
Pd3Cl
Scheme 1. Synthesis of the Pd3Cl catalyst
The structure of Pd3Cl was determined by Xꢀray crystallography (see the crystal
data in Table S1) and is shown in Figure 1. UVꢀvis. absorption measurements show
that the Pd3Cl cluster exhibits multiple bands centered at 485, 418 and 340 nm
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(Figure S1). Squareꢀwave voltammetry in CH2Cl2 shows a reversible oxidation wave
at +1.13 V vs. Ag+/Ag and a reversible reduction wave at – 1.32 V vs. Ag+/Ag. This
shows stability extended to the reduced and oxidized cluster states at least at the
cyclovoltammetry time scale; it also highlights the existence of a very large
HOMOꢀLUMO energy gap (vide infra) that is consistent with a great robustness,
including in aerobic conditions.
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Figure 1. Crystal structure of the Pd3Cl catalyst [Pd3Cl(PPh2)2(PPh3)3]+ [SbF6]ꢀ.
Pd3Cl has a single positive charge on the cluster, thus an average oxidation state
of +4/3 for the three Pd atoms and an electron count of 16.67 valence eꢀ per Pd. There
are 74 valence electrons in Pd3Cl modelized here by the cluster
[Pd3(PH2)3(PH3)3]Cl+, in which each Pd, P, H atoms contribute 10, 5 and 1
valence electrons, respectively. According to the Adaptive Natural Density
Partitioning (AdNDP) analysis, as shown in Figure 2 (B, C and D) using this
simplified model cluster, there are twelve loneꢀpair 4d orbits (yz,zx,x2ꢀy2,z2) of
three Pd atoms with occupancy numbers (ON = 1.93ꢀ1.99 |e|), fifteen PꢀH
bonds (ON = 1.99 |e|), nine PꢀPd bonds (ON = 1.97ꢀ1.98 |e|), and one
threeꢀcenter twoꢀelectron (3cꢀ2e) delocalized PdꢀPdꢀPd bond (ON = 1.98 |e|).
The P atoms are in sp3 hybridization, and the symmetry of the PꢀPd and
PdꢀPdꢀPd bonds shows that Pd is in dsp2 hybridization (4dxy, 5s and 5pxy). The
square dsp2 hybridization of Pd leads to the planarity of the cluster; the 3
centerꢀ2 electron PdꢀPdꢀPd bond and the bridging ligands are the reason for the
stability of the Pd3 triangle. The PPh3 and PPh2 ligands of Pd3Cl provide a bulky
protection making the approach of the Pd centers by large substrates difficult.
DFT calculation was also used to calculate the HOMOꢀLUMO energy gap
of Pd3Cl, Pd3Br and Pd3Ar. Figure 2E shows the DFT optimized structures of
these three clusters at the TPSSh/6ꢀ31G*/LANL2DZ level, where the starting
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coordinates are from the singleꢀcrystal structure of Pd3Cl. The HOMOꢀLUMO
energy gap of Pd3Cl is calculated to be 2.61 eV in good agreement with the
experimental value (~2.49 eV, Figure S2; compare with 2.45 eV in
squareꢀwave voltammetry, vide supra), which indicates that the results of these
calculations are reliable. The molecular structure of Pd3Br is very similar to
that of Pd3Cl without distortion, and its HOMOꢀLUMO energy gap is also very
large (2.55 eV), indicating the similar stability of these two clusters. However,
the HOMOꢀLUMO energy gap of Pd3Ar is obviously lower (2.19 eV), which
indicates that Pd3Ar is somewhat less stable than the other two structures.
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Figure 2. Optimized geometry (A) and AdNDP chemical bonding (B, C, D)
of [Pd3(PH2)3(PH3)3]+. E: Optimized structures of Pd3Cl
, Pd3Br and Pd3Ar
at TPSSh/6ꢀ31G*/LANL2DZ level (the calculated HOMOꢀLUMO energy
gaps are indicated in parentheses at the bottom of the figure).
Suzuki-Miyaura reactions catalyzed by Pd3Cl
The cluster Pd3Cl was utilized to catalyze the Suzuki–Miyaura crossꢀcoupling
reaction between various aryl bromides and phenylboronic acid (Table 1). All the
substituted substrates with either electronꢀdonating groups or electronꢀwithdrawing
substituents gave rise to high yield products in 15ꢀ60 min reaction time
(turnover frequency (TOF): 1100ꢀ5000 hꢀ1).
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Table 1. Catalytic activity of [Pd3Cl(PPh2)2(PPh3)3]+ in a Suzuki coupling reaction[a]
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Time
(min)
Yield[b]
(%)
Entry
R
R’
Product
TON
TOF(h-1)
1
2
H
CH3
CH3O
t-Bu
CH3CO
NO2
CF3
H
H
H
30
30
30
60
60
60
60
15
15
15
30
30
97.2
93.3
94.1
99
1220
1200
1220
1230
1160
1170
1240
1240
1200
1230
1190
1210
2450
2390
2440
1230
1160
1170
1240
4950
4820
4920
2370
2420
3
H
4
H
5
H
91.1
90.0
96.2
96.4
93.8
95.6
92.3
94.1
NO2
6
H
7
H
8
CH3
CH3O
t-Bu
CF3
Cl
9
H
10
11
12
H
H
H
[a] Reaction conditions: bromobenzene (1.64 mmol), arylboronic acid (1.64 mmol),
0.08 mol% catalysts, K2CO3 (3.28 mmol),
yields.
iꢀpropanol (5 mL), water (5 mL). [b] GC
The UVꢀvis. spectra of the solution in the reaction process are represented
in Figure 3A, showing that the Pd3Cl absorption spectrum almost remains the
same, indicating the absence of apparent cluster size change or decomposition.
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The 31PꢀNMR analysis (Figure 3B) shows that the initial Pd3Cl gives signals at
11.25, 11.75, 12.25, 18.5 and 19 ppm. During the course of the catalytic
reaction, all the peaks move to lower field due to the Cl to Br exchange on the
Pd3 skeleton. Both ESI and Xꢀray analyses confirmed that Clꢀ in Pd3Cl was
replaced by Brꢀ, explaining why all the PꢀNMR peaks moved to lower field
31
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(
vide infra).
Figure 3. (A) UVꢀvis. spectrum and (B) 31PꢀNMR tracking of the
SuzukiꢀMiyaura reaction.
ESIꢀMS was performed in order to analyze the catalyst and gain insight into
the reaction intermediates of the Pd3Cl catalyzed SuzukiꢀMiyaura cross
coupling reaction. Canary and coꢀworkers observed catalytic intermediates in
the “classic” SuzukiꢀMiyaura reaction mechanism by using ESIꢀMS.45 Pd3Cl
was mixed with the two reactants, respectively, and the reaction process was
monitored by ESIꢀMS. As shown in Figure S3, Pd3Cl showed the absence of
discernable change when only bromobenzene and K2CO3 were added, but
Pd3Cl reacted with phenylboronic acid and K2CO3 in 2h. Pd3Cl would interact
with phenylboronic acid under alkaline conditions, and a new substance at m/z
1553.11 was observed (Figure 4A, profile c). Then 0.006 mL bromobenzene
was added, the 1553.11 Da peak disappeared and a new peak at 1557.06 Da
resulted (Figure 4A, profile d). This indicates that Pd3Cl first reacts with
phenylboronic acid to generate an intermediate of mass 1553.11 Da. The
structure of this intermediate is assigned to [Pd3Ar(PPh2)2(PPh3)3]+ (theoretical
mass: 1553.13 Da, denoted Pd3Ar in short), and the 1557.06 Da species is due
to [Pd3Br(PPh2)2(PPh3)3]+ (theoretical mass: 1557.00 Da, denoted Pd3Br in
short; see the crystal structure in Figure S4 and the crystal data in Table S2).
The experimental isotope patterns are consistent with the simulations based
upon the two formulas (Figure 4B). Although the mass difference is small (~4
Da only), ESI readily distinguishes Pd3Ar from Pd3Br. In order to further
prove this evolution, iodobenzene was used to replace bromobenzene in this
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process. As showed in Figure S5, when iodobenzene was added, the 1553.11
Da peak disappeared, and a new peak at 1603.03 Da resulted, which is due to
[Pd3I(PPh2)2(PPh3)3]+ (Pd3I, theoretical mass: 1602.99 Da). This experiment
further demonstrates that Pd3Ar is an intermediate of the SuzukiꢀMiyaura
reaction catalyzed by Pd3Cl. The reaction between Pd3Cl and phenylboronic
acid is in a stationary state without bromobenzene, but upon adding
bromobenzene the reaction system breaks the equilibrium, and the intermediate
Pd3Ar reacts with bromobenzene to generate Pd3Br. Before adding
bromobenzene, the only addition of phenylboronic acid and K2CO3 leads to
partial decomposition of Pd3Cl in the reaction (Figure 4A), but it does not if
both reactants are introduced at the same time (Figure S6). Hence,
bromobenzene plays a role of protecting the catalysis in this process.
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Figure 4. (A)ESIꢀMS tracking of [Pd3Cl(PPh2)2(PPh3)3]+ reacts with
phenylboronic acid under alkaline condition: 0.0072 g (0.06 mmol)
phenylboronic acid, 2 mg (1.32x10ꢀ3 mmol) catalyst, 0.0164 g (0.12 mmol)
K2CO3 and 3 mL CH2Cl2, rt. a:0 h. b:1 h. c:2 h. d: adding 0.006 mL
bromobenzene on the basis of c. (B)The evolution of [Pd3Ar(PPh2)2(PPh3)3]+ to
[Pd3Br(PPh2)2(PPh3)3]+. (C)EXAFS k2χ(k) oscillation functions, and (D) their
Fourier transform spectra of [Pd3Cl(PPh2)2(PPh3)3]+ reacting with
phenylboronic acid under alkaline condition during 2 h, with or without adding
bromobenzene. The corresponding spectra of the solution of
[Pd3Cl(PPh2)2(PPh3)3]+, solution of H2PdCl4 and Pd bulk are also displayed for
reference.
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Additional information on the structural evolution of the clusters during the
Suzuki reaction was provided by the Pd Kꢀedge XAFS measurements. We
compare the EXAFSk2χ(k) oscillation functions (Figure 4C) and their Fourier
transform spectra (Figure 4D) of Pd3Cl reacting with phenylboronic acid under
alkaline conditions, with or without the addition of bromobenzene. As a
reference, the spectra of Pd3Cl in CH2Cl2 solution, solution of H2PdCl4 and Pd
bulk are also plotted. The Fourier transform curve of the starting Pd3Cl solution
shows two characteristic peaks located at around 1.7 and 2.7 Å. As the PdꢀCl
peak of H2PdCl4, the main peak at 1.7 Å is attributed to PdꢀP and PdꢀCl that
could not be discriminated by EXAFS. The weaker peak at 2.7 Å is ascribed to
the PdꢀPd pairs, but located at higher position than the PdꢀPd peak of bulk Pd,
indicating the atomic arrangements of the triangle Pd3 skeleton of the cluster.
For the catalysis by Pd3Cl of the SuzukiꢀMiyaura reaction, regardless of the
addition time of bromobenzene, the overall EXAFS spectral shape remains
almost unchanged. This provides strong evidence that the triangle Pd3 skeleton
of the Pd3Cl catalyst appears intact during the Suzuki reaction, in agreement
with the UVꢀvis., NMR, and ESI analyses. Closer inspection of the XAFS
spectra suggests that there are still some discernible differences caused by the
different reaction times of the bromobenzene addition. Observed from the
kꢀspace EXAFS oscillation functions k2χ(k), the oscillation amplitude in the
low kꢀregion (k<5 Åꢀ1) is considerably reduced if no bromobenzene is added.
Taking into account the different scattering amplitudes of C, Cl/P, and Br atoms
to Xꢀray, this is due to the coordination of Pd atoms by lighter atoms than Cl/P,
which could only be C and/or O atoms coming from the aromatic or hydroxyl
group respectively. Combining the XAFS result with the ESIꢀMS analysis, the
formation of Pd3Ar intermediate in the reaction is ascertained. In contrast, in
the presence of bromobenzene the oscillation amplitude in the low kꢀregion is
only slightly weakened, but the oscillations in the medium kꢀregion (k~8 Åꢀ1)
are intensified. Moreover, the first coordination peak exhibits some notable
shift to higher Rꢀposition. These changes could only come from the
coordination of heavier neighbors than Cl/P to Pd atoms, consistent with the
generation of Pd3Br, rather than Pd3Ar, as suggested by the ESIꢀMS analysis.
We investigated the fate of the catalyst under the reaction conditions in the
presence of the base, but in the absence of the reagents. The MALDIꢀMS
(Figure S8) showed the presence of Pd3OH and other peaks that have not been
identified. This reactivity contrasts with the clean behavior of the catalyst in the
presence of the substrates for which only the sequence Pd3ClꢀPd3ArꢀPd3Br is
observed along the catalytic process.
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Finally the kinetic analysis of the catalytic reaction shows the absence of
induction time (Figure S9) and of linear correlation with the Hammet
parameters of the substituents of the aryl bromide and arylboronic acid (Figure
S10).
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Nevertheless, under the same condition as with bromobenzene (Table 1,
25°C) the reaction of chlorobenzene with PhB(OH)2 in 3 h yielded the SM
coupling product in 25% yield, whereas during the same time the yields raised
to 71.7% and 76.7% % at 60°C and 80°C respectively.
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Discussion
The cationic clusters Pd3X (X = Cl or Br) have been shown here to be excellent
catalysts for the SuzukiꢀMiyaura reaction of a variety of bromoarenes and aryboronic
acids in hydroꢀalcoholic medium in the presence of K2CO3 under ambient aerobic
conditions. This catalyst contrasts with the usual PdCl2/phosphine or
Nꢀheterocyclic
carbene (NHC) systems that have been shown to generate Pd(0) catalysts in situ and
for which the wellꢀknown cycle involving the succession of the three steps, oxidative
additionꢀtransmetallationꢀreductive elimination has been well established.1,2,14ꢀ28
Indeed in these classic catalysts, the generated square planar species such as PdL2S2
(L = phosphine or NHC ligand; S = labile solvent ligand) readily undergoes oxidative
addition onto the Pd(0) center as the first step of the catalytic cycle.1 On the other
hand, the present catalysts are exceptionally robust. For instance they are stable up to
170°C as shown by the thermogravimetric analysis (Figure S11). Such a great
robustness is rather exceptional in cluster chemistry and contrasts with the thermal
lability of transition metal nanoparticules that easily give rise to Oswald ripening
upon heating. Their molecular peak is easily found in the ESI mass spectrum without
loss of any ligand or other impurity. Thus these features show that the integrity of the
Pd3 cluster frame is preserved during the catalysis process, and catalysis by
mononuclear Pd species subsequent to cluster dissociation is clearly discarded. On the
contrary the first catalytic step must occur on the intact Pd3X catalyst itself. This
situation contrasts with that of many late transition metal triphenylphosphine
complexes such as WilkinsonꢀOsborn catalyst46 for which the large phosphine cone
angle47 and electronic requirement allows facile reversible phosphine dissociation
from the metal center creating free coordination sites.4 In Pd3X, the robustness is
insured by the great stability of the tetraꢀtriangular framework containing the 9
coplanar Pd3P5X atoms and in particular three bridging ligands around the Pd3 triangle
as witnessed by the theoretical calculations confirming the great stability and a very
large HOMOꢀLUMO gap. The overall positive cluster charge brings about aerobic
stability. Not surprisingly, the cationic clusters are completely unreactive with
arylbromides alone in spite of their great catalytic activity, discarding the possibility
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of such a primary step involving aryl bromide oxidative addition in the catalytic
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On the other hand and interestingly, the cluster Pd3Ar is formed as the only
observable intermediate during the catalytic reaction before the final formation of
Pd3Br when the reaction is conducted with aryl bromides or Pd3I in the presence of
aryl iodides. This shows that transmetallation is the first step proceeding before the
reaction of aryl bromide or aryl iodide.
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Transmetallation as the first step of the Suzuki reaction has to our knowledge
already been suggested in only one case with a PCsp3P pincer Pd(II) catalyst by
Olsson and Wendt in 2009, but the SuzukiꢀMiyaura reaction was conducted at 160°C,
and the product yields were mediocre to modest.48
The transmetallation step of the SuzukiꢀMiyaura reaction that is the second step
following the oxidative addition of the aryl halide onto Pd(0) has been the subject of
considerable discussion in the literature.14ꢀ28 In early works, it had been proposed that
arylboronates generated from the reaction between the arylboronic acid and the
nucleophile MeOꢀ, EtOꢀ or HOꢀ were more active than arylboronic acid in the
transmetallation step.1,14,15 Two competitive mechanisms have later been computed
and much been debated by the groups of Maseras and LloydꢀJones.16ꢀ21 (Scheme 2).
“oxo-Pd” pathway
Pd OH
HO-
Ar
B(OH)2
Pd X
[Ar-B(OH)3]-
“boronate”
Ar
B-(OH)2
Pd O+
pathway
H
B(OH)3
Pd Ar
+
Scheme 2. The two possible mechanisms for the transmetallation step of the
SuzukiꢀMiyaura reaction in Pd catalysis. With Pd3Cl the “oxo pathway” is priviledged
by the observation of the PdꢀOH species in the absence of the substrates.
The first mechanism involves substitution of the halide by the boronate in the
Pd(II)ꢀX intermediate resulting from oxidative addition (“boronate” pathway), and the
second mechanism involves substitution of the halide in Pd(II)ꢀX by the nucleophilic
base OHꢀ or ORꢀ followed by bond formation between the oxygen atom of the
hydroxo or alkoxo Pd ligand and the boron atom of the neutral borane
(“oxoꢀpalladium” pathway, Scheme 2). The priviledged mechanism has been proposed
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to depend on the reagents and reaction conditions.21 This latter mechanism was
favored, however, on the basis of kinetic and NMR studies by Carrow and Hartwig22
and by Amatore and Jutand23ꢀ26 for coupling of aryl boronic acids with simple aryl
halides in aqueous solvents with [PdLn] (L = PPh3 and PCy3) as catalyst and M2CO3
as base (producing OHꢀ by reaction with water)
An insightful review of this
.
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competition between these mechanisms has recently been published by Lennox and
LloydꢀJones.21 Amatore and Jutand also showed some negative roles of OHꢀ that are
the formation of unreactive arylborate and complexation of the OH ligand by the
counter cation of OHꢀ.26 In a recent article in Science by Thomas and Denmark, three
different species containing palladiumꢀoxygenꢀboron linkages were proposed, a
tricoordinate boronic acid complex, and two tetracoordinate boronate complexes with
2: 1 and 1: 1 stoichiometry with respect to palladium. All of these species transfer their
boronꢀbearing aryl groups to a coordinatively unsaturated palladium center in the
critical transmetallation step.27 All these investigations refer to the Pd(II) intermediate
species that follows the classic oxidative addition and show the complexity of this step.
Concerning the presently reported catalysis by the Pd3Cl species, the attack of an
anion is favored by the positive charge, and this significant electrostatic effect is
consistent with the priority of the transmetallation step before the reaction of ArBr.
The present situation is therefore different from the classic mechanism of PdL2 species,
because the transmetallation occurs here with a Pd species that has not yet reacted with
the aryl halide. Note, however, that the chloro ligand is present in the catalyst, which is
reminiscent of the halogenoꢀalkyl Pd(II) intermediate following oxidative addition of
the PdLn catalysts (but the Pd oxidation states are different). The ClꢀPd bond is actually
the only active site in the otherwise very bulky Pd3Cl catalyst. The two options of the
“oxoꢀpalladium” vs. “boronate” mechanism pathway both also remained open here,
however (Scheme 2).
Indeed, the bridging chloro ligand of Pd3Cl could be substituted either by OHꢀ
resulting from the reaction of carbonate with water or with boronate resulting from the
nucleophilic attack of OHꢀ onto the B atom of RB(OH)2. The experiment conducted
with the base under the reaction conditions (presence of excess water, 5 mL) in the
absence of substrate showing the formation of some Pd3OH species involved in the
“oxoꢀpalladium” mechanism strongly favors this later transmetallation mechanism.
Kinetic studies showed the absence of induction time, consistent with the absence of
cluster dissociation, but no correlation could be established with the Hammet
parameters of the different substituents to determine the rateꢀlimiting step of the
reaction (see SI, Figures S9 and S10). Computational studies are called for and should
confirm the “oxoꢀPd” pathway of the transmetallation step and investigate the reaction
mechanism of the aryl halide with the Pd3Ar intermediate. A priori, an oxidative
addition would be difficult given the steric crowding around the Pd centers and the
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oxidation state of +4/3, and a
σ
ꢀbond metathesis is more likely for this step. So far the
order of the steps is now well established. Presently the overall mechanism is indicated
in Scheme 3. After one cycle the species Pd3Br is formed and becomes the catalyst for
further cycles as confirmed also using this complex as the starting catalyst.
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Scheme 3. Mechanism of Suzuki coupling reaction catalyzed by the Pd3Cl
cluster [Pd3Cl(PPh2)2(PPh3)3]+ [SbF6]ꢀ.
Conclusion
A
Pd3Cl cluster has been designed and characterized here as a very robust
framework, stable up to 170°C. This cluster has been used as catalyst for the
SuzukiꢀMiyaura reaction in air whereby short reaction times, room temperature
and low Pd concentrations have been applied with excellent results. The catalytic
reaction was followed in order to investigate its mechanism. The crystal
structures of the catalysts before and after the reaction were also resolved, and
the identity of the structures suggests that the reaction is catalyzed by the intact
Pd3 frame. During the reaction Pd3Cl first reacts with phenylboronic acid to
generate the intermediate Pd3Ar. This difference is essential with the “classic”
mechanism of SuzukiꢀMiyaura reactions under similar room temperature
conditions (using alkylphosphineꢀPd catalysts)49 that also proceed in which
oxidative addition is the first step, as summarized in Scheme 4.
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Pd(0)
ArX
Pd
Pd X
5
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8
Ar'B(OH)2
+ OH-
Ar Ar'
Ar
Ar
Pd
B(OH)3
9
ArX
X
Ar'
Ar'
Pd
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Ar'B(OH)2
B(OH)3
+ OH-
“Classic mechanism” with PdLn
New mechanism with Pd3X
Scheme 4. Comparison between the “classic” mechanism of the SuzukiꢀMiyaura
reaction and the mechanism shown with the Pd3Cl cluster as catalyst (see the
mechanistic details in Schemes 2 and 3).
In the absence of substrate, the catalysts reacts with a base under the
reaction conditions of the catalytic reaction (rt) to form Pd3OH resulting from
the substitution of the Cl ligand by OH. The formation of the species Pd3OH at
rt strongly argues in favor of its intermediacy in the catalytic reaction, i.e. in
favor of the “oxoꢀpalladium” mechanism of the transmetallation step that is the
first step of the SuzukiꢀMiyaura reaction catalyzed by Pd3Cl. The substrate
Ar,ꢀBr attacks the intermediate Pd3Ar to generate the final product, while Brꢀ
replaces Arꢀ in the intermediate Pd3Ar to form Pd3Br that continues the role of
Pd3Cl as catalyst in the SuzukiꢀMiyaura reaction. Novel insights into the
catalysis of various cross CꢀC coupling reactions catalyzed by metal clusters or
nanoparticles can further be envisaged at the light of this new catalyst type and
mechanistic operation. In conclusion, the new catalyst Pd3X (X = Cl or Br) are
very different from both the mononuclear and nanoparticle families of Pd
catalysts due to their exceptional robustness with only open reactive site at the
PdꢀX bond of the rigidly held Pd3 cluster framework.
Experimental Section
General data
All the chemicals and reagents are commercially available and were used
as received. Thin plate chromatography (TLC) commercial Merck Silica Gel 60
F254 was used for analytical TLC, and Merck Kieselgel 200ꢀ300 was used in
1
preparative column chromatography. H NMR spectra were acquired on a
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Brucker AM 400 operating at 400 MHz (solvent: CDCl3 or CD2Cl2). C NMR
spectra were acquired on a Bruker AM 400 operating at 100 MHz (solvent:
CDCl3). 31P NMR spectra were acquired on a Bruker AM 400 operating at 100
MHz (solvent: CD2Cl2). Conversion and was measured with Shimadzu GC
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2010 plus. The crystal structure was determined via a CCD Xꢀray
diffractometry. UVꢀvis. spectra were recorded on a Techcomp UV1000
spectrophotometer. Mass spectrometry analyses, including electrospray
ionization mass spectrometry (ESIꢀMS) and matrixꢀassisted laser desorption
ionization mass spectrometry (MALDIꢀMS), were performed on Agilent and
Bruker instruments, respectively. Square wave voltammetry measurements
(SWV) were performed on an electrochemical workstation (CHI 660D) at a
scan rate of 0.05 V·sꢀ1 under ambient conditions. Quick Xꢀray absorption fine
structure (QXAFS) measurements at Pd Kꢀedge (24 350 eV) were performed in
transmission mode at BL14W1 beamline of the Shanghai Synchrotron
Radiation Facility (SSRF), China. The storage ring of SSRF worked at 3.5 GeV
with a maximum current of 210 mA. In the QXAFS measurements, the Si (311)
monochromator moved continuously, so that quick measurement (about 15 s
per scan) became possible. Electrochemical measurements were performed with
an electrochemical workstation (CHI 700E) using a Pt working electrode (diameter 0.4
mm), a Pt wire counter electrode, and a Ag wire quasiꢀreference electrode in 0.1 M
Bu4NPF6/CH2Cl2. Prior to use, the working electrode was polished with 0.05 mg·mL−1
Al2O3 slurries and then cleaned by sonication in dilute CH3CH2OH and nanopure
water successively. The electrolyte solution was deaerated with ultraꢀhigh purity
nitrogen for 40 min and blanketed with a nitrogen atmosphere during the entire
experimental procedure.TGA was conducted by SDTQ600 (USA), ramp at 10 °C/min
to 800 °C.
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Preparation of the [Pd3Cl(PPh2)2(PPh3)3]+ clusters.
All the procedures were conducted at room temperature (rt, 21ꢀ24°C).
PdCl2 (1.42 g, 8 mmol) was dissolved in an aqueous solution of HCl acid (16
mmol, 2 equiv. per palladium), then the solution was diluted to 10 mL. An
aliquot of the above solution (0.6 mL, 0.48 mmol) was added into 10 mL THF,
followed by addition of triphenylphosphine (0.313 g, 1.2 mmol). After vigorous
stirring for 8 minutes, NaBH4 (0.08 g, 2.1 mmol, dissolved in 5 ml ethanol) was
added, and the reaction solution was kept under vigorous stirring for 60 minutes.
Then, centrifugation was performed to remove undissolved (excess)
triphenylphosphine, and the left solution was rotaevaporated to dryness in
vacuo. To exclude sodium salt, the solid was redissolved in 10 mL methylene
dichloride, and washed with water, then evaporated to dryness again. The
remaining solid was extracted by ethanol several times, and the ethanol solution
obtained was rotaevaporated to dryness. The solid was dissolved in minimum
ethanol, and the solution was added dropwise to hexane. The precipitate was
collected by centrifugation, and the product was Pd3Cl obtained.
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For crystallization, the cluster Pd3Cl was dissolved in ethanol (1.5ꢀ2 mL), and
NaSbF6 was added to the solution, followed by centrifugation. The solid was
collected and redissolved in methylene dichloride, followed by centrifugation
again. Finally, the solution was crystallized via layering with hexane at rt for
two days.
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Typical procedure for the Suzuki reaction
To a roundꢀbottom flask phenylboronic acid (0.2 g, 1.64 mmol) and
halogenobenzene (1.64 mmol), 2 mg (1.32×10ꢀ3 mmol) cluster, K2CO3 (0.453 g,
3.28 mmol) and 5 mL/5 mL H2O/isoꢀpropanol were successively added. After
stirring at r.t., the reaction was tracked by TLC analysis. The mixture was
rotaevaporated to dryness in vacuo and purified by silica gel column
chromatography (see text and Table 1).
NMR spectra, ESIꢀMS and MALDIꢀMS tracking of the Suzuki reaction: a
roundꢀbottom flask was charged with 0.0072 g (0.06 mmol) phenylboronic acid
and 0.006 ml (0.06 mmol) bromobenzene, 2 mg (1.32×10ꢀ3 mmol) cluster,
0.0164 g (0.12 mmol) K2CO3 and 3 ml CH2Cl2 (NMR tracking was carried out
for the reaction in CD2Cl2) or H2O/isoꢀpropanol with stirring at rt.
DFT calculation
The structures and energies were calculated using DFT as implemented in
GAUSSIAN 09.50 Geometry optimization and frequency analysis were
performed using the metahybrid functional of Tao, Perdew, Staroverov, and
Scuseria (TPSSh).51 Structural minima were verified by the absence of
imaginary vibrational modes. A modest 6ꢀ31G* basis set was used for all atoms
except Pd, for which the LANL2DZ basis set was used. In order to gain insight
into the nature of the bonding in Pd3 cluster, we apply a tool named Adaptive
Natural Density Partitioning (AdNDP)52 to obtain the patterns of chemical
bonding.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications
website at DOI: xxx
Additional observations and data (PDF)
Crystallographic information (CIF)
Crystallographic information (CIF)
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AUTHOR INFORMATION
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Corresponding Author
pengꢀli@ahu.edu.cn; zmz@ahu.edu.cn; sqwei@ustc.edu.cn
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Author Contributions
§These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support from NSFC 21571001, 21372006&U1532141, the Ministry
of Education, and the Education Department of Anhui Province, China, 211
Project of Anhui University is gratefully acknowledged.
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