NMR approach for the identification of dinuclear and mononuclear complexes: The first detection of [Pd(SPh)2(PPh3)2] and [Pd2(SPh)4(PPh3)2] - The intermediate complexes in the ca

Article abstract of DOI:10.1016/j.jorganchem.2010.10.012

In the present study we have analyzed the nature of palladium complexes in the catalytic system for selective carbon-sulfur bond formation via the addition of S-S and S-H bonds to alkynes. For the first time the mononuclear and dinuclear palladium complex

Full text of DOI:10.1016/j.jorganchem.2010.10.012

Journal of Organometallic Chemistry 696 (2011) 400e405
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
NMR approach for the identification of dinuclear and mononuclear complexes:
The first detection of [Pd(SPh)2(PPh3)2] and [Pd2(SPh)4(PPh3)2] e The
intermediate complexes in the catalytic carbonesulfur bond formation reaction
, ,
,
,
,
Valentine P. Ananikov a 1, Sergey S. Zalesskiy a 1, Vadim V. Kachala a 1, Irina P. Beletskaya b
*
**
a Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect 47, Moscow 119991, Russia
b Lomonosov Moscow State University, Chemistry Department, Vorob’evy gory, Moscow 119899, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present study we have analyzed the nature of palladium complexes in the catalytic system for
selective carbonesulfur bond formation via the addition of SeS and SeH bonds to alkynes. For the first
time the mononuclear and dinuclear palladium complexes were clearly detected by DOSY NMR under
the catalytic conditions. It was demonstrated that the concentration of these palladium complexes
strongly depends on the amount of phosphine ligand available under reaction conditions.
Ó 2010 Elsevier B.V. All rights reserved.
Received 2 August 2009
Received in revised form
1 October 2009
Accepted 5 October 2010
Available online 13 October 2010
Keywords:
Catalysis
Palladium complexes
DOSY NMR
Diffusion
Mechanism
1. Introduction
involvement of dinuclear and larger metal species in catalysis
[1e5]. Generation of dinuclear complexes may take place via
Transition metal catalysis has dramatically changed the face of
modern organic chemistry and fine organic synthesis. A number of
outstanding catalytic systems have been developed in recent years
for stereo-, regio-, chemo- and enantio selective transformations of
all classes of organic compounds. Amazingly, most of the catalytic
systems were discovered by variation and screening of catalysts
and ligands (sometimes by chance). Although this approach is still
widely utilized, on the moment it became clear that empirical
search should be accomplished by rational tuning of the catalytic
systems based on mechanistic knowledge. It is the progress in
understanding reaction mechanisms that will determine our
success in design of new catalytic systems in the nearest future.
Most of the classical mechanisms related to homogeneous
catalysis were assumed to involve mononuclear molecular
complexes of transition metals. However, recent mechanistic
studies have provided increasing number of evidence about
dissociation of one of the ligands and formation of coordination
vacancy, followed by coordination of a lone pair of heteroatom
(Scheme 1). Depending on the nature of the system, the reaction
may stop on the formation of complex with one bridging ligand (1).
If the second heteroatom is available for complexation, the complex
with two bridging ligands (2) is most likely to be obtained.
As representative examples related to catalysis, different dinu-
clear transition metal complexes are shown on Chart 1 [1e5]. Using
Pd catalysts dinuclear complexes with bridged halogens were
formed after oxidative addition of organic halides to the metal
center (A, Chart 1). The oxidative addition of organic halides is the
initial step of several cross-coupling reactions, Heck reaction, etc.
and possible involvement and the role of dinuclear complexes have
been considered [6]. Coordination of the lone pair of the nitrogen
atom may lead to dinuclear complex with the metal atoms con-
nected by the NR2 groups (B, Chart 1). Formation of such complexes
was anticipated in the catalytic hydroamination reactions [7].
Coordination of the oxygen group OR in bridging mode gave rise to
dinuclear species (C, Chart 1) important for hydroalkoxylation
reactions and Wacker-type chemistry [7].
* Corresponding author. Fax: þ7 499 1355328.
** Corresponding author. Fax: þ7 495 9393618.
E-mail addresses: val@ioc.ac.ru (V.P. Ananikov), beletska@org.chem.msu.ru
(I.P. Beletskaya).
Dinuclear, trinuclear and polynuclear complexes are of partic-
ular importance in the reactions involving metal complexes and
1
Fax: þ7 499 1355328.
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.012

V.P. Ananikov et al. / Journal of Organometallic Chemistry 696 (2011) 400e405
401
of Pd complexes in carbon-heteroatom bond formation through the
addition of X-H and X-X bonds to alkynes (X ¼ S, Se) has been
revealed [8,9].
L
L
X
L
X
X
X
L
+
+
LnM
LnM
LnM
MLn
MLn
MLn
LnM
MLn
-L
-L
X
X
X
X
1
2
As exemplified for the series of chalcogen derivatives of Pd and
Pt, 31P NMR chemical shifts showed minor changes in the
complexes of completely different types, with different metals and
bridging groups (Chart 2) [10e12]. The attempts to distinguish
a mixture of these complexes by 31P NMR of the phosphine ligands
were unsuccessful. Insufficient sensitivity of 31P NMR to the
structural changes was noticed with as small as 27.9e30.4 ppm
chemical shifts range reported for the palladium complexes and
confirmed again with rather small difference of 18.7e21.5 ppm for
the platinum analogs (Chart 2). In addition, 31P NMR chemical shifts
strongly depended on the solvent. For example, the 31P chemical
shift for the common contaminant - triphenylphosphine oxide
Ph3P]O - changed from 25.6 ppm in C6D6 to 28.6 ppm in CDCl3 and
showed a similar difference (Dd ¼ 3 ppm) compared to those
changes observed in the transition metal complexes (Chart 2) [13].
Not surprisingly, nowadays most of the complexes were first
isolated in the crystalline form and characterized by X-ray analysis,
then the data was extrapolated to solution [14]. This approach
limits the scope of the mechanistic study only to the certain types
of stable metal complexes available as high quality single crystals.
Moreover, this may provide wrong information about reaction
mechanism, since very often it is unclear whether polynuclear
complex was present in the original solution or it was formed
during crystallization and stabilized by interactions in the crystal.
Undoubtedly, an appropriate NMR technique is of vital importance
to monitor metal complexes directly in solution and to distinguish
mononuclear and dinuclear derivatives in situ.
Scheme 1. Formation of dinuclear complexes with one and two bridging ligands.
R2
N
R
O
R
L
X
X
L
R'
X = I, Br, Cl
M
M
LnM
MLn
LnM
M
M = Pd, Ni, Rh, Ir, Ru, etc.
R
N
O
R
L
R2
A
B
C
R
X
R
X
R
RX
L
L
RX
X
L
X = S, Se, Te
M = Ni, Pd, Pt
M
M
M
M
M
X
R
XR
L
X
R
X
R
XR
n
D
E
Chart 1. Representative examples of dinuclear and polynuclear metal complexes
involved in catalysis [1e9].
chalcogen derivatives (D and E, Chart 1) [8]. Strong binding of
chalcogen groups XR to the metal centers can link mononuclear
species into a polymeric chain (E, Chart 1). High stability of such
polymers was found as the main origin of catalyst poisoning by
chalcogen species, for example, by thiols RSH and disulfides RSSR
[9]. Splitting of the polymeric metal derivatives by phosphine
ligands to smaller size metal complexes was of principal impor-
tance for preventing poisoning and maintaining catalytic activity in
carbonesulfur bond formation [8,9].
It should be pointed out that, depending on the nature of
reaction, catalyst and reagents involved, dinuclear and polynuclear
metal complexes may act as intermediates of the catalytic cycles,
represent resting states, or block the catalytic cycle via formation of
stable unreactive species. In any case the structure and the role of
these di(poly)nuclear metal complexes should be clearly revealed
in details.
In spite of great mechanistic importance, there are no useful
methods available to distinguish mononuclear and dinuclear
complexes in solution in the spectral monitoring of catalytic reac-
tions. The NMR signals of peripheral groups are usually not sensi-
tive enough to the nature of the complex. Spectral data taken from
the bridging heteroatom could provide more information, but,
unfortunately, there are no suitable NMR active nuclei available for
high resolution measurements with X ¼ I, Br, Cl, O, S, etc. For other
nuclei, like 15N, the measurements are time consuming at natural
abundance and are hardly possible under catalytic conditions.
For our study we have chosen metal chalcogen derivatives with
2. Results and discussion
In the present study we have carried out diffusion ordered NMR
spectroscopy (DOSY) measurements to resolve the presence of
mononuclear and dinuclear Pd complexes directly in solution in the
catalytic reaction. DOSY is a well-known NMR technique, which has
proven the application in various chemical systems including
transition metal complexes [15,16]. The idea of DOSY NMR is to
differentiate the content of solution according to translational
diffusion coefficients. Diffusion coefficients are related to the speed
of molecular motions in liquids and depend on the size of dissolved
compounds. According to the StokeseEinstein equation, the diffu-
sion coefficient for a molecule is inversely proportional to the
hydrodynamic molecular radius [15]. Particularly, for the studied
system we can expect that mononuclear Pd complexes of smaller
size and weight should exhibit different diffusion coefficients as
compared to dinuclear complexes, which possess larger size and
weight.
phosphine ligands as
a suitable model system. Such metal
complexes are very well characterized by X-ray analysis in the solid
state and by NMR studies in solution [10e12]. The catalytic activity
Pt complexes
Th
Ph3P
PhSe
Ph3P
ThSe
Ph3P
Ph3P
SePh
SePh
SePh
PPh3
SeTh
PPh3
Ph3P
ThSe
Se
SeTh
PPh3
Pt
Pt
Pt
Pt
Pt
Se
Th
18.7 - 19.1 ppm
20.8 ppm
21.5 ppm
20.4 ppm
Th = 2-thienyl
Pd complexes
Ph
S
Ph
Th
Se
R
S
S
Ph3P
PhS
Ph3P
PhSe
Ph3P
SPh
PPh3
Se
SePh
PPh3
SeTh
PPh3
S
S
Ph3P
R
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
Pd
S
Se
ThSe
Se
PPh3
Ph
Ph
Th
30.4 ppm
27.9 ppm
29.3 ppm
28.5 ppm
Chart 2. 3P NMR chemical shifts of the PPh3 ligand coordinated to mononuclear and dinuclear chalcogenide complexes of platinum and palladium [10,11].

402
V.P. Ananikov et al. / Journal of Organometallic Chemistry 696 (2011) 400e405
PhS
P(Oi-Pr)3
PhS
SPh
Ph2S2
P(Oi-Pr)3
Pd(P(Oi-Pr)3)4
Pd2dba3
Pd
+
Pd
+
-P(Oi-Pr)3
(i-PrO)3P
(i-PrO)3P
P(Oi-Pr)3
SPh
3
3
cis-
trans-
Ph
S
Ph
S
PhS
P(Oi-Pr)3
PhS
SPh
+
Pd
Pd
Pd
Pd
(i-PrO)3P
SPh
(i-PrO)3P
P(Oi-Pr)3
S
S
Ph
trans-
Ph
cis-
4
4
Scheme 2. Oxidative addition of Ph2S2 in the system Pd/P(Oi-Pr)3 (dba ¼ dibenzylideneacetone).
First, we have defined the model system to examine reliability of
data is very useful since signals belonging to the compounds of
similar size appear aligned with the same diffusion coefficient
value. Particularly, the signals of both mononuclear complexes
were aligned on the same horizontal line of the DOSY spectrum, as
well as the signals corresponding to dinuclear complexes were
aligned on another horizontal line. The mononuclear (3) and
dinuclear (4) complexes were clearly resolved in the diffusion NMR
study. Larger self-diffusion coefficient 5.5 ꢀ 0.5 ꢁ 10ꢂ10 m2 sꢂ1 was
measured for the smaller complexes 3 (MW ¼ 741 Da), while larger
complexes 4 (MW ¼ 1066 Da) possessed smaller self-diffusion
DOSY NMR to distinguish between the mononuclear and dinuclear
Pd complexes with sulfur ligands in solution. For this purpose we
have studied oxidative addition of diphenyl disulfide to Pd(P(Oi-
Pr)3)4 species generated from Pd2dba3 and the P(Oi-Pr)3 ligand
(Scheme 2). The choice of ligand was crucial for the model system.
With tri(isopropyl)phosphite both the mononuclear and dinuclear
complexes 3 and 4 were isolated and characterized by X-Ray
analysis, and 1H and 31P{1H} NMR signals were assigned for trans-3,
cis-3, trans-4 and cis-4 [17].
The 31P{1H} NMR spectrum of the studied mixture (Fig. 1A)
coefficient 4.5 ꢀ 0.5 ꢁ 10‑10 m2 sꢂ1
.
contained four signals of the complexes trans-3 (
d
¼ 102.7 ppm),
After this successful application of DOSY NMR to the known
model system we have studied the system with PPh3 ligand, which
may contain complexes 5 and 6 (Chart 3). The 1H NMR spectrum
can not be used to distinguish the nature of palladium complexes in
this mixture, neither 31P{1H} NMR spectrum (Fig. 2A) can provide
the required information. In the X-Ray structure analysis the Pd
complex 5 has never been obtained and structurally characterized,
only corresponding dinuclear species 6 have been isolated [10,11].
The possibility of formation of mononuclear species in the system
remained a puzzling mechanistic question, raised in several studies
related to catalysis (see reviews [9]).
cis-3 110.5 ppm), trans-4 98.8 ppm) and cis-4
(
d
¼
(
d
¼
(
d
¼ 101.2 ppm). High intensity resonances correspond to ther-
modynamically more stable trans-isomers.
This sample was a subject of DOSY NMR analysis with pulsed
field gradient strength varied from 5.5 G/cm to 662.5 G/cm in 128
steps. The experiment was carried out in 31P nuclei detection with
the delays
D
¼ 20 ms and ¼ 1.6 ms set in the pulse sequence. The
d
resulting 2D DOSY data showed conventional 31P NMR spectrum in
one dimension and the diffusion spectrum was displayed in the
second dimension (Fig. 1A and B). The DOSY way to edit diffusion
In the DOSY NMR spectrum we have clearly resolved two
patterns of the signals belonging to mononuclear 5 (MW ¼ 849 Da)
and dinuclear 6 (MW ¼ 1174 Da) complexes with self-diffusion
coefficients 6.1 ꢀ 0.5 ꢁ 10ꢂ10 m2
s
ꢂ1 and 4.9 ꢀ 0.5 ꢁ 10ꢂ10 m2 sꢂ1
,
respectively (Fig. 2B). Larger self-diffusion coefficient of
9.1 ꢀ 0.5 ꢁ 10ꢂ10 m2 sꢂ1 was measured for the O]PPh3
(MW ¼ 278 Da). Although cis-5, trans-5 and O]PPh3 showed close
31P NMR shifts, they were clearly resolved on the diffusion axis
(Fig. 2B) [19].
In the present study for the first time we have revealed the
presence of mononuclear [Pd(SPh)2(PPh3)2] complexes, and have
determined their 31P NMR chemical shifts
d
¼ 26.2 and 25.5 ppm. In
a similar manner as discussed above, cis- and trans- complexes
were observed in each case and we can propose that more intense
signals correspond to thermodynamically more stable trans-
isomers.
With the characteristic NMR parameters at hands we have
studied the catalytic reaction of Ph2S2 addition to the triple bond of
alkynes, catalyzed by Pd/PPh3 system (Scheme 3). The reaction is
unique due to exceptional stereoselectivity of the transformation
(Z/E > 99/1) and high product yields (>90%). It was proposed that
[Pdn(SPh)2n(PPh3)m] complexes formed after oxidative addition of
the SeS bond to zerovalent palladium are the intermediates of the
catalytic reaction, however, the nature of these metal species
Fig. 1. The 31P{1H} NMR spectrum (A) and DOSY NMR spectrum (B) of the mixture of
complexes 3 and 4 (C6D6; 400 MHz 1H, 160 MHz 31P) [18].
Ph
S
Ph
S
PhS
PPh3
SPh
PhS
PPh3
SPh
PhS
SPh
PhS
SPh
Pd
Pd
Pd
Pd
Pd
Pd
Ph3P
Ph3P
Ph3P
PPh3
S
S
Ph3P
PPh3
Ph
trans-6
Ph
cis-6
trans-5
cis-5
Chart 3. Mononuclear and dinuclear Pd complexes with triphenylphosphine ligand.

V.P. Ananikov et al. / Journal of Organometallic Chemistry 696 (2011) 400e405
403
Fig. 2. The 31P{1H} NMR spectrum (A) and DOSY NMR spectrum (B) of the mixture of
complexes 5 and 6 (C6D6; 600 MHz 1H, 240 MHz 31P); the signal marked by the asterisk
corresponds to triphenylphosphine oxide.
Fig. 3. The plot of reaction yield in the catalytic Ph2S2 addition to hexyne-1 at different
Pd:PPh3 ratios measured after a short period of time (1 h; 80 ꢃC).
R
[Pd]/PPh3
toluene, 80oC
R
+ PhS-SPh
PhS
SPh
90-99%, Z/ E > 99:1
R = CH2OH, CH2CH2OH, CH2NMe2,
nBu, C6H10(OH)
Scheme 3. The catalytic reaction of stereoselective Ph2S2 addition to alkynes in the
Pd/PPh3 system [20].
remained unclear [9]. Involvement of the [Pdn(ER)2n(PPh3)m]
complexes in the catalytic RE-H bond addition to alkynes in the
presence of phosphine ligands was also discussed for E ¼ S and Se
[8b,9].
We have measured 31P{1H} NMR spectrum in the conditions of
catalytic reaction and have detected the signals of mononuclear
complexes 5 with the chemical shifts
dinuclear complex 6 with the chemical shifts
d
¼ 26.2, 25.5 ppm and
d
¼ 29.5, 30.9 ppm.
The performance of the catalytic reactions was evaluated at
different Pd: phosphine ligand ratios. The plot reflecting the yield of
the reaction after 1 h of heating at 80 ꢃC showed an increase from
10% at 1:1 ratio to 33% at 1:8 ratio, with the further increase of the
yield to 46% at 1:32 ratio (Fig. 3). Clearly, an excess of the ligand
facilitated product formations and enhanced the performance of
the catalytic reaction [8,9]. After 4 h of heating at 80 ꢃC the reaction
reached 86%, 63% and 29% yields at 1:4, 1:2 and 1:1 metal:ligand
ratios, respectively (>95% conversion for the other ratios). Inter-
esting to point out that at 1:64 ratio the large excess of the ligand
decreased the performance of the catalytic system (Fig. 3).
With the NMR spectroscopic study we have analyzed the nature
of the catalytic system and have determined the amounts of
mononuclear and dinuclear palladium complexes under catalytic
conditions (Fig. 4). The amount of dinuclear complexes increased at
1:1e1:12 metal:ligand ratios due to suppression of polymerization
of palladium sulfur species. Indeed, the systems with 1:1 and 1:2
ratios were heterogeneous and contained insoluble [Pd(SPh)2]n
species, the system at 1:4 ratio was nearly homogeneous with
minor amount of precipitate, and the systems with 1:8e1:64 ratios
were completely homogeneous. The NMR study has shown that
splitting of the metal polymers resulted in formation of soluble
metal complexes of smaller size, simultaneously, an increase of the
reaction yield was observed.
Fig. 4. The plot of amounts of mononuclear (5) and dinuclear (6) complexes in the
catalytic reaction of Ph2S2 addition to hexyne-1 at different Pd:PPh3 ratios (measured
after 1 h at 80 ꢃC).
Interesting to point out that the curve corresponding to dinu-
clear complexes showed a maximum at 1:12 ratio, while the
amount of mononuclear complexes increased monotonously
(Fig. 4). Obviously, at some point (>1:12 ratio) the amount of
dinuclear complexes was decreased in favor of formation of
mononuclear species. Thus, a series of equilibriums were pre-
sented in the studied catalytic system and the overall composition
of the mixture depended on the amount of phosphine ligand
(Scheme 4).
Comparing the performance of the catalytic reaction (Fig. 3) and
the amount of metal species (Fig. 4) at different Pd:PPh3 ratios we
may point out that reaction yield did not increase monotonously as
the curve for the mononuclear species. The maximum values were
observed at different Pd:PPh3 ratios - 1:32 (Fig. 3) and 1:12 (Fig. 4).
Thus, the contributions of both types of intermediate metal
complexes to the product formation in the studied catalytic reac-
tion should be taken into account. Of course, this is rather
preliminary assumption, which should be further rationalized and
studied in more details.

404
V.P. Ananikov et al. / Journal of Organometallic Chemistry 696 (2011) 400e405
Scheme 4. Different palladium species in the studied system.
To conclude, the present study has demonstrated that mono-
3.3. Sample preparation and DOSY analysis of Pd/PPh3 system (5
and 6)
nuclear and dinuclear complexes can be clearly distinguished in
solution by DOSY NMR. The study of the Pd sulfur complexes with
the triphenylphosphine ligand resolved both complexes in situ in
the reaction mixture. The mechanism of the catalytic reaction
should be reconsidered to take into account possible formation of
the mononuclear and dinuclear metal complexes and to find out
their contribution into the product formation.
Pd2dba3 (30.0 mg, 0.029 mmol), Ph2S2 (12.7 mg, 0.058 mmol),
PPh3 (7.6 mg, 0.029 mmol) and 0.6 mL of C6D6 were added sequen-
tially into an NMR tube. After shaking for 15e20 min a brown
suspension was formed. 31P NMR spectrum of the resulting suspen-
sion showed formation of palladium complexes. Sample was filtered
through celite into a Shigemi tube, total sample height was 1 cm.
The 2D DOSY spectrum was acquired on a Bruker Avance II 600
spectrometer equipped with a GAB gradient amplifier using
a 5-mm BBO Bruker gradient probe. Temperature was stabilized at
303 K, VTU air flow was set to 670 l/h. An experiment was per-
formed using a standard stegp1s pulse program (stimulated
gradient spin-echo with one spoil gradient). Durations of gradient
3. Experimental part
3.1. General procedures
All experiments were carried out under an argon atmosphere
with dry and degassed solvents. All reagents were purchased from
commercial sources (Acros, Aldrich), unless otherwise stated, and
used as received after purity control with NMR spectroscopy. Tris
(dibenzylideneacetone)dipalladium was prepared as reported
before [21]. Deuterated solvents were stored over molecular sieves
(4 Å, 8e12 mesh). 1H and 31P spectra were acquired on a Bruker
Avance II 600 spectrometer equipped with a 5 mm BBO probe.
pulse (d) and diffusion time (D) were 6 and 100 ms, respectively.
Sine gradient shape was applied. Gradient strength was calibrated
relative to self-diffusion coefficient of HDO (1.9$10ꢂ9 m2/s) for 1%
H2O in D2O sample at 298 K and was 6.50 G/cm A for BBO probe.
Data acquisition was established in 256 steps with linear gradient
evolution from 0.91 to 43.23 G/cm (2e95% of maximum gradient
coil current) with 32 scans per step. 2D DOSY spectra were gener-
ated using Bruker Topspin 2.1 software.
Durations of 90ꢃ pulses for 1H and 31P were 10.5 and 19.0
ms,
respectively. Chemical shifts are reported in parts per million
relative toTMS (1H) or 80% H3PO4/H2O (31P) as internal and external
standards, respectively. First, DOSY experiment was carried out
with dedicated gradient probe, followed by the measurements of
the same samples with routine gradient probe. The results
measured on both probes were in total agreement with each other
and confirmed reliability of routine hardware for these measure-
ments. The procedure for the measurements with dedicated
diffusion probe and routine probe are described below for phos-
phite and phosphine systems, respectively.
3.4. Catalytic addition of diphenyl disulfide to 1-heptyne with
various PPh3:Pd ratios
Pd2dba3 (15.5 mg, 0.015 mmol) was placed into an NMR tube.
A corresponding amount of PPh3 (3.9 mg; 7.9 mg; 15.7 mg;
31.5 mg; 47.2 mg; 62.9 mg 125.9 mg and 251.8 mg for the
experiments with 1:1; 2:1; 4:1; 8:1; 12:1; 16:1; 32:1 and 64:1
PPh3:[Pd] ratios, respectively) followed by Ph2S2 (109.2 mg,
0.5 mmol) and 1-heptyne (48.1 mg, 0.5 mmol) were added.
Reagents were dissolved in 0.6 mL of C6D6 and the sample was
heated at 80 ꢃC. 1H and 31P NMR spectra were recorded after 1 h,
4 h, and 8 h of heating. In certain cases a strong overlap of
mononuclear complexes signals and signal of triphenylphosphine
oxide was observed. In such cases, the peaks were resolved by
addition of a small amount of CDCl3 (ca. 0.2 mL) to the sample. The
addition of CDCl3 induced downfield shift of PPh3O resonance,
while the signals of palladium complexes did not change notice-
ably (known chemical shifts of triphenylphosphine oxide in C6D6
and CDCl3 are 25.6 and 28.6 ppm, respectively).
3.2. Sample preparation and DOSY analysis of the Pd/P(Oi-Pr)3
system (3 and 4)
Pd2dba3 (20.0 mg, 0.019 mmol), Ph2S2 (8.4 mg, 0.038 mmol), P
(Oi-Pr)3 (4.0 mg, 0.019 mmol) and 0.6 mL of C6D6 were added
sequentially into an NMR tube. After shaking for 15e20 min
a brown suspension was formed. 31P NMR spectrum of the resulting
suspension showed formation of mono- and dinuclear complexes 3
and 4 identified according to the published data. Sample was
filtered through celite into a Shigemi tube, total sample height
was 1 cm.
Acknowledgement
The 2D DOSY spectrum was acquired on a Bruker Avance 400
spectrometer using Diff60 gradient probe with 31P selective
insert. Sample was not spinning and the temperature was regu-
lated at 303 K, VTU air flow was set to 670 l/h. An experiment was
The research was supported in part by the Russian Foundation
for Basic Research (Project No. 07-03-00851), Program No 1 of
Division of Chemistry and Material Sciences of RAS and Program No
20 of RAS.
performed using
program. Durations of the gradient pulse (
) were 1.6 and 20 ms, respectively. Sine gradient shape was
a
basic diffse (gradient spin-echo) pulse
d
) and diffusion time
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gradient evolution from 5.5 to 662.5 G/cm with 88 scans for each
step. 2D DOSY spectra were generated using Bruker Topspin 2.1
software.
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[14] Electrospray ionization mass spectrometry (ESI-MS) can be also of help for
identification of metal complexes in solution; for the known advantages and
limitations see the reviews: (a) Electrospray ionization mass spectrometry
(ESI-MS) can be also of help for identification of metal complexes in solu-
tion; for the known advantages and limitations see the reviews: M.N. Eberlin
Eur. J. Mass Spectrom. 13 (2007) 19;
̃
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[7] Selected representative examples: (a) Selected representative examples:
A. Dzudza, T. Marks Chem. Eur. J. 16 (2010) 3403;
(b) M.S. Hadfield, A.-L. Lee, Org. Lett. 12 (2010) 484;
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(f) C. Bruneau, P.H. Dixneuf, Angew. Chem. Int. Ed. 45 (2006) 2176;
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Chem. 71 (2006) 4270;
(h) K. Alex, A. Tillack, N. Schwarz, M. Beller, ChemSusChem 1 (2008) 333;
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[18] For the O]P(Oi-Pr)3 (MW
¼
208 Da) the 31P NMR chemical shift of
139.8 ppm and self-diffusion coefficient of 10.3 ꢀ 0.5 ꢁ 10‑10 m2 sꢂ1 were
observed (corresponding signal is omitted on the 2D spectrum).
[19] Uncoordinated ligand or ligand oxide may serve as an internal reference in
DOSY experiment, if a sharp peak on diffusion axis was obtained.
[20] For the experimental conditions of catalytic reactions see Refs. [9a,10e,f].
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N.Y.-Toronto, 1985.
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